Abstract:
A method of welding materials includes obtaining a first material having a first thermoplastic layer formed thereon; obtaining a second thermoplastic layer; positioning the first material and the second thermoplastic layer such that the first thermoplastic layer faces the second thermoplastic layer; applying energy to the first thermoplastic layer and the second thermoplastic layer at a first location to establish a first weld; applying energy to the first thermoplastic layer and the second thermoplastic layer at a second location, the second location being offset from the first location, a portion of the second location overlapping a portion of the first location.

Description:
BACKGROUND 
       [0001]    This invention relates generally to welding materials and in particular, to welding thermoplastic materials, or thermoplastics and non-thermoplastics together, to form a seam having enhanced seam strength. 
         [0002]    There exists in the art a number of applications for textiles having a thermoplastic material adhered thereto. For example, U.S. Pat. No. 6,350,709 discloses a textile substrate having a polymeric film, such as polyamide, polyolefin, or polyurethane laminated thereto. This textile substrate may be woven of nylon, polyester, or other synthetic fibers. U.S. Pat. No. 6,350,709 also discloses a method for heat sealing sheets of the laminated material to form an automotive air bag. 
         [0003]    When forming structures from materials having a thermoplastic layer thereon, seams are formed by placing the materials between dies and applying energy. The polymeric films are bonded through melting and curing. A drawback to conventional seam forming techniques is the lack of techniques to enhance seam strength, especially with materials that form inherently weak bonds together. Regardless of the structure (e.g., air bag, clothing) enhanced seam strength is beneficial in meeting product performance requirements. Thus, there is a need in the art for methods for enhancing seam strength in products formed from textiles having a polymeric sheet laminated thereto. 
       SUMMARY 
       [0004]    Embodiments of the invention include a process for welding materials including obtaining a first material having a first thermoplastic layer formed thereon; obtaining a second thermoplastic layer; positioning the first material and the second thermoplastic layer such that the first thermoplastic layer faces the second thermoplastic layer; applying energy to the first thermoplastic layer and the second thermoplastic layer at a first location to establish a first weld; applying energy to the first thermoplastic layer and the second thermoplastic layer at a second location, the second location being offset from the first location, a portion of the second location overlapping a portion of the first location. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a cross-sectional view illustrating a step in a process of welding materials. 
           [0006]      FIG. 2  is a cross-sectional view illustrating a step in the process of welding materials. 
           [0007]      FIG. 3  is a cross-sectional view illustrating a step in the process of welding materials. 
           [0008]      FIG. 4  is a cross-sectional view illustrating a step in the process of welding materials. 
           [0009]      FIG. 5  is a plan view illustrating an exemplary welding pattern. 
           [0010]      FIG. 6  is a plan view illustrating another exemplary welding pattern. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  is a cross-sectional view illustrating a step in a process of welding materials. At this initial stage, a thermoplastic layer  20  is formed on a surface of a material  10 . The thermoplastic layer  20  may be a polymeric sheet film such as a polyurethane film. Alternatively, the thermoplastic layer  20  may be formed by applying a resin, rather than a film. The thermoplastic film may be formed from a blown resin, extruded resin, molded resin, etc. The material  10  shown in  FIG. 1  is a 3-layer laminate that is moisture permeable while being air-permeable or non air-permeable. In exemplary embodiments, the 3-layer laminate includes a woven layer  12 , a non-woven layer  14  and a knit layer  16 . The material  10  may be an expanded polytetrafluoroethylene (PTFE) material similar to part No WAAZ 103604A, or part No. WCJX 145103D from W.L. Gore &amp; Associates, Elkton, Md. This laminate  10  may be prepared generally according to the teachings of U.S. Pat. No. 4,194,041. It is understood that other materials may be used for material  10 . In the next step shown in  FIG. 2 , the polymeric film  20  is bonded to the material  10  by applying energy through a heat press. It is understood that other techniques may be used to bond layer  20  to material  10  such as RF or ultrasonic welding. A variety of thermoplastic welding techniques may be used including RF, ultrasonic, impulse, hot plate, hot air, etc. The welding process heats up the thermoplastic layer  20  to the point that it flows into material  10 . As shown in  FIG. 2 , during the heat press process, the thermoplastic layer  20  melts and becomes embedded in the knit layer  16  of material  10 . This forms a secure mechanical bond between the thermoplastic layer  20  and material  10 . 
