PROPERTIES OF PLASTICS [1], [2]

In order to make proper designs with any material, including plastics, it is necessary to know certain physical, chemical, electrical and mechanical properties of the material. The following are the terms that are important in specifying the properties of a plastic.

Mechanical Properties

Plastics have the characteristics of both a viscous liquid and a spring-like elastomer, traits known as a viscoelasticity. These characteristics are responsible for many of the characteristic material properties displayed by plastics. Under mild loading conditions, such as short-term loading with low deflection and small loads at room temperature, plastics usually react like springs, returning to their original shape after the load is removed. Under long-term heavy loads or elevated temperatures many plastics deform and flow similar to high viscous liquids, although still solid.

Creep is the deformation that occurs over time when a material is subjected to constant stress at constant temperature. This is the result of the viscoelastic behavior of plastics.

Stress relaxation is another viscoelastic phenomenon. It is defined as a gradual decrease in stress at constant temperature.

Recovery is the degree to which a plastic returns to its original shape after a load is removed.

Specific gravity is the ratio of the weight of any volume to the weight of an equal volume of some other substance taken as the standard at a stated temperature. For plastics, the standard is water.

Water absorption is the ratio of the weight of water absorbed by a material to the weight of the dry material. Many plastics are hygroscopic, meaning that over time they absorb water.

Tensile strength at break is a measure of the stress required to deform a material prior to breakage. It is calculated by dividing the maximum load applied to the material before its breaking point by the original cross-sectional area of the test piece.

Tensile modulus (modulus of elasticity) is the slope of the line that represents the elastic portion of the stress-strain graph.

Elongation at break is the increase in the length of a tension specimen, usually expressed as a percentage of the original length of the specimen.

Compressive strength is the maximum compressive stress a material is capable of sustaining. For materials that do not fail by a shattering fracture, the value depends on the maximum allowed distortion.

Flexural strength is the strength of a material in bending expressed as the tensile stress of the outermost fibers of a bent test sample at the instant of failure.

Flexural modulus is the ratio, within the elastic limit, of stress to the corresponding strain.

Izod Impact is one of the most common ASTM tests for testing the impact strength of plastic materials. It gives data to compare the relative ability of materials to resist brittle fracture as the service temperature decreases.

For finding hardness, Rockwell Number is the net increase in depth of impression as the load on a penetrator is increased from a fixed minimum load to a high load and then returned to a minimum load.

Coefficient of thermal expansion is the change in unit length or volume resulting from a unit change in temperature. Commonly used unit is 10^-6 cm/cm/°C.

Thermal conductivity is the ability of a material to conduct heat; a physical constant for the quantity of heat that passes through a unit cube of a material in a unit of time when the difference in temperature of two faces is 1°C.

Heat Deflection temperature (HDT) test is one in which a bar of the polymer is heated uniformly in a closed chamber while a load of 66psi or 264psi is placed at the center of the horizontal bar. The HDT is the temperature at which a deflection of 0.25mm is noted at the center. The HDT indicated how much mass the object must be constructed of to maintain the desired form. It also provides a mesure of the rigidity of the polymer under a load as well as temperature.

Limiting oxygen index is a measure of the minimum oxygen level required to support combustion of the polymer.

Absorption. Polymers have a potential to absorb various corrodents the come to contact with, particularly organic liquids. This can result in swelling, cracking and penetration to the substrate of the component.

Chemical Properties

Many applications require that plastics retain critical properties, such as strength, toughness, or appearance, during and after exposure to natural environmental conditions. Some of the environmental effects that may damage plastic materials are as follows:

Corrosion of metallic materials takes place via an electrochemical reaction at a specific corrosion rate. However, plastics do not have such specific rates. They are usually completely resistant to a specific corrodent or they deteriorate rapidly.polymers are attacked either by chemical reaction or solvation. Solvation is the penetration of the polymer by a corrodent, which causes softening, swelling, and ultimate failure. Corrosion of plastics can be classified in the following ways as to attack mechanism:

1. Disintegration or degradation of a physical nature due to absorption, permeation, solvent action, or other factors.

2. Oxidation, where chemical bonds are attacked

3. Hydrolysis, where ester linkages are attacked

4. Radiation

5. Thermal degradation involving depolymerization and possibly repolymerization

6. Dehydration (this is not so common)

7. Combinations of the above

The absorption of UV light, mainly from sunlight, degrades polymers in two ways. First, the UV light adds thermal energy to the polymer as in heating, causing thermal degradation. Second, the UV light excites the electrons in the covalent bonds of the polymer and weaken the bonds. Hence the plastic becomes more brittle.

