CN113444315B

CN113444315B - Low-forming-pressure polypropylene foamed product and preparation method thereof

Info

Publication number: CN113444315B
Application number: CN202010216280.5A
Authority: CN
Inventors: 蓝小琴; 郑文革; 吴飞; 沈斌; 罗海斌; 崇云凯
Original assignee: Ningbo Institute of Material Technology and Engineering of CAS
Current assignee: Ningbo Institute of Material Technology and Engineering of CAS
Priority date: 2020-03-25
Filing date: 2020-03-25
Publication date: 2022-02-25
Anticipated expiration: 2040-03-25
Also published as: CN113444315A

Abstract

The invention relates to a low-forming-pressure polypropylene foamed product and a preparation method thereof, wherein the preparation method comprises extrusion granulation, heating and pressurizing foaming, pressure loading and molding forming. The low-molding-pressure polypropylene foamed product prepared by the invention has the tensile strength of more than or equal to 0.75MPa and the density of less than or equal to 0.11g/cm³The foam has uniform foam cell structure, excellent mechanical property, controllable foaming density and high foaming multiplying power.

Description

Low-forming-pressure polypropylene foamed product and preparation method thereof

Technical Field

The invention relates to the technical field of foamed products, in particular to a polypropylene foamed product with low forming pressure and a preparation method thereof.

Background

The Polypropylene foaming product (EPP) has certain mechanical property, sound insulation property, green and easy degradation, and the like, thereby being widely applied to the fields of automobiles, packaging, buildings, sound insulation, heat insulation and the like.

The polypropylene foamed article having higher mechanical properties also has a higher melting point, which requires higher steam pressure during molding. For example, a polypropylene foamed product having a melting point of 150 ℃ usually requires a molding pressure of 0.35MPa or more, which exceeds the upper limit of the pressure resistance of the conventional molding machine, and a special high-pressure molding device is required for the molding machine, so that the cost is high.

In addition, various additives in the preparation process affect the particle properties of the polypropylene foamed product, such as surface blocking, cell collapse, and the like, so that the cell structure of the polypropylene foamed product is not uniform.

Disclosure of Invention

In view of the above, there is a need to provide a low-forming-pressure polypropylene foamed product and a preparation method thereof, wherein the low-forming-pressure polypropylene foamed product prepared by the preparation method has a uniform cell structure and excellent mechanical properties, and the molding forming pressure is low, so that the preparation cost can be effectively reduced.

According to a first aspect of the present invention, there is provided a method for producing a polypropylene foamed article of low molding pressure, comprising the steps of:

mixing a first resin, a second resin and a foaming auxiliary agent, and extruding and granulating to obtain blended beads, wherein the first resin comprises a polypropylene resin, the melting point of the first resin is 140-165 ℃, and the melting point of the second resin is 15-45 ℃ lower than that of the first resin;

mixing the blended beads, a dispersing agent, a foaming agent and a dispersing medium, and heating and pressurizing to prepare foamed beads;

carrying out pressure loading treatment on the foaming beads to prepare prefabricated beads;

supplying steam to the preformed beads, and molding the preformed beads by the steam to obtain the low molding pressure polypropylene foamed article,

wherein the pressure of the steam is 0.12MPa to 0.25MPa, and the dispersing agent comprises an organic dispersing agent and an inorganic dispersing agent.

In one embodiment, the organic dispersant comprises at least one of stearic acid monoglyceride, vinyl bis stearamide, calcium stearate and sodium benzenesulfonate, and the inorganic dispersant comprises at least one of nanoclay, montmorillonite, kaolin and alkali metal phosphate.

In one embodiment, the mass ratio of the organic dispersant to the inorganic dispersant is (0.3-2): 1.

In one embodiment, the material of the first resin includes at least one of a polypropylene-based resin homopolymer, a first polypropylene random copolymer, and a polypropylene block copolymer;

and/or the material of the second resin comprises at least one of a second polypropylene random copolymer, low-density polyethylene, low-density linear polyethylene, ethylene-octene copolymer and propylene-ethylene copolymer.

In one embodiment, the foaming aid comprises at least one of an antioxidant, a stabilizer, an anti-ultraviolet agent, a nucleating agent, a lubricant, and a color masterbatch;

and/or the foaming agent comprises at least one of carbon dioxide, nitrogen, propane, butane and pentane.

