Kutz M. Handbook of materials selection

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598 CHARACTERIZATION AND IDENTIFICATION OF PLASTICS

Fig. 3 Digital programmable viscometer. (Courtesy Brookﬁeld Engineering Laboratories)

polymer melt processing operations. A rheometer is a precision instrument that

provides the accuracy and reproducibility necessary for polymer characterization

test.

A quality control test to qualify the incoming material is quick, simple, and

accurate. The capillary rheometer can be used to assure that a material complies

with the ﬂow tolerances set for that grade of material. A quality control graph

of a polymer ﬂow curve can be generated with upper and lower critical viscosity

limits over a wide range of shear rates by running multiple tests on a standard

material. The thermal stability of a polymer and resulting processing limitations

are of extreme importance to a processor. Capillary rheometer can accurately

predict the thermal stability of the polymer. Materials such as polyvinyl chloride

(PVC) can begin to cross-link and exhibit a dramatic increase in viscosity if left

in the process too long. This can lead to rejected or failure-prone parts and

wasted time and materials. The capillary rheometer allows the residence time

capabilities of materials to be studied at wide range of processing temperatures

and shear rates so the optimal processing temperature can be determined. The

data generated from capillary rheometer measurements can be used to study the

processibility of regrind material as well as process optimization. The capillary

rheometer consists of an electrically heated cylinder, a pressure ram, temperature

controllers, timers, and interchangeable capillaries. The plunger can be moved

1 MATERIAL CHARACTERIZATION TESTS 599

at a constant velocity, which translates to a constant shear rate. The force to

move the plunger at this speed is recorded, which determines the shear stress.

Alternately, a weight or constant pressure can be applied to the plunger, which

generates a constant shear stress, and the velocity of the plunger can be deter-

mined by cutting and weighing the output. Shear rate can be calculated by

knowing the melt density.

The sample material is placed in the barrel of the extrusion assembly, brought

to temperature, and forced out through a capillary. The force required to move

the plunger at each speed is detected by a load cell. Shear stress, shear rate, and

apparent melt viscosity are calculated. Shear stress versus shear rate and apparent

viscosity versus apparent shear rate curves are plotted.

1.3 Viscosity Tests

Viscosity is deﬁned as the property of resistance to ﬂow exhibited within the

body of a material expressed in terms of a relationship between the applied

shearing stress and the resulting rate of strain in shear. In the case of ideal or

Newtonian viscosity, the ratio of shear stress to shear rate is constant. Plastics

typically exhibit non-Newtonian behavior, which means that the ratio varies with

the shearing stress. There are two different aspects of viscosity. Dynamic or

absolute viscosity, best determined in a rotational type of viscometer with a small

gap clearance, is independent of the density or speciﬁc gravity of the liquid

sample and is measured in poises (P) and centipoises (cP). Kinematic viscosity,

usually determined in some form of efﬂux viscometer equipped with a capillary

bore or small oriﬁce that drains by gravity, is strongly dependent on density or

speciﬁc gravity of the liquid and is measured in stokes (S) and centistokes (cS).

The relationship between two types of viscosity is

stokes

⫻ specific gravity ⫽ poises

The measurement and control of rheological properties are usually performed

with simple devices called ‘‘viscometers’’ or ‘‘viscosimeters,’’ which do not mea-

sure true viscosities of either the dynamic or kinematic type but make relative

ﬂow comparisons. Viscosity of plastic materials is measured in three basic ways,

employing principles involving liquid deformation due to various forces.

1. Downward rate of gravity ﬂow through capillary bores and small oriﬁces

(capillary viscometers).

2. Upward speed of a trapped air bubble.

3. Torque developed by the liquid drag between moving and stationary sur-

faces.

The ﬁrst method is basically used for thermoplastic materials. The latter two

are more commonly used for thermosetting materials, plastisols, and organosols

and are discussed in Section 1.2.

1.4 Gel Permeation Chromatography

Quite often traditionally used tests such as melt index or viscosity tests do not

provide enough information about the processibility of the polymer. Such tests

600 CHARACTERIZATION AND IDENTIFICATION OF PLASTICS

only measure an average value and tell us nothing about the distribution that

makes up the average. Take, for instance, average daily temperatures reported

for a particular city. These reports can be very misleading since they lack more

useful information about extreme high and extreme low temperatures. In a very

similar manner, the melt index and viscosity tests relate very well to the average

molecular weight of the polymer but fail to provide the necessary information

about the molecular weight distribution of the polymer. Two batches of resin

may have the same melt index, which simply indicates that their viscosity av-

erage molecular weights are similar. Their molecular weight distributions, the

number of molecules of various molecular weights that make up their averages,

can be signiﬁcantly different. If an excessively high-molecular-weight fraction

is present, the material may be hard and brittle. Conversely, if an excessive

amount of low-molecular-weight fraction is present, the material may be soft or

sticky. When evaluating the nature of the incoming plastics material is extremely

important to a processor, he must look for a more reliable technique that will

provide the necessary information, to qualify the material, such as the measure-

ment of molecular weight distribution. The molecular weight distribution is the

single most fundamental property of the polymer. The molecular weight distri-

bution not only provides basic information regarding the processibility of the

polymer but also gives valuable information for predicting its mechanical prop-

erties.

