Binders are used in the production of composite panels and plywood. The binders can be made by reacting formaldehyde with urea and/or melamine, referred to herein as simply" (M) UF binders". The formaldehyde reacts with amine groups in the urea and/or melamine.

The formaldehyde acts as a crosslink to form polymers.

The binders are used to make composite panels from lignocellulosic materials such as wood, bagasse, straw and agricultural residues. Composite panels may include particleboard, fibreboards including medium density fibreboard (MDF), waferboard, strandboard including oriented strandboard (OSB), plywood and other products made from combinations of particles, fibres, wafers, strands and veneers. Throughout the following disclosure the reference to"composite panels"will include such boards, plywood and other products.

Composite panels made using a binder of the type to which this invention relates commonly emit gaseous

formaldehyde. The formaldehyde emissions are related to the quantity of formaldehyde used to manufacture the binder. The quantity of formaldehyde in the binder may be expressed relative to the amount of amine functional groups as the formaldehyde-amine molar ratio (F: NH2). In general, the lower the F: NH2 ratio the lower the quantity of formaldehyde which is emitted There are a number of recognised methods of measuring the emission of formaldehyde. One method is to extract the formaldehyde with toluene. The total extractable formaldehyde is related back to the mass of the composite panel. The amount of formaldehyde expressed as mg/lOOg of bone dry composite panel is commonly referred to as the perforator value. The test method is described in the CEN Standard EN120.

Other test methods/standards include J1S A5905-1994 Fibreboards and J1S A5908-1994 Particleboard, F1 formaldehyde emission as measured by JAS Structural Plywood and LVL Standards.

These methods measure the amount of formaldehyde emitted from pieces of composite panel placed in a desiccator.

The emitted formaldehyde is absorbed into water and expressed as mg/litre of water. The methods are commonly referred to as the Japanese desiccator method.

In accordance with the present invention, a formaldehyde emission less than or equal to EO as measured by the Japanese desiccator method or the CEN Standard EN120 equivalent perforator value is the objective.

The F: NH2 ratio of the binders used in the construction of composite panels influerces some of the physical and mechanical properties of the composite panel. By way of example, the tensile stress perpendicular to the plane of the panel (internal bond) and the thickness swell caused by immersion of the composite panel in water typically deteriorates as the F: NH2 ratio decreases. Therefore, when making composite panels which have very low emissions of formaldehyde (ie less than or equal to EO) the F: NH2 ratio is often so low that the binder is unable to impart the desired physical and mechanical properties to the composite panel.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a binder which can be used to make composite panels with ultra-low formaldehyde emissions yet with desired physical and mechanical properties.

An object of the present invention is thus the production of composite panels having ultra-low formaldehyde emissions.

According to one broad aspect the present invention provides a binder for composite panels as herein defined, the binder being formed by reaction of formaldehyde with urea and/or melamine to achieve a molar ratio which will result in a formaldehyde emission in a composite panel of the desired level and adding an isocyanate to reverse loss in physical and mechanical properties arising from use of the binder in the composite panel.

A second broad aspect of the invention provides a method of producing a composite panel as herein defined, the method being characterised by using a binder of the first broad aspect to bind the material of at least a core layer of the composite panel.

According to a third broad aspect there is provided by the invention a composite panel as herein defined made using a binder method of the type set forth in the first or second broad aspect.

In the preferred form the molar ratio is selected to result in a formaldehyde emission of equal to or less than EO.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention in a preferred form is based on the selection of a binder with a F: NH2 ratio which gives the desired ultra-low formaldehyde emission. To this binder

is added a quantity of the isocyanate commonly known as MDI or PMDI. The resultant (M) UF/MDI blend provides a binder for composite panels made of lignocellulosic materials which exhibits the desired end use physical and mechanical properties. This is achieved due to the MDI component reversing the losses in the physical and mechanical properties arising out of the use of the selected (M) UF binder.

The quantity of (M) UF binder, expressed as the percentage of binder solids based on the bone dry mass of the lignocellulosic material, may be between substantially 1 and 20% depending on the nature of the composite panel and the desired physical and mechanical properties. More commonly, the quantity of (M) UF binder is between 3 and 18%. Preferably, the amount used is between substantially 5 and 15%.

The quantity of MDI added depends on the desired physical and mechanical properties and the desired level of formaldehyde emission. With the objective of meeting formaldehyde emission less than or equal to EO, the quantity of MDI added will typically be between substantially 0.1 and 4% of the bone dry mass of lignocellulosic material. More commonly, it is between 0.3 and 3% preferably between substantially 0.5 and 2%.

Furthermore, the MDI which is added may be used to substitute for some or all of the melamine component of

(M) UF binders without incurring a significant decline in the moisture resistant properties of the composite panels.

To more fully describe the invention reference will now be made to the following examples.

Example 1 Laboratory-scale composite panels with surface layers made using fibre containing an MUF binder alone and a core layer made with randomly oriented strands of nominal size 75x5xO. 5mm using different combinations of the MUF binder and MDI were evaluated for formaldehyde emissions and thickness swell following immersion in water. A formaldehyde scavenger containing urea was incorporated as part of the MUF binder to give different F: NH2 ratios.

The composite panels were 30mm thick after sanding and the nominal density was 700g/m3.

The fibre for the surface layers was blended with 15% of MUF binder. The strands for the core layer were coated with combinations of MUF binder and MDI as noted in Table 1. Some physical properties including formaldehyde emissions are included in the table.

The properties are summarised in Table 1.

