JP2005271499A - Method for manufacturing thermoplastic resin expansion molded body and molded body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a box-shaped thermoplastic resin expansion molded body by which the expansion molded body showing successful injection expansion moldability, a high expansion ratio, excellent lightweight properties and rigidity can be obtained at a low cost, and a foam molded article. <P>SOLUTION: This foam molded body is constituted of at least a standing wall and a bottom surface, and the standing wall has a hollow part, while the bottom surface has a foam part. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an injection foam molded article made of a thermoplastic resin. Specifically, the present invention relates to a thermoplastic resin foam molded article having excellent lightness and rigidity and a method for producing the same.

  Conventionally, a thermoplastic resin has been widely used as a resin molded product in home appliances, automobiles, buildings, housing facilities, and the like. These resin molded products have been used in many fields in place of metal parts because of their low density, light weight, and non-corrosiveness compared to metals.

  In particular, polypropylene resin has excellent physical properties and molding processability. Moreover, because of its advantages that it does not generate harmful gases during incineration and can be recycled, it can be rapidly used as an environmentally friendly material. The range of use is expanding. In addition, the demand for polypropylene resin products for automobile-related materials is high, and in particular, social demands for weight reduction, high strength and rigidity are increasing.

  For the purpose of weight reduction, cost reduction, and prevention of warping and sink marks of a molded article, so-called injection foam molding is performed in which foaming is performed by impregnating a conventional injection molding with a foaming agent. However, the polypropylene resin is crystalline and has a low melt tension (melt tension), and it has been difficult to generate voids in the molded body due to the destruction of bubbles during foaming or to increase the expansion ratio. In addition, since the bubbles were non-uniform and large, the resulting molded article was not sufficiently rigid.

  As a method for improving foamability, a method has been proposed in which a crosslinking agent or a silane-grafted thermoplastic resin is added to increase the melt tension of a polypropylene resin (for example, Patent Document 1 and Patent Document 2). However, in this method, although a foamed molded product having a high expansion ratio can be obtained, the viscosity at the time of melting is excessively increased, and injection molding becomes difficult, and the surface properties of the obtained molded product are also poor.

  In addition, by introducing long chain branching by irradiation, the melt tension is higher than that of a normal linear polypropylene resin, and the viscosity increases as the stretch distortion of the melt increases. A polypropylene resin is commercially available from Sun Allomer as HMS-PP (High Melt Strength Polypropylene) (Patent Document 3). It is known that a foam molded article can be obtained by using such HMS-PP as a base resin for injection foam molding (Patent Document 4). However, the HMS-PP used here has a melt flow rate of only about 4 g / 10 minutes, has a low fluidity at the time of melting, and has a thin part with a mold cavity clearance of about 1 to 2 mm. There was a problem with short shots. On the other hand, HMS-PP (30 g / 10 min) with a high melt flow rate is also known, but although it exhibits strain hardening, it has a melt tension of only about 0.3 cN and obtains a foamed molded article with a high expansion ratio. Was difficult. And since manufacture of these HMS-PP uses an expensive radiation equipment, manufactured HMS-PP also becomes expensive and it is difficult to provide the product obtained from it at low cost.

  In addition, a method has been proposed in which a polypropylene resin having a high melt flow rate and high melt tension mixed with polyethylene having a specific intrinsic viscosity is used for injection foam molding (Patent Document 5). However, since such a polypropylene resin does not show a remarkable strain hardening property like the HMS-PP having a long-chain branch, the bubbles are destroyed at a high expansion ratio such that the expansion ratio exceeds 2. As a result, internal voids tend to occur, and it was not possible to meet the needs for high rigidity and light weight.

  On the other hand, fiber reinforcing agents such as glass fibers and carbon fibers and inorganic fillers such as talc are actively added for the purpose of improving the strength and rigidity of the thermoplastic resin molded body itself. However, these additives have a relatively high specific gravity and satisfy the rigidity as a molded article, but go against the weight reduction of the molded article. And in order to reduce the weight, it is necessary to reduce the thickness of the molded body, but in order to reduce the thickness, the fluidity of the molten resin is necessary, and when the thickness is reduced, the strength, There is a problem that the rigidity is greatly reduced.

  For this reason, in order to reduce the weight while ensuring strength and rigidity, a molded body design is provided in which reinforcing ribs are provided.

  As a method of providing a polypropylene resin product having excellent rigidity, in a box-shaped molded body composed of a bottom surface portion consisting of a bottom surface and a peripheral wall portion rising from the bottom surface in the periphery, the bottom surface is foamed and the peripheral wall portion is made non-foamed Methods have been proposed (Patent Documents 6 and 7). However, in this method, since the peripheral wall portion is non-foamed, there is no contribution to the weight reduction, and a further weight reduction method is required.

  In addition, a molded body having a ribbed hollow structure that connects the front and back surfaces can be used for gas injection using a mold having a movable core that protrudes into the mold cavity and can be retracted from the protruding position. A method of forming by performing from the core has been proposed (Patent Document 8). However, since this method uses a mold structure member called a movable core to form the ribs, the number of ribs is naturally limited, so that it is difficult to obtain the required strength and rigidity unless the thickness of the front and back surfaces is increased. Become.

As described above, it has been difficult to obtain a polypropylene resin product having high lightness and excellent rigidity at low cost.
JP 61-152754 A Japanese Patent Laid-Open No. 7-109372 Japanese Patent Laid-Open No. 62-121704 JP 2001-26032 A JP 2003-128854 A JP 2001-114148 A JP-A-10-128795 JP-A-5-64829

  An object of the present invention is to provide a thermoplastic resin foam molded article having good injection foam moldability, excellent lightness and rigidity, and a production method capable of obtaining the foam molded article at low cost.

