POLYESTER/POLYCARBONATE BLENDS HAVING IMPROVED CLARITY AND IMPACT STRENGTH

Abstract: Disclosed are clear thermoplastic molding compositions comprising an aromatic polycarbonate, a polyester of terephthalic acid and 1,4-cyclohexane-dimethanol, and the copolymeric transesterification product of the polycarbonate and polyester.

Description: POLYESTER/POLYCARBONATE BLENDS HAVING IMPROVED CLARITY AND IMPACT STRENGTH Technical Field The present invention relates to polyester/ polycarbonate blends having improved clarity and impact strength. The blends comprise the transesterification product of polyester and polycarbonate. Background of the Invention Polycarbonate is a useful engineering plastic for parts requiring clarity, high toughness, and, in some cases, good heat resistance. However, polycarbonate also has some important deficiencies, among them poor chemical and stress crack resistance, poor resistance to sterilization by gamma radiation, and poor processabilit . One method to improve these properties is to blend polycarbonate with another resin, a useful family of such resins being the thermoplastic polyesters. This invention relates to a blend comprised of poly(1,4-cyclohexylenedimethylene terephthalate) [referred to herein sometimes as "Polyester C], bisphenol A polycarbonate, and copolymers of this polyester and polycarbonate which have surprisingly good low temperature toughness compared to known polyester/polycarbonate blends . In addition, the processability, chemical resistance, and radiation resistance of the polycarbonate are also improved. Research Disclosure 22921, May, 1983, concerns blends of polycarbonate with copolyesters based on terephthalic acid and a mixture of 1,4-cyclohexane- dimethanol and ethylene glycol. Similar blends are disclosed in U.S. Patent 4,786,692. Blends of polycarbonate with another family of copolyesters, those based on a mixture of terephthalic and isophthalic acids with 1,4-cyclohexanedimethanol, are disclosed in U.S. Patents 4,188,314 and 4,391,954. British Patent Specification 1,599,230 (published January 16, 1980) discloses blends of polycarbonate with polyesters of 1,4-cyclohexanedimethanol and a hexacarbocyclic dicarboxyliσ acid. Mohn, et. al., reported on thermal properties of blends of polyesters based on 1,4-cyclo¬ hexanedimethanol and terephthalic acid or terephthaliσ/isophthalic acid mixtures with polycarbonate [J. Appl. Polym. Sci., 23, 575 (1979)], concluding that there were only slight differences in behavior between the two systems, and that these results were not unexpected. The first known disclosure of blends of Polyester C and polycarbonate is attributed to Mohn, et. al, [J. Appl. Polym. Sci., 23, 575 (1979)]. Their work suggested that chemical reactions did not occur during the melt mixing of polycarbonate with polyesters of 1,4-cyclohexanedimethanol and terephthalic acid or terephthalic/isophthaliσ acid mixtures. Thus, it is indeed surprising that applicants found that a chemical reaction, transesterification, does in fact occur, and that certain physical properties of the blend are improved. Mohn et al. further concluded that due to the chemical similarities of the two polyesters employed, the similarities observed in the blend properties resulting from their respective blending with polycarbonate were not unexpected. In British Patent Specification 1,599,230, blends of polycarbonate with polyesters of 1,4-cyclohexane¬ dimethanol and a hexacarbocyclic dicarboxylic acid were disclosed. They also reported no significant differences in the polyester/polycarbonate blends that were claimed. Further, no indication of the addition to the blend of or the formation in situ of any copolymeric adduσts was reported. The present invention is a new thermoplastic composition comprised of Polyester C, bisphenol A polycarbonate, and copolymers of Polyester C and polycarbonate. In their unmodified forms, the blends of this invention are clear. This, combined with their excellent impact strengths, make them useful in ophthalmic and safety shield applications. Additional applications include medical parts, appliance parts, and housings. Description of the Invention According to the present invention, there is provided a clear thermoplastic molding composition comprising a compatible blend of (a) 10-90% by weight aromatic polycarbonate, (b) 10-90% by weight polyester consisting essentially of repeat units from terephthalic acid and 1,4-cyclohexanedimethanol, and (c) 1-50%, preferably 1-20%, by weight of the copolymeric reaction products of (a) and (b), wherein the total of (a), (b) and (c) is 100%. The polycarbonate consists of the polycarbonate of 4,4' -isopropylidenediphenol (bisphenol A). The I.V. of the polycarbonate is 0.3-1.2. The polycarbonate portion of the blends is prepared in the melt, in solution, or by interfacial polymerization techniques well known in the art. Many such polycarbonates