PLASTIC GRANULATE

Abstract: The invention relates to a plastic granulate on the basis of a thermoplastic polymer and natural fibres. A bundle of the natural fibres, predominantly oriented in the longitudinal direction of the granulate, is provided with a sheath of the thermoplastic polymer. The invention also relates to a process for the preparation of such a granulate, as well as to a moulding made thereof.

Description: PLASTIC GRANULATE The invention relates to a plastic granulate on the basis of a thermoplastic polymer and natural fibres. The invention also relates to a process for the preparation of such a granulate, as well as to a moulding made thereof. Such a granulate is known from EP-A-865,891 , which describes how the granulate is obtained by mixing chopped natural fibres with a thermoplastic polymer, followed by production of a granulate. The granulate thus obtained is for instance used as a starting material for mouldings, produced with application of techniques such as injection moulding. During the preparation of the granulate and in the moulding process the material is brought to a high temperature, i.e. the processing temperature of the thermoplastic polymer, which temperature is mostly 40-100 °C above the melting temperature of the polymer. During such treatments the natural fibres are also exposed to these high temperatures. At those temperatures the natural fibres are exposed to a high thermal and mechanical burden, which results in degradation and decomposition of the fibres and thus ultimately loss of the properties of the moulding. The aim of the invention is a plastic granulate on the basis of a thermoplastic polymer and natural fibres, obtained with much less thermal and mechanical burdening of the natural fibres, due to which the formation of a moulding from such a granulate gives an object having improved properties. The plastic granulate according to the invention is characterized in that the granulate comprises a bundle of natural fibres, predominantly oriented in the longitudinal direction of the granulate, the bundle being provided with a sheath of the thermoplastic polymer and the fibre bundle being as long or virtually as long as the granulate, and wherein the decomposition temperature of the natural fibres is 20 to 80 °C higher than the melting point of the thermoplastic polymer. The granulate according to the invention thus contains a bundle of natural fibres (in the following also referred to as 'rope') provided with a sheath made of a thermoplastic polymer. The type of rope as such is not really relevant to the invention, much less than the fact that the rope is provided with a sheath, i.e. the outer surface of the rope is sheathed with the thermoplastic polymer. The thermoplastic polymer can at most be partially present between the fibres of the rope, i.e. there is no full impregnation of the rope. The fact is that the characteristic feature of the invention is that during the preparation of the granulate the fibre bundle as a whole, i.e. the rope, is only subjected virtually exclusively to a thermal treatment for a relatively short time, while in the case of full impregnation (like in the prior art) this thermal effect affects every fibre of the bundle, resulting in much more fibre degradation. The fibres in the granulate according to the invention thus preserve their original properties to a high extent or even completely. The fibre material used for the rope present in the granulate according to the invention can be any natural material, of vegetable as well as animal origin. The fibre material is preferably selected from the group comprising jute, flax, kenaf, sisal and hemp fibres, but cotton and silk fibres are also quite suitable. Mixtures of two or more of these fibres can also be present, either in one mixed rope, or in the form of separate ropes. The granulate can also comprise a mixture of natural and other fibres. The latter can be for instance plastic fibres (such as polyolefin, aramid or polyester fibres), inorganic fibres (such as glass or metal fibres) or carbon fibres. Preferably, as 'other fibres', plastic fibres and/or glass fibres are used. Such other fibres can be present in one mixed rope together with the natural fibres, but also be present separately in the plastic granulate. The fibres can optionally be provided with an oil or with a sizing, in combination with an adhesion improvement agent if required. These agents, which are known perse to the person skilled in the art, promote the dispersion in and/or the adhesion of the fibres to the thermoplast of the moulding to be produced ultimately. For a plastic granulate according to the invention it is important that the difference between on the one hand the temperature at which the natural fibres begin to degrade or decompose significantly and on the other hand the melting point of the thermoplastic polymer is sufficiently large. The decomposition temperature of the natural fibres must therefore be 20 to 80 °C, preferably 35-70 °C, higher than the melting point of the thermoplastic