Method for improving stress whitening phenomenon of impact-resistant co-polypropylene

Abstract

The invention relates to the field of preparation of impact-resistant co-polypropylene, in particular to a method for improving stress whitening of impact-resistant co-polypropylene. By optimizing the ethylene content of the rubber phase in the copolymerization process, the compatibility between the matrix and the rubber phase is improved during the production of the impact-resistant co-polypropylene product, so that the stress whitening behavior of the impact-resistant co-polypropylene product is improved, meanwhile, other properties of the product are not reduced, and the impact-resistant co-polypropylene product has excellent mechanical properties in the processing and forming stage. An oriented ellipsoidal rubber phase with a smaller size is formed in an injection molding sample by applying a flow field with proper strength, and the oriented ellipsoidal rubber phase can effectively inhibit the growth of micropores or crazes when the injection molding sample deforms. Besides, after a flow field with proper strength is applied, the size of a rubber phase is remarkably reduced, and when an injection molding sample deforms, the smaller the size of the rubber phase is, the less cavities are generated in the injection molding sample and the less crazes are generated around the injection molding sample, so that the stress whitening behavior of the anti-impact co-polypropylene product is further improved.

Description

Technical Field

[0001] The invention relates to the field of preparation of impact-resistant copolymer polypropylene, in particular to a method for improving stress whitening phenomenon of impact-resistant copolymer polypropylene. Background Art

[0002] Industrially, impact-resistant copolymer polypropylene is mainly produced through a two-step process. The first step is the self-polymerization of propylene monomer. After the self-polymerization is completed, the uninactivated initiator remaining in the propylene homopolymer can still initiate monomer polymerization. The second step is to simultaneously introduce ethylene monomer and propylene monomer into the propylene homopolymer. The uninactivated initiator remaining in the propylene homopolymer in the previous step will initiate the copolymerization of the two monomers to form an EPDM rubber phase. Therefore, impact-resistant copolymer polypropylene is an in-vessel alloy with a good balance of rigidity and toughness. It has been widely used in housings and other parts in the home appliance industry. However, when impact-resistant copolymer polypropylene is used in the production of injection molded parts, such as washing machine housings, when the injection molded part is ejected by the ejector pin, due to local stress concentration, whitening will occur at the contact point with the ejector pin, seriously affecting the product appearance and sales. Therefore, it is very necessary to improve the stress whitening resistance of impact-resistant copolymer polypropylene materials.

[0003] The stress whitening phenomenon that occurs in impact-resistant polypropylene during deformation is caused by the formation of internal voids or crazing with sizes comparable to the wavelength of visible light. When subjected to external stress, tiny voids or crazing may form within the matrix and rubber phase, or at the interface between them. These voids or crazing are particularly prevalent within the rubber phase and at the interface between them. These voids or crazing are similar in size to the wavelength of visible light, scattering visible light and reducing the transparency of the material. Therefore, the better the compatibility between the polypropylene matrix and the rubber phase, and the smaller the rubber phase size, the fewer voids are generated within the rubber phase or at the interface, and the better the stress whitening resistance of 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 matrix molecular weight to bring it closer to the viscosity of the rubber phase. However, reducing the rubber phase content reduces the impact performance of the product, and adjusting the matrix molecular weight affects the melt index, which directly affects the processing flowability of the product in the melt state. Therefore, it is necessary to find a method to improve the compatibility between the polypropylene matrix and the rubber phase and reduce the size of the rubber phase without changing the impact performance and melt flow properties of the product, thereby improving its stress whitening resistance. Summary of the Invention

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

[0005] To achieve the above object, the present invention provides the following solutions:

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

[0007] Step 1, subjecting propylene gas, hydrogen, a main catalyst, and a co-catalyst to a gas-phase homopolymerization reaction to obtain homopolypropylene;

[0008] Step 2, subjecting the homopolypropylene to a gas-phase copolymerization reaction with propylene gas, ethylene gas and hydrogen to obtain a mixed powder of homopolypropylene and ethylene-propylene copolymer (the weight-average molecular weight of the ethylene-propylene copolymer is 300,000-500,000);

[0009] Step 3, after uniformly mixing the mixed powder and the antioxidant, extruding and granulating, 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 content of the rubber phase in the impact-resistant copolymer polypropylene is 10 wt%-20 wt%; the content of ethylene in the rubber phase is 50 wt%.

[0012] If the ethylene content in the rubber phase is lower than 50 wt%, the glass transition temperature of the impact copolymer polypropylene will increase, resulting in a decrease in its low-temperature toughness, which will affect 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 field, failing to significantly reduce its size and effectively suppressing stress whitening. When the injection pressure is above 6 MPa, the rubber phase size decreases further. When its size is below 150 nm, the impact copolymer polypropylene cannot induce cavitation when subjected to impact loads, resulting in a decrease in the material's impact toughness. Therefore, the present invention preferably limits the injection molding pressure to 1 MPa to 4 MPa.

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

[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 the hydrogen gas to the 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] The invention regulates the weight average molecular weight of the ethylene-propylene copolymer by regulating 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 / (ethylene gas+propylene gas) is 0.05-0.35; the mass ratio of hydrogen gas / (hydrogen gas+propylene gas) is 0.005-0.1.

[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-1.7 MPa, temperature 70° C.-85° C., and polymerization time 0.3 h-0.8 h.

[0021] The present invention regulates the content of the rubber phase in the impact-resistant copolymer polypropylene by regulating the polymerization time of the gas phase copolymerization reaction. The longer the polymerization time and the more ethylene-propylene copolymer there is, the higher the rubber phase content is.

[0022] In a preferred embodiment of the present invention, 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 800 ppm. Other antioxidants commonly used in the art, such as 1076 and 626, are also suitable for use in the present invention.

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

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

[0025] The present invention does not impose any particular limitation 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 are set to: a rotational speed of 300 rpm to 500 rpm, and a processing temperature of 180° C. to 220° C. in each zone of the extruder.

[0026] The present invention improves the compatibility between the matrix (homopolypropylene) and the rubber phase by adjusting the ethylene content in the copolymer during the gas phase polymerization stage without changing the matrix molecular weight, rubber phase content, or product melt index. This allows the prepared impact-resistant copolymer polypropylene to significantly improve its blushing resistance while maintaining good mechanical and processing properties. During the injection molding stage, the rubber phase elongates along the flow field direction, reaches a critical aspect ratio, and ultimately breaks apart to form several smaller ellipsoidal rubber phases with their major axes oriented along the flow field direction. These smaller oriented ellipsoidal rubber phases can effectively inhibit the growth of micropores or streaks, further improving the blushing resistance of the impact-resistant copolymer polypropylene.

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

[0028] The method for improving the stress whitening phenomenon of impact-resistant copolymerized polypropylene in the prior art includes chemical modification and physical modification. The commonly used chemical modification method is to reduce the rubber phase content in the copolymerization stage, and sometimes even need to improve the polymerization device or directly replace the polymerization device, resulting in increased costs; physical modification is to blend a portion of polyethylene or covering agent into the impact-resistant copolymerized polypropylene, which will affect the original mechanical properties or performance of the product. Compared with the prior art, the present invention improves the compatibility between the matrix and the rubber phase when producing the impact-resistant copolymerized polypropylene product by optimizing the ethylene content of the rubber phase in the copolymerization process, thereby improving its stress whitening behavior, while not reducing other product properties. The inventive method is applicable to the Spherizone process device in the current prior art and will not increase the transformation cost and energy consumption of the device. In the processing and forming stage, by applying a flow field (injection molding pressure) of suitable intensity, a smaller oriented ellipsoidal rubber phase is formed inside the injection molded sample. These oriented ellipsoidal rubber phases can effectively suppress the growth of micropores or silver streaks when the injection molded sample is deformed. In addition, after applying a flow field of appropriate intensity, the size of the rubber phase is significantly reduced. When the injection molded sample is deformed, the smaller the size of the rubber phase, the fewer the voids produced inside it and the silver streaks produced around it, which further improves the stress whitening behavior of the impact-resistant copolymer polypropylene product.

