RU2160284C2 - Ethylene/vinyl acetate copolymer production process - Google Patents

Abstract

FIELD: polymer production. SUBSTANCE: process including feeding monomers into high- pressure reactor and bulk polymerization at 300 C and pressure 120-250 MPa in presence of peroxide initiator dissolved in organic solvent is distinguished by that the latter is vinyl acetate or vinyl acetate/polyglycol mixture containing no more than 15 wt % polyglycol, initiator solution being introduced into ethylene/vinyl acetate mixture before the reaction inlet. Resulting copolymers show improved physico- chemical properties and form products with improved strength characteristics. Film manufactured from these copolymers is more transparent, strong, and resistant to atmospheric effects. EFFECT: reduced consumption of peroxide initiator and reduced flowability of copolymer melt. 6 ex

Description

 The invention relates to the field of chemical industry, in particular, to methods for producing copolymers of ethylene with vinyl acetate by high pressure in the presence of peroxide initiators. Such copolymers are widely used in the production of various types of films, profiles, cable sheaths, an adhesive layer for coating steel pipes, conveyor belts, for coating paper, cardboard, fabrics, foil and in other areas of technology.

Known (USSR Author's Certificate N 1024451. MKI C 08 F 10/02, publ. 07. 12.82) a method for producing copolymers of ethylene with vinyl acetate by copolymerization of these monomers by high pressure in the presence of peroxide initiators fed to the reactor in a solvent consisting of molecular organosiloxane with molecular mass of 50,000-440000 and a cyclic organosiloxane with the number of silicon atoms in a cycle of 3-6 with a mass ratio of 1: 1-1: 9 or in a mixture consisting of polyorganosiloxane with a molecular weight of 5000-440000 and a hydrocarbon solvent with tempera boiling range 28-220 ^o C with their mass ratio of 1: 1-1: 5.

The copolymerization is carried out in an autoclave or tubular reactor at 200-300 ^o C and a pressure of 120-300 MPa. As the peroxide initiator, for example, tert-butyl or lauryl peroxides, tert-butyl perbenzoate are used at an initiator concentration of up to 30% by weight in the solvent.

The copolymers of ethylene with 10-22 wt.% Vinyl acetate are obtained by the described method. The amount of extractable substances in the copolymer is 0.66-0.70 wt. % The specified method for producing copolymers of ethylene with vinyl acetate has the following disadvantages:
1. High flow rate initiator.

 Based on the data given in the description of the copyright certificate N1024451 (examples 5 and 6), the consumption of the initiator-tert-butyl perbenzoate of the copolymer is 0.54-0.55 g per 1 kilogram of copolymer.

2. The high value of the melt flow rate (MFR). So for a copolymer with a content of 10-22 wt.% Vinyl acetate obtained at a pressure of 160 MPa and a temperature of 220 ^o C, the MFI is 18-21 g / 10 min, which significantly limits the scope of its application, in particular from a copolymer with such MFI impossible get durable bulk products, profiles and high-quality film.

 3. Low physical and mechanical properties of the copolymer. So, according to our data, for a copolymer with a content of 10.2 wt.% Vinyl acetate, the tensile strength at break is 9 MPa (see our control example No. 6). According to TU6-05-1636-77, a copolymer of grade 11306-075 with a vinyl acetate content of 10-14 wt.% Should have a value of 9.8 MPa).

Closest to the claimed method according to the set of essential features is a method for producing copolymers according to USSR author's certificate N979371 (MKI C 08 F 10/02, publ. 07.12.82), according to which a mixture of these monomers is fed into a high pressure reactor and copolymerized in bulk at a pressure of 120-250 MPa and a temperature of up to 300 ^o C in the presence of a peroxide initiator fed to the reactor in a neutral dispersed medium consisting of a hydrocarbon solvent and a polyethylene wax with a molecular weight 600-6000, dropping point 64-115 ^o C and 3.5-11.0 double bonds per 1000 hydrocarbon atoms with a mass ratio of wax and hydrocarbon solvent from 1: 3 to 1: 9. The copolymers obtained in accordance with the prototype method (ed. Certificate of the USSR N 979371) have a rather low content of extractable substances, for example, a copolymer of ethylene with 10 wt.% Vinyl acetate contains 0.51 wt.% Extractable (see example 14 of the description prototype), as well as higher physical and mechanical properties: tensile strength for the specified copolymer is 10.1 MPa.

