GB2065735A - Fibre-cement boards - Google Patents

Abstract

The fibres used to reinforce products made from hydraulic binders and casting resins are split fibres made from films which are manufactured from a polymer of acrylonitrile having a molar concentration of the acrylonitrile units not less than 33%. These fibres show a high affinity to set cement, so that the tensile bending strength is increased.

Description

SPECIFICATION Product manufactured using hydraulic binders and/or plastics The present invention relates to a product manufactured using hydraulic binders and/or plastics.

Amongst building materials, fibre-reinforced cement products, based on asbestos and cement, have been known for decades. In the asbestos cement industry, processes of manufacture using the L.

Hatschek winding process (Austrian Patent Specification 5,970) are still the most widely used.

These known processes for the manufacture of, for example, asbestos cement pipes and sheets are based on the use of vat forming machines. In these, a very dilute asbestos cement suspension is transferred, via a machine-chest and perforated cylinder, in the form of a web onto a felt and is wound, by means of former drums and pipe cores, until the desired thickness is obtained. For the manufacture of corrugated sheets, the asbestos cement fleece can, after reaching the desired thickness, be cut from the former drum and be allowed to harden between oiled corrugated metal plates.

In the course of recent years it has been found that asbestos, a well-proven material, will no longer be available in unlimited amounts and must be regarded as one of those natural materials whose supplies are likely to be exhausted most rapidly. The deposits of asbestos which warrant mining are furthermore only to be found in a few countries, which in turn can lead to undesirable dependence, and this, even today, already manifests itself in rising prices.

It is therefore desirable to find cheap new fibres for cement reinforcement, which are suitable for the production of fibre-reinforced cement products, having the desired mechanical properties, on the production installations widespread in the asbestos cement industry.

The literature already contains innumerable publications concerning the use of the most diverse natural, synthetic, organic and inorganic fibres. Wool, cotton, stink, polyamide, polyester, polyacrylonitrile, polypropylene and polyvinyl alcohol fibres have already been investigated for the reinforcement of cement. Work with glass fibres, steel fibres and carbon fibres is also known. Amongst all these fibres, however, none could hitherto take the place of asbestos in cement, in respect of price and reinforcing action.

The requirements which fibres suitable for cement reinforcement have to meet are extremely high: Amongst chernical requirements, it is, above all, alkali resistance in saturated calcium hydroxide solution at temperatures of not less than 800C which is an absolute precondition. As regards the chemical structure of a suitable fibre, it can be said that the concentration of polar functional groups present should be as high as possible, to ensure affinity to the cement.

The physical fibre data should match, in important properties, the physical data of the hydraulic binders. In the case of cement it is known that this material has a certain brittleness and, for example, can break at an elongation of even as low as about 0.3%. As far as reinforcing fibres in cement are concerned, it follows that those fibres which oppose the greatest forces to minimal elongation exhibit the best reinforcing action.

In addition to the properties of lowest possible elongation at break, coupled with highest possible strength, the preconditions include good dispersibility, and good distribution in a dilute, aqueous cement slurry, if the fibres are to be convertible to fibre-cement products by dewatering processes.

If the commercially available fibres are scrutinised for the properties mentioned above, all textile fibre types such as polyester, polyacrylonitrile, polyamide, viscose, cotton and wool fibres must be ruled out since the physical fibre data differ excessively from the physical properties of the hydraulic binders.

Organic high-modulus fibres based on polyester, polyvinyl alcohol o. rayon, such as are employed in the tyrecord industry, are superior, in their mechanical properties, to the textile type of fibres, but the involved manufacture of such fibrcs correspondingly leads to higher prices. Other high-modulus fibres known in industry, such as glass fibres, carbon fibres and Aramid fibres are either not alkali-resistant, or the prices rule out the products for use as cement-reinforcing fibres.

It would therefore be desirable to find a new fibrous material which opposes even a low elongation with highest possible resistance and also has an acceptable price.

It is known that one of the cheapest types of processes for the manufacture of fibres are production processes employing film fibre technology. Starting from a film, fibres are produced by a mechanical fibrillation process.

A survey of film fibre technology is given in an article by H. Krässig, J. Polym. Sc. Macromolecular Review, Vol. 12, pages 321-410 (1977). By optimisation of mechanical strength in the manufacture of the film, it is possible to obtain, after fibrillation, fibrous materials which prove superior to textile fibres in respect of elongation at break and of strength when used as reinforcing fibres. At the present time, polypropylene is one of the most advantageously priced thermoplastics. Hence, split fibres based on polypropylene have already reached the stage of industrial manufacture. The use of polypropylene split fibres for reinforcing of mortar has already been described by Shell Oil (U.S. Patent Specification 3,591,395).The use of polypropylene split fibres for the manufacture of fibre-reinforced cement by the technology of dewatering processes is also known (German Auslegeschrift 2,819,794).

However, it is known to those skilled in the art that the manufacture of cement products, reinforced with polypropylene fibres, which is carried out by means of dewatering processes based on dilute, aqueous fibre/cement slurries, for example Hatschek processes, suffers from numerous disadvantages.

The hydrophobic character of polypropylene results in poor affinity to cement, so that the fibre properties in respect of reinforcement also do not come into play and the resulting cement products have only a low strength. In dilute cement slurries, the polypropylene fibres separate out because of their low specific gravity. This manifests itself in poor fibre distribution of products manufactured by the winding processes. The fibre concentrations are increased at the sheet surface, and this can cause great problems in subsequent covering and coating processes.

