All you need to know about adhesives

Everybody at some time has to stick two things together. Papers in the office, ceramic tiles in the kitchen, ceiling tiles in the bathroom, wood in the workshop, and metal in the garage all have to be joined with what is generally known as glue. But the name 'glue' is only a vague term for a wide range of different adhesives manufactured for different purposes. It is important to know how to select the right glue for a particular job, and how to use it for best results.

There is no such thing as an all-purpose glue. There is only a very wide range of different substances which, when used in various special ways, will join together pieces of materialpaper, wood and plastics, metal and glass and a range of other materials. These adhesives, as
they should properly be known, are themselves made from a wide range of different substances and are applied in different ways. To make the best use of them, you need to know not just what will stick two things, but how best to make them stick-in other words, the right way to use the adhesive.

The more similar the properties of the two objects to be glued together, the stronger will be the bond between them after the glue has set. Two pieces of softwood glued together, for example, or two pieces of hardwood, are in perfect harmony and the glue will produce a very strong bond. If, however, you glue a piece of softwood to a piece of hardwood, the different rates at which they expand and contract as they absorb or lose moisture from the air may cause the wood to warp, and the strength of the glued joint will not be high. After a while, the stresses at the glue line may even cause the glue to break up and the pieces to come apart.

This also applies to metals. If the characteristics of the two pieces are the same, and the adhesive is of a suitable type, then a strong bond results that will resist quite a lot of wear and tear. If they differ, the glued joint not only has to resist wear and tear, but also the devastating effects of different rates of expansion and contraction. After a time, the adhesive bond may separate or, in some cases, the surface of the metal may undergo a structural change due to the stresses in the glue line, and the metal itself will break.
There are times, though, when materials which are very different have to be bonded together-rubber to metal, for instance, glass to stone, or plastic to wood.
between different materials, the strengt bond is almost nil if the wrong adhesive Any adhesive used has to be of a ty special characteristics that will quite bridge the gap between the different mat

Types of adhesive

Basically, adhesives can be classifie five headings which include almost al readily-available handyman's glues. The, 1. Traditional glues; the main membE group is animal glue (brown glue) wh come in a solid form which has to be h dissolve it, such as pearl wood glue, c liquid such as Croid, Duraglue and Scot Other glues in this group include blood i (formerly used in the manufacture of r and now largely superseded by s resins), and casein, which is still widE for woodwork. This consists of ground curds plus lime or sodium salt. To get results from casein you must use it can described below. Under perfect cot casein is an extremely strong adhesive. 2. Clear acetate-based glues are cc and quick-drying.

They are quite vers general-purpose use and are suitable 1 diverse substances as broken china, cai fabrics and leather. Examples include Uhu and balsa wood cement. 3. Contact adhesives are usually based and form an immediate bond I two previously-coated surfaces (whi~ been allowed to dry for the specified t instant they areis applied. These include Evo-stik Impact, Durofast, Unistik, Superstik, Dunlop Thixofix, Tretobond 404 and Clam 3.
4. Polyvinyl acetate (pva) is a relatively new type of adhesive. Because it is easy to use and fairly inexpensive, it is now one of the most popular general-purpose adhesives. Brands include Bondfast, Evo-stik Wood adhesive, Unibond, Bostik Carpentry, Borden Wood Glue, Clam 7, Bondcrete and Timbabond.

A recent development is an adhesive called School Glue which can be washed out of fabric and clothing even after it has set hard.
5. Synthetic resins are plastic-like substances that come in many different types, some of which are used as adhesives. These are extremely good for bonding a wide range of substances. Those based on urea formaldehyde include Aerolite and Cascamite, the epoxy resins include Araldite, Bostik 7, Dunlop Epoxy and Borden Power Pack. Another brand, Aerodux, is based on a resorcinol resin. This group of adhesives, particularly the epoxy resins, are the strongest and most versatile of all adhesives.

Characteristics and use

Animal glues are not gap-filling. This means that you cannot disguise poor carpentry by pouring thick glue into a badly-cut joint. The reason is that although the glue sets hard, it shrinks if it is applied too thickly. This causes the glue to crack and produces what is known technicallyas a 'dryjoint'.
The best animal glues-such as brown glue, at one time extensively used by carpentersare generally bought in cake form or in small bright beads called pearl glue. Either type is dissolved in a heated glue pot before use and must be applied while hot, preferably to wood surfaces that have been pre-heated.

This prevents the glue chilling and thickening before the joint is closed up and cramped in position. Some brown glues are available in permanently liquid or semi-liquid form. They will keep freerunning even when cold, until exposed to the air, which causes them to set.
Casein comes in powder form and must be mixed carefully with just the right amount of water. It is important to ensure that lumps do not form. After mixing, the glue should be allowed to stand for a few minutes to let air bubbles come to the surface. This type of glue is even less gap-filling than hot brown glue and the thinner the glue line the stronger it is, so careful and even cramping is needed to give the maximum bond strength.
Casein does have its drawbacks. It stains some hardwoods, tends to deteriorate in damp conditions, and, in some situations, may go mouldy. Dubious though some of these traits may seem, it is worth emphasizing that, when properly used and properly protected, it is a very strong and inexpensive glue.

