PROCESS FOR PRODUCING THERMOPLASTIC PRODUCTS HAVING ORIENTED COMPONENTS

Abstract: There is disclosed a process for the continuous production of a thermoplastic product having oriented components. The thermoplastic compound may include filler. Preferably the filler material is characterized by high aspect ratios (length to diameter). The filler may be an inorganic filler such as mica and talc flakes, glass and carbon short fibers or organic cellulosic materials derived from wood/forest by-products and agricultural by-products. Plastics such as polyolefins, vinyls, styrenics, polyesters and polycarbonates may be used. The process may be used to manufacture both integral foam products and substantially solid products of various shapes which may have flexural/bending strength and modulus 2-10 times higher than conventionally produced products made of the same plastic/polymer material (foamed and unfoamed). The product produced by the process may have filler concentrations as high as 75 percent. The product is made by an extrusion process which orients the thermoplastic polymer and the filler, when present, in the longitudinal direction during extrusion, and which prevents relaxation of the thermoplastic polymer thereby substantially preserving the imparted orientation. With respect to the integral foamed core embodiment, the die assembly is designed to facilitate orientation of polymer molecules and filler particles, when present, in order to produce an oriented skin; to form a cavity in the thermoplastic compound; to foam thermoplastic compound into the cavity due to the action of a foaming agent; and to cool the thermoplastic compound to form an integrally foamed product.

Description: PROCESS FOR PRODUCING THERMOPLASTIC PRODUCTS HAVING ORIENTED COMPONENTS FIELD OF THE INVENTION The present invention relates to processes and apparatus for the manufacture of thermoplastic compounds. More particularly, it relates to processes for manufacture of thermoplastic compounds having longitudinally oriented polymer molecules. The products formed by the present process can be solid or hollow or can have an integral foamed core. BACKGROUND OF THE INVENTION Thermoplastic polymers have been used for years in a variety of applications. However, one of the disadvantages of thermoplastic polymers which are extruded using conventional methods can be their inferior mechanical properties, such as load bearing ability, in comparison to other materials such as wood and metal. These inferior mecha- nical properties have limited the range of applications in which most thermoplastic poly¬ mers can be utilized. Much effort has been directed at developing polymers with improved mechanical properties. While these polymers, which are generally referred to as engineering polymers, have better mechanical properties than what are commonly referred to as the commodity thermoplastic polymers (polyethylene, polypropylene, polystyrene, polyvinyl- chloride), the improvement in mechanical properties is typically accompanied by higher prices. Alternatively, various means have been proposed to increase the mechanical properties of the commodity thermoplastic polymers. One of the most common methods is to add reinforcing fillers to the thermoplastic polymers. Materials which are typically used as reinforcing fillers have aspect ratios (ratio of length to effective diameter) substantially greater than one, such as fibers or thin platelets, and have properties, such as stiffness, which are greater than that of the thermoplastic polymer . As a result of the reinforcing properties of the filler, the resulting composite material typically has significantly superior mechanical properties as compared to the pure thermoplastic polymer. Reinforcing fillers can either be manufactured, such as glass, carbon, metal or ceramic fibers, or occur naturally, such as cellulosic fibers, asbestos, mica or talc. A variety of polymers in a range of grades are commercially available which contain various fibrous and mineral fillers. Another method for increasing the mechanical properties of thermoplastic poly¬ mers is to align or orient a substantial number of the polymer molecules in the same di- rection. Thereby exploiting the strong carbon-carbon backbone of the polymer where it is present. The orientation of the polymer molecules increases the strength and stiffness of the resulting product. Various methods have been proposed which increase the mechanical properties of thermoplastic polymers through orientation. In fiber spinning, the polymer fibers are drawn after exiting the die, consequently orienting the polymer molecules in the direction of drawing. In film blowing, biaxial orientation (in both parallel and perpendicular directions to the machine direction) is achieved by expanding an annular extrudate with air under high pressure. This expansion of the extrudate yields circumferential and axial drawing. In both of these processes, the resulting product has dimensions which allow for rapid cooling which preserves the imparted molecular orientation. For larger profiles, the ability to cool the profile in order to preserve any orientation which may be imparted is substantially more difficult, in particular, because conventional extrusion processes generally employ temperatures well above the melting or softening point of the thermoplastic polymer. One solution to the problem of sufficient cooling is solid phase extrusion, in which the orientation of the polymer molecules takes place below the melting point or softening point of the thermoplastic polymer. For example, UK 2 207 436 (Ward et al, 1989) describes a process to orient the molecules of linear polyethylene by extruding it in the solid phase. This solid phase deformation process is conducted in a hydraulic ram extruder and requires pressures of the order of 150 to 250 MPa (22,000 to 36,000 psi) to forge a polyethylene billet through a die at temperatures below the melting point of the polyethylene. However, ram extrusion is a slow and non-continuous process, which involves high pressure, and is uneconomical for most commercial applications. Now in accordance with the present invention, it has been found that thermoplastic polymers of any molecular weight can be continuously extruded to produce various profiles having substantial orientation. This increases the strength and modulus of the resulting product in comparison with conventional extrusion processes. In addition, the extrusion process of the present invention can orient both the thermoplastic polymer molecules and the filler particles that may be present in the same direction during extrusion and substantially prevent the relaxation of the imparted orientation. SUMMARY OF THE INVENTION The present invention provides a process for continuous production of a thermoplastic product having oriented components. The process comprises the steps of providing a thermoplastic compound comprising thermoplastic polymer. The thermoplastic compound is brought to a temperature in a predetermined range just above and including the softening point temperature of the thermoplastic compound thereby producing a molten thermoplastic compound. The molten thermoplastic compound is forced through a die, the thermoplastic compound undergoing converging flow through the die to impart longitudinal orientation to at least some of the thermoplastic polymer. The thermoplastic compound is cooled after imparting longitudinal orientation to a temperature below its softening point temperature to preserve the imparted orientation therein and to solidify the thermoplastic compound. The products can be solid or hollow profiles. In another aspect of the invention there is provide a process for continuous production of a thermoplastic product having oriented components and a foamed core. The process comprises the steps of providing a thermoplastic compound comprising thermoplastic polymer and foaming agent. The thermoplastic compound is brought to a temperature in a predetermined range just above and including the softening point temperature of the thermoplastic compound thereby producing molten thermoplastic compound. The molten thermoplastic compound is forced through a die and an interior cavity is formed therein. The thermoplastic compound undergoes converging flow through the die to impart longitudinal orientation to at least some of the thermoplastic polymer. The imparted longitudinal orientation in the outer layer of the thermoplastic compound is preserved thereby forming an oriented skin. The thermoplastic compound is foamed into the cavity. The thermoplastic compound is cooled below the softening point temperature of the thermoplastic compound to solidify the thermoplastic compound. The products produced according to this aspect of the invention are integral foam pro- files consisting of a solid oriented skin and a foamed core. In another aspect of the invention there is provided a device for manufacturing thermoplastic product having oriented components from a thermoplastic compound having thermoplastic polymer. The device comprises a means for bringing the thermoplastic compound to a temperature in a predetermined range just above and including the softening point of the thermoplastic compound. Downstream thereof is a die having internal walls, an inlet, an outlet and a passage communicating between the inlet and outlet, the diameter of the inlet being greater than the outlet, a portion of the internal walls converging and defining a converging passageway wherein in use the thermoplastic compound undergoes converging flow through the die to impart longitudinal orientation to at least some of the polymer molecules. The device also comprises a forcing means for forcing the heated thermoplastic compound through the die and a cooling means downstream of the converging passageway for cooling the thermoplastic compound. Optionally the device may include a mandrel positioned in the die and wherein the mandrel causes a cavity to be formed in the thermoplastic compound. In another aspect of the invention there is provide a thermoplastic product comprising a thermoplastic compound comprising thermoplastic polymer and having a rigid solid outer layer with longitudinally oriented thermoplastic polymer and an integral low density foamed interior. BRIEF DESCRIPTION OF THE DRAWINGS The process of the present invention will now be described, by way of example only, reference being made to the accompanying drawings, in which: FIGURE 1 is an elevational view of an apparatus for producing thermoplastic products in accordance with the present invention; FIGURE 2 is an enlarge detailed cross-sectional view of a portion of the apparatus contained within encircled portion 2 in FIGURE 1 configured to produce solid products; FIGURE 3 is an enlarge detailed cross-sectional view of a portion of the apparatus contained within encircled portion 2 in FIGURE 1 configured to produce hollow and integral foam products; FIGURE 4 is a view taken along line 4-4 of FIGURE 3; FIGURE 5 is a cross-sectional view of a circular oriented hollow profile which may be produced with the die assembly illustrated in FIGURE 3; and FIGURE 6 is a cross-sectional view of a circular integral foam profile with an oriented solid skin which can be produced with the die assembly illustrated in FIGURE 3. DETAD ED DESCRIPTION OF THE INVENTION In the present process, a thermoplastic compound is continuously extruded from an extruder through a die assembly comprised of an adapter, reservoir, die and calibrator, and in the case of hollow and integral foam products, a mandrel. The adapter serves to connect the die assembly to the extruder. The reservoir, which is either heated or cooled to achieve the desired temperature, serves two functions. Firstly, the reservoir is used to homogenize the bulk temperature of the thermoplastic compound, and second, to ensure that lubricant, which may be injected at interfaces between the thermoplastic compound and the inner surfaces of the die assembly, is evenly dispersed before entering the die. The thermoplastic compound first passes through the adapter and reservoir due to the conveying action of an extruder screw and is then pushed through a converging die. The lubricant is used to promote elongational flow and reduce shear flow through the die. The die is designed to produce longitudinal orientation of polymer molecules and filler particles (if present). The processing conditions are chosen to facilitate orientation in the die and to subsequently preserve the imparted orientation and solidify the resulting product. Vacuum calibrators or sizers, cooling baths, caterpillar type pullers and any additional downstream apparatus known to those skilled in the art may be used downstream of the calibrator to further improve product quality and increase productivity. It is understood that thermoplastic compound refers to a mixture of a thermoplastic polymer and any additives such as colorants, stabilizers, flame retardants, lubricants, processing aids, and the like. In addition, the thermoplastic compound may or may not contain fillers, particularly reinforcing fillers, compatibilizing agents and physical and/or chemical foaming agents. Embodiments of the apparatus for producing solid, hollow and integral foam thermoplastic products with a degree of longitudinal molecular orientation and filler orientation (when filler is present) using the process disclosed herein will first be discussed followed by a discussion of the types of raw materials and pretreatment thereof which may be used to produce the high modulus products. APPARATUS AND METHOD FOR PRODUCING SOLID PRODUCTS Referring first to FIGURES 1 and 2 there is illustrated an embodiment of apparatus 10 for producing solid, high modulus thermoplastic products using the process of the present invention. The apparatus consists of, in sequence, an extruder 12, a die assembly 14, a cooling bath 16, a puller 18 and a cut-off saw 20. Extruder 12, which may be one of many different types of single or twin screw extruders known to those skilled in the art, is used to melt and convey the thermoplastic compound through a passageway 17 in die assembly 14 shown in detail in FIGURE 2. The processing conditions in extruder 12 are chosen to ensure that the thermoplastic compound is melted. FIGURE 2 shows a longitudinal cross-section of die assembly 14 configured for producing solid or non-foamed oriented thermoplastic products with a circular profile using the process of the present invention. Those skilled in the art will appreciate that the expression "non-foamed" products refers to substantially solid products. Of course it will be understood that depending on the materials, natural foaming agents (such as water) may be present, which may cause some degree of foaming. It is also understood that this process can be used to produce solid products of any section (profile) having a cross-section with constant dimensions given that die assembly 14 is configured to produce the desired profile. Die assembly 14 comprises an adapter section 30 attached to the end of extruder barrel 24, with a breaker plate 28 interposed between the end of barrel 24 and section 30. Die assembly 14 further includes, in sequence from upstream to downstream, reservoir 40 adjacent to adapter section 30, die 60 and calibrator 100. The conveying action of extruder screw 26 located in extruder barrel 24 forces the molten thermoplastic compound through breaker plate 28 into adapter section 30. The shape of adapter section 30 is designed so that it can be attached at its upstream end 32 to extruder barrel 24 by any means known in the art so that there is a gradual transition from the upstream section 32 to the downstream or exit section 34 of adapter section 30 adjacent the upstream inlet of reservoir section 40. It is understood that adapter section 30 and reservoir 40 may be constructed as a single section in certain cases. One or more channels 52 are provided in the side of reservoir 40 through which lubricant can be injected to the interface between the wall of the reservoir and the ther¬ moplastic compound. Channel(s) 52 are preferably shaped, dimensioned and spaced to ensure that the lubricant is distributed evenly around the inside wall of reservoir 40 thereby evenly coating the outside of the thermoplastic compound as it is forced through channel 17. The thermoplastic compound may also contain lubricant which was added to the compound prior to extrusion. As illustrated in FIGURE 1, the lubricant may be injected by a device 56 such as a metering pump, syringe pump, gear pump or other ap¬ paratus known in the art which can consistently deliver the necessary amount of lubricant at a sufficiently high pressure to overcome the pressure inside reservoir 40. The lubri¬ cant is used to promote elongational flow and reduce shear flow through die 60. Suitable lubricants for injection include the silicone oils, liquid paraffins, glycerol, fatty amides and other such lubricants in liquid form. The length of reservoir 40 is chosen so that there is sufficient time for the lubri- cant to spread evenly around the outside perimeter of the thermoplastic compound. In addition, reservoir 40 must be of sufficient length to ensure that the thermoplastic compound enters die 60 at the desired temperature and that the temperature of the com¬ pound is as uniform as possible throughout its cross-section. The design of die 60 is important to the success of the process. First, the exit section of die 60 should correspond to the section of the desired profile. Second, the profile of the die should be carefully designed with consideration for the resulting degree of molecular orientation and filler orientation (if filler is present), the throughput and the surface appearance of the product. In order to promote the orientation of the polymer molecules and filler particles (if present) in the flow direction, it is desirable to force the thermoplastic compound through a die with a converging profile such that the die imparts a permanent deformation to the molten compound. Die 60 is shaped to provide a con¬ verging flow of the thermoplastic compound therethrough. It is understood that reservoir 40 and die 60 may be constructed as a single section in certain cases. The draw ratio of the die, which is defined as the ratio of the cross-sectional areas of the entrance and exit of the die, must be sufficiently large so that the resulting elongational flow is sufficient to orient polymer molecules and filler particles (if present) in the product. However, if the draw ratio is too large, the pressure drop through the die will be unacceptably large and defects may appear on the surface of the extrudate due to melt fracture. Draw ratios of 3:1 up to 15:1 are typical, however larger draw ratios may be possible with certain thermoplastic compounds given appropriate processing conditions. In addition larger draw ratios are typically expected to produce a greater degree of orientation in the product than smaller draw ratios, for the same thermoplastic compound. The contour of the die may be mathematically correlated to the viscoelastic defor¬ mation of the molten thermoplastic compound in terms of the elongational strain rate or elongational stress in the converging zone of the die. Studies have shown that a constant elongational strain rate is usually effective. The elongational strain rate is defined as the rate of change in length per unit length and thus the resulting elongational strain rate is a function of the volumetric flowrate of the thermoplastic compound through the die. The best results are expected for die profiles which impart a decreasing elongational strain rate, for example hyperbolic profiles. In addition to hyperbolic profiles, parabolic or conical profiles would suffice. However, to increase the maximum acceptable flowrate of the thermoplastic compound through the die, the converging die angles should be small, so that the resulting elongational strain rate in the die zone does not exceed the strain rate associated with the onset of melt fracture (i.e. poor extrudate surface quality). Thus the maximum acceptable flowrate of the thermoplastic compound through the die is determined by the choice of the die profile as well as the molecular weight of the polymer resin, the filler type and concentration and the temperatures of the die and the thermoplastic compound as it enters the die. External lubricants, which can be injected in reservoir 40 or added to the thermo¬ plastic compound, aid the elongational deformation in die 60 by reducing friction between the compound and interior surface 94 of die 60. Increasing the lubricity of interior surface 94 of die 60 by highly polishing its interior surface or by applying coatings designed to increase lubricity can also aid in minimizing friction. The final element of die assembly 14 is calibrator 100, which has the same cross- section as the adjacent exit portion of die 60, and consequently, the same section as the desired profile of the product. The main function of calibrator 100 is to maintain the dimensional stability of the product and to provide the cooling necessary to preserve the orientation of polymer molecules and filler particles (if present) in the thermoplastic compound. The length of calibrator 100 is chosen so that there is sufficient cooling of the thermoplastic compound, to preserve the imparted orientation therein. The temperature of calibrator 100 is important in determining the length of the calibrator. Alternative embodiments of apparatus 10 may comprise more than one temperature control zone for the calibrator or include a calibrator consisting of several sections in order to achieve a temperature profile which would allow for a gradual or programmed cooling of the thermoplastic compound. The temperature profile requirements for the apparatus 10 will be discussed in more detail below. It is understood that die 60 and calibrator 100 may be constructed as a single section in certain cases. The temperature settings for extruder barrel 24 and die assembly 14 (adapter sec¬ tion 30, reservoir 40, die 60 and calibrator 100) must be carefully selected. For instance, the temperature of extruder barrel 24 should be set high enough to melt the thermoplastic compound and to avoid weld lines or striations which may be seen in the product due to breaker plate 28. The temperature in extruder barrel 24 is also carefully controlled to prevent excessive torque. However, if the resulting temperature of the thermoplastic compound is too high, then reservoir 40 may be too short to ensure that the temperature across the cross-section of the compound is sufficiently uniform. The temperature of die 60 should be high enough to facilitate orientation, but low enough to substantially prevent the polymer molecules and filler particles (if present) from relaxing from their oriented state. The die temperature is typically set 0-10°C above the melting point (softening point) temperature for semi-crystalline polymers and 10-60°C above the glass transition temperature (i.e. above the softening point temperature) for amorphous polymers. The temperature of calibrator 100 is chosen such that the thermoplastic compound is cooled sufficiently in order to preserve the orientation and to substantially solidify the product. Therefore the temperature of the calibrator 100 must be below the softening point temperature of the thermoplastic compound. Referring again to FIGURE 1, the extrudate, upon exiting die assembly 14, can be routed through cooling bath 16 as illustrated in FIGURE 1 to further cool the extrudate in order to facilitate handling of the finished product. Cooling bath 16 may also be used to wash away any residual lubricant. After passing through cooling bath 16, a caterpillar type puller 18 may be used to pull the extrudate. The resulting tension can aid the extrusion process by preventing any deformation of the extrudate due to bending or sagging which might occur downstream of the die assembly. Finally, an in¬ line cut-off saw 20 can be used to cut the finished product to the desired length. APPARATUS AND METHOD FOR PRODUCING HOLLOW PRODUCTS FIGURE 3 shows a longitudinal cross-section of a die assembly 14' configured for producing hollow thermoplastic products with an annular profile using the process of the present invention. It is understood that it is possible to produce hollow products of any section (profile) having a cross-section with constant dimensions given that die assembly 14' is configured to produce the desired profile. The most significant difference between die assembly 14' illustrated in FIGURE 3 and die assembly 14 illustrated in FIGURE 2 is the presence of mandrel 46 which is necessary in order to produce hollow products. All other elements of die assemblies 14 and 14' are the same. Mandrel 46 is supported at its upstream end 49 by primary mandrel support section 42 which is located between adapter section 30 and reservoir 40. A cross-section of mandrel support section 42 is shown in FIGURE 4 and there are three supports 44 to which mandrel 46 is attached. The number and placement of supports 44 are chosen to provide adequate support for mandrel 46 without significantly disrupting the flow of the thermoplastic compound, while the shape of supports 44 is chosen to promote streamlined flow of the thermoplastic compound around supports 44. At the centre of one or more of supports 44 is provided a channel 50 through which lubricant can be injected to the interface between mandrel 46 and the thermoplastic compound. This may be done by a similar means as shown at 56 in FIGURE 1 and described above. It is understood that mandrel 46 and mandrel support section 42 may be constructed as one section or in several sections. Located between reservoir 40 and die 60 is a second mandrel support section 62. The cross-section of mandrel support section 62 is similar to that of mandrel support section 42 shown in FIGURE 4, but in this case mandrel 46 is not attached to support section 62 so that die assembly 14' may be readily disassembled for servicing. As before, the number and placement of the supports within mandrel support section 62 are chosen to provide adequate support for the mandrel without significantly disrupting the flow of the thermoplastic compound while the supports are shaped and dimensioned to ensure streamlined flow of the thermoplastic compound around the supports. A second mandrel support section may or may not be necessary depending on the size and length of the mandrel. All of the considerations made in designing die assembly 14 for producing solid products must also be made in designing die assembly 14' for producing hollow products. For instance, the length of reservoir 40 and calibrator(s) 100, the shape of die 60, the temperature settings of the die assembly 14' and extruder 12, etc., should be chosen in consideration of the resulting degree of orientation, throughput and surface appearance of the extrudate, as discussed above. In producing hollow products the shape/size and length of mandrel 46 are also of considerable importance. The cross- section of mandrel 46 in combination with the cross-section of die 60 and calibrator 100 will determine the shape as well as the wall thickness of the product. The length of mandrel 46 is chosen so as to provide dimensional stability to the inside perimeter of the resulting hollow profile. It may also be desirable to heat and/or cool the mandrel by any appropriate means known in the art. The processing conditions, including the temperature settings for the mandrel if heating and/or cooling is used, are selected to facilitate the orientation of the thermoplastic compound in the converging die and to subsequently preserve the imparted orientation and to substantially solidify the resulting hollow product. FIGURE 5 illustrates the cross-section of a circular hollow profile which can be produced by using die assembly 14' in apparatus 10 using the present process. The thickness of the wall of the hollow profile 124 will be determined by the design details of die assembly 14'. APPARATUS AND METHOD FOR PRODUCING INTEGRAL FOAM PRODUCTS Apparatus 10 illustrated in FIGURE 1 using die assembly 14' of FIGURE 3 may also be used to produce integral foam products with a circular profile having a solid oriented skin and a foamed core. It is understood that it is possible to produce integral foam products of any section having a cross-section with constant dimensions given that die assembly 14' is configured to produce the desired profile. In this case, extruder 12, which is used to melt and convey a thermoplastic compound containing a foaming agent through die assembly 14' illustrated in FIGURE 3, is also used to initiate the foaming action necessary to produce the foamed core by activating the foaming agents and by creating the necessary pressure to perform the process. The processing conditions in extruder 12 are chosen to melt the thermoplastic compound and to activate and substantially homogeneously distribute the foaming agents throughout the compound. It is understood that foaming agent in this case refers to either physical or chemical foaming agents or a combination thereof. The configuration of the die assembly needed to produce the integral foam products is similar to the configuration of die