GB2265914A

GB2265914A - Method of plasticising medium density fibre board

Info

Publication number: GB2265914A
Application number: GB9207418A
Authority: GB
Inventors: Nicholas Hardy
Original assignee: HOFFMAN THORNWOOD PLC; WOOD BENDERS SYSTEMS Ltd; WOODBENDERS SYSTEMS Ltd
Current assignee: HOFFMAN THORNWOOD PLC; WOOD BENDERS SYSTEMS Ltd; WOODBENDERS SYSTEMS Ltd
Priority date: 1992-04-04
Filing date: 1992-04-04
Publication date: 1993-10-13
Also published as: GB9306627D0; AU3899593A; GB9207418D0; WO1993019911A1

Abstract

In a method of plasticising medium density fibre board (MDF) the MDF is impregnated with anhydrous ammonia and then reshaped. Prior to said impregnation, the MDF may be exposed to a partial vacuum in an enclosed reaction chamber. Apparatus for carring out the method is also shown. That apparatus comprises a reaction chamber connected to a variable volume reservoir chamber, (defined by, for example, a collapsible bag), and to a vacuum source. As the MDF absorbs ammonia from the reaction chamber, the volume of the reservoir chamber reduces, enabling the pressure of ammonia in the reaction chamber to be maintained.

Description

METHOD OF PLASTICISING MEDIUM DENSITY FIBRE BOARD 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.

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 the conventional techniques that are used on wood and wood products. Unlike wood in which the fibres extend longitudinally and are cross-linked, the fibre board comprises a compressed mass of 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.

It is a general objective of the present invention to provide a method of plasticising MDF effectively to allow bending or other deformation to be carried out without significant weakening or damage to the material.

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

Advantageously the anhydrous ammonia is gaseous and the medium density fibreboard 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.

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

Advantageously the temperature of the reaction chamber is monitored during the plasticisation process to determine when the process 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 wood 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.

Description of the Preferred Embodiments 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 sub atmospheric 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, the reaction is energetically efficient at ambient temperature and, therefore, heating is not normally necessary. Heated room temperatures of 20"e or thereabouts are preferred to economise on ammonia usage.

For further economy the gaseous ammonia in the chamber may be recycled 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. For thicknesses up to 2 cm the reaction time is generally of the order of one and a half hours or less. An example of this is given below: MDF meeting BS1142 (1.1 MPa internal bond strength) and having length, width and thickness dimensions of 850 mm, 150 mm and 19 mm respectively was treated in an 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 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.

The method of the present invention is extremely efficient, economical and versatile. Tightly bended complex configurations including multiple spirals may be achieved within comparatively short reaction times of 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 refrigeration 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 substantial breakthrough in handling of medium density fibre board and will greatly increase the range of application of the material.

Claims

1. A method of plasticising medium density fibre board, which method comprises impregnating the medium density fibre board with anhydrous ammonia.

2. A method as claimed in Claim 1, wherein the anhydrous ammonia is gaseous and the medium density fibreboard is impregnated therewith in an enclosed reaction chamber.

3. A method as claimed in Claim 2, wherein 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.

4. A method as claimed in Claim 1, 2 or 3, wherein the gaseous anhydrous ammonia is substantially pure.

5. A method as claimed in Claim 1, 2, 3 or 4, wherein the temperature of the reaction chamber is monitored during the plasticisation process to determine when the process is substantially effectively complete.

6. A method of altering the shape, texture or configuration of medium density fibre board with method comprises impregnating the medium density fibre board with anhydrous ammonia and forming the wood to a desired shape, texture or configuration.

7. A method as claimed in Claim 6, wherein the anhydrous ammonia is gaseous and the medium density fibreboard is impregnated therewith in an enclosed reaction chamber.

8. A method as claimed in Claim , 7, wherein the step of forming the medium density fibre board is carried out by manipulation of the board shortly after its removal from the chamber.