GB2118565A - Polyester production - Google Patents

Abstract

Highly condensed polyester, e.g. polyethylene terephthalate, is produced by a continuous, two- stage treatment of a granulate (intrinsic viscosity at least approximately 0.35). In a first stage, water vapour is used. The second stage involves condensation by conventional methods. The highly condensed polyethylene terephthalate which can be obtained is eminently suitable for use in the food industry, in particular for bottles, owing to its considerably reduced total acetaldehyde content.

Description

SPECIFICATION Polyester production The present invention relates to a process for the continuous production of polyester, in particular polycondensates of a (cyclo)aliphatic diol and, preferably, an aromatic dicarboxylic acid. The invention is of especial applicability in the production of polyethylene terephthalate.

High molecular weight polyesters, and particularly high molecular weight polyethylene terephthalate, which have been condensed in the solid phase, are of growing commercial importance. Polyethylene terephthalate of this type is used in granule form for the production of bottles or containers in the food industry; US-A-3733309 describes its production.

The main problem in the condensation of polyethylene terephthalate is the removal of readilyvolatile and often also olfactory substances, in particular the acetaldehyde which is present as a chemically-bonded vinyl ester and is also physically dissolved. Their presence makes the product unsuitable for use in the food industry.

Various processes have been proposed for removing these volatile constituents from the polyester. Thus, for example, US-A-4263425 describes the gassing of polyethylene terephthalate at 200-220"C with inert gases. The polyethylene terephthalate granules must contain less than 25 ppm of ester interchange catalysts or 20 ppm of phosphoric acid in this process. US-A4223128 proposes the treatment of the solid polyethylene terephthalate with air at about 180-220"C. However, the products of both these procedures have a total content of physicallydissolved and chemically-bonded acetaldehyde which is generally too high for present-day requirements.

According to the present invention, a polyester in the solid phase is first treated with water vapour at 100-245"C and is then condensed at 200-245"C under an inert gas and/or air under ambient or reduced pressure.

The polyester is preferably in granulate form. It may be derived from aliphatic or cycloaliphatic diol and dicarboxylic acid (preferably aromatic) components. Examples of suitable diols are alkylene or cycloalkylene glycols, in particular a,w-alkylene glycols such as ethylene, 1,3propylene or 1 ,4-butylene glycol, cyclohexane-1 4-dimethylol, and cyclohexane-1 4-diol.

Examples of suitable dicarboxylic acids are terephthalic acid, isophthalic acid, naphthalenedicarboxylic acids, adipic acid and sebacic acid. Mixtures of diols and dicarboxylic acids may be used. Small contents of trifunctional and polyfunctional alcohols and carboxylic acids may be present, for cross-linking purposes. The most preferred polyester for use in the invention is polyethylene terephthalate, and this will be used for the purposes of further illustration of the invention.

The degree of removal of physicaily-dissolved acetaldehyde from polyethylene terephthalate may be determined in known manner, by the diffusion of the aldehyde from the polyethylene terephthalate granulate. In order to remove the chemically-bonded acetaldehyde, the vinyl ester groups in the polyethylene terephthalate must first be converted into free acetaldehyde, and apparently this has not completely occurred hitherto. It appears that, in the invention, the presence of water vapour allows the physically-dissolved and chemically-bonded acetaldehyde to be removed almost quantitatively from the polyester granules without adversely affecting the polyester.

A new method of analysis has been developed to determine quantitatively the total content of physically-dissolved and chemically-bonded acetaldehyde. In this method, a polyester sample is ground (20-50 mesh), mixed in a glass bottle with a 10-fold quantity of oxygen-free water and sealed with a polytetrafluoroethylene-coated rubber septum. The bottle is then heated for 1 7 hours to 120"C. After cooling the bottle, a sample of the aqueous solution is removed and the content of acetaldehyde is analysed colorimetrically or by head-space gas chromatography.

Methods of analysis for determining the acetaldehyde content in polyester are described in DE-A-2834162 and US-A-4223128. These known methods are similar. In the former, the ground polyester is sealed in a glass flask with a rubber septum. The air contained in the flask is replaced by nitrogen. The flask is then heated for 90 minutes to 150"C and a sample of the acetaldehyde-nitrogen mixture is analysed for acetaldehyde by gas chromatography. This procedure gives only a proportion of the acetaldehyde in the polyester.

