A MOLDABLE PELLET BASED ON A COMBINATION OF NATURAL FIBERS AND THERMOPLASTIC POLYMER

Abstract: A moldable material comprising a core of sized natural fibers (1), forming a continuous natural fiber strand sheathed in a sheath of thermoplastic (10), or optionally, may be chopped into pellets (14). Also disclosed are sized natural fiber products, natural fiber-containing products, and processes for making natural-fiber containing products and fiber-reinforced composite articles.

Description: A MOLDABLE PELLET BASED ON A COMBINATION OF NATURAL FIBERS AND THERMOPLASTIC POLYMER TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION The present invention relates generally to a moldable material suitable for use in current technologies for molding fiber-reinforced composites. The invention further relates to moldable pellets based on a combination of natural fibers and a thermoplastic polymer material. More particularly, the invention relates to moldable pellets, each comprising a core of natural fibers that has been coated with a sheath of a thermoplastic material. The resulting pellets provide for the retention of fiber length at a high level during the molding process and impart a high level of mechanical performance. BACKGROUND OF THE INVENTION Fiber-reinforced plastics or composites, as they are known, are well known as materials that are lightweight, generally non-metallic, and extremely strong. Accordingly, these materials are used in a variety of applications where impact-resistance, high load- bearing capacity and resilience are desired without the drawbacks of using materials such as metals, which may be too heavy, or which may be susceptible to atmospheric degradation such as corrosion. Many methods are available for the manufacture of fiber-reinforced composites. Typically, making such composites includes first molding or shaping a suitable molding medium comprised of lengths of a fiber reinforcement and a polymeric molding material into the desired shape, then curing the shaped material, thereby causing it to set or harden into a toughened fiber-reinforced article. While numerous processes utilizing different types of molding media and molding and curing processes have been developed, all generally require combining a fiber reinforcement with a polymeric molding material to form a moldable mixture, so that the fiber reinforcement may enhance the strength properties of the resulting product. Moreover, all composite manufacturing processes generally share the goal of maximizing the dispersion of the fiber reinforcement throughout the polymer, thereby ensuring that when the molded composite is formed, it does not include areas with lesser amounts of fiber dispersion, which can cause the composite to experience performance failures, such as bursting or cracking. While it is desirable that the fiber reinforcement be dispersed evenly throughout the composite matrix, it has been found that this goal is often difficult to achieve. Accordingly, the process of combining the fiber reinforcement with the molding polymer typically requires mixing to blend these ingredients, and this in turn causes shear or breakage of the fiber reinforcement into shorter lengths. The shortened fiber length in some respects reduces the physical strength of the composite, in that there is less interweaving of the lengths of fiber reinforcement in the composite matrix, and therefore the load-bearing capacity and impact resistance is reduced. It is therefore desirable that molding materials for making composites contain fiber reinforcements of maximal length, and it is also desirable that such molding materials facilitate retention of fiber length during the molding process. One means of increasing the dispersion of the fiber reinforcement in the composite matrix is to apply a thin layer of sizing to the surfaces of the fiber reinforcement. Typically, the sizing contains ingredients that chemically modify the surfaces of fiber reinforcements to both aid in their dispersion and to promote bonding and adhesion between the fiber reinforcements and the molding polymer. In this regard, a more stable combination of fiber reinforcement and molding polymer is achieved. Additionally, the sizing provides some protection to the fiber reinforcement, making it less susceptible to breakage into shorter lengths. Considerable efforts have been made in the development of sizing agents that are effective to aid in dispersibility and reduce breakage of fiber reinforcements. In addition to the need to develop molding materials and sizings that promote fiber length retention during molding, there is also a need for fiber-reinforced composites that include environmentally friendly and cost-efficient alternatives to conventional fiber reinforcements such as glass fiber. Glass fiber, while being lighter and less susceptible to corrosion than metal reinforcements, carries some inherent processing difficulties. For example, during mechanical processing of glass fibers, loose fibers or fragments of fiber, also known as fuzz, may become airborne. These loose fibers or fragments may become finely dispersed in the processing environment, or they may collect on equipment surfaces. Additionally, even though glass fiber is lighter than metal reinforcements, it adds more weight to the composite product than alternative reinforcements such as polymer fibers or natural fibers. Accordingly, these alternative reinforcements, in particular natural fibers, are extremely desirable options for use as fiber reinforcements in composite manufacture. However, attempts at using natural fibers in composite manufacture have encountered some difficulties. While conventional molding processes, such as extrusion molding or compression molding, have successfully used glass and even polymer reinforcements in the manufacture of fiber-reinforced composites, the pre-molding conditions that are routinely required, such as mixing and compounding at high temperatures, have placed significant demands on the materials that are utilized. In particular, natural fibers have generally been excluded from use in composite molding