Polyurethanes: Alternative crosslinking mechanisms
Due to the proposed restriction, diisocyanates have been in the spotlight and the search for alternative crosslinking mechanisms in the focus of R&D. We spoke to Lars Ossenschmidt, Technical Service Manager Paint, Building and Industrial Coatings of Worlée-Chemie, about the issue and approaches.
What is the current situation regarding the tightening up of isocyanate labelling?
Lars Ossenschmidt: Like any other hazardous substances, isocyanates should be handled safely by the user. Restrictions on mixtures containing diisocyanates have been proposed under REACH. For the sake of proper handling, multi-level user training is recommended.
Given that these products are used in so many diverse areas, a large number of users may have to be trained. This expected complexity is inducing users to explore substitute products. Due to the excellent properties of the polyurethanes used, it is of course difficult to find products that offer comparable or even superior properties. Our silane-functional polyurethanes, which contain urethane and urea groups, could close this gap because they provide excellent properties without requiring isocyanates for crosslinking.
What alternative crosslinking mechanisms is Worlée developing?
Lars Ossenschmidt
Worlée-Chemie
Ossenschmidt: The topic of alternative crosslinking mechanisms has been on our minds since 2015. In this context, we have looked at various possibilities but have ultimately decided to stick with the sol/gel silane process. The reasons for this were the necessary availability of the raw materials, the fundamental suitability of this technology and the ease of implementation in our production process.
Apart from being able to use our knowledge of the synthesis of the base polyols, it quickly became clear that it would make sense to further strengthen the backbone of the polymer via urethane groups.
Could you talk about the problem of the balance between drying and processing time?
Ossenschmidt: Classic 2K polyurethane coatings consist of a first coating component which contains polyols that have hydroxyl or amine groups. The second component is the corresponding isocyanate. These two components are mixed just prior to the application and can then react with each other. This reaction takes place within a certain period of time and is accompanied by a rise in viscosity. Depending on the reactivity, fast-drying coating systems can be obtained which, however, then also have short processing times, i.e. their viscosity rises rapidly. The usual processing times here are between one and 24 hours.
Our silane-functional polyurethanes allow a certain degree of disconnection between the drying time and the processing time. When the catalyst has been added, the reaction can start. This requires the silane groups to by hydrolysed by moisture and to split off the blocking agent. This is an equilibrium reaction, which takes place quickly only after application and not in the container.
It is possible to formulate coatings that exhibit very rapid drying and crosslinking, yet still have a long processing time. Typical processing times are between 24 hours and several weeks.
For which application areas are solutions available?
Ossenschmidt: As a manufacturer of binders and additives, we basically have a broad portfolio for various application areas and cooperate with a large number of customers. Since our silane-functional polyurethanes can be based on different polyols, such as polyesters, alkyd resins, acrylates, etc., it is of course possible to cover a wide range of applications. These currently range from industrial wood coatings and high-quality industrial topcoats for construction machinery and vehicles to glass and tile coatings, paints, varnishes and wood oils.
