WO2002008316A1

WO2002008316A1 - Fibre-filled polymer composite

Info

Publication number: WO2002008316A1
Application number: PCT/GB2001/003316
Authority: WO
Inventors: Henry Robert George Aykroyd
Original assignee: Biofibres Limited
Priority date: 2000-07-22
Filing date: 2001-07-23
Publication date: 2002-01-31
Also published as: GB0017970D0; AU2001272663A1

Abstract

There is provided a process for producing a fibre-filled polymer composite, said process comprising providing a suspensive annually-grown fibres in a liquid, adding particulate polymer to suspension, and removing said liquid from the suspension to form a dried composite. The annually-grown fibres may be hemp, jute or flax. The method produces a fibre-filled polymer composite using natural fibres and wherein the long fibre length is maintained during processing thus enhancing the strength of the polymer composite material. Also provided is a method of producing a fibre-filled polymeric article, said method comprising moulding or thermoforming the composite, for example by injection moulding crumb or shredded pieces of the composite, or by vacuum forming sheet. Further provided is a fibre-filled polymer composite which comprises a mixture of annually-grown fibres and particulate polymer. The composite may comprise 1-60 % (w/w) and further may comprise 20-40 % (w/w) polymer to fibre ratio. The composite may be used in the manufacture of tiles, furniture, interior window frames, car dashboards, door panels, non-structural panels, computer housings, and the like.

Description

FIBRE-FILLED POLYMER COMPOSITE The present invention relates to composite materials, and in particular fibre-filled polymer composites.

The strength and stiffness of thermoplastic and thermosetting polymers can be enhanced by the incorporation of fibres which themselves have higher strength and stiffness than the base polymer. Examples of such reinforcing fibres include synthetic (e.g. glass, steel, aramid, etc.) and natural fibres (e.g. cellulose). The technology for mixing fibres and polymers has been developed over time to produce the least damage to the individual fibres in order to obtain the maximum strength. For a reasonable reinforcing effect the aspect ratio (ratio of the length of fibre to the diameter of the fibre) should be greater than 200:1 and ideally around 1000:1. It is important that this should be maintained throughout processing to final product. This is relatively easily achieved in the case of thermosetting resins where the fibres are typically layered in sheet or laminate form, and the resin a low viscosity liquid. In such cases there is no need for high temperature/high shear compounding. However, with thermoplastic polymers used for injection moulding the fibres have to be compounded with polymer in a primary compounding process at high temperature. This can result in the fibres being damaged, even with the use of modern technology and glass fibres. In addition to the above limitations of the current processes, natural fibres have additional problems which have hitherto limited their use. Wood fibres generally do not have sufficient length to meet the above criteria. However, certain annually grown crops such as hemp etc. are potentially useful. The machinery used to produce the composite material is usually designed for glass fibres and not natural fibres. Natural fibres can be bulky and difficult to feed through hoppers designed for glass fibres. Furthermore, if the fibres are compounded with polymer and pelleted into a form suitable for injection moulding the compounding process can reduce the fibre length through the shearing and mixing action. Moreover, the pelleting process damages the fibres and reduces their length to the point where the benefits of long fibres are lost.

W098/31626 describes wet-laid non-woven webs made from unpulped long natural fibre bundles. Mixing of synthetic fibres with the natural fibres is described, prior to conventional drying on a web-forming screen. This document specifically excludes treating the fibres with any postformation bonding agent, and does not include the incorporation of polymer matrix material (s) during web formation except where it is in the for of man-made fibres.

W096/05347 describes jute and kenaf fibre composite materials utilising compatiblisers/bonding agents namely maleated polypropylene. This document specifically mentions powdered polypropylene but only in the context of its reactive modification by maleic anhydride.

It would be environmentally beneficial to use natural fibres in place of artificial glass fibres currently used in thermosetting and thermoplastic polymers.

Therefore, an object of the present invention is to provide a process for producing a natural fibre-filled polymer composite which obviates or mitigates at least one of the aforementioned disadvantages associated with existing processes.

According to a first aspect of the present invention there is provided a process for producing a fibre-filled polymer composite, which comprises:

- providing a suspension of annually-grown fibres in a liquid;

- adding particulate polymer to the suspension; and

- removing liquid from the suspension to form a dried composite.

The term "annually-grown fibres" is understood to refer to fibres from plants which are grown and harvested within a single growing season. Examples of such plants are hemp, jute and flax. Wood fibres such as those used in paper-making are excluded.

