Gluing of Wood and Wooden Materials

Today for the assembly gluing of wood, for gluing veneers, plastic sheets, and films, and for the manufacture of wood-based materials (chipboard, plywood, hardboard, profiles), synthetic adhesives are used almost exclusively. The traditional use of adhe­sives based on natural products (glutin and casein glues) is confined to a few special cases only (for example, violin making).

Poly(vinyl acetate) emulsion "white glues" are used widely for the gluing of wood and wood-based materials. They generally contain small quantities of solvents and/or plasticizers to adjust the film-forming temperature and chalk as a pH buffer and filler. They have a solids content of approximately 50 — 65 % and a film-forming temperature of 0 to 15 °С. Special components may be added to increase the setting rate or to
prolong the open time. The application weight is 100-200g/m2, the open time 5-25 min, and the clamping time 5-45 min for a compressive force of 5 — 20 N/cm2. The exact figures strongly depend on the glue coating weight and the type of substrate.

Bonds with higher water resistance can be achieved by using special blends or two-component emulsion glues. The second component may be a diisocyanate or particularly an inorganic salt (usually, aluminum nitrate). These hardeners, added in a quantity of at least 5 %, improve the moisture resistance of the joint. However, their addition limits the pot life.

Normal glues develop EN 204/205 Class D2 strengths which are adequate for all indoor applications, for example, for the assembly gluing of furniture and for lami­nating plastic sheets. Self-cross-linking one — or two-component poly(vinyl acetate) glues, which develop Class D3 and D4 strengths according to the same European standard, may be used for gluing in moist atmospheres (bathrooms) and for exterior assembly gluing (windows, doors).

Furniture Foil Adhesives. These adhesives consist primarily of ethylene-vinyl acetate copolymer emulsions. Rigid PVC and melamine-paper foils, generally 0.2 — 0.4 mm thick, are applied to wood-based materials for decorative purposes. The adhesives must exhibit high tack and high resistance to heat and cold and must counteract the shrinkage of the film (application weight 60 -100 g/m2).

Hot-melt adhesives are mainly used for automatic edge-banding applications to bond edge materials to particle board, as well as for profile wrapping with foils, paper, and veneer.

Increasingly, hot-melt adhesives also are being used for the postforming process. Immediately after surface lamination, the overhanging laminate is bonded to the contoured panel edge without an additional adhesive joint.

Hot-melt adhesives are applied on edge-banding machines, profile-wrapping ma­chines, and postforming machines. The adhesive is applied to the substrates with special rollers or slot nozzles. The joint between the cover and the core material is achieved by applying pressure with a roller system while the adhesive sets.

In assembly processes, such as furniture or door manufacture, the hot-melt adhesive is applied in spots or lines by hand or with automatic guns. Often the hot-melt adhesive is exclusively used as an additional assembly support due to its high initial strength, while poly(vinyl acetate) glue is used for achieving the final strength (two-shot process). Hot-melt adhesives for the edge-banding and postforming process are highly viscous products, often filler-free or with a filler content of up to 50 %. For profile wrapping, low-viscosity hot-melt adhesives are used, sometimes with fillers. For assembly bonding, a wide range of assembly hot-melt adhesives varying in viscosity, setting time, and open time is available for different equipment and uses.

As adhesives based on ethylene — vinyl acetate copolymers cover a very wide range of applications, they are used in large quantities. In the case of higher demands on the bond, polyamide — or polyolefin-based hot-melt adhesives are recommended. Highest
bonding quality regarding heat and cold resistance, bonding strength, and waterproof­ness are achieved by using polyurethane-based, moisture-reactive hot-melt adhesives.

EVA hot-melt adhesives have heat resistance up to 85 °С, polyolefin and polyamide hot-melt adhesives up to 130 °С, and reactive polyurethane hot-melt adhesives up to 150 °С. The cold resistance lies between — 20 and — 30 °С; in the case of polyurethane — based hot-melt adhesives, it is even lower than — 30 °С.

Urea formaldehyde (UF) resin glues are the most widely used adhesives in wood­working and are preferably applied by the hot-glue technique. They are marketed as precondensates in the form of 50-66% solutions or as dry powders that must be dissolved in water. Curing aids can be added. In veneering, contact times as short as 30 s are possible for press temperatures of 100 -120 °С. The most common additive is ammonium chloride. The amount of hardener governs both setting time and pot life. Glue joints in thick boards can be set by high-frequency heating.

