Kutz M. Handbook of materials selection

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578 PLASTICS TESTING

between them that is distributed through the volume of the specimen. Or, simply

stated, the volume resistance is the resistance to leakage through the body of

the material. Volume resistance generally depends upon the material. The term

most commonly used by designers is volume resistivity. It is deﬁned as the ratio

of the potential gradient parallel to the current in the material to the current

density or, simply stated, the volume resistivity of a material is the electrical

resistance between the opposite faces of a unit cube for a given material and at

a given temperature.

High-volume resistivity materials are desirable in applications requiring su-

perior insulating characteristics.

The surface resistance of a material is deﬁned as the ratio of the direct voltage

applied to the electrodes to that portion of the current between them that is

primarily in a thin layer of moisture or other semiconducting material that may

be deposited on the surface. Simply stated, surface resistance is the resistance

to leakage along the surface of an insulator. The surface resistance of a material

depends upon the quality and cleanliness of the surface of the product. A product

with oil or dirt particles on it gives lower surface resistance values.

The test procedures to determine electrical resistance values are rather com-

plex. ASTM D257 (IEC 93) describes the procedures as well as the complex

electrodes and apparatus required to carry out the test in detail.

Temperature and humidity both seem to affect the insulation resistance ap-

preciably. As a rule, the higher the temperature and humidity, the lower the

insulation resistance of a material.

3.4 Arc Resistance (ASTM D495)

Arc resistance is the ability of a plastic material to resist the action of a high-

voltage electrical arc, usually stated in terms of time required to form material

electrically conductive. Failure is characterized by carbonization of the surface,

tracking, localized heating to incandescence, or burning. In all applications in

which conducting elements are brought into contact, arcing is inevitable.

Switches, circuit breakers, and automotive distributor caps are a few good ex-

amples of applications where arcing is known to cause failure. Another term

that is generally associated with arcing is tracking. Tracking is deﬁned as a

phenomenon where a high-voltage source current creates a leakage or fault path

across the surface of an insulating material by slowly but steadily forming a

carbonized path appearing as a thin, wiry line between the electrodes. Tracking

is accelerated by the presence of surface contaminants such as dirt and oil and

by the presence of moisture. Resistance to arcing or tracking depends upon the

type of plastic materials such as phenolics that tend to carbonize easily and

therefore have relatively poor arc resistance. Plastics such as alkyds, melamines,

and ﬂuorocarbons are excellent arc-resistant materials. The failures due to arcing

are not always because of carbonization or tracking. Many plastics such as acryl-

ics simply do not carbonize. However, they do form ignitable gases that cause

the product to fail in a short time. Arc resistance of plastics can be improved

substantially by the addition of ﬁllers such as glass, mineral, wood ﬂour, asbes-

tos, and other inorganic ﬁllers. Table 3 lists the arc resistance of some common

plastic materials.

The determination of the arc resistance of plastics using a standard test

method has always been a problem because of the numerous ways the test can

3 ELECTRICAL PROPERTIES 579

Table 3 Arc Resistance of Various Plastics

Plastics

Arc Resistance

(sec)

Polycarbonate (10–40 percent glass ﬁlled) 5–120

Polycarbonate 10–120

Polystyrene (high impact) 20–100

ABS 50–85

Polystyrene (general purpose) 60–80

Rigid polyvinyl chloride 60–80

Polysulfone 75–190

Urea formaldehyde 80–150

Ionomer 90–140

SAN 100–150

Epoxy 120–150

Acetal (homopolymer) 130

Polyethylene (low-density) 135–160

Polypropylene 135–180

PTFE

⬎200

Acrylics No track

be conducted. ASTM has developed four basic test methods to satisfy this con-

cern. ASTM D495, which is high-voltage, low-current, dry arc resistance of solid

electrical insulation, has been the most widely used and accepted. This test is

only intended for the preliminary screening of materials, for detecting the effect

of changes in formulations, and for quality control testing. Since the test is

conducted under clean and dry laboratory conditions that are rarely encountered

in practice, the prediction of materials from test results is next to impossible.

Figure 18 illustrates a typical setup for an arc resistance test. The voltage is

applied intermittantly and severity is increased in steps until the failure occurs.

Arc resistance is measured in seconds to failure.

ASTM method D2132 outlines the procedure for determining dust and fog

tracking and erosion resistance of electrical insulating materials. The test is car-

ried out in a fog chamber with a standardized dust applied to the sample surface.

