WO2000043202A1

WO2000043202A1 - Coextruded sheets used in heat sealed thermoformed articles

Info

Publication number: WO2000043202A1
Application number: PCT/US1999/027964
Authority: WO
Inventors: Roldolfo J. Salmang
Original assignee: The Dow Chemical Company
Priority date: 1999-01-21
Filing date: 1999-11-24
Publication date: 2000-07-27
Also published as: CO5111027A1; AU1633800A; AR022155A1

Abstract

In one aspect, the present invention is a multilayer coextruded sheet which comprises: a) a sheet layer comprising: I) from 40 to 60 weight percent of an olefin polymer; II) from 25 to 45 weight percent of an impact modified monovinylidene aromatic polymer, comprising from 1 to 25 weight percent of a rubber and 75 to 99 weight percent of a monovinylidene aromatic polymer matrix having a molecular weight (Mw) from 50,000 to 400,000, said weight percents being based on the total weight of said impact modified, vinylaromatic polymer; III) from 8 to 25 weight percent of a compatibilizing polymer, which acts to increase interfacial adhesion between components I) and II); and b) a sheet layer comprising a monovinylidene aromatic polymer.

Description

COEXTRUDED SHEETS USED IN HEAT SEALED THER OFORMED ARTICLES The present invention relates to coextruded sheets of monovinylidene aromatic polymers.

There continues to be a need in the food packaging industry for a sealable, peelable lid/container system, wherein the lid peels off easily, while retaining its physical integrity. There are many food packaging applications for plastic containers comprising a monovinylidene aromatic polymer composition, which are heat sealed with a special thin film material. For example, the dairy industry commonly uses aluminum foil with a hot melt seal lacquer as a lid or sealing structure for many types of plastic containers, such as cups for yogurt and other dairy products. Aluminum is resistant to high temperatures, allowing for short cycle times, however aluminum has low tear resistance, thereby resulting in tearing of the lid during peeling. Other food packaging systems have utilized lids of polyvinyl chloride (PVC) or polyesters, such as biaxially oriented polyethylene terephthalate (PET). These lids typically employ hot melt adhesive coatings to ensure adhesion to the container. Another type of system utilizes a lid which comprises one or more polystyrene-compatible resin layers coextruded with one or more polyester top or substrate layers to improve the strength of the lid, and to prevent the hot-seal bar from sticking to the lid. A disadvantage of lids employing PVC or polyesters is that these systems contain polymers which are not fully compatible with the polystyrene commonly employed in the container to which the lid is attached. Therefore, recycling of dairy containers containing diverse polymeric materials requires a preliminary, expensive separation step.

Another attempt to solve this problem utilizes polyolefin films such as low density polyethylene. Such polyolefins are desirable due to their transparency, food compatibility and low cost. Typically, special thermoformable sheets are used to produce containers which are more compatible with the desired polyethylene film lids. Such sheets have been produced from compositions such as polystyrene coextruded with ethylene-vinyl acetate and polyethylene. However, the regrind of this composition is not fully or totally compatible for recyclability into any of the coextruded layers without affecting or changing their properties.

It would be desirable to have a food packaging system having an easily sealed plastic lid for thermoformed food containers that peels off easily, while retaining its physical integrity and regrind compatibility. In one aspect, the present invention is a multilayer coextruded sheet which comprises: a) a sheet layer comprising:

I) from 40 to 60 weight percent of an olefin polymer,

II) from 25 to 45 weight percent of an impact modified monovinylidene aromatic polymer, comprising from 1 to 25 weight percent of a rubber and 75 to 99 weight percent of a monovinylidene aromatic polymer matrix having a molecular weight (Mw) from 50,000 to 400,000, said weight percents being based on the total weight of said impact modified, vinylaromatic polymer;

III) from 8 to 25 weight percent of a compatibilizing polymer, which acts to increase interfacial adhesion between components I) and II); and b) a sheet layer comprising: a monovinylidene aromatic polymer.