Polymers suitable for use in the present invention include thermoplastic and thermosetting polymers, such as polyolefins, including polyethylene and polypropylene, poly(vinyl chloride) (PVC) , styrenic polymers including polystyrene, high impact polystyrene, styrene- acrylonitrile, and acrylonitrile-butadiene-styrene, polyamides, polyurethanes, polyesters, polyacrylates, polycarbonates and the like. Polymer may either be powdered, granular or otherwise in particulate form. Typically, the particle size is in the range 0.01 to 10mm. Preferably, the suspension comprises of 0.5 - 20% (w/w) dry solids content, more preferably 1.5 - 10% (w/w) dry solids content, and most preferably 2 - 5% (w/w) dry solids content. The liquid is usually water. Additives, such as stabilisers and colourings may be added to the suspension prior to removal of the liquid.

Preferably, the liquid is removed mechanically in a first stage to 20 - 80% (w/w) liquid content, preferably 20 - 60% (w/w) , more preferably 40 - 60% (w/w) liquid content and most preferably, 45 - 55% (w/w) liquid content. Liquid contents of 25 - 35% (w/w) may also be used. The liquid may be removed mechanically from the suspension by compression means or forming a partial vacuum on the opposite side of a porous substrate, the substrate being designed to retain the polymer fibre composite but allow liquid to pass through. Examples of suitable means include a hydraulic press, a belt press or paper-former known in the paper-making industries. Higher levels of compression can result in a lower liquid content after the first drying stage. Preferably, the liquid is further removed by heating in a second stage to obtain a composite of 0 - 20% (w/w) liquid content, more preferably 3 - 15% (w/w) liquid content and most preferably 5 - 9% (w/w) liquid content. Since the purpose of the second stage is to remove as much liquid as possible, achieving a value as close to 0% liquid content is preferable. Therefore, the range of liquid content in the composite may preferably be 0 - 15% (w/w) liquid content and more preferably 0 - 9% (w/w) liquid content. The liquid may be removed using hot air. The hot air is preferably of a temperature range of 100 - 200°C, more preferably 130 - 170°C and most preferably 140 - 160°C. The composite may be further heated using a radiation source, such as an infra red heater, to at least partially melt the polymer (in the case of a thermoplastic polymer) . It will be obvious to those skilled in the art that the exact temperature to which the composite is heated will depend on the melting temperature of the polymer used.

An additional coating of polymer may be applied to produce a desired surface finish, for example after partial melting of the thermoplastic polymer in the composite.

The fibre-filled polymer composite may be formed into laminated sheets or shredded into individual small pieces, the small pieces being generally 2 - 3 millimetres wide and 3 - 10 millimetres long.

The laminated sheets or shredded pieces may then be used for moulding either with or without the addition of further polymer. Thus, a second aspect of the invention provides a method of producing a fibre-filled polymeric article, which comprises moulding or thermofor ing the composite, for example by injection moulding the shredded pieces or by vacuum forming a sheet. However, in an alternative form of the invention, the suspension is formed directly into dried composite in the form of particles or "crumb" by employing a screw press (or other shear-compression device) to remove liquid from the suspension. A screw press is well known in the paper making field for removing water from slurry. It generally comprises an elongate screw rotatably mounted within a cylindrical vessel. Suspension is introduced into one end of the screw and is transported by rotation thereof to a second end provided with a flange, which is opened periodically to remove the dried crumb. The flights of the screw decrease in spacing towards the flange end so as to compress and shear the suspension. Liquid is forced out through holes in the cylindrical vessel walls. The screw press may, for example, be a Beloit press. Dewatering is typically achieved down to 30% (w/w) as before.

This results in the polymer-composite having a crumblike texture containing a range of particles sizes. To improve the flow characteristics of the crumb, the particles sizes are preferably mechanically reduced in size (eg_* by sieving) until they pass through, for example, a screen with 10 mm diameter or less holes. It may then be further dried according to any of the methods described above. The final product may be further blended with polymer and then compounded into conventional polymer pellets.

The dried crumb may be used directly for compounding for injection moulding purposes. The crumb typically comprises a mass of polymer particles having fibres intimately wrapped around each particle.

According to a third aspect of the present invention, there is provided a fibre-filled polymer composite which comprises a mixture of annually-grown fibres and particulate polymer. The composite may be in the form of a sheet, shredded particles or crumb.

