Patent Publication Number: US-2018036936-A1

Title: Apparatus and method of processing a continuous sheet of polymer material

Description:
BACKGROUND 
     The present disclosure relates generally to film capacitors and, more specifically, to an apparatus and method of forming polymeric thin films for use in film capacitors. 
     At least some known metalized film capacitors include two metal foil electrodes separated by a layer of polymer film. For example, some capacitors include two layers of metallic foil interleaved with two layers of polymer film, and the interleaved structure is wound about a spindle in a manner such that the two layers of metallic foil are electrically separated from each other. The layer of polymer film is typically fabricated from a dielectric material, such as polypropylene. Other high temperature resistant and high capacitance materials may also be suitable for use as the layer of polymer film. However, commercially available high temperature resistant and high capacitance materials, such as polyetherimide, are too thick for effective use in thin film and foil capacitors and may have manufacturing defects, such as wrinkling, thickness non-uniformity, surface defects, and residual solvent. 
     BRIEF DESCRIPTION 
     In one aspect, a method of processing a continuous sheet of polymer material is provided. The method includes routing the continuous sheet of polymer material from a first spool and along at least a first heated roller and a second heated roller, heating the continuous sheet of polymer material to a first temperature on the first heated roller and the second heated roller, and controlling a rotational speed of the first heated roller and the second heated roller such that the continuous sheet of polymer material is stretched when routed from the second heated roller to the first heated roller. 
     In another aspect, an apparatus for use in processing a continuous sheet of polymer material is provided. The apparatus includes a first spool mount configured to receive a first spool having an unprocessed portion of the continuous sheet of polymer material wound thereon, a first heated roller, and a second heated roller. The continuous sheet of polymer material routed from the first spool and along at least the first heated roller and the second heated roller. The apparatus also includes a heating system and a drive system. The heating system is thermally coupled with at least one of the first heated roller and the second heated roller, and the heating system heats the continuous sheet of polymer material on the first heated roller and the second heated roller to a first temperature. The drive system actuates the first heated roller and the second heated roller, and controls a rotational speed of the first heated roller and the second heated roller such that the continuous sheet of polymer material is stretched when routed from the second heated roller to the first heated roller. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a block diagram of an exemplary apparatus for use in processing a continuous sheet of polymer material; and 
         FIG. 2  is a side view of an exemplary roller assembly that may be used in the apparatus shown in  FIG. 1 . 
     
    
    
     Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of the disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
     DETAILED DESCRIPTION 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged. Such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     Embodiments of the present disclosure relate to an apparatus and method of forming polymeric thin films for use in film capacitors, for example. More specifically, the apparatus and method described herein facilitate uni-axially stretching a pre-formed polymer film fabricated from an amorphous, rather than semi-crystalline, polymer material. The polymer film is preheated, heated, stretched, annealed, and cooled in a continuous process such that a thin dielectric film having a thickness of less than about 4 microns is formed. The apparatus includes a series of roller elements that perform one or more of the process functions articulated above when the polymer film is routed therethrough. For example, the polymer film is stretched when routed between a pair of heated roller elements that operate at different rotational speeds. As such, manufacturing defects are mitigated in the polymer film, and the polymer film is stretched to a desired thickness in an efficient and continuous manner. 
       FIG. 1  is a block diagram of an exemplary apparatus  100  for use in processing a continuous sheet (not shown in  FIG. 1 ) of polymer material. In the exemplary embodiment, apparatus  100  includes a roller assembly  102  including a plurality of rollers. More specifically, roller assembly  102  includes heated rollers  104  and cooled rollers  106 . The continuous sheet of polymer material is routed along roller assembly  102 , and heated rollers  104  and cooled rollers  106  perform one or more processing functions, as will be described in more detail below. For example, apparatus  100  further includes a heating system  108 , a cooling system  110 , and a drive system  112  coupled to roller assembly  102 . Drive system  112  independently controls a rotational speed of heated rollers  104  and cooled rollers  106  to facilitate stretching the continuous sheet of polymer material, as will be described in more detail below. In addition, heating system  108  is thermally coupled to heated rollers  104  and cooling system  110  is thermally coupled to cooled rollers  106  to facilitate controlling the temperature of the continuous sheet of polymer material. In an alternative embodiment, apparatus  100  includes a film slitting system that trims the side edges of the continuous sheet before being routed to roller assembly  102 . 