  That is, the present invention relates to a foamed molded article comprising at least a standing wall part and a bottom part, wherein the standing wall part has a hollow part, and the bottom part has a foaming part. .

As a preferred embodiment,
(1) The thermoplastic resin is (A) 50 parts by weight or more and 95 parts by weight or less of a linear polypropylene resin having a melt flow rate of 10 g / 10 min to 100 g / 10 min and a melt tension of 2 cN or less, B) Modified polypropylene resin having a melt flow rate of 0.1 g / 10 min or more and less than 10 g / 10 min, a melt tension of 5 cN or more, and exhibiting strain-hardening property, 5 parts by weight or more and 50 parts by weight or less (however, linear) 2. The thermoplastic resin foam molded article according to claim 1, comprising: 100 parts by weight of the total of the polypropylene resin (A) and the modified polypropylene resin (B).
(2) The modified polypropylene resin (B) is a modified polypropylene resin obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator, and a conjugated diene compound.
(3) The foaming ratio is 2 having a foamed layer having an average cell diameter of 500 μm or less on the foamed bottom surface part and a non-foamed layer having a thickness of 10 μm or more and 1000 μm or less formed on at least one surface of the foamed layer. More than 10 times and less than
(4) The hollow ratio of the hollow portion of the standing wall portion is 10% or more and 50% or less.
It relates to the above-mentioned thermoplastic resin foamed molded article.

The second aspect of the present invention is: (a) foaming thermoplasticity in a cavity provided in a mold having a standing wall portion, which is composed of a fixed mold and a movable mold capable of moving forward and backward at an arbitrary position. Injecting resin,
(C) A step of injecting a pressurized fluid into the standing wall portion during the injection of the thermoplastic resin or after completion of injection, and a step of hollowing the standing wall portion, (d) movable after injecting the pressurized fluid into the standing wall portion The method includes the step of retreating the mold and foaming the bottom surface portion. The method for producing the thermoplastic resin foam-molded article as described above is characterized in that (a) the injection step and (c) pressurization are preferable. (B) A precooling step is provided between the fluid injection steps, and the method for producing a thermoplastic resin foam molded article as described above.

  The thermoplastic resin foam molded article of the present invention has a non-foamed layer so-called skin layer on the bottom surface and the standing wall surface, and the foamed layer so-called core layer inside the bottom surface has uniform fine bubbles at a high expansion ratio. Moreover, since the standing wall portion has a hollow portion, a box-shaped injection foamed molded article having excellent lightness and rigidity can be obtained. In addition, since a relatively expensive modified polypropylene resin is diluted with an inexpensive linear polypropylene resin, it is possible to provide such an excellent quality foamed molded article at a low cost.

Hereinafter, embodiments of the present invention will be described.
(1) Molded body The thermoplastic resin foam molded body of the present invention is a foam molded body comprising at least a standing wall portion and a bottom surface portion, and has a hollow portion in the standing wall portion and a foamed portion in the bottom surface portion. At least one or more standing wall portions may be provided. For example, when the bottom surface portion is a quadrangle, it is preferable to have four standing wall portions rising from each side.

  The thermoplastic resin foam molded article of the present invention preferably has a foam layer having an average cell diameter of 500 μm or less, more preferably 200 μm or less, and a thickness formed on the surface of at least one side of the foam layer of 10 μm. And a non-foamed layer having a thickness of 100 μm or more and 500 μm or less. When the average cell diameter of the foam layer exceeds 500 μm, excellent rigidity may not be obtained. If the thickness of the non-foamed layer is less than 10 μm, the rigidity tends to decrease, and if it exceeds 1000 μm, it may be difficult to obtain light weight.

  The expansion ratio of the bottom portion of the thermoplastic resin foam molded article of the present invention is preferably 2 to 10 times, more preferably 3 to 6 times, and the wall thickness is preferably 20 mm or less, Preferably it is 10 mm or less. If the expansion ratio is less than 2 times, it may be difficult to obtain light weight, and if it exceeds 10 times, the rigidity tends to be significantly reduced. The expansion ratio is a value obtained from the ratio of specific gravity between the foamed molded product and the non-foamed molded product injection-molded under the same conditions except that no foaming agent is added to the thermoplastic resin.

The thermoplastic resin foam molded article of the present invention has a hollow portion in the standing wall portion. In the present invention, the hollow portion refers to a void in which a pressurized fluid is injected and grown. In the present invention, a preferable hollow ratio is 10% or more and 50% or less, and more preferably 20% or more and 40% or less. This is because if the hollow ratio is high, the surface appearance is damaged by a hesitation mark or the like, and if it is low, the effect on weight reduction and sink and warp is impaired. The hollow ratio is defined by the following formula.
Hollow ratio (%) = [weight of standing wall of non-hollow molded body (g) −weight of standing wall of molded body of the present invention (g)] / [weight of standing wall of non-hollow molded body (g)] × 100
The standing wall weight of the molded body of the present invention is the weight of the standing wall of the molded body according to the present invention, and the non-hollow molded body standing wall weight is the same as the molded body weight and molding conditions, And it is the weight of the standing wall portion of the molded body in which the pressurized fluid is not press-fitted into the standing wall portion, that is, the hollow portion is not formed.

(2) Thermoplastic resin The thermoplastic resin used in the present invention can be used without particular limitation as long as a known thermoplastic resin is a main component, and may contain various additives as necessary. .
Specific examples of the thermoplastic resin include polyolefin resins such as polyethylene and polypropylene, polystyrene resins, polyamide resins, polyethylene terephthalate resins, polycarbonate resins, polyphenylene ether resins, modified polyphenylene ether resins, ABS resins, polyoxymethylene resins, Etc. Among these, polyolefin resins such as polyethylene and polypropylene are preferable because of the need to reduce the weight of the structure.

