Second Part covers Tear Resistance, Ozone Resistance, Low Temperature Resistance.
Tear Resistance
The tearing of rubber is a mechanical rupture process started where forces are concentrated in an area usually caused by a cut, defect or deformation.
How to Test Tear Resistance
Tear resistance is tested on a tensometer in the same manner as the tensile strength test except the specimen is one of 5 specific shapes: Type A, B, C, T or CP. A graph is produced in the same manner as the stress-strain curve except the Tear Strength graph is force over jaw separation length. Tear strength is calculated by taking the maximum force divided by the median thickness of the specimen (Ts = F/d).
Type A – Crescent shaped specimen with a nick or cut
Type B – Tab End specimen with a nick or cut
Type C – Right Angle specimen with a nick or cut
Type T (Trouser)– Molded block, 150 X 15 X 2mm, with a 40mm cut
Type CP (Constrained path)– Molded specimen 125 X 28.5 X 5.33mm. This is a special molded shape with fabric reinforcement molded in the mid-plane of the sample. The specimen has a narrows groove down the length in the center.
Ozone Resistance
Ozone (O3), resistance is used to test the relative ability of the rubber compound to resist outdoor weathering or ozone chamber testing. Some applications like door and window trim would be subject to weathering so testing would give an estimation of how the rubber compound will react to weathering. Other sources of ozone exposure include air purifiers and ozone generators used to purify, deodorize, disinfect and kill bacteria in just about everything from air to food.
How to Test Weathering/Ozone Resistance
ASTM Method D1171 addresses how to test weathering and ozone resistance. In D1171, rectangular cross section samples are wrapped around a wooden mandrel and left in the sun or placed in an ozone chamber. After a period of time either method A or method B is used to grade the samples. In method A no cracking is permitted under 2X magnification and in method B, three samples are checked and graded depending on the severity of cracking and given a quality retention value (expressed as a percentage) derived from Table 1 in ASTM D1171.
ASTM Method D1149 is used to test the effects of specific levels of ozone concentration on specimens that are under dynamic or static surface strain conditions.
Low Temperature Resistance
There are two low temperature tests that are used in testing low temperature properties of elastomers, ASTM D2137, Low Temperature Brittleness, and ASTM D1379, TR-10/TR-70 Temperature Retraction test. Low Temperature Brittleness is the most common low temperature test you will see on a physical properties data sheet. The temperature retraction test is not as common but will give you more accurate continuous operating low temperature results and a better indication of the viscoelastic and crystallization effects at low temperature.
ASTM D2137 – Low Temperature Brittleness
The Low Temperature Brittleness test is use to determine the lowest temperature at which a rubber specimen will not exhibit fractures or cracks when subject to a specific impact condition. There are two tests methods, A and B. Test Method A is for rubber volcanizates and Test Method B is for rubber coated fabrics. This test is useful for development purposes but may not necessarily indicate the lowest temperature at which the compound will operate. The TR-10/TR-70 Temperature Retraction Test is more effective in determining the lowest temperature at which a compound will continue to operate.
How to Test Low Temperature Brittleness
Specimens are cut from a die and placed into a fixture. The specimens are immersed into a liquid bath at the specified test temperature for a determined length of time. After immersion deliver a single impact to the specimen and note any cracks, fissures or holes visible to the naked eye. Repeat the test at the next highest temperature (usually 10°C increments) until the specimen passes with no cracks, fissures or holes.
ASTM D1379 – TR-10/TR-70 Temperature Retraction
The TR-10/TR-70 Temperature Retraction test is used to evaluate the crystallization effects and viscoelastic properties of the rubber specimen at low temperature. This test will give you a better indication of compounds lowest temperature at which it will continuously operate.
How to Test TR-10/TR-70 Temperature Retraction
This test is performed by stretching a die cut specimen in a special fixture to 250% elongation or 50% of the ultimate elongation if 250% can not be obtained. The stretched specimens are immersed in in a liquid at -70°C for 10 minutes freezing the sample to a state of reduced elasticity. Now, after releasing the specimens, slowly raise the temperature of the samples and measure the temperature and length of the specimens at 2 minute intervals. Report the temperature at which the sample retracted 10% (TR10), 30% (TR30), 50% (TR50) and 70% (TR70).
The TR10 value can be used to indicate the low temperature at which it will continuously operate, and it also correlates with the brittle point. The greater the temperature difference between the TR10 and TR70 the greater the tendency of the rubber to crystallize. TR70 also correlates with low-temperature compression set.
Understanding the physical properties of rubber will help you determine what properties are important to your application.