processes because they did not adequately withstand the high temperature processing conditions, and have been extremely susceptible to breakage. Additionally, it has been observed that extrusion, compounding and subsequent molding degrades the length and chemical composition of the natural fibers, while compression molding results in an undesirable felt or mat-type product, which cannot be processed by high speed and high throughput molding processes. In view of the deficiencies in the art, it is an object of the present invention to provide a moldable material for use in a composite molding process, that has the advantages of incorporating environmentally and economically desirable natural fibers as the fiber reinforcement, while generating this moldable material in a form that allows it to be processed readily without losing significant fiber length during a subsequent molding process. Such a process should be capable of producing moldable materials suitable for forming composites from natural fibers without causing any loss of physical properties or processability of such fibers because of prolonged exposure to high heat. Such a process should also be capable of high-speed throughput to ensure rapid and cost-efficient manufacturing of the moldable, natural fiber-containing product. There is also a need for a natural fiber-reinforced moldable material having better fiber dispersion qualities, which will then aid in the improvement of its performance. Additionally, there is a need for a natural fiber-reinforced moldable material that has low environmental impact, in that the fiber component is of natural origin and is therefore readily biodegradable and recyclable, is formed by a process that requires less energy, and eliminates the problem of glass fuzz buildup on processing equipment. Moreover, there is a need for a natural fiber-reinforced moldable product that is prepared in a manner such that unpleasant odors that are normally caused by degradation of natural fibers during processing to form the moldable material have been minimized or eliminated. These needs are met by the products and process of the present invention. SUMMARY OF THE INVENTION It has now been discovered that materials suitable for use in the manufacture of fiber-reinforced composites may be prepared using a natural fiber reinforcement. In one aspect, therefore, the invention is a moldable material comprising a core of natural fibers forming a natural fiber strand that has been sheathed in a thermoplastic material. The moldable material may optionally be chopped into pellets for use in composite molding. In another aspect, the invention comprises a multi-filament fiber reinforcement product comprising a strand of natural fiber reinforcement and a coating of a sizing composition disposed on the surfaces of the fibers in the strand. The sizing composition may be aqueous or non-aqueous, in either variation comprising a coating ingredient selected from thermoplastic polymers, thermosetting polymers, and hydrocarbon oils or waxes, and further comprising one or more ingredients selected from coupling agents, lubricants and other conventional additives. The sized natural fiber strand may be sheathed in a thermoplastic material to form a moldable material according to the invention. The invention further comprises a process of making a moldable natural fiber- containing material comprising: a) providing a multi-filament natural fiber strand; and b) sheathing the natural fiber strand in a thermoplastic material. The invention additionally comprises a process of making a fiber-reinforced composite article comprising: a) providing a moldable material comprised of a natural fiber strand sheathed in a thermoplastic material; b) heating to melt the thermoplastic material; c) working the material to filamentize the strand and disperse the filaments thereof in the thermoplastic material; d) molding the moldable material to form an article; and e) cooling the article to form a fiber-reinforced composite article. The inventive concept also extends to fiber-reinforced articles formed according to such a process. The present invention, which has low environmental impact and is fully recyclable, overcomes the limitations of the previous processes by minimizing the exposure of the fibers to excessively high temperatures, thus virtually eliminating chemical degradation and any associated odors. In addition, the limitations caused by poor dispersion of the natural fibers in a composite molding matrix have been surmounted. Rather, improved dispersion of the fibers and increased composite part performance have been obtained through application of a sizing composition on the surfaces of the natural fibers. The present invention also overcomes limitations that have previously prevented or restricted the use of natural fibers as reinforcements in composite molding, in particular, the mechanical degradation of the fiber structure during conversion of the natural fiber material into moldable reinforcement materials. In contrast, the present invention provides pellets containing long and undamaged fibers, which can be molded to form high quality composites. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a process of manufacturing moldable pellets comprised of a natural fiber strand material and a thermoplastic sheathing material according to the invention. Fig. 2 is a three-dimensional representation of a pellet formed according to the invention. Fig. 3 is a transverse section of a pellet formed according to the invention. DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION The present invention utilizes natural fibers as the basis for forming a natural fiber strand, which subsequently acts as a substrate upon which a thermoplastic material is disposed to form a moldable material. Accordingly, in one embodiment, the moldable material of the present invention comprises a multi-filament natural fiber strand in combination with a thermoplastic material such that the thermoplastic material is formed as a sheath around the natural fiber strand. The natural fiber strand may optionally further