GB1584387A - Wood substitute - Google Patents

Description

(54) WOOD SUBSTITUTES (71) We, WILKINSON SWORD LIMITED, a British Company, of Sword House, High Wycombe, Buckinghamshire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to wood substitutes.

Various types of wood substitute are known, e.g. hardboard and chipboard, comprising cellulosic filler fibres or particles, e.g. sawdust, wood chips, straw, bagasse, paper, cardboard and many other types of naturally occurring, or synthetic or semi-synthetic fibrous cellulosic material which would otherwise be regarded as a waste product, in a binder, often a phenolic or aminoplast resin.

In according with the present invention we provide a method of forming a wood substitute of high strength and toughness, of good formability whether by a moulding operation, including both extrusion and calendering, or by a cutting or planing operation, and at an extremely low cost.

In the method of this invention a wood substitute is formed by dispersing into an aqueous binder, in an amount sufficient to provide a fibre content in the final wood substitute of from 3080% on a dry solids basis, a mixture consisting of hydrophilic cellulosic fibres and hydrophobic fibres in major and minor proportions respectively, said binder comprising a) water, b) a mixture consisting of, on a dry weight basis, from 3095% of granular starch and from 570% of a hydrophilic polymer, and c) a water-soluble or dispersible cross-linking agent capable of chemically cross-linking said cellulosic fibres, and/or chemically bonding them to the starch component of the binder, component c) being present in an amount up to 60/, by weight, based on the dry weight of said fibres and said mixture of starch and hydrophilic polymer, shaping the dispersion so formed so as to orientate the fibres within the binder along a common axis, and thereafter drying the shaped dispersion.

Preferably the product wood substitute will contain, on a dry solids basis, from 50--70 by weight of said blend of hydrophilic and hydrophobic fibres, the balance of the substitute being made up of the starch and hydrophilic polymer components of the binder together with the cross-linking agent and the optional ingredients, as hereinafter discussed, which may be incorporated into the wood substitute for specific purposes. Generally speaking also, the wood substitutes made in accordance with this invention will contain some water due to the hydrophilic nature of the fibre reinforcement and the binder, this water resulting partly from the process of manufacture and incomplete drying of the product, but also from uptake of moisture from the atmosphere.The actual water content of the wood substitute will therefore depend to a large extent on the relative humidity and temperature of the surrounding atmosphere, but will usually be in the range 2 to 14% by weight of the total composition at relative humidities and temperatures in the ranges 30-60% and 15-250C respectively.

The hydrophilic fibres used in compositions of this invention may be any natural, synthetic or semi-synthetic cellulosic fibre, such as wood pulp, cotton, rayon, jute, sisal, flax, hemp, abaca (manila hemp) etc. By reason of low cost, preferred are waste cellulosic materials such as paper, board, newsprint, straw, sawdust, bagasse and cotton linters and also waste jute, sisal, hemp or flax.

The hydrophobic fibres, which impart toughness to the wood substitute, may be any natural or synthetic, organic or inorganic, fibrous material having sufficient hydrophobicity not to enter into any significant chemical bonding reaction with the binder components. Thus naturally occurring cellulosic fibres such as sisal or jute may be used which retain a significant proportion of the original lignin content to render their surfaces hydrophobic. Alternatively, synthetic hydrophobic fibres may be used e.g. polyesters, polyolefins and nylon and other polyamides.

For maximum strength and toughness, the fibres should be as long as possible and in the final product the length of the fibres will be governed by practical considerations rather than anything else. Thus, in the manufacture of the wood substitutes of this invention it is essential that, before shaping and drying, the fibres should be thoroughly mixed with the binder materials under conditions of sufficiently high shear, e.g. in a triple roll mill, in order to obtain maximum wetting of the cellulosic fibres by the binder materials, and these conditions will inevitably result in chopping of the fibres into shorter lengths.In the case of the hydrophobic fibres, which in any case do not undergo any substantial chemical bonding with the binder components, intimate contact with the binder materials is less important, and so chopping of the hydrophobic fibres during mixing can be kept to a minimum by adding the hydrophobic fibres towards the end of the mixing process consistent with obtaining a substantially uniform distribution thereof throughout the binder.

Generally speaking, the cellulosic fibres in the final product should have a minimum length of 0.1 mm, preferably greater than 1.0 mm, and an aspect ratio (length to diameter) minimum of at least 10, preferably greater than 200.

To give toughness to the final product, the hydrophobic fibres should be as long as possible and generally should have a minimum length of 0.5 mm, preferably greater than 5 mm. The diameters of these fibres need to be as small as is practicable, typically 1--100 y in diameter.

The major portion, i.e. above 50 /n by weight, of the fibre blend used in this invention should be constituted by the hydrophilic, cellulosic fibres and the minor position by the hydrophobic fibres. The preferred fibre blends contain from 90- 99.9% by weight hydrophilic fibre and 10.1 by weight hydrophobic fibre.

The solid granular starch component of the binder may be derived from a variety of sources, e.g. cereals such as wheat, maize, barley, rye, sorghum, millets, oats and rice, and root crops such as potato, tapioca and sweet potato.

A variety of hydrophilic polymers of natural or synthetic origin may be used as the hydrophilic polymer component of the binder. Preferred are naturally occurring hydrophilic polymers which may be proteinaceous materials of vegetable origin e.g. gluten or zein, or of animal origin, e.g. casein, or starch derived materials e.g. soluble starch, or solubilised starch derivatives such as dextrin and dialdehyde starch. Whilst water solubility of the hydrophilic polymer is not necessary good dispersibility in water is essential. Combinations of two or more different types of starch may be used.

The hydrophilic polymer may, however, be derived from the granular starch by mechanical means, either prior to or during the mixing of the fibres into the binder. Thus, for example, if maize starch is the only added binder component, then if sufficient mechanical energy is imparted to the dispersion during the mixing stage, enough damaged starch may be formed to act in the same manner as if soluble starch had been added initially. This method may be preferred on cost grounds.

As synthetic and semi-synthetic hydrophilic polymers there may be used organic polymers such as poly(vinyl alcohol), poly(acrylic acid), carboxymethylcellulose and hydroxyethylcellulose, and inorganic hydrophilic polymers such as the sodium silicates.

The granular starch and hydrophilic polymer components of the binder will generally be used in amounts of 30 to 95 /n by weight and from 5-70% by weight respectively, preferably from 70 to 95% and from 5 to 30% respectively.

Where a cereal or root crop flour, e.g. wheat flour or tapioca, is used as the source of the granular starch, this may contain sufficient hydrophilic polymer, e.g.

gluten in the case of wheat flour, to avoid the necessity for any additional hydrophilic polymer. Also, up to 50 /n by weight of cereal flour by-products may be incorporated in the binder in order to reduce the cost. A preferred source of the granular starch component is biscuit flour. Another preferred source of granular starch, by reason of cheapness, is waste or animal feed grade tapioca.

Other typical binder combinations for use in the present invention are: wheat starch/gluten (9:1 wt ratio) maize starch/soluble starch (4:1 wt ratio) potato starch/soluble starch (7:1 wt ratio) maize starch/dextrin (4:1 wt ratio) potato starch/dextrin (7:1 wt ratio) maize starch/Na > SiO3 (4:1 wt ratio) potato starch/Na2SiO3 (9:1 wt ratio) The cross-linking agent used in preparing wood substitutes in accordance with this invention to increase the strength of the composition at high humidities, may be any water-soluble or dispersible compound capable of reacting chemically with the cellulosic fibres to cross-link them together and/or to bond them chemically to the starch component of the matrix.Suitable cross-linking agents are water-soluble aldehydes such as glutaraldehyde, acetaldehyde, formaldehyde, paraformaldehyde and amine aldehyde condensates such as melamine formaldehyde and urea formaldehyde. The preferred cross-linking agents are formaldehyde and paraformaldehyde. The cross-linking agents may be used in amounts up to 6% by weight (dry solids basis). Higher amounts are unnecessary and uneconomic.

Amounts in the range 1--3% by weight are preferred. In addition, a small amount of an acid or acid salt e.g. aluminium sulphate or ferric sulphate may be added to catalyse the reaction of the aldehyde cross-linking agent with the cellulosic fibres and/or the starch.

In addition to the essential ingredients listed above the wood substitutes prepared in accordance with this invention may also contain: 1. Fire retardants; e.g. ammonium sulphate or ammonium dihydrogen phosphate, up to 20% by weight dry solids basis, and 2. Fungicides; e.g. sorbic acid, up to 1% by weight dry solids basis.

As already indicated, in the manufacture of the wood substitutes- in accordance with this invention, it is essential that the fibrous reinforcement and the matrix materials be thoroughly milled together under conditions of high shear to obtain maximum wetting of the cellulosic fibres by the matrix materials before shaping the composition to obtain orientation of the fibres. Blending may be done on any conventional blender able to provide mixing of the components under conditions of sufficiently high shear. Based on the weight of the solids, from 30- 100% by weight of water, preferably 40 to 80% will be added to the mix in the blender.

In the blender, the fibre reinforcement, the matrix materials, the water, and any optional minor ingredients may be mixed in any order, although as indicated, it is preferred to add the hydrophobic fibres towards the end of the mixing operation to minimise break-up and chopping of the fibres into short lengths.

Following blending the mixture may then be shaped by any suitable means, e.g. by extrusion, calendering or injection moulding, which will obtain a degree of orientation of the fibres in the matrix, after which the shaped composition is dried.

Dyring may be accomplished by any suitable means e.g. air drying, oven drying, infra-red heating, dielectric heating or microwave heating. Although, with some cross-linking agents, e.g. formaldehyde and paraformaldehyde, drying at elevated temperatures, preferably above 100"C, is necessary to achieve adequate reaction.

A particular advantage of the wood substitutes of this invention is their formability by extrusion into a variety of shapes and sizes e.g. sheets, strips, rods, tubes and the like, with good conformation to the die shape and good uniformity in the crosssectional shape of the extrudate.

The wood substitutes made by the method of this invention may be used in a variety of applications where their strength, toughness, good formability, particularly by extrusion, and above all low cost are advantageous. Applications which are envisaged include acoustic panelling, pencil casings, strip mouldings, match splints, low cost packaging and crating materials and thermal insulation.

Typical wood substitute compositions made according to this invention and the manufacture thereof are illustrated in the following examples, all percentages are by weight. In each of Examples 1, 2 and 4, the biscuit flour and waste tapioca is such as to provide a mixture of granular starch and hydrophilic polymer in accordance with the requirements of this invention.

EXAMPLE 1 A composite of the following composition was prepared:-- biscuit flour (37.2 by weight of solids), waste newspaper (57.2% nylon fibre (1 cm long 50 ,um diameter, 1%), sorbic acid (0.3%), glutaraldehyde (3.8%), Al2(SO4)16 H2O (0.5%). Water was added at the proportion of 57 g per 100 g of solids. The mixture was mixed on a laboratory triple roll mill for 10 minutes, with the nylon fibres being added during the last stages of mixing and then extruded by hand through a circular die of 2.1 mm diameter. The extrudate was baked in an oven at 1300C for 2 hours.

After equilibration for 3 days to laboratory ambient it was found that the density of the material was 0.88 gcm-3 and that the strength and modulus (measured in a conventional three point bend test with a span of 28.6 mm, a crosshead speed of 5 mm/min and with an indentor radius of curvature of 4 mm) were 93+16 and 6560+1660 MNm-2 respectively.

EXAMPLE 2 A composite was made up as in Example 1 above from low grade biscuit flour (33.8%), waste newspaper (62.0%), nylon fibre (1.0%), paraformaldehyde (1.9%), Fe(NO3)39 H2O (1.0%), with 58 g of water added per 100 g solids. After mixing the dough was extruded through a 2.2 mm square section die and baked in an oven for 2 hours at 1500C.

The material had a strength of 56*9 MNm-2, a modulus of 4070*1170 MNm-2 and a density of 0.85 gcm-3.

EXAMPLE 3 A composite was prepared from maize starch (39.2%) soluble starch (9.8%), polyethylene terephthalate fibres (1 cm long, 670 elm diameter, 5%), ground wood pulp (43.9%), melamine/formaldehyde laminating resin (2.0%), All36 H2O (0.1%), with 60 g of water being added per 100 g of solids. After extrusion through a 2.1 mm diameter round die, the extrudate was allowed to dry slowly at 200C. The material had a density of 1.14 gcm-3 and a strength and modulus of 118*9 and 5960+800 MNm-2 respectively.

EXAMPLE 4 A composite was prepared from ground waste tapioca (46%), mixed waste paper (50%), nylon fibre (1%), paraformaldehyde (2%), Al2(SO4)316 H2O (2%) with 60 g of water being added per 100 g of solids. After mixing, extrusion was through a 2.2 mm square die and the extrudate was heated to 1600C for 2 hours. The material had a density of 0.88 gcm-3 and a strength of 60*9 MNm-2.

EXAMPLE 5 A composite was prepared as in Example 4 above, but substituting waste newspaper for the waste mixed paper. This resulted in material of density 0.8 gcm-3 and a strength of 63+11 MNm-2.

EXAMPLE 6 A composite was prepared from wheat starch (38.3%), gluten (9.6%), polypropylene fibres (1 cm long, 60--70 Mm diameter, 2%), sisal (2.3 cm lengths, 22%), straw (24.0%), urea/formaldehyde resin (3.8%), A1C136 H2O (0.10/,), sorbic acid (0.2%, with 60 g of water being added per 100 g of solids. After mixing, extrusion was through a 2.1 mm diameter round die, and the extrudate was dried at 600C for I hour. This resulted in material of density 0.85 gcm-3 and with strength and modulus of 57*9 and 2930+460 MNm-2 respectively.

WHAT WE CLAIM IS: 1. A method of forming a wood substitute, which comprises dispersing into an aqueous binder, in an amount sufficient to provide a fibre content in the final wood substitute of from 30--800/, on a dry solids basis, a mixture consisting of hydrophilic cellulosic fibres and hydrophobic fibres in major and minor proportions respectively, said binder comprising a) water, b) a mixture consisting of, on a dry weight basis, from 3-95 of granular starch and from 5-70% of a hydrophilic polymer, and c) a water-soluble or dispersible cross-linking agent capable of chemically cross-linking said cellulosic fibres, and/or chemically bonding them to the starch component of the binder, component c) being present in an amount up to 6% by weight, based on the dry weight of said fibres and said mixture of starch and hydrophilic polymer, shaping the dispersion so formed so as to orientate the fibres within the binder along a common axis, and thereafter drying the shaped dispersion.

2. A method according to Claim 1, wherein the cross-linking agent is used in an

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (16)

**WARNING** start of CLMS field may overlap end of DESC **. 1%), sorbic acid (0.3%), glutaraldehyde (3.8%), Al2(SO4)16 H2O (0.5%). Water was added at the proportion of 57 g per 100 g of solids. The mixture was mixed on a laboratory triple roll mill for 10 minutes, with the nylon fibres being added during the last stages of mixing and then extruded by hand through a circular die of 2.1 mm diameter. The extrudate was baked in an oven at 1300C for 2 hours. After equilibration for 3 days to laboratory ambient it was found that the density of the material was 0.88 gcm-3 and that the strength and modulus (measured in a conventional three point bend test with a span of 28.6 mm, a crosshead speed of 5 mm/min and with an indentor radius of curvature of 4 mm) were 93+16 and 6560+1660 MNm-2 respectively. EXAMPLE 2 A composite was made up as in Example 1 above from low grade biscuit flour (33.8%), waste newspaper (62.0%), nylon fibre (1.0%), paraformaldehyde (1.9%), Fe(NO3)39 H2O (1.0%), with 58 g of water added per 100 g solids. After mixing the dough was extruded through a 2.2 mm square section die and baked in an oven for 2 hours at 1500C. The material had a strength of 56*9 MNm-2, a modulus of 4070*1170 MNm-2 and a density of 0.85 gcm-3. EXAMPLE 3 A composite was prepared from maize starch (39.2%) soluble starch (9.8%), polyethylene terephthalate fibres (1 cm long, 670 elm diameter, 5%), ground wood pulp (43.9%), melamine/formaldehyde laminating resin (2.0%), All36 H2O (0.1%), with 60 g of water being added per 100 g of solids. After extrusion through a 2.1 mm diameter round die, the extrudate was allowed to dry slowly at 200C. The material had a density of 1.14 gcm-3 and a strength and modulus of 118*9 and 5960+800 MNm-2 respectively. EXAMPLE 4 A composite was prepared from ground waste tapioca (46%), mixed waste paper (50%), nylon fibre (1%), paraformaldehyde (2%), Al2(SO4)316 H2O (2%) with 60 g of water being added per 100 g of solids. After mixing, extrusion was through a 2.2 mm square die and the extrudate was heated to 1600C for 2 hours. The material had a density of 0.88 gcm-3 and a strength of 60*9 MNm-2. EXAMPLE 5 A composite was prepared as in Example 4 above, but substituting waste newspaper for the waste mixed paper. This resulted in material of density 0.8 gcm-3 and a strength of 63+11 MNm-2. EXAMPLE 6 A composite was prepared from wheat starch (38.3%), gluten (9.6%), polypropylene fibres (1 cm long, 60--70 Mm diameter, 2%), sisal (2.3 cm lengths, 22%), straw (24.0%), urea/formaldehyde resin (3.8%), A1C136 H2O (0.10/,), sorbic acid (0.2%, with 60 g of water being added per 100 g of solids. After mixing, extrusion was through a 2.1 mm diameter round die, and the extrudate was dried at 600C for I hour. This resulted in material of density 0.85 gcm-3 and with strength and modulus of 57*9 and 2930+460 MNm-2 respectively. WHAT WE CLAIM IS:

