Woodglue test

This article was originally printed in the March 2008 edition The Woodworker magazine but I have updated it today; February 2013

This isn’t an A-Z of all available glues, mostly it’s the glues I had in stock, and a few more interesting adhesives I bought in. There are so many makes of each type of glue that it wasn’t practical for me to try them all, even so I  spent a whole week mixing and using various glues.

In order to compare their relative qualities I made a number of sample joints. I took the simplest approach possible and made lap joints which I pulled apart to see to what extent the glue line failed. As it didn’t rely on making dozens of perfectly identical joints this was the most consistent approach to testing the glue I could think of. However the trouble with testing wood joints is that wood itself is hardly constant and there was quite a bit of variance with the results. In reality I would need to make hundreds of joints with each glue to produce definitive results, which is obviously not a practical proposition for me on my own. However one thing that has come out very strongly from doing these test joints has been that with all the adhesives good practice is vital to get a good bond.

The test

I made about a dozen joints with each type of glue, using hardwood, softwood and for some of the water resistant glues I made a joint using tanalised timber. Additionally I made some end grain to end grain joints using 2×2 PAR softwood. Straight butted end grain joints are a real challenge for any glue and should be avoided at all costs; this is because the capillaries in the wood draw the glue out of the joint dramatically weakening the bond. However I thought it would be a useful exercise to perhaps more noticeably demonstrate the differences in the glues.

For the test I used PAR (planed all round) softwood which I bought in specifically to try to get some consistency, and oak which I machined up from some flooring I had left over. Before gluing I took a shaving off each face and lightly abraded the surface with 100 grit sandpaper. The actual test was done in early autumn with a workshop temperature of about 20 degrees. The joints were left for two weeks to ensure the glue had completely cured before I pulled them apart. For the end grain joints I applied a good quantity of glue to both ends, which I left to soak in for a few minutes before applying more glue and clamping the joint.

Results

PVA

PVA is the probably the most commonly used wood glue as it is so cheap, easy and pleasant to use. There are various formulations which offer differing setting times but the one I normally use is Evo-Stik Resin W. Formulated for indoor use it sets clear, sands well and does not damage tool edges. It has a relatively short open time and will partially set in 5- 10 minutes, although joints should be clamped for several hours, with it then taking 24 hours for it to achieve its full strength. Evo-Stik also makes a weatherproof version which has very similar properties to standard Resin-W except that it is water resistant.

All PVA adhesives set by loss of water from the glue line. The rate at which moisture is lost and hence the rate at which it sets is determined by the wood moisture content, the spread rate, and ambient conditions of humidity and temperature.

In use with softwood standard Resin W gave fantastic results, the wood broke around the joint but the joint itself stayed intact. With hardwood it was less convincing with typically 50% of the fibres adhered. The end grain joint was surprisingly hard to break but when it did go it broke cleanly through the glue line.

Weatherproof Resin W oddly gave the opposite results, all the hardwood joints remained intact, with the wood snapping before the joint failed, yet with softwood the joints failed through the glue line. The end grain joint performed the same as standard Resin W.

Titebond Original

This is an Aliphatic resin glue, often referred to as yellow glue, the added resins give the glue a stronger initial tack than a standard PVA. It does not dry quite as clear as a PVA with its slightly yellow hue. In use it is very similar to PVA.

I expected this to work better than standard PVA but the results were rather disappointing in that all the joints I made failed through the glue line with typically about 25% of the fibres adhering.

Titebond 2

A water resistant Aliphatic resin glue; technically known as a cross linking PVA, which cures through chemical reaction rather than evaporation. This glue has been approved for indirect food contact so can for instance be used for chopping boards. It dries to a light orangey tan colour.

This glue performed almost identically to weather resistant Resin W with the softwood joints failing through the glue line  whereas the wood around the hardwood joints snapped before the joint failed. The end grain joint snapped relatively easily through the glue line.

Titebond 3

Like Titebond 2 this is another cross linking PVA, however rather than just a slightly different formulation to make it more water resistant this is substantially different to Titebond 1 or 2. It has a thinner consistency and a longer open time. It is marketed as an all-round waterproof glue but it must not be used if the joint is to be continually submerged, nor should it be used for structural applications This glue has been approved for indirect food contact so can for instance be used for chopping boards. It dries to a dark tan colour.

Titebond 3 gave good consistent results with both softwood and hardwood; the wood fibres broke apart either side of the glue line  on every joint I tried. Even the end grain joint was reasonably strong and split the wood rather than breaking cleanly . The joint I made in tanalised timber fared equally well with the wood shattering around the joint.