comprise a sizing that protects the filaments in the strand and improves the compatibility of the filament surfaces with the thermoplastic material of the sheath. The resulting sheathed strand may be used in continuous form, or as chopped segments, in a molding process to form fiber-reinforced composites. The term "natural fiber", as used in conjunction with the present invention, refers to cellulosic plant fibers extracted from any part of a plant, including the stem, seeds, leaves, roots or bast. Any natural fibers meeting the requirements of the manufacturing process described herein may be used in the present invention. Suitable natural fibers may include, but are not limited to, jute, bamboo, ramie, cotton, bagasse, hemp, coir, linen, kenaf, sisal, flax, henequen, or any combination thereof. In addition, the term "substrate" is used in reference to the natural fiber strand in its capacity to function as the base element of the present invention, upon which the sizing composition and/or thermoplastic material may be disposed. Any arrangement of a plurality of gathered, natural fiber filaments forming a multi-filament strand may be used as a substrate in the present invention. The filaments may be linearly disposed as a collection in parallel alignment, or they may be formed as a twisted strand comprised of one or more plies. Preferably, the natural fiber filaments are in the form of a one-ply twisted strand. However, since different types of natural fibers have different shapes or configurations and varying densities, there may be a wide range in the numbers of fibers required to form a suitable strand. Accordingly, the natural fiber strand's characteristics may best be described and measured in terms of parameters such as twist factor and weight per unit length, due to the wide array of dimensional characteristics of natural fibers. The twist factor is a measure of the average amount of helical twisting imparted to the strand along its length. Twist is an important aspect of the present invention because it allows the formation of a continuous strand that preserves sufficient structural integrity to serve as a substrate for the coating processes used to apply the sizing composition and/or the thermoplastic material to the strand according to the invention. Natural fibers are discontinuous or staple fibers, in that they naturally exist in relatively short lengths, typically in the range of about 2 inches (5.08 cm) to about 6 feet (1.83 meters). A continuous coating process, for example the process of sheathing the natural fiber strand in a thermoplastic material, requires a continuous strand. Therefore, in order to properly utilize that process, the natural fibers must be entwined together to form a strand having a minimum amount of structural integrity. However, in addition to increasing the strength of the fiber strand, the amount of twist can also decrease the dispersibility of the natural fibers in the subsequent molding process. Consequently, where the twist is increased, more severe processing of the fiber strand may be required, either by means of increased temperature or increased shear forces applied to the fiber strand to attain proper dispersion. Increasing either one of these processing conditions typically results in degradation of the fibers. Accordingly, since the fewer twists there are per inch (2.54 cm) correlates to an increased ease of dispersion of the fibers, generally a range of about one twist per 1 inch (2.54 cm) to about one twist per 4 inches (10.16 cm) of strand length has been found to provide for optimal dispersion, while retaining sufficient tensile strength to survive the sizing and strand-sheathing processes. The basis weight of the natural fiber strand should also be sufficient to facilitate use in a continuous coating process. Preferably, the weight of the natural fiber strand should be in the range of from about 1 gram per yard (1.093 gram per meter) length to about 4 grams per yard (4.372 grams per meter) length. For example, sisal twine having a weight of about 3 grams per yard (3.279 grams per meter) may be used as the natural fiber strand. The natural fiber strand that serves as the substrate for sheathing with a thermoplastic material according to the invention may optionally be sized with a suitable sizing composition before the thermoplastic material is applied. This sizing composition may comprise a coating ingredient that provides an even film or layer on the surfaces of the filaments in the strand. This coating ingredient may be selected from thermoplastic polymer, thermosetting polymers and hydrocarbon oils or waxes. The proportion of each type of coating ingredient in the sizing composition will vary, depending on which is used. The proportion of the coating ingredient will also depend on whether the sizing is an aqueous or non-aqueous formulation. In one embodiment of the invention, the coating ingredient may be a thermoplastic polymer. Any suitable thermoplastic polymer may be used. The thermoplastic polymer may be added in emulsified form to an aqueous sizing composition, or it may be used without prior emulsification in a non-aqueous sizing composition. Examples of such polymers include maleated polypropylenes, hydrocarbons, waxes, wax emulsions and polyurethanes. Preferably, the thermoplastic polymer that functions as the coating ingredient is a maleated polypropylene that is not in emulsified form. An example of such a polymer is "E-43", available commercially from Eastman Chemicals Inc., which has a molecular weight of about 9100 atomic mass units, and softens at about 153°C (307°F). Typically, when the coating ingredient is a thermoplastic polymer, it may be used in an aqueous sizing composition at a concentration that provides a proportion of the sized strand weight of from about 0.1% by weight to about 10% by weight, based on the total weight of the sized natural fiber strand; or in a non-aqueous sizing composition to provide a proportion of from about 0.1% by weight to about 25% by weight, based on the total weight of the sized natural fiber strand. The coating ingredient may also