In another aspect, the present invention is a multilayer coextruded sheet which comprises: a) a sheet layer comprising: I) from 40 to 60 weight percent of an olefin polymer,

III) from 8 to 25 weight percent of a compatibilizing polymer, which acts to increase interfacial adhesion between components I) and II); and b) a sheet layer comprising: a monovinylidene aromatic homopolymer, wherein the sheet layer of a) is from 3 to 20 percent of the total thickness of the sheet and the sheet layer of b) is from 80 to 97 percent of the total thickness of the sheet. In yet another aspect, the present invention is a multilayer coextruded sheet which comprises: a) a sheet layer comprising: I) from 40 to 60 weight percent of an olefin polymer,

II) from 25 to 45 weight percent of an impact modified monovinylidene aromatic polymer, comprising from 1 to 25 weight percent of a rubber and 75 to 99 weight percent of a monovinylidene aromatic polymer matrix having a molecular weight (Mw) from 50,000 to 400,000, said weight percents being based on the total weight of said impact modified, vinylaromatic polymer; III) from 8 to 25 weight percent of a compatibilizing polymer, which acts to increase interfacial adhesion between components I) and II); and b) a sheet layer comprising: an impact modified monovinylidene aromatic polymer having a Mw of from 200,000 to 230,000 and a polydispersity of from 2.0 to 2.7. wherein the sheet layer of a) is from 3 to 20 percent of the total thickness of the sheet and the sheet layer of b) is from 80 to 97 percent of the total thickness of the sheet.

In another aspect, the present invention is a multilayer coextruded sheet which comprises: a) a sheet layer comprising:

I) from 40 to 60 weight percent of an olefin polymer,

III) from 8 to 25 weight percent of a compatibilizing polymer, which acts to increase interfacial adhesion between components I) and II); and b) a sheet layer comprising: a monovinylidene aromatic polymer foam, wherein the sheet layer of a) is from 1 to 5 percent of the total thickness of the sheet and the sheet layer of b) is from 95 to 99 percent of the total thickness of the sheet.

In another aspect, the present invention is a thermoformed article made from the coextruded sheets described above. In another aspect, the present invention is a food packaging system comprising a thermoformed article produced from the coextruded sheets described above and a polyolefin or polyolefin coated lid.

These packaging systems offer excellent peelable, sealable behavior, have improved environmental stress crack resistance and can be used in recycling efforts without costly separation procedures.

According to the present invention there is provided thermoformable coextruded sheet produced from polymer blends having good impact properties and peelable, sealable behavior. The coextruded sheet is typically produced from at least two polymer compositions. The first polymer composition comprises a rubber modified monovinylidene polymer, an olefin polymer and a compatibilizerfor said monovinylidene and olefin polymers.

Suitable impact modified, monovinylidene aromatic polymers include rubber modified homopolymers of C₆-2o monovinylidene aromatic monomers, copolymers of two or more such monomers and copolymers of one or more such monomers with up to 25 weight percent of a copolymerizable comonomer other than a monovinylidene aromatic monomer. Vinyl aromatic monomers useful for producing the monovinylidene aromatic polymers used in the present invention include, but are not limited to those described in US-A-4,666,987, US-A-4,572,819 and US-A-4,585,825.

Preferably, the monomer is of the formula:

R' I Ar— C=CH₂ wherein R is hydrogen or methyl, Ar is an aromatic ring structure having from 1 to 3 aromatic rings with or without alkyl, or halo substitution, wherein any alkyl group contains 1 to 6 carbon atoms. 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, and vinyl anthracene, and mixtures thereof. The vinyl aromatic monomers may also be combined with other copolymerizable monomers. Examples of such monomers include, but are not limited to alpha-methylstyrene, all isomers of vinyl toluene, especially paravinyltoluene, all isomers of ethyl styrene, t-butylstyrene, n-propyl styrene, vinyl biphenyl, vinyl naphthalene, and vinyl anthracene. The monovinylidene aromatic polymer is preferably polystyrene.

Suitable rubbers used to modify the impact properties of the foregoing monovinylidene aromatic polymers are those having Tg less than 0°C, preferably less than -20°C. Examples of suitable rubbers are homopolymers of C .₆ conjugated dienes, especially butadiene or isoprene; copolymers of one or more monovinylidene aromatic monomers and one or more C .₆ conjugated dienes, especially butadiene or isoprene; copolymers of ethylene and propylene or ethylene, propylene and a nonconjugated diene, especially 1 ,6-hexadiene or ethylidene norbomene; homopolymers of C₄.₆ alkyl acrylates; copolymers of C₄.₆ alkyl acrylates and a copolymerizable comonomer, especially a monovinylidene aromatic monomer or a Cι- alkyl methacrylate. Also included are graft polymers of the foregoing rubbery polymers wherein the graft polymer is a monovinylidene aromatic polymer. A preferred monovinylidene aromatic monomer for use in all of the foregoing rubbery polymers is styrene. A most preferred rubbery polymer is polybutadiene or a styrene/butadiene copolymer. The above rubbers may be prepared by anionic solution polymerization techniques or by free radically initiated solution, mass, emulsion or suspension processes. The rubbery elastomers prepared by emulsion polymerization may be agglomerated to produce larger particles having a bimodal or trimodal, etc. particle size distribution if desired.

