Patent Publication Number: US-2006019078-A1

Title: Multi-layer fiber reinforced thermoplastic and method for making the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to a panel made up of fiber reinforced thermoplastic sheets bonded together with a thermoplastic-compatible connecting layer.  
      2. Description of Related Art  
      Bonding thermoplastic materials may be accomplished using a variety of surface treatments that chemically modify the surface of the material, including chemical treatments of primers, flame treating or corona treating.  
      Two problems arise from using these treatments. First, the treatment uses a second processing step that requires additional equipment and cost. Second, the bond is a chemical bond that occurs only at the surface of the interface of the two materials.  
      The sheets may also be secured to one another with mechanical fasteners such as screws or, on the other hand, utilizing glue between the two sheets. However, in utilizing these techniques, it is difficult, if not impossible, to secure together two sheets such that there is no gap between them. Such a feature is especially important when the two fiber reinforced thermoplastic sheets are being utilized as the wall and associated scuff pad within the storage compartment of a refrigerator trailer. Under these circumstances, and with the existence of gaps, moisture will build up in gaps between the two sheets and will produce mildew and the associated undesirable odor.  
      A panel is needed whereby two sheets may be bonded together in a relatively simple and inexpensive process such that there are no gaps between these two sheets that would be susceptible to moisture and the formation of mildew.  
     SUMMARY OF THE INVENTION  
      One embodiment of the subject invention is directed to a multi-layer fiber reinforced thermoplastic panel comprised of a first fiber reinforced thermoplastic sheet, wherein the thermoplastic has a first melting point. The panel is also comprised of a second fiber reinforced thermoplastic sheet, wherein the thermoplastic has a second melting point. There is a connecting layer between the first sheet and the second sheet, wherein the connecting layer is made of a thermoplastic-compatible material having a third melting point less than the first and second melting points and wherein the connecting layer is bonded to the thermoplastic in the first sheet and in the second sheet.  
      Another embodiment of the subject invention is directed to a method of forming a multi-layer fiber reinforced thermoplastic panel with a first fiber reinforced thermoplastic sheet bonded to a second fiber reinforced thermoplastic sheet, wherein the thermoplastic in the first sheet has a first melting point and the thermoplastic in the second sheet has a second melting point. The method comprises the steps of: 
          a) introducing a first connecting film upon the first sheet, wherein the first connecting film is made of a thermoplastic-compatible material having a third melting point, which is less than the first melting point, to define a first arrangement;     b) heating the first arrangement to a temperature above the first melting point to bond the first connecting film to the first sheet;     c) introducing a second connecting film upon the second sheet, wherein the second connecting film is made of a thermoplastic-compatible material having a third melting point less than the second melting point to define a second arrangement;     d) heating the second arrangement to a temperature above the second melting point to bond the second connecting film to the second sheet;     e) placing the first arrangement against the second arrangement such that the first connecting film is contacting the second connecting film to define a composite panel; and     f) heating the composite panel to a temperature above the third melting point but below the first and second melting points to bond the connecting films to one another to form a connecting layer and thereby bond the first sheet to the second sheet.        

      Yet another embodiment of the invention is directed to a method of making a pair of identical composite panels, wherein each composite panel has a longitudinal axis and is made of a first fiber reinforced thermoplastic sheet with a second fiber reinforced thermoplastic sheet attached thereto by a connecting film bonded to each sheet, wherein the second sheet has a width less than half that of the first sheet and wherein the second sheet is positioned at the edge of the first sheet, wherein the method comprises the steps of: 
          a) placing one composite panel against the other composite panel such that the second sheet of each panel contacts the first sheet of the other panel to form a symmetrical shape when viewed along the longitudinal axis;     b) compressing the first sheet and second sheet against the connecting film; and     c) heating the pair of composite panels to a temperature above the melting point of the connecting film but below the melting point of the thermoplastic in each of the first and second thermoplastic sheets, such that the connecting film of each first and second sheet in each composite panel fuse to one another thereby forming a connecting layer and bonding together the first sheet and the second sheet of each composite panel.       