  Examples of the polyolefin resin include olefin homopolymers, olefin copolymers, and copolymers of olefins with a small amount of monomers other than olefins. In the case of a copolymer, it may be a random copolymer or a block copolymer. Specific examples of the olefin include ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene and the like, preferably 2-20 carbon atoms, 2-8 alpha olefins etc. are mention | raise | lifted. Examples of monomers other than olefins include vinyl compounds copolymerizable with α-olefins. These monomers can also be used individually by 1 type, and can also be used in combination of 2 or more type.

  Specific examples of the polyolefin resin include polyethylene resin, polypropylene resin, poly 1-butene resin, poly 4-methyl-1-pentene resin and the like. Among these, a polyethylene resin and a polypropylene resin are preferable, and a polypropylene resin is particularly preferable. Examples of the polypropylene resin include a homopolymer of propylene, a random copolymer or a block copolymer of propylene and a small amount of other α-olefin such as 10 mol% or less. Examples of the polypropylene resin include a homopolymer of propylene, a random copolymer or a block copolymer of propylene and a small amount of other α-olefin such as 10 mol% or less.

  Among the thermoplastic resins, it is more preferable to use a polypropylene resin, and two kinds of polypropylene resins, linear polypropylene resin (A) and modified polypropylene resin (B) having different melt flow rates and melt tensions, respectively. ) Are most preferably used as a mixture.

  The linear polypropylene resin (A) has a melt flow rate of preferably 10 g / 10 min to 100 g / 10 min, more preferably 15 g / 10 min to 50 g / 10 min, and preferably a melt tension. 2 cN or less, more preferably 1 cN or less. When the melt flow rate is less than 10 g / 10 min, when producing an injection foam molded article, the molding cavity has a thin portion with a clearance of about 1 to 2 mm, and it tends to be a short shot and is continuously stable. Molding tends to be difficult. When the melt flow rate exceeds 100 g / 10 min, bubbles tend to be destroyed at the time of foaming, and there is a tendency that a high foaming ratio cannot be obtained, and the rigidity of the molded body tends to be lowered. On the other hand, when the melt tension exceeds 2 cN, the transferability to the mold surface tends to be poor, and a foam molded article having a beautiful surface appearance may not be obtained.

  The melt flow rate is a value measured in accordance with ASTM D-1238 and measured under a load of 230 ° C. and 2.16 kg. When a strand lowered at a piston descending speed of 10 mm / min is drawn at 1 m / min from a die having a hole of φ1 mm and a length of 10 mm at 230 ° C., and the take-up speed is increased at 40 m / min after stabilization. This refers to the take-up load of a pulley with a load cell when it breaks.

The linear polypropylene resin (A) here is a polypropylene resin having a linear molecular structure, and is obtained from a usual polymerization method, for example, a transition metal compound supported on a carrier and an organometallic compound. Obtained in the presence of a catalyst system such as a Ziegler-Natta catalyst. Specific examples include propylene homopolymers, block copolymers, and random copolymers, which are crystalline polymers. As a copolymer of propylene, a copolymer containing propylene in an amount of 75% by weight or more is preferable in that the crystallinity, rigidity, chemical resistance, etc., which are characteristics of the polypropylene resin, are maintained. The copolymerizable α-olefin is ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3,4-dimethyl-1-butene. , 1-heptene, 3-methyl-1-hexene, 1-octene, 1-decene, and the like, α-olefin having 2 or 4 to 12 carbon atoms, cyclopentene, norbornene, tetracyclo [6,2,1 ^1,8 , 1 ^3,6 ] -4-dodecene and other cyclic olefins, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 1,4-hexadiene, methyl-1,4-hexadiene, 7-methyl-1,6 -Diene such as octadiene, vinyl chloride, vinylidene chloride, acrylonitrile, vinyl acetate, acrylic acid, methacrylic acid, maleic acid, ethyl acrylate, acrylic Butyl Le, methyl methacrylate, maleic anhydride, styrene, methyl styrene, vinyl toluene, and vinyl monomers such as divinylbenzene. Among these, ethylene and 1-butene are preferable in terms of improving cold brittleness resistance and low cost.

  The modified polypropylene resin (B) used in the present invention preferably has a melt flow rate of 0.1 g / 10 min or more and less than 10 g / 10 min, more preferably 0.3 g / 10 min or more and 5 g / 10 min or less. The melt tension is preferably 5 cN or more, more preferably 8 cN or more, and it is preferable to exhibit strain hardening. When the melt flow rate is less than 0.1 g / 10 min, the dispersibility in the linear polypropylene resin (A) is deteriorated, the expansion ratio and the bubbles are not uniform, and the surface property tends to be deteriorated. When the melt flow rate is 10 g / 10 min or more, the concentration of the modified polypropylene resin (B) on the surface of the molded body becomes too high, and a beautiful surface appearance may not be obtained. On the other hand, when the melt tension is less than 5 cN, there is a possibility that a foamed molded article of 2 times or more may not be obtained, and uniform fine bubbles may not be obtained.

  Strain hardening here is defined as that the viscosity increases as the stretch strain of the melt increases, and is usually the method described in Japanese Patent Application Laid-Open No. 62-121704, that is, the elongation measured by a commercially available rheometer. This can be determined by plotting the relationship between viscosity and time. Further, for example, strain hardening can be determined from the breaking behavior of the molten strand at the time of melt tension measurement. That is, when the take-up speed is increased, the melt tension increases abruptly, and when cutting, strain hardening is exhibited. When the modified polypropylene-based resin (B) does not exhibit strain-hardening properties, it tends to be difficult to obtain a foamed molded article having a high foaming ratio exceeding 2 times even if the melt tension is high.

