GRANULAR COMPOSITE CONTAINING CRIMPED FIBRES AND PLASTIC ARTICLES MADE THEREFROM

Abstract: This granular composite is obtained by chopping granules from a composite strand which contains at least one bundle of gear crimped fibres embedded in a polymer. The fibres can have reinforcing or electrically conductive properties. The gra­ nular composite is mixed with another polymer granulate and softened, whereby the fibres are dispersed in the polymer mass, which is then shaped for example by injection molding into plastic articles.

Description: The invention relates to a granular composite material containing crimped fibres, and to plastic articles made from such material. In the manufacture and shaping of plastic articles, plastic granules containing additives are often used, whereby these master batch granules are then plastified and mixed intensively with an amount of resin thus forming a viscous mass. This mass can then be shaped into articles by extrusion and/or moulding. In applicant's U.K. patent GB-B-2.150.936 there is described the production of a granular composite containing electrically conductive fibres, e.g., short stainless steel fibres. Such fibres are used for the shaping of thermoplastic articles with antistatic properties or shielding properties against electromagnetic radiation. According to this patent, fibres are introduced and uniformly distributed through a plastic by the use of an intermediate granular composite. In addition to conductive fibres, nonconductive fibres, e.g., glass fibres, can also be introduced in the same manner to reinforce the plastic. Although the dispersion attained according to this patent is good, the injection moulding process conditions must be controlled very accurately. In particular, the shear forces in the warm plastified mass to be injection molded must be controlled to achieve a sufficiently uniform dispersion without excessive fibre breakage. This causes the rate of production to be relatively low. According to the present invention, a uniform fibre dispersion without significant fibre breakage is achieved under widely varying molding process conditions, such as pressures, temperatures, resin viscosities and throughput, if the fibres in the granular composite are present in an ordered but not too dense packing. This helps to control the gradual release of the fibres by the shear forces occuring during the warm kneading of the granulate and makes this less dependent upon e.g. the kind of fibre, its composition, thickness and modulus. A preferred way to realize the voluminous, loose packing in the granular composite according to the invention, is by using a composite plastic strand into which continuous filaments or staple fibres have been embedded as bundles and wherein at least one bundle consists of crimped fibres, preferably gear crimped fibres. This composite strand is then chopped in a per se known manner into composite granules of desired length, e.g. from 3 to 15 mm, whereby the fibres predominantly run from one end of the granule to its opposite end. Such granular composite is then mixed in the desired ratio with ordinary resin granules, whereupon the mixture is hot kneaded and shaped into articles with a uniform distribution of the fibres through the entire article or through predetermined parts of the article. Gear crimped fibres offer the excellent advantage of allowing a highly permanent shape of crimping wave with an accurately controlled degree of crimp or voluminosity by an appropriate choice and adjustment of the intermeshing gears. Stuffer box crimping, air jet crimping (Taslan), false twist and edge crimping lead to a high voluminosity, but the regularity of the crimp and particularly the entangling of adjacent filaments can hardly be avoided. Gear crimping is known per se. The more or less regular wave introduced into the bundles by the gears is converted into a permanent crimp by a suitable means of fixation. For synthetic fibres this is usually a heat treatment. Metal fibres, e.g. stainless steel fibres, can be advantageously crimped. Straight bundles of metal fibres can e.g. be crimped by impregnating said bundles with resin and applying gear crimping to the stiffened bundles and fixing this at an elevated temperature before winding. Straight bundles of glass or carbon fibres can be processed in the same way. A number of the composite bundles thus obtained can be subsequently combined to form a strand by sheathing them with a plastic coating as will be described below. A regular crimp can also be obtained by knitting (e.g. simple warp knitting) followed by a fixation of the knitted fabric to form a durable crimp in the bundles and by finally unravelling the fixed knitted fabric. It is, however, recommended to use nontwisted fibre bundles regardless of which the crimping process is used. In order to obtain an effective, loose packing of the fibres in the granulate as described above, a simple, almost sinusoidal zigzag crimp in at least one of the fibre bundles may suffice, whereby the wavelength W is between 2 and 30 mm and the amplitude A is chosen between 0.2 and 7 mm in such a way that W/A is larger than 2 and preferably at least 4. If so desired, a more complicated crimp profile can be used. A planar sinusoidal crimp is for example applied in a first stage of gear crimping, whereupon in a second stage, after fixation of the first crimp, the bundle is passed through the nip of a second set of gears. The wavelength and/or the amplitude of the second crimping stage may differ from those of the first stage. Both superimposed crimp waves are of course fixed in a suitable manner. The crimp wave can also result from a larger number of superimposed zigzag crimps. In this way, a definite three-dimensional crimp can be achieved, e.g. when the axes of rotation of at least one set of gear rollers form an angle of e.g. between 30° and 90° with those of a previous set. The limits for both wavelength and amplitude in the case of a more complex crimp profile resulting from the use of several sets of gear roller pairs will preferably be as defined above, whereby