JP4276489B2 - Method for producing polypropylene resin expanded particles and polypropylene resin expanded particles - Google Patents

Description

  The present invention relates to a method for producing expanded polypropylene resin particles and the expanded polypropylene resin particles.

  In recent years, polypropylene-based resins are expanding their fields of use because of their excellent mechanical strength, heat resistance, processability, price balance, and excellent properties such as easy incineration and easy recyclability.

  Similarly, in-mold foam molded articles made of non-crosslinked polypropylene resins have added cushioning, heat insulation and other characteristics without losing the excellent properties of the above polypropylene resins. Widely used as materials, heat insulating materials, etc.

  In recent years, there has been a demand for an in-mold foam-molded article made of a non-crosslinked polypropylene resin that is lightweight and highly rigid, particularly in the automotive field, and has been studied using a highly rigid polypropylene resin. Higher-rigidity polypropylene resins tend to have higher melting points as the rigidity is higher. Most of the high-rigidity polypropylene resins have a melting point of 145 ° C. or higher. In particular, when the melting point of the polypropylene resin is 145 ° C. or higher, the steam pressure required for in-mold molding is too high. As a result, it exceeds the pressure resistance (0.45 MPa (G)) of the conventional molding machine. Only molded articles with insufficient fusion between the expanded particles could be obtained. Therefore, in order to mold the conventional expanded polypropylene resin particles in a mold, a special molding apparatus that can withstand high steam pressure is required. Furthermore, in the case of expanded particles in which the melting point of the polypropylene resin is 145 ° C. or higher, there has been a problem that the amount of steam used at the time of molding becomes enormous.

  Attempts have been made to solve this problem and obtain a foamed molded product that can be molded even within the pressure resistance of conventional molding apparatuses. For example, the applicant of the present invention, as previously shown in Patent Document 1, from a first polypropylene resin granular foam having a melting point of 140 ° C. or higher and a second polypropylene resin foam closely adhered to the surface thereof Therefore, a polypropylene resin expanded particle having a specific surface area with a melting point of the second polypropylene resin 2 to 10 ° C. lower than the melting point of the first polypropylene resin was proposed. However, although the foamed particles can be molded with a low steam pressure, the foamed molded product obtained has low rigidity such as compression strength because the core layer of the foamed particles has low rigidity.

  Patent Document 2 discloses a foamed core layer made of a crystalline thermoplastic resin, an ethylene polymer having a melting point lower than that of the thermoplastic resin, and the like, which is substantially non-foamed. Expanded particles composed of layers have been proposed. However, the molded body obtained using the expanded particles has a problem of low heat resistance such as low bending characteristics under heating conditions.

JP 58-145739 A JP-A-10-77359

  The present invention is capable of being heat-molded at a steam pressure lower than the steam pressure required for heat-molding conventional polypropylene resin foam particles, and obtains a foam-molded article having sufficient rigidity and heat resistance. An object of the present invention is to provide expanded polypropylene resin particles that can be used.

  As a result of intensive studies to solve the above problems, the present inventors have foamed multilayer resin particles in which the outer layer polypropylene resin has a lower melting point than the core layer polypropylene resin and the outer layer thickness has a specific value. Foamed particles impregnated with an agent to expand the foamed resin impregnated multilayer resin particles in a heat-softened state to form a substantially non-foamed surface layer portion are substantially It is found that the foamed particles having a non-foamed surface layer part can be fused with each other at a low steam pressure without losing the inherent properties such as rigidity of the polypropylene resin when molding. It came to complete.

  That is, according to this invention, the manufacturing method and expanded particle of the expanded particle shown below are provided.

[1] A core layer formed from a polypropylene resin and an outer layer formed from a polypropylene resin, the melting point (ts) of the polypropylene resin of the outer layer, and the melting point (ti) of the polypropylene resin of the core layer The foaming agent-impregnated multilayer resin particles in a heat-softened state are foamed by impregnating the foaming agent into the multilayer resin particles having the relationship of (1) satisfying the formula (1) and the outer layer thickness being 30 μm or less. A method for producing expanded polypropylene resin particles.
(Equation 3)
1.5 (° C.) ≦ ti-ts ≦ 30.0 (° C.) (1)
(However, the unit of ti and ts in the formula is ° C.)

[2] The method for producing expanded polypropylene resin particles according to [1], wherein the tensile modulus of the polypropylene resin of the core layer is 1200 MPa or more.

[3] Foamed particles obtained by foaming multilayer resin particles composed of a core layer formed of a polypropylene resin and an outer layer formed of a polypropylene resin, the foamed particles being a polypropylene resin of the core layer Of the outer layer and a substantially non-foamed surface layer portion made of the polypropylene resin of the outer layer, and the extrapolated melting start temperature (Ts) of the surface layer portion obtained by micro differential thermal analysis measurement is A polypropylene resin expanded particle having a temperature lower by at least 2 ° C. than an extrapolated melting start temperature (Ti) of an inner layer portion.

[4] The relation between the extrapolated melting start temperature (Ts) of the surface layer portion and the extrapolated melting start temperature (Ti) of the inner layer portion satisfies the formula (2), Polypropylene resin foam particles.
(Equation 4)
3 (° C.) ≦ Ti−Ts ≦ 40 (° C.) (2)
(However, the unit of Ti and Ts in the formula is ° C.)

[5] The DSC curve obtained by differential scanning calorimetry of the polypropylene resin expanded particles shows at least an endothermic curve peak specific to the polypropylene resin and an endothermic curve peak on the higher temperature side than the endothermic curve peak, and The expanded polypropylene resin particles according to [3] or [4] above, wherein the heat quantity of the endothermic curve peak on the high temperature side is 15% to 70% with respect to the total heat quantity of all endothermic curve peaks.

[6] The expanded polypropylene resin particles according to any one of [3] to [5], wherein the expanded polypropylene resin particles have a cylindrical shape.

The foamed particles obtained by the method of the present invention have a relationship between the melting point (ts) of the polypropylene resin in the outer layer and the melting point (ti) of the polypropylene resin in the core layer, with the thickness of the outer layer in the multilayer resin particles being a specific value. However, by making the specific relation hold, the surface layer portion of the expanded particles is substantially non-expanded. As a result, the obtained foamed particles are excellent in fusion property between the foamed particles at a low steam pressure even if the melting point of the polypropylene resin of the core layer is higher than the melting point of the polypropylene resin of the outer layer.

  Furthermore, the expanded particles obtained by the method of the present invention have a specific value of the tensile modulus of the polypropylene resin of the core layer, and therefore when used as an energy absorbing material such as a bumper, an excellent energy absorption amount is obtained by foam molding. Can be given to the body.

  The foamed particle of the present invention is a foamed particle obtained by foaming multilayer resin particles comprising a core layer formed from a polypropylene resin and an outer layer formed from a polypropylene resin, and the foamed particles are An outer layer melting temperature of the outer layer obtained by micro differential thermal analysis measurement (inner layer portion formed by foaming the inner layer portion of the polypropylene resin and a substantially non-foamed surface layer portion comprising the outer layer polypropylene resin ( Since Ts) is at least 2 ° C. lower than the extrapolated melting start temperature (Ti) of the inner layer portion, even if the melting point of the polypropylene resin of the core layer is higher than the melting point of the polypropylene resin of the outer layer, the steam pressure is low Even if it is heat-molded, it is a foamed particle having excellent fusion properties between the foamed particles.

  Furthermore, the expanded particles of the present invention have a relationship between the extrapolated melting start temperature (Ts) of the surface layer portion of the expanded particles obtained by micro differential thermal analysis measurement and the extrapolated melting start temperature (Ti) of the inner layer portion of the expanded particles. Is a foamed particle that can be molded at a lower steam pressure without reducing the heat resistance of the foamed molded product.

  Furthermore, when the DSC curve obtained by differential scanning calorimetry of the foamed particles is a specific configuration, the expanded particles of the present invention have a low open cell ratio, excellent compressive strength, and easily. Internal pressure can be applied, and foamed particles with good moldability can be obtained.

  Furthermore, when the foamed particles of the present invention adopt a configuration in which the shape of the foamed particles is a cylindrical shape, the foamed particles can be fused to each other without breaking the cylindrical shape when the foamed molded article is formed. Therefore, a polypropylene resin foam molded article having high voids can be obtained.

  The method for producing expanded polypropylene resin particles (hereinafter, simply referred to as “expanded particles”) of the present invention includes a foaming agent applied to multilayer resin particles comprising a core layer formed from a polypropylene resin and an outer layer formed from a polypropylene resin. The foamed resin-impregnated multilayer resin particles in a heat-softened state are foamed.

  The polypropylene resin of the core layer of the multilayer resin particles used in the production method of the present invention includes, for example, a propylene homopolymer or a propylene component unit of 60 mol% or more (preferably a propylene component unit of 80 mol% or more). And a copolymer of propylene and other comonomers, or a mixture of two or more selected from these resins.

  Examples of the copolymer of propylene containing 60 mol% or more of propylene component units and other comonomers include, for example, propylene-ethylene random copolymer, propylene-ethylene block copolymer, propylene-butene random copolymer, propylene-ethylene-butene random copolymer. Etc. are exemplified.

