Abstract: Nano-silica prepared by gas phase method is the best filler for reinforcing high temperature vulcanized silicone rubber. The effect of nano-silica structure on the properties of silicone rubber is studied. The results show that the silica aggregates dispersed to the 100-200 nm scale have a good reinforcing effect on the silicone rubber. The nano-silica powder is added to the silicone rubber to form a crystallite region with silica as a nucleus, which increases the physical cross-linking point and is more susceptible to crystallization.
Key words: nano-silica, silicone rubber, reinforcement

The mechanical strength of pure silicone rubber is very low. When mixed with reinforcing filler, the tensile strength of vulcanized rubber can be increased from 0.35MPa to 14MPa, and the reinforcing rate is up to 40 times, which is far higher than the reinforcing rate that other rubber can achieve. 1.4-10 times), it can be seen that the use of fillers plays a decisive role in the final properties of silicone rubber [1]. Wang research shows that the formation of inter-particle network structure increases the effective volume of filler reinforcement, and thus the modulus of the elastomer increases [2]. Wu Jihuai of Huaqiao University studied talc, quartz, α-silica fossil, β-silver fossil, kaolin, pyrophyllite, eucalyptus and heavy calcium as fillers to reinforce silicone rubber vulcanizate, and found quartz in crystalline structure. The tensile strength and rebound rate of the vulcanizate as filler are higher than that of silica prepared by precipitation method, but lower than that of silica prepared by gas phase method [3], indicating that fumed silica is excellent for silicone rubber. Reinforcement performance. In this paper, different methods of gas phase nano-silica were selected to study the effect of nano-silica structure on the properties of silicone rubber.
Experimental part
1.1 main raw materials
    Methyl vinyl silicone rubber (VMQ), molecular weight 600,000, vinyl content 0.17%, Dongjue Fine Chemical Co., Ltd. products. Nano-silica A-200 prepared by gas phase method, Degussa company product of Germany; M-5, Cabot company product of USA; ECUST, pilot product of East China University of Science and Technology. Hydroxy silicone oil, containing 10% hydroxyl, Changshu reagent factory products. Vulcanizing agent bis-25, the Aczo company in the Netherlands.
1.2 sample preparation
    According to the formula ratio, the raw rubber, hydroxy silicone oil and fumed nano-silica are uniformly mixed on the double-roll mill, and the rubber mixture is thinly passed through the sheet. After heat treatment at 170 ° C for 2 hours, the refining and vulcanizing agent is added. The film was molded on the vulcanizer the next day. The vulcanization conditions were 175 ° C × t90. T90 is a positive vulcanization time of the vulcanizate measured by an LH-90 vulcanizer.
1.3 performance test
    The hardness is determined according to the national standard GB/T531. Using the AG-2000A Japan Shimadzu Material Universal Testing Machine, the tensile speed was (500±50) mm/min, and the tensile and tear properties were measured according to GB/T528 and GB/T529. The vulcanization time and vulcanization temperature of the vulcanizate were measured using an LH-90 rubber type vulcanizer. The particle size distribution of the powder in the particle size distribution range (0.04 to 2000 μm) was analyzed by ultrasonic using an LS-230 Particle Analysis particle size analyzer.
2. Results and discussion
2.1 The structural form of gas phase nano-silica
    As shown in Fig. 1, the primary particles of nano-silica are spherical particles of 2-20 nm, and the spherical particles are chemically bonded to form a branched aggregate of a pearl string structure of 50-500 nm, and the aggregate of the structure cannot be cut by shearing. The mechanical force dispersion is the most basic unit of reinforcing silicone rubber [4], and the aggregates are loosely connected by a hydrogen bond to form a loose agglomerate.
Effect of 2.2 nano-silica aggregates on reinforcing effect
As shown in Fig. 2, after the A-200, M-5, and ECUST agglomerates are depolymerized by ultrasonic dispersion, the peaks of the aggregates form a normal distribution with uniform distribution, and the particle size is between 100-200 nm, which is reinforcing silicon. The effective particle size of the rubber. The wider the particle size distribution range of the powder, the smaller the effective reinforcing powder ratio and the lower the performance. αF indicates the difference in the agglomerated structure of the silica powder, which is closely related to the morphology of the filler, and indicates the structure of the existing filler in the vulcanizate after structural damage occurring during the kneading and vulcanization process. It can be seen from Table 1 that the higher the peak shape of the aggregate, the more complete and effective the filler structure exists in the vulcanizate, the larger the vulcanized rubber torque, the larger the αF, and the better the mechanical properties such as tensile and tear properties.
2.3 Effect of specific surface area on reinforcing effect
The rigid chain structure of the nano-silica powder is the basic skeleton of the reinforcing action. As shown in Fig. 3, the hardness increases linearly with the increase of the specific surface area, and the tear strength reaches the maximum at about 250 specific surface area. The strength is the largest at around 200 specific surface area. However, the larger the specific surface area, the smaller the particle size, the greater the binding energy between the powder surfaces, and the more difficult the dispersion of the powder in the silicone rubber, and thus the reinforcing effect is lowered.
Effect of 2.4 nano-silica on the crystallization properties of silicone rubber
    The chain structure of silicone rubber is relatively simple, the main chain is composed of -Si-O-bonds, and two methyl groups are symmetrically linked on the side chain of Si atoms. The stereoregularity of the chains is better, and the steric hindrance of the substituents is smaller. The rate of crystallization is relatively large. Due to the powder addition of the powder, the hydroxyl groups bound to the surface of the powder and the oxygen atoms of the silicone rubber are hydrogen-bonded to form a microcrystalline region in which the powder is a nucleus, dispersed in the matrix, and the physical crosslinking point is increased. So that crystallization is more likely to occur. It can be seen from Fig. 4 that as the amount of addition increases, the melting endotherm of the microcrystalline region decreases from 27.84 J/g of raw rubber to 16.92 J/g of 35 parts; in addition, ECUST powder Compared with the A-200 powder, the melting endotherm of the microcrystalline region is larger than that of the A-200 powder, indicating that the A-200 powder is easier to devitrify at low temperature, and it is difficult to harden the silicone rubber at low temperature. The elasticity disappears.

3. Conclusion
    a. The chain structure of nano-silica is the basic skeleton of reinforced silicone rubber, the specific surface area is 200-250 m2/g, and the aggregates have a particle size distribution of 100-200 nm after depolymerization, and the uniform distribution of nano-silica Powder, reinforced silicone rubber has the best tensile and tear properties.
    b. DSC analysis shows that the addition of nano-silica powder forms a microcrystalline region with silica as the nucleus, which increases the physical cross-linking point, which makes crystallization more likely to occur.