Lignocellulosic fibre-filled composites are widely used in a broad spectrum of structural as well as non-structural applications including automotive, building and construction, furniture, sporting goods and the like. This is because of the advantages offered by organic fibres compared to conventional inorganic fillers, and includes:    1. plant fibres have relatively low densities compared to inorganic fillers;    2. plant fibres result in reduced wear on the processing equipment;    3. plant fibres have the advantages of health and environmental issues;    4. plant fibres are renewable resources and their availability is more or less unlimited;    5. composites reinforced by plant fibres are CO2 neutral;    6. plant fibres composites are recyclable and are easy to dispose; and    7. complete biodegradable composite product can be made from plant fibres if used in combination with biopolymers.
There is extensive prior art in the field of lignocellulosic fibre reinforced composite materials. Notably, Zehner in U.S. Pat. No. 6,780,359 (2004) describes a method of manufacturing a component mixing cellulosic material with polymer, forming composite granules and molding granules into a component, utilizing a selection of thermoplastic resins, cellulose, additives, and inorganic fillers as feedstock and specifying a preference of wood flour over wood fibre in order to achieve a sufficient coating of cellulose by the plastic matrix.
Hutchison et al. in U.S. Pat. No. 6,632,863 (2003) teaches manufacturing of a pellet comprising at least 55% cellulosic fibre, blending the pellet with more polymer to form a final composition of at least 35% fibre and molding said pellet into articles.
Snijder et al. in U.S. Pat. No. 6,565,348 (2003) describes a multi-zone process involving melting the polymer, feeding the fibre continuously into the melt and kneading the mixture to produce fibres of the highest aspect ratio, and extruding the mixture and form granules.
Sears et al. in U.S. Pat. No. 6,270,883 (2001) describes use of a twin-screw extruder blending of fibre granules or pellets with the polymer and additives.
Medoff et al. in U.S. Pat. No. 6,258,876 (2001) teaches a process for manufacturing a composite comprising shearing cellulosic of lignocellulosic fibre to the extent that its internal fibres are substantially exposed to form texturized fibres, and combining them with a resin. Medoff et al. in U.S. Pat. No. 5,973,035 (1999) teaches a similar cellulosic composite.
Mechanical properties of the lignocellulosic fibre-filled polymer composites are mainly determined by the: (i) length of the fibres in the composite; (ii) dispersion of the fibres in the polymer matrix; (iii) interfacial interaction between the fibres and the polymer matrix (in conventional lignocellulosic fibre composites fibre agglomeration has been observed, which is the main constraint of developing structural materials; and (iv) the chemical nature of the fibre.
The prime challenges allied with the development of a manufacturing process for high performance structural materials from lignocellulosic and inorganic fibre-filled thermoplastic materials include retention of the fibre length which may be desirable for the effective stress transfer from the matrix to the fibre and well dispersion of fibres in the matrix to avoid stress concentrating agglomerates in addition to a good fibre matrix interfacial adhesion which enhances the stress transfer to the fibre. Lignocellulosic fibres are rich in hydroxyl groups and because of the strong hydrogen bonds between these hydroxyl groups it is extremely difficult to get a homogeneous dispersion of these fibres in the hydrophobic thermoplastic matrix. The highly hydrophilic cellulosic fibres are highly incompatible with the hydrophobic thermoplastic matrix and this also leads to poor wetting and dispersion of the fibres. Use of proper interface modifiers can improve the wetting and dispersion to a certain extent and improve the performance of the composites. Research has been done to improve dispersion and interfacial adhesion and hence to improve the properties of the lignocellulosic composites.
For example, in U.S. Pat. No. 4,250,064 (1981) Chandler describes the use of plant fibres in combination with fine or coarse inorganic filler such as CaCO3 to improve the dispersion of fibres in the polymer matrix. Methods such as pretreatment of cellulosic fibres by slurrying them in water and hydrolytic pre-treatment of cellulosic fibres with dilute HCl or H2SO4 was described by Coran et al. and Kubat et al. in U.S. Pat. No. 4,414,267 (1983) and U.S. Pat. No. 4,559,376 (1985), respectively. Pretreatment of cellulosic fibres with lubricant to improve dispersion and bonding of the fibres in the polymer matrix was disclosed by Hamed in U.S. Pat. No. 3,943,079 (1976).
Use of functionalised polymers and grafting of cellulosic fibres with silane for improving dispersion and adhesion between fibre and matrix have been disclosed by Woodhams in U.S. Pat. No. 4,442,243 (1984) and Besahay in U.S. Pat. No. 4,717,7421 (1988) respectively. Raj et. al in U.S. Pat. No. 5,120,76 (1992) teaches a process for chemical treatment of discontinuous cellulosic fibres with maleic anhydride to improve bonding and dispersability of the fibres in the polymer matrix. Beshay in U.S. Pat. No. 5,153,241 (1992) explained the use of titanium coupling agent to improve bonding and dispersion of cellulosic fibres with the polymer.