  The melting point of the polypropylene resin in the core layer is preferably 145 ° C. or higher in order to increase the heat resistance of the final polypropylene resin in-mold foam molded product (hereinafter referred to as “foam molded product”). It is more preferably 155 ° C. or higher, further preferably 158 ° C. or higher, and most preferably 160 ° C. or higher. The upper limit of the melting point is usually about 170 ° C.

  In addition, the polypropylene-based resin of the core layer is superior in that the compression strength of the foam molded article is large, and further the energy absorption amount when the foam molded article is used as an energy absorbing material such as a bumper is excellent. The tensile yield strength is preferably 31 MPa or more, and more preferably 32 MPa or more. The upper limit of the tensile yield strength is not particularly specified, but is usually 45 MPa at most.

  In addition, the polypropylene-based resin of the core layer prevents the bubble film from being broken when foaming the multilayer resin particles, and further prevents the bubble film from being broken at the time of heating in the mold. The tensile elongation at break is preferably 20% or more, more preferably 100% or more, and further preferably 200 to 1000%.

  The tensile yield strength and tensile elongation at break are both based on the measurement method described in JIS K 6758 (1981).

  The foamed particles obtained by foaming the surface layer portion described in Patent Document 1 have low rigidity such as compressive strength in the foamed molded article obtained from the low rigidity of the core layer. In order to make the rigidity higher, the polypropylene resin of the core layer in the method of the present invention preferably has a tensile elastic modulus of at least 1200 MPa. When the foamed molded product obtained with such a configuration is used as an energy absorbing material such as a bumper, an excellent energy absorption amount is imparted. From such a viewpoint, the pressure is more preferably 1250 MPa or more, further preferably 1300 MPa or more, and particularly preferably 1360 MPa to 2500 MPa. As a high-rigidity polypropylene resin having a tensile modulus of 1200 MPa or more, most of the propylene homopolymer exhibits such high rigidity, and even if it is a copolymer of propylene and another comonomer, the comonomer component is contained. Those having an extremely small proportion tend to exhibit such high rigidity.

The tensile elastic modulus is a value obtained by measuring the resin under the following conditions in accordance with JIS K 7161 (1994).
Test piece: Test piece 1A type described in JIS K 7162 (1994) (direct molding by injection molding),
Tensile speed: 1 mm / min

The polypropylene resin of the core layer preferably has a melt flow rate (test condition 14 of JIS K7210 (1976)) abbreviated as MFR of 1 g / 10 min to 100 g / 10 min. If the MFR is less than 1 g / 10 min, the effect of lowering the molding steam temperature during in-mold molding may be insufficient. Moreover, when MFR exceeds 100 g / 10min, there exists a possibility that the obtained foaming molding may become weak. From such a viewpoint, the MFR is more preferably 10 g / 10 min or more and 70 g / 10 min or less.
Polypropylene resins having the above characteristics can be obtained from commercially available polypropylene resins produced by various methods.

  To the polypropylene resin of the core layer, other synthetic resins, synthetic rubbers and / or elastomers other than the polypropylene resin can be added within a range that does not impair the intended effect of the present invention. The amount of the synthetic resin other than polypropylene resin, synthetic rubber and / or elastomer added is preferably at most 35 parts by weight, more preferably 20 parts by weight or less, per 100 parts by weight of the polypropylene resin. More preferred is 10 parts by weight or less, and most preferred is 5 parts by weight or less.

  Synthetic resins other than the polypropylene resin include high density polyethylene, medium density polyethylene, low density polyethylene, linear low density polyethylene, linear ultra-low density polyethylene, ethylene-vinyl acetate copolymer, ethylene- Examples thereof include ethylene resins such as acrylic acid copolymers and ethylene-methacrylic acid copolymers, and styrene resins such as polystyrene and styrene-maleic anhydride copolymers.

  Examples of the synthetic rubber include ethylene-propylene rubber, ethylene-1-butene rubber, propylene-1-butene rubber, styrene-butadiene rubber and hydrogenated products thereof, isoprene rubber, neoprene rubber, and nitrile rubber. Examples of the elastomer include styrene-butadiene block copolymer elastomers and hydrogenated products thereof.

  In addition, various additives can be contained in the polypropylene resin of the core layer as desired. Examples of such additives include antioxidants, ultraviolet light inhibitors, antistatic agents, flame retardants, metal deactivators, pigments, dyes, nucleating agents, and bubble regulators. Examples of the air conditioner include inorganic powders such as zinc borate, talc, calcium carbonate, borax, and aluminum hydroxide.

  The content of these additives is preferably 20 parts by weight or less, and particularly preferably 5 parts by weight or less, per 100 parts by weight of the base resin composed of a polypropylene resin or the like forming the core layer. In particular, the amount of the bubble regulator is preferably 0.005 to 1 part by weight when the average bubble diameter is 20 to 300 μm.

  As the outer layer polypropylene resin of the multilayer resin particle used in the production method of the present invention, the relationship between the melting point of the outer layer polypropylene resin and the melting point of the core layer polypropylene resin satisfies the formula (1) described later. Except for the above, the same thing as the polypropylene resin of the core layer is exemplified.

  The melting point of the polypropylene resin in the outer layer is preferably 130 ° C. or higher, more preferably 140 ° C. or higher, from the viewpoint of heat resistance. On the other hand, the upper limit is preferably 155 ° C. or less, more preferably 150 ° C. or less, and further preferably 145 ° C. or less from the viewpoint of fusing the expanded particles with a low molding pressure.

  In addition, in the polypropylene resin of the outer layer, various additives can be contained as necessary, like the polypropylene resin of the core layer. Examples of such additives include antioxidants, ultraviolet inhibitors, antistatic agents, flame retardants, metal deactivators, pigments, dyes, and crystal nucleating agents. Among them, additives that impart functionality such as antioxidants, UV inhibitors, antistatic agents, flame retardants, metal deactivators, pigments, dyes, or crystal nucleating agents can be used only by adding them to the outer layer. Is preferable in that it is obtained.

  The content of these additives is preferably about 20 parts by weight or less, and particularly preferably 5 parts by weight or less, per 100 parts by weight of the base resin composed of a polypropylene resin or the like of the outer layer. This lower limit is approximately 0.01 parts by weight.

In the method of the present invention, the relationship between the melting point (ts) of the polypropylene resin of the outer layer and the melting point (ti) of the polypropylene resin of the core layer satisfies the following formula (1), and the thickness of the outer layer is specified: Configured to be the value of. With the multilayer resin particles having such a structure, the expanded foamed particles can be foamed particles even if the melting point of the polypropylene resin of the core layer is higher than the melting point of the polypropylene resin of the outer layer or heat-molded at a low steam pressure. It is excellent in mutual fusion property.
Furthermore, it is preferable to satisfy the following formula (3) from the viewpoint of obtaining foamed particles that can be molded at a lower steam pressure without reducing the heat resistance of the foam molded article, and to satisfy the following formula (4). More preferably, it is more preferable that the following expression (5) is satisfied. However, the units of (ti) and (ts) in the formula are both ° C.
(Equation 5)
1.5 (° C.) ≦ ti-ts ≦ 30.0 (° C.) (1)
(Equation 6)
1.5 (° C.) ≦ ti-ts ≦ 25.0 (° C.) (3)
(Equation 7)
1.5 (° C.) ≦ ti-ts ≦ 20.0 (° C.) (4)
(Equation 8)
1.5 (° C.) ≦ ti-ts ≦ 15.0 (° C.) (5)

As the melting point (ts) of the polypropylene resin of the outer layer, a value obtained by a measuring method based on JIS K7122 (1987) is adopted.
When producing multi-layer resin particles, 2 to 4 mg of polypropylene resin used as a raw material for the outer layer is collected, and temperature rise measurement is performed at 10 ° C./min from room temperature (10 to 40 ° C.) to 220 ° C. using a differential scanning calorimeter. Do. Thereafter, the temperature is lowered to 40 ° C. at a rate of 10 ° C./min, and after reaching 40 ° C., the temperature rise measurement is performed again to 220 ° C. at 10 ° C./min. The peak of the DSC curve obtained by the second temperature increase obtained by this measurement is defined as the melting point. In addition, when there are two or more peaks, the peak of the melting peak having the largest amount of heat is adopted.

  For the melting point (ti) of the polypropylene resin of the core layer, a value obtained in the same manner as the measuring method of the melting point (ts) of the polypropylene resin of the outer layer is adopted.

In the method of the present invention, the relationship between the melting point (ti) of the polypropylene resin of the core layer in the multilayer resin particles and the melting point (ts) of the polypropylene resin of the outer layer satisfies a specific formula, and the thickness of the outer layer The multilayer resin particles are formed so that the thickness becomes 30 μm or less. With this configuration, when the multilayer resin particles are foamed, the core layer is foamed but the outer layer is not foamed. Thus, foamed particles that can be molded at a lower steam pressure than the foamed particles in which the outer layer is foamed are obtained.
The mechanism by which the outer layer of the multilayer resin particle does not foam is not clear, but when the multilayer resin particle foams, a certain amount of resin thickness is required to retain the foaming agent in relation to the foaming force and the melt viscosity of the resin. The thickness of the outer layer of the multilayer resin particles is a specific value, and the relationship between the melting point (ti) of the polypropylene resin of the core layer and the melting point (ts) of the polypropylene resin of the outer layer described above However, it is considered that the core layer of the multilayer resin particles can be foamed and the outer layer can be substantially non-foamed by satisfying a specific formula.