be suitably selected from the group consisting of thermosetting polymers. Examples of such thermosetting polymers include unsaturated polyesters, epoxy resins and polyurethanes. The thermosetting polymer, when used a coating ingredient, may be used in an aqueous sizing composition at a concentration that provides a proportion of the sized strand weight of from about 0.1% by weight to about 10%) by weight, based on the total weight of the sized natural fiber strand; or in a non- aqueous sizing composition to provide a final proportion of from about 0.1 % by weight to about 25%o by weight, based on the total weight of the sized natural fiber strand. In an alternative embodiment, the coating ingredient may be selected from the group consisting of hydrocarbons, which includes, for example, liquid hydrocarbon oils and waxes that may be either in solid, amorphous or fluid form at room temperature. The hydrocarbon oil is preferably a mineral oil, such as "WHITEREX 425", which is commercially available from Mobil Chemicals. An example of a suitable hydrocarbon wax is "SHELL WAX 100", which is commercially available from Shell Chemical Co. The hydrocarbon coating ingredient may be used in an aqueous sizing composition to provide a proportion of the sized strand weight of from about 0.1 % by weight to about 10%) by weight, based on the total weight of the sized natural fiber strand; or in a non- aqueous sizing composition to provide from about 0.1 % by weight to about 25%o by weight, based on the total weight of the sized natural fiber strand. Preferably, the coating ingredient used in the aqueous or non-aqueous sizing compositions of the present invention is a hydrocarbon oil or wax having a molecular weight in the range of about 250 atomic mass units (amu) to about 4000 amu. The amount of this hydrocarbon coating ingredient that is incorporated into an aqueous sizing composition before it is applied to the natural fiber strand may vary from about 0.5% by weight to about 10% by weight, based on the total weight of the aqueous sizing composition. Preferably, the amount that is added to an aqueous-based sizing is from about 1%> by weight to about 3% by weight, based on the total weight of the aqueous sizing composition. In non-aqueous sizing compositions according to the invention, the amount of the hydrocarbon coating ingredient may range from about 0.5%o by weight to about 25%o by weight, and is preferably from about 10%o by weight to about 15%> by weight, based on the total weight of the non-aqueous sizing composition. The sizing composition may also include a suitable coupling agent. Coupling agents promote linking between the natural fiber strand and the sizing composition, thereby enabling better adhesion of the sizing composition to the strand surfaces. Examples of such coupling agents include organosilanes, titanates, zirconates, aluminates, zirco-aluminates and chromium methacrylates. Certain organosilane coupling agents, which may polymerize during use in the presence of natural fibers, display some adhesive properties by promoting the wetting of the natural fiber surfaces, which in turn causes the molecules on the fiber surfaces to interact and adhere together. As a result, an adhesive effect is observed with the use of such organosilanes. In this regard, such organosilane coupling agents demonstrate thermosetting properties resembling the thermosetting coating ingredients which may also be included in the sizing compositions of the invention. A suitable organosilane that acts as a coupling agent in the aqueous or non- aqueous sizing compositions of the invention is gamma-amino propyltriethoxy silane, which is an aminosilane that is available commercially under the tradename "A-l 100" from C.K. Witco Inc. The coupling agent is used in an amount effective to provide the coupling effect necessary to adhere the sizing composition to the surfaces of the natural fiber strand. Additionally, one or more conventional additives selected from processing aids, lubricants, viscosity-modifiers, surfactants, odor inhibitors, fragrances, fungicides, biocides and polymeric compatibihzers may also be included. One skilled in the art may determine the selection and amount of each of these additives commensurate with the desired effect in the sizing composition. The sizing composition, if used, may therefore be applied to form a multi-fiber reinforcement product comprised of a strand of natural fiber reinforcement that has been soaked or coated with the sizing composition. The sizing composition may be applied by any conventional means, including dip-draw immersion baths, rollers, pads or sprayers. The sizing composition may be applied using an in-line applicator in a continuous process, or it may be applied separately off-line. Preferably, the sizing is applied at a temperature varying from about 10°C (50°F) to about 200°C (392°F), depending on the type of natural fiber being used. However, the application temperature should generally not exceed 200°C (392°F) for an extended period, as this would cause cellulose fiber degradation. The sizing may be applied, for example to a plurality of individual filaments, which after treatment with the sizing composition may be gathered and twisted into a strand, or, alternatively, the sizing composition may be applied to a previously formed strand. In the latter process, the sizing composition should be of a viscosity that permits flow of the sizing composition to soak or penetrate through the filaments in the natural fiber strand, so as to coat substantially all of the surfaces of the individual filaments in the strand. For both aqueous and non-aqueous sizing compositions, the viscosity may range from about 1 cP (.001 Pa«s) to about 200 cPs (.2 Pa«s). Aqueous sizing compositions may preferably range in viscosity from about 1 cP (.001 Pa*s) to about 20 cPs (.02 Pa«s) at approximately 60°C (140°F), depending on the amount of mix solids, and, more preferably, are of a viscosity of from about 1 cP (.001 Pa»s) to about 10 cPs (.01 Pa*s) at 60°C (140°F). Where the sizings are non-aqueous, the viscosity may range from about 