JP2009286899A - Polypropylene resin foamed particle and in-mold foaming molded product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide polypropylene resin foamed particles fusible comparatively excellently even at low temperature for allowing molding at wide-ranged temperature when the polypropylene resin foamed particles are loaded inside a mold to be heat-molded with steam into an in-mold foaming molded product and capable of giving an in-mold foaming molded product, which has excellent surface characteristics because few clearances are formed between the foamed particles on the surface and the in-mold foaming molded product has few wrinkles. <P>SOLUTION: The polypropylene resin foamed particles obtained by foaming a polypropylene resin with a melting point of 125-160&deg;C have a true magnification ranging from 20-40 times. In the polypropylene resin foamed particles, internal mean cell diameters of the foamed particles are 150-1,000 &mu;m, a maximum cell diameter of surface thin-film cells is 500 &mu;m or less, and an area occupancy of the surface thin-film cells on the polypropylene resin foamed particle surface is 0-40%. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to expanded polypropylene resin particles. Specifically, polypropylene-based foamed particles that can be suitably used for the production of polypropylene-based resin-molded in-mold foam molded articles used for buffer packaging materials, boxes, heat insulating materials, automobile bumper core materials, and the like, and in-mold foam-molded articles comprising the same About.

  In-mold foam moldings obtained by filling polypropylene resin foam particles in molds and heat-molding with water vapor are the advantages of in-mold foam moldings, such as shape flexibility, lightness, and heat insulation. have. Compared to in-mold foam moldings using similar synthetic resin foam particles, compared with in-mold foam moldings obtained using polystyrene resin foam particles, chemical resistance, heat resistance, strain recovery rate after compression It is excellent in (mechanical characteristics) and has excellent dimensional accuracy, heat resistance, and compressive strength as compared with an in-mold foam molded article using polyethylene resin expanded particles. Due to these characteristics, in-mold foam molded articles obtained using polypropylene resin foam particles are used in various applications such as heat insulating materials, shock-absorbing packaging materials, automobile interior members, and automobile bumper core materials.

  In-mold foam moldings used for applications such as general cushioning packaging materials, automobile interior members, and returnable boxes that are used in places where the user can see, often have surface properties as important. In these applications, good surface properties are required in addition to physical properties such as rigidity, lightness, and heat insulating properties normally required for in-mold foam molded products. Since the in-mold foam molded body is made by fusing the foam particles together to form an in-mold foam molded body, there is a gap between the foam particles in the in-mold foam molded body, or there is a fine wrinkle on the surface of the in-mold foam molded body. May be seen, and many products that emphasize the appearance dislike these. In order to make the gaps between the expanded particles inconspicuous, generally, the air pressure in the expanded particles is previously increased to an atmospheric pressure or higher at the time of in-mold expansion molding, and / or the heating steam pressure of the molding machine is increased, A method such as increasing the expansion force to eliminate the gap between the expanded particles and promoting fusion is adopted, but if such a method is used, the gap disappears, but conversely on the surface of the in-mold foam molded body There is a tendency to generate fine wrinkles. In general, the “surface particle gap” and the “surface wrinkle” of the in-mold foam molded product are considered to have a contradictory relationship.

  Conventional foamed particles have been molded with the heating conditions determined by a balance between the two, but such a molding temperature range is narrow, and each time the molding machine or the mold is changed, the conditions are investigated and the optimum conditions are complicated. It was. In addition, depending on the mold, there is a part that is easy to pass steam and a part that is difficult to pass, so in practice it is difficult to homogenize the temperature in the mold. There has been a problem that “gaps” and “surface wrinkles” occur, and there has been a demand for an in-mold foam molded product in which “surface particle gaps” and “surface wrinkles” do not exist at a wide range of molding temperatures.

  In addition, expanded particles that can be molded with steam as low as possible are desired because of recent demands for reducing carbon dioxide emissions due to energy consumption, reducing steam utility costs in terms of reducing molding processing costs, and reducing molding cycles. . Further, when molding with high-temperature steam is required, there is a problem that it is necessary to use a molding machine or a mold having a high pressure resistance specification, and there is a problem that the equipment cost becomes high. Therefore, molding with low-temperature steam is also desired.

As a study on the surface film of polypropylene resin expanded particles, Patent Documents 1 and 2 show that the cell thickness of the polypropylene resin expanded particle skin is made 2 to 4 times or more thicker than the cell thickness inside the expanded particles. Further, it is described that the repeated compression strength of the in-mold foam molded article is improved and the self-expanding ability is imparted. Regarding molding, most of the molding results are disclosed only with relatively high-temperature steam of 3.2 to 3.3 kgf / cm ² (gauge pressure), but there is no mention of defects, and there is no description regarding the molding temperature range. Absent.
JP 58-61128 A Japanese Patent Laid-Open No. 2-14225

  The purpose of the present invention is to compare the foamed polypropylene resin foam particles even when the fusion temperature of the polypropylene resin foam particles is low when filling the polypropylene resin foam particles in a mold and heat-molding with water vapor to form an in-mold foam molded product. In the mold with excellent surface properties, wide molding temperature range, few gaps between the foamed particles on the surface of the in-mold foam molded product, and few in-mold foam molded products An object of the present invention is to provide polypropylene-based resin expanded particles capable of obtaining an expanded molded article.

  As a result of intensive studies by the present inventors with respect to the above-mentioned problems, the surface properties when trying to obtain a high-magnification in-mold foam molded product of a polypropylene-based resin, the cell of a thin-film cell existing on the surface of the foamed particle It was found that the diameter, the area occupied by the thin film cell, and the internal average cell diameter (hereinafter referred to as the average cell diameter) are related.

  That is, in-mold foam molding of the polypropylene resin expanded particles is performed in a state where the polypropylene resin is softened by heating, but because the polypropylene resin is a crystalline polymer, a state in which crystals and amorphous are mixed, Although foaming in a semi-molten state, that is, expansion and stretching of the cell membrane, if the average cell diameter is too small, stretching strain occurs in the entire cell membrane, and shrinkage force tends to work during foaming. In the case of insufficient heating at the time of molding, the particle gap cannot be filled, and even in the case of excessive heating, the cell film is likely to be distorted at the time of molding, and may be relaxed by the curing process or aging, resulting in a gap between the expanded particles.

