Wood structure and coatings : an electron microscope study of Dacrydium cupressinum, Podocarpus dacrydioides and Pinus radiata weatherboard substrates (1990)
AuthorsMiddleton, Tina Mariashow all
A scanning electron and transmission electron microscope study of wood substrate anatomy and ultrastructure, involving over one thousand micrographs, was made on weatherboards of two endemic timbers of New Zealand and one exotic timber, to determine their response to eight different coating systems currently recommended for exterior use. Dacrydium cupressinum Lamb. (rimu), Podocarpus dacrydioides A. Rich. (kahikatea) and Pinus radiata D.Don. (radiata pine) were the taxa chosen and three water based primer systems, four solvent based primer systems and a specially formulated Ti02 tagged alkyd novel coating system were used to demonstrate wood/coating relationships. The key elements present in the coatings were detected by energy dispersive X-ray analysis and used for tracing coating penetration in the weatherboards. The primer coated weatherboards were studied in transverse section. Penetration limit of the seven primers was three cells deep. Ti02 tagged alkyd coated weatherboards were also examined longitudinally as they exhibited much deeper substrate penetration than the primers. behaviour of this substrate coating system. The essence of this thesis is therefore the The entire cross-sectional length of each weatherboard was photographed under the microscope and quantitative information on the diverse nature of the coating was obtained by counting the number of filled cells along rows of tracheids inwards from the coating interface, measuring the slope of the tracheid lengths relative to the board surface, determining the influence of the tracheid cell growth directions and cut of board as seen in profile view. Statistical analyses showed that with reference to combination comparisons of the two types of mounted weatherboard profiles, namely, 'u'-type and 'n'type cuts, the mean gross coating penetration depth was significantly different among the three taxa. The maximum gross coating penetration limit recorded along the top surface for D. cupressinum '0' and 'n'-type boards is nine cells; and for P. dacrydioides it is six cells in 'u'-type and thirteen cells in 'n'-type boards. Coating penetration appears shallower for P. radiata, with the maximum depth being eight cells for 'u'-type and four cells for 'n'-type boards. Extreme maximum penetration occurred along the sides of coated weatherboards. The mean angle made by the tracheid cell length and the substrate face is greater in 'u'-type boards than 'n'-type boards. The performance of the Ti02 tagged alkyd coating and the wood substrates were tested using accelerated weathering techniques involving the Xenon-arc Weather-Ometer. Coated and uncoated boards were weathered for one thousand, two thousand and three thousand hours. Coating durability and substrate behaviour were qualitatively assessed microscopically. Spectrocol orimetrican d spectroscopic techniques including the L *a* b* and chromaticity colour systems, fourier transform infrared and attenuated total reflectance methods were used to obtain supplementary quantitative information. Theories on observed coating/substrate behaviour are discussed.