Moisture resistant frozen food packaging using an over-print varnish

This invention relates to moisture resistant frozen food packaging using highly-sized paperboard and press applied moisture resistant over-print varnishes. Such structures of this type, generally, employ a moisture-resistant coating which is placed between the food product and the paperboard in order to provide a barrier for the food from the board and also to prevent the paperboard from absorbing moisture. Also, edge-wick moisture absorption is minimized by the use of the highly-sized sheet.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to moisture resistant frozen food packaging using 
highly-sized paperboard and press applied moisture resistant over-print 
varnishes. Such structures of this type, generally, employ a 
moisture-resistant coating which is placed between the food product and 
the paperboard in order to provide a barrier for the food from the board 
and also to prevent the paperboard from absorbing moisture. Also, 
edge-wick moisture absorption is minimized by the use of the highly-sized 
sheet. 
2. Description of the Related Art 
Cartons which are used for distributing, marketing and, in some cases, 
heating portions of prepared foods are fabricated with a paperboard 
structural substrate. In some applications, the paperboard may be coated 
with a polymeric or similar material for barrier or aesthetic purposes. A 
water-resistant coating is placed between the food product and the 
paperboard to provide a barrier for the food from the board and, also, to 
prevent the paperboard from absorbing moisture from the food. In cases 
where the product is continuously kept frozen, this is usually adequate. 
However, markets in developing countries where electricity is expensive 
and freezers are turned off at night, the outside and inside surfaces of 
the packages are often exposed to liquid water from condensation and 
thawing which can ruin the functionality of the package and its 
appearance. Different methods of rectifying this problem have led to 
trade-offs in print quality, ease of printing and water resistance of the 
package. 
Current frozen food packaging using paperboard consists of paperboard with 
extruded polymers or wax on the food contact side. This allows the outside 
of the carton to be used as a printing surface for the graphics desired by 
the retailer. However, under some conditions as discussed above, the 
package is subjected to high moisture or liquid water that penetrates the 
paperboard through the printed surface and through "edge-wicking". Water 
absorption can ruin the package appearance by warping the board, thereby 
causing the coating/ink surface to flake and reducing package integrity 
through paperboard delamination. 
Another package design utilizes an extruded polymer or wax application on 
both sides of the paperboard. This design prevents water penetration 
through the paperboard, but does not address edge-wicking. It is also less 
desirable since it requires the package to have a printed overwrap or the 
printing be performed on a polymer or wax surface. Also, it is well known 
that printing on a polymer or wax surface may result in the graphic 
quality being compromised as compared to printing on a clay-coated 
paperboard surface. It is also less desirable for the printer/converter to 
print on a polymer/wax surface because this requires special inks, 
equipment and other infrastructure. 
A further package design involves laminating a film on the outside over the 
printed or unprinted paperboard. This requires an additional step beyond 
printing which increases cost and rejection rate on the package. Exemplary 
of such designs are U.S. Pat. No. 4,595,611 ('611) to Quick et al., 
entitled "Ink-Printed Ovenable Food Containers" and U.S. Pat. No. 
4,830,902 ('902) to Plantenga et al., entitled "Paper Object Printed With 
Ink and Coated With A Protective Layer". While the '611 patent discloses 
the use of an outside film over the printed or unprinted paperboard, the 
'611 reference is primarily concerned with the use of a sulfonated 
polyester to improve adhesion of a food-contact, heat sealable polyester 
layer. The use of an over-print varnish to protect printing is discussed, 
but the use of this coating to improve the functioning of the package in 
terms of watering resistance is not mentioned. 
With respect to the '902 patent, again while it discusses the use of a film 
on the outside over the printed paperboard, it does not teach any mention 
of moisture resistance of the applied coating, the sizing level of the 
paperboard substrate or any type of packaging. Also, the '902 patent 
discloses the use of an over-print varnish to protect the printing on a 
paper substrate, but not as a functional characteristic of a frozen-food 
package. Finally, ovenability is not mentioned. Therefore, a more 
advantageous package, then, would be presented if the over-print varnish 
would improve the water resistance of the package, while at the same time 
protecting the printing on the paper substrate. 
