Cement-fibre material.

The invention relates to a reinforced with polyvinyl alcohol fibers, cementitious material.

Asbestos-reinforced cement phases are nowadays in the production of eg pipes, roof tiles, wall coverings, flower pots, house, roof and chimney liners. used.

Asbestos-containing cementitious materials also have a relatively high strength, even with a small thickness. However, the conventional asbestos-containing cementitious substrates have a relatively low impact resistance and are easily broken, for example, when a stone hits. This fact plays a role above all when the asbestos-containing cementitious substrate is used for the manufacture of pipes, roof tiles, wall coverings, etc. It has already been proposed to increase the asbestos content in order to increase the strength. However, this measure has only a small effect.

But it is also not recommended from the point of view of the waste of raw materials.

It has therefore already been proposed to use natural or synthetic fibers for reinforcing the cement matrix. In the case of an addition of natural fibers, such as cellulose, cotton and silk, as well as synthetic fibers, such as polyamide, polyester and polypropylene fibers, this measure has indeed led to certain successes; an improvement to the desired extent was not achieved. Relatively good results are obtained when adding glass fibers to the cementitious substrate. However, it is hiebei to use an alkali-resistant glass fiber (see, for example, JP-OS 104918/1974), which results in expensive products because of the high cost price of such a fiber. In addition, long-term tests have shown that in such an approach after a relatively short time with a decrease in strength must be expected.

It has also been proposed to reinforce cementitious masses with mixtures of inorganic and organic fibers. For example, 3P-OS 104917/1974 shows how reinforced cement products can be produced with mixtures of glass fiber and polyvinyl alcohol fibers. However, the behavior of such products is also not ideal, especially as regards flexural strength.

It has now been found that one obtains very flexurally strong and impact resistant cementitious substrates which do not have the disadvantages mentioned, if the cementitious materials 2 to 20 vol .-% short cut polyvinyl alcohol fibers added, the stretch of 4 to 8% and a modulus of greater than 1.28 N / dtex (130 g / dtex).

The invention thus relates to a reinforced with Polyvinylalkohlfasern, cementitious, hardened material, which is characterized in that it contains 2 to 20 vol .-% of polyvinyl alcohol fibers of the second generation with a cutting length of 3 to 12 mm, an elongation of 4 to 8 % and an A modulus in excess of 1.28 N / dtex, and optionally other natural or synthetic fibrous materials such as glass fibers, metal fibers or asbestos, and / or fillers, such as groundwood, pulp wastes, fibrids and the like. contains, wherein the polyvinyl alcohol fibers preferably have a titer of 1 to 5 dtex.

The polyvinyl alcohol fibers (PVA fibers) are PVA fibers of the so-called second generation. They are prepared with the help of additives such as boric acid to the spinning solution and subjected to a thermal aftertreatment. The fibers thus produced differ from the so-called first generation PVA fibers in terms of particularly high strength, water insolubility, low elongation and high initial modulus.

By a cementitious material is meant a material containing an inorganic cement and / or an inorganic binder or adhesive which is hardened by hydration. Particularly suitable binders which are cured by hydration include, for example, Portland cement, enamel cement, Eisenportland cement, trass cement, blast furnace cement or gypsum.

PVA fibers of the so-called first generation are polymers of the general formula [-CH₂-CH (OH) -]_n with molecular weights of, for example, 13,000 to 100,000 and a density of, for example, 1.23 to 1.30; They are generally prepared by saponification of polyvinyl acetate (see, for example, Rompps Chemie Lexikon, 7th Edition, page 2776). The PVA fibers of the so-called second generation mentioned are also known per se and can be produced, for example, according to US Pat. No. 3,987,140.

PVA fibers according to the invention become the cementitious substrate in such proportions

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3, the product obtained contains 2 to 20% by volume of these fibers. Fiber mixtures below 2% bring according to the experience, no significant improvements in terms of eg flexural and impact resistance. Fibrous admixtures in excess of 20% by volume make the production of the desired products more expensive, without appreciably improving the respective strengths.

The fibers are preferably mixed in cut lengths of 3 to 12 mm, wherein both uniformly long individual fibers and a mixture of fibers of different lengths can be used. Ground fibers can also be used. The titer of the individual fibers can vary within a wide range, but titers of 1 to 5 dtex are preferred.

The fibers may be evenly distributed in the cement paste or, as in the case of

Shaped pieces, in places that are particularly exposed to the effects of mechanical forces, present in increased concentration. The fibers may also be in the form of nonwoven fabrics, yarns, ropes, nets, fabrics and the like. be used.

