Study on the Preparation and Properties of Dealcoholized Transparent RTV Silicone Rubber

ZENG Xiang-lei，ZHANG Shao-hong, DENG Li-hua，HU Xin-song

(Research Center of Guangdong Adhesive and Sealing Materials (Glorystar) Engineering, Guangzhou Glorystar Chemical Co. Ltd., Guangzhou 510450，Guangdong)

Abstract: The dealcoholized transparent RTV-1 silicone rubber was prepared by adding crosslinker, fumed silica and tin catalyst in alkoxyl terminated 107 silicone rubber. The effects of the amount of crosslinking agent, the types and amount of catalyst on the storage stability of silicone rubber, and the types of coupling agent on adhesion and surface drying time were investigated. Results showed that the optimum preparation conditions are as follows: add 4 phr of methyltrimethoxysilane in 100 phr of alkoxy terminated 107 silicone rubber, add 9 phr methyl trimethoxy silane in 100 phr of alkoxy terminated silicone rubber, 90 phr of fumed silica, 1.2 phr of self-made composite coupling agent, 0.6 phr of silane and 0.3 phr of tin complex. The silicone rubber prepared under these conditions has better comprehensive properties, whose tensile strength is 1.35 MPa, and the elongation at break is 173%, the surface drying time is 9 min. After storage for 100℃×5 d, the tensile strength of silicone rubber is 0.51 MPa, the surface drying time is 60 min, and the complete vulcanization time is 2 d.

Keywords: alkoxy terminated; RTV; dealcoholized; transparent

With the prosperity and development of the electronic industry, the demand for silicone rubber in this field is increasing. General deacetic acid type and deketoxime type silicone rubbers are corrosive to metals and are not suitable for electronic components. Deacetone type silicone rubber has superior performance but relatively expensive. The dealcoholized silicone rubber is non-corrosive to metals and has a low price, which can be used in large quantities in electronic products^[1]. Generally dealcoholized silicone sealant is based on α,ω-dihydroxy polydimethylsiloxane (107 silicone rubber); methyltrimethoxysilane is used as crosslinking agent; titanium complex is used as catalyst, and it is combined with fillers and mixed with additives. However, the above formulas have obvious shortcomings, mainly due to poor adhesion and viscosity peaks during the production process. In this experiment, alkoxy-terminated 107 silicone rubber was used as the base rubber, and the crosslinking agent, fumed white carbon black, other additives and catalysts were added in a certain order to obtain a transparent dealcoholized room temperature vulcanization (RTV) silicone rubber. What’s more, good storage stability and excellent bonding performance are needed in order to meet the requirements of electronic and electrical adhesives.

1 Experiment

1.1 Main raw materials and equipment

Here are experiments about mixing silicone rubber and relevant issues.

α,ω-dihydroxy polydimethylsiloxane (107 silicone rubber): viscosity 20 000 mPa.s, Dow Corning (China) Co., Ltd. Alkoxy-terminated polydimethylsiloxane (tetramethoxy polydimethylsiloxane): viscosity 20 000 mPa·s, self-made^[2]. Fumed white carbon black: LM-150, Cabot (China) Company. Methyltrimethoxysilane, vinyltrimethoxysilane: Hubei New Lantian New Materials Co., Ltd. γ-aminopropyltriethoxysilane (KH 550), γ-glycidoxypropyltrimethoxysilane (KH 560), N-(2-aminoethyl)-3-aminopropyltrimethoxy Silane (KH 792): Jingzhou Jianghan Fine Chemical Co., Ltd. Tin acetate, dibutyl tin dilaurate: Beijing Zhengheng Chemical Co., Ltd. Tetraisopropyl titanate: Nanjing Quanxi Chemical Co., Ltd. Tin complex: organotin and polyethyl silicate are compounded in an appropriate ratio and self-made^[3]. Compound coupling agent: KH 550, KH 560, KH 792 are compounded according to the same mass ratio and self-made. Silazane: Wuxi Sailiwei Biotechnology Co., Ltd. Capping catalyst: Sodium hydroxide and methanol are mixed at a mass ratio of 1:1, self-made. Acetic acid: Tianjin Fuyu Fine Chemical Co., Ltd.

