Polyethylene moulding composition for coated steel pipes

FIELD: chemistry.

SUBSTANCE: composition with multimodal distribution of molecular weight has density between 0.94 and 0.95 g/cm3 at 23°C and melt flow index (MFI190/5) between 1.2-2.1 dg/min in accordance with ISO 1133. The composition contains 45-55 wt % low molecular weight homopolymer A of ethylene, 30-40 wt % high molecular weight copolymer B of ethylene and another olefin containing 4-8 carbon atoms, and 10-20 wt % ultrahigh molecular weight copolymer C of ethylene and another olefin containing 4-8 carbon atoms. The composition has high processibillity and resistance to mechanical loads and breaking, especially at temperatures below 0°C.

EFFECT: flawless coating for steel pips has mechanical strength properties combined with high hardness.

10 cl, 1 tbl, 1 ex

 

The present invention relates to a polyethylene molding compositions with a multimodal distribution of molar masses, which is particularly suitable for producing protective coatings on pipes, as well as to a method for producing such a molding composition in the presence of a catalytic system containing a catalyst of the Ziegler and socialization, through a multi-stage sequence of reactions involving successive stages of polymerization.

Polyethylene is widely used in industry, in cases where, to ensure a long life time even at elevated temperatures requires a material with high mechanical strength and high resistance to thermal-oxidative degradation. For such applications it is an advantage if the polyethylene moulding composition has a low permeability for water vapor and oxygen, as in this case, the steel tube is then effectively protected from corrosion when in contact with moisture and air. In addition, the special advantages of polyethylene are good chemical resistance, low own weight and the fact that this material is easy to process in the melt.

In WO 97/03139 described based on the polyethylene coating composition having a bimodal distribution of molar masses, suitable for receiving coating the metal pipe and which is covered with the substrate increased resistance against corrosion, oxidative aging, the influence of weather conditions of all types and mechanical stress.

Known plastic molding compositions with unimodal distribution of molar masses are characterized by defects in the region of their processability, resistance to decay when exposed to environmental and mechanical strength. Molding compositions with a bimodal distribution of molar masses represent a technical improvement compared to compositions with unimodal distribution. They are more easily processed and have (as indicated by the density) increased ratio of stiffness/resistance to decay under the action of the environment.

Thus, the present invention was to develop a polyethylene molding composition which retains high technology, but with significant advantages relative resistance to decay under the action of the environment and resistance to mechanical loads, particularly at temperatures below 0°C.

This problem is solved by means mentioned in the beginning of the molding compositions of the General type, the distinctive feature of which is that it contains from 45 to 55 wt.% low molecular weight homopolymer ethylene And 30 to 40 wt.% high molecular weight copolymer of these is Jena and another olefin, containing from 4 to 8 carbon atoms, and from 10 to 20 wt.% ultracytochemical copolymer of ethylene, where all percentages are given relative to the total weight of the molding composition.

Also the invention provides a method of obtaining such molding compositions in the process of cascading suspension polymerization and defect-free coating of steel pipes by means of this composition having excellent properties of mechanical strength in combination with high rigidity.

Polyethylene molding composition according to the invention is characterized by a density in the range from 0.94 to 0.95 g/cm3at 23°C and wide tri-modal distribution of masses. High molecular weight copolymer b contains a proportion of additional units of olefin monomer containing from 4 to 8 carbon atoms, namely from 5 to 8 wt.%. Examples of such comonomers are 1-butene, 1-penten, 1-hexene, 1-octene and 4-methyl-1-penten. Ultrasonometry copolymer of ethylene With a similar image contains one or more of the above-mentioned comonomers in amounts in the range from 7 to 11 wt.%.

In addition, the molding composition according to the invention has a melt flow index in accordance with ISO 1133, expressed as MFI190/5in the range from 1.2 to 2.1 DG/min and a viscosity number VNoverallmeasured in accordance with ISO/R 1191 in on the Kalina at a temperature of 135°C, in the range from 260 to 340 cm3/g, in particular from 280 to 320 cm3/year

Trimodality as a measure of the positions of the centers of gravity of three individual distributions of molar mass can be described by using the viscosity number VN in accordance with ISO/R 1191 polymers formed at successive stages of polymerization. The width of the bands of the polymers formed in the individual stages of the reaction, the following:

The viscosity number VN1measured for the polymer after the first stage polymerization, identical to the viscosity number VNAlow-molecular-weight polyethylene and according to the invention is from 70 to 90 cm3/year

The viscosity number VN2measured for the polymer after the second stage polymerization, does not correspond to VNBrelatively high molecular weight polyethylene formed in the second stage of polymerization, which can only be calculated mathematically, but corresponds to the number of the viscosity of the mixture of polymer a and polymer Century According to the invention VN2is in the range from 150 to 180 cm3/year

The viscosity number VN3measured for the polymer after the third stage polymerization, does not correspond to VNCultracytochemical copolymer, formed at the third stage polymerization, which similarly can only be calculated mathematically, but corresponds to the number wask the particular mixture of the polymer And, polymer and polymer C. According to the invention VN3is in the range from 260 to 340 cm3/g, in particular from 280 to 320 cm3/year

The polyethylene produced by polymerization of monomers in suspension at temperatures in the range from 70 to 90°C., preferably from 80 to 90°C., a pressure in the range of 2 to 10 bar and in the presence of a highly active Ziegler catalyst composed of a transition metal compounds and alumoorganic connection. Polymerization is a three-stage, i.e. it is carried out in three successive stages, with each stage molar mass is regulated by the addition of hydrogen.

