JP2006240051A - Foamed polypropylene resin molding and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for easily and inexpensively producing a box-shaped foamed molding excellent in lightweight properties, rigidity, and surface smoothness which is capable of thin-wall injection of a high expansion ratio. <P>SOLUTION: The method for producing the injection-foamed article includes the first foaming process in which immediately after a polypropylene resin comprising 50-95 pts.wt. of a linear polypropylene resin (A) with an MFR of 10-100 g/10 min and an MT of 2 cN or below and 5-50 pts.wt. of a modified polypropylene resin (B) having an MFR of at least 0.1 g/10 min and below 10 g/10 min and an MT of at least 5 cN and indicating strain hardening properties is packed in a mold cavity having a clearance t<SB>0</SB>smaller than a mold cavity clearance t<SB>2</SB>corresponding to a final product wall-thickness, a movable mold 1 is moved back to a clearance t<SB>1</SB>smaller than t<SB>2</SB>and larger than t<SB>0</SB>and the second foaming process in which after the clearance t<SB>1</SB>is kept for a prescribed time, the movable mold 1 is moved back to the mold cavity clearance t<SB>2</SB>at a final product shape position. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an injection foam molded body made of polypropylene resin and a method for producing the same.

  In the injection molding of polypropylene-based resins, so-called injection foam molding has been conventionally performed in which foaming is performed in order to reduce weight, reduce costs, and prevent warping and sink marks of the molded body (for example, Patent Document 1). However, the polypropylene resin is crystalline and has a low melt tension (melting tension), and bubbles are easily destroyed. As a result, appearance defects called silver streaks (or swirl marks) are likely to occur on the surface of the molded body, and voids are easily generated inside, making it difficult to increase the expansion ratio. In addition, since the bubbles were non-uniform and large, the resulting molded article was not sufficiently rigid. In addition, the void as used in the present case is a coarse bubble generated by internal bubbles communicating with each other and substantially means a bubble having a diameter exceeding 1.5 mm.

  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 2 and Patent Document 3). 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.

  By introducing long-chain branching by radiation irradiation, the melt tension is higher than that of ordinary linear polypropylene resin, and the viscosity increases as the stretch distortion of the melt increases. The resin is commercially available from Sun Allomer as HMS-PP (High Melt Strength Polypropylene) (Patent Document 4). 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 5). Usually, in order to achieve significant weight reduction while maintaining rigidity, the mold cavity clearance thickness (thickness before foaming) at the time of injection filling is significantly reduced compared to the non-foamed injection molded body before weight reduction. It is necessary to make it highly foamed. However, since the HMS-PP used here has a melt flow rate of only about 4 g / 10 min and has low fluidity at the time of melting, it is greatly thinned, for example, a molding having a thin portion of about 1 to 2 mm. However, there was a problem that a short shot was likely to occur. 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.

  Also, a polypropylene resin (Patent Document 6) having a high melt flow rate and a high melt tension mixed with polyethylene having a specific intrinsic viscosity, and a high melt tension containing a component having a specific intrinsic viscosity by multistage polymerization. A method has also been proposed in which a mixture of a polypropylene resin and a high melt flow rate polypropylene resin (Patent Document 7) is used for injection foam molding. 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, there is known a method for obtaining an injection foam molded article having a high expansion ratio of 2 times or more by a method including a foaming step of opening a mold in two stages (Patent Document 8, Patent Document 9, and Patent Document 10). In particular, the method described in Patent Document 8 is a method of performing a primary mold expansion step after a predetermined time after completion of filling, and then stopping the expansion of the cavity for a predetermined time, and then expanding the cavity to the final expansion width. It is preferable because a high expansion ratio is easily obtained. However, the base resin used here is HMS-PP having the above-mentioned long-chain branch, and since it has poor fluidity, it is difficult to injection-fill into a thin cavity of less than 3 mm, and a significant weight reduction cannot be expected. In addition, in order to maintain the shape at the time of filling for a predetermined time after injection filling and perform the cavity expansion speed in the primary mold expansion process at a low speed such as 2 to 5 mm / second, Since the resin temperature in the mold is equal to or lower than the temperature suitable for foaming, it is difficult to achieve high foaming, and the resulting molded article is also inferior in surface smoothness. In particular, when the foam-molded product is a box-shaped one, there is a problem that voids are likely to occur at the bottom surface.

As described above, until now, it has been difficult to obtain an injection foam molded article excellent in surface smoothness and rigidity, which has good injection foam moldability and can be significantly reduced in weight at a high expansion ratio. It was.
JP-A-6-198668 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 2003-268145 A JP 2001-341154 A JP-A-7-88878 JP-A-4-73114

  An object of the present invention is to provide a production method that can easily and inexpensively obtain a box-shaped foamed molded article that is excellent in lightness, rigidity, and surface smoothness because of its thin foam injection filling and high foaming ratio. Is to provide.

