GB2028841A - Method of producing composite material - Google Patents

Abstract

A composite material in the form of a board, block or the like is produced by mixing together particles of resinous particulate material such as wood chips, chopped branches, tree bark, leaves and pine needles or peat and chopped rubber, with a thermosetting resin and thereafter hot pressing the mix to set the composite. The resinous material may be scrap wood particles from sawmills and it would normally require drying to a moisture content of around 7% by weight. The product is a strong composite with good machinability and other physical properties.

Description

SPECIFICATION Method of producing composite material This invention relates to a method of producing a composite material; more specifically, but not exclusively the invention relates to the manufacture of wood-based composite material of the "particle board" type.

In previous manufacturing processes for the production of particle board, wood is ground, chopped, milled or otherwise reduced to particles which are then screened to fractions of various sizes.

The graded wood particles selected for use are carefully dried in a current of hot air to a moisture content of, approximately, 5% or less. The dried particles are mixed with a quantity of thermosetting resin and subjected to a cold pre-press roughly to form a sheet of desired dimensions. The preformed sheet is then pressed in a hot press to set the resin and bind the particles together.

The conventional process normally uses particles of the consistency of sawdust or sometimes lays down layers of particles to form a sandwich structure with a layer of relatively large particle size sandwiched between layers of sawdust sized particles.

The prior processes produce oversized boards with rough edges and these have to be subsequently sawn to size. The boards, particularly at the edges and corners are friable and are therefore easily damaged during handiing.

In use, for example, in the manufacture of laminated boards for furniture, veneers or plastics lamina are applied not only to the faces of the boards but to the edges and any damages on the edges can lead to insufficient adhesion and subsequent delamination of the edging strip.

Furthermore, conventional particle board has a relatively low breaking strength which makes it unsuitable for use in high load bearing situations.

An object of the present invention is to obviate or mitigate the aforesaid disadvantages.

According to the present invention there is provided a process for the manufacture of composite material comprising hot pressing a mixture of resinous particulate material and a thermosetting resin as a binder in a mould to compact same in the mould and to combine and harden the resins.

The most preferred resinous particulate material is wood particles. The wood may be waste timber from wood processing plant chopped branches, bark and twigs, pine needles, peat, leaves but other resinous materials, mainly of natural origin, such as rubber, may also be used.

Prior to mixing with thermosetting resin binder the particulate material may be heat-treated in a current or dry air to reduce its moisture content to from 5 to 12%, preferably about 7%.

The pressure applied during pressing is to be sufficient to bring adjacent particles in the mix into close abutment, the resin being thereby forced into thin films surrounding the particles, and it is presently thought, the resin is forced into the fibrous structure of the wood itself. A pressure of around 100 p.s.i. is suitable for the production of wood particle board.

The thermosetting resin may be, for example, a urea-formaldehyde resin, phenol formaldehyde, melamine formaldehyde or any other readily available resin.

The amount of resin used is related to the character, amount and particle size of the particulate material. For wood chips of a size which passes a 25 mm screen the weight ratio of chips to resin is optimally 9:1 but this may be altered one way or the other depending on, for example, the natural resin content of the wood. Wood particles with high natural resin content require less synthetic resin; wood with low natural resin content need more. Reduction of the particle size normally necessitates the using of more synthetic resin, not only because the smaller size increases the surface area of the particles but also because the mechanical size reduction process tends to remove a proportion of the natural resin from the particles on to the cutting or grinding elements of the size reduction machinery.

The temperature applied during hot pressing is determined by the choice of thermosetting resin to be at or above its setting temperature, a suitable range being from 100 to 1 500C. For ureaformaldehyde a temperature of about 1 2OOC is suitable. It is believed, but not confirmed experimentally, that there is an optimum temperature at which interaction of the natural and synthetic resins occurs instantaneously and irreversibly to form the hard cured polymeric resinous product. This temperature is somewhat above the normal curing temperature of the synthetic resin and it varies a little with the type of wood used. The optimum can be determined very easily by trial and error on test mouldings.

The period of time for which the particles must be pressed is determined by the quantity of particles used, as sufficient time must be given to allow heat to penetrate into the board to cure the resins. For a 0.625 inch thick board a period of from 1.5 to 2.0 minutes is sufficient.

The application of heat and pressure has the effect of causing the resinous material to exude its ovvn natural resin on to the surfaces of the particles. Since, for practical purposes, the content of natural resin will be insufficient to enable the loose particles to be bound by the application of heat and pressure alone it is necessary to add a proportion of synthetic thermosetting resin to achieve the necessary degree of binding.

The composition may contain other materials for decorative effect or to improve the physical properties of the product. Additives include sand, cement, gravel, rubber particles and dust and reinforcing members of metal, wood or plastics material.

While the main intended use of this invention is in the production of boards there is no intended restriction to this product. Since the process uses a moulding technique the invention can be used to produce moulded articles of virtually any shape or form normally produced by moulding. Blocks, bricks, tiles, planks, rods and poles can be produced as simply as boards.

The end uses of the product are mainly in the building, furniture-making, and construction industries. Boards may be used as flooring and walling. Elongate members may be used as structural elements in the construction of buildings. Pit chocks made by this invention may be substituted for the timber used conventionally.

The product may be laminated with decorative plastics laminae or it may be painted or varnished.

