text: one aspect of the invention is a method for producing plastic / natural fiber composite structures which are reinforced with smaller amounts of glass fibers ( preferably less than 15 % by weight ) and the formulation includes a coupling agent such as one based on maleic anhydride , which acts as a bonding agent for both the natural cellulosic fibers and the glass fibers with the plastic matrix . this material can be produced by using any of the standard techniques for producing these composites , including extrusion , injection molding , compression molding etc . in one embodiment of the invention the plastic is a thermoplastic composite . in another embodiment of the invention the plastic is chosen from the olefin group of plastics . in a further embodiment of the invention the plastic is a high density polyethylene . another aspect of the invention is a method for producing plastic / natural fiber composite with a cellular structure which is reinforced with smaller amounts of glass fibers and the formulation includes a coupling agent such as one based on maleic anhydride , which acts as a bonding agent for both the natural cellulosic fibers and the glass fibers with the plastic matrix . this material can be produced by using any of the standard techniques for producing these composites in extrusion or injection molding . the preferred method of producing these composites in extrusion is described in u . s . pat . no . 6 , 936 , 200 . in one embodiment of the invention the plastic is a thermoplastic composite . in another embodiment of the invention the plastic is chosen from the olefin group of plastics . in a further embodiment of the invention the plastic is a high density polyethylene . the invention herein is a wood plastic composite that includes glass fibre as filler . the glass fibre is preferably less than 15 % by weight . preferably the plastic is a thermoplastic composite and more specifically preferably the plastic is chosen from the group of olefin plastics . preferably the cellulosic filler is between 10 % and 60 % by weight of the material . the invention herein is described below by way of example only . the plastic material used in this study was hdpe ( 2710 , mi = 17 , nova chemical ), and the wf used was standard softwood ( pine ) grade 12020 , supplied by american wood fibers . the coupling agent used for improving the adhesion between the hydrophobic hdpe and the hydrophilic wf was the maleic anhydride - g - hdpe ( mah - g - pe , fusabond mb - 100d , 0 . 8 ˜ 1 . 1 wt % of mah , mi 0 . 2 . 0 g / 10 min , dupont canada ) and in all compositions it was 3 wt % of mixed hdpe and wf . the glass - fiber ( gf ) used was owens corning milled fiber , 737 - bd , 1 / 32 ″ in length and 16 micron diameter . the coupling agent used also improved adhesion between plastic and gf . preparation of samples : hdpe , wf and the coupling agent were mixed in a kinetic mixer from werner & amp ; pfleiderer till the temperature rose to 180 ° c . and allowed to cool down . it was then granulated using a c . w . brabender granulator into small pellets , which were then oven dried , overnight , under a vacuum using an advantec - vacuum drying oven at a temperature of 80 ° c . the mixture was then extruded in c . w . brabender extrusion system and pelletized in a c . w . brabender pelletizer . in the case of hdpe - wf / gf composite , the milled gf were added and mixed with the oven - dried mixture before being extruded and pelletized . compression molding : the pellets were then compression - molded into sheets using a carver hydraulic press . the mold was compressed at 160 ° c . under 4 . 5 metric tons for one minute . after forming , the sheets where cooled , under pressure , to ambient temperature and cut into dog - bone shaped sample conforming to astm d638 type - v specifications . mechanical property testing : the dog - bone shaped samples cut from the compression - molded sheets were tested on a lloyd instruments ls100 universal testing machine , till fracture , according to astm d638 , but with the cross - head speed of 1 mm / min , and the results were obtained using nexygen mt software . for each composition 10 samples were tested and the average values were used for interpreting the results . density : the densities of the samples were determined using astm d792 - 00 . scanning electron microscope ( sem ) characterization of fracture surface : the samples were cut near the fracture surface and sem micrographs of fracture surfaces were taken . for this , each sample was first gold - coated , using a sputter coater ( e 50000c ps3 ), and the microstructure was examined using a hitachi 510 sem . effect of gf on stress at fracture : one advantage of using the maleic anhydride as a coupling agent is that it promotes adhesion between plastic and both wf and gf so that only one coupling agent can be used for both materials . fig1 shows the result of adding 5 % gfr in wpc . as expected the fracture strength of wpc increased from 16 to 31 % for different compositions of wpc , due to the addition of gfr . the strong gf did contribute to increasing the load bearing ability of the wpc . this also indicates that maleic - ahydride coupling agent successfully promoted adhesion between gf and the plastic matrix . effect of wf content on stress at fracture : fig1 also shows the effect of wf content on the stress level at which fracture