Reactive Vs Unreactive Hot Melt Adhesive
Hot Melt Adhesive are adhesives which are based on thermoplastic polymers. These polymers are solid at room temperature, soften on heating to give viscous liquids and can therefore be applied as a melt. In contrast to the so-called warm melt adhesives, which have a pasty consistency and are applied at slightly elevated temperatures, typically in the range from 40 to 80° C., the application of the hot melt adhesive is effected at temperatures from 85° C. On cooling to room temperature, hot melt adhesives solidify with simultaneous buildup of the adhesive strength.
Classical hot melt adhesives are unreactive adhesives. On heating, they soften or melt again, with the result that they are not suitable for use at elevated temperature. In addition, classical hot melt adhesives often also tend to creep even at temperatures well below the softening point (cold flow).
These disadvantages were substantially eliminated in the case of the so-called reactive hot melt adhesives by introducing into the polymer structure reactive groups leading to crosslinking. In particular, reactive polyurethane compositions are suitable as hot melt adhesives. They are also referred to as PU-RHM for short. They generally consist of polyurethane polymers which have isocyanate groups and are obtained by reacting suitable polyols with an excess of diisocyanates. After their application, they rapidly build up a high adhesive strength by cooling and acquire their final properties, in particular their heat distortion resistance and resistance to environmental influences, by the postcrosslinking of the polyurethane polymer as a result of reaction of the isocyanate groups with moisture.
Owing to the molar mass distribution resulting during the preparation of the polyurethane polymers having isocyanate groups, however, such PU-RHM generally contain significant amounts of unreacted monomeric diisocyanates which are partly expelled in gaseous form at the application temperatures of 85° C. to 200° C., typically 120° C. to 160° C., which are usual in the case of hot melt adhesives and, in the form of irritant, sensitizing or toxic substances, they constitute a health hazard for the processor. For this reason, various efforts have been made to reduce the content of monomeric diisocyanates in reactive polyurethane compositions in general.
An obvious approach is the physical removal of the monomeric diisocyanate by distillation or extraction. These methods require complicated apparatus and are therefore expensive, in addition, they cannot be readily used for all diisocyanates.
Another approach consists in the use of special diisocyanates having isocyanate groups of different reactivity. However, a disadvantage of this process is the insufficient availability of suitable diisocyanates on an industrial scale, associated with a high price. In addition, it is necessary to make sacrifices in the crosslinking rate since mainly only the isocyanate groups having the lower reactivity are available for the crosslinking reaction.
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