GB2306177A

GB2306177A - Fibrous board

Info

Publication number: GB2306177A
Application number: GB9520913A
Authority: GB
Inventors: Emily Gina Bullock; Adrian Michael Steer; Peter John Tonge
Original assignee: Tenmat Ltd
Current assignee: Tenmat Ltd
Priority date: 1995-10-12
Filing date: 1995-10-12
Publication date: 1997-04-30
Also published as: WO1998022266A2; GB9520913D0; WO1998022266A3

Abstract

A board product comprises a matrix of a plastic clay, reinforced with a glassy inorganic fibre, the amounts of clay and inorganic fibre being in the respective ranges 29 to 80 per cent by weight and 15 to 55 per cent by weight, characterised in that the board product is compacted after formation and whilst still wet, to a density of at least 1200 kg/mâ. The clay may include both china and ball clay. The board is suitable for thermal insulation.

Description

Fibrous Sheet Material This invention relates to fibrous sheet materials such as board based on inorganic fibres.

Conventional asbestos-based millboards consist of 50 to 97% of asbestos fibre together with various fillers and binders. They have excellent properties owing to the high strength and temperature resistance of asbestos fibres and a density of about 1000 kg/m3. They can be made by paper or board making processes in which an aqueous suspension of the ingredients is applied to a sieve or filter medium through which the water drains to leave a moist tissue or felt. These felts are usually pressed or rolled into sheets, either individually or plied together to give a laminated structure.

It is desirable to replace the asbestos fibres in these materials, but direct replacement of the fibres is very difficult since asbestos fibres can be processed like textile fibres and yet have the temperature resistance, non-flammability and strength of mineral fibres.

The properties demanded from conventional millboards, and which must be matched by any replacement are:1) Good high temperature properties, ie, low organic content to ensure non-combustibility and good retention of properties after exposure to elevated temperatures.

2) Adequate strength and flexibility (tensile and flexural strengths in excess of 2 MPa and 4 MPa respectively are usually required).

3) A sufficiently low density (less than 1200 kg/m3) to provide adequate insulation properties.

4) Ideally the ability to soften and reshape the product by moistening.

5) A reasonably low price.

6) The ability to be rapidly formed into sheets - preferably by a continuous process. Ideally this should be possible using existing asbestos processing equipment.

7) The ability to be punched into various shapes with clean-cut edges and without delamination.

It has been found that by judicious choice of materials and their amounts replacement products can be made with very promising properties. For example, it is known from GB-A-2,031,043 to manufacture a board product comprising a matrix of a plastic clay reinforced with a glassy inorganic fibre the amounts of clay and inorganic fibre being in the respective ranges 29 to 80 per cent by weight and 15 to 55 per cent by weight and the density of the product being at least 500 kg/m2.

Preferably the board product contains also a complementary binder with minimal combustibility. Ball clay is a well-known highplasticity clay which is widely used in the ceramics industry, and is usually fired to produce the final product.

Boards based on this general formulation and incorporating inorganic fibres such as mineral wool have outstanding utility as a replacement for asbestos-based millboard in many applications.

But there are certain applications where it has proved hitherto impossible to replace traditional asbestos-cement products. In applications where cyclic pressures and severe thermal shock are encountered, millboard products are not satisfactory and normal practice has been to use an asbestos cement slab or block. Where such an asbestos cement block or slab is used as insulation between an induction heater and a brazing station which presents articles to the heater in rapid succession and under pressure, its ability to withstand both pressure and thermal shock are clearly crucial. Until the present invention, it was thought necessary to use conventional asbestos-cement products for this type of service.

It has now been discovered that by changing the processing conditions disclosed in GB-A-2,031,043, significantly improved performance can be achieved without resorting to the use of asbestos fibres. Thus according to the present invention, a board product comprising a matrix of a plastic clay reinforced with a glassy inorganic fibre, the amounts of clay and inorganic fibre being in the respective ranges 29 to 80% by weight and 15 to 55%, weight is compacted after formation and whilst still wet, to a density of at least 1200 kg/m3. Preferably the compaction step is carried out to a density of from 1200 to 2000 kg/m3 and, more preferably, in the range 1400 to 1600 kg/m3.

Surprisingly, it has been found that such compaction increases the strength of a millboard product such that it will withstand compressive thermal cycling, so that it can be directly substituted for conventional asbestos-cement materials, even in severe applications. Thus it has proved possible to replace with a material according to the present invention the asbestos cement blocks used in repetitive brazing with induction heaters.

In order that the invention be better understood, a preferred embodiment of it will now be described by way of example. Thus a board formulation in the form of an aqueous suspension was made up to contain: Constituent % bv wt Colloidal silica 7.0 China clay 15.0 Ball clay 30.0 Ceramic fibres 44.0 Farina starch 4.0 To this was added a minor amount of a flocculating agent, prior to forming into board.

This latter was done on a Hatschek machine and, before drying the sheet product, it was compacted to a density of 1500 Kg/m3 by pressing between platens. The resultant product had a flexural strength of 10 MPa and was resistant to thermal shock, as determined empirically by using it to replace an asbestos cement product in a cyclic brazing operation in which articles to be brazed together were presented to an induction heater by a press table, the board product of this invention constituting the only insulation between the heater and the article to be brazed.

By contrast, a conventionally process non-asbestos millboard product was too easily damaged by the combination of repeated thermal shock and pressure.

If desired, and where the product of the invention may be subject in use to impact and/or abrasion, it may be sprayed or otherwise coated to minimise the risk of dust formation.

Claims

1. A board product comprising a matrix of a plastic clay, reinforced with a glassy inorganic fibre, the amounts of clay and inorganic fibre being in the respective ranges 29 to 80 per cent by weight and 15 to 55 per cent by weight, characterised in that the board product is compacted after formation and whilst still wet, to a density of at least 1200 kg/m3.

2. A board product according to claim 1, characterised in that the compaction is to a density in the range 1200 to 2000 kg/m3.

3. A board product according to claim 1 or 2 characterised in that the compaction is to a density in the range 1400 to 1600 kg/m3.

4. A board product according to any of claims 1 to 3 further including a surface coating to minimise the risk of dust formation due to impact and/or abrasion encountered in use.

5. The use of a product according to any preceding claim as insulation material between an induction heater and a substrate to be brazed.