Fibre-containing products manufactured with hydraulic binders

Fibre-reinforced cement products

Fatex Division Establishment

Fibre-containing products manufactured with Hydraulic binders

The present invention relates to fibre-containing products manufactured with hydraulic binders, which are characterised in that they contain fibres made of acrylonitrile polymers having a molar concentration of acrylonitrile units of 98 to 100%, as reinforcing fibres and/or fillers.

Moreover, it relates to a process for the manufacture of such products, as-def-in-Pafeeat-Glaira-S_r

Amongst the conventional construction materials, fibre-reinforced cement products manufactured starting from asbestos and cement have already been known for decades. In the asbestos cement industry, processes based on the winding process of L. Hatschek (Austrian Patent Specification 5,970) are still the most widespread of the processes for the manufacture of construction elements. The technology of this production process is described exhaustively, for example, in the book by Harald Klos entitled ״Asbestzement ( Asbestos Cement ), Springer Verlag, 1967.

These known processes for the manufacture of, for example, asbestos cement pipes and sheets are based on the use of cylindrical screen machines. In this process, a dilute asbestos cement suspension is transferred in the form of a mat to a felt, via a pulp chest and a sieve cylinder, and is wound up to the desired thickness with the aid of forming rolls or pipe mandrels. For the manufacture of corrugated sheets, the asbestos cement mat can be cut from the forming roll, after the desired thickness has been reached, and can be left to harden between oiled corrugated metal sheets.

In the course of recent years, it has become clear that asbestos, which is of proven value, will no longer continue to be available in unlimited amounts and must be counted amongst those natural materials the reserves of which will probably be exhausted most rapidly. In addition, the

־ 2 ־ deposits of asbestos worth extracting are distributed over only a few countries, and this in turn can lead to undesirable dependencies which, today, have already resuited in increasing prices.

It is thus desirable to use new fibres as reinforcing fibres and also as fillers for hydraulic binders, for example for reinforcing cement, which fibres are capable of giving fibre-containing products^ the desired mechanical propertiesחי the type of production plants which are widespread in the processing industry, for example the asbestos cement industry.

For the sake of simplicity, reference will be made to cement as the preferred binder in the present description. All other hydraulically setting binders, can, however,be employed in place of cement. Suitable hydraulically. setting binders are to be understood as meaning a material which contains an inorganic cement and/ or an inorganic binder or adhesive which is hardened by hydration. Particularly suitable binders which are hardened by hydration include, for example, Portland cement, high-alumina cement, iron Portland cement, trass cement, slag cement, gypsum, calcium silicates formed on autoclave treatment, and combinations of individual binders.

The most diverse fillers and additives, which, for example, can have a beneficial influence on the pore structures of a cement block or, for example, can improve the drainage behaviour of the suspensions on the draining machines, are frequently also added to the binders. Possible additives of this type are materials such as fly ash, pyrogenic silica, ground quartz, ground rock, kaolins, blast-furnace slags, pozzolanas and the like.

The literature already contains innumerable publocations on the use of various natural, synthetic, organic and inorganic fibres. Fibres made of wool, cotton, silk, polyamide, polyester, polyacrylonitrile, polypropylene and polyvinyl alcohol, inter alia, have already been investigated for reinforcing cement. Likewise, work with fibres made of glass, steel, aramide and carbon is known. Of all these fibres, none has hitherto proved useful in practice, especially in cement.

The requirements to be satisfied by fibres which are suitable for reinforcing cement and other hydraulically setting binders are exceptionally great:

In the case of the chemical requirements, alkali resistance in saturated calcium hydroxide solutions at elevated temperatures is, in particular, an absolute prerequisite. On the subject of the chemical structure of a suitable fibre, it can be said that the highest possible concentration of polar functional groups must be present so as to achieve an adequate affinity for the cement.

Furthermore, the physical data on the fibres should correspond, in important properties, to the physical data on the hydraulic binders. In the case of cement, it is known that this material has a certain brittleness and, for example, can already break at an elongation of about 0.3%. For reinforcing fibres in cement, it is found that the fibres showing the best reinforcing effect are those which most strongly resist a minimum elongation. However, it must be taken into account here that fibres which are treated with an aqueous cement suspension can change their properties, and the extent of such a change cannot be predicted. Thus, this means that, despite good initial mechanical properties of a fibre, the fibre may not have the desired effect in the cement if the fibre properties change during the hydration processes of the cement.

