CN115073797A - Method for optimizing foaming behavior of polypropylene in supercritical fluid through molecular structure regulation and control - Google Patents

Method for optimizing foaming behavior of polypropylene in supercritical fluid through molecular structure regulation and control Download PDF

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CN115073797A
CN115073797A CN202210795491.8A CN202210795491A CN115073797A CN 115073797 A CN115073797 A CN 115073797A CN 202210795491 A CN202210795491 A CN 202210795491A CN 115073797 A CN115073797 A CN 115073797A
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polypropylene
foaming
supercritical fluid
green body
temperature
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龚鹏剑
金碧辉
李光宪
吴炳田
王素真
洪江
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Long Chain Light Material Nanjing Technology Co ltd
Jiangsu Jitri Advanced Polymer Materials Research Institute Co Ltd
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Long Chain Light Material Nanjing Technology Co ltd
Jiangsu Jitri Advanced Polymer Materials Research Institute Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
    • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
    • C08J9/122—Hydrogen, oxygen, CO2, nitrogen or noble gases
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    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
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    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/0061—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof characterized by the use of several polymeric components
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2203/00—Foams characterized by the expanding agent
    • C08J2203/06—CO2, N2 or noble gases
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2203/00—Foams characterized by the expanding agent
    • C08J2203/08—Supercritical fluid
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    • C08J2205/00—Foams characterised by their properties
    • C08J2205/04—Foams characterised by their properties characterised by the foam pores
    • C08J2205/052—Closed cells, i.e. more than 50% of the pores are closed
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
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    • C08J2423/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2423/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
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Abstract

The invention provides a method for optimizing polypropylene foaming behavior in a supercritical fluid through molecular structure regulation, which comprises the following steps: (1) introducing high melt strength polypropylene containing a long branched chain structure into the high crystallinity polypropylene, and pressing and forming to obtain a polypropylene green body; (2) the supercritical fluid is adopted to swell the polypropylene green body and release pressure for foaming, in the swelling process, the long-chain branched structure of the polypropylene with high melt strength in the polypropylene green body can regulate and control the melt strength of the polypropylene green body, and simultaneously can partially destroy the regularity of a polypropylene molecular chain, hinder the free movement of the polypropylene molecular chain, regulate and control the crystallization degree of the polypropylene and widen the melting limit, and a microcrystalline cross-linked network is constructed in the polypropylene green body, so that the foaming temperature interval of the polypropylene in the supercritical fluid is widened, and the cell structure of the polypropylene foaming material is improved. The invention can reduce the temperature control difficulty in the production process of the polypropylene foaming material, and is beneficial to stabilizing and improving the quality of the polypropylene foaming material.

Description

Method for optimizing foaming behavior of polypropylene in supercritical fluid through molecular structure regulation and control
Technical Field
The invention belongs to the technical field of polypropylene foaming, and relates to a method for optimizing polypropylene foaming behavior in a supercritical fluid through molecular structure regulation.
Background
As general-purpose plastics, polypropylene (PP) has higher rigidity and lower glass transition temperature (Tg) than room temperature, so that the mechanical property of the PP is stronger than that of Polyethylene (PE) and the impact resistance of the PP is stronger than that of general-purpose polystyrene (GPPS). And simultaneously, the PP is easy to recover and is an environment-friendly material. The introduction of the cellular structure can endow the PP with the advantages of light weight, high specific strength, high wave permeability, heat preservation and insulation, compressive absorption performance and the like. Therefore, the PP foaming material has application advantages. However, the traditional chemical foaming method is difficult to obtain the foaming material with small pore diameter, large foaming ratio and uniform pore diameter, which is not only unfavorable for improving the mechanical property of the foaming material, but also makes the foaming material unable to exert the advantage of wave transmission. The microcellular foam material with large foaming multiplying power, small aperture and uniform aperture can be prepared by the supercritical fluid foaming process.
PP has a low melt strength due to its linear nature, which is provided entirely by the physically cross-linked network of crystalline domains. However, the very narrow melting limit of PP makes the foaming temperature range very narrow. The foaming effect is completely different only by the difference of the foaming temperature of 0.1-1 ℃: the temperature is 0.1-1 ℃ lower than the optimal foaming temperature, the crystalline region of PP is hardly melted, and CO is enabled to be generated 2 Extremely difficult to penetrate into the amorphous regions of PP; meanwhile, the growth of the foam holes is hindered by the crystal region, so that the size of the foam holes of the foaming material is extremely small, the foaming multiplying power is very low and is only 1-3 times, and the foamed product does not have the performance advantage brought by the high foaming multiplying power; and when the temperature is 0.1-1 ℃ higher than the optimal foaming temperature, the crystalline region of the PP is almost completely melted, so that the strength of the matrix melt is rapidly reduced, the growth of the foam holes is difficult to support, and the problems of foam hole collapse and combination and the like are further caused. Thus, the PP has a suitable foaming temperatureThe interval is very narrow, the foaming temperature interval is usually not more than 1 ℃, the requirement on the temperature control precision in the foaming process is extremely high, and the requirement on the temperature control fault tolerance of equipment is extremely high. The foaming temperature is required to be controlled at a quite stable level in the preparation process, the slight fluctuation of the temperature can obviously influence the appearance of cells of the PP foaming material, the stability of the temperature control even relates to whether PP can be successfully foamed, the stable quality of the PP foaming material is difficult to ensure, and the large-scale production and the preparation of the PP foaming material with high performance in the industry are difficult to realize.
Based on the above technical problems, if the foaming performance of PP in the supercritical fluid foaming process can be improved through the molecular structure design, the foaming temperature range of PP is widened, the cell structure of the foamed material obtained by foaming is improved, and the positive effects are generated for reducing the difficulty of producing the PP foamed material by adopting the supercritical fluid foaming process, stabilizing and improving the quality of the PP foamed material, and promoting the industrial scale-up production of the PP foamed material.
Disclosure of Invention
The invention provides a method for optimizing polypropylene foaming behavior in supercritical fluid through molecular structure regulation and control, aiming at the problems that in the prior art, when a PP foaming material is prepared, the requirement on the control precision of the foaming temperature is high and the large-scale production is difficult to realize in industry because the proper foaming temperature range of PP is very narrow, and the quality stability of the PP foaming material is difficult to ensure.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for optimizing polypropylene foaming behavior in a supercritical fluid through molecular structure regulation and control comprises the following steps:
(1) introducing high melt strength polypropylene into the high crystallinity polypropylene to obtain polypropylene alloy, and performing compression molding on the polypropylene alloy to obtain a polypropylene green body; the high melt strength polypropylene is high melt strength polypropylene containing a long branched chain structure;
(2) and swelling the polypropylene green body by using a supercritical fluid and releasing pressure for foaming, wherein in the swelling process, the long-chain branched structure of the polypropylene with high melt strength in the polypropylene green body can regulate and control the melt strength of the polypropylene green body, can partially destroy the regularity of a polypropylene molecular chain, hinder the free movement of the polypropylene molecular chain, regulate and control the crystallization degree of the polypropylene, widen the melting limit of the polypropylene green body, and construct a microcrystalline cross-linked network in the polypropylene green body, so that the foaming temperature range of the polypropylene in the supercritical fluid is widened, and the cell structure of the polypropylene foaming material prepared by foaming is improved.
In the technical scheme of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation, the raw material high-crystallinity polypropylene adopted in the step (1) is structurally-unmodified polypropylene and comprises isotactic polypropylene, syndiotactic polypropylene and atactic polypropylene; regulating and controlling the melt strength of the polypropylene billet in the step (2), namely improving the melt strength of the polypropylene raw material in the supercritical fluid swelling process by using high-melt-strength polypropylene; the control of the crystallization degree of the polypropylene in the step (2) means that the crystallization degree of the polypropylene raw material in the supercritical fluid swelling process is properly reduced and a microcrystalline cross-linked network is constructed by the high melt strength polypropylene.
In the technical scheme of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation, the content of high-melt-strength polypropylene in the polypropylene blank prepared in the step (1) and the swelling temperature in the step (2) are required to ensure that the melting limit of a DSC (differential scanning calorimetry) secondary heating DSC (differential scanning calorimetry) curve of the polypropylene blank is widened, and meanwhile, the area of a high-temperature melting peak of the DSC curve of the polypropylene foaming material obtained through supercritical fluid foaming in the step (2) is moderate, so that the polypropylene has proper crystallinity to maintain a cellular structure in the swelling process, but the growth of cells is not limited.
Further, the content of the high melt strength polypropylene in the polypropylene green body in the step (1) is related to the crystallinity of the polypropylene raw material, generally, if the crystallinity of the polypropylene raw material is relatively low, the content of the high melt strength polypropylene in the polypropylene green body is lower, and if the crystallinity of the polypropylene raw material is relatively high, the content of the high melt strength polypropylene in the polypropylene green body is higher. In practical application, the content of high melt strength polypropylene in the polypropylene billet can be determined according to the crystallinity of the polypropylene raw material actually adopted, the foaming ratio of the polypropylene foaming material, the mechanical property and other requirements.
Further, if a high-strength, low-dielectric polypropylene foam having an expansion ratio of 20 times or less is desired, the content of the high-melt-strength polypropylene is preferably 10 to 30 wt%, and if a polypropylene foam having an expansion ratio of 20 times or more, high thermal insulation performance and ultra-low dielectric performance is desired, the content of the high-melt-strength polypropylene should be more than 30 wt%, more preferably 55 to 80 wt%, and still more preferably 60 to 70 wt%.
In the step (2) of the technical scheme of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation, one feasible operation of swelling, pressure relief and foaming the polypropylene blank body by using the supercritical fluid is as follows:
placing the polypropylene green body in a high-pressure cavity, introducing gas serving as a physical foaming agent into the high-pressure cavity, controlling the temperature of the high-pressure cavity to be 149-156 ℃, controlling the pressure of the high-pressure cavity to convert the gas serving as the physical foaming agent into a supercritical fluid state, keeping the temperature and the pressure until the physical foaming agent achieves swelling balance in the polypropylene green body, and then decompressing and foaming to obtain the polypropylene foaming material.
Further, in the step (2) of the technical scheme of the method for optimizing the foaming behavior of the polypropylene in the supercritical fluid through molecular structure regulation, the pressure of the high-pressure cavity is controlled to be 7.32-30 MPa.
Further, in the technical solution of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation, the gas as the physical foaming agent can be N 2 、CO 2 Or an inert gas.
Based on the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation, the invention also provides a preparation method of the polypropylene foaming material, which comprises the following steps:
(1) introducing high melt strength polypropylene into the high crystallinity polypropylene to obtain polypropylene alloy, and performing compression molding on the polypropylene alloy to obtain a polypropylene green body; the high melt strength polypropylene is high melt strength polypropylene containing a long branched chain structure;
(2) and swelling and releasing pressure to foam the polypropylene green body by using supercritical fluid to obtain the polypropylene foam material.
Further, the content of high-melt-strength polypropylene in the polypropylene green body prepared in the step (1) of the method and the swelling temperature in the step (2) are ensured to widen the melting limit of a DSC (differential scanning calorimetry) secondary heating) DSC curve of the polypropylene green body, and the area of a high-temperature melting peak of the DSC curve of the polypropylene foaming material obtained by supercritical fluid foaming in the step (2) is moderate. For example, the polypropylene preform in step (1) contains the high melt strength polypropylene in an amount of 30 wt% to 80 wt%, preferably 55 wt% to 80 wt%, and more preferably 60 wt% to 70 wt%.
Further, the operation of swelling and pressure-relief foaming the polypropylene green body by using the supercritical fluid in the step (2) comprises the following steps: placing the polypropylene green body in a high-pressure cavity, introducing gas serving as a physical foaming agent into the high-pressure cavity, controlling the temperature of the high-pressure cavity to be 149-156 ℃, controlling the pressure of the high-pressure cavity to convert the gas serving as the physical foaming agent into a supercritical fluid state, keeping the temperature and the pressure until the physical foaming agent achieves swelling balance in the polypropylene green body, and then decompressing and foaming to obtain the polypropylene foaming material. Preferably, the pressure of the high-pressure cavity is controlled to be 7.32-30 MPa in the step (2); the gas as a physical blowing agent may be N 2 、CO 2 Or an inert gas.
After the method disclosed by the invention is adopted to optimize the foaming behavior of polypropylene in the supercritical fluid, the foaming temperature range of the polypropylene foaming material can be widened from not more than 1 ℃ to about 8 ℃, the prepared polypropylene foaming material has uniform closed-cell structure foam cells, the size and distribution of the foam cell structure are uniform, the highest foaming multiplying power can reach about 30 times, usually 1-30 times, the supercritical fluid foaming is carried out in the temperature range suitable for foaming, the size of the foam cells of the polypropylene foaming material is slowly increased along with the increase of the foaming temperature, and the suitable foaming temperature and the content of the high-melt-strength PP can be selected according to the requirement of the polypropylene foaming material in practical application.
