Background Art Reconstituted wood panels are used for similar purposes to wood panels prepared from timber, such as a building material or in the manufacture of furniture such as cupboards, tables and desks.

Reconstituted wood panels are prepared by binding cellulosic material using a thermosetting resin.

The cellulosic material may be wood or derived from wood. The wood can come from a variety of sources including forest lumber (often called roundwood), sawmill waste, waste from other mills such as plywood facilities and urban waste (such as pallets, packing crates, old furniture etc). Other cellulosic materials can also be used, such as wheat straw or bagasse from sugar cane (collectively called agri-fibres), or cellulosic material obtained from bamboo, jute, oil palms etc.

The cellulosic raw material is mechanically reduced in size to produce cellulosic material having the appropriate size for the intended use. For the manufacture of particleboard, the cellulosic material is in the form of wood flake. For the manufacture of medium density fibreboard, the cellulosic material is in the form of cellulosic fibres.

To prepare a reconstituted wood panel, the cellulosic material is mixed with a thermosetting resin. The main thermosetting resins used in the manufacture of reconstituted wood panels are urea formaldehyde (UF) resin, melamine urea formaldehyde (MUF) resin, phenol formaldehyde (PF) resin and monomethyldiisocyanate (MDI). Other components are often also added to the mixture of the resin and the cellulosic material. Such components include wax which provides moisture resistance to the reconstituted wood panel. Other components which may be added include catalysts to catalyse the curing of the resin, fungicides, insecticides, colouring agents and fire retardants.

The mixture of the cellulosic material, the resin and other components (called "resinated flake", "resinated fibre"etc. depending on the form of the cellulosic material

used) is then mechanically formed into a mat, and the mat is then hot pressed in a hot press to cure the resin under heat and pressure. The mat is hot pressed for a sufficient time to cure the resin throughout the mat. A hot press is a press which applies above atmospheric pressure and above ambient temperature to an article. Such presses have hot plates which are applied to the surface of an article, applying pressure to the article and heating the article. Two main types of hot presses are generally used in the manufacture of reconstituted wood panels: platen presses and continuous presses.

Platen presses may be multi or single opening platen presses. Typically, during the manufacture of particleboard or medium density fibreboard, the mat is hot pressed at a temperature of about 150-200°C and at a pressure of about 200-400 psi (1500-3000 kPa). The temperature during hot pressing is given as the temperature of the hot plates applied to the article in the press. All pressures referred to herein are the pressures on the mat and not line pressures.

In some instances, a prepress may be used to prepress the mat prior to the mat being placed in the hot press to cure the resin.

For most uses of reconstituted wood panels, it is preferable to have the highest density part of the reconstituted wood panel near the major surfaces of the panel (the top and bottom surfaces). However, when the uncured mat is placed in a hot press to cure the resin, the top and bottom surfaces of the mat are exposed to the heat of the hot plates of the hot press and the resin at the surfaces is cured, before the maximum pressure in the hot press is achieved. This results in a layer on the top and bottom surfaces of the cured mat which has been cured at less than the maximum pressure achieved during pressing and which is therefore of low density and quality. This is known as"precure"and the surface of the cured mat which has been subject to precure is called the"precured surface".

For many uses of reconstituted wood panels, this low quality precured surface has to be removed by sanding the cured mat to produce a finished reconstituted wood panel which has suitable surface properties. The precured surface can amount to as much as 20% of the total raw material used in producing the finished product. The sanding of the surfaces of the cured mat is also an additional step in the process of preparing the reconstituted wood panel, and adds to the cost of preparing the reconstituted wood panel. Further, the dust generated by the sanding process needs to be disposed of. Typically, the dust is either burnt as fuel in the factory's heat generating plant, recycled back into the production of other panels, or dumped. However, as a fuel the dust is expensive and difficult to use. Recycling the dust back into the production process can have negative effects on the properties of the reconstituted wood panels produced. Dumping the dust incurs additional costs.

Mechanical advances in presses have to some extent reduced the severity of precure in modern plants. This has been achieved by having press lines that are continuous which avoids the long closing periods of multi daylight platen presses.

Multi daylight presses are presses having several"daylights". Such presses are designed to press several mats simultaneously, with one mat in each daylight. Modern continuous presses can achieve as little as 0.2 mm of precure on the top and bottom surfaces of the cured mat, that is, a precured surface having a depth of as little as 0.2 mm. Older multi daylight presses typically result in about 0.5 to 1. 5 mm of precure on the top and bottom surfaces of the cured mat.