 The disadvantages of the prototype method are the high consumption of the initiator-tert-butyl perbenzoate (0.54 g per 1 kilogram of copolymer) and the high MFI of the obtained product (for an ethylene copolymer with 10 wt.% Vinyl acetate, the MFI is 21 g / 10 min) durable solid products, profiles and high-quality film.

 The technical result, the achievement of which the present invention provides, is to reduce the consumption of peroxide initiator in the production of ethylene vinyl acetate copolymers and to reduce the melt flow index of the resulting copolymer.

The specified technical result is achieved by the fact that in the method for producing copolymers of ethylene with vinyl acetate, comprising feeding a mixture of these monomers into a high-pressure reactor and copolymerizing them in bulk at a pressure of 120-250 MPa and a temperature of up to 300 ^o C in the presence of a peroxide initiator used as a solution in an organic solvent, vinyl acetate is used as the organic solvent, and the solution of the peroxide initiator in vinyl acetate is fed into a mixture of ethylene and vinyl acetate at the inlet of the reactor.

 To date, vinyl acetate has not been used as a peroxide solvent due to the generally accepted opinion that the introduction of peroxide initiators in vinyl acetate or other liquid comonomers will cause their homopolymerization, which, under the conditions of production of ethylene copolymers by high pressure method, will lead to precipitation of the homopolymer in the supply pipelines and their clogging.

 We found that when initiating solutions are supplied not to the reactor, but to a mixture of ethylene and vinyl acetate at the inlet to the reactor, there is practically no homopolymerization of the comonomer. This allows the supply of peroxide solutions in vinyl acetate at the inlet of the reactor in the industrial production of copolymers. The use of solutions of peroxides in vinyl acetate in the process of copolymerization of ethylene with vinyl acetate provides a low content of extractable substances in the copolymer and at the same time leads to an unobvious technical result - a decrease in the consumption of peroxide initiator and a decrease in the MFI value of the copolymer. The possibility of producing copolymers with a reduced MFR value is of great practical importance, since such copolymers have improved physical and mechanical properties and, based on them, products with improved strength characteristics and a high-quality film with increased transparency, strength, and weather resistance can be obtained.

In accordance with the proposed method, copolymerization can be carried out both in a tubular and in an autoclave reactor at temperatures of 200-300 ^o C and pressures of 120-250 MPa in the presence of peroxide initiators such as, for example, tert-butyl peroxide, tert-butyl perbenzoate, lauryl peroxide, etc. It is advisable to use solutions of peroxide initiators with a concentration of 3-20 wt. % When using multi-zone reactors, when a mixture of ethylene with vinyl acetate is fed into the high pressure reactor in several streams, a solution of the peroxide initiator in vinyl acetate is supplied to each stream of ethylene with vinyl acetate at the inlet of the reactor.

 Solutions of peroxides in vinyl acetate have a low viscosity (~ 0.5 cP). In the case when existing pumps cannot pump liquids with such a low viscosity at specific plants, it is advisable to introduce thickeners, for example, synthetic oils of the Laprol or Orits-210 type, into vinyl acetate. The amount of thickener in the solution is determined by the quality of the plunger pair of pumps, but should not exceed 15 wt.%, Since the use of large quantities of oil leads to a deterioration in the quality of the resulting copolymers (see our control example 5).

 According to the proposed method, it is advisable to obtain ethylene copolymers with a vinyl acetate content of up to 40 wt.%.

 The invention is illustrated by the following examples.