Failure is characterized by the erosion of the specimen or tracking. ASTM

D2302 also describes the test for differential wet tracking resistance of electrical

insulating materials with controlled water-to-metal discharges. The inclined

specimen is partially immersed in a water solution of ammonium chloride and

a wetting agent. Failure is characterized by tracking. ASTM D2303 describes

the test for liquid–contaminant inclined plane tracking and erosion of insulating

materials. In this test, the specimen is inclined at 45

⬚, and the electrolyte is

discharged onto the surface at a controlled rate, increasing the voltage at the

same time. The failure is marked by erosion and tracking.

Electromagnetic Interference/Radio Frequency Interface Shielding

The explosive growth of computers, telecommunication, and other electronic

devices has created an urgent need for understanding and developing test and

measurement techniques and standards for electromagnetic interference (EMI)

and radio frequency interface (RFI) shielding. All electronic devices are required

to have electromagnetic compatibility. It is a phenomenon caused by unrestricted

580 PLASTICS TESTING

Fig. 18 Arc resistance test. (Courtesy Bayer Corporation)

electrical and magnetic energy that escapes from any electrical device and

reaches a second unintended device. It is called EMI if the receiving device

malfunctions as a result of this pollution. Any electrical signal falling between

direct current (DC) electricity and the gamma-ray frequency region of the elec-

tromagnetic spectrum can be sources of EMI. For most practical purposes, the

majority of EMI problems are limited to a part of spectrum from 1 kHz to 10

GHz. This portion of electromagnetic spectrum is known as the radio frequency

interface (RFI) band and covers the radio and audio frequencies. EMI of some

form is experienced by us everyday. Some of the most common sources of EMI

are lightning, static electricity, television and radio receivers, electric motors,

electric appliances, paging system, and radar transmitters. Some of the most

common receptors of EMI are microprocessors, computers, hi-ﬁ equipment, elec-

tronic measuring equipment, radio and television receivers, remote control units,

aircraft navigation systems, and humans and animals.

The fact that electronic devices are both sources and receptors of EMI creates

a twofold problem. Since electromagnetic radiation penetrating the device may

cause electronic failure, manufacturers need to not only protect the operational

integrity of their product but also must comply with the regulations aimed at

reducing electromagnetic radiation emitted into the atmosphere.

The switch from metal to plastics as a housing material for electronic equip-

ment has contributed to the EMI shielding issue. Plastics are insulators to elec-

trical energy, so EMI waves pass freely through unshielded plastic without any

3 ELECTRICAL PROPERTIES 581

impedance or resistance. Metals, being conductors, reﬂect or absorb electromag-

netic energy, and unwanted electrical energy is easily grounded. Shielding, the

term used to describe the effective ‘‘blocking’’ of EMI, is provided by a con-

ductive barrier that harmlessly reﬂects or transmits electrical interference to

ground.

EMI/RFI shielding of electronic devices is achieved in many different ways.

Two of the most common ways to achieve shielding is either to coat the plastic

housing with conductive material or to make the housing material itself con-

ductive. The second method of using conductive plastics for shielding effective-

ness is relatively new and for the most part in research and development stage

with limited commercial applications. Coating methods on plastic parts are by

far the most popular and the most varied shielding techniques. Basically, they

all involve coating the ﬁnished plastic housing with a layer of conductive ma-

terial to cerate a faraday cage (a concept that an enclosed conductive housing

has a zero electrical ﬁeld), resulting in a EMI shield. Electroless plating method

is also gaining popularity as an alternate method to conductive coatings.

Shielding Effectiveness Measurement

Electromagnetic energy is most effectively measured in terms of attenuation.

Attenuation is the amount by which the intensity of an electromagnetic signal

is reduced by the introduction of a shielding medium. The attenuation is mea-

sured in decibels (dB) and is the ratio of the ﬁeld strength without the shield to

the ﬁeld strength with the shield. Shielding effectiveness measured in decibels

is a logarithmic scale, which means that an attenuation measurement of 50 dB

is ten times more effective than a measurement of 40 dB.

Four basic techniques for measuring shielding effectiveness have been devel-

oped:

Open ﬁeld testing

Shielded box

Coaxial transmission line

Shielded room

Conductivity Measurement

In production, conductivity testing becomes an important means for quality con-

trol of individual parts as well as for part-to-part testing of assembled enclosures.

Surface resistivity, volume resistivity, and ohms-per-square measurement are

common tests for both conductive coating and conductive plastics and used as

a quick and practical quality check.

Surface Resistivity. Surface resistivity measures the resistance of a material

to moving or distributing a charge over its surface. The lower the surface resis-

tivity value, the easier a material will redistribute an electrical charge over its

entire surface area. Surface resistivity test procedure is described in ASTM

D257.

