Drying and gas or vapor contact with solids – Apparatus – Gravity flow type
Reexamination Certificate
2001-08-30
2004-06-08
Bennett, Henry (Department: 3743)
Drying and gas or vapor contact with solids
Apparatus
Gravity flow type
C034S112000, C264S328100
Reexamination Certificate
active
06745492
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a process for manufacturing a preform obtained by injecting into a mould at least one thermoplastic resin, in a substantially crystalline state, as well as to an installation for implementing this process. It also relates to the plastic recipient obtained by drawing and blowing in a mould a preform manufactured according to the process of the invention. The invention resides more precisely in an improvement made to the operation of drying the resin in the form of granules, which drying operation is carried out prior to injecting the resin into the mould. The invention will primarily, but not exclusively, find an application in the manufacture of plastic preforms and recipients, based on at least one polyethylene terephthalate resin (PET), that have a low acetaldehyde (AA) content, which makes them primarily, although not exclusively, suitable for storing mineral water.
In the packing industry, and more especially in the field of bottling, the use of plastic recipients that are obtained by drawing and blowing a preform in a mould is very widespread. Usually, the preform is obtained by injecting one or more thermoplastic resins into a mould. Those thermoplastic resins most commonly used to date, on account of their low cost price, are polyethylene terephthalate (PET) homopolymer or copolymer based resins. These PET resins have, however, the major drawback of being permeable to gases, in particular to oxygen and carbon dioxide. When it is wished to obtain recipients having specific functional properties that cannot be obtained with PET resins, more specific thermoplastic resins having the properties sought after are used to manufacture the preform. For example, when it is wished to produce a plastic recipient having improved gas barrier properties, it is known to use specific thermoplastic resins such as polyamide resins containing m-xylylene groups and commonly referred to as “MX-nylon”, resins based on a vinyl alcohol and ethylene copolymer and, for example, EVOH resins, or again, polyethylene 2,6-naphthalene dicarboxylate (PEN) resins, etc. These specific resins can be used by being mixed, prior to injection, with a PET or similar resin, the mixture obtained usually being injected into a mould to produce the preform. More commonly, these specific resins are used for the manufacture of preforms having a multilayer structure, by sequential and/or parallel co-injection of a specific resin and of a PET or similar resin.
Usually, the thermoplastic resin or resins, which are used to manufacture a preform, have a degree of crystallisation greater than 50% and initially take the form of solid granules. These granules, which are in a substantially crystalline state, are obtained in a known manner by subjecting granules of resin in an amorphous state to a heat treatment in the granule manufacturing step. Granules of resin in an amorphous state have the drawback of tending to stick together and form agglomerates. This preliminary step of crystallisation of these resin granules, which is implemented during the granule manufacturing step, advantageously makes it possible to obtain resin granules which, owing to their crystalline state, no longer tend to agglomerate together. One example of a process of crystallisation of polyester resin granules is described, in particular, in European patent application EP-A-0 379 684. In this publication, the polyester resin granules in a crystalline state are obtained by subjecting polyester resin granules in an amorphous state to a heat treatment taking place in two successive steps.
To manufacture a preform from a thermoplastic resin substantially in a crystalline state and in the form of granules, the following successive operations are performed:
melting and mixing the resin granules by passing them through a screw type extruder;
injecting the melted resin from the extruder into a mould.
As a rule, the aforementioned step of melting and mixing the granules of resin at the time of manufacturing a preform is preceded by a step of storing these resin granules in a substantially crystalline state, which may vary in length, depending on the particular case. The thermoplastic resins most commonly used to manufacture the preforms and, in particular, PET resins, absorb the moisture of the atmosphere when stored in the form of granules for a certain time. This moisture present in the resin, because of the high temperatures used to produce the melting of the resin granules and to inject the melted resin into the preform mould, generates degradation through hydrolysis of the resin. This degradation by hydrolysis is all the more marked the greater the moisture content of the resin and/or the higher the resin heating temperatures. This hydrolytic degradation of the resin undesirably results in a reduction in the weight of the molecule of the resin, thus reducing the intrinsic viscosity of the resin and the associated properties. Now, it is known that an excessive loss of intrinsic viscosity can impair the quality of the preform obtained and, in particular, can impair the transparency of the preform and can lead to a loss of the mechanical properties of the bottle obtained from the preform. It is thus essential for preform manufacturers to reduce this phenomenon of degradation through hydrolysis of the resin as far as possible. For this purpose, prior to transformation of the resin by passing through the extruder screw, the granules of resin in a crystalline state are dried in a preliminary operation, which makes it possible to reduce the relative moisture content of the resin granules. To date, this drying operation has been carried out by passing the resin granules through a drying hopper, the output of which is connected to the input of the extruder, and which delimits an internal drying chamber brought to a given temperature, by means of a flow of dry, hot air that is continually renewed.
During the process of manufacturing a preform, the resin or resins used also undergo thermal degradation, which occurs chiefly when the resin passes through the screw extruder. This thermal degradation is harmful as it leads, on one hand, to a loss of the physical properties of the resin, reflected, in particular, in a loss of intrinsic viscosity, the appearance of traces of crystallinity on the preform or again, a change in the colour of the preforms and, on the other hand, to the formation of undesirable by-products. In particular, in the case of a PET resin, one of the undesirable by-products resulting from the thermal degradation of the resin is acetaldehyde (CH
3
CHO), which is produced chiefly in gaseous form, and which is characterized by a fruity smell and taste. The acetaldehyde ends up in the walls of the recipient that is produced from the preform, and it migrates, during storage, in contact with the product stored in the recipient. If the acetaldehyde content is too high, the result is a substantial deterioration in the taste of the product stored, a deterioration in taste that can be undesirably perceived by the consumer. In the case of a PET resin, it is thus important to ensure that as little acetaldehyde as possible is generated. It is vital to maintain a very low acetaldehyde content when the product stored in the recipient is one with little taste, such as, for example, mineral water. This is why, in the particular field of mineral water storage, bottlers impose on their suppliers of PET based bottles a maximum acetaldehyde content that must not be exceeded.
One of the difficulties of the operation of drying granules of resin in a crystalline state lies in the choice of temperature and the duration of drying in the hopper. To ensure that the water absorbed by the resin is efficiently removed, and to bring the relative moisture content of the resin down to acceptable levels, the temperature and the drying time have to be sufficient. Conversely, the temperature and the drying time must be sufficiently reduced to limit as far as possible any thermal and hydrolytic degradation of the resi
Bennett Henry
Harness & Dickey & Pierce P.L.C.
Nguyen Camtu
Schmalbach-Lubeca AG
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