Bi-component synthetic fibres for application in cement-bonded building materials

The invention relates to special plastic fibres, witch are suitable for the application in concrete with a largest grain diameter of 4 mm and more, and thereby decisively improve the tensile strength and post-failure behaviour of these building materials.

The tensile strength of concrete is lower than its compressive strength by a factor of approx. 10. The failure is effected in a relatively brittle manner. Concrete therefore needs to be reinforced when accommodating tensile forces or shear forces on the building construction. Safety considerations are often at the forefront. A concrete component on exceeding the maximal load should not break in a catastrophic and abrupt manner, but firstly absorb a certain amount of energy, thus display a ductile behaviour. Both are conventionally achieved by way of steel reinforcement. The type of reinforcement is planned in a detailed manner and then the reinforcements are applied in costly manner mostly by hand. In certain cases, one may do away with conventional steel reinforcement completely or partly by way of admixing shortly cut steel fibres. Steel fibres however have decisive disadvantages. They are prone to corrosion which often leads to ugly strips of rust or spots on the concrete. They further entail the danger of injury. They also have poor dosing and mixing properties as a result of their rigidity. Large dosing weights arise as a result of the large density, witch shows up in the costs. The admixing of steel fibres also leads to a relative high scatter of the material properties due to the non-uniform distribution. Other fibre types, such as glass fibres, have decisive disadvantages, for example a limited resistance to alkali.

The use of plastic fibres offers an alternative. The fibres thereby on the one hand need to have a relatively large tensile strength, and on the other hand need to have a high bonding strength with the concrete. In the case of loading, the static friction on the complete surface of the fibre is to remain effective, so that the fibre is uniformly pulled out and is in the position of absorbing a lot of failure energy. Inexpensive fibre types, in particularly also compared directly to steel fibres, may be manufactured on the basis of polyolefin\'s (polypropylene, polyethylene) or other thermoplastic plastics. Whilst one succeeds with this, in achieving notable tensile strengths with values which to some extent are better than steel, the modulus of elasticity and the bonding strength to concrete is generally low with these types of fibres. An improvement of the static friction may be achieved by increasing the E-modulus of the fibres, manufactured of relatively expensive raw materials.

Multi-layered, thermoplastic plastic fibres for reinforcing concrete are known from EP 1350773. There, it is particularly emphasised that the polymers of the different layers have different melting points. The polymer with the lower melting point lies in the core, that with the higher one lies in the casing, wherein the difference is to be 10° C. to 20° C. This measure is to serve for the stretching after heating in a special oven, in that the inner layer is likewise adequately heated with the heating of the outer layer, so that a stretching by the factor 3 to 12 is possible. By way of the stretching, the plastic molecules are orientated longitudinally. The strength in the plastic is firstly achieved by way of this. These plastic fibres are provided on their outer side with structures before the stretching, for increasing the adhesive force in the concrete. In detail, these filaments are manufactured such that a double-layered or multi-layered film is created by way of co-extrusion. Afterwards, this film is provided with an embossing by way of calendar. The film is subsequently cut into narrow strip lets. At the end, the two-dimensional striplets are yet stretched, by which means the bulges or thin- and thick locations effectively arise.

It is however important to ascertain that the embossing and stretching take place in a coherent process. Partial material accumulations arise due to the embossing in the undrawn condition, as is taught by EP 1350773 A2. The polymer which is displaced by the structuring is still amorphous. If one draws thereafter, then firstly the zone with the smallest material accumulation is stretched. It is generally known and also obvious that firstly the locations with smallest resistance are drawn during each stretching process. In this case, this is clearly the thin locations. For this reason previously embossed striplets may not be uniformly drawn at the end, thus after a stretching after the effected embossing. It would be difficult or even impossible whilst maintaining favourable production conditions, which means whilst avoiding filament breakages, to be able to completely stretch filaments embossed in such a manner at all. The thin locations would be completely stretched, whilst at the thick locations, the degree of stretching and thus also the orientation of the molecules must necessarily be smaller. By way of this, the bulges which are manufactured according to this method are softer than the other locations of the filament, and accordingly they have an insufficiently high modulus of elasticity. This means that the bulges—on pulling out—are slightly worn. Furthermore, it is basically not possible to obtain sharp-edged bulges by way of stretching after the embossing, since the profile of the bulges are “blurred” due to the drawing, which is clearly evident in FIG. 1 of EP 1,350,773 A2. Since each chain is only as strong as its weakest member, this method also entails a certain amount of material wastage, since an over-proportional amount of polymer needs to be used, in order to achieve the designed strength values in the thin locations. FIG. 1(A) in EP 1,350,773 A2 likewise makes this point clear. With the embossing before the stretching, one may only achieve bulges which have a very large distance to one another. Thus in [0041] of EP 1350773 A2 it is mentioned that the stretch ratio is to be between 3:1 to 12:1 and preferably between 5:1 to 10:1. With a pyramid embossing as is shown in FIG. 1(B), and after a subsequent minimal total drawing (stretch factor) of 5, the distances from bulge to bulge is 5 mm, with a total drawing of 10, which represents the absolute minimum with a high-strength PP or HDPE-filament, thus a distance of 10 mm from bulge to bulge results, without taking into account the embossing gap! FIG. 1(A) of EP 1350773 A2 furthermore very clearly illustrates the profile of the filaments which one obtains by way of an embossing before the stretching. The fibres are above-averagely thin in the web (thin locations). The thickenings continuously increase towards the bulges (thick locations) and after become increasing flatter. Thus to a certain extent a cone is formed on both sides by each bulge. This particularity is always repeated with the method as is described under EP 1,350,773 A2, independently of which embossing type is selected, whether a pyramid, wave or angular profile, or a single-side or double-sided embossing. The bulges necessarily and always on both sides run out at a very acute angle to the diameter of the next thin location. The sliding out of the concrete, in comparison to a sharp embossing with marked transitions from the thin to the thick locations, is significantly more unfavourable.

