Wood Laminates—Advantages and Limitation.
By way of explanation this paper was planned to be one aspect of modern applications of plastics. At the outset, I must join forces with other wood technologists, the world over, who are indignant about the popular misconception engendered by the terms “plastic planes” and “plastic houses.” The plastics play a useful though minor role in the combinations. Accordingly I will pass on first of all to a consideration of some properties of wood, and incidentally of timbers.
The fear of exhaustion of basic materials—of oil and coal and metallic ores—is ever present in the modern world. They are materials formed before the advent of man, and current formations apparently cannot keep pace with usage. We, too, have been apt to regard wood as a material to be mined, as one whose imminent exhaustion is to be regarded as inevitable. There is little doubt that supplies of some timbers, as instance our own kauri, are likely to become scarce, but in the light of our present knowledge, it is unthinkable that the material “wood” should become exhausted. Some timbers are more easily farmed as a crop than others and the trend must be towards replacement of the multiplicity of timbers having special inherent qualities with a more restricted range of species. It will be enforced by gradual working out of easily accessible supplies of mahogany, lignum vitae, ebony, etc., or by reasons of nationalist self-sufficiency, or the circumstances of war—e.g., British and German timber use during the
war, when beech and other home-grown species replaced many tropical woods. Such replacement in normal times will be slow, as very many advantages are contained in the very remarkable assemblages of cells which go into the make-up of the different timbers.
The cell, or rather its skeleton, in wood has particular significance in this discussion. In woods produced by coniferous trees, known to us as “softwoods,” only two important types of cells are present—a rather thick-walled type (forming the bulk of the wood and combining in the tree the functions of conduction of food solutions and of mechanical strength to support the massive column which is the tree trunk) and thin-walled storage cells. Softwoods include the timbers best known to our western civilisation. The other vast group, “hardwoods,” have thin-walled cells forming tubes for conduction, thick-walled mechanical-strength cells, and thin-walled storage cells. Leaving aside the inherent differences in wood structure which distinguish the various timbers and determine to a considerable extent their properties and uses, I wish to stress the important fact that the bulk of the cells in woods are long and slender, with the long axis standing vertically in the trunk of the standing tree. The long axis of the cells is, in other words, along the grain of the wood. Wood has high tensile and compressive strength along the grain and low tensile and compressive strength across the grain. An individual cell may be likened to a rolled tube of paper in-so-far as these strength properties are concerned. If we increase the number of layers on the inside of the tube the enclosed space will be decreased, the wall thickness increased, and the weight and strength of the tube increased. It is mainly the ratio of air spaces to cell-wall volumes which determines the weight of a given volume of any timber, as the material constituting the walls is basically the same in all timbers. Deposits or secretions of gums, resins, oils, etc., do contribute in a small measure to the dry weight of the heavier timber—e.g., lignum vitae and black maire. The extremes in weight for a cubic foot of dry timbers of commercial importance are 6 lb. for balsa and over 80lb. for lignum vitae. In general it may also be said that strength varies directly with specific gravity.
I have said the cell-wall material, which I will call “wood substance,” is basically the same in all timbers. It consists principally of cellulose (50%) and lignin (26%), and has a specific gravity of 1.54 (i.e., it weighs about 96lb. per cub. ft.). It will be appreciated, therefore, that timbers weighing over 80lb. per cubic foot have not much room left in the cells for air space.
It will assist our understanding of a number of phenomena connected with timber if we have a brief glance at modern ideas concerning the makeup of the cell wall. Cell walls are made up of concentric layers which can be dissected into long, slender fibrils arranged spirally and forming a meshwork. A smaller unit of the cellulose is the crystallite, which in turn is composed of groups of cellulose molecules. This molecule has a length many hundreds of times its diameter—it is exceedingly strong in the longitudinal direction—but adjacent molecules are not strongly united crosswise. Lignin is generally believed to be amorphous and to exist interspersed between cellulose crystallites.