Some reconstituted wood panels are prepared having multiple layers. For example, particleboard typically comprises three layers, a coarse core layer which provides mechanical strength and fine surface layers to provide a good finish. Because of the multilayered formation, the core and surface layers can be resinated separately with resins that cure at different rates. The middle of the mat is the slowest part to heat up since it is further from the hot plates of the hot press. The resin used in the core layer is therefore typically adapted to cure more quickly than the resin used in the surface layers. Nevertheless, precure on the outer surface of the surface layers still remains a problem in the manufacture of such reconstituted wood panels.

Many plants for producing reconstituted wood panels have a spray system for spraying water onto the uncured mat as it is transported into the hot press. Water is sprayed onto the mat primarily to stop the surfaces of the mat drying out since uneven moisture content between the top and bottom surfaces of the mat can result in a reconstituted wood panel which warps. The water also assists heat transfer to the centre of the mat during the curing in the hot press. In the hot press, the water is converted to steam. The steam pressure at the surface of the mat forces the steam into the core of the mat transferring heat to the core of the mat and thus decreasing the time taken to cure the resin throughout the mat. This can be very advantageous as wood and other cellulosic material are fairly good thermal insulators so heat transfer through a mat in the hot press can be slow.

This spray system also reduces precure to some extent. The water sprayed on the surface of the mat acts as a heat sink thus slowing down the rate at which the resin at the surface heats up when the mat is first exposed to the heat of the hot press.

The spraying of water onto the uncured mat before curing can however create an additional problem of causing the mat to stick to the hot plates of the hot press. To address this problem, a release agent is sometimes added to the water spray to inhibit the mat sticking to the press. Typical release agents include neutralised and acid phosphate esters, waxes, silicones and oils.

It would be advantageous to provide a method for reducing precure during the manufacture of reconstituted wood panels.

Summary of the Invention In a first aspect, the present invention provides a method for reducing precure during the curing of a thermosetting resin in an article containing the resin, the method comprising the steps of treating at least part of the surface of the article to chemically inhibit the curing of the resin at the treated surface, and then curing the resin by hot pressing the article in a hot press. The inventors have found that chemically inhibiting the curing of the resin at the treated surface allows additional time for the pressure of the hot press to be applied to the article before the resin at the surface of the article cures, thereby reducing the incidence of precure at the treated surface.

The article may be any article comprising a thermosetting resin which is subject to precure during the curing of the resin in a hot press. Typically the article comprises a thermosetting resin in admixture with other components. Typically the thermosetting resin is an acid curing resin, that is, a resin that cures more quickly at lower pH than at higher pH. The article may for example be a mat for manufacturing a reconstituted wood panel, the mat comprising a mixture of an acid curing thermosetting resin, cellulosic material and optionally other components.

Typically the resin is an acid curing thermosetting resin, and the surface of the article is treated to chemically inhibit the curing of the resin by treating the surface of the article to increase the pH of the article at the treated surface. Typically the surface of the article is treated to increase the pH of the article at the treated surface by applying to the surface one or more agents for increasing the pH of the article at the surface treated. Preferably the agent is a basic compound having a boiling point below 300°C or a compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C.

Accordingly, in a preferred embodiment, the present invention provides a method for reducing precure during the curing of an acid curing thermosetting resin in an article containing the resin, the method comprising the steps of applying to at least part of the surface of the article one or more compounds selected from the group consisting of basic compounds having a boiling point below 300°C and compounds that when heated to less than about 300°C form a basic compound having a boiling point below 300°C, in an amount sufficient to inhibit the curing of the resin at the treated surface when the article is hot pressed in a hot press, and then curing the resin by hot pressing the article in a hot press.

As used throughout this specification including the claims, the term"basic

compound"refers to a compound that reacts with a protonic acid to give water and a salt or that dissolves in water to produce hydroxide ions, and includes compounds such as soluble hydroxides, ammonia and organic amines.

Typically the basic compound having a boiling point below 300°C or the compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C, is applied to the surface by applying to the surface an aqueous solution of the basic compound having a boiling point below 300°C or the compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C.

The present invention has particular application for reducing precure during the manufacture of reconstituted wood panels, such as particleboard and medium density fibreboard.