In one embodiment, the mass ratio of the first resin, the second resin and the foaming auxiliary agent is 100 (40-100) to 1-12.

In one embodiment, the mass ratio of the blended beads, the dispersing agent, the foaming agent and the dispersing medium is 100 (2-5): 10-20): 300-500.

In one embodiment, a first pressure and a first temperature are used in the pressure heating treatment, wherein the first pressure is 1.8MPa to 7.0MPa, and the first temperature is 130 ℃ to 160 ℃.

In one embodiment, the pressure loading treatment uses a second pressure, the second pressure is 0.3 to 0.8MPa, and the time of the pressure loading treatment is 6 to 12 hours.

According to a second aspect of the present invention, there is provided a low molding pressure polypropylene foamed article, which is prepared by the preparation method of any one of the above, and has a tensile strength of 0.75MPa or more and a density of 0.11g/cm or less³。

Compared with the prior art, the preparation method has the beneficial effects that:

firstly, the prefabricated bead prepared by the first resin with a higher melting point and the second resin with a lower melting point is difficult to deform in a molding process, and can keep higher uniformity of cells, and the surface of the prefabricated bead can be fused and bonded under lower steam pressure, so that the steam pressure in the molding process is greatly reduced, a traditional molding machine can meet the preparation requirement, and extra cost is not needed.

Secondly, the organic dispersant and the inorganic dispersant are compounded for use, the inorganic dispersant can improve the surface tension of a dispersion medium and provide a mechanical isolation effect, and the organic dispersant can form a protective barrier effect on the foaming beads, so that the foaming beads form a uniform system in the dispersion medium, the foaming beads can form a uniform cell structure under the conditions of heating and pressurizing, the surfaces of the foaming beads are not bonded, agglomerated and not collapsed, and the foaming of the resin with higher melting point difference is ensured under lower forming pressure.

Thirdly, the prepared beads after the heating, pressurizing, foaming and pressure-carrying treatment of the blended beads have higher foaming multiplying power and higher uniformity of foam holes, and the surfaces of the foam holes are smooth, regular and round, thereby being beneficial to forming in the molding forming process.

The low-molding-pressure polypropylene foamed product prepared by the invention has the tensile strength of more than or equal to 0.75MPa and the density of less than or equal to 0.11g/cm³The foam has uniform foam cell structure, excellent mechanical property, controllable foaming density and high foaming multiplying power.

Drawings

FIG. 1 is a flow chart of a method for preparing a low molding pressure polypropylene foamed article according to one embodiment of the present invention;

fig. 2 is a scanning electron micrograph of a polypropylene foamed article with a low molding pressure according to embodiment 1 of the present invention;

fig. 3 is a DSC scan graph of a low molding pressure polypropylene foamed article provided in embodiment 1 of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

The terms "first," "second," and the like herein are used for distinguishing between different objects and not necessarily for describing a particular order. Furthermore, the terms "including" and "having," as well as any variations thereof, are intended to cover non-exclusive inclusions.

The foaming temperature of the resin is generally close to the melting point of the resin, and the preparation of a foamed product with high rigidity usually requires a higher melting point corresponding to the resin, but if the melting point of the resin is too high, higher steam pressure and larger steam quantity are required to realize melt bonding in the molding process. The applicant has long and intensively studied to find that the vapor pressure during molding can be reduced by adding a resin having a low melting point. However, if the difference between the melting points of the low-melting-point resin and the polypropylene resin is small, the vapor pressure in the molding process still cannot be effectively reduced, the vapor pressure is usually above 0.25MPa, and if the melting point of the low-melting-point resin is too low, the mixed resin is very easy to deform in the molding process, the uniformity of the cell structure cannot be maintained, so that the foaming performance of the polypropylene foamed product with low molding pressure cannot be ensured, and the problems of surface adhesion, cell collapse and the like can occur.

In order to prepare a low-molding-pressure polypropylene foamed product with a uniform cell structure and excellent mechanical properties by using a lower molding pressure, referring to fig. 1, the present invention provides a low-molding-pressure polypropylene foamed product and a preparation method thereof.

Specifically, the preparation method comprises the following steps:

step S1: mixing a first resin, a second resin and a foaming auxiliary agent, extruding and granulating to prepare a blended bead, wherein the first resin comprises a polypropylene resin, the melting point of the first resin is 140-165 ℃, and the melting point of the second resin is 15-45 ℃ lower than that of the first resin;

step S2: mixing the blended beads, a dispersing agent, a foaming agent and a dispersion medium, pressurizing and heating at a first pressure and a first temperature, and then decompressing to prepare foamed beads;

step S3: providing compressed gas to the expanded beads, and carrying pressure on the expanded beads by the compressed gas at a second pressure to prepare prefabricated beads;

step S4: supplying steam to the preformed beads, and molding the preformed beads by the steam to obtain the low molding pressure polypropylene foamed article.

Wherein the vapor pressure is 0.12 MPa-0.25 MPa, and the dispersant comprises an organic dispersant and an inorganic dispersant.

Because the melting range of the first resin and the second resin adopted by extrusion granulation is large, the particles are easy to fuse in the process of preparing the expanded beads by heating and pressurizing, and the organic dispersant and the inorganic dispersant are compounded in the process of preparing the expanded beads, so that the surfaces of the formed expanded beads are not adhered to each other.

Specifically, in the heating and pressurizing treatment process, the inorganic dispersing agent is dispersed in the dispersion medium, so that the surface tension of the dispersion medium can be improved, the uniform dispersion of the expanded beads in the dispersion medium is facilitated, the bonding risk of the expanded beads is reduced, and the inorganic dispersing agent can be adsorbed and distributed on the surfaces of the expanded beads, so that the adjacent expanded beads can be mechanically isolated from each other, and the agglomeration phenomenon caused by mutual fusion is avoided.

The lipophilic group of the organic dispersant can be adsorbed on the surface of the expanded beads, and the hydrophilic group can be solvated by the dispersion medium and spread into the dispersion medium, thereby forming a charged protective barrier around the expanded beads, surrounding the expanded beads and generating electrostatic repulsion between adjacent expanded beads, so that the whole reaction kettle is kept stable and the expanded beads are not bonded. In addition, in the pressure relief process, the organic dispersant forms a large amount of foam, so that the expanded beads are effectively protected in the foam, the foam holes of the expanded beads cannot collapse due to large difference between the internal temperature and the external temperature, and the uniformity of foaming is further ensured.

According to the invention, the organic dispersant and the inorganic dispersant are compounded for use, the inorganic dispersant can improve the surface tension of a dispersion medium and provide a mechanical isolation effect, and the organic dispersant can form a protective barrier effect on the foaming beads, so that the foaming beads form a uniform system in the dispersion medium, and the foaming beads are beneficial to forming a uniform cell structure under the conditions of heating and pressurizing, and the surfaces of the foaming beads are not bonded, agglomerated and collapsed, thereby ensuring that the resin with higher melting point difference is foamed under lower forming pressure.

Considering that too much inorganic dispersant will affect the molding process, is not good for adhesion and will affect the mechanical properties of the polypropylene foamed product, the mass ratio of the organic dispersant to the inorganic dispersant is preferably (0.3-2): 1, and preferably the mass ratio of the organic dispersant to the inorganic dispersant is (0.5-1): 1.

Further, the organic dispersing agent comprises at least one of stearic acid monoglyceride, vinyl bis stearamide, calcium stearate and sodium benzenesulfonate.

The inorganic dispersant comprises at least one of nano clay, montmorillonite, kaolin and alkali metal phosphate.

Specifically, in step S1, the first resin, the second resin, and the foaming aid are primarily mixed and melted by extrusion granulation to obtain a blended bead with good dispersibility and stable structure, and the blended bead is easy to be foamed subsequently, wherein the blended bead has a solid particulate structure, the diameter of the blended bead is 0.5 mm-2 mm, the length-diameter ratio of the blended bead is 1-2: 1, and the surfaces of the particles of the blended bead are not adhered to each other.

Wherein the material of the first resin comprises at least one of polypropylene resin homopolymer, first polypropylene random copolymer and polypropylene block copolymer. It should be noted that the material of the first resin includes polypropylene-based resin, and has a melting point of 140-165 ℃ to make polypropylene have high mechanical properties, it is understood that the first resin may be a homopolymer, or may be a copolymer and include other high molecular materials, and the high molecular chain may be regular or irregular, and the invention does not limit the molecular structure of the first resin.

The material of the second resin comprises at least one of a second polypropylene random copolymer, low density polyethylene, low density linear polyethylene, ethylene-octene copolymer, propylene-ethylene copolymer. The second resin may include polypropylene or other olefins, and the olefin polymer and the polypropylene have good miscibility and are easy to blend.

Specifically, in order to facilitate nucleation and obtain a blended bead having a stable structure, the foaming aid includes at least one of an antioxidant, a stabilizer, an anti-uv agent, a nucleating agent, a lubricant, and a color masterbatch.

Furthermore, the mass ratio of the first resin, the second resin and the foaming auxiliary agent is (40-100) to (1-12) 100.

In order to obtain a polypropylene foamed product with a more uniform cell structure and to allow the foaming aid and the foaming agent to be sufficiently mixed in the first resin and the second resin and to achieve sufficient foaming through temperature and pressure control, in step S2, the blended beads, the dispersing agent, the foaming agent, and the dispersion medium are mixed, and subjected to pressure and heat treatment at a first pressure and a first temperature, and then subjected to pressure release to prepare foamed beads.

The pressure-heating treatment is usually carried out in an autoclave, and the dispersion medium includes, but is not limited to, water. The first pressure is 1.8MPa to 7.0MPa, and the first temperature is 130 ℃ to 160 ℃. The bulk density of the expanded beads thus obtained was 30 to 90 g/L.

Specifically, the blowing agent includes at least one of carbon dioxide, nitrogen, propane, butane, and pentane.

Before the expanded beads are prepared by pressurization, pressure relief treatment is needed to fully disperse redundant foaming agent, a plurality of cellular structures with uniformly distributed sizes and shapes are formed in the expanded beads, the surfaces of the expanded beads are not bonded, and the yield of the expanded beads is high.

Specifically, the mass ratio of the blending beads, the dispersing agent, the foaming agent and the dispersing medium is 100 (2-5): 10-20): 300-500.

In one embodiment, the above preparation method further comprises, after the heat-pressure treatment of the blended beads and before the pressure-loading treatment of the expanded beads, drying the expanded beads at a second temperature to remove the dispersion medium in the expanded beads, wherein the second temperature is 60 ℃ to 80 ℃.

Further, the pressure loading treatment of the expanded beads in step S3 can make the closed cells inside the expanded beads more regular and smooth, and the surface thereof more flat, which is helpful for forming a more uniform cell structure and a higher expansion ratio, and the obtained polypropylene foamed product is more uniform.

The pressure loading treatment is usually carried out in a pressure loading tank, and the second pressure corresponding to the gas in the pressure loading tank is 0.3MPa to 0.8MPa and the pressure loading treatment time is 6h to 12h, considering that the lower pressure loading pressure and the longer pressure loading time are helpful for slowly homogenizing the cells of the expanded beads at a uniform speed, but the lower pressure loading pressure and the overlong pressure loading time can influence the preparation efficiency.

In step S4, the preformed beads are fused and bonded to each other by steam molding, and are cooled and fixed to form a polypropylene foamed article having a fixed shape. The steam is usually water vapor, the pressure and temperature of the water vapor are determined by the components and the proportion of the preformed beads, and the pressure and temperature of the water vapor also affect the melt bonding effect of the polypropylene foamed product.

If the pressure and temperature of the water vapor are too high, the surfaces of the preformed beads will collapse rapidly, destroying the foaming effect of the polypropylene foamed article, and if the pressure and temperature of the water vapor are too low, the surfaces of the preformed beads will not adhere well to each other. Preferably, the temperature of the water vapor is a third temperature, the third temperature is between the melting point of the first resin and the melting point of the second resin, i.e., the third temperature is less than the melting point of the first resin, and the third temperature is greater than the melting point of the second resin. In the process of molding through steam molding, the higher melting point of the first resin can keep the cellular structure free from the influence of steam, so that the cellular structure is kept unchanged, and the lower melting point of the second resin can promote the surfaces of the prefabricated beads to be melted and bonded with each other, so that a polypropylene foamed product with uniform cellular structure and better mechanical property is obtained.

The invention also provides a polypropylene foaming product with low forming pressure, which is prepared by the preparation method, and the foam holes are regular and smooth, have smooth surfaces and have uniform foam hole structures.

The polypropylene foamed product with low forming pressure has tensile strength greater than or equal to 0.75MPa and elongation at break greater than or equal to 15%, so that the polypropylene foamed product has excellent mechanical properties. The low-molding-pressure polypropylene foamed product has a foaming ratio of 10-30 times and a high foaming ratio.

The density of the polypropylene foaming product with low molding pressure is less than or equal to 0.11g/cm³. Preferably, the density of the polypropylene foamed article of low molding pressure is 0.03g/cm³～0.09g/cm³。

Hereinafter, preferred embodiments and comparative embodiments are enumerated for better understanding of the present invention. However, the following embodiments are merely illustrative of the present invention, and are not limited thereto or thereby.

Embodiment 1:

and (3) extruding and granulating: 100 parts by weight of polypropylene resin with a melting point of 148 ℃, 60 parts by weight of LDPE resin with a melting point of 118 ℃ and 5 parts by weight of foaming auxiliary agent are placed in an extruder and are subjected to mixing, extrusion, cooling, wire drawing and grain cutting to form blended beads, and the prepared blended beads have the diameter of 2mm and the length-diameter ratio of 2: 1.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, and filling 15 parts by weight of CO₂And heating to 2.3MPa and 149 ℃ under pressure, and instantaneously depressurizing to release the pressure in the autoclave to atmospheric pressure to obtain expanded beads.

Carrying out pressure loading: and (3) placing the dried expanded beads into a pressure-bearing tank, filling compressed air, carrying out pressure bearing at a second pressure and standing for a certain time to obtain the prefabricated beads.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.25MPa, and the temperature of the water vapor is 120 ℃. The steam molding machine was a small-scale molding machine capable of withstanding a saturated steam pressure of 0.5MPa, and heat molding was performed by completely closing the mold and filling the mold with preformed beads in a molding die having a size of 500mm × 60mm, and supplying pressure to the mold. And after heating and forming, taking out the formed body through water cooling and air cooling, annealing for 12 hours in a drying room at the temperature of 80 ℃, and cooling to room temperature to obtain the polypropylene foamed product with low forming pressure.

The proportions and the qualities of the foaming agent, the foaming aid and the dispersant are shown in the following table 2.

Embodiment 2:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of polypropylene resin with the melting point of 150 ℃, 50 parts by weight of LDPE resin with the melting point of 118 ℃ and 10 parts by weight of foaming auxiliary agent are added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, and filling 10 parts by weight of CO₂And heated to 2.2MPa and 151 ℃ under pressure.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.23MPa, and the temperature of the water vapor is 120 ℃.

Embodiment 3:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of a polypropylene resin having a melting point of 143 ℃, 70 parts by weight of an LLDPE resin having a melting point of 126 ℃ and 10 parts by weight of a foaming aid are added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, and filling 20 parts by weight of N₂And heated to 7.0MPa and 143 ℃ under pressure.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.22MPa, and the temperature of the water vapor is 120 ℃.

Embodiment 4:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of polypropylene resin with a melting point of 163 ℃, 40 parts by weight of LDPE resin with a melting point of 118 ℃ and 5 parts by weight of foaming auxiliary agent are added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, charging 12 parts by weight of pentane, pressurizing and heating to 2.0MPa and 150 ℃.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.18MPa, and the temperature of the water vapor is 120 ℃.

Embodiment 5:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of a polypropylene resin with a melting point of 163 ℃, 90 parts by weight of a POP resin with a melting point of 121 ℃ and 10 parts by weight of a foaming aid are added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, and filling 17 parts by weight of CO₂And heated under pressure to 2.6MPa and 163 ℃.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.15MPa, and the temperature of the water vapor is 120 ℃.

Embodiment 6:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of polypropylene resin with a melting point of 148 ℃, 100 parts by weight of r-PP resin with a melting point of 133 ℃ and 7 parts by weight of foaming auxiliary agent are added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, and filling 11 parts by weight of N₂And heated to 5.0MPa and 147 ℃ under pressure.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.25MPa, and the temperature of the water vapor is 120 ℃.

Embodiment 7:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of polypropylene resin with a melting point of 148 ℃, 50 parts by weight of POP resin with a melting point of 107 ℃ and 12 parts by weight of foaming auxiliary agent are added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, and filling 10 parts by weight of CO₂And heating to 2.8MPa and 148 ℃ under pressure.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.12MPa, and the temperature of the water vapor is 120 ℃.

Embodiment 8:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of polypropylene resin with a melting point of 145 ℃, 65 parts by weight of POE resin with a melting point of 125 ℃ and 8 parts by weight of foaming auxiliary agent are added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, filling 19 parts by weight of pentane, pressurizing and heating to 1.8MPa and 135 ℃.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.19MPa, and the temperature of the water vapor is 120 ℃.

