IE42223B1 - Wood substitute and preparation thereof - Google Patents

Abstract

1505833 Forming wood substitutes PLASTIC RESEARCH LABORATORIES Ltd 23 Dec 1975 [24 Dec 1974] 55831/74 Heading B5A A substitute for wood comprises a mass of natural organic fibres bonded together by a thermoset resin with at least some of the fibres being aligned from a mass of loose separate fibres, e.g. by carding, before being bonded together by a moulding and curing operation. The fibres have a minimum length of one inch. The fibres exemplified are jute, sisal, waste ropes, bale-twine string, manila, flax, hemp, ramie, straw or bark. The resins may be phenolic or amine resins and the properties of the "wood" may be altered by incorporation of fillers and by the proportion of the fibres which are aligned.

Description

This invention relates to a wood substitute and to a process for its preparation.

The cost of virtually all types of natural woods and timbers, and also such substitutes as plywood and blockboard, has increased considerably of late and they are becoming increasingly difficult to obtain.

The present invention provides for a wood substitute designed to overcome these difficulties and which utilises natural organic fibrous materials in conjunction with synthetic resins. The invention provides a wood substitute comprising a mass of natural organic fibres bonded together by a thermoset resin, at least some of the fibres having been physically aligned before being bonded together and having a minimum length of one inch.

The invention also provides a process for preparing a wood substitute which process comprises physically aligning at least some of the fibres in a mass of natural organic fibres having a minimum length of one inch, applying an uncured thermosetting resin to the at least partially aligned fibres, procuring the resin on the fibres, and moulding and curing the precured material.

The natural organic fibres, sometimes referred to as vegetable fibres, in tho wood substitute should be at least partially aligned before tho resin is applied. The alignment of the fibres provides tho wood substitute with directional strength and grain, both properties of natural woods, and provides the wood substitute with an appearance which can be remarkably similar to that of natural woods and aesthetically pleasing. In general the higher is the degree of alignment the better is the strength of the wood substitute. The wood substitutes prepared in accordance with the invention can be prepared in order to simulate the appearance and physical properties of a wide variety of natural woods, ranging from high strength hard woods to low strength soft woods, while at the same time providing alternatives to such commonly available products as plywood. Indeed wood substitutes can b® provided at a reasonable cost to have physical properties matching or surpassing the physical properties of many common hard woods.

The fibres' used are natural organic materials which are commonly available either as natural raw materials or wastes. It is especially preferred to use jute or sisal in waste or natural form, either by themselves or in combination. Jute cuttings or card waste and sisal waste are readily available, low cost, materials which are commonly available in bale form. Other organic fibrous materials which may be used include waste ropes, bale-twine, string, manila, flax, hemp, ramie, straw Or bark. These materials are readily available in substantial quantities and provide wood substitutes having properties, including appearance, which simulate those of natural wood.

The fibres used to prepare the wood substitute can also comprise a minor proportion of inorganic fibres, for example glass or mineral wool, glass fibre wastes and asbestos, the natural organic fibres predominating in order to impart the necessary woodlikc character to the wood substitute.

The fibres can be aligned by methods known in the textile industries, for example by carding or combing. Electrostatic methods of alignment could also be used.

The at least partially aligned fibres preferably are chemically sized before the uncured resin is applied thereto. Suitable sizing agents are starch, natural or synthetic waxes, or synthetic sizing materials such as cationic ketene dimer emulsions. The sizing of the fibres serves to seal or size them before the uncured resin is applied and prevents excessive resin pick-up and improves the wood-like proper ties of the final product.

