Kutz M. Handbook of materials selection

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568 PLASTICS TESTING

correlation in the overlapping regions is possible. However, due to differences

in elasticity, creep, and shear characteristics between different plastics, a general

correlation is not possible.

Durometer Hardness (ASTM D2240, ISO 868)

The Durometer hardness test is mostly used for measuring the relative hardness

of soft materials. The test method is based on the penetration of a speciﬁed

indentor forced into the material under speciﬁed conditions.

The Durometer hardness tester consists of a pressure foot, an indentor, and

an indicating device. The indentor is spring loaded and the point of the indentor

protrudes through the hole in the base. The test specimens are at least thick

–

-in.

and can be either molded or cut from a sheet. Several thin specimens may be

piled to form a -thick specimen but one piece specimens are preferred. The

–

-in.

poor contact between the thin specimens may cause results to vary considerably.

The test is carried out by ﬁrst placing a specimen on a hard, ﬂat surface. The

pressure foot of the instrument is pressed onto the specimen, making sure that

it is parallel to the surface of the specimen. The durometer hardness is read

within 1 sec after the pressure foot is in ﬁrm contact with the specimen.

Two types of durometers are most commonly used—type A and type D. The

basic difference between the two types is the shape and dimension of the in-

dentor. The hardness numbers derived from either scale are just numbers without

any units. Type A durometer is used with relatively soft material while type D

durometer is used with slightly harder material.

2 THERMAL PROPERTIES

Thermal properties of plastic materials are equally as important as mechanical

properties. Unlike metals, plastics are extremely sensitive to changes in temper-

ature. The mechanical, electrical, or chemical properties of plastics cannot be

looked at without looking at the temperature at which the values were derived.

Crystallinity has a number of important effects upon the thermal properties of a

polymer. Its most general effects are the introduction of a sharp melting point

and the stiffening of thermal mechanical properties. Amorphous plastics, in con-

trast, have a gradual softening range. Molecular orientation also has a signiﬁcant

effect on thermal properties. Orientation tends to decrease dimensional stability

at higher temperatures. The molecular weight of the polymer affects the low-

temperature ﬂexibility and low-temperature brittleness. Many other factors such

as intermolecular bonding, cross-linking, and copolymerization all have a con-

siderable effect on thermal properties. From the above discussion, it is very clear

that the thermal behavior of polymeric materials is rather complex. Therefore,

in designing a plastic part or selecting a plastic material from the available

thermal property data, one must thoroughly understand the short-term as well

as the long-term effect of temperature on properties of that plastic material.

2.1 Tests for Elevated Temperature Performance

Designers and material selectors of plastic products constantly face the challenge

of selecting a suitable plastic for elevated temperature performance. The difﬁ-

culty arises due to the varying natures and capabilities of various types and

grades of plastics at elevated temperatures. Many factors are considered when

selecting a plastic for a high-temperature application. The material must be able

2 THERMAL PROPERTIES 569

to support a design load under operating conditions without objectionable creep

or distortion. The material must not degrade or lose necessary additives that will

cause drastic reduction in the physical properties during the expected service

life.

All the properties of plastic materials are not affected in a similar manner by

elevating temperature. For example, electrical properties of a particular plastic

may show only a moderate change at elevated temperatures, while the mechan-

ical properties may be reduced signiﬁcantly. Also, since the properties of plastic

materials vary with temperature in an irregular fashion, they must be looked at

as a function of temperature in order to obtain more meaningful information.

From the foregoing, it is quite clear that a single maximum-use temperature that

will apply to all the important properties in high-temperature applications is

simply not possible.

One of the most important considerations while studying the performance of

plastics at elevated temperatures is the dependence of key properties such as

modulus, strength, chemical resistance, and environmental resistance on time.