  In the method of the present invention, the lower limit of the thickness of the outer layer of the multilayer resin particles is preferably 1 μm or more, more preferably 2 μm or more, and particularly preferably 3 μm or more from the viewpoint of excellent fusion properties between the expanded particles. On the other hand, the upper limit is preferably 25 μm or less, more preferably 18 μm or less, and particularly preferably 15 μm or less from the viewpoint of preventing foaming.

  The thickness of the outer layer in the multilayer resin particles of the present specification was divided into two equal parts and the cross section was enlarged as much as possible so that the entire cross section could be entered under a microscope, and the multilayer resin particles were further divided into two equal parts. It is measured from a photograph taken with an optical microscope so that the outer layer appears on the entire circumference in a vertical section. Specifically, a straight line is drawn on the photograph so that the cross section is approximately bisected, and a straight line is drawn so as to be perpendicular to the straight line. The average is obtained as the thickness of the outer layer of one multilayer resin particle. This operation is performed for 10 foamed particles, and the arithmetic average value is taken as the thickness of the outer layer in the multilayer resin particles. When it is difficult to understand the thickness of the outer layer in the multilayer resin particles, it is preferable to produce the multilayer resin particles by adding a colorant to the resin constituting the outer layer in advance.

When the foamed particles obtained in the method of the present invention are molded as compared with ordinary foamed particles (foamed particles not having a multilayer structure), the foamed particles are inferior in secondary foamability, and the resulting foamed molded product has the same shape as the mold. The average bubble diameter is preferably 20 μm or more because the mold transferability is low. From the viewpoint described above, it is more preferably 25 μm or more, and further preferably 30 μm or more. On the other hand, the upper limit value is preferably 300 μm or less from the viewpoint that the foamed molded product obtained does not break bubbles due to compressive stress and can be used repeatedly without distortion. From the above viewpoint, 250 μm or less is more preferable, and 200 μm is further preferable.
In addition, the measurement method of an average bubble diameter divides an expanded particle into two equal parts, expands as much as possible so that a cross section may enter all under a microscope, and the cross section is image | photographed. Based on the photograph, a straight line is drawn on the photograph so that the cross section is approximately bisected, and the value obtained by dividing the length of the straight line by the number of all bubbles in contact with the straight line is taken as the average bubble diameter of one foamed particle. Thus, 20 foam particles were obtained, and the arithmetic average thereof was adopted as the average cell diameter of the foam particles.

  Examples of the shape of the multilayer resin particles used in the method of the present invention include a cylindrical shape, a rugby ball shape, a spherical shape, and a cylindrical shape. Foamed particles obtained by foaming such multilayer resin particles have a cylindrical shape, a spherical shape, a rugby ball shape, or a cylindrical shape according to the shape before foaming. Among these shapes, when a cylindrical shape is selected, a foam molded body having a high porosity can be obtained, and such a foam molded body is excellent in water permeability.

  In the present invention, the shape of the polypropylene resin expanded particles described above is cylindrical. The shape having one or two or more through holes penetrating in the vertical direction of the columnar expanded particles such as a cylinder, an elliptical column, and a prism. In addition to the shape having the above-mentioned through-holes as well as the one (for example, Japanese Patent Laid-Open No. 7-137063 (A) to (F)), a hollow shape having a blade-like protrusion on a part of the outer surface In addition to the shape having the through hole in addition to the shape having the through hole (for example, Japanese Patent Laid-Open No. 7-137063). In addition to the shape having a through hole in addition to the shape having a through hole in FIG. 2 of FIG. 7-137063 (for example, FIG. 2 in FIG. 2 of JP-A-7-137063). ).

  Next, an example of a method for producing the multilayer resin particles of the present invention will be described.

  First, the polypropylene resin constituting the core layer and other resins and additives blended as necessary are supplied to one extruder, heated and kneaded to form a first mixed melt for core layer formation Form a resin. At the same time, the polypropylene resin constituting the outer layer and other resins and additives blended as necessary are supplied to another extruder, heated and kneaded to obtain a second mixed molten resin for forming the outer layer. Form.

  Next, the first mixed molten resin and the second mixed molten resin are supplied to the co-extrusion die, and the flow of the second mixed molten resin is a strand-shaped flow of the first mixed molten resin in the die. The flow of the first mixed molten resin and the flow of the second mixed molten resin are merged so as to cover the periphery of the substrate, and both are laminated. Next, multilayer resin particles can be produced by extruding the laminated mixed molten resin into a strand shape from a die, cooling it, and then cutting.

  In addition, the adjustment of the thickness of the outer layer in the multilayer resin particles is performed by adjusting the balance between the discharge amount of the core layer and the discharge amount of the outer layer and adjusting the speed at which the strand-shaped extrudate is taken.

  Thus, it is preferable to cut the material after coextrusion into a strand shape, because multilayer resin particles in which the outer layer is laminated on the core layer can be obtained. However, in the present invention, a method may be employed in which a multilayer sheet in which a resin for forming an outer layer is laminated on a resin for forming a core layer is extruded and the sheet is cut into granules.

  The area occupied by the outer layer in the surface area of the multilayer resin particles is that the foamed particles obtained by foaming the multilayer resin particles are imparted with excellent fusing properties, and furthermore, a foam molded article with excellent bending strength can be obtained. It preferably accounts for 50% or more of the entire surface area, more preferably 60% or more, and particularly preferably 70% or more. Note that the outer layer on the surface area of the multilayer resin particles may be linearly coated.

  Next, a preferred example of a method for foaming foamed particles using the multilayer resin particles will be described.

  First, the multilayer resin particles are dispersed in an aqueous medium such as water or alcohol in a closed container such as an autoclave together with a foaming agent, and heated to a temperature equal to or higher than the softening temperature of the polypropylene resin that forms the core layer. Is impregnated with a blowing agent. Next, while maintaining the pressure in the sealed container at a pressure equal to or higher than the vapor pressure of the foaming agent, one end below the water surface in the sealed container is opened, and the multilayer resin particles and the aqueous medium are simultaneously at a lower pressure than in the container. Released downward (hereinafter referred to as dispersion medium release foaming method). Usually, from the viewpoint of handling, the above-mentioned aqueous medium is preferably water.

  In the dispersion medium releasing foaming method, it is preferable to add a dispersant to the dispersion medium so that the multilayer resin particles are not fused to each other in the container when heated in the container. As such a dispersant, any agent that prevents fusion of the multilayer resin particles in the container may be used, and it can be used regardless of whether it is organic or inorganic. Is preferred. For example, natural or synthetic clay minerals such as amusnite, kaolin, mica, clay, aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, iron oxide and the like in one kind or a combination of several kinds May be used. In addition, a dispersing agent is normally used about 0.001-5 weight part per 100 weight part of multilayer resin particles.

Furthermore, in the dispersion medium releasing foaming method, a dispersion strengthening agent that strengthens the dispersing power of the dispersing agent (the function of preventing the fusion of the multilayer resin particles in the container even when the amount of the dispersing agent is small). May be added to the dispersion medium. Such a dispersion strengthening agent is an inorganic compound that can be dissolved in at least 1 mg or more in 100 cc of water at 40 ° C., and at least one of the anion or cation of the compound is a divalent or trivalent inorganic substance. preferable. Examples of such inorganic substances include magnesium chloride, magnesium nitrate, magnesium sulfate, aluminum chloride, aluminum nitrate, aluminum sulfate, iron chloride, iron sulfate, and iron nitrate. When producing low-expanded expanded particles having an apparent density of 100 g / L or more, it is preferable to use a dispersion strengthening agent.
The dispersion strengthener is usually used in an amount of about 0.0001 to 1 part by weight per 100 parts by weight of the multilayer resin particles.

Examples of the blowing agent used in the dispersion medium releasing foaming method include aliphatic hydrocarbons such as propane, butane, hexane, and heptane, cyclic aliphatic hydrocarbons such as cyclobutane and cyclohexane, chlorofluoromethane, trifluoromethane, Organic physical foaming agents such as halogenated hydrocarbons such as 1,2-difluoroethane, 1,2,2,2-tetrafluoroethane, methyl chloride, ethyl chloride, methylene chloride, nitrogen, oxygen, air, carbon dioxide, A so-called inorganic physical foaming agent such as water is exemplified. An organic physical foaming agent and an inorganic physical foaming agent can also be used in combination. The physical foaming agent described above is preferable because low-density foamed particles can be easily obtained.
Among the above physical foaming agents, those mainly composed of one or more inorganic physical foaming agents selected from the group of nitrogen, oxygen, air, carbon dioxide and water are preferred. Further, among these, nitrogen and air are preferable in consideration of the stability of the apparent density of the expanded particles, the environmental load and the cost. When water is used as the foaming agent, the water used as a dispersion medium for dispersing the multilayer resin particles in the sealed container may be used as it is.

