The Technique of FURNITURE MAKING

Synthetic resin glues

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.

Separate application resin glues

All plastic (synthetic resin) glues are formed by the reaction at elevated temperatures of the components (phenol or urea, etc. with formal­dehyde, for PF or UF glues). This reaction or polymerization provides resins for moulded and extruded plastic-ware, etc., but if carefully controlled and arrested, adhesives are com­pounded which can be supplied either as fine­grained powders or water-based thick syrups. However, the reaction cannot be halted com­pletely, and therefore all resin glues have a limited working life only, which should always be stated on the container. To promote the ultra-rapid setting of these glues a suitable 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.

Mixed application glues

With these (Aerolite KL, etc.) both glue and liquid or powder hardener are carefully weighed or measured in the correct propor­tions, and mixed together before application. They are thus ideal veneering glues whose setting time can be greatly accelerated by the application of heat. Glues are also available in which a suitable powder hardener is incorporated with the powder glue (Cascamite One Shot, etc.), and mixing with water releases the acidifying action of the hardener and initiates the set. These glues are principally gap - filling assembly glues, but are also quite satisfactory for veneered work, although they are not so responsive to heat as straight
veneering glues. They require no elaborate weighing or measuring out, are easy to mix, and are sufficiently strong and moisture resistant for most purposes. The storage life in closed containers is the same as for most other powder resin glues, i. e. one year or more.

(not impact glues) are thermosetting, and once set cannot be resoftened by heat. The accelera­tion of the set by applied heat is very marked, as the following tables show:

Miscellaneous synthetic glues

Epoxy resin glues and hardeners (Araldite, etc.) are in the form of stiff creams which are intimately mixed together prior to use; thus there is very little initial solvent present, and no volatile matter is evolved during the reaction. These glues have wide applications in bonding notoriously difficult surfaces, metal, glass, china, plastics, wood, rubber, etc., and with a suitable filler can be used as a waterproof mastic for bonding plastic working-tops to steel and porcelain sinks, etc. Polyvinyl acetate emulsion glues are white creams which form transparent glue-lines and set by evaporation of the water content. They are ready-mixed general-purpose assembly glues, not water-resistant but with considerable tensile strength and durability, and a curious affinity for brick, cement, glazed tiles, paint, etc. They have a tendency to creep from open joints in humid conditions, and are therefore not suitable for tropical conditions. Cellulose (vegetable) glues with added syn­thetic resins (quick-setting cements, balsa cement, etc.) have practical uses in the workshop, but not for general assembly-work.

So-called impact glues are synthetic or natural rubber latex glues in suitable solvents, and are widely used for bonding decorative, laminated plastics on site where cramping/ clamping or pressing facilities are not available; the natural resilience of the rubber gives the necessary amount of flexibility between the inert plastic and the creep or movement of the wood. They are sometimes used for assembly - work and bonding small strips or inlays of veneers, but this is not their rightful function, and must be regarded as an expedient only, not suitable for permanent work.