Abstract:
The present invention relates to an improved composite barrier laminate for an improved container for citrus juices and other liquids. The container utilizes a paperboard barrier laminate for the containment of essential oils and the prevention of losses of vitamin C. The laminate makes use of an inner barrier layer of a heat-sealable oxygen and flavor oil barrier material to enhance the barrier properties of the laminate as well as an extremely thin low density polyethylene food contact layer to provide a resultant secure homogenous non-polar heat seal.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to heat-sealable barrier laminates for the containment of essential oils and the prevention of loss of vitamin C in paperboard cartons, as well as to a process for making such laminates. More particularly, this invention relates to barrier laminates which are comprised of an improved heat-sealable composite structure which retains the vitamin C in juice at nutritional-claims levels throughout the normal expected shelf life when the carton/package is made as well as improving the retention of citrus juice flavor oils therein. 
     Heat-sealable low-density polyethylenes are well known to be components of current paperboard citrus juice carton which provide little barrier to absorption and/or transmission of citrus juice essential flavor/aroma oils. Additionally, it is well known that impermeable materials such as aluminum foil, polar materials such as: polyamides, polyethylene terephthalates,polyvinylidene chlorides,polyvinyl chlorides, etc., and highly crystalline non-polar materials such as high-density polyethylene and polypropylene provide varying degrees of barrier to the absorption and/or transmission of non-polar citrus juice flavor oils such as d-Limonene, et al. However, these materials in the past have required a thick liquid contact low density polyethylene layer for heat sealability as well as an expensive tie layer to provide consistent adhesion between the LDPE and the barrier material. The thick LDPE liquid contact layer (1.5 mil) generally required in such cartons also absorbs citrus flavor/aroma oils, with resultant deleterious effects on product quality. 
     The existing commercial structure for a paperboard carton for juice and similar products has utilized an easily heat-sealable barrier laminate composed of paperboard sandwiched between two layers of low density polyethylene (LDPE). The LDPE is an inexpensive heat-sealable moisture barrier. The conventional structure falters in that the thick LDPE layer absorbs the essential oils of the juice after short periods of time causing integrity decay of heat seals, stress cracking of the layer and allows transmission of the essential oils into the paperboard and to the atmosphere. Additionally, the conventional structure provides virtually no barrier resistance to oxygen causing the juice to lose vitamin C in large amounts. 
     One other conventional structure adds two additional layers to the structure identified above, namely a foil layer and an additional LDPE layer. The improved conventional structure has poor barrier properties relating to the absorption of essential oils and aromas, since the interior contacting layer is still a thick layer of LDPE. 
     The object of the present invention is to produce an improved juice packaging heat-sealable laminate material for a juice carton which does not transmit flavor/odor ingredients of citrus and other juices and still exhibits an improved heat-seal structure. 
     The present invention is related to applicants&#39; novel structures outlined in U.S. Pat. 4,698,246, issued Oct. 6, 1987, U.S. Pat. 4,701,360, issued Oct. 20, 1987, patent application Ser. No. 060,199, filed June 10, 1987 and in patent application Ser. No. 055,629, filed May 29, 1987. 
     Applicants&#39; novel structure is an improvement on prior structures in the food contact layers. 
     SUMMARY OF THE INVENTION 
     The preferred embodiment of the present invention reveals an improved composite structure providing a substantial barrier to the loss of vitamin C and an almost complete barrier to the loss of essential flavor oils over an extended shelf life period of the carton (eight weeks) and possibly beyond the eight week period as well. The preferred embodiment comprises from the outer atmosphere exposed surface to the inner surface contacting the essential oils and/or flavors (liquid juice): a first thin exterior layer of a low density polyethylene polymer; a paperboard substrate; an interior laminate coating comprising a Plexar tie layer, an aluminum foil barrier layer, and another Plexar tie layer and a second thin layer of a low density polyethylene polymer coated onto the laminate coating layer of Plexar/foil/Plexar in contact with the juice rendering the laminate heat-sealable. 
     A second embodiment of the present invention reveals a non foil composite structure, providing a substantial barrier to the loss of vitamin C and an almost complete barrier to the loss of essential flavor oils; which comprises from the outer atmosphere exposed surface to the inner surface contacting the essential oils and/or flavors (liquid juice); a thin layer of a low density polyethylene polymer, a paperboard substrate, a tie layer of Plexar or ethylene acrylic acid, a interior layer of polyethylene terephthalate polymer (PET) and a thin layer of a low density polyethylene polymer coated onto the interior layer of PET in direct contact with the juice, rendering the laminate heat-sealable. 
     A third and fourth embodiment of the present invention reveal a non-foil composite structure, providing a substantial barrier to the loss of vitamin C and an almost complete barrier to the loss of essential flavor oils; which comprise from the outer atmosphere exposed surface to the inner surface contacting the essential oils and/or flavors (liquid juice): a thin layer of a low density polyethylene polymer, a paperboard substrate, an interior layer of a glycol-modified polyethylene terephthalate (PETG) or acid-modified polyethylene terephthalate (PETA) and a thin layer of a low density polyethylene polymer coated onto the interior layer of the modified PET, in direct contact with the liquid, rendering the laminate heat-sealable. 
     A fifth embodiment, of the present invention reveals a non-foil composite structure, providing a substantial barrier to the loss of vitamin C and an almost complete barrier to the loss of essential flavor oils; which comprises from the outer atmosphere exposed surface to the inner surface contacting the essential oils and/or flavors (liquid juice): a thin layer of a low density polyethylene polymer, a paperboard substrate, an interior layer of a polyvinylidene chloride copolymer (PVDC) and a thin layer of a low density polyethylene polymer coated onto the interior layer of PVDC, in direct contact with the liquid, rendering the laminate heat-sealable. 
     The cartons constructed of the laminate of the present invention enable significant flavor oil retention of the citrus juice contained, and also significant prevention of loss of vitamin C, resulting in a substantial extension of the shelf life thereof and permits replacement of the costly aluminum foil barrier. 
     The present invention has produced a suitable container which has excellent barrier properties utilizing a laminate which can be securely heat-sealed with its exterior and interior layers being a non-polar constituent (LDPE) and a like non-polar constituent (LDPE) from front to back. The liquid juice components are insoluble in the polar FOIL, polyethylene terephthalate, glycol-modified polyethylene terephthale or polyvinylidene chloride copolymer material, preventing flavor oil absorption and resulting swelling, stress cracking, plasticization and heat seal degradation as occurs with a thick LDPE as the contact layer. 
     The preferred laminates of the present invention not only exhibit significant barrier properties to extend the shelf life of the juice, but the laminates can be produced using conventional extrusion equipment. 
     The preferred embodiments of the present invention are constructed as follows: 
     Stepwise, the paperboard is flame treated on both sides. Second, a thin layer of molten LDPE is then placed onto the paperboard substrate by extrusion coating. 
     Thirdly, the web is turned over and a sandwich layer of aluminum foil surrounded by Plexar tie layers, or one Plexar tie layer and foil, or foil alone is coextruded onto the uncoated exposed side of the paperboard substrate. This newly placed layer is corona discharge treated or flame treated to facilitate adhesion of a subsequent food contact layer. 
     Lastly, an extremely thin layer of LDPE is extruded onto the interior sandwich layer of Plexar-Foil-Plexar, or Plexar-Foil layer or FOIL layer. The completed laminate can now be heat-sealed from front to back (LDPE to LDPE) at conventional temperatures (250° F. to 500° F.). 
     The newly formed laminate can then be scored, cut into blanks, folded and side-seam heat-sealed thereon for transport. 
     Once transported, the prepared blanks can be filled onto conventional equipment, such as a PurePak® machine made by Ex-Cell-O. The blanks are formed heat-sealed at the bottom, filled and heat-sealed at the top by the PurePak® machine to complete the filled carton. 
