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5 Testing and Meaning of Test Data Introduction This chapter will present examples of test- ing and their behavior that influence deci- sions to be made by designers when analyzing data. Testing methods have been prepared and used for over 2,000 years with probably more in the past century that in all the past. Examining and properly applying property test data of plastics is important to designers. The technology of manufacturing the same basic type or grade of plastics (as with steel and other materials) by different suppliers may not provide the same results. In fact a supplier furnishing their material under an initial batch number could differ when the next batch is delivered and in turn could ef- fect the performance of your product. Taking into account manufacturing tolerances of the plastic, plus variables of equipment and pro- cedure, it becomes apparent that checking several types of materials from the same or from different sources is an important part of material selection and in turn their use. Based on past performances it has been proven that the so-called interchangeable grades of plastics have to be evaluated care- fully by the designer as to their affect on the performance of a product. An important consideration to include as far as equivalent grade of material is concerned is its process- ing characteristics. There can be large differ- ences in properties of a product and test data if the manufacturing features vary from grade to grade or batch to batch. This situation in most cases does not effect the product per- formances but could require changing equip- ment process controls to maximize the prod- uct performances and minimize cost. Overall Responsibility Should the designer have this type of re- sponsibility. That person gets the credit for a successful product. If the product fails in ser- vice regardless of the reason the responsibil- ity should be the designer who did not meet the product’s entire requirement. In speci- fying the specific plastic and/or process to use, their test requirements should have been more complete and/or meet closer require- ments. This action would include factors such as the limits on the variabilities of the de- sign configuration (dimensions, etc.), plastic, and process. As an example, quality control (QC) on the plastic and fabricated product is required even if all that is required is limit- ing (*) the weigh the material and fabricated product. 5 Testing and Meaning of Test Data 297 The problem of acquiring complete knowl- edge and control of candidate material grades should be resolved in cooperation with the raw material suppliers. Having the material supplier meet specific performance require- ments is important. In turn it may be nec- essary for testing incoming material even if the supplier provides data you requested. It should be recognized that selection of the favorable material is one of the three basic elements in producing a successful product namely design, material selection, and con- version into a finished product. How to re- solve processing problems uses the same ap- proach as reviewed with material control. So one can say the designer should not carry all these responsibilities. True but if the responsibility is not delegated to one person, you allow for problems to develop. Perhaps a certain qualified manager (the designer’s boss) should have the responsibility. For a small operation it is usually its owner who may or may not properly delegate specific responsibilities. Destructive and Nondestructive Testing Testing yields basic information about any materials (plastics, steels, etc.), its properties relative to another material, its quality with reference to standards or material inspec- tions, and can be applied to designing with plastics. Examples of static and dynamic tests are reviewed in Chapter 2. There are destructive and nondestructive tests (NDTs) (2). Most important, they are essential for determining the performance of plastic materials to be processed and of the finished fabricated products. Testing refers to the determination by technical means prop- erties and performances. This action, when possible, should involve application of es- tablished scientific principles and procedures. It requires specifying what requirements are to be met. There are many different tests (thousands) that can be conducted that re- late to practically any material or product requirement. Usually only a few will be ap- plicable to meet your specific application. Ex- amples of these tests will be presented. In the familiar form of testing known as destructive testing, the original configuration of a test specimen and/or product is changed, distorted, or usually destroyed. The test pro- vides information such as the amount of force that the material can withstand before it ex- ceeds its elastic limit and permanently dis- torts (yield strength) or the amount of force needed to break it. These data are quanti- tative and can be used to design structural products that would withstand a certain load, heavy traffic usage, etc. NDT examines material without impairing its ultimate usefulness. It does not distort the specimen and provides useful data. NDT allows suppositions about the shape, severity, extent, distribution, and location of such internal and subsurface residual stresses; defects such as voids, shrinkages, and cracks; and others. Test methods include acoustic emission, radiography, IR spectroscopy, x-ray spectroscopy, magnetic resonance spectro- scopy, ultrasonic, liquid penetrant, photo- elastic stress analysis, vision system, holog- raphy, electrical analysis, magnetic flux field, manual tapping, microwave, and bire- fringence (Fig. 5-1). There is usually more than one test method to determine a performance because each test has its own behavior and meaning. As an example there are different tests used to determine the abrasion resistance of materi- als. There is the popular Taber abrasion test. It determines the weight loss of a plastic or other material after it is subjected to abrasion for a prescribed number of the abrader disk rotations (usually 1000). The abrader consists of an idling abrasive speed controlled rotat- ing wheel with the load applied to the wheel. The abrasive action on the circular specimen is subjected to a rotary motion. Other abrasion tests have other type ac- tions such as back and forth motion, one di- rection, etc. These different tests provide dif- ferent results that can have certain relations to the performance of a product that will be subjected to abrasion in service. A method of evaluating the adhesive bond to a plastic coating substrate is a tape test. Pressure-sensitive adhesive tape is applied to an area of the adhesive coating, which is 298 5 Testing and Meaning of Test Data Sample log-in terminal Program development terminal Gas , . . chromatography Atomic spectroscopy Manual data entry LIMW2000 Analytical disciplines data stations Report management terminal Program development terminal UV/VIS spectroscopy F luorescence spectroscopy 1600 Fl Infrared spectroscopy Elemental analysis 1 4 Manual data entry Fig. 5-1 Examples of plastics evaluation in a computer-aided chemistry laboratory. sometimes crosshatched with scratch lines. Adhesion is considered to be adequate if the tape pulls off no coating when it is removed. A bearing strength test method is used for determining the behavior of materials sub- jected to edgewise loads such as those ap- plied to mechanical fasteners (plastics, etc.). For plastics, one of the tests uses a flat rectan- gular specimen with a bearing hole centrally located near one end. It is loaded gradually either in tension or compression. Load and longitudinal deformation of the hole are mea- sured frequently or continuously to rupture with resulting data plotted as a stress-strain curve. For this purpose, strain is calculated by dividing change in the hole diameter in the direction of loading by the original hole di- ameter. Bearing stress is calculated by divid- ing the load by the bearing area being equal to the product of the original hole diameter and specimen thickness. Test results are influ- enced by the edge-distance ratio, that is the ratio between the distance from the center of the hole to the nearest edge of the spec- imen in the longitudinal direction and hole diameter. A different type of evaluation is the po- tential of a material (plastic, etc.) that comes in contact with a medical patient to cause or incite the growth of malignant cells (that is, its carcinogenicity). It is among the issues ad- dressed in the set of biocompatibility stan- dards and tests developed as part 3 of ISO- 10993 standard that pertain to genotoxicity, carcinogenicity, and reproductive toxicity. It describes carcinogenicity testing as a means 5 Testing and Meaning of Test Data 299 to determine the tumorigenic potential of de- vices, materials, and/or extracts to either a single or multiple exposures over a period simulating the total life span of the devices. The circumstance under which such an inves- tigation may be required is given in part 1 of Interesting that in this highly scientific world, there are what appear to be non- scientific test methods. As an example early during the 1930s, the US navy in Dalgren, VA developed a very successful and useful air- craft canopy “chicken” impact test called the Dalgren test. This test continues to be used providing the required test results to ensure the proper performance of a canopy in ser- vice. Basically a 4 lb (2 kg) chicken is fired out of a cannon-like device and is used to evaluate the impact damage on aircraft win- dows. Your author attempted to develop a replacement, highly scientific test, to replace the Dalgren test without success other than making one more intricate and costly to con- duct that provided the same results. In order to determine the strength and en- durance of a material under stress, it is neces- sary to characterize its mechanical behavior. Moduli, strain, strength, toughness, etc. can be measured microscopically in addition to conventional testing methods. These param- eters are useful for material selection and de- sign. They have to be understood as to ap- plying their mechanisms of deformation and fracture because of the viscoelastic behavior of plastics (Chapter 2). The fracture behavior of materials, especially microscopically brit- tle materials, is governed by the microscopic mechanisms operating in a heterogeneous zone at the crack tip or stress raising flow. In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measure- ments in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are im- portant microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the forma- tion of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction ISO-10993. techniques, scanning electron microscope, and transmission electron microscopy. Conditioning procedures of test specimens and products are important in order to obtain reliable, comparable, and repeatable data within the same or different testing labora- tories. Procedures are described in various specifications or standards such as having a standard laboratory atmosphere [50 f 2% relative humidity, 73.4 f 13°F (23 f l”C)] with adequate air circulation around all spec- imens. The reason for this type or other con- ditioning is due to the fact the temperature and moisture content of plastics can affect different properties. Testing and Classification Properties of plastics such as physical, me- chanical, and chemical are governed by their molecular weight, molecular weight distribu- tion, molecular structure, and other molecu- lar parameters (Chapters 2 and 8); also the additives, fillers, and reinforcements that en- hance certain processing and/or performance characteristics. Properties are also effected by their previous history (includes recycled plastics), since the transformation of plastic materials into products is through the appli- cation of heat and pressure involving many different fabricating processes. Thus, varia- tions in properties of products can occur even when the same plastic and processing equip- ment are used. Conducting tests such as those related to molecular characteristics provides a means of classifying them based on test results (2). Testing and Quality Control Testing and QC are discussed but often the least understood. Usually it involves the inspection of materials and products as they complete different phases of processing. Products that are within specifications pro- ceed, while those that are out of specifica- tion are either repaired or scrapped. Possibly the workers who made the out-of-spec prod- ucts are notified so “they” can correct “their” mistake. 300 5 Testing and Meaning of Test Data The approach just outlined is after-the-fact approach to QC; all defects caught in this manner are already present in the product being processed. This type of QC will usually catch defects and is necessary, but it does lit- tle to correct the basic problem(s) in produc- tion. One of the problems,with add-on QC of this type is that it constitutes one of the least cost-effective ways of obtaining quality products. Quality must be built into a prod- uct from the beginning of the design that fol- lows the FALL0 approach (Fig. 1-3); it can- not be inspected into the process. The target is to control quality before a product becomes defective. Testing and People Personnel or operators involved in testing from raw materials to the end of the fabricat- ing line develop capability via proper training and experience. Experience and/or develop- ing the proper knowledge are required to en- sure that the correct test procedure is being conducted and test results are accurate and not interrupted incorrectly. At times, with new problems developing on-line, different tests are required that may be available or have to be developed. Unfortunately a great deal of “reinventing the wheel” can easily oc- cur so someone should have the responsibil- ity to be up to date on what is available. Another unfortunate or fortunate situa- tion exists that a very viable test was at one time developed and used within the industry. In time it was changed many times by dif- ferent companies and organizations (ASTM, ISO, etc.) to meet new industries needs con- cerning specific requirements. One studying the potential of using that particular test may not have the access to the basic test that prob- ably is all that is required. Basic vs. Complex Test Choosing and testing a plastic when only a few existed that could be used for spe- cific products would prove relatively simple if the selection were limited, but the variety of plastics has proliferated. Today’s plastics are also more complex, complicating not only the choice but also the necessary tests. Fillers and additives can drastically change the plas- tic’s basic characteristics, blurring the line be- tween commodity and engineering plastics. Entirely new plastics have been introduced with esoteric molecular structures. There- fore, plastic suppliers now have many more sophisticated tests to determine which plas- tic best suits a product design or fabricating process. For the product designer, however, a sim- ple basic test, such as a tensile test, will help determine which plastic is best to meet the performance requirements of a product. At times, a complex test may be required. The test or tests to be used will depend on the product’s performance requirements. To ensure quality control material suppli- ers and developers routinely measure such complex properties as molecular weight and its distribution, crystallinity and crystalline lattice geometry, and detailed fracture char- acteristics (Chapter 6). They use complex, specialized tests such as gel permeation chromatography (2, 3), wide- and narrow- angle X-ray diffraction, scanning electron mi- croscopy, and high-temperature pressurized solvent reaction tests to develop new poly- mers and plastics applications. Specification and Standard The industry specifications and standards are regularly updated to aid designers and processors in controlling quality and to meet safety requirements, and thus they will prove useful to anyone who must choose tests and QC procedures. For example, the ASTM, UL, ISO, and DIN (see below) tests are among the most popular and important ones. Or- ganizations involved directly or indirectly in preparing or coordinating specifications, reg- ulations, and standards include the following: ASTM. American Society for Testing and UL. Underwriters Laboratories. ISO. International Organization for Stan- Materials. dardization. 