A kind of foamed polypropylene beads, a preparation method thereof and a molded part

Abstract

The present invention provides a foamed polypropylene bead, a preparation method thereof and a molded part, relating to the technical field of foamed polypropylene; the foamed polypropylene resin comprises a foamed core layer and a non-foamed skin layer coated on the outer side of the foamed core layer; the foamed core layer comprises polypropylene A, polyethylene B, ethylene-acrylic acid copolymer, a cell nucleating agent and a co-expanding agent; the non-foamed skin layer comprises polyethylene C, polypropylene A and ethylene-acrylic acid copolymer. The foamed polypropylene bead provided by the present invention improves the pressure parameters in the EPP foaming process through the components in the foamed core layer, and can better control the stability of the foaming process during the preparation of EPP beads with a low foaming ratio, obtaining foamed beads with uniform particle volume size and density, which helps to reduce the fluctuation of processing parameters during the molding process and improve the appearance of the molded part.

Description

Technical Field

[0001] The invention relates to the technical field of foamed polypropylene, in particular to a foamed polypropylene bead and a preparation method and a molded product thereof. Background Art

[0002] Expanded polypropylene (EPP) beads and their molded parts have excellent mechanical properties and energy absorption, and the structural shape of the parts can be designed by mold, which is suitable for making lightweight special-shaped parts. EPP is not only widely used in automotive parts and packaging fields, but also suitable for external shell structural parts and internal insulation structural parts of some electronic and household appliances.

[0003] When EPP is used as a structural part, it needs to have a large enough load-bearing capacity. Users generally use EPP products with a relatively low foaming ratio, such as EPP parts with a density greater than 120g / L. However, when EPP beads with a bulk density greater than 120g / L are currently manufactured by a high-temperature and high-pressure autoclave method, there is a serious problem of uneven particle size and density, which leads to large fluctuations in processing parameters during molding and poor product appearance.

[0004] In view of this, how to improve the uniformity of volume size and density distribution of low expansion ratio EPP beads is a technical problem that urgently needs to be solved. Summary of the invention

[0005] The technical problem to be solved by the present invention is: in order to solve the problem of uneven volume size and density distribution of low foaming ratio EPP beads in the prior art, the present invention provides a foamed polypropylene bead, the core layer of the foamed polypropylene bead contains a specific content of polyethylene B, which improves the pressure parameters of the EPP foaming process. In the process of preparing the low foaming ratio EPP beads, the stability of the foaming process can be better controlled to obtain foamed beads with uniform particle volume size and density, thereby solving the problem of uneven particle size distribution of low foaming ratio EPP beads in the prior art; in addition, the skin layer of the foamed polypropylene bead contains specific contents of polypropylene A and polyethylene C, which gives it the characteristics of low molding energy consumption and not easy to stick to the mold.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A foamed polypropylene bead comprises a foamed core layer and a non-foamed skin layer covering the outer side of the foamed core layer;

[0008] The foamed core layer comprises the following components in parts by weight:

[0009]

[0010] The non-foamed skin layer comprises the following components in parts by weight:

[0011] 40 - 75 parts of polyethylene C;

[0012] 20 - 50 parts of polypropylene A;

[0013] 5 - 15 parts of ethylene - acrylic acid copolymer.

[0014] Optionally, the melt index of the polyethylene B is 0.5 - 3 g / 10 min, the melting point is 110 - 130 °C, and the elongation at break is 300% - 1000%.

[0015] Optionally, the melt index of the polypropylene A is 6 - 9 g / 10 min, and the melting point is 137 - 150 °C.

[0016] Optionally, the melt index of the polyethylene C is 4 - 15 g / 10 min, and the melting point is 105 - 125 °C.

[0017] Optionally, the average particle size of the cell nucleating agent is 3 - 10 μm.

[0018] Optionally, the cell nucleating agent is selected from at least one of polytetrafluoroethylene powder, zinc borate, talc powder, barium sulfate, calcium carbonate, calcium stearate, and silicon dioxide.