  In addition, the thin-film cell part softens and stretches easily during molding and heating, stretches to the limit and becomes a permanent strain, and if the cell diameter of the thin-film cell is large or occupies a large area, the entire foamed molded body in the mold is foamed. The cell of the particles is loose or the strength of the in-mold foam molded body is weakened due to the defect of the thin film cell, which easily leads to generation of wrinkles.

  From the above, in order not to cause deterioration of the surface property of the in-mold foam molded article, the surface cell membrane of the foamed particles related to the surface property, the remaining force to be stretched to the cell membrane at the time of cell membrane stretching at the time of molding (Thick film), stretches well without resistance, the cell film is relatively uniform, and it is necessary to use a low melting point polypropylene resin to mold at a low molding pressure. The present invention has been completed. In the present invention, the ratio of the skin part (surface part) of the foamed particles and the internal cell thickness as described in JP-A-58-61128 and JP-A-2-14225 is not related, and the skin part This is technically different in that a beautiful foamed molded article is obtained at a wide range of molding temperatures by controlling the distribution state of the thin film present on the (surface portion) to a specific state.

  That is, the first aspect of the present invention is a polypropylene resin foamed particle having a true magnification of 20 times to 40 times obtained by foaming a polypropylene resin having a melting point of 125 ° C. or higher and 160 ° C. or lower. The average cell diameter is 150 μm or more and 1000 μm or less, the maximum cell diameter of the surface thin film cell is 500 μm or less, and the area occupation ratio of the surface thin film cell on the surface of the expanded polypropylene resin particles is 0% or more and 40% or less. It is related with the polypropylene resin expanded particle characterized by these.

  As a preferable embodiment, the polypropylene resin described above obtained by foaming a polypropylene resin having a melt viscosity at 170 ° C. of 7500 poise or more and 12000 poise or less and a melt tension of 0.5 g or more and 1.8 g or less. Relates to expanded particles.

  The second aspect of the present invention is that the above-mentioned polypropylene resin foamed particles are filled in a mold and heated to fuse the particles with each other, and the foaming ratio in the polypropylene resin mold is 30 to 50 times. It relates to a molded body.

  The polypropylene resin foamed particles of the present invention have good fusion properties even at relatively low heat molding pressures, and the particle gaps and wrinkles on the surface of the in-mold foam molded product obtained at a wide molding temperature. An in-mold foam-molded article having a low surface property and excellent surface properties can be obtained.

  The polypropylene resin used as the base resin in the present invention is a resin containing 50 mol% or more of propylene as a monomer. Examples of monomer components that can be used other than propylene include ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, and 3,4-dimethyl-1. Α-olefins having 2 or 4 to 12 carbon atoms such as butene, 1-heptene, 3-methyl-1-hexene, 1-octene and 1-decene, cyclopentene, norbornene, tetracyclo [6,2,11,8, Cyclic olefins such as 13,6] -4-dodecene, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 1,4-hexadiene, methyl-1,4-hexadiene, 7-methyl-1,6 -Dienes such as octadiene, vinyl chloride, vinylidene chloride, acrylonitrile, vinyl acetate, acrylic acid, methacrylic acid, maleic acid Ethyl acrylate, butyl acrylate, methyl methacrylate, maleic anhydride, styrene, methyl styrene, vinyl toluene, and vinyl monomers such as divinylbenzene. Among these, it is preferable to use ethylene and 1-butene from the viewpoint of improving cold brittleness resistance and low cost.

  The melting point of the polypropylene resin of the present invention is 125 ° C. or higher and 160 ° C. or lower. Preferably, it is 125 degreeC or more and 150 degrees C or less. The melting point here refers to melting the resin particles by heating 5 to 6 mg of polypropylene resin particles from 40 ° C. to 220 ° C. at a rate of 10 ° C./min using a differential scanning calorimeter (DSC). Then, after crystallizing by lowering the temperature from 220 ° C. to 40 ° C. at 10 ° C./min, and further raising the temperature from 40 ° C. to 220 ° C. at 10 ° C./min, the DSC obtained at the second temperature rise It is the melting peak temperature in the curve. When the melting point is higher than 160 ° C., the fusion between the polypropylene resin expanded particles becomes insufficient at a low thermoforming pressure. When the melting point is lower than 125 ° C., the heat resistance may be inferior depending on the application.

  The melt viscosity at 170 ° C. of the polypropylene resin used for the expanded polypropylene resin particles of the present invention is preferably 7500 poise or more and 12000 poise or less. More preferably, it is 8000 poise or more and 12000 poise or less. In the present invention, the melting point is the temperature of the melting peak obtained by the DSC curve, but actually shows a “distribution” in which crystal components at lower and higher temperatures exist than the melting point. Therefore, a temperature of 170 ° C. for a polypropylene resin is a temperature at which melt stretching processing is possible but there is a slight unmelted crystal, and the melt viscosity at that time is during the production of polypropylene resin foam particles. It indirectly reflects the viscosity at the time of semi-melting during the foam molding in the mold and the tenacity of the resin. When the melt viscosity is less than 7500 poise, the resin does not have sufficient tenacity at the time of foaming, and the closed cell ratio of the polypropylene resin foam particles becomes low, making molding difficult, and maintaining the shape of the molded body during in-mold foam molding. Even if it is deformed and cured, there may be a case where wrinkles at the time of deformation remain. If the melt viscosity exceeds 12000 poise, the resin at the time of semi-melting may be too sticky to obtain high-magnification polypropylene-based resin expanded particles.