In a second aspect, the present invention provides a method for preparing a reconstituted wood panel, the method comprising the steps of : a) preparing a mat comprising a mixture of an acid curing thermosetting resin and cellulosic material; b) treating at least part of the surface of the mat to increase the pH of the mat at the treated surface by an amount sufficient to inhibit the curing of the resin; and then c) curing the resin by hot pressing the mat in a hot press.

Typically the surface of the mat is treated to increase the pH of the mat at the treated surface by applying to the surface one or more agents for increasing the pH of the mat at the treated surface. Preferably the agent is a basic compound having a boiling point below 300°C or a compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C.

In a preferred embodiment, the present invention provides a method for preparing a reconstituted wood panel, the method comprising the steps of : a. preparing a mat comprising a mixture of an acid curing thermosetting resin and cellulosic material; b. applying to at least part of the surface of the mat one or more compounds selected from the group consisting of basic compounds having a boiling point below 300°C and compounds that when heated to less than about 300°C form a basic compound having a boiling point below 300°C, in an amount sufficient to inhibit curing of the resin at the treated surface when the mat is hot pressed in a hot press; and then c. curing the resin by hot pressing the mat in a hot press.

Typically the basic compound having a boiling point below 300°C or the

compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C, is applied to the surface by applying to the surface an aqueous solution of the basic compound having a boiling point below 300°C or the compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C.

In a third aspect, the present invention provides a composition for use in reducing precure during the curing of an acid curing thermosetting resin in a hot press, the composition comprising one or more basic compounds having a boiling point below 300°C and/or compounds that when heated to less than about 300°C form a basic compound having a boiling point below 300°C in an aqueous carrier.

In a fourth aspect, the present invention provides a composition comprising one or more basic compounds having a boiling point below 300°C and/or compounds that when heated to less than about 300°C form a basic compound having a boiling point below 300°C in an aqueous carrier, when used in the method according to the first or second aspects of the present invention.

Detailed Description of the Invention The present invention may be used to reduce precure during the curing of a thermosetting resin in a hot press in any article containing a thermosetting resin. The present invention has particular application for reducing precure during the manufacture of reconstituted wood panels, such as particleboard and medium density fibreboard. As used herein, the terms"reduce precure"and"reducing precure"encompass the elimination of precure as well as a reduction in the amount of precure.

The invention is described below in more detail in relation to the manufacture of reconstituted wood panels. However, the invention is not limited to the manufacture of reconstituted wood panels, and the method of the invention may be used to reduce precure during the curing of a thermosetting resin in a hot press in any article containing a thermosetting resin.

In accordance with the method for preparing a reconstituted wood panel according to the present invention, a mat comprising an acid curing thermosetting resin and cellulosic material is prepared.

Typically, the acid curing thermosetting resin is an acid curing thermosetting organic resin. Examples of acid curing thermosetting organic resins include urea formaldehyde resin, melamine urea formaldehyde resin, melamine formaldehyde resin and tannin modified formaldehyde resins. Most typically, the acid curing thermosetting resin is urea formaldehyde resin or melamine urea formaldehyde resin. A mixture of acid curing thermosetting resins may be used, for example a mixture of urea

formaldehyde resin and melamine urea formaldehyde resin, or a mixture of urea formaldehyde resin, melamine formaldehyde resin and penol formaldehyde resin.

The cellulosic material can be any cellulosic material. Typically the cellulosic material is wood or is derived from wood. The wood may be obtained from any source, including forest lumber, sawmill waste, waste from other mills, or urban waste such as pallets, packing crates, furniture etc. Other cellulosic material can be used such as wheat straw or bagasse from sugar cane, or cellulosic material derived from bamboo, jute, oil palm, etc. For the manufacture of particleboard, the cellulosic material is in the form of wood flake. For the manufacture of medium density fibreboard, the cellulosic material is in the form of cellulosic fibres.

The mat may include other components in addition to the acid curing thermosetting resin and the cellulosic material. Such other components may include wax to provide moisture resistance to the finished product. Other components which may be included in the mat include fungicides, insecticides, colouring agents or fire retardants. In some embodiments of the invention, the mat includes a catalyst to catalyse the curing of the resin and thus reduce the time taken to cure the resin. A salt of a weak base and a strong acid can be used as the catalyst. Other suitable catalysts include ammonium salts of strong acids such as ammonium chloride, ammonium sulphate or ammonium nitrate. When such ammonium salts are heated during the hot pressing of the mat, ammonia is produced and driven off leaving the acid which catalyses the curing of the resin in the mat.

