Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-06-13
2002-05-14
Wilson, D. R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
active
06388019
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a biaxially stretched styrenic resin sheet having high transparency which can uniformly be coated with silicone oil and exhibits satisfactory blocking resistance when stacked either as sheeting or as formings.
BACKGROUND OF THE INVENTION
A biaxially stretched styrenic resin sheet is widely used in the field of food wrapping because of its transparency and nerve. However, it shows a strong blocking tendency, and formings obtained therefrom such as containers are hardly separated apart when stacked.
In order to overcome the blocking problem, it has been proposed to incorporate rubber particles into a styrenic resin as disclosed in JP-A-50-74649 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and to apply silicone oil on the surface of the stretched styrenic resin sheet as disclosed in JP-A-52-8080. The rubber particles added to the styrenic resin matrix are projected on the surface on biaxial stretching to improve blocking resistance of the stretched sheet. However, the rubber particles are flattened on biaxial stretching so that the proportion of the rubber projected on the surface is hardly constant. An increased amount of rubber incorporated to ensure satisfactory blocking resistance tends to cause reduction in transparency. In addition, with an increase of the rubber content, foreign matter may be incorporated into the resin composition because of scorching of rubber during long run operation or scorching of recycle rubber added, resulting in poor appearance of the sheet and causing a sheet break.
It was suggested to decrease the rubber content by increasing the size of the rubber particles to, e.g., about 6 to 10 &mgr;m, but this method is accompanied with surface roughening or gloss reduction of the resulting biaxially stretched sheet.
It has also been proposed to incorporate synthetic silicon dioxide (JP-B-6-856, the term “JP-B” as used herein means an “examined published Japanese patent appplication”) or a combination of silicon dioxide and styrene-grafted rubber (JP-B-6-855) to a styrenic resin so that the silicon dioxide may be projected on the surface of the sheet on biaxial stretching thereby to improve blocking resistance of the sheet. In these systems, however, the biaxially stretched sheet suffers from reductions in transparency and gloss because of a difference in a refractive index between silicon dioxide (refractive index: 1.3 to 1.45) and the styrenic resin matrix (refractive index: 1.55 to 1.59). Further, because silicon dioxide has poor wettability with the styrenic resin matrix, it falls off the matrix. Therefore, the resulting sheet has reduced physical properties and is of no practical use.
JP-B-47-2180 discloses a process of preparing a high molecular film (a polymer film) containing a fine particle having at least one functional group selected from an amino group, a hydroxyl group, a carboxyl group, an epoxy group and a maleic anhydride unit, but its application to a biaxially stretched styrenic resin sheet would not provide an article having an excellent transparency.
JP-B-55-7471 discloses a resin-composition having a fine particle polymer dispersed therein which provides an opaque, translucent, or delustered sheet but does not provide a biaxially stretched transparent styrenic resin sheet as disclosed in the present invention.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a biaxially stretched styrenic resin sheet which has improved silicone oil coating properties, and does not suffer from deterioration of appearance, and breaks of the sheet.
It is another object of the present invention to provide a biaxially stretched styrenic resin sheet which insures enhanced blocking resistance while inhibiting deterioration of transparency of the sheet, so that even when a pile of the sheets is blanked, each sheet can easily be separated apart.
A further object of the present invention is to provide a process for manufacturing a biaxially stretched styrenic resin sheet which has such meritorious characteristics as mentioned above, in a simple and easy manner.
To accomplish these objects, the inventors of the present invention did much investigation, and have found that a resin composition comprising a styrenic resin as a matrix and fine particles of a crosslinked styrene-based resin whose refractive index is close to that of the styrenic resin matrix provides, upon being biaxially stretched, a biaxially stretched styrenic resin sheet having fine unevenness uniformly on the surface thereof. The crosslinked fine particles are not flattened on stretching and form fine projections on the surface of the stretched sheet. As a result, the biaxially stretched styrenic resin sheet has improved silicone oil coating properties, does not suffer from deterioration of appearance, breaks which tend to occur due to silicon dioxide added or scorch of the rubber added, and reduction of transparency, and exhibits improved blocking resistance so that even when a pile of a plurality of the sheets is blanked each sheet can easily be separated apart.
The biaxially stretched styrenic resin sheet of the present invention comprises a resin composition comprising (A) a styrenic resin and (B) crosslinked styrenic fine particles having an average particle size of 0.5 to 15 &mgr;m in an amount of from 0.001 to 1% by weight (10 to 10,000 ppm) based on the resulting resin composition (a first embodiment of the invention).
In the biaxially stretched styrenic resin sheet of the present invention, the resin composition may further comprise (C) a rubber modified styrenic resin containing rubber component particles (a second embodiment of the invention). The rubber component particles may, for example, have an average particle size of about 0.1 to 4 &mgr;m. The rubber modified styrenic resin is added such that the particulate rubber component concentration is from 0.005 to 0.1% by weight (50 to 1000 ppm) based on the total weight of the resulting resin composition. In this embodiment, the crosslinked styrenic fine particles may be added in an amount of about 0.001 to 1% by weight (10 to 10,000 ppm), preferably about 0.002 to 0.1% by weight (20 to 1,000 ppm) based on the resin composition.
The present invention further relates to a styrenic resin composition for the biaxially stretched sheet, a process for producing the biaxially stretched styrenic resin sheet, and a molded container as produced with the use of the biaxially stretched sheet.
DETAILED DESCRIPTION OF THE INVENTION
Component (A)
The amount of the styrenic resin is usually from about 90 to about 99.999% by weight (e.g., about 90 to about 99.9% by weight) based on the total weight of the resin composition.
The styrenic resin that can be used as a matrix in the present invention comprises at least one monomer unit derived from styrene monomers, such as styrene, an alkylstyrene (e.g., o-, m- or p-methylstyrene, p-ethylstyrene, p-isopropylstyrene, butylstyrene or p-t-butylstyrene); an &agr;-alkylstyrene (e.g., &agr;-methylstyrene or &agr;-ethylstyrene); a halostyrene (e.g., o-, m- or p-chlorostyrene, bromostyrene or fluorostyrene); a di-, tri-, tetra- or pentasubstituted chlorostyrene, bromostyrene or fluorostyrene; and an &agr;- or &bgr;-halostyrene (e.g., &agr;-chlorostyrene, &agr;-bromostyrene, &bgr;-chlorostyrene or &bgr;-bromostyrene).
The styrenic resin may further comprise other monomers copolymerizable with the styrene monomer, such as acrylonitrile, methyl methacrylate and maleic anhydride.
These styrenic resins can be employed singly or in combination.
A preferred styrenic resin includes a resin having high transparency, inclusive of polystyrene, a styrene-vinyltoluene copolymer, a styrene-&agr;-methylstyrene copolymer, a styrene-methyl (meth)acrylate copolymer. Polystyrene can advantageously be used among others. Incidentally, the styrenic resin has a visible light transmittance of about 70 to 95%, preferably about 80 to 95%.
Methods for preparing the styrenic resin are not particularly limite
Yamamoto Syozo
Yanai Hiroyuki
Yoshioka Shuji
Daicel Chemical Industries Ltd.
Sughrue & Mion, PLLC
Wilson D. R.
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