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An extruded composite adapted for use as a building material includes a core having a base polymer and a natural fiber in a substantially homogeneous mixture and an ionomer capstock. To improve adherence of the ionomer to a base polymer, the ionomer can be mixed with a similar or substantially similar base polymer prior to coextrusion with the core. Additionally, various additives may be mixed with the capstock material to improve visual aesthetics of the product and performance of the building material, especially over time.
WOOD-PLASTIC COMPOSITES UTILIZING IONOMER CAPSTOCKS AND METHODS OF MANUFACTURE
CROSS-REFERENCE TO RELATED APPLICATION
 This application claims priority to and the benefit of U.S. Provisional Application Serial No. 61/139,205, filed December 19, 2008, the disclosure of which is hereby incorporated by reference herein in its entirety.
FIELD OF THE INVENTION
 This invention relates to systems and methods for fabricating extruded wood-plastic composites and, more particularly, to systems for fabricating extruded wood-plastic composites that utilize ionomer cap stocks.
BACKGROUND OF THE INVENTION
 In the past 25 years, a new type of material has entered the plastics products market. Commonly referred to as wood-plastic composites (WPCs), fiber-plastic composites, or plastic composites (PCs), the new materials have been accepted into the building products markets in applications such as outdoor decking and railing, siding, roofing and a variety of other products. The market for the wood-plastic composite has grown and WPCs now are used in automotive applications, as well as in the building products sector of the economy.
 A wood-plastic composite is a blended product of wood, or other natural fibers, and a thermoplastic material. The products can be produced with traditional plastics processes such as extrusion or injection molding. For example, many building products are produced using extrusion processing similar to conventional plastics processing. The wood and plastics materials are blended before or during the extrusion process. The wood-plastic composites often compete with wood in the building products market. The current WPC materials are most often compounds of wood, or natural fibers, and polyethylene, polypropylene, or polyvinyl chloride (PVC). Presently available WPCs, however, suffer from certain drawbacks. For example, if the composite contains too high or too low of a ratio of plastic to wood, the finished product may not have the desired visual appearance or structural performance characteristics. Such products are less desirable in the marketplace. Additionally, WPCs may be expensive to produce, due to the high cost of the thermoplastic materials and other additives used in manufacture.
 Ironically, many consumers expect WPCs to appear similar to wood, but also expect WPCs to perform as a robust plastic compound. To increase performance, manufacturers often incorporate UV stabilizers, antioxidants, biocides, color, fire retardants, or other additives into the WPC formulation. These additives, however, can increase manufacturing costs of the product, even though certain additives provide noticeable benefit only on a limited location on the product (e.g., in the case of UV stabilizers, the benefit only effects the exterior of the product that is exposed to sunlight). To reduce the amount of additives that are incorporated into the product, capstocking is often used. In general, capstocks are coextruded with the core material to form a thin layer of polymer over the core extruded material. Various additives may be incorporated into the capstock, thus reducing the total amount of additives per linear foot of product. These capstocks, however, may suffer from delamination from the underlying WPC and may crack or otherwise fail, causing an unsightly appearance, impaired performance, and consumer dissatisfaction.
 With certain capstocks, to improve adhesion, a discrete tie layer is typically placed between the core material and capstock, but this tie layer can present a number of problems. For example, the bond formed by the tie layer may separate from one or both of the capstock and core material over time, leading to product failure. This may occur because the capstock and core material may expand and contract at different rates, due to differences in material properties, which may cause failure of the bond. Also, water, ice, or other hazards related to installed environmental conditions can still penetrate the capstock layer, for example, via gaps at the edges of discrete capstock sections. Additionally, manufacturing costs of capstocked products utilizing a discrete tie layer tend to be high, since the tie layer must be applied to finished capstock and core materials.
SUMMARY OF THE INVENTION
 In one aspect, the invention relates to an extruded composite adapted for use as a building material, the extruded composite including a core having a base polymer and a natural fiber in a substantially homogeneous mixture and a capstock having an ionomer. In an embodiment, the base polymer is selected from the group consisting of polypropylene, polyethylene, HDPE, MDPE, LDPE, LLDPE, and combinations thereof. In another embodiment, the natural fiber is selected from the group consisting of wood chips, wood flour, wood flakes, sawdust, flax, jute, hemp, kenaf, rice hulls, abaca, and combinations thereof. In yet another embodiment, the capstock further includes a capstock polymer, wherein the ionomer and the capstock polymer are a substantially homogeneous mixture. In still another embodiment, the base polymer a first polymer and the capstock polymer is the first polymer, which may be HDPE.
 In another embodiment of the above aspect, the capstock further includes an additive. In an embodiment, wherein the additive includes at least one of a colorant, a UV stabilizer, an antioxidant, a biocide, and a fire retardant. In another embodiment, the colorant is a variegated colorant. In yet another embodiment, the core includes about 1% to about 100% base polymer, by weight. In still another embodiment, the core includes about 46% base polymer, by weight.
 In another embodiment of the above aspect, the capstock includes about 1% to about 100% ionomer, by weight; about 20% to about 80% ionomer, by weight; or about Tl 5% ionomer, by weight. In another embodiment, the capstock includes about 0% to about 99% capstock polymer, by weight; about 20% to about 80% capstock polymer, by weight; or about 72.5% capstock polymer, by weight. In yet another embodiment, the capstock has a thickness of about 0.012 inches to about 0.040 inches, or about 0.015 inches to about 0.020 inches. In still another embodiment the capstock is bonded directly to at least one side of the core via coextrusion.
0010] In another aspect, the invention relates to a method of manufacturing an extruded composite adapted for use as a building material, the method including the steps of providing a base polymer, providing a natural fiber, mixing and heating the base polymer and the natural fiber to produce a base mixture including a substantially homogeneous melt blend, providing an ionomer, and coextruding the ionomer onto at least a portion of the base mixture through a die to form an extruded profile. In an embodiment, the method further includes the steps of providing a capstock polymer, and mixing and heating the ionomer and the capstock polymer to produce a capstock mixture having a substantially homogeneous melt blend. In another embodiment, the base polymer is a first polymer and the capstock polymer is the first polymer, which may be selected from the group consisting of polypropylene, polyethylene, HDPE, MDPE, LDPE, LLDPE, and combinations thereof. In one embodiment, the first polymer is HDPE. In yet another embodiment, the method further includes the steps of providing an additive, and mixing and heating the ionomer, the capstock polymer, and the additive to produce a capstock mixture having a substantially homogeneous melt blend. In still another embodiment, the additive is at least one of a colorant, a UV stabilizer, an antioxidant, a biocide, and a fire retardant. In another embodiment, the method further includes the step of cooling the extruded profile, which may include passing the extruded profile through a liquid, which maybe at least one of a water and a coolant.