CN112341662B

CN112341662B - Gray antistatic polypropylene composite foaming bead with skin-core structure and molded product thereof

Info

Publication number: CN112341662B
Application number: CN202010667091.XA
Authority: CN
Inventors: 熊业志; 何若虚; 曾佳; 刘缓缓; 蒋璠晖
Original assignee: Wuxi Hi Tec Environmental Material Co ltd
Current assignee: Wuxi Hi Tec Environmental Material Co ltd
Priority date: 2020-07-10
Filing date: 2020-07-10
Publication date: 2022-03-11
Anticipated expiration: 2040-07-10
Also published as: CN112341662A

Abstract

The invention discloses a gray antistatic polypropylene composite foaming bead with a skin-core structure, which consists of a foaming core layer and a non-foaming skin layer, wherein the foaming core layer consists of polypropylene, a foam cell nucleating agent, an antioxidant, coloring carbon black and a lubricant, and the skin layer consists of polypropylene, a polyether block copolymer antistatic agent, maleic anhydride graft modified polypropylene and a multi-walled carbon nanotube; the composite foaming bead is obtained by extruding and granulating through a co-extrusion extruder and then foaming through a high-pressure foaming kettle. The skin layer contains the composite antistatic agent and the low-melting-point polypropylene, so that good sintering, apparent quality and stable permanent antistatic property of the molded product are ensured; the core layer has excellent foaming effect, and ensures the adjustability of the foaming multiplying power and excellent mechanical property of the molding product. The moldings have not only permanent antistatic properties but also good rigidity.

Description

Gray antistatic polypropylene composite foaming bead with skin-core structure and molded product thereof

Technical Field

The invention belongs to the technical field of high polymer material processing, and particularly relates to a gray antistatic polypropylene composite foaming bead with a skin-core structure and a molding product thereof.

Background

Expanded polypropylene beads (EPP) and a material for a molded article thereof have excellent mechanical properties, are easier to recover than conventional expanded materials, play a significant role in the field of expanded plastics, and have been widely used in the fields of packaging protective materials, sound and heat insulating materials, automobile part materials, and the like.

The surface resistance of the EPP material is reduced, the concentration of electrons on the surface of the EPP material is favorably dispersed, and particularly, the anti-static EPP material is more needed for packaging materials of precision electronic equipment, liquid crystal display screens and the like. At present, the antistatic EPP material is mainly realized by two ways, one is that an amphiphilic organic molecule adding way is adopted, but the antistatic performance of the EPP material is greatly changed along with the environmental humidity and temperature, and the surface resistance is generally only 10^9-12Omega; secondly, conductive carbon black, graphite, carbon nano tubes and other substances are adopted as conductive effective components, although the surface resistance of the product can reach 10^5-10Omega, but the addition amount is large and the color is black, so that the foaming performance of EPP is weakened to a certain extent, and excessive inorganic powder filling easily causes powder falling, so that the risk of damaging the surface of an electronic device exists in the transportation process.

The invention aims to obtain a gray permanent antistatic polypropylene composite expanded bead with less inorganic powder addition and a molded product thereof. The surface resistance of the molded article was stabilized at 10^6-9Omega, good internal fusion, excellent apparent quality, and no fear of powder falling when used as an antistatic packaging material.

Disclosure of Invention

The invention aims to overcome the defects of the existing antistatic foamed polypropylene material, and produces a gray antistatic polypropylene composite foamed bead with a skin-core structure and a molded product thereof, wherein the fusion pressure of the gray antistatic polypropylene composite foamed bead is lower than 2.0bar, the surface quality of the molded part is full and beautiful, and the surface resistance is 10^6-9Omega and does not change with temperature and humidity.

