The subject of adhesives is grossly misunderstood. initially I shall try to clarify the uses and methods relating to the commonest woodworking materials.
The most generally used material these days seems to be the white glue known as PVA . (Polyvinyl Acetate) While this is a very useful material it has some limitations.
The material consists of an emulsion of the plastic, plus in some cases, various fillers . The glue dries to a more or less hard film and can be tailored to suit its end use to some degree. Some dry clear but are usually rubbery.. If you power sand them they clog up the paper very quickly. There are some, formulated for the production industry, which dry very hard and don’t clog up the abrasive paper. The main consideration for the small user is that the solid content should be fairly high. This usually means buying the more expensive ones.
PVA glue is usually used direct from a squeezy bottle and only needs to be applied to one side of the joint except in very special cases. Where there is a danger of it being wiped off the surface during the assembly of a joint Such as a well fitting mortise and tencon one should carefully apply to all faces of the joint.
The main disadvantage of PVA is that it suffers from “creep” This means that if put under constant tension it will tend to fail after time by just in plain terms stretching. There is a side effect of this that if you glue a solid table top together with it. after a while you will be able to feel the joints.. just a tiny ridge appears after a while. I therefore don’t recommend using PVA for gluing solid tops. Never use PVA for items in constant tension such as musical instruments. PVA should not be used for woodwork destined for places with high temperature or humidity.
There are a group of PVA materials which claim to be waterproof.. It is only safe to consider them as damp resistant. What ever the manufacturers say protect yourself and use one of the several very definitely waterproof materials that are available for any critical applications where water resistance is important.
Basically a good and useful material which should be on everyone's workshop shelf if only for general use not related to your main product.
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Animal Glue (Gelatine)
Animal Glue is made of gelatin and has several unique qualities that make it still a very useful material even with the multitude of more modern materials available today. It grabs the work as it cools and doesn’t have to wait for a long chemical process or for the material to dry. This makes possible what are called “Rubbed Joints” Where pieces of wood are carefully planed to a good fit and just glued together. They are rubbed backwards and forwards to squeeze out execs glue and just carefully left to dry for 8 hours or so. There are also applications in musical instrument , where for instance a Violin must be opened for repair now and then, where nothing else will do.
Ordinary gelatine glue which I have since my student days referred to as “Animal Glue “ comes in several different forms. It may be supplied as granules , “beads”, slabs or as a liquid, well a jelly. The dry forms are better from a shelf life point of view but the jelly form only needs heating and possibly diluting a little.
The dry forms of the material need to be soaked in water over night if possible .I place the required amount in my pot and just cover the granules with water. The glue is heated in a thermostatically controlled “Glue pot” or in a water jacket pot like a bain marie. The glue should be hot but not boiling for use. The glue should be about the consistency of olive oil when prepared. It should run in an unbroken stream from the brush when it is lifted from the glue.
There are glues made from fish and “ cold ” glues formulated to be used at room temperature but the cold glue has really been superseded by PVA
Gelatine is also used in the “sizes” Used in “Water Gilding” and the making of Gesso.
Urea formaldehyde is a synthetic resin glue developed during the second world war and used extensively during the war for wooden aircraft and boat construction. It comes in several different forms and can be specially formulated for different applications.
The material takes the form of a resin and a hardener. The hardener can be a powder or a liquid and is added to the resin in a fairly strict ratio. The hardener can also be applied to one side of a joint while the resin is applied to the other. This can work OK but has limitations. The material is also supplied as a powder to which water is added. This is a good system but if you use a lot of glue it is very expensive. The glue was a resin which was then dried. The material can also be supplied with the hardener included in the powder so the wetting of it starts the setting process.
The Material can be quite weather and waterproof but shouldn’t be used for high performance applications such as underwater parts in boats. Resorcinol Formaldehyde is the material to use here. Having said that the better Urea Formaldehyde glues are very durable.
Urea formaldehyde resins can be thickened with various fillers for gluing joints etc. in hand workshops. This can also make them more “gap filling”.
