Package having improved barrier properties

An effective oxygen barrier which can substantially extend the shelf life of oxygen sensitive products comprising at least outer and inner plies of a polymeric material and a substantially anaerobic space between the two or more plies.

FIELD OF THE INVENTION 
The present invention generally relates to packages employing gas-permeable 
films and is particularly useful in packaging oxygen-sensitive products 
such as food products. 
BACKGROUND OF THE INVENTION 
Many products are sensitive to gases commonly found in the air, with oxygen 
tending to be the most problematic of these gases. For instance, many 
foods tend to be adversely affected by oxygen because they undergo 
chemical changes in the presence of oxygen that degrade their taste or 
color. One example of such a chemical change is the tendency of fats to 
react with oxygen and become rancid. Oxygen may also promote the growth of 
bacteria and the like which will cause food to spoil. 
When commercially packaging oxygen-sensitive food products or the like for 
extended storage before sale or ultimate use by a consumer, care must be 
taken to minimize the product's contact with oxygen during storage. When 
canning food products, the cans are hermetically sealed to keep air from 
coming into contact with the food. In commercial canning processes, the 
containers tend to be formed entirely out of metal, with seams being 
welded or mechanically crimped closed. In home canning processes and in 
some other commercial processes, glass containers having metal lids are 
used and the lids commonly include a compressible sealing strip of a 
rubber-like material to form an air-tight seal between the glass jar and 
the lid. In some home canning processes, an additional wax barrier is 
placed between the product and the food to limit contact with any oxygen 
which may leak through the seal between the jar and the lid. 
Both of these types of containers tend to be quite effective in keeping 
oxygen out of the interior, and hence out of contact with the food stored 
inside, because metal and glass are essentially absolutely 
oxygen-impermeable. So long as the seal between the component parts of the 
containers remains intact, there is little chance that oxygen will enter 
the container and affect the food. 
In modern packaging, plastic materials have in many instances entirely 
replaced metal or glass as the primary component of the package due to the 
lower cost of plastics. For instance, frozen pizzas and high-fat products 
such as potato chips and the like are commonly sold in an entirely plastic 
container, perhaps using a label formed of paper or some other readily 
printed material. Unlike metal or glass, though, virtually all polymeric 
materials used in packaging food are at least slightly permeable to 
oxygen, with the permeability varying from one plastic material to 
another. Although plastics tend to be significantly cheaper than metal or 
glass in most food packaging applications, the oxygen permeability of 
plastic films can also reduce the effective shelf life of the product 
contained in the package. 
Many attempts have been made to develop materials for use in the packaging 
industry that minimize oxygen transmission; these attempts have 
encompassed both development of homogenous polymeric films with new 
plastics and composite films that may include layers of different 
plastics. Among the polymeric films most commonly used in the packaging 
industry are polyvinylidene chloride (PVDC, sold under the trade name 
"Saran"), which has a relatively low gas permeability or transmittance; 
biaxially oriented nylon, which exhibits moderate oxygen transmittance; 
and polyethylene, which transmits oxygen more freely. For instance, a 13 
micron film of a PVDC will transmit about 4.0 cm.sup.3 of oxygen/m.sup.2 
of surface area/atmosphere/day, while a 1 mil (25 micron) film of nylon 6 
will transmit oxygen at about ten times that rate (about 40 cm.sup.3 of 
oxygen/m.sup.2 of surface area/atmosphere/day). 
Current composite materials may include a layer of a metal foil, e.g. a 25 
.mu.m aluminum foil, disposed between a pair of plastic films, which may 
be formed of different polymers if so desired. One of the advantages of 
using a plastic/metal composite material is that the metal layer can, if 
thick enough, make the composite material substantially totally oxygen 
impermeable. 
Unfortunately, though, materials which provide better resistance to oxygen 
transmission also tend to be more expensive. Plastic/metal composite films 
are generally much more expensive than a film formed solely of the plastic 
material and are also opaque, preventing a consumer from seeing the 
contents of the package at the point of sale. There are also significant 
cost differences between different polymeric film materials. As a general 
rule of thumb, polymeric films which have lower rates of oxygen 
transmittance tend to be more expensive than films with higher oxygen 
transmission rates. 
The oxygen transmittance of a polymeric film of a given composition is 
generally inversely proportional to its thickness--a film which is twice 
as thick will transmit about half as much oxygen. Polymeric films used as 
walls of containers also have to meet certain other physical requirements, 
such as minimum tensile strength, to provide a suitable commercial 
container. Accordingly, it is frequently more cost-effective to use a 
thicker film of a cheaper plastic material than a thinner film of a more 
expensive plastic to achieve the same net oxygen transmittance. 
Numerous attempts have been made to provide a more cost effective oxygen 
barrier. For instance, U.S. Pat. No. 4,105,818, issued to Scholle, sets 
forth an alleged improvement in packages using polymeric films. According 
to Scholle's teachings, one can improve the barrier properties of a 
plastic film by splitting a single thicker ply of plastic into a pair of 
thinner films, with each of the thinner films having a thickness about 
half that of the thicker film. Scholle claims that, at steady-state 
conditions, a single ply of 0.5 mil PVDC transmits about twice as much 
oxygen as a single ply of 1.0 mil PVDC, (as one would expect), yet a 
composite film consisting of two plies of 0.5 mil PVDC transmits less than 
half the oxygen transmitted by the 1.0 mil film. 
This is counterintuitive in that one would expect such a film to transmit 
at about the same rate as the 1.0 mil film since its total thickness is 
the same. As explained more fully below, though, this simply is not 
believed to be the case. 
