- mayo, 2016
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The EC FP7 funded three year project, Servowood, is currently looking at the performance of exterior wood
coatings. The results should help improve the sustainability of the use of exterior wood in buildings and help not only coating manufacturers but also building owners, architects, specifiers and users of coated exterior wood. Servowood should improve the confidence of consumers in the performance of external wood products, therefore protecting the market share of wood coatings, as well as providing improved maintenance guidelines and, hence, confidence in guarantees and warranties and statements about environmental impact.
The project’s main objective is to develop standards that will facilitate the prediction of service life for exterior wood coatings across different climatic zones in Europe. This should improve the capability of short term laboratory tests, including accelerated weathering, to predict the behaviour of coatings under field conditions. The project is also conducting a large natural weathering test programme to assist in the correlation between the observed coating break-down against the laboratory tests being conducted.
Previously Jon Graystone (PRA) gave an overview of the Servowood project in the article entitled ‘Predicting the service life of exterior wood coatings’ (PPCJ, p 21, October 2015). We now publish an update on this work from the other research partners, which is being conducted on behalf of the SME Associations (CEPE, BCF, BWF, CCS, DFL and FEMIB). Here we ask a number of the Research Partners, EMPA (Martin Arnold), FCBA (Laurence Podgorski), CATAS (Franco Bulian) and HFA (Gerhard Gruell), involved in the project to outline the tests that they are carrying out, in addition to the natural weathering (EN 927-3*) and artificial weathering (EN 927-6*), in the Servowood project to help predict the life of exterior wood coatings.
WHAT TESTS ARE YOU CONDUCTING?
EMPA: The five RTD partners in the Servowood project have joined their resources to share the work for an extensive series of exposure experiments (see table below). These experiments were defined to characterise and quantify specific factors, which are expected to affect the service life of coated wood. In total, more than 3800 coated wood panels (common and home sourced) are involved in the exposure tests, which for capacity reasons have to be implemented sequentially during the course of the project. EMPA is involved in all the exposures except the substrate experiment.
CATAS: We are conducting a wide range of tests on both coated panels and also on free films. These are:
- Thermal analysis
- Tensile tests
- Infrared and UV-Visible Spectroscopy
- Contact angle measurements
- Surface investigation techniques
- (Scanning Electron Microscope (SEM)
- Atomic-force microscopy (AFM)
- Matrix Assisted Laser Desorption/Ionisation
- Time of Flight (MALDI-TOF) etc.
FCBA: Besides artificial and natural weathering tests and conventional assessments (cracking, flaking, chalking, blistering, gloss and colour changes, adhesion), at FCBA we are studying mechanical properties of wood coatings and their variations owing to weathering. Despite their significant influences on performance, mechanical properties are not assessed in the standards of the EN 927 series. More specifically we are testing Persoz hardness of coatings applied on wood directly (not on glass as prescribed by EN ISO 1522*) and are studying its variations versus artificial and natural weathering time. In addition, we are studying mechanical properties of coatings using tensile tests made at different temperatures (both elevated and below zero)
HFA: One very central work package is the substrate set, which is a set of samples in artificial and natural weathering that consists of panels of different wood species. It includes pine, spruce, larch, oak and meranti with different growth ring orientation. This set is observed by HFA and FCBA in collaboration. At HFA, maintenance indicators are produced, distributed and evaluated. These indicators show a clearly visible colour change, if a certain threshold of cumulative weathering stress is reached. This allows a comparison of the weathering stress at different weathering sites. Additionally, the maintenance indicators serve as a quality assessment tool to ensure equal artificial weathering conditions amongst the laboratories of the RTD partners. Further, a moisture sensor has been developed and tested at HFA. This sensor measures the moisture content at the wood/coating interface. Cracks that reach through the coating can be clearly detected by this device due to a pronounced increase of the interface moisture content. Additionally, ATR-FTIR (Fourier Transformation Infrared Spectroscopy, equipped with an Attenuated Total Reflection device) measurements during selected artificial and weathering trials are conducted at HFA. With this method, the photo-oxidation/photo-degradation process of the coatings can be monitored and further quantified. As an addition, HFA is working on a colour model that allows the prediction of the photo-induced discolouration of semi-transparent wood coatings during weathering. Furthermore, the colour change as a result of the application of maintenance coatings are modelled.
HOW AND WHY WILL THIS HELP PREDICT THE PERFORMANCE OF THE COATING?
EMPA: A particularly important experiment regarding laboratory exposures is part of the vulnerability experiment. Here we observe coating performance under rather different exposure doses by running tests with systematically modified exposure cycles. In this experiment we are deliberately using ‘low’ and ‘high’ settings for irradiance, temperature and moisture to explore the boundaries of possible exposure conditions.