Horn disclosed, in U.S. Pat. No. 5,288,772 (1994), the use of pre-treated high moisture cellulosic materials for making composites. A hydrolytic treatment of the fibres at a temperature of 160-200 degrees Celsius using water as the softening agent has been claimed by Pott et. al in a Canadian Patent No. CA 2,235,531 (1997). Sears et. al disclosed a reinforced composite material with improved properties containing cellulosic pulp fibres dispersed in a high melting thermoplastic matrix, preferably nylon as described in U.S. Pat. No. 6,270,883 (2001) and European Patent No. 1,121,244 (2001).
Performance of a discontinuous fibre-filled composite is also dependent on fibre length. For example, longer discontinuous fibres have the capacity to withstand greater stress and hence have greater tensile properties than shorter fibres of similar nature, as larger fibres can absorb more stress prior to failure than a shorter fibre. Jacobsen disclosed in the U.S. Pat. No. 6,610,232 (2003) the use of long discontinuous lignocellulosic fibres for thermoplastic composites.
Another technique to improve the dispersion of the lignocellulosic fibres is to use high shear during melt blending of the fibres with plastics. Since the fibres are prone to break down, the high shear results in small fibres in the resultant compound where the fibres are not effective to carry the load from the matrix. Or, in other words, due to the high shear, the fibre length goes down to less than a critical fibre length. In order to achieve a high performance material from lignocellulosic thermoplastic composites, it is therefore necessary to well disperse the fibres in the matrix while preserving the critical fibre length.
An earlier patent application of the inventors of the present invention, namely Canadian Patent application 2,527,325 filed on Nov. 18, 2005, discloses a process to obtain high performing recyclable lignocellulosic fibre-filled thermoplastic composites with improved dispersion of fibres.
Hybridization is another technique to improve the performance properties of lignocellulosic composites and makes it suitable for high strength applications where conventional glass fibre-filled materials are used. Though there is extensive prior art relating the use of inorganic hybrid fibre system, there are few references directed at the preparation and development of hybrid thermoplastic composites using lignocellulosic fibre in combination with other organic or inorganic fibres.
The mechanical properties of the organic-inorganic hybrid fibre reinforced composites are highly dependant on the fibre length and dispersion of individual fibres in the polymer matrix and the interfacial compatibility between the individual fibres and the matrix. Fibre length is more critical, as the inorganic fibres are more prone to break down compared to organic fibres which adversely prevent the exploitation of the full potential of the composite materials.
The inventors of the present invention already reported that glass fibre composites processed by similar conditions to that of lignocellulosic fibre composites showed comparatively lower properties because of the extensive fibre breakage. (See: M. Sain, S. Law, F. Suhara and A. Boullioux, Journal of Reinforced Plastics and Composites, 24, 121 (2005).)
An earlier patent application of the inventors of the present invention, namely United States Publication No. 20050225009 and application Ser. No. 11/005,520, filed on Dec. 6, 2004, discloses a process to obtain high performing cellulosic and glass fibre-filled thermoplastic composites with improved dispersion of the cellulosic fibres. There is a need for a process for producing glass fibre-filled thermoplastic composites that includes microfibres, thereby providing enhanced interaction between the organic and inorganic fibres thereby causing an overall increase in the strength of the composite. In addition, there is a need for a process that enables the production of said fibre-filled thermoplastic composite in a single mixing apparatus, with accompanying time and energy savings.
There is extensive prior art regarding the manufacture of long glass-filled thermoplastic composites, where the long glass fibre in the form of strands or rovings are impregnated with thermoplastic resin compositions using different processing techniques. This can be either produced in the form of pellets or used directly for further processing by any techniques such as injection, compression, compression injection, extrusion, blow molding and press molding. The general methods basically involve coating or impregnation of the glass fibre with thermoplastic resin, either by dip-coating where the fibre bundles are dipped either in a powdery thermoplastic resin which is floating in the atmosphere or suspended in a liquid state, or dipped in a thermoplastic resin in a molten state, in a manner that is well-known.
For example, U.S. Pat. No. 5,409,763 (1995) to Serizawa et al. teaches a method of making glass fibre reinforced thermoplastic where a roving of glass fibre bundles is heated and then passed through a cross head die where the molten thermoplastic resin composition is fed at a specified ratio so that the glass rovings is dipped in the molten resin in the cross head die, where after the impregnated strands coming out of the die pass through a shaping die and then to a pelletizer. The content of glass fibre and the resin can be adjusted by controlling the speed of the glass rovings and the feed rate of the molten polypropylene.
Hawley disclosed in the U.S. Pat. No. 5,169,941 (1992) and U.S. Pat. No. 5,185,117 (1993) a process of making long glass fibre thermoplastic composite pellets using multiple extruder apparatus. In this method, the melted thermoplastic resin is introduced into the compounding extruder at a point downstream of the inlet point for the reinforcing fibres, so that the fibres are mechanically worked and heated before coming into contact with heated molten thermoplastic resin, and the hot mixture from the extruder may be fed directly into preform-making equipment to produce a measured preform of desired size, weight, and shape.
