* The terms "improved" or "densified" timber mean "timber changed mech-anically by impregnation or compression or both". It may be described as an assembly of timber veneers usually first treated with synthetic resins and improved or densified by a further application of heat and high pressure.
Many processes have been devised for treating timber in this manner to improve its mechanical and electrical properties and its dimensional stability under varying conditions. Apart from normal conversion to plywood, these processes however, always involve one or more of the three basic treatments—
synthetic resin impregnation, heating and application of pressure. Improved timber is sometimes known as compregnated timber or compreg, but in Australia it is known generally as densified timber.
Densification is usually affected by pressures up to about a ton per square inch, applied at temperatures of about 300°F. The specific gravity of timber substance is practically the same for all species, being about 1 -5, whereas the specific gravities of the various species themselves, as they occur in nature, range from less than 0-2 to more than 1-0 due to the presence of interstices or voids in their structure. There is thus plenty of room for increasing the density by reducing the void space, and it has been found that as the density is increased the strength also is increased. Prior impregnation of the veneers with a synthetic resin further contributes additional strength.
Not all strength properties are improved (that of izod may be reduced) and of those that are, compressive strength is the only important property which consistently increases very much more rapidly than the density.
Spectacular increases in shear strength, for example, are related more to the resin used and its distribution than to increase in density.
Temperature of pressing has a critical effect on the degree of compression.
While it has long been known in a general way that the plasticity of timber
* For information contained in this chapter, acknowledgements are made to The Australian Timber Journal, March/April 1940, and September 1942 (contributed by Division of Forest Products), D. of F.P. Newsletter, no. 161, and to W. C. Steanes, Densified Wood Pty. Ltd.
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may be increased by raising its temperature, it has recently been shown that for dry timber, of some species at least, this effect is greatly accentuated at temperatures in the vicinity of 350°F., which appear to correspond with the softening point of dry lignin. This material which is one of timber's main constituents is thought to bind together the cellulose fibres and to be largely responsible for the compressive strength of timber.
Material built up from laminations of timber and bonded with synthetic resin glues comes under the heading of "densified timber" since high pressure is always used in bonding, causing a compression of the surface cells of the veneers, and possibly assisting the penetration of the resin into the timber.
The bonding pressure is held very high in order to increase the compression of the timber.
The usual method of bonding laminated stock is by soaking the plies in synthetic resin solutions (with or without special impregnating pressures) and subsequent hot pressing. So high a proportion of synthetic resin may be contained by this material that with thin plies it may perhaps more fittingly be regarded as resin reinforced with timber.
The advantages of densified laminated timber are many. Local weaknesses of the solid timber are practically eliminated; rapid seasoning can be carried out without deterioration, owing to the thinness of the plies; moisture changes are reduced to a minimum; the resin acts to restrain recovery from compression; mechanical strength, especially in compression and shearing, is greatly increased. Used as a bearing surface it has much greater resistance to surface wear.
Disadvantages could be the presence of unsuspected weaknesses in gluing and the difficulty encountered in subsequent gluing and working the finished material by orthodox methods. The weight of timber is materially increased but is still substantially below that of metal members which it may be called upon to replace (See Table 23).
It is sometimes desired to give laminated timber a more uniform strength in the plane of the laminations. For this purpose the grain of adjacent plies is oriented at different angles, the most usual angle of course being 90 degrees.
As in plywood, this can be done in any kind of laminated stock, including compressed and impregnated stock, though in many cases, especially in aircraft construction, the strength desired in compressed and impregnated laminated stock is in one predominant direction.
In building up the laminations for densified wood the thickness of the veneers is decided by the nature of the material required. The thinner the veneers the greater the uniformity, degree of impregnation, compression and density. The degree of impregnation also depends in some measure on the characteristics of the resin and the manufacturing process.
It is thus necessary that (a) the timber chosen will yield well cut veneers economically, and (b) that these can be impregnated with resin to the degree required for the product under manufacture.
Although improved timber has been made with timbers having an air dry (12 per cent moisture content) density as high as about specific gravity 0-8, timbers between about 0-5 to 0-6 are preferable, with an air dry weight per
TABLE 23
cubic foot between 30 and 40 pounds. Hoop pine has been found to be ideal for this purpose.
A most important field for the use of laminated timber of moderate thick-ness is in aircraft construction, where the good mechanical properties of natural timber are highly desirable, especially if the most serious defect of non-conformity can be removed.
Strengths in shear and compression of densified laminated timber are from 60 to 120 per cent higher than solid timber parallel to the grain and from 100 to 150 per cent higher perpendicular to the grain. The stiffening of the cells and vessels has its chief result in a considerable increase in compressive strength. The great differences between compressive and tensile strength met with in solid timber are nearly eliminated in densified timber.
