Authored by:
Jon Malinoski
Sabic Innovative Plastics
Pittsfield, Mass.
Edited by Kenneth J. Korane

Polycarbonate materials based on bisphenol-A (BPA) are noted for their inherent optical clarity, low color, toughness, and ductility, along with high heat, chemical, and flame-resistance properties. For these reasons, BPA polycarbonate is used in a wide array of applications, including optical media, protective lenses and face shields, electronics housings, appliance components, medical devices, as well as engineered films and sheets.

As is typical with most amorphous thermoplastic polymers, the melt viscosity of polycarbonate at a given temperature is primarily a function of its chain-molecular weight (assuming a linear polymer architecture and relatively monomodal molecular-weight distribution). However, mechanical properties such as toughness and lowtemperature ductility are also proportional to molecular weight, with ductility improving as average chain length increases.

Thus, product engineers working with polycarbonate often have to balance the competing demands of molding efficiency — the ability to fill increasingly longer flow paths and thin-wall part designs — with the ductility and toughness that a particular application requires.

Since the commercial introduction of BPA polycarbonate over 50… years ago, numerous innovations have improved the flow and ductility balance of standard polycarbonate (PC). For example, PC can be blended with other polymers such as ABS to form tough alloys with better flow and processing capabilities. Another approach is to incorporate other monomers or oligomers (molecules that contain just a few monomers), such as polydimethylsiloxane, into the polymer backbone to form polycarbonate copolymers with enhanced processing and toughness properties. One potential drawback, however, is the resulting materials can range from slightly translucent to completely opaque, thus limiting their use in applications that demand light transmission or clarity.

A new range of polycarbonate copolymers, made from bisphenol-A and a bio-sourced comonomer derived from castor-bean oil, overcomes these drawbacks. The copolymers better balance flow and ductility, compared to standard PC, yet retain the same optical clarity and lighttransmission properties. The copolymer materials have been commercialized as Lexan HFD resins. Here’s a look at how they compare to standard polycarbonate across a range of mechanical, impact, rheological, and injectionmolding properties. In addition, we evaluate glass fiberfilled formulations of HFD copolymer and standard polycarbonate for mechanical and impact properties, as well as surface gloss.

Experimental background

The materials studied are standard bisphenol-A polycarbonate and Lexan HFD resins. They are labeled according to their composition followed by their melt-flow rate (MFR) at 300°C/1.2 kgf. Standard polycarbonate examples are termed PC and the new copolymer is labeled HFD. For example, a 25 MFR standard polycarbonate material is designated PC-25 and a 40 MFR HFD material is HFD-40. Glass-fiber-filled formulations are labeled according to composition and weight percent of glass loading. For instance, a standard polycarbonate sample with 20% by weight of glass fiber is called PC-GF-20.

All formulations were thoroughly compounded in a 30-mm corotating twin screw (Werner & Pfleiderer ZSK-30) extruder using a melt temperature of 300°C with a rate of 20 kg/hr, 20-in.-Hg vacuum, and screw speed of 400 rpm. The extruded material was cooled under water, pelletized, and dried at 120°C for 4 hr with a desiccant-bed dryer.

To make test specimens, the dried pellets were injection molded using a Van Dorn 80T molding machine at 300°C melt temperature to form test parts for impact and mechanical testing. Among the various tests performed:

  • Melt-flow rate (MFR), melt-volume rate (MVR), and density measurements were conducted on pellet samples following ASTM D1238, ISO1133, and ASTM D792 Standards.
  • Light transmission, heat-deformation temperature (HDT), Izod impact, instrumented impact, flexural, and tensile tests were performed on 3.2-mm-thick injection-molded parts according to ASTM Standards D1003, D648, D256, D3763, D790, and D638.
  • Viscosity versus shear-rate measurements were performed at 300°C using a Kayeness capillary rheometer.
  • Spiral-flow molding studies were conducted at a melt temperature of 290°C and channel depths of 1.5, 2.3, and 3.0 mm.
  • Mold-release studies used a 1° draft-angle cup tool at 300°C melt temperature, 60°C mold temperature, and 56-sec injection-molding cycle time.
  • Surface gloss for glass-fiber-filled PC and HFD samples was measured on 100-mm square plaques 2.5-mm thick. 60° gloss measurements were recorded using a BYK Microgloss meter.
  • For spiral-flow and surface-gloss molding studies, the injection speed, injection pressure, and cycle time were held constant for all standard polycarbonate and HFD copolycarbonate samples.

Ductility

A series of HFD copolymer and standard polycarbonate samples were prepared spanning a range of melt viscosities from 7-gm/10-min MFR (300°C, 1.2 kgf ) to 65 gm/10 min. Ductility of these samples was measured by notched-Izod impact testing over a range from room temperature (23°C) down to –40°C. The ductile-brittle transition temperature, defined as the temperature where the samples transition from 100% ductile to 100% brittle in the notched Izod impact test, was recorded for these samples. It’s plotted as a function of melt flow in the “Transitiontemperature” graph.

For any given melt-flow rate, the HFD samples show around 10°C lower ductile/brittle transition temperatures compared to the standard polycarbonate materials. Conversely, for a given ductile/brittle transition temperature, the HFD copolymers show between 40 to 60% higher MFR than standard polycarbonate.

A closer examination of physical and mechanical properties of 25 MFR PC, 25 MFR HFD, and 40 MFR HFD is shown in the “Property-comparison” table. The three materials have identical optical properties, as measured by percent light transmission (89% at 3.2-mm thickness) and refractive index (1.58), as well as similar densities (1.20 gm/cm3). Mechanical properties such as modulus and tensile elongation at break are also comparable between the PC and HFD materials.

The HFD samples show roughly 15°C lower softnotchedening temperatures compared to the PC material as measured by heat-deflection temperature (HDT) at 0.45 and 1.82-MPa loads. All three materials are 100% ductile at room temperature in notched-Izod impact testing, and the PC-25 and HFD-40 samples show comparable ductility at 0°C. The HFD-25 material has 100% ductility in 0°C notched Izod impact testing and even shows slightly higher room temperature impact energy (789 J/m) compared to the standard 25 MFR PC at 23°C (759 J/m).

Melt viscosity

The melt viscosity (measured in Pa-sec) for PC-25 at 300°C was measured using a capillary rheometer and compared to the melt viscosity of HFD-40 at 280° and 300°C melt temperatures. The “Viscosity versus shear” graph show the results. Over the shear range typical for injection-molding processes (1,000 to 5,000 sec-1) the HFD-40 material has roughly 30% lower melt viscosity compared to PC-25 at 300°C. The viscosity-versus-shearrate curve for HFD-40 at 280°C nearly overlaps the curve for PC-25 at 300°C.

Thus, in practical compounding or injection-molding conditions, we would expect the HFD-40 material at 280°C melt temperatures to process nearly the same as PC-25 at 300°C. Lowering the processing temperature of a polycarbonate material while maintaining ductility can be particularly important in applications involving thermally unstable additives or colorants that can degrade and lose efficiency under normal PC injection-molding or compounding temperatures.

To validate the higher MFR and lower melt viscosity of HFD-40 compared to PC-25 in a practical injection-molding application, the two materials were molded at 290°C melt temperature in a spiral-flow tool with a channel width of 16 mm and channel depth that can vary from 0.75 to 4.5 mm. The “Spiral-flow molding results” graph contains a plot of spiral-flow length at 290°C melt temperature for PC-25 and HFD-40 at three different channel depths. For each depth, the HFD-40 has 25 to 35% l ong e r f l ow length compared to the PC-25 sample. These results indicate that without sacrificing ductility or mechanical properties, the HFD- 40 material can improve processing compared to PC-25 in large or thin-wall parts, and also reduce residual molded-in stresses in challenging part designs.

Mold release

The mold-release performance of PC-25 compared to HFD-40 samples was measured using a cylindrical cup tool with a 1° core draft angle at 300°C melt temperature, 60°C mold temperature, and 56-sec cycle time. Both HFD-40 and PC-25 samples were compounded with an identical loading of a commercially available mold-release agent commonly used with general-purpose polycarbonate products.

Results of the test are shown in the “Mold release comparison” chart. Here, the HFD-40 sample showed a 75% lower ejector-pin force as the part was ejected from the tool. This significant improvement in mold-release performance can also contribute to an overall reduction in residual molded-in stresses for HFD parts. Based on these results, the HFD-40 material should improve processing characteristics and reduce molded-in stress and optical birefringence compared to the 25 MFR standard polycarbonate materials.

Glass-filled materials

Chopped or milled glass fibers are often added to polycarbonate formulations to increase stiffness or modulus of the material at the expense of impact and mechanical properties as well as surface gloss and appearance. Formulations of HFD copolymer and standard polycarbonate were compounded under identical conditions with 10 to 40% by weight of glass-fiber loadings. The melt viscosity of the PC and HFD samples were matched as close as possible for a given glass loading to determine if HFD materials hold any ductility advantages compared to PC. The “Property comparison – glass-filled materials” table shows the results of physical and mechanical tests.

Similar to the unfilled HFD and PC examples, glass-filled PC materials show 12 to 14°C higher HDT values compared to the HFD samples. There is not a significant difference in notched-Izod and instrumented impact properties between HFD and PC. Both HFD and standard polycarbonate formulations from 10 to 40% glass content show 0% ductility in room-temperature notched-Izod impact tests.

At 10% weight-glass loading, both HFD and PC material are 100% ductile in unnotched-Izod impact as well as instrumented impact tests. The PC-GF-10 material shows marginally higher impact energies in this test compared to the HFD-GF-10 sample. Unnotched-Izod and instrument impact tests report 0% ductility for both PC and HFD materials at 20% and higher glass loadings, and there is no significant advantage or disadvantage in impact energy for the HFD or PC formulations. The glass-filled HFD formulations show between 2 to 5% lower tensile and flexural modulus compared to comparable glass-filled standard polycarbonate materials. Based on this data, there is no significant difference in physical, mechanical, or impact properties between the glass-filled HFD and glass-filled PC samples, other than a small drop in heat-deflection temperature.

Surface gloss

Square plaques (100-mm × 2.5-mm thick with polished, smooth surfaces) were molded from the 10% glass content PC-GF-10 and HFD-GF-10 formulations under identical conditions to compare the surface quality of the two materials. Carbon black, 1% by weight, was added to each formulation to increase the surface contrast for visual comparison and gloss measurements.

The accompanying images show the results. The HFDGF- 10 samples have a considerably higher surface gloss compared to the PC-GF-10 samples. Analyzing the surface using optical microscopy (50× magnification) shows a substantial amount of exposed glass fibers in the PC-GF-10 sample, whereas the glass fibers in the HFD-GF-10 sample appear to be hidden beneath a thin layer of resin. The exposed glass fibers in the PC-GF-10 sample result in a rough surface and, thus, lower surface gloss.

The 20 to 40% glass-fiber HFD and PC samples showed similar results when molded in the same tool under similar conditions. The surface gloss of the glass-filled PC and HFD formulations was measured at a 60° angle, and as a function of glass-fiber loading. Results are shown in the “Surface-gloss” chart.

For all glass loadings, the HFD materials improve 60° surface gloss by 35 to 45% compared to the glass-filled standard polycarbonate samples. While melt temperature, mold temperature, injection speed, and cycle time can all play a role in determining the surface appearance of a glass-filled polycarbonate material, from these results it is clear that the glass-filled HFD materials offer a marked improvement in surface gloss compared to standard glassfilled PC — without a significant sacrifice in modulus or impact properties.

Overall, HFD copolymer materials offer a better balance of melt flow and ductility, compared to standard polycarbonate without sacrificing optical clarity. The HFD copolymers can improve toughness and low-temperature ductility at the same melt flow, or offer longer flow lengths and lowertemperature processing capability at the same impact as standard PC. In addition, HFD resins significantly improve mold-release properties, all of which can lower molded-in stresses in injection-molded parts. Glass-fiber-filled compositions of HFD copolycarbonate have mechanical and impact properties comparable to glass-filled standard PC materials, but with higher surface gloss in injection-molded parts due to fewer exposed glass fibers.

© 2012 Penton Media, Inc.