CN110551312B - Polyethylene resin foamed particles, method for producing same, and polyethylene resin foamed molded article - Google Patents

Polyethylene resin foamed particles, method for producing same, and polyethylene resin foamed molded article Download PDF

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CN110551312B
CN110551312B CN201810565643.9A CN201810565643A CN110551312B CN 110551312 B CN110551312 B CN 110551312B CN 201810565643 A CN201810565643 A CN 201810565643A CN 110551312 B CN110551312 B CN 110551312B
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polyethylene resin
particles
expanded
bulk density
pressure
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CN110551312A (en
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福泽淳
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Zhong Hua Suzhou Cushioning Material Co ltd
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Zhong Hua Suzhou Cushioning Material Co ltd
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
    • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
    • C08J9/122—Hydrogen, oxygen, CO2, nitrogen or noble gases
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
    • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
    • C08J9/125—Water, e.g. hydrated salts
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/16—Making expandable particles
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/22—After-treatment of expandable particles; Forming foamed products
    • C08J9/228—Forming foamed products
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2203/00—Foams characterized by the expanding agent
    • C08J2203/06—CO2, N2 or noble gases
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2203/00—Foams characterized by the expanding agent
    • C08J2203/10—Water or water-releasing compounds
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2323/04—Homopolymers or copolymers of ethene
    • C08J2323/08—Copolymers of ethene

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  • Chemical Kinetics & Catalysis (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
  • Organic Chemistry (AREA)
  • Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)

Abstract

The present invention relates to polyethylene resin foamed particles, a method for producing the same, and a polyethylene resin foamed molded article. The polyethylene resin foamed particles have a bulk density of 8 to 20g/L and a heat shrinkage of 30 to 50%. The expanded beads can give a foamed molded article having a wide range of molding conditions, a beautiful appearance, and a light weight, and are particularly suitable for producing a complicated foamed molded article having a thin wall portion.

Description

Polyethylene resin foamed particles, method for producing same, and polyethylene resin foamed molded article
Technical Field
The invention relates to polyethylene resin foamed particles for manufacturing a thin-wall buffer material, a preparation method thereof and a polyethylene resin foamed molded body obtained by carrying out in-mold foaming molding on the polyethylene resin foamed particles, belonging to the technical field of polymer foamed materials.
Background
Polyethylene resin foamed molded articles are widely used for various purposes such as transport boxes for postal services and logistics services as cushioning packaging materials and heat insulating materials having excellent flexibility and heat insulating properties. In recent years, polyethylene resin foamed molded articles have been gradually developed to have more complicated three-dimensional shapes. For example, in order to increase the number of articles that can be packed in each transport box as much as possible to reduce the transport costs, it is generally required to reduce the thickness of the frames and partitions as much as possible to the minimum extent that the contents are not damaged, so the fixed frames and partitions of the molding body are generally formed of thin-walled portions having a thickness as small as about 8 mm; for example, a projection is required to absorb the impact force when a fall occurs. In addition, the foamed molded article is required to have a light weight for easy transportation.
In general, a foamed molded article is obtained by filling foamed particles into a mold of an in-mold forming machine and performing in-mold foaming. Since the bulk density of the expanded beads reflects the density of the molded article, in order to obtain a foamed molded article having a light weight and a complicated shape such as a thin portion and a convex portion, it is necessary to use expanded beads having a low bulk density and excellent in-mold formability.
In the production of polyethylene resin foamed particles, an inorganic gas such as carbon dioxide is generally used as a blowing agent, and the resin particles are impregnated with the inorganic gas under steam heating to form expanded foamed particles (see patent document 1). Patent document 2 discloses a method for producing expanded linear low-density polyethylene resin particles by dispersing linear low-density polyethylene resin particles together with an organic volatile blowing agent in a dispersion medium, heating and pressurizing the dispersion medium to impregnate the resin particles with the organic volatile blowing agent, and then releasing the resin particles from a low-pressure region to foam the resin particles. Patent document 3 discloses that a decompression foaming step is performed using carbon dioxide as a foaming agent, and the resultant expanded beads are more environmentally friendly than organic volatile foaming agents, but the foaming ability is reduced, and when it is difficult to obtain primary foaming of expanded beads having a high expansion ratio, secondary foaming can be performed on the obtained expanded beads. Patent document 4 discloses that selecting polyethylene-based expanded particles having a specific DSC curve after at least 2 foaming steps can solve the problems of poor fusion at the ends of an in-mold foamed article and poor appearance. In order to obtain a polyethylene resin foamed molded article having a small shrinkage rate with respect to the mold size, a small deformation, a good surface tension, and antistatic properties, patent document 5 also discloses that a non-crosslinked polyethylene resin foamed molded article satisfying the requirements can be obtained by performing in-mold foam molding using non-crosslinked polyethylene resin foamed particles having appropriate shrinkage properties and containing an antistatic agent, and specifically, in this document, shrinkage is expressed by using a shrinkage rate obtained from a bulk density BD of the non-crosslinked polyethylene resin foamed particles at 23 ℃ and 0.1MPa and a bulk density VBD of the non-crosslinked polyethylene resin foamed particles at 23 ℃ and under a reduced pressure of 0.002MPa or less, as represented by the following formula (a): shrinkage rate ═
(BD-VBD)÷VBD×100(A)。
However, there are difficulties in producing a polyethylene foamed molded article having a complicated shape and a light weight by using the polyethylene foamed particles obtained by the method disclosed in the above document, and further improvement in the in-mold moldability thereof is required. This is because, when in-mold molding is performed using a mold of a complicated shape, the foamed particles produced by the above method tend to be difficult to fill in the thin wall portion of the molding die and in a portion away from the mold inlet, and since the foamed particles are easily expanded in the mold, it is difficult for steam to uniformly pass through the mold, which easily causes uneven heating of the steam, and fusion failure between partially expanded particles occurs. The heating temperature range suitable for forming is difficult to regulate and control, and various forming defects are easy to occur on the thin-wall forming body. In addition, in general, in order to reduce the bulk density, the art also generally performs two-stage foaming using foamed particles having a relatively large average particle weight. Expanded beads having a large average particle weight have a relatively large particle diameter, and therefore, the filling property into the thin portion is deteriorated, the heating temperature range during in-mold molding becomes narrower, and it becomes more difficult to obtain a molded article having a good appearance. However, if expanded beads having a small average particle weight are used to improve the filling property into the thin wall portion, on the one hand, the expansion ratio is relatively low, and on the other hand, the shrinkage ratio of the in-mold expanded molded article tends to increase, and wrinkles on the surface become conspicuous, resulting in deterioration of the surface appearance.
