From chemistry perspective, polyurethane coating is a polymer formed by the polymerization reactions of isocyanates and polyethers. It is processed by blending the prepolymer with catalysts, solvents and other additives to achieve specific properties.
Polyurethane synthesis reations
In actual applications, PU coating is a type of protective finish or layer applied to various surfaces, such as wood, metal, concrete floor, construction element, protecting them from wear, abrasion, chemicals and other environmental factors. Apart from serving as a barrier to protect the substrates, polyurethane coating can also be formulated into a customized finish that enhance the aesthetic appeal. They could be glossy, matte, transparent or in any color depending on the desired effect.
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Polyurethane (PU) coatings are classified into one-component (1K) and two-component (2K) based on their formulation and application method.
One-component polyurethane coatings include urethane oil coatings, moisture-curing and blocked polyurethane coatings. With the Isocyanate Polyol Reaction already occured during the manufacturing process, these coatings are pre-mixed formulations with no additional mixing required before applications. And thus one-component PU coatings come ready-to-use directly from the container, significantly enhancing the efficiency of coating operations. These coatings find common applications in areas like floor coatings, anti-corrosive coatings, and coatings for pre-coated materials.
Two-component PU coatings were formed by a chemical reaction between the two components after mixing, forming a robust crosslinking network that offers higher abrasion resistance, chemical and weather resistance. In contrast, one-component PU coatings do not require crosslinking; they rely on solvent evaporation to form a film (a physical reaction). While 1K coatings provide simple installation, they exhibit limited properties compared to 2K PU coatings.
Two-component polyurethane coatings are typically composed of two parts: isocyanate prepolymers (also known as low molecular weight urethane polymers) and hydroxyl-containing resins, commonly referred to as curing agent component and main agent component. Depending on the hydroxyl-containing component, they can be categorized as acrylic polyurethane, alkyd polyurethane, polyester polyurethane, polyether polyurethane, epoxy polyurethane, and more. They generally exhibit excellent mechanical properties, high solid content, and overall good performance.
Two-component polyurethane coatings find wide applications in wood coatings, automotive OEM and refinish, industrial anti-corrosive coatings, floor coatings, polyurethane waterproof coatings, and more. Compared to 1K PU coatings, two-component polyurethane coatings require strict environmental conditions and include complex application process.
PU coatings feature excellent resistance to abrasion and chemical, strong adhesion, wide range of hardness to protect the substrates in different applications, ensuring long-lasting protection against various environmental factors. For applications that need to withstand the challenging environments, PU coatings are the most ideal choice with the ability to maintain a long-term performance. Besides, these coatings can be formulated to achieve custom properties and decorative performance.
Although polyurethane coatings are suitable for a wide range of applications, it still comes with some shortcomings such as sensitivity to moisture. Moisture exposure can lead to reduced adhesion and form bubble that leave the substrate uneven on the surface. And thus polyurethane coatings are not suitable for applications with a higher level of humidity.
Furthermore, PU coatings contain free isocyanates which can bring harmful effect to human health during applications such as respiratory issues and allergic reactions. It’s essential for workers involved in isocyanate exposure to appropriate personal protective equipment to minimize the potential effect.
Epoxy and polyurethane are both popular coating solutions, but they are suitable for different applications with unique properties. Epoxy coatings, which are made from epoxy resins, tend to provide a higher level of strength and adhesion, making it a better choice for priming on harder floor. Polyurethane coatings, on the other hand, can create finish that is more flexible with enhanced decorative properties.
Epoxy resin contains polar functional groups such as hydroxyl and ether bonds, creating strong intermolecular forces and, in some cases, chemical bonds with adjacent surfaces. This inherent molecular structure provides epoxy coatings with exceptional adhesion and ability to create a strong and seamless bonding with concrete floor both chemically and mechanically.
Epoxies form a durable and thick coats with superior impact and abrasion resistance. These coatings can protect the floor surface against the heavy impact from vehicles, constant movement of industrial machinery, engineering facilities. Moreover, epoxy coatings remain crack-free even amidst temperature fluctuations and other challenging environmental factors.
The installation of Epoxy coatings is very straightforward as it can cure at room temperature, highly adaptable to diverse environments and suitable for various substrates. Upon application, epoxy coatings can easily form a durable and smooth surfaces on concrete flooring with shorter curing time. Featuring a more advantageous price than PU coatings, epoxies provide a much more cost-effective coating solution as primer or topcoats.