Environmental Stress Failure Resistant Polycarbonate

This invention relates generally to polycarbonates, and more particularly, to environmental stress failure resistant and impact resistant polymer blends comprising a polycarbonate and a copolymer of ethylene and carbon monoxide.

Polycarbonates are well-known commercially available resinous materials having a variety of applications. They are typically prepared by the reaction of dihydroxy compounds with a carbonate precursor, such as phosgene, in the presence of a catalyst. Methods of direct phosgenation, interfacial condensation and transesterification, for the preparation of polycarbonates, are described in detail in "The Chemistry and Physics of Polycarbonates", by H. Schnell, John Wiley & Co., N.Y., 1964.

Polycarbonates are high temperature, high performance engineering thermoplastics having a combination of good thermal and mechanical properties, especially when prepared from one or more aromatic diols. The blending with polycarbonates of additional compounds, such as for example, other thermoplastic resins, copolymer rubber compounds, and the like, is commonly practiced in order to improve one or more properties of the homopolymer polycarbonate.

The applications for polycarbonates as engineering materials are limited because they generally exhibit severe environmental stress failure. By environmental stress failure is meant surface crazing or cracking, or disintegration of the molded piece resulting from its contact with organic solvents while under stress, such as may occur for example when solvents are used to clean or degrease stressed parts fabricated from polycarbonates or polycarbonate blends.

U.S. Pat. No. 3,431,224 discloses a blend of a polycarbonate and a polyolefin such as polyethylene or an olefin copolymer such as ethylene/propylene, in order to improve the environmental stress failure resistance of the homopolycarbonate. U.S. Pat. No. 3,780,140 discloses a terpolymer of ethylene, carbon monoxide and a third component such as vinyl acetate, for blending with a polycarbonate to improve impact resistance.

U.S. Pat. No. 3,248,359 discloses the use of an ethylene/carbon monoxide copolymer for improving the environmental stress failure resistance of polyolefins such as polyethylene. Finally, U.S. Pat. No. 2,541,987 discloses a copolymer of ethylene and carbon monoxide as a plasticizer in chlorinated vinyl polymers such as polyvinyl chloride. The prior art does not suggest that an ethylene/carbon monoxide copolymer would be an effective blend component for increasing the environmental stress failure resistance and impact resistance of polycarbonates.

The present invention is directed toward a novel polymer blend, comprising an aromatic polycarbonate and a copolymer of ethylene and carbon monoxide. The blend surprisingly is highly impact resistant, and is likewise resistant to environmental stress failure.

The polymer blends of the present invention exhibit high impact resistance, chemical resistance, temperature stability, and excellent thermoplastic engineering properties, making them particularly suitable for producing molded plastic components for the automotive and electrical housing manufacturing industries.

The aromatic polycarbonates suitable for use in the present invention are produced by any of the conventional processes known in the art for the manufacture of polycarbonates. Generally, aromatic polycarbonates are prepared by reacting an aromatic dihydric phenol with a carbonate precursor, such as for example phosgene, a haloformate or a carbonate ester.

A preferred method for preparing the aromatic polycarbonates suitable for use in the present invention involves the use of a carbonyl halide, such as phosgene, as the carbonate precursor. This method involves passing phosgene gas into a reaction mixture containing an activated dihydric phenol, or a nonactivated dihydric phenol and an acid acceptor, such as for example pyridine, dimethyl aniline, quinoline and the like. The acid acceptor may be used undiluted or diluted with inert organic solvents, such as methylene chloride, chlorobenzene or 1,2-dichloroethane. Tertiary amines are advantageous since they are good solvents as well as acid acceptors during the reaction.

The temperature at which the carbonyl halide reaction proceeds may vary from below 0.degree. C. to about 100.degree. C. The reaction proceeds satisfactorily at temperatures from room temperature to 50.degree. C. Since the reaction is exothermic, the rate of phosgene addition may be used to control the temperature of the reaction. The amount of phosgene required will generally depend upon the amount of dihydric phenols present. Generally speaking, one mole of phosgene will react with one mole of dihydric phenol to form the polycarbonate and two moles of HCl. The HCl is in turn taken up by the acid acceptor.

Another method for preparing the aromatic polycarbonates useful in the present invention comprises adding phosgene to an alkaline aqueous suspension of dihydric phenols. This is preferably done in the presence of inert solvents such as methylene chloride, 1,2-dichloroethane and the like. Quaternary ammonium compounds may be employed to catalyze the reaction.

Yet another method for preparing such aromatic polycarbonates involves the phosgenation of an agitated suspension of an anhydrous alkali salt of an aryl diol in a nonaqueous medium such as benzene, chlorobenzene or toluene. The reaction is illustrated by the addition of phosgene to a slurry of the sodium salt of, for example, Bisphenol A in an inert polymer solvent such as chlorobenzene.

Generally speaking, a haloformate such as the bis-haloformate of Bisphenol A may be used in place of phosgene as the carbonate precursor in any of the methods described above.

When a carbonate ester is used as the carbonate precursor in the polycarbonate forming reaction, the materials are reacted at temperatures in excess of 100.degree. C., for times varying from 1 to 15 hours. Under such conditions, ester interchange occurs between the carbonate ester and the dihydric phenol used. The ester interchange is advantageously consummated at reduced pressures on the order of from about 10 to about 100 millimeters of mercury, preferably in an inert atmosphere such as nitrogen or argon.

Although the polymer forming reaction may be conducted in the absence of a catalyst, one may, if desired, employ a typical ester exchange catalyst, such as metallic lithium, potassium, calcium or magnesium. The amount of such catalyst, if used, is usually small, ranging from about 0.001% to about 0.1%, based on the weight of the dihydric phenols employed.

In the solution methods of preparation, the aromatic polycarbonate emerges from the reaction in either a true or pseudo solution depending on whether an aqueous base or pyridine is used as an acid acceptor. The copolymer may be precipitated from the solution by adding a polymer nonsolvent, such as heptane or isopropanol. Alternatively, the polymer solution may be heated, typically under reduced pressure, to evaporate the solvent.

A preferred aromatic polycarbonate is characterized by repeated units corresponding to the general formula: ##STR1## wherein X is a divalent C.sub.1 -C.sub.15 hydrocarbon radical, a single bond, --O--, --S--, --S.sub.2 --, --SO--, --SO.sub.2 --, or --CO--. Each aromatic ring may additionally contain 1 or 2 substituents such as C.sub.1 -C.sub.4 alkyl hydrocarbon radicals or halo radicals. A most preferred aromatic polycarbonate is prepared form 2,2-bis-(4-hydroxyphenyl)propane (Bisphenol A).

The aforementioned methods of preparing aromatic polycarbonates are more fully set forth in U.S. Pat. Nos. 2,999,846, 3,028,365, 3,148,172, 3,153,008, 3,248,414, 3,271,367, and 4,452,968, which are hereby incorporated in their entirety by reference thereto.

Also included in the term aromatic polycarbonate are the polycarbonate/polyester copolymers of the types disclosed in U.S. Pat. Nos. 3,169,121, 4,105,633, 4,156,069, 4,260,731 and 4,287,787 which are incorporated herein in their entirety by reference thereto, as well as mixtures of polycarbonates and polycarbonate/polyester copolymers.

The olefin/carbon monoxide copolymers of the present invention may be prepared by polymerizing ethylene with carbon monoxide, at an elevated temperature and pressure, in the presence of a catalyst such as a peroxy compound or azo compound. Specific procedures for preparing the copolymers are more fully set forth in U.S. Pat. Nos. 2,495,285, 2,541,987, 4,024,325 and 4,024,326 which are incorporated herein in their entirety by reference thereto. The amount of carbon monoxide in the copolymer may be from about 0.1% to about 50% of the total weight of the copolymer; preferably it is about 2% to about 15% by weight.

The polymer blend of the present invention comprises about 50% to about 95%, preferably about 75% to about 95%, of an aromatic polycarbonate; and about 5% to about 50%, preferably about 5% to about 25%, of an ethylene/carbon monoxide copolymer. The recited percentages are in relation to the total weight of the resinous blend. The components may be mixed using any conventional mixing apparatus.

The polymer blends of polycarbonate and ethylene/carbon monoxide copolymer may contain additional elastomeric impact modifiers, such as for example acrylonitrile-butadiene-styrene copolymers (ABS copolymers), methylmethacrylate-butadiene-styrene copolymers (MBS rubbers), hydrogenated styrene-butadiene copolymers, functionalized EPDM copolymers, acrylic latexes, polybutadiene, polyisoprene, polybutene, and the like.

The polymer blends of the present invention may furthermore contain conventional thermoplastic polymer additives, such as for example fillers, thermal stabilizers, dyes, flame retarding agents, reinforcing agents, softeners, mold-release agents, seed-forming agents, pigments, plasticizers, antistatic agents, ultraviolet radiation absorbers, lubricants, and the like, in conventional amounts generally not exceeding 50% of the total weight.

The invention is more easily comprehended by reference to specific embodiments which are representative of the invention. It must be understood, however, the the specific embodiments are provided only for the purposes of illustration and understanding, and that the invention may be practiced otherwise than as specifically illustrated and described without departing from its spirit and scope.

Dry blends comprising polycarbonate resin and ethylene/carbon monoxide copolymer were prepared in the proportions described in Table I (as were comparison compositions), and subsequently melt-mixed in a co-rotating twin screw extruder. The resulting polymer blend strands were quenched and pelletized, and the pellets injection molded into test specimens whose mechanical properties are described in Tables II and III.

Table II illustrates that the environmental stress failure resistance for blends of polycarbonate and ethylene/carbon monoxide was greater than for blends of polycarbonate and other thermoplastic polyolefin materials such as polypropylene and copolymers of ethylene and acrylic acid, as evidenced by the retention of % elongation at break values after exposure to the solvent.

LDPE, when blended with polycarbonate, acted similarly to the copolymer of ethylene and carbon monoxide in that both blends retained their % elongation at break values after exposure to the solvent. However, delamination of the polycarbonate and LDPE blends was severe, in contrast to the minimal delamination observed for polycarbonate blends containing the copolymer of ethylene and carbon monoxide. Additionally, lower initial % elongation at break values were observed for the polycarbonate blends containing LDPE than for blends containing the copolymer of ethylene and carbon monoxide. Likewise, the initial dart impact values were lower for polycarbonate blends containing LDPE than for blends containing the copolymer of ethylene and carbon monoxide, as illustrated in Table III.