Table of contents (click on the titles to go to the chapters directly)

Introduction

In general, 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 fiber surface which prevents satisfactory fibre/polymer intertwining.

When natural fibres are used it is important that during the extrusion process these fibres 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, while the length of the fibres should be as large as possible.

Prior art shows that a high aspect ratio of the fibres in the final product has not been achieved up to now. 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 results in fibre filled plastic composites that can not compete with glass fibre reinforced materials on performance.

Hence, a substantial need exists for a continuous process for the production of polymer/cellulosic fiber 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.

Our invention

We have invented a process that results in natural fibre reinforced plastic composites that can compete with glass fibre reinforced plastic composites on performance and price. The design of this process is such that during the continuous mixing the natural fibres are opened up to elementary fibres with a high aspect ratio, which are homogeneously distributed in the polymeric melt. This process can be carried out by all extruders with two separate feeding ports and a degassing port. The preferred extruder for performing the process of the present invention is a co-rotating twin-screw extruder.

The thermoplastic polymers that can be applied in the invention comprise "commodity" plastics like polypropylene (homopolymer and copolymer), low density polyethylene, high density polyethylene, polystyrene (homopolymer, copolymer and terpolymers) and ABS. In addition engineering plastics can be applied. Besides the virgin polymeric material, the recycled grades of the above-mentioned plastics are also applicable in the present invention.

As natural fibers, preferably annual plant fibres or bast fibres, like flax, hemp, jute and kenaf are used. According to a different embodiment paper fibers such as recycled fibres from newspaper are used. It is also possible to use a combination of different types of fibres such as paper fibres and bast fibres.

According to the present invention it is preferred to add a coupling agent to the polymeric material. With a coupling agent is meant a polymer which can be mixed with the polymer matrix and which can be chemically coupled onto the fibres.

Technology benefits

• The compounding process is unique as it results in an optimal mixing of thermoplast and cellulosic fibre without shortening the natural fibre too much. The resulting granules are homogeneous in composition, which makes the resulting PP-fibre composite strong and stiff.

• The process can be carried out with conventional extruder equipment, using the following set-up of screw elements:

• positive screw elements

• at least one mixing element

• at least one RSE

• This continuous process can be scaled-up to bulk production.

• For compounding industries, the technology has the advantage that it opens an opportunity to produce bulk granules on existing extruder equipment for new materials with competing cost price. The specs of the materials depend on the type of natural fiber and applied thermoplastics. For instance, PP filled with 50% flax fibres has a flexural modulus of ~6.000 MPa, a flexural strength of ~100 MPa and a Charpy impact of ~20 kJ/m².

• The resulting compounds can either compete on price with glass fibre reinforced PP, or compete with virgin PP on performance. Moreover, the compounds have the following advantages:

• The production of granules with natural fibres does not expose operators to potential safety or health problems as is the case with glass fibers.

• In inside automotive applications our composites have the advantage of not injuring passengers when an accident occurs because the natural fibres are soft and not harsh as glass fibres

The potential applications of the natural fibre reinforced plastic are countless. Some areas of interest are:

• Automotive industry: both for inner and outer parts

• Packaging industry: for instance for light weight pallets

• Consumer products: any product containing composites

• Building and construction industry: replacement of hardwood

The mechanical properties of the compounded material according to the present invention compared with the properties of the polymer matrix are shown below.

1 reflected prices are indicative (2005); they are subjected to purchase volumes

In a Round Robin test of natural fibre reinforced plastics, carried out by the Nova-Institut in Germany, it turned out that the composites that were produced by us via this invention, had the best scores. Out of 8 applicants, our samples had the best Tensile Strength, the best Tensile stiffness, the best Flexural Strength and the best Flexural Stiffness. Only for the elongation and Impact properties other samples were better. [Ref: Die Ergebnisse des Nuturfaser-Spritzguss-Rundtests 2002 im Detail; Tagungsunterlagen zur nova-Fachkonferenz Naturfaser-Spitzguss für Verbundwerkstoffe in der Automobilindustrie, 01. Oktober 2002, RWE-Konferenz-Zentrum, Hürth].

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This new extrusion process was partly funded by the Common Fund for Commodities of the United Nations. The compounding technology has been demonstrated on pilot scale (40 kg/hr). Commodity plastics such as PP and (HD)PE are successfully reinforced by several natural fibres types such as flax, hemp, jute, kenaf and sisal; in addition reject streams from the pulp & paper/fibre processing industries turned out excellent cheap raw material as fillers in plastics. Also other plastics like ABS, PS, and PBT can be reinforced by these natural fibers.

Pilot scale tests (processing for more than 8 hours continuously and at MT scale) have been carried out and show that the process is stable. In addition the composite shows reproducible properties since 7 years of testing. Granules have been utilized on product-specific (injection) molding machines for the French and German automotive industry to produce interiour door trims (850*600*3 mm, 140 pieces/hr). The products were subjected to automotive-specific mechanical ageing tests, and the industry concluded that the price/performance was such that it can compete with commercial references.