IE48578B1 - Process for the production of non-combustible asbestos-free board products - Google Patents

Description

This invention relates to the production pf noncombustible asbestos-free boarej products,.

Conventional non-combustible board products consist of 50 to 97% of asbestos fibre together with various fillers (predominantly kaolin) and binders. They have excellent properties owing to the high strength and temperature resistance of asbestos fibres, and a density of about 1000 kg/m . They can be made by paper or board making processes in which an aqueous suspension of the ingredients is applied to a sieve or other 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. Non-flat shapes, which could be difficult to make by moulding flat sheets, can be produced directly using foraminous formers of the desired shape.

It is desirable to replace the asbestos fibres in these board products, but direct replacement of the fibres is very difficult since asbestos fibres combine many useful properties: thus they can be processed like textile fibres, and yet they have the temperature resistance, non-flammability and strength of mineral fibres.

The properties which are ‘demanded of conventional asbestos-based board products and which must be matched by any replacement, are:25 - 3 10 1) Good high temperature properties i.e. low organic content to ensure non-combustibility and good retention of properties after exposure to elevated temperatures. 2) Adequate strength and flexibility. 3) A sufficiently low density (e.g. less than 1200 Kg/m ) to provide adequate insulation properties. 4) The ability to soften and reshape the product by moistening (rewetting).

) 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.

We have now found that by judicious choice of materials and their proportions, replacement products can be made with very useful properties.

According to the invention, a process for the production of a non-combustible asbestos-free board product comprises (a) dewatering on a sieve or other filter medium 25 an aqueous suspension containing the following ingredients in the following proportions by weight of total solids: Unfired Ball Clay Glassy inorganic fibre Starch Organic web-forming fibre - 67% 18 - 42% 1.5 - 5% up to 7% the content of unfired ball clay in the suspension being greater than that of the glassy inorganic fibre? and (b) drying the dewatered suspension to form a board product of tensile strength greater than 2 MPa and a density in the range 5003 1200 kg/m , under conditions such as to leave the ball clay in the unfired state and the starch content in the range 1.5-5% by weight, so that the board product is remouldable when re-wetted with water .

We have found that the ball clay, a high-plasticity clay which is widely used in the ceramics industry and is usually fired to produce the final product, has several advantages over other types of clay, such as kaolin or china clay, in that it gives a board product having better dry strength, improved wet mouldability, better interlaminar adhesion during manufacture and better interlaminar adhesion when dry. Clearly, for the board product to be remouldable when rewetted, ball clay must not be fired during preparation of the board.

The preferred glassy inorganic fibres are glassy wools including so-called glass wool, mineral wool and ceramic fibre. Of these materials (all of them expensive in comparison with ball clay) mineral wool offers the most attractive combination of temperature resistance and cost. As produced, however, mineral wools contain too much non-fihrous shot to give a satisfactory board product, and a substantial part of the shot must be removed either before or during board production. The glass or mineral wool may be given a silane pre-treatment to promote adhesion to the matrix material, and preferably - 5 is treated with a dispersing agent, e.g. a surfactant.

The length of the inorganic fibres is usually reduced during the manufacture of the board and it is difficult to lay down specific limits for the length required. The fibre must be short enough to be initially dispersed and to give an even distribution in the product and to avoid balling-up of the fibre. On the other hand the fibre should be long enough to provide adequate reinforcement in the board.

These inorganic fibres lack the web-forming properties of asbestos fibres, but this is compensated for by including web-forming fibre in the aqueous suspension to be dewatered, e.g. in the form of a pulp, to give wet and dry strength, flexibility and the ability to use conventional paper or board making machinery. Cellulose pulp is preferred, wood pulp with long, fine fibres being the best although other pulps can give satisfactory results at lower cost.

During board making the organic fibres form a web on the sieve or other filter medium and support the other materials deposited from the suspension; and after the board is formed they give green strength to the dewatered material before it is dried.