  The filling amount of the physical foaming agent in the dispersion medium releasing foaming method is appropriately selected according to the type of foaming agent to be used, the foaming temperature, and the apparent density of the intended foamed particles. Specifically, for example, when nitrogen is used as the foaming agent and water is used as the dispersion medium, the pressure in the sealed container immediately before the start of foaming, that is, the pressure in the space in the sealed container (gauge pressure) However, it is preferable to select so that it may become 0.6-6 MPa. In general, the smaller the apparent density of the target foam particles, the higher the pressure in the space in the container. The higher the apparent density of the target foam particles, the lower the pressure in the space. It is desirable.

  In the dispersion medium releasing foaming method, the physical foaming agent may be filled into the container at the same time as the temperature rise, filled in the middle of the temperature rise, or filled in a stable state immediately before the start of foaming. It does not matter if the particles are impregnated with a foaming agent.

  The method for heat-softening and foaming the multilayer resin particles impregnated with the foaming agent in the method of the present invention is not limited to the dispersion medium releasing foaming method described above. As described in Japanese Patent No. 372630, a method may be used in which foamable multilayer resin particles impregnated with a foaming agent are foamed with a heating medium such as heated steam or hot air.

The foamed particles of the present invention are preferably produced by the method described above, and are foamed particles obtained by foaming multi-layer resin particles composed of a core layer formed from a polypropylene resin and an outer layer formed from a polypropylene resin, The foam particles are composed of an inner layer portion formed by foaming the polypropylene resin of the core layer and a substantially non-foamed surface layer portion formed of the polypropylene resin of the outer layer, and the surface layer obtained by micro differential thermal analysis measurement. The extrapolated melting start temperature of the part is at least 2 ° C. lower than the extrapolated melting start temperature of the inner layer part.
With such a configuration, the melting point of the polypropylene resin in the core layer is higher than the melting point of the polypropylene resin in the outer layer, and even if it is heat-molded at a low steam pressure, the foam particles can be fused to each other. Excellent expanded particles.

  In the present specification, the substantially non-foamed surface layer portion made of a polypropylene resin as an outer layer is taken by photographing 100 cross-sectional photographs (magnification 200 times) of the surface layer portion of 100 expanded particles, Among them, the cross-sectional photograph in which the surface layer portion is foamed is preferably 10 sheets or less, and more preferably 5 sheets or less.

  Examples of the polypropylene resin that forms the outer layer of the expanded particles of the present invention include the same polypropylene resins as the polypropylene resin that forms the outer layer of the expanded particles used in the above-described method of the present invention. Examples of the polypropylene resin that forms the core layer of the expanded particles of the present invention include the same polypropylene resin as the polypropylene resin that forms the core layer of the expanded particles used in the above-described method of the present invention.

  The expanded particles of the present invention are preferably a resin different from the polypropylene resin in the inner layer portion and the polypropylene resin in the surface layer portion. The different resin referred to in this specification means that any one of melting point, melting start temperature, MFR and Vicat softening temperature is different. The values of the melting point, extrapolation melting start temperature, and Vicat softening temperature described above are such that the value of the surface layer portion is smaller than the value of the inner layer portion. Further, the value of MFR is such that the value of the surface layer portion is larger than the value of the inner layer portion. For the physical property values, a resin obtained by cutting the surface layer portion of the expanded particles with a cutter or the like is used, and for the inner layer portion of the expanded particles, a resin that is cut and defoamed so that the surface layer portion does not enter is used.

In the expanded particle of the present invention, when micro differential thermal analysis measurement is performed, the relationship between the extrapolated melting start temperature (Ts) of the surface layer portion and the extrapolated melting start temperature (Ti) of the inner layer portion satisfies the following equation: The expanded particles are preferable from the viewpoint of forming expanded particles that can be molded at a lower steam pressure without reducing the heat resistance of the expanded molded article. However, the unit of Ti and Ts in the formula is both ° C.
(Equation 9)
3 (° C.) ≦ Ti−Ts ≦ 40 (° C.) (2)

From the viewpoint of being able to be heat-molded even at a low steam pressure and preventing the heat resistance of the resulting foamed molded product from being lowered, it is preferable to satisfy the following formula (6), and to satisfy the following formula (7) Is more preferable, it is more preferable that the following formula (8) is satisfied, and it is particularly preferable that the following formula (9) is satisfied. However, the unit of Ti and Ts in the formula is both ° C.
(Equation 10)
3 (° C.) ≦ Ti−Ts ≦ 40 (° C.) (6)
(Equation 11)
5 (° C.) ≦ Ti−Ts ≦ 40 (° C.) (7)
(Equation 12)
7 (° C.) ≦ Ti−Ts ≦ 40 (° C.) (8)
(Equation 13)
9 (° C.) ≦ Ti−Ts ≦ 40 (° C.) (9)

  In order to configure such that the relationship between the extrapolation melting start temperature (Ts) of the surface layer part and the extrapolation melting start temperature (Ti) of the inner layer part satisfies the above-described formula (2), etc., the multilayer resin particles It is preferable to measure the melting point of the polypropylene resin forming the core layer and the outer layer with the above-mentioned differential scanning calorimeter and select a preferable resin combination as shown in the above-described equation (1).

  The micro differential thermal analysis (μDTA) in this specification uses a micro thermal analysis system “2990 type micro thermal analyzer” manufactured by T.A. Instruments Japan, Inc., and the temperature rising rate is 10 ° C. from 25 ° C. to 250 ° C. Measured under the conditions of / sec.

  FIG. 3 and FIG. 4 show an example of the μDTA curve with respect to the surface of the expanded particles, and these figures will be used to explain how to determine the extrapolated melting start temperature of the surface layer portion of the expanded particles. FIG. 3 shows an example of the μDTA curve for each of the surface layer portion of the multilayer expanded particle and the surface layer portion of the single layer expanded particle. In FIG. 3, the curve Cm is an example of a μDTA curve for the surface layer portion of the multilayer foamed particle, the Pm point on the curve Cm is the melting start temperature, and the Pme point is the intersection of the base line (BL) and the tangent line (TL). Is the extrapolated melting start temperature. On the other hand, the curve Cnm is an example of a μDTA curve with respect to the surface layer portion of the single-layer expanded particle, the Pnm point on the curve Cnm is the melting start temperature, and the Pnme point is the intersection of the base line (BL) and the tangent line (TL). Is the extrapolated melting start temperature. FIG. 4 shows an example of a μDTA curve with respect to the surface of the multilayer foamed particles (those having a surface layer part extrapolation melting start temperature slightly higher than that of FIG. 3). In FIG. 4, the curve Cm is a μDTA curve, the Pm point on the curve Cm is its melting start temperature, and the Pme point is the extrapolation melting start temperature at the intersection of the baseline (BL) and the tangent (TL). is there.

The melting start temperature here means the temperature at which the μDTA curve starts to change downward from the baseline (BL) in the μDTA curve obtained by micro differential thermal analysis (specific heat per hour starts to change). The extrapolated melting start temperature is a straight line obtained by extending the baseline (BL) of the μDTA curve to the high temperature side, and a tangent drawn from each point on the μDTA curve on the high temperature side from the melting start temperature. The temperature at the intersection of the tangent and the tangent (TL) that maximizes the angle between the tangent and the straight line extending from the base line (BL) to the high temperature side.

  In the micro differential thermal analysis, the expanded particles are fixed to the sample stage of the apparatus (if one expanded particle is too large as it is, it is fixed to an appropriate size, for example, by cutting in half), and then The probe tip (the part to be in contact with the surface layer part of the foamed particle has a tip of 0.2 μm in length and width) is lowered to contact the surface layer part of the foamed particle toward the randomly selected location on the surface of the foamed particle. It is carried out in the state of letting.

As the extrapolated melting start temperature of the surface layer portion of the multilayer expanded particle by the micro differential thermal analysis, an arithmetic average value of 8 points excluding the maximum value and the minimum value is adopted from the measurement results of 10 different measurement points. In addition, when there are a plurality of maximum values and minimum values, an arithmetic average value of several points excluding them is adopted. In addition, when all 10 measured values on the average are the same, or when only the maximum and minimum values are obtained and the difference between the maximum and minimum values is within 10 ° C., the 10-point phase An arithmetic mean value is adopted. If only the maximum and minimum values were obtained, and the difference between the maximum and minimum values exceeded 10 ° C, the same procedure as described above was performed by measuring 10 points on different surfaces. The arithmetic average value can be obtained by and adopted. If the condition is still not met, repeat the same operation.
The above μDTA results indicate that the decrease in the extrapolated melting start temperature of the surface layer portion of the multilayer foamed particles contributes to the decrease in the minimum fusion temperature required during molding.

  In addition, the mechanism by which the extrapolation melting start temperature of the surface layer portion of the multilayer foam particles is not clear is not certain, but when the multilayer resin particles are foamed, the foam is foamed based on the melting point of the resin constituting the core layer. Since the outer layer of the multilayer resin particles is rapidly cooled from a temperature higher than the melting point of the resin constituting the outer layer, the Smeltica structure of low-melting-point crystals increases, and the extrapolated melting start temperature of the surface layer portion of the multilayer foam particles decreases. I think that.