(Prior art document)
Patent document 1: JP Kokai Ping 4-372630A (published: 1992, 12 and 25)
Patent document 2: JP Kokai Hei 2-53837A (published: 1990 2 months and 22 days)
Patent document 3: JP Kokai 2000-17079A (published: 1/18/2000)
Patent document 4: WO2011-086937A1 (published: 2011 year 7 month 21 day)
Patent document 5: WO2013-011951A1 (published: 24.1.2013)
Disclosure of Invention
Problems to be solved by the invention
In order to solve the above problems of the prior art, a main object of the present invention is to provide polyethylene resin expanded beads which can be used for producing a foamed molded article having an aesthetic appearance, a light weight and a wide molding processing condition, particularly a complicated molded article having a thin portion, and which have a low bulk density and excellent in-mold moldability.
Another object of the present invention is to provide a method for preparing the above expanded polyethylene resin beads, which can obtain expanded beads having a low bulk density and a small average particle weight without changing the existing equipment excessively, and which can be operated easily and can be produced efficiently.
It is another object of the present invention to provide a foamed molded article having a wide range of molding conditions in-mold foaming, a beautiful appearance, and a light weight, and particularly a complicated foamed molded article having a thin portion.
Means for solving the problems
As a result of diligent research directed toward solving the above problems, the present inventors have found that by using polyethylene resin expanded beads having a suitable heat shrinkage ratio and bulk density of the expanded beads and performing in-mold expansion molding, a foamed molded article having a wide range of molding conditions, an attractive appearance, and a light weight can be obtained, and that the present invention is particularly suitable for producing a complicated foamed molded article having a thin portion.
The invention comprises the following technical scheme:
[1] a polyethylene resin expanded bead having a bulk density of 8 to 20g/L and a heat shrinkage ratio of 30 to 50% as determined by the following formula (1),
heat shrinkage (%) of expanded beads ═ Va-Vb)/Va×100 (1)
Wherein, VaRefers to the packing volume of the expanded particles; vbThe method is characterized in that the foaming particles are heated for 1 hour at the temperature of 100 ℃, and then the stacking volume is measured after the foaming particles are placed for 1 hour at the temperature of 23 ℃; bulk density, Va、VbAll were measured at 23 ℃ and 0.1 MPa.
[2] The polyethylene resin expanded beads according to [1], wherein the expanded beads have a bulk density of 9 to 18 g/L.
[3] The expanded polyethylene resin beads according to claim 1 or 2, wherein the expanded beads have an average bead weight of 0.3 to 3.0 mg.
[4] The expanded polyethylene resin beads according to any one of [1] to [3], wherein the expanded beads have an average bead weight of 0.5 to 2.5 mg.
[5] The expanded polyethylene resin particle according to any one of [1] to [4], wherein the expanded polyethylene resin particle is obtained by three or more times of expansion.
[6] The polyethylene resin foamed particles according to [5], wherein the bulk density of the foamed particles obtained by the final foaming is: the bulk density of the expanded particles obtained in the previous expansion is 1 (1.1-2.5).
[7] The expanded polyethylene resin beads according to any one of items [1] to [6], wherein the base resin of the expanded polyethylene resin beads mainly comprises a linear low density polyethylene resin and a high density polyethylene resin in an amount of 0 to 10% by weight based on the total weight of the polyethylene resin.
[8]According to [7]The polyethylene resin foamed particles are characterized in that the linear low-density polyethylene resin has a density of 0.92 to 0.94g/cm3And the melt index is 1.0-4.0 g/10 min.
[9] A method for producing polyethylene resin foamed particles, comprising:
1) adjusting the size of polyethylene resin particles so that the average particle weight of finally obtained expanded particles is adjusted to 0.3-3.0 mg, dispersing the polyethylene resin particles and a foaming agent in a dispersion medium in a closed container, heating and pressurizing, and releasing the polyethylene resin particles to a pressure area lower than the internal pressure of the closed container to obtain first-stage expanded polyethylene resin particles;
2) the polyethylene resin first-stage expanded particles are treated by the following steps: pressurizing the inorganic gas by the inorganic gas to increase the internal pressure of the inorganic gas impregnated into the particles to be more than the atmospheric pressure, and then performing heating treatment on the particles for at least 20 seconds by using water vapor with the pressure of 0.02MPa to 0.07MPa to further foam the particles to obtain polyethylene resin second-stage foamed particles, wherein the pressure value of the water vapor is the gauge pressure;
3) repeating n times the step 2) to obtain expanded polyethylene resin particles, wherein n is an integer of 1 or more, and controlling the internal pressure of the inorganic gas impregnated into the particles and the pressure of the steam so that the bulk density of the expanded polyethylene resin particles obtained by repeating n times: the bulk density of the polyethylene resin expanded beads obtained by repeating the n-1-pass was 1 (1.1 to 2.5).
[10] The process for producing expanded polyethylene resin beads according to [9], wherein the expanded polyethylene resin beads obtained by the process have a bulk density of 8 to 20g/L and a heat shrinkage ratio of 30 to 50% as determined by the following formula (1),
heat shrinkage (%) of expanded beads ═ Va-Vb)/Va×100 (1)
Wherein, VaRefers to the packing volume of the expanded particles; vbThe method is characterized in that the foaming particles are heated for 1 hour at the temperature of 100 ℃, and then the stacking volume is measured after the foaming particles are placed for 1 hour at the temperature of 23 ℃; bulk density, Va、VbAll were measured at 23 ℃ and 0.1 MPa.
[11] The process according to [9] or [10], wherein the ratio of the bulk density of the polyethylene resin expanded beads obtained by repeating n cycles in step 3) to the bulk density of the polyethylene resin expanded beads obtained by repeating n-1 cycles is 1 (1.5-2.2).
[12] A polyethylene resin foamed molded article obtained by filling the polyethylene resin foamed particles according to any one of [1] to [8] or the polyethylene resin foamed particles produced by the production method according to any one of [9] to [11] in a mold and then performing in-mold foam molding.
ADVANTAGEOUS EFFECTS OF INVENTION
The polyethylene resin expanded beads according to the present invention have a low bulk density and excellent in-mold moldability, although the average bead weight is small. The expanded beads can be efficiently obtained without changing the existing equipment much, and even when used for producing a complicated foamed molded article having a thin wall portion, the expanded beads can be molded under a wide range of molding conditions, and an in-mold foamed molded article having a good appearance and substantially no wrinkles on the surface can be obtained.
Detailed Description
The following describes embodiments of the present invention, but the present invention is not limited to these embodiments. The present invention is not limited to the configurations described below, and various modifications are possible within the scope of the claims, and embodiments and examples obtained by appropriately combining the technical means disclosed in the respective embodiments and examples are also included in the technical scope of the present invention. All documents described in this specification are incorporated herein by reference.
In the present specification, the term "foaming material" refers to a material obtained through a foaming step. In particular, a foam obtained by in-mold foaming is referred to as an "in-mold foamed molded article".
"bulk volume" means the volume of the particles under natural packing, and in the present specification means the volume measured by filling the expanded particles into a volumetric cylinder at 23 ℃ and 0.1 MPa.
< polyethylene resin pellets >
The polyethylene resin particles of the present invention are used for producing the polyethylene resin expanded particles of the present invention. That is, the polyethylene resin foamed particles of the present invention are produced by foaming the polyethylene resin particles of the present invention. The polyethylene resin particles in the present invention include a polyethylene resin as a base resin and an additive such as a hydrophilic compound.