     Heating system  108  and cooling system  110  are embodied as any temperature control devices that enable apparatus  100  to function as described herein. For example, heating system  108  includes at least one heating element  114  coupled to heated rollers  104 . In one embodiment, heating element  114  is an inductive heating device coupled directly to heated rollers  104 . In addition, heated rollers  104  are fabricated from a thermally conductive material. As such, heat generated by heating element  114  is conducted through heated rollers  104  and transferred to the continuous sheet of polymer material. Heating system  108  further includes a control device  116  for independently controlling the temperature of each heated roller  104 . In an alternative embodiment, heating element  114  is embodied as an external heater that transfers heat to the continuous sheet of polymer material through convection. 
     Cooling system  110  includes at least one cooling element  118  coupled to cooled rollers  106 . In one embodiment, cooling element  118  is a heat exchange device that channels a flow of cooling fluid therethrough. Similar to heated rollers  104 , cooled rollers  106  are likewise fabricated from a thermally conductive material. As such, as will be explained in more detail below, heat transferred from the continuous sheet of polymer material when routed from heated rollers  104  is conducted through cooled rollers  106  and dissipated in the working fluid channeled through cooling element  118 . Cooling system  110  further includes a control device  120 , such as a flow controller, for independently controlling the temperature of each cooled roller  106 . In an alternative embodiment, cooling element  118  is any cooling device capable of maintaining cooled rollers  106  at a temperature for cooling the continuous sheet of polymer material. 
     Apparatus  100  further includes a first spool mount  122  and a second spool mount  124 . Drive system  112  is coupled to first spool mount  122  and second spool mount  124  for independently controlling a rotational speed thereof. As will be described in more detail below, first spool mount  122  is operable for feeding the continuous sheet of polymer material towards roller assembly  102 , and second spool mount  124  is operable for collecting the continuous sheet of polymer material received from roller assembly  102 . 
       FIG. 2  is a side view of roller assembly  102  that may be used in apparatus  100  (shown in  FIG. 1 ). In the exemplary embodiment, heated rollers  104  include a first heated roller  126 , a second heated roller  128 , a third heated roller  130 , and a fourth heated roller  132 . In addition, cooled rollers  106  include a first cooled roller  134  and a second cooled roller  136 . First spool mount  122  receives a first spool  138  thereon, and second spool mount  124  receives a second spool  140  thereon. First spool  138  has an unprocessed portion  142  of a continuous sheet  144  of polymer material wound thereon. Continuous sheet  144  of polymer material is routed along roller assembly  102  and collected on second spool  140 . More specifically, continuous sheet  144  of polymer material is processed when routed along roller assembly  102 , and a processed portion  146  of continuous sheet  144  of polymer material is collected on second spool  140 . 
     In the exemplary embodiment, the polymer material is an amorphous, high temperature resistant, and high capacitance material having a glass transition temperature greater than or equal to 140° C., for example. Exemplary polymer materials include, but are not limited to, polyetherimide, polytetrafluoroethylene, polycarbonate, polysulfone, polyethersulfone, modified high temperature polycarbonate, fluorine polyester, and polyvinylidene fluoride-polytetrafluoroethylene copolymers. In addition, continuous sheet  144  is a pre-formed sheet of polymer material formed in a melt-extrusion, solvent cast, or blow molding process, for example. 
     In operation, continuous sheet  144  is routed from first spool  138 , through roller assembly  102 , and collected on second spool  140 . More specifically, continuous sheet  144  is routed from first spool  138  to fourth heated roller  132 , from fourth heated roller  132  to third heated roller  130 , from third heated roller  130  to second heated roller  128 , from second heated roller  128  to first heated roller  126 , from first heated roller  126  to first cooled roller  134 , from first cooled roller  134  to second cooled roller  136 , and from second cooled roller  136  to second spool  140 . Heating system  108  is thermally coupled with at least one of first heated roller  126  and second heated roller  128 . Heating system  108  heats continuous sheet  144  of polymer material on first heated roller  126  and second heated roller  128  to a first temperature. More specifically, heating system  108  heats first heated roller  126  and second heated roller  128  to the first temperature and heat is transferred to continuous sheet  144 . In the exemplary embodiment, heating system  108  heats continuous sheet  144  of polymer material to the first temperature that is greater than a glass transition temperature of the polymer material. As such, continuous sheet  144  is softened for elastic deformation when stretched in roller assembly  102 . 
