JPS6134392B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to structures consisting of multiple layers of polymers. Multilayer polymer structures are suitable for flexible and rigid food packaging. Flexible multilayer film
Used for pouches, bags and wraps. The thick multilayer structure can be formed into containers of shapes and dimensions as are common for metal cans. be sufficiently oxygen-impermeable for the expected storage time of the packaged article;
is necessary. Many of the polymers suitable for economical practical use in packaging have relatively poor oxygen barrier properties. Therefore, it is desirable to use an inner layer that is impermeable to oxygen. Among the preferred candidate polymers for oxygen barrier layers are ethylene vinyl alcohol (EVOH) copolymers and polyvinyl alcohol (PVOH). Although both are excellent oxygen barriers, both exhibit significant sensitivity to moisture. Above relatively low water contents, both begin to rapidly lose their oxygen barrier properties. There are several opportunities for moisture ingress into the barrier layer, including from the packaged food itself, from storage in humid air, and from water vapor present in commercial sterilization processes. Vinyl alcohol polymers and copolymers are excellent oxygen barrier materials. Substantially fully hydrolyzed ethylene vinyl alcohol copolymers (EVOH) have rheological properties that allow them to be coextruded with common packaging polymers, thus
EVOH is a polymeric material suitable for use as an oxygen barrier layer in rigid and flexible multilayer polymeric structures for packaging. EVOH is relatively expensive, as much as eight times more expensive than other polymers commonly used for packaging. The high cost of such EVOH necessarily makes the use of thin layers such an economic imperative. Even layers lower than one thousandth of an inch (1 mil = 0.025 mm) of dry EVOH can provide protection from oxygen that allows some foods to be stored for more than a year. be. EVOH and PVOH are moisture sensitive polymers. The oxygen barrier performance of an EVOH or PVOH layer is degraded by the presence of moisture in the layer. 1st
The figure is a graph plotting the oxygen permeability of EVOH as a function of water content. In this figure, oxygen permeability (vertical axis) is expressed in cubic centimeters of O ₂ per day over a 100 square inch film area at an O ₂ pressure gradient of 1 atmosphere per mil of film thickness. cc・mil/100 (inch) ²・day・a
tm Moisture content is expressed as weight percent water in EVOH (lower horizontal axis of Figure 1) and also as "water activity" (upper horizontal axis of Figure 1). Water activity can be defined as the ratio of the water vapor pressure produced by a substance divided by the vapor pressure produced by pure water at the same temperature and pressure.
When expressed as a percentage, this value is often referred to as "relative humidity." The precise thermodynamic definition of "water activity" is discussed in standard texts such as Lewis and Randall's "Thermodynamics" (1923). It should be noted that in FIG. 1, oxygen permeability is plotted on a logarithmic scale (vertical axis). Approximately 3% of water content and 7
%, the permeability increases by a factor of 10, and about 3%.
Between 12% and 12%, there is an increase of over 100 times. Therefore, when the moisture content exceeds about 7%, a sharp drop in oxygen barrier performance occurs. From the viewpoint of food packaging economics, an oxygen permeability of 0.1 or less in the above unit (the unit of the vertical axis in FIG. 1) is preferable. The price of EVOH typically forces the use of thin layers such that very small amounts of water cause a significant increase in water activity. This may therefore cause the oxygen permeability to exceed the desired limit. EVOH is hydrophilic and draws moisture from its surroundings. To protect the EVOH layer from moisture, the EVOH layer is embedded between inner and outer layers of a moisture barrier polymer such as polyethylene, polypropylene, or a blend of both. FIG. 2 is an enlarged cross-sectional view of a typical multilayer container wall. Structural layers 1 and 2 of polyolefin are bonded by adhesives 4 and 5.
It is adhered to the EVOH oxygen barrier layer 3. The adhesive layers 4 and 5 are manufactured by Rolling, Illinois, USA, for example.
It may be a modified (or modified) polyolefin such as that sold under the trademark "Plexay" by Chemplex of Meadows. Suitable EVOH polymeric materials with a vinyl alcohol content of about 70 mol% are sold by Kuraray Co., Ltd. under the trademark "EVAL" and by Nippon Gosei Co., Ltd. as "GLD" and "GLE".
Sold under the trademark . Polyolefins are good moisture barriers at normal predictable outside temperatures and/or below, and therefore
It acts to isolate the EVOH layer from moisture. However, at high temperatures, polyolefins become increasingly permeable to moisture. Many food products are retorted in packaging or wrapping materials or packaging containers for commercial sterilization.
