Source from :http://fennerschool-associated.anu.edu.au/fpt/plywood/sawingsanding.html
ADHESIVES USED IN PLYWOOD MANUFACTURING
Application of Adhesive
Preparation of Veneers for Gluing
Glue and Gluing Procedures
Mixing and Spreading
APPLICATION OF ADHESIVE
Application of adhesive to layons can be achieved by any one of the following methods.
- Roller coaters
Roller coaters (see Pic 1) as their name suggest applies adhesive to the layons by means of rollers. Both sides of the layon are coated. The rollers are often corrugated to control the spread of the adhesive.Picture 1:-Roller coater (Courtesy of Dr. P.D.Evans)
- Curtain coaters
These apply adhesive in a thin film or curtain of adhesive to one side of the veneer as it passes through the coater.
- spray coaters
These use air sprays to atomise the adhesive and apply a thin film of the adhesive on to one side of the veneer.
- Liquid and foam extruders
These apply beads or rods of adhesive to the veneer surface which coalesce to form a complete covering of adhesive during the pressing process.
The aim of all the coaters above is to deposit a regulated even cover of adhesive on to the veneer surface. Different wood species and grades of veneer require different levels of adhesive. Generally dense wood species and smooth cut veneers requires less adhesive than porous wood or rough veneers(Evans pers.con.1996, Baldwin 1981). Glue spread on the veneer surface is usually expressed in terms of grams of adhesive per square meter of veneer.
PREPARATION OF VENEERS FOR GLUING
Depending on the type and quantity of plywood to be produced, the steps to be taken in preparing the veneer stock for gluing and pressing, may range from direct transfer of the dry veneer. To the glue spreader, and then a series of operations (redrying, grading and matching, dry clipping, jointing taping and splicing, inspecting and repairing, conditioning) as required in the manufacture of high quality furniture plywood.
In general however most veneers wether rotary cut or sliced, are cut to size and squared by a dry clipper, edge jointed, taped or spliced and graded into cores, cross bands, or face veneers prior to being assembled for gluing and pressing.
Sliced veneers are processed, basically the same as rotary cutting. However, difficulties are aggravated by usually greater working lengths. Also more waste and loss are more serious when working with expansive material and large sized veneers(Baldwin 1981) .
GLUE AND GLUING PROCEDURES
Synthetic resign glues replaced protein and starch glues for plywood manufacture, with urea formaldehyde predominating because of price, properties and ease of application. Australian mills once used tannin formaldehyde resigns or casein and soybean glues.Boas (1947)
- Urea formaldehyde glues are extensively used for interior and intermediate grade bonding, which covers the majority of hardwood plywood produced. Formulas differ somewhat and can be adapted to meet specific conditions.
- Polyvinyl acetate glues and also polyvinyl acetate/urea formaldehyde glues are used for edge jointing and veneering. Polyvinyl acetate glues resistant to boiling water are available.
- Melamine formaldehyde glues are not extensively used for plywood gluing but are used where a high grade bond is required, and where black phenolic glues cannot be tolerated. They are used to fortified urea-formaldehyde glues to increase the weathering resistance of the bond. The largest application of the melamine formaldehyde resin is in the production of decorative overlays.
- Phenol-formaldehyde glues are standard for exterior bonds. With a glue properly formulated for exterior use and suitably employed, no exposure conditions or laboratory testes are known which will degrade hot pressed phenolic glue bonds without destroying the adjacent wood layers.(Baldwin 1981) Phenolic glues are also used for impregnating veneers and paper overlays for plywood.
- Resorcinol-formaldehyde and phenol/resorcinol-formaldehyde glues are similar to phenolic glues in quality of performance but being more reactive, can be cured at room temperature. They are more expensive than phenolic glues, and therefore limited in use to special applications.
Fillers may be used to assist in reducing penetration of the glues into the wood, increase its viscosity at the critical time before setting, and tend to fill small cavities between veneers, thus preventing starved joints. Fillers are usually finely ground inert minerals such as clay or wood/grain flour finer than 180 mesh.
Cereal flours are used as extenders. These reduce the resign content of glue to the minimum necessary for the required bond but generally require a heavier spread of glue mixture.
MIXING AND SPREADING
Glues are cheapest if bought in liquid form in bulk and handled by pump to storage and factory. They can be purchased in containers in syrup or dry form, if stored in a cool dry place they will keep for several months even in the tropics.
Glue mixers (see Pic 2) are much the same for all kinds of glue, and usually consist of a mechanically driven paddle device for stirring the mixture in a container. It is general practice to mix the ingredients on a weight basis. A glue with a short working life must be prepared in small batches. When mixing several types of glue, special care should be taken when cleaning between mixers, since even traces of one type may seriously affect the prosperities of another.
Figure 2 :- Glue mixer (Courtesy of Dr. P.D.Evans)
Glue spreading machines usually have power driven rollers, ether of steel or rubber, with groove patterns consistent with the type of glue being applied. The amount of glue spread is controlled by one or more doctor rolls and varies with the type of glue, the species and quality of the veneer, and other factors.
Figure3:-Glue roller (Courtesy of Dr. P.D.Evans)
It is essential to the successful production of plywood that there should be a strong glue bond between plies. Not all woods behave equally well in this connection. A strong bond is difficult to obtain with some of the heavier timbers and here a strong bond is needed most. As the greater stiffness of the denser wood causes increased strain on the bond with shrinkage and swelling tendencies in the plywood, as a result of changing atmospheric conditions(Baldwin 1981). Where gluing to the surface of plywood is necessary, as in the case of overlays, the face veneer should exhibit good gluing properties.
Some species absorb glue much more readily than others and glue viscosity has to be adjusted to meet the required of the wood being used. This can cause problem when absorbent and non-absorbent species are used in combination(Baldwin 1981).. Some woods are inherently bad for gluing and finishing standpoints, because of natural constituents in the wood for example, oil or waxy materials may bleed to the surface during drying or storage and present a poor gluing surface, or result in uneven staining or painting characteristics. Steaming of logs prior to veneer manufacture, control of drying conditions, and their use as soon as possible after the production of the veneer can help in some cases.