All issues / Volume 2 (2008) / Issue 7 (July)
The current submission concerns an editorial note.
Thermally induced shape-memory effect (SME) in tensile mode was investigated in binary and ternary blends of two ethylene-1-octene copolymers with a degree of branching of 30 and 60 CH3/1000C and/or nearly linear polyethylene cross-linked after melt mixing with 2 wt% of liquid peroxide 2,5-dimethyl-2,5-di-(tert.butylperoxy)-hexane at 190°C. The average cross-link density estimated by means of the Mooney-Rivlin equation on the basis of tensile test data was characterized between 130 and 170 mol•m–3 depending on the blend composition. Thermal analysis points out multiple crystallization and melting behavior of blends caused by the existence of several polyethylene crystal populations with different perfection, size and correspondingly different melting temperature of crystallites. That agrees well with the diversity of blends phase morphology characterized by atomic force microscopy. However, triple- and quadruple-SME could be observed only after two- and accordingly three-step programming of binary and tertiary blends, respectively, at suitable temperatures and strains. Compared to performances obtained for the same blend after single-step programming above the maximal melting temperature the significantly poorer characteristics of SME like strain fixity and strain recovery ratio as well as recovery strain rate occurred after multi-step programming.
The present study investigated the effect of temperature on tensile strength and modulus of injection moulded ABS polymer reinforced with both short fibres (GF) and spherical glass beads (GB) over the temperature range 25 to 100°C. Tensile strength, σh and modulus, Eh, of ABS/GF/GB hybrids increased as volume fraction of the total glass in the hybrids increased. A linear increase in σh and Eh was found with increasing the hybrid ratio of the glass fibre, χf, over the entire temperature range studied. Results indicated that the tensile properties of the ABS/GF/GB hybrid composites at any temperature, T, can be estimated from the rule-of-hybrid-mixtures of the form Ph = Pcfχf + Pcb(1 – χf) where Pcf and Pcb are the tensile properties of the ABS/GF and ABS/GB composites at temperature T, respectively. It was found also that σh and Eh both decrease with increasing temperature in a linear manner. The rate at which σh and Eh decreased with temperature was dependent upon the hybrid ratio of the glass fibre, χf, and the total concentration of the glass in the hybrids.
New polyimide nanocomposites containing organically modified montmorillonite (MMT), synthetic silicate (chrysotile) nanotubes (SNT), and zirconium dioxide (ZrO2) were prepared to investigate the influence of the nanoparticle morphology on the nanocomposite rheology and mechanical properties under selected conditions that the materials are likely to encounter during use. The efficiency of homogeneous dispersion of the nanoparticles in the polyimide matrix was studied by measuring the rheology of model oligoimides (OI) dispersions containing the desired amounts of the nanoparticles. The OI/nanoparticles dispersions showed significant increase in complex viscosity with increasing concentration of the nanoparticles that depended strongly on the nanoparticle morphology and aspect ratio. Polyimide nanocomposite films (PI-PM) prepared from the poly(amic acid) of poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PM) filled with the desired concentration of the nanoparticles showed an increase in tensile modulus with increasing nanoparticle concentration in the order MMT>SNT>ZrO2. In contrast to the PI-PM/MMT films, the PI-PM films filled with 10 vol% of SNT and ZrO2 showed higher sample failure strains, suggesting that the SNT and ZrO2 may be more effective in improving the ductility of the polyimide nanocomposites for applications where the relatively brittle polyimide/MMT nanocomposites films are not useable.
In this study, thermal and mechanical properties of nanocomposites containing SC-15 epoxy resin and polyhedral-oligomeric-sil-sesquioxanes (POSS) have been studied. Dynamic Mechanical Analysis (DMA) results show that the addition of 5 wt% of POSS yielded a 13% increase in the storage modulus and a 16°C enhancement in Tg. Thermo gravimetric Analysis (TGA) results show that the thermal stability of epoxy increased with higher POSS content. Tension tests were used to evaluate the mechanical properties of materials. Both modulus and tensile strength are linear functions of POSS content. Scanning Electric Microscopy (SEM) pictures of fracture surfaces show that the roughness of the fracture surfaces of epoxy increased after adding POSS. Based on experiment results, a three-parameter nonlinear constitutive equation was developed to describe the strain-softening stress-strain relationship behavior of materials. The parameters in this model are the elastic modulus, a strain exponent, m, and a compliance factor, β. Their relationships to the POSS weight fraction were obtained from the experiment results. The simulated stress-strain curves from the model agree with the test data. Analysis of the model shows that both the strain exponent, m, which controls the strain softening and hardening effect of the material, and the compliance parameter, β, which controls the flow stress level of the material, increase with higher POSS content.
