Patent Publication Number: US-6702133-B1

Title: Plastic retorable container system having a closure with an improved conformable liner

Description:
FIELD OF THE INVENTION 
     The current invention is directed to a closure for a container. More specifically, the current invention is directed to a plastic closure having a seal adapted to withstand retorting. 
     BACKGROUND OF THE INVENTION 
     Certain beverages and food stuffs, for example, liquid pediatric nutritional products, must be sterilized after being hermetically sealed in the containers in which they will ultimately be sold. Sterilization is typically accomplished by a retorting process in which the container is heated to relatively high temperatures, such as 260° F. or above. Although steam is used to subject the container to external pressurization during retorting in order to reduce the effects of the increased internal pressure generated by the heating, there is nevertheless a pressure imbalance that imposes additional forces on the container and closure. Thus, the closures for such containers, which are typically referred to as “retortable closures,” must be capable of maintaining a hermetic seal when subjected to the temperatures and pressures associated with retorting. 
     Although closures for many applications, such as carbonated beverages, are made entirely from plastic, in the past, closures for retortable containers were made from metal or, more recently, composites of metal and plastic, such as polypropylene. Such a composite retortable closure is disclosed in U.S. Pat. No. 4,813,561 (Ochs). In such closures, sealing between the closure and container neck is provided by a bead of sealant, such as plastisol, applied to the metallic portion of the closure. The use of metal in a retortable closure was thought necessary because the high temperatures associated with retorting weakens and distorts plastic. It was thought that unless metal was used to stiffen and stabilize the closure, especially the portion supporting the sealant, the high temperatures and pressure resulting from retorting would result in loss of the hermetic seal. 
     Consequently, it would be desirable to provide a retortable plastic closure with improved sealing capability. 
     SUMMARY OF THE INVENTION 
     It is an object of the current invention to provide a retortable plastic closure, especially an all-plastic retortable closure. This and other objects is accomplished in a retortable container system, comprising (i) a container having a neck portion forming an approximately cylindrical inner wall, the upper portion of the inner wall having a diameter d 2 , (ii) a closure, the closure comprising (i) a generally circular plastic top portion having upper and lower surfaces, and (ii) a generally cylindrical plastic skirt portion extending downwardly from the top portion, the skirt portion having means for securing the closure to the container neck portion, (iii) a seal for sealing the closure to the container. The seal comprises a (i) circumferentially extending ridge integrally formed in the plastic top portion so as to project downwardly from the lower surface thereof, the ridge forming a first structural wall extending downwardly from the lower surface of the plastic top portion, and (ii) a flexible liner bonded to the lower surface of the plastic top portion, a first portion of the flexible liner covering at least the first structural wall so as to form a downwardly extending liner wall, the liner wall having a diameter that is greater than the diameter of the container neck inner wall upper portion, whereby the container neck inner wall compresses the liner wall when the closure is applied to the container. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation of a closure according to the current invention as applied to a conventional retortable plastic container. 
     FIG. 2 is a plan view of the closure according to the current invention shown in FIG.  1 . 
     FIG. 3 is a cross-section taken along line III—III shown in FIG.  2 . 
     FIG. 4 is an isometric view of a portion of a cross-section of a closure according to the current invention showing the stiffening ribs. 
     FIG. 5 is a detailed view of a portion of a cross-section of the closure shown in FIGS. 2 and 3. 
     FIG. 6 is a detailed view of a portion of a longitudinal cross-section through the neck portion of the container shown in FIG.  1 . 
     FIG. 7 is a detailed view of a cross-section of the closure shown in FIGS. 2 and 3 after installation on a container. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A retortable plastic closure  2  according to the current invention is shown applied to a conventional retortable plastic container  60  in FIG.  1 . As shown in FIGS. 2 and 3, the closure  2  is comprised of a circular plastic top portion  4 , having upper and lower surfaces  5  and  7 , respectively, and a cylindrical skirt  6  extending downwardly from the top portion and integrally formed with it. Axially extending ribs  12 , which aid gripping of the closure, are circumferentially distributed around the skirt  6 . As is conventional, a thread  50  is formed in the inner wall of the skirt portion  6  that mates with the thread  51  formed on the outer wall of the neck portion  40  of the conventional retortable container, shown in FIG.  6 . Although threads are used in the preferred embodiment, those skilled in the art will recognize that other methods of securing the closure to the container may also be utilized, such as a snap-on configuration. 
     As shown in FIG. 6, the neck  40  of the container  60  forms an approximately cylindrical inner wall  42  that is preferably slightly conical so as to be oriented at an angle A 2  to the container axis that, for example, may be about 8°, so that the neck inner wall tapers outwardly (i.e., become thicker) as it extends downwardly. The topmost portion  45  of the straight section of the inner wall  42  has a diameter d 2 , while the uppermost portion  44  of the container neck  40  is formed by a convex segment, the top of which has a diameter d 3 . 
     Although not necessary for the practice of the current invention, as is conventional, a tamper evident band  10  may be formed on the lower portion of the skirt  6  and may include ratchet teeth that engage mating ratchet teeth formed in the container neck, such as disclosed in the aforementioned U.S. Pat. No. 4,813,561, hereby incorporated by reference herein. 
     Preferably, the top portion  4 , skirt  6 , and tamper evident band  10  are integrally formed by injection molding a plastic, such as polypropylene. The polypropylene is of a grade having a sufficiently high heat distortion temperature, typically referred to as HDT, so as to be able to withstand retorting, such as SV 954 grade polypropylene available from Montell U.S.A. Inc. of Wilmington, Del. In the preferred embodiment, the closure is made entirely of plastic and no metal is used to strengthen the top portion  4  or skirt  6 . 
     A number of half-moon shaped openings  18  are formed in the periphery of the top portion  4 . The openings  18  allow the threaded portions of the closure and container neck to be washed after the closure  2  has been secured to the container, for example, to remove fluid that spilled onto the threaded portions during filling and capping. 
     According to one important aspect of the current invention, a circumferentially extending ridge  24  is integrally formed in the lower surface  7  of the top portion  4 . The ridge  24  serves to resist radially inward pressure resulting from compression of a liner  20  by the container neck, as discussed below. Preferably, the ridge  24  projects downwardly from the adjacent portions of the lower surface  7  by at least about 0.075 inch (1.9 mm). 
     As shown best in FIG. 5, the inner surface of the ridge  24  forms a circumferentially extending wall  23  that is oriented at an angle C to the axis of the closure  2  that is at least about 45°, and preferably about 60°. The angling of the wall  23  allows the liner material to flow over the lower surface  7  during compression molding, as discussed below. The outer surface of the ridge  24  forms a circumferentially extending structural wall  25  that is preferably disposed an at angle A 0  to the axis of the closure  2  so that the wall tapers inwardly as it extends downwardly. Preferably, the angle A 0  is at least about 5° but not more than about 15° greater than the angle A 2  of the container neck inner wall  42 . Most preferably, A 0  is about 7° greater than A 2 . In one embodiment of the invention, the angle A 0  is approximately 15° and the angle A 2  is about 8°. A circumferentially extending structural wall  26  is also integrally formed in the lower surface  7  disposed adjacent the ridge wall  25 . Preferably, the wall  26  is oriented approximately radially. Finally, an axially oriented, circumferentially extending structural wall  29  is formed adjacent the wall  26 . 
     If no additional modifications are made to the design, the presence of the ridge  24  would increase the difference between the thick and thin portions of the top portion  4 . In one particular embodiment of the invention, the thickness of the central panel portion of the top portion  4  is about 0.06 inch (1.5 mm) and the ridge  24  projects downwardly about 0.077 inch (2.0 mm). This would result in a thickness in the ridge area of approximately 0.137 inch (3.5 mm) so that the ratio of the maximum to minimum thickness of the top portion was almost 2.3. Such a nonuniform thickness profile can result in temperature gradients that cause distortion of the closure  2  as it cools following injection molding, creating sink marks. 
     Therefore, in a preferred embodiment of the invention, a circumferentially extending groove  14  is formed in the upper surface  5  of the top portion  4  opposite the ridge  24  so that the distance between the bottom of the groove and the surface of the ridge defines the thickness of the top portion in the vicinity of the ridge, thereby resulting in a more uniform thickness throughout the top portion. For example, in the particular embodiment discussed above, the depth of the groove  14  is about 0.047 inch, so that the thickness of the top portion  4  in the vicinity of the ridge  24  is only 0.090 inch (2.29 mm). This reduces the maximum to minimum thickness ratio in the top portion  4  to about 1.5 and prevents distortion of the closure  2  following injection molding. 
     Unfortunately, the presence of the groove  14  can also reduce the strength of the closure  2 , especially in light of the presence of the washing openings  18  nearby. Thus, in a preferred embodiment of the invention, a network of ribs  16  and  17  are integrally formed within the groove  14 , as shown best in FIGS. 2 and 4. Preferably a first portion  17  of the ribs are distributed around the circumference of the groove  14  at 45° intervals and extend in the approximately radial direction. A second portion  16  of the ribs extend at an angle B to the radial direction so as to form crosses. Preferably, the angle B is about 20° to 30°. 
     According to the invention, a flexible sealing liner  20  is attached to the lower surface  7  of the top portion  4 . The liner  20  is made from a flexible plastic material, preferably a conventional thermoplastic elastomer, such as Kraton™, developed by Shell Chemical Company and available from GLS Corporation&#39;s Thermoplastic Elastomers Division. Most preferably, the liner  20  is a thermoplastic elastomer with oxygen barrier properties, such as Trefsin™, available from Advanced Polymer Systems LP of Akron, Ohio, or Polyliner™, available from D.S. Chemie of Germany. 
     Preferably, the liner  20  is formed by extruding a bead of thermoplastic elastomer onto the center of the top portion lower surface  7  and then compression molding it so as to form a liner bonded to the portion of the lower surface  7  encompassed by the axially extending wall  29 . Preferably, the liner  20  is compression molded so as to have a thickness of about 0.030 inches (0.76 mm) in the central portion of the lower surface  7  and about 0.040 inches (1 mm) in the vicinity of the ridge  24 —that is, the portion of the liner  20  covering the ridge  24  should be about ⅓ thicker than the central portion of the liner in order to minimize the amount of liner material utilized. Although additional increases in the thickness differential by further thinning of the central portion of the liner  20  would reduce material usage even more, it may require the application of excessively large pressures by the punch during compression molding and, therefore, is preferably avoided. 
     As shown best in FIG. 5, the liner  20  covers the entirety of the ridge  24  formed in the lower surface  7 , including the inner ridge wall  23  and the outer ridge wall  25 . The portion of the liner  20  covering the inner ridge wall  23  forms a downwardly extending liner inner wall. The portion of the liner  20  covering the outer ridge wall  25  forms a circumferentially extending and downwardly projecting outer liner wall  27 . Preferably, the length of the downwardly projecting wall is at least about 0.075 inch (1.9 mm) long. Preferably, the downwardly projecting liner wall  27  is oriented at an angle A 1  that approximately matches the angle A 0  of the structural wall so that the liner wall tapers inwardly as it extends downwardly to a lowermost portion  41 . Thus, the angle A 1  to is greater than the angle A 2  of the container neck wall  42 , preferably by at least about 5° and more preferably by at least about 7° but not more than about 15°. In addition, the diameter D 2  of the uppermost portion  47  of the downwardly projecting liner wall  27  is greater than the diameter d 2  of the upper portion of the container neck wall  42 . 
     Since the diameter D 2  of the upper liner wall is greater than the diameter d 2  of the upper neck wall, as the closure  2  is threaded onto the container neck  40 , the upper portion  47  of the liner wall  27  will initially make line contact with the neck wall  42 . Under the action of the threads, further rotation of the closure  2  drives it downward onto the container neck  40 . This causes the liner  20  to compress so that the upper surface  47  of the liner wall  27  conforms to the upper portion of the neck inner wall, resulting in surface contact between the liner wall  27  and the container neck inner wall  42  at the upper portion of the liner wall. As the closure  2  is tightened further onto the container neck  40 , the surface of the liner wall  27  conforming to the neck outer wall  42  spreads downward so that middle surface of the liner wall then conforms to the neck wall. Preferably, this is followed by the lower portion  41  conforming to the container neck inner wall  42  when the closure  2  is fully tightened onto the container neck. Thus, the diameter D 2  is sufficiently greater than d 2  so that, despite the fact that A 1  is greater than A 2 , the compression of the liner  20  will eventually cause a major portion, and most preferably the entirety, of the downwardly projecting liner wall  27  to conform to the surface of the neck outer wall  42 . Preferably, the diameter D 2  of the top of the liner wall  27  is at least about 0.010 inches (0.25 mm) greater than the diameter d 2  at the top of the container neck wall. In one embodiment of the invention, D 2  is about 0.014 inch (0.36 mm) greater than d 2 . 
     Causing the compression of the liner  20  to spread downward as described above causes the majority of elastic deformation or flow of the liner to occur downwardly, thereby maximizing the compression of the liner that results from a given magnitude of torque applied to the closure  2 —that is, the amount of compression that is ultimately achieved is greater than it would have been if the initial contact had been along the entire surface of the liner wall  27  so that further compression required the liner to flow elastically not only downwardly, but upwardly, where there is greater resistance to such elastic flow. 
     As also shown in FIG. 5, the portion of the liner  20  covering the radially extending wall  26 , which is adjacent the downwardly extending liner wall  27 , forms a radially extending upper liner wall  28 . Since the liner  20  preferably extends to the axial wall  29  of the top portion, the diameter of the periphery of liner wall  28  is also D 3 . Preferably, D 3  is greater than the diameter d 3  of the top of the uppermost portion  44  of the container neck  40  so as to ensure that a major portion, and preferably, substantially the entirety, of the upper liner wall  28  is compressed by, and conforms to, the surface of the uppermost portion  44  of the container neck  40  when the closure is threaded onto the neck. 
     As shown in FIG. 7, preferably, when the closure  2  of the current invention is threaded onto the container neck  40 , substantially the entirety of the liner downwardly projecting wall  27  is compressed radially inward by the upper portion of the neck inner wall  42 . In addition, substantially the entirety of the liner upper wall  28  is compressed axially upward by the uppermost portion  44  of the container neck  40 . (Note that the uncompressed location of the liner walls  27  and  28  is shown by the dashed line in FIG. 7.) This compression ensures that flexible liner  20  creates sealing contact over a large surface so as to provide a robust hermetic seal between the closure  2  and the container neck  40 . The compression also causes a portion  21  of the liner  20  to extrude outward through the gap formed between the upper portion of the container neck outer wall  43  and the closure axial wall  29 , as also shown in FIG.  7 . This provides further sealing contact between the container neck  42  and the closure  2 . Thus, a seal is provided not only along the uppermost portion  44  of the container neck  40  but also along the upper portion of its outer wall  43 . 
     As a result of the extensive and robust sealing discussed above, and despite the fact that the closure  2  is preferably made entirely of plastic and is, therefore, subject to distortion and weakening during retorting, retorting will not result in a failure of the hermetic seal. Rather, such distortion will be accommodated by variation in the compression to which the liner  20  is subjected along both its downwardly projecting wall  27  and its upper wall  28 . 
     Although the present invention has been illustrated in a system in which an outward facing liner wall contacts and conforms to the inner container neck wall, the invention could also be practiced by arranging the ridge on a larger diameter so that the inner facing liner wall contacted and conformed to the outer wall of the container neck. Thus, the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.