Polyurethanes (PUs) have many favourable characteristics which make them suitable for various applications. They have good resistance to chemicals and abrasions, and are able to produce tough, hard finishes. In the paint industry, PUs can exist as PU lacquers, two-component PU systems or oil modified PUs. PUs are formulated as varnishes and paints and are used as finishing coats to protect and seal substrates such as wood, metals and plastics. Their use results in a durable, strong, abrasion-resistant coating that can be used in hardwood floors.
PU provides favourable properties for paints as they result in tough, durable coatings that are highly resistant to extreme weather conditions. They also offer good glossy properties and a consistent finish which improve the overall aesthetic of the paints. Higher levels of gloss lead to a property known as ‘self-cleaning’, in which grime and dust washes off easily during rain and residual dirt present can be removed with ease using soap and water. PU-based paint is therefore a suitable option for long-term protection of numerous substrates. PUs can also be applied on a straight oil finish, but the curing time for oils is relatively slow and volatile byproducts are formed during curing. Therefore, there is a need for the oil to be exposed to oxygen and the oils must be sufficiently cured to properly accept the PU. Water-borne or solvent-borne oil-modified PUs are currently the most widely used wood floor finishes.
Most transparent and translucent paints, varnishes and film-polymer coatings are susceptible to deterioration through UV radiation/ light exposure to different degrees. Some pigments in paints protect the film from damage by UV light, but in the case of PU varnishes, UV-absorbers are included as additives to act against the destruction. PUs usually have the most resistance to properties that poorly affect the performance of paints such as temperature extremes, moisture, humidity and fungus.
PUs are adopted as finish coats for industrial coating systems which are applied on top of primer and intermediate coats, that combine individual coating properties to form long-term protective coating systems. PU paint finish coats usually display the following characteristics and advantages:
Finishes have high tint and glossy properties that withstand the loss of gloss and colour
They form hard, tough and abrasion-resistant films which resist wear and tear
These coatings have good weatherability, they are resistant to extreme weather conditions
They can be utilized in a wide range of temperatures (upto 110°C)
They are resistant to chemicals, solvent-based spray paints and strong graffiti-removing agents
They can be recoated easily, after required surface preparation and they do not require solvents for sanding, removal, or surface reactivation
Water-borne systems are those systems which have water as the active component. They can also be cleaned up or thinned with water. These systems consist of PUs, epoxies, acrylic lacquers, latex coatings and enamels. They may be one or two-component, baked or air-dry systems. Currently, water-borne technology is advancing significantly. Water-based lacquers are being developed due to the environmental concerns of solvent-based lacquers. They are more eco-friendly and less toxic in nature. Due to their numerous advantages, solvent-based clear and colored lacquers are being replaced by water-based colored lacquers in industrial applications, especially wood furniture finishes. PU technology used in the paint industry is categorized into reactive and non-reactive coatings and these coatings exist as one-component and two-component systems. One-component systems cure when the film is exposed to oxygen, heat or moisture. Two-component systems require additives to be mixed prior to application and curing is carried out by direct crosslinking at room temperature. Crosslinking improves the hardness of the system, and also enhances resistance to abrasion, extreme temperature, weathering, water and other solvents. However, if it is carried out to a very high extent, it may lead to poorer flexibility.
Technology of Reactive Coatings: Reactive PU paint systems are categorized into the following categories: 1. One-component PU Systems: One-component PUs, cured by moisture, are easy to apply and are utilized for repair and maintenance because of their excellent mechanical properties. 2. Two-component PU Systems: The initial primary reaction occurs between the isocyanate and the polyols. After these two components are mixed, they start to react which results in an increase in viscosity, leading to gelation. The curing process takes place at lower temperatures, which provides advantages in processing and cost and the resulting coatings have many favourable properties. Once the two resins are combined, this kind of coating has a rapid curing reaction and high storage stability. These formulations may be water-borne, solvent-free and solvent-based systems. Water-borne two-component paints: Adding to the numerous favourable properties present in solvent-borne systems, water-borne systems emit fewer volatile organic compounds and are overall much more eco-friendly. These systems are used in numerous industries including machinery, technical textiles, wooden furniture, transportation and protective metal coating applications. Some water-based paint formulations may require up to 10% solvent to help the formation of the homogeneous polymer film. Coatings are cured at temperatures ranging from 20°C to 80°C. 3. Powder Coatings: These coatings do not require solvents and have good coating properties, but their application is relatively limited. Powder coatings provide durability, low-temperature flexibility, abrasion resistance and good aesthetics. 4. Stoving or Oven-curing PU Systems: Oven-curing paints are fabricated by forming a one-component pseudo mixture which is stable at ambient temperatures. When this mixture is heated to 100-200°C, which is its activation temperature, the isocyanate gets unblocked and reacts with the polyol, which results in coat formation.
Technology of Non-Reactive Coatings: Non-reactive PU systems do not contain free isocyanates, but completely formed polymers with urea or urethane linkages. The performance of the paints is a function of hydroxyl value of the polyol used and the level of branching. These coatings may also contain different additives such as solvents, thickening agents, pigments, flattening agents, extenders, levelling agents and catalysts for various applications, which are used to obtain specific properties. C&E formulates cutting-edge water-borne PUDs to fit coating requirements. We aim to create excellence by meeting our customers' requirements and manufacturing exceptional products. Information on our ‘Puthane’ range of water-borne PUDs can be found on our website.
By Ashni Arun on March 16th, 2022