polymer. A suitable thermoplastic material for the plastic granulate of the invention is in principle any thermoplastic polymer which satisfies the above- mentioned condition concerning the melting point in relation with the decomposition temperature of the natural fibre. Particularly suitable are polyolefins and polyolefin-based polymer systems (such as blends of a polyolefin and a non- cross-linked or cross-linked rubber, as in a thermoplastic elastomer vulcanizate). The polyolefin is preferably chosen from the group of polyethylene and polypropylene, both the homopolymers and the copolymers being suitable. A person skilled in the art is familiar with such materials. Thermosetting materials which are thermoplastic during the preparation of the plastic granulate and are only cured in the final processing stage are also suitable as material for the granulate according to the invention. The length and the diameter of the granulate are not critical as such. Generally, the length of the granulate is < 50 mm, the diameter < 15 mm. For practical purposes it is preferred for the granulate to have a length of 5-40 mm and a diameter of 1-10 mm. The fibre content of the plastic granulate according to the invention can in principle be chosen freely, depending on the further use of the granulate. If the granulate is supplied in the form of a masterbatch, the fibre content will generally be high, while if the granulate is used as such a lower fibre content will mostly be used. On the whole the granulate preferably contains 20-85 wt.% fibres. Since the fibres are not well dispersed in the granulate and it is mostly a requirement that mouldings made of the granulate do have a good dispersion, it is of advantage if the granulate already has features which will promote said dispersion in the moulding. One such feature is that the rope is a bundle of the natural fibres and readily flowing plastic fibres (i.e. fibres which melt in the moulding process and have a low viscosity then) so that the thermoplastic material spreads well through the fibres. Another, preferred option is that the sheath of the plastic granulate, viewed from the core of the granulate, consists of a first layer of a low- viscous thermoplastic polymer and a second layer of a high-viscous thermoplastic polymer, the ratio of the melt indices of the two polymers lying between 10 and 100. The melt index of the first layer preferably lies between 50 and 250 and the melt index of the second layer preferably between 0.1 and 25, both values being determined in accordance with ISO 1133. In order to ensure good properties of the moulding ultimately obtained, the thermoplastic polymers are preferably of the same type or compatible with each other. It should also be ensured, for instance by means of an adhesion promoter, that in the moulding there is good contact between the original sheath material and the fibres. The plastic granulate according to the invention can further contain the ingredients that are required for plastic objects, such as antioxidants, UV stabilizers, fillers, colorants, etc. Such ingredients are known to the person skilled in the art. The invention also relates to a process for the preparation of a plastic granulate as described above. In such a preparation process it is important that the fibres are supplied in such a form that in the sheathing procedure a granulate is obtained in which the fibres are predominantly oriented in the longitudinal direction of the granulate. To that end the fibres should be combined to form one or more continuous bundles or ropes. The manufacture of such a bundle or rope as such is known to the person skilled in the art. The ropes can be of a single natural or of another fibre, they can also be a mixture of natural fibres, a mixture of one or more natural fibres with other fibres, etc. The process according to the invention is characterized in that one or more continuous bundles of the natural fibres, optionally mixed or jointed with other fibres, is sheathed with a melt of the thermoplastic polymer, after which the product obtained (the extrudate) is cooled and cut up to the desired length. In this process use can be made of sheathing techniques known per se, such as for the sheathing of glass or metal fibres. For this purpose the thermoplastic polymer is heated, using an extruder for instance, to a temperature above the melting point and as such supplied to the sheathing apparatus. In the process according to the invention the continuous bundle(s) of (natural) fibres is preferably sheathed in a first step with a low-viscous thermoplastic polymer and subsequently or simultaneously with a high-viscous polymer, with the ratio of the melt indices of the two polymers lying between 10 and 100. The melt index of the low-viscous thermoplastic polymer preferably lies between 50 and 250, while the melt index of the high-viscous thermoplastic polymer lies between 0.1 and 25, both measured according to ISO 1133. The residence time, in particular that of the natural fibres, during the sheathing process is relatively short, without the occurrence of mechanical degradation of these fibres and only minor, if any, thermal degradation of the natural fibres. The contact times in the sheathing process are in general from 0.5 and 1.0 second; then the product obtained is cooled down already. According as the ratio of the mass of the sheath to the mass of the fibres decreases, the effect of the invention, i.e. diminishing/eliminating the degradation of the natural fibre, increases. The proportion of fibres is therefore preferably between 20 and 85 wt.