  Moreover, the melt tension at 170 ° C. of the polypropylene resin used for the expanded polypropylene resin particles of the present invention is preferably 0.5 g or more and 1.8 g or less. More preferably, it is 0.5 g or more and 1.6 g or less. When the melt tension is less than 0.5 g, the force for holding the bubbles is insufficient at the time of producing the polypropylene resin foamed particles, the bubbles are easily broken, and it may be difficult to keep the closed cells. When the melt tension exceeds 1.8 g, the cell membrane of the polypropylene resin foamed particles tends to be excessively stretched when the polypropylene resin foamed particles are produced or foamed, and as a result, latent distortion occurs in the cell membrane. Tend. As a result, shrinkage immediately after foaming or shrinkage due to heating during curing occurs, and the particle gap of the in-mold foamed molding may open, resulting in deterioration of surface properties, deterioration of dimensional accuracy, and recovery of curing may take a long time. .

The melt viscosity and melt tension of the present invention were measured by using a die having a diameter of 1 mmφ × land length of 10 mm and performing extrusion at 170 ° C. and a shear rate of 122 sec ⁻¹ , take-up speed of 6 m / min, die tip and melt tension. The contact distance of the measurement pulley is a value when measured under the condition of 35 cm. At this time, the ambient atmosphere has a humidity of 50% at 25 ° C. The melt tension has an amplitude on the chart, but in the present invention, the median value of the amplitude is the melt tension.

  In order to make the internal average cell diameter and surface thin film cell of the polypropylene resin expanded particles of the present invention have a specific property, the polypropylene resin has an appropriate strength when semi-molten and has no resistance to stretching. One of the important requirements is that it is difficult to generate latent distortion. As such a resin, it is preferable that the entanglement between the polymers is small, but as long as the melt tension is within an appropriate range, the high molecular weight component, long chain branching, and partial crosslinking may be present in the polypropylene resin. . This characteristic may be uniquely determined depending on the polymerization conditions of the polypropylene resin, or may be intentionally controlled.

  It is important that the foamed polypropylene resin particles of the present invention have moderate viscosity and strength when semi-molten, have no resistance to stretching, are less susceptible to latent distortion, and have less cell non-uniformity on the surface of the expanded particles. become. As such foamed particles, the surface layer of the foamed particles has a certain degree of melting characteristics of the polypropylene resin to be used and a thickness capable of extending during expansion of the foamed particles during molding, regardless of the thickness of the internal cell membrane. It will be possible. Although it is considered that the melting characteristic of the polypropylene copolymer resin is originally determined by the molecular weight distribution and composition distribution during the polymerization, it can be intentionally controlled by a melt tension adjusting agent. Moreover, the cell structure of the surface and the inside of the expanded particle of the present invention is not limited to this, although it is easily expressed in the resin having the above-described melt viscosity and melt tension. Moreover, the cell structure of the surface and the inside of the foamed particle of the present invention can be adjusted by the kind of foamed cell forming agent and foaming agent, the amount added, the foaming pressure and the like.

  As a method for intentionally controlling the melt viscosity and melt tension of the polypropylene resin, there is a method using a melt tension adjusting agent. Examples of melt tension adjusting agents include organic peroxides and polypropylene oligomers. Specifically, a method of reducing the melt tension by decomposing a polypropylene resin with an organic peroxide and a method of reducing the melt tension by adding a polypropylene oligomer to the polypropylene resin can be mentioned.

  For example, in order to decompose a polypropylene resin with an organic peroxide, it is generally performed by adding an organic peroxide to a polypropylene resin heated and melted in an extruder. The amount of the organic peroxide used is preferably in the range of 0.001 to 0.1 parts by weight with respect to 100 parts by weight of the polypropylene resin. Some commercially available polypropylene resins having a narrow molecular weight distribution are those in which the molecular weight distribution is adjusted by this method, which is called rheology control or visbreaking. A polypropylene resin having a narrow molecular weight distribution has few high molecular weight components, so that there is little entanglement between molecules at the time of foaming, that is, half melt stretching, and the melt tension becomes an appropriate value. In the peroxide treatment, it is preferable to appropriately select the molecular weight of the polypropylene resin to be used so that the melt viscosity at 170 ° C. is in the range of 7500 poise to 12000 poise. Examples of the organic peroxide to be used include 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, t-butylperoxylaurate, 2,5-dimethyl2,5di (benzoylperoxide). Oxy) hexane, t-butylperoxybenzoate, dicumyl peroxide, 1,3-bis (t-butylperoxyisopropyl) benzene, t-butylperoxyisopropyl monocarbonate and the like.

  Polypropylene resin foamed particles are prepared using the polypropylene resin as described above, and it is preferable to add a foam cell forming agent to the polypropylene resin. When a volatile foaming agent such as propane, isobutane, normal butane, isopentane, or normal pentane is used as the foaming agent, an inorganic nucleating agent such as talc, silica, or calcium carbonate is used as the foaming cell forming agent. It is preferable to add 0.005 parts by weight or more and 0.1 parts by weight or less with respect to 100 parts by weight of the resin. When an inorganic foaming agent such as air, nitrogen, carbon dioxide, or water is used as the foaming agent, it is preferable to use the above inorganic nucleating agent and / or water-absorbing substance.

  Water-absorbing substances include water-soluble inorganic substances such as sodium chloride, calcium chloride, magnesium chloride, borax and zinc borate, melamine (chemical name 1,3,5-triazine-2,4,6-triamine), ammelin (same as 1, 3,5-triazine-2-hydroxy-4,6-diamine), ammelide (1,3,5-triazine-2,4-hydroxy-6-amine), cyanuric acid (1,3,5-triazine) -2,4,6-triol), isocyanuric acid (1,3,5-triazine-2,4,6 (1H, 3H, 5H) -trione), acetoguanamine (1,3,5-triazine-) 2,4-diamine-6-methyl), benzoguanamine (1,3,5-triazine-2,4-diamine-6-phenyl), tris (methyl) isocyanurate, tris (ethyl) Compounds having a triazine skeleton such as sosocyanurate, tris (butyl) isocyanurate, tris (2-hydroxyethyl) isocyanurate, melamine / isocyanuric acid condensate, polyethylene glycol, alkali metal salt of ethylene (meth) acrylic acid copolymer, Hydrophilic properties such as alkali metal salt of butadiene (meth) acrylic acid copolymer, alkali metal salt of carboxylated nitrile rubber, alkali metal salt of isobutylene-maleic anhydride copolymer, alkali metal salt of poly (meth) acrylic acid Polymers. When using a water-soluble inorganic substance, it is preferable that the addition amount is 0.01 to 1 part by weight with respect to 100 parts by weight of the polypropylene resin. When using a compound having a triazine skeleton, the amount added is preferably 0.001 part by weight or more and 1 part by weight or less with respect to 100 parts by weight of the polypropylene resin. When using a hydrophilic polymer, it is preferable that the addition amount is 0.1 to 5 parts by weight with respect to 100 parts by weight of the polypropylene resin. Two or more of these water-soluble inorganic substances, compounds having a triazine skeleton, and hydrophilic polymers may be used in combination.

  When the addition amount of these foamed cell forming agents is inappropriate, the internal average cell diameter of the foamed particles and the cell diameter and the occupation ratio of the surface thin film cell may be out of the scope of the present invention.

  In addition, if necessary, melt-strength modifiers, nucleating agents, water-absorbing agents, surfactant-type or polymer-type antistatic agents, pigments, flame retardant improvers, conductivity improvers, etc. can be used as polypropylene resins. As an addition method, it is usually preferable to add to a molten resin in the production process of polypropylene resin particles.

  Polypropylene resin is usually melt-processed in advance using an extruder, kneader, Banbury mixer, roll, etc. so that it can be easily used for foaming, and is desired to have a cylindrical shape, elliptical shape, spherical shape, cubic shape, rectangular parallelepiped shape, etc. It can be set as the polypropylene resin particle of the shape. The production of polypropylene resin particles is usually performed by adding desired additives to polypropylene resin by dry blending or masterbatch blending, melt-kneading in the extruder, and extruding into a strand from the die at the tip of the extruder, water tank For example, a method of cutting the strand sufficiently cooled by a method such as cutting into particles, or a method of underwater cutting in which the resin is directly discharged from a die into water while being cut into particles. As the extruder, a single screw extruder, a twin screw extruder or the like is used. The resin temperature during processing is preferably a melting point of the polypropylene resin + 30 ° C. or more and 250 ° C. or less. If it exceeds 250 ° C., the polypropylene resin may be degraded and deteriorated. When the resin temperature is lower than the melting point of the polypropylene resin + 30 ° C., the kneading may not be sufficient, or an excessive load may be applied to the extruder.

  The produced polypropylene resin particles may undergo shrinkage deformation in the flow direction of the extruder during reheating, particularly foaming, and depending on the shape of the foamed particles after shrinkage deformation, the filling of the mold during molding becomes worse. Therefore, it is preferable to predict the heat shrinkage and appropriately adjust the shape such as the length and thickness of the polypropylene resin particles.

  The average particle weight of the polypropylene resin particles is preferably 0.5 to 3.0 mg, more preferably 0.5 to 2.0 mg, and still more preferably 0.5 to 1.5 mg.

  The polypropylene resin expanded particles of the present invention can be obtained by expanding the above-described polypropylene resin particles under the conditions described later.

  The expanded polypropylene resin particles of the present invention are prepared by placing a dispersion containing polypropylene resin particles, a foaming agent, water, a dispersing agent and a dispersion aid in a pressure vessel, heating to a predetermined temperature, Originally, it is obtained by discharging the dispersion into a low-pressure atmosphere rather than in the pressure vessel. Specifically, polypropylene resin particles are dispersed in a pressure-resistant container together with a foaming agent, water, a dispersant, and a dispersion aid, and the dispersion is preferably a melting point of the polypropylene resin of −25 ° C. or higher and a melting point of + 25 ° C. or lower. The dispersion is heated under pressure while maintaining the temperature and pressure in the container constant while heating to a temperature in the range of -10 ° C and more preferably in the range of -10 ° C to -10 ° C and impregnating the foaming agent. Polypropylene-based resin expanded particles are produced by discharging in a low-pressure atmosphere than in the container.

  There is no particular limitation on the pressure vessel used when producing the expanded polypropylene resin particles, and any vessel that can withstand the pressure and temperature in the vessel at the time of producing the expanded polypropylene resin particles can be used. For example, an autoclave type pressure vessel can be used. It is done.

  There are no particular restrictions on the foaming agent used to produce polypropylene resin foamed particles, and volatile foaming agents such as propane, isobutane, normal butane, isopentane, and normal pentane; inorganic foams such as air, nitrogen, carbon dioxide, and water An agent can be illustrated and these can be used individually or in combination of 2 or more types. The amount of the foaming agent used is different in order to obtain polypropylene resin foamed particles having a desired expansion ratio, but it is usually preferably 5 parts by weight or more and 100 parts by weight or less with respect to 100 parts by weight of the polypropylene resin. If the amount is less than 5 parts by weight, the foamed polypropylene resin particles having a desired expansion ratio may not be obtained. If the amount exceeds 100 parts by weight, the amount of the foaming agent dissolved in the polypropylene resin exceeds the saturation amount and does not dissolve. May be wasted. In addition, when the amount of the foaming agent is not appropriate, the average cell diameter of the polypropylene resin foamed particles may become fine.

  Examples of the dispersant that can be used in the present invention include tribasic calcium phosphate, basic magnesium carbonate, calcium carbonate, basic zinc carbonate, aluminum oxide, iron oxide, titanium oxide, aluminosilicate, barium sulfate, and kaolin. These inorganic dispersants can be used, and these can be used alone or in combination.

  Examples of the dispersion aid that can be used in the present invention include alkyl sulfonic acid soda, dodecylbenzene sulfonic acid soda, n-paraffin sulfonic acid soda, and α-olefin sulfonic acid soda. Can be used in combination. The combination of the dispersant and the dispersion aid can be appropriately selected depending on the foaming agent and the like.