[0019] Optionally, the co - blowing agent is selected from at least one of polyvinyl alcohol, polyethylene glycol, and potassium alum.

[0020] Optionally, the mass ratio of the foamed core layer to the non - foamed skin layer is (90 - 97):(3 - 10).

[0021] Another object of the present invention is to provide a method for preparing the foamed polypropylene beads as described above, comprising the following steps:

[0022] S1: According to the formulation amount of the foamed core layer, polypropylene A, polyethylene B, ethylene - acrylic acid copolymer, cell nucleating agent, and co - blowing agent are mixed and then extruded and pelletized through a twin - screw extruder to obtain a core - layer masterbatch;

[0023] S2: According to the formulation amount of the non - foamed skin layer, polyethylene C, polypropylene A, and ethylene - acrylic acid copolymer are mixed and then extruded and pelletized through a twin - screw extruder to obtain a skin - layer masterbatch;

[0024] S3: The core - layer masterbatch and the skin - layer masterbatch are respectively added to the core - layer extruder and the skin - layer extruder of a twin - single - screw co - extrusion machine set. After plasticization, they are simultaneously extruded through a co - extrusion die to form filaments and pelletized to obtain composite microparticles with a skin layer covering the core layer;

[0025] S4: The composite microparticles, dispersant, and water are added to an autoclave, sealed, and then CO 2, stir, raise the temperature of the reaction kettle to the foaming temperature, adjust the pressure in the kettle to the foaming pressure, hold for 5 - 30 min, relieve the pressure and discharge the material to atmospheric pressure to obtain foamed polypropylene beads.

[0026] Another object of the present invention is to provide a molded article prepared by steam molding of the foamed polypropylene beads as described above.

[0027] The beneficial effects of the present invention are as follows:

[0028] The foamed polypropylene beads provided by the present invention have a foamed core layer containing a specific content of polyethylene B, which increases the elasticity and viscosity of the material during the foaming process, improves the pressure parameters in the EPP foaming process, and can better control the stability of the foaming process during the preparation of low-foaming ratio EPP beads, obtaining foamed beads with uniform particle volume and density, which helps to reduce the fluctuations in processing parameters during the molding process and improve the appearance of the molded article; combined with the structure of a non-foaming skin layer covering the foamed core layer, and the skin layer material contains a specific content of polyethylene C and polypropylene A, the low-melting-point polyethylene C promotes the sintering between the bead skins, effectively reducing the molding energy consumption of the foamed beads, and at the same time the polypropylene A component in the skin layer prevents the bead skins from melting excessively during the molding process, and can effectively inhibit the adhesion of the surface of the molded article to the metal mold during the molding process with high-temperature steam. Detailed Embodiments

[0029] The present invention will now be described in further detail. The embodiments described below are exemplary and are intended to explain the present invention and should not be construed as limiting the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0030] To solve the problem of uneven particle volume and density distribution of low-foaming ratio EPP beads in the prior art, the present invention provides a foamed polypropylene bead, which includes a foamed core layer and a non-foaming skin layer covering the outside of the foamed core layer; wherein,

[0031] By weight, the foamed core layer includes the following components:

[0032]

[0033] By weight, the non-foaming skin layer includes the following components:

[0034] 40 - 75 parts of polyethylene C;

[0035] 20 - 50 parts of polypropylene A;

[0036] 5 - 15 parts of ethylene-acrylic acid copolymer.

[0037] The expanded polypropylene beads provided by the present invention have a foamed core layer containing a specific content of polyethylene B, which increases the elasticity and viscosity of the material during the foaming process, improves the pressure parameters of the EPP foaming process, and can better control the stability of the foaming process during the preparation of low-foaming-ratio EPP beads, obtaining foamed beads with uniform particle volume and density, which helps to reduce the fluctuations of processing parameters during the molding process and improve the appearance of molded parts; combined with the structure of a non-foaming skin layer covering the foamed core layer, and the skin layer material contains a specific content of polyethylene C and polypropylene A, the low-melting-point polyethylene C promotes the sintering between the bead skins, can effectively reduce the molding energy consumption of the foamed beads, and at the same time the polypropylene A component in the skin layer prevents the bead skins from melting excessively during the molding process, and can effectively inhibit the adhesion of the molded part skin to the metal mold during the molding process with high-temperature steam.