Impact modified monovinylidene aromatic polymers are well known in the art and are commercially available. Suitable impact modified polymers are prepared by blending the rubbery polymer with previously prepared matrix polymer having the desired chemical composition, by graft polymerizing the matrix in the presence of a predissolved rubber, or by a combination of such techniques.

Preferred impact modified monovinylidene aromatic polymers are prepared by dissolving the rubber in the monomer(s), optionally in the presence of a solvent or diluent and polymerizing the resulting solution, desirably while agitating the solution so as to prepare a dispersed, grafted, impact modified polymer, having rubbery domains containing occlusions of matrix polymer dispersed throughout the resulting polymeric matrix. Such rubber modified polymers, known as mass or solution polymerized, high impact polymers, are previously well known in the art and are commercially available. Additional quantities of rubbery polymer, especially emulsion grafted rubbery polymers may be blended into the impact modified monovinylidene aromatic polymer if desired.

A highly preferred monovinylidene aromatic monomer used in producing the impact modified polymer is styrene. A very highly preferred high impact polystyrene is prepared by the solution or mass polymerization technique and contains from 5 to 15 (more preferably from 6 to 9) weight percent polybutadiene rubber. Most highly preferred high impact polystyrenes are those wherein the polystyrene matrix has a molecular weight from 60,000 to 225,000 (preferably from 100,000 to 225,000 and more preferably from 150,000 to 225,000). When measuring such molecular weights, the technique employed is that of gel permeation chromatography employing a polystyrene standard.

In another embodiment, it can be particularly advantageous to employ high impact polystyrene (HIPS) of a sort which has a broadly distributed range of different sized polystyrene-grafted rubber particles dispersed therein. Such material may, for example, have a very broad or relatively flat rubber particle size distribution wherein the dispersed rubber particles individually range in size from 0.1 micron to 8 or 10 micron or more and wherein the total amount by weight of rubber contained within such HIPS material is more or less evenly (or randomly) distributed throughout the entire size range indicated. On the other hand, said broad particle size distribution may instead take the form of either a broad monomodal distribution or a multi-modal (for example, bimodal, trimodal, etc.) distribution within the above-noted broad size distribution range.

As is well known, in the case of a typical monomodal distribution a predominant portion by weight or volume of the dispersed rubber particles are located or concentrated at or around a particular peak size within the above-specified range and the weight or volume percentage of particles contained in the other portions of the entire size range incrementally diminish as one proceeds from said peak particle size toward either end of the overall size range.

As is also well known, multi-modal particle size distributions are typified by two or more distinct peaks being discernible in the overall particle size distribution (that is, when the entire particle population is plotted either by number fraction, weight fraction or volume fraction as a function of size over the entire size range in question).

One particular type or category of HIPS resin which can be used herein, are HIPS resins which have a bimodal particle size distribution wherein the majority (for example, from 55 to 95, preferably from 70 to 90 and more preferably from 80 to 90 weight percent) of the dispersed rubber particles have a cellular occlusion morphology and in combination have a volume average particle size (and form a volume-based particle size distribution peak) in the range of from 1.0 to 2.5 micron and wherein a minority of the dispersed rubber particles (for example, from 5 to 45, preferably from 10 to 30 and more preferably from 10 to 20 weight percent) have a substantially solid non-occluded morphology and form a particle size distribution peak or mode in the range of from 3 to 8 micron. Exemplary of this type of high impact polystyrene resin is Styron™ 484 which is available from The Dow Chemical Company.