  By using a specific polypropylene resin as a base resin and opening a mold cavity at a specific mold opening speed in two stages, the present inventors can achieve a significant weight reduction with a high expansion ratio. In addition, the present inventors have found that a box-shaped foamed molded article having excellent rigidity and surface smoothness and particularly difficult to produce has been obtained at low cost, and the present invention has been completed.

That is, a first aspect of the present invention is a method for producing a foamed molded article by injecting a melt of a polypropylene resin composition comprising a polypropylene resin and a foaming agent into a mold, wherein the polypropylene resin (A ) 40 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) a melt flow rate of 0.1 g / 10. Min. To less than 10 g / 10 min, melt tension of 5 cN or more, and 5 to 60 parts by weight of a modified polypropylene resin exhibiting strain hardening (however, linear polypropylene resin (A) and modified polypropylene resin) Resin (B) total is 100 parts by weight), and the mold is composed of a fixed mold and a movable mold that can move forward and backward, An injection step of injecting and filling the molten mixture into a mold cavity having a mold cavity clearance t ₀ smaller than the mold cavity clearance t ₂ corresponding to the shape position of the product, and immediately after that, smaller than t ₂ ; The first stage foaming step of retracting the movable mold to the mold cavity clearance t ₁ larger than t ₀ , and then the mold cavity corresponding to the shape position of the final product after holding the clearance of t ₁ for a predetermined set time. - a process for the production of injection-foamed molded, characterized in that it comprises a second stage foaming step of retracting the movable mold up to the clearance t _2.

As a preferred embodiment,
(1) The modified polypropylene resin (B) is obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator, and a conjugated diene compound,
(2) The mold opening speed in the first stage foaming step is 7 mm / second or more and 100 mm / second or less,
(3) The mold opening speed in the second stage foaming step is 0.5 mm / second or more and 20 mm / second or less,
(4) The relationship between t ₀ , t ₁ and t ₂ is expressed by the following equation:

ここで、ｔ₀は０．５ｍｍ以上３ｍｍ未満である。
（５）前記第二段発泡工程における金型キャビティ・クリアランスｔ₁を保持する設定時間が１秒以上２０秒以下であること、
を特徴とする前記記載の射出発泡成形体の製造方法に関する。

Here, t ₀ is 0.5 mm or more and less than 3 mm.
(5) The set time for maintaining the mold cavity clearance t ₁ in the second stage foaming step is 1 second or more and 20 seconds or less,
A method for producing the injection foam molded article as described above.

The second aspect of the present invention relates to a foamed molded article produced by the above-described method.
(1) The foaming ratio is 2 times or more and 10 times or less, having 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. That
(2) The weight reduction rate (L) represented by the following formula is 20% or more,

ここで、Ｗ_Sは同じ剛性を有する非発泡射出成形体の重量、Ｗ_Eは前記発泡成形体の重量である。
（３）前記発泡成形体が底面部と、該底面部と一体的に成形された立壁部とからなる箱形状であること、
を特徴とする前記記載の発泡成形体に関する。

Here, W _S is the weight of the non-foamed injection molded body having the same rigidity, and W _E is the weight of the foam molded body.
(3) The foamed molded body has a box shape including a bottom surface portion and a standing wall portion formed integrally with the bottom surface portion,
This relates to the above-mentioned foamed molded article.

  The method for producing an injection-foamed molded article of the present invention is capable of thin-wall injection filling necessary for significant weight reduction by using a polypropylene-based resin that has high fluidity at the time of melting and high melt tension. Immediately after the polypropylene resin in the molten state is injected and filled into the mold cavity, the movable mold is retracted and foamed at a specific speed in two stages, so that it has high foaming ratio, but it is lightweight, rigid, A foamed molded article having excellent surface smoothness can be obtained. This effect is particularly prominent in a box-shaped foam molded article.

  The first feature of the method for producing a polypropylene resin injection foam molded article of the present invention is that two types of polypropylene resins (A) and (B) having different melt flow rates and melt tensions are used.

  The polypropylene resin used in the present invention comprises a linear polypropylene resin (A) and a modified polypropylene resin (B).