In addition to the abovementioned physical properties of fibres, it is likewise important that the fibres can readily be dispersed in a dilute aqueous cement suspension and also remain uniformly dispersed on adding further additives, if these fibres are to be processed by draining processes to give fibre cement products. Fibres or fibre mixtures in a range of lengths up to 30 mm have proved useful, it being possible for the cut fibres to be employed in uniform lengths, for example of 3 to 24 mm, or in mixed lengths. In particular cases, it has proved advantageous to pretreat the fibres by means of a cutting and/or fibrillating grinding operation.

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Fibres having agauge-of 0.1 to 15 dtex, especially 0.5 to 15 dtex, can be used for fibre material.

If the commercially available fibres are now examined for the abovementioned properties, all known types of textile fibre, such as fibres made of polyester, polyacrylonitrile, polyamide, viscose, cotton and wool, must be discarded because their mechanical behaviour is too different from that of the hydraulic binders.

High-tenacity organic fibres based on polyester, polyvinyl alcohol or rayon, such as, for example, those employed in the tyre industry, are admittedly superior to the types of textile fibre in their mechanical properties. However, these valuable properties are greatly impaired under the wet-alkaline process conditions in the manufacture of fibre cement. Other heavy-duty fibres known in industry, such as glass fibres, carbon fibres and aramide fibres, are either also non-resistant to alkali or uneconomical; in addition, their affinity for the cement matrix leaves something to be desired. They do not therefore come into consideration for use as reinforcing fibres for cement.

The object of the invention is thus to use a fibrous material which offers the greatest possible resistance even to a low elongation, is changed as little as possible by a cement suspension and, after hardening, imparts increased mechanical strength to the cement/fibre composite.

It is known that the most widespread fibres with polar functional groups include polyacrylonitrile fibres. These fibres are produced in large amounts and employed, in particular, in the clothing industry. However, with all the types of polyacrylonitrile fibres commercially available hitherto, it has still not been possible to achieve an adequate reinforcing effect for hydraulically setting binders. It may well be that the cause is to strength be found in the relatively low -tenacity-and the high elongation at break of these fibres. To improve the dyeability and the textile handle and also to facilitate the filament-forming process, all commercially available polyacrylonitrile fibres contain 4 to 15% of one or more comonomers such as vinyl acetate, methyl acrylate, methyl methacrylate and vinyl derivatives containing carboxyl, sulfo or pyridine groups. It is admittedly possible to improve the mechanical properties of these fibres to a certain extent, that is to say to reduce the elongation at break and increase the tenacity. This can be accomplished by the optimisation, well known amongst fibre manufacturers, of the stretching processes of the fibres after the filament-forming process, downstream of the spinneret. However, this optimisation is subject to practical limitations because of the inherent properties of the fibre materials. If a cement matrix is reinforced with such fibres, a certain, but still unsatisfactory, improvement in the reinforcing effect is indeed found, compared with a cement matrix containing the conventional acrylic fibres mentioned.

It has now been found, surprisingly, that polyacrylonitrile fibres manufactured using a polymer, having a molar concentration of at least 98% of acrylonitrile units and a relative viscosity of at least 2.60 (measured as a 0.5% strength solution in dimethylformamide) can be used for the desired purposes. In a cement charge, such fibres are superior to other high-tenacity polyacrylonitrile fibres having the conventional composition described above, in that they retain their original properties in the aqueous alkaline cement suspension, in particular their tensile properties, such as, for example, a high initial modulus from, for example, within the range of 1,000 cN/tex to 2,000 cN/tex.

The invention thus relates to fibre-containing solid products manufactured with hydraulic binders, which are characterised in that they contain fibres made of acrylonitrile polymers having a molar concentration of acrylonitrile units of 98 to 100%, as reinforcing fibres and/or fillers.

