M.M. Reboredo

Modified woodflour as thermoset fillersPart I. Effect of the chemical modification and percentage of filler on the mechanical properties

Composites made from an unsaturated polyester/styrene thermoset matrix and esterified woodflour have been prepared and tested. Different degrees of esterification of the wood particles with maleic anhydride were obtained by using different times of reaction, which lead to materials with varied final properties. Water absorption performed on treated particles indicates that they are more hydrophobic than the untreated ones. Flexural, compression and dynamic-mechanical tests were performed on composites to find out an optimum level of chemical modification of the woodflour. Moreover, the relationship between the filler content and the composite final properties was also studied for a selected filler treatment. An important increment in particle dispersion was obtained by modifying the woodflour with maleic anhydride.

Modified woodflour as thermoset fillers: II. Thermal degradation of woodflours and composites

Abstract Composites made from an unsaturated polyester/styrene thermoset matrix and maleic anhydride esterified woodflour have been prepared and tested. Different degrees of esterification of the wood particles with maleic anhydride (MAN) were obtained by using different times of reaction, which led to materials with varied final properties. X-ray diffraction studies have been carried out on raw and chemically treated woodflours. The degree of crystallinity of modified woodflours is lower than that of the untreated one and decreases as the MAN content of the samples increases. The thermal degradation of the woodflours, matrix, and derived composites was estimated by thermogravimetry (TG) in nitrogen environment. Thermogravimetry was shown to be sensitive enough to detect differences between woodflour samples with different treatments and degrees of esterification and between composites with different woodflour contents.

Moisture diffusion in polyester-woodflour composites

Abstract The understanding of water–polymer interactions in polymeric composite materials is critical to the prediction of their behavior in applications where they are exposed to water or humid environment. Moisture diffusion in unsaturated polyester–woodflour composites exposed to environments kept at room temperature and different relative humidities was investigated. Equations obtained from microscopic mass balances for vapor diffusion in solids were used to determine the effective diffusion coefficients of the different woodflours, neat resin and composites. The effect of the size and shape of the composite specimens used in the experimental work on the final moisture content was also evaluated. Different models were used to predict the composite effective diffusion coefficients as a function of filler concentrations.

Dielectric behaviour of CaCu3Ti4O12-epoxy composites

The dielectric behavior of composite materials (epoxy resin - barium titanate and epoxy - CCTO) was analysed as a function of ceramic amount. Composites were prepared by mixing the components and pouring them into suitable moulds. In some compositions, the matrix was reduced by tetrahydrofuran (THF) incorporation. Samples containing various amounts of ceramic filler were examined by TG/DTA and scanning electron microscopy analysis. Dielectric measurements were performed from 20 Hz to 1 MHz and 30 to 120 °C. It was demonstrated that the epoxy - CCTO composites possessed higher permittivity than classic epoxy - BaTiO3 composites. However, the low resin permittivity prevailed in the composite dielectric performance.

FTIR spectroscopy applied to woodflour

The features of infrared spectra of woodflour obtained by transmission and diffuse reflectance (DRIFT) are discussed. Intrinsic differences between the two techniques, as well as differences in sample preparation for transmission (pressed pellet) or DRIFT (simple mixing with powered KBr), affect the appearance of the spectra recorded. Other important differences arise from the water content in the sample, as well as in the KBr powder utilized as reference and dilution. Some of these differences are discussed since they occur in regions usually utilized to check modifications in the wood. The effect of sample concentration is investigated for both techniques. In transmission, the opacity of the pellet limits the sample concentration to about 2 wt% in KBr in order to obtain useful spectra. While no concentration limitations are present for DRIFT, distortion of the intensities of bands occur, mainly at low wavelengths. Finally, the use of DRIFT subtraction to detect small surface variations in the woodflour ...

Lignocellulosic materials and unsaturated polyester matrix composites: Interfacial modifications

The addition of particulate fillers (woodflour) and short fibers (sisal) into an unsaturated polyester matrix was performed and analyzed. The efficiency of the filler treatment was carefully investigated, in particular, esterification with two different anhydrides, maleic anhydride (MAN) and an alkenyl succinic anhydride (ASA). The reaction with MAN was performed under different experimental conditions in order to reach different degrees of esterification. The efficiency of the reactions was assessed by FTIR, titrimetric techniques and moisture absorption values. Esterification improves the wettability of the fillers by the resin, so that higher concentrations of filler could be incorporated in the composite. The reaction of the unsaturations of the attached anhydrides with the styrene co-monomer was confirmed by FTIR. This reaction was frequently invoked in the literature, but most times was not confirmed. Scanning electron microscopy study confirmed the improved adhesion of the fillers to the matrix. Surfaces of fractured esterified woodflour composites showed that the resin fills the hollow central regions of the woodflour cells (lumens) reducing capillarity effects during humidity or water sorption by the composites. Mechanical tests were performed on some of the composites to illustrate the effect of the covalent linkages created between the esterified lignocellulosics and the matrix. Tests in which the characteristics of the interphase played an important role were chosen in order to highlight differences resultant of the chemical co-reaction.

Evaluation and modelling of integral capacitors produced by interdigitated comb electrodes

Integral capacitors (IC) of one or two-layer printed wiring board (PWB) circuits were produced using comb electrodes fixtures and dielectric composites as the inter-electrode material. ICs were fabricated at laboratory scale, using copper comb electrodes and BaTiO3-epoxy composite materials deposited on a glass-Epoxy FR4 board. They were experimentally tested in order to obtain their electrical response. Furthermore, ICs behaviour was modelled through 2-dimensional models applying finite element method (FEM). Results showed that by this laboratory technique it was possible to obtained integral capacitors with low dielectric losses. Moreover, acceptable agreement was found between numerical and experimental capacitance results for all the different analysed ICs. In conclusion, 2D FEM models are a suitable tool to predict electric response of IC devices.

The durability of Kevlar-epoxy interfaces

Pull-out experiments have been carried out with Kevlar fibres embedded in epoxy resin. Friction accompanied debonding, and had to be allowed for in the analysis. The debonding stress was about equal to the matrix strength for 80°C cured epoxies. However, debonding appears to be a brittle fracture process, and the works of fracture corresponding to the apparent interface strengths are very low, ranging from ca. 20-40 Jm-2 depending on the surface treatment and degree of cure of the resin. Water immersion for 2300 h at room temperature reduced the apparent strengths and works of fracture with some of the surface treated fibres, but not with the untreated fibres. Interface pressures during debonding were 10-15 MPa for the 20°C cured specimens and 20-30 MPa for the 80°C cure. Water soaking markedly reduced the friction coefficients. Post-debonding friction was high, but estimates of the parameters was probably unreliable due to the fibre having a somewhat thick end due to fibrillation when being cut.

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.

Nanofibers obtained by electrospinning of BaTiO3 particles dispersed in polyvinyl alcohol and ethylcellulose

Barium titanate particles (100-300 nm) synthesized by hydrothermal method were dispersed in both polyvinyl alcohol (PVA) and ethylcellulose (EC) solutions. These suspensions were processed by electrospinning. When no particles were added, homogeneous polymeric nanofibers were obtained. Under certain conditions, polymeric suspensions of barium titanate particles were electrospun generating polymeric fibers with BT particles. The effect of a surfactant was also assessed over the formation of nanofibers. The BaTiO