occurs in wpc and wpc + 5 % gfr samples . the wpc did not exhibit any remarkable change in fracture stress levels with varying wf contents . however , when 5 % gf was added , not only did the fracture stress level increased in general , but it also increased with increasing wf content , increasing from 16 % compared to wpc when wf content was 10 %, to 31 % compared to wpc when the wf content was 300 %. this result is somewhat surprising . in all compositions , the coupling agent constituted 3 parts per 100 parts of mixture by weight . therefore , one possible explanation for the observed behavior is that with increasing wf content , the concentration of coupling agent in the plastic matrix increased , as it would not be dissolved in the solid wood particles . this increased concentration of coupling agent in the plastic matrix resulted in improvement in adhesion between the gf and the plastic matrix resulting in higher mechanical properties . effect of wf and gf content on elongation at fracture : fig2 shows these effects clearly . addition of gf decreases the elongation at fracture due to the increased stiffness provided by these fibers . increasing the wf content also reduces the elongation at failure due to reduction in the amount of ductile plastic matrix in the composite . effect of wf and gf content on modulus : fig3 shows the secant modulus of wpc and gfr - wpc . the modulus increases with increasing wf contents for all composites . however , the inclusion of gf resulted in the very significant increase of about 50 % in the value of the modulus . this is directly due to the high value of the reinforcing material &# 39 ; s modulus . effect of wf and gf content on density : fig4 shows the density of the composites . addition of gf increased the density of the composite by about 2 to 3 %. increasing the wf composition by 10 % also resulted in similar amount of density increases . microstructure of the fracture surfaces : fig5 show the sem micrographs of xx % wpc and wpc + 5 % gf . in the wpc with gf , the glass fiber can be seen clearly and apparently the coupling agent has not fully acted on the glass surface as adhesion is not observed . whereas , the wf are not distinguishable indicating that the amount of coupling agent used is sufficient for fully compatibilizing the wf but not the gf . additional work is needed to clarify the amount of coupling agent required for full compatibilization of gf . the inclusion of 5 % gf reinforcement resulted in substantial improvements in both strength and modulus of wpc . however , inspection of sem micrographs indicates that perhaps the gf have not achieved full adhesion with the plastic matrix and there may be room for further improvement in properties . in this study , tensile tests of wpc specimen with and without gfr were carried out for varying amounts of wf content . density and surface characteristics of the fractured specimen were also studied . significant improvements in properties were observed . a common problem in production of natural fiber composites is the control of volatiles emitted by the cellulosic fibers at the elevated processing temperatures . in normal composites , the volatile emissions can cause unwanted voids which can severely hamper the mechanical properties of the composites . in foamed composites , these volatiles can deteriorate the cell structure of the obtained foams , leading to poor mechanical properties . one way of alleviating this problem is to pre - dry the cellulosic natural fibers by using one of the standard drying techniques and limiting the duration and temperature of the process to a low value as taught by the u . s . pat . no . 6 , 936 , 200 issued aug . 30 , 2005 to park et al . the invention herein is related to the use of glass fibre filler with wood plastic composite foams . as well the invention herein has determined that the use of a coupling agent , or a blend of coupling agents , that is compatible with both the wood filler and glass fibre filler improves the properties of the wood plastic composite . as used herein , the terms “ comprises ” and “ comprising ” are to construed as being inclusive and opened rather than exclusive . specifically , when used in this specification including the claims , the terms “ comprises ” and “ comprising ” and variations thereof mean that the specified features , steps or components are included . the terms are not to be interpreted to exclude the presence of other features , steps or components . it will be appreciated that the above description related to the invention by way of example only . many variations on the invention will be obvious to those skilled in the art and such obvious variations are within the scope of the invention as described herein whether or not expressly described . 2 c . clemons , forest products j , 54 , 6 , 10 ( 2002 ). 3 r . g . raj , b v kokta , j d nizio , j appl polym sci , 45 , 91 , ( 1992 ). 4 r . g . raj , b . v . kokta , g . groleau , and c . daneault , plast . rubber proc . appl ., 11 , 215 ( 1989 ). 6 f . a . shutov , in handbook of polymeric foams and foam technology , d . klempner and k . c . frisch , eds ., hanser , new york ( 1991 ) 7 n . p . cheremisinoff , p . n . cheremisinoff , fiberglass reinforced plastics , © 1995 noyes 8 h . jiang , d . p . kamden , b . bezubic and p . ruede , j vinyl & amp ; 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