Process for the dimensional stabilization of containers in polyethylene terephthalate

[0002] The present invention is directed to an improved process for the stabilization of heat deformation in polyethylene terephthalate containers obtained by the injection blow-molding technique.

[0003] It is known that containers (bottles) in PET obtained by injection blow molding are subject to deformation. In addition, they are subject to deformation of a considerable extent when they are employed under high temperature conditions such as those which occur during hot-filling and sterilization operations.

[0004] The temperature of such treatments can reach 80°-90° C. and the material tends to soften and shows shrinkage once cooled, with a volume reduction which can reach 15-250%.

[0005] With the aim of reducing or eliminating such inconveniences, the preforms of the containers obtained by injection molding are subjected to heat stabilization treatment (heat-setting), which is carried out during the blowing phase of the preform.

[0006] The treatment temperature is higher than 130° C. and generally is between 130° and 180° C. with a duration of is a few seconds. The preforms thus obtained give the desired dimensional heat set qualities to the containers.

[0007] During the treatment, however, olygomers are formed which lead to fouling of the mold, and consequently, the necessity of cleaning it and therefore frequent interruptions in the production cycle.

[0008] It has now been found that it is possible to reduce or diminish the precipitation of olygomers in the preform blow molds and therefore to be able to operate with production cycles which do not require mold cleaning operations. This can be achieved by subjecting to heat setting, preforms obtained from polyethylene terephthalate (PET) or its copolymers (COPET), in which up to 15% by moles of units deriving from terephthalic acid are substituted by units deriving from isophthalic acid and/or naphthalene dicarboxylic acid, previously subjected to upgrading or polycondensation in the solid state (SSPA) in the presence of small percentages of a dianhydride of an aromatic tetracarboxylic acid.

[0009] Pyromellitic dianhydride is the preferred dianhydride. The dianhydrides are employed in quantities from 0.01 to 0.2% by weight, preferably 0.04-0.1%.

[0010] The polycondensation treatment in the solid state is conducted at temperature generally between 180° and 220° C. for sufficient time to increase the intrinsic viscosity of the polymer to the desired value.

[0011] The dianhydride is added to the PET or COPET in the melt state. It is convenient to dissolve it in ethylene glycol.

[0012] It was found that, employing upgraded PET and COPET as above indicated, the heat treatment of the preforms can be carried out at temperatures notably lower than those employed in the hitherto known processes. The heating temperatures of the preform blow mold are between 80° and 130° C. and are sufficient on confer to the containers the necessary dimensional stability. Operating under such heating conditions, mold fouling phenomena do not occur.

[0013] The duration of the treatment is of a few seconds, generally less than 10 seconds.

[0014] The container thus obtained is cooled in air, after extraction from the mold.

[0015] The preform enters the preheated mold at temperatures between 100° and 140° C.

[0016] The stabilized containers obtained in this way. are suitable for applications such as hot-filling.

[0017] The following examples are provided to illustrate but not to limit the invention.

[0018] 20 kg/h of PET (IV=0.57 dl/g) are fed continuously from the melt polycondensation section of a PET production pilot plant to a counter-rotating non-intermeshing twin screw extruder equipped with a degassing system.

[0019] To the extruder are fed 40 g/h of a mixture of crystalline PET containing 20% in weight of PMDA, equal to a quantity of PMDA in the PET of 0.04% by weight.

[0020] The test conditions were:

[0021] The product was extruded in the form of chips of diameter 5 mm and 5 mm length. The IV was 0.62 dl/g.

[0022] The granules were then subjected to SSP in a continuous pilot plant operating as follows:

[0023] The polymer obtained according to Example 1 is transformed, after drying, into preforms using an injection molding machine BMB 270 with the following procedure:

[0024] The preforms thus obtained are blown into bottle form using a Krupp Corpoplast blow molding machine.

[0025] Conditions were the following:

[0026] Preforms obtained from the granules obtained according to Example 1 were blown in a machine having molds equipped with a heating system. The temperature of the mold was 120° C. with a duration of treatment of 5 seconds, after which the preforms were cooled in air. The starting temperature of the preform in the mold was 100° C. No fouling of the molds was noted during the entire production cycle.

[0027] The bottles of Example 2 and those of Comparison Example 1 were subjected to the following temperature conditions:

[0028] The dimensional variations of the bottles are reported in Table 1.

[0029] The tests at 85°, 90° and 95° C. were carried out on bottles aged in water at 50° C. for 6 hours.