A METHOD FOR THE PRODUCTION OF WOOD-POLYMER COMPOSITES

Abstract: A method for the production of wood-polymer composites by impregnating solid wood, or a material on the basis of wood, with a polymerisable monomer by the use of vacuum and/or pressure, and immersing the impregnated wood in a liquid regulated to a temperature sufficient to start the polymerization reaction. The liquid is an aqueous salt solution which comprises between 0.1 mole/liter and saturation at said temperature of one or more salts dissolved in water.

Description: A METHODFOR THEPRODUCTION OF WOOD-POLYMER COMPOSITES TECHNICAL FIELD This invention relates to a method for the production of wood- polymer composites. In particular it relates to the impregnation of wood or wood based materials with polymerisable monomers and the use of an agueous salt solution as the heat transfer medium at the polymerization of the monomers. BACKGROUND ART Species of wood offering superior properties such as hardness, tensile and compressive strength, are invariably tropical hard¬ woods that grow slowly. They would be prohibitively expensive, and it is also considered ecologically unacceptable to harvest them. Many alternative materials that grow faster, are more plentiful and in many cases more aesthetically pleasing, need upgrading in their mechanical properties, which may be achieved by the impregnation of wood with curable or polymerisable liquids followed by a curing or polymerising reaction to obtain wood- polymer composites. Advantages of wood-polymer composites over native wood include: improved hardness, toughness, abrasion resi¬ stance, dimensional stability and mechanical properties (tensile and compression); reduced anisotropy in mechanical properties; improved moisture exclusion; better fire retardant properties; improved decay resistance and improved weatherability. The principles of impregnation of wood are described clearly by W.E. Mott and CJ. Rotariu in "Impregnation and polymerization methods and systems used in the production of wood-polymer materials. Impregnation of fibrous materials". International Atomic Energy Agency, Vienna, 1968, p. 83-91. The polymerization of polymerisable materials impregnated in wood can be initiated by methods well known in the art. Polymerization by high intensity irradiation from radioactive sources is relatively expensive, reduces the mechanical strength of the wood and also involves a potential hazard to workers, cf. Zesz. Probl. Postepow Nauk Roln. , volume 299, 1987, J. Raczkowski, "Effect of the gamma-radiation rate on styrene polymerization in wood and on some properties of the composite", p. 91-102. Electron beam irradiation can also be used, but the penetration depth of electrons is limited. Thermal initiation of radical polymerisable materials can be efficiently facilitated by the addition of free radical initiators, for example peroxides, azo compounds, or by redox systems. The temperature required to facilitate polymerization of a practical rate is system dependent in that initiators have differing decomposition temperatures and different monomers show different rates of polymerization. For some polymerization systems, such as epoxy resins or urethane resins, the rate of cure is temperature dependent. Polymerization processes are exothermic and it is often desirable to remove heat from the wood during polymerization hence limiting the peak in temperature from the exotherm. Wood begins to degrade at a temperature of 150 °C, cf. Carbon, Vol. 2, 1964, p. 211, and therefore, for many products, it is preferred that the peak temperature in the center of the wood does not exceed this temperature for extended periods of time. GB 1.281.419 discloses the use of a hot press to facilitate the polymerization in a wood-polymer composite. Heat transfer is efficient, but items of complex geometry cannot be cured by this method. Further, the equipment is expensive. US 4.009.150 utilises the wrapping of impregnated wood pieces in aluminium foil to reduce the loss of monomer from the wood surface during heating. This method though functional is labour intensive and not suited for commercial production. FR 2 270 064 discloses the method of enclosing the impregnated wood with a wax coating followed by heat treatment under nitrogen at high pressure. Wax residues must be thoroughly cleaned from the wood surfaces as small traces can cause problems with subsequent painting or gluing. EP 45 828 discloses the use of heated nitrogen under pressure to facilitate polymerization. Heat transfer from the gas to the wood is