Method of separating polymers from mixtures thereof

Abstract: A method of separating at least two immiscible thermoplastic polymer components from a liquid mixture thereof, wherein a first of the at least two immiscible thermoplastic polymer components has a first melt viscosity and a second of the at least two immiscible thermoplastic polymer components has a second melt viscosity, wherein the second melt viscosity is higher than the first melt viscosity, wherein the method involves flowing the liquid mixture through a shear zone, wherein the liquid mixture is subjected to shear in the shear zone at a shear rate and a shear temperature sufficient to separate the at least two immiscible thermoplastic polymer components into substantially discrete and continuous phases, wherein a first of the phases substantially contains the thermoplastic polymer component having the first melt viscosity and a second of the phases substantially contains the thermoplastic polymer component having the second melt viscosity. Also provided are methods of recycling a liquid polymer mixture using the separation method.

Description: This invention relates to a method of separating immiscible polymers from mixtures thereof. More particularly, this invention relates to a method of separating immiscible polymers from mixtures thereof based on melt viscosity differences between the polymers,and further to the use of said method in recycling mixed polymer waste. Recycling waste into useful materials is a growing goal in modern society, particularly in the plastics and fibers industries. Landfills are becoming filled to capacity and new sites are hard to find because of zoning regulations and neighborhood concerns. Recycling waste is also important because of the growing global depletion of raw materials needed to make fresh material. Polymer waste, often made from petroleum products, is a fertile area for recycling. Man-made polymers generally do not degrade quickly and petroleum will eventually be depleted. Thus far, mixed polymer waste, particularly mixed polymer waste containing immiscible polymers, have posed unique recycling problems. For example, while nylon and PET individually can be readily recycled, mixtures of nylon and PET are not easily recycled as even blends of virgin nylon and PET blends do not readily spin into yarn. Immiscible polymer blends, such as most nylon 6/PET blends, exhibit multiple melting points and form liquid dispersions which appear as a heterogenous mixture when viewed under a microscope. Such immiscible mixtures often have poor adhesion between the phases. The morphology of these dispersions can present certain challenges in forming the mixture into articles such as fibers as the morphology adversely affects the spinning and drawing processes. For example, some dispersions, e.g., nylon/PET liquid dispersions, display a morphology where one polymer forms globules that are surrounded by the other polymer. The size and shape of these globules affect the properties of the final yarn. Excessively large globules can make fiber spinning virtually impossible and, even if spinning is achieved, can make drawing the fibers quite difficult. In general, the globules must be significantly smaller than the spinneret capillary diameter for the mixture to flow in a nearly homogenous manner. In the best case, the spun fiber will contain long fibrils that deform along with the matrix polymer. In the worst case, the blend will contain large globules that are rigid during drawing. Methods of recycling polymer blends are known and are discussed, for example, in Clemson University Professional Development Seminar, "Thermoplastic Waste Reclamation", February 9-10, 1993; Kaminsky, W., "Recycling of Polymeric Materials by Pyrolysis", Makromol. Chem., Macromol.Symp. 48/49, pp. 381-393 (1991); and Chemical Week, "Honing Technology to Improve Economics", December 18/25, 1991. "Plastic lumber" extruded from a mixture of post-consumer polymers is known, but such usage is limited and does not solve the problem of excessive amounts of plastic waste. Incineration or "thermal recycling" produces energy from polymer mixtures but at the same time destroys the resource and may present environmental and health hazards. Because recycling polymer mixtures according to current methods is both difficult and expensive, it would be desirable to provide a mixed polymer recycling method that is less expensive and avoids the problems encountered in the current methods. Because recycling the individual polymers is easier than recycling the mixture of the polymers, it would be further desirable to provide a method of recycling the mixture that involves recycling the individual polymers. Recycling the individual polymers in a polymer mixture would involve separating the polymers from the mixture. Current polymer separation processes usually involve mechanical separation, chemical separation or a mixture of these approaches. However, these processes are difficult and expensive to operate. Mechanical separation of polymer solids involves size and density-based techniques using instruments such as cyclones and screen classifiers. These instruments require solids to be physically distinct particles. Thus, solid polymer mixtures must be ground to sizes that will readily break into the individual polymer component particles. While there are instruments which can do this, grinding polymer mixtures to the individual polymer particles is difficult and generally expensive, often requiring cryogenic processes. It would be desirable, therefore, to provide a method of recycling polymer mixtures which involves separation of the individual polymers from the mixtures, wherein the polymer separation does not require grinding the polymer mixture into particles of the individual polymers. One drawback to many chemical separation processes is the use of significant amounts of organic solvent, which is undesirable for environmental reasons. Methods of chemically separating nylon and PET from mixtures thereof are disclosed, for example, in U.S. patent Nos. 5,241,066 and 5,264,536. Therefore, it is desirable to provide a method of recycling polymer mixtures which involves separation of the individual polymers, wherein polymer separation does not require significant amounts of organic solvent. Current extrusion processes use a single extruder. In these processes, extruding a two component blend would require a second extruder and, in the case of fiber spinning, modification of the spinning beam, to accommodate two polymer flows. It would be desirable to provide a more economical two-component extrusion process which would require only a single extruder and, in fiber spinning processes, no modification of