WO2002000408A2 - Method for the production of thermoplastic plastics with natural fibre components - Google Patents

Abstract

The invention relates to a method for the production of thermoplastic plastics with natural fibre components and improved mechanical properties and is particularly suitable for the production of granulates of said plastic material as semi-finished product for products manufactured using the injection moulding method. Natural fibres are thus used in the form of fibre cuttings, fibre pellets, fibre pre-pregs, fibre granules or small fabric sections. The fibres are introduced in a mixing chamber into a polymer or polymer mixture, in which the pitch of the kneading screw is 10 to 50 % lower than in kneading screws for the mixing of glass fibres. Said introduction is at a temperature 1 to 5 °C above the melting point of the modified polymer or polymer mixture, the above are melted and mixed. After a residence time of 2.0 - 3.0 minutes from the beginning of the mixing of the fibres, the fibre-polymer mixture is drawn off from the mixing device in the form of a, preferably several, strand(s). The above method permits the addition of between 4 and 30 % natural fibres to the modified polymer or polymer mixture based on the end-product.

Description

Process for the production of thermoplastics with parts of native fibers

description

The invention relates to a process for the production of thermoplastic materials with fractions of native fibers and improved mechanical properties and is particularly suitable for producing granules of plastics modified in this way as semi-finished products for products produced by the injection molding process.

While natural fibers are incorporated for flat molded articles made of plastics with proportions of native fibers such as interior trim parts for motor vehicles, in that, as described for example in DE OS 19836313.3, natural fiber composite mats which have at least one natural fiber layer and at least one thermoplastic plastic layer by heating and pressing in a press tool to form a molded part are formed, it is to be processed predominantly in injection molding processes in the case of more complicated shaped parts of useful fiber-reinforced polymers, in particular fiber-reinforced, thermoplastically processable polymers.

Such a process generally requires semi-finished products which already have the finished mixture of fibers and polymer, these semi-finished products being generally used as granules, agglomerates or pellets for injection molding processes. For this purpose, the fibers have to be mixed with the polymer in a compounding process, fused and shaped into granules, agglomerates or pellets.

Extruders are often used to manufacture and process such fiber-reinforced semi-finished products made of polymer. Free-flowing fibers are metered into a melt of a thermoplastically processable polymer. B. in glass fibers gives a very homogeneous polymer-fiber mixture.

This compounding is for smooth fibrous materials in DE PS 43 14 852.2 using the example of a method for producing flame-retardant thermoplastic

Described plastics with improved mechanical properties and easy to control when using glass fibers or similar smooth fibers.

When using natural fibers, such a procedure is only possible to a very limited extent, since the natural fibers are only partially free-flowing due to their irregular shape and their rough surface. In order to achieve good flowability with natural fibers, these have to be shortened to a length that allows them to be dosed evenly. In this case, however, their length is no longer sufficient to achieve a satisfactory reinforcing effect on the composite material. Some suggestions have already been made to incorporate longer fiber sections.

For this purpose, PS 19822051.0 presents a device for the continuous dosing of light cut fibers with moderate to poor flowability for the production of thermoplastics or fiber-reinforced compounds by means of extrusion or compounding. With the device, cut fibers (carbon fibers, aramid fibers, LCP fibers, conventional synthetic fibers such as PA, PAN, PP PET and others, as well as cellulose regenerator and natural fibers) are drawn off with a largely constant amount per unit of time from the lower end of a storage container by a speed-controlled pin roller , The withdrawal device of the cut fibers is controlled by a constant weighing process of a differential balance, on which the metering device stands, so that the cut fiber removal is kept largely constant. The fibers drawn off by a pin roller should then fall through a chute into an extruder or compounding system. So that bridges are not immediately created again in this inevitably funnel-shaped shaft Preventing absorption by the extruder, as can easily be seen, the area of use is restricted to relatively strong and smooth fibers. The selection of natural fibers in particular is hereby very limited.

Rough or strongly wavy bevels such as hemp or cotton fibers can only be processed to a limited extent or in small quantities.

It would be conceivable to treat the fiber properties by a method as proposed in OS 19835983.7, and to coat the cellulosic natural fibers with another organic material before incorporating them into the thermoplastic, in order to bring strength and surface smoothness close to synthetic fibers in which one Coating is carried out with at least one resinous thermoset which hardens on the cellulosic natural fibers.

