JPH1121368A - Foamed polyolefinic resin molding and its preparation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a foamed polyolefinic resin molding having a small difference between the density of the surface part and that of the central part and being excellent in properties such as a rate of fusion, mechanical strengths and appearance. SOLUTION: Pre-expanded particles which have such a crystal structure that two melting points appear in the measurement with a differential scanning calorimeter and in which the heat of the endothermic peak of the high- temperature side melting point is 0.3-6.0 cal/g, at least 300, per mm<2> , fine cells having a diameter of 0.5-50 μm are present on the surface part of the pre- expanded particles, and the mean cell diameter in the central part is 100-1,000 μm are subjected to in-mold molding to obtain an expanded polyolefin resin molding having a rate of fusion of at least 50%, having such a crystal structure that two melting points appear in the measurement with a differential scanning calorimetry and having a ratio of the density on the surface part to that of the central part of 0.8-1.0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin resin used for automobile parts such as heat insulating materials, cushioning packaging materials, boxes, core materials for bumpers, pillars, platforms, side impact materials, pallet materials, tool boxes and the like. And a method for producing the same. More specifically, the obtained molded article has a small difference in density between the surface layer portion and the central portion, and has excellent properties such as a fusion rate, mechanical strength, and appearance, and a polyolefin resin foam molded article and excellent in-mold molding. The present invention relates to a method for producing the foamed molded article in a cycle.

[0002]

2. Description of the Related Art Polyolefin-based resin-molded foams are superior to polystyrene-based resin-molded foams in chemical resistance, heat resistance, and strain recovery after compression. Widely used for automobile parts such as mail boxes, bumper core materials, pillars, platforms, side impact materials, pallet materials, tool boxes, etc.

The following method is known as a method for producing the above-mentioned polyolefin-based resin-molded in-mold foam.

(A) Polyolefin pre-expanded particles are pressurized with an inorganic gas and filled into a mold which can be closed but cannot be closed while the internal pressure of the particles is 1.18 atm or more, and is steamed. (2) Fill polyolefin pre-expanded particles into a mold that can be closed but not hermetically sealed, and heat-fused with steam or the like. (C) Crosslinked polyolefin by heating and curing while the volume is 70% to 110% of the volume of the mold to obtain a molded article according to the mold. A method in which the pre-expanded particles are compressed by gas pressure to 80% or less of the original apparent bulk volume, filled into a molding die, and heat-fused to form a molded product according to the mold (JP-B-53-33996). Gazette) (d) Pre-foaming of polypropylene resin Prefoaming having two melting peaks as measured by differential scanning calorimetry of the polymer, and having a melting peak calorific value QH based on the melting peak on the higher temperature side of the two melting peaks of 0.5 to 2.3 cal / g. A method in which the particles are filled in a mold that can be closed but cannot be sealed, and heated and fused with steam or the like to form a shaped product (Japanese Patent Application Laid-Open No. 63-107516).

[0005]

However, (A)
Each of the methods (1) to (4) leaves a problem in at least one of the molding cycle, the surface properties and appearance of the molded article, the dimensional accuracy, the mechanical strength, and the like.

In order to obtain pre-expanded particles used for molding, dichlorodifluoromethane, difluorotetrafluoroethane, 1,1-difluoro-1-chloroethane,
Halogenated hydrocarbons such as 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane and pentafluoroethane; aliphatic hydrocarbons such as propane-butane-pentane; cyclic compounds such as cyclobutane and cyclopentane Aliphatic hydrocarbons; difficult to handle due to flammability due to use of volatile blowing agents such as inorganics such as carbon dioxide, destruction of ozone layer and global warming, very expensive Have one or more of the following problems:

Further, the pre-expanded particles produced by using the volatile foaming agent are rapidly cooled from the outside in a state where the cooling speed of the surface layer is higher than the cooling speed of the inside of the particles at the time of foaming. , High density on the surface layer of the pre-expanded particles,
When a so-called particle skin layer is formed and molded in a mold, the particle skin layer is pressed against the wall surface of the mold, so that a higher-density molded body skin layer is formed. The center difference becomes large, and the mechanical strength apparently decreases. In addition, if it is attempted to reduce the difference in surface density of the compact by the conventional molding method, there is no other method than reducing the inner pressure of the mold, and it is difficult to secure the pressure required for internal fusion. The fusion rate of the body is reduced, and the reduction of the mechanical strength characteristics cannot be prevented.

[0008] Further, in the case of such a molded article having a large difference between the surface and the core, for example, when the surface layer is sliced and discarded, the waste weight per unit molded article becomes large, and the yield is low. descend.

Further, even during molding, the permeation time of water vapor and the like in the particle skin layer present on the fusion surface of the particles becomes longer, resulting in a longer mold cooling time, a longer molding cycle, and a longer molding cycle. It also has the disadvantage that the properties are reduced.

[0010]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned disadvantages of the prior art, has a small difference in surface density between cores, has excellent mechanical strength characteristics, and has a beautiful appearance. An object of the present invention is to provide a method for producing the molded article which has excellent adhesion and can shorten a molding cycle.

