WO1999009081A1

WO1999009081A1 - High gloss high impact monovinylidene aromatic polymers

Info

Publication number: WO1999009081A1
Application number: PCT/US1998/016501
Authority: WO
Inventors: Mehmet Demirors
Original assignee: The Dow Chemical Company
Priority date: 1997-08-13
Filing date: 1998-08-07
Publication date: 1999-02-25
Also published as: AU8698098A; AR016608A1

Abstract

The present invention is a rubber modified monovinylidene aromatic polymer having a bimodal particle size distribution comprising: a) rubber particles of a star or branched low viscosity rubber having a volume average particle size of from 0.1 to 1.5 ν, and a cellular or core shell morphology or mixture thereof, and b) rubber particles of a star or branched low viscosity rubber, linear diene rubber or block copolymer rubber having a volume average particle size of from 1.5 to 10 ν, and a cellular morphology, with the proviso that when the volume average particle size of the particles of a) is greater than 1 ν, the volume average particle size of the particles of b) is at least 2 ν, wherein the particles of a) are from 50 to 99 wt. percent of the total diene rubber content.

Description

HIGH GLOSS HIGH IMPACT MONOVINYLIDENE AROMATIC POLYMERS The present invention relates to rubber modified monovinylidene aromatic polymers, particularly polymers containing two distinct rubber volume average particle sizes, herein referred to as bimodal compositions. Rubber modified monovinylidene aromatic polymers have typically been prepared from vinyl aromatic monomers by polymerizing the vinyl aromatic monomer in the presence of a dissolved rubber. The vinyl aromatic monomer polymerizes, forming a discontinuous phase dispersed throughout a continuous phase of dissolved rubber in monomer. As the vinyl aromatic monomer continues to polymerize, the discontinuous polymer phase becomes larger in volume, thus forming a continuous phase, while the rubber forms a discontinuous phase dispersed throughout. This phenomenon, referred to as "phase inversion", is, therefore, the conversion of the polymer from a discontinuous phase dispersed in the continuous phase of the rubber/monomer solution, through the point where there is no distinct continuous or discontinuous phase in the polymerization mixture, to a continuous polymer phase having the rubber dispersed throughout.

Various bimodal compositions containing two distinct rubber particle sizes have been produced to attempt to effectively balance and achieve high gloss, while maintaining high impact properties using several types of rubbers. U.S. 4,334,039 issued to Dupree et al. and U.S. 4,153,645 issued to Lanza et al. disclose the use of butadiene rubbers to obtain polymers having a bimodal rubber particle size distribution. Such polymers, while having good toughness, do not have the desired level of gloss. EP-048,389 by Echte discloses the use of styrene/butadiene block copolymer rubbers wherein small particles are made from a 40/60 styrene/butadiene block copolymer and are of core-shell type morphology. While such products have the desired balance of gloss and impact, they are economically disadvantageous because of higher cost due to the use of the block copolymer rubbers. Additionally, higher amounts of block copolymer must be used in order to obtain a given polybutadiene level, additionally increasing cost. It is well known in the art that the balance of gloss and impact properties of high impact monovinylidene aromatic polymers is dependent on rubber particle size, rubber level and flow properties of the product. Typically, smaller particle containing resins have higher gloss and lower impact, while larger rubber particle containing resins have lower gloss and higher impact properties. Standard rubbers, usually known as butadiene homopolymer type, cannot be sized small enough to make the desired high gloss products. Block copolymers usually lead to small rubber particles but are more expensive. Other approaches have been used to solve the problem of obtaining small particles using polybutadiene rubbers. EP-277,687 discloses a rubber modified polymer containing radial or branched polybutadiene rubber having volume average diameter of 0.1 to 1.2 microns (μ) and rubber particles containing either radial, branched or linear rubber having a volume average particle diameter of from 1 to 5μ. However, while these compositions have high gloss, they do not have sufficient toughness. Therefore, there remains a need to produce rubber modified monovinylidene aromatic polymers having gloss and impact properties similar to those products which use block copolymer rubbers, but without the high cost of block copolymer rubbers. The present invention is a rubber modified monovinylidene aromatic polymer having a bimodal particle size distribution comprising: a ) rubber particles of a star or branched low viscosity rubber having a volume average particle size of from 0.1 to 1.5μ, and a cellular or core shell morphology or mixture thereof, and b) rubber particles of a star or branched low viscosity rubber, linear diene rubber or block copolymer rubber having a volume average particle size of from 1.5 to 10 microns, and a cellular morphology, with the proviso that when the volume average particle size of the particles of a) is greater than 1 μ, the volume average particle size of the particles of b) is at least 2μ, wherein the particles of a) are from 50 to 99 wt. percent of the total diene rubber content

