Adhesives

Historical context

Ancient records show that the use of glue was known to the Egyptians as long ago as 3000 BC. In the tomb of the Pharaoh Rekhmara, wall paintings and carvings were discovered illustrating the manufacture of an elaborate veneered cabinet, the whole process faithfully depicted, including the heating of the glue pot on the fire, the spreading of the glue and the laying of the veneer. Centuries later the rubber tree was tapped for its sap, yielding natural rubber, which is still used as the base for contact adhesives.

Casein glue, made from cow's milk, was the most water-resistant adhesive available until the advent of waterproof resins during World War II. The famous moulded wooden framed and plywood covered 'Mosquito' fighter-bombers were initially assembled with casein adhesives until it was found that in the tropics these glues were weakened by fungal attack. This problem stimulated research into synthetic rubber and resin adhesives, which led to the development of glues for almost every purpose. These glues are often stronger and more durable than the materials they join!

Why does glue stick?

The fact that two porous surfaces could be bonded together by glue was once attributed solely to the keying of the adhesive round the fibres and into the crevices of the materials. However, scientists, particularly in the field of polymer chemistry, now claim that this could only account for a small fraction of the joint strength, and that molecular attraction is primarily responsible. Even though there is a very wide range of woodworking glues now on the market, not all of these glues are suitable for the small workshop. Some, like the film-type formaldehydes, require carefully controlled temperatures or special apparatus. Some are available only in large quantities. Some have too limited a shelf-life; that is, they do not keep long enough without deteriorating to be of value to the small user.

It is advantageous to know the characteristics of the various adhesives generally available.

PVA (polyvinyl acetate)

This is an excellent woodworking glue. It is a white, ready-to-use emulsion with a creamy consistency produced by reacting acetylene with acetic acid, which dries clear. Though not water­ resistant, it does resist fungi and bacteria, has excellent gap-filling properties, is non-staining, can be cleaned up easily with a damp cloth, is non-flammable and cheap to purchase. It is a good general-purpose adhesive. However, it will not bond a non-porous surface to any other surface and, therefore, is not suitable for joining metal to timber. PVA is a cold-setting glue, but drying can be accelerated by the application of heat, which should not exceed 65 °C or the glue will melt. A waterproof version is available. Even when dry, it is slightly flexible and will 'creep' under tension.

Epoxy resin

These are waterproof and very strong. They will bond wood to wood, metal to metal, rubber to metal, glass to glass, glass to metal and plastics to plastics (e.g. fibreglass). They will not, however, join thermoplastics such as perspex, PVC or polyethylene. They are set by a chemical reaction when two parts (a glue and a hardener) are mixed together. Some glues require the adhesive to be put on one joining surface and the hardener on the other. There are epoxies that come in a powder form with the hardener already included-simply add water prior to using. Depending on the formulation, the glue remains workable for between five minutes and four to five hours and dries clear.

Hot melt

These are generally supplied in stick form and are used in conjunction with a glue gun that heats the glue and applies it to the job. These multipurpose adhesives are suitable for mounting dry timber on a wood lathe for face-plate work and bonding a variety of materials (for example, particle board, many plastics, paper products and fabrics). In a school environment hot-melt adhesives can only be used in small areas because the gun can only deliver small amounts of molten glue at a time. Industrial applications can handle much larger areas. Hot glues eliminate the need for clamping or pressing, and the piece that has been glued is ready to use within minutes-after the glue has cooled down.

Urea-formaldehyde

This is probably the most commonly used adhesive in industrial situations, and it comes in both powder and liquid forms. In the powder type, a hardener is available if wood flour or walnut shell flour is added and this improves its gap-filling properties. Where exceptionally long open working times are necessary, resin may be applied to one part and hardener to the other so that setting does not take place until the parts are brought into contact with each other. There are concerns over the emission of gases from board products that utilise this glue.

Other formaldehyde glues

Phenol formaldehyde

This is a waterproof glue that is used primarily in the industrial manufacture of plywood intended for use in extremely moist conditions. Available as a liquid or a powder, this adhesive is more commonly used in the form of a film, which is made by impregnating a porous paper and allowing it to dry. This film is placed between the layers of ply before applying a high-pressure hot pressing drying process.

Melamine formaldehyde

This waterproof, heat-resistant, non-staining adhesive is prepared for use by mixing the powdered resin with water then adding the liquid hardener to form a thick, smooth glue. As with urea formaldehyde, separate application is also possible, making melamine ideally suited to caravan construction and boat building.

Resorcinol

This is supplied as a dark-coloured liquid with, usually, a powder hardener. It is generally accepted as the best adhesive for the construction of boats and aircraft (and wherever an extremely durable glue is required). Resorcinol formaldehyde has outstanding resistance to cold or boiling water, heat, solvents, mould and fungi. Its only disadvantage is that its dark colour stains timber.

Contact

Although not generally used to bond wood to wood, this is very good for edge veneering and well-suited to securing plastic laminates to timber. Adhesive is applied to both surfaces (smooth surfaces should be roughened first) and allowed to dry for 10 to 20 minutes before the pieces are brought into contact, when the bond is immediate. Strength increases as the glue cures, which can take several days. Clamping is unnecessary, but care must be taken to position the panels exactly and press them from the centre outwards to avoid trapping air.

Pot life: The maximum amount of time you have to apply two-component glues to your work after they have been mixed.

Open time: The maximum amount of time glued pieces can be left open to the air before assembly.

Clamp time: The minimum amount of time required before you can remove the clamps from a glue-up assembly. (Note: PVAs and hide glue offer only 30-50% of full strength at stated time. If a joint is under stress, maintain clamping pressure for the full cure time.)

Cure time: The time it takes for a glue joint to achieve full 100% bonding strength.

Shelf life: The period of time that glue remains useable. (Note: Excessive heat, humidity, or repeated freeze/thaw cycles will cause glues to spoil prematurely. Under ideal conditions, adhesives remain useable for longer time periods. If a glue appears abnormal, test first, or throw it away.)