Embodiment 9:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of polypropylene resin with a melting point of 140 ℃, 80 parts by weight of r-PP resin with a melting point of 122 ℃ and 1 part by weight of foaming auxiliary agent are added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, filling 13 parts by weight of butane, pressurizing and heating to 6.2MPa and 130 ℃.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.20MPa, and the temperature of the water vapor is 120 ℃.

Embodiment 10:

this embodiment is basically the same as embodiment 1, except that:

and (3) extruding and granulating: 100 parts by weight of a polypropylene resin having a melting point of 165 ℃, 75 parts by weight of an LLDPE resin having a melting point of 140 ℃ and 3 parts by weight of a foaming aid were added.

Heating and pressurizing for foaming: placing the blended beads, a dispersing agent and deionized water into a high-pressure reaction kettle, filling 16 parts by weight of propane, pressurizing and heating to 3.5MPa and 155 ℃.

Molding: and (3) placing the preformed beads subjected to pressure loading in a molding machine, and molding by using water vapor, wherein the pressure of the water vapor is 0.17MPa, and the temperature of the water vapor is 120 ℃.

Comparative mode 1:

this embodiment is basically the same as embodiment 1, except that: the expanded polypropylene beads are prepared from a polypropylene copolymer material with a melting point of 134 ℃.

Comparative mode 2:

this embodiment is basically the same as embodiment 1, except that: expanded polypropylene beads were prepared from a homopolypropylene having a melting point of 163 ℃ and a polypropylene expanded article was prepared by steam molding.

Comparative mode 3:

this embodiment is basically the same as embodiment 1, except that: the dispersant in the heating and pressurizing foaming process is montmorillonite.

Comparative example 4:

this embodiment is basically the same as embodiment 1, except that: the dispersant in the heating and pressurizing foaming process is sodium benzenesulfonate.

The following performance tests were performed on the polypropylene foamed articles obtained in embodiments 1 to 10 and comparative embodiments 1 to 4:

1. characterization of mechanical Properties

The tensile properties and elongation at break of the test materials were measured according to GB1040-2006 using an Instron5567 universal materials tester.

2. Testing of the bulk Density of expanded beads

Injecting the prepared foaming beads into a 1L measuring cylinder, scraping the surface of the measuring cylinder, weighing the mass of the foaming beads in the measuring cylinder, and keeping the sample in a loose state in the experimental process to prevent any vibration, so that the mass of the foaming beads in unit volume can be obtained.

TABLE 1

TABLE 2

Microscopic analysis and characterization were performed on the polypropylene foamed products obtained in the above embodiments 1 to 10 and comparative embodiments 1 to 4. Specifically, Hitachi TM-1000 desktop scanning electron microscope was used for microscopic analytical characterization.

It can be found that the polypropylene foamed products obtained in embodiments 1 to 10 have regular and smooth cells, flat surfaces, and relatively uniform cell structures. Fig. 2 shows the characterization results of the polypropylene foamed article obtained in embodiment 1.

However, the polypropylene foamed products obtained in comparative examples 1 to 4 were bonded to different degrees and could not be subjected to steam molding.

Further, the polypropylene foamed product obtained in embodiment 1 is subjected to DSC (differential scanning calorimetry) to obtain a first temperature rise curve by heating from 40 ℃ to 200 ℃ at a heating rate of 10 ℃/min, and then the temperature is maintained for 3 minutes, and then the temperature is reduced from 200 ℃ to 40 ℃ at a cooling rate of 10 ℃/min, and then the temperature is maintained for three minutes, and then the polypropylene foamed product is heated from 40 ℃ to 200 ℃ at a heating rate of 10 ℃ to obtain a second temperature rise curve, and the test results refer to fig. 3.

As can be seen from fig. 3, a triple melting peak structure appears in the DSC melting curve due to the addition of the low melting point second resin. The first melting peak is the melting peak formed by the low-melting-point resin, the second and third melting peaks are the melting peaks formed by the foaming temperature reaching to the vicinity of the melting point of the polypropylene in the foaming process, the unmelted crystals in the particles are rearranged under the induction of the foaming agent gas to form a new crystal structure, and the melting points of the two melting peaks are arranged at two sides of the melting point of the original polypropylene particles.

The existence of the tri-melting peak structure can lead the vapor pressure to have a wider range interval and lower vapor pressure in the molding process, and the molding pressure value is lower.