  The amount of dispersant and dispersion aid used varies depending on the type and the type and amount of polypropylene resin used, but usually 0.2 parts by weight to 3 parts by weight of dispersant with respect to 100 parts by weight of water, The dispersion aid is preferably 0.001 part by weight or more and 0.1 part by weight or less. Moreover, in order to make the polypropylene resin particles have good dispersibility in water, it is usually preferable to use 20 to 100 parts by weight with respect to 100 parts by weight of water.

  The true magnification of the expanded polypropylene resin particles obtained by the above production method is 20 times or more and 40 times or less, and more preferably 25 times or more and 35 times or less. If polypropylene resin expanded particles having a true magnification of less than 20 times are used, an in-mold foam-molded article having the desired expansion ratio cannot be obtained. When the true magnification is larger than 40 times, the molded body is easily contracted and deformed during in-mold foam molding, and a desired shape cannot be obtained.

  In the present invention, a dispersion liquid containing polypropylene-based resin particles, foaming agent, water, dispersant, and dispersion aid is placed in a pressure vessel, heated to a predetermined temperature, and dispersed under pressure. In some cases, the liquid is discharged in a low-pressure atmosphere from the pressure-resistant container and foamed, and this is referred to as “one-stage foaming”, and the polypropylene-based resin foam particles obtained by one-stage foaming are sometimes referred to as “single-stage foam particles”.

  Even when single-stage foamed particles having a foaming ratio less than the desired foaming ratio are obtained by single-stage foaming, the single-stage foamed particles are placed in a sealed container and subjected to pressure treatment to impregnate nitrogen, air, etc. After making the pressure higher than normal pressure, the first-stage expanded particles are heated with steam or the like and further expanded to obtain polypropylene-based resin expanded particles having an expansion ratio of 20 to 40 times.

  Here, the further expansion of the single-stage expanded particles is sometimes referred to as “two-stage expansion”, and the expanded particles obtained by the two-stage expansion may be referred to as “two-stage expanded particles”.

  When performing two-stage foaming, the preferable expansion ratio of the first-stage foamed particles is 5 times or more. When the magnification of the first-stage expanded particles is less than 5 times, the target magnification may not be reached even if the second-stage expansion is performed, or the variation in the expansion ratio may be increased, resulting in poor quality.

  The pressure in the first-stage expanded particles when the two-stage expansion is performed is preferably 0.20 to 0.70 MPa, and particularly preferably 0.3 to 0.55 MPa. When the pressure in the first-stage expanded particles is less than 0.20 MPa, the effect of the second-stage expansion is small and the magnification may hardly increase. When the pressure is 0.70 MPa or more, the variation in the expansion ratio may increase.

  In the present invention, unless otherwise specified, the polypropylene-based resin expanded particles refer to those immediately before being subjected to in-mold expansion molding. For example, when two-stage expansion is performed, it refers to two-stage expanded particles.

Here, the true magnification of the polypropylene resin expanded particles is obtained by determining the weight w (g) and the ethanol submerged volume v (cm ³ ) of the polypropylene resin expanded particles, and the density d (g / cm ³ ) of the polypropylene resin particles before expansion. ) From the following equation.
True magnification = d × v / w

  The internal average cell diameter of the expanded polypropylene resin particles of the present invention is 150 μm or more and 1000 μm or less, preferably 200 μm or more and 500 μm or less. When the internal average cell diameter is less than 150 μm, the cell membrane is thinly stretched and stretched greatly, which causes shrinkage / deformation and deterioration of surface properties during in-mold foam molding. On the other hand, if it exceeds 1000 μm, the variation of the internal average cell diameter increases, the number of surface thin film cells having a cell diameter larger than 500 μm that causes wrinkles increases, and the appearance of the in-mold foam molded article becomes poor.

  In order to obtain a polypropylene resin foamed particle having an internal average cell diameter of the polypropylene resin foamed particle in the above range, it can be obtained by appropriately adjusting the type, amount of use, and foaming pressure of the additive or foaming agent in the polypropylene resin. .

The average internal cell diameter of the expanded polypropylene resin particles is obtained by observing and photographing a cut surface that includes almost the diameter of the expanded polypropylene resin particles with a microscope, and drawing a straight line passing through the approximate center of the expanded polypropylene resin particles. From the distance (L) between the two intersections with the resin foam particle surface and the number of bubbles (n) through which the straight line penetrates, the following is obtained.
Internal average cell diameter = L / n

  The maximum cell diameter of the surface thin film cell of the polypropylene resin expanded particles of the present invention is 500 μm or less, preferably 350 μm or less. When the maximum cell diameter of the surface thin film cell diameter is 500 μm or more, wrinkles are likely to occur in the in-mold foam molded article, which is not preferable.

  In the present invention, the area occupation ratio of the surface thin film cell on the surface of the expanded polypropylene resin particles is 0% or more and 40% or less. Preferably they are 0% or more and 30% or less. When the area occupation ratio of the surface of the surface thin film cell on the surface of the expanded particles is larger than 40%, wrinkles are generated in the in-mold expanded molded body.

  In order to obtain a polypropylene resin foamed particle having a surface thin film cell in the above range, it can be obtained by appropriately adjusting the type and amount of the additive or foaming agent in the polypropylene resin and the foaming pressure.

  The cell diameter and occupied area of the surface thin film cell are determined by observation with a low vacuum SEM. In the low-vacuum SEM, the contrast due to the difference in the density of the substance distributed on the sample surface is obtained by the reflected electrons, so that the high density portion in the thick film is observed as white and the low density portion in the thin film is observed as black. Under the observation condition of the low vacuum SEM used in the present invention, a thin portion having a film thickness of several μ or less appears black. This is defined as a surface thin film cell. The maximum cell diameter of the thin film cell refers to the longest diameter of the portion that appears black in the low vacuum SEM image. The area occupied by the thin-film cell is the ratio of the area of the black portion to the entire surface image area. FIG. 1 shows an image diagram, and FIGS. 2 to 4 show low-vacuum SEM observation diagrams of Examples and Comparative Examples.

