A method for improving stress whitening in impact copolymer polypropylene

Abstract

This invention relates to the field of preparing impact-resistant copolymer polypropylene, and particularly to a method for improving the stress whitening phenomenon in impact-resistant copolymer polypropylene. By optimizing the ethylene content of the rubber phase during copolymerization, this invention improves the compatibility between the matrix and the rubber phase in the production of impact-resistant copolymer polypropylene products, thereby improving their stress whitening behavior without reducing other product properties. During the molding stage, applying a flow field of appropriate intensity causes the formation of smaller oriented ellipsoidal rubber phases within the injection-molded sample. These oriented ellipsoidal rubber phases effectively inhibit the growth of micropores or streaks when the injection-molded sample deforms. Furthermore, after applying a flow field of appropriate intensity, the size of the rubber phase is significantly reduced. The smaller the size of the rubber phase during deformation of the injection-molded sample, the fewer voids are generated within it and the fewer streaks are formed around it, further improving the stress whitening behavior of the impact-resistant copolymer polypropylene product.

Description

Technical Field

[0001] This invention relates to the field of preparing impact-resistant copolymer polypropylene, and in particular to a method for improving the stress whitening phenomenon in impact-resistant copolymer polypropylene. Background Technology

[0002] Industrially, impact-resistant copolymer polypropylene is mainly produced through a two-step process. The first step involves the self-polymerization of propylene monomers. After self-polymerization, the residual initiator in the propylene homopolymer can still initiate monomer polymerization. The second step involves simultaneously introducing ethylene and propylene monomers into the propylene homopolymer. The residual initiator from the previous step will then initiate the copolymerization of the two monomers to form an ethylene-propylene rubber phase. Therefore, impact-resistant copolymer polypropylene is an in-reactor alloy with good rigidity-toughness balance and has been widely used in the housings and other components of household appliances. However, when impact-resistant copolymer polypropylene is used in injection molding, such as in washing machine housings, whitening occurs at the contact point with the ejector pin due to localized stress concentration when the injection molded part is ejected, severely affecting the product's appearance and sales. Therefore, improving the stress-whitening resistance of impact-resistant copolymer polypropylene materials is essential.

[0003] The stress whitening phenomenon that occurs in impact-resistant polypropylene during deformation originates from the formation of voids or streaks within its structure, whose size is similar to the wavelength range of visible light. Under external stress, tiny voids or streaks may form between the matrix and the rubber phase, or at their interface, in impact-resistant polypropylene, particularly within the rubber phase and at the interface. These voids or streaks, with sizes similar to the wavelength of visible light, scatter visible light, leading to a decrease in the material's transparency. Therefore, the better the compatibility between the polypropylene matrix and the rubber phase, and the smaller the size of the rubber phase, the fewer voids will form within the rubber phase or at the interface, resulting in better stress whitening resistance in the impact-resistant copolymer polypropylene. In actual production, the compatibility between the polypropylene matrix and the rubber phase, as well as the size of the rubber phase, can be improved by reducing the rubber phase content or adjusting the molecular weight of the matrix to make its viscosity closer to that of the rubber phase. However, reducing the rubber phase content will decrease the product's impact resistance, while adjusting the matrix molecular weight will affect the product's melt index, directly impacting the product's processing fluidity in the molten state. Therefore, it is necessary to find a way to improve the compatibility between the polypropylene matrix and the rubber phase and reduce the size of the rubber phase without changing the product's impact performance and melt flow properties, thereby improving its stress whitening resistance. Summary of the Invention

[0004] Based on the above, this invention provides a method for improving the stress whitening phenomenon of impact-resistant copolymer polypropylene. This invention improves the compatibility between the impact-resistant copolymer polypropylene matrix and the rubber phase and reduces the size of the rubber phase by modifying the polymerization process and processing conditions, thereby improving the stress whitening phenomenon of the impact-resistant copolymer polypropylene.

[0005] To achieve the above objectives, the present invention provides the following solution:

[0006] This invention provides a method for improving the stress whitening phenomenon of impact-resistant copolymer polypropylene, comprising the following steps:

[0007] Step 1: Propylene gas, hydrogen, main catalyst, and co-catalyst are subjected to a gas-phase homopolymerization reaction to obtain homopolymer polypropylene;

[0008] Step 2: The homopolymer polypropylene is subjected to a gas-phase copolymerization reaction with propylene gas, ethylene gas and hydrogen to obtain a mixed powder of homopolymer polypropylene and ethylene-propylene copolymer (the weight average molecular weight of the ethylene-propylene copolymer is 300,000-500,000).

[0009] Step 3: After the mixed powder and antioxidant are mixed evenly, the mixture is extruded and granulated to obtain impact-resistant copolymer polypropylene;

[0010] Step 4: Injection molding the impact-resistant copolymer polypropylene; the injection molding pressure is 1MPa to 4MPa.

[0011] The impact-resistant copolymer polypropylene contains 10wt%-20wt% of the rubber phase; the ethylene content in the rubber phase is 50wt%.

[0012] If the ethylene content in the rubber phase is less than 50 wt%, the glass transition temperature of the impact copolymer polypropylene will increase, leading to a decrease in its low-temperature toughness and affecting the low-temperature impact toughness of the product.

[0013] When the injection pressure is below 1 MPa, the rubber phase only deforms slightly along the flow direction, failing to significantly reduce its size and thus failing to effectively suppress stress whitening. When the injection pressure is above 6 MPa, the rubber phase size further decreases. When its size is below 150 nm, the impact-resistant copolymer polypropylene cannot induce voiding in the rubber phase under impact loads, thereby reducing the material's impact toughness. Therefore, the present invention preferably limits the injection pressure to 1 MPa to 4 MPa.

[0014] In a preferred embodiment of the present invention, the injection pressure of the injection molding is 2MPa to 4MPa; more preferably, it is 3MPa to 4MPa.

[0015] In a preferred embodiment of the present invention, in step 1, the main catalyst is a Zn catalyst; the co-catalyst is triethylaluminum; and the mass ratio of hydrogen to propylene gas is 0.07 to 0.15.

[0016] In a preferred embodiment of the present invention, in step 1, the conditions for the gas-phase homopolymerization reaction are set as follows: reaction pressure of 2.65 MPa to 2.95 MPa, reaction temperature of 65°C to 75°C, and polymerization time of 0.6 h to 1 h.

[0017] This invention controls the weight-average molecular weight of ethylene-propylene copolymer by adjusting the polymerization time.

[0018] In a preferred embodiment of the present invention, the weight-average molecular weight of the ethylene-propylene copolymer is 100,000 to 300,000.

[0019] In a preferred embodiment of the present invention, in step 2, the mass ratio of ethylene gas to (ethylene gas + propylene gas) is 0.05 to 0.35; and the mass ratio of hydrogen to (hydrogen gas + propylene gas) is 0.005 to 0.1.

[0020] In a preferred embodiment of the present invention, the conditions for the gas-phase copolymerization reaction are set as follows: reaction pressure 1.0 MPa to 1.7 MPa, temperature 70°C to 85°C, and polymerization time 0.3 h to 0.8 h.

[0021] This invention regulates the rubber phase content in impact copolymer polypropylene by controlling the polymerization time of the gas-phase copolymerization reaction. The longer the polymerization time and the more ethylene-propylene copolymers there are, the higher the rubber phase content.

[0022] In a preferred embodiment of the present invention, in step 3, the antioxidant includes antioxidant 168 and antioxidant 1010; the content of antioxidant 168 in the mixed powder is 1000 ppm, and the content of antioxidant 1010 is 800 ppm. Other antioxidants commonly used in the art, such as 1076 and 626, are also applicable to the present invention.

[0023] In a preferred embodiment of the present invention, the injection molding conditions are set as follows: injection temperature of 180℃~230℃, holding pressure temperature of 45℃~65℃, holding pressure of 1MPa~4MPa, injection time of 3s~8s, and holding time of 15s~35s.

[0024] The melt index of the impact-resistant copolymer polypropylene prepared by the method of the present invention is 20 g / 10 min to 50 g / 10 min.

[0025] This invention does not impose any special limitations on the parameter settings for extrusion granulation. The extrusion and granulation parameter settings commonly used by those skilled in the art for preparing impact-resistant copolymer polypropylene can be adopted. For example, the parameters of the extruder can be set as follows: rotation speed of 300 rpm to 500 rpm and processing temperature of each zone of the extruder of 180℃ to 220℃.

[0026] This invention improves the compatibility between the matrix (homopolymer polypropylene) and the rubber phase by adjusting the ethylene content in the copolymer during the gas-phase polymerization stage, without altering the molecular weight of the matrix, the rubber phase content, or the melt index of the product. This results in a significantly improved whitening resistance of the prepared impact-resistant copolymer polypropylene while maintaining good mechanical and processing properties. During injection molding, the rubber phase elongates along the flow direction. After reaching the critical aspect ratio, it eventually breaks down to form several smaller ellipsoidal rubber phases oriented along the flow direction. These smaller, oriented ellipsoidal rubber phases effectively inhibit the growth of micropores or streaks, further improving the whitening resistance of the impact-resistant copolymer polypropylene.

[0027] The present invention discloses the following technical effects:

[0028] Existing methods for improving stress whitening in impact-resistant copolymer polypropylene include chemical and physical modification. Common chemical modification methods involve reducing the rubber phase content during copolymerization, sometimes requiring modifications to the polymerization unit or even its replacement, leading to increased costs. Physical modification involves blending polyethylene or a masking agent into the impact-resistant copolymer polypropylene, which can affect the product's original mechanical or performance properties. Compared to existing technologies, this invention optimizes the ethylene content of the rubber phase during copolymerization, improving the compatibility between the matrix and the rubber phase in the production of impact-resistant copolymer polypropylene, thereby improving its stress whitening behavior without reducing other product properties. This method is applicable to existing Spherizone process equipment without increasing equipment modification costs or energy consumption. During the molding stage, applying a suitable flow field (injection pressure) causes the formation of smaller oriented ellipsoidal rubber phases within the injection-molded sample. These oriented ellipsoidal rubber phases effectively inhibit the growth of micropores or streaks when the injection-molded sample deforms. Furthermore, after applying a flow field of appropriate strength, the size of the rubber phase is significantly reduced. When the injection-molded sample undergoes deformation, the smaller the size of the rubber phase, the fewer voids are generated inside it and the fewer silver streaks are generated around it. This further improves the stress whitening behavior of the impact copolymer polypropylene product.

[0029] This invention provides a new approach to improving the stress whitening behavior of impact copolymer polypropylene. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1Two-dimensional small-angle X-ray diffraction patterns of impact-resistant copolymer polypropylene injection-molded tensile specimens after deformation, prepared in Examples 1-2 and Comparative Examples 1-2; wherein, (a) is Example 1, (b) is Example 2, (c) is Comparative Example 1, and (d) is Comparative Example 2.

[0032] Figure 2 SEM images of the cross sections of the impact-resistant copolymer polypropylene injection-molded tensile specimens prepared in Examples 1-2 and Comparative Examples 1-2 after etching; wherein, (a) is Example 1, (b) is Example 2, (c) is Comparative Example 1, and (d) is Comparative Example 2.

[0033] Figure 3 The injection molded part obtained by using the impact-resistant copolymer polypropylene sample prepared in Comparative Example 1 shows obvious stress whitening at the stress point of the ejector pin.

[0034] Figure 4 The injection molded part obtained by using the impact-resistant copolymer polypropylene sample prepared in Example 2 did not show stress whitening at the force point of the ejector pin. Detailed Implementation

[0035] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0036] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0037] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0038] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0039] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0040] Unless otherwise specified, the "%" mentioned in this invention refers to a percentage by mass.

[0041] Unless otherwise specified, the technical solutions described in this invention are all conventional solutions in the field, and the reagents or raw materials used are all purchased from commercial channels or are publicly available unless otherwise specified.

[0042] The Spherizone process apparatus is used in this embodiment of the invention.

[0043] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0044] Example 1

[0045] (1) Propylene gas, hydrogen gas, main catalyst (ZN catalyst), and co-catalyst (triethylaluminum) were added to the first reactor for gas-phase homopolymerization. The mass ratio of hydrogen gas to propylene gas was 0.1. Reaction conditions: reaction pressure was 2.7 MPa, reaction temperature was 70 °C, polymerization time was 0.8 h, and homopolymer polypropylene powder with a weight average molecular weight of 250,000 was obtained.

[0046] (2) The homopolymer polypropylene powder prepared in step (1) was added to the second reactor along with propylene, ethylene, and hydrogen for gas-phase copolymerization. The mass ratio of propylene gas, ethylene gas, and hydrogen in the second reactor was controlled as follows: ethylene / (ethylene + propylene) = 0.15, hydrogen / (hydrogen + propylene) = 0.02. The reaction pressure was 1.4 MPa, the temperature was 75 °C, and the polymerization time was 0.5 h. A mixed powder of homopolymer polypropylene and ethylene-propylene copolymer (the weight average molecular weight of the ethylene-propylene copolymer was 450,000) was obtained.

[0047] (3) After the above mixed powder and antioxidant 168 and antioxidant 1010 are mixed evenly, they are extruded by an extruder and granulated by a granulator to obtain an impact copolymer polypropylene with an ethylene content of 50% in the rubber phase (the rubber phase content in the impact copolymer polypropylene is 15%); wherein, the addition concentration of antioxidant 168 is 1000ppm and the addition concentration of antioxidant 1010 is 500ppm.

[0048] (4) The above-mentioned impact copolymer polypropylene was injection molded into dumbbell-shaped tensile specimens and long strip-shaped impact specimens with V-shaped notches using an injection molding machine. The injection temperature was 220℃, the holding temperature was 60℃, the injection pressure was 1MPa, the holding pressure was 1MPa, the injection time was 5s, and the holding time was 30s.

[0049] (5) By combining a small-angle X-ray diffractometer with a Linkam stretching stage, the change in the void volume fraction of the dumbbell-shaped stretched specimen prepared in step (4) during the stretching process can be detected, thereby evaluating the degree of whitening of the specimen. The stretching rate of the in-situ stretching experiment was 5 μm / s, and the stretching temperature was 25℃. The exposure time for each small-angle X-ray image was 180 s. Figure 1 (a) is the second two-dimensional small-angle plot of the in-situ tensile test (strain = 12%). The scattering invariant Q value can measure the void volume fraction in the spline. Figure 1 The scattering invariant value is obtained by performing a sector integral with an angle of 0-90 degrees on the two-dimensional small-angle plot.

[0050] (6) The impact performance of the impact copolymer polypropylene impact specimens was tested; the melt index of the impact copolymer polypropylene was determined in accordance with GB / T3682-2000.

[0051] (7) The impact-resistant copolymer polypropylene dumbbell-shaped sample was immersed in liquid nitrogen for 30 min, and then fractured along the injection direction to take the middle part. The fractured sample was placed in xylene solution at 60℃ for 6 h to etch away the rubber phase. The surface of the etched sample was sputtered with gold, and then the cross section of the sample was scanned with a scanning electron microscope. The size of the rubber phase was analyzed and statistically analyzed using software.

[0052] Example 2

[0053] The only difference from Example 1 is that the injection pressure in step (4) is changed to 4 MPa; the other steps and parameters are the same as in Example 1.

[0054] Comparative Examples 1-2

[0055] The only difference from Example 1 is that the ethylene content in the rubber phase and the injection pressure are different from those in Example 1; see Table 1 for details.

[0056] Table 1. Analysis Results of Impact-Resistant Copolymer Polypropylene Process, Processing Conditions, and Corresponding Whitening Indicators

[0057]

[0058] Figure 1Two-dimensional small-angle X-ray diffraction patterns of impact-resistant copolymer polypropylene injection-molded tensile specimens after deformation, prepared in Examples 1-2 and Comparative Examples 1-2; wherein, (a) is Example 1, (b) is Example 2, (c) is Comparative Example 1, and (d) is Comparative Example 2. Figure 1 It can be seen that Example 1 exhibits a significant meridional scattering signal, indicating the formation of voids perpendicular to the long axis of the tensile direction. After increasing the injection pressure from 1 MPa to 4 MPa, the scattering signal representing the volume fraction of voids in the system in Example 2 significantly weakens. Although Comparative Example 1 and Example 2 have the same flow field intensity (injection pressure), the scattering signal intensity of Comparative Example 1 is significantly higher than that of Example 2, indicating that increasing the ethylene content in the rubber phase (ethylene-propylene copolymer) leads to more voids in the system, thus exhibiting more pronounced stress whitening behavior. Comparative Example 2 is similar to Examples 1 and 2, with an ethylene content of 50% in the copolymer. However, no flow field was applied to Comparative Example 2, and its two-dimensional scattering signal intensity is significantly higher than that of Examples 1 and 2 after applying a flow field.

[0059] Figure 2 SEM images of the cross-sections of the impact-resistant copolymer polypropylene injection-molded tensile specimens prepared in Examples 1-2 and Comparative Examples 1-2 after etching; wherein, (a) is Example 1, (b) is Example 2, (c) is Comparative Example 1, and (d) is Comparative Example 2. Figure 2 SEM images from Example 1 and Comparative Example 2 show that when the ethylene content in the rubber phase is the same, applying a suitable flow field will cause the rubber phase particles to orient along the flow field direction. Oriented rubber phase can effectively inhibit the growth of micropores or streaks. The minor axis dimension of the oriented rubber particles is smaller than that of the rubber particles in Comparative Example 2 without a flow field. Therefore, the scattering invariant value of the sample in Example 1 is lower, and the voiding and stress whitening behaviors are weaker. For Examples 1-2, when the injection pressure is increased from 1 MPa to 4 MPa, the rubber phase begins to crack during elongation along the flow field direction, and the minor axis dimension of the rubber phase decreases from 0.37 μm to 0.18 μm. This further illustrates the important role of flow field intensity on the size of rubber phase particles during processing and molding. Comparing the SEM images from Example 2 and Comparative Example 1, it can be seen that when the injection pressure is the same, increasing the ethylene content in the ethylene-propylene copolymer will increase the minor axis dimension of the rubber phase and reduce the compatibility between the matrix and the rubber phase, making the impact-resistant copolymer polypropylene more prone to voiding and more severe whitening during stretching.

[0060] Figure 3 The injection molded part obtained by using the impact-resistant copolymer polypropylene sample prepared in Comparative Example 1 shows obvious stress whitening at the stress point of the ejector pin.

[0061] Figure 4Injection-molded parts obtained using the impact-resistant copolymer polypropylene sample prepared in Example 2 did not exhibit stress whitening at the ejector pin stress point. (Comparison) Figure 3 and Figure 4 It can be seen that, under the same injection molding process conditions, the impact copolymer polypropylene with 50% ethylene content in the copolymer is more resistant to whitening than the impact copolymer polypropylene with 56% ethylene content during the ejection process.

[0062] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for improving the stress whitening phenomenon of an impact copolymerized polypropylene, characterized in that, Includes the following steps: Step 1: Propylene gas, hydrogen, main catalyst, and co-catalyst are subjected to a gas-phase homopolymerization reaction to obtain homopolymer polypropylene; Step 2: The homopolymer polypropylene is subjected to a gas-phase copolymerization reaction with propylene gas, ethylene gas and hydrogen to obtain a mixed powder of homopolymer polypropylene and ethylene-propylene copolymer. Step 3: After the mixed powder and antioxidant are mixed evenly, the mixture is extruded and granulated to obtain impact-resistant copolymer polypropylene; Step 4: Injection molding the impact-resistant copolymer polypropylene; the injection molding pressure is 1MPa to 4MPa. The impact-resistant copolymer polypropylene contains 10wt%-20wt% of the rubber phase; the ethylene content in the rubber phase is 50wt%. The conditions for the gas-phase copolymerization reaction were set as follows: reaction pressure 1.0 MPa to 1.7 MPa, temperature 70°C to 85°C, and polymerization time 0.3 h to 0.8 h.

2. The method of improving stress whitening in an impact copolymerized polypropylene according to claim 1, characterized in that, In step 1, the main catalyst is a Zn catalyst; the co-catalyst is triethylaluminum; and the mass ratio of hydrogen to propylene gas is 0.07 to 0.

15.

3. The method of improving stress whitening in an impact copolymerized polypropylene according to claim 1, wherein, In step 1, the conditions for the gas-phase homopolymerization reaction are set as follows: reaction pressure of 2.65 MPa to 2.95 MPa, reaction temperature of 65°C to 75°C, and polymerization time of 0.6 h to 1 h.

4. The method of improving stress whitening in an impact copolymerized polypropylene according to claim 1, wherein, The homopolymer polypropylene has a weight-average molecular weight of 100,000 to 300,000.

5. The method of improving stress whitening in an impact copolymerized polypropylene according to claim 1, wherein, In step 2, the mass ratio of ethylene gas to (ethylene gas + propylene gas) is 0.05 to 0.35; the mass ratio of hydrogen gas to (hydrogen gas + propylene gas) is 0.005 to 0.

1.

6. The method of improving stress whitening in an impact copolymerized polypropylene according to claim 1, wherein, In step 3, the antioxidants include antioxidant 168 and antioxidant 1010; the content of antioxidant 168 in the mixed powder is 1000 ppm, and the content of antioxidant 1010 is 500 ppm to 800 ppm.

7. The method of improving stress whitening in an impact copolymerized polypropylene according to claim 1, wherein The injection molding conditions are set as follows: injection temperature is 180°C to 230°C, holding pressure temperature is 45°C to 65°C, holding pressure is 1MPa to 4MPa, injection time is 3s to 8s, and holding time is 15s to 35s.

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