IES84698Y1 - A process for preparing a wood-polyolefin composite - Google Patents

Description

A process for preparing a wood-polyolefin composite Introduction The present invention relates to a process for preparing a wood-polyolefin composite having reduced creep; the composite comprising a cellulose-based filler in the amount of between 20% and 80% by weight, a polyolefin in the amount of between 80% and % by weight wherein at least 25% of the polyolefin is recycled polyolefin and an amount of creep reducing agent. The invention also relates to a process for preparing a wood-polyolefin composite article and to a wood-polyolefin composite and wood- polyolefin composite article prepared by these processes.

Composite materials consisting of a mixture of cellulose-based filler and a polymeric material such as a polyolefin are well known. In the specification the term “cellulose- based filler" refers to all types of material containing cellulose and includes but is not limited to wood flour, fibre and particles and other natural materials. A polyolefin is a polymer of the alkene family of hydrocarbons. In the specification the term “by weight” refers to by weight of the wood-polyolefin composite unless where otherwise specified.

Wood-polyolefin composites generally comprise at least a portion of recycled polyolefin and are therefore a cost effective way of utilising recycled polymers. Wood- polyolefin composites are beneficial in that they can be easily substituted for a number of applications requiring wood. Furthermore, in some aspects they are more advantageous than wood in that they have both a longer life and require lower maintenance than wood and do not splinter or crack.

One of the main problems with wood-polyolefin composites however is that they experience problems caused by creep of the polyolefin component. “Creep“ is defined as the time dependent deformation of a material in response to an applied stress and thus can be caused by a number of different factors. Creep in the wood- polyolefin composites usually occurs when unsuitable polyolefins such as lower molecular weight and lower density polyolefins or recycled polyolefins are used.

Recycled polyolefins in general have reduced molecular weight which is caused by UV light and oxygen present breaking the carbon bonds within the polymeric chains during the initial use of the polyolefin and by heat processing during recycling. One of the main problems with using recycled polyolefins is that due to their lower molecular weight and a higher melt index, extrusion downstream is difficult to control. A further problem with using recycled polyolefins is that it can be difficult to obtain the exact type of polyolefins required and more often than not only recycled polyolefins such as lower molecular weight and low density recycled polyolefins which are not particularly suitable for wood-polyolefin composites are available. One particular type of unsuitable polyolefin is linear low density polyethylene made initially for blown film use. As this type of polyolefin is both low molecular weight and low density, it has a very low creep resistance and therefore is unsuitable for wood-polyolefin composites.

An obvious solution to this problem would be to use virgin polyolefins only and in particular virgin polyolefins having a high molecular weight. The disadvantage of this solution however would be that these polyolefins are costly and difficult to process and therefore their use in wood-polyolefin composites would not be cost effective.

Another solution to this problem is to add agents during the processing of the wood- polyolefin composites to reduce creep. US Patent Publication No. US2004/0204519 discloses a wood-filled thermoplastic composite for use in the decking industry. A number of additives are added during processing to reduce creep. These additives include coupling agents such as chlorinated paraffin waxes and lubricants such as ethylene bis-stearamide, stearate esters or fatty acid esters to increase the bond strength and improve the processing of the composites. One of the drawbacks of this thermoplastic composite and its process for preparation is that due to the amount of additives required and the cost of these additives, the overall cost of preparing the thermoplastic composite can exceed the value of the final product. Thus in order to reduce cost, cheaper processing aids which are not as effective sometimes have to be used.

A further disadvantage of these thermoplastic composites is that they do not take into account the variability of the starting materials such as the polyolefins. This can lead to excessive amounts of processing aids being used when not required, or insufficient amounts being used when needed.

Thus there is a need for a process for preparing a wood-polyolefin composite comprising at least a portion of recycled polyolefin and which has a higher resistance to creep, but which can be prepared at minimum cost.

Statements of Invention According to the invention, there is provided a process for preparing a wood—polyo|efin composite having reduced creep; the composite comprising a cellulose-based filler in the amount of between 20% and 80% by weight, a polyolefin in the amount of between 80% and 20% by weight wherein at least 25% of the polyolefin is recycled polyolefin and an amount of creep reducing agent; characterised in that the process comprises: preparing a sample wood-polyolefin composite by compounding an amount of polyolefin and cellulose-based filler in a ratio corresponding to the amount of polyolefin and cellulose—based filler in the wood-polyolefin composite and at a temperature above the melting temperature of the polyolefin to form the sample wood-polyoiefin composite; analysing the sample wood-polyolefin composite; detennining the amount and type of creep reducing agent required to reduce creep in the wood-polyolefin composite; and compounding the cellulose—based filler and polyolefin with the determined amount of creep reducing agent in an extruder at a temperature above the melting point of the polyolefin to form the wood-polyolefin composite.

