2009

Today’s polyurethane formulators face a number of hurdles when trying to achieve the desired film properties and appearance for their systems. Whether the target is a high gloss finish for a VOC-compliant coating, a fast-curing adhesive or a strong, flexible defect-free sealant, the issues facing PU formulators when trying to develop new technologies include:
• retaining and improving performance cure and physical properties of PU systems;
• meeting legislative demands such as volatile organic component (VOC) reduction; and
• reducing toxicity by decreasing isocyanate concentration.

A major issue is the ongoing battle to deal with moisture, which is often attracted through hygroscopic polyols and solvents or present in pigments, fillers and plasticisers. The reaction of moisture with isocyanate can seriously compromise performance (such as film strength) and also appearance, through the generation of carbon dioxide.

Oxazolidine technology, available in the Incozol range of products, offers an alternative low-toxicity solution to formulators of both one and two-component PU systems in overcoming formulation issues related to moisture. All oxazolidine reactions are triggered by contact with moisture: it is their preferential reaction with water that helps formulators limit any moisture-isocyanate reaction problems.

Oxazolidines can be used in aliphatic prepolymers and ones based on TDI (toluene diisocyanate), and a number of grades can be used in MDI (methylene diphenyl diisocyanate) systems. However, oxazolidine products offer additional benefits other than simply inhibiting the reaction with moisture, some of which are described below.

Improvements in performance
Formulators of sealants, adhesives and high-build elastomeric coatings are always striving to improve performance properties. Traditionally, polyurethane systems incorporate a high level of isocyanate so that they cure at an effective rate, often with the incorporation of toxic catalysts such as lead, mercury and more recently tin types.

Oxazolidine technology, however, offers a number of routes forward. For example, high-solids one-component systems can incorporate oxazolidines in a number of ways. The oxazolidine presence not only reduces problems associated with carbon dioxide generation, but enables PU manufacturers to reduce the level of isocyanate in the formulation.

This is achieved by mixing the oxazolidine, initially latent in the formulation, with the formulation components to give in-can stability. On application, and subsequent exposure to moisture, the oxazolidine-moisture reaction yields amino alcohol functionality, which can effect a thorough cure at much lower isocyanate concentrations, by a so-called ‘moisture-triggering’ route.

This ‘moisture-trigger’ approach allows formulators to reduce prepolymer isocyanate contents from typically 10-15 percent to much lower levels of 2-5 percent.

Incozol 4, Incozol HP and Incozol NC are oxazolidines that can help formulators achieve this aim. Users can select specific products based on property needs. Furthermore, the oxazolidine latent-curing agent increases the level of crosslinking through its four-functionality (two secondary amine and two primary hydroxyl groups). This helps formulators improve cure rate and raise the tensile strength to the coating, adhesive or sealant.

Oxazolidines offer formulators other routes. The reaction of a hydroxyl-functional oxazolidine such as Incozol 3 enables PU formulators to partially end-cap the isocyanate groups in the prepolymer to further reduce isocyanate content. This approach can also impart other benefits such as early tensile strength development through latent in-situ crosslinking on exposure to moisture.

Reducing VOCs, improving appearance
Legislative pressure continues to force formulators to find ways to reduce solvent use to make products more environmentally benign.

Low-viscosity oxazolidine reactive diluents such as Incozol LV allow the design of 2K PU high-solids systems, with the added benefit of eliminating gassing. Incozol LV is a four-functional bis-oxazolidine curing agent that significantly reduces hydrogen bonding through its carbonato-link and has a low viscosity of 50cps, allowing formulators to reduce product viscosity very effectively. This allows formulators to continue to use the same application tools. It also eliminates the need to retrain operators or dispose of contaminated waste associated with using alternative water-based products.

As mentioned, oxazolidines can improve coating appearance by eliminating carbon dioxide pinholes which occur as a result of the moisture-isocyanate reaction. Pinhole defects can result in down-glossing. More significantly they can compromise film integrity, and lead to loss of strength or ease of attack by chemical reagents.

Recent developments
Recent developments at Incorez include a high purity urethane bis-oxazolidine, required for applications where stability problems between the oxazolidine and prepolymer have occurred. A catalyst-free grade has also been developed to address these stability issues.

In addition, a non-crystallising grade of urethane bis-oxazolidine that remains liquid at temperatures down to -10°C, thereby reducing the energy and handling costs associated with melting the product, has also been developed.

Main applications
Oxazolidine technology has been widely adopted in PU coatings, sealant and adhesives. The coatings sector has adopted the technology for 2K, high-solids systems for automotive, marine, wind turbine and aerospace uses, while in 1K PU coatings it is used in industrial maintenance.

Oxazolidines are also widely used in 1K aliphatic and aromatic PU cartridge sealants. These high-performance elastomeric sealants require no mixing and typically no priming to adhere to concrete and masonry.

There is also growing interest in the use of oxazolidines in 1K reactive hot-melt adhesives, where they accelerate cure rate, and improve both green strength and crosslink density.