Field of the invention

The present invention relates to a method of plasticising medium density fibre board to enable it to be altered to a desired shape, texture or configuration; to a piece of medium density fibre board which has been plasticised and to apparatus for plasticising medium density fibre board.

Background to the invention

Medium density fibre board (MDF) is a composite material that has been commercially available for at least two decades. In that time it has proven extremely popular for a number of reasons, including its high strength, homogeneous nature, absence of grain and consequent ease of working by machining to provide a smooth high quality finish.

MDF does, however, have a major drawback in that it is notoriously difficult to plasticise effectively for altering its shape, by for example bending the board or imprinting an external surface of the board with with a pattern or design. The conventional techniques that are used on wood and wood products have been found to be inappropriate for MDF. Unlike wood, in which the fibres extend longitudinally and are cross-linked, the fibre board comprises a compressed mass if disordered fibres which are bonded together by a synthetic resin such as urea formaldehyde or melamine urea formaldehyde. In MDF little or no concerted longitudinal stretch or compression

of the fibres can occur and, therefore, the material does not readily deform without significant loss of tensile strength when subjected to the conventional treatment of applying pressure with or without heat and/or moisture. The widely used wood plasticisation technique of applying steam or boiling water is especially detrimental to the integrity and strength of the MDF apparently because it causes swelling of the fibres and degradation of the bonding resin.

Tt is a general objective of the present invention to provide a method of plasticising MDT ⁷ effectively to allow beding or other deformation to be carried out without significant weakening or damage to the material.

Summary of the invention

According to a first aspect of the invention there is provided a method of plasticising medium density fibre board, which method comprises impregnating at least part of the medium density fibre board with anhydrous ammonia.

Where an imprint is to be made in an external surface of the board, it may only be necessary to impregnate the outer layers of the board in the region of that surface. If, however, the board is to be re-shaped by being bent curved or twisted, the board is preferably impregnated across substantially the entire thickness of the board at least in the region of the board to be deformed.

Advantageously the anhydrous ammonia is gaseous and the medium density fibre board is impregnated therewith in an enclosed reaction chamber. Preferably the ammonia is applied to the medium density fibre board at a pressure of

less than about five bars and more preferably less than three bars .

Preferably, prior to impregnation of the board with anhydrous ammonia, the board is subjected to an at least partial vacura by at least partially evacuating the reaction chamber.

In this case, the chamber may then be filled with gaseous anhydrous ammonia conveniently substan ially at atmospheric pressure.

The reaction chamber may with advantage be connected to a reservoir chamber of a variable volume, which contains anhydrous ammonia, ^' and is so arranged that the absorption by the board of gaseous anhydrous ammonia in the reaction chamber leads to the reduction of the volume of the reservoir chamber so as to allow the transfer gaseous anhydrous ammonia from the reservoir chamber to the reaction chamber to maintain the desired pressure in the reaction chamber.

The reservoir chamber is preferably collapsible, and is convieniently defined by a flexible bag.

The anhydrous ammonia supplied to the reaction chamber is preferably substantially oure. The ammonia is suitably not less than 98% and preferably more than 99% pure.

Advantageously the temperature of the reaction chamber is monitored during the plas icisation process to determine when the process is substantially effectively complete. It has been found that the temperature in the reaction chamber increases during the reaction between the

anhydrous ammonia and the board and stabilises when the reaction is substantially effectively complete.

According to a further aspect of the present invention there is provided a method of altering the shape, texture or configuration of medium density fibre board which method comprises impregnating the medium density fibre board with anhydrous ammonia and forming the board to a desired shape, texture or configuration.

Preferably the step of forming the medium density fibre board is carried out by manipulation of the board shortly after its removal from the chamber.

The invention also lies in a piece of medium density fibre board which has been plasticised by a method according to the first aspect of the invention, and to a piece of medium density fibre board which, having been plasticised, has been deformed.

Brief description of the drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings which ^' show in diagramatic form a reaction chamber and reservoir chamber for carrying out a method according to the first aspect of the invention, and in which:-

Figure 1 shows the apparatus with the reservoir chamber in its inflated, filled state; and

Figure 2 shows the apparatus with the reserviour in a partially collapsed state.

Detailed description

Medium density fibre board to be bent, expanded compressed, impressed or otherwise formed is placed in a suitably dimensioned reaction chamber. The chamber is closed and may be at least partially evacuated before high purity gaseous anhydrous ammonia is piped into the chamber to fill the chamber to a suitable pressure. The gaseous ammonia is suitably obtained from a source of greater than 99.7% purity.

The conditions of temperature and pressure of the process of plasticisation of the MDF by the gaseous ammonia may be varied within a substantial range. However, pressures of gaseous anhydrous ammonia within the reaction chamber of above five bars have a notable effect in causing expansion of the fibre board whereas pressures below three bars appear not to induce swelling to any significant extent. This effect is especially marked with MDF of low internal bond strength. No deleterious effects are obtained by use of subatmospheric pressures. However, supply of markedly sub atmospheric pressure of gaseous ammonia will substantially extend the reaction time. Pressures of between one and three bars are, therefore, believed to be most appropriate. With regard to temperature and, the reaction is energetically efficient at ambient temperature and, therefore, heating is not normally necessary. Heated room temperatures of 20°C or thereabouts are preferred to economise on ammonia usage.

For further economy the gaseous ammonia in the chamber may be recyled by provision of suitable pipework and one or more further chambers.