limited as the heat capacity of the gas is low, and a degree of evaporation of low boiling point monomers results in a drying out of the surface. WO 93/03896 utilises a similar principle, but the gas is not circulated. This is claimed to reduce the loss of monomer from the wood surface. The problems are however essen- 5 tially the same. The use of liquids for the purpose of heat transfer has been reported in several documents. K. Kubiaczyk and M. Lawniczak in Zesz. Probl. Postepow Nauk Roln. , volume 260, 1983, p. 95 - 107, o have used machine oil; while DE 24 35 122 discloses the use of polyethylene glycol; and M. Lawniczak in Holzforsch. Holz- verwert., Vol. 24, 1972, No. 3, p. 51-3, has used paraffin wax. Cleaning of the wood after curing can be difficult and contamin¬ ation of surfaces detrimental to the adhesion of paint and 5 adhesive. DE 27 18 770 discloses the use water at 85 to 90 °C to polymerize acrylate and methacrylates in porous media such as stone, wood or metals. EP-A-0.626.240 teaches the use of water as a heat ° transfer medium for the polymerization of wood-polymer com¬ posites. Preferably, the water is hot (40-200 °C) and pressuri¬ zed. Both patents take advantage of the excellent heat transfer properties of water to give good temperature control. Typical monomers have a low solubility in water and therefore the loss 5 of monomer is low. However, practical usage of this invention has shown that an unacceptable level of splitting of the final wood product will take place. SUMMARY OF THE INVENTION 0 it has now unexpectedly been found that the splitting and dis¬ tortion of a final wood-polymer composite can be highly reduced or eliminated by the use of an aquous salt solution as the temperature control medium during the polymerization reactions. Thus, the present invention provides a method for the production of wood-polymer composites by impregnating solid wood or a material on the basis of wood with a polymerisable monomer by the use of vacuum and/or pressure, and then immersing the impregnated wood in a liquid regulated to a temperature sufficient to start the polymerization reaction, wherein there is used as said liquid an aqueous salt solution at curing temperature, which salt solution comprises an amount between 0.1 mole/liter and satura¬ tion at said temperature of one or more salts dissolved in water. The wood-polymer composites obtained by said method can suitably be used in the manufacturing of construction materials, such as flooring, paneling, door or window frames, skirting boards, mouldings, thresholds, tubes, and sinks; leisure or sports articles, such as clubs, club heads, cues, bats, skis, toboggans, boathulls, masts, rudders and handles; households articles, such as furniture, ornaments, utensils, toys and buttons; and for materials used in direct contact with wet and soily environments; and for the preserving of articles. FIGURES Figure 1 shows the cupping of final polymer-impregnated solid birch samples at various temperatures and salinities. Figure 2 shows the bow of final polymer-impregnated solid birch samples at various temperatures and salinities. Figure 3 is a photograph of an end section of a birch sample not prepared according to the invention. Figure 4 is a photograph of a section of the surface of a birch sample not prepared according to the invention. Figure 5 is a photograph of an end section of a Canadian maple sample not prepared according to the invention. Figure 6 is a photograph of a section of the surface of a Canadian maple sample not prepared according to the invention. Figure 7 is a photograph of an end section of a birch sample prepared according to the invention. Figure 8 is a photograph of a section of the surface of a birch sample prepared according to the invention. Figure 9 is a photograph of an end section of a Canadian maple sample prepared according to the invention. Figure 10 is a photograph of a section of the surface of a Canadian maple sample prepared according to the invention. DETAILED DESCRIPTION OF THE INVENTION Any type of suitable wood or wood based material can in principle be used by the method of the present invention for the production of wood-polymer composites. However, a prerequisite is that the wood material is able to absorb at least a minor quantity of a polymerisable monomer. Soft and/or porous materials will obtain the greatest improvements in desired properties by the present method. It may be used solid wood from coniferous trees, such as spruce and pine; or from deciduous trees, such as birch, maple, ash, oak, elm and the like. Wood based materials like prefabric¬ ated boards, such as plywood and chipboards, may also be used. Moreover, prefabricated and manufactured articles may also be impregnated and hardened by the present method. The polymerisable monomer impregnated in the wood material is preferably a free radical polymerisable monomer. A mixture of different monomers may also be used. Free radical polymerisable substances suitable for the impregnation of wood materials according to the present method comprise: vinyl monomers, such as (meth)acrylates, styrenes, maleic anhydride; macro monomers; polyfunctional monomers such as divinyl benzene, diallyl phthalate, triallyl phosphate, di- and tri-(meth)acrylates and unsaturated polyesters. Suitable not free radical polymerisable substances comprise epoxy resins; phenol/formaldehyde, melamine- /formaldehyde and urea/formaldehyde resins; and polyisocyanates. The wood or wood based material may be impregnated with the polymerisable substances by any method known in the art. Impregnation of wood is normally performed on seasoned wood. A large proportion of the water present in the original living material is removed and the wood optionally placed under vacuum in an autoclave to remove air from its cells. The polymerisable material is then introduced to the autoclave to a level that covers the material. An external pressure may be applied to accelerate the uptake of the substance. To assist in the absorption of the polymerisable substance the material may be dispersed in a non-polymerisable organic or inorganic solvent. Non-limiting examples of such solvents are alcohols, such as methanol, ethanol, propanols, butanols and pentanols; ketones, such as acetone, methylethylketone and diethylketone; hydrocar¬ bons, such as pentanes, hexanes, heptanes, octanes and nonanes; acids, such as formic acid and acetic acid; water and dimethyl¬ sulfoxide. Impregnation is preferably performed at ambient temperature or lower. In the end excess polymerisable material is drained off and the impregnated wood subjected to conditions to initiate polymerization or hardening. Curing is performed by immersing the impregnated wood in the salt solution either by submerging the impregnated wood into a bath containing the salt solution, by showering the impregnated wood with the salt solution or by pumping the salt solution into an enclosed tank or autoclave containing the impregnated wood, with the curing water at the desired temperature. A closed tank or autoclave enable the use of excess pressure and thus temperatures in excess of 100 °C if desired. The solubility of most polymeris- able materials suited for wood-polymer composites in water is low, but some polymerization or curing in the aqueous phase will occur. This polymer tends to flocculate in the salt solution and can be removed by filtration or another suitable method. After the polymerization or curing the excess salt on the product may be rinsed off with cold water and then the wood-polymer composite is dried. Drying may be carried out at ambient or elevated tempe¬ rature and may involve the use of reduced pressure, and/or microwave or infrared radiation. The optimum polymerisation temperature is dictated by the used polymerization or curing system. Thus, the salt solution in which the impregnated wood is immersed has a curing temperature preferably of 40 to 200 °C. However, temperatures higher than 150 °C can lead to the onset of degradation of the cellulose in the wood and are not favoured unless the polymerisable material dictates a higher temperature. To achieve acceptable polymerisa¬ tion reaction rates the curing temperature should not be lower than approx. 40 °C. A preferred curing temperature is therefore within the range of from 40 to 150 °C. The salt solution may be comprised of a single salt or a mixture of salts dissolved in fresh water, or optionally sea water. Preferred salts are halides of metals from group I and group II of the Periodic Table of the elements. A mixture comprising two or more such salts may also be used. More preferably a mixture of salts includes sodium chloride as the major salt component. The most preferred salt is sodium chloride because of its low price and its availability. Sea water, optionally concentrated or diluted, is favoured for the same reasons of low price and availability. Advantages from the addition of salt to the water occur from very low concentrations, e.g. a 0.1 molar solution. However, higher concentrations result in better and more reliable results. A preferred salt solution