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
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).