5 Testing and Meaning of Test Data 301 DIN. Deutsches Instut, Normung. ACS. American Chemical Society. AMs. Aerospace Material Specification. ANSI. American National Standards ASCE. American Society of Chemical ASM. American Society of Metals. ASME. American Society of Mechanical AWS. American Welding Society. BMI. Battele Memorial Institute. BSI. British Standards Institute. CPSC. Consumer Product Safety Commis- CSA. Canadian Standards Association. DOD. Department of Defense. DODISS. Department of Defense Index & Specifications & Standards. DOT. Department of Transportation. EIA. Electronic Industry Association. EPA. Environmental Protection Agency. FMRC. Factory Mutual Research Corpo- FDA. Food and Drug Administration. FMVSS. Federal Motor Vehicle Safety FTC. Federal Trade Commission. JAPMO. International Association of IEC. International Electrotechnical Com- IEEE. Institute of Electrical and Elec- IFI. Industrial Fasteners Institute. IPC. Institute of Printed Circuits. ISA. Instrument Society of America. JIS. Japanese Industrial Standards. MIL-HDBK. Military Handbook. NADC. Naval Air Development. NACE. National Association of Corrosion NAHB. National Association of Home Institute. Engineers. Engineers. sion. ration. Standards. Plumbing & Mechanical Officials. mission. tronic Engineers. Engineers. Builders. NEMA. National Electrical Manufactur- ers’ Association. NFPA. National Fire Protection Associa- tion. NIST. National Institute of Standards & Technology (previously the National Bureau of Standards). NIOSH. National Institute for Occupa- tional Safety & Health. OSHA. Occupational Safety & Health Administration. PLASTEC. Plastics Technical Evaluation Center. PPI. Plastics Pipe Institute. QPL. Qualified Products List. SAE, Society of Automotive Engineers. SPE. Society of Plastics Engineers. SPI. Society of the Plastics Industry. STP. Special Technical Publications of the TAPPI. Technical Association of the Pulp ASTM. and Paper Industry. These test procedures and standards are subject to change, so it is essential to keep up to date if one has to comply with them. It may be possible to obtain the latest issue on a specific test (such as a simple tensile test or a molecular weight test) by contact- ing the organization that issued it. For exam- ple, the ASTM issues new annual standards that include all changes. Their Annual Books of ASTM Standards contain more than seven thousand standards published in sixty-six vol- umes that include different materials and products. There are four volumes specifically on plastics: 08.01-Plastics 1; 08.02-Plastics 11; 08.03-Plastics 111, and 08.04-Plastic Pipe and Building Products. Other volumes include in- formation on plastics and RPs. The complete ASTM index are listed under different cate- gories for the different products, types of tests (by environment, chemical resistance, etc.), statistical analyses of different test data, and so on (56,128,129). The ASTM issues other useful informa- tion for the designer that are included in its Special Technical Publications (STPs). Some examples of STPs are STP 701, Wear Tests 302 5 Testing and Meaning of Test Data for Plastics: Selection and Use, R. Bayer, ed., 1980,106 pages; STP 736, Physical Testing of Plastics, R. Evans, ed., 1981, 142 pages; STP 816, Behavior of Polymeric Materials in Fire, E. L. Schaffer, ed., 1983,121 pages; STP 846, Quality Assurance of Polymer Materials and Products, Green, Miller and lhrner, ed., 1985, 142 pages; and STP 936, Instrumental Impact Testing of Plastics and Composite Materials, s. L. Kessler, ed. 376 pages. works only with those plastics that can be at- tacked by a specific solvent. Immersed prod- ucts in a temperature controlled solvent for a specific time period identifies external and internal stresses. After longer time period's products could self-destruct. Stress and crack formations can be calibrated using different samples subjected to different loads. There is the brittle coating system applied on the surface of a product that identifies con- ditions such as stressed levels, cracks, etc. A lacquer coating is applied, usually sprayed, on the surface of the product. It provides experimental quantitative stress-strain mea- surement data. As the product is loaded in proportion to loads that would be encoun- tered in service, cracks begin to appear in the coating. The extent of cracks is noted for each increment of load. Prior to this action, the coating is calibrated by sprayingit on a simple beam and observing the strain at which cracks appear. This nondestructive test method can be used to aid in placing strain gauges for fur- ther measurements. Photoelastic measurement is a very useful method for identifying stress in transparent plastics. Quantitative stress measurement is possible with a polarimeter equipped with a calibrated compensator. It makes stresses vis- ible (Fig. 5-2). The optical property of the in- dex of refraction will change with the level of stress (or strain). When the photoelastic Stress Analysis There are different techniques to evaluate the quantitative stress level in prototype and production products. They can predict poten- tial problems. Included is the use of electri- cal resistance strain gauges bonded on the surface of the product. This popular method identifies external and internal stresses. Their various configurations are made to identify stresses in different directions. This technique has been extensively used for over a half century on very small to very large products such as toys to airplanes. There is the optical strain measurement system that is based on the principles of optical interference. It uses Moire, laser, or holographic interferometry (2,3320). Another very popular method is using sol- vents that actually attack the product. It Fig. 5-2 Photoelastic stress patterns for these two molded products during the same production run shows that the processing conditions have changed; right view relates to why the product fails in service. 5 Testing and Meaning of Test Data 303 material is stressed, the plastic becomes bire- fringent identifying the different levels of stress via color patterns (2,3,216). This photoelastic stress analysis is a tech- nique for the nondestructive determination of stress and strain components at any point in a stressed product by viewing a transparent plastic product. If not transparent, a plastic coating is used such as certain epoxy, polycar- bonate, or acrylic plastics. This test method measures residual strains using an automated electro-optical system. This concept has been known for over a century. Expressed as Brewster’s Constant law, it states that the index of refraction in a strained material becomes directional, and the change of the index is proportional to the magnitude of the stress (or strain) present. Therefore, a polarized beam in the clear plas- tic splits into two wave fronts in the X and Y directions that contain vibrations oriented along the directions of principal stresses. An analyzing filter passes only vibrations par- allel to its own transmitting plane (Chap- ter 4, TRANSPARENT AND OPTICAL PRODUCT, Polarized Lighting). The constructive and destructive interfer- ence creates the well known colorful pat- terns seen when stressed plastic are placed between two polarized filters. Some informa- tion about the stress gradients comes from observations of the patterns that provide qualitative analysis. The index of refraction in these directions is different and the differ- ence (or birefringence) is proportional to the stress level. When light that has experienced such re- tardation is viewed by a polarizer oriented at 90” to the original plane of light polariza- tion, the two components of the original light beam interfere with one another. This results in a change in color and intensity of the ob- served light. Observed colors correspond to different levels of retardation at that point, which in turn correspond to stress levels. To solve the measurement problem and ob- tain quantitative results (retardation, magni- tude of the residual strain, etc.), various tech- niques are used. An example is using a very simple device known as a wedge compensator (ASTM D 4093). It is placed between the light coming through the sample and the an- alyzing filter. The compensator reverses the retarding action of the induced strains in the plastic. Strain is calculated in the compen- sator by multiplying the birefringence (retar- dation per unit thickness) by a strain-optic re- sponse of the plastic being tested. Equal but opposite retardation is established and when superimposed on the retardation caused by the induced strain that restores a null. The in- tensity of the transmitted light becomes zero; revealed by a visible black fringe. A scale on the compensator supplies a quantitative read- ing of retardation. Flat surfaces that are not readily conducive to stress evaluation by other means can be tested by the nondestructive Moire fringe analysis. Measurements of strains both elas- tic and plastic as well as evaluation of high temperature effects on the part are possible. A transparent film with a grid of equidis- tant lines is initially deposed on the product. Deformation in the product due to stresses changes the spacing between the grid lines. When a test grid is superimposed on a non- deformed grid the superposition produces an optical effect known as Moire fringes. If the test product is not strained and the grids are precisely aligned, no fringes will be ob- served. Visible fringes can be precisely mea- sured to determine the degree of strain in a product. Flaw Detection Test methods are used to detect flaws. As an example when flaws or cracks “grow” in plastic, minute amounts of elastic energy are released and propagated in the material as an acoustic wave. A nondestructive acous- tic emission test has sensors placed on the surface that can detect these waves provid- ing information about location and rate of flaw growth. These principles form the basis for nondestructive test methods such as sonic testing. The nondestructive electrical eddy current test is a method in which eddy current flow is induced in the test object. Changes in the flow by variations in the test specimen 304 5 Testing and Meaning of Test Data are reflected into a nearby coil or coils for subsequent analysis by suitable instrumen- tation and techniques. With the nondestruc- tive electromagnetic test methods, different wavelength regions of electromagnetic en- ergy having frequencies less than those of visible light yields information regarding the quality of materials. A frequently used test has x-rays or gamma rays passing through a structure that absorbs distinctive flaws or inconsistencies in the ma- terial so that cracks, voids, porosity, dimen- sional changes, and inclusions can be viewed on the resulting radiograph. The nondestructive temperature differen- tial test by infrared is used. In this method, heat is applied to a product and the surface is scanned to determine the amount of infrared radiation is emitted. Heat may be applied continuously from a controlled source, or the product may be heated prior to inspection. The rate at which radiant energy is diffused or transmitted to the surface reveals defects within the product. Delaminations, unbonds, and voids are detected in this manner. This test is particularly useful with RPs. With nondestructive ultrasonic test back and forth scanning of a specimen is accom- plished with ultrasonics. This NDT can be used to find voids, delaminations, defects in fiber distribution, etc. In ultrasonic testing the sound waves from a high frequency ul- trasonic transducer are beamed into a mate- rial. Discontinuities in the material interrupt the sound beam and reflect the energy back to the transducer, providing data that can be used to detect and characterize flaws. It can locate internal flaws or structural discontinu- ities by the use of high frequency reflection or attenuation (ultrasonic beam). Of historical interest may be the use of a half dollar coin (the lighter weight 25e not as efficient). During the early 1940s the coin tap test was used very successfully in evaluating the performances of plastics, particularly RP primary aircraft structures. With a good ear (human hearing ear) there was (and is) a def- inite different sound between a satisfactory and unsatisfactory RP product. The unsatis- factory product would contain voids, delam- inations, defects in fiber distribution, etc. In the mean time the more elaborate and accu- rate sonic testing equipment were developed and used. There is the microtoming optical analy- sis test. In this procedure thin slices (under 30 Fm) of the plastics are cut from the prod- uct at any level and microscopically examined under polarized light transmitted through the sample. Rapid quality and failure analysis examination occurs by this technique. This technique has been used for many years in biological studies and by metallurgists to de- termine flaws, physical and mechanical prop- erties. Examination can be related to stress patterns, mechanical properties, etc. Limitation of Test When working with tests it becomes (logically) obvious in most cases that options exist as to how the test is to be conducted. This is true for the different materials or products (plastics, steels, etc.). Different sizes (thicknesses, widths, and/or lengths) and/or shapes of test specimens are usually required with plastics such as those rigid to flexible to brittle. Different speed of testing are used, and so on. The explanation in the test pro- vides a guideline as to what specific test condi- tions and specimen are used for the different types of materials. Another potential variable relates to specimen shrinkage, which results from the preparation of specimens. After be- ing processed or upon cooling, specimens can develop nonuniform shrinkage (sink marks). These test methods and the number and complexity of the variables present is related to the level of sophistication of the test. The combination that can influence test data de- fines the test limitations. Variables are found not only in test methods, but also in other non-test-related areas affecting data gener- ation. Examples include misinterpretation, misuse, or misapplication of the test or any of its integral parts (test setup, test procedure, reporting, etc.) contribute to their limitations (2 to 11,64,208). Test variables are a primary contributor to test limitations. These limitations are 5 Testing and Meaning of Test Data 305 determined by the variables within a standard test method (STMs from ASTM) and within statistical analysis. Variables are also associ- ated with materials (Chapter 6), and process- ability (Chapter 8). Adding to test limitations is a general lack of understanding regarding the language of testing. As usually reported, some people do not know the definition of a test, nor do they know the difference be- tween physical properties (length, tempera- ture, density, etc.) and mechanical proper- ties (tensile, flex, impact, etc.). Also, the term sample and specimen are sometimes used interchangeably that is not correct. Let it be known that one can say the tests are essential and all provide useful functions. However like design, material and process variables that exist, there are also testing vari- ables. In order to apply them to a design, the designer should understand their meanings and purpose for existing. The result will be the proper use of tests. Meaning