1. A method of forming a wood substitute, which comprises dispersing into an aqueous binder, in an amount sufficient to provide a fibre content in the final wood substitute of from 30--800/, on a dry solids basis, a mixture consisting of hydrophilic cellulosic fibres and hydrophobic fibres in major and minor proportions respectively, said binder comprising a) water, b) a mixture consisting of, on a dry weight basis, from 3-95 of granular starch and from 5-70% of a hydrophilic polymer, and c) a water-soluble or dispersible cross-linking agent capable of chemically cross-linking said cellulosic fibres, and/or chemically bonding them to the starch component of the binder, component c) being present in an amount up to 6% by weight, based on the dry weight of said fibres and said mixture of starch and hydrophilic polymer, shaping the dispersion so formed so as to orientate the fibres within the binder along a common axis, and thereafter drying the shaped dispersion.

2. A method according to Claim 1, wherein the cross-linking agent is used in an

amount of from 1--3 by weight, based on the dry solids weight of the fibres, the granular starch and the hydrophilic polymer.

3. A method according to Claim 1 or 2, wherein the cross-linking agent used is a water-soluble or dispersible aldehyde or a water-soluble or dispersible melamine formaldehyde or urea formaldehyde condensate.

4. A method according to Claim 3, wherein the cross-linking agent used is formaldehyde or paraformaldehyde.

5. A method according to any one of Claims 1 to 4, wherein the fibre blend constitutes from 4060% of the product wood substitute (dry solids basis).

6. A method according to any one of Claims 1 to 5, wherein the fibre blend used contains, on a weight basis, from 9099.9% hydrophilic fibres and from 0.110% of hydrophobic fibres.

7. A method according to any one of Claims 1 to 6, wherein the hydrophilic fibres are of chopped waste paper, board, newsprint, straw, sawdust, bagasse, cotton, jute, sisal, hemp or flax.

8. A method according to any one of Claims 1 to 7, wherein the hydrophobic fibres are polyester, polyamide or polyolefin fibres.

9. A method according to any one of Claims 1 to 8, wherein the hydrophilic fibres are at least I mm in length and have a length:diameter ratio of at least 200:1.

10. A method according to any one of the preceding claims, wherein the hydrophobic fibres have a diameter of from l-l00 and a length of at least 5 mm.

11. A method according to any one of the preceding claims, wherein the granular starch content of the binder is from 795% based on the combined weights of the granular starch and the hydrophilic polymer.

12. A method according to any one of the preceding claims, wherein the hydrophilic polymer binder is of natural origin.

13. A method according to Claim 12, wherein the hydrophilic polymer binder is gluten, zein, casein, soluble starch, dextrin or dialdehyde starch.

14. A method according to any one of Claims 1--11, wherein the granular starch and the hydrophilic polymer components of the binder are both derived wholly or in part from a cereal flour or from a root crop flour.

15. A wood substitute when made by a method claimed in any one of the preceding claims.

16. A match splint formed from a wood substitute as claimed in Claim 15.