The mechanism for optimizing the foaming behavior of the polypropylene in the supercritical fluid is as follows:
according to the invention, the high melt strength polypropylene with the long chain branch structure is introduced into the high-crystallinity polypropylene raw material, so that on one hand, the melt strength of the polypropylene raw material is improved by utilizing the high viscoelasticity caused by molecular chain entanglement of the high melt strength polypropylene, the melt strength and mechanical property lost by the polypropylene crystallinity damaged by the long chain branch structure are compensated, and the cell structure is optimized to obtain the cells with uniform pore diameter. On the other hand, the regularity of a polypropylene molecular chain is damaged by utilizing a long branched chain structure part contained in the high-melt-strength polypropylene, and the free movement of the polypropylene molecular chain is hindered, so that the crystallization perfection degree of the polypropylene raw material in the swelling process is weakened, the melting limit is widened, and the foaming temperature interval is widened; the physical crosslinked network of the microcrystal, which is constructed by the undamaged crystal region, can still provide certain melt strength in the swelling process and maintain certain cell structure and mechanical property. Meanwhile, the two-phase matrixes of the polypropylene alloy are the same to achieve a complete eutectic state, so that various performance reductions caused by the existence of the interface are avoided. Based on the reasons, the foaming temperature interval of the polypropylene in the supercritical fluid is widened, the cell structure of the polypropylene foaming material prepared by foaming is effectively improved, and the foaming behavior of the polypropylene in the supercritical fluid is optimized.
Compared with the prior art, the technical scheme provided by the invention can produce the following beneficial technical effects:
1. the invention provides a method for optimizing foaming behavior of polypropylene in supercritical fluid through molecular structure regulation, which has the core technical conception that a proper amount of high-melt-strength polypropylene with a long-chain branch structure is introduced into a high-crystallinity polypropylene raw material to improve the melt strength and the crystallization degree of the polypropylene raw material in the swelling process of the supercritical fluid, thereby widening the temperature range suitable for foaming of the polypropylene, and simultaneously reducing the foaming temperature and improving the cell structure of the polypropylene foaming material. The expansion of the foaming temperature interval can greatly reduce the problem that obvious difference occurs in the foam morphology and the foaming multiplying power of the foaming material caused by unstable equipment temperature control in production, can improve the fault tolerance of equipment and reduce the temperature control difficulty, and meanwhile, the foaming temperature moves towards low temperature, so that the use temperature of the equipment can be reduced, the energy consumption is reduced, and the expansion of the foaming temperature interval is very significant for the product quality control in the actual production process. The invention can provide technical support for the large-scale production of the polypropylene foaming material by utilizing the supercritical fluid foaming technology, and promote the industrial process of the supercritical fluid foaming of the polypropylene foaming material.
2. Experiments prove that the foaming temperature range of the polypropylene foaming material can be widened from not more than 1 ℃ to about 8 ℃ by the method, the prepared polypropylene foaming material has uniform closed-cell structure cells, the size and the distribution of the cell structure are uniform, the foaming multiplying power can reach about 30 times, usually between 1 and 30, the supercritical fluid foaming is carried out in the temperature range suitable for foaming, the cell size of the polypropylene foaming material is slowly increased along with the increase of the foaming temperature, the suitable foaming temperature can be selected according to the requirement of the polypropylene foaming material in practical application, and the polypropylene foaming material with narrower cell size distribution range is prepared. The invention can solve the problems of difficult control of foam holes, wide distribution range of the size of the foam holes and lower foaming ratio of the polypropylene foaming material prepared by the existing chemical foaming method, can improve the mechanical property and the dielectric property of the existing polypropylene foaming material and improve the quality of the polypropylene foaming material.
3. The method has simple process and good process controllability, particularly widens the suitable foaming temperature range of the polypropylene, reduces the temperature control difficulty, and is favorable for popularization and application in industrial practice.
Drawings
FIG. 1 is a DSC curve of a series of polypropylene preforms prepared in example 1, wherein the graphs (a), (b) and (c) are a primary temperature rise curve, a secondary temperature rise curve and a primary temperature drop curve, respectively, and PPB, PPB/HMSPP 70/30, PPB/HMSPP 50/50, PPB/HMSPP 30/70 and HMSPP 140 represent polypropylene preforms having HMSPP contents of 0 wt%, 30 wt%, 50 wt%, 70 wt% and 100 wt%, respectively.
FIG. 2 is a DSC curve of a series of polypropylene foams prepared in example 2, wherein the graphs (a) to (e) are DSC curves of polypropylene foams prepared by foaming polypropylene preforms having HMSPP contents of 0 wt%, 30 wt%, 50 wt%, 70 wt% and 100 wt%, respectively, at different temperatures.
FIG. 3 is an SEM image of a series of polypropylene foams prepared in example 2, wherein (a) the image is an SEM image of a foam prepared by foaming a polypropylene green body with an HMSPP content of 0 wt% at 154 ℃, 155 ℃ and 156 ℃ respectively, from left to right; (b) the figure is an SEM picture of the foaming material prepared by respectively foaming a polypropylene blank with the HMSPP content of 30 wt% at 153 ℃, 154 ℃ and 156 ℃ from left to right; (c) the figure is an SEM picture of the foaming material prepared by respectively foaming a polypropylene blank with the HMSPP content of 50 wt% at 152 ℃, 153 ℃ and 154 ℃ from left to right; (d) the figure is an SEM figure of the foaming material prepared by respectively foaming a polypropylene blank with the HMSPP content of 70 wt% at 149 ℃, 150 ℃, 151 ℃ and 152 ℃ from left to right; (e) the figure is an SEM image of the foaming material prepared by respectively foaming a polypropylene blank with the HMSPP content of 70 wt% at 148 ℃, 149 ℃, 150 ℃ and 151 ℃ from left to right.
FIG. 4 shows the results of cell size and cell density tests on a series of polypropylene foams prepared in example 2, wherein the graphs (a) to (e) show the cell size and cell density of the foams prepared by foaming a polypropylene green body having HMSPP contents of 0 wt%, 30 wt%, 50 wt%, 70 wt% and 100 wt%, respectively.
Fig. 5 shows the results of the foaming ratio test of a series of polypropylene foams prepared in example 2, wherein 100% PPB, 70% PPB, 50% PPB, 30% PPB and 0% PPB represent the foaming ratios of polypropylene foams prepared by foaming polypropylene blanks having HMSPP contents of 0 wt%, 30 wt%, 50 wt%, 70 wt% and 100 wt% at different temperatures, respectively.
Detailed Description
The method for optimizing the foaming behavior of polypropylene in supercritical fluid by molecular structure regulation according to the present invention is further illustrated by the following examples. The following described examples are only a part of the embodiments of the present invention, and not all of them. Other embodiments, which can be derived by those skilled in the art from the summary and examples of the invention without creative efforts, are within the protection scope of the present invention.
In each of the following examples, a polypropylene (PP) material having a model number of PPB-M02(J340) and a density of 0.9g/cm was used 3 Melt flow rate at 230 ℃ and 2.16kg load of 2.2g/10min, in the examples and figures below, the polypropylene material is referred to as PPB; the high melt strength polypropylene (HMSPP) used is WB140 with a density of 0.95g/cm 3 Melt index at 230 ℃ and 2.16kg load is 1.8g/10min, and in the examples and figures below, the high melt strength polypropylene is designated as HMSPP or HMSPP 140.
Example 1
In this example, different contents of HMSPP were introduced into the PPB to prepare a polypropylene blend alloy, and the influence of HMSPP content on the melting point and crystallization behavior of the PPB was examined.
And putting the PPB granules and the HMSPP granules into a double-screw extruder, carrying out melt blending at 180 ℃ and carrying out extrusion molding to obtain polypropylene co-mixed gold granules, putting the polypropylene co-mixed gold granules into a vacuum drying oven, and drying at 80 ℃ for 2h to remove moisture. And carrying out hot press molding on the dried polypropylene blended alloy to obtain a flat-plate-shaped polypropylene blank. By adjusting the feeding proportion of the PPB granules and the HMSPP granules, polypropylene blanks with the HMSPP contents of 0 wt%, 30 wt%, 50 wt%, 70 wt% and 100 wt% are prepared.