  The linear polypropylene resin (A) has a melt flow rate of 10 g / 10 min to 100 g / 10 min, preferably 30 g / 10 min to 60 g / 10 min, and a melt tension of 2 cN or less, preferably 1 cN or less. It is. When the melt flow rate is in the range of 10 g / 10 min or more and 100 g / 10 min or less, when producing an injection-foamed molded article, in the molding having a thin portion with a mold cavity clearance of about 0.5 mm or more and less than 3 mm. Is less prone to short shots and enables continuous and stable molding. In addition, since the foaming ratio is high and bubbles are not easily destroyed during foaming, a foamed molded article having a beautiful surface appearance can be obtained. Moreover, if the melt tension is 2 cN or less, the transferability to the mold surface is good, and a foamed molded article having a beautiful surface appearance can be obtained.

The melt flow rate is a value measured under a load of 2.16 kg at 230 ° C. in accordance with ASTM D-1238. Used, a strand lowered at a piston descending speed of 10 mm / min was taken out 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 was increased at 40 m / min ² after stabilization. Sometimes, it refers to the take-up load of the 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 has a melt flow rate of 0.1 g / 10 min or more and less than 10 g / 10 min, preferably 0.3 g / 10 min or more and 5 g / 10 min or less. The tension is 5 cN or more, preferably 8 cN or more, and exhibits strain hardening. When the melt flow rate is 0.1 g / 10 min or more and less than 10 g / 10 min, the dispersibility in the linear polypropylene resin (A) is good, the foaming ratio is high, the bubbles are uniform, and the surface property is high. A good foamed molded product of the present invention is obtained. In addition, transferability to the mold surface is good, and a beautiful surface appearance can be obtained. In addition, when the melt tension is 5 cN or more, a foamed molded product having uniform and fine cells twice or more can 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. The modified polypropylene resin (B) exhibits strain-hardening properties, and when the melt tension is high, a foamed molded product with a high foaming ratio of 2 times or more can be obtained and broken at the molten resin flow front end during injection molding. A foam molded article having excellent surface smoothness can be obtained for the reason that silver streaks that occur due to easy foaming are less likely to occur.

  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 can be manufactured at low cost because it does not require expensive equipment. It is preferable from the point. Examples of the raw material polypropylene resin used for 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 parts by weight or more and 20 parts by weight or less, and more preferably 0.05 parts by weight or more and 5 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 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) -3-hexyne and benzoyl peroxides Oxide, t-butyl peroxyoctate, t-butyl peroxyisobutyrate, t-butyl peroxylaurate, t-butyl peroxy 3,5,5-trimethylhexanoate, t-butyl peroxyisopropyl carbonate 1, peroxyesters such as 2,5-dimethyl-2,5-di (benzoylperoxy) hexane, t-butylperoxyacetate, t-butylperoxybenzoate, di-t-butylperoxyisophthalate A seed | species or 2 or more types is 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.

  Of the total 100 parts by weight of the linear polypropylene resin (A) and the modified polypropylene resin (B) used in the present invention, the linear polypropylene resin (A) is 40 to 95 parts by weight, preferably Is 50 parts by weight or more and 80 parts by weight or less. The modified polypropylene resin (B) is 5 to 60 parts by weight, preferably 10 to 50 parts by weight. If it is the said compounding quantity, the foaming molding with a foaming ratio of 2 times or more which has a uniform fine bubble will be obtained. In addition, short shots do not occur in molding having a thin portion, continuous production can be performed stably, and a foam molded article having excellent surface smoothness can be provided at low cost.

  The polypropylene resin for injection foam molding used in the present invention can be obtained by mixing a linear polypropylene resin (A) and a modified polypropylene resin (B). The mixing method is not particularly limited and can be performed by a known method. For example, dry blending of pellet-shaped resin using a blender, mixer, etc., melt mixing, melting and mixing in a solvent, etc. Can be mentioned. In the present invention, the method of dry blending and then subjecting to injection foam molding is preferable because it requires less heat history and decreases the melt tension.

  The second feature of the method for producing a foamed molded article in the present invention is that immediately after injection-filling a melt of a polypropylene resin composition comprising the polypropylene resin for injection foam molding and a foaming agent into a mold cavity, In two stages, the movable mold is retracted and foamed at a specific speed.

  The polypropylene resin for injection foam molding is supplied to an injection molding machine in a state containing a foaming agent. 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, normal injection molding machines can be used safely, and uniform fine bubbles are easily obtained. Chemical foaming agents are inorganic chemical foaming agents, physical foaming agents are nitrogen and carbon dioxide. Inorganic gas such as air is preferable. These foaming agents include, for example, foaming aids such as organic acids such as citric acid and inorganic fine particles such as talc and lithium carbonate, in order to stably and uniformly make the foamed foam air bubbles. A nucleating agent such as may be added. Usually, 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 a polypropylene resin.