In order to achieve the purpose, the invention adopts the following technical scheme:

the gray antistatic polypropylene composite foaming bead with the skin-core structure consists of a foaming core layer and a non-foaming skin layer coated on the surface of the foaming core layer, wherein the foaming core layer consists of polypropylene, a foam cell nucleating agent, an antioxidant, coloring carbon black and a lubricant; the cortex consists of polypropylene, a polyether block copolymer antistatic agent, maleic anhydride graft modified polypropylene and a multi-walled carbon nanotube;

the composite foaming beads are obtained by adopting a co-extrusion extruder to extrude and granulate and then foaming the extruded composite foaming beads through a high-pressure foaming kettle, wherein the co-extrusion extruder comprises 2 extruders I and II and a co-extrusion die head III; the method comprises the following steps that materials of a foaming core layer are melted and plasticized into a melt by an extruder I and then led into a co-extrusion die head III, materials of a skin layer are melted and plasticized into a melt by an extruder II and then led into the co-extrusion die head III, the melt materials of the skin layer are coated on the surface of the melt materials of the foaming core layer through a flow channel structure in the co-extrusion die head III, then the melt materials of the skin layer are discharged through a circular die with the diameter of 0.5-3.5mm, and after traction and cooling, filament strips are cut by a granulator to obtain composite particles with the skin-core structure, wherein the length of the composite particles is 1.0-2.5mm, and the weight of the composite particles is 0.5-1.5 mg;

the foaming process of the composite particles with the skin-core structure is as follows: (1) putting the composite particles with the skin-core structure, the aqueous dispersion medium and the dispersion auxiliary agent into a high-pressure foaming kettle and sealing; (2) under the action of mechanical stirring, the high-pressure foaming kettle is heated to a foaming temperature T2, and CO is introduced into the closed dispersion system through a booster pump₂Until the foaming pressure P is between T1 and 8 ℃ and T2 and T1+15 ℃, and P is between 0.5MPa and 5.5 MPa; (3) after the composite particles with the skin-core structure are soaked by gas, instantly discharging the dispersion system to an air pressure environment lower than P to obtain expanded primary foaming beads; (4) and (3) carrying out air pressure loading on the primary foamed beads through a pre-pressing tank, and then expanding the beads with the internal pressure under the heating action of hot air to obtain the gray antistatic polypropylene composite foamed beads with the skin-core structure, which are lower in density and 20-50 times in foaming multiplying power.

Furthermore, the polypropylene of the foaming core layer is random copolymerization polypropylene, the melting point is 145-158 ℃, and the melt index is 5-9.5 g/10 min; the foam cell nucleating agent is talcum powder or zinc borate with the grain diameter of 5-10 microns; based on the total weight of the foaming core layer, the polypropylene content is 95.0-99.50%, the coloring carbon black content is 0.5-2.4%, the foam cell nucleating agent content is 0.1-0.5%, the antioxidant content is 0.1-1.5%, and the lubricant content is 0.1-1.5%.

Further, the polypropylene of the skin layer is random copolymerization polypropylene, the melting point is 120-150 ℃, and the melt index is 6-10g/10 min; the melting point of the maleic anhydride grafted modified polypropylene is 120-145 ℃, and the melt index is 6-15g/10 min; the length-diameter ratio of the multi-wall carbon nano tube is more than 1000; based on the total weight of the cortex, the polypropylene content is 60-80%, the polyether block copolymer antistatic agent content is 12-24%, the maleic anhydride graft modified polypropylene content is 8-18%, and the multi-walled carbon nanotube content is 0.2-1.5%.

Furthermore, the melting point of polypropylene of the foaming core layer is 148-153 ℃, and the melt index is 6-8g/10 min; based on the total weight of the foaming core layer, the content of the polypropylene is 97.0 to 98.0 percent, the content of the coloring carbon black is 0.8 to 1.2 percent, and the content of the antioxidant is 0.05 to 0.5 percent; (ii) a The melting point of the polypropylene of the skin layer is 125-140 ℃, and the melt index is 7-9g/10 min; the melting point of the maleic anhydride grafted modified polypropylene is 125-140 ℃, and the melt index is 7-12g/10 min; based on the total weight of the cortex, the polypropylene content is 60-75%, the maleic anhydride grafted modified polypropylene content is 10-15%, and the content of the multi-walled carbon nano-tube is 0.4-1.2%.