In addition to polyethylene polyethylene molding composition according to the invention may further contain additional additives. These include, for example, stabilizers, antioxidants, UV absorbers, light stabilizers, metal decontamination officers, compounds, decomposing peroxide, basic co-stabilizers in the amount of from 0 to 10 wt.%, preferably from 0 to 5 wt.%, as well as fillers, reinforcing materials, plasticizers, lubricants, emulsifiers, pigments, optical brighteners, flame retardants, antistatics, blowing agents, or combinations of these compounds, in a total amount of from 0 to 50 wt.% with respect to the total weight of the mixture.

Moulding composition according to the invention is particularly applicable for the Holocene coatings on metal pipes by extrusion, when the polyethylene moulding composition of the first plastification in the extruder at a temperature in the range from 200 to 250°C., and then extruded through a suitable outlet on the pipe surface and cools down.

Moulding composition according to the invention for obtaining coatings can be treated particularly well by the extrusion process because it is characterized by toughness specimen notched (ISO) in the range from 8 to 14 kJ/m2and resistance to cracking when exposed to the environment (ESCR) in the range of >200 hours

The impact strength of the specimen with notch ISO measured at -30°C in accordance with ISO 179-1/leA/DIN 53453. The sample size is 10×4×80 mm, and it has a V-shaped notch with an angle of 45°, a depth of 2 mm and a radius at the base of a cut of 0.25 mm

Resistance to cracking when exposed to the environment (ESCR) for the moulding compositions according to the invention is determined by measuring the internal state and expressed in hours (h). This laboratory method is described in M.Fleiβner in Kunststoffe 77 (1987), p.45 ff, and complies with ISO/CD 16770, which is currently valid. The report says that there is a relationship between the determination of slow crack growth in creep testing on the test rods with annular notch and the examination of the fragility in the analysis of the internal giving the program according to ISO 1167. Reducing the time to failure is achieved by reducing the time of crack initiation by applying a cut (blade to 1.6 mm) in ethylene glycol as a medium, inducing cracking when exposed to the environment at a temperature of 80°C and the stress at elongation of 3.5 MPa. Getting samples is performed by cutting out three samples of size 10×10×90 mm from the pressed plate thickness of 10 mm On centers of the samples in turn cause cuts when using a blade in the apparatus of self-constructed for this purpose (Figure 5 in publication). The depth of cut of 1.6 mm

Example 1

Polymerization of ethylene was carried out continuous process in three series-connected reactors. In the first reactor download the Ziegler catalyst obtained by the method described in WO 91/18934, example 2, indicated in this application 2.2., in the amount of 0.08 mmol/h, and sufficient environment for suspension (hexane), triethylaluminium as socializaton in the amount of 0.08 mmol/h, ethylene and hydrogen. The number of ethylene (65 kg/h) and the amount of hydrogen (68 g/h) is chosen so that the proportions in the gas space of the first reactor ranged from 25 to 26% vol. ethylene and 65% hydrogen; the remainder is a mixture of nitrogen and evaporating the suspension medium.

The polymerization in the first reactor PR the lead at a temperature of 84°C.

Then the suspension from the first reactor is directed to the second reactor, in which the proportion of hydrogen in the gas space is reduced to 7-9%, and that was added ethylene in the number of 48.1 kg/h and 1-butene in the number 2940 g/h reduction in the number of hydrogen is achieved by intermediate depressurization H2. Concentration in the gas space of the second reactor were 73% vol. for ethylene, 8% vol. for hydrogen and 0.82% for 1-butene; the remainder is a mixture of nitrogen and evaporating the suspension medium. Additionally impose the suspension medium and triethylaluminium.

The polymerization in the second reactor is carried out at a temperature of 83°C.

The slurry from the second reactor is sent to the third reactor through the following intermediate depressurization H2whereby the amount of hydrogen in the gas space of the third reactor is 2.5%vol.

In the third reactor is injected ethylene in the amount of 16.9 kg/h and 1-butene in the amount of 1500 g/h of the gas space of the third reactor proportions are 87% for ethylene, a 2.5% vol. for hydrogen and 1.2% vol. for 1-butene, the remainder is a mixture of nitrogen and evaporating the suspension medium. Additionally impose the suspension medium and triethylaluminium.

