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CN-121975050-A - High-environmental stress cracking resistance polyolefin resin for rotational molding storage tank and preparation method thereof

CN121975050ACN 121975050 ACN121975050 ACN 121975050ACN-121975050-A

Abstract

The invention discloses a high environmental stress cracking resistance polyolefin resin for a rotational molding storage tank and a preparation method thereof, and relates to the field of polyethylene resin preparation. The polyolefin resin has a weight average molecular weight of 6-10 x 10 4 g/mol, a molecular weight distribution index of 5.5-8.5, a density of 0.937-0.948 g/cm 3 , a crystallinity of 49-60%, a melting point of 125-129 ℃, a melt flow rate of 3.5-7.5 g/10min, an environmental stress cracking resistance of not less than 1000 h, and a tensile strength of 21-28 MPa, and is prepared by copolymerizing ethylene with a C4-C8 alpha-olefin comonomer under the action of a catalyst, and regulating the molecular weight by adding hydrogen. The polymer developed by the invention is suitable for rotational molding processing technology, realizes high-strength mechanical property and excellent environmental stress cracking resistance, and effectively solves the problem that the existing polyolefin resin is easy to crack and even explode in storage tank application.

Inventors

  • LI WEI
  • LI NUO
  • DI YUTAO
  • SHI XUANYU
  • WANG JINGDAI
  • YANG YONGRONG

Assignees

  • 浙江大学

Dates

Publication Date
20260505
Application Date
20260130

Claims (8)

  1. 1. A highly environmentally resistant, crack resistant polyolefin resin for rotomolded storage tanks, characterized in that the polyolefin resin is a copolymer of ethylene and an α -olefin, wherein the α -olefin is selected from at least one of C4-C8 α -olefins and has a mole fraction in the polymer structural units of 1.0% to 5.0%, the copolymer having a tie molecule formation probability of greater than 3.2 and simultaneously satisfying the following properties: Weight average molecular weight of 6-10×10 4 g/mol; the molecular weight distribution index is 5.5-8.5; the density is 0.937-0.948 g/cm3; The crystallinity is 49-60%; melting point of 125-129 ℃; 190. melt flow rate at C.at 2.16 kg load conditions is 3.5-7.5 g/10min.
  2. 2. The polyolefin resin according to claim 1, wherein it has an environmental stress crack resistance time of not less than 1000 hours when tested at 50 ℃ according to GB/T1842-2008 standard.
  3. 3. The polyolefin resin according to claim 1, wherein the tensile speed is 50 mm/min and the tensile strength is 21-28 MPa, as measured according to GB/T1040.2-2022 standard, using type 1A test specimens.
  4. 4. A process for producing the polyolefin resin according to any one of claims 1 to 3, characterized by employing a gas-liquid-held polymerization process in a reactor, which comprises: firstly, injecting an inert alkane condensing agent into the reactor, then sequentially injecting an alpha-olefin comonomer, a cocatalyst and a main catalyst, finally introducing ethylene, and controlling reaction conditions to form a liquid-holding polymerization environment in which a gas-solid two-phase region and a gas-liquid-solid three-phase region coexist in the reactor; The injection amount of the condensate enables the reaction system to be in a liquid-holding mode, namely, a part of the condensate is held on the surfaces of polymer particles in a liquid phase form to form wet particles for slurry polymerization, and a part of the condensate exists in a gas phase form to form dry particles for gas phase polymerization; in this state, the polymer particles circulate between the gas-solid two-phase region and the gas-liquid-solid three-phase region, so that the active catalyst center alternately undergoes gas-phase and liquid-phase polymerization microenvironments; The composition and the injection amount of the condensate meet the conditions that the reaction temperature is 70-85 ℃ and the reaction pressure is 1.8-2.2 MPa, the condensate rate of the condensate in the reactor is 15-50% through simulation calculation by a CHAO-SEA physical property method, and the condensate rate is defined as the proportion of the mass of the condensate condensed and attached to polymer particles in a liquid phase form to the mass of the total injected condensate.
  5. 5. The method of claim 4, wherein the operating temperature of the gas-liquid-solid three-phase zone is more than 10 ℃ lower than the operating temperature of the gas-solid two-phase zone, and wherein the gas-liquid-solid three-phase zone is located in the lower portion of the reactor and the gas-solid two-phase zone is located in the upper portion of the reactor.
  6. 6. The process according to claim 4, wherein the process is carried out in the presence of a Ziegler-Natta catalyst system, the conditions of the copolymerization being such that the partial pressure of hydrogen is from 0.15 to 0.25 MPa and the molar feed ratio of alpha-olefin comonomer to ethylene is from 0.05:1 to 0.30:1.
  7. 7. The method according to claim 4, wherein the inert alkane condensing agent is n-hexane and the injection amount is 50-200 mL/liter of reaction volume, and the mass ratio of n-hexane to alpha-olefin in the condensed liquid is 1:1-10:1.
  8. 8. The method according to claim 4, wherein the Ziegler-Natta catalyst system comprises a main catalyst and a cocatalyst, the cocatalyst is triethylaluminum, and the catalyst is added by adding the triethylaluminum cocatalyst into the reaction system, mixing thoroughly, and adding the main catalyst.

