Fillers silicone adhesives

Fillers can also be used to promote or enhance the thermal stability of the silicone adhesive. Normal silicone systems can withstand exposure to temperatures of 200 C for long hours without degradation. However, in some applications the silicone must withstand exposure to temperatures of 280 C. This can be achieved by adding thermal stabilizers to the adhesive formulations. These are mainly composed of metal oxides such as iron oxide and cerium oxide, copper organic complexes, or carbon black. The mechanisms by which the thermal stabilization occurs are discussed in terms of radical chemistry. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fillers, plasticizers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utilize diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, silicones, or polysulfide, are also used. 

The rheological properties of adhesives and sealants are important in many applications. When these products must be pumped or applied through automated equipment, the flow characteristics at pertinent shear rates are critical. Sophisticated rheological measurements can be performed to predict performance. The rheology of silicone adhesives and sealants can be tailored through adjustment of polymer viscosity, filler loading, and incorporation of various additives. 

These versatile polymers can be compounded with other chemicals and fillers into an enormous variety of products that serve in a multitude of applications. This chapter will describe the product types, chemistry, and applications of only one major class of silicone products, the adhesive sealants. In addition, the class of silicone adhesive products will be described and discussed these materials are employed where release characteristics are desired, rather than adhesion. 

Another type of sealant uses a combination of chemistry to yield a product useful as a sealant. This chemistry is a marriage of silicone and organic chemistry known as a polyether silicone. The basic polymer for this type of sealant is the reaction product of a polyether diol with a diisocyanate followed by further reaction with an amino-functional alkoxy silane. Thus, the central portion of this polymer is a polyether that is terminated with a urethane linkage that leads to the terminal group which is an alkoxy silane. This polymer can cure by the methods described above for a standard silicone but it contains the properties of a polyether. As with the normal silicones described above, these materials also have to be reinforced in order to obtain the tensile properties desired for a sealant. The same list of fillers as described above can be used in polyether silicones. Adhesion promoters such as silane coupling agents, diluents of various sorts, plasticizers, and catalysts are also added to these materials. 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

Fumed silicas (Si02). Fumed silicas are common fillers in polychloroprene [40], natural rubber and styrene-butadiene rubber base adhesives. Fumed silicas are widely used as filler in several polymeric systems to which it confers thixotropy, sag resistance, particle suspension, reinforcement, gloss reduction and flow enhancement. Fumed silica is obtained by gas reaction between metallic silicon and dry HCl to rend silica tetrachloride (SiCU). SiC is mixed with hydrogen and air in a burner (1800°C) where fumed silica is formed ... 

On the other hand, the alkoxide system presented several problems in formulation. The system first chosen as a model consisted of a trimethoxymethyl silane crosslinker, 8000 centistoke HEB siloxane, and a catalyst. A number of catalysts were used and each exhibited different cure rates and electrical properties. DuPont tetraalkoxytitante-Tyzor appears to he one of the better catalysts used in this type of curing system. Fillers are usually incorporated into the silicone formulation to improve mechanical properties, promote adhesion, and to serve as light screening and pigment agents. Cab-o-sil, a form of fumed silica, carbon-black, titanium dioxide and calcium carbonate are then used as RTV fillers. 

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. 

At present industry widely manufactures glue sealant Elastosil based on low-molecular silicone elastomers. Besides elastomers, the compositions of these sealants include fillers, plasticisers, solidifiers and adhesive components in some cases pigments are used to give them a colour. 

Polyphenylsilsesquioxane varnishes are used as binding agents for heat-resistant, protective and decorative enamels. To increase low adhesion, which is characteristic of silicone varnishes, enamels are often supplemented with various organic modifiers, pigments and fillers used in the vamish-and-paint industry. 

The analogous large-volume series ZSK MEGAvolume with 8.7 Nm/cm3, a diameter ratio of 1.8, and screw speeds of up to 1800 rpm is used for compounds with a high solid matter content, such as those often found in the chemical/food/pharmaceutical industries. The maximum speeds of up to 1800 rpm are used mainly for low-viscosity products, such as pressure-sensitive adhesives and silicon sealants because they provide better elastomer and filler dispersion. Other processes often require large volume only and operate at very moderate speeds of 100 to 600 rpm to protect the product or for direct extrusion into films, sheets, or profiles. 

The chemicals used for coating and laminating are polymeric materials, either naturally occurring or produced synthetically. These include natural and synthetic rubbers, polyvinyl chloride, polyvinyl alcohol, acrylic, phenohc resins, polyurethanes, silicones, fluorochemicals, epoxy resins and polyesters." Coating formulations typically include auxiliaries such as plasticizers, adhesion promoters, viscosity regulators, pigments, fillers, flame retardants, catalysts and the like. ... 

Both one-component and two-component silicone sealants contain polydimethyl siloxane as the base polymer along with fillers such as calcium carbonate and/or fumed silica fillers, plasticisers (silicone oil) and a variety of cross-linking agents and adhesion promoters. Two-component sealants utilise a catalyst such as dibutyl tin dilaurate, alkyl silicate esters and metallic salts (Maslow, 1982). 

The microstructure of a silicone resin emulsion paint is shown in Fig. 3. The overview even at low magnification (Fig. 3a) indicates that one binder forms thick films (black arrows) into which sinks some of the approx. 300-nm-sized white pigment (titanium dioxide). This is the organic polymeric binder. Because of the deficit of organic polymer, pore voids are open and voids between fillers are only partly filled (Fig. 3c), i.e., only small adhesive bridges are present (black arrows in Figs. 3a,c), which determine the binding power. 

Filler-free formulations of two-component silicone rubbers crosslinkable by condensation or addition are utilized, with or without added solvent, for the coating of papers and plastic foil, optionally as an aqueous emulsion, e.g. for strippable paper for self-adhesive labels or packaging foil for bitumen. Vulcanization is carried out at high temperature (100 to 180°C), to attain as short hardening times as possible. 

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