Abstract:
Positive temperature coefficient polymer (PTC) materials of high loading with uniformly distributed electrically conductive material are manufactured by first producing an intimate mixture (pre-mix or master batch) of a crystalline polymer (e. g. high density polyethylene) with an electrically conductive filler (e. g. carbon black) in proportions that is less rich in the filler than the final PTC product. The intimate mixture, pre-mix or master batch, is utilized in a subsequent step in a molten liquid form for admixing with more filler material in quantities to produce the desired loading of the filler in the polymer matrix. The resulting the admixture is extruded, shaped and formed by conventional equipment to provide a positive temperature coefficient polymer composition that, when equipped with electrodes, is suitable for forming positive temperature coefficient circuit protecting devices. Positive temperature coefficient polymer composition having electrically conductive filler, usually carbon black in ratios as high as 55 to 60 percent by weight can be obtained by the process.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit, under 35 U.S.C. Section 119(e), of Provisional Application Ser. No. 60/101,892; filed Sep. 25,1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to manufacturing methods for preparing positive temperature coefficient polymer compositions. More particularly, the present invention relates to a manufacturing method for preparing positive temperature coefficient polymer compositions of high loading and uniform distribution of filler. 
     2. Brief Description of the Prior Art 
     Positive temperature coefficient (PTC) polymer compositions or materials are well known in the art, and have found wide-spread use as PTC devices in numerous electrical and electronic apparatus. The primary function of the PTC devices incorporating these polymer compositions is to prevent overloading and/or overheating of an electronic circuit. PTC devices are capable of serving this function because of the characteristic of the PTC polymer compositions to abruptly increase their electrical (ohmic) resistance with a relatively small rise in temperature within a predetermined (tripping) temperature range. Thus the PTC devices act as a type of fuse that trips when the circuit in which it is incorporated in series draws more than a predetermined amperage, and allow normal operation again after the cause for increased amperage in the circuit has ceased to exist and the heat generated in the PTC device by the excess current is dissipated. 
     The positive temperature coefficient polymers incorporated in the PTC devices typically comprise a crystalline polymer matrix in which an electrically conductive material (filler) is uniformly distributed. A typical and frequently used polymer for the matrix is high density polyethylene (HDPE), and a typical and frequently used electrically conductive material for the filler is carbon black. The PTC polymer materials typically include a high loading of the filler, for example a typical composition may contain approximately 45 percent by weight of carbon black. 
     A detailed description of the principles of operation of PTC devices, materials and methods of manufacturing the same, can be found in published PCT application WO 97/06660 (published on Feb. 27, 1997) assigned to the same assignee as the present application, and in published PCT application WO 97/39461 (published on Oct. 23, 1997). 
     Several problems encountered in the prior art in connection with the manufacturing of the PTC polymers relate to or are caused by the high loading of the polymer matrix with the carbon black or other filler. Briefly summarized, the high loading renders it difficult to consistently achieve uniform distribution of the filler, and therefore difficult to achieve uniform resistivity. The high loading also results in relatively high wear and tear of the machinery utilized for manufacturing the PTC polymer material, causes frequent jamming and difficult clean-up after jamming of the machinery. 
     The present invention provides an improved process for manufacturing the PTC polymeric materials. The manufacturing process of the present invention results in more consistently uniform distribution of the filler in the polymer matrix, reduces wear and tear in the machinery, eases the task of cleaning-up the machinery, and allows for manufacturing PTC materials with higher loading of conductive filler than was consistently possible in the prior art. 
     SUMMARY OF THE INVENTION 
     Positive temperature coefficient polymer materials are manufactured in accordance with the present invention by first producing an intimate mixture (pre-mix or master batch) of a crystalline polymer, such as high density polyethylene, with an electrically conductive filler, such as carbon black, in proportions that is less rich in the filler than the final PTC product. The intimate mixture, pre-mix or master batch, is utilized in a subsequent step in a molten liquid form for admixing with more filler material in quantities to produce the desired loading of the filler in the polymer matrix. Thereafter, the admixture is extruded, shaped and formed by conventional equipment to provide a positive temperature coefficient polymer composition that, when equipped with electrodes, is suitable for forming positive temperature coefficient circuit protecting devices. 
