Patent Publication Number: US-2011052904-A1

Title: Pultrusion process and related article

Description:
FIELD OF THE INVENTION 
     This invention relates generally to continuous profile molding methods and products made using such methods. More particularly, the present invention relates to processes for manufacturing pultrusion articles that may be used, for example, as tool handles, having a construction which significantly increases the strength of such articles without a significant corresponding increase in weight. 
     BACKGROUND OF THE INVENTION 
     The process of pultrusion generally involves the manufacture of articles having a continuous profile of a single selected cross-section matching that of a die. Usually the manufactured article comprises a thermosetting type resin (i.e., polyesters, epoxies, phenolics, etc.), reinforced with such materials as reinforcing fibers, including boron, Kevlar, hemp, cotton, sisal, etc. Pultrusion manufacturing processes have a significant number of applications, but there is also a significant limitation, i.e., the articles produced have only one continuous profile (round, square, hollow, channel, etc.) in cross-section. 
     In recent years, pultrusion manufacturing processes have been adapted to manufacture composite rod assemblies that may be used as handles for hand tools such as shovels, rakes, hoes and the like. The basic technique for running filaments through a resin bath and then through an elongated heated die chamber to produce a cured composite rod of the same shape as the die chamber has been known for some time. See, for example, U.S. Pat. Nos. 2,948,649 and 3,556,888. This method, however, produces a solid extruded product which is unacceptably heavy and/or too rigid for many tool handle applications. The weight problem can be alleviated by means of an existing process to extrude hollow chambers utilizing a die chamber with the center filled, leaving an annular cross-section through which the resin-coated fibers are pulled. This weight reduction is achieved, however, at the cost of significantly reduced bending or flexural strength in comparison with a solid rod, resulting in a tool handle which would not be suitable for use in certain high-stress applications such as general purpose shovel handles. Further, to increase interlaminar strength of the chamber-forming fibers, a substantial percentage of fibers running other than in a longitudinal direction have been thought to be required. 
     In an attempt to improve the bending strength of tool handles, the fiber-resin rods, which are manufactured to be substantially hollow throughout a major portion of their length, have been reinforced at areas of expected high stresses during tool use. Such improved tool handles and related methods are shown in U.S. Pat. No. 4,570,988. These composite tool handles have further been improved by the introduction of one or more reinforcing beads of fiber-resin material extending the length of the load-bearing rod. Such tool handles are shown in U.S. Pat. No. 4,605,254, the contents of which are incorporated herein by reference. 
     Although such above-described composite tool handles are generally superior to wooden handles, the competitive pressures of the marketplace have encouraged tool handle manufacturers to seek new processes, materials and construction techniques to further increase the strength of composite tool handles. 
     It is well known that utilizing unidirectional strands of resin-coated glass fibers in a pultrusion process is the most economical process for manufacturing a composite rod assembly. In many cases, glass fibers such as a fabric mat veil have been introduced into the pultrusion process to reduce interlaminar failure or to increase the hoop strength of the rod assembly by providing cross-fibers within the cured fiber-resin composite load-bearing jacket. The use of cross-fibers, however, typically and undesirably increases the costs associated with manufacture of composite rod assemblies and decreases tensile strength along the length thereof. 
     Composite rod assemblies are far stronger in tension (due to the strength characteristics of the fiber materials), whereas the compressive loads are borne almost entirely by the interfiber resinous material. 
     Accordingly, there has been an on-going need for improved composite assemblies and related manufacturing processes to provide significantly increased tensile and flexural strength without a corresponding increase in weight. Such a manufacturing process preferably permits use of relatively low-cost fiber and resin materials, and utilizes unidirectional fibers in a pultrusion manufacturing process. Additionally, there exists a need for a composite assembly having increased interlaminar and hoop strength without the use of cross-fibers. Moreover, a novel composite assembly is needed which has greatly-improved resistance to shear failure through the resin, as exhibited in prior composite rod assemblies. The present invention fulfills these needs and provides other related advantages. 