assembly 14' used to produce hollow products. As discussed above, lubricant is injected at the interfaces between mandrel 46 and inner wall 41 of reservoir 40 and the thermoplastic compound to promote elonga¬ tional flow and reduce shear flow through annular die 60. The difference in die assembly 14' in producing hollow products and integral foam products is a matter of the design and function of mandrel 46. In producing hollow products, the function of mandrel 46 is to define the inner perimeter of the resulting extrudate and is designed accordingly. In producing integral foam products, the function of mandrel 46 is to produce a cavity in the thermoplastic compound into which the compound can expand, in order to form the foam core of the integral foam product. Therefore the cross-section of mandrel 46 is chosen to facilitate the formation of a foam core which is substantially homogeneous in both density and cell size. Equally important is the thickness of gap 90 between the outer wall 92 of mandrel 46 and inner wall 94 of die 60. If gap 90 is too small and if the temperature of die 60 is sufficiently low, the thermoplastic compound may completely solidify in the gap before any foaming takes place, thereby resulting in a hollow product instead of an integral foam product. Too small a gap might also yield an undesirably high pressure drop across die 60 while too large a gap may not yield a sufficiently high pressure drop so as to cause substantially all of the foaming to occur beyond the end portion 47 of mandrel 46. The length of mandrel 46 is also important in determining when the foaming will take place, and as a result, its length will determine in part the thickness of the oriented solid skin, as well as the properties of the foam core. In order to achieve the desired skin thickness and degree of foaming, the length of mandrel 46 typically extends several inches downstream of the converging portion of die 60. In addition, die assembly 14' is designed to maintain the a sufficiently high pressure along die assembly 14' so that substantially all of the foaming will occur only beyond end portion 47 of mandrel 46. In this process, the main function of calibrator 100 is to maintain the dimensional stability of the product and to provide the cooling necessary to preserve the orientation of the polymer molecules and filler particles (if present) in the solid skin. Calibrator 100 also prevents the foaming action from deforming the profile. The length of calibrator 100 is chosen so that oriented skin is sufficiently solidified and that the foaming action is complete. The temperature profile of calibrator 100 is therefore important in determining the length of the calibrator. The temperature settings of extruder barrel 24 and die assembly 14' (adapter section 30, reservoir 40, mandrel 46, die 60 and calibrator 100) must be carefully selected in order to produce integral foam products. For instance, the temperature of extruder barrel 24 should be set high enough to melt the thermoplastic compound and to avoid weld lines or striations which may be seen in the material due to breaker plate 28. The temperature of die 60 should be high enough to facilitate orientation, but low enough to substantially prevent the oriented polymer molecules and filler particles (if present) in the outer portion of the forming product from relaxing from their oriented state. The temperature of calibrator 100 is chosen such that the thermoplastic compound is cooled sufficiently in order to produce the oriented solid skin without significantly inhibiting the formation of the foam core. In addition, it may be desirable to heat the end of mandrel 46 by any appropriate means known in the art in order to maintain the interior temperature of the thermoplastic compound in order to facilitate the formation of the foam core. Thus, the bulk temperature of the thermoplastic compound (which is determined by the temperature of extruder 12 and reservoir 40), the temperatures of die 60, mandrel 46 and calibrator 100 and the length of mandrel 46 in combination determine the thickness of the oriented solid skin of the extrudate. Similarly, the draw ratio, the profile of die 60, the amount and type of lubricant which is injected, the bulk temperature of the compound and the temperature of die 60 determine the degree of orientation in the solid skin. The density of the resulting foam core is determined by the composition of the thermoplastic compound, the concentration and type (physical and/or chemical) of foaming agent, the temperature of the material to be foamed, the length and shape of mandrel 46 and the thickness of the oriented solid skin. Referring now to FIGURE 6 there is illustrated the cross-section of a circular integral foam profile which may be produced by the present process. The product includes a foam core 120 and a solid outer surface portion or skin 122. As discussed above, the thickness of oriented solid skin 122, the degree of orientation of the thermoplastic polymer molecules and filler particles (if present) in the solid skin, the surface appearance of the product and finally the density and the cell size of the foam core 120 will depend on the design details of die assembly 14' (particularly die 60 and mandrel 46), the processing conditions, (i.e., the temperature profiles for the various components of extruder 12 and die assembly 14'), the type and amount of lubricant used and the throughput of the process, and finally the formulation of the thermoplastic compound including the choice of foaming agents. The overall density of the integral foam product will be determined by the thickness of solid skin 122 and the densities of solid skin 122 and foamed core 120. The properties of the integral foam products produced via the process of this invention are desirable in comparison to the properties ordinary integral foam products in several aspects. First, the integral foam products produced by the process of this invention have increased flexural strength and modulus (on the order of 2 to 10 times of those of ordinary integral foam products). These improvements are the result of the oriented polymer molecules and filler particles (if present) in the solid skin, thus, maximizing the effectiveness of the concept of an integral foam since the strength and stiffness of integral foam products is limited by the strength and stiffness of the solid skin surrounding the foam core. Second, the thicker skin of the integral foam products produced by the process of this invention allows them to be nailed or screwed without damage, unlike typical integral foam products which cannot take nails or screws because the solid skin is thin and the foamed core is brittle. Third, the integral foam products produced by the process of this invention may contain higher concentrations of filler (up to 80 percent by weight in certain cases) than typical integral foam products which contain very little filler (less than 30 percent by weight) if any fillers are used at all. In addition to the reduction in material costs, the increased filler content also contributes to the enhanced mechanical properties if reinforcing fillers are used. FORMULATION AND PREPARATION OF THE THERMOPLASTIC COMPOUNDS The polymer component of the thermoplastic compound may comprise thermo¬ plastic polymer being selected from the group consisting of polyolefins (polyethylenes, polypropylenes and copolymers thereof), vinyl chloride homopolymers and copolymers, styrenics (polystyrene, ABS and styrene/maleic anhydride copolymers), polyesters, poly- amides, polycarbonates and the like. Moreover, this process excludes thermosetting polymers such as phenolics, urea-formaldehyde resins, epoxy resins and the like. The process can be carried out with virgin or recycled (waste) thermoplastic polymers (plastics). While the process can be carried out with mixed (commingled) recycled plastics, the quality of the resulting extruded products will depend substantially on the composition of the recycled plastic supply, more specifically on the types and concentrations of different polymers. For economic reasons, granulated (chipped) plastics recovered from bottles or film (prior to pelletizing) are preferred since pelletizing can significantly increase the cost of the resulting material. In choosing between various grades of a particular polymer, the average molecular weight of a given grade can significantly affect the degree of orientation which is achieved with the process of this invention. Typically, for a given polymer, grades with lower average molecular weights will lose their orientation due to relaxation quicker than grades with higher average molecular weights. Thus, higher molecular weight grades of a given polymer are preferred in order to help preserve the greatest degree of the orientation imparted by the converging die. In addition, higher molecular weight grades can typically be extruded with higher elongational strain rates in the converging die and may therefore be processed at higher flowrates. However, the choice of molecular weight of the polymer may be limited depending on the ability of the latter to be mixed with a reinforcing filler. Therefore, a compromise is usually necessary between the degree of relaxation of the orientation, the processability and the ease of mixing. Therefore the highest molecular weight consistent with ease of mixing is normally preferred. The filler component of the thermoplastic compound may comprise materials which are typically used as a reinforcing filler. A filler is typically considered to be a reinforcing filler if the aspect ratio, which is defined as the ratio of the length to the effective diameter, is substantially greater than one. Inorganic reinforcing fillers include glass fibers, carbon fibers, metal