  Specifically, the polypropylene resin expanded particles are divided into two so as to substantially include the center, and observation is performed in a stereoscopic image mode so that the entire surface portion of the expanded particles enters the screen without performing a process such as vapor deposition. When Hitachi Seiki Type N, manufactured by Hitachi Keiki Co., is used as the measuring device, the observation conditions are: degree of vacuum 30 Pa, acceleration voltage 15 kV, emission current about 30 μA, objective movable aperture 3, input signal BSE2, observation magnification 30 times, 3D mode observation To do. The state of the thin film cell is observed from the obtained foamed particle surface observation image, and the measurement of the longest diameter of the thin film cell and the area occupation ratio of the thin film cell are determined.

The polypropylene resin expanded particles of the present invention have two melting peaks on the low temperature side and the high temperature side in the DSC curve when measured with a differential scanning calorimeter (DSC), and the DSC high temperature side melting peak calorie ratio (hereinafter, (It may be simply referred to as DSC peak ratio) is preferably in the range of 10 to 50%. In the DSC curve obtained when the sample is heated at a rate of 10 ° C./min from 40 ° C. to 220 ° C., the DSC peak ratio is measured between the low temperature side peak, the low temperature side peak, and the high temperature side peak. The amount of heat surrounded by the tangent to the melting start baseline from the local maximum point, the melting peak calorie QL on the low temperature side, and the melting end from the local maximum point between the high temperature side peak, the low temperature side peak, and the high temperature side peak of the DSC curve From the high temperature side melting peak calorific value QH that is the amount of heat surrounded by the tangent to the baseline
DSC peak ratio (%) = QH / (QH + QL) × 100
As required. When the DSC peak ratio is within this range, it is easy to obtain an in-mold foam molded product having a high surface beauty.

  When the polypropylene resin foamed particles of the present invention are used for in-mold foam molding, a) a method of using as it is, b) a method of previously injecting an inorganic gas such as air into the polypropylene resin foamed particles to impart foaming ability. C) Conventionally known methods such as a method in which polypropylene resin expanded particles are filled in a mold in a compressed state and molded may be used.

  For example, in the case of using the method (b), in order to form an in-mold foam molded article from the polypropylene resin foam particles of the present invention, the polypropylene resin foam particles are preliminarily air-pressurized in a pressure resistant container, and air is introduced into the particles. A foaming force is imparted by press-fitting, and this is filled in a mold that can be closed but cannot be sealed, and with a water vapor pressure of about 0.15 MPa to 0.33 MPa (gauge pressure) using water vapor or the like as a heating medium. Molded with a heating time of about 3 to 30 seconds to fuse polypropylene resin foamed particles together, and then to the extent that deformation of the in-mold foam molding after taking out the in-mold foam molding by water cooling can be suppressed After cooling, an in-mold foam-molded product can be obtained by opening the mold.

  Since the polypropylene resin expanded particles of the present invention use polypropylene resin expanded particles having a melting point of 125 ° C. or higher and 160 ° C. or lower, in some cases, molding can be performed even when the heat molding pressure of about 0.15 MPa is low. It has the characteristics.

  For fusion, after cutting about 2 mm in the thickness direction of the in-mold foam molded body with a cutter knife or the like, the in-mold foam molded body is broken by hand from the cut portion, and the fracture surface is observed, The ratio was evaluated based on the ratio of broken polypropylene resin foam particles.

  In the present invention, a polypropylene resin in-mold foam molded product can be obtained by in-mold foam molding. The expansion ratio of the obtained in-mold foam molded article is preferably 30 to 50 times.

The expansion ratio of the in-mold foam molded product is obtained by dividing the ethanol submerged volume (cm ³ ) of the in-mold foam molded product by the weight (g) and multiplying by the resin particle density (g / cm ³ ) before foaming. It is.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention in detail, this invention is not limited to these.

  Table 1 shows polypropylene resins used in Examples and Comparative Examples, and Table 2 shows various additives. In addition, Table 3 shows the production conditions of each polypropylene resin expanded particle, and Table 4 shows various physical properties of the expanded particle.

Various evaluations were performed as follows.

<Measurement of melting point>
Using a DSC6200 type differential scanning calorimeter manufactured by Seiko Instruments Inc., 5-6 mg of polypropylene resin particles are heated from 40 ° C. to 220 ° C. at a heating rate of 10 ° C./min. From the DSC curve obtained by melting and then crystallizing by lowering the temperature from 220 ° C. to 40 ° C. at 10 ° C./min, and then increasing the temperature from 40 ° C. to 220 ° C. at 10 ° C./min. The melting peak temperature when the temperature was raised was defined as the melting point.

<True magnification of expanded polypropylene resin particles>
The weight w (g) and the ethanol submerged volume v (cm ³ ) of a bulk volume of about 50 cm ³ of the polypropylene resin foam particles used for molding were determined, and the following formula was obtained from the density d (g / cm ³ ) of the resin particles before foaming. Determined by
True magnification = d × v / w
When the used expanded particles were two-stage expanded particles, the true magnification of the first-stage expanded particles was also described.

<Internal average cell diameter>
20 polypropylene-based resin expanded particles, each of the cut surfaces including the diameter of each of the 20 is observed and photographed under a microscope, and a straight line passing through substantially the center of each polypropylene-based resin expanded particle is drawn. It calculated | required as follows from the distance (Li) of the intersection of one place, and the number of bubbles (ni) which this straight line penetrated, and also set it as the arithmetic mean of 20 pieces.
Cell diameter Ri = Li / ni
20 arithmetic averages of internal average cell diameter = Ri

<Maximum cell diameter of surface thin film cell and area occupation ratio of surface thin film cell>
Low-vacuum observation was performed on 20 polypropylene-based resin expanded particles using a SEMEDX Type N manufactured by Hitachi Keiki Co., Ltd. The foamed particles were cut so as to pass through almost the center of the foamed particles, the surface of the foamed particles was used as an observation surface, and the sample height was adjusted and observed so that the working distance was 20 without performing processing such as vapor deposition. . The observation conditions were a degree of vacuum of 30 Pa, an acceleration voltage of 15 kV, an emission current of about 30 μA, an objective movable diaphragm 3, an input signal BSE2, an observation magnification of 30 times, and 3D mode observation. The state of the thin film cell is observed from the obtained foamed particle surface observation image, the longest diameter of the thin film cell is measured, and the area occupancy of the thin film cell is visually observed. If the area occupancy is 20% or less, “◎”, 20% A larger value of 40% or less was “◯”, and a value larger than 40% was “x”.