Surface resistivity meters that allow direct reading in ohms/square measure-

ment are most commonly used. One of the meters uses a probe with an inner

and an outer ring. A charge is applied to the inner ring, which travels over the

582 PLASTICS TESTING

Fig. 19 Point-to-point resistance measurement. (Courtesy Enthone-EMI Corporation)

surface of the material and then is recorded by the outer ring. In addition to

these resistivity meters, manufactures often use a digital ohmmeter to measure

the point-to-point resistance between two probes in contact with metallic coating.

This method utilizes two spring-loaded probes that are placed at designated

positions on the part or between two areas on mating parts. The conductivity

between the two points is measured. This method is illustrated in Fig. 19.

Volume Resistivity. Volume resistivity measures the ability of a material to

resist moving charge through its volume. The lower the volume resistivity of a

material, the more conductive the material. This test procedure is also described

in detail in ASTM D257.

Ohms-per-Square Testing. Ohms-per-square testing involves the placement

of a probe on a surface of the part. This probe has two outside blocks of equal

geometry and a space of equivalent dimension between the blocks. The probe

is simply placed on the part and conductivity is measured. The test is extremely

useful in quick checking the coated parts to verify minimum thickness of a

conductive ﬁlm. However, the test is not very effective for determining the over-

all conductivity of a coating across an actual part.

4 WEATHERING PROPERTIES

The increased outdoor use of plastics has created a need for a better understand-

ing of the effect of the environment on plastic materials. The environmental

factors have signiﬁcant detrimental effects on appearance and properties. The

severity of the damage depends largely on factors such as the nature of the

environment, geographic location, type of polymeric material, and duration of

exposure. The effect can be anywhere from a mere loss of color or a slight

crazing and cracking to a complete breakdown of the polymer structure. Any

attempt to design plastic parts without a clear understanding of the degradation

mechanisms induced by the environment would result in a premature failure of

the product. The major environmental factors that seriously affect plastics are:

4 WEATHERING PROPERTIES 583

1. Solar radiations—ultraviolet (UV), infrared (IR), X-rays

2. Microorganisms, bacteria, fungus, mold

3. High humidity

4. Ozone, oxygen

5. Water: vapor, liquid, or solid

6. Thermal energy

7. Pollution: industrial chemicals

The combined effect of the factors mentioned above may be much more severe

than the effect of any single factor, and the degradation processes are accelerated

many times. Many test results do not include these synergistic effects, which

almost always exist in real-life situations.

UV Radiation. All types of solar radiation have some sort of detrimental

effect on plastics. Ultraviolet radiation is the most destructive of all radiations.

The energy in ultraviolet radiation is sufﬁciently strong enough to break molec-

ular bonds. This activity in the polymer brings about thermal oxidative degra-

dation that results in embrittlement, discoloration, and an overall reduction in

physical and electrical properties. Xenon arc lamps, ﬂuorescent lighting, sun

lamps, and other artiﬁcial sources also emit a similar type of harmful radiation.

Other factors in the environment such as heat, humidity, and oxygen accelerate

the UV degradation process.

One of the best methods of protecting plastics against UV radiation is to

incorporate UV absorbers or UV stabilizers into the plastic materials. The UV

absorbers provide preferential absorption to most of the incident UV light and

are able to dissipate the absorbed energy harmlessly. Thus, the polymer is pro-

tected from harmful radiation at the cost of UV absorbers which are destroyed

in the process with time. Several types of organic and inorganic UV absorbers,

are developed for this purpose. Almost all inorganic pigments absorb UV radi-

ation to a certain extent and provide some degree of protection. Perhaps the

most effective pigments are certain types of carbon black that absorb over the

entire range of UV and visible radiation and transform the energy into less

harmful radiation.

UV stabilizers, unlike UV absorbers, inhibit the bond rupture by chemical

means or dissipate the energy to lower levels that do not attack the bonds. The

effectiveness of such additives when incorporated with the polymer can be de-

termined by various test methods discussed in this chapter.

Microorganisms. Polymeric materials are generally not vulnerable to mi-

crobial attack under normal conditions. However, low-molecular-weight addi-

tives such as plasticizers, lubricants, stabilizers, and antioxidants may migrate

to the surface of plastic components and encourage the growth of microorgan-

isms. The detrimental effect can be seen readily through the loss of properties,

change in aesthetic quality, loss of transmission (optical), and increase in brit-

tleness. The rate of growth depends upon many factors, such as heat, light, and

humidity. Preservatives, also known as fungicides or biocides, are added to plas-

tic materials to prevent the growth of microorganisms. These additives are highly

toxic to lower organisms but do not affect the higher organisms.