    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is prior art and illustrates the manner by which two fiber reinforced thermoplastic sheets may be bonded to one another;  
       FIG. 2  illustrates a side view of the multi-layer fiber reinforced thermoplastic in accordance with the subject invention;  
       FIG. 3  illustrates the manner by which two fiber reinforced thermoplastic sheets may be bonded to one another in accordance with the subject invention;  
       FIG. 4  illustrates the two fiber reinforced thermoplastic sheets illustrated in  FIG. 3  contacting one another;  
       FIG. 5  is a cross-sectional view along arrows  5 - 5  in  FIG. 4 ;  
       FIG. 6  is the arrangement illustrated in  FIG. 5  after heat is applied;  
       FIGS. 7-12  illustrate one method whereby a multi-layer fiber reinforced thermoplastic panel is fabricated;  
       FIG. 13  illustrates a cross-sectional view of two fiber reinforced thermoplastic panels positioned to form a symmetrical arrangement;  
       FIG. 14  illustrates the use of a double belt press to process the symmetrically arranged panels illustrated in  FIG. 13 ; and  
       FIG. 15  illustrates a final product of two multi-layer fiber reinforced thermoplastic panels which were produced after the symmetrical arrangement illustrated in  FIG. 13  was processed by the double belt press illustrated in  FIG. 14 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates a first fiber reinforced thermoplastic sheet  10  having a first face  15  and a second fiber reinforced thermoplastic sheet  20  having a first face  25  which, as indicated by arrow  30 , are intended to be mated with one another.  
       FIG. 2  illustrates a multi-layer fabric reinforced thermoplastic panel  40  comprised of a first fiber reinforced thermoplastic sheet  10  having a front face  15  and a second fiber reinforced thermoplastic sheet  20  having a first face  25 . A connecting layer  35  is between the first sheet  10  and the second sheet  20 . The first sheet  10  has thermoplastic therein having a melting point MP 1 . The second sheet  20  has thermoplastic therein having a melting point MP 2 . The connecting layer  35  is made of a thermoplastic-compatible material having a third melting point MP 3 . A thermoplastic-compatible material is a material that when heated together with a thermoplastic base will bond with the thermoplastic base. The connecting layer  35  is bonded to the thermoplastic in the first sheet  10  and the second sheet  20  such that the first sheet  10  and the second sheet  20  are connected to one another through the connecting layer  35 . The third melting point MP 3  is less than the melting points MP 1  and MP 2 .  
      The connecting layer  35  makes substantially full contact with the front face  15  of the first sheet  10  and the front face  25  of the second sheet  20  such that there are no gaps between the sheet  10  and the connecting layer  35  and between sheet  20  and connecting layer  35 . This feature is very beneficial when the panel  40  is exposed to humidity or moisture that could accumulate in any gaps and promote the growth of bacteria such as mildew.  
      The connecting layer  35  may be a thermoplastic such as polypropylene or polyolefin. Additionally, the connecting layer  35  may be a co-polymer such as a modified polyolefin. A key distinction between the material for the connecting layer  35  and the thermoplastic within the first sheet  10  and the second sheet  20  is that the connecting layer  35  is a thermoplastic-compatible material that will melt at a lower temperature than the thermoplastic within the first sheet  10  and within the second sheet  20 .  
      The first sheet  10  and the second sheet  20  may be identical materials or, depending upon the application, may be different materials in as much as the connecting material  35  is capable of bonding to the thermoplastics within these materials.  
      The first sheet  10  and the second sheet  20  may have identical thicknesses; however; depending upon the application for the panel, the thicknesses of each sheet may be different. Furthermore, as a general guideline, the thickness T 1  ( FIG. 2 ) of the first sheet  10  and the thickness T 2  of the second sheet  20  are at least twice the thickness T 3  of the connecting layer  35 , although, as seen in  FIG. 2 , these proportions are exaggerated for clarity. In one embodiment, the thickness T 1  of the first sheet  10  and the thickness T 2  of the second sheet  20  are approximately 0.1 inch while the thickness T 3  of the connecting layer  35  is approximately 0.05 inch.  
      In one application, the panel  40  may be used in refrigeration trucks such that the first sheet  10  acts as a wall while the second sheet  20  acts as a scuff pad.  