Examples of the polyethylene resin used as the base resin in the present invention include a high-density polyethylene resin, a medium-density polyethylene resin, a low-density polyethylene resin, and a linear low-density polyethylene resin. In order to increase the expansion ratio and mechanical strength, linear low density polyethylene (or linear low density polyethylene, sometimes abbreviated as "LLDPE") is preferably used.
The density of the linear low-density polyethylene resin used in the present invention is preferably 0.920g/cm3～0.940g/cm3More preferably 0.925g/cm3Above and below 0.940g/cm3. If the density is less than 0.920g/cm3In the case of the polyethylene resin foam molded article, the shrinkage rate tends to increase, and when the density is 0.940g/cm3In the above case, the range of the heating temperature for molding in-mold molding tends to be narrow, and the density is defined in JIS K7112The value measured as a reference. The linear low-density polyethylene resin preferably has a melt index (abbreviated as "MI") of 1.0 to 4.0g/10min, more preferably 1.5 to 2.5g/10min, where MI is a value measured at 190 ℃ under a load of 2.16kg according to JIS K7210. The MI is high, the fluidity in the molten state is relatively good, and the surface appearance of the molded article is good, but a long water cooling time is required to control the shape of the molded article, and the production cycle is long. In the present invention, if the MI of LLDPE is less than 1.5g/10min, the in-mold foamed molded article may suffer from poor appearance; when the MI of LLDPE exceeds 4.0/10min, the in-mold foamed article tends to have a large percentage of cells connected and to easily shrink.
In the present invention, a plurality of linear low-density polyethylene resins having different densities and molecular weights may be mixed and used, or a high-density polyethylene may be blended with LLDPE, and in order to expand the pressure range at the time of in-mold molding, it is preferable to blend 0 to 10% by weight of the high-density polyethylene based on the total weight of the polyethylene resins.
The composition of the linear low-density polyethylene resin used in the present invention includes homopolymers of ethylene and copolymers of ethylene and an α -olefin having 4 to 10 carbon atoms. Examples of the α -olefin having 4 to 10 carbon atoms include 1-butene, 1-pentene, 1-hexene, 3-dimethyl-1-butene, 4-methyl-1-pentene, 4-dimethyl-1-pentene, 1-octene, and the like. Without limitation, in one embodiment of the invention the α -olefin is selected from 4-methyl-1-pentene. The content of these alpha-olefins in the entire polyethylene resin is preferably 1 to 20% by weight, and particularly preferably 5 to 9% by weight. When the content of the α -olefin exceeds 20% by weight, the molded article tends to have a low mechanical strength.
In addition to the base resin, a hydrophilic compound is preferably added to the polyethylene resin particles. By adding the hydrophilic compound, expanded beads having a high expansion ratio can be easily obtained even when an inorganic gas is used as a foaming agent.
The hydrophilic compound used in the present invention is a compound containing a hydrophilic group such as a carboxyl group, a hydroxyl group, an amino group, a sulfo group, or a polyoxyethylene group in its molecule, or a derivative thereof, and also includes a hydrophilic polymer. Specifically, examples of the compound having a carboxyl group include lauric acid and sodium laurate, and examples of the compound having a hydroxyl group include ethylene glycol and glycerin. Examples of the other hydrophilic organic compound include organic compounds having a triazine ring such as melamine and isocyanuric acid. These can be used alone, also can be used in combination of more than 2. Among these hydrophilic compounds, glycerin is preferred, which makes it easy to obtain expanded particles having a high expansion ratio when an inorganic gas is used as a foaming agent, and which can reduce the foaming pressure for obtaining expanded particles having a desired expansion ratio.
The hydrophilic polymer is a polymer having a water absorption rate of 0.5 wt% or more as measured in accordance with ASTM D570, and includes so-called hygroscopic polymers; a water-absorbent polymer which is a polymer that is insoluble in water, absorbs water several times to several hundred times its own weight, and is difficult to dehydrate even when pressure is applied; and a water-soluble polymer which is a polymer dissolved in water at normal temperature or high temperature. Examples of the hydrophilic polymer include carboxyl group-containing polymers such as ethylene- (meth) acrylic acid copolymers; polyamides such as nylon-6, and copolymerized nylon; nonionic water-absorbent polymers such as polyethylene glycol and polypropylene glycol; polyether-polyolefin resin block copolymers represented by PELESTAT (trade name, manufactured by Sanyo chemical Co., Ltd.); crosslinked polyethylene oxide polymers typified by AQUACALK (trade name, manufactured by Sumitomo refining Co., Ltd.) and the like. These can be used alone, also can be used in combination of more than 2. Among these hydrophilic polymers, nonionic water-absorbent polymers and polyether-polyolefin resin block copolymers are preferable because they have relatively good dispersion stability in a pressure-resistant container and exhibit water absorption when added in a small amount.
The polyethylene resin particles of the present invention preferably contain the hydrophilic compound in an amount of 0.01 to 10 parts by weight, more preferably 0.1 to 1 part by weight, based on 100 parts by weight of the polyethylene resin. If the content of the hydrophilic compound is less than 0.01 parts by weight, the water content in the obtained expanded beads is relatively small, and the heat shrinkage rate of the expanded beads tends to be too small, and if the content is more than 10 parts by weight, the surface appearance and mechanical properties of the in-mold foamed molded article may be impaired.
The polyethylene resin particles may further contain a foam nucleating agent for promoting the formation of cell nuclei. Examples of the foam nucleating agent used in the present invention include inorganic nucleating agents such as talc, calcium stearate, calcium carbonate, silica, kaolin, titanium oxide, bentonite, barium sulfate, and zinc borate. These can be used alone, also can be used in combination of more than 2. Of these foaming nucleating agents, talc, calcium carbonate and calcium stearate are preferable, which are inexpensive and facilitate uniform cell formation. The amount of the foam nucleating agent of the present invention to be added varies depending on the type of the foaming agent to be used, and is generally 0.005 to 2 parts by weight, more preferably 0.01 to 1 part by weight, based on 100 parts by weight of the polyethylene resin. When the amount of the foam nucleating agent added is less than 0.005 part by weight, there is a tendency that a molded article of low bulk density cannot be obtained. When the amount of the foam nucleating agent added exceeds 2 parts by weight, the average cell diameter of the polyethylene resin foamed particles tends to be too small, and the in-mold foam moldability tends to be poor.
Additives such as antistatic agents, colorants, flame retardants, heat stabilizers, light stabilizers, and radiation heat transfer inhibitors may be added to the polyethylene resin particles as necessary without impairing the effects of the present invention. Examples of the heat stabilizer include a hindered amine compound, a phosphorus compound, and an epoxy compound. Examples of light stabilizers include hindered amines, phosphorous stabilizers, epoxy compounds, phenolic antioxidants, nitrogen-containing stabilizers, sulfur stabilizers, benzotriazoles, and the like.
