Patent Publication Number: US-2009223920-A1

Title: Abuse resistant preform and container neck finish

Description:
TECHNICAL FIELD 
     The present invention is directed generally to plastic preforms and containers, and more specifically to a preform and container neck finish able to withstand the mechanical abuse and rigors of multiple use cycles, while maintaining a secure seal with a closure. 
     BACKGROUND OF THE INVENTION 
     Conventional manufacture of plastic (e.g., polyethylene terephthalate (PET)) containers employs an initial step of forming a substantially amorphous and unoriented parison or preform by an injection molding process. The process of crystallizing the finish of such preforms is sometimes used to thermally stabilize the dimensions of the finish, and in particular the finish threads, for high temperature applications such as hot filling and pasteurization. 
     In hot filling and pasteurization applications, the sealed container undergoes significant pressure changes, i.e., the pressure increase due to filling with a heated liquid and immediate capping or the exposure to a high temperature sterilizing bath/spray, followed by a decrease in pressure (vacuum) as the product in the container cools. These internal pressure changes cause the threads on the cap to push up or down on the warm threads of the finish, distorting the finish threads. If the warm finish threads distort, the cap becomes loose and there is a loss of product quality (e.g., exposure to the atmosphere and possible contamination) and/or leakage. 
     There is another class of cold-filled carbonated beverage containers, known as returnable/refillable containers (also known as refillable containers). As distinguished from one way or disposable bottles which are intended for a single use (i.e., only one cycle of manufacture, filling, distribution, consumption and disposal), refillable containers are designed to withstand the rigors of multiple pressurized cold filling and use cycles, including a hot caustic wash between each use and refilling. Refill containers are uncapped when cleaned (by the hot caustic wash), so the finish threads are not exposed to the capping forces at high temperatures, as previously described. However, refill containers are subjected to greater opportunities for mechanical abuse. Not only must they typically withstand at least ten cycles of re-use to be commercially viable as refillable containers, but after each use they are potentially subject to unintended conditions of storage, handling and misuse by the consumer (e.g., filling with other liquids, crushing, and/or use for other purposes, such as a candle holder) which can seriously affect their ability to survive the intended number of commercial re-use cycles. In particular, refill containers have a substantially unoriented and amorphous neck finish of relatively low mechanical strength. As such, the finish can easily be nicked or dented during such uncontrolled handling by the consumer prior to their return to the manufacture for cleaning and re-use. This denting of the neck finish is a particular problem because the neck finish is one area of the container with particularly tight dimensional tolerances necessary to ensure a tight seal with the closure. With carbonated beverages, a tight seal is particularly important to avoid a loss of carbonation pressure which reduces product quality and/or shelf life. For these reasons, refillable containers are subject to a leakage test after refilling and a substantial number of refillable containers are rejected for failure to withstand the leakage test. 
     It would thus be desirable to provide a neck finish for a refillable pressurized container able to withstand the rigors of multiple use cycles and maintain a tight closure seal. As always, the commercial viability of such an improved neck finish will be determined by the relative costs of materials, processing steps, and required apparatus for achieving the improved finish. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the invention, there is provided a neck finish of a preform or container for receiving a closure and adapted to withstand the abuse of multiple use cycles without leakage failure. The neck finish includes a thread, bead or snap-on mechanism for attaching the closure, and an open upper end with a top sealing surface (TSS) which forms a liquid tight seal with the closure, wherein the TSS is crystallized. 
     The neck finish may be for a pressurized returnable and refillable container. The crystallized TSS provides a more secure finish-closure seal, thus increasing the service life of the container. Preferably, the container can withstand at least 10 refill cycles, including a hot caustic wash at a temperature of at least 60° C. and pressurized filling of at least 3 atmospheres, while maintaining a secure seal with the closure. More preferably, the container can withstand at least 20, and still more preferably at least 25 refill cycles. 
     The crystallized portion of the neck finish extend from the TSS downwardly and terminates above the thread, bead or snap-on mechanism. The neck finish may be made of a crystallizable polymer selected from the group consisting of polyesters, polyolefins and polyamides. The neck finish in one embodiment comprises polyethylene terephthalate homopolyer or copolymers. The neck finish may be injection molded. Apart from the crystallized TSS which is rendered opaque, the remainder of the neck finish may be substantially transparent. The refillable container may be substantially transparent below the opaque TSS. 
     The neck finish of the container may engage a closure having a sealing fin or liner for engaging the TSS. 
     In another embodiment, a container is provided comprising a pressurized returnable and refillable container which is cold filled with a carbonated beverage, and able to withstand at least 10 refill cycles including a hot caustic wash at a temperature of at least 60° C. and pressurized filling of at least 3 atmospheres, the container having a neck finish made of a substantially transparent, amorphous and unorientied crystallizable polymer, and a crystallized opaque top sealing surface (TSS). 
     The container may include a closure which forms a liquid tight seal with the TSS. The polymer may comprise polyethylene terephthalate homopolymer of copolymers. The container can preferably withstand at least 20 refill cycles and more preferably 25 refill cycles. 
     In another embodiment, a method is provided of making a neck finish of a preform or container for withstanding the abuse of multiple use cycles. The method includes providing a neck finish having a thread, a bead or snap-on mechanism for attaching a closure and above the attaching mechanism a top sealing surface (TSS) for forming a liquid tight seal with a closure, the method including crystallizing the TSS. 
     The neck finish may comprise a substantially transparent, amorphous and unoriented crystallizable polymer and the TSS is heated to crystallize and render the TSS opaque. 
     The neck finish may be for a pressurized returnable and refillable container. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
         FIG. 1  is a partial perspective view of a top end of a preform according to one embodiment of the invention, having a neck finish with a crystallized top sealing surface (TSS); 
         FIG. 2  is a partial perspective view of a top end of a container, blown from the preform of  FIG. 1 , and sectioned vertically into two halves, showing the neck finish of the container with a crystallized TSS; 
         FIG. 3  is a schematic illustration of one embodiment of a neck finish and closure; 
         FIG. 4  is a schematic illustration of another embodiment of a neck finish and closure; 
         FIG. 5  is a schematic cross sectional view of one embodiment of a preform for making a refillable container; 
         FIG. 6  is a schematic elevational view of a 1.5 liter PET refillable carbonated beverage bottle produced from the preform of  FIG. 5 , which illustrates the blow molding process by showing a partially blown article formed as the preform is expanded to form the container. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with various embodiments of the invention, a neck finish of a preform or container is provided having a crystallized top sealing surface for withstanding the abuse of multiple use cycles. 
     One exemplary embodiment of a preform and container having a neck finish with a crystallized top sealing surface (TSS) according to the invention is illustrated in  FIGS. 1-6 .  FIGS. 1-2  show a sample preform having a crystallized TSS, and the resulting container finish having a crystallized TSS.  FIGS. 3-4  show two examples of closures applied to the finish, illustrating the seal between the crystallized TSS and interior top surface of the closure.  FIGS. 5-6  show cross sections of a preform and the resulting blow molded refillable container. 
       FIG. 1  shows the top end of a preform  10  including a tapered shoulder portion  16 , an increased diameter capping flange  11 , and above the capping flange, a neck finish  14 . The neck finish  14  includes exterior screw threads  15  and above the threads a top sealing surface  13 . The entire preform is substantially transparent, except for the top sealing surface  13  which has been rendered milky white or opaque, due to crystallization. 
       FIG. 2  is a cut away view of an upper end of a container formed from the preform of  FIG. 1 . The container has been vertically sliced in half, producing a left half  20 A and a right half  20 B. As shown, the container includes an upper tapered dome or shoulder portion  21 A,  21 B, which has been expanded by blow molding the corresponding section  16  of the preform. The result is a substantial reduction in wall thickness and increase in orientation. In contrast, the capping flange  11 A,  11 B and neck finish  14 A,  14 B have not been expanded, and retain the same dimensions as in the preform  10 . Again, the entire refillable container  20  is substantially transparent, except for the crystallized top sealing surface  13 A,  13 B. 
     The TSS has been crystallized to improve its nick resistance and durability. In a returnable/refillable container, the finish is susceptible to nicks, abrasion and wear through repeated usage. The present invention is directed to crystallizing only the top sealing surface of the finish in order to reduce the mechanical damage, such as nicks, abrasion, etc. during the return and refill cycles. Reducing the mechanical damage to the finish will reduce or prevent carbonation loss and increase the service life of the container by providing a more secure finish-closure seal. In particular, an improvement in caustic stress crack resistance has been found, i.e., the ability to withstand 10 returnable and refillable cycles (a hot caustic wash of at least 60° C. followed by pressurized filling of at least 3 atmospheres), more preferably 20 cycles, and still more preferably 25 cycles, without leakage of the closure (see the TSS/closure leakage test described below). 
       FIG. 3  illustrates schematically and in cross section a typical threaded closure  52  applied to a container neck finish  14 . The closure includes a top wall  53  and a depending annular skirt  54  extending down from the top wall. The interior wall of the skirt includes an internal thread  55  for engaging a complimentary external thread  15  on the container neck finish. The closure may be made of, for example, polypropylene (PP) and/or polyethylene (PE) material. The closure further includes a liner  56  covering at least a portion of the underside of the top wall which engages the top sealing surface (TSS)  13  of the finish. The liner may be made of, for example, ethylene vinyl alcohol (EVA) and/or a thermoplastic elastomer material such as an olefin or styrene butadiene styrene (SBS). As shown, the liner is intended to provide a tighter seal with the TSS. The carbonation pressure, illustrated by arrows  5  exerting an upward force on the closure liner, acts to push away (separate) the cap liner from the TSS of the neck finish. Maintaining a tight seal of the neck finish enclosure requires maintaining a sufficiently high downward force exerted by the closure threads on the finish threads to maintain a seal between the TSS and cap liner, effectively counteracting the upward carbonation pressure. By providing a crystallized TSS  13 , the TSS can withstand a higher capping force, as well as maintaining resistance to nick abrasion and wear due to repeated usage. 
     One measure of the improved seal provided by the crystallized TSS is to conduct a leakage test as is known in the art. The test for leakage is performed in a “Secure Seal Tester” distributed by SecurePak, P.O. Box 1210, Maumee, Ohio 43537, USA. The Secure Seal Tester (SST) is used for checking thread integrity and a proper seal on glass or plastic carbonated beverage containers that use an aluminum roll-on, twist crown, or plastic cap. By detecting gas leakage, rather than liquid leakage, the SST gives more sensitive and accurate readings (see www.secure-pak.com). According to one embodiment, if the container sample does not leak at 175 psi, then it passes the leakage test. 
     To perform the test, a finish is cut from a plastic bottle or preform and placed in a fixture. The fixture is placed in a water tank and a hose attached for increasing the internal pressure until the closure leaks (i.e., bubbles are seen in the tank) and the air pressure of leakage is recorded. In one embodiment, a refillable two liter carbonated beverage container is leak tested at 175 psi for 60 seconds. If it can withstand this pressure and time, it passes the leakage test. The closure used in the test is the same closure as is used commercially for the sample container. 
       FIG. 4  shows another embodiment of a closure  70  on a neck finish  14 . Closure  70  includes a top wall  72  having an interior downwardly extending annular sealing fin  74  (rather than a liner) for forming a liquid tight seal with the TSS  13  of the finish. 
     The TSS may be crystallized by any of the known methods used in the art. Generally, a finish portion may be thermally crystallized by placing the finish portion adjacent a heating element, such as a radiant heater, at a suitable temperature and for sufficient time to crystallize the polymer in the area of the TSS. For example, for a PET refillable carbonated beverage container, the heater may be positioned in a range of about ⅜ to ½ inches from the TSS, the heater being in a temperature range of about 500 to 1250° F., and the crystallizing taking about 30 to 75 seconds. Adjustments to the time and temperature can be made for other preform materials and preforms of varying dimensions, including the desired depth of crystallization of the TSS. In accordance with the present invention, it is intended to crystallize only the area of the TSS, above the threads of the neck finish. The crystallization may be substantially uniform or graded (e.g., of decreasing crystallization levels moving away from the TSS). 
     A particular example of a refill preform and container will now be described, this being one application in which a container neck finish is subjected to increased opportunities for damage, including cracking, at the TSS. 
       FIG. 6  shows a typical 1.5 liter PET refillable carbonated beverage bottle. Generally, the refillable container has a relatively thin and highly oriented sidewall and a relatively low orientation and thickened base portion. This combination enables the container to withstand the demands of multiple returnable/refillable cycles. 
     More specifically, the bottle  20  is of a unitary blow-molded construction, including a bi-axially oriented hollow body having a closed bottom end  26  and an open top end  12  with a neck finish  14 , extending above a capping flange  11 . The neck finish has external screw threads  15  for receiving a screw-on closure (not shown). Between the neck finish and base is a substantially vertically disposed sidewall  25 , including an upper domed shoulder portion  21  above a substantially cylindrical panel portion  22  (as defined by a longitudinal center line CL) of the bottle. The base  26  has a central outwardly-concave dome  23  with a central gate portion  24 , and an inwardly-concave chime  28  including a standing ring on which the container rests. A radially increasing outer base portion  30  provides a smooth transition from the chime  28  to the sidewall  22 . 
     A preform of the type suitable for making the container of  FIG. 6  is shown in  FIG. 5 . The preform  10  has the same open top end  12  and unexpanded neck finish  14  with threads  15 , as the container  20 . The preform has a substantially elongated tubular body portion, including a tapered shoulder forming portion  16 , a substantially cylindrical sidewall forming portion  17 , and a thickened base forming portion  18 , which tapers to a reduced thickness end cap  19 . The cylindrical preform sidewall  17  has a thickness of about 6.1 mm and is stretched at an average planar stretch ratio of about 10:1 to produce a bottle sidewall (panel  22 ) of about 0.6 mm. The planar stretch ratio is the ratio of the average thickness of the panel forming portion of the preform to the average panel thickness of the bottle. The preferred planar stretch ratio for polyester refillable beverage bottles of about ½ to 2.5 liters/volume is about 7-14:1, and more preferably about 8-13:1. The hoop stretch is preferably from 3-4:1 and the axial stretch from 2-3.5:1. This produces a container sidewall panel with the desired abuse resistance, and an injection molded preform sidewall with desired visual transparency. The sidewall thickness and stretch ratio selected are based on the dimensions of the specific bottle, the internal pressure of the product (e.g., 3 atmospheres for beer, 4 atmospheres for soft drinks), and the processing characteristics of the particular material (as determined, for example, by the intrinsic viscosity). 
       FIG. 6  includes schematic profiles of the preform  10  prior to and during expansion to form the container  20 . The preform body is placed in a blow mold cavity (not shown) and the area beneath the capping flange  11  is axially and radially expanded to conform to the surface of the mold cavity. A stretch rod (not shown) axially extends the preform such that the preform end cap  19  is brought into engagement with the bottom of the blow mold cavity, centering the preform in the cavity. The axial extension of the stretch rod occurs simultaneously and/or sequentially with radial expansion in the preform.  FIG. 6  shows an intermediate partially expanded body  32  during the blow molding process. As illustrated, the thinner preform sidewall  17  is more easily stretched and thus undergoes greater stretch than the base  18 , resulting in a relatively thin, highly oriented bottle sidewall  22  and much thicker and low orientation base  26 . 
     The 1.5 liter container of  FIG. 6  is about 335 millimeters in height and about 93.5 millimeters in widest diameter. The container has varying levels of crystallinity corresponding to the extent to which the bottle wall portion is stretched to produce strain-induced crystallization and optionally heated to produce thermal-induced crystallization. The panel portion  22  is blown to the greatest extent and has the highest average percent crystallinity of 25-35%. The tapered shoulder  21  has an average percent crystallinity of 20-30%. In contrast, the substantially thicker and lesser blown base  26  has 0-2% crystallinity in the central gate  24 , 2-8% in the chime  28 , and a range of crystallinity there between the dome  23 . The outer base  30  has a crystallinity range between that of the chime  28  and the panel  22 . The neck finish  14  is not expanded and remains substantially amorphous and unoriented, at 0-2% crystallinity. 
     To maintain transparency, below the TSS of the neck finish, any thermal induced crystallinity should be at relatively low temperature, e.g., contact with a mold at a mold temperature of 110-140° C. for PET. The percent crystallinity is determined according to ASTM D1505 as follows: 
       % crystallinity=[( ds−da )/( dc−da )]×100 
     where ds=sample density in g/cm 3 , da=density of an amorphous film of 0% crystallinity (for PET 1.333 g/cm 3 ) and dc=density of the crystal calculated from unit cell parameters (for PET 1.455 g/cm 3 ). 
     A refillable container is generally substantially transparent to enable contaminant inspection. In accordance with the present invention, the crystallized TSS will not substantially affect contaminant inspection, and if necessary the TSS can be inspected for contaminants by another measure, such as visually. One measure of transparency is the percent haze for transmitted light through the wall (H T ) which is given by the following formula: 
         H   T   =[Y   d ÷( Y   d   +Y   s )]×100 
     where Y d  is the diffuse light transmitted by the specimen, and Y s  is the specular light transmitted by the specimen. The diffuse and specular light transmission values are measured in accordance with ASTM method D 1003, using any standard color difference meter such as model D25D3P manufactured by Hunterlab, Inc. A commercial refillable container would generally have a percent haze (through the wall) of less than 15%, preferably less than 10%, and more preferably less than about 5%. 
     The orientable crystallizable materials useful in making returnable/refillable containers include thermoplastic polyester materials such as those based on polyalkylene and, in particular, polyethylene terephthalate (PET). PET is meant to include the use of copolymers of PET in which a minor proportion, for example, up to about 10% by weight, of the ethylene terepthalate units are replaced by compatible monomer units. Thus, as used herein, PET means PET homopolymer and PET copolymers of the grades suitable for making containers, which are well known in the art. For example, the glycol moiety of the monomer may be replaced by aliphatic or alicyclic glycols such as cyclohexane dimethanol (CHDM). The dicarboxylic acid moieties may be substituted with, for example, aromatic dicarboxylic acid such as isopthalic acid (IPA). 
     The PET polymers may contain other compatible additives and ingredients which do not adversely affect the performance characteristics of the container. Examples of such ingredients include thermal stabilizers, light stabilizers, dyes, pigments, plasticizers, fillers, anti-oxidants, lubricants, extrusion aids, residual monomer scavengers and the like. 
     The intrinsic viscosity (IV) affects the processability of the polymer resins. PET having an intrinsic viscosity of about 0.8 is widely used in the carbonated soft drink industry. Resins for various applications may range from about 0.55 to about 1.04, and more particularly from about 0.65 to 0.85. Intrinsic viscosity measurements are made according to the procedure of ASTM D-2857, by employing 0.0050±0.0002 g/ml of the polymer in a solvent comprising o-chlorophenol (melting point 0° C.) at 30° C. 
     The preform for making the transparent refillable bottle should be substantially amorphous, which for PET means up to about 10% crystallinity, preferably no more than about 5% crystallinity, and more preferably no more than about 2% crystallinity. The substantially amorphous or transparent nature of the preform may alternatively be defined by a percent haze (HT) of about no more than 20%, preferably no more than about 10%, and more preferably no more than about 5%. The substantially amorphous preform may be a single layer or a multi-layer preform made according to well known injection molding processes. 
     A commercial refillable container can generally withstand at least ten (10) refill cycles and, more preferably twenty (20) refill cycles, while maintaining its aesthetic and functional features. A test procedure for simulating such a refill cycle without leakage is described below. 
     Each container is subjected to a typical commercial caustic wash solution prepared with 3.5% sodium hydroxide by weight and tap water. The wash solution is maintained at a designated wash temperature of at least 60° C. The bottles are submerged uncapped in the wash for fifteen (15) minutes to simulate the time/temperature conditions of a commercial bottle wash system. After removal from the wash solution, the bottles are rinsed in tap water and then filled with a carbonated water solution at 4±0.2 bar (4.0±0.2 atmospheres) to simulate the pressure in a carbonated soft drink container (CSD) and then capped and placed in a 38° C. convection oven at 50% relative humidity for twenty-four (24) hours. This elevated oven temperature is selected to simulate longer commercial storage periods at lower ambient temperatures. Upon removal from the oven, the containers are emptied and again subjected to the same refill cycle, until failure. 
     Although several preferred embodiments of the invention have been specifically illustrated and described herein, it is to be understood that variations may be made in the preform and container construction, materials, and method of forming the same without departing from the scope of the invention as defined by the appended claims.