     The barrier laminates produced by the present invention not only exhibit excellent barrier properties and can be easily constructed but also meet FDA approval for use in food packaging. 
     Thus, until the advent of the present invention few suitable containers for the containment of citrus juices have been developed which retain the advantages of using paperboard as the base material as well as having a relatively inexpensive barrier layer which can be fabricated using conventional extrusion coating equipment. 
     The present invention described herein is particularly useful as a paperboard laminate employed in the manufacture of citrus juice or other liquid product containers. Such containers make use of heat-seals for seaming and closing, for example, cartons, folding square or rectangular containers or boxes, and/or even fabricating cylindrical tubes. 
     The packaging material is printed, converted into flat blanks, side seamed, and then formed, filled, and sealed using conventional carton filling machines. Specifically, the preferred embodiment of the new packaging material, as used to produce 10-ounce, quart, half-gallon and three-quart GABLE-TOP® cartons, comprises an exterior coating of 0.8 mil LDPE, paperboards of varying thickness (depending on carton size), a coextruded sandwich layer of an adhesive tie material (Plexar 177), 0.00285 inches aluminum foil, an adhesive tie layer material (Plexar 177)], and a product contact coating layer of 0.5 mil LDPE. 
     The second embodiment of the present invention comprises an exterior coating of 0.8 mil LDPE, paperboards of varying thickness (depending on carton size), 0.5 to 0.8 mil adhesive tie layer, 0.000285 inches aluminum foil, and 0.5 mil LDPE (product-contact). 
     The third embodiment of the present invention comprises an exterior coating of 0.8 mil LDPE, paperboards of varying thickness (depending on carton size), 0.00285 inches of aluminum foil and 0.5 mil LDPE (product-contact). 
     Prior cartons containing aluminum foil are often expensive relative to the performance problems they present when being form fill-sealed by the juice packager. Also, the usual thick (1.5-2.01 mil) LDPE product-contact layer generally required in such cartons absorbs citrus flavor/aroma oils, with subsequent deleterious effects on product quality. 
     The present invention makes use of a barrier layer of aluminum foil, PET, PETG, PETA or PVDC, reducing loss of essential oils and vitamin C, while functioning as an excellent oxygen barrier. Provided is a very thin (0.5 mil or less) product-contact layer of LDPE thereby reducing juice absorption in the product-contact layer and resultant degradation of the contact layer, as well as creating a laminate which allows for a strong front to back homogeneous heat-seal. 
     Additionally, applicants&#39; novel structures are applicable tonon-paperboard blow-molded container applications. The burial of a barrier material next to a high density polyethylene (HDPE) structural substrate would yield positive results and a structure wherein the food contact layer is a thin layer of low density polyethylene would complete the structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional elevation of an existing commercial structure of a laminate; 
     FIG. 2 is a cross-sectional elevation of an existing commercial structure of a laminate; 
     FIG. 3 is a cross-sectional elevation of an existing commercial structure of a laminate; 
     FIG. 4 is a cross-sectional elevation of an existing commercial structure of a laminate; 
     FIG. 5 is a cross-sectional elevation of the preferred embodiment of the laminate of the present invention; 
     FIG. 6 is a cross-sectional elevation of an alternate embodiment of the laminate of the present invention; 
     FIG. 7 is a cross-sectional elevation of an alternate embodiment of the laminate of the present invention; and 
     FIG. 8 is a cross-sectional elevation of an alternate embodiment of the laminate of the present invention. 