the spinning beam. An object of this invention is to provide a less expensive method of separating immiscible polymers from a mixture thereof. Another object of this invention is to provide a method of separating immiscible polymers from a mixture thereof wherein separation does not require the use of significant amounts of organic solvent. A further object of this invention is to use the separation method described in the foregoing objects to recycle mixed polymers. A still further object of this invention is to provide a method of recycling immiscible polymer mixtures which takes advantage of the ease associated with recycling individual polymers. Another object of this invention is to provide a method of making bicomponent articles, such as sheath/core bicomponent fibers, which requires the use of only one extruder. These and other objects which are achieved according to the present invention can be readily discerned from the following description. The present invention is based in part on the discovery that immiscible polymers can be separated from mixtures thereof by means of their melt viscosity differences. This invention is also based in part on the discovery that the above polymer separation method can be used in recycling mixed polymer waste. This invention is also based on the surprising discovery that factors which previously have been considered obstacles to recycling polymer mixtures, e.g., heterogenous polymer dispersions and poor adhesion between the polymer phases, can actually facilitate the substantial separation of the polymers and the recycling of polymer mixtures under proper conditions. Accordingly, one aspect of this invention is directed to a method of separating at least two immiscible thermoplastic polymer components from a liquid mixture thereof, wherein a first of said at least two immiscible thermoplastic polymer components has a first melt viscosity and a second of said at least two immiscible thermoplastic polymer components has a second melt viscosity, wherein said second melt viscosity is higher than said first melt viscosity, wherein said method comprises flowing said liquid mixture through a shear zone, wherein said liquid mixture is subjected to shear in said shear zone at a shear rate and a shear temperature sufficient to separate said at least two immiscible thermoplastic polymer components into substantially discrete and continuous phases, wherein a first of said phases substantially comprises the thermoplastic polymer component having the first melt viscosity and a second of said phases substantially comprises the thermoplastic polymer component having the second melt viscosity. A second aspect of the present invention is directed to a method of recycling a liquid polymer mixture comprising at least two immiscible thermoplastic polymers with different melt viscosity values, comprising subjecting the mixture to the separation method described above and forming articles from either the individual polymers or separated mixture thereof. A method of recycling polymer mixtures that involves separating polymers from mixtures thereof as described above has the added advantage of imparting uniformity to the final product. The method of this invention also provides the advantage of requiring only one extruder to form bicomponent articles, such as bicomponent sheath/core fibers. Two or more immiscible polymers are fed into a single extruder to form a mixture, and the resulting mixture is passed through the shear zone. Mechanical separation of the polymers is not required because the polymers are spun directly from the shear zone. Another benefit provided by the method of this invention is that it allows for the use of a single extruder in a process wherein two or more immiscible polymers are mixed, separated and then recombined to form an article, e.g., a fiber, with a desirable cross-section. FIG. 1 is a schematic illustration showing a useful shear zone in the form of a capillary used in the present invention. FIG. 2 represents a perspective, cut-away view of the capillary represented in FIG. 1 showing a means useful to separate the individual polymers from each other. FIGS. 3-5 represent cross-sections of a two-polymer mixture at various stages of the shearing process in the capillary represented in FIG. 1. FIG. 6 is a schematic representation of the devices used in the method of the present invention, including a polymer separation detection device and a second polymer metering pump. FIG. 7 is a schematic representation of an optical microscopic photograph taken of the cross- section of a monofilament sample prepared according to the method of this invention, wherein FIG. 7 shows separation into a sheath/core structure. FIG. 8 is a schematic representation of an optical microscopic photograph taken of the cross- section of a monofilament sample prepared according to the method of this invention, wherein FIG. 8 shows failure of separation, resulting in a mixed structure. FIG. 9 is a schematic representation of an optical microscopic photograph taken of the croSS- sect ion of a monofilament sample prepared according to the method of this invention, wherein FIG. 9 shows separation into a ring structure. FIG. 10 is a schematic representation of an optical microscopic photograph taken of the cross-section of a monofilament sample prepared according to the method of this invention, wherein FIG. 10 shows separation into a partial sheath/core structure and a partial side-by-side structure. In the method of this invention, at least two immiscible thermoplastic polymer components having different melt viscosity values are separated from a liquid mixture thereof. The term "immiscible" as used herein means that the polymer components are insoluble or substantially insoluble with one another. The term "polymer component" as used herein refers to one or more polymers of the same class having similar melt viscosities. The liquid mixture is passed through a shear zone at a shear rate and a shear temperature sufficient to separate the polymer components having different melt viscosity values into substantially discrete and continuous phases. The formation of the phases is the result of the shearing action, which generally causes one polymer component to move away from the central region of the shear zone and the other polymer component to move toward the central region of the shear zone. Preferably, in the method of this invention, the shear temperature and the shear rate are sufficient to separate the immiscible thermoplastic polymer components into a sheath/core configuration having a core phase and a sheath phase, the sheath phase surrounding the core phase. Most preferably, the core phase substantially contains the