W/A will be larger than 2. For the predominant or primary zigzag crimp, W can be between 4 and 20 mm or between 4 and 15 mm, whereupon a second (and if required, a third) zigzag crimp wave with a shorter wavelength is superimposed upon the primary crimp. It is, however, important to design the crimp in such a way that practically each segment between two subsequent crests in the crimp does not make too large an angle with the neutral axis (main direction) of the crimped bundle. Such excessively large angles, e.g. larger than 45°, can result in poor chopping behavior of the strand, because the chopping plane must preferably be as perpendicular to the fibre direction at the chopping point as possible. If this is not the case, there is a risk of producing granules with unravelling fibres at the chopped sections. Angles that are too large, resulting in excessive voluminosity and elasticity, can also cause problems during the embedding of the bundles in the resin to form the composite strand, as will be illustrated in the examples. The degree of voluminosity (lack of compactness) can be accurately controlled, in particular by combining a number of parallel bundles with different crimp characteristics and possibly different thicknesses into strands. This difference of crimp characteristics can be achieved by selecting the planes of crimp of adjacent bundles so that they are not parallel. It is also possible to shift the crimp wave axially (phase shift) with respect to the crimp in the adjacent bundle. Bundles with different crimp profiles can, of course, be combined, e.g. bundles with different wavelengths and/or amplitudes, W/A ratios, sinusoidally crimped bundles and threedimensionally crimped bundles. The desired voluminosity in the strand can also be achieved by positioning crimped bundles next to one or more bundles containing almost straight, noncrimped fibres. Such straight and/or crimped bundles can be multifilament bundles or stapel fibre slivers. Multifilament bundles and staple fibre slivers can be combined if so desired. Granular composites made from strands incorporating staple fibre slivers usually disperse better. This holds also when these slivers stem from previously crimped filament bundles. Of course, a combination of the above means of controlling the degree of voluminosity can be used as well. Bundles containing gear crimped metal fibres can for instance be combined side-by-side with almost straight carbon or glass fibre bundles since these are more difficult to crimp. It is also possible to use a combination of gear crimped metal fibres and synthetic fibres (for instance made from low melting polymers) which may be crimped or not, in order to produce granules analogues to the embodiments as described on page 13 of the aforementioned U.K. patent No. 2.150.936. The strand and the composite granule made thereof must contain electrically conductive fibres as for instance metal fibres or metallized fibres, if the plastic article produced therefrom is to possess antistatic properties, shielding properties against electromagnetic radiation or electric conductivity. Metal fibres with a conductivity of at least 0.5 % of the copper standard, as for example stainless steel fibres, are particularly suitable. The fibres preferably have a diameter of at most 15 microns. Carbon fibres and metallized (e.g. nickel coated) carbon fibres are also suitable. The strand may contain bundles of fibres of different conductivities, for instance stainless steel fibres and carbon fibres. A combination of bundles containing conductive fibres with bundles containing nonconductive fibres can also be used. Finally, it may be preferable to incorporate into the strand only nonconductive or poorly conductive fibres, for instance reinforcement fibres such as glass or carbon fibres. The strand contains preferably between 1000 and 35000 fibres. In order to obtain a proper dispersion of the fibres into the polymer matrix during the hot molding process, the volume percentage of the fibres in the strand (and thus in the composite granulate derived therefrom) will preferably be between 20 and 80 %. The plastic into which the bundles have been embedded to form a strand may have a rather low melt viscosity. The melt viscosity will, for instance, be lower than that of the polymer constituting the main part of the article to be formed and thus of the granular resin to be mixed with the composite granulate according to the invention. Similarly, the melt index of the strand polymer can be chosen at a higher value than the article's main polymer. If so desired, very finely divided highly polar organic compounds or conductive compounds can be added to the polymer as for instance carbon black or metal powder. These encourage the formation of electrically conducting bridges between the adjacent dispersed fibres in the article. Similarly, the addition to the polymer of the strand of certain coupling, bonding or wetting agents, such as for example silanes, titanates and zirconates, can be considered in order to control the adhesion of the fibre surfaces to the polymer matrix into which these fibres are to be dispersed. Finally, the resin impregnated bundles as described above may be extrusion coated with a further polymer layer, which layer may have the same or almost the same composition as the polymer used for the impregnation of the fibre bundles. This additional polymer may in certain cases also have the same or almost the same composition as the main (first) polymer constituent of the plastic article, if for instance polycarbonate resin is used. Similarly, the composition of the impregnating resin of the fibre bundles may correspond to the main polymer of the plastic article and said fibre bundles may be optionally coated with an additional polymer layer. The strand itself may have a circular or elliptical cross section, or rectangular whereby its width w is larger than its thickness t. Example 1 Several polymer compositions were prepared by mixing various polymer granules with granular composite according to the invention for the injection molding