The mat is typically prepared by adding the resin and the other components of the mat, if any, to the cellulosic material and mixing to prepare a mixture of the resin, the cellulosic material and the other components. The resultant mixture is then mechanically formed into a mat.

In accordance with the method of the present invention, at least part of the surface of the mat is treated to increase the pH of the mat at the treated surface by an amount sufficient to inhibit the curing of the resin at the treated surface. By"inhibit the curing of the resin"it is meant that the resin cures more slowly at a given temperature and pressure. Typically all or substantially all of the top and bottom surfaces of the mat are treated to increase the pH of the mat at the top and bottom surfaces.

Preferably the surface of the mat is treated to increase the pH of the mat at the treated surface by applying to the surface one or more agents for increasing the pH of the mat at the treated surface. Preferably the agent is a basic compound having a boiling point below 300°C or a compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C. Such agents are preferred as when such agents are used and the mat is placed in the hot press and exposed to the

heat of the hot press, a basic compound is present at the surface of the mat with suitable vapour pressure and mobility to penetrate into the surface of the mat increasing the pH of the mat in a region adjacent to the treated surface. In some embodiments, the pH of the mat is increased to a depth of about 0.5 to 2.0 mm, more typically about 0.5 to 1. 5 mm, from the surface of the mat. The agent and the amount of the agent applied to the mat are preferably selected such that the pH of the mat is increased to a depth sufficient to eliminate precure when the mat is hot pressed in the hot press. As will be apparent to a person skilled in the art, a basic compound having a boiling point below 300°C will be present at the surface of the mat at some vapour pressure, even if the temperature of the hot plates of the hot press is below the boiling point of the agent or the temperature at which all the agent decomposes to form, or otherwise forms, a basic compound having a boiling point below 300°C. In some embodiments, the agent applied to the surface of the mat is a basic compound having a boiling point below 220°C, and in some embodiments is a basic compound having a boiling point below 140°C.

Typically the compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C, is a compound that decomposes to form ammonia at a temperature below about 300°C.

The agent for increasing the pH of the mat applied to the surface of the mat may for example be ammonia. Ammonia is a basic compound having a boiling point below 300°C. However, because of the safety procedures which must to be followed when handling ammonia, it is preferred that the agent applied to the surface of the mat is a basic compound having a boiling point below 300°C other than ammonia, or a compound that when heated to less than about 300°C forms a basic compound having a boiling point below 300°C.

In some embodiments of the invention, the basic compound having a boiling point below 300°C is an organic amine. The organic amine may, for example, be an alkanolamine or morpholine.

Basic compounds having a boiling point below 300°C include for example: diethylamine-NH (C2H5) 2 triethylamine (ATE)-N (C2H5) 3 monoethanolamine (MEA)-NH2CH2CH2OH diethanolamine (DEA)-NH (CH2CH20H) 2 dimethylethanolamine (DMEA) morpholine-C4H9NO 2-methyl-2-amino-l-propanol (AMP).

Compounds that decompose to form a basic compound having a boiling point below 300°C, such as ammonia, when the compound is heated to a temperature below 300°C include: hexamine- (CH2) 6N4 urea-CO (NH2) 2.

Other compounds that form a basic compound having a boiling point below 300°C, such as ammonia, when heated to a temperature below 300°C include ammonium hydroxide, and ammonium salts of weak acids such as: ammonium bicarbonate-NH4HCO3 ammonium carbamate-NH2COONH4 ammonium carbonate- (NH4) 2CO3.

Typically the agent for increasing the pH of the mat is applied to the surface of the mat immediately prior to the curing of the resin in the hot press.

In some embodiments of the invention, the mat is prepressed prior to the mat being hot pressed in the hot press. The mat may be prepressed before or after the agent for increasing the pH of the mat at the treated surface is applied to the surface of the mat.

Typically, the agent for increasing the pH of the mat is applied to the surface of the mat by applying an aqueous solution of the agent, typically a dilute aqueous solution, to the surface. The solution is typically applied by spraying the solution onto the surface of the mat. The solution may be applied to the surface of the mat using conventional apparatus used in the manufacture of reconstituted wood panels for spraying water or water and a release agent onto the surface of mats. Such apparatus typically comprise a spray head. The spray head is typically a small airless spray with a nozzle diameter of about 125-250 microns, and a fan spread of 10-30°. Multiple spray heads are typically used to ensure even distribution of the solution. The aqueous solution may also include components in addition to water and the agent for increasing the pH of the mat at the treated surface, such as one or more release agents to prevent the mat sticking to the plates of the hot press.

