WO2001094094A1

WO2001094094A1 - Composite thermoplastic material, and method and installation for making same

Info

Publication number: WO2001094094A1
Application number: PCT/FR2001/001720
Authority: WO
Inventors: Christophe Dupic; David Weiller; Eric Prevost; Emmanuel Kelbert; Richard Piercy
Original assignee: Epiplast Sa
Priority date: 2000-06-07
Filing date: 2001-06-05
Publication date: 2001-12-13
Also published as: AU7418901A; FR2809985A1

Abstract

The invention concerns a composite thermoplastic material comprising a synthetic polymer matrix and a filler of particles of vegetable materials which constitute 10 to 50 wt. % of the material and consists essentially of surface particles of cereal and/or oleaginous origin with average size ranging between 1 and 3 mm. The material is obtained by mixing in a screw extruder (10) granules of compacted surface particles of cereal and/or oleaginous products, with the melted plastic material and in continuously extracting at an output end (20) the resulting mixture in the form of an extruded string (3). The granules are introduces in a point distant from the output end so as to ensure that the granules are sufficiently stirred in the extruder to disintegrate and that the particles are homogeneously mixed with the melted thermoplastic material.

Description

Thermoplastic composite material, and method and installation for manufacturing this material.

The present invention relates to a new thermoplastic composite material, as well as to the method and installation for manufacturing this material.

In the field of composite materials combining a synthetic plastic material with additional elements intended to improve certain properties of the plastic material, the idea has long been known to integrate vegetable fibers as reinforcement, in particular in a thermoplastic matrix such as polypropylene. .

This integration is conventionally done by introducing the fibers into an extrusion machine of known type, commonly with one or two extrusion screws, where the fibers are hot mixed with the molten plastic material serving as a matrix, to obtain at the outlet of the extruder is an extruded rod of composite material, which can then be cut to form granules which can be used in machines for manufacturing various parts of such plastics. From an economic point of view, the use of vegetable fibers instead of mineral fibers such as, for example, well-known glass fibers, makes it possible to reduce costs considerably, on the one hand because of the generally lower price of natural fibers and also because of the lightening of the composite structures produced, and on the other hand because of the reduction in recycling costs.

With regard to the general performance of composites reinforced with vegetable fibers, these fibers make it possible to reduce the density of the final product, the vegetable fibers being approximately half as dense as glass fibers for example.

Typically, in known composites combining a thermoplastic matrix and plant fibers, these fibers are so-called "long" fibers, that is to say generally of length greater than 10 mm. These fibers are conventionally obtained by a defibering operation carried out on the vegetable raw material, consisting for example, most commonly, of flax, sisal, hemp, kenaf, jute, but can also be generally obtained from the stems of plants. annuals or grains or their husk. The defibration operation aims to extract the vegetable fibers, that is to say the cellulosic part, from vegetable matter, by chemical type processes conventionally used in the paper industry, or by steam explosion, or by mechanical processes. such as carding.

The defibration operations of the vegetable raw material are therefore preparation operations which have a non-negligible cost, thereby entailing a significant cost of the fibers which can be used during their integration as reinforcement in the plastic material.

In addition, the fibers obtained are unitary and have a high expansion coefficient. This results in several drawbacks, in particular a large storage volume for temporary storage of the fibrous material between defibration and its integration into the plastic material, and transport problems due to the low densities of the fibrous material. In addition, this high expansion coefficient makes it difficult to handle the fibers to introduce them into the conventional production tools of the plastics industry, such as extrusion screws.

Another drawback results from the virtual necessity of stoving the fibers just before their introduction into the extruder. Indeed, the very nature of the fibers and their Originally, they inevitably originally contain a high proportion of water, which is unacceptable when hot mixing with the plastic in the extruder. Conventionally, the fibers are steamed during their manufacture, which reduces their original water content. But it is almost impossible to prevent a water uptake later, during storage, transport, or even simply the manipulations necessary to feed the fibers in one extruder. However, excessively high humidity of the fibers can cause problems in feeding the machine, by phenomena of adhesion of the fibers to each other, leading to jams in the supply and metering means at the input of the extruder, or at least to imprecise and irregular dosages.

Furthermore, the fibers are conventionally introduced into the extruder towards its exit, this being considered necessary to prevent them from being deteriorated by a long passage time at the high temperatures encountered in the extruders, combined with intense stirring caused by the extrusion screws over their entire length. This requirement leads to the need to facilitate the best possible mixing of the fibers with the molten plastic material, only in the exit zone of the extruder. The mixing of the plastic-fiber mixture being consequently limited, it follows from this the need to have a perfect regularity and homogeneity of the fiber supply.

All these imperatives make it essential to oven the fibers continuously just before their introduction into the extruder, which requires in the immediate vicinity of the extruders of the pre-treatment facilities for bulky fibers, and the costs of pre - important treatment. Document GB 2278365 illustrates such a known process, according to which straw is crushed or chopped to reduce the size of the fibers, then a first mixture is produced with granulated plastic, intended to form a sufficiently aerated premix to allow sufficient subsequent drying by dry air caused to pass through this premix. The premix is then immediately re-mixed in adequate proportion with granulated plastic material, then introduced into a conventional type screw extruder. It is easy to see the complexity of the installation required in the immediate vicinity of the extruder in order to dry the plant material and avoid its re-absorption of moisture before the final mixing is carried out in the extruder. In addition, if the presence of plastic granules in the premix is favorable to increase the speed of dehumidification, it can also be a cause of moisture uptake between the outlet of the dryer and the moment when the plant fibers are finally coated. plastic in the extruder.

Yet another drawback encountered in the manufacture of plastic composites with vegetable fibers is poor bonding of the fibers with the matrix, which moreover seems to be linked to the very nature of the fibers, to the problems of introduction into the 'extruder mentioned previously, and especially the presence of water on the surface of the fibers. In an attempt to solve this problem, it has been proposed, in particular in the aforementioned document GB 2278365, to use a matrix of cellulose-based polymer, the compatibility of which with vegetable material is better. However, this forces us to limit ourselves to the use of cellulose-based polymers which, moreover, are less frequently used, have a higher cost and have less mechanical properties.

The object of the present invention is to propose a new manufacturing process, and a new material. composite resulting therefrom, which makes it possible to solve the problems mentioned above and to eliminate the drawbacks of the prior techniques.

It aims in particular to provide a new composite material combining a plastic material, in particular a thermoplastic such as polyolefin polymers (polypropylene, polyethylene, ...), or styrenics (polystyrene, ABS and derivatives) or vinyl

(PVC, etc.) or polyesteramides and derivatives, and a material of plant origin acting as a filler or as a structural reinforcement, which is less expensive than the currently known composite materials, while offering a significant increase in the mechanical rigidity of the product produced in such a material compared to a product consisting of the single plastic material.

It also aims to provide such a lighter composite material than composites based on polypropylene and mineral fibers or other mineral fillers such as talc. It also aims to provide such a composite material, serving as an intermediate product for the manufacture of various objects, by injection or extrusion, these objects having a particular visual appearance, attractive and natural. It also aims to provide a composite material which, even after shaping in the form of the objects mentioned above, is easily recyclable and / or incinerable.

It also aims to propose a process for obtaining a composite which is easily industrializable, while minimizing the need for adaptation of preexisting production facilities or of the conventional type, in particular by using production tools of the type commonly used in conventional plastics, without the need for additions of devices specifically adapted for the use of raw materials of plant origin.

With these objectives in view, the subject of the invention is a thermoplastic composite material comprising a matrix of a synthetic polymer and a filler of particles of plant materials, characterized in that said filler constitutes from 1 to 50% by weight of the material and consists essentially of surface particles of cereal and / or oilseed origin, with average dimensions between 1 and 5 mm. When the filler constitutes from 1 to approximately 10% by weight of the material, an aesthetic effect is particularly obtained due to the visible presence of said plant particles in the plastic matrix. When the said load constitutes more than approximately 10%, it is essentially the mechanical aspects and the physical properties which are influenced.

