Ciba has developed a new, patented photolatent-base curing technology for the automotive and industrial coatings markets.  

To date the number of photolatent compounds known to be inactive and stable in the dark but capable of efficiently generating a basic catalyst on exposure to light is very small, however, this novel technology allows previously inaccessible curing mechanisms to be used to design completely new resin types with improved coating properties.

It also opens up new opportunities for conventional coating systems. Furthermore, it allows economically and environmentally more attractive radiation curing.

• New photolatent base technology allows cure on demand of conventional systems  
• Adjustable-speed UV and daylight curing  
• Post cure of poorly exposed and shadow areas on 3-D objects  
• Extended pot life of 2K premix systems, reduced bake times  

Ciba's new photolatent-base curing technology is especially useful for 3-D objects for example in automotive and industrial sectors.  

Ciba has developed several new classes of photolatent bases, which are stable and inactive in the dark when incorporated in coating systems, but efficiently release tertiary amines or amidines on irradiation with UV or daylight. Some photolatent catalysts have specially been designed for hybrid-type curing, providing radicals and active base catalyst simultaneously.  

Strong bases, which are released by light from the photolatent bases, are true catalysts that can initiate polymerization of numerous different coating systems and enable cure on demand of these systems. These novel bases can be used in a wide variety of existing and completely new resin types. The new technology allows adjustable-speed UV and daylight curing, as well as post cure of poorly exposed and shadow areas on three-dimensional objects. It also extends the shelf life of coatings and the pot life of 2K systems, which have to be mixed prior to application.  

Recently, several new classes of photolatent bases have been developed at Ciba, which efficiently release tertiary amines or amidines upon irraditation with UV or daylight. Curing can be initiated in various coating systems, which cure for example via epoxy-epoxy crosslinking, Michael reaction or via polyol-polyisocyanate additions.

Some photolatent catalysts have specifically been designed for curing of hybride-type resins, providing radicals and active base catalyst simultaneously. The photo-generation of these bases proceeds via reactive radical intermediates which can at first initiate crosslinking among suitable unsaturated resin components, such as acrylic modified resins, unsaturated polyesters and unsaturated monomers. Subsequently the concomitantly generated reactive tertiary amine bases can act as suitable catalyst to provide additional base catalyzed crosslinking in suitably designed dual cure systems.

Photolatent bases for hybride type resins
Industrial significance for this type of photolatent bases has been attained in imaging systems such as resist materials for the electronic industry. After irradiation of the coating film through an image forming mask radicals are formed in a fast first step from the photolatent precursor, e. g. Ciba® IRGACURE™ 907, initiating an instantaneous radical crosslinking reaction among the unsaturated components of the resin. Within seconds the exposed spots on the substrate form a cured film; unexposed parts can easily be washed off by suitable developing processes. Thus an image corresponding to the mask is created.

After UV exposure in the first step the radicals have partially been consumed by concommitant hydrogen abstraction. Via steric release a reactive base catalyst is set free initiating a much slower crosslinking among carboxy and epoxy functionalized units of the resin in the second step. The rate of crosslinking can significantly be increased by thermal aftertreatment (post baking) to obtain the desirable perfomance of the resist material for subsequent processes in the electronic industry.

The post curing process occurs “in the dark” without further exposure to UV radiation. The fractions of released tertiary amine base are already sufficient to initiate the dark-curing process and to allow for a significant temperature reduction and an overall much more economic process.

Photolatent bases for polyol isocyanate crosslinking
Another type of photolatent catalysts has been developed at Ciba for curing coatings solely via base catalyzed crosslinking. On illumination with suitable UV or visible radiation strong amidine type bases such as DBN, DBU or TMG can effectively be released from the blocked precursor molecule on demand. The absorption spectra of this type of photolatent catalysts can be tuned to different wavelengths by introducing suitable modifications.

In parallel, properties such as solubility or the physical properties may be adapted from liquid to relatively high-melting compounds allowing to meet the requirements for various applications. Industrial relevance for this type of photolatent bases can be attained in coating systems based on polyol isocyanate crosslinking mechanisms. These resins are poly-addition systems widely used in the coating industry. Suitable catalysts can accelerate these systems, e. g. metal catalysts such as dibutyltin dilaurate (DBTL) or basic catalysts such as diazabicyclo octane (DABCO) or diazabicyclo nonene (DBN).

In our experiments we have employed a photolatent DBN allowing for the first time to cure a polyol-isocyanate system ”on demand” within a few hours at room temperature. In this case the latent precursor has deliberately been optimized to be sensitive towards daylight as trigger. Thus harmless fluorescent lamps (“artificial daylight”) could be employed to initiate curing.

Catalysis significantly speeds up the crosslinking among polyol and isocyanate. Uttermost advantage of the photolatent DBN vs DBTL or unblocked DBN is realized by its photolatency. A flexible window for application of the coating is provided to the end user and curing within a few hours can be initiated ”on demand” after triggering with daylight. Optimization of the photolatent base to radiation sources with strong emission in the UV region can further significantly reduce the curing time.

This benefit of photolatency and cure “on demand” is well documented in a increased pot-life of the photolatent vs. active formulations (DBTL, DBN) when stored in the dark. The formulation containing DBTL has lost its applicability at room temperature already after one hour whereas the photlatent formulation provides an extra application window vs catalyzed and uncatalyzed formulations of ca. 4 hours. The “ready-to-apply” paint can be applied for a relevant period of one working day and subsequently be triggered to cure on demand by the end user.

Depending on crosslinking mechanism and application requirements further photolatent bases of appropriate reactivity and absorption characteristics are available on request. On top, the sensitivity towards daylight or UV radiation and secondary properties such as solubility or the physical appearance may be fine tuned from liquid to relatively high-melting compounds allowing to meet the requirements for various applications on your request.

For further information and R&D samples, please contact
photobases@cibasc.com