Classification of Adhesives:
There are two main classes of adhesives: thermoplastic and thermosetting.
Thermoplastic sets either my loss of solvent or by cooling. It will soften again by applying the solvent to the glue line or by re-heating.
Thermosetting sets and solidifies through a chemical reaction and is irreversible.
Adhesives that set at room temperature are known as 'cold-setting' and those that require heating up to a temperature perhaps in the case of pearl glue 40 and others up to 100 degrees C are known as 'hot-setting'.
Types of Adhesives:
Animal. Known as scotch glue (yes glue!), pearl or hide glue. Made from the hides, hoofs and bones of animals. Available now in cake or more likely supplied in small beads. It is essential for restoration work, veneering and other applications where compatibility with original artefacts is needed. Used extensively in the musical instrument industries. It is for internal use only and has high gap filling properties.
Fish glue is also available which is made out of fish offal and skins. Good for small repairs. Not recommended for structural work.
Casein. Manufactured from soured, skimmed milk curbs which are dried and crushed into a powder form. General joinery and woodwork use. It may stain some hardwoods and oak is particularly prone to darken. To use the powder it is mixed with cold water into a smooth creamy paste. Developed in WW2 for the manufacture of the Mosquito plane which was largely made of fabricated plywood panels. Internal (INT) use only.
Modern synthetic resins.
• Phenol formaldehyde (PH) ~ used where great strength is required, durable and water resistant. An 'engineering adhesive' used for heavy structural work and quality plywood. Durability is WBP.
• Resorcinol formaldehyde (RF) ~ outstanding durability qualities under the severest weather conditions and used mainly for external construction work. Decorative work is not possible because it has a dark glue line. Durability is WBP.
• Urea formaldehyde (UF) ~ Perhaps more familiar to the cabinet and woodworker under the trade name 'Humbrol Cascamite' now apparently called 'Extramite'. Used when a high quality but stable fix is required. Sets at normal room temperatures and is relatively cheap and available. Suitable for laminating, fabricating and veneering when large presses or good cramping facilities are available. It is pre-mixed to a smooth paste with cold water in a non-metallic container. Those little plastic water cups are fine. Durability is BR.
• Melamine formaldehyde (MF) ~ A more expensive adhesive usually used as an additive. Used in the production of hard decorative plastic laminates. (MR)
• Polyvinyl acetate (PVA). (White Glue) A one part emulsion with high bond strength. It can be used on all woods, soft, hard, chipboards, plywood, etc. A good general purpose woodwork adhesive for indoor use only. There is an external quality made but I would not rate it greater than MR. - Evo Stick Resin W ~ INT
• Aliphatic resin glues (Yellow Glue) - Possible has an upper hand on PVA especially for the brown hardwoods. It provides a strong initial tack with fast setting to help reduce the clamping time. Offers excellent 'sand ability' and is unaffected by finishes. Titebond - INT
• Polyurethane Glue - A waterproof adhesive for multi-purpose applications. Ideal for metal, plastic, ceramics and other porous or non-porous materials. Although it foams up it does not expand or contract in the glue line. MR
Contact. ( natural and synthetic rubber) A rubber based adhesive supplied ready for use. Used for bonding plastic laminates, sheet flooring and other fabrics, etc. to wood or other materials. It is applied to both surfaces with a toothed spatula and allowed to dry to the touch. The two surfaces are then carefully brought together under pressure to remove any air bubbles. Immediate contact is obtained so accuracy is essential. In practice the laminate or fabric is 'rolled' on from the far edge with both surfaces held against a straight edge to provide a guide. There are some contact adhesives available that provide limited amount of adjustment. It should be used in a well ventilated workshop with no smoking or naked lights because the vapours are flammable and toxic. Internal use only. Evo Stick 528 ~ INT
Hot Melts - ethylene vinyl acetate (EVA) was the original polymer ~ Heated in a gun or applicator with electronically controlled elements. The flow is controlled by a trigger. Not recommended for jointing because of the thick glue line that quickly goes off. Ideal for 'tacking' jobs such as upholstery, and packaging etc. specially smaller components. Some hot melt glue guns are powered by a gas cartridge that gives a hotter and thinner glue line. A warm atmosphere with warmed components increase the working time and a thinner glue line. Care must be taken to avoid the hot glue on hands and fingers. A very bad burn results - the author has first hand experience and scars to show. - INT
Epoxy Resins ~ Two part epoxy resins when mixed together produces after curing an incredible bond on many different types of materials. The adhesive is mixed with equal proportions on a tile or card until it becomes a whitish stiff paste. A rapid version is obtainable that may cure after a 90 seconds bond. Applying heat, hot air or 'cooking' in an oven accelerates the process.
Cyanoacrylate Adhesives ~ or 'Super Glue' ~ used extensively throughout for repairing and manufacturing small objects. Not used in great quantities in the woodworking industry but very useful for repairing and 'tacking' small objects when strength and speed is required. Excellent for model making particularly the 'Zap' varieties. Requires a moisture to work.
How Adhesives work
• By loss of solvent ~ either evaporation of the solvent as in the case of contact adhesives or by the absorption into the timber in the case of emulsion (PVA) adhesives.
• By cooling. Adhesives applied in a molten condition solidify on cooling. This has the advantage of a very fast set. Animal and hot melt adhesives are good examples.
• By chemical reaction such as two part adhesives with a resin and a hardener or catalyst. Mixing the two together activates the adhesive. Synthetic resins, epoxy resins, Cascamite, etc are good examples. A good clean well measured mix is essential.
• Some adhesives set by a combination of one or more of these processes.
Durability grades.
WBP ~ Weather and Boil Proof
BR ~ Boil Resistance
MR ~ Moisture Resistance
INT ~ Internal only
Storage or Shelf life
Time able to be stored in original containers.
Pot life
Effective working time after mixed - depends on temperature.
Pressing or Cramping time
Minimum time glued surfaces should remain under pressure. or Curing or Assembly (closed) time
After application and contact with wood - usually in cramps - time depends on temperature. Heat including RF accelerates the time. Refer to manufacturers instructions.
Applying adhesives
The joint area must be clean, free from resin, oils, etc. It should be dry. The joint should not be roughed or sand papered but prepared from the saw, chisel, plane etc. so that it fits well square and free from wind.
Adhesive should be applied to both surfaces with a brush, spatula or applicator taking care to 'wet' all surfaces of the joint area. Sufficient adhesive should be applied but not over spilled. Adhesive that 'bleeds' through the joint must be removed immediately and thoroughly with a wet cloth otherwise the adhesive will effect the staining and finishing coats. Some allow the overspill to 'gel' then remove it carefully with an old but sharp chisel. This practice may not be acceptable for quality finishing. The remedy is not to use to much adhesive in the first place.
Glue line. The actual glue layer of the joint is called the 'glue line' and although it is essential to apply sufficient adhesive on both surfaces, care should be taken to keep this thin as possible ~ thicker glue lines weaken the joint and are unsightly.
Temperature. Check the temperature range of the adhesive you are using and make sure the workshop is warm or cool enough for the process. Adhesives are responsive to temperature and the curing time may well be reduced by applying heat of some form.
Radio frequency (RF) acceleration is very effective but requires specialist knowledge and equipment. It is also potentially dangerous.
Gap filling properties gap filling capacities - gaps = 1.3 mm
Adhesive safety:
1. always follow manufactures instructions
2. use a barrier cream or disposable gloves
3. do not use flammable types near naked lights
4. always provide adequate ventilation
5. avoid inhaling toxic fumes or powders
6. thoroughly wash hands, before eating, smoking and after work with soap and water and appropriate hand cleaner.