DE2834162A1 - PROCESS FOR THE MANUFACTURE OF POLYAETHYLENE TEREPHTHALATE PLASTIC FOR PACKAGING OR FILLING OF FOOD - Google Patents

Description

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Process for the manufacture of polyethylene terephthalate plastic for packaging or filling food

The invention relates to a method for the manufacture of linear polyethylene terephthalate plastic for packaging or bottling food for human consumption where the food comes into direct contact with the plastic.

Linear, d. H. thermoplastic polyesters have hitherto been used for the production of textile thread and fibers been. In addition, there is a considerable market and a wide range of possible uses for polyester in the film industry. For example, linear polyesters are used as films for wrapping and packaging as well as for magnetic tapes, films, lacing material, Electrical insulation, drawing paper, etc. used. Where foils are used for packaging purposes, come in most cases the foils do not come into contact with the food. The shrinkage properties and the mechanical strength of polyester films used for food packaging, have been used in combination with other materials such as paper or polyethylene foils.

In the special case of containers such as bottles that are used to store carbonated beverages, such as Liquor or beer, linear polyesters are preferably used as a manufacturing material.

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It is known that all uses are special Requires properties of the material used. It is also known that special applications can have special requirements.

A prerequisite for using polyester for food packaging is optimal Polymer homogeneity. Other characteristics such as the viscosity of the product and the average molecular weight must be chosen carefully as this has an influence have essential properties such as mechanical strength and crystallinity. For many Purposes such as B. the production of bottles, is optimal transparency and minimal coloring of the polymer required. In general, properties that are suitable for a specific use of are of the greatest importance, are of absolutely secondary importance for another purpose.

For use as a packaging material for food or beverages, another factor is paramount Meaning. There must be no taste or odor from the polymer on the food stored in the containers. or ignore stimulants. It is known that polyesters, as a result of decomposition or degradation, acetaldehyde which is perceptible even in the smallest concentrations due to its strong specific odor,

The presence of acetaldehyde indicates some degradation, which ultimately leads to coloration. the Color formation has a detrimental effect, i. H. the polymer may only show minimal coloring. It is necessary that bottles ^ which are used for the storage of

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soaks are intended to have an exceptionally low concentration of acetaldehyde, as this is of penetrates the bottle walls into the liquid. The permissible limit values depend on various factors from, such as B. on the type of drink, shelf life, etc., but also on the method of determination of the acetaldehyde present.

Acetaldehyde can be partially, but not quantitatively, removed from a polyester melt. For example, acetaldehyde can be partially eliminated when a melt is treated with stirring at high temperatures under vacuum will. When using this method, the limit values are not reached because the temperature of the melt is too high, whereby, parallel to the removal of the previously formed acetaldehyde, a breakdown occurs, through which new acetaldehyde is created. In addition, the removal of acetaldehyde is technically feasible Plant more difficult within an economic period, as the viscosity increases and the melt layer gets thicker.

Machines for the production of bottles, other hollow bodies or other packaging can be made with melt are fed from a condensation reactor. For the most part, these machines are made with polyester shreds loaded. The choice of the starting material for Bottle-making machines, however, depend on the geographical location of the plant and thus on the transport problems certainly. Even if a melt is essentially free of acetaldehyde and those made from it Schnitzel re-used for bottle production.

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are melted, acetaldehyde is produced again through degradation. The acetaldehyde concentration in the for that The melt used in bottle manufacture depends on the remelting conditions of the chips. In each In this case, the aldehyde concentration or the aldehyde content is unbearably high. Hence the amount of acetaldehyde that can pass into a liquid that comes into contact with the bottle, also too high.

In addition, until recently there was no standard test method for determining acetaldehyde in bottle manufacture certain polymers or in the bottles themselves. The processes used were based on the Gas chromatography (J.A. Meyer, J. Chromatog. 99, 709-720 (1974)). The difference between the test methods, which are now used by institutes or individuals lies in the procedures used as well as in the way in which the acetaldehyde content in the polymer is calculated.

After the problem of acetaldehyde in the polymer became known, many attempts have been made to solve it. The aim of these experiments was to find conditions that would reduce the re-formation of acetaldehyde during the remelting of cossettes for bottle manufacture. It was assumed that polyesters _s would be used as a starting material for j bottles in most cases, and that the chips are not manufactured in the same plant as the bottles. Therefore, in practically all cases where polyester chips form the raw material, these chips have to be dried before being processed into bottles.