20 cPs (.02 Pa»s) to about 200 cPs (.2 Pa*s) at 60°C (140°F), and is preferably from about 20 cPs (.02 Pa*s) to about 60 cPs (.06 Pa«s) at 60°C (140°F). These preferred viscosities typically promote sufficient penetration and coating of the filaments in the natural fiber strand by the sizing composition. As described above, one embodiment of the present invention is a moldable material comprising a strand of a continuous natural fiber reinforcement comprised of a plurality of natural fibers, which is substantially sheathed in a casing of thermoplastic material. The thermoplastic material is preferably one that is suitable for use as a molding matrix resin in the formation of fiber-reinforced composites. The thermoplastic material typically has a molecular weight of at least about 7000 atomic mass units, up to a molecular weight of several hundred thousand atomic mass units, and may be chosen from the group including, but not limited to, polyolefins, polyamides, thermoplastic polyesters, vinyl polymers, polymer-modified asphalts and mixtures thereof. Examples of such thermoplastic materials include polypropylene, polyethylene, polypropylene modified asphalt, recycled polypropylene, polyvinyl chloride, polyethylene terephthalate, polybutylene terephthalate, polypropylene co-polymers polyamides, and mixtures thereof. The amount and type of the thermoplastic material may be such that the moldable material formed in combination therewith may be molded in a conventional molding process, without further addition of any other molding resin, to form a fiber-reinforced composite article. Optionally, the moldable material may be further combined with an additional, conventionally known molding resin during the molding process. Such additional molding resins include polyolefins, polyamides or any other thermoplastic polymers suitable for molding purposes, as would be apparent to one skilled in the art. The process of making the molding materials of the invention comprises providing a multi-filament natural fiber strand, and sheathing the strand in a sheath of the thermoplastic material. Before sheathing with the thermoplastic material, the natural fiber strand may optionally be sized with a sizing composition as hereinbefore described. Fig. 1 is illustrative of one embodiment of a process of manufacturing a moldable material, using a nonaqueous sizing, according to the present invention. According to Fig. 1, one or more ends of natural fiber strand 1 are unwound from collets 2 and passed over one or more rollers 3, after which the natural fiber strand ends 1 are immersed in and drawn through a dip-bath 4. The dip-bath 4 contains a sizing composition of a sufficient viscosity to permit soaking or penetration thereof through the filaments in the natural fiber strand ends 1. For example, the sizing composition may comprise a coating ingredient such as a hydrocarbon oil as well as one or more other conventional additives, as are herein described. After exiting the dip-bath 4, the strand ends 1 may optionally be drawn over one or more roller bars 5 to provide tensioning and gathering of the ends to form a consolidated end la comprised of multiple filaments, around which a layer of thermoplastic material is applied to form a sheath. In an alternative embodiment, where an aqueous sizing is used, the consolidated end 1 a may be passed through one or more ovens (not shown) before the coating of thermoplastic material is applied. A particularly preferred method of applying a thermoplastic material around a continuous fiber material to form a sheath is described in U.S. Patent No. 5,972,503. According to this method, the natural fiber strand may be pulled or otherwise passed through a suitable coating device. The coating device suitably includes a means to provide a source of molten thermoplastic material for coating the strand, such as an extruder. The coated strand may then be passed through a die, which regulates the amount and thickness of the layer of molten thermoplastic material on the surface of the strand, and smoothes the molten thermoplastic material to form a sheath that encases the strand. A plurality of sheathed strands can be formed by pulling or otherwise passing a number of the coated strands through a corresponding number of dies, with each die having an orifice sized to form the coating into a thermoplastic sheath of the desired thickness. Preferably, the coating device is a wire coater, which is device or group of devices capable of coating or sheathing one or more strands with a thermoplastic material so as to form a sheath of relatively uniform thickness on each strand. Preferably, the wire coater also includes a die that shapes the sheath to the desired uniform thickness and/or cross-section. The strand is fed or passed through the coating device using a suitable mechanism such as a puller, which pulls the strand through the wire coater. The puller can be separate from or part of the wire coater. The wire-coated strand may thereafter be wound onto a spool or its equivalent, or may otherwise be passed through a chopper to be segmented into pellets. A chopper may be adapted to also function as a puller, or to aid the puller in pulling the strand through the wire coater. The speed of the natural fiber strand through the wire coater die may vary between 100-300 feet per minute (.508-1.524 meters per second). This rapid throughput speed permits exposure of the natural fiber strand to the thermoplastic material while it is at a relatively high temperature, yet minimizes the exposure time such that the temperature of the natural fiber strand itself is not elevated to the point where degradation of the fibers would occur. As a result, the natural fiber strand has a better heat history and is better able to form a moldable material that provides an excellent reinforcement for composite manufacture. This technique is represented in Fig. 1, wherein a consolidated end la formed by gathering the individually sized natural fiber strand ends 1 is drawn through a wire coater 6 as the coating device. The wire coater 6 is equipped with a shaped die 7, which