Clear glues, which are usually supplied in tubes and tins, are good, general-purpose household adhesives which can be used to repair china, to refix loose tiles and to cope with a lot of miscellaneous jobs around the house. But this does not necessarily mean that these glues are the best for every application, and certain jobs that they can be used for may be done better by other adhesives, as set out in the table below. These glues contain solvents, which are nearly always highly inflammable; furthermore, it can be injurious to the health to inhale the fumes for too long. When applying these glues there should be no naked flame in the same room and the area must be well ventilated.
For bonding non-absorbent surfaces such as between them because this will split the bond. A good way of avoiding this difficulty is describ. Do not use these adhesives anywhere near naked flame.

Many substances, including plastic laminate, are so strongly fixed by contact adhesive that for all practical purposes they can never be removed. Any attempt to pull the joint apart will usually break the laminate. Where a carpet has been stuck down to the floor with this adhesive, the bond is so strong that if you try to pull the carpet off, you will tear it and drag through the pile. These points should be considered before deciding to use this type of glue.

The pva adhesives for woodworking have done much to take the mess and inconvenience out of glueing. They are white in colour and generally available in plastic squeeze bottles with spouts. These adhesives are ideal for use in dry, indoor situations but have poor resistance to damp conditions. Again, they are not gapfilling, but they have the advantage that they require only light holding pressure to develop their best strength.

Synthetic resins have several advantages over other adhesives. They are gap-filling, so they can be used to disguise inaccurate jointing and cutting. They also have very good resistance to wet conditions, particularly the epoxy resins. These are supplied in a two-part pack and the right amount for the job must first be mixed. Once mixed, the glue remains usable only for about 30 minutes, and must be used within this
time. It may, however, take up to three days to gain its full strength.

Epoxy resins are so versatile that you can use them to bond almost anything. The pretreatment of the surfaces is very important, however, and it differs from material to material.For many materials it is sufficient simply to remove all grease and roughen the surface of the material. The following materials can be treated in this way: ceramic, glass, nylon, clear acrlyic sheet, fibreglass and other synthetic resin products, carbon, zinc, lead, tin, solder, nickel and most other metals. The pre-treatment of other materials is set out in Table 1 on page 1582.

Special-purpose adhesives

There are quite a few special-purpose adhesives which cannot be grouped under the headings already given. Common types include ceiling-tile adhesive and various kinds of paper-hanging paste.
A number of adhesives are made for polystyrene tiles to enable them to be stuck to ceilings or walls. It used to be recommended that five dabs of adhesive on the back of each tile, arranged like the five spots on dice, were sufficient. But it has now been found that this method of sticking the tiles is dangerous because, if a fire breaks out, the tiles catch fire and the insecure fixing allows large areas of molten and burning tile to drip from the ceiling and spread the fire.
The technique now recommended is to comb the adhesive evenly over the entire back of the
tile with a special toothed applicator usually ~ supplied with the adhesive. To press the tiles to the ceiling, use a flat piece of wood or a plasterer's hawk or float to press them into place-this avoids dents and dirt marks from your finger-tips.
For ceramic wall and floor tiles, other special adhesives are made. . Pearl glue is an
animal glue that has to be dissolved before use. This is done by heating it in a glue pot. Above, lower picture. Some epoxy glues come in two parts, a hardener and the adhesive.
G. Ceiling cement
H. Floor tile cement
I. Wall tile cement
f. Urea formaldehyde adhesives
K. Wall and ceiling paper pastes

very carefully. They also encouraged the growth of fungus. Nowadays, thanks to water-soluble cellulose, starch and dextrine, a number of different types and grades of wallpaper paste are available; all are easy and clean to use and at least one will suit any type of wallpaper. provide full details on paper-hanging; the only general rule is that these pastes must be mixed slowly. The powder is added to the proper quantity of water and the mixture is stirred to avoid lumps.

General hints

The success of a glued joint using any glue on any substance depends on the glue being able to produce the best possible bond to the surfaces. For this reason, the pieces to be glued must be clean and perfectly dry; so must your hands. If there is oil or grease on wood when it is glued, the glue will not work. The grease must be brought out by repeated applications of clean newspaper and a very hot iron followed by a wash in methylated spirit to remove the last traces from the surface fibres.

After wood is sanded, the pores are clogged with dust. Wood dust will soak up glue and prevent it from penetrating the surface, so you should dust off freshly-sanded surfaces thoroughly with a stiff brush; a vacuum cleaner helps as well.
Paint forms an excellent barrier between underlying surface and glue, and flakes off so that the glue will not hold. If you are trying to glue wood which has been painted, remove all traces of paint first with a scraper, sandpaper,rotary abrasive disc or wire brush-but do not use a chemical paint stripper as this may affect the subsequent attempt at glueing.

If one of two different surfaces to be joined is more absorbent than the other as, for instance, in the case of sticking felt on to wood, spread the adhesive on the wood, not the felt, or the adhesive will soak into the absorbent material and "starve' the joint.
Glued joints in wood are always strongest when the glued surface runs parallel to the grain of the wood. End-grain glueing makes for a very weak joint and this should be avoided wherever possible.

Hot weather, high humidity, and extreme cold all have an effect on the process of glueing. Particularly with synthetic resins, the hotter the room used for glueing, the quicker the glue will set-though the shorter the glue will remain workable. Animal glues, particularly the casein cements, are very dependent on ideal workshop conditions for their strength. Usually it is unwise for the amateur to attempt to glue anything in a temperature lower than 55°F (13"C) or higher than 80°F (27°C). Avoid glueing during very humid conditions out of doors, and keep all excessive heat and moisture away from the joint until it is properlyset.

All you need to know about adhesives