[0038] Specifically, the present invention preferably uses polyethylene B as linear low-density polyethylene, preferably a metallocene-catalyzed product; and further preferably, the melt index of this polyethylene B is 0.5-3 g / 10 min, the melting point is 110-130 °C, and the elongation at break is 300%-1000%.

[0039] By introducing polyethylene B into the foamed core layer, the present invention helps to increase the autoclave foaming pressure of the formulated material. Specifically, when producing EPP beads with a bulk density of 120-150 g / L by autoclave foaming, the foaming pressure is not less than 1.2 MPa, so that it is easier to obtain EPP beads with uniform particle volume and stable and controllable density; further, on the one hand, to avoid the weak effect on the increase of foaming pressure when the content of polyethylene B is too low, and on the other hand, to avoid the weakening of the heat resistance or mechanical properties of the material when the content of polyethylene B is too high, the present invention preferably adds 3-20 parts of polyethylene B.

[0040] The present invention preferably uses polypropylene A in both the foamed core layer and the non-foaming skin layer as propylene-butene or ethylene-propylene random copolymer polypropylene, such as SEETEC PP R3410 of LG Chemical; preferably, the melt index of polypropylene A is 6-9 g / 10 min, and the melting point is 137-150 °C.

[0041] The present invention preferably has a melt index of 5-10 g / 10 min for the ethylene-acrylic acid copolymer in both the foamed core layer and the non-foaming skin layer, and preferably the mass content of acrylic acid is 5-15%; by introducing the ethylene-acrylic acid copolymer, it helps to improve the stability of the material extrusion process.

[0042] The present invention preferably has an average particle size of 3-10 μm for the cell nucleating agent, and preferably the cell nucleating agent is selected from at least one of polytetrafluoroethylene powder, zinc borate, talc powder, barium sulfate, calcium carbonate, calcium stearate, and silicon dioxide.

[0043] The preferred blowing aid of the present invention is selected from at least one of polyvinyl alcohol, polyethylene glycol, and potassium alum.

[0044] In the present invention, polyethylene C is preferably linear low-density polyethylene or low-density polyethylene, preferably a metallocene-catalyzed product; the melt index of polyethylene C is preferably 4-15 g / 10 min, and the melting point is 105-125 °C.

[0045] The specific combination of the foamed core layer and the non-foamed skin layer provided by the present invention enables the foamed polypropylene beads to have the characteristic of easy sintering of the skin, reduces the molding energy consumption, and the skin of the sintered part is not easily adhered to the mold, avoiding the difficulty of demolding the EPP part, and contributing to improving the appearance of the part; the mass ratio of the foamed core layer to the non-foamed skin layer is preferably (90-97):(3-10).

[0046] In addition, if necessary, other plastic additives such as antioxidants, lubricants, antistatic agents, flame retardants, and colorants may also be included in the foamed polypropylene beads provided by the present invention.

[0047] Another object of the present invention is to provide a method for preparing the foamed polypropylene beads as described above, and the preparation method includes the following steps:

[0048] S1: According to the formulation amount of the foamed core layer, polypropylene A, polyethylene B, ethylene-acrylic acid copolymer, cell nucleating agent, and blowing aid are mixed and then granulated by twin-screw extrusion to obtain a core layer masterbatch.

[0049] S2: According to the formulation amount of the non-foamed skin layer, polyethylene C, polypropylene A, and ethylene-acrylic acid copolymer are mixed and then granulated by twin-screw extrusion to obtain a skin layer masterbatch.

