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Nylon 6.6 accelerating aging studies: II. Long-term thermal-oxidative and hydrolysis results
Abstract
Long-term (greater than 5 year exposures), low-temperature (as low as 37 °C) accelerated oven aging results were obtained for Nylon 6.6 fibers under thermo-oxidative conditions (air aging with an oxygen partial pressure of 13.2 cmHg in Albuquerque). To assess the importance of humidity on aging, experiments were also conducted under a combination of 100% RH plus 13.2 cmHg of oxygen partial pressure at temperatures ranging from 138 °C to 64 °C plus an additional experiment at 70% RH and 80 °C. The low-temperature tensile strength results showed that the Arrhenius activation energy under the pure oxidative degradation conditions dropped from ∼96 kJ/mol above ∼100 °C–∼30 kJ/mol below this temperature, indicative of a transition in the oxidative chemistry at low temperatures. Earlier work by our group on the same material concluded that hydrolytic degradation effects dominated oxidation effects at higher aging temperatures. However, the current long-term, low-temperature comparisons lead to the conclusion that humidity is not an important aging factor below ∼50 °C. By extrapolating time–temperature superposed oxidative degradation data using the low-temperature activation energy, we obtain predictions at 21 °C. At this temperature, we estimate that a tensile strength loss of 50% takes on the order of 70 years. The 21 °C predictions are shown to be reasonably consistent with long-term (up to 38 year) ambient results on similar Nylon materials removed from field-aged parachutes. Although the estimated average exposure temperature varies from parachute to parachute, the highest average temperature is estimated to be on the order of 21 °C.
... Up to now, several experimental studies on thermal degradation of PAs have been reported in the literature [5][6][7][8][9][10][11][12]. In these studies, some important factors affecting the mechanism of thermal degradation of PAs, such as temperature, additives, and polymer structures, were investigated. ...
... However, due to the complex physical scenario and the inherently limited resolution of experimental investigations, current techniques based on testing and past experience cannot provide sufficient physical insight that is essential to fully understand polymer degradation mechanisms. Another major drawback of the experimental investigations is the long time (up to several years) required to perform the tests [11,12] and the difficulty of isolating the influence of each parameter. The development of numerical procedures based on atomistic simulation methods is therefore indispensable to explore degradation processes. ...
... Firstly, a polymer chain with = n 20 monomers (C H N O ) 12 22 2 2 shown in Fig. 1, containing 40 amide groups and ethyl-and propyl-terminated, is built. The corresponding molecular weight (M n ) is around 4500 g/mol. ...
Thermal degradation under wet conditions is considered as an important aging mechanism in polyamide 6,6 (PA 6,6). The effect of water on thermal degradation of amorphous PA 6,6 is investigated at relatively low temperatures, varying from 1000 to 2000 K, using reactive force field molecular dynamics (MD) and collective variable-driven hyperdynamics simulations. The simulation of the related long-term chemical reactions is made possible thanks to the self-learning accelerated MD concept of hyperdynamics in combination with the corresponding accurate reproduction of the correct dynamics, consistent with unbiased MD simulations. The kinetics of cleavage reactions of the amide bonds in the backbone of the polymer chains, responsible for the thermal degradation of the polymer, is studied, and the influence of water content on the activation energy and pre-exponential factor of the cleavage reactions is explored. The results show that activation energy and pre-exponential factor are in agreement with experimental data. The proposed simulation framework not only estimates kinetic properties of thermal degradation that are consistent with experimental observations but also provides a predictive tool for studying long-term thermal degradation of PA 6,6.
... The second is the plastification effect of water and temperature. This paper do not investigate hygrothermal ageing of thermoplastic, as illustrated in Boubakri et al. [3], Goncalves et al. [4], Bernstein and Gillen [5], Bergeret et al. [6]. We only focus on the initial effects of water and temperature, namely the plastifying effects. ...
... The good agreement between both experimental points and the model (plain line) illustrated in Fig. 7, with D 1 ¼ 6:1 and D 2 ¼ 2:5%, validates Equation (6). As for the WLF equation, W c ref is an arbitrary choice and can be changed in the same way as modifying T ref in Equation (5). The expression of the shift factor on the hygroeffect, logb Wc , proposed here is in accordance with the study made on epoxy and PA6 specimens by Ishisaka and Kawagoe [45]. ...
... Initially, the WLF time-temperature equivalence was verified at large strains on uploading-unloading tensile tests and results are depicted in Fig. 9. At constant water content W c ¼ 0:1%, for a specimen tested at T 1 ¼ 80 + C, the equivalent strain rate _ ε 1 at _ ε 2 ¼ 9:8 Â 10 À5 s À1 and T 2 ¼ 70 + C was calculated using Equation (5) and values from Table 2, and the result was _ ε 1 ¼ 6:8 Â 10 À3 s À1 .The reader can notice that mechanical responses are equivalent as expected. ...
... PAs hydrolysis in neutral or acidic water solutions [7e20] and in humid atmospheres [21e23] has been extensively studied at the molecular [8e12, 21], macromolecular [7,8,13e19], morphological [13,19] and macroscopic scales [7,20,22,23] since the last half century. It has been the object of specific sections or chapters in handbooks devoted to organic chemistry [24,25], chemical ageing of polymers [26,27] and organic composite materials [28,29], polyamides and their industrial applications [30e33] such as fibres [32] or pipes [33]. ...
... From the early 60s, Mikolajewski et al. have observed that the presence of oxygen in a humid atmosphere could accelerate noticeably the overall degradation rate of PAs [34]. But it is only recently that this result has been reconsidered by the scientific community and interpreted as a combination of the effects of thermal oxidation and hydrolysis [22,23,35]. Is there a simple additive effect or rather a real coupling between both types of chemical processes? ...
... In the case of PA 6-6, however, the effects of oxygen would be significant only at relatively low temperatures close to room temperature [21,23]. Through a careful examination of degradation products by 17 O NMR spectroscopy, Alam [21] has shown that an almost pure hydrolysis process proceeds between 65 and 125 C in 92e96% RH. ...
PA 6-6 hydrolysis at 60, 70, 80 and 90 °C in distilled water has been studied by Fourier transform infrared spectroscopy, viscometry in molten state, differential scanning calorimetry and uniaxial tensile testing. The molar mass decreases sharply from the early periods of exposure to reach an equilibrium value of about MnE ≈ 10−11 kg mol−1 almost temperature independent. Hydrolytic chain scissions destroy the entanglement network in the amorphous phase and liberate small macromolecular segments which rearrange locally and initiate a chemicrystallisation. As expected, the embrittlement occurs at a very low conversion of the hydrolysis, in particular when the number average molar mass becomes lower than a critical value of about MnF ≈ 17 kg mol−1, i.e. very close to its initial value. A new kinetic model has been derived from the classical mechanistic scheme of reversible hydrolysis. This model describes satisfyingly all the kinetic characteristics of the reversible hydrolysis of PA 6-6 not controlled by water diffusion: decrease in molar mass, increase in crystallinity ratio and decrease in ultimate elongation, but also of other types of polyamides previously studied, such as PA 11. Moreover, when it is used as an inverse method, this model gives access to the rate constants of hydrolysis and condensation reactions. It is thus an interesting tool for elucidating structure/rate constant relationships in common families of hydrolysable polymers.
... P olyamides, in particular nylons, are commercially available and used in automotive parts, carpets, ropes, food packaging, barrier materials, clothing, etc. Certain high-reliability applications require that nylon materials are either stored or used for durations up to several decades [1,2]. Therefore, it is critical to enhance the current knowledge base of nylon aging, in particular nylon aged under thermal-oxidative conditions, as a means to correlate degradation product formation to changes in physical properties. ...
... In addition, physical properties that alter the performance and possibly the application of a material can change as a function of temperature and time. Consequently, our experimental accelerated aging temperature and times were chosen such that nylon 6.6 degradation products were collected and detected at multiple time points that correlated to a loss of tensile strength up to 100 % [2]. C-14 labeling was previously used to study the mechanism of the photooxidation of amides [23]. ...
... Scheme 1. Structures of nylon 6.6 monomers and polymers used in these studies studies that focused on changes in tensile strength (i.e., physical properties) [1,2]. The aging temperature was chosen to achieve reasonable levels of physical property degradation in an acceptable amount of time. ...
Aged materials, such as polymers, can exhibit modifications to their chemical structure and physical properties, which may render the material ineffective for its intended purpose. Isotopic labeling was used to characterize low-molecular weight volatile thermal-oxidative degradation products of nylon 6.6 in an effort to better understand and predict changes in the aged polymer. Headspace gas from aged (up to 243 d at 138 °C) nylon 6.6 monomers (adipic acid and 1,6-hexanediamine) and polymer were preconcentrated, separated, and detected using cryofocusing gas chromatography mass spectrometry (cryo-GC/MS). Observations regarding the relative concentrations observed in each chromatographic peak with respect to aging time were used in conjunction with mass spectra for samples aged under ambient air to determine the presence and identity of 18 degradation products. A comparison of the National Institute of Standards and Technology (NIST) library, unlabeled, and isotopically labeled mass spectra (C-13 or N-15) and expected fragmentation pathways of each degradation product were used to identify the location of isotopically labeled atoms within the product's chemical structure, which can later be used to determine the exact origin of the species. In addition, observations for unlabeled nylon 6.6 aged in an O-18 enriched atmosphere were used to determine if the source of oxygen in the applicable degradation products was from the gaseous environment or the polymer. Approximations for relative isotopic ratios of unlabeled to labeled products are reported, where appropriate.
... This interaction explains the material sensitivity resulting in plasticisation and physical degradation when in service [15,55,56]. The second type of degradation is chemical and irreversible from the mechanical point of view, resulting from the hydrolysis process [5,57]. The determined molar weight and distributions by Le Gac et al. [58] in unaged, aged in seawater for six months and also dried after ageing specimens did not show relevant changes. ...
... Formulated differently, there is no moisture content in the material, or its amount is very little and negligible. Prolonged exposure may lead to continuous material degradation due to thermal-oxidative ageing (if the oxygen is present) [57]. However, within the framework of the current research, we assume that no chemical damage occurs due to relatively short desorption time, and corresponds to the value on the plateau. ...
The present work , pursued in collaboration with CETIM , is focused on the mechanical performance of preloaded bolted composite joints for automotive and aeronautical fields. Owing to the controlled preload application , bolted joints occupy a significant segment in the industry. Working environmental conditions tend to vary over time, affecting sensitive to temperature and humidity thermoplastic composites. An estimation of out - of - plane elastic properties of composites is , therefore, essential for an accurate dimensioning of bolted joints. The objective of the t hesis is to take into consideration and analyse the environme ntal impact on woven thermoplastic composite materials in order to ameliorate an analytical model for bolted composite joints. Conditioning protocols are proposed for an accurate evaluation of m aterial state at several Relative Humidity levels. Effect of humid ageing is investigated through the performed mechanical characterisation of a neat thermoplastic matrix and two woven composite materials. Numerical simulations provide the out - of - plane environment - related properties enabling the material compliance estimation. Mechanical testing of bolted composite joints is proposed to determine the loss of preload over time and to relate composite mechanical properties to the durability of joints.
... It results in a chain scission of the polymer followed by the irreversible reduction of mechanical characteristics. Bernstein et al. [31] investigated the effect of time of thermo-oxidative ageing and hydrolytic ageing in oxygen on the tensile strength of PA66. Indeed, the mechanical performance of the resin is strongly reduced at high and low temperatures, moisture and the duration of exposure. ...
... Once the moisture content reaches the plateau and stabilises, the specimen is assumed to be in the dry state. Prolonged exposure may lead to continuous material degradation [31]. However, within the framework of the current research, we assume that no chemical damage occurs due to relatively short desorption time, and C glob corresponds to the value on the plateau. ...
The polyamide-based composite materials are widely employed in various industrial domains. Along with the numerous advantages of these engineering materials, an important sensitivity to hygrothermal conditions represents a major drawback. Hence, this study aims to thoroughly evaluate the variation of physical and mechanical properties in moist and thermal environments of three stacking sequences. An extensive tensile testing campaign is performed in order to create a rich experimental database. It is therefore preceded by a proper establishment of desiccation/absorption protocol for the composite material. The results demonstrate that the humid ageing parameters are highly affected by the temperature. The mechanical response is dependent on the glass transition temperature, or Tg-T criterion, due to the occurring material plasticisation as a result of absorbed moisture. The shear modulus, the shear yield stress and strength deteriorate with the increase of temperature and moisture content. The changes of properties for the longitudinal and transversal orientations are not considerably pronounced.
... Moreover, the presence of water in PA6 can lead to a significant chemical degradation of the polymer. Amide groups react with water through a hydrolysis process [120] when immersed in water at high temperatures and/or for a long duration [121]. ...
... To prevent hydrolysis at this temperature, the samples were tested as soon as saturation was reached. Also, Bernstein [121] showed that a longer time of exposure is needed for hydrolysis to occur. From this result, it is clear that there is a very significant decrease in T g from 66 ± 1 °C in the dry state to -9 ± 1 °C when the specimen is fully saturated with 9% of water. ...
The composite materials used at sea are today nearly all based on thermoset resins (polyester, epoxy). However, there is an increasing number of thermoplastic matrix polymers available on the market (PP, PA, PPS, PEEK…), which offer possibilities for forming by local heating, attractive mechanical properties and the potential for end of life recycling. The reasons for the slow adoption of these materials are that they require a completely different manufacturing route compared to the current materials, they have more complex microstructure, and that there is little experience with them, particularly for thick structures. The aim of this study is to design, manufacture and test thermoplastic composite pressure vessels for 4500 meter depth, in order to establish a technical, economic and ecological assessment of the use of these materials to replace traditional composites underwater. Various material options exist but there are questions concerning the water sensitivity of less expensive thermoplastic composites (C/PA6) for these applications, more especially concerning its effect on the mechanical properties. In this study, a specific water diffusion model has been developed and semi-empirical relationships have been proposed to account for the effect of water on the mechanical properties. The effect of processing conditions have also been addressed and have shown a strong effect on the mechanical properties. Finally, thick thermoplastic cylinders were manufactured and tested until implosion. Results showed that it is possible to use C/PA6 thermoplastic composite cylinders for deep sea applications as these imploded at pressures higher than 600 bar.
... Moreover, the presence of water in PA6 can lead to a significant chemical degradation of the polymer. Amide groups react with water through a hydrolysis process [9] when immersed in water at high temperatures and/or for a long duration [10]. The latter leads to chain scissions that induce an embrittlement of the polymer and so lead to a large decrease in the mechanical properties. ...
... To prevent hydrolysis at this temperature, the samples were tested as soon as saturation was reached. Also, Bernstein [10] showed that a longer time of exposure is needed for hydrolysis to occur. ...
... During accelerated weathering, temperatures ranged between 50 and 60 °C, compared to an average temperature of 10 °C (min − 5 °C, max + 25 °C) during outdoor weathering. Bernstein and Gillen [30] showed that humidity is not a critical aging factor below 50 °C. Thus, a hydrophobic coating can slow down hydrolytic degradation during periods of 100% humidity in accelerated weathering. ...
Nylon 6 is one of the most widely used polymers in the world. For some nylon products, hydrophobic coatings are used for impregnation. However, it has been suggested in the literature that coating could accelerate aging. Therefore, in this paper, we focused on the degradation process of dyed nylon yarns with and without perfluorinated coating under accelerated weathering conditions. To monitor the degradation process, we used methods such as tensile test, molecular weight analysis, ultraviolet–visible spectroscopy, Fourier-transform infrared spectroscopy, differential scanning calorimetry, energy dispersive X-ray analysis, and scanning electron microscopy. We found that the hydrophobic coating is unlikely to have a negative effect on the degradation process. However, the coating decomposes during weathering, and its concentration on the fiber's surface decreases. The type of dye used was identified as the most significant factor influencing the degradation rate. This was explained by the screening effect of dyes in the UVA region of the light spectrum. Manufacturers of nylon products, which require a pleasant appearance and safety, should, therefore, consider a careful selection of dyes.
... 28 At higher temperatures it has been reported that elevated humidity increases the rate of degradation with Bernstein reporting that in the case of PA6,6 ageing, that thermooxidative degradation dominates below 50 C, while the effect of hydrolytic degradation is increasing important above this temperature. 29 An ageing study conducted in an oxygen free environment found that submerging PA11 samples in waters at temperatures of 110-140 C caused a significant loss in inherent viscosity due to chain scission and a resulting build-up in low molecular weight fragments. [30][31][32] The FTIR results are presented in Figure 6. ...
Additive manufacturing is traditionally used to manufacture either prototypes or very small-scale demonstrators. In recent years though, it is being increasingly used to make low volume parts for the aeronautical and defence industry. One concern with laser sintered parts is that their relatively porous nature, means that they may be more susceptible to ageing than injection moulded parts. Parts were aged for 6 months in at different temperatures (18°C, 40°C, 50°C, 60°C, 80°C and 100°C) and in a humidity chamber at 60°C and 80% relative humidity. Each month samples were removed for characterisation. The testing included tensile testing, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and gas pycnometry. During ageing the samples displayed visible discolouration and embrittlement over the 6-month test period. This embrittlement was not observed in those samples aged at room temperature or an elevated humidity. The observed yellowing in the samples aged above ambient temperature is likely a result of the build-up unsaturated degradation products. No significant differences as a result of ageing were observed via DSC, TGA, SEM or gas pycnometry.
... Additionally, the simultaneous effect of humidity, oil and fuel must be considered, since all together they can significantly reduce the performance of the PA materials. 15 It is well known from the literature that, at specific experimental conditions such as elevated temperature, neutral or acidic water solutions, [16][17][18][19][20][21][22][23][24] humid atmosphere 25,26 and cooling fluids, e.g. ethylene glycol/ water solutions, 12,13,15,[27][28][29][30] PAs are able to undergo reversible hydrolysis. ...
Two types (with and without a hydrolysis stabilizer) of polyamide 6.6 reinforced with 30 w/w % glass fibers were examined against the influence of automotive cooling fluids, e.g. ethylene glycol aqueous solutions. The overall goal is to give a methodology to compare the performance of PA 6.6 materials against the impacts of hydrolysis environment. The stabilizer effect on the hydrolytic resistance of the materials was assessed by aid of tensile tests according to ISO 527, and more detailed the strain-at-break values of them were evaluated. The degradation mechanism of both polyamide (PA) types was monitored by infrared (IR) spectroscopy and scanning electron microscopy (SEM). The material lifetime was described by the Arrhenius equation. The results show that the hydrolysis stabilizer operates effectively at low temperature but exhibits weak performance above 130°C, which was explained by faster consumption of the stabilizing agent.
... These potential problems are widely investigated by several researchers (Celina et al., 1998;Gillen et al., 1997). The accelerated aging method can also be conducted using various mediums such as air, distilled water, and salinated water (Bernstein and Gillen, 2010;Simar et al., 2014). Accordingly, Rabanizada et al. (2015) investigated the effects of different aging media on the natural rubber (NR). ...
A multi-physics model is proposed to predict the changes in the constitutive behavior of cross-linked polymeric systems due to damages induced by deformation, oxidation, moisture, and temperature. Coupling the concept of network evolution, hydrolysis, and thermo-oxidation aging, we hypothesize that the synergized effects of deformation-induced damage, as well as different environmental factors such as humidity, temperature, and oxygen, can be taken into account through a generic model. Deformation-induced damage is due to the chain breakage while hygrothermal aging results from the interaction of elastomeric components with moisture in the presence of oxygen and temperature. Results indicate that hygrothermal aging can be considered as a consequence of damage accumulation of two concurrent aging mechanisms, namely; (i) thermo-oxidative, and (ii) hydrolytic aging. In order to capture the mutual effects of thermo-oxidative and hydrolytic aging, the assumption has been made that each of the aging phenomena can be superimposed upon each other. In fact, each of them acts independently, and as a result, they can propagate hygrothermal aging in collaboration. In view of multiple kinetics involved in hygrothermal aging, its effect has been induced by the concurrent effects of three micro-mechanisms; (i) chain scission due to the presence of temperature, (ii) reduction of cross-links attributed to the attendance of water, and (iii) formation of cross-links as a repercussion of oxygen present in the environment. Utilizing the theory of network decomposition, all phenomena and their correlation were modeled, and thus, the strain energy function of the polymer matrix is written with respect to four independent mechanisms; (i) the shrinking original matrix that has neither been attacked by water nor oxygen, (ii) conversion of the first network to two new networks due to reduction and formation of cross-links, and (iii) energy loss from network degradation due to water attrition to polymer active agents. The proposed model cannot consider variation in oxygen density since it has been derived for cases where there is abundant oxygen. Apropos, the model is valid for a slow aging process that occurs in super-thin samples. Finally, the model is validated with respect to extensive sets of our experimental data. In view of its interoperability, precision, and deep insight it provides into the nature of the aging phenomenon, the model is a good choice for advanced implementation in FE applications.
... This chemical degradation leads to chain scission in the polymer and also results in significant changes in mechanical properties [9e11], but this degradation mechanism will not be considered in this paper. In fact, ageing times are short enough here to avoid any hydrolysis of the polymer, in order to focus only on plasticization [9,10]. ...
This paper considers the effect of water content on mechanical properties of polyamide 6 when used in a humid environment. First an experimental section presents the tensile behavior of polyamide with different amounts of water, with and without a through-thickness water gradient. Water profiles are introduced by immersion in sea water at 25 °C. The effect of testing temperature is also investigated in order to consider the influence of macromolecular chain mobility on the mechanical behavior. Then, in a second section, modeling of yield stress is proposed based on physical considerations. This can be used for life time prediction of PA6 in humid environment when plasticization is the main degradation mechanism.
... Such a non-linearity of the Arrhenius plot has been observed for the mechanical properties of various polymeric materials (Celina et al. 2005;Ding and Wang 2008). Gillen et al. (2005) and Bernstein and Gillen (2010) ensured the existence of a non-Arrhenius behavior by comparing the naturally aging polymer samples with the samples treated at widely ranged temperatures. Or, as applied for a lot of accelerated deterioration tests, lower a T of the naturally aging samples could be due to an accelerated change because of the moisture in the cell wall -which induces the hydrolysis -the repetition of drying and wetting, and the repetition of hot and cold periods. ...
Color changes of wood during natural aging are important from the aesthetical point of view and as criteria of degradation. The color properties of aged keyaki wood samples were compared with those of recent wood samples and those of heat treated wood samples in the previous paper. The values of L* and b* of aged wood were lower than those of recent wood, which was similar to those of heat treated wood. The values of a* of aged wood were larger than those of recent wood and heat treated wood, showing a different trend compared to heat treated wood. The results of kinetic analysis showed that the color changes during natural aging occurred more rapidly than the prediction estimated from the color changes of heat treated wood. These results were different from those of hinoki wood, which suggested that the responses to natural aging would differ depending on wood species.
... Note that Nylon 1010 is often used at relatively high temperatures, so the thermooxidative degradation of this thermoplastic material is liable to occur, including discoloration and deterioration of the mechanical properties. Although several groups have studied the thermooxidative aging of Nylon 6 [5][6][7][8][9] and Nylon 66 [10][11][12][13], to the best of our knowledge there are very few reports on that of Nylon 1010. In order to understand the actual performances of Nylon 1010, it is of great urgency to study its thermo-oxidative aging behaviors. ...
The thermo-oxidative aging behaviors of Nylon 1010 films were studied by various analytical methods, such as measuring the chromaticity, relative viscosity, carbonyl index, UV absorbance at 280 nm and elongation at break of the aged films. The thermo-oxidative aging plots of the results obtained via these various methods at different temperatures are subjected to the time–temperature superposition analysis, which are found to be well superposed. The b* values are used as X axis and the other results, i.e., relative viscosity, carbonyl index, UV absorbance at 280 nm and elongation at break, are used as Y axis, respectively. The relationship between the b* values and the other results is obtained, from which we can derive the changes of physical and chemical properties at different b* values. Since the b* values can be quickly determined by using a portable spectrophotometer, the on-line evaluation of the thermo-oxidative aging of Nylon 1010 can be realized.
... Such a non-linearity of the Arrhenius plot has been observed for the mechanical properties of various polymeric materials (Celina et al. 2005;Ding and Wang 2008). Gillen et al. (2005) and Bernstein and Gillen (2010) ensured the existence of a non-Arrhenius behavior by comparing the naturally aging polymer samples with the samples treated at widely ranged temperatures. Or, as applied for a lot of accelerated deterioration tests, lower a T of the naturally aging samples could be due to an accelerated change because of the moisture in the cell wall -which induces the hydrolysis -the repetition of drying and wetting, and the repetition of hot and cold periods. ...
The color properties of aging wood samples from historical buildings have been compared with those of recent wood samples that were heat treated at temperatures ranging from 90 degrees C to 180 degrees C. The results of kinetic analysis obtained by the time-temperature superposition method showed that the color change during natural aging was mainly due to a slow and mild oxidation process. In other words, heat treatment could accelerate the changes in wood color that occur during aging. In one sample, the color change (Delta E-ab*) after 921 years at ambient temperature was almost equivalent to that of heating (artificial aging) approximately for 6.7 h at 180 degrees C. The results have been interpreted that the aging and the subsequent change in wood color begin at the time of tree harvesting.
... The sudden transition of E a at a certain temperature or the non-Arrhenius behavior have been reported in many previous literatures [33e37]. Decrease of activation energy usually implies a lower activation energy chemical degradation pathway dominating at high temperatures [38]. The reason for the transition was discussed. ...
Abstract This study was carried out with an intention to remove the oxide scale on hot-rolled steel by gaseous reduction instead of traditional acid pickling method with an aim to reduce the pollution. The reduction of iron oxide scale by hydrogen–argon mixture was studied by thermogravimetric tests in the temperature range of 370–550 °C. The rate controlling process was discussed according to the Avrami–Erofe'ev equation generalized method. The analysis suggests that the reduction of scale is controlled by two- and/or three-dimensional growth of nuclei in the whole temperature range investigated. The apparent activation energy exhibit a sudden decrease from 78.8 to 31.8 kJ/mol at temperature higher than 410 °C. Morphological structure of the reduced scale was investigated by scanning electron microscope.
... At low to moderate temperature (typically below 200 C), polymer oxidation is mainly initiated by the thermal decomposition of hydroperoxide groups [32]. In general, two distinct modes compete: unimolecular and bimolecular modes [35]: Polymers ÀCH 2 ÀCH 2 À 1.5 Â 10 10 73 4.0 Â 10 À3 PE >NÀCH 2 À or eOÀCH 2 À 1.8 Â 10 9 63 2.5 Â 10 À2 PAs, PET, PBT ÀCH¼CHÀCH 2 À 4.0 Â 10 9 68 7.7 Â 10 À3 PBD ...
Nylon, also known as polyamides (PAs), undergoes local environmental degradation, leading to a decline in its mechanical properties over time. The degradation process is strongly influenced by the surrounding environment in which the polymer is utilized. PAs can experience various forms of degradation, such as thermal degradation, oxidation, hydrothermal oxidation, UV oxidation, and hydrolysis. It is crucial to understand each of these degradation mechanisms individually to better comprehend the degradation process of nylon. Although extensive research has been conducted on hydrolysis over the past few decades, there is currently no comprehensive review that consolidates the latest findings. This review analyzes the available characterization data and evaluates the changes in molecular weight, crystallinity, chemical structure, and mechanical properties of PAs that have aged in oxygen-free water at high temperatures. The primary objective is to gain a comprehensive understanding of the aging process of PAs in water without oxygen. Additionally, secondary information is provided, including the influence of fibers and additives in the polymer, hydrolysis susceptibility among PAs with longer aliphatic chains, correlation between different parameters, impact of acids, and important factors for determining the point at which PAs become brittle when subjected solely to hydrolysis.
Polymer degradation under aggressive environmental stressors often develops heterogeneities due to diffusion-limited reaction phenomena. This is well established for diffusion-limited oxidation (DLO), which is known to occur for most polymeric materials at elevated temperatures but has been less summarized for the conditions of diffusion-limited hydrolysis (DLH). An overview of hydrolysis for several materials and a computational model, analogous to the underlying equations for DLO, able to define this diffusion-reaction system is presented. A systematic study of the influence of various parameters, such as water diffusivity, reaction rate and order, and a more in-depth focus on residual isocyanate hydrolysis in a polymeric methylene diphenyl diisocyanate (pMDI) based polyurethane (PU) foam is given. For this system, we present experimental data for model ‘input’ parameters and discuss predictions for different conditions. We conceptually compare the behavior of diffusion-limited oxidation to that of diffusion-limited hydrolysis (DLH). With the mathematical framework and key material properties presented herein, any DLH phenomena following Fickian diffusion behavior can be understood, modeled, and predicted.
Fire-protective clothing are manufactured using heat and flame resistant fibers that are made of high-performance polymers. These polymers exhibit a gradual reduction in their performance after long-term exposure under various aging conditions. This study investigates the thermal aging behavior of a high-temperature resistance polymer, polyetherimide (PEI), at elevated temperatures in air. Changes in the ultimate tensile strength (UTS), glass transition temperature, and surface properties are observed. In addition, infrared spectroscopy identifies chemical bonds in PEI being consumed because of chain scission. Applying the time–temperature superposition principle to the UTS data gives an activation energy of 112 kJ mol⁻¹ for the effect of thermal aging on the mechanical strength of PEI. This value is similar to what has been reported for fire-protective clothing fabrics, which opens the possibility to use PEI as a sensing material for the development of end-of-life sensors for fire-protective fabrics.
Polyamide 6 films were aged aged in two environments inducing either only oxidation or only hydrolysis of the polymer for up to two years. Ageing temperatures ranged from 80°C to 140°C. Samples were characterized periodically in terms of both chemical structure at the macromolecular scale, using SEC, DSC, SASX and WAXS, and mechanical behaviour through tensile tests. Both degradation mechanisms lead to chain scission within the polymer, an increase in crystallinity ratio, a decrease in the amorphous layer thickness and an embrittlement of the polymer. First a decrease in the strain at break is observed while the maximal stress remains unchanged. Then a drop in maximal stress is identified. Using these experimental results, the origin of the embrittlement and the factors governing embrittlement are discussed. The decrease in strain at break is attributed for the first time in polyamide to the decrease in concentration of tie molecules determined through a theoretical approach. The loss in entanglements is associated with the drop in maximal stress. Furthermore, it is shown that the crystallinity ratio does not govern the embrittlement of polyamide. However, both the molar mass and the amorphous layer thickness are faithful indicators of this embrittlement whatever the degradation mechanism.
The lightness of design
Lines and curves make the pavilion a dynamic and trimmed sculpture whose characteristics derive from the brand image of the vehicle manufacturer. Uniform, matt stainless steel sheets wrap the body shell seamlessly. In a similar way to monocoque design, which is used in lightweight construction in the automotive and aircraft industries, the space-creating shell of the building takes over the load-bearing function. A total of 620 stainless steel covering sheets with welded-on stiffening ribs were prefabricated in a shipyard in Stralsund and assembled on site
Table of contents conference proceedings
5
Anspruchsvolle Oberflächen und Leichtbau in der Instrumententafel des neuen Volkswagen Tiguan
Mielke, R. / Dierks, P. | 201
19
3D Simulation für den Leichtbau in der kunststoffverarbeitenden Industrie
Kurz, M. | 2016
31
Neuartige PUR-Oberflächen. Selbstheilend und mehr
Kleba, I. / Emig, J. | 2016
47
Mono-polymer lift-gate solution cuts CO2 emissions
Liraut, ...
Thermo-oxidative aging of dipentaerythritol (DPE) stabilized, glass fibre reinforced PA66 compounds was studied at 200°C. The influence of DPE as a thermo-oxidation stabilizer on the retention of tensile properties, molecular weight, crystallinity, oxidized layer thickness as well as the specimen surface and fracture morphology was analysed. Moreover, the impact of DPE on the intrinsic humidity inside the test specimens before aging was examined due to its hygroscopic behaviour. The reduction of the extractable amount of DPE from test granules over the aging time proved the existence of chain extension or crosslinking reactions between the polyamide or its degradation products and DPE. Whereas DPE improves the retention of mechanical characteristics during aging only slightly, the combination of DPE and glass fibres reaches retention of mechanical characteristics above 50 % after 1000 h of aging at 200°C in air. DPE raises the intrinsic humidity during processing and before aging resulting in an increase of PA66 degradation during processing and in the first aging phase due to additional hydrolysis. Both the ensuing shorter chain length and water-induced post-crystallisation give rise to higher crystallinity after processing, increasing the oxygen diffusion limitation which results in slower thermo-oxidation. In addition, DPE accelerates the formation of the oxidized layer on the PA66 surface but slows down its growth during the later aging phase.
This work deals with the effects of long‐term storage on the properties of polyamide 12 filled with nanoclay mass fractions of 0, 3, and 5%. More precisely, such nanocomposites were prepared using internal mixer, and then stored in a room under ambient conditions, for 6 years, protected from light and temperature variations. The changes in their thermal, mechanical, and rheological properties were analyzed, in relationship with the microstructure changes. Various analyses and tests were conducted, including wide‐angle X‐ray diffraction experiments, water mass loss evaluation, differential thermal analyses, strain sweep tests on solid‐state specimens, as well as time sweep and oscillatory shear measurements on melt‐state specimens. The results mainly revealed the effects of the absorbed water on the changes of the aged nanocomposite performances and highlighted the positive role of nanoclay platelets in the preservation of processability and end‐use properties. Long time storage effects on the properties of PA12/C30B nanocomposites. Modification of the material viscoelastic properties after ageing. Positive role of nanoclay in protection of PA12 matrix against time‐ageing. Modification of the material viscoelastic properties after ageing. Positive role of nanoclay in protection of PA12 matrix against time‐ageing.
The first aim of this work is to study the degradation of an industrial polyesterurethane (PURm) by radio-oxidation and then by leaching in an alkaline aqueous solution. The second aim is to measure the complexing power of hydrosoluble degradation products (HDP) with actinides.
To reach these goals, PURm was first characterized and then radio-oxidized at room temperature with gamma rays up to 10 MGy. Second, it was leached at pH 13.3 at different temperature values. Numerous analytical techniques were employed in order to characterize the HDP. Europium(III) was used as an analogue of actinides(III) and the behavior of HDP with europium(III) was analyzed by time-resolved luminescence spectroscopy (TRLS).
Whatever the received dose by PURm, the two main HDP are adipic acid and butane-1,4-diol. The leaching data acquired at 40 and 60°C, on the 1 MGy radio-oxidized PURm, correlate with the model given by Yoon et al. (1997). On the contrary, the data at room temperature (22°C in average) are not in agreement with the model. Nevertheless, it seems that the plateau which was reached after a long period of leaching is the same whatever the temperature used in this study.
The results allow to conclude that the predominant mechanism occurring during the leaching of unirradiated and radio-oxidized PURm in an alkaline medium is the hydrolysis of the soft segments ester groups.
The complexation of europium(III) by HDP in alkaline medium was demonstrated. The measurement of the complexing power and the identification of ligands was achieved under certain conditions.
This article focuses on the accelerated aging behavior of a glass fiber-reinforced and heat stabilized polyamide grade. Accelerated aging of injection molded specimens was achieved by hot air exposure (HAE) and pressure cooker testing (PCT) at elevated temperatures. For both aging routines, characteristic aging indicators were found. For hot air exposure, the carbonyl index obtained from infrared spectra revealed superficial thermo oxidation. For pressure cooker testing, the development of a regular crack pattern and successive surface roughening were observed by light microscopy and laser confocal microscopy. This was accompanied by a significant reduction in molar mass assessed by viscometry. In addition, progressing degradation of mechanical performance was ascertained by tensile testing experiments. At the onset of material cracking, the molar mass was about 25% of the initial value.
The Institut de Police Scientifique (IPS) of Lausanne has developed and routinely employs a profiling method for heroin that allows establishing links between seizures of different cases. The comparison is based on chemical characteristics of samples. During the distribution, the samples are transported, stored and diluted, and the chemical characteristics are then subject to change. The objective of this study was to evaluate the consequences of an important dilution on the reliability of the established links and to observe the influence of temperature and humidity on the chemical profile of heroin samples submitted to accelerated ageing conditions. It was noticed that dilution does not interfere directly with profiling, however the inference of a common source between the specimens depends on the difference between the concentrations of the samples compared. Concerning the ageing study, all samples could be linked and no significant variation of the profile was observed. However, the quantities of some compounds are more likely to vary and could cause modifications in the profile in the long-term.
The likelihood of successful system operation following long periods of dormancy can be increased by assessing the long-term environmental effects on the performance of materials and subsystems. Because of the long experimental times often required in many accelerated aging programs, the need for faster and more reliable methods for investigating the degradative aging of polymeric materials under various environments becomes important. In this study, a micro-calorimeter was used in monitoring the heat flow as a function of time for a Nylon 6.6 polymer. Arrhenius model was used in fitting the data, following which activation energy was determined. The results obtained are consistent with published data for other sensitive measurement methods, but at a fraction of the samples and time. Implementation of technique and mitigation of weaknesses inherent in it will be presented and discussed.
The time temperature superposition method based on Arrhenius model was applied to predict the service life of nylon 6 with single temperature aging factor. In addition, the combination of Eyring model and Arrhenius model was further used to predict the service life of nylon 6 at different temperatures and humidity by introducing humidity aging factor, and the service life prediction model under hydrothermal ageing for nylon 6 was established. The results show that the thermal-oxidative degradation activation energy (Ea) declines with decrease of the ageing temperature, indicating of the change of degradation mechanism. There is a critical temperature below which the hydrolytic degradation doesn't dominate. The service life prediction model of nylon 6 under hydrothermal ageing is proved to be effective by ageing experiments. ©, 2015, Chengdu University of Science and Technology. All right reserved.
PA 6,6 is now considered for the elaboration of valve parts in the domestic network of drinking water distribution. However, this material will be used only if its long-term durability, in real use conditions, is clearly demonstrated. The kinetic analysis of the main chemical ageing mechanisms which may occur and interact in the domestic network of drinking water distribution: hydrolysis, thermal oxidation and chemical attack by chlorine dioxide, has allowed to finally build a general kinetic model for PA 6,6 degradation. This model gives access to PA 6,6 structural modifications at the different pertinent scales (molecular, macromolecular, morphological and macroscopic scales) in use conditions. It predicts the embrittlement of PA 6,6 when the number average molar mass reaches a critical value of the order of 17 kg/mole. Many research prospects are presented such as the introduction of this kinetic model in common mechanical calculation codes.
In this study, measurements of nylon 11 pipes subjected to ageing at 40 °C, 70 °C and 100 °C in water, methanol and xylene using both a constant pressure of 200 bar and a cyclic pressure regime are reported. Gravimetric measurements indicate the rate at which the solvent is absorbed by the polymer, and differential scanning calorimetry follows the changes in the crystallinity as the materials are aged. Dramatic changes in the tensile properties are observed when the pipes are subjected to high pressure and reflect a relaxation of the stress in the matrix introduced by the quenching process when the pipes were extruded. The magnitude of the change varies with the fluid and reflects their relative abilities to be absorbed by the polymer. Measurements of the relative viscosity for the polymer indicated that in the case of water and methanol, hydrolytic degradation of the polymer is time-dependent. The impact of the morphological changes in the dynamic mechanical properties revealed the movement of elasticity transition to higher temperatures and reduction in the chain mobility with time. Changes in the mechanical properties are a function of the initial stress relaxation and chain scission as a consequence of degradation and thermodynamically driven morphological changes increasing the crystallinity and embrittling the polymer. As the pipes aged the burst pressures progressively decreased. Examination of the failure surfaces indicated brittle failure and clear evidence of environmental stress cracking. Whilst the data indicate that the pipes might be used effectively for hydrophobic fluids, hydrophilic fluids and in particular methanol can significantly shorten their effective life.
The long-term hydrothermal aging behavior of polyamide 6 (PA6), with and without stabilizers at 100% relative humidity/90ºC was studied. It was found that the first stage of aging was the Fickian process and corresponded to the physical absorption of water until equilibrium, resulting in slight change of reduced viscosity and chemical structure of the sample of PA6. The second stage of aging was the initiation process of hydrolytic degradation of PA6, which resulted in a decrease of reduced viscosity and increase of end group content. In the final stage of aging, the relative weight gain (Wr) dropped, the reduced viscosity decreased monotonically, and end groups increased continuously, resulting from the hydrolysis of amido links and molecular degradation of PA6. The presence of four types of stabilizers effectively weakened the oxidation reaction and resulting molecular degradation of PA6 during aging. The lifetime of PA6 in a hydrothermal environment was predicted through a method based on the Arrhenius model by considering both temperature and humidity as environmental factors, and, furthermore, was predicted through the time-temperature superposition method.
Elastomers are often degraded when exposed to air or high humidity for extended times (years to decades). Lifetime estimates normally involve extrapolating accelerated aging results made at higher than ambient environments. Several potential problems associated with such studies are reviewed, and experimental and theoretical methods to address them are provided. The importance of verifying time temperature superposition of degradation data is emphasized as evidence that the overall nature of the degradation process remains unchanged versus acceleration temperature. The confounding effects that occur when diffusion-limited oxidation (DLO) contributes under accelerated conditions are described, and it is shown that the DLO magnitude can be modeled by measurements or estimates of the oxygen permeability coefficient (P-Ox) and oxygen consumption rate (4)). P-Ox and phi measurements can be influenced by DLO, and it is demonstrated how confident values can be derived. In addition, several experimental profiling techniques that screen for DLO effects are discussed. Values of phi taken from high temperature to temperatures approaching ambient can be used to more confidently extrapolate accelerated aging results for air-aged materials, and many studies now show that Arrhenius extrapolations bend to lower activation energies as aging temperatures are lowered. Best approaches for accelerated aging extrapolations of humidity-exposed materials are also offered.
Accelerated thermal-oxidative aging tests on Nylon 66 automotive airbag material are designed and performed in this paper. After the environmental profile of airbag was analyzed, Nylon 66 airbag yarns were reasonably selected as the test samples, and its tensile strength was chosen as the significant character of their aging stability. By heat aging、humidity aging、cycle ageing and comprehensive process aging experiments, 350dtex Nylon 66 filament were sampled and the tensile strength loss of samples was measured, to determine the mechanisms and factors that influence aging stability of the Nylon 66 airbag materials. The experiment results show that Nylon 66 has excellent aging stability, and that water molecules can accelerate its degradation. The experiment data can be used to predict the airbag lifetime in a benign store environment, and as references in selecting airbag materials.
Regarding the underlying special relaxation feature of water-plasticized hydrophilic polymer during performance evolution with water content change, we report the water desorption kinetics and periodic creep responses of Poly (vinyl alcohol) (PVA) films subsequent to rejuvenation by above-glass transition relative humidity (RH) annealing and following RH-jump at various rates. Moisture Sorption Analyzer and Dynamic Mechanical Analyzer are utilized to control RH and to capture data to probe the evolving relaxation towards equilibrium regarding two temperature-RH conditions. The result reveals an evident jump rate dependence of desorption kinetics and recoverable creep deformation. The different target RH yields the different change pattern of normalized water content and retardation time. PVA manifests a rapid relaxation stage with the special viscoelastic response before experience of usual physical aging. By analysis of the superposition principle and the relevant characteristic parameters, the relaxation of hydrophilic polymer after water desorption through the glass transition is generalized as three successive phases.
Monomer casting (MC) nylon-6 was polymerically modified by grafting 4-amino-2,2,6,6-tetramethylniperidine (TEMP), as a reactive hindered amine, onto its chain so as to improve the compatibility and stability of the stabilizer in the polymer matrix. The long-term aging of MC nylon-6 under UV irradiation indicated that the grafted sample of MC nylon-6 had lower gel content than that of the pure sample, and only a slow change in the reduced viscosity was observed. The carboxyl concentration of the grafted sample was lower than that of the pure sample, and its terminal amine group concentration was relatively higher during the whole aging process, presenting a more stable chemical structure. The delayed oxidation exothermic peak and increase of oxidation induction time (OIT) suggested that the grafted TMEP was stable in the matrix of nylon-6 after long-term UV aging. Decreasing crystallization ability after aging was observed for the pure sample of MC nylon-6, while a relatively stable crystallization behavior was observed for the grafted samples, indicating that TEMP can inhibit the structural change of MC nylon-6 during UV irradiation.
Nanoindentation using atomic force microscopy was used to examine the mechanical properties of fibers at nanoscale. The effect of accelerated aging of nylon fibers under ultraviolet (UV) and thermal-oxidative conditions on mechanical property gradients across the fiber cross-section was investigated. It was identified that UV degradation produces bulk degradation up to 48 h of exposure and later surface degradation predominates with lower values of Young's modulus at surface compared to center. Wide angle X-ray scattering shows a decrease of crystallinity with UV exposure. Unlike UV degradation, the thermal degradation was predominant at the surface for the first 3 weeks of exposure and was homogeneous after 4 weeks of exposure. These results support the idea that property gradients arise at high levels of degradation of nylon fiber as a result of UV and thermal-oxidative conditions, and ultimately lead to embrittlement of the fibers. These results are in contrast to polypropylene, where the nanoindentation modulus increased with degradation and increased from the core to the surface after degradation. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers
Nylon 6.6 containing 13C isotopic labels at specific positions along the macromolecular backbone has been subjected to extensive thermal-oxidative aging at 138 °C for time periods up to 243 days. In complementary experiments, unlabeled Nylon 6.6 was subjected to the same aging conditions under an atmosphere of 18O2. Volatile organic degradation products were analyzed by cryofocusing gas chromatography mass spectrometry (cryo-GC/MS) to identify the isotopic labeling. The labeling results, combined with basic considerations of free radical reaction chemistry, provided insights to the origin of degradation species, with respect to the macromolecular structure. A number of inferences on chemical mechanisms were drawn, based on 1) the presence (or absence) of the isotopic labels in the various products, 2) the location of the isotope within the product molecule, and 3) the relative abundance of products as indicated by large differences in peak intensities in the gas chromatogram. The overall degradation results can be understood in terms of free radical pathways originating from initial attacks on three different positions along the nylon chain which include hydrogen abstraction from: the (CH2) group adjacent to the nitrogen atom, at the (CH2) adjacent the carbonyl group, and direct radical attack on the carbonyl. Understanding the pathways which lead to Nylon 6.6 degradation ultimately provides new insight into changes that can be leveraged to detect and reduce early aging and minimize problems associated with material degradation.
Thermal ageing studies over relative high temperature range (from 250 °C to 85 °C) have been conducted on polydimethylsiloxane (PDMS) rubber with the aim of investigating ageing behavior. Tensile elongation, compression set and creep measurements are introduced to monitor the ageing process. Time temperature superposition is performed and the linear trends are found in the Arrhenius plots for all the approaches, revealing an identical process dominates the degradation process. Activation energies are found to be depended on the methods used, which are 88.5 kJ/mol for tensile tests, 67.7 kJ/mol for compression set measurements and 75.3 kJ/mol for creep measurements. The phenomenon that activation energy obtained from tensile elongation is always higher than other approaches is rationalized by the sensitive modification of thermal degradation on the rubbery network. Moreover, correlations between the results from different approaches have been examined. The linear relationships have been found between tensile elongation and creep or compression set measurements for the three methods are all influenced by the network structure.
Thermal degradation of a high temperature epoxy network is studied in terms glass transition temperature (T-g) reduction over a temperature window encompassing the T-g of the network. The T-g is shown to decrease as the network is thermally aged at elevated temperatures in air and in argon. The duration of the aging experiments is extended to long time such that the absolute T-g reduction approaches a long time reduction plateau. Degradation is dominated by non-oxidative pyrolysis with a small contribution from diffusion limited thermal oxidative degradation at the surface. A time-temperature superposition is constructed from the extent of T-g reduction of samples aged in air and the thermal shift factors are shown to have Arrhenius scaling behavior. An activation energy is extracted that agrees with previous activation energy measurements derived from other property measurements of the same network aged under similar conditions. The agreement of the activation energy with past results shows that T-g reduction is controlled by the same degradation mechanism and may be used as an observable for lifetime estimates when thermal degradation is pyrolytic in nature. The extent of T-g reduction is modeled with an autocatalytic rate expression and compared to previous property measurements to show the difference in sensitivity of observable material properties on degradation.
Oxidation of organic materials typically results in the outgassing of degradation compounds. The most abundant outgassing thermal-oxidative degradation species of nylon 6.6 are known to be carbon dioxide (CO2) and ammonia (NH3). By performing accelerated aging experiments under thermal-oxidative conditions on unlabeled, 13C, and 15N isotopically labeled nylon 6.6 polymers, we identified the origins of CO2 and NH3 as a means to gain insight of the underlying chemical pathways which lead to their formation. Additionally, an isotopically enriched oxygen environment (18O2) was used in experiments tailored to discriminate between oxygen originating from the carbonyl carbon in nylon 6.6 and oxygen originating from the ambient air environment. To our knowledge, this work is the first ever account which quantitatively distinguishes oxygen containing degradation species originating from the polymer backbone and oxygen species coming from the ambient air during the oxidation process. Cryofocusing gas chromatography-mass spectrometry (cryo-GC/MS) performed on the outgassed products demonstrated the presence of CO2, 13CO2, CO18O, C18O2, NH3, and 15NH3. We show that approximately 42% of the CO2 formed comes from the carbonyl carbon atoms in the interior of the macromolecular chain. About 15% of the CO2 originates from the methylene groups adjacent to the nitrogen atoms within the chain, while about 25% originates from all of the other methylene carbons in the nylon repeat unit. Approximately 18% of the CO2 came from chain-end carboxylic acid groups in the nylon, indicative of end group concentration. The agreement of isotopic labeling between nylon and ammonia confirms that the source of ammonia in the degradation experiments is nylon and not some nitrogen-containing impurity (e.g., solvent) in the material. Identification of these species was pertinent in developing an enhanced understanding of the chemical degradation processes. Most importantly, the methodologies employed in this work may be extended to other organic materials and likely leveraged towards future sensor development in applications that aim to provide condition monitoring of aging materials.
The influence of various standard (ASTM, ISO) and experimental moisture conditioning methods on mechanical performance of injection molded nylon 6 is discussed as a result of an in-depth, comprehensive investigation.
These methods covered a wide range for two basic process parameters for conditioning: temperature (from 23 to 100 C) and relative humidity (from 50% RH to water immersion). The variation of these parameters may result in significantly different moisture absorption rates, equilibrium levels and mechanical properties. The kinetics of mechanical performance and microstructure were evaluated prior to tests and during conditioning in this comprehensive analysis.
The results from this investigation may provide comprehensive, up-to-date information and recommendations concerning accelerated nylon conditioning methods for test specimens and various molded parts, preselection of nylon based plastic for design, and prediction and optimization of mechanical performance.
Testing of plastics generally involves accelerated methods whose greatest merit is their ability to yield experimental data rapidly. The results may in some cases be of no help in determining the useful life-time of the plastics end-product. This chapter critically examines the accelerated testing procedures used to assess the stability of plastics to thermo- oxidation and photo-oxidation.
Thermal analysis techniques such as DSC are discussed in detail. It is shown that although DTA/DSC is excellent for quality control purposes, it is of no value in the prediction of oven aging in the solid state. In the case of light stability, the results obtained in various accelerated exposure devices for multifilaments, tapes, films and injection molded plaques are compared with the corresponding data from out-door weathering, and the best correlation is found with exposure devices equipped with lamps that do not emit light of wavelengths below 295nm, e.g. filtered xenon arcs. However, even with such devices, correlation may be restricted for reasons related to the physical behavior of the light stabilizer system. This is demonstrated clearly for PP tapes.
The thermal oxidative aging of a crosslinked hydroxy-terminated polybutadiene (HTPB)/isophorone diisocyanate (IPDI) based polyurethane rubber, used as a polymeric binder in solid propellant grain, was investigated at temperatures from 25 C to 125 C. The changes in tensile elongation, polymer network properties and chain dynamics, mechanical hardening and density were determined with a range of techniques including modulus profiling, solvent swelling, NMR relaxation and Oâ permeability measurements. We critically evaluated the Arrhenius methodology that is commonly used with a linear extrapolation of high temperature aging data using extensive data superposition and highly sensitive oxygen consumption experiments. The effects of other constituents in the propellant formulation on aging were also investigated. We conclude that crosslinking is the dominant process at higher temperatures and that the degradation involves only limited hardening in the bulk of the material. Significant curvature in the Arrhenius diagram of the oxidation rates was observed. This is similar to results for other rubber materials.
We have been working for many years to develop improved methods for predicting the lifetimes of polymers exposed to air environments and have recently turned our attention to seal materials. This paper describes an extensive study on a butyl material using elevated temperature compression stress-relaxation (CSR) techniques in combination with conventional oven aging exposures. The results initially indicated important synergistic effects when mechanical strain is combined with oven aging, as well as complex, non-Arrhenius behavior of the CSR results. By combining modeling and experiments, we show that diffusion-limited oxidation (DLO) anomalies dominate traditional CSR experiments. A new CSR approach allows us to eliminate DLO effects and recover Arrhenius behavior. Furthermore, the resulting CSR activation energy (E{sub a}) from 125 C to 70 C is identical to the activation energies for the tensile elongation and for the oxygen consumption rate of unstrained material over similar temperature ranges. This strongly suggests that the same underlying oxidation reactions determine both the unstrained and strained degradation rates. We therefore utilize our ultrasensitive oxygen consumption rate approach down to 23 C to show that the CSR E{sub a} likely remains unchanged when extrapolated below 70 C, allowing very confident room temperature lifetime predictions for the butyl seal.
Although it has long been known that the glass transition temperature (T-g) of nylons changes significantly with moisture content, literature to date has lacked reliable measurements of this type, primarily due to the changes in moisture during the T-g scanning experiments. Introduction of the new dynamic DSC technique has allowed us to quantify the dependence of T-g on % relative humidity for nylon 6 and nylon 66. By eliminating the possibility of moisture pickup, we have been able to separate the effect of crystallinity as well as orientation on the T-g of nylon 6. The methodology described here should be applicable to other moisture sensitive polymers as well, e.g., poly(vinyl alcohol).
We have carried out oven aging studies on eight different commercial chlorosulfonated polyethylene cable jacket materials at temperatures ranging from 80°C to 150°C utilizing ultimate tensile elongation as the degradation parameter. For each material, the elongation results were time–temperature superposed at the lowest aging temperature. When the resulting empirical shift factors were tested for Arrhenius behavior, it was found that the eight materials were Arrhenius at ∼100°C and higher with very similar activation energies averaging ∼107kJ/mol. Longer-term aging results at temperatures lower than 100°C for three of the materials provided evidence for curvature to lower activation energies. For one of these materials, we conducted oxidation rate measurements at six temperatures ranging from 37°C to 108°C. The results offered further evidence for a small drop in activation energy below 100°C. Chemical evidence supporting this change in activation energy was derived from analysis of the production rates of CO2 during oxidation. As the temperature was lowered, the amount of CO2 produced relative to the O2 consumed dropped substantially, implying that the chemistry leading to CO2 becomes less important at lower temperatures.
High-temperature oven aging exposures from 110°C to 80°C have been conducted on a commercial chloroprene rubber cable jacketing material where the time-dependent degradation at each temperature is monitored by following the ultimate tensile elongation. By time–temperature superposing the results at the lowest experimental temperature (80°C), empirical shift factors are first derived and then analysed using the conventional Arrhenius approach and finally extrapolated using the derived activation energy Ea of 96kJ/mol in order to make predictions at 25°C. To test the constant Ea assumption underlying the Arrhenius extrapolation, we conducted oxygen consumption measurements at six temperatures ranging from 25°C to 95°C and found evidence for a slow drop in Ea from ∼96kJ/mol above 80°C to an average value of ∼82kJ/mol below 80°C. This curvature predicts a 50% reduction in room temperature lifetime and suggests that certain higher Ea degradation processes become less important as room temperature is approached. Analyses of the production rates for CO2 show that CO2 becomes less important relative to the oxygen consumed as the temperature is lowered, evidence in accord with the above suggestion. Further evidence for a similar drop in Ea (from 89kJ/mol above 70°C to 71kJ/mol below 70°C) comes from comparisons of accelerated aging elongation data taken ∼25 years ago on another chloroprene rubber cable jacketing material with recent elongation results taken on samples that have aged for ∼24 years at ∼24°C.
Unstabilized and phenolic antioxidant stabilized PP films were aged in draft air ovens at temperatures between 150 and 40°C. The Arrhenius plots of the failure times reveal quite distinctive features. Thus, unstabilized PP films show a marked downward curvature of the plot at temperatures below 80°C. The role of Ti catalyst residues in this behavior is discussed.Phenolic antioxidant stabilized films show a similar bent in the Arrhenius plot at significantly higher temperatures. Furthermore, these plots can be superposed onto the curve obtained for the unstabilized film by a shift parallel to the reciprocal absolute temperature axis. The numerical value of the shift is a function of the chemical nature of the phenolic antioxidant and of its concentration. Remarkably, the phenol stabilized films show a second curvature in the Arrhenius plot at temperatures below 80°C. This second curvature is an upward bent so that the Arrhenius plot becomes a typical sigmoid. This low temperature curvature depends also on the nature of the phenolic antioxidant and its concentration. It is attributed to some kind of complexation of the Ti catalyst residues by the phenols. Comparison with PP films stabilized with Hindered Amine Stabilizers (HAS) and with PE-LD films lends additional support to the conclusions concerning the role of transition metals.
The thermal hydrolysis of nylon 6,6 between 338 and 398 K was investigated using solution 17O NMR spectroscopy. By performing the hydrolysis with isotopically 17O-enriched H2O, it is possible to easily identify the non-volatile oxygen-containing degradation products formed during the hydrolysis of nylon. For the aging temperature range investigated, the dominant oxygen-containing degradation species are carboxylic acids, consistent with the hydrolytic cleavage of the amide bond. These 17O NMR studies allowed the temperature variation for the hydrolysis of the amide bond in nylon 6,6 to be determined with of the initial rate of carboxylic acid concentration production giving an energy of activation of ∼87±1 kJ/mol.
The determination of the glass transition temperature of semi-crystalline polymers is a controversial problem in the literature, because of the complexity of the phenomenon and of the different methods used for its measurement. In this work the glass transition temperatures of five commercial nylons (nylon-6, nylon-6,6, nylon-6,10, nylon-11, nylon-12) have been measured by both thermal and mechanical methods. The behaviour observed during thermal measurements is analogous to that observed by Gordon, who found that the transition detected in the heating cycle disappeared in the subsequent cooling cycle and appeared again only after a sufficient rest period of the samples, and at a temperature different from the initially measured one. He attributed this behaviour to the structure of the amorphous regions of the material, where the hydrogen bonding groups form an irregular network. The delay in reforming the above mentioned network is the main cause of the dependency of the observed transition on the thermal history imposed on the samples. Mechanical measurements give results that are quite insensitive to the thermal treatment of the materials, and thus provide very reproducible values of the transition. This feature allows the possibility of attributing to the transition obtained the character of a true glass transition where the main cause of the phenomenon is the increased mobility of the chain backbone in the amorphous regions of the materials with increasing temperature. This character was also confirmed by dilatometry, with results in agreement with Boyer's criteria for a true glass transition temperature.
Conventional high-temperature compression stress–relaxation (CSR) experiments (e.g., using a Shawbury–Wallace relaxometer) measure the force periodically at room temperature. In this paper, we first describe modifications that allow the force measurements to be made isothermally and show that such measurements lead to more accurate estimates of sealing force decay. We then use conventional Arrhenius analysis and linear extrapolation of the high-temperature (80–110 °C) CSR results for two commercial butyl o-ring materials (Butyl-A and Butyl-B) to show that Butyl-B is predicted to have approximately three times longer lifetime at room temperature (23 °C). To test the linear extrapolation assumed by the Arrhenius approach, we conducted ultrasensitive oxygen consumption measurements from 110 °C to room temperature for the two butyl materials. The results indicated that linear extrapolation of the high temperature CSR results for Butyl-A was reasonable whereas a significant curvature to a lower activation energy was observed for Butyl-B below 80 °C. Using the oxygen consumption results to extrapolate the CSR results from 80 °C to 23 °C resulted in the conclusion that Butyl-B would actually degrade much faster than Butyl-A at 23 °C, the opposite of the earlier conclusion based solely on extrapolation of the high-temperature CSR results. Since samples of both materials that had aged in the field for ∼20 years at 23 °C were available, it was possible to check the predictions using compression set measurements made on the field materials. The comparisons were in accord with the extrapolated predictions made using the ultrasensitive oxygen consumption measurements, underscoring the power of this extrapolation approach.
Experiments were performed to elucidate the degradation mechanism of hot-pressed polyamide 66 upon exposure to water. For films exposed to water over the temperature range 25°C–90°C, degradation was monitored using FTIR and solid-state 13C NMR spectroscopies. The data are consistent with a mechanism in which (1) a radical is formed on the methylene carbon adjacent to the amide nitrogen, (2) this radical reacts with oxygen to form a hydroperoxide, and (3) the hydroperoxide decomposes to form an imide or a hydroxylated amide, both of which may cleave leading to chain scission. Water appears to facilitate degradation by increasing the flexibility of the polymer matrix through swelling rather than acting as a reactive species, at least at the early stages of the process. An apparent activation energy of 15±2kJ/mol is observed for the early stages of degradation, suggesting that segmental motions in the polymer associated with water and oxygen sorption or inter-chain radical reactions are indeed key components of the degradation process.
The sorption and transport of water in nylon 6,6 films as functions of the relative humidity (RH) and temperature were studied. Moisture-sorption isotherms determined gravimetrically at 25, 35, and 45°C were described accurately by the GAB equation. Water-vapor transmission rates were enhanced above ≈ 60–70% RH, primarily due to the transition of the polymer from glassy to rubbery states. The glass transition temperatures (Tg's) of nylon 6,6 were measured at various moisture contents using differential scanning calorimetry. The results showed that the sorbed water acted as an effective plasticizer in depressing the Tg of the polyamide. Fourier transform infrared spectroscopy (FTIR) was utilized to characterize the interaction of water and the nylon. Evidence from FTIR suggested that the interaction of water with nylon 6,6 took place at the amide groups. Based on the frequency shift of the peak maxima, moisture sorption appeared to reduce the average hydrogen-bond strength of the NH groups. However, an increase was seen for the CO groups. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 197–206, 1999
The service life of ABS polymer, stabilized by 2-(3,5-di-tert-butyl-4-hydroxyanilino)-4,6-bis(octylthio)-1,3,5-triazine and containing 50% of a modifying rubber component, was estimated from oxidative induction times measured by DSC in isothermal mode in the temperature interval 140–170C. The lifetime of ABS powder at the actual temperature of drying was predicted by linear extrapolation according to Arrhenius. However, the extrapolated value was much longer than the real lifetime determined from the long-term oven aging tests at 70 and 90C, simulating the industrial drying process. The effect of changes in the apparent activation energy of oxidation due to antioxidant consumption during polymer aging is discussed.
The influence of temperature and of residues of a TiCl3-based polymerization catalyst on the oxidation rate of polypropylene was determined. The plots of the logarithm of the induction period versus the reciprocal temperature (Arrhenius plot) are curved. The influence of the concentration of the polymerization catalyst on the stability depends on the degradation temperature. At 50°C polymers containing less than 2 or 8 ppm of a killed Ti polymerization catalyst show a longer induction period than polymers containing 64 and 180 ppm Ti. At 130°C there is almost no difference in the stability of these four different polymers.The different influences for the different polymers at high and low temperatures as well as the curvature of the Arrhenius plots can be explained assuming a change in the hydroperoxide decomposition mechanism. The hydroperoxide decomposition is probably thermal at high temperature and Ti-catalysed at low temperature.
Lifetime prediction of polymeric materials often requires extrapolation of accelerated aging data with the suitability and confidence in such approaches being subject to ongoing discussions. This paper reviews the evidence of non-Arrhenius behaviour (curvature) instead of linear extrapolations in polymer degradation studies. Several studies have emphasized mechanistic variations in the degradation mechanism and demonstrated changes in activation energies but often data have not been fully quantified. To improve predictive capabilities a simple approach for dealing with curvature in Arrhenius plots is examined on a basis of two competing reactions. This allows for excellent fitting of experimental data as shown for some elastomers, does not require complex kinetic modelling, and individual activation energies are easily determined. Reviewing literature data for the thermal degradation of polypropylene a crossover temperature (temperature at which the two processes equally contribute) of ∼83 °C was determined, with the high temperature process having a considerably higher activation energy (107–156 kJ/mol) than the low temperature process (35–50 kJ/mol). Since low activation energy processes can dominate at low temperatures and longer extrapolations result in larger uncertainties in lifetime predictions, experiments focused on estimating Ea values at the lowest possible temperature instead of assuming straight line extrapolations will lead to more confident lifetime estimates.
We present a general approach for more confidently correlating accelerated aging results with aging under service conditions using the Arrhenius methodology. We first show that, as a result of complex diffusion-limited oxidation effects, time/temperature correlations may occur for some properties but not for others. To rigorously extrapolate high temperature results to low temperatures, we sought an ultrasensitive technique correlated to macroscopic degradation and capable of measurements at or near service temperatures. We achieved this objective by monitoring oxygen consumption rates at high (accelerated) temperatures, to establish the necessary correlation, and at low temperatures (down to 23 °C), to determine their temperature-dependence in the extrapolation region. Because easily measurable oxygen consumption rates of 10−13 mol/g s correspond to decades of predicted lifetime for most elastomers, this approach increases confidence in long-term predictions and therefore provides a means of testing Arrhenius extrapolations.
Accelerated aging of Nylon 6.6 fibers used in parachutes has been conducted by following the tensile strength loss under both thermal–oxidative and 100% relative humidity conditions. Thermal–oxidative studies (air circulating ovens) were performed for time periods of weeks to years at temperatures ranging from 37 °C to 138 °C. Accelerated aging humidity experiments (100% RH) were performed under both an argon atmosphere to examine the ‘pure’ hydrolysis pathway, and under an oxygen atmosphere (oxygen partial pressure close to that occurring in air) to mimic true aging conditions. As expected the results indicated that degradation caused by humidity is much more important than thermal–oxidative degradation. Surprisingly when both oxygen and humidity were present the rate of degradation was dramatically enhanced relative to humidity aging in the absence of oxygen. This significant and previously unknown phenomena underscores the importance of careful accelerated aging that truly mimics real world storage conditions.
Predicting life expectancy and simulating age of complex equipment using accelerated aging techniques. Sandia National Laboratories report
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Gillen KT, Mead KE. Predicting life expectancy and simulating age of complex equipment using accelerated aging techniques. Sandia National Laboratories report; January, 1980. SAND79e1561.
Predicting life expectancy and simulating age of complex equipment using accelerated aging techniques
- K T Gillen
- K E Mead
Gillen KT, Mead KE. Predicting life expectancy and simulating age of complex
equipment using accelerated aging techniques. Sandia National Laboratories
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Polymeric encyclopedia
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Boca Raton, FL: CRC Press; 1996.