     SUMMARY OF THE INVENTION 
     The present invention resides in an improved process for manufacturing pultrusion articles that may be used, for example, as a tool handle, and a pultrusion die molding process for making such articles. The manufacturing process comprises, generally: feeding fibers into a pultrusion die, such that the fibers are aligned about the periphery of the die chamber; injecting thermoset material and a foaming agent into the die, such that the thermoset material and the foaming agent are mixed, distributing the thermoset material and foaming agent such that a solid skin is formed about the fibers and a foamed core is formed therebetween; and advancing the fibers and the core through the die to allow the article to cure, such that a reinforced skin, which has increased tensile strength, is bonded to the core, which has increased compressive strength. 
     An object of the invention is to provide a continuous article which can be quickly and easily manufactured with a foamed, lightweight, crush-proof core and a reinforced shell. 
     Another object is to provide a trouble-free and reliable method which is more economical than existing alternatives, and the end product of this invention is relatively strong due to the strong tensile strength of the shell and the strong compressive strength of the core. 
     Another object is to provide an article the strength of which is enhanced by the perfect molded fit and strong bonding of the shell to the core. 
     Another object is to provide a method for producing a pultruded article which utilizes a foaming agent to allow for uniform cells to form, with a good structure to provide for increased strength and reduced weight of the core. 
     Another object is to provide an article with increased tensile and flexural strength without a corresponding increase in weight by using unidirectional fibers in a pultrusion manufacturing process. 
     Another object is to provide an article with increased interlaminate and hoop strength without the use of cross fibers. 
     Another object is to provide an article with improved resistance to shear failure through the thermoset material. 
     One aspect of the invention is directed to a method for manufacturing a pultrusion article. The method includes: pulling fibers through a die chamber, with the fibers being distributed about the periphery of the die chamber; injecting thermoset material and a foaming agent into the die chamber; and distributing the thermoset material and foaming agent through the die chamber, whereby the thermoset material and foaming agent cooperate with the fibers about the periphery of the die chamber to form a shell about an inner core of thermoset material and foaming agent. 
     Another aspect of the invention is directed to an article having a core of cured thermoset material and foaming agent. Unidirectional fibers are distributed about the periphery of the article. A shell of cured thermoset material encapsulates the unidirectional fibers, with the shell having a greater density than the core. The core and the shell are bonded together. 
     Another aspect of the invention is directed to a pultruded article having a core and a shell. The core is of cured thermoset material and foaming agent. The shell is of cured thermoset material encapsulating pultruded unidirectional fibers. The pultruded article provides increased strength without the use of cross-fibers. 
     Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of the pultrusion machine adapted to perform the process described herein. 
         FIG. 2  is an enlarged view of a portion of the machine, showing a mandrel and die chamber with fibers and thermoset resin positioned therein. 
         FIG. 3  is a cross-section view of one embodiment of an article which is manufactured using the process described herein. 
         FIG. 4  is a cross-section view of a second embodiment of an article which is manufactured using the process described herein. 
         FIG. 5  is a perspective view of a shovel having a handle manufactured in accordance with the method described. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in the exemplary drawings, the present invention is embodied in a composite fiber-resin article  10  with foamed core  12 , and a pultrusion process or method for its production. However, the invention is not limited to a composite fiber-resin rod. The process described herein can be used to manufacture many different articles, including articles with cross-sectional shapes which are not round or uniform and articles which have a relatively large cross-sectional area or articles which have a relatively long length. 
     In exemplary  FIG. 1 , the method of this invention is schematically illustrated. A fiber material  14  is drawn off of a series of spools  16  or bales and through a carding disc  18  into a die chamber  20 . Alternatively, fiber mats (not shown) may be drawn into the die chamber  20 . A thermoset material or resin mixture  22  is introduced into the die chamber  20  proximate a first end thereof. The thermoset resin  22  may be of the type in which an exothermic reaction occurs when the components  24 ,  26  of the thermoset resin  22  are mixed together. The thermoset resin includes a foaming agent therein which allows the resin to foam or expand to create the foamed core  12  with air gap  28  and a solid skin  30  on the outside of the article being formed (adjacent the surface of the die chamber), the article being reinforced with the fibers  14 . For particular applications, the thermoset resin may have fillers provided therein, such as, but not limited to, wood flour and calcium carbonate. The fibers  14  are pulled through the die chamber  20 , causing the thermoset resin  22  to coat the fibers  14 . If a sufficient exothermic reaction occurs, sufficient heat may be generated to allow the thermoset material  22  to cure and bond to the fibers  14  as the fibers  14  are pulled through the die chamber  20 . Additional heat, if required, may be supplied by a heating element (not shown) which surrounds the die chamber  20 . If the exothermic reaction generates too much heat, cooling may be supplied by a cooling element (not shown) which surrounds the die chamber  20 . In any case, the die chamber  20  is configured so that as the article is pulled out of the die chamber  20 , by tractor type pullers  31  or other known devices, the article is fully cured. The fully cured article may then be cut into the desired length by a conventional cutting device  32  or have additional coatings applied thereto in other stations. 
