Multiscale Reinforcement of Polymers Combining Cellulosic Wood Pulp with Layered Silicate Nanoplatelets

Synergies resulting from the combination of discontinuous reinforcing elements at two different size scales are examined in two polymer types — rubbery matrices comprising acrylonitrile-co-butadiene (NBR) elastomer and a high density polyethylene plastic. The latter is derived from a recycled post-consumer waste stream that is upgraded by the reinforcement to compensate for any degradation experienced during prior use or impurities introduced during recycling. The two reinforcements are wood pulp at the microscale and exfoliating layered silicate clays at the nanoscale. Appropriate compatibilizing agents are employed to allow wetting of the reinforcement with the polymer matrix, promote dispersion and provide a strong interface. In general, the microscale elements provide mechanical strengthening in tension, while the nanoscale reinforcements enhance stiffening and reduce failure propagation by tearing. The use of natural reinforcements and recycled feedstocks imparts environmental acceptability to such formulations.     

Determination of the interface strength between two polymer materials by a new curved interface tensile test: Preliminary experimental results

Recently, the authors have proposed a new experimental method for the determination of adhesion strength between two different materials. A curved interface and special arrangement of materials is used for the tensile test of bimaterial specimens to avoid singular stress fields around corners and edges. The main advantage of the test consists in the fact that the strength is determined under conditions of a uniform tensile stress field normal to the interface in the region where debonding starts. The present paper presents experimental results for two bimaterial systems - PMMA/TPE and PC/TPE (two stiff standard polymers with a thermoplastic elastomer). The expected failure behaviour was observed during the experiments, thus enabling the estimation of adhesion strength by using calculated stress concentration factors. The influence of the radius of curvature is discussed in detail.     

Injection Molding of Wood–Fiber/Plastic Composite Foams

This paper investigates the feasibility of injection-molded wood–fiber/high-density polyethylene (HDPE) composite foams that can replace injection-molded HDPE solids in industrial applications. The study applies injection foam molding technology using a physical blowing agent to a wood–fiber/HDPE composite, and examines the effects of the processing parameters on the dimensional and mechanical properties and cell density of the composite foams. In addition, the physical properties and cost of wood–fiber/HDPE composite foams are compared with those of solid HDPE. The experimental results show that wood–fiber/HDPE composite foams that have a 20% weight reduction have superior physical properties, such as density, dimensional properties (68% decrease of shrinkage and 91% decrease of warpage) and mechanical properties (28% increase of Young's modulus). Furthermore, the cost analysis confirms that wood–fiber/HDPE composite foams are much less expensive (by 40%) than HDPE. Therefore, it is concluded that wood–fiber/HDPE composite foams are strong candidates for replacing current injection-molded HDPE products.     

Micromechanical Behavior of Carbon Nanotube-Covered SiC Fibers in Polymer Matrix Composites

The incorporation of nanotube-covered fibers in continuous fiber/epoxy composites has been shown to influence the mechanical, electrical, and thermal properties of the composite. Increased interlaminar shear stress, flexural strength and modulus have been reported in such composites over composites containing bare fibers. In this study, the microstructure and interfacial shear strength (ISS) of continuous silicon carbide fiber/epoxy composites with and without nanotubes grown from the SiC fiber surface were investigated with micro-Raman spectroscopy (MRS) and microscopy. The fibers with nanotubes grown from the surface were found to have a reduced ISS compared with the bare fibers. Electron microscopy showed good wetting of epoxy in the nanotube forests, but poor attachment of the nanotube forests to the fibers. These results suggest that the mechanism leading to improvements in bulk composite properties is not due to an improvement in the fiber/matrix ISS.     

Dielectric breakdown phenomena in Al2O3 filled DGEBA/MDA/SN composite system

Dielectric breakdown phenomena by electrical treeing deterioration was investigated in the new epoxy resin system DGEBA/MDA/SN filled with Al₂O₃. As the filler content increased, the maximum electric field at breakdown increased and then decreased with increase of defects such as voids, impurities and delamination or peeling between filler and matrix. As the electrode separation increased, the breakdown voltage increased, but the breakdown strength decreased and then saturated to 17 kV/mm. Also, the maximum electric field at the tip when the system was failed increased. The electrical tree initiated from the side of the needle electrode was not from the tip where the reinforced field is the highest. The electrical tree was blocked by the filler. The interface condition of filler and polymer matrix played an important role in the electrical treeing resistance. The final breakdown phenomena showed fan-type crack as observed in the non-filled system.     

In situ observation of interfacial debonding process induced by matrix crack propagation in two-dimensional unidirectional model composites

Interfacial debonding behavior is studied for unidirectional fiber reinforced composites from both experimental and analytical viewpoints. A new type of two-dimensional unidirectional model composite is prepared using 10 boron fibers and transparent epoxy resin with two levels of interfacial strength. In situ observation of the internal mesoscopic fracture process is carried out using the single edge notched specimen under static loading. The matrix crack propagation, the interfacial debonding growth and the interaction between them are directly observed in detail. As a result, the interfacial debonding is clearly accelerated in specimens with weakly bonded fibers in comparison with those with strongly bonded fibers. Secondary, three-dimensional finite element analysis is carried out in order to reproduce the interfacial debonding behavior. The experimentally observed relation between the mesoscopic fracture process and the applied load is given as the boundary condition. We successfully evaluate the mode II interfacial debonding toughness and the effect of interfacial frictional shear stress on the apparent mode II energy release rate separately by employing the present model composite in combination with the finite element analysis. The true mode II interfacial debonding toughness for weaker interface is about 0.4 times as high as that for a stronger interface. The effect of the interfacial frictional shear stress on the apparent mode II energy release rate for the weak interface is about 0.07 times as high as that for the strong interface. The interfacial frictional shear stress and the coefficient of friction for weak interface are calculated as 0.25 and 0.4 times as high as those for strong interface, respectively.     

Inorganic-organic interpenetrating frameworks: oxi-hydroxide iron (III) inside poly-4-vinylpyridine-methacrylic acid-N,N'-methylenebisacrylamide. Part I: Synthesis and macrostructure

A carboxylic gel-type resin, poly-4-vinylpyridine-methacrylic acid-N, N'-methylenebisacrylamide is designed and synthesized in order to be able to incorporate both Fe^III and, potentially, also Au^III in its polymer framework. Along with the gradual substitution of H⁺ with Na⁺, of Na⁺ with Fe³⁺/3 and after a very controlled and gradual precipitation of Fe^III as oxihydroxides induced by the hydrolysis of urea directly inside the nanoporous domains of the metalated resin particles, an organic-inorganic composite is obtained. The material is characterized with TGA, ESEM, XRMA and Mössbauer spectroscopy.     

Interfacial adhesion of cellulose fiber and natural fiber filled polypropylene compounds and their effects on rheological and mechanical properties

Rayon fiber (RN) and pine wood fiber (PW) filled polypropylene (PP) compounds, PP/RN (90/05 and 75/25 wt%) and PP/PW (90/05, 75/25 and 50/50 wt%), are investigated for their interfacial adhesion, rheological properties, morphology, nucleation and mechanical properties. The interfacial adhesion of the RN-filled PP compounds is better than that of the PW ones. As the concentration of the RN and the PW particles is increased, the dynamic viscosity, the crystallization temperature, and the tensile modulus are increased; however, the tensile strain is decreased. The viscosity of the RN-filled compounds is higher than that of the PW ones at the same loadings. Significant differences are found in the elongation yield test. As the concentration of the particles is increased, the elongation yield stress of the RN compounds is increased. Elongation yield stress of the PW compounds is decreased and more spherulites are locally developed on the RN surface than the PW surface. The interfacial adhesion of the RN surface with PP is better than that of the PW surface. The elimination of extractives on the PW surface improves the mechanical property of the PW/PP compounds; however, it reduces processability of the PW/PP compounds.     

Shungite - a carbon-mineral filler for polymeric composite materials

Shungite rock material after fine grinding was investigated as a filler for polymeric composites. Shungite carbon structural and physico-chemical peculiarities predetermine shungite filler behaviour in polymeric matrixes. Specific interaction between the network of globules (the main structural units of shungite carbon of nanoscale size) and the network of mineral phase structural units in every shungite filler particle leads to the amphiphylic surface properties of the filler. Influence on rheological properties of different polymeric composite materials was demonstrated as an example of advanced potential possibilities of shungite filler. The metastable structure of shungite carbon and its reactivity seemed to be the key to understanding the observed effect.     

Resin–Sisal and Wood Flour Composites Made from Unsaturated Polyester Thermosets

Short fibers and wood flour were selected as fillers in the production of two types of unsaturated polyester composites (bisphenolic and isophthalic-based thermosets). Sisal fibers were subjected to washing in order to remove the organic coating on the fibers (which were originally prepared for cord manufacture) and to maleic anhydride (MAN) esterification. The effect of these treatments on the thermomechanical properties of the composites, as well as on the mechanical properties (flexural and compression) and water absorption was investigated. All the results are coincident in showing the improved interfacial adhesion obtained by washing and mainly by esterification of the fibers. Additionally, hybrid wood flour sisal composites were prepared and their mechanical properties compared to those of the one-filler composites. The hybrid composites showed improved modulus and maximum stress.