of plastic articles with shielding properties against electromagnetic radiation. The granular composite was substantially prepared according to the process as described in Example 1 of the aforementioned U.K. patent. Each granule contained about 20.000 continuous stainless steel (AISI 316 L) filaments with a diameter of 0.0065 mm embedded into a linear polyester resin (Dynapol® L850) and coated with a modified alkyd resin with adequate frictional (smearing) properties. The cylindrical composite strand had a diameter of about 2 mm and was chopped into 6 mm long granules. This granular composite was dry mixed with the usual polyphenylene oxide based granulate (Noryl®) so that the mixture contained 10 wt% (i.e. 1.5 vol%) metal fibres. It was then extruded using a Samafor 45 extruder into an almost circular strand of 4 mm diameter in which the metal fibres were dispersed. This strand was chopped into 10 mm long granules which were fed to a Stubbe® injection molding machine as described in Example 6 of the aforementioned U.K. patent. The extrusion nozzle temperature was controlled at 280-300°C and the screw speed was 60 rpm. Injection speeds and pressures were as recommended by the resin manufacturer. The injection moulded square plaques (150 x 150 mm) had a thickness of 3 mm. This process flow was performed on two types of granular composites : one was a conventional granular composite containing a number of straight filament bundles, whereas the other was a granular composite containing the same number of bundles, gear crimped according to the invention. The injection molded plaques with the straight and crimped fibres had about the same average reflection value R of about 90 % at 10 GHz in the far field. They therefore provided about the same shielding against electromagnetic radiation. The plaques containing crimped fibres exhibited, however, a remarkably more uniform dispersion of the fibres than those containing straight fibres. Each bundle was crimped in the same way in that two zigzag deformations with wavelengths of 7.5 and 5 mm respectively and amplitudes of 1 and 0.7 mm respectively were superimposed. The bundles were assembled and allowed of excellent impregnated with polyester resin solution, passing smoothly through the circular stripping orifice and displaying a very good resin take-up. Possibly, the voluminosity of the crimped bundles is responsible for this phenomenon because of the many voids between fibres and sub bundles and also because of a certain degree of elasticity of the sub bundles which prevents breakage when passing through the stripping orifice, which action of course exerts a certain stress and stress resistance on the bundles. Impregnating a more compact arrangement of straight fibres is found to lead to a much more difficult resin take-up. When passing such bundles through the orifice, they do not yield elastically which leads to more breakage. This causes broken-off fibre bundle ends to accumulate at the orifice or get entangled around the other bundles which prevents an uninterrupted composite strand formation and an easy release of the fibres out of the chopped granular composite. The improved resin take-up and more voluminous embedding in composite granules also promotes a rapid dispersion of the granulate during hot working. A too pronounced crimp, on the other hand, has also been found to hamper a rapid impregnation. Then, the impregnated bundles show a tendency to open up in the transverse direction (to expand) after having been passed through the stripping orifice and during the drying stage (stiffening) which endangers the internal cohesion of the composite strand. This also leads to the risk of granules splitting along their longitudinal axes and thus to the formation of irregularly shaped granules. Example 2 Granular composite was made according to Example 1, but with a granule length of 4 mm. The granules were mixed with ABS-granules (Cycolac® KJBE) to form a master batch with 6 wt% (1 vol%) metal fibres, which was fed directly to the same Stubbe® injection molding machine to form plaques of 150 x 150 x 3 mm. Orifice temperature was controlled at 220-240°C and the screw speed was again 60 rpm. Injection pressure and back pressure were controlled as prescribed by the resin manufacturer. When the granular composite with crimped fibres according to the invention was used, various injection molding conditions invariably led to a better appearance of the injection molded plaques (i.e. a better dispersion) than when granular composite containing straight stainless steel fibres was used. Further with an increased injection pressure, a better dispersion (and thus appearance) was observed for both types of composites. The electromagnetic shielding values determined by reflection measurements as in Example 1, were almost the same : R = 85 % (at 10 GHz). In general, it can thus be concluded that crimped fibres lead to a better dispersion and thus to an improved appearance of the molded articles. This last aspect especially is gaining more importance. In addition, a better dispersion leads to a more homogeneous composition resulting in articles with more uniform physical, chemical and mechanical properties throughout their mass, which is a highly valued characteristic. The invention also applies to a granular composite containing a water-soluble resin. Such a granulate will gradually disintegrate when agitated in water or in an aqueous suspension to form a uniform fibre dispersion, which can then be poured onto a permeable sieve to form a fibre mat (e.g. a metal fibre mat) by a wet process. When the aqueous suspension is a paper pulp, a paper with a predetermined content of crimped (e.g. conductive) fibres results which fibres are homogeneously dispersed in the paper sheet. In this instance, it will be advantageous to choose a composite granule length of less than 5 mm and preferably less than 3 mm. Of course, the invention can also be applied to disperse fibres in thermosetting resins by the use of granular composites, for instance as described in Example 4 of the aforementioned U.K. patent.

