Microstructural and Physical Aspects of Heat Treated Wood: Part 2. Hardwoods

Abstract

Heat treatment of wood is an effective method to improve the dimensional stability and durability against biodegradation. Optimisation of a two-stage heat treatment process at relatively mild conditions (<200°C) and its effect on the anatomical structure of hardwoods were investigated by means of a light and scanning electron microscopic analysis. Hardwood species such as beech and poplar, were predominantly sensitive to collaps of the vessels and some deformation of the libriform fibres directly near the vessels. In treated beech and birch radial cracks were observed near the rays. Optimisation of the heat treatment process conditions including the application of a steam hydro thermolysis stage reduced such damages to a minimum. Broken cell walls perpendicular to the fibre direction resulting in transverse ruptures has been noticed in heat treated hardwood species. This contributes to abrupt fractures of treated wood as observed in bending tests which can lead to considerably different failure behavior after impact of mechanical stress. In some treated hardwood species maceration (small cracks between tracheids) was noticed after heat treatment. Heat treatment did not reveal damage to the ray parenchyma pit membranes, bordered pits and large window pit membranes; and the margo fibrils appeared without damage.

Microstructural aspects of thermally modified Eucalyptus grandis wood

“…Abundant literature can be found about this process's effect on wood's chemical composition, but the same cannot be said about anatomy (Awoyemi and Jones 2011). Of the few studies about the cell dimensions of thermally modified wood (Andersson et al 2005, Awoyemi and Jones 2011, Bakar et al 2003, Boonstra et al 2006a, 2006b, Hietala et al 2002, none has focused on eucalyptus, which is forest species planted the most in Brazil.…”

“…In another study the ray parenchyma cells of untreated Fagus sylvatica wood were clean and open, but a brown-red extractive appeared after thermal modification (Boonstra et al 2006b). …”

“…Other studies have also revealed that thermal modification causes changes in wood microstructure, which are strongly associated with the species and the characteristics of the process. But the major changes are a result of drying defects (such as collapse and cracks), caused by the drying stresses provoked by the drastic water removal (Boonstra et al 2006a(Boonstra et al , 2006b). …”

Método ultrassônico para estimativa do módulo de elasticidade de madeiras de Pinus taeda tratadas termicamente

“…ESTEVES et al (2008b) (2005) observaram, para a mesma determinação, coefi ciente de determinação de 0,89 (lenho juvenil), 0,94 (lenho adulto) e 0,89 (lenho juvenil e lenho adulto). As menores determinações encontradas no presente estudo podem ser explicadas pelas modifi cações anatômicas ocorridas na madeira após as termorretifi cações, já que, ao analisar essa propriedade, BOONSTRA et al (2006) concluíram que tratamentos a quente podem gerar o colapso dos vasos e fi bras, o que, consequentemente, afeta a continuidade da propagação da onda ultrassônica.…”

“…Dessa forma, verifi ca-se que fatores como as altas temperaturas empregadas no processo de termorretifi cação, além de causar variações físicas (KARTAL et al, 2007;ESTEVES et al, 2007), mecânicas (ESTEVES & PEREIRA, 2009), químicas (INARI et al 2007), anatômicas (BOONSTRA et al, 2006), superfi ciais (HUANG et al, 2012) Assim, o aprimoramento e a quantifi cação de outros fatores infl uentes nos processos de termorretifi cação, como temperatura e tempo do processo, devem ser avaliados em estudos futuros, a fi m de determinar com mais clareza e precisão o Ec0 através do Ed.…”