Furthermore, the multi-walled carbon nanotube and maleic anhydride are grafted and modified with polypropylene to prepare a modified foaming master batch, and the specific steps are as follows: mechanically mixing 90-95wt% of maleic anhydride grafted modified polypropylene and 5-10wt% of multi-walled carbon nanotubes, extruding by a double screw, drawing by a granulator, cutting and granulating to obtain 1-5mg of microparticle master batch with single weight; weighing 15kg of microparticle master batch, 20kg of deionized water, 0.2kg of kaolin and 0.1kg of sodium dodecyl benzene sulfonate, and adding the materials into a high-pressure foaming kettle with the volume of 60L; under the stirring action, the temperature in the kettle is raised to be 3-8 ℃ lower than the melting point of the maleic anhydride grafted modified polypropylene, and 8-10MPa of CO is pumped in by a booster pump₂The dispersion is at high temperature and high pressureKeeping for 20min, opening a discharge valve, discharging the materials to a normal pressure environment to obtain an expansion body with the foaming multiplying power of 20-45 times, washing the expansion body with clear water, drying, defoaming by a double screw, extruding and granulating to obtain the modified foaming master batch.

Further, the skin layer is made of 60 wt% -75 wt% of polypropylene, 10wt% -15 wt% of modified foaming master batch, 12 wt% -24 wt% of polyether block copolymer antistatic agent, antioxidant and lubricant.

Further, the proportion of the skin layer is 8-15 wt% and the proportion of the inorganic filler is less than 1.5 wt% based on the total weight of the composite expanded bead.

The gray permanent antistatic foamed polypropylene molding product is obtained by adopting gray antistatic polypropylene composite foamed beads with a skin-core structure through water vapor sintering molding, and comprises the following specific steps: filling the composite foamed beads subjected to pressure loading by the pre-pressing tank into a mold cavity through a vacuum pipeline, expanding the composite foamed beads and fusing the surface skin under the action of high-temperature steam, cooling and shaping through a cooling medium on the surface of the mold, and demolding; further curing and shaping the molded part in a hot air curing chamber at 50-110 ℃ to obtain a gray permanent antistatic foamed polypropylene molded product.

Further, the pressure of the water vapor sintering molding is lower than 2.0bar, and the surface resistance of the molded part is 10^6-9Omega, does not change with temperature and humidity.

Compared with the prior art, the invention has the beneficial effects that:

(1) the polypropylene resin in the core layer has high melting point, so that the EPP molded product can obtain high-rigidity mechanical property, and the EPP molded product packaging material with high rigidity and difficult deformation has better anti-twisting or bending effect on electronic devices with larger sizes. The sheath layer polypropylene resin has low melting point, is more beneficial to the forming and sintering of EPP molded products and has excellent apparent quality.

(2) The skin layer does not need to add a coloring agent at all, and the apparent gray of the composite expanded beads is generated by the light transmittance of the expanded beads of the core layer; and the inorganic filling amount of the foaming beads is low, so that the foaming and antistatic properties of the molded product are ensured, the molding pressure of the molded product is low, the energy consumption is low, the apparent quality is excellent, and more importantly, the molded product is used as a packaging material, the powder removal phenomenon of inorganic powder is avoided, and the antistatic protective packaging material is suitable for antistatic protective packaging of precise electronic devices and liquid crystal display screens.

(3) The modified master batch is prepared by grafting the multi-walled carbon nanotube and maleic anhydride to the modified polypropylene, and the foaming stripping and blocking effects of the supercritical carbon dioxide are utilized in the process, so that the dispersion of the carbon nanotube in a matrix is facilitated, the re-agglomeration phenomenon is greatly reduced, a conductive network is more easily formed due to the advantage of high length-diameter ratio, and the good conductivity is still realized under the condition of low carbon nanotube filling amount.