According to the invention there is provided a product manufactured using hydraulic binders and/or plastics, characterised in that, as a reinforcement and/or as a filler, fibres are provided which are produced by fibrillation processes and/or cutting processes from films or tapes of polymers of acrylonitrile having a molar concentration of the acrylonitrile units of not less than 33%, and preferably not more than 93%.

Surprisingly, it has in fact been found that fibres which are produced from films of polymers of acrylonitrile by fibrillation processes and/or cutting processes, exhibit a low elongation at break coupled with high strength. Furthermore, because of the specific gravity and the inherent hydrophilic character, such fibres show good dispersibility in aqueous, dilute cement slurries, and show no tendency to demix.

The present invention enables the reinforcement of building materials based on hydraulic binders and fibrillated plastic fibres to be improved in such a way that the fibres disperse uniformly in the fibre/cement slurry during production on dewatering machinery, and that the fibres furthermore possess a high affinity to the binder in the hydrated end product.

Fibres according to the present proposal can be incorporated into water/sand/cement pastes by means of the mixers widespread in the concrete industry. Equally, these fibres can also be used to reinforce, for example, a casting resin. As an example of their use, we will here described the manufacture of a thin-walled fibre/cement sheet using a Hatschek machine, since the use of polypropylene split fibres for cement reinforcement, corresponding to the prior art, presents problems in execution. The technology of this production process has been described in detail in the book by Harald Klos, Asbestzement (Asbestos Cement) 1967, Springer-Verlag. To carry out the experiment, the following cement slurries were prepared and were fed to a Hatsc;lek machine via a stirred vat.

Mixture 1: 153 kg of asbestos (grade 4: grade 5 = 1:3) were ground with 62 litres of water for 30 minutes in a pan grinder. The digested asbestos was then introduced into a fast-running vertical mixer, containing 1.5 m3 of water. After stirring for 10 minutes, the mixture was pumped into a horizontal mixer and one tonne of cement having a specific surface area of 3000-4000 cm2/g was admixed. This asbestos cement slurry was then fed, via a stirred vat, to the Hatschek machine, for further processing.

Mixture 2: 80 g of waste paper (free from glazed paper) were pulped for 10 minutes in 1 m3 of water in a Solvopulper. This fibre suspension was diluted to 2.5 m3 and 22 kg of fibrillated polypropylene fibres of 8 mm staple length, which had been produced from a 32 y thick film, were added, after which pulping was continued for 5 minutes. After transferring the mixture into a cement mixer by pumping, 1000 kg of cement having a specific surface area of about 3000-4000 cm2/g were admixed in the course of 10 minutes.

To o improve the flocculation, 80 g of polyacrylamide in the form of an 0.2% strength solution were additionally admixed to the fibre/cement s,lurry. The mixture thus obtained was fed to a Hatschek machine via a stirred vat.

Mixture 3: Mixture 3 was prepared analogously to mixture 2, with the difference that instead of polypropylene split fibres, 22 kg of split fibres which consisted of a copolymer containing 71 parts by weight of acrylonitrile, 21 parts by weight of methyl acrylate and 8 parts by weight of butadiene/acrylonitrile copolymer were employed.

6 mm sheets were produced from these three mixtures on a Hatschek machine with 7 revolutions of the former drum, and these sheets were pressed to a thickness of 4.8 mm between oiled metal sheets for 45 minutes in a stack press under a specific pressure of 250 bar. The sheets were tested following a setting time of 28 days, after which the sheets had additionally been soaked for 3 days. The results of the experiments are summarised in the table which follows: TABLE

Tensile Specific Density bending strength impact strength of the sheets Mixture No. N /mm2 N.mm /mm2 g /cm3 1 Asbestos 27.8 1.8 1.80 2 Polypropylene 20.1 2.3 1.60 split fibres 3 Split fibres of 25.2 2.7 1.70 acrylonitrile polymers The results show that when using asbestos as the reinforcing fibres for cement reinforcement, products which have high strength can be manufactured, but these show a certain brittleness, which manifests itself in the values of the specific impact strength. If polypropylene split fibres are employed, it is found that this type of fibre does not make any particular contribution to the strength. Polypropylene fibres merely give an improvement in the impact strength of cement products. When using spilt fibres based on a thermoplastic acrylonitrile-containing polymer, both a genuine contribution to the tensile bending strength and an improvement in the impact strength values can be found.

Whilst we have here particularly singled out the fibres produced from films, because they exhibit advantages due to their geometrical shapes, it is also conceivable to use spun fibres, of a certain staple length, made from acrylonitrile polymers, the mechanical textile properties of these fibres being at least comparable.

Claims (5)

1. Product manufactured using hydraulic binders and/or plastics, characterised in that, as a reinforcement and/or as a filler, fibres are provided which are produced by fibrillation processes and/or cutting processes from films or tapes of polymers of acrylonitrile having a molar concentration of the acrylonitrile units of not less than 33%, and preferably not more than 93%.

2. Product according to patent claim 1, characterised in that it is a building element, produced from hydraulic binders, which contains the reinfo. cing fibres and/or fillers.

3. Product according to patent claim 2, characterised in that the building element containing the hydraulic binders is produced by a dewatering process, using winding machinery, a forming vat, a wire web, injection equipment, filter presses or a continuous mono-extrusion process.

4. Product according to patent claim 2, characterised in that the building element is in the form of a sheet, corrugated sheet, pipe or moulding.

5. A pioduct as claimed in claim 1 and substantially as herein'before described.