The family of isocyanates (symbol NCO) is very broad, encompassing a range of compounds that act as cross-linking partners for resins with hydroxyl groups, forming urethanes. This is why products made with these compounds are called polyurethane paints or coatings. However, the isocyanate family is vast, and its members can vary significantly, often allowing for different combinations, resulting in a wide range of end products with varying costs and performance.
Definition of “Polyurethane”
The term “polyurethane” by itself doesn’t reveal much, as the number of urethane bonds formed can vary significantly. For example, forming three urethane bonds is not the same as forming twelve, and the polymerization times can differ accordingly. An essential parameter is the hydroxyl value of the base resin, along with the number of NCO groups in the isocyanates used for the reaction. When all possible bonds are formed, the system reaches stoichiometry, achieving optimal performance. Correctly calculating stoichiometry is complex and should be left to experienced chemists. It’s best to avoid changing the usage percentages unless one has a deep understanding of the underlying chemistry.
Families of Isocyanates
Isocyanates are primarily divided into two main categories: aromatic and aliphatic. Aromatic isocyanates, like toluene diisocyanate (TDI), are more reactive, so they polymerize more quickly and are more cost-effective. They offer good hardness and chemical resistance but are susceptible to ultraviolet rays, leading to yellowing, dullness, and flaking when exposed to prolonged sunlight. Therefore, aromatic isocyanates are typically used for primers that will be covered with a topcoat and for interior finishes.
Aliphatic isocyanates, like hexamethylene diisocyanate (HDI), are UV-resistant and do not yellow or lose gloss. However, they are more expensive and are generally considered higher quality. There are also mixed catalysts on the market, which combine the characteristics of both types, offering variable performance based on the ratio of the two types of isocyanates.
Types of Isocyanates: Flexible and Rigid
Isocyanates can also be classified as flexible or rigid. Rigid isocyanates (generally belonging to the aromatic or isocyanurate family) are typically used for sandable primers due to their superior sandability. Flexible isocyanates are more suitable for topcoats because their flexibility reduces the risk of future cracking or spider-webbing in the paint film.
Residue and Solid Levels
Another factor to consider is the solid content of the catalysts, which determines the amount of isocyanate excluding the solvent. The solid content levels are typically classified into MS (medium solid), HS (high solid), and UHS (ultra-high solid), with values ranging from 25% to over 42%. Higher solid levels can lead to specific uses in textured products, but they can cause curing issues, particularly in hot environments.
Safety Regulations
The European Union has recently recognized the high toxicity of diisocyanates used in auto body shops and industrial coatings. As a result, diisocyanates with high levels of free monomers are gradually disappearing, being replaced by similar diisocyanates with lower free monomer content. This transition has led to increased prices due to the difficulty of sourcing these substances on the international market.
If you have any doubts or questions about this complex topic, which is of critical importance to applicators, feel free to contact us for further clarification.
Article written by: Ind. Exp. Massimo Rubbi – A.I.T.I.V.A. member, Emilia Romagna Region – CTU n. 48, Piacenza Court. Tel: 0039 3289205637.
