Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Patent
1997-08-13
2000-03-07
Kuhns, Allan R.
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
264 459, 264 54, 264321, B29C 4420, B29D 2200
Patent
active
060337486
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to plastic articles and to a method of manufacturing them. More particularly, it relates to a process for producing extruded poly (ethylene terephthalate) sheet having a foamed structure suitable for subsequent thermoforming into "lightweight" packaging and food containers, especially dualovenable food trays used for reheating food in microwave or conventional ovens.
It is well established that crystalline poly(ethylene terephthalate), CPET, is very suitable for the manufacture of trays and other similar articles which are used in applications such as food containers where it is required to reheat the contents to high temperatures, typically 175.degree. C. to 200.degree. C., and many millions of these are already in use. However, the density of `solid` CPET trays is high and it is desirable that the density of trays, and hence the weight of material used for their manufacture, is reduced. It is also well-known that by inducing a foaming action into an extrusion process, the density of sheet extruded therefrom is lower than the density of solid (i.e., non-foamed) material. As a consequence of foaming, the amount of polymer resin needed to produce an article having a particular thickness is reduced accordingly. Unfortunately, it is recognised that the foaming action also reduces the melt viscosity of extrudate and this, particularly in the case of PET, causes difficulties in controlling the extrusion process and sheet thickness. The resultant extruded sheet suffers from a lack of consistency and the impact strength of trays and other articles produced from it is usually too low.
EP-A-0372846 claims to overcome these aforementioned difficulties by the incorporation, into the molten polymer resin composition of certain carboxylic acid anhydride branching agents, eg pyromellitic dianhydride. Unfortunately, articles obtained from such compositions tend to have relatively high levels of unreacted branching agents which is undesirable when such articles are intended as food trays. WO 94/17131 discloses the use of a premix concentrate containing polyester and anhydride branching agents in order to overcome this problem of retained unreacted anhydride in the final articles. Although the use of this concentrate does bring about a reduction in the final level of unreacted branching agents, some still remains in the final articles.
An object of the present invention is to eliminate the reliance on acid anhydride branching agents to improve the properties of foamed CPET, particularly the melt viscosity in an extrudate, the consistency and foam structure, such that articles produced have a good compromise of physical properties such as low density and high impact strength.
According to the invention there is provided a method of making a foamed polyester resin article comprising forming a molten mixture containing poly(ethylene terephthalate), having an intrinsic viscosity (I.V.) greater than 0.7 and a moisture content of not greater than 0.03%. a melt viscosity modifier and a heat-activatable foaming agent having a decomposition temperature greater than 150.degree. C. heating the mixture in the barrel of an extruder and extruding the mixture through the die of the extruder to form a foamed extruded product.
We have found that by using the combination of the foaming agent and the melt viscosity modifier we have been able to produce extruded poly(ethylene terephthalate) sheet and food trays formed therefrom having a density which is 35-40% less than that of conventional, non-foamed CPET. The reduction in density represents a saving in the quantity of the polyester resin used in the manufacturing process. Furthermore, the final products do not contain unacceptable acid anhydride branching agents.
The polymer resin used in the present invention is crystallizable poly(ethylene terephthalate), having an intrinsic viscosity (I.V.) greater than 0.7. Poly(ethylene terephthalate) having an I.V. of 0.7 or less is not suitable for use in carrying out the present invention since it is unlikely to provid
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Database WPI, Seciton Ch., Week 8503, Derwent Publications Ltd., Class A23, AN 85-015055 (Undated).
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Dunning Peter James
Sparkes Andrea
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