Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1998-09-11
2001-05-15
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
Reexamination Certificate
active
06232358
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an expandable rubber-modified styrene resin composition to obtain a molded foam having an excellent impact resistance and pliability as well as a high expansion ratio and also relates to a molded foam obtained by said expandable rubber-modified styrene resin composition.
2. Description of the Related Art
A molded foam made from a polystyrene resin is employed widely as a container and a heat insulating material for its excellent shock-absorbing properties and heat-insulating properties as well as easiness in molding into an article. However, because of its insufficient impact resistance and pliability which may cause formation of cracks and breaks, it involves problems such as difficulty in applying to a package for a precision instrument.
On the other hand, a molded foam made from a polypropylene resin requires a large-scaled device for molding although it has excellent impact resistance and pliability. Further, its resin characteristics also requires a transportation from a manufacturer to a molding company in a form of pre-expanded beads. As a result, a production cost becomes disadvantageously high.
Recently, a rubber-modified styrene resin molded foam which is easy to be molded and has improved impact resistance and pliability compared with a polystyrene foam has been proposed in Japanese Laid-Open Patent Publication Nos.3-182529, 5-116227, 7-11043 and 7-90105.
Nevertheless, the impact resistance and the pliability of a conventional molded foam is improved only to a limited and insufficient degree, and attempts to achieve a high expansion ratio result in obtaining a molded foam having a poor appearance due to shrinkage and a reduced strength. Accordingly, it is impossible to use a molded foam as being foamed at a high expansion ratio, resulting in a limitation in package resource saving.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an expandable rubber-modified styrene resin composition that is enabled to obtain a molded foam having a high expansion ratio, excellent impact resistance and pliability and satisfactory appearance.
The present invention is an expandable rubber-modified styrene resin composition comprising 85 to 99% by weight of a rubber-modified styrene resin containing a rubber particle of a diene polymer dispersed in a continuous phase of a styrene resin and 1 to 15% by weight of a volatile blowing agent whose boiling point is 80° C. or lower, wherein:
the weight-average molecular weight of said continuous phase of the styrene resin is 150,000 to 300,000;
the graft ratio of the styrene resin to the diene polymer is 70 to 135%; and,
the swelling index of the gel fraction of said diene polymer in toluene at 250C. is 12 to 25.
According to the present invention, an expandable rubber-modified styrene resin composition that is enabled to obtain a molded foam having a high expansion ratio, excellent impact resistance and pliability and satisfactory appearance can be provided.
DETAILED DESCRIPTION OF THE INVENTION
The styrene resin described above can for example be produced by dissolving a diene polymer in an aromatic vinyl compound such as styrene, p-methylstyrene and &agr;-methylstyrene followed by effecting radical polymerization in the presence of an azo compound such as azobisisobutyronitrile or a peroxide such as benzoyl peroxide and t-butylperoxybenzoate with employing bulk polymerization, solution polymerization, suspension polymerization, bulk-suspension polymerization or the like.
The weight-average molecular weight of the styrene resin described above is 150,000 to 300,000. When the weight-average molecular weight is less than 150,000, strength of a molded foam may become lower, while that exceeding 300,000 causes difficulty in producing a molded foam. The weight-average molecular weight is preferably 180,000 to 250,000.
The diene polymer described above can be a polymer of a diene compound such as butadiene and isoprene or a copolymer with an aromatic vinyl compound such as styrene capable of being copolymerized with a diene compound. Preferably, a butadiene polymer is employed.
Preferably, the rubber-modified styrene resin described above preferably contains 5 to 20% by weight of a diene polymer.
A content less than 5% by weight may cause difficulty in obtaining a molded foam having satisfactory impact resistance and pliability. On the other hand, a content exceeding 20% by weight may not provide an increase in strength corresponding to an increase in the amount and results in adverse effects on moldability such as melting of the surface of the molded foam.
More preferably, the content is in a range of 7 to 15% by weight.
The graft ratio of a styrene resin to a diene polymer is in a range of 70 to 135%. A graft ratio less than 70% may not provide a molded foam having sufficient impact resistance and pliability. On the other hand, a graft ratio exceeding 135% may not cause much orientation of the rubber particle in a foam membrane, resulting in difficulty in foaming, which may lead to shrinkage or deformation in a molded foam of a high expansion ratio. Preferably, the graft ratio is 80 to 130%. More preferably, the ratio is 100 to 120%.
The graft ratio described above is a value defined as follows.
[Graft ratio]=[Gel fraction-Diene polymer content]×100/[Diene polymer content]
The content described above is the weight (%) based on the rubber-modified styrene resin.
The swelling index of the gel fraction of the diene polymer in toluene at 25° C. is 12 to 25. A swelling rate of the gel fraction less than 12 may not cause much orientation of the rubber particle in a foam membrane, resulting in difficulty in foaming, which may lead to shrinkage or deformation in a molded foam of a high expansion ratio. On the other hand, a swelling index exceeding 25 does not allow a molded foam having sufficient impact resistance and pliability. Preferably, the swelling index is 13 to 20. More preferably, the swelling index is 15 to 20.
The swelling index of the gel fraction of a diene polymer is a value defined as follows.
[swelling index]=[Weight of gel swollen in toluene at 25° C.]/[Dry gel weight]
In an expandable rubber-modified styrene resin composition, 85 to 99% by weight of a rubber-modified styrene resin described above is contained.
A volatile blowing agent having a boiling point of 80° C. or lower is employed.
In an expandable rubber-modified styrene resin composition, the volatile blowing agent described above is contained in an amount of 1 to 15% by weight. A content less than 1% by weight results in an insufficient foaming capability, and accordingly a predetermined expansion ratio can not be achieved. On the other hand, a content exceeding 15% by weight involves a problem that shrinkage and deformation may occur when producing a molded foam.
The volatile blowing agent described above can be an organic compound having a boiling point of 80° C. or lower such as propane, butane, isobutane, pentane, isopentane, neopentane, cyclopentane, hexane, trichlorofluoromethane, dichlorofluoromethane, dichlorotetrafluoroethane, chloromethane, dichloromethane, methanol, diethylether and the like, which may be used singly or in combination.
Preferably, the expandable rubber-modified styrene resin composition described above contains a plasticizer for the purpose of imparting the molded foam with pliability.
Such a plasticizer can be esters such as dioctyl phthalate and dioctyl adipate, hydrocarbons such as toluene, xylene and cyclohexane, fatty esters of polyhydric alcohols such as glycerin tristearate and glycerin trioctoate as well as mineral oils, liquid paraffin and the like.
The expandable rubber-modified styrene resin composition described above may also contain one or more additives such as inorganic fillers, lubricants, flame retardants, antioxidants, antistatic agents, UV absorbers and carbon black.
Thus, an expandable rubber-modified styrene resin composition
Haraguchi Kenji
Kajihara Taketoshi
Michl Paul R.
Mitsubishi Chemical Foam Plastic Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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