For rubber products, raw rubber is required to have a certain plasticity. For example, for rubber of molded industrial products, the plasticity of raw rubber is required to be around 0.25-0.35. For calendering, extrusion, sponge rubber, glue and other rubber materials, the plasticity is required to be around 0.4-0.6. If the rising rubber does not reach the required plasticity, it will bring great difficulties to mixing, and the quality of the product cannot be guaranteed. Therefore, for raw rubber with high Mooney viscosity, plasticization must be carried out and the required plasticity must be obtained to ensure the smooth progress of subsequent processes. What adverse effects will excessive or low plasticity have on rubber processing and products? If the raw rubber is not mixed enough and the plasticity does not meet the requirements, mixing will be difficult, and phenomena such as roll-off will occur; in addition, the shrinkage rate of the rubber increases. If the raw rubber is over-plasticized, the hardness and tensile strength of the mixed rubber will decrease, and the resistance to media and aging will decrease. Therefore, it is very important to plasticize the raw rubber according to the requirements of different products.
Not every raw rubber needs to be plasticized. This depends on the rubber plasticizing characteristics, Mooney viscosity, etc. In general, except for special-purpose rubbers with a Mooney viscosity (ML100℃1+4) below 80, plasticizing is not necessary. Mixing can be done by thinning several times before mixing. Raw rubber with a Mooney viscosity (ML100℃1+4) above 80 needs to be plasticized, but this is not always the case. For example, although the Mooney viscosity of fluororubber is 65-180, its molecular chain is rigid and stable. After long-term plasticization of raw rubber, not only does the viscosity decrease very little, but sometimes it increases. Therefore, fluororubber can be directly mixed without plasticizing.
From the perspective of the domestic rubber processing industry, the main plasticizers are unsaturated natural rubber and hard nitrile rubber. For EPDM, butyl rubber, chloroprene rubber, butadiene rubber and acrylic rubber, if used for molded products, plasticization is not required and they can be directly mixed. However, for calendering, extrusion, and sponge products, in order to ensure that the shrinkage rate of the products is small and the surface of the products is smooth, even if the raw rubber can be mixed without plasticizing, it is necessary to perform multi-stage plasticizing to improve the plasticity of the raw rubber and produce qualified products. In recent years, soft butyl rubber imported from abroad, such as Japanese nitrile 240S and Russian CKH-26, has a low Mooney viscosity. Generally, it is not necessary to plasticize, but directly mix.
The following is a brief description of the plasticizing methods and precautions for natural rubber and nitrile rubber.
Natural rubber has a large raw rubber Mooney viscosity and high mechanical strength. If it is not plasticized, it is very difficult to mix (here mainly refers to 1#~5# smoke sheet rubber). Therefore, natural rubber must be plasticized before mixing in order to obtain appropriate plasticity.
Since natural rubber is a crystalline rubber, especially in winter, if the crystallization is not removed, it will bring great difficulties to rubber cutting and damage the equipment. Therefore, before cutting the rubber, it is necessary to bake the rubber. Glue baking is generally carried out in a glue baking room at a temperature of about 60°C for 48 hours.
After the raw rubber is taken out from the drying room, first remove the impurities on the surface of the raw rubber, and then use a rubber cutter to cut it into small pieces of about 5 kg for standby use. It is best to separate the rubber blocks with an isolation agent to prevent them from sticking to each other and keep them clean. The cut rubber blocks need to be broken, and generally factories do this on a rubber mixer. Taking a 14-inch rubber mixer as an example, first adjust the narrow baffle (the working surface accounts for about 2/3 of the roller length) and reduce the roller distance (0.5㎜) to prevent the rubber mixer from being overloaded, causing "tripping" and damage to the equipment. Then put the rubber blocks into the rubber mixer one by one against one end of the large gear wheel, and the amount of rubber loaded is about 20 kg. When loading the rubber, you should face the rubber mixer sideways or stand on the side of the rubber mixer to prevent the rubber from popping out and injuring people, and at the same time turn on the cooling water. The broken raw rubber should be plasticized in time and not parked for a long time. Because if it is parked for a long time, the raw rubber will stick together again. It is more troublesome to cut and break the rubber again.
During plastication, put the raw rubber into the plasticator and let it fall directly into the material tray through the roller gap naturally without wrapping the roller. Repeat the process for about 30 minutes, and keep the roller temperature below 50℃. If the temperature is too high, false plasticity will occur (once the plasticized rubber cools down, it will recover). In order to avoid this phenomenon, the lower the roller temperature, the better. At the same time, the increase in mechanical shear force is conducive to the degradation of rubber macromolecules, thereby obtaining the plasticization effect.
In the first ten minutes after the start of plastication, the plasticization effect of natural rubber is very obvious. As time goes by, the increase in plasticity will slow down. If the mixing is stopped at this time, it will be quickly reflected during the mixing: the plasticized rubber is not easy to wrap the roller, and even if it is wrapped, the surface is not smooth, and powder cannot be added. At this time, the plasticized rubber should be re-thinned and plasticized. Until a satisfactory effect is obtained.
From the above situation, although the time is constantly extended, the plasticity of natural rubber is not as significant as in the first ten minutes. But it does not mean that only ten minutes of plasticization is enough. On the contrary, plasticizing is very necessary. Otherwise, the labor intensity will increase and the mixing time will be prolonged, and the quality of the mixed rubber cannot be guaranteed.