Therefore, the short-term heat resistance data alone is not adequate for designing

and selecting materials that require long-term heat resistance. For the sake of

convenience and simplicity, we divide the elevated temperature effects into two

categories:

1. Short-term effects

a. Heat deﬂection temperature

b. Vicat softening temperature

c. Torsion pendulum

2. Long-term effects

a. Long-term heat resistance test

b. Underwriters Laboratory (UL) temperature index

c. Creep modulus/creep rupture tests

Short-Term Effects

Heat Deﬂection Temperature (HDT)(ASTM D648, ISO 75-1, ISO 75-2).

Heat deﬂection temperature is deﬁned as the temperature at which a standard

test bar (5

⫻⫻in.) deﬂects 0.010 in. under a stated load of either 66 or

––

264 psi. The heat deﬂection temperature test, also referred to as the heat distor-

tion temperature test, is commonly used for quality control and for screening

and ranking materials for short-term heat resistance. The data obtained by this

method cannot be used to predict the behavior of plastic materials at elevated

temperature nor can it be used in designing a part or selecting and specifying

material. Heat deﬂection temperature is a single-point measurement and does

not indicate long-term heat resistance of plastic materials. Heat distortion tem-

perature, however, does distinguish between those materials that lose their rigid-

ity over a narrow temperature range and those that are able to sustain light loads

at high temperatures.

The apparatus for measuring heat deﬂection temperature consists of an en-

closed oil bath ﬁtted with a heating chamber and automatic heating controls that

raise the temperature of the heat transfer ﬂuid at a uniform rate. A cooling

570 PLASTICS TESTING

Fig. 15 DTUL/ vicat tester. (Courtesy Atlas Electric Devices Company)

system is also incorporated to fast cool the heat transfer medium for conducting

repeated tests. The specimens are supported on steel supports, 4 in. apart with

the load applied on top of the specimen vertically and midway between the

supports. The contact edges of the support and of the piece by which pressure

is applied is rounded to a radius of in. A suitable deﬂection measurement

–

device, such as a dial indicator, is normally used. A mercury thermometer is

used for measuring temperature. The unit is capable of applying 66 or 264 psi

ﬁber stress on specimens by means of a dead weight. A commercially available

heat deﬂection measuring device with a closeup of a specimen holder is illus-

trated in Fig. 15.

More recently, automatic heat deﬂection temperature testers have been de-

veloped. These testers typically replace conventional temperature and deﬂection

measuring devices with more sophisticated electronic measuring devices with

digital read-out system and a chart recorder that prints out the results. Such an

automatic apparatus eliminates the need for the continuous presence of an op-

erator and thereby minimizes operator-related errors.

The test specimens consist of test bars 5 in. in length, in. in depth by any

–

width from to in. The test bars may be molded or cut from extruded sheet

––

as long as they have smooth, ﬂat surfaces and are free from excessive sink marks

or ﬂash. The specimens are conditioned employing standard conditioning pro-

cedures.

The specimen is positioned in the apparatus along with the temperature and

deﬂection measuring devices and the entire assembly is submerged into the oil

bath kept at room temperature. The load is applied to a desired value (66 or 264

psi ﬁber stress). Five minutes after applying the load, the pointer is adjusted to

zero and the oil is heated at the rate of 2

Ⳳ 0.2⬚C/min. The temperature of the

2 THERMAL PROPERTIES 571

oil at which the bar has deﬂected 0.010 in. is recorded as the heat deﬂection

temperature at the speciﬁed ﬁber stress.

Long-Term Effects

The long-term effects of elevated temperature on properties of plastics are ex-

tremely important, especially when one considers the fact that the majority of

applications involving high heat are long-term applications. During long-term

exposure to heat, plastic materials may encounter many physical and chemical

changes. A plastic material that shows little or no effect at elevated temperature

for a short time may show a drastic reduction in physical properties, a complete

loss of rigidity, and severe thermal degradation when exposed to elevated tem-

perature for a long time. Along with time and temperature, many other factors

such as ozone, oxygen, sunlight, and pollution combine to accelerate the attack

on plastics. At elevated temperatures, many plastics tend to lose important ad-

ditives such as plasticizers and stabilizers, causing plastics to become brittle or

soft and sticky.

Three basic tests have been developed and accepted by the plastics industry.

If the application does not require the product to be exposed to elevated tem-

perature for a long period under continuous load, a simple heat resistance test

is adequate. The applications requiring the product to be under continuous sig-

niﬁcant load must be looked at from creep modulus and creep rupture strength

test data. Another one of the most widely accepted methods of measuring max-

imum continuous use temperature has been developed by Underwriters Labo-

ratories. The UL temperature index, established for a variety of plastic materials

to be used in electrical applications, is the maximum temperature that the ma-

terial may be subjected to without fear of premature thermal degradation.

Long-Term Heat Resistance Test (ASTM D794). The long-term heat resis-

tance test was developed to determine the permanent effect of heat on any prop-

erty by selection of an appropriate test method and specimen. In ASTM

recommended practice, only the procedure for heat exposure is speciﬁed and not

the test method or specimen.

Any specimen, including sheet, laminate, test bar, or molded part may be

used. If a speciﬁc property, such as tensile strength loss is to be determined, a

standard tensile test bar specimen and procedures must be used for comparison

of test results before and after the test. The test requires the use of a mechanical

convection oven with a specimen rack of suitable design to allow air circulation

around the specimens. The test is carried out by simply placing the specimen in

the oven at a desired exposure temperature for a predetermined length of time.

The subsequent exposure to temperatures may be increased or decreased in steps

of 25

⬚C until a failure is observed. Failure due to heat is deﬁned as a change in

appearance, weight, dimension, or other properties that alter plastic material to

a degree that it is no longer acceptable for the service in question. Failure may

result from blistering, cracking, loss of plasticizer, or other volatile material that

may cause embrittlement, shrinkage, or change in desirable electrical or me-

chanical properties.

Many factors that affect the reproducibility of the data. The degree of tem-

perature control in the oven, the type of molding, cure, air velocity over the

572 PLASTICS TESTING

specimen, period of exposure, and humidity of the oven room are some of these

factors. The amount and type of volatiles in the molded part or specimen may

also affect the reproducibility.

UL Temperature Index. The increased use of plastic materials in electrical

applications such as appliances, portable electrically operated tools and equip-

ments, and enclosures has created a renewed interest in the ability of plastics to

withstand mechanical abuse and high temperatures. A serious personal injury,

electric shock, or ﬁre may occur if the product does not perform its intended

function. Underwriters Laboratories, an independent, not-for-proﬁt organization

concerned with consumer safety, has developed a temperature index to assist UL

engineers in judging the acceptability of individual plastics in speciﬁc applica-

tions involving long-term exposure to elevated temperatures. The UL tempera-

ture index correlates numerically with the temperature rating or maximum

temperature in degrees centigrade above which a material may degrade prema-

turely and therefore be unsafe.

Relative Thermal Indices. The relative thermal index of a polymeric material

is an indication of the material’s ability to retain a particular property (physical,

electrical, etc.) when exposed to elevated temperatures for an extended period

of time. It is a measure of the material’s thermal endurance. For each material,

a number of relative thermal indices can be established, each index related to a

speciﬁc thickness of the material.

The relative index of a material is determined by comparing the thermal-

aging characteristic of one material of proven ﬁeld service at a particular tem-

perature level with the thermal-aging characteristics of another material with no

ﬁeld service history. A great deal of consideration is given to the properties that

are evaluated to determine relative thermal index. For the relative thermal index

to be valid, the properties being stressed in the end product must be included in

the thermal-aging program. If, for any reason, the speciﬁc property under stress

in the end product is not part of the long-term aging program, the relative thermal

index may not be applicable to the use of the material in that particular appli-

cation.

Relative Thermal Index Based upon Historical Records. Through experience

gained from testing a large volume of complete products and insulating systems

over a long period, UL has established relative thermal indices on certain types

of plastics. These fundamental temperature indices are applicable to each mem-

ber of a generic material class. Table 2 lists the temperature indices based on

past ﬁeld test performance and chemical structure.

Relative Thermal Index Based upon Long-Term Thermal Aging. The long-

term thermal-aging program consists of exposing polymeric materials to heat for

a predetermined length of time and observing the effect of thermal degradation.

To carry out the testing, an electrically heated mechanical convection oven is

preferred, however, with some provisions, a noncirculating static oven may be

employed. The speciﬁc properties to be evaluated in the thermal-aging program

are to be as nearly as possible representative of the properties required in the

end application.

2 THERMAL PROPERTIES 573

Table 2 Relative Thermal Indices Based upon Past

Field Test Performance and Chemical Structure

Material

Generic Thermal

Index (⬚C)

Nylon (type 6, 11, 6 / 6, and 6 / 10)

Polycarbonate

Molded phenolic

c,d

150

Molded melamine

c,d

130

Molded melamine-phenolic

c,d

130

Fluorocarbin resins

(1) Polytetraﬂuoroethylene 150

(2) Polychlorotriﬂuoroethylene 150

(3) Fluorinated ethylene propylene 150

Silicone rubber 105

Polyethylene terephthalate ﬁlm 105

Urea formaldehyde 100

Molded alkyd

c,d

130

Molded epoxy

c,d

130

Molded diallyl phthalate

c,d

130

Molded polyester

c,d

(thermosetting) 130

From UL 746 B. Reprinted with permission from Under-

writers Laboratories, Inc.

Includes glass ﬁber-reinforced materials.

Includes simultaneous heat and high-pressure matched metal

die-molded compounds only. Excludes low-pressure or low-

temperature curing processes such as open-mold (hand lay-

up, spray-up, contact bag, ﬁlament winding), encapsulation,

lamination, etc.

Includes materials having ﬁller systems of ﬁbrous (other than

synthetic organic) types but excludes ﬁber reinforcement sys-

tems using resins that are applied in liquid form.

Compounds having a speciﬁc gravity of 1.55 or greater (in-

cluding those having cellulosic ﬁller material) are acceptable

at temperatures not greater than 150

⬚C (302⬚F).

The most common mechanical properties include tensile strength, ﬂexural

strength, and Izod impact strength. The electrical properties of concern are di-

electric strength, surface or volume resistivity, arc resistance, and arc tracking.

The test specimens are standard ASTM test bars, depending upon the type of

test. The UL publication, ‘‘Polymeric Materials—Short-Term Property Evalua-

tions, UL 746A,’’ describes the specimen and test procedures to determine me-

chanical and electrical properties.

To determine the relative thermal index, a control material with a record of

good ﬁeld service at its rated temperature is selected. The control material of

the same generic type and the same thickness as the candidate material is pre-

ferred. At least four different oven temperatures are selected. The highest tem-

perature is selected so that it will take no more than 2 months to produce end

of life of the material. The next two lower temperatures must produce the an-

ticipated end of life of 3 and 6 months, respectively. The lowest temperature

selected will take 9 to 12 months for the anticipated results.

The end of life of a material is based upon the assumption that at least a

2:1 factor of safety exists in the applicable physical and electrical property re-

574 PLASTICS TESTING

quirements. The end of life of a material is the time at each aging temperature,

when a property value has decreased 50% of its unaged level. A 50% loss of

property due to thermal degradation is not expected to result in premature, unsafe

failure.

To avoid underrating the material’s relative temperature index, UL publishes

the ratings in three categories: applications involving electrical properties only,

applications involving both electrical and mechanical properties, and applications

involving both electrical and mechanical properties without impact resistance.

UL publishes such data in its semiannual Plastics Recognized Component Di-

rectory. Since the long-term heat-aging resistance of plastics is dependent on the

thickness of the part or test specimen, UL requires that the thermal index testing

be carried out over a wide range of thicknesses. Finally, it is important to un-

derstand that the UL temperature index program recognizes that the upper tem-

perature limits of plastics are dependent upon the stresses applied on the end

product in use. Consequently, the temperature index of a particular plastic qual-

iﬁes it only for those UL applications that UL has speciﬁcally approved.