 Example 1

The experiment was conducted on a high pressure ethylene copolymerization unit equipped with a tubular reactor with an inner diameter of 6 • 10 ^-3 m and a length of 25 m. A mixture of ethylene with 10.5 wt.% Vinyl acetate was supplied to the reactor with a speed of 50 kg / h and a pressure of 150 MPa .

A 6 wt.% Solution of tert-butyl perbenzoate in vinyl acetate was used as an initiator, which was supplied by a plunger pump to a mixture of ethylene with vinyl acetate in the amount of 40 g / hr before entering the reactor. The maximum copolymerization temperature was 245 ^° C. The unreacted mixture of ethylene and vinyl acetate was separated from the copolymer according to a traditional scheme consisting of high and low pressure recycles. The pressure in the high-pressure recycle system was maintained equal to 220 MPa, in the low-cycle recycle -0.2 MPa. The unreacted mixture of ethylene with vinyl acetate was fed to the inlet of the intermediate and reaction compressors, and the resulting copolymer was fed to a granulator. The copolymer yield was 5 kg / h; the consumption of tert-butyl perbenzoate was 0.48 g / kg of copolymer. The copolymer had the following properties:
- the content of vinyl acetate is 10.9 wt.%;
- melt flow rate - 7 g / 10 min;
- tensile strength - 11.2 MPa (114 kg / cm ² );
- elongation at break - 620%;
- the content of extractable - 0.50 wt.%.

 The content of extractable substances in the copolymer was determined according to GOST 16337-77.

 Example 2

The experiment was carried out on a high pressure ethylene copolymerization unit equipped with a 0.006 m ³ autoclave reactor and a high-speed stirrer. A mixture of ethylene with 10.3 wt.% Vinyl acetate was supplied to the reactor by compressor with a space velocity of 30 kg / hr under a pressure of 180 MPa. A 6 wt.% Solution of tert-butyl perbenzoate in vinyl acetate, which was fed into the reactor in an amount of 28.5 g / h, was used as an initiator. The maximum copolymerization temperature in the autoclave was 250 ^° C. The separation of the obtained copolymer from the unreacted mixture was carried out in accordance with Example 1. The yield of the copolymer was 3.8 kg / h, the consumption of tert-butyl perbenzoate was 0.45 g / kg of copolymer. The product had the following quality characteristics:
the content of vinyl acetate is 10.0 wt.%;
- melt flow rate - 1 g / 10 min;
- tensile strength - 12.2 MPa (124 kg / cm ² );
- elongation at break - 610%;
- the content of extractable - 0.52 wt.%.

 Example 3

The experiment was carried out under the conditions of Example 1, but a mixture of vinyl acetate with 15 wt.% Orites-210 synthetic oil was used as the initiator solvent. The concentration of tert-butyl perbenzoate in the mixture was 5.3 wt. %, solution viscosity (vinyl acetate - glycol oil - tert-butyl perbenzoate) 10 cP. 44 g / hr of an initiating mixture was fed into the reaction stream of ethylene with vinyl acetate, the yield of the copolymer was 4.9 kg / hr, and the initiator flow rate was 0.48 g / kg of the copolymer. The resulting copolymer had the following quality indicators:
- the content of vinyl acetate is 10.8 wt.%;
- melt flow rate - 7.3 g / 10 min;
- tensile strength - 10.8 MPa (110 kg / cm ² );
- elongation at break - 610%.

 - the content of extractable - 0.53 wt.%.

 Example 4

A 600 kg / h ethylene mixture with 13 wt.% Vinyl acetate, under a pressure of 145 MPa, was fed into a tubular reactor with an internal diameter of 0.034 m and a length of 320 m. A solution with a viscosity of 4 cP and containing 3 wt.% Tert-butyl perbenzoate, 7 wt.% Orits-210 oil and the rest was vinyl acetate was used as the initiating mixture. The feed of the initiating mixture was 9 kg / h, the maximum copolymerization temperature was 245 ^° C. The copolymer yield was 562 kg / h, the flow rate of the initiator was 0.48 g / h. The resulting copolymer had the following quality indicators:
- the content of vinyl acetate is 13.9 wt.%;
- melt flow rate - 9 g / 10 min;
- tensile strength - 9.9 MPa (114 kg / cm ² );
- elongation at break - 650%.