Advantageously, the fibres used are pretreated by the process according to Swiss Patent Application No. 1297/79-0 (german Offenlegungsschrift 3.002,484), which is herein incorporated by reference.

The fibres used according to the invention, which can optionally be employed together with other fibres, are advantageously admixed in an amount such that the total proportion of fibres in the hardened product is from 0.1 to 30 per cent by weight and preferably from 1 to 12 per cent by weight, and, when used as pure reinforcing fibres, is especially from 1 to 8 per cent by weight. The fibres are generally used in a range of lengths up to 30 mm, it being possible to employ the cut fibres in uniform lengths, for example of 3 to 24 mm, or in mixed lengths. In particular cases, it has proved advantageous to pretreat the fibres by means of a cutting and/or fibrillating grinding operation.

Fibres having a titer of 0.1 to 15 dtex, especially 0.5 to 15 dtex, can be used for fibre material.

The processing of such fibres to give products according to the present invention is carried out in that hydraulic binders are mixed with water, the customary supplementary materials and additives and with fibres of acrylonitrile polymers having a molar concentration of acrylonitrile units of 98 to 100% which retain their tensile properties during and after the setting process as reinforcing fibres, and in that the mix/Ure is optionally partially drained, converted to the desired shape and allowed to harden. Said process is performed, for example, on a Hatschek machine.

6a

The manufacture of the fibres according to the present invention is not a subject of the present patent application. It is carried out, for example, by a known dry spinning process or, preferanly, by a wet-spinning process. These high-tenacity fibres with a low elongation at break can be manufactured ,for example, as follows:

1,700 g of a polymer consisting of 99.5% of acrylonitrile and 0.5% of methyl acrylate and having a relative viscosity of 2.85 (measured as a 0.5% strength solution in di methyl forma mi de (DMF) were dissolved in 8,300 g of DMF to give a homogeneous spinning solution. After filtration, this solution was forced through a spinneret possessing one hundred orifices of diameter 0.06 mm, at a rate of 16.2 ml/ minute, into a coagulation bath which consisted of 50% of DMF and 50% of water and was at a דtemperature of 50°c ,

After immersion over a length of 50 cm, the resulting filaments were drawn off at a speed of 5.5 m/minute. They were stretched, at a speed of 29.3 m/minute and at a temperature of 99°C, in two successive stretching baths consisting of 60% of DMF and 40% of water, washed in water and finished in further baths and then dried on two heated pairs of rollers with surface temperatures of 140 and 185°C, respectively, a shrinkage of 0.7 m/minute being allowed. The residence time on the first pair of rollers at a temperature of 140°0 was chosen so that the filament was glossy, that is to say no longer had vacuoles, on leaving the pair of rollers. The filament was drawn off from the second pair of rollers at 3,3.3 m/minute and stretched over four heated plates which contacted the filament alternately from below and above, at temperatures of 145, 145, 165 and 180°C, at 95 m/minute, with the aid of an unheated pair of rollers, and then wound up onto spools. The effective total stretching ratio was 1:17.3; the mechanical properties of the filaments thus obtained (type A) are shown in Table I.

Particularly suitable types of fibre can also be obtained by means of an additional setting treatment, for example with hot contact surfaces, hot air, hot water, steam or the like, after the contact stretching.

For the type B fibres used in the following illustrative embodiment, the setting was carried out on two heated pairs of rollers without allowing shrinkage. The surface temperatures of the pairs of rollers were 210 and 230°C. The textile-mechanical data of these fibres are recorded in Table I. By means of the setting treatment, the shrinkage on boiling could be reduced from 9.5% to 2.0%.

Using the spinning process described above (variant A), a further polymer which can be used according to the invention, consisting of 99% of acrylonitrile units and 1% of methyl acrylate units and having a relative viscosity of 2.84 (type C) and, by way of comparison, a conventional polymer containing 96% of acrylonitrile units and 4% of methyl acrylate units and having a relative viscosity of 2.78 (type D), were also processed to give fibers. By way of comparison, a commercially available polyacrylonitrile fiber for textile purposes (type E), having the following composition: 93.5% of acrylonitrile units, 6% of methyl acrylate units and 0,5% of methallyl sulphonate, was also examined.

