Effect of thermal aging on the electrical properties of crosslinked polyethylene

This article reports on the effect of thermal aging on the electrical properties of crosslinked polyethylene (XLPE) used in high voltage cables. It was shown that thermal aging modifies the electrical properties of the polymer. The degradation is accelerated when the temperature is increased. This degradation is due to a thermooxidation, followed by a loss of antioxidant, and a change in color of the material. At 80 and 100°C, the antioxidant is practically efficacious during the aging, whereas at 120 and 140°C, it is consumed after 1500 and 1000 h, respectively, which results in an increase in the dielectric losses and a diminution in the volume resistivity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Accelerated aging versus realistic aging in aerospace composite materials. II. Chemistry of thermal aging in a structural composite

Samples of an aerospace structural epoxy composite (8552/IM7) were subject to long‐term (≈ 1 year) thermal aging at temperatures of 70°, 120°, 170°, and 200°C (in air). The changes to the chemical and physicochemical structure of the composite were analyzed by a range of different techniques, including gravimetric analysis, Fourier transform infrared (FTIR), and dynamic mechanical analysis (DMA) to compare the effects of different severities of degradation treatment. The results highlighted the large differences in chemical effects between the surface and the interior of the composite with very minor changes in the latter even at quite high aging temperatures and long aging times. The oxidative changes at the surface, however, varied from highly selective molecular changes for particular chemical groups at the lower aging temperatures (70° and 120°C), to quite general and extensive oxidative degradation at the higher aging temperatures (170° and 200°C). The results indicated that the mechanical changes in an aged composite of this type will vary greatly with the material thickness and surface protection as well as the aging temperature the composite is exposed to. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3221–3232, 2006     

聚丙烯热氧老化机理的研究

利用熔体流动速率法和红外光谱法研究了纯聚丙烯（PP）粉料的热氧老化机理。结果表明,纯PP粉料的热氧老化降解机理服从指数函数,且函数的拟合度高,说明在热氧老化降解前期,需要聚集一定的能量,这个过程中PP只发生微弱降解,出现“诱导期阶段”,而当能量积聚到一定程度后,PP开始大幅度降解,这时会出现热氧老化降解的“加速阶段”。     

Effects of the additive package on the thermal properties of a commercial polyoxymethylene homopolymer

The present study worked out the effects of the additive package on both the degradation behavior and the thermal properties of a commercial polyoxymethylene (POM) homopolymer (Delrin® 900P) during an artificial thermooxidative aging. The POM pellets were investigated as received and after a storage duration of 5 weeks at 140°C in an oven. Additionally, the pellets for the initial and aged conditions were purified to remove the additives. The degradation behavior was determined by means of thermogravimetric analysis (TGA), while the thermal properties were studied with differential scanning calorimetry (DSC). Further, optical microscopy and Fourier transform infrared spectroscopy were used to follow changes occurring during the artificial aging. Dynamic TGA in nitrogen and air showed that even after a severe aging procedure, some thermooxidative stabilizer was still present. However, its effectiveness was no longer good, as the aged POM showed a lower degradation temperature than the initial material. This result is evidence that thermally sensitive and more volatile compounds were formed. Purification of the aged pellets led to a material that degraded immediately during melting. Isothermal DSC experiments carried out at several temperatures showed that the elimination of the additives increased the crystallization rate. The crystallization kinetics could be described by using the Avrami model. J. VINYL ADDIT. TECHNOL, 2008. © 2008 Society of Plastics Engineers     

Accelerated aging versus realistic aging in aerospace composite materials. I. The chemistry of thermal aging in a low‐temperature‐cure epoxy composite

Samples of an aerospace‐grade epoxy composite (M20/IM7) are subject to long‐term (～ 1 year) thermal aging at temperatures of 70°C, 120°C, 170°C, and 200°C (in air) and the changes to the chemical and physicochemical structure of the composite are analyzed by a range of different techniques, including gravimetric analysis, FTIR, DSC, and DMA to compare the effects of different severities of degradation treatment. The results show that at the lower temperatures, the oxidative degradation changes are very selective for chemical defect groups, particularly near the sample surfaces. However, at the higher temperatures, combinations of further cure reactions and generalized oxidative degradation changes (again from the surface inwards) make for a highly complex ageing pattern for this particular composite material. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4291–4303, 2006     