The advantage of preparing a sample wood-polyolefin composite and analysing the sample composite in order to determine the amount and type of creep reducing agent required is that the most effective amount and type of creep reducing agent can be chosen.

Each of the different types of creep reducing agents has different levels of effectiveness. Thus different amounts of each of the agents can produce the same effect. Furthermore the difference in cost between each of the creep reducing agents is substantial. By determining the level of creep that would be present in the wood- polyolefin composite, based on the level of creep in the sample wood-polyolefin composite it is now possible to determine what the most effective creep reducing agent would be at the lowest cost. As the correct amount and type of creep reducing agent or agents can be more accurately determined, a sufficient but not excessive amount of creep reducing agent can be added to prepare the final wood-polyolefin composite, and thus the overall cost will be reduced.

Preferably, analysing the sample wood-polyolefin composite comprises: measuring one or more of flexural modulus, flexural strength and creep of the sample wood-polyolefin composite to yield one or more of a flexural modulus value, a flexural strength value and a creep value.

Further preferably, determining the amount and type of creep reducing agent required comprises the steps of: comparing the or each measured flexural modulus value, flexural strength value and creep value with a corresponding acceptable value; and calculating the amount and type of creep reducing agent required based on the or each value.

The advantage of measuring either or both of the flexural modulus and flex strength is that they can be measured directly after preparation of the sample and thus the amount of creep reducing agent required can be determined in less time. The advantage of measuring the flexural modulus is that it is easier to measure than flexural strength when the sample wood-polyolefin composites are in thin sections.

The advantage of measuring the creep directly is that the exact creep performance can be determined and thus this is the most accurate analysis. The advantage of measuring more than one of the flexural modulus, flexural strength and creep is that more exact results can be obtained.

In one embodiment of the invention, the creep reducing agent is a silane grafted polyolefin. in another embodiment of the invention, the creep reducing agent is a mixture of silane and peroxide which grafts the polyolefin to form a silane grafted polyolefin during compounding.

In a further embodiment of the invention, the creep reducing agent is a high molecular weight polyolefin selected from the group consisting of one or more of ultra high molecular weight polyethylene (UHMPE) and high molecular weight polyethylene (HMWPE). In a still further embodiment of the invention, the creep reducing agent is a long chain branched polyolefin. The advantage of using either high molecular weight polyolefin or long chain branched polyolefin is that a substantial portion of this can be obtained from recycled sources and thus can be obtained at a low cost.

In another embodiment of the invention, the creep reducing agent is a maleic anhydride grafted polyolefin. The advantage of using this type of polyolefin is that it has been found to be the most effective type of creep reducing agent and thus lower amounts are required to achieve the same effect as other creep reducing agents.

In a further embodiment of the invention, the creep reducing agent is a natural or synthetic silicate modified nanoclay selected from the group consisting of one or more of montmorillonite, saponite, beidellite, nontronite, hectorite, bentonite and synthetic fluoromica or any analogue thereof. in one embodiment prior to adding the nanoclay to the polyolefin, preparing a masterbatch by adding between 10% and 50% of the modified nanoclay by weight to between 50% and 90% of a masterbatch polyolefin by weight of the masterbatch; and adding the masterbatch in the amount of between 5% and 40% by weight to the polyolefin and the cellulose-based filler to provide the wood-polyolefin composite.

Preferably, the masterbatoh polyolefin is grafted with at least one monomer which is capable of reacting with the polyolefin in a molten condition. In one embodiment of this invention, the masterbatoh polyolefin is grafted prior to adding the nanoclay to the masterbatoh polyolefin. In another embodiment of this invention, the nanoclay is added to the masterbatoh poiyolefin during grafting of the masterbatoh poiyolefin.

Ideally, the masterbatoh poiyolefin is a recycled poiyolefin.

Preferably, the cellulose-based filler is selected from the group comprising one or more of hardwoods, softwoods, plywood, peanut hull, bamboo, straw, ground up chip board, ground up medium density fibre board, and cardboard. l n one embodiment of the invention, the polyolefin is homopolymeric poiyolefin selected from the group consisting of one or more of polyethylene and polypropylene.