At steady-state, the two-ply film will indeed transmit oxygen at 
substantially the same rate as a single-ply film having the same total 
thickness. Furthermore, if air is permitted to remain between the two 
plies of the composite film it has been found that the transmission rate 
of such a composite film is actually substantially greater than that of 
the single-ply film of the same thickness, at least initially. Only after 
some time has passed will such a composite film approach the transmission 
rate of the single-ply film of the same thickness; such a film cannot 
reduce the transmission rate below that of a single-ply of the same 
thickness. 
Other attempts have been made to extend the shelf life of oxygen-sensitive 
films by providing "oxygen absorbing" materials in the container with the 
product. Such oxygen absorbing materials operate on the principle that 
they are more reactive with oxygen than the product and therefore will 
consume oxygen entering the package before it can react with the food 
product. For instance, an oxygen absorbing product sold by Mitsubishi Gas 
Chemical Company under the trademark AGELESS utilizes finely divided iron 
powders to scavenge oxygen from the atmosphere. 
However, there are concerns with placing such a material in direct physical 
contact with food products. In addition to obvious risks of degradation of 
taste and color of the foods, there are concerns regarding the possible 
reactions of these powders with the food products themselves. Accordingly, 
if such oxygen absorbing products are used in packaging foods, they must 
generally be physically isolated from the food. This adds further 
complexity, and hence cost, to these packages. 
It would be advantageous to provide a cost-effective polymeric film 
material for packaging oxygen-sensitive products which does not suffer 
from the problems associated with prior art materials. In particular, such 
a film is desirably translucent or substantially transparent to permit 
consumers to see the food products at the point of sale. It should not 
introduce potential contaminants into the inner cavity of the container 
wherein the product is stored. And, perhaps most importantly, it should 
provide an effective barrier to oxygen transmission to enhance the shelf 
life of oxygen-sensitive products without unduly increasing the expense of 
the package. 
SUMMARY OF THE INVENTION 
The present invention provides an effective oxygen barrier which can 
substantially extend the shelf life of oxygen-sensitive products without 
significantly increasing the cost of packages using the film. The barrier 
of the invention consists of at least inner and outer plies of a polymeric 
material and a substantially anaerobic space between the two or more 
plies. In one preferred embodiment, the space between the plies is filled 
with a substantially anaerobic gas, such as nitrogen. In an alternative 
embodiment, at least one of the two plies is provided with a textured 
surface and a vacuum is drawn on the space between the plies. The texture 
of the ply or plies will serve to physically space one film from the 
other, providing a substantially anaerobic space between the two plies. 
The present invention also encompasses a variety of package designs using a 
barrier of the invention. In one embodiment, the package includes a 
plurality of sides and at least a portion of one of these sides is 
provided with a substantially transparent or translucent barrier in 
accordance with the invention. Such a package may be used where it is 
desired to provide a window in an otherwise opaque package to permit 
consumers to view its contents. In another embodiment, all of the sides of 
the container except for one is formed of a barrier film of the invention 
while the remaining side is formed of a material having barrier properties 
at least equal to those of the barrier of the invention. Such a package 
may be used, for instance, where it is preferred that the package include 
an opaque area for carrying a label or other printed material yet permit a 
consumer to see the product which they are buying.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 depicts a composite film barrier according to one preferred 
embodiment of the invention. In this schematic drawing, the composite film 
10 comprises an inner ply of a polymeric material 20, an outer ply of a 
polymeric material 30, and a substantially anaerobic space 40 disposed 
therebetween. Each of the two plies is formed of a sheet of polymeric 
material; these two plies may be formed of the same or different polymers. 
For reasons explained more fully below, though, it is generally preferred 
that the two plies be formed of the same polymeric material and have 
approximately the same thickness so that their individual oxygen 
transmission rates are approximately equal. 
In some applications, the two plies may be formed by simply folding a 
single sheet of polymeric material to provide two plies adjacent one 
another and joined together along one edge thereof. In such a design, the 
inner and outer plies of the barrier may be sealed together, such as by 
heat sealing or the like, to fully define the anaerobic space 
therebetween--the space should not be open to the atmosphere. 
The plies may be formed of any known polymer having sufficient barrier 
properties for use in packaging applications. Films currently used in the 
art include those made from polyester, polypropylene, PVDC, nylon, and 
polyethylene, as well as multi-layer laminate films formed of contiguous, 
bonded layers of these and other polymers. Any of these films may be 
advantageously used in the present invention. One of the goals of the 
invention, though, is to provide a particularly cost effective oxygen 
barrier for use in packaging applications. In order to further that end, 
in many situations it will be advantageous to use a cheaper polymeric 
film. 
The thickness of these two plies of polymeric material may be varied as 
desired. it is contemplated that the thickness of each of these plies will 
be dependent on the particular application for which the barrier ten of 
the invention is being used. This thickness may also depend upon the 
composition of the plies because, as explained above, the oxygen 
transmittance of a polymeric film will depend to a very large extent upon 
the material from which it is formed. It is generally contemplated, 
though, that the thickness of these films will be at least about 10 .mu.m, 
and they may be significantly thicker than that, depending on the 
particular application. For instance, a highly flexible package may be 
formed of a 10 .mu.m film while a relatively rigid package design may 
require a much thicker film. 
In an alternative embodiment of the invention, at least one of the plies 
20,30 of the composite barrier film is formed of an edible polymeric 
barrier material. Although both of the of the plies may be formed of such 
an edible barrier, the other of the two plies is desirably formed of an 
ordinary, inedible polymeric material, as in the embodiment set forth 
above. It is particularly preferred that the inner ply 20 be the one 
formed of the edible material and the outer film be formed of the 
polymeric material. This will permit a consumer to remove the outer, 
polymeric ply from a package utilizing a barrier of the invention to 
arrive at an edible food product, eliminating the need to completely 
remove two separate plies before reaching the product stored within the 
package. 