And, due to the addition of the low melting point second resin, the melting peak width is widened. When the difference between the melting points of the first resin and the second resin is small, the melting peaks of the second resin are combined, and the DSC melting curve has a double melting broad-peak structure; when the difference between the melting points of the first resin and the second resin is large, three melting peak structures which are more obvious appear in the DSC melting curve. In the process of molding and forming the expanded polypropylene beads by steam, the three melting peaks enable the range of steam pressure to be larger, the temperature of forming steam is between the lowest melting peak and the highest melting peak, the middle melting peak enables the selection of forming temperature and pressure to be more accurate, bonding between particles is facilitated, the melting recrystallization of the lowest temperature melting peak promotes the bonding of the surfaces of the particles, and the highest temperature melting peak can keep the shape of cells unchanged, so that a foamed product with excellent mechanical properties is obtained.

In addition, the steam temperature is between the high and low melting peaks, which is beneficial to the bonding between the preformed beads, the low-temperature melting peak is melted and recrystallized to promote the bonding of the surfaces of the preformed beads, and the high-temperature melting peak can keep the shape of the cells unchanged, so that the foam product with excellent mechanical property is obtained.

The features of the above-described embodiments may be arbitrarily combined, and for the sake of brevity, all possible combinations of the features in the above-described embodiments are not described, but should be construed as being within the scope of the present disclosure as long as there is no contradiction between the combinations of the features.

The above embodiments only express a few embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. A preparation method of a polypropylene foaming product with low forming pressure is characterized by comprising the following steps:

mixing a first resin, a second resin and a foaming auxiliary agent, extruding and granulating to prepare a blended bead, wherein the mass ratio of the first resin to the second resin to the foaming auxiliary agent is (40-100) to (1-12), the first resin is made of polypropylene resin, the melting point of the first resin is 140-165 ℃, and the melting point of the second resin is 15-45 ℃ lower than that of the first resin;

mixing the blended beads, a dispersing agent, a foaming agent and a dispersion medium, and heating and pressurizing to obtain foamed beads, wherein the mass ratio of the blended beads to the dispersing agent to the foaming agent to the dispersion medium is 100 (2-5) to (10-20) to (300-500);

the steam pressure is 0.12-0.25 MPa, the dispersing agent comprises an organic dispersing agent and an inorganic dispersing agent, the mass ratio of the organic dispersing agent to the inorganic dispersing agent is (0.3-2): 1, the organic dispersing agent comprises at least one of stearic acid monoglyceride, vinyl bis stearamide, calcium stearate and sodium benzenesulfonate, and the inorganic dispersing agent comprises at least one of nano clay, montmorillonite, kaolin and alkali metal phosphate.

2. The method for producing a low molding pressure polypropylene foamed article according to claim 1, wherein the material of the first resin includes at least one of a polypropylene-based resin homopolymer, a first polypropylene random copolymer, and a polypropylene block copolymer.

3. The method for producing a low molding pressure polypropylene foamed article according to claim 1 or 2, wherein the material of the second resin includes at least one of a second polypropylene random copolymer, a low density polyethylene, a low density linear polyethylene, an ethylene-octene copolymer, and a propylene-ethylene copolymer.

4. The method for preparing a polypropylene foamed article with low molding pressure as claimed in claim 1, wherein the foaming auxiliary agent comprises at least one of antioxidant, anti-ultraviolet agent, nucleating agent, lubricant and color masterbatch.

5. The method for preparing a low molding pressure polypropylene foamed article according to claim 1 or 4, wherein the blowing agent comprises at least one of carbon dioxide, nitrogen, propane, butane, pentane.

6. The method for preparing a polypropylene foamed product with low molding pressure according to claim 1, wherein a first pressure and a first temperature are adopted in the heating and pressurizing treatment, wherein the first pressure is 1.8MPa to 7.0MPa, and the first temperature is 130 ℃ to 160 ℃.

7. The method for preparing a polypropylene foamed product with low molding pressure according to claim 1, wherein a second pressure is used in the pressure loading treatment, the second pressure is 0.3MPa to 0.8MPa, and the time of the pressure loading treatment is 6h to 12 h.

8. A low-molding-pressure polypropylene foamed product, which is prepared by the preparation method of any one of claims 1 to 7, has a tensile strength of 0.75MPa or more, and has a density of 0.11g/cm or less³。