NO850819L - FIBER MIXTURE FOR REINFORCEMENT PREPARATIONS, PARTICULAR REINFORCEMENT OF CEMENT PRODUCTS - Google Patents

Description

The present invention relates to a fiber mixture for the production of fibre-reinforced construction materials, such as e.g. hydraulically hardening materials, in particular a cement material, which contains polyacrylonitrile fibers and polyvinyl alcohol fibers as reinforcement fibers. The invention further relates to the use of such fiber mixtures in the production of shaped bodies after the shaping of hardening materials.

The polyacrylonitrile fibers are referred to below as PAN-

fibers and the polyvinyl alcohol fibers are referred to as PVA fibers.

It is common knowledge that from watery slurries, consisting for example of asbestos and cement, with the help of dewatering machines, e.g. Hatschek machines or by the injection method can produce shaped building articles in different ways, such as roof sheets, corrugated sheets, pipes or garden articles. In recent years, such building articles have secured a dominant place among building materials. In recent years, however, it has become increasingly clear that the most important raw material for the production of these products, namely asbestos, for various reasons such as availability, price trends and due to health considerations, will not be available for long for this application. On this advantageous combination of properties that asbestos fibers exhibit, not only are the well-known excellent performance properties of asbestos cement due, but they also form the basis of the worldwide Hatschek dewatering method.

All of the above-mentioned conditions have in recent years initiated an intensive research activity which has had the task of finding replacement fibers that can fully satisfactorily replace asbestos in the existing production processes in connection with the dewatering methods. It soon became clear, however, that no single type of fiber could be found that combined all the positive properties of asbestos.

It is in particular the following properties that distinguish asbestos as both an excellent processing and reinforcing fiber:

- high specific surface area

- good dispersibility

- excellent chemical resistance

- high cement retention capacity

- good lane-forming ability

- high tensile strength

- high modulus of elasticity

- low elongation at break

As a process aid, asbestos shows a very good distribution ability in an aqueous cement slurry. During the dewatering step, due to its good filtering ability and good cement affinity, asbestos is able to retain the cement in the formed composite material. In the hydrated end product, the high tensile strength in connection with the high E-modulus and the low elongation at break has a positive effect, which results in the well-known high flexural tensile strength of the asbestos-cement products.

As neither natural nor synthetic fibers with asbestos's combination of properties could be found, the research efforts with the aim of finding possible replacement products have resulted in fiber mixtures corresponding to asbestos' two main functions having to be used so that fiber cement can be produced with these new fibers in existing facilities (see e.g. .DE-PS 3 002 484, Amiantus). The filtering properties of asbestos can be simulated by adding cellulose and/or synthetic fibers to the fiber mixtures. For the strengthening effect, reinforcing fibers are added. Such fibers can be organic or inorganic high-modulus fibres, which are usually added in average lengths of 4-12 mm.

There are hardly any synthetic fibers that have not been tested as cement reinforcement fibers for this application purpose. However, most fibers have been found unsatisfactory for various reasons, such as insufficient chemical resistance, poor cement affinity, insufficient mechanical properties or due to excessive cost. Of the overall fiber offer, only two synthetic fiber types have so far been shown to satisfy the requirements for a cement reinforcement fiber. One of the fibers was developed on the basis of polyacrylonitrile and is commercially available, e.g. by the company Hoechst (BRD) under the trade name "Dolan 10"

(see also Swiss patent application 1 919/81-8). The other fiber is

constructed on the basis of polyvinyl alcohol and is available from the company Kurarai, Japan, under the trade name "Kuralon" (DE-PS

28 50 337). The most important properties of these fibers are listed in Table I.

A comparison between the textile mechanical properties of the PVA and PAN fibers shows that PVA fibers exhibit the best mechanical properties. If 6 mm fibers of these two types are now distributed in an aqueous cement slurry and processed into small fiber cement boards using a filter press, it is clear from the measured strengths of the boards that the best fiber properties also give the end product higher strength (table 2).

TABLE 2

Strength values for small fiber cement boards made from Portland cement and the high modulus fibers based on PVA and PAN using a filter press Textile mechanical properties of the fibers used

The fracture work is a very important material technology property. It provides information on the product's brittleness and impact resistance. In practice, this can result in such outcomes as, for example, when laying roof slabs with different high values for breaking capacity, but with identical bending strengths, the load exerted by the roofer can in one case result in the slabs suddenly breaking without warning (cracking), while the load in another case, on the other hand, is taken up by a higher deflection.

