Fabrication of nonextruded recycled rubber–polyethylene composites

This study was conducted to investigate the possibility of one-stage molding process skipping compounding extrusion for the fabrication of cross-linked polyethylene (PE)-ground tire rubber (GTR) composites. The process resulted in a wide range of composites with various properties. Response surface methodology technique based on central composite design was employed with variables: polyethylene content (PE: % per polymer fraction), dicumyl peroxide (DC: % per polymer fraction), molding residence time (RT: min), and filler content (F: % per total mass). A quadratic model was able to significantly describe tensile strength, elongation at break (EB), and impact resistance/energy of the composites as a function of PE, DC, RT, and F. Tensile strength (TS) was positively affected by PE, DC, and RT; however, it was negatively affected by the filler content. Tensile EB and impact resistance of the composites were improved by DC and RT, while reduced by PE and filler increment. Composites with TS, ultimate elongation, and impact resistance of 11.5 MPa, 140%, 244 MJ/m², respectively, were obtained under optimized conditions. The nonextrusion molding process is recommended for the fabrication of PE-GTR composites due to the higher stiffness/tensile modulus and a slightly lower strength of nonextruded composites compared to the extruded composites.     

Resistivity–temperature characteristics of filler-dispersed polymer composites

Composites containing carbon nano tube (CNT) or carbon black (CB) conductive particle filler have the special characteristics of positive-temperature-coefficient (PTC) effects of resistivity. We quantitatively studied the relationship between poly(vinylidene fluoride) (PVDF) polymer's thermal volume expansion and the PTC effects of PVDF/CNT and PVDF/CB. The equation to revise filler content at each temperature due to the considerable thermal volume expansion rate of PVDF polymer indicates that filler content decreased with rising temperature. The graphs of filler content at room temperature plotted against apparent filler content with PTC effect were linear and their slopes were constant. From these graphs, we can determine the filler content necessary to occurring PTC effects. For example, the CNT content was 89% at room temperature, and the CB content was 93%. To our knowledge, this study is the first to report such phenomena.     

Production and investigation of structure and properties of polyethylene–polylactide composites

Under conditions of shear deformations, low-density polyethylene (LDPE) and polylactide (PLA) composites are obtained in rotor disperser. The production of these composites allows one to use polymers derived from natural raw and to reduce the cost of the materials on their base. The addition of rigid PLA leads to increase in elastic modulus from 200 for LDPE to 1190 for LDPE–PLA (50:50 wt %) composites and in tensile strength from 13.3 for LDPE to 17.8 for LDPE–PLA. By differential scanning calorimetry method, it is shown that LDPE and PLA are incompatible. Using X-ray diffraction analysis, it is found that degree of crystallinity of composites decreases from 46.1 at 50:50 wt % to 36.9 at 80:20 wt % component ratios with the rise in LDPE content. Tests on fungus resistance show that the composites containing 50 wt % PLA are more resistant than the composites containing 30 wt % PLA. First by gel-permeation chromatography method, it is shown that composite degradation after exposure in soil is accompanied by the PLA chain scission and depolymerization with formation of monomers and dimers (M _w of PLA decreases from 118,860 to 80,100). The obtained composites can be applied as packaging materials. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47598.     

Aluminum oxide particles/silicon carbide whiskers’ synergistic effect on thermal conductivity of high-density polyethylene composites

Aluminum oxide (Al₂O₃) particles and silicon carbide (SiC) whiskers improved the thermal conductivity of high-density polyethylene (HDPE). To improve the dispersion of inorganic fillers in the matrix, 5 wt% of maleic anhydride-modified polyethylene was added into HDPE as a compatibilizer, and the hybrid matrix was denoted as mHDPE. The thermal conductivity, heat resistance, and tensile properties of resulting HDPE composites were characterized. The results showed that the thermal conductivity reached its maximum value of 0.8876 W/(m K) at 1/4 weight ratio of Al₂O₃/SiC, which was 110.3, 54.8, and 8.8% higher than that of pure HDPE, mHDPE/Al₂O₃, and mHDPE/SiC composites, in the order given, indicating that hybrid fillers have synergistic effect on the thermal conductivity of HDPE composites. Moreover, they also have a synergistic effect on the heat resistance and Young’s modulus. As the SiC content increases, the heat resistance of the composites increases at first and then falls, and the maximum VST is reached at an Al₂O₃/SiC weight ratio of 3/2, which is 5.4 °C higher than that of HDPE. The maximum Young’s modulus of the composites (1160 MPa) is obtained at an Al₂O₃/SiC weight ratio of 1/4, and the yield strength increases gradually as the SiC whiskers’ content increases.     

