Wear tests for plastics

Budinski, K. G., Grove. T. H., "Predicting Polymer Serviceability for Wear Applications", in STP 701 Wear Tests for Plastics Selection and Use, ASTM, West Conshohocken, PA, 1983, p 59-75. 

It is the purpose of this paper to review available plastic ww tests and to comment on their applicability to various tribosystems and to improve the comfort level that users can have that iheir plastic wear tests are producing valid material comparisons. The objective is to supply potential plastic users with gui Iines for the selection of appropriate wear tests for various industrial situations. We will start with a discussion of PV measurement, then wear factors, abrasivity testing, abrasion of plastics by other surfaces, plastic bushings, tests on plastic films, and erosion tests. These tests will be put into a selection matrix for plastic users. 

Table 2. Plastic wear tests for various applications.

Commonly used wear and abrasion tests for plastic materials are discussed in the following sections. 

Where fluid pressure is carried by a capillary tube, such as with the thermostatic expansion valve or pressure switches, the tube should be installed with due attention to the risk of it chafing against metal edges and wearing through. Tubes to manometers are usually in plastic, but may be copper. These must be carefully tested for leaks, as they are transmitting very low pressures. 

Radioactive nuclides have important commercial applications. For example, americium-243 is used in smoke detectors. Its role is to ionize any smoke particles, which then allow a current to flow and set off the alarm. Carbon-14 is incorporated into steel samples to test for wear and into plastic samples to check for uniformity. Exposure to radiation is also used to sterilize food and inhibit the sprouting of potatoes. The radiation kills the bacteria that spoil food but does not produce harmful substances. 

Wear face shield, goggles, laboratory coat, and nitrile rubber gloves. Cover spill with a 1 1 1 mixture by weight of sodium carbonate or calcium carbonate, clay cat litter (bentonite), and sand. Using a plastic shovel, scoop into a pail of water in the fume hood (about 66 mL/g). Cautiously add aqueous 5.5% ceric ammonium nitrate (4 volumes per volume of aqueous solution) and stir for an hour. If the solution remains orange, an excess of ceric ammonium nitrate is present and the azide has been completely destroyed. The solution can be washed into the drain with at least 50 times its volume of water.6,7 The solid residue is treated as normal refuse. A spot test for checking if azide is completely destroyed is as follows Place a drop of the test solution in the depression of a spot plate and treat with 1 or 2 drops of dilute hydrochloric acid. Add a drop of ferric chloride solution and gently heat the spot plate. A red color indicates hydrazoic acid and incomplete decomposition. 

Haluska, L. A. Wear Testing of Abrasion - Resistant Coated Plastics, Special Technical Publication 769, American Society for Testing and Material, (1982). 

There is currently only one standard test method in ISO devoted to the measurement of abrasion resistance for plastics materials, this being the "Taber test standardized in ISO 9352 [101]. In this test a specimen is clamped onto a turntable that rotates at either 60 rev per min (where the electrical mains frequency is 50 Hz) or 72 rev per min (where the electrical mains frequency is 60 Hz) and weighted abrasive wheels press against the surface of the te.st piece. The center of rotation of the test piece is offset from the line of contact of the wheels, and so as the former is rotated this caiuses the wheels to rotate and to wear an annular track into the specimen surface. Figure 19 gives an illustration of the test piece... 

Tests have been ran to broaden this database to include the behavior of TP composites against several metals with different surface finishes. Also, wear factors for the metal surfaces, caused by the rubbing of plastic composites, have been studied. Certain metals chosen for stationary counterface materials were cold-rolled 1141 steel, 304 and 440 stainless steel, 70/30 brass, 2024 aluminum alloy, and phosphor bronze (ICI-LNP) [323]. 

Ignoring counterface wear is another of the fundimental weaknesses of both PV and wear factor. Nobody will argue that a wear system contains two or more members, yet only the wear of the plastic members is presented in manufacture s literature. A cemented carbide counterface could be used for all tests and it may not wear, but this would not simulate most real-life tribosytems. Counterface wear is particularly important with glass and carbon fiber (CF) reinforced plastics. Counterface wear is often the cause of plastic bearing failure in reinforced and unreinforced bearings. 

Rubbing a flat-ended pin against a drum covered with bonded abrasive (sandpaper) is a standard test for metals (ASTM G 141), but it could be used for plastics. The ASTM test standard allows the option of rotation of the pins about their axis. The pins traverse the drum in a spiral path (Figure 3) and a reference pin material is run on the same abrasive in a path parallel to the test pin. Wear is expressed as a wear ratio. The use of a reference sample with every test sample mitigates differences in lots of abrasive media. This test simulates tribosystems with severe abrasion, for example, a plastic snow shovel. In use, it will be rubbed on concrete or simularly abrasive paving. 

Table 1. Plastic wear tests used for simulating sliding contact with other rigid surfaces.

The pin-on-disk test (13) is not a standard for plastic wear testing, but it is the only wear test apparatus t many laboratories have and thus it will be used for plastic, metal, ceramic, etc. It works quite well if this tribosystem simulates the real-life tribosystem of interest. The rider in the pin-on-disk test is usually a ball or a hemispherical-ended pin. Flat-ended pins are used, but they often hydroplane and do not wear flat. Plastic testing laboratories often have molds for flex bw and tensile samples. Ball cavities can easily be put in these same molds to provide molded spheres as riders for a pin-on-disk test. The disk can be made from any counterface of interest. It is particularly easy to measure wear volume from spherical surfaces. A simple calculation using the diameter of the flat worn on the sphere will produce a wear volume measurement that is usually more accurate that can be obtained by mass change measures. This te excels for simulating tribosystems that involve very small normal forces on the plastic member. 

NF C26-251 October 1980 Methods of testing insulating materials. Method of test for coefficients of friction of plastic film and sheeting for use as electrical insulatioiL NF T51 -110/Al August 1977 Plastics. Plane to plane contact wear and fiiction test, altemating linear friction. 

Table 12.13 Tribology Data for Mitsui Chemicals, Inc., PES Plastics by Suzuki Friction/Wear Test (Ring-on-Disk Configuration)...

---