Another physical property of wood requiring mention is the relationship of wood and water. Some timbers when freshly sawn contain as much as 200% moisture content based upon the oven dry weight of the wood. Most of this water is in the cell cavities, but about 25% is in the cell walls actually between the crystallites. In drying out a great many problems arise both during loss of water from the cell cavities and the subsequent loss from the cell walls. The principal point to be made, however, is that when water is removed from between the crystallites the wood shrinks. Shrinkage is very slight along the grain—i.e, along the length of the cellulose molecules, but considerable across the grain, as the molecules are allowed to come closer to one another through the loss of the water films. In air-drying timber a point is reached at which the moisture content is in equilibrium with the average relative humidity. As an example this E.M.C. for timber out of doors in Wellington but not subject directly to rain wetting is usually assumed to be about 17%. Timber in use indoors has a lower E.M.C., which is easily attained only by kiln-drying. Winter conditions in a steam-heated building may require timber to be dried to less than 10% M.C. Shrinkage commences about 25% M.C. and continues until the wood is free from moisture. The most important point to make is that wood remains hygroscopic. With an increase in relative humidity of the atmosphere water is reabsorbed between the crystallites and swelling takes place. In passing it should be noted that both cellulose and water are polar, a fact which is well illustrated
by the adhesion between two blocks of wood frozen together. Even in small particles wood remains hygroscopic and cannot make a coherent mass with a matrix which is non-hygroscopic. You may be aware of the current endeavours to make a light-weight concrete, using sawdust in conjunction with cement, and also woodwool-cement building slabs. The problem of dimensional instability is probably the most troublesome of any connected with wood use. I need not emphasise the innumerable good properties of wood as we are usually better equipped to improve a material if its faults are recognised. Some of those troublesome properties are:—
1. Wood is hygroscopic and the dimensions of a piece vary with change of relative humidity, being negligible normally along the grain but considerable across grain.
2. Wood is not strong enough across the grain for many purposes.
3. Boards are frequently not wide enough or else they contain defects marring their strength or their appearance.
4. Wood in large sizes dries slowly under normal seasoning practice with a gradation of moisture-content from the outside to the centre.
5. In those large sizes also defects may develop in seasoning—e.g., warping and checking.
6. Many of our timbers with special inherent qualities are in short supply.
Plywood: By cutting the wood into veneers and assembling and gluing them so that alternate layers have their grains crossing at right angles we can achieve:—
(a) An equalisation of strength properties along the length and width of the panel.
(b) Greater resistance to checking and splitting and less change in dimensions with changes in moisture content. This presupposes that the assembly is of balanced construction so that on either side and equidistant from the core there is an opposite, similar and parallel ply.
(c) A wide sheet of wood can be obtained with any desired number of plies.
(d) Diversification can be extended further by orienting the grain of adjacent plies not only at right angles but at various angles according to the redistribution of strength which is required.
(e) Because of the non-splitting quality of plywood nails may be used close to the ends or edges of the sheets.
(f) Even with the redistribution of strength in plywood, it is interesting to note the favourable strength-weight factors in comparison with the metals, especially in stiffness factor.
In discussing the technical problems associated with extensive use of plywood it is evident that very satisfactory bonding with casein glues enables it to fill a useful function for interior or fully protected locations. A slightly different formulation of casein glue, giving a moderate degree of water resistance, permits an extension of its uses under conditions of moderate exposure in both plywood and laminated structural members. However, the advent of artificial-resin bonding, especially phenol-formaldehyde, has opened up far greater possibilities as the glue-line is extremely durable under severe exposure and eliminates failures due to deterioration by fungus or bacterial agencies. This was the stage at which the wood itself rather than the glue-line became the weaker member on account of the old bogey of shrinkage and swelling with alternate drying and wetting. The wood veneers could not shrink across their widths, and the net result of alternating stresses was to cause a disruption of the wood itself, manifested as surface checks. Several methods have been devised for reducing or eliminating that surface checking. One was simply the application of sealing coats of artificial-resin varnish, and a group of materials designed for the purpose grew up under the title of “water repellents”. A more costly product employs a resin-impregnated paper hot-pressed to the faces of the plywood sheet. Locally, during the war, neither artificial-resin glues nor artificial-resin water repellents were available when considerable quantities of plywood were required for exterior work; by using water-resistant casein glues and a copper-oleate wax hot-dip a fairly useful article was obtained. Subsequently liquid phenol-formaldehyde resin glues and film P.F. glues have both come into use in so-called waterproof plywood. It is useful for applications ranging from bus and trailer bodies, cool chambers, drying chambers, to boat building and house
construction. The need for a water-repellent sealer was apparent in one house erected in Auckland. One or two such sealers are likely to be available from both British and American sources in the near future.
