This invention relates to polyisocyanate compositions and, in particular, to polyisocyanate compositions for use in binding lignocellulosic material used in the manufacture of wafer board (known extensively as oriented strand board), medium density fϊberboard and particle board (also known as chipboard).

The use of organic polyisocyanates as binders for lignocellulosic material in the manufacture of sheets or molded bodies such as wafer board, chipboard, fϊberboard and plywood is well known and is commercially desirable because the resulting composites have high adhesive and cohesive strength, flexibility to changes in wood species, versatility with respect to cure temperature and rate, excellent structural properties of the resulting composites and the ability to bond with lignocellulosic materials having higher water content than typically used for condensation resins such as phenol formaldehyde. In a typical process the organic polyisocyanate, optionally in the form of a solution, dispersion or aqueous emulsion, is applied to the lignocellulosic material which is then subjected to heat and pressure.

Preferred isocyanates are aromatic polyisocyanates of functionality two or higher such as pure diphenylmethane diisocyanate (MDI) or mixtures of methylene bridged polypheny 1 polyisocyanates containing diisocyanates, triisocyanates and higher functionality polyisocyanates. Methylene bridged polyphenyl polyisocyanates are well known in the art. They are prepared by phosgenation of corresponding mixtures of polyamines obtained by condensation of aniline and formaldehyde. For convenience, polymeric mixtures of methylene bridged polyphenyl polyisocyanates containing diisocyanate, triisocyanate and higher functionality polyisocyanates are referred to hereinafter as polymeric MDI.

These polyisocyanate binder compositions should be able to provide for composites with superior dimensional stability when in contact with moisture.

However the dimensional stability achieved (thickness swell in the presence of moisture and shrinkage) is not optimal when standard polymeric MDI compositions are used. Standard polymeric MDI generally has a difunctional MDI content of between 30 and 50 wt%, preferably about 40 wt%. US 4528117 describes aqueous isocyanate emulsions and their use as binders in the production of shaped articles. These emulsions contain from 80 to 20 wt% of water, from 18 to 79 wt% of an organic polyisocyanate, e.g. polymeric MDI containing from 35 to 70 wt% of difunctional MDI, from 0.5 to 15 wt% of a sulfonic acid as release agent and from 0 to 10 wt% of a nonionic surface-active agent as an emulsifϊer.

AU 677060 describes an isocyanate-based composition useful as binder for the production of composite materials. The composition is made up of an aromatic polyisocyanate and a polyester having an average molecular weight of from 600 to 5000 which polyester is obtainable by self-condensation of ricinoleic acid, optionally using a C2-C20 starter polyol, and optional additives. Preferably the polyisocyanate is a polymeric MDI in which the proportion of diphenylmethane diisocyanates is between 35 and 75 wt%.

It is an object of the present invention to provide polyisocyanate compositions for binding lignocellulosic materials that minimise thickness swell without adversely affecting other performance characteristics such as bonding strength.

The present invention provides a polyisocyanate composition for binding lignocellulosic materials comprising methylene bridged polyphenyl polyisocyanates having a content of difunctional diphenylmethane diisocyanate isomers of between 50 and 70 wt%, preferably between 55 and 70 wt% and even more preferably between 60 and 70 wt%.

By using polymeric MDI with such a high content of difunctional species a substantial reduction (5 to 18 %) in thickness swell of the lignocellulosic bodies bound with said polymeric MDI is achieved at the same overall resin loading.

Alternatively a substantial reduction (25 to 40 %) in resin loading is possible while maintaining the swelling behaviour at the same performance level, hence leading to an economic benefit. And at the same time all the other properties remain similar or at least are not detrimentally affected. The polymeric MDI composition of the present invention has a difunctional MDI isomer content of between 50 and 70 wt%. This difunctional MDI content can consists of any of the MDI isomers (4,4'-, 2,2'- 2,4'-) either on its own or as a mixture of one or more. Preferably a mixture of the 4,4 '-isomer and the 2,4 '-isomer will constitute the difunctional MDI.

The polyisocyanate composition of the present invention containing such high content of difunctional species can be obtained by suitable modification of the ratio of the ingredients and/or of the separation techniques in the process for making the precursor polyamines and/or the subsequent phosgenation process. For example, by selection of a suitable molar ratio of aniline to formaldehyde (in whatever physical form) in the manufacture of the polyaromatic polyamine (MDA) a polymeric MDI with a high content of difunctional MDI species can be obtained.