     As better shown in  FIG. 2 , the fibers  14  are inserted into the die chamber  20  around the periphery thereof. The fibers  14  may be fiberglass or any other material that has the desired physical and tensile strength characteristics desired. A die insert or mandrel  34  is positioned proximate a first end of the die chamber  20  and extends partially therein. The mandrel  34  is dimensioned to have a cross-sectional shape which is similar to, but smaller than, the cross-sectional shape of the die chamber  20 . This allows the fibers  14  to be inserted into the die chamber  20  around the periphery thereof. The mandrel  34  has two delivery chambers  36 ,  38  which extend therethrough. In the embodiment shown, the two delivery chambers  36 ,  38  extend from one end toward a second end. Portions of the first delivery chamber  36  and the second delivery chamber  38  are essentially parallel to each other. Proximate the second end of the mandrel  34 , the two chambers  36 ,  38  converge. The particular configuration of the delivery chambers  36 ,  38  and their relative position to each other can vary without departing from the scope of the invention. The second end of the mandrel  34  has a concave arcuate portion  40 . The delivery chambers  36 ,  38  extend through the second end such that the convergence of the two chambers  36 ,  38  occurs proximate the center of the concave arcuate portion  40 . 
     As is shown in the Figures, the two delivery chambers  34 ,  36  are used to deliver the components  24 ,  26  of thermoset resin  22  or mixture to the die chamber  20 . The thermoset resin  22  may be a polyurethane base or any other material having the desired compressive strength, physical and curing characteristics required. A first component  24  of the thermoset resin  22  may be introduced into a mixing chamber (not shown) prior to entering either of the delivery chambers. A second component  26  or hardener may be supplied to the mixing chamber. In addition, a foaming agent may be introduced into the mixing chamber. The mixture of the first component  24 , second component  26  and foaming agent is introduced into the delivery chambers and flows through the delivery chambers to the die chamber  20 . In this embodiment, the mixing chamber must be provided in close proximity to the die chamber, so that the mixture can be supplied through the delivery chambers before the chemical reaction caused by mixing the components increases viscosity and inhibits the movement of the mixture through the delivery chambers. Also in this embodiment, the delivery chambers need not converge prior to entering the die chamber  20  and, therefore, may be spaced at different locations along the concave portion  40  of the die chamber  20 . 
     Alternatively, as shown in  FIG. 2 , the first component  24  of the thermoset resin  22  may flow through one of the delivery chambers  36 , and the second component  26  may flow through the alternate delivery chamber  38 . In this embodiment, the first component  24  and the second component  26  do not mix until they are introduced into the die chamber  20 . The foaming agent may be introduced into either the first component  24  or the second component  26  prior to the introduction of either into the respective delivery chamber  20 . This allows the chemical reaction caused when the components  24 ,  26  are mixed together to be properly controlled in the die chamber  20 . In this embodiment, the first and second components  24 ,  26  are forced through the delivery chambers  36 ,  38  under high pressure. The high pressure causes the first and second components  24 ,  26  to properly mix in the die chamber  20  as they exit the delivery chambers  36 ,  38 . This method is sometimes referred to as impingement mixing. 
