SUPERIOR RESISTANCE TO THERMO-OXIDATIVE AND CHEMICAL DEGRADATION IN POLYAMIDES AND POLYPHTHALAMIDES

Transcription

1 SUPERIOR RESISTANCE TO THERMO-OXIDATIVE AND CHEMICAL DEGRADATION IN POLYAMIDES AND POLYPHTHALAMIDES Steven Mok E.I. du Pont de Nemours and Company, Troy, MI Gary P. Kozielski, Coreen Y. Lee, Jennifer L. Thompson E.I. du Pont de Nemours and Company, Wilmington, DE Avinash Malshe E.I. du Pont India Limited, Chennai, India Klaus W. Bender Du Pont de Nemours Deutschland GmbH, Neu-Isenburg, Germany Abstract DuPont SHIELD Technology allows polyamide and polyphthalamide resins to be used at higher temperatures than could be previously achieved. This SHIELD Technology combines several innovations, including a new polymer backbone, polymer modifications and a special set of additives, to enhance performance. The resistance to thermo-oxidative damage and chemical degradation is highly superior to standard polyamide polyamide and polyphthalamide resins. Examples of improved performance include: Improved air oven aging - retaining >50% of initial mechanical properties after at least 1000 hours at 210 C Improved fluid aging resistance - maintaining >75% of its impact strength after 5000 hrs at 150 C in hot oil. Improved CaCl2 resistance, resisting cracks three times the number of cycles of standard glass-reinforced nylons. Introduction One global megatrend is the pursuit of reduced fossil fuel dependency and emissions. A significant portion of fossil fuel is consumed in automobiles, so a significant amount of research and development is devoted to improving their fuel efficiency. One way to increase fuel efficiency is to reduce the overall vehicle weight by replacing metal with high-performance polymers. DuPont has introduced a new technology for polyamides that pushes the boundaries of polymer performance. This SHIELD Technology which combines modifications to the base polymer with synergistic additives to give superior thermal oxidative performance to molded parts. The products using SHIELD Technology also maintain the processing advantages of standard nylon grades.

2 The SHIELD Technology functions to slow the diffusion of oxygen into the sample.[1] Cross sections of an aged, molded test bar show that the rate of oxidation is significantly inhibited. As seen in Figures 1 and 2, the depth of the oxidation layer is minimal in the SHIELD Technology product (Fig 2) compared to the traditional PA66 product (Fig 1) despite the 20 C higher temperature. Figure 1. Microtomes of tensile bars aged at 210 C in air, of standard heat-stabilized glass-reinforced PA66 Figure 2. Microtomes of tensile bars aged at 230 C in air, of glass-reinforced PA with SHIELD Technology. Polyamides are already used in a number of automotive applications such as in engine cooling and intake systems but are limited in applications that require higher temperatures due to proximity to the engine and exhaust systems, like charge air coolers, resonators, and mufflers. Traditionally these higher temperature applications use metal or a high-temperature polymer like polyphenylene sulfide (PPS). The new SHIELD Technology allows polyamides to be considered for these high temperature applications. Materials The polymers used in these studies were Zytel HTN polyphthalamide (PPA) [2] and a partially aromatic copolyamide designed to have melt point and processing very similar to PA66. The compounded polymers are glass-reinforced with standard additives and processing aids as well as the proprietary additives. Specifically in this paper, several designations are used. The polymers are designated by two sets of acronyms. The first part of the acronym references the base polymer: PA6 for polyamide 6 and, PA66 for polyamide 66, stabilized with conventional technology that is used for most of the commercially available materials today. PA-S denotes the proprietary copolyamide with PA66 melt point, stabilized with new SHIELD Technology. PPA refers to polyphthalamide polymers and PPA-S is polyphthalamide resin with SHIELD Technology. The second part of the acronym refers to the amount of glass fiber in the resin. For example GF35 describes a resin with 35% glass fiber. Therefore a resin described PA66 GF35 is a polyamide 66 with 35% glass fiber reinforcement.

3 Heat Aging Discussion Resistance to oxidation becomes critical to long-term performance as operating temperatures are increased. Typically polymers exposed to high temperature, oxidative environments will degrade starting from the outside surface. As the oxidation affects a higher percentage of the cross section, properties become severely compromised. With some polymers, especially polyamides, this loss in properties is accompanied by substantial change of the surface layer due to discoloration or char formation [3,4,5]. Property retention of polyamides can be significantly extended at higher temperatures by inhibiting and retarding the reaction with oxygen at the surface of the molded part. Products stabilized with SHIELD Technology exhibit this protective effect. Figure 3 shows stress at break after aging at 210 C. Samples compounded with 45% GF demonstrated the highest, initial tensile strength while the PPA samples stabilized with SHIELD Technology demonstrated the higher strength after aging, when compared with PA-S GF35. At both 35 and 45% GF loading, SHIELD stabilized grades retained tensile strength twice as long as standard polyamides. PPS tensile strength decreased by 25% in the first 500 hours, then remained relatively constant. Figure 3. Stress at break after oven aging at 210 C. Measured at 23 C, per ISO 527. Degradation of plastics over time can be represented by an Arrhenius relationship based on air oven aging at different temperatures. The reaction is measured by the loss of a given property to a specified level. For simple systems, this provides a way to represent long term retention of properties, through accelerated testing at elevated temperatures. As shown in Figure 4, 50% retention of tensile strength is typically used as a reference. Two SHIELD Technology grades are plotted versus a standard PA66, confirming that the time for the PA-S grades to reach 50% retention of strength is at least twice that of standard heat stabilized PA66. While changing glass level from 35% to 50% increases absolute tensile strength, it does not change percent retention so both grades fall on the same curve.

4 Figure 4. Arrhenius plot, based on 50% retention of stress at break measured at 23 C, per ISO 527 using 4mm bars. Temperature Index (TI) is another common reference to quantify air oven aging performance. Several test methods [6, 7, 8] show calculations for TI based on the Arrhenius relationship between time for property loss and the reciprocal of the absolute temperature. Figure 5. Comparison of Temperature Index for 3000 hours, based on 50% retention of stress at break using 4mm thick bars, per ISO 2578 The TI is shown for 3000 hours, based on air oven aging at several temperatures per ISO The chart shows that materials with SHIELD Technology demonstrate C higher TI than equivalent grades that incorporate standard heat stabilization technology. PPA-S grades demonstrated higher temperature index value over the temperature range of the data.

5 While these calculations based on Arrhenius methodology provide useful guidance, applying these estimates to predict actual service life of parts should be treated carefully due to additional stresses and other variables with parts in use conditions [7, 8]. In particular, extrapolation outside the range of temperatures measured can sometimes lead into a region of non-arrhenius behavior where the dominant reaction changes. This has been reported in the case of diffusion-limited oxidation [10], which appears to occur with SHIELD Technology resins. Separate tests designed to confirm service life in specific applications are recommended. High Temperature Properties Properties after aging are typically measured at room temperature. However, along with the need to resist oxidation is the requirement to maintain strength and stiffness at elevated temperature. Creep modulus provides an indication of stability of stiffness over time at constant temperature and load. Figure 6 compares the creep modulus of SHIELD Technology grades at 200 C, showing that these materials are slightly lower but comparable to standard grades. PPA resins show the highest creep resistance. Figure 6. Accelerated Flexural Creep by DMA for 200 C, 7 MPa load Tensile modulus at elevated temperature is shown in Figure 7. Here the PA-S GF35 is lower than the standard PA66 GF35, but the PPA with SHIELD Technology was higher at 210 C, and showed only a slight decline when measured at 230 C.

6 Figure 7. Tensile modulus measured at both 210 C and 230 C, per ISO 527. Fluid Aging Chemical resistance is one of the key factors in material selection for automotive applications. Similar to the air oven aging results, the new PA-S grades provide better retention of properties for aging in various fluids compared to the incumbent commercial polyamide grades. The fluids chosen represent typical automotive fluids. Fluid aging with motor oil was conducted by immersion of molded test bars at 150 C up to 5,000 hours. Figure 6 shows the change for unnotched charpy impact over time for PA-S GF35 compared to standard PA66 GF35 commercial grades. The impact property of the PA-S GF35 grade demonstrated approximately 80% retention after 3000 hours, which is significantly higher than standard PA66 GF35 resin. The higher impact properties at elevated temperature with motor oil suggest that the PA-S GF35 grade may be suitable for oil pan applications. Tensile properties were also measured and PA-S GF35 provided higher retention of strain at break which is consistent with the higher unnotched impact.

7 Figure 6. Unnotched charpy impact test of PA-S polymer (top line) and standard glass-reinforced PA66 after aging in motor oil at 150 C at various times. Automotive transmission fluid (ATF) resistance by immersion was also tested at 150 C for 3,000 hours as shown in Figure 7. Various properties were measured after fluid aging using ATF fluid. Both of the SHIELD stabilized resins had higher tensile strength after 3000 hours aging. A plot of percent retention (not shown) demonstrated very similar values for both of the SHIELD grades. In comparison, the commercial 35% glass filled PA66 demonstrated the most deterioration in tensile properties. Figure 7. Stress at break measured after immersion in automotive transmission fluid at 150 C.

8 Environmental stress cracking occurs by a different failure mechanism compared to thermal oxidation. Stress cracking of polyamides with various metal chloride salts has been discussed in the literature [11]. Calcium chloride is commonly used for road salts and resistance to stress cracking is a key property for various automotive parts. Resistance to calcium chloride (CaCl2) was conducted using a special cycle exposure test method. For the calcium chloride testing, the samples are initially conditioned at for 4 hours in boiling water. The following steps are followed for testing: (1) the test bars are clamped horizontally in a test rig, on one end, and 20 MPa stress is applied by attaching a weight to the opposite end, (2) CaCl2 solution (10% in water) is applied to a cloth that wraps the test bars, (3) the bars are exposed at 100 C for 2 hours, (4) the test bars are cooled at 23 C for one hour, (5) the bars are washed and inspected for cracks. The test cycle is repeated by clamping the test bars and applying the stress. The number of cycles to cracking and subsequent breakage is graphed in Figure 8. The PA-S GF35 resin required 60 cycles before visible cracks appears, in contrast to the conventional material which lasted only 20 cycles. In addition, the PA-S GF35 grade lasted more than 100 cycles (test was stopped before failure), in contrast to the PA66 and PA6 grades which lasted only 70 cycles and 38 cycles, respectively. Figure 8. CaCl 2 crack resistance test both cycles to first observation of a crack and cycles to break. Conclusions SHIELD Technology provides a step change increase to the thermo-oxidative resistance of various polyamides, at least doubling the time to reach the same retention of a given property after air oven aging. Due to their aromatic nature, PPA resins with SHIELD stabilization have demonstrated especially superior performance at high temperatures, creating the possibility for PPS replacement in some situations. Compared to aliphatic polyamides, the SHIELD Technology also improves the resistance to calcium chloride stress cracking and aggressive fluids like hot engine oil and automotive transmission fluid. In demanding environments, these improvements would be expected to extend the life of components produced from these materials.

9 DuPont has initially incorporated the SHIELD Technology into a family of Zytel HTN PPA and Zytel PLUS PA products. We expect these new, highly oxidation-resistant, light weight materials will create opportunities for cost effective material solutions in 1) parts designed for improved life expectancy, and 2) replacing metal components in harsh environments such as exhaust systems, turbo charging systems, transmissions, and engine cooling systems, where performance and durability are required. In fact, the SHIELD Technology incorporated in our PA have successfully delivered performance cost effectively in charge air coolers and acoustic cover applications for passenger cars equipped with powerful turbo charged engines. Legal Disclaimer: The information set forth herein is furnished free of charge and is based on technical data that DuPont believes to be reliable and falls within the normal range of properties. It is intended for use by persons having technical skill, at their own discretion and risk. This data should not be used to establish specification limits nor used alone as the basis of design. Handling precaution information is given with the understanding that those using it will satisfy themselves that their particular conditions of use present no health or safety hazards. Since conditions of product use and disposal are outside our control, we make no warranties, express or implied, and assume no liability in connection with any use of this information. As with any product, evaluation under end-use conditions prior to specification is essential. Nothing herein is to be taken as a license to operate or a recommendation to infringe on patents. DuPont, and Zytel are registered trademarks or trademarks of E. I. du Pont de Nemours and Company or its affiliates. Acknowledgements The authors wish to acknowledge the contributions of others whose efforts provided much of the basic data for this paper, including Robert Palmer, Marv Martens, Toshikazu Kobayashi, Guillaume Doy, Philippe Miniou, and Patricia Tooley. Additional valuable feedback and analysis was provided by Robert Lawton, Mitsunobu Nakatani, and Yasuhiko Ohashi. References 1. Audoin, L.; Langlois, V.; Verdu, J.; de Bruijn, J.C.M. J. Mater. Sci., 29, 569 (1994). 2. Per ASTM D5336, a PPA is a polyamide in which the diacid residues are at least 55mole percent terephthalic or isophthalic acid [10]. 3. Gijsmann, P., Tummers, D., & Janssen, K., Polymer Degradation and Stability, 49, 121 (1995). 4. Billingham, N.C., in Oxidation Inhibition in Organic Materials, Vol. II, ed. J.Pospisil & P.P. Klemchuk, CRC Press, Boca Raton, Florida, 1990, Holland, B.J. & Hay J.N., Polymer International 49, 943 (2000). 6. ISO 2578:1993, Plastics-Determination of time-temperature limits after prolonged exposure to heat, International Organization for Standardization. 7. IEC : 2006, Thermal endurance properties Part 3: Instructions for calculating thermal endurance characteristics, International Electrotechnical Commission. 8. ASTM D , Standard Practice for Heat Aging of Plastics Without Load, ASTM International. 9. ASTM D , Standard Specification for Polyphthalamide (PPA) Injection Molding Materials, ASTM International. 10. Gillen, K.T., Celina, M., Clough R.L., and Wise, J., Trends in Polymer Science, 5, 250 (1997). 11. Kohan, M.I, Nylon Plastics Handbook 1995,