OS 19834132.6 proposes another solution that involves a relatively high level of mechanical engineering. This device for the production and preparation of composite materials, in particular of thermoplastic and natural fibers, has a feed device for strand-like material. This feed device opens axially into an annular space which is delimited on its circumference by an annular perforated die which is coated on the outside by at least one knife. At least one friction vane rotates in the annular space, the effective flank of which forms an all-round plasticizing chamber with the inner wall of the perforated die. The plasticizing chamber narrows against the direction of rotation of the friction wing and is closed off in the direction of rotation of the friction wing by a sealing segment. The ring-shaped die is designed to rotate around the friction vane and the speed and direction of rotation of the die and the friction vane can be set independently of one another. The fiber strand and plastic are mixed in the interior by the conveyors and pressed through the perforated die. The fibers are not shortened until the strands of granules are separated. In addition to the effort involved in producing an executable fiber strand, there is a great risk of winding and / or ball formation at higher fiber contents, which can then lead to malfunctions. The object of the invention is therefore to provide a process for the production of thermoplastics, in particular those based on modified polypropylene and modified polyamides made of D-aminocaprolactam or polyoxymethylene and / or acrylic butyldiene styrene (ABS), by which these plastics are mixed by admixture native Fibers, in particular of the bast fibers type, can be equipped with good mechanical properties with a simultaneous reduction in mass. In particular, the method should enable relatively long-fiber fibrils to be incorporated into the plastic in order to achieve sufficient strengths. The mechanical engineering effort should be comparable to the processing of glass fibers.

According to the invention, this object is achieved by a method according to the features of claim 1. The further claims 2 to 7 describe meaningful refinements of the method by specifying the materials and parameters that are preferably to be used in the method.

The process according to the invention for the production of thermoplastic materials with portions of native fibers and improved mechanical properties is based on the assumption that the free-flowing native fibers or fiber agglomerates are freed of any residual moisture by a pre-drying process.

According to the invention, in addition to cut vegetable fibers in free-flowing fiber lengths that may be used, spent grain or bagasse fibers are also proposed according to the invention. In particular, the use of native fibers in the form of free-flowing fiber agglomerates has to be considered as a particularly effective step for the integration of larger textile components. In addition to the thread sections, fiber pellets, fiber prepegs, fiber granules or small fabric sections made of native fibers, which are claimed as free-flowing fiber agglomerates, all fiber connections that are not listed can be used, in which the fibers in free-flowing bodies are predominantly mechanically connected in such a way that the dressing has a small surface that is as smooth as possible. Another prerequisite is that the dressing can be completely dissolved by the mechanical stress that occurs when mixing and homogenizing.

After drying, the fibers and free-flowing fiber agglomerates are fed into a known tamping machine via gravimetric metering

Mixing chamber supplied and melted and mixed there with a polymer or polymer mixture optionally modified by adding additives with the addition of an adhesion promoter in an extruder at a temperature which is 1 to 5 ° C. above the melting point of the modified polymer or polymer mixture.

As a rule, the fibers are fed through the tamping unit to the kneading zone of the extruder and mixed with the modified polymer or polymer mixture to form structures which are interlaced in a mesh-like manner. The fact that the pitch of the kneading screw is 10 to 50% less than that of kneading screws for the addition of glass fibers ensures that the time in which the fibers are exposed to the highest thermal load can be significantly reduced.

By homogenizing the fiber-polymer mixture in the heating stages following the kneading zone at a steadily falling temperature, the viscosity of the polymer portion and thus the mechanical effect for the dissolution of the fiber agglomerates is improved.

The fiber-polymer mixture then leaves the mixing device after a dwell time of 2.0-3.0 minutes since the beginning of the mixing of the fibers into one or preferably several strands. This significantly reduces the throughput time and thus the thermal relief of the chamfers if a nozzle plate is used which has nozzle openings which are at least 10% larger than those used in the production of conventional polymer granules. The strands thus spun can be granulated with known devices. With the proposed method it is possible to add between 4 and 30% native fibers calculated on the end product to the modified polymer or polymer mixture.

Although other tamping mechanisms are also conceivable for the method and are to be regarded as being within the scope of the invention, the invention can be implemented in particular by using a known screw conveyor with tamping rails arranged around the screw conveyor and running predominantly parallel to the axis of the screw conveyor as the tamping mechanism.

Depending on the intended use of the product, the modified polymer or polymer mixture may contain one or more of the substances polyamide, polypropylene, polyoxymethylene and / or acrylic butyldiene styrene (ABS) as the basic substance or mixture component.

When using the method, it has also proven to be useful if maleic anhydride is used as an adhesion promoter. The invention is described below in the form of three exemplary embodiments.

example 1

A polypropylene-hemp fiber blend is made as follows to produce injection molded parts. Modified polypropylene with a melting temperature of 190 ° C is mixed with maleic anhydride in a ratio of 18: 1, fed via a first metering unit to the first heating zone of a twin-screw extruder and melted there at 193 ° C.

Pressed hemp fiber pellets are fed into the extruder via a second dosing unit, which have previously been dried to a residual moisture content of less than 0.3%. The fiber pellets are dissolved into fiber by the kneading screw and the formation of mesh-like structures mixed with the polypropylene. The pitch of the kneading screw is 30% less than that of kneading screws for adding glass fibers By homogenizing the fiber-polymer mixture in the heating stages following the kneading zone at a steadily falling temperature, the viscosity of the polymer portion and thus the mechanical effect for the dissolution of the fiber agglomerates is improved.