(4) The polyether block copolymer antistatic agent containing strong polar chain segments or groups has excellent conductivity and surface resistance of 10⁷Omega, the proportion of the omega in the weight of the skin layer is 15-24 percent, and the dosage of the omega is lower than that of the polyether block material conductive agent in the prior art. On one hand, in the melting and blending process of the skin layer formula, the maleic anhydride modified PP conductive master batch tends to uniformly exist at the interface of polypropylene and a polyether block conductive agent due to excellent interface compatibility, part of the carbon nanotube network in the modified master batch can migrate to a polypropylene phase and an antistatic agent phase due to the shearing and mixing action of mixing, and the carbon nanotube network tends to be closer to the antistatic agent due to the strong polar chain segment or group existing in the polyether block copolymer antistatic agent phase containing the strong polar chain segment or group; the carbon nano tube network promotes the stable connection and the electric conduction of the antistatic agent in the cortex, and the electric conduction performance of a few carbon nano networks in a tightly connected area is better than that of the antistatic agent. The carbon nano tube with good dispersibility and the polyether block copolymer antistatic agent generate a synergistic antistatic effect, so that the addition amount of the polyether block copolymer antistatic agent is reduced; on the other hand, the reduction of the addition amount of the polyether block copolymer antistatic agent is more beneficial to the forming and sintering firmness of a molded product and improves the mechanical property and the apparent quality of the molded product.

(5) The composite foaming bead skin layer accounts for 8-15% of the total weight, and can stabilize the surface resistance of the molded product to 10^6-9Omega, andcan meet the mechanical property of a molding product and improve the stability of the process of co-extruding and manufacturing particles. CN 103724653A also prepared antistatic 10 using similar techniques^5-10According to European composite foamed particles and molded bodies thereof, the conductive layer of the composite foamed particles is added with a large amount of conductive carbon black, the apparent color of the product is black under high carbon content, and the composite foamed particles also have the risk of powder falling to damage the surface of a liquid crystal screen when packaged as a high-end display screen.

(6) The peak area of the highest melting point peak of the first DSC temperature rising curve of the core layer of the composite foaming bead is controlled to be 15-20J/g through the optimized selection of the polypropylene raw material and the accurate control of the foaming process. The molded product produced by the expanded beads in the range not only has better rigidity, namely good compression performance, but also has moderate molding pressure and reduced energy consumption. A large number of experiments show that the area of the highest melting point peak of the first DSC temperature rising curve of the core layer of the composite foaming bead and the secondary foaming easiness degree of the foaming bead have correlation and are closely related to the apparent mass of a molded product. The composite foaming bead has the advantages that in the steam molding process, the numerical value of pressure (KPa) of 10% compression strain of a 45P molded part/apparent density (g/L) of a molded product is more than 5, the molded product has permanent antistatic property and good rigidity, and the molded product can show good buffer protection effect on electronic screens with larger sizes and heavier weights.

Drawings

FIG. 1 is a sectional SEM photograph of gray antistatic polypropylene composite foamed beads with a skin-core structure according to the present invention;

FIG. 2 is a schematic view of the internal structure of the molded article of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The main raw materials used in the present invention are shown in Table 1.

TABLE 1

The antioxidant is a commercially available general-purpose 1076 antioxidant, and the lubricant is a commercially available general-purpose 7001 lubricant.

The production method of the master batch F comprises the following steps: mechanically mixing maleic anhydride branched modified polypropylene D and a multi-walled carbon nanotube (the weight content is 5-10%), extruding by a double screw, drawing by a granulator, and cutting and granulating to obtain 1-5mg of microparticle master batch E. Weighing 15kg of E micro-particles, 20kg of deionized water, 0.2kg of kaolin and 0.1kg of sodium dodecyl benzene sulfonate, and adding the E micro-particles, the deionized water, the kaolin and the sodium dodecyl benzene sulfonate into a high-pressure resistant reaction kettle with an internal volume of 60L; raising the temperature in the kettle to 3-8 ℃ lower than the melting point of the maleic anhydride branched modified polypropylene D under the action of stirring, and introducing CO of about 8-10MPa by using a booster pump₂And keeping the dispersion at high temperature and high pressure for 20min, namely opening a discharge valve of the kettle body, directly discharging the materials to normal pressure through a high-pressure resistant metal pipeline at the bottom of the kettle to obtain an expansion body of 20-45 times, washing the expansion body with clean water, drying, and carrying out twin-screw extrusion granulation through defoaming to obtain the modified F master batch.