The cross-sections of the fibers may be of various shapes due to physical and chemical variations in the manufacturing process (spinning solvent and coagulation bath, as well as the selection of appropriate spinneret bores). It can be in this way, for example, round fibers, multilobal fibers, hollow fibers, porous fibers and the like. produce. The outer fiber surface may also have been roughened, fissured or felted by physical aftertreatment processes. PVA fibers are, chemically speaking, a very reactive substrate, so that they can easily be chemically modified. It is possible to use all customary functional groups, such as carboxyl groups, amide groups, nitrile groups, phosphate groups, sulfate groups and the like. via addition, substitution or radical reactions with the appropriate reagents introduce. Finishes or bonding agents can be applied to or into the fibers without actual chemical reactions and can perform specific functions for the anchoring of the PVA fibers in the mass to be reinforced by purely physical forces. With the methods mentioned above, PVA fibers, for example, flameproof, hydrophobic, etc., can be equipped or crosslinked. All of the thus modified PVA fibers can also be used according to the invention for the purpose in question.

Polyvinyl alcohol fibers can be added exclusively to the cementitious substrate to improve the desired properties in the process according to the invention. However, it is usually preferred to add, in addition to the specified PVA fibers, asbestos, glass or other artificial or natural fibers, alone or in combination.

Frequently, in addition to the reinforcing fibers, adjuvants, such as pulp waste, groundwood, fibrids, for example, of polypropylene and other fillers are frequently added to the mass to be reinforced. The essence of the invention will be explained in more detail with reference to Examples 1 to 3, which are not intended to limit the invention in any way. The comparative example shows the superiority of the cement compositions according to the invention over those according to the prior art.

Examples 1 to 4: Portland cement aqueous slurries were prepared containing the appropriate vol% of appropriately cut fibers. In Example 1, 40% total 12% by weight of chrysotile asbestos was added. In Examples 2 and 3, 50% by volume of this asbestos (density 2.7) was replaced by the relevant% by volume of the PVA fibers used according to the invention (both fibers having a density of 1.3).

In the winding process (the slurry is deposited on a wire mesh of a layer machine in the form of a layer so that the water flows off.) The solid components on the wire mesh are then transferred to an endless felt belt in the form of a wet layer and the latter is subsequently wound up on a drum ), webs were prepared and hydrated at 100% relative humidity and 21 ° C. After 28 days, flexural tensile strength and impact strength were measured in the wetted state. The results are given in the table below.

The conventional PVA fibers used according to the invention had the following characteristic data:

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<td></td><td>Conventional fibers</td><td>Fibers used according to the invention</td>

<td>cutting length</td><td>6 mm</td><td>6 mm</td>

<td>elongation</td><td>22%</td><td>6.5%</td>

<td>A module</td><td>0.64 N / dtex</td><td>2.16 N / dtex</td>

<td>tear strength</td><td>0.044 N / dtex</td><td>0.083 N / dtex</td>

<td>example</td><td>asbestos Vol .-%</td><td>PVA Vol .-%</td><td>flexural strength N / cm²</td><td>Schlagzähig- ness cm.N / cm²</td><td>density g / cm³</td>

<td>1</td><td>100</td><td>-</td><td>3139 ± 294</td><td>12.8</td><td>1.8</td>

<td>2</td><td>50</td><td>50</td><td>3335 ± 294</td><td>22.6</td><td>1.8</td>

<td>3</td><td>90</td><td>10</td><td>3551</td><td>19.6</td><td>1.8</td>

<td>encryption gleichs- example</td><td>50</td><td>50</td><td>2011 ± 392</td><td>19.6</td><td>1.8</td>

The comparative example shows the strengths obtained with 50% replacement of asbestos by conventional PVA fibers, while in Examples 2 and 3 the corresponding values ​​obtained with the PVA fibers used according to the invention are recorded.

It can be seen that a 50% replacement of asbestos by the inventively used PVA fibers results in a considerably higher impact strength than when using as additive exclusively asbestos. The flexural strength remains virtually unchanged. On the other hand, the comparative example with 50% replacement by a conventional PVA fiber shows considerably lower values ​​with regard to flexural tensile strength. The impact strength in question is also significantly lower than when the PVA fiber used according to the invention is used. When adding fibers with a

Cutting length of 3 or 12 mm leads to analogous test results.

The invention accordingly relates to a fiber-reinforced cementitious material characterized by containing from 2 to 20% by volume of short-cut polyvinyl alcohol fibers having an elongation of 4 to 8% and a modulus of more than 1.28 N / dtex, or a method for producing such a fiber-reinforced, cementitious material, or the use of polyvinyl alcohol fibers having the mentioned characteristics for producing such a fiber-reinforced cementitious material.