Products sensitive to oxygen, particularly foods, beverages and medicines, deteriorate or spoil in the presence of oxygen. One approach to creating an oxygen free environment is to package such products in a container comprised of at least one layer of a so-called “passive” gas barrier film that is a physical barrier and reduces or eliminates the transmission of oxygen through the container wall but does not react with oxygen. For instance, layers of thermoplastic polyester (PET) are often supplemented with additional layers of packaging material to prevent oxygen permeation.
Adding a barrier layer of gas barrier film increases the overall passive-barrier of the package to other gases as well. Ethylvinyl alcohol (EVOH), polyvinylidene dichloride (PVDC), and polyamides like poly(m-xylylene adipamide) (MXD6), are examples of films commonly used for this purpose due to their excellent gas barrier properties. These materials are often used in beer packaging because the low permeation rate (high passive barrier) keeps the oxygen away from the product as well as keeping the carbon dioxide in the beverage. Distinct layers of differing materials are not preferred because the multilayer construction adds cost.
It is therefore preferable to blend all the components into a single layer, called a monolayer construction. The monolayer construction does not work for most compositions due to poor aesthetics. For example, a monolayer of poly(m-xylylene adipamide) (MXD6) blended into PET (polyethylene terephthalate or its crystallizable copolymers) has unacceptable haze at desirable barrier levels of the MXD6. This is particularly the case for the walls of stretch blow molded and reheat stretch blown containers or stretched film wraps. In the reheat stretch blown container, for example, the MXD6 is dispersed into the PET and the composition subsequently injection molded into an amorphous article called a preform. The preform is a container itself and is generally tube shaped with one closed end and optionally threads to receive the screw cap around the opening of the other end. In the case of the preform for a jar, the length of the tube is so small that the preform resembles a diaphragm or disk.
The preform can then be stretched, also known as orienting the material, into the shape of the container. Depending upon the process, the preform can be stretched after injection or extrusion molding but before it cools below the glass transition temperature or the major component (usually the polyester). Alternatively, the cold molded preform can be stretched after being reheated to above the glass transition or softening temperature of the composition making up the preform. The unstretched preform of MXD6 dispersed into PET is typically transparent with very little haze. However, upon stretching, or being pulled, pushed out or even squished, the subsequent thinned wall has a tremendous amount of haze. This haze is such that it renders an uncoloured monolayer bottle with stretched walls made from polyester-polyamide blends unmarketable.
According to U.S. Pat. No. 6,288,161, haze and colour are caused by a change in the refractive index of the materials when the polymers are oriented. Orientation enlarges the size of MXD6 domains so that the size of enough domains are greater than the minimum wavelength of visible light (approximately 400 nm) resulting in increased light scattering.
Table 1 of U.S. Pat. No. 6,288,161 shows a fourfold increase in haze when the article is oriented (stretched) to a 9 drawdown ratio. Tables 2 and 3 of U.S. Pat. No. 6,288,161 show even larger haze differences caused by the reheat stretch blow molding of the container wall.
U.S. Pat. No. 6,288,161 teaches overcoming the haze by using a low stretch process to limit the degree of orientation so that the MXD6 domains are less than the minimum wavelength of visible light. Low stretch is achieved through a process called extrusion blow molding. Low stretch extrusion blow molding is inferior to high stretch processes such as reheat blow and injection stretch blow mold because of the economics and the significantly greater barrier properties associated with the higher stretch ratios (drawdowns). There exists therefore a need for a low haze mono-layer high stretched wall comprised of a polyester and a polyamide, preferably MXD6.
U.S. Pat. No. 6,444,283 discloses that the haze of a film of polyester blended with a polyamide increases with increasing amounts of polyamide. U.S. Pat. No. 6,444,283 teaches that the reduction of the haze of a film comprising polyester blended with a polyamide can be reduced by using a polyamide with a number average molecular weight less than 15000 and an amino to carboxyl end group ratio greater than or equal to 1.0. At present, there is no commercial grade of MXD6, the preferred polyamide, available at the required molecular weight taught in U.S. Pat. No. 6,444,283.
Japanese Patent Application H10-7893 to Harada et al, teaches that good transparency of a polyester/mx polyamide blend can be obtained without a compatibilizer when the end groups satisfy the equations of50<a−b<300 and   1)a+b<300,   2)where a is the number of amino end groups in micro equivalents per gram and b is the number of carboxyl end groups in micro equivalents per gram. Harada et al teaches that if in formula 1) a−b is less than 50 a resin with a clearly improved transparency of the blended resin composition can not be obtained. For a−b to be greater than 50, the number of amino end groups must therefore exceed the number of carboxyl end groups. When the number of amino groups exceeds the number of carboxyl groups, the amino to carboxyl end group ratio is by definition greater than 1.0.