The mechanical properties of the resulting fibres are summarised in Table I.

Fibres made of acrylonitrile polymers having a molar concentration of at least 98% of acrylonitrile units, as are used according to the invention, have an adequate resistance to alkali even at elevated temperatures to satisfy the requirements mentioned at the outset.

TABLE

Mechanical properties of high-lAfet^^olyacrylonitrile fibres with different molar concentrations of acrylonitrile (.ex^mtn-etf on individual

<td></td><td> fibres)</td>

<td> Type A 99,5% of Examination A * Titer Sacrge״, dtex 2.9</td><td> Type B Type C Type D Type E 99,5% 99,0% 96,0% 93,5% Of AN of AN of AN of AN_________ 2.9 2.9 2.9 2.9</td>

<td> Strengthj^^uy cN/tex 83</td><td> 74 82 85 35</td>

<td> Initial modulus cN/tex 1490</td><td> 1420 1510 1410 1100</td>

<td> Elongation at break, % 8.0</td><td> 10.0 8.1 7.9 20</td>

<td> Examination B ** Strength Terra-city־ cN/tex 81</td><td> 74 80 70 23</td>

<td> Initial modules cN/tex 1400</td><td> 1350 1490 810 630</td>

<td> Elongation at break, % 9,1</td><td> 10,8 8,3 12,0 35</td>

<td colspan="2"> Examination A * : Examination of individual fibers under normal condi-</td>

<td> tions at</td><td> 20°C and 65&amp; relative humidity.</td>

<td> Examination B ** : In order</td><td> to test the resistance of the fibers to ce-</td>

<td colspan="2"> ment-water previously to the normal examination A, the fibers were immerged into a aqueous slurry of 50 g Portland cement in one liter water at 80°C for 24 hours and then conditioned for examination A.</td>

To assess these five types of fibres חיו respect of their suitability as reinforcing fibers in cement, the following examples and comparison experiments were compared under analogous conditions, as described below.

Preparation of the mixtures for processing on a Hatschek machine

Mixture 1 (comparison example)

In an edge mill, 153 kg of asbestos (grade 4 : grade 5 = 1:3) were milled with 62 litres of water for 30 minutes. The disintegrated asbestos was then introduced into a high-speed vertical mixer containing 1.5 m of water. After stirring for ten minutes, the mixture ............................

־ 10 ־ was pumped over into a horizontal mixer and 1 tonne of cement having a specific surface area of 3,000 to 4,000 cm²/g was mixed in. For further processing, this asbestos/cement suspension was then fed to the Hatschek machine via a pulp chest.

Mixtures 2 to 4 and Comparison Examples 5 and 7

In a Solvopulper, 80 kg of waste paper (without glazed paper) and 15 kg of aluminium sulphate were pulped in 1 m³ of water for 10 minutes. This fibre suspension was diluted to 2.5 m³ , 20 kg of the polyacrylonitrile fibre samples to be tested, having a cut length of 6 mm, were added and the mixture was then pulped for a further 5 minutes. 45 kg of pulverulent calcium hydroxide were then added and the mixture was pulped for a further 12 minutes. After the mixture had been pumped over into a cement mixer, 1,000 kg of cement having a specific surface area of about 3,000 to 4,000 cm^?/g were mixed in for 15 minutes.

To improve the cement retention capacity, 80 g of a polyacrylamide (Separan NP-10, Dow Chemical Corporation), in the form of a 0.2% strength aqueous solution, was mixed into the fibre-cement suspension. The resulting mixture was fed to a Hatschek machine via the pulp chest.

Mixture 7 was prepared only from waste paper and cement, without polyacrylonitrile fibres.

Production of the test sheets

With the above Mixtures 1 to 7, 6 mm thick sheets were produced on a Hatschek machine, with seven revolutions of the forming roll, and these sheets were compressed for 45 minutes between oiled metal sheets in a stack press under a specific press loading of 250 bars, to give a thickness of 4.8 mm. The sheets were tested after a setting time of 28 days, after the sheets had been watered for a further 3 days. The experimental results are summarised in Table II.