The duration of the process of plasticisation by impregnation with the gaseous anhydrous ammonia is proportional to the size of fibre board being treated. An example of the use of the process which employs anhydrous ammonia at a greater pressure than atmosphere is given below:

MDF meeting BS1142 (1.1 MPa internal bond strength) and having length, width and thickness dimensions of850 mm, 150 mm and 19 mm respectively was treated in and enclosed reaction chamber at an ambient temperature of 8.5° centigrade with gaseous anhydrous ammonia at two and half bars pressure for a period of 90 minutes. The temperature of the reaction chamber was monitored throughout and rose by 6° centigrade above the starting temperature and stabilised at that level. The sample of fibre board was removed and bent to a curve of 400 mm radius of curvature and held in that state by clamps for a period of 24 hours. When subsequently released, the sample retained the curvature without significant spring back and detailed laboratory analysis indicated that no significant deterioration of the fibre board had occurred.

From repeated trials it has been found that the temperature of the reaction chamber in which the fibre board is plasticised increases during the reaction and stabilises when the reaction is substantially effectively complete. The observed temperature rise is generally of the order of 6" centigrade.

In a modified version of the process, the MDF is exposed to ammonia substantially at atmospheric pressure, and to that end the appartus shown in Figures 1 and 2 may be

emp l oye d .

The reaction chamber 1 includes an entrance through which a piece of MDF (not shown) may be placed in the reaction chamber 1. kfter the MDF has been placed in the reaction chamber 1, the entrance is closed by means of a suitable door (not shown) which, when closed, seals the entrance to prevent the passage of gas therethrough.

The reaction chamber 1 communicates with a vacuum pump 2 via a valve 4 and with the reservoir chamber 6 via valve 8, and the temperature inside the reaction chamber 1 can be monitored by a temperature sensor (not shown). The reservoir chamber 6 is bounded and hence defined by a flexible bag 10, which can be inflated with gaseous anhydrous ammonia supplied from a pressurised source (not shown) through a valve 12.

In use, the MDF is placed in the reaction chamber 1, the door to the reaction chamber 1 is closed and the reaction chamber 1 is then partially evacuated by the pump 2 which draws air out of the reaction chamber 1 through the valve 4, the valve 8 being maintained in its closed position. The valve 4 may then be closed so that the reaction chamber 1 is maintained in a partially evacuated state for an appropriate length of time (discussed below).

During this stage, the bag 10 is inflated with gaseous anhydrous ammonia, and, the valve 12 is then closed.

Subsequently the valve 8 is opened to allow gaseous anhydrous ammonia to fill the reaction chamber 1 from the bag 10, which partially collapses, into the condition shown in Figure 2, in which the pressure of gaseous

anhydrous ammonia in the bag 10 and reaction chamber 1 is substantially 1 atmosphere.

As the MDF reacts with gaseous anhydrous ammonia it absorbs the gas from the reaction chamber 1, causing the bag 10 to collapse further, permitting more gaseous anhydrous ammonia to be transferred to the reaction chamber 1 so as to maintain the pressure of the anhydrous gaseous ammonia in the latter.

Three examples of the use of the process are given below:

Example 1

Boards meeting EMB standards cut to

300mm x 500mm x 12mm and 300mm x 500mm x 19mm

were place in the reaction chamber 1 which was then evacuated to 27 inches of mercury, and then left for a period of 10 minutes. The boards were then treated with anhydrous gaseous ammonia at atmospheric pressure within the reaction chamber for 4 hours. They were then placed in a press heated to 50°C and formed within the press for 5 minutes to a curve having a diameter of 860mm. During this stage the ammonia was sufficiently boiled off for them permanently to retain their altered shape

These boards were tested by the manufacturer of the board for internal bond strength and swelling changes. The tests proved negative there was no change from the original specifications of the board.

Examp l e 2

Boards cut to 160mm x 200mm x 18 mm were placed in a reaction chamber which was then evacuated as in Example 1. The boards were then treated with anhydrous ammonia at atmospheric pressure within the reaction chamber, for a period of 24 hours.

They were then placed in a press heated to 50°C and a Prince of Wales feather pattern measuring 150mm x 110mm was impressed into the boards to a depth of 7mm, causing the ammonia to be sufficiently boiled off for the boards to retain their new shape.

Example 3

Boards cut to 650mm x 450mm x 1.8mm were placed in a reaction chamber which was then evacuated as before. The boards were then treated with anhydrous ammonia at atmospheric pressure, within the reaction chamber, for a period of 24 hours.

They were then placed in a press heated to 50°C and formed, to give a raised and fielded panel effect, for a period of 4 minutes at a pressure of 70 P.S.I during which time the ammonia was sufficiently boiled .off for the boards to retain their new shape. X ^* 7hen the panels were laid on a flat surface, it was found that the boards had been formed to a depth of 12mm.

The method of the, present invnetion is extremely efficient, economical and versatile; Tightly bended complex configura ions including multiple spirals may be achieved within comparatively short reaction times of as

little as between 1 and 2 hours and with thicknesses of board of 1-2 cm or more. No input of heat is required and substantial cost-savings may be obtained through use of gaseous anhydrous ammonia which does not require refigeration and which does not need to be supplied at super atmospheric pressures.

It will be appreciated from the foregoing that the present invention represents a subtantial breakthrough in handling of medium density fibre board and will greatly increase the range of application of the material.