DE2617412C2 - Process for the production of copolymers of ethylene - Google Patents

Description

The invention relates to a method of manufacture of copolymers of ethylene containing up to 30 percent by weight of vinyl acetate in polymerized form, by copolymerization of ethylene and vinyl acetate in a reaction zone at pressures above 800 atm, temperatures between 150 and 350 ° C and average residence times up to a maximum of 100 Seconds, the reaction mixture obtained after the polymerization after leaving the reaction zone through a cooling zone in which a pressure below 500 atm and a temperature between 150 and 250 prevail, in the direct to the cooling zone subsequent high pressure product separation zone (A) is passed, and wherein the mean residence time of the Reaction product in the cooling zone and the high pressure product separation zone (A) is 2 to 10 minutes.

In such a process, which is analogous, for example in DE-OS 19 62 254, one obtains Ethylene-vinyl acetate copolymers that contain only small amounts of free acetic acid.

It has been found, however, that especially when the reactor is running for a long time in apparatus which the reactor downstream, the formation of inhomogeneous proportions in the ethylene polymer cannot be ruled out is. So it is not possible according to known methods, with running times of the reactor of more than 400 hours to obtain a completely homogeneous polymer that is not affected by discoloration.

The present invention is based on the object of the polymerization process described at the outset using a cooling zone so that you can even after long running times of the Reactor predominantly receives products with very good homogeneity.

The object is achieved according to the invention in that the high pressure product separation zone (A) leaving the reaction mixture through a low pressure product separation zone (B), in which pressures below 10 atm prevail, is passed into a discharge extruder (C), B and C directly with one another without a shut-off device are connected and the fill level of the reaction mixture in B is adjusted by the discharge of C.

The temperature in the cooling zone is preferably 200 to 235 ° C.

In a preferred process, the fill level of the reaction mixture in the low-pressure product separation zone fluctuates (B) by a maximum of ± 3%.

After the known copolymerization of ethylene with vinyl acetate leaves the reaction zone

jo the reaction mixture the reaction zone with a temperature which is generally above 285 ° C, and enters a jacket pipe, where it is heated to a temperature between 150 and 250 ° C, preferably 200 and 235 ° C, is cooled. This process step is under

r> simultaneous relaxation of the reaction mixture Pressures made below 500 atm. The cooled reaction mixture is then poured into a High pressure product separation zone (A), also called high pressure separator, passed, in which the same pressure

4 »prevails as in the cooling zone in front of the Inside the jacket pipe, namely below 500 atm. In this separation zone, in which there is also a temperature is between 150 and 250 ° C, preferably 200 and 235 ° C, is that formed in the reaction tube

4 ^r > polymer separated from the unpolymerized monomers. The mean residence time of subsequent to the polymerization reaction mixture obtained after cooling down to a temperature between 150 and 25O ⁰ C is 2 to 10 minutes. It is crucial

ίο that the resulting polymer after leaving the Reaction zone is brought to the temperature below 250 ° C as quickly as possible. To this end is between the polymerization zone and high pressure product separation zone (A) the cooling zone, consisting of one Jacketed pipe, arranged. It is also essential that there is now one in the high pressure product separation zone (A) a constant temperature between 150 and 150 over all areas of the product which remains for 2 to 10 minutes 250 ° C prevails. The temperature in the high pressure

For this purpose, product separation zone (A) is preferably kept constant at ± 2 ° C. From the high-pressure product separation zone (A) the copolymer, which still contains smaller amounts of monomers, is passed into a downstream low-pressure product separation zone (B), also known as the low-pressure separator, in which pressures below 10 atm prevail. From there the copolymer is fed into a discharge extruder (C).
Under the direct connection of B and C

Without a shut-off device, the coupling of B and C is understood without interruption, so that none Any shut-off device designed to stop or inhibit the flow of the reaction mixture from B to C There is therefore no shut-off device between B and C to regulate the level of the reaction product in B used, but the filling level is determined exclusively by the discharge speed of the extruder C. The discharge speed of the extruder C is regulated by the speed of the screw of the extruder C ι ο The level of the reaction mixture in B, the so-called filling level, can therefore be determined by the speed of the screw regulated and preferably kept constant to ± 3%.

It is an advantage of the process according to the invention that the reactor can run for a long time predominantly receives products with very good homogeneity and thus film quality

The melt index of the ethylene-vinyl acetate copolymers according to the invention is 0.1 to 50 g / 10 min (ASTM-D-1238-65Tbei 190 ⁰ C and 2.16 kg). The copolymers can contain up to 30 percent by weight of vinyl acetate in copolymerized form. The copolymers obtained are particularly suitable for the production of films: 5

The parts given in the following examples are parts by weight.

example

A tube reactor is supplied with a mixture compressed to 2100 atm, which consists of 10,000 parts of ethylene, 1,250 parts of vinyl acetate and 14 moles, per 1 million moles of ethylene, of oxygen. The heat transfer means of the reactor jacket is maintained during the polymerization at a temperature of 190 "C. The reaction mixture obtained by the liberated heat of reaction in the reaction zone a peak temperature of 305 ⁰ C. The average residence time of the reaction mixture in the Reaktiunszone is 50 seconds.