Other U.S. Patents from Hawley (U.S. Pat. No. 6,186,769 (2001) and U.S. Pat. No. 6,875,385 (2005)) teach an inline compounding process where continuous strands of fibre from supply spools are entrained with pressurized, molten resin flowing through a coating device and coated with the resin. The movement of the fibre and resin through the coating die may be controlled to provide a predetermined quantity of molding material to the feed screw for an injection molding machine, or simply a plate movable to and from a compression molding machine. The fibre strands may also be cut into predetermined lengths by a cutting device positioned downstream of the coating device.
Wilson in U.S. Pat. No. 5,540,797 (1996) describes a pultrusion apparatus and process for impregnation of multiple fibre tows with a thermoplastic resin. The process comprises an impregnation vessel, having an entrance and an exit end with a control device for fibre insert and resin flow, and wherein a pulling mechanism pulls the fibre tows through melted resin contained within the impregnation vessel and into a stepped decreasing diameter passageway in the resin meter and profile die to remove excess resin and shape the impregnated fibre into a useful structure before it is cooled below the melting point of the resin by the cooling die and then cut the cooled impregnated fibre structure into the desired lengths.
Shirai et al. disclosed in U.S. Pat. No. 5,718,858 (1998) an apparatus and methods for producing long fibre-reinforced thermoplastic resin compositions by initially loosening a continuous fibre bundle by a fibre loosening device to form a moving web-like continuous fibre bundle which passes through the die and is coated with a thermoplastic resin melt extruded through a slit disposed in the die by an extruder, and the impregnated web-like continuous fibre bundle may then be shaped to form a final product.
U.S. Pat. No. 6,482,515 (2002) to Berndt et al. teaches a process for producing a colored long-fibre-reinforced polyolefin structure for molding by injection molding, extrusion, blow molding or plastic compression molding, wherein fibre bundles are passed through a flat die which has been charged with a melt made from thermoplastic composition with additives, and the immersed fibre bundles are passed through a shaping die, cooled, and cut perpendicular to their running direction to give the length of the structure.
U.S. Pat. No. 7,048,431 (2006) to Sieverding et al. describes an extruder for producing fibre-containing thermoplastics using a scale assembly having a weighing plate for determining an amount of fibre to be supplied to the extruder and a fibre feeding device comprising at least two fibre guide units so as to have a good control of the fibre feeding.
U.S. Pat. No. 5,110,275 to Scheuring teaches an extruder where the glass fibres are added in the form of a fibre strand or fibre roving in such a way that adequate wetting of the fibres with molten plastics takes place in the extruder.
U.S. Pat. No. 5,879,602 to Scheuring describes an impregnating device in which the fibre rovings to be added to the extruder are pre-impregnated with plastics and a process for making the long fibre thermoplastic composite by feeding the pre-impregnated rovings to the extruder.
European Patent No. 0 056 703 B2 to Cogswell et al. details a so-called pultrusion method where continuous fibre strands are pulled through a melt of plastics to produce a composite material.
Further, Scheuring et al. in U.S. Pat. No. 6,419,864 teaches a method of preparing long fibre reinforced thermoplastics of an average length of 10 to 50 mm using a twin screw extruder in a single manufacturing process, where the fibre rovings are fed in to the molten thermoplastic composition, and cut into long fibre sections in a fibre incorporation zone.
Few prior art references disclose producing long organic fibre reinforced thermoplastic pellets. For example, Snijder et al. in U.S. Pat. No. 6,565,348 (2003) teaches a multi-zone process involving melting the polymer, feeding the organic fibre continuously into the melt and kneading the mixture to produce fibres of the highest aspect ratio, and extruding the mixture and form granules.
WO03091006 (2003) to Breard et al. discloses a method of producing a novel material comprising organic fibre bundles that are pre-impregnated with organic resin and thereafter taking the form of spools such as a yarn or ribbon.
Moreover, German Patent No. 19711247 (1997) to Mieck et al. describes a process of preparing long organic fibre reinforced polymer pellets where the hybrid slivers of reinforcement fibres and matrix fibres were heated and compacted by twisting and forming a continuous strand.
In sum, although the prior art shows processing of thermoplastic composites containing different lignocellulosic fillers and/or inorganic fillers in combination with organic fillers, with different combinations of thermoplastics, coupling agents, and fibre treatments, they are deficient in producing high strength performance cellulosic filled thermoplastic composite materials. What is needed is high performance structural composite materials where both the organic as well as the inorganic fibres have an effective fibre length and well dispersed and bonded with the thermoplastic matrix materials. What is further needed is hybrid composite materials suitable in applications where thermal resistance is important in addition to the other performance properties.
Additional Prior art publications concerning lignocellulosic thermoplastic materials that may include inorganic fibres include: