CA1261304A - Method of obtaining acceptable configuration of a plastic container after theremal food sterilization process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a high oxygen barrier thermally sterilizable plastic container for packaging food com-prised of a high oxygen barrier layer and one or more structural layer(s) which consist(s) essentially of polyolefin(s), which container has been annealed and thereby shrunk such that It has a residual shrinkage of about 2% or less at the temperature(s) at which the container will be thermally sterilized when It Is filled with food and sealed.

Description

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Thls Inventlon ~enerally relates to contalners used for packa~lng foods and, In one aspect, It relates to a method of Improvlng the confl~uratlon of packed plastlc contalners after thermal processlng of the contalner and Its content. In another aspect, the present Inventlon Is concerned wlth attalnlng accept-able confl~uratlon of such contalners after thermal processlng.
In stlll another aspect, the present Inventlon relates to proper desl~n of plastlc contalner to Improve thelr confl~uratlon after thermal processlng.
Thls appllcatlon Is a dlvlslonal appllcatlon of copend-lng appllcatlon No. 444,658 flled January 4, 1984.

It is common knowledge ln the food packaging industry that after a container is filled with certain foods and is clo-sed, the container and its content must be thermally processed to sterlllze the food so that it will be safe for human consumption.

Thermal processln~ of such containers Is normally car-rled out at temperatures hl~her than about 190 F. In varlous equlpment such as rotary contlnuous cookers, stlll retorts and the llke, and the contalners are subJected to varlous cook-cool cycles before they are dlscharged, stacked and packed for shlp-ment and dlstrlbutlon. Under these thermal processln~ condl-tlons, plastlc contalners tend to become dlstorted or 3~

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deformed due to sidewall panelling (buckling of the containersidewall) and/or distortion of the container bottom wall, some-times referred to as ~bulging~ or ~rocker bottom~. These defor-mations and distortions are unslghtly, and interfere with proper stacking of the containers during their shipment, and also cause them to roc~ and to be unstable when placed on counters or table tops. In addition, bottom bulging is, at times, considered to be a posslble indicat~on of spoilage of the food thus resulting in the re;ection of such containers by consumers.
One reason for the distortion of the container is that during thermal processing the pressure within the container exceeds the external pressure, i.e., the pressure in the equip-ment in which such process is carried out. One solution to this problem is to assure that the external pressure always exceeds the lnternal pressure. The conventional means of achieving this condition is to process the filled container in a water medium with an overpressure of air sufficient to compensate for the internal pressure. This is the means used to process foods packed in glass ~ars and in the well-known "retort pouch". The chief disadvantage of this solution is that heat transfer in a water medium is not efficient as heat transfer in a steam atmo-sphere. If one attempts to lncrease the external pressure in a steam retort by adding air to the steam, the heat transfer effi-ciency will also be reduced relatlve to that in pure steam.

Several factors contribute to the increase in internalpressure within the container. After the container is filled with food and hermeticall~ closed, as a practical matter, a small amount of air or other gases will be present in the headspace above the food level in the container. This headspace of alr or gas is present even when the container is sealed under partial vacuum, in the presence of steam (flushing the container top with steam prior to closing) or under hot fill conditions (l90OF).
When the container is heated during thermal processing, the headspace gases undergo significant increases in volume and 1~ 6~ ~3~

pressure. Additional internal pressures will also develop due to thermal expansion of the product, increased vapor pressures of the products, the dissolved gases present within the container product and the gases generated by chemical reactions in the product during its cooking cycle. Thus, the total internal pressure within the container during thermal processing is the sum total of all of the aforementioned pressures. When this pressure exceeds the external pressure, the container will be distorted outwardly tending to çxpand the gases in the headspace thereby reducing the pressure differential. When the container is being cooled, the pressure within the container will decrease.
Consequently, the sidewall and/or the bottom wall of the container will be distended inwardly to compensate for the reduction in pressure.

It has been generally observed that such thermally pro-cessed plastic containers may remain distorted because of bulging in the bottom wall and/or sidewall panelling. Unless these deformities can be eliminated, or substantially reduced, such containers are unacceptable to consumers.

It must also be noted that it is possible to make a container from a highly rigid resin with sufficient thickness to withstand the pressures developed during thermal processlng and thus alleviate the problems associated therewith. However, 3~

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practlcal conslderatlon and economy rnllltate agalnst the use of such contalners for food packaglng.

Accordlngly, the present Inventlon Improves the conflg-uratlon of a plastlc contalner after thermal processlng.

The present Inventlon also allevlates the problems assoclated wlth bottom bulglng and slde wall panelllng of a plas-tlc contalner whlch result from thermal processlng.

The present Inventlon agaln attalns an acceptable con-talner confIguratlon after such contalner Is packed wlth food, hermetlcally closed and thermally processed.

The present Inventlon further provldes methods, and contalner conflguratlons whlch permlt plastlc contalners to have acceptable conflguratlons desplte thelr havlng been subJected to thermal food processlng condltlons.

The present Inventlon agaln facllltates thermal food processlng of plastlc contalners packed wlth food.

Accordlng to one aspect of the Inventlon of the copend-lng applIcatlon No. 444,658, there Is provlded a method of ther-mal sterlllzatlon of a plastlc contalner packed wlth food to obtaln a thermally sterlllzed packed contalner havlng an accept-able conflguratlon, comprlslng pre-shrlnklng the contalner, flll-lng the pre-shrunk contalner wlth food, seallng the contalner, elther or both of sald fllllng and seallng steps Includlng selectlng an Inltlal contalner headspace volume and an amount of gas, taklng Into account an Inltlal vacuum level, if any, at sealIng such as to cause bulglng of the contalner bottom wall and subsequent reformatlon of the contalner bottom wall wlthout slg-nlflcant slde wall panelllng, thermally sterlllzlng the packed contalner at a temperature and pressure a tIme suffIclent to sterllize the contalner and food and to cause the contalner bot-0~

tom wall to bulge, and, reforming the bulged contalner bottom wall by provldlng that the plastlc of the bulged contalner bottom wall Is at a reformable temperature at whlch the plastlc Is soft whlle provldlng a pressure dlfferentlal such that the pressure external of the contalner exceeds the pressure Internal the con-talner. Sultably sald pre-shrlnklng Is attalned by anneallng sald contalner at an elevated temperature untll the contalner becomes essentlally non-shrlnkable upon further anneallng at sald temperature. Preferably sald annealIng temperature Is from about 1900F to about 270F Deslrably sald pre-shrlnklng step Is effected durlng the contalner maklng operatlon.

Accordlng to another aspect of the copendlng applIca-tlon there Is provlded a method of thermal sterlllzatlon of a plastlc contalner packed wlth food to obtaln thermally sterlllzed packed contalner havlng an acceptable conflguratlon, comprlslng fllllng the contalner wlth food, seallng the contalner, elther or both of sald fllllng and seallng steps Includlng selectlng an Inltlal contalner headspace volume and an amount of gas, taklng Into account an Inltlal vacuum level, If any, at seallng such as to permlt bulglng and subsequent reformatlon of the contalner bottom wall wlthout slgnlflcant slde wall panelllng, thermally sterlllzlng the packed contalner In a retort operated at a tem-perature and pressure for a tlme suffIclent to sterlllze the con-talner bottom wall, coollng the contalner and Its contents, anddurlng the coollng step, reformlng the contalner bottom wall to attaln an acceptable contalner conflguratlon by controlllng ambl-ent pressure external of the contalner and the coollng condl-tlons, sald controlllng step Includlng provldlng that plastlc of the bottom wall of the contalner Is at a reformable temperature at whlch the plastlc Is soft, whlle provldlng a pressure dlffer-entlal such that the pressure external of the contalner exceeds the pressure Internal the contalner. Sultably, sald reformlng Is achleved whlle the bottom wall of sald contalner Is at a 3~ reformable temperature. Deslrably, sald reformlng Is achleved by provldlng a pressure wlthln the contalner. Preferably, sald - 4a -L;~O~

reformlng Is achleved by providing a pressure exterlorly of sald contalner whlch exceeds the Internal pressure wlthin the con-talner. More preferably, sald reformlng Is achleved by gradually coollng sald contalner and reduclng the Internal pressure In the contalner and reduclng the Internal pressure In the contalner relatlve to the external pressure.

Th~ inventlon of the copendlng applIcatlon agaln pro-vldes a method of thermal sterlllzatlon of a plastlc contalner packed wlth food, to obtaln a thermally sterlllzed packed con-talner havlng an acceptable conflguratlon, comprlslng, fllllng the contalner wlth food, sealIng the contalner, elther or both of sald fllllng and sealIng steps Includlng selectlng an Inltlal headspace volume and an amount of gas In the contalner and taklng Into account an Inltlal vacuum level, If any, at sealIng such as to permlt bulglng and subsequent reformatlon of the contalner bottom wall wlthout slgnlflcant slde wall panelllng, thermally sterlllzlng the packed contalner In a retort operated at a tem-perature and pressure for a tIme suffIclent to sterlllze the con-talner and Its contents to cause bulglng of the contalner bottomwall, coollng the contalner and Its contents, and, durlng the coollng step, reformlng the contalner bottom wall to attaln an acceptable contalner conflguratlon by subJectlng the exterlor of the contalner to gas pressure, and controlllng sald pressure and the coollng condltlons, sald controlllng step Includlng provldlng that the plastlc of the bottom wall of the contalner Is at a reformable temperature at whlch the plastlc Is soft whlle provld-lng a pressure dlfferentlal such that the pressure external of the contalner exceeds the pressure Internal the contalner. Sult-ably, the Inltlal vacuum level at seallng of the contalner Isfrom about 10 to about ~0 Inches of mercury. More preferably, after sealIng, and before the thermal sterlllzatlon step a vacuum Is present In sald contalner and a headspace of gases Is maln-talned In the contalner upward end such that the arlthmetlc prod-uct of the Inltlal vacuum level In the contalner and theheadspace volume Is from about 400 Inches Hg x cc to about 800 - 4b -30~

Inches Hg x cc Deslrably after thermal ~terlllzatlon of the contalner, there Is Included the step of reformlng the contalner bottom wall to substantlally attaln an acceptable contalner conflguratlon.

The Inventlon of the copendlng appllcatlon In a stlll further aspect thereof provldes a method of thermal sterlllzatlon of a plastlc contalner packed wlth food to obtaln a thermally sterlllzed packed contalner havlng an acceptable conflguratlon, comprlslng selectlng and utlllzlng a plast1c contalner whose bot-tom wall has portlons of less stress reslstance relatlve to other portions o$ the bottom wall and relatlve to the slde wall to al-low controlled bulglng of the bottom wall durlng thermal sterll-lzatlon, fllllng the contalner wlth food, sealIng the packed con-talner, elther or both of sald fllllng and seallng steps Includ-lng selectlng an Inltlal vacuum level, If any, at seallng such as to permlt bulglng and subsequent reformatlon of the contalner bottom wall wlthout slgnlflcant slde wall panelllng, thermally sterlllzlng the packed contalner In a retort oPerated at a tem-perature and pressure for a tIme suffIclent to sterlllze the con-talner bottom wall, coollng the contalner and Its contents, and, durlng the coollng step, reformlng the bottom wall to obtaln a contalner havlng an acceptable conflguratlon by controlllng the amblent pressure external of the contalner and the coollng condl-

2~ tlons, sald controlllng step Includlng provldlng that the plastlcof the bottom wall of the contalner Is at a reformable tempera-ture at whlch the plastlc Is soft whlle provldlng a pressure dlf-ferentlal such that the pressure external of the contalner exceeds the pressure Internal the contalner.
The Inventlon of the copendlng applIcatlon agaln pro-vldes a method of provldlng a thermally sterlllzed plastlc food contalner havlng a bottom wall and havlng an acceptable conflgu-ratlon whlch comprlses, thermally pre-shrlnklng sald contalner, fllllng the pre-shrunk contalner wlth food, sealIng the packed contalner, elther or both of these steps Includlng, selectlng an - 4c -0 ~

Inltlal headspace amount and a volume of gas, taklng Into account an Inltlal vacuum level, If any, at seallng such as to permlt bulglng and subsequent reformatlon oF the contalner bottom wall wlthout slgnlflcant slde wall panelllng, thermally sterlllzlng the packed contalner In a retort operated at a temperature and pressure for a tlme suffIclent to sterlllze the contalner and Its contents, coollng the contalner, and durlng the coollng step, reformlng the contalner bottom wall by controlllng the amblent pressure external the contalner and the coollng condltlons, sald controlllng step Includlng provldlng that the plastlc of the bot-tom wall of the contalner Is at a reformable temperature at whlch the plastlc Is soft whlle provldlng a pressure dlfferentlal such that the pressure external of the contalner exceeds the pressure Internal the contalner.
The Inventlon of the copendlng aPplIcatlon stlll fur-ther provldes a method of thermal sterlllzatlon of a plastlc con-talner packed wlth food to obtaln a thermally sterlllzed Packed contalner havlng an acceptable conflguratlon, whlch comprlses, fllllng the contalner wlth food, sealIng the contalner, elther or both of these steps Includlng selectlng an Inltlal headspace vol-ume and an amount of gas, taklng Into account a vacuum level, If any, at sealIng such as to permlt bulglng and subsequent reforma-tlon of the contalner bottom wall wlthout slgnlflcant slde wall panelllng, and thermally sterlllzlng the packed contalner at a temperature and pressure for a tIme suffIclent to sterlllze the contalner and food and so that the bottom wall bulges, and reformlng the bulge of the bottom wall by provldlng a pressure dlfferentlal whereln the pressure external of the contalner exceeds the pressure Internal the contalner whlle provldlng that the plastlc of the bulge Is at a reformable temperature at whlch the plastlc Is soft, to thereby obtaln an acceptable contalner conflguratlon. Sultably In the method there Is Included the step of pre-shrlnklng the contalner. Deslrably the pre-shrlnklng Is effected thermally.

- 4d -The Inventlon of the copendlng appllcatlon agaln pro-vldes a method of thermal sterlllzatlon of a plastlc contalner packed wlth food to obtaln a thermally sterlllzed packed con-talner havlng an acceptable conflguratlon, whlch comprlses, flll-Ing the contalner wlth food, sealIng the contalner, elther orboth of these steps Includlng selectlng an Inltlal headspace vol-ume and an amount of gas, taklng Into account a vacuum level, If any, at seallng such as to permlt bulglng and subsequent reforma-tlon of the container bottom wall wlthout slgnlflcant slde wall panelllng, thermally sterlllzlng the packed contalner In a retort at a temperature and pressure for a tlme su~fIclent to sterlllze the contalner and food, sald sterlllzlng step causlng bulglng and creep of plastlc of the bottom wall, provldlng that the plastlc of the bulged contalner bottom wall Is at a reformably tempera-ture at whlch the plastlc Is soft whlle provldlng a pressure dlf-ferentlal such that the pressure external the contalner exceeds the pressure Internal the contalner, thereby reformlng the bottom wall wlthout slgnl~lcant slde wall panelllng.

The Inventlon of the copendlng applIcatlon further pro-vldes a method of thermal sterlllzatlon of a plastlc contalner packed wlth food to obtaln a thermally sterlllzed packed con-talner havlng an accePtable conflguratlon, comprlslng fllllng the contalner wlth food, seallng the contalner, elther or both of sald fllllng and sealIng steps Includlng selectlng an Inltlal contalner headspace volume and an amount of gas, taklng Into account an Inltlal vacuum level, If any, at seallng such as to permlt bulglng and subsequent reformatlon of the contalner bottom wall wlthout slgnlfIcant slde wall panelllng, thermally sterlllz-Ing the packed contalner In a retort havlng a steam envlronment operated at a temperature and pressure for a tIme suffIclent to sterlllze the contalner and Its contents and to cause bulglng and creep of plastlc of the contalner bottom wall, coollng the con-talner and Its contents, and durlng the coollng step, reformlng the contalner bottom wall to attaln an acceptable contalner con-flguratlon by controlllng the amblent pressure external of the - 4e -30~

contalner and the coollng condltlons and utlllzlng the amblent pressure external the contalner to reform the bulged contalner bottom wall.

The Inventlon of the copendlng appllcatlon agaln pro-vldes a method of thermal sterlllzatlon of a contalner whlch has a plastlc end wall and Is packed wlth food to obtaln a thermally sterlllzed packed contalner havlng an acceptable conflguratlon, whlch comprlses. fllllng the contalner wlth food, sealIng the contalner, elther or both of these steps Includlng selectlng an Inltlal headspace volume and an amount of gas, taklng Into account a vacuum level, If any, at seallng such as to permlt bulglng and subsequent reformatlon of the contalner end wall wlthout slgnlfIcant slde wall panelllng, thermally sterlllzlng the packed contalner at a temperature and pressure for a tlme suffIclent to sterlllze the contalner and food and so that the end wall bulges, and reformlng the bulge of the end wall by con-trolllng the amblent pressure external of the contalner and the coollng condltlons, and utlllzlng the amblent pressure external of the contalner at a level whlch exceeds the employed durlng thermal sterlllzatlon to reform the contalner end wall whlle pro-vldlng that the plastlc of the bulge Is at a reformable tempera-ture at whlch the plastlc Is soft, to thereby obtaln an accept-able contalner conflguratlon.
The Inventlon of the copendlng appllcatlon agaln pro-vldes a method of thermal sterlllzatlon of a plastlc contalner packed wlth food to obtaln a thermally sterlllzed pac~ed con-talner havlng an acceptable conflguratlon, whlch comprlses flll-Ing the contalner wlth food, seallng the contalner, elther or both of these steps Includlng selectlng an Inltlal headspace vol-ume and an amount of gas, taklng Into account an Inltlal vacuum level, If any, at sealIng such as to permlt reformatlon of the contalner bottom wall wlthout slgnlflcant slde wall panelilng, thermally sterlllzlng the packed contalner In a retort operated at a temperature and pressure for a tIme sufflclent to sterlllze the contalner and Its contents and to cause bulglng of the con-talner bottom wall. coollng the contalner and Its contents, and, durlng the coollng step, reformlng the bulged contalner bottom wall to attaln an acceptable contalner conflguratlon by estab-llshlng an pre-selected amblent gas pressure In the retort at the concluslon of thermally sterlllzlng, and controlllng the amblent pressure and the coollng condltlons, sald reformlng step belng effected In the retort at an Inltlal pressure level hlgher than that employed durlng the sterlllzatlon step, sald controlllng step Includlng effectlng coollng gradually such that as the pres-sure Internal the contalner decreases, reformlng occurs when the plastlc of the bottom wall Is at a reformable temperature at whlch the plastlc Is soft.

The Inventlon of the copendlng applIcatlon agaln pro-vldes a method of thermal sterlllzatlon of a plastlc contalner packed wlth food to obtaln a thermally sterlllzed packed con-talner havlng an acceptable conflguratlon, whlch comprlses flll-Ing the contalner wlth food, seallng the contalner, elther or both of these steps Includlng selectlng an Inltlal headspace vol-ume and an amount of gas, taklng Into account an Inltlal vacuum level, If any, at seallng such as to permlt bulglng and subse-quent reformatlon of the contalner bottom wall wlthout slgnlfl-cant slde wall Panelllng. thermally sterlllzlng the packed con-talner In a retort operated at a temperature and pressure for atlme suffIclent to sterlllze the contalner and Its contents and to cause bulglng and creep of plastlc of the contalner bottom wall, coollng the contalner and Its contents, and, durIng the coollng step, reformlng the bulged contalner bottom wall to attaln an acceptable contalner conflguratlon by controlllng the pressure external of the contalner and the coollng condltlons, sald controlllng step Includlng provldlng that the pressure Is hlgher than that employed durlng the sterlllzlng step, provldlng that the plastlc of the bulged contalner bottom wall Is warm ~5 whlle provldlng a pressure dlfferentlal such that the amblent 31)~

pressure external of the container exceeds the pressure Internal the contalner, and utlllzlng the ambient pressure whlle sald plastlc Is warm to reform the bulged bottom wall.

The Inventlon of the copendlng applIcatlon further pro-vldes a method of Improvlng the conflguratlon of a plastlc con-talner whlch Is fllled wlth food, sealed and thermally sterll-lzed, sald method comprlslng pre-shrlnklng sald contalner prlor to fllllng, malntalnlng a headspace of gases In sald contalner after seallng, and reformlng the contalner bottom wall after thermal sterlllzatlon of sald contalner.

The Inventlon of the copendlng applIcatlon provldes a generally cyllndrlcal Plastlc contalner for use In packaglng and thermal sterlllzatlon of foods, sald contalner comprlslng slde walls and a bottom wall deflnlng a bottom closure for the con-talner. sald bottom wall belng conflgured to Include portlons whlch are less reslstant to stress relatlve to other portlons of the bottom wall and relatlon to the slde walls.
In accordance wlth thls Inventlon, a method Is provlded for Improvlng the conflguratlon of thermally processed plastlc contalner whlch Is packed wlth food. ObJectlonable dlstortlons and deformatlons (I.e., rocker bottom and/or slde wall panelllng) In the contalner are ellmlnated, or substantlally reduced, by proper contalner deslgn, by malntalnlng proper headspace of gases) In the contalner durlng thermal processlng, by controlllng reformlng of the contalner bottom wall after thermal processlng and/or by pre-shrlnklng the empty contalner prlor to fllllng and seallng.

Accordlng to one aspect thereof the present Inventlon provldes a hlgh oxygen barrler thermally sterlllzable plastlc contalner for packaglng food comprlsed of a hlgh oxygen barrler layer and one or more structural layer(s) whlch conslst(s) essen-tlally of polyolefln(s), whlch contalner has been annealed and - 4h -313~
thereby shrunk such that It has a resldual shrlnkage of about 2%
or less at the temperature(s) at whlch the contalner wlll be thermally sterlllzed when It Is ~IIIed wlth food and sealed.
Sultably, the thermal sterlllzatlon temperature Is from about 1900F. to about 2700F. for from a few mlnutes to about several hours. Deslrably, the thermal sterlllzatlon Is effected at from about 212F. to about 270F. for from a few mlnutes to about sev-eral hours.

In a partlcular aspect thereof the present Inventlon provldes a hlgh oxygen barrler thermally sterlllzable plastlc contalner for packaylng food comprlsed of a hlgh oxygen barrler layer and one or more structural layer(s) whlch conslst(s) essen-tlally of polyolefln(s), whlch contalner has been annealed and shrunk at about 250X F. for about 15 mlnutes or the equlvalent, sald contalner thereby havlng enhanced thermal sterlllzatlon characterlstlcs In that, by vlrtue of Its resldual shrlnkage when the contalner Is f I I led wlth food, sealed and thermally sterll-lzed at from about 190F. to about 270 for a few mlnutes to about several hours, It wlll shrlnk about 2% of less durlng the thermal sterlllzatlon. Sultably, the contalner has multlple lay-ers and Is InJectlon molded or InJectlon blow molded. Deslrably, the contalner has a bottom wall whlch has portlons of less stress reslstance relatlve to other portlons of the bottom wall and rel-atIve to the slde wall.

In one embodlment of the present Inventlon the con-talner has a bottom wall whlch, by vlrtue of Its portlons of less stress reslstance, wlll bulge due to the bulIdup of contalner Internal pressure and the Increase In the contalner's volume dur-lng thermal sterlllzatlon, and whose bulged bottom wall has approxlmately the same surface area as would a spherIcal cap whose volume Is the same as that of the undeformed volume of the bottom wall of the contalner Plus the deslred volume Increase,

3~ whereln the volume (V) Is determlned by V=(1/6) ~ h(3a2~h2) where Uh" Is the helght of the dome of the spherlcal cap, and Ua~ Is the radlus of the contalner at the Intersectlon of the slde wall and bottom wall of the contalner, the surface of the spherlcal cap can be calculated as follows: S2= rr(a2+h2) where S2 Is the surface area of the spherlcal cap, and Ua~ and Uh" are as deflned above, and whereln the ratlo of the "h" dlmenslon to the ~a~
dlmenslon Is expressed as: k=h/a or h=ka where ~h" and ~a~ are as defIned above, and "k" Is about 0.47. Sultably, the contalner has a bottom wall whlch has portlons of less stress reslstance relatlve to other portlons of the bottom wall and relatlve to the slde wall.

In another embodlment of the present Inventlon the con-talner's bottom wall In Its normal posltlon Is deslgned to have approxlmately the same surface area as would a spherlcal cap whose volume Is the same as that of the undeformed volume of the bottom wall of the contalner plus the deslred volume Increase, whereln the volume (V) Is determlned by V=(1/6) ~r h(3a2~h2) where Uh~ Is the helght of the dome of the spherlcal cap, and ~a~ Is the radlus of the contalner at the Intersectlon of the slde wall Z0 and bottom wall of the contalner, the surface of the spherlcal cap can be calculated as follows: S2= ~ (a2~h2) where S2 Is the surface area of the spherlcal cap, and ~a~ and ~h~ are as defIned above, whereln the deslred volume Increase Is 5% of the orlglnal volume of the contalner.
In another aspect thereof the present Inventlon pro-vldes a hlgh oxygen barrler thermally sterlllzable Plastlc con-talner for packaglng food comprlsed of a hlgh oxygen barrler layer and one or more structural layer(s) whlch conslst(s) essen-tlally of polyolefln(s), whlch contalner has been annealed andshrunk at a temperature approxlmately the same or hlgher than the temperature at whlch It wlll be thermally sterlllzed to achleve a resldual shrlnkage of 2% or less, sald contalner thereby havlng enhanced thermal sterlllzatlon characterlstlcs In that, by vlrtue of the contalner's reslduai shrInkage, the contalner when fllled wlth food, sealed and thermally sterlllzed at from about 190F.

- 5a -~6~3()~

to about 270 for a few mlnutes to about several hours, the con-talner wlll shrIn~ about 2% or less durlng the thermal sterillza-tlon. Sultably, the contalner has been annealed untll no slgnif-lcant shrlnkage In the contalner volume Is realIzed upon further anneallng.

The present Inventlon further provldes a thermally sterlllzable multl-layer plastlc contalner for packaglng food whlch when fllled wlth food, sealed and thermally sterlllzed, wlll shrlnk about 1 1/2% or less.

The present Inventlon agaln provldes a thermally ster-lllzable multl-layer plastlc contalner for packaglng food whlch when heat treated at 2500F. for 15 mlnutes or the equlvalent, wlll shrlnk about 1 1/2% or less.

The present Inventlon also provldes a plastlc contalner for packaglng food. whlch contalner Is thermally sterlllzable to render shelf stable food packed and sealed In the contalner whlch comprlses: a slde wall and a bottom wall, the bottom wall belng adapted, when the contalner Is fllled wlth food and sealed to deform and accommodate Increases In Internal pressure and Increases In volume of the contalner wlthout burstlng durlng thermal sterlllzatlon, sald bottom wall havlng portlons of less stress reslstance relatlve to other portlons of the bottom wall and relatlve to the slde wall, and havlng approxlmately the same surface area as would a spherlcal cap whose volume Is the sum of the undeformed volume of the bottom wall plus the deslred volume Increase, the volume "V" of sald cap belng determlnable by the followlng equatlon: V = 1/6 r~ h (3a2 ~ h2) where Uh~ Is the helght of the dome of the spherlcal cap, and "a" Is the radlus of the contalner at the Intersectlon of the slde wall and the bottom wall, whereln the surface area "S2" of the cap may be calculated by the followlng equatlon: S2 = ~ (a2 ~ h2) whereln the ratlo of the "h" dlmenslon to the ~au dlmenslon Is expressed as: k =
h/a or h = ka, where k = about .47. Sultably, the contalner by - 5b -vlrtue of Its havlng been pre-shrunk, has a resldual shrlnkage of 2% less such that when fllled wlth a foodstuff. hermetlcally sealed and thermally sterillzed at temperatures of from about 190F. to about 270F. for from a few mlnutes to several hours, the contalner wlll shrlnk 2X or less. Deslrably, the resldual shrlnkage Is less than 1.7%. and when fllled~ sealed and so ther-mally sterlllzed, wlll shrlnk 1.7X or less. Desirably, the por-tlons o~ less stress res~stance are selected from the group con-slstlng of thlnner portlons, undulatlons, segmented Indented por-tlons, and comblnatlons thereof. Preferably, the portlons ofless stress reslstance are undulatlons and thlnner portlons whlch provlde excess materlal and whlch unfold when the contalner Internal pressure exceeds the contalner external pressure In the retort durlng thermal sterlilzatlon.
The present Inventlon also provldes a plastlc contalner for packaglng food, whlch contalner Is thermally sterlllzable to render shelf stable food packed and sealed In the contalner whlch comprlses: a slde wall and a bottom wall, the bottom wall belng Z0 adapted, when the contalner Is fllled wlth food and sealed to deform and accommodate Increases In Internal pressure and Increases In volume of the contalner wlthout burstlng durlng thermal sterlllzatlon, sald bottom wall havlng approxImately the same surface area as would a spherlcal cap whose volume Is the sUm of the undeformed volume of the bottom wall plus the deslred volume Increase, the volume UV" of sald cap belng determlnable by the followlng equatlon: V ~ 1/6 rfh (3a2 + h2) where ~h~ Is the helght of the dome of the spherlcal cap, and "a~ Is the radlus of the contalner at the Intersectlon of the slde wall and the bottom wall, whereln the surface area ~S2~ of the cap may be calculated by the followlng equatlon: S2 = rr(a2 ~ h2) whereln the ratlo of the ~hu dlmenslon to the "a~ dlmenslon Is expressed as: k = h/a or h = ka, where k = about .47.

The present Inventlon agaln provldes a plastlc con-talner for packaglng food, whlch contalner Is thermally sterlllz-- 5c -able to render shelf stable food packed and sealed In the con-talner whlch comprlses: a slde wall and a bottom wall, the bottom wall belng adapted, when the contalner Is fllled wlth food and sealed to deform and accommodate Increases In Internal pres-sure and Increases In volume of the contalner wlthout burstlngdurlng thermal sterlllzatlon, sald bottom wall havlng portlons of less stress resistance relatlve to other portlons of the bottom wall and relatlve to the slde wall and havlng approxlmately the same surface area as would a spherlcal cap whose volume Is the sum of the undeformed volume of the bottom wall plus the deslred volume Increase, the volume V" of sald cap belng determlnable by the followlng equatlon: V = 1/6 ~r h (3a2 ~ h2) where ~h~ Is the helght of the dome of the spherical cap, and Ua~ Is the radlus of the contalner at the Intersectlon of the slde wall and the bottom wall, whereln the surface area "S2 of the cap may be calculated by the followlng equatlon: S2 =~r (a2 ~ h2) whereln the deslred volume Increase Is about 6% of total volume of the contalner.

The present Inventlon wlll be further Illustrated by way of the accompanylng drawlngs, whereln llke numerals are employed to deslgnate llke parts and In whlch:-Flgure lA Is a front el evatlonal vl ew partly In sec-tlon, of a cyllndrlcal contalner of thls Inventlon before the contalner Is packed wl~h food and sealed;

F/gure lB Is a front elevatlonal vlew partly In sec-tlon, of the contalner shown In Flgure lA after the contalner has been fllled wlth food and sealed under partlal vacuum;
F/gure lC Is a front elevatlonal vlew partly In sec-tlon. of the contalner shown In Flgure lB durlng thermal process-lng but before reformlng, showlng bulglng of the contalner bottom wall;

- 5d -~2~
Flgure lD ls a front elevatlonal vlew partly In sec-tlon, of the contalner shown In Flgure lC ll lustratlng rocker bottom after thermal processlng;

Flgure lE ls a front elevatlonal vlew partly In sec-tlon, of a contaln~r slmllar to F/gure 1D but wherein the con-talner slde walls are panelled;

Flgure IF ls a cross sectlonal vlew of the contalner taken along the llne 1F-lF In Flgure lE;

Flgure lG ls a front elevatlonal vlew partly In sec-tlon, of the contalner shown In F/gure lA ll lustratlng slde wall panelllng and bottom bulglng;

3~

Il i 1 1;~3~4 ; Figure lH is a front elevational view partly in section, of ¦ the container shown in Figure lA after thermal processing, . according to the present invention;
. . Figure 2 is an enlarged vertical section schematicaliy 51 illustrating the cylindrical container of Figure lA;
Figure 3 is a partial elevational fragmentary sectional view of a multi-layer thermoformed container similar to that shown in Pigure 2, showing wall portions having different thicknesses;
Figure 4 is a partial elevational fragmentary sectional view of a multi-layer in~ection blow molded container similar to that shown in Figure 2, showing wall portions having different thicknesses;
Figure 5 is a partial elevational fragmentary sectional lS view of a container similar to Figure 3 but showing the dimensions of a multi-layer thermoformed container;
Figure 6 is a partial elevatiohal fragmentary sectional view of a contalner similar to Figure 3 but showing the dimensions of a multi-layer injection blow molded container;
~igure 7 is a schematic representation illustrating container bottom wall geometry before and after bulging;
Figure 7A is an elevational view of the container shown in Figure 6;
Figure 7B is a bottom view of the bottom wall of the container of P-igure 7A;
Figure 8 is a partial elevational fragmentary sectional . view of the container of Figure 7 showing the container bottom ! ,'~ wall in neutral bulged and inwa dly distented portions;
i~ ~

L3(~4 ~ Lgure 9 is a gra~hical representation illustrating bottom reforming and sidewall panelling as functions of temperature ~ and pressure;

.- Figure` 10 Ls a graphic representation of experimental data illust~ating the relationship between the initial headspace of . . gas.qes in the container and sealing vacuum in the container.
':;' j ' :
.j ' Figure 11 is a graphical repreqentation o~ calculatlons : . . ;
defining the relationshlp bet~een the initial headspace of : . ¦ gases in the container and the sealing vacuum in the contalnerS
` ! DETAILED DESCRIPTION OF ~E INVENTION.

:;- In a typical operation involving food packaglng, the . . plastic.containers are fllled wlth oods and each container is : then hermetically sealed by a top closure. As it was : ! . , . I previously mentioned, the container is typically either sealed .`~ . under vacuum or in an atmosphere o steam created by . hot-filling or by passing steam at the container top while `. sealing. As~it was also mentloned previously, aftec the container is sealed,.there invariably ls ,a headspace of - gases.in the container. Next, the sealed conta'.ner is therm~ally processed at a temperature which i9 usually about l9a . : F. or higher depending on the food, in order to sterilize the ~. container and its content, and thereafter cooled to ambient : temperature. After thermal erocessing and cooling, the ~ containers are removed from the thermal processing equipment, `.. ~ stored and then shipped for distribution.
. During the cooking cycle of- the thermal sterilization ~;. process, the pressure within the container will rise due to .::; -7-- . , "' ` "
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1;~6~
increased pressure of headspace gases, the vapor pressures of the products, the dissolved gases in the products as well as the gases which may sometimes be generated from chemical reactions in the container's content, and due to thermal expansion of the product. The reversible thermal expansion of the conta~ner will tend to lower the pressure within the container; however, the net effect of all the factors will be an increase in pressure.
Therefore, during the cook cycle, the pressure within the con-tainer will exceed the external pressure and, consequently, the container bottom wall will dlstend outwardly, l.e., it wlll bulge. As it was also previously mentioned, after thermal pro-cessing and cooling, the pressure within the container is decreased and the container bottom wall will flex inward to com-pensate for this reductlon of pressure. Frequer.tly, however, the container bottom does not fully return to an acceptable position or configuration and remains bulged to varying degrees.

The containers to which the present invention is well suited are plastic containers which are made of rigid or semi-rigid plastic materials wherein the container walls are prefer-ably made of multilayer laminate structures. A typical laminate structure may consist of several layers of the following materi-als; outer layer of polypropylene or a blend of polypropylene with high density polyethylene, adhesive layer, barrier layer such as ethylene-vinyl alcohol copolymer layer, adhesive layer, and an inner layer of polypropylene or a blend of polypropylene with high density polyethylene.

: ~x~

The adhesive is usually a graft copolymer of maleic anhydride and propylene whereln the maleic anhydride moleties are grafted onto the polypropylene chain.
¦ It must be underYtood, however, that the nature o the ; 5 ¦ different layers are not per se critical since the advantages of this invention can be realized for containers made of other plastLc materials as well, including those havlng less or more than five layers, including single layer containers.
Referrlng now to the drawings, there i9 shown in Figure lA
a plastic contalner 1 having sidewalls 3 and a bottom wall 5 which includes a substantlally flat portion 7 and outer and inner convex annular rings 9 and 9a with an interstitial ring 9b.
After the contalner is filled, it is sealed with a top closure ll as shown in Figure lB. As it was previously mentioned, after the contalner ls filled and sealed, there will be a headspace of gases at the container top generally deslgnated as 13.
Figure lC shows the container 1 durlng thermal processing, or after thermal processlng but before bottom reforming. As shown in thi~ figure, the container bottom is outwardly distended because the pressure within the container exceeds the external pressure. If no proper prior measures are taken, after the container is cooled, the bottom wall may remain deformed as shown in Figure lD. Such container configuration is unstable or undesirable due to rocker bottom. As will hereinafter be explained, rocker bottoms (Flgure lD~ and sidewall panelling as shown in ~igures lE and lF, or both (Figure lG), may be minimized or prevented by pre-shrinklng the _g_ ."~';.

container prioc to filling and closlng, by reforming the container bottom wall, by ad~usting the headspace of gase~ ~n the contaLner at each vacuum level, by proper container design, or by combinations of these factors. Figure 1~ represents the 5! desired container configuration ater theEmal processing and ¦ reforming of the container because it has no rocker bottom or I sLdewall panelling this container configuration is the same or nearly the same as the configuration shown in ~Lgure lB.
As it was previously mentloned, during the cooking cycle, the pressure within the container will rise due to the aforemen~ioned factor~, and the container bottom wall will be outwardly distended. Unless proper measures are taken, the container may burst due to excessive pressure in the container. The container must be designed to deform outwardly lS at a container internal pressure below the pressure which causes bursting of the container at the particular cooking temperature. Por example, at 250 F., a temperature commonly used for sterilizing low acid foods ~e.g., vegetables), of the container will burst if the internal pressure of the container exceeds its external pressure by approximately 13 p.s.i It will be understood, oi' course, that this pressure will be dLfferent at other cooking temperatures and for other ; i container sizes and designs.
The amount of outward distention of the container bottom wall, and hence the volume increase in the container, during the cooking cycle, must be sufficient as to prevent bursting of the container by reducing the internal pressure. It has been found that this volume increase depends on several factors, such as, the initLal vacuum level Ln the container headspace, 3~

the initial headspace, thermal expansion of the product and the container, the container design and its dimensions. Table I
below sets forth the volume change for a multi~layer injection blow molded container (303 x 406) at two different thermal pro-cessing conditions.

Table 1 ~onditlon Exam~le A Example B
Steam Temperature F 230 240 Content Temperature at filling, F 70 70 Content av. temperature, end of cook, F 225 235 Max~ inside metal end wall temp., F 228 238 Pressure at closing, psia 6.7 6.7 Internal Pressure assuming no bulge ~Pl), psia 27.4 32.6 Internal Pressure after bulge (P2) psia 23.7 28.0 Internal Pressure minus External Pressure Unbulged Container P1~14.7, psi 12.7 17.9 Bulged Container P2-14.7, ~psi 9.0 13.3 Burst Strength of container, psi at 25 process temperature 19 16 Head Space Volume, Initial Volume, cu. in. 1.~8 1.48 Volume After Bulge, cu. in. 3.10 3.11 Volume Increase, cu. in. 1.62 1.63 Example B of Table I illustrates that if the container does not bulge sufficiently to reduce the pressure differential to below 16 p.s.i. the container would burst. On the other hand, Example A represents conditions under which bottom bulging is 6~04 I
! not required to prevent burstlng. It should be recognized that ¦ bursting oE a container can occur through a failure of the sealing means as well as by a rupture of container wall. It should also be recognized that the decrease in pressure 5! differential as a result of bottom bulging is beneficial even ¦ if the container would not burst at the higher pressure. Such ¦ a reduction in pressure differential will reduce the amount of creep- or ~permanent deformatlon~ which the container will undergo durlng the thermal process. As will be discussed later, such creep makes it more difficult to reform the bottom wall later ln the thermal process.
¦ In order to attain the desired increase in volume of the container, lt has been found that the container bottom wall l must be so deslgned as to provide a significant deformation of 15¦ the bottom wall of the container. Such bottom wall design is a ¦¦ significant consideration during the cook cycle and reforming as wlll hereafter be explained.
It has been dlscovered that ln order to accommodate the l requirementq of volume increase of the container without bursting durlng the cook cycle, and lnward distention oE the bottom wall on reform to attain an acceptable bottom configuration, the container must be appropriately designed.
Thus, the contalner bottom wall must be so designed and l configured as to include portlon-q which have lower stress resistance relative to other portions of the bottom wall, as well as relative to the container sidewall. Such container configuration is shown in Figure 2 wherein the bottom wall includes portions such as shown at lS, 17, 19 and 21 which are conflgured to have lower stress resistance than the portion of the bottom wall designated by 7, and the sidewalls as shown at 23 and 25.

Although the bottom wall of the container may be made to include portions of less stress resistance by varying the bot-tom configuration, such lower stress resistant areas can be formed by varying the material distributions of the container so that its bottom wall include wea~er or thinner portions. Thus, as shown in Figure 4, the thicknesses of the bottom wall at T5 and T6 are less than T7, the thickness of the remainlng segment of the bottom wall. Similarly, T5 and T6 are less than T2, T3 and T4, the thicknesses at different portions of the sidewall.
Similar differences in material distrlbution are shown in Figure .

Another example of a bottom configuration which includes portions of less stress resistance is one having seg-mented indented portions preferably equal, such as a cross con-figuration wherein the indented portions have less stress resis-tance than the remainder of the bottom wall, e.g. remaining seg-ments thereof, and than the container sidewall. Preferably the indented segments of the cross meet at the axial center of the bottom. Deeper lndentations assist reformation, and while shal-lower ones help to prevent excess of bulging.

A large outward deformation of the container bottom wall is usually best achieved by unfolding of "excess" material in the container bottom rather than by simple stretching of the plastic wall. The preferred container bottom wall should there-fore be designed so as to have approximately the same surfacearea as would a spherical zap whose volume is the sum of the undeformed volume of the bottom of the container plus ' 1c:6~3V4 .' l .
the desired vol~me increase. The volume of the hemispherical cap shown in Figure 7 can be determined from the equation (1) as follows: 2 2 V - lJ6 h(3a + h ) ~1) 5 where ~V~is the volume, ~h~ is the height of the dome of the spherical cap and ~a~ is the radius of the container at the intersection of the sidewall and bottom wall of the container.
The surface of the spherical cap may be calculated from equat~on 2 as follows:
S2 ~ (a2 + h2) (2) where ~S2~ is the surface area of the spherical cap, and ~a' and ~h~ are as discussed above. ~
The design volume and the surface area of the spherical cap required for ~atisfactory bulge and reform over a wide range of food processing conditions or a container of any given size (within a wide range of sizes) may be calculated by the following procedure:
The ratio of the ~h~ dimension to the ~a~ dimension is expressed as k ~ h/a or h ~ ka i where ~h- and ~a~ are as described above. It has been diqcovered that ~k~ is about .47 for satisfactory containers.
Therefore the required volume and surface area of the spherical cap required for a satisfactory container of a given size may be calculated as follows:
V - 1/6 (.47)a (3a2 + (.47a)2) 52. _ (a2 + (.47ap4/32) - .. ,~
where ~52 " ~V~, and ~a~ are as discussed above for the given size contalner.

~'i .
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I~ ~Z613~4 The bottom is designed to have a surface ~Sl~, in the folded portion so that ~Sl~, is approximately equal to 52 As it was previously explained, at the conclusion of the thermal steril~zatlon cycle, the contalner bottom wall is distended outwardly and must therefore be reformed to attain an acceptable bottom configuration. The bulged bottom will not return to its original configuration merely by eliminating the pressure differential across the container wall. This failure to return to its original configuration is a result of ~creep~ l or ~permanent deformation~ of the plastic material. Creep i8 a ¦
well-known property of many polymeric materials. The bottom wall can be reformed by imposing added external pressure, or reducinq the internal Dressu~e in the container~ so that the pressure outside the container exceeds the pressure within the ' 15 container. This reformatlon can best be effected while the bottom wall i9 at ~reformable temperature~. This temperature will of course vary depending on the nature of the plastic used to form the bottom wall but, for polyethylene -polypropylene blend, thiq temperature is about 112 F.
` 20 Reformatlon by imposing an ~overpressure~ can be readily attained by introducing air, nitrogen, or some other inert gas I at the conclusion of thermal processing but before cooling.
;`; Where the contents can be degraded by oxidation, it is preferable to use nitrogen or another inert gas rather than oxygen since at the prevailing reform temperatures, the oxygen ¦ and moisture barrier properties of the plastic are reduced. I
The advantages of adequate overpressure during reforming of the container bottom wall is illustrated in the following ¦ series of tests. -15-~ I

: .~

I ~ 4 1 ~
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¦ Several thermoformed plastic containers (401 x 408 i.e.
; 4-1/16 inches in diameter and 4-8/16 inches high) were filled with water to a gross headspace of 10/32 inch, closed at atmospheric conditions and thermally processed in a still retort under an atmoqpheEe of steam at 240 ~. for 15 mlnutes.
At the conclusion of the thermal sterilization process, air was ¦ introduced into the retort to increase the pressure from 10 to 15 p.s.i.g. Thereafter, the container contents were cooled to 160F. by introducing water into the retort. The resul-lo tlng containers were observed to have severely bulged bottom and sidewall panelllng.
The foregoing procedure was repeated for another set of i identical thermoformed plastic containers under the same conditions except that the pressure during reform was increased to 25 p.s.i.g. prior to introduclng the cooling water. The resulting container~ had no rocker bottoms oc sidewall panelling and the containers had an acceptable configuration.
The results ar ahown ln Table :I below.
.
.. ~ . I
. .

,~,, . . .
, . :;, 1;~613~4 -;, . ~

TA~L3 II
I ~
'¦¦ COORING CYCLE (1) ~EFCRH CYCLE t2) CcNn~un~ER CCNPIG~RA$ICN I caKKEN~s I l _ ' plll Pres~ure Pres3ureSlde~all j Bottc~
i TemQ., (p.s.l.g.) at 160 P.Panalllng (3) Bulgo (4) , ( F.) (p.3.1.9.) . . I
i 160 P. 10 lS Severo Sever~ I All 10160 F. 10 15 Sevor~ Seqer~ I Containers I 160 P. 10 15 Sever~ Severe I ~ad 175 ~ 10 15 Severe Severe I Ob~ectlonable l ~ 17S p 10 15 severo Severe ~ Configuration ¦ ~ 175 F. 10 15 Severe Severe . ~ .
i 160 P. lo 2; ooR-l ! OR-125 , All , 160 F. 10 2; OCR-2 ! CR-120 j Containers 160 p. lo . 2s OOR~l I aR-l4s : 3ad 1 175 F. 10 25 Ca~l o~-245 ! Acceptable lS175 e. lo 25 OOR-l OR-168 Configuration D 5 F. 10 25 OoR-l oX-140 (1) Steam cook at 240 P. mou~ms~m te~perature.
t2~ Air pre~sur~ durlnq coollng m~lntaLned untll contalner content wa3 cooled to 160 F.
(3) OC~' deslgnatQJ out o~ roundDe3s wlth O~R of 1 LndicntiDg almost per~ect rouDdDe33 and OaR o~ S indlcatlnq aLm3st panelled.

(4) N~ rs ~ollc~lnq CR measure center pan~l depth in mlla. T~us oR-L25 iDdicate3 ~ inwnrd botto~ distention o~ l~a Lnch.
i . I
. I
~ ~5~

'.`., I . I

¦ . I

~L~6~3(34-..
;.-. 'rllus, as illust~ated in Table II, an adcquate overpre~sure . m~st be mai~tai~ed during reform in o~er to obtain acceptable : container configur~tion. From tlle above, it can be seen that . ... . "overp~essure" herein mea~ t.he retort coolin9 pressure is usually : gre~ter tllan the retort co~lingpressure- OverpressUre does not ~ ~ refer to the pressure outside the container relative to the pressure.. inqlde tlle container.

n anotn~r serles o~ teqts, plastic contalnees ~3a3 x 4U6) were ~llled wlth 8.3 ounces of green beans cut to 1-1/4 to lo 1-1~2 lnches ln slze. A~small quantity of concentrated salt solutionJ was added to each contalner and the container was Silled to over~low wlth water a~ 2~0P. to Z05 P. Each contalnar was topped to approximately 6/32 inch headspace and then steam ~low closed wlth a mekal end. The containers were then stacked in a still retort, metal ends down, with each stack sepaeated from tbe next by a perorated divider plate.
; Two batche3 o~ contalners (lOa containees pee batch) weee ; cooked ln steam at 250~F ~or 13 minute~. ~t the conclu310n of~
; the cooking cycle air was introd~ced into the retort to ; 20 increase the pr~saure from lS p.s.i.g. to 25 p.s.i.g. and the container was then cooled by water Sor 5-1/2 minutes. The retort was then vented to atmospherlc pre~sUre and coolLn~
contlnued for an additional 5-1/2 minutes. Examinatlons of the ,~ I containers showed no rocker bottom or sidewall panelling and all the co~tainers had accsptable con~lguratlon3.
~ ~ In ancther serleq o~ ~ests plastic containers ~3~3 x 4a6) : were ~illed with 10.2 ounce o~ blanched ~ancy pea~. A small suantlty o~ a cQncentrat~d salt solution was added to each container and the : container ~ j'W~ Eilled to overilow with water ¦ at 200~ to 205QF. Each container wa3 topped to approximatsly 6/32 inch headspace and then ~team flow closed with a metal ~nd. ~he containsrs were stacked in a stlll Eetort~ metal ends down, in 4 layer~, with 25 container3 in eac~ layer . ~, .

~13~4 I . 1.,, separated by a perforated divider plate. The containers were then cooked with steam at 250~P. for 19 minutes. One batch of the containers was cooled with water at the retort pressure of lS - 16 p.s.l.g.. The resulting containers did not reform properly due to bottom socker and sidewall panelling. Another ¦ batch was reformed at 25 p.s.i.g. by passing air into the retort and then cooled with cold water for approximately 6 minutes after which the retort was vented to ambient pressure and cooled for another 6 minutes. No rocker bottom or ~dewall panelling was observed and all the containers in this batch had acceptable configuration.
As has been discussed a container which is subjected to a ¦ normal thermal processing cycle will bulge outwardly at the end I of the heating cycle. If at that time the container were to be !
I punctured so that the inside to outside pressure difi'erential across the container wall would be eliminated and the container then cooled, the bulged condltion would persist and the bottom ¦ would not reform. In order to reform the container, the I pressure outside the container must exceed the Pressure inside Z0 1 the container.
¦ Fi~ure 9 shows the pressure diferential re~uired to reform the bulged bottom wall o a particular multi-layer injection blow molded container tcurve A) and alao the pressure differential above which the ~idewall panels (curve 8). This relationship is shown over the range of 33F to 250 ~.
The data for Figure 9 were developed by heating the container in an atmosphesic hot air oven to 250F and , sub~ecting it to an internal pressure of about 6 psig ~or'a few ¦
¦ minutea. The container temperature was the~ ad~usted to/the ~ -19-various temperature values shown on the graph and the internal pressure was then decreased until reform and panelling occurred and the corresponding pressure differentials were recorded.

From Figure g it is noted that if the container mate-rial is lsooF or above and a pressure differential ~P outside - P
inside) is applied across the container walls, the container will reform satisfactorily whereas if the container wall is at 7sOF or lower, and a pressure differential is applied it will panel at a lower pressure than is necessary to produce bottom reform. In addition it is noted that for this design, and in the 150F to 2500F temperature range, there is a difference between the pres-sure differential required for proper reform and that which causes sidewall panelling.

It is further noted that curves "A" and "B" cross at about 112F, indicating a temperature below which satisfactory reform cannot be accomplished. In observing the containers dur-ing testing it was noted that at 150F or above, reforming appeared to occur gradually and proportionally with the pressure change. At 75F and below reform and panelling occurred abruptly.

The increase in external pressure while the plastic is warm can be readily accomplished in most still retorts by intro-ducing air or nitrogen at the end of the steam heating cycle but before the cooling water is introduced. Although air and nitro-gen are equally effective in reforming the container, the use of air could result in some undesired permeation of oxygen into the container since the oxygen barrier properties of some containers are reduced by the high temperatures and moisture conditions dur-ing retort. We have found that the introduction of such an air or nitrogen overpressure is also effective in many continuous rotary cookers.

In other cases, it is impractical to impose such an ,~

.......

3~

added gas overpressure, either because there is no provision for maintaining such a pressure during cooling or because the pres-sure limitations of the equipment are such that the pressure required for reforming exceeds the allowable equipment pressure limits. It has been found that under certain conditions, the desired reformation can be achieved even without such an exter-nally applied pressure or with an external pressure insufficient for reformation at the internal pressures existent at the end of the heating cycle. The key to proper reformation under these restrictions is to cool gradually the container in such a manner that the plastic will still be relatively soft at the time when the container contents have cooled sufficiently to reduce the internal pressure below the external pressure. This can be accomplished with the use of relatively warm cooling water, at least during the initial stages of cooling.

f;

)4 In connection w1th the above, l t has been found that under certain conditions less than the previously mentioned large overpressure of about 10 to 15 psig ;s sufficient to obtain successful reformation. It has been found that the retort or external pressure during cooling can be moderately hlgher, about the same as, or even below the retort cook pressure. This would apply whether the retort is still or continuous.
.:
., .. ~- .
The following series of tests will further illustrate this aspect of the invention.
. ' ~:;
Several in; ectlon blow mol ded mul ti-l aye r plastlc containers (211 X 215, l.e. 2-11/16 ~nches in dlameter and 2-15/16 inches high) were filled with 135F water to leave a serles of different headspaces, closed by a double seam w1th a steel end at 20 inches of vacuu:n and thermal1y processed in a still retort at 250F ~15.3 psig equilibrium steam pressure) for 90 minutes. At the conclus;on of the thermal sterilization process, air was lntroduced to attain an air pressure of about 15 psig. Thereafter, the container content was ; cooled for 12 mlnutes to below 165F wlth water sprayed onto the .. . .
: i: plastic end of tlle container while the contalner was resting on i ts metal end. Table IIA below shows that plastlc containers having a headspace in the six through ten cc range when stlll retorted as above ~, were successfully reformed with a pressure during cooling about the same as pressure during coollng.

. . .

.. .. ,:;-, ......................................................... .
......
; ~ - 21a -. '.i' ' ~
; .
. . .

. - TABLE 1 l A ~ 3~)'i CONrAlNER CONFIGURATION
llead space Volume (cc) After Retorting 2 Rocker 2 Rocker 2 Success 2 Success 2 Success 4 Success 4 Success 4 Success 4 Rocker 4 Rocker 4 Rocker 6 Success 6 Success 6 Success 6 Success 6 Success 6 Success C Success B Success ~' ) Success S Success .;
r~ Success Success l O Success l O Success l O Success Success Success ; 10 Panel 12 Success 12 Kink*
lZ Kink*
1 2 Success l 2 Success lZ Success l 4 Success 14 Success 1 4 Panel 1 4 Panel 1 4 Panel 1 4 Panel *Kink A (listortl;on oF ththbti95 of the bottom- ht liSa~Jrsel)ace and ~, vacuum.

- 21b-,J
. .~, i L3~

The retort pressure and pressure of a container pro cessed under the conditions of Table IIA during thermal process-ing are shown below in Table IIB.

Table IIB
Condltion Time, Container Retort in Retorts minutes ~sla psia Mid Cook 50 21.5 15,0 End Cook 93 21.0 15.0 Cooling Before Reform 95 18.5 14.5 Container Reform 98 13.0 14.0 End of Cooling 109 13.0 14.0 Pressure Released 110 -0.3 0 *The successfully reformed container whose history is shown in Table IIB had a headspace of 8 cc.
In another test, a container packed as in the previous case was thermally sterilized and cooled under "overpressure"
cooling. The results aré shown in Table IIC below., Table IIC
Condltlon Tlme, Contalner Retort Re~orts mlnutes psi~ psiq Mid Cook 55 15.2 10.5 End of Cook 109 15.2 10.5 Start overpressure 109.5 21.0 17.0 Start Water Spray 113.5 20.0 19.5 Container Reform 118.5 18.0 19.0 End Overpressure Cool 130 18.0 19.2 Pressure Released 131 -0.2 0 * The successfully reformed contalner whose history is shown in Table IIC had a headspace of 8 cc.
As shown in Table IIB, the retort pressure during the cooling cycle may be less than the retort pressure durlng cooking cycle. This is evident by comparlng the pressure of 15.0 psig at the end of the cooking cycle with the pressure of 14.0 psig dur-ing cooling cycle (container reform).

- 21c -lX~O~
In case of overpressure cooling as it is seen from rigure IIC, the retort pressure in the cooling cycle (container reform) is 19.5 psig compared to a retort pressure of 10.6 psig at the end Or the cook-ing cycle. This indicates that tlle retort pressures during cooling and reform need not be as much as 15 psig hiyher than the retort pressure during cooking.

In both cases the resulting containers had acceptable container configuration.
. '. .
Results similar to Table IIC were attained by packing the container with Chili and ~eans instead of water. These results are sl)own in Table IID below.

Condition in ContainerRetort ~: Retorts _ Time minutes psig ~ 9_ End ofCook 16s 17 210 6 Start overpressure 115.5 19.8 19.5 Start water spray 119 21.020.8 Container Reformed 123.5 18.5 End overpressure cool 130.5 1~3.0 19.5 : Pressure released 131 1.2 ~The multi-layer plastic containers successfully rerormed under the conditions shown in Table IID were Zll X 215 inclles and closed with a steel end.

As showll in Table IID the retort pressure at the end of the cook is 10.6 psig and during cooling (container reForm) the retort pressure ;s 19.5 psig. Once again it is noted that this dirference ;s less than 15 psig but the container configuration was still acceptable an(lllad no rocker bottom or sidewall panelling. ~:

- 21d -:S~ t . ~ ~

~X613(14 While lhe above test results indicate that accel)table container configurations are readily obtainable with a still retort, acceptable container configuratlons are also readily attainable with a Steritort and with continuous retorts . The follow;ng test results show successful reformation of containers in a Steritort cooker/cooler.
Several inject-iorl blow molded multi-layer plastic conta;ners (211 X 215) were f;lled with 135DF water to leave a series of different headspaces, closed by a double seam with a steel end at 20 inches of vacuum and thern-ally processed ;n a Ster;tort at 250F
(15.3 psig equilibrium steam pressure) for 30 minutes. At the conclusion oF the thelmal sterilization process, air was introduced to obtain an air pressure of 13.3 psig. Thereafter, the container content was cooled for 5 minutes at that air pressure by continually or intermittently submerging the containers in water during rotation of the Steritort reel on which the contalners are mounted and during the rotation of tlle container in tlle water in tlle lower portion of tlle Steritort shell housing. The container content was cooled to below 165F, were then additional1y cooled to below llO~F in the same nlanner but at atmospheric pressure.
Table IIE below shows that plastic containers having a lleadspace in the four through ten cc range when Steritort processed in tlle manner described above were successFully reformed with a cool pressure about 2 psig below tlle cook pressure.

- 21e -~ .

` '1 . ' , . "

TAûL~ 1I E
Container Configuration ~leacspace Volume (cc) _ After Retortin~
2 Success 2 Rocker 2 Success 2 Rocker 2 Success 2 Success 4 Success 4 Success ; 4 Success 4 Success 4 Success 4 Success 6 Success 6 Success 6 Success 6 Success 6 Success 6 Success S Success 8 Success Success a Success : 8 Success 8 Success Success 1 0 Succ~ss Succcss . 10 Success Success Panel 12 Success 12 Panel 12 Success 12 Panel . 12 Success 12 Success 14 Panel 1 4 Panel 14 Panel 14 Panel 14 Success 14 Success : .
- 21f -:, ' ' .
:' :

1~1304 :
Wh;le the above test results show plastic containers can be successfully reformed using a Steritort process, they also indicate plastic containers can be successfully reformed in continuous retorts, since it ls well known that ster1torts are used in laboratories to simulate, and predict performance of containers thermally processed in, commercial continuous, e.g. rot ar~ retorts.
Although the test results demonstrate successful container reformation with containers filled to within certain headspace ranges, it is to be noted that the headspace range may be different and may be wider than reported above, since, as discussed herein, bottom bulglng, panelling and succesful reformation will depend on various factors such as container size, wa11 thlcknesses, design, and materlal properties, initial vacuum level in the contalner headspace, initia1 headspace, thermal expanslon of the product and the container, whether the container has been pre-shrunk, and, as ; .. :
will be discussed in detail, the cooling process including the type employed, and especially the rate and uniformity of cooling.
In addition to achieving a condition, however obtained, during the coollng cycle whereln the pressure outside of the container (Po) is greater than pressure inside the contalner (Pl) to obtain successful reformation, it has been found that the type, rate and uniformity of cooling of the container body also are Yery important factors to be considered for successful reformatlon, particularly in relation to how and when the aforementioned pressure differential will occur. These coollng factors affect the headspace range in which successful reformation can be attalned, given other factors suFh as the contalner's characteristics and its contents.

'.
- 219 - !~' lX6130~
~ s previously stated, reformation is best eFfected at a temperature at which the plastic is reformable. In reforlllation during cooling it is desirable that Pi be reduced below Po when the plastic is reformable, preferably soft. Since cooling the plast;c afFects its softness and reformabi1ity, the coo1ing factors are important. During cooling, Pi, which ;n the cook cycle exceeded Po, will initially be about the same as or slight1y above Po. When the container is gradually cooled, Pi drops below Po primarily because the vapor lressure in the conta;ner decreases as the contents are cooled. Tllis pressure differential provides the dr;v;ng force for container reformat;on. Thus, under the cooling conditions, the rerormatioll process beg;ns and the bottom buige begins to reform or invert.

In certain applications the more gradua1 the cooling rate the w;der the lleadspace range will be. It has been found tl1at w;th a still retort, the cooling rate of the plastic body may be faster, cooling is less uniform and the headspace range for reforlllation to acceptable configurations may be narrower, than with Steritort and continuous retorts.

, .
:' ' :'' - 21h -t . ,1.', 13~4 In a still retort, in which water flows onto the plas-tic container bottom adjacent to which is any headspace, since the container is inverted and rests on the metal end which usu-ally is its top end. Not being in direct contact with the heated contents, the plastic bottom wall cools and stiffens relatively more quickly than it does in a Steritort where the water contact is different. Cooling of the container body is less uniform than in a Steritort in the sense that the container's bottom which is in first contact with the water and is not in contact with the heated contents, cools more rapidly than the sidewall whlch is ln direct contact with the heated contents. The above will occur in any still retort in which containers are so inverted during the thermal processing.

In a Steritort, and increasingly so for a continuous retort, cooling of the plastic is more gradual. In a Steritort, the containers are in a horizontal position on the Steritort reel and the containers are rotated about the axis of the reel and about their axes as they are repeatedly submerged in the water at the bottom portion of the shell housing. The heated contents are more uniformly mixed or agitated and more uniformly in contact with the container sidewalls and bottom wall, and the container is more uniformly cooled than in a still retort. Thus, the plas-tic of the container, particularly, is bottom stays warmer longer, is in reformation condition longer and stiffens later.
This is particularly desirable because it has been found that in any coollng cycle, it ls particularly important that cooling be effected in a manner that when the internal pressure of the con-tainer drops below the pressure exterior of the container, e.g.
in the cooler, the temperature of the plastic bottom not be so much cooler than that of the sidewall such that the bottom would be stiff and more stable than the sidewalls and the sidewalls would panel before the bottom reforms, sucks in or inverts.
Thus, in a Steritort or continuous cooling process this condition is avoided since conditions can be such that a significant tem-perature differential between the bottom and sidewall temperature - 21i -~61;~0~

is avoided, and their temperatures are more unlform during cool-ing.

- 21; -.

1~6~

As it was prevlously described, the bottom bulge w111not properly reform unless the relative rigldity of the bulged bottom wall is less than that of the sidewalls. This relative rigidity depends on the temperature of the plastic walls at a time when the external pressure exceeds the internal pressure.
Even if this r;gidity relationshlp is such that the bottom does reform ~nwardly from its bulged position, it will not always reform far enough to form an acceptable container at the end of the cooling phase of the process. In particular, it has been found that iF the initial vacuum level in the , I
' ,i, , ~, !, ;:':1 ', : . .
. ',"`' ''.
~, .: .

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. ..

' - 21~-.' :
. ~ .
~ y ~ ~130~

container is not sufficient, the bottom wall will not always be uniformly reformed. Thus, the bottom wall will in many cases be distended inwardly in one area of the bottom while still remain-ing distended outwardly in another portion, thereby producing a "rocker" bottom. Even when the more extended portion does not extend beyond the base of the sidewall so as to form a "rocker"
bottom, the appearance of such an unevenly formed bottom is unde-sirable. This non-uniform reformation is believed to result pri-marily from non-uniformities in the plastic thickness as formed in the container manufacturing process.

We have discovered, however, that we can produce satis-factorily uniform reformation of the bottom even with such imper-fect containers by filling the containers under conditlons whlch will result in all areas of the bottom being largely inverted.
In particular, we have found that for a given fill height and hence a given initial headspace volume, there is a given minimum vacuum level required for full inversion. For a smaller initial headspace volume, the minimum vacuum level required would be greater. We have found that the proper relationship of these two variables can be defined by how much inward deflection of the bottom would be required to increase the pressure in the final headspace to nearly atmospheric. If the deflection required to compress the headspace is too low, the bottom will not fully invert and rocker bottoms can result. For the preferred con-tainer shown in Figure 6, the headspace and initlal vacuum levels should be sufficient to invert the bottom of the container by at least 14 cubic centimeters before the headspace gases would be compressed, at room temperature, to approximately atmospheric pressure.

1~:13~4 ;`.' ' It will be obvious to one skilled in the art that any gasses dissolved in the product will alter this relationship in the ~ame way as lf those dissolved gasses had been present initially in the headRpace. Curve A on figure 11 represents the relationship between headspace and initial vacuum level in the container in cases where there are no significant amount of dissolved gasses (i.e. water)in the container content.
It wlll further be recognized that the initial vacuum can be generated either with a vacuum closing machine or by displacing ~ome of the air in the headspace with steam by impinging steam into the headspace volume while placing the closure onto the container by the well known ~steam flow closure~ method.
If the vacuum level in the container is very high, the bottom wall will distend inwardly as long as it continues to be ¦
les~ resistant to deflectlon than is the sidewall. once lt has distended inwardly to the point where it has Formed a concave dome, it will start to become more resiatant to further deflection than i~ the sidewall. If there is still sufficient vacuum remaining at that point, the sidewall will panel giving an undesirable appearance. As in the minimum allowable vacuum level described previously, the maximum allowable vacuum level ; depends on the fill height. Again it has been found that the proper relationship of these two variables can be defined by how much deflection of the bottom would be required to increase the pressure in the final headspace to atmospheric. For the preferred container shown in ~igure 11, the headspace and initial vacuum levels should be sufficient to invert the bottom of the container by no more than 26 cubic centimeters. CurYe B

~. ~ . I

~1 1;26130~ 'I
I ... I

on figure 11 epresents the relat~onship betwee~ these tw~variables for the case in which there is not a sifnigicant amount of dissolved gasses; i.e. water.
At values of initial vacuum and headspace volume S falling below curve A, the containers will form rocker bottoms and at values above curve B~ the containers wlll panel. Values falling between curves A and B are therefore desired.
~ he above calculated relationships correspond approxlmately to the experimental results for a group o containers which have been specially treated by a process of this invention known as annealing. The data on these ¦ containers are represented by the curves marked A' and B' in figure 10. For containers which have not been 90 treated, rocker bottoms are observed under conditions which would be ctlculated to lnvert acceptably. Data on these containers are representèd by the curves A " and B'' in the figure 10.
We have found that this increased tendency to form rocker bottoms after thermal processing is the result of a shrinkage which occurs in these contalners at the temperatures experienced ~n the food sterili~ation process. As a result of this shrinkage, the volume of the container after processing will be less than would otherwise be expected.
Correspondlngly, the amount of bottom deflection which would be required to compress the headspace to approximately atmospheric pressure is reduced and the bottom will no longer fully invert under conditions which would have achieved full inversion without such shrinkage. As will be apparent from the above discussion and from the experiment results presented below, improved container configuration after processing can be . . ~

~ L30qL
.
I
.

achieved by annealing or pre-shrinking the containers before filling or sealing.
The pre-shrinking of the container may be achieved by annealing the empty container at a temperature which is approximately the same, or prefeLably higher, than the thermal processing temperature. The temperature and time required for thermal sterilization of food will vary depending on the type ¦ of food but, generally, for most packaged foods, thermal processing is carrled at a temperature of from about 190 F.
lOI ~for hot-filling) to about 270JF., for a few minutes to about several hour~. It ls understood , of course, that this time need only to be long enough to sterilize the food to meet the commercial demands.
For each container, at any given annealing lS temperature, there is a corresponding annealing time beyond which no significant shrinkage in the container volume can be detected. Thus, at a given temperature, the container is annealed until no significant shrinkage in the container volume ¦
! ,' , i~ realized upon further annealing.
In addition to pre-shrinking the container by a separate heat treatment step conducted in an oven or similar device, it is possible to achieve the same results by pre-shrinking the container as a part of the container making operation. By adj~sting mold cooling times and/or mold temperatures, so that the container is hotter when removed from the mold, a container which shrinks less during thermal processing can be obtained. This is shown below for a series of 303 x 406 containers made by multi-layer injection blow molding in which the residence time in the ~low mold was !
l ll II-J ~ 3~
i . I

! . deliberately varied to show the effect of removing the . ¦ container at different temperatures on the container's - . performance during thermal processing.
., Shrinkage .
. Mold Cloqed Temp. on ~ 250~F, Container Time-Sec. Leavinq Mold 15 Minutes `Desiqnation CapacitY-cc cc.
` 1 510 2.4 Lowest 10.2 2.0 2 505 1.2 Intermediate 8.5 1.7 !
.~. . I 8 498 0.1 ~ighest 4.4 0.9.¦
., I . I
. ¦ ~ote that the container 3 had partially shrunk on . ¦ cooling to room temperature and had less shrinkage at 250 F
¦ than containers 1 and 2. All these containers were filled with j water at a range of headspace, and a 20~ closing vacuum, and retorted at 250F for 15 minutes to determine the range of . ¦ headspace that would be used to achieve good container ,, I coniguration.
:~, 20 ; ~igh TemperatureAllowable ~eadspace Container Annealin~ cc . I¦ 1. No 39-40 ! , 25 1 Yes 20-40 2 ~o 25-40 : 2 Yes 18-40 3 No 22-40 3 Yes 17-40 ', .
.' ~`'~

~ 30~

Note that container #l when unannealed had only a 1 cc range in headspace. Containers #2 and #3 without annealing had a much larger range. Of particular importance is the fact that container #3, without a separate heating step, had virtually as broad a range as container #1 had with a separate high tempera-ture annealing step.

The amount of residual shrinkage in the container when it is filled and closed has a ma~or effect on the range of allow-able headspace and vacuum levels. When shrinkage exceeds about1-1/2% (at 250F for 15 minutes) it becomes ex$remely difficult to use the containers commercially unless they are deliberately pre-shrunk. The containers discussed above were made by either injection blow molding or thermoforming and had shrinkage of 1.4 and 4~, respectively. There are other plastic containers being developed for thermal processed foods which have about 9% resid-ual shrinkage and will also benefit from this pre-shrinking invention.

These containers are the Lamicon Cup made by Toyo Seikan in Japan using a process called Solid Phase Pressure Form-ing, and containers made using the Scrapless Forming Process by Cincinnati Milacon who is developing this process.

The advantages of using an annealed container in the process of the present invention can be further appreciated by reference to Figure 10. As shown in this Figure, the use of annealed containers increases the headspace range which may be maintained in the container at closlng. Thus, for example, for a typical multi-layer in~ection blow molded container of 303 x 406, filled with 70F deionized water, of the container is closed at an initial sealing vacuum of 20 inches, usable f;

1, headpsace which can be tolerated at reform for an unannealed container is 26-40 cc. This corresponds to a headspace range fF 14cc. If, however, the container is annealed, the usable headspace is 21-4G cc, then measuring the headspace range to 19 cc.
The increased usable headspace range allows for less ¦ accuracy during the filling step. since commercial filling and clo~ing equipment are generally de~igned within an accuracy of ~ 8 cc, the annealed container will not require much modification of such equipment.
It has also been discovered that further improvements ~ in container reformation may be realized by using a container il which has been pre-shrunk prior to thermal processing. The use ~ of pre-shrunk container permits greater range of filling conditions as will hereinater be explained.
For each container, at any given annealing temperature, there is a corresponding time beyond which no significant shrinkage is attained in the container volume. Thus, at any 20¦ given temperature, the container is annealed until no further significant shrinkage in the container volume is detected upon further annealing. Obviously, this will vary with the different resins used to make the container and the relative thicnkess of the container wall.
Instead of pre-shrinking the container by annealing as aforesaid, it is possible to use a pre-shrunk container wherein the container volume has been reduced during the container making operation. Thus, whether container is made by injection blow molding or by thermoforming, the container made may be -2a-~ 3~4 essentially non-shrinkable since its volume has been reduced dur-ing container making operation.

The following Examples will serve to further illustrate the present advantages of the use of annealed (pre-shrunk) con-tainers.

Two sets of thermoformed multilayered plastic contain-ers t303 x 406, i.e., 3-3/16 inches in diameter and 4-6/16 inches high) were used in this Example. The first set was not annealed but the second set was annealed at 250F for 15 minutes in an air oven, resulting in 20 cc volume shrinkage of the container mea-sured as follows:
A Plexiglass (a trademark) plate having a central hole is placed on the open end of the container and the container is filled wlth water until the surface of the Plexlglass plate is wetted with water. The fllled contalner and Plexlglass plate are weighed and the weight of the empty container plus the Plexiglass plate is subtracted therefrom to obtain the weight of water. The volume of the water is then determined from the temperature and density at that temperature.

The above procedure was carried out before and after anneallng of the contalner. The overflow volume shrlnkage due to anneallng was 20 cc, or 3.9 volume percent, based on a container volume of 502 cc.

Both sets of contalners were filled with 75F deionized water and the containers were sealed by double seaming a metal end using a vacuum closing machine at 20 inches of vacuum. All containers were then ,~r~

!1 ; ~:6~3~
. . Il . ' . ~ reto~ted in a Steritort at 250F. for 20 minutes and then . ~ cooled at 25 p.s.i. The results are shown in Table III below, .
wherein ~Rocker~ signlfies that the container is unsatisfactory due to bulglng in the container bottom, ~Panel- designates .
sidewall panelling and, again, an unsatisfactory con- .
tainer, and ~OK~ indicates that the container is satis-. . factory because it has no significant bottom bulging or ¦ sidewall panelling. .

. TABLE III
: 10 .

. Condition After Condition After ; ~eadspaceClosinq ~achine _ Retortinq ' . ¦ Volume, cc Annealed Not Annealed Annealed Not Anneal d . ~ 15 .. ¦ 16 OR OR Rocker Rocker . ¦ 18 OR OR OR Rocker : I 20 OR OR OR Rocker . 22 OR OR OR Rocker : Z0 24 OR OR OR Rocker 26 OR OR OK. Rocker 23 Og OR OK Rocker . 30 OR OR OR Rocker 32 OR OR OK ~ocker 25¦ 34 Panel Panel OR Rocker 36 Panel Panel Panel Panel As shown in Table III, the annealed, and hence, pre-shrunk containers are free ~rom bottom bulging or sidewall panelling, ¦ wherea~ the non-annealed containers largely fail due to rocker . I . ` . I

. I 1~3~ ~

or panel effects. In addition, the use of annealed containers permits greater range of headspace volume as compared to the ' containers which were not annealed prior to thermal processing.

. I
Example 1 was repeated under similar conditions exceptthat the plastic containers used had been obtained by injection I
blow molding. Shrinkage due to annealing was 7.9 cc or 1.6 10 volume percent. The results are shown in Table IV. I
TABLE IV
. ,' ..~
Condition After Condition AEter 15 BeadspaceClosinq MachlneRetorting ;` Volume, cc Annealed Not Annealed Annealed Not Anneal~d . ' I
16 OK OR Rocker Rocker 18 OR OR OR Rocker OR OR OR Rocker 22 OR OR OK ~ocker 24 OR OR OR Rocker ¦
26 OR OR OR Rocker ¦

OR OR OK OR

¦ 34 Panel Panel OK OK
, 36 Panel Panel Panel Panel ~LX6~

The results in this example also illustrate the advantages whlch result from annealing of the containers prior to retorting.

This example wa~ ~imilar to Example l except that retorting was carrLed out at 212F. for 20 minutes. As shown in Table V , ~imilar results were obtained as in the previous examples.

` TA~LE V
Condltion Ater Condition After ~eadspace Closina Machine Retorting _ Volume, cc Annealed Not Annealed Annealed Not Anneal~ d OR OR Rocker Rocker 16 OR OR Rocker Rocker 17 OR OR OR Rocker 18 OR OR OR Rocker 19 OR OR OR Rocker OR OR OK Rocker 21 OR OK OR Rocker " 22 OR OR OR Rocker 23 OR OR OR Rocker 24 OR OR OR Rocker OR OR OR Rocker 26 OR OR OR Rocker 27 OR OR OR Rocker 2Ao OR OR OR Rocker 29 OR OR OR Rocker OR OR OR Rocker 31 OR OR OR Rocker 32 OR OR OR Rocker 33 OR OR OR Rocker 34 Panel Panel OR OR
Panel Panel Panel Panel .
The procedure o Exa~ple 3 was repeated except that the containers had been obtained by injection blow molding. Table VI shows the same type of advantageous results as 1n the previous examples.

: ~.

i!

lZ6130~ ~
.'' .' I
~ TABL~ VI
.
Condition After Condition After ; ~eadspace Closing Machine Retorting-; 5 Volume, ccAnnealed Not Annealed Annealed Not Annealed ! 15 OK OK RockerRocker I 17 OR OK RockerRocker : 19 OK OK RockerRocker 21 OK OK OK Rocker 23 OR OK OK Rocker ¦ 25 OK o~ o~ Rocker 27 OK OK og OK

33 Panel Panel OK OK
Panel Panel Panel Panel ,. . I
.The increased usable headspace range allows for less accuracy in the filling steps. since commercial filling and closing equipment are generally designed within an accuracy of + 8 cc, the annealed container will not require much modiflcation of such equipment.
In the foregoing examples the advantages of pre-shrinking of the container by annealing are illustrated utilizing containers filled with water because of experimental simplicity. These advantages can also be realized, however, in other cases where the container is filled with fruits, vegetable or other edible products. Por example, injection blow molded multilayer plastic containers (303 x 406) were filled with fresh pears and syrup (130~., 20~ sugar solution) and retorted at 212~F. for 20 minutes. Prior to filling, a set of the containers was annealed at 250~F. for 15 minutes, while the other set was not annealed. When 7500 containers were annealed prior to retorting, the success rate was as high as 95 ' ~ .

.. ,...... I

¦ percent, with only about 5 percent reform failure. In the case ¦ of non-annealed containers, the success rate was considerably less since reorm failures were observed in most retorted containers, S l ,'`.

. l ...

Claims (26)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A high oxygen barrier thermally sterilizable plas-tic container for packaging food comprised of a high oxygen bar-rier layer and one or more structural layer(s) which consist(s) essentially of polyolefin(s), which container has been annealed and thereby shrunk such that it has a residual shrinkage of about 2% or less at the temperature(s) at which the container will be thermally sterilized when it is filled with food and sealed.

2. The container of claim 1 wherein the thermal ster-ilization temperature is from about 190°F. to about 270°F. for from a few minutes to about several hours.

3. The container of claim 2 wherein the thermal ster-ilization is effected at from about 212°F. to about 270°F. for from a few minutes to about several hours.

4. A high oxygen barrier thermally sterilizable plas-tic container for packaging food comprised of a high oxygen bar-rier layer and one or more structural layer(s) which consist(s) essentially of polyolefin(s), which container has been annealed and shrunk at about 250°F. for about 15 minutes or the equiva-lent, said container thereby having enhanced thermal steriliza-tion characteristics in that, by virtue of its residual shrinkage when the container is filled with food, sealed and thermally sterilized at from about 190°F. to about 270°F for a few minutes to about several hours, it will shrink about 2% of less during the thermal sterilization.

5. The container of claim 4 wherein the container has multiple layers and is injection molded or injection blow molded.

6. The contalner of clalm 4 whereln the contalner has a bottom wall whlch has portlons of less stress reslstance rela-tive to other portions of the bottom wall and relative to the side wall.

7. The container of claim 4 wherein the container has a bottom wall which, by virtue of its portions of less stress resistance, will bulge due to the buildup of container internal pressure and the increase in the containers volume during ther-mal sterilization, and whose bulged bottom wall has approximately the same surface area as would a spherical cap whose volume is the same as that of the undeformed volume of the bottom wall of the container plus the desired volume increase, wherein the vol-ume (V) is determined by V=(1/6) .pi.h(3a2+h2) where "h" is the height of the dome of the spherical cap. and "a" is the radius of the container at the intersection of the side wall and bottom wall of the container. the surface of the spherical cap can be calculated as follows: S2= .pi.(a2+h2) where S2 is the surface area of the spherical cap, and "a" and "h" are as defined above, and wherein the ratio of the "h" dimension to the "a" dimension is expressed as: k=h/a or h=ka where "h" and "a" are as defined above, and "k" is about 0.47.

8. The container of claim 7 wherein the container has a bottom wall which has portions of less stress resistance rela-tive to other portions of the bottom wall and relative to the side wall.

9. The container of claim 4 wherein the thermal ster-ilization is at from about 212°F. to about 270°F., and the con-tainer shrinkage will be about 1 1/2% or less during thermal sterilization.

10 . The container of claim 4 wherein the thermal ster-ilization is at from about 212°F. to about 250°F., and the con-tainer shrinkage will be about 1 1/2% or less during thermal sterilization.

11. The container of claim 10 wherein the container shrinkage will be about 1% or less during thermal sterilization.

12. The container of claim 4 wherein the container's bottom wall in its normal position is designed to have approxl-mately the same surface area as would a spherical cap whose vol-ume is the same as that of the undeformed volume of the bottom wall of the container plus the desired volume increase, wherein the volume (v) is determined by V=(1/6) .pi.h(3a2+h2) where "h" is the height of the dome of the spherical cap, and "a" is the radius of the container at the intersection of the side wall and bottom wall of the container, the surface of the spherical cap can be calculated as follows: S2= .pi.(a2+h2) where S2 is the sur-face area of the spherical cap, and "a" and "h" are as defined above, wherein the desired volume increase is 5% of the original volume of the container.

13. A high oxygen barrier thermally sterilizable plas-tic container for packaging food comprised of a high oxygen bar-rier layer and one or more structural layer(s) which consist(s) essentially of polyolefin(s), which container has been annealed and shrunk at a temperature approximately the same or higher than the temperature at which it will be thermally sterilized to achieve a residual shrinkage of 2% or less, said container thereby having enhanced thermal sterilization characteristics in that, by virtue of the containers residual shrinkage, the con-tainer when filled with food, sealed and thermally sterilized at from about 190°F. to about 270°F for a few minutes to about sev-eral hours, the container will shrink about 2% or less during the thermal sterilization.

14. The container of claim 13 wherein the container has been annealed until no significant shrinkage in the container volume is realized upon further annealing.

15. A thermally sterilizable multi-layer plastic con-talner for packaging food which when filled with food, sealed and thermally sterilized, will shrink about 1 1/2% or less.

16. The container of claim 15 wherein the thermal sterilization is at 250°F. for 15 minutes, or the equivalent.

17 . A thermally sterilizable multi-layer plastic con-tainer for packaging food which when heat treated at 250°F. for 15 minutes or the equivalent, will shrink about 1 1/2% or less.

18. A plastic container for packaging food, which con-tainer is thermally sterilizable to render shelf stable food packed and sealed in the container which comprises: a side wall and a bottom wall. the bottom wall being adapted, when the con-tainer is filled with food and sealed to deform and accommodate increases in internal pressure and increases in volume of the container without bursting during thermal sterilization, said bottom wall having portions of less stress resistance relative to other portions of the bottom wall and relative to the side wall, and having approximately the same surface area as would a spheri-cal cap whose volume is the sum of the undeformed volume of the bottom wall plus the desired volume increase, the volume "V" of said cap being determinable by the following equation: V =
1/6 .pi. h (3a2 + h2) where "h" is the height of the dome of the spherical cap, and "a" is the radius of the container at the intersection of the side wall and the bottom wall, wherein the surface area "S2" of the cap may be calculated by the followlng equation: S2 = .pi.(a2 + h2) wherein the ratio of the "h" dimen-sion to the "a" dimension is expressed as: k = h/a or h = ka, where k = about .47.

19. The container of claim 18 wherein the container by virtue of its having been pre-shrunk, has a residual shrinkage of 2% less such that when filled with a foodstuff, hermetically sealed and thermally sterilized at temperatures of from about 190°F. to about 270°F. for from a few minutes to several hours, the container will shrink 2% or less.

20. The container of claim 19 wherein the residual shrinkage is less than 1.7%, and when filled, sealed and so ther-mally sterilized, will shrink 1.7% or less.

21. The container of claim 19 wherein the residual shrinkage is less than 1%, and when filled, sealed and so ther-mally sterilized, will shrink 1% or less.

22. The container of claim 19 wherein the container is injection blow molded.

23. The container of claim 18 wherein the portions of less stress resistance are selected from the group consisting of thinner portions, undulations, segmented indented portions, and combinations thereof.

24. The container of claim 23 wherein the portions of less stress resistance are undulations and thinner portions which provide excess material and which unfold when the container internal pressure exceeds the container external pressure in the retort during thermal sterilization.

25. A plastic container for packaging food, which con-tainer is thermally sterilizable to render shelf stable food packed and sealed in the container which comprises: a sidewall and a bottom wall, the bottom wall being adapted, when the con-tainer is filled with food and sealed to deform and accommodate increases in internal pressure and increases in volume of the container without bursting during thermal sterilization, said bottom wall having approximately the same surface area as would a spherical cap whose volume is the sum of the undeformed volume of the bottom wall plus the desired volume increase, the volume "V"
of said cap being determinable by the following equation: V =

1/6 .pi. h (3a2 + h2) where "h" is the height of the dome of the spherical cap, and "a n is the radius of the container at the intersection of the side wall and the bottom wall, wherein the surface area "S2" of the cap may be calculated by the following equation: S2 =.pi.(a2 + h2) wherein the ratio of the "h" dimen-sion to the "a" dimension is expressed as: k = h/a or h = ka, where k = about .47.

26. A plastic container for packaging food, which con-tainer is thermally sterilizable to render shelf stable food packed and sealed in the container which comprises: a side wall and a bottom wall, the bottom wall being adapted, when the con-tainer is filled with food and sealed to deform and accommodate increases in internal pressure and increases in volume of the container without bursting during thermal sterilization, said bottom wall having portions of less stress resistance relative to other portions of the bottom wall and relative to the side wall and having approximately the same surface area as would a spheri-cal cap whose volume is the sum of the undeformed volume of the bottom wall plus the desired volume increase. the volume "V" of said cap being determinable by the following equation: V =
1/6 .pi.h (3a2 + h2) where "h" is the height of the dome of the spherical cap, and "a" is the radius of the container at the intersection of the side wall and the bottom wall, wherein the surface area "S2" of the cap may be calculated by the following equation: S2 - .pi.(a2 + h2) wherein the desired volume increase is about 5% of total volume of the container.