NMR imaging of curing of alkyds and moisture transport through coatings on wood
Başlık çevirisi mevcut değil.
- Tez No: 544122
- Danışmanlar: Prof. OLAF ADAN
- Tez Türü: Doktora
- Konular: Ağaç İşleri, Metalurji Mühendisliği, Wood Products, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2018
- Dil: İngilizce
- Üniversite: Technische Universiteit Eindhoven
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 157
Özet
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Özet (Çeviri)
The awareness of the environmental risks of several coating ingredients has increased much in the last decades. Therefore, more environmental friendly constituents in the coating formulations have gained interest. One is the tendency to replace cobalt‐based alkyd driers with more environmentally friendly alternatives. Another environmental issue is the regulations concerning the usage and emissions of volatile organic compounds (VOCs) in coatings, especially in wood coatings. Alkyd based paints are widely used as air drying coatings. Without any catalysts, the natural drying time of alkyd based coatings would be weeks or months, which is not desirable from the practical point of view. The autoxidation process can be accelerated by adding catalysts, the so‐called driers. Regarding the function of driers, they are commonly divided into two main classes: primary and secondary driers. Primary driers promote oxygen uptake, hydroperoxide formation and decomposition processes, whereas secondary driers are said to enhance the crosslinking of the film. As primary driers, cobalt‐based driers are the most used drying catalysts as they lead to highly crosslinked and hard films. There is a clear driver to replace cobalt‐based driers by eco‐friendlier alternatives as a result of its potential health implications. In Chapter 2, we investigated the influence of cobalt and alternative metals ‐ an iron based and two manganese based driers ‐ on the oxidative drying of alkyd coating films using a combination of techniques. High spatial resolution NMR profiling allows tracing the depth‐resolved crosslinking during drying, and is used for the first time in conjunction with time‐resolved ATR‐FTIR and standard methods like the Beck‐ Koller (BK) drying test, König hardness and DMA. Based on the results from these different techniques, we found correlations between the drying pattern of driers and properties such as the drying rate, the overall and depth‐resolved crosslink density and hardness development. During the drying in the presence of cobalt‐based drier, a skin layer is formed that acts as a front and moves towards the bottom of the film. On the other hand, cobalt alternatives exhibit more uniform, i.e. homogeneous, crosslinking throughout the film. Further analyses show that cobalt gives a lower overall drying rate, significantly higher final crosslink density and hardness development compared to its alternatives. These results can be explained by the balance between the speed of crosslinking reactions and oxygen diffusion, which determines the dominating type of drying. In order to enhance homogeneous drying of the film, primary driers are used in combination with secondary driers. However, each combination of primary and secondary driers leads to a different oxidative drying pattern resulting in variations in the crosslink density and hardness development. In Chapter 3, we investigated the curing behavior of secondary driers, calcium and zirconium, using high spatial resolution NMR. In the beginning of drying, there is an inhibition period present. The addition of secondary driers reduces the inhibition period. Calcium seems to be more efficient than zirconium in reducing the inhibition period. Furthermore, we observed increased drying speeds and higher crosslink densities when adding secondary driers. While calcium showed to promote the front speed, zirconium showed to increase the measured crosslink density. In a diagram showing both the crosslink density and front speed, the effect of calcium and zirconium can be described as a vector addition, i.e. they have additive effects. These findings provide an overall update for the paint industry and the ongoing research of cobalt alternatives. For further optimization of cobalt alternatives, the focus of the research should be on understanding the influence of the driers on drying pattern of the film and the role of inhibition period on the drying rate. Front drying works best to obtain densely crosslinked hard films. For a fast drying film with a reasonable crosslink density and hardness, a primary drier in combination with secondary driers promoting homogeneous drying without an inhibition period should be preferred. Another driver for coating innovation is due to stricter regulations concerning the usage and emission of VOCs. With the regulations on VOCs and new technical developments, waterborne coatings are becoming more and more common in the wood coatings sector. However, waterborne coatings are more sensitive to water compared to solventborne coatings. It has led to an increased interest in studying the influence of coatings on the changes of wood moisture content. Understanding the changes of wood moisture content is of practical importance, since the mechanical properties or the dimensional stability of wood are influenced by the moisture content. Moreover, a high moisture content may result in durability loss due to fungal growth and/or delamination of an applied protective coating. Since protection of the substrate is a main feature of coatings, getting knowledge on how a coating resists and interacts with water is helpful in predicting the service life of the wood substrate and the coating. As a first step, the influencing factors on the coating performance against water should be identified. These factors can be the wood substrate, the type of binder, pigmentation, film thickness, etc. To investigate water transport through coatings on wood, we started to analyze transport through the wood itself in Chapter 4. Oak, teak and pine sapwood were selected as the wood types covering a whole range of low to high density wood. The first experimental part on uncoated wood was to do the calibration of moisture content for each type with the low resolution NMR. The calibration results allow us to directly quantify the amount of water from the NMR signal during water uptake and drying of wood samples. Relaxation analyses were performed to understand the state of water within the wood, e.g. bound water in the cell wall or free water in lumen or voids. For all wood types, water transport appeared to be internally (i.e. wood) limited during both uptake and drying. This means that the water movement is limited by water transport within the wood. We further found that there is a unique coupling between the MC and the fractions of bound and free water during uptake and drying. This fact allows to obtain the ratio between each state at any period, just by knowing the MC. For conceptual understanding of transport mechanism, we compared experimentally determined diffusion constants with those derived by the diffusion models. We found that diffusion in the cell wall fibers plays a critical role in the transport process. While Chapter 4 only deals with wood, coated wood has been studied in Chapters 5 and 6. In all our formulations used in this thesis, we have studied transparent coatings in order to avoid the influence of pigments, especially on the water permeability. Three types of coatings were formulated: a solventborne (SB) alkyd, a waterborne (WB) alkyd and a waterborne (WB) acrylic, specifically for this project. To understand the influence of the wood substrate on the permeability, we compared water permeability of free films and wood supported films in Chapter 5. We studied free films in two pre‐conditioned states, whereas the free films with water treatment are considered as leached and without water treatment as non‐leached. This allows to study the differences in the permeability, solubility and diffusion. Non‐leached WB films show a significant higher permeability and solubility compared to leached ones. After removal of the hydrophilic material (such as surfactants) in the films by leaching with water, the permeability and solubility values decrease significantly for the WB coatings, but the leaching has no significant influence on the values for the SB alkyd. We found that the permeability of the coatings is similar for leached free films and films on wood. Apparently, the interaction of the coating with the wood has no influence on the permeability for the considered combinations. These results show that wet‐cup is a good indication for testing permeability of these coatings, when applied on wood. The films, especially WB systems, should undergo, prior to a wet cup experiment, a leaching process. Finally, we have demonstrated the direct relation of the permeability with the average water activity inside the coating, which is connected to the activities of both sides of the coating, in Chapter 6. The permeability rises when the average activity increases during uptake, and drops when the average activity decreases during drying. Further, the correlation between the sorption isotherms and the permeability is found to be remarkably well, which proves that the permeability variations are due to the amount of water present in the coating. In overall, the permeability is not about the type of water vapour or liquid present at one side of the coating; it is all about the moisture content of the coating.
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