That is, according to the present invention, the fusion rate obtained by molding the pre-expanded polyolefin resin particles in a mold is 50%.
The above-mentioned compact has a crystal structure exhibiting two melting points as measured by a differential scanning calorimeter, and the ratio of the center density to the surface layer density of the compact is 0.8 or more.
0 or less, wherein the polyolefin-based resin foam molded article (Claim 1) has a diameter of 0.5 to 50 μm on the surface layer.
The polyolefin-based resin foam molded article according to claim 1, wherein 300 or more fine bubbles per 1 mm ² are present, and the density is 45 to 200 g / L. The polyolefin resin foam molded article according to claim 1, 2 or 3, wherein the polyolefin resin is a polypropylene resin (claim 3), the polyolefin resin pre-expanded particles can be closed but sealed. A method for producing a polyolefin-based resin foam molded article according to a mold, wherein the pre-foamed particles show two melting points in a differential scanning calorimeter measurement. Having an endothermic peak calorie at the high-temperature side melting point of 0.3 to 6.0 ca.
1 / g, and at least 300 fine bubbles having a diameter of 0.5 to 50 μm are present per 1 mm ^{2 in} the surface layer of the pre-expanded particles, and the average bubble diameter at the center is 100 μm or more and 1000 μm or less. The present invention also relates to a method for producing a polyolefin resin foam molded article characterized by the following (claim 5), and the method for producing according to claim 5 (claim 6), wherein the heat fusion is heat fusion with steam.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is generally known that there is a positive correlation between the density and the mechanical strength of a foamed molded product. When comparing the compressive strength characteristics of a conventional compact with a large conventional compact with the same density of the entire compact including the surface layer, the conventional compact shows that
Since the core layer with low density supports the surface layer with high density, high strength can be maintained by the surface layer strength in the elastic deformation region where the strain amount is small, but as the strain amount increases, the buckling of the bubble film of the core layer becomes large. Since it progresses rapidly and cannot support the surface layer, the amount of increase in the compressive strength with respect to the amount of strain in the plastic deformation region becomes small.

On the other hand, in the molded article of the present invention, since the difference in the surface core density of the molded article is small, the difference in characteristics between the elastic deformation region and the plastic deformation region is small. In addition, because there are many fine bubbles having a diameter of 0.5 to 50 μm in the surface layer where the particle skin layer is likely to gather, it is equivalent to the conventional product despite the low surface layer density compared to the conventional foamed molded product. A degree of elastic deformation region strength can be maintained. Further, since the difference in the surface core density is small, the increase in the compressive strength with respect to the increase in the amount of strain in the plastic deformation region is larger than that of the conventional foamed molded body. The strength in the deformation region is improved, and the appearance is beautiful with pearl luster.

The improvement in compressive strength due to the presence of the surface fine bubbles is quite surprising. For example, JS Colton, Plastics Engineering, August, 88
(1988), pp53-55 or Shinbo, Polymer Processing, Vol. 45, N
o.7 (1996), pp13-18, etc., similar to the mechanical strength characteristics of foams commonly called microcellular foam or mucellular plastic, etc. It is thought to be related to the stretch orientation.

[0015] Further, the foamed molded article of the thermoplastic resin is often not only a rectangular parallelepiped shape but also a complicated shape. In such a case, when measuring the compressive strength, the density is high and the heat-sealing property is high. Except for the surface layer portion whose physical properties are easily affected by the molding conditions at the time, it is common to cut a test specimen for measurement, perform measurement after sampling, and use the obtained measurement value as the compressive strength value of the molded body in general. Is being done.

Therefore, in the case of a compact having the same density,
Since the density of the test piece for measuring the compressive strength is low in the conventional molded body having a high density of the surface layer, the compressive strength value using the test piece is such that the density of the surface layer of the present invention is low, The result of the test is that the density of the test piece is relatively low as compared with the compressive strength value of the formed body. That is, by reducing the difference in surface core density of the foamed molded article, apparently, the compressive strength value of the molded article having the same density is improved, and the mechanical strength of the foamed molded article is measured by such a measuring method. If you want,
The excellent effects of the foamed molded article of the present invention are exhibited.

The polyolefin resin foam molded article of the present invention is a molded article having a fusion rate of 50% or more obtained by molding the polyolefin resin pre-expanded particles in a mold.

The polyolefin resin used in the present invention has an olefin monomer unit content of 50% or more, more preferably 70% or more.
% Or more and 100% or less, and a resin containing 50% or less, and more preferably 30% or less and 0% or more of a monomer unit copolymerizable with an olefin monomer. Since the olefin monomer unit is contained in an amount of 50% or more, a molded article that is lightweight and has excellent mechanical strength, workability, electrical insulation, water resistance, and chemical resistance can be obtained. Monomers that can be copolymerized with olefin monomers are used to modify adhesiveness, transparency, impact resistance, gas barrier properties, antistatic properties, improve moldability, shorten molding cycles, etc. It is a component to be used, and in order to obtain the effect of using it, it is preferable to use 2% or more, more preferably 5% or more.

Specific examples of the olefin monomer include:
Ethylene, propylene, butene, pentene, hexene,
Examples thereof include α-olefin monomers having 2 to 8 carbon atoms such as heptene and octene, and cyclic olefins such as norbornene-based monomers. Of these, ethylene and propylene are preferred because they are inexpensive and the physical properties of the obtained polymer are improved. These may be used alone or in combination of two or more.

Specific examples of the monomer copolymerizable with the olefin monomer include vinyl alcohol esters such as vinyl acetate, and alkyl groups having 1 to 6 carbon atoms such as methyl methacrylate, ethyl acrylate and hexyl acrylate. Examples thereof include (meth) acrylic acid alkyl esters, vinyl alcohol, methacrylic acid, and vinyl chloride. Of these, vinyl acetate is adhesive, flexible,
Methyl methacrylate is preferred in terms of low-temperature properties, and methyl methacrylate is preferred in terms of adhesiveness, flexibility, low-temperature properties, and thermal stability.
These may be used alone or in combination of two or more.