         [0012]      FIG. 3  illustrates the next step in which a second thermoplastic-coated material  30  (e.g., part No. PS 8010 from Deerfield Urethane Inc, Whately, Mass.) is placed on thermoplastic layer  20 . The second thermoplastic-coated material  30  may include a textile  34  and a thermoplastic (e.g., polymeric) layer  32  bonded thereto. The thermoplastic layer  32  may be polyurethane in exemplary embodiments. The thermoplastic layer  32  is placed so as to face the thermoplastic layer  20 . In alternate embodiments, the second material  30  may omit the textile layer so as to only include a thermoplastic polymer layer  32 . 
         [0013]    As shown in  FIG. 4 , a welded seam is then formed by applying, for example, radio frequency energy and pressure using equipment such as a Thermatron 15KW Model KF 122. A variety of welding techniques may be used to apply energy including RF, ultrasonic, impulse, hot plate, hot air, etc. As depicted in  FIG. 4 , the RF tooling die  40  welds the thermoplastic layers  20  and  32 . This process forms a bond between the thermoplastic layers  32  and  20 . The welding process pushes the thermoplastic (meltable) material into the pores in material  10  and causes a mechanical bond between the infused thermoplastic layers and the material  10 . 
         [0014]    As shown in  FIG. 4 , the tooling die  40  is controlled so as to set an offset distance between multiple welds. An exemplary offset welding technique is illustrated in  FIG. 5  which shows a top view of an exemplary weld. To form the multiple welds, a tooling die is used to form a first weld  50 . The material is welded between the electrode and base plate as shown in  FIG. 4 . Then a second weld  52  is formed, offset by offset distance d and overlapping the first weld  50 . This double weld technique drives the thermoplastic material from layers  20  and  32  further into the material  10  (whether a non-woven or porous woven surface is used) and creates an additional mechanical bond on top of the cohesive bond between the polymeric molecular chains, thus increasing weld strength. 
         [0015]    An alternate offset weld is shown in  FIG. 6 . In  FIG. 6 , the first weld  50  has a larger size than subsequent welds  52 . The subsequent welds  52  are offset from the edge of the first weld  50  and offset from each other. The multiple, offset welding provides a higher seam strength than conventional techniques. 
         [0016]    Other techniques may be used to further increase the weld strength. One technique is to perform spot impregnation of thermoplastic material plastic into the material. By-preheating the welding tools, the thermoplastic layers  20  and  32  can be pushed further into the top knit layer  16  of material  10 . With this thermoplastic/fabric construction it is possible to distribute tension over a greater surface area. This also improves weld strengths if material more resistant to tearing and fracturing when stressed is used. Lastly, with the thermoplastic material infused into the fibers of the fabric layer  16 , the fabric stiffens and loses some of its stretch. The pre-heating can be done with heated tools, buffer materials, or double hits using the same tool die. 
         [0017]    Using spot impregnation the molten thermoplastic is forced into a porous material and cured in place around material structure (e.g. threads). In some cases this structure is lubricious, or of a chemistry that has a tendency to reject the impregnation of the molten material. In such embodiments, the material  10  can also be prepared with various solvents or abrasives to make the fibers in the material (e.g., fibers in knit layer  16 ) more porous. The molten thermoplastic material can enter the pores of the fibers to more securely bond the polymeric layer to the fabric layer. 
         [0018]    By prepping the substrate first, by heat or chemistry, it is easier to intertwine the thermoplastic layers  20  and  32  and the material  10 . That is to say, the cured thermoplastic materials may wrap around structures in material  10 , but their chemistry remains distinct. 
         [0019]    In alternate embodiments, molecular chains on the surface of material  10  may be broken, for example with solvents, prior to welding the thermoplastic layer to the material  10  to promote better bonding. In these cases, a slurry is formed on the surface of the materials where there may be a mixing of the chemistry (at or not at the molecular level), but the thermoplastic layers  20  and  32  and the material  10  are cured together in the same general space. 
         [0020]    In additional embodiments, the process is used to secure a thermoplastic layer to a first material that does not include a thermoplastic. For example, the double welding process may be used to secure a thermoplastic layer to a material (e.g. a fabric such as a woven fabric or non-woven fabric) without the need for a second thermoplastic layer. 
         [0021]    While this invention has been described with reference to one or more embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention.