Adhesives chemically reactive systems

The adhesive may be solvent or water-based, hot-melt, coldseal or heatseal and pressure sensitive or chemically reactive. So the solidification process may occur via drying of water or solvent-based adhesives, by cooling of hot-melt and heat-seal adhesives, or by curing of chemically-reactive systems. With two notable exceptions - self-adhesive labels used on items of fruit or vegetables, and heat-sealable layers on packaging films - adhesives are in general not intended to touch the packaged food directly. 

Epoxy resin Adhesives. Epoxies [47], [48] are among the most widely used structural adhesives. These chemically reactive systems include two-component systems that cure at room temperature or elevated temperature and one-component systems, which usually require heat for curing. 

Phenolics or phenol-formaldehyde structural adhesives are chemically reactive systems that cure to form thermosets. In one-component systems, meltable powders (resols) are used as binders for particle board or as alloys (including nitrile-phenolics, vinyl - phenolics, and epoxy-phenolics), which are used in the structural bonding of metals. In two-component systems, the resin and catalyst are mixed and then heated to initiate curing. Both systems cure by a condensation reaction that produces a byproduct. 

Adhesives and sealants are manufactured from a variety of polymers. Their selection and their combinations used impact solvent selection. Most solvent systems are designed to optimize the solubility of the primary polymer. Adhesives can be divided into ones which bond by chemical reaction and ones which bond due to physical processes. Chemically reactive adhesives are further divided into three more categories for those that bond through polymerization, polyaddition, or polycondensation. Physically bonding adhesives include pressure sensitive and contact adhesives, melt, or solution adhesives, and plastisols. Polymerization adhesives are composed of cyanoacrylates (no solvents), anaerobic adhesives (do not contain solvents but require primers for plastics and some metals which are solutions of copper naphthenate), UV-curable adhesives (solvent-free compositions of polyurethanes and epoxy), rubber modified adhesives (variety solvents discussed below). 

Thermoplastic linear polyurethanes which are usually chain-terminated so that no unreacted free NCO groups remain available. Environmental considerations direct growing attention to these newer non-polluting urethane adhesive forms, e.g. powders, films, aqueous dispersions and 100% solids reactive systems. Some systems do possess blocked diisocyanates which are activated on heating to produce chemically reactive solid systems. 

By loss of liquid, an adhesive applied as a true solution or a dispersion of solids will dry through loss of water or another solvent, leaving behind a film of adhesive. A reactive adhesive system will form internal chemical bonds through... 

Chemically reactive polyurethanes include both one- and two-component systems. One-component systems are usually based on a polyether polyol treated with a polyisocyanate to give an isocyanate-terminated polymer. A one-component system cures when exposed to moisture at room temperature. One-component polyurethane hot-melt adhesives are also cured by moisture after application. Two-component systems result from the reaction of low molecular mass polyols and isocyanates or from isocyanate-terminated prepolymers with either polyols or polyamines. Two-component systems cure at room and/or elevated temperatures. 

In order to achieve this state, many of these adhesives are reactive, i.e. multi-component systems which, after application to the adherend, change their physical format from liquidus to infusible solid by chemical reaction. These polymerisation and cross-linking reactions, often thermally induced, are achieved using one or more of the following reaction mechanisms ... 

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