Examples of the colorant used in the present invention include inorganic pigments such as carbon black, ketjen black (ケッチェンブラック), iron black, cadmium yellow, cadmium red, cobalt violet, cobalt blue, prussian, ultramarine blue, chrome yellow, zinc yellow, barium yellow and the like; organic pigments such as polyazo, quinacridone, phthalocyanine, perinone, anthraquinone, thioindigo, diwuqin oxazine, isoindolinone and quinophthalone.
The antistatic agent used in the present invention is not particularly limited, and examples thereof include low-molecular antistatic agents such as fatty acid ester compounds, aliphatic ethanolamine compounds and aliphatic ethanolamine compounds, and high-molecular antistatic agents. These antistatic agents may be used alone, or 2 or more of them may be used in combination. For example, a commercially available mixture of octadecyl diethanolamine monostearate and octadecyl diethanolamine may be exemplified by Electro Stripper TS-11B (manufactured by Kao corporation), and a commercially available mixture of octadecyl diethanolamine monostearate, octadecyl diethanolamine and aliphatic alcohol may be exemplified by Electro Stripper TS-15B (manufactured by Kao corporation). The antistatic agent is preferably contained in an amount of 0.1 to 3 parts by weight, particularly preferably 0.2 to 2 parts by weight, based on 100 parts by weight of the polyethylene resin. When the content of the antistatic agent is less than 0.1 part by weight, antistatic performance tends to be hardly exhibited, and when it exceeds 3 parts by weight, the deformation shrinkage of the obtained foamed molded article tends to be large, and the stretching of the surface of the molded article tends to be deteriorated.
The present invention is used for a radiation heat transfer inhibitor (a substance having a property of reflecting, scattering, or absorbing light in the near infrared ray or infrared ray region (for example, in a wavelength range of 800 to 3000 nm)), for example, graphite, graphene, activated carbon, carbon black, titanium dioxide, metal aluminum, or the like.
< Process for producing polyethylene resin pellets >
Examples of the method for producing the polyethylene resin pellets of the present invention include the following production steps (also referred to as "granulation step"). First, a polyethylene-based resin and additives such as a hydrophilic compound, a foam nucleating agent and the like as needed are blended by a mixing method such as a dry blending method, a masterbatch method and the like. Next, the obtained mixture is melt-kneaded by an extruder, a kneader, a banbury mixer, or the like, extruded, and cut by a cutter, a pelletizer, or the like, to obtain polyethylene resin pellets having a desired shape such as a cylindrical shape, an elliptical shape, a spherical shape, a cubic shape, or a rectangular parallelepiped shape. Alternatively, the blend may be extruded from a die directly into water, cut into a pellet shape immediately after extrusion, and cooled.
Without limitation, in one embodiment of the invention an extruder (e.g., a twin screw extruder) process is used. Specifically, a polyethylene resin is blended with a hydrophilic compound, a foam nucleating agent, and other additives, if necessary, and the blend is fed into an extruder, melt-kneaded at a resin temperature of about 280 ℃, extruded through a die attached to the tip of the extruder, water-cooled, and then chopped to obtain polyethylene resin pellets. Alternatively, when a liquid hydrophilic compound is used, the liquid hydrophilic compound may be added to a molten polyethylene resin in the middle of an extruder and kneaded. Further, the hydrophilic compound may be supplied in a liquid state in a fixed amount to a hopper portion into which the polyethylene resin is fed in an extruder.
The weight of each polyethylene resin particle (also referred to as "average particle weight") in the present invention is preferably 0.3 to 3.0mg, more preferably 0.5 to 2.5mg, and still more preferably 0.5 to 2.0 mg. When the average particle weight of the polyethylene resin particles is less than 0.3mg, the shrinkage of the in-mold foamed molded article tends to increase, and when the average particle weight is more than 3.0mg, it may be difficult to fill the thin portion of the mold. Here, the average particle weight of the polyethylene resin particles can be measured by the following method: 500 polyethylene resin particles were randomly selected and divided into 5 groups of 100 particles, the weight (mg) of each group of resin particles was measured by an electronic balance, and then divided by the average particle weight of 100 groups, the highest value and the lowest value were removed, and the average of the average particle weights of the middle 3 groups was taken as the average particle weight of the polyethylene resin particles. In addition, the weight of each polyethylene resin particle hardly changes even after the foaming step, and there is no problem in setting the average particle weight of the polyethylene resin particles as the average particle weight of the polyethylene resin foamed particles.
< polyethylene resin expanded beads >
The polyethylene resin foamed particles of the present invention have a bulk density of 8 to 20g/L and a heat shrinkage of 30 to 50% as determined by the following formula (1).
Heat shrinkage (%) of expanded beads ═ Va-Vb)/Va×100 (1)
Wherein, VaRefers to the packing volume of the expanded particles; vbThe method is characterized in that the foaming particles are heated for 1 hour at the temperature of 100 ℃, and then the stacking volume is measured after the foaming particles are placed for 1 hour at the temperature of 23 ℃; bulk density, Va、VbAll were measured at 23 ℃ and 0.1 MPa. The bulk volume is measured by filling a predetermined amount of the foamed particles into a measuring cylinder or the like.
The method for measuring the bulk density of the invention is as follows: the internal volume (internal volume VL) of the container was measured at 23 ℃ under 0.1MPa, and then expanded particles were added to the container, vibrated for 30 seconds with a vibrator and filled into the container, the expanded particles were added so that the upper surface of the expanded particles was above the top end surface of the container, and after the expanded particles were filled, the top end surface of the container was scraped with a straight plate in an upright state to make the top end surface of the container flush with the surface of the expanded particles. The weight W g of the expanded beads remaining in the container is weighed. The bulk density of the polyethylene resin expanded beads at 23 ℃ and 0.1MPa can be calculated from the formula (2).
Bulk density (g/L) ═ W/V (2)
The polyethylene resin foamed particles of the present invention are obtained by foaming polyethylene resin particles. The present inventors have found that, while polyethylene resin foamed particles shrink due to heating and pressurization during in-mold molding, the present inventors have simultaneously controlled the heating shrinkage rate and bulk density of the foamed particles, and have been effective in obtaining a complex molded article having a thin portion, excellent in appearance, free from wrinkles, wide in molding conditions, and light in weight.
The present inventors have conducted intensive studies on various physical properties of expanded beads in order to find a suitable characterization method of the heat shrinkage property of expanded beads by simulating the shrinkage process by heating under pressure which the expanded beads undergo during in-mold molding. The shrinkage rate disclosed in patent document 5 can reflect the heat shrinkage performance of the expanded beads to some extent, but when a complicated in-mold molded article having a thin portion is produced using the expanded beads disclosed in the above documents, there is a possibility that excessive local heating may occur in the thin portion and the surface of the molded article tends to be wrinkled, and these problems need to be further improved.
The present inventors have found through studies that the heat shrinkage ratio of the expanded beads represented by formula (1) can better exhibit the heat shrinkage ratio possessed by the resin expanded beads, and can reflect the in-mold moldability possessed by the expanded beads in producing a complex molded article having a thin-walled portion.
In the present invention, when the heat shrinkage rate of the polyethylene resin expanded beads is less than 30%, there are problems that the filling property of the expanded beads into a mold is poor, the partial shrinkage rate is increased, and the surface smoothness is deteriorated when an in-mold expanded molded article having a thin portion is formed; when the heat shrinkage rate exceeds 50%, the foamed molded article tends to shrink or deform, and it is difficult to obtain a molded article having a desired size. In short, if the heat shrinkage rate is not within the range defined in the present invention, the surface of the polyethylene resin foamed molded article having a complicated shape obtained by in-mold foaming is wrinkled, and the surface appearance is poor.
Further, it is necessary to control the bulk density of the polyethylene resin foamed particles of the present invention to be 8 to 20g/L, and more preferably 9 to 18 g/L. When the bulk density is less than 8g/L, the shrinkage rate under heating becomes large, and therefore, the polyethylene resin foamed molded article tends to be easily shrunk or deformed, and the mechanical properties are deteriorated. When the bulk density exceeds 20g/L, the advantage of the lightweight property of the molded article is lost and the heat shrinkage tends to be too low, so that it is difficult to obtain a molded article having an excellent surface appearance.
The present inventors have found that the magnitude of the heat shrinkage of the polyethylene resin expanded beads depends not only on the bulk density of the expanded beads but also on the average weight of the expanded beads, the process of expanding the expanded beads, and the like.
The average weight of the polyethylene resin foamed particles is preferably 0.3 to 3.0mg, more preferably 0.5 to 2.5mg, and still more preferably 0.5 to 2.0 mg. When the average particle weight is less than 0.3mg, variations in weight are liable to occur in production, and when it exceeds 3.0mg, the particle diameter of the foam particles tends to become too large to deteriorate the mold filling property, and wrinkles tend to occur on the surface of the molded article. The average particle weight may be determined by the same method as the average particle weight of the polyethylene resin particles or may be directly determined as the average particle weight of the polyethylene resin expanded particles.
In the present invention, a method for obtaining polyethylene resin foamed particles having a heat shrinkage ratio of 30 to 50% comprises foaming polyethylene resin particles three times (or three times) or more. Preferably, the bulk density of the foamed particles obtained by the last foaming is adjusted by adjusting parameters such as pressure in the foaming process: the bulk density of the expanded beads obtained in the previous expansion was 1: (1.1-2.5). From the viewpoint of saving the production cost and improving the heat shrinkage property of the expanded beads, the ratio of the bulk density of the expanded beads obtained by the last expansion to the bulk density of the expanded beads obtained by the previous expansion is further preferably 1 (1.1 to 2.2), more preferably 1: (1.5-2.2). If the bulk density of the expanded beads obtained by the previous expansion is 2.5 times or more the bulk density of the expanded beads obtained by the last expansion, the filling property of the expanded beads tends to be deteriorated, and wrinkles tend to be easily formed on the surface of the molded article.
< method for producing expanded polyethylene resin beads >
The method for producing the polyethylene resin particles of the present invention may comprise the steps of:
1) adjusting the size of polyethylene resin particles so that the average particle weight of finally obtained expanded particles is adjusted to 0.3-3.0 mg, dispersing the polyethylene resin particles and a foaming agent in a dispersion medium in a closed container, heating and pressurizing, and releasing the polyethylene resin particles to a pressure area lower than the internal pressure of the closed container to obtain first-stage expanded polyethylene resin particles;
2) the polyethylene resin first-stage expanded particles are treated by the following steps: pressurizing the inorganic gas by the inorganic gas to increase the internal pressure of the inorganic gas impregnated into the particles to be more than the atmospheric pressure, and then performing heating treatment on the particles for at least 20 seconds by using water vapor with the pressure of 0.02MPa to 0.07MPa to further foam the particles to obtain polyethylene resin second-stage foamed particles, wherein the pressure value of the water vapor is the gauge pressure;
3) repeating n times the step 2) for the obtained expanded beads, wherein n is an integer of 1 or more, preferably n is 1 or 2, and controlling the internal pressure of the inorganic gas impregnated into the beads and the pressure of the steam so that the bulk density of the polyethylene resin expanded beads obtained by repeating n times: the bulk density of the polyethylene resin expanded beads obtained by repeating the n-1-pass was 1 (1.1 to 2.5).
The blowing agent used in step 1) is, without limitation, water and/or an inorganic gas such as carbon dioxide, air, nitrogen and the like, which may be used alone or in combination of plural, which have a small environmental load and no risk of combustion, from the viewpoint that foamed particles having a low bulk density are easily obtained, and carbon dioxide is most preferable.
The dispersion medium in step 1) is preferably an aqueous dispersion medium, and the aqueous dispersion medium used in the present invention is preferably water alone, but a dispersion medium in which methanol, ethanol, ethylene glycol, glycerin, or the like is added to water may be used. In the case where the hydrophilic compound is contained in the present invention, water in the aqueous dispersion medium also functions as a foaming agent, and contributes to an increase in expansion ratio. In order to prevent the polyethylene resin particles from adhering to each other, a dispersant is preferably used in the aqueous dispersion medium. Examples of the dispersant used in the present invention include inorganic dispersants such as calcium phosphate, magnesium phosphate, basic magnesium carbonate, calcium carbonate, barium sulfate, kaolin, talc, and clay. These dispersants may be used alone, or 2 or more kinds may be used in combination. The present invention further preferably uses a dispersing aid together with the dispersant. Examples of the dispersing aid used in the present invention include carboxylic acid salt type surfactants such as N-acylamino acid salts, sulfonic acid salt type surfactants such as alkylbenzenesulfonic acid salts, sulfate type surfactants such as alkylallyl ether sulfate, maleic acid copolymer salts, and polycarboxylic acid type polymeric surfactants such as polyacrylic acid salts. These dispersing aids may be used alone, or 2 or more of them may be used in combination. Of these, it is preferable to use at least 1 kind selected from calcium phosphate, magnesium phosphate, barium sulfate or kaolin as a dispersant and at least 1 kind selected from sodium n-alkane sulfonate, sodium alkylbenzenesulfonate (e.g., DBS) as a dispersion aid in combination. The amount of the dispersant and the dispersing aid used in the present invention varies depending on the kind thereof, the kind of the polyethylene resin particles used and the amount used, and usually, the dispersant is preferably added in an amount of 0.2 to 2.5 parts by weight, more preferably 0.005 to 0.08 part by weight, based on 100 parts by weight of the aqueous dispersion medium. In order to improve the dispersibility of the polyethylene resin particles in the aqueous dispersion medium, it is preferable to add 25 to 90 parts by weight of the polyethylene resin particles to 100 parts by weight of the aqueous dispersion medium.