  In the in-mold molding of the foamed particles, the fusion of the foamed particles is performed between the surfaces of the foamed particles. Therefore, it is significant to perform a thermal analysis only on the surfaces of the foamed particles. It seems impossible to know the tendency of melting start only on the surface of the expanded particles by the DSC method. It is μDTA that makes this possible. Moreover, although the temperature rising rate is 10 ° C. per second with μDTA, this speed is close to the temperature rising rate when heating the foamed particles in actual in-mold molding (such a high temperature rising rate). Is difficult with the DSC method). Therefore, it is significant to analyze at a heating rate approximate to such actual in-mold molding. For this reason, the present invention employs micro differential thermal analysis (μDTA) for the surface layer portion of the multilayer foamed particles. The extrapolated melting start temperature based on this measurement may not indicate the exact melting start temperature, but the tendency of the extrapolated melting start temperature to be higher or lower than the molding temperature Match. In addition, the extrapolation melting start temperature is excellent in reproducibility since there are few errors.

  As a result of the above μDTA, the extrapolated melting start temperature of the surface layer portion of the expanded particles is lower than the extrapolated melting start temperature of the inner layer portion. This shows that it contributes to the lowering of the minimum fusing temperature required at the time of molding.

  The foamed particles of the present invention preferably have an apparent density of 10 g / L to 500 g / L. When the apparent density is less than 10 g / L, the foamed particles become open-celled and there is a possibility that a foamed molded product cannot be obtained. On the other hand, when the apparent density exceeds 500 g / L, the density of the obtained foamed molded product is too high, and there is a possibility that physical properties peculiar to the foam such as heat insulating properties, buffer properties, and lightness may be lost.

The apparent density (g / L) of the expanded particles is calculated by dividing the weight (g) of the expanded particles by the apparent volume (L). The apparent volume (L) of the expanded particles is excluded when about 5 g of expanded particles left at 23 ° C. and atmospheric pressure for 48 hours or more are submerged in water in a measuring cylinder containing 100 cm ³ of water at 23 ° C. It is obtained by reading the volume and converting this to liters. For this measurement, a plurality of foamed particles are used in such an amount that the weight of the foamed particles is 0.5000 to 10.0000 g and the apparent volume of the foamed particles is 50 to 90 cm ³ .

  The expanded particles of the present invention are substantially non-crosslinked. The term “substantially non-crosslinked” refers to a case where the ratio of insoluble matter to boiling xylene under a specific condition is 1% by weight or less of the sample.

  That is, regardless of whether the core layer polypropylene resin, the outer layer polypropylene resin, the foam particles composed of the inner layer portion and the surface layer portion, or the foam molded body obtained by heat molding are used as samples (per 100 g of xylene). 1 g of sample) was immersed in boiling xylene at normal pressure for 8 hours, and then quickly filtered through a 74 μm wire mesh defined in JIS Z 8801 (1966), and the remaining boiling xylene insoluble matter remaining on the wire mesh When the ratio of the insoluble matter is 1% by weight or less of the sample, it is called substantially non-crosslinked.

The content P (%) of the insoluble matter is expressed by the following formula (10).
(Equation 14)
P (%) = (M ÷ L) × 100 (10)
However, M is the weight (g) of insoluble matter, and L is the weight (g) of the sample.

  In the present invention, the DSC curve obtained by differential scanning calorimetry of the expanded particles includes an endothermic curve peak inherent to the polypropylene resin (hereinafter simply referred to as “inherent peak”), and an endothermic curve peak higher than the endothermic curve peak. (Hereinafter, simply referred to as “high temperature peak”), and the amount of heat of the endothermic curve peak on the high temperature side is preferably 15% to 70% with respect to the total amount of heat of all endothermic curve peaks. Such foamed particles have a high closed cell ratio and are suitable for heat molding.

  When the amount of heat at the high temperature peak is less than 15% with respect to the total amount of heat at all the endothermic curve peaks, the steam pressure at the time of molding can be lowered, but the compression strength, energy absorption amount, etc. of the resulting foam molded article May decrease. If it exceeds 70%, the air pressure that must be applied to the foamed particles prior to molding the foamed particles may become too high, or the molding cycle may be prolonged.

  From this point of view, when the polypropylene resin forming the inner layer portion is a propylene-ethyne copolymer, the heat amount of the high temperature peak is preferably 20% or more, and preferably 25% or more with respect to the total heat amount of all endothermic curve peaks. Is more preferable, and 30% or more is more preferable. In addition, the upper limit is preferably 60% or less, more preferably 50% or less from the above viewpoint.

  Further, from this viewpoint, when the polypropylene resin forming the inner layer portion is a propylene homopolymer, the heat amount of the high temperature peak is preferably 20% or more, and preferably 25% or more with respect to the total heat amount of all endothermic curve peaks. Is more preferable, and 30% or more is more preferable. Further, the upper limit is preferably 60% or less, and more preferably 50% or less.

  The total heat amount (total heat amount) of all endothermic curve peaks of the expanded particles in the present invention is preferably 60 J / g to 150 J / g. When the amount of heat is less than 60 J / g, physical properties such as compression may be reduced. On the other hand, when it exceeds 150 J / g, the secondary foamability at the time of molding may be poor and a foamed molded product having many gaps may be formed.

Furthermore, when the polypropylene resin forming the inner layer portion is a propylene-ethylene copolymer, the total heat quantity of the endothermic curve peak is preferably 60 J / g to 100 J / g.
Moreover, when the polypropylene resin forming the inner layer portion is a propylene homopolymer, the total heat quantity of the endothermic curve peak is preferably 60 J / g to 150 J / g.

The total calorific value of the endothermic curve peak and the calorific value of the high temperature peak are measured as follows by a measuring method based on JIS K7122 (1987).
First, 2 to 10 mg of expanded particles are collected, and the temperature rise is measured at 10 ° C./min from room temperature (10 to 40 ° C.) to 220 ° C. using a differential scanning calorimeter. An example of a DSC curve obtained by such measurement is shown in FIG.

  The DSC curve in FIG. 1 shows an intrinsic peak a derived from the polypropylene resin constituting the expanded particles and a high temperature peak b, and the amount of heat of the high temperature peak b corresponds to the peak area. Can be obtained as follows.

  First, a straight line (α−β) connecting a point α corresponding to 80 ° C. on the DSC curve and a point β on the DSC curve corresponding to the melting end temperature T of the expanded particles is drawn. The melting end temperature T is the intersection of the DSC curve on the high temperature side of the high temperature peak b and the high temperature side baseline.

Next, a straight line parallel to the vertical axis of the graph is drawn from the point γ on the DSC curve corresponding to the valley between the intrinsic peak a and the high temperature peak b, and the point intersecting with the straight line (α−β) is defined as δ. . The area of the high-temperature peak b is the portion surrounded by the curve of the high-temperature peak b portion of the DSC curve, the line segment (δ-β), and the line segment (γ-δ) (the portion hatched in FIG. 1). This is the area, which corresponds to the amount of heat at the high temperature peak.
Further, the total heat quantity of all endothermic curve peaks referred to in the present invention is the area of the portion surrounded by the DSC curve and the straight line (α-β) of the intrinsic peak a and the high temperature peak b in FIG. It corresponds to the total heat of the curve peak.

  The high temperature peak b is observed in the first DSC curve measured as described above, but not in the DSC curve obtained by raising the temperature in the second time. In the second DSC curve, as shown in FIG. 2, only the endothermic curve peak (inherent peak a) unique to the polypropylene resin constituting the expanded particles is observed.

  In the measurement of the intrinsic peak and the high temperature peak of the expanded particles with the differential scanning calorimeter as described above, when the weight per expanded particle is less than 2 mg, a plurality of the total weight is 2 to 10 mg. The foamed particles may be used as they are for the measurement, and when the weight per foamed particle is 2 to 10 mg, the foamed particles may be used for the measurement as they are. When the weight exceeds 10 mg, one cut sample with a weight of 2 to 10 mg obtained by cutting one expanded particle into a plurality of particles may be used for the measurement. However, this cut sample is obtained by cutting one foamed particle using a cutter or the like, but the non-foamed part of the whole foamed particle, such as intentionally containing many non-foamed parts of the foamed particle. Of course, it should be avoided to cut the sample so that the ratio between the foamed portion and the foamed portion changes greatly. As an example of preparation of a cut sample, when the weight per expanded particle is 18 mg, if the expanded particle oriented in an arbitrary direction is cut horizontally from the middle of the vertical direction, two approximately 9 mg of approximately the same shape can be obtained. A cut sample is obtained, and the ratio between the surface layer portion and the inner layer portion of the expanded particles that each cut sample has from the beginning does not change. One of the two cut samples thus obtained may be used for the measurement of the intrinsic peak and the high temperature peak as described above.

  Next, the technical meaning of the high temperature peak in the expanded particles of the present invention and the relationship between the heat quantity of the high temperature peak and the configuration of the present invention will be described.