[0050] S3: The core layer masterbatch and the skin layer masterbatch are respectively added to the core layer extruder and the skin layer extruder of a twin-single screw co-extrusion unit, plasticized and then extruded and granulated through a co-extrusion die at the same time to obtain composite particles with the skin layer covering the core layer.

[0051] The preparation of the composite particles with the average length and average single weight within a specific range is also beneficial to the uniformity of the volume size and density of the foamed beads. The average length of the composite particles is preferably 1.2-3 mm, and the average single weight is 1-3 mg. More preferably, the average length of the composite particles is 1.5-2.8 mm, and the average single weight is 1.2-2.5 mg; when the single weight is less than 1.0 mg, the particles tend to be finer, and the volume uniformity between the particles is more difficult to control in the process, and the variation tendency of the dispersion degree of each material among different particles is greater, affecting the foaming uniformity of the beads; when the single weight is higher than 3 mg, the foamed beads have a larger volume, which in turn affects the filling uniformity of the mold cavity during the molding process.

[0052] S4: Add the composite particles, dispersant, and water into an autoclave. After sealing, add CO 2 , stir continuously, raise the temperature of the reaction kettle to the foaming temperature of 145°C - 155°C, adjust the pressure in the kettle to the foaming pressure of 1.2 - 1.8 MPa, maintain for 5 - 30 min under these temperature and pressure conditions, relieve the pressure and discharge the material to atmospheric pressure to obtain foamed polypropylene beads with a bulk density of 120 - 150 g / L; the endothermic enthalpy value of the melting peak above the inherent melting point in the first DSC melting curve of the foamed beads is 8 - 15 J / g (hereinafter referred to as the "endothermic enthalpy value of the high-temperature melting peak").

[0053] The preparation method of the foamed polypropylene beads provided by the present invention uses polypropylene A, polyethylene B, ethylene-acrylic acid copolymer, cell nucleating agent, and co-blowing agent as the raw materials for the foaming core layer, and uses polyethylene C, polypropylene A, and ethylene-acrylic acid copolymer as the raw materials for the non-foaming skin layer; the foaming core layer contains a specific content of polyethylene B, which improves the pressure parameters in the EPP foaming process. During the preparation of low-foaming ratio EPP beads, the stability of the foaming process can be better controlled, and foamed beads with uniform particle volume and density can be obtained, which helps to reduce the fluctuations in processing parameters during the molding process and improve the appearance of the molded parts; combined with the structure of the non-foaming skin layer covering the foaming core layer, and the skin layer material contains specific contents of polyethylene C and polypropylene A, the low-melting polyethylene C promotes the sintering between the bead skins, which can effectively reduce the molding energy consumption of the foamed beads. At the same time, the polypropylene A component in the skin layer prevents the bead skin from melting excessively during the molding process, and can effectively inhibit the adhesion of the molded part skin to the metal mold during the molding process with high-temperature steam.

[0054] Another object of the present invention is to provide a molded part prepared by steam molding of the foamed polypropylene beads as described above.

[0055] The molded part provided by the present invention uses the above-mentioned foamed polypropylene beads with uniform particle volume and density as raw materials, which helps to reduce the fluctuations in processing parameters during the molding process and improve the appearance of the molded part; combined with the structure of the non-foaming skin layer covering the foaming core layer, it reduces the molding energy consumption and is not easy to stick to the mold.

[0056] To make the above objects, features, and advantages of the present invention more obvious and understandable, the specific embodiments of the present invention will be described in detail below.

[0057] The performance parameters of the materials selected in the examples and comparative examples of the present invention are shown in Table 1:

[0058] Table 1

[0059]

[0060] The melt index of the ethylene-acrylic acid copolymer is 7.5 g / 10 min, and the mass content of acrylic acid is preferably 10%.

[0061] The formulations and parameters of the composite particles S1-S6 and composite particles D1-D4 are shown in Table 2.

