Functionalized-graphene/ethylene vinyl acetate co-polymer composites for improved mechanical and thermal properties

The surface functionalization of graphene and the preparation of functionalized graphene/ethylene vinyl acetate co-polymer (EVA) composites by solution mixing are described. Octadecyl amine (ODA) was selected as a surface modifier for the preparation of functionalized graphene (ODA-G) in an aqueous medium. The ODA-G was characterized by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, which confirm the modification and reduction of graphite oxide to graphene. Atomic force microscopy shows that the average thickness of ODA-G is ca. 1.9 nm. The ODA-G/EVA composites were characterized by X-ray diffraction and transmission electron microscopy, which confirms the formation of ODA-G/EVA composites. Measurement of tensile properties shows that the tensile strength of the composites (with 1 wt.% ODA-G loading) is ∼74% higher as compared to pure EVA. Dynamic mechanical analysis shows that the storage modulus of the composites is much higher than that of pure EVA. The thermal stability of the composite with 8 wt.% of ODA-G is ∼42 °C higher than that of pure EVA. The electrical resistivity has also decreased in the composites with 8 wt.% of ODA-G.     

Effect of vinyl acetate content and electron beam irradiation on the flame retardancy, mechanical and thermal properties of intumescent flame retardant ethylene-vinyl acetate copolymer

Ethylene vinyl acetate copolymer (EVA) flame retarded by ammonium polyphosphate (APP) and pentaerythritol (PER) was cross-linked by electron beam irradiation. The effects of vinyl acetate content and electron beam irradiation on the flame retardancy, mechanical and thermal properties of EVA composites were investigated. The volatilized products of EVA/APP/PER composites were characterized by thermogravimetric analysis/infrared spectrometry. As VA content increased, the volatilized products increased in the second decomposition step, but decreased in the third decomposition step. For all samples, the increase of irradiation dose could improve both the gel content and the Limit Oxygen Index (LOI, the minimum oxygen concentration by volume for maintaining the burning of a material) values of irradiated composites. The mechanical and thermal properties of the irradiated EVA composites were also evidently improved at appropriate irradiation dose as compared with those of unirradiated EVA composites, whereas these properties decrease at higher irradiation dose because of the electron beam irradiation-induced oxidative degradation or chain scission.     

The rheological behavior of ethylene vinyl acetate copolymer/rectorite nanocomposites during the melt extrusion process

To optimize the preparation process for ethylene vinyl acetate (EVA)/rectorite nanocomposites during the melt extrusion, the effect of rectorite on the rheological property of molten polymer has been explored in this paper. The dispersion of rectorite in EVA was probed by X‐ray diffraction, and the rheological behaviors of EVA copolymer and EVA/rectorite nanocomposites during the extrusion process were investigated by means of HAAKE minilab. The positron results reveal that introducing the rectorite in EVA matrix increases the interfaces in composites. And the rheological results indicate that the viscosity of EVA and EVA/rectorite nanocomposites in the molten state was influenced by the processing temperature, processing time and shearing rate. For all the samples, the viscosity increases with increasing the shear rate, and decreases with increasing extrusion temperature. Moreover, compared with the pure EVA, the EVA/rectorite nanocomposite presents a lower viscosity at the same processing condition. Copyright © 2016 John Wiley & Sons, Ltd.     

Study on microstructure and mechanical properties relationship of short fibers/rubber foam composites

Research on short fibers/rubber foam composites is rarely found in the literature. In this paper, microcellular rubber foams unfilled (MF), strengthened by pretreated short fibers (MFPS) and untreated short fibers (MFUS) are prepared, respectively. The microstructure and mechanical properties of the three composites have been studied via scanning electron microscope (SEM) and mechanical testing, respectively. The SEM results show that both pretreated and untreated short fibers disperse uniformly in the composites and in bidimensional orientation. Moreover, the pretreated short fibers have much better adhesion with the rubber matrix than untreated ones. The experimental results also indicate that the introduction of short fibers is mainly responsible for the great enhancement of most mechanical properties of the microcellular rubber foams, and the good interfacial adhesion of the short fibers with the matrix contributes to the more extensive improvement in the mechanical properties. It is also found that the reinforcement effect of short fibers to compressive modulus strongly depends on the density of microcellular rubber foams, the orientation of short fiber and the deformation ratio. The compressive modulus of microcellular rubber foams at the normalized density less than 0.70 and beyond 0.70 is predicted by the modified Simple Blending Model and the Halpin-Kerner Model, respectively. The theoretically predicted values are in good accordance with the experimental results.     

Effect of thermal history on the mechanical properties of three polypropylene impact-copolymers

The effect of thermal history on static mechanical properties and impact fracture behavior of three reactor polypropylene impact-copolymers (ICPPs) was investigated for three ICPPs prepared using commercial Innovene^®, Unipol^® and Spheripol^® polymerization technologies. Multiple extrusion employing a co-rotating twin-screw extruder resulted in a significant reduction of the molecular weight of the PP homopolymer phase evidenced by the increasing melt flow index (MFI). Neither cross-linking of the ethylene-propylene rubber (EPR) phase nor EPR particle coarsening was detected for any of the ICPPs after 5 consecutive extrusions. Decreasing molecular weight of the PP homopolymer phase caused change in the crystalline morphology of injection molded specimens due to the change in crystallization kinetics and reduction of the number of tie molecules, however, the overall degree of crystallinity did not change, significantly. The static tensile mechanical properties (E, σ_y, ?_b), critical strain energy release rate, G_c, and the Charpy notched impact strength, a_k, decreased with increasing MFI in a monotonous manner for all the ICPPs investigated. Despite significant differences between the absolute values of the mechanical properties for the three ICPPs, the MFI dependence of the σ_y and G_c relative to that for the unaffected ICPP fell on a single master curve for all of them. High-speed digital camera, used to follow the fracture process during the instrumented impact test, revealed no significant change of the small scale yielding fracture process with increasing MFI. This was in an agreement with the negligible change in the size of the crack tip plastic zone, R_p, predicted using simple mixed mode fracture model. The plane strain value of the critical strain energy release rate, G_1c, calculated from the measured G_c for the INN (2.4 kJ/m²), UNI (2.8 kJ/m²) and SPH (3.5 kJ/m²) using a simple LEFM model did not exhibit significant dependence on the number of extruder passes. The observed differences between the three ICPPs were ascribed to the significantly larger EPR content in UNI compared to the other two ICPPs and significantly larger content of isotactic PP homopolymer in the INN compared to the remaining two ICPPs.     

Polystyrene prepared by reactive extrusion: kinetics and effect of processing parameters

The conversion and residence time were investigated during the bulk polymerization of styrene in a twin screw extruder. It was found that polymerization mainly occurred in the zone between 400 and 1000 mm along the screw axis in the extruder, corresponding to the residence time of the reactants ranging from 1 to 4 min in the extruder. Furthermore, the processing conditions (feed rate, screw rotation rate) and average molecular weight of the polymer have a great effect on the residence time. Based on dimensionless analysis, a model of the residence time has been built‐up, which has been confirmed by the results of realistic measurements. A kinetic model of the polymerization has also been established under the assumption that the screw extruder can be regarded as an ideal plug flow reactor. Copyright © 2004 John Wiley & Sons, Ltd.     

Fracture morphology and mechanical properties of ethylene/vinyl acetate rubber vulcanizates reinforced by in situ prepared sodium methacrylate

Ethylene/vinyl acetate rubber (EVM) was reinforced by sodium methacrylate (NaMAA) that was in situ prepared through the neutralization of sodium hydroxide and methacrylic acid in EVM during mixing. The mechanical properties of EVM vulcanizates with different NaMAA loadings and at different crosslink densities were studied and compared with those of high abrasion furnace carbon black (HAF) filled EVM vulcanizates. The fracture surfaces of gum and filled EVM vulcanizates were observed with scanning electron microscopy. The results showed that NaMAA‐reinforced EVM vulcanizates had better mechanical properties than HAF/EVM vulcanizates. When the NaMAA loading was 50 phr, the tensile strength of the NaMAA/EVM vulcanizate was 30 MPa, the tear strength was 102 kN/m, and the elongation at break was over 400%. Fourier transform infrared analysis confirmed that NaMAA formed in the compounding process and underwent polymerization during vulcanization. Scanning probe microscopy analysis revealed that nanoscale particles dispersed in the NaMAA/EVM vulcanizates. The mechanical properties were correlated with the fracture morphology of all the vulcanizates. The tensile rupture of NaMAA‐filled EVM vulcanizates occurred through tearing from a crack in the bulk of the samples. Tear deviation occurred with the addition of NaMAA and resulted in a rough surface, leading to an improvement in the tear strength of NaMAA‐filled EVM vulcanizates. The micrographs of the tear surfaces of the vulcanizates indicated that the different fracture modes depended on the NaMAA loading and the crosslink density. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1715–1724, 2004     

Influence of fillers on mechanical properties of ATH filled EPDM during ageing by gamma irradiation

The presence of a significant content of fillers accelerates the degradation of ATH filled EPDM subjected to gamma irradiation at room temperature. Above the melting temperature of the EPDM, this induces a decrease in the apparent mechanical reinforcement of the fillers. This also promotes de-cohesion mechanisms which leads to an increase in the strain at break with irradiation dose. It is not clear whether the use of a filler treatment attenuates this accelerating effect or not; however, part of this treatment remains efficient at a high dose and seems also to delay but not suppress the occurrence of de-cohesion mechanisms at large strain. Moreover, at room temperature, i.e. below the melting temperature, all the consequences of ageing by gamma irradiation are strongly attenuated by the presence of a semi-crystalline microstructure, the morphology of which is not too strongly modified by irradiation.     

Morphological and mechanical properties of carbon nanotube/polymer composites via melt compounding

The mechanical properties and morphology of multiwall carbon nanotube (MWNT)/polypropylene (PP) nanocomposites were studied as a function of nanotube orientation and concentration. Through melt mixing followed by melt drawing, using a twin screw mini‐extruder with a specially designed winding apparatus, the dispersion and orientation of MWNTs was optimized in PP. Tensile tests showed a 32% increase in toughness for a 0.25 wt % MWNT in PP (over pure PP). Moreover, modulus increased by 138% with 0.25 wt % MWNTs. Transmission electron microscopy and scanning electron microscopy demonstrated qualitative nanotube dispersion and orientation. Wide angle X‐ray diffraction was used to study crystal morphology and orientation by calculating the Herman's orientation factor for the composites as function of nanotube loading and orientation. The addition of nanotubes to oriented samples causes the crystalline morphology to shift from α and mesophase to only α phase. Furthermore, the addition of nanotubes (without orientation) was found to cause isotropization of the PP crystal, and drawing was shown to improve crystal orientation through the orientation factor. In addition, differential scanning caloriometry qualitatively revealed little change in overall crystallinity. In conclusion, this work has shown that melt mixing coupled with melt drawing has yielded MWNT/PP composites with a unique combination of strength and toughness suitable for advanced fiber applications, such as smart fibers and high‐performance fabrics. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 864–878, 2006     

Molecular structure and properties of cellulose acetate chemically modified with caprolactone

Cellulose acetate (CA) with a degree of substitution of 1.7 was modified with caprolactone (CL) under various reaction conditions in an internal mixer. Processing temperature changed from 120 to 220 °C, while reaction time varied between 5 and 45 min. The composition and structure of the polymer was analyzed by various methods including FTIR, MALDI-TOF and NMR spectroscopy and its mechanical characteristics were determined by dynamic mechanical analysis and tensile testing. The results indicate that homopolymerization occurs under relatively mild conditions, while grafting requires higher temperatures and longer times. Grafted polycaprolactone (gPCL) chains are attached mainly to positions 2 and 6 of the glucose ring and their length increases with increasing reaction time and temperature, but the chains are always much shorter than those obtained in solution polymerization. Changes in the degree of substitution during grafting are small indicating that homopolymerization proceeds easier than grafting. Grafting seems to be easier in cellulose acetate with a larger degree of substitution in spite of the smaller number of active -OH groups present. Internal plasticization is more efficient than the external plasticizing effect of monomeric caprolactone. Plasticization results in a decrease of stiffness and strength, but deformability increases only slightly.     

Compatibilisation effect of PP-g-MA copolymer on iPP/SiO2 nanocomposites prepared by melt mixing

In the present study, a series of iPP/SiO₂ nanocomposites, containing 1, 2.5, 5, 7.5, 10 and 15 wt% SiO₂ nanoparticles, were prepared by melt mixing in a twin screw co-rotating extruder. Poly(propylene-g-maleic anhydride) copolymer (PP-g-MA) containing 0.6 wt% maleic anhydride content was added to all nanocomposites at three different concentrations, 1, 2.5 and 5 wt%, based on silica content. Mechanical properties such as tensile strength at break and Young’s modulus were found to increase and to be mainly affected by the content of silica nanoparticles as well as by the copolymer content. For the tensile strength at break as well as for yield point, a maximum was observed, corresponding to the samples containing 2.5-5 wt% SiO₂. At higher concentrations, large nanosilica agglomerates are formed that have as a result a decrease in tensile strength. Young’s modulus increases almost linearly on the addition of SiO₂, and takes values up to 60% higher than that of neat iPP. Higher concentrations of PP-g-MA resulted in a further enhancement of mechanical properties due to silica agglomerate reduction. This finding was verified from SEM and TEM micrographs. Evidently the surface silica hydroxyl groups of SiO₂ nanoparticles react with maleic anhydride groups of PP-g-MA and lead to a finer dispersion of individual SiO₂ nanoparticles in the iPP matrix. The enhanced adhesion in the interface of the two materials, as a result of the mentioned reaction, has been studied and proved by using several equations. The increased Vicat point of all nanocomposites, by increasing the PP-g-MA content, can also be mentioned as a positive effect.     

The role of microstructure on the mechanical properties of polyurethane foams containing thermoregulating microcapsules

Rigid polyurethane foams with up to 50 wt% of microcapsules from LDPE-EVA containing Rubitherm^®RT27 were synthesized. The influence of microcapsules on the foams density, microstructure and mechanical resistance was studied. Cell size and strut and wall thicknesses were analyzed by SEM. The relationships between densities and foam microstructures with their Young's moduli and collapse stress were found by the Gibson and Ashby formulations and the Kerner equation for mechanical properties of composites. It was found a cell structure change from polyhedral closed-cells to spherical or amorphous open-cells. A good agreement between the experimental and theoretical data was observed but requiring a cell form factor. Thus, Fitting parameters confirmed the high trend of these microcapsules to be incorporated into the foam cell walls and the form factors depicted the abrupt change of cell morphology. Thus, these equations are suitable for predicting the mechanical properties of foams containing fillers of low mechanical resistance.     

Effect of organoclay on thermal and dynamic mechanical properties of novel thermoplastic polyurethane nanocomposites prepared by melt intercalation technique

Polymer nanocomposites based on thermoplastic polyurethane (TPU) containing organophilic montmorillonite (OMMT) were prepared by melt compounding method followed by compression molding. Different percentage of organically modified nanoclays (1, 3, 5, 7, and 9 wt%) was incorporated into the TPU matrix in order to examine the influence of the nanofillers on nanophase morphology and materials' properties. The microstructure morphology of the nanocomposites was examined by transmission electron microscopy (TEM), energy dispersion X‐ray analysis (EDX), wide angle X‐ray diffraction (WAXD), and atomic force microscope (AFM). The observation established that the organoclay is homogeneously dispersed and preferentially embedded in the TPU soft segment phase. Significant enhancement in the thermal stability of the nanocomposites was observed with the addition of the OMMT under thermogravimetric analysis (TGA). Dynamic mechanical properties of the TPU nanocomposites were analyzed using a dynamic mechanical thermal analyzer (DMTA), which confirms that the addition of OMMT has a strong influence on the storage and loss modulus of the TPU matrix. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of melt temperature on the phase morphology, thermal behavior and mechanical properties of injection-molded PP/LLDPE blends

<正>The phase morphology and thermal behavior of various isotactic polypropylene(PP)/linear low density polyethylene(LLDPE) blends were investigated with aid of scanning electron microscopy(SEM) and differential scanning calorimetry(DSC),respectively.The effect of barrel(melt) temperature on the morphology,thermal behavior and the resultant mechanical properties of the injection molded bars was the research focus,and the influence of LLDPE composition was also taken into account.It was found that the mechanical properties,especially the tensile ductility and the impact strength,were greatly affected by the processing temperature.The samples obtained at low temperatures had the highest elongation at break and impact strength,while those molded at high temperatures had the poorest toughness.Two reasons were responsible for that:first,the phase size in the samples increased with the processing temperature;second, possible orientation existed in the samples obtained at low processing temperatures.     

Morphological,rheological, crystalline and mechanical properties of ethylene-vinyl acetate copolymer/linear low-density polyethylene/amphiphilic graphene oxide nanocomposites

Chemical modification of graphene oxide has become a popular method for imparting unique properties to extend its application. Here, we show a simple way to synthesize amphiphilic graphene oxide (AGO) by grafting quaternary ammonium salt onto GO sheets. The AGO sheets not only showed high thermal stability and good dispersion in many polar and non-polar solvents in comparison to GO sheets but also the chemical modification maintained the two-dimensional structure. As a result, the AGO sheets improve the interfacial interaction between ethylene-vinyl acetate copolymer (EVA) and linear low-density polyethylene (LLDPE). Because of the large size of AGO, the location of AGO is very dependent on the mixing strategy. The AGO was dispersed in the EVA phase when AGO was mixed first with EVA and then with LLDPE, whereas it was confined in the LLDPE phase when AGO was mixed first with LLDPE and then with EVA. AGO sheets were found at the interface of LLDPE and EVA when AGO, EVA, and LLDPE were mixed together, suggesting that AGO has a high interfacial interaction with both LLDPE and EVA. These high interfacial interactions lead to high tensile strength, Young's modulus, complex viscosity and crystallization temperature in comparison to the EVA/LLDPE blends without AGO sheets.     

Influence of silica on shape memory effect and mechanical properties of polyurethane-silica hybrids

Polyurethane block copolymer (PU) was synthesized and was followed by a sol-gel reaction with tetraethoxysilane (TEOS) to prepare high performance polyurethane-silica hybrids with shape memory function. Their tensile and shape memory properties were compared as a function of TEOS content and PU hard segment content. A tensile test showed that the mechanical properties were largely influenced by TEOS content, and the maximum elongation-at-break as well as maximum breaking stress and modulus were obtained when TEOS at 10 wt% was used. Shape memory of hybrids was also obtained from a thermomechanical test, and showed good shape retention and shape recovery of more than 80% for all samples. Consequently, by silica hybridization, an improvement in the mechanical properties and shape recovery force of PU could be achieved without any decrease in their shape recovery effect.     

Study on mechanical properties of elastomers reinforced by zinc dimethacrylate

In this work, mechanical properties of various elastomers with the representative structural features reinforced by zinc dimethacrylate (ZDMA) were studied. The results showed that there is great difference in reinforcing effects of ZDMA for different elastomers. Strain-stress curves revealed that the tensile-induced crystallization of chains might be the main reason for high strength of ZDMA/rubber composites at room temperature. The saturation and regularity of rubber chains are two essential features to determine the mechanical properties of the composites at room temperature. The kind of ZDMA makes considerable impact on performances of ZDMA/rubber composites, except for ZDMA/hydrogenated nitrile-butadiene rubber (HNBR) composite. The high temperature strengths of ZDMA/rubber composites are not high and are dominated by the polarity and saturation of matrix. Basing on the previous researching, a possible model for microstructure of ZDMA/rubber composites was put forward, which could well interpret the observed phenomenon.     

Effect of long fiber thermoplastic extrusion process on fiber dispersion and mechanical properties of viscose fiber/polypropylene composites

Viscose fiber reinforced polypropylene (PP/VF) composites were manufactured using long fiber thermoplastic (LFT) extrusion techniques with two different methods namely LFT‐l and LFT‐2. The compatibilizer [maleated polypropylene (MAPP)] and dispersing agent [stearic acid (SA)] were added to the PP/VF in order to improve the fiber dispersion and interfacial adhesion. The PP/VF composites manufactured using LFT‐2 showed better fiber dispersion with higher tensile and flexural properties compared to the composites manufactured using LFT‐1 method. Similarly, the impact strength and toughness of the LET‐2 composites showed an improvement of 36 and 20% than LFT‐1 whereas the average fiber length of composites was decreased from 6.9 mm to 4.4 mm because of the increase in shear energy as a result of residence time. Further, the addition of SA and MAPP to LFT‐2 process has significantly improved the fiber dispersion and mechanical performance. The fiber dispersion and fracture behavior of the LFT‐1 and LFT‐2 composites were studied using scanning electron microscopy analysis. The Fourier transformation infrared spectra were also studied to ascertain the existence of type of interfacial bonds. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of crystal polymorphism on mechanical and barrier properties of poly(l-lactic acid)

The effect of crystal polymorphism on barrier and mechanical properties of PLLA is detailed in this contribution. PLLA films containing different amounts of α and α′ crystal forms were prepared by annealing quenched PLLA at different temperatures. The polymorphic structure of the films was analyzed by X-ray diffraction. Mechanical properties and permeability to water vapor were investigated as a function of degree of crystallinity (w_C) and related to crystal polymorphism developed during annealing. The polymorphic structure of PLLA significantly affects mechanical and barrier properties. The α crystal modification provides a better barrier to water vapor and a higher Young’s modulus, compared to films containing the α′ modification, but a lower elongation at break. The varied barrier and mechanical properties were correlated to the different packing of PLLA chains in the two analyzed polymorphs. The conformational disorder of the α′ form makes this structure a mesophase (condis crystal), with remarkable effects on material properties.     

Micellization and morphological characterization of Ag-micelles prepared by poly(vinyl acetate)–silver nitrate in solvent/nonsolvent system

The inducing method for preparing Ag-micelle solution with the use of mixed solvent/nonsolvent, and the morphological characterization of the generated metal–micelles were investigated and reported in this paper. In this method, an Ag containing metal chelate polymer (MCP) raw solution was preprepared by dissolving poly(vinyl acetate) (PVAc)–silver nitrate (AgNO₃) MCP in conc. formic acid, and a mixed solvent of HCOOH/H₂O with specific water composition was then added to induce the micellization of the MCP chain. The critical water concentration (CWC) that was needed for inducing the formation of the Ag-micelles, and the water concentration at which the flocculation of the Ag-micelles occurred in micellar solution, were studied by measuring the transmittance of the dilute MCP solution; the results showed that a long-lasting MCP solution with stable micelles might be prepared by using a H₂O/HCOOH solvent of specific weight ratio 1:1.2. The effect of the AgNO₃ concentration on the morphology of the Ag-micelles was also investigated by transmission electron microscopy (TEM). At AgNO₃ concentration below 0.5 wt%, the Ag-micelles displayed a variety of core-shell structure; but as the AgNO₃ concentration was increased to 1.0–2.0 wt%, micelles that had Ag-solid embedded in the micellar core were observed.