Another specific category of HIPS resin which can be used herein, is a resin which has a bimodal particle size distribution, but which has a substantially smaller overall average particle size. In this latter type of HIPS resin, a predominant portion by weight (for example, from 65 to 99, preferably from 70 to 95 and more preferably from 80 to 95 weight percent) of the dispersed rubber particles individually have a particle size in the range of from 0.1 to 0.8 (preferably from 0.2 to 0.6 and more preferably from 0.2 to 0.4) micron and collectively form a volume fraction-based particle size distribution peak within the above-stated size range. The remaining portion (for example, from 1 to 35, preferably from 5 to 30 and more preferably from 5 to 20 weight percent) of the dispersed rubber particles in such HIPS resin individually have particle sizes which are 1 micron or more (but typically less than 10 micron) and collectively form a volume fraction-based size distribution peak in the 1.1 to 8 (preferably 1.2 to 3 and more preferably 1.2 to 2.5) micron size range.

The relatively smaller sized dispersed rubber particles of this latter HIPS resin can have either a single occlusion (that is, polystyrene core and rubber shell) type of particle morphology or can instead be of the multiple occlusion (for example, cellular) variety. However, it will typically be of the single occlusion type, particularly in those instances wherein the volume average size of the indicated small particle fraction is in the 0.1 to 0.6 (especially 0.2 to 0.5) micron size range. On the other hand, the relatively larger size portion of the particular HIPS resin's dispersed rubber particles will typically be of the multiple occlusion/cellular variety. One exemplary HIPS resin of this latter type is available from The Dow Chemical Company as XU70007.

In another embodiment of the present invention, both of the above-described types of bimodal HIPS resins are employed in combination with each other to provide what is essentially a trimodal HIPS formulation. When so employed, the resulting formulation can be generally characterized as having a substantial population (for example, from 15 to 92, preferably 25 to 85, more preferably from 30 to 80 and most preferably from 50 to 70 weight percent on a total rubber content weight basis) of relatively small sized (for example, 0.1 to 0.8 or 1 , preferably 0.2 to 0.6 and more preferably 0.2 to 0.4 micron) grafted rubber particles (preferably having a single occlusion structure) in combination with (a) from 5 to 80 (preferably 10 to 70, more preferably from 10 to 60 and most preferably from 20 to 45) weight percent (on a rubber weight basis) of medium sized (for example, from 1.2 to 3 and preferably from 1.2 to 2.5 micron) particles, typically having a cellular morphology, and (b) from 2 to 20 (preferably from 4 to 15 and more preferably from 5 to 10) weight percent of large sized particles in the greater than 3 up to 10 (especially the 4 to 8) micron size range. When such trimodal HIPS resins are prepared by blending the above-described separate bimodal HIPS resins, the indicated very large grafted rubber particle component will have a relatively dense, non-occluded rubber morphology of the sort which has been noted above. However, as will be readily apparent to those skilled in this art, such large particle grafted rubber component can alternatively be prepared by known, conventional means so as to have a highly occluded cellular morphology or structure.

In those instances wherein it is desired to employ a trimodal HIPS resin ingredient and wherein it is desired to obtain such ingredient by blending or compounding separately prepared bimodal HIPS resins of the types described above, the individual small size/medium size and medium size/large size bimodal HIPS blendstock resins can typically be blended in a 10:90 to 90:10 weight ratio relative to each other but will preferably be blended in a 20:80 to 80:20 (more preferably 25:75 to 75:25 and most preferably 35:65 to 65:35) weight ratio. Suitable olefin polymers for use herein include, but are not limited to, enhanced polyethylene resins prepared using metallocene catalysts such as ELITE™ resins, high and low density polyethylenes as well as linear low density polyethylene, for example, copolymers of ethylene and one or more C .₈ α-olefins. A preferred olefin polymer is high density polyethylene having a density from .945 to .970, more preferably from .955 to .965. Such high density polyethylene resins are preferred due to the stiffness and chemical resistance which they impart to the final, thermoformable, resin blend. Enhanced polyethylene resins are also preferred for their improved sealability due to lower heat seal initiation temperature. It is preferred that the olefin polymer have a melt viscosity, which is matched, or nearly matched to that of the monovinylidene aromatic resin, thereby enabling the resulting blend to achieve thorough melt mixing due to high shear stresses between the components and appropriate phase domain size reduction. Preferred are the use of monovinylidene aromatic polymers and olefin polymers having viscosities at the temperature of blending wherein the ratio of VvANop is from 1 :10 to 1 :0.05, more preferably from 1 :2.0 to 1 :0.1. In the foregoing formula VV_A is the vinylidene aromatic polymer melt viscosity and V₀p is the olefin polymer melt viscosity. Such melt viscosities are measured by dynamic mechanical spectroscopy at a shear rate of 0.1 sec^"1.

