Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2002-06-27
2004-08-03
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S139000, C521S140000
Reexamination Certificate
active
06770682
ABSTRACT:
TECHNICAL FIELD
The present invention relates to foamable modified polystyrene resin particles, a process for preparation of the resin particles, a foamed article which is produced by using the resin particles and has excellent balance of break resistance and cushioning property and a process for production of the foamed article.
BACKGROUND ART
It is well known that a foamed article produced by expanding foamable polystyrene resin particles obtained by impregnating polystyrene resin particles with a blowing agent is broken easily. To improve such a defect, there has been proposed a process for mechanically blending a styrene-butadiene block copolymer with a polystyrene resin (cf. JP-B-47-17465 and JP-A-54-158467), a process for mixing a styrene-butadiene block copolymer and a polybutadiene resin in a solvent to obtain a phase-separated microstructure by a solvent casting method (cf. JP-A-56-67344) and a process employing particles of a resin, so-called high impact polystyrene resin (HIPS) particles which are excellent in impact strength and are prepared by polymerizing a styrene monomer in the presence of conjugated diene polymer (JP-B-47-18428 and JP-A-7-90105).
However it is necessary that those foamable polystyrene resin particles are subjected to pelletizing through extrusion molding, which results in increase in cost. Further in case of a commercially available high impact polystyrene resin, a molecular weight of its polystyrene cannot be increased beyond a certain level.
As a method for improving break resistance at low cost, an impregnation polymerization method for polymerizing by impregnating styrene polymer particles with a conjugated diene monomer, or the like.
For example, JP-A-6-49263 discloses foamable polystyrene resin particles prepared by copolymerizing a monomer mixture of a styrene monomer and a conjugated diene monomer in a state of the monomer mixture being impregnated around the surface portion of polystyrene resin particles to give polystyrene resin particles containing the obtained conjugated diene copolymer gathering densely around the surface portion of the resin particle, and then impregnating the polystyrene resin particles with a blowing agent. Also it is disclosed that the center portion of the foamable resin particle is rich with polystyrene and, since the blowing agent is kept in the center portion, the resin particles having high expansion ratio can be obtained. Further it is disclosed that the monomer mixture is a mixture of 10 to 55% by weight of styrene monomer and 90 to 45% by weight of conjugated diene monomer. However in all the examples thereof, the conjugated diene monomer is used in an amount of not less than 1.5 times that of the styrene monomer and an object of that invention is to obtain polystyrene resin particles, in which a conjugated diene copolymer gathers densely around the surface portion of the resin particle and the center portion of the foamable resin particle is rich with polystyrene. From that point of view, only a monomer mixture comprising the conjugated diene monomer in a larger proportion essentially is disclosed.
The process mentioned above does not require a pelletizing step and is excellent from the viewpoint of cost. However the obtained foamed article, particularly the foamed article produced by molding pre-expanded particles within a mold is low in percentage of fused particles (fusion rate) and is insufficient in break resistance (in impact strength by a falling weight method) because the conjugated diene polymer particles are present densely around the surface portion of the resin particle.
Also in WO98/29485, there is disclosed foamable modified polystyrene resin particles obtained by polymerizing a conjugated diene monomer in polystyrene resin particles to form rubber particles of conjugated diene polymer in a uniformly dispersed state in the polystyrene resin particles, impregnating and polymerizing a styrene monomer in the obtained polystyrene resin containing rubber particles and then impregnating the obtained modified polystyrene resin with a blowing agent. Those foamable resin particles are characterized in that when they are expanded, there is substantially no deformation of the rubber particles before and after the expansion and an Izod impact strength of the modified polystyrene resin itself before expanded is lower than that of conventional resin called high impact polystyrene (HIPS). Also though the publication WO98/29485 discloses impregnation and polymerization of a styrene monomer and a conjugated diene monomer, a mixing ratio thereof disclosed is only 5:10 (weight ratio) in example of the publication. In this case, too, a monomer mixture being rich with the conjugated diene monomer is used.
The process mentioned above does not necessitate a pelletizing step and is excellent from the viewpoint of cost. Also a fusion rate of pre-expanded particles in a foamed article obtained by expanding them in mold is high and a break resistance (impact strength by a falling weight method) is improved as compared with the foamed article disclosed in the above-mentioned JP-A-6-49263 but an obtained foamed article is not equal to a foamed article of commercially available high impact polystyrene (HIPS) in case of expanding at high expansion ratio (about 50 times or more) into a foamed article having a density of not more than 0.02 g/cm
3
.
As mentioned above, the impregnation polymerization method does not necessitate a pelletizing step and can enhance break resistance of polystyrene resin foamed article at low cost. However it was difficult to attain break resistance equal to that of a foamed article obtained by using high impact polystyrene (HIPS) particles.
An object of the present invention is to provide foamable modified polystyrene resin particles which can be produced without a pelletizing step and gives a foamed article having excellent break resistance and cushioning property even at high expansion ratio, a process for preparation thereof and a foamed article obtained by expanding the resin particles.
DISCLOSURE OF INVENTION
Namely the present invention relates to foamable modified polystyrene resin particles which are foamable modified polystyrene resin particles (d) comprising modified polystyrene resin particles (c) containing a blowing agent (e); the modified polystyrene resin particles (c) comprise rubber polymer particles (b) dispersed in a polystyrene resin,
wherein the rubber polymer particles (b) are dispersed uniformly throughout a continuous phase of the polystyrene resin or in a state of a density of the particles (b) being higher in a center portion of the resin particle than in a surface layer portion thereof, and
the modified polystyrene resin particles (c) are modified polystyrene resin particles (c1) obtained by impregnating a monomer mixture consisting essentially of a styrene monomer and a conjugated diene monomer into polystyrene resin particles (a) and carrying out copolymerization, said monomer mixture being containing the styrene monomer in a larger amount than the conjugated diene monomer.
The modified polystyrene resin particles (c) may be modified polystyrene resin particles (c2) obtained by impregnating the monomer mixture consisting essentially of the styrene monomer and the conjugated diene monomer in the polystyrene resin particle (a) and carrying our copolymerization; said monomer mixture containing the styrene monomer in a larger amount than the conjugated diene monomer to give the modified polystyrene resin particles (c1) and further subjecting the obtained polystyrene resin particles to impregnation polymerization of a styrene monomer.
It is preferable that the above-mentioned rubber polymer particles (b) consists essentially of a copolymer of the styrene monomer and the conjugated diene monomer.
It is preferable that the modified polystyrene resin particles (c1) are prepared by impregnating 40 to 10 parts by weight of the monomer mixture into 60 to 90 parts by weight of polystyrene polymer particle (a) and carrying out copolymerization; said monomer mixture comprisin
Maruhashi Shotaro
Oohara Hidekazu
Ueda Yuichi
Wakishige Katsuhiko
Foelak Morton
Kaneka Corporation
Leydig , Voit & Mayer, Ltd.
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