fibers, ceramic fibers, asbestos, talc, mica and the like. Organic materials can also be used as reinforcing fillers. For instance, it is possible to use fibers made with a polymer with a much higher softening point than the polymer component of the thermoplastic compound, such as nylon or polyester fibers with poly¬ ethylene. The filler concentration may be varied, but mixing of the filler and polymer may become difficult at high filler concentrations (greater than 70% by weight) for various filler and polymer combinations. Cellulose fibers or particles comprising substantially cellulose fibers can also be used as reinforcing filler. However, cellulose is prone to decompose at temperatures above 220 degrees C, and therefore polymers which must be processed above this tem¬ perature are necessarily excluded if cellulose based fillers are used. Thus, a majority of the so-called engineering polymers cannot be employed in the process of the present invention with cellulosic fillers since their softening temperatures are too high. Cellulosic fillers may be derived from wood/forest by-products and wastes such as wood flour or ground wood, sawdust, ground paper (newspaper, magazine, cardboard), wood pulps (chemical, chem-mechanical, mechanical, bleached or unbleached) and agricultural by-products such as rice hulls, rice/wheat straws, corn husks, coconut husks, the shells of various nuts and the like having a substantial cellulose component. Techniques to produce fine, but free flowing cellulosic filler particles which can readily be used for the process of this invention are well known. Thus, the supply of cellulosic fillers is almost unlimited which offers economic advantages. For instance, short cellulose fibers may be usefully employed in the process of this invention that may otherwise be considered of no commercial value, for example fines from pulp mills. In addition, tests have shown that thermoplastic compound containing cellulosic fillers may be reground and re¬ processed numerous times without significant reduction in the mechanical properties of the resulting product. This remarkable durability is attributed to the extraordinary flexi¬ bility and toughness of the cellulose fibers which resist further breakage during reprocessing. In preparing the thermoplastic compound, it may be necessary to employ dispers¬ ing/coupling agents in order to disperse and compatibilize non-polar polymers with high¬ ly polar fillers such as cellulose fibers. These surfactants preferentially wet the surface of the filler particles (thereby, increasing the degree of dispersion) and provide increased adhesion (coupling) between the surface of the filler particles and the polymer. It has been found useful to employ carboxylated polyolefins as dispersing/coupling agents with polyolefin polymers. For example, maleated polyethylenes are effective dispersing agents when polyethylene is used, whereas maleated polypropylenes are more effective when polypropylene is used as the polymeric component of the thermoplastic compound. The quantity of dispersing agent required depends upon the surface area of the filler par- tides, and is usually 1 to 5 parts in a hundred of the thermoplastic compound by weight. The optimum amount is readily determined by experiment. Other compounds, such as fatty acids, titanates, zirconates, silanes and the like can also be used as compatibili- zing/dispersing agents. In order to produce a thermoplastic compound containing a reinforcing filler, a weighed quantity of filler is first admixed with an appropriate polymer, an appropriate compatibilizing/dispersing agent (if necessary) and any other additives such as colorants, stabilizers, flame retardants and the like which may be used to enhance the properties of the product. The mixture is then subjected to intensive mixing in a twin screw extruder, a thermokinetic mixer such as a Gelimat mixer (Draiswerke) or K-Mixer (Synergistics), or any similar type of mixing equipment. Thermokinetic mixers are particularly effective for the preparation of thermoplastic compounds with high filler contents, as they have been found to effectively disperse the filler in the compound. In addition, it is desirable to use thermokinetic mixers to prepare thermoplastic compounds containing cellulosic fillers. The high intensity mixing action of the thermokinetic mixers not only reduces the size of unacceptably large cellulosic particles, it can also separate loosely bonded cellulose fibers thereby increasing the reinforcing capacity of the cellulosic filler. When producing foamed products as with apparatus 10 in FIGURE 1, the choice of foaming agent will depend upon several factors including the extrusion condition, the type of resin and cost of the foaming agent. Foaming agents are generally categorized as either chemical or physical foaming agents. Chemical foaming agents decompose when heated to produce gasses which can then foam the thermoplastic compound. Com- mon chemical foaming agents include sodium bicarbonate which decomposes to produce carbon dioxide and azodicarbonamide which decomposes to produce nitrogen. Physical foaming agents are typically compounds which vaporize or are gaseous at the processing temperatures and can produce the necessary pressure to foam the thermoplastic com¬ pound. Water, carbon dioxide, nitrogen and chlorofluorocarbons are commonly used as physical foaming agents and may be mixed with the compound prior to extrusion or may be injected directly into the barrel of the extruder. While chemical foaming agents are typically more expensive than physical foaming agents, they generally produce finer cell structures in the foamed product which is often desirable. Consequently, a combination of chemical and physical foaming agents have been used to achieve the desired cell structure while reducing the cost of the foaming agent. When cellulosic fillers are used, water absorbed by the cellulose may be suffi¬ cient to impart the degree of foaming required by the process so that additional foaming agents need not be employed. For example, under normal circumstances wood fibers in equilibrium with air may contain up to 10 percent water depending upon the humidity of the air. Some of the water absorbed by the cellulose fibers will be vaporized during mixing, but a sufficient proportion may be retained to foam the thermoplastic compound. In addition, the thermoplastic compound can absorb water during the compounding process if, for example, an underwater pelletizer is used. However, the use of water as a foaming agent will be limited by the fact that polycondensation polymers, such as poly¬ esters and polycarbonates, depolymerize at processing temperature in the presence of just a few ppm of water. Thus, the use of water as foaming agent with these polymers is not recommended. EXAMPLES The following non-limiting examples will illustrate the method of manufacture with the process of this invention using the apparatus in FIGURES 1 to 3. Flexural properties were measured using the procedure of ASTM D-790. For products with circular profiles the jig was modified as in ASTM D-4476 to accommodate the curvature of the test specimens. The fracture toughness measurements followed the ASTM D-256 Izod test procedure. The properties of several substantially unoriented solid samples (with and without filler) which were prepared by injection molding are presented in Table 1 for comparison with the properties of the various oriented samples. The following notation is employed in the examples: Polymers HIPS High impact polystyrene, MI =13.5 MB Recycled HDPE milk bottles MC Recycled mixed colour HDPE MIPS Medium impact polystyrene, MI =19 PE1 Blow molding grade high density polyethylene, MI =0.4 PE2 Injection molding grade high density polyethylene, MI =5 PP Polypropylene, MI =0.8 PVC Polyvinylchloride, K value=58 Cellulosic Fillers CS Corn stalks DIN Deinked newsprint GC Ground cardboard GN Ground newsprint GWP Ground wood pulp RH Rice hulls SD Sawdust TMP Thermo-mechanical pulp WF Wood flour WS Wheat straw Example 1: This example describes the production of a circular solid polypropylene products 0.33 inches in diameter. (a) The thermoplastic compound used in this example comprised extrusion grade polypropylene (Profax 6631, MI = 1, Himont). 1 part per hundred by weight of silicone oil (Dow Corning 200, 12,500 cs, Dow Corning) was added to the compound to act as a lubricant. (b) The apparatus used in this example was a 0.75 inch single screw extruder (L/D ratio 24: 1) equipped with a die assembly with proper dimensions to produce a circular solid profile 0.33 inches in diameter. The reservoir and die were constructed as one section with an inlet and outlet diameters of 0.75 and 0.33 inches, respectively. The calibrator was approximately 7 inches long. The draw ratio of the die assembly was 5: 1. (c) The processing conditions were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 155, 170, 175°C. Settings for the die assembly temperature control zones (adapter, reservoir/die, calibrator): 165, 140, 90°C. Speed of extruder screw rotation: 8 RPM. (d) The circular solid samples produced had an outer diameter of 0.33 inches. The mechanical properties of the samples are presented in Table 2. Example 2: This example describes the production of a circular solid polystyrene products 0.33 inches in diameter. (a) The thermoplastic compound used in this example comprised extrusion grade polystyrene (Crystal Polystyrene 202, MI = 3.0, Huntsman Chemical Corp.). 