Polyurethane

I tested two polyurethane glues; the well known Gorilla Glue, and Elch Pro Rapid Set polyurethane adhesive from Henkel. Although the ElchPro is marketed as a rapid set adhesive it has a longer open time (30 minutes) than the Gorilla glue (15 minutes). There are some very fast setting polyurethanes available which set in just a few minutes, for instance, Wudcare Fast Grab, just as there are some that take over an hour, such as Regular Balcotan 100.

Polyurethane is far less pleasant to use than PVA/Aliphatic glues, it is an irritant and harmful by inhalation. It has a solvent odour and marks the skin indelibly so wearing disposable gloves is recommended. The fact it foams out of the joints makes things worse as the glue seems to get everywhere. That said it is relatively easy to clean up from the timber if it is pared away just as it starts to cure.

I have seen polyurethane glue recommended because of its gap filling properties, so to test how well it would perform in a loose fitting joint my first joint was clamped lightly together. When I tested this it snapped incredibly easily through the glue line, leaving a microscopic layer of glue on both faces. It would appear that although it may fill the gap, foamed glue, even when it is a very thin layer, imparts little strength.

For the remainder of the test I applied a strong clamping pressure across the joints and the results with both softwood and hardwood were then very good with both glues, Gorilla glue and Elch Pro performing almost identically throughout. The end grain joints were strong enough to split the wood before the glue failed.

More specialist glues

All the remaining glues need mixing before use which is a distinct disadvantage as it is nearly always necessary to mix more glue than is needed which then goes to waste. Plus if only a small amount of glue is needed it is difficult to mix small quantities accurately.

Cascamite

The name Cascamite implies the glue is casein based (i.e. derived from milk solids) whereas this glue is actually a urea formaldehyde adhesive.

Cascamite comes in powder form and is mixed with water before use. It is important to mix it accurately according to the instructions, as using too much water will reduce the strength of the glue; this is because it allows the glue to be absorbed into the timber drawing it away from the glue line. Although this is a water resistant glue it shouldn’t be permanently immersed in water or the resin will eventually break down.

Cascamite has little odour and is relatively pleasant to use although it may cause dermatitis with prolonged contact so it is advisable to wear gloves. It dries clear and does not creep so is a particularly good choice for veneering.

This glue gave really excellent results with hardwood but the softwood joints broke through the glue line with about 50% of the fibres still adhered in place. The end grain joint was surprisingly strong but when it did break it broke cleanly through the glue line.

Aerolite 306

Originally developed just prior to World War 2, this is the adhesive that was used to make the laminated timber frame for the Mosquito bomber; sixty years later it is still the most common glue approved for use in wood aircraft building in the UK.

Aerolite 306 is another urea formaldehyde adhesive but where this differs from Cascamite is that it uses an acid hardener to cure the glue. The acid catalyst alters the chemistry of the urea formaldehyde, lengthening the molecules and forming a stronger adhesive than urea formaldehyde alone. It is resistant to cracking and will fill gaps of over a millimetre and still form a strong bond. A further advantage with this adhesive is that it does not require the joint to be as firmly clamped as other adhesives to achieve a reliable bond.

The resin mixes easily and has little odour, it can cause skin irritation and like all powers should not be inhaled. The acid catalyst is corrosive and has a very strong odour, gloves, eye protection and good ventilation are a must.

To use Aerolite 306 the powered resin is mixed with water, normally in the ratio 2:1 by weight, although less water can be used if a thicker adhesive is require. The mixed resin will stay usable for at least a day if it remains uncontaminated with hardener. One surface is coated with resin the other with hardener and the joint brought together whilst the hardener is still moist. Cure times are temperature dependent, ranging from 5 minutes at 30 degrees C to 25 minutes at 15 degrees C, clamping times range from 11/4 to 31/2 hours.

This glue gave the most impressive results of all the glues I tried. Every joint was consistently stronger than the surrounding wood even the end grain joint resisted all attempts to break the bond.

Polyproof (Also known as Cascophen and Extraphen)

Originally known as Cascophen it was renamed Extraphen when Humbrol was sold off in 1994. Now it is made by the Polyvine group they have changed its name again, this time to Polyproof. More apt perhaps as this is the most “proof” wood adhesive I know which is readily available in small quantities.

Polyproof is a resorcinol phenol formaldehyde glue which comes in two parts, resin and hardener. When set the glue has no known solvent, it is totally waterproof and fully proof against moulds and fungi. It is also resistant to heat so can be used for fire doors. The major down side of this glue apart from the price, it is the most expensive glue on test, is that it dries to a very dark red-brown colour  which leaves a very dark noticeable glue line. Its main uses are for plywood, laminating structural timbers, hull construction, garden furniture, gates and gluing preservative treated timber; however a limiting factor for exterior work is that the glue should be allowed to set at normal room temperature (15-25 degree C), low temperatures will slow the setting time and should be avoided.

Polyproof has a strong unpleasant odour, the resin gives off a flammable vapour and can cause dermatitis so it is wise to provide adequate ventilation and wear gloves when using it. The powder and resin mix easily together and once mixed the glue remains usable for 3-4 hours. Setting time is temperature dependent but roughly 12 hours; I keep the waste mixed glue in a pot so that I can see when it has completely set and I can safely remove the clamps.

This glue is sometimes used when it is impossible to get a really tight fitting joint, so my first joint was not clamped; I applied plenty of glue and loosely laid one piece of wood on another. This joint was incredibly strong, the wood shattered well away from the glue line. The tightly clamped joints did not fair quite so well with about 80% hardwood and 60% softwood fibres adhering. The end grain joint was very strong, the wood shattered well before the joint failed.

West system epoxy

Although West System Epoxy is primarily aimed at the boating market (it is approved by Lloyds Register of Shipping), this adhesive will find many uses onshore. Neat epoxy is usually too thin to work well as a wood adhesive but the West System comprises various hardeners and fillers which can be combined with the resin to produce a versatile range of woodworking adhesives.

West System epoxy dries clear but adding a filler will change its colour; for instance both colloidal silica and microfibre fillers are off-white, whilst the low density filler is dark brown. One advantage of using epoxy is that it can be coloured with a pigment if required.

To use the adhesive, the resin is mixed with one of the available hardeners (standard, slow, extra slow and clear) This makes a very thin liquid which is brushed onto both surfaces and allowed to soak in. A filler is then added to the remaining mixed adhesive which thickens the mix; the makers recommend a consistency somewhere between mayonnaise and peanut butter for general bonding. This is then applied to one surface whilst the priming coat is still tacky and the joint should be clamped up with enough pressure to create squeeze-out but without forcing all the thickened adhesive out of the joint. Cure time is variable and will depend on the temperature and the type of hardener used.

Unthickened epoxy is useful for filling cracks as it is very searching and will fill the entire crack.

Epoxy has a very strong unpleasant odour; it also gives off a flammable vapour plus the hardener in particular can cause dermatitis, so ventilation and gloves are a necessity.

As I assume that joints will sometimes be made without using a filler so I made one set of joints with filler and the other without. With hardwood (oak) I got excellent results with both filled and unfilled epoxy. The tanalised timber joints also gave identical results, however adding a filler made the softwood joints appreciably stronger as I suspect the porosity of the softwood has drawn some of the glue out of the joint.

However it was the end grain joints which demonstrated this effect the most as I was able to break the unfilled joint by hand; it just snapped cleanly through the glue line. Conversely the end grain joint made with filled (with colloidal silica) epoxy was one of the strongest on test.

Despite getting similar results with the hardwood joints, I would always recommend using a filler when using epoxy as a wood glue as it removes the risk of a glue starved joint and increases the gap filling properties of the adhesive.

Finally of all a couple of specialist glues which you may well find of use.

Titebond liquid hide glue is particularly useful for making a traditional repair to antique furniture without all the hassle associated with hide glue. Normally using hide glue involves the use of a heated glue pot, but this adhesive has been formulated for use straight out of the bottle. It has a quite specific shelf life (about 12 months) after which the glue will not set. As there is no noticeable change to the appearance, this is one of the few adhesives with a use by date on the container.

Pros, very easy to use for a hide glue, traditional, application of gentle heat will melt the glue

Cons; for specialist use, short shelf life.

Titebond moulding and trim wood glue is a very thick “sticky” adhesive formulated for a very fast initial tack and is designed particularly for gluing mouldings in place. As it is so thick it doesn’t run and will fill small gaps

Pros; fast drying, strong initial tack, water clean up, non toxic

Cons; Too thick for everyday use

Test results

This is my score table. I scored the glues on the optically perceived percentage of wood fibres still adhered once the joint was broken. (This means I looked at it and made my best guess)

Adhesive Softwood Hardwood Tanalised
Resin W s/w 100 H/w 50 –
Resin W weatherproof s/w 30 h/w100 –
Titebond 1 s/w 30 h/w 35 –
Titebond 2 s/w30 h/w100 –
Titebond 3 s/w70 h/w70 tan 100
Gorilla h/w70 s/w80
Elch Pro rapid set poly s/w60 h/w90
Extramite s/w50 h/w90 tan 40
Extraphen s/w 60 h/w 80 tanalised 40
Aerolite s/w 95 h/w 100
West system epoxy s/w 50 h/w 90 tanalised 70
West epoxy plus filler s/w 60 h/w 90 tanalised 70

s/w= softwood  h/w= hardwood   tan= tanalised

The higher the score the better the glue bond