[0029] The present invention provides a new idea for improving the stress whitening behavior of impact-resistant copolymer polypropylene. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1These are two-dimensional small-angle X-ray diffraction patterns of the impact-resistant copolymer polypropylene injection-molded stretch strips 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 These are SEM images of the cross-sections of the impact-resistant copolymer polypropylene injection-molded tensile strips 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 copolymer polypropylene sample prepared in Comparative Example 1 has obvious stress whitening at the force-bearing point of the ejector pin.

[0034] Figure 4 The injection molded part is obtained by using the impact-resistant copolymer polypropylene sample prepared in Example 2, and no stress whitening occurs at the force-bearing point of the ejector pin. DETAILED DESCRIPTION

[0035] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting 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 terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0037] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0038] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

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

[0040] Unless otherwise specified, the "%" in the present invention refers to mass percentage.

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

[0042] The Spherizone process device is used in the embodiment of the present invention.

[0043] The technical solutions provided by the present invention are 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, a primary catalyst (ZN catalyst), and a co-catalyst (triethylaluminum) were added to a first reactor for gas-phase homopolymerization. The mass ratio of hydrogen to propylene gas was 0.1. Reaction conditions were: reaction pressure of 2.7 MPa, reaction temperature of 70°C, and polymerization time of 0.8 h. A homopolymerized polypropylene powder with a weight-average molecular weight of 250,000 was obtained.

[0046] (2) The homopolypropylene powder prepared in step (1) was added to a second reactor 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 to be: ethylene / (ethylene + propylene) = 0.15, and 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 homopolypropylene and ethylene-propylene copolymer (the weight average molecular weight of the ethylene-propylene copolymer was 450,000) was obtained.

[0047] (3) The mixed powder, antioxidant 168 and antioxidant 1010 are uniformly mixed, extruded through an extruder and granulated by a granulator to obtain an impact-resistant copolymer polypropylene having an ethylene content of 50% in the rubber phase (the rubber phase content in the impact-resistant copolymer polypropylene is 15%); wherein the added concentration of antioxidant 168 is 1000 ppm, and the added concentration of antioxidant 1010 is 500 ppm.

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

[0049] (5) The small-angle X-ray diffractometer is combined with the Linkam stretching stage to detect the change in the void volume fraction of the dumbbell-shaped tensile specimen prepared in step (4) during the stretching process, thereby evaluating the whitening degree of the specimen. The stretching rate of the in-situ stretching experiment is 5 μm / s and the stretching temperature is 25°C. The exposure time of each small-angle X-ray image is 180 s. Figure 1 (a) is the second two-dimensional small-angle image of the in-situ tensile test (strain = 12%). The scattering invariant Q value can measure the volume fraction of the voids in the spline. Figure 1 The two-dimensional small-angle graph is subjected to a sector integration with an angle of 0-90 degrees, and the integral result is the scattering invariant value.

[0050] (6) The impact performance of the impact-resistant copolymer polypropylene impact specimen is tested; the melt index of the impact-resistant copolymer polypropylene is determined according to GB / T3682-2000.

[0051] (7) A dumbbell-shaped specimen of impact-resistant copolymer polypropylene was immersed in liquid nitrogen for 30 min and then fractured along the injection direction. The middle portion of the specimen was taken. The fractured specimen was placed in a 60°C xylene solution for 6 h to etch away the rubber phase. The etched specimen surface was gold-sprayed and then scanned using 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 remaining steps and parameters are the same as those in Example 1.

[0054] Comparative Example 1-2

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

[0056] Table 1 Analysis results of impact copolymer polypropylene process, processing conditions and corresponding whitening related indicators

[0057]

[0058] Figure 1The two-dimensional small-angle X-ray diffraction patterns of the impact-resistant copolymer polypropylene injection-molded stretch strips prepared in Examples 1-2 and Comparative Examples 1-2 after deformation; 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 has obvious meridian direction scattering signals, indicating that it forms voids in the direction perpendicular to the long axis stretching. After the injection pressure is increased from 1MPa to 4MPa, the scattering signal representing the volume fraction of voids in the system in Example 2 is significantly weakened. 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) will cause the system to produce more voids, thereby exhibiting more obvious stress whitening behavior. Comparative Example 2 is the same as Example 1 and Example 2, and the ethylene content in the copolymer is 50%, but Comparative Example 2 does not apply a flow field, and its two-dimensional graph scattering signal intensity is significantly higher than that of Example 1 and Example 2 after applying a flow field.

