Rubber-modified monovinylidene aromatic polymers such as high impact polystyrene (HIPS) are widely used to thermoform articles such as disposable food containers, including cups, margarine tubs, and the like. However, due to trimming of the thermoformed parts, significant dust can be generated, which must be removed by dust extraction equipment, causing downtime for equipment cleaning and maintenance and productivity losses. Additionally, dust accumulates on the molded parts and may contaminate foodstuffs in applications such as form, fill and seal, while also causing problems in printing.

Therefore, there is a need for a method for reducing the amount of dust generated during the thermoforming process.

The present invention uses polybutene to reduce the amount of dust generated in the thermoforming process of rubber-modified monovinylidene aromatic polymer resins. At least 1 weight percent polybutene is incorporated into the high impact monovinylidene aromatic resin composition prior to thermoforming, wherein the weight percent is based on the total weight of the monovinylidene aromatic resin composition.

Polybutene has been used in polymer compositions for flow improvement as in NL 6513430A, and for environmental stress crack resistance (ESCR) improvement, such as disclosed in U. S. Patent No. 5,834, 126. The present invention is for a new use of polybutene in reducing the dust generated in thermoforming processes of rubber- modified monovinylidene aromatic polymers.

Rubber-modified monovinylidene aromatic polymers include any rubber modified polymer derived from a vinyl aromatic monomer, including, but not limited to styrene; alkyl substituted styrenes such as alpha-alkyl-styrenes (for example, alpha methylstyrene and alpha-ethyl-styrene) and ring alkylated styrenes and isomers thereof (for example, ortho ethyl styrene, 2, 4-dimethyl styrene and vinyltoluene, particularly, ortho or para vinyl toluene) ; ring substituted halo-styrenes such as chloro-styrene, and 2,4-dichoro-styrene ; and styrenes substituted with both a halo and alkyl group such as 2-chloro-4-methylstyrene ; and vinyl anthracene. In general, the preferred monovinyl aromatic monomers are styrene, alpha-methylstyrene, one or more of the vinyl toluene isomers, and/or mixture of two or more of these, with styrene being the most preferred monovinyl aromatic compound.

The monomer may optionally comprise minor amounts of one or more additional comonomers, preferably in an amount less than 10 percent by weight of the polymerizable monomer mixture. Suitable comonomers are unsaturated nitriles, for example acrylonitrile ; alkyl acrylates and alkyl methacrylates, for example methyl methacrylate or n-butylacrylate ; ethylenically unsaturated carboxylic acid monomers; and ethylenically unsaturated carboxylic acid derivative

monomers including anhydrides and imides such as maleic anhydride and N-phenyl maleimide.

Rubber-modified monovinylidene aromatic polymers and methods for their preparation are well-know in the art and well described in the patent art such as, for example, in U. S. Patent Nos.

6,441, 090; 6,211, 298; 6,323, 282; 6,221, 471; 6,350, 813; 4,666, 987; 4,572, 819; and 4,585, 825.

The polymerization mixture may also contain additive materials and/or polymerization aids such as plasticizers or lubricants, such as mineral oil, butyl stearate or dioctyl phthalate ; stabilizers including antioxidants (for example, alkylated phenols, such as di-tert-butyl-p-cresol or phosphites such as trisnonyl phenyl phosphite); chain transfer agents, such as an alkyl mercaptan, such as n-dodecyl mercaptan; or mold release agents, for example, zinc stearate; all of which additives and/or polymerization aids are added to the reaction mixture where appropriate including before, during or after polymerization.

The use of a chain transfer agent is optional and is usually employed only in the production of a composition or prepolymer containing larger size rubber particles (for example, having an average particle size of at least one micrometer). If employed, the chain transfer agent is generally employed in an amount of from 0.001 to 0.5 weight percent based on the total weight of the polymerization mixture to which it is added.

Crosslinking of the rubber in the resulting product and removal of the unreacted monomers, as well as any reaction diluent, if employed, and other volatile materials is advantageously conducted employing conventional techniques.

In order to reduce dust generation in the trimming of parts produced by thermoforming of the rubber-modified monovinylidene aromatic polymer, at least 1 weight percent of polybutene is added to the rubber-modified monovinylidene aromatic polymer composition. Generally, the polybutene is present in the rubber-modified monovinylidene aromatic polymer composition in an amount of at least 1 weight percent, preferably at least about 2 weight percent to about 5 weight percent, preferably to about 4.5, more preferably to about 4, and most preferably to about 3.5 weight percent, based on the total weight of the rubber-modified monovinylidene aromatic polymer composition.

Typically, the polybutene number average molecular weight (Mn) is from 900 to 1300, preferably from 900 to 1200, more preferably from 900 to 1100 and most preferably from 900 to 1000.

The polybutene can be incorporated in the rubber-modified monovinylidene aromatic polymer composition at any time prior to thermoforming, but is preferably added in the reactor during polymerization of the vinyl aromatic monomer. Alternatively the polybutene can be added during the fabrication step by feeding into the extruder and melt blending with the rubber-modified polymer composition.

The process of the present invention will benefit any type of trimming/grinding step of a thermoforming process, provided that polybutene can be added to the feed if necessary (if not already added during polymerization).