  As such a modified polypropylene resin (B), for example, the linear polypropylene resin is irradiated with radiation, or the linear polypropylene resin, radical polymerization initiator, conjugated diene compound is melt mixed. Examples thereof include a modified polypropylene resin containing the resulting branched structure or high molecular weight component. Among these, in the present invention, a modified polypropylene resin obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator, and a conjugated diene compound is inexpensive because it does not require expensive radiation irradiation equipment. This is preferable because it can be manufactured. Examples of the raw material polypropylene resin used in the production of the modified polypropylene resin (B) include the same as the linear polypropylene resin (A).

  Examples of the conjugated diene compound include butadiene, isoprene, 1,3-heptadiene, 2,3-dimethylbutadiene, 2,5-dimethyl-2,4-hexadiene, and these may be used alone or in combination. Good. Among these, butadiene and isoprene are particularly preferable because they are inexpensive and easy to handle and the reaction easily proceeds uniformly.

  The addition amount of the conjugated diene compound is preferably 0.01 to 20 parts by weight and more preferably 0.05 to 5 parts by weight with respect to 100 parts by weight of the linear polypropylene resin. If the amount is less than 0.01 parts by weight, the effect of the modification may be difficult to obtain, and if the amount exceeds 20 parts by weight, the effect is saturated, which may not be economical.

  Monomers copolymerizable with the conjugated diene compounds, such as vinyl chloride, vinylidene chloride, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl acetate, acrylic acid, methacrylic acid, maleic acid, maleic anhydride, metal acrylate Salt, metal methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, and other acrylic esters, methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-methacrylic acid 2- Methacrylic acid esters such as ethylhexyl and stearyl methacrylate may be used in combination.

  The radical polymerization initiator generally includes peroxides, azo compounds, and the like, but those having a capability of extracting hydrogen from a polypropylene resin or the conjugated diene compound are preferable. Generally, ketone peroxides, peroxyketals, hydroperoxides are used. Organic peroxides such as oxides, dialkyl peroxides, diacyl peroxides, peroxydicarbonates, and peroxyesters are listed. Of these, those having particularly high hydrogen abstraction ability are preferred, such as 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, 1,1-bis (t-butylperoxy) cyclohexane, Peroxyketals such as n-butyl 4,4-bis (t-butylperoxy) valerate, 2,2-bis (t-butylperoxy) butane, dicumyl peroxide, 2,5-dimethyl-2,5 -Di (t-butylperoxy) hexane, α, α'-bis (t-butylperoxy-m-isopropyl) benzene, t-butylcumyl peroxide, di-t-butylperoxide, 2,5-dimethyl Diacyl peroxides such as dialkyl peroxides such as -2,5-di (t-butylperoxy) hexyne-3 and benzoyl peroxides Oxide, t-butyl peroxyoctate, t-butyl peroxyisobutyrate, t-butyl peroxylaurate, t-butyl peroxy 3,5,5-trimethylhexanoate, t-butyl peroxyisopropyl carbonate , 2,5-dimethyl-2,5-di (benzoylperoxy) hexane, t-butyl peroxyacetate, t-butyl peroxybenzoate, and peroxyesters such as di-t-butylperoxyisophthalate Or 2 or more types are mentioned.

  The addition amount of the radical polymerization initiator is preferably 0.01 parts by weight or more and 10 parts by weight or less, more preferably 0.05 parts by weight or more and 2 parts by weight or less with respect to 100 parts by weight of the linear polypropylene resin. If the amount is less than 0.01 part by weight, the effect of reforming may be difficult to obtain, and if the amount exceeds 10 parts by weight, the effect of reforming may be saturated and not economical.

  The apparatus for reacting the linear polypropylene resin, the conjugated diene compound, and the radical polymerization initiator includes a roll, a kneader, a Banbury mixer, a Brabender, a single screw extruder, a kneader such as a twin screw extruder, a twin screw, etc. Examples of the surface renewal machine include a horizontal stirrer such as a biaxial multi-disk device, and a vertical stirrer such as a double helical ribbon stirrer. Among these, a kneader is preferably used, and an extruder is particularly preferable from the viewpoint of productivity.

  There is no particular limitation on the order and method of mixing and kneading (stirring) the linear polypropylene resin, the conjugated diene compound, and the radical polymerization initiator. The linear polypropylene-based resin, the conjugated diene compound, and the radical polymerization initiator may be mixed and then melt-kneaded (stirred). After the polypropylene-based resin is melt-kneaded (stirred), the conjugated diene compound or the radical initiator may be mixed. They may be mixed simultaneously or separately, collectively or divided. The temperature of the kneading (stirring) machine is preferably 130 to 300 ° C. in that the linear polypropylene resin melts and does not thermally decompose. The time is generally preferably 1 to 60 minutes. In this way, the modified polypropylene resin (B) used in the present invention can be produced. There is no restriction | limiting in the shape and magnitude | size of polypropylene resin (A) and (B), and a pellet form may be sufficient.

  In the present invention, the mixing ratio of the linear polypropylene resin (A) and the modified polypropylene resin (B) is 100 parts by weight in total, and the linear polypropylene resin (A) is 50 parts by weight or more and 95 parts by weight or less. It is preferable that it is 60 parts by weight or more and 90 parts by weight or less. The modified polypropylene resin (B) is preferably 5 to 50 parts by weight, more preferably 10 to 40 parts by weight. If the amount of the linear polypropylene resin (A) is more than 95 parts by weight, that is, if the modified polypropylene resin (B) is less than 5 parts by weight, it may not be possible to obtain a foamed product of twice or more, and uniform fine bubbles. It may be difficult to become. When the amount of the linear polypropylene resin (A) is less than 50 parts by weight, that is, when the amount of the modified polypropylene resin (B) is more than 50 parts by weight, there is a tendency that a short shot is likely to occur in a molding having a thin part. In some cases, it is difficult to provide a foamed molded article at low cost.