is an aqueous 1 molar NaCl solution. Sodium chloride may be partly replaced by other salts, e.g. potassium chloride. Even ammonium chloride may be used with an acceptable result. The upper concentration limit is the saturated aqueous solution. The main purpose of adding salt to the aqueous heat transfer medium, here also called the curing water, is to achieve a salting-out effect which reduces the water uptake in the cells of the wood, thus avoiding splitting and cracking of the final wood product. We will now briefly explain this salting-out phenomenon. In the case of a free radical polymerization of electrically neutral monomers all the species involved will be non-electrolytes: the initial monomer, the propagating macro- radical and the produced polymer chain. In initiator systems comprising redox initiators the initiating species will carry charges and hence be electrolytes. This initiator is however present in only a minute amount and evenly distributed throughout the impregnated material. Electrolytes and non-electrolytes will be solvated to different degrees in water, the electrolytes being capable of bonding the water molecules in its vicinity more tightly in a solvation sphere than the non-electrolyte molecules do. In a multicomponent system consisting of a polar solvent and one or more electrolyte and non-electrolyte solutes the solvent will be preferentially solvating the electrolyte molecules. Consequently, the non-electrolyte solute will not experience as great a solvating power by the solvent as it would in a solvent void of any electrolyte ions. This effect, i.e. that the solubility of a non-electrolyte solute in water decreases in the presence of an electrolyte solute, is known as a salting-out effect. In embodiments of the present invention the solvent is water, while the electrolyte solute is the salts being added to the water, and the non-electrolyte solutes primarily of concern are the liquid or gaseous monomers and the growing macro- radicals. It is believed that a macro-radical that is precipi- tated or salted out while still being within the pores of the impregnated wood-based material will hamper the transport of liquid and gaseous monomer molecules out of, and water molecules into, the material. It is also believed that the lower the solubility of monomer in the curing water, the lower the driving force for the transport of monomer from the wood-based material into the curing water will be. A more theoretical explanation for the reduced solubility of a non-electrolyte in an aqueous electrolyte solution as compared to an aqueous solution without any electrolyte, may be deduced on the basis of activity coefficients. In an ideal solution, both the solvent and solute activity coefficients,γ α , γ 2 , respec¬ tively, are unity. Hence the activity, α, for a small solute molecule is directly proportionial to its mole fraction, x, in the solution, as given by a = γx. For macromolecules, the volume fraction, φ, is commonly used in place of the mole fraction. For non-ideal solutions, including most strong electrolyte solutions, the activity coefficient deviate non-linearly from unity as a function of composition or ionic strength of the solution, as given by α=p/p*, where α is the activity of the solute, p is the vapour pressure of the solution and p* is the vapour pressure of the pure solvent. At 18 °C sea water has p=19.02 kPa and pure water p * =19.38 kPa, thus the activity of the solvent (water) is α=0.98 (Atkins, Oxford Univ. Press, 4th Ed., 1990, p. 179), and hence reduced by 2 % from pure water. In embodiments of the invention a series of preparations of birch wood-polymer composite and Canadian maple wood-polymer composite were investigated. Five different salinities for the curing aqueous solution were utilized: 0, 0.4, 1, 2, and approx. 6.1 molar aqueous salt solutions. Improved quality of the resulting wood-polymer composite was observed when saline water was utilized. After curing in circulating fresh water at 90 °C, all the samples had cracks, both in the core and on the surface. For a similar experiment, except that the curing water was a 0.4 or 1 molar aqueous salt solution, a minority of the samples expe¬ rienced cracking of the core while the majority showed no signs of cracking what so ever. For yet another set of similar ex- periments, except that the curing solution was a 2 molar or saturated aqueous sodium chloride aqueous solution, none of the wood-polymer composite veneers showed any signs of cracking. For birch, Canadian maple, spruce, chipboard and plywood, there were no signs of cracking after curing for 1 hour at 70 °C in a circulating 2 molar salt solution, while for birch and Canadian maple the vast majority of the produced wood-polymer composite samples had clear cracks in the core after curing for approx. 1 hour at 60 °C in circulating fresh water. EXAMPLES General procedure Pieces of veneers approx. 500 mm long, 70 mm wide and 10 mm thick with a humidity content of approx. 6% by weight were placed in a stainless steel autoclave. The autoclave was then closed and evacuated by the use of a single-stage oil-sealed rotary vane pump until a constant pressure of not higher than 20 kPa. A polymerizing solution at approx. 8 °C consisting of technical grade methylmethacrylate containing approx. 0.5% by weight of dissolved azobisisobutyronitrile was then fed into the evacuated autoclave until the wood pieces were completely submerged. The polymerizing solution was further forced into the wood by applying approx. 650 kPa pressurized air above the surface of the solution for approx. 20 minutes. Subsequently, the autoclave was drained for excess polymerizing solution, whereupon heated water of a specified temperature was circulated through the autoclave from a thermostated reservoir. The water contained a specific concentration of dissolved salt(s). The water was introduced into the autoclave at a higher tem¬ perature than the desired curing temperature to alleviate the heat loss to the stainless steel autoclave. The circulating curing water was heated in the reservoir, and after the pre¬ determined curing temperature had been reached, the water was allowed to circulate at approx. constant temperature for 1 hour, after which the autoclave was drained for water and the wood- polymer composite samples were removed and quickly flushed under cold water. The wood-polymer composite was wiped with tissue before weighing and recording the dimensions. The obtained wood-polymer composites were visually examined for core and surface cracking. The cupping and the bow were measured as the depth of the curvature across and along the surface of the veneer, respectively. Experimental details and results are recorded in the table below. The polymer content (P%) of wood samples was determined according to P% = 100- ( tt- upc - m 0 ) /m w , c , where m^ and m 0 is the weight of wood-polymer composite and wood based substrate, respectively. The weights were determined after drying for at least 6 hours at 105 °C. Examples 1 and 2 (Comparative examples ) Veneers of birch and Canadian maple were impregnated and cured according to the general procedure outlined above. The water contained no salt and the curing temperature was approx. 60 C C. All three birch samples and two out of three Canadian maple samples had core cracking. Examples 3 and 4 (Comparative examples) Veneers of birch and Canadian maple were impregnated and cured according to the general procedure outlined above, with the exception that the impregnating liquid contained 85% by weight of methyl methacrylate and 5% by weight of styrene as polymer¬ isable components with 0.5% by weight of azobisisobutyronitrile, and 10% by weight of isopropanol as a non-polymerisable solvent. The curing water had no salt, and the curing temperature was 80 °C. None of the samples showed clear cracking immediately after curing, but after being dried at 60 °C for 22.5 hours, all the samples showed clear core cracking. Examples 5 and 6 (Comparative examples) The procedure of Examples 1 and 2 was repeated, except that the curing temperature was 90 °C. All samples showed core and surface cracking. Examples 7 and 8 Veneers of birch and Canadian maple were impregnated and cured according to the general procedure outlined above. The water contained 1 mole per liter of NaCl, and the curing temperature was 90 °C. None of the samples showed any surface cracking, and only one out of three birch samples showed some minor core cracking. Examples 9 and 10 The procedure of Examples 7 and 8 was repeated, except that the NaCl concentration was 2 moles per liter. None of the samples showed any cracking. Examples 11 to 15 veneers of birch, Canadian maple and spruce, and pieces of chipboard and plywood were submitted to the procedure of Examples 9 and 10, except that the curing temperature was 70 C C. None of the samples showed any cracking. Examples 16 and 17 Veneers of birch and Canadian maple were impregnated and cured according to the general procedure outlined above. The curing temperature was 80 °C in a saline solution consisting of approx. 