The melt index (MI) of the polyolefin resin is, for example, 0.5 to 30 g / 10 min for a polypropylene resin, and more preferably 1 to 10 g / min.
10 minutes is preferable, and the flexural modulus (JIS K
7203) as 5000 to 20000 kgf / c
m ² , further 6000-16000 kgf / cm ² , melting point 125-165 ° C., further 130-165
° C is preferred. When the MI is less than 0.5 g / 10 min, the melt viscosity is too high to obtain pre-expanded particles having a high expansion ratio, and when the MI is more than 30 g / 10 min, the melt viscosity with respect to the elongation of the resin during foaming. Of the pre-expanded particles tends to increase.

Specific examples of the polyolefin resin include, for example, an ethylene-propylene random copolymer,
Ethylene-propylene-butene random terpolymer,
Polypropylene resins such as polyethylene-polypropylene block copolymer and homopolypropylene; polyethylene resins such as low density polyethylene, medium density polyethylene, linear low density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer ;
Examples include polybutene and polypentene. These polymers may be used alone or in combination of two or more. Further, the polyolefin-based resin may be used in a non-crosslinked state, or may be used after being crosslinked by peroxide or radiation. Among these, a polypropylene resin is preferable because the obtained foamed molded article has a good balance of heat resistance, strength, and cost.

The polyolefin resin is prepared by adding 0.01 to 5% of a hydrophilic polymer so that the total amount is 100% with respect to 95 to 99.99% (% by weight, hereinafter the same) of the polyolefin resin. You may make it contain. By adding the hydrophilic polymer, the pre-expanded particles, and thus the fine bubbles in the surface layer of the expanded molded article, can be generated stably, shortening the molding cycle during in-mold molding, and further obtaining the molded article. Can be reduced.

The above-mentioned hydrophilic polymer is a polymer having a hydrophilic group such as a carboxyl group, a hydroxyl group, an amino group, an amide group and an ester group in the molecule. Classified as water-soluble polymer.

The hygroscopic polymer is ASTM D5
A polymer having a water absorption of 0.5% or more and an upper limit of 100% as measured according to No. 70.

Representative examples of the hygroscopic polymer include a carboxyl group-containing polymer, a polyamide, a thermoplastic polyester elastomer, and a cellulose derivative.

Specific examples of the carboxyl group-containing polymer include, for example, a terpolymer of ethylene-acrylic acid-maleic anhydride and a copolymer of ethylene- (meth) acrylic acid with sodium ions and potassium ions. Examples include an alkali ion, an ethylene ionomer crosslinked with a transition metal ion such as a zinc ion, and an ethylene- (meth) acrylic acid copolymer.
These may be used alone or in combination of two or more. Among these carboxyl group-containing polymers,
Ethylene ionomer in which ethylene- (meth) acrylic acid copolymer is cross-linked with alkali metal ions such as sodium ion and potassium ion is excellent in dispersibility in polyolefin resin, and polyolefin resin in relatively small amount. It is particularly preferably used in the present invention because it can stably generate a large number of the fine cells in the surface layer portion of the pre-expanded particles and reduce the difference in the surface core density of the expanded molded article.

Specific examples of the polyamide include, for example, nylon-6, nylon-6,6, and copolymerized nylon (EMS-CHEMIE A
G), trade name Grilltex, etc.). These may be used alone or in combination of two or more.

Specific examples of the thermoplastic polyester elastomer include, for example, a block copolymer of polybutylene terephthalate and polytetramethylene glycol. These may be used alone or in combination of two or more.

Specific examples of the cellulose derivative include:
For example, cellulose acetate, cellulose propionate and the like can be mentioned. These may be used alone or in combination of two or more.

The water-absorbing polymer refers to a polymer that absorbs water several times to several hundred times its own weight without being dissolved in water, and is hardly dehydrated even when pressure is applied.

Specific examples of the water-absorbing polymer include a starch-acrylic acid graft copolymer, a cross-linked polyvinyl alcohol-based polymer, a cross-linked polyethylene oxide-based polymer, and an isobutylene-maleic acid-based copolymer. These may be used alone or in combination of two or more.

Specific examples of the crosslinked polyvinyl alcohol-based polymer include, for example, Nippon Synthetic Chemical Industry Co., Ltd.
Various crosslinked polyvinyl alcohol-based polymers represented by the trade name Aqua Reserve GP and the like can be mentioned. These may be used alone or in combination of two or more.

Specific examples of the crosslinked polyethylene oxide polymer include various crosslinked polyethylene oxide polymers represented by Aquacork (trade name, manufactured by Sumitomo Seika Co., Ltd.). These may be used alone or in combination of two or more.

Specific examples of the isobutylene-maleic acid copolymer include Kuraray Co., Ltd., trade name K
Various isobutylene-maleic acid copolymers represented by I-gel and the like can be mentioned. These may be used alone or in combination of two or more.

The water-soluble polymer is a polymer that is soluble in water at normal or high temperature.

Specific examples of the water-soluble polymer include, for example, poly (meth) acrylic acid-based polymer, poly (meth)
Examples include acrylate polymers, polyvinyl alcohol polymers, polyethylene oxide polymers, and water-soluble cellulose derivatives. These may be used alone or in combination of two or more.

Examples of the poly (meth) acrylic acid-based polymer include polyacrylic acid, acrylic acid-ethyl acrylate copolymer, polyhydroxyethyl methacrylate and the like. These may be used alone.
More than one species may be used in combination.