The polymer is in a cooled central tube under a pressure of 300 atm to a temperature cooled from 235 ° C and into a high pressure separator (A), which is connected downstream of the jacket pipe initiated The temperature in the high pressure separator

(A) is held constant at ± 2 ° C; the mean residence time in the cooling zone and in the high pressure separator (A) is 10 minutes.

The reaction mixture leaving the high pressure separator (A) arrives in the low pressure separator (B), in which there is a pressure of approx. 1 atm

The low-pressure separator (B) of the polymerization apparatus is connected directly to the discharge extruder (C) without a shut-off device. The stand in the separator

(B) is controlled with the speed of the extruder (C) and kept constant.

An ethylene copolymer is obtained which contains 12.4 percent by weight of vinyl acetate in copolymerized form. The content of free acetic acid is only 0.012 percent by weight (measured on freshly granulated product). The ethylene-vinyl acetate copolymer has a melt index (190 ° C / 2.16 kg) of 4.2 g / 10 minutes, a density of 0.9328 g / cm ³ (measured according to DIN 53 479 / 7.2), has a good homogeneity and has good film properties.

The quality of the copolymers obtained with increasing running time of the reactor is given in Table I. shown:

Table I.

Term of the

Reactor

(Hours.)

Quality of the copolymer

Homogeneity Number of specks

(0.5-1mm)

(1-2 mm)

St.

(> 2 mm)

Discoloration

Brittle lcil

100

200

400

600

800

1200

1600

> 2000

equally good

<20
<20
<20
<20
<20
<20
<20
<20
<20

no
no
no
no
no
no
no
no
no

0
0
0
0
0
0
0
0
0

Comparative experiment

The procedure is as described in the example, but the low-pressure separator (B) is the polymer apparatus via a 5 m long product line with a shut-off device to regulate the level in the Low pressure separator (B) connected to the extruder (C).

The homogeneity of the copolymers obtained, prepared according to the comparative experiment, deteriorates the longer the reactor runtime, as shown in Table II.

Table II Quality of the copolymer Number of specks II III Discoloration Brittle portion Term of the homogeneity I. (1-2 mm) (> 2 mm) Reactor
(SId.) (0.5 mm) <1 <0, l <20 <1 <0, l no 0 Well <20 <1 <0, l no 0 100 Well <20 1-2 0.2 no 0 200 Well 20-25 sporadically isolated 400 good-medium 3 0.3 yellowish occurring 600 25th 4th 0.3 the same the same middle 25th 6th 0.5 the same the same 800 middle 43 the same isolated 1000 medium-bad 10 2-3 until regularly 1200 50 yellowish regularly bad > 12 2-3 big amount of 1600 > 60 yellowish the same bad > 2000

The homogeneity of the ethylene copolymers indicated in the example and in the comparative experiment was determined on foils with a thickness of 40 μm.

The films were produced by adding the ethylene-vinyl acetate copolymer at a melt temperature at 150 ° C. The take-off speed is 6.5 m / min, the neck length 250 Tim, das Inflation ratio 1: 2. The extruded film tube passes through an optical device for continuous Measurement of inhomogeneities in the film. From the continuous double-layed film, the one Width of 23 cm, an area of 5 cm is recorded during the measurement. Any inhomogeneity (specks or Lump in the foil) generates an electrical impulse.

The number of impulses, which corresponds to the number of specks, is added up over a measuring time of, for example, 400 seconds (corresponding to a measured film area of 4.3 m ² ). By setting the sensitivity of the measurement differently

jo can split the inhomogeneities into 3 groups be, namely

I Speck size from 0.5 to 1 mm

II Speck size from 1 to 2 mm
j5 III speck size larger than 2 mm.

The results are summarized in Tables I and II.

Claims (2)

Patent claims: 1. Process for the production of copolymers of ethylene containing up to 30% by weight of vinyl acetate contain polymerized, through copolymerization of ethylene and vinyl acetate in a reaction zone at pressures above 800 atm, temperatures between 150 and 350 ° C and average residence times up to a maximum of 100 seconds, with the ι ο Following the polymerization reaction mixture obtained after leaving the reaction zone through a cooling zone in which a pressure below 500 atm and a temperature between 150 and 250 ° C prevails, in the high pressure product separation zone (A) immediately following the cooling zone is passed and wherein the mean residence time of the reaction product in the cooling zone and in the High pressure product separation zone (A) is 2 to 10 minutes, characterized in that the the reaction mixture leaving the high pressure product separation zone (A) through a low pressure product separation zone (B), in which pressures below 10 atm prevail, are passed into a discharge extruder (C) is, where B and C are directly connected to each other without a shut-off device and the fill level of the Reaction mixture in B is adjusted by the discharge of C. 2. The method according to claim 1, characterized in that the fill level of the reaction mixture in B fluctuates by a maximum of ± 3%.