【摘要】 用原硅酸乙酯为交联剂和二月桂酸二丁基锡酯为催化剂使聚二甲基硅氧烷在室温下进行熟化变为三向结构的弹性体(硅橡皮)时,在有水的情况下,原硅酸乙酯用量越多,熟化越快,无水时,结果相反。我们认为在空气中,原硅酸乙酯会经水解缩合变为聚硅酸乙酯,因此室温熟化的真正的交联剂不是象文献所说的原硅酸乙酯而是聚硅酸乙酯。 室温熟化所得的硅橡皮在高温下,机械强度会迅速下降然后逐渐恢复。将硅橡皮分别在空气及氮气中进行高温处理,并测定其溶胀度的变化的结果,证明这现象是由于硅橡皮在高温下发生降解及氧化所致。降解是由于水在二月桂酸二丁基锡酯或它的水解产物月桂酸催化下使聚二甲基硅氧烷主链的Si—O—Si键水解所引起的。更多还原

【Abstract】 Polydimethylsiloxane containing hydroxyl end groups can be vulcanized at room temperature by using ethyl orthosilicate as crosslinking agent and dibutyltin dilaurate as catalyst. The generally accepted mechanism of the vulcanization is given in Eq. (1). However, in actual practice, the relative quantity of ethyl orthosilicate to hydroxysiloxane used is always many times larger than the stoichiometric ratio. The excess of ethyl orthosilicate should render the chain terminal unreactive toward the crosslinking reaction as shown in Eq. (2), thus questioning the validity of the proposed mechanism.In our experiments carried out in moist air (Table 1), the rate of vulcanization increased with the increase of the amount of ethyl orthosilicate, and the reverse was true when vulcanization took place in dry sealed tubes (Table 2). This indicates that under dry condition, the excess ethyl orthosilicate retards the crosslinking action as expected, while, in the presence of water, ethyl orthosilicate is hydrolysed to form polyethylsilicate having increased opportunity for the crosslinking. Consequently, the vulcanization should proceed according to Eqs. (3) and (4) where polyethylsilicate, instead of ethyl ortho-silicate, is the real crosslinking agent, as is generally proposed.When heated to 250癈 in air, the tensile strength of the room temperature vulcanized silicone rubber dropped to almost zero within an hour, and then gradually recovered on prolonged heating (Fig. 1b). The recovery was not observed in N2 atmosphere (Fig. 1c). Similar results were obtained when polydimethylsiloxane alone was heated with dibutyltin dilaurate. In air, the viscosity at first dropped and then rose again (Fig. 3a), whereas, in N2 , there was no such increase in viscosity after the initial drop (Fig. 3b). A small amount (0.1%) of dibutyltin dilaurate caused a significant decrease of molecular weight of polydimethylsiloxane on heating, while further increase of the catalyst produced only a small effect (Fig. 4). However, when water was added, great decrease in molecular weight was noted (Fig. 5). The effect of increasing amount of lauric acid on the molecular weight is shown in Fig. 6. All these seem to indicate that the initial drop of the tensile strength of the room temperature vulcanized silicone rubber is due to the breaking of the Si-O-Si bond of polydimethylsiloxane by a trace of water in the system under the catalytic action of lauric acid or of dibutyltin dilaurate (Eq. 5). The recovery of the tensile strength when heated in air for a long time is caused by the oxidation of the methyl group of the dimethylsiloxane in the presence of dibutyltin dilaurate to form new crosslinking units.更多还原