The composite may comprise 1 - 60% (w/w) polymer to fibre preferably 10-50% (w/w), and most preferably 20 - 40% (w/w) . However, higher ratios of polymer to fibre may be achieved by, for example, a continuous process wherein the partially formed sheet may be transferred to a melting section, or where additional polymer is applied as a surface coating in a separate operation. In the situation where the sheet is pelletised, additional polymer can be added in the final compounding.

The composite may be used in the manufacture of tiles, furniture, interior window frames, car dashboards, door panels, non-structural panels, computer housings, and the like. It will be appreciated that the principal advantage of the present invention is that the process produces a fibre- filled polymer composite employing natural fibres wherein the fibre length is maintained during processing. The long fibre length, typically 4 - 6mm long, enhances the strength of the polymer composite material. Costs are reduced since the need for conventional melt compounding is eliminated. In some instances the particulate polymer may be sourced direct from the polymerisation reactor, again eliminating the extrusion compounding and allowing access to raw commodity prices. The fibre lengths are maintained when embedded in laminates and at the largest length consistent with the size of the diced pieces is substantially maintained in the final injection moulded product. The capital cost of the equipment is modest given the potential throughputs and in many cases may be carried out by a conventional paper making plant. The process uses natural fibres instead of environmentally damaging glass fibres and further environmental benefits include savings in electricity compared to other compounding processes.

These and other aspects of the present invention will become apparent from the following Examples.

Example 1

A fibre-filled composite sheet was produced by hand as follows.

15g of harvested hemp fibre and shive in sheet form was shredded by hand and then thoroughly mixed with 2 litres of water in a plastic container to form a uniform pulp with an electric hand mixer. 30g of granulated polypropylene (as used for rotational moulding and produced by grinding - supplied by

Webco) was added together with detergent (optional) . The mixture was thoroughly agitated until the fibre and polymer were in uniform suspension.

A hand forming wire mesh was placed into the container and lifted slowly to collect a layer of fibres and polymer thereon. Water was allowed to drain leaving a uniform A4 sheet. The sheet was removed from the forming wire and placed between two appropriate size felts. The fibre/polymer sheet sandwiched between the felts was placed in a laboratory hydraulic press and pressed at 15 tonnes.

At this point the sheet contained approximately 50% moisture and had sufficient strength to be handled. It was then placed in an oven at around 100°C to dry to about 7% water content.

Once the sheet was thoroughly dry, it was transferred to a hot press and pressed at 180°C and 15 tonnes force. Finally, the sheet was cooled to ambient temperature.

The sheet could be used for thermoforming or shredded into pieces for use in an injection moulding process. Example 2

Fibres suitable for use in the present invention are obtainable from a process which involves the biomechanical pulping of natural fibres to produce fibres of 5 - 15 millimetre in length. The process comprises maintaining fibre crops, such as hemp, flax and jute, under anaerobic conditions so as to allow ensiling and subjecting the ensiled material to mechanical impacting in a substantially dry environment; so as to bio-mechanically pulp the material.

Following the biomechanical process described above, the fibres produced are then subjected to a washing and refining stage in which cleaned fibres are separated and the shive length reduced. This process is continued until the fibre and shive meet the required specification. This is carried out using standard paper washing and refining equipment.

Once the desired specification has been achieved, the product is delivered to a mixing chest at approximately 3% dry matter and 97% water (w/w) . The polymer is added at this stage in powder or granule form and thoroughly mixed with the fibres and water using a pump or stirrer. Polymers in non-granulated form may also be used. Fillers and other additives such as stabilisers and/or colourants may be added at this stage.

The mixed product is pumped at approximately 3% dry matter (w/w) into a head box prior to sheet formation on a belt press or a paper former. The product is pressed to approximately 50% (w/w) dry matter on the belt press before passing onto a tunnel air dryer.

The tunnel air dryer dries the product to approximately 7% (w/w) liquid content. This is achieved by drawing hot air, which has been heated by gas to around 150°C, through a drying wire mesh. It has been observed that at temperatures greater than 220°C, the fibre may degrade. This results in a composite product wherein the fibres form a uniform co-mingled substrate around polymer granules. The resultant sheet is sufficiently bonded to enable it to be delivered to a melting tunnel.

An infra red melting tunnel melts the polymer (s) into the fibre substrate. An additional coating of polymer may be applied at this point to produce the desired surface finish. The product is then passed to a cooling section.