1 part per hundred by weight of silicone oil (Dow Corning 200, 12,500 cs, Dow Coming) was added to the compound to act as a lubricant. (b) The apparatus used in this example was the same as used in Example 1. (c) The processing conditions were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 155, 180, 180°C. Settings for the die assembly temperature control zones (adapter, reservoir/die, calibrator): 160, 140, 90°C. Speed of extruder screw rotation: 6 RPM. (d) The circular solid samples produced had an outer diameter of 0.33 inches. The mechanical properties of the samples are presented in Table 2. Example 3: This example describes the production of a circular solid polyethylene products 1 inch in diameter. (a) The thermoplastic compound used in this example comprised blow molding grade high density polyethylene (Sclair 58A, MI =0.4, Du Pont). (b) The apparatus used in this example was a 2.5 inch single screw extruder (L/D ratio 24: 1) equipped with a die assembly with proper dimensions to produce a circular solid profile 1 inch in diameter. The reservoir was 2 inches in diameter and 12 inches in length. The calibrator consisted of two sections, each 10 inches long, with separate temperature control systems which provided air cooling. The draw ratio of the die assembly was 4:1. (c) The processing conditions were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 135, 140, 141, 142°C. Settings for the die assembly temperature control zones (adapter, reservoir, die, calibrators): 140, 138, 136, 125, 90°C. Speed of extruder screw rotation: 20 RPM. Lubrication: 200 cs silicone oil (Dow Coming) injected through two injection ports in the reservoir at a rate of 20 ml per hour. Production rate: 24 in/min. (d) The circular solid samples produced had an outer diameter of 1.0 inch. The mechanical properties of the samples are presented in Table 1. Example 4: This example describes the production of various circular solid cellulose filled polyethylene products 0.33 inch in diameter. (a) The thermoplastic compounds used in this example comprised blow molding grade high density polyethylene (Sclair 58A, MI=0.4, Du Pont), 4% by weight of the cellulosic filler of coupling agent (Fusabond MB 226D, Dupont), with a variety of cellulosic fillers for a range of filler concentrations. The cellulosic fillers used were: thermo-mechanical pulp (TMP), ground wood pulp (GWP), deinked newsprint (DIN), ground newsprint (GN), wood flour (WF), ground cardboard (GC), ground com stalks (CS), ground wheat straw (WS), and ground rice hulls (RH). Typically, 1 part per hundred by weight of silicone oil (Dow Coming 200, 12,500 cs, Dow Coming) was added to the compounds to act as a lubricant. The filler type and concentration for the various compounds is presented in Table 3. (b) The apparatus used in this example was the same as used in Example 1. (c) Representative processing conditions for all of the compounds were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 135, 165, 150°C. Settings for the die assembly temperature control zones (adapter, reservoir/die, calibrator): 145, 140, 120°C. Speed of extruder screw rotation: 20 RPM. (d) The circular solid samples produced had an outer diameter of 0.33 inches. The mechanical properties of the various samples are presented in Table 3. Example 5: This example describes the production of various circular solid cellulose filled polyethylene products 0.33 inch in diameter. (a) The thermoplastic compounds used in this example comprised 50 parts by weight of various grades of polyethylene, 4% by weight of the cellulosic filler maleated polyethylene (Fusabond MB 226D, Dupont) as a coupling agent, and 50 parts by weight of either thermo-mechanical pulp or deinked newsprint. The different polyethylenes used were: injection molding grade polyethylene (Sclair 2907, MI = 5, Du Pont), recycled high density polyethylene from milk bottles and recycled (post-consumer) mixed colour high density polyethylene. The polymer type as well as filler type and concentration for the various compounds is presented in Table 4. Typically, 1 part per hundred by weight of silicone oil (Dow Coming 200, 12,500 cs, Dow Coming) was added to the compounds to act as a lubricant. (b) The apparatus used in this example was the same as used in Example 1. (c) Representative processing conditions for all of the compounds were similar to those given in Example 4. (d) The circular solid samples produced had an outer diameter of 0.33 inches. The mechanical properties of the various samples are presented in Table 4. Example 6: This example describes the production of various circular solid cellulose filled polypropylene products 0.33 inch in diameter. (a) The thermoplastic compounds used in this example comprised 50 parts by weight of extrusion grade polypropylene (Profax 6631, MI = 1, Himont), 4% by weight of the cellulosic filler of maleated polypropylene (Epolene E-43, MW = 4500, Eastman Chemicals), and 50 parts of either thermo-mechanical pulp or deinked newsprint. The filler type and concentration for the various compounds is presented in Table 4. 1 part per hundred by weight of silicone oil (Dow Coming 200, 12,500 cs, Dow Coming) was added to the compounds to act as a lubricant. (b) The apparatus used in this example was the same as used in Example 1. (c) Representative processing conditions for all of the compounds were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 165, 190, 180°C. Settings for the die assembly temperature control zones (adapter, reservoir/die, calibrator): 180, 165, 130°C. Speed of extruder screw rotation: 20 RPM. (d) The circular solid samples produced had an outer diameter of 0.33 inches. The mechanical properties of the various samples are presented in Table 4. Example 7: This example describes the production of various circular solid cellulose filled polyvinyl chloride products 0.33 inch in diameter. (a) The thermoplastic compound used in this example comprised 70 parts by weight of of rigid polyvinyl chloride (K value = 58) containing suitable stabilizers and processing aids and 30 parts of wood flour. 1 part per hundred by weight of silicone oil (Dow Coming 200, 12,500 cs, Dow Coming) was added to the compound to act as a lubricant. (b) The apparatus used in this example was the same as used in Example 1. (c) Processing conditions for all of the compounds were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 140, 190, 180°C. Settings for the die assembly temperature control zones (adapter, reservoir/die, calibrator): 170, 150, 105 °C. Speed of extruder screw rotation: 6 RPM. (d) The circular solid samples produced had an outer diameter of 0.33 inches. The mechanical properties of the samples are presented in Table 4. Example 8: This example describes the production of various circular solid cellulose filled polystyrene products 0.33 inch in diameter. (a) The thermoplastic compounds used in this example comprised either medium impact polystyrene (MI = 19) or high impact polystyrene (13.5) as the polymer component, thermo-mechanical pulp or wood flour as the filler component and reactive polystyrene or styrene maleic anhydride at 4% by weight of the cellulosic filler. The polymer type as well as filler type and concentration for the various compounds is presented in Table 5. 1 part per hundred by weight of silicone oil (Dow Coming 200, 12,500 cs, Dow Coming) was added to the compounds to act as a lubricant. (b) The apparatus used in this example was the same as used in Example 1. (c) Representative processing conditions for all of the compounds were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 100, 160, 145°C. Settings for the die assembly temperature control zones (adapter, reservoir/die, calibrator): 130, 120, 110°C. Speed of extruder screw rotation: 6 RPM. (d) The circular solid samples produced had an outer diameter of 0.33 inches. The mechanical properties of the various samples are presented in Table 5. Example 9: This example describes the production of various circular solid mica filled polyethylene products 0.33 inch in diameter. (a) The thermoplastic compounds used in this example comprised blow molding grade high density polyethylene (Sclair 58A, MI=0.4, Du Pont), mica (Mica White 200, average particle size = 35 microns, L.V. Lomas), and ionomer resin (Surly 9950, MI = 5.5, Du Pont) as the compatibilizing agent. The filler and compatibilizer concentrations for the various compounds are presented in Table 6. 1 part per hundred by weight of silicone oil (Dow Coming 200, 12,500 cs, Dow Corning) was added to the compounds to act as a lubricant. (b) The apparatus used in this example was the same as used in Example 1. (c) Representative processing conditions for all of the compounds were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 135, 155, 155°C. Settings for the die assembly temperature control zones (adapter, reservoir/die, calibrator): 145, 140, 120°C. Speed of extruder screw rotation: 10 RPM. (d) The circular solid samples produced had an outer diameter of 0.33 inches. The mechanical properties of the various samples are presented in Table 6. Example 10: This example describes the production of a circular solid sawdust filled polyethylene products 1 inch in diameter. (a) The thermoplastic compound used in this example comprises 50 parts by weight high density polyethylene (Sclair 58A, MI=0.4, DuPont), 50 parts sawdust and 4 parts maleated polyethylene (Fusabond MB 226D, MI = 2, DuPont). (b) The same apparatus was used in this example as in Example 3. (c) The processing conditions were as follows: Settings for the extruder barrel temperature control zones (upstream end to downstream end): 140, 142, 145, 148°C. Settings for the die assembly temperature control zones (adapter, reservoir, die, calibrators): 144, 140, 136, 100, 50°C. Speed of extruder screw rotation: 14 RPM. Lubrication: 200 cs silicone oil (Dow Corning) injected through two injection ports in the reservoir at a rate of 1 ml per hour. Production Rate: 20 in/min. (d) The circular solid samples produced had an outer diameter of 1.0 