Preferably, the polyethylene is selected from the group consisting of one or more of High Density Polyethylene (HDPE), Medium Density Polyethylene (MDPE), Low Density Polyethylene (LDPE), Linear Low Density Polyethylene (LLDPE), Low Density Low Molecular Weight Polyethylene (LDLMWPE).

In another embodiment of the invention, the polyolefin is copolymeric polyolefin selected from the group consisting of one or more of recycled butene-1, recycled hexene-1, and recycled 4 methylpentene-1 (4MP-1).

Preferably, the molecular weight of the polyolefin is in the region of between 50,000 and 350.000 daltons.

The invention also relates to a wood-polyolefin composite prepared by the process of the invention.

According to the invention, there is also provided a process for preparing a wood~ polyolefin composite article, comprising extruding the wood-poiyolefin composite of the invention at a temperature in the region of between 150°C and 240°C to form the composite article. According to the invention, there is still further provided a wood-polyolefin composite article prepared by the process of the invention Detailed Description of the Invention The invention will now be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the figures, wherein: Figure 1 outlines in flow diagram form the process according to the invention; Figure 2 depicts the displacement of HMWtPE. HMWtPE. LLDPE and MMWtPE as described in the Example; Figure 3 depicts the displacement of LLDPE, LLDPE + HMWtPE, LLDPE + clay and LLDPE + MAgPE as described in the Example; and Figure 4 depicts the displacement of MMWtPE, MMWtPE + HMWtPE, MMWtPE + MAgPE, and MMWtPE + silane as described in the Example.

All of the equipment used is well known equipment and accordingly does not require any further description.

Referring to Fig. 1 in step 1 a sample of polyolefin comprising a portion of recycled polyolefin is obtained. In step 2, a sample of cellulose-based filler is obtained and is compounded with the sample polyolefin in step 3 at a temperature above the melting temperature of the polyolefin to form a sample wood-polyolefin composite in step 4.

The sample wood-polyolefin composite is analysed in step 5 and the amount and type of creep reducing agent required to reduce creep in the wood-polyolefin composite is determined in step 6. in step 7, the determined amount of creep reducing agent is added to a further portion of polyolefin in step 8 and cellulose- based filler in step 9 in an extruder where they are compounded in step 10 at a temperature above the melting point of the polyolefin to form the wood-polyolefin composite in step 11.

Preparation and analysis of sample wood-polyolefin composite The sample wood-polyolefin composite is made up by compounding an amount of polyolefin and cellulose-based filler in the same ratio as the amounts used to make up the wood-polyolefin composite. The type of polyolefin used in the sample wood- polyolefin composite will be the same as that which will be used to make up the wood-polyolefin composite. As the sample will preferably be small, the compounding of the test samples can be carried out using a lab Brabender mixer or other suitable mixing equipment such as a small extruder. It will be appreciated however that if larger amounts are used compounding could also be carried out in a manufacturing extruder.

Analysis of the sample wood-polyolefin composite may be carried out by measuring one or more of the flexural modulus, flexural strength and creep of the sample wood- polyolefin composite. The flexural modulus and flexural strength are measured according to the protocols laid out in the American Society for Testing Materials (ASTM) tests under lSO1'/'8.

The creep is measured by hanging a weight from the sample wood-polyolefin composite for a period of time and measuring the displacement of the composite over time, as outlined in the accompanying example.

An acceptable flexural modulus value would be any value greater than 2.0GPa and any value greater then 22MPa would be considered acceptable for flexural strength.

An acceptable value for creep would be a displacement of less than 9mm in 1000 hours.

It has been found that if the measured flexural modulus of the sample wood- polyolefin composite is between 1.8GPa and 2.0GPa or the flexural strength is between 18MPa and 22MPa that this will result in a slightly higher level of creep in the final wood-polyolefin composite that would normally be acceptable for wood replacement uses. All of the following creep reducing agents have been found to be suitable to reduce creep in the final wood-polyolefin composite, if it is present at this level in the sample composite.

All of the percentages expressed below are by weight of the polyolefin used to make the wood-polyolefin composite.

Silane grafted polyolefin: 2% - 50% High molecular weight polyolefin: 20% - 50% Long chain branched polyolefin: 0.5% - 5% Maleic anhydride grafted polyolefin: 1% - 2% Modified nanoclay: 1% - 5% Each of these creep reducing agents can work independently or in combination. It will be appreciated that if any of these agents are combined that amounts at the lower end of the ranges given can be used.