[0059] Figure 2 The following are SEM images of the cross-section of the impact-resistant copolymer polypropylene injection-molded tensile strips 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 The SEM images of Example 1 and Comparative Example 2 show that when the ethylene content in the rubber phase is the same, the rubber phase particles will be oriented along the flow field direction after applying a flow field of appropriate intensity. Orienting the rubber phase can effectively suppress the growth of micropores or silver streaks. The minor axis size of the rubber particles after orientation is smaller than the rubber particles in Comparative Example 2 without the flow field. Therefore, the scattering invariant value of the sample in Example 1 is lower, and the cavitation and stress whitening behavior are weaker. For Examples 1-2, when the injection pressure is increased from 1MPa to 4MPa, the rubber phase begins to break during elongation along the flow field direction, and the minor axis size of the rubber phase decreases from 0.37μm to 0.18μm. This further illustrates the important role of flow field intensity on the size of the rubber phase particles during the processing and molding process. Comparing the SEM images of 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 size of the rubber phase and reduce the compatibility between the matrix and the rubber phase, making the impact copolymer polypropylene more prone to cavitation and more severe whitening during stretching.

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

[0061] Figure 4The injection molded part obtained by using the impact copolymer polypropylene sample prepared in Example 2 has no stress whitening at the force point of the ejector pin. Figure 3 and Figure 4 It can be seen that under the same injection molding process conditions, the impact copolymer polypropylene with an ethylene content of 50% in the copolymer is more resistant to whitening during the ejection process than the impact copolymer polypropylene with an ethylene content of 56%.

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

Claims

1. A method for improving the stress whitening phenomenon of impact copolymer polypropylene, characterized in that: The following steps are involved: Step 1, subjecting propylene gas, hydrogen, a main catalyst, and a co-catalyst to a gas-phase homopolymerization reaction to obtain homopolypropylene; Step 2, subjecting the homopolypropylene to a gas-phase copolymerization reaction with propylene gas, ethylene gas and hydrogen to obtain a mixed powder of homopolypropylene and ethylene-propylene copolymer; Step 3, after uniformly mixing the mixed powder and the antioxidant, extruding and granulating, to obtain impact-resistant copolymer polypropylene; Step 4, injection molding the impact-resistant copolymer polypropylene; the injection molding pressure is 1MPa to 4MPa; The content of the rubber phase in the impact-resistant copolymer polypropylene is 10 wt%-20 wt%; the content of ethylene in the rubber phase is 50 wt%.

2. The method for improving stress whitening of impact copolymer 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 the hydrogen gas to the propylene gas is 0.07 to 0.

15.

3. The method for improving stress whitening of impact copolymer polypropylene according to claim 1, characterized in that: 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 for improving stress whitening of impact copolymer polypropylene according to claim 1, characterized in that: The weight average molecular weight of the homopolypropylene is 100,000 to 300,000.

5. The method for improving stress whitening of impact copolymer polypropylene according to claim 1, characterized in that: In step 2, the mass ratio of ethylene gas / (ethylene gas+propylene gas) is 0.05-0.35; the mass ratio of hydrogen gas / (hydrogen gas+propylene gas) is 0.005-0.

1.

6. The method for improving stress whitening of impact copolymer polypropylene according to claim 1, characterized in that: The conditions of the gas phase copolymerization reaction are set as follows: reaction pressure 1.0 MPa-1.7 MPa, temperature 70° C.-85° C., and polymerization time 0.3 h-0.8 h.

7. The method for improving stress whitening of impact copolymer polypropylene according to claim 1, characterized in that: 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 500 ppm to 800 ppm.

8. The method for improving stress whitening of impact copolymer polypropylene according to claim 1, characterized in that: The injection molding conditions are set as follows: injection temperature of 180° C. to 230° C., holding temperature of 45° C. to 65° C., holding pressure of 1 MPa to 4 MPa, injection time of 3 seconds to 8 seconds, and holding time of 15 seconds to 35 seconds.

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