Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2001-09-13
2003-05-06
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S275000, C528S279000, C528S286000, C528S302000, C528S308000, C528S308600, C525S437000, C525S443000, C524S081000, C524S706000, C524S710000, C428S035700
Reexamination Certificate
active
06559271
ABSTRACT:
The invention relates to a method for producing polyesters with a reduced content of free acetaldehyde from terephthalic acid and ethylene glycol via a catalysed polycondensation in the melt and the use of this method.
Polyesters made of dicarboxylic acids and diole, in particular those of the type of polyethylene terephthalate and the copolyesters thereof with other dicarboxylic acids and alkylene glycols and also higher valent polycarboxylic acids and polyols, are used to a significant extent for producing containers and bottles for food and beverages.
For this reason, plant capacities for producing such polyester granulate grades have increased enormously, In the case of large plants, polyester can be produced more economically than previously. The competition of the producers put pressure on the prices. On the other hand, this in turn stimulates the development of new application possibilities and supersedes other materials such as PVC. Packaging polyester has thus become a bulk product, in the production and sale of which only small margins can be realised. In these circumstances, any improvement or simplification in the production process which increases the margins is of interest to the producers of packing polyesters.
Such a simplification in the production process resides in the fact that polyester with an average molar mass required for packaging purposes, measured via the intrinsic solution viscosity (IV), is produced by polycondensation in the melt. The intrinsic viscosity is measured thereby at 25° C. in phenol/dichlorobenzene (1:1) (see A. Horbach, R. Binsack, H. Müller, Angew. Makromol. Chem. 98 (1981) 35-48). As a consequence, the step of solid-state postpolycondensation is omitted, which is nowadays necessary without exception. This step is associated with significant expense in apparatus and energy; first of all the polyester melt is converted into an amorphous granulate. This granulate must be heated again and be treated by crystallisation in at least two steps, which are controlled precisely according to temperature and dwell time, for the subsequent solid-state post-polycondensation (SSP). Without this complex preparation, the result is baking or agglomeration of the granulate in the SSP, which implies production interruption, repair operations and product loss. The actual SSP requires dwell times of between approximately 6 and 15 hours and sweeping with inert gas which, for economic reasons, must be treated after use and returned into the process. This results in large reactor dimensions and a large number of auxiliary equipment and units for the gas cleaning and gas treatment with a corresponding energy requirement.
The spatial requirement and the building height should be emphasised in particular. In addition there is the additional cost of the supply, metering and removal of the solid material for the reason that the SSP and the gas treatment have to be executed at a high temperature and in the exclusion of atmospheric oxygen.
This large outlay on equipment and energy is unnecessary if the polycondensation is implemented in the melt up to the required molar mass. The mentioned molar masses concern number averages M
n
which was determined from the IV according to Horbach et al. (literature cited above). According to the state of the art to date it is possible to have a molar mass increase in the final stage of polycondensation in the melt, starting from approximately 6,000 g/mol to approximately 20,000 g/mol. An extension to higher molecular masses of 25,000 g/mol to 35,000 g/mol, maximum 40,000 g/mol has to date not been considered for two reasons;
1. No finishing reactors were available, with which higher molecular masses of 25,000 g/mol to 35,000 g/mol, maximum 40,000 g/mol could be achieved and which were able thereby to deliver an acceptable product quality with respect to colour, free acetaldehyde and content of vinyl ester end groups, (a measure of the thermal damage and of the potential for re-formation of acetaldehyde in further processing for example so as to form bottles).
2. Only SSP was able to produce a polyester granulate which had the low concentration of free acetaldehyde (AA) necessary for processing into beverage bottles and a low AA re-formation during processing of the granulate into bottles (injection moulding, stretch blow moulding).
In the meantime the development of the “DISCAGE” end reactors of the Inventa-Fischer company has progressed so far that the first reason no longer plays a decisive role. Reference is made hereby to EP 0 719 582 and the reactor type described there with all of the embodiment variants is included in this application.
It is hence possible to maintain colour, AA and the concentration of vinyl ester end groups at a level which makes application of AA-bonding additives useful. In addition it has become possible to achieve the required molar mass increase with a single end reactor. By using two end reactors which are connected in series, the molar mass increase could indeed be achieved more easily but it would be necessary to accept the disadvantages of a longer dwell time in the melt (and hence increased formation of colour, AA and vinyl ester groups) and the greater expense of equipment.
Polyesters are produced according to the state of the art by melt polycondensation from low molecular esters of dicarboxylic acids with alkylene diols at increased temperatures by separating water and alkane diols. The separation of the volatile products of the polycondensation is effected by the application of vacuum and an intensive mixing of the melt. By adding special catalysts, in particular metal compounds such as antimony trioxide, the polycondensation is accelerated and the attainment of high molecular masses is made possible, such as are required for production of these containers.
The high temperatures during production and processing of the melt are the cause of decomposition reactions of the polyesters which lead to the release of acetaldehyde via several steps, said acetaldehyde remaining in the melt and escaping gradually after processing into containers and bottles from there, diffusing into the enclosed food and beverages and affecting their smell and taste disadvantageously.
The thermal decomposition of the polyesters and hence the formation of acetaldehyde is however favoured by the known polycondensation catalysts. They all have a limited selectivity, i.e. they catalyse not only the mole mass structure (chain lengthening) but also the molar mass decomposition—in varying degrees—by thermal ester cleavage.
The manner used almost exclusively to date for production comprises the polycondensation of the raw materials terephthalic acid and ethylene glycol (with supplements of smaller quantities of comonomers such as isophthalic acid, diethylene glycole or cyclohexamethylene diole for improving the processing properties in the melt up to an average molar mass of approximately 20,000 g/mol (IV 0.63). In order to further increase the average molar mass to the values required for packaging purposes of between 25,000 and 30,000, max. 40,000 g/mol, there is used nowadays exclusively solid-state post polycondensation.
For this purpose, polyester melt is converted into a solid granulate after achieving an average molar mass of approximately 20,000 which is not yet sufficient for the production of bottles. Subsequently, this granulate is further condensed in a solid-state polycondensation at temperatures below the melting point until the required average molar mass is achieved.
The solid-state polycondensation offers the advantage that, at lower temperatures, the above-mentioned decomposition reactions do not occur or only to a greatly reduced extent and furthermore that already present acetaldehyde escapes from the granulate particles under the conditions of the solid-state polycondensation and is removed. In this way, a high molecular polyester granulate with an acetaldehyde content under 3 ppm and up to under 1 ppm is obtained, which is used for further processing into containers and bottles. Further processing of
Hagen Rainer
Rafler Gerald
Schaaf Eckehart
Acquah Samuel A.
Inventa-Fischer GmbH & Co. KG
Marshall & Melhorn LLC
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