     Advantages of the two-step admixing process of the present invention include the ability to provide uniform distribution of the filler in the polymer matrix, reduced wear and tear in the machinery that performs the second admixing with the filler and extrusion, and ability to attain a higher percentage of loading with the filler than what was normally achievable in the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow-chart showing the steps performed in the preferred embodiment of the manufacturing process of the present invention. 
     FIG. 2 is a schematic representation of the preferred mode of performing a second admixing and extrusion step in accordance with the present invention. 
     FIG. 3 is a side view of a laminated positive temperature coefficient composition produced in the process of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A number of polymers, copolymers and mixtures of polymers are known in the art as suitable for the manufacture of positive temperature coefficient (PTC) compositions, and are described for example in the publications WO 97/39461 (see pages 17-18) and WO/970660 (see page 14). Although in the preferred embodiment of the process of the present invention high density polyethylene (HDPE) is used, the process of the present invention is applicable for the manufacture of positive temperature coefficient (PTC) compositions with any one of the polymers, copolymers or polymer mixtures which are otherwise known in the art to be suitable for this purpose. Another example for a preferred polymeric material used in the process of the present invention is polyvinylidene fluoride (PVDF). The descriptive portions and drawings of the publications WO 97/39461 and WO/970660 are expressly incorporated herein by reference. 
     Conductive filler components suitable for the manufacture of positive temperature coefficient (PTC) compositions are also well known in the art, and are also described in the publications WO 97/39461 (see page 18) and WO/970660. Generally speaking, the art prefers the use of carbon black (C.B.) as the filler material, and carbon black is also the preferred electrically conductive filler used in the process of the present invention. 
     Generally speaking, the objective of the present invention is to attain a consistently uniform (and preferably also high) loading of the filler (carbon black) in the polymer (preferably HDPE) matrix, with minimum wear-and-tear on the machinery that manufactures the positive temperature coefficient (PTC) compositions. The process of the invention that accomplishes these objectives is hereinafter described with reference to the drawing figures and in connection with the use of high density polyethylene (HDPE) and carbon black. Nevertheless, it should be kept in mind that other polymers and fillers known in the art, as noted above, can also be used in accordance with the present invention. The process of the invention is capable of producing positive temperature coefficient (PTC) compositions with a loading of carbon black (or other filler) as high as approximately 55 to 60 percent by weight, a more typical and presently preferable loading in accordance with the process is 50 to 55 percent by weight. 
     FIG. 1 schematically shows the step wherein the HDPE material is ground to provide particles typically in the range of 0.05 to 0.5 mm, preferably in the range of 0.1 to 0.2 mm. The grinding of the HDPE material can be performed by conventional techniques, utilizing conventional machinery suitable for this purpose. In the presently preferred embodiment of the invention the grinding of the polymer particles is performed in a conventional pellet grinder. 
     In the next step of performing the process of the invention, desired quantities of the ground HDPE and carbon black are measured by weight and admixed, so as to provide an admixture of predetermined desired proportions of HDPE and carbon black. The carbon black utilized for this purpose is commercially available. Typically, carbon black utilized for the manufacture of PTC compositions has a particle size of 20 to 100 nm, the carbon black presently utilized in the preferred embodiment of the process of the invention is in the particle size range of 50 to 100 nm. The weighing of the two components, HDPE and carbon black and their mixing or blending can be performed batch-wise, or in a continuous process. The mixing or blending step is indicated in FIG. 1 as “dry blending”, signifying that the first mixing of these components is at ambient temperature, without melting of the polymer material. When the weighing and blending is performed in a continuous process, then the components may be fed into a mixing apparatus (not shown) by feeder hoppers, (not shown) conventional in the art. 