Adding wheat flour or corn flour (corn starch) to the resin when used in high pressure heated veneer presses stops it bleeding through the veneer. The heat from the press plates lowers the viscosity of the glue and so the starch sets it to stop it reaching the surface of the veneer while the chemical reaction continues to set the glue fully.
Once these materials are mixed they have what is called a pot life. This is the time they remain usable after mixing. We usually consider about 20 to 30 minutes about right for hand workshops. This time depends on formulation and temperature, The pot life of the materials is dramatically reduced at high temperatures and a slower formulation must be used. When using the material in industrial glue spreaders for heated presses and in heated assembly jigs a much longer pot life may be chosen allowing the glue to remain workable for perhaps a whole 8 hour shift.
Epoxy resins did not have much influence on woodwork until three Canadian boat building brothers developed what they called the “WEST SYSTEM” of boat building using specially formulated epoxies , wood and various reinforcement fibres. If you are seriously interested in Epoxy wood applications I can do no better than recommend their book “ The Gougeon Brothers on Boat Construction” ISBN; 0-87812-166-8 printed by Pendell Printing Inc. Midland MI.
Or go to their web site.
I use wood epoxies for all sorts of jobs but it is an expensive material for general workshop use and can cause skin problems if not used carefully.
Resorcinol formaldehyde is a very important adhesive. It is totally resistant to water and will also glue difficult timbers such as Teak. It may be used in underwater applications with complete confidence and will glue wood to cement based materials which Urea formaldehyde glues (which rely on an acidic reaction to set) won’t because of the alkalinity of the cement.
Resorcinol formaldehyde comes as either a resin with a powder hardener or two resins to be mixed together .. I prefer the latter as the measuring is easier and the mixing ratio is usually one to one.
Resorcinol formaldehyde is varies in colour from light pink to almost black so this has to be taken into account if the woodwork being glued is visible.. However if the joints are good it should be acceptable except on very light coloured woods.
Hot Melt Glue
Hot melt glue comes in the form of solid “plastic” which is heated up by a device of some sort and applied to the material to be glued. The joining surfaces are then bought together quickly before the glue cools down. It may be supplied in pellets, slugs, or for the common hot melt gun, in rod form. Hot melt adhesive is used extensively for edging man made board panels whether with veneer, plastic strip or even solid wood lipings. There are many very ingenious (and usually very expensive) machines to do this.
Hot melt has limited use for the hand workshop but I find it useful for holding things temporally while I work on them and I even use it to hold small pieces of wood on the face plate of the lathe sometimes.
The hot glue can burn your fingers quite badly so be careful if you do get it on your skin. In pulling bits of set adhesive off burned skin you can take a layer of skin with them.
Contact adhesive consists of a material which is applied to both mating surfaces of a job and allowed to dry .. then when the surfaces are bought into contact with one another they bond instantly .
The material comes either as solvent based solution or as an water based emulsion. My experience is that the water based material is less reliable than the solvent based on. However I recommend the use of the water based one as the solvents in the other sort are very nasty.
The most important use of the contact adhesive is in laying plastic laminate. ( Formica etc. ). It is not the only way to do it though . If you have access to a veneer press it is much better to use an ordinary woodworking adhesive if you can. Urea formaldehyde or PVA. However if you don’t have a press or the plastic laminate has to be laid “on site “ for some reason then you will have to use contact adhesive.
It is important to follow the manufacturers instructions about drying times.. I find that the time you can leave a glued job before closing the joined surfaces varies from about 15 minutes to 6 hours depending on the material. spread the material as evenly as possible with a serrated spreader and allow to dry thoroughly. When bringing the two surfaces together you must be very sure to get them properly lined up as once the glue touches it wont come away. Let your adhesive dry thoroughly so there are no wet spots. I find this is easier if you use a proper glue spreader designed for contact adhesives as you get a much more even spread which dries more evenly (it also uses much less of what is a fairly expensive material). Then put either battens of wood or sheets of newspaper on your glued surface and place the laminate on top and line it up carefully. Now remove the battens or the paper one piece at a time pressing down the laminate as you go. I then use a small roller to make sure there is good contact between the glued surfaces. Some people do this with a block of wood and a hammer.
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