Effects of temperature on cure kinetics and mechanical properties of vinyl–ester resins

The relationships among cure temperature, chemical kinetics, microstructure, and mechanical performance have been investigated for vinyl–ester resins. Fourier transform infrared spectroscopy was used to follow the reactions of vinyl–ester and styrene during isothermal curing of Dow Derakane 411‐C‐50 at 30 and 90°C. Reactivity ratios of vinyl–ester and styrene vinyl groups were evaluated using the copolymer composition equation. The results indicate that the ratio of vinyl–ester to styrene double bonds incorporated into the network is greater for 30 than for 90°C cure. Mechanical properties were obtained for systems subjected to isothermal cures at 30 and 90°C and postcured above ultimate T_g. The results show that the initial cure temperature significantly affects the mechanical behavior of vinyl–ester resin systems. In particular, values of strength and fracture toughness for postcured samples initially cured isothermally at 30°C are significantly higher than those obtained for samples cured isothermally at 90°C. Examination of fracture surfaces using atomic force microscopy revealed the existence of a nodular microstructure possessing characteristic nodule dimensions that are affected by the temperature of cure. Such features suggest the existence of phase separation during cure. A binary interaction model in conjunction with chemical kinetic data and estimated solubility parameters was used to evaluate enthalpic interactions between the growing polymer network and monomers of the vinyl–ester system. The results indicate that the interaction energy becomes increasingly endothermic as cure progresses and that this energy is affected by the temperature of cure through differences in copolymerization behavior. Hence, in addition to entropic factors, the changes in enthalpic contribution to the Gibbs free energy suggest that the probability of phase separation increases with extent of cure and that its onset is potentially affected by cure temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 725–744, 1999     

Aging effect on the mechanical properties of hybrid gels

Sol-gel derived unsupported films and thin rods have been obtained from co-hydrolysis of triethoxysilane and methyldiethoxysilane. The materials are flexible, dense and transparent. Films and rods have been aged for different periods of time in air at room temperature. The elastic modulus has been measured by means of tensile or flexural tests. Measurements showed an increase of elastic modulus with aging time and showed different values for films and rods. The observed evolution of mechanical properties has been related to a corresponding structural modification as highlighted mainly by MAS-NMR studies. Analyses pointed out the crucial role of condensation processes and showed that the stiffness increase arises from the formation of relatively few bonds which link and constrain pre-existing mobile network regions.     

Thermal and mechanical properties of Th-30Zr alloy

Journal of Radioanalytical and Nuclear Chemistry - Literature studies reveal apaucity of data on the Th-Zr system. Investigations on the Th-30Zr alloy and its synthesis, microstructure and...     

The effect of sugars on the mechanical properties of processed cereals

Breakfast wheat-flake materials of different composition have been reconstituted as barshaped test pieces to reduce geometry and structure effects and allow better comparison of the matrix mechanical properties. The ground flakes comprised a control formulation and others in which components had been subtracted or substituted. The aim was to compare the mechanical properties of pressed specimens of multiple-component systems with those published for simpler one- and two-component materials. Sucrose or fructose, present in the ratio sugarwheat 15.9–6.1, lowered the modulus of wheat-flake material, but by progressively lesser extent with decreasing water content below 22% (wet-weight basis, w.w.b), the difference becoming negligible at water contents of 7 to 10% (w.w.b). However, the energy to break wheat-flake samples and their fracture toughness were reduced more by fructose than sucrose addition to a control formulation sample at these water contents. The energy to break and fracture toughness increased markedly with increasing water content for all formulations.This work was supported by a MAFF-DTI LINK grant under the Food Processing Sciences Programme involving Weetabix Ltd, APV Baker, Campden Food and Drink Research Association, University of Nottingham and Novo Nordisk (UK) Ltd. The authors acknowledge the scientific and financial support of the MAFF and LINK participants. They would also like to thank Dr. Simon Livings (University of Cambridge, Cavendish Laboratory) for his advice on preparation of hot pressed-samples and Mr. David Steer for the design of the hot press.     