     FIG. 9 is a cross-sectional elevation of an alternate embodiment of the laminate of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The existing commercial structure for a paperboard carton for juice and similar products has made use of an easily heat-sealed barrier laminate composed of paperboard 4 (FIG. 1) sandwiched between two thick layers of low density polyethylene (LDPE) 2,6 (1.5-2.0 mil). The LDPE is an inexpensive heat-sealable material which acts to a limited extent as a moisture barrier to prevent loss of essential oils (flavor) and aroma. The problem encountered with th conventional laminate structure has been that the essential oils of the juice (namely--d-Limonene) have, after short periods of time, been absorbed into the thick (1.5-2.0 mil) LDPE layer causing heat seal decay, stress cracking, and swelling while stripping the juice of the essential oils. Additionally, the conventional structure (FIG. 1) provides virtually no barrier resistance to oxygen which causes the juice to lose vitamin C in great quantities after a relatively short period of time. To illustrate, the conventional paperboard 1/2 gallon juice carton will lose 60.5% of its essential oil (d-Limonene) and 84% of its vitamin C content in a storage period of six weeks (SEE TABLE 1). 
     One conventional existing paperboard carton (FIG. 2) utilizes two extra layers in addition to the layers disclosed in FIG. 1 to add greater barrier resistance to the passage of oxygen and resultant loss of vitamin C. Aluminum foil 14 has been added to the laminate structure to increase the barrier&#39;s resistance to the flow of oxygen. An additional thick layer of LDPE 16 in the past has been needed to allow the laminate to be heat-sealed from front to back with the exterior LDPE 8 layer. The structure of the barrier laminate (FIG. 2) has poor barrier properties relating to the absorption of essential oils and aromas, since the heat-sealable contacting layer is a thick (1.5-2.0 mil) layer of low density polyethylene. The shelf storage life of the juice carton made up of the barrier laminate of FIG. 2 still exhibits a percentage loss of essential oils (d-Limonene) of 35.5%, while greatly improving its barrier properties with respect to the percentage loss of vitamin C, 24% (See Table 1). The addition of the foil layer allows the laminate to exhibit excellent O 2  barrier properties. FIGS. 3 and 4 disclose structures of barrier laminates described in U.S. Pat. No. 4,513,036. FIG. 3 discloses a barrier laminate comprising a sandwich of LDPE 18-paperboard 20-High Density Polyethylene (HDPE) 22-LDPE 24. The laminate disclosed exhibits large losses of essential oils during its shelf life of six weeks, namely 60.5%, while also exhibiting large losses of vitamin C during the six week period, namely 87% (see Table 1). The economics and ease of fabrication of the laminates of FIG. 3 are outweighed by the poor barrier properties exhibited. 
     FIG. 4 discloses the preferred embodiment of U.S. Pat. No. 4,513,036, namely a barrier laminate comprising LDPE 26-Paperboard 28-Polypropylene 30-LDPE 32. The additional polypropylene layer 30 adds to the barrier properties at relatively low additional costs. The barrier properties still are extremely deficient in resistance to the passage of oxygen and loss of vitamin C, namely 71% after six weeks. The polypropylene laminate structure loses 39.5% of its essential oils (d-Limonene) after six weeks (see Table 1). 
     Both embodiments disclosed in the patent cited above do not adequately preserve the flavor/aroma and vitamin C content of the juice. The structure of the existing commercial constructions have all faced the same problem due to the necessity for heat sealing the seams and closures while forming the carton blank and while filling the cartons with juice or the like. The necessity of forming a heat seal from the front to the back of the laminate has resulted in the use of a thick exterior layer of LDPE and a thick interior layer of LDPE, both non-polar compounds which exhibit excellent heat-sealing characteristics to one another (see FIGS. 1-4). 
     Referring to FIG. 5, the preferred embodiment of the laminate of the present invention is shown as comprising a paperboard substrate 36 of varying thickness which is most suitably high-grade paperboard stock, for example, 282 lb Milk Carton Board, to which is applied on one side a thin coating of low density polyethylene (LDPE) 34, in a coating weight ranging from about 9 to about 15 pounds per ream. Any commercial extrusion coating grade LDPE is suitable for use herein. On the back or interior portion of the laminate, namely onto the paperboard layer 36, is applied a sandwich 44 of a 4.5 tie material (Plexar 177) 38/0.000285 inches foil 40/4.5 lb tie material (Plexar 177) 42, and a product contact coating layer of 0.5 ml LDPE. 