The polymerization in the second reactor is carried out at a temperature of 83°C.

Long-term activity of the catalyst of polymerise the AI, necessary for the above-described cascade way of response, provide specially designed Ziegler catalyst, the composition of which is described in the application WO mentioned in the beginning. Measure applicability this catalyst is its unusual extremely high response to hydrogen and its high activity, remains constant over a longer period of from 1 to 8 hours.

The suspension medium is separated from the polymer suspension, leaving the third reactor, the powder is dried and sent to the granulation.

The viscosity number and proportion of WA, WBand WCpolymers a, b and C for polyethylene moulding composition obtained according to example 1 below in table 1.

Example1
WA[wt.%]50
WB[wt.%]37
WC[wt.%]13
VN1[cm3/g]80
VN2[cm3/g]165
VNoverall[cm3/g]304
FNCT [h]220
AFM (-30°C)of 3.8 kJ/m2
ACN (+23°C)13 kJ/m2

Abbreviations physical properties of table 1 have the following meanings:

FNCT - resistance to cracking when exposed to the environment (Full Notch Creep Test, creep testing with full cut), some method of measuring the internal state described M.Fleiβner, [h], conditions: 95°C, 3.5 MPa, water/2% Arkopal.

AFM (-30°C) impact strength of the specimen with notch, determined according to ISO 179-1/leA/DIN 53453 in [kJ/m2] at -30°C.

ACN (+23°C) impact strength of the specimen with notch, determined according to ISO 179-1/leA/DIN 53453 in [kJ/m2] at +23°C.

1. Polyethylene molding composition intended for the manufacture of protective coatings on the pipe, which has a multimodal distribution of molar mass and density at 23°C in the range from 0.94 to 0.95 g/cm3and the value of the melt flow index (MFI190/5), in accordance with ISO 1133 in the range from 1.2 to 2.1 DG/min, and contains from 45 to 55 wt.% low molecular weight homopolymer And ethylene, 30 to 40 wt.% high molecular weight copolymer of ethylene and another olefin containing from 4 to 8 carbon atoms, and from 10 to 20 wt.% ultracytochemical with the polymer of ethylene and another olefin, containing from 4 to 8 carbon atoms, all percentages are given relative to the total weight of the molding composition.

2. Polyethylene molding composition according to claim 1, and high molecular weight copolymer b contains comonomers containing from 4 to 8 carbon atoms, in an amount of from 5 to 8 wt.% in relation to the weight of the copolymer, and ultrasonometry copolymer With ethylene contains comonomers in amounts of from 7 to 11 wt.% in relation to the weight of copolymer C.

3. Polyethylene molding composition according to claim 1, in which the comonomers are 1-butene, 1-penten, 1-hexene, 1-octene, 4-methyl-1-penten or mixtures thereof.

4. Polyethylene molding composition according to claim 1, which has a viscosity number VNoverallmeasured in accordance with ISO/R 1191 in decaline at a temperature of 135°C, in the range from 260 to 340 cm3/g, preferably from 280 to 320 cm3/year

5. Polyethylene molding composition according to any one of claims 1 to 4, which has an impact strength of the specimen with notch AFM (-30°C) in the range from 3.5 to 4.5 kJ/m2and impact strength of the specimen with notch ACN (+23°C) within 12 to 16 kJ/m2and resistance to cracking when exposed to the environment (FNCT) in the range from 150 to 250 h

6. A method of obtaining a polyethylene molding composition according to any one of claims 1 to 5, in which the polymerization of the monomer and the monomer is carried out in suspension at temperatures in the range from 20 to 120°C, pressure in the range of 2 to 10 bar and in the presence of a highly active Ziegler catalyst composed of a transition metal compounds and alyuminiiorganicheskikh connection, and polymerization is a three-stage polymerization, when the molar mass of the polyethylene produced at each stage is regulated in each case by means of hydrogen.

7. The method according to claim 6, in which the concentration of hydrogen in the first stage of polymerization is set so that the viscosity number VN1low molecular weight comopolitan And is in the range from 70 to 90 cm3/year

8. The method according to claim 6, in which the concentration of hydrogen in the second stage of polymerization is set so that the viscosity number VN2a mixture of homopolymer and copolymer In the range from 150 to 180 cm3/year

9. The method according to any of PP-8, in which the concentration of hydrogen in the third stage of polymerization is set so that the viscosity number VN3a mixture of homopolymer And, copolymer, and the copolymer is in the range from 260 to 340 cm3/g, in particular from 280 to 320 cm3/year

10. The use of polyethylene molding composition according to any one of claims 1 to 5 for application of protective coatings on steel pipes and plastic molding composition first plastificator in the extruder when the tempo is Torah in the range from 200 to 250°C, and then ekstragiruyut through the hole on the pipe surface and there cool.



 

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