It is apparent from the above that there exists a need in the art for an 
over-print varnish which is capable of being coated on a frozen-food 
package, and which at least equals the printing protection characteristics 
of the known over-print varnishes, but which at the same time is able to 
increase the water resistance and reduce the edge-wicking of the package. 
It is a purpose of this invention to fulfill this and other needs in the 
art in a manner more apparent to the skilled artisan once given the 
following disclosure. 
SUMMARY OF THE INVENTION 
Generally speaking, this invention fulfills these needs by providing a 
composite structure for a frozen-food paperboard package having decreased 
edge-wicking and including an over-print varnish for increasing moisture 
resistance, comprising a water resistant, over-print varnish layer, a 
print graphics layer located interior to the varnish layer, a layer of 
particulate minerals located interior to the print graphics layer, a 
paperboard substrate located interior to the layer of particulate 
minerals, and a food-contact polymer layer located interior to the 
paperboard substrate. 
In certain preferred embodiments, the water-resistant, over-print varnish 
is press-applied over the graphics layer. Also, the paperboard substrate 
is a highly-sized paperboard to prevent edge-wicking. Finally, a layer of 
a hold-out varnish may be located between the water-resistant varnish 
layer and the print graphics layer to further increase water-resistance. 
In another further preferred embodiment, moisture resistance and 
edge-wicking of the paperboard package composite structure are 
substantially improved through the use of the over-print varnish and the 
highly-sized paperboard. 
The preferred package, according to this invention, offers the following 
advantages: lightness in weight; ease of assembly; good stability; 
excellent durability; good economy; reduced edge-wicking; and increased 
moisture resistance. In fact, in many of the preferred embodiments, these 
factors of ease of assembly, durability, reduced edge-wicking, and 
increased moisture resistance are optimized to the extent that is 
considerably higher than heretofore achieved in prior, known package 
composite structure.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention focuses on providing moisture/water resistance of a 
packaging material by addressing the water absorption through the edge 
(edge-wick) and through both surfaces separately. The food-contact surface 
may provide water resistance by using a multitude of polymers, waxes or 
coatings that are appropriate for this intended use (heat resistant for 
ovenable cartons, etc.). This is no different than the methods previously 
described. Edge-wick moisture absorption is minimized by use of a 
highly-sized substrate such as liquid packaging paperboard. 
Moisture absorption from the outside of cartons prepared from the packaging 
material is minimized, according to the present invention, by using a 
water-resistant varnish that is applied to a paperboard substrate 
subsequent to printing the desired graphics, preferably, on the same 
printing press. This methodology allows for a clay-coated paperboard to be 
used so that printability is enhanced. The clay-coating will also provide 
some "hold-out" for the water-resistant varnish so that a continuous film 
may be applied while using low coat weights of the varnish, thus, 
resulting in a lower cost package. It also avoids the costly extra step of 
laminating a water-resistant film to the printed side. The structure of 
the package is described more completely below with reference to FIG. 1. 
With reference to FIG. 1, there is illustrated an advantageous coated 
substrate for use in a frozen-food package including an over-print varnish 
for increased moisture resistance and decreased edge-wicking. More 
particularly, composite structure 2 includes, in part, from exterior to 
interior, over-print varnish layer 4, print graphics layer 6, particulate 
mineral layer 8, highly-sized paperboard substrate 10, and food contact 
polymer layer 12. 
Over-print varnish layer 4, preferably, is applied by a printing press at 
an application rate or coat weight of at least 1 pound per 1000 ft.sup.2. 
