The chemistry of polyurethane
Polyurethane dispersions (PUDs) are often the preferred polymer type of the base coating formulators water to enhance the performance properties, such as hardness, scratch resistance and chemical resistance in a wide variety of applications. PUDs can bring these advantages and often even combining with other traits such as low temperature flexibility and reduced "tacky," in which other polymers such as acrylic fail to match these properties with the same ease of PUD's.
Polyurethanes were developed by Otto Bayer in 1937, becoming a fantastic success story and a business of many billions of dollars in today's world. They are produced by polyaddition reaction of a diisocyanate (di- or polyfunctional) with a polyol and other reagents as curing agents or chain extenders containing two or more reactive groups; catalysts; blowing agents; surfactants; fillers; anti-aging agents, colorants & pigments, flame retardants, mold release agents, etc.
The isocyanates may be aliphatic or aromatic. The hydroxy compounds may vary in molecular weight, chemical nature and functionality. The polyols may be polyethers, polyesters or possess hydrocarbon structure. The chemical nature and the functionality of the reactants must be chosen according to the desired final properties. This flexibility makes it possible to obtain materials with different physical and chemical properties, and makes the PUs occupy important position in the world market of high-performance synthetic polymers.
The commercial development of PU's began in Germany in the late 1930s, initially with the production of rigid foams, adhesives, and paints. Elastomers had their origin in the 1940s, Germany and England. During World War II the development of PUs has been discontinued, but since 1946 its market has shown tremendous growth.
Polyurethanes are formed by the polycondensation reaction between polyisocyanates and polyols, usually diisocyanates and diols give the urethane group,
Reaction formation of the polyurethane.
The main polyisocyanates used in the polyurethane production may be aliphatic or aromatic. Based on this choice is possible to obtain various properties of the final product by a combination of isocyanates chosen with other raw materials such as polyols and additives.
As isocyanates, polyols play a key role in defining the final properties of the polyurethane obtained, directly influencing on various properties. The differences between the properties of polyurethanes made with different types of polyols is shown below.