      Unlike the prior art, which utilized mechanical fasteners or glue to secure one sheet to another sheet, the subject invention utilizes a connecting layer. Directing attention to  FIGS. 2 and 3 , the connecting layer  35  ( FIG. 2 ) is produced by depositing upon the front face  15  of the first sheet  10  a connecting film  45  and, furthermore, by depositing upon the front face  25  of the second sheet  20  a connecting film  50 . The connecting film  45  and the connecting film  50  are heated and melted into a single connecting layer  35 . These connecting films  45 ,  50 , as illustrated in  FIG. 4 , are placed against one another and as indicated in  FIG. 5 , heat is applied to the exposed surfaces of the first sheet  10  and the second sheet  20  at a temperature above the melting point MP 3  of the connecting films  45 ,  50 , but below the melting points MP 1 , MP 2  of the thermoplastic within the first sheet  10  and the second sheet  20 . As a result, the connecting film  45  and the connecting film  50  melt together to form a single connecting layer  35  between the first sheet  10  and the second sheet  20  as shown in  FIG. 6 .  
      While typically identical materials would be used for the connecting film  45  and connecting film  50 , it is possible for the connecting layers  45 ,  50  to be made of different materials as the circumstances warrant and have different melting points, but lower than the melting points of their respective sheets  10 ,  20 .  
      What has so far been described is the melting of the carrier film  45  and carrier film  50  between the first sheet  10  and the second sheet  20  to form a connecting layer  35  between the first sheet  10  and the second sheet  20 . However, for the connecting layer  35  to properly bond to the first sheet  10  and the second sheet  20 , it is necessary to first secure each connecting film  45 ,  50  to its respective sheet  10 ,  20 .  
      A method of forming a multi-layer fiber reinforced thermoplastic panel  40  is illustrated in  FIGS. 7-12 . In particular, fabrication of the panel  40  ( FIG. 2 ) with the first fiber reinforced thermoplastic sheet  10  bonded to the second fiber reinforced thermoplastic sheet  20  will be described. The thermoplastic in the first sheet  10  has a first melting point MP 1  and the thermoplastic in the second sheet  20  has a second melting point MP 2 .  
      Briefly stated, the connecting layers of each sheet must first be fused to each sheet by heating the connecting layer and the sheet to a temperature not only above the melting point of the connecting layer, but also above the melting point of the thermoplastic in the sheet. This is best performed at the time of fabrication of the sheet because heating the sheet to a temperature above the melting point of the thermoplastic in the sheet after the sheet is fabricated will cause the sheet to warp.  
      Furthermore, while the material of each connecting film typically will be identical and therefore each film will have the same melting point as the connecting layer, it is possible for each connecting layer to be a different material. Therefore, under these circumstances, the connecting films will have different melting points, but still have melting points less than the melting points of the thermoplastic within the sheets.  
      Directing attention to  FIG. 7 , a first connecting film  45  is introduced upon the first sheet  10 , wherein the first connecting film  45  is made of a thermoplastic-compatible material having a third melting point MP 3 , which is less than the first melting point MP 1 , defining a first arrangement  60  as illustrated in  FIG. 8 . The first arrangement  60  is heated to a temperature above the first melting point MP 1  of the first sheet  10 , which is also above the melting point MP 3  of the connecting film  45 , to bond the first connecting film  45  to the first sheet  10 . It is important to exceed the melting point MP 1  of the first sheet to fully fuse the connecting film  45  thereto. The mating of the connecting film  45  with the first sheet  10  should occur at the time the first sheet  10  is fabricated, since after the first sheet has cooled from fabrication, subsequent heating above the melting point of the sheet will cause the sheet to warp.  
      Directing attention to  FIG. 9 , a second connecting film  50  is introduced upon the second sheet  20  to define a second arrangement  70  as illustrated in  FIG. 10 . The second connecting film  50  is made of a thermoplastic-compatible material having a third melting point MP 3  which is less than the second melting point MP 2 . The second arrangement  70  is heated to a temperature above the second melting point MP 2  and the third melting point MP 3  to bond the second connecting film  50  to the second sheet  20 . It is important to exceed the melting MP 2  of the second sheet to fully fuse the connecting film  50  thereto. The mating of the connecting film  50  with the second sheet  20  should occur at the time the second sheet  20  is fabricated, since after the second sheet has cooled from fabrication, subsequent heating above the melting point of the sheet will cause the sheet to warp.  