In a comparison of the two given methods of analysis, a conventional commercial polyethylene terephthalate bottle granulate (Hoechst M81 E) is examined. By the known method, the acetaldehyde content was 2.6 ppm, corresponding to the given specification of 2-3. By the new method, the total acetaldehyde content was 9.4 ppm.

The first stage of the novel process is preferably extended until the total content of physicallydissolved and chemically-bonded acetaldehyde is less than 20, more preferably less than 10, and most preferably less than 5, ppm. This figure can be determined by the new analytical method, by removing the sample from the apparatus at the relevant point.

In the first stage, pure water vapour is preferably used. However, it is also possible to use a mixture of water vapour and air and/or inert gas.

The second condensation stage of the novel process is suitably conducted under inert gas and/or air, as is conventional. Preferably an inert gas, in particular nitrogen, is used, Ambient pressure or vacuum is used.

Conventional apparatus may be used. The first and second stages may be carried out in one integrated, two-zone reactor, or, preferably, in a reactor comprising separate parts for the two stages. Suitable apparatus is illustrated in the accompanying, schematic, drawing. Its use will now be described by way of example.

A polyethylene terephthalate granulate is introduced through an inlet 1 into a reactor section 2 in which the first stage of the process takes place, and fall under gravity in a counter-flow of water vapour. The water vapour flows into the reactor section 2 at 3, strips acetaldehyde and flows out again at 4. A sealable opening 5 is provided for the removal of samples for analysis.

The granulate then passes through a flow regulator 6 and subsequently enters a second reactor section 7, in which condensation takes place in a counter-flow of inert gas and/or air.

The reactor section 7 comprises an inlet 8 and outlet 9 for the inert gas and/or air as well as a flow reguiator 10 for the outflow of product.

By the process according to the invention, condensed polyethylene terephthalate with a total content of physically-dissolved and chemically-bonded acetaldehyde of less than 3 ppm and with an intrinsic viscosity normally of at least 0.7 (initially usually at least 0.35) can be obtained, without any significant extension of the condensation time. Acetaldehyde (and other volatiles) can be removed by the novel process much more effectively (almost quantitatively) than by known methods. Thus, polyethylene terephthalate can be obtained which is eminently suitable for use in the food industry, in particular for the production of bottles and films.

In Examples which illustrate the invention, polyethylene terephthalate granulate (intrinsic viscosity 0.668; total acetaldehyde content 76 ppm) was introduced at 50 kg/h into the upper reactor section 2 of apparatus as shown in the drawing, and was processed in the manner described above with reference to the drawing. Results are shown in the following Table.

In the first stage, the amount of water vapour was 0.5 kg per kg of granulate. In the second stage, a nitrogen counter-flow was used. Intrinsic viscosity was measured for a 0.5 g sample in 100 ml of a 1:1 w/w mixture of phenol and tetrachloroethane at 20"C. In Example 6, 10% v/v air was added to the water vapour. Example 7 is comparative.

First stage Water vapour granulate Acetaldehyde Nitrogen Second stage Acetaldehyde admission residence content after admission residence content after temperature time first stage temperature time second stage Intrinsic Example ("C) (min) (ppm) ( C) (min) (ppm) viscosity 1 240 15 4.0 240 240 0.4 0.810 2 230 15 4.5 230 360 0.6 0.822 3 220 15 5.0 230 360 0.7 0.837 4 190 120 4.5 230 360 0.6 0.845 5 160 120 17.5 235 300 1.92 0.896 6 190 120 4.5 235 360 0.45 0.830 7 240 360 4.4 0.860

Claims (8)

1. A process for the production of a high molecular weight polyester, which comprises treating polyester in the solid phase first with water vapour at 100-245"C, and then by condensation at 200-245 C with inert gas and/or air under ambient or reduced pressure.

2. A process according to claim 1, in which the polyester is derived from an aliphatic or cycloaliphatic diol and an aromatic dicarboxylic acid.

3. A process according to claim 1, in which the polyester is polyethylene terephthalate.

4. A process according to claim 3, in which the product contains less than 3 ppm acetaldehyde.

5. A process according to claim 3 or claim 4, in which the acetaldehyde content after the water vapour treatment is ;es,, can 20 ppm.

6. A process according lo claim 5, in which the acetaldehyde content after the water vapour treatment is less than 10 ppm.

7. A process according to any preceding claim, substantially as described herein with reference to the accompanying drawing.

8. A process according to claim 1, substantially as described in any of Examples 1 to 6.