By surface particles should be understood here particles generally having a relatively large surface with respect to their largest linear dimension, unlike fibers which, as everyone knows, are essentially relatively long in relation to their cross section. By way of comparison, the surface particles considered according to the invention have, for example, a form of flakes having faces whose average dimension is of the order of a few millimeters, 1 to 5 mm, preferably 1 to 2 mm, or a few mm ² of surface, for a thickness less than 1 mm, or even from one to a few tenths of a millimeter only, while fibers, in particular such as the long vegetable fibers used according to the prior art generally have a length of more than 10 mm , for a cross section of less than 1 mm ² . These dimensions are however given here for explanatory and comparative purposes only, and should in no way be considered as limiting their the present invention.

The material according to the invention has the advantage of using as a raw material, in addition to the thermoplastic material, particles of cereal material which can be easily obtained from straw or grains, or from oleaginous material, without the need for defibration, and therefore at a lower cost, comparatively of the order of 1000 French Francs per ton, and even less, for granules leaving a pellet press fed with chopped and ground straw, against 2500 French Francs per ton for vegetable fibers coming from pulping.

The particles of cereal origin used according to the invention come from the following cereal species: wheat, barley, oats, rye, triticale, sorghum, corn, the stem of which can be used to give the straw after harvesting the grain, but also the grain and its envelopes which in particular give milling sounds. It is also possible to use flax or hemp or by-products or co-products of the flax and hemp industry, apart from the fiber itself. It should be clearly understood that the expression used above of “particle of cereal and / or oil origin” encompasses in particular the various above materials, used alone but also any mixture of these in suitable proportions.

For example, the average composition of a cereal straw is:

- cellulose 35 to 50%

- hemicellulose 30 to 30% - lignin 15 to 20%

- mineral matter 4 to 7%

- proteins 4 to 5%

- fat 0.5 to 1%

- humidity 10 to 15%

For big sounds, the composition is - cellulose 9 to 12%

- hemicellulose 30 to 40%

- lignin 4 to 6%

- starch 10 to 15% - protein 12 to 16%

- ash 4 to 8%

- fat 2 to 3.5%

- humidity 10 to 15%

For fine sounds, the composition is:

- cellulose 6 to 10%

- hemicellulose 20 to 30%

- starch 15 to 25%

- protein 12 to 16% - ash 4 to 7%

- fats 2.5 to 4%

- humidity 10 to 15%

Typically, the material according to the invention comprises, by weight:

40 to 80% of thermoplastic material, chosen from polyolefin polymers (polypropylene, polyethylene, etc.), or styrene polymers (polystyrene, ABS and derivatives) or vinyl polymers (PVC, ..) or polyesteramides and derivatives;

- 10 to 50% of particles of vegetable matter,

- from 5 to 25% of the weight of plant material in coupling agent, such as preferably polypropylene (PP) grafted with maleic anhydride in the case of a composite with PP matrix, or any other thermoplastic grafted with maleic anhydride of the same base as matrix (for example maleized styrene), or any other product capable of interacting with the OH sites of cellulose, or of other coupling agents such as fatty acids of plant origin (saturated or unsaturated natural fatty acids) having the advantage of improving potential future degradability,

- additives chosen according to needs from: dyes, anti-ultraviolet, foaming agents, flame retardants, lubricant, etc. The coupling agent advantageously influences the mechanical properties and the humidity resistance of the composites according to the invention.

On the one hand, the thermoplastic matrices particularly targeted by the invention have a chemical origin different from that of plant materials, which are based on cellulose. This chemical difference results in different physical behaviors. Polyolefins, for example, are apolar and hydrophobic, while plant materials are polar and hydrophilic. It is in fact almost impossible to obtain optimal chemical compatibility between the cereal or oilseed materials and the various synthetic matrices targeted by the invention.