584 PLASTICS TESTING

It is necessary to evaluate the effectiveness of various antimicrobial agents

both on a laboratory scale as well as in actual outdoor exposure. Many such

methods have been devised to perform these tests.

Oxygen, Moisture, Thermal Energy, and Other Environmental Factors. In

addition to UV radiation and microorganisms, there are many other relevant

factors that can add to polymer degradation. Even though most polymers react

very slowly with oxygen, elevated temperatures and UV radiation can greatly

promote the oxidation process. Water is considered relatively harmless; however,

it can have at least three kinds of effects leading to early polymer degradation:

a chemical effect, that is, the hydrolysis of unstable bonds; a physical effect,

that is, destroying the bond between a polymer and ﬁller resulting in chalking;

the third is a photochemical effect. Thermal energy plays an indirect role in

polymer degradation by accelerating hydrolyis, oxidation, and photochemical

reactions induced by other factors. Many other factors such as ozone, atmos-

pheric contaminants including dirt, soot, smog, sulfur dioxide, and other indus-

trial chemicals, have a signiﬁcant effect on polymers.

4.1 Accelerated Weathering Tests

Most data on the aging of plastics are acquired through accelerated tests and

actual outdoor exposure. The latter being a time-consuming method, accelerated

tests are often used to expedite screening the samples with various combinations

of additive levels and ratios. A variety of light sources are used to simulate the

natural sunlight. The artiﬁcial light sources include carbon arc lamps, xenon arc

lamps, ﬂuorescent sun lamps, and mercury lamps. These light sources, except

ﬂuorescent, are capable of generating a much higher intensity light than natural

sunlight. Xenon arc lamps can be operated over a wide range from below peak

sunlight to twice the sunlight levels. Quite often, a condensation apparatus is

used to simulate the deterioration caused by sunlight and water as rain or dew.

Modern instruments have direct specimen spray on front and/or back side of

the specimen.

There are three major accelerated weathering tests:

1. Exposure to carbon arc lamps

2. Exposure to xenon arc lamps

3. Exposure to ﬂuorescent UV lamps

The xenon arc, when properly ﬁltered, most closely approximates the wavelength

distribution of natural sunlight.

Exposure of Plastics to Fluorescent UV Lamps and Condensation

(ASTM G53, ISO 4892)

This method is meant to simulate the deterioration caused by sunlight and dew

by means of artiﬁcial ultraviolet light and condensation apparatus (Figs. 20 and

21). Solar radiation ranges from ultraviolet to infrared. Ultraviolet light of wave-

lengths between 290 and 350 nm is the most efﬁcient portion of terrestrial

sunlight that is damaging to plastics. In the natural sunlight spectrum, energy

below 400 nm accounts for less than 6% of the total radiant energy. Since the

4 WEATHERING PROPERTIES 585

Fig. 20 UV light and condensation apparatus. (Courtesy Q-Panel Lab Products)

Fig. 21 Cross section of a UV light and condensation apparatus.

(Courtesy Q-Panel Lab Products)

586 PLASTICS TESTING

special ﬂuorescent UV lamps radiate between 280 and 365 nm, they accelerate

the degradation process considerably. In recent years, the UVA-340 lamps have

increased in popularity because of the poor results of the conventional FS-40

lamps.

The test apparatus basically consists of a series of UV lamps, a heated water

pan, and test specimen racks. The temperature and operating times are indepen-

dently controlled both for UV and the condensation effect. The test specimens

are mounted in specimen racks with the test surfaces facing the lamp. The test

conditions are selected based on requirements and programmed into the unit.

The specimens are removed for inspection at a predetermined time to examine

color loss, crazing, chalking, and cracking.

Exposure of Plastics to Carbon Arc-Type Light and Water (ASTM D1499,

ISO 4892)

