Influence of the fatty acid pattern on the drying of linseed oils
Abstract: The interest in renewable resources due to environmentalfactors has increased the interest to use new VOC-free linseedoil qualities together with reactive diluents for coatingapplications. The drying of two linseed oils, oilAwith a high content (74,2 %) of linoleic acid (C18:2)and oilB, a more traditional linseed oil with a high amount(55,2-60,4 %) of linolenic acid (C18:3), was followed in orderto reveal how the structural variations of the oils fatty acidpattern and the addition of the fatty acid methyl ester of oilAas a reactive diluent (0, 20 40 wt%) can change thedrying performances of the oils and their final filmproperties.The influence of the drying temperature and the influence ofdriers was investigated. The drying performance of thedifferent oil formulations applied on pinewood substrates wasbriefly investigated. Two different analytical techniques,chemiluminescence (CL), and real-time infrared spectroscopy,(RTIR), were shown to be versatile tools for the analysis ofthe drying process. Chemiluminescence is shown to be a usefultechnique to follow oxidative drying measurements on wood.The final properties of the dried film, depends on the fattyacid composition of the oil. Linseed oil (oilB) high in linolenic acid (C18:3) has more problems withresidual unconjugated cis-unsaturations in the drying film.Surface sealing effects were achieved when driers were added,or when the reaction temperatures were increased. The?skin?will act as a diffusion barrier for oxygen andfurther drying. The drying then proceeds at a very slow rate,leading to residual unconjugated cis-unsaturations affectingthe long-term durability and the colourfastness. Reaction ratesare influenced for both wanted intermolecular cross-linkingreactions building up the polymer matrix, and unwantedintramolecular degradation reactions, leading to volatileemission of low molecular species as well as photon emissionfrom the auto-oxidation process. The incorporation of reactivediluents, fatty methyl esters of oilAreduce the effects of surface sealing for oilBat higher temperatures and therefore increase theoxidative drying rate. Slightly softer final coatings areobtained. The drying of oilAdoes not tend to be as sensible as for oilBto driers and raised temperatures, but the addition ofreactive diluents influences the final coating propertiesgiving softer final coatings.CL-measurements when oilAand oilBwere applied to pinewood substrates resulted inshorter induction periods for the oxidation reactions. It showsthat the oxidative drying is influences by a chemicalreactivity between the wood substrate and the linseed oils.This would have a great affect on the ability of the oil topenetrate deep into the wood structure. The higher thereactivity of the oil, the lesser the oil will penetrate thewood. These results greatly improve the possibility to producea coating for wood protection with acceptable properties usingoils high in linoleic acid with fatty acid methyl esters addedas reactive diluents. This shows that studies on othersubstrates than wood might be misleading.
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