WATER ABSORPTION IN COATED WOOD ‐
INFLUENCE OF DIFFERENT WOOD TYPES AND COATINGS
Mari Sand Sivertsen, Mycoteam Per Otto Flæte, Norwegian Institute of Wood Technology
Introduction
• Absorption of liquid water – important for performance (technical service life)
Photo: Mycoteam Photo: Mycoteam • Known differences between waterborne acrylic
coatings and solvent‐borne alkyd coatings as to water uptake.
• Wood properties are important for water uptake in uncoated wood – and to some extent in coated wood
as well.
• Effects of different wood properties in combination with different coating types need more study.
• Previous study on a sub‐sample of the material:
– No difference between high‐density and low‐density
spruce within the 72 hours indicated in the EN 927‐5 standard.
– Prolongation to four weeks brought out differences
between these two wood types.
Sivertsen and Flæte 2011: Water absorption in coated Norway spruce (Picea abies) cladding boards. Eur. Journ. of Wood and Wood Products.
• Objectives:
– Study the effect from different wood types in interaction with different coatings on long‐term water absorption .
– Investigate the effect of reporting results as mass of water absorbed or moisture content gain.
Material and methods
• Norway spruce, high and low density. Good correlation between density and annual ring width.
• Scots pine heartwood, density in between the spruce types.
• Sealing; ends, edges, rear faces.
• Long‐term floating based on 927‐5 (3000 h)
• Weight measured throughout the water exposure
• Results expressed as
– MWA: mass of water (g)/area (m2)
– MCg: mass of water in percent of dry mass of wood
Results – wood material • High‐density spruce highest water uptake, low‐density spruce highest moisture content gain
• Uncoated: Low‐density absorbed more water, and increased more in MC
Results ‐ coating
• Waterborne acrylic highest absorption.
• Solvent‐borne alkyd lowest absorption.
• WB with primer: High initially, lower as floating progressed. Discussion
• Water uptake seems to slow as wood approaches FSP; MCg levels out. – Confirms the notion that FSP is an upper limit for water penetration through coating (Derbyshire and Robson 1999). • Difference in MWA between SL and SH specimens shown earlier to be a density effect.
– Same amount of water absorbed in low‐density wood causes larger MC increase – and FSP is reached more rapidly.
Discussion
• The density effect cannot explain the difference between spruce specimens and pine heartwood.
– PH and SL more different regarding density, PH and SH more different regarding water uptake.
– Lower FSP in pine heartwood; lower moisture concentration gradient.
– Lower diffusivity => lower water flux.
Thank you for your attention!