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Effect of Surface of PPT Silica on Properties of Rubber Products
(1) Moisture
The water content of precipitated silica includes free water and bound water. Free water on the surface of silica can be easily removed when heated. Free water is too little, indicating that the surface of silica has little light base, which reduces the activity of silica, and promotes the aggregation of silica during mixing, so it is difficult to distribute in the compound. With less surface water, not only the vulcanization rate is affected, but also the reinforcing effect is reduced accordingly. Free water is slightly higher, mixing rubber has good operation performance, fast powder feeding speed, less dust flying, roll wrapping property is also good, vulcanization speed is slightly faster. However, the surface of vulcanizate tends to bubbles due to excessive water content, high heat generation, easy scorching, agglomeration and uneven dispersion, and the physical and mechanical properties of vulcanizate also decrease accordingly. Generally, surface water at 6-8% can play the best isolation role on silica molecule, so as to prevent the re-aggregation of particles and evenly distribute in the compound during mixing, which is conducive to improving the properties and accelerating the vulcanization speed. The bound water is "inside" of silica, which is relatively stable, and can be removed only when the temperature is higher than 400 C. The general content is 4.0-7.0%. If too much water is combined, it shows that the gel content in the reaction process is high, the finished product has little activity, and the reinforcing effect on the rubber is also small. The excessive dehydration and sanding tendency in the reaction process are also unfavorable to the reinforcement.
(2) pH value
The acidity and alkalinity of silica have a great influence on the vulcanization of rubber. The acidity of silica delays the vulcanization of rubber, while the alkalinity promotes the vulcanization, but the excessive alkalinity is unfavorable to the reinforcement. PH is slightly acidic silica, which is beneficial to the tension resistance and wear resistance of rubber materials and products. Therefore, the PH value of precipitation silica for rubber is generally 6-8, which is close to neutrality.
(3) Surface modification
Surface modification of silica refers to the addition of coupling agent in silica to achieve its hydrophobicity. With the addition of coupling agent, CH3O-1 (methoxy group) at one end of the coupling agent reacts easily with the silanol group on the surface layer of silica, and at the other end reacts with the molecule of image rubber, that is, the bridging effect of coupling agent between silica and rubber. There are many ways to modify the surface of silica. At present, silane coupling agent (such as Si-69) is most commonly used to treat silica.
(4) Impurities
In the preparation of precipitated silica, impurities such as Cu, Fe and Mn are easily brought in. Although these trace impurities have little effect on the physical properties of rubber compound, the content of Cu, Fe and Mn is still required to be reduced to a certain limit in order to make rubber products have excellent antioxidant and aging resistance. In view of this, GB 10517 stipulates that the precipitation method of silica is copper < 30 mg/kg, Mn < 50 mg/kg and Fe < 1000 mg/kg.Sodium salt is unavoidable in the production of precipitation silica, but it is not suitable to be used as reinforcing filler for natural or synthetic rubber because of its high sodium content, and the transparency of the compound is obviously poor. However, in the production process, the reaction synthesis process is controlled, the gel production of over Na+ is prevented, and the rinsing of semi-finished products slurry is strengthened. The content of sodium salt can be reduced to less than 0.5%, so the precipitated silica has little effect on the properties of the rubber.
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JINSHA PRECIPITATED SILICA MANUFACTURING CO.LTD.
Email.: jk@jksilica.com
Add.: Gaosha Industrial Zone, Shaxian, Fujian, China
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