Critical stress of high‐density polyethylene during stress and photo‐oxidative aging

The long‐term photo‐oxidative aging behavior of high‐density polyethylene (HDPE) under different tensile stress was studied using a stress‐aging apparatus. The aging behavior was investigated through the methods of the surface morphology observation, gel content measurement, Fourier transform infrared spectroscopy, and creep behavior. It was found that stress has influence on the development of cracks and stress induces cracking through creep deformation. With increasing stress, the cracking time decreases in a reversed S‐shape curve way, and there is a critical stress near 7 MPa where the cracking time has a maximum decreasing rate. Meanwhile, the creep deformation increases rapidly when the stress exceeds the critical stress. The critical stress of HDPE is about 20–25% of breaking strength, and HDPE with low comonomer content has good dimensional stability when the stress is less than the critical stress, while HDPE with high comonomer content has a good performance when the stress exceeds the critical stress. This study may be useful for the rational selection of HDPE for the sheath material of bridge cable. POLYM. ENG. SCI., 55:2277–2284, 2015. © 2015 Society of Plastics Engineers     

Dynamic mechanical properties of extruded nylon–wood composites

Dynamic mechanical properties determine the potential end use of a newly developed extruded nylon–wood composite in under‐the‐hood automobile applications. In this article, the dynamic mechanical properties of extruded nylon–wood composites were characterized using a dynamic mechanical thermal analyzer (DMTA) to determine storage modulus, glass transition temperature (T_g), physical aging effects, long‐term performance prediction, and comparisons to similar products. The storage modulus of the nylon–wood composite was found to be more temperature stable than pure nylon 66. The T_g range of the nylon–wood composite was found to be between 23 and 56°C, based on the decrease in storage modulus. A master curve was constructed based on the creep curves at various temperatures from 30 to 80°C. The results show that the relationship between shift factors and temperature follows Arrhenius behavior. Nylon–wood composites have good temperature‐dependent properties. Wood fillers reduced the physical aging effects on nylon in the wood composites. The comparison of the nylon–wood composite with other similar products shows that nylon–wood composites are a promising low cost material for industrial applications. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Investigation on morphology and properties of melt compounded polyoxymethylene/carbon nanotube composites

Polyoxymethylene nanocomposites containing different contents of carbon nanotubes were produced by a two‐step melt compounding process using a twin‐screw extruder. The dispersion quality, thermal and mechanical properties, and the creep as well as the tribological behaviors of the nanocomposites were investigated. Morphological investigations show that the masterbatch dilution process significantly improves the dispersion quality of carbon nanotubes within polyoxymethylene matrix, and as a consequence, enhanced mechanical properties and creep resistance are gained. Furthermore, to predict the long‐term property based on the short‐term experimental data, the time–temperature superposition principle and Findley model were used. Master curves with extended time scale are constructed using time–temperature superposition principle to horizontally shift the short‐term experimental data. The simulated results confirm the reinforced creep resistance by incorporation of the carbon nanotubes into the polymer matrix even at extended long time scale. By contrast, the tribological performance of polyoxymethylene was remarkably impaired after adding carbon nanotubes. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42639.     

Performance of natural‐fiber–plastic composites under stress for outdoor applications: Effect of moisture,temperature, and ultraviolet light exposure

The effects of moisture, temperature, and ultraviolet (UV) light on performance of natural‐fiber–plastic composites (NFPC) were assessed. We conducted short‐term tests in the laboratory and long‐term tests under natural exposure and measured changes in mechanical properties and color in samples of the composite. Chemical changes of the composite's materials were measured by X‐ray photoelectron spectroscopy to elucidate the mechanisms of chemical transformations on the material surface. Relative humidity highly affected the modulus of rupture (MOR) and the modulus of elasticity (MOE), and had a greater effect than temperature and UV exposure on performance of the composite. The lightness of the composite was increased by the UV effect in the short‐ and the long‐term tests. The X‐ray photoelectron spectroscopy (XPS) analysis suggested that the composite was protected by the UV absorber. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 2570–2577, 2006     

