GB2090261A - Sheets or moulded bodies and methods for their manufacture - Google Patents

Abstract

Sheets or moulded bodies, for example particle board, are manufactured by hot pressing a lignocellulosic material in contact with an organic polyisocyanate and an animal wax having a melting point of at least 60 DEG C. Preferably the animal wax is applied to the lignocellulosic material in the form of a dispersion in an aqueous emulsion of the polyisocyanate. The process facilitates the release of the sheets or moulded bodies from the caul plates or platens of the press.

Description

SPECIFICATION Sheets or moulded bodies and methods for their manufacture The invention relates to sheets or moulded bodies and methods for their manufacture.

The use of organic polyisocyanates as binders for ligno-cellulosic material in the manufacture of sheets or moulded bodies such as chipboard, fibreboard and plywood is well known.

In a typical process, the organic polyisocyanate, optionally in the form of a solution or an aqueous emulsion, is applied to the ligno-cellulosic material which is then subjected to heat and pressure. The high quality of the resulting articles is due, to a very large extent, to the excellent adhesive properties of organic polyisocyanates. At the same time, however, these adhesive properties can be a disadvantage in that they cause severe sticking of the ligno-cellulosic material to the hot metal surfaces with which it comes into contact during the hot-pressing operation. The product can be harmed in this way or even rendered entirely worthiess and the subsequent cleaning of the contaminated surfaces is difficult, time-consuming and costly.

Several attempts have been made to overcome this adhesion problem but no completeiy satisfactory solution has been found. Thus, the conventional release agents such as cils, wax polishes, silicones and polytetrafluoroethylene have proved unsatisfactory as have the specialised urethane release agents including those used in structural foam applications.

In our United States Patent No. 4110397 it has been proposed to apply a metallic soap at the interface of the ligno-cellulosic material and the mould surface. The metallic soaps are, in fact, excellent release agents but in practice their efficient application to the appropriate surfaces causes problems and limits their usefulness.

A technique that has been used in the manufacture of particleboard is to provide a core of isocyanate treated wood chips with outer layers of chips separately treated with a formaldehyde condensate adhesive resin. The adhesion problem is reduced in this way but the quality of the product is less satisfactory. The use of release papers has also been tried but causes process difficulties and adds to costs.

It has beer: reported by Thomas M. Maloney in "Modern Particleboard and Dry-Process Fiberboard Manufacturing" (Miller-Freeman Publications) that the inclusion of a paraffin wax in a particleboard formulation together with formaldehyde based resins to improve waterrepellency helps to keep the board from sticking to the caul plates and hot press (page 401). Paraffin waxes do not have this effect when polyisocyanates are used as binders and it is acknowledged at page 377 of the same book that the adhesion of isocyanates to caul plates and press parts is a serious problem.

It has now surprisingly been found that excellent release from metal and other surfaces is obtained if, in addition to being treated with an organic polyisocyanate, the ligno-cellulosic material is treated with certain animal waxes as hereinafter defined.

Thus, according to the invention, there is provided a process for the manufacture of sheets or moulded bodies which comprises hot pressing a ligno-cellulosic material in contact with an organic polyisocyanate and an animal wax having a melting point of at least 600 C.

Whilst the process of the invention is particularly suitable for the manufacture of the particleboard known extensively as chipboard and will be largely described with reference to such manufacture, it is not to be regarded as limited in that respect and can also be used in the manufacture of medium density fibre board, wafer board, mouldings by the Werzalit process and plywood. Thus, the ligno-cellulosic material used can include wood chips, wood fibres, shavings, veneers, wood wool, cork, bark, sawdust and like waste products of the woodworking industry as well as other materials having a ligno-cellulosic basis such as bagasse, straw, flax, sisal, hemp, rushes, reeds and grasses. Additionally, there may be mixed with the ligno-cellulosic material, other particulate or fibrous materials such as glass fibre, mica, asbestos, rubber and plastics materials.

Organic polyisocyanates which may be used include diisocyanates and high functionality isocyanates, particularly aromatic polyisocyanates. Mixtures of polyisocyanates may be used of which the crude mixtures of di- and higher functionality polyisocyanates produced by phosgenation of aniline,f & maldehyde condensates, known as crude MDI, are especially suitable. The organic polyisocyanates may be isocyanate-ended prepolymers made by reacting an excess of a diisocyanate or higher functionality polyisocyanate with a polyol. The organic polyisocyanate may advantageously be used in the form of an aqueous emulsion by stirring the isocyanate with water in the presence of an emulsifying agent. Suitable emulsions have been described in U.K. Patent Specification No.

1444933, U.K. Patent Application 201 8796A and European Patent Publication No. 13112.

Animal waxes which may be used in the process of the invention include beeswax.

Mixtures of animal waxes having melting points of at least 600C may be used if desired.

The process of the invention is readily carried out by applying the organic polyisocyanate and the wax to the ligno-cellulosic material and then subjecting the treated material to heat and pressure. The wax may be applied before or after the polyisocyanate but preferably the two are applied together. To facilitate even distribution, the wax may be applied in the form of a solution in an organic solvent which may be the polyisocyanate but preferably it is applied in the form of an aqueous dispersion, preferably a dispersion in an aqueous emulsion of the polyisocyanate. If desired, other conventional binding agents, such as formaldehyde condensate adhesive resins, may be used in conjunction with the polyisocyanate. Conventional additives, for example, water-repellency aids, may be used.For example a paraffin wax water-repellency additive may be incorporated in the dispersion of the animal wax.