     Referring to FIG. 6, the alternate embodiment of the laminate of the present invention is shown as comprising a paperboard substrate 54 of varying thickness which is suitably high grade paperboard stock, i.e., 282 lb milk carton board, to which is applied on one side a very thin 0.8 mil coating of low density polyethylene (LDPE) 52, in a coating weight range averaging 12 lbs per 3000 square feet. Any commercial extrusion coating grade LDPE is suitable for use herein. On the back or interior portion of the laminate, namely onto the paperboard layer 54, a (Plexar 177) tie layer 56 and a layer of 0.000285 inches of foil 56 are applied. Extrusion coated thereon is a very thin 0.5 mil or less layer 58 of a non-polar LDPE. 
     Referring to FIG. 7, a third embodiment of the laminate of the present invention is shown as comprising a paperboard substrate 66 of varying thickness which is suitably high grade paperboard stock, i.e. 282 lb milk carton board, to which is applied on one side a very thin 0.8 mil coating of LDPE 64, a 0.000285 inches layer of foil 68 is applied. Extrusion coated thereon is a thin 0.5 mil or less layer 70 of a non-polar LDPE. 
     Referring to FIG. 8, an alternate embodiment of the present invention is shown which discloses a non-foil laminate structure comprising a paperboard substrate 74 of varying thickness which is suitably high grade paperboard stocks, i.e., 282 lb. milk carton board, to which is applied on one side a very thin 0.8 mil coating of LDPE 72. Buried in the structure is a barrier material layer 76 of either polyethylene terephthalate, glycol-modified polyethylene terephthalate, acid-modified polyethylene terephthalate or polyvinylidene chloride copolymers. Extrusion coated thereon is a thin 0.5 mil or less layer 78 of a non-polar LDPE. 
     FIG. 9 is an embodiment of the invention wherein a blow-molded container has incorporated therein a buried layer of a barrier material such as polyethylene terephthalate, glycol-modified polyethylene terephthalate, acid-modified polyethylene terephthalate o polyvinylidene chloride copolymers. In contact with the juice, food or non-food layer is a thin layer of a low-density polyethylene polymer. 
     Although specific coating techniques have been described, any appropriate technique for applying the layers onto the paperboard substrate can be suitably employed, such as extrusion, coextrusion, or adhesive lamination or single and/or multilayer films to paperboard to achieve the stated inventions of this patent. 
     The unique barrier effect provided by the laminate of the present invention to the % loss of essential oils and to the % loss of vitamin C is clearly demonstrated by the following example outlined in Table 1. 
     Standard 1/2 gallon juice containers were prepared and filled with juice. A typical essential oil in the juice was d-Limonene. The filled cartons were stored for a test period of six weeks after which the juice was analyzed to determine the percentage loss by weight of the essential oil d-Limonene and the percentage loss by weight of vitamin C. 
     All weights in lbs are given in lbs per 3000 square feet. 
     
                       TABLE 1______________________________________                 %Test Sample           Loss of1/2Gallon Juice       Essential                          % Loss ofContainer             Oil      Vitamin C______________________________________LDPE/BOARD/LDPE (FIG. 1)                 60.5     84LDPE/BOARD/LDPE/FOIL/ 35.5     24LDPE (FIG. 2)LDPE/BOARD/HDPE/LDPE  60.5     87(FIG. 3)LDPE/BOARD/POLYPROPYLENE/                 39.5     71LDPE (FIG. 4)LDPE/BOARD/PLEXAR-FOIL-                 16.5     46PLEXAR/LDPE (FIG. 5)______________________________________ 
    
     It can be clearly seen that the container prepared from a laminate of the present invention provides an almost complete barrier to the loss of essential oils far greater than has been present in existing structures. Additionally, the oxygen passage or percentage loss of vitamin C has been greatly reduced over all prior laminates not containing aluminum foil. 
     The effectiveness of the laminate of the present invention as a barrier to migration of essential oils and flavors, as well as a barrier to a loss of vitamin C permits a significant extension of shelf life of containers constructed therefrom.