Abstract: Nano-silica prepared by fumed process is the best filler for reinforcing high-temperature vulcanized silicone rubber. This paper studied the effect of nano-silica structure on the properties of silicone rubber. The results show that silica aggregates dispersed to 100-200nm have an excellent reinforcing effect on silicone rubber. Adding nano-silica powder into silicone rubber forms microcrystalline regions with silica as the crystal nucleus, increases physical cross-linking points, and makes crystallization easier.
Key words: Nano-silica; Silicone rubber; Reinforcement
Pure silicone rubber has very low mechanical strength. After mixing with reinforcing fillers, the tensile strength of vulcanizate can be increased from 0.35MPa to 14MPa, with a reinforcement rate as high as 40 times, far higher than that of other rubbers (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's research shows that the formation of interparticle network structure increases the effective volume of filler reinforcement, thus increasing the modulus of elastomer
[2]. Wu Jihuai et al. from Huaqiao University studied talc, quartz, α-cristobalite, β-cristobalite, kaolin, pyrophyllite, sericite, ground calcium carbonate and other powders as fillers to reinforce silicone rubber vulcanizates. It was found that the tensile strength and resilience of vulcanizates with crystalline quartz as filler were higher than those of silica prepared by precipitation process, but lower than those prepared by fumed process
[3], indicating that fumed nano-silica has excellent reinforcing performance for silicone rubber. In this paper, fumed nano-silica with different properties was selected to study the effect of nano-silica structure on the properties of silicone rubber.
1. Experimental Part
1.1 Main Raw Materials
Methyl vinyl silicone rubber (VMQ), molecular weight 600,000, vinyl content 0.17%, product of Dongjue Fine Chemical Co., Ltd. Fumed nano-silica A-200, product of Degussa, Germany; M-5, product of Cabot, USA; ECUST, pilot product of East China University of Science and Technology. Hydroxy silicone oil, containing 10% hydroxyl group, product of Changshu Reagent Factory. Vulcanizing agent Bis-2,5, product of Aczo, Netherlands.
1.2 Sample Preparation
According to the formula ratio, raw rubber, hydroxy silicone oil and fumed nano-silica were mixed uniformly on a two-roll rubber mill. The compound was passed through thin sheets, heat-treated at 170℃ for 2h, then remilled with vulcanizing agent, and sheeted out. The next day, it was molded on a vulcanizing press. The vulcanization condition was 175℃×t90. t90 is the optimum vulcanization time of vulcanizate measured by LH-90 vulcameter.
1.3 Performance Test
Hardness was measured according to national standard GB/T531. AG-2000A universal material testing machine (Shimadzu, Japan) was used with a tensile speed of (500±50) mm/min. Tensile and tear properties were measured according to GB/T528 and GB/T529 respectively. The optimum vulcanization time and vulcanization temperature of vulcanizate were measured by LH-90 rubber vulcameter. LS-230 Particle Analysis particle size analyzer was used to analyze the particle size distribution range (0.04～2000μm) of the powder under ultrasonic condition.
2. Results and Discussion
2.1 Structural Form of Fumed Nano-Silica
The primary particles of nano-silica are spherical particles of 2-20nm. Spherical particles are connected by chemical bonds into branched chain aggregates with pearl string structure of 50-500nm. The aggregates of this structure cannot be dispersed by mechanical forces such as shearing, and are the most basic unit for reinforcing silicone rubber [4]. Aggregates form loose network agglomerates through hydrogen bonds.
2.2 Effect of Nano-Silica Aggregates on Reinforcement
After ultrasonic dispersion and depolymerization of A-200, M-5 and ECUST agglomerates, the peaks of aggregates present a normal distribution with uniform distribution, and the particle size is between 100-200nm, which is the effective particle size for reinforcing silicone rubber. The wider the particle size distribution range of the powder, the smaller the proportion of powder with effective reinforcement, and the performance decreases. αF represents the difference due to the agglomeration structure of silica powder, which is closely related to the morphology of the filler, and represents the structure of the existing filler in the vulcanizate after structural damage during mixing and vulcanization. Combined with Table 1, it can be seen that the higher the peak shape of the aggregate, the more complete and effective the filler structure in the vulcanizate, the greater the torque of the vulcanizate, the larger the αF, and the better the mechanical properties such as tensile and tear properties.
2.3 Effect of Specific Surface Area on Reinforcement
The rigid chain structure of nano-silica powder is the basic framework of reinforcement. As shown in Figure 3, hardness increases linearly with the increase of specific surface area. Tear strength reaches the maximum at about 250 specific surface area, and tensile strength reaches the maximum at about 200 specific surface area. However, the larger the specific surface area, the smaller the particle size, the greater the binding energy between powder surfaces, and the more difficult the dispersion of powder in silicone rubber, so the reinforcement effect decreases.
2.4 Effect of Nano-Silica on 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 connected to the side chain of Si atom. The stereoregularity of the chain is good, the steric hindrance of substituents is small, and the crystallization speed is relatively high. Due to the addition of powder, the hydroxyl groups bonded on the powder surface combine with the oxygen atoms of silicone rubber through hydrogen bonds to form microcrystalline regions with powder as crystal nucleus, which are dispersed in the matrix, increasing physical cross-linking points and making crystallization easier. As can be seen from Figure 4, with the increase of filling amount, the melting heat absorption of microcrystalline regions decreases from 27.84J/g of raw rubber to 16.92J/g of 35 parts; In addition, under the same addition amount, the melting heat absorption of microcrystalline regions of ECUST powder is higher than that of A-200 powder, indicating that A-200 powder is easier to crystallize at low temperature, and it is not easy to harden silicone rubber and lose elasticity at low temperature.
3. Conclusion
a. The chain structure of nano-silica is the basic framework for reinforcing silicone rubber. Nano-silica powder with specific surface area of 200-250m²/g, aggregate particle size distribution of 100-200nm after depolymerization and uniform distribution has the best tensile and tear properties for reinforced silicone rubber.
b. DSC analysis shows that the addition of nano-silica powder forms microcrystalline regions with silica as crystal nucleus, increases physical cross-linking points, and makes crystallization easier.