In a further aspect, the present invention provides a composition for use in reducing precure during the curing of an acid curing thermosetting resin in a hot press, the composition comprising:

a. one or more release agents selected from the group consisting of phosphate esters, waxes, silicones and oils; b. one or more compounds selected from the group consisting of ammonia, organic amines having a boiling point below 300°C or that when heated to less than about 300°C form aminonia, and ammonium salts of a weak acid; c. water; and wherein the pH of the composition is greater than 7.

The quantity of the agent for increasing this pH of the mat applied to the surface of the mat will vary depending on the particular agent and resin used and the intended pressure and temperature during the curing process. A person skilled in the art would readily be able to determine the quantity of the agent that can be applied to the surface of the mat to inhibit the curing of the resin at the treated surface without preventing the resin from curing during the hot pressing of the mat.

The resin is cured by hot pressing the mat in a hot press. The mat is typically hot pressed at a platen temperature from about 140°C to about 220°C, more typically from about 150°C to about 200°C, and a pressure from about 200 to 400 psi (about 1500 to about 3000 kPa), for a time sufficient to cure the resin throughout the mat.

Examples The present invention will now be described in further detail by reference to the following non-limiting examples.

The release agent used in the following examples is an aqueous solution containing about 20% by weight of a neutralised phosphate ester (i. e. the pH of the release agent is about 7).

Example 1 A composition having the following components (expressed as a percentage by weight) was prepared: Release agent 10% Hexamine 16% Dimethylethanolamine 10% Water 64% A mat comprising a thermosetting resin (urea formaldehyde resin) and

cellulosic material (fibres obtained from Pinus radiata) was prepared. The composition was sprayed at 15 to 100 grams per square metre onto the fibre mat both on the top and bottom surfaces, before the mat was placed in a multi-daylight hot press to cure the resin. The mat was hot pressed at 145°C and 250psi (about 1900 kPa) for 30 seconds to cure the resin, producing a reconstituted wood panel having a thickness of 3mm.

Two identical mats were also prepared and hot pressed under the same conditions. One mat was hot pressed without the surface of the mat being treated in any manner, and the other mat was sprayed on the top and bottom surfaces with a mixture of water and the release agent prior to the mat being hot pressed.

The mat sprayed with the composition described above exhibited significantly less precure than the other two mats as assessed by a visual comparison and by a density scan using a Gracon density scanner.

To prepare similar reconstituted wood panels having greater thickness, longer curing times and higher temperatures would be used. For thicker panels the mat would typically be hot pressed at 250-400psi (about 1900-3000kPa) and 180-200°C for approximately 7-10 seconds per millimetre of panel thickness.

Example 2 A composition having the following components (expressed as a percentage by weight) was prepared: Release agent 10% Monoethanolamine 20% Water 70% The composition was sprayed on the top and bottom surfaces of a particle board mat comprising a urea formaldehyde resin and wood flake. The composition was applied at a rate of 15-100 grams per square metre. The mat was then placed in a hot press to cure the resin. The mat was hot pressed at 2Mpa and 190°C for 150 seconds.

Two identical mats were also prepared and hot pressed under the same conditions. One mat was hot pressed without the surface of the mat being treated in any manner, and the other mat was sprayed on the top and bottom surfaces with a mixture of water and the release agent prior to the mat being hot pressed.

The mat sprayed with the composition described above exhibited significantly less precure than the other two mats as assessed by a visual comparison and by a density scan using a Gracon density scanner.

The present invention advantageously provides a method for reducing precure during the curing of a thermosetting resin in a hot press. The method can be used to reduce precure during the manufacture of reconstructed wood panels. When the method of the present invention is used to reduce precure during the manufacture of reconstituted wood panels, the method can, by reducing precure, reduce waste during the manufacture of the reconstituted wood panels and thus can reduce the costs of producing the reconstituted wood panels. An advantage of the method of the present invention is that it can be used to reduce precure during the manufacture of reconstituted wood panels without requiring modification of conventional hot presses currently used in the manufacture of reconstituted wood panels.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. All such variations and modifications are to be considered within the scope of the present invention the nature of which is to be determined from the foregoing description.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word"comprise"or variations such as"comprises"or"comprising"is used in an inclusive sense, i. e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.