Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2002-02-14
2004-01-06
Johnstone, Adrienne C. (Department: 1733)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C156S133000, C426S127000
Reexamination Certificate
active
06673865
ABSTRACT:
The present invention is directed to mixtures of adhesive components, in particular mixtures of acrylic or acrylic/vinyl latex adhesive components, one of which latex adhesive components is formed from a substantial portion of chlorinated vinyl monomers. The mixture of adhesive components provides packaging material with controlled gas barrier properties.
BACKGROUND OF THE INVENTION
For many types of food packaging applications, such as packaging meats and cheeses, it is desirable that the packaging material transmit as little gas as possible. For such purposes, high gas barrier materials, such as nylon, ethylene/vinyl alcohol, or polyvinylidene chloride film or coatings may be employed.
On the other hand, certain foods, particularly fruits and vegetables, may maintain their freshness longest if the packaging material provides for some gas transmittance. Fruits and vegetables, though harvested, continue respiratory functions for an extended period of time, and packaging is preferably provided which allows the produce to breathe. Everyone is well acquainted with the nylon mesh bags in which citrus fruits are commonly sold, the porous bags allowing for complete exposure of the fruit to the atmosphere. Were citrus fruits to be packaged in air-tight bags, rapid rotting would occur.
Harvested fruits emit gases, particularly ethylene oxide, which hasten the fruits rotting. Air-tight packaging of fruits would, of course, result in build-up of concentrations of such gases. In fact, it is known that fruits are genetically programmed to produce such gases. A fruit is, after all, a vehicle for the species-regenerating seed, and if the fruit is not eaten and the seed thereby transported by an animal, rotting of the fruit is necessary for the seed to develop into a plant. Likewise vegetables continue some respiratory function after harvesting, though generally less than fruit.
Research has shown that different fruits and vegetables have different “breathing” requirements, and each fruit or vegetable may have its longest shelf life in packaging of specific gas barrier properties. There is a need therefore, for producers of packaging material to be able to adjust the gas barrier property of packaging material according to the specific packaging applications.
One proposed approach for adjusting gas barrier properties is to vary the thickness or gauge of the film or of specific layers of a film laminate. Manufactures of packaging materials, particularly of multilayer film laminates, have difficulty with this approach as it is difficult from standpoints both of material handling and apparatus adjustment considerations to change film layer thicknesses from one packaging application to another. The present invention is directed to mixtures of adhesive components, the proportions of which can be easily adjusted, to provide for controlled gas barrier properties.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a mixture of a first, higher-barrier latex adhesive component comprising a polymer formed of monomers comprising acrylic monomers and chlorinated vinyl monomers and optionally, other vinyl monomers, and a second, lower-barrier latex adhesive component comprising a polymer formed of acrylic monomers and optionally, vinyl monomers, including chlorinated vinyl monomers. The gas barrier property of a film formed entirely from the higher-barrier latex adhesive component has an oxygen transmission (OTR) rate at least 50 cc O
2
/100 in.
2
/day (775 cc O
2
/(m
2
/day), preferably at least 100 cc O
2
/100 in.
2
/day (1550 cc O
2
/(m
2
/day) and most preferably at least 200 cc O
2
/100 in.
2
/day (3100 cc O
2
/m
2
/day) lower than does a film formed entirely from the lower-barrier adhesive component. Herein, the OTRs are based upon a coating weight of 3.5 lbs/ream (3.5 lbs/3000 ft
2
) (5.71 kg/1000 in
2
), this being a common coating weight. The polymer of the first latex adhesive component comprises between about 5 and about 95 wt % of the combined weight of the polymers of the first and second latex adhesives, and the polymer of the second latex adhesive component comprises between about 95 and about 5 wt % of the combined weight of the polymers of the first and second latex adhesives, whereby the gas barrier properties of a film formed from the latex adhesive mixture is intermediate of a film formed from either the first or the second latex adhesive component alone.
Another aspect of the invention is a method of producing a multi-layer laminate with an adhesive layer having controlled gas barrier properties by adhering two or more film layers having low gas barrier properties with a latex adhesive having greater gas barrier properties. The composition of the latex adhesive is prepared by using an appropriate amount of a first latex adhesive component having higher-barrier properties and an appropriate amount of a second latex adhesive component having lower-barrier properties so as to provide an adhesive layer with an OTR ranging anywhere from that of the higher-barrier latex adhesive component to that of the lower-barrier latex adhesive component.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
Herein the gas barrier properties of an adhesive layer are described in terms of its oxygen transmission rate (OTR) based on a coating weight of 3.5 lbs/3000 ft
2
. Although other gases, such as CO
2
, ethylene, and ethylene oxide, are also factors in food product freshness, the transmission rate of these gases will vary generally in the direction that the oxygen transmission rate varies. The gas barrier property of a multi-layer (2 or more layers) packaging laminate depends, of course, not only on the gas barrier property of the adhesive layer or layers, but on the gas barrier properties of the film layers as well as the adhesive. However, the present invention is primarily applicable to laminates in which all of the film layers have poor gas barrier properties relative that which is to be provided by to the adhesive layer. Most particularly, the invention is directed to multi-layer laminates formed of sheets of polyolefins, such as polyethylene and polypropylene. Polyolefins have poor gas barrier properties, typically having OTRs in the range of 50-500 cc O
2
/100 in.
2
/day/mil (775-7750 cc O
2
/m
2
/day/mil 0.025 mm). Such sheets may be of the same or different polymers. Herein, therefore, gas barrier property is discussed in relation only to that of the adhesive layer provided by a latex adhesive applied at a coating weight of 3.5 lbs/3000 ft
2
. The producer of the packaging material will also have to factor in the gas barrier properties of the film layers in adapting the packaging to specific applications.
The terms “higher-barrier” and “lower-barrier” as used herein to describe the latex adhesive components, are relative terms, one adhesive component having significantly greater gas-barrier properties than the other.
The optimal OTR for most fruits and vegetables ranges from about 50 to about 300, particularly from about 70 to about 225 cc/100 in
2
/day (775-4650, preferably 1085-3488 cc O
2
/m
2
/day), and the invention is primarily directed to providing OTRs in these ranges, although the OTR may be varied over any range to which latex adhesive components of the type described herein are capable of providing.
Suitable acrylic monomers for forming both the higher-barrier and lower-barrier adhesive components include, but are not limited to acrylic acid, methacrylic acid, and C
1
-C
12
-alkyl esters of acrylic and methacrylic acid. The higher-barrier latex adhesive component also is formed in part from chlorinated vinyl monomers, such as vinyl chloride and, preferably, vinylidene chloride. It is believed that the large chlorine molecules contribute to dense polymer packing and therefore enhanced gas barrier properties. Vinylidene chloride is preferred to vinyl chloride for forming the higher-barrier latex adhesive component because it contains two chlorine molecules and is a liquid, as opposed to gaseous vinyl chloride, at ambient temperatures.
Lamber Robert M.
Sherman H. Dale
Falk Stephen T.
Johnstone Adrienne C.
Rohm and Haas Company
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