     The “dry blended” admixture of HDPE and carbon black is thereafter extruded and pelletized, to provide a “pre-mix” or “master batch” (M.B.) of intimately admixed HDPE and carbon black material. The extrusion and pelletizing of this pre-mix or master batch (M.B.) can be performed in machinery which per se is conventional in the art. Presently, the steps of extrusion and pelletizing of this pre-mix are preferably performed in a co-rotation twin screw compounding extruder having a diameter of 46 mm, and a length to diameter (L/D) ratio of approximately 40. The pelletizing machinery is preferably equipped with double vent ports and a water bath is preferably employed in the pelletizing step. 
     Generally speaking, the pre-mix or master batch of HDPE and carbon black contains approximately 25 to 45 percent (by weight) of carbon black; presently it is preferred to produce a pre-mix or master batch that has approximately 30 to 35 percent by weight of carbon black. 
     The pre-mix or master batch admixture of HDPE and carbon black may be stored for substantial time periods without absorbing moisture, and is therefore better suited for storage than carbon black which is hygroscopic and absorbs moisture. Absorbed moisture is known to adversely affect the electrical characteristics of the PTC devices containing carbon black. Because the pre-mix or master batch admixture of HDPE and carbon black can be stored for substantial time, it can be manufactured well in advance before it is utilized in the subsequent steps of the process of the present invention, and can be manufactured at location different than where the subsequent steps are performed. The precise composition of the pre-mix or master batch can be deduced from the quantities of materials added and can also be analyzed by conventional methods. 
     In the next step of the process of the invention, as is schematically shown in the drawing figures, the pre-mix or master batch is fed into a compounding extruder  10  through its main feed hopper which in this example is a single screw gravitational feeder  12 , as shown in the more detailed schematic drawing of FIG.  2 . Carbon black, in the amount calculated to provide the desired carbon black loading in the PTC composition, is also fed into the compounding extruder  10  through another feeder hopper that is shown in the drawing as twin screw gravitational feeder  14 . The compounding extruder  10  provides heat to melt the HDPE material, and applies its twin screws to admix the master batch with the newly added carbon black, and to supply the admixture through a gear pump  16  into a die  18 . The quantity of carbon black added to the compounding extruder in this step of the process raises the loading of the resulting mixture to the typical approximately 50 to 55 percent by weight, and if desired may raise it to approximately 55 to 60 percent by weight. The compounding extruder  10  used in this step may apply its twin screws in co-rotation, or counter-rotation, presently however the use of counter-rotating screws is preferred. 
     Whereas the material, pre-mix or master batch, utilized in the compounding extruder  10  is new and gives rise to new and beneficial results in the process, the compounding extruder  10  and the rest of the apparatus shown in FIG. 2 per se is known in art. Preferably it is constructed in accordance with the teachings of published PCT application WO 97/0660. Briefly summarizing the conventional or prior art part of the apparatus shown in FIG. 2, the gear pump  16  produces a substantially constant volumetric output of the compounded “melt phase” material at sufficiently high pressure for delivery into the sheet die  18 . The sheet die  18  forms the compounded material, while still in the melt phase, into a high tolerance continuous web  20 . The formed polymeric web  20 , while at a temperature just below the melt temperature of the polymeric material, is fed into a lamination mechanism  22 . The lamination mechanism  22  receives from first and second pay-off reels  24  and  26  first and second continuous webs or sheets  28  and  30  of conductive metal foil. These foil sheets  28  and  30  are passed through first and second foil pre-heaters  32  and  34 , and the foils are laminated between a pair of lamination rollers  36  onto each side of the polymeric web  20 . The resulting continuous web of laminate  38  may then be cut into measured lengths, prior to forming individual active PTC elements (not shown). The conductive foils  28  and  30  on each side of the laminate  38  act as electrodes in the active PTC elements. The cutting of the laminate  38  and the formation of individual active PTC device elements is not shown in FIG.  2 . The laminate  38  itself is schematically shown in FIG.  3 . 
