HOW TO TEST PLASTIC MATERIAL PROPERTIES?

Tensile Impact is comparatively new and there is relatively little industry data available to adequately assess its accuracy and utility. However, possible advantages over the notched Izod test are immediately apparent: the notch sensitivity factor is eliminated, and energy is not used in pushing aside the broken portion of the specimen.

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 which halts the crosshead clamp. The pendulum head continues 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 a brittle or ductile type.

ASTM D-1822.

Flexural properties of plastics are obtained by placing a specimen on two supports spaced 4 in. apart. A load is applied in the center at a specified rate and the loading at failure (psi) is the flexural strength. For materials which do not break, the flexural property usually given is Flexural Stress at 5% strain.

In bending, a beam is subject to both tensile and compressive stresses.

Because most thermoplastics do not break in this test even after being greatly deflected, the flexural strength cannot be calculated. Instead, stress at 5% is calculated, i.e., the loading in psi necessary to stretch the outer surface 5%.

ASTM D-790.

Izod Impact testing is done by clamping a specimen 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 consumed in breaking the sample is calculated from the height the pendulum reaches on the follow-through.

The Izod impact test indicates the energy required to break notched specimens under standard conditions. It is calculated as ft. lb. per inch of notch and is usually calculated on the basis of a one inch specimen although the specimen used may be thinner in the lateral direction.

The Izod value is useful in comparing various types of grades of a plastic. In comparing one plastic with another, however, the Izod impact test should not be considered a reliable indicator of overall toughness or impact strength. Some materials are notch-sensitive and derive greater concentrations of stress from the notching operation. The Izod impact test may indicate the need for avoiding sharp corners in parts made of such materials. For example, nylon and acetal-type plastics, which in molded parts are among the toughest materials, are notch sensitive and register relatively low values on the Izod impact test.

ASTM D-256.

Deflection Temperature shows the temperature at which an arbitrary amount of deflection occurs under established loads. It is not intended to be a direct guide to high temperature limits for specific applications. It may be useful in comparing the relative behavior of various materials in these test conditions, but it is primarily useful for control and development purposes.

A specimen is placed on supports 4 inches apart and a load of 66 or 264 psi is placed on the center. The temperature in the chamber is raised at the rate of 2 + 0.2 °C per minute. The temperature at which the bar has deflected 0.010 inch is reported as "deflection temperature at 66 (or 264) psi fiber stress."

ASTM D-648.

Rockwell hardness can differentiate the relative hardness of different types of a given plastic. But since elastic recovery is not involved as well as hardness, it is not valid to compare the hardness of various kinds of plastic entirely on the basis of this test. Rockwell hardness is not an index of wear qualities or abrasion resistance. For example, polystyrenes have high Rockwell hardness values but poor scratch resistance.

A steel ball under a minor load is applied to the surface of the specimen. This indents slightly and assures good contact. The gauge is then set to zero. The major load is applied for 15 seconds and removed, leaving the minor load still applied. The indentation remaining after 15 seconds is read directly off the dial.

The size of the balls and the loadings vary, and values obtained with one set cannot be correlated with values from another set. See ASTM D 785 for further details

Vicat Softening Point is a good way to compare the heat softening characteristics of polyethylenes. Also, it may be used with other thermoplastics.

The apparatus for testing Vicat softening point consists of a temperature-regulated oil bath with a flat-ended needle penetrator so mounted as to register the degree of penetration on a gauge.

A specimen is placed with the needle resting on it. The temperature of the bath (preheated to about 50 °C lower than anticipated Vicat softening point) is raised at the rate of 50 °C/hr. or 120 °C/hr. The temperature at which the needle penetrates 1 mm. is the Vicat Softening Point.

ASTM D-1525.

Shear Strength data is obtained by mounting a specimen in a punch type shear fixture and the punch (1 in. D ) is pushed down at a rate of 0.005 in. per min until the moving portion of the specimen clears the stationary portion. Shear strength is calculated as the force/area sheared.

Shear strength is particularly important in film and sheet products where failures from this type of load may often occur. For the design of molded and extruded products, it would seldom be a factor. Plastic sheets or molded plastic discs measuring 0.005 to 0.500 in. thick are used in the test.

ASTM D-732.

Compressive properties are obtained by mounting in a compression tool between testing machine heads which exert a constant rate of movement. Indicator registers loading.

The compressive strength of a material is calculated as the psi required to rupture the specimen or deform the specimen a given percentage of its height. It can be expressed as psi either at rupture or at a given percentage of deformation.

The compressive strength of plastics is of limited design value since plastic products (except foams) seldom fail from compressive loading alone. The compressive strength figures, however, may be useful in specifications for distinguishing between different grades of material, and also for assessing, along with other property data, the overall strength of different kinds of materials.

ASTM D-695.

Deformation Under Load indicates the ability of rigid plastics to withstand continuous short-term compression without yielding and loosening when fastened as in insulators or other assemblies 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.

The specimen is placed between the anvils of the testing machine and loaded at 1000 psi. (Sometimes other loadings may be specified). The gauge is read 10 seconds after loading, and again 24 hours later. The deflection is recorded in mils. The original height is calculated after the specimen is removed from the testing machine by adding the change in height to the height after testing. By dividing the change in height by the original height and multiplying by 100, the percent deformation is calculated. This test may be run at 73.4, 122, or 158 °F.

ASTM D-621.

Durometer Hardness instrument has a pointed indenter projecting below the base (face) of the pressure foot. When the indenter is pressed into the plastic specimen so that the base rests on the plastic surface, the amount of indentation registers on the dial indicator.

This test measures the indentation into the plastic of the indenter under load, according to a scale of 0 to 100. There is no unit of measurement. Readings taken immediately after application may vary from those taken after pressure has been held for a time, because of creep. This test is preferred for polyethylene because the Rockwell test loses meaning when excessive creep is encountered. For other materials (acetate, acetal, etc.) the Rockwell hardness test is still the standard.

ASTM D-1706.

Specific Gravity is a strong element in the price factor and thus has great importance. Beyond the price/volume relationship, however, specific gravity is used in production control, both in raw material production and molding and extrusion.

Polyethylenes, for example, may have density variation, depending upon the degree of "packing" during molding, or the rate of quench during extrusion. Although specific gravity and density are frequently used interchangeably, there is a very slight difference in their meaning. Specific 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 temperature.

It is properly expressed as Specific Gravity, 23/23 °C. Density is the weight per unit volume of material at 23 °C and is expressed as D23C, g/cm3. The discrepancy enters from the fact that water at 23 °C has a density slightly less than one.

ASTM D-792.

Stiffness In Flexure is determined by clamping a specimen into the apparatus shown here and a 1% load applied manually. The deflection scale is set at zero. The motor is engaged and the loading increased, with deflection and loading figures recorded at intervals. A curve is drawn of deflection versus load, and from this is calculated stiffness in flexure in pounds per square inch.

This test does not distinguish the plastic and elastic elements involved in the measurement and therefore a true elastic modulus is not calculable. Instead, an apparent value is obtained and called "stiffness in flexure." It is a measure of the relative stiffness of various plastics and taken with other pertinent property data; is useful in material selection.

ASTM D-747.