High Pressure Oxidative Induction Time Research Articles

Antioxidant-stabilizer depletion of 4 HDPE geomembranes with high HP-OIT in MSW leachate

The antioxidant-stabilizer depletion of four 1.5-mm HDPE geomembranes from the same manufacturer each with a different resin and additive package is examined in air and a synthetic municipal solid waste leachate at a range of temperatures (40–95°C) for 7.5 years. Two were formulated for high temperatures and used polyethylene of raised temperature resistance (PE-RT) resins while two used more conventional HDPE geomembrane formulations. The depletion of protective antioxidants and stabilizers was monitored using standard and high-pressure oxidative induction time (OIT) tests and the notably different depletion times for both OIT tests implied they were detecting different groups of AO-S. Although both PE-RT GMBs showed significantly slower AO-S depletion at 85°C in air compared to the conventional PE GMBs, only one PE-RT GMB maintained this status in 85°C leachate, highlighting the limitation of air aging tests (and importance of fluid immersion tests). The importance of running immersion tests long enough to reveal the residual HP-OIT value is stressed. The roles of stabilizer mobility and solubility in polyethylene and their suspected involvement in residual HP-OIT behavior are also illustrated.

Long-term performance of a HDPE geomembrane stabilized with HALS in chlorinated water

A high density polyethylene geomembrane (GMB) stabilized with hindered amine (light) stabilizers (HALS or HAS) is immersed in four chlorinated water solutions with a simulated free chlorine concentration range of 0.5–5 ppm at five different temperatures (25, 40, 65, 75, and 85 °C) for 70 months. Standard and high pressure oxidative induction time (OIT) tests are performed to monitor antioxidants depletion while melt flow index, tensile, and stress crack resistance (SCR) tests are conducted to monitor degradation in physical and mechanical properties. Degradation in the GMB properties occurred shortly after immersion in chlorinated water at all temperatures except at 25 °C. Increasing the free chlorine concentration resulted in faster degradation of the tensile properties and SCR. The predicted time to nominal failure based on SCR ranges between 25 years at 40 °C and 5 years at 85 °C in chlorinated water (with 0.5 ppm free chlorine). A comparison between the degradation in SCR of this GMB and a GMB with a different resin and without HALS shows significant difference in their performance in chlorinated water but not in other incubation media.

Effect of texturing on the longevity of high-density polyethylene (HDPE) geomembranes in municipal solid waste landfills

The effect of texturing (co-extrusion using blowing agent) on the longevity of a geomembrane (GMB) when immersed in synthetic municipal solid waste leachate is investigated over a ∼3 year period. Based on data at four temperatures (40, 55, 75, and 85 °C), the time to antioxidant depletion of the textured portion of a 1.5 mm core thickness high-density polyethylene (HDPE) GMB is 40% (standard oxidative induction time) and 9% (high-pressure oxidative induction time) faster compared to the 1.5 mm smooth edge of the GMB. However, despite this, stress crack resistance results show that texturing may have no significant effect on the time to nominal failure for this GMB. It is also shown that HDPE GMBs made from nominally the same resin but from different production lots have different rates of stress crack resistance degradation and hence time to nominal failure; this should be considered both in landfill design and landfill construction quality assurance.

Long-term performance of high-density polyethylene (HDPE) geomembrane seams in municipal solid waste (MSW) leachate

The effect of a synthetic municipal solid waste leachate on the long-term performance of dual-wedge welds in a 1.5 mm thick high-density polyethylene geomembrane (GMB) is reported based on 4 years of testing at 40, 65, 75, and 85 °C. The effect of leachate on the GMB well away from the weld, in the heat-affected zone (HAZ) beside the weld, and in the welded zone are investigated. The slowest antioxidant depletion rate was in the weld itself and the fastest rate for the HAZ adjacent to the weld. The shear break and peel break properties started to decrease after the standard oxidative induction time had depleted to residual, but before the high-pressure oxidative induction time had reached residual. Failures occured at the HAZ adjacent to weld in both the shear and peel tests. No failure of the seam itself was observed. The times to nominal failure of the GMB in the critical HAZ are predicted. The rate of degradation in the weld and sheet are compared.

Effects of a very low pH solution on the properties of an HDPE geomembrane

The effects of a very low pH solution (pH = 0.5) on a high-density polyethylene (HDPE) geomembrane (GMB) are evaluated using accelerated ageing tests. The GMB specimens were incubated at 65°C, 75°C, and 85°C for up to 19 months. They were periodically removed from the immersion media and tested for retained chemical, mechanical and physical properties. The antioxidant depletion rates obtained from standard oxidative induction time (Std-OIT) tests were consistently lower than that obtained from high-pressure oxidative induction time (HP-OIT) tests at the three temperatures. This difference led to two substantially different estimates of antioxidant depletion time. The relationship between Std-OIT and HP-OIT was non-linear. It indicates the importance of using both methods to assess antioxidant depletion time of GMBs. Crystallinity, melt flow index (MFI) and tensile test results show that physical ageing has occurred, but no significant changes in physical properties due to oxidative degradation were observed after 19 months’ immersion.