Effect of thermal-oxidative aging on mechanical,chemical, and thermal properties of recycled polyamide 66

This article addresses the effect of thermal aging on unreinforced and glass-reinforced recycled polyamide 66. As an accelerated test, injection-molded test bars were aged at 110, 140, and 170°C for up to 4000 h in air to simulate service life. FTIR spectroscopy demonstrated that the oxidative degradation primarily occurred between the surface and a depth of 0.5 mm. Furthermore, the degradation in the surface region was more pronounced with recycled as well as unstabilized materials. Reprocessing resulted in a faster increase of carbonyl groups, a decrease in melting peak temperature, and elongation at break during subsequent aging. Because of process-induced fiber shortening, however, the elongation at break of recycled reinforced samples was always at least as high as that of virgin samples for up to 4000 h of aging at 140°C. The decrease in melting peak temperature as determined by differential scanning calorimetry (DSC) indicated that the surface or boundary regions of the crystallites in the material are affected by aging. The loss in elongation at break for the reinforced material was shown to correlate with the reduction in melting peak temperature of material taken from the surface region of aged samples. The contribution of the degraded surface region to the properties was studied by removal of surface layers prior to testing. The degradation in the surface region was the sole cause, even of glass fiber-reinforced polyamide, for the embrittlement of aged samples. Furthermore, aging-induced changes in tensile strength and modulus were independent of the removal of the surface region, indicating that these properties are controlled by changes occurring in the bulk of the material. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1619–1630, 1997     

Polymer degradation initiated via infectious behavior

Understanding polymer degradation phenomena is of critical importance for material reliability. A novel dual stage chemiluminescence detection system has been developed and applied to probe for material interaction effects during polymer degradation. Evidence is presented for the first time that in an oxidizing environment a degrading polymer A (in this case polypropylene, PP) is capable of infecting a different polymer B (in this case polybutadiene, HTPB) over a relatively large distance. In the presence of the degrading material A, the thermal degradation of polymer B is observed over a significantly shorter time period. Infectious intermediate volatiles from material A are able to initiate and shorten the degradation processes in material B. This observation documents infectious behavior that will have significant consequences for materials interactions, understanding material degradation processes and long-term aging effects in combined material exposures.     

Aging behavior of a hot‐cured epoxy system

The thermal and hydro‐thermal aging of a hot‐cured epoxy system (diglycidylether of bisphenol A (DGEBA) + dicyandiamide (DDA)) in the glassy state is revisited using DSC and IR attenuated total reflection spectroscopy. Because of the diffusion of DDA from the solid particles into the liquid DGEBA matrix, curing produces a highly crosslinked amorphous matrix that contains low crosslinked amorphous regions. After full curing, the network possesses a relatively low molecular mobility and no residual reactive groups. Thermal and hydro‐thermal loading is performed at 60°C, well below the principal glass transition temperature (T_g1 = 171°C). Both aging regimes cause significant chemical and structural changes to the glassy epoxy. It undergoes a phase separation of relatively mobile segments inside the low mobile matrix, providing a second glass transition that shifts from T_g2 = 86–114°C within 108 days of aging. This phase separation is reversible on heating into the viscoelastic state. Hydro‐thermal aging leads to a reversible and a nonreversible plasticizing effect as well. On thermal aging, no chemical changes are observed but hydro‐thermal aging causes significant chemical modifications in the epoxy system. These modifications are identified as a partial degradation of crosslinks produced by the cyano groups of the DDA and correspond to the nonreversible plasticitation. These changes in the cured epoxy should exert an influence on the mechanical properties of an adhesive bond. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Electrical insulation properties of silicone rubber under accelerated corona and thermal aging

A contactless method based on energy shift of high-energy cut-off of the x-ray bremsstrahlung, the so-called Duane Hunt Limit and a conventional low voltage electrical technique (three-probes technique) is applied on thermal and corona aged silicone rubber (SiR) to measure, respectively, the surface potential, V_s, and the surface resistivity, ρ_s. The effect of aging on these quantities, representing the dielectric properties, is studied. The results are highly reproducible and highlight a good correlation between V_s and ρ_s. It was observed that thermal aging combined with electrical aging deteriorates more the electrical properties of the polymer than thermal aging alone. Explanations for electrical characteristics (V_s, ρ_s) change with aging are supported by attenuated total reflection Fourier transform infrared spectroscopy spectra analysis and a chemical mechanism of aging in three steps (i.e., oxidation-polycondensation, degradation, and thermal cracking). The surface degradation of the polymer is revealed by images of surface morphology obtained by using scanning electron microscopy (SEM). Roughness is greater for combined thermal and corona aging mode compared to thermal aging alone. In addition, the surface degradation of SiR polymer is confirmed by the loss of its hydrophobicity.     

Effect of Thermal Aging on the Optical Properties of ABS Plastics

Poly(acrylonitrile/butadiene/styrene) (ABS) is a two-phase material consisting of elastomer particles in a glassy polymer matric of styrene and acrylonitrile (SAN) [1]. The photooxidation of ABS was the subject of several studies [2, 3]. It was suggested that several processes will take place during photooxidation. These changes include the formation of hydroperoxide [4], chain breakage of the polystyrene, and the oxidation of the polymer as it was monitored by IR spectroscopy [4–6]. Also photooxidation affect the polybutadiene in ABS and oxidizes it, which results in the formation of hydroperoxide. There are no data available on the thermal degradation of ABS. In a previous study the thermal aging of recycled high-impact polystyrene was studied using UV-vis spectroscopy [7]. It was found that this method provided very useful information about the degradation of several industrial polymers [8–12]. In this paper thermal degradation of ABS is investigated by UV-vis and IR spectroscopy.     

Environmental effects on poly‐p‐phenylenebenzobisoxazole fibers. I. Mechanisms of degradation

This is the first of a two‐part series investigating the degradation mechanisms of PBO fiber and approaches to alleviating degradation and improving fiber properties. Poly‐p‐phenylenebenzobisoxazole (PBO) fiber is a high strength and modulus fiber with remarkable thermal stability. Recent in‐service failures of this fiber have revealed that the fiber degrades rapidly in relatively mild environmental conditions of moisture and heat. In this work the mechanisms of degradation due to moisture, the presence of acid, and the effect of radiation from the UV–vis spectrum are investigated. It is found that exposure to moisture results in the loosening of the fiber morphology leading to an increase in the number and size of defects. The presence of aqueous acid causes both loosening of the fiber structure and hydrolysis of the oxazole ring structure. The effect of UV–vis radiation is primarily hydrolysis of the material near the fiber surface with attendant formation of amide linkages. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3517–3525, 2006     

Oxidation and crosslinking processes during thermal aging of low‐density polyethylene films

This work analyzes the influence of thermal degradation on the microstructure and the mechanical properties of low‐density polyethylene subjected to aging at 70°C in the dark for times up to 21 months. It is found that the polymer shows a gradual increase of its elastic modulus and a dramatic reduction of its ductility, due to secondary crystallization. Infrared spectroscopy (FTIR) reveals the autoaccelerated oxidation of the polymer after 5 months aging. It is observed that the unsaturated vinylidene groups initially present in the material are gradually overridden by vinyl groups and, eventually, by t‐vinylene groups. Nuclear magnetic resonance (¹³C NMR) shows that the initial butyl chain branches are progressively completed by shorter ramifications, namely ethyl branches. These results are discussed in term of macromolecular mechanisms: (i) oxidation, (ii) chain scission, and (iii) crosslinking. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of thermal degradation on the mechanical properties of a diglycidyl ether of bisphenol A/1,3-bisaminomethylcyclohexane (DGEBA/1,3-BAC) epoxy resin system

The effect of thermal degradation on the mechanical properties of a diglycidyl ether of bisphenol A (DGEBA)/1,3-bisaminomethylcyclohexane (1,3-BAC) epoxy system, cured with two different curing cycles—a short cycle and a long cycle—were studied using tensile and Izod impact experiments and scanning electronic microscopy, SEM, observations. From these experiments it can be noted a loss of mechanical properties of the material cured with both cycles with aging time, although the material cured with the long cycle presents better properties at any aging time. This better behavior can be explained from the time temperature transformation, TTT, diagram of this system. A good correlation was observed between the decrease in the intensity of the peak of β transition in tan δ curve obtained by dynamic mechanical analysis, DMA, and the decrease of the Izod impact strength when thermal aging is increasing. Also, a good correlation can be found between the increase in the fragility of the material with aging time and the morphology of fractured surfaces observed by SEM. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1841–1849, 1997     

Degradation assessment of LDPE multilayer films used as a greenhouse cover: Natural and artificial aging impacts

This article focuses on the assessment and understanding of the mechanism of natural and artificial aging processes of a triple‐layer film made of low‐density polyethylene (LDPE) used as greenhouse cover. The film material contains color dye and ultraviolet–A (UV–A) and infrared (IR) stabilizers and antioxidant. The combined effect of temperature variations and UV–A radiations, of the natural and artificial aging, on the physical properties (free surface energy and yellow color measurements), mechanical behavior (tensile tests), thermal stability (TGA and DSC analysis), and structural stability (FTIR analysis) was investigated. The natural aging was conducted on a greenhouse, located in northern Algeria, over a period of 7 months. However, the artificial aging was performed at four different agricultural greenhouse simulating conditions of temperature and UV–A radiation (namely, at 40°C, 40°C with UV–A, 50°C, and 50°C with UV–A) for periods of aging up to 5486 h (7.6 months). The results revealed that, the maximum loss of the yellow color additives occurs at 2981 h under the natural aging process and at 2440, 1096, 1340, and 121 h under the four artificial aging conditions, respectively. There was an observed increase in the films free surface energy and a significant degradation in the mechanical properties with aging time. This can be correlated with the film material structural changes. The natural aging of the film in North Africa is almost equivalent to artificial aging at 40°C. The concurrent effect of temperature and UV–A radiations induced polymer chains scission leading to faster degradation in the film material and consequently a reduction in its durability and service lifetime. The results show also that the measured parameters are directly related to the limit of use criterion for evaluating the lifespan of agricultural greenhouse LDPE covers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Application of computed X-ray tomography scanning in the study of thermo-oxidative degradation of thick-walled filled natural rubber vulcanizates

The aging of five thick-walled natural rubber compounds has been studied by computed X-ray tomography scanning and crosslink density measurements. The compounds were compouded as ordinary carbon-black-filled rubbers with sulfur and peroxide as curing agents. The rubber samples were aged in air at 70, 100, and 150°C for 1000h. The relatively new technique of computed X-ray tomography scanning proved to be a good method for studying the aging procedure, and especially for following the crack propagation in the surface. Antioxidants (TMQ and 6PPD) had a low effect on the resistance toward oxidative degradation and crosslinking under these conditions. Surprisingly, the efficient sulfur-vul-canized material had a poor resistance toward thermal degradation. When the crosslink density and the computed X-ray tomography scanning results were compared, it was assumed that the computed X-ray tomograph detected oxygen in the surface, both as elementary oxygen and as oxygen in degradation products, i.e., in carbonyls. The results agree well with the theory that oxidative aging is limited by the ability of the oxygen to diffuse into the material.     

Investigation of long-term thermal aging-induced damage in oxide/oxide ceramic matrix composites

Long-term thermal aging is a typical factor affecting the thermo-mechanical fatigue life for hot-end components in the gas turbine. The present work focuses on the development of thermal aging-induced damage in 2-D woven oxide/oxide ceramic matrix composites from micro-mechanism and macroscopic mechanical performance. The porosity evolution and mechanical performance after long-term thermal aging were characterized through mercury intrusion measurements and uniaxial compressive tests, respectively. The results show that the decrease of micro-porosity directly reflects the irreversible evolution of material microstructure in the thermal aging process, and the decrease of compressive strength after aging is the macroscopic reflection of the microstructure variation. The porosity increment of matrix was thus used to characterize the thermal aging-induced damage, establishing a unique analysis model between the increment of micro-porosity under thermal aging and the corresponding degradation of material compressive strength. The experimental results are in good agreement with the established model.     

Study of epoxy and epoxy–cyanate networks thermal degradation to predict materials lifetime in use conditions

Composite materials are used more and more for aeronautical applications and they have to be as thermally stable as possible. The thermostability of carbon‐fiber/epoxy–cyanate composites elaborated with an autoadhesive and autoextinguish prepreg were tested. Dynamical and isothermal aging tests were carried out to evaluate the composite thermal stability. Thermal degradation products were identified by chromatography/mass spectrometry analysis and the results obtained were compared with data known on the material network structure. The physicochemical network structure evolutions and the thermal aging data are correlated with the interlaminar shear strength (ILSS) mechanical results. For epoxy–cyanate composites, cracking appears after a longer time of aging than for epoxy composites. This new epoxy–cyanate material isothermal stability seems to be good and particularly good if it was postcured after processing. The comparison of chemical, mechanical, and crack formation results obtained by accelerated aging tests allowed us to determine models to predict long‐term behavior. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3142–3153, 2000     

Thermal stability of radiation‐grafted and dyed poly(ethylene terephthalate) fabric

The effect of degree of grafting (GY) and degree of dye uptake on the thermal stability of polyethylene terephthalate (PET) fabric was studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X‐ray diffraction analysis (XDA). TGA showed that the degradation process was composed of three overlapping stages. The first and second stages were studied in detail. Methacrylic acid (MAA)‐grafted PET fabric was dyed using Rhodamine Red (RR) and Astrazonrot Violet (AV) basic dyes. It was found that grafting deteriorated the thermal stability of both stages. The first stage showed the formation of two new steps at low and high temperatures. Both steps are heating rate and graft yield dependent. The deteriorating effect of grafting was followed by the changes in the kinetic parameters. AV dyeing of grafted samples accelerated the degradation of both stages whereas RR dyeing improved the thermal stability to reach that of ungrafted fabric. XDA showed that the crystalline nature of AV dye stuff is responsible for the rapid degradation in both stages via the introduction of highly incompatible crystalline phase in the polymer back bone, which resulted in the formation of internal stresses that enhanced the degradation process. DSC measurements supported TGA results. The improvement in the thermal stability by RR dyeing is attributed to the amorphous nature and the high thermal stability of the RR dye stuff. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101:1007–1020, 2006     

天然橡胶硫化胶的热氧老化性研究

用热分析法(TGA、DSC)和裂解气相色谱-质谱分析(PGC-MS)研究了天然橡胶硫化胶的耐老化性能及其机理。结果表明,天然橡胶硫化胶的热氧老化初期力学性能和交联密度增加,热失重起始温度上升,随着老化过程的进行,降解效应占优势,硬度和交联密度下降,热氧稳定性变差。其热解机理可能是主链中键的断裂,产物主要是异戊二烯单体和1,4-二甲基-4-乙烯基环己烯。     

In vitro degradation of dry‐jet‐wet spun poly(lactic acid) monofilament and knitted scaffold

In vitro degradation behavior of dry‐jet‐wet spun poly(lactic acid) (PLA) monofilament and knitted scaffold were studied at three different pH i.e., at 4.6, 7.4, and 8.0 at 37°C for 20 weeks. Characterization of PLA by intrinsic viscosity, thermal properties, and scanning electron microscopy (SEM) was carried out. It is observed that the pH of the medium has significant role on degradation behavior of PLA. The degradation at pH 4.6 is observed to be maximum, which is confirmed by the drop of 52% in intrinsic viscosity. The degradation process has effect on the hydrophobicity of the PLA. The decrease in contact angle from 73° to 48° indicates that the PLA surface tends to become more hydrophilic as the degradation proceeds. The SEM analysis showed that with the degradation, surface deterioration takes place. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2006–2012, 2007