About The Properties Of Plastics

Thermoplastic materials can be divided into two main categories: amorphous and semi-crystalline. Amorphous polymers are materials that are inherently transparent and are predominantly unreinforced grades. Semi-crystalline polymers are opaque and are usually blended with certain additives such as glass fibers, minerals and impact modifiers. Ultra-high performance polymers offer some of the higher material properties in the field and can be either amorphous or semi-crystalline. They are often defined by their superior overall performance.

Typical Properties

When selecting a high-performance plastic, it is important to understand the nature of the plastic, its properties and the corresponding test methods. Only with this knowledge will you be able to evaluate the strengths and limitations of a particular resin to determine if it meets your application requirements. The following discussion will help design engineers unfamiliar with plastics to understand and appreciate the importance of this knowledge in the material selection process. It is not intended to be exhaustive and is intended only as a preliminary reference.

Thermal Properties

The reliable performance of a material at elevated temperatures is often a key consideration for designers. Thermal properties provide a reference point for two important aspects of a material's performance in a high temperature environment. The first aspect is the immediate softening effect that heat imparts to plastics. This effect limits the ambient temperature to which the plastic is exposed, even if only for a short period of time. The second aspect is the long-term thermal stability of the material. Since prolonged exposure to high temperatures results in a degradation of material properties, it is essential to understand the effects of long-term thermal environments on the material properties that are critical in your application.

Heat Deflection Temperature (HDT) is a relative measure of a plastic's ability to work under high temperature loads. At this temperature and a load of 1.8 MPa, the sample produces a specific deformation. It is generally accepted that the maximum working temperature must be 5-10 degrees below the heat deflection temperature.

The Relative Thermal Index (RTI) is a relative measure of a plastic's ability to continue working at high temperatures. The index is defined as a temperature at which a material retains 50% of its specified properties after 100,000 hours of exposure to air. The values of the Relative Thermal Index given in this manual are based on retention of tensile strength. The Relative Thermal Index (RTI) can be used as a conservative basis when considering maximum continuous use temperatures. For applications requiring less time, data sheets with RTI values for 5,000 and 10,000 hours are available upon request.

The glass transition temperature (Tg) is the temperature at which a significant change in polymer properties occurs and the polymer transforms from a glassy to a rubbery state. For amorphous polymers, this temperature is generally about 10∶ higher than the heat deflection temperature (HDT) and is usually used as an upper temperature limit for short-term use of the material. Semi-crystalline polymers lose some of their rigidity when they reach this temperature, but retain their serviceable properties below the melting point of the material.

The melting point (Tm) is the temperature at which the crystalline regions within a semi-crystalline polymer soften. The melting point usually represents the absolute upper temperature at which a semi-crystalline polymer remains in solid form.

Mechanical Properties

Since most applications will be under some degree of mechanical loading, it is important to understand the changes that occur in materials under the influence of load. Design engineers often change the load carrying capacity or deformation of a component under load by varying the thickness of the cross-section. Tensile strength can be measured by the process of fixing one end of a specimen and loading it at a specific rate at the other end until the specimen yields or breaks.

Elongation is a measure of how much a specimen can be stretched before it yields or breaks. A high elongation indicates that the material is tough and ductile. A low elongation usually indicates a rigid and brittle material. Glass fiber reinforced materials generally exhibit low elongation due to the addition of glass fibers, thus low elongation values do not always indicate brittleness. The flexural modulus can be measured by loading the middle of a specimen supported by two points. This modulus is defined as the slope of the stress/strain curve and is a useful indicator of stiffness or hardness.

When making material comparisons, the higher the tensile strength of a material, the smaller the required section thickness if the same load carrying capacity requirements are met. Similarly, the higher the flexural modulus of a material, the lower the required section thickness for the same deformation. For some applications, the cross-section may already be the smallest thickness possible given the practicalities of the injection molding process, and relative strength may not be a consideration. Impact resistance can be broadly defined as the ability of a material to resist breakage when struck by an object or dropped onto a hard surface. The Izod impact is the most common test method for evaluating this property of a material, and can be performed using either notched or unnotched strips.

The results of the unnotched Izod impact test give a good indication of the actual impact resistance of the material. A result of NB indicates that the specimen did not break under the experimental conditions. The notched Izod impact test is used to detect the tendency of a material to crack when the surface is scratched or notched. A material with a high non-notched Izod value and a low notched Izod value indicates a tough material with high notch sensitivity. When considering the use of this type of material, it is important to allow for the largest possible radius at all corners.

Electrical Properties

Most plastics are good electrical insulators. The electrical properties listed here - dielectric strength, volume resistivity, and surface resistivity - provide basic information about a material's ability to act as an electrical insulator. Material grades that contain large amounts of carbon fiber or carbon powder are generally not suitable for this type of application. When designing a plastic part whose primary function is electrical insulation, a number of other electrical properties must be considered before a material is finally selected.

General Properties

Weight reduction is the primary driver for many applications where plastics are used in place of metals. Specific gravity, the density of the resin divided by the density of water, can be used to estimate the weight of a part. The material with the lowest specific gravity will produce the lightest part. Specific gravity also affects the material cost of a part. On a per unit weight basis, more parts can be built from a material with a lower specific gravity than from a material with a higher specific gravity.

Water absorption can be measured by weighing a part before and after 24 hours of exposure to water. Water absorption can cause changes in the dimensions and properties of a material, and different materials are affected in different ways. While low water absorption is generally desirable, special attention should be paid to the effect of water absorption on the material's properties, rather than just considering the absolute amount of water drawn in.

Chemical compatibility

Exposure to chemical environments affects the working performance of materials, and for each specific application, the compatibility of the material with the chemicals in the environment of the application to which it belongs is tested. Chemical compatibility grades are listed in this manual in the hope of establishing an idea of which types of chemicals are compatible with which types of materials, and which types of materials they may be incompatible with. These grades are assigned based on prolonged exposure, and some materials defined as lower grades may be suitable for applications with shorter exposure times. Some chemical/material combinations that are classified as superior may also not be suitable for a particular reagent, temperature, stress level, and material combination.

Processing and Manufacturing

The properties listed here illustrate the range of processing temperatures required for each type of material. Melt and mold temperature data can assist in the selection of processing equipment. The molding shrinkage values listed were obtained by

standard test methods and may not be relevant to some specific parts. However, this value is valuable in material comparisons to help determine if a mold used to mold one material can be used to mold another material and make a part of the same size.

Melt flow rates are used to characterize our amorphous plastics, and these values reflect how easily the material flows. When comparing the melt flow rates of amorphous plastics offered by other manufacturers, it is important to determine whether the temperatures and loads used in their tests are consistent with those used by us. We have listed the typical processing of each type of product within each product line. Most of our products are processed by injection molding, but some grades of sheet, profiles and other shapes can be processed by extrusion. Extruded sheets can be thermoformed. Producing coatings and films can be done by solution processing methods.