may be of round configuration, or of any other desired configuration depending on the desired cross-sectional profile of the wire-coated strand after it exits the die 7. The wire coater 6 is supplied with a molten thermoplastic material that is derived from melting of a solid thermoplastic source material such as pellets in a headbox 8. The molten thermoplastic material is then fed into an extruder 9. From the extruder 9, a stream of the molten thermoplastic material is forced through a nozzle opening (not shown) onto the consolidated end la as it passes through the wire coater 6. As the consolidated end la is drawn through the die 7, the thermoplastic material is caused to flow around the consolidated end 1 a, thereby forming a thermoplastic-sheathed strand 10, in which the natural fiber filaments are bundled together to form a core surrounded by an outer layer of thermoplastic material. The thermoplastic-sheathed strand 10 exits the wire coater 6 and is drawn through a water bath 11 at ambient temperature, which serves as a cooling means. Any appropriate alternative cooling means may be used, for example, air-drying, refrigeration units, or water sprayers. The cooled thermoplastic-sheathed strand 10 may be wound and stored for subsequent molding in continuous form, or it may be chopped into pellets and packaged for subsequent use in molding applications. The chopping of the continuous strand into discrete pellets may be performed via in-line or off-line processes. In this regard, the apparatus can include means such as a chopper for segmenting the continuous strand into a plurality of discrete pellets. In the embodiment represented by Fig. 1, the thermoplastic- sheathed end 10 may be tensioned over one or more rollers 12, then drawn through a chopper 13, which segments the strand 10 into approximately equal lengths of the desired size. Preferably, the thermoplastic-sheathed strand 10 is chopped into pellets 14 of from about 0.25 inch (.635 cm) to about 2 inches (5.08 cm) in length. Optionally, the thermoplastic-sheathed end 10 may be pulled through the wire coater by a puller (not shown) located downstream of the wire coater and before the chopping means. Alternatively, the chopping means itself may be adapted to perform the function of the puller or to aid the puller in pulling the sized natural fiber strand through the wire coater. The resulting pellets 14 preferably vary in length from about 0.25 inches (.635 cm) to about 2 inches (5.08 cm), with the most preferred length being from about 0.5 inches (1.27 cm) to about 1 inch (2.54 cm). However the lengths of the pellets may be adjusted to longer lengths or shorter lengths as required for the appropriate applications. Accordingly, the pellet sizes may be selected to provide for the proper fiber length retention, as well as to provide an optimal aspect ratio for automated handling and processing equipment. As shown in Fig. 1, the pellets 14 may be collected in a bin 15 and stored for future packaging or processing, or may be packaged directly by integrating an in-line packaging device into the forming operation (not shown). The pellets formed according to the invention are typically cylindrical in shape, although the shape may be modified by changing the configuration of the die through which the wire coated strand is pulled after the sheath of thermoplastic material is applied. Fig. 2 represents a pellet 14 that is approximately cylindrical in shape, which comprises a tubular sheath 16 of thermoplastic material that forms a casing around a core 17 comprised of a plurality of natural fiber filaments derived from one or more roving ends. A transverse section of this embodiment of the pellets formed by the invention, as shown in Fig. 3, indicates that the core 17 of natural fiber filaments is substantially surrounded by the sheath 16, such that the sheath 16 provides a layer of thermoplastic material that is of substantially consistent thickness around the core 17. The pellets may contain varying proportions, by weight, of the thermoplastic sheath material. In one preferred embodiment, the thermoplastic sheath material may provide all of the molding resin used to form the composite matrix during molding of the molding materials to form a composite article, such that compounding with an additional molding resin before molding is unnecessary. Accordingly, the thickness of the thermoplastic sheath may be varied to increase or decrease the proportion of the thermoplastic material in the moldable material. The present invention further relates to the manufacturing of fiber-reinforced composite articles from the moldable materials of the invention, using a molding process selected from injection molding, compression molding, extrusion-compression molding, extrusion-injection molding, compression-injection molding, or any combination thereof. A preferred process for making a fiber-reinforced composite article comprises molding pellets of the invention in a molding process that comprises heating and/or extrusion, to render the moldable material thermoformable, and to disperse the natural fiber segments of the pellet throughout the thermoplastic material. The mixture of molten thermoplastic material and fiber is then introduced into a suitable mold by a conventionally known process, such as injection or extrusion, or may be otherwise formed or shaped into an article and cooled. During the cooling process, the mixture of the molten thermoplastic material and natural fiber segments hardens into a tough, resilient composite. EXAMPLES Example 1. Non-aqueous sizing method. Commercial sisal twine (Ambraco 16000 Untreated Baler Twine) with a weight of approximately 3.2 grams per yard (3.50 grams per meter) and about one twist per inch (2.54 cm) was used. The number of sisal fibers in the twine was about 120. Three samples of non-aqueous size, samples A-C, were prepared: A. 1000 grams of mineral oil (WHITEREX 425). B. 750 grams of mineral oil and 250 grams of maleated PP wax (EPOLENE E-43) were combined and heated to approximately 150°C (302°F) with stirring. C. 675 grams of mineral oil, 225 grams of maleated