2.2 Brittleness Temperature (ASTM D746, ISO 974)

At low temperatures, all plastics tend to become rigid and brittle. This happens

mainly because, at lower temperatures, the mobility of polymer chains is greatly

reduced. In the case of the amorphous polymers, brittle failure occurs at a tem-

perature well below glass transition temperature. The polymer tends to get

tougher as it reaches glass transition temperature. By contrast, in crystalline

polymers the toughness is mainly dependent on the degree of crystallinity, which

generally creates molecular inﬂexibility resulting in only moderate impact

strength. The size of the crystalline structure formed also has signiﬁcant effect

on the impact strength of the polymer. The larger the crystalline structure, the

lower the impact strength. The polymers exhibiting ductile failure generally

show high plastic deformation characterized by material stretching and tearing

before fracturing. Brittleness temperature is deﬁned as the temperature at which

plastics and elastomers exhibit brittle failure under impact conditions. Yet an-

other way to deﬁne brittleness temperature is the temperature at which 50% of

the specimens tested exhibit brittle failure under speciﬁed impact conditions.

3 ELECTRICAL PROPERTIES

The unbeatable combination of characteristics such as ease of fabrication, low

cost, light weight, and excellent insulation properties have made plastics one of

the most desirable materials for electrical applications. Although the majority of

applications involving plastics are insulation related, plastics can be made to

conduct electricity by simply modifying the base material with proper additives

such as carbon black.

Until recently, plastics were considered a relatively weaker material in terms

of load-bearing properties at elevated temperatures. Therefore, the use of plastics

in electrical applications was limited to non-load-bearing, general-purpose ap-

plications. The advent of new high-performance engineering materials has al-

tered the entire picture. Plastics are now speciﬁed in a majority of applications

requiring resistance to extreme temperatures, chemicals, moisture, and stresses.

The primary function of plastics in electrical applications has been that of an

3 ELECTRICAL PROPERTIES 575

Fig. 16 Schematic of dielectric strength test.

insulator. This insulator or dielectric separates two ﬁeld-carrying conductors.

Such a function can be served equally well by air or vacuum. However, neither

air nor vacuum can provide any mechanical support to the conductors. Plastics

not only act as effective insulators but also provide mechanical support for ﬁeld-

carrying conductors. For this very reason, the mechanical properties of plastic

materials used as insulators become very important. Typical electrical applica-

tions of plastic material include plastic-coated wires, terminals, connectors, in-

dustrial and household plugs, switches, and printed circuit boards.

The key electrical properties of interest are dielectric strength, dielectric con-

stant, dissipation factor, volume and surface resistivity, and arc resistance.

3.1 Dielectric Strength (ASTM D149, IEC 243-1)

The dielectric strength of an insulating material is deﬁned as the maximum

voltage required to produce a dielectric breakdown. Dielectric strength is ex-

pressed in volts per unit of thickness such as V/mil. All insulators allow a small

amount of current to leak through or around themselves. Only a perfect insulator,

if there is such an insulator in existence, can be completely free from small

current leakage. The small leakage generates heat, providing an easier access to

more current. The process slowly accelerates with time and the amount of volt-

age applied until a failure in terms of dielectric breakdown or what is known as

puncture occurs. Obviously, dielectric strength, which indicates electrical

strength of a material as an insulator, is a very important characteristic of an

insulating material. The higher the dielectric strength, the better the quality of

an insulator. Three basic procedures have been developed to determine dielectric

strength of an insulator. Figure 16 schematically illustrates the basic setup for a

dielectric strength test. A variable transformer and a pair of electrodes are nor-

mally employed. Specimens of any desirable thickness prepared from the ma-

terial to be tested are used. Specimen thickness of in. is fairly common. The

––

ﬁrst procedure is known as the short-times method. In this method, the voltage

is increased from zero to breakdown at a uniform rate. The rate of rise is gen-

576 PLASTICS TESTING

erally 100, 500, 1000, or 3000 V/s until the failure occurs. The failure is made

evident by actual rupture or decomposition of the specimen. Sometimes a circuit

breaker or other similar device is employed to signal the voltage breakdown.