It is time now to look more critically at the position in New Zealand. A long-term study of conversion of N.Z. logs by peeling has established the unfortunate fact that high-quality peeler logs are in insufficient supply. Unless the wastage figure at the lathe and in the drying process can be brought down, one cannot be very optimistic about future supplies until some of the exotic coniferous woods are available in large enough sizes. Peeler logs are specially selected for cylindrical form, absence of wind, bark seams and knot defects, but a conversion factor for green veneer sheets is not very greatly superior to that obtained by sawing logs of a more average quality. Loss in subsequent factory operations, too, is not negligible, although it may show slight advantages over dressed board products. Hence it is incorrect to anticipate an early supply of plywood for exterior sheathing of houses.
Coreboards: This second group of laminated product covers “blockboard” and battenboard also. The construction normally has a solid timber core of glued up strips accurately dressed for the application to both faces of a cross-banding veneer with grain at 90° to that of the core, followed by the finishing veneers (both faces) laid with grain parallel to that of the core. Casein glue is the normal adhesive in New Zealand and insignis pine has practically superseded totara and redwood as a light weight core of good stability.
In comparison with plywood the advantages of coreboard lie in those uses where a greater degree of stiffness is required; it is also regarded as having superior qualities of flatness. From the timber-economy viewpoint coreboard is a useful way of using up short lengths of timber and conserving veneers. Solid-core doors, furniture (especially tabletops), solid panelling, instrument-mounting boards, printers' blocks and various workbench tops are familiar examples. It is unfortunate in New Zealand that manufacture of these products is too widely dispersed to enable the price to be brought down to the extent that will permit of its wider use. In Australia large-scale manufacture permits wide use for coffins.
Glued laminated timbers for construction: As indicated by the title this construction refers to two or more layers of wood glued together with the grain of all layers approximately parallel. The laminations may vary as to species, number, size, shape and thickness. The properties of laminated members are essentially the same as those of solid wood, but if well-constructed, they are usually more uniform in strength properties and less apt to change shape with variations in moisture content. Other advantages over large natural-timber members are:—
(a) Seasoning of board sizes is much quicker and avoids most of the warping and checking involved in seasoning of large sizes.
(b) Larger and longer members than are generally available as single pieces are made possible.
(c) Laminations can be so arranged as to minimise the effect of defects—up to 60% of defective-grade boards have been used in interior laminations.
(d) Curved members may be provided.
(e) Tapering of members so that their cross-sections at various points in the length are not greater than required by the imposed loads, results in economy of material.
Water-resistant casein glues have been very satisfactory in European use over a fairly long period. Until recently there has been little occasion to think in terms of improved glues, as most applications were in interiors of buildings. New Zealand has lagged very badly in applying this type of construction, and I am able to quote only one example of our rimu having been used—it was in Australia. It may be urged that we lose timber by sawing up the laminations, and that the careful workmanship required in manufacture, gluing and assembly is beyond our capacity, but with more careful husbanding of timber supplies in the future and the use of artificial-forest-grown timber with more defects, some more attention to design is essential. By careful selection of boards, and their assembly so that the inevitable defects are distributed so that they shall not occur at places of maximum stress, we shall be able to use the timber to advantage. I have not touched on members using plywood for the web in conjunction with board laminations.
Aircraft structural members have been one of the more spectacular uses of laminated timber in which artificial [ unclear: ] bonding has been required. Instead
of simple clamping for the requisite time to secure adhesion of casein glue it is necessary to apply heat to cure the resin glues. Various ingenious methods have been employed—e.g., banks of infra-red lamps, immersion in boiling water, radio-frequency heating and partial curing amounting to spot-gluing, with electric wires inserted at intervals along the glue-lines. The glue manufacturers for their part have been developing resin glues to be used with catalysts so that curing can be done at temperatures below 80° F. with clamping periods of from 5 to 8 hours. The curing can be hastened by subjecting the assemblies to heat in an ordinary timber-drying kiln. Some resorcinol glues are well adapted for this type of curing. For heavily stressed structures, such as laminated oak keels, curing cycles should be at higher temperatures—190° F. is the recommendation for these nominal 75° F. glues. Numerous laminated items are coming into every-day use, and I cannot hope in the time available to quote any further examples.