The thermosetting resin used to prepare the wood substitute preferably is a phenolic resin, that is a condensation product of a phenolic compound and an aldehyde, for example a phenolformaldehyde resin, cresol-formaldehyde resin or resorcinol-formaldehyde resin, or an amine resin, that is a condensation product of an amine and an aldehyde, such as urea-formaldehyde resin or a melamine-formaldehyde resin. Thermosetting resins containing a natural resixi ingredient such as lignin could also be used. When using thermosetting resins which undergo successive stages of cure, such as phenolic resins it is preferred to - 4 42323 apply the synthetic resin as a Stage A resin to the aligned organic fibres, to pecure the resin to a Stage B resin after first removing as much of the Stage A resin as does not readily remain on the surfaces of the aligned fibres, and to cure the resin to a final Stage C resin under conditions of elevated temperature and pressure during the moulding operation. The procuring stage serves to attach the resin firmly to the aligned fibres and to avoid undue loss during the moulding operation.

The preparation of a wood substitute in accordance with the invention can be on a continuous or batch basis. If a continuous method is employed then long lengths, or planks, of substitute wood can be produced using a continuous pressing operation such as is described in British Specification No. 27642.

A preferred process of the invention for preparing a wood substitute will now be described in more detail: A mass of the fibres, for example a bale, is pretreated, suitably by teasing, in order to loosen or separate the fibres. If the fibres are available only in long lengths then they can be chopped to a desired length which should not be below 1 inch, preferably not below 2 inches.

Desirably the fibres are from 9 to 18 inches long since then they impart good directional properties to the wood substitute and can readily be aligned The pretreated fibres preferably then are subjected 42333 to a carding operation, for example using a rough carding machine, in order to achieve the desired degree of alignment. When it is desired to obtain a higher strength substitute wood, simulating a hard wood, the organic fibres suitably are formed into slivers in which approximately 70 to 75 percent of the fibres are aligned in one direction. When the intention is to provide a lower strength substitute wood, simulating a soft wood, then the degree of alignment need not be as high and can be as low as say 50 percent. It will bo appreciated that different grades of wood substitutes can readily be prepared, from the same starting materials in accordance with the invention merely by using different arrangements of the fibres. Thus a high grade substitute plywood, having equal strengths in two directions at right angles to each other, can be prepared by laminating superimposed layers comprising fully aligned fibres in such a way that the directions of alignment are at right angles to each other in successive layers. A laminate in which the fibres in successive layers are only partially aligned provides a medium grade plywood substitute. A product comprising a core of partially aligned fibres and a surface layer of fully aligned fibres provides an acceptable softwood substitute, while a product in which substantially all of the fibres are aligned can provide a hardwood substitute. - 6 42223 Tho fibres having the desired degree of alignment preferably then are sized and dried to .fasten the sizing agent and remove excess water or solvent. The drying temperature should not exceed the temperature at which degradation of the fibres occurs. In general the drying temperature should be below 140°C, preferably below 120°C.

The sized fibres are then treated with the thermosetting resin, preferably a Stage A thermosetting resin, by any suitable method, such as impregnation in a bath, spraying, vacuum impregnation or by roller coating or other standard techniques used in thermosetting resin technology. The resin suitably is applied in the form of a solution in a suitable solvent, for example as a solution containing 20% by weight resin solids and 80% by weight ethanol. Surplus resin is removed, as by nip squeezing, and any excess resin can then be recycled. The resin on the fibres should then be dried and precured, in tne case of thermosetting resin to a Stage B resin, in order to attach the resin firmly to the fibres and to avoid undue loss during the subsequent moulding operation.

Precuring can be brought about by allowing the resin coated fibres to air dry in a warm ambient temperature to allow solvent to evaporate and leave the resin in a tacky state. At this stage the moisture content of the matrix is important, since if it is too high the matrix will shrink unduly upon moulding at elevated temperatures and pressures. If necessary the moisture content can he reduced by further drying. The precured material can, if necessary, then be cut into desired lengtls for layering into a mould. The amount of precured material introduced into the mould will clearly be such as to provide a moulded product or wood substitute having the required dimensions and density. The precured material introduced into the mould is pressed and finally cured, in the case of the thermosetting resin to a Stage C resin, the conditions of the moulding operation being suited to the particular synthetic resin being employed but usually involving pressures below 250 p.s.i. and temperatures below 135°C. The product of the moulding operation can then be trimmed to size, as by sawing, should this be necessary.

