Disclosed is a mass polymerized rubber-modified monovinylidene aromatic copolymer composition with an excellent balance of physical and mechanical properties and high intrinsic gloss, and methods for preparing such a composition.

FIELD OF THE INVENTION

This invention relates to mass polymerized rubber-modified monovinylidene aromatic copolymer compositions with an excellent balance of physical and mechanical properties and high intrinsic gloss, and methods for preparing such compositions.

BACKGROUND OF THE INVENTION

Monovinylidene aromatic copolymers reinforced with rubber, in particular with diene rubber, represent a well known class of commercially available engineering polymers widely described in the literature. Specific examples of the copolymers are for example styrene and acrylonitrile copolymers, generally referred to as SAN resins, containing rubber particles, for example butadiene, dispersed in the polymeric matrix, generally known as ABS resins.

The rubber-modified monovinylidene aromatic copolymers can be prepared by continuous or batch processes and by various polymerization processes such as bulk, mass-solution, or mass-suspension, these are generally known as mass polymerization processes. A continuous mass polymerization process is known and described for example in U.S. Pat. Nos. 2,694,692; 3,243,481 and 3,658,946, and in published EP 400,479. This process consists of dissolving the rubbery material in the monovinylidene aromatic monomer and ethylenically unsaturated nitrile monomer mixture, adding possibly a radical polymerization initiator and an inert diluent, and then polymerizing the resulting solution. Immediately after the polymerization reaction commences, the rubbery material in the monomer mixture separates into two phases, of which the former, consisting of a solution of the rubber in the monomer mixture, initially forms the continuous phase, whereas the latter, consisting of a solution of the resultant copolymer in the monomer mixture, remains dispersed in form of droplets in said continuous phase. As polymerization and hence conversion proceed the quantity of the latter phase increases at the expense of the former. As soon as the volume of the latter phase equals that of the former, a phase change occurs, generally known as phase inversion.

When this phase inversion takes place, droplets of rubber solution form in the polymer solution. These rubber solution droplets incorporate by themselves small droplets of what has now become the continuous polymer phase. During the process, grafting of the polymer chains on the rubber takes place, too.

Generally, the polymerization is carried out in several stages. In the first polymerization stage, known as prepolymerization, the solution of the rubber in the monomer mixture is polymerized until phase inversion is reached. Polymerization is then continued up to the desired conversion.

Mass polymerization affords rubber-modified monovinylidene aromatic copolymers with a good balance of physical and mechanical properties, however the surface gloss of such copolymers is not always quite satisfactory. It is well know that surface gloss of a fabricated product is a function of copolymer composition as well as how the article is fabricated, e.g., the conditions under which the copolymer is molded or extruded. Less than favorable fabricating conditions result in lower surface gloss sometimes referred to as the intrinsic gloss of the copolymer.

The intrinsic gloss of diene rubber-modified monovinylidene copolymers can be improved by reducing the size of the rubber particles to less than 1 micrometer, e.g., by vigorous stirring during polymerization. However, this approach has not been successful because the usually available linear polybutadiene rubbers have a rather high molecular weight and thus a high solution viscosity, so that even with a strong agitation, it is not possible to achieve satisfactory rubber sizing, at least for the rubber concentrations commonly used in these copolymers (5 to 15 percent). Linear polybutadiene rubbers of low molecular weight and hence of reduced solution viscosity could be easily sized under stirring, but these rubber suffer from the known cold flow draw back, which introduced additional problems in their storage and handling.

U.S. Pat. No. 4,421,895 discloses the use of a diene rubber with a solution viscosity of 80 centipoise (cps) or less, when measured as a 5 weight percent solution in styrene at 25 C., in ABS production. Specifically, the diene rubber proposed in this patent is a styrene and butadiene linear block copolymer. This type of block rubber does not suffer from cold flow and easily affords the formation of small particles. Using this type of linear block rubber and operating in accordance with the process described in this patent, an ABS with rubber particles of less than 0.7 micrometers ( m) is obtained. However, by using the above styrene and butadiene linear block copolymer, the intrinsic gloss improvement is achieved at the expense of the other physical properties and mechanical properties, in particular the impact strength, so that the ABS obtained does not offer the desired combination of good physical and mechanical properties and intrinsic gloss.

Further, rubber-modified monovinylidene aromatic copolymers prepared from rubber mixtures of linear butadiene and linear block copolymer rubbers are disclosed for example in U.S. Pat. No. 5,756,579. However, while mechanical properties are improved these ABS compositions do not achieve the desired intrinsic gloss.

It is also known from the literature that rubber-modified monovinylidene aromatic copolymers can be prepared by using as the rubber a star-branched or radial block polymer. The use of such star-branched rubbers in rubber-modified monovinylidene aromatic copolymer production is discussed for example in U.S. Pat. Nos. 5,191,023; 4,587,294 and 4,639,494, in published EP 277,687 and in JP 59-232,140 and 59-179,611. However, while compared with other known diene rubbers, these star-branched rubbers afford copolymers with an improved balance of gloss and physical and mechanical properties they still have the drawback of lower intrinsic gloss.

Attempts to provide rubber-modified monovinylidene aromatic copolymers with an improved balance of gloss and physical and mechanical properties prepared from mixtures of star-branched and linear rubbers are disclosed for example in published JP 5-194,676; 5-247,149; 6-166,729 and 6-65,330 and in published EP 160,974. However, these compositions have large rubber particle sizes resulting in lower intrinsic gloss than desired.

In view of the deficiencies of the rubber-modified monovinylidene aromatic copolymer compositions thus obtained by utilizing any of such methods it would be highly desirable to provide an economical rubber-modified monovinylidene aromatic copolymer composition which exhibits an improved balance of physical and mechanical properties combined with high intrinsic gloss.

SUMMARY OF THE INVENTION

Accordingly, the present invention is such a desirable economical rubber-modified monovinylidene aromatic copolymer composition having a desirable balance of high intrinsic gloss and good impact resistance. The composition comprises a continuous matrix phase, comprising a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer, with a rubber dispersed therein as discrete rubber particles, said composition having an intrinsic gloss of at least 70 percent and an Izod impact strength of at least 150 Joule per meter (J/m). The rubber-modified copolymer is prepared using bulk, mass-solution or mass-suspension polymerization techniques.

In another aspect, the present invention is a process for preparing a mass polymerized rubber-modified monovinylidene aromatic copolymer composition comprising the steps of mass polymerizing in the presence of a dissolved rubber a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer, optionally in the presence of an inert solvent, to the desired degree of conversion and subjecting the resultant mixture to conditions sufficient to remove any unreacted monomers and to cross-link the rubber, said composition having an intrinsic gloss of at least 70 percent and an Izod impact strength of at least 150 J/m.

In a further aspect, the present invention involves a method of molding or extruding a mass polymerized rubber-modified monovinylidene aromatic copolymer composition comprising a continuous matrix phase, comprising a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer, with a rubber dispersed therein as discrete rubber particles, said composition having an intrinsic gloss of at least 70 percent and an Izod impact strength of at least 150 J/m.

In yet a further aspect, the invention involves molded or extruded articles of a mass polymerized rubber-modified monovinylidene aromatic copolymer composition comprising a continuous matrix phase, comprising a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer, with a rubber dispersed as discrete rubber particles, said composition having an intrinsic gloss of at least 70 percent and an Izod impact strength of at least 150 J/m.

The mass polymerized rubber-modified monovinylidene aromatic copolymer compositions of the present invention are especially useful in the preparation of molded objects notably parts prepared by injection molding techniques where a balance of high surface gloss and good impact resistance is desired. Such properties are particularly appropriate for household appliances, toys, automotive parts, extruded pipe, profiles and sheet for sanitary applications, power tool housings, telephone housings, computer housings, copier housings, etc.

EXAMPLES

To illustrate the practice of this invention, examples of preferred embodiments are set forth below. However, these examples do not in any manner restrict the scope of this invention.

The compositions of Examples 1 to 9 and comparative examples A to E are mass produced acrylonitrile butadiene styrene terpolymer resins wherein the rubber was dissolved in a feed stream of styrene, acrylonitrile, and ethyl benzene to form a mixture. The mixture was polymerized in a continuous process while agitating said mixture. The polymerization occurred in a multi staged reactor system over an increasing temperature profile. During the polymerization process, some of the forming copolymer grafts to the rubber particles while some of it does not graft, but instead, forms the matrix copolymer. The resulting polymerization product was then devolotalized, extruded, and pelletized.

The pellets were used to prepare test specimens on a DEMAG injection molding machine model D 150-452 having the following molding conditions: Barrel temperature settings of 220, 230, and 240 C.; Nozzle temperature of 250 C., Hot runner tip temperature of 245 C., Mold temperature of 50 C.; Injection pressure: 70 bar; Holding pressures 1/2/3: 60/50/35 bar; Back pressure: 5 bar; Injection time: 10 seconds; Follow-up pressure 1/2/3: 5/4/2 seconds; Cooling time: 20 seconds; and injection speed: 18 cubic centimeters per second (cm 3 /s).

The formulation content and properties of Examples 1 to 9 and comparative examples A to E are given in table 1 below in percent based on weight of the total rubber-modified monovinylidene aromatic composition. In Table 1:

Linear rubber-1 is a 70/30 butadiene/styrene block copolymer commercially available as SOLPRENE 1322 from Industries Negromex having a 5 percent solution viscosity in styrene of 25 cps;

Linear rubber-2 is a 15/85 styrene and butadiene block copolymer commercially available as SOLPRENE 1110 from Industries Negromex having a 5 percent solution viscosity in styrene of 35 cps;

Star-branched rubber-1 is a butadiene homopolymer commercially available as BUNA HX565 from Bayer having a 5 percent solution viscosity in styrene of 44 cps, a Mooney viscosity of 59, a M w of 200,000, and a Mn of 110,000;

Star-branched rubber-2 is an anionically polymerized 95/5 butadiene/styrene block copolymer coupled with a tetrafuctional coupling agent having a 5 percent solution viscosity in styrene of 20 cps;

PBD is the percent polybutadiene in the rubber;

SAN is the styrene and acrylonitrile matrix copolymer;

M w is the matrix copolymer (e.g., SAN) weight average molecular weight which was measured by gel permeation chromatography using polystyrene standards, determinations were made with a UV detector set at 254 nanometers;

M n is the matrix copolymer (e.g., SAN) number average molecular weight which was measured by gel permeation chromatography using polystyrene standards, determinations were made with a UV detector set at 254 nanometers;

% AN is the percent acrylonitrile in the SAN;

PDMS is a polydimethylsiloxane available as DC 200 (50 centistokes) from Dow Corning;

RPS is the rubber particle size reported as volume average particle diameter (Dv) determined by the analysis of transmission electron micrographs.

Samples prepared from melt flow rate strands produced by means of an extrusion plastometer at 220 C. and 3.8 kg load were cut to fit a microtome chuck. The area for microtomy was trimmed to approximately 1 square millimeter (mm 2 ) and stained in OsO 4 vapor overnight at 24 C. Ultrathin sections were prepared using standard microtomy techniques. 70 nanometer thin sections were collected on Cu grids and were studied in a Philips CM12 Transmission Electron microscope at 120 KV. The resulting micrographs were analyzed for rubber particle size distribution and rubber phase volume by means of a Leica Quantimet Q600 image analyzer. Images were scanned with a resolution of 0.005 micometer/pixel in auto contrast mode in which the white level was adjusted first to give full-scale output on the whitest part of the image then black level was adjusted to give zero output on the darkest part of the image. Unwanted artifacts in the background were removed by a smooth white morphological transform.

Micrographs show particles which are not cut through the middle. A correction method developed by Scheil (E. Scheil, Z. Anorg. Allgem. Chem. 201, 259 (1931); E. Scheil, Z. Mellkunde 27(9), 199 (1935); E. Scheil, Z. Mellkunde 28(11), 240 (1936)) and Schwartz (H. A. Schwartz, Metals and Alloys 5(6), 139 (1934)) is slightly modified to take the section thickness (t) into account. The measured area of each rubber particle (a i ) is used to calculate the equivalent circle diameter n i : this is the diameter of a circle having the same area as the rubber particle. The distribution of n i is divided into discrete size groups of 0.05 micometer d i from 0 to 1 micometer. N i = n i + j = i + 1 m N j d j 2 - d i 2 - d j 2 - d i - 1 2 t + d i 2 - d i - 1 2

where

N i : number of particles in class i after correction

d i : maximum diameter of class i

m: total number of classes.

n i : number of particles in class i before correction

Once Ni versus d i is obtained, the volume average diameter (Dv) of the rubber particles is calculated as follows: Volume average diameter D v = i = 1 m N i d i 3 N 3

LAR is the light absorbance ratio determined using a Brinkmann model PC 800 probe calorimeter equipped with a 450 nm wavelength filter, from Brinkmann Instruments Inc., Westbury, N.Y., or equivalent, is used. In a first vial, a 0.4 gram (g) sample of rubber-modified copolymer is dissolved in 40 milliliters (ml) of dimethylformamide (DMF). From the first vial, 5 ml of the resulting DMF solution is added to a second vial containing 40 ml of DMF. From the first vial, 5 ml of the resulting DMF solution is added to a third vial containing 20 ml of dichloromethane (DCM). The probe is zeroed in neat DMF. The absorption of the DMF solution in the second vial and the absorption of the DCM solution in the third vial are determined. The light absorbance ratio is calculated by the following equation: LAR = ( Absorbance of Sample in DMF ) ( Absorbance of Sample in DCM ) ;

The following tests were run on Examples 1 to 9 and comparative examples A to E and the results of these tests are shown in Tables 1:

intrinsic Gloss was determined by 60 Gardner gloss on specimens prepared from molded samples, 30 minutes after molding, according to ISO 2813 with Dr. Lange RB3 reflectometer.

The dimensions of the molded plaque are 64.2 mm 30.3 mm 2.6 mm. Intrinsic gloss is measured in the center of the plaque on the surface at which the pressure is measured. The materials are injected through one injected point located in the middle of the short side of the mold. During injection molding, the injection pressure switches to holding pressure when the cavity pressure reaches the pre-set value. The pressure transducer is located at a distance of 19.2 mm from the injection point. By using a constant pre-set cavity pressure value, the weight of the molded plaques is the same for materials with different flow characteristics.

The polishing of the mold is according to SPI-SPE1 standard of the Society of Plastic Engineers.

Charpy impact resistance was determined according to DIN 53453 at 23 C.;

Izod impact resistance as measured by the Notched Izod test (Izod) was determined according to ISO 180/4A at 23 C.;

MFR melt flow rate was determined according to ISO 1133 on a Zwick 4105 01/03 plastometer at 220 C. and an applied load of 10 kg, samples were conditioned at 80 C. for 2 hours before testing; and

Tensile property testing was done in accordance with ISO 527-2. Tensile Type 1 test specimens were conditioned at 23 C. and 50 percent relative humidity 24 hours prior to testing. Testing was performed at room temperature using an Zwick 1455 mechanical tester.