It has been found that the best combination when the creep is at this level in the sample composite is a combination of the high molecular weight polyolefin and the maleic anhydride grafted polyolefin. Modified nanoclay could also be added to each of these polyolefins either alone or in combination to further improve creep in the final wood-polyolefin composite.

Another successful combination which has been found is the combination of high molecular weight polyolefin, maleic anhydride grafted polyolefin and long chain branched polyolefin.

It has been found that if the measured flexural modulus of the sample wood- polyolefin composite is between 1.6GPa and 1.8GPa or the flexural strength is between 14MPa and 18 MPa that this will result in a greater percentage of creep in the final wood-polyolefin composite. In this case the following creep reducing agents have been found to be most suitable when creep is present at this level in the sample composite: Silane grafted polyolefin: 2% - 50% High molecular weight polyolefin: 35% — 50% Long chain branched polyolefin: 0.5% - 5% Maleic anhydride grafted polyolefin: 1% — 5% Modified nanoclayr 1% — 5% in this case it has been found that all of the above creep reducing agents with the exception of maleic anhydride grafted polyolefin need to be combined with at least one other creep reducing agent in order to reduce creep to an acceptable level.

Maleic anhydride grafted polyolefin is an extremely effective creep reducing agent and if added on its own during processing of the final wood-polyolefin composite at the higher end of the range specified can reduce creep sufficiently.

In addition to this the following combinations of the other creep reducing agents can successfully reduce creep when the flexural modulus and or flexural strength values are within these ranges: % - 5% silane grafted polyolefin, 35% - 50% high molecular weight polyolefin and/or 0.5% - 2% long chain branched polyolefin % - 50% high molecular weight polyolefin, 1% — 2% maleic anhydride grafted polyethylene and/or 0.5% - 5% long chain branched polyolefin % - 5% modified nanoclay, 35% - 50% high molecular weight polyolefin and/or 0.5% - 5% long chain branched polyolefin It has been found that if the measured flexural modulus of the sample wood- polyolefin composite is below 1.6GPa and the flexural strength is less than 14MPa that this will result in an even greater percentage of creep in the final wood-polyolefin composite. In this case the following creep reducing agents in the following amounts have been found to be most suitable when creep is present at this level in the sample composite: Silane grafted polyolefin: 2% - 50% High molecular weight polyolefin: 20% - 50% Long chain branched polyolefin: 0.5% — 5% Maleic anhydride grafted polyolefin: 1% - 5% Modified nanoclay: 2% - 5% In the above case, it has been found that each of the creep reducing agents must be combined with at least one of the other creep reducing agents in order to reduce creep in the final wood-polyolefin composite. The most successful combination has been found to be the following: % - 55% high molecular weight polyolefin, 2.5% - 7.5% maleic anhydride grafted polyolefin and 0.5% - 1.5% long chain branched polyolefin.

The sample wood-polyolefin composite used for analysis can be of any size provided that sufficient test pieces can be cut out for the flexural or creep tests, depending on which test or tests are used.

Preparation of wood-polyolefin composite It will be appreciated that the addition of each of the polyolefin, cellulose-based filler and creep reducing agent to the extruder to form the wood-polyolefin composite can be in any order.

Optionally, additives such as adhesion promoters, biocides, antioxidants, pigments, processing aids, colorants, coupling agents, reinforcing agents, foaming agents and lubricants can also be added to extruder. These additives can either be added at the same time as the cellulose-based filler, polyolefin and creep reducing agent or some time afterwards during compounding.

The cellulose-based filler may be wood flour, fibres or particles obtained from a number of sources such as a lumber yard or furniture factory. Hardwoods, softwoods and plywoods can be used, however hardwood is preferable. It is also possible to use wood fibre particles from other sources such as peanut hull, bamboo and straw.

Other cellulose containing materials such as ground up chip board, ground up medium density fibre board, and cardboard can also be used. Prior to compounding with the polyolefin and creep reducing agent, the cellulose-based filler generally undergoes a size reduction step in a suitable means such as a hammer mill which results in the filler having an average particle size of less than 100 mesh. The targeted mesh size of the cellulose-based filler is dependent upon the end-use application.

As at least a portion of the polyolefins used is recycled, they more often than not comprise a mixture of polyolefins having different melting points. The polyolefin with the highest melting point should have a melting point below 170°C. The melting point of this polyolefin should be low enough so as not to require compounding or extrusion temperatures sufficiently high so as to cause degradation of the cellulose-based filler.