Plasticized and unplasticized PLA/organoclay nanocomposites: Short‐ and long‐term thermal properties,morphology, and nonisothermal crystallization behavior

The short‐ and long‐term thermal properties, organoclay dispersion state, and the nonisothermal crystallization kinetics of organoclay based nanocomposites of poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) plasticized PLA were investigated. Differential scanning calorimetry analyses showed that plasticization of PLA/PEG blend was diminished due to physical aging by the time. The change in thermal properties such as glass transition temperature, cold crystallization temperature, and melting temperature was monitored. It was revealed from X‐ray diffraction analyses that in long term, the exfoliated and/or intercalated organoclay structure of nanocomposites observed in short term (just after processing) was differentiated to a tactoidal form (i.e., nonseparated clays). The nonisothermal crystallization behavior and kinetics were examined by using Avrami, Ozawa, and combined Avrami–Ozawa models. Moreover, the nucleating effect of clays was investigated in terms of Gutzow and Dobrewa approaches. It was found out that clays did not act as nucleating agents in plasticized PLA nanocomposites, which was also in good agreement with activation energy values obtained from Kissinger and Takhor models. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Thermal aging of bisphenol-A polycarbonate/acrylonitrile–butadiene–styrene blends

Thermal aging of immiscible bisphenol-A polycarbonate/acrylonitrile–butadiene–styrene (PC/ABS) blends containing 25, 60, and 75% PC and the PC and ABS blend components have been studied. Changes in Izod impact properties and dynamic mechanical spectra are reported following aging at 90, 110, and 130°C for times up to 1500 h. PC/ABS blends containing 60 and 75% PC were found to retain high impact performance following aging at elevated temperatures, compared to the PC blend component. Dynamic mechanical spectroscopy is an effective probe for investigating the structure–property changes occurring and the mechanisms of aging. For PC and ABS, the changes were mainly due to physical aging of the amorphous polymers when aged below the glass-transition temperature. For the PC/ABS blends, oxidative degradation additionally contributes to loss of toughness. Although structure–property changes are related to the behavior of the blend components, additional factors of potential importance for multiphase polymer–polymer systems have been identified, including a redistribution of stabilizers during the blend manufacture. © 1995 John Wiley & Sons, Inc.     

Physical aging and thermal aging resistance of polycarbonate/poly(styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene) blends

The effect of thermal annealing on the mechanical properties of polycarbonate (PC) was investigated by tensile testing, Izod impact testing, and differential scanning calorimetry (DSC). An endothermic peak appeared in the DSC curve by annealing at various temperatures below T_g. The peak area, as a measure of the degree of physical aging, increased with annealing time. The Izod impact strength decreased suddenly just before an endothermic peak became detectable. The results imply that at the very early stage of physical aging, a trigger for the ductile‐to‐brittle transition may be initiated. By blending hydrogenated styrene‐butadiene‐styrene block copolymer (SEBS), the embrittlement by annealing was prevented. That is, in the PC/SEBS blends, the physical aging of PC matrix proceeded in the same way as in neat PC; however, the Izod impact strength did not decrease. This thermal aging resistance seems to originate from the negative pressure effect of SEBS particles that provides dilational stress fields for PC matrix to enhance the local segment motions. POLYM. ENG. SCI., 52:1958–1963, 2012. © 2012 Society of Plastics Engineers     

PETN Coarsening – Predictions from Accelerated Aging Data

Ensuring good ignition properties over long periods of time necessitates maintaining a good level of flow porosity in powders of initiator materials and preventing particle coarsening. To simulate flow porosity changes of such powder materials over long periods of time a common strategy is to perform accelerated aging experiments over shorter time spans at elevated temperatures. In this paper, we examine historical accelerated‐aging data on powders of pentaerythritol tetranitrate, an important energetic material, and make predictions for long‐term aging under ambient conditions. We develop an evaporation–condensation‐based model to provide some mechanistic understanding of the coarsening process.     

