Plastic article or earthenware shaping or treating: apparatus – With separate and distinct upstream agitating or kneading means – Agitator or kneader comprises means rotating about a...
Reexamination Certificate
2001-02-17
2003-05-20
Cooley, Charles E. (Department: 1723)
Plastic article or earthenware shaping or treating: apparatus
With separate and distinct upstream agitating or kneading means
Agitator or kneader comprises means rotating about a...
C366S083000, C366S088000
Reexamination Certificate
active
06565348
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for continuously manufacturing composites of polymer and cellulosic fibres and to the compounded material obtained therewith. It also relates to an extruder to be used in this process.
BACKGROUND OF THE INVENTION
It is known to produce fibre reinforced plastics. For instance GB 1,151,964 describes a method for obtaining a plastics material reinforced with brittle fibres such as glass fibres. According to this method a brittle fibre substance is supplied as a continuous strand to the other components of the mixture in such a manner that the fibre breaks to a predetermined length. The apparatus used for this process comprises several mixing and kneading members which are not specified.
Lately, the focus on reinforcing fibres is shifting from glass fibres to certain kinds of cellulosic fibres which have outstanding intrinsic mechanical properties. These have the potential to compete with glass fibres as reinforcing agents in plastics. The specific strength of these agrofibres is 50 to 80 percent of glass fibres, whereas the specific modulus can exceed that of glass fibres. Supplementary benefits include low cost, low density, renewability and (bio)degradability. In addition, they are less abrasive during processing with thermoplastics and do not expose operators to potential safety or health problems.
A major disadvantage of cellulosic fibres is the limited temperature at which they should be processed without losing their additional mechanical properties. Further, it has been proven difficult to obtain a homogeneous mixture of polymer and fibre. This is mainly due to the non-polar polymeric surface versus the highly polar fibre surface which prevents satisfactory fibre/polymer intertwining.
EP 426,619 describes a method of producing panels from a thermoplastic polymer and a thermosensitive filler by means of an extruder having at least three feedingly effective helical extrusion sections and at least two non-feeding kneading sections. The extruder thus comprises at least two kneading zones and at least three extrusion zones. The filler is preferably fed into the second extrusion zone.
When cellulosic fibres are used it is important that during the extrusion process these fibers obtain and maintain a high aspect ratio so as to obtain a compounded material with mechanical properties comparable with those of materials containing glass fibres. This means that the diameter should be as small as possible, preferably so called elementary fibres are used. Further the length of the fibres should be as large as possible.
Obtaining such a high aspect ratio of the fibres in the final product has up to now not been achieved. The problem with extrusion processes according to the state of the art is that the high shear forces in the extruder often result not only in a smaller diameter of the fibres but also in a smaller length of the fibres. This reduces the strength properties of the compounded material considerably, relative to the glass fibre reinforced materials.
Accordingly, a substantial need exists for a continuous process for the production of polymer/cellulosic fibre composites with substantially improved mechanical properties, i.e. its rigidity and its strength. In addition, these resulting property improvements should be valid for multiple fibre sources at broad polymer processing temperature ranges and independent from the polymeric melt behaviour.
SUMMARY OF THE INVENTION
The above objects are achieved by the present invention. The present invention relates to a process for continuously manufacturing composites of polymer and cellulosic fibres, comprising the steps of:
a) feeding a polymer upstream into an extruder;
b) melting and mixing the polymer in a zone (A) of the extruder; wherein zone (A) comprises at least one positive transportation screw element,
c) feeding cellulosic fibres into the extruder in a zone (B) of the extruder, which zone (B) is located downstream of zone (A);
d) transporting the mixture of polymer and cellulosic fibres obtaining in zone (B) through a degassing zone (C), which zone (C) is located downstream of zone (B), wherein zone (C) comprises at least one positive transportation screw element and
e) transporting the mixture obtained in zone (C) through a pressure building zone (D) of the extruder, which zone (D) is located downstream of zone (C), wherein zone (D) comprises at least one positive transportation screw element,
f) releasing the mixture obtained in zone (D) into a die,
characterized in that zone (B) comprises at least one positive transportation screw element, at least one kneading section and at least one negative transportation screw element such that in zone (B) of the extruder the cellulosic fibres are fibrillated to obtain cellulosic fibres with an aspect ratio as high as possible, while simultaneously mixing the cellulosic fibres with the melted polymer.
The design of this process is such that during the continuous mixing the cellulosic fibres are opened up to elementary fibres (fibrillation) with a high aspect ratio, which are homogeneously distributed in the polymeric melt. The process results in a compounded material with improved rigidity and strength.
The present invention also relates to an extruder which can be used to carry out the process. The present invention comprises all extruders with two separate feeding ports and a degassing port. The preferred extruder for performing the process of the present invention is a corotating twin-screw extruder. An example of such an extruder is a Berstorff ZE corotating twin-screw extruder with a length to diameter ratio varying from 35 to 40.
As indicated above, the extruder comprises four zones: a zone (A) where a polymer fed to the extruder is melted and mixed; a zone (B) where cellulosic fibres are fed to the extruder, fibrillated to elementary fibres and simultaneously mixed with the polymer; a zone (C) wherein the mixture of polymer and cellulosic fibres obtained in zone (B) is degassed and a zone (D) where pressure is built up.
According to the invention zone (A), which is the polymer melting and mixing zone, comprises at least one positive transportation screw element. Preferably zone (A) further comprises at least one kneading section and at least one negative transportation screw element. Preferably, zone (A) begins at a distance of 20×D calculated from the beginning of the die, wherein D stands for the diameter of the extrusion screw. Generally zone (A) ends at a distance of 38×D.
In this application the location of the zones is calculated from the beginning of the die, i.e. from the end of the extruder. This is contrary to general practice in which these distances are calculated from the beginning of the extruder. This is done because according to the invention it is important that the feeding port for the fibres is located as close as possible to the end of the extruder.
Zone (A) can further be divided into four temperature zones. Zone (A
1
) defines the feeding of the polymer. In this zone, which is generally located between 34×D and 38×D, the polymeric material is fed to the extruder. A conventional feeder in combination with a hopper are generally used for this purpose. After being fed to the extruder, the material is transported to zone (A
2
).
In zone (A
2
), which is generally located between 30×D and 34×D, the polymeric material starts to melt, predominantly by shear forces. From (A
2
) the material is transported to (A
3
), which is located between 24×D and 30×D. From this zone the polymer is transported to zone (A
4
) located between 24×D and 20×D. Both zone (A
3
) and (A
4
) serve to further melt and mix the polymer.
Zone (B), which is the fibre fibrillation and mixing zone, comprises at least one positive transportation screw element, at least one kneading section, preferably at least two kneading sections, and at least one negative transportation screw element, Zone (B) is preferably located at a distance between 8×D and 20×D. The
Bos Harriette Louise
Snijder Martinus Hendricus Bernard
Van Kemenade Mathea Johanna Joseph Maria
Cooley Charles E.
Instituut Voor Agrotechnologisch Onderzoek (ATO-DLO)
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