Novel laminates for paperboard cartons and a process of forming said laminates

The present invention relates to 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. Also disclosed is a process of making the laminate. The laminate makes use of a layer of a heat-sealable glycol-modified polyethylene terephthalate to enhance the barrier properties of the laminate.

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 product contact material which does 
not absorb or transmit flavor or odor ingredients of citrus and other 
juices. 
Heat-sealable low-density polyethylenes are well known to be components of 
current paperboard citrus juice cartons which provide no 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 
terphthalates, 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 could not be substituted for 
low density polyethylene since they lacked the requisite heat sealability 
over a practical temperature range, necessary FDA clearance for direct 
contact to foods stress cracking resistance and cutability during the 
scoring, and/or die cutting conversion processes. Due to the failures of 
these impermeable materials, past efforts have concentrated on using a 
combination of these flavor oil resistant materials with low density 
polyethylene as the heat-sealable component. 
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 to prevent loss of essential oils and flavors. The conventional 
structure falters in that the LDPE layer absorbs the essential oils of the 
juice after short periods of time causing integrity decay of heat seals 
and stress cracking of the layer. 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 expensive foil layer increases barrier resistance to the flow 
of oxygen, while the additional LDPE allows for ultimate heat-sealability 
of the laminate. The improved conventional structure has poor barrier 
properties relating to the absorption of essential oils and aromas, since 
the interior contacting layer is still LDPE. 
The object of the present invention is to produce an improved juice 
packaging heat-sealable laminate material which does not absorb or 
transmit flavor/odor ingredients of citrus and other juices. 
SUMMARY OF THE INVENTION 
The preferred embodiment of the present invention reveals a heat-sealable 
barrier laminate providing a substantial barrier to the loss of Vitamin C 
and an almost complete barrier to the loss of essential flavor oils over 
the shelf life period of the carton and far beyond the six week period as 
well. The preferred embodiment comprises from the outer surface to the 
inner surface contacting the essential oils and/or flavors: an exterior 
layer of a low density polyethylene, a paperboard substrate, an interior 
layer of a low density polyethylene and a layer of a glycol-modified 
polyethylene terephthalate (PETG) coated onto the interior layer of the 
low density polyethylene, in contact with the juice 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 significant 
extension of the shelf life thereof and permits replacement of the costly 
aluminum foil barrier. 
The preferred PETG, Kodak Kodabond Copolyester 5116 is available from 
Eastman Chemical Products, Inc., Kingsport, Tenn. 
The present invention has produced a suitable container which has excellent 
barrier properties utilizing a laminate which can be heat-sealed with its 
exterior and interior layers being a non-polar (LDPE) and a polar (PETG) 
from front to back. The conventional theories have been that the laminate 
could not be heat-sealed on conventional apparatus at practical 
temperatures without having non-polar constituents on its ends. The liquid 
juice components are insoluble in the polar PETG material, preventing 
flavor oil absorption and resulting swelling, stress cracking, and 
plasticization, heat seal degradation as occurs with LDPE as the contact 
layer. 
The preferred laminate of the present invention not only exhibits 
significant barrier properties to extend the shelf life of the juice, but 
the laminate is produced using conventional extrusion equipment. 
Stepwise, a layer of molten LDPE is placed onto the paperboard substrate by 
extrusion coating, the newly formed layer of LDPE is then corona discharge 
treated or flame treated in preparation for heat-sealing later in the 
process. 
Secondly, the web is flipped over down-line and a layer of LDPE is 
extrusion coated onto the other an exposed side of the paperboard 
substrate. This layer is also corona discharge treated or flame treated to 
facilitate adhesion to subsequent PETG layer. 
Thirdly, a molten layer of PETG is extrusion coated onto the interior layer 
of LDPE. The completed laminate can now be heat-sealed from front to back 
(LDPE to PETG) at conventional temperatures (250.degree. F. to 500.degree. 
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 placed onto conventional 
equipment, such as a PurePak.RTM. machine made by Ex-Cell-O. The blanks 
are heat-sealed at the bottom, filled and heat-sealed at the top by the 
PurePak.RTM. machine to complete the filled carton. 
The barrier laminate produced by the present invention not only exhibits 
excellent barrier properties and can be easily constructed but also meets 
FDA approval for use in food packaging. Kodak's Copolyester 5116 is FDA 
approved for direct food contact and the preferred PETG of the invention. 
Other PETG's which heat seal at low temperatures (250.degree. F. to 
500.degree. F.) and which can be cut on conventional machinery could also 
be used as the contacting barrier. 
Thus, until the advent of the present invention no suitable containers for 
the containment of citrus juices have been developed which retain the 
advantages of using paperboard as the base material and is an FDA approved 
heat-sealable barrier laminate which is economical and can be formed 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 containers. Such containers which make use of a heat-seal for 
seaming and closing such as folding boxes, square or rectangular 
containers or cartons, or even cylindrical tubes.

