Efeito do envelhecimento e da rugosidade nas propriedades da superfície e na aderência do verniz em madeiras de eucalipto

Abstract

The general objective of this study was to evaluate the effects of aging and surface roughness on wettability, surface energy and adhesion of a water-borne varnish on eucalypt woods. The specific objectives were: (1) to evaluate the effect of aging time on color change and surface inactivation and its correlations; (2) to evaluate the effect of the aging time of the surface after machining on the performance of a water-borne varnish and its correlation with wettability; and (3) to evaluate the effect of machining process (sanding and planing) on 3D roughness, wettability, surface energy, and adhesion of a water-borne varnish on eucalyptus woods. To meet these objectives, the thesis was divided into three chapters. In the first chapter, color and contact angle measurements were performed for seven eucalypt species (Corymbia citriodora, Eucalyptus camaldulensis, Eucalyptus pellita, Eucalyptus robusta, Eucalyptus saligna, Eucalyptus urophylla and Eucalyptus grandis) on the fresh surface (freshly machined) and aged after 7, 14, 21 and 28 days. The colorimetric analyzes were performed in the CIE 1976 L*a*b* color space and contact angle determined by a DSA 100 drop shape analyzer in static mode. In the second chapter, Corymbia citriodora, Eucalyptus camaldulensis, and Eucalyptus pellita woods were studied. Contact angle analysis and adhesion tests of a water-borne varnish were performed on the fresh surface (freshly machined) and aged surfaces after 4, 8, 24 and 48 h of planing. In the third chapter, 3D roughness parameters, wettability, surface energy and varnish adhesion on the sanded and planed surfaces of Eucalyptus grandis and Eucalyptus robusta woods were evaluated. The roughness was evaluated by 3D method using a MicroMeasure CHR 150 optic white light roughness meter. The wettability and surface energy were determined by contact angle analysis performed with FTÅ D200 goniometer. The surface energy was calculated by the Lifshitz-van der Walls/ Lewis acid-base theory using three different liquids: distilled water, formamide, and diiodomethane. The adhesion of a water-borne varnish to the wood was determined by the tensile strength test (pull-off). The results of the first chapter permitted to classify woods into three density classes: very high, high, and low; and two color groups: yellowish and reddish. The storage of the eucalypt woods caused darkening (lower luminosity - L*) and increased chromaticity on the surface. The storage time increased the contact angle, resulting in surfaces more hydrophobic. The variables L*, a*, b* and C*ab had a greater impact on the wettability of the surface than the density. The results of the second chapter showed a decrease in surface wettability and loss of coating adhesion with increasing aging time. Loss of adhesion usually occurs shortly after 4 h of planning, however, a more important effect is observed after 48 h. Also, the results show that in some eucalypt species, loss of adhesion is faster than in others. No correlation was found between surface wettability and varnish adhesion on Eucalyptus camaldulensis and Eucalyptus pellita woods. However, in Corymbia citriodora wood, the tensile strength decreased with increasing mean contact angle, showing a significant correlation. The results of the third chapter showed that the planing increased the surface roughness showing higher values for the amplitude parameters (Sa, Sq, Sv, and Sz) than sanding. The sanded surfaces presented greater wettability and surface energy, however, this machining operation did not favor the penetration of the varnish. The planed surfaces of Eucalyptus grandis showed better adhesion of the coating than sanded surfaces. The rougher surfaces presented lower wettability and surface energy and higher adhesion of the coating, therefore, the roughness has a more important impact on the adhesion of the water-borne varnish to eucalypt woods than the wettability and surface energy properties.