FIG. 1 is a DSC curve of a series of polypropylene preforms prepared in this example, wherein the graphs (a), (b) and (c) are the primary temperature-rising curve, the secondary temperature-rising curve and the primary temperature-decreasing curve, respectively. From the temperature rise curve, the melting point of the polypropylene preform tends to decrease and the melting limit becomes wider as the HMSPP content increases. The main reason is that the interior of HMSPP molecules has a long-chain branch structure, which destroys the regularity of PPB molecular chains, simultaneously hinders the free movement of PPB molecular chains, and reduces the movement capacity of PPB molecular chains, thereby weakening the crystallization perfection degree of polypropylene. The temperature reduction curve shows that the crystallization temperature of the polypropylene billet is obviously increased along with the increase of the content of the HMSPP, which indicates that the long branched chain structure of the HMSPP can effectively promote crystallization nucleation.
Example 2
In this example, supercritical CO was performed based on a series of polypropylene preforms of different HMSPP content prepared in example 1 2 Foaming, the effect of different contents of HMSPP on the foaming behaviour of the PPB was investigated.
Respectively placing the polypropylene blanks with different HMSPP contents prepared in the example 1 into a high-pressure reaction kettle, and introducing CO serving as a physical foaming agent into the high-pressure reaction kettle 2 Exhausting air from the high-pressure reactor several times, setting the temperature of the high-pressure reactor, and introducing CO into the high-pressure reactor when the temperature of the high-pressure reactor is increased to be close to the set temperature 2 And (3) controlling the pressure of the high-pressure reaction kettle to be 16MPa, controlling the temperature of the high-pressure cavity to be a set temperature and the pressure to be 16MPa, swelling until the foaming agent reaches the swelling balance in the polypropylene green body (the swelling time is about 2 hours), and then quickly releasing pressure and foaming to obtain a series of polypropylene foaming materials.
In this embodiment, for polypropylene blanks with different HMSPP contents, different foaming temperatures are set, specifically as follows:
setting the temperature of a high-pressure reaction kettle to be 150 ℃, 152 ℃, 154 ℃, 155 ℃, 156 ℃ and 157 ℃ respectively for a polypropylene green body with the HMSPP content of 0 wt%;
setting the temperature of the high-pressure reaction kettle to be 150 ℃, 152 ℃, 153 ℃, 154 ℃, 155 ℃ and 156 ℃ respectively for the polypropylene green body with the HMSPP content of 30 wt%;
setting the temperature of a high-pressure reaction kettle to be 150 ℃, 152 ℃, 153 ℃, 154 ℃ and 155 ℃ respectively for a polypropylene green body with the HMSPP content of 50 wt%;
setting the temperature of the high-pressure reaction kettle to be 149 ℃, 151 ℃, 152 ℃ and 153 ℃ respectively for the polypropylene green body with the HMSPP content of 70 wt%;
for a polypropylene preform with a HMSPP content of 100 wt%, the autoclave temperature was set to 148 deg.C, 149 deg.C, 150 deg.C and 151 deg.C, respectively.
FIG. 2 is a DSC curve of a series of polypropylene foams prepared in this example, wherein the graphs (a) to (e) are DSC curves of polypropylene foams prepared by foaming polypropylene preforms having HMSPP contents of 0 wt%, 30 wt%, 50 wt%, 70 wt% and 100 wt% at different temperatures, respectively. As can be seen from fig. 2, with the increase of the foaming temperature, the DSC curve of each polypropylene foam shows a bimodal phenomenon, because the high-temperature foaming process is equivalent to high-temperature annealing, and the crystalline regions of PP are melted and regenerated during the high-temperature foaming process. As the foaming temperature increased, the area of the high temperature peak decreased, indicating that under the corresponding temperature conditions, the crystalline regions of the PPB matrix had mostly melted during the swelling process, with only a small amount of crystalline regions being present. For pure PPB, some second melting peak needs to be maintained to maintain the cell structure, but too large a second melting peak can affect cell growth. As can be seen from the graph (a) in FIG. 2, when pure PPB foams at 154-156 ℃, a DSC curve shows double peaks, but the two peaks are mainly the second melting peak, the excessive second melting peak can limit the growth of cells, and when pure PPB foams only at 157 ℃, the relatively small second melting peak is shown, so that the foaming temperature range of the pure PPB is narrow and difficult to control.