     Several means for controlling the quality and characteristics of the PTC composition, and that of the resulting laminate  38  may be utilized as part of, or in conjunction with the present invention. For example, as is described in published PCT application WO 97/0660, a closed loop feedback control system, under the control of a microprocessor (not shown) may be used to control the feeders  12  and  14 , as well as the pressure applied in the compounding extruder  10 . 
     The microprocessor may use an algorithm that uses the inlet pressure of the gear pump  16  as the controlling parameter, with a predetermined inlet pressure as the set point. Thus, by means of a pressure transducer (not shown) at the inlet of the gear pump  16  the pump inlet pressure is measured, and a measured pressure signal is fed to the microprocessor. The microprocessor then periodically compares the value of the measured pressure signal with the stored nominal or set point pressure value. The comparison yields a pressure difference signal that is fed to a feeder control mechanism (not shown) and an extruder control mechanism (not shown) respectively, to synchronize the feed rates of the gravimetric feeders  12 ,  14 , and the rotation rate of the screws in the twin screw extruder  10  in such a manner as to minimize the absolute value of the pressure difference signal. Since the pump inlet pressure is a function of the feed rates and extruder screw rotation rate, the pump inlet pressure can be maintained at or very close to the set point pressure by controlling the feed rates and the extruder screw rotation rate. 
     If a thickness gauge (not shown) is used to measure the thickness of the laminate  38  then another closed loop feedback system may be employed to maintain the thickness of the laminated web  38  to within close tolerances. The thickness control system employs a measured thickness signal generated by the thickness gauge and fed to the microprocessor that may control the thickness of the laminate  38  in analogy to the control mechanism described above, and specifically as disclosed in published PCT application WO 97/0660. 
     Still another important control mechanism that is used in the process of the present invention is illustrated in the flow chart drawing of FIG.  1 . In accordance with this control mechanism the resistivity of the PTC composition produced in the process is measured, and the measured value is compared to a predetermined desired resistivity value. The amount of carbon black and/or the amount of pre-mix or master batch material is then adjusted in the feeders  12  and  14  to obtain a laminate  38  having a corrected or upgraded resistivity. The steps of measuring resistivity, comparing the measured values to a predetermined desired resistivity value, and adjusting the feeding mechanism in accordance with the difference, can be performed periodically, or continuously and automatically by using a microprocessor (not shown) The microprocessor receives as its input a signal representing the measured resistivity, compares it to the stored predetermined desired resistivity value, and produces a difference signal that is used in an algorithm to produce signals for controlling the feeders  12  and  14 . The principles of controlling the feeder in a feedback system using the measured resistivity of the PTC product, are described in published PCT application WO 97/39461. 
     Several advantages of the above-described novel process of the invention include the ability to prepare and store the master batch HDPE-carbon black mixture without the disadvantage caused by the hygroscopic properties of carbon black. Another advantage of the invention is the ability to consistently attain uniform distribution of the filler material in the polymer matrix and therefore attain uniform resistivity and other physical characteristics in the PTC composition. 
     Still another advantage is that relatively high torques are avoided during operation of the compounding extruder, whereas high torques frequently occur in the prior art when a PTC composition having a high loading of carbon black is prepared. As the high torques approach the design limit in the extruders in accordance with the prior art, the mixing process is less stable and results in material inconsistency and poor yield of the product. The high torques in the prior art also result in extensive wear of the extruder machinery. High loading with carbon black in the prior art also leads frequently to stiff carbon black aggregates that jam the machinery, resulting in difficult and expensive interruption in the operation of the machinery and tedious clean-up. All of the foregoing is avoided in accordance with the present invention. Yet another advantage of the present invention is the ability to produce PTC compositions of such high carbon black loading (approximately 55 to 60 percent by weight) with consistently uniform electrical characteristics that was hitherto very difficult or impossible in accordance with the prior art.