Description

High-environmental stress cracking resistance polyolefin resin for rotational molding storage tank and preparation method thereof Technical Field The invention belongs to the technical field of high polymer material synthesis, and particularly relates to a high environmental stress cracking resistance polyolefin resin for a rotational molding storage tank and a preparation method thereof. Background Polyethylene resin is widely applied to manufacturing of rotational molding storage tanks due to excellent flexibility, low temperature resistance and chemical stability (acid and alkali resistance), and is used for containing mediums such as water, chemical liquid and the like. However, high strength/high modulus polyethylene tanks present a serious risk of Environmental Stress Cracking (ESC) over prolonged use, leading to tank cracking (commonly known as "blow out"). If corrosive liquid is contained, the explosion tank can cause liquid leakage, so that safety accidents such as soil pollution, personal injury and the like are caused. The nature of Environmental Stress Cracking (ESC) is brittle fracture of semi-crystalline polyethylene under the synergistic action of polar solvent and continuous low stress, firstly, the applied or residual stress generates microscopic stress concentration at the interface of the wafer and amorphous area to induce micro-voids, then, surfactant or organic solvent is permeated to reduce the surface energy of crack tip to promote slow expansion of crack along the amorphous area between the wafers, under the continuous stress, the number of tie-molecules (tie-molecules) connecting adjacent wafers is gradually reduced due to relaxation and untwisting, so that the stress transmission capability is continuously weakened, when the residual connecting molecules are insufficient to maintain the integral structure between the wafers, the molecular chains are broken, the crack is penetrated rapidly, and the material finally shows brittle failure without obvious plastic deformation. This process is the "crack growth slowness" (SCG) mechanism. According to the Slow Crack Growth (SCG) model, environmental Stress Cracking (ESC) of polyethylene is essentially due to failure of tie molecules connecting adjacent wafers, which is embodied by slipping of tie molecules from crystalline regions under stress and disentanglement of amorphous regions under the action of penetrating solvents. The ribbon molecules act as critical structures for the wafer attachment, and their content and distribution are determining factors in the ability of the material to resist ESCs. The lacing molecules consume the fracture energy by effectively transmitting and dispersing the localized stresses at the crack tip, forming a dense network that impedes the penetration of the solvent into the amorphous region, thereby slowing crack propagation, and requiring higher energy to disentangle, thereby significantly improving the Environmental Stress Crack Resistance (ESCR) of the material. Thus, a core strategy to enhance ESCR performance is to maximize lacing molecule content. The ESCR performance of the polymer is comprehensively influenced by factors such as molecular weight, molecular weight distribution, branching degree, crystallinity and the like, the molecular weight is improved to help increase the length of a lacing molecular chain and enhance the capability of the lacing molecular chain penetrating through a wafer, under the condition of similar molecular weight, the wider molecular weight distribution is favorable for simultaneously forming high molecular weight chain segments and low molecular weight chain segments, the density of connecting points among the wafers is increased, so that the number of lacing molecules is increased, the content of short branched chains is increased, the thickness of the wafer is limited, the probability of forming the lacing molecules is improved, and the disentanglement resistance of amorphous molecular chains is enhanced. The polyolefin resin for the rotational molding storage tank ensures the slow crack growth resistance of the material and also has the requirements on the processing technology of rotational molding, namely the processing requirements on the resin, namely the moderate molecular weight (melt index 2-8g/10 min). While the low molecular weight polymer has excellent processability, the thermal stability and strength are low, while the high molecular weight polymer has higher strength and excellent ESCR performance, but the low molecular weight polymer has poor fluidity and high viscosity, and is difficult to plasticize and form. However, the prior art has difficulty in combining high cracking resistance with rotational molding processing requirements. For example, chinese patent CN 116057083A discloses a method for preparing polyethylene resin. The polymer has a melt flow rate MFR of 0.4 to 2.0 g/10 min measured at 190 ℃ under a load of 2.16 kg and an Environmental Stre