%. The process of the present invention has the further advantage that, compared to prior art processes, the moisture content of the fibres is less critical; pre-drying of the fibres is now optional, instead of necessary. The continuous bundle of natural fibres can also contain fibres of a thermoplastic, preferably a plastic which will melt in a subsequent shaping process and have a low viscosity, which promotes the dispersion of the natural fibres in the moulding. With a view to promoting the later dispersion of the fibres in the moulded object it can be of advantage to spread the natural fibre-bundle slightly already before or while they are fed into the sheathing apparatus. An alternative for obtaining better dispersion is to heat the fibres before they are fed into the sheating apparatus; this improves the wetting of the fibre by the thermoplastic polymer. These steps should be done with care, however, because this enhanced spreading and/or wetting entails an enhanced risk of thermal degradation of the natural fibres in the sheathing process. The invention also relates to a moulding made out of a plastic granulate according to the invention, for instance by injection moulding, by in- mould decorating, by extrusion (optionally followed by deep drawing) or by extrusion followed by compression. In such a moulding process the granulate is fed to a moulding machine, is heated to the moulding temperature, and the moulding is obtained. An alternative to this moulding process can be to feed the extrudate from the sheating process directly (i.e. without the extrudate being cooled and cut up to granulate) to the moulding machine, a machine as described before. In such a process a second heating step is avoided. The invention is illustrated below; the Examples and comparative experiments are however not intended to restrict the scope of the invention. Examples I and II and comparative experiments A and B Example I A bundle of jute fibres was sheated in an extruder with a polypropyle polymer. The bundle comprised 3 strands of jute, having a total tex of 2000 grams/1000 mtr. The polypropylene polymer was a homopolymer with a melt-index (at 230 °C/2.16 kg) of 47 dg/min and a density of 905 kg/m 3 . The polymer also comprised Polybond®, a maleic acid anhydride modified polypropylene from Uniroyal (GB) as adhesion improver. The extruder was a single screw extruder from Schwabenthan (DE) having a screw diameter of 30 mm. The extruder was provided with a fibre bundle guide (U4SCC) from Unitek (AT), together with pinoles having a diameter of 2.9 mm, mounted at a right angle to the extruder. The jute bundle was passed through the extruder head at a speed of 50 m/min and sheated with the polypropylene having a melt temperature of 249 °C. The so obtained strand was fed through a waterbath, cooled to 50 °C and cut into granulate having a length of 12.5 mm. The granulate had a fibre content of 35 wt.%. This granulate was injection moulded to test bars with a moulding machine from Netstal (DE), and a force of 130 ton. The temperature applied was 230 °C, the injection time was 4 sec. Comparative experiment A The granulate of Example I was fed to a ZSK 30 twin-screw extruder from Werner and Pfleiderer (DE); the extruder had a mild screw design, appropriate for dispersing the fibres in the polymer melt. The screw speed was 250 RPM, the melt temperature 230 °C, the through-put 12 kg/hour. The obtained strand was cut into granulate and used for injection moulding as per Example I. Example II Example I was repeated, but now with polyethylene (Vestolen® 1640LO from DSM), said polymer also comprising Yparex®, a maleic acid anhydride modified polyethylene from DSM, as adhesion improver. The temperature of the melt during the sheating process was 239 °C, and during the moulding process 225 °C. Comparative experiment B Comparative experiment A was repeated, but now with the granulate obtained in Example II. The temperature of the melt in the ZSK was 211 °C; the throughput was 5 kg/hour. The test bars were tested for several properties: - Tensile strength, according to ISO 527 1 B - Flexural strength, according to ASTM D790 - Impact strength, Izod, according to ISO 180/4A - Linear coefficient of expansion (LCE), according to ASTM D696 (23-80 °C) - Falling dart impact strength (FDI; VEM), according to ISO 66030-2V The results of the Examples and comparative experiments are given in Table I. Table I Remarks: The test bars from comparative experiments A and B showed black discoloration.

Claims: CLAIMS 1. Plastic granulate on the basis of a thermoplastic polymer and natural fibres, characterized in that the granulate comprises a bundle of natural fibres, predominantly oriented in the longitudinal direction of the granulate, the bundle being provided with a sheath of the thermoplastic polymer and the fibre bundle being as long or virtually as long as the granulate, and wherein the decomposition temperature of the natural fibres is 20 to 80 °C higher than the melting point of the thermoplastic polymer. 2. Plastic granulate according to claim 1 , characterized in that the natural fibres have been selected from the group comprising jute, flax, kenaf, sisal and hemp fibres, or mixtures thereof. 3. Plastic granulate according to any one of claims 1 -2, characterized in that the decomposition temperature of the natural fibres is 35 to 70 °C higher than the melting point of the thermoplastic polymer. 4. Plastic granulate according to any one of claims 1-3, characterized in that the thermoplastic polymer is a polyolefin. 5. Plastic granulate according to claim 4, characterized in that the polyolefin is chosen from the group of polyethylene and polypropylene. 6. Plastic granulate according to any one of the claims 1-5, characterized in that the granulate has a length of 5-40 mm. 7. Plastic granulate according to any one of claims 1-6, characterized in that the granulate has a diameter of 1 -10 mm. 8. Plastic granulate according to any one of claims 1-7, characterized in that the granulate contains 20-85 wt.% of fibres. 9. Plastic granulate according to any one of claims 1-8, characterized in that the sheath, viewed from the core of the granulate, consists of a first layer of a low-viscous thermoplastic polymer and a second layer of a high- viscous thermoplastic polymer, the ratio of the melt indices of the two polymers lying between 10 and 100. 10. Plastic granulate according to claim 9, characterized in that the polymer of the first layer and the polymer of the second layer are the same polymers or polymers which are compatible with each other. 1 1. Plastic granulate according to any one of claims 1 -10, characterized in that plastic fibres and/or glass fibres are additionally present in the granulate. 12. Plastic granulate according to claim 11 , characterized in that the plastic fibres and/or the glass fibres are present in the bundle of natural fibres. 13. Process for the preparation of a plastic granulate according to any one of claims 1-12, characterized in that that a continuous bundle of natural fibres is sheathed with a melt of a thermoplastic polymer, after which the product obtained is cooled and cut up to the desired length. 14. Process according to claim 13, characterized in that the continuous bundle of natural fibres is in a first step sheathed with a low-viscous thermoplastic polymer and subsequently or simultaneously sheathed with a high-viscous polymer, with the ratio of the melt indices of the two polymers lying between 10 and 100. 15. Process according to any one of claims 12-14, characterized in that plastic fibres and/or glass fibres are additionally present in the continuous bundle of natural fibres. 16. Moulding made of a plastic granulate according to any one of claims 1 -12 or obtained by means of a process according to any one of claims 13-15. 17. Process for the preparation of a mould on the basis of a thermoplastic polymer and natural fibres, characterized in that a continuous bundle of natural fibres, sheated with a melt of a thermoplastic polymer, is prepared, and without being cooled and/or cut being fed to a moulding machine.

Inventors: BOYENS JOSEPH PAULUS HUBERTUS, HANSEN OCTAVE WILLEM MARIE, BOYENS, JOSEPH, PAULUS, HUBERTUS, HANSEN, OCTAVE, WILLEM, MARIE
Applicants: BOYENS JOSEPH PAULUS HUBERTUS, DSM NV, HANSEN OCTAVE WILLEM MARIE, BOYENS, JOSEPH, PAULUS, HUBERTUS, DSM N.V., HANSEN, OCTAVE, WILLEM, MARIE
Application year: 2001
Main classification: B29B
IPC codes: B29B9, B29K21, B29K105