The polyisocyanate may be applied in such an amount to give a weight ratio of polyisocyanate to ligno-cellulosic material in the range 0.1:99.9 to 20:80, preferably 0.5 :99.5 to 7.0:93. A suitable weight ratio of wax to ligno-cellulosic material is in the range 0.01:99.99 to 10:90, preferably 0.1:99.9 to 2.0:98.0.

In particleboard manufacture, the lignocellulosic material and polyisocyanate may be conveniently mixed by spraying the polyisocyanate on to the particulate lignocellulosic material while it is being agitated in an enclosed mixer either of a single batch type or a continuous type. The polyisocyanate is preferably employed in the form of an aqueous emulsion and since the wax is conveniently applied in the form of an aqueous dispersion, the polyisocyanate and wax are most suitably sprayed on to the particulate ligno-cellulosic material in a single aqueous system.

The treated ligno-cellulosic material may then be sprinkled on to caul plates, usually made of aluminium or steel, which serve to carry the "furnish" into the press where it is compressed to the desired extent, usually at a temperature between 1400 and 230or. In the absence of an efficient release agent the ligno-cellulosic material adheres strongly to the caul plates and/or the platens of the press. In the process of the present invention, the adhesion of particles of the lignocellulosic material to the metal surfaces is reduced to a negligible extent. This is so even when the surfaces are already contaminated with resinous materials or their degradation products. At the start of a manufacturing run it is helpful, but not essential, to condition the caul plates using a wax of the type used in the Drocess of the invention.

Thus the caul plates are suitably sprayed with an aqueous dispersion of the wax release agent. The conditioned plates may then be used a large number of times in the process of the invention without further treatment.

Other methods of manufacturing particleboard include depositing the treated ligno-cellulosic material on to conveyor belts for transfer to the press and here again the process of the invention is advantageous in minimising adhesion of the ligno-cellulosic material to the belts and press.

More detailed descriptions cf methods of manufacturing particleboard and similar products based on ligno-cellulosic material are available in the prior art. The techniques and equipment conventionally used can be adapted for use in the process of the invention.

In some manufacturing situations, the treated ligno-cellulosic material may come into contact with materials other than those specifically mentioned above, for example brass, chrome plated surfaces and plastics materials. In such situations, the process of the invention is again remarkably effective in minimising unwanted adhesion.

The sheets and moulded bodies produced in accordance with the 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 by the following Example in which all parts and percentages are by weight unless otherwise stated.

EXAMPLE 1 An aqueous dispersion of beeswax was prepared by dissolving 2 parts of Lubrol W and 2 parts of a condensate of cetyl/oley! alcohol with 17 mols of ethylene oxide as emulsifying agent in 86 parts of water at 95-980C and then adding 10 parts of beeswax at the same temperature with constant stirring. After addition of the wax, the dispersion was stirred whilst cooling to room temperature.

An emulsifiable isocyanate was prepared by reacting 3 parts of methoxy polyethylene glycol (molecular weight 650) with 97 parts of crude MDI having an NCO content of 30% and containing approximately 50% of diphenylmethane diisocyanates, the remainder being polymethylene polyphenyl polyisocyanates of higher functionality.

6 parts of the emulsifiable isocyanate were thoroughly mixed into 12 parts of the wax dispersion to give an isocyanate emulsion which was sprayed on to 102 parts of a 50/50 spruce/pine wood chip mixture (moisture content 2.0% particle size 5 mesh) using a laboratory rotary blender. Two caul plates 50 cm x 50 cm, one aluminium the other steel, were conditioned with a beeswax emulsion made as described above. The blended treated chip was then placed on the bottom aluminium caul plate to give a furnish of depth 8 cm. The caul plate was then transferred to a single daylight press with platen temperatures 1 750C, the steel caul plate placed on top of the furnish and 19 mm metal stops placed in position. The press was closed and a ram pressure of 900-11 00 psi (i.e. board pressure 350-550 psi) was maintained for 4 minutes 45 seconds. The press was then opened and the particle board released from both surfaces without any noticeable adhesion. The operation was repeated 20 times without any further conditioning of the caul plates, the result being the same in each case. The particle boards were still releasing satisfactorily when the run was terminated and the caul plates could have been used an even larger number of times without further conditioning.

Claims (11)

1. A process for the manufacture of sheets or moulded bodies which comprises hot pressing 3 ligno-cellulosic material in contact with an organic polyisocyanate and an animal wax having a melting point of at least 600 C.

2. A process according to claim 1 wherein the animal wax is beeswax.

3. A process according to the preceding claims wherein the organic polyisocyanate is applied to the ligno-cellulosic material in the form of an aqueous emulsion.

4. A process according to claim 3 wherein the wax release agent is applied together with the organic polyisocyanate in the form of a dispersion in the aqueous emulsion of the polyisocyanate.

5. A process according to any of the preceding claims wherein the ratio of the organic polyisocyanate to the ligno-cellulosic material is in the range 0.1:99.9 to 20:80 by weight.

6. A process according to claim 5 wherein the ratio of the organic polyisocyanate to the lignocellulosic material is in the range 0.5:99.5 to 7.0:93.0 by weight.

7. A process according to any of the preceding claims wherein the ratio of the wax release agent to the ligno-cellulosic material is in the range 0.001:99.9 to 10:90 by weight.

8. A process according to claim 7 wherein the ratio of the wax release agent to the lignocellulosic material is in the range 0.1:99.9 to 2.0:98.0.

9. A process substantially as described with reference to any one of the Examples.

10. Sheets or moulded bodies whenever prepared by a process according to any of the preceding claims.

11. An aqueous emulsion of an organic polyisocyanate having dispersed therein an animal wax having a melting point of at least 600 C.