Various additives can bo incorporated in the wood substitute in order to modify its properties.

Thus the wood substitute can be softened somewhat by the addition of such additives as polyvinyl acetate, rubbers and leather scrap. These additives can be incorporated in a resin formulation or introduced separately at the stage of applying the resin to the aligned fibres.

Embossed or moulded finishes can be obtained using appropriately designed mould tools, while flat substitute woods can be laminated with natural veneers or artificial surface layers to provide decorative laminates. Thus the resin content of the wood substitute prepared in accordance with the invention is such that surface films or sheets, such as of kraft paper impregnated with a thermosetting resin such as a phenolic resin, will laminate satisfactorily to the wood substitute.

When applying such a surface film the matrix of aligned fibres and precured resin should desirably be pressed between stainless steel platens, the surfaces of the cured wood substitute sanded and the surface film laminated under suitable conditions, for example not more than 250 p.s.i. for at least 10 minutes at 135°C and then cooled below 90°C before ejection from the press and trimming.

The thickness of the moulded wood substitute can vary considerably, say from 1/16 inch up to 2 inches, but it is preferred to mould in thicknesses up to 1.5 inches. The surfaces of the resulting substitute wood can be sanded and treated in the usual manner for natural woods with stains or polishes to achieve a high quality finish. The wood substitutes prepared in accordance with the invention tend to be harder than natural woods but can be machined, sawn and drilled as with natural wood provided the tips of the working tools are hardened. If nailing and screwing qualities arc required then tho resin matrix can bo softened by the addition of materials such as polyvinyl acetate and rubbers. _ 9 _ Other mechanical operations which can be adopted during the npwrsg., pf—producing the wood substitute include imparting a twist to a sliver of aligned fibres to provide additional strength, and ' also the formation of separate layers of precured material by a preliminary pressing operation, which layers are used to build up a laminate of several such layers in which the directions of alignment of successive layers can be different, and preferably are at right angles to each other, the laminate subsequently being cured and pressed, to provide a product similar to a plywood.

Wood substitutes prepared in accordance with the invention have a high modulus of rupture and possess an external appearance closely similar to that of natural woods. The physical properties of the wood substitutes are determined to a very large extent by the physical properties of the natural organic fibres used in their preparation and by the extent to which the fibres are aligned. The resin contributes relatively little to the properties of the wood substitute and, in this connection, it should be noted that the wood substitute comprises only a minor proportion, usually 5 to 30% by weight, preferably 15 to 25% by weight, of the resin. The wood substitutes are thus quite unlike reinforced resin structures in which the resin is usually present in a major proportion, sometimes providing as much as 75% by weight of the reinforced rosin structure. In the wood substitutes prepared in accordance with the invention the ffisin, in the uncured state initially coats the surface of the fibres, the extent to which the resin impregnates the fibres being reduced by sizing, and the individual fibres are then bonded together by the curing of the resin. The resin is thus bonding the fibres together rather than the fibres reinforcing the resin.

The conditions of the moulding and curing step .in fhe process of the invention will generally need to be such as to provide a cured product having properties rendering it suitable for use as wood or timber substitute. Typical such properties for a substitute softwood are: a density of at most 55 lbs/cu.ft., preferably at most 50 lbs/cu.ft; and a modulus of rupture of at most 11000 p.s.i and for a substitute hardwood are: a density of 55 to 75 lbs/cu.ft. and a modulus of rupture of 20,000 to ,000 p.s.i. Typical modulus of rupture values for a soft wood such as deal or larch are from 10,000 to 14,000 p.s.i. Similarly a keruing hardwood typically has a modulus of rupture of about 18,000 p.s.i. while,a substitute wood prepared from jute and having a density of 60 pounds per cubic foot can have a modulus of rupture of 26,700 p,s.i. and a substitute wood prepared from string having a density of 72 pounds per cubic foot can have a modulus of /rupture of from 20,000 to 30,000 p.s.i.

