Wednesday 24 September 2008
Robert Ingham talks us through the main types of glue
Urea formaldehyde resinsUrea formaldehyde (UF) resins are essentially of a two-part formula. The bulk is made up of a resin, to which a curing agent is added. This is certainly the case with industrial application. However, the introduction of Cascamite One-Shot back in the 1950s saw the resin and hardener mixed together in powder form. The addition of water released the chemical action of the hardener or curing agent resulting in a stated pot life of about 30 minutes. The pot life, however, was further affected by room temperature. Warmer temperatures reduced the pot life and open assembly time. For most situations the open assembly time is adequate. The plus side of heat-accelerated curing was that UF resins can be thermally set by a variety of methods.
Radio Frequency (RF) heating is used to cure bonds rapidly for joints and direct heating through metal plates is used for veneering. Not only is the glue set by the application of heat but the bond is also cured in a very short period of time. It's ideal for large-scale production.
I stopped using UF resins some time ago when I found that the shelf life was rather short. The shelf life was also affected by the container lid being removed from time to time, in a shared situation, especially if the lid was not replaced quickly, causing the powder to absorb atmospheric moisture. Joint failure and veneers lifting were common problems associated with moisture absorption. I came to the conclusion that UF was an uneconomical and inconvenient adhesive. It had to be mixed and there was almost always some left over, which was wasted.
There is no doubt that UF resins are strong, but I have found that the water held in the wood fibres after laminating takes some time to evaporate. This results in changes in the shape of the laminated form, which can lead to problems. In fact, any adhesive with a water base suffers from this problem. My final criticism of UF resins is the hardness of the cured resin: it dulls cutting edges. Planing the edge of a laminated component, results in the immediate loss of edge sharpness, blunting the blade.
Polyvinyl acetatesPVAs are probably the most popular adhesives. A white suspension of chemicals in a water base, they are available ready-mixed and ready to use. Whereas UF resins were developed mainly in the UK, the development of PVA was accelerated in Germany, mainly for industrial use. The bond is formed partly by the application of pressure and partly by evaporation of the water base.
The resultant glue layer is actually thermoplastic, but it is difficult to reverse the bond in the same way you can with animal glues. The cured glue is quite flexible and in some situations the subsequent movement of timber produces a phenomenon known as 'creep', when the glue can actually be felt as a line on the surface. This flexibility can be a positive advantage when dynamic movement, such as that experienced when sitting in a chair, is essential to the structure.
I have found in practice that the shelf life and pot life, which are essentially the same, make PVA a good, all-round glue. The open assembly time is rather short, so if you use PVA you need to be well organized and apply it quickly. It is slightly pressure-sensitive but there is time to manipulate joints before the bond starts. Clamps can be removed after a couple of hours but, as with all adhesives, the working strength is not achieved until the glue is cured. The cured glue film is opaque and if it is not cleaned off the surface it will eventually show as a pale mark when the wood oxidizes and changes colour.
Aliphatic resinsMy favoured general-purpose workshop glue is an aliphatic resin. For me, it has the primary advantage of convenience, being ready to use, with no need for mixing, no problems with ratios and no waste as a result of trying to judge how much to mix. As the solvent used to keep the glue liquid is water, it is necessary to keep it in a reasonably airtight container to benefit from its quite extensive shelf life. Kept in the containers in which it is supplied the lids ensure a good seal when stored. However, as it is available in different-sized containers I find it most economical to buy one gallon at a time and decant smaller quantities into the dispensers that I use to apply the glue.
The next advantage that attracted me to aliphatic resin is that it requires a short pressure time (although sometimes this can also be a disadvantage). Even after a few minutes the bond is strong enough to prevent repositioning as in the case of a joint that has to slide to a final position. The short pressure time, which is the result of the glue being distinctly pressure sensitive, is an advantage for rapid turnover of assembled components - ideal for batch production. I make batches of small boxes, which would be slowed down in terms of production if I were using a UF resin. With this glue I can clean up a set of dovetail joints quite safely after the box has been glued up for half an hour. Here, it is necessary to make a distinction between the setting time and curing time for the effective handling of assembled components. Although it is possible to remove a joint from clamp pressure after as little as 15 minutes and, with care, work on the assembled components, the final strength of the glue will not be realized until it has cured. In my experience a curing time of 24 hours seems to apply to all the glues generally available.
To reduce the problems experienced with a joint that needs to slide to final position, I try to design it in such a way that its location is positive in two directions, needing movement only in the third direction for assembly. As aliphatic resins are water-based, dispensers, brushes, rollers and any excess squeezed from joints can be easily cleaned off with water.