Analysis of the results shows that the PVA fibers with the better textile mechanical properties are not only capable of

to give the fiber cement boards a higher bending tensile strength, but also that the work of breaking is very much higher than in the case of the PAN fibres. In these experiments, the work at break is defined as the area under the stress-elongation curve to the point where the maximum bending tensile strength is reached, i.e. the disc undergoes fracture (Fig. 1).

With knowledge of the connection between the fiber data and

the resulting product properties of fiber cement boards, it would be easy to produce fiber cement products with very high requirements in terms of flexural strength, impact strength and breaking performance, exclusively using PVA fibres. However, this solution is counteracted by the high price of PVA fibres. Due to the high raw material costs associated with a very expensive thread production process, the costs for the production of PVA fibers are approx. 50-100% higher than for PAN fibres. If you also take into account that the price of asbestos is currently only a fraction of the price of synthetic fibres, it is obvious that the fiber price must be given a decisive importance, so that fiber cement products at all can be manufactured industrially. It would therefore be highly desirable for the fiber cement industry to find a fiber with the properties of the PVA fibers described, but which has a tolerable price, i.e. are not significantly more expensive than the PAN fibres.

According to the mixing rule valid for fiber-reinforced composite materials, one should expect in the case of fiber mixtures that the strengths, respectively the work of fracture resulting for the reinforced material, follow the mixing ratio linearly and proportionally (cf. "Fiber-Reinforced Cement Composites", Technical Report 51.067, The Concrete Society, Terminal House, Grosvenor Gardens, London 1973 and

H. Krenchel "Fiber Reinforcement", Akademisk Forlag, Copenhagen, 1964).

It was now surprisingly found that a very high proportion of PVA fibers can be replaced with cheap PAN fibers without the fiber cement product's positive properties needing to deteriorate.

The mixture according to the invention of PVA and PAN fibers is characterized in that it contains as little as possible of, however at least 10% PVA fibers.

The fiber mixtures according to the invention will now be elucidated in more detail with the help of the following practical experimental examples.

As suitable fibers for mixtures of PAN and PVA fibers, high-modulus polyacrylonitrile fibers can be used which exhibit an E-modulus of at least 13.0 cN/dtex, an elongation at break of at most 16% and a tensile strength of at least 6 cN/dtex.

Suitable polyvinyl alcohol fibers are high-modulus fibers with the following specifications: E-modulus of at least 175 cN/dtex, elongation at break of a maximum of 15% and a tensile strength of at least 10 cN/dtex.

Both fiber types can be used with uniform titer values or as mixtures of fibers with different titer values. Preferably, however, fibers in the range 0.5 - 5.0 dtex are used. The fibers can either be cut to precise, uniform lengths, or they can be ground or mixed in different lengths. Preferably, the PVA fibers are used in lengths of 4 - 15 mm and the PAN fibers in lengths of 2 - 12 mm.

The following examples will show that it is particularly advantageous if the PVA fibers used are approx. 1/3 longer than the mixed PAN fibres.

As possible production methods for structural parts where the fiber mixtures according to the invention are used, dewatering methods can be used, for example, using round or long wire machines, but also monostring systems, injection systems or filter presses.

Mixtures which are suitable for the processing in the aforementioned facilities contain in an aqueous suspension, in addition to the fiber mixtures according to the invention, also a binder, such as e.g. cement and possibly further fibrous materials with filter properties, as well as various fillers or aggregates.

Suitable as binders are hydraulic inorganic binders such as cement, gypsum, alkaline earth metal silicates or alkaline earth metal aluminates. However, organic binders such as synthetic resins can also be used. As fill-

and aggregates, for example, quartz sand, blast furnace slag, fly ash, pozzolans, mica, stone flour can be used.

As auxiliary fibers that serve to retain binder and fillers on the wire, cellulose fibers in the form of sulphate cellulose, sanding pulp, thermomechanical pulp and/or synthetic fibrids based on synthetic resins, such as e.g. polyethylene. The retention ability can also be further improved by using flocculating agents, for example based on polyacrylamides.

Products that can be produced with these mixtures

on the dewatering plants, are for example flat plates, corrugated plates, pipes or shaped articles such as garden supplies.