  The mixing method of the linear polypropylene resin (A) and the modified polypropylene resin (B) is not particularly limited, and can be performed by a known method. For example, the pellet-shaped resin is dried using a blender, a mixer, or the like. Examples thereof include blending, melt mixing, melting in a solvent and mixing. In the present invention, the method of dry blending and then subjecting to injection foam molding is preferred because it requires less heat history and decreases the melt flow rate and melt tension.

  The thermoplastic resin foam molded article in the present invention is a molded article obtained by supplying the thermoplastic resin and the foaming agent to an injection molding machine and then injecting them into a mold to perform foam molding.

  The foaming agent that can be used in the present invention is not particularly limited as long as it can be usually used for injection foam molding, such as a chemical foaming agent and a physical foaming agent. The chemical foaming agent is mixed with the resin in advance and then supplied to the injection molding machine, and decomposes in the cylinder to generate a gas such as carbon dioxide. Examples of the chemical foaming agent include inorganic chemical foaming agents such as sodium bicarbonate and ammonium carbonate, and organic chemical foaming agents such as azodicarbonamide and N, N′-dinitrosopentatetramine. A physical foaming agent is injected into a molten resin in a cylinder of a molding machine as a gaseous or supercritical fluid, dispersed or dissolved, and functions as a foaming agent by being released from pressure after being injected into a mold. It is a thing. Physical foaming agents include aliphatic hydrocarbons such as propane and butane, alicyclic hydrocarbons such as cyclobutane and cyclopentane, halogenated hydrocarbons such as chlorodifluoromethane and dichloromethane, nitrogen, carbon dioxide, air, etc. Inorganic gas. You may use these individually or in mixture of 2 or more types.

  Among these foaming agents, an inorganic chemical foaming agent that can use a normal injection molding machine and easily obtain uniform fine bubbles is preferable. These inorganic chemical foaming agents include, for example, foaming aids such as organic acids such as citric acid and inorganic fine particles such as talc, in order to stably and finely form the foamed foam. A nucleating agent such as may be added. In general, the inorganic chemical foaming agent is preferably used as a masterbatch of a polyolefin resin having a concentration of 10 to 50% by weight from the viewpoints of handleability, storage stability, and dispersibility in polypropylene resin.

  The amount of the inorganic chemical foaming agent to be added varies depending on the type, concentration in the masterbatch, and desired foaming ratio. It is used in the range of 0.5 parts by weight or less, more preferably 0.5 parts by weight or more and 10 parts by weight or less. By using in this range, it is easy to obtain a foamed molded article having a foaming ratio of 2 times or more and uniform fine cells economically.

  Further, if necessary, as long as the effects of the present invention are not impaired, antioxidants, metal deactivators, phosphorus processing stabilizers, UV absorbers, UV stabilizers, fluorescent brighteners, metal soaps, antacid adsorption An additive such as a stabilizer such as an agent, a crosslinking agent, a chain transfer agent, a nucleating agent, a lubricant, a plasticizer, a filler, a reinforcing material, a pigment, a dye, a flame retardant, and an antistatic agent may be used in combination. These additives that are used as necessary are naturally used within a range not impairing the effects of the present invention, but are generally preferably 0.01 with respect to 100 parts by weight of the thermoplastic resin of the present invention. More than 10 parts by weight is used.

(3) Molding Method of Thermoplastic Resin Foam Molded Article Next, an injection foam molding method will be specifically described. Examples of the molding method of the molded body of the present invention include the following methods. That is, a mold having a standing wall portion, a heating cylinder, and a pressurized fluid injection device, each of which includes a stationary mold and a movable mold capable of moving forward and backward at an arbitrary position, are provided with a foamable thermoplastic resin. Using an injection molding machine, (a) foaming thermoplasticity in a cavity provided in a mold having a standing wall portion, which is composed of a fixed mold and a movable mold capable of moving forward and backward at an arbitrary position A step of injecting a resin (injection step), and (c) a step of injecting a pressurized fluid into the standing wall portion during or after the injection of the thermoplastic resin to hollow the standing wall portion (pressurized fluid injection step) ) And (d) After injecting the pressurized fluid into the standing wall portion, the molded body is formed by injection foam molding by a method including a step of retracting the movable mold and foaming the bottom surface portion (foaming step). (D) After the foaming step, (e) a step of cooling the foam (cooling step) is preferably performed. In some cases, foaming inside the standing wall portion is prevented, and the hollow portion of the standing wall portion is formed. In order to promote, between (a) the injection step and (c) foaming step, (b) a step of preventing the foaming inside the standing wall portion and promoting the formation of the hollow portion of the standing wall portion (preliminary cooling) Step) may be provided.
In addition, the mold configuration may be a mold having a standing wall portion constituted by a fixed mold and a movable mold capable of moving forward and backward at an arbitrary position. In view of the relationship, a fixed die / movable die is preferably a die having an inlay structure, and a die having a three-plate structure is preferred in terms of the cooling process.
(A) Injection process In this process, the thermoplastic resin material melted in the heating cylinder is injected and filled into the cavity of the mold. A known method can be applied to the injection process itself, and the molding conditions may be appropriately adjusted depending on the MFR of each thermoplastic resin, the type of foaming agent, the type of molding machine, or the shape of the mold. Usually, in the case of polypropylene resin, it is performed under conditions such as a resin temperature of 170 to 250 ° C., a mold temperature of 10 to 100 ° C., a molding cycle of 1 to 60 minutes, an injection speed of 10 to 300 mm / second, and an injection pressure of 10 to 200 MPa. Is preferred.
(B) Pre-cooling step After the foaming thermoplastic resin molding material is injected and filled in the cavity in the above step, before the surface layer in contact with the standing wall portion mold surface is completely solidified, (c) pressurized fluid injection When the step and (d) the foaming step are performed, 1) the pressurized fluid breaks through the molded body, or the pressurized fluid press-fitted from the standing wall portion mold surface enters the bottom portion, and the bottom portion 2) The standing wall portion is also slightly foamed, and the hollow portion may not be formed. Therefore, in order to prevent foaming inside the standing wall portion and promote formation of a hollow portion in the standing wall portion, (b) a pre-cooling step is provided between (a) the injection step and (c) the foaming step. be able to. Specifically, using a mold cooling mechanism, after injection filling is completed, the state of a cavity having a volume of 50 to 95% of the volume of the target molded body can be maintained for a certain period of time. . The holding time can be determined empirically depending on the type of thermoplastic resin molding material to be used, the size of the molded body, etc., but is usually about 1 to 5 seconds. If it is less than 1 second, the thickness of the surface layer is insufficient, and a molded article with excellent rigidity may not be obtained. In addition, a defoaming trace is formed on the standing wall due to the press-fitting of pressurized fluid. There is. If it exceeds 5 seconds, solidification of the inner layer of the standing wall portion is completed, and there is a possibility that the pressurized fluid cannot be press-fitted.