Gel permeation chromatography (GPC) is the method of choice for determin-

ing the molecular weight distribution of a polymer. This technique has gained

wide acceptance among the plastic material manufacturers and the processors

because of its relatively low cost, simplicity, and its ability to provide accurate,

reliable information in a very short time. GPC reveals the molecular weight

distribution of a polymer compound. It detects not only the resin-based mole-

cules such as polymer, oligomer, and monomer but most of the additives used

in plastic compounds and even low-level impurities. The molecular weight dis-

tribution curve is plotted for a well-characterized standard material, and the

proﬁle of the curve is compared with the test sample. In this manner, batch-to-

batch uniformity can be checked quickly as a means of quality assurance.

The separation of polymer molecules by GPC is based upon the differences

in their ‘‘effective size’’ in solution. (Effective size is closely related to molecular

weight.) Separation is accomplished by injecting the polymer solution into a

continuously ﬂowing stream of solvent that passes through highly porous, tiny,

rigid gel particles closely packed together in a tube. The pore size of the gel

particles may vary from small to very large. As the solution ﬂows through the

gel particles, molecules with small effective size (low molecular weights) will

penetrate more pores than molecules with larger effective sizes and, therefore,

take longer to emerge than the larger molecules. If the gel covers the right range

of molecular sizes, the result will be a size separation with the largest molecules

exiting the gel-packed tubes (columns) ﬁrst.

A test sample is prepared by dissolving a small amount of polymer in the

solvent and ﬁltering the solution to remove the undissolved impurities. The next

step is to select the proper size columns, connect them, set the sensitivity setting

on the detectors, and allow the instrument to equilibrate. A trial analysis is done

by injecting the polymer solution into the instrument. The chromatogram is

1 MATERIAL CHARACTERIZATION TESTS 601

Fig. 4 GPC equipment. (Courtesy Waters Corporation)

carefully analyzed. If the chromatogram shows all the desired information, the

ﬁnal analysis is carried out. If not, the operating parameters, such as column

size, ﬂowrate, and number of columns are optimized. The trial step is repeated

before proceeding to the ﬁnal analysis.

During the ﬁnal analysis, as the sample ﬂows through the column, the mol-

ecules are separated according to size by a simple mechanical effect. Because

of their smaller size, the smaller molecules enter into the gel pores more readily

and, therefore, take extra time to reach the bottom of the column. The various

molecular weight species are separated by the difference in travel time through

the column and pass through a detector in descending order of size. The detector

measures the concentration of each molecular size and plots the molecular

weight distribution of the sample on a strip chart. Commercially available GPC

equipment is shown in Fig. 4.

A GPC curve can reveal a great deal of information. The regions of the

chromatogram can generally be easily and quickly correlated with the molecular

weights of the components of the polymer. The most direct—and sometimes the

most informative—use of the GPC curve is in the comparison of different ma-

terials by overlaying their chromatograms. Differences in molecular weight dis-

tributions, peak shapes, shifts, and tailing are readily observable. Comparisons

of additives and other lower molecular weight species are straightforward.

Frequently, a master chromatogram representing the acceptable range of GPC

proﬁles is established with samples from ‘‘good’’ batches. All subsequent

batches are then chromatographed and compared with the master curve as a

rapid quality control method. A number of correlations between the GPC curve

and the physical and processing behaviors of a polymer have been developed.

602 CHARACTERIZATION AND IDENTIFICATION OF PLASTICS

Fig. 5 Thermal analysis of plastics. (Courtesy Perkin-Elmer Corporation)

1.5 Thermal Analysis Techniques

Thermal analysis (TA) consists of a family of analytical techniques in which a

property of the sample is monitored against time or temperature while the tem-

perature of the sample is programmed. Properties include weight, dimension,

energy take-up, differential temperature, dielectric constant, mechanical modulus

evolved gases, and other, less common attributes. The application of thermal

analysis is widespread within the polymer and elastomer industries. Thermal

analyzers are used in industry to qualify a material for ﬁtness of use and to

troubleshoot processing problems. Thermal analysis consists of three primary

techniques that may be used individually or in combination:

1. Differential scanning calorimetry (DSC)

2. Thermogravimetric analysis (TGA)

3. Thermomechanical analysis (TMA)

The maximum beneﬁt of thermal analysis can be gained by using combination

of all three techniques to characterize a polymer. Figure 5 shows the three most

common techniques and the test methods for each. The following sections de-

scribe the more common techniques in terms of analytical capability and stan-

dard methodology.