Electrical response of microcellular EPDM rubber composites: complex dielectric modulus formalism and current–voltage characteristics

Electrical response of conductive carbon black (Vulcan XC 72)-reinforced microcellular EPDM rubber composites has been studied as a function of variation in blowing agent and filler loading in the frequency range of 10–10⁵ Hz. The data was analyzed by dielectric modulus formalism. The examined system exhibit a strong dependence of dielectric modulus on the applied frequency. A gradual increase of real part of dielectric modulus with frequency is observed for all fillers and blowing agent loadings. The imaginary part of the dielectric modulus exhibited one relaxation peak with frequency at each filler and blowing agent loading. With increase in filler loading the peak shifts toward higher frequency whereas, with blowing agent loading the relaxation peak shifts toward lower frequency. The relationship between real and imaginary part of dielectric modulus shows a semicircular trend followed by a linear increase for all filler and blowing agent loadings. Hence, the presence of non-Debye type of relaxations has been confirmed. The effect of variations in filler and blowing agent loading on current–voltage characteristics has also been investigated. It is observed that with increase in filler and blowing agent loading, the nonlinearity of the curves increases and the point from which this nonlinearity starts decreases to lower voltage values. It is also observed that the electrical current is free from time when the measuring voltage is low. But as the applied voltage increase to 30 and 40 V, the electrical current changes with time.     

Influence of forming conditions on characteristics of alumina–PSZ composites

Abstract

The purpose of the work reported in the present paper was to establish the correlation between the physical, mechanical, and microstructural properties of alumina matrix composites reinforced with (CeO₂, Nd₂ O₃, Y₂O₃ )–PSZ (partially stabilised zirconia) depending on the processing and thermal treatment conditions. The composites obtained from fine powder mixtures were formed by hydraulic pressing, ceramic injection moulding, and hot pressing under various temperature and pressure conditions. The samples were fired at 1550–1770°C in an oxidising atmosphere and in vacuum depending on the forming conditions. Comparative microstructure investigations were made by TEM on sample surfaces. The XRD results were in accordance with the determined properties of the investigated compositions. The results highlighted that the best physical and mechanical properties and homogenous microstructure for the ZTA composites were obtained by firing in vacuum.     

Effect of nanosized reinforcement particles on mechanical properties of high density polyethylene–hydroxyapatite composites

Abstract

The effect of nanoscaled hydroxyapatite (HA) filler particles on the mechanical properties of the high density polyethylene–hydroxyapatite (HDPE–HA) composite samples has been investigated. Nanosized HA particles with an average size in the range of 40–50 nm were synthesised by mechanical milling method. The composite samples with various amounts of nanoscaled HA particles were produced by mixing the ceramic and high density polyethylene particles using a single screw extrusion system. The results of the mechanical testing on the composite samples showed an increase in the fracture strength and the young's modulus values with increasing volume fraction of HA content in the composite samples. At the same time, there were decreases in both the fracture strain and toughness values with increasing volume fraction of the ceramic filler particles. In addition the comparison of the results obtained in this study with the mechanical properties of the commercially available composite samples (HAPEX) shows that similar mechanical properties can be reached at a much lower ceramic content, if nanoscaled HA particles are used in the fabrication of these composite biomaterials.     

A commercially viable compatibilizer system for wood–polyethylene composites

A commercially viable compatibilizer system containing poly(diphenylmethane diisocyanate) (PMDI) and stearic acid was developed for improving the interfacial adhesion between wood and polyethylene (PE). The treatment of PE with PMDI before mixing with wood increased both the modulus of rupture (MOR) and the modulus of elasticity (MOE) of the resulting wood–PE composites. Addition of stearic acid at certain dosages further increased MOR of the resulting composites. The PMDI–stearic acid compatibilizer system was more effective in increasing both the MOR and MOE of the resulting wood–PE composites than maleic anhydride-grafted polyethylene (MAPE), a commonly used commercial compatibilizer. The compatibilization mechanisms for the PMDI–stearic acid system were investigated by Fourier transform infrared (FT-IR) spectroscopy. The water-resistance test revealed that the composites with the PMDI–stearic acid system were statistically more water-resistant than those with MAPE.     