High-density plywood or “Improved Wood”: Dealing only with the laminated products, which are infinitely more important than any proven impregnated solid-timber proposition, there are numerous interesting developments in which plastics play a very important part. By the forcing of synthetic resins throughout the cell structure of wood a far greater degree of dimensional stability is achieved. Resin-forming constituents of low molecular weight having an affinity for the wood substance can be used. This is quite distinct from mechanical deposition of solid material in the wood which would thereby have merely improved hardness and compressive strength, and has been done only with a few specific resinoids under specific treating conditions. The water-soluble P.F. resinoids used penetrate the cell-wall structure, which I went to some pains to explain and bond intimately with the wood; the water solvent is then evaporated and when heat is applied the resin is set permanently, giving us a material which may conveniently be called “Impreg”. An assembly in which only the surface veneers are impregnated has been shown to be practicable. The resultant product has increased stability, durability, electrical and acid resistance and compressive strength and hardness; impact strength is the property adversely affected.
A natural step from “Impreg” is compression of the assembled veneers treated with the water-soluble resinoids as outlined above. With a pressure of 1,000lb. per square inch, the specific gravity can be increased to 1.3 or 1.4 in this form of improved wood, called Compreg. Both Impreg and Compreg have been developed by the Madison Forest Products Lab., U.S.A., but developments in Britain produced a number of products which were more immediately useful in the war. Untreated veneers were laid up as in normal plywood assemblies with interleaved sheets of film glue; with compression of 2,500 lb. to 3,000 lb. per square inch, the cured assemblies had specific gravities approximating to that of Compreg. The glue penetrates some distance into the veneers from each glue-line, but the wood is not completely impregnated. The products had the immediate advantages of lower resin content and ease of manufacture; consequent upon the lower resin content were lower costs, and impact strength superior to that of Compreg. This last feature is, of course, explained by the low impact strength of phenolic resins as compared with that of untreated wood. Dimensional stability was very satisfactory for most purposes, but much inferior to that of Compreg.
Other methods of producing improved wood have been used also. As to the products, they comprise a formidable list employing orientation of the grains of constituent veneers to suit the applications. Handles such as are produced in New Zealand employ only parallel laminations, so also do picker sticks, sporting goods, rollers and bearing plates. Aeroplane propellers are a very important item. By interleaving shorter laminations decreasing in length progressively towards the centre among the longer ones in a propellor assembly and compressing it, a high specific gravity at the hub end and a lower S.G. at the tip could be obtained. Different given orientations are required for gears. New Zealand's requirements in the industrial field are extremely varied, but the over-all requirements for any one item are small; in consequence prospects of large-scale manufacture are not favourable. There is, too, a major local difficulty in securing a timber equal in calibre to birch, mahogany, walnut, or Australian coachwood for the supply of very thin, even-textured veneers free from defects. Southland silver beech approaches the standard, but peeler logs are very few.
Double curvature construction: While metal will draw or stretch to form a cup, wood possesses this characteristic to a much smaller degree. In general
it will compress more safely than it will stretch, and the thinner the layers the more compression can be secured. The technique of compound curvature in plywood is largely built around the use of fluid pressure applied with flexible bags during the curing operation. Narrow strips of sliced instead of peeled veneers are usually cut to pattern shapes which will fit closely together around the curves of the mould without overlapping. The angular directions of the grain of successive veneer layers are determined by the particular curvature required. One typical shell-like shape used in aircraft construction is often called monocoque construction. When braces or stiffeners are added in this shell construction, it is called semi-monocoque.
Actually the fuselage of the Mosquito plane employed a much simpler method than the tailored, moulded shapes already referred to. For a half fuselage one skin of pre-cut plywood was placed over a wood or concrete form and scarf jointed, using casein glue, then a layer of balsa wood, followed by another skin of plywood. Curvature of this sandwich was maintained by placing over it a number of steel bands held in tension by clamps during curing.
It will be interesting to watch the establishment of glider construction in New Zealand, as it is probable that the double curvatures will introduce the tailoring and moulding problems. Another incipient industry, laminated cask staves, will probably be meeting its curvature problem by simpler machining methods after assembly and curing of the resin-bonded laminations.
Inevitably one attempts to cover too broad a field. Many of the developments are only in their incipient stages in New Zealand, but they are bringing numerous problems relating to wood. We are looking hopefully for veneering logs in some of the less remote Pacific Islands to supplement our own logs, whose shortcomings are fairly widely appreciated. Good veneers as well as sawn cores or sawn laminations are yielded by forest-grown insignis pine, but substantial supplies of large logs of this species and locally grown Douglas fir are still some distance off.