An important aspect of the present invention is to have succeeded in ensuring this compatibility thanks to the coupling agent, which makes it possible to use common polymer matrices, and especially capable of being worked at low temperature, which is a necessary condition so as not to degrade the particles of vegetable matter. The coupling agent makes it possible to create a chemical bridge between the constituents of the composite, to guarantee an efficient charge transfer between the matrix and the plant particles, and to obtain a composite exhibiting optimal mechanical properties. The coupling agents used generally consist of a reactive "head", belonging to the acid family, at the end of a long aliphatic chain, generally belonging to the chemical family of the matrix. The reactive end of the chains reacts chemically with the hydroxyl sites present on the surface of plant particles. So there is an anchor strong chemical between the coupling agent and said particles. The long "body" formed by said chain will in turn become entangled in the macromolecular network of the thermoplastic matrix. All coupling agents capable of ensuring such functions by the chemical process described above can be used. For example coupling agents based on isocyanate or organo-slianes.

Furthermore, the composites produced according to the present invention are characterized by a moisture content of less than 1% measured by the Karl Fisher method (standard ISO-760).

The coupling agent acts as a factor for reducing sensitivity to humidity. Indeed, the chemical reaction of the end of the coupling agent with the hydroxyl sites present on the surface of the plant particles brings a significant improvement in the resistance to humidity of the composites according to the invention. The OH sites intrinsically present on the surface of the particles are the main cause of their hydrophilicity, since water molecules can very easily bind to these hydroxyl groups. Thus, the blocking of these reactive sites by the coupling agent makes it possible to limit the presence of water molecules at the periphery of the particles, and thus makes it possible to significantly reduce the uptake of moisture from the composites. The coupling agent therefore prevents increased moisture uptake, swelling of the plant particles, as well as a premature drop in the mechanical properties of the composites according to the invention.

The material according to the invention is obtained from plant parts, in particular cereals, raw or very roughly ground, and all of the ground material can be used, from the largest to the finest particles. This has an important advantage compared to the use of fibers which need to be sorted and therefore lead to more waste of raw material.

In addition, the appearance of the products and objects obtained by injection or extrusion from the material according to the invention reveals the surface particles of plant material, which give an attractive and natural appearance to the said products.

Also, the composite material is easily recyclable and / or incinerable, greatly minimizing the amount of ultimate non-recoverable waste.

Compared to fillers of mineral fibers, such as glass fibers, or talc, the density of the material is about 20% lower. In addition, tests have shown that the product according to the invention has the advantage of being less abrasive for the sleeves, screws and tools, in comparison with the mineral fillers and reinforcements.

Furthermore, if the mechanical properties of the material according to the invention are less than those of glass fiber reinforced composites or certain other mineral fibers, they are clearly 1.5 to 2.5 times better than those of plastic. alone, thereby improving the rigidity in bending and in traction. The flexural and tensile strengths are also improved by the presence of vegetable matter, at levels however less than those stated in the case of rigidity. In all cases, the increase in proportion of cereal or oilseed products has the effect of increasing the properties of the composite, compared to using the material of the thermoplastic matrix alone, and this at lower cost.

The material according to the invention is preferably in the form of thermoplastic granules of dimensions between 2 and 4 mm, obtained by cutting a rod conventionally obtained by implementation of an extruder, but could also be supplied in the form of said rod or thermoformable plates.

The subject of the invention is also a method of manufacturing a thermoplastic composite material comprising a matrix of a thermoplastic material and a filler of particles of plant materials, according to which plant particles are mixed with plastic material in a screw extruder. melted and the mixture obtained is continuously extracted at one outlet end in the form of a rod, characterized in that the plant material is introduced into the extruder in the form of granules formed of surface particles of compacted cereal and / or oil products , at a point distant from said outlet end so as to ensure, in the extruder and before its outlet, mixing of said granules sufficient to deagglomerate them and a substantially homogeneous mixture of said particles with the molten thermoplastic material.

