Glass transition temperature glue

Poly(vinyl acetate) is used for adhesives and as a wood glue (40% solution), as a raw material in lacquers and varnishes (dispersions), and as a concrete additive (in the form of a fine, dispersible powder obtained by spray drying). Poly(vinyl acetate) grades that are more resistant to hydrolysis are obtained by copolymerization with vinyl stearate or vinyl pivalate, since the saponification rate is reduced by the bulkier side groups. Pure poly(vinyl pivalate) has too high a glass transition temperature, 78 C, for most poly(vinyl ester) applications. Other copolymers of vinyl acetate are produced with ethylene (see Section 25.2.1) or vinyl chloride (see Section 25.7.5.3). 

Melt glues are amorphous and/or partially crystalline polymers above their glass transition temperatures or melt temperatures. Their viscosities should not be too high so that they can wet surfaces well, and not too low so that they do not flow away from where they are applied. Best results are obtained for viscosities of about 10-1 000 Pa s. Poly(ethylenes), poly(ethylene-co-vinyl acetate), poly(vinyl butyrals), versamides, polyamides, aromatic copolyesters, polyurethanes, bitumens, and asphalts, for example, are used as melt glues. The adhesive effect is produced by solidification of the melt glue. 

For the following discussion it will be assumed that the glue layer deforms more readily than the material to be glued. It must also be assumed that there are no chemical bonds between the material and the glue. The adhesion which is thus to be discussed depends mainly on adsorption and diffusion. The adhesive should be a clean material, and also it should be above its glass transition temperature (if amorphous) or above its melting point (if partly crystalline). The lower the molar mass of the adhesive, the more rapidly it can diffuse into the material to be bonded. 

Melt glues are used above their melting or glass-transition temperatures. They are used, for example, in the textile industry to bond cover and support materials. Poly(ethylene), poly(ethylene-co-vinyl acetate), and terpolyamides are typical melt glues. 

Dependence of glass transition temperature on degree of curing (DSC) in a highly filled EP-resin glue [636]... 

These materials are based on polymer latexes made by emulsion polymerization. They flow easily while the continuous water phase is present and dry by evaporation of the water, leaving behind a layer of polymer. In order that the polymer particles coalesce to form a continuous joint and be able to flow to contact the adherend surfaces, the polymers used must be above their glass transition temperature at use temperature. These requirements are similar to those for latex paints, so it is not surprising that some of the same polymers are used in both applications, for example, styrene-butadiene copolymers and poly(vinyl acetate). Nitrile and neoprene mbbers are used for increased polarity. A familiar example of a latex adhesive is white glue, basically a plasticized poly(vinyl acetate) latex. Latex adhesives have displaced solvent-based adhesives in many applications because of their reduced pollution and fire hazards. They are used extensively for bonding pile and backing in carpets. 

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