A variety of edible films are known in the art. A number of different 
edible films, as well as parameters useful in selecting such films, are 
set forth by S. Guilbert in "Technology and Application of Edible 
Protective Films", Chapter 19 (pp. 371-394) of Food Packaging, Takashi K., 
editor (1990), the teachings of which are incorporated herein by 
reference. In general, edible films are formed from: proteins; cellulose, 
starches, dextrins or their derivatives; plant hydrocolloids; waxes, fat 
products, monoglycerides or their derivatives; or mixtures of these 
materials. For instance, these films may comprise a simple coating of a 
gelatin material applied directly to the product. 
The manner in which an edible film will be formed for use as the inner ply 
20 of the invention will vary depending upon the nature of the material 
used in the film. In many applications, though, a film-forming agent (e.g. 
gelatin or the like) will be carried in a water-based solution. This 
solution may be applied directly to the product to be packaged and the 
solution will be dried to produce the inner ply 20. This coated product 
may then be sealed within the outer ply 30 with an anaerobic space 40 
disposed between the two plies. As explained hereinafter in connection 
with FIG. 6, it is preferred that the transmittance of the inner ply be 
about the same as the transmittance of the outer ply. Accordingly, it is 
desirable that the composition and thickness of each of the two plies be 
chosen such that the oxygen transmittance of these two plies be about the 
same. 
As explained in some detail below, the anaerobic space 40 between the two 
plies of the barrier will gradually build a partial pressure of oxygen 
when stored in air. Nonetheless, it is preferred that the space 40 be 
generally free of oxygen when the barrier is initially formed. Providing 
such an anaerobic space may be accomplished in any of a wide variety of 
manners. 
In one preferred embodiment, the space 40 is flushed with a supply of an 
anaerobic gas. Nitrogen gas is particularly preferred due to its 
relatively low cost and substantial lack of any toxicity, but virtually 
any suitable anaerobic gas may be used. Equipment for flushing the 
headspaces of packages are known in the art and need not be disclosed in 
detail here. It should also be recognized that it is virtually impossible 
to completely remove all oxygen from this space. If an anaerobic gas is 
flushed through the space after the space is already defined, some oxygen 
will unavoidably remain within the space because no amount of flushing 
will serve to remove 100% of the oxygen which may be present between the 
films. The amount of residual oxygen between the two plies 20, 30 may be 
reduced, though, if the plies are assembled in anaerobic atmospheres, such 
as by assembling the two plies to form the composite film barrier 10 of 
the invention within a nitrogen atmosphere. As used herein, the phrase 
"substantially anaerobic" or "substantially oxygen-free" is intended to 
take into account these process limitations on economically removing 
oxygen from the space during commercial packaging of goods. 
The anaerobic space 40 may, in another embodiment, be provided by drawing a 
vacuum on the space 40 rather than filling that space with an anaerobic 
gas. There are a wide variety of textured polymeric films available on the 
market. If one or both of the plies 20, 30 of the invention shown in FIG. 
1 were made of such a textured film, the texturing of one or both of these 
plies would tend to define a space between the two films even after a 
vacuum is drawn on this space. If so desired, only one of the inner 
surface 22 of the inner film or inner surface 32 of the outer film may be 
provided with such a texturing. This should still provide a sufficient 
space between the films if the texturing is appropriately chosen. 
The texture which is applied to the inner faces 22, 32 of the plies may be 
chosen as desired. In most applications where a vacuum is to be drawn, 
though, it is preferred that the texturing be relatively dense rather than 
more spread out. By "dense", it is meant that the texture of the surface 
will vary relatively frequently so that there will not be large surface 
areas of the film which are smooth and untextured. By way of example, if 
the texturing of the inner surface of the ply or plies is in a generally 
grid-shaped pattern, the lines of the grid should be fairly close to one 
another so that there will not be large, smooth panes between the lines. 
This will add some structural support to the plies and ensure that the 
pressure will not collapse one ply fully against the other ply, which 
could substantially eliminate the anaerobic space 40 between the plies. 
In one particular embodiment utilizing the vacuum drawn on the space 40, 
both plies 20, 30 of the barrier 10 are provided with a series of 
elongated, substantially linear protrusions on its surface. The linear 
protrusions of the inner surface 22 of the inner ply and those on the 
inner surface 32 of the outer ply are desirably oriented generally 
perpendicularly to one another to essentially produce a grid of abutting 
protrusions between the films. This will help to maintain an anaerobic 
space between the two plies 20, 30 when a vacuum is drawn on the anaerobic 
space 40. 
The advantages of the present invention over a conventional, single-film 
package structure are shown in FIGS. 2-7. FIG. 2 is a simplified schematic 
diagram of a conventional prior art package using a polymeric film. In 
this conventional package an oxygen-consuming product 15' is contained 
within a single layer 20' of a polymeric film. FIG. 3 shows a schematic 
diagram similar to FIG. 2 illustrating a barrier 10 of the present 
invention using a packaging application. Once again, the oxygen-consuming 
product 15 is retained within the inner ply 20. In accordance with the 
present invention, though, a second outer ply 30 is disposed around the 
inner ply 20 with an anaerobic space 40 being formed between the inner and 
outer plies. 
The oxygen transmission properties of the prior art package illustrated in 
FIG. 2 are well known in the art. Many products will essentially consume 
all of the oxygen which may be transmitted through the single barrier 20'. 
Thus, over time, the differential pressure of oxygen between the external 
atmosphere (which is assumed to be air with a partial pressure of oxygen 
of about 0.20 atm) and that within the enclosure defined by the film 20' 
will remain constant. Under these conditions, one would expect the rate of 
transmission of oxygen through the film 20' to remain substantially 
constant. 