  What is necessary is just to set the usage-amount of the said foaming agent suitably with the foaming magnification of the final product, the kind of foaming agent, and the resin temperature at the time of shaping | molding. For example, usually in the case of an inorganic chemical foaming agent, it is preferably 0.5 to 20 parts by weight, more preferably 1 to 10 parts by weight with respect to 100 parts by weight of the polypropylene resin of the present invention. Used in range. By using in this range, it is easy to economically obtain a foamed molded article having a foaming ratio of 2 times or more and uniform fine cells.

  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. Of course, these additives used as necessary are used within the range not impairing the effects of the present invention, but generally 0 parts by weight with respect to 100 parts by weight of the polypropylene resin composition of the present invention. 0.01 parts by weight or more and 10 parts by weight or less is used.

In the method for producing an injection-foamed molded article of the present invention described below, when the mold is box-shaped, each cavity clearance t ₀ , t ₁ , t ₂ represents the cavity clearance of the bottom surface portion.

A resin composition for injection foam molding comprising a foaming agent and the polypropylene resin for injection foam molding is movable so that the mold can move forward and backward with a fixed mold after being melt-kneaded in an injection molding machine cylinder. A mold cavity having a mold cavity clearance t ₀ smaller than the mold cavity clearance t ₂ corresponding to the shape position of the final product is injection-filled (injection process). Here, t ₀ is preferably 0.5 mm or more and less than 3 mm. When t ₀ is less than 0.5 mm, it may be difficult to stably inject and fill even if the base resin of the present invention is used, and it is also difficult to achieve high foaming of 2 times or more. Further, when t ₀ is 3 mm or more, there is a tendency that the effect of weight reduction is difficult to obtain.

Immediately after the injection process, the movable mold is retracted to a mold cavity clearance t ₁ smaller than t _{2 and} greater than t ₀ (first stage foaming process). Here, “immediately after the injection filling process” means that the movable mold is retracted, that is, the mold cavity is opened substantially simultaneously with the completion of filling. The set holding time from the completion of filling to the opening of the mold cavity is preferably less than 0.5 seconds, more preferably less than 0.3 seconds. If the time to open the mold cavity after filling is 0.5 seconds or more, the resin temperature in the mold cavity tends to fall below the foaming suitable temperature when injection filling into the thin cavity as in the present invention. Yes, it tends to be difficult to obtain a high expansion ratio of 2 times or more in a part or the whole of the molded body. Further, it is preferable that the mold cavity clearance t ₁ after the movable mold retreats in the first-stage foaming step has a relationship of the following formula with t ₀ and t ₂ .

ｔ₁が、この範囲内にある場合は、内部ボイドが少なく、表面平滑性の優れた高発泡成形体が得られやすい。

When t ₁ is in this range, there are few internal voids, and a highly foamed molded article having excellent surface smoothness can be easily obtained.

  The mold opening speed in the first stage foaming step is preferably 7 mm / second or more and 100 mm / second or less, and more preferably 10 mm / second or more and 70 mm / second or less. When the mold opening speed is less than 7 mm / second, the bubbles are not uniform, and voids tend to be generated inside the molded body, so that the rigidity tends to decrease. When the mold opening speed exceeds 100 mm / second, the surface smoothness tends to be inferior because the base resin foam is difficult to follow the mold opening speed.

After the first foaming step, the movable mold is stopped, and the mold is smaller than the mold cavity clearance t ₂ corresponding to the shape position of the final product and larger than the mold cavity clearance t ₀ at the time of injection filling. After holding the cavity clearance t ₁ for a predetermined set time, the movable mold is retracted to the mold cavity clearance t ₂ corresponding to the shape position of the final product (second stage foaming step).

Here, the set time for maintaining the mold cavity clearance t ₁ is preferably 1 second to 20 seconds, and more preferably 3 seconds to 10 seconds. When the set time for maintaining t ₁ is less than 1 second, voids in the molded body are likely to be generated, and when it exceeds 20 seconds, it is difficult to achieve high foaming and the surface smoothness tends to deteriorate.

  Furthermore, the mold opening speed in the second stage foaming step is preferably 0.5 mm / second or more and 20 mm / second or less, more preferably 1 mm / second or more and 10 mm / second or less. When the mold opening speed is less than 1 mm / second, foaming may be poor and high foaming may be difficult. On the other hand, if it exceeds 20 mm / sec, foaming of the base resin may be difficult to follow the mold opening speed, and the surface smoothness may be inferior.

  Other molding conditions may be appropriately adjusted according to the MFR of each polypropylene resin, the type of foaming agent, the type of molding machine, or the shape of the mold. Usually, the resin temperature is 170 to 250 ° C., the mold temperature is 10 to 100 ° C., the molding cycle is 1 to 60 minutes, the injection speed is 10 to 300 mm / second, the injection pressure is 10 to 100 MPa, and the like.