FIG. 3 is an SEM image of a series of polypropylene foams prepared in the present example, wherein (a) is an SEM image of a polypropylene green body with HMSPP content of 0 wt% foamed at 154 deg.C, 155 deg.C and 156 deg.C respectively; (b) the figure is an SEM picture of the foaming material prepared by respectively foaming a polypropylene blank with the HMSPP content of 30 wt% at 153 ℃, 154 ℃ and 156 ℃ from left to right; (c) the figure is an SEM picture of the foaming material prepared by respectively foaming a polypropylene blank with the HMSPP content of 50 wt% at 152 ℃, 153 ℃ and 154 ℃ from left to right; (d) from left to right, the figure is an SEM image of the foamed material prepared by foaming a polypropylene blank with the HMSPP content of 70 wt% at 149 ℃, 150 ℃, 151 ℃ and 152 ℃ respectively; (e) the figure is an SEM image of the foaming material prepared by respectively foaming a polypropylene blank with the HMSPP content of 70 wt% at 148 ℃, 149 ℃, 150 ℃ and 151 ℃ from left to right.
As can be seen from fig. 3, when the content of HMSPP in the polypropylene preform is low, such as shown in (a), (b) and (c) of fig. 3, the long-chain branch of HMSPP has a limited ability to control the cell structure, and the presence of the second melting peak still mainly restricts the cell structure (cell morphology, cell size, foaming ratio, etc.). However, compared with the graph (a), the cell structure in the graph (b) and (c) is improved to some extent, but the foaming temperature interval is still narrow and is only about 2-4 ℃. When the content of HMSPP in the polypropylene green body is further increased, as shown in (d) (e) of fig. 3, the regulation and control capability of the long-chain branch of HMSPP on the cell structure is further improved, the foaming temperature is reduced, the cell structure is more obviously improved, and the foaming temperature range is further widened. This is mainly due to the introduction of more HMSPP, whose long chain branch structure partially destroys the crystalline structure of the PPB, hampers the free movement of the PP molecular chain, and at the same time provides the melt strength needed for foaming.
The results of cell size and cell density tests on a series of polypropylene foams prepared in this example are shown in FIG. 4, wherein the graphs (a) to (e) are the results of cell size and cell density tests on foams prepared by foaming polypropylene blanks having HMSPP contents of 0 wt%, 30 wt%, 50 wt%, 70 wt% and 100 wt%, respectively.
As can be seen from the graphs (b) and (c) of fig. 4, when the HMSPP content in the polypropylene preform is 30 wt% and 50 wt%, the cell diameter of the polypropylene foam is within 1 ℃, (b) 153 ℃ to 154 ℃ of the graph, and (c) 152 ℃ to 153 ℃ of the graph, a tendency of sudden increase occurs, which means that the second melting peak exists in a large amount to almost completely melt within the 1 ℃. This is rather disadvantageous for the control of the foaming temperature interval and it is difficult to obtain continuous cells of a specific cell size. As can be seen from the graphs (d) and (e) in fig. 4, as the content of HMSPP in the polypropylene green body is further increased, the change of the cell diameter of the polypropylene foam material is continuously adjustable, and the foaming temperature range is widened relative to the situations in the graphs (a) to (c) in fig. 4.
The results of the expansion ratio test of a series of polypropylene foams prepared in this example are shown in fig. 5. As can be seen from the combination of FIGS. 3 to 5, the suitable foaming temperature range for pure PPB foam materials is very narrow, and is only close to 2 ℃; at 154 ℃, because a large number of crystal regions exist in the system, the foaming ratio is only 1.74, and the cells are about 5 μm; after the temperature is increased by 2 ℃, the foam holes are enlarged, and the foaming multiplying power is increased to 15; after the temperature is continuously increased, the mixture is in a molten state and cannot be foamed. With the increase of the HMSPP content in the polypropylene green body, the foaming temperature moves to a low temperature, but when the HMSPP content in the polypropylene green body is 30 wt% and 50 wt%, the temperature range suitable for foaming is still in a state of about 2-4 ℃, and when the HMSPP content in the polypropylene green body is increased to 70 wt%, the temperature range suitable for foaming is further widened, and the temperature range suitable for foaming is about 6 ℃.