FIG. 7 depicts the total volume of oxygen which would be transmitted 
through a variety of different barriers as a function of time for a 
package having a surface area of approximately 700 cm.sup.2. Two of the 
lines in that figure are directed to prior art packages which utilize a 
single film barrier, such as that shown in FIG. 2. The line labelled as 
T=4 designates a single layer film having a transmittance (T) of about 4.0 
cm.sup.3 O.sub.2 /m.sup.2 /atm/day (designated hereinafter as T=4.0); the 
line in FIG. 7 designated T=20 depicts the total amount of oxygen one 
would expect to be transmitted through such a single layer film having a 
transmittance of about 20 cm.sup.3 of O.sub.2 /m.sup.2 /atm/day (T=20). 
These two films may be exemplified by a PVDC film of about 13 .mu.m, which 
has a transmittance of about T=4.0, and a 50 .mu.m nylon 6 film which has 
a transmittance of about T=20. As can be seen from FIG. 7, both of these 
lines are substantially linear, indicating the rate of oxygen transmission 
would be substantially constant over a period of more than 200 days. 
FIG. 4 plots the concentration of oxygen in a package such as that shown in 
FIG. 2 wherein the product 15 is replaced with dead air. Once again, the 
surface area of the package is assumed to be about 700 cm.sup.2, the 
external atmosphere is assumed to be air which has a partial pressure of 
oxygen of about 0.20 atm, and the volume within the package is assumed to 
be about 530 cm.sup.3. At ideal equilibrium, the partial pressure will be 
substantially the same within the package as it is outside the package, 
i.e., 0.20 atm (shown as the limit in FIG. 4). The rate of change of the 
partial pressure of oxygen can be expressed by the following formula 1: 
EQU (dP/dt)=rate/V 
wherein P is the partial pressure of oxygen in the container, t is time, 
and V is the volume within the container, in this case 530 cm.sup.2. The 
rate of oxygen transmission can be expressed as rate=TA(P.sub.o -P), 
wherein T is the transmittance of the barrier, A is the surface area of 
this barrier, P.sub.o is the partial pressure of oxygen outside of the 
container (assumed to be that of air at about 0.20 atm) and P is the 
partial pressure of oxygen in the container. Substituting this expression 
for the rate in formula 1 and solving the differential equation, the 
partial pressure of oxygen within the package as a function of time can be 
expressed as the following formula 2: 
EQU P(t)=P.sub.o {1-e.sup.(-TAt/V) }. 
FIG. 4 plots the partial pressure of oxygen within the prior art container 
of FIG. 2 as determined in accordance with this formula. 
A half-life (t.sub.1/2) of a container, as used herein, designates the time 
it takes for the partial pressure of oxygen to reach one-half of its 
equilibrium level. Obviously, at equilibrium t=.infin., the partial 
pressure within the container (P) will be the same as the partial pressure 
of oxygen outside of the container (P.sub.o). As explained above, in FIG. 
4 the partial pressure of oxygen outside of the container is assumed to be 
about 0.20 atm, which is average for ambient air. Accordingly, the 
half-life of the film will be the time at which the partial pressure of 
oxygen within the container reaches about 0.10 atm. As shown in FIG. 4, 
the t half-life of that film is approximately 130 days for that prior art 
package, which initially was filled with 530 cm.sup.3 of an anaerobic gas. 
As noted above, the composite film barrier 10 of the invention includes a 
substantially anaerobic space 40 between the two plies 20, 30 of the 
barrier. In a barrier of the invention, the rate of transmission of oxygen 
into the space 40 can be expressed as the rate of oxygen transmission 
through the outer ply 30 into the space 40 (rate.sub.1) minus the rate of 
oxygen transmission from the interior space 40 through the inner ply 20 
(rate.sub.2). By analogy to formula 1, formula 3 can be stated as follows: 
EQU (dP/dt)=(rate.sub.1 -rate.sub.2)/V, 
wherein V is the volume of the space 40 between the inner ply 20 and outer 
ply 30 of the barrier 10. As the rate of transmission through each of 
these barriers can be expressed as TA(P.sub.o -P), formula 3 can be 
expressed as the following formula 4: 
EQU (dP/dt)={T.sub.1 A.sub.1 (P.sub.o -P)-T.sub.2 A.sub.2 (P-P.sub.in)}/V, 
wherein P.sub.in is the partial pressure of oxygen within the container, 
i.e. on the side of the inner ply 20 farthest away from the outer ply 30. 
It is assumed that the product 15 in the package shown in FIG. 3 will 
consume any oxygen which is transmitted through the barrier into the 
interior of the package. Accordingly, P.sub.in will always be zero. 
Integration of the resulting formula yields the following formula 5: 
EQU P.sub.s =(a/b)P.sub.o {1-e.sup.(-bt) }, 
wherein a=T.sub.1 A.sub.1 /V and b=(T.sub.1 A.sub.1 +T.sub.2 A.sub.2)/V. 
When the barrier 10 is at true equilibrium conditions, i.e., when 
t=.infin., the partial pressure of oxygen within the space 40 between the 
plies can be expressed as (a/b)P.sub.o. 
The optimum balance between respective transmittances of the inner ply and 
the outer ply, i.e., T.sub.1 and T.sub.2, respectively, is important in 
maximizing the overall barrier properties of the composite film barrier 10 
of the invention. As noted above, it has been determined that the optimum 
balance is achieved when these transmittances are about equal to one 
another. In order to demonstrate this, one can determine when the 
half-life of the composite film barrier 10 is at its maximum; since this 
means that the rate of oxygen build-up in the space 40 is at its slowest, 
the barrier effect of the invention will be at its maximum. 
The half-life of the barrier 10 can be expressed as t.sub.1/2 =ln(2)/b. 
