Mechanical and thermal properties of graphite platelet/epoxy composites

Anhydride-cured diglycidyl ether of bisphenol A (DGEBA) reinforced with 2.5-5% by weight graphite platelets was fabricated. The structural, mechanical, viscoelastic and thermal properties of these composites were studied and compared. XRD studies indicated that the processing of composites did not change the original d-spacing of pure graphite. Tensile property measurements of composites indicated higher elastic modulus and tensile strength with increasing concentration of graphite platelets. The storage modulus and glass transition temperatures (T_g) of the composites also increased with increasing platelet concentration, however, the coefficient of thermal expansion decreased with the addition of graphite platelets. The thermal stability was determined using thermogravimetric analysis. The composites showed higher thermal stability in comparison with pure epoxy and increased char concentration for higher graphite concentration. The effects of reinforcement on the damage mechanisms of these composites were investigated by scanning electron microscopy.     

The role of adhesion in the mechanical properties of filled polymer composites

Filled polymer composites have been prepared in which the energetics of the filler surfaces was systematically varied in order to investigate the dependence of the mechanical properties of the composite on the interfacial strength as predicted by the thermodynamic work of adhesion at the filler-matrix interface. A high-purity silica filler was used, treated with three different organofunctional silane coupling agents (two alkylsilanes and an aminosilane) to varying degrees from zero to complete coverage. The surface energetics of the modified fillers was characterized using both inverse gas chromatography (IGC) and dynamic contact angle analysis (DCA). While the surface energy assessments from IGC were higher than those obtained with wetting measurements, as expected, the trends with fractional coverage of silane were the same for each method, and were used to evaluate the thermodynamic work of adhesion. Highly filled polymer composites were prepared by dispersing the variously treated silica fillers into the amorphous thermoplastic matrix polymers: poly(methyl methacrylate) and poly(vinyl butyral). Specimens of the composites were tested mechanically to give the yield stress. The poly(methyl methacrylate) composites all failed cohesively in the matrix, unaffected by any of the filler surface treatments. The poly(vinyl butyral) composites, however, all displayed purely interfacial failure, with the yield stress strongly dependent on the type and extent of the filler surface treatment. While all three silanes were found to decrease the filler surface energy, and consequently the thermodynamic work of adhesion, with higher surface coverage, corresponding decreases in the yield stress were found only for the alkylsilanes. For the aminosilane, the measured yield stress was found to increase with surface coverage and therefore to decrease with the work of adhesion. The difference in behavior between the two types of coupling agent is explained in terms of acid-base effects.     

The effect of nanosilica on mechanical,thermal and morphological properties of epoxy coating

In this study, nanosilica of very high specific surface area is used as reinforcing filler for preparing an epoxy-based nanocomposite coating. For appropriate dispersion of nanoparticles in the polymer matrix, ultrasound waves were applied after mechanical mixing. The resulting perfect dispersion of nanosilica particles in epoxy coating revealed by transmission electron microscopy ensured the transparency of the nanocomposite. Nanoindentation was used to determine some mechanical properties such as hardness and elastic modulus. The obtained results show 26 and 21% increases in hardness and elastic modulus, respectively for resin filled with 5% nanosilica compared to neat epoxy. DMA results show that the glass transition temperature of samples is increased with increasing silica nanoparticles. The result of TGA shows significant improvement of the thermal decomposition temperature of epoxy coating containing 5% nanosilica compared to neat epoxy. Scanning electron microscopy (SEM) micrographs of fractured surfaces show increased roughness with nanosilica addition.     

The kinetics, thermal and mechanical properties of epoxy-polycarbonate blends cured with aromatic amine

PC-DER331 blends are transparent and homogeneous due to the transesterification taking place during hot melting process. This transesterification reaction does not occur or occurs insignificantly during the preparation of PC-DER332 blends. PC is separated from the PC-DER332 melt mixture by slow cooling to room temperature but the mixture remains a single phase by quenching with an ice bath. Two systems of PC-epoxy blends, PC-DER331 blends and PC-DER332 blends are cured by the m-phenylene diamine (MPDA) in a stoichiometric ratio. Curing kinetics have been carried out by differential scanning calorimeter (DSC). The presence of PC accelerates the curing reaction. Infrared spectroscopy (IR) indicates the occurrence of transesterification in the PC-DER331/MPDA blends during curing. The flexural modulus increases with the increase of the PC content while the notched Izod impact strength decreases with the increase of the PC content for all the blending systems. The fracture surfaces of the PC-DER331/MPDA blends are smooth, an indication of a homogeneous morphology. The fracture surfaces of the PC-DER332/MPDA blends are rough, an indication of a heterogeneous morphology.     