  The foamed molded article of the present invention thus obtained has an average cell diameter of preferably 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, preferably 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 surface is not beautiful and the rigidity tends to decrease. If it exceeds 1000 μm, the lightness may not be obtained.

  The expansion ratio of the foamed molded article of the present invention is preferably 2 times or more and 10 times or less, more preferably 2.5 times or more and 6 times or less. If the expansion ratio is less than 2 times, it is 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 the specific gravity of the foamed molded product and the non-foamed molded product that was injection molded under the same conditions except that the foaming agent was not added to the polypropylene resin composition for injection foam molding.

  Furthermore, the weight reduction rate L shown by the following formula of the foamed molded product of the present invention is preferably 20% or more, and more preferably 25% or more.

ここで、Ｗ_Sは同じ剛性を有する非発泡射出成形体の重量、Ｗ_Eは前記発泡成形体の重量である。軽量化率が２０％未満の場合には本発明の特徴である大幅な軽量化が得られない。

Here, W _S is the weight of the non-foamed injection molded body having the same rigidity, and W _E is the weight of the foam molded body. When the weight reduction rate is less than 20%, the significant weight reduction characteristic of the present invention cannot be obtained.

  The method for producing an injection-foamed molded article in the present invention can produce various foam-molded articles, and in particular, creates a box-shaped foam-molded article comprising a bottom wall portion and an upright wall portion integrally molded. It is effective when you do. Usually, when a box-shaped highly foamed molded article is manufactured with an enlarged mold cavity clearance, voids are likely to occur on the bottom surface, which often causes a reduction in rigidity. According to the production method of the present invention, even a box-shaped foamed molded article can be highly foamed more than twice.

  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) Melt flow rate: Measured in accordance with ASTM 1238 at a temperature of 230 ° C. and a load of 2.16 kg.
(2) 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.
(3) 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 breakage, it indicates “strain hardenability”; It does not show sex ".
(4) Injection foam 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.
(5) Surface smoothness: The degree of surface irregularities of the foamed molded product was evaluated in the following three stages.

No surface irregularities ... ○
Some irregularities on the surface ... △
Many surface irregularities ...
(6) Foaming ratio: A specimen including a non-foamed layer on the surface was cut out from the bottom part of the foam molded article, and the ratio of specific gravity to the bottom part of a separately produced non-foamed molded article having a thickness of 3 mm (Reference Example 1). I asked for it.
(7) Average cell diameter, non-foamed layer thickness: It was determined from a micrograph of a cross section obtained by cutting the bottom surface of 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.
(8) Internal Void: A cross section obtained by cutting the bottom surface of the foamed molded product 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: Flexural modulus measured on a specimen cut to a width of 10 mm from the bottom surface of the foamed molded product so that the longitudinal direction of the specimen is perpendicular to the injection resin flow direction in accordance with JIS-K6911 ( The bending stiffness (G) was determined from the following equation using E) and the secondary moment of inertia (I).

ここで断面二次モーメント（Ｉ）は、試片の巾（ｂ）および厚み（ｈ）から次式で表される。

Here, the cross-sectional secondary moment (I) is expressed by the following equation from the width (b) and thickness (h) of the specimen.

（１０）軽量化率：軽量化率を求める発泡成形体の重量をＷ_E、後述の参考例にしたがって作製した、これと同じ剛性Ｇを有する非発泡射出成形体の重量をＷ_Sを予め図１から求めておき、式３から軽量化率Ｌを計算した。

(10) Weight reduction rate: W _{E represents} the weight of the foam molded body for which the weight reduction rate is obtained, and W _{S represents} the weight of the non-foamed injection molded body having the same rigidity G produced according to the reference example described later. The weight reduction ratio L was calculated from Equation 3 and calculated from Equation 3.