Antioxidant depletion in high-density polyethylene (HDPE) geomembrane with hindered amine light stabilizers (HALS) in low-pH heap leach environment

Antioxidant depletion from a high-density polyethylene (HDPE) geomembrane with hindered amine light stabilizers (HALS) immersed in seven different low pH solutions is examined over a 3 year period. The examined solutions had the range of pH (0.5, 1.25, and 2.0) likely to encompass the pH of the leach solutions found in copper, nickel, and uranium heap leach pads. The metal concentration for these solutions is adopted from copper raffinate solutions. Additional solutions are investigated to examine the effects of field practices such as using surfactants in the leach solutions and pre-curing of the ores used to improve the metallurgical response of the ore. For the antioxidants detected by standard oxidative induction time (Std-OIT), there was a depletion to residual value of about 20% of the initial Std-OIT that varied depending on the incubation temperature and pH of the solution whereas decreasing the pH from 2 to 0.5 did not significantly affect the depletion rates of Std-OIT. The antioxidants detected by high-pressure oxidative induction time (HP-OIT) exhibited the fastest depletion in pH = 1.25 with the highest residual values followed by pH 2.0 and the slowest HP-OIT depletion was in pH = 0.5, but with the lowest residual values. Arrhenius modelling is used to predict the length of the antioxidant depletion stage for each solution based on both Std-OIT and HP-OIT.

Ageing of exposed geomembranes at locations with different climatological conditions

The degradation of high-density polyethylene (HDPE) geomembranes exposed to the elements depends, inter alia, on the climatological conditions. This study investigates the degradation in the properties of HDPE geomembranes installed at two mine facilities after almost 16 years of exposure in a warm–hot climate and at a research site after 6 years of exposure in a mild–cold climate. Samples were exhumed at the field sites from different locations and the properties of the geomembrane were measured in the laboratory. The depletion of antioxidants detected by the standard oxidative induction time (Std-OIT) test for the geomembrane installed at the research site was faster on the slope than on the base; however, there was negligible difference in the depletion of antioxidants–stabilizers detected by the high-pressure oxidative induction time (HP-OIT) test between the slope and base. Under the field conditions described, the antioxidant depletion time for the exposed geomembrane installed at the research site (mild–cold climate) was inferred to be 20 to 54 years. The exposed HDPE geomembranes installed at the mine facilities (warm–hot climate) have reached the time of nominal failure based on their stress crack resistance. However, they have not ruptured under the exposure conditions even though the stress crack resistance has dropped to as low as 70 h at some locations.

Effect of High pH Found in Low-Level Radioactive Waste Leachates on the Antioxidant Depletion of a HDPE Geomembrane

AbstractAntioxidant depletion from a high-density polyethylene (HDPE) geomembrane with hindered amine light stabilizers (HALS) immersed in three different high pHs solutions is examined over a 3-year period. The three solutions had the same concentration of inorganic salts but a range of pH (9.5, 11.5, and 13.5) likely to encompass those found in low-level radioactive waste leachate and other geoenvironmental applications. Increasing the pH from 9.5 to 13.5 increased the antioxidant depletion rates detected by both standard and high-pressure oxidative induction time tests (Std-OIT and HP-OIT) and also increased the residual HP-OIT values. Arrhenius modeling is used to predict the length antioxidant depletion stage for each solution.

Effect of high temperatures on antioxidant depletion from different HDPE geomembranes

The effect of elevated temperatures, typically 95–115 °C, on antioxidant depletion from a high-density polyethylene (HDPE) geomembrane (GMB) incubated in air, water and synthetic leachate is examined. It is shown that the antioxidant depletion in synthetic leachate at 95–115 °C is consistent with what would be expected from Arrhenius modeling based on data from lower temperatures (25–85 °C). A similar finding is reached for incubation in air. However, when incubated in water the antioxidant depletion is more complicated. At temperatures above 100 °C a four-parameter exponential model was needed to fit oxidative induction time data that exhibited quite different early-time and later-time depletion rates. The early-time depletion rate decreases with an increase of the temperature while the later-time depletion rates follow the more typical pattern of increasing with increasing temperature. Three additional HDPE GMBs with different antioxidant packages are examined at elevated temperatures in air. The GMB with the lowest initial standard (Std) oxidative induction time (OIT) and without hindered amine light stabilizer (HALS) has the longest antioxidant depletion stage based on Std-OIT at these elevated temperatures. GMBs stabilized with HALS showed only a slight change in their high pressure OIT during the current study. It is shown also that degradation in physical properties can start at Std-OIT values above the residual OIT values.

Degradation behaviour of HDPE geomembranes with high and low initial high-pressure oxidative induction time

The degradation of three high density polyethylene geomembranes (GMBs) (denoted xA, xB and xC) when immersed in simulated landfill leachate at 85 °C is examined. All three GMBs met the requirements of the generic industry specification GRI-GM13 with respect to their performance properties. The large high-pressure oxidative induction time (HP-OIT) (i.e., 790 min for xB and 960 min for xC) combined with the relatively high level of trace nitrogen detected, suggest the presence of hindered amine light stabilizers (HALS) as part of the antioxidant package in these GMBs, whereas, the relatively low initial HP-OITs (i.e., 260 min) and the low level of trace nitrogen for xA, suggest the absence of HALS in xA. Although xA had the lowest initial standard Std-OIT (Std-OIT was 115, 158 and 175 min for xA, xB and xC, respectively) it exhibited the longest time to antioxidant depletion based on Std-OIT. For the three GMBs, the HP-OIT depleted following exponential decay model until reaching a residual value. xB had the slowest HP-OIT depletion (0.016 month−1) and was still depleting without reaching a residual value at the end of this study (after 46 months). xA experienced the fastest HP-OIT depletion (∼62 times faster than of xB) and reached a residual value of 78 min. For xC, HP-OIT depleted 40 times faster than for xB reaching a residual value (∼610 min) that was still higher than the HP-OIT of 400 min specified by GRI-GM13 for a new GMB. xB had the longest time to nominal failure despite having the lowest initial SCR value (330 h for xB compared to 910 and 800 h for xA and xC, respectively) and not having the highest OITs values. Although xC had a residual HP-OIT of 610 min, the SCR, melt index (MI) and tensile properties for xC had decreased to the point that xC was at nominal failure, indicating that the degradation can take place without the total depletion of antioxidants/stabilizers captured by the HP-OIT.

Antioxidative properties of hydrogenated cardanol for cotton biodiesel by PDSC and UV/VIS

Biodiesel is a non-toxic biodegradable fuel that consists of alkyl esters produced from renewable sources, vegetal oils and animal fats, and low molecular mass alcohols, and it is a potential substitute for petroleum-derived diesel. Depending on the raw materials used, the amount of unsaturated fatty acids can vary in the biodiesel composition. Those substances are widely susceptible to oxidation processes, yielding polymeric compounds, which are harmful to the engines. Based on such difficulty, this work aims to evaluate the antioxidant activity of cashew nut shell liquid (cardanol), as additive for cotton biodiesel. The oxidative stability was investigated by the pressure differential scanning calorimetry (PDSC) and UV/Vis spectrophotometer techniques. The evaluated samples were: as-synthesized biodiesel — Bio T0, additivated and heated biodiesel — Bio A (800 ppm L−1 of hydrogenated cardanol, 150°C for 1 h), and a heated biodiesel — Bio B (150°C, 1 h). The oxidative induction time (OIT) analyses were carried out employing the constant volume operation mode (203 psi oxygen) at isothermal temperatures of 80, 85, 90, 100°C. The high pressure OIT (HPOIT) were: 7.6, 15.7, 22.7, 64.6, 124.0 min for Bio T0; 41.5, 77.0, 98.6, 106.6, 171.9 min for Bio A and 1.7, 8.2, 14.8, 28.3, 56.3 min for Bio B. The activation energy (E) values for oxidative processes were 150.0±1.6 (Bio T0), 583.8±1.5 (Bio A) and 140.6±0.1 kJ mol−1(Bio B). For all samples, the intensities of the band around 230 nm were proportional to the inverse of E, indicating small formation of hyper conjugated compounds. As observed, cardanol has improved approximately four times the cotton biodiesel oxidative stability, even after the heating process.

Antioxidant Depletion Lifetime in High Density Polyethylene Geomembranes

A major issue in the use of geomembranes for waste containment is an estimate of the material's durability (i.e., its lifetime) to various aging phenomena. For high density polyethylene geomembranes, which are the focus of this study, there are three stages in assessing lifetime: depletion of antioxidants, induction time, and time to reach half-life of a relevant engineering property. This paper addresses the first stage of these three sequential processes. Twenty laboratory incubation devices were made to simulate landfill conditions. Four sets of five columns were maintained at elevated temperatures of 85, 75, 65 and 55°C. Samples were retrieved at various intervals over a 24-month incubation period. Various physical, mechanical, and chemical properties were evaluated. The depletion of antioxidants in the incubated samples was monitored using both standard and high pressure oxidative induction time tests. Arrhenius modeling was used on the data to extrapolate the antioxidant lifetime to a typical landfill site temperature of 20°C. The resulting predicted time was 200 to 215 years. Also, it should be emphasized that within this period of time the physical and mechanical properties of the incubated samples remained unchanged.