      As illustrated in  FIG. 11 , the first arrangement  60  is placed against the second arrangement  70  such that the first connecting film  45  is contacting the second connecting film  50  to define a composite panel  40 . The composite panel  40  is then heated to a temperature above the third melting point MP 3  but below the first melting point MP 1  and second melting point MP 2  to bond the connecting films  45 ,  50  to one another to form a connecting layer  35  as illustrated in  FIG. 12 , and thereby bonding the first sheet  10  to second sheet  20  and forming the composite panel  40 .  
      The method may be further comprised of compressing the first arrangement  60  against the second arrangement  70  to promote bonding of the connecting films  45 ,  50  to their respective sheets  10 ,  20 . The step of compressing may occur during or after heating the first arrangement  60  and the second arrangement  70 . Once the connecting films  45 ,  50  have been heated above their melting points and fused into a single connecting layer  35 , the first arrangement  60  and the second arrangement  70  may be cooled in a controlled fashion utilizing any number of different technologies known in the art of thermoplastic fabrication.  
      Additionally, during or after heating the composite panel  40 , the panel  40  may be compressed to promote adhesion of the connecting films  45 ,  50 .  
      One preferred application of the sheet in accordance with the subject invention is as a wall of a refrigerated trailer. Briefly returning to  FIG. 2 , the first sheet  10  may extend seven feet high along the wall of a trailer while the second sheet  20  may extend a distance of approximately three feet from the edge of sheet  10  which is resting upon the floor of the trailer. In this manner, the second sheet  20  acts as a scuff plate to deflect impact or other movement of the contents within the trailer that would normally scuff or puncture the trailer walls. As a result, the shape of such a panel may generally be described as L-shaped.  
      As a result, as illustrated in  FIGS. 13-15 , a pair of identical composite panels  40 ,  40 ′ may be made simultaneously. As illustrated in  FIGS. 4 and 13 , each panel  40  has a longitudinal axis L and is made of a first fiber reinforced thermoplastic sheet  10  and a second fiber reinforced thermoplastic sheet  20  with connecting film  45 ,  50  therebetween and bonded to each sheet  10 ,  20 . The second sheet  20  has a width W 1  which is less than half that of the width W 2  of the first sheet  10 . The second sheet  20  is positioned at one edge  12  of the first sheet  10 . An identical composite panel  40 ′, wherein identical reference items are indicated with a prime sign, is placed against the composite panel  40  such that the second sheet  20 ,  20 ′ of each panel  40 ,  40 ′ contacts the first sheet  10 ,  10 ′ of the other panel  40 ,  40 ′ to form a symmetrical shape when viewed along the longitudinal axis L. The arrangement of the pair of panels  40 ,  40 ′ illustrated in  FIG. 13  is also illustrated in  FIG. 14  and identified as reference number  80 . It should be noted that this arrangement in  FIG. 14  is viewed from the side as shown by arrows  14 - 14  in  FIG. 13 . Oriented in this fashion, the second sheets  20 ,  20 ′ ( FIG. 13 ) provide vertical support to the first sheets  10 ,  10 ′ such that this arrangement  80  may be acted upon a double belt press  100  to assist in bonding each first layer with each second layer.  
      Directing attention to  FIG. 14 , the arrangement  80  when introduced within the double belt press  110  is compressed. The double belt press  100  also contains heaters  105  which act upon the arrangement  80  and raise the temperature of the connecting films  45 ,  50  above their melting point MP 3  but below the melting points MP 1 , MP 2  of the thermoplastic within the first sheet  10  and the second sheet  20  such that the connecting layer  35  is formed. Once the connecting layer  35  is formed, it is desirable to cool the connecting layer  35  and coolers  110  are provided downstream of the heaters  105  to accomplish this task. The double belt press  100  along with the heaters  105  and coolers  110  used therein are commercially available products. Once the arrangement  80  is passed through the double belt press  100 , the connecting layer  35  is formed, thereby bonding the first sheet  10  to the second sheet  20  on each of the panels  40 ,  40 ′, such that, as illustrated in  FIG. 15 , the panels may be separated and the respective first sheet  10  and the second sheet  20  are bonded through the connecting layer  35 .  
      While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. The presently preferred embodiments described herein are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.