The creep reducing agents added can either be silane grafted polyolefins, high molecular weight polyolefins, long-chain branched polyolefins, maleic anhydride grafted polyolefins or nanoclay either added directly or in the form of a masterbatch.

In the specification a high molecular weight polyolefin refers to a polyolefin having a molecular weight of greater than 200,000 and a long chain branched polyolefin refers to a low density polyolefin having a molecular weight in the region of between 100,000 and 150,000 with long side chain branching, wherein each branch has greater than 10 carbons.

The presence of any of these creep reducing agents in the wood-polyolefin composite provides a higher resistance to creep in the final composite and extruded composite article. Other types of agents which have been shown to reduce creep such as chlorinated resins and chlorinated paraffin waxes could also be used.

The nanoclay used should also have been modified by cation exchange with an alkyl ammonium ion. The monomer used to graft the masterbatch polyolefin may be any one of ethylenically unsaturated carboxylic acids, ethylenically unsaturated carboxylic acids and anhydrides and mixtures thereof. The monomer used is generally maleic anhydride and is in the amount of less than 2% by weight of the masterbatch polyolefin.

Creep is defined by the following formula: A(t) oc O’/E0 t" where: _13_ A is the amount of deformation E0 is the short term modulus of the material n is the time-dependency of subsequent deformation described by a power law, Therefore the deformation of the material is inversely proportional to E0 and proportional to the time under stress, 0, to the power index n. In order to reduce creep therefore the short term modulus should either be increased and/or the time- dependency index n should be decreased.

One way of increasing the short term modulus (E0) of the composite is to incorporate reinforcements such as fibres or flakes made from stiff inorganic materials such as glass or clay into the composite. Thus the addition of nanoclays either directly or in the form of a masterbatch serves to increase the short term modulus (E0) and thus reduce creep in the composite. The resultant composites comprising nanoclay have been found to have a significant reduction in creep in particular in the short term.

The time dependency (n) has been found to be influenced by the polymer structure and in particular by the amorphous regions of a crystalline structure. Increasing the crystallinity has been found to reduce the proportion of amorphous network which leads to a reduction in the time dependency index (n). In order to increase the crystallinity, it is known that the molecular weight and/or chain branching in the polymer must be reduced. However, this reduction would lead to a composite susceptible to creep as the polymeric chains would slip readily from the crystals and through the amorphous regions. It has however been found that that by crosslinking the polymeric chains and thus connecting the branches to other molecules and forming a network structure that this will also lead to a reduction in the time dependency index n with the resultant composite having minimal or no creep.

Crosslinking can be achieved by adding a creep reducing agent in the form of polyolefins which promote crosslinking. The polyolefins can be in the form of long chain branched polyolefins which reduce creep due to their complex molecular architecture, producing a more tangled network, which reduces the molecular mobility of the polyolefins. The polyolefins can also be high molecular weight polyolefins wherein between 10% and 30% of its molecules are greater than 106 in molecular weight. UHMPEs generally have a molecular weight of greater than 500,000 daltons and HMWPEs generally have a molecular weight of between 200,000 and 500,000 daltons. different lengths wherein the length of their molecules result in a similar effect to the Both have a broad distribution of molecules at long-chain branched polyolefins.

The maleic anhydride grafted polyolefins reduce creep by the maleic anhydride end bonding to the cellulose-based filler and the polymeric chains of the polyolefin. This results in a bridge forming between the polyolefin and the cellulose-based filler.

These bridges assist creep resistance by increasing the short term modulus of the composite and by improving the time dependency by tying the polyolefin to the cellulose-based filler.

It will be appreciated that adding either a nanoclay or a polyolefin which promotes crosslinking or a maleic anhydride grafted polyolefin will lead to a reduction in creep. It will further be appreciated however, that by adding both a combination of two or more of a nanoclay, a polyolefin which promotes crosslinking, or a maleic anhydride grafted polyolefin that a significantly higher reduction in creep will be expected as the short term modulus (E0)will be increased and the time dependency index (n) will be decreased. This will also result in a prolonged reduction in creep over long periods.

The following example is given by way of illustration only and should not be construed as limiting the subject matter of the invention.