Magnetically processed carbon nanotube/epoxy nanocomposites: Morphology, thermal, and mechanical properties

The processing-structure-property relationships of multiwalled carbon nanotubes (MWNTs)/epoxy nanocomposites processed with a magnetic field have been studied. Samples were prepared by dispersing the nanotube in the epoxy and curing under an applied magnetic field. The nanocomposite morphology was characterized with Raman spectroscopy and wide angle X-ray scattering, and correlated with thermo-mechanical properties. The modulus parallel to the alignment direction, as measured by dynamic mechanical analysis, showed significant anisotropy, with a 72% increase over the neat resin, and a 24% increase over the sample tested perpendicular to the alignment direction. A modest enhancement in the coefficient of thermal expansion (CTE) parallel to the alignment direction was also observed. These enhancements were achieved even though the nanotubes were not fully aligned, as determined by Raman spectroscopy. The partial nanotube alignment is attributed to resin a gel time that is faster than the nanotube orientation dynamics. Thermal conductivity results are also presented.     

Effects of graphene oxide sheets-zirconia spheres nanohybrids on mechanical,thermal and tribological performances of epoxy composites

In this work, graphene oxide sheets-zirconia spheres (ZrO₂-rGO) nanohybrids were fabricated by Schiff base or Michael addition reaction. Their structure was characterized by FT-IR spectroscopy, UV–vis absorption spectra, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and atomic force microscope in detail. The reaction process of PDA-capping on rGO and APTES treatment on ZrO₂ nanoparticles were verified and it was proved that the ZrO₂ nanoparticles were successfully adhered onto the wrinkled surface of the graphene oxide. As a new multifunctional nanofillers, the ZrO₂-rGO nanohybrids were introduced into epoxy matrix and the mechanical, thermal properties and tribological performances of the fabricated composites were also detailedly investigated. Compared with the neat EP composites, the tensile strength and elongation at break of 0.1?wt% ZrO₂-rGO/EP are improved by 33% and 40%, respectively. Besides, the propagation of decomposition reactions in the composites could be impeded by anchoring ZrO₂ nanoparticles on the lamellar skeleton of graphene oxide. Furthermore, the lubricating effect and strong interfacial interaction contributed by the ZrO₂-rGO nanohybrids result in efficient load transfer from the matrix to the hybrids, which enables the ZrO₂-rGO/EP composites to have a fairly high wear resistance performance. This novel and effective approach using ZrO₂-rGO nanohybrids as multifunctional nanofillers could be beneficial to promote the development of high performance composites.     

Morphology, thermal relaxations and mechanical properties of layered silicate nanocomposites based upon high-functionality epoxy resins

This paper investigates the possibility of improving the mechanical properties of high-functionality epoxy resins through dispersion of octadecyl ammonium ion-modified layered silicates within the polymer matrix. The different resins used are bifunctional diglycidyl ether of bisphenol-A (DGEBA), trifunctional triglycidyl p-amino phenol (TGAP) and tetrafunctional tetraglycidyldiamino diphenylmethane (TGDDM). All resins are cured with diethyltoluene diamine (DETDA). The morphology of the final, cured material was probed by wide-angle X-ray scattering, as well as optical and atomic force microscopy. The α- and β-relaxation temperatures of the cured systems were determined using dynamic mechanical thermal analysis. It was found that the presence of organoclay steadily decreased both transition temperatures with increasing filler concentration. Further, the effect of different concentrations of the alkyl ammonium-modified layered silicate on the toughness and stiffness of the different epoxy resins was analyzed. All resin systems have shown improvement in both toughness and stiffness of the materials through the incorporation of layered silicates, despite the fact that it is often found that these two properties cannot be simultaneously achieved.     

Enhanced mechanical and thermal properties of polyurethane/functionalised graphene oxide composites by in situ polymerisation

ABSTRACT

Isocyanate-functionalised graphene (iGO) was prepared and incorporated into a thermoplastic polyurethane via an in situ polymerisation. Firstly, graphene oxide was successfully modified using a mixture of isocyanate- and diisocyanate-containing compounds, leading to the formation of good dispersions of resulting functional graphene oxide in organic solvents, such as N,N-dimethylacetamide and N,N-dimethylformamide. The addition of iGO into polyurethane matrix improved both mechanical and thermal properties in the polyurethane/iGO composites relative to neat polyurethane. An addition of only 0.03?wt-% of functionalised graphene into the polyurethane increased Young’s modulus by 1.4 times and tensile strength by two times. Meanwhile, the elongation at break was similar to that of the neat polymer. In addition, dynamic mechanical analysis also confirmed the improvement in storage modulus of the polymer composites especially at high-temperature range. We believe that the developed modification approach for graphene oxide and polyurethane/graphene composites presented herein could be useful in polymer/graphene composite development.     

Enhanced mechanical,thermal and adhesion properties of polysilsesquioxane spheres reinforced epoxy nanocomposite adhesives

ABSTRACT

Epoxy-based systems serve as excellent adhesives to join a wide range of substrates such as metal, ceramics, plastics, etc. The mechanical properties of such systems can be improved considerably by the addition of filler to the epoxy matrix. Herein, polymethylsilsesquioxane (PMS) and poly(methyl/vinyl)silsesquioxane (PMVS) nanosphere were synthesised by hydrolytic condensation of oraganosilane as a precursor in aqueous phase. The epoxy nanocomposite adhesives were prepared by adding different weight percentages (1–4 wt%) of the PS nanospheres. Tensile and compressive strength of the adhesive formulations were studied using the universal testing machine (UTM) and it was observed that the mechanical properties of the composites showed an increasing trend on increasing the filler loading. Adhesive strength of the epoxy composites on mild steel substrate was studied by conducting the lap shear test and EPV-4 exhibited a 31% increase in adhesive strength on the mild steel compared to the neat epoxy. Surface morphology of the epoxy composites were visualised from the SEM images and the composites also showed enhanced thermal conductivity. Higher mechanical and adhesive strength indicates the potential of the prepared nanocomposites to be used as an effective formulation in adhesive-based systems.     

Piezoelectric properties of polypeptide-PMMA molecular composites fabricated by contact charging

The most common piezoelectric materials (PM) in use today are ceramic crystals which are heavy and brittle despite high piezoelectricity. Polymer-based PM is an alternative to ceramic crystals but they have lower piezoelectric coupling constants which deteriorates quickly at high temperature. In an effort to develop non-brittle and light PM with stable piezoelectric properties, we explored fabrication of composite materials comprising piezoelectric α-helical poly(α-amino acids), poly(γ-benzyl α,l-glutamate) (PBLG) and matrix polymer, poly(methylmethacrylate) (PMMA). Thick composite disks were created by contact charging of PBLG-MMA solution mixture followed by curing the MMA matrix in a designed mold. Compared to our prior method of corona discharge, this new method allowed the application of predefined electrical fields to the PMMA solution with little MMA evaporation. This communication presents the fabrication and characterization of a series of PBLG-PMMA composite disks with various PBLG compositions prepared under different poling conditions. The results show for the first time that all PBLGs can be poled in the direction normal to the disk surface and that the poled PBLGs within the PMMA matrix are directly responsible for the piezoelectricity of the composite materials. The two-polymer composite system allows independent modulation of film’s mechanical properties and piezoelectricity at a molecular level.     

Performances of an epoxy-amine network after introducing the MWCNTs: rheology,thermal and electrical conductivity,mechanical properties

In order to compare the performance of the neat epoxy-amine network and the same network with dispersed multi-walled carbon nanotubes. The curing reaction, rheology, thermal and electrical conductivity, mechanical performance and microstructure of epoxy-amine systems were investigated in this study. The curing schedule of the epoxy-amine systems was formulated depends on the parameters of the curing reaction. The processing windows and processing time of epoxy-amine systems were provided by using rheological analysis. The thermal and electrical conductivity of epoxy-amine network with dispersed multi-walled carbon nanotubes were enhanced. The mechanical performance of the epoxy-amine network was improved after introducing the dispersed multi-walled carbon nanotubes.     

Miscibility, morphology, thermal, and mechanical properties of a DGEBA based epoxy resin toughened with a liquid rubber

Epoxy resin based on diglycidyl ether of bisphenol A and varying content of hydroxyl terminated polybutadiene were cured using an anhydride hardener. The ultimate aim of the study was to modify the brittle epoxy matrix by liquid rubber to improve the toughness characteristics. Chemorheological analysis of the modified network was performed to understand the physical transformations taking place during the cure polymerization reaction. The delay in gel time on inclusion of rubber can be explained by lower reactivity due to dilution and viscosity effect. Tensile, flexural, and fracture toughness behaviors of neat as well as modified networks have been studied to observe the effect of rubber modification. The morphological evolution of the toughened networks was examined by scanning electron microscope, and the observations were used effectively to explain the impact properties of the network having varying content of liquid rubber. Acoustic emission studies were performed on neat and certain modified systems. Based on acoustic emission results and morphological characteristics, toughening and failure mechanisms were discussed. The behavior of the relaxation peaks were evaluated by dynamic mechanical analysis and tried to explain the composition of networks. Thermal stabilities of the toughened epoxies were studied using thermogravimetric analysis (TGA). The activation energy for decomposition of neat and modified epoxies has been estimated and compared. The reduction in cross-linking density of the thermoset upon modification has been confirmed and explained.     

Dynamic mechanical and morphological properties of polycarbonate/multi-walled carbon nanotube composites

Dynamic mechanical and morphological properties of the polycarbonate (PC)/multi-walled carbon nanotube (MWNT) composites were studied by dynamic mechanical thermal analysis (DMTA) and X-ray diffractometry, respectively. For the without annealed PC/MWNT composites containing the higher content of the MWNT (≥7.0 wt%), double tan δ peaks were observed, which could be explained by the phase separation morphology model. For the annealed PC/MWNT composites, a broad single tan δ peak was observed. From the X-ray diffraction of the annealed PC/MWNT composites, it was observed that more regular structure of the PC was obtained, which was consistent with the result of the thermal analysis of the annealed PC/MWNT composites. From the dynamic mechanical properties, thermal analysis, and X-ray diffraction of the annealed PC/MWNT composites, it is suggested that PC/MWNT composites show a broad single tan δ peak and partially crystalline structure of the PC in the PC/MWNT composites by annealing.     

Effect of hybrid nanofillers on the thermal, mechanical, and physical properties of polypropylene composites

In this study, the effect of single and hybrid nanofillers on the thermal, mechanical, and physical properties of polypropylene composites were carried out. This nanocomposite was compounded using two-roll mill mixing method and the filler content was fixed at 4 vol % loading. The single filler used is synthetic diamond (SD), boron nitride (BN), and carbon nanotube (MWNT). The hybrid system was composed by addition of MWNT into single SD and BN. The prepared samples were characterized by thermal properties, tensile and flexural, and these results were supported by the morphology, void content, and melt flow index values. The result showed that the hybrid composite with combination of BN and SD with MWNT indicate higher thermal conductivity and thermal stability and lower thermal expansion. However, no significant improvements in tensile and flexural strengths were observed due to large formation of agglomeration as being captured by SEM micrographs. Furthermore, the existence of higher percent void content suggests low adhesion and poor compatibility between hybrid filler and matrix. This caused detrimental effect of strength of hybrid composites rather than single filler composites.     

Mechanical and fracture mechanical characterization of building materials used for external thermal insulation composite systems

Fracture mechanical investigations of building materials applied for external insulation composite systems have been performed in order to provide material data for the numerical simulation of mechanical failure. For this purpose a wedge splitting procedure according to Tschegg has been employed and modified for the investigation of material layers with a thickness of approx. 5 mm. The testing method proved to be suitable for investigation of both the materials themselves and the compound of two layers. Results of commercially available building materials for external insulation composite systems are shown. From their dependence on different preparation procedures, it may be concluded how temperature and humidity may affect material properties under conditions of actual service.     

Maleimide-epoxy resins: preparation, thermal properties, and flame retardance

Maleimide modified epoxy compounds were prepared through reacting N-(4-hydroxylphenyl)maleimide (HPM) with diglycidylether of bisphenol-A. Triphenylphosphine and methylethylketone were utilized in the reactions as a catalyst and a solvent, respectively. The resulting compounds possessed both oxirane ring and maleimide group. The kinetics of the curing reactions of the maleimide-epoxy compounds and amine curing agents, 4,4-diaminodipheylmethane (DDM) and dicyandiamide (DICY), were studied. Incorporation of maleimide groups into epoxy resins provided cyclic imide structure and high cross-linking density to the cured resins, to bring high glass transition temperatures (179 °C) and good thermal stability (above 380 °C) to the cured resins. High char yields in the thermogravimetric analysis and high limited oxygen index values (25.5-29.5) were also observed for the cured resins to impy their good flame retardance.     

Physical,mechanical, and thermal properties of polypropylene composites filled with walnut shell flour

Injection molded specimens were prepared from the walnut shell flour and polypropylene with and without maleic anhydride-grafted polypropylene at 40, 50, and 60% (weight) contents of the walnut shell. The bending and tensile modulus of the composites significantly increased with increasing the filler content while the bending and tensile strengths significantly decreased. Water absorption and thickness swelling of the composites increased with increasing filler content. The MAPP improved the interfacial adhesion between walnut shell flour and polymer matrix. A 40/57/3 formulation of the walnut shell flour/polypropylene/MAPP can be used in outdoor applications requiring a high dimensional stability.     

Effect of sub-micron silica fillers on the mechanical performances of epoxy-based composites

Solid state thermo-mechanical properties, as well as low and large strain mechanical behaviour, of epoxy composites filled with sub-micron pyrogenic silica are discussed in this paper. The reinforcement mechanisms involved are investigated. Two distinct series of pyrogenic silica were used: hydrophilic silica with various specific surface areas and silica grafted with various organo-modifications. Furthermore, two series of networks, having either a high or low crosslink density, and resulting thus either in glassy or rubbery materials at room temperature, were considered. Dynamic mechanical analysis, uniaxial tensile tests and fracture mechanic tests were performed.All our results showed that pyrogenic silica leads to an improvement of network mechanical properties both in the glassy and rubbery states. The simultaneous increase of stiffness and toughness was observed, demonstrating the great potential of pyrogenic silica for the reinforcement of thermosetting systems. This exceptional behaviour has been interpreted in terms of the interactions and morphology developed.     

Rheological,mechanical, thermal,and morphological properties of polypropylene/ethylene‐octene copolymer blends

Rheological and morphological studies were performed on polymer blends of ethylene‐octene copolymer [polyethylene elastomer (PEE)] and polypropylene (PP). The viscosities of PEE, PP, and PEE/PP blends were analyzed using an Instron capillary rheometer and a Rheometrics Dynamic Stress Rheometer, SR 200. A non‐Newtonian flow behavior was observed in all samples in the shear rate range from 27 to 2700 s⁻¹, whereas at shear rates in the range from 0.01 to 0.04 s⁻¹, a Newtonian flow behavior was verified. The scanning electron micrographs showed that dual‐phase continuity may occur between 50 and 60 (wt %) of PEE. This result is consistent with the Sperling's model. The mechanical analysis showed that PEE/PP, with 5 wt % of PEE, presented an increase on the mechanical properties and as the PEE content increased, a negative deviation in relation to an empirical equation was observed. Thermal analysis showed that there were no change in the crystallization behavior of the matrix when different elastomer contents were added. Dynamic mechanical thermal analysis showed that samples with low PEE contents presented only one peak, indicating a certain degree of miscibility between the components of these blends. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 692–704, 2000     

Assessment of morphological,thermal, and viscoelastic properties of epoxy vinyl ester coating composites: Role of glass flake and mixing method

In this work, glass flake (GF)/epoxy vinyl ester resin composites were fabricated with various compositions and mixing methods. The effect of GF on thermal and mechanical behavior of these composites was investigated using different techniques such as differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and thermogravimetric analysis (TGA). The results showed that the presence of GF in epoxy vinyl ester formulation could obviously affect the cure temperature, reaction enthalpy value, and degradation temperature. DMTA results also exhibited that the tan δ peak area decreased and storage modulus increased with increasing GF content and this effect seemed to be different depending on the initial epoxy vinyl ester compositions. The scanning electron microscopy (SEM) images showed that mixing method had a strong effect on the surface morphology, size, and distribution of glass flake. The effect of mixing method on properties of produced composite was also studied.