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The object of this invention is to produce bottles with the lowest possible acetaldehyde content for Prevents the transfer of taste or smell to the liquids in polyester bottles. In order to achieve this goal, we believe that the upper limit of the acetaldehyde concentration in polymer chips must not be higher than 2.5 ppm. In reducing acetaldehyde levels, factors must be taken such as the quality of the polymer as well as the manufacturing conditions of the same, the treatment of the chips • and the conditions necessary for the manufacture of the bottles are taken into account. The invention task also includes the concern to obtain a polymer or a bottle of one hundred percent transparency, whereby the lowest possible color is to be achieved. Another part of the invention is manufacturing a polymer of great mechanical strength to make bottles that are light in weight but strong and thus save packaging material.

The acetaldehyde contents given here were chromatographed as follows with the aid of head-space analysis Determined: A sample of milled polyester (20-50 mesh) was placed in an appropriately sized glass flask and closed with a rubber septum. The air contained in the flask was replaced with nitrogen. The flask was heated to 150 ° C. in an oil bath. After 90 minutes, a sample of the acetaldehyde-nitrogen mixture was taken withdrawn through the stopper with a gas syringe, injected into a gas chromatograph and at

160 ⁰ C by means of a column of 1 m SS, 1/8 "inner diameter, Porapak Q 80/100 mesh, analytically examined.

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The stated object has been achieved on the basis of the initially indicated type in that chips from linear polyethylene terephthalate having an intrinsic viscosity from 0.5 to 0.85 to a temperature between about 180 and 230 ⁰ C for a period of about 4-12 hours heated to less than 2.5 ppm to reduce the acetaldehyde content.

In the course of our attempts at making bottles from different types of polymers Surprisingly, it turns out that the acetaldehyde concentration in bottles from different polymer batches showed significant differences. An examination of the possible causes for these differences found that before the bottle was made, the schnitzel used was dried in different ways was. Therefore, the drying conditions were examined to determine which reactions occurred during the Result in the drying process in the chips.

It is known that acetaldehyde is removed during drying by diffusion from the chips, and therefore the concentration depends according to the rules of physical chemistry _s ie after Fick'sehen law, the viscosity of the polymer and largely on the temperature. In addition to viscosity and temperature, the time as well as the shape and size of the chips have an influence on the elimination of acetaldehyde during drying.

The specific surface area of the chips increases with decreasing particle size Surface of cylindrical chips with a diameter /

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Langeη ratio of 1 according to the formula 6: diameter calculated. Another factor in the fuck's eye law is the diffusion path, which increases with decreasing particle size

size decreased. ^

If the mass transfer, ie the diffusion of the acetaldehyde from the chips, is ended at low temperatures, there is no risk of further thermal or thermo-oxidative degradation. It is also known that the amount of acetaldehyde diffusing from the chips increases within certain limits with increasing temperature. It was also found that even drying polyester chips at temperatures of 160 ^° C. over an uneconomical period of 60 hours is not sufficient to produce chips which are suitable for the production of bottles with a sufficiently low acetaldehyde content. To investigate this problem, we carried out series tests, the description of which follows below and in the course of which the influence of the drying temperature on the formation of acetaldehyde and its concentration in the polymer was examined.

An examination of the analytical results showed that apart from the thermal or thermo-oxidative degradation and in addition to the acetaldehyde already contained in the polymer before drying, there is another source of the origin of the acetaldehyde. These are the vinyl esters in the polymer, which react with the OH end groups of the polyester chains and thereby form acetaldehyde. This reaction, in which acetaldehyde is obtained, takes place mainly at 220 ^° C. If the drying

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If the temperature is high enough, the vinyl end groups react in this way. This reaction occurs does not occur during the drying process, then it takes place during the remelting of the Schnitzel with accumulation of large amounts of acetaldehyde. This additional amount of acetaldehyde is formed and carried through Eliminates diffusion during drying, then the acetaldehyde content in the bottles is reduced to the amount which was not removed during the drying process plus the amount that was due to thermal or thermo-oxidative Degradation was formed during the remelting of the chips. Therefore, the drying temperature is also allowed not too high to avoid the formation of too high amounts of acetaldehyde by thermal or thermo-oxidative Avoid degradation during the drying process. In addition, great care must be taken with the terms used under which the dried polymer is melted again.

Accordingly, the invention relates to a special process for drying polyester chips intended for the production of bottles or food packaging. First, the intrinsic viscosity of the polyester chips must be between 0.5 and 0.85, the Intrinsic viscosity ^{is measured at 25 0} C in a solvent with a content of 60 % phenol and 40 % tetrachloroethane at a concentration of the polymer of 0.5 g / 100 ml. Second, nitrogen or another inert gas is used as the heating medium in a continuously or discontinuously operated dryer. The drying process must be carried out within 4-12 hours, the exact time depending on the temperature, the intrinsic

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Sic viscosity of the chips and the size and shape of the granulate is selected. Within the workable Intrinsic viscosity. Range between approx. 0.40 and 0.80 and with one schnitzel size - with regard to the smallest Dimensions - between 1 and 4 mm, it has been found that the heating time preferably corresponds to the equation

T = 2.74 (D) (IV) ° ' ⁶ + 2,

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should correspond, where T is the heating time (hours), D is the smallest chip size and IV is the intrinsic viscosity defined above. The temperature must not be less than 180 ° C. in order to initiate the reaction of the vinyl end groups with OH groups and must ^{not exceed 230 °} C. in order to avoid severe degradation.

