Curing 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.). 

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. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

Polystyrene Although polystyrene is usually bonded by solvent cementing, it can be bonded with vinyl acetate/vinyl chloride solution adhesives, acrylics, polyurethanes, unsaturated polyesters, epoxies, urea-formaldehyde, rubber-base adhesives, polyamide (Versamid-base), polymethylmethacrylate, and cyanoacrylates. The adhesives should be medium-to-heavy viscosity and room-temperature and contact-pressure curing. An excellent source is a Monsanto Company technical information bulletin which recommends particular commercial adhesives for bonding polystyrene to a number of different surfaces. Adhesives are recommended in the fast-, medium-, and slow-setting ranges (10). 

Polysulfone A number of adhesives have been found useful for joining polysufone to itself or to other materials. These include 3M Company s EC 880 solvent-base adhesive, EC 2216 room-temperaturecuring epoxy two-part paste, Bloomingdale Division, American Cyana-mid Company BR-92 modified epoxy with DICY curing agent, or curing agent "Z" (both spreadable pastes), vinyl-phenolics, epoxy-nylons, epoxies, polyimide, rubber-based adhesives, styrene polyesters, resorcinol-formaldehyde, polyurethanes, and cyanoacrylates. The EC 880, EC 2216, and the two BR-92 adhesives are recommended by the polysulfone manufacturer. Union Carbide (16) (17). 

Chem. Desarip. Modified dehydrogenated (disproportionated) rosin CAS 8050-09-7 EINECS/ELINCS 232-475-7 Uses Thermoplastic resin in hot-melt-applied adhesives and coating tor paper and paperboard substrates as tackifier and processing aid tor rubber-based adhesives and molding compds. emulsifier tor emulsion polymerization plasticizer, softener, tackifier tor use in contact with food Features Pale, oxidation-resistant si. retards cure Pro rties USDA Rosin N solid, flakes sol. in alcohols, esters, ketones, min. spirits, and aromatic hydrocarbons dens. 1.058 kg/l R B soften, pt. 73 C flash pt. (COC) 209 C acid no. 154 sapon. no. 159... 

Solvent solutions and latex cements require the removal of the solvent from the adhesive before bonding can take place. This is accomplished by simple or heat-assisted evaporation. Some of the stronger or more environmentally resistant rubber-based adhesives require an elevated-temperature cure. Only slight pressure is usually required with pressure-sensitive adhesives (PSAs) to obtain a satisfactory bond. These adhesives are permanently tacky and flow under pressure, thus they provide intimate contact with the adherend surface. 

Adhesives used to bond acetal homopolymer to itself and to other materials, such as aluminum, steel, natural rubber, neoprene rubber, and Buna rubber, include polyester with isocyanate curing agent, rubber-based adhesives, phenolics, epoxies, modified epoxies, and vinyls. Solvent cementing... 

Suggested adhesives include modified acrylics, epoxies, polyesters, resorcinol-formaldehyde, furane, phenol-formaldehyde, polyvinyl formal-phenolic, polyvinyl butyral, nitrile rubber-phenolic, polyisobutylene rubber, polyurethane rubber, reclaimed rubber, melamine-formaldehyde, epoxy-phenolic, and cyanoacrylates. For maximum adhesion primers should be used. Nitrile-phenoUcs give excellent bonds if cured under pressure at temperatures of 149 C. Lower-strength bonds are obtained with most rubber-based adhesives. 

The curing of CR adhesives is different from that for most rubber-based adhesives as it involves the labile chlorine atoms. This is reflected in the compounding ingredients. The most common room curing agent is zinc oxide, but isocyanates are also very common. Further discussion is given in Polychloroprene rubber adhesives modifiers and additives. 

Most Rubber-based adhesives may be cured by a sulphur-based vulcanizing system (see Rubber-based adhesives compounding), however, as mentioned in Polychloroprene rubber adhesives applications and properties, CR adhesives are cross-linked by various reactions involving the labile chlorine atoms in the repeat unit. This is reflected in the additives used, as discussed below. ... 

Uses Thermopiastic resin in hot-meit-appiied adhesives and coatings for paper and paperboard substrates as tackifler and processing aid for rubber-based adhesives and moiding compds. emuisifier for emuision poiymerization piasti-cizer, softener, tackifler for use in contact with food Features Paie, oxidation-resistant si. retards cure... 

Commonly used adhesives include epoxy, rubber, acrylic or vinyl emulsions, and urethanes. Epoxies are high-priced, but they have better chemical resistance and durability than the others, and they have dominated the market in outdoor applications. Significant advantages of the epoxy-based adhesives are that they have no solvents and, therefore, exhibit little shrinkage. They cure relatively fast and, therefore, are not as exposed to inclement... 

Blends of elastomers are routinely used to improve processability of unvulcanized rubbers and mechanical properties of vulcanizates like automobile tires. Thus, cis-1,4-polybutdiene improves the wear resistance of natural rubber or SBR tire treads. Such blends consist of micron-sized domains. Blending is facilitated if the elastomers have similar solubility parameters and viscosities. If the vulcanizing formulation cures all components at about the same rate the cross-linked networks will be interpenetrated. Many phenolic-based adhesives are blends with other polymers. The phenolic resins grow in molecular weight and cross-link, and may react with the other polymers if these have the appropriate functionalities. As a result, the cured adhesive is likely to contain interpenetrating networks. 

Non-structural adhesives used on CELCXDN arc usually one-component, room-temperature-curing systems based either on thermoplastic resins or elastomeric materials dispersed in solvents. They are normally used in applications which will not be exposed to temperatures over 180°F. Neoprene rubber adhesives are examples. (7). 

Natural-rubber-based pressure-sensitive adhesives can be cured by standard mbber curatives, eg, sulfur plus an accelerator (see Rubber, natural) ... 

As previously noted, the primary use for this elastomer has been as solvent-based adhesives and sealants. Solvent-based products are losing market share to water-based poly-chloroprenes, to other polymer types such as acrylics and polyurethanes, and to hot melt adhesives. However, where the processing facility is able to contain the vapor emissions, a solvent-based adhesive or sealant is preferred because of better wetting of surfaces, faster drying, and higher performance of the cured or dried product. Many rubber bonding... 

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