  As another pre-cooling method, there is a case where a unit for locally cooling the standing wall portion is provided in the mold, so that the solidification of the standing wall portion is accelerated. In addition, it is preferable to form a heat insulating layer made of a material having a lower thermal conductivity than metal on the surface of the bottom surface of the mold.

As a constituent material of the heat insulating layer, it is preferable to use a heat resistant polymer having a glass transition point of 200 ° C. or higher and a breaking elongation of 10% or higher. Specifically, a polyimide resin is preferable, and further, a heat resistant polymer is used. A linear thermosetting polyimide having excellent heat resistance and cold heat cycle resistance is preferred.
Further, by controlling the injection amount of the pressurized fluid and controlling the formation of the hollow portion of the standing wall portion, it is possible to suppress deterioration of physical properties and surface appearance of the standing wall portion.

(C) Pressurized fluid injection process After the injection molding process, in some cases after preliminary cooling, pressurized fluid is injected into the molten resin in the standing wall cavity from the pressurized fluid injection port provided in the mold standing wall section. Press fit, thereby forming a hollow in the standing wall.

Usually, the pressurized fluid inlet is provided on the fixed side. When the pressurized fluid injection port is provided on the fixed side, it is preferable to provide a mechanism for injecting the injection port into the mold cavity to inject into the mold cavity at the time of fluid injection and storing in the mold before the foaming step. . The position is preferably provided in the center of the stationary mold standing wall portion, and is preferably provided at the base of the standing wall portion.
In addition, when the pressurized fluid inlet is attached to the movable mold, the position is not particularly limited as long as the position is the leading edge of the movable wall, but the mold structure is preferably a three-plate structure, and the center of the leading wall It is preferable to open in the vicinity.
As the pressurized fluid used in the present invention, a fluid which is usually gaseous or liquid at normal temperature and normal pressure and hardly reacts or dissolves with the resin used under the temperature and pressure at the time of injection is used. For example, nitrogen, carbon dioxide, helium, water, glycerin, liquid paraffin and the like are preferably used. The injection pressure of the pressurized fluid to be press-fitted is not particularly limited as long as it is equal to or higher than normal pressure.

(D) Foaming step In the above step, the movable mold is retracted simultaneously with, during or after the pressurization of the pressurized fluid, the volume of the cavity is expanded to the volume of the molded body, and the molded body bottom portion in the molten state Foam the layer. That is, the foamable thermoplastic resin molding material in the molten state starts foaming due to the pressure drop accompanying the expansion of the volume, but the already solidified surface layer does not foam even if the pressure drop occurs, and the foaming agent is included. Even if it is, it maintains a dense non-foamed state. In the standing wall portion, the surface layer is solidified and in a non-foamed state, and a hollow portion is already formed in the inner layer.

(E) Cooling step In this step, after the foaming step, the molded body is taken out of the mold after cooling. The cooling time is not particularly limited as long as the molded body can be taken out. In this way, a non-foamed layer is formed on the surface layer, a foamed layer in which uniform fine bubbles are formed at a high foaming ratio on the bottom inner layer, and a hollow portion is formed on the standing wall portion. An excellent box-shaped foamed molded article is obtained.

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

In the examples and comparative examples, the test methods and criteria used in various evaluation methods are as follows.
(1) Foaming ratio of bottom surface part: A specimen including a non-foamed layer on the surface was cut out from the foamed molded product, and obtained from the ratio of specific gravity with a separately produced non-foamed molded product having a thickness of 3 mm (Reference Example 1). .
(2) Average cell diameter of bottom surface foamed layer, non-foamed layer thickness: It was determined from a micrograph of a cross section obtained by cutting the foamed molded product in the thickness direction. The average cell diameter was an average value of 20 arbitrarily selected. The non-foamed layer was an average value of the movable mold side and the fixed mold side.
(3) Molded body thickness of bottom surface portion: For a cross section cut in the thickness direction, an average value of three points of both end portions and the central portion was used.
(4) Melt flow rate: Measured according to ASTM 1238 at a temperature of 230 ° C. and a load of 2.16 kg.
(5) Melt tension: A capilograph (manufactured by Toyo Seiki Seisakusho) with an attachment for measuring melt tension was used. When a strand lowered at a piston descending speed of 10 mm / min was drawn at 1 m / min from a die having a hole of φ1 mm and a length of 10 mm at 230 ° C., it broke when the take-up speed was increased at 40 m / min ² after stabilization. The take-up load of the pulley with the load cell at that time was taken as melt tension.
(6) Strain hardenability: When measuring the above-mentioned melt tension, when the take-up speed is increased, the take-up load suddenly increases, and when it reaches rupture, “strain hardenability” is indicated; None ”.
(7) Injection foaming moldability: The number of short shots (number of defects) when 20 shots were continuously molded was determined and evaluated in the following three stages.
The number of defects is 0 ...
The number of defects is 1-2.
The number of defects is 3 or more.
(8) Internal void: A cross section of the foamed molded product cut in the thickness direction was observed, and the presence or absence of a void having a size of 1 mm or more in the foamed layer was examined.
No internal voids ... ○
Something ...
(9) Rigidity: A predetermined sample was cut out from the molded body and subjected to a bending test in accordance with JIS-K6911 to obtain the maximum bending load, which was taken as one of the indices of the rigidity of the molded body. The greater the maximum bending load value, the better the rigidity. In addition, the rigidity was evaluated in the following two stages from a comparison with the rigidity obtained from a specimen cut out from a separately produced non-foamed molded article having a thickness of 3 mm (Reference Example 1).
Same as or better than non-foamed molded body ...
Inferior to non-foamed molded body
(10) Hollow ratio of standing wall: From the definition of the hollow ratio, the hollow ratio of the standing wall was obtained.

Next, polypropylene resins and inorganic chemical foaming agents used in Examples and Comparative Examples are shown below.
(A) Linear polypropylene resin PP-1: J707 manufactured by Grand Polymer Co., Ltd. (propylene / ethylene / block copolymer, melt flow rate 23 g / 10 min, melt tension 1 cN or less)
PP-2: PM600A manufactured by Sun Allomer (homopolymer, melt flow rate 7.5 g / 10 min, melt tension 1 cN or less)
(B) Modified polypropylene resin MP-1: 100 parts by weight of a polypropylene homopolymer having a melt flow rate of 3 g / 10 min as a linear polypropylene resin, and 0.3 wt. Of t-butyl peroxyisopropyl carbonate as a radical polymerization initiator Part of the mixture is fed from a hopper of a 44 mmφ twin screw extruder (L / D = 38) at 50 kg / hour, and isoprene monomer is fed at a rate of 0.25 kg / hour from the introduction part provided in the middle using a metering pump. Modified polypropylene resin obtained by supplying, cooling the strands with water, and chopping (melt flow rate 0.5 g / 10 min, melt tension 12 cN, with strain hardening)
MP-2: As a polypropylene-based linear resin, a polypropylene homopolymer having a melt flow rate of 9 g / 10 min, the amount of radical polymerization initiator used is 0.4 parts by weight, and the supply rate of isoprene monomer is 1 kg / hour. Modified polypropylene resin obtained in the same manner as MP-1 (melt flow rate 4 g / 10 min, melt tension 9 cN, with strain hardening)
MP-3: PF814 manufactured by Sun Allomer (homopolymer, melt flow rate 3 g / 10 min, melt tension 10 cN, strain hardening)
MP-4: PF611 manufactured by Sun Allomer (homopolymer, melt flow rate 30 g / 10 min, melt tension 1 cN or less, with strain hardening)
MP-5: FH6000 manufactured by Chisso Corporation (homopolymer, melt flow rate 0.5 g / 10 min, melt tension 7 cN, no strain hardening)
(C) Inorganic chemical foaming agent CB-1: Polyslen EE275 manufactured by Eiwa Chemical Co., Ltd. (decomposition temperature 155 ° C., decomposition gas amount 40 ml / g, master batch of low density polyethylene)
CB-2: Polyslene PEM30S manufactured by Eiwa Kasei Co., Ltd. (decomposition temperature 155 ° C., decomposition gas amount 30 ml / g, low density polyethylene master batch)

(Examples 1-6)
The linear polypropylene resin (A), the modified polypropylene resin, and the inorganic chemical foaming agent were dry blended at a composition ratio shown in Table 1 to obtain a polypropylene resin composition for injection foam molding.
The resin composition pellets were injection-foamed in a box shape of 310 mm long × 190 mm wide × 95 mm deep using a MD350S-IIIDP type injection molding machine manufactured by Ube Industries, Ltd. The mold has a structure including a fixed mold and a movable mold capable of moving forward and backward at an arbitrary position. Moreover, while providing the protrusion part on the fixed type | mold standing wall part surface, the pressurized fluid injection port was provided in the center. The mold temperature was 50 ° C. for both the fixed mold and the movable mold. The bottom part mold clearance during injection is 2 mm, the standing wall part clearance is 3 mm, and after completion of injection filling, nitrogen gas at a pressure of 70 kg / cm ² is supplied from the pressurized fluid inlet to the mold cavity. The movable mold was retracted at the same time as it was press-fitted into the molten resin on the inner wall, and the final mold clearance was adjusted so that the bottom of the molded body had a predetermined thickness, and foamed. Other molding conditions were a cylinder temperature of 200 ° C., an injection pressure of 100 MPa, an injection speed of 160 mm / second, a movable mold retraction speed of 50 mm / second during foaming, and a cooling time of 90 seconds.

