Tensile rupture of short fibre filled thermoplastic elastomer

Tensile rupture in short silk fibre filled thermoplastic elastomer blends from low-density polyethylene and natural rubber (NR) containing pre-cuts of different lengths has been studied. _b, the tensile strength of the blends, was found to decrease with increase in the cut length in accordance with the Griffith's theory of fracture. However, unlike in the case of vulcanized NR, no critical cut length (at which the strength drops abruptly) was found in the case of both blends and unvulcanized NR. The energy to fracture per unit volume,W _b, also varied inversely with the length of the pre-cut. Values of inherent flaw size,I _O, of the composites, determined by extrapolation ofW _b to the value obtained when no initial pre-cut was present, were found to increase with fibre loading. Blends with longitudinally oriented fibres showed higherI _O values than those with transversely oriented fibres.     

木质纤维填充改性聚氨酯弹性体性能研究

为了改善聚氨酯弹性体拉伸和热稳定性能,本文以环氧树脂改性木质纤维,再填充至聚氨酯(PU)弹性体中制得生物基复合材料,采用万能试验机、扫描电镜和热分析仪分别对复合材料的拉伸性能、断面形貌和热稳定性进行测试分析。结果表明：与未改性填充木质纤维复合材料相比,经水性环氧和E-51环氧树脂改性木质纤维分别填充4wt.%和2wt.%时,复合材料的拉伸强度分别由18.2 MPa提高至22.4和23.7 MPa,断裂伸长率分别由898%提高至1 124%和2 269%。其中,采用2wt.%E-51环氧树脂溶液改性木质纤维填充2wt.%时,与未改性木质纤维相比,其拉伸强度提高了6.3%,断裂伸长率提高4倍以上,这是由于环氧改性木质纤维提高了木质纤维与树脂基体间的界面相容性。失重率为5%时,与未填充PU相比,复合材料的分解温度由269℃升高至279℃,提高了3.7%。填充木质纤维可提高PU拉伸性能与热稳定性。     

Improvement of electrostrictive properties of a polyether-based polyurethane elastomer filled with conductive carbon black

The electrostrictive properties of a polyether-based polyurethane elastomer and its corresponding composites filled with conductive carbon black (CB) were studied by measuring the thickness strain S_Z induced by external electric fields E. For films with thicknesses of approximately 50 μm, the apparent electrostrictive coefficient M was measured at low electric fields, E ? 4 V/μm, and different CB contents (up to a volume fraction of 2%). Dielectric measurements in AC mode were performed in order to determine the percolation threshold f_c, which was 1.25 v%. This optimal volume fraction yielded a remarkable threefold increase in M, associated with an increase of the dielectric constant by a factor 7, in comparison with pure PU. This enhancement of the electric field-induced strain and apparent electrostriction was mainly triggered by an increase of the dielectric constant, even if the intrinsic electrostriction coefficient Q was decreased. The nanocomposites thus seem to be very attractive for low-frequency electromechanical applications. Above f_c, their conductivity was raised and their electrostrictive activity lost. Finally, there is a good agreement between the experimentally determined dependence on the CB content of the M coefficient and the theoretical estimation calculated from dielectric and mechanical measurements.     

Study of the reinforcing mechanism and strain sensing in a carbon black filled elastomer

The mechanical and electrical properties of a styrene-butadiene rubber (SBR) matrix reinforced with carbon black, at various weight fractions were experimentally studied. Τhe electroconductive composites were used for strain sensing, under tension, by measuring together strain and electrical resistance. The storage and loss modulus decrement with strain were also investigated, on the basis of Payne effect, while a micromechanical model developed elsewhere was employed for the interpretation of the reinforcing mechanism of the composites examined.     

Preparation and dielectric properties of a polyurethane elastomer filled with resol-derived ordered mesoporous carbon

This study deals with the preparation and dielectric properties of polyurethane (PU) elastomer films by resol-derived ordered mesoporous carbon (OMC) nanopowder incorporation in the PU polymer matrix. Resol-derived OMC with a 2D hexagonal mesoporous carbon framework is used as conducting fillers to achieve homogeneous dispersion and favorable interfacial interactions in the polymer matrix. The dielectric properties depend on the applied field frequency and the carbon filler weight fraction. The carbon fraction has little effect on the relative permittivity. The relative permittivity of all the PU-OMC composites increases with the decline of frequency. Incorporating a small amount of OMC into the PU polymer had no influence on the dielectric loss. Along with the increasing carbon fraction above the percolation threshold, dielectric loss of PU-OMC composites increases exponentially in the low frequency range. PU-0.75 wt% OMC composite possesses the best dielectric properties, and the obtained relative permittivity and dielectric loss at 1 kHz is 9.59 and 0.03018, respectively.     