Power mixer: DLH-5, Foshan Jinyinhe Machinery Equipment Co., Ltd..  Universal tensile testing machine: CMT4304, Shenzhen Sansi aspect Technology Co., Ltd.. Blower oven: 101-0H, Guangzhou Kangheng Instrument Co., Ltd.

1.2 Preparation of dealcoholized transparent RTV silicone rubber

Add 100 phr of alkoxy-terminated 107 silicone rubber and 2~6 phr of methyltrimethoxysilane to the power mixer at room temperature, vacuum and stir for 10 min, after unloading the vacuum, add 9 phr of gas phase dried at 120℃ for 4 hours. French white carbon black is evacuated to -0.08 MPa, high-speed stirring for 30 minutes, adding other additives and 0.2 to 0.5 phr of tin catalyst, vacuum stirring for 30 minutes, and then discharge.

1.3 Performance test

Tensile strength and elongation at break: tested according to GB/T 528-2009. Surface drying time: tested according to GB/T 13477.5-2002. Storage stability: sealed the prepared silicone rubber in an oven at 100℃ Test after placing it for 5 d. Adhesion: Test according to GB/T7124-2008. Complete curing time: Cut a 6 mm diameter rubber strip every 24 hours to observe whether it is completely cured.

2 Results and discussion

2.1 The influence of the amount of crosslinking agent on the properties of silicone rubber

To 100 phr of alkoxy-terminated 107 silicone rubber, add 9 phr of fumed white carbon black, 2~6 phr of methyltrimethoxysilane crosslinking agent, 1.2 phr of composite coupling agent, 0.6 phr of hydroxyl scavenger silicium Alkane^[4], 0.3 phr of tin complex catalyst, the effect of the amount of crosslinking agent on the performance of silicone rubber was investigated, and the results are shown in Table 1.

It can be seen from Table 1 that the amount of crosslinking agent has little effect on the surface drying time of silicone rubber, but has a greater impact on the tensile strength, elongation at break and the surface drying time after storage at 100℃ for 5 d. When the amount of crosslinking agent is 3 to 4 phr, the storage stability is better, and the tensile strength and elongation at break are also higher. If there is too much crosslinking agent, the tensile strength and elongation will decrease instead. The more the amount of crosslinking agent is, the shorter the surface drying time after storage at 100℃ for 5 d. The storage stability of the rubber compound is relatively better. Maybe it’s because a large amount of methyltrimethoxysilane reacts with residual moisture to form silicon. Oxane oligomers maintain a low water content during the storage of the rubber compound. Considering comprehensively, the preferred amount of crosslinking agent in this experiment is 4 phr.

Table 1 The effect of contents of different crosslinking agent on the performance of silicone rubber

2.2 The effect of the type and amount of catalyst on the storage stability of silicone rubber

Adding 9 phr of fumed white carbon black in 100 phr of alkoxy terminated 107 silicone rubber, adding 4 phr of methyltrimethoxysilane also called methyltrichlorosilane, 1.2 phr of composite coupling agent, 0.6 phr of silazane, 0.2~0.5 phr of tin catalyst. The effect of the type and amount of catalyst on the storage stability of silicone rubber was investigated. The results are shown in Table 2 and Table 3. Table 2 shows the effect of catalyst types on the storage stability of silicone rubber. Table 3 shows the effect of the amount of tin complex catalyst on the storage stability of silicone rubber.

Table 2 The effect of types of different catalysts on the storage stability of silicone rubber

Note: The amount of catalyst is 0.3 phr.

