Adhesive applications, thermosetting

Adhesive applications, thermosetting epoxy resin based on N-cyanourea-terminated oligomer, 107... 

Many commercial thermosets are quite often used as complex mixtures of several co-reacting monomers and prepolymers, specifically formulated to suit a given end-use application. For example, in coating and adhesive applications, often mixtures of different epoxies, differing in chemical structure and/or molecular weights, are used as required. Even two different thermoset-... 

Quite frequently, copolymerization is used to optimize the properties of polyacrylates. For example, copolymers of ethyl acrylate with methyl acrylate provide the required hardness and strength, while small amounts of comonomers with hydroxyl, carboxyl, amine, and amide functionalities are used to produce high-quality latex paints for wood, wallboard, and masonry in homes. These functionalities provide the adhesion and thermosetting capabilities required in these applications. Monomers with the desired functional groups most often used in copolymerization with acrylates are shown in Table 15.8. 

Polyester resins (alkyds) were commercialized for coatings use in 1926, and unsaturated polyesters were used as thermoset fiberglass composite matrix resins in the 1940s, but the early resins made poor adhesives. When flexibilized resins appeared in the 1950s, they were used as adhesives. Today, unsaturated polyesters are widely used as adhesives for thermoset plastics bonding, and even for metal bonding in most countries, but are seldom used as adhesives in the United States, where the more expensive epoxy adhesives are used in similar applications. The saturated polyesters, used as thermoplastic hot-melt adhesives, seem to have appeared in the literature first in the 1954—1957 period. 

Another application of condensed tannin extracts that has proved technically successful is as tire cord adhesives. Both thermosetting tannin formulations [68] and tannin-resorcinol-formaldehyde formulations have been experimented with successfully. 

Tapes for component supply in loading installations usually consist of an acrylate- or rubber-based adhesive (also thermosetting) on a paper or polyester foil carrier (dependent on the mass of the components). Depending on the type of application, different materials are used for the adhesive and the carrier for covering tapes. Most important carrier materials are foils (PVC, polypropylene, cellulose, polyester, polyimide), papers, and woven and nonwoven fabrics (cotton, glass). Adhesives frequently used are based on rubbers, silicones, and acrylates. Carrier less adhesive foils are used, for example, for the bonding of copper foils and polyimide films to get special base materials for circuit boards. 

These are materials that cannot be heated and melted after the initial cure. Curing takes place by chemical reactions at room temperature or at an elevated temperature, depending on the type of adhesive. Some thermosetting adhesives require considerable pressure, while others require only contact pressure. Solvents are sometimes added to facilitate application. These adhesives are usually available as solvent-free liquids, pastes, and solids. 

Contact adhesives have relatively high shear and peel strengths. Strength and durability approach those of structural thermosetting adhesives. A typical contact adhesive application is the bonding of decorative surface materials to wood for kitchen countertops. Parts must be correctly positioned before bonding because repositioning is difficult or impossible after the bond is made. 

Application and utilisation of these film adhesives Structural thermosetting film adhesives The film is cut to the desired size and laid on one of the substrates (refer to Fig. 92). Parts are assembled and hot pressed in an autoclave or a hot press, for instance for a one component epoxy adhesive for 30 min at 120°C, under a pressure of 100-300 kN/m for REDUX 312, or 60 min at 170°C with REDUX 308 or REDUX 322. 

As mentioned above, both thermoplastic and thermosetting type acrylic resins are commercially available for coatings, elastomers, sealants and adhesive applications. These resins are supplied in many different delivery forms such as powders or pellets, solutions in organic solvents and aqueous dispersions. These resin types are briefly described below. 

Adhesives can be broadly classified as being thermoplastic, thermosetting, elastomeric, or alloy blend. These four adhesive classifications can be further subdivided by specific chemical composition as described in Tables 7.20 through 7.23. The types of resins that go into the thermosetting and alloy adhesive classes are noted for high strength, creep resistance, and resistance to environments such as heat, moisture, solvents, and oils. Then-physical properties are weU suited for structural-adhesive applications. 

In joining reinforced thermosetting pipe it is particularly important that the pipe be cut without chipping or cracking it. It is also important to sand, file, or grind any mold-release agent from the surfaces to be cemented. Joints are built up layer by layer of adhesive-saturated reinforcement by following the manufacturer s recommended procedure. Application of adhesive to the surfaces to be joined and assembly of these surfaces shall produce a continuous bond and provide an adhesive seal to protect the reinforcement from attack by the contents of the pipe. Unfilled or unbonded areas of the joint are considered defects and must be repaired. 

The applications of rubber as an engineering material almost invariably involve bonding to a rigid substrate or reinforcement. In some instances these bonds need to be established to a fully cured thermoset rubber or a molded thermoplastic rubber, and a wide variety of adhesives suitable for this purpose are available. In... 

Structural applications of rubber base adhesives were also obtained using rubber-thermosetting resin blends, which provided high strength and low creep. The most common formulations contain phenolic resins and polychloroprene or nitrile rubber, and always need vulcanization. 

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