In a further embodiment it is desirable that the crystalline melting point of the olefin polymer be less than the thermoforming temperature of the monovinylidene aromatic polymer. Monovinylidene aromatic polymers are known to be highly amenable to thermoforming due to the fact that such polymers exhibit a melt rheology over a range of temperatures (referred to as the rubbery plateau or thermoforming range) such that the viscous and elastic properties of the melt are properly balanced for good thermoformability. Below such temperature the polymer melt has an excessive elastic modulus and the polymer retains a "memory" permitting excessive snap back after forming of the desired thermoformed shape. Above this temperature, the melt possesses insufficient elastic modulus, and parts experience shear thinning during thermoforming. Thus it is necessary that at the thermoforming temperature of the monovinylidene aromatic polymer, the olefin polymer no longer retains a crystalline structure or otherwise excessively affects thermoforming properties of the monovinylidene aromatic polymer. That is, the crystalline melting point, Tc, of the olefin polymer must be less than the thermoforming range of the monovinylidene aromatic polymer. Preferably the thermoforming range of the monovinylidene aromatic polymer and ultimately the polymer blend is from 130 to 200°C, more preferably from 135 to 190°C. Suitable olefin polymers for use herein are HDPE 12165 and ELITE™ 5400, available from DuPont Dow Elastomers and The Dow Chemical Company, respectively.

The impact modified monovinylidene aromatic monomer and the olefin polymer are typically from 68 to 96 percent by weight of the total blend, preferably from 70 to 95 weight percent, more preferably from 75 to 90 percent and most preferably from 80 to 90 percent.

These polymers are typically used at a polyolefin to impact modified monovinylidene aromatic polymer weight ratio of from 0.35 to 4.5, preferably from .5 to 3.0, more preferably from .8 to 2.5, and most preferably from 1.0 to 2.0. The compatibilizing polymer is a polymer, or mixture of polymers, having the ability to increase interfacial adhesion between the monovinylidene aromatic polymer and the olefinic polymer. As such, the compatibilizing polymer may be thought of as a polymeric surfactant, having a portion thereof that is compatible with the monovinylidene aromatic polymer and another portion that is compatible with the olefinic polymer. Suitable compatibilizing polymers are readily determined by preparing a blend of components I) and II) and comparing the physical properties, especially the impact resistance and ductility of such blend, with a similar blend containing the compatibilizing polymer. Satisfactory compatibilizing polymers produce an increase in both impact resistance and ductility. Preferably such increase in both properties is at least 10 percent, more preferably 20 percent.

Desirably, such polymers are elastomers, that is, polymers having a Tg less than 0 °C, preferably less than -20 °C, and having a molecular weight from 10,000 to 150,000, more preferably from 20,000 to 100,000, and most preferably from 50,000 to 100,000 as determined by gel permeation chromatography using a polystyrene standard. Preferred compatibilizing polymers are elastomeric polymers containing a monovinylidene aromatic monomer and a C₂-ιβ -olefin or conjugated or nonconjugated diolefin. Especially preferred are thermoplastic, elastomeric block copolymers of one or more monovinylidene aromatic monomers and one or more C .₆ conjugated dienes. Such block copolymers include diblock, triblock, multiblock and radial block copolymers whether tapered, partially tapered (that is, tapered between less than all blocks) or hydrogenated, and mixtures of the foregoing. A most preferred compatibilizing polymer is a triblock copolymer or hydrogenated triblock copolymer of the monovinylidene aromatic monomer or monomers employed in component I), and either butadiene, isoprene or a mixture thereof. Thus for use in high impact polystyrene containing blends, the preferred compatibilizer is a styrene/butadiene or styrene/isoprene triblock copolymer, containing 25-45 (preferably 30 to 45, more preferably 35 to 45 and most preferably 40 to 45) weight percent styrene. One such block copolymer for use herein is VECTOR™ 4411 , available from Dexco Polymers. A preferred block copolymer for use herein is a styrene/isoprene/styrene triblock copolymer which contains from 42 to 44 weight percent styrene and 56 to 58 weight percent isoprene and which has a weight averaged molecular weight (Mw) of 89,000 and a number average molecular weight (Mn) of 86,000. These and other block copolymers suitable for use herein will typically have a fairly narrow molecular weight distribution, with the Mw:Mn ratio thereof typically being in the range of from 1.0 to 1 .3 (preferably from 1.0 to 1.2 and more preferably from 1.0 to 1.1 ).

