MONOVINYLIDENE AROMATIC RESINS

Abstract: The present invention is a monovinylidene aromatic resin having a bimodal weight average molecular weight (Mw) distribution such that 85 to 95 percent by weight of the resin has an average Mw of from 250,000 to 400,000 and from 2 to 15 percent by weight of the resin has a Mw of 1,000 to 20,000, wherein each fraction has a polydispersity (Mw/Mn) of from 1.5 to 3.0.

Description: MONOVINYLIDENE AROMATIC RESINS The process of the present invention relates to monovinylidene aromatic resins. Monovinylidene aromatic resins have been used in applications such as foam sheet, extrusion and oriented articles. Plasticizers have typically been used in such resins to increase the flow and increase the rate of production of molded or extruded articles. However, plasticizers such as mineral oil, significantly lower the glass transition temperature (Tg) of the resin. This can slow the production time, since the products produced must be cooled to a lower temperature for adequate setting to occur prior to mold removal. In addition, the use of plasticizers such as limonene add significant cost to the resins, making their use undesirable. JP-08-172959 discloses a method of modifying aromatic plastics by compounding styrene oligomers having a molecular weight of 2200-3800 and a polydispersity of 1.6 to 2.1 with aromatic plastics such as polystyrene. However, the polystyrene compositions disclosed still have significantly reduced Tg due to the presence of mineral oil plasticizer in Styron 666 and the flexural strength is significantly reduced. Therefore, there remains a need for a monovinylidene aromatic resin having good flowability and Tg without the need or presence of mineral oil or other plasticizer. The present invention is a monovinylidene aromatic resin having a bimodal weight average molecular weight (Mw) distribution such that 85 to 95 percent by weight of the resin is a fraction having an average Mw of from 250,000 to 400,000 and from 2 to 15 percent by weight of the resin is a fraction having a Mw of 1 ,000 to 20,000, wherein each fraction has a polydispersity (Mw/Mn) of from 1.5 to 3.0. This resin has good flow and Tg properties and can be used in various applications, such as injection molding, foam sheet and oriented polystyrene extruded sheet, resulting in increased production rates. The present invention is a monovinylidene aromatic resin having a specific Mw profile having an improved balance of melt flow and strength without the addition of a traditional plasticizer. The monovinylidene aromatic resin has a bimodal Mw distribution such that 85 to 95 percent by weight of the resin has an average Mw of from 250,000 to 400,000 and from 2 to 15 percent by weight of the resin has a Mw of 1 ,000 to 20,000, wherein each fraction has a polydispersity (Mw/Mn) of from 1.5 to 3.0. Monovinylidene aromatic polymers suitable for the process of the present invention are those produced by polymerizing a vinyl aromatic monomer. Vinyl aromatic monomers 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=CH 2 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, 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 acrylic monomers such as acrylonitrile, methacrylonitrile, methacrylic acid, methyl methacrylate, acrylic acid, and methyl acrylate; maleimide, phenylmaleimide, and maleic anhydride. In addition, the polymerization of the vinyl aromatic monomer may be conducted in the presence of predissolved elastomer to prepare impact modified, or grafted rubber containing products, examples of which are described in US-A-3,123,655, US-A-3,346,520, US-A-3,639,522, and US-A-4,409,369. Polymerization processes and process conditions for the polymerization of vinyl aromatic monomers are well known in the art. Although any polymerization process can be used, typical processes are continuous bulk or solution polymerizations as described in US-A-2,727,884 and US-A-3,639,372. Cationic polymerization processes can also be used, especially in preparing the low molecular weight component. Such processes are well known in the art such as in US-A-4,087,599, US-A-4,161 ,573 and US-A-4,112,209. In one embodiment of the present invention, the major proportion of the monovinylidene aromatic resin is a high molecular weight fraction. Typically at least 85 percent, preferably at least 90 percent and most preferably at least 95 percent of the resin has a molecular weight from 250,000, preferably from 260,000, more preferably from 270,000 and most preferably from 280,000 to 400,000, preferably to 375,000, more preferably to 350,000 and most preferably to 325,000. The resin also contains a lower molecular weight fraction which is typically from 2 percent, preferably from 2.5 percent, more preferably from 3 percent and most preferably from 3.5 percent, and even more preferably from 5 percent to 15 percent, preferably to 12 percent and most preferably to 10 percent, and even more preferably to 8 percent by weight. This lower molecular weight fraction has a Mw which is typically from 1 ,000, preferably from 1 ,500, more preferably from 2,000 and most preferably from 2,500 