The PVA/PAN fiber mixtures according to the invention must i

the following is explained in more detail with the help of some application examples.

aj Production of fiber cement boards for experimental purposes

The experiments were carried out on a round wire dewatering machine, type Hatschek.

In a separate pulper (pulpers), the fiber cement slurries were prepared with a solids content of 80 g/l and then continuously pumped to the pulp bin for a Hatschek machine.

Before the supply to the pulp bin, 200 ppm of a flocculating agent of the polyacrylamide type was also dosed to improve cement retention. On the machine, plates of approx.

6 mm, which were pressed to a thickness of 4.8 mm between oiled plates for 60 minutes in a stack press at a specific pressing pressure of 250 bar. Of all variants, unpressed samples were also produced and tested. The curing of the fiber cement boards took place over 25 days in a humidity chamber with 100% relative humidity at 20°C. After the plates had additionally been stored for 3 days under water, they were tested in a wet state.

The experiments, in which fiber cement boards consisting only of the fiber mixture according to the invention and cement, i.e. without filter aids, is carried out on a filter press.

b) Mixtures used

For production on a filter press:

The following varieties were used as fiber mixtures:

The textile technical properties of the fibers used were:

- PAN

Titer, 1.5 dtex, tensile strength 7.2 cN/dtex,

E-modulus 140 cN/dtex, elongation at break 9%

- PVA

Titer, 2 dtex, tensile strength 12.5 cN/dtex

E-module 250 cN/dtex, elongation at break 6.5%

PAN fibers and PVA fibers were used in different combinations in lengths of 4 and 6 mm.

For the experiments on the filter press, mixtures consisting only of Portland cement and the PVA/PAN fibers were prepared in water.

c) Testing of the fiber cement boards

The testing of the test plates was carried out with the help of

a Wolpert testing machine with three-point storage using 25 x 25 cm plates. The storage distance was 167 mm and the sample speed 26 mm/Min. The assessment of the results was carried out using a connected calculator.

d) Results

The results are shown in tables 3-7.

Claims (13)

1. Mixture of polyacrylonitrile and polyvinyl alcohol fibers as reinforcing fibers for construction material that hardens after forming, especially for hydraulically hardening materials, characterized in that the fiber mixture contains 50 - 90% polyacrylonitrile fibers and 50 - 10% polyvinyl alcohol fibers. 2. Mixture according to claim 1, characterized in that the PAN fibers exhibit an E-modulus of at least 130 cN/dtex, an elongation at break of at most 16% and a strength of at least 6 cN/dtex. 3. Mixture according to claim 1, characterized in that the PVA fibers exhibit an E-modulus of at least 175 cN/dtex, an elongation at break of at most 15% and a strength of at least 10 cN/dtex. 4. Mixture according to one or more of the preceding claims, characterized in that the PVA and PAN fibers used have a titer of 0.5 - 10 dtex. 5. Mixture according to one or more of claims 1-3, characterized in that the length of the PAN fibers is in the range 2-12 mm. 6. Mixture according to one or more of claims 1-3, characterized in that the length of the PVA fibers is in the range 4-15 mm. 7. Mixture according to one or more of the preceding claims, characterized in that the preferred ratio between the cut lengths for the PVA and PAN fibers respectively is 4:3 to 3:2. 8. Mixture according to one or more of the preceding claims, characterized in that the reinforcing fiber mixture also contains aggregate materials, such as cellulose fibers and/or synthetic pulp (pulp) as processing aids. 9. Mixture according to claim 8, in combination with fillers/aggregates, such as quartz sand, amorphous silicic acid, blast furnace slag, fly ash, pozzolans, limestone and mica. 10. Use of the mixture according to one or more of the preceding claims, for strengthening hydraulic binders, such as cement, gypsum, alkaline earth metal silicates or alkaline earth metal aluminates. 11. Use according to claim 10, for the production of fibre-reinforced molded parts, such as e.g. sheets, corrugated sheets or pipes, whereby a diluted aqueous slurry is prepared with all the mixture components and this is placed in the desired shape and then brought to harden. 12. Form parts, such as e.g. sheets, corrugated sheets or pipes, produced using a reinforcing fiber mixture according to one or more of claims 1-9. 13. Molded parts according to claim 12, characterized in that they contain 1-5%, preferably 1.5-2.5% of the reinforcing fiber mixture.