Examples of the poly (meth) acrylate polymer include sodium polyacrylate, sodium polymethacrylate, potassium polyacrylate, potassium polymethacrylate and the like. These may be used alone or in combination of two or more.

Examples of the polyvinyl alcohol-based polymer include polyvinyl alcohol and a vinyl alcohol-vinyl acetate copolymer. These may be used alone or in combination of two or more.

Examples of the polyethylene oxide polymer include polyethylene oxide having a molecular weight of tens of thousands to several millions. These may be used alone or in combination of two or more.

Examples of the water-soluble cellulose derivative include carboxymethyl cellulose and hydroxyethyl cellulose. These may be used alone or in combination of two or more.

The above-mentioned hygroscopic polymer, water-absorbing polymer and water-soluble polymer may be used alone or in combination of two or more.

Further, the polyolefin resin includes:
Based on the total amount of the polyolefin resin and the hydrophilic polymer added if necessary, talc, kaolin, clay,
Mica, sodium carbonate, calcium carbonate, sodium borate, calcium hydroxide, etc. have an average particle size of 0.1.
A filler of 1 to 50 μm, furthermore 0.1 to 20 μm, may be added in a range of 0.001 to 10%, more preferably 0.001 to 5%. When a filler is added, the average cell diameter at the center of the pre-expanded particles is made uniform, which is preferable in that the mechanical strength, heat resistance and the like of the obtained foamed molded article are improved.

The polyolefin-based resin pre-expanded particles are particles of the above components at a concentration of 0.1 to 5 mg / particle, for example, have a crystal structure showing two melting points as measured by a differential scanning calorimeter, and The endothermic peak calorie indicating the melting point is 0.3 to 6.0 cal / g, and more preferably 1.0 to
6.0 cal / g, and a diameter of 0.5 to
50 μm, and fine bubbles of 0.5 to 30 μm are 1 m
m ² per 300 or more, more present than 500, expansion ratio 1.5 to 30 times, more 2-20 times, 60% to 100% closed cell ratio, and further 70% to 100%, except the surface layer portion ( (Central part) average cell diameter 100-1000μ
m, and more preferably 100 to 600 μm.

As described above, since the pre-expanded particles have a crystal structure exhibiting two melting points, preferably 5 ° C. or more, and more preferably 5 to 30 ° C. separated by differential scanning calorimetry, water vapor is generated during fusion molding. When the pre-expanded particles are heated by, for example, it has an appropriate secondary expandability and a resin film strength enough not to cause foam shrinkage, and has a wide temperature range (width of molding conditions) in which good fusion moldability is obtained. It becomes pre-expanded particles.

The two melting points are usually 110 to 155 ° C. and 120 to 170 ° C. for a polypropylene resin.
In the case of polyethylene resin, usually 70 to
Located at 110 ° C and 90-130 ° C.

When the endothermic peak calorific value indicating the high-temperature side melting point is less than 0.3 cal / g, the strength of the pre-expanded particles is insufficient, and the strength characteristics of the obtained foamed molded article are reduced, and 6.0 cal. / G, the secondary foamability of the pre-expanded particles at the time of fusion molding is reduced, so that the fusion ratio is 50% or more, and the density of the central portion with respect to the density of the surface layer portion of the foamed molded product. Is not preferable because the range of molding conditions for setting the ratio of 0.8 to 1.0 or less is extremely narrow.

The endothermic peak calorie of the lower melting point of the two melting points depends on the crystallinity of the polyolefin resin, but is usually 2.0 to 20.0 cal / g.

The melting point and endothermic peak calorific value were measured by a differential scanning calorimeter (SSC5200 manufactured by Seiko Instruments Inc.).
Using a pre-expanded polyolefin resin particle of about 5
A DSC curve having two melting points obtained when 10 mg was collected and measured at a heating rate of 10 ° C./min from 40 ° C. to 220 ° C. showed a baseline between each peak indicating the two melting points. , A tangent to the DSC curve is drawn from the point closest or coincident to the high-temperature side, and this is the area of the portion between the tangent and the DSC curve.

When the diameter of the fine bubbles in the surface layer of the pre-expanded particles is larger than 50 μm, there is a case where the distinction between the bubbles in the center and the fine bubbles in the surface layer is not clear, and it is less than 0.5 μm. In this case, since the wavelength of visible light is about 0.4 to 0.7 μm, the presence of bubbles cannot be confirmed optically (the bubbles become transparent), and therefore, they are not considered as bubbles in the present invention.

If the number of fine bubbles per 1 mm ² is too small, the distribution of fine bubbles in the surface layer becomes sparse, the density of the skin layer on the surface of the pre-expanded particles increases, and the effect of shortening the molding cycle is diminished. Not only that, the density of the obtained foamed molded product surface layer portion is large, the density of the central portion is small, and as a result, the ratio of the central portion density to the surface layer portion density is less than 0.8, and the mechanical strength characteristics are reduced, It is not preferable because excellent appearance with pearl luster is impaired.
The upper limit of the number of microbubbles per 1 mm ² is about 1,000,000 considering that microbubbles having a diameter of 0.5 μm are densely arranged in a single layer within the above range.

Further, the average bubble diameter at the center is 100
When the thickness is less than μm, the mechanical strength characteristics are reduced, and the foam is easily broken at the time of heat fusion molding. When the thickness exceeds 1000 μm, the mechanical strength is also reduced.