are commercially available, and are normally made by reacting bisphenol A with phosgene, dibutyl carbonate, diphenyl carbonate, etc. The polyester portion of the molding compositions of the present invention is prepared by conventional polycondensation procedures well known in the art. Conventional catalysts such as, for example, tin or titanium are generally used in amounts of 10-150 ppm of the polymer. These catalysts remain in the polymer. The polyester, poly(l,4-cyclohexylene dimethylene terephthalate) contains repeat units from a dicarboxylic acid component and a glycol component. The dicarboxylic acid component, a total of 100 mol %, consists essentially of terephthalic acid and the glycol component, a total of 100 mol %, consists essentially of 1,4-cyclohexanedimethanol. The polyesters can be prepared by direct condensation of the acid with the glycol or ester interchange using a lower alkyl ester such as dimethyl terephthalate. The essential components of the polyester, e.g., terephthalic acid or dimethyl terephthalate and 1,4-cyclohexanedimethanol, are commercially available. The polyesters described above should have an I.V. (inherent viscosity) of from 0.4 to 1.2. The copolymeric reaction products of the polycarbonate and polyester can be any combination of terephthalic acid (TPA) , polycarbonate (PC), or 1,4-cyclohexanedimethanol (CHDM) repeat units as defined below, linked to any combination of TPA, PC, or CHDM by either an ether [-0-] or a carbonate [-OCO -] unit. Repeat Units PC- —• 0 • • o « \ / I \ / CHDM: -CH 2 —^ ^—CR^ •—• The transesterification reaction can be schematically represented as follows : TPA-0-CHDM + PC-0C0 2 -PC (PCT) I I (Polycarbonate) For each transesterification reaction that occurs, a new unit in the polymer chain is formed as defined by the reaction scheme. The residual catalyst used in forming the polyester acts as catalyst for transesterification. As the transesterification process proceeds, these same products are formed randomly along the polymer chain again in any combination previously discussed. It should be apparent that there are a large number of possible combinations of repeat units in the transesterification reaction products . The transesterification reaction (i.e., the degree of transesterification) can be monitored by using conventional nuclear magnetic resonance (NMR) techniques. By using the analytical method, the average number of units formed, via the transesterification process, per chain can be determined. Various NMR spectroscopy techniques are known in the art for measuring the amount of transesterification. Among them are the following articles: E.V. Gouinlock, R. A. Wolfe, J. C. Rosenfeld, J. Appl. Polym. Sci., 20, 949-958, (1976) J. Devaux, et. al. J. Polym. Sci. , Polym. Phys. Ed. 20, 1881-1894, (1982) Transesterification of the polyester and polycarbonate can be carried out by mixing them at a temperature of 290-325°C for a time sufficient to attain the desired transesterification reaction product, generally 1-10 minutes. Of course, it should be understood that the reaction time is dependent upon the temperature, residual catalyst level, and the reaσtants' inherent viscosities. The blends may be made by melt compounding the components, for example, in a single or twin screw extruder. The copolymeric components can be produced in the molten state, either separately from the blend and then subsequently added to the polyester/ polycarbonate premix prior to blending or, given sufficient exposure to processing conditions in the presence of active residual polyester polymerization catalyst(s), in situ. Melt temperatures typically will be in the range of 295- 315°C; it is desirable that they not exceed 325°C. Of course it is understood that the copolymeric components of this invention can be prepared by alternative methods such as solid phase ester-carbonate interchange reactions and then subsequently added to the polyester/polycarbonate premix prior to blending. The range of composition of the blends is from 10 to 90 wt % Polyester C and from 1 to 50 wt % copolymeric components, with the remaining composition being comprised of polycarbonate. The preferred compositional range is from 40 to 60 wt % Polyester C and from 1 to 20 wt % Polyester C/polycarbonate copolymeric components. The composition may also contain other additives such as impact modifiers, colorants, stabilizers, processing aids, flame retardants, fillers, and the like. The following examples are submitted for a better understanding of the invention. Example 1 (Control) A 50/50 blend of polycarbonate with Polyester A (a copolyester having repeat units from 100 mol % terephthalic acid, 70-85 mol % 1,4-cyclohexane¬ dimethanol and 30-15 mol % ethylene glycol), I.V. = 0.75, is prepared. The properties are shown in Table 1. Example 2 (Control) A similar blend is prepared from Merlon M40 polycarbonate with Polyester B, a copolyester having repeat units from 100 mol % 1,4-cyclohexanedimethanol with a mixture of 85 mol % terephthalic and 15 mol % isophthalic acids. The properties are also shown in Table 1. Example 3 Two 50/50 blends of polycarbonate and Polyester C are similarly prepared. A blend consisting of 50% polycarbonate, 49.75% Polyester C, and 0.25% stabilizer was also prepared. The stabilizer is a distearyl pentaerythritol diphosphite compound commonly used as a melt stabilizer that acts to reduce the activity of the residual catalyst in the Polyester C. All blends are prepared as previously outlined, allowing for sufficient residence time for the necessary transesterification reactions to occur at the given processing temperature. As evidenced in Table 2, the 50/50 Polyester C/ polycarbonate premix undergoes transesterification reactions during the melt processing, forming in situ 7 and 14% by weight Polyester C/polycarbonate copolymeric components respectively as determined by NMR spectroscop . At similar processing conditions, " the blend containing the stabilizer is similarly shown to contain no copolymeric components. All of the blends prepared in Examples 1, 2, and 3 display high notched Izod impact strengths at 23°C, but, unexpectedly, the value at 0°C remains high for only the Polyester C/polycarbonate blends containing at least some copolymeric fraction. Example 4 The blends of Table 3 are made from Polyester C, I.V. = 0.77, and polycarbonate, and demonstrate that across the entire composition range unexpectedly high values of the notched Izod impact strength at low temperatures (0°C) are obtained. In addition, the compositions have a useful balance of other properties (modulus, tensile strength, and heat deflection temperature) . TABLE 1 Polycarbonate/ Polycarbonate/ Pol ester A Pol ester B Notched Izod Impact Strength, 23°C ft-lb/in. (J/meter) 0°C Tensile Strength at Yield, psi (MPa) Tensile Strength at Break, psi (MPa) Elongation at Break, (%) I Flexural Modulus, psi (MPa) I Flexural Strength, psi (MPa) Heat Deflection Temperature, (°C) 105 106 TABLE 2 Notched Izod Impact, 23°C. ft-lb/in. (J/meter) 0°C Tensile Strength at Yield, psi (MPa) Tensile Strength at Break, psi (MPa) Elongation at Break (%) o Heat Deflection Temperature (°C) 94 94 87 I Concentration of Polyester C/ 14 PC Copolymers (%) TABLE 3 Flexural Modulus, 235,000 241,000 248,000 271,000 281,000 287,000 306,000 312,000 336,000 psi (MPa) (1619) (1660) (1708) (1867) (1936) (1977) (2108) (2150) (2315) Flexural Strength, 9520 9800 10540 11340 11620 12060 12500 13020 13570 psi (MPa) (65.6) (67.5) (72.6) (78) (80) (83) (86) (90) (93.5) HDT at 66 psi 84 88 94 101 106 111 120 124 132 ( .455 MPa) (°C) As used herein, the inherent viscosity (I.V.) is measured at 25°C using 0.50 g of polymer per 100 mL of solvent consisting of 60% by weight phenol and 40% by weight tetrachloroethane. The following tests are used herein: Flexural Modulus: ASTM-D790 Flexural Strength: ASTM-D790 Tensile Strength: ASTM-D638 Elongation: ASTM-D638 Izod Impact Strength: ASTM-D256 Heat Distortion Temperature: ASTM-D648 Unless otherwise indicated, all parts, percentages, ratios, etc. are by weight. The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be affected within the spirit and scope of the invention.

Claims: CLAIMS 1. A thermoplastic molding composition characterized as containing (a) 10-90% by weight aromatic polycarbonate, (b) 10-90% by weight polyester consisting essentially of repeat units from terephthalic acid and 1,4-cyclohexanedimethanol, and (c) 1-50% by weight of the copolymeric reaction products of (a) and (b) wherein the total of (a), (b) and (c) is 100%. the composition when consisting only of (a), (b) and (c) being clear. 2. A molding composition according to Claim 1 wherein the I.V. of the polycarbonate is 0.3-1.2 and the I.V. of the polyester is 0.4-1.2. 3. A molding composition according to Claim 1 comprising 40-60% by weight polycarbonate, 40-60% by weight polyester and 1-20% by weight of said copolymer, the total of said polycarbonate, polyester and copolymer being 100%. 4. A molding composition according to Claim 1 wherein said polyester and polycarbonate are reacted by mixing at a temperature of 290-325°C. 5. A thermoplastic molding composition characterized as being produced by mixing 10-90 weight % polyester consisting essentially of repeat units from terephthalic. acid and 1,4-cyclohexane- dimethanol, and 90-10 wt % aromatic polycarbonate under reaction conditions such that transesterifi¬ cation occurs to the extent that 1-50% of the polyester moieties are chemically reacted with a polycarbonate moiety.

Inventors: FLORA THOMAS EDUARD, SEYMOUR ROBERT WILLIAM, SMALL JAMES DOUGLAS JR, FLORA, THOMAS, EDUARD, SEYMOUR, ROBERT, WILLIAM, SMALL, JAMES, DOUGLAS, JR.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1990
Main classification: C08L67
IPC codes: C08L67, C08L69