Synthesis and mechanical behavior of functionally gradient polyisocyanurate materials based on hydroxy-terminated butadiene rubber

Functionally gradient polyisocyanurate-based structural materials in which the modulus of elasticity could be arbitrarily varied over a continuos range from 3 to 2000 MPa were prepared from hydroxy-terminated butadiene rubber and diphenylmethane diisocyanate. The materials are synthetically obtainable both via bulk polymerization (molding) and as composite materials with fillers of any type, including both highly porous compliant fillers that have no effect on the mechanical properties of the polymer matrix and reinforcing fillers, such as carbon and glass clothes. The trends in the main properties were studied; it was found that, over the entire range of elastic moduli relevant to the glass-to-rubber transition, the materials retain the elastic behavior inherent in polymer glasses, not the viscoelastic behavior characteristic of the transitional region between the glassy and rubbery states.     

Effects of molecular weight,rubber toughening,and environment on fatigue crack propagation behavior of block amide copolymers

The Fatigue Crack Propagation (FCP) behavior of block amide copolymers is investigated as a function of molecular weight, rubber toughening as well as environmental conditions. The enhancement of FCP resistance with increasing average molecular weight is shown and correlated to features observed on the fracture surface. Particular attention is paid to hysteretic heating, measured with an infrared camera, in the crack tip zone of different average molecular weight copolymers and rubber-toughened copolymer. A FCP approach of stress-cracking in an aqueous solution of zinc chloride is proposed here. An improvement in FCP resistance as the average molecular weight increases, similar to that exhibited in normal environment, appears. The shift in da/dN values over the tested range of can be approximated by an exponential function:      

Dispersing behavior of recycled rubber particles

The hydrophobic nature of recycled rubber particles presently limits their use only in non-aqueous media. Recycled rubber particles were chemically modified by preparation of amphiphilic semi-interpenetrating polymer networks using poly(acrylic acid) as the hydrophilic polymer. The resulting composite particles are water dispersible and suitable for various aqueous media applications.     

Origin and propagation of tears in plastic materials

Zusamenfassung Bei Temperaturen über dem Glasübergangspunkt haben hochpolymere Körper einen so niedrigen Elastizitätsmodul, daß die Kräfte, die zum Aufspalten chemischer Bindungen notwendig sind, nicht ohne extrem große Deformationen des Materials entstehen können. Bei kautschukelastischen Polymeren ist dies auch tatsächlich der Fall. Viele viskoelastische Polymere können jedoch fast ohne Deformation zerrissen werden. Bei genauerer Betrachtung ergibt sich, daß die starken Kräfte, die erforderlich sind zur Trennung chemisch gebundener Atome, eine Folge der hohen Deformationsgeschwindigkeit sind. Die scharfe Spitze des Rißwinkels bildet nämlich eine Diskontinuität in der Spannungsverteilung. Jeder infinitesimale Fortgang des Risses ist deswegen mit einem Umklappen der Scherdehnung verbunden und trotz kleiner Rißfortpflanzungsgeschwindigkeiten ist die Deformationsgeschwindigkeit des Materials groß in der Nähe dieser Spitze. Es ist diese hohe, aber örtlich sehr beschränkte Dehnungsgeschwindigkeit, welche dem Material die notwendige Härte gibt, so daß Reißen möglich wird.

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With 5 figures in 6 details     

Crack initiation and evolution in vulcanized natural rubber under high temperature fatigue

The nanoscaled crack initiation and evolution of natural rubber under high temperature (85 °C) and small strain amplitude (strain maximum α = 1) fatigue condition were investigated. It was shown by scanning electron microscopy (SEM) images that cracks and cavities with dimensions in nanoscale in the NR matrix appear during the high temperature fatigue. FTIR study indicated that thermal oxidation effect leads to the crosslinking structure destruction. According to the combined analysis of SEM, energy-dispersive X-ray (EDX) spectrometer and small angle X-ray scattering investigations, it was deduced that the destruction of crosslinking structure mainly locates in the vicinity of the ZnS particles with a diameter of 20.2 nm. The ZnS particles are generated as a byproduct in the vulcanization process. Further, the real-time SAXS analysis revealed that the cracks are primarily initiated at relative higher strains (0.7<α < 1) in the region of ZnS aggregations and larger cavities are derived from the enlargement of the cracks.     

Composite materials based on modified butadiene-styrene rubber

Effect of fabrication procedure of rubber stocks on the strength and fatigue performance of composite materials based on modified butadiene-styrene rubber was examined.     

Fatigue testing of rubber materials and articles

The mechanism of rubber fatigue in tension, based on fracture mechanics, and the relationship between crack growth measurements and fatigue tests are revised. Deviations from linearity are considered. The importance of test conditions, such as constant stress or constant strain, in development work is stressed.

Poor precision of fatigue results is noted. As frequency distributions of test results often do not follow a Gaussian pattern, fitting a Weibull distribution function to experimental fatigue results is recommended as it allows estimation of confidence intervals of parameters such as median, mean, etc., and the statistical analysis of results.

The use of master curves for extrapolation of fatigue results to low tearing energies close to fatigue limit is revised. Extension of fatigue results to other deformation modes is briefly considered.     

Crack propagation from a preexisting flaw at a notch root

We study crack kinking from a preexisting crack initiated at a notch root. This makes it necessary to evaluate the stress intensity factors at the tip of the initial crack, as a function of the “stress intensity factor of the notch” (the multiplicative coefficient of the singular stress field at the notch root in the absence of the crack), the length of the crack, the aperture angle of the notch and the angle between its bisecting line and the direction of the crack. Applying Goldstein and Salganik's well-known principle of local symmetry yields then the prediction of the kink angle of the crack extension. It is found that although the notch is always predominantly loaded in “mode I”, this angle is generally not large enough for the crack tip to get closer to the bisecting line of the notch.     

Fault propagation behavior study of CSTR in HAZOP

This paper discusses the role of process modeling in safety analysis. Process modeling is applied in the fault propagation behavior study of CSTR chemical production. For that purpose, HAZOP methodology and continuation analysis were used. The proposed hazard identification methodology involves analysis of steady-state multiplicity and safe operating conditions as well as those which can shift process units from one steady state to another. All presented case studies are also supported by system dynamic simulations, essential to detect oscillatory thermal instability. In this paper, N-oxide alkylpyridines production process was chosen to identify potential hazard and operational problems. Presented dynamic simulations represent an analysis of the system response to step changes in the key operating parameters. The effect of deviations of three key parameters on the reactor safe operation was investigated. The proposed numerical algorithms represent a mathematical engine of the simulation module within an automated model-based HAZOP analysis tool.     

Unique rheological behavior of rubber based nanocomposites

Montmorillonite clay (N) based nanocomposites were prepared using three different grades of acrylonitrile butadiene rubber (NBR) (19%, 34%, and 50% acrylonitrile contents), styrene butadiene rubber (SBR), and polybutadiene rubber (BR). Rheological study was carried out on these nanocomposites at three different temperatures (110 °C, 120 °C, and 130 °C) over a range of shear rates for comparison. The results showed that the shear viscosity decreased with increasing shear rate and incorporation of the unmodified (N) and the modified (OC) fillers up to a certain loading, when the results were compared with the gum rubber. This effect became more prominent with increasing polarity of the rubber. The die swell, on the other hand, decreased with loading of N and OC. With increasing filler volume fraction, the die swell further decreased. Decrease of viscosity with concomitant decrease of die swell is unique in such systems. Consecutive runs of the same sample over different shear rates increased the viscosity. The results were explained with the help of X‐Ray Diffraction (XRD) data and Transmission Electron Microscopy (TEM).© 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1854–1864, 2005     

Compressive-tensile fatigue behavior of cords/rubber composites

Cord/rubber composites are used to build complex structures which may be submitted to cyclic loads, sometimes leading to critical fatigue failure. The focus of this study is to investigate the cyclic compressive/tensile strain behavior of polyester, polyamide and hybrid polyaramid/polyamide cords. For that, the cords were embedded in rubber belts to be used in a specially designed rotating pulley equipment that allows monitoring and controlling of tensile force, frequency and strain level. All fatigue tests were performed using stress-control mode, and tensile residual strength of the cords was measured as a function of material type, number of cycles and compressive/tensile strain level. The results show that compressive and tensile cyclic strains decrease residual properties. Hybrid cords showed higher residual strength than polyester and polyamide cords when subject to high compressive strain or high number of cycles. Moreover, morphological evaluation indicated failure to be associated with microbuckling and extensive fibrillation.     

Thermomechanical behavior of rubberlike materials

A simple form of nonisothermal free energy function A(λ₁, λ₂, λ₃, T) for rubberlike materials results from postulating that the entropy is a separable symmetric function of the extension ratios λ_i along the principal strain directions and considering the fundamental properties of rubberlike materials, i.e., that rubber elasticity is associated primarily with changes in entropy and the variation of elastic tension with changes in temperature is linear. The explicit representation of A is reduced to the Valanis-Landel strain energy function for isothermal cases.     

Kinetic behavior of UV-sensitive polymers from modified natural rubber

Photosensitive copolymers have been obtained by chemical modification of natural rubber. The introduction of the photoreactive cinnamoyl group in different conversions has been carried out by the reaction of natural polyisoprene with maleic anhydride, followed by ring opening and condensation with some oxyalkylcinnamate esters. These reactions gave rise to five polymer series. Each series, for which the anhydride content was kept constant, is constituted of five copolymeric products, differing from each other in the size of the aliphatic chain between the chromophore and the main chain. The photosensitive products were submitted to UV irradiation, and their kinetic behavior has been investigated in order to correlate structural dissimilarities with the dimerization rate constants.     

Annealing effect on semi-crystalline materials in creep behavior

Three glass reinforced polymers of Nylon 66, PET and HT-Nylon with glass transitions of 58, 72, and 132°C are selected for checking the necessity of high temperature annealing and degree of its impact on creep strains. The improvement after annealing depends on the polymer type of whether it is fast or slow crystallizing at molding process and whether it has hydrogen bonded sheets. The physical property changes observed of these materials before and after annealing support the explanation of crystal reorganization through crystallization, free volume reduction through densification and crystal perfection through better chain packing. The prediction of long-term creep strains up to 10 years using time-temperature superposition technique has been the practice for last 13 years in our laboratory. The accuracy of the prediction are shown in confidence level of about 90%.This revised version was published online in November 2005 with corrections to the Cover Date.     

Photochemical behavior of lanthanide-containing polymer materials

Light-transforming polymer materials activated by compositions based on the mixed-ligand europium carboxylates and anthranilic acid exhibiting an intense luminescence in the spectral region 400–700 nm are prepared. The photolysis of the polymer materials is investigated. For the obtained compositions the flaring of intensity of luminescence of europium ion and anthranilic acid was detected.     

Electrorheology and characterization of acrylic rubber and lead titanate composite materials

Oxide one‐pot synthesis was used to synthesize a polymer precursor to lead titanate, PbTiO₃. Perovskite lead titanate, PbTiO₃, was synthesized via the sol–gel process. The dielectric constant, electrical conductivity and loss tangent of our acrylic rubber (AR71)–lead titanate (PT) composite material (AR/PT_8) were 14.15, 2.62 × 10^?7/Ω m, and 0.093, respectively, measured at 27 °C and 1000 Hz. SEM micrographs of composites between the AR71 elastomer and PbTiO₃ showed that the particles were reinforced within the matrix. The electrorheological properties of the AR71/PT composites were investigated as functions of electric field strength from 0 to 2 kV/mm and PbTiO₃ particle volume fraction. The storage modulus increased linearly with particle volume fraction, with or without an electric field. Without an electric field, the particles merely acted as a filler to absorb or store additional stress. With the electric field on, particle‐induced dipole moments were generated, leading to interparticle interactions, and thus a substantial increase in storage modulus. With PbTiO₃ particle volume fractions as small as 10^?4 embedded in the elastomer matrix, the modulus increased by nearly a factor of 2 as the electric field strength varied from 0 to 2 kV/mm. Copyright © 2008 John Wiley & Sons, Ltd.     

Evaluation of shock absorption properties of rubber materials regarding footwear applications

A challenge that faces the footwear market is the improvement of casual shoe comfort, being shock absorption one of the most significant properties for comfort. However, the methodologies available for the quantification of the shock absorption properties in materials for footwear applications are limited. With this work, an evaluation methodology of the shock absorption properties of rubber materials is proposed and the relationship between different test methods is established. Eight different commercial compact heeled rubber materials at six different thicknesses were prepared and characterized according to their physical and impact absorption properties. From the experimental values, the following correlations were defined: energy compression/hardness and energy compression/maximum deceleration. Shock absorption prediction curves were also determined from the experimental maximum deceleration values. Results present an excellent tool for industrial applications since they allow correlation of shock absorption properties and heel thickness, and also to predict the contribution of rubber material composition to the final product shock absorption properties.