  Since the high temperature peak heat amount appears in the first DSC curve as described above, it is caused by the crystal structure of the polypropylene resin, and this high temperature peak heat amount, that is, the crystal structure is caused by the difference between the melting point of the resin and the foaming temperature. Experience has shown that it is strongly influenced.

  In general, when filling the foamed particles into the mold and heat-molding with steam, the minimum saturated steam pressure (hereinafter referred to as the minimum) Steam pressure) is known to exist. The temperature corresponding to the lowest steam pressure is called the lowest fusion temperature.

  It has been empirically known that the amount of heat at the high temperature peak is closely related to the minimum fusing temperature and acts as a factor for determining the minimum fusing temperature. Further, when the same polypropylene resin is used, the minimum fusing temperature tends to decrease as the high-temperature peak heat value decreases. In addition, the value of the calorific value of the high temperature peak is strongly influenced by the level of the foaming temperature in the production stage of the foamed particles, and when the same polypropylene resin is used, the calorific value of the high temperature peak tends to decrease as the foaming temperature increases. There is.

  However, when foam moldings are thermoformed using foam particles with a small amount of heat at the high temperature peak, the minimum fusing temperature tends to be relatively low, but the strength properties such as compression strength (rigidity) of the foam moldings, etc. Tends to decrease relatively. On the other hand, when a foam molded article is heat-molded using foam particles having a high high-temperature peak heat, strength properties such as compression strength of the foam molded article tend to be relatively high, but the minimum fusion temperature is relatively As described above, a problem arises in that a high steam pressure is required when producing a foamed molded article.

  That is, the most preferred foamed particles are those having the contradictory properties that the strength properties such as the compression strength of the foamed molded product are relatively high although the minimum fusing temperature is low. The foamed particles of the present invention satisfy the contradictory properties at the same time, and the minimum fusing temperature of a polypropylene resin excellent in strength properties and the like is effectively reduced. Accordingly, if the foamed molded article is heat-molded using the foamed particles of the present invention, a molded article having practical strength in mechanical properties such as compressive strength can be produced using a conventional molding apparatus.

  Next, a method for adjusting the amount of heat at the high temperature peak of the foamed particles of the present invention in the above-described dispersion medium releasing foaming method will be described. That is, as described above, the foamed particles are obtained by a method in which the multilayer resin particles are dispersed in water together with the foaming agent in a sealed container, heated, impregnated with the foaming agent in the multilayer resin particles, and then released under low pressure. be able to.

  When the multilayer resin particles are foamed by such a dispersion medium releasing foaming method, if the heating temperature and heating time are set based on the melting point of the polypropylene resin constituting the core layer, the amount of heat at the high temperature peak can be increased. The obtained expanded particles can be excellent in strength properties such as compressive strength.

  As a specific method for adjusting the high temperature peak in the dispersion medium releasing foaming method, when the multilayer resin particles are dispersed in an aqueous medium and heated, the melting end temperature (tie) of the polypropylene resin in the core layer is not exceeded. The temperature is raised and stopped at an arbitrary temperature (Ta) within a range of 20 ° C. lower than the melting point (ti) of the resin and less than the melting end temperature (tie), preferably at that temperature (Ta), preferably Hold for about 10 to 60 minutes, then heat to a temperature (Tb) in the range of 15 ° C. lower than the melting point (ti) to the end of melting temperature (tie) + 10 ° C., stop at that temperature, It is preferable to hold the foam for a sufficient period of time, preferably about 10 to 60 minutes, and then release the multilayer resin particles from the sealed container under low pressure to cause foaming.

  In the dispersion medium releasing foaming method, it is preferable to set the temperatures Ta and Tb and the holding time as described above, because the amount of heat at the high temperature peak of the foamed particles is mainly the resin particles when producing the foamed particles. This is because it depends on the temperature Ta and the holding time at the temperature, the temperature Tb and the holding time at the temperature, and the temperature rising rate.

  In general, the amount of heat at the high temperature peak of the expanded particles tends to increase as the temperature Ta or Tb decreases within the above temperature range or as the holding time increases. Usually, the temperature rising rate is 0.5 to 5 ° C./min. By repeating the preliminary experiment in consideration of these points, it is possible to easily know the production conditions for the expanded particles exhibiting a desired high-temperature peak heat quantity.

  In addition, the temperature range at the time of foaming demonstrated above is a suitable temperature range at the time of using an inorganic type physical foaming agent as a foaming agent. When an organic physical foaming agent is used in combination, the appropriate temperature range tends to shift to a lower temperature side than the above temperature range depending on the type and amount of use.

  In the foamed particles of the present invention, after aging under atmospheric pressure, it is preferable to increase the bubble internal pressure as necessary, and then heat molding to obtain a foamed molded article having no gap between the foamed particles. It is preferable because physical properties such as compression strength of the obtained foamed molded article are improved.

  In order to increase the bubble internal pressure of the foamed particles, the foamed particles can be placed in a sealed container and left for an appropriate period of time in a state where pressurized air is supplied into the container to allow the pressurized air to penetrate into the foamed particles. That's fine. The apparent density of the foamed molded product produced by such a method can be arbitrarily selected depending on the purpose, but is usually in the range of 9 g / L to 600 g / L. In addition, the apparent density of a foaming molding is calculated | required by the external dimension (L) and its weight (g) of a test piece.

  Moreover, it is preferable to make the foamed particles having an increased bubble internal pressure into foamed particles having a higher expansion ratio by heating with water vapor or hot air. When heat molding is performed using the expanded particles having a high expansion ratio, a foamed molded body having a high expansion ratio can be easily obtained.

  The foamed molded article is filled with a foam that can be heated and cooled and opened and closed and sealed after increasing the internal pressure of the foamed particles as necessary. It is preferable to manufacture by adopting a batch-type in-mold thermoforming method in which is heated and expanded to be fused, and then cooled and taken out from the mold.

  As a molding machine used in the batch-type in-mold heating molding method, many molding machines already exist all over the world, and the pressure resistance is 0.41 MPa (G) or 0.45 MPa although it varies slightly depending on the country. There are many things of (G). Therefore, the pressure of the saturated steam when the expanded particles are expanded and fused is preferably 0.45 MPa (G) or less, and more preferably 0.41 MPa (G) or less.

The foamed particles of the present invention can be made into a foamed molded article by a continuous molding method.
In the continuous molding method, the foamed particles are continuously supplied between belts that continuously move along the upper and lower sides of the passage after increasing the bubble internal pressure as necessary, and a saturated steam supply region (heating The foamed particles are expanded and fused with each other when passing through the region), and then cooled by passing through the cooling region, and then the obtained molded product is taken out from the passage and sequentially cut to an appropriate length. The body is obtained. Such continuous molding methods are described, for example, in JP-A-9-104026, JP-A-9-104027 and JP-A-10-180888.

  Further, the foamed molded article obtained using the foamed particles of the present invention preferably has an open cell ratio of 40% or less based on ASTM-D2856-70 procedure C, more preferably 30% or less, It is particularly preferably 25% or less. A molded body having a smaller open cell ratio is excellent in mechanical strength.

  In addition, a surface decoration material can be laminated and integrated on at least a part of the surface of the foamed molded article obtained by using the foamed particles of the present invention. US Pat. No. 5,928,776, US Pat. No. 6,096,417, US Pat. No. 6,033,770, US Pat. No. 5,474,841, European Patent 477476, WO 98/34770 , WO98 / 00287, Japanese Patent No. 3092227 and the like.

  Further, in the foamed molded article obtained by the present invention, the insert material can be combined and integrated such that all or part of the insert material is embedded. A method for producing such an insert composite type in-mold foam molded article is described in detail in US Pat. No. 6,033,770, US Pat. No. 5,474,841, JP-A-59-1277714, Japanese Patent No. 3092227, and the like. Has been.

  Hereinafter, the present invention will be described in detail with reference to examples and comparative examples.

  Table 1 shows the types of polypropylene resins used in Examples 1 to 8 and Comparative Examples 1 to 3, the heat of fusion (J / g) of the resin by differential scanning calorimetry, and the melt flow rate (MFR).

Examples 1-8
100 parts by weight of a polypropylene resin (resin (y) in the table) and 0.05 parts by weight of zinc borate powder (cell regulator) are supplied to an extruder, heated and melt-kneaded for core layer formation. A first mixed molten resin was formed. At the same time, the polypropylene resin (resin (x) in the table) shown in Table 2 was supplied to another extruder and heated, melted and kneaded to form a second mixed molten resin for outer layer formation.

Next, the first mixed molten resin for forming the core layer and the second mixed molten resin for forming the outer layer are supplied to a coextrusion die, and in the die, the second mixed molten resin is the first mixed molten resin. The second mixed molten resin was laminated on the first mixed molten resin so as to cover the periphery of the mixed molten resin strand. The ratio (y _A / x _B ) between the discharge amount (y _A ) of the first mixed molten resin and the discharge amount (x _B ) of the second mixed molten resin was set to 20/1.