The following Table provides a comparison between the properties of keruing and wood substitutesin accordance with the invention: Material Density (lbs/cu.ft) Modulus Flexural of ruptureModulus (p.s.i.) (106psi) Water absorption (% by weight in 48 hours) Keruing 55.57 17,500 2.0 6.9 (A) Wood substitute 55.4 19,800 - 4.5 (B) Wood substitute 60 26,700 4.5 2.0 (The modulus of rupture values given in this specification were obtained by a three point bend test carried out on specimens measuring 300mm x 20mm x 20mm. The specimens were supported over a 280mm span in trunnions carried on roller bearings. R load was applied midway along the span at a constant speed. The load deflection was recorded automatically until a specimen failed to support one tenth of the maximum recorded load or deflected by more than 60mm, whichever occurred first).

The wood substitutes of the invention are expected to find utility in a wide variety of applications, for example as cladding, decking, flooring, panelling, beams and load bearing members, door and window frames, and mouldings,in such industries as the building industry, furniture manufacture, motor vehicle, ship and aircraft industries, and the packaging industry.

The wood substitutes of the invention tend to swell and deform less than natural woods and. can therefore be used in many applications in which dimensional stability is highly desirable, e.g, as concrete shuttering and in other building applications. They also have very good resistance to sea water and could ic-placi. valuable hardwoods, such as oak, in marine applications, e.g, as a replacement for groynes. When a phenolic resin is used the wood substitute of the invention is resistant to attack by, for example, fungus, wood worm, insects or termites. This provides a further advantage over natural timbers which are particularly prone to mould growths and wood worm growth.

The following Examples serve to illustrate the inventions ' EXAMPLE 1 A quantity of carded jute was immersed in a resin bath containing 2 parts CL 151/76 phenolic resin to 1 part methylated spirits by inLurne. A quantity of waste sisal string cut into hanks was separately immersed in tho same resin bath. The jute and sisal ware then passed through a pair of steel nip rollers to squeeze out surplus resin and then placed on expanded metal gauze between press plattens to precure the phenolic resin to the 'B* stage. The squeezed jute and sisal string were approximately twice as heavy as the dry materials. A steel mould having inside measurements of 19 inches x '18 inches was then charged as follows: (1). 1 pound of 19 inch lengths of jute and precured resin was layered onto the bottom of the mould. (2). 11 pounds of .19 inch lengths of sisal string and precured resin were then hand layered as evenly as possible on the jute layer to form the core of the potential sample of substitute wood. (3) A further 1 pound of the 19 inch lengths of jute and precured resin was layered on top of the string core.

The mould was then closed to stops to give approximately a 1 inch thick moulding and the sample was heated under pressure for 7/0 minutes at 160°C.

A quantity of resin exuded from the- mould during pressing and cured on the edges and outside of the mould which suggests that insufficient resin was removed by squeezing. However, a tough, woodlike board was ejected from the mould measuring 18 inches x 19 inches and being an average of 0.342 inches thick and weighing exactly 10 pounds.

This represents a density of 59 pounds/ft^.

EXAMPLE 2 When Example 1 was repeated but the mould stops were moved to give a thinner sample, there was obtained a woodlike board having an average thickness of 0.428 inches, representing a density of 71.4 pounds/ft^. - 14 4 2323 EXAMPLE 3 A quantity of carded jute was impregnated with resin, squeezed and the resin precured as in Example 1. A layer of 19 inch lengths of the jute and precured resin was placed in the bottom of the mould.

A quantity of jute card waste was passed through Masson cutters (1 inch screen) to tease and flocculate it as was a quantity of sisal string. 3 pounds of the teased jute and 3 pounds of the chopped string were taken and passed through the Masson cutters once more but this time in equal handfuls together to mix the jute and string into a composite chopped mixture.

The 6 pounds of mixed jute and string was then charged into a small Gardner ribbon mixer and to the mixture was added 500 gms. of CS 52 powdered phenolic resin into which 60 grams of Hexemine had previously been incorporated in a mortar and pestle.