CATAS: The main goal of our studies is to detect possible physical and chemical changes indicating a progress of the coating decay during weathering. Such data could be elaborated and possibly used as indicators of the coating degradation at an early stage. Two main problems of the coating formulators and users are, in fact, related to the length of time required for testing a new natural weathering exposure at EMPA coating (almost one year according to EN 927-3*) and the lack of real measurements. The evaluation of the coating performance is, in fact, carried out by visual assessment being subjective and normally based on a descriptive scale. The use of ‘real’ physical measurements could help formulators to a clearer understanding and improvement of the performance of their formulations and, from the other side, to allow a more objective selection by the coating user. Another important goal of this part of the Servowood project is related to the possibility to correlate natural and artificial weathering experiments in a more scientific way.
FCBA: Our results have shown that during weathering an increase in hardness is observed for any coatings, probably due to subsequent crosslinking (for alkyd coatings), loss in surfactants and/or cosolvents (for waterborne coatings). We have shown a clear link between high hardness and cracking development, and, therefore, service life. Our contribution to the project has also shown that there is a clear correlation between Persoz hardness and Young’s modulus: the higher the hardness, the higher the Young’s modulus. This correlation shows that the use of the Persoz pendulum is an easy technique to give evidence of mechanical properties of exterior wood coatings. It is a simple tool and the test is cheap and non-destructive. Tensile tests are sometimes tricky because making free films may be very difficult for some coatings. However, tensile tests that FCBA are able to perform, at different temperatures (-30°C to 70°C), are very useful to the formulator and complement the Persoz hardness test because they inform about the nature of coatings (brittle or ductile).
HFA: The substrate set will deliver data on the influence of wood species and growth ring orientation on coating durability. This is a very important effect because it is well known that a flat sawn panel is prone to cracking, due to extensive swelling and shrinking, while a rift sawn panel does not. These experiments, within Servowood, will gain data to include wood species and quality effects in the model. The maintenance indicators may serve as a simple tool in order to estimate the cumulative local weathering conditions on a specific site. This sensor could have a great practical benefit by helping customers in determining the right time for maintenance of coatings at their site. The developed moisture sensor can serve as a more advanced evaluation tool to determine the state of the coating system. In case a coating failure is imminent, rapid increased moisture content can be observed. This gives a clear indication that the coating is starting to fail and the application of a maintenance coat is strongly advised. With ATR-FTIR measurements, the state of photo-oxidation of the coatings can be determined. However, it leads only to an indirect measurement of the change of properties of the coatings. It is, therefore, of great use in a laboratory in order to compare the relative photo-oxidation rates. For practical use, a correlation between the state of photo-oxidation/degradation and eg mechanical properties would be necessary.
HAVE YOU ANY EARLY INDICATIONS OF HOW IMPORTANT THIS TEST IS?
EMPA: By introducing extended periods of condensation we are able to increase moisture levels in the wood panels to values typical in outdoor exposures. Thus, we can avoid the progressive drying out of the wood panel present in the standard EN 927-6* exposure cycle with dominating irradiance at elevated temperature. However, this experiment is a typical example where we can only assess the full consequences on coating performance when all the eight necessary runs (three factors with each ‘low’ and ‘high’ settings) of three months each will be completed. Since these experiments are still running, it is too early to draw final conclusions.
CATAS: Yes, we are collecting a lot of data, which explain very well the evidence of the coating degradation differences. We are also using some of the test methods presented above to study the possible correlation between natural and artificial test methods. However, we would like to finally add that although we are satisfied and even excited by the work done, we are also conscious that the success of our work depends on the practical usability of our experiences.
FCBA: Results confirm findings of another recent study at FCBA: coatings with the best performance have a low initial hardness (lower than 60sec) with few variations during weathering. Therefore, the Persoz pendulum should be used to a larger extent by wood coatings manufacturers to develop formulations with better performances to weathering. In the Servowood project we have also shown that the coating pigmentation contributes to minimise hardness variation during weathering and, therefore, improves service life.
HFA: The maintenance indicators work very well in artificial weathering devices. In exterior weathering, the colour change is much slower than expected. The moisture sensor is working very precisely and reveals the smallest defects of coatings, even before they are visible on the surface.
It can be seen from the tests that the Research Partners are conducting, that useful information improving prediction of coating performance is emerging from this project. However, it is expected that in the final year, Servowood should gather its most useful information for manufacturers, specifiers and users of exterior wood coatings, and this will be reported on when available.
- EN 927-3 Paints and varnishes. Coating materials and coating systems for exterior wood. Natural weathering test.
- EN 927-6 Paints and varnishes. Coating materials and coating systems for exterior wood. Exposure of wood coatings in artificial weathering using fluorescent UV lamps and water.
- EN ISO 1522 Paints and varnishes. Pendulum damping test.
Article published in PPCJ- April 2016.