The polymers of the present invention have excellent gloss and impact properties similar to those obtained using block copolymer rubbers, while being more economical. Such products are highly desirable in replacing higher cost products in injection molding and extrusion applications. The present invention is related to rubber modified monovinylidene aromatic polymers. The monovinylidene aromatic rubber modified polymers are derived from one or more vinyl aromatic monomers. Representative vinyl aromatic monomers include styrene, alkyl substituted styrenes such as alpha-alkyl-styrenes, for example aipha-methylstyrene, alpha-ethylstyrene; ring substituted styrenes, for example, vinyltoluene, particularly p-vinyltoluene, o-ethylstyrene and 2,4-dimethylstyrene; ring substituted halo-styrenes such as chlorostyrene, and 2,4-dichloro-styrene; styrene substituted with both halo and alkyl groups, such as 2-chloro-4-methylstyrene, vinyl anthracene; and mixtures thereof. Preferably styrene and or alpha-methyl-styrene is used as the vinyl aromatic monomer, with styrene being most preferred. Comonomers may also be used in combination with the vinyl aromatic monomer, preferably in an amount of up to 40 percent by weight of the polymerizable monomer mixture. Representative comonomers include unsaturated nitriles, such as acrylonitrile; alkyl acrylates and alkyl methacrylates such as methyl methacrylate or n- butylacrylate; ethylenically unsaturated carboxylic acids; and ethylenicaily unsaturated carboxylic acid derivatives including anhydrides and imides, such as maleic anhydride and N-phenyl maleimide.

The rubber suitable for use to produce rubber particles having a volume average particles size of from 0.1 to 1.5μ as recited in a), is a low viscosity rubber having a solution viscosity (5 percent in styrene at 20°C) in the range 20 to 120 centipoise (cps) and a Mooney viscosity (ML+1, 100°C) of 30 to 80. Suitable rubbers include both so-called radial or star rubbers having three or more polymer segments bonded to a single polyfunctional element or compound, or a branched rubber having a cis content of less than 75 percent and at least one, or a significant number of subordinate chains of sufficient length such that the viscosity of the rubber is less than the viscosity of a linear polymer of the same monomeric components and same molecular weight. Such rubbers useful in a), typically have a relatively high average molecular weight, a relatively low solution viscosity and a medium to high Mooney viscosity. In general, the solution viscosity for the rubber will be below 120 cps while the Mooney viscosity will be less than 80 cps.

The radial or branched rubber preferably employed in a) of the present invention typically exhibits a second order transition temperature not higher than 0°C, and preferably not higher than -20°C. Suitable rubbers include alkadienes which include 1 ,3-conjugated dienes such as butadiene, isoprene, chloroprene or piperylene. Most preferred are homopoiymers prepared from 1 ,3-conjugated dienes, with homopolymers of 1 ,3-butadiene being especially preferred. Alkadiene copolymer rubbers containing small amounts, for example up to 10 or 15 weight percent, of other monomers such as vinyl aromatics can also be employed if the rubbers meet the other qualifications described herein.