For gluing plywood, the UF resins are generally mixed with extenders such as rye flour, followed by the addition of curing aids, the type and amount of which governs the setting characteristics at the processing temperature and pressure. The glues thus prepared are applied to the dry veneers (moisture content 6 -12 %) by a glue spreader (application weight 120 — 200 g/m2), and the glue-coated veneers are assembled and then glued in multilayer hot presses or high-speed single-layer presses at 100 -160 °С and 60 — 200 N/cm2, depending on the type of wood. For particle board production, unextended UF resins are used preferably. The particles are coated with glue by spraying, the particles of the center layer being coated with less resin than the particles of the two surface layers. Average glue resin content is ca. 10% (i. e., 6-8% for the center layer and 8-12% for the surface layers). The glue-coated particles are first spread to form a three-layer cake, precompressed, and then compressed in hot presses to form particle board.

UF glues ideally satisfy the requirements for interior use but are unsuitable for exterior application. The requirement for low formaldehyde content must be consid­ered by all producers of particle board. As plywood and veneer glues, with regard to water resistance, UF resins satisfy the requirements of DIN 68705 and EN 314, category IF, and as particle-board binders they meet the requirements of DIN 68763, category V 20.

The glues are cheap, easy to apply, and therefore suitable for the mass production of wood-based materials.

Melamine resin glues are important in the woodworking field when weatherproof glue joints according to DIN 68705, category AW at relatively low press temperatures (under 100 °С) are required. With added extenders moisture-proof glue joints can be obtained that meet the requirements of the same DIN category IF, at press temper­atures above 75 °С.

Melamine-Formaldehyde (MF) Resins. The largest field of application for mela­mine formaldehyde (MF) resins is the manufacture of decorative laminates and the surface finishing of chipboard, hardboard, and profiles. Special papers with weights of 18-150g/m2 are impregnated with MF resins, dried, and then pressed onto the substrates at 130-180 °С. Their colorless transparency, hardness, chemical resistance, and absence of yellowing have made these resins particularly suitable for surface finishing. For technical applications including, e. g., concrete form-work boards, they are also applied by the wet-resin process, in which the resin solution is applied to the surface and cured under heat and pressure.

Phenolic resin glues, for cost reasons generally derived from phenol cresol mixtures, are used for the hot gluing of plywood, the manufacture of chipboard, and as binders for fiberboard. They are marketed in liquid form. Their shelf life is limited to about six months. They set at 130-180 °С, generally with addition of hardeners. For setting at room temperature, the addition of a special hardener (usually p-toluenesulfonic acid) is essential. The use of phenolic resins requires a relatively low moisture content of the wood (6-10%).

Joints based on phenolic resin glues are of high quality and weatherproof. They are recommended for gluing wood to metal (metal-clad wood).

Resorcinol resin glues set without the assistance of heat under mildly alkaline conditions. Therefore, no damage is caused to the wood by acid hardeners, and there are no restrictions on the moisture content of the wood up to 18 %. The resins can be cured with paraformaldehyde. Wood-to-wood glue joints with resorcinol resins are among the strongest and are weatherproof and resistant to solvents, oils, and fats. For reasons of economy, mixed condensates of resorcinol and phenol are used instead of pure resorcinol.

Contact Adhesives. In countries with a high standard of mechanization, contact adhesives based on polychloroprene rubber for woodwork are used only for on-site repairs and for special purposes, e. g., for gluing plastic sheets or veneers onto rounded substrates. Here, contact adhesives are widely replaced by poly(vinyl acetate)-based wood glues. However, in less developed countries, contact adhesives are still widely used for veneering and laminating plastic sheets to wooden substrates. Their advantage over all other wood glues is the high bonding strength achieved virtually immediately after joining under short-term pressure (several seconds) without the need for sophisticated equipment.

Polyurethane Adhesives. The search for formaldehyde-free binders resulted in a new generation of polyurethane-based glues for chipboard production. The usage of polyurethane based adhesives for assembly of wooden materials has also increased. These liquid polyurethane adhesives are single-component adhesives that harden by a chemical cross-linking reaction which is initiated by moisture in the material. The water
resistance of wooden joint fulfills the requirements of EN 204, D4. Furthermore, the adhesive joint shows high resistance to a variety of organic solvents, weak acids, and alkalis.

Overfall Quality Standards for Wood Glues. There are several national and European standards for the adhesives used in woodworking, of which the following are the most important: EN 204, EN 205, DIN 53255, DIN 68705, and DIN 68761-765.

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