Effect of recycling on rheological and mechanical properties of poly(lactic acid)/polystyrene polymer blend

A continued increase in the use of plastics has led to an increasing amount of plastics ending up in the waste stream; and the increasing cost of landfill disposal and public interest in support of recycling has meant that plastics recycling must increase. In this work, the effect of multiple extrusion and injection of poly(lactic acid)/polystyrene polymer blend (PLA/PS) on its rheological and mechanical properties is presented. Rheological properties were studied using a capillary rheometer, apparent shear rate (γ _a), apparent shear stress (τ _a), apparent viscosity (η _a), and flow activation energy were determined. The mechanical properties of the blend were investigated on dog bone-shaped samples obtained by injection molding, tensile tests were performed, stress at break, strain at break, and Young’s modulus were determined. The results showed that the apparent viscosity of PLA/PS blend decreases monotonously with increasing the processing number. Also it was found that stress and strain at break of the blend decrease sharply after two processing cycles, whereas the processing number has a little effect on Young’s modulus.     

How the work of adhesion affects the mechanical properties of silica-filled polymer composites

An equation correlating work of adhesion (W _a) with Young's modulus and tensile strength of silica-filled polymer composites is derived. It shows that the logarithms of Young's modulus and tensile strength are inversely proportional to W _a. Fourier transform infrared (FT. i.r.) results of the composites show that the silica interphase thickness increases with increased W _a ^h values (the hydrogen bond component of W _a). The logarithmic correlation between interphase thickness and W _a is similar to that found for both Young's modulus and tensile strength. These similarities suggest that W _a can be used to quantify interfacial bonding. Our study further shows that the composite with the lowest W _a value follows the Guth-Smallwood equation for predicting Young's modulus. However, as the interphase layer becomes thicker (increased W _a value), Young's modulus increases more than predicted from the Guth-Smallwood equation. Thus, an extension of the Guth-Smallwood equation is introduced to account for the effect of W _a on the Young's modulus value.     

Viscoelastic properties of hybrid copolymers based on methacryloxypropyl-grafted nanosilica and methyl methacrylate

The linear viscoelastic behavior of “model” hybrid materials based on methyl methacrylate and methacryloxypropyl-grafted nanosilica was investigated. As unique features, the materials under study present an excellent dispersion of silica within the polymer matrix and are almost free of uncross-linked chains. In addition, very progressive changes in network architecture are available, resulting from changes in particle diameter, d, volume fraction of filler, Φ, number of methacryloyl units grafted per surface unit of silica particle, n, and nature of the grafting agent. The influence of these parameters on the characteristics of the mechanically active relaxations α and β was examined. Emphasis was put on the storage modulus, E′, on the loss modulus, E′′, and on their dependence on filler volume fraction. E′′ values were shown to simply account for the reduction of the mechanical energy lost within the material, in connection to the occurrence of polymer molecular motions. Analysis of E′ variations as a function of Φ was based on the theoretical models available in the literature to account for the contribution of the spherical filler particles. In the glassy state, Kerner’s and Christensen and Lo’s models yielded comparable results. In the rubbery state, Guth and Gold’s model was shown to prevail on Kerner’s model.     

Static and dynamic mechanical properties of poly(vinyl chloride) loaded with aluminum oxide nanopowder

A series of nanocomposites from poly(vinyl chloride) loaded with different concentrations of Al₂O₃ nanopowder was prepared. The tensile mechanical properties of these composites were studied at different temperatures namely; stress–strain curves. The elastic modulus was calculated and found to decrease with increasing both filler loading and temperature. The strain at a certain stress at different temperatures was studied and the thermal activation energy for polymer chains was calculated. The complex viscosity as well as the storage modulus was found to decrease with increasing the filler loadings at different frequencies. The relaxation time of the polymer matrix was calculated and found to independent on the concentration of the filler but it decreased linearly with increasing frequency. The glass transition temperature was found to increase with increasing both filler loading and frequency.     

