Method for improving impact strength of bimodal HDPE resin

Abstract

The invention relates to a method for improving the impact strength of bimodal HDPE (High-Density Polyethylene) resin, which is characterized in that a high-boiling-point polar solvent is preferably selected and added into a bimodal HDPE resin solution, so that the interaction between the bimodal HDPE resin and the solvent can be adjusted, and the effective enrichment of multi-branched-chain components in the bimodal HDPE resin is realized. By using the method disclosed by the invention, the impact strength of the bimodal HDPE product can be greatly improved. Compared with methods reported in previous patents or literatures, the method can be used as a supplement of an existing high-impact-strength double-peak HDPE production technology.

Description

Technical Field

[0001] The invention belongs to the technical field of HDPE resins, and in particular relates to a method for improving the impact strength of bimodal HDPE. Background Art

[0002] High-density polyethylene (HDPE) is a type of polymer material made by self-polymerization of ethylene monomer or copolymerization with 1-butene, 1-hexene and other monomers under the action of polymerization catalysts such as titanium, chromium or cyclopentene. It usually has a density of 0.940 g / cm 3 Thanks to its high density and crystallinity, HDPE has the advantages of high mechanical strength and corrosion resistance. It can be made into daily necessities or industrial products through blow molding, extrusion, injection molding, cast film and other processing methods. It has been widely used in pressure pipes, hollow products, building materials, cables and film packaging products.

[0003] In order to further improve the processing performance of HDPE resin when used in products such as films or pipes, a production technology of multi-stage series or parallel connection of reactors is usually used to make the molecular weight of HDPE present a bimodal distribution. Among the mechanical properties of bimodal HDPE resin, impact strength ( ) is an important indicator to measure its toughness, which is defined as the energy absorbed by a unit cross-sectional area when the test sample breaks or fractures under the impact load. In order to improve the toughness or impact resistance of HDPE resin, the usual method is to add a small amount of comonomers such as 1-hexene or improve the distribution of resin molecular chains during the production process of the resin. However, when the copolymerization ability of the polymerization catalyst system used is insufficient, it is difficult to effectively adjust the distribution of comonomers on the HDPE molecular chain, or the molecular weight distribution does not meet expectations, it is easy to have problems with insufficient impact performance of bimodal HDPE, but there are few reports on further improving the impact resistance of bimodal HDPE resins by post-processing. Therefore, in view of the problem that the existing bimodal HDPE resin cannot meet the use requirements due to its impact performance, it is very necessary to develop a method for post-processing bimodal HDPE to improve its impact performance. Summary of the invention

[0004] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a method for improving the impact strength of bimodal HDPE. In order to achieve the above purpose, the present invention first adds a bimodal HDPE resin to a stirring tank 1, adds an organic solvent A to the stirring tank 1, and sets the temperature to T1 so that the resin is completely dissolved. The obtained solution is pumped into a stirring tank 2, and then a certain amount of polar solvent B is added to the stirring tank 2, and the reaction temperature of the stirring tank 2 is controlled to cool to T2, so that the low-branching, low-molecular-weight, easy-to-crystallize component PE resin gradually precipitates. Further, the slurry in the stirring tank 2 is filtered to remove the precipitate, and the clarified liquid is spray-dried to obtain a PE powder, and the tail gas is condensed to collect the solvent and recycle it. Further, the obtained PE powder is purged with an inert gas to remove the residual trace organic solvent, and finally a bimodal HDPE resin product with stronger impact capacity is obtained.

[0005] To achieve the above object, the present invention adopts the following technical solution: A method for improving the impact strength of bimodal HDPE, mainly comprising the following steps: (1) h parts of bimodal HDPE resin are transported to a stirred tank 1 through a pipeline, a parts of solvent A are added thereto, the stirring speed is set to 30-500 r / min, and refluxed at 140-160°C for 10-60 min until the resin is completely dissolved; (2) Pumping the obtained solution into stirring tank 2, introducing b parts of polar solvent B into stirring tank 2, controlling the temperature of the solution in stirring tank 2 to drop to 70-105°C, setting the stirring speed to 20-100 r / min, and controlling the mixing time to 0.5-60 min, so that some PE resins with low branching degree and low molecular weight components are gradually precipitated; (3) filtering the slurry in the stirred tank 2 to remove the precipitate; (4) The clarified liquid obtained in (3) is spray dried to obtain PE powder, and the tail gas is condensed to collect and recycle the solvent.

[0006] (5) Further, the obtained PE powder is purged with an inert gas to remove the residual trace organic solvent, and finally a bimodal HDPE resin product with stronger impact capacity is obtained. The tail gas generated in this step is incorporated into (4) for centralized treatment.

[0007] In the step (1), the amount h of the bimodal HDPE resin is 1-40 parts, preferably 4-35 parts.

[0008] The volume of the solvent A is 50-150 parts, which ensures that the bimodal HDPE resin powder is evenly dispersed in the solvent A.

[0009] The added amount of the solvent B is 1-50 parts, preferably 5-20 parts.

[0010] The solvent A is one or more of toluene, mixed xylene, o-xylene, m-xylene, and p-xylene, preferably o-xylene, m-xylene, or p-xylene.

[0011] The polar solvent B and the solvent A have good mutual solubility under the operating conditions, and the polar solvent B and the solvent A should be prevented from stratifying due to poor compatibility.

[0012] By selecting polar solvent B and adding it to the solution of bimodal HDPE resin, the interaction between polar solvent B and PE resin can be adjusted, and then by combining with cooling crystallization, some PE molecules with low branching degree and strong crystallization ability can be limitedly precipitated. In this way, the effect of enriching some components with small molecular weight and many branches in PE can be achieved to a certain extent, which can better improve the impact resistance of bimodal HDPE resin.

[0013] The solvent B is an alcohol substance with strong polarity and relatively high boiling point, and can be one or a mixture of n-pentanol, isopentanol, tert-pentanol, 1,2-pentanediol, neopentyl glycol, 1,4-pentanediol, 3-methyl-1,5-pentanediol, 1-hexanol, cyclohexanol, 2-ethylhexanol, 1,2-hexanediol, 1,5-hexanediol, 1,6-hexanediol, 1-heptanol, 1,7-heptanediol, 1-octanol, 2-octanol, 3-octanol, 1,2-octanol, 1,8-octanol, 1-nonanol, 1-decanol, 1-undecanol, 2-undecanol, 3-undecanol, etc.

[0014] The temperature of the spray drying process in step (4) can be set at 80-110°C.

[0015] In step (5), the inert gas may be a mixture of one or more of nitrogen, argon and helium, and the temperature of the inert gas is controlled at 60-80°C.

[0016] Cooling crystallization is a method of grading according to the crystallization ability of molecular chains. The relationship between the crystallization temperature of the precipitated components and the wafer thickness during the cooling process is given by the following equation: in, T m is the melting point of the polymer; T c is the crystallization temperature; is the melting point of a crystal with infinite wafer thickness; v is a parameter related to the structure. From this formula, we can see that T c and T mThere is a positive correlation between the melting point and the crystallization temperature, that is, a polymer with a specific melting point can be obtained by adjusting the crystallization temperature. The higher the melting point, the thicker the crystal, and the stronger the crystallization ability of the precipitated crystal. Therefore, the crystal with a larger crystal thickness has the highest crystallization temperature and can be precipitated first during the cooling process.

[0017] By adding polar solvent B to the bimodal HDPE resin solution, the interaction parameters between HDPE and solvent A can be adjusted, and then the PE molecules with low branching degree and strong crystallization ability are preferentially precipitated and separated by cooling crystallization, enriching the multi-branched components in PE to a certain extent, thereby improving the impact resistance of HDPE.

[0018] The significant advantages of the present invention are: In view of the deficiencies of the prior art, the purpose of the present invention is to provide a method for improving the impact strength of bimodal HDPE. By preferably selecting an organic solvent B with a higher boiling point and a stronger polarity and adding it to a bimodal HDPE resin solution, the interaction between the bimodal HDPE resin and the solvent A can be adjusted, and then the low-branching, low-molecular-weight, easily crystallized components in the bimodal HDPE resin are preferentially precipitated and separated, thereby achieving effective enrichment of the multi-branched components in the bimodal HDPE resin. The use of a post-treatment method can greatly improve the impact strength of the product and improve the product quality. Compared with the methods reported in previous patents or literature, the present invention can be used as a supplement to the existing high-impact strength bimodal HDPE production technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 GPC test results of the sample of Example 1; Figure 2 GPC test results of sample 2; Figure 3 GPC test results of sample 3; Figure 4 GPC test results of the samples in Example 4; Figure 5 GPC test results of the sample of Comparative Example 1. DETAILED DESCRIPTION

[0020] The following examples further illustrate the method for improving the impact strength of bimodal HDPE of the present invention. The examples are implemented based on the technical solution of the present invention. Detailed implementation methods, specific operating steps, technical parameters, etc. are given in the examples and comparative examples, but the protection scope of the present invention is not limited to the examples. Unless otherwise specified, the experimental methods in the examples are conventional operations in the art, and the experimental materials or reagents used in the examples can be purchased from the market.

[0021] The molecular weight and molecular weight distribution of PE samples were determined using a Polymer Char high temperature gel permeation chromatograph (GPC-IR). The specific operation was to first weigh about 6 mg of the sample and dissolve it in 15 ml of trichlorobenzene at 160 °C. After it was completely dissolved, about 2 ml of the solution was taken into a sample bottle. The test temperature was 160 °C. The test was carried out in accordance with the provisions of "SH / T 1759 Determination of Molecular Weight Distribution of Solution Polymers by Gel Permeation Chromatography".

[0022] The ZwickRoell HIT 5.5P pendulum impact tester was used to test the impact strength of the injection molded samples. The test was carried out in accordance with the provisions of "GB / T1043 Determination of the impact properties of plastics with a simple supported beam" and the test temperature was 23°C.

[0023] Example 1 A method for improving the impact strength of bimodal HDPE comprises the following steps: (1) 3 parts of bimodal HDPE resin were transferred to stirred tank 1 through a pipeline, 100 parts of p-xylene were added thereto, the stirring speed was set to 50 r / min, and refluxed at 140°C for 10 min until the resin was completely dissolved; (2) The obtained solution was pumped into stirring tank 2, and 5 parts of 1-decanol were introduced into stirring tank 2. The temperature of the solution in stirring tank 2 was controlled to drop to 101°C, the stirring speed was set to 25 r / min, and the mixing time was controlled to 2 min. Part of the PE resin was gradually precipitated; (3) filtering the slurry in the stirred tank 2 to remove the precipitate; (4) The clarified liquid obtained in (3) is spray-dried at 100° C. to obtain PE powder, and the tail gas is condensed to collect and recycle the solvent.

[0024] (5) Further, the obtained PE powder is purged with nitrogen at 65°C to remove the residual trace organic solvent, and finally a bimodal HDPE resin product with stronger impact capacity is obtained. The tail gas generated in this step is incorporated into (4) for centralized treatment.

[0025] The molecular weight distribution diagram of the obtained bimodal HDPE is as follows: Figure 1 The molecular weight, molecular weight distribution index (PDI), kilomethyl number and impact strength test results are shown in Table 1.

[0026] Example 2 A method for improving the impact strength of bimodal HDPE comprises the following steps: The amount of 1-decanol in step (2) of Example 1 was changed to 11 parts, and the rest of the experimental operations were consistent with Example 1. The molecular weight distribution of the obtained bimodal HDPE is as follows: Figure 2The molecular weight, PDI, kilomethyl number and impact strength test results are shown in Table 1.

[0027] Example 3 A method for improving the impact strength of bimodal HDPE comprises the following steps: The solvent A used in step (1) of Example 1 was changed to o-xylene, and the 1-decanol in step (2) was changed to cyclohexanol. The rest of the experimental operations were the same as those in Example 1. The molecular weight distribution of the obtained bimodal HDPE is as follows: Figure 3 The molecular weight, PDI, kilomethyl number and impact strength test results are shown in Table 1.

[0028] Example 4 A method for improving the impact strength of bimodal HDPE comprises the following steps: (1) 5 parts of bimodal HDPE resin were transferred to the stirred tank 1 through a pipeline, and 110 parts of mixed xylene (n 邻二甲苯 / n 对二甲苯 / n 间二甲苯 The molar ratio was about 1 / 1 / 1), the stirring speed was set to 60 r / min, and refluxed at 145°C for 10 min until the resin was completely dissolved; (2) The obtained solution was pumped into stirring tank 2, and 6 parts of 1,2-pentanediol were introduced into stirring tank 2. The temperature of the solution in stirring tank 2 was controlled to drop to 98°C, the stirring speed was set to 20 r / min, the mixing time was controlled to 5 min, and part of the PE resin was gradually precipitated; (3) filtering the slurry in the stirred tank 2 to remove the precipitate; (4) The clarified liquid obtained in (3) is spray-dried at 105°C to obtain PE powder, and the tail gas is condensed to collect and recycle the solvent.

[0029] (5) Further, the obtained PE powder is purged with nitrogen at 60°C to remove the residual trace organic solvent, and finally a bimodal HDPE resin product with stronger impact capacity is obtained. The tail gas generated in this step is incorporated into (4) for centralized treatment.

[0030] The molecular weight distribution of the obtained bimodal HDPE is as follows Figure 4 The molecular weight, PDI, kilomethyl number and impact strength test results are shown in Table 1.

[0031] Comparative Example 1 The molecular weight, molecular weight distribution and impact strength of the untreated bimodal HDPE resin were directly tested. The molecular weight distribution results are as follows: Figure 5 The molecular weight, PDI, kilomethyl number and impact strength test results are shown in Table 1.