The ability to subject filled food packages to retort processing conditions of 250 °C (approximately 121 °C) or higher in the presence of steam or water (under pressure) for half an hour or more is not a practical requirement. Ru. Under retort processing conditions, the moisture permeability of the polyolefin increases significantly, and a situation may occur where moisture penetrates into the EVOH layer. When the package is brought to room temperature, the moisture impermeability of the polyolefin is restored, allowing moisture to escape.
Becomes restrained in the EVOH layer. The amount of water bound in this way can easily exceed a water content of about 7% (water activity of about 0.9 at room temperature), thus compromising the oxygen barrier performance of the EVOH layer for many purposes. It will make it appropriate. FIG. 3 is a plot of the water absorbed by the EVOH layer embedded between the inner and outer layers of polyethylene in a can-shaped multilayer rigid container. The moisture content is plotted as a function of time in the retort at 250°C. 0.1 after less than half an hour has passed.
(Table 1) It is observed that the excess water content gives greater oxygen permeability. A desiccant extracts moisture from its surroundings. The incorporation of desiccant agents (including agents not conventionally recognized as desiccant agents) into the multilayer film structure ensures that a reasonably low moisture content is maintained in the moisture-sensitive layer. ,discovered. In particular, we believe that by incorporating a desiccant in the contact agent layers 4 and 5 adjacent to the EVOH barrier layer 3, the water activity of EVOH can be reduced to 0.8 or 0.9 or less (this corresponds to a water content of 6 to 7% or less). It was determined that the oxygen barrier properties of EVOH could be maintained by maintaining the oxygen permeability at about 0.1 or less. FIG. 4 is a generalized isothermal hygroscopic curve for hydrate-forming desiccants such as dibasic sodium phosphate and calcium chloride. The dry capacity of the desiccant (C=weight of water/weight of desiccant) is plotted against the water activity (a) of the surrounding material. When the surrounding material is air, water activity is usually determined by relative humidity (R.
H.). No water is absorbed until the water activity humidity reaches the value (a ₁ ) at which the first hydrate is formed. In the case of calcium chloride, the first hydrate occurs at relative humidity below 2% (a<0.02).
Water is absorbed up to point C ₁ which represents the completion of the first hydrate. Beyond that, no further water is absorbed until the water activity humidity reaches the value A ₂ (where a second hydrate forms and absorbs water up to point C ₂ ). Such a process begins at a point (a) where the substance begins to dissolve and a saturated solution is formed.
_s ) for the number of types of hydrates that the substance forms. Saturated solutions continue to absorb water at and beyond the saturation limit ( _Cs ). FIG. 5 is an isothermal hygroscopic curve of a desiccant that does not form hydrates. Common salt is a typical desiccant of this type. Salt does not absorb water at humidity levels below about 75%. When the relative humidity reaches 75%, a saturated solution is formed, which continues to absorb moisture. FIG. 6 is an isothermal hygroscopic curve for a water-soluble desiccant such as gelatin that absorbs water at all water activities. At high water activity, it dissolves and the solution continues to absorb water. FIG. 7 is an isothermal hygroscopic curve for bentonite viscosities that absorb water at all water activities and water-insoluble desiccants such as silica gel. Since no solution is formed, moisture absorption does not proceed further once the voids are filled. From the description of the various desiccants with reference to FIGS. 4-7, it will become clear that different types of materials have different water absorption effects from their surroundings. Furthermore,
The special temperature hygroscopicity of a particular substance depends on temperature. The most available data are reported for room temperature. Individual substances can behave quite differently at high temperatures, such as those found during food retorting. For example, dibasic sodium phosphate forms three hydrates at room temperature, but at 250°C (121°C) it does not form any hydrates. Table 1 shows that 250〓(121
Several desiccant agents are exemplified that provide favorable oxygen permeability after 2 hours of retorting at temperatures (°C). Desiccant was incorporated into layers 4 and 5 of the multilayer structure. Before use, the desiccants were ground in a warming blender and sieved. The desiccant was blended with the adhesive polymeric material by melt blending. The oxygen permeability data for each desiccant is the average of several measurements. The EVOH used was GLD. The adhesive material is
It was Plexar. The olefin was high density polyethylene.

【table】

Table 2 shows some other desiccants that are not as effective as the desiccants in Table 1. The multilayer structure was similar to that in Table 1 except for the desiccant.

[Table] Sheave
(Linde 4A)
Bentonite viscosity 20 1.7 2.8 14.8 0.51
soil
Na ₂ SO ₄ 20.6 1.6 2.5 14.6 0.99

Claims (1)

[Scope of Claims] 1. A multilayer polymer structure having a layer of a moisture sensitive polymer, characterized in that the moisture sensitive layer is protected by incorporating a desiccant. 2. A multilayer polymer structure having an inner layer of a moisture sensitive polymer, characterized in that a desiccant is blended in the layer closest to the moisture sensitive layer. 3. The multilayer structure according to claim 2, wherein the moisture-sensitive polymer is an ethylene-vinyl alcohol copolymer or polyvinyl alcohol. 4. The multilayer structure according to claim 1, wherein the desiccant is calcium chloride. 5. The multilayer structure according to claim 1, wherein the desiccant is sodium chloride. 6. The multilayer structure according to claim 1, wherein the desiccant is dibasic sodium phosphate. 7. The multilayer structure according to claim 1, wherein the desiccant is ammonium chloride. 8. The multilayer structure according to claim 1, wherein the desiccant is sucrose. 9. The multilayer structure according to claim 1, wherein the desiccant is potassium carbonate. 10. The multilayer structure according to claim 1, wherein the desiccant is potassium alum. 11. The multilayer structure according to claim 1, wherein the desiccant is magnesium sulfate. 12. The multilayer structure according to claim 1, wherein the desiccant is magnesium chloride. 13. The multilayer structure according to claim 1, wherein the desiccant is sodium nitrate. 14. The multilayer structure according to claim 1, wherein the desiccant is ammonium nitrate. 15. The multilayer structure according to claim 1, wherein the desiccant is monoammonium phosphate. 16. The multilayer structure according to claim 1, wherein the desiccant is potassium bromide. 17 consisting of an inner layer of a moisture-sensitive oxygen barrier polymer, outer layers of structural polymers, and intermediate layers of adhesive polymers located between the inner and outer layers; A multilayer polymer structure, characterized in that the structure contains a desiccant. 18 consisting of an inner layer of moisture-sensitive oxygen barrier polymer, outer layers of polyolefin, and intermediate layers of adhesively modified polyolefin located between the inner and outer layers, the modified polyolefin containing a desiccant. A multilayer polymer structure characterized by: 19 Multilayer structure consisting of an inner oxygen barrier layer of ethylene-vinyl alcohol copolymer, an outer layer of polyolefin, and an intermediate layer of adhesively modified polyolefin containing a desiccant. 20. The multilayer structure according to claim 19, wherein the desiccant is calcium chloride. 21. The multilayer structure according to claim 19, wherein the desiccant is sodium chloride. 22. The multilayer structure according to claim 19, wherein the desiccant is ammonium chloride. 23. The multilayer structure according to claim 19, wherein the desiccant is dibasic sodium phosphate. 24. The multilayer structure according to claim 19, wherein the desiccant is sucrose. 25. The multilayer structure according to claim 19, wherein the desiccant is potassium carbonate. 26. The multilayer structure according to claim 19, wherein the desiccant is potassium alum. 27. The multilayer structure according to claim 19, wherein the desiccant is magnesium sulfate. 28. The multilayer structure according to claim 19, wherein the desiccant is magnesium chloride. 29. The multilayer structure according to claim 19, wherein the desiccant is sodium nitrate. 30. The multilayer structure according to claim 19, wherein the desiccant is monoammonium phosphate. 31. The multilayer structure according to claim 19, wherein the desiccant is potassium bromide. 32. The multilayer structure according to claim 19, wherein the desiccant is ammonium sulfate. 33. A multilayer polymer structure having a layer of a moisture-sensitive oxygen barrier polymer, characterized in that a desiccant is blended to protect the oxygen barrier properties of the structure.