In the wood coating industry, water-borne PUDs have significant potential in various applications and they are currently gaining a lot of attention. As they are easy to use, eco-friendly, free of formaldehyde and emit low volatile organic compounds, they are effective substitutes for solvent-borne PUDs.
Water-borne PUDs have good heat resistance, quick drying technology, atomization and high bonding strength. Their superior qualities make them suitable to match the natural characteristics of wood. PUDs have low molecular weights which allow them to penetrate into the cell walls of the wood. The free isocyanate group present in PU resins forms strong chemical bonds with the hydroxyl groups present in the wood. They are preferred for wood coatings as they preserve the color of the wood, dry quickly and emit fewer volatile organic compounds. As PUDs are low viscous liquids, more coats have to be applied. They also require high ventilation during application.
When used in the topcoats of wooden furniture, the favorable characteristics required for good performance include resistance to chemicals and scratches, hardness, good leveling and adhesion. Coatings for wooden furniture and wood-based flooring also require resistance to water, UV and chemicals, good optical transparency, maximum durability, excellent abrasion, resistance, high flexibility and glossy properties.
Some favorable properties of water-borne PUD wood coatings include low VOC-emitting, good sandability, abrasion resistant, favorable lifting resistance, high drying speed, resistance to yellowing, good pore filling capacity, suitable speed of water release, ability to withstand wear and tear, resistance to extreme weather conditions, NMP-free, resistance to UV radiation and chemical agents, good balance between flexibility and strength, low-temperature crack resistance, good adhesion to various surfaces, suitable mechanical resistance and good water vapor permeability.
Properties of wood significantly affect the performance of coatings. Some characteristics that should be studied when understanding and studying the performance of wood coatings are flexibility, wettability, density, moisture content, surface texture, grain properties, wood extractives and knots.
Flexibility- When wood absorbs moisture and is subsequently dried, it leads to swelling and shrinking. Hence the coating’s flexibility plays an important role when determining its durability. Measuring the glass transition temperature (Tg) gives a definition for the measure of coating flexibility.
Density and Moisture Content- Weathering is caused by moisture, wind and UV radiation exposure and has effects on the wood surface. Wood surface coatings can significantly reduce weathering degradation. Coatings have better performance on woods with lower density than on woods with higher density, as they possess lesser swelling and shrinkage. The moisture content of the wood also has an effect on the performance of wood coating. Coatings should not be applied on wet wood as it may fail due to adhesion problems, because when the wood dries, it will be shrinking and moisture migrates out of the wood surface. Wood, in response to surrounding relative humidity conditions, adsorbs and desorbs moisture and eventually reaches an equilibrium moisture content which is typically around 12-15% MC. The coating should be applied when the wood reaches a moisture content which is somewhat close to its average equilibrium moisture content.
Wettability- It is the capability of any liquid to spread on a substrate due to surface tension. Testing the wettability of the coating is an important step to understand its adhesion to wood or any substrate. For the coating to possess good wetting properties, they should possess much lesser surface tensions, compared to the substrate surface energy.
PUD wood coatings are classified into one-component and two-component coatings. One-component PUD wood coatings comprise one ready-to-use product. The two-component coatings are made by mixing two products (resin and hardener) together. One-component PUD coatings are selected as they do not require any additives and hence they are easy to use. Two-component PUD coatings are still preferred over one-component coatings as they give better results due to higher abrasion and chemical resistance, good durability, and larger conservation of coating film. The two-component PUD coatings also possess greater surface hardness, higher resistance to other atmospheric agents and provide better adhesion on substrates. When two-component PUD coatings are selected, the type of hardener and thinner should be selected as well.
An issue with water-borne PUDs is that, when compared to organic solvents, they have a low evaporation rate of water. To decrease the time required for drying, and make sure the film formed is dry, with no defects, small amounts of additives are added to the formulations. They are therefore usually used as two-component PUD coatings. Surfactants, wetting agents, UV-stabilizers, co-solvents, mildewcides, dispersing agents, driers, coalescing agents and defoamers are usually used as additives in water-borne wood coatings and can modify coating properties.
Different properties of the coating, interactions between the components of the coating and wood and changes in surface characteristics of wood after modification must be considered to formulate superior, long-lasting wood coatings. C&E’s ‘Puthane’ range of water-borne PUD coating formulations are fabricated to provide clear, strong and durable wood coatings. Contact us to learn more about how our products can enhance and improve the properties of wood.
By Ashni Arun on Feb 11th, 2022