  Next, the laminated mixed molten resin is extruded in a strand form from a coextrusion die and cut so that the diameter is about 1 mm and the length is about 1.5 of the diameter, and the average weight per particle is 2 mg of multilayer resin particles were obtained. The surface area of the outer layer of the multilayer resin particles was 86% of the entire surface of the multilayer resin particles. The thickness of the outer layer of the multilayer resin particle is shown in Table 2. The thickness of the outer layer of the multilayer resin particles was measured according to the measurement method described above.

Using the multilayer resin particles, foamed particles were produced as follows.
In a 400 liter autoclave, 100 parts by weight of the multilayer resin particles, 220 parts by weight of water, 0.05 parts by weight of sodium dodecylbenzenesulfonate (surfactant) and 0.3 parts by weight of kaolin (dispersant) are shown in Table 2. Carbon dioxide gas (foaming agent) was added, the temperature was raised to a temperature 5 ° C. lower than the foaming temperature shown in Table 2 with stirring, and the temperature was maintained for 15 minutes. Next, the temperature was raised to the foaming temperature and held at that temperature for 15 minutes. Next, one end of the autoclave was opened, and the contents of the autoclave were released under atmospheric pressure to obtain expanded particles.

  During the release of the multilayer resin particles from the autoclave, the release was performed while supplying carbon dioxide gas into the autoclave so that the pressure in the autoclave was maintained at the pressure in the autoclave immediately before the release.

  The obtained foamed particles are washed with water and subjected to a centrifuge, and then left to stand at atmospheric pressure for 24 hours. After curing, the apparent density of the foamed particles, the high temperature peak heat, the ratio of the high temperature peak heat to the total heat, the average Bubble diameter, surface layer extrapolation melting start temperature (Ts), inner layer extrapolation melting start temperature (Ti), surface layer extrapolation melting start temperature (Ts) and inner layer melting start temperature (Ti) Differences ("Ti-Ts" in Table 3) were measured. The results are shown in Table 3.

  In addition, the measuring method of the apparent density (g / L) of a foamed particle group, the high temperature peak calorie | heat amount (J / g) of a foamed particle, and an average bubble diameter was performed according to the method mentioned above.

  The methods for measuring the extrapolated melting start temperature (Ts) of the surface layer portion and the extrapolated melting start temperature (Ti) of the inner layer portion were measured by the methods described above.

  A multilayer resin particle colored by adding a colorant to the outer layer was prepared, and foamed in the same manner as in Examples 1 to 8 and the foamed particle was observed with a microscope. A portion corresponding to the colored outer layer (surface layer portion of the foamed particle) ) Was not foamed.

  Using the foamed particles obtained in Examples 1 to 8, foamed molded articles were molded as follows. After increasing the internal pressure at the time of heat molding shown in Table 4 using pressurized air in a pressure vessel, using a small-scale molding machine capable of withstanding a saturated steam pressure of 0.59 MPa (G), 250 mm × 200 mm × After filling the mold with a molding space of 50 mm with a small gap (about 1 mm) without completely closing the mold, and then exhausting the air in the mold with steam pressure completely The mold was clamped and heat-molded by supplying the steam pressure shown in Table 4 into the mold.

  After heat molding, after water cooling until the surface pressure of the molded body in the mold reaches 0.059 MPa (G), the molded body is taken out from the mold, cured at 60 ° C. for 24 hours, then cooled to room temperature and foamed. A molded body was obtained.

  Table 4 shows the steam pressure and the fusion rate at the time of heat molding in this example.

  The specific measurement of the fusion rate described above is as follows. First, the obtained foamed molded product was cut with a cutter knife in the thickness direction of the molded product, and then the foamed molded product was broken by hand from the cut portion. did. Next, the number (n) of the expanded particles existing on the fracture surface and the number (b) of the expanded material particles were measured, and the ratio (b / n) × 100 (%) of (n) and (b). Was the fusion rate.

Measurement of the internal pressure of the foamed particles during the heat molding shown in Table 4 was performed as follows.
Within 70 seconds after taking out the foam particles just before heat molding with increased internal pressure from the pressurized tank, 70mm x 100mm with many needle holes of a size that allows the foam particles to pass but not allow the air to pass freely. It is housed in a polyethylene bag with a temperature of 23 ° C. and moved to a temperature-controlled room under atmospheric pressure with a relative humidity of 50%. Subsequently, the weight was read on the balance in the constant temperature. The measurement of the weight was 120 seconds after taking out the above expanded particle group from the pressurized tank. The weight at this time was defined as Q (g). Subsequently, the bag was left in the same greenhouse for 48 hours. Since the pressurized air in the expanded particles permeates through the cell membrane and escapes to the outside as time passes, the weight of the expanded particles decreases accordingly, and after 48 hours, the weight has reached equilibrium. Stable. After 48 hours, the weight of the bag was measured again, and the weight at this time was defined as U (g). Subsequently, all of the foam particles were taken out of the bag immediately in the same temperature chamber and the weight of the bag alone was read. The weight was defined as Z (g). Any of the above weights were read to 0.0001 g. The difference between Q (g) and U (g) is the increased air amount W (g), and the internal pressure P (MPa) of the expanded particles is calculated from the following equation. The internal pressure P corresponds to a gauge pressure.
(Equation 15)
P = (W ÷ M) × R × T ÷ V (11)

  However, in the above formula, M is the molecular weight of air, and here, a constant of 28.8 (g / mol) is adopted. R is a gas constant, and here, a constant of 0.0083 (MPa · L / (K · mol)) is adopted. T means an absolute temperature, and since an atmosphere of 23 ° C. is adopted, it is a constant of 296 (K) here. V means the volume (L) obtained by subtracting the volume of the base resin in the expanded particle group from the apparent volume of the expanded particle group.

  The compression strength, apparent density, and heat resistance of the foamed molded products obtained in Examples 1 to 8 were measured, and the results are shown in Table 4.

  From Table 4, in Examples 1-8, the steam pressure of the heat molding conditions was 0.45 MPa (G) or less, and the fusion rate was 50% or more. Furthermore, in Examples 3 to 8, the steam pressure under the heat molding conditions was 0.41 MPa (G) or less. For this reason, it was the expanded particle which can be fully shape | molded with the conventional molding machine.

The compression strength of the foamed molded product was measured as follows.
First, a test piece having a length of 50 mm, a width of 50 mm, and a thickness of 25 mm was cut out from the obtained foamed molded article (the entire skin was cut). Next, the test piece was subjected to a compression test according to JIS Z 0234-1976 A method at a test piece temperature of 23 ° C. and a load speed of 10 mm / min until the strain reached 55%, and the obtained stress-strain diagram Further, the stress at the time of 50% strain was read, and this was taken as the compressive strength.

  The apparent density of the foamed molded product was measured according to the method described above.

The heat resistance of the foamed molded articles obtained using the foamed particles of Examples 1 to 8 was evaluated.
Cut the specimen size thickness 10 mm × width 40 mm × length 200 mm from the obtained foamed molded article, and the jig used for three-point bending with a support base radius R of 1.25 mm and a height of 100 mm and a span distance of 150 mm. A test piece was placed and a weight of 5 g of a cylindrical rod having a diameter of 5 mm and a length of 50 mm was placed on the center of the test piece and left in an oven at 140 ° C. for 22 hours.
The deformation rate is obtained by the following equation (12), where Ha is the height after heating, where Hb is the height from the lower surface of the jig before heating to the central portion in the length direction of the test piece, and evaluation is performed based on the value. It was. The results are shown in Table 4.
(Equation 16)
Deformation rate (%) = (Hb−Ha) / Hb × 100 (12)
* ... Deformation rate is 10% or less.
O ... Deformation rate exceeds 10% and is 30% or less.
X: Deformation rate exceeds 30%.

  From the results of Example 3 and Example 4, Example 5 and Example 6, even when the same multilayer resin particle is used, if the foaming temperature is high, the extrapolated melting start temperature of the inner layer part and the compensation of the surface layer part in the obtained foamed particle are obtained. There was a tendency for the difference from the external melting start temperature to become wide.

  When Example 3 is compared with Example 8, the apparent density of the expanded particles is almost the same, but the expanded particles of Example 8 have a high melting point between the inner layer portion and the core layer portion even though the high-temperature peak heat amount is large. Since the difference was large (Table 2, Example 3: 2.1 ° C., Example 8: 21 ° C.), molding was possible at a steam pressure lower than that of the expanded particles of Example 3.

Comparative Examples 1-3
Using the polypropylene resin (resin (y) in the table) and the polypropylene resin (resin (x) in the table) shown in Table 5, the thickness of the outer layer of the multilayer resin particles shown in Table 5 is used. Example 1 except that the ratio (y _A / x _B ) between the discharge amount (y _A ) of the mixed molten resin and the discharge amount (x _B ) of the second mixed molten resin was set to 3.5 / 1 Multilayer resin particles were obtained in the same manner as in -8. The surface area of the outer layer of the multilayer resin particle was 86% of the entire surface of the multilayer resin particle.

