An Evaluation of Polypropylene by Dynamic Mechanical Tests

Bryce Maxwell and James E. Heider Princeton University Plastics Laboratory, Princeton, N . J .

e significance of dynamic properties will be dis-

testing will be described. The most important of these are the significance of the limit of linear viscoelastic response and the effects of processing temperature and pressure history on the mechanical response of poly- meric materials. The common practice of testing over a narrow range of stress or strain magnitude in order to determine if the response is of a linear viscoelastic nature does not prove the point unless loading is ex- tended to approach zero stress or strain. Similarly, much valuable information is lost if test specimens are con- ditioned to some standard state prior to evaluation of dynamic properties. The important and useful varia- tions in properties which can be achieved by con- trolled pressure and temperature history, particularly for polycrystalline materials, must be studied in a sys- tematic manner.

Th cussed and some overlooked aspects of dynamic Loss Factor

Upon the application of an oscillatory load to a poly- meric material, two mechanisms of response will be observed. Some of the distortional mechanisms will respond in-phase with the applied loading and will store energy in an elastic manner. Some distortional mechanisms will not be able to respond in-phase with the loading and will produce an out-of-phase compo- nent of response. The former gives rise to the storage or in-phase modulus, E, and the latter gives rise to the out-of-phase or loss modulus, E2. The ratio of loss modulus to the storage modulus, EJE, is called the loss factor. It has been shown ( 3 ) that the value of both components of the complex modulus are dependent upon temperature and the rate of change of mechanical load, or the frequency of the oscillatory loading. A typical set of dynamic data is shown in Figures 1 and 2 using rigid polyvinyl chloride as an example. The

Figure 1. Dynamic Figure 2. Dynamic three dimensional d a t a three dimensional d a t a using rigid poly (vinyl using rigid poly (vinyl h chloride). chloride).

174 SPE TRANSACTIONS, APRIL, 1962

Any evaluation of the response characteristics of high polymeric materials to me- chanical loading must involve consideration of the e#ects of both the time scale of loading and temperature. I t is not possible to obtain either a n understanding of the engineering application characteristics or insight into the relationship between prop- erties and internal structure without obtaining data on properties which are a func- tion of the temperature dependence of the spectrum of relaxation times. Dynamic mechanical tests involving the application of sinusoidal loading have been found to be a convenient method for obtaining such information. (1, 2)

data was obtained using previously described equip- ment (4) with which it is possible to cover both a wide frequency and tcmperature range.

The evaluation of the modulus of elasticity of poly- meric materials is often obtained by a standard ASTM test method at a single rate of straining and a standard testing temperature. Such an evaluation would give data for only one point on the three dimensional surface of Figure 1. Similarly many dynamic studies to evaluate the properties of polymers are made as a function of only one of the two significant variables, frequency or temperature. It is obvious that both variables, must be controlled and studied in order to properly evaluate the material. It is particularly dangerous to control just one variable, for example, temperature, and allow the fre- quency to change with the response of the material as is often done in dynamic studies using a free oscillation system. This means that a single curve of properties as a function of temperature is obtained along some arbi- trary diagonal of the solid plot. The importance of the control and study of these two variables is even more apparent if one considers a polycrystalline material such as isotactic polypropylene as shown in Figure 3. This very complex loss factor behavior can not be evaluated unless care is taken to control both frequency and temperature.

Viscoelastic Response Limit Another very important variable that is often ne-

glected in dynamic testing is the limit of linear visco- elastic response. Since the response of these materials combine both viscous and elastic behavior it is neces- sary to make sure that the magnitude of the oscillatory stress or strain applied in the test does not change the number and character of structure distortion mechan- isms in the material. That is, as the magnitude of strain is increased, the dynamic modulus will remain constant for any given frequency and temperature up to the point where the strain magnitude induces additional structural mechanisms to enter into the materials re- sponse. At this point the in-phase modulus will de- crease. Figure 4 illustrates this point with polypropyl- ene. As the magnitude of the applied oscillatory strain is increased from zero, the slope of the dynamic stress- strain curve remains constant up to the limit of linear viscoelastic response. Once the limit is reached, there is a distinct break in the curve followed by a region which again exhibits constant slope but of lower value. The common method of determining if the data taken in a dynamic test is linear viscoelastic is to test over a range OK values of stress or strain on each side of that used to obtain the data. If the storage modulus varies only slightly with the magnitude of stress or strain the data

Figure 3. Dynamic three dimensional doto using Figure 4. Stress-strain curve for polypropylene; at point A isotactic polypropylene. slcpe changes.

ov L 0 so 100 IS0 200

BTAAIN'K.

SPE TRANSACTIONS, APRIL, 1962 175

0 0 20 .D 1" L O 100 TEUPLl l lTUl l ,

Figure 5. Effect of quench temperature o n the compliance 1 /E.

I0 t

TT"PT".IY"L .C

Figure 6. Effect o f quench temperature o n the loss factor E2/E1.

L 0 PO 40 80 nn 100

TEMPERATURE -0

Figure 7. Effects of pressure dur ing crystallization; quench temperature IS 125°C.

is said to be of a linear viscoelastic nature. It is ap- parent from the data of Figure 4 that if such a deter- mination was made above the break in the curve, one would find an essentially constant storage modulus and would think the material to be linear viscoelastic. The only true test of linear viscoelasticity must involve tests approaching zero strain as a limit and the modulus must be absolutely independent of strain.