Rheological characterization of polypropylene filled with magnesium hydroxide

Consideration is given to the dynamic viscoelastic and shear flow properties of magnesium hydroxide-filled polypropylene, at a filler concentration of 60 wt%. Five variants of magnesium hydroxide were used, one surface-treated with magnesium stearate. The results reported illustrate the effects of filler particle size, morphology and surface coating on the rheology of the composites. The presence of magnesium hydroxide caused a significant increase in the shear viscosity of polypropylene relative to unfilled polymer, although this was much less pronounced using surface-treated filler, particularly at low shear rates. Complex viscosity and storage modulus data, obtained at very low shear rates (0.002 s^–1), demonstrated the presence of a critical shear yield stress for flow to occur, which was greatest for compositions containing uncoated fillers with small particle size. These observations are discussed in terms of structure formation between the particles. Results obtained from capillary and dynamic measurements of melt flow were found to follow the Cox-Merz rule.     

Transient shear flow of model lithium lubricating greases

This paper deals with the analysis of the transient shear flow behavior of lithium lubricating greases differing in soap concentration and base oil viscosity. The shear-induced evolution of grease microstructure has been studied by means of stress-growth experiments. With this aim, different lubricating grease formulations were manufactured by modifying the concentration of lithium 12-hydroxystearate and the viscosity of the base oil, according to a RSM statistical design. Moreover, atomic force microscopy (AFM) observations were carried out. The transient stress response can be successfully described by the generalized Leider-Bird model based on two exponential terms. Different rheological parameters, related to both the elastic response and the structural breakdown of greases, have been analysed. In this sense, it has been found that the elastic properties of lithium lubricating greases were highly influenced by soap concentration and oil viscosity. The stress overshoot, τ _max, depends linearly on both variables in the whole shear rate range studied, although the effect of base oil viscosity on this parameter is opposite at low and high shear rates. Special attention has been given to the first part of the stress-growth curve. In this sense, it can be deduced that the “yielding” energy density not only depends on grease composition, but also on shear rate. Moreover, an interesting asymptotic tendency has been found for both the “yielding” energy density and the stress overshoot by increasing shear rate. The asymptotic values of these parameters have been correlated to the friction coefficient obtained in a ball-disc tribometer.     

Effect of particle size ratio and volume fraction on shear strength of binary granular mixture

This paper deals with the mechanical properties of a binary granular mixture: a mixture of large and small frictional particles. The binary mixture is characterized by the particle size ratio (α = D _L /D _S ≥ 1), where D _L and D _S denote the diameter of large and small particles, and the volume fraction of the small particles W _S . In order to evaluate the shear strength of such a system, a transition range (W_S^a ￡ W_S ￡ W_S^b{W_{S}^{a} \le W_{S} \le W_{S}^{b}}), where W_S^a{W_{S}^{a}} and W_S^b{W_{S}^{b}} are minimum and maximum W _S values of the range, respectively, is defined as the range in which the interaction between the small and the large particles cannot be negligible. Then a simplified packing structure model is proposed to estimate W_S^a{W_{S}^{a}} and W_S^b{W_{S}^{b}} with respect to α. A series of 2D Discrete Element simulation and physical experiment proved that the proposed method can successfully describe the shear strength transition of the densely packed granular material both in 2D and 3D. As a general trend, it also turns out that the contribution of the small particles cannot be negligible even in their small content, and the contribution of large particles disappears when their average spacing with respect to the small particle size is around 2 both in the simulation and the experiment.     

Some electrical properties of the defectiveγ phases of spinel structure during their transformation to the rhombohedral α phases

During the transformation of “cubic” iron sesquioxides, γ-Fe₂O₃, substituted by divalent or trivalent ions, to rhombohedral α phases, the electrical conductivity yields discontinuities and a change in plots of logσ againstf(1/T). The average temperature of these discontinuities is influenced by the particle size, and extent of oxidation and substitution. For divalent substituted defective spinels, the activation energy for the haematite precipitation appears to depend on the extent of substitution and the size of the particle.     

Nonequilibrium Casimir force on a particle in cold vacuum background near a heated plate

General expressions for a nonequilibrium Casimir force in a stationary situation for the “small particle-plate” system with arbitrary local dielectric properties of materials are obtained for the given particle (T ₁) and plate (T ₂) temperatures and a “cold” (T ₃ = 0) vacuum background over the plate.     

Rheological and dynamic mechanical properties of polymer-bonded magnets based on Sm2Co17 and polyamide-12

The rheological and dynamic mechanical properties of polymer-based composites of Sm₂Co₁₇ and polyamide-12 with different particle loadings, sizes, and surface treatments are reported. Sm₂Co₁₇ particles were surface-treated with three different silanes: 3-glycidoxy(propyl)trimethoxysilane, 3-amino(propyl)trimethoxysilane (APTMS), and methyltrimethoxysilane (MTMS). It was shown, for the composites with untreated particles, that the viscosity and storage modulus increased with increasing filler content (0–60 vol%) and decreasing filler particle size. In addition, the glass transition temperature increased significantly and the damping decreased with increasing filler content. Of the silanes, the MTMS, which yielded only a thin surface layer, had in general the least effect on the rheological properties of the composite. The composite containing the APTMS-coated filler showed the highest storage modulus. The results give new insights on how to prepare polymer-bonded magnets with optimal process conditions (rheology) and dynamic mechanical properties, by varying the amount of particles, their size, and surface treatment.     

Rheological behaviors of fumed silica filled polydimethylsiloxane suspensions

Rheological behaviors of fumed silica filled hydroxylated polydimethylsiloxane suspensions were investigated in both static and dynamic shear modes. Both viscosity and modulus increase with filler’s concentration and specific surface area, however, they decrease with the improved dispersion and proper surface modification. In addition to the effective volume effect of filler’s excluded volume and polymer–filler interaction, the polymer-mediated filler–filler interaction contributes significantly. Such an interaction was classified according to the particle distance, and the concept of “inter-particle excess energy” was proposed. A combination of effective volume effect and inter-particle excess energy can be used to interpret the rheological behaviors of the nanocomposites.     

The role of friction in mixing and segregation of granular material

We use discrete element modelling to investigate the processes of mixing and size segregation in a polydisperse mixture of spherical particles in a three-dimensional rectangular box and analyze the influence of friction between the particles on segregation. The packed bed is stirred by a rectangular bar moving periodically in the horizontal direction. The parameters were introduced to characterise the segregation and mixing intensities, and a differential equation was proposed to describe the evolution of segregation intensity approaching exponentially a certain steady state value. It was found that the dynamic friction coefficient has a non-monotonous influence on the processes of mixing and size segregation in poly-disperse granular systems. Critical value of the dynamic friction coefficient μ_crit was identified. For the values of friction μ > μ_crit, behaviour of granular material can be characterised as a “laminar” flow with dominating convective motion of packed bed. For values of friction μ < μ_crit, behaviour of granular mattter can be characterised as “turbulent” flow with dominating “local” mixing inside the packed bed.     

Analysis of the influence of crushing on the behavior of granular materials under shear

The behavior of granular materials mainly depends on the mechanical and engineering properties of particles in its structural matrix. Crushing or breakage of granular materials under compression or shear occurs when the energy available is sufficient to overcome the resistance of the material. Relatively little systematic research has been conducted regarding how to evaluate or quantify particle crushing and how it effects the engineering properties of the granular materials. The aim of this study is to investigate the effect of crushing on the bulk behavior of granular materials by using manufactured granular materials (MGM) rather than using a naturally occurring cohesionless granular material. MGM allow changing only one particle parameter, namely the “crushing strength”. Four different categories of MGM (with different crushing strength) are used to study the effect on the bulk shear strength, stiffness modulus, friction and dilatancy angle “engineering properties”. A substantial influence on the stress–strain behavior and engineering properties of granular materials is observed. Higher confining stress causes some non-uniformity (strong variations/jumps) in volumetric strain and a constant volumetric strain is not always observed under large shear deformations due to crushing, i.e. there is no critical state with flow regime (with constant volumetric strain).     

A unified model of initiation and growth of fatigue macrocracks. part 3. stage of growth of a macrocrack

From a common viewpoint, fatigue fracture of materials is simulated by the process of initiation of an initial macrorack of lengtha _i =d ^* (d ^* is the constant of the material), which is successively (stepwise) repeated at the stage of its growth. As a result, the diagram “range of local stresses-period of initiation of an initial macrocrack” or ”range of local strains-period of initiation of an initial macrocrack,” which was determined for notched specimens, can be used for the construction of the diagram “growth rate of a macrocrack-range of the stress intensity factor” or “growth rate of a macrocrack-range of local strains,” respectively, if the crack is presented as a sharp notch with effective rounded radius ρ_eff=d ^* of the tip. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 3, pp. 5–14, May–June, 1999.     

Peculiarities of mechanical response of heavily filled polypropylene composites

Temperature dependences of dynamic moduli of polypropylene filled with flame retardant Mg(OH)₂ or with CaCO₃ were investigated. The filler content varied from O to 50 vol% and both storage (G) and loss (G) shear moduli were measured from + 50 to + 300 °C at frequencies 1 and 90 Hz. The influence of the filler particle shape was dominant for the origin of the physical network. Chemical adhesion aid was superfluous in composites with subcritical particle size. Two step decrease of G and two peaks on G temperature dependences were observed above the critical filler concentration. The explanation of this phenomenon was based on the proposal that polypropylene immobilized on the filler surface creates a new continuous phase, while primary polypropylene exists only in separated domains. Destruction of the physical network was explained using results of thermoanalysis, providing the catalytic effect of Mg(OH)₂ on the thermal oxidation of polypropylene in the interlayer above 240 °C.     

Electron-beam enhanced fusion of binary metals and alloying in melts

Binary mixing and alloying during electron-beam fusion in bi-metal joints have been investigated and presented in this paper. In case of beam-power densities greater than about 10¹⁰ Wm⁻², rapid fusion of metals is accompanied by violent agitation of the melts. The resulting mass transport in the melt is described here from an enhanced mass diffusion coefficientD =D _s(W′/a ₀ η)^1/2, whereD _s is mass diffusivity in solid,W′ is beam-power density,a ₀ is acceleration in melt growth andη is the coefficient of viscosity. Moreover, a uniformly mixed central-widthw′ is predicted in the fusion zone (total widthw), where,w′ is significantly dependent onη andw throughW′. For these conditions, the continuity of mass transport in stationary and travel modes of the source has been solved using line and plane source models and conservation principles. The predictions from these calculations have been validated through quantitative analyses of experiments.     

Review of the role of the interphase in the control of composite performance on micro- and nano-length scales

In fiber reinforced composites (FRCs), exhibiting heterogeneous structure at multiple length scales, the interphase phenomena at various length scales were shown to be of pivotal importance for the control of the performance and reliability of such structures. Various models based on continuum mechanics were used to describe effects of the macro- and meso-scale interphase on the mechanical response of laminates and large FRC parts, satisfactorilly. At the micro-scale, the interphase is considered a 3D continuum with ascribed average properties. Number of continuum mechanics models was derived over the last 50 years to describe the stress transfer between matrix and individual fiber with realtively good success. In these models, the interphase was characterized by some average shear strength, τ _a, and elastic modulus, E _a. On the other hand, models for tranforming the properties of the micro-scale interphase around individual fiber into the mechanical response of macroscopic multifiber composite have not been generally successfull. The anisotropy of these composite structures are the main reasons causing the failure of these models. The strong thickness dependence of the elastic modulus of the micro-scale interphase suggested the presence of its underlying sub-structure. On the nano-scale, the discrete molecular structure of the polymer has to be considered. The term interphase, originally introduced for continuum matter, has to be re-defined to include the discrete nature of the matter at this length scale. The segmental immobilization resulting in retarded reptation of chains caused by interactions with solid surface seems to be the primary phenomenon which can be used to re-define term interphase on the nano-scale. Thus, the Rubinstein reptation model and a simple percolation model were used to describe immobilization of chains near solid nano-particles and to explain the peculiarities in the viscoleastic response of nano-scale “interphase.” It has also been shown that below 5 nm, Bernoulli–Euler continuum elasticity becomes not valid and higher-order elasticity along with the proposed reptation dynamics approach can provide suitable means for bridging the gap in modeling the transition between the mechanics of continuum matter at the micro-scale and mechanics of discrete matter at the nano-scale.     

On laws governing the splitting failure of flat metallic specimens subjected to thermal shock

Computed and experimental data on the splitting failure of copper, nickel, titanium, molybdenum, brass, and bronze metallic foils from 0.005 to 1 mm thick under thermal shock initiated by the x-radiation of a nuclear explosion are presented. It is proposed that the concepts “average energy liberated over the thickness (mass) of the specimen” ε, “specific absorbed energy” W, and “splitting strength of the material” σ be used as criterial characteristics of failure thresholds of optically thin flat metallic specimens (foils). It is demonstrated that the critical average energy liberation ε_*, which results in splitting, decreases logarithmically (ε_* =A _*-B _*log Δ) with increasing thickness Δ of the irradiated specimens in the interval Δ≈0.001–1 mm, and the critical specific potential energy W_* reguired to effect splitting increases with increasing optical mass m of the specimen under the law W_*=−αmlog (βm), where A_*, B_*, α, and β are certain parameters. It is shown that the longevity of the copper, nickel, titanium, molybdenum, brass, and bronze under radiation-induced thermal shock decreases exponentially with increasing amplitude of the failing stress (splitting strength) and can be described on the basis of the kinematic concept of strength. Deceased. Translated from Problemy Prochnosti, No. 1, pp. 37–47, January–February, 1997.     

Development of a biocomposite based on green polyethylene biopolymer and eggshell

In this investigation a fully biobased composite material has been obtained using a biobased polyethylene obtained from sugar cane as matrix and eggshell (ES) as filler. ES was studied in order to replace mineral carbonate calcium as polymer filler, which is commonly used. In order to do this the ES has been chemically modified and then its potential for the development of a biocomposite was evaluated. The filler adhesion to the polymer matrix has been improved using titanate particle treatment which has been chosen between silane and zirconate. The use of titanate as coupling agent enlarges the range of operating temperatures and also improves the interfacial bonding as it is displayed in impact fracture surface. Mechanical, thermal and rheological analysis were carried out in order to analyze the effect of the modified ES loading percentage. Thermal analysis showed a proportional effect of the filler load over the degradation temperature and an inversely effect over the enthalpy. Effect of the modified ES content on mechanical properties of PE/ES was also studied. The results showed that the modified CaCO₃ can effectively improve the mechanical properties of bioPE, improving stiffness, hardness, flexural and tensile modulus. The amount of filler increases the viscosity, this fact specially hinders the processing processes which work with low shear rates.     

Dynamic moduli of CaCO3- and Mg(OH)2-filled polypropylene

Fillers suppressed the temperature dependence of storage modulus and caused the flattening of the temperature dependence of the loss modulus in the glass transition region of polypropylene (PP). The glass transition temperature (T _β) of PP did not change with filler content (v _f). This indicates that none of the fillers affect the mobility of PP in the bulk. A new loss maximum appeared at 50 °C forv _f>0.2. This maximum became more prominent when increasing either the filler content or filler specific surface area. Interparticle interactions, leading to the space network of weakly bonded particles, affected PP mobility indirectly. The enhanced interfacial adhesion led to a further decrease of PP mobility nearT _β and to the increase of the new loss maximum at 50 °C.