Observations of physical aging in a polycarbonate and acrylonitrile–butadiene–styrene blend

The effects of physical aging of a 75 : 25 PC/ABS blend have been studied using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). From DSC, two distinct peak endotherms at about 90°C and 110°C, which are associated with the glass transition of ABS (T_g,ABS) and PC (T_g,PC) components, respectively, were observed. When progressive aging was monitored at 80°C for over 1000 h, the changes in enthalpic relaxation, glass and fictive temperatures for the blend followed similar trends to those already seen in the literature for PC aged between 125 and 130°C. The rate of enthalpy relaxation was also comparable. The plot of peak endotherm against logarithmic aging time for the PC blend constituent, however, behaved quite differently from the linear relationship known for highly aged PC. The ABS peak component also appeared to be insensitive to aging. Both observations were confirmed to be statistically significant using analysis of variance methods. Using temperature modulated‐DSC, there is evidence that aging increases the blend miscibility as the T_g,PC shifts toward the stationary T_g,ABS during aging. Parallel FTIR investigations found oxidation of butadiene during aging to be even at this relatively low temperature, forming hydroxyl and carbonyl degradation products. The presence of ABS in the blend also appeared to have prevented the shifting from the trans‐cis to trans‐trans arrangement of the carbonate linkage, which is a well‐known phenomenon during elevated temperature aging of PC alone. Moreover, the carbonate linkage appears to have been at the lower energy, trans‐trans, arrangement prior to the aging process. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Linear Hygrothermal Viscoelastic Characterization of Nafion NRE 211 Proton Exchange Membrane

The tensile relaxation modulus of a commercially available proton exchange membrane, Nafion® NRE 211, was obtained over a range of humidity levels and temperatures using a commercial dynamic mechanical analyzer (DMA). Hygral stress relaxation master curves were first constructed, followed by a hygrothermal master curve using the time temperature moisture superposition principle. The hygrothermal master curve was fitted using a 10‐term Prony series and validated using longer term stress relaxation tests. To validate the results from the stress relaxation experiments, short and long‐term creep compliance was converted into stress relaxation modulus using a well‐known viscoelastic conversion formula, and compared with the relaxation modulus obtained under identical conditions. Good agreement was found between the two datasets. It was evident that relaxation data at 2% RH at the test temperatures was not superposable with the master curves obtained at higher relative humidity (10% < RH < 90%) at the temperature range 70 °C < T < 90 °C. It was observed that the longer term relaxation modulus under humid conditions matched well with the hygrothermal master curve; however, the longer term relaxation modulus under dry conditions was significantly higher than the relaxation master curve obtained under dry conditions, raising the possibility of a physical aging process in the ionomer and/or irreversible morphological changes in the membrane under dry conditions.     

Closed‐loop recycling of postused PP‐filled garden chairs using the restabilization technique. Part 2: Material performance during accelerated heat aging

The long‐term thermal stability of postused CaCO₃‐filled Polypropylene was evaluated through accelerated heat aging tests for about 2000 h. The material from old garden chairs was recycled by applying the remelting‐restabilization technique while using different stabilization systems in selected concentrations. To study the effect of heat aging on the material properties, crystallinity content, tensile strength, and tensile impact strength together with time to embrittlement were monitored. The results illustrate that the restabilization recipe is of crucial importance for improving the long‐term thermal stability of the postused chair material, ensuring its reuse in the original application. Best long‐term performance is achieved by combining a filler deactivator with appropriate heat and light stabilizer systems. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3033–3044, 2003     

Prediction of creep of polymer concrete

Polymer concrete possesses viscoelastic properties conditioned by relaxation processes in the polymer binder. Their acceleration with an increase of temperature (principle of time–temperature equivalence) is used in predicting the long‐term creep of polymer concrete. Physical aging of the polymer binder influences the creep of polymer concrete. To predict the long‐term creep accounting for the aging process, an attempt to improve the time–temperature equivalence principle was undertaken. As a result of the experimental study of polyester resin‐based concrete and its structural components (a resin unfilled and filled with diabase flour), it has been established that the changes in the creep compliance of the material follow according to the principle of the time–aging time equivalence with the reduction function depending on aging temperature. To predict the long‐term creep of polymer concrete, a function of the time–temperature–aging time reduction was applied. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1949–1952, 1999     

Humidity sensing properties of the vinylpyridine–butyl acrylate–styrene copolymers

In this article, the humidity sensors based on the vinylpyridine (VP)–butyl acrylate (BA)–styrene (St) copolymers are developed. The influencing factors of the copolymer's humidity sensing properties, such as the mol percentage of the fed monomers and the quaternization reagent ratio (namely, dibromobutane : butyl bromide ratio), are studied, and the long‐term stability of the copolymers is investigated as well. The results show that as the content of BA increases and the content of St decreases, the copolymer's hysteresis and response time decreases, and with the increasing of the quaternization reagent ratio, the copolymer's hysteresis and response time decreases. Also, the sensors based on the copolymers show 2–3% RH reproducibility under various long‐term test conditions. These results demonstrate an overall excellent performance in the reproducibility and long‐term stability. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1992–1996, 1999     

A viscoelastic–plastic constitutive model for uniaxial ratcheting behaviors of polycarbonate

In this paper, a modified viscoelastic–plastic constitutive model has been proposed on the framework of Anand's work to describe the uniaxial ratcheting behavior of polycarbonate (PC) under tension–tension cyclic loading. The experimental observation illustrates that the previously accumulated deformation has an assignable influence on the subsequent material response during the ratcheting process of PC. Thus, the deformation resistance in the viscoelastic micromechanism is assumed to be evolving with the local accumulated inelastic strain rather than keeping unchanged in the original Anand's model. The proposed model is validated firstly by the monotonic tension and creep experiment results of PC. Then, its capability to describe the uniaxial ratcheting behaviors is compared with Anand model. Finally, the modified model is adopted to study the effect of mean stress, stress amplitude, loading rate, and peak holding time on the ratcheting behaviors of PC. It is shown that the proposed model can predict reasonably the uniaxial tension–tension ratcheting behavior of polymer. POLYM. ENG. SCI., 55:2559–2565, 2015. © 2015 Society of Plastics Engineers     

Long‐term performance of environmentally‐friendly blown polyurethane foams

We studied the long‐term performance of new environmentally‐friendly blowing agents for polyurethane foams. Several blowing agents, hydrofluorocarbons, hydrocarbons, and a possible hydrochlorofluorocarbon substitute (dimethoxymethane), as well as hydrochlorofluorocarbons, were analyzed. The determination of effective diffusion coefficients (knowledge of which is required to study long‐term performance) was performed by means of a classical gas chromatographic technique and by a new method based on infrared spectroscopy. The reliability of the experimental procedure used is showed by comparing experimental and predicted aging, as the slope of the aging curve (i.e., thermal conductivity vs. time) depends only on effective diffusion coefficients. Our study of long‐term performance of foams blown with alternative blowing agents shows that hydrofluorocarbons represent a proper alternative to hydrochlorofluorocarbons, as the foams show similar initial thermal conductivity and a slower aging rate (i.e., better long‐term performance). POLYM. ENG. SCI. 45:260–270, 2005. © 2005 Society of Plastics Engineers.     

A review of aspects affecting performance and modeling of short‐natural‐fiber‐reinforced polymers under monotonic and cyclic loading conditions

The use of short natural fibers as reinforcing fibers was hampered by uncertainties associated with the performance of these developed short‐fiber‐reinforced composites. Much of this uncertainty comes from an unclear understanding of different aspects controlling the properties and the behavior of natural fibers and their developed composites. This study provides a benchmark review that highlights several factors affecting the performance of short‐natural‐fiber‐reinforced polymers (SNFRPs). Additionally, the study also reviews the researches related to the short term (monotonic) and the long‐term (cyclic) behaviors as well as the potential monotonic and life prediction models and techniques suited for SNFRPs. POLYM. COMPOS. 36:397–409, 2015. © 2014 Society of Plastics Engineers     

A new protocol for accelerated screening of long‐term plasticity‐controlled failure of polyethylene pipe grades

In this study, a new experimental protocol to evaluate long‐term, plasticity‐controlled failure using short‐term testing is validated on a high‐density polyethylene (PE100) pipe grade. The protocol starts with the assessment of the strain rate dependence of the yield stress in uniaxial extension at various temperatures. Additional uniaxial compression tests are performed to characterize the influence of hydrostatic stress. The plastic flow kinetics are subsequently captured using a Ree‐Eyring modification of the pressure‐modified Eyring flow equation. In combination with the hypothesis that failure occurs at a critical amount of accumulated plastic strain, a versatile tool to predict time‐to‐failure is obtained. POLYM. ENG. SCI., 56:676–688, 2016. © 2016 Society of Plastics Engineers     

Effect of cycling frequency and self‐heating on fatigue behavior of reinforced and unreinforced thermoplastic polymers

An experimental study was conducted to evaluate the effect of frequency and self‐heating on fatigue behavior of two unreinforced and two short glass fiber reinforced thermoplastic polymers. Load‐controlled fatigue tests were conducted under fully reversed (R = ?1) and R = 0.1 conditions with specimens loaded in either longitudinal or transverse direction to the mold flow direction. Effect of frequency on fatigue life was evaluated at 23 and 125°C and for a range of frequencies between 0.063 and 20 Hz. Incremental step frequency tests were also performed at different stress ratios and stress levels. Surface temperature rise was found to be material, frequency, and stress level dependent. Three energy‐based models were applied to the incremental step frequency data and relationships were developed for each material to estimate surface temperature rise as a function of test frequency and stress level. Relationships were also developed to assess critical frequency for the unreinforced thermoplastics at a given stress level above which surface temperature does not stabilize. POLYM. COMPOS., 55:2355–2367, 2015. © 2015 Society of Plastics Engineers     

Effects of fiber size on short‐term creep behavior of wood fiber/HDPE composites

When natural fiber‐thermoplastic composites are used in long‐term loading applications, investigating creep behavior is essential. The creep behavior of high‐density polyethylene (HDPE)‐based composites reinforced with four sizes of wood fibers (WFs) (120–80, 80–40, 40–20, and 20–10 mesh) was investigated. The instantaneous deformation and creep strain of all WF/HDPE composites increased at a fixed loading level when the temperature was increased incrementally from 25 to 85°C. At a constant loading level, composites containing the larger‐sized WFs had better creep resistance than those containing smaller‐sized fibers at all measured temperatures. The creep properties of composites with smaller‐sized WFs were more temperature‐dependent than those with larger‐sized WFs. Two creep models (Burger's model and Findley's power law model) were used to fit the measured creep data. A time–temperature superposition principle calculation was attempted for long‐term creep prediction. The Findley model fitted the composite creep curves better than the four‐element Burger's model. From the predicted creep response of the WF/HDPE composites, two groups of small fibers (120–80 and 80–40 mesh) had the lowest creep resistance over long periods of time at the reference temperature of 25°C. The largest WFs (10–20 mesh) provided the best composite creep resistance. POLYM. ENG. SCI., 55:693–700, 2015. © 2014 Society of Plastics Engineers     

Microindentation into an epoxy composition to assess the influence of aging on mechanical properties

This work aims to apply the microindentation technique for assessing the influence of two aging factors—long‐term weathering and long‐term laboratory aging—on the time‐dependent mechanical properties of an epoxy composition. The linear theory of viscoelasticity was taken as a background for the problem, and the time‐independent Poisson ratio value was assumed to simplify the assessment. Two nano/microindenters were used (Hysitron Triboscan and Nano XP Indenter) at two different laboratories. Four special time‐dependent loading histories were applied: indentation under a step load, indentation under a constant load rate, indentation with a fixed depth of penetration, and indentation under a constant rate of penetration. The short‐term histories of the viscoelastic compliance of a common epoxy composition, affected by 5‐year weathering or laboratory aging, measured using a microindentation technique were compared to the data derived from standard macro measurements. The findings suggest that a qualitative assessment of the influence of the investigated aging effects on mechanical properties can be handled using short‐term microindentation data, but the data has to be freed of possible attendant factors, especially of the influence of polishing procedures accompanying the microindentation techniques, before comparing it with standard measurement data. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012