Preferably, the granules of cereal and / or oilseed products are obtained by granulation and compacting, in a pellet press, of particles of plant material of dimensions between 2 and 6 mm. This makes it possible to obtain granules of vegetable material at a lower cost, which can be used directly at the inlet of the extruder.

The particles of cereal or oilseed material are obtained either by chopping and then crushing straw, or from bran.

The surface particles of plant material are moreover easily obtained from the raw materials, stems or grains, by conventional methods of chopping, grinding, etc., and their agglomeration and compacting is also carried out by conventional methods and tools, such as pellet presses type known per se.

The use of granules of compacted cereal or oleaginous material makes it possible to avoid all the aforementioned problems encountered with the use of fibers. In particular, it makes it possible to simplify the storage and transport of the materials to be introduced into the extruder, by reducing the volume of the plant material relative to the volume occupied by conventional long vegetable fibers. The water uptake of the compacted granules, during their storage and handling, is relatively low compared to the fibers, which makes it possible to eliminate the need for steaming of the plant materials just before their introduction into the extruder.

Indeed, the granulation step according to the invention, although carried out substantially at room temperature, by means of a pellet press of known technology, does not give rise to phenomena of significant friction within the particles, thus resulting in localized elevations of the temperature. This rise in temperature makes it possible to extract a significant amount of moisture intrinsically present within the plant charges, while remaining clearly below the degradation temperature of said particles.

In addition, the densified form of the particles after compacting has a much lower surface area directly in contact with the ambient air than that of the particles or fibers used in the processes according to the prior art, which makes it possible comparatively to greatly reduce the recovery rate. possible humidity. The extraction of moisture during granulation and the limitation of moisture uptake mentioned above makes it possible to achieve, for the plant material, moisture levels lower than those required to allow easy mixing with the thermoplastic material, and avoids therefore the pre-baking step systematically necessary in previous processes. The introduction of the granules far enough from the exit of the extruder also makes it possible to ensure their deagglomeration during mixing resulting from the movement of the screws of the extruder. In addition, the amount of heat thus supplied facilitates the vaporization of the residual water contained in the granules, without the need to steam them beforehand.

Preferably, the granules of vegetable products are introduced into the extruder simultaneously with granules of plastic material and any additives, in the first part of the extrusion screws, that is to say approximately in the first third of their length, and without prior baking. According to additional provisions: the additives comprise at least one coupling agent serving to improve the adhesion of the particles of cereal and / or oil-bearing material with the plastic material. - During the mixing carried out in the extruder, degassing by suction is carried out, in the mixture contained in the extruder, of the vaporized moisture, initially present in the plant material.

- after leaving the extruder, the rod obtained is dried and cooled by an air knife.

The subject of the invention is also a manufacturing installation for implementing the method defined above, the installation being characterized in that it comprises:

a supply system comprising means for dosing granules of cereal and / or oilseed products, means for dosing granules of thermoplastic material, and optionally means for dosing additives,

- a screw extruder having an end zone inlet to which the supply system is connected, an outlet end opposite the inlet end and comprising an extrusion die, an area open to ambient air, located downstream of the end d inlet, and a vacuum degassing zone located between the zone open to ambient air and the outlet end,

- cooling, drying and cutting means, located downstream from the outlet of the extruder to solidify the extruded rod respectively, dry it and cut it into granules of composite material according to the invention.

Other characteristics and advantages will appear in the description which will be given of an embodiment of the process for obtaining the material according to the invention.

Reference will be made to the accompanying drawings in which: FIG. 1 is a schematic view of the extruder and its supply means, FIGS. 2 and 3 schematically illustrate two alternative embodiments of the means for cooling the extruded rod, and for granulation ,

- Figures 4 and 5 illustrate, respectively in section and in front view, a third variant of the cooling means of the extruded rod, and granulation.