Polymers having random branching, as well as methods for their preparation, are known in the art and reference is made thereto for the purpose of this invention. Representative branched rubbers and methods for their preparation are described in Great Britain Patent No. 1,130,485 and in Macromolecules, Vol. II, No. 5, pg. 8, by R. N. Young and C. J. Fetters.

Radial or star polymers, commonly referred to as polymers having designed branching, are conventionally prepared using a polyfunctional coupling agent or a polyfunctional initiator. Methods for preparing star or radial polymers having designed branching are well-known in the art. Methods for preparing a polymer of butadiene using a coupling agent are illustrated in US-A-4,183,877; US-A-4,340,690; US-A-4,340,691 and US-A-3,668,162, whereas methods for preparing a polymer of butadiene using a polyfunctional initiator are described in US-A-4,182,818; US-A-4,264,749; US-A-3,668,263 and US-A-3, 787,510. As known by those skilled in the art, various techniques such as control of the branching and molecular weight control can be used to adjust and tailor these polymers to achieve the necessary solution and Mooney viscosities, as well as the ratio of these two. The rubber suitable for use to produce the rubber particles having a volume average particle size of 1.5 to 10μ, as recited in b), can be the same rubber as described previously in a), a different rubber, or a mixture thereof. Typically the rubber can be any rubbery polymer which can be dissolved in the vinyl aromatic monomer. Preferred rubbery polymers include a homopolymer or copolymer of an alkadiene or a ethylene- propylene copolymer optionally containing a non-conjugated diene. More preferably, the rubber is a homopolymer of a 1 ,3-conjugated diene such as butadiene, isoprene, piperylene, and chloroprene, or a copolymer of a conjugated diene with one or more vinyl aromatic monomers such as styrene; alpha, beta-ethylenically unsaturated nitriles such as acrylonitrile; and alpha-olefins such as ethylene or propylene. Most preferred rubbers are homopoiymers of 1 ,3-butadiene and block or random copolymers of at least 30, more preferably from 50 to 90, weight percent 1 ,3-butadiene and up to 70, more preferably from 5 to 50, weight percent of a vinyl aromatic compound, preferably styrene. The rubber utilized to produce large rubber particles of b) is preferably a polybutadiene. The rubber utilized to produce small rubber particles of a) is preferably a star or branched polybutadiene or a poly(butadiene-styrene)block copolymer. The small rubber particles of a) typically have a core-shell (single, major occlusion) or cellular (multiple, minor occlusions) morphology or mixture thereof. The large particles of b) typically have a cellular or similar multiple-occlusion morphology. The amount of rubber initially dissolved in the vinyl aromatic monomer is dependent on the desired concentration of the rubber in the final rubber-reinforced polymer product, the degree of conversion during polymerization and the viscosity of the solution. The rubber is typically used in amounts such that the rubber-reinforced polymer product contains from 2 to 20 percent, preferably from 3 to 17 percent, and more preferably from 3 to 15 weight percent rubber, based on the total weight of the vinyl aromatic monomer and rubber components, expressed as rubber or rubber equivalent. The term "rubber" or "rubber equivalent" as used herein is intended to mean, for a rubber homopolymer, such as polybutadiene, simply the amount of rubber, and for a block copolymer, the amount of the copolymer made up from monomer which when homopolymerized forms a rubbery polymer, such as for a butadiene-styrene block copolymer, the amount of the butadiene component of the block copolymer.

The polymerization is preferably conducted in one or more substantially linear stratified flow or so-called plug-flow type reactors, as described in UUS-A-2,727,884. The techniques of mass-polymerization and the conditions needed for producing the desired average particle sizes are well known to one skilled in the art.

The temperature at which the polymerization is conducted will vary according to the specific components, particularly initiator, but will generally vary from 60 to 190°C. The polymerization can be conducted in the presence of an initiator. Suitable initiators include any initiator capable of imparting the desired grafting of polymer to the rubber particle under the conditions of polymerization and accelerating the polymerization of the vinyl aromatic monomer. Representative initiators include peroxide initiators such as peresters, for example tertiary butyl peroxybenzoate and tertiary butyl peroxyacetate, dibenzoyi peroxide, dilauroyl peroxide, 1.1 -bis tertiarybutyl peroxycyclohexane, 1-3-bis tertiarybutyiperoxy-3,3,5-trimethyi cyciohexane, and di- cumyl peroxide. Photochemical initiation techniques can be employed if desired. Preferred initiators include dibenzoyi peroxide, tertiarybutylperoxy benzoate, 1,1-bistertiarybutylperoxy cyciohexane and tertiarybutylperoxy acetate. initiators may optionally be employed in a range of concentrations dependent on a variety of factors including the specific initiators employed, the desired levels of polymer grafting and the conditions at which the mass polymerization is conducted. Specifically, initiators may be employed in amounts from 0 to 2000, preferably from 100 to 1500, parts by weight per million parts by weight of vinyl aromatic monomer. Additionally, a solvent may be used in the polymerization process. Acceptable solvents include normally liquid organic materials which form a solution with the rubber, vinyl aromatic monomer and the polymer prepared therefrom. Representative solvents include aromatic and substituted aromatic hydrocarbons such as benzene, ethylbenzene, toluene, xylene or the like; substituted or unsubstituted, straight or branched chain saturated aliphatics of 5 or more carbon atoms, such as heptane, hexane, octane or the like; alicyclic or substituted aiicyclic hydrocarbons having 5 or 6 carbon atoms, such as cyciohexane. Preferred solvents include substituted aromatics, with ethylbenzene and xylene being most preferred. In general, the solvent is employed in amounts sufficient to improve the processability and heat transfer during polymerization. Such amounts will vary depending on the rubber, monomer and solvent employed, the process equipment and the desired degree of polymerization. If employed, the solvent is generally employed in an amount of up to 35 weight percent, preferably from 2 to 25 weight percent, based on the total weight of the solution.

Other materials may also be present in the polymerization process, including plasticizers, for example mineral oil; flow promoters, lubricants, antioxidants, catalysts, mold release agents, or polymerization aids such as chain transfer agents, including alkyl mercaptans, for example n-dodecyl mercaptan. If employed, a chain transfer agent is typically present 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.

Crossiinking of the rubber in the resulting product and removal of the unreacted monomers, as well as any solvent, if employed, and other volatile materials is advantageously conducted employing conventional techniques, such as introducing the polymerization mixture into a devolatilizer, flashing off the monomer and other volatiles at elevated temperature, for example, from 200 to 300°C under vacuum and removing them from the devolatilizer.

In bimodal compositions, two distinct volume average particle sizes are produced and combined. In particular, the rubber particles have different volume average particle sizes, wherein one contains small rubber particles having a volume average particle size of from 0.1 to 1.5 micrometers, and the other contains large rubber particles having a volume average particle size of from 1.5 to 10 micrometers, with the proviso that when the volume average particle size of the small particles is greater than 1 μ, the large particles have a volume average particle size of at least 2μ.

In bimodal compositions, the desired ratio of the small to large particles is dependent upon the properties desired in the final rubber-reinforced polymer. Typically the amount of small particles ranges from 50 to 99, preferably from 75 to 96, and most preferably from 80 to 95 percent of the total amount of rubber particles in the rubber- reinforced polymer. For products which require high gloss properties, the amount of small particles is from 85 to 98 percent and the amount of large particles is 2 to 15 percent. Other polymers in which higher impact strength is desirable can have 65 to 75 percent small particles and 25 to 35 percent large particles.

As used herein, the volume average particle size refers to the diameter of the rubber particles, including all occlusions of vinyl aromatic polymer within the rubber particles. Volume average particle sizes and distributions may be measured using conventional techniques such as a Coulter Counter™ or, transmission electron microscopy image analysis. Large particles are measured using a 50μ tube and smaller particles are measured by using a 20 micron tube. In one embodiment of the present invention, a high impact polystyrene (HIPS) composition is produced comprising a polymerized vinyl aromatic monomer, with dispersed particles of rubber having a bimodal particle size distribution. The size of the rubber particles are dependent upon the desired gloss and impact properties of the polymer product For bimodal HIPS compositions, the small rubber particles are typically in the range of from 0.1 to 1.5μ, preferably from 0.2 to 1.3, more preferably from 0.3 to 1.2 and most preferably from 0.2 to 1.1 μ, and the large rubber particles are typically from 1.5 to 10, preferably from 1.8 to 8, more preferably from 1.8 to 7 and most preferably from 2.0 to 6μ. Alternatively, the process may be utilized in the preparation of acrylonitrile- butadiene-styrene (ABS) type compositions, in which an alkenyl nitrile, generally acrylonitrile is used as a comonomer. For bimodal ABS compositions the small particles are typically in the range of from 0.2 to 1 , preferably from 0.3 to 1 , more preferably from 0.4 to 0.9 and most preferably from 0.5 to 0.8μ and the large rubber particles are typically from 1.5 to 10, preferably from 1.5 to 6, more preferably from 1.5 to 5 and most preferably from 1.5 to 4μ.

Due to the excellent balance of gloss and toughness properties, these rubber- reinforced bimodal compositions are useful in a wide variety of applications such as consumer electronics, small household appliances, toys and furniture. These polymers are also useful in extrusion applications such as in the preparation of a gloss layer using coextrusion techniques for refrigerator liners.

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 parts by weight unless otherwise indicated. EXAMPLES

Two feeds, a) and b) are prepared for each Example as listed in Table 1. The following rubbers are available from Bayer AG of Germany: HX528, a polybutadiene rubber. HX565, a low cis star branched rubber.

BL65533, a 60/40 butadiene/styrene block rubber. 200 kg of Feed was prepared as given in below table 1 and a) is fed to the top of a 2.5 liter agitated stirred tube reactor with a volume of 2.5 liters at the rate given in Table I. As the first polymerization feed travels down the reactor, the temperature of the polymerization is increased to maintain the rate of polymerization, typically from 112 to 130°C.

50 kg of Feed was prepared as given in table 1 and b) is fed to the top of a second agitated 1.1 liter stirred tube reactor. As the second polymerization feed travels down the reactor, the temperature of the polymerization is increased to maintain the rate of polymerization, typically from 120 to 135°C. The first and second reactors are operated to yield a solids content (polystyrene and rubber together) of 25 to 45 percent. The partially polymerized mixture from each of the first and second reactors are then combined and fed into a third reactor identical to that used for Feed a). The third reactor is operated to yield a solids content of 50 to 70 percent. The partial polymer coming out of the third reactor is then fed into a fourth reactor wherein they are further polymerized to 70 to 90 percent solids and fed into a devolatilizing extruder to remove unreacted monomers and diluents. The resultant polymer melt is then cut with a rotating knife into granules. TABLE I

PB=Polybutadiene HX528 Block=Block copolymer rubber BL65533 Star-Branched=low cis star branched rubber HX565 Initiator is tertiary-butylperbenzoate NDM=n-dodecyl mercaptan ^♦COMPARATIVE EXAMPLES

Feed rates are optimized for each run to produce product of optimum balance and gloss for the rubber used. Tensile testing is done on an Instron Tensile machine, using a pulling speed of 5 mm/min. according to ASTM D-638.

Notched Izod Impact testing is also done on injection molded specimens according to ASTM test method D-256.

Charpy Impacts are determined using injection molded bars according to the test method ISO-179-2C.

Gloss measurements are done using two sets of molding conditions reflecting 1) molding conditions for optimized gloss and 2)molding conditions for optimized output or reduced cycle time to produce the molded part on an Arburg All Rounder 170CM. Molding conditions for optimized gloss (1) are high mold temperature, high melt temperature and high injection speed. Molding conditions for optimized output

(2) are those that lead to a low level of gloss, including low melt temperature, low mold temperature and low injection speed. The difference between the gloss measurements under both of these conditions generally reflect the gloss sensitivity of the material. A greater difference between the measurements indicates a more gloss sensitive product and vice versa. For high gloss applications a low gloss sensitivity is desired. Sixty degree Gardner measurements are done using a Dr. Lange device.

The following molding conditions are given for the two different gloss measurements. TABLE II

¹Temperatures at four sites in the injection molding machine along the barrel representing molten polymer mass temperature. The first value is taken from the nozzle of the molding machine.

TABLE III

RESULTS

PB=Polybutadiene HX528 Block=Block copolymer rubber BL65533 Star-Branched= >w cis star branched rubber HX565 'COMPARATIVE EXAMPLES

Examples III and IV have high impact, excellent gloss and low gloss sensitivity. Thus excellent balance of impact and gloss is achieved without the use of expensive block copolymers.

Claims

CLAIMS:

1. A rubber modified monovinylidene aromatic polymer having a bimodal particle size distribution comprising: a ) rubber particles of a star or branched low viscosity rubber having a volume average particle size of from 0.1 to 1.5μ, and cellular or core shell morphology or mixture thereof, and b) rubber particles of a star or branched low viscosity rubber, linear diene rubber, or block copolymer rubber having a volume average particle size of from 1.5 to 10μ, and a cellular morphology, with the proviso that when the volume average particle size of the particles of a) is greater than 1 μ, the volume average particle size of the particles of b) is at least 2μ, wherein the particles of a) are from 50 to 99 wt. percent of the total diene rubber content 2 . The composition of Claim 1 wherein the rubber of a) is a homopolymer of butadiene.

3 . The composition of Claim 1 wherein the rubber of b) is a homopolymer of butadiene.

4 . The composition of Claim 1 wherein the polymer is a HIPS polymer and the rubber particles of a) are from 0.2 to 1.3μ.

5 . The composition of Claim 4 wherein the rubber particles of a) are from 0.3 to 1.2μ

6 . The composition of Claim 5 wherein the rubber particles of a) are from 0-2 to 1.1 μ. 7 . The composition of Claim 1 wherein the polymer is a HIPS polymer and the rubber particles of b) are from 1.8 to 8μ.

8 . The composition of Claim 7 wherein the rubber particles of b) are from 1.8 to 7μ.

9 . The composition of Claim 8 wherein the rubber particles of b) are from 2 to 6μ.

10 . The composition of Claim 1 wherein the polymer is an ABS polymer and the rubber particles of a) are from 0.2 to 1 μ.

11 . The composition of Claim 10 wherein the rubber particles of a) are from 0.3 to 1 μ. 12 . The composition of Claim 11 wherein the rubber particles of a) are from 0.4 to 0.9μ.

13 . The composition of Claim 12 wherein the rubber particles of a) are from 0.5 to 0.8μ.

14 . The composition of Claim 1 wherein the polymer is an ABS polymer and the rubber particles of b) are from 1.5 to 10μ.

15 . The composition of Claim 14 wherein the rubber particles of b) are from 1.5 to 6μ. 16 . The composition of Claim 15 wherein the rubber particles of b) are from 1.5 to 5μ.

17 . The composition of Claim 16 wherein the rubber particles of b) are from 1.5 to 4μ. lδ . The composition of Claim 1 wherein the rubber particles of a) have a core shell morphology.

19 . The composition of Claim 1 wherein the rubber particles of a) have a cellular morphology.

20 . The composition of Claim 1 wherein the rubber particles of a) have a mixture of core shell and cellular morphology. 21 . The composition of Claim 1 wherein the polymer is a HIPS polymer wherein the volume average particle size of the particles of a) is from 0.3 to 0.8μ and the volume average particle size of the particles of b) is from 5 to 6μ.

22 . Articles produced from the composition of Claim 1.