The method of pressure-releasing foaming used in step 1) is specifically a method in which polyethylene resin particles, an aqueous dispersion medium, a dispersant and a dispersing aid as required are charged into a closed container, the closed container is evacuated as required, a foaming agent is introduced into the closed container, the container is heated to a temperature not lower than the softening temperature of the polyethylene resin particles or the softening temperature of the polyethylene resin, the pressure in the closed container is increased to about 1.5 to 4MPa (gauge pressure), preferably 2.5 to 4MPa (gauge pressure), the foaming agent is further added to adjust the pressure to a desired foaming pressure after heating as required, the foaming temperature can be finely adjusted, the foaming temperature is generally in the range of 112 to 135 ℃, and the polyethylene resin particles impregnated with the foaming agent are released to a pressure range lower than the internal pressure of the closed container (generally, atmospheric pressure) to obtain the polyethylene resin particles Polyethylene resin first-stage expanded beads. Generally, the bulk density of the expanded beads in the first stage is about 55 g/L.
In order to further reduce the bulk density, it is necessary to perform a step (also referred to as a second-stage foaming step) of impregnating the polyethylene resin first-stage foamed particles with an inorganic gas to apply an internal pressure higher than the normal pressure, and then bringing the polyethylene resin first-stage foamed particles into contact with steam of a specific pressure to heat and foam the polyethylene resin first-stage foamed particles to obtain polyethylene resin second-stage foamed particles having a low bulk density. The inorganic gas is inert inorganic gas, and commonly comprises air, nitrogen, carbon dioxide and the like, and the inorganic gas is air in view of saving cost. In the invention, the heating time of the water vapor is at least 20 seconds, preferably 60 to 90 seconds, and the temperature of the water vapor is preferably 80 to 115 ℃. The pressure of the steam is adjusted depending on the bulk density of the resultant two-stage expanded beads, and is preferably adjusted
The gauge pressure is preferably 0.02 to 0.07MPa, more preferably 0.025 to 0.04 MPa. When the pressure of the water vapor is less than 0.02MPa (gauge pressure), the expansion ratio tends to be difficult to increase, and when it exceeds 0.07MPa (gauge pressure), the expanded beads tend to agglomerate with each other. The internal pressure of the inorganic gas impregnated into the primary expanded beads can be adjusted depending on the bulk density of the secondary expanded beads and the steam pressure in the secondary expansion step, and is preferably 0.22 to 0.56MPa (absolute pressure), more preferably 0.30 to 0.40MPa (absolute pressure). If the internal pressure is less than 0.22MPa (absolute pressure), the pressure of the water vapor needs to be increased in order to obtain a desired bulk density, and therefore the expanded beads tend to aggregate, whereas if the internal pressure exceeds 0.56MPa (absolute pressure), the percentage of continuous expansion of the second-stage expanded beads tends to increase, gas in the beads tends to escape, the foaming force during molding tends to deteriorate, and the molded article tends to have appearance defects and a reduced compressive strength. In general, the bulk density of expanded beads can be greatly reduced by the secondary expansion step, but the shrinkage rate of the secondary expanded beads upon heating is generally not 30%, and wrinkles are likely to appear on the surface of a molded article when a complicated molded article having a thin portion is produced by directly using the secondary expanded beads.
The present inventors have found that if the two-stage expanded beads are expanded again, that is, the expansion process a described in step 2) is repeated, the pressure of the water vapor and the internal pressure of the inorganic gas impregnated into the expanded beads are adjusted so that the bulk density of the three-stage expanded beads prepared: the bulk density of the two-stage expanded particles is 1 (1.1-2.5), and preferably, the ratio of the bulk densities of the two is 1: (1.1-2.2), more preferably 1: (1.5-2.2) to easily obtain three-stage expanded beads having a heat shrinkage ratio required in the present invention. If the bulk density of the three-stage expanded beads has not reached the conditions required in the present invention, the expansion process A may be repeated again as appropriate. In other words, after step 2), the obtained expanded beads may be repeatedly subjected to n times of the expansion step a) described in step 2), where n is an integer of 1 or more, and the pressure of the water vapor and the internal pressure of the inorganic gas impregnated into the expanded beads may be adjusted so that the bulk density of the polyethylene resin expanded beads obtained by repeating n times: the bulk density of the polyethylene resin expanded beads obtained by repeating the n-1-pass was 1 (1.1 to 2.5). Preferably, from the viewpoint of saving production costs and improving the heat shrinkage property of the expanded particles, the ratio of the bulk density of the expanded particles obtained by the last expansion to the bulk density of the expanded particles obtained by the previous expansion is further preferably 1: (1.5-2.2). If the bulk density of the expanded beads obtained by the previous expansion is 2.5 times or more the bulk density of the expanded beads obtained by the last expansion, the surface of the molded article tends to be wrinkled.
< foamed molded article of polyethylene resin and method for producing the same >
In the present invention, a polyethylene resin foamed molded article can be obtained by performing in-mold foaming as follows: the polyethylene resin foamed particles obtained in the above manner are filled in a mold and heated with steam or the like to fuse the foamed particles to each other.
As the in-mold foaming method, for example, the following methods can be used:
1) a method in which polyethylene resin foamed particles are subjected to a pressure treatment with an inorganic gas (for example, air, nitrogen, carbon dioxide, or the like) to impregnate the polyethylene resin foamed particles with the inorganic gas, a predetermined internal pressure is applied to the polyethylene resin foamed particles, and then the polyethylene resin foamed particles are filled in a mold and heated and fused with steam; or the like, or, alternatively,
2) a method of filling a mold with polyethylene resin foamed particles compressed under gas pressure to increase the internal pressure of the particles, and heating and fusing the particles with steam; or the like, or, alternatively,
3) a method of filling expanded polyethylene resin beads in a mold and heating and fusing the beads with steam without any particular pretreatment.
For example, in the case of 1), a polyethylene resin in-mold foamed article can be produced by pressurizing air in a pressure-resistant container in advance, pressing the air into the polyethylene resin foamed particles to make the pressure inside the foamed particles to be 0.08MPa (absolute pressure) or more and 0.3MPa (absolute pressure) or less, thereby imparting foaming ability, filling the foamed particles into a sealable but not sealable molding die, molding the foamed particles under the conditions of a heating water vapor pressure of about 0.05MPa (gauge pressure) or more and 0.4MPa (gauge pressure) or less and a heating time of about 1 to 120 seconds by using water vapor as a heating medium, welding the polyethylene resin foamed particles to each other, and then cooling the foamed particles with water until the deformation of the in-mold foamed article after the in-mold foamed article is extracted from the molding die can be suppressed. In order to further improve the surface properties, mechanical properties, dimensional stability, and the like of the polyethylene resin in-mold foamed molded article, the internal pressure of the expanded beads is preferably 0.1MPa (absolute pressure) or more and 0.25MPa (absolute pressure) or less, the heating water vapor pressure is preferably 0.05MPa (gauge pressure) or more and 0.25MPa (gauge pressure) or less, and the heating time is preferably 5 seconds or more and 60 seconds or less.
