László Százdi

Electrochemical oxidation of carbon fibres: adsorption of the electrolyte and its effect on interfacial adhesion

A preliminary study indicated the adsorption of sodium hydroxide electrolyte to carbon fibre oxidised in a continuous anodic electrochemical process. The adhesion of the oxidised fibres to epoxy resin decreased with increasing amount of adsorbed NaOH. In this study, experiments were carried out to confirm the preliminary observations. Bundles of fibres were soaked in water and both the solution and the fibre were analysed. The results proved that sodium hydroxide is removed slowly from the fibre surface, more than 20 h is needed to create a relatively clean surface. The adsorbed NaOH blocks reactive groups and hinders coupling between the matrix and the fibre. Its removal increases both the chemical and the electrochemical activity of the fibre and leads to a significant improvement of interfacial adhesion.

Surface characterization of electrochemically oxidized carbon fibers: surface properties and interfacial adhesion

Carbon fibers are oxidized in two electrolytes at different electrolyte concentrations and potentials. The chemical composition of the fiber surface changes considerably in both cases. The oxidation in H2SO4 results in the formation of sulfur containing groups, but quinoidal compounds are also detected on the surface. The concentration of all functional groups increases with increasing electrolyte concentration at 5 V, but does not change as a function of oxidation potential in 20 wt% solutions below this value. Carboxylic functional groups are formed on the fiber in NaOH, but some adsorbed NaOH also remains on the surface after oxidation. Cyclic voltammetry reflects the modification of the surface differently. Peak potentials remain basically constant, while peak currents depend on the type of the treatment. Good correlation has been found among peak current, the chemical composition of the fiber surface and IFSS of epoxy composites in the case of H2SO4. The adsorbed NaOH interferes with the electrochemical reaction that takes place on the fiber surface. Moreover, IFSS decreases as the amount of adsorbed NaOH increases.

Preparation, structure, and properties of PVC/montmorillonite nanocomposites

Much less information is available on the preparation and properties oflayered silicate PVC nanocomposites than on composites prepared with other polymers and practically all of this information is limited to plasticized PVC (pPVC). In one study the presence of the silicate layers was shown to hinder the migration of the plasticizer [ 17]. Others found that the incorporation of the silicate into plasticized PVC improves mechanical properties and decreases flammability [ 18, 19, 20]. Occasionally partial exfoliation was observed, which was accompanied by an improvement of optical clarity and mechanical properties [21]. Contrary to most attempts published in literature, the goal of our study was to prepare layered silicate nanocomposites from an unplasticized PVC (uPVC) formulation and to determine their structure as well as their properties. An attempt was made to estimate the extent of exfoliation and to fmd a correlation between the structure of the composites and their properties. The uPVC layered silicate nanocomposites were prepared from a commercial organophilized montmorillonite (OMMT) at three temperatures 170, 180, and 190 °C using a two-roll mill. The sheets produced were compression molded into I mm thick plates. Composites containing sodium montmorillonite (NaMMT) were used as reference. The silicate content ofthe composites changed from 0 to I 0 vol% in 7 steps. The structure of the composite and the extent of exfoliation were characterized by a wide angle X-ray scattering (WAXS) and by the measurement of light transmission. The stability was determined by the measurement of color. The glass transition temperature and the tensile properties of the composites were also measured. The preparation of PVC nanocomposites is difficult; the quaternary ammonium salts used for the organophilization of the montmorillonite accelerates the degradation of PVC. However, under appropriate conditions nanocomposites can also be prepared from this polymer. Processing led to the partial exfoliation of the organophilic silicate shown by the considerable change in transparency. As Fig. 5 shows, the light transmission, of compression molded plates, decreases drastically when N aMMT is used as filler, while the transparency

Effect of interfacial interaction on the structure and rheological properties of polyamide-6/clay nanocomposites

A series of polyamide-6 (PA6)/layered silicate (clay) nanocomposites were prepared via direct melt compounding using a conventional single screw extruder, and then the effect of interfacial interaction on the characteristic internal structure and rheological properties of PA6/clay nanocomposites was investigated. XRD diffractograms indicated a large extent of exfoliation of the layered silicate entering into a strong interaction with PA6. The formation of such morphology was further supported by TEM images. In addition, various rheological properties were interpreted in conjunction with morphological characteristics depending on interfacial interaction between PA6 and the layered silicate.

Surface chemistry and adhesion in carbon fiber reinforced epoxy microcomposites

In an extensive series of experiments, PAN based carbon fiber was subjected to electrochemical oxidation under a wide variety of conditions. In this part of the study, the activity and chemical composition of fibers oxidized in sulfuric acid were characterized by cyclic voltammetry (CV), diffuse reflectance infrared spectroscopy (DRIFT) and X-ray photoelectron spectroscopy (XPS). Interfacial adhesion was measured in epoxy microcomposites by fragmentation. The results proved that the type and amount of functional groups formed on the surface of the fiber depends very much on the conditions of oxidation. In sulfuric acid, a large number of sulfoxides also form besides the usual functional groups containing carbon and oxygen. The surface concentration of all functional groups increases with the intensity of oxidation, but to a different extent. Only a few of them increase interfacial interaction. Sulfur-containing groups and quinoidal compounds are not very reactive, while carboxyl groups seem to be the most active in the improvement of adhesion. Chemical reactions take place on the surface of the fiber during the curing of the resin, which depend on the chemical composition of both the surface and the resin system. The chemical activity of the fiber can be characterized by cyclic voltammetry, because the number of electroactive and reactive groups increase with the intensity of oxidation in a similar way. Although the removal of a weakly adherent layer during oxidation may contribute to the improvement of adhesion, chemical coupling plays a similarly important role.