     In another alternative, the first component  24  of the thermoset resin  22  may flow through one of the delivery chambers  36 , and the second component  26  may flow through the alternate delivery chamber  38 . In this embodiment, the delivery chambers  36 ,  38  converge into a mixing chamber (not shown) before they reach the second end of the mandrel  34 . This allows the first component  24  and the second component  26  to mix immediately prior to being introduced into the die chamber  20 . The foaming agent may be introduced into either the first component  24  or the second component  26  prior to the introduction of either into the respective delivery chamber  36 ,  38 . This allows the chemical reaction caused when the components  24 ,  26  are mixed together to be properly controlled in the mixing chamber of the mandrel  34 . A static mixer (not shown) may be provided in the mixing chamber to facilitate the proper mixing of the components  22 ,  24  in the mixing chamber. The mixed components are then introduced into the die chamber  20 . The components  24 ,  26  may be forced through the delivery chambers at a relatively low pressure, as the configuration of the mixing chamber facilitates the proper mixing of the components  24 ,  26  prior to their introduction into the die chamber  20 . 
     The term “foaming agent” is used to describe any substance which, alone or in combination with other substances, is capable of producing a cellular structure in a plastic or rubber mass. Thus, foaming agents include soluble solids that leave pores when pressure is released, soluble solids that leave pores when leached out, liquids which develop cells when they change to gases, and chemical agents that decompose or react under the influence of heat to form a gas. An endothermic foaming agent is a foaming agent that absorbs heat, and an exothermic foaming agent is a foaming agent that generates heat. While an exothermic foaming agent is described herein, the invention is not limited to the use of an exothermic foaming agent. A number of foaming agents suitable for use in the method described herein are described below. In no way should the description of these foaming agents be construed as limiting the scope of the invention. Any foaming agent having the appropriate properties is suitable. 
     Solid foaming agents are typically employed in pellet form. The actual foaming agent may dust a carrier pellet, such as a low-density polyethylene bead. Liquid foaming agents are generally employed in a carrier, such as a fatty acid ester, a mineral oil or a polyol. Known liquid foaming agents include certain aliphatic and halogenated hydrocarbons, low boiling alcohols, ethers, ketones, and aromatic hydrocarbons. Chemical foaming agents range from simple salts such as ammonium or sodium bicarbonate to complex nitrogen-releasing agents, of which azobisformamide is an important example. 
     Foaming agents are generally classified as physical or chemical. Chemical foaming agents undergo a chemical transformation when producing gas, while physical foaming agents undergo a generally reversible physical change of state, e.g., vaporization. Physical foaming agents include liquid agents. Liquid physical foaming agents include volatile liquids which produce gas through vaporization. Chemical foaming agents are generally solids that liberate gas(es) by means of a chemical reaction or decomposition when heated. They are necessarily selected for specific applications or processes based on their decomposition temperatures. 
     As described above, the foaming agent is added to the thermoset mixture. Typically, foaming agents are added in an amount greater than 0 to about 5 percent by volume of the thermoset mixture. 
     In operation, the fibers  14  are continuously pulled into the die chamber  20  about the periphery thereof. As previously described, the thermoset resin mixture  22  with the foaming agent is caused to flow into the die chamber  20 . The rate at which the mixture flows into the die chamber  20  is governed based on the speed at which the fibers  14  are pulled through the die chamber  20  and the desired density required for the core  12 . The concave arcuate portion  40  of the mandrel  34  is configured to insure that the thermoset resin mixture is properly mixed as the components  24 ,  26  exit the delivery chambers  36 ,  38 . The configuration of the concave arcuate portion  40  of the mandrel  34  also facilitates the delivery of the thermoset resin  22  mixture to coat the fibers  14 , as a portion of the thermoset resin mixture  22  will flow along the surface of the concave arcuate portion  40  and be deposited on the fibers  14 . 
     As the thermoset resin mixture  22  is continually fed into the die chamber  20 , the mixture  22  is continually advanced along the longitudinal axis of the die chamber  20 , in a direction away from the mandrel  34 . Simultaneously, the fibers  14  are pulled through the die chamber  20  using conventional pultrusion techniques. As this occurs, the fibers  14  and thermoset resin mixture  22  are heated, by heat generated by the exothermic reaction, by a conventional heating element which surrounds the die chamber  20 , or both, to accelerate the curing of the thermoset resin mixture. The cured article is pulled from the die chamber  20  by tractor-type pullers  31 , or other known means, and cut to length using known cutting devices  32 . Prior to or after cutting, the article may be subjected to additional operations, such as co-extruding a cap stock layer, injection molding, coating or other such operations, to provide enhanced features based on the use of the article. 
