Several coating properties are the subject of improvement. Most often those properties are directly linked to the composition of the resin system and to the specific application of the system. It turns out that polysiloxanes (EU), often called silicones (EU), can be used to improve a variety of system properties. This article is about improving the outdoor durability of coatings by combining organic resins and polysiloxane.
Outdoor durability of coatings, often called weatherability or exterior durability, refers to the resistance to degradation during outdoor exposure. Photo-oxidation by the UV component of sunlight and hydrolysis of the resin system are important processes during the deterioration of coatings during outdoor exposure. A coating can yellow, crack, lose gloss or lose adhesion because of degradation.
The speed of degradation of a coating during outdoor exposure depends, amongst others, upon two key factors. First, the strength of the bonds in the resin system is important. The higher the bond energy, the more difficult it is to break the system down. Secondly, the amount of UV absorption by the resin governs how many radicals can be formed. Radicals act as the ‘scissors’ for chemical bonds during photo-oxidative degradation of coatings.
Key Properties of Polysiloxane
Two key properties of polysiloxanes can be directly linked to the excellent outdoor durability of these polymers. First, the bonds in the polymer are strong, implying that a high amount of energy is needed to break down the backbone. Secondly, polysiloxane is nearly transparent towards UV radiation.
How to Use Polysiloxanes
The principle that is often used when outdoor durability must be improved is that part of the organic resin is substituted by polysiloxane. In general it can be said that the outdoor durability of a resin improves when the polysiloxane content is increased.
Polysiloxanes of low molecular weight, so-called oligomers (EU), can be used as intermediates in resin synthesis. These branched, 3-dimensional polysiloxanes contain functional groups that can react with raw materials used in organic resins. This implies that polysiloxane can be used as a building block in the synthesis of resins that are used for coatings. Most often such polysiloxane oligomers contain hydroxyl (EU) (-OH) and/or methoxy (EU) (-OCH3) groups. The functional groups of the polysiloxane copolymerise with reactive groups, most often hydroxyl groups, of the organic resin.
Several companies supply polysiloxanes that can be used as intermediates in the synthesis of resins to obtain binder systems with improved durability. SILRES® IC 368 (EU) of Wacker Chemie AG (EU), being a global leader in silicone chemistry, is an example of such a product.
IC 368 is a liquid material, nearly free of organic solvent, that can be used as a building block in condensation polymers, such as alkyds and polyesters, and in radical polymerisation products, such as acrylates.
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