In the example described here, the material according to the invention is obtained by mixing granules of surface particles of cereal material with the thermoplastic material. This mixing is carried out in a single-screw or twin-screw extruder 10, of a type known per se, in which the granules of cereal products, the plastic granules and the additives are introduced towards an inlet end 11, and a rush composite material comes out of it by rod dies located at the outlet end 20.

The granules of cereal products can be obtained in two ways, from straw, or from cereal grains.

In the method of obtaining from straw, the stalks of cereals are, at the time of harvest, bundled and pressed, commonly on the field of harvest. They can then be stored under cover in this form. The straw bales are then conveyed to a grinding and granulation unit, of a type known per se, in particular for the preparation of livestock feed. The straw is first chopped, in a chopping bowl, to reduce the straw strands to a length of the order of 20 to 40 mm. Then the strands are conveyed pneumatically, or mechanically by screws, bucket elevators, or the like, to a hammer mill which reduces the strands to particles of a few millimeters, for example from 2 to 6 mm or even less, from 1 to 3 mm for example.

The ground straw then obtained is conveyed to an annular pellet press, of well known type, to be transformed into compacted straw granules, granules with a diameter of the order of 3 to 8 mm. This operation makes it possible to densify the product with a view to its storage, transport and use in the extruder 10.

In the method of obtaining granules from grains, the cereal grains are first subjected to a milling process which performs the separation of the elementary constituents of the grains, namely the starchy parts, or flour, and the ligno- cellulosic, or bran, which are the seed coat or husk of the grain. The bran is then granulated by passing through an annular pellet press, to form granules of particles with dimensions of 3 to 10 mm. The flour resulting from the milling process can be partially or entirely introduced into the pellet press with bran, or be used for other applications, or even be introduced directly into the extruder as an additional filler in the composite material, this which can further lower the cost by total use of the raw material.

The granules of cereal products obtained as indicated above can then be stored before use in the extruder. Whatever the raw material used, straw or grain, a notable advantage of granulation is that it allows a substantial increase in the apparent density of the product, hence ease of transport and storage,

The extruder of FIG. 1 schematically represents an extruder of the conventional type in plastics processing, which can be an extruder with two counter-rotating or co-rotating screws, or an onov screw extruder.

The introduction of the granules can be done vertically or laterally, in the first third of the length of the screws, for example directly at an inlet end 11. The granules of cereal material are fed and metered by a first metering device 21, for example consisting of a volumetric predoser 21a and a belt weight metering device 21b. The thermoplastic material is introduced, also in the form of granules, by a second metering device 22, and the additives by one or more additional metering units 23.

It will be noted that, in the case of the manufacture of a composite comprising a mixture of plant materials of different natures, these will preferably be dosed and introduced substantially simultaneously into one extruder. It would also be possible to pre-mix granules of original materials different, or granules and a complement of non-granulated plant materials, or else to use granules formed of different plant materials compacted together. As can be seen in the drawing, the inclination of the thread of the screws of the extruder varies over their length, in a manner known per se, in particular to cause a greater mixing effect of the materials introduced.

In the first zones 11, 12 and 13, at the entrance to the extruder, the polymer is melted and the mixing of the various constituents introduced begins. The screws can be provided at this point with notches to promote the deagglomeration of the cereal granules.

A special zone 14, open to the air, is provided to allow a partial evacuation of the moisture contained in the granules of cereal products in the form of vapor resulting from the heating of all the materials under the effect of the screw mixing. In the following zones 15, 16, 17, the constituents are intimately mixed following the significant kneading caused by the screws.

In zone 17, a thorough degassing is carried out, under a vacuum of more than 0.5 bars, approximately 0.8 bars for example, to extract the remainder of water vapor. A special element, of the inverted mixer type, is located in zone 16, between the degassing zone and the venting zone 14, to generate a seal of molten material whose role is to seal zone 16 and thus avoid any risk of communication between the opening 14a of the zone 14 vented to the open air and the degassing zone.