The polyethylene resin in-mold foamed molded article thus obtained can be used for applications such as heat insulating materials, cushioning packaging materials, and interior parts for automobiles. The polyethylene resin foamed molded article of the present invention is particularly suitable for use as a cushioning packaging material for producing articles having various shapes, since the foamed molded article having a complicated shape with a thin portion can be produced.
The invention is further illustrated, but not limited, by the following examples.
Examples
The evaluation methods carried out in examples and comparative examples will be described.
< measurement of melt flow index >
Melt flow index (i.e., melt index, MI) is measured at the orifice diameter using an MI measuring instrument described in JIS K7210
Figure BDA0001684350420000181
The measurement was carried out under the conditions of a well length of 8.000. + -. 0.025 mm, a load of 2160g and a temperature of 190. + -. 0.2 ℃.
< measurement of bulk Density >
The internal volume (internal volume VL) of the container was measured at 23 ℃ under a standard atmospheric pressure (0.1 MPa). Then, the expanded beads were added to the container, vibrated with a vibrator for 30 seconds and filled into the container, the expanded beads were added so that the upper surface of the expanded beads was above the top end surface of the container, and after the expanded beads were filled, the top end surface of the container was scraped with a straight plate in an upright state so that the top end surface of the container and the surface of the expanded beads were flush. The weight W g of the expanded beads remaining in the container is weighed. The bulk density of the polyethylene resin expanded beads at 23 ℃ and 0.1MPa (standard atmospheric pressure) can be calculated from the formula (2).
Bulk density (g/L) ═ W/V (2)
< measurement of Heat shrinkage percentage of expanded beads >
Placing a certain amount of foamed particles into a measuring cylinder at 23 deg.C and 0.1MPa, and vibrating with a vibrator for 30s to read the corresponding scale on the upper surface of the foamed particles as Va(ii) a Placing the measuring cylinder filled with the foaming particles into an oven preheated to 100 ℃, heating for 1 hour, taking out, placing for 1 hour at 23 ℃ and 0.1MPa, vibrating for 30 seconds by using a vibrator, and reading the scale corresponding to the upper surface of the foaming particles in the measuring cylinder as Vb. The heat shrinkage of the expanded beads was determined by the following formula (1).
Heat shrinkage (%) of expanded beads ═ Va-Vb)/Va×100 (1)
< shrinkage Rate >
The shrinkage rate was determined from the following formula (A) by a method for evaluating the shrinkage rate of expanded beads disclosed in WO2013-011951A 1.
Shrinkage factor ═ BD-VBD ÷ VBD × 100 (a)
Here, BD is the bulk density of non-crosslinked polyethylene resin foamed particles at 23 ℃ and 0.1MPa (normal atmospheric pressure), and VBD is the bulk density of non-crosslinked polyethylene resin foamed particles at 23 ℃ and 0.002 MPa.
< surface beauty of foam molded article >
The in-mold foamed molded article obtained was visually observed for a surface of 350mm in the vertical direction × 450mm in the horizontal direction, and the surface appearance was evaluated according to the following criteria.
Excellent surface appearance, smooth surface, no wrinkles and few particle gaps
Good surface appearance and slight wrinkles
The surface of the X-shaped steel sheet is unqualified in appearance, and the surface of the X-shaped steel sheet has more wrinkles.
In examples and comparative examples, the substances used were as follows, and were used without particular purification or the like.
Glycerol [ purified glycerol D manufactured by LION K.K. ]
Talc [ TalCAN PK-S, manufactured by Linchen chemical Co., Ltd ]
Calcium phosphate [ manufactured by Taiping chemical industry Co., Ltd ]
Sodium alkylsulfonate (sodium n-alkane sulfonate) [ LaEMUL PS, produced by Kao corporation ]
(examples 1 to 2)
[ preparation of polyethylene resin particles ]
The MI is 2g/10min, the melting point is 123 ℃, and the density is 0.926g/cm3As a base resin, a linear low density polyethylene containing 8.2% by weight of 4-methyl-1-pentene as a comonomer. 100 parts by weight of a base resin was taken, 0.25 part by weight of glycerin, a hydrophilic compound, and 0.05 part by weight of talc, a foaming nucleating agent, were added, and they were dry-blended. The dry-blended mixture was fed into a twin-screw extruder, melt-kneaded at a resin temperature of about 280 ℃, extruded into a strand through a circular die attached to the tip of the extruder, cooled with water, and cut with a cutter to obtain polyethylene resin pellets. The average particle weight of the polyethylene resin particles was 1.8 mg.
[ preparation of expanded polyethylene resin beads for one stage ]
At a capacity of 0.3m3The autoclave was charged with 100 parts by weight of the obtained polyethylene resin pellets (80 kg), 200 parts by weight of water, 0.6 part by weight of calcium phosphate (i.e., tricalcium phosphate) as a dispersant, 0.04 part by weight of a surfactant, sodium alkylsulfonate as a dispersion aid, and 7 parts by weight of carbon dioxide as a foaming agent were added under stirring.
The contents of the autoclave were heated to a foaming temperature of 123 ℃. Then, carbon dioxide was added to increase the pressure of the autoclave to 3.3MPa (gauge pressure), the above foaming temperature and pressure were maintained for 30 minutes, and then a valve at the lower part of the autoclave was opened to release the contents of the autoclave through an open hole (single hole) having a diameter of 3.6mm in an atmosphere of 100 ℃. The bulk density of the resultant expanded beads was measured and the results are shown in Table 1.
[ preparation of polyethylene resin two-stage expanded beads ]
The polyethylene resin first-stage expanded beads obtained were dehydrated, placed in a pressure-resistant container, impregnated with air under pressure, the internal pressure of the first-stage expanded bead-impregnated gas was adjusted, and heated with steam to carry out second-stage expansion, thereby obtaining polyethylene resin second-stage expanded beads. The water vapor pressure shown in table 1 (i.e., the vapor pressure shown in table 1) and the internal pressure of the gas for impregnating the expanded particles (referred to simply as the internal pressure in table 1) were used. The bulk density of the obtained polyethylene resin two-stage expanded beads was measured, and the results are shown in table 1.
[ preparation of three-stage expanded polyethylene resin particles ]
The obtained polyethylene resin two-stage expanded beads were dehydrated, and then placed in a pressure vessel, and the two-stage expanded beads were impregnated with air under pressure, the internal pressure of the impregnation gas of the two-stage expanded beads was adjusted, and the polyethylene resin three-stage expanded beads were obtained by heating with steam and performing three-stage foaming. The water vapor pressure (i.e., the vapor pressure shown in table 1) and the internal pressure shown in table 1 were used. The bulk density and the heat shrinkage of the obtained polyethylene resin three-stage expanded beads were measured, and the results are shown in table 1.
The shrinkage ratios of the expanded beads of examples 1 and 2 were 5% as determined by the method for evaluating the shrinkage ratio of the expanded beads disclosed in WO2013-011951A 1.