Table 1 Treatment No 1 2 3 4 Strand binder MUF (%) 10 10 10 10 Scavenger (%) 0 0 0.1 0.2 F: NH2 0.43 0.43 0.425 0.42 MDI (%) 0 1 1 1 Formaldehyde emission (mg litre) 0.62 0.73 0.48 0.41 Thickness swell (20°C. 24h) 11 7 8 10 Thickness swell (100°C, 10min) 68 59 46 55 Example 2 Composite panels 30mm thick were manufactured in the laboratory. The surface layers of the composite panels were made from wood fibre containing an MUF binder alone.

The core layer of the composite panels consisted of randomly oriented wood strands of nominal size 75x5xO. 5mm blended with different combinations of MUF binders and MDI. The melamine content of the MUF binders used in the core layer varied between 3 and 30%. A formaldehyde scavenger containing urea was incorporated as part of the core layer MUF binder to give different F: NH2 ratios.

The composite panels were 30mm thick after sanding and the nominal density was 700kg/m3.

The fibre for the surface layers was blended with 15% of MUF binder. The strands for the core layer were coated with combinations of MUF binder and MDI as noted in Table

2. Some physical properties including formaldehyde emissions are included in the table.

Table 2 Treatment No 1 2 3 4 5 Strand binder MUF (%) 10 10 10 10 10 Scavenger (%) O O 0.1 0.2 0.2 F: NH2 0.43 0.43 0.425 0.42 0.42 Melamine content (%) 30 30 30 10 3 MDI (%) 0 1 1 1 1 Formaldehyde emission (mg/litre) 0.95 0.81 0.60 0.54 0.51 Thickness swell (20°C, 24h) 10 9 8 7 5 Thickness swell (70°C, 2h) 28 23 22 31 24 Example 3 A series of composite panels were manufactured in a composite panel plant. The composite panels had surface layers made using wood fibre containing an MUF binder and a core layer made from randomly oriented wood strands of nominal size 75x5x0. 5mm using different combinations of MUF resins and MDI. A formaldehyde scavenger containing urea was incorporated as part of the core layer MUF binder to give different F: NH2 ratios. The thickness of the composite panels after sanding was 30mm and the nominal density was 600kg/m3.

The fibre for the surface layers was blended with 15% of the MUF binder in the blowline. The strands for the core layer were coated with combinations of (M) UF binders and MDI in a rotary drum blender as noted in Table 3. The internal bonds of the composite panels and their swelling characteristics in water under various conditions were measured (see Table 3).

Table 3 Treatment No 1 2 3 4 5 6 Strand binder MUF (%) 10 10 10 10 10 10 Scavenger (%) 0 2 2 2 1 1 F: NH2 0.43 0.34 0.34 0.34 0.38 0.38 Melamine content (%) 30 10 10 30 30 10 MDI (%) 0 1 2 2 2 2 Formaldehyde emission (mg/litre) 0.82 0.24 0.21 0.26 0.39 0.34 Internal bond (kgf/cm2) 0.36 0.16 0.26 0.37 0.41 0.41 Thickness swell (20°C, 24h) 4. 7 7.7 5.8 3.9 4.2 4.9 Example 4 A series of composite panels was manufactured in the laboratory using (M) UF binders containing from 0 to 50% melamine based on solids and MDI. The surface layers of

the composite panels consisted of wood fibres with an (M) UF alone as the binder. The core layer of the composite panels consisted of wood strands of nominal size 75x5x0. 5mm, randomly oriented and with various combinations of (M) UF binder and MDI.

The fibre for the surface layers was blended in a laboratory blender with 15% of an MUF binder. The strands for the core layer were coated with combinations of (M) UF and MDI as noted in Table 4. The internal bonds of the composite panels and their swelling characteristics under various conditions were measured (see Table 4).

Table 4 Treatment No 1 2 3 4 5 6 7 8 Strand binder MUF (%) 15 10 10 10 10 10 10 15 Melamine content (%) 30 0 0 3 13 30 50 30 MDI (%). 0 0 1 1 1 1 1 0 Internal bond<BR> (kg/cm) 1.23 0.77 0.99 1.00 1.12 1.03 1.10 1.24 Thickness swell (20°C, 24h) 9.6 13.6 10.4 8.1 9.6 8.8 8.3 10.6 Thickness swell (70°C, 2h) 18.8 37.3 21.9 28.6 18.9 17.3 16.6 18.2 Thickness swell (100°C, 10min) 16.0 31.7 9.7 16.0 11.1 14.2 14.3 14.1 The present invention thus provides a melamine-urea- formaldehyde/diphenylmethane-di-isocyanate (MUF/MDI) blend as a binder for composite panels made from

lignocellulosic materials with a formaldehyde emission level of less than EO.

The use of PMDI may lead to a reduction in the amount of melamine in the (M) UF resin component without loss of moisture resistance properties. It is possible to obtain thickness swells better than those obtained with a 50% melamine resin BOS with a resin containing less than 50% melamine.

It is also possible to use an (M) UF resin and PMDI to improve the thickness swells of composite panels.

The PMDI addition compensates for the loss of product properties brought about by the low quantities of formaldehyde in the (M) UF resins.

(M) UF may contain an added quantity of UF resin and/or urea solution and/or other formaldehyde scavenger with a preferred option being an (M) UF resin.

By use of the binder according to the present invention, production of composite panels having ultra-low formaldehyde emissions (emissions less than or equal to EO) can be achieved without adversely impacting on the physical and mechanical properties of the composite panel.