Silicone polymers sealants

Silicones are probably best known for their application as sealants and as release materials for pressure sensitive adhesives [107]. The silicone polymer combines an inorganic backbone made from silicon-oxygen bonds with organic substitution on the silicon atom. This repeating unit, shown below is called a siloxane. 

This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

When formulating a silicone adhesive, sealant, or coating, based on hydrosilylation addition cure, one must consider the following properties of the uncured product pot life, dispensing technique, rheology, extrusion rate, cure performance. These characteristics directly affect the processing properties of the polymer base or crosslinker parts. The degree of cure conversion at the temperature of interest is determined by properties such as tack free time, cure profile and cure time. Once... 

The surface energy of silicones, the liquid nature of the silicone polymers, the mechanical properties of the filled networks, the relative insensitivity to temperature variations from well below zero to very high, and the inherent or added reactivity towards specific substrates, are among the properties that have contributed to the success of silicone materials as adhesives, sealants, coatings, encapsulants, etc. 

Moisture-Curing Silicones. The formulation of moisture-curing silicones includes a silicone polymer, filler, a moisture-reactive cross-linker, and sometimes a catalyst. The most common silicone polymer used in sealant formulations is an alternating silicon—oxygen backbone with methyl groups attached to the silicon such as the silicone polymer (1). 

Silicone polymer plasticizers have historically been used in many formulations. These plasticizers (qv) are of the same Si—O backbone as the functional polymers but generally are terminated with trimethyl groups which are unreactive to the cure system. This nonreactivity means that, if impropedy used, the plasticizer can migrate from the sealant and stain certain substrates. Staining has been a widely publicized flaw of silicone sealants, but the potential of a formulation to stain a substrate can be minimized or eliminated with proper formulation work. In general, this is accomplished by not using plasticizers for formulations developed for stain-sensitive substrates. 

Silicone polymers when cured into elastomers by themselves are weak, gel-like materials. For this reason, fillers must be used to provide reinforcement. The type of fillers (qv) used in silicone sealants varies widely two of the most common fillers are fumed silica and calcium carbonate. 

Acrylics. There are two principal classes of acrylic sealants latex acrylics and solvent-release actylics. High molecular weight latex acrylic polymers are prepared by emulsion polymerization of alkyl esters of acrylic acid, The emulsion polymers are compounded inlo sealants by adding fillers, plasticizers, freeze-thaw stabilizers, thickeners, and adhesion promoters. As is true of the silicone lalex sealants, die acrylic latex sealants are easy to apply and clean with water. 

Fig. 7 shows the content of low molecular weight siloxanes in the silicone polymer. As shown in the figure, almost all of the low molecular weight siloxanes can be removed from the original polymer through refining process. Nowadays, refined polymers are used for RTV rubber for electrical and electronic applications. Refined silicone sealants are used as clean room sealants in the field related to semiconductor manufacturing. 

Outgasslng of Silicone Polymers. The percent condensable versus time curves at 150°C for the heat vulcanized (HV) preformed silicone seals, and for the room temperature vulcanized (RTV) silicone sealants are shown in Figure 2. Samples of each class of silicone were obtained from several sources for evaluation in this study. For ease of discussion, however, only the high and low extremes for each class of silicone are plotted in Figure 2. All of the other silicones evaluated fell between these extremes. 

Sealants made with this silicone polymer are extrudable over a wide temperature range they can be applied in winter on the Alaskan North Slope or in the heat of an Egyptian summer. When the polymer is then crossllnked into a three-dimensional network to form the cured sealant, the rubberlness, or extensibility, does not change significantly with temperature. A sealant used in Alaska will continue to stretch easily and seal when temperatures are far below freezing. 

The silicone polymer backbone is composed of Si-O-Si bonds. This bond is very strong and stable with a bond energy of 87 Kcal/ mole. The polymer can tolerate 250°C to 300°C without decomposing.— The fully compounded silicone sealant, when cured to a rubber, can withstand 200°C for sustained periods of time with no special additives and even higher temperatures with polymer modifications and/or heat stability additives.— The Sl-O-Si molecular structure is also transparent to U.V., so silicone sealants are virtually unaffected by weather. Samples of silicone sealants used in exterior construction applications have been tested after 20 years of actual performance. These samples exhibited essentially no change in physical properties or adhesion during that time period. 

The exceptional intrinsic performance of silicone sealants is due to the silicone polymer, the filler, and the unique crossllnk-Ing systems. 

The key feature in these reactions is that silane cross-linkers form slloxane bonds, the same as the polymer backbone. Thus, the stability and uniqueness of the silicone polymer has not been significantly disturbed by the crosslinking system. The crosslink density of a sealant system is often modified to achieve a desired stress-strain performance profile. 

Tock, W., Dinivahi, M. V. R. N., and Chew, C. H., Viscoelastic Properties of Structural Silicone Rnbber Sealants, Advances Polymer TechnoL, 8(3) 317-324 (1988)... 

Dimethyl silicone polymers are, because of their relatively low polarities, relatively compatible with hydrocarbons and easily swollen by fuels. Substitution for the methyl functionality by polar cyanoethyl and trifiuoropropyl groups imparts significant polarity to these elastomers and makes them resistant to swelling by nonpolar substances these specialized polymers can be compounded into very useful adhesives and sealants for applications requiring fuel resistance. 

Silicone sealants of this type are available as colorless (translucent) compositions if desired. Silicone polymer content is in the 60-70% range (40-50% polysiloxane plus up to 20% dimethyl silicone or methylphenylsilicone plasticizer). Pigments and fillers include titanium dioxide, silicas, calcium carbonate, or dried clays. Pyrogenic silicas are used for rheological control to produce nonslump sealants. 

Silane polyurethane hybrids are urethane-based polymers which have been end-capped with reactive silane groups. Urethane-based and silicone-based sealants are two major, single component sealant technologies useful in many applications. For instance, they are used for sealing and bonding cement-containing compounds, metals, plastics, and glass. 

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