THERMAL AGEING PERFORMANCE OF POLYOLEFINS UNDER DIFFERENT TEMPERATURES

THERMAL AGEING PERFORMANCE OF POLYOLEFINS UNDER DIFFERENT TEMPERATURES Huang Wu, Yuming Lai, Ye (Jessica) Huang, Sharon Wu, Stacy Pesek, and Stefan Ultsch The Dow Chemical Company, Midland, MI Abstract Heat stability of polyolefin materials is of great interest as the need for long lifetimes is expected for certain applications. Accelerated tests are often used where materials are tested under elevated temperatures, in which unrealistic degradation may occur. This paper aims to demonstrate the importance of choosing adequate temperatures for accelerated ageing test. Also, a nondestructive surface chemistry tracking method is employed to provide insight into degradation as a fast and convenient alternative to mechanical testing. A comparison is made between the two tracking metric results under different temperatures, which revealed the importance of selecting an adequate ageing temperature for comparing materials with different melting temperatures. Above the polymer melting temperature the decrease in crystallinity allows more oxygen to diffuse into the polymer and may cause unrealistic failure, resulting in invalid comparisons under high testing temperatures. benefits from shorter turnaround time, these tests are commonly questioned as to whether they have altered the failure mode. From a simple Arrhenius law stand point, when comparing two materials A and B, a testing result showing that A lasts longer than B at higher temperature used in accelerated ageing does not guarantee that A will last longer than B in the field, as demonstrated in Error! Reference source not found.. Due to the possible difference in the activation energy of degradation, comparison at only one elevated temperature may not reflect the actual performance of the material. Thus, multiple testing temperatures are generally preferred. Introduction Typical approaches for longevity evaluation of polyolefins include natural and artificial weathering techniques. The natural weathering evaluation is the most accurate measure for the actual product performance in the field. However such tests are limited by the long turnaround time and difficulty in translating results from one geographical location to another. Artificial weathering techniques have thus been developed to evaluate polymer longevity in various applications. Compared to the natural weathering approach, artificial weathering possesses two major advantages: 1) acceleration of the degradation process through an increased stressor level, resulting in faster turnaround time and 2) controllable ageing conditions that make cross-comparison easier and enable modeling of the degradation rate under different environmental conditions. Obviously, the biggest drawback of artificial weathering is its deviation from the real application conditions. Such tests are usually a simplification of the type of stressors (e.g., only heat or UV and heat) and increased stressor levels. Although artificial weathering Figure 1. Schematic of an Arrhenius plot for materials A and B, where A has a lower activation energy (E a ) than B resulting in ranking at testing temperature mismatching the reality. Moreover, raising the temperature of heat ageing can also cause the simple Arrhenius relation to fail due to the change of degradation reaction kinetics. The most frequently considered limitation is diffusion limited oxidation (DLO) [1-6]. As temperature rises, the consumption rate of oxygen also increases as the degradation of the polymer is accelerated. For a thick sample, the core will reach the high temperature but may be short of oxygen exposure as compared to the surface, especially for semi-crystalline polymers in which the gas permeability is limited due to the crystalline phases. This consideration needs to be kept in mind when choosing the right condition for evaluating longevity of polymer materials, especially when high acceleration of degradation is demanded. One should expect that the results may change once the ageing temperature SPE ANTEC Anaheim 2017 / 987

approaches the melting point of the polymer due to a dramatic change in the oxygen diffusion rate. In this paper we aim to explore the temperature dependence of several kinds of polylefin resins. Retention of mechanical properties, a representation of bulk physical characteristics, and change in chemical structure will be characterized and discussed in order to test the validity of high temperature ageing results. Surface chemistry changes in the materials are also tracked by Fourier transformation infrared spectrometry and compared to bulk mechanical property retention. Inconsistencies between the two tracking metrics will be discussed. Materials Six polyolefin (PO) resins were tested and the material characteristics are listed in Error! Reference source not found.. The antioxidant (AO) package was part of the original processing aid and there were no additional stabilizers added to these resins. Although the three AO packages were not exactly the same, their composition and loading were very similar, therefore the effect of AO to material weathering resistance was considered negligible. Resins were compression molded into thin sheets with a nominal 0.8 mm thickness, and placed in ovens for accelerated thermal ageing according to ASTM D573. In order to understand the temperature effects on degradation, thermal ageing at 96 C, 106 C, 116 o C, 125 C and 135 C was tested. Samples were removed from the ovens at different intervals depending on the ageing temperatures and cut into strips (nominal dimension, length: 2.5, width: 0.5 ) for ASTM D882 tensile test and carried out by MTS Insight with a speed of 20 in/min. Resin MI Density (g/cc) AO package PO-1 0.5 0.877 AO package 1 PO-2 5 0.877 AO package 1 PO-3 30 0.880 AO package 1 PO-4 0.67 0.888 AO package 2 PO-5 0.67 0.867 AO package 2 PO-6 0.6 0.88 AO package 3 Table 1. Materials information of resins. MI stands for melt flow index. FTIR spectra Infrared spectra were acquired with a Thermo Scientific Nicolet is50 FTIR and its built-in ATR accessory at a resolution of 4 cm -1. Sixteen scans (47- second acquisition time) were collected for each spectrum. The ATR accessory was equipped with a single bounce diamond ATR crystal. The oxidation index (OI) was calculated by the ratio of the integration of spectra, referring to Error! Reference source not found. and the [( OH, NH, CH) -( CH)] equation was expressed as OI = ( CH). [7] The OI value at each thermal ageing was normalized to its unaged OI value. The normalized OI value becomes larger when more polymer is oxidized. Figure 2. An example of FTIR and OI calculation. Small Angle X-ray Scattering (SAXS) SAXS measurements were performed using beamline 5ID-D in the DuPont-Northwestern-Dow (DND-CAT) Synchrotron Research Center at the Advanced Photon Source, Argonne National Laboratory. An energy of 17 kev, corresponding to a wavelength of 0.729 Å, was selected from an undulator beam using a double-crystal monochromator and then collimated using two sets of adjustable slits. For in-situ experiments, samples were placed in DSC pans which were loaded in a Linkam hot stage. The heating rate was 10 C/min and the samples were kept at each temperature for 10 min before collecting X-ray data. Samples were analyzed in a normal beam transmission mode with a CCD detector that was spatially calibrated with silver behenate. The data were collected using a CCD detector (MAR) having a maximum resolution of 2048 x 2048 pixels (78.75 x 78.75 µm 2 pixel size) with a 16-bit intensity scale and a circular active area of 133 mm diameter. Linear representation (one dimensional plots) of the SAXS data was achieved using the Dow-developed SCATTER visualization and data SPE ANTEC Anaheim 2017 / 988

reduction platform. Plots were generated using radial integration procedures. Results and Discussions Figure 3 shows resin breaking stress retention after thermal ageing. The data points were limited in PO-4 at 125 o C and 135 o C due to the appreciable cracks formed and the specimens were untestable. Figure 3 shows, in general, the breaking stress retention was decent at lower temperature and became poor toward higher temperature. In other words, the degradation was faster at higher temperature which was expected. To compare the resin performance, data was processed by Tukey pairwise comparison and given a rank at each testing temperature. These results are presented in Error! Reference source not found.. The higher rankings represent better retention at selected temperatures and ageing duration. Interestingly, PO-3 had the highest ranking at 96 o C and 106 o C, whereas PO-6 was dominant at 125 o C and 135 o C. At 116 o C, PO-3 and PO-6 were comparable. This data suggested that the selection of