Influence of Silica on Process Properties of Compounds

1. Compounding and dispersing

Due to its large specific surface area, silica tends to aggregate twice, and it is very easy to absorb moisture in the air, resulting in a strong hydrogen bond association between hydroxyl groups, which further improves the cohesion between particles. It is much more difficult to mix and disperse than carbon black, and when a large amount is mixed, it is easy to generate gel, which hardens the rubber compound and generates heat during mixing. In order to obtain good dispersion, it is required to maintain the shear force as high as possible during initial mixing, so that these aggregate particles of silica can be destroyed as much as possible without causing excessive mechanical degradation of the rubber molecular chain. . For this reason, silica should be added in small amounts in batches to reduce heat generation. Appropriately increasing the mixing temperature is beneficial to remove part of the adsorbed moisture on the surface of silica, reduce the cohesion between particles, and help the dispersion of silica in the rubber compound.

2. Structural control in silica reinforced silicone rubber compound

Silica, especially fumed silica, is the best reinforcing agent for silicone rubber, but has a problem of hardening the compound, commonly referred to as the "structuring effect". Its structure increases with the extension of the storage time of the rubber compound, and it is even serious to the extent that it cannot be reworked or scrapped. There are two explanations for this. One is that the end groups of silicone rubber are condensed with hydroxyl groups on the surface of silica;

There are two ways to prevent structuring. One is to add some substances that can react with the hydroxyl groups on the surface of silica, such as hydroxy silicone oil, diphenylsilicon diol, silazane, etc. during mixing. When using diphenylsilyl diol, it should be treated at 160~200℃ for 0.5~1h after mixing. This prevents the structuring of the silica-filled silicone rubber. Another way is to modify the surface of silica in advance, first remove part of the surface hydroxyl groups, and fundamentally eliminate the structure.

3. Mooney viscosity of rubber compound

The ability of silica to generate gel is comparable to that of carbon black. Therefore, when mixing silica, the Mooney viscosity of the compound increases, which deteriorates the processing performance. Therefore, in the compound formulation containing silica The choice and amount of softener is important. Paraffin hydrocarbons, naphthenic hydrocarbons and aromatic hydrocarbons are often added to IIR, and the amount varies depending on the amount of silica and the Mooney viscosity, generally up to 15-30%. In NR, the softening effect of vegetable softeners such as rosin oil and tall oil is the best, the softening effect of synthetic softeners is not great, and the softening effect of mineral oil is the lowest.

4. The vulcanization speed of the rubber

There are a large number of micropores on the surface of silica particles, which have a strong adsorption effect on the vulcanization accelerator, so the vulcanization is obviously delayed. In order to avoid this phenomenon, on the one hand, the amount of the accelerator can be appropriately increased; on the other hand, the active agent can be used, so that the active agent is preferentially adsorbed on the surface of silica, thus reducing its adsorption to the accelerator.

The active agents are generally nitrogen- or oxygen-containing amines, alcohols, and alcoholamines low-molecular-weight compounds. For NR, amines are more suitable, such as diethanolamine, triethanolamine, butanediamine, hexamethylenetetramine and the like. For SBR, alcohols are more suitable, such as hexanetriol, diethylene glycol, glycerol, polyethylene glycol, etc. The dosage of the active agent depends on the amount of silica, pH value and rubber type, and the general dosage is 1~3% of the silica.

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