Fine adhesives

Adhesives of the aminoplastic (see Step polymerization) and phenol formaldehyde (see Phenolic adhesives single-stage resoles and Phenolic adhesives two-stage novolacs) types are most widely used. Although basically similar, an adhesive for plywood manufacture will require a different formulation to one for particle board, or medium-density fibre board (MDF) since methods of application and processing differ. Thus, in plywood, large sheets of veneer must be uniformly coated with adhesive, usually by a roller or curtain coater in particle board, chips or wafers must be coated with very fine adhesive droplets, while small bundles of wet fibres must be sprayed with adhesive in the manufacture of MDF. Hence, formulation and production of resins has become a mixture of art and science, with resin manufacturers able to produce resins tailored for use in a particular board-manufacturing plant, or with a particular species of timber. 

Many of these features are interrelated. Finely divided soHds such as talc [14807-96-6] are excellent barriers to mechanical interlocking and interdiffusion. They also reduce the area of contact over which short-range intermolecular forces can interact. Because compatibiUty of different polymers is the exception rather than the rule, preformed sheets of a different polymer usually prevent interdiffusion and are an effective way of controlling adhesion, provided no new strong interfacial interactions are thereby introduced. Surface tension and thermodynamic work of adhesion are interrelated, as shown in equations 1, 2, and 3, and are a direct consequence of the intermolecular forces that also control adsorption and chemical reactivity. 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

Adhesives. Poly(vinyl alcohol) is used as a component in a wide variety of general-purpose adhesives to bond ceUulosic materials, such as paper and paperboard, wood textiles, some metal foils, and porous ceramic surfaces, to each other. It is also an effective binder for pigments and other finely divided powders. Both fully and partially hydrolyzed grades are used. Sensitivity to water increases with decreasing degree of hydrolysis and the addition of plasticizer. Poly(vinyl alcohol) in many appHcations is employed as an additive to other polymer systems to improve the cohesive strength, film flexibiUty, moisture resistance, and other properties. It is incorporated into a wide variety of adhesives through its use as a protective coUoid in emulsion p olymerization. 

Poly(vinyl alcohol) is used as an additive to dry-wall joint cements and stucco finish compounds. Rapid cold-water solubiUty, which can be achieved with finely ground PVA, is important in many dry mixed products. Partially hydrolyzed grades are commercially available in fine-particle size under the name S-grades. The main purpose of the poly(vinyl alcohol) is to improve adhesion and act as a water-retention aid. 

Woodflour, a fine sawdust preferably obtained from softwoods such as pine, spruce and poplar, is the most commonly used filler. Somewhat fibrous in nature, it is not only an effective diluent for the resin to reduce exotheim and shrinkage, but it is also cheap and improves the impact strength of the mouldings. There is a good adhesion between phenol-formaldehyde resin and the woodflour and it is possible that some chemical bonding may occur. 

The reaction of water with isocyanate is shown in the third item of Fig. 1 [5]. The water/isocyanate reaction is the major curing mechanism for the one-component urethane adhesives. Most one-component urethanes are based on an isocyanate-terminated prepolymer (I). Usually, the moisture in the air is used to cure the adhesive, but in some instances, a fine mist of water may be introduced on top of the adhesive before the bond is closed, in order to facilitate cure ... 

In other parts of the world, plywood adhesive fillers are obtained from local sources and may be quite different than those used in North America. In Southeast Asia, banana flour is quite important. In Europe, calcium carbonate (chalk) is often used. Nearly any fibrous material or fine particulate material capable of forming a functionally stable suspension can be made to work if the formulator is sufficiently skillful. However, the mix formulator will be very specific about the type and grade of filler to be used in a particular mix. Substitutions may lead to serious gluing problems. 

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