Assuming that the surface areas of the inner and outer plies (20, 30 
respectively) are approximately the same, i.e., A.sub.1 .apprxeq.A.sub.2 
and substituting for b, the half-life can be expressed as t.sub.1/2 
.apprxeq.V.multidot.ln(2)/A(T.sub.1 +T.sub.2). The resistance of an oxygen 
barrier is defined as the inverse of its transmittance, i.e., resistance 
R=1/T. If one were to set a constant total resistance for the barrier 
(i.e. R.sub.1 +R.sub.2 =R, where R is constant), one could define a 
proportional thickness parameter .alpha. such that R.sub.1 =.alpha.R and 
R.sub.2 =(1-.alpha.)R. Substituting these values in for T.sub.1 and 
T.sub.2, respectively, one obtains the following formula: 
EQU t.sub.1/2 ={VR.alpha.(1-.alpha.)ln(2)}/A. 
Setting all of the other variables as constants and varying only .alpha., 
one will obtain the generally parabolic curve shown in FIG. 6. As that 
graph shows, the maximum half-life is obtained when .alpha. is between 
about 0.4 and about 0.6, with the peak being at .alpha.=0.5. Thus, when 
the transmittance of the two plies 20,30 are approximately equal to one 
another, the half-life (and hence the barrier properties of the barrier 
10) is maximized. This may be accomplished in any desired manner, such as 
by choosing the appropriate thicknesses of two different polymeric 
materials to obtain approximately equal transmittances for the two films. 
In many instances, though, it may be easier to make both of the plies of 
the barrier from the same material at the same thickness, such as by 
forming both plies from the same stock of sheet material, as explained 
above. 
The bottom curve of FIG. 5 depicts the partial pressure of oxygen within 
the space 40 as a function of time. As explained above, when the system is 
at equilibrium, the partial pressure of oxygen within this space is 
P=(a/b)P.sub.o. When the transmissivity of both films is the same, b=2TA/V 
so formula 5 may be rewritten as the following formula 6 for the 
particularly preferred embodiment wherein the transmittance of the two 
films is substantially equal: 
EQU P.sub.s =1/2P.sub.o {1-e.sup.(-bt) }. 
From this formula it is clear that the equilibrium partial pressure P is 
1/2P.sub.o, or about 0.10 atm when the package is in air with a partial 
pressure P.sub.o of about 0.20 atm. Accordingly, the limit of the curve is 
where P=0.10 and the half-life of the barrier is determined at P=0.05, or 
half of the change before reaching equilibrium. 
FIG. 5 also illustrates another important point about the present 
invention. The top curve in that graph illustrates the partial pressure of 
oxygen in the space 40 if the space is initially filled with air rather 
than having an anaerobic gas such as is employed in a barrier 10 of the 
invention. If the structure of a barrier of the invention is virtually 
identical to that of the barrier set forth above with the exception that 
the anaerobic space 40 is replaced with ambient air, the initial partial 
pressure of oxygen in the space 40 would be about 0.20 atm but would 
approach the same equilibrium pressure of about 0.10 atm, i.e. the partial 
pressure of oxygen in the space will decrease over time rather than 
increase over time as in a barrier of the invention. 
Since the other parameters of formula 6 remain the same, the analogous 
formula for the barrier having aerobic air in the space 40 would be 
P.sub.s =1/2P.sub.o {1+e.sup.(-bt) }; the upper curve in FIG. 5 is based 
upon this formula. It should also be noted that the half-life of this 
air-filled barrier would be the same as that for a barrier of the 
invention; in the case of the model package used to generate the data of 
FIG. 5 (T.sub.1 =T.sub.2 =40, A=700 cm.sup.2 0.07 m.sup.2, and V=530 
cm.sup.3) t.sub.1/2 =65.6 days for both of these barriers. 
The formula for determining the half lives of these films may be further 
simplified. If one assumes that the spacing between the two plies 20,30 of 
the barrier 10 will remain substantially constant across the entire 
barrier, the volume V of the space 40 will be a function of the areas of 
the plies. The same conclusion is reached if, rather than assuming a 
constant distance between the plies, the distance between the plies varies 
but the average distance between the plies is known. In particular, the 
volume V equals the product of the surface area A of the plies (assumed to 
be about the same for both plies) times the distance between the plies, X. 
By substitution, the half-life of the film may be expressed as t.sub.1/2 
={Xln(2)}/2T or t.sub.1/2 =0.347X/T. (It is important that the units of 
the surface area A and the volume V be expressed in the same units. If 
they are not, such as where the area is expressed in square meters and the 
volume is expressed in cubic centimeters, appropriate corrections in units 
must be made.) Using the same model container used to generate FIG. 5, the 
half-life is once again 65.6 days and x is 0.757 cm for that particular 
design. 
In determining the total oxygen transmitted through a barrier 10 of the 
invention, the relevant rate of transmission is that of the inner ply 20. 
Although that rate is dependent upon the partial pressure of oxygen in the 
space 40 between the plies, it is clear that only oxygen which actually 
passes through the inner ply 20 will actually enter the interior of the 
package and come into contact with the product 15. Hence, the total oxygen 
transmitted through a barrier of the invention may be expressed as 
follows: 
EQU Total O.sub.2 =.intg.rate.sub.2 dt=AT.intg.P dt. 
Integrating this formula yields the following formula 7: 
EQU Total O.sub.2 (t)=1/2AT P.sub.o {t+[1-e.sup.(-bt) ]/b}. 
The curve labelled 10 in FIG. 7 shows the total oxygen transmitted through 
a barrier 10 of the invention. In generating that plot, a container having 
substantially the same parameters as those of the model container set 
forth above (i.e. T.sub.1 =T.sub.2 =40, A=700 cm.sup.2 =0.07 m.sup.2, and 
V=530 cm.sup.3) was assumed. 
