Post 173 ⇒ by Gautam Shah →
Film forming substances constitute substantial mass of coating systems like paints and varnishes. Even if other components, such as the colourants, are not present, a coating can still be created. The materials form a film after application, and become a solid surface. The coating materials are in solid or liquid form, but convert to a liquid phase before the application. Film forming substances develop inter-facial adhesion between the coating and the substrate and also provide a film that binds all the ingredients. Wax and Bitumen are heat melting Film forming substances. Some macromolecular polymers, such as polyethylene or fluoro-polyethylene resins applied in powder-form by spraying also form films.
Convertible (irreversible) film systems
In the FIRST case, the film forming substances on evaporation or removal of its solvent or catalyse to polymerize further and convert into a different substance. After such a change the new substance or the film is insoluble, the process is irreversible.
Lime-wash dries out by evaporation of water, but the carbonation helps convert the CaO to CaCo3. After carbon dioxide absorption the film becomes a ‘converted’ mass. Linseed oil when double boiled (DB Oil) readily polymerises on exposure to air. Similar process occurs for oils modified with Rosins and Alkyds Resins(long oil length). The change occurs at ambient air temperature. Some of the higher Length Alkyds (mid and short oil length resins) need a slightly elevated temperature (‘baking’) for the polymerization. All these substances as film are converted into a non-soluble mass – a ‘convertible coating system’. Epoxy esters, Urethanated oils, Epoxy esters, Urethane resins, epoxy resins, alkyd-amino resins, silicones and some acrylics, provide convertible coatings. Some of the resins (such as epoxy and PU resins) form a film by catalyst action.
Non-Convertible (reversible) film systems
In the SECOND case the film formation takes place by simple change of a phase, which occurs on solvent removal or evaporation. The film remains soluble and the process reversible. One of the simple examples is Postal stamp glue, which can be wetted and re-fixed. Other Solvent evaporation film forming systems are chlorinated rubbers, cellulose nitrate (NC Lacquers), vinyls, some acrylics, and bitumens.
Film forming materials for coatings are also classified according to their technique of film formation.
Techniques of film formation
- Solid-thermo plastic film-former: The solid material is melted for application and solidifies after application. E.g. wax.
- Lacquer type film former: The material dries by solvent evaporation.
- Oxidizing film former: Oxygen from air enters the coating, and cross-links it, to form a solid substance.
- Room temperature catalysed film former: Chemical (catalyst) agents blended into coating, just before application cause cross linking into a solid polymer at room temperature.
- Heat cured film former: Heat causes cross linking of the film former or activates a catalyst that is not active until heat has been applied.
- Emulsion type film former: The solvents evaporate, and droplets of plastic film-former floating in it flow together (coagulate) to form a film. The droplets of plastic are not soluble in the solvent-medium, which is usually water.