<Foamed particle DSC peak ratio>
The polypropylene resin expanded particles were measured with a differential scanning calorimeter (DSC). In the DSC curve obtained when 3-6 mg of expanded particles are heated from 40 ° C. to 220 ° C. at a rate of 10 ° C./min, the melting start base from the maximum point between the low temperature side peak, the low temperature side peak, and the high temperature side peak Surrounded by the tangent to the melting end baseline from the maximum point between the high temperature side peak and the low temperature side peak and the high temperature side peak of the DSC curve, which is the amount of heat surrounded by the tangent to the line From the high temperature side melting peak calorific value QH, which is the amount of heat generated,
DSC peak ratio (%) = QH / (QH + QL) × 100
As sought.

<Shrinkage against mold (shrinkage)>
A 400 × 300 × 22 mm plate-shaped in-mold foam molded body is formed with polypropylene resin foam particles, cooled, removed from the mold, left at 25 ° C. and 50% rh for 1 hour, and then cured at 75 ° C. for 8 hours. After that, the sample was left again at 25 ° C. and 50% rh for 12 hours, and the vertical, horizontal, and thickness dimensions were measured to determine the shrinkage ratio with respect to the mold dimensions. Thereafter, the average of the shrinkage ratios of length, width, and thickness was defined as the average shrinkage ratio, and this was evaluated. The average shrinkage ratio of 3.0% or less was evaluated as ◯, the average shrinkage greater than 3.0% and 4.0% or less as Δ, and the case where it was greater than 4.0% as x. When the average shrinkage rate is larger than 3.0%, it is considered that the dimensional accuracy of the in-mold foam molded article is poor, and there is a problem in practical use.

<In-mold foam molding deformation evaluation (deformation)>
The appearance of the plate-like in-mold foam-molded product whose mold shrinkage was measured was visually observed, and those that had almost no sink marks (dents) or large wrinkles (streaks of 1 cm or more) that would be derived from sink marks were ○. In the case where there is a wrinkle but there is no sink, Δ is marked, and a case where there are many wrinkles and the entire shape is waved is marked with ×.

<Evaluation of molded body side elongation (elongation)>
The 12 side parts of the plate-shaped (rectangular) in-mold foam molded body subjected to the deformation evaluation of the molded body were observed, and the number of gaps formed because the expanded particles did not extend sufficiently on the sides, and the expanded particle shape remained as it was The following judgment was made by observing that no edge appeared.
No gap ... ◎
Less than 5 ... ○
5 or more and 10 or less ... △
The edge of the side does not come out ... ×

<Small evaluation of compact surface (small)>
The surface of the plate-like in-mold foam molded body for which the molded body deformation evaluation was performed was observed to observe whether or not there were small wrinkles (1 mm or more and streaks less than 10 mm).
No gavel ... ◎
There is a gavel ... ×

<Fusability evaluation (fusion)>
The plate-shaped in-mold foam molded body for which surface properties were evaluated was cut with a cutter knife in the thickness direction of the in-mold foam molded body, and then the in-mold foam molded body was broken by hand from the cut portion. The fracture surface was observed, and the ratio of the broken polypropylene resin expanded particles was determined and determined. The forming vapor pressure 1 in Table 5 describes the vapor pressure at which the fusion rate has reached 60% as the lower limit of forming pressure.

<Foaming ratio of in-mold foam molding>
A block having a bulk volume of about 50 cm ³ is cut out from the plate-like in-mold foam molded body subjected to the evaluation of fusion property, and its weight W (g) and ethanol submerged volume V (cm ³ ) are obtained. It calculates | requires by following Formula from the density d (g / cm < ³ >) of this.
Foaming ratio = d × V / W

Example 1
As a polypropylene resin, an ethylene-propylene random copolymer (K3) having a melting point of 137.2 ° C. obtained by using an organic peroxide as a melt tension adjusting agent in 100 parts by weight of an ethylene-propylene random copolymer (K2). ) And 0.1 parts by weight of zinc borate as a cell nucleating agent was dry blended and then melt-kneaded in a 50 mm single-screw extruder (20VSE-50-28 type, manufactured by Osaka Seiki Co., Ltd.). The melt-kneaded resin was extruded in a strand shape from a circular die having a diameter of 2 mm, cooled with water, and cut with a pelletizer to obtain polypropylene resin particles having a weight of 1.2 mg / grain.

  100 parts by weight of the obtained polypropylene resin particles, 300 parts by weight of water, 0.8 part by weight of kaolin (ASP-170 manufactured by Engelhard) as a dispersant, 0.02 part by weight of sodium dodecylbenzenesulfonate as a dispersion aid Into a pressure-resistant autoclave having a capacity of 10 L, 7 parts by weight of carbon dioxide gas was added as a blowing agent with stirring. The autoclave contents were heated up and heated to a foaming temperature of 143 ° C., and then carbon dioxide gas was added to adjust the internal pressure of the autoclave to 3.3 MPa (gauge pressure). Then, after holding for 30 minutes, the valve | bulb of the autoclave lower part was opened, the autoclave content was discharge | released under atmospheric pressure through the 4.0 mm diameter opening orifice, and the polypropylene resin polypropylene resin expanded particle was obtained. The resulting polypropylene-based resin expanded particles (single-stage expanded particles) had a true magnification of 14.5 times and a DSC peak ratio of 23.9%. Furthermore, the obtained polypropylene resin expanded particles are subjected to air pressure treatment, the air pressure inside the polypropylene resin expanded particles is set to 0.36 MPa, and heated and expanded by steam of 0.12 MPa (gauge pressure). Foaming was performed to obtain polypropylene resin (two-stage) expanded particles (K3C1) having a true magnification of about 30 times.