Table 2 shows the moldability at this time and the shape and physical properties of the obtained foamed molded article. Since the polypropylene resin composition of the present invention is excellent in fluidity, short shots during continuous molding are unlikely to occur, and injection foam moldability is good. The obtained foamed molded product has a thickness of the bottom of the molded product in the range of 4.2 to 8.4 mm and a foaming ratio of 2.1 to 4.1 times, and has a high foaming ratio and excellent lightness. Yes. The foam layer had an average cell diameter of 200 μm or less and a non-foamed layer (skin layer) of 200 to 400 μm, and no voids were observed inside the molded body. As a result, it can be seen that the thickness of the bottom surface of the molded body separately produced by injection molding is equal to or higher than that of the non-foamed molded body reference example 1 having a thickness of 3 mm. Moreover, the hollow rate of a standing wall part is 30% or more, and it turns out that 43% of weight reduction compared with the non-hollow molded object is achieved.

(Reference Example 1)
In the examples, the non-foamed molded article was taken out by cooling for 90 seconds after completion of injection filling without using the modified polypropylene resin and the inorganic chemical foaming agent. The results are shown in Table 2.
(Comparative Example 1)
The same operation as in Example 3 was performed except that the modified polypropylene resin was not used. The results are shown in Table 2. When the expansion ratio was doubled, voids were generated in the molded foam layer, and the rigidity was lowered. Further, a sufficient hollow portion was not obtained for the standing wall portion.
(Comparative Example 2)
The same operation as in Example 3 was performed except that PP-2 was used as the linear polypropylene resin. The results are shown in Table 2. During 20 shots in continuous molding, short shots occurred in 4 shots (20% defective rate), and it was found that the injection foam moldability was poor.
(Comparative Example 3)
The same operation as in Example 3 was performed except that MP-4 (melt tension was outside the range of the present invention) was used as the modified polypropylene resin. The results are shown in Table 2. When the expansion ratio was doubled, voids were generated in the molded foam layer, and the rigidity was lowered.
(Comparative Example 4)
The same operation as in Example 3 was performed except that the modified polypropylene-based resin used MP-5 (not having the strain hardening property of the present invention). The results are shown in Table 2. When the expansion ratio was doubled, voids were generated in the molded foam layer, and the rigidity was lowered.
(Comparative Example 5)
It implemented like Example 3 except not having used linear polypropylene resin. The results are shown in Table 2. In 20 continuous shots, short shots occurred in 5 shots (defective rate 25%), and it was found that the injection foam moldability was poor. Further, the surface of the obtained foamed molded article was uneven, and a sufficient hollow portion was not formed in the standing wall portion.

  The thermoplastic resin composition for injection foam molding of the present invention has good injection foam moldability, and the resulting box-shaped foam molded article is excellent in light weight and rigidity. Can be widely used for home appliances and building materials.

It is sectional drawing seen from the side surface of the metal mold | die used in the present Example. In FIG. 1, the mold is closed and molded (unfoamed state). In FIG. 2, the pressurized fluid is pressed into the standing wall. In FIG. 3, the mold is retracted and the bottom surface is foamed. It is the state which closed and formed. It is a surface perspective view of the foaming molding obtained in the present Example. It is sectional drawing cut | disconnected by AA and BB of FIG. It is a perspective view of the sample for a rigidity test cut | disconnected by AA of FIG. It is sectional drawing of an example of the metal mold | die implemented by this case.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Movable mold 2 Fixed mold 3 Gate 4 Molding machine nozzle 5 Cavity 6 Pressurized fluid injection port 7 Unfoamed molding 8 Hollow part (standing wall part)
9 Foam layer (bottom surface)
10 Standing wall 11 Bottom

Claims (7)

  A thermoplastic resin foam-molded article comprising at least a standing wall part and a bottom part, wherein the standing wall part has a hollow part and the bottom part has a foam part.   The thermoplastic resin is (A) 50 to 95 parts by weight of a linear polypropylene resin having a melt flow rate of 10 g / 10 min to 100 g / 10 min and a melt tension of 2 cN or less, and (B) melt 5 to 50 parts by weight of a modified polypropylene resin having a flow rate of 0.1 g / 10 min or more and less than 10 g / 10 min, a melt tension of 5 cN or more and exhibiting strain hardening (however, a linear polypropylene resin) 2. The thermoplastic resin foam molded article according to claim 1, wherein the total of (A) and the modified polypropylene resin (B) is 100 parts by weight.   The modified polypropylene resin (B) is a modified polypropylene resin obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator, and a conjugated diene compound. 2. The thermoplastic resin foam molded article according to 2.   The foamed bottom surface portion has a foam layer having an average cell diameter of 500 μm or less, and a non-foam layer having a thickness of 10 μm or more and 1000 μm or less formed on at least one surface of the foam layer, and a foaming ratio of 2 to 10 The thermoplastic resin foam-molded article according to any one of claims 1 to 3, wherein the thermoplastic resin foam-molded article is not more than double.   The thermoplastic resin foam-molded article according to any one of claims 1 to 4, wherein a hollow ratio of the hollow portion of the standing wall portion is 10% or more and 50% or less. (A) a step of injecting a foamable thermoplastic resin into a cavity provided in a mold having a standing wall portion, which is composed of a fixed mold and a movable mold capable of moving forward and backward at an arbitrary position;
(C) A step of injecting a pressurized fluid into the standing wall during or after the injection of the thermoplastic resin to hollow the standing wall.
(D) after injecting the pressurized fluid into the standing wall, the step of causing the movable mold to retreat and foaming the bottom surface;
The method for producing a thermoplastic resin foam-molded article according to any one of claims 1 to 7, wherein:   The method for producing a thermoplastic resin foam molded article according to claim 6, wherein (b) a preliminary cooling step is provided between the (a) injection step and (c) pressurized fluid injection step.

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