Analysis of Reasons for Yellowing of Transparent Silicone Rubber

Silicone rubber is a highly elastic organic polymer material. Compared with ordinary rubber, it has excellent properties such as high temperature resistance, low temperature resistance, weather resistance, chemical resistance, breathability, and non-toxicity. It is widely used in electronic and electrical, civil medical, automotive industry, textile industry and medical equipment fields. As a reinforcing agent for silicone rubber, gas-phase silica accounts for more than 60% of the total consumption of gas-phase silica. Therefore, silicone rubber can be said to be a promising application field for high-end gas-phase silica.

In the production of silicone rubber, the problem of yellowing of transparent silicone rubber has long plagued the formula designers, production and quality inspection personnel of enterprises. For high-temperature vulcanized silicone rubber, it is necessary to undergo one vulcanization and secondary vulcanization to remove the residual decomposition products of peroxides and further crosslink the silicone rubber to obtain the best physical properties. However, the phenomenon of yellowing often occurs in the silicone rubber after secondary vulcanization, which affects the transparency of the products and requires a perfect solution.

Impurities and low pH of carbon black impurities in transparent silicone rubber

It has been reported that under certain conditions, when the pH value of carbon black is acidic, it can cause transparent silicone rubber to visibly turn yellow, and the degree of yellowing is inversely proportional to the pH value. When the pH is less than 5.5, this inverse proportionality relationship is more obvious. This is because the di-alkyl peroxide contained in carbon black is catalytically cleaved under acidic conditions to produce a keto compound, which is condensed into an aldehyde alcohol, and then dehydrated to form a conjugated compound. This compound will absorb the spectral band of blue light, which will cause the silicone rubber after secondary vulcanization to turn yellow.

In addition, the gas-phase carbon black manufacturing process contains chloride ions and a small amount of multivalent metal ions such as Fe and Cr. Chloride ions will become highly oxidizing hypochlorite ions in the vulcanization process of silicone rubber, and when they encounter metal ions, the originally low-valence metal ions are easily oxidized into colored high-valence ions. This can also cause transparent silicone rubber to turn yellow.

Surface treatment agent HMDZ makes transparent silicone rubber rubber solution alkaline

In the production process of transparent liquid silicone rubber, hexamethyldisilazane (HMDZ) is often used as a surface treatment agent to treat gas phase carbon black. Hexamethyldisilazane can only generate trimethyl silanol for treating carbon black after reacting with water, and it also releases ammonia. If ammonia remains in the rubber solution, it will produce strong alkalinity and cause silicone rubber to turn yellow.

Vulcanization temperature and time of transparent silicone rubber

When transparent silicone rubber is undergoing heat treatment, if the time is too long or the temperature is too high, the vinyl group in the rubber solution will oxidize to form conjugated diene compounds, which will cause the silicone rubber to turn yellow after secondary vulcanization.

Impurities introduced into the reaction vessel of silicone rubber

Generally, the reaction vessels for rubber materials are made of materials such as stainless steel, which may introduce trace metal impurities such as iron and chromium. Even trace metal impurities can cause transparent silicone rubber to turn yellow after secondary vulcanization.

In summary, to prevent yellowing of silicone rubber after secondary vulcanization, the following preventive measures can be taken :

  • When designing the formula, try to make the mixing system close to neutral ;

  • Control reasonable vulcanization temperature and time ;

  • Use a curing agent such as polymethylhydrogensiloxane with hydrogen-containing side chains to inhibit the oxidation reaction of impurities contained in organosilicon and prevent yellowing.

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