It can be seen from Table 2 that the type of catalyst has a great influence on the storage stability of the rubber compound. The stability of silicone rubber prepared from tin acetate and dibutyl tin dilaurate is relatively poor. The surface drying time changes drastically before and after storage for 100℃ × 5 d. The storage stability of the silicone rubber prepared by the tin complex is good. This may be due to differences in the catalytic activity of different types of catalysts^[5]. The catalytic activity of tin acetate and tin laurate is higher, so the surface drying time of the prepared silicone rubber is shorter. However, the surface drying time is greatly prolonged after storage for 100℃ × 5 d, and the complete vulcanization time is not less than 8 d, and the stability is relatively poor. The catalytic activity of tin complexes is low, but the prepared silicone rubber has the best storage stability.Considering comprehensively, this experiment chooses tin complexes as catalysts.

Table 3 The effect of content of tin complex on the storage stability of silicone rubber

It can be seen from Table 3 that with the increase of the amount of tin complex, the surface drying time of the rubber compound is gradually shortened, and the tensile strength changes little. After storage for 100℃ × 5 d, the surface drying time of silicone rubber first decreased and then increased, and the tensile strength decreased significantly. The amount of catalyst determines the reaction rate. There are a series of complex reactions in the rubber during storage, which affect the moisture vulcanization of the rubber. The accelerated aging experiment can amplify the difference in the reaction rate of the rubber. On the whole, it is more appropriate to use 0.2~0.3 phr of catalyst, and the surface drying time is no difference between before and after storage for 100℃ × 5 d. When 0.4~0.5 phr of catalyst is added, the reaction rate in the rubber compound becomes very fast and the surface drying time is shortened. The tensile strength of the vulcanized rubber after storage for 100℃ × 5 d dramatically drop. Therefore, the preferre type of catalyst in this experiment is tin complex, and the preferred amount of catalyst is 0.3 phr.

2.3 The influence of coupling agent type on the adhesiveness of silicone rubber

To 100 phr of alkoxy-terminated 107 silicone rubber, add 9 phr of fumed white carbon black, 4 phr of methyltrimethoxysilane, 1.2 phr of coupling agent, 0.6 phr of silazane, and 0.3 Part of the tin complex catalyst, the effect of coupling agent types on the adhesiveness of silicone rubber was studied, and the results are shown in Table 4.

It can be seen from Table 4 that when KH 550 and KH 560 are used as a compound coupling agent, the adhesion of silicone rubber is poor and the surface drying time is longer. When KH 792 is used as the coupling agent alone, the adhesion of silicone rubber is better than that of KH 550 and KH 792. When used in conjunction, the effect is slightly worse; when the self-made composite coupling agent is used, the cohesive damage area of silicone rubber on the above-mentioned substrates is the largest, the bonding performance is the best, and the surface drying time is shorter. This may be because the diamine structure of KH 792 exerts a viscosity-increasing effect^[6], and its strong catalytic effect can speed up the cross-linking reaction and shorten the surface drying time. The thickening effect of KH 550 and KH 560 compound coupling agent is relatively poor. In addition to considering the adhesion to the substrate when selecting the coupling agent, the surface drying time of the rubber compound also needs to be considered. Therefore, self-made composite coupling agent is preferred in this experiment.

Table 4 The effect of types of different coupling agent on the adhesion of silicone robber

Note: 1) KH 550, KH 560, KH 792 are compounded according to the mass ratio of 1:1:1.

3 Conclusion

With alkoxy-terminated 107 silicone rubber as the base rubber, a crosslinking agent, fumed white carbon black, tin catalyst and other additives are added to prepare a dealcoholized transparent one-component RTV silicone rubber with good storage stability. The preferred preparation conditions are to add 4 phr of methyltrimethoxysilane agent, 9 phr of fumed white carbon black, 1.2 phr of composite coupling agent, and 0.6 phr to 100 phr of alkoxy-terminated 107 silicone rubber. With Silazane and 0.3 phr tin complex, the overall performance of the silicone rubber prepared under these conditions is better, the tensile strength is 1.35 MPa, the elongation at break is 173%, and the surface drying time is 9 min; After being stored for 100℃ × 5 d, the tensile strength of the silicone rubber is 0.51 MPa, the surface drying time is 60 min, and the complete vulcanization time is 2 d.

References

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