Example 1 Materials: Recycled polyethylene: The recycled polyethylene was from one of the following sources: A) High molecular weight mixed bottle grade polyethylene (HMWtPE) which had a melt index of circa. 8.0 HLMI (High Load Melt Index) (21.6kg) Flex modulus: 2.56 GPa, Flex strength: 26.9MPa B) Medium molecular weight polyethylene (MMWtPE) which was obtained from used green rotomoulded tanks.

Flex modulus: 1.35 GPa, Flex strength: 20.7MPa C) Linear low density polyethylene (LLDPE) which was obtained from used LLDPE film.

Flex modulus: 1.26 GPa, Flex strength: 13.8MPa Virgin polyethylene: The virgin polyethylene was from one of the following sources: D) Commercially available High Molecular Weight Polyethylene (HMWtPE), Lupolen 52612 High Density Polyethylene (HDPE) ex Bassel. This polyethylene was a high molecular weight blow moulding grade of polyethylene which had a melt index of circa 2.0 high load (21 .6kg) Flex modulus: 2.57 GPa, Flex strength: 35.5MPa E) Cellulose-based filler: The wood used was commercially available wood flour dried to less than 5%.

Creep reducing agents: F) Maleic anhydride grafted polyethylene: Fusbond MB10OD ex DuPont.

G) Nanoclay: The nanoclay used was clay masterbatch obtained from Crownstone Limited.

H) Silane: Silane grafted polyethylene ex Micropol The different formulations were made up using the above materials in the amounts shown in Table 1.

Table 1: Formulations: (% parts by weight) Run D A C B E G F H HMWtPE HMWtPE LLDPE MMWtPE Wood Clay MAgPE Silane D 50 50 A 50 50 C 50 50 B 50 50 C+D 40 30 30 C+G 44 44 12 C+F 47.5 47.5 5 (5%) C+F 49.5 49.5 1 (1%) B+D 20 30 50 B+F 45 50 5 (5%) B+F 49 50 1 (1%) B+H 30 50 20 Each of the formulations were added to a Brabender Plastograph EC at 180°C over a period of 2 minutes. Each mix was then mixed for a further 8 minutes at 60 rpm. The resultant mixes were then compression moulded to produce flat sheets circa 3.5mm thick.

All of the samples (wood-polyolefin composites comprising creep reducing agents and wood-polyolefin composites with no creep reducing agents) were then tested for creep as follows. Samples were cut from the pressed plaques to give a strip of wood- polyolefin composite approximately 25mm x 10mm x 3.5mm. These were then conditioned by leaving in a room at 23°C for a day and then placed in a rig with support only at the two ends. A weight of 150g was then hung from the middle of the sample and the displacement or “sag” was measured over a period of time and these results are illustrated in Figs 2 to 4.

Fig. 2 shows the creep of each of the polyolefin resins A, B, C and D. A creep of over 9mm at 1000 hrs on this creep test predicts that products made from this material are likely to have unacceptable creep in practice. Figs. 3 and 4 show the effect of various creep reducing agents.

In this specification the terms “comprise, comprises, comprised and comprising” and the terms “include, includes, included and including” are all deemed totally interchangeable and should be afforded the widest possible interpretation.

The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail within the scope of the claims.

Claims (4)

1. A process for preparing a wood-polyolefin composite having reduced creep; the composite comprising a cellulose-based filler in the amount of between 20% and 80% by weight, a polyolefin in the amount of between 80% and 20% by weight wherein at least 25% of the polyolefin is recycled polyolefin and an amount of creep reducing agent; characterised in that the process comprises: preparing a sample wood-polyolefin composite by compounding an amount of polyolefin and cellulose-based filler in a ratio corresponding to the amount of polyolefin and cellulose-based filler in the wood- polyolefin composite and at a temperature above the melting temperature of the polyolefin to form the sample wood-polyolefin composite; analysing the sample wood-polyolefin composite; determining the amount and type of creep reducing agent required to reduce creep in the wood-polyolefin composite; and compounding the cellulose-based filler and polyolefin with the determined amount of creep reducing agent in an extruder at a temperature above the melting point of the polyolefin to form the wood- polyolefin composite.

2. A process for preparing a wood~polyo|efin composite substantially as described hereinbefore with reference to the accompanying example and drawings.

3. A wood-polyolefin composite as prepared by the process as claimed in any of claims 1 or 2. A process for preparing a wood-polyolefin composite article, comprising extruding the wood-polyolefin composite as claimed in claim 3 at a temperature in the region of between 150°C and 240°C to form the composite article. A wood-polyolefin composite article prepared by the process as claimed in claim

4.