A layer of the mixture of chopped string, chopped jute and powdered resin was charged to the mould on top of the layer of jute and precured resin. A further layer of jute and precured resin was then placed on top to form the top surface. The mould was then closed to 1 1/2 inch stops to give a potential sample thickness of 1 inch and pressed for 25 minutes at 160°C.

The resulting board was a tough, woodlike - 15 42223 material 18 inches x 19 inches with an average thickness of 0.973 inches.

The board weighed 9.5 pounds and had a 3 density of 49.8 pounds/ft .

EXAMPLE 4 After being carded, quantities of fully aligned jute fibres and partially aligned jute cutting fibres of length 6 to 12 were sized in a 10% solution of Aquapel 3 (which is a cationic ketene dimer emulsion from the Hercules Powder Co.

Ltd). and fully dried at a temperature not exceeding 140°C. 7 lbs of partially aligned sised jute and 1.25 lbs of fully aligned sized jute were coated with a 20% solution of phenol-formaldehyde resin (grade CL 151/76 from Sterling Moulding Materials Ltd), the excess resin being squeezed out by nip rollers, and the fibres allowed to dry in a warm ambient temperature of 30°C to allow the solvent (ethanol) to evaporate and leave the uncured resin in a tacky state. Tho fully aligned fibres were divided in two, half being placed in the bottom of a metal mould, followed by the whole of the partially aligned fibres and finally the second half of the fully aligned fibres being placed in the same direction as the first half. This sandwich was then pressed for 30 minutes at 135°C and 250 p.s.i. pressure.

The resulting board had a thickness of 0.5 inch, a density of 53 lbs/ft and a modulus of - 16 42223 rupture of 11,000 lbs p.s.i.

EXAMPLE 5 Fully aligned carded jute cuttings were sized in a 3.5% solution of starch (grade Jalan B37 ex Laing-National Ltd) and fully dried at a temperature not exceeding 140°C. gms of the sized fibres were coated in a 50% solution of phenol-formaldehyde resin (grade CL 151/76 ex Sterling Moulding Materials), the excess being squeezed out by nip rollers, and the fibres allowed to dry in a warm ambient temperature of 30°C to remove the solvent and leave the uncured resin in a tacky state. The fully aligned fibres were then superimposed in a 4 x 3 metal mould and pressed for 20 minutes at 150°C and 500 lbs. p.s.i. pressure.

The resulting board had a thickness of 0.25 inch, a density of 60 lbs/cu.ft and a modulus of rupture of 26,700 lbs. p.s.i.

Claims (12)

1. A wood substitute comprising a mass of natural organic fibres bonded together by a thermaset resin, at least some of the fibres having been physically aligned before being bonded together and having a minimum length of one inch.

2. A wood substitute according to claim 1 wherein the fibres have a minimum length of 2 inches.

3. A wood substitute according to claim 1 or 2 wherein the fibres are of sisal or jute.

4. A wood substitute according to any one of the preceding claims wherein the thermoset resin is a phenolic or amine resin.

5. A wood substitute according to claim 1 substantially as described in any one of the Examples.

6. A process for preparing a wood substitute which process comprises physically aligning at least some of the fibres in a mass of natural organic fibres having a minimum length of one inch, applying an uncured thermosetting resin to the at least partially aligned fibres, precuring the resin on the fibres, and moulding and curing the precured material.

7. A process according to claim 5 wherein the fibres are physically aligned by carding.

8. A process according to claim 6 or 7 wherein the at least partially aligned fibres are chemically sized before the uncured resin is applied thereto.

9. A process according to any one of claims 6 to 8 wherein the fibres have a minimum length of 2 inches. - 18 42223

10. A process according to any one of claims 6 to 9 wherein the thermosetting resin is a phenolic or amine resin.

11. A process according to claim 10 wherein the resin is applied to the at least partially aligned fibres as a Stage A resin, excess resin is removed from the fibres by pressing, the resin is precured to a Stage B resin, and the resin is cured to a final Stage C resin under conditions of elevated temperature and pressure during the moulding operation.

12. A process according to claim 6 substantially as described in any one of the Examples.