The amount of compatibilizing polymer used in the present invention is typically within a polyolefin to compatibilizing polymer ratio of 3/1 to 8/1 by weight, preferably from 3.5/1 to 6/1 , more preferably from 4/1 to 6/1 , and most preferably from 4.5/1 to 5.5/1 .

The first polymer composition of a) may also contain other additives such as antioxidants, and mold release agents

The second polymer composition b)comprises a monovinylidene aromatic polymer. The monovinylidene aromatic polymer may be in the form of a homopolymer, or a rubber modified polymer. Monovinylidene aromatic homopolymers typically have a weight average molecular weight of 270,000 to 320,000. The rubber modified monovinylidene aromatic polymer used in the second polymer composition is preferably a rubber modified polymer as described previously in the first polymer composition a). The rubber modified monovinylidene aromatic polymer of the second polymer b) composition typically has a weight average molecular weight (Mw) of from 180,000 to 250,000, preferably from 200,000 to 230,000 and most preferably from 210,000 to 220,000. This polymer also has a typical polydispersity (Mw/Mn) of from 1.8 to 3.0, preferably from 2.0 to 2.7 and most preferably form 2.2 to 2.5. The polymer typically contains rubber particles having a volume average particle size of from 3 to 7 microns, preferably from 4 to 6 microns, wherein the rubber particles have a cellular or multiple occlusion morphology. The monovinylidene aromatic polymer (homopolymer or rubber modified) of b) can also be in the form of a polymer foam.

Methods of producing monovinylidene aromatic homopolymers, rubber modified polymers and foams thereof, are well known in the art.

When a foam is used as the layer of b), the thickness of b) will typically be from 95 to 99 percent of the total thickness of the multilayer extruded sheet. In a preferred embodiment the multilayer coextruded sheet consists essentially of a layer of a) consisting essentially of: I) from 40 to 60 weight percent of an olefin polymer,

II) from 25 to 45 weight percent of an impact modified monovinylidene aromatic polymer, comprising from 1 to 25 weight percent of a rubber and 75 to 99 weight percent of a monovinylidene aromatic polymer matrix having a molecular weight (Mw) from 50,000 to 400,000, said weight percents being based on the total weight of said impact modified, vinylaromatic polymer; and

III) from 8 to 25 weight percent of a compatibilizing polymer, which acts to increase interfacial adhesion between components I) and II); and a sheet layer of b) consisting essentially of: an impact modified monovinylidene aromatic polymer having a Mw of from 200,000 to 230,000 and a polydispersity of from 2.0 to 2.7. wherein the sheet layer of a) is from 3 to 20 percent of the total thickness of the sheet and the sheet layer of b) is from 80 to 97 percent of the total thickness of the sheet. In one aspect of the present invention, the two polymer compositions described previously are coextruded to produce multilayer thermoformable sheets. In a preferred embodiment, the multilayer sheet consists essentially of a layer of the first polymer composition comprising from 3 to 20 percent of the total sheet thickness, and a layer of the second polymer composition comprising from 80 to 97 percent of the total sheet thickness.

The multilayer sheet can be produced using known techniques in the art such as multilayer extrusion and blow molding. Multilayer sheet thicknesses are typically from 0.2 to 1.6 millimeters (mm) preferably from 0.3, more preferably from 0.4, and most preferably from 0.5 to 1.8, preferably to 1.7, more preferably to 1.6, and most preferably to 1.5 mm. Multilayer sheets comprising a foam layer will typically have thicknesses of from 2 to 4 mm. Multilayer sheets can be further processed by thermoforming into articles which have good impact strength. When thermoformed, it is preferable that the first polymer composition is in the inside position such that it forms the inside surface of the thermoformed article. Thermoformed articles include things such as cups, trays, foam containers, and foam trays. The thermoformed articles of the present invention are useful in food packaging applications wherein thin films of polyolefin or polyolefin coated material are heat sealed thereto. This unique food packaging substrate provides excellent sealable, peelable behavior for the polyolefin films sealed thereto.

Typical films which can be used to seal the thermoformed articles of the present invention include polyolefin films, such as polyethylene or a polyolefin film coated material such as aluminum foil, paper, or thin cardboard.

