Gluing Wood with Epoxy Resins
Everybody in Ireland knows about epoxy glue, but did you know that epoxy glue is great for gluing wood? There are various glues suitable for gluing wood. PVA wood glue, polyurethane wood glue and epoxy wood glue, are the three glues of choice. The decision to use one type depends on various factors, chief if which is the final use for the bonded parts. In this blog, we will talk about the use of two component epoxy glues. With our supply partner’s, we have a range of epoxy glues, each with clearly defined performane characteristics, but all sharing three pack sizes, 50 ml, 200 ml and 400 ml cartridges.
These epoxy glues are extensively used in a whole host of applications.
First of all, in mixing two-component epoxy glues, it is important that the two components be thoroughly mixed, if not, the bonded parts will be physically weak when the glue has cured. One of the most dependable methods of ensuring complete mixing of the epoxy components, is to use the static mixers sold with each pack size. Don’t try to save a few cents by neglecting to purchase static mixers. For two component glues to work correctly, they must be mixed in a static mixer.
Before gluing the wood, it is important to remember that surface preparation is at least 50% of adhesive bonding technology, so follow the suggestions carefully.
First of all, ensure that the wood is clean and dry, and has a water contant of below 13%. Several types of wood may present gluing problems. As an example, oily woods such as teak may need to be solvent wiped before glue is applied. We always suggest the use of fast evaporating solvents as slow evaporating solvents are absorbed by the wood and will cause the epoxy bond to fail. Even solvent cleaning hardwoods after gluing (while the glue is still wet) has in some cases, caused glue-line failures. Some woods, in particularly ebony, contain a wax rather than oils. Saw cutting or dry sanding can smear this wax over the surface, making gluing difficult, especially on end grain or 45 degree bevels. Wet sanding with P180 or even P120, can clean such material off the surface to be glued and has been found effective in improving the bond strength of such joints.
For the best results the epoxy adhesive should be applied to both of the surfaces to be glued. Reason being is that if the epoxy is placed on one side only, it may be absorbed into both surfaces too quickly, thus leading to a glue starved joint. It is best to allow the glue to sit long enough on the wood surface, enabling the surface fibres to soak up some adhesive. In doing this, after the pieces have been assembled, the wood will not absorb the glue that would otherwise fill the gap between the pieces. Both scarf and butt joints in particular, are especially prone to soaking glue out of the joint, wicking into the end grain. Edges of plywood are notorious for soaking up liquids. Care is needed.
As in all wood bonding projects, it is important that the joints being glued, are well fitting. Badly manufactured timber joints, which have been clamped to bend them into contact, will have tremendous spring-back forces pulling in opposite directions. Wood joints which are badly fitting, will tear away from their parallel neighbors, often within hours to days after the clamps are removed.
Just because the wood joints are flat, for example in flat lamination, does not mean that less caution is needed. Straight, smooth, well-fitting wood elements can be made to fail by using excessive clamping force. Squeezing a glue joint down to zero glue-line thickness forces out almost all the glue from between the pieces, and the natural porosity of wood wicks away the remaining microscopic residue of glue. The result is a joint which is starved of glue. This type of joint will often fail when the press is open or the clamps are removed. If the failed joint shows no divots of wood pulled out of the opposite side, the cause of the failure is almost certainly excessive clamping pressure and not enough time allowed before clamping. The wood element should be clamped gently. Just enough to squeeze out the excess glue and bring the wood pieces into contact at the microscopic high points of the joint. Thick pads of soft rubber under the clamp faces ensure gentle, even clamping forces
Curved beams are best made by steam-bending the individual laminations, letting them dry in a fixture that will set the new shape, permanently.
Steam-bent wood holds its new shape without any stress. Steam-bent ribs for boats have been used for at least hundreds and perhaps thousands of years. If steam bending is not an option, cross-grain fasteners or splines (tenons, biscuits) should be screwed or glued at each end, because the curved structure will want to straighten, and the glue joint will fail by cleavage.
In all wood gluing jobs, it is important to remember that wood is a natural product and varies tremendously.
Please contact our laboratories for specific advice.