The surface layer portion of the polyolefin resin pre-expanded particles refers to a portion extending from the surface of the pre-expanded particles to 50 μm, and all the single-layer fine bubbles located at the outermost layer of the pre-expanded particles are covered by this surface layer portion. Is included in the heart
This is the part of the pre-expanded particles excluding the surface layer.

The diameter of the fine bubbles in the surface layer is defined as
This is the diameter (so-called equivalent diameter) obtained by determining the cross-sectional area of the cells observed in an enlarged micrograph of the surface of the pre-expanded particles and assuming that the cross-sectional area is equivalent to a circular area. Further, the average cell diameter at the central portion is obtained by drawing a line corresponding to a length of 1 mm on a portion excluding the surface layer in the enlarged micrograph of the cross section of the pre-expanded particles, and calculating the number of cells passing through the line. Afterwards, it is the average bubble diameter determined based on the procedure described in ASTM D3576.

The polyolefin resin molded article of the present invention comprises:
The pre-expanded particles as described above are molded in a mold, specifically, filled in a mold that can close the pre-expanded particles but cannot seal them, and for example, steam (0.1 to 6.0 kg / cm ² G). The fusion rate is 50% by any conventional molding method such as heat fusion with water vapor or the like.
% Or more, and more preferably 70% or more of a shaped polyolefin-based resin molded article. The fusion rate is 5
If the amount is less than 0%, the mechanical strength such as the tensile strength and the bending strength of the foamed molded article is reduced, and in extreme cases, the pre-expanded particles are peeled off or missing, and the shape cannot be maintained.

The fusion rate is measured by the method described in Example 1 described later.

The polyolefin resin molded article of the present invention comprises:
It has a crystal structure exhibiting two melting points as measured by differential scanning calorimetry, and the ratio of the density of the center part to the density of the surface layer part of the molded product is 0.8 or more, more preferably 0.85 or more, and particularly 0.1 or more. It is a polyolefin-based resin foam molded article of 88 or more and 1.0 or less. Because it is formed from a material having a crystal structure showing two melting points by the differential scanning calorimeter measurement, the melting peak becomes broad, and a stepwise decrease in strength due to crystal melting during heating is suppressed, and heat resistance is reduced. Since the ratio of the density of the central portion to the density of the surface layer is 0.8 or more and 1.0 or less, the hermetic distribution of the entire molded body including the surface layer becomes uniform, and the apparent appearance is improved. This leads to a feature that the mechanical strength is improved. In addition, when the density ratio is less than 0.80, the effect of improving the mechanical strength is small, and when a foam molded article having a uniform central portion density is obtained by using the in-mold molding method, the surface layer is in principle required. Since the partial density is equal to or higher than the central density, it is not necessary to exceed 1.0.

If it is attempted to produce a foamed molded article having a small difference in surface density and excellent mechanical strength characteristics by the conventional molding method, the fusion rate of the foamed molded article is reduced, which is not preferable. .

That is, according to the differential scanning calorimeter method, it has a crystal structure exhibiting two melting points, has a high-temperature endothermic peak calorie of 0.3 to 6.0 cal / g, and has fine bubbles in the surface layer. When the in-mold molding is performed using the conventional pre-expanded particles having no, the surface layer portion having the large density is not formed, and the surface-to-core ratio of the molded body density is 0.8 or more, and 1.0 or more.
In order to make the following, it is necessary not to strongly press the pre-expanded particles against the mold wall surface during the in-mold molding, that is, not to increase the maximum mold inner surface pressure. It is necessary to change the molding conditions such as reducing the compression ratio and decreasing the molding temperature. However, in this case, the fusion rate of the molded article is reduced to less than 50% in any case, so that the mechanical strength characteristics are reduced. In a remarkable case, the cross section of the foam molded article is rubbed with a finger. Even with a certain degree of surface wear, the particles can detach and drop.

On the other hand, when in-mold molding is performed using pre-expanded particles having fine cells in the surface layer as in the present invention, the pre-expansion can be performed without changing the molding conditions as described above. Since the fine bubbles in the surface layer of the particles expand due to heating, a foam molded article having a small density difference can be easily obtained.

Further, when in-mold molding is performed using pre-expanded particles having a large number of fine bubbles in the surface layer as in the present invention, a high-density particle skin layer is not formed on the surface of the pre-expanded particles, Perhaps because the pre-expanded particles, and eventually the water vapor in the expanded molded article, easily permeates to the outside of the molded article, the depressurization at the time of cooling the mold proceeds rapidly, and the in-mold molding cycle is shortened.

[0063] Polyolefin type resin foamed molded product of the present invention, the fine bubbles having a diameter 0.5~50μm said such a surface layer portion 1 mm ² per 300 or more, preferably 500
Since it is a molded product obtained by molding the pre-expanded particles present in the mold in a mold, similarly to the case of the pre-expanded particles, the surface layer portion has a diameter of 0.5 to 50 μm, and more preferably 1.0 to 30 μm.
The molded product has 300 or more, more preferably 500 or more, and an upper limit of 1,000,000 or less micrometer microbubbles per 1 mm ² . In addition, since the particle skin layer is included in the center of the molded body, it is difficult to determine the average cell diameter.However, the average cell diameter of the portion corresponding to the center of the pre-expanded particles is usually
100-1000 μm, further 150-800 μm,
The average cell diameter of the portion corresponding to the particle skin layer portion is 0.5
５０50 μm, and more preferably 1.0-50 μm.