As explained above, the transmittance of a polymeric film is generally 
proportional to the thickness of the film, at least for films of the same 
material. As noted above, the line in FIG. 7 labeled as T=20 roughly 
corresponds to the oxygen transmitted by a single ply film made of nylon 6 
at a thickness of approximately 50 .mu.m. By splitting the thickness of 
that film in half, such as to about 25 .mu.m of nylon 6, one would expect 
the transmittance of that single film to be about T=40. Conversely, if one 
were to place two films having a transmittance of T=40 together to 
function as a single-ply barrier, one would expect the net transmittance 
of this laminated film to be about T=20. Since the barrier of the 
invention used in generating the line 10 in FIG. 7 is composed of two 
plies each having a transmittance of T=40, one would expect this barrier 
to transmit oxygen at almost an identical rate to that of the line labeled 
T=20 in FIG. 7. 
FIG. 7 shows that the slope of the curve 10, i.e., the rate at which oxygen 
is being transmitted through the barrier 10, approaches the same slope as 
the prior art single-ply film having a T=20. However, it is clear from 
this figure that the total amount of oxygen transmitted by the barrier 10 
of the invention is substantially less than that transmitted by a 
single-ply film having the same thickness as the total thickness of the 
two plies 20, 30 of the barrier 10. In particular, the barrier 10 of the 
invention transmits oxygen at a very low rate in the beginning. As a 
matter of fact, for about 38 days, the barrier 10 utilizing two plies 
having a T=40 transmits oxygen at a lower rate than a single-ply film 
having a T=4. It is only after the partial pressure of oxygen within the 
space 40 of the barrier increases over time that the transmittance of the 
barrier will begin to approach that of a single-ply film having the same 
total thickness as the two plies of the invention. 
Food and other products which are sensitive to oxygen generally have a 
shelf life which is limited by the amount of oxygen which is permitted to 
come into contact with the food. At least for food products, the shelf 
life of the product tends to be relatively short. For instance, in the 
case of refrigerated dough products, the shelf life is limited; for meats 
and some other products the shelf life may be noticeably less. 
Accordingly, if one can substantially improve the barrier properties of 
the package for these products, one can substantially extend the 
anticipated shelf life of the product. Alternatively, one could use a more 
transmissive polymer in the plies 20, 30 of the invention than is 
necessary for a single-ply film such as is currently used, yet achieve the 
same product shelf life. Although this may not extend the shelf life of 
the product, this can significantly reduce the cost of the package itself. 
As mentioned above, Scholle's U.S. Pat. No. 4,105,818 alleges that the 
barrier properties of a film may be improved simply by splitting a single 
film into two plies with each ply having a thickness about half that of 
the original single ply. However, as explained above, this is, at best, 
only a part of the truth. If one were to simply laminate the two films on 
top of one another, such as by wrapping a package in the first ply and 
then tightly wrapping the second ply over the first ply, one would achieve 
a transmittance essentially equivalent to that of the single-ply film of 
the same thickness. Perhaps more importantly, though, Scholle does not 
even suggest that there is any importance in removing oxygen from any 
space which may be present between the plies. 
The line identified as 10" in FIG. 7 is a plot of the total oxygen 
transmittance of a barrier having a structure substantially the same as 
the barrier 10 of the invention but wherein the space 40 is initially 
filled with air rather than being a substantially anaerobic space. By 
comparison of the curve 10" and the line labeled T=20 in FIG. 7, it is 
clear that such a barrier containing air would vastly underperform even a 
single-ply film having the same total thickness. 
The reason for this can be understood a little more fully by reference to 
FIG. 5. As shown in that graph, the initial partial pressure of oxygen in 
the space 40 would be the same as that of air, i.e., about 0.20 atm. 
Hence, the barrier would initially behave as though the inner ply were in 
direct contact with the ambient environment. The initial transmission rate 
is therefore that of the inner ply alone, which in this case is about 
twice that of the single-ply film having a T=20. Over time, the partial 
pressure of oxygen within the space 40 will approach its equilibrium 
partial pressure of about 0.10 atm and the barrier will behave as a 
single-ply film having the same total thickness of the polymer. This is 
borne out in FIG. 7 by the fact that the slope of the curve 10" gradually 
approaches that of the line labeled T=20. 
Thus, Scholle's teachings are defective in at least three ways. First, 
there is no indication that there should be any space whatsoever between 
the two plies of the polymeric film. Second, even if such a teaching were 
present, there is no indication that this space should be anaerobic. 
Finally, Scholle's assertions that, at "steady state", the transmittance 
of two 0.5 mil films is less than half that of a single 1 mil ply simply 
are not true--the slope of the line 10" approaches that of the T=20 line 
toward equilibrium. FIG. 7 clearly bears out the fact that at steady state 
the rate of transmission of the barrier is dependent almost solely upon 
the total thickness of the polymeric film or films used in forming the 
barrier. 
It is interesting to note that Guilbert mentions the use of an exterior 
over-packaging to protect an edible film. (See "Technology and Application 
of Edible Protective Films" at p. 375.) However, there is no recognition 
by Guilbert that an anaerobic space must be provided between these two 
films. Accordingly, the packing technique suggested by Guilbert will 
suffer from the same defects one would observe in a package in accordance 
with Scholle's limited teachings. 
Turning once again to formula 7 and realizing the fact that the variable b 
can be rewritten in terms of the distance X between the two plies 20, 30 
as explained above, it is clear that the total oxygen transmitted by a 
barrier 10 of the invention is at least in part a function of the distance 
X. In particular, if one were to significantly increase X, one would 
expect to significantly increase the half life of the barrier and 
significantly decrease the transmittance of the barrier and the total 
oxygen transmitted through the barrier over time. It should be noted that, 
eventually, the barrier 10 will reach an equilibrium state wherein the 
total transmittance of the barrier is essentially the same as that of a 
single-ply film having the same total thickness as the two plies 20, 30, 
regardless of the distance X. For a greater distance X, though, the half 
life will be increased and the barrier properties during the early life of 
the film will be improved significantly. 
