GLUES AND THEIR USES
Glues perform such an important function in construction work that we consider it a privilege to be able to present to our readers the following authentic information extracted from the Constructional Research Bulletin issued by the Director of Constructional Research, Timber Development Association Limited.
The use of glues for joining une piece of wood to another is of extreme antiquity, and a classical and oft-quoted example of its early employment is in the elaborate mummy-cases of the Pharaohs. For many centuries, however, the glues them- selves and the method of using them were closely-guarded craft secrets, and only recently have they become part of the general technique of industry.
To-day there are many kinds of adhesive freely available, and the potential user may well find difficulty in selecting the The aim of this Bulletin is one most suited to his purpose. to assist him in this maller.
In so small a space, only an outline can be given and no can be Such information fine details can be discussed. obtained from the adhesive makers, and the user is advised to avail himself of their services.
Uses of Glues in the Woodworking Industry
Glues are used for two main purposes: for the manu- In the former, facture of plywood and in assembly work. large sheets of thin veneer are spread with the adhesive and bonded together in a hydraulic press, the platens of which are heated by steam or hot water. The process is under exact control at every stage, and the machinery involved is of the precision type. In assembly work-a term applied to the gluing together of pieces of timber to make such articles as furniture, houses, aircraft and boats the areas to be spread are comparatively small, the methods of applying pressure can seldom compare with those of the hydraulic press in uniformity or intensity, and the operations are usually carried out at ordinary workshop temperatures.
The process of applying ornamental veneers to plywood is generally similar to that for the manufacture of plywood itself, but in some cases it approximates more nearly to assembly gluing.
The function of pressure in the making of a glued joint is often misconstrued. Its sole use is to bring the two surfaces into contact, and the amount required depends on the If the stiffness of the wood and whether or not it is flat. surfaces are uneven, as with rough sawing or because of excessive twisting, close continuous contact cannot be effected by practical means, and the resultant glue-line, instead of being uniformly thin, will vary from place to place. This is of little moment with some types of adhesive, but vastly important with those that will perform efficiently only if the glue-line is thin.
Generally speaking, the character and process of plywood manufacture autoinatically produce thin glue-lines, but in assembly work trouble is often encountered-particularly where nailing is relied upon to perform the function of pressure. The matter is not serious providing an adhesive has been selected that meets the conditions, namely with "gap filling" characteristics, for "close-contact" types should only be used when thin glue-lines can be obtained with certainty.
Descriptions of Glues in Common Use
Wood adhesives can be classified into six main types according to their origin and constitution. Mixtures of the various types are made for particular purposes, but their uses and properties vary so much with their formulation, that they cannot be described in general terins, and are omitted from this bulletin.
The names and principal uses of the six types are summarised below, and a more detailed description follows of their general characteristics. Three of the glues are not actually used in this country, but as they are found in imported plywood, the potential buyer should be familiar with their properties.
(i) Animal glue-Assembly work and veneering, very
seldom used nowadays for plywood.
but
now albumin-Plywood manufacture, (ii) Blood
very little used in this country. (iii) Casein-Assembly work, plywood and veneering. (iv) Soya-bean derivatives-Not used in Great Britain,
but much employed by foreign plywood makers. (v) Starch derivatives-Not used in Great Britain, but
sometimes used by foreign plywood makers.
(vi) Synthetic resins-Assembly work, plywood and
veneering.
(i) Animal Glue. A general name given to adhesives obtained from bones, sinews, hides ог skins. Another commonly-used name is Scotch Glue and an allied variety is known as fish glue. The adhesive is sold as a solid varying in size from powder to the so-called cake, as an undried jelly, and in liquid form. The solid is prepared for use by first swelling it with one to three times its weight of cold water and then warming to a liquid at 60°C. (140°F.). The common practice of boiling the mix is entirely wrong and one that causes rapid deterioration. Jelly glues are prepared by warming alone and require no addition of water, whilst the liquid form is sold ready for immediate use.
21
Where the surfaces to be spread are large (as in veneering work), animal glues are applied by a roller machine Sited with heated storage troughs, but in assembly work a brush is employed, or special grid-covered dipping vessels if produc- tion warrants these.
Most animal glues form their bond (or set) by a com- bined cooling and drying action which transforms the liquid into a solid.
This type of adhesive has been in use for centuries and has a high reputation for strength and durability under dry conditions, for its non-staining characteristics, and for the ease with which joints can be machined or finished with hand tools. It has no irritant effects on the skin of operatives. The main drawback of animal glue is its complete lack of resistance to water and damp.
(ii) Blood Albumin. This adhesive is derived from blood obtained from slaughterhouses, and is purchased as a dry flake and mixed with water and such chemicals as lime or ammonia by the user. It is not available as a ready-to-use glue, and is in fact no longer of any great importance in this country. Blood albumin is still popular with foreign plywood makers, and its description is therefore included in this bulletin.
The adhesive requires heat for its setting, the action being similar to that which changes the form of the white of an egg during boiling.
Blood glues are only moderately strong but have a high resistance to water, even at boiling point. They are, however, very liable to attack by moulds and bacteria, which can destroy them completely.
(ii) Casein, Casein, the basic material from which the glue is made, is derived from soured milk. It is possible for a user to prepare his own adhesive from published formulae, but unless the quantities are large and he can put the work into the hands of a competent chemist, he would be well advised to purchase the prepared glue-powder from firms specialising in its production.
This prepared powder contains all the necessary chemicals and only needs to be mixed with water to be ready for use. The makers issue full instructions and these should be followed to the very letter, for although the glue will apparently tolerate a surprising amount of abuse in mixing, such practice is unfair to the adhesive, and is reflected in its subsequent performance.
Casein glues are available for assembly work, but the strongest and most water-resistant types are alkaline and tend to stain the wood. There are non-staining varieties that also set at room temperatures, which are useful where dis- coloration is objectionable, as in veneered work. A third type of casein glue is made especially for veneering and is used in a heated press.
The cold-setting glues resemble condensed milk in con- sistency, and for assembly work involving small areas, are must conveniently applied with a spatula or hand roller. Their alkaline nature is trying to the operatives' skin, and rubber gloves should be worn if contact with the adhesive is pro- longed. The strength of the joint is improved if the spread surfaces are allowed to dry for a few minutes before being clamped together. Casein has quite good gap-filling character- istics, but should not be used if the glue lines are likely to rise in thickness to more than 0.03 in. Its setting action is mainly one of drying but some chemical change occurs.
Generally speaking, casein glues are easy to mix and apply, are not sensitive to temperature and so can be used in almost any workshop. They tend to have an abrasive action on edged tools. Joints made with casein glue are strong in the dry state, lose much but by no means all of their strength when thoroughly wetted, but regain it completely on re-drying. There is, however, some danger of attack on the wet glue by bacteria and moulds.
(iv) Soya Bean Derivatives. The soya bean is now an important crop. Grown in the United States and Far East for its oil, the residue is used for glue-making, particularly in the Douglas Fir plywood industry. It is not available in Britain, but so much imported plywood is bonded with soya glue that users should know of its properties, which resemble those of casein. There are many similar protein residues that can be employed for the same purpose, notably those of ground nuts, castor and rape seed, and it is more than likely that glues made with these will soon be found in plywood exported from countries where such crops are grown.
(v) Starch Derivatives. Here again is a class of adhesive that is used in America but not by the wood-working industries in this country. These vegetable glues, as they are commonly called, are derived from cassava and from starch obtained from corn and potatoes. They are essentially plywood adhesives, and give bond moderately strong when dry but with little or no resistance to water.