The present invention is based on the surprising discovery that the addition of polybutene significantly decreases the level of dust creation during the trimming process of monovinylidene aromatic polymer resins such as high impact polystyrene (HIPS). Surprisingly, the production of dust during trimming has been found to relate directly to the shear strength of the polymer material as measured by ASTM D732-99, wherein a high shear stress product produces more dust during trimming. This behavior becomes more pronounced at lower trimming temperatures.

It is important to note that the shear strength of rubber-modified monovinylidene aromatic polymer decreases with increasing rubber content, and given the correlation we have determined between shear strength and dust creation, the increase in rubber content would thus lower dust creation as well. However, increasing the rubber content leads to higher polymer viscosities which is detrimental to the extrusion process, and lowers the modulus (which is detrimental to part stiffness) which may cause jamming in the trim die/punch. We have found however, that the addition of polybutene allows one to decrease the shear strength without significantly changing the modulus and at the same time decreasing the resin viscosity. Accordingly, the addition of polybutene makes it possible to optimize resin viscosity, resin modulus and shear strength at the same time.

The following experiments are set forth to illustrate the present invention and should not be construed to limit its scope. All parts and percentages are by weight unless otherwise indicated.

The abbreviations used in the tables include Mw (weight average molecular weight), Mn (number average molecular weight) and Mw/Mn Ratio (polydispersity).

EXAMPLES High impact polystyrene is prepared in continuous stirred tank reactors in series. All raw materials are added in the feed to the first reactor.

Two feed streams are added simultaneously to the first reactor, l) 3% by weight of a high cis polybutadiene rubber and 4 percent by weight of polybutene (viscosity of 220 cSt) dissolved in styrene; and 11) 0.8 percent by weight of mineral oil (viscosity of 70 cSt) in ethylbenzene (50: 50).

N-dodecylmercaptan is added during the reaction (after phase inversion) to control the molecular weight of the matrix. Zinc stearate (500ppm) and antioxidant (2,6 di-tert-butyl-4- (octadecanoxycarbonylethyl)-phenol) (1 OOOppm) are also added. Characterization of the synthesized polymers are given in Table 1.

Table 1 MO PB RC R. P. S. Mw Mn Mw/Swell % % % m g/mol/mol Mn Index Comparative 0. 8 0 3 3. 59 175000 64200 2. 72 11. 3 Example 1. Example 1 0.8 4 3 3. 36 172300 63700 2.70 13.1 Example 2 1.5 2 6 3. 14 168500 62300 2.70 11.6 Comparative 0 0 9 2.44 176400 63200 2.79 10.2 Example 2 Comparative 0.8 0 9 2. 15 174000 61900 2.81 10.5 Example 3 Example 3 0.8 4 9 1. 99 167200 60000 2.78 12.3 Example 4 0 4 9 2. 07 169400 60800 2.78 11.6 MO is mineral oil.

PB is polybutene.

RC is rubber content.

RPS is mean rubber particle size determined using a Coulter Counter.

These samples are extruder in a Brabender lab extruder to a 1 mm thick sheet. For runs of 12 hours or more, it is noted that the cutter requires cleaning for the samples which did not contain polybutene, due to dust buildup.

Shear stress force is measured according to ASTM D732-99 at 50 mm/s. The modulus is measured according to ASTM D638. The Melt flow rate (MFR) is measured according to ASTM D1238 at 200°C/5Kg per 10 min.

Example 1 The effect of polybutene is observed in the two samples below. By adding 4 percent polybutene the shear strength of the sample is decreased by 1000 N while the modulus does not decrease significantly. In addition, the flow is improved which results in easier processing. MO PB RC Shear M. F. R. Tensile Cutter % % % (MPa) g/10 Modulus/Cleaning* min N/mm2 Comparative 0.8 0 3 49.4 10 2759 yes Example 1 Example 1 0. 8 4343. 7 19. 3 2523 no *Cutter cleaning required within 12 hour period due to dust build-up.

Example 2 By adding polybutene the shear stress force can be decreased to allow a good cutting with much less rubber needed. In addition the flow is not decreased.

MO PB RC Shear M. F. R Tensile Cutter % % % (MPa) g/10 Modulus/Cleaning* min N/mm2 Comparative 0.8 0 3 49.4 10 2759 yes Example 1 Example 2 1. 5 2 6 37. 6 9. 3 2170 no Comparative 0.8 0 9 37. 5 5.3 2019 yes Example 3 *Cutter cleaning required within 12 hour period due to dust build-up.

Example 3 MO PB RC Shear MFR Tensile Cutter % % % (MPa) g/l0mi Modulus Cleaning* n/N/mm2 Comparative 0.8 0 3 49. 4 10.1 2759 yes Example 1 Example 3 0. 8 4 9 37. 7 6. 8 1890 no *Cutter cleaning required within 12 hour period due to dust build-up. Example 4 The reduction in shear stress force while adding melt flow without changing the modulus. M PB RC Shear MFR Tensile Cutter O % % (MPa) g/l0mi Modulus/cleaning* % nN/mm2 Comparative 0 0 9 40.8 3.9 1963 yes Example 2 Example 4 0 4 9 37. 1 6.5 1915 no *Cutter cleaning required within 12 hour period due to dust build-up.