The surface layer portion of the molded body refers to a portion having a thickness of up to 50 μm from the surface. The method of determining the number of fine bubbles and the method of determining the bubble diameter are the same as in the case of the pre-expanded particles. .

The density of the polyolefin resin foam molded article of the present invention thus obtained is preferably 45 to 2
00 g / L, and more preferably 45 to 150 g / L.

Next, a method for producing the pre-expanded polyolefin resin particles used in the present invention will be described.

In order to obtain the pre-expanded polyolefin-based resin particles, for example, polyolefin-based resin particles as a raw material, preferably polyolefin-based resin particles containing a hydrophilic polymer, are dispersed in an aqueous dispersion medium in a closed container. After heating to a temperature equal to or higher than the melting point of the system resin particles and equal to or lower than the melting end temperature, in a method of producing polyolefin resin pre-expanded particles by opening one end of a container, a volatile foaming agent is not used, and a substantially aqueous system is used. It is produced by using a dispersion medium, usually water, as a blowing agent.

There are no particular restrictions on the closed container, the aqueous dispersion medium in which the polyolefin resin particles are dispersed, the ratio of the polyolefin resin particles to the aqueous dispersion medium, and the like. It can be adopted as long as the ratio with the aqueous dispersion medium is used.

However, the foaming agent used is not a usual volatile foaming agent but water which is substantially used as a dispersion medium. When producing the polyolefin resin composition pre-expanded particles, when using a volatile blowing agent such as a halogenated hydrocarbon, a lower aliphatic hydrocarbon, carbon dioxide, nitrogen, air as a blowing agent, The diameter of the fine bubbles in the surface layer portion of the pre-expanded particles exceeds 50 μm, or the number of fine bubbles of 0.5 μm or more and 50 μm or less is 300
Either less than less than one piece is generated, and little or no microbubbles are seen in the surface layer of the pre-expanded polyolefin resin particles.

On the other hand, if water having an extremely high boiling point and a large latent heat cooling during evaporation is used as a foaming agent as compared with a normal volatile foaming agent, the growth of bubbles will occur when the particles become less than 100 ° C. during foaming. It is considered that the suspension stops and fine bubbles are generated on the surface layer of the pre-expanded particles.

The microbubbles in the surface layer portion of the pre-expanded particles are foamed by the above-described method, and then, the foamed particles are provided with a gas such as air or nitrogen to impart foaming ability. It does not disappear even if foaming in the second and subsequent stages (multi-stage foaming) is performed with heated air or steam.

As compared with the conventional method using a volatile foaming agent,
Since water is used substantially as a foaming agent, in the present invention, the expansion ratio tends to be relatively difficult to be obtained only by the first-stage foaming method. The disadvantages can be overcome. 2 above
For foaming after the stage, any of the conventionally known methods can be applied.However, when the multistage foaming method is used, a unit cell is required so as not to lower the closed cell rate of the obtained pre-expanded particles. It is preferable to take care that the maximum value of the same stress acting per film thickness is not more than a certain fixed position.

The melting point and melting end temperature of the polyolefin resin particles in the present invention are measured as follows. That is, polyolefin resin particles (5 to 10
mg) in the differential scanning calorimeter measurement.
The temperature is raised and lowered at a rate of 10 ° C./min, and then, when the temperature is raised again at a rate of 10 ° C./min, the peak of the maximum endothermic peak that appears is the melting point. The point at which the temperature coincides with the baseline is set as the melting end temperature.

In order to produce pre-expanded particles by the above-mentioned method, it has a crystal structure showing two melting points in a differential scanning calorimeter measurement, and has an endothermic peak calorie showing a high-temperature side melting point of 0.3 to 6. 0 cal / g, and at least 300 microbubbles having a diameter of 0.5 to 50 μm are present in the surface layer of the pre-expanded particles per 1 mm ² , and the average bubble diameter at the center is 100 μm or more and 1000 μm or less. Certain pre-expanded particles are obtained.

[0075]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1 An ethylene-propylene random copolymer ("Noblen FM" manufactured by Sumitomo Chemical Co., Ltd.) was used as the polyolefin resin.
321B ": density 0.90 g / cm ³ , melting point 145 ° C.
For 98% of MI = 5.5 g / 10 min), an ethylene ionomer (“Himilan 1707” manufactured by Mitsui-DuPont Polychemicals, Ltd.): sodium metal salt of ethylene-methacrylic acid copolymer, MI = 0.9 g / 10 Min, melting point 89
℃) 1 part of talc (average particle size: 0.9 μm) as an inorganic filler was added to 100 parts (parts by weight, hereinafter the same) of a polymer component to which 2% was added, and fed to a 50φ single screw extruder to melt. After kneading, the mixture was extruded from a cylindrical die having a diameter of 1.5 mmφ, cooled with water, and cut with a cutter to obtain columnar polyolefin resin particles (1.8 mg / particle). The melting point of the obtained polyolefin resin particles was 145 ° C., the melting end temperature was 161 ° C., and the density measured by JIS K 7112 was 0.91 g / cm ³ .

The obtained polyolefin resin particles 100
Parts were placed in a pressure-resistant closed container together with 300 parts of water, 1.5 parts of tribasic calcium phosphate and 0.04 parts of α-olefin sodium sulfonate, and then stirred at 154.5 ° C.
Heated. The pressure at this time was about 6 kg / cm ² G. After that, the internal pressure of the pressure-resistant closed container was increased to 30 kg / cm ² G by air pressurization, and the valve at the bottom of the closed container was immediately opened to allow the aqueous dispersion (resin particles and aqueous dispersion medium) to pass through the orifice having a diameter of 4 mmφ under atmospheric pressure. To give pre-expanded particles having a closed cell structure. At this time, the pressure was maintained with air so that the pressure in the container did not decrease during the discharge.