Claims: 1. A composite strand comprising fibres embedded as a plurality of bundles in a polymer, characterized in that at least one of said bundles comprises crimped fibres with an almost sinusoidal zigzag crimp with a wavelength W between 2 and 30 mm and an amplitude A between 0.2 and 7 mm and in which W/A is larger than 2. 2. A strand according to claim 1, in which the crimp in said crimped fibres is formed by a plurality of superimposed zigzag deformations. 3. A strand according to claim 2, in which said crimped fibres have a three-dimensional crimp. 4. A strand according to claim 1, in which said strand contains a number of adjacent fibre bundles with different crimp profiles. 5. A strand according to claim 1, in which said strand contains at least one additional bundle of almost straight fibres. 6. A strand according to claim 1, in which said strand contains metal fibres having a conductivity of at least 0.5 % of the copper standard. 7. A strand according to claim 6, in which said metal fibres are stainless steel fibres. 8. A strand according to claim 1, in which said strand contains metallized fibres. 9. A strand according to claim 1, in which said strand contains carbon fibres. 10. A strand according to claim 1, in which said strand contains fibres of different electrical conductivities. 11. A strand according to claim 6, in which said strand contains fibres of different electrical conductivities. 12. A strand according to claim 1, in which said strand contains only nonconducting fibres. 13. A strand according to claim 1, in which the volume percentage of the fibres is between 20 and 80 %. 14. A strand according to claim 1, in which said polymer has a relatively low melt viscosity. 15. A strand according to claim 1, in which said polymer contains finely divided, electrically conducting compounds. 16. A strand according to claim 1, in which said polymer contains coupling agents to control the adhesion of the fibre surfaces to said polymer matrix. 17. A strand according to claim 1, in which said strand contains a number of polymer impregnated fibre bundles coated with an additional polymer layer. 18. A strand according to claim 17, in which the additional polymer layer has the same or almost the same composition as the polymer used to impregnate the fibre bundles. 19. A strand according to claim 1, having an elliptical or rectangular cross-section in which the strand width w is larger than its thickness t. 20. A granular composite obtainable by chopping granules from a strand as defined in claim 1, in which the fibres predominantly extend from one end of the granule to the other. 21. A granular composite according to claim 20 in which said strand contains a number of polymer impregnated fibre bundles coated with an additional polymer layer. 22. A composite molding compound to be used for molding plastic articles comprising a mixture of granular composite as defined in claim 20 and another polymer granulate. 23. A molding compound according to claim 22 wherein the polymer of the granular composite is substantially the same as that of the polymer granulate of the plastic article. 24. A molding compound according to claim 22 comprising the granular composite of claim 21. 25. A molding compound according to claim 24 wherein said additional polymer layer has the same or almost the same composition as that of the polymer granulate of the plastic article. 26. A plastic article obtained by shaping the compound according to any of the claims 22 or 25, in which the fibres have been uniformly distributed in predetermined parts of the article or in the entire article.

Inventors: ADRIAENSEN, LUDO, VERHAEGHE, FRANS
Applicants: N.V. BEKAERT S.A.
Application year: 1988
Main classification: C08J5
IPC codes: C08K7, C08J3, B29C70, B29B9, B29C45, C08K9, B29B11, B29K105