The composite particles are produced by adopting a customized extrusion unit for co-extruding to form particles and a special extrusion structure die head, wherein the extrusion unit comprises 2 extruders I and II and a shared extrusion structure die head III; the extruder I is responsible for the melting plastification of foaming sandwich layer formula materials, the fusant flows into the die head III through the guide pipe, the extruder II is responsible for the melting plastification of skin layer formula materials, the fusant flows into the die head III through the guide pipe, the fusant of I and II passes through a special flow channel structure in the die head, the fusant of II can be coated on the surface of the fusant of I, then the fusant is discharged through a circular die with the diameter of 0.5-3.5mm, and after traction and cooling, the filament strip is cut by a granulator to obtain composite particles. And adjusting the discharge flow rates of the first and second layers by an extrusion process to realize the weight ratio of the composite particle skin layer to the core layer. The composite particle skin layer accounts for 8-15% of the weight of the particle.

The production process of the composite expanded beads of the present invention is that firstly, the preferable composite particles with the length of 1.0-2.5mm and the single weight of 0.5-1.5mg are carried out according to the following processes: (1) putting the composite particles, the aqueous dispersion liquid and the dispersing auxiliary agent into a high-pressure-resistant closed kettle according to a certain weight ratio and sealing; (2) under the action of mechanical stirring, the kettle body is heated to foaming temperature T2, and CO2 is introduced into the closed dispersion system through a booster pump to reach foaming pressure P; (3) after the composite fine particles are impregnated with the gas, the dispersion is instantaneously discharged to an atmosphere of a pressure lower than P to obtain expanded primary expanded beads. The specific temperature and pressure parameters were: t1-8 ℃ and T2 ℃ and T1+15 ℃, 0.5MPa and P5.5 MPa; (4) and (3) carrying out air pressure loading on the primary expanded beads through a pre-pressing tank, and then expanding the particles keeping the internal pressure under the heating action of hot air to obtain secondary composite expanded beads with lower density and 20-50 times of magnification.

The forming process of the gray antistatic foamed polypropylene molding product is as follows: the composite foaming beads after being loaded and pressed by the pre-pressing tank are filled into a mold cavity through a vacuum pipeline, the beads expand and the skin is sintered under the action of high-temperature steam, and then the composite foaming beads are cooled and shaped by flowing medium water on the surface of the mold and are discharged from the mold. Further curing and shaping the molded part in a hot air curing chamber at 50-110 ℃ to obtain a gray antistatic foamed polypropylene molded product.

The components and their weight percentages of examples 1-5 and comparative examples 1-5 are shown in Table 2.

TABLE 2

The method comprises the following steps of 1, adding no master batch F but increasing the using amount of a polyether block conductive agent C, 2, adding only the master batch F but not adding the polyether block conductive agent C, 3, 4 and 5, weighing maleic anhydride branched modified polypropylene D and carbon nano tubes, directly mixing the weighed materials with other components of a skin layer, matching with a particle core layer material, producing composite particles by adopting a customized extruder set for co-extruding and manufacturing particles and a special extrusion structure die head, obtaining foamed beads through a subsequent kettle type foaming process, and carrying out comparison and test on performance after molding.

The process parameters and the properties of the molded articles of examples 1 to 5 and comparative examples 1 to 5 are shown in Table 3.