次に、実施例、比較例で使用したポリプロピレン系樹脂、発泡剤を以下に示す。
（Ａ）線状ポリプロピレン系樹脂
ＰＰ−１：プロピレン・エチレン・ブロックコポリマー、メルトフローレート４５ｇ／１０分、メルトテンション１ｃＮ以下
ＰＰ−２：プロピレン・エチレン・ブロックコポリマー、メルトフローレート３０ｇ／１０分、メルトテンション１ｃＮ以下
ＰＰ−３：プロピレン・エチレン・ブロックコポリマー、メルトフローレート５ｇ／１０分、メルトテンション１ｃＮ以下
（Ｂ）改質ポリプロピレン系樹脂
ＭＰ−１：線状ポリプロピレン系樹脂としてメルトフローレート４ｇ／１０分のポリプロピレン・ホモポリマー１００重量部と、ラジカル重合開始剤としてｔ−ブチルパーオキシイソプロピルカーボネート０．４重量部の混合物を、ホッパーから５０ｋｇ／時で４６ｍｍφ二軸押出機（Ｌ／Ｄ＝４０）に供給してシリンダ温度２００℃で溶融混練し、途中に設けた圧入部よりイソプレンモノマーを定量ポンプを用いて０．３ｋｇ／時の速度で供給し、ストランドを水冷、細断することにより得た改質ポリプロピレン系樹脂（メルトフローレート０．５ｇ／１０分、メルトテンション１６ｃＮ、歪硬化性を示す）
ＭＰ−２：線状ポリプロピレン系樹脂としてメルトフローレート１５ｇ／１０分のポリプロピレン・ホモポリマー、ラジカル重合開始剤の混合量として０．７重量部、イソプレンモノマーの供給量を１ｋｇ／時とした以外はＭＰ−１と同様にして得た改質ポリプロピレン系樹脂（メルトフローレート３ｇ／１０分、メルトテンション１４ｃＮ、歪硬化性を示す）
ＭＰ−３：チッソ社製ＦＨ６０００（ホモポリマー、メルトフローレート０．５ｇ／１０分、メルトテンション７ｃＮ、歪硬化性を示さない）
（Ｃ）発泡剤
ＢＡ−１：化学発泡剤マスターバッチ（永和化成社製ポリスレンＥＥ２７５、分解ガス量４０ｍｌ／ｇ）
ＢＡ−２：炭酸ガス（純度９９％以上）
（実施例１〜７）
線状ポリプロピレン系樹脂（Ａ）、改質ポリプロピレン系樹脂（Ｂ）、発泡剤（Ｃ）を表１に示す組成比でドライブレンドし、射出発泡成形用ポリプロピレン系樹脂組成物を得た。

Next, polypropylene resins and foaming agents used in Examples and Comparative Examples are shown below.
(A) Linear polypropylene-based resin PP-1: propylene / ethylene block copolymer, melt flow rate 45 g / 10 min, melt tension 1 cN or less PP-2: propylene / ethylene block copolymer, melt flow rate 30 g / 10 min, Melt tension 1 cN or less PP-3: Propylene / ethylene block copolymer, melt flow rate 5 g / 10 min, melt tension 1 cN or less (B) Modified polypropylene resin MP-1: Melt flow rate 4 g / g as linear polypropylene resin A mixture of 100 parts by weight of a polypropylene homopolymer of 10 minutes and 0.4 parts by weight of t-butylperoxyisopropyl carbonate as a radical polymerization initiator was transferred from a hopper at a rate of 50 kg / hr to a 46 mmφ twin screw extruder (L / D = 40 ) Supplied, melt-kneaded at a cylinder temperature of 200 ° C., isoprene monomer was supplied at a rate of 0.3 kg / hr from a press-fitting part provided in the middle using a metering pump, and the strand was cooled with water and shredded. Polypropylene resin (melt flow rate 0.5 g / 10 min, melt tension 16 cN, showing strain hardening)
MP-2: Polypropylene homopolymer having a melt flow rate of 15 g / 10 min as a linear polypropylene resin, 0.7 parts by weight as the mixing amount of the radical polymerization initiator, and the supply amount of isoprene monomer being 1 kg / hour Modified polypropylene resin obtained in the same manner as MP-1 (melt flow rate 3 g / 10 min, melt tension 14 cN, showing strain hardening)
MP-3: FH6000 manufactured by Chisso Corporation (homopolymer, melt flow rate 0.5 g / 10 min, melt tension 7 cN, no strain hardening)
(C) Foaming agent BA-1: Chemical foaming agent master batch (Polyslen EE275 manufactured by Eiwa Kasei Co., Ltd., decomposition gas amount 40 ml / g)
BA-2: Carbon dioxide (purity 99% or more)
(Examples 1-7)
The linear polypropylene resin (A), the modified polypropylene resin (B), and the foaming agent (C) were dry blended at the composition ratio shown in Table 1 to obtain a polypropylene resin composition for injection foam molding.

After melt-kneading the resin composition containing the foaming agent at a resin temperature of 200 ° C. and a back pressure of 5 MPa using an injection molding machine of “MD350S-IIIDP type” (shutoff nozzle specification) manufactured by Ube Industries, Ltd., 40 ° C. A box-shaped cavity having a pin gate of φ2 mm and configured to be a fixed type and a movable type that can be moved forward and backward (vertical wall portion: vertical wall portion: 10 ° inclination) In a mold having a clearance of 3 mm and a bottom surface portion: clearance t ₀ ), injection filling was performed at an injection speed of 100 mm / second (injection process). Immediately after the completion of injection filling (set holding time 0 second), the movable mold was retracted to the mold cavity clearance t ₁ at the bottom to foam the resin in the cavity (first stage foaming step).

