Ageing mechanism and mechanical degradation behaviour of polychloroprene rubber in a marine environment: Comparison of accelerated ageing and long term exposure

Polymers are widely used in marine environments due to their excellent properties and good weathering resistance. Despite this extensive use, their long term behaviour in such an aggressive environment is still not well known. To assess the polymer durability within reasonable durations, it is essential to perform accelerated ageing tests to accelerate the degradation kinetics but without any modification of the degradation process. This study therefore proposes and validates accelerated ageing tests to study marine ageing of a silica-filled chloroprene rubber (CR) used for offshore applications. Several accelerated ageing protocols are investigated for temperatures ranging from 20 to 80 °C in renewed natural seawater. The ageing consequences are characterized using physical measurements (FTIR, solid state NMR) and mechanical testing based on monotonic tension tests. Instrumented micro-indentation tests are also employed, in order to describe accurately the ageing gradients through sample thickness. The measurements obtained on the samples cut from accelerated specimens are compared to those obtained from the topcoat of an offshore flowline aged under service conditions for 23 years. For both kinds of specimens, polychloroprene develops rapid material changes most clearly represented by a considerable increase in stiffness, which allows the accelerated ageing protocols to be validated.     

Description and modeling of polyurethane hydrolysis used as thermal insulation in oil offshore conditions

Polymers are widely used for passive thermal insulation coatings on steel pipe in offshore oil and gas production. In this industry, structures used in deep sea have to be reliable, as they are in service for more than 20 years in a very severe environment: sea water, hydrostatic pressure and temperature gradient. One of the main questions is how to test and predict the lifetime of such structures in the laboratory? This study presents one approach that has been developed to characterize and predict the degradation of polymers used as thermal insulation materials.This paper is dedicated to polyurethane (polyether based) degradation in sea water at high temperature. Ageing has been performed in natural sea water under hydrostatic pressure at temperatures ranging from 70 to 120 °C on 2 mm thick samples. Water diffusion in the material and hydrolysis have been characterized using mass evolution and tensile tests. Based on these results, a model for the urethane hydrolysis reaction is proposed.     

Accelerated artificial ageing of new dimer fatty acid-based polyamides

Replacing petroleum-based raw materials with renewable resources is now a major challenge in terms of economical and environmental viewpoints. Vegetable oils are expected to be an interesting alternative to produce a new generation of bio-based polymers. Due to the possible outdoor exposure in diverse applications, knowledge of the resistance to weather of these new materials is an important issue, not only for aesthetic aspects as in rapid yellowing, but also for changes in their properties. New innovative dimer acid-based bio-polyamides (DAPA) were exposed to an artificial ageing environment produced by a UV/condensation weathering device for different times. To follow and to evaluate this material ageing, several techniques were employed to correlate the chemical modifications, with the morphology and the mechanical properties. The formation of peroxide was found to be very rapid and accompanied by high gel fraction formation and chain breaking. Infrared spectroscopy showed a build-up of carbonyl absorption in the range 1700-1780 cm⁻¹, due to primary and secondary photo-oxidation products. In correlation with these morphological evolutions and in agreement with the high decrease of fusion enthalpy by differential scanning calorimetry, X-Ray diffraction showed an amorphous solid state after ageing. Strain-stress measurements revealed a change in DAPA behaviour, varying from a ductile to an elastomeric material, before and after ageing, respectively.     

Accelerated ageing to study the degradation of cellulose nitrate museum artefacts

Cellulose nitrate is susceptible to hydrolysis as well as loss of plasticiser when left in a humid atmosphere. A comparison of the ageing behaviour of cellulose nitrate samples prepared from cotton linters was used to simulate the artefacts studied in a previous study. Certain artefacts were also subjected to accelerated ageing at 12%, 55% and 75% relative humidity at 70 °C. The rate of degradation was observed to vary with the RH, indicating the connection between the absorption of moisture and the hydrolysis process. The effect of varying the sulphate level on the rate of hydrolysis was studied using concentrations similar to those detected in artefacts. The study was carried out using <0.1 mg g⁻¹, 2 mg g⁻¹ and 5 mg g⁻¹ of sulphate and distinct differences were observed which is consistent with the conclusions drawn for a study of a number of artefacts. The observed rates of degradation are consistent with previous studies on cellulose nitrate. The degradation was studied using a combination of infrared spectroscopy, ion chromatography and gel permeation chromatography. The analysis was complemented by a study of the weight changes which occur during ageing.     

Residual stability of polyurethane automotive coatings measured by chemiluminescence and equivalence of Xenotest and Solisi ageing tests

Degradation of poly(ester-urethanes) and poly(acrylic-urethanes), as a base for automotive paintings in interior applications, has been studied by chemiluminescence. The samples were clearcoat and black-pigmented paints, unstabilised and stabilized with HALS Tinuvin 292 and UV absorber Tinuvin 1130, exposed to various doses of artificial weathering in Xenotest and Solisi equipment. Chemiluminescence has appeared a powerful tool to evaluate the oxidation stability of various polyurethane systems. From the dependences of oxidation onset temperature on heating rate, the kinetic parameters describing the dependence of induction periods on temperature have been obtained. The kinetic parameters enabled us to calculate the length of the induction period for a chosen temperature, the protection factors of various additives and the residual stability of the polymer after an artificial ageing stress. It has been found that the loss of residual stability with ageing dose obeys a first-order relationship. Equivalence between the two methods of artificial ageing has been determined. The results indicate that the equivalence depends on the polymer composition. The procedure presented here can also be applied for the determination of equivalence of accelerated and field tests so contributing to establishing a reliable correlation between them.     

Accelerated oxidative ageing of polypropylene fibers in aqueous medium under high oxygen pressure as studied by thermal analysis

Stabilised polypropylene fibres of 28 μm diameter have been exposed at 80 °C under 5 MPa oxygen pressure, in dry conditions, in pure water and in alkaline medium. Their residual stability is determined from the temperature T_ox of the onset of the oxidation exotherm, measured by differential scanning calorimetry at 10 K min⁻¹ scanning rate. The time t_ex to reach the characteristics of unstabilised samples (T_ox=175 °C) is of the order of 220 (dry conditions), 160 h (water) and 80 h (alkaline medium), whereas it is of the order of 750 h at 110 °C in classical tests made at atmospheric pressure. This increase of ageing rate is discussed in terms of kinetic effect of O₂ pressure and efficiency of the liquid medium to extract stabilisers.     

Accelerated ageing and degradation in poly-l-lactide/hydroxyapatite nanocomposites

Dry, compression molded films of medical grade poly-l-lactide (PLLA) showed a marked reduction in tensile strength and strain after accelerated ageing in aqueous NaOH at 50 °C, accompanied by mass loss, surface erosion, increased hydrophilicity and, in the case of the initially amorphous films, cold crystallization owing to the plasticizing effect of the ageing medium. Addition of well dispersed nanosized hydroxyapatite (nHA) particles resulted in increases in the rate of mass loss during ageing, identified with accelerated degradation at the matrix/particle interfaces. However, the associated decreases in tensile strength and strain to fail with ageing time were far less marked in the presence of the nHA than in the unmodified films. This implied that nHA acts as an effective toughener of the bulk material, consistent with TEM observations of the deformed films, which indicated failure of the particle-matrix interfaces to promote plastic deformation of the PLLA.     

Time–temperature superposition method for predicting the permanence of paper by extrapolating accelerated ageing data to ambient conditions

In this paper we present a time-temperature superposition method for predicting the permanence of paper by extrapolating accelerated paper ageing data to ambient conditions. The presented method includes a test for the presence of shift factors to superpose all of the raw accelerated ageing data over the temperature range studied to obtain a master curve, a numerical fit of the master curve for producing a master equation representing the kinetics of paper degradation, a critical examination of applying Arrhenius equation for explaining the relationship between the empirically determined shift factors and the accelerated ageing temperature, and a verification of the Arrhenius activation energy extrapolation assumption. Unlike the usual approach that extrapolates the Arrhenius relationship between lifetime and temperature, without corroborating evidence, to ambient temperatures, we test the Arrhenius activation energy extrapolation assumption by determining the influence of acidity on cellulose hydrolysis reactions, and have found that detection and identification of the acid-sensitive linkages in cellulose substances is an ultra-sensitive and reliable method to measure degradation of cellulose and paper in what is normally the extrapolation region (ambient temperatures). Taking the examples of natural ageing data in literature from 18 bleached kraft dry-lap pulps for 22 years under ambient conditions and three handsheet samples for 22 years under controlled conditions, comparison of the predictions with natural ageing results has been addressed.     

Mechanical properties of solvolysis lignin-derived polyurethanes

In order to study the tensile properties of solvolysis lignin polyurethanes, polyurethane (PU) films were prepared from the solvolysis lignin (SL)–polyethylene glycol (PEG)–diphenylmethane diisocyanate (MDI) system, and in addition to this further PU films were also prepared from the SL–MDI system. In the SL–PEG-MDI system, the SL content, the molecular weight of PEG and the NCO:OH ratio were varied in order to control the physical properties. The tensile stress and Young's modulus of the PUs of the SL–PEG–MDI type increased with increasing SL content and NCO:OH ratio. The tensile properties of the PUs from the SL–MDI system showed no NCO:OH ratio dependency; i.e. the mechanical properties of PUs are not influenced by the change in crosslinking. It is possible to control the tensile properties of PUs of the SL–PEG–MDI type by changing the content of PEG and SL at a constant NCO:OH ratio.     

Prospecting exopolysaccharides produced by selected bacteria associated with marine organisms for biotechnological applications

In this study, bacteria associated with marine organisms were screened for the production of exopolysaccharides(EPSs) on MY media containing sea salts(2.5%-10%). Three selected isolates were identified as Alteromonas sp. PRIM-21, Nitratireductor sp. PRIM-24 and Enterobacter sp. PRIM-26 using 16 S r RNA gene sequencing. Optimization of the growth and EPS production kinetics in relation to incubation time were assessed. The purified EPS yield was 590, 650 and 540 mg·L-1 culture media respectively in Alteromonas sp. PRIM-21, Nitratireductor sp. PRIM-24 and Enterobacter sp. PRIM-26. Biochemical and FTIR analyses revealed the presence of biologically important functional groups in the EPS produced by all the three isolates. The EPS produced by Nitratireductor sp. PRIM-24 and Alteromonas sp. PRIM-21 showed 2.0% sulfate content. These bacterial EPS also showed antioxidant and emulsifying activities and the EPS produced by Enterobacter sp.PRIM-26 showed significantly higher antioxidant activities in terms of superoxide(IC50 0.33 mg·mL -1) and DPPH(IC50 0.44 mg·mL -1) radical scavenging. It also showed higher emulsifying activities against selected hydrophobic substrates with EI24 values above 60%. From the results of the study, it can be concluded that the isolated bacteria produce EPS that can be investigated in detail for biotechnological applications.     

Thermal properties of solvolysis lignin-derived polyurethanes

Thermal properties of polyurethane (PU) films prepared from solvolysis lignin (SL), polyethylene glycol (PEG) and diphenylmethane diisocyanate (MDI) were examined by differential scanning calorimetry and thermogravimetry. In the SL—PEG—MDI system, the SL content, the molecular weight (M_n) of PEG and the NCO/OH ratio were changed in order to control the thermal properties. Glass transition temperatures (T_g's) of the prepared PU's were dependent on the SL content, the M_n of PEG and the NCO/OH ratio. However, the T_g of PU was significantly influenced by the SL content: the increment of T_g was ca. 150 K when the SL content in PEG increased from 0 to ca. 50%. The decomposition of the PU's was markedly dependent on the content of SL. Other factors, such as the NCO/OH ratio and the M_n of PEG, are less dominant compared with the SL content. This fact suggests that the dissociation between the isocyanate groups and the phenolic OH groups in SL may be the major factor in the whole process of the thermal decomposition of the PU containing SL     

Characterization of seawater and weather aged polyurethane elastomer for bend stiffeners

A bend stiffener grade polyurethane (PU) elastomer was physically and mechanically characterized by attenuated total reflectance Fourier transform infrared spectroscopy, thermogravimetric analysis and tensile tests. The material was then exposed to artificial seawater and weather up to 12 months to evaluate its stability as bend stiffeners are exposed to this type of environment during offshore operation. The characterization of aged samples was performed comparing the ageing effects on the chemical structure, thermal stability and mechanical properties with those of the non-aged material. The mass variation of aged samples immersed in artificial seawater was also measured. A slight change in the chemical structure led to a color change from dark green to brown in the samples exposed to natural weathering for 12 months. Increases in thermal stability, stiffness and strength characteristics were also verified, which may be associated to additional crosslink formation. In contrast, a significant mechanical property drop was observed for the artificial seawater aged PU, being attributed to a plasticizer effect induced by the ageing fluid. The stress-strain curves were adjusted with the Mooney-Rivlin model allowing the crosslink density estimation. The weather aged PU presented higher crosslink density than seawater aged and non-aged samples.     

Toward developing accelerated stress tests for proton exchange membrane electrolyzers

Proton exchange membrane water electrolysis is technically the most suitable technology for the production of green hydrogen on a large scale. Although it is still more expensive than hydrogen produced from fossil sources, it has already been commercialized. Novel components with cost-effective materials and efficient manufacturing processes are being rapidly developed. However, these components must endure durability tests that can guarantee a lifetime of at least 50,000 operation hours. Consequently, there is an urgent need to develop accelerated stress test protocols based on a deep understanding of degradation mechanisms of stack components. Recent reports show that the main degradation mechanisms are associated to anode catalyst dissolution, membrane chemical decomposition, and formation of semiconducting oxides on the metal components. These mechanisms can be accelerated by stressors such as high current density, dynamic operation, and shutdown modes. On the basis of these reports and knowledge of the operational requirements for large-scale proton exchange membrane water electrolysis, we propose an accelerated stress test protocol for the fast evaluation of newly developed cost efficient and durable components.     

Modification of aqueous polyurethanes by forming latex interpenetrating polymer networks with polystyrene

A series of latex particles with interpenetrating polymer network structure have been synthesized from waterborne polyurethane (PU) and polystyrene (PS). The effect of PU/PS composition, cross-linking density in the PS domain as well as in PU have been studied in terms of dispersion size, transmission electron microscopy morphology, mechanical and dynamic mechanical properties in addition to swellability in water and toluene of the dispersion cast film. It was found that inverted core (PS)–shell (PU) morphology was well defined and that the domain size as well as the film properties were well controlled by the latex composition and cross-linking density of both phases. Received: 15 March 2000 Accepted: 21 February 2001     

Correlations between structure and accelerated artificial ageing of XLPE

The elucidation of the structural and morphological changes that occur during the crosslinking of polyethylene and the correlation of these changes with the physical, chemical and ageing resistance is very important to forecast the properties and the performance of the final material.The accelerated ageing of XLPE samples with various degrees of crosslinking has been carried out using two types of artificial weathering equipments, with UV or Xenon light bulbs with different cycles of temperature and humidity, varying time of exposition for both systems. The changes in the properties of the materials have been analyzed by Tensile Tests, Scanning Electron Microscopy (SEM), Dynamic-Mechanical Analysis (DMA), X-Ray Diffraction (XRD). The main degradation effects for each level of crosslinking is presented for the pristine XLPE.     

Studies on the effect of extraction of isophorone diisocyanate-based segmented polyurethanes

Linear segmented polyurethane was synthesised, using isophorone diisocyanate (IPDI), poly(tetramethylene oxide) glycol (PTMG) and 1,4 butanediol, for use in biomedical applications. The chemical stability of this polyurethane in hot methanol during Soxhlet extraction was studied by viscosity measurements, thermal studies, mechanical tests, ultraviolet and infrared spectral studies, and gel permeation chromatography. Hot methanol degrades the polymer at the allophanate linkages, while extracting low molecular weight polyurethane fractions. More urea linkages are formed in the extracted polymer.     

Accelerated wear testing of polyurethane hydraulic seal

The wear characteristics of a polyurethane (PU) hydraulic seal were investigated using a hydraulic seal tester and a pin-on-plate reciprocating tribo-tester, and the results were compared to field data with the aim of developing an accelerated wear test method for hydraulic seals. Tests using a hydraulic seal tester and a pin-on-plate reciprocating tribo-tester were found to reproduce abrasive wear of PU from the field. However, a significant compression set was observed from the test using the hydraulic seal tester. Motivated by the occurrence of abrasive wear from the field, the discolored lubricant and the lubricant with alumina particles were further used for testing using the pin-on-plate reciprocating tribo-tester. The height decrease data of the sealing surface showed that the wear was accelerated by factors of 2.1–3.4 using these degraded lubricants. The outcomes of this work are expected to aid in the design of reliable accelerated life testing for hydraulic seals.     

Accelerated aging and aging mechanism of acrylic sealant

One-component acrylic latex sealants with different moduli were subjected to accelerated aging tests, heat aging at different temperatures, xenon arc and UV exposure. The effects of various aging factors on sealant properties were studied by reviewing the appearance and mechanical properties. The aging mechanism was evaluated by thermal analysis, FTIR and solvent swell. The results show that a good relation exists among mechanical properties after aging; mild accelerated aging conditions and short aging times are enough to lead to obvious degradation of acrylic sealants with low modulus. Fillers with ingredients within high modulus may resist UV radiation effectively and enhance their durability but the quantity must be proper. The degree of chain breaking caused by UV is larger than that by heat aging. The study also shows that there are the same or similar aging mechanisms for different aging conditions and that the cure reaction by water volatilisation dominates the initial period of aging while degradation will be induced when aging time is longer and it can result in changes of molecular structure.     

Accelerated weathering of polyaramid and polybenzimidazole firefighter protective clothing fabrics

Exposure to simulated ultraviolet sunlight at 50 °C and 50% relative humidity caused a significant deterioration in the mechanical performance of polyaramid and polyaramid/polybenzimidazole based outer shell fabrics used in firefighter jacket and pants. After 13 days of exposure to these conditions the tear resistance and tensile strength of both fabrics decreased by more than 40%. The polybenzimidazole containing fabric was less impacted by these conditions as it maintained approximately 20% more of its mechanical properties. These conditions also significantly degraded a water repellant coating on the fabric, which is critical to the water absorption performance of the outer shell fabrics. However, these conditions had little impact on the ultraviolet light protection of the outer shell as both fabrics still blocked 94% of ultraviolet light after 13 days of exposure. Confocal microscopy showed these conditions caused significant surface decomposition of and the switch from ductile to brittle failure of the polyaramid fibers. Cleavage of the amide linkages and the formation of oxidation species (as observed by Infrared spectroscopy) suggested these conditions caused photo-oxidation of the polyaramid fibers. There was little evidence of polybenzimidazole fiber degradation.     

Critical considerations for the accelerated ageing of high-density polyethylene potable water materials

Accelerated ageing conditions with chlorinated water were identified that minimize variations in solution chemistry and water sorption, and that also enable the interpretation of HDPE physical and chemical changes during 20 week (3884 h) immersion periods. Of the ten conditions tested, three conditions with an alkalinity concentration of 50 ppm as CaCO₃ at pH 6.5 and at 23 and 37 °C performed best. These three conditions exhibited stable pH, free available chlorine, and alkalinity concentration during 20 weeks of HDPE immersion with 72 h changes of ageing solution. HDPE was periodically characterized using differential scanning calorimetry, thermogravimetric analysis, tensile analysis, a density gradient column, moisture analysis, and optical and infrared spectroscopy. Formation of surface carbonyl bonds and gradual reductions in oxidation induction time were detected, as well as water sorption into HDPE. Ageing solution and water sorption monitoring recommendations from this work should be incorporated into accelerated ageing protocols and considered when characterizing aged PE drinking water pipe.