of Data It is evident that in order to use the data from those you perform to those from mate- rial suppliers data sheets, it is imperative to have a thorough understanding of how the data are evolved and what caution is to be exercised when applying the data to product designs or other evaluations. They can eas- ily be interpreted incorrectly to mean some- thing one desire's in their design approach. Interpretations are always made and pro- vide excellent logical approaches to develop- ing a design however they require dedicated concentrations and relationships to the basic meaning of the test. In reality tests have only certain meanings. The following information provides exam- ples of guides as to the meaning to a test. Tests reviewed are based on ASTM standards. Very limited reviews are provided in each of the following examples regarding size of specimens, speed of testing, and so forth that are provided in the specific standards or spec- ifications. Information in the standards or specifications generally provide much more details on what they mean. When reviewing a test that is to be used in a purchase order, etc. make sure you have proper identification of the test including where required specimen configuration with test's date of issue. What is important to the designer is if the test relates to the product performance requirements. Physical Property Specijic Gravity/Density Specific gravity and density are frequently used interchangeably; however, there is a very slight difference in their meaning. Spe- cific gravity is the ratio of the weight of a given volume of material at 73.4"F (23°C) to that of an equal volume of water at the same temper- ature. Density is the weight per unit volume of material at 73.4"F (23°C) (Table 5-1). Wa- ter is the standard where it has a specific grav- ity of 1. The density of water is at 62.4 Ib/ft3. The discrepancy enters because water at 73.4"F (23°C) has a specific gravity slightly less than one. To convert density to spe- cific gravity, the following factor can be used (ASTM D 792): Density, g per cm3 = specific gravity x 0.99756 (5-1) To the designer, the specific gravity is use- ful in calculating strength-to-weight and cost- to-weight ratios, and as a means of identifying a material. Specific volume is a conversion of specific gravity into cubic inches per pound. Since the volume of material in a product is the first bit of information established after its shape is formulated, the specific volume is a conve- nient conversion factor for weight: Specific volume (in3/lb) = 27.7/specific gravity (5-2) Many different additives, fillers, and/or re- inforcements are used in plastic materials. The weight of the compounds change accord- ing to the amount included. Figure 5-3 pro- vides a guide to determining their specific gravities. 306 5 Testing and Meaning of Test Data Table 5-1 Specific gravity and density comparisons of different materials Specific Density, Materials Gravity 1b.lcubic in. Thermoplastics ABS Acetal Acrylic Cellulose Acetate Cellulose Acetate Cellulose Propionate Ethyl Cellulose Methyl Methacrylate Nylon, Glass-Filled Nylon Polycarbonate Polyethylene Polypropylene Polybutylene Polystyrene Polyimides PVC-Rigid Polyester Thermosets Alkyds, Glass-Filled Phenolic-G.P. Polyester, Glass-Filled Butyrate Rubber Metals Aluminum Brass-Yellow (#403) Steel-CR Alloy (Strip & Bar) Steel-Stainless 304 Magnesium AZ-91B Iron-Pig, Basic Zinc-S AE-903 SAE-309 (360) 1.06 1.43 1.19 1.27 1.19 1.21 1.10 1.20 1.40 1.12 1.20 0.94 0.90 0.91 1.07 1.43 1.20 1.31 2.10 1.40 2.00 1.25 2.64 8.50 7.85 7.92 1.81 7.10 6.60 0.0383 0.0516 0.0430 0.0458 0.0430 0.0437 0.0397 0.0433 0.0505 0.0404 0.0433 0.0339 0.0325 0.0329 0.0386 0.0516 0.0433 0.0473 0.0758 0.0505 0.0722 0.0451 0.0953 0.3070 0.2830 0.2860 0.0653 0.2560 0.2380 The number of grams per cubic centimeter is the same as the specific gravity. For example, if the specific gravity is 1.47, that substance has a density of 1.47 gms/cm3. Water Absorption The data should indicate the temperature and time of immersion and the percentage of weight gain of a test specimen. The same ap- plies to data at the saturation point of 73.4"F (23"C), and, if the material is usable at 212°F (lOO°C), also to saturation at this tempera- ture. Moisture or water absorption is an impor- tant design property. It is particularly signif- icant for a product that is used in conjunc- tion with other materials that call for fits and clearances along with other close tolerance dimensions. The moisture content of a plastic affects such conditions as electrical insulation resis- tance, dielectric losses, mechanical proper- ties, dimensions, and appearances. The effect on the properties due to moisture content de- pends largely on the type of exposure (by immersion in water or by exposure to high humidity), the shape of the product, and the inherent behavior properties of the plastic material. The ultimate proof for tolerance of moisture in a product has to be a prod- uct test under extreme conditions of usage in which critical dimensions and needed proper- ties are verified. Plastics with very low water- moisture absorption rates tend to have better dimensional stability. Water Vapor Transmission There are substantial differences in the rates at which water vapor and other gases can permeate different plastics. For instance, PE is a good barrier for moisture or water vapor, but other gases can permeate it rather readily. Nylon, on the other hand, is a poor barrier to water vapor but a good one to other vapors. The permeability of plastic films is re- ported in various units, often in grams or cu- bic centimeters of gas per 100 in.2 per mil of thickness (0.001 in.) of film per twenty-four hours. The transmission rates are influenced by such different factors, as pressure and tem- perature differentials on opposite sides of the film. The effectiveness of a vapor barrier can be rated in a term such as perms. An effective vapor barrier in buildings should have a rat- ing no greater than, say, 0.2 perm. A rating of one perm means that one ft2 of the barrier is penetrated by one gram of water vapor per hour under a pressure differential of one in. of mercury. One in. of mercury equals virtu- ally 0.5 psi; one gram is one seven-thousandth of a pound. 5 Testing and Meaning of Test Data - 4.0 4.0 -- 3.0 3.0 : 2.5 2.5: -: 2.0 2.0 307 .- :- -. 0.10 -. 0.09 -' 0.00 - 0.07 -- 0.3 - 0.06 0.5 -' 0.05 compound A similar problem is presented by vehi- cle tires and certain blow molded bottles, which must be virtually impermeable to air and other gases. An example of the use of a very impermeable elastomers is butyl rubber. Because of its impermeability to gases, butyl rubber is used as a roof coating. With plas- tic bottles, different layers of both coinjected and coextruded plastics (Chapter 8) can be used to fabricate the bottle to make it imper- meable to different vapors and gases depend- ing on the barrier plastic included. Water Vapor Permeability The material to be tested is fastened over the mouth of a dish that contains either wa- ter or a desiccant. This assembly is placed in an environment of constant humidity and temperature. The gain or loss in weight of the assembly is used to calculate the rate of water vapor movement through the speci- men under prescribed conditions of humid- ity inside and outside of the dish. The re- sults are reported in grams per 100 square inches during 24 hours, or equivalent metric units. It should be recognized that all plastic ma- terials over a time period allow a certain amount of water vapor, organic gas, or liq- uid to permeate the thickness of the material. It is only a matter of degree of permeation between various materials used as barriers against vapors and gases. It has been found that the permeability coefficient is a func- tion of the solubility coefficient and diffusion coefficient. The process of permeation is ex- plained as the solution of the vapor into the incoming surface of the barrier, followed by diffusion through the barrier thickness, and evaporation on the exit side. 308 5 Testing and Meaning of Test Data The problem of permeability exists when- ever a plastic material is exposed to va- por, moisture, or liquids. Typical cases are electrical batteries, instruments, components installed underground, encapsulated electri- cal components, food packaging, and vari- ous fluid-material containers. In these cases, a plastic material is called upon to form a bar- rier either to minimize loss of vapor or fluid or to prevent the entrance of vapor or fluid into a product. From the designers’ viewpoint, the tolerable amount of permeation established by test under conditions of usage with a pro- totype product of correct shape and material is the only direct answer. Different factors influencing permeation. (1) The composition of the barrier, including additives, fillers, colorants, plasticizers, etc. Even when data on permeability are avail- able for a specific industry grade of a plastic, they cannot be used for evaluation because different commercial grades can contain in- gredients that can change the values. (2) Crys- talline plastics are better vapor barriers than amorphous plastics. Also, TSs have better barrier properties than TPs, especially when the fillers are nonmoisture absorbing. (3) An increase in temperature brings about an in- crease in permeability. Additionally, an in- crease in vapor pressure of the permeating agent also causes acceleration of transmis- sion. (4) Product thickness is inversely pro- portional to permeation, i.e., with double the thickness, there is one-half of the evapora- tion. (5) Coatings such as epoxy-based fin- ishes will improve resistance to permeation. (6) In the case of organic vapors, the perme- ation will depend not only on the composition of the barrier, but also on the molecular con- figuration of both the barrier material and the permeating agent. Shrinkage The use of correct shrinkage information is very important, not only for having the desired proportions of a product, but also for functional purposes. The shrinkage data can be shown in a range of two values. The lower figure is intended to apply to thin parts, whereas the higher figure would involve thicker parts. Your interest is in your spe- cific thickness(s). Determining the shrinkage to occur when products (plastics, steels, etc.) are fabricated is not an easy task even when similar products are to be manufactured. The choice of shrinkage for a selected ma- terial and a specific product is the responsibil- ity initially of the designer but also involves the mold or die designers and the fabrica- tors. When the product designer has limited knowledge on how shrinkage is effected by the mold or die during fabrication, these peo- ple have to be included in the design. If ex- perience with the selected grade of plastic is limited, the design should be submitted to the material supplier for recommendations or someone how is knowledgeable, and the data coordinated with the interested parties. Tolerance Where very close tolerances are involved, preparing a prototype of the full size product may be necessary to establish critical dimensions. If this step is not practi- tal, it may be necessary to test a mold or die during various stages of cavity or die open- ing manufacture with allowances for correc- tion in order to determine the exact shrinkage needed. Considering the factors that can contribute to variations in shrinkage during the fabrica- tion of the products, it will be fully appreci- ated how significant it is to select the appro- priate numbers. The data on shrinkage have to be approached with much care if one is to avoid dimensional problems with the plas- tic product. Examples of how shrinkage is in- fluenced by different processes will provide some insights on how critical it is to ensure a degree of repeatability in the materials be- havior and the process. Chapters 6 and 7 pro- vide more information. As an example the shrinkage and in turn its tolerance of injection molded TPs will be affected as follows. (1) Higher cavity pres- sures will cause lower shrinkages. (2) Thick sections will shrink more than thin ones. (3) A cooler at the time of the product being ejected from the mold cavity will bring about a lower shrinkage. (4) A melt temperature of the ma- terial at the lower end of the recommended 5 Testing and Meaning of Test Data I 309 I Strength Formability Rigidity Toughness range will produce a lower shrinkage. (5) A longer cycle time, above the required solidifi- cation point, will partially conform the prod- uct closer to the mold dimensions, thereby bringing about lower shrinkage. (6) Open- ings in a product (holes and core shapes) will somewhat interfere with shrinkage in the cav- ity and thus will bring about lower and vary- ing shrinkages than would be the case in a product without openings. (7) Larger feeding gates to the product will cause lower shrink- age permitting higher pressure buildup in the cavity. (8) Materials that are crystalline have dual shrinkage so they are higher in the di- rection of material flow and lower perpen- dicular to it. In a symmetrical part; when cen- ter gated, the shrinkage will average out and be reasonably uniform. (9) Most TPs attain their full shrinkage after 24 hours, but there are some which may take weeks to stabilize their dimensions fully (manufacturer of the material usually indicates whether there is a delayed shrinkage effect present). (10) Glass fiber-reinforced or otherwise filled TPs have considerably lower shrinkages than the basic plastic. The TS plastic compression-molded prod- ucts will have a higher shrinkage when: (1) cavity pressure is on the low side, (2) when mold temperature is on the high side, (3) when cures are shorter, (4) when products are thicker, (5) when a material is soft flowing (highly plasticized), (6) when a material is preheated at relatively low heat, andor (7) when a high moisture content is present in the raw material. Durability Mechanical Property As summarized in Fig. 5-4 mechanical pro- perties encompass different behaviors. Tensile Property It should be recognized that tensile prop- erties would most likely vary with a change of speed of the pulling jaws and with vari- ation in the atmospheric conditions. Figure 2-14 shows the variation in a stress-strain curve when the speed of testing is altered; also shown are the effects of temperature changes on the stress-strain curves. When the speed of pulling force is increased, the material reacts like brittle material; when the temperature is increased, the material reacts like ductile material. The tensile data show the stress necessary to pull the specimen apart and the elonga- tion prior to breaking. A moderate elonga- tion (about 6%) of a test specimen generally implies that the material is capable of absorb- ing rapid impact and shock. The area under the stress-strain curve is indicative of over- all toughness except for reinforced plastics (Fig. 2-9). A material of very high strength, high rigidity, and little elongation would tend to be brittle in service. For applications where almost rubbery elasticity is desirable, a high ultimate (over 100%) elongation is an asset. The tensile test and the calculated property data from it provide a most valuable source of 310 5 Testing and Meaning of Test Data information for the designer in determining product dimensions. The important consid- eration is that use conditions compare rea- sonably closely to the test conditions as far as speed of load, temperature, and moisture are concerned. Should use conditions dif- fer appreciably, a test should be requested for data that are comparable to service re- quirements, which would thereby ensure that applicational needs will be based on more exacting data. The test is relatively inexpen- sive, and, where critical uses are encountered, it will eliminate interpolation and guessing. The tensile data are also useful for compar- ing various materials in this property. It is to be noted that tensile data should only be ap- plied to short-term