Silicone resin varnish
Silicone resin paints or silicone resin paints are paints in which silicone resin is used as the main binder. In contrast to most other paints, these are based on an inorganic binder matrix. The main applications are in areas that require high temperature resistance, e.g. B. in ovens.
Silicone resin paints are to be distinguished from silicone resin paints . These are aqueous emulsion paints whose properties have been optimized by adding small amounts of silicone resin.
composition
Like all paints, silicone resin paints contain binders , pigments , fillers , additives and solvents .
Crosslinked, high molecular weight polysiloxanes are used as film formers . The basic structure therefore consists of silicon - oxygen bonds. Organic residues (e.g. alkyl or aryl ) are always bonded to silicon atoms. The most frequently used substituents are methyl or phenyl groups . With the so-called modified silicone resins, silicone resins and classic paint binders such as polyester , alkyd resins , acrylates or epoxies are combined in an early phase of the synthesis.
Most of the raw materials suitable for other paint systems are theoretically also suitable for silicone resin paints. Due to the field of application of silicone resin lacquers, however, mainly temperature-stable raw materials are used in practice. Mainly metallic effect pigments , titanium dioxide , carbon black or iron oxide red are used for pigmentation .
Only a few substances can be used as solvents, since good solubility is only present in aromatics and halogenated hydrocarbon compounds, as well as esters . Solubility is limited in aliphatic solvents and ketones .
properties
In general, silicone resin varnishes offer a property profile that differs from all classic varnishes, i.e. varnishes based on organic binders. In particular, the extremely high temperature stability and good weather resistance, especially against chalking and yellowing , stand out. The low proportion of organic groups results in only a small change in the film volume during curing. This results in a very dense layer, which in turn produces good film strength.
Methyl silicone resin lacquers are incompatible with almost all organic binder systems. They create an extremely hydrophobic lacquer surface that is also repellent to adhesives . These systems are mostly matt and extremely brittle. They only have a low pigment absorption capacity. Resistance to solvents is good, but not to alkalis . The temperature stability is good, so that up to 500 ° C can be reached briefly and up to 200 ° C permanently. Lacquers based on methyl silicone resin lacquers can physically dry by splitting off water . Due to the oxidative cleavage of methyl groups when exposed to high temperatures, the paint layer is reduced to an extremely stable silicon dioxide framework. In combination with metallic pigments, an extremely heat-stable layer is created. These systems are known as silver bronze .
Phenyl silicone resin lacquers have a better pigment absorption capacity than methyl silicone resin lacquers, higher temperature stability (briefly up to 700 ° C, permanently up to 280 ° C) and better compatibility with organic lacquer binders, but have a softer surface. These systems are also extremely resistant to temperature changes. Paints based on phenyl silicone resins create a sticky surface when the solvent evaporates, so that these systems are exclusively stoving paints.
With silicone-modified binders, the weather resistance is higher than with the corresponding purely organic binders, even if only a little silicone resin is contained. The other properties then essentially depend on the other binder component. If the silicone resin makes up the majority, the temperature resistance is only slightly restricted, but an improvement in the application behavior is often noticeable.
Applications
Silicone resin paints are mainly used to coat stoves, ovens and other devices from the electrical industry. In general, these systems are suitable for all applications that require high temperature and / or weather resistance. Silicone-modified systems are used for highly weather-resistant coatings that do not require additional permanent temperature resistance.
processing
Heat-resistant protective and top coatings require thorough substrate preparation. As a rule, a metallic substrate is coated with a primer , also based on silicone resin , which is followed by a multilayer structure. Since the higher layers would no longer adhere when fully cured, the lower layers only cure incompletely, so that a compromise between adhesion and crosslinking is achieved.
In the case of highly heat-resistant corrosion protection systems, the adhesive strength to the substrate is decisive, so that mechanical pretreatment by blasting is common here .
Individual evidence
- ↑ a b c H. Römpp; Römpp Lexicon; Lacquers and printing inks; 1998; Thieme; Stuttgart; ISBN 9783137760016 .
- ↑ a b c d e f A. Goldschmidt, B. Handtschke, E. Knappe, G. Vock; Glasurit manual for paints and varnishes; 11th edition; Vincentz Publishing House; Hanover; 1984; ISBN 3878701926 .
- ↑ H. Kittel; Textbook of paints and coatings Volume I, Part 2: Basics, binders; Page 287ff; WA Colomb Publishing House; Berlin; 1973; ISBN 3879030421 .
- ↑ a b c d e f H. Kittel; Textbook of paints and coatings Volume IV: Paint and coating systems, formulation; Page 486ff; WA Colomb Publishing House; Berlin; 1976; ISBN 3879030472 .