to 20,000, preferably to 18,000, more preferably to 16,000 and most preferably to 14,000. Molecular weight values are measured using gel permeation chromatography techniques (GPC) which are well known in the art. The polydispersity (weight average molecular weight(Mw))/(number average molecular weight (Mn)) for each fraction described above is typically from 1.5, preferably from 1.6, more preferably from 1.7 and most preferably from 1.8 to 3.0, preferably to 2.8, more preferably to 2.7 and most preferably to 2.6. It has been discovered that resins falling within this bimodal Mw profile have excellent flow and Tg properties without using plasticizers such as mineral oil. Typical polymer additives can also be included in the resin of the present invention, including antioxidants, stabilizers, mold release agents, and chain transfer agents Typically, the high Mw fraction and the low Mw fraction are made in separate reactors and combined in the appropriate proportions to produce the resin of the present invention. The individual resins can be made simultaneously and combined within the same process or the two resins can be blended using any blending method including solution or dry blending. The resin of the present invention can also be produced within one continuous process wherein the low molecular weight fraction is produced in situ while producing the high molecular weight fraction within the same process. This can be accomplished using methods described in EP-797,600. In one embodiment of the present invention, the monovinylidene aromatic polymer is a polystyrene resin having a high molecular weight fraction of at least 320,000 and a low molecular weight fraction of from 1 ,000 to 20,000, which is used to produce foam sheet. Methods of producing foam sheet are well known in the art, any of which can be used to produce foam sheet from the resin of the present invention. Typical blowing agents include carbon dioxide, hydrochlorofluorocarbons, and alcohols, mixtures thereof. The lower molecular weight fraction increases the flow of the polystyrene resin without substantially lowering the Tg, thus production can occur at higher rates compared to resins containing plasticizers which lower the Tg. The monovinylidene aromatic resin of the present invention can also be used in blends with other polymers, such as high impact monovinylidene aromatic polymers. These blends can be used in applications such as form-fill-seal products, especially yogurt type containers. The monovinylidene aromatic resin of the present invention can also be used in various applications including foam sheet, meat trays, egg cartons, cups, biaxially oriented sheet, injection molded articles and oriented products. 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 solution of 3.5 weight percent low molecular weight polystyrene (Mw 1 ,400, Mn 660), 3.0 percent ethylbenzene, 93.5 percent styrene, 75 ppm of 1 ,1-bis(tert- butylperoxy)cyclohexane initiator and 25 ppm sulfoethyl methacrylate is passed through a series of stirred tube reactors with a temperature gradient from 128 to 150°C. A slurry of zinc stearate in ethylbenzene is added halfway through the reactors such that the final concentration of zinc stearate in the product is approximately 1200 ppm. The partial polymer is devolatilized at 230°C and 10 mm, followed by pelletization, to give a bimodal polystyrene resin with 94.5 percent of the resin having a Mw of 328,000 (Mn of 134,000) and 5.5 percent having a Mw of 1 ,400. This resin exhibits an enhanced flow rate (MFR of 2.5 vs. 1) over monomodal high MW prepared from styrene under identical polymerization conditions while decreasing the Tg of the resin only slightly (106°C vs. 108°C). The following molding conditions are used for ASTM specimen molding: 28 ton Arburg injection molding machine settings: barrel (200, 220,210, 205°C), mold (50°C), b27= 17.3 cm, b29= 21.5 cm, b28= 21.5 cm, b26= 13.9 cm, hold press.= 3.5 MPa, back press= 0.42MPa, recover time= 17 sec, hold time= 5 sec, fill time= 5 sec, cooling time= 20 sec, cycle time = 31.5 s The following ASTM methods are used: Tensile Testing ASTM D638-94b Flex Testing ASTM D790-92 Un-notched Testing ASTM D256-93 The following molding conditions are used for molding of audio cassettes: 180 ton Sumitomo injection molding machine settings: barrel (245, 255, 250, 235, 220 °C), mold (30 °C), screw back = 73.54 mm, switch-over = 10.4 mm, injection velocity = 95, 100, 95, 80 mm/s to 60, 30, 15, 10.5 mm hold pressure = 19, 20, 47 kgf/cm 2 for 0.2, 0.3, 0.2 seconds Properties of the molded resin is listed in Table I. Control 1 and 2 are Styron™ 685 D available from The Dow Chemical Company. TABLE I Resin Control 1 Control 2 Example I Mw 305,000 305,000 328,000 Low Mw PS (Mw= 1400)(%) 5.5 Tensile Strength (MPa) 51.9 51.9 53.6 Tensile Break Elong. (%) 2 2 3.5 Tensile Modulus (MPa) 3054 3054 3027 Flexural Strength (MPa) 105.5 105.5 105.5 Flexural Strain @ Fail (%) 4.2 4.2 5 Flexural Modulus (MPa) 3296 3296 3372 Un-notched Izod (kg-cm/cm) 20 20 37 Fill time (seconds) 0.7 unavailable 0.49 Hold time (seconds) 0.6 unavailable 0.7 Cooling time (seconds) 2.6 2.2 2 Plastication time (seconds) 3.87 4.05 3.41 Cycle time (seconds) 7.2 7.4 6.75 Plastication savings (sec) 0 -0.18 0.46 Cycle time savings (sec) 0 -0.2 0.45 Cooling time savings (sec) 0 0.4 0.6 Fill time savings (sec) 0 0.7 0.21 Fill & cool savings (sec) 0 1.1 0.81 The bimodal resin is also used to produce foam sheet and meat trays therefrom. Meat trays are manufactured using a continuos roll fed thermoformer where a roll of the bimodal resin foam sheet (produced using a tandem extrusion process) is passed through a heated oven and is heated to a temperature sufficient to soften and expand the sheet. The expanded foam sheet is vacuum formed into meat trays and cooled. The formed sheet containing the meat trays is mechanically cut to form the trays. The remaining portion of the sheet or web is fed into a grinder, re-ground and later extruded into additional foam sheet. The typical gauge and density of foam sheet used to make meat trays is 110 to 125 mils and 48 to 56 g/l, respectively. A resin blend of 80 percent high heat general purpose polystyrene (STYRON™ 685D) and 20 percent STYRON™ 685D regrind and a resin blend of 80 percent bimodal polystyrene resin as prepared above and 20 percent bimodal polystyrene resin regrind are extruded and compared using a commercial tandem extrusion process (two extruders in series). To assure a valid comparison between the two resins, foam sheet samples are made with the same gauge, density, basis wt. and cell size. These parameters are controlled by adjusting the percent blowing agent, percent talc, and the rate at which the sheet is pulled. The blowing agent is a blend of hydrocarbon and carbon dioxide. The carbon dioxide was the lesser of the two blowing agent components. TABLE II Initial Sheet Results Resin Gauge (mils) Density (a/\) Basis Wt.(α/613cm 2 ) Cellist (mm) 685D 116 54.4 9.6 0.34 Example 1 1 17 52.8 9.6 0.38 ASTM-D638 shaped tensile foam specimens are prepared and tested. The results are listed in Table III. TABLE III % Tensile Elongation Machine Direction/Cross Direction Resin 3 days 10 davs 17 days 31 days 685D 8/2 6/2 7/4 9/4 Bimodal 8/2 6/3 7/3 8/4 Post expansion is the amount the foam sheet expands versus its initial gauge within a thermoformer. Post expansion of the meat trays is measured by measuring stacks of 125 number 2 type meat trays for their overall average heights. The results are listed in Table IV. The bimodal resin shows a 7 percent increase in stack height. TABLE IV Stack Height of 125 Number 2 Type Meat Trays Resin Stack Height (cm) 685D 74.2 Bimodal 79.2

Claims: CLAIMS: 1. A monovinylidene aromatic resin having a bimodal weight average molecular weight (Mw) distribution such that 85 to 95 percent by weight of the resin has an average Mw of from 250,000 to 400,000 and from 2 to 15 percent by weight of the resin has a Mw of 1 ,000 to 20,000, wherein each fraction has a polydispersity (Mw/Mn) of from 1.5 to 3.0. 2. The monovinylidene aromatic resin of Claim 1 wherein the resin is a polystyrene. 3. The monovinylidene aromatic resin of Claim 1 wherein 85 to 95 percent by weight of the resin has an average Mw of from 260,000 to 375,000. 4. The monovinylidene aromatic resin of Claim 1 wherein 85 to 95 percent by weight of the resin has an average Mw of from 270,000 to 350,000. 5. The monovinylidene aromatic resin of Claim 1 wherein 85 to 95 percent by weight of the resin has an average Mw of from 280,000 to 325,000. 6. The monovinylidene aromatic resin of Claim 1 wherein 2 to 15 percent by weight of the resin has a Mw of from 1 ,500 to 18,000. 7. The monovinylidene aromatic resin of Claim 1 wherein 2 to 15 percent by weight of the resin has a Mw of from 2,000 to 16,000. 8. The monovinylidene aromatic resin of Claim 1 wherein 2 to 15 percent by weight of the resin has a Mw of from 2,500 to 14,000. 9. The monovinylidene aromatic resin of Claim 1 wherein 2 to 15 percent by weight of the resin has a Mw of from 1 ,500 to 18,000. 10. The monovinylidene aromatic resin of Claim 1 wherein the polydispersity is from 1.6 to 2.8. 11. The monovinylidene aromatic resin of Claim 1 wherein the polydispersity is from 1.7 to 2.7. 12. The monovinylidene aromatic resin of Claim 1 wherein the polydispersity is from 1.8 to 2.6. 13. Foam sheet produced from the monovinylidene aromatic resin of Claim 1. 14. Injected molded articles produced from the monovinylidene aromatic resin of Claim 1. 15. Biaxially oriented extruded sheet produced from the monovinylidene aromatic resin of Claim 1. 16. Extruded thermoformed sheet produced from the monovinylidene aromatic resin of Claim 1. 17. Thermoformed articles produced from the monovinylidene aromatic resin of Claim 1. 18. Form-fill-seal articles produced from the monovinylidene aromatic resin of Claim 1.

Inventors: BOSIERS LUC, DEMIRORS MEHEMET, HAHNFELD JERRY L, KOPPI KURT A, PIKE WILLIAM C, SALAMON BRENT A, SUGDEN JOHN L, WELSH GARY C, BOSIERS, LUC, DEMIRORS, MEHEMET, HAHNFELD, JERRY, L., KOPPI, KURT, A., PIKE, WILLIAM, C., SALAMON, BRENT, A., SUGDEN, JOHN, L., WELSH, GARY, C.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1999
Main classification: C08L25
IPC codes: C08L25