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It has been documented that the addition of short-fiber glass to polycarbonate materials results in an increase in modulus, improved dimensional stability and enhanced flame resistance.2-4 However, several disadvantages are involved when adding fiber glass to polycarbonate materials, particularly loss of transparency, decrease in ductility and toughness, and increase in melt viscosity. Numerous studies have focused on investigating the affect of glass fiber surface coating, fiber aspect ratio, and fiber orientation on the mechanical properties of thermoplastic compositions.5-7
A new range of polycarbonate copolymers composed of BPA and a bio-sourced co-monomer derived from castor bean oil has recently been introduced.8-14M These materials, designated LEXAN™ high-flow ductile (HFD) resins,* show an improved flow and ductility balance compared to standard PC while retaining similar optical clarity and light transmission properties.
Some demonstrated advantages of the HFD copolymer resins compared to standard BPA polycarbonate include improved molding capability for long flow-length or thin-wall parts, lower temperature processing capability, improved low-temperature ductility, and superior mold release performance. In addition, preliminary studies have shown that fiber glass-filled formulations of HFD copolymers result in improved visual appearance and higher surface gloss in injection molded parts.14
Study SpecificsA study was undertaken to examine the physical and mechanical properties of fiber glass-filled HFD copolymer compositions at 10, 20, and 30% glass loadings, and to compare the performance of these materials to comparable glass-filled standard polycarbonate products. The comparison covers typical ASTM testing parameters such as melt flow rate (MFR), heat deflection temperature (HDT), izod and multi-axial impact, as well as tensile and flexural modulus. In addition, the melt viscosity and spiral flow molding performance of the glass-filled HFD and standard PC materials are presented.
Injection molding studies using a polished square plaque tool were performed in order to evaluate the effect of resin melt temperature, mold temperature, and injection speed on surface gloss for the glass-filled HFD and PC materials. Finally, the influence of melt viscosity, as well as formulation heat deflection temperature on molded plaque surface gloss, was evaluated by screening a range of HFD copolymer compositions with varying HDT and melt viscosity rate (MVR) properties.
All formulations were thoroughly compounded in a 30 mm co-rotating twin screw (Werner & Pfleiderer, ZSK-30) extruder using a melt temperature of 300°C with a rate of 20 kgs/hr, 20 in. of mercury vacuum and a screw speed of 400 rpm. Chopped fiber glass was added during the compounding step via a downstream feeder. The extrudate was cooled under water, pelletized and dried at 120°C for four hours 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. Physical and mechanical properties of the test materials in this study were measured using ASTM and ISO test standards. Viscosity vs. shear rate measurements were performed using a Kayeness capillary rheometer. Spiral flow molding studies were performed at a melt temperature of 310°C; mold temperature of 95°C; and channel depths of 1.5, 2.3, and 3.0 mm. The surface gloss for glass fiber-filled PC and HFD samples was measured on 100 mm square plaques at 2.5 mm thickness; 60° gloss measurements were recorded using a BYK Microgloss gloss meter.
Results and DiscussionThe physical and mechanical properties of fiber glass-filled HFD copolymer and standard PC materials were screened using a battery of standard ASTM and ISO test methods. The results of this comparative study are listed in Table 1.
The HFD copolymer samples show significantly higher MFR and MVR compared to the standard polycarbonate examples for 10, 20 and 30 wt % glass loadings. The increase in melt flow between HFD and standard PC materials becomes larger as glass content increases. For example, GFHFD-10 shows 2.5 times higher MVR compared to GFPC-10. However, GFHFD-30 shows four times higher MVR compared to GFPC-30. (The relative differences in melt viscosity between the glass-filled HFD and PC materials will be further evaluated using capillary rheology and spiral flow molding in upcoming sections.)
The impact and mechanical properties of the fiber glass-filled HFD copolymer and standard PC materials were analyzed using 23°C notched and un-notched izod, as well as multi-axial impact test methods. The GFHFD and GFPC materials do not show any significant differences in impact energy for notched izod impact across the range of fiber glass loadings. At 10% glass content, the GFPC-10 sample did demonstrate higher impact energy for un-notched izod impact (2020 J/m) compared to GFHFD-10 (1390 J/m). At 20 and 30% glass loadings, the difference between GFPC and GFHFD un-notched izod impact becomes negligible.
A comparison of the mechanical properties of glass-filled polycarbonate and glass-filled HFD copolymer materials shows very similar flexural and tensile modulus values. As expected, the modulus or stiffness of these materials increases with increasing fiber glass content. Tensile strength results were also nearly identical between the GFPC and GFHFD samples. These results indicate that there are no significant differences in resin-glass adhesion, and no significant differences occur in glass attrition during processing between the HFD copolymer and standard PC materials. Density, mold shrinkage performance, and thermal expansion properties were nearly identical between the PC and HFD samples across the 10-30% fiber glass content range.
It is important to note that the glass-filled PC samples in this study are capable of achieving a UL94 V-0 rating, whereas the glass-filled HFD copolymer samples are only formulated to achieve an HB rating. Future studies on the glass-filled HFD materials will focus on evaluating various formulation options for improving flame resistance capability.
The capillary melt viscosity at a constant shear rate of 1500 s-1 for 10, 20, and 30% glass-filled HFD and PC materials is shown in Figure 2. At a melt temperature of 300°C, the glass-filled HFD materials show 40-65% lower melt viscosity compared to the GFPC samples. As is expected based on the MFR and MVR data, the difference in melt viscosity between HFD and PC materials is larger at higher glass content.
Spiral flow molding is a tool often used to compare the practical flow length of materials under similar processing conditions. The spiral flow tool used in this study has a fixed channel width of 16 mm and a channel depth that can be varied from 1.5-3.0 mm. The results of spiral flow molding at a melt temperature of 310°C and a mold temperature of 85°C are shown in Figure 3.
Longer flow lengths and the opportunity for part thickness reduction are desirable attributes for GFHFD materials, particularly for applications such as mobile phone housings and frames. In addition, the higher flow properties of glass-filled HFD materials enable the use of higher fiber glass loadings for a given part geometry compared to standard glass-filled PC, which results in increased part stiffness and dimensional stability.
It has been previously documented that fiber glass-filled compositions of HFD polycarbonate copolymer show higher surface gloss in injection molded plaques compared to comparable glass-filled standard polycarbonate materials.10,14 An example of this is shown in Figure 4, where, under a given set of molding conditions, the GFHFD materials show 30-50% higher 60° gloss compared to the GFPC samples. When the surfaces of the plaques generated in this study are analyzed by optical microscopy, it is clear that the reduction in gloss for glass-filled materials (compared to an unfilled sample) is caused by exposed glass fibers at the part surface (see Figure 5). In the case of the GFHFD-10 example, the glass fibers are buried underneath a thin layer of resin, resulting in higher gloss and overall improved surface aesthetics.
An injection molding study was completed using a polished-surface square plaque tool to illustrate the effects of melt temperature, mold temperature, and molding injection speed on surface gloss. The results of this study, which compares results for the 20% glass-filled GFHFD and GFPC materials, are shown in Table 2. From the data, it is clear that increasing resin melt temperature, increasing mold temperature, and increasing injection speed all result in an increase in surface gloss.
In all cases, the GFHFD-20 material showed higher surface gloss than the GFPC-20 samples. The ability to achieve higher surface gloss is particularly important for parts requiring a good aesthetic appearance or applications where multiple coats of paint are normally required to hide any exposed glass fibers. In addition, certain part geometries or designs do not allow for extremely high melt or mold temperatures to achieve a high surface gloss. The glass-filled HFD materials therefore offer a broader processing window to achieve a higher gloss finish compared to a standard glass-filled PC.
As stated previously, the two major physical property differences between the GFPC and GFHFD compositions are melt flow and HDT. It is conceivable that either or both properties could contribute to producing higher gloss surface appearance for the GFHFD materials.
However, it was noted during these experiments that, as MVR increases, the pressure inside the mold decreases accordingly. When the injection speed was raised (125 mm/s), a consistently high surface gloss was observed across the entire MVR range. These results indicate that the in-mold pressure, which is a function of both resin melt viscosity as well as injection molding speed, is a key contributor to achieving higher surface gloss. A material with a higher MVR, such as the glass-filled HFD compositions, allows for larger/thinner wall parts and faster injection speeds, which can potentially improve molding cycle time and productivity.
In the second set of experiments, a range of 20% glass-filled HFD copolymer compositions were compounded at the same MVR target (11 cm3/10 min at 300°C/1.2 kg) but with varying HDT targets. Injection molding studies on these materials at a fixed set of process conditions resulted in a clear trend, showing decreasing surface gloss with increasing formulation HDT (see Figure 7).
From these results, it is clear that, at a given processing condition, the GFHFD materials with lower HDT remain in a molten state for a longer time during injection molding, which allows the resin to flow around the glass fibers and yield a more resin-rich surface. Therefore, the combination of lower HDT and higher melt flow (ability to mold at faster injection speeds) enables the glass-filled HFD copolymer compositions to yield a higher gloss surface compared to an equivalent glass-filled standard polycarbonate material.
ConclusionIn summary, the fiber glass-filled HFD polycarbonate copolymer materials are characterized by significantly higher melt flow compared to their equivalent glass-filled standard polycarbonate samples. The GFHFD compounds show 11-18°C lower HDT compared to the GFPC products, but have relatively similar impact, mechanical, and dimensional stability properties.
The glass-filled HFD copolymer materials allow for longer practical injection molding flow lengths and part thickness reduction. In addition, the glass-filled HFD samples yield improved surface gloss in injection molded plaques compared to the standard glass-filled PC materials.
In general, increasing resin melt temperature, mold temperature, and injection speed results in an increase in surface gloss and improved aesthetic appearance. Due to their higher flow and lower HDT properties, the glass-filled HFD copolymer materials offer a broader injection molding process window to achieve a higher gloss surface.
For more information, contact SABIC Innovative Plastics at One Plastics Ave., Pittsfield, MA 01201; call (413) 448-7110; or visit the website at www.sabic-ip.com
Author's NotesThe author would like to acknowledge Don Wedding and Mike Oeth for their work in formulating/compounding the test materials and performing the injection molding studies. Figures and tables are provided for general information only and are not for the purpose of warranty or specification. In all applications, testing of a part in end use conditions is strongly recommended.
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