TABLE

Test results for small sheets made of cement reinforced with polyacrylonitrile fibres

<td> Mixture No.</td><td> Flexural strength N/mm²</td><td> Specific impact strength N/mm/mm²</td><td> Density of the sheets g/cm³</td>

<td> 1) Asbestos (comparison example)</td><td> 29.2</td><td> 1.8</td><td> 1.76</td>

<td> 2) Type A PAN fibres containing 99.5% of acrylonitrile units</td><td> 26.3</td><td> 2.7</td><td> 1.76</td>

<td> 3) Type A PAN fibres containing 99.5% of acrylonitrile units, fixed</td><td> 26.2</td><td> 2.7</td><td> 1.77</td>

<td> 4) Type C PAN fibres containing 99.0% of acrylonitrile units</td><td> 25.9</td><td> 2.6</td><td> 1.74</td>

<td> 5) Type D PAN fibres (comparison example) containing 96.0% of acrylonitrile units</td><td> 21.8</td><td> 2.7</td><td> 1.76</td>

<td> 6) Type E PAN fibres (comparison example) containing 93.5% of acrylonitrile units</td><td> 20.2</td><td> 2.6</td><td> 1.75</td>

<td> 7) Cellulose/cement mixture without PAN fibres (comparison example)</td><td> 18.5</td><td> 2,2</td><td> 1.74</td>

The flexural strengths of the fibre-reinforced, small cement sheets show that, on using polyacrylonitrile fibres having largely identical initial mechanical properties, surprisingly only the types of polyacrylonitrile used according to the invention, namely type A and type B containing 99.5% of acrylonitrile units and type C containing 99.0% of acrylonitrile units, are capable of making a substantial contribution to reinforcement in a cement matrix. The specific impact strength is not influenced by the nature of the polyacrylonitrile fibres used. The specific impact strength of the small asbestos cement sheets is greatly exceeded by that of the small fibre cement sheets. Apart from the impact strength, the flexural strength is also of decisive importance for practical use. As can be seen from the above table, the fibres which can be used according to the invention give substantially higher values than the type D and type E comparison fibres.

In additional experimental examples, it will be shown how the fibres used according to the invention also prove useful in different fibre dimensions and in combination with conventional fillers. The experiments were carried out in an identical manner to that already described for Mixtures 2 to 7, the additional fillers being introduced into the cement mixer after the introduction of the cement. The fibres used according to the invention were employed as follows: Mixture 8

Portland cement 81.5%

Silica filter dust (SiO^ content = 98.8%, average particle size = 0.5μ) 12.0%

Pulp fibres (45°SR) 4.0%

Type B PAN fibres: 2.5%

These fibres were cut beforehand to a length of 18 mm and then ground further by means of a cutting mill (Condux, type CS 500/600-4). The following fibre length distribution resulted:

mesh 10.2% mesh 19.6% mesh 33.2%

100 mesh 26.9%

200 mesh 9.7% >200 mesh 0.4% (mesh in this context means: number of openings/inch).

Mixture 9

Portland cement 82%

Blast-furnace slag 8%

Rock wool 4%

PAN fibres/pulp mixture 6% parts of PAN fibres having a cut length of 8 mm, prepared in accordance with variant A and containing 98 mol % of acrylonitrile units, were ground beforehand, in a fibrillating manner, together with 2 parts of sulphate pulp in a Jordan refiner. This mixture was added in an amount of 6%.

The two Mixtures 8 and 9 were processed on a Hatschek machine, as described above, to give test sheets and tested after hardening for 28 days. The results are summarised in Table III.

TABLE

Test results for small sheets made of cement reinforced with polyacrylonitrile fibres and containing fillers.

<td> Mixture No.</td><td> Flexural strength N/mm²</td><td> Specific impact strength N/mmyfmm²</td><td> Density of the sheets g/cm³</td>

<td> 8)</td><td> 26.4</td><td> 2.3</td><td> 1.76</td>

<td> 9)</td><td> 26.6</td><td> 2.4</td><td> 1.85</td>

The results in Table III show that the fibres used strength according to the invention also give good-tensiie values when prepared differently and when in combination with different additives. Mixture 8 could be particularly easily processed and Mixture 9 made it possible to achieve higher densities.