Next, after holding for a predetermined set time in the state of the clearance t _{1 on} the bottom surface of the mold, the movable mold is moved backward again to the mold cavity clearance t ₂ on the bottom surface corresponding to the shape position of the final product. Foaming was performed (second stage foaming step). After completion of foaming, the foamed molded product was taken out after cooling for 60 seconds.

  Table 2 shows the clearance at the bottom of each mold, the retreat speed of the movable mold, and the set holding time in each process. Further, Table 3 shows injection foam moldability, rigidity and weight reduction ratio of the obtained foamed molded article.

  Since the polypropylene resin of the present invention is excellent in fluidity, even when the mold cavity clearance at the time of injection filling is 2 mm or less, short shots at the time of continuous molding hardly occur and the injection foam moldability is good. Further, the box-shaped foamed foam molded article obtained by such a molding method has almost no surface irregularities and excellent surface smoothness, and is within a range of foaming ratio of 2.4 to 4.8 times. High foaming ratio. The average cell diameter was 150 μm or less and had a non-foamed layer (skin layer) of 200 to 400 μm, and there was almost no void inside the molded body. As a result, a weight reduction rate of 27 to 41% was achieved with respect to the non-foamed molded body having the same rigidity in spite of the box-shaped foamed molded body.

(Reference example)
In Examples, the non-foamed molded article was taken out after injection-filling without using a modified polypropylene resin and a foaming agent and cooling for 60 seconds. At this time, by changing the clearance of the initial mold bottom surface, molded bodies having different bottom wall thicknesses were obtained.

(Comparative Example 1)
The same operation as in Example 6 was performed except that foaming by expanding the cavity was performed in one step. The results are shown in Table 3. The expansion ratio caused voids in the molded body, and the surface smoothness and rigidity decreased.

(Comparative Example 2)
The same operation as in Example 6 was performed, except that there was no set holding time in the second stage foaming step. The results are shown in Table 3. The expansion ratio caused voids in the molded body, and the surface smoothness and rigidity decreased.

(Comparative Example 3)
The same operation as in Example 6 was performed except that the modified polypropylene resin was not used. The results are shown in Table 3. The expansion ratio was only about 2 times, voids were generated inside the molded body, and the surface smoothness was poor. Since the rigidity was significantly reduced, the weight reduction rate was less than 20%.

(Comparative Example 4)
The same operation as in Example 6 was conducted except that the modified polypropylene resin used was MP-3 (not showing the strain hardening property of the present invention). The results are shown in Table 3. The expansion ratio was only about 2 times, voids were generated inside the molded body, and the surface smoothness was poor. Since the rigidity was significantly reduced, the weight reduction rate was less than 20%.

(Comparative Example 5)
The same operation as in Example 6 was carried out except that PP-3 (melt flow rate outside the range of the present invention) was used as the linear polypropylene resin. The results are shown in Table 3. During 20 shots in continuous molding, short shots occurred in 2 shots, and it was found that the injection foam moldability was poor.

(Comparative Example 6)
It implemented like Example 6 except not having used linear polypropylene resin. The results are shown in Table 3. During 20 shots in continuous molding, short shots occurred in 5 shots, and it was found that the injection foam moldability was poor.

(Examples 8 to 10)
The chemical foaming agent masterbatch B-1 as a foam nucleating agent is dry blended into the linear polypropylene resin (A) and the modified polypropylene resin (B) at a composition ratio shown in Table 4 to produce a polypropylene system for injection foam molding. A resin mixture was obtained.

  The injection molding machine used in Examples 1 to 7 was changed to a vent type specification, and carbon dioxide gas could be supplied at a constant pressure using “CO2 gas supply device MAC-100” manufactured by Asahi Engineering Co., Ltd. The amount of carbon dioxide dissolved in the molten resin was controlled by the carbon dioxide supply pressure as shown in Table 4. Table 5 shows the molding conditions. Other molding conditions were the same as in Examples 1-7.

  Table 6 shows the injection foamability at this time, the rigidity and the weight reduction rate of the obtained foamed molded article. Since the polypropylene resin of the present invention is excellent in fluidity, short shots do not easily occur during continuous molding, and injection foam moldability is good. In addition, the box-shaped foamed foam molded article obtained by such a molding method has almost no surface irregularities and excellent surface smoothness, and has a foaming ratio of 2.5 to 3.1 times. The foaming ratio. The average cell diameter was 160 μm or less and had a non-foamed layer (skin layer) of 300 to 400 μm, and there was almost no void inside the molded body. As a result, a weight reduction rate of 27 to 41% was achieved with respect to the non-foamed molded body having the same rigidity in spite of the box-shaped foamed molded body.

