High heat PC

HIGH HEAT POLYCARBONATESGary F. Smith, James Mahood, Matt Pixton, Adam S. Zerda and Mark A. SannerGE Plastics1 Lexan LaneMt. Vernon, IN 47620AbstractA series of new high heat polycarbonate copolymers have been prepared and commercialized. These transparent resins extend polycarbonate heat capability beyond 200 o C while maintaining excellent melt processability, good mechanical properties and strength as well as practical toughness and flame resistance. In addition, the copolymers demonstrate useful optical properties including low birefringence and high refractive index, which may be useful in lighting and lens applications. The new resins may be blended with a variety of additives, such as fiberglass, to make a family of amorphous high heat resins.IntroductionPolycarbonate resins are amorphous engineering thermoplastics with an excellent balance of toughness, strength, optical properties, and heat resistance properties. Bisphenol A (BPA) polycarbonate (BPA-PC), the industry standard, is used in such diverse applications as CDs, DVDs, commercial and automotive lighting, instrument panels, water bottles and ophthalmic lens. However, many new applications require thermal capabilities greater than that of BPA-PC, which has a glass transition temperature (T g) of 150°C and heat deflection temperature (HDT) of 132o C at 1.8 MPa.Polyestercarbonates, which have a T g of 175°C and HDT of 148o C at 1.8 MPa can increase thermal capabilities by 20o C, however they have found limited use in high heat applications. Some applications such as commercial and automotive lighting, headlamp reflectors, automotive bezels, fire helmets, cookware and medical applications all require higher heat capability than currently achievable by BPA-PC and Polyestercarbonates materials. Many higher heat polycarbonate resins have been prepared over the years. In most cases, there is a significant number of trade-offs made between mechanical and physical properties with melt processability in order to obtain additional thermal capabilities. Impact properties are decreased with reduced water white color (more yellowness) and melt processing is made more difficult with reduced melt flow of the material. A new family of polycarbonate copolymer resins (Lexan® XHT grade resins) have been developed, which give an excellent balance of thermal capabilities, melt flow and ductility. These resins will be denoted as XHT followed by the Tg o CExperimentalPolycarbonate resins were prepared via proprietary polymerization processes and extruded on single or twin screw extruders. ASTM, ISO and UL procedures were followed for physical testing.Results & DiscussionA series of new polycarbonate copolymers. Lexan® XHT grade resins (XHT PC), have been prepared by replacing varying amounts of the Bisphenol A (BPA) with new comonomers. As the level of comonomers is increased the T g increases in a linear fashion to over 280°C as shown in Figure 1. This extends the opportunity for higher heat applications significantly. Primary focus in this discussion will be materials in the 170-220o C range. This results in a corresponding increase in the heat deflection temperatures (HDT) from 138o C for BPA polycarbonate to 208o C at 0.45 MPa.As the heat capability of resins go up the toughness or impact strength of the material typically drops considerably. These new copolymers were designed to exhibit impact strength better than commercially available high heat polycarbonates and other resins with similar glass transition temperatures. The ASTM notched izod impact data is shown in Figure 2. These resins are compared to polycarbonate copolymers based on BPA and isophorone bisphenols (denoted as BPI followed by the Tg o C) and polyetherimides (PEI). The notched izod impact is nearly 2 to 4 times greater than these high heat resins.This practical impact advantage is observed for both low flow, high molecular weight resins and high flow, low molecular weight versions of the resin.Higher viscosity is often observed with materials of high T g. The higher viscosity and corresponding higher processing temperatures often lead to difficulties with compounding or injection moldings. The new high T g polycarbonates described here exhibit improved processability over comparable resins of similar T g. Spiral flow curves (Figure 3) demonstrate these processing advantages by exhibiting longer flow lengths at comparable processing temperatures and injection pressure. Alternatively, comparable flows lengths can be achieved at lower temperatures. The ability to process at lower temperatures can reduce the potential for resin degradation and reduced cycle time while increasing productivity.The resins demonstrate good optical properties. The refractive index is 1.60 for the 185°C XHT 185 resin and 1.611 for the XHT220 resin. These materials have a reduced birefringence versus BPA polycarbonate. The yellowness index (YI) ranges from 0.2 to 8 while the transmittance ranges from 83 to 89%. The % Haze ranges from 0.3 to 0.8.Higher heat resins will naturally be exposed to higher temperatures in their applications. Many polycarbonates are susceptible to significant color changes on thermal aging. The color stability of the new high-heat polycarbonate resins show excellent retention of color versus the isophorone bisphenol based materials. Heat aging studies at 160°C (Figure 4) show a three-fold reduction in yellowness index over time with these new resins. The more stable color then translates to a smaller change in the optical transmission characteristics of the polymers.Automotive bezels and reflectors often require the molded parts to be metallized. The copolymers are readily metallized with both evaporative vapor deposition and sputtering techniques. Good surface is maintained during heat exposures within 10°C of the heat deflection temperature.3 Good surface reflectivity is maintained even with the use of up to 100% metallized regrind. Minimal impact on the metallized gloss is observed upon the addition of metallized regrind as shown in Figure 5. Regrind studies using 20% non metallized regrind showed no effect of gloss measurements in non metallized parts demonstrating excellent melt stability. (Figure 6)Flame resistance versions of these copolymers can be formulated. We have demonstrated UL V-0 behavior has been demonstrated in transparent versions of these resins at 2.0 mm as compared to 3.0 mm in comparable BPA-PC gradesBasic physical properties are shown in Table 1.ConclusionA new high heat polycarbonate family of copolymers has been developed and commercialized giving an excellent balance of heat, flow and ductility. The resins show good aging characteristics, flame resistance capability and the ability to undergo secondary operations such as metallization.References1.Schmidhauser, J and Sybert, P, Ch 5, pp 61-95 inLegrand, D.G, Bendler, John T. Handbook ofPolycarbonate Science and Technology, 2000,Marcel Dekker Inc., New York2.“Polycarbonates” by J. DeRudder, N.Rosenquist, B. Sapp, P Sybert, EngineeringPlastics Handbook, Chapter 14, pp 327-383,2005, J. Margolis, Editor, McGraw Hill3.Zerda, Adam, Guggenheim, K., Grau, Beatrice,Hoeks, Theo, “Heat Aging Performance ofMetallized Thermoplastics for AutomotiveLighting”, ANTEC, 2007 Presentation.Figures and tables are provided for general information and are not for the purpose of warranty or specification.Any flame or ignition resistant rating presented herein is NOT intended to reflect hazards presented by this or any other material under actual fire conditions.All resins and mixtures discussed herein should be thoroughly tested in actual parts under end use conditions before incorporation into any device.Table 1. High heat polycarbonate physical propertiesFigure 1. Glass Transition Temperature vs. copolymer content.XHT High Heat PC Tg vs Copolymer Level150175200225250275300020406080100% CopolymerT g , CTypical High Heat Polycarbonate Resin Properties TestsUnitsXHT 185XHT 185XHT 195XHT205XHT215XHT220BPI 175BPI 182BPI 206BPI 225HF FLOWMelt Flow Rate, 330C, 2.16 kgg/10 min 241099107281595ASTM D 1238THERMAL Tg°C 183185195205215220175182206225DSCHDT @ 0.45 MPa °C 175177179195204208163172190210ASTM D 648HDT @ 1.8 MPa °C 163164169177185194151159176194ASTM D 648Vicat B/120°C 182183189203212213170179197215ASTM D 1525MECHANICAL Flexural modulus MPa 2670265025502590270028202510251025702550ASTM D 790Flexural strength MPa 121123110111116135115115122124ASTM D 790Tensile modulus MPa 2750275027302770293028802320232023502290ASTM D 638Tensile strengthMPa 78787378838772727777ASTM D 638Tensile elongation @ brk%65676466682595958538ASTM D 638IMPACTNotched Izod (RT)J/m 91163153116848377644847ASTM D 256Un-notched Izod (RT)J/m NB NB NB NB NB NB NB NB NB NB ASTM D 4812Multiaxial impact (RT)J 72726560525268787472ASTM D 3763Charpy Impact StrengthkJ/m2141612111096656ISO 179/1eAFigure 2. Comparison of notched izod impact of the high heat polycarbonates with isophorone based bisphenols (BPI) copolymers and polyetherimides.Figure 3. Spiral flow at 3.0 mm of high heat polycarbonates vs. processing temperatureNo t c h e d I z o d (J /m )Spiral Flow vs. Melt Temperature(0.12" wall, 1800 psi Inj. Pressure, 220F mold)0510152025303540620640660680700720740oFS p i r a l F l o w - I n c h e sFigure 4. Change in yellowness index (YI) after heat aging in air at 160o CFigure 5. Effects of metallized regrind on metallized surface glossHigh Heat PC 160C Heat Aging Delta YI010203040506070040080012001600Hrs @ 160CD e l t a Y IMetallized Regrind Effects on 20oGloss150015501600165017001750180018501900195020000255075100% Metallized Regrind20o G l o s sFigure 6. Effect of repeated use of 20% non-metallized regrind on surface gloss758595105115125012345Regrind cycles20o G l o s s。