thermoplastic polymer component having the second melt viscosity and the sheath phase substantially contains the thermoplastic polymer component having the first melt viscosity. Exiting the shear zone are two or more output streams each substantially containing one of the discrete phases or a single output stream containing substantially uniform separated phases. The term "substantially uniform" as used herein is meant to convey that each polymer in the mixture is contained in a separate continuous phase as opposed to a plurality of discrete islands in a matrix. Most preferably, the liquid mixture subjected to shear in this invention contains two immiscible thermoplastic polymers having different melt viscosity values. Generally, the more immiscible the polymers and the greater the difference in their melt viscosity values, the easier it is to separate the polymers according to the method of this invention. Thermoplastic polymer mixtures which can be sheared according to the method of this invention include, for example, those polymer mixtures found in scraps resulting from fiber, yarn, fabric and plastics processing procedures, discarded consumer products, and other waste materials which are desired to be recycled. Classes of thermoplastic polymers suitable for separation in the method of this invention include those useful in fiber spinning processes. Such polymers include, for example, polyamides, polyesters, and polyolefins. Non-limiting examples of polymer mixtures which can be separated include, e.g., polyamide/polyester mixtures, polyamide/polyolefin mixtures, and polyester/polyolefin mixtures. Polycaproamide/polyethylene mixtures are readily separated in the practice of the method of this invention. Other particular polymer mixtures which can be separated according to the method of this invention include, for example, polyamide/polyethylene terephthalate such as polycaproamide/polyethylene terephthalate, polyamide/polypropylene such as polycaproamide/polypropylene, polyamide/polyethylene such as polyamide/high density polyethylene and polycaproamide/high density polyethylene; polyethylene terephthalate/polyethylene and polyethylene terephthalate/polypropylene mixtures. Such mixtures are not intended to limit the present invention as it is believed that any immiscible thermoplastic polymer mixture having melt viscosity differences can be separated according to the present invention. Preferably, in the method of this invention, the thermoplastic polymer component having the first melt viscosity is high density polyethylene and the thermoplastic polymer component having the second melt viscosity is polycaproamide. The weight ratio of each polymer component contained in the liquid mixture to be separated can vary widely from 99:1 to 1:99. The mixture to be separated is in a liquid state prior to passing through the shear zone. The term "liquid" as used herein refers to a molten state or a molten state in admixture with a small amount of solvent so as to increase the fluidity of a portion of the polymer mixture. Preferably, the mixture is in a purely molten state with no added solvent. Typically, the molten mixture is prepared by means of extrusion. For example, a molten polymer mixture can be prepared by feeding the mixture, typically in the form of a finely divided or granulated mixed solid, to a single screw extruder with a metering section or pump. The extruder can be any conventional laboratory-scale or full-scale screw extruder commonly used in processing thermoplastic polymers, e.g., in mixing, homogenizing, extruding, granulating, spinning and similar operations. An example of a suitable laboratory-scale extruder is an extruder having a screw diameter of one inch and depth of channel in the metering section of 0.031 inch. An example of a suitable full-scale extruder is one having a screw diameter of 3.5 inches and a metering section channel depth of 0.0938 inch. In the extruder, the mixture is subjected to a temperature sufficient to melt all components of the mixture (i.e., melting temperature). For nylon/polyester, nylon/polyolefin, and polyester/polyolefin mixtures, melting temperatures ranging from about 265°C to about 300°C have been found useful. In the method of this invention, the extruded liquid mixture is passed through a shear zone, preferably in a laminar flow. Turbulence in the flow of the mixture through the shear zone should be avoided. The Reynolds value (a measure of the turbulence in the flow) of the flow is preferably less than about 4000, more preferably less than about 3000, and most preferably less than about 2000. To minimize or prevent turbulence in the flow, the inner wall of the shear zone should be smooth. The shear temperature preferably ranges from about 250°C to about 300°C, and more preferably from about 270°C to about 285°C. The shear rate preferably ranges from about 200 l/sec to about 1100 l/sec, more preferably from about 200 l/sec to about 700 l/sec, and most preferably from about 300 l/sec to about 500 l/sec. The immiscible thermoplastic polymers in the mixture preferably have a melt viscosity ratio of from about 2:1 to about 7:1, more preferably from about 2:1 to about 3:1, and most preferably from about 2.3:1 to about 2.7:1. Preferably, in the method of this invention, the thermoplastic polymer component having the first melt viscosity is high density polyethylene and the thermoplastic polymer component having the second melt viscosity is polycaproamide, the melt viscosity ratio of said second melt viscosity to said first melt viscosity ranges from about 2:1 to about 7:1, more preferably from about 2:1 to about 3:1, most preferably from about 2.3:1 to about 2.7:1, the liquid mixture is in a molten state, the shear temperature ranges from about 250°C to about 300°C, more preferably from about 270 to about 285°C, the shear rate ranges from about 200 l/sec to about 1100 l/sec, more preferably from about 200 l/sec to about 700 l/sec, and most preferably from about 300 l/sec to about 500 l/sec, the shear zone is a capillary in laminar fashion at a Reynolds value of less than about 4000, more preferably less than about 3000, and most preferably less than about 2000. The shear zone preferably comprises a passage containing a liquid-contacting "shear contact region". The term "shear contact region" as used herein refers to a region in the shear zone which is in substantial contact with the mixture as the mixture passes through the shear zone, wherein the contact between the shear contact region and the passing mixture induces shear in the shear zone. The shear contact region may be, for example, a fluid or a solid surface. More preferably, the shear zone comprises a hollow orifice having an inner