Silicone copolymers bearing reactive vinyl and hydride functionalities: Synthesis, characterisation and particulate composite thereof for specialty applications

Highlights

• High molecular weight vinyl terminated poly(dimethyl-co-diphenyl)siloxane synthesised.

• The polymer is syntheisized via anionic polymerization followed by chain termination.

• The crosslinker is synthesized via cationic ring opening polymerization.

• The filled polymer system shows good adhesive and mechanical characteristics.

• They can be used for aerospace applications owing to low outgassing characteristics.

Abstract

High molecular weight vinyl terminated poly (dimethyl-co-diphenyl)siloxane polymer (V-PDMPS) was synthesised by a multistage process including hydrolysis of precursor chlorosilanes and their anionic polymerization followed by chain termination. The low molecular weight hydride functional crosslinker for V-PDMPS, trimethylsilyl terminated poly (dimethyl-co-methylhydrogen-co-diphenyl)siloxane, (TMS-PDMHS) was synthesised by ring opening polymerisation of cyclic siloxanes. These two novel addition curable polymers were characterized by FTIR, NMR and chromatographic methods. Further, the role of catalyst and moderator in determining the rheological characteristics of the system was studied in detail. V-PDMPS was compounded with both pristine quartz and surface functionalised quartz (TMCS- quartz) in different compositions and the effect of these fillers in determining the adhesive, mechanical and rheological properties of the elastomer was investigated. The addition curable silicone polymer based formulations with low outgassing characteristics and having adequate mechanical and rheological properties developed in this study can find potential applications in aerospace, especially as adhesive.

Introduction

The recent past has witnessed great advancements in the development of high performance silicone elastomers. A wide range of industries including electrical, electronics, pharmaceutical, medical, optical, automotive and aerospace depend on silicone elastomers to meet a variety of applications. Silicones are used extensively as sealant materials, coatings, adhesives, potting compounds and foams [1], [2], [3], [4], [5]. The inherent elastic properties of silicones make them a natural choice for sealing applications. Silicones are revolutionising the technology of adhesion, thereby opening up advanced design possibilities. They can bond materials together providing good strength along with exceptional lightness, and can be designed for permanent or temporary adhesion. Silicones can bind together materials that were traditionally difficult to join such as glass and stone. The excellent adhesion characteristics originate from the chemical structure of silicones [6]. Their useful properties include good resistance to fire, heat, ageing and weathering as well as good low temperature flexibility.

Poly (diphenyl siloxane) (PDPS) homopolymer has a higher decomposition temperature than poly (dimethyl siloxane) (PDMS) homopolymer, the most common member of silicone family [7], [8], [9]. PDPS has mechanical properties that are substantially different compared to that of PDMS and copolymers derived with the two units (diphenyl and dimethyl) in the structure exhibit properties intermediate between the two homopolymers. The incorporation of methyl-phenyl siloxane unit or diphenyl siloxane unit as a copolymer with PDMS has been shown to increase the onset temperature of degradation to nearly 400 °C from 300-350 °C of PDMS [7], [10]. Because of the high thermal stability and appreciable mechanical properties, poly(dimethyl-co-diphenyl siloxanes) are generally considered for application as adhesives for high temperature service [11].

In spite of the several advantages, silicones are considered unsuitable for applications where exposure to high stresses occur; primarily because of their poor mechanical properties. Hence, these elastomers are reinforced with fillers that enhance the properties such as tear strength, tensile strength, hardness and abrasion resistance [12], [13], [14]. Reinforcement by inclusion of a solid phase such as synthetic silica, quartz and metal oxide possessing large surface area have been shown to be very effective in improving the mechanical properties of silicones [15]. Among these solid reinforcements, amorphous silica with surface area ranging from 150 m²/g to 400 m²/g has been reported to offer the highest reinforcement [16]. Synthetic silica fillers, for example, precipitated silica, possess siloxane and silanol groups as reactive functionalities in their structure [17] and the silanol groups (SiOH) are reported to be acidic in nature [18]. In order to improve the bonding with PDMS, the approach of modifying silica surfaces, mostly with alkoxy silanes, has also been attempted by many researchers [19], [20]. Barthel et al. [21] reported that trimethylsilylation is an effective approach for reducing the thickening effect of fumed silica when incorporated in to siloxane matrix.

This paper describes the synthesis and characterisation of vinyl terminated poly(dimethyl-co-diphenyl)siloxane and trimethylsilyl terminated hydride functional silicone copolymer, and investigation on the role of catalyst and cure moderator in deciding the rheological characteristics of the elastomer resulting by blending these polymers. Apart from that, the role of surface treatment in crystalline silica (quartz) in determining the mechanical and rheological properties of the elastomers is investigated. The optimised composition derived from the study has been evaluated as adhesive for specialty applications.