Rubber is a polymer, thus it will show three mechanical states: viscous-fluid, highly elastic, and glassy, respectively, in different temperature ranges when the external forces are applied.
The polymer chain of rubber has two types of moving units, i.e. the whole molecular chain and the individual chain segments in the chain. In a certain temperature range, although the whole chain cannot move, some segments of the chain are displaced due to the internal rotation of the chain, so the shape of the molecule can change, stretching or curling, and when stress is applied, slow deformation occurs, and after the stress is removed, it slowly returns to its original state with good elasticity, which is the high elastic state of rubber. After reaching the glass transition temperature, the entire macromolecule and chain links are completely frozen, and only the molecules vibrate in their original positions, and the elasticity of the polymer disappears, which is the glass state of rubber.
As a rubber polymer in practical use is the use of its outstanding high elasticity, rubber is expected to maintain a high elastic state in a wide range of temperatures, requiring as low a glass transition temperature as possible, i.e. good cold resistance. Some rubber products are subjected to tensile, compression, impact, bending, shear, and torsion respectively at low temperatures, so the appropriate test method should be selected as far as possible according to the working state and technical conditions of the rubber product so that there is a good correlation between the test results and the actual use of performance. Commonly used low-temperature test methods are temperature retraction test (TR test), determination of low-temperature rigidity, brittleness temperature test, determination of tensile cold resistance coefficient, determination of compression cold resistance coefficient, determination of glass transition temperature of rubber, determination of low-temperature hardness, low-temperature compression permanent deformation test and stress relaxation test.
To carry out a low-temperature test, it is first necessary to establish low-temperature conditions that meet the test requirements. The general requirements for obtaining high and low temperatures for testing rubber and other elastic materials are laid down in the international standard ISO 383-85. It specifies the general principles for the construction and operation of temperature-controlled boxes for specimen parking and testing.
Low-temperature test devices
To achieve the temperature of the low-temperature control box, a variety of heat transfer media can be used, of which there are liquid heat transfer media and gas heat transfer media. Internationally, the liquid heat transfer media used for low-temperature testing are acetone, methanol, ethanol, butanol, hexane, etc. The gas heat transfer media used are air, carbon dioxide, liquid nitrogen vapor, etc. The refrigerants used are dry ice, liquid chlorine, Freon, liquid air, etc. Liquid heat transfer media have the fastest heat transfer rate but may have harmful effects on the rubber if immersed for too long. The use of small glass bead flow beds offers multiple advantages of liquid heat transfer combined with chemical inertness.
The general requirements for low-temperature control boxes are as follows.
- The heat transfer medium in the box should have no obvious effect on the performance of the rubber.
- In the part of the control box where the specimen is placed, the temperature should be controlled within the permissible tolerance range specified in the relevant test method.
- The heat transfer medium in the box can be made to circulate well by using a fan or stirrer.
- It is advisable to use automatic means of temperature control.
- After the specimen has been placed in the apparatus, the temperature should be brought back to the specified temperature as quickly as possible, and in any case in no more than 15 minutes, especially for gaseous media, following the minimum overshoot or minimum loss of adjustment required.
- The dimensions of the control box are not specified as long as a uniform temperature can be maintained in the space where the specimen is located.
- The chamber should be insulated to prevent condensation and frost on the external surfaces during low-temperature testing.
- The construction of the chamber depends on the type of heat transfer medium.