Alternatively and preferably the polyisocyanate composition of the present invention is obtained by blending a standard polymeric MDI having a lower difunctional MDI content (usually in the range 30 to 50 wt%, preferably 38 to 48 wt%, most preferably 40 to 45 wt%) with another polymeric MDI composition having a higher difunctional MDI content or with a polyisocyanate composition containing solely one or more of the difunctional MDI isomers, the latter combination being preferred.

The polymeric MDI composition of the present invention can also be a water-emulsifϊable one as described, for example, in GB 1444933 and EP 516361, incorporated herein by reference. The polymeric MDI is made water-emulsifϊable by reaction with a compound (preferably a monoalkyl ether of polyethylene glycol) such that after reaction a non-ionic surface-active agent devoid of hydroxy, amino and carboxylic acid groups is obtained.

Especially for applications such as medium density fϊberboard the use of emulsifϊable polyisocyanate compositions is preferred.

These emulsifϊable polymeric MDI compositions of the present invention having a content of difunctional MDI isomers in the presently claimed ranges can preferably be obtained by mixing an emulsifϊable polymeric MDI with low difunctional MDI content with a composition containing solely difunctional isomers. Or alternatively standard polymeric MDI with low difunctional MDI content is blended with a difunctional MDI composition and subsequently this blend is modified in the standard ways so as to become emulsifiable.

Modified polyisocyanates containing isocyanurate, carbodiimide or uretonimine groups may be employed as well. Further blocked polyisocyanates, like the reaction product of a phenol or an oxime and a polyisocyanate, may be used, having a deblocking temperature below the temperature applied when using the polyisocyanate composition.

The organic polyisocyanate may also be an isocyanate-ended prepolymer made by reacting an excess of a diisocyanate or higher functionality polyisocyanate with a polyol. Preferably however such a prepolymer is not used, especially not a polyisocyanate composition made from a polyester obtainble by self-condensation of ricinoleic acid, as described in AU 677060.

The polyisocyanate composition for use according to the present invention may be produced in accordance with any of the techniques known in the art. The difunctional content of the polymeric MDI composition may be brought within the required ranges, if necessary, by any technique well known in the art.

The polyisocyanate binder composition may further contain any of the additives generally known in the art. Conventional release agents such as polysiloxanes, saturated or unsaturated fatty acids or fatty acid amides or fatty acid esters or polyolefin wax can be added to the polyisocyanate composition of the present invention. By doing so the release performance from the press platens is improved; pre-treatment of the press platens with external release agents is another way to improve the release. In a preferred embodiment sulfonic acids such as described in US 4528117 are not present as release agent in the polyisocyanate composition of the present invention. In order to further improve either the storage stability of the polyisocyanate composition or the cost effectiveness of the present invention a diluent may be added to the composition. Examples of preferred diluents are phthalates, aliphatic carboxylates, fatty acid esters, linseed oil, soybean oil and propylene carbonate.

The composition further may comprise conventional additives like flame retardants, lignocellulosic preserving agents, fungicides, bacteriocides, biocides, waxes, fillers, surfactants, thixotropic agents, curing aids, emulsifiers, wetting agents, coupling agents and other binders like formaldehyde condensate adhesive resins and lignins, neat or modified in some way such as formaldehyde polycondense, polypropoxylated or ethoxylated. The additives can be used in the amounts commonly known in the art.

A particularly useful additive is a sizing wax further improving the thickness swell. These sizing waxes are typically used in an amount of 0.5 to 2 wt% on dry weight of wood. Examples of suitable sizing waxes include fatty acids, parafϊn waxes, Fischer-Tropsch waxes and Hydrowax, such as Hydrowax 730 available from Sasol.

This sizing wax can be premixed with the polyisocyanate composition or, preferably, applied separately to the lignocellulosic material. In this latter case it is particularly preferred that first the polyisocyanate composition is added to the lignocellulosic material and then subsequently the sizing wax.

The polyisocyanate composition of the present invention can be made by simply mixing the ingredients at room or elevated temperature or, when necessary, in case one of the ingredients is solid at room temperature, above the melting point of such an ingredient or by prior solubilisation in an appropriate solvent unless otherwise required as a suspension.

The present invention is primarily concerned with a process for preparing lignocellulosic bodies by bringing lignocellulosic parts into contact with the present polyisocyanate composition and by pressing this combination.