wall which functions as the shear contact region. Most preferably, the shear zone is a capillary, the inner wall of which acts as the shear contact region. As stated previously herein, the flow of the polymer mixture through the shear zone is preferably laminar and non-turbulent. The shear properties of the shear zone depend on the length, diameter and cross-section shape thereof. The term "diameter" as used herein refers to the diameter of a round cross-section or the average diameter of a non-round cross-section. The term "average diameter" as used herein refers to the average of the total diameter values in a non- round cross-section. High shear properties are typically achieved by making the shear zone long and narrow. Preferably, the shear zone will have a length-to-diameter ("L/D") ratio of from about 30:1 to about 100:1, and preferably of about 100:1. The shear zone can have either a round or a non-round cross-section. Non-round cross-sections impart higher shear to the shear zone than do round cross- sections because the non-round cross-sections have greater surface area for contact with the mixture as the mixture passes through the shear zone. The greater the surface area in contact with the mixture as the mixture passes through the shear zone, the greater the shear. Examples of suitable non-round shapes include slot shapes, curved shapes, triangular shapes, square shapes, crescent shapes, cross shapes, star shapes, Y shapes, and the like. The end of each shear zone preferably has a small gap and a narrow opening roughly the diameter of the center concentration of the polymer mixture. Moreover, the shear zone need not necessarily be hollow as other solid surfaces over which the mixture to be separated flows can cause shear and eventual separation of the polymer phases. Thus, flat, roughened flat, corrugated, etc. surfaces can be used. Again, the degree of shear will depend on length of the shear zone and surface area of the solid surface. Furthermore, turbulence in the flow should be avoided. Parameters which appear to be particularly important to the separation of the polymer phases in accordance with the method of this invention include the miscibility of the polymers; the melt viscosities of the polymer components; the percent volume of the polymer components; the shear rate (the linear velocity gradient through the shear zone, in reciprocal seconds, assuming viscosity is constant); the shear temperature; and the length of time for which the mixture undergoes shear. The melt viscosity differences between the polymers at a particular temperature can be characterized as the differences in the "coefficient of friction" between the polymers. The greater the viscosity difference, the higher will be the differences in the coefficient of friction. With higher coefficient of friction differenceS between the polymers, lower shear rates will generally be effective to separate the polymers. With lower coefficient of friction differenceS, higher shear rates are generally required to separate the polymers. The length of the shear zone will depend on the coefficient of friction differences and on the shear rate used in the shear zone. Assuming shear rate is the same, a longer shear zone is usually required to separate polymers with a lower coefficient of friction difference than is required to separate polymers having a higher coefficient of friction difference. However, for a given coefficient of friction difference, higher shear rates will usually allow the use of shorter shear zones since the increase in shear rate will generally compensate for the corresponding decrease in shear zone length. It is believed that one or both of two mechanisms are at work in producing the sheath/core configuration of the polymer mixture as the mixture passes through the shear zone in the method of this invention. These mechanisms are referred to herein as the "viscosity separation mechanism" and the "continuous/disperse separation" mechanism. The viscosity separation mechanism comes into play at high shear rates and large viscosity differences and results in the lower viscosity polymer encapsulating the higher viscosity polymer. The viscosity separation mechanism is described in greater detail in J.L. White and B. Lee, Trans. Soc. Rheology, 19:3, 457-479 (1975), which is hereby incorporated by reference herein. The continuous/disperse separation mechanism occurs at low viscosity differences and lower shear rates, where the polymers are highly immiscible and the higher viscosity polymer forms the continuous phase and the lower viscosity polymer forms the disperse phase. The higher viscosity polymer will form the continuous phase if the polymer is predominant in the mixture, i.e., has a volume of greater than 50%. The disperse phase collects into a single domain due to the immiscibility of the polymers. The driving force for the disperse/continuous separation mechanism is to reduce the area of contact, i.e., the interface, between the two polymer phases. The component with the lower volume in the mixture is the "dispersed" phase. The dispersed phase should be in the center of the flow because that geometry reduces the area of contact. In spite of the forces to push the disperse phase to the outside, the force to minimize the interface between the phases is greater. on the other hand, if the volume of the dispersed phase in the mixture is greater than 50%, the dispersed phase would move toward the outside. Thus, whether a high shear rate or a low shear rate should be used to separate the polymers in a mixture according to the method of this invention will depend on the particular mixture. The examples hereinbelow show the most complete separation of a polymer mixture containing 80% by weight nylon 6 and 20% by weight high density polyethylene is achieved by means of the disperse/continuous separation mechanism, which uses low viscosity differences and low shear rates. The viscosity separation mechanism, which uses high viscosity differences and high shear rates, results in incomplete separation of the polymers and unstable configurations such as the ring geometry, wherein a nylon 6 surrounds and is incorporated into the center of a ring of HDPE. Although the melt viscosity values of the polymers are important to the method of this invention, the relative viscosity values (e.g., in the case of polycaproamides and polyesters) or the melt index values (e.g., in the case of polyolefins) are sometimes easier to measure and can be frequently relied on herein instead of melt viscosity values to determine the parameters of shear necessary for separation. Moreover, the relative viscosity of a polymer is also a measure of the polymer's molecular weight and, therefore, differences