stress conditions, such as operating a switch or shifting a clutch gear, etc. The yield point is the first point on the stress-strain curve at which an increase in strain occurs without an increase in stress. The stress at which a material exhibits a spe- cial limiting deviation from the proportion- ality of stress-to-strain is the yield strength. A material whose stress-strain curve ex- hibits points of zero slope may be considered to have a yield point such as described in Fig. 2-11. The data sheets usually omit the yield strength when there is a zero slope point on the stress-strain curve in the yield region. In reinforced plastic materials, the values of the yield strength and the tensile strength are very close to each other. The important tensile modulus (modulus of elasticity) is another property derived from the stress-strain curve. The speed of testing, unless otherwise indicated is 0.2 in./min, with the exception of molded or laminated TS materials in which the speed is 0.05 in./min. The tensile modulus is the ratio of stress to corresponding strain below the proportional limit of a material and is expressed in psi (pounds per square inch) or MPa (mega- Pascal) (Fig. 2-7). The proportional limit is the greatest stress that a material is capable of sustaining with- out any deviation of the proportionality law. It is located on the stress-strain curve below the elastic limit. The elastic limit is the great- est stress that a material is capable of sustain- ing without any permanent strain remaining upon complete release of the stress. For materials that deviate from the pro- portionality law even well below the elastic limit, the slope of the tangent to the stress- strain curve at a low stress level is taken as the tensile modulus. When the stress-strain curve displays no proportionality at any stress level, the secant modulus is employed instead of the tensile modulus (Fig. 2-2). The secant modulus is the ratio of stress to correspond- ing strain, usually at 1% strain or 85% from the initial tangent modulus. The tensile modulus is an important prop- erty that provides the designer with informa- tion for a comparative evaluation of plastic material and also provides a basis for predict- ing the short-term behavior of a loaded prod- uct. Care must be used in applying the tensile modulus data to short-term loads to be sure that the conditions of the test are comparable to those in use. The longer-term modulus is treated under the creep test (Chapter 2). The tensile data can be applied to the de- sign of short-term (such as l or 2 hour dura- tion) or intermittent loads in a product pro- vided the use temperature, the humidity, and the speed of the load are within 10% of the test conditions outlined under the procedure. The intermittent specification merely indi- cates that there be sufficient time for strain recovery after the load has been removed. The next step is to determine an allow- able working stress. This is done by using a safety factor usually of I'/Z to 2'/2 on the yield strength or tensile strength. If the type of stress is clearly defined, the 11/2 factor is adequate; otherwise, it should be higher (Chapter 2, Safety Factor). The final step is to calculate the elonga- tion that the product would experience un- der the selected allowable working stress to see if such an elongation would permit the proper functioning of the product. The elon- gation could conceivably become the limit- ing component, and the working stress can be calculated from: Modulus = stress/strain = E (5-3) 5 Testing and Meaning of Test Data 311 If product use conditions vary appreciably from those of the standard test, a stress-strain curve, derived using the procedure of antici- pated requirement, should be requested and appropriate data developed. Flexural Property These properties apply to products sub- jected to bending, and, since many plastic products are involved in uses where bending stresses are generated, this deserves close at- tention, especially in view of the viscoelastic nature of the materials. It should be noted that test information would vary with specimen thickness, temper- ature, atmospheric conditions, and different speed of straining force. This test is made at 73.4"F (23°C) and 50% relative humidity. For brittle materials (those that will break below a 5% strain) the thickness, span, and width of the specimen and the speed of crosshead movement are varied to bring about a rate of strain of 0.01 in./in./min. The appropriate specimen size are provided in the test speci- fication. The flexural strength is the maximum stress that a material sustains at the moment of break. For materials that do not fail, the stress that corresponds to a strain of 5% is frequently reported as the flexural strength (Fig. 2-15). As a matter of interest it should be stated that in this test the force of bending and as- sociated amount of deflection is recorded. A formula gives the relationship between de- flection and strain: The flexural yield strength is determined from the calculated data of load-deflection curves that show a point where the load does not increase with an increase in deflection. The flexural modulus is the ratio, within the elastic limit, of stress to corresponding strain. It is calculated by drawing a tangent to the steepest initial straight-line portion of the load-deflection curve and using an appro- priate formula. In many plastic materials, as is the case with metals, when performing the flexural tests, increasing the speed of deflecting force makes the specimen appear more brittle and increasing the temperature makes it appear more ductile. This is the same relationship as in tensile testing. When materials are evaluated against each other, the flexural data of those that break in the test cannot be compared unless the condi- tions of the test and the specimen dimensions are identical. For those materials (most TPs) whose flexural properties are calculated at 5% strain, the test conditions and the spec- imen are standardized, and the data can be analyzed for relative preference. For design purposes, the flexural properties are used in the same way as the tensile properties. Thus, the allowable working stress, limits of elon- gation, etc. are treated in the same manner as are the tensile properties. Compressive Property The compressive data are of limited design value. They can be used for comparative ma- terial evaluation and design purposes if the conditions of the test approximate those of the application. The data are of definite value for materials that fail in the compressive test by a shattering fracture. On the other hand, for those that do not fail in this manner, the compressive information is arbitrary and is determined by selecting a point of compres- sive deformation at which it is considered that a complete failure of the material has taken place. About 10% of deformation are viewed in most cases as maximum. The test can provide compressive stress, compressive yield, and modulus. Many plas- tics do not show a true compressive mod- ulus of elasticity. When loaded in compres- sion, they display a deformation, but show almost no elastic portion on a stress-strain curve; those types of materials should be compressed with light loads. The data are de- rived in the same manner as in the tensile test. Compression test specimen usually requires careful edge loading of the test specimens otherwise the edges tend to flourhpread out resulting in inacturate test result readings (2-19). 312 5 Testing and Meaning of Test Data Shear Strength The specimen is mounted in a punch-type shear fixture, and the punch (1 in. diameter) is pushed down at a rate of 0.05 in./min un- til the moving portion of the sample clears the stationary portion. Shear strength is cal- culated as the force per area sheared. Shear strength is particularly important in film and sheet products where failures from this type of load may occur (Fig. 2-21). This property can be used for comparison with other ma- terials and for determination of the forces needed for punching openings (holes, etc.). Izod Impact The popular Izod impact tester can use different size specimens depending on the type of plastic and their method of fabrica- tion. The specimen is usually 1/8 in. x 7 2 in. x 2 in.; other sizes are also used. Specimens can be notched or unnotched. A notch is cut in a specified manner on the narrow face of the specimen. The sample is clamped in the base of a pendulum testing machine so that it is cantilevered upward with the notch facing the direction of impact. The pendulum is released, and the force expended in breaking the sample is calculated from the height the pendulum reaches on the follow-through. The speed of the pendulum at impact is controlled. The impact test, with its usual notch, indi- cates the energy required to break notched specimens under standard conditions. It is calculated as foot-pounds per inch (J/m) of notch and is usually calculated on the basis of 1 in. wide specimen, although the specimen may be thinner in the lateral direction. The Izod value is useful in comparing var- ious types of grades of a plastic within the same material family. In comparing one plas- tic with another, however, the Izod impact test should not be considered a reliable indi- cator of overall toughness or impact strength. Some materials are notch-sensitive and de- velop greater concentrations of stress from the notching operation. It should be noted that the notch serves not only to concentrate the stress, but also to present plastic defor- mation during impact. The lzod impact test may indicate the need to avoid inside sharp corners on parts made of such materials. For example, nylon and acetal-type plastics, which in molded prod- ucts are among the toughest materials, are notch-sensitive and register relatively low values on the notched Izod impact test. Tensile Impact Small and long specimens of tensile bar shape specimens have their major change in dimensions in the necked-down section. The specimen is mounted between a pendulum head and crosshead clamp on the pendulum of an impact tester. The pendulum is released and it swings past a fixed anvil that halts the crosshead clamp. The pendulum head con- tinues forward, carrying the forward portion of the ruptured specimen. The energy loss (tensile impact energy) is recorded, as well as whether the failure appeared to be of a brittle or ductile type. This test has possible advantages over the notched Izod test. The notch sensitivity fac- tor is eliminated, and energy is not used in pushing aside the broken portion of the spec- imen. The test results are recorded in ft-lb/in2 (kJ/m2). This allows for minor variations in dimensions of the minimum in cross-section area. Two specimens are used, S and L (short and long), so that the effect of elongation on the result can be observed. A ductile failure (best observed on the L specimen) results in a higher elongation and, consequently, in a higher total energy absorption than a brittle failure (best observed on the S specimen) in any specific material. The energy for speci- men fracture is a function of the force times the distance it travels. Thus two materials showing the same energy values in the ten- sile impact test (all elements of the test being the same) could consist of two different fac- tors, such as a small force and a large elon- gation compared to a large force and a small elongation. If one is to consider the application of these data to a design, the size of the force and its rate of application would have to be obtained and compared with the design requirement. The breakdown of energy into components 5 Testing and Meaning of Test Data 313 of force and speed becomes possible by the addition of electronic instrumentation to the testing apparatus, thus enabling the supplier of the material to furnish additional informa- tion for material selection. Impact Strength The data from the lzod and tensile im- pact tests can be comparatively evaluated es- pecially when experience has been acquired with any one type of material (Table 5-2). One should keep in mind their individual limitations. Impact resistance is a significant characteristic of a material in many product designs. Associated with impact resistance is the term material toughness. Neither one can be measured in a way that is meaningful to the designer. Per ASTM test procedures test specimens can be of different thicknesses. With certain plastics from different manufac- turers the impact strength test result for one thickness can be higher than the other using a different thickness. However in actual ser- vice, products from these materials can show the tougher material to have the lower im- pact strength result; this is a rare situation (Chapter 7, SELECTING PLASTIC, Prop- erty Category, Impact). The term impact implies a very high speed of the acting force, whereas toughness is not related to any specific speed. Since the two terms are used in conjunction with each other in describing resistance to impact, it appears desirable to correlate those readily obtain- able properties that would reflect on speed of impact and toughness. At a 73.4"F (23°C) test temperature and a (ASTM) speed of acting force listed in each test category, the following results prevail: (1) a high modulus and high lzod points to a very tough material, (2) a high modulus and low Izod points to a brittle material, and (3) a low modulus and high Izod points to a flexible and ductile material. When use conditions differ from those ap- plied to data sheet tests, certain comparative evaluation can be made. Selecting an estab- lished high impact plastic such as polycarbon- ate as the standard, a tensile test would be made on this material at use speeds of strik- ing force and end use environmental con- ditions. This provides a modulus and stress- strain curve. The same kind of test would be made on the materials being evaluated. The area under a tensile stress-strain curve is a measure of toughness. It thus becomes possible to compare the modulus and areas under the curve and thereby estimate im- pact strength as a percentage of the standard. The notch sensitivity factor is eliminated and a judgment element is introduced that can prove accurate if the information is diligently analyzed. Where critical design areas involv- ing safety to humans or protecting valuable devices are concerned, the simulation of end use with full size prototype products (includ- ing extremes of conditions) is the most desir- able way to test selected materials. There are other types of impact tests for shock loading where energy is required to cause complete failure is reported. Each has their specific behaviors that can be related to specific product performance requirements. Tests include ball burst, ball or falling dart us- ing different weights and heights, bag drop, bullet-type instantaneous impact, Charpy, dart drop, Mullen burst, tear resistance, and tub (2). Hardness Hardness basically is the resistance to in- dentation as measured under specific con- ditions such as depth of indentation, load applied, and time period. Different tests re- late to different hardness behaviors of plas- tics. They include Barcol, Brinell, durom- etei, Knoop, Mohs, Rockwell, Shore, and Vicat (2). Hardness is closely related to strength, stiffness, scratch resistance, wear resistance, and brittleness. The opposite characteristic, softness, is associated with ductility. There are different kinds of hardness that measure a number of different properties (Fig. 5-5). The usual hardness tests are listed in three categories: (a) to measure the resistance of a material to indentation by an indentor; some measure indentation with the load applied, some the residual indentation after it is re- moved, such as tests using Brinell hardness, 314 5 Testing and Meaning of Test Data Table 5-2 Room temperature impact resistance data for several plastics Izod Impact Energy for 0.318 cm (0.125 in.) Tensile-impact Energy thick, Notched Short Long Type Trademark Grade Jim kJ/m2 kJ/m2 Generic Material Specimens, Specimens, Specimens, ABS Cycolac Acetal copolymer Celcon Acetal Delrin homopolymer Acrylic Plexiglas Nylon (DAM) Zytel (0.2% moisture) Nylon (50% RH) Zytel (2.5% moisture) Phenolic Durez Polycarbonate Lexan Polyethylene Dow Phenylene ether Prevex Polypropylene Pro-fax copolymer Polystyrene Fostarene Hostyren Poly sulfone Udel DH GSM KJB L M25 M90 100 500 900 V052i045 MI-7 DR 101 ST 801 158 L 21 1 101 ST 801 158 L 29053d 152d 18441d 141 940 08064N 10062N 04052N 08035N PQA VKA 6523 7523 8523 50 360 760 840 P-1700 P-1710 P-1270 235 99-131" 374 102-1 1 5" 214 95 400 100-12oa 85 75 123 74.7 69.4 216 32' 64b 53 907 53 80 112 1068 75 19 17 14 640-850 640 53 48 80 130 267 293 42.7 133.5 379 21 54 97 161 69 341 69 421 69 336 157 153 231 218 473-631 526 113" 86" 190 150 350 200 150 504 588 611 525 1470 1155 945 88 105 140 81 "0.159 cm (A in.) thick specimens. bMolded notch. '0.635 cm (0.250 in.) thick specimen with molded notch. dInjection-molded specimens. 