Preparation of synthetic resin glues is more exact and the makers' instructions must be closely followed. Two types are used: (1) gap-filling assembly glues for general construction work permitting glue thicknesses of up to V20 in (1.27 mm);(2) veneering glues which have little if any gap-filling properties, and require intimate contact between the surfaces. Some veneering glues (Aerolite KL, etc.) can have gap-filling properties imparted to them with suitable powder hardeners, and are thus dualpurpose glues. Additionally, synthetic resin glues are divided into (a) separate application glues; (b) mixed application glues. With the exception of polyvinyl acetate glues all synthetic resins require an acid accelerator or catalyst hardener, and set by chemical action plus evaporation or absorption of the solvent, thus they cannot be reconstituted. They have no natural tack, and require sufficient clamping pressure to bring both surfaces into intimate contact.
hardener, often known as an 'accelerator' or 'catalyst', is added, which sets the glue into an intensely hard, infusible solid, and at the same time imparts water-resistant properties. Beyond that, and in the nature of all true catalysts, this hardener has no other function than promotion of the reaction, and therefore the simplest method of using resin glues is to coat one surface with the resin and the other with the hardener. In this way there is no mixing to be done. no waste, and the minimum amount of hardener is used, for all solvents used to liquify the glue so that it can be spread on the wood surfaces, and all liquid hardeners added, must disperse either by evaporation or absorption, and if present in excessive quantities will become trapped within the body of the glue during the rapid chemical set, and may eventually set up crazing or fracturing. Single application UF glues with spirit hardener of the type Aerolite 300 (CIBA [ARL] Ltd), and obtainable either in powder form for long storage life or as a ready-for-use syrup (effective life three to four months), are amongst the strongest in this class, and will accommodate glue-line thicknesses of up to V20 in (1.27 mm) in assembly-work without fracturing. This type of glue cannot be used for veneering work as the application of either hardener or syrup to the veneers will immediately curl them; nor must the hardener be premixed with the syrup, as the setting is much too rapid.
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