Dielectric constant enhancement in a silicone elastomer filled with lead magnesium niobate–lead titanate

Lead magnesium niobate–lead titanate (PMN–PT) ferroelectric powder was used to develop a particulated composite based on a silicone elastomer matrix, with improved dielectric permittivity. The filler was characterised by X-ray diffraction and scanning electron microscopy. Complex dielectric permittivity (10–10⁸ Hz) and tensile mechanical properties (elastic modulus and ultimate stress) of composites at various filler contents (up to 30% by vol.) were compared with those of the neat silicone elastomer. Both the dielectric constant and loss factor regularly increased with the filler content. The elastic modulus increased with a lower rate than that of the dielectric constant. Even though the addition of filler resulted in a detriment of both toughness, ultimate stress and elongation at break, a good stretchability was still retained, as elongation ratios greater than 3 were possible at the highest filler content. Several dielectric models were compared to the experimental data and the best match was achieved by the Bruggeman model, which can be used as a predictive rule for different volume contents of filler.     

Micromechanical analysis of an elastomer filled with particles organized in chain-like structure

Organization of iron filler particles inside an elastomer is obtained by curing the polymer in presence of a magnetic field. We have studied the effect of structuring the particles in chains on the quasistatic behavior in elongation in the absence of magnetic field. The effect of a coupling molecule between the surface of the particles and the elastomer is also analyzed. It is shown that the modulus of the first loading curve is strongly increased by structuring the particles, and also by the use of a coupling agent. Using an effective medium approach we well reproduce the experimental behavior of the elastic modulus and we deduce that a thick layer of elastomer is still present between the particles. A finite element calculation allows to distinguish between two modes of rupture at high strains, depending on the strength of the coupling between the particles and the matrix.     

Effect of elastomer interfacial agents on tensile and impact properties of CaCO3 filled HDPE

This work focuses on the modification of the tensile yield strength of CaCO3 filled HDPE brought about by the incorporation of SEBS elastomers. Two types of SEBS elastomers were used, grafted and ungrafted with maleic anhydride functions. The grafted elastomer encapsulates the filler particles in-situ and creates an adhesive interphase. The tensile yield stress was increased with increasing content of grafted elastomer until a maximum value. The influence of the interfacial area and the volume fraction of filler were studied. It was shown that the relative increase in tensile yield stress when increasing amount of interfacial agent was added, both depends on the volume of filler and the interfacial area.     

Effects of particle size and structure of carbon blacks on the abrasion of filled elastomer compounds

The effects of the particle size and structure of various carbon blacks on friction and abrasion behavior of filled natural rubber (NR), styrene–butadiene rubber (SBR) and polybutadiene (BR) compounds were investigated using a modified blade abrader. The effect of particle size and structure on abrasion resistance should be considered for the optimum design of desired wear properties. Characteristic parameters were introduced from the particle size and the structure of carbon blacks, with a linear relationship between the Young’s modulus and these characteristic parameters. The frictional coefficient depended not only on the particle size, but also on the structure of carbon black. The rates of abrasion were decreased with increasing surface area and developing structure of carbon blacks. Abrasion rates of the compounds were found to be proportional to a power n of the applied frictional work input. It was also observed that BR compounds caused much slower wear than NR and SBR compounds. The worn surfaces of the rubber compounds filled with carbon black having smaller particle size and a more developed structure showed narrower spaced ridges and better abrasion resistance. It means that smaller particle size and better structure development of carbon black resulted in improved abrasion resistance.     

Solvent effect on ageing of silica gels

The structure of silica gels derived from tetraethoxysilane (TEOS) with molar compositions of TEOSH₂OHNO₃=1100.4 and then aged in various solvents, was studied. The effect of various solvents having different physical properties on the gel structure, as well as the relationship between the solvent properties and the dried gel structure, were investigated. The density, surface area and pore-size distribution were measured. The results of the pore structure and SEM showed the aged gels to have a slit-shaped micropore, narrow pore-size distribution, and homogeneous microstructure. The density, surface area, pore size and pore volume of dried gels changed as the gels were subjected to ageing in various solvents. The surface area could be related to the polarity parameter of the ageing solvent.     

The effect of particle shape on the mechanical properties of filled polymers

The existing models for predicting the elastic moduli of polymers dispersed with particles of shape other than spheres and continuous fibres are reviewed. The applicability and limitation of these equations are discussed. The emphasis of the review is to seek a unified understanding and approach to the effect of particle shape at finite concentration on the elastic moduli, thermal expansion coefficient, stress concentration factor, viscoelastic relaxation modulus and creep compliance of filled polymers. The effects of anisotropic particle shape on mechanical properties of polymeric composites are clearly illustrated. Attention is also drawn to the relationship between elastic moduli, thermal expansion, creep elongation and stress relaxation moduli.     

The effect of thermal ageing on carbon fibre-reinforced polyetheretherketone (PEEK)

The static and dynamic mechanical properties of carbon fibre-reinforced PEEK (APC-2) laminates subjected to long-term thermal ageing and cycling treatments have been studied using three-point bend flexure tests. Results are discussed with respect to morphological changes and degradation analysis. S/N curves were modelled using fatigue modulus degradation data. Ageing laminates at high temperatures, for long time periods, between the glass transition temperature, T _g, and the melting temperature, T _m, caused a significant reduction in mechanical properties. However, for short ageing periods, a crystal-perfection process occurs which enhanced the low stress level fatigue resistance of both laminate geometries.     

The effect of thermal ageing on carbon fibre-reinforced polyetheretherketone (PEEK)

The morphological properties of carbon fibre-reinforced PEEK (APC-2) laminates subjected to long-term thermal ageing and cycling treatments have been studied using differential scanning calorimetry and wide-angle X-ray diffraction techniques. This work sought provided a structural explanation for measured changes in mechanical properties. Annealing at 120 ° had no effect but significant changes in crystal content and crystal perfection occurred at 250 ° and 310 °C. At 250 °C, for short ageing periods, both crystal growth and perfection processes were observed. For longer ageing periods, there was a multiple increase in capital perfection. Initial ageing at 310 °C caused rapid crystal growth, but further ageing resulted in a reduction in measured crystal contents due to thermal degradation.     

The effect of resin/crosslinker ratio on the mechanical properties and fungal deterioration of a maxillofacial silicone elastomer

Variation of the crosslinker/resin ratio of a room temperature condensation cure maxillofacial silicone elastomer has caused considerable changes in the mechanical properties and deterioration by Candida albicans. Increasing the crosslinker/resin ratio caused a decrease in the tensile strength and stiffness of the elastomer. However, tear strength appeared to show an optimum value at the recommended crosslinker/resin ratio. These effects were due to the low molar mass silicone polymer that acts as a carrier for the actual crosslinking additive. The general decrease in mechanical properties was accompanied by an increase in the hexane extractables content and an increase in the Si–H content of the elastomer. The unbound polymer (extractable material) content of the elastomer was found to influence the colonization of the material by C. albicans. An increase in the unbound polymer content corresponded to an increasing number of hyphae and blastospores observed penetrating into the elastomer. The data obtained in this study have significant implications concerning the degree of control of elastomer formulation and the deterioration of maxillofacial appliances.     

The effect of accelerated ageing on performance properties of addition type silicone biomaterials

The UV-protection provided to addition type silicone elastomers by various colorants, such as conventional dry earth pigments, as well as the so called “functional or reactive” pigments, was investigated. Moreover, the effect of a UV light absorber and a silica filler was also explored. Under the experimental parameters of this work, the exposure of silicone to UV radiation resulted in some changes of the IR absorbance, thermal decomposition after 400°C, T_g and tensile properties, whereas the storage modulus of samples was not affected. The obtained spectroscopic data, as well as the results of TGA and storage modulus, were interpreted by assuming that chain scission takes place during aging, whereas the improvement of tensile strength allows the hypothesis of a post-curing process, initiated by UV radiation. Therefore, the increase of T_g could partly be due to the above reason and, furthermore, to the contribution of a rearrangement of chain fragments within the free volume of the elastomeric material. Regarding the evaluation of various coloring agents used in this work, the obtained results show that dry pigments are more sensitive to accelerated ageing conditions in comparison with functional liquid pigments. Moreover, the hydrophobic character of silicone matrix is enhanced, with the addition of this type pigments because of the vinyl functional silanes groups present in their chemical structure. Finally, it should be noted that the incorporation of silica nanofiller did not seem to prevent the silicone elastomer from degradation upon UV irradiation, but showed a significant reinforcing effect.     

The effect of rotating cylinder on the heat transfer in a square cavity filled with porous medium

The heat transfer in a square cavity filled with clear fluid or porous medium is numerically investigated in the present study. To change the heat transfer in the cavity a rotating circular cylinder is placed at the centre of the cavity. The ratio of cylinder diameter to cavity height is chosen as 0.8. Depending on the angular velocity of the cylinder the convection phenomena inside the cavity becomes natural, mixed, and forced. To keep the number of data low the Grashof number, Gr, is set to 10⁶, while the parameter defining the convection regime in the cavity, Gr/Re², is changing from 0.0625 to 10². The Darcy number in the cavity is set to 10⁻², 10⁻³, and 10⁻⁴. Galerkin finite element method is used to solve the Navier–Stokes equations with Brinkman–Forcheimer extended Darcy’s law, and energy equation in 2-D non-dimensional form. The solution methodology is compared and validated with the literature for a similar problem, and good agreement is achieved. The results are presented in terms of Nusselt numbers, velocity profiles and temperature contours. The results show that rotation is more effective in the forced convection regime than in mixed and natural convection regimes, and at high spin velocities the heat transfer is almost independent from the Darcy number.