Extrusion of plant materials encapsulated in a thermoplastic

Plastic and nonmetallic article shaping or treating: processes – With severing – removing material from preform mechanically,... – Forming continuous work followed by cutting

Reexamination Certificate

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C264S102000, C264S177200, C264S211230

Reexamination Certificate

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06479002

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and an apparatus for the continuous production of shaped articles or mouldings, which preponderantly comprise small-sized plant material encapsulated in a thermoplastic material.
2. Description of the Related Art
Such shaped articles from wood shavings encapsulated in a thermoplastic material have long been used in the most varied fields, because they offer numerous advantages when compared with wood. More particularly, these relate to the dimensional stability and weather resistance, together with the insensitivity to fungal and insect attacks, while at the same time providing strength and processing characteristics comparable to those of wood. This also makes it possible to appropriately recycle plant waste materials such as wood shavings and flour.
It is known to produce in a discontinuous manner, shaped articles preponderantly, i.e., more than 50%, comprising small-sized plant material. The masses to be moulded are filled into dies or moulds, in which the-desired shape is produced under pressure and temperature.
German reference No. DE-OS 22 45 871, discloses the mixing of wood shavings and a plastics material, followed by the introduction of the mixture into an extruder. In the extruder the mixture is very rapidly heated-to an extrusion temperature and dehumidified by venting. In this method, undesired water vapour can still be present in the extruded product and must be removed from the latter by a special pressing device following the extruder.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method and an apparatus for the production of shaped articles or mouldings, with which from moulding materials with a preponderant proportion of small-sized plant materials mouldings, can be continuously produced in a dimensionally accurate and very economic manner.
The invention provides a method for the continuous production of mouldings, which preponderantly comprise small-sized plant materials, which are encapsulated in a thermoplastic material, in which the small-sized plant material is introduced into an extruder, the plant material and the thermoplastic material are mixed and compacted under pressure, while setting a clearly defined temperature, which is above the melting point of the thermoplastic material, the pressure in at least one section of the same extruder is reduced to a value in which residual moisture of the plant material is transformed into water vapour, which is removed from the extruder and in a subsequent extruder section the heated, dehumidified material mixture of the plant material and the melted thermoplastic material is compacted and extruded to the desired moulding.
According to a fundamental concept of the invention, for a good extrudability of the material, a good bond between the thermoplastic material and the surface of the plant particles is essential. According to the invention, a bond is achieved in that through the process sequence a good heating of the plant material of up to 250° C. is obtained. A possible reason for the positive effect obtained is that the melted thermoplastic material can wet in an almost optimum manner the microscopically rough and fibrous surface of the process caused by, heated plant parts in conjunction with the intense mechanical working in the extruder. This leads to very high Van der waals bonding forces between the plant material and the thermoplastic material. This ensures the necessary cohesion when the moulding material passes out of the extruder and up to the time that the material has been completely solidified.
It is also necessary to remove to the greatest possible extent air inclusions and steam formed from residual moisture during processing in the extruder and this can be achieved by venting several times. It has proved advantageous in this connection to subject the small-sized plant material in the extruder to at least one processing cycle prior to the supply of the thermoplastic material and which covers the method steps compaction, as well as subsequent stress relief-and venting.
A particularly reliable removal of the residual moisture from the plant material is ensured if the small-sized plant material undergoes this processing cycle three times prior to the supply of the thermoplastic material.
As a result of the method according to the invention it is possible to produce mouldings with a proportion of above 50% small-sized plant material in a continuous, stable process. The plant material can be constituted by plant waste produced in large quantities such as wood flour, wood shavings, cereal chaff, straw bagasse (sugar cane straw), cellulose, cocoa, coconut or palm fibres, etc., which can be appropriately recycled.
The comminuted plant material usable for this method comprises particles in a size preferably between 0.5 and 10 mm, butt the use of smaller and larger particles is not excluded.
According to the invention, the plant material is introduced loose or as precompacted compacts and separately from the thermoplastic material into the extruder. The supply of these materials through separate supply or metering devices permits a high metering precision. When using a relatively moist plant material it is particularly important to extract moisture therefrom by compaction and heating, together with subsequent pressure reduction and without the thermoplastic material and only subsequently is the thermoplastic material supplied to the heated material. Good results can be obtained when using a plant material with a residual moisture content of less than 25% at normal or ambient temperature.
According to the invention, the plant material is used in the largely natural state in which it occurs. Such an untreated material or at the most precompacted by pressing or squeezing can have a relatively high moisture content between 10 and 20%. According to the invention, the moisture content of the plant material is reduced during preheating and/or in the extruder during compaction and stress relief.
The plant material temperature is below the carbonization temperature and above the melting point of the thermoplastic material, particularly between 120 and 200° C. For an adequate compaction of high-volume material, in the first extruder section through a corresponding construction of the extruder screw helixes, a pressure is set which is preferably between 5 and 50 bar. If the desired pressure is not obtained as a result of the strong compaction of the extrusion material, then on the extruder casing and in the extruder screw can additionally be provided heating elements for temperature control purposes. On reaching the desired temperature and pressure, values in a subsequent extruder section the pressure is suddenly dropped significantly, preferably to atmospheric pressure or even an underpressure. As a result of this brief, drastic pressure reduction suddenly can consequently without significant temperature losses the residual moisture in the plant material is transformed into steam, which is removed from the extruder at numerous points. The extrusion material is subsequently compacted and completely extruded to the desired shape.
Besides the improved strength of the mouldings produced according to the invention, the method of the invention is also particularly economically advantageous. Thus, the method can be carried out in only one extrusion machine. Moreover, the energy requirements are very low, because it is only necessary to melt the thermoplastic material once.
In order to produce particularly firm and strong mouldings, according to the invention the plant material in a proportion of 70 to 90% and the thermoplastic material in a proportion of 10 to 30% form the material supplied to the extruder. A relatively high proportion of plant material leads to an inexpensive end product, because for the inventive method it is possible to use conventional natural materials, which are, e.g., obtained as waste products in the wood-processing industry. The thermoplastic materials which can be used are in particul

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