wax and 100 grams of stearic acid were combined and heated to about 175°C (347°F) with stirring. A ball of sisal twine was unwound from the inside and immersed using a dip bath in each of the sizes and then rewound on a winding machine. Size A was applied at room temperature, but Size B and Size C were heated in the sizing bath and applied at elevated temperature (100°C ± 20°C) (212°F ± 68°F) to the sisal. The amount of sizing picked up by the sisal ranged from about 15-20%o by weight, based on the total weight of the sized strand (unless noted otherwise, all proportions in the examples described herein are designated as percentage by weight, based on the total weight of the sized strand). The small wound packages of sized sisal twine were then ready to be wire-coated as the next step. The sized natural fibers were threaded through a cross-head extrusion (wire- coating) die with a 3 mm diameter exit hole attached to a Killion 2 inch (5.08 cm) extruder. Set point temperatures on the extruder were all 225°C (437°F). The extruder was used to feed molten Huntsman P4C6Z-059 (MFI=35) polypropylene which had been previously dry blended with 2% Polybond 3200 polymeric coupling agent around the fibers as they passed through the die. After exiting the die the coated strand passed immediately into a 2 meter long water bath maintained at 10°C (50°F) and into a Killion 4-24 double belt puller. In this example the puller speed was set at 100 feet per minute (.508 meter per second) and the extruder output was set so as to obtain a strand with a weight of 10 gram per yard (10.93 gram per meter) which was calculated to give a (dry) fiber content of 30%> by weight. In this example the strand was fed directly from the puller into a Conair Jetro 2047 chopper which cut the strand into pellets of 12 mm length suitable for feeding into an injection molding machine. The pellets from each of samples A, B and C were pre-dried overnight at 88°C (190°F) and then injection molded to form disks using a Van Dorn 300-RS-25 moulding machine. Molding set point temperatures were 150°C-171°C (302°F-340°F), screw speed was 60 rpm (6.28 radian per second) and back pressure 80 psi (551.58 kPa). The die cavity was 3 mm thick and 177 mm in diameter. The die temperature was set at 88°C (190°F). The molded disks were evaluated by visual examination. It was observed that all the molded disks emitted minimum odors due to the low molding temperatures. Additionally, the sisal fibers maintained their natural light brown color, which indicated that no more than a minimal level of degradation had occurred during processing. Disk A (molded from the pellets of sample A) clearly showed a collection of undispersed fiber bundles distributed in the PP matrix; based on a visual estimate only about 10%) of the fibers were dispersed in the matrix. Disk B showed a high level of dispersion. It was estimated that only less than about 10% of the fiber bundles had not dispersed in the matrix. Disk C showed substantially complete fiber dispersion; no bundles were visible. Example 2. Aqueous sizing method. An aqueous size consisting of 21.5 grams of gamma-aminopropyltriethoxy silane and 100 grams of a maleated PP emulsion (CHEMCOR 43N40) were added to demineralized water such that the final amount of size made was 8823 grams. The mix solids were calculated to be 0.6%> by weight. An amount of 1135 grams of sisal twine (same type as Example 1) was unwound into a 5- gallon (18.93 liter) capacity plastic bucket. It was laid loosely in the bucket. The aqueous size was poured on top of the sisal twine and allowed to soak for ten minutes. The top end of the sisal was found and the twine was transferred by hand and excess size was squeezed off as the twine was being transferred. The weight of the sized sisal after sizing was 2853 grams, which indicated that the sisal had picked up approximately 150% of its weight in sizing composition. Other experiments have shown that the amount of water picked up by sisal when dipped in water is about 150%o of the weight of sisal. The pail was placed in a forced air oven at 105°C (221°F) and the water was removed after about 30 hours drying time. The amount of dried sizing composition on the dried sisal was about 0.9% by weight. The sisal was then wire coated as described in Example 1. It is believed that Applicants' invention includes many other embodiments which are not herein specifically described, accordingly this disclosure should not be read as being limited to the foregoing examples or preferred embodiments.

Claims: WHAT IS CLAIMED IS: 1. A moldable material comprising: a natural fiber strand comprising a plurality of entwined natural fibers, and a sheath of thermoplastic material. 2. The moldable material of claim 1, wherein the natural fibers are selected from the group consisting of jute, cotton, bagasse, hemp, coir, linen, bamboo, ramie, kenaf, sisal, henequen, abaca, flax and mixtures thereof. 3. The moldable material of claim 1, wherein said thermoplastic material is selected from the group consisting of polyolefms, polyamides, thermoplastic polyesters, vinyl polymers, polymer-modified asphalts and mixtures thereof. 4. The moldable material of claim 3, wherein said thermoplastic material is selected from the group consisting of polypropylene, polyethylene, polypropylene modified asphalt, recycled polypropylene, polyvinyl chloride, polyethylene terephthalate, polybutylene terephthalate, polypropylene co-polymers, polyamides and mixtures thereof. 5. The moldable material of claim 1, further comprising a sizing on the surfaces of the natural fiber strand. 6. The moldable material of claim 5, wherein the sizing comprises a thermoplastic polymer, a thermosetting polymer or a hydrocarbon as a coating ingredient. 7. The multi-filament fiber reinforcement product of claim 1, in the form of pellets of from about 0.25 inches (.635 cm) to about 2 inches (5.08 cm) in length. 8. A multi-filament fiber reinforcement product comprising: a natural fiber strand comprised of a plurality of natural fiber filaments formed into one or more plies; and a coating of a sizing composition disposed on the surfaces of the filaments. 