Differential Scanning Calorimetry (ASTM D3417, ASTM D3418)

Differential scanning calorimetry (DSC), the most widely used thermal analysis

technique, is a technique in which the heat ﬂowrate to the sample (differential

power) is measured while the temperature of the sample, in a speciﬁed atmo-

sphere, is programmed. Because all materials have a ﬁnite heat capacity, heating

or cooling a sample specimen results in a ﬂow or heat in or out of the sample.

1 MATERIAL CHARACTERIZATION TESTS 603

Fig. 6 Two most common types of DSC measuring cells. (Courtesy Perkin-Elmer Corporation)

The two most common types of commercial DSC measuring cells are shown in

Fig. 6. The heat ﬂux DSC employs a disk containing sample and reference

positions that are heated by a common furnace. The differential heat ﬂow to the

sample is proportional to the temperature difference that develops between

sample and reference junctions of a thermocouple. The power compensations

approach controls a temperature enclosure around the sample and reference in-

dividually. Through ampliﬁed feedback from platinum resistance thermometers,

it records the differential energy ﬂow necessary to maintain the sample on the

speciﬁed temperature program. With either approach the output is heat ﬂow,

normally expressed in milliwatts, watts/gram (normalized) or watts/gram-degree

(speciﬁc heat units).

The test procedure is simple. A small quantity of sample, usually 5–10 mg,

is weighed out into a inert capsule (usually made of aluminum). The encapsu-

lated sample is placed in the DSC sample holder or onto the sample platform

of a DSC cell disk. In the attached control module or computer the operator

selects a temperature range and heating rate or perhaps a more complex tem-

perature program. The test is started. (A hypothetical DSC curve showing both

endothermic and exothermic changes in a polymer is shown in Fig. 7.) Initially,

constant energy input is required to heat the sample at a constant rate. This

establishes a baseline. At a transition point, the sample requires either more or

less energy depending on whether the change is endothermic or exothermic. For

example, when the glass transition point is reached, the heat capacity increases.

The midpoint is taken as the glass transition temperature, T

. Adding plasticizers

to a formulation lowers T

. When a polymer reaches the melting point, it requires

more energy (endothermic) to melt the crystalline structure. The area of the peak

in units of energy is the enthalpy of fusion, the heat of melting. The temperature

dependence of the peak and its shape give information about degree of crystal-

linity, the molecular weight distribution, degree of branching, copolymer blend

604 CHARACTERIZATION AND IDENTIFICATION OF PLASTICS

Fig. 7 Typical DSC thermogram. (Courtesy Perkin-Elmer Corporation)

ratio, and/or processing history. Often quality procedures involve comparing the

melting proﬁle to that of a standard ‘‘good’’ material.

When a sample cures, more energy is usually released, and the change is

exothermic. The area of the curing peak is proportional to the number of cross-

links that were formed. This indicates degree of cure. The shape of the curing

curve can be analyzed to obtain the reaction kinetic parameters.

Thermogravimetric Analysis (TGA)

Thermogravimetric analysis is a test procedure in which changes in weight of a

specimens is progressively heated. The sample weight is continuously monitored

as the temperature is increased either at a constant rate or through a series of

steps. The components of a polymer or elastomer formulation volatilize or de-

compose at different temperatures. This leads to a series of weight loss steps

that allow the components to be quantitatively measured. A typical high-

performance apparatus consists of an analytical balance supporting a platinum

crucible for the specimen, the crucible situated in a electric furnace. Variations

in instrumentation include horizontally mounted furnaces and top loading bal-

ances.

A simple thermal separation can be seen in the determination of mineral ﬁller

in polypropylene (Fig. 8). The sample is heated in an air atmosphere to com-

pletely decompose the polypropylene such that the remainder is mineral ﬁller.

The amount of volatiles (here, none), polymer, and ﬁller is obtained by analysis

of the weight loss curve. The small weight loss (3.5%) occurring over the tem-

perature range of 750–800

⬚C is characteristic of the particular mineral ﬁller used

in this sample. An unﬁlled polypropylene thermogram is also run to compare

with the ﬁlled formulation. TGA is very useful in characterizing polymers con-

taining different levels of additives by measuring the degree of weight loss. TGA

can also be used to identify the ingredients of blended compounds according to

the relative stability’s of individual components. The thermal stability of a pol-

ymer can be obtained through a kinetic analysis of the decomposition proﬁle.