Some plastics that are originated from natural products, or plastics that have natural products mixed with them, are potentially susceptible to degradation by microorganisms. This is not a desired property in the use stage of the plastic product. However, at the end of their life cycle, disposal of plastics become an important issue. Plastics disposal and recycling will be discussed in another chapter.

Oxidation is a degradation phenomena when the electrons in a polymeric bond are so strongly attracted to another atom or molecule (here, oxygen) outside the bond that the polymer bond breaks. The results of oxidation are loss of mechanical and physical properties, embrittlement, discoloration, etc.

Environmental stress cracking occurs when the plastic is exposed to hostile environment conditions and mechanical stresses at the same time. It is different from polymer degradation because stress cracking does not break polymer bonds. Instead, it breaks the secondary linkages between polymers. These are broken when the mechanical stresses cause minute cracks in the polymer and they propagate rapidly under harsh environmental conditions. Nevertheless, the plastic material would not fail that fast if exposed to either the damaging environment or the mechanical stresses separately.

Crazing. In some cases, an environmental chemical embrittles the plastic material even when there is no mechanical stress applied. Cracks may also appear when the plastic part is stresses (usually in tensile) with no apparent environmental solvent present. These phenomena are called crazing and differ from environmental stress cracking in both the direction of the cracks and the extent of the cracking. The crack direction in environmental stress cracking is in the direction of moleculer orientation in the part, while in crazing the cracks are much more numerous in a small area but are much shorter than environmental stress cracks

Permeation is molecular migration through microvoids either in the polymer or between polymer molecules. Permeability is a measure of how easily gases or liquids can pass through a material. All materials are somewhat permeable to chemical molecules, but plastic materials tend to be an order of magnitude greater in their permeability than metals. However, not all polymers have the same rate of permeation. In fact, some polymers are not affected by permeation.

Flame spread classification ratings are defined as follows:

Flame spread rating	Classification
0-25	Noncombustible
25-50	Fire retardant
50-75	Slow-burning
75-200	Combustible
Above 200	Highly combustible

Most untreated plastic materials will burn. Combustion properties of polymers can be improved by the use of additives.

Electrical Properties

Resistivity of a material is the resistance that a material presents to the flow of electrical charge.

Dielectric strength is the voltage that an insulating material can withstand before breakdown occurs. It usually depends on the thickness of the material and on the method and conditions of the test.

Arc resistance is the property that measures the ease of formation of a conductive path along the surface of a material, rather than through the thickness of the material as is done with dielectric strength.

Dielectric constant or permitivity is a measure of how well the insulative material will act as a dielectric capacitor. This constant is defined as the capacitance of the material in question compared (by ratio) with the capacitance of a vacuum. A high dielectric constant indicates that the material is highly insulative.

Dissipation factor of a material measures the tendency of the material to dissipate internally generated thermal energy (i.e. heat) resulting from an applied alternating electric field.

Optical Properties

Light transmission. Plastics differ greatly in their ability to transmit light. The materials that allow light pass through them are called transparent. Many plastics do not allow any light to pass through. These are called opaque materials. Some plastic materials have light transmission properties between transparent and opaque. These are called translucent.

Surface reflectance. The reflection of light off the surface of a plastic part determines the amount of gloss on the surface. The reflectance is dependent upon a property of materials called the index of refraction, which is a measure of the change in direction of an incident ray of light as it passes through a surface boundary. If the index of refraction of the plastic is near the index of air, light will pass through the boundary without significant change in direction. If the index of refraction between the air and the plastic is large, the ray of light will significantly change direction causing some of the light to be reflected back toward its source.