Waterborne polyurethane dispersions
Waterborne polyurethane dispersions
Keywords Dispersions,Polymers, USA, Waterborne
Waterborne polyurethane dispersion polymers have been available in the USA since the early 1970s, when they were introduced specifically for plastics and leather finishes. Over the past 25 years, the acceptance of waterborne polyurethane dispersions throughout various industries such as glass fibre sizing, car coatings, printing inks, adhesives, textiles, leather and wood coatings has continued an accelerated growth pattern. Over 40 million pounds of waterborne polyurethane dispersions are consumed a year, with the largest application segment being wood coatings.
The rapid growth and continued use of waterborne polyurethane dispersions is driven primarily by the exceptional performance properties in the areas of abrasion, chemical resistance, toughness, flexibility and adhesion. Another beneficial aspect is the low VOC content associated with coatings that utilize waterborne polyurethane dispersions.
Basically there are two types of isocyanates used in waterborne polyurethane dispersions: aliphatic and aromatic. And three types of polyols: polyesters, polyethers and polycarbonates.
Polyurethane dispersions based on aliphatic isocyanates are the most common and, therefore, are produced in the largest quantity and are available in the widest number of variations. The primary benefit of aliphatic isocyanates is light fastness.
Commercial aliphatic waterborne polyurethane dispersions are available based on H12MDI,IPDI, TMXDI and HDI. The specific choice of isocyanate is often critical in determining the final properties of the polymer.
Polyurethane dispersions based on aromatic isocyanates are used to a lesser extent, typically for primer and adhesive applications where colour retention is not required. While aromatics do not provide the same level of light fastness as aliphatics, they do offer good chemical resistance and are available at a lower cost.
The other main chemical difference between polyurethane dispersions is the polyols used to produce them.
Polyester-based dispersions provide good adhesion, good UV resistance and fair hydrolysis stability. They are moderately expensive and are used as coatings on a variety of substrates, including wood, plastic and textiles requiring only modest hydrolysis resistance.
Polyether-based dispersions offer excellent hydrolysis resistance and moderate adhesion, yet provide poor UV resistance. While they are the least expensive of the polyols,polyether-based dispersions generally exhibit lower tensile strength than those based on polyesters. They are primarily used on substrates that require good hydrolysis resistance, such as textile coatings.
Polycarbonate-based dispersions are employed in applications that require optimum overall performance because they provide excellent toughness, adhesion, hydrolytical stability and UV resistance. They are, however, very expensive and used only in specialty applications where the highest degree of durability is required.
Waterborne polyurethane dispersions provide many unique performance benefits. Coatings formulators have found that they can often utilize the unique properties of a waterborne polyurethane dispersion in a blend with another polymer and provide the benefits of a waterborne polyurethane dispersion at a lower cost per gallon.
Waterborne polyurethane dispersions are utilized in blends with polyesters, alkyds, acrylics and other waterborne polymers. The most common technique is to blend with acrylic emulsions, which results in coating systems with enhanced adhesion, gloss,hardness and hardness development properties.
To ensure the greatest success when blending these types of polymers, formulators must evaluate both the acrylic and polyurethane choices for compatibility. To test for compatibility, one must look for coagulum generation and viscosity stability over time.
However, particularly with clear wood finishes, a clear, haze-free coating should be achieved. Drawing the coating down on black glass is one of the best test methods employed to evaluate this characteristic.
The first waterborne polyurethane dispersions were developed in the early 1970s. While these"conventional" dispersion polymers are still widely used throughout industry, a series of technology advancements has led to the introduction of new polymer types. They include self-crosslinking waterborne polyurethane dispersions,interpenetrating polymer networks (IPBs) and, more recently, polymer hybrids.
Self-crosslinking waterborne polyurethane dispersions were introduced during the 1980s and incorporate reactive functional groups within the polymer backbone, bringing improvements in abrasion, chemical and scuff resistance.
The introduction of IPN chemistry, sometimes referred to as polymer alloys, created polyurethane acrylic systems that provide a balance of properties similar to those of a polyurethane dispersion aqueous acrylic emulsion "cold blend". However, the IPN chemistry eliminates the extra step of cold blending, making it easier for the coatings formulator to use a single IPN polymer.
Most recently, there is a growing development of interacting, or hybrid, systems. These systems combine polyurethane dispersions and acrylic emulsion materials that interact with self-crosslinking mechanisms. The development of these chemistries has resulted in polymers with greater performance enhancements over conventional systems. These enhancements include improved abrasion and chemical resistance, mar and scuff resistance, and hardness development. They also provide the same ease-of-formulating as IPN polymers.