5 Testing and Meaning of Test Data 315 Hardness Scales tube tread Fig. 5-5 Hardness of different materials using different test methods. Vickers and Knoop indentors, Barcol hardness, and Shore durometers (2); (b) to measure the resistance of a material to scratching by another material or by a sharp point, such as the Bierbaum hardness or scratch-resistance test and the Moh one for hardness; and (c) to measure rebound efficiency or resilience, such as the various Rockwell hardness tests. The various tests provide different behavior characteristics for plastics, as described by different ASTM standards such as D 785. The ASTM and other sources provide different degrees of comparison for some of these tests. Some ductile plastics, such as PC and ABS, can be fabricated like metals with punching and cold-forming techniques. These process- ing techniques are analogous to the hardness tests in that a rigid “indentor” is pressed into a sheet of a less-rigid plastic. Durometer hardness An arbitrary numer- ical value that measures the resistance to intention of a blunt indenter point of the durometer. The higher the number, the greater indention hardness. Barcol hardness Also called Barcol im- presser. It is a measure of the hardness of a plastic, that includes laminate or rein- forced plastic, using a Barber Coleman spring loaded indenter. Gives a direct reading on a 0 to 100 scale; higher number indicates greater hardness. This test is often used to measure the degree of cure for plastics, particularly TS plastics. Brinell hardness A common test used to determine the hardness of a material by in- dentation of a specimen. Pressing a hardened steel ball generally 10 mm diameter down on a specimen carries out the test, and the di- ameter of the subsequent impression formed provides a basis for calculating hardness. Knoop hardness It is a measure of hard- ness is measured by a calibrated machine that forces a rhomb-shape, pyramidal diamond in- denter having specified edge angles under specific small loading conditions into the sur- face of the test material; the long diagonal in the material is measured after removal of the load. Mohs hardness It is a measure of the scratch resistance of a material. The higher the number, the greater scratch resistance with number 10 being termed diamond. Rockwell hardness Sheets or plaques at least 0.25 in. thick are used. This thickness may be built up of thinner pieces, if necessary. A steel ball under a minor load is applied to the surface of the specimen. This indents the specimen slightly and assures good contact. The gauge is then set at zero. Basically the major (higher) load is applied for 15 seconds and removed, leaving the minor load still ap- plied. The indentation remaining after 15 sec- onds is read directly off the testing equipment dial. The size of the balls used and loadings vary, and values obtained with one set cannot 316 5 Testing and Meaning of Test Data be correlated with values from another set. Rockwell hardness can differentiate the rel- ative hardness of different types of a given plastic; but, since elastic recovery is involved as well as hardness, it is not valid to compare the hardness of various types of plastic en- tirely on the basis of this test. Hardness usually implies resistance to abrasion, wear, or indentation (penetration). In plastics it only means resistance to inden- tation. The scales range from; (1) “R” with a major load of-60 kg; indenter of 0.5 in., (2) “L” with a major load of 60 kg; indenter of 0.25 in., (3) “M” with a major load of 100 kg; indenter of 0.25 in., (4) “E” with a major load of 100 kg; indenter of 0.125 in., and (5) “K” with a major load of 150 kg; indenter of 0.125 in. The hardness is of limited value to the de- signer, but can be of some value when com- paring these data between materials. Scleroscope hardness It is a dynamic in- dentation hardness test using a calibrated in- strument that drops a diamond-tipped ham- mer from a fixed height onto the surface of the material being tested. Shore hardness It is the indentation hard- ness of a material as determined by the depth of an indentation made with an in- denter of the Shore type durometer. The scale reading on this durometer is from zero (corresponding to 0.100 in. depth) to 100 for zero depth. The Shore A indenter has a sharp point, is spring-loaded, and is used for the softer plastics. The Shore B indenter has a blunt point, is spring-loaded at a higher value, and is used for harder plastics. Vicut hardness It is a determination of the softening point for TPs that have no definite melting point. The softening point is taken the temperature at which the specimen is usually penetrated to a depth of 1 mm2 (0.0015 in2) circular or square cross section, under a 1,000 g load. Deformation Under Load The specimen is a small cube, either solid or composite. It is placed between the anvils of the testing machine, and loaded at 1000, 2000, or 4000 psi. The gauge is read 10 s after loading, and again 24 h later. The deflection is recorded in mils. Calculation is made after the specimen is removed from the testing ma- chine. By dividing the change in height by the original height and multiplying by 100, the percentage deformation is calculated. This test may be run at different temperatures. This test on rigid plastics indicates their ability to withstand continuous short-term compression without yielding and loosening when fastened as in insulators or other assem- blies by bolts, rivets, etc. It does not indicate the creep resistance of a particular plastic for long periods of time. It is also a measure of rigidity at service temperatures and can be used as identification for procurement. Data should indicate stress level and the tempera- ture of the test. Fatigue Strength The fatigue strength is defined as that stress level at which the test specimen will sustain “N” cycles prior to failure. The data are gen- erated on a machine that runs at 1800 cycles per minute. This test is of value to material manufacturers in determining consistency of their product (Chapter 2). Long- Term Stress RelaxatiodCreep This review concerns the long-term behav- ior of plastics when exposed to conditions that include continuous stresses, environ- ment, excessive heat, abrasion, and contin- uous contact with liquids. This subject has been reviewed in Chapter 2 (LONG-TERM LOAD BEHAVIOR) but since it is a very im- portant subject the review is continuing. Tests such as those outlined by ASTM D 2990 that describe in detail the specimen preparations and testing procedure are intended to pro- duce consistency in observations and records by various manufacturers, so that they can be correlated to provide meaningful informa- tion to product designers. The procedure under this heading is in- tended as a recommendation for uniformity 5 Testing and Meaning of Test Data 317 of making setup conditions for the test, as well as recording the resulting data. The rea- son for this action is the time consuming na- ture of the test (many years duration), which does not lend itself to routine testing. The test specimen can be round, square, or rectangu- lar and manufactured in any suitable manner meeting certain dimensions. The test is con- ducted under controlled temperature and at- mospheric conditions. The requirements for consistent results are outlined in detail as far as accuracy of time interval, of readings, etc., in the proce- dure. Each report of test results should indi- cate the exact grade of material and its sup- plier, the specimen’s method of manufacture, its original dimensions, type of test (tension, compression, or flexure), temperature of test, stress level, and interval of readings. When a load is initially applied to a speci- men, there is an instantaneous strain or elon- gation. Subsequent to this, there is the time- dependent part of the strain (creep), which results from the continuation of the constant stress at a constant temperature. In terms of design, creep means changing dimensions and deterioration of product strength when the product is subjected to a steady load over a prolonged period of time. All the mechanical properties described in tests for the conventional data sheet proper- ties represented values of short-term appli- cation of forces, and, in most cases, the data obtained from such tests are used for com- parative evaluation or as controlling specifi- cations for quality determination of materials along with short-duration and intermittent- use design requirements. The visualization of the reaction to a load by the dual component interpretation of a material is valuable to the understanding of the creep process, but meaningless for design purposes. For this reason, the designeris in- terested in actual deformation or part failure over a specific time span. This means making observations of the amount of strain at cer- tain time intervals which will make it possible to construct curves that could be extrapolated to longer time periods. The initial readings are 1,2,3,5,7,10, and 20 h, followed by read- ings every 24 h up to 500 h and then readings every 48 h up to 1000 h (Chapter 2). The time segment of the creep test is com- mon to all materials, i.e., strains are recorded until the specimen ruptures or the specimen is no longer useful because of yielding. In either case, a point of failure of the test specimen has been reached. The stress levels and the temperature of the test for a material is determined by the manufacturer. The guiding determinants are the continuous allowable working stress at room temperature and the continuous allow- able working stress at temperatures of poten- tial applications. The strain readings of a creep test can be more convenient to a designer if they are pre- sented as a creep modulus. In a viscoelas- tic material, strain continues to increase with time while the stress level remains constant. Since the modulus equals stress divided by strain, we have the appearance of a changing modulus. The creep modulus, also known as ap- parent modulus or viscous modulus when graphed on log-log paper, is normally a straight line and lends itself to extrapolation for longer periods of time. The apparent mod- ulus should be differentiated from the mod- ulus given in the data sheets, because the lat- ter is an instantaneous value derived from the testing machine. The method of obtaining creep data and their presentation have been described; how- ever, their application is limited to the ex- act same material, temperature use, stress level, atmospheric conditions, and type of test (tensile, compression, flexure) with a toler- ance of &lo%. Only rarely do product re- quirement conditions coincide with those of the test or, for that matter, are creep data available for all grades of material that may be selected by a designer. In those cases a creep test of relatively short duration such as 1000 h can be instigated, and the information can be extrapolated to the long-term needs. It should be noted that reinforced thermo- plastics and thermosets display much higher resistance to creep (Chapter 2). Creep information is not as readily avail- able as short-term property data sheets are. From a designer’s viewpoint, it is im- portant to have creep data available for products subjected to a constant load for 318 5 Testing and Meaning of Test Data prolonged periods of time. The cost of per- forming or obtaining the test in comparison with other expenditures related to product design would be insignificant when consid- ering the element of safety and confidence it would provide. Furthermore, the proving of product performance could be carried out with a higher degree of favorable expecta- tions as far as a plastic material is concerned. Progressive material manufacturers can be expected to supply the needed creep and stress-strain data under specified use condi- tions when requested by designer; but, if that is not the case, other means should be utilized to obtain required information. In conclusion regarding creep testing, it can be stated that creep data and a stress- strain diagram indicate whether plain plastic properties can lead to practical product di- mensions or whether a RP has to be substi- tuted to keep the design within the desired proportions. For long-term product use un- der continuous load, plastic materials have to consider creep with much greater care than would be the case with metals. Summation Throughout this book all the different types of mechanical properties are presented and reviewed. These mechanical properties include a tremendous range of different types that can usually be characterized by their stiffness, strength, and toughness. Stiffness The same factors that influence thermal expansion dictate the stiffness of plastics. Thus in a TS the degree of cross- linking and amount of overall flexibility are important. As an example, in a TP its crys- tallinity and secondary bond’s strength con- trol its stiffness. Strength The subject of strength is much more complex than stiffness, since so many different types exist such as short or long term, static or dynamic, and torsion or impact strengths. Some strength aspects are interre- lated with those of toughness. This section re- views certain simplified concepts of strength that are important influences on strength based on long and short term exposure. The crystallinity of TPs is important for their short term yield strength. Unless the crystallinity is impeded, increased molecu- lar weight generally also increases the yield strength. However, the cross-linking of TSs increases their yield strength substantially but has an adverse effect upon toughness. Long term rupture strengths in TPs are in- creased much more readily by increasing the secondary bonds’ strength and crystallinity than by increasing the primary bond strength. Fatigue strength is similarly influenced, and all factors that influence thermal dimensional stability also affect fatigue strength. This is a result of the substantial heating that is often encountered with fatigue, particularly in TPs. Toughness The subject of toughness is usually the most complex factor to define and understand. Tough plastics are usually described as ones having a high elongation to failure or ones in which a lot of energy must be expended to produce failure. For high toughness a plastic needs both the ability to withstand load and the ability to elongate substantially without failing except in the case of reinforced TSs that are tough, which may have high strengths with low elongation. It may appear that factors contributing to high stiffness are required, but this is not true, because there is an inverse relationship between flaw sensitivity and toughness: the higher the stiffness and the yield strength of a Tp, the more flaw sensitive it becomes. How- ever, because some load bearing capacity is required for toughness, high toughness can be achieved by a high trade off of certain factors. Crystallinity increases both stiffness and yield strength, resulting usually in decreased toughness. This is true below Tg in most amor- phous plastics, and below or above the