     As described, the core  12  is foamed using a foaming agent to create the foam core before the core solidifies. The pressure created from the foaming agent causes the foamed material to flow toward the periphery of the die chamber  20 . As the material flows outward, the walls of the die chamber  20  prevent or constrain the further flow of material beyond the walls. Due to the constrained flow of the foamed material at the periphery, the material is compressed, minimizing the effect of the foaming agent, and thereby forming a solid skin  30  on the outside of the article. As the fibers  14  are positioned about the periphery of the die chamber  20 , the solid skin  30  forms about the fibers  14 . This causes the fibers  14  to reinforce the solid skin  30 , thereby providing extra strength to the shell  42  of the article. The thickness of the skin  30  of the foamed material can be controlled by the speed at which the thermoset material  22  is introduced into the die chamber  20 , by the speed at which the fibers  14  are pulled through the die chamber  20 , and/or by the amount of foaming agent  28  introduced into the thermoset material  22 . By allowing the thermoset material  22  to advance more slowly or by using a greater proportion of foaming agent to thermoset resin, the pressure developed in the core  12  will cause more material to be pushed toward the periphery, causing the skin  30  to become thicker. These same factors also affect the density and compressive strength of the core  12 . The more thermoset resin  22  that is pushed to the periphery, the less thermoset resin remains in the core  12 , causing the density of the core  12  to be reduced. However, due to the foaming agent and the resulting air voids  28 , the compressive strength of the core  12  is increased. 
     Examples of articles manufactured by the process are shown in  FIGS. 3 ,  4  and  5 .  FIG. 3  is a cross-sectional view illustrating a portion of the essentially round article, showing the fiber-resin composite shell  42  bonded to the foamed core  12 .  FIG. 4  is a cross-sectional view illustrating a portion of the essentially rectangular article, showing the fiber-resin composite shell  42  bonded to the foamed core  12 . In each illustration, the air voids  28  are shown in the foamed core  12 .  FIG. 5  is a perspective view of a shovel  44  having a handle  46  manufactured in accordance with the method described.  FIGS. 3 ,  4  and  5  are shown as representative examples and are not meant to limit the scope of the invention to these particular embodiments. 
     By this method, a continuous article  10  can be quickly and easily manufactured with a foamed, lightweight, crush-proof core  12  and a reinforced shell  42 . The method of this invention is trouble-free and reliable in use, is more economical than existing alternatives, and the end product of this invention is relatively strong due to the strong tensile strength of the shell and the strong compressive strength of the core. The strength is enhanced by the perfect molded fit and strong bonding of the shell  42  to the core  12 . 
     The use of the foaming agent allows for uniform cells to form, with a good structure. This provides for increased strength and reduced weight of the core  12 . The use of the foaming agent also produces a repeatable and consistent structure, as the foaming agent is uniformly activated through the core. 
     Foaming agents also tend to remain homogenized when added to the thermoset mixture. Using exothermic foaming agents also allows for faster production. Since the exothermic foaming agent adds heat, the thermoset mixture can cure more quickly, thereby allowing the fibers and the thermoset mixture to be pulled through the die chamber at a higher rate of speed, such as, for example, at rates of 16 feet/minute. 
     The use of the foaming agent in the pultrusion process described allows this process to be used for relatively large articles. As the thermoset material cures, the foaming agent expands, negating the thermoset material&#39;s tendency to shrink and form voids. This reduces or eliminates the sink marks and other poor aesthetics associated with larger articles having thick cross-sections. This also allows the process to be used with any shape article. 
     The use of the core  12  and shell  42  described herein provides significantly increased tensile and flexural strength without a corresponding increase in weight. The article uses relatively low-cost fiber and resin materials, and utilizes unidirectional fibers in a pultrusion manufacturing process. This provides an article with increased interlaminate and hoop strength without the use of cross fibers. Additionally, the article has greatly improved resistance to shear failure through the resin, when compared to prior articles. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.