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ASTM Testing

The American Society for Testing Materials has created a variety of tests in order to quantify the properties of plastics. It is important to understand that these tests are not material performance tests for predicting real life results over a period of time, but are actually short term material quality tests. They indicate to a resin buyer whether the batch of resin  he buys today is in the same formulation and quality range he bought before.

The ASTM tests are essentially short term tests with all variable fixed except the one being measured. Therefore, the ASTM data should act only as a guide to material selection, with actual parts being tested in real life application. When discussing properties, it is important to understand the conditions of the ASTM test, particularly the temperatures at which the tests were made.

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Copolymer

Two different polymers react together in polymerization so that each polymer chain is composed partly of monomer A and partly of monomer B. If three different polymers are reacted, the polymer chain is called a terpolymer. (Homopolymer identifies the single polymer monomer type of chain).

Crystalline and Amorphous Polymers 

Molecular chains may become compact and uniform. As the symmetrical molecules approach each other within critical distances, crystals begin to form in the areas of the densest packing, and the materials becomes crystalline. A material whose molecules remain random, are loosely packed, and form no crystals is called an amorphous material.

Crystalline materials are characterized by good chemical resistance, greater stiffness and creep resistance, higher tensile strengths, high melt temperatures, but tend to shrink and warp. Examples of crystalline materials include polyethylene, polypropylene, Nylon, acetals, kynar, PVC, and TPX.

Amorphous materials include ABS, polycarbonate, modified PPO (Noryl), polystyrene, acrylic, polysulfone, polyethersulfone, and ULTEM. Amorphous materials typically have more flexibility and good impact strength, but have poor chemical resistance.

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Electrical Properties

I. Resistance

Resistance to the passage of electrical current is expressed in Ohms. Insulating materials, when dry, offer high resistance to current (they are poor conductors). Thermoplastics are used extensively as insulators, and the various electrical properties typically are measures of the plastics ability to stand up to or resist electrical current.

A. Volume Resistivity

Even good insulators will allow a small amount of electrical current to “leak” through. The smaller the amount of leakage, the better                      the materials resistance to electrical current (resistivity). The measure of this leakage is volume resistivity, expressed in Ohms                                  per cubic centimeter (the test measures the Ohms ressitance between the opposite faces of a centimeter cube). The quantities will be                      in the millions, so they are stated in powers of 10. The higher the value, the better the insulating ability.

Materials can be classified in three categories:

Categories                                 Volume Resistivity (OHM/CM)

Conductors                               10^-6 – 10^3

Semi Conductors                    10^3 – 10^8

Insulators                                   10^8 – 10^19

An emerging market is to add conductive fillers (such as carbon) to thermoplastics so that they will act as semi-conductors. They are used where it is necessary to conduct static build up away from the particular area (computer housing, floor mats in electrically sensitive areas, etc.). Semi-conductive plastics are also used as shielding for electrical equipment to conduct away electromagnetic and radio frequency interference.

B. Surface Resistivity

The same concept as volume resistivity, except that it measures the resistance between two opposite edges of a surface film 1 cm square, and is expressed in Ohms, or Ohms per square.

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Hardness

The indentation hardness (as opposed to surface hardness) of plastic can be measured in several ways, but the most common ways are the Brinell and Rockwell methods. In both cases, a hardened steel ball is pressed into the smooth surface of the material. In the Brinell method, the spherical area of the indentation is measured and the Brinell number is calculated from this area and applied load. In the Rockwell system, the depth of the indentation is measured by means of an arbitrary scale.

The size of the ball and the load may be varied with the hardness of the material, with the different combinations of load and ball being designated by a letter proceeding the hardness number. Thus, a Rockwell hardness may be given as M-90 or R-80. In all cases, the higher index number means a harder material. The “R” scale is usually used for softer materials and the “M” scale for the harder ones

These systems and numbers may not be an indication of surface hardness or resistance to scratching of the various materials.

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 Light Transmittance

The percentage of light that is transmitted through a transparent material at a specific thickness. Haze is the percentage of light which, when passed through a material, is scattered from the normal straight light beam.

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Molecular Weight

A molecule is made up of atoms. Each type of atom has a different atomic weight (i.e. hydrogen = 1, carbon = 12). A simple organic compound such as Methane (CH4) has a low molecular weight because it has one carbon atom and four hydrogen atoms (the molecular weight equals 16).

However, in polymerization where a long chain of chemical units are formed into one molecule, the molecular weight can be varied by making longer and longer chains. For example low density polyethylene is C100H202, and has a molecular weight of 1402. A high density polyethylene molecule adds more carbon and hydrogen atoms to the long chain, and the molecular weight will be in the 250,000 range. And ultra high molecular weight polyethylene has a molecular weight of over 4,000,000.

The longer the chain, the more atoms in the molecule, and the higher the molecular weight. The major effects of of increasing chain length are increased toughness, creep resistance, stress-crack resistance, melt viscosity, and difficulty in processing. The chains resemble long intertwined bundles of spaghetti, with no physical connectors between chains.

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Specific Gravity

The density (weight) of any material divided by the density (weight of equal volume) of water.

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Thermoplastic

Because there is no physical connection between the long chains of molecules (except a weak electro-static connection), the polymer can be reshaped through heating; this is called a thermoplastic. As long as the chains remain linear, the polymer can be reshaped over and over.

Thermoset

Once a thermoset has taken its shape, it can never again be reshaped through heating, because the linear molecule chains go through a three dimensional chain combination and actually link together (called “cross lining”). This cross linking process can be created by heat, chemical agents, irradiation, or a combination of these. Cross linked materials have improved resistance to heat, chemical attack, stress cracking, and creep. However, they are more brittle, more difficult to process, are not transparent, and are difficult to color. Some thermoplastics can be cross linked to take on properties that are thermoset in nature.

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Water Absorption

The ability of a material to absorb water from an environment. Measure in two ways: 1) As a percent of the weight of the water absorbed in a 24 hour period, relative to the weight of the dry material, at 730° F. 2) The total amount of water a material can absorb (saturation point) expressed as a percent of the weight of water absorbed relative to the weight of the dry material. This measure is called water absorption at equilibrium.

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