Reactive wetting, evolution of interfacial and bulk IMCs and their effect on mechanical properties of eutectic Sn-Cu solder alloy

Lead free solders are increasingly being used in electronic applications. Eutectic Sn-Cu solder alloy is one of the most favored lead free alloys used for soldering in electronic applications. It is inexpensive and principally used in wave soldering. Wetting of liquid solder on a substrate is a case of reactive wetting and is accompanied by the formation of intermetallic compounds (IMCs) at the interface. Wettability of Sn-0.7Cu solder on metallic substrates is significantly affected by the temperature and the type of flux. The wettability and microstructural evolution of IMCs at the Sn-0.7Cu solder/substrate interfaces are reviewed in the present paper. The reliability of solder joints in electronic packaging is controlled by the type and morphology of interfacial IMCs formed between Sn-0.7Cu solder and substrates. The formation and growth mechanisms of interfacial IMCs are highlighted. Mechanical behavior of bulk solder alloy and solder joint interfaces are analyzed. The characteristics of the IMCs which have marked effect on the mechanical properties and fracture behavior as well as reliability of solder joints of the alloy are discussed. An attempt has also been made to discuss the effect of cooling rate and strain rate on shear strength, tensile properties and creep resistance of the solder alloy. It is recommended that future work should focus on evolving a standard procedure involving sequential assessment of wetting behavior, evolution of IMCs and mechanical properties.     

Electrochemical properties of Mg2Ni and Mg2Ni2 prepared by mechanical alloying

Crystallized Mg₂Ni and Mg₂Ni₂ amorphous alloys synthesized by mechanical alloying at room temperature were found to present first discharge capacities of 270 mAh/g and 500 mAh/g, respectively. These capacities decrease upon subsequent cycling to reach 30 mAh/g and 70 mAh/g after 60 charge/discharge cycles. The largest initial capacity, measured for the Mg₂Ni₂ composition, is ascribed to its amorphous nature, while its poor capacity retention upon cyding appears to originate from a fine Ni dispersion within the Mg/Ni matrix. This dispersion enables a better protection of the Mg against oxidation during cycling. We show, however, that this protection of Mg by Ni is not sufficient to avoid a strong corrosion of Mg in the KOH electrolyte during cycling, leading to the formation of Mg(OH)₂. Received: 8 October 1997 / Accepted: 20 January 1998     

Enhanced thermoelectric properties of bismuth sulfide polycrystals prepared by mechanical alloying and spark plasma sintering

Bismuth sulfide powders were synthesized by mechanical alloying (MA) and then consolidated by spark plasma sintering (SPS) technique. In order to improve the electrical transport properties of bismuth sulfides, the carrier concentration was optimized by modifying chemical composition of sulfur through producing sulfur vacancies, and the carrier mobility was enhanced by a two-step SPS as a hot-forging process through increasing grain orientation. The electrical resistivity of bismuth sulfides was reduced to 10⁻⁴ from 10⁻² Ω m by optimizing sulfur content, and further lowered by hot-forging, whereby the power factor was significantly increased from 91 to 254 μW/mK². The hot-forged Bi₂S_2.90 sample showed the highest ZT=0.11 (at 523 K), which is higher than the reported value. The present work revealed that bismuth sulfide compounds as a promising candidate of thermoelectric materials can be synthesized by a simple process.     

The effect of densification on the mechanical properties of amorphous glassy polymers

Four polymers, viz. polystyrene, polycarbonate, poly(methyl methacrylate), and poly-(vinyl chloride), were cooled from the melt to room temperature under hydrostatic pressures of 30 and 1000 atm. Cooling under high pressure increased the density by 0.4–0.6%, and the effect of this has been examined for the torsional creep properties, the dynamic properties at 1 Hz, the Charpy impact strength, the thermal expansivity, and the torsional yield stress. It turned out that, in general, densification affects the thermomechanical properties only slightly.     

The effect of mechanical activation on the hydration properties of nanodispersed silica

The hydration of highly dispersed nanosize silica was studied by low-temperature ¹H NMR spectroscopy before and after the mechanical activation in a ball mill.     

The effect of water on the physicochemical and mechanical properties of gelatin

Strips of gelatin have been prepared by extrusion at different water contents varying from 20 to 50% H₂O (dry weight basis, d.w.b.). The processes of subsequent hydration or dehydration of these strips were followed by dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction and NMR relaxation measurements. A comparison of the calculated dependence of theT _g of gelatin (T _g anhydrous, 200?C) on water content (using the Ten Brinke and Karasz equation) with experimental results derived from DMTA showed that freshly extruded material followed the theoretical plot below 25% H₂O (d.w.b.), but at higher water contents, the7 _g deviated positively, probably due in part to the effect of delayed re-equilibration of water content after thawing of separated ice crystals. The experimental results determined after storage for one week fell on a different line, with aT _g of 145?C for anhydrous gelatin Possibly, theT _g is elevated by crystallization — a view supported by the WAXS spectra. The NMR relaxation results also showed a profound mobilization of the gelatin protons at water contents greater than 25% d.w.b.     

The effect of the alkyl chain length on the mesomorphic properties of new lactic acid derivatives

New series of lactic acid derivatives with alkyl terminal chain have been synthesised and their mesomorphic properties studied. We have varied the length of chiral and non-chiral terminal alkyl chains and found that prolonging both chains has a strong effect on the SmA*–SmC* phase transition. Most of the new materials exhibit only paraelectric SmA* phase; for homologues with a longer non-chiral chain (m ≥ 10), the ferroelectric (SmC*) phase appears below the SmA* on cooling and persists down to a room temperature. The role of the chiral terminal chain in the molecule is quite opposite – only its short length supports the existence of ferroelectric phase. Additionally, a hexatic phase appeared below the SmA*–SmC* phase sequence for several homologues at low temperatures. All materials have been studied using standard experimental techniques (differential scanning calorimetry (DSC), texture observations, polarisation and tilt angle measurements, etc.). Liquid crystalline properties of new materials have been compared with the previously prepared and studied lactic acid derivatives.     

The effect of the mechanical properties of the molecular network on the wool-water isotherm

Summary The present position of theoretical investigation of the water-content versus relative humidity isotherm is briefly indicated. Previous workers have anticipated an effect on equilibrium water-content arising from the mechanical properties of the keratin network. Such an effect is reported for supercontracted wool fibres and for reduced and S-methylated fibres, for which an increased saturation water-content is observed. In the case of S-methyl wool, saturation water-content shows little or no dependence on the number of disulphide cross-links, however it does depend on the number of S-methyl groups present. The results are considered in terms ofFlory-Huggins solution theory, and it is indicated that this theory can be usefully applied for relative humidities in excess of 85%. In normal wool solution absorption is permitted only in the noncrystalline component, but in supercontracted wool or S-methyl wool takes place in both components. Even when saturation water-content is greatly increased the isotherm is not appreciably changed below 85% R. H., indicating that no additional sites for strong binding of water become available; apparently all such sites are accessible in normal wool at approximately 85% relative humidity.

---

Zusammenfassung Der gegenwärtige Stand theoretischer Untersuchungen für den isothermen Wassergehalt gegen relative Feuchtigkeit ist kurz kritisch dargelegt. Die bisherigen Bearbeiter haben einen Einfluß auf das Gleichgewicht des Wassergehaltes seitens der mechanischen Eigenschaften des Keratinnetzwerkes angenommen. Ein solcher Effekt wird für superkontrahierte Wollfasern und für solche, reduziert und mit Schwefelmethyl behandelt, mitgeteilt, für die eine erhöhte Wassersättigung zu beobachten ist. Im Fall der schwefelmethylierten Wolle zeigt die Sättigung geringe oder keine Abhängigkeit von der Zahl der Disulfidvernetzungen, sie hängt jedoch von der Zahl der Schwefel-Methylgruppen ab. Diese Resultate werden mit derFlory-Huggins-Lösungstheorie betrachtet, und es zeigt sich, daß diese Theorie gut für relative Feuchtigkeiten größer als 85% angewendet werden kann. In normaler Wolle ist Lösungsabsorption nur in den nichtkristallinen Anteilen möglich, aber in superkontrahierter oder schwefelmethylierter Wolle findet sie auch in den Kristalliten statt. Auch wenn der Sättigungswert der Isotherme gegen Ende stark beeinflußt wird, ändert sich der Verlauf der Isotherme unterhalb 85% r. F. nicht. Das zeigt, daß kerne zusätzlichen Plätze für starke Wasserbindungen entstehen. Offensichtlich sind alle diese Plätze in normaler Wolle bei etwa 85% r. F. zugänglich.

     

The effect of thiolation on the mechanical and protein adsorption properties of polyurethanes

Segmented polyurethanes are important polymers for a number of industrial and technological applications. The purpose of this work was to synthesize polybutadiene-based polyurethanes and subsequently graft carboxylate and sulfonate side chains via thiol-ene reaction. Spectroscopic investigations showed that grafting yielded good conversion for the vinyl unsaturation of the polybutadiene soft segment. DSC and tensile testing revealed that grafted polyurethanes had a better segmental compatibility and superior mechanical properties than the control polyurethane without grafting. The carboxylic side chains of the soft segment were responsible for the observed improved mechanical properties. Initial protein adsorption tests on these polymers were found to be higher than the control surface. The polyurethanes of the current study could be used for biomedical applications where protein attachment to the surface is needed for specific cell adhesion and tissue repair.     

The effect of temperature on the mechanical properties of a protein-based biopolymer network

Several studies have shown that eggshell membrane (ESM) is a suitable biomaterial with potential applications in biomedicine such as wound repair and cell culture. In order to control and improve the use of ESM for biomedical applications their physical and structural properties must be known. In this paper, we have studied the effect of temperature on the mechanical properties of the ESM. Atomic force microscopy was used to assess the morphology of the ESM. The mechanical properties of the membranes were studied by means of uniaxial tensile tests carried out at four different temperatures. Differential scanning calorimetry and thermo-gravimetrical analysis were used to assess the thermal transitions of the ESM and the influence of the water content on its thermal behavior. The Young’s modulus showed a linear inverse dependence with regard to the temperature of the sample. A peak associated to the thermal denaturation of collagen was observed in the DSC tests of the membrane. These peaks showed a dependence on the water content of the specimens.     

具有阳极氧化膜的镁合金电极性能的研究

镁合金电极上生成阳极氧化膜的保护技术，使电极耐大气腐蚀的性能大大优于用铬酸盐膜保护的镁电极。阳极氧化工艺条件不同。镁电极的耐腐蚀性和电压滞后性能不同，本文讨论了膜的组成，形态，厚度等对性能的影响。     

Preparation of Fe/B powders by mechanical alloying

Steels with a high boron content are valuable as a neutron shield in waste containers and as control absorbers in nuclear reactors. The purpose of this study was to obtain by mechanical alloying an iron powder with 50% boron (by weight) and then powder-metallurgy materials. The elementary powders were mixed in a high-energy mill for 36 h in an inert atmosphere. Samples were withdrawn at intervals, and the powder was characterized by differential thermal analysis, X-ray diffraction and electron microscopy. The Fe/B powders withdrawn at different intervals of milling were diluted with further additions of iron up to a final content of 10% boron. The mixtures were uniaxially compacted at 500 MPa; their green density was verified, and they were sintered in argon at 1150°C. Their physical properties (density and dimensional change) and bending strength were evaluated and microstructural studies and fracture tests were performed.     

Thermal stability of solid solutions obtained by mechanical alloying

Summary The heats of ordering for mechanochemically synthesized nano-sized supersaturated solid solutions were demonstrated for Ni-In, Ni-Sn, Ni-Al, Cu-Sn, Cu-Hg systems. It is shown that increasing concentration of doping element leads to decreasing of decomposition temperature and to increasing of decomposition enthalpy. The concentration heterogeneity of doping elements in mechanochemically synthesized supersaturated solid solutions was found. The reactivity of alloys Cu 20 mass % Sn for commercial powder and MA powder was investigated.     

The effect of curing conditions on the physical and mechanical properties of styrenated polyester

The curing behavior of unsaturated polyester resin was investigated. In the present study, styrene, methyl ethyl keton peroxide (MEKP) and cobalt octoate (CO) have been selected as solvent (monomer), catalyst and accelerator, respectively. The effect of the concentration of both MEKP and CO on the curing exotherm, curing time, hardness and compressive strength of the prepared copolymers was investigated.     

The effect of planar molecular orientation on the mechanical properties of rigid-chain polymide films

The effect of biaxial orientation on the mechanical properties and supramolecular structure of the films based on rigid-chain polyimides and copolyimides was studied. The films were oriented in two different ways: by hot drawing and by cold drawing at the prepolymer stage and the subsequent cyclization according to the specified temperature regime. It was shown that the curing of the precursor films after their preliminary biaxial drawing is accompanied by self-orientation, although its degree is well below that after uniaxial drawing under the same conditions. As compared with homopolyimide films, the films based on rigid-chain copolyimide exhibit a lower tendency toward orientation under biaxial drawing. The reasons for such behavior are discussed.