Strength in impact is somewhat reduced by the addition of the rigid synthetic resin.
Improved timber of high density and high resin content is extremely moisture resistant and prolonged immersion in water causes only negligible absorption or change in volume. With its excellent mechanical properties, it occupies an important place between natural timber and non-ferrous metallic materials. In illustration, some mechanical properties of improved timber may be compared with those of duralumin.
From these figures, it is evident that on a strength/weight basis, densified timber is comparable to duralumin in certain mechanical properties. Also these mechanical properties can be manipulated by varying the veneer timber, the type of resin, the resin content and the density of the materials.
The extent to which individual properties may be improved is shown in Table 24. The figures represent the average of the maximum values
ascer-TABLE 24
MAXIMUM PROPERTIES (PARALLEL TO GRAIN)
Ultimate Tensile Strength—lb./sq. in. 55,000 Modulus of Elasticity—lb./sq. in 5,500,000 Ultimate Compressive Strength—lb./sq. in. 40,000 Modulus of Rupture parallel to laminations—lb./sq. in 45,000
Ultimate shear strength—lb./sq. in.—
Parallel to laminations 6,100 Perpendicular to laminations 9,000 Izod Parallel Laminations—ft. lb 20
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turned for the respective properties as determined by tests of various types of densified timber. The materials were made and tested by the Division of
Forest Products. It should be stressed that these properties do not refer to any one single product.
Densified timber has been made commercially in England, the Continent of Europe, the United States and Australia for several years and has found many uses. An interesting application during the last war was in the manu-facture of the variable pitch wooden airscrews which were made in Australia as well as in England, America and Germany. For this type of airscrew, natural timber is not sufficiently strong to withstand the stresses at the blade root but airscrews with a root section of densified timber were developed successfully for use in certain types of aircraft.
Improved timber has many other uses where good mechanical or electrical properties arc required. The following examples illustrate the diversity of uses for which this material is suitable.
General Uses: Bearings (including windmill bearings), ball and roller bear-ing cages, gear wheels, brake linbear-ings, pulleys, rollers, trolley wheels, press tools and substitutes for certain metallic parts in the munitions industry, carriage, lift and escalator floors, and band-saw guides.
Aircraft Industry: Variable pitch wooden propellers, joysticks, instrument panels, fuselage stringers, spar webs, seaplane float struts, wireless masts, spar booms for main and tail planes and packing blocks.
Ship and Boat Building Industry: Bearings for stern tubes, rowlock blocks, pulley blocks and sheaves.
Textile Industry: Shuttles, bobbins and picking sticks and a substitute for laminated canvas rollers.
Electrical Uses: Instrument mounting panels, terminal boards, mounting blocks for switches, fuses, cut-outs, relays, and so on; switch operating rods and handles, insulating stools, radio and telephone insulators, barriers, core spacers, slot wedges, busbar supports, bushes, brush gear bases, channelling and clips for cables, fish plates for electric railways and generally in high
tension electrical insulation.
The Australian made product should find many other uses for which it is ideally suited and also should provide a useful substitute for some materials often unobtainable or which must be conserved for urgent requirements. It must not, however, be regarded generally as a substitute material since it has definite and highly valuable applications, and is proving superior to many materials previously used for the same purpose.
B I B L I O G R A P H Y
Boas, I. H. Commercial Timbers of Australia, Their Properties and Uses. Australia—
C.S.I.R.O. Melbourne Government Printer. 1947.
Gunn, A. L. "Improved Wood": a survey of the literature available. Australian Timber Journal, 6(3): 143. Mar./Apr. 1940. Sept. 1940.
Lynch, C. "Developments in improved woods in Australia." Timber of Canada, 6(4):
50-51 Dec. 1945.
Meyer, L. H. Plywood, What It Is—What It Does. New York, McGraw-Hill. 1947.
Tamblyn, N. "Australian made improved wood, its properties and uses." Aust. Timber Journal. 8:904. Sept. 1942.
Wood, A. D. and Linn, T. G. Plywoods, Their Developments, Manufacture and Appli-cation. Rev. ed. Edinburgh, W. and A. Johnston. 1950.
Wood, A. D. Plywoods of the World, Edinburgh & London. W. & A. K. Johnston &
G. W. Bacon Ltd. 1963.
"Improved Woods": the manufacture and characteristics of compressed woods, with special reference to "Permali". Wood (London), 12: 10-12. Jan. 1947.
United States. Forest Products Laboratory, Madison. Forest Products Laboratory resin-treated wood (Impreg.) Rev. ed. 1950. Report, 1380 Forest Products Laboratory resin-treated, laminated, compressed wood (Compreg.) Rev. ed. 1951. Report, 1381.
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