Further details of the invention are explained in more detail below with the aid of method examples. In each of the following examples were 13 g of polyethylene terephthalate chips with an intrinsic viscosity of 0.85 and a carboxy1 group content of 15 eq. / 10 g

dried for a period of 3 hours at 200 ⁰ C in a nitrogen stream to adjust the water content to less than 0.003 weight to be reduced.%.

The chips were then quickly transferred to a stainless steel tube with a diameter of 10 mm, preheated to 280 ° C., under a nitrogen blanket. After this tube was quickly filled, it was quickly closed with a plunger. 10 minutes after it had been melted and ^{heated to 280 °} C., the polymer was extruded into a vessel flushed with nitrogen in order to obtain a cable 1 mm in diameter.

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example 1

Under a nitrogen blanket, 8 g of a freshly extruded cable were cut into pieces 2-3 cm long and placed in an Erlenmeyer flask. The Erlenmeyer flask was provided with a silicone disk (septum) which was attached to the flask by means of a screw cap, through which gas samples were taken for gas analysis. The flask was also equipped with a stirring disk, the holding rod of which protruded through a glass stopper and was moved up and down at intervals of 4 seconds by means of a magnet. The Erlenmeyer flask was placed in an oil bath which reached to the bottom of the stopper. The oil was heated to 160 ⁰ C. The nitrogen kept in motion over the pieces of cable (chips) was kept at a constant temperature of 160 ^° C.! 1 C held. After 1 1/2 hours, three gas samples were taken from the gas space above the chips at an interval of 3 minutes each and examined for their acetaldehyde content by gas chromatography.

The gas space above the chips was then flushed with nitrogen for 90 seconds. After a residence time of 1 1/2 hours Another 3 gas samples were taken at 160 ° C. and tested analytically for their acetaldehyde content. This procedure was repeated twice, each time with an interval of 1 1/2 hours. From this resulted after 1 1/2, 3, 4 1/2 and 6 hours heating at 160 ° C four mean values. The results are shown in Table I. The amount of acetaldehyde found in the gas space was related to the weighted amount of polyester (8 g) in the calculation and expressed in ppm / weight.

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Example 2

Example 1 was repeated except that the freshly extruded cable or the chips for a period of 6 hours at 180 ° C instead were heated to 160 ⁰ C. The test results are shown in Table I.

Example 3

Example 1 was repeated with the exception that polyester chips were or 6 hours to a temperature of 200 t 1 ⁰ C instead of 160 ° C heated cables. The test results are shown in Table 1.

Example 4

Example 1 was repeated except that the polyester chips or cables were heated to 220 for 6 hours

1 1 ° C instead of 160 ° C were heated. The test results are shown in Table I.

The gas chromatographic determination of the acetaldehyde in the gas space above the schnitzel was carried out with a Perkin bucket Analysis device (version F20 B, with FID) using a filled with Porapak Q and kept at 160 C. 1 m column carried out. Gas samples of each

2 ml was injected into the analyzer. The calibration was carried out using an exponential gas diluter. The mean values of the aldehyde contents found and based on the polyester are shown in Table I.

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example Table I.

Temperature (C) 160 180 200 220 Acetaldehyde (ppm) after 1. 1 1/2 hours 23.6 30.4 45.3 51.6 2. 4.9 6.0 7.9 9.4 3. " 2.4 3.2 4.6 5.6 4th 1.3 1.8 3.2 4.2 Total (ppm) 32.2 41.4 61.0 70.8

In these examples it was assumed that the acetaldehyde content and other characteristics of the polyester cable were identical in each of Examples 1-4. A detailed examination of the plotted in Fig. 1 and in The values given in Table I show that the acetaldehyde content of the sample is basically made up of three components composed as follows:

(A) Starting acetaldehyde content of the sample of approx. 25 ppm.

(B) Newly formed acetaldehyde due to the 6 hour heating time the thermal degradation of the polyester.

This newly formed acetaldehyde is present in an amount that is approximately proportional the 4 »measurement is and can be calculated on this basis.

(C) Amount of acetaldehyde used during the 6 hour Heating time, especially during the first 1 1/2 hours due to the reaction of Intermediate products are formed which were already contained in the sample used.

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This last-mentioned portion C is decisive for the subject matter of this invention. With the intermediate products it is probably primarily vinyl esters, caused by thermal cleavage of polyester chains.

