Polyurethane dispersions (PUDs) are the market leader for coating substrates in various applications today. Water-borne PUDs are currently gaining a lot of attention as they are eco-friendly and emit very few volatile organic compounds without compromising coating performance. Many plastics may be sensitive to certain types of coating solvents. In many industries, water-borne PUDs are therefore used as effective substitutes for solvent-borne PUDs. Water-borne PUDs, due to their flexibility and durability, are extremely advantageous when used as coatings for plastic substrates. They have good abrasion resistance, excellent gloss and are able to adapt to different base materials to form superior coatings.
Coating performance can be subdivided into five segments- processing applications, chemical resistance, mechanical performance, UV and heat resistance and coating appearance.
Plastic coatings require durability, adherence, non-cracking and non-yellowing properties, flexibility, water vapour permeability, good lubricity properties and resistance to chemicals, abrasions, humidity, UV radiation and heat. Depending on the application of the plastic, soft/ medium-hard/ hard/ very-hard coatings can be formulated. Coatings formulated for plastic substrates may also need additional specific characteristics, when compared to coatings designed for metal or wood substrates. For example, plastic is usually non-conductive. Most paint lines possess high transfer efficiency and use an electrostatic spray gun for application. Plastic substrates therefore have to be prepared with a conductive filler or pigment, or coated with a conductive precoat layer before they can be applied using the electrostatic spray gun. These coatings must also be flexible and have similar elongation to the plastic substrate.
Plastics have a low value of surface tension, which leads to more difficult levelling and wetting of coatings on plastic substrates. Plastics also melt and deform at higher temperatures. Painted plastic parts are therefore usually cured at room temperature, or at lower temperatures than metal substrates (250°F for high bake plastics, 180°F for low bake plastics). Specific preparation may be required before coating to obtain good surface evenness of the plastic.
The coloured surface finish of plastic products are achieved by mixing the resins with solid pigments or liquid colourants during manufacturing. Painted finishes are useful in applications where a large amount of surface aesthetic qualities such as colour matching, gloss, and distinctness of image are needed.
When PVC, a plastic polymer, is taken into consideration, water-borne PUDs act as very good coatings as they have excellent abrasion resistance, are extremely flexible, have superior tensile strength, and display good resistance to plasticizer migration from the PVC. They can be used on their own, or can be crosslinked with additives such as defoamers, wetting agents and thickeners to improve their overall properties.
Due to environmental advantages, water-borne PUDs are the dominant choice for plastic coatings. We, at C&E, create excellence by providing durable water-borne PUDs under our ‘Puthane’ range for high-performance applications. Contact a C&E representative or visit our website to learn more!
By Ashni Arun on March 9th, 2022