Preferably, any suitable acrylic-based type of over-print varnish can be 
used. Particulate mineral layer 8 is, preferably, a fluidizied bed of 
minerals such as coating clay, calcium carbonate, and/or titanium dioxide 
with starch or adhesive which is smoothly applied to a traveling web 
surface. Successive densification and polishing by calendering finishes 
the mineral coated surface to a high degree of smoothness and a superior 
graphics print surface. Substrate 10 is, typically, paperboard constructed 
from an 0.018" thick solid bleached sulfate (SBS) sheet. Definitively, the 
term paperboard describes paper within the thickness range of 0.008 to 
0.028". The invention is relative to the full scale of such a range as 
applied to packaging and beyond. Substrate 10, preferably, is sized 
according to conventional techniques and at a sizing application rate of 
approximately 0.8% of rosin size or 0.4% alkyl ketene dimer size. Food 
contact polymer layer 12 can include any suitable food contact polymer 
such as, but not limited to, polyethylene terephthalate, polypropylene, 
polyethylene, and nylon. Finally, layer 12, preferably, is applied at a 
rate of approximately 19 pounds per 1000 ft.sup.2. 
With respect to FIG. 2, there is illustrated another embodiment of a 
composite structure 20. Structure 20 includes, in part, water-resistant 
varnish layer 4, a second hold-out varnish layer 22, print graphics layer 
6, particulate mineral layer 8, paperboard substrate layer 10, and food 
contact polymer layer 12. Layers 4, 6, 8, 10, and 12, preferably, are 
constructed of the same materials as their corresponding layers in 
composite structure 2. 
However, composite structure 20 includes an additional hold-out varnish 
layer 22 located between water-resistant varnish layer 4 and print layer 
6. Layer 22, preferably, is constructed of any suitable overprint varnish. 
Also, layer 22 is applied at a coat weight of approximately 1 pound per 
1000 ft.sup.2. It has been determined that by applying layer 22 prior to 
the application of layer 4, a significant improvement in water resistance 
can be obtained. The improvement is shown in the Table, below. 
Composite structures 2 and 20 are preferable to most printers/converters 
since they involve printing on a clay coated surface rather than on a 
polymer surface. As discussed above, printing on polymers involves special 
inks, equipment and requires extra drying time to pass through. It also 
often involves extra warehouse space to allow pallets of printed substrate 
to dry prior to converting or additional printing. Without additional 
warehousing to allow drying, printing on polymers often excludes the use 
of two passes through the printing press which reduces the type of presses 
that can be used and the number of colors that can be applied to a 
package. Printing on a clay-coated surface, then applying the 
water-resistant varnish or a water resistant varnish and a hold-out 
varnish, according to the present invention, allows the printer to pursue 
various options in printing graphics. This is because the press 
application of the varnishes eliminates an extra converting step that is 
currently necessary with many packages. 
Laboratory trials using water-resistant varnishes on ovenable paperboard 
(clay-coated solid bleached sulfate (SBS) with a polyethylene 
terephthalate coating) and liquid packaging paperboard with polyethylene 
were conducted. The results, in the Table below, demonstrate how the 
water-resistant varnish and the use of a highly-sized paperboard have 
superior water-resistance as compared to standard ovenable paperboard. 
TABLE 
______________________________________ 
Sample % water pick-up* 
______________________________________ 
PET-coated paperboard (control) 
51% 
PET-coated liquid packaging paperboard (control) 
52% 
PET-paperboard with varnish 
28% 
PET-liquid packaging paperboard with varnish 
13%** 
Polymer-coated both side paperboard 
&gt;5% 
______________________________________ 
*% water pick up is defined as the weight of water absorbed by the 
paperboard after 1 hour of submersion divided by the initial weight of th 
paperboard. 15% is the limit established by a typical customer/converter. 
**13% is an average value taken from values ranging from 7% to 25%. The 
25% data point is believed to be due to inadequate coat weight resulting 
in a discontinuous film layer. 
As can be seen from the data in the TABLE, the use of water resistant 
varnish with various types of paperboard decreases the percentage of water 
pick-up, i.e., increase the water-resistance. Even though the water 
pick-up of the varnished aseptic paperboard is not as low as the 
polymer-both-side paperboard, the converting and printing advantages of 
the present invention make it more desirable for the customer and the 
ultimate end user. As discussed above, this is due to the cost, ease of 
manufacturing and superior graphics when printing on a clay surface as 
compared to polymer-on-wax surface. 
Once given the above disclosure, many other features, modifications or 
improvements will become apparent to the skilled artisan. Such features, 
modifications or improvements are, therefore, considered to be a part of 
this invention, the scope of which is to be determined by the following 
claims.