Polymer Blends of Polycarbonate, PCTG and ABS

Disclosed herein are polymer blends of polycarbonate, polyester and graft rubber compositions which are useful as engineering thermoplastic molding resins. Also disclosed are methods of making and using such polymer blends.

As used herein compositions are described by weight percent. To facilitate an understanding of this invention the variety of polymers discussed herein shall be referred to by their common acronyms. However in the examples the acronyms refer to the specific polymers identified therein.

Except for the specific polymer identified as used in the examples, the term "PC" as used herein means an aromatic polycarbonate derived from the reaction of bisphenol-A and phosgene; such PC is commercially available from General Electric Company under the LEXAN trademark, from the Dow Chemical Company under the CALIBRE trademark and from Mobay Corporation under the MAKROLON trademark, including modifications of such polymers such as halogenated polycarbonate.

Unless indicated otherwise the term "PETG" as used herein means an amorphous polyester of terephthalic acid and a mixture of predominately ethylene glycol and a lesser amount of 1,4-cyclohexanedimethanol; such PETG was commercially available from Eastman Chemical Company under the KODAR trademark and is currently sold under the EKTAR trademark.

Except for the specific polymer identified as used in the examples, term "PCTG" as used herein means an amorphous polyester of terephthalic acid and a mixture of predominately 1,4-cyclohexanedimethanol and a lesser amount of ethylene glycol, such PCTG was commercially available from Eastman Chemical Company under the KODAR trademark and is currently sold under the EKTAR trademark. Unless indicated otherwise the term "PET" as used herein means a crystallizable polyalkylene terephthalate, i.e. a polyester of terephthalic acid and ethylene glycol; such PET is commercially available from The Goodyear Tire & Rubber Company under the CLEARTUF trademark.

Unless indicated otherwise the term "PBT" as used herein means a crystallizable polyalkylene terephthalate, i.e. a polyester of terephthalic acid and butanediol; such PBT is commercially available from General Electric Company under the VALOX trademark.

Except for the specific polymer identified as used in the examples, the term "ABS" as used herein means a graft rubber composition comprising a substrate of butadiene or copolymers of butadiene and styrene or acrylonitrile which substrate is grafted with styrene and acrylonitrile or derivatives thereof, e.g. .alpha.-methyl styrene and methacrylonitriles. ABS typically also comprises extractable styrene-acrylonitrile copolymer, i.e. coplymer not grafted to the rubber; such ABS is commercially available from Monsanto Company under the LUSTRAN trademark.

Unless indicated otherwise the term "ASA" as used herein means a graft rubber composition comprising a crosslinked acrylate rubber substrate e.g. butyl acrylate grafted with styrene and acrylonitrile or derivatives thereof, e.g. .alpha.-methyl styrene or methacrylonitrile, such ASA is commercially available from General Electric Company under the GELOY and/or LURAN trademark.

Unless indicated otherwise the term "MBS" as used herein means a graft rubber composition comprising a substrate of butadiene or mixtures of butadiene and styrene or acrylonitrile grafted with styrene and methacrylate; such MBS is commercially available from Rohm & Haas Company under the PARALOID trademark.

Unless indicated otherwise the term "AIM" as used herein means an acrylate impact modifier such as a graft rubber composition comprising a crosslinked acrylate rubber grafted with an acrylate thermoplastic; such AIM is as commercially available from Rohm & Haas Company under the PARALOID trademark.

Nakamura et al. disclose in U.S. Pat. No. 3,864,428 blends of crystallizable polyester, such as polyalkylene terephthalates, e.g. PET or PBT, or polyalkylene naphthalates, PC and graft copolymers, e.g. MBS, having lower melt viscosity than PC alone.

Cohen et al. disclose in U.S. Pat. No. 4,257,937 blends of PBT (optionally PET), PC and polyacrylate.

Fromuth et al. disclose in U.S. Pat. Nos. 4,180,494 and 4,264,487 blends of PET, PC and AIM.

Lohmeijer et al. disclose in U.S. Pat. No. 4,654,400 blends of PC, PBT and AIM.

Blends of PET and/or PBT, PC and styrene-butadiene block copolymers are disclosed in U.S. Pat. Nos. 4,220,735, 4,239,677 and 4,111,895.

Chung et al. disclose in European Patent Application EP 0 108 996 A1 that impact properties of thermoplastic blends of polyalkylene terephthalate, e.g. PET, PC and an elastomeric based graft copolymer, e.g. ABS, are increased by the addition of a modifying oil. More specifically, Chung et al. illustrated that the addition of naphthenic oil (3.11%) improved the toughness of a blend of 64.2% PET, 14.2% PC and 21.4% ABS.

Allen et al. disclose in European Patent Application EP 0 111 810 A2 that a disadvantage associated with the use of polyalkylene terephthalate is its relatively low notched impact strength which carries over into blends of the polyester with PC. It is further disclosed that amorphous copolyesters e.g. PETG or PCTG, can be used in polycarbonate blends to improve in impact strength, transparency, processability, solvent resistance and environmental stress cracking resistance.

Boutni discloses in U.S. Pat. No. 4,628,074 blends of PC, PCTG and EPDM.

Ishihara discloses in U.S. Pat. No. 4,775,717 blends of PC and (graft modified) polystyrene having high heat resistance; improvements in melt viscosity are at the expense of toughness (e.g. see Table 1).

Liu discloses in U.S. Pat. No. 4,604,423 improving the impact strength of glass fiber-filled PC with a minor amount of amorphous polyester, e.g. PETG or PCTG and an olefin acrylate polymer.

Key properties for assessing the polymer blends of this invention are: melt flow into a mold as characterized by "Spiral Flow" (determined according to procedures set forth hereinbelow); heat resistance as characterized by heat distortion temperature under load (HDTUL) measured in accordance with ASTM D648; and impact resistance as characterized by notched Izod impact resistance of 3.2 cm (1/8 inch) thick samples (IZOD-1/8) and thick section notched Izod impact resistance of 6.4 cm (1/4 inch) thick samples (IZOD-1/4), measured at room temperature (23.degree. C.) in accordance with ASTM D-256.

Although polymer blends are prepared to provide a more desirable balance of properties as compared to the properties of the component polymers, there are often deficiencies in one or more properties of the polymer blend. For instance, a popular commercial blend comprises PC and ABS have higher heat resistance than ABS and reduced notch sensitivity in thick sections than PC; but a limitation of such blends have less than desired flow properties for many injection molding applications. Commercially available blends of PC and amorphous polyester have desirable heat resistance but low impact resistance, especially in thick sections, and less than desired flow properties for many injection molding applications. Blends of amorphous polyester and ABS have adequate flow properties but are generally so deficient in heat and impact resistance as to be undesirable as a molding resin.

I have discovered that polymer blends comprising PC, amorphous polyester, i.e. PCTG, and graft rubber composition, i.e. ABS or ASA or mixtures thereof, can be provided with a surprisingly advantageous balance of properties which makes such polymer blends especially useful as thermoplastic molding resins. Certain polymer blends of this invention comprise at least 10% PCTG and have improved melt flow into a mold, e.g. Spiral Flow of at least about 27 cm, compared to blends of PC and ABS having Spiral Flow of about 24 cm, without adverse reduction in thick section impact resistance and heat resistance properties common to blends of PC and ABS; more preferred blends comprise at least about 20% PCTG and more preferably at least about 30% PCTG. Certain other polymer blends of this invention comprise at least 10% PC and have substantially improved heat resistance, e.g. HDTUL of 75.degree. C., and substantially improved thick section impact resistance, e.g. IZOD-1/4 of about 275 Joules/meter (J/m) (about 5 foot-pounds/inch), as compared to blends of ABS and PCTG having HDTUL of 70.degree. C. and IZOD-1/4 of 180 J/m, without adverse effect on the high melt flow into a mold common to blends of ABS and PCTG. Still other polymer blends of this invention comprise at least about 10% graft rubber composition, i.e. ABS or ASA, and have substantially improved melt flow into a mold, e.g. Spiral Flow of about 28 cm, and substantially improved impact resistance, e.g. IZOD-1/8 of 875 J/m and IZOD-1/4 of 130 J/m, as compared to blends of PC and PCTG having Spiral Flow of less than 24 cm, IZOD-1/8 of about 85 J/m and IZOD-1/4 of about 70 J/m, without significant adverse effect on high heat resistance common to blends of PC and PCTG. Preferred blends of this invention exhibit high a melt flow into a mold, e.g. Spiral Flow of at least 27.5 cm; other preferred blends exhibit improved heat resistance, e.g. HDTUL of least 75.degree. C. and still other preferred blends exhibit substantially improved impact resistance, especially thick sections impact resistance i.e. IZOD-1/4, of at least 130 J/m preferably at least 300 J/m more preferably at least 500 J/m.

A useful rheological characterization for melt flow into a mold of a molten polymer blend can be measured by injecting a molten resin into a mold having a small, fixed cross-sectional area over an elongated flow path. A moldability index is the length of the elongated flow path that is filled before solidification of the injected polymer at specified molding conditions, i.e. volumetric injection rate, mold temperature, polymer melt temperature and mold volume. Mold configurations commonly used include half round profiles extended in spiral or serpentine shapes. The moldability index of the polymer blends of this invention exemplified hereinbelow was characterized by "Spiral Flow" which is the length of a spirally-elongated, half-round mold that is filled with molten polymer under specified injection molding conditions. Numerical values of parameters used to define "Spiral Flow" apparatus and methods are nominal. For "Spiral Flow" the mold is cut into a flat surface with a round broach having a radius of 2.77 mm (0.109 inch) to a depth of 2.77 mm and a length of at least 42 cm; a 5 to 15 cm long runner connects from a 4 mm (diameter) sprue to the mold at the beginning of the spiral; the runner has a cross sectional area between 2 and 4 times the cross-sectional area of the half round mold. The mold is maintained at 40.degree. to 50.degree. C. (110.degree. F.); the polymer or blend of polymers is at a temperature between 250.degree. to 280.degree. C. (510.degree. F.) as measured in an "air shot" of molten polymer and is injected in a mold volume (i.e. volume of the sprue, runner and half round length) between 20% and 60% of the maximum shot volume of the injection molder and at an injection rate between 3 and 7 cm.sup.3 /second.

To avoid variations, "Spiral Flow" is measured after equilibrating the molding cycle (e.g. after about 20 cycles) and is determined as the average half round injection length of 12 consecutive moldings having a molding cycle between 10 and 30 seconds. A typical 21 second molding cycle comprises the following time sequence: delayed injection--1 second, injection--6 seconds, hold--6 seconds, cooling--6 seconds, and mold open--2 seconds. The throttle valve is wide open and the injection pressure is adjusted to provide a calibration half round mold length (i.e. base line Spiral Flow) between 20 and 24 cm for a 50/50 blend of the PC and PCTG which are used in the blends. Improvements in melt flow into a mold are indicated by Spiral Flow mold lengths greater than the base line Spiral Flow.

Grafted rubber composition used in the blends of this invention comprises ABS, ASA or a mixture thereof.

ABS used in the polymer blends of this invention comprises from 5 to 80 weight percent of a substrate butadiene-based rubber having a glass transition temperature below 0.degree. C. and from 95 to 20 weight percent of a grafted styrene-acrylonitrile copolymer consisting essentially of from 20 to 75 parts by weight of a styrene monomer and from 80 to 25 parts by weight of acrylonitrile or methacrylonitrile, said copolymer having been polymerized in the presence of and grafted onto the substrate rubber, e.g. by well known methods such as graft emulsion polymerization. Typically, some of the styrene-acrylonitrile copolymers is extractable, i.e. not grafted to the rubber. Styrene monomers can comprise styrene, substituted styrene such as alpha-methyl styrene, chlorostyrene, bromostyrene, para-methyl styrene and the like or a mixture thereof. The butadiene substrate rubber can be crosslinked, and in many cases is preferably crosslinked, and can comprise a butadiene-acrylonitrile rubber or styrene-butadiene rubber. Preferably, ABS comprises from 20 to 45 weight percent of substrate butadiene rubber and from 55 to 80 parts by weight of styrene-acrylonitrile copolymer. Preferred styrene-acrylonitrile copolymer comprises from 50 to 75 parts by weight of styrene monomer units and from 25 to 50 parts by weight of said polar monomer units. Especially preferred ABS comprises styrene and acrylonitrile in the ratio of about 70/30.

ASA used in the polymer blends of this invention can comprise about 20 to 60 weight percent of a substrate acrylate rubber, e.g. about 45 weight percent cross linked butyl acrylate rubber, grafted with styrene and acrylonitrile in the proportion and manner of ABS, as deserved above.

In addition to the above components the polymer blends of this invention can advantageously contain other additives common to polymer blends of engineering thermoplastics such as antioxidants, stabilizers, flame retardants, dyes, pigments, fillers such as fibers and minerals, plasticizers and the like.

The component polymers of the polymer blends of this invention can be assembled by any convenient mixing process, e.g. extrusion blended in a single or twin screw extruder or in a high intensity mixer such as a Banbury Mixer or Farrell Continuous Mixer.

The polymer blends of this invention are especially useful as engineering thermoplastics for applications warranting a balance of properties inherent in such blends, including chemical resistance, e.g. to mineral acids such as muriatic acid; low mold shrinkage, e.g. about 6 mils/inch (0.6%); good mechanical properties, e.g. tensile strength and elongation, high toughness, e.g. good notched Izod impact resistance even at low temperatures.

The invention is now described with reference to the following examples which are for purposes of illustration only and are not intended to imply any limitation on the scope of the invention.

The polymer blends described in the following examples contained about 0.75 percent by weight of an antioxidant mixture comprising 2 parts by weight alkylated phenol obtained from Ethyl Corp as Ethanox 330 and 1 part by weight dilaurylthiodipropionate obtained from Morton Thiokol Company.

In the following examples components were dried before mixing under vacuum or nitrogen, e.g. PC at about 110.degree. C., PETG at about 70.degree. C., and ABS and ASA at about 65.degree. C. When VAMAC was used, it was preblended with ABS intimately mixed at about 100 rpm in a Killion single screw extruder (3.8 cm diameter). The blended components were fed to a Leistritz twin screw extruder equipped with a die-faced pelletizer. The polymer blend was extruded into a water bath and pelletized.

For characterization the polymer blends were injection molded into notched specimen bars for Izod impact resistance testing according to ASTM D-256 where the notch radius was 0.01 inches and the measurements were conducted at room temperature (about 23.degree. C.). In the following examples the term "IZOD 1/8" refers to the notched Izod impact resistance measured on a 1/8 inch thick specimen bar; and the term "IZOD 1/4", on a 1/4 inch thick specimen bar. Izod impact resistance is reported in Joules/meter (J/m). The term "HDTUL" refers to heat distortion temperature measured on 1/8 inch thick specimen bars stressed at 264 psi according to ASTM D 648. HDTUL is reported in degrees Celcius (.degree.C.). The term "Spiral Flow" refers to a polymer melt flow characterization where defined herein above. Spiral Flow is reported in centimeters (cm).

The following specification identifies the specific polymers used in the examples:

ABS--38 weight percent nitrile rubber particles (average particle size 0.2 microns) having a butadiene/acrylonitrile ratio of 93/7; 62 weight percent styrene-acrylonitrile copolymer having a styrene/acrylonitrile ratio of 73/27.

PC--Calibre.TM. M300-6 polycarbonate obtained from Dow Chemical Company.

PCTG--KODAR.TM. PCTG obtained from Eastman Chemical Company.

This example serves to illustrate that the incorporation of ABS in prior art polymer blends of PC and PCTG provides polymer blends according to this invention having improved melt flow and impact resistance and retaining a desirably high heat resistance. The results reported in Table 1 show that the incorporation of only 10 percent ABS substantially improves melt flow, e.g. by about 25 percent over blends of PC and PCTG; thick section toughness is about doubled. Further increases in ABS, i.e. an additional 10 percent increment, provides blends with spiral flow number above 30 cm and about a 500 percent increase in thick section toughness.

This example serves to illustrate that the incorporation of PCTG in prior art polymer blends of PC and ABS provides polymer blends according to this invention having improved melt flow and retaining desirably high impact and heat resistance. The results reported in Table 2 show that the incorporation of 30 percent PCTG into blends of PC and ABS improves melt flow, e.g. to a Spiral Flow number of about 28. Exceptionally high thick section toughness and heat resistance common in blends of PC and ABS are retained.

This example serves to illustrate that the incorporation of PC in prior art polymer blends of PCTG and ABS provides polymer blends according to this invention having improved heat and thick section impact resistance while retaining desirably high melt flow. The results reported in Table 3 show that the incorporation of only 10 percent PC with substantially improved heat resistance, i.e. an increase of about 5.degree. C. over the heat resistance of blends of PETG and ABS; thick section toughness is increased about 50%. Further increases in PC, e.g. in additional 10 percent increments, provides blends which retain exceptionally high melt flow properties with substantial improvements in thick section toughness and heat resistance.