inch. The mechanical properties of the samples are presented in Table 7. Example 11: This example describes the production of a circular hollow sawdust filled polyethylene products 1 inch in diameter. (a) The same thermoplastic compound was used in this example as in Example 10. (b) The apparatus used in this was a 2.5 in single screw extmder (L/D ratio 24: 1) equipped with a die assembly with proper dimensions to produce a circular hollow profile 1 inch in diameter. The reservoir was 2 inches in diameter and 12 inches long. The calibrator consisted of two sections, each 10 in long, with separate temperature con¬ trol systems which provided air cooling. The die assembly was equipped with a circular mandrel 0.7 inches in diameter which extended 8 inches past the end of the die. The draw ratio of the die assembly was 7:1. (c) The processing conditions were as follows: Settings for the extmder banel temperature control zones (upstream end to downstream end): 140, 145, 148, 151 °C. Settings for the die assembly temperature control zones (adapter, mandrel support, reservoir, die, calibrators): 149, 146, 142, 135, 124, 63°C Speed of extmder screw rotation: 6 RPM. Lubrication: 200 cs silicone oil (Dow Coming) injected through two injection ports in the reservoir at a rate of 2 ml per hour. Production Rate: 16 in/min. (d) The circular hollow samples produced had a outer diameter of 1 inch. The mechanical properties of the samples are shown in Table 7. Example 12: This example describes the production of a circular integral foam sawdust filled polyethylene products 1 inch in diameter. (a) The same thermoplastic compound was used in this example as in Example 10. A combination of water (moisture in the compound) and sodium bicarbonate was used as the foaming agent (approximately 2 parts in 100 by weight of water, 1 part sodium bicarbonate). (b) The apparatus used in this example was a 2.5 in single screw extmder (L/D ratio 24: 1) equipped with a die assembly with proper dimensions to produce a circular integral foam profile 1 inch in diameter. The reservoir was 2 inches in diameter and 12 inches long. The calibrator consisted of two sections, each 10 inches long, with separate temperature control systems which provided air cooling. The die assembly was equipped with a circular mandrel 0.7 inches in diameter which extended 2.5 inches beyond the end of die. The draw ratio of the die assembly was 7. (b) The processing conditions were as follows: Settings for the extmder banel temperature control zones (upstream end to downstream end): 135, 142, 144, 146°C. Settings for the die assembly temperature control zones (adapter, mandrel support, reservoir, die, calibrators): 140, 135, 124, 122, 100, 44°C Speed of extmder screw rotation: 15 RPM. Lubrication: 200 cs silicone oil (Dow Coming) injected through two injection ports in the reservoir at a rate of 1 ml per hour. Production Rate: 20 in/min. (c) The circular integral foam samples produced had an outer diameter of 1 inch and had a rigid, glossy skin and a cellular intemal stmcture. The mechanical properties of the samples are presented in Table 7. Example 13: This example describes the production of a rectangular solid sawdust filled polyethylene products 1.5 inches by 2.5 inches. (a) The same thermoplastic compound was used in this example as in Example 10. (b) The apparatus used in this example was a 2.5 in single screw extmder (L/D ratio 24: 1) equipped with a die assembly with proper dimensions to produce a solid rectang¬ ular profile 1.5 inches by 2.5 inches. The reservoir consisted of two sections, each 7.5 inches long with intemal dimensions of 3 inches by 5 inches, with separate temperature controls for the two sections. The calibrator also consisted of two sections, each 10 inches long, with separate temperature control systems which provided air cooling. The draw ratio of the die assembly was 4: 1. (c) The processing conditions were as follows: Settings for the extmder barrel temperature control zones (upstream end to downstream end): 140, 142, 143, 146°C. Settings for the die assembly temperature control zones (adapter, reservoirs, die, calibrators): 144, 142, 138, 136, 115, 80°C Speed of extmder screw rotation: 6 RPM. Lubrication: 200 cs silicone oil (Dow Corning) injected through two injection ports in the first reservoir at a rate of 16 ml/hr. Production Rate: 3.25 in/min. (d) The solid rectangular samples produced were 1.5 inches by 2.5 inches. The properties of this sample are presented in Table 8. Example 14: This example describes the production of a rectangular hollow sawdust filled polyethylene products 1.5 inches by 2.5 inches. (a) The same thermoplastic compound was used in this example as in Example 10. (b) The apparatus used in this example was a 2.5 inch single screw extmder (L/D ratio 24:1) equipped with a die assembly with proper dimensions to produce a rectang¬ ular profile 1.5 inches by 2.5 inches. The reservoir was 9.0 inches long and had intemal dimensions of 3 by 5 inches. The calibrator also consisted of two sections, each 10 inches long, with separate temperature control systems which provided air cooling. The die assembly was equipped with a rectangular mandrel with external dimensions of 0.96 inches by 1.96 inches which extended approximately 8 inches beyond the end of the die. The draw ratio of the die assembly was 7:1. (c) The processing conditions were as follows: Settings for the extmder banel temperature control zones (upstream end to downstream end): 139, 141, 142, 144°C. Settings for die assembly temperature control zones (adapter, reservoir, die, calibrators): 140, 127, 136, 100, 70°C Speed of extmder screw rotation: 8 RPM. Lubrication: 200 cs silicone oil (Dow Coming) injected through two injection ports in the first reservoir at 10 ml/hr. Production Rate: 4 in/min. (d) The hollow rectangular samples produced were 1.5 inches by 2.5 inches with a wall thickness of 0.27 in. The properties of these samples are presented in Table 8. Example 15: This example describes the production of rectangular integral foam sawdust filled polyethylene products 1.5 inches by 2.5 inches. (a) The same thermoplastic compound was used in this example as in Example 10. Water was used as a foaming agent, and consequently the moisture content of the thermoplastic compound was carefully adjusted to 4 parts per 100. (b) The apparatus used in this example was a 2.5 in single screw extmder (L/D ratio 24:1) equipped with a die assembly with proper dimensions to produce a rectangular integral foam profile 1.5 inches by 2.5 inches. The reservoir consisted of two sections, each 7.5 inches long with intemal dimensions of 3 by 5 inches, with separate temperature controls for the two sections. The calibrator also consisted of two sections, each 10 in long, with separate temperature control systems which provided air cooling. The die assembly was equipped with a rectangular mandrel with extemal dimensions of 0.96 inches by 1.96 inches which extended approximately 2 inches beyond the end of the die. The draw ratio of the die assembly was 7:1. (c) The representative processing conditions are as follows: Settings for the extmder barrel temperature control zones (upstream end to downstream end): 140, 141, 143, 143°C. Settings for the die assembly temperature control zones (adapter, reservoirs, die, calibrators): 143, 138, 135, 133, 110, 90°C. Speed of extmder screw rotation: 8 RPM. Lubrication: 200 cs silicone oil (Dow Coming) injected through two injection ports in the first reservoir at 24 ml/hour. Production Rate: 3.2 inch per minute. (d) The rectangular integral foam samples produced were 1.5 inches by 2.5 inches and had a rigid, glossy skin and a cellular intemal stmcture. The properties of these samples are shown in Table 8. Table 1. Mechanical Properties of Injection Molded Samples The thermoplastic compound comprises 50 parts by weight injection molding grade high density polyethylene, 50 parts thermo-mechanical pulp and 4 parts maleated polyethylene (Fusabond MB 226D, MI = 2, DuPont). The thermoplastic compound comprises 50 parts by weight polypropylene (Profax 6631, MI = 2, Himont), 50 parts thermo-mechanical pulp and 4 parts maleated polypropylene (Epolene E-43, MW=4500, Eastman Chemicals). Table 2. Flexural Modulus of Various Solid Unfilled Samples 1. 1.0 inch diameter sample. 2. 0.33 inch diameter sample. Table 3. Mechanical Properties of Solid Cellulose Filled Polyethylene Samples 1. All compounds contained 4 percent by weight (of the filler) of compatibilizing agent. Table 4. Mechanical Properties of Various Solid Cellulose Filled Samples Table 5. Mechanical Properties of Solid Cellulose Filled Polystyrene Samples 1. The thermoplastic compound contained 4% by weight (of the filler) of compatibilizing agent (when it was added). Table 6. Mechanical Properties of Solid Mica Filled Polyethylene Samples 1. The mica (Mica White 200, L. V. Lomas) had an average particle size of 35 microns and an aspect ratio of approximately 5. 2. Ionomer resin (Surlyn 9970, DuPont) was used as the compatibilizing agent. Table 7. Mechanical Properties of Various 1" Diameter Cellulose Filled Polyethylene Samples 1. The thermoplastic compound comprises 50 parts by weight high density polyethylene (Sclair 58A, MI=0.4, DuPont), 50 parts sawdust and 4 parts maleated polyethylene (Fusabond MB 226D, MI = 2, DuPont). 2. The density of material forming the solid wall of the profile. Table 8. Mechanical Properties of Various 1.5" x 2.5" Cellulose Filled Polyethylene Samples 1. The thermoplastic compound comprises 50 parts by weight high density polyethylene (Sclair 58 A, MI =0.4, DuPont), 50 parts sawdust and 4 parts maleated polyethylene (Fusabond MB 226D, MI = 2, DuPont). 2. The density of material forming the solid wall of the profile.

