1. Matching of rubber:
(1) Selection of raw rubber:
The raw rubber of low hardness silicone material should be selected with high viscosity or high ethylene content to accept a large amount of filling oil, so as to obtain high rubber viscosity and cohesive strength, improve the processing property of the rubber compound and the physical machinery of the vulcanized rubber. performance. However, when using these high-viscosity raw rubber to produce low-hardness rubber, excessive oil addition may cause difficulty in mixing. Therefore, it is better to use the rubber grade which is oil-filled in the polymerization stage in the low-hardness rubber compound. The performance of the mixing process is improved. At the same time, the ultra-high molecular weight of the raw rubber itself makes the viscosity of the rubber compound not excessively reduced when the oil is mixed in a large amount, thereby ensuring the processing performance of the rubber compound. The high molecular weight of oil-filled silica gel enables high rubber strength even with low-reinforcing fillers, which reduces the problems associated with the addition of large amounts of oil during mixing, shortens the mixing time and enables low-hardness compounds. High viscosity is made possible, and the strength of the rubber compound is also improved. For the above reasons, the low hardness rubber is preferably oil-filled silica gel. The second is high viscosity raw rubber.
(2) Selection of reinforcing agent:
If the low-hardness rubber is not subjected to the special requirements of tensile strength, tear resistance, wear resistance, etc., it is generally possible to use medium-reinforcing or low-reinforcing carbon black, which has better dispersibility.
(3) Selection of softener:
The amount of softener in the low-hardness rubber is generally large, so the filler oil which has good compatibility with the raw rubber should be selected to prevent the oil from oozing out. Various softeners have certain solubility in the raw rubber. In order to effectively reduce the hardness without squirting, a variety of softeners are generally used. The use of the grease in the low-hardness compound has a poor softening effect, but has a remarkable effect on improving the stiffness of the rubber and preventing the production of pores in the product. The effect and the solubility (dosage) of other softeners in the compound can be improved.
(4) Selection of vulcanizing agent:
The low molecular weight softener added in the low hardness compound can reduce the crosslinking efficiency of the rubber compound, and the dilution effect on the vulcanizing agent is obvious, so the vulcanization system in the low hardness rubber compound should be appropriately strengthened. Especially for the peroxide vulcanization. Rubber compound, because a large number of low molecular softeners will compete with macromolecules for active free radicals, thus reducing the crosslinking efficiency, so the amount of peroxide in the low hardness compound should also increase accordingly. Generally the same formulation of peroxide curing The compound is one or two degrees lower than the sulfur vulcanizate.
Second, the process points:
Since the low-hardness rubber compound generally needs to add a large amount of softener to adjust the hardness, the shearing force is small when the rubber compound is mixed, the filler is easy to agglomerate or the dispersion is poor, so the conventional feeding sequence and the mixing method are very Difficult to meet the requirements of process and performance, so we use a more efficient mixing method - anti-mixing method for low-hardness compounds.
(1). Roller temperature <50
Low-hardness rubber compound should use low-temperature mixing and open cooling water because of low viscosity and low shear force.
(2). Feeding sequence
A small amount of bulk (carbon black) ------- vulcanizing agent, anti-aging agent, dispersing agent --- large material and softener.
(3). Main method: inverse mixing method
Low-hardness rubber generally does not need to be separately masticated. It can be mixed with a little pressure and can be mixed. The roller distance is 4-6mm, and the glue is evenly glued. When forming a smooth continuous roll rubber (should have proper stacking glue) You can add a small amount of large material first; then add vulcanizing agent, anti-aging agent, dispersing agent; After the small material is ready to be eaten, it is packaged to make it evenly dispersed. This process is generally easier to operate; then, the bulk black carbon black is started. The liquid softener is added by mixing, the feeding must be slow, and the method of feeding from the middle roller is adopted. When the material is basically finished, the bulk material in the cleaning disk is cleaned by the "eight-knife" method; then the roller is adjusted. Thin pass <1.5mm 4 times; the last roll is the next piece.
A. The roller temperature is generally as low as possible;
B. Do not open the knife when adding small materials, the speed is faster;
C. Low-hardness rubber compound must be slow when refueling due to the addition of a large amount of softener. It is not allowed to open a large knife or a knife.
D. It is more difficult to mix the rubber below 30 degrees. The oil and the large material can be added twice, once in half, and after 8 hours, the other half.