Elastomeric conductive composites based on conducting polymer–modified carbon black

Thermally stable elastomeric composites were prepared via melt processing from poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) and conducting polymer-modified carbon black (CPMCB) additives. CPMCB additives represent a novel thermally stable conductive compound made via “in-situ” deposition of polyaniline or polypyrrole on carbon black particles. Incorporating CPMCB is advantageous to the melt processing of composites, as it reduces the melt viscosity in comparison to the use of pure carbon black. Thermogravimetric analyses (TGA) showed that the composites are thermally stable with no appreciable degradation at temperatures as high as 300°C. In addition, the electrical conductivity of the composites was found to be very stable at high temperatures. Polym. Compos. 25:617–621, 2004. © 2004 Society of Plastics Engineers.     

Tuning the interphase adhesion in high-density polyethylene–silica nanocomposites with ionic liquids

This work addresses the use of ionic liquids (ILs) as multifunctional additives in the formation of high-density polyethylene (HDPE)/sol–gel silica nanocomposites via the melt-mixing process. Different approaches for nanocomposite formation were studied and compared, including (1) silica modification with ILs during the sol–gel process and further addition into the polymer matrix, (2) reactive mixing in which silica-IL filler was formed in situ, and (3) direct IL application in the melt-mixing chamber with nonmodified silica xerogel and HDPE. The nanocomposites were characterized by thermogravimetric analyses, differential scanning calorimetry, scanning electron microscopy, dynamic thermomechanical analyses, transmission electron microscopy, and contact angle analyses. To improve the silica compatibilization with HDPE, we investigated imidazolium ILs that presented at least one nonpolar ionic counterpart. This permitted control of the silica structure, morphology, dispersion, and interfacial interactions, providing enhancements in thermomechanical properties. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47366.     

Synthesis and characterization of conducting polymer composites based on polyaniline–polyethylene glycol–zinc sulfide system

Hybrid semiconducting polymer composites containing polyaniline, polyethylene glycol and zinc sulfide have been prepared in various combinations by in-situ polymerization of aniline using ammonium per disulfate in acidic medium. A biomimetic approach of controlled precipitation has been used. A mechanism of formation of these hybrid materials has been suggested in which polyethylene glycol works as a medium for diffusion-limited growth of various components during their precipitation. These materials have been characterized by a variety of spectroscopic techniques, differential scanning calorimetry, X-ray diffraction, scanning electron microscopy and alternating current impedance spectroscopy. Equivalent circuits for different contributions from grain, grain boundary and electrode for different systems have been determined with the help of complex non-linear least square analysis software. The microstructure-property correlation have been discussed along with the possible conduction mechanisms from the temperature dependence of conductivity as variable-dimension variable-range hopping for different compositions of single, double and triple composite materials.     

Development of spark plasma sintered conductive SiC–TiB2 composites for electrical discharge machining applications

Highly dense electrically conductive silicon carbide (SiC)–(0, 10, 20, and 30 vol%) titanium boride (TiB₂) composites with 10 vol% of Y₂O₃–AlN additives were fabricated at a relatively low temperature of 1800°C by spark plasma sintering in nitrogen atmosphere. Phase analysis of sintered composites reveals suppressed β→α phase transformation due to low sintering temperature, nitride additives, and nitrogen sintering atmosphere. With increase in TiB₂ content, hardness increased from 20.6 to 23.7 GPa and fracture toughness increased from 3.6 to 5.5 MPa m^1/2. The electrical conductivity increased to a remarkable 2.72 × 10³ (Ω cm)^–1 for SiC–30 vol% TiB₂ composites due to large amount of conductive reinforcement, additive composition, and sintering in nitrogen atmosphere. The successful electrical discharge machining illustrates potential of the sintered SiC–TiB₂ composites toward extending the application regime of conventional SiC-based ceramics.     

Highly conductive coatings of carbon black/silica composites obtained by a sol–gel process

Conductive submicronic coatings of carbon black (CB)/silica composites have been prepared by a sol–gel process and deposited by spray-coating on glazed porcelain tiles. Stable CB dispersions with surfactant were rheologically characterized to determine the optimum CB-surfactant ratio. The composites were analyzed by Differential Thermal and Thermogravimetric Analysis and Hg-Porosimetry. Thin coatings were thermally treated in the temperature range of 300–500 °C in air atmosphere. The microstructure of the coatings was determined by scanning electron microscopy and the structure evaluated by confocal Raman spectroscopy. The electrical characterization of the samples was carried out using dc intensity–voltage curves. The coatings exhibit good adhesion, high density and homogeneous distribution of the conductive filler (CB) in the insulate matrix (silica) that protects against the thermal degradation of the CB nanoparticles during the sintering process. As consequence, the composite coatings show the lowest resistivity values for CB-based films reported in the literature, with values of ～7 × 10^?5 Ωm.     

Effect of blend ratio on the dynamic mechanical and thermal degradation behavior of polymer–polymer composites from low density polyethylene and polyethylene terephthalate

Microfibrillar polymer–polymer composites (MFCs) based on low-density polyethylene (LDPE) and polyethylene terephthalate (PET) were prepared by cold drawing-isotropization technique. The weight percentage of PET was varied from 5 to 45 %. Microfibrils with uniform diameter distribution were obtained at 15 to 25 wt% of PET as evident from the scanning electron microscopy (SEM) results. Dynamic mechanical properties such as storage modulus (E′), loss modulus (E″) damping behavior (tan δ) were examined as a function of blend composition. The E′ values were found to be increasing up to 25 wt% of PET. An effort was made to model the storage modulus and damping characteristics of the MFCs using the classical equations used for short-fiber reinforced composites. The presence of PET microfibrils influenced the damping characteristics of the composite. The peak height at the β-transitions of loss modulus was lower for MFCs with 25 % PET, showing that they had superior damping characteristics. This phenomenon could be correlated with the PET microfibrils morphology. The thermal degradation characteristics of LDPE, neat blends and microfibrillar blends (MFBs) were compared. The determination of activation energy for thermal degradation was carried out using the Horowitz and Metzger method. The activation energy for thermal degradation of microfibrillar blends was found to be higher than that for the corresponding neat blends and MFCs. The long PET microfibrils present in MFBs could prevent the degradation and enhance the activation energy.     

Microstructure,interfacial characteristics,and wear performances of Cu–Fe–SiC cermet composites

The Cu–Fe metal-based ceramic grinding wheel material with SiC as abrasive was prepared by the powder metallurgy process of ball milling and hot pressing sintering. Cu–Fe–SiC cermets with Cu:Fe mass ratios of 4:1, 1:1, and 1:4 were designed by changing the composition of metal binder. The phase composition, microstructure, mechanical properties, and grinding properties of Cu–Fe–SiC cermets were systematically studied. The effect of Cu–Fe binder ratio on the microstructure and properties of cermets was analyzed. The results show that with the increase of Fe content, the density and hardness of cermets increase gradually, indicating that the mechanical properties are improved. Because the Fe in the adhesive can react with the abrasive SiC to form the reaction bonding interface, the Cu–80Fe–SiC cermets with higher Fe content have better adherence. The grinding test results of Cu–80Fe–SiC cermet show that the friction coefficient is .341, the surface roughness is 6.64 μm, the residual stresses parallel to the grinding direction are 353.3 MPa, and the residual stresses perpendicular to the grinding direction are 140.9 MPa. With the increase of Fe content, the wear mechanism changes from ploughing and cutting to friction.     

The role of electrical current mode in calcium carbonate deposition on conductive carbon nanofiber–epoxy coating

Calcium carbonate is one of the most common scaling minerals. In this paper we have used different electrical current modes (direct current [DC], pulsed DC, and alternating current [AC]) to control the amount, morphology, and distribution of calcium carbonate deposit on electroconductive epoxy/carbon nanofiber (CNF) coating. The effect of different current modes on surface scaling was visualized using scanning electron microscopy. It has been shown that both AC and DC anodic polarization limited scale deposition on epoxy/CNF coated surfaces, although the mechanisms of scale inhibition during AC and DC polarization were different. DC polarization of the coating at +2 V resulted in the smallest scale buildup without leading to coating degradation, while DC polarization at potentials as high as +5 V caused the coating to degrade. Interestingly, application of pulsed DC with high pulse frequency (50 Hz) inhibited the degradation. The type of current applied affected also the morphology of the precipitate at the cathode. The results presented in this work show, for the first time, how different modes of electrical current applied to electroconductive composite coatings can be used to control the morphology and distribution of calcium carbonate scale, and how the organic coating degradation at high polarization potentials can be avoided.     

Powder characteristics,sintering behavior and microstructure of sol–gel derived ZTA composites

A series of alumina/zirconia composites of varying compositions of zirconia were prepared through the sol–gel technique. Precursors were calcined at different temperatures ranging from 300 to 1400°C and sintered at 1530°C for 3 h. Compacts made from the powder calcined at 950°C yielded density up to >99% of theoretical density by pressureless sintering. Pore size distribution and the densification behavior were explained with respect to calcination temperature. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia grains in the alumina matrix. It is also observed that the faceted intergranular zirconia grains are at the grain junctions and the corners of the alumina matrix.     

Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites

ZrB₂–SiC–graphite composites with 0–35 vol% graphite flakes were densified via hot-pressing route at the temperature of 1800 °C under the uniaxial pressure of 40 MPa for 1 h. Consolidation, mechanical properties, and microstructure of hot-pressed composites were investigated by variation of graphite content. By the addition of graphite, the relative density of composites increased, and at this hot pressing condition, fully densified composites were fabricated. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% graphite, while the maximum Vickers hardness resulted in 2.5 vol% graphite doped one, and its value was equal to 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the Zr₃C₂ and B₄C phases besides the main existing ZrB₂, SiC, and graphite phases. The newly carbide phases formed at the surface of ZrB₂ grains. The addition of graphite into the ZrB₂–SiC composites improved the sintering process and caused a fine-grained microstructure.     

Mullite–molybdenum composites fabricated by pulse electric current sintering technique

Pulse electric current sintering of monolithic mullite and mullite/0–100 vol.% Mo composites was performed in vacuum of 4.5×10⁻⁵ Torr at temperatures and pressures of 1500 °C and 20 MPa, respectively. No traces of additional phases were observed by SEM and XRD for these composites. Microstructural observations reveal that Mo (molybdenum) particles dispersed uniformly at lower Mo contents and exhibited flaky and elongated structure at higher content. Simultaneous increase of fracture strength and toughness occurred with increase in Mo content. It attained a maximum of 1.1 GPa and 9.2 MPa m^1/2, respectively for 90 vol.% Mo composites. The increase in flexural strength is due to smaller initial flaws in mullite/Mo composites for lower Mo contents and due to plastic deformation of Mo phase for higher Mo contents. Similarly, frontal process zone toughening and crack bridging are expected to be the responsible mechanisms for enhanced toughness in these composites. Partial debonding in the mullite–Mo interface, giving rise to plastic deformation of Mo phase also contributes in the increase of toughness values.     

Oxidation behavior of electrically conductive α/β SiAlON composites with segregated network of TiCN

The oxidation behavior of novel electrically conductive α/β SiAlON composites with a continuous network of 2.5–10 vol% TiCN particulates was investigated. Composites, produced by coating spray dried granules with nano TiCN particles by a simple blending method, were gas pressure sintered at 1990 °C for 1 h under 10 MPa N₂ pressure. Oxidation tests were carried out between 800 °C and 1200 °C in air for 2 and 48 h in atmosphere of dry air. Below 1000 °C, the formation of TiO₂ crystals on the surfaces of TiCN particles was observed. Before the glass transition temperature of intergranular phase (T_g < 1000 °C), it was revealed that oxidation is controlled by the diffusion of oxygen into pre-formed TiO₂ particles. Above T_g, liquid glass dissolves the intergranular phase elements such as Ti, Y, and Si at the interface between TiCN and SiAlON particles. Migration of Ti towards the (opening point of the TiCN network) surface was found to be the main reason for the formation of subsurface porosity that slows down Ti diffusion through the surface. Moreover, it was detected that at high temperatures surface porosity filled by the intergranular glassy phase. Consequently, the oxidation rate was found to be decreased due to the slower oxygen diffusion.