in molecular weight can be used to determine shear parameters. In general, the discrete and continuous phases can exit the shear zone as either (a) a single output stream containing both of the phases; (b) two or more divided output streams, wherein at least one divided output stream substantially contains the first phase and at least one divided output stream substantially contains the second phase; or (c) four or more divided output streams, wherein at least two divided output streams substantially contains the first phase and at least two divided output streams substantially contains the second phase. Preferably, (a) said single output stream; (b) said two or more divided output streams; or (c) said four or more divided output streams can be recycled, comprising forming a filament or article therefrom. If desired, any output stream containing the first and second polymers in a sheath/core configuration can be manipulated to cause the core polymer and the sheath polymer to switch positions, i.e., the core polymer becomes the sheath polymer and the sheath polymer becomes the core polymer. The method of this invention may also include the steps of passing the single output stream (a), at least one of the two or more divided output streams (b), or at least one of the four or more divided output streams through one or more additional shear zones at a shear rate and a shear temperature sufficient to increase the concentration of the thermoplastic polymer component having the first melt viscosity in the first phase and to increase the concentration of the thermoplastic polymer component having the second melt viscosity in the second phase. In addition, the method of this invention may also include (i) combining the at least one divided output stream substantially containing the first phase and the at least one divided output stream substantially containing the second phase to form a mixture thereof; (ii) combining the at least two divided output streams substantially containing the first phase to form a mixture thereof; and/or (iii) combining the at least two divided output streams substantially containing the second phase to form a mixture thereof. The output stream or streams can be recycled in various ways. For example, the polymer stream(s) may be directed into a spinneret and spun into thermoplastic fibers or filaments; extruded into a die and cut into resin chips; undergo continuous extrusion to form plastic parts; or undergo a combination of the foregoing methods. It is to be understood that the ways in which the polymer stream(s) can be recycled are not limited to the foregoing recited methods. Filaments may be formed by directing the output stream(s) or output stream mixture(s) described above into a spinneret to form fibers and yarns. Plastic articles may be formed by extruding the output stream(s) or output stream mixture(s) described above into a die and cutting the extruded stream into resin chips, or by subjecting the output stream(s) or output stream mixture(s) to continuous extrusion or injection molding and the like to form plastic articles. one or more output streams may be fiber- spun to form bicomponent or multicomponent fibers of various structures including but not limited to sheath/core fibers, side-by-side fibers, islands-in-sea fibers, and very fine fibers such as microfibers. A broad embodiment of a method of forming a sheath/core bicomponent fiber within the scope of this invention, comprises: (A) extruding in a single extruder a liquid mixture comprising at least two immiscible thermoplastic polymer components, wherein a first of said at least two immiscible thermoplastic polymer components has a first melt viscosity and a second of said at least two immiscible thermoplastic polymer components has a second melt viscosity, wherein said second melt viscosity is higher than said first melt viscosity; (B) passing said extruded liquid mixture through a shear zone, wherein said liquid mixture is subjected to shear in said shear zone at a shear rate and a shear temperature sufficient to separate said at least two immiscible thermoplastic polymer components into substantially discrete and continuous phases, wherein a first of said phases substantially comprises the thermoplastic polymer component having the first melt viscosity and a second of said phases substantially comprises the thermoplastic polymer component having the second melt viscosity; (C) removing said discrete and continuous phases from the shear zone as (a) a single output stream comprising both of said phases; (b) two or more divided output streams, wherein at least one divided output stream substantially comprises the first phase and at least one divided output stream substantially comprises the second phase; or (c) four or more divided output streams, wherein at least two divided output streams substantially comprises the first phase and at least two divided output streams substantially comprises the second phase; and (D) spinning said (a) single output stream, (b) said two or more divided output streams, or said (c) four or more divided output streams, to form said sheath/core bicomponent fiber. A particularly useful method of fiber spinning two or more output streams to form bicomponent or multicomponent fibers is disclosed in commonly assigned U.S. Patent No. 5,162,074 to Hills. Briefly, Hills teaches a method of making plural component fibers wherein a plurality of separate dissimilar molten/solution polymer components are flowed in a structure containing several parts, including a spinneret plate and at least one distributor plate. Each distributor plate has an upstream surface and a downstream surface, either or both surfaces having multiple distribution flow paths etched thereon. Each separate component is distributed through the multiple distribution flow paths to an array of inlet holes for multiple spinneret orifices in the spinneret plate. Each component flows into each inlet hole to provide in each spinneret orifice a combined flow containing each of the plural components. Through the spinneret orifices, fibers are issued from the structure. The use of the etched distributor plate can allow various intricate and complex arrangements of the polymer components to be formed in the multicomponent fiber product in addition to the conventional side-by-side arrangements. In a preferable embodiment the spinning is carried out in a spinning apparatus comprising a spinneret plate and at least one distributor plate, wherein each distributor plate has an upstream surface and a downstream surface, either or both surfaces having multiple distribution flow paths etched thereon. The separation method of this invention can be further understood by