Production process of pre-foamed particles

Abstract: Pre-foamed particles are produced by dispersing resin particles, which contain nucleocells, in a dispersing medium in a closed vessel, heating the contents to a temperature of the softening point of the particles or higher and holding the con­ tents at the temperature for a predetermined period of time while pressurizing the contents with an inorganic gas, and then opening the vessel at one end thereof so as to release the par­ ticles and medium into an atmosphere of a pressure lower than the internal pressure of the vessel, thereby causing the resin particles to foam. The above production process provides good pre-foamed particles with ease and without failure. It is also possible to avoid deleterious effects due to the swelling action of foaming agents.

Description: This invention relates to a process for producing pre-foamed particles. Foamed-in-place articles which are obtained by filling pre-foamed particles in molds and then heating the thus-filled particles to cause them to expand in the molds have been used for a wide range of applications such as heat-insulating materials, cushioning materials, packaging materials, buoyancy materials, etc. These pre-foamed particles have conventionally been prepared, as described for example in Japanese Patent Publication No. 1344/1981, by dispersing resin particles, which contain a volatile organic solvent, in water in a closed vessel, heating the contents to a temperature of the softening point of the resin or higher while holding the internal pressure of the vessel at the vapor pressure of the foaming agent or higher, and then opening the vessel at one end thereof, said one end being lying under the water, to simultaneously release the resin particles and water from the vessel into an atmosphere of a pressure lower than the internal pressure of the vessel. In this prior art process, propane, butane, heptane, hexane, cyclobutane, cyclopentane, trichlorofluoromethane, dichlorodifluoromethane or the like is used as the volatile organic foaming agent. These volatile organic foaming agents however serve as good solvents for resins and cause resin particles to swell. They are thus accompanied by such drawbacks that the suitable foaming temperature range is narrow upon effecting the pre-foaming, the expansion ratio is significantly affected by the foaming temperature, and difficulties are encountered in controlling the expansion ratio. Besides, some of these volatile organic foaming agents are dangerous due to their toxicity or inflammability and even if some other volatile organic foaming agents are not accompanied by too much problems in terms of dangerousness, they are however too expensive. Additionally, these volatile organic foaming agents develop problems of environmental contamination such as destruction of the ozonosphere. Despite of the above-mentioned various problems of such volatile organic foaming agents, they are still continuously used because no effective foaming agent capable of replacing them have been developed. GB-A-2 080 813 is directed to a process for producing a foamed article of a polyolefin resin. This document discloses a feature that the above-mentioned foamed article is obtained by using, as pre-foamed particles, resin particles containing a gaseous mixture consisting of an inorganic gas and a volatile organic blowing agent in their cells and causing the pre-foamed particles to foam. As mentioned above, the use of a volatile organic blowing agent is disadvantageous. GB-A-1 047 341 relates to a method for preparing a cellular polymeric thermoplastic material and the document discloses the feature that a foamed polymeric material is obtained by extruding a polymeric thermoplastic material admixed with a decomposable foaming agent or the like from an extruder. However, the cells contained in the polymeric material obtained by the prior art teaching are fine cells and are strictly cells obtained by causing the polymeric raw material to foam by a foaming effect of the decomposable foaming agent or the like. Since the cells differ from the nucleocells of the invention of the present application, which are formed with a view toward forming cells serving as nuclei for cells to be formed in the pre-foamed particles, without intentionally causing resin particles to foam, the foamed polymeric material obtained by the process of GB-A-1 047 341 is apparently different from the resin particles of the present invention, which are not yet foamed and contain the nucleocells. US-A-4 360 484 is concerned with a process for decreasing the density (an expansion process) of a polyolefin resin foam material by two-step expansion and the document discloses a technique that a lower-density foamed product is obtained by filling a primarily foamed resin material having a closed-cell structure together with nitrogen gas into a closed vessel and then heating, pressurizing and decompressing the contents so as to secondarily expand the resin material. A