Abstract
A one-pot, enzyme-catalyzed bulk polymerization method for direct production of highly branched polyesters has been developed as an alternative to currently used industrial procedures. Bio-based feed components in the form of glycerol, pentaerythritol, azelaic acid, and tall oil fatty acid (TOFA) were polymerized using an immobilized Candida antarctica lipase B (CALB) and the potential for an enzymatic synthesis of alkyds was investigated. The developed method enables the use of both glycerol and also pentaerythritol (for the first time) as the alcohol source and was found to be very robust. This allows simple variations in the molar mass and structure of the polyester without premature gelation, thus enabling easy tailoring of the branched polyester structure. The postpolymerization crosslinking of the polyesters illustrates their potential as binders in alkyds. The formed films had good UV stability, very high water contact angles of up to 141° and a glass transition temperature that could be controlled through the feed composition.
Highlights
IntroductionWood has been widely used for outdoor applications. Without protection, wooden structures degrade through various different processes, such as exposure to UV light, moisture, and microorganisms, leading to severe deterioration in properties [1,2]
For many years, wood has been widely used for outdoor applications
Glycerol was provided from Emmelev A/S (Otterup, Denmark), Tall oil fatty acid (TOFA) and pentaerythritol were from PPG Industries (Søborg, Denmark)
Summary
Wood has been widely used for outdoor applications. Without protection, wooden structures degrade through various different processes, such as exposure to UV light, moisture, and microorganisms, leading to severe deterioration in properties [1,2]. The most widely used coating system for exterior wood applications is based on alkyds. An alkyd binder can be considered a hyperbranched unsaturated polyester prepared from a ternary mixture of polyhydric alcohols, bifunctional acids and fatty acids, where the unsaturation provides the drying properties to the final coating formulation [4]. Pentaerythritol is more photo-stable than glycerol due to the lack of secondary carbons in its structure [6], which makes this a very favorable component for binders used in exterior coatings that are under constant attack by solar radiation. Considering the movement towards bio-based products, a traditional alkyd formulation already contains a number of bio-based components such as bio-based fatty acids and glycerol. The challenge, lies in Polymers 2016, 8, 363; doi:10.3390/polym8100363 www.mdpi.com/journal/polymers
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