Further, the pre-expanded particles are put into a closed container, and the pressure is adjusted to 2.3 kg / cm ² G by air pressure, and the mixture is left at room temperature for 24 hours. Filled in a mold (270 × 290 × 60) that can be closed but cannot be sealed as 2.0 atm (abs), molded at a heating vapor pressure of 3.0 kg / cm ² G, and has excellent pearl luster A polyolefin resin foam molded article with a beautiful appearance was obtained.

The physical properties and moldability of the obtained pre-expanded particles and expanded molded article were evaluated by the following methods. Table 1 shows the results.

(Expansion ratio of pre-expanded particles) About 3 to 10 g of pre-expanded particles were dried at 60 ° C. for 6 hours, and after measuring the weight w, the volume v was measured by a submerged method. The specific gravity ρb = w / v is determined, and the density ρ of the raw resin particles is determined.
The expansion ratio K = ρr / ρb was determined from the ratio to r.

(Open cell ratio) The closed cell volume of the obtained pre-expanded particles was determined using an air-comparison type hydrometer (manufactured by Tokyo Science Co., Ltd., type 1000), and the apparent closed cell volume was separately determined by a submerged method. Closed cell rate (%) obtained by dividing by volume
Was determined by subtracting from 100.

(Number of Melting Points and Heat Absorption on High Temperature Side) After thoroughly weighing 5 to 10 mg of sufficiently dried pre-expanded particles, a differential scanning calorimeter (SSC5500 manufactured by Seiko Instruments Inc.)
To a temperature of 40 ° C. to 220 ° C. at a rate of 10 ° C. /
The measurement was carried out under the measurement conditions for minutes, and the number of endothermic peaks that appeared was defined as the number of melting points. In each case, two melting points appeared, so the calorific value of the endothermic peak on the high temperature side was measured (FIG. 1), and the measured value was defined as the high-temperature endothermic amount QH.

(Fine Cell Size and Number of Surface Layer Part of Pre-expanded Particles) Five pre-expanded particles were arbitrarily taken out, and two enlarged microphotographs (× 1000) of the surface layer part were taken using an optical microscope. Draw a square corresponding to 100 μm on each side on the obtained 10 micrographs, and calculate the cross-sectional area of all the bubbles including at least a part thereof,
When this was assumed to be a circle, an equivalent diameter that would have the same area was obtained, and the diameter was defined as the surface fine bubble diameter. In addition, the number of surface fine bubbles having a surface layer fine bubble diameter of 0.5 μm or more and 50 μm or less is measured, the total number thereof is determined (a total of 10 sheets of 0.1 mm ² ), and this is multiplied by 10. 1 mm ²
The number of fine bubbles per unit was calculated.

(Average cell diameter at the center of the pre-expanded particles) In a magnified micrograph (× 50) of a cross section of the pre-expanded particles cut with a knife, a line segment corresponding to 1 mm was formed at a portion excluding the surface layer portion. After the number of bubbles passing through the line segment was determined, the number was determined based on the procedure described in ASTM D3576.

(Fused ratio of foam molded article) A crack having a depth of about 5 mm was formed on the surface of the foam molded article with a knife, the molded article was split along the crack, the fracture surface was observed, and the total particle size was observed. The ratio of the number of broken particles to the number was determined and defined as the fusion ratio of the foamed molded product.

(Number of Melting Points of Polyolefin Resin Foam Molded Article) A weight of 5 to 1 was set so as not to include the surface layer portion in the molded article.
The number of melting points was determined in the same manner as the number of melting points of the pre-expanded particles except that a sample of 0 mg was collected.

(Density of foamed molded article) After the molded article was dried at 80 ° C. for 48 hours and then measured for weight w, the volume v was determined by the submerged method, and the density ρ = w / v was determined.

(Surface Layer Density) Surface (6 faces) of foamed molded article
5 mm thick measurement sample containing
After measuring the weight w, the volume v was determined by the submergence method, and the density w / v was calculated.

(Central Density) After measuring the weight w of the remaining portion of the foamed molded product after collecting the surface layer density measurement sample, the volume v was determined by the submerged method, and the density w / v was calculated.

It was calculated from (density surface-to-core ratio) (center density) / (surface layer density).

(Fine cell diameter and number of surface layer portion of polyolefin resin foam molded article) Five samples of 5 mm x 100 mm x 100 mm including the surface layer of the rectangular parallelepiped molded article were collected, and the pre-expanded particles were The surface layer microbubble diameter and number of the molded article were determined in the same manner as the surface layer microbubble diameter and number were determined.

(Measurement of strength with skin) A rectangular parallelepiped measurement test piece having a thickness of 25 mm including the surface (50 × 50 mm) of the foamed molded product was taken (n = 5), and was subjected to 23 ° C. and 50% R.
After being left in the standard state of H for one week, the weight w and the three-dimensional dimensions were measured, the volume and the test piece density were determined, and then, in the same standard state, a compression strength test was performed at a test speed of 10 mm / min. The compressive stress [kg / cm ² ] at the time of 5% strain and 50% strain in the thickness (25 mm) direction was determined, and the measured values at five points were averaged. Here, after the measurement, as a result of examining the strain-stress curves in each test, it was confirmed that all of them were within the range of plastic deformation at the time of 5% strain. Thereafter, the strength improvement ratio was measured from (compression stress at 50% strain) / (compression stress at 5% strain).

