Modificação superficial de polpa de celulose para melhoria de durabilidade de fibrocimentos

Abstract

Cementitious materials tend to break when subjected to tensile stresses due to their brittleness. For this reason, reinforcing agents may be introduced into them. Asbestos is a material widely used for this purpose because it presents good physical mechanical properties, low cost, good adhesion to the matrix and high durability, but it can be associated with the occurrence of some respiratory diseases, which led to its banishment in Brazil and prohibition in some countries of the world such as Belgium and Japan. Synthetic fibers such as polypropylene are satisfactory for this purpose, but their use implies in a higher cost as compared to fiber cement produced with vegetable fibers or asbestos fibers. Vegetable fibers are composed of cellulose, hemicellulose, lignin and extractives. After Kraft pulping of vegetable material, such as eucalyptus, lignin and extractives are removed, as it adversely affects the cement cure process, remaining cellulose (which has good mechanical resistance) and a large amount of hemicellulose. Thus, cellulosic pulps can be used for fiber cement production by extrusion process, since the use of reinforcements of vegetable origin in fiber cement is economically feasible, as well as being renewable materials that have equivalent mechanical strength to that of inorganic fibers. However, the vegetable fibers undergo degradation, mineralization and dimensional instability when exposed to the alkaline medium provided by cement matrix, and with the continuity of the curing process, there is an impair of the physical and mechanical characteristics of the composite. An alternative to overcome this condition would be the modification of the hydrophilic surface of cellulose pulp by insertion of hydrophobic functional groups in the cellulose structure, improving the fiber cement interaction, even after being submitted to aging cycles. In this work, we first approached the modification of the cellulose pulp from the application of silane (OTES) and waterproofing agent of fabrics (PFBS), through hydrolysis and direct application, respectively, in the pulp dry mass in concentrations of 0 (control group), 5, 10, 15, 20 and 25%. To confirm the surface modification of the pulps, scanning electron microscopy analyzes were performed with energy dispersive X-ray spectroscopy, atomic force microscopy, Fourier transform infrared spectroscopy, tests of moisture absorption, contact angle/surface energy analysis. Subsequently, the production of the composites with 5% of pulp (control group, OTES and PFBS according to the previously established concentrations), 60% of Portland cement, 33% of agricultural limestone, 1% of HPMC, 1% of ADVA and water / cement ratio of 0.4 were approached. The physical, mechanical and microstructural properties of the composites were analyzed after 28 days of curing, 10 and 20 cycles of accelerated aging. In this context, the objective of this work was to evaluate the physical-mechanical performance and durability of extruded cement composites produced with Kraft pulp after superficial modification with OTES and PFBS, in different concentrations. The best results were obtained for the treatment with 5% OTES, due to the good fiber-matrix interaction after the aging cycles.