1 MATERIAL CHARACTERIZATION TESTS 605

Fig. 8 Typical TGA thermogram. (Courtesy Perkin-Elmer Corporation)

Thermomechanical Analysis (TMA)

When a sample is heated, its dimensions change because of thermal expansion,

stress reorientation, and deformation under an imposed stress. Thermomechan-

ical analysis consists of measuring these properties using a constant force on

the sample. Under a no-load or ﬁxed load, it measures dimensional change in

the vertical direction as the sample temperature is controlled. TMA equipment

(Fig. 9) consists of a probe mechanically connecting three entities: (1) a force

transducer to control the force applied to the sample, (2) a position transducer

to measure the displacement, and (3) a temperature-controlled sample specimen.

Samples to be examined are cut to a deﬁned geometry and then deformed by

the TMA probe in a deﬁned manner. Sample deformation includes compression,

tension, and bending geometries. Once mounted, the sample is surrounded by a

furnace and monitored by a thermocouple.

A typical TMA thermal curve is shown in Fig. 10. Here a fused silica rod is

used as a compression probe. The probe is placed onto the sample, and the

thermal expansion or contraction is recorded as a function of temperature. The

polymer used in the test contains a chemical blowing agent. Over the ﬁrst 60

⬚C,

the TMA probe is slightly pushed up by the expansion of the polymer solid. As

the melting point is reached, the lightly loaded probe penetrates the sample at

a rate related to the viscosity. This penetration is later reversed by the expansion

of the foam as the blowing agent is released. Polymer characterization through

the use of TMA is accomplished by determining the glass transition temperature,

the coefﬁcient of expansion, and the elastic modulus. TMA data also correlates

with the Vicat softening point and the heat distortion temperature.

606 CHARACTERIZATION AND IDENTIFICATION OF PLASTICS

Fig. 9 Testing by TMA (schematic). (Courtesy Perkin-Elmer Corporation)

Fig. 10 Typical TMA thermal curve. (Courtesy Perkin-Elmer Corporation)

2 IDENTIFICATION ANALYSIS OF PLASTIC MATERIALS 607

1.6 Spectroscopy

Infrared spectroscopy is one of the most widely used material analysis tech-

niques for over 70 years. An infrared spectrum represents a ﬁngerprint of a

sample with absorption peaks that correspond to the frequencies of vibrations

between the bonds of the atoms making up the material. Because each different

material is a unique combination of atoms, no two compounds produce the exact

same infrared spectrum. This fact allows a positive identiﬁcation of polymeric

materials. By studying the size of the peaks in the spectrum, one can also de-

termine the amount of material present.

A newly developed technique called Fourier transform infrared (FT-IR) spec-

trometry overcomes the limitations encountered with the traditional infrared

technique. The original infrared instruments were of dispersive type, which sep-

arated the individual frequencies of energy emitted from infrared source using

a prism or grating. These instruments measured each frequency individually,

making the entire process painfully slow. Modern FT-IR instruments can process

as many as 100 samples per day compared to only 2–4 samples per day. FT-IR

spectroscopy is fast, precise, and simple, requiring a very small amount of sam-

ple for a successful amalysis. In infrared spectroscopy, IR radiation is passed

through a sample. Some of the infrared radiation is absorbed by the sample and

some of it is passed through (transmitted). The resulting spectrum represents

molecular absorption and transmission, creating a molecular ﬁngerprint of the

sample. Figure 11 graphically illustrates this process.

The sample analysis process is quite simple with the use of modern spec-

trometer coupled with a powerful computer. Infrared energy is emitted from a

glowing black-body source. This beam passes through an interferometer where

the ‘‘spectral encoding’’ takes place. The resulting interferogram signal then exits

the interfermeter. Next, the beam enters the sample compartment where it is

transmitted though or reﬂected off the surface of the sample. This is where the

speciﬁc frequencies of energy, which are uniquely characteristic of the sample,

are absorbed. The beam ﬁnally passes through a detector and the signal is sent

to a computer where a mathematical technique called the Fourier transformation

takes place. The infrared spectrum is displaced on the CRT for analysis and

interpretation.

The computer is equipped with a collection of thousands of known polymer

and additive FT-IR spectra for easy comparison and identiﬁcation. A spectral

library search can identify a polymer, additive, or a contaminant within minutes,

making the entire process very fast and efﬁcient. Figure 12 illustrates the entire

FT-IR process.

2 IDENTIFICATION ANALYSIS OF PLASTIC MATERIALS

Plastic products are manufactured using a variety of processing techniques and

materials. It is practically impossible to identify a plastic material or product by

a visual inspection or a simple mechanical test. There are many reasons that

necessitate the identiﬁcation of plastics. One of the most common reasons is the

need to identify plastic materials used in competitive products. Defective prod-

ucts returned from the ﬁeld are quite often put through rigorous identiﬁcation

analysis. Sometimes it is necessary to identify a ﬁnished product at a later date