The glassy inorganic fibres preferably form 23 to 42 percent by weight of the aqueous suspension. The organic web-forming fibres are to be kept to minimum proportions owing to their combustibility and 3 to 7 percent of such fibres is the preferred proportion by weight of total solids. Cellulose fibre is preferred as the organic fibre because of its superior web-forming properties in the wet state and its low combustibility in comparison with synthetic pulp fibres such as polyolefines (e.g. polypropylene, polyethylene). 578 - 6 The ball clay is included in the board product to keep down cost, to contribute to various properties of the board including temperature resistance and mechanical strength both during and after manufacture. The ball clay also provides a cohesive matrix in the material owing to its plastic nature.

The ball clay preferably forms 42 to 62 percent by weight of the board product. The greater the length of the glassy inorganic fibre content, the higher the proportion of ball clay that can be employed within the stipulated range.

Reinforcing fillers such as fibrous wollastonite, mica and waste calcium silicate insulation can be included in amounts up to a total of less than 10% by weight.

The starch, which in the product is present in the so-called hydrolysed (gelatinised) state, gives the product dry strength but softens when wet to facilitate remoulding of the product on rewetting. The preferred starch is farina starch.

Bentonite clay may be incorporated in the aqueous suspension to assist in controlling the drainage rate of water therefrom through the sieve or other filter medium during board production. It may be used in an amount up to 5 percent by weight.

Dry strength resins of the type used in the paper industry can be used in small quantities e.g. up to 0.5% by weight, to improve the strength of the product.

In order to counteract the combustibility of the organic constituents, it may sometimes be useful to incorporate small quantities of flame retardant. These should be selected to suit the particular organic materials employed but ammonium hydrogen phosphates. - 7 boric acid or other borates, have been found to be effective.

The plant employed to carry out the process of the invention may be the same paper or board making plant as is used for making asbestos-based boards.

The invention is illustrated by the following Examples.

Examples 1 to 6 Sample boards were made to six different formulat10 ions. In each case the boards were prepared by mixing the ingredients as an aqueous suspension (each being added in the order shown except the Silane which was applied to the mineral wool as a pre-treatment), applying to a sieve, dewatering, pressing at 7 MPa pressure and drying at 12O-13O°C. The six formulations are tabulated below in Table 1. The (dry) density and tensile and flexural strengths of the sample boards were measured and the results obtained are also tabulated below in Table 1.

LD O ω CD in m CM in O cn ω in m cm o cn cm cn cn in ι ι I cm cn cn in O cm Λ rd cm in LD in cm cn m ill I h m cn ,.,, ro in cm ld cn III ι CD P •rf H G a β ♦H S (Short fibre nJ β •pH CO •H •pH £ •«s CD «•-I P P P CM 0 nJ Φ Ό CM Ul «*·» cu CL fo > Ή fo XJ CQ cn Qj r-f nJ P c nJ 3 Ό CD TJ 1“f CP >i TJ CD -H Ul ft CD X fi CD C 0 β nJ 0 P £ CO 0 nJ P 0 0 CD ι—{ £ •H 0 Ό TJ >i P r-f nJ •Η P P ϋ >1 β Q) 0 H nJ p rf Ρ CD P ία 0 JS CJ 0 0 P H β fo (0 ι—1 **** P •H Ch ω nJ 0 CD'-' i—l β P Ό 0 p P >H nJ Φ CD •pH X CD P CQ « fo w SJ h fo Q Properties 3 1100 815 740 950 843 681 density (kg/ιη ) Tensile strength (MPa) 11.2 4.2 4.1 7.1 6.7 5.3 Flexural strength (MPa) - - 4.5 12 11 3 o >—i O CM - 9 Examples 7 to 10 To investigate the influence of starch content on the mechanical strength of the material four different boards were made using different furnishes on a pilotscale Hatscheck board-making machine. In the latter the finish is slurried in water and fed to a vat in which a drum having a surface of a fine sieve material such as wire mesh is partially immersed. The drum ends are provided with face seals against the sides of the vat, and water is withdrawn from the interior of the drum through the drum end so that water flows inwardly into the drum depositing a layer of the solids from the slurry on the mesh sieve. This is carried out of the vat by rotation of the drum and deposited from the drum onto a felt which conveys the layer and on which the layer is dewatered. From the felt the layer is passed onto a drum upon which it is wound until a predetermined thickness has built up. Then the material on the drum is slit and removed as a sheet.

