Abstract:
The use of coextruded or integral sealing ribs on zipper flanges enhances the quality of the sealing between the flanges and the walls of a reclosable package. The coextruded sealing ribs may be made from sealing resins or similar materials. Additionally, grip strips are placed on the opposite side of the flanges from the ribs in order to improve the quality of the sealing even further and to inhibit inner flange sealing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the use of coextruded sealing ribs on zipper flanges to enhance the quality of the sealing between the flanges and the walls of a reclosable package. The present invention further relates to placing grip strips on the opposite side of the flanges from the ribs in order to improve the quality of the sealing even further. 
     2. Description of the Prior Art 
     Most currently manufactured flexible packaging is constructed from films including polypropylene. However, the application or sealing of reclosable zippers to polypropylene has been problematic at best. In general, polypropylene is minimally compatible with the polyethylene-based sealant materials that are typically found on a reclosable zipper. The resulting bond strengths between the polyethylene flange and the polypropylene film are typically not as strong as can be expected in polyethylene laminate packaging, and the temperatures required to form these weak bonds are very near to the temperatures which induce distortions in the film. 
     While forming zippers from polypropylene can result in stronger bonding between the zipper flanges and the film, these zippers are typically stiff and less flexible than polyethylene zippers, and therefore not as desirable. 
     Moreover, in thick film, gusseted, transverse direction package constructions, the zipper is prone to inner flange sealing due to the small sealing window of this zipper film combination. Similarly, when the slider zipper is notched, zipper transport can be problematic as the notches create sections of zipper that are more prone to deformation due to drag or tension transients. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to improve the bonding between polyethylene zipper components, such as flanges, and polypropylene web or film in the field of reclosable packages. 
     It is therefore a further object of the present invention to improve the stiffness of zippers, particularly notched zippers, in order to improve the reliability of the transport of these zippers. 
     These and other objects are attained by providing a flanged zipper with coextruded ribs on the sealing surfaces of the flanges. The ribs can be a continuous extrusion with the same material in a thin layer extruded in the space between the ribs, or the ribs can be discrete and separate from each other on the zipper flange. The ribs act as points of pressure concentration that enhance the sealing characteristics of the zipper flange to the film or substrate. Additionally, the addition of extruded grip strips on the zipper flange directly opposite the ribs further enhances the pressure concentration. 
     This configuration improves the heat transfer to the sealing surface of the zipper flange, improves the stiffness of the zipper flanges and improves the temperature difference between the onset of sealing initiation and the onset of inner flange sealing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further objects and advantages of the invention will become apparent from the following description and from the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional, partially exploded view, of a flanged zipper of the present invention, including the sealing ribs. 
         FIG. 2  is a cross-sectional view of a first embodiment of the sealing ribs opposite the grip strips on a zipper flange. 
         FIG. 3  is a cross-sectional view of a second embodiment of the sealing ribs, wherein the ribs are formed discretely on the flange. 
         FIG. 4  is a cross-sectional view of a third embodiment of the sealing ribs, wherein the ribs are coextruded and formed as a separate layer of material from the underlying flange. 
         FIG. 5  is a cross-sectional view of a fourth embodiment of the sealing ribs, wherein the ribs and the flange are formed or extruded as a single layer of material. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail wherein like numerals refer to like elements throughout the several views, one sees that  FIG. 1  is a cross-sectional, partially exploded view of flanged zipper  10  within the first and second walls  12 ,  14  of reclosable package  100 . First and second walls  12 ,  14  are typically formed of polypropylene. Flanged zipper  10 , typically formed from polyethylene includes first and second interlocking profiles  16 ,  18  with respective first and second flanges  20 ,  22 . The exterior surfaces of first and second flanges  20 ,  22  are sealed to the interior surface of respective first and second walls  12 ,  14 , as is well known to those skilled in the art. Slider  23  operates in a conventional manner to separate first and second interlocking profiles if moved in a first direction and interlocking first and second interlocking profiles  16 ,  18  if moved in a second direction. 
     While flanged slider zippers are disclosed, this invention is likewise applicable to other styles of zippers, such as press-to-close or webless. 
     The exterior surfaces of first and second flanges  20 ,  22  include ribs  30 . Ribs  30  are typically coextruded in the formation of first and second flanges  20 ,  22  and are formed parallel to each other through the length of flanged zipper  10 . The interior surfaces of first and second flanges  20 ,  22  include grip strips  32 , preferably positioned immediately opposite the ribs  30 , and likewise formed parallel to each other through the length of zipper  10 . 
     As shown in  FIG. 2 , this positioning increases the thickness of the structure between the apex  31  of rib  30  and the apex  33  of grip strip  32 . This increase in thickness results in further increased pressure between the apex  31  of rib  30  and the first or second walls  12 ,  14  thereby resulting in an increased quality of seal. 
       FIGS. 3 ,  4  and  5  show different configurations for the formation of the ribs  30 .  FIG. 3  shows ribs  30  formed as discrete from each other and coextruded with the base flange resin layer  40  forming the flanges  20 ,  22 .  FIG. 4  shows ribs  30  formed from a common layer  42  coextruded with the base flange resin layer  40  forming the flanges  20 ,  22 .  FIG. 5  shows ribs  30  and flanges  20 ,  22  formed as a single integral layer. 
     Ribs  30  may be formed of any of many different cross sections, including, but not limited to triangles, squares, rectangles and trapezoids. 
     In  FIGS. 3 and 4 , ribs  30  are typically formed of materials with a lower melting point than that of flanges  20 ,  22 , thereby lowering the effective seal initial temperature of the zipper assembly due to the enhanced heat transfer into the smaller mass of ribs  30 . The material used for ribs  30  can include typical sealant layer resins, such as EVA resins; metallocene catalyzed resins; plastomers; very low density polyethylenes; or terpolymers of ethylene, propylene and a third component, but is not limited thereto. Further, sealant resins can be applied between the ribs  30  to further enhance the sealing. 
     This enhanced sealing due to ribs  30  is particularly beneficial with difficult substrates, such as when the package walls  12 ,  14  are made from polypropylene. 
     Improved sealing qualities are also achieved when ribs  30  are made from the same material as the flanges  20 ,  22  as shown in  FIG. 5 . This is important in such applications as retort applications, where zipper are produced from the same materials as the package film, and no sealant layers are coextruded onto the zippers. 
     The size and spacing of the ribs  30  on zipper  10  has an optimum range of flange coverage in the area that the seal bar touches. Typically, the ribs should cover between fifteen and fifty percent of the flange area that will be under the heat sealing operation. In this way, the effect of pressure over this area is multiplied by the fraction of the surface area covered by the ribs. Typically, if the surface area covered by ribs is less than fifteen percent of the flange area under the sealing operation, there is not enough sealant to create a satisfactory bond to the film. Conversely, typically, if the ribs cover over fifty percent of the flange area under the sealing operation includes ribs, then the effect of pressure concentration is reduced. 
     The lowering of the seal initiation temperature described above effectively improves the temperature differential between the point that the slider zipper is adhered to the film substrate and the point where the inside surfaces of the zipper flanges will seal to themselves, due to the amount of heat required to attach the zipper to the package walls. The creation of a larger temperature differential on a slider zipper is especially important in transverse bag or package applications with thick films, metallized films, gusseted constructions, and combinations thereof. 
     Moreover, grip strips  32  opposite ribs  30  on flanges  20 ,  22  improve the resistance to inner flange sealing as heat transfer is minimized when the grip strips  32  of opposing flanges  12 ,  14  come into contact with each other. This can be further enhanced by coextruding grip strip  32  with a melting point which is higher than that of the base flange resin. 
     Additionally, ribs  30  increase the stiffness or modulus of the flanges  20 ,  22  of flanged zipper  10 , aiding in the transport of the flanged zipper  10  during manufacture. 
     Thus the several aforementioned objects and advantages are most effectively attained. Although preferred embodiments of the invention have been disclosed and described in detail herein, it should be understood that this invention is in no sense limited thereby and its scope is to be determined by that of the appended claims.