Tg in a substantially crystalline plastic. However, above the Tg in a plastic having onlymoderate crystallinity increased crystallinity improves its toughness. Furthermore, an increase in molecular weight from low values increases toughness, but with continued increases, the toughness begins to drop. Cross-linking produces some dimensional stability and improves toughness in a non- crystalline type of plastic above the Tg, but 5 Testing and Meaning of Test Data 319 high levels of cross-linking lead to embrittle- ment and loss of toughness. This is one of the problems with TSs for which an increase in Tg is desired. Increased cross-linking or stiffen- ing of the chain segments increases the Tg, but it also decreases toughness. A popular way to increase toughness is to blend, compound, or copolymerize a brittle plastic with a tough one. Although some loss in stiffness is usu- ally encountered, the result is a satisfactory combination of properties. Thermal Property Figure 5-6 and Tables 5-3 to 5-5 provide an introductory guide to the different thermal properties of plastics. Heat resistance prop- erties of plastics retaining 50% of properties obtainable at room temperature with plas- tic exposure and testing at elevated temper- atures are shown in Fig. 5-6 for the general family or group type. Zone 1: acrylic, cellulose esters, crystal- lizable block copolymers, LDPE, PS, vinyl polymers, SAN, SBR, and urea- formaldehyde. Zone 2: acetal, ABS, chlorinated poly- ether, ethyl cellulose, ethylene-vinyl ac- etate copolymer, furan, ionomer, phe- 1.650 I 1 I I I I Time. h Fig. 5-6 Guide to heat resistance with 50% re- tention of properties. noxy, polyamides, PC, RDPE, PET, PP, PVC, and urethane. Zone 3: polychlorotrifluoroethylene, and vinylidene fluoride. Zone 4: alkyd, fluorinated ethylene- propylene, melamine-formaldehyde, phenol-furfural, and polysulfone. Zone 5: acrylic, diallyl phthalate, epoxy, phenol-formaldehyde, TP polyester, and polytetrafluoroethylene. Zone 6: parylene, polybenzimidazole, polyphenylene, and silicone. Zone 7: polyamide-imide, and polyimide; Zone 8: plastics now being developed us- ing rigid linear macromolecules rather than crystallization and cross-linking, etc. Specific family or group of plastics (polyethylene, polyvinyl chlorides, etc.) are compounded or alloyed to provide different properties and/or processing behaviors. Thus a plastic listed in Fig. 5-6 could have different heat resistance properties. Deflection Temperature Under Load The DTUL, also called the heat distortion temperature (HDT) of a plastic is a method to guide or assess its load-bearing capac- ity at an elevated temperature. Details on the method of testing are given in ASTM D648. Basically a 1.27 cm ('/z in.) deep plas- tic test bar is mounted on supports 10.16 cm (4 in.) apart and loaded as a beam. A bending stress of either 66 psi or 264 psi (455 gPa or 1,820 gPa) is applied at the center of the span. The test is conducted in a bath of oil, with the temperature increased at a constant rate of 2°C per minute. The DTUL is the temper- ature at which the sample attains a deflec- tion of 0.0254 cm (0.010 in.). This test is only a guide. It represents a method that could be correlated to product designs, but as with most other tests conducted on test specimens and not on a finished product, it is just a guide (Fig. 5-7). In this test, if the specimen contains inter- nal stresses the value will be lower than a specimen with no stresses. In fact, the test can 320 5 Testing and Meaning of Test Data Table 5-3 Examples of plastics in elevated temperature applications Polymer Comments Polyphenyls Polyphenylene oxide Polyphenylene sulfide Polybenzyls; polyphenethyls Parylenes (poly-p-xylylene) Polyterephthalamides Polysulfanyldibenzamides Polyhydrazides Poly oxamides Phenolphthalein polymers Hydroquinone polyesters Polyhydroxybenzoic acids Polyimides Polyarylsiloxanes Carboranes Polybenzimidazoles Polybenzothiazoles Polyquinoxalines Polyphenylenetriazoles Polydithiazoles Polyoxadiazoles Pol yamidines Pyrolyzed polyacrylonitrile Polyvinyl isocyanate ladder polymer Poly amide-imide Poly sulfone Polybenzaylene benzimidazoles Polybenzoxazoles (pyrrones) Ionomer Decompose at 530°C (986°F); infusible, insoluble polymers. Decomposes close to 500°C (932°F); heat cures above 150°C (302°F) to elastomer; usable heat range -135-185°C (-211-365°F). Melts at 270-315°C (578499°F); crosslinked polymer stable to 450°C (842°F) in air: adhesive and laminating applications. Fusible, soluble, and stable at 400°C (752°F); low molecular weight. Melt above 520°C (968°F); insoluble; capable of forming films; poor thermal stability in air; stable to 400-525°C (752-977°F) in inert atmosphere. elongation, modulus. Melting points up to 455°C (851°F); fibers have good tenacity, Melting points up to 330°C (626°F); soluble; good fiber properties. Dehydrate at 200°C (392°F) to over 400°C (752°F) to form Some melting points above 400°C (752°F); give clear, flexible films. Melting points of 300°C (572°F) to over 400°C (752°F); formable Soluble polymers with melting points of 335°C (635°F) to over Films melt at 380450°C (7164342°F); stable to oxidation but not Commercial film, coating, and resin stable up to 600°C (1112°F); Good thermal stability 400-500°C (752-932°F); coatings, adhesives. Stable in air and nitrogen at 400-450"C (752442°F); elastomeric properties for silane derivatives up to 538°C (1OOO"F); adhesives. Developmental laminating resin, fiber, film; stable 24 hours at 300°C (572°F) in air. Stable in air at 600°C (1112°F); cured polymer soluble in concentrated sulfuric acid. Stable in air at 500°C (932°F); tough, somewhat flexible resins; make film, adhesive. Thermally stable to 400-500"C (752-932°F); make film, fiber, coatings. Decompose at 525°C (977°F); soluble in concentrated sulfuric acid. Decompose at 450-500°C (842-932°F); can be made into fiber or film. Stable to oxidation up to 500°C (932°F); can make flexible elastomer. Stable above 900°C (1625°F); fiber resists abrasion with low tenacity. Soluble polymer that decomposes at 385°C (725°F); prepolymer Service temperatures up to 288°C (550°F); amenable to fabrication. Thermoplastic; use temperature -102°C (-152°F) to greater than Thermally stable to 600°C (1112°F); insoluble in common solvents; polyoxadiazoles; good fiber properties. into fiber and film. 400°C (752°F). to hydrolysis; tough, flexible films; good thermal stability. continuous use up to 300°C (572°F). melts above 405°C (761PF). 150°C (302°F); acid and base resistant. good mechanical properties. Stable in air to 500°C (932°F); insoluble in common solvents except High melt and tensile strength; tough; resilient; oil and solvent sulfuric acid; nonflammable; chemical resistant; film. resistant; adhesives, coatings. (Continues) 5 Testing and Meaning of Test Data Table 5-3 (Continued) 321 Polymer Comments Diazadiphosphetidine Thermoplastic up to 350°C (662°F); thermosetting at 357°C (707°F); cured material has good thermal stability to 500°C (932°F); amenable to fabrication. stability. Phosphorous amide epoxy Phosphonitrilic Metal polyphosphinates Phenylsilesesquioxanes Soluble B-staged material; amenable to fabrication; good thermal Retention of properties in air, up to 399°C (750°F). Polymers stable to better than 400°C (752°F). Soluble; high molecular weight; infusible; improved tensile strength (phenyl-T ladder polymers) high thermal stability to 525°C (977°F) in air; film forming. be used to determine the degree of internal stress. Since a stress and the deflection for a certain depth of test bar are specified, this test may be thought of as establishing the temper- ature at which the flexural modulus decreases to particular values: 35,000 psi (240 MPa) at 66 psi load stress, and 140,800 psi (971 MPa) at 264 psi. CoefJicient of Linear Thermal Expansion The specimen can be square or round to fit a dilatometer test tube in a free sliding man- ner. The length is governed by the sensitivity of dial gauge, the expected expansion, and the accuracy desired. The specimen is mounted in the dilatometer and placed in a bath of either -22°F (-30°C) or 87°F (+30"C) un- til the temperature of the bath is reached. When this takes place, the indicator dial is read showing the expansion or contraction of the specimen. These readings are compared with measurement of specimen length prior to placing it in the dilatometer. With the application of plastics in combi- nation with other materials, the coefficient of expansion plays an important role in mak- ing design allowances for expansions (also contractions) of various materials at different temperatures so that satisfactory functions of products are ensured. Table 5-4 Examples of ignition and flash temperatures ~ ~ ~~~ Self Ignition Flash Ignition "F "C "F "C Polyethylene 662 350 644 340 Polypropylene 1022 550 968 520 Polytetrafluoroethylene 1076 580 1040 560 Polyvinyl chloride 842 450 734 390 Polyvinyl fluoride 896 480 788 420 Polystyrene 914 490 662 350 SBR (Styrene Butadiene Rubber) 842 450 680 360 ABS (Acrylonitrile Butadiene Styrene 896 480 734 390 Polymethyl methacrylate 806 430 572 300 PAN (Polyacrylonitrile) 1040 560 896 480 Cellulose (paper) 446 230 410 210 Cellulose acetate 878 470 644 340 66 Nylon cast 842 450 788 420 66 Nylon spun and drawn 986 530 914 490 Polyester 896 480 824 440 322 5 Testing and Meaning of Test Data 2 ' 8 G Table 5-5 Effects of elevated temperature and chemical agents on stability of plastics Environment Cchylcnt mpolymrs "FE' Flumnated ethylene pmpylmes (FEP) PerRuoroslkoxier (PFA) Polychlomuifluom- ethylenes (CTFE) Polvarrafluoro- 77 200 % Chan&e Pla~tic Malcrial (2S"C) (93.3"c) 77 200 77 200 77 200 77 200 77 200 77 200 77 2W by Weigh1 Acclals 1-4 2-4 1 2 1-2 4 1-3 2-5 1-5 2-5 5 5 5 5 1 2-3 0,224.25 Acrylics 5 5 2 3 5 5 1 3 2 5 4 4 - 5 5 5 5 5 0.2-0.4 Acrvlonilrilc-Bu~diem- 4 5 2 3-5 3-5 5 1 2 4 I 2-4 1-4 5 1-5 5 3-5 5 0.1-0.4 Sryrencs (ABS) Aramids (arurnalic 1 1 1 1 1 1 2 3 4 5 3 4 2 5 1 2 0.6 2 3 2 3 3 4 2 3 3 5 3 5 3 4 5 I 3 3 4 2 4 3 5 3 5 3 4 5 I 3 3 4 1 2 3 5 3 5 3 1-2 2-4 2 3 2 4 2 3 2 4 1-2 2-3 2 1 2 1 2 1-2 2-4 1 1-2 1 2 2-3 3 4 4 5 5 5 5 5 5 1 2 1 5 1 5 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1. . I I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I ~hylcnc.s@(TFE) Furans 1 lonomers 2 Melunincs (filled) 1 Niailes (high barrier nIlOyS 1 of ABS or SAN) Nylons I Phenolics (filled) I Poly allomcrs 2 Araung of 1 Indicates grealesl Slabilily 5 5 5 4 4-5 5 1 5 5 5 5 5 5 3 4 4-5 2 3 - 4 2 5 1 1 1 1 1 1 1 1 1 1 2-7 0.9-2.0 1.2-2.1 0.2-0.7 0.01-0.10 0.05-0.13 C0.03 co.01 G0.03 0.01-0.10 0 l 1 1 l 1 2 2 2 2 1 1 5 5 1 1 0 . 0 1 - 0 . 2 0 4 1 4 4 4 1 4 I 4 2 4 1 5 1 4 0 . 0 1 - 1 . 4 1 1 1 1 1 2 3 2 3 2 3 2 3 1 2 0.01-1.30 4 I 2-4 1-4 2-5 1 2-4 I 2-4 2-5 5 3-5 5 1-5 5 0.2-0.5 1 I 1 1 2 1 2 2 3 5 5 5 5 1 1 0.2-1.9 1 1 1 1 1 2 3 3 5 1 1 4 5 2 2 0.1-2.0 4 2 4 4 5 1 1 1 1 1 3 1 4 1 3 co.01 The difference in thermal expansion be- tween the usual commodity plastics and steel is very large. It is to be noted that some plas- tic material changes in length rather abruptly at some temperatures, beyond the limits of the test condition. In such cases, a special investigation should be instigated, and the coefficient of expansion established under temperatures of usage. However there are plastics that can be compounded to match or even have less thermal expansion than steel, etc. This test shows the reversible linear ther- mal expansion. The accuracy of these results may be affected by factors in certain plastics such as loss of plasticizer, solvent, relieving of stresses, etc. When a product demands most precise data, the factors mentioned should be considered for their possible influence on the information. Brittleness Temperature The conditioned specimens are can- tilevered from the sample holder in the test apparatus, which has been brought to a low temperature (that at which the speci- mens would be expected to fail). When the 5 Testing and Meaning of Test Data Table 5-5 (Continued) 323 Environment Polyamide-imides 1 I 1 1 2 3 1 1 3 4 2 3 2 3 1 1 0.22-0.za Polysylsulfones (PAS) 4 5 2 3 4 5 1 2 2 2 1 1 2 4 3 4 1.2-1.8 Polycstera (Ihumoplaslic) 2 5 1 3 - 5 3 5 1 - 2 5 3 4 - 5 2 3 - 5 2 3 - 4 o . I M o . 0 9 Polybutylmu (PE) 3 5 I 5 4 5 1 2 1 3 1 3 1 4 I 3 < 0 . 0 1 - 0 . 3 Polrcprbonatcs (Po 5 5 1 1 5 5 1 5 5 5 1 I 1 1 5 5 O . l 5 - 0 . 3 5 Polvcsten(Ihermostt-glass 1-3 3-5 2 3 2 4 2 3 3 5 2 3 2 4 3-4 4-5 0.01-2.3 fiber filled) HDPE-low density lo high dmsity) (UHMWPE-ultnhigh molecular wcrght) PolyimiBs Polyphcnylene oxides (PW) (modified) Polyphenylene sulfides (PPS) Polyphenylsulfones Polypropylenes (PP) Polystyrenes (PS) Polysuifoner Polyurcthsnes (PUR) Polyvinyl chlorides (PVC) Polyvinyl chlorides- chlorinated (CPV0 Polyvinylidene fluorides (PVDR Silicones Styrene acrylonitrilts (SAN) Umas (filled) Vinyl esters (glass-fiber Polyehylenes (LDPE- Polyethylenes fillcd) 4 5 4 5 4 5 1 1 1 1 1-2 1-2 1-3 3-5 2 3 3 4 3 4 3 4 1 1 1 1 1 1 1 1 3 4 1 1 1 1 1 1 2 3 4 5 3 4 2 5 1 1 4 5 2 3 4 5 1 1 1 1 1 2 1 2 2 3 1 2 1 1 1 2 1 1 1 1 1 1 1 2 1 1 4 4 1 1 5 5 1 1 1 1 1 1 1 1 3 4 4 5 4 5 5 5 1 5 1 5 4 5 4 5 4 5 4 4 1 1 5 5 1 1 1 1 1 1 1 1 3 4 3 4 2 3 4 5 2-3 3-4 2-3 3-4 2-3 3-4 4 4 4 5 4 5 1 5 5 5 1 5 1 5 1 5 2 5 4 5 4 4 I 2 5 5 1 2 1 2 1 2 2 3 4 5 1 1 1 1 1 1 I 1 1 2 1 2 1 ? 3 5 4 4 2 3 4 5 1 2 4 5 3 4 4 5 2 4 4 5 3 4 3 5 1 3 1 3 1 3 3 4 4 5 1 3 1 3 1 3 2 3 2 3 4 5 2 3 1 2 1 3 1-2 2-4 1-2 4 1 3 1 3 1 2 2 3 3-4 4-5 2 4 2 4 2-3 6 5 1 1 1 1 1 2-3 2-3 4-5 2 4 0.oao.Ol <0.01 0.3-0.4 0.06-0.07 <o.os 0.5 0.01-0.03 0.03-0.60 0.2-0.3 0.02-1.50 0.04-1.00 0.04-0.45 0.04 0.1-0.2 0.20-0.35 0.44.8 0.01-2.50 specimens have been in the test medium for 3 minutes, a single impact is administered, and the samples are examined for failure. Failures are total breaks, partial breaks, or any visible cracks. The test is conducted at a range of temperatures producing varying percentages of breaks. From these data, the temperature at which 50% failure would oc- cur is calculated or plotted and reported as the brittleness temperature of the material according to this test. This test is of some use in judging the relative merits of various materials for low- temperature flexing or impact. However, it is specifically relevant only for materials and conditions specified in the test, and the val- ues cannot be directly applied to other shapes and conditions. The brittleness temperature does not put any lower limit on service temperature for end use products. The brittleness tempera- ture is sometimes used in specifications. Thermal Aging Section UL 746B provides a basis for se- lecting high-temperature plastics and pro- vides a long-term thermal-aging index, the RTI or relative thermal index. The testing procedure calls for test specimens in selected thicknesses to be oven aged at certain ele- vated temperatures (usually higher than the expected operating temperature, to acceler- ate the test), then be removed at various in- tervals and tested at room temperature. 324 5 Testing and Meaning of Test Data Polyimidffiraphite PolyimiddGlass Silicone Fluocoplastics/Gl~ Poiyetherketoneketone (PEEK)Kiraphile Liquid CfystaVPo er PolvesterT&ss TS Polvestw/Glau Poly&?rirnide/Giass PolybutadiendG lass Siliione Melamine-Formaldehyde SiliconwGlass Polybenzimidazole (PBI) Bimleimide (BMI)/Carbon PolyimidelGlass PolyetherketOoelGb Bismaleimide (BMWlass PolyketondGlass Polyetheretherketone (PEEKJGlass Polyphenylene Sulfidffilass Polyimide Polhide-lmide/Glass Phenol-Formaldehyde Cyanate Esterffilass PoiysulionelGlass PolyarcinaticTF’-EpoxyTS/G/TsK;lass Polyethylene TerepMhabte/Glan Silicon-Polvearbonate/Glass VinyVGlasS Polyurethane Fig. 5-7 Guide to heat resistance based on the heat-distortion temperature per ASTM D 648 at 264 psi. Another reason for using higher temper- atures is that for an application requiring long-term exposure a candidate plastic is of- ten required to have an RTI value higher than the maximum application temperature. The properties tested can include mechani- cal strength, impact resistance, and electrical characteristics. A plastic’s position in a test’s RTI is based on the temperature at which it still retains 50% of its original properties. The time required to produce a 50% re- duction in properties is selected as an arbi- trary failure point. These times can be gath- ered and used to make a linear Arrhenius plot of log time versus the reciprocal of the ab- solute exposure temperature. An Arrhenius relationship is a rate equation followed by many chemical reactions. A linear Arrhenius plot is extrapolated from this equation to pre- dict the temperature at which failure is to be expected at an arbitrary time that depends on the plastic’s heat-aging behavior, which are usually 11,000 hours, with a minimum of 5,000 hours. This value is the RTI. As practiced by the UL, the procedure for selecting an RTI from Arrhenius plots usu- ally involves making comparisons to a control standard material and other such steps to cor- rect for random variations, oven temperature variations, condition of the specimens, and others. The stress-strain and impact and elec- trical properties frequently do not degrade at the same rate, each having their own separate RTIs. Also, since thicker specimens usually take longer to fail, each thickness will require a separate RTI. The UL uses RTIs as a guideline to qual- ify materials for many of the standard appli- ances and other electrical products it regu- lates. This testing is done in a conservative manner qualified by judgments based on long experience with such devices; UL does not apply indexes automatically. In general, these RTIs are very conservative and can be used as safe continuous-use temperatures for low- load mechanical products. Other Heat Test There are different heat tests, some be- ing specific to a product environment. There are those for temperature and also humidity. With certain materials, humidity combined with elevated temperatures has a significant effect on the material’s behavior. This effect would not be evident in the conventional heat distortion test (HDT). Test specimens can also be used to simulate some degree of warpage. Figure 5-8 compares unreinforced and reinforced glass fiber-TS polyester flexural-type specimens at differ- ent temperatures in a droop test (with a cen- ter support), sag test (end supports), and an expansion test (bolted at three points). The study for this particular test is conducted at various temperatures. Thus by analyzing the thermal limits of the various materials available, starting with the maximum and minimum environmental temperatures under which a product must operate and adding any thermal increase 5 Testing and Meaning of Test Data 325 with glass without glass 260" with glass without glass 220" with glass without glass 160" with glass without glass Ambient v Droop Test Sag Test Expansion Test Fig. 5-8 Example of droop, sag, and expansion tests using RP samples. from hysteresis heat that develops from useful mechanical properties in the 149 to flex or vibration and so on will at least 260°C (300 to 500°F) range (Fig. 5-10). Their tell the designer which materials cannot be costs are usually high, but so is their perfor- used. mance. The ratings given the designer will also pro- High-temperature plastics fall into the vide some idea of the short-term stiffness to usual categories of TSs and TPs. The TSs are be expected of various materials at elevated used principally by the aircraft and aerospace temperatures, as well as their thermal aging markets but also for automotive, industrial, resistance with regard to certain properties. medical, and electronic products. Epoxies are Establishing two parameters, ASTM D 648 principally used, with other plastics being TS and UL 746B, for a variety of materials pro- polyesters, phenolics, and urethanes. These vides the designer with a reasonable start- plastics are usually reinforced with the high ing point for initially assessing materials for strength fibers seen in Fig. 5-9, individually or high-temperature applications. Most high- in combination with S-glass, graphite, aramid, performance plastics are filled compounds, and others. About 85wt% of all RPs used since fillers and reinforcements (Fig. 5-9) gen- in conventional-temperature environments erally enhance high temperature strength and only require E-glass (14). stiffness. High-temperature TPs are available to A general definition for a high-tempera- compete withTSs, metals, ceramics, and other ture plastic is one having a thermal value in nonplastic materials. The heat-resistant TPs terms of ASTM D 648 and UL 746B higher include polyetheretherketone (PEEK) and than 149°C (300°F). There are numerous polyethersulfone (PES), polyamideimide, plastics that are both processable and have liquid crystal polymer (LCP) and others. a 0 I= a L .- - Q m LL - I I I I I I I I 1 I I I I 500,Ooo - 400,000 - .- a 5 i GLASS t; 300,000 - - p: t- v) 200mo ‘ \ \ S T E E L \ALUMINUM - TITANIUM 100,Ooo - 0 I I I I I I I I I I 1 I I 1 0 400 800 1200 1600 2000 2400 2800 Temperature PVC- PC PP PMMA 559- Nylon - PTFE 500 - PS - CA -ABS - PUR -Red Oak 4% - PVC 400 Fig. 5-10 Basic guide to flash ignition and self- ignition temperatures (per ASTM D 1929) for plastics and red wood. 5 Testing and Meaning of Test Data 327 Electrical Property The resistance of most plastics to the flow of direct current is very high. Both surface and volume electrical resistivities are impor- tant properties for applications of plastics in- sulating materials. The volume resistivity is the electrical resistance of the material mea- sured in ohms as though the material was a conductor. Insulators will not sustain an in- definitely high voltage; as the applied voltage is increased, a point is reached where a drastic decrease in resistance takes place accompa- nied by a physical breakdown of the insula- tor. This is known as the dielectric strength, which is the electric potential in volts, which would be necessary to cause the failure of a 1/8-in. thick insulator (Chapter 4, ELEC- TRICAWELECTRONICS PRODUCT). Electrical Resistance Specimens for these tests may be any prac- tical form, such as flat plates, sheets, or tubes. These tests describe methods for determin- ing the severaI properties defined below. Two electrodes are placed on or embedded in the surface of a test specimen. Different proper- ties are obtained. Insulation resistance is the ratio of direct voltage applied to the electrodes to the total current between them; dependent upon both volume and surface resistance of the speci- men. In materials used to insulate and sup- port components of an electrical network, it is generally desirable to have insulation resis- tance as high as possible. Volume resistivity is the ratio of the poten- tial gradient parallel to the current density. Surface resistivity is the ratio of the po- tential gradient parallel to the current along its surface to the current per unit width of the surface. Knowing the volume and surface resistivity of an insulating material makes it possible to design an insulator for a specific application. Volume resistance is the ratio of direct volt- age applied to the electrodes to that portion of current between them that is distributed through the volume of the specimen. Surface resistance is the ratio of the direct voltage applied to the electrodes to that por- tion of the current between them that is in a thin layer of moisture or other semicon- ducting material which may be deposited on the surface. High volume and surface resis- tance are desirable in order to limit the cur- rent leakage of the conductor that is being insulated. Arc Resistance This test shows the ability of a material to resist the action of an arc of high voltage and low current close to the surface of the insu- lation in tending to form a conducting path therein. The arc resistance data are of rela- tive value only for distinguishing materials of nearly identical composition, such as for qual- ity control, development, or identification. Dielectric Strength Specimens are thin sheets or plates hav- ing parallel plane surfaces and are of a size sufficient to prevent flashing over. Dielec- tric strength varies with thickness and, there- fore, specimen thickness must be reported. The dielectric strength varies inversely with the thickness of the specimen. The dielectric strength of plastics will drop sharply if holes, bubbles, or contaminants are present in the specimen being tested. Since temperature and humidity affect re- sults, it is necessary to condition each type of material as directed in the specification for that material. The test for dielectric strength must be run in the conditioning chamber or immediately after removing the specimen from the chamber. The specimen is placed between heavy cylindrical brass electrodes, which carry elec- trical current during the test. There are two ways of running this test for dielectric strength. In the short-time test the voltage is increased from zero to breakdown at a uni- form rate. The precise rate of voltage rise is specified in the governing material speci- fications. In the step-by-step test the initial 328 5 Testing and Meaning of Test Data voltage applied is 50% of breakdown voltage shown by the short-time test. It is increased at rates specified for each type of material, and the breakdown level is noted. Breakdown by these tests means passage of sudden excessive current through the specimen and can be ver- ified by instruments and by visible damage to the specimen. This test is an indication of the electrical strength of a material as an insulator. The dielectric strength of an insulating material is the voltage gradient at which electric fail- ure or breakdown occurs as a continuous arc (the electrical property analogous to tensile strength in mechanical properties). The di- electric strength of materials varies greatly with several conditions such as humidity and geometry, and it is not possible to directly apply the standard test values to field use unless all conditions, including specimen di- mensions, are the same. Because of this, the dielectric strength test results are of relative rather than absolute value as a specification guide. Dielectric Constant and Dissipation Factor The specimen may be a sheet of any size convenient to test, but should have uniform thickness. The test may be run at standard room temperature and humidity, or in special sets of conditions as desired. In any case, the specimens should be preconditioned to the set of conditions used. Electrodes are applied to opposite faces of the test specimen. The capacitance and dielectric loss are then mea- sured by comparison or substitution methods in an electric bridge circuit. From these mea- surements and the dimensions of the speci- men, dielectric constant and loss factor are computed. The dissipation factor is a ratio of the real power (in-phase power) to the reactive power (power 90" out of phase). It is also defined as: (1) IT is the ratio of conductance of a capac- itor in which the material is the dielectric to its susceptance, (2) IT is the ratio of its paral- lel reactance to its parallel resistance; it is the tangent of the loss angle and the cotangent of the phase angle, and (3) IT is a measure of the conversion of the reactive power to real power, showing as heat. The dielectric constant is the ratio of the capacity of a condenser made with a particu- lar dielectric to the capacity of the same con- denser with air as the dielectric. For a mate- rial used to support and insulate components of an electrical network from each other and ground, it is generally desirable to have a low level of dielectric constant. For a material to function as the dielectric of a capacitor, on the other hand, it is desirable to have a high value of dielectric constant, so that the capac- itor may be physically as small as possible. The loss factor is the product of the di- electric constant and the power factor, and is a measure of total losses in the dielectric material. Optical Property Examples of plastics' transparent proper- ties are shown in Table 5-6. A basic behav- ior of the appearance of a transparent ma- terial, that is one that transmits light, is its transmittance: the ratios of the intensities of light passing through and the light incident on the specimen. Similarly, the appearance of an opaque material (one which may reflect light but does not transmit it) is characterized by its reflectance, the ratio of the intensities of the reflected and incident light. A translucent substance is one that transmits part and re- flects part of the light incident on it. Gloss is the geometrically selective reflection of a sur- face responsible for its shiny or lustrous ap- pearance. This property may be measured by the use of various photoelectric instruments or simply by observation. Haze and Huminous Transmittance In this test, haze of a specimen is defined as the percentage of transmitted light that, in passing through the specimen, deviates more than 2.5" from the incident beam by forward scattering. Basically it is defined as the ratio of transmitted to incident light. 5 Testing and Meaning of Test Data 329 Table 5-6 Examples of plastics' transparent properties Methyl Methyl Methacrylate ASTM Methacrylate Polystyrene Styrene Properties Method Units (Acrylic) (Styrene) Polycarbonate Copolymer Refractive index (no) Abbe. value ( v ) dnldt x 10-sIoC Haze (%) Luminous transmittance thickness) (0.125-in. Critical angle (ic) Deflection temperature 3.6 Flmin., 264 psi 3.6 FImin., 66 psi Coefficient of linear thermal expansion Recommended max. cont. service temp. Water absorption (immersed 24 hrs. at 73°F) Specific gravity (density) Hardness (0.25-in. sample) Impact strength (Izod Notch) Dielectric strength Dielectric constant 60 HZ 106 Hz Power factor 60- lo6 Hz Volume resistivity D 542 D 542 D1003 % D1003 % degree D648-56 "F D 696-44 in.Iin./"F x 10-6 "F D570-63 % D 792 D 785-62 D 256 ft.4b.h. D149-64 Vlmil D 150 D 150 D257 ohm-cm 1.491 1.590 57.2 30.9 8.5 12.0 1 2 t 3 92 88 42.2 39.0 198 180 214 230 3.6 3.5 198 180 0.3 0.2 1.19 1.06 M97 M 90 0.3-0.5 0.35 500 500 3.7 2.6 22.2 2.45 0.05 0.0002 0.03 0.0002-0.0004 10'8 >lo16 1.586 34.7 14.3 <3 89 39.1 280 270 3.8 255 0.15 1.20 M 70 12-17 400 2.90 2.88 0.0007 0.0075 8 x 10l6 1.562 35 14.0 t 3 90 39.6 212 3.6 200 0.15 1.09 M 75 450 3.40 2.90 0.006 0.013 1015 These qualities are considered in most ap- plications for transparent plastics, forming a basis for directly comparing the transparency of various grades and types of plastic. The data are of value when a material is consid- ered for optical purposes. Many transparent plastics do not have water clarity, and, for this reason, the data should indicate whether the material was natural or tinted when tested. Luminous Reflectance Opaque specimens should have at least one plane surface. Translucent and transparent 330 5 Testing and Meaning of Test Data specimens must have two surfaces that are in Chapter 4, TRANSPARENT AND plane and parallel. This test is the primary OPTICAL PRODUCTS, Properties, Perfor- method for obtaining colormetric data. The mances, and Products. property determined that is of design interest is luminous transmittance. Abrasion and Mar Resistance Opacity and Transparency Opacity or transparency is important when the amount of light to be transmitted is a con- sideration. These properties are usually mea- sured as haze and luminous transmittance. As reviewed haze is defined as the percentage of transmitted light through a test specimen that is scattered more than 2.5" from the incident beam. Luminous transmittance is the ratio of transmitted light to incident light. Table 5-7 provides the optical and various other prop- erties of different transparent plastics. Some definitions of key terms used in identifying optical conditions are reviewed In this test for transparent plastics, the loss of optical effects is measured when a specimen is exposed to the action of a special abrading wheel. In one type of test the amount of material lost by a specimen is determined when the specimen is exposed to falling abrasive particles or to the action of an abrasive belt. In another test, the loss of gloss due to the dropping of loose abrasive on the specimen is measured. The results produced by the different tests may be of value for research and development work when it is desired to improve a material with respect to one of the test methods. The variables that enter into tests of this type are Table 5-7 Notable behaviors of some transparent plastics Generic Family Notable Characteristics Transparent ABS Acrylic (PMMA) Good impact properties, good processibility Excellent resistance to outdoor exposure, crystal clarity Allyl diglycol carbonate Cellulosics Good abrasiodchemical resistance, thermoset Heat sensitive, limited chemical resistance, good toughness Nylon, amorphous PET, PETG Polyarylate Polycarbonate Excellent abrasion resistance, moisture sensitive Good barrier properties, not weatherable, clarity dependent Excellent UV resistance, high heat distortion Excellent toughness, good thermaUflammability characteristics on processing, orientation greatly increases physical properties Polyetherimide Polyphthalate carbonate Poly ethersulfone Poly-4-methylpentene-1 Good chemicaVso1vent resistance, good thermal/flammability Good thermal properties, autoclavable Excellent thermal stability, resists creep UV/moisture sensitive, high crystalline melting point, lowest density of all thermoplastics properties, inherent high color Poly phenylsulfone Polystyrene Poly sulfone PVC, rigid Excellent thermal stability, resists creep, inherent high color Excellent processibility, poor UV resistance, brittle Excellent thermaUhydrolytic stability, poor weatherability/ Excellent chemical resistance/electrical properties, weatherable, impact strength decomposition evolves HCl gas Styrene acrylonitrile Styrene butadiene Styrene maleic anhydride Styrene methyl methacrylate Thermoplastic urethane, rigid Good stress-crack and craze resistance, brittle Good processibility, no stress whitening Higher-heat styrenic, brittle Good processibility, slightly improved weatherability Excellent chemical/solvent resistance, good toughness 5 Testing and Meaning of Test Data 331 so numerous that it is questionable how the information from such tests could be used. Some of the factors are type of abrasive, shape of abrading particle, nature of plastic material, speed of action on the plastic, shape of the part and its temperature, the manner in which the abrasive is attached to the backing, and the bonding agent. Currently, these tests are of no practical value to the designer and the only approach to the problem of scratch, mar, and abrasion resistance is to simulate actual performance needs. For optical purposes, a cast sheet in the allyl family of plastics known as CR39 has been used as a standard of comparison in evaluating scratch and mar resistance of a material. The CR39 is used for eye lenses and other optical products where the advantages of plastics are a consideration. Coatings have been developed for polycarbonate, acrylics, and other plastics that dramatically improve the scratch and mar resistance of these materials. Weathering Outdoor Weathering The specimen has no specified size. Spec- imens for this test may consist of any stan- dard fabricated test specimen or cutlpunch pieces of sheet or machined sample. Speci- mens are mounted outdoors on racks slanted at 4.5” and facing south. It is recommended that concurrent exposure be carried out in many varied climates to obtain the broadest, most representative total body of data. Sam- ple specimens are kept indoors as controls and for comparison. Reports of weathering describe all changes noted, areas of exposure, and period of time. Outdoor testing is the most accurate method of obtaining a true picture of other resistance. The only drawback of this test is the time required for several years’ exposure that are usually located in different climatic zones around the world. A large number of specimens are usually required to allow peri- odic removal and to run representative labo- ratory tests after exposure. Accelerated Weathering The specimen may be any shape. Artifi- cial weathering has been defined by ASTM as “The exposure of plastics to cyclic labora- tory conditions involving changes in temper- ature, relative humidity, and ultraviolet (UV) radiant energy, with or without direct water spray, in an attempt to produce changes in the material similar to those observed after long-term continuous outdoor exposure.” Three types of light sources for artificial weathering are in common use: (1) enclosed UV carbon arc [7.5 UV energy output, ap- prox. (x sunlight)], (2) open-flame sunshine carbon, and (3) water-cooled xenon arc. Se- lection of the light source involves many con- ditions and circumstances, such as the type of material being tested, product service condi- tions, previous testing experience, or the type of information desired. Since weather varies from day to day, year to year, and place to place, no precise cor- relation exists between artificial laboratory weathering and natural outdoor weathering. However, standard laboratory test conditions produce results with acceptable reproducibil- ity and in general agreement with data ob- tained from long-time outdoor exposures. Fairly rapid indications of weatherabil- ity are therefore obtainable on samples of known materials that through testing expe- rience over a period of time have general correlations established. There is no artifi- cial substitute for precisely predicting out- door weatherability on materials with no previous weathering history. Weatherome- ters produce conditions to accelerate effects that would be observed in specimens exposed outdoors. Accelerated Exposure to Sunlight The Atlas Type FDA-IR Fadeometer is used primarily to check and compare color stability. Besides determining the stability of various pigments needed to provide both standard and custom colors, the Fadeometer is helpful in preliminary studies of various stabilizers, dyes, and pigments compounded in plastics to prolong their useful life. 332 5 Testing and Meaning of Test Data It is primarily for testing materials to be used in products subject to indoor exposure and to sunlight. Exposure in the Fadeometer cannot be related directly to exposure in di- rect sunlight, partially because other weather factors are always present outdoors. Conditioning Procedure As reviewed it is important that test speci- mens or products be properly prepared based on available specifications andor standards that provide controlled conditioning proce- dures when conducting weathering as well as all other tests. The following is one example. There are other conditions set forth to pro- vide for testing at higher or lower levels of temperature and humidity. Procedure for conditioning test specimens can call for the following periods in a standard laboratory atmosphere [50 f 2% relative hu- midity, 73.4 f 13°F (23 f 1°C): Adequate air circulation around all specimens must be pro- vided. The reason for this test is due to the fact the temperature and moisture content of plastics affects different properties such as the physical and electrical properties. In or- der to get comparable test results at different times and in different laboratories a standard has been established. Harmful Componenl Review in Chapter 2, WEATHERING/ ENVIRONMENT, Weather Resistance. Environmental Stress Cracking This test wasprepared and is limited to type 1 (low-density) polyethylenes. Specimens are annealed in water or steam at 212°F (100°C) for 1 h and then equilibrated at room tem- perature for 5-24 h. After conditioning the specimens are nicked according to directions given. The specimens are bent into a U shape in a brass channel and inserted into a test tube that is then filled with fresh reagent (Igepal). The tube is stoppered with an aluminum- covered cork and placed in a constant tem- perature bath at 122°F (50°C). These specimens are inspected periodically and any visible crack is considered a failure. The duration of the test is reported along with the percentage of failures. The crack- ing obtained in this test is indicative of what may be expected from a wide variety of stress-cracking agents. The information can- not be translated directly into end-use service prediction, but serves to rank various types and grades of polyethylene categories of re- sistance to environmental stress cracking. Though restricted to type 1 polyethylene, this test can be used on high and medium density PE materials as well as other plastics, in which case it would be considered a modified test. Fire Fiamma bility Underwriters' Laboratories (UL) Test 94 can be used. The placement of the speci- men, the size of the flame, and its position and location with respect to the specimen are described in detail in this important UL specifications. Depending on their nonburn- ing to burning capabilities, results of tests are reported as being materials classed 94V-0, 94V-1, 94V-2, 94-5V, etc. (Chapter 2, HIGH TEMPERATURE, Flammability). Oxygen Index The test method describes a procedure for measuring the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will support glowing combus- tion of plastics. The oxygen index is the min- imum concentration of oxygen expressed as a volume percent in a mixture of oxygen and nitrogen that will just support glowing com- bustion of a material initially at room tem- perature under the conditions of this method. From this description, it is apparent that the lower the oxygen index the more the plastic contributes to the support of combustion. Many of the basic plastics require additives that will improve their resistance to support- ing combustion. These improvements vary in degree, and the designer must be cautioned not to over specify the requirement for 5 Testing and Meaning of Test Data 333 flammability. It should be recognized that the highest protection against burning can be costly, can contribute to higher specific gravity, and can adversely affect mechanical properties. Analyzing Testing and Quality Control Designers and processors should keep quality under control and demand consistent materials that can be used with minimum of uncertainty. Basically involves inspection and testing of raw materials to the finished prod- ucts. Plant QC is as important to the end result as selecting the best processing and control conditions with the correct grade of plastic, in terms of both properties and appearance. After the correct plastic has been chosen, any blending, reprocessing, and storage stages of operation need to be fre- quently or continuously updated. The pro- cessor should set up specific measurements of quality to prevent substandard products reaching the customer. QC involve those quality assurance actions which provide a means to control, measure, and establish re- quirements of the characteristics of plastic materials, processes, and products. From a practical aspect, when the expres- sion “quality control” is use, we tend to think in terms of a good or excellent product. In industry, it is one that fulfills customer’s ex- pectations. These expectations or standards of performance are based on the intended use and selling price of the product. Con- trol is the process of regulating or direct- ing an activity to verify its conformance to a standardhpecification and to take correc- tive action if required. Therefore QC is the regulatory testing process for those activi- ties that measure a product’s Performance, compare that performance with established standards/specifications, and pursue correc- tive action regardless of where those activi- ties occur. There are three phases in the evolution of most QC systems; (1) defect detection where an “army” of inspectors tries to identify de- fects; (2) defect prevention where the process is monitored, and statistical methods are used to control process variation, enabling adjust- ments to the process to be made before de- fects are produced; and (3) total quality con- trol where it is finally recognized that quality must extend throughout all functions and it is management’s responsibility to integrate and lead the various functions towards the goals of commitment to quality and customer-first orientation (3). When using the defect-detection approach to quality control certain problems develop. Inspection does nothing to improve the pro- cess and is not very good at sorting good- from-bad. Also, sampling plans developed to support an acceptable quality level (AQL) of 5%, for example, say that a company is con- tent to deliver or reject 5% defects. There are different methods to apply QC on-line. An example is with infrared mea- surement. The ability to record IR spectra of plastic melts provides for process moni- toring and control in the manufacture pro- cess. Precise information on quality can be obtained rapidly. Furthermore, it is also pos- sible to make measurements on unstable in- termediates of importance. Although spec- troscopy on melts is considerably different from that on solid materials, this does not limit the information content. IR has for many years been an important aid to inves- tigating the chemical and physical properties of molecules. It gives qualitative and quanti- tative information on chemical constituents, functional groups, impurities, etc. As well as its use in studying low molecular weight com- pounds, it is used with equal success for char- acterizing plastics. It is a highly informative method of applying testing. To ensure that QC and testing proce- dures are followed a quality control man- ual should be implemented. It is a docu- ment usually setup in a computer’s software program that states and provides the de- tails of the plant’s quality objectives and how they will be implemented, documented, and followed. Statistical Process Control and Quality Control The term statistics basically is a summary value calculated from the observed values in 334 5 Testing and Meaning of Test Data a sample or product. It is a branch of math- ematics dealing with the collection, analysis, interpretation, and presentation of masses of numerical data. The word statistic has two generally accepted meanings: (1) a collection of quantitative analysis data (data collection) pertaining to any subject or group, especially when the data are systematically gathered and collated and (2) the science that deals with the collection, tabulation, analysis, in- terpretation, and presentation of quantitative data. Statistical process control (SPC) is an im- portant on-line method in real time by which a production process can be monitored and control plans can be initiated to keep quality standards within acceptable limits. Statistical quality control (SQC) provides off-line analysis of the big picture such as what was the impact of previous improvements. It is important to understand how SPC and SQC operate. There are basically two possible ap- proaches for real-time SPC. The first, done on-line, involves the rapid dimensional mea- surement of a part or a non-dimensional bulk parameter such as weight that is the more practical method. In the second approach, contrast to weight, other dimensional mea- surements of the precision needed for SPC are generally done off-line. Obtaining the fi- nal dimensional stability needed to measure a part may take time. As an example, amor- phous injection molded plastic parts usually require at least a half-hour to stabilize. The SPC system starts with the premise that the specifications for a product can be defined in terms of the product’s (customer’s) requirements, or that a product is or has been produced that will satisfy those needs. Gen- erally a computer communicates with a series of process sensors and/or controllers that op- erate in individual data loops. The computer sends set points (built on which performance characteristics of the product must have) to the process controller that constantly feeds back to the computer to signal whether or not the set of points are in fact maintained. The systems are programmed to act when key variables af- fecting product quality deviate beyond set limits (3).