The expansion of the foaming temperature interval can effectively improve the problem that obvious differences appear in the foam morphology and the foaming multiplying power of the foaming material caused by unstable equipment temperature control in production, can improve the fault tolerance of equipment and reduce the temperature control difficulty, and meanwhile, the foaming temperature moves towards low temperature, so that the equipment use temperature can be reduced, the energy consumption can be reduced, and the expansion of the foaming temperature interval is very significant for the product quality control in the actual production process.
As can be seen from the disclosure of this example, when HMSPP is introduced into PP, the long branched chain structure of HMSPP can provide sufficient melt strength for the polypropylene billet during swelling, and can also control the degree of crystallization of polypropylene during swelling. In the swelling process, when the crystallization degree of the polypropylene is proper, the foaming temperature interval can be widened, the cell structure of the polypropylene foaming material can be improved, and the foaming multiplying power of the foaming material can be easily regulated and controlled; however, if the matrix of the foam material is completely free of the fine crystals, the expansion ratio of the foam material is not easily controlled, and the mechanical strength of the foam material is also reduced. Therefore, for the blended alloy of PPB and HMSPP, when the content of HMSPP is about 70 wt%, for example, the content of HMSPP in a polypropylene blank is 60 wt% to 70 wt%, the foaming temperature process interval can be effectively widened, cells with continuous cell aperture, foaming multiplying power and better cell morphology can be obtained, and the excellent mechanical properties of the foaming material can be endowed.

Claims (6)

1. The method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation is characterized by comprising the following steps:
(1) introducing high-melt-strength polypropylene into the high-crystallinity polypropylene to obtain a polypropylene alloy, and performing compression molding on the polypropylene alloy to obtain a polypropylene blank; the high melt strength polypropylene is high melt strength polypropylene containing a long branched chain structure;
(2) and swelling the polypropylene green body by using a supercritical fluid and releasing pressure for foaming, wherein in the swelling process, the long-chain branched structure of the polypropylene with high melt strength in the polypropylene green body can regulate and control the melt strength of the polypropylene green body, can partially destroy the regularity of a polypropylene molecular chain, hinder the free movement of the polypropylene molecular chain, regulate and control the crystallization degree of the polypropylene, widen the melting limit of the polypropylene green body, and construct a microcrystalline cross-linked network in the polypropylene green body, so that the foaming temperature range of the polypropylene in the supercritical fluid is widened, and the cell structure of the polypropylene foaming material prepared by foaming is improved.
2. The method for optimizing the foaming behavior of polypropylene in a supercritical fluid through molecular structure control according to claim 1, wherein the polypropylene billet in the step (1) contains the high melt strength polypropylene in an amount of 55 wt% to 80 wt%.
3. The method for optimizing the foaming behavior of polypropylene in a supercritical fluid through molecular structure control according to claim 2, wherein the high melt strength polypropylene is contained in the polypropylene body in the step (1) in an amount of 60 wt% to 70 wt%.
4. The method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation and control according to any one of claims 1 to 3, wherein the operation of swelling and pressure-relief foaming of the polypropylene green body by using the supercritical fluid in the step (2) is as follows: placing the polypropylene blank in a high-pressure cavity, introducing gas serving as a physical foaming agent into the high-pressure cavity, controlling the temperature of the high-pressure cavity to be 149-156 ℃, controlling the pressure of the high-pressure cavity to convert the gas serving as the physical foaming agent into a supercritical fluid state, keeping the temperature and the pressure until the physical foaming agent achieves swelling balance in the polypropylene blank, and then releasing pressure for foaming to obtain the polypropylene foaming material.
5. The method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation and control as claimed in claim 4, wherein the pressure of the high pressure cavity in the step (2) is controlled to be 7.32-30 MPa.
6. Method for optimizing the foaming behavior of polypropylene in supercritical fluid through molecular structure control according to claim 4 or 5, wherein the gas as physical foaming agent is N 2 、CO 2 Or an inert gas.
CN202210795491.8A 2022-07-06 2022-07-06 Method for optimizing foaming behavior of polypropylene in supercritical fluid through molecular structure regulation and control Pending CN115073797A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116333423A (en) * 2023-03-03 2023-06-27 广东奔迪新材料科技有限公司 High strength and high toughness polyolefin plastomer expanded bead articles and methods of making the same

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116333423A (en) * 2023-03-03 2023-06-27 广东奔迪新材料科技有限公司 High strength and high toughness polyolefin plastomer expanded bead articles and methods of making the same

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