two-step expansion process is of disadvantage. Furthermore, the primarily foamed resin material obtained according to the process known from US-A-4 360 484 differs from the product which is obtained according to this invention. EP-A-0 095 109 relates to a process for the preparation of expanded particles, in which a mixture of water and of polyolefin resin particles, impregnated with a volatile blowing agent, is released from the interior of a pressure-resistant container under a constant temperature and constant pressure, preferably keeping the partial pressure of the blowing agent constant, through at least one orifice having an area of 0.07 to 300 mm², into a low-pressure zone. EP-A-0 113 903 (Art. 54(3) EPC) describes a process for producing prefoamed polymer particles, which comprises a step of impregnating carbon dioxide in polymer particles, a step of dispersing the polymer particles into a dispersion medium in a sealed vessel, a step of heating the polymer particles to a temperature higher than the softening point of said polymer particles, and a step of opening one end of said vessel and simultaneously releasing the polymer particles and dispersion medium from inside the vessel to an atmosphere held at a pressure lower than that of the vessel. DE-B-21 55 775 relates to a process for producing foamed polyolefin particles which are formed with cavities with volume shrinkage accompanying cristallization of the polymer, a dispersing agent being absorbed in these cavities. With the foregoing in view, the present inventors have carried out an investigation for many years on pre-foamed particles useful in molding foamed-in-place articles. As a result, they succeeded in developing a novel production process which can produce with ease and good efficiency pre-foamed particles having excellent physical properties, leading to completion of this invention. This invention relates to a process for producing pre-foamed particles, wherein resin particles are dispersed in a dispersing medium in a closed vessel, the contents are heated to a temperature of the softening point of the resin particles or higher and held at the temperature for a predetermined period of time while pressurizing the contents and the vessel is then opened at one end thereof so as to release the thus-heated and pressurized resin particles and dispersing medium into an atmosphere of a pressure lower than the internal pressure of the vessel, thereby causing the thus-heated and pressurized resin particles to foam, the improvement comprising the steps of: using, as the resin particles, substantially unfoamed resin particles containing nucleocells formed by kneading a decomposable foaming agent and optionally a void-forming agent together with a resin at a temperature of the decomposition temperature of the decomposable foaming agent or higher in an extruder so as to decompose the decomposable foaming agent partly or in its entirety, both in an extruder, when extruding and pelletizing the resin from the extruder to produce the resin particles, or formed by kneading a resin and a volatile foaming agent in an extruder to gasify the volatile foaming agent when foaming and pelletizing the resin from the extruder to produce the resin particles, which serve as nuclei for cells to be formed in the prefoamed particles, and having a volume about 1.01 to 2.0 times the volume which the resin particles are supposed to have before the nucleocells are formed therein; and holding the contents for at least 1 minute under a pressure in the range of 25.5-40.2 bar (25-40 kg/cm²) while pressurizing the contents with an inorganic gas, thereby causing the unfoamed resin particles to pre-foam to an expansion ratio of 2 to 40 times. The term "nucleocells" as used herein means cells which serve as nuclei for cells to be formed in prefoamed particles. These nucleocells are formed in resin particles by the methods as will be mentioned next. Namely, upon extruding a resin through an extruder and then pelletizing the resultant extrudate to produce resin particles, (1) to knead a decomposable foaming agent together with the resin at a temperature of the decomposition temperature of the foaming agent or higher to decompose the foaming agent partly or in its entirety; (2) to knead the resin and a volatile foaming agent so as to gasify the volatile foaming agent. The kneading operations are all effected in the extruder. As illustrative of the above-mentioned decomposable foaming agent, may be mentioned azodicarbonamide, monosodium citrate, dinitrosopentamethylenetetramine, p,p'-oxybis(benzenesulfonylhydrazide), diazoaminobenzene, azobisisobutyronitrile, paratoluenesulfonyl semicarbazide, benzenesulfonyl hydrazide, sodium hydrogencarbonate, etc. Exemplary volatile foaming agents may include aliphatic hydrocarbons such as propane, butane and hexane, alicyclic hydrocarbons such as cyclobutane and cyclopentane, halogenated hydrocarbons such as trichlorofluoromethane, dichlorodifluoromethane, dichlorotetrafluoroethane, methyl