Influence of silica on the process properties of rubber and vulcanizate properties

(1) the influence of silica on the process properties of rubber

1. Mixing and dispersion of rubber materials
White carbon black due to the specific surface area is very big, always tends to be the second aggregation, and easy to absorb moisture in the air, which is easy to produce strong hydrogen bond association between hydroxyl, further improve the cohesion between the particles, so the mixing and dispersing of white carbon black is much more difficult than carbon black, and in more quantity match, also easy to generate gel, make the glue material hardening, mixed with heat production. In order to obtain good dispersion, it is required to maintain as high shear force as possible at the beginning of mixing, so that the aggregate particles of silica can be destroyed as much as possible without causing excessive mechanical degradation of the rubber molecular chain. Therefore, silica should be added in small amounts in batches to reduce heat generation. Appropriately increasing the mixing temperature is conducive to removing part of the adsorption moisture on the surface of silica, reducing the cohesion between particles, and contributing to the dispersion of silica in the rubber.

2. Structural control of silica black reinforced silicone rubber compound
Silica, especially gas phase silica, is the best reinforcer for silicone rubber, but there is a problem of hardening the rubber mix, commonly known as "structural effect". The structure of the compound increases with the extension of the parking time, and even serious enough to return to refining and scrap. There are two explanations for this. One is the condensation of the silicone rubber end group with the surface hydroxyl group of silica black. On the other hand, the hydrogen bond is formed between the silicon oxygen chain of silicone rubber and the hydroxyl group on the surface of white carbon black. There are two ways to prevent structuration. One is to add some substances that can react with the hydroxyl group on the surface of white carbon black, such as hydroxyl silicone oil, diphenyl silicondiol, silazane, etc. When using diphenyl silicondiol, after mixing should be treated at 160~200℃ for 0.5~1h. In this way, silica black filling silicone rubber can be prevented from structuring. The other approach is to modify silica surface in advance by removing some surface hydroxyl groups to fundamentally eliminate structuralization.

3. Mooney viscosity of adhesive material
The ability of silica gel generation is similar to that of carbon black, so when mixing silica, the Mooney viscosity of rubber material is improved, so that the processing performance is deteriorated, so the selection and dosage of softener in the rubber formula containing silica is very important. Paraffin hydrocarbons, cycloalkanes and aromatic hydrocarbons are often added to IIR. The amount varies depending on the amount of white carbon black and the viscosity of Mooney, generally up to 15-30%. In NR, the softening effect of plant softeners such as rosin oil and tall oil is the best, but the softening effect of synthetic softeners is not good, and the softening effect of mineral oil is the lowest.

4. Vulcanization rate of rubber
There are a lot of micropores on the surface of silica particles, which have a strong adsorption effect on the curing accelerator, so the curing is delayed obviously. In order to avoid this phenomenon, on the one hand, the dosage of accelerant can be appropriately increased; On the other hand, active agent can be used to preferentially adsorb on the surface of silica, so as to reduce its adsorption of accelerant.

Active agent is generally containing nitrogen or oxygen amine, alcohols, alcohols amine low molecular compounds. Amines are more suitable for NR, such as diethanolamine, triethanolamine, butylene diamine, hexamethylenetetramine and so on. Alcohols are more suitable for SBR, such as hexanol, diethylene glycol, glycerol, polyethylene glycol, etc. The amount of active agent should be determined according to the amount of silica, PH value and rubber varieties, and the general amount is 1~3% of silica.

(2) the influence of silica on the properties of vulcanized rubber

Silica has significant reinforcing effect on all kinds of rubber, especially on silicone rubber.
White carbon black is a kind of white reinforcing agent second only to the corresponding furnace carbon black. The vulcanized rubber containing a certain amount of white carbon black and the corresponding furnace method of carbon black (such as HAF) reinforced vulcanized rubber, has the advantages of high strength, large elongation, high tear strength, high hardness, good insulation, etc.. Generally, the combination of carbon black and white carbon black can obtain better comprehensive properties.