TABLE 3

As can be seen from the parameters and performance tables of examples 1 to 5, the melting point and the melt index of the polypropylene A raw material of the foaming core layer are optimally selected, and the optimal kettle type foaming process parameters ensure that the peak areas of the highest melting points of the first DSC temperature rise curves of the composite foaming beads are all between 15 and 20J/g. Under the condition, the composite expanded bead has good secondary expansibility, and the ratio of the 10 percent compressive strain pressure of the 45P molded product to the apparent density of the molded product is more than 5, so that the EPP expanded molded product has good mechanical property. In addition, in the skin layer component, part of the carbon nanotube network in the master batch F can migrate to the polypropylene B phase and the polyether block conductive agent C phase, and the well-dispersed carbon nanotubes and the polyether block conductive agent C have synergistic antistatic effect, so that the addition amount of the polyether block conductive agent is reduced, the addition amount of the whole inorganic filler is also greatly reduced, the forming pressure of a molded product is reduced, the energy consumption is reduced, particularly, the surface resistance of the molded product is obvious, and the surface resistance is not obviously reduced even after 30 days. FIG. 1 shows a schematic view of the distribution of the polymer components in the skin of a molded article. The foamed core layer of the polypropylene composite foamed bead has a good closed-cell structure as shown in figure 2, and the skin layer has almost no cells or micro-foaming.

As can be seen from the performance of comparative example 1 in which the masterbatch F was not added, but only the amount of the polyether block conductive agent was increased in a large amount, the surface resistance of the molded article was relatively large because no carbon nanotube was added, and the molding pressure was high due to excessive addition of the polyether block conductive agent. From the performance of comparative example 2, in which substantially the same amount of masterbatch F as in example was added without polyether block conductive agent C, it can be seen that the low carbon nanotube-filled system did not achieve good permanent antistatic effect without the synergistic effect of the compounded conductive region. It can be seen from comparative examples 1 and 2 that the present invention utilizes the synergistic antistatic effect of the well-dispersed carbon nanotube network formed in the masterbatch F and the polyether block conductive agent to produce good permanent antistatic performance of the molded article prepared by the present invention.

From comparative examples 3 to 5, it can be seen that when the carbon nanotubes and the polyether block conductive agent C are simply dispersed in the skin layer of the polypropylene composite expanded beads, the surface resistance of the molded article does not meet the antistatic requirement, because the two conductive agent components may not exhibit a good synergistic antistatic effect due to the uneven dispersion or more agglomeration of the carbon nanotubes. Therefore, in the technology of the invention, the carbon nanotube master batch F modified by supercritical carbon dioxide expansion and dispersion is easier to embody the conductivity, and is more favorable for forming stable connected conductivity between the carbon nanotubes and the polyether block conductive agent in the cortex system. Therefore, under the condition of less adding amount of the carbon nano tube and the polyether block conductive agent, good permanent antistatic performance can be still achieved; moreover, the carbon nano tube content in the surface matrix of the molded product is very low, and the powder removal phenomenon of the molded product in the using process is eliminated.

In light of the foregoing description of the preferred embodiment of the present invention, many modifications and variations will be apparent to those skilled in the art without departing from the spirit and scope of the invention. The technical scope of the present invention is not limited to the content of the specification, and must be determined according to the scope of the claims.

Claims

1. The gray antistatic polypropylene composite foaming bead with the skin-core structure is characterized in that: the foaming core layer consists of polypropylene, a foam cell nucleating agent, an antioxidant, coloring carbon black and a lubricant; the cortex consists of polypropylene, a polyether block copolymer antistatic agent, maleic anhydride graft modified polypropylene and a multi-walled carbon nanotube;

the composite foaming beads are obtained by extruding and granulating through a co-extruder and then foaming through a high-pressure foaming kettle, wherein the co-extruder comprises an extruder I, an extruder II and a co-extrusion die head III; the method comprises the following steps that materials of a foaming core layer are melted and plasticized into a melt by an extruder I and then led into a co-extrusion die head III, materials of a skin layer are melted and plasticized into a melt by an extruder II and then led into the co-extrusion die head III, the melt materials of the skin layer are coated on the surface of the melt materials of the foaming core layer through a flow channel structure in the co-extrusion die head III, then the melt materials of the skin layer are discharged through a circular die with the diameter of 0.5-3.5mm, and after traction and cooling, filament strips are cut by a granulator to obtain composite particles with the skin-core structure, wherein the length of the composite particles is 1.0-2.5mm, and the weight of the composite particles is 0.5-1.5 mg;

