Method for producing polyethylene resin

Abstract

The present invention provides a method for producing a polyethylene resin in which gel or fish eye formation is greatly reduced, by controlling various reaction conditions such as the supply flow rate ratio of the monomers to each reactor, the temperature range of each reactor, and the temperature range of the mixing region, in the method for producing a polyethylene resin which is applied to produce a polyethylene resin for a protective film of display.

Claims

1. A method for producing a polyethylene resin comprising: conducting radical polymerization while supplying an initiator containing oxygen gas and an organic peroxide, and an ethylene monomer to a first and a second polymerization reactor connected in series with a mixing region disposed between them, wherein the radical polymerization is carried out under polymerization conditions of the following Formulas 1 to 3:
0.4<S/R<0.5  [Formula 1] in Equation 1, R represents a total flow rate of the ethylene monomer supplied to the first and second polymerization reactors, and S represents a flow rate of the ethylene monomer supplied to the first polymerization reactor,
θ1>0.5  [Formula 2] wherein, θ1 represents (a maximum temperature in the first polymerization reactor—a temperature of a mixing region where a polyethylene resin-containing product formed in the first polymerization reactor and a newly introduced ethylene monomer are mixed)/the maximum temperature in the first polymerization reactor,
θ2>0.8  [Formula 3] wherein, θ2 represents (a maximum temperature in the second polymerization reactor—the temperature of the mixing region where the polyethylene resin-containing product formed in the first polymerization reactor and the newly introduced ethylene monomer are mixed)/the temperature of the mixing region.

2. The method for producing a polyethylene resin according to claim 1, wherein the radical polymerization is carried out under reaction conditions further satisfying the following Formula 4:
θ3>0.95  [Formula 4] wherein θ3 represents (the maximum temperature in the first polymerization reactor an inlet temperature of the first polymerization reactor)/the inlet temperature of the first polymerization reactor.

3. The method for producing a polyethylene resin according to claim 1, wherein the radical polymerization is carried out by high-pressure polymerization at a temperature of 150° C. or more under a pressure of 2200 bar or more.

4. The method for producing a polyethylene resin according to claim 1, wherein the flow rate of the ethylene monomer supplied to the first polymerization reactor is 20,000 to 30,000 kg/hr, and a total flow rate of the ethylene monomer and polyethylene resin-containing product supplied to the second polymerization reactor is 48,000 to 62,000 kg/hr.

5. The method for producing a polyethylene resin according to claim 1, wherein the maximum temperature in the first polymerization reactor is 270 to 340° C., and the maximum temperature in the second polymerization reactor is 260 to 320° C.

6. The method for producing a polyethylene resin according to claim 1, wherein the temperature of the mixing region where the polyethylene resin-containing product formed in the first polymerization reactor and the newly introduced ethylene monomer are mixed is 140 to 180° C.

7. The method for producing a polyethylene resin according to claim 2, wherein the inlet temperature of the first polymerization reactor is 140 to 170° C., and an inlet temperature of the second polymerization reactor is 170 to 220° C.

8. The method for producing a polyethylene resin according to claim 1, wherein the polyethylene resin-containing product includes a polyethylene resin, an unreacted ethylene monomer, and an oxidized polyethylene gel, and a number of the oxidized polyethylene gel is less than 10 of the entire polyethylene resin-containing product.

9. The method for producing a polyethylene resin according to claim 8, wherein the unreacted ethylene monomer is re-introduced and reused into the first and second polymerization reactors.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The FIGURE is a scheme illustrating a schematic configuration of a reaction apparatus in which a method for producing a polyethylene resin of one embodiment is carried out.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(2) Hereinafter, preferred examples are presented to facilitate understanding of the present invention. However, the following examples are given for illustrative purposes only, and the scope of the present invention is not intended to be limited to or by these examples.

Examples 1 to 4, Comparative Examples 1 and 2: Preparation of Polyethylene

(3) A polyethylene resin was produced in a reaction apparatus that has a tubular reactor of the first and second reactors, and has a mixing region connected to these first and second reactors in which the polyethylene resin-containing products formed in the respective first and second reactors were mixed.

(4) First, the ethylene monomer compressed through a compressor was preheated to the inlet temperature of each reactor, and then injected into the first and second reactors, respectively. Oxygen gas and organic peroxide were injected together with these monomers were injected into each reactor (substance name: tert-butyl peroxypivalate; please fill in the specific substance name), and the total amount used was oxygen gas: 0.5 kg/h, organic peroxide: 40 L/h.

(5) In the first and second reactors, the flow rate of the ethylene monomer supplied to the first and second reactors, the inlet temperature of the first and second reactors, the maximum temperature in the first and second reactors, and the temperature of the mixing region were controlled as summarized in Table 1 below. The reaction conditions of Formulas 1 to 4 calculated therefrom are summarized and shown together in Table 1 below.

(6) Thereby, radical polymerization was carried out in the first and second reactors, respectively, and the progressing pressure of this polymerization reaction was 2300 bar. Through the control of these reaction conditions, polymerization reaction progressed in each reactor to obtain a product containing unreacted monomer, polyethylene resin and gel. These products were mixed in a mixing region and then discharged. The number of oxidized polyethylene gels (fish eyes) contained in the total product from the discharged products was calculated by a method of forming the film of the polyethylene resin sample and then counting by using an optical detector and a laser analyzer. The results are shown in Table 1 below.

(7) More specifically, using a single screw extruder (Dr. Collin Co. Teachline E20T), the pellet of the product containing the polyethylene resin was produced into a casting film (54 mm*33 m; thickness: 500 μm) for gel analysis at 190° C. for 10 minutes. Except for the edge of the film, the number of gels generated in a 1 m.sup.2 area (about 30 mm*about 33 m) was measured with an optical detector and laser analyzer attached to the extruder. This process was repeated three times, and the average value was taken as the number of gels. At this time, the gel was visually confirmed by the optical detector, while in the laser analyzer, the region where the difference in refractive index from the rest of the film occurred was defined as gel. The number of gels having a particle diameter of 250 μm or more and less than 350 μm was counted.

(8) On the other hand, the unreacted ethylene monomer was separated from the polymerization product by a method of volatilizing the monomer in a high temperature and depressurizing process, and re-introduced and reused into the first and second reactors. The conversion rates of the polyethylene resins obtained through the above reaction process are shown in Table 1 below.

(9) TABLE-US-00001 TABLE 1 Comparative Example Example Process condition 1 2 3 4 1 2 Monomer supply flow 24 25 25 26 29 30 rate (first reactor; ton/h) Monomer supply flow 31 30 30 29 26 25 rate(second reactor; ton/h) Inlet temperature of 154 147 153 155 163 157 first reactor (° C.) Inlet temperature of 193 206 195 190 211 216 second reactor (° C.) Maximum temperature 308 308 306 306 316 311 of first reactor(° C.) Maximum temperature 275 281 284 282 285 272 of second reactor(° C.) Temperature of mixing 148 148 144 144 161 165 region Formula 1 0.44 0.45 0.45 0.47 0.53 0.55 Formula 2(θ1) 0.52 0.52 0.53 0.53 0.49 0.47 Formula 3(θ2) 0.86 0.90 0.97 0.96 0.77 0.65 Formula 4(θ3) 1.0 1.1 1.0 0.98 0.94 0.98 Fish eye(number/g) 3.3 1.8 2.0 2.5 24.3 41.8

(10) Referring to Table 1, it was confirmed that the generation of polyethylene oxide gels (fish eyes) can be minimized in the polymerization method of Examples in which the reaction conditions are optimized.