Surprisingly, we found that these intermediate products react more or less completely, and in fact ^{sufficiently quickly at temperatures above 200 °} C., as is expressed in the formation of acetaldehyde. This can be seen from Fig. 1.

The dashed line in Fig. 1 shows the respective residual content of intermediate compounds, expressed in acetaldehyde. If the polyester is melted again, they react these intermediate products and generate the acetaldehyde amounts specified at the drying temperatures in question. These amounts are added to the amount created by thermal degradation of the polyester.

With increasing treatment temperature from 200 C to z. B. 240 C, undesirable reactions or conditions occur to an increasing extent. This includes:

(A) New formation of acetaldehyde due to progressive thermal and possibly thermal oxidative Degradation;

(B) the tendency of the chips to stick together, what

5 the use of very expensive equipment

required for this procedure;

(C) Growing consumption of materials and energy for heating nitrogen and cleaning and the circulation of the inert gas.

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For this reason, the drying conditions must be chosen so that a maximum amount of intermediate products that can form acetaldehyde react and that the aldehyde formed is removed from the chips during drying. This is the only way to minimize the amount of newly formed aldehyde during the remelting of chips for bottle production. Furthermore, the drying temperature must be chosen so that thermal or thermal-oxidative degradation is largely avoided ₉ since such degradation leads to a deterioration in quality, especially with regard to the coloration of the material.

Example 5

^{Polyester chips with the dimensions 3.1 × 2.5 × 2.1 were heated to 220 °} C. for 6 hours in a cylindrical tube in a stream of nitrogen (1 kg nitrogen / 1 kg polyester).

The acetaldehyde determination was carried out analogously to the method described above, the chips being ^{heated twice to a temperature of 180 °} C. for 11/2 hours under a fresh nitrogen blanket. The acetaldehyde content, based on polyester, was only 1.08 and 0.61 ppm, while untreated chips gave values of 21.8 and 4.8 ppm aldehyde.

Example 6

Example 5 was repeated with the exception that the chips were heated to 200 ° C. in a stream of nitrogen, all in one tube of 10 mm diameter preheated to 270 ° C

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stainless steel - melted as in Examples 1 to 4 and a few seconds later it was spun into a cable 1 mm in diameter. Afterward the acetaldehyde content was determined as described in Example 5. It was 4.2 and 2.2 ppm.

New acetaldehyde had been formed, but in an amount well below the acetaldehyde content of the original schnitzel lay.

In summary, the test results can be summarized as evaluate as follows: It was surprisingly found that within a certain temperature range of the Acetaldehyde content of the final product, d. H. of bottles, is almost constant and minimal.

This minimum content is achieved at temperatures which are higher than those normally used for drying polyester, but at the same time lower than those used in industry for solid polycondensation processes. In the temperature range between 180 and 230 ° C., preferably between 200 and 220 ^° C., there are polycondensation reactions which also have a positive effect on our invention. This additional positive effect consists in an increase in the intrinsic viscosity by 0.05-0.15 intrinsic viscosity units, the exact increase value depending on the temperature, chip size, initial viscosity and residence time. The increase in intrinsic viscosities improves the strength of the material and reduces the tendency to crystallize. Crystallization has a detrimental effect as it can lead to clouding of the end product, ie the bottles.

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The present process is used for the production of PoIy merem that meets the strict rules and regulations of control authorities such . B. FDA complies and is therefore suitable for use in the food industry «

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Claims (5)

EWALD OPPEkMANN 883731 OFFENBACH (MAIN) KAISERSTRASSE 9 TELEPHONE (OSIl) . EWOPAT CABLE 2nd August 1978 Op / ef 1/7703 Zimmer Aktiengesellschaft Borsigallee 1
6000 Frankfurt 60 Expectations 1. A process for the production of linear polyethylene terephthalate plastic for packaging or filling into bottles of foodstuffs intended for human consumption, the foodstuffs coming into direct contact with the plastic, characterized in that chips made of linear polyethylene terephthalate have an intrinsic effect -Viscosity of 0.5-0.85 can be heated to a temperature between about 180 and 230 ° C for a period of about 4-12 hours to reduce the acetaldehyde content to less than 2.5 ppm. 909808/0843 Zimmer Aktiengesellschaft 1/7703 2. The method according to claim 1, characterized in that the chips are heated to a temperature between about 200 and 220 ⁰ C. 3. The method according to claim 1 or 2, characterized in that the chips are heated in the presence of inert gas. 4. The method according to claim 3, characterized in that the inert gas used is nitrogen. 5. The method according to any one of claims 1 to 4, characterized in that the heating time is determined by the size and intrinsic viscosity of the polyethylene terephthalate chips according to the relationship T = 2.74 (D) (IV) ⁰ ^ + 2.41 where T is the heating time, D is the smallest chip size and IV is the intrinsic viscosity of the polymer. 909808/0843