This method is very useful in determining the resistance of plastic materials

when exposed to radiation produced by carbon arc lamps. There are basically

two different types of carbon arc lamps used as the source of radiation. The ﬁrst

type is an enclosed carbon arc lamp. The second type is known as an open-

ﬂame sunshine carbon arc. The enclosed carbon arc apparatus basically consists

of either single or twin enclosed carbon arc lamps mounted in a chamber. The

ﬂame portion of the carbon arc lamp is surrounded by a bell-shaped borosilicate

glass globe. The globe ﬁlters out UV radiation below 275 nm not found in direct

sunlight and creates a semisealed atmosphere in which the arc burns more ef-

ﬁciently. The open-ﬂame sunshine carbon arc lamp operates in a free ﬂow of

air. This lamp accommodates three upper and three lower electrodes that are

consumed during 24–26 h of continuous operation. The lamp is typically sur-

rounded by Corex glass ﬁlters that cut off light below 255 nm. The lamp is

designed to run with ﬁlters in place; however, they can be removed to increase

the available UV energy below 300 nm and thereby increase signiﬁcantly the

rate of photodegradation of UV-sensitive materials. The enclosed arc-type ap-

paratus uses an open-ended drum-type cylinder, whereas the sunshine arc type

uses a cylindrical framework. Both apparatuses revolve around centrally

mounted arc lamps. The provision is also made to expose the specimen to water,

which is sprayed through nozzles. The light-on, light-off, and water-spray cycles

are independent of each other, and the apparatus can be programmed to operate

with virtually any combination. A black panel temperature inside the test cham-

ber can be monitored and controlled by a sensor mounted directly on the re-

volving specimen rack.

Exposure of Plastics to Xenon Arc-Type Light and Water (ASTM D2565,

G26, ISO 4892)

A ﬁltered xenon arc-type light source is one of the most popular indoor exposure

tests since it exhibits a spectral energy distribution of sunlight at the surface of

the earth. The xenon arc lamp consists of a burner tube and a light ﬁlter system

with interchangeable glass ﬁlters used in combination to provide a spectral dis-

tribution that approximates natural sunlight exposure conditions. The apparatus

typically has a built-in recirculating system that recirculates distilled or deionized

water through the lamp. The water cools the xenon burner and ﬁlters out long

4 WEATHERING PROPERTIES 587

wavelength infrared energy. For air-cooled lamps, this is accomplished by the

use of optical ﬁlters.

Two basic procedures are recommended. Procedure A is a normal operating

procedure for comparative evaluation within a series exposed simultaneously in

one instrument. Procedure B is used for comparing results among instruments.

Both procedures are described in detail in the ASTM Standards Manual.In

either case, it is highly recommended that a controlled irradiance exposure sys-

tem be used. This is best accomplished through the use of a continuously con-

trolled monitor that can automatically maintain uniform intensity at preselected

wavelengths or wavelength range, when broadband control is being used.

Interpretations and Limitations of Accelerated Weathering Test Results

There is a general agreement among the researchers, manufacturers, and users

that the data from accelerated weathering tests cannot be correlated exactly with

the results of natural weathering. However, accurate ranking of the weatherabil-

ity of most material is possible by using improved test methods and sophisticated

equipment. Accelerated weathering tests were devised to study the effect of

actual outdoor weather in a relatively short time period. These tests often pro-

duce misleading results that are difﬁcult to interpret or correlate with the results

of actual outdoor exposure. The reason for such a contradiction is that in many

laboratory exposures, the wavelengths of lights are distributed differently than

in normal sunlight, possibly producing effects different from those produced by

outdoor weathering. All plastics seem to be especially sensitive to wavelengths

in the ultraviolet region. If the accelerated device has unusually strong emission

at the wavelength of sensitivity of a particular polymer, the degree of accelera-

tion is disproportionately high compared to outdoor exposure. The temperature

of the exposure device also greatly inﬂuences the rate of degradation of a poly-

mer. The higher temperature may cause oxidation and the migration of additives,

which, in turn, affects the rate of degradation. One of the limitations of accel-

erated weathering devices is their inability to simulate the adverse effect of most

industrial environments and many other factors present in the atmosphere and

their synergistic effect on polymers. Some of the newly developed gas exposure

cabinets have partially overcome these limitations. These units are capable of

generating ozone, sulfur dioxide, and oxides of nitrogen under controlled con-

ditions of temperature and humidity.

Improved ultraviolet sources and more knowledge of how to simulate natural

wetness now make it possible to achieve reliable accelerated weathering results.

Before drawing any ﬁnal conclusions concerning the ability of a polymer to

withstand outdoor environment based on accelerated weathering tests, one must

conduct actual outdoor exposure tests for a reasonable length of time.

4.2 Outdoor Weathering of Plastics (ASTM D1435)

The test is devised to evaluate the stability of plastic materials exposed outdoors

to varied inﬂuences that comprise weather exposure conditions that are complex

and changeable. Important factors are climate, time of year, and the presence of

industrial atmosphere. It is recommended that repeated exposure testing at dif-

ferent seasons and over a period of more than one year be conducted to conﬁrm

exposure at any one location. Since weathering is a comparative test, control