 - the content of extractable - 0.52 wt.%.

 Example 5 (control).

 The experiment was carried out under the conditions of example 1, but using a mixture of vinyl acetate with 20 wt.% Orits-210 oil as an initiator solvent. The initiating solution (tert-butyl perbenzoate - vinyl acetate - Orits-210) had a viscosity of 20 cP; the solution was fed into the reactor at 45 g / h. The copolymer yield was 4.7 kg / h; the consumption of tert-butyl perbenzoate was 0.54 g / kg of copolymer.

The resulting copolymer had the following quality indicators:
- the content of vinyl acetate is 10.9 wt.%;
- melt flow rate - 10 g / 10 min;
- tensile strength - 9.8 MPa (100 kg / cm ² );
- elongation at break - 600%.

 - the content of ecotrained - 0.56 wt.%.

 Example 6 (similar).

The experiment was carried out under the conditions of example 1, but as a solvent of tert-butyl perbenzoate, a mixture consisting of polymethylsiloxane with a molecular weight of 150,000 and white spirit was used in a mass ratio of 1: 1. The concentration of tert-butyl perbenzoate in the solution was 3 wt.%, The viscosity of the solution at 50 ^° C. was 6 cP. The initiating solution was supplied to the beginning of the reactor in an amount of 84 g / h. The initiator consumption in the experiment was 0.55 g / kg of copolymer. Received 4.6 kg / h of a copolymer of ethylene with vinyl acetate with the following quality characteristics:
- the content of vinyl acetate is 10.9 wt.%;
- melt flow rate - 14 g / 10 min;
- tensile strength - 9.0 MPa (92 kg / cm ² );
- elongation at break - 610%.

 - the content of extractable - 0.65 wt.%.

 Example 6 (prototype).

The experiment was carried out under the conditions of example 1, but a dispersion of unoxidized polyethylene wax with a molecular weight of 2000 in white spirit was used as a solvent of tert-butyl perbenzoate. The dispersion was prepared in accordance with example 1 of the description of the prototype, the content of the components in the initiating composition is the following, wt.%:
tert-butyl perbenzoate - 3.0
polyethylene wax - 20.0
white spirit - 77.0
The initiator consumption in the experiment was 0.56 g / kg of copolymer.

Received a copolymer of ethylene with vinyl acetate with the following quality characteristics:
- the content of vinyl acetate is 10.9 wt.%;
- melt flow rate - 14 g / 10 min;
- tensile strength - 9.8 MPa (100 kg / cm ² );
- elongation at break - 630%.

 - the content of extractable - 0.56 wt.%.

 As follows from the above examples, the use of vinyl acetate peroxides as a solvent makes it possible to reduce the initiator consumption by 11–12 wt.% When producing copolymers of ethylene with vinyl acetate, which in large-scale production facilities (modern production capacities amount to several tens of thousands tons of copolymers per year) significant economic value. The possibility of obtaining copolymers with reduced MFI values is of great practical importance, since such copolymers have improved physical and mechanical properties and, based on them, products with improved strength characteristics and a high-quality film with increased transparency, strength, and weather resistance can be obtained. At the same time, other qualitative indicators of the copolymers (tensile strength, elongation at break, content of extractables) do not deteriorate.

Claims (1)

A method of producing ethylene vinyl acetate copolymers, comprising feeding a mixture of these monomers into a high pressure reactor and copolymerizing them in bulk at a pressure of 120 - 250 MPa and a temperature of up to 300 ^o C in the presence of a peroxide initiator used as a solution in an organic solvent, characterized in that vinyl acetate is used as an organic solvent, and a solution of peroxide initiator in vinyl acetate is fed into a mixture of ethylene and vinyl acetate at the inlet of the reactor.