Claims: THEREFORE WHAT IS CLAIMED IS: 1. A process for continuous production of a thermoplastic product comprising a thermo¬ plastic compound having oriented components, comprising the steps of: (a) providing a thermoplastic compound comprising thermoplastic polymer; (b) bringing the thermoplastic compound to a temperature in a predetermined range just above and including the softening point temperature of the thermoplastic compound thereby producing molten thermoplastic compound; (c) forcing the molten thermoplastic compound through a die, the thermoplastic compound undergoing converging flow through the die to impart longitudinal orientation to at least some of the thermoplastic polymer; and (d) cooling the thermoplastic compound after imparting longitudinal orientation to a temperature below the softening point temperature of the thermoplastic compound to preserve the imparted orientation therein and to solidify the thermoplastic compound. 2. A process according to claim 1 wherein the thermoplastic compound undergoes converging flow through the die which imparts a decreasing elongational strain rate with the elongational strain rate not exceeding the strain rate associated with the onset of melt fracture. 3. A process according to claim 1 wherein the thermoplastic compound undergoes converging flow through the die which imparts a substantially constant elongational strain rate with the elongational strain rate not exceeding the strain rate associated with the onset of melt fracture. 4. A process according to claim 1 wherein the thermoplastic compound includes a filler intimately mixed therewith. 5. A process according to claim 4 wherein the filler has an aspect ratio greater than unity. 6. A process according to claim 5 wherein the converging flow through the die imparts longitudinal orientation to at least some of the filler. 7. A process according to claim 4 wherein the filler is selected from the group consis¬ ting of cellulosic fibers, cellulosic particles, mica flakes, talc flakes, asbestos fibers, glass fibers, metal fibers and carbon fibers. 8. A process according to claim 4 wherein the thermoplastic compound has up to 75% filler by weight. 9. A process according to claim 4 wherein the thermoplastic compound undergoes converging flow through a die which imparts a decreasing elongational strain rate with the elongational strain rate not exceeding the strain rate associated with the onset of melt fracture. 10. A process according to claim 4 wherein the thermoplastic compound undergoes converging flow through a converging die which imparts a substantially constant elonga¬ tional strain rate with the elongational strain rate not exceeding the strain rate associated with the onset of melt fracture. 11. A process according to claim 1 including forming an interior cavity within the thermoplastic compound as it passes through said die to produce a hollow profile and whereby during cooling the hollow profile is solidified. 12. A process according to claim 4 including forming an interior cavity within the thermoplastic compound as it passes through said die to produce a hollow profile and whereby during cooling the hollow profile is solidified. 13. A process for continuous production of a thermoplastic product comprising a thermoplastic compound having oriented components and a foamed core, comprising the steps of: (a) providing a thermoplastic compound comprising thermoplastic polymer and foaming agent; (b) bringing the thermoplastic compound to a temperature in a predetermined range just above and including the softening point temperature of the thermoplastic compound thereby producing a molten thermoplastic compound; (c) forcing the molten thermoplastic compound through a die and forming an interior cavity therein, the thermoplastic compound undergoing converging flow through the die to impart longitudinal orientation to at least some of the thermoplastic polymer; (d) preserving the imparted longitudinal orientation in the outer layer of the thermoplastic compound thereby forming an oriented skin; (e) the thermoplastic compound being foamed into the cavity; and (f) cooling the thermoplastic compound below the softening point temperature of the thermoplastic compound to solidify the thermoplastic compound. 14. A process according to claim 13 wherein the thermoplastic compound undergoes converging flow through the die which imparts a decreasing elongational strain rate with the elongational strain rate not exceeding the strain rate associated with the onset of melt fracture. 15. A process according to claim 13 wherein the thermoplastic compound undergoes converging flow through the die which imparts a substantially constant elongational strain rate with the elongational strain rate not exceeding the strain rate associated with the onset of melt fracture. 16. A process according to claim 13 wherein the thermoplastic compound includes a filler intimately mixed therewith. 17. A process according to claim 16 wherein the filler has an aspect ratio greater than unity. 18. A process according to claim 17 wherein the converging flow through the die imparts longitudinal orientation to at least some of the filler. 19. A process according to claim 16 wherein the filler is selected from the group consisting of cellulosic fibers, cellulosic particles, mica flakes, talc flakes, asbestos fibers, glass fibers, metal fibers and carbon fibers. 20. A process according to claim 16 wherein the thermoplastic compound has up to 75% filler by weight. 21. A process according to claim 16 wherein the thermoplastic compound undergoes converging flow through the die which imparts a decreasing elongational strain rate with the elongational strain rate not exceeding the strain rate associated with the onset of melt fracture. 22. A process according to claim 16 wherein the thermoplastic compound undergoes undergoes converging flow through the die which imparts a substantially constant elongational strain rate with the elongational strain rate not exceeding the strain rate associated with the onset of melt fracture. 23. A device for manufacturing thermoplastic product having oriented components from a thermoplastic compound comprising thermoplastic polymer, comprising; (a) means for bringing the thermoplastic compound to a temperature in a predetermined range just above and including the softening point temperature of the ther¬ moplastic compound; (b) a die downstream of (a) having intemal walls, an inlet, an outlet and a passage communicating between the inlet and outlet, the area of the inlet being greater than that of the outlet, a portion of the intemal walls converging and defining a converging passageway wherein in use the thermoplastic compound undergoes converging flow through the die to impart longitudinal orientation to at least some of the thermoplastic polymer; (c) forcing means for forcing the thermoplastic compound through the die; and (d) cooling means downstream of the converging passageway for cooling the thermoplastic compound. 24. A device according to claim 23 wherein the die has a draw ratio defined as the ratio of the cross-sectional area of the inlet and the outlet and wherein the draw ratio is between 3:1 and 15: 1. 25. A device according to claim 24 wherein the shape of the converging passageway has a substantially hyperbolic profile. 26. A device according to claim 23 wherein the forcing means comprises a standard screw type extmder. 27. A device according to claim 23 wherein (a) is a reservoir adjacent to the die. 28. A device according to claim 23 wherein the cooling means comprises a calibrator having an intemal wall defining a channel, the calibrator positioned adjacent to the die, the intemal wall being configured to have the same cross-sectional profile as the outlet of the die. 29. A device according to claim 23 wherein the die further comprises a mandrel positioned therein and extending through the converging passageway wherein in use the mandrel causes a cavity to be formed in the thermoplastic compound. 30. A device according to claim 29 wherein the distance the mandrel extends downstream of the converging passageway is chosen so that the thermoplastic compound is sufficiently cooled before it reaches the end of the mandrel so that the cavity formed therein is preserved. 31. A device according to claim 29 wherein the distance the mandrel extends downstream of the converging passageway is chosen so that the longitudinal orientation of at least some of the thermoplastic polymer is preserved in the outer layer of the thermoplastic compound and that the thermoplastic compound foams into the cavity to form a solid profile with an integral foamed core. 32. A device according to claim 29 wherein the die has a draw ratio defined as the ratio of the cross-sectional area of the inlet and the outlet and wherein the draw ratio is between 3:1 and 15: 1. 33. A device according to claim 32 wherein the shape of the converging passageway has a substantially hyperbolic profile. 34. A device according to claim 29 wherein the forcing means comprises a standard screw type extmder. 35. A device according to claim 29 wherein (a) is a reservoir adjacent to the die. 36. A device according to claim 29 wherein the cooling means comprises a calibrator having an intemal wall defining a channel, the calibrator positioned adjacent to the die, the intemal wall being configured to have the same cross-sectional profile as the outlet of the die. 37. A thermoplastic product produced by the process of claim 1 wherein at least some of the thermoplastic polymer has longitudinal orientation imparted thereto. 38. A thermoplastic product produced by the process of claim 5 wherein at least some of the thermoplastic polymer and filler of the thermoplastic compound have longitudinal orientation imparted thereto. 39. A thermoplastic product comprising a thermoplastic compound comprising ther¬ moplastic polymer and having a rigid, solid outer layer with longitudinally oriented thermoplastic polymer and an integral low density foamed interior. 40. A thermoplastic product according to claim 39 wherein the thermoplastic com¬ pound includes a filler intimately mixed therewith, the filler located in the solid outer layer being longitudinally oriented with the longitudinally oriented thermoplastic polymer. 41. A thermoplastic product according to claim 40 wherein the filler has an aspect ratio greater than unity. 42. A thermoplastic product according to claim 41 wherein the filler is selected from the group consisting of cellulosic fibers, cellulosic particles, mica flakes, talc flakes, asbestos fibers, glass fibers, metal fibers and carbon fibers. 43. A thermoplastic product according to claim 41 wherein the thermoplastic compound has up to 75% filler by weight. 44. A thermoplastic product according to claim 39 wherein the thermoplastic polymer is chosen from the group consisting of polyethylene, polypropylene, polystyrene, polyvinyl and ethylene propylene copolymer. 45. A thermoplastic product according to claim 41 wherein the thermoplastic polymer is chosen from the group consisting of polyethylene, polypropylene, polystyrene, polyvinyl and ethylene propylene copolymer.

Inventors: PABEDINSKAS ARUNAS A, SUWANDA DEDO, WOODHAMS RAYMOND T, ZHOU VINCENT W, PABEDINSKAS, ARUNAS, A., SUWANDA, DEDO, WOODHAMS, RAYMOND, T., ZHOU, VINCENT, W.
Applicants: SRP IND LTD, SRP INDUSTRIES LTD.
Application year: 1993
Main classification: B29C47
IPC codes: B29C47, B29C55, B29K23, B29K105