chloride, ethyl chloride and methylene chloride, and so on. On the other hand, illustrative of the void-forming agent may include clay, diatomaceous earth, talc, calcium carbonate, aluminum hydroxide, aluminum oxide and the like. When producing nucleocell-containing resin particles by using a decomposable foaming agent or volatile foaming agent, the foaming agent is used in such an amount that the resultant nucleocell-containing resin particles have a volume about 1.01-2.0 times, preferably, 1.01-1.5 times the volume which the resin particles are supposed to have before the nucleocells are formed therein. Among these various nucleocell-forming methods, it is particularly preferred to use a decomposable foaming agent. Such a decomposable foaming agent may preferably be used in an amount of 0.005-5 wt.%, or especially, 0.01-1.0 wt.%, both based on the corresponding resin. It is also feasible to use both decomposable foaming agent and void-forming agent in combination. The existence of nucleocells is essential in the present invention. Without nucleocells, it is only possible to obtain pre-foamed particles with low expansion ratios. If one dares to obtain pre-foamed particles having a high expansion ratio without making use of nucleocells, there is no choice for him other than effecting the inorganic gas pressurization at an elevated pressure [for example, 59.8 bar (60 kg/cm²(G)) or so]. This is certainly disadvantageous from the economical viewpoint. As base materials of resin particles useful in the practice of this invention, may be mentioned styrene-base resins such as styrene homopolymers, styrene-maleic anhydride copolymers, styrene-acrylonitrile copolymers and styrene-acrylonitrile-butadiene copolymers; and polyolefins such as ethylene homopolymers, propylene homopolymers, ethylene-propylene block copolymers, ethylene-propylene random copolymers. Of these base materials, polyolefins are preferred with low-density polyethylene and ethylene-propylene random copolymers being particularly preferred. They may be either crosslinked or not crosslinked. In the present invention, resin particles containing nucleocells are dispersed in a dispersing medium in a closed vessel. Here, any dispersing medium may be employed so long as it does not dissolve the resin. As exemplary dispersing media, may be mentioned water, ethylene glycol, glycerin, methanol, ethanol and the like. These dispersing media may be used either singly or in combination. However, water is used usually. Upon dispersion of resin particles in such a dispersing medium, it may also be possible to add a dispersing agent such as fine aluminum oxide particles, fine titanium oxide particles, fine basic magnesium carbonate particles, fine basic zinc carbonate particles, fine calcium carbonate particles or the like as needed. The dispersing agent may usually be employed in an amount of 0.01-5 parts by weight per 100 parts by weight of resin particles. In the present invention, the resin particles which have been dispersed in the dispersing medium in the closed vessel are then heated to a temperature of the softening point of the resin particles or higher. At the same time, an inorganic gas is fed into the vessel, whereby the resin particles are held under an elevated pressure. As to the heating temperature, no particular problem or inconvenience will arise so long as it is above the temperature at which the resin particles are softened. However, the heating temperature may preferably be 15°C above the softening point of the resin particles or higher. Here, the term "softening point" as used herein means a value determined at a load of 4.6 kp/mm² in accordance with ASTM D643. As exemplary inorganic gases useful for the pressurization, may be mentioned air, nitrogen, carbon dioxide, hydrogen, helium, neon, oxygen and so on. Two or more of these gases may be employed as a mixture. The pressure at which the resin particles are pressurized by the inorganic gas is 25.5-40.2 bar (25-40 kg/cm² (G)). The holding time of the resin particles at such an elevated pressure may be 1 minute or longer, or preferably, 5-15 minutes or longer. Foaming ability can be imparted to the resin particles by heating the resin particles to a temperature of the softening point of the resin particles or higher and then holding the resin particles at the temperature while pressurizing same by an inorganic gas as mentioned above. Thereafter, by opening one end of the vessel to release the resin particles and dispersing medium into an atmosphere of a pressure lower than the internal pressure of the vessel (normally, atmospheric pressure), the resin particles are allowed to foam. During this releasing period, the internal pressure of the vessel is usually held at the same level as the internal pressure before the initiation of release of the resin particles. In the above manner, pre-foamed particles having an expansion ratio of 2-40 times can be obtained. As has been described above, the present invention has made it possible