In this study, the cellulose crystals, prepared by acid hydrolysis of flax fiber, consisted of slender rods with lengths ranging from 100 to 500 nm and diameters ranging from 10 to 30 nm, respectively. After mixing the suspension of flax cellulose nanocrystals (FCNs) and plasticized starch (PS), the nanocomposite films were obtained by the casting method. The effects of FCNs loading on the morphology, thermal behaviour, mechanical properties and water sensitivity of the films were investigated by means of wide-angle X-ray diffraction, differential scanning calorimetry, tensile testing, and water absorption testing. Scanning electron microscopy photographs of the failure surfaces clearly demonstrated a homogeneous dispersion of FCNs within the PS matrix and strong interfacial adherence between matrix and fillers, which led to an increase of glass transition temperature ascribed to the starch molecular chains in the starch-rich phase. In particular, these nanocomposite films exhibited a significant increase in tensile strength and Young’s modulus from 3.9 to 11.9 MPa and from 31.9 to 498.2 MPa, respectively, with increasing FCNs content from 0 to 30 wt%. Also, with a loading of FCNs, the resulting nanocomposite films showed a higher water resistance. Therefore, FCNs played an important role in improving the mechanical properties and water resistance of the starch-based materials.
An industrial scale measuring system was set up to investigate the pressure arising in the mould cavity during polyurethane integral skin foaming. The system is able to measure the pressure arising in the mould cavity and the pressure distribution using a piezoresistive pressure sensor. The pressure distribution was measured at 18 points along the mould surface at constant production parameters. Then six production parameters, which affect the pressure, were investigated in detail with the Taguchi method of experimental design. The results of the design were processed by ANOVA (analysis of variance). Three major influencing parameters were estimated by regression analysis. Finally an equation was developed to give a good estimation to the pressure arising in the mould cavity.
A peroxide curable hydrogenated nitrile rubber (HNBR) was modified by cyclic butylene terephthalate oligomer (CBT), added in 100 parts per hundred rubber (phr). CBT polymerization was expected to occur simultaneously with that of the curing of the HNBR rubber (T = 190°C, t = 25 min). Differential scanning calorimetry (DSC) indicated that only a minor part of CBT has been polymerized (pCBT) in the hybrid. Dynamic mechanical thermal analysis (DMTA) revealed that HNBR formed the continuous whereas (p)CBT the dispersed phase. Mechanical properties (hardness, tensile modulus, ultimate tensile strength and strain, tear strength) of the HNBR and HNBR/CBT were determined and collated. Tribological properties were investigated with pin(steel)-onplate(rubber) (POP), with roller(steel)-on-plate (rubber) (ROP), with oscillating steel cylinder on rubber plate (Fretting) test configurations. Coefficient of friction (COF) and specific wear rate of the HNBR-based systems were determined. It was found that the resistance to wear increases with CBT hybridization. On the other hand, COF did not change much with CBT content. The friction and wear characteristics strongly depended on the test configurations. The worn surface of the HNBR systems was inspected in scanning electron microscope (SEM) to conclude the typical wear mechanisms. SEM investigation showed that the CBT was predominantly recrystallized from its molten state under the curing conditions set. The well developed prism- and platy-like, micron-scaled CBT crystals were made responsible for the reinforcing effect observed.
Polystyrene (PS) based nanocomposites were prepared by melt mixing in a Brabender Plasticoder with calcium phosphate as the nanofiller. These nanocomposites showed improvement in almost all physical properties. Contact angle measurements of the composites with water and methylene iodide were carried out. Various contact angle parameters such as total solid surface free energy, work of adhesion, interfacial free energy and spreading coefficient were analysed. The interaction parameter between the polymer and the liquids has been calculated using the Girifalco-Good’s equation.