This is not considered a positive indication of voltage breakdown since other

factors such as ﬂashover, leakage current, corona current, or equipment mag-

netizing current can inﬂuence such indicating devices.

The second method is known as the slow-rate-of-rise method. The test is

carried out by applying the initial voltage approximately equal to 50% of the

breakdown voltage as determined by the short-time test or as speciﬁed. Next,

the voltage is increased at a uniform rate until the breakdown occurs.

The step-by-step test method requires applying initial voltage equal to 50%

of the breakdown voltage as determined by the short-time test and then increas-

ing the voltage in equal increments and held for speciﬁed time periods until the

specimen breaks down. In almost all cases the dielectric strength values obtained

by step-by-step method corresponds better with actual use conditions. However,

the service failures are generally at voltage below the rated dielectric strength

because of the time factor involved.

3.2 Dielectric Constant and Dissipation Factor (ASTM D150, IEC 250)

Dielectric Constant (Permittivity)

Dielectric constant of an insulating material is deﬁned as the ratio of the charge

stored in an insulating material placed between two metallic plates to the charge

that can be stored when the insulating material is replaced by air (or vacuum).

Deﬁned another way, the dielectric constant is the ratio of the capacitance by

two metallic plates with an insulator placed between them and the capacitance

of the same plates with a vacuum between them. Simply stated, the dielectric

constant indicates the ability of an insulator to store electrical energy. In many

applications, insulating materials are required to perform as capacitors. Such

applications are best served by plastic materials having a high dielectric constant.

Materials with a high dielectric constant have also helped in reducing the phys-

ical size of the capacitors. Furthermore, the thinner the insulating material, the

higher the capacitance. Because of this fact plastic foils are extensively used in

applications requiring high capacitance.

One of the main function of an insulator is to insulate the current-carrying

conductors from each other and from the ground. If the insulator is used strictly

for this purpose, it is desirable to have the capacitance of the insulating material

as small as possible. For these applications, one is looking for the materials with

very low dielectric constant.

The dielectric constant test is fairly simple. The test specimen is placed be-

tween the two electrodes, as shown in Fig. 17, and the capacitance is measured.

Next, the test specimen is replaced by air and once again the capacitance value

is measured. The dielectric constant value is determined from the ratio of the

two measurements. Dielectric constant values are affected by factors such as

frequency, voltage, temperature, humidity, etc.

Dissipation Factor

In all electrical applications, it is desirable to keep the electrical losses to a

minimum. Electrical losses indicate the inefﬁciency of an insulator. The dissi-

3 ELECTRICAL PROPERTIES 577

Fig. 17 Schematic of dielectric constant test. (Courtesy Bayer Corporation)

pation factor is a measure of such electrical inefﬁciency of the insulating ma-

terial. The dissipation factor indicates the amount of energy dissipated by the

insulating material when the voltage is applied to the circuit. The dissipation

factor is deﬁned as the ratio of the conductance of a capacitor in which the

material is the dielectric to its susceptance or the ratio of its parallel reactance

to its parallel resistance. Most plastics have a relatively lower dissipation factor

at room temperature. However, at high temperatures, the dissipation factor is

quite high, resulting in greater overall inefﬁciency in electrical system. Loss

factor, which is the product of dielectric constant and the dissipation factor, is

a frequently used term since it relates to the total loss of power occurring in

insulating materials.

3.3 Electrical Resistance Tests

As stated earlier, the primary function of an insulator is to insulate current-

carrying conductors from each other as well as from ground and to provide

mechanical support for components. Naturally, the most desirable characteristic

of an insulator is its ability to resist the leakage of the electrical current. The

higher the insulation resistance, the better the insulator. Failure to recognize the

importance of insulation resistance values while designing products such as ap-

pliances and power tools could lead to ﬁre, electrical shock, and personal injury.

Insulation resistance can be subdivided into:

1. Volume resistance

2. Surface resistance

Volume resistance is deﬁned as the ratio of the direct voltage applied to two

electrodes that are in contact with a specimen to that portion of the current