[0062] Table 2

[0063]

[0064]

[0065] Composite particles S1-S6 and composite particles D1-D5 were used to prepare foamed beads, which were then used to prepare molded parts. The parameters in the preparation process of Examples 1-7 and Comparative Examples 1-5 and the properties of the molded parts are shown in Table 3.

[0066] Table 3

[0067]

[0068] Among them, the minimum molding pressure is the minimum steam sintering pressure required for the proportion of particles with destroyed cells on the broken surface of the foamed bead molding to be more than 95%.

[0069] Apparent quality of the product: "ⅰ" indicates that there are many pits or gaps on the surface of the product; "ⅱ" indicates that there are a small number of pits or gaps on the surface of the product; "ⅲ" indicates that there are no pits or gaps on the surface of the product or very few or very small pits or gaps.

[0070] Whether to stick to mold: It means that the parts often stick to the mold surface, making demoulding difficult, or the mold sticking rate is not less than 10%, that is, the parts stick to the mold at least 10 times out of 100 moldings; "*" means that the mold sticking occurs occasionally, or the mold sticking rate is greater than 1% and less than 10%, that is, the number of mold sticking occurs between 1 and 10 times out of 100 moldings; It means that there is basically no mold sticking, or the mold sticking rate is less than 1%, that is, after 100 molding cycles, mold sticking occurs at most once or never.

[0071] Bulk density range of foam beads: indicates the maximum density fluctuation range that can be controlled for the density stability of foam beads in the case process parameters; the narrower the range, the more controllable the volume size and density uniformity of the foam beads; the wider the range, the more difficult it is to control the volume size and density uniformity of the foam beads, which is not conducive to foaming uniformity.

[0072] Appearance uniformity of expandable beads: "☆" indicates that when more than 10,000 expandable beads are laid flat for observation, obvious size non-uniformity is found among the particles; "☆☆" indicates that when more than 10,000 expandable beads are laid flat for observation, a small amount of size non-uniformity is found among the particles; "☆☆☆" indicates that when more than 10,000 expandable beads are observed with the naked eye, no obvious size non-uniformity is found among the particles.

[0073] From the data in the above table, it can be seen that each embodiment of the present invention can obtain EPP beads with uniform particle volume size, uniform density, a stable and controllable range of 120 - 150 g / L, and low molding energy consumption. The molded parts have excellent appearance and are easy to demold.

[0074] In Comparative Example 1, polyethylene B was not added to the foaming core layer. During the autoclave foaming process, the foaming pressure was lower than 1.2 MPa. During the pressure relief process of high-pressure autoclave foaming, it was difficult to stably control the pressure fluctuation in the autoclave. During different time periods of the pressure relief process, the density fluctuation of the expandable beads was very large, and the bulk density range of the expandable beads widened to 100 - 150 g / L, resulting in a wide density range of the molded parts. It was difficult for the molding factory to obtain molded parts with a foaming ratio and weight within the controllable process range; and the minimum compressive stress at 10% strain of the parts was low, that is, the compressive stress of the part with a density close to 115 g / L was small; at the same time, during the molding process, because the density range of the expandable beads was wide, the expansion and contraction ratio differences between the beads were large, resulting in poor appearance of the parts, that is, there were more pits or cracks on the surface of the parts.

[0075] In Comparative Example 2, the average single weight of the composite particles was less than 1 mg. It was more difficult to control the volume uniformity among the particles in terms of process, and the variation tendency of the dispersion degree of each material among different particles was greater, which had a certain impact on the density uniformity of the expandable beads. The bulk density range of the obtained expandable beads was relatively wide, in the range of 120 - 140 g / L; during the molding process, the expansion and contraction ratio differences between the beads were slightly large, resulting in a small amount of pits or cracks on the surface of the parts.

[0076] In Comparative Example 3, the skin layer material of the expandable beads did not contain easily sintered polyethylene C. The steam pressure during the molding process of the expandable beads was high, which was not conducive to maintaining the closed-cell structure inside the beads. There were a small amount of pits or cracks on the surface of the molded parts, and the minimum compressive stress at 10% strain decreased.