The table II below gives details of the compositions from which the boards are made, and their density and tensile strength. Satisfactory boards were obtained at all the starch concentrations illustrated (2, 3, 4 and 5 percent). There is a need to limit the amount of starch needed in order to keep down the content of potentially combustible organic material in the board, but a minimum amount is needed to achieve the necessary binder function. The content is 1.5% to 5.0% by weight of the board as indicated previously. - 10 Table II Example No. 7 8 9 10 Constituent % Dry Weight Wood Pulp 5.0 5.0 5.0 5.0 Mica 5.0 5.0 5.0 5.0 Bentonite 2.5 2.5 2.5 2.5 Ball Clay 55.5 54.5 53.5 52.5 Swedish Rockwool 30.0 30.0 30.0 30.0 Farina Starch 2.0 3.0 4.0 5.0 Percol E24 0.005 0.005 0.005 0.005 (Retention Aid) Properties Density Kg/m3 995 740 993 985 Tensile Strength MPa 4.4 5.4 5.1 6.4 Examples 11 to 14 To illustrate the influence of mineral wool content on the mechanical strength of boards made on a pilotscale Hatschek machine a series of boards were made to four different furnishes. The compositions of the four different furnishes and the density and tensile strength of the boards made from them are given in Table III below. - 11 Table III Example No. 11 12 13 14 Constituent % Dry Weight Wood Pulp 5.0 5.0 5.0 5.0 Mica 5.0 5.0 5.0 5.0 Bentonite 2.5 2.5 2.5 2.5 Ball Clay- 64.5 59.5 49.5 44.5 Swedish Rockwool 20.0 25.0 35.0 40.0 Farina Starch 3.0 3.0 3.0 3.0 Percol E24 0.005 0.005 0.005 0.005 (Retention Aid) Properties Density Kg/m^ 1010 1140 1130 1080 Tensile Strength MPa 4.5 6.5 6.8 5.6

Claims (7)

1. A process for the production of a non-combustible asbestos-free board product, comprising (a) dewatering on a sieve or other filter medium an aqueous suspension containing the following ingredients in the following proportions byweight of total solids Unfired ball clay Glassy inorganic fibre Starch Organic web-forming fibre 42 - 67% 18 - 42% 1.5 - 5% up to 7% the content of unfired ball clay in the suspension being greater than that of the glassy inorganic fibre; and 15 (b) drying the dewatered suspension to form a board product of tensile strength greater than 2 MPa and a density in the range of 500-1200 kg/m , under conditions such as to leave the ball clay in the unfired state 20 and the starch content in the range 1.5-5% by weight, so that the board product is remouldable when rewetted with water.

2. A process according to Claim 1, in which said aqueous suspension contains, by weight of total solids, 25 42 - 62% of unfired ball clay and 23 - 42% by weight of glassy inorganic fibre.

3. A process according to Claim 1 or 2, in which the glassy inorganic fibre is mineral wool.

4. A process according to Claim 1, 2 or 3, in which said aqueous suspension contains, by weight of - 13 total solids, from 3 to 7% by weight of organic web-forming fibre.

5. A process according to any one of Claims 1 to 4, in which the organic web-forming fibre is cellulose fibre. 5

6. A process according to any one of Claims 1 to 5, in which the starch is farina starch.

7. A non-combustible asbestos-free board product whenever produced by a process as claimed in any of Claims