WO 2004/069909 teaches that the haze can be visually masked by adding a colorant to absorb the light at the wavelengths corresponding to the size of incompatible domains, such as polyamide, dispersed in the polyester. Because this technique requires a colorant its use is limited to coloured bottles.
There exists, therefore, a need for an uncoloured polyester-polyamide composition which does not produce an unacceptable visual haze when stretched.
U.S. Patent Application 2004/0013833 A1 discloses a compatibilized polymer blend comprising polyamide, PET or PET containing copolymer, and at least one compatibilizer. Preferred compatibilizers of U.S. Patent Application 2004/0013833 A1 include, but are not limited to, polyester ionomers, are the polycondensation reaction products of an aromatic dicarboxylic acid or its ester-forming derivative, a diol or its ester forming derivative, and an ester forming compound comprising an ionic sulfonate group disclosed in U.S. Pat. No. 6,500,895 B1.
Japanese Patent Application 0 3181246 A, is aimed at improving the impact resistance of multi-layer structures. The application discloses a composition of the sulfonated polyester and MXD6 and claims:
1) a multilayer structure consisting of at least two layers of a m-xylilenediamine group containing polyamide (A) layer and a thermoplastic polyester (B1) or polycarbonate (B2) layer, which contains, with respect to the total diol and/or the total dicarboxylic acid, 0.1-20 mol. % of a diol and/or dicarboxylic acid represented by the formula X—R.
Where X is a diol or dicarboxylic acid, and R is —SO3Y, —COOY, —OY, —PO(OY)2, —PO3Z,
Where Y is a monovalent metal and Z is a bivalent metal.
The authors of Japanese Patent Application 0 3181246 state that the composition relates to multilayer structures, especially to co-injection stretched multilayer structures and compositions, with considerably improved impact-resistant delamination properties and that the divalent metals perform better than the monovalent metals.
U.S. Pat. No. 5,300,572 discloses a moldable polyester resin composition and molded articles there from include, based on the total weight of all resin compositions: A) between 2 to 98% by weight of a compatabilizing metal sulfonate group-containing aromatic polyester copolymer which is the polycondensation reaction product of (a) an aromatic dicarboxylic acid or its ester-forming derivative, (b) a diol compound or its ester-forming derivative, and (c) an ester-forming compound containing a metal sulfonate group; B) between 2 to 98% by weight of an additive resin which is one of (B-I) an olefin copolymer which is the copolymerization reaction product between an olefin with a least one of an a,b-unsaturated carboxylic acid or its derivative and a vinyl alcohol or its ester, (B-II) a polyamide resin; and optionally (C) between 0 to 96% by weight of a non-compatabilizing aromatic polyester resin, and again the bivalent metal is the preferred compatibilizer.
U.S. Pat. No. 5,300,572 discloses neither the role of the end groups nor the use of the modified polyester and polyamide composition in a stretched article such as the wall of a reheat stretch or even extrusion blow container.
There exists therefore a need for a polyester/polyamide composition that can create a low haze stretched wall monolayer container.
Japanese Patent Application JP 2663578-B2 to Kamatsu advocates the use of sodium isophthalate sulfonate copolymerized into the polyester to compatibilize polyester and polyamide blends. The examples provided in Japanese Patent Application JP 2663578-B2 are all based on sodium sulfoisophthalate and show only a reduction in haze and increased haze with increased amounts of MXD6.
PCT Application WO 2005/023530 claims the use of cobalt and zinc to reduce the increased color from blending a polyester, an ionic compatibilizer and partially aromatic polyamide. The application teaches that the divalent metal is more effective than monovalent metals and has examples showing that the domains of MXD6 dispersed into a copolymer with sodium isophthalic acid are approximately 200 nm.
The cobalt compounds useful in WO 2005/023530 include cobalt acetate, cobalt carbonate, cobalt chloride, cobalt hydroxide, cobalt naphthenate, cobalt oleate, cobalt linoleate, cobalt octoate, cobalt stearate, cobalt nitrate, cobalt phosphate, cobalt sulphate, cobalt (ethylene glycolate), and mixtures of two or more these, among others. As a transition metal catalyst for active oxygen scavenging, a salt of a long chain fatty acid is preferred, cobalt octoate or stearate being the most preferred. For colour control WO 2005/023539 prefers cobalt acetate. However, information in the market is that beer companies for example, do not desire cobalt in their packages for perceived marketing concerns.
None of the above mentioned prior art discloses a technique to achieve dispersed particles with average particle size less than 200 nm when stretched, a good colour composition which will not experience substantially increased haze with increasing amounts of dispersed material, or has an acceptable haze upon manufacturing and has good colour, particularly in the absence of cobalt.