Water-based coatings for the prevention of discolouration by knot extractives on pine wood
In the last decades there has been an increasing development of water- based wood coatings following the environmental concerns to lower the use of toxic solvents and to reduce the emissions of Volatile Organic Compounds. The opaque paints of this nature used in pine wood suffer from undesired disco...
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| Format: | masterThesis |
| Language: | English |
| Published: |
2019
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| Subjects: | |
| Online Access: | https://hdl.handle.net/20.500.12008/31608 |
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| Summary: | In the last decades there has been an increasing development of water- based wood coatings following the environmental concerns to lower the use of toxic solvents and to reduce the emissions of Volatile Organic Compounds. The opaque paints of this nature used in pine wood suffer from undesired discoloration, related to the presence of knots which have an outstanding high concentration of extractives, mainly resin acids, stilbenes and lignans, which migrate through the polymeric structure. This phenomenon is known as Knot Bleeding. Within the framework of this thesis, the physicochemical nature of the extractives present on knots of Pinus cembra was studied and the substances responsible for the yellowing of white opaque paints were identified. Two stilbenes and five lignans were detected in the polar knot extracts and their presence was confirmed in the discolored coating over the knots. The influence of the ageing conditions on the diffusion of knot extractives through a reference coating was studied. Both temperature and humidity have a key role in the migration of colored and non-colored compounds. The latter get degraded to colored substances after the exposure to sunlight radiation. The Knot Bleeding resistance was tested according to the Standard Procedure for Knot Staining CEN/TC 139/WG 2 and the color of the coating over the knot was measured using CIELab coordinates From the work presented in this thesis a deep understanding of the causes of the discoloration of water-based wood coatings is provided. |
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