Film thicknesses are typically from 0.012 to 0.08 mm, preferably from 0.018 more preferably from 0.020 and most preferably from 0.026 to 0.07, preferably to 0.06, more preferably to 0.05 and most preferably to 0.04 mm. Sealing can be achieved using any conventional heat-sealing or impulse sealing machines which are commonly used for styrene polymer cups. As is well- known to those skilled in the art, settings for sealing machines are determined by the temperature of the seal bar, pressure of the seal bar applied to the rim of the containers, the seal time, and the thickness of the lidding structure. Advantageous sealing conditions are at temperatures between 90 and 250°C, pressures between 1 and 10 bar, and seal times between 0.5 and 1 .5 seconds.

The present invention provides a polystyrene heat sealable structure; sealable by a wide variety of polyethylene film lidding such as LDPE, aluminum coated with PE, paper coated with PE and transparent polyethylene; having a high closure integrity and peelable seal which does not tear. Additionally, thermoformed articles of the present invention offer high environmental stress crack resistance, and regrind can be reused due to it's high compatibility.

In another embodiment of the present invention, the first composition of a) can be used as a single component in a single layer extruded sheet. This sheet can be used in forming flexible products such as flexible lidding structures. The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted. Amounts are in weight parts or weight percentages unless otherwise indicated. EXAMPLE 1

A high impact polystyrene (HIPS) (Styron™ 484 available from The Dow Chemical Company) is coextruded with a HI PS/Polyethylene composition (XU72551.08 (available from The Dow Chemical Company) wherein the XU composition is 8 percent of the total sheet thickness. The coextruded sheet is then thermoformed into a tub shape wherein the XU layer forms the inner surface of the tub. The tub is heat sealed at 180 to 220 °C at 3 Bar and a one second dwelltime with a low density polyethylene film. The same structure is thermoformed into a shallow tray and heat sealed with a low density polyethylene film.

Both articles exhibit excellent peelability without tearing with a peel strength of 240 gr/cm.

Claims

WHAT IS CLAIMED IS:

1. A coextruded multilayer sheet which comprises: a) a sheet layer comprising:

I) from 40 to 60 weight percent of an olefin polymer, II) from 25 to 45 weight percent of an impact modified monovinylidene aromatic polymer, comprising from 1 to 25 weight percent of a rubber and 75 to 99 weight percent of a monovinylidene aromatic polymer matrix having a molecular weight (Mw) from 50,000 to 400,000, said weight percents being based on the total weight of said impact modified, vinylaromatic polymer; and III) from 8 to 25 weight percent of a compatibilizing polymer, which acts to increase interfacial adhesion between components I) and II); and b) a sheet layer comprising: a monovinylidene aromatic polymer.

2. The multilayer coextruded sheet of Claim 1 wherein a) is from 3 to 20 percent of the total thickness of the sheet and the sheet layer of b) is from 80 to 97 percent of the total thickness of the sheet.

3. The multilayer coextruded sheet of Claim 1 wherein the monovinylidene aromatic polymer of b) is an impact modified monovinylidene aromatic polymer having a Mw of from 200,000 to 230,000 and a polydispersity of from 2.0 to 2.7.

4. The multilayer coextruded sheet of Claim 1 wherein the monovinylidene aromatic polymer of b) is a monovinylidene aromatic homopolymer.

5. The multilayer coextruded sheet of Claim 1 wherein the monovinylidene aromatic polymer of b) is in the form of a foam and the sheet layer of b) is from 95 to

99 percent of the total thickness of the multilayer sheet.

6. The coextruded polymer sheet of Claim 1 wherein the olefin polymer is a high density polyethylene.

7. The coextruded polymer sheet of Claim 1 wherein the olefin polymer is an enhanced polyethylene resin.

8. The coextruded polymer sheet of Claim 1 wherein the impact modified monovinylidene aromatic polymer of II) is a high impact polystyrene.

9. The coextruded polymer sheet of Claim 1 wherein the compatibilizing polymer is a styrene/butadiene or styrene/isoprene triblock copolymer.

10. The coextruded polymer sheet of Claim 1 wherein the impact modified monovinylidene aromatic polymer of b) is a high impact polystyrene.

11. A thermoformed article produced from the coextruded sheet of Claim 1.

12. A food packaging system comprising the thermoformed article of Claim 7 and a polyolefin or polyolefin coated lid.