Using the multilayer resin particles, expanded particles were produced in the same manner as in Examples except for the addition amount of carbon dioxide gas and the expansion temperature shown in Table 5.
The obtained foamed particles were washed with water and centrifuged in the same manner as in Examples 1 to 8, and then allowed to stand at atmospheric pressure for 24 hours. After that, the apparent density of the foamed particles, the high temperature peak heat amount, Ratio of high-temperature peak heat quantity to total heat quantity, average bubble diameter, extrapolated melting start temperature (Ts) of surface layer part, extrapolated melting start temperature (Ti) of inner layer part, melting start temperature (Ts) of inner layer part and inner layer part The difference from the extrapolated melting start temperature (Ti) of (Ti-Ts in Table 6) and the like were measured. The results are shown in Table 6.

  A multilayer resin particle colored by adding a colorant to the outer layer was prepared, and the foamed particles were observed in a microscope in the same manner as in the comparative example. When the foamed particles were observed with a microscope, Comparative Examples 1 to 3 were portions corresponding to the colored outer layer (foamed) The surface layer portion of the particles was foamed.

Heating was carried out in the same manner as in Examples 1 to 8 except that the foamed particles obtained in Comparative Examples 1 to 3 were used, and the internal pressure at the time of heat molding shown in Table 7 was increased, and the heat molding was performed at the steam pressure shown in Table 7. Molding was performed.
As a result, in Comparative Example 1, the fusion rate was 80%, and the saturated steam pressure that the molding machine could withstand was 0.50 MPa (G), which exceeds the pressure resistance of the conventional molding machine, 0.45 MPa (G). It was.
In Comparative Example 2, when the foamed particles of Comparative Example 1 were used and the saturation steam pressure was 0.43 MPa (G), the foamed particles were not fused when the mold was opened after molding. An expanded molded article was not obtained.
In Comparative Example 3, the high-temperature peak heat amount is as low as 26 J / g, so the steam pressure was 0.43 MPa (G), which is less than the pressure resistance of the conventional molding machine, 0.45 MPa (G). Compared with the compressive strength of Comparative Example 1 in which the compression strength is the same, the compressive strength of Comparative Example 3 was about 22% lower.
In addition, the * mark of the fusion rate of Comparative Example 3 in Table 7 indicates that although the obtained foamed molded product is fused to the foamed particles, the foamed molded product is After making an incision of about 10 mm in the thickness direction, the foamed molded body was broken by hand from the incision part, and the fracture surface was observed with an electron microscope. As a result, it was found that peeling occurred at the interface between the core layer part and the surface layer part. I understood.

Example 9
Using the polypropylene resin (resin (y) in the table) and the polypropylene resin (resin (x) in the table) shown in Table 8, the outer layer is made of polypropylene resin (x), and the cross section is cylindrical. A cylindrical multilayer having an average weight of 2 mg per particle, extruded from a coextrusion die in a strand shape and cut to a diameter of about 1.0 mm and a diameter to length ratio of about 2.0. Resin particles were obtained. The surface of the obtained cylindrical multilayer resin particles was laminated at the surface layer portion. The surface area of the outer layer of the multilayer resin particles was 89% of the entire surface of the multilayer resin particles.

Comparative Example 4
Using polypropylene resin shown in Table 9, extruded from an extrusion die having a cylindrical cross section into a strand shape, cut so that the diameter is about 1.0 mm and the ratio of diameter to length is about 2.0. Thus, cylindrical multilayer resin particles having an average weight per particle of 2 mg were obtained.

Example 9, Comparative Example 4
Using the cylindrical multilayer fat particles, cylindrical foam particles were produced as follows.
In a 400 liter autoclave, 50 parts by weight of the above cylindrical multilayer resin particles, 220 parts by weight of water, 0.05 parts by weight of sodium dodecylbenzenesulfonate (surfactant) and 0.3 parts by weight of kaolin (dispersant), Table 8 The carbon dioxide gas (foaming agent) shown in FIG. 5 was added, and the temperature was raised to a temperature 5 ° C. lower than the foaming temperature shown in Tables 8 and 9 while stirring, and then kept at that temperature for 15 minutes. Next, the temperature was raised to the foaming temperature and held at that temperature for 15 minutes. Next, one end of the autoclave was opened, and the contents of the autoclave were released under atmospheric pressure to obtain cylindrical expanded particles. Other than that was performed like Example 1-8.

  The obtained cylindrical foamed particles were washed with water and centrifuged, and then allowed to stand at atmospheric pressure for 24 hours. After curing, the apparent density of the cylindrical foamed particles, the high-temperature peak heat quantity, and the high-temperature peak relative to the total heat quantity. Heat ratio, average bubble diameter, surface extrapolation start temperature (Ts), inner layer extrapolation start temperature (Ti), surface extrapolation start temperature (Ts) and inner layer extrapolation A difference from the starting temperature (Ti) (in Table 10, “Ti-Ts”) and the like were measured. The results are shown in Table 10 and Table 11.

  Next, a foamed molded article was produced using the cylindrical foamed particles obtained in Example 9 and Comparative Example 4.

  The bubble internal pressure was raised to the value shown in Table 12, and the mold was not completely closed and filled with a small gap (about 1 mm), and then the mold was completely exhausted after the air in the mold was exhausted with steam. Fastening was performed in the same manner as in Examples 1 to 8 except that heating was performed by supplying steam having a pressure shown in Table 12 into the mold. Table 12 shows the results of measuring the compressive strength, apparent density, and porosity of the obtained foamed molded article.

The porosity was measured as follows.
The porosity (%) of the foamed molded product is a (cm ³ ) based on the volume obtained from the external dimensions (25 mm × 25 mm × 100 mm) of the foamed molded product, and the sample is placed in the alcohol of a graduated container containing alcohol. The true volume of the sample obtained from the increment of the scale when submerged was defined as b (cm ³ ) and obtained from the following formula.
(Equation 17)
Porosity (%) = {1- (b / a)} × 100 (13)

  From Table 12, when Example 9 and Comparative Example 4 were compared, in Example 9, the steam pressure was low and the fusion between the expanded particles was good. In Comparative Example 4, the steam pressure was 0.55 MPa (G), which exceeded 0.45 MPa (G) of the conventional molding machine. Further, the foamed molded product obtained in Example 9 had a higher porosity than the foamed molded product obtained in Comparative Example 4.

Comparative Example 5
Although it carried out similarly to Examples 1-8 except having used the polypropylene resin (resin (y) in a table) and polyethylene resin (resin (x) in a table) shown in Table 13, it was laminated. When the mixed molten resin is extruded into a strand from a coextrusion die and cut to have a diameter of approximately 1 mm and a length of approximately 1.5 of the diameter, the core layer polypropylene resin and the outer layer polyethylene resin Since the adhesiveness with the resin was low, the polypropylene resin in the core layer was lost and multilayer resin particles could not be obtained.

It is a figure which shows an example of the chart of the 1st DSC curve of the expanded particle of this invention. It is a figure which shows an example of the chart of the 2nd time DSC curve of the expanded particle of this invention. It is a figure which shows an example of the chart of the curve obtained by micro differential thermal analysis measurement. It is a figure which shows an example of the chart of the curve obtained by micro differential thermal analysis measurement.

Claims (6)

A core layer formed from a polypropylene resin and an outer layer formed from a polypropylene resin, and the relationship between the melting point (ts) of the polypropylene resin of the outer layer and the melting point (ti) of the polypropylene resin of the core layer In which the foaming agent-impregnated multilayer resin particles in a heat-softened state are foamed by impregnating the foaming agent into the multilayer resin particles having the outer layer thickness of 30 μm or less. A method for producing resin foam particles.
(Equation 1)
1.5 (° C.) ≦ ti-ts ≦ 30.0 (° C.) (1)
(However, the unit of ti and ts in the formula is ° C.) The method for producing expanded polypropylene resin particles according to claim 1, wherein the tensile modulus of the polypropylene resin of the core layer is 1200 MPa or more. Foamed particles obtained by foaming multi-layer resin particles composed of a core layer formed from a polypropylene resin and an outer layer formed from a polypropylene resin, wherein the foamed particles are foamed from the polypropylene resin in the core layer. And an outer layer melting point of the outer layer obtained by micro-differential thermal analysis measurement is the outer layer melting start temperature (Ts) of the inner layer part. Polypropylene-based resin expanded particles characterized by being at least 2 ° C. lower than the extrapolated melting start temperature (Ti). 4. The polypropylene resin foam according to claim 3, wherein the relationship between the extrapolation melting start temperature (Ts) of the surface layer portion and the extrapolation melting start temperature (Ti) of the inner layer portion satisfies the formula (2). particle.
(Equation 2)
3 (° C.) ≦ Ti−Ts ≦ 40 (° C.) (2)
(However, the unit of Ti and Ts in the formula is ° C.) The DSC curve obtained by differential scanning calorimetry of the polypropylene resin expanded particles shows at least an endothermic curve peak unique to the polypropylene resin and an endothermic curve peak higher than the endothermic curve peak, and 5. The expanded polypropylene resin particle according to claim 3, wherein the heat quantity of the endothermic curve peak is 15% to 70% with respect to the total heat quantity of all the endothermic curve peaks. The expanded polypropylene resin particles according to any one of claims 3 to 5, wherein the expanded polypropylene resin particles are cylindrical.

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