The mixing continues in the zones 18, 19 at the end of the screw, before the extruded rod 3 is pushed through the die 20, giving the rod a diameter of 2 to 4 mm. The extrusion temperature is around 170 ° C at 200 ° C maximum, so as not to thermally degrade cereal products. If necessary, to further avoid this risk of degradation, it will be possible to add wax at the entry of the screw, in order to reduce the friction which generates internal heating in the extruder.

Note also that the degassing allowing the evacuation of most of the moisture from the cereal particles is facilitated at substantially two-thirds of the length of the screws, thanks to the heating produced by the kneading of the material. There is thus practically no need for an additional supply of energy to remove the water inevitably contained in the granules, which thus makes it possible to avoid the necessary baking before extrusion in the processes using fibers. At the outlet of the die, the rod 3 typically comprises less than 1% of humidity.

Another particular advantage of the granulation of cereal materials is that it facilitates the metering and its precision during the introduction of the granules into the extruder, and consequently it makes it possible to ensure better homogenization with the matrix of polymeric material. It will be noted in fact that, during the mixing caused by the screws, the granules of cereal products are disaggregated in the first part of the screw, thus allowing good homogenization of the mixture with the matrix of thermoplastic material during the kneading undergone subsequently. In addition, this mixing can also have an effect of decreasing the average size of the particles, without however decomposing them completely, so that the particles still appear visibly in the composite material and in the molded or injected objects made from said material. .

At the exit of the extruder, the rod 3 is cooled and then cut to form granules 31 of composite material able to be reused in various parts manufacturing processes, by injection molding, or profiles, by extrusion.

According to a first method, illustrated in FIG. 2, the rod is cooled immediately at the outlet of the extruder by a short-term soaking, for example less than 2 seconds, in a water tank 32, then subjected to drying by a jet. laminar air against the current 33a supplied by a pressurized air generator 33, the air flow completing the cooling and expelling the water from the surface of the rod. Such a method is suitable for an extrusion with a high production throughput, for example of 500 kg / hour, with a low or medium particle rate, typically less than or equal to 30% by weight.

The rod then passes through a granulator 34 which cuts the rod into granules 31, and which also serves to drive the rod by traction, this driving being made possible by the fact that the rod is sufficiently solidified by its passage through the tank. water.

According to a second method, represented in FIG. 3, the rod 3 at the outlet of the extruder is subjected only to cooling by a laminar air flow supplied by the pressurized air generator 33. A belt conveyor 35 is used to support and convey the rod from the die 20 until it is sufficiently solidified and resistant to be taken up by the granulator, and cut into granules as in the previous case. This second method, although requiring more time for cooling, and therefore a longer installation length, has the advantage of avoiding any absorption of moisture by the extruded rod. It is more particularly indicated for a low flow extrusion, of the order of 50 kg / hour for example, and whatever the level of particles. According to a third method, shown in Figures 4 and 5, the rod 3 is cut and cooled by spraying water directly at the outlet of the extruder. To this end, a box 36 for cutting and cooling is fitted to the die 20. This box comprises a rotor 37 provided with knives which cut the rod into granules 31, which granules are immediately cooled inside the box by means of l pressurized water from a sprayer 38, then dried. The latter method is suitable for high production rates, for example more than 500 kg / hour, and with a high level of cereal particles.

Cooling by nebulization of water, that is to say by fine droplets projected onto the surface of the extruded rod, could constitute a favorable compromise between the requirements of high production throughput and low moisture uptake during cooling.

By way of example, the table below shows some characteristics of the material according to the invention, compared with polypropylene alone and with composite materials loaded with minerals or vegetable fibers. Note the significant increase in the value of the tensile and flexural and threshold stress modules for the material according to the invention

(columns 2 to 4) compared to polypropylene alone

(column 1), and the significantly lower density compared to the mineral-charged composites (columns 5 and

6). The essential gain compared to composites based on vegetable fibers (columns 7 and 8) results from the lower cost price and the ease of implementation.

The implementation of the composite according to the invention can be carried out in a conventional manner on traditional injection presses. The vegetable origin of the reinforcing fillers of the material nevertheless requires respect certain rules to preserve the physical and mechanical integrity of the composite. Thus, the temperature during the injection will be kept below 200 ° C, preferably between 160 and 200 ° C, approximately to avoid carbonization of the cereals. Steaming before injection is required, for example 4 hours at 85 ° C or 2 hours at 110 ° C, to guarantee optimal physical and mechanical properties of the parts injected. Given the size of the cereal particles, the thickness of the pieces must be sufficient, for example greater than 1 mm.

Furthermore, in order to limit the thermal degradation of the cereal particles, it is also important to limit the shear of the material during injection, this shear being a source of localized temperature rises. Thus, the geometry of the mold supply system must be able to limit these shears within the molten material. The nozzles, channels and injection thresholds will preferably have a circular cross section and a diameter of approximately 1 mm, to avoid obstruction by cereal particles.

Claims

1. Thermoplastic composite material comprising a matrix of a synthetic polymer and a filler of particles of plant materials, characterized in that said filler constitutes from 10 to 50% by weight of the material and consists essentially of surface particles of cereal origin and / or oilseed medium size between

1 and 5 mm.

2. Composite material according to claim 1, characterized in that it comprises a coupling agent in a proportion of between 5 and 25% of the weight of the plant particles.

3. Material according to claim 2, characterized in that the coupling agent is a thermoplastic resin with the same base as the matrix, grafted with maleic anhydride.

4. Material according to claim 1 characterized in that it is in the form of thermoplastic granules of dimensions between 2 and 4 mm.

5. Material according to claim 1, characterized in that it has a moisture content of less than 1%.

6. A method of manufacturing a thermoplastic composite material comprising a matrix of a thermoplastic material and a filler of particles of plant materials, according to which vegetable particles are mixed in a screw extruder (10) with molten plastic material and continuously extracts at the outlet end the mixture obtained in the form of a rod (3), characterized in that the plant material is introduced into the extruder in the form of granules formed from surface particles of compacted cereal and / or oilseed products, at a point distant from said outlet end (20) so as to ensure in the extruder and before leaving it, mixing of said granules sufficient to desagglomerate them and a substantially homogeneous mixture of said particles with the molten thermoplastic material.

7. Method according to claim 6, characterized in that the granules are introduced into the extruder without prior baking.

8. Method according to claim 6, characterized in that the granules of cereal and / or oilseed products are obtained by granulation and compacting, in a granulating press, of particles of plant material of dimensions between 2 and 6 mm.

9. Method according to claim 8, characterized in that the particles of cereal and / or oleaginous material are obtained by chopping then grinding of straw.

10. Method according to claim 8, characterized in that the particles of cereal material are obtained from bran.

11. Method according to claim 6, characterized in that the granules of cereal and / or oil products are introduced into the extruder simultaneously with plastic granules and any additives.

12. Method according to claim 11, characterized in that the additives comprise at least one coupling agent serving to improve the adhesion of the particles of cereal and / or oilseed material with the plastic material.

13. The method of claim 6, characterized in that during the mixing carried out in the extruder, degassing is carried out by suction of the moisture present in the mixture contained in the extruder.

14. Method according to claim 6, characterized in that after leaving the extruder, the rod (3) obtained is dried and cooled by an air knife (33a).

15. Manufacturing installation for implementing the method according to one of claims 6 to 14, characterized in that it comprises:

a feed system comprising metering means (21) of granules of cereal and / or oilseed products, metering means (22) of granules of thermoplastic material, and optionally metering means (23) of additives, a screw extruder (20) having an inlet end region (11) to which the feed system is connected, an outlet end (20) opposite the inlet end and having a die extrusion, a zone (14) open to ambient air, located downstream of the inlet end, and a degassing zone (17) under vacuum situated between the zone open to ambient air and the end of outlet, means (32 to 38) for cooling, drying, and cutting, located downstream of the outlet of the extruder to respectively solidify the extruded rod, dry it and cut it into granules of composite material according to one of claims 1 to 5.