[0032] Comparative Example 2 The 1-decanol in step (2) of Example 1 was replaced with methanol, and the rest of the experimental operations were consistent with Example 1. After methanol and solvent A were mixed, the two had poor miscibility and were easily volatile. No precipitate was found after filtering in step (3). The molecular weight, PDI, thousand carbon methyl number, and impact strength test results of the finally obtained bimodal HDPE are shown in Table 1.

[0033] Table 1 Summary of molecular weight and molecular weight distribution, carbon methyl number, and impact strength test results From the test results of Comparative Example 1 in Table 1, it can be seen that for the bimodal HDPE resin without any treatment, its Mw is about 248500 g / mol, the molecular weight distribution index (PDI) is about 18.5, the number of thousand carbon methyl groups is about 12.31, and its molecular weight presents a bimodal distribution state, in which the low molecular weight component is more, which is significantly higher than the content of the high molecular weight component, and the impact strength is only 28.6MPa.

[0034] In Comparative Example 2, when the polar solvent used is methanol, it may be because methanol has a too low boiling point and poor miscibility with solvent A, and is easily volatilized in stirred tank 2, making it difficult to effectively regulate the interaction between the bimodal HDPE resin and solvent A and unable to quickly precipitate the easily crystallized PE component. The resulting bimodal HDPE resin is basically consistent with the starting material in weight average molecular weight, PDI, and thousand carbon methyl number, and the impact strength is basically unchanged from before treatment and has not been improved.

[0035] It can be seen from Examples 1-3 that after the bimodal HDPE resin is subjected to steps such as dissolution and crystallization to remove some of the easily crystallized, low molecular weight components of the PE resin, the relative content of the large molecular weight components of the bimodal HDPE resin is slightly increased, and the number of thousand-carbon methyl groups of the large molecular weight components in the resin is further increased. The impact strength of the resin is improved to varying degrees, with the increases being 37.7%, 70.9% and 93.7% respectively.

[0036] It can be seen from Example 4 and Comparative Example 1 that by further changing the type, amount, operating process conditions, etc. of the solvent used, after the bimodal HDPE resin is treated by dissolution, crystallization, filtration, drying, etc., the weight average molecular weight Mw of HDPE is increased to 305500 g / mol, and the molecular weight distribution index is reduced to about 15.0. At this time, the impact strength is increased to 64.1 MPa, and the increase is as high as about 124.1%.

[0037] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for improving the impact strength of bimodal HDPE resin, characterized in that: First, the bimodal HDPE resin is transported to the stirring tank 1 through a pipeline, and solvent A is added thereto, and the resin is completely dissolved under heating and stirring; then the obtained solution is pumped into the stirring tank 2, and polar solvent B is introduced into the stirring tank 2, and the temperature of the solution in the stirring tank 2 is controlled to be lowered to a certain temperature, so that some PE resins with low branching degree and low molecular weight components are gradually precipitated; then the slurry in the stirring tank 2 is filtered to remove the precipitate, and the obtained clarified liquid is spray-dried to obtain PE powder; finally, the obtained PE powder is purged with an inert gas to remove the residual trace organic solvent, and finally a bimodal HDPE resin product with stronger impact capacity is obtained.

2. The method according to claim 1, characterized in that The bimodal HDPE resin is used in an amount of 1-40 parts, the solvent A is used in an amount of 50-150 parts, and the polar solvent B is used in an amount of 1-50 parts.

3. The method according to claim 1, characterized in that The solvent A is one or more of toluene, mixed xylene, o-xylene, m-xylene and p-xylene.

4. The method according to claim 1, characterized in that: The solvent A is one of o-xylene, m-xylene and p-xylene.

5. The method according to claim 1, characterized in that The polar solvent B is one of n-pentanol, isopentanol, tert-pentanol, 1,2-pentanediol, neopentyl glycol, 1,4-pentanediol, 3-methyl-1,5-pentanediol, 1-hexanol, cyclohexanol, 2-ethylhexanol, 1,2-hexanediol, 1,5-hexanediol, 1,6-hexanediol, 1-heptanol, 1,7-heptanediol, 1-octanol, 2-octanol, 3-octanol, 1,2-octanol, 1,8-octanol, 1-nonanol, 1-decanol, 1-undecanol, 2-undecanol, and 3-undecanol.

6. The method according to claim 1, characterized in that The spray drying process temperature is set at 80-110°C.

7. The method according to claim 1, characterized in that The inert gas is a mixed gas of one or more of nitrogen, argon and helium.

8. The method according to claim 1, characterized in that The temperature of the inert gas is controlled at 60-80°C.

9. The method according to claim 1, characterized in that: The solution in stirred tank 2 was cooled to 70-105°C.

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