Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
1998-09-04
2001-04-24
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S327000, C428S519000, C428S521000, C428S523000, C428S910000, C525S086000, C525S316000
Reexamination Certificate
active
06221471
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to blends of rubber modified monovinylidene aromatic polymers.
Rubber modified monovinylidene aromatic polymers, such as high impact polystyrene (HIPS), have found numerous applications which require high impact strength. One application wherein HIPS is typically used is food packaging products which specifically require good impact strength and transparent properties. Although toughness is achieved by HIPS resins, the transparency of thermoformed articles, such as thermoformed cups, is achieved using an expensive HIPS product having rubber particles with a specific capsule morphology.
Many attempts have been made to acquire the necessary balance of toughness and transparency in thermoformed articles using various materials and techniques. Transparent resins, such as polystyrene, polyethylene terephthalate and polypropylene have been used to produce transparent packaging via in-line thermoforming. However, in-line thermoforming processes require oversize extruders which run slower and are therefore less productive than conventional lines. Additionally, these polymers do not have the impact properties that rubber modified monovinylidene aromatic polymers can achieve. Alternatively, transparent packaging has been prepared by extruding and thermoforming blends of butadiene-styrene copolymer or styrene-butadiene-styrene copolymer with polystyrene. However, the copolymers are expensive and increase cost. Transparent packaging has also been made using optical grade general purpose polystyrene which is produced using tenter frames. However, such products can only be obtained to a maximum sheet thickness of 0.6 mm due to process constraints, which is not thick enough for some thermoforming applications. Polyvinylchloride has also been used to produce transparent packaging, but is thermally unstable and undesirable for food packaging.
Therefore, there remains a need for polymers which can produce cost effective transparent packaging, which can be used in food packaging and non-food packaging markets, and can be produced by conventional thermoforming lines.
SUMMARY OF THE INVENTION
The present invention is a blend of a monovinylidene aromatic polymer and a rubber modified monovinylidene aromatic polymer, which can be used to produce cost effective, transparent packaging that can be used in food packaging and non-food packaging markets, and can be produced using conventional thermoforming equipment. The blend comprises:
a) from 50 to 88 wt. percent based on the total weight of the blend, of a monovinylidene aromatic polymer having a weight average molecular weight (Mw) of from 265,000 to 400,000 and a melt flow of less than 2.5 g/10 min.; and
b) from 12 to 50 wt. percent based on the total weight of the blend, of a rubber modified monovinylidene aromatic polymer comprising a monovinylidene aromatic polymer matrix having rubber particles dispersed therein, wherein the rubber particles comprise, based on the total weight of the rubber particles, from 25 to 100 weight percent rubber particles having a capsule morphology and a volume average particle size of from 0.1 to 0.4 microns; and from 75 to 0 weight percent rubber particles having an entanglement morphology having a volume average particle size of from 0.25 to 1 micron.
This blend can be used to produce a monolayer sheet or film, or as a core layer in a multilayer sheet or film. These blends offer significant impact resistance properties and economic benefit as well by using general monovinylidene aromatic polymers to partially replace the expensive rubber modified polymers which are typically used in such applications.
Another aspect of the present invention is a multilayer sheet or film comprising at least one layer of the blend of the present invention.
The blend of the present invention can be used to produce mono- or multilayer sheet or film using conventional extrusion, blow molding or blown film techniques. The sheet or film can be used to prepare transparent packaging materials which are cost effective and have good impact properties.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In one embodiment, the present invention is a blend of a monovinylidene aromatic polymer and a rubber modified monovinylidene aromatic polymer.
Monovinylidene aromatic polymers are produced by polymerizing vinyl aromatic monomers such as those described in U.S. Pat. Nos. 4,666,987, 4,572,819 and 4,585,825, which are herein incorporated by reference. Preferably, the vinyl aromatic monomer is of the formula:
wherein R′ is hydrogen or methyl, Ar is an aromatic ring structure having from 1 to 3 aromatic rings with or without alkyl, halo, or haloalkyl substitution, wherein any alkyl group contains 1 to 6 carbon atoms and haloalkyl refers to a halo substituted alkyl group. Preferably, Ar is phenyl or alkylphenyl, wherein alkylphenyl refers to an alkyl substituted phenyl group, with phenyl being most preferred. Typical vinyl aromatic monomers which can be used include: styrene, alpha-methylstyrene, all isomers of vinyl toluene, especially paravinyltoluene, all isomers of ethyl styrene, propyl styrene, vinyl biphenyl, vinyl naphthalene, vinyl anthracene and the like, and mixtures thereof.
The monovinylidene aromatic polymers used in the blend of the present invention has a typical Mw of from 265,000 to 400,000 and a melt flow rate of less than 2.5 g/10 min. Typically the Mw is from 265,000, preferably from 270,000, more preferably from 275,000 and most preferably from 280,000 to 400,000, preferably to 375,000, more preferably to 350,000 and most preferably to 305,000. The melt flow rate is typically less than 2.5, preferably less than 2.3, more preferably less than 2.1, and most preferably less than 1.8 g/10 min.
The blend of the present invention also comprises a rubber modified monovinylidene aromatic polymer. Rubber modified monovinylidene aromatic polymers are produced by polymerizing vinyl aromatic monomers in the presence of a predissolved elastomer, examples of which are described in U.S. Pat. Nos. 3,123,655, 3,346,520, 3,639,522, and 4,409,369, which are incorporated by reference herein. In particular, a preferred rubber modified monovinylidene aromatic polymer used in the blends of the present invention and the method for making, is disclosed in U.S. Pat. No. 4,428,106 and U.S. Pat. No. 5,491,195, which are incorporated herein by reference.
Typically, the rubber modified monovinylidene aromatic polymer comprises from about 92 to about 55 weight percent based on the total weight of the polymer composition, of a monovinylidene aromatic polymer matrix and, dispersed within said matrix, from about 8 to about 45 weight percent of grafted and occluded 1,3-alkadiene-based rubber particles, said particles being composed, on a rubber particle weight basis, of:
from 25 to about 100 weight percent of rubber particles having a capsule morphology and a volume average particle size of from 0.1 to 0.4 micron.
Typically, the free (i.e., ungrafted) monovinylidene aromatic polymer matrix formed during polymerization will have a weight averaged molecular weight (Mw) of from about 130,000 to about 250,000 (preferably from about 150,000 to about 25 220,000); a number averaged molecular weight (Mn) from about 40,000 to about 100,000 (preferably from about 45,000 to about 75,000); and Mw:Mn ratio of from about 2 to about 5 (preferably from about 2.2 to about 3.5).
Rubber materials which are suitable for use in the rubber modified monovinylidene aromatic polymers of b) include rubbery 1,3-alkadiene polymers having a second order transition temperature of 0° C., or less, preferably −20° C. or less. Suitable rubbers include 1,3-alkadiene homopolymers and copolymers containing, in polymerized form and on a elastomeric polymer weight basis, from 60 to 100 percent of a 1,3-alkadiene monomer, such as butadiene, isoprene, and from 0 to 40 weight percent of one or more monoethylenically unsaturated comonomers such as styrene, acrylonitrile, alpha-methylstyrene, methacrylonitrile, methyl methacrylate, ethyl acrylate
Salmang Rodolfo J.
Sugden John L.
The Dow Chemical Company
Thibodeau Paul
Zacharia Ramsey
LandOfFree
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