ageing temperature played a crucial role on determining the resin performance. Although a higher ageing temperature is often selected for shorter turnarounds, there are risks to consider, such as higher ageing temperature can lead to results that disagree with results performed at lower or ambient temperatures. Therefore, material evaluation (rankings) can vary widely depending on ageing temperature and care should be taken to ensure valid results are considered. Breaking stress retention Temperature 96 o C 106 o C 116 o C 125 o C 135 o C Age duration (hrs) Resin PO-1 PO-2 PO-3 PO-4 PO-5 PO-6 Figure 3. Relationship of resin breaking stress retention to thermal ageing temperature and duration. In addition to mechanical properties, the change of resin surface chemistry was characterized by FTIR. Figure 5 shows the relationship between surface oxidation index (OI) and tensile breaking stress retention to thermal ageing. The normalized OI was reversed in order to keep it the same scale (zero to one) as the tensile retention. It was apparent that PO-5 had a strong correlation between OI and breaking stress retention across all five testing temperatures, whereas the correlation was poor or absent in PO-4 and PO-6. For PO-1, PO-2 and PO-3, the correlation was poor at lower temperatures but became more pronounced at 125 o C and 135 o C. Ranking Temperature ( o C) Figure 4. Ranking of resin breaking stress retention. Data was compared at 96 o C 2690 hr, 106 o C 3371 hr, 116 o C 2688 hr, 125 o C 676 hr, and 135 o C 402 hr. Higher ranking represents better retention at selected temperature and ageing time. Breaking stress retention & Reverse Normalized OI Temperature 96 o C 106 o C 116 o C 125 o C 135 o C Normalized time Resin PO-1 PO-2 PO-3 PO-4 PO-5 PO-6 Figure 5. Normalized reverse oxidation index (red) and resin specimen breaking stress retention (blue) as a function of normalized thermal ageing time. The smoothing curves were used to guide the trend of change. For easier visual comparison, the thermal ageing time was normalized to the maximum ageing time at each temperature, which was 5545 hr for 96 o C, 5392 hr for 106 o C, 4910 hr for 116 o C, 2876 hr for 125 o C, and 1691 hr for 135 o C. Unlike the tensile test, which in nature is a measurement of a bulk mechanical property, the OI by FTIR is a surface chemistry characterization. During SPE ANTEC Anaheim 2017 / 989

thermal accelerated ageing, the polymer could be oxidized by reaction with ambient oxygen. The amount of surface polymer oxidation was quantitatively realized as OI. When oxygen diffused further into the bulk, more polymer was degraded and mechanical properties became compromised. Therefore, we believe that when a resin undergoes homogenous oxidation, similar extent of oxidation occurs at the surface and in the bulk, the OI and the tensile breaking stress retention should reach a good correlation (as seen in the case of PO-5). Error! Reference source not found. shows the degree of crystallinity of resins characterized at different elevated temperatures. A negative 100 % change in crystallinity with respect to room temperature, or zero crystallinity, represents the testing temperature is above the melting point. All resins, except for PO-6, show a steady reduction of crystallinity with increasing temperature. With lower degree of crystallinity in the sample, ambient oxygen can readily permeate through surface and further diffuse into bulk to cause more polymer oxidation. Indeed, referring to Error! Reference source not found., the results in Figure 5 became self-explanatory. For examples, PO-5 has the lowest melting point (Tm 96 o C) and the strongest correlation in OI and mechanical property retention at all five testing temperatures. The lack of correlation in PO-6 was due to its higher crystallinity. The correlation in PO- 1, PO-2 and PO-3 became decent when testing was above melting point (Tm ~ 120 o C). chosen for testing. For example, when comparing PO-6 and PO-3 at 125 C and 135 C, one may conclude that PO-6 is superior than PO-3 in terms of thermal stability. However, PO-3 performs better than PO-6 under lower temperatures which are closer to application conditions in field. The false ranking at higher temperatures is caused by the melting of PO-3 at 125 C while PO-6 still maintains most of its crystalline phase. Another observation worth pointing out is that the OI approach can be useful and limited at the same time, depending on the type of material and the metrics of interest. It is, after all, a surface sensitive technique. For semi-crystalline polymers where oxygen diffusion is limited by the crystalline phase, the FTIR method will only detect the surface change which does not necessarily represent the bulk properties, resulting a mismatch between OI and mechanical property retention. In this case, OI can still be useful if the surface characteristics are of interest, such as color and gloss changes, but is not adequate to be used as a predictor for the change of bulk mechanical properties. On the other hand, when testing amorphous polymers, OI is potentially a good alternative to mechanical testing due to the tight correlation between surface and bulk chemistry change. One advantage of OI over mechanical testing is that the OI is a non-destructive test and can be repeated on the same specimen, which can both save time and improve the measurement accuracy. Percentage of crystallinity change w.r.t. RT Temperature ( o C) Figure 6. Relationship of crystallinity and temperature. The Y-axis was plotted as the percentage of crystallinity change with respect to room temperature. Crystallinity was measured by holding the specimens at elevated temperatures for at least 20 min prior to collecting data. A negative 100 % change in crystallinity indicates that the testing temperature is above the melting point. Combining Figure 5 and Error! Reference source not found., it is obvious that when comparing materials with different melting points, the temperature of heat ageing needs to be selected carefully to avoid the impact from material phase change. It is even more important when only a limited number of testing conditions are Conclusions In this study, the thermal ageing performance of different polyolefin materials has been evaluated. Six polyolefin samples were thermally aged under five temperatures, among which some are above the melting temperature of certain polymers. The change of material properties was characterized in two ways: the bulk mechanical properties by tensile testing and the surface chemistry change by FTIR. A comparison between the two tracking metric results under different temperatures revealed the importance of selecting an adequate ageing temperature for comparing materials with different melting temperatures, as the decrease in crystallinity at higher temperature allows more oxygen to diffuse into the polymer and cause unrealistic failure, resulting in invalid comparisons under high testing temperatures. Additionally, OI is suggested to be a good tracking metric for surface changes of semi-crystalline polymers and a non-destructive alternative to the mechanical tests for amorphous polymers. SPE ANTEC Anaheim 2017 / 990

Acknowledgment Portions of this work were performed at the DuPont- Northwestern-Dow Collaborative Access Team (DND- CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by Northwestern University, E.I. DuPont de Nemours & Co., and The Dow Chemical Company. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Data was collected using an instrument funded by the National Science Foundation under Award Number 0960140. References 1. Celina, M., et al., Thermal degradation studies of a polyurethane propellant binder. Rubber Chemistry and Technology, 2000. 73(4): p. 678-693. 2. Celina, M., et al., Oxidation profiles of thermally aged nitrile rubber. Polymer Degradation and Stability, 1998. 60(2-3): p. 493-504. 3. Celina, M., et al., Correlation of chemical and mechanical property changes during oxidative degradation of neoprene. Polymer Degradation and Stability, 2000. 68(2): p. 171-184. 4. Audouin, L., et al., ROLE OF OXYGEN DIFFUSION IN POLYMER AGING - KINETIC AND MECHANICAL ASPECTS. Journal of Materials Science, 1994. 29(3): p. 569-583. 5. Wise, J., K.T. Gillen, and R.L. Clough, Time development of diffusion-limited oxidation profiles in a radiation environment. Radiation Physics and Chemistry, 1997. 49(5): p. 565-573. 6. Wise, J., K.T. Gillen, and R.L. Clough, Quantitative model for the time development of diffusion-limited oxidation profiles. Polymer, 1997. 38(8): p. 1929-1944. 7. Oreski, G. and G.M. Wallner, Evaluation of the aging behavior of ethylene copolymer films for solar applications under accelerated weathering conditions. Solar Energy, 2009. 83(7): p. 1040-1047. SPE ANTEC Anaheim 2017 / 991