(Measurement of Strength without Skin) A rectangular parallelepiped measuring test piece of 50 × 50 × 25 mm was taken so as not to include the surface of the foamed molded article (n = 5), and the same as in the above-mentioned strength measurement with skin. A compressive strength test was performed. However, the compressive stress was obtained only at the time of 50% strain, and the measured values at five points were averaged.
The compressive strength was used.

Example 2 Except that the foaming pressure during the production of the pre-expanded particles was changed to 15 kg / cm ² G, the pre-expanded polyolefin-based resin particles and foam molding having excellent pearly luster were obtained in the same manner as in Example 1. I got a body. Table 1 shows the physical properties and moldability of the obtained pre-expanded particles and expanded molded article.

Comparative Example 1 As a polyolefin resin, 0.01 part of talc was added to 100 parts of the same ethylene-propylene random copolymer as in Example 1, and polyolefin resin particles ( (1.8 mg / particle) (melting point: 14)
5 ° C., melting end temperature 161 ° C., density 0.90).

100 parts of the polyolefin resin particles are
300 parts of water, 1.5 parts of tribasic calcium phosphate and 0.04 parts of α-olefin sodium sulfonate were put into a pressure-tight container, and isobutane was used as a volatile blowing agent at 144.5 ° C. and 19.5 kg / cm ^2. The mixture was foamed with G to obtain pre-expanded particles. The pre-expanded particles were molded under the same conditions as in Example 1 to obtain an expanded molded article. Table 1 shows the physical properties and moldability of the obtained pre-expanded particles and expanded molded article.

When this is compared with Example 1, fine bubbles are not present in the surface layer of the pre-expanded particles. Therefore, when molding is performed under the same conditions as in Example 1 (the same inner surface pressure of the mold),
The molding cycle was prolonged, the surface-to-core ratio of the density was less than 0.8, and the appearance had a dull luster. Therefore, when the compact density is the same, the compressive strength in the elastic region (at 5% strain) is almost the same in the compressive strength measurement using the test piece with the skin, but the compressive strength is increased in the plastic region. Accompanying strength improvement rate (= (compression stress at 50% strain) /
(5% strain compressive stress)), and the compressive stress at 50% strain decreases. In addition, in the normal measurement of compressive stress using a test piece without skin, the strength of the test piece was significantly reduced, probably due to the small specific gravity of the test piece, and the effectiveness of the molded article and the method for producing the molded article in the present invention was confirmed. Is done.

Comparative Example 2 The foaming temperature was 141.0 ° C. and the foaming pressure was 19.8 kg / c.
Pre-expanded polyolefin resin particles were obtained in the same manner as in Comparative Example 1 except that m ² G was used. Also, a polyolefin-based resin foam molded article was obtained in the same manner as in Comparative Example 1 except that the heating steam pressure was 2.2 kg / cm ² G. Table 1 shows the results
Shown in The high-temperature endotherm of the pre-expanded particles is 6.2 cal /
g, and the heating steam pressure during molding is 2.2.
When reduced to kg / cm ² G, the secondary expandability of the pre-expanded particles was reduced, and the surface-to-core ratio of the density of the molded article was 0.83, but the fusion ratio was 0%, By rubbing the fracture surface with a finger, the foamed particles were peeled off and missing, and a satisfactory foamed molded article was not obtained.

[0099]

[Table 1]

[0100]

The in-mold foam molded article of the polyolefin resin obtained according to the present invention has a small difference in density between the surface layer and the central part, and is excellent in properties such as fusion rate, mechanical strength and appearance. These are resin-based foamed moldings, which are suitable for heat insulating materials, cushioning and packaging materials, mail boxes, core materials for car bumpers, pallet materials, and the like. Further, the production method of the present invention can easily provide such an in-mold foam molded article of a polyolefin resin.

[Brief description of the drawings]

FIG. 1 is a DSC curve of a differential scanning calorimeter measured to determine the number of melting points and the amount of endotherm in Examples of the present invention.

Claims (6)

[Claims] 1. A molded article having a fusion rate of 50% or more obtained by molding in-mold polyolefin-based resin pre-expanded particles, and having a crystal structure showing two melting points as measured by a differential scanning calorimeter. And a ratio of the density of the center portion to the density of the surface layer portion of the molded product is 0.8 or more and 1.0 or less. 2. The polyolefin resin foam according to claim 1, wherein at least 300 microbubbles having a diameter of 0.5 to 50 μm are present in the surface layer per 1 mm ² . 3. The polyolefin resin foam according to claim 1, having a density of 45 to 200 g / L. 4. The polyolefin resin foam according to claim 1, wherein the polyolefin resin is a polypropylene resin. 5. A method for producing a polyolefin resin foam molded article according to claim 1, wherein the polyolefin resin pre-expanded particles are filled in a mold that can be closed but cannot be sealed, and heat-fused. The pre-expanded particles have a crystal structure showing two melting points in a differential scanning calorimeter measurement, and have an endothermic peak calorie at a high-temperature side melting point of 0.3 to 6.0 cal / g.
And the surface layer portion of the pre-expanded particles has a diameter of 0.5 to
There are 300 or more microbubbles of 50 μm per 1 mm ² , and the average bubble diameter at the center is 100 μm or more,
A method for producing a polyolefin-based resin foam molded article, which is not more than 0 μm. 6. The method according to claim 5, wherein the heat fusion is heat fusion with steam.