9. The multi-filament fiber reinforcement product of claim 8, wherein the natural fibers are selected from the group consisting of jute, cotton, bagasse, hemp, coir, linen, bamboo, ramie, kenaf, sisal, henequen, abaca, flax and mixtures thereof. 10. The multi-filament fiber reinforcement product of claim 8, wherein the natural fiber reinforcement is one ply, having a twist factor of from about one twist per 1 inch (2.54 cm) to about one twist per 4 inches (10.16 cm) of strand length, and having a weight of from about 1 gram per yard to about 4 grams per yard length of strand. 11. The multi-filament fiber reinforcement product of claim 8, wherein the sizing composition comprises a coating ingredient selected from the group consisting of thermoplastic polymers, thermosetting polymers, hydrocarbon oils and waxes. 12. The multi-filament fiber reinforcement product of claim 11 , wherein the sizing composition comprises a hydrocarbon oil or a wax. 13. The multi-filament fiber reinforcement product of claim 12, wherein the sizing composition comprises a hydrocarbon oil or wax having a molecular weight in the range from about 250 amu to about 4000 amu. 14. The multi-filament fiber reinforcement product of claim 12, wherein the sizing composition has a viscosity of from about 1 cP (.001 Pa»s) to about 200 cPs (.2 Pa«s) at a temperature of about 60°C (140°F). 15. The multi-filament fiber reinforcement product of claim 8, wherein the sizing composition further comprises a coupling agent selected from the group consisting of organosilanes, titanates, zirconates, aluminates, zirco-aluminates and chromium mefhacrylates. 16. The multi-filament fiber reinforcement product of claim 8, wherein the sizing composition further comprises one or more compounds selected from the group consisting of processing aids, lubricants, viscosity-modifiers, surfactants, odor inhibitors, fragrances, fungicides, biocides and polymeric compatibilisers. 17. A process of making a moldable natural fiber-containing material comprising: a) providing a multi-filament natural fiber strand; and b) sheathing the natural fiber strand in a sheath of thermoplastic material. 18. The process of claim 16, wherein the step of sheathing the natural fiber strand is a wire coating process that forms a thermoplastic-sheathed composite strand. 19. The process of claim 17, further comprising cutting the thermoplastic-sheathed natural fiber strand into pellets. 20. The process of claim 17, wherein the thermoplastic material used for sheathing the natural fiber strand has a molecular weight of at least 7000 amu. 21. The process of claim 17, wherein said thermoplastic material is selected from the group consisting of polyolefms, polyamides, thermoplastic polyesters, vinyl polymers, polymer-modified asphalts and mixtures thereof. 22. The process of claim 21, wherein said thermoplastic material is selected from the group consisting of polypropylene, polyethylene, polypropylene modified asphalt, recycled polypropylene, polyvinyl chloride, polyethylene terephthalate, polybutylene terephthalate, polypropylene co-polymers, polyamides and mixtures thereof. 23. The process of claim 17, wherein the natural fiber strand is sized with a sizing composition before it is sheathed with the thermoplastic material. 24. The process of claim 23, wherein the sizing composition comprises a coating ingredient selected from the group consisting of thermoplastic polymers, thermosetting polymers, hydrocarbon oils and waxes. 25. The process of claim 24, wherein the sizing composition comprises a hydrocarbon oil or wax that is solid at room temperature. 26. The process of claim 25, wherein the sizing composition has a viscosity of from about 1 cP (.001 Pa»s) to about 200 cPs (.2 Pa«s) at a temperature of about 60°C (140°F). 27. The process of claim 25, wherein the hydrocarbon oil or wax has a molecular weight in the range of from about 250 amu to about 4000 amu. 28. A process of making a fiber-reinforced composite article comprising: a) providing a moldable material comprised of a natural fiber strand sheathed in a thermoplastic material; b) heating to melt the thermoplastic material; c) working the material to filamentize the strand and disperse the filaments thereof in the thermoplastic material; d) molding the moldable material to form an article; and e) cooling the article to cause it to harden. 29. The process of claim 28, wherein the moldable material is in the form of pellets. 30. The process of claim 29, wherein the step of molding the pellets further comprises combining the pellets with a resinous matrix material. 31. The process of claim 26, wherein said thermoplastic material is selected from the group consisting of polyolefms, polyamides, thermoplastic polyesters, vinyl polymers, polymer-modified asphalts and mixtures thereof. 32. The process of claim 31, wherein said thermoplastic material is selected from the group consisting of polypropylene, polyethylene, polypropylene modified asphalt, recycled polypropylene, polyvinyl chloride, polyethylene terephthalate, polybutylene terephthalate, polypropylene co-polymers, polyamides and mixtures thereof. 33. A fiber-reinforced composite article made by the process of claim 26.

Inventors: ADZIMA LEONARD J, BARSOTTI LUANN E, BRANDT LUC J L, CHENEY TERRY L, KASHIKAR SANJAY P, MICHAEL RAJ S, PONN FREDERICK H, THOMASON JAMES L, ADZIMA, LEONARD, J., BARSOTTI, LUANN, E., BRANDT, LUC, J.,L., CHENEY, TERRY, L., KASHIKAR, SANJAY, P., MICHAEL, RAJ, S., PONN, FREDERICK, H., THOMASON, JAMES, L.
Applicants: ADZIMA LEONARD J, BARSOTTI LUANN E, BRANDT LUC J L, CHENEY TERRY L, KASHIKAR SANJAY P, MICHAEL RAJ S, OWENS CORNING COMPOSITES SPRL, OWENS CORNING FIBERGLASS CORP, PONN FREDERICK H, THOMASON JAMES L, ADZIMA, LEONARD, J., BARSOTTI, LUANN, E., BRANDT, LUC, J.,L., CHENEY, TERRY, L., KASHIKAR, SANJAY, P., MICHAEL, RAJ, S., OWENS CORNING, OWENS CORNING COMPOSITES S.P.R.L., PONN, FREDERICK, H., THOMASON, JAMES, L.
Application year: 2001
Main classification: C08J5
IPC codes: B29B9, B29B11, B29C70, B29K105, C08J3, C08J5, C08K7, C08L97, C08L101