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 layers of low 
density polyethylene (LDPE) 2, 6. The LDPE is an inexpensive heat-sealable 
material which acts only to a limted extent as a moisture barrier to 
prevent loss of essential oils (flavor) and aroma. The problem encountered 
with conventional laminate structure has been that the essential oils of 
the juice (namely - D Limonene) has, after short periods of time, been 
absorbed into the LDPE layer causing heat seal decay, stres 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 84.5% of its Vitamin C content in a storage 
period of six weeks (SEE TABLE 1). 
One conventional existing paperboard carton (FIG. 2) utilizes two 
additional 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's resistance to the flow of oxygen. The additional 
layer of LDPE is 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 
still 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.sub.2 barrier properties. Although, the use of a foil 
layer is extremely beneficial, the increased expense makes the use of foil 
economically less desirable. 
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 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 its resistance to the passage of oxygen 
and its 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 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 an exterior layer of LDPE and an 
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 which 
is most suitably high-grade paperboard stock, for example, 282 lb. Milk 
Carton Board, to which is applied on both sides a coating of low density 
polyethylene (LDPE) 34, 38 in a coating weight ranging from about 5 to 
about 400 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 LDPE layer 38 is applied a layer of PETG 40. The PETG being a 
heat-sealable layer composed of Kodak's KODABOND Copolyester 5116. 
Referring now to FIG. 9, wherein a block diagram discloses the method of 
forming the heat-sealable barrier laminate of FIG. 5. 
The laminate can be easily fabricated. In Step A, a molten layer of the 
LDPE 34 is extrusion coated onto the paperboard substrate 36. Step B, the 
LDPE layer 34 is corona discharge or flame treated in preparation for 
subsequent heat-sealing Step C, the web is flipped over to facilitate Step 
D, which has a layer of molten LDPE 38 extrusion coated onto the 
paperboard substrate 36. Step E, LDPE layer 38 is corona discharge treated 
to facilitate the adhesion of a subsequent PETG coating, and lastly, Step 
D, a layer of PETG 40 is extrusion coated onto LDPE layer 38 to complete 
the sandwich. 
Referring now to FIG. 6, an alternate embodiment of the laminate of the 
present invention is shown. The embodiment adds an additional barrier 
layer which not only provides total containment of flavor oils, but also 
provides significant containment of Vitamin C. In this alternate 
embodiment, the paperboard substrate 44 is coated on the external surface 
thereof with a web of heat-sealable LDPE 42. On the internal surface of 
the paperboard substrate 44 is applied a coating of LDPE 46. Overlying the 
LDPE web 46 is a layer of an oxygen barrier material 48. The oxygen 
barrier material can be any of the following group: foil, 
polyacrylonitrile, polyethylene terephthalate, polyvinylidene chloride, 
polyamide, polyethylene/vinyl alcohols or polyvinyl chloride. Overlying 
the barrier material is a layer of heat-sealable PETG 50 which will 
ultimately form the internal surface of the container constructed 
therefrom. 
To enhance the adhesion of the additional barrier layer 48 to the LDPE 
layer 46 and the PETG layer 50, tie layers 47, 49 can be interposed 
therebetween. Adhesives which are extrudable or coextrudable, such as 
Dupont's C.times.A's or Norchem's PLEXARS, are suitable choices. 
FIG. 7 discloses another embodiment of the present invention. The 
embodiment replaces the exterior LDPE layer 34 of FIG. 5 with an exterior 
layer 52 of PETG extrusion coated onto the paperboard board substrate 54. 
The interior layers namely layer 56 (LDPE) and interior layer 58 (PETG) 
resemble the structure of FIG. 4. The addition of a heat-sealable layer of 
PETG on the exterior of the carton allows for numerous additional uses of 
the product. Such uses would be for packages requiring an enhanced oxygen 
barrier or ease of front to back sealing. 
FIG. 8 reflects a further embodiment of FIG. 7 wherein the laminate 
comprises from front to back: a layer of PETG 60, a layer of LDPE 62, the 
paperboard substrate 64, a layer of LDPE 66, and an innermost layer of 
PETG 68. 
FIG. 9 reflects an embodiment similar to that of FIG. 8. Replacing the 
outer layer of a heat-sealable PETG 60, is a blend layer 70 of PETG and 
LDPE. The blend is made up of between five (5) to thirty (30) percent PETG 
and the rest being LDPE. The laminate is comprised from outside to inside: 
LDPE/PETG Blend 70-LDPE 72-PAPERBOARD SUBSTRATE 74-LDPE 76-PETG 78. The 
advantage of the LDPE/PETG blend layer is that it eliminates the need for 
surface treatment prior to heat sealing. 
Although these specific coating techniques have been described, any 
appropriate technique for applying the layers onto the paperboard 
substrate can be suitably employed. 
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. 
The six cartons tested were those shown in FIGS. 1-6 and described herein. 
TABLE 1 
______________________________________ 
Test Sample % Loss of 
1/2 Gallon Juice Essential % Loss of 
Container Oil Vitamin C 
______________________________________ 
LDPE-BOARD-LDPE 60.5 84 
(FIG. 1) 
LDPE-BOARD-LDPE-FOIL- 
35.5 24 
LDPE (FIG. 2) 
LDPE-BOARD-HDPE-LDPE 60.5 87 -(FIG. 3) 
LDPE-BOARD-POLYPROPYLENE- 
39.5 71 
LDPE (FIG. 4) 
LDPE-BOARD-LDPE-PETG 0* 47 
(FIG. 5) 
LDPE-BOARD-LDPE-FOIL- 
0* 24 
PETG (FIG. 6) 
______________________________________ 
*Less than one percent 
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 
comprising LDPE heat seal layers. 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.