Fillers rubber-based adhesives

Some rubber base adhesives need vulcanization to produce adequate ultimate strength. The adhesion is mainly due to chemical interactions at the interface. Other rubber base adhesives (contact adhesives) do not necessarily need vulcanization but rather adequate formulation to produce adhesive joints, mainly with porous substrates. In this case, the mechanism of diffusion dominates their adhesion properties. Consequently, the properties of the elastomeric adhesives depend on both the variety of intrinsic properties in natural and synthetic elastomers, and the modifying additives which may be incorporated into the adhesive formulation (tackifiers, reinforcing resins, fillers, plasticizers, curing agents, etc.). 

In rubber base adhesives, fillers may affect properties such as cohesion, cold flow, rheology and peel adhesion. Most fillers increase cohesion and reduce cold flow. In some formulations, even a small addition of filler dramatically reduces peel strength either because of interactions with the tackifier or because filler particles at the surface reduce the area of contact between the adhesive and the substrate. 

Pigments and fillers are added to rubber base adhesives for various purposes ... 

For viscosity or sag control. When the rubber base adhesive is applied on a vertical surface, addition of a filler prevents the adhesive from running down the wall. In solvent-borne formulations, fumed silica can be used as anti-sag filler. In water-borne systems, clays impart yield stress and excellent sag control. 

Moisture. The presence of water in a filler is not usually beneficial. Most fillers added to adhesives have a moisture content lower than 1 wt%. Only precipitated silicas and sepiolite contain about 5-10 wt% moisture. For some applications, fillers must be completely dried to exhibit adequate performance. Moisture absorbed on the surface of fillers impacts the rate and extent of curing of rubber base adhesives. 

Clay and talc are the most common fillers in rubber base adhesive formulations. Both have platy shapes favouring the interactions with elastomers. 

Although natural quartz, cristobalite and opal are used as fillers, only synthetic products (fumed and precipitated silicas) find use as fillers in rubber base adhesives. 

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 ... 

Titanium dioxide is a common pigment, and zinc oxide and magnesium oxide are common fillers in rubber base adhesives. 

In solvent-borne rubber adhesives, a variety of solvents can be chosen to control drying rate, adjust viscosity and dissolve important ingredients. Resins can be added to improve tack, wetting properties, heat resistance, bond strength and oxidation resistance. The most common resins nsed in rubber-based adhesives are rosins, rosin esters, and terpene, coumarone-indene, hydrocarbon and phenobc resins. Plasticizers and softeners reduce hardness, enhance tack and decrease cost of rubber adhesive formulations. Paraffinic oils, phthalate esters and polybutenes are typical plasticizers. Fillers are not often added to rubber adhesive formulations because they reduce adhesion. However they are sometimes used because they decrease cost and increase solution viscosity. Carbon black and titanium dioxide are also used to provide colour to the adhesives. Clays, calcium carbonate and silicates are also common fillers in rubber adhesive formulations. For water-borne adhesives, typically protective colloid, preservative, defoamers, wetting agents and emulsifiers are included in the formulations. 

Carbon Black (Queen, 1978) - Carbon black is the most widely used filler for reinforcing rubber compounds and, as such, is used in the manufacture of black rubber based adhesives and sealants. Carbon black reinforcing fillers and pigments are made by the incomplete combustion of organic hydrocarbons under controlled conditions. 

Density. Most fillers added in rubber base formulation have a density between 2 and 2.7 g/cm-, except barium sulphate (4-4.9 g/cm- ) and zinc oxide (5.6 g/cm ). Addition of filler increases the free volume of the polymer and, in general, there is a critical concentration of filler at which the density of the formulation increases. The method of incorporation of filler in the adhesive formulation is important because air voids may appear when a poor dispersion is produced. 

PMDA has also been claimed to improve the quality of polyester based adhesives(26), acrylics(27), and epoxies(29). The inclusion of PMDA into thermoplastic and copolyester rubber, with alkaline earth filler, is claimed to make an excellent hot-melt adhesive(30). 

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