Another positive advantage is that, although the glue sets hard, providing a rigid bond, the glue line is not hard enough to damage cutting edges when being worked on later. It is also available in a water-resistant version suitable for outdoor work but, it is not suitable for immersion in water or in situations of continuous wetting and is therefore not suitable for boat-building.
Epoxy resinsEven with the potential increased open-assembly time of aliphatic resin, I doubt if it would be long enough for laminating. The complexity of the process, coating several layers that have then to be positioned on a mould before pressure is sequentially applied, either with cramps or a vacuum bag press, demands a considerable open assembly time. In these circumstances I favour an epoxy resin. However, I must add that I do all of my flat veneering with aliphatic resin, speeding up the application of glue with a reservoir-fed roller and thus reducing the open-assembly time. The panels can be taken out of the press after as little as half an hour. I have not as yet researched some of the aliphatic resins currently available in America, which are reported to have long open-assembly times that are suitable for laminating. One further technical advantage makes epoxy resins preferable for laminating: there is no water to dry out after the chemical curing process and as a result little distortion to the shape being formed.
Epoxy resins are two-part systems consisting of a resin and a hardener or curing agent. The shelf life, though quite extensive, is critical and the duration is clearly marked on the container. Mixing is also critical so the manufacturer supplies a metering device, usually in the form of a simple hand-operated pressure pump that is inserted into the container in which the pieces are stored. In theory it should be straightforward to use but I have found that if the pumps are not fully primed before delivering the resin or curing agent, the wrong ratio of mix occurs, resulting in potential failure. Also, for small quantities, it can be difficult to measure the amount that is needed.
When failures were experienced by my students in the past due to lack of cure, my assumption was that insufficient curing agent had been used in the mix. In such cases, my solution was to add more, but I found that it did not improve the result. After contacting the technical help line for the product I discovered that too much curing agent is worse than too little. The ratio is critical, so I took the advice of the scientist I spoke to and now I use a set of digital kitchen scales, which allows me to mix amounts as small as 0.2oz (5g) with accuracy. For even smaller amounts I use hypodermic syringes.
Why am I so critical about quantities? It's a question of cost and waste. The long open-assembly time makes it easier to mix up a little more if you run out rather than mixing too much in the first place and throwing away whatever is left over.
Exothermic reactionsEpoxy resins applied straight from the bottle are too thin for most timbers, meaning that the glue soaks into the wood to such an extent that the actual surface-to-surface bond is starved of resin. This can be overcome by the addition of a colloidal silica filler, which thickens the mix and retards excessive penetration. The technical information supplied with these products stresses the fact that once mixed the resin is subjected to an exothermic reaction, producing intense heat, and dramatically shortening the pot life if held in bulk. However, it isn't until this actually happens and you finish up with a solid block of hardened plastic in a few minutes that the danger and waste of this phenomenon is fully appreciated!
The easiest way to overcome this problem and extend the pot life is to pour the mix into a shallow dish, so the heat produced is dissipated over a larger surface area. I use a small Teflon-coated baking dish, which can be cleaned out by peeling off any remaining glue once it has cured, so that I can use the dish again. Although West System produces a solvent for cleaning uncured resin off surfaces, containers and applicators, it is possible to use methylated spirits for this purpose. Meths is not suitable as a thinner but as I mentioned earlier thinning is not necessary.
Finally, there is the question of applying the resin to surfaces. For small surfaces economy paint brushes are suitable but when you need to cover a large surface, for example when laminating, a thin-walled plastic foam paint roller is ideal. Don't even try to clean them after use - buy them in bulk and throw them away when they have done their job.
Cyanoacrylate adhesivesBetter known generally as 'Superglue', cyanoacrylate adhesives are probably the most versatile adhesive system available today, capable of bonding dissimilar materials quickly and with great strength. They are available in different viscosities, from water thin for rapid penetration to very thick for longer open-assembly times, meaning that there is a viscosity for almost any range of bonding requirements. Bonding times vary from 'instantly' to 90 seconds. A spray-on accelerator is also available to produce an immediate cure. Because of the cost and the speed of bond, Superglue is mainly suitable for bonding small areas and is ideal for repairs.
The main drawback, particularly with the super thin viscosity, is its tendency to migrate beyond the area being glued and stick to other surfaces, usually your fingers! This requires very careful metering of the amount necessary: very often just one drop is enough. I use an old scalpel blade onto which I apply one drop and then carefully offer it up to the place that is being glued. Often it is possible to assemble the components and flush the glue in, relying on capillary action to draw it into the joint, rather like soldering. If pressure needs to be applied, use a piece of polythene sheet between the pressure block and the work piece as any squeeze-out will not adhere to the polythene. I even make my own filler for repairing surface damage with Superglue and fine sanding dust to match the colour. It sets hard in seconds and can be planed down and sanded in a couple of minutes.
The resultant repair hardly shows after a finish has been applied.