6.11 moles NaCl per liter water. (A saturated aqueous NaCl solution at 25 °C will contain approx. 6.11 moles NaCl per liter water according to the Merck Index, volume 10, 8430. ) None of the samples showed any sign of cracking. Examples 18 and 19 The procedure of Examples 3 and 4 was repeated for veneers of birch and Canadian maple, except that the curing water contained 1.5 moles sodium chloride (NaCl) and 0.5 mole potassium chloride (KCl) per liter water. None of the samples showed any sign of cracking immediately after curing or after being dried at 60 °C for 22.5 hours. Example 20 Veneers of Canadian maple were submitted to the general procedure outlined above The curing temperature was 80 °C and the curing water was synthetic sea water having the composition of sea water from the North Sea. The saline solution consisted of 373 mmoles of sodium chloride (NaCl), 1 mmole of sodium bicarbonate (NaHC0 3 ) , 7 mmoles of potassium chloride (KCl) , 1 mmole of potassium bromide (KBr), 6 mmoles of calcium sulphate (CaS0 4 ), 14 mmoles of magnesium chloride (MgCl 2 ) and 8 mmoles of magnesium sulphate (MgS0 4 ) per liter water, giving a 0.4 molar salt solution. None of the samples showed any sign of cracking after impreg- nation and curing. Table Polymer Cracking Example Sample Curing con¬ observed No. water, tent, temp. wt% °C cone, of salt, molar Comp 1 Birch 60 0 33 core Comp 2 Can.maple 60 0 35 core Comp 3 Birch 80 0 43 core Comp 4 Can.maple 80 0 39 core Comp 5 Birch 90 0 46 core and surface Comp 6 Can.maple 90 0 38 core and surface 7 Birch 90 1 NaCl 48 none 8 Can.maple 90 1 NaCl 37 none 9 Birch 90 2 NaCl 49 none 10 Can.maple 90 2 NaCl 35 none 11 Birch 70 2 NaCl 49 none 12 Can.maple 70 2 NaCl 36 none 13 Spruce 70 2 NaCl 35 none 14 Chipboard 70 2 NaCl 44 none 15 Plywood 70 2 NaCl 35 none 16 Birch 80 6.1 NaCl 55 none 17 Can.maple 80 6.1 NaCl 35 none 18 Birch 80 1.5 NaCl 48 none 0.5 KCl 19 Can.maple 80 1.5 NaCl 39 none 0.5 KCl 20 Can.maple 80 0.4 33 none synt. sea water

Claims: P a t e n t c l a i m s 1. A method for the production of wood-polymer composites by impregnating solid wood or a material on the basis of wood with a polymerisable monomer by the use of vacuum and/or pressure and immersing the impregnated wood in a liquid regulated to a temperature sufficient to start the polymerization reaction, characterised in that there is used as said liquid an aqueous salt solution, which salt solution comprises between 0.1 mole/liter and saturation at said temperature of one or more salts dissolved in water. 2. The method of claim 1, characterised in that said wood is a prefabricated article or prefabricated board such as plywood or chipboard. 3. The method of claim 1 or 2, characterised in that said polymerisable monomer is a free radical polymerisable monomer. 4. The method of claim 1 or 2, characterised in that said polymerisable monomer comprises a mixture of different monomers, which may also comprise multifunctional monomers. 5. The method of claims 1 to 4, characterised in that said polymerisable monomer when introduced into the wood is dissolved or dispersed or emulsified in a non-polymerisable solvent. 6. The method of claims 1 to 5, characterised in that said salt solution in which the impregnated wood is immersed has a curing temperature of 40 to 200 °C. 7. The method of claim 6, characterised in that said salt solution has a temperature of 40 to 150 °C. 8. The method of claims 1 to 7, characterised in that the salt or the mixture of salts are selected from group I and group II metal halides, and mixtures thereof. 9. The method of claim 8, characterised in that the mixture of salts contains sodium chloride as the major salt component. ιo. A use of wood-polymer composites obtained by the method of any of the preceding claims, in the manufacturing of wood based construction materials, such as flooring, paneling, door or window frames, skirting boards, mouldings, thresholds, tubs, and sinks; leisure or sports articles, such as clubs, club heads, cues, bats, skis, toboggans, boathulls, masts, rudders and handles; households articles, such as furniture, ornaments, utensils, toys and buttons; and for materials used in direct contact with wet and soily environments; and for the preserving of articles.

Inventors: FELIX JOHAN, JAHR DAG MORTEN, REDFORD KEITH, FELIX, JOHAN, JAHR, DAG, MORTEN, REDFORD, KEITH
Applicants: FELIX JOHAN, JAHR DAG MORTEN, NORSKE SKOG FLOORING A S, REDFORD KEITH, FELIX, JOHAN, JAHR, DAG, MORTEN, NORSKE SKOG FLOORING A/S, REDFORD, KEITH
Application year: 1997
Main classification: B27K3
IPC codes: B27K3