A history of UV powder coatings
The use of unsaturated monomers and polymers, including polyesters, is successfully established in liquid coating resins as well as in Radcure technology for nearly 30 years. Their cross-linking reaction by polymerization leads to formation of stable carbon-carbon single bonds and results in resilient networks. The polymerization reaction is started by thermal heat using so-called initiators or by UV light in the presence of photo-initiators.
In the mid nineties the idea was born to transfer this technical approach into the powder coatings business. The intention was to enlarge the scope of powders into new fields of application using heat sensitive substrates like engineered or natural wood, medium density fiberboards (MDF), composites and plastic substrates. Such heat sensitive substrates were previously completely out of reach for the powder coatings technology. This led to the design and development of solid unsaturated polymers to enable UV technology as an alternative.
What are the advantages of UV curing powders?
As with other powder coatings resins, UV curing resins are solvent-free, emit little or no VOC and can be handled at the formulation and application stage as normal thermosetting powders. Whereas classical powders rely on temperature only for the melting and curing process, these processes for UV curing powders can be regarded as being separate. At the first step after electrostatic application, the melting process can be best performed by IR irradiation in very short times and at temperatures as low as 110°C. During the melting process there is no viscosity increase by the curing reaction resulting in much improved flow. Cure then follows by exposure to UV light within several seconds. Increased productivity and smaller footprint of the coating line makes UV curing powders an attractive alternative to other low bake systems.
Solutions based on UV curing powder coating resins are available for clear coats, pigmented powders for indoor and outdoor applications.
The curing of this group of unsaturated binders by Electron-Beam (EB), Laser and UV-LED will certainly further enlarge the application window in future.
UV curing powder coating resins for metal applications
UVECOAT® 2100 has been developed to allow application direct to metal substrates for industrial coatings and is a proven resin providing standard outdoor durability. UVECOAT® 2200 as another option in this category satisfies all requirements for architectural coatings; this is supported by Florida exposure data that demonstrates its superdurable character. UVECOAT® 2200 is superior in flow and smoothness of final finishes, but with lower degree of flexibility compared to UVECOAT® 2100 . Both grades have shown good corrosion resistance in ASTM B 117 tests of up to 2000 hrs.
For application on metal substrates it is crucial to fine tune reactivity of the unsaturated binder to avoid loss of adhesion due to shrinkage during the cross-linking process. A blend with UVECOAT® 9539, which was especially developed to improve adhesion at higher film thicknesses, enables formulators to tackle these issues with success. Furthermore, the flow out and wetting of the substrate surface can be improved by adding the specialty resin UVECOAT® 9010. