AIR3-CT94-2463 Performance and Durability of Wooden Window Joinery Painted with New Types of Paints with Low Organic Solvent Content

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AIR Cluster VIII - Wood Products : Paints/Coatings/Plastics : Solid Wood Products : Wood (Lignocellulose)

	Proposal No:	AIR3-CT94-2463
	Date Prepared:	September 1999
	Source:	Final periodic report April 1998

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Final periodic report April 1998

Summary

Objectives The overall objective of the research is to generate knowledge in order to improve and optimised performance and durability of paints with low VOC for window joinery made from European softwood species. This includes durability of the modem joinery paints in various climatic conditions in Europe as well as design of window joinery typical to participating countries. The project aims to clarify those crucial factors of wooden joinery painted with low VOC paints which affect performance and which differ from those of conventional solvent-based paints. These factors are moisture-related properties of painted wood such as permeability, moisture-content and dimensional changes of painted wood. Surface phenomena such as penetration of the first coat into wood, degradation of wood under a paint film, adhesion, and resistance of paints against microorganisms are also performance-determining factors included in the studies.

Activities

The main activities were:

• the evaluation of the longer-term performance of the various wood specimens painted with the low VOC coatings and the solvent-based reference coatings
• determination and discussion of moisture-related properties such as moisture transmission, moisture content and dimensional changes vs natural exposure
• evaluation of the effectiveness of the model coatings in protecting the interface
• the role of low VOC paints in development of blue-stain and the efficacy of fungicides in protecting the acrylate and alkyd paint systems against fungi

The results were presented to industry sponsors and partners in a one day seminar as well as in a technical meeting for the partners where the results were discussed and the completion of the project including reporting was planned.

Results

Task 1. Durability of finishes with low volatile organic content on window joinery The performance and durability properties, of 9 low VOC paints and 2 solvent-based reference coatings on panels, L-joints and on national windows or window corner joints, were determined after 18 and 24 months’ exposure, and final analysis of the performance of coating systems was carried out. Changes due to ageing of 18 months weathering in some of the specimens were monitored more closely by means of various analytical techniques. Performance differences between the coatings were apparent, with most changes in L-joints. Final analysis of 12 months’ and 24 months’ assessment data has been performed using prEN927-2. During this second year coating defects increased for all substrate type.

Task 2. Moisture-related properties of modern joinery paints The effect of weathering on the moisture transmission values of the coatings and moisture content of the coated panels and L-joints during the period were determined. The relationship between the moisture transmission properties and moisture content during service was discussed. Information on the influence of coating permeability on dimensional stability was obtained through testing:

• the changes in dimensions of joinery size timber during service
• the dimensional changes of smaller painted blocks exposed to different relative humidities
• shrinkage on micro-scale of thin painted strips of pine sapwood in order to observe differences between late-wood and early-wood

Liquid water absorption values for the model coatings were determined after one year of natural weathering, using the floating method described in prEN927-5. The results were compared with the values determined on the samples prior to exposure. Weathering caused a general reduction in water absorption values of the coatings; this was more marked for the water-based coatings.

In the outdoor exposure test, as well as in the laboratory trials, a linear correlation independent of coating type was found between dimensional changes and moisture content. However, the magnitude of the dimensional changes was clearly influenced by the coating's moisture permeability.

Task 3. Enhancement of the properties of wood-coating interface Coating parameters affecting paint penetration into wood substrate were determined. Fluorescence microscopy in combination with staining of the wood following painting was found to be a valuable method for evaluation of paint penetration. Image analysis was applied in order to quantify penetration. The potential of confocal laser microscopy to investigate paint penetration was also evaluated. Adhesion measurements were carried out after 12-months natural weathering and compared with readings taken at the initial inspection.

The influence of wood-coating interactions on wood strength was investigated and the mechanical properties of the coatings systems characterised. The effect of weathering and of temperature change on coating extensibility was determined. Artificial weathering tests were carried out in which thin strips of wood were exposed to a radiation source behind detached coating films. Eight of the model systems were included in the tests. A parallel trial was conducted in which the coatings were applied to the strips by brushing. The results showed that the detached coating films gave good protection to the wood against degradation by radiation for a period of six weeks artificial weathering in the QUV apparatus. After 12 weeks of exposure, there were some indications of strength loss in strips shielded by the solvent-based stain. Tests carried out using brush applied coatings showed that wood-coating interactions affected the tensile strength of the wood. The effect appeared to be linked to coating type. There was also evidence that the anti-blue stain preservative treatment could adversely affect wood strength. Detached films of six model coatings and two commercial systems were subjected to tensile tests to determine the Young's modulus and strain to break. The water-based acrylic model systems differed considerably in modulus and extensibility. The solvent-based alkyd systems were both more extensible than the commercial alkyd system. A sharp increase in elastic modulus and a reduction in coating extensibility were observed when the test was carried out at a lower temperature. Similar changes, though less marked, were also observed after the period of weathering.

Task 4. The resistance of modern wood finishes to blue-staining and mould Two testing techniques was used to evaluate development of blue-stain. Many variables concerning paint formulation were studied in order to obtain a better understanding of the efficacy of low VOC paints to inhibit or promote blue-stain fungi both as surface phenomena or as barrier system. Information was also obtained concerning the need for incorporation of blue-stain fungicides in all types of wood coating primers for exterior use in order to prevent surface blue-stain and about the influence of penetration on blue-stain in surface colonisation.

The painted specimens for mould tests were weathered for six months. Blue-stain, cracking and flaking were assessed after weathering and the specimens were conditioned before the mould test. The incubation test was carried out in the same way as the unweathered specimens, with the exception that the test was 23 weeks. A part of the samples were analysed by means of light microscopy, while moisture content and analysis of loss of mass were determined after incubation.