The present invention relates to plastic containers adapted to contain food and beverages whose taste and/or odor may be affected by the materials of which the containers are made as well as by contaminants generated in the course of their manufacture and use of the containers. In particular, the invention relates to containers produced from polymers subject to degradation in the course of their conversion into containers, with the products of such degradation remaining entrapped within their walls and capable of diffusing into the contents, thereby affecting the taste and odor thereof.
Similarly, the polymers may contain oligomers i.e., molecular fractions that have insufficiently polymerized, as well as unreacted monomers, both of which may diffuse into the contents of the containers, with the same affect, as above. These conditions are encountered particularly in polymers obtained by condensation reactions, such as polycarbonate, nylon, and also polyethylene terephthalate (PET), the polymer predominantly used for beverage bottles and food jars.
To be suitable for use in food and beverage containers, PET must provide protection of the contents against deterioration such as, in the case of soft drinks, loss of carbonation or chemical reactions due to exposure to ambient conditions. Further, depending on their intended use, containers may be subject to considerable internal pressure and forces due to handling and storage. Therefore, the grades of PET to be used must be able to impart physical properties for withstanding these forces.
As is known, PET is a polymer that may be obtained in stepwise condensation of terephthalic acid and ethylene glycol. Polymerization is more complete the longer it is carried out at the appropriate temperature, and it may be repeatedly interrupted. Up to a certain stage of condensation, the reaction is carried out in the "molten" phase i.e., low-viscosity state, and is therefore termed a melt phase reaction, with the product designated as melt-phase PET. As this phase of the reaction progresses, the viscosity of the heated reacting mass increases to a degree, beyond which continuing manufacture is impractical. The polymer obtained at that stage, and even before, has many uses, notably for the production of textile fibers and film. However, melt-phase PET has insufficient physical properties and permeation resistance for use in large beverage bottles and certain other important packaging containers. Even more important, melt-phase PET entrains acetaldehyde (AA), a noxious product of thermal degradation, and also some oligomers, down to as yet unreacted monomer constituents, notably ethylene glycol. AA, which abbreviation stands for acetaldehyde, CH.sub.3 CHO, is a liquid of pungent-fruity odor that desorbs readily out of the walls of a container that is made from PET, in which the AA is entrained, into the contents to spoil the odor and taste thereof, even in very small concentrations. Thus, typical specifications for soft-drink bottles call for an AA limit of 1 ppm, and even less for drinking water. Likewise, unreacted monomers and oligomers may enter the content of the container and may, apart from interference with taste, constitute a health hazard as determined by prevailing laws regulating the same.
Accordingly, since AA is present in melt phase PET, the use of melt phase PET is not accepted for many forms of packaging, including some of the most important ones, i.e. beverage bottles. For the same reason melt-phase PET cannot be used even when its physical properties suffice, as in the case of small bottles that are subject to lesser stresses than large ones, since these stresses rise in proportion with size.
In order to obtain PET without the drawbacks associated with melt-phase PET, polymerization/condensation has to be continued. Since it is impractical to continue in the molten state, additional, expensive processes are added, wherein the melt-phase product is cooled, comminuted to a particle size suitable to be suspended and heated in and by a stream of hot air. The product is heated in a solid-state post-condensation reaction, preceded by crystallizing the melt-phase PET. Naturally, the cost of the finished PET increases substantially by these steps.
The cost of polymerizing the feedstock into the bottle-grade PET nearly doubles, compared with limiting the process to melt-phase polymerization. Particularly when the physical properties of containers made of melt-phase PET would otherwise suffice, as for small bottles, the increase in cost is accepted solely to eliminate excessive amounts of AA and other contaminants. In view of the fact that some 40% or more of the total manufacturing cost of a PET bottle is represented by the cost of the PET, it is readily seen that the use of melt-phase resin would represent great economic benefits.
There exists, therefore, a need for a container and process for manufacturing the container wherein melt-phase or scrap PET can be used in the container having unrestricted application to food and beverage by not contaminating the same.