[0077] In Comparative Example 4, the skin layer material of the expandable beads did not contain polypropylene A. Although the steam pressure during the molding process of the expandable beads was low, the situation that the surface of the parts adhered to the metal mold surface occasionally occurred during the molding process, affecting demolding.

[0078] The physical properties of polyethylene B added to the foam core layer in Comparative Example 5 are not within the required range of the present invention, and the foaming pressure in the autoclave foaming process cannot be effectively improved, that is, the foaming pressure is still lower than 1.2 MPa, and the bulk density range of the obtained foamed beads is widened to 100-150 g / L. There are more pits or gaps on the surface of the molded parts, and the density range of the molded parts is wide. It is difficult for the molding plant to obtain molded parts with foaming ratio and weight within the controllable process range; and the minimum compressive stress of the part at 10% strain is low, that is, the compressive stress of the part with a density close to 115 g / L is small.

[0079] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A foamed polypropylene bead, characterized in that, it comprises a foamed core layer and a non-foamed skin layer coated on the outside of the foamed core layer; By weight, the foamed core layer comprises the following components: 70-94 parts of polypropylene A; 3-20 parts of polyethylene B; 2-8 parts of ethylene-acrylic acid copolymer; 0.02-0.5 part of cell nucleating agent; 0.05-2 parts of auxiliary blowing agent; By weight, the non-foamed skin layer comprises the following components: 40-75 parts of polyethylene C; 20-50 parts of polypropylene A; 5-15 parts of ethylene-acrylic acid copolymer; The melt index of the polyethylene B is 0.5-3 g / 10 min, the melting point is 110-130 °C, and the elongation at break is 300%-1000%; The melt index of the polypropylene A is 6-9 g / 10 min, and the melting point is 137-150 °C; The melt index of the polyethylene C is 4-15 g / 10 min, and the melting point is 105-125 °C; The foamed polypropylene bead is prepared by the following method: S1: According to the formulation amount of the foamed core layer, polypropylene A, polyethylene B, ethylene-acrylic acid copolymer, cell nucleating agent, and auxiliary blowing agent are mixed and then granulated by twin-screw extrusion to obtain a core layer masterbatch; S2: According to the formulation amount of the non-foamed skin layer, polyethylene C, polypropylene A, and ethylene-acrylic acid copolymer are mixed and then granulated by twin-screw extrusion to obtain a skin layer masterbatch; S3: The core layer masterbatch and the skin layer masterbatch are respectively added to the core layer extruder and the skin layer extruder of a twin-single screw co-extrusion unit, plasticized and then extruded through a co-extrusion die to form filaments and granulated to obtain composite microparticles with the skin layer coating the core layer; S4: Add the composite particles, dispersant and water into an autoclave, seal it and then add CO 2 , stir, raise the temperature of the reaction kettle to the foaming temperature, adjust the pressure in the kettle to the foaming pressure, maintain for 5 - 30 min, relieve the pressure and discharge the material to atmospheric pressure to obtain foamed polypropylene beads; The average length of the composite microparticles is 1.2-3 mm, and the average single weight is 1-3 mg.

2. The foamed polypropylene bead according to claim 1, characterized in that, the average particle size of the cell nucleating agent is 3-10 μm.

3. The foamed polypropylene bead according to claim 2, characterized in that, the cell nucleating agent is selected from at least one of polytetrafluoroethylene powder, zinc borate, talc powder, barium sulfate, calcium carbonate, calcium stearate, and silicon dioxide.

4. The foamed polypropylene bead according to claim 1, characterized in that, the auxiliary blowing agent is selected from at least one of polyvinyl alcohol, polyethylene glycol, and potassium alum.

5. The foamed polypropylene bead according to any one of claims 1-4, characterized in that, the mass ratio of the foamed core layer to the non-foamed skin layer is (90-97):(3-10).

6. A molded article, characterized in that, it is prepared by steam molding of the foamed polypropylene bead according to any one of claims 1-5.

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