The product is easily worked on conventional wood working machinery; it may be sawn, ground, drilled or sanded by the same technique as for hardwood.

The mechanism by which the particles are bound together to form an article is not fully understood; several explanations are possible. Probably-it is a combination of more than one of these which is actually responsible. A tentative theory is that as the particles of wood are heated in the press the natural resin of the wood exudes on to the surface of the particles. Also that heating and pressing forces even more of natural resin from the wood into contact with the added synthetic resin with which it interacts to form a hard coherent polymeric material binding the particles together.It is possible to bind wood particles together by the application of heat and pressure alone thus using only the natural wood resin as a binder and this fact demonstrates that the wood resin can participate in the bonding process; however, a board so produced is rather less than satisfactory and it is necessary to add synthetic resin to obtain a commercially acceptable product. Other possible effects at play are that (a) the synthetic resin impregnates the fibres of the wood where it hardens and (b) the pressure compacts the fibres of the wood together making it much more dense.

The accompanying drawing is a flow diagram showing a typical sequence of operations involved in the manufacture of the composite material of this invention on an industrial scale.

Referring to the Figure, wood particles are passed sequentially through stages I to Ill where they are graded, dried and stored in a stockpile.

Resin is prepared in the resin mixer IV and then sprayed in a rotary sprayer V on to chips withdrawn from the stockpile Ill.

Resin coated chips are passed on through a way up hopper and a prepacker unit to a hot platen press (stages VI and VIII) from where they are off-loaded, stage IX, as product It has been found in the development of this invention that the heat-setting of a combination of a synthetic urea-formaldehyde resin with natural wood resin produces a hard coherent polymeric product of presently unkknown composition. The binding and adhesive properties of the heat-set product appear to be much better than many commerically available products intended for similar purposes. A heatcurable resinous product may be provided, in accordance with this invention, which comprises a mixture of natural wood resin extracted from timber and a thermosetting synthetic resin preferably of the urea formaldehyde type.Such a product may be produced by extracting natural resin from timber waste, by a solvent extraction process for example, to yield a viscous mass of the natural resin, this mass being then intimately admixed with a synthetic thermosetting resin.

The invention will now be described by way of illustration in the following Example.

EXAMPLE Waste wood from a sawmill, consisting of tree bark, chippings, scrap timber pieces and sawdust was fed to a chipping machine to produce a random mix of sizes. The mix was then screened on a 25 mm screen to remove the largest particles. The material passing the screen was further screened on a 5 mm screen. The material held on the 5 mm screen (sizes between 5 and 25 mm) is referred to as "large chip" and that which passes the screen (less than 5 mm) is called "small chip.

Several mixes of large and/or small chip and urea-formaldehyde resin were made up and pressed in a platen press according to the details given in the following Table.

TABLE Mix No. I II Ill IV

Large Chip 100% ~~~ ~~~ 65 Small Chip ~~~ 100 100 35 Wt. % Chips 90 80 85 82 Wt. % Resin 10- 20 15 18 Pressure (PSI) 100 100 100 100 Temperature ( C) 120 120 120 120 Time (Min) 1.5 1.5 1.5 1.5 Prior to mixing with resin the wood chips were pre-dried to a moisture content of 7 weight percent. The resin was in the form of uncured crystalline grains of the consistency of table sugar.

On application of heat and pressure the mix of chips and resin were closely bound together to form hard finished boards which on testing were found to have breaking strengths of around 0.3 ton.

The boards were extremely hard and were easily worked on ordinary woodworking machinery.

Claims (14)

1. A process for the manufacture of composite material comprising hot pressing a mixture of resinous particulate material and a thermosetting resin as a binder in a mould to compact same in the mould and to combine and harden the resins.

2. A process according to Claim 1, in which the resinous particulate material is wood particles.

3. A process according to Claim 2, in which the wood particles are of timber waste, chopped branches, bark, twigs, pine needles or leaves.

4. A process according to Claim 1, in which the particulate material is peat or chopped rubber.

5. A process according to any preceding claim, in which before mixing with the thermosetting resin the particulate material is heat treated in a current of dry air to reduce the moisture content of the particles to from 5 to 1 2% by weight.

6. A process according to any preceding claim, in which the temperature of hot pressing is from 100to1500C.

7. A process according to any preceding claim, in which the pressure of hot pressing is from 80 to 120 p.s.i.

8. A process according to any preceding claim, in which the particle of the particulate material are of a size which pass a 25 mm screen.

9. A process according to any preceding claim, in which the ratio of particulate material to binder is 9:1.

10. A process according to any preceding claim, in which the binder is a urea-formaldehyde resin, phenol formaldehyde or melamine formaldehyde.

1 A process for the manufacture of a wood-based product comprising drying wood particles in a current of dry air to a moisture content of 7% by weight, screening the particles on a 25 mm screen and removing the retained particles, mixing the dry, screened particles with urea-formaldehyde resin in a weight ratio of 9:1, hot pressing the mix in a mould at a pressure of 100 p.s.i. and at a temperature of 1 200C for a period of 1.5 minutes to set the mix.

12. A process for the manufacture of a composite material, substantially as hereinbefore described with reference to the accompanying drawing.

1 3. A process for the manufacture of a composite material according to the Example.

14. A composite material whenever produced by the process claimed in any of Claims 1 to 13.