Due to the special use, hard rubber has obvious differences in raw material selection and formulation design compared with other rubber products.
1） Glue selection of high-unsaturated raw rubber such as natural, butylbenzene, butadiene and chlorine
Ding et al. apply because high hardness comes from intensive cross-linking, while low-unsaturated rubbers (such as butyl and EPDM) do not. Waste rubber powder can also be used, but it can only be used as an additive and cannot replace raw rubber. Its advantages are reflected in two aspects: first, it helps to make rubber; second, it can reduce the quality problems of common pores, sponges and bursts of hard rubber. Recycled rubber can replace part of the raw rubber, but also reduce the shrinkage of the rubber, which is beneficial to extrusion and calendering, but it is not good against impact and bending resistance. In addition, synthetic rubber is advantageous for heat resistance and hardness improvement.
2） Vulcanizing agent Sulfur is the most commonly used vulcanizing agent. The theoretical amount should be 37 parts (100 parts by weight of rubber), and it is usually controlled in the range of 35 to 40 parts. A sulfur promoter can also be used in place of sulfur. Like soft rubber, the sulfur bond structure is also divided into single sulfur, disulfide and polysulfide.
3） Promoter accelerators are generally less effective than soft gels in hard gels, and have anomalous phenomena. For example, the promotion effect of TMTD is not as good as accelerator D. More commonly used is the combination of an inorganic accelerator and an organic accelerator. Application examples are 5 parts of magnesium oxide + accelerator 808 #1.5 parts.
4） Softeners can improve the process of mixing, extrusion, etc., but are not conducive to the impact resistance, bending and heat resistance of the product. Applicable varieties are mineral oil, asphalt, paraffin, ancient horse and so on.
5） Fillers are good for hardness improvement and shrinkage reduction, but they are not good for physical properties and insulation properties, so it is necessary to control the dosage, except for hard rubber powder, which can reduce shrinkage during processing and reduce heat generation. The only drawback is the reduced heat resistance.
6） Hard rubber Because almost all double bonds have been saturated by cross-linking, there is no aging problem, and anti-aging agents are generally not used.
(2) Manufacturing process
From the rubber to the molding, the manufacturing process is similar to other rubber products, but the vulcanization has a larger difference. In general there are three types of vulcanization methods:
1） Direct vapor vulcanization is suitable for thin products with low surface quality requirements, such as chemical linings and smaller products.
2） Flat vulcanization is suitable for model products, such as battery tanks and various types of plates, tubes, rods and other profiles.
3) Water bath vulcanization is suitable for products with high gel content and products with hydrous silicic acid as filler, such as microporous separator.
The vulcanization of hard rubber is suitable for high crosslink density, and the process generally takes:
A longer curing time, such as the battery cover vulcanization time up to 165min;
B two or three stages of vulcanization;
C Gradually warm up.
(3) Performance indicators
Hard rubber performance evaluation items are also different from soft rubber, generally including the following four items: breaking strength (MPa); impact strength (J∕m3); Martin heat resistance (°C); weight gain rate in acid (% ).