reference to FIGS. 1-5. In these figures, a molten thermoplastic immiscible polymer mixture containing a first polymer 1 and a second polymer 2 is fed through entry zone 3 into capillary 4 and passed therethrough. During passage through the capillary 4, the mixture contacts the inner wall 5 of the capillary and undergoes shear in accordance with this invention. The shear causes the mixture to separate into discrete and continuous phases 6 and 7, wherein phase 6 substantially contains the first polymer 1 and phase 7 substantially contains the second polymer 2. Entry zone 3 of the capillary may have either a conical profile or a hyperbolic profile. Preferably, the entry zone will have a hyperbolic profile, because such a profile provides a uniform rate of increase in shear. As shown in FIG. 1, phase 6 is concentrated in central section 8 of capillary 4 and phase 7 is concentrated near inner wall 5 of the capillary. Whether central section 8 contains the higher melt viscosity polymer or the lower melt viscosity polymer appears to vary according to the particular polymer mixture used. For example, separation of nylon and high density polyethylene from nylon/high density polyethylene blends carried out according to the method of this invention generally results in the nylon migrating toward central section 8 and the high density polyethylene migrating toward inner wall 5. An output stream 9 substantially containing phases 6 and 7 exits capillary 4 via outlet 10 and, optionally, can be directed to a distribution plate 11 or spun directly to form a fiber. As can be seen in FIG. 2, plate 11 includes a central hollow orifice 12, which has a smaller diameter than outlet 10 of capillary 4, and sidestream distribution channels 13 and 14. Plate 11 divides the output stream 9 into a central core stream 15 and an outer sheath stream which can be removed as sidestreams 16 and 17 in sidestream distribution channels 13 and 14. The size of orifice 12 of distribution plate 11 will determine the concentration of the separated polymer streams. For example, a smaller orifice causes central core stream 15 to contain a higher concentration, e.g., 100%, of polymer 1 and a lower or zero concentration of polymer 2. A larger orifice 12 will result in central core stream 15 containing an increasing portion of polymer 2. Central core stream 15 and the outer sheath stream such as through sidestreams 16 and 17 may each be directed into additional capillaries (not shown) for further separation of polymer 1 and polymer 2. It is to be understood that although FIG. 1 depicts removal of the polymer mixture from the shear zone at the last stage, i.e., the stage represented at line 20-20, of the process, the mixture can be removed at earlier stages, i.e., stages represented at lines 18-18 and 19-19, respectively, of the process if one of the polymers is present as a discrete, and-continuous phase containing none or insubstantial amounts of the second polymer. The discrete polymer phase can be removed from the mixture, and the remaining portion of the mixture can be subjected to further shearing. Furthermore, a distribution plate, such as plate 11, can be used at an earlier stage of the shear zone if a discrete and continuous phase has been formed by that stage and is desired to be removed from the mixture. FIGS. 3-5 respectively depict crosssections of the polymer mixture at an early stage shown at line 18-18, an intermediate stage shown at line 19-19 and the final stage shown at line 20-20 of the shearing process in the capillary 4. FIG. 3 shows the cross-section obtained at the early stage shown at line 18-18, wherein the first polymer 1 is present as relatively small islands in a sea of the second polymer 2. FIG. 4 shows the cross-section obtained at the intermediate stage shown at line 19-19, wherein the first polymer 1 is present as large globules in a matrix of the second polymer 2. FIG. 5 shows the cross-section obtained at the final stage shown at line 20-20, wherein the first polymer 1 is present as a core surrounded by a sheath of the second polymer 2. The method of this invention preferably includes the steps of using a polymer separation detection device or sensor on one or more output streams, and using a second polymer metering pump in contact with the sensor through a speed control device to control the flow rate of the output streams. The use of such a sensor and second polymer metering pump is illustrated in FIG. 6. In FIG. 6, polymers fed into an extruder 30 through a hopper are extruded to form a molten polymer mixture. After extrusion, the polymer mixture passes through a first polymer metering pump 31, which controls the flow rate of the polymer mixture. The mixture enters assembly 32, usually a spin pack assembly, which may contain a filtration system typically made up of wire screens, sand, shattered metal, or sintered metal. The assembly 32 further contains a shear zone preferably made up of one or more capillaries. The polymer mixture undergoes shear in the shear zone to form separate phases (preferably sheath/core phases) in accordance with the method of this invention. Output stream 33 and output stream 35 may exit the shear zone. The sheath polymer and the core polymer can be in either output stream. A sensor 34 may be in contact with one of the output streams, e.g., output stream 35. An additional sensor (not shown) is preferably also in contact with the other output stream. The sensor 34 can be connected to a speed control device 36 which in turn is connected to a second polymer metering pump 37. The second polymer metering pump 37 controls the flow rate of the core and sheath polymer streams. The sensor 34 detects the amount of each polymer in an output stream, e.g., output stream 35. If the composition of the output stream needs to be adjusted, the sensor signals the speed control device 36 to cause the second polymer metering pump 37 to increase or decrease, as needed, the flow rates of the polymer streams. output stream 38 should then be made up of substantially either the sheath or the core polymer. For example, if a mixed polymer stream containing 60 grams of sheath-forming polymer and 40 grams of core-forming polymer flows into the shear zone at a flow rate of 100 grams/minute (which is achieved by means of the first polymer metering pump 31), and the core output stream (e.g., output stream 35) is detected by the sensor as containing 40 grams of core polymer and 5 grams of sheath polymer for a flow rate of 45 grams/minute, the second polymer metering pump 37 will decrease the flow rate of the core output stream to 40 grams/minute which in turn will cause the sheath output stream to increase by 5 grams per minute (the 5 additional grams being