The actual distance X chosen for a package of the invention such as that 
shown in FIG. 3 can be varied as desired. In the embodiment noted above 
wherein a textured ply is used and a vacuum is drawn on the space 40, the 
distance X will obviously be relatively small. If an anaerobic gas, such 
as nitrogen, is used to fill the space 40, the distance X between the 
plies can be increased at will. However, there will be some practical 
limitations on the distance X in commercial packaging situations. 
For instance, the barrier 10 must meet certain other physical requirements 
in addition to oxygen transmittance, such as tensile and shear strengths, 
in order to be commercially useful. If one were to greatly increase X, 
this would permit one to significantly reduce the thickness of the plies 
20, 30. However, if the plies are made too thin, they will not be able to 
withstand even normal abuse during shipping and handling. The distance X 
would therefore vary dependent upon a number of different factors, 
including the product being packaged and the anticipated storage 
conditions of that package. However, as a general rule it is believed that 
the distance X should be between about 100 and about 10,000 times the 
average thickness of the two plies. In the preferred embodiment noted 
above, wherein the thickness of the plies is desirably at least about 10 
.mu.m, this will lead to a minimum distance of about 1000 .mu.m (1 mm, or 
about 4 mils). 
FIG. 8 schematically illustrates another embodiment of a barrier 10 of the 
invention. In this embodiment, the barrier includes an inner ply 20 and an 
outer ply 30, as described above in connection with FIG. 1. However, the 
embodiment shown in FIG. 8 also utilizes a third, intermediate ply 25 
disposed between the inner and outer plies. As illustrated in that 
drawing, this will divide the space 40 between the inner and outer plies 
into a first anaerobic space 42 and a second anaerobic space 44. 
In many respects, the barrier illustrated in FIG. 8 will perform much like 
the barrier shown in FIG. 1. However, by adding an additional ply to the 
barrier one can further increase the half life of the barrier 10 by 
creating two anaerobic spaces within which the partial pressure of oxygen 
must be increased. Thus, the barrier shown in FIG. 8 would outperform the 
barrier shown in FIG. 1, at least until equilibrium is reached. If the 
total thickness of the various plies used in these two different 
embodiments is the same, at equilibrium one would expect the total 
transmittance of both of these barriers to be virtually identical. The 
embodiment of FIG. 8 simply further delays the equilibrium conditions, 
thereby reducing the total oxygen transmittance of the barrier. 
FIG. 9 illustrates yet another embodiment of a multiple-ply barrier. This 
barrier utilizes an inner ply 20 and an outer ply 30 with an anaerobic 
space therebetween. However, the intermediate ply 25 shown in FIG. 8 is 
replaced with a plurality of generally tubular members 25'. These tubular 
members are formed of a polymeric film and are desirably filled with an 
anaerobic gas. In order for oxygen to be transferred through this barrier, 
it will first have to pass through the outer ply into the series of 
divided anaerobic spaces 42' adjacent the outer ply 30. The oxygen will 
then be transmitted through the polymeric film of the tubular members 25' 
into the anaerobic space 46' therein. The oxygen must then pass from the 
anaerobic space 46' into the series of anaerobic spaces 44' adjacent the 
inner ply 20. Only then will the oxygen be able to proceed into the 
interior of the package into contact with the product. 
This barrier shown in FIG. 9 therefore essentially behaves as a 4-ply 
barrier in accordance with the invention. It should be understood that any 
number of plies may be used in the barrier of the invention provided that 
there is an inner ply 20, an outer ply 30, and at least one anaerobic 
space 40 disposed between these two plies. If additional plies are used, 
the anaerobic space 40 may be subdivided into a series of smaller spaces, 
but the principle of the invention remains the same. 
FIG. 10 illustrates yet another embodiment of a composite film barrier of 
the invention. As in the previously described embodiments, the present 
embodiment includes an inner ply 20, an outer ply 30 and an intermediate 
ply 25 disposed between the inner and outer plies. However, in this 
embodiment the intermediate ply is corrugated to define a serpentine cross 
section, as shown in FIG. 10. This intermediate ply abuts the inner and 
outer plies at the apexes of its corrugations and, in a particularly 
preferred embodiment, the intermediate ply is physically attached to the 
inner and outer plies along some or all of these lines of contact. This 
attachment may be accomplished in any suitable fashion such as adhesion 
with a cementitious material of by heat sealing the plies to one another. 
The construction shown in FIG. 10 provides a three-ply barrier, i.e. a 
barrier which will function in much the same manner as that shown in FIG. 
8. In accordance with the invention, the spaces 42,44 between the plies 
may be filled with an anaerobic gas, such as nitrogen. In one preferred 
construction of this barrier, though, the anaerobic gas in the spaces 
42,44 is greater than ambient pressure. This will lend structural strength 
to the barrier, making the barrier useful in forming "semi-rigid" 
packages. Obviously, the pressure within the spaces 42,44 should not be so 
great as to cause the plies to rupture during normal shipping and 
handling. Hence, the pressure should be greater than ambient pressure but 
not too great; a pressure differential of between about 0.1 and about 0.5 
atm should work well. 
A somewhat different embodiment of the invention is shown in FIG. 11. 
Whereas all of the previous embodiments utilize a only polymeric plies to 
define anaerobic spaces, the invention depicted in FIG. 11 utilizes a 
foamed polymeric material sandwiched between two plies 20,30 to accomplish 
a similar end. In particular, the package 100 of this embodiment includes 
a polymeric material which is foamed to define a polymeric matrix 110 
having a plurality of anaerobic spaces 120 dispersed throughout this 
matrix. 