     When heated to the first temperature, drive system  112  (shown in  FIG. 1 ) actuates first heated roller  126  and second heated roller  128 . Drive system  112  controls a rotational speed of first heated roller  126  and second heated roller  128  such that continuous sheet  144  of polymer material is stretched when routed from second heated roller  128  to first heated roller  126 . More specifically, as described above, drive system  112  is capable of controlling the rotational speed of first heated roller  126  and second heated roller  128  independently of each other. Drive system  112  rotates first heated roller  126  at a greater rotational speed than second heated roller  128  such that an output rate of first heated roller  126  is greater than second heated roller  128 , thereby stretching continuous sheet  144  of polymer material in a uni-axial direction defined along the routing path. In addition, rotating second heated roller  128  at a lower rotational speed than first heated roller  126  facilitates increasing the residence time of continuous sheet  144  on second heated roller  128 , thereby allowing sufficient time for the temperature of continuous sheet  144  to increase to the first temperature. In one embodiment, a difference in rotational speeds of first heated roller  126  and second heated roller  128  is defined within a range between about 10 percent and about 30 percent. In addition, in one embodiment, drive system  112  controls the rotational speed of at least one of first heated roller  126  and second heated roller  128  such that continuous sheet  144  of polymer material is stretched to a thickness of less than or equal to about 4 microns. 
     In the exemplary embodiment, continuous sheet  144  is routed along fourth heated roller  132  and third heated roller  130 , before being routed to second heated roller  128 , such that continuous sheet  144  of polymer material is preheated to a temperature lower than the first temperature. More specifically, heating system  108  heats fourth heated roller  132  to a first preheated temperature and heats third heated roller  130  to a second preheated temperature, and heat is transferred from fourth heated roller  132  and third heated roller  130  to continuous sheet  144 . The first preheated temperature and the second preheated temperature are both less than the first temperature. Preheating continuous sheet  144  of polymer material facilitates gradually increasing the temperature of the polymer material to reduce the formation of thermal stress within continuous sheet  144 . 
     As described above, heating system  108  is capable of controlling the temperature of heated rollers  104  independently of each other. In some embodiments, heating system  108  operates such that a temperature difference between fourth heated roller  132  and third heated roller  130 , and between third heated roller  130  and second heated roller  128  is less than a predetermined threshold. As such, a thermal gradient within continuous sheet  144  is reduced when increasing the temperature of continuous sheet  144 , such that the formation of thermal stress within continuous sheet  144  is also reduced. In one embodiment, the predetermined threshold is less than or equal to about 200° F. (93.3° C.). In an alternative embodiment, a single preheated roller is implemented prior to routing continuous sheet  144  to second heated roller  128 . 
     As described above, first cooled roller  134  receives continuous sheet  144  of polymer material routed from first heated roller  126 . Cooling system  110  (shown in  FIG. 1 ) is thermally coupled with first cooled roller  134 , and cooling system  110  anneals continuous sheet  144  of polymer material on first cooled roller  134  to a third temperature lower than the first temperature. More specifically, cooling system  110  cools first cooled roller  134  to the third temperature, and heat is transferred from continuous sheet  144  to first cooled roller  134 . In the exemplary embodiment, the third temperature is less than the glass transition temperature of the polymer material such that continuous sheet  144  is hardened on first cooled roller  134 . 
     Moreover, second cooled roller  136  receives continuous sheet  144  of polymer material routed from first cooled roller  134 . Cooling system  110  cools continuous sheet  144  of polymer material on second cooled roller  136  to a fourth temperature lower than the third temperature. More specifically, cooling system  110  cools second cooled roller  136  to the fourth temperature, and heat is further transferred from continuous sheet  144  to second cooled roller  136 . As such, retraction of continuous sheet  144  is limited when processed portion  146  is collected on second spool  140 . 
     The apparatus and method of forming polymeric thin films from pre-formed polymeric material, as described above, facilitating correcting deficiencies in known apparatuses and methods. More specifically, the apparatus includes heated rollers, cooled rollers, independent heating and cooling systems, and a drive system for selectively processing a continuous sheet of polymer material routed through the apparatus. The heating system heats the polymer material to greater than its glass transition temperature, and the rollers uni-axially stretch the polymer material after it has been heated. As such, the thickness of the continuous sheet is reduced and manufacturing defects in the pre-formed polymeric material is mitigated. 
     An exemplary technical effect of the apparatus and method described herein includes at least one of: (a) reducing the thickness of a continuous sheet of polymer material; (b) reducing and mitigating manufacturing defects typically found in a commercially available pre-formed polymeric film; and (c) enabling the use of high temperature resistant and high capacitance material in thin film capacitors. 
     Exemplary embodiments of an apparatus and method of processing a continuous sheet of polymeric material, and related components are described above in detail. The system is not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the configuration of components described herein may also be used in combination with other processes, and is not limited to practice with only turbine assembles and related methods as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many applications where stretching a film is desired. 
     Although specific features of various embodiments of the present disclosure may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of embodiments of the present disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the embodiments of the present disclosure, including the best mode, and also to enable any person skilled in the art to practice embodiments of the present disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the embodiments described herein is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.