the 5 grams of sheath polymer originally in the core output stream). The sum of the flow rates of the output streams will equal the flow rate of the polymer mixture into the shear zone. Thus, the use of the sensor and second polymer metering pump provides control of the composition of the output streams and thereby ensures that on-line quality control of the degree of separation is possible. With the sensor and second polymer metering pump, different separation geometries (e.g., channel size or pressure drop experienced by the polymer components) will not be required for different ratios of viscosity and component ratios. In other words, without the sensor and second pump, the amount of each polymer in the output streams will depend on the rheology of the polymer mixture and the conditions within the shear zone. The use of a sensor and a second polymer metering pump to control output stream composition is less time-consuming, more flexible and less expensive. The sensor can be an infrared or near-infrared spectrometer. To determine output stream composition, the sensor may measure color, capacitance, pressure drop, infrared spectrum, or other like properties of the output stream. Preferably, in the method of this invention, wherein a detection device is in communication with a speed control device, and said speed control device is in communication with a polymer metring pump which controls flow rate of one or more output streams, (1) said detection device (a) measures amounts of the first of at least two immiscible thermoplastic polymer components and of the second of said at least two immiscible thermoplastic polymer components in (i) a single output stream, (ii) one of two or more divided output streams, or (iii) one of four or more divided output streams and (b) signals said speed control device to signal said polymer metering pump to increase or decrease the flow rate of the one or more output streams to an extent needed to increase or decrease said amounts to desired levels; (2) said speed control device signals said polymer metering pump to increase or decrease said flow rate to said extent; and (3) said polymer metering pump increases or decreases said flow rate to said extent. The present invention will be described in more detail by way of the following examples but it should not be construed to be limited thereto. EXAMPLES 1-32 Examples 1-32 illustrate the effect of various parameters on the degree to which immiscible polymers in a mixture will separate into a sheath/core structure. The following terms used in the examples and tables below are defined as follows: "N6" - nylon 6 "HDPE" - high density polyethylene "RV" - relative viscosity "MV" - melt viscosity "Sep. Desc." - separation description "S/C" - sheath/core structure, wherein the core is nylon 6 and the sheath is HDPE, see FIG. 7 for illustration "M" - mixed, see FIG. 8 for illustration "R" - ring structure - see PIG. 9 for illustration "p" - structure which is partially sheath/core and partially side-by-side in nature, see FIG. 10 for illustration The HDPE/nylon 6 mixture samples used in Examples 1-32 were obtained as follows. Two mixtures of high density polyethylene (HDPE) and nylon 6 were prepared by blending HDPE (24 grams, 20% by weight) and dry nylon chips (96 grams, 80% by weight) in a clean, clear plastic bag. The nylon 6 had a relative viscosity (RV) of about 2.7 in one mixture and a relative viscosity of about 3.3 in the other mixture. The HDPE in both mixtures had a melt flow index of about 31.5. Sixteen samples were taken from each mixture, for a total of 32 samples. Each sample was placed in a glass bottle (one bottle per rheometer experiment) and dried in a vacuum oven at 100°F (37°C) for 24 hours. A piston rheometer having an L/D ratio of 30:1 and a 0.5mm die was set to a temperature of 270°C or 285°C. After the rheometer reached the set temperature, the dry polymer mixture samples were placed therein and allowed to melt for two minutes longer than usual. The samples were then extruded in the rheometer at the shear rates recited in Tables I-IV hereinbelow. Fibers were directly collected from several of the extruded samples and optical microscopic photographs were taken of the fiber cross-sections. Schematic illustrations of the photographs taken of most of the fiber-cross sections obtained in the examples are presented in FIGS. 7-10. The relative viscosity of the nylon 6 was 2.7 in Examples 1-16 and 3.3 in Examples 17-32. Examples 1-8 and 17-24 used a shear temperature of 270°C, while Examples 9-16 and 25-32 used a shear temperature of 285°C. Tables I-IV below further set forth other parameters used in the examples. In addition, Tables I-IV recite the figure which represents the schematic illustration of fiber cross-section photograph obtained in the corresponding example. Examples 1-8: N6-RV:2.7 270°C Ex. # Shear Rate (l/sec) MV N6 (Pa-Sec) MV HDPE (Pa-Sec) MVR N6:HDPE MV N6/HDPE Mixture Sep. Des. Fig. # 1 200 270 102 2.65 93.96 S/c 7 2 300 269 101 2.66 105.75 s/c 7 3 500 251 95 2.64 96.3 S/c 7 4 700 235 89 2.64 85.52 S/c 7 5 900 217 83 2.61 75.75 P 10 6 1100 205 78 2.63 66.97 P 10 7 1300 197 74 2.66 58.59 P 10 8 2000 180 68 2.65 49.71 P 10 Examples 9-16: N6-RV:2.7: 285°C Ex. # Shear Rate (l/sec) MV N6 (Pa-Sec) MV HDPE (Pa-Sec) MVR N6:HDPE MV N6/HDPE Mixture Sep. Des. Fig. # 9 200 225 96 2.34 80.05 S/c 7 10 300 228 92 2.48 74.09 sic 7 11 500 221 83 2.66 59.94 sic 7 12 700 214 79 2.71 57.46 S/c 7 13 900 202 73 2.77 70.97 S/c 7 14 1100 194 64 3.03 64.23 S/c 7 15 1300 187 62 3.02 58.03 P 10 16 2000 165 61 2.70 44.7 P 10 Examples 17-24: N6-RV:3.3 270°C Ex. # Shear Rate (l/sec) MV N6 (Pa-Sec) MV HDPE (Pa-Sec) MVR N6:HDPE MV N6/HDPE Mixture Sep. Des. Fig. # 17 200 780 102 7.65 131.34 P 10 18 300 703 101 6.96 121.86 S/c 7 19 500 595 95 6.26 89.41 R 9 20 700 529 89 5.94 82.54 S/c 7 21 900 475 83 5.72 71.18 R 9 22 1100 424 78 5.44 67.35 R 9 23 1300 412 74 5.57 54.09 R 9 24 2000 369 68 5.72 47.99 R,s/c - Examples 25-32: N6-RV:3.3 285°C Ex. # Shear Rate (l/sec) MV N6 (Pa-Sec) MV HDPE (Pa-Sec) MVR N6:HDPE MV N6/HDPE Mixture Sep. Des. Fig. # 25 200 553 96 5.76 159.1 M 8 26 300 472 92 5.13 131.44 s/c 7 27 500 353 83 4.25 82.68 P 10 28 700 303 79 3.84 75.83 P,R - 29 900 280 73 3.84 65.84 R 9 30 1100 276 64 4.31 61.57 P,R - 31 1300 251 62 4.05 49.93 R 9 32 2000 242 61 3.97 42.25 P,R - The results shown in Tables I-IV and in FIGS. 7-10 indicate that the most complete polymer separation to form bicomponent fiber sheath/core structures occurs at low melt viscosity values and low shear rates. Higher melt viscosity values and higher shear rates tend to result in incomplete separation as evidenced by fiber cross-sections having a partial sheath/core, ring, or mixed structure.