In a preferred embodiment, the foamed polymer is disposed between inner and 
outer plies (20 and 30, respectively) of a polymeric material such as that 
set forth above. The anaerobic spaces 120 may be essentially under vacuum 
or may be filled with an anaerobic gas, not unlike the anaerobic spaces 40 
in the previous embodiments. In most instances, it is anticipated that the 
spaces 120 will be filled with an anaerobic gas rather than utilizing 
vacuum in order to avoid undue structural stress on the foam structure. 
Utilizing the foam matrix 110 of this embodiment will serve to further 
reduce the oxygen transmittance of a barrier 10 of the invention because 
oxygen must migrate through the matrix 110 before entering the anaerobic 
spaces 120, further delaying the increase in oxygen concentration in the 
anaerobic space. This foam may also increase the structural properties of 
a barrier in accordance with the invention by supporting the two plies in 
a spaced-apart relationship and, perhaps, adding structural rigidity to 
the barrier. 
The foamed matrix 110 of the embodiment shown in FIG. 11 may be formed of 
any suitable material that includes substantially anaerobic spaces 120 
dispersed relatively uniformly throughout. Care should be taken, though, 
that the spaces do not become filled with air or any other aerobic gas; if 
air is present in these spaces 120, the barrier would not perform very 
well, as suggested by FIG. 7. One possible material for forming the foam 
matrix 110 is polystyrene. A sheet of foamed polystyrene may be injected 
between the plies 20,30 by extruding a bulk polystyrene material, such as 
polystyrene beads, and injecting a low-boiling-point hydrocarbon, such as 
Freon 12 or pentane, into the molten polystyrene. This type of 
manufacturing process is commonly utilized in forming foamed polystyrene 
products such as containers for eggs, hot beverage containers and the like 
by molding such a polystyrene sheet into the desired shape. If so desired, 
the matrix 110 may be formed of polystyrene or the like as a sheet 
material and this sheet material may be laminated with a ply of polymeric 
material on either side to form a barrier such as that shown in FIG. 11. 
As noted above, the present invention also encompasses a variety of package 
designs incorporating a barrier of the invention. One embodiment of such a 
package is shown in FIG. 3, wherein a product is completely enclosed in 
the inner ply 20 and the outer ply 30 fully encloses the inner ply and the 
product. Two additional embodiments of packages according to the present 
invention are illustrated in FIGS. 12 and 13. 
In the embodiment of FIG. 12, the package 200 comprises a plurality of 
sidewalls 210 defining an inner cavity 215 for receiving a product P. Most 
of the sidewalls are formed of a barrier 10 of the invention. As explained 
above, this barrier is desirably formed from a transparent, or at least 
translucent, material to permit a consumer to view the contents of the 
package. 
At least a portion of one sidewall 212 of the package, though, is desirably 
formed of another material, which may be opaque. Forming this side of an 
opaque material permits a label or the like to be imprinted directly on 
the package. The material used in this sidewall 212 should have an oxygen 
transmittance no greater than that of the barrier 10 of the invention, and 
preferably has a transmittance lower than that of the barrier 10. This 
sidewall 212 may, for instance, be formed of a laminated material that 
incorporates a metal layer, such as an aluminum film, which would make the 
sidewall 212 essentially oxygen impervious. This structure utilizes 
material quite efficiently in producing a package which permits a consumer 
to see the product at the point of purchase. 
As explained previously, it is important that the barrier 10 of the 
invention be essentially sealed from direct contact with the ambient 
environment. Accordingly, when the sidewalls 210 of the package shown in 
FIG. 12 is formed, the plies of the barrier should be sealed together to 
seal off the anaerobic space therebetween. This may be accomplished by 
sealing the plies of the barrier together while leaving excess material 
extending beyond the seal, with the other sidewall 212 being attached to 
this excess material after the product P is placed in the inner cavity 215 
of the container. Alternatively, the sealing of the plies may take place 
when the sidewall 212 is attached to the remaining sidewalls, such as 
where the sidewall 212 is attached to the rest of the package by heat 
sealing or the like. 
Another embodiment of a package 200' according to the invention is shown in 
FIG. 13. In this embodiment, a barrier 10 of the invention comprises only 
a single sidewall 212' of the package while the remaining sidewalls 210' 
are formed of another material. Once again, it is preferred that the 
material comprising the other sidewalls 210' have an oxygen transmittance 
no less than that of the barrier. This embodiment has particular utility 
providing a window or a visible side in a package which is otherwise 
opaque or translucent. 
For instance, many food products, such as frozen foods, are sold in trays 
which are sized to hold a single serving of a product. The trays must 
generally be formed of an opaque material that has sufficient structural 
strength to contain the product and function as a plate or bowl for the 
consumer. In accordance with the invention, a tray such as that commonly 
used in packaging frozen foods may be provided with a substantially 
transparent composite film barrier of the invention as a cover on the 
tray. The barrier 10 should be sealingly attached to the tray about the 
periphery of the upstanding sidewalls 210' in such a manner as to 
effectively seal the plies of the barrier to one another and seal the 
barrier to the sidewalls; heat sealing should work well for many polymeric 
materials. 
Alternatively, it may be desirable to provide a package that has a very low 
oxygen transmittance and utilize a barrier of the invention as a 
relatively small window in only a portion of a single sidewall. Although 
this embodiment is not specifically illustrated in the drawings, it is 
very similar to the embodiment of FIG. 13 except that only a portion of 
the sidewall 212' would be formed of a barrier 10, with the balance of 
that sidewall being formed of a material having a transmittance of no less 
than that of the barrier. Such a package may be used, for instance, when 
there is a need to protect the product from the effects of excessive 
ultraviolet light, yet it is desirable to allow consumers to see the 
product at the point of sale. 
While a preferred embodiment of the present invention has been described, 
it should be understood that various changes, adaptations and 
modifications may be made therein without departing from the spirit of the 
invention and the scope of the appended claims.