with ease to impart foaming ability to resin particles by making use of resin particles containing nucleocells, dispersing the particles in a dispersing medium in a closed vessel, heating the contents to the softening point of the particles or higher and holding the particles under an elevated pressure with an inorganic gas. It has thus become feasible to produce good pre-foamed particles with ease and without failure. The present invention has also made it possible to avoid all deleterious effects which will otherwise be given to resin particles due to the swelling action of their foaming agent. This invention will hereinafter be described in further detail by the following Examples and Comparative Examples. Examples 1-6: In an extruder, 100 parts by weight of an ethylene-propylene random copolymer (Vicat softening point: 124°C) and the decomposable foaming agent or the decomposable foaming agent and void-forming agent, which are given in Table 1, were molten and kneaded. After extruding the resultant melt from the extruder, the extrudate was chopped to obtain resin particles. Cross-sectional areas of those resin particles were microscopically observed to determine whether nucleocells were contained or not. Observation results are also given in Table 1. Then, 300 parts by weight of water and 1 part by weight of fine aluminum oxide particles as a dispersing agent were charged together with 100 parts by weight of the above resin particles into a closed vessel. After heating with stirring the contents to the temperature given in Table 2, the contents were kept pressurized at the pressure and for the time period, both given in the same table, with air. One end of the vessel was then opened to release the resin particles and water under atmospheric pressure, while maintaining the internal pressure of the vessel at the same level. Properties of the resulting pre-foamed particles are also shown in Table 2. Comparative Examples 1-2: After melting an ethylene-propylene random copolymer (Vicat softening point: 124°C) in an extruder, the resultant melt was extruded and then chopped to obtain resin particles. Existence of nucleocells in the resin particles were not recognized. Then, those resin particles were charged, with the same composition as in Examples 1-6, in a closed vessel. The contents were heated and pressurized under the conditions given in Table 2, followed by their release under atmospheric pressure. As shown in Table 2, they were not allowed to undergo any substantial foaming when the holding time under the elevated pressure was short. Even when held under the elevated pressure for the long period of time, the expansion ratio was far smaller than those of the pre-foamed particles obtained respectively in the Examples.

Claims: A process for producing pre-foamed particles, wherein resin particles are dispersed in a dispersing medium in a closed vessel, the contents are heated to a temperature of the softening point of the resin particles or higher and held at the temperature for a predetermined period of time while pressurizing the contents, and the vessel is then opened at one end thereof so as to release the thus-heated and pressurized resin particles and dispersing medium into an atmosphere of a pressure lower than the internal pressure of the vessel, thereby causing the thus-heated and pressurized resin particles to foam, the process comprising the steps of: using, as the resin particles, substantially unfoamed resin particles containing nucleocells formed by kneading a decomposable foaming agent and optionally a void-forming agent together with a resin at a temperature of the decomposition temperature of the decomposable foaming agent or higher in an extruder so as to decompose the decomposable foaming agent partly or in its entirety, both in an extruder, when extruding and pelletizing the resin from the extruder to produce the resin particles, or formed by kneading a resin and a volatile foaming agent in an extruder to gasify the volatile foaming agent when foaming and pelletizing the resin from the extruder to produce the resin particles, which serve as nuclei for cells to be formed in the prefoamed particles, and having a volume about 1.01 to 2.0 times the volume which the resin particles are supposed to have before the nucleocells are formed therein; and holding the contents for at least 1 minute under a pressure in the range of 25.5 - 40.2 bar (25 - 40 kg/cm²) while pressurizing the contents with an inorganic gas, thereby causing the unfoamed resin particles to pre-foam to an expansion ratio of 2 to 40 times. A process according to Claim 1, wherein the decomposable foaming agent is used in an amount of 0.005 to 5 wt.% based on the resin.

Inventors: AKIYAMA HIROYUKI, IIZUKA KOJI, KUBOTA TAKASHI, OKABE SHIGERU, TOKORO HISAO, AKIYAMA, HIROYUKI, IIZUKA, KOJI, KUBOTA, TAKASHI, OKABE, SHIGERU, TOKORO, HISAO
Applicants: JAPAN STYRENE PAPER CORP, JAPAN STYRENE PAPER CORPORATION
Application year: 1985
Main classification: C08J9
IPC codes: C08J9