The lignocellulosic bodies are prepared by bringing the lignocellulosic parts into contact with the polyisocyanate composition like by means of mixing, spraying and/or spreading the composition with/onto the lignocellulosic parts and by pressing the lignocellulosic parts, preferably by hot-pressing, normally at 12O ⁰ C to 300°C, preferably 14O ⁰ C to 27O ⁰ C and 2 to 6

MPa specific pressure.

Such binding processes are commonly known in the art.

In wafer board manufacture the lignocellulosic material and the polyisocyanate composition may be conveniently mixed by spraying the present polyisocyanate composition on the lignocellulosic material while it is being agitated. In medium density fϊbreboard the lignocellulosic material and the polyisocyanate composition may be conveniently mixed by spraying the present polyisocyanate composition on the lignocellulosic material in a blowline as commonly used.

In one manufacturing process the lignocellulosic material after treatment with the polyisocyanate composition is placed on caul plates made of aluminum or steel which serve to carry the resinated furnish into a press where it is compressed to the desired extent (thickness or density specified) usually at a temperature between 12O ⁰ C and 300 ⁰ C, preferably between 14O ⁰ C and 27O ⁰ C. At the start of a manufacturing run it may be helpful, but not essential, to condition the press platens by spraying their surfaces with an external release agent or to increase the cycle time of the first press load. A preconditioned press may then be used many times in the process of the invention without further treatment.

While the process is particularly suitable for the manufacture of wafer board known extensively as oriented strand board and will be largely used for such manufacture, the process may not be regarded as limited in this respect and can also be used in the manufacture of medium density fϊberboard, particle board (also known as chipboard) and plywood.

Thus the lignocellulosic material used can include wood strands, woodchips, wood fibers, shavings, veneers, wood wool, cork, bark, sawdust and like waste products of the wood working industry as well as other materials having a lignocellulosic basis such as paper, bagasse, straw, flax, sisal, bamboo, coconut fibers, hemp, rushes, reeds, rice hulls, husks, grass, nutshells and the like. Additionally, there may be mixed with the lignocellulosic materials other particulate or fibrous materials such as grinded foam waste (for example, grinded polyurethane foam waste), mineral fillers, glass fiber, mica, rubber, textile waste such as plastic fibers and fabrics. These materials may be used in the form of granulates, shavings or chips, fibers, strands, spheres or powder.

When the polyisocyanate composition is applied to the lignocellulosic material, the weight ratio of polyisocyanate/lignocellulosic material will vary depending on the bulk density of the lignocellulosic material employed. Therefore, the polyisocyanate compositions may be applied in such amounts to give a weight ratio of polyisocyanate/lignocellulosic material in the range of 0.1:99.9 to 25:75 and preferably in the range of 0.5:99.5 to 10:90 and most preferably in the range 2:98 to 8:92 or even 1.5:98.5 to 6:94.

By using the presently claimed polyisocyanate composition lower resin loadings (25 to 40 % lower than standard loadings) can be used without dramatically deteriorating the thickness swell performance of the boards.

If desired, other conventional binding agents, such as formaldehyde condensate adhesive resins, may be used in conjunction with the polyisocyanate composition.

More detailed descriptions of methods of manufacturing wafer board and medium density fϊbreboard and similar products based on lignocellulosic material are available in the prior art. The techniques and equipment conventionally used can be adapted for use with the polyisocyanate compositions of the present invention.

The sheets and molded bodies produced from the polyisocyanate compositions of the present invention have excellent mechanical properties and they may be used in any of the situations where such articles are customarily used.

The invention is illustrated but not limited by the following examples. In these examples the following ingredients were used:

ISO 1: emulsifϊable polymeric MDI modified with 3 % of monomethyl ether of polyethylene glycol of MW 750 and having a difunctional MDI content of 44.2 wt%.

ISO 2: difunctional MDI containing about 50 wt% of 2,4'-MDI and about 50 wt% of 4,4'-

MDI. ISO 3: polymeric MDI having a difunctional MDI content of 43.6 wt%.

ISO 4: difunctional MDI containing about 2 wt% of 2,4'-MDI and about 98 wt% of 4,4'-

MDI.

ISO 5: polymeric MDI having a difunctional MDI content of 40.3 wt%.