Once the composite product is sufficiently cooled, it is cut into sheets or diced into individual pieces approximately 2 - 3 millimetre wide and 3 - 10 millimetres long. The shredded pieces are similar in size and handling properties to conventional compounding pellets.

The resultant diced pieces are observed to be uniformly filled with natural fibres which are suitable for injection moulding either with or without the additional of further polymer.

Claims

CLAIMS 1. A process for producing a fibre-filled polymer composite, which comprises:

- providing a suspension of annually-grown fibres in a liquid;

- adding particulate polymer to the suspension; and

- removing liquid from the suspension to form a dried composite.

2. A process according to claim 1 wherein the annually-grown fibres are obtained from hemp, jute or flax.

3. A process according to claim 1 wherein said particulate polymer is a thermoplastic or a thermosetting polymer.

4. A process according to claim 3 wherein said thermoplastic and thermosetting polymers are selected from the group consisting of polyolefins, polyvinyl chloride, styrenic polymers, polyamides, polyurethanes, polyesters, polyacrylates and polycarbonates.

5. A process according to claim 1 or claim 3 wherein said particulate polymer is powdered or granular.

6. A process according to claim 5 wherein said powdered or granular polymer has a particle size in the range of 0.01 - 10mm.

7. A process according to claim 1 wherein said suspension comprises of 0.5 - 20% (w/w) dry solids content.

8. A process according to claim 7 wherein said suspension comprises 2 - 5% (w/w) dry solids content.

9. A process according to claim 1 wherein said liquid is water.

10. A process according to claim 1 wherein stabilisers or colourings are added to said suspension prior to removal of said liquid.

11. A process according to claim 1 wherein said liquid is removed mechanically in a first stage to 10 - 80%

(w/w) liquid content.

12. A process according to claim 11 wherein said liquid is removed mechanically to 25 - 35% (w/w) liquid content.

13. A process according to any one of claims 1, 11 or 12 wherein said liquid is removed mechanically from the suspension by a shear-compression means to produce dried composite.

14. A process according to claim 13 wherein said dried composite is in the form of crumbs.

15. A process according to claim 14 wherein said crumbs are mechanically reduced to a size of 10mm or less.

16. A process according to any one of claims 1, 11 or 12 wherein said liquid is removed mechanically from said suspension by forming a partial vacuum on the opposite side of a porous substrate which retains the polymer fibre composite which allows liquid to pass through.

17. A process according to any preceding claim wherein said liquid is further removed by heating in a second stage to obtain a composite of 0 - 20% (w/w) liquid content.

18. A process according to claim 17 wherein said liquid is further removed by heating to obtain a composite of 0 - 9% (w/w) liquid content.

19. A process according to claims 17 or 18 wherein said liquid is removed using hot air.

20. A process according to claim 19 wherein said hot air is of a temperature range of 100 - 200°C.

21. A process according to claim 20 wherein said hot air is of a temperature range of 140 - 160°C.

22. A process according to any one of claims 1 or 11 - 21 wherein said composite is further heated using a radiation source to partially melt the thermoplastic polymer.

23. A process according to claim 22 wherein said radiation source is an infra red heater.

24. A process according to any preceding claim wherein an additional coating of polymer is applied to produce a desired surface finish.

25. A process according to any preceding claim wherein said fibre-filled polymer composite is formed into laminated sheets or is shredded into individual small pieces.

26. A process according to claim 25 wherein said laminated sheets are shredded into individual small pieces which are 2 - 3 millimetres wide and 3 - 10 millimetres long.

27. A fibre-filled polymer composite which comprises a mixture of annually-grown fibres and particulate polymer.

28 . A composite according to claim 27 which comprises 1 - 60% (w/w) polymer to fibre ratio .

29. A composite according to claim 28 which comprises 20 - 40% (w/w) polymer to fibre ratio.

30. A composite according to any one of claims 27 - 29 wherein said composite is in the form of laminated sheets or sheet shredded into individual small pieces.

31. A fibre-filled polymer composite which comprises a mixture of annually-grown fibres and particulate polymer, the composite being the form of crumb.

32. A composite according to any one of claims 27 - 31 wherein said composite is used in the manufacture of tiles, furniture, interior window frames, car dashboards, door panels, non-structural panels, or computer housings.

33. A method of producing a fibre-filled polymeric article, said method comprising moulding or thermoforming the composite according to claim 30 or 31 by injection moulding said shredded pieces or crumb, or by vacuum forming said laminated sheet.