Claims: A method of separating at least two immiscible thermoplastic polymer components from a liquid mixture thereof, wherein a first of said at least two immiscible thermoplastic polymer components has a first melt viscosity and a second of said at least two immiscible thermoplastic polymer components has a second melt viscosity, wherein said second melt viscosity is higher than said first melt viscosity, wherein said method comprises flowing said liquid mixture through a shear zone, wherein said liquid mixture is subjected to shear in said shear zone at a shear rate and a shear temperature sufficient to separate said at least two immiscible thermoplastic polymer components into substantially discrete and continuous phases, wherein a first of said phases substantially comprises the thermoplastic polymer component having the first melt viscosity and a second of said phases substantially comprises the thermoplastic polymer component having the second melt viscosity. A method according to claim 1, wherein the shear temperature and the shear rate are sufficient to separate said at least two immiscible thermoplastic polymer components into a sheath/core configuration having a core phase and a sheath phase, said sheath phase surrounding said core phase. A method according to claim 2, wherein the core phase substantially comprises the thermoplastic polymer component having the second melt viscosity and said sheath phase substantially comprises the thermoplastic polymer component having the first melt viscosity. A method according to claims 1 to 5, wherein the shear zone comprises a passage containing a liquid-contacting shear contact region. A method according to claim 4, wherein said shear zone comprises a hollow orifice having an inner wall, wherein said inner wall is said liquid-contacting shear contact region. A method according to claim 1, wherein the liquid mixture flows through said shear zone in a laminar fashion. A method according to claim 1, wherein said liquid mixture is in a molten state. A method according to claim 1, wherein the at least two immiscible thermoplastic polymer components are each selected from the group consisting of polyamides, polyesters, and polyolefins. A method according to claim 1, wherein said discrete and continuous phases exit the shear zone as (a) a single output stream comprising both of said phases; (b) two or more divided output streams, wherein at least one divided output stream substantially comprises the first phase and at least one divided output stream substantially comprises the second phase; or (c) four or more divided output streams, wherein at least two divided output streams substantially comprises the first phase and at least two divided output streams substantially comprises the second phase. A method according to claim 9, further comprising recycling said (a) said single output stream; (b) said two or more divided output streams; or (c) said four or more divided output streams, comprising forming a filament or article therefrom. A method of forming a sheath/core bicomponent fiber, comprising: (A) extruding in a single extruder a liquid mixture comprising at least two immiscible thermoplastic polymer components, wherein a first of said at least two immiscible thermoplastic polymer components has a first melt viscosity and a second of said at least two immiscible thermoplastic polymer components has a second melt viscosity, wherein said second melt viscosity is higher than said first melt viscosity (B) passing said extruded liquid mixture through a shear zone, wherein said liquid mixture is subjected to shear in said shear zone at a shear rate and a shear temperature sufficient to separate said at least two immiscible thermoplastic polymer components into substantially discrete and continuous phases, wherein a first of said phases substantially comprises the thermoplastic polymer component having the first melt viscosity and a second of said phases substantially comprises the thermoplastic polymer component having the second melt viscosity (C) removing said discrete and continuous phases from the shear zone as (a) a single output stream comprising both of said phases; (b) two or more divided output streams, wherein at least one divided output stream substantially comprises the first phase and at least one divided output stream substantially comprises the second phase; or (c) four or more divided output streams, wherein at least two divided output streams substantially comprises the first phase and at least two divided output streams substantiallY comprises the second phase; and (D) spinning said (a) single output stream, (b) said two or more divided output streams, or said (c) four or more divided output streams, to form said sheath/core bicomponent fiber. A method according to claim 11, wherein spinning is carried out in spinning apparatus comprising a spinneret plate and at least one distributor plate, wherein each distributor plate has an upstream surface and a downstream surface, either or both surfaces having multiple distribution flow paths etched thereon.

Inventors: DAVIS MICHAEL A, DUGAN JEFFREY S, HELMS CHARLES F JR, HOYT MATTHEW B, DAVIS, MICHAEL A., DUGAN, JEFFREY S., HELMS, CHARLES F., JR., HOYT, MATTHEW B.
Applicants: BASF CORP, BASF CORPORATION
Application year: 1995
Main classification: C08F6
IPC codes: B29C47, B29B17, C08F6, B29B13