ISO 6: emulsifϊable polymeric MDI modified with 3 % of monomethyl ether of polyethylene glycol of MW 750 and having a difunctional MDI content of 39.7 wt%. EXAMPLE l:

Emulsified compositions containing various polyisocyanates as identified below in Table 1 and water (50/50 wt/wt) were prepared. These compositions were used to make medium density fibreboards using a dry blending method wherein the wood fibres are deballed, charged into the drum blender whereupon resin is sprayed onto the wood whilst it is tumbling using an air assisted spray nozzle. Commercially produced Eastern European mixed softwood fibres having a moisture content of 12% were used. Resin loadings of 3 or 4 wt% on total wood composite were used.

Wood panels with dimensions 40 x 40 x 1.2 cm were produced using a single step press to thickness press profile with press platens at 22O ⁰ C; the total pressing time was 150 seconds. After producing the panels they were conditioned at 23 ⁰ C and 50% relative humidity for a minimum of 7 days. The samples were then sanded and cut using a circular saw to 5 x 5 x 1.1 cm. They were allowed to continue conditioning in the same conditions for a further minimum of 7 days.

Thickness swell was measured according to standard BS 317. The number represented in Table 1 below is the average results of 8 cut samples. Also internal bond strength IB V20 (dry) (according to standard BS 319 modified RH 50±5 %, temp 23±2°C) and IB VlOO (cooked) (according to standard BS 319 modified RH 50±5 %, temp 23±2°C / EN 1087-1) was measured.

The results presented in Table 1 below show that by using polymeric MDI compositions having higher difunctional MDI content than the standard polymeric MDI compositions (Ref 1 and Ref 2) leads to boards with improved swelling performance and in most cases also improved bond strength.

Table 1

EXAMPLE 2:

Compositions containing various polyisocyanates as identified below in Table 2 were prepared.

These compositions were used to make medium density fibreboards using a dry blending method wherein the wood fibres are deballed, charged into the drum blender whereup resin is sprayed onto the wood whilst it is tumbling using an air assisted spray nozzle.

Western European source of mixed softwood fibres having a moisture content of 12% were used.

Resin loadings of 4 wt% on total wood composite were used.

Wood panels with dimensions 40 x 40 x 1.2 cm were produced using a single step press to thickness press profile with press platens at 22O ⁰ C; the total pressing time was 150 seconds. After producing the panels they were conditioned at 23 ⁰ C and 50% relative humidity for a minimum of 7 days.

The samples were then sanded and cut using a circular saw to 5 x 5 x 1.1 cm. They were allowed to continue conditioning in the same conditions for a further minimum of 7 days. Thickness swell was measured according to standard BS 317. The number represented in Table 2 is the average results of 8 cut samples. Also internal bond strength IB V20 (according to standard BS 319 modified RH 50±5 %, temp 23±2°C) and IB VlOO (according to standard BS 319 modified RH 50±5 %, temp 23±2°C / EN 1087-1) was measured.

The results presented below in Table 2 show that by using polymeric MDI compositions having higher difunctional MDI content than the standard polymeric MDI compositions (Ref 3) leads to boards with improved swelling performance and also improved bond strength. Table 2

EXAMPLE 3

Compositions containing various polyisocyanates as identified below in Table 3 were prepared.

These compositions were used to make medium density fibreboards using pilot scale blow line (2 cm diameter and 20 m long).

Fibres were produced in situ from Western European source of mixed softwood fibres, in this case provided as wood chips (chips have been cooked at 15O ⁰ C, 5 bar for 5 min.). The resinated fibres were then dried to a moisture content of 7-9 %.

Resin loadings of 4 wt% on total wood composite were used. Wood panels with dimensions 50 x 50 x 1.2 cm were produced using a two step press profile in which the panel is first pressed rapidly to 13 mm and after 75 seconds the panel is pressed to final thickness of 12 mm for a further 75 seconds. Again the press platen temperature was 22O ⁰ C.

The samples were sanded, conditioned and cut to 5 x 5 x 1.2 cm dimensions. Thickness swell was measured according to standard BS 317. The number represented in Table 3 is the average results of 8 cut samples. Also internal bond strength IB V20 (according to standard BS 319 modified RH 50±5 %, temp 23±2°C) was measured.

The results presented below in Table 3 show that by using polymeric MDI compositions having higher difunctional MDI content than the standard polymeric MDI compositions (Ref 4) leads to boards with improved swelling performance and improved bond strength. Table 3

EXAMPLE 4

Similarly as in Example 3 medium density fϊbreboards were made from polyisocyanate compositions containing various polyisocyanates as identified in Table 4 except that in this case the polyisocyanates were emulsified in water (50/50 wt/wt).

Results are reported in Table 4.

Table 4