Thus one may only achieve laterally flatted or rounded bulges which have a large distance to one another, with the embossing before the stretching, and furthermore it is clear that a structure conversion in the inside of the fibres is accepted as a result of the stretching which follows the embossing with regard to time. During each stretching process, firstly the locations with the smallest resistance are drawn. This in this case is clearly the thin locations produced on account of the structuring. For this reason, the fibres stretched after an embossing no longer have homogeneous molecule structures. Rather, the thin locations are completely stretched, whilst the degree of stretching at the thick locations and thus also the uniform orientation of the molecules is inevitably smaller. For this reason, an over-proportional amount of polymer is used, in order to achieve the desired strength values in the thin locations. Furthermore, the thick locations are soft, which likewise worsens the bonding to the concrete, and leads to a sliding out of the cement stone matrix, which is much more likely when compared to a hard polymer surface.

Against this background, it is the object of the present invention to provide plastic fibres for the application in cement-bonded building materials, in particular in concrete with a largest grain of greater than 4 mm in diameter, by way of which the mechanical properties of these building materials are significantly improved in that they comprise a homogeneous molecular structure, as well as denser embossing on their surface. At the same time, these plastic fibres should be more practical in their handling and admixing, should achieve their tensile strength with minimal masses, and be capable of competing with steel fibres with regard to cost.

This object is achieved by a plastic fibre for the application in concrete, with a largest grain diameter >4 mm, with an average diameter of 0.15 to 2 mm, corresponding to approx. 160 to 28,000 dtex (Dezitex=gram per 10,000 running meters) which is characterised in that it is a bi-component fibre which is stretched by the factor 5 to 15 and is manufactured by way of a co-extrusion method from a central core and a casing which envelops this, of differently pure polymers or polymer mixtures, and that after the stretching has been effected, a continuously or interruptedly structured or grooved surface is embossed onto this continuously stretched bi-component fibre, wherein the depth of this structuring is more than 10% of this average fibre diameter, and the maximal distances of their structure tips within attached structures in the axial direction lie between 0.5 mm and 3 mm.

Due to the division into a core and a casing, on the one hand the casing polymer with respect to the workability (rheology) and the bonding strength between fibre and concrete, furthermore the degree of stiffness, the dimensional stability and the wear strength, and the one hand the core polymer with regard to the high tensile strength and a small extension at break, may be optimised independently of one another. By way of this, one may not only achieve fibres with very new, improved characteristics, but also reduce the costs, since it is not the complete fibre which must consist of expensive universal polymers, as is the case with full fibres. Furthermore, there results the possibility of applying the expensive components to a lesser extent, for example only in the casing. The casing polymer may be optimised to the desired bonding to the cement, on the one hand by way of embossing, and on the other hand by way of chemical modification on the surface. Thereby, the combination of both measures has been shown to be extremely effective.

Different constructions of fibres which are suitable for incorporation into cement-bonded building material are shown in the accompanying drawings, and the effect in the concrete is displayed by way of measurement diagrams.

There are shown in:

FIG. 1: a force-path diagram for representing the bonding strength, i.e. the force per fibre surface of different fibre types in a cement-bonded building material;

FIG. 2: an intermittently embossed fibre with an initially round cross section;

FIG. 3: a device for embossing with two rollers arranged parallel to one another, represented schematically;

FIG. 4: an embossing type of the fibres, seen from the side;

FIG. 5: a further embossing type of the fibres, seen from the side;

FIG. 6: an embossed fibre with a fibre core and fibre casing of different materials, with fine particles or nanoparticles in the core and/or casing polymer;

FIG. 7: a force-path diagram for representing different bonding strengths with identical plastic fibres, wherein nanoparticles were applied in the casing polymer with the one fibre;

FIG. 8: a force-path diagram for representing the composite strength of fibres with nanoparticles and embossing in the casing polymer, in comparison to a fibre without nanoparticles and embossing in the casing polymer;

FIG. 9: an individual bundle with thousands of plastic fibre sections, for incorporation into the cement-bonded material to be mixed.

Some basic facts are explained here before dealing with the individual figures. Fibres which may be applied into concrete, in contrast to fibres in fibre cement products, have a significantly larger diameter of 0.15 to 2 mm, since otherwise with the usual fibre dosages for mechanical reinforcement, which is to say in the region of 0.3 to 2% by volume, one may not achieve an adequate workability of the building material. With such thick fibres, the bonding strength between the fibre and the building material, in particular based on inexpensive low-modular polymers such as polyolefin\'s, has hitherto been inadequate, since the cross section of the fibre reduces in the case of loading, and thus the fibre may easily slip out of its embedding. An increase of the bonding strength by way of a suitable polymer selection or by way of the increase of the adhesion to the concrete due to the increase of the surface tension of the fibres or due to a corona-, plasma- or fluoride treatment, or also by way of depositing wax dispersions or softenings, has been shown to be insufficient with low-molecular fibres and therefore also with bi-component fibres. With the use of high-modular plastics, the tapering of the cross section is only adequately reduced when the fibre for the large part or completely, is manufactured from these expensive raw materials. A bi-component fibre provided with an embossed structure now opens very new perspectives for the applications in concrete.