2. The gray antistatic polypropylene composite expanded bead with a skin-core structure as claimed in claim 1, wherein: the polypropylene of the foaming core layer is random copolymerization polypropylene, the melting point is 145-158 ℃, and the melt index is 5-9.5 g/10 min; the foam cell nucleating agent is talcum powder or zinc borate with the grain diameter of 5-10 microns; based on the total weight of the foaming core layer, the polypropylene content is 97.0-98.0%, the coloring carbon black content is 0.8-1.2%, the foam cell nucleating agent content is 0.1-0.5%, the antioxidant content is 0.05-0.5%, and the lubricant content is 0.1-1.5%.

3. The gray antistatic polypropylene composite expanded bead with a skin-core structure as claimed in claim 1, wherein: the polypropylene of the skin layer is random copolymerization polypropylene, the melting point is 120-150 ℃, and the melt index is 6-10g/10 min; the melting point of the maleic anhydride grafted modified polypropylene is 120-145 ℃, and the melt index is 6-15g/10 min; the length-diameter ratio of the multi-wall carbon nano tube is more than 1000; based on the total weight of the cortex, the polypropylene content is 60-75%, the polyether block copolymer antistatic agent content is 12-24%, the maleic anhydride graft modified polypropylene content is 10-15%, and the multi-walled carbon nanotube content is 0.4-1.2%.

4. A skin-core structured gray antistatic polypropylene composite expanded bead as claimed in any one of claims 1 to 3, wherein: the polypropylene melting point of the foaming core layer is 148-153 ℃, and the melt index is 6-8g/10 min; the melting point of the polypropylene of the skin layer is 125-140 ℃, and the melt index is 7-9g/10 min; the melting point of the maleic anhydride grafted modified polypropylene is 125-140 ℃, and the melt index is 7-12g/10 min.

5. The gray antistatic polypropylene composite expanded bead with a skin-core structure as claimed in claim 1, wherein: the preparation method comprises the following steps of preparing a modified foaming master batch by grafting the multi-walled carbon nano tube with maleic anhydride and modified polypropylene, and specifically comprises the following steps: mechanically mixing 90-95wt% of maleic anhydride graft modified polypropylene and 5-10wt% of multi-walled carbon nano-tube, extruding by a double screw, drawing by a granulator, cutting and granulating to obtain single particles with the weight of 1-5mgMicroparticle master batches; weighing 15kg of microparticle master batch, 20kg of deionized water, 0.2kg of kaolin and 0.1kg of sodium dodecyl benzene sulfonate, and adding the materials into a high-pressure foaming kettle with the volume of 60L; under the stirring action, the temperature in the kettle is raised to be 3-8 ℃ lower than the melting point of the maleic anhydride grafted modified polypropylene, and 8-10MPa of CO is pumped in by a booster pump₂And (3) maintaining the dispersion at high temperature and high pressure for 20min, opening a discharge valve, discharging the material to a normal pressure environment to obtain an expansion body with the foaming multiplying power of 20-45 times, washing the expansion body with clear water, drying, defoaming by a double screw, extruding and granulating to obtain the modified foaming master batch.

6. A gray permanently antistatic expanded polypropylene molding characterized by: the gray antistatic polypropylene composite foaming bead with the skin-core structure of any one of claims 1 to 5 is obtained by water vapor sintering molding, and comprises the following specific steps: filling the composite foamed beads subjected to pressure loading by the pre-pressing tank into a mold cavity through a vacuum pipeline, expanding the composite foamed beads and fusing the surface skin under the action of high-temperature steam, cooling and shaping through a cooling medium on the surface of the mold, and demolding; further curing and shaping the molded part in a hot air curing chamber at 50-110 ℃ to obtain a gray permanent antistatic foamed polypropylene molded product.

7. A gray permanently antistatic expanded polypropylene molding as claimed in claim 6, wherein the pressure of the steam fusion molding is less than 2.0bar and the surface resistance of the molding is 10^6-9Omega, does not change with temperature and humidity.