  The obtained polypropylene resin expanded particles (K3C1) were preliminarily air pressure inside the polypropylene resin expanded particles in a mold 300 mm long × 400 mm wide × 22 mm thick using a polyolefin foam molding machine KD-345 manufactured by Daisen Corporation. Filled with expanded polypropylene resin particles (K3C1) adjusted to 0.20 MPa and compressed in the thickness direction by 5% with 0.21 MPa (gauge pressure) and 0.24 MPa (gauge pressure) steam in the mold By performing foam molding, a foamed molded product in a polypropylene resin mold was obtained. The water vapor pressure at the time of thermoforming was selected based on the fusion rate. The obtained in-mold foam molded product was left to stand at 25 ° C. × 50% rh for 1 hour, then cured and dried in a constant temperature room at 75 ° C. for 8 hours, and again left at 25 ° C. × 50% rh for 1 hour to make various evaluations. Observed. Table 5 shows the evaluation results of the in-mold foam molding.

(Examples 2-11, Comparative Examples 1-5)
Polypropylene resin having a true magnification of about 30 times (two times) in the same manner as in Example 1 except that the resin type, additive formulation, and foamed particle production conditions shown in Table 2 were used, and the two-stage foaming conditions were appropriately adjusted. Step) Expanded particles (K3C2 to K1C11, K3C12 to K3C16) were obtained.

  For these polypropylene-based resin (two-stage) expanded particles (K3C2 to K1C11, K3C12 to K3C16), the fusion rate is determined from the thermoforming pressures 0.21, 0.24, 0.27, and 0.30 MPa (gauge pressure). Molding was carried out by appropriately selecting the above, and a polypropylene resin in-mold foam molded product was obtained in the same manner as in Example 1. The obtained in-mold foam molded article was evaluated in the same manner as in Example 1. Table 5 shows the evaluation results of the in-mold foam molding.

Example 12
As a polypropylene resin, an ethylene-propylene random copolymer (K1) having a melting point of 141.5 ° C. shown in Table 1 was used, and 0.1 parts by weight of talc was dry blended as a cell nucleating agent. The mixture was melt kneaded in a machine (Osaka Seiki Kogyo Co., Ltd. 20VSE-50-28 type). The melt-kneaded resin was extruded in a strand shape from a circular die having a diameter of 2 mm, cooled with water, and cut with a pelletizer to obtain polypropylene resin particles having a weight of 1.2 mg / grain.

  3. 100 parts by weight of the obtained polypropylene resin particles, 300 parts by weight of water, 2 parts by weight of tricalcium phosphate (manufactured by Taihei Chemical Industrial Co., Ltd.) as a dispersing agent, and 0.04 part by weight of sodium alkylsulfonate as a dispersing aid. Into a 5 L pressure-resistant autoclave, 28 parts by weight of isobutane was added as a blowing agent with stirring. The autoclave contents were heated to a foaming temperature of 136.0 ° C. Then, after holding for 30 minutes, the valve | bulb of the autoclave lower part was opened, the autoclave content was discharge | released under atmospheric pressure through the opening orifice of diameter 4.0mm, and the polypropylene resin expanded particle was obtained. The resulting expanded polypropylene resin particles (K1B1) had an expansion ratio of about 30 times, a cell diameter of 290 μm, and a DSC peak ratio of 23.5%.

  The obtained polypropylene resin expanded particles (K1B1) were heat-molded with water vapor of 0.27 and 0.30 MPa (gauge pressure) to obtain a polypropylene resin in-mold expanded foam. The obtained in-mold foam molded article was evaluated in the same manner as in Example 1. The results are shown in Table 5.

  In Examples, all of the in-mold foam molded article had good shrinkage ratio to the mold, in-mold foam molded article deformation, elongation, small wrinkle, and fusion property.

  In Comparative Examples 1 to 4, the expansion ratio and internal average cell diameter of the polypropylene resin foamed particles are within the range of the present invention, but the surface thin film cell ratio is out of the range, and the mold shrinkage rate of the in-mold foam molded product , Any of elongation, gavel and plural were insufficient. In Comparative Example 5, both the average internal cell diameter and the surface thin film cell rate of the polypropylene resin foamed particles were out of the range, and the shrinkage rate, elongation and small wrinkles of the in-mold foam molded product were insufficient. Even if the molding steam pressure was changed, an in-mold foam molded article having a good quality balance could not be obtained.

  The in-mold foam-molded product obtained by molding the polypropylene resin foam particles of the present invention in-mold is highly deformed and has little dimensional stability even in a shape that easily deforms or shrinks for buffer packaging materials, etc., and has excellent dimensional stability. The elongation between them is excellent, the surface of the molded body has no fine wrinkles, and the surface properties are beautiful. Moreover, it can be molded with a relatively low water vapor pressure, and can be produced industrially and economically.

It is a figure showing the image about the foam particle surface thin film cell part. It is the cross-sectional photograph which observed the cross section of the polypropylene resin expanded particle of Example 1 with the low vacuum SEM. The black part is the surface thin film cell. It is the cross-sectional photograph which observed the cross section of the polypropylene resin expanded particle of Example 6 with the low vacuum SEM. The black part is the surface thin film cell.

Claims (3)

  In a polypropylene resin expanded particle having a true magnification of 20 times or more and 40 times or less obtained by foaming a polypropylene resin having a melting point of 125 ° C. or more and 160 ° C. or less, the internal average cell diameter of the expanded particles is 150 μm or more and 1000 μm or less. And the maximum cell diameter of the surface thin film cell is 500 μm or less, and the area occupation ratio of the surface thin film cell on the surface of the polypropylene resin expanded particle is from 0% to 40%, .   The expanded polypropylene resin particles according to claim 1, obtained by foaming a polypropylene resin having a melt viscosity at 170 ° C of 7500 poise or more and 12000 poise or less and a melt tension of 0.5 g or more and 1.8 g or less.   The polypropylene resin foamed particles according to any one of claims 1 and 2 are filled in a mold and heated, and the particles are fused to each other, and the expansion ratio of the polypropylene resin mold is 30 to 50 times. Foam molded body.