(Comparative Example 7)
The same operation as in Example 8 was performed except that foaming by expanding the cavity was performed in one step. The results are shown in Table 6. The expansion ratio caused voids in the molded body, and the surface smoothness and rigidity decreased.

(Comparative Example 8)
The same operation as in Example 8 was performed except that the modified polypropylene resin was not used. The results are shown in Table 6. The expansion ratio was only about 2 times, voids were generated inside the molded body, and the surface smoothness was poor. Since the rigidity was significantly reduced, the weight reduction rate was less than 20%.

  The method for producing an injection-foamed molded article of the present invention enables thin-injection filling necessary for significant weight reduction by using a polypropylene-based resin having high fluidity at the time of melting and high melt tension. Moreover, although the obtained foaming molding is high foaming magnification, it is excellent in lightweight property, rigidity, and surface smoothness. Furthermore, according to the manufacturing method of the present invention, since the effect is exhibited by the box-shaped foamed molded article, it includes automobile interior materials such as luggage boxes, console boxes, tool boxes, food packaging containers, and household appliance housings. It can be widely used for daily miscellaneous goods boxes.

It is the figure which showed the relationship between the molded object weight of the non-foaming injection molded object in a reference example, and the rigidity of the test piece cut out from the bottom face part. In the reference examples and examples, a box-shaped mold is filled with a polypropylene resin. In the embodiment, the movable mold of the box-shaped mold is moved to enlarge the cavity, and the filled resin is foamed.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Movable type | mold 2 Fixed type | mold 3 Gate 4 Nozzle tip part of injection molding machine 5 Bottom face part 6 Standing wall part 7 Resin injection-filled in mold cavity 8 Foam molding

Claims (10)

In a method for producing a foamed molded article by injecting a melt of a polypropylene resin composition comprising a polypropylene resin and a foaming agent into a mold, the polypropylene resin (A) has a melt flow rate of 10 g / 10. Min. To 100 g / 10 min, linear polypropylene resin having a melt tension of 2 cN or less and 40 parts by weight to 95 parts by weight, and (B) a melt flow rate of 0.1 g / 10 min to 10 g / 10 min. 5 to 60 parts by weight of a modified polypropylene resin having a melt tension of 5 cN or more and exhibiting strain hardening (however, the total of the linear polypropylene resin (A) and the modified polypropylene resin (B) is 100) The mold is composed of a fixed mold and a movable mold that can be moved forward and backward, and corresponds to the shape position of the final product. An injection step of injecting and filling the molten mixture into a mold cavity having a mold cavity clearance t ₀ smaller than the mold cavity clearance t ₂ , immediately thereafter, a mold cavity smaller than t _{2 and} larger than t ₀ The first stage foaming step for retracting the movable mold to the clearance t ₁ , and then holding the clearance of t ₁ for a predetermined set time, and then the mold cavity corresponding to the shape position of the final product is further moved to the clearance t ₂ A method for producing an injection-foamed molded article comprising a second-stage foaming step of retreating.   The injection-foamed molded article according to claim 1, wherein the modified polypropylene resin (B) is obtained by melt-mixing a linear polypropylene resin, a radical polymerization initiator, and a conjugated diene compound. Manufacturing method.   3. The method for producing an injection-foamed molded article according to claim 1, wherein a mold opening speed in the first stage foaming step is 7 mm / second or more and 100 mm / second or less.   The method for producing an injection foam molded article according to any one of claims 1 and 2, wherein a mold opening speed in the second stage foaming step is 0.5 mm / second or more and 20 mm / second or less. The method for producing an injection-foamed molded article according to any one of claims 1 to 4, wherein the relationship among the t ₀ , t ₁ , and t ₂ is expressed by the following equation.

Here, t ₀ is 0.5 mm or more and less than 3 mm. Injection foam molded article according to any one of claims 1 to 5, wherein the set time for holding the mold cavity clearance t ₁ in the second step expansion process is less than 20 seconds 1 seconds Manufacturing method. The foaming molding manufactured by the method as described in any one of Claims 1-6.
  The foaming ratio is 2 times or more and 10 times or less, having a foamed layer having an average cell diameter of 500 μm or less 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. The foamed molded product according to claim 7. The foam molded article according to claim 7 or 8, wherein a weight reduction ratio (L) represented by the following formula is 20% or more.

Here, W _S is the weight of the non-foamed injection molded body having the same rigidity, and W _E is the weight of the foam molded body.   The foamed molded product according to any one of claims 7 to 9, wherein the foamed molded product has a box shape including a bottom surface portion and a standing wall portion molded integrally with the bottom surface portion.

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