Polypropylene When isotactic polypropylene is cooled from the

melt, crystallization takes place by a process of nuclea- tion in regions where the polymer chains lie close to- gether and parallel followed by the growth of crystal- lites as the amorphous chains are drawn into the lower energy state of the crystal. The nucleation and growth processes may be varied by the rate of cooling and the applied pressure. Rapid cooling may produce extensive nucleation but reduced growth due to decreased ther- mal energy in the amorphous chains as a result of de- creased temperature. Pressure on the other hand has been shown to cause nucleation at temperatures above the atmospheric melting point (5) . Therefore, it is pos- sible to cause nucleation throughout the mass of a poly- meric material at elevated temperatures, but even though the thermal energy may be high the growth rate may be retarded by the pressure increased internal viscosity of chain motion. As a result of these compet- ing effects it is not immediately apparent what effects variation in temperature and pressure history will have on the crvstalline structure and hence on the dvnamic

i , properties of polypropylene. Therefore, it is necessary to study temperature and pressure history as controlled variables.

Referring to Figure 3 , we see that polypropylene's dynamic properties are essentially independent of fre- quency hut very depcndent on test temperature. At all frequencies studied a maximum or peak in the loss fac- tor is found at or below room temperature. This peak indicates that as temperature is decreased from room temperature a change in the internal structural response of the material takes place such that at around 0°C: some mechanism of structural response is no longer contributing to the dynamic behavior. Observations of the mechanical behavior of this material indicate that over this temperature range the material changes from being ductile to brittle. Hence, the loss factor peak is an indication of this transition in properties and any changes that can be made in the height and location of this peak by controlling the temperature and pressure history during crystallization will indicate changes in the ductile-brittle transition.

Figures 5 and 6 show the effect of quench tempera- ture on the compliance, l/El, and the loss factor, E,/ E,, as a function of test temperature at 10 cycles per second. The term quench temperature is used here to denote the temperature of the mold and is related in- versely to the rate of cooling. Varying the quench tem- perature produces marked changes in the dynamic properties. For example, it is possible to vary the stor- age modulus from 46,000 psi. to 83,000 psi. or an 80% change at a test temperature of 100°C. The effect of quench temperature on the low temperature loss factor

SPE TRANSACTIONS, APRIL, 1962 176

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= t O L .so 0 so 100 Is0 zoo *h

OUCNCM TCYICRbTURC 'C

Figure 8. The effect of quench temperature an visco- elastic response limit.

peak is even more significant. When a low quench tem- perature is used, which would be analogous to the rapid cooling encountered in commercial injection and extru- sion operations, the loss factor peak has a high value of 5.5% at 20°C test temperature. As the quench tem- perature is increased the low temperature loss factor peak decreases in value and shifts to lower test tem- peratures. I t is, therefore, possible to vary the physical properties of this material over a wide range by vary- ing the temperature history during crystallization.

Figure 7 shows the effects of pressure during crystal- lization at a quench temperature of 125°C. on the dy- namic properties as a function of temperature. The in-phase modulus is changed slightly by an increase in hydrostatic pressure during molding while the loss fac- tor is increased over essentially the entire test tempera- ture range. Therefore, it is evident that both the ther- mal history and pressure history affect the dynamic properties of this polycrystalline material and a com- plete evaluation requires an extensive study of the inter- relationship of these two variables with physical prop- erties.

The thermal history also affects the value of the limit of linear viscoelastic response as shown in Figure 8. As the quench temperature is increased the strain limit decreases. This is consistent with crystallization theory in that the higher the quench temperature the greater the thermal energy of the amorphous chains, thus in- creasing the opportunity for crystal growth. As the dc- gree of crystallinity increases the amorphous content decreases thereby decreasing the amount of structure contributing to the linear viscoelastic response, result- ing in a decrease in the strain limit. If the material is

Figure 9. Effects of testing far loss factor and in- Dhase modulus above and below the strain limit.

tested above the limit, the in-phase modulus will be decreased as shown in Figure 9 and the loss factor increased over the entire test temperature range.

Conclusions It has been shown that in order to evaluate the

mechanical properties of a polycrystalline polymer the dynamic properties must be studied under the follow- ing conditions:

1. As a function of both controlled frequency and test temperature.

2. The strain limit of linear viscoelastic response must be determined and the test data obtained with strain magnitude as a controlled variable.

3. The dynamic properties must be determined as :I

function of the pressure and temperature history during crystallization. If this is not done important evaluation information such as the possibility of elimination of the low temperature loss factor peak in polypropylene may be overlooked.

Literature References 1. See for example: Viscoelastic Properties of Polymers,

Ferry, John D., John Wiley and Sons, Inc., New York, 1961.

2. Maxwell, Bryce, S . P . E . Journul, 15, 6, June (1959). 3. See for example: Mechanical Behavior of High Poly-

mers, Alfrey, T., Jr., Interscience Publishers, New York, 1948.-

4. Maxwell, Bryce, ASTM Bulletin, N o . 215, Tidy I ,

( 1956). 5. Matsuoka, S., and Maxwell, Bryce, J . of Poly. Sci.,

32, 124, Oct. (1958). T H E END

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