[ production of foamed molded article ]
After removing moisture from the obtained polyethylene resin three-stage expanded beads, an internal pressure of 0.15MPa (gauge pressure) was applied by air pressurization treatment, and the resulting mixture was filled in a mold having a length of 450mm, a width of 350mm and a thickness of 8mm, and then subjected to in-mold expansion molding under a steam pressure of 0.11MPa (gauge pressure), to obtain a polyethylene resin expanded molded article. Further, after the obtained molded body was left to stand under atmospheric pressure for about 1 hour, it was dried for 24 hours using an oven set to 75 ℃ to obtain a polyethylene resin thin-walled foamed molded body. The surface appearance of the obtained polyethylene resin foamed molded article was evaluated, and the results are shown in table 1.
(examples 3 to 4)
Polyethylene resin particles and polyethylene resin foamed particles were produced in the same manner as in example 1-2 by controlling a cutter, changing the average particle weight of the polyethylene resin particles, and changing the internal pressure and vapor pressure of two-stage foaming and three-stage foaming, and a polyethylene resin foamed molded article was obtained. The evaluation results of the polyethylene resin expanded beads and the polyethylene resin foamed molded article are shown in table 1.
Comparative examples 1 and 2
Polyethylene resin beads and polyethylene resin expanded beads were obtained in the same manner as in example 1-2 except that the internal pressure and the steam pressure in the two-stage foaming and the three-stage foaming were changed, and a polyethylene resin expanded molded article was obtained. The evaluation results of the polyethylene resin expanded beads and the polyethylene resin foamed molded article are shown in table 1. The shrinkage of the three-stage expanded beads obtained in comparative examples 1 and 2 was 7% as determined by the method for evaluating the shrinkage of expanded beads disclosed in WO2013-011951a 1.
Comparative example 3
Polyethylene resin particles and polyethylene resin expanded particles were obtained as polyethylene resin expanded molded articles in the same manner as in example 1-2, except that the three-stage foaming step was not performed and the internal pressure and vapor pressure of the two-stage foaming were changed. The evaluation results of the polyethylene resin expanded beads and the polyethylene resin foamed molded article are shown in table 1.
Comparative examples 4 to 5
The average particle weight of the polyethylene resin particles was varied by controlling the cutter. At the same time, the three-stage foaming process is not carried out and the internal pressure and vapor pressure of the two-stage foaming are changed. Polyethylene resin beads and polyethylene resin expanded beads were used in the same manner as in example 1-2 to obtain a polyethylene resin expanded molded article. The evaluation results of the polyethylene resin expanded beads and the polyethylene resin foamed molded article are shown in table 1.
As is clear from examples 1 to 4, when the polyethylene resin expanded beads of the present invention were used, even when a mold having a thickness of only 8mm was used, a foamed molded article having good appearance and no wrinkles was obtained, and it was found that the shrinkage rate disclosed in the prior art did not satisfactorily reflect the performance of in-mold foaming of polyethylene expanded beads in a mold having a small thickness, and the shrinkage rates of comparative examples 1 and 2 were within the shrinkage rate range disclosed in WO2013-011951a1, but the surface appearance of the foamed molded article was poor, and therefore, foamed beads having the heat shrinkage rate of the foamed beads of the present invention were relatively easy to obtain a molded article having excellent appearance or good quality.
As is clear from comparison of comparative examples 1 and 2 with examples 1 and 2, since the bulk density of the expanded beads is out of the range of the present invention, even if the polyethylene resin beads are similarly expanded three times, the expanded beads having the heat shrinkage ratio of the present invention cannot be obtained, and the surface appearance of the molded article is not satisfactory.
In each of comparative examples 3 to 5, the polyethylene resin beads were foamed only twice, and although the bulk densities of comparative examples 3 to 5 were within the range of the bulk density required in the present invention and the average bead weights of the expanded beads of comparative example 3 were within the preferable range of the present invention, the expanded beads having the heat shrinkage ratio of the present invention could not be obtained in comparative examples 3 to 5, and the surface appearance of the molded article was unsatisfactory.
TABLE 1
Figure BDA0001684350420000231

Claims (8)

1. A polyethylene resin expanded bead having a bulk density of 8 to 20g/L and a heat shrinkage ratio of 30 to 50% as determined by the following formula (1),
heating shrinkage (%) of expanded beads = (V)a-Vb）/Va×100 （1）
Wherein, VaRefers to the packing volume of the expanded particles; vbThe method is characterized in that the foaming particles are heated for 1 hour at the temperature of 100 ℃, and then the stacking volume is measured after the foaming particles are placed for 1 hour at the temperature of 23 ℃; bulk density, Va、VbAll are measured at 23 ℃ and 0.1MPa,
the average particle weight of the foaming particles is 0.3-3.0 mg.
2. The polyethylene resin foamed particles according to claim 1, wherein the bulk density of the foamed particles is 9 to 18 g/L.
3. The expanded polyethylene resin beads according to claim 1, wherein the expanded beads have an average bead weight of 0.5 to 2.5 mg.
4. The expanded polyethylene resin particle according to claim 1 or 2, wherein the expanded polyethylene resin particle is obtained by three or more times of expansion.
5. The expanded polyethylene resin beads according to claim 4, wherein the bulk density of the expanded beads obtained by the final expansion is: the bulk density of the expanded particles obtained from the previous expansion =1 (1.1-2.5).
6. A method for producing the polyethylene resin foamed particles according to any one of claims 1 to 5, comprising:
1) selecting polyethylene resin particles with a proper size so that the average particle weight of finally obtained expanded particles is adjusted to 0.3-3.0 mg, dispersing the polyethylene resin particles and a foaming agent in a dispersion medium in a closed container, heating and pressurizing, and releasing the polyethylene resin particles to a pressure area lower than the internal pressure of the closed container so as to obtain polyethylene resin primary expanded particles;
2) the polyethylene resin first-stage expanded particles are treated by the following steps: pressurizing the inorganic gas by the inorganic gas to increase the internal pressure of the inorganic gas impregnated into the particles to be more than the atmospheric pressure, and then performing heating treatment on the particles for at least 20 seconds by using water vapor with the pressure of 0.02 MPa-0.07 MPa to further foam the particles to obtain polyethylene resin second-stage foamed particles, wherein the pressure value of the water vapor is the gauge pressure;
3) repeating n times the step 2) to obtain expanded polyethylene resin particles, wherein n is an integer of 1 or more, and controlling the internal pressure of the inorganic gas impregnated into the particles and the pressure of the steam so that the bulk density of the expanded polyethylene resin particles obtained by repeating n times: the bulk density of the polyethylene resin expanded beads obtained after repeating n-1 cycles =1: (1.1-2.5).
7. The method for producing expanded polyethylene resin beads according to claim 6, wherein the ratio of the bulk density of expanded polyethylene resin beads obtained by repeating n cycles in step 3) to the bulk density of expanded polyethylene resin beads obtained by repeating n-1 cycles is 1: (1.5-2.2).
8. A polyethylene resin foamed molded article obtained by filling the polyethylene resin foamed particles according to any one of claims 1 to 5 or the polyethylene resin foamed particles produced by the production method according to claim 6 or 7 into a mold and then performing in-mold foam molding.
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