The fiber-polymer mixture is passed over 8 heating zones while the temperature is reduced to 193 ° C. and then leaves the extruder after a dwell time of 2.0 minutes since the fibers were added. Through a nozzle plate with eight nozzles with a diameter of 5 mm, the mixture is formed into cylindrical strands, cooled and chopped into granules with a particle size of 6 * 6 mm. The technical parameters to be achieved in this way are comparable to those of a mixture using glass fibers both in the tensile modulus of elasticity and in the Charpy impact strength, and are summarized in the following table in addition to the parameters of the following exemplary embodiments.

Beispiel 2

Example 2

A polypropylene-hemp fiber blend is made as follows to produce injection molded parts.

Polypropylene with a melting temperature of 170 ° C is fed via a first metering unit to the first heating zone of a twin-screw extruder and melted there at 175 ° C. Coarse fibers from the hemp beer production, which have been wetted with an adhesion promoter, are fed into the extruder via a second dosing unit and have previously been dried to a residual moisture content of less than 0.3%. The fibers are fed into the 6th heating zone of the extruder through two laterally arranged feed screws and mixed with the polypropylene to form structures which are swirled like a net. The pitch of the kneading screw is 20% lower than that of kneading screws for adding glass fibers

By homogenizing the fiber-polymer mixture in the heating stages following the kneading zone at a steadily falling temperature, the viscosity of the polymer portion and thus the mechanical effect for dissolving the fiber agglomerates is improved.

The fiber-polymer mixture then leaves after a residence time of 2.4 minutes since the beginning of the admixing of the fibers. Through a nozzle plate with eight nozzles with a diameter of 4.5 mm, the mixture is formed into cylindrical strands, cooled and chopped into granules with a particle size of 6 * 6 mm. The technical parameters to be achieved are summarized in the table above. Example 3

For the production of injection molded parts, a polyamide-flax fiber mixture is produced as follows.

Polypropylene with a melting temperature of 215 ° C is mixed with maleic anhydride in a ratio of 16: 1, fed via a first metering unit to the first heating zone of a twin-screw extruder and melted there at 220 ° C.

Via a second dosing unit, chips from a chinenized linen fabric with an average edge length of 8 mm are fed to the extruder. The fibers are introduced into the 7th heating zone of the extruder and form structures with the polypropylene which are intermingled in a network mixed. The pitch of the kneading screw is 30% lower than that of kneading screws for the addition of glass fibers and processed as in Example 1. The fabric sections used are extremely free-flowing due to the surface treatment of the fabric. Particularly long fiber sections can be worked in with this.

Claims

Process for the production of thermoplastic materials with parts of native fibers 1. A process for the production of thermoplastics with portions of native fibers and improved mechanical properties, characterized in that - free-flowing native fibers or fiber agglomerates are freed of residual moisture by a pre-drying process, and fed to a tamping unit via gravimetric metering, that a polymer or polymer mixture modified by known processes by adding additives with the addition of an adhesion promoter in an extruder at a temperature which is 1 to 5 ° C. above the melting point of the modified polymer or polymer mixture lies, is melted and - the fibers are fed through the tamping unit to the kneading zone of the extruder and mixed with the modified polymer or polymer mixture to form mesh-like swirled structures, the pitch of the kneading screw being 10 to 50% less than that of kneading screws for the admixture of glass fibers is executed the fiber-polymer mixture is homogenized in the heating stages following the kneading zone at a steadily falling temperature, and after a dwell time of 2.0-3.0 minutes since the beginning of the addition of the fibers to one or preferably several strands, using a nozzle plate which has at least 10% larger nozzle openings than the ones used in the production of conventional polymer granules, - the spun strands are granulated with known devices. 2. The method according to claim 1, characterized in that one of the modified molten polymer or polymer mixture - Plant fibers cut as native fibers, bagasse or spent grain in free-flowing fiber lengths or in the form of free-flowing Fiber agglomerates are used. 3. The method according to claim 2, characterized in that. As a free-flowing fiber agglomerates - thread sections, - fiber pellets, - fiber prepegs - fiber granules or - Small sections of tissue made of native fibers are used. 4. The method according to any one of the preceding claims, characterized in that between 4 and 30% native fibers calculated on the end product are added to the modified polymer or polymer mixture. 5. The method according to any one of the preceding claims, characterized in that a known screw conveyor is used as the tamping device with guide rails arranged around the screw conveyor and running predominantly parallel to the axis of the screw conveyor. 6. The method according to any one of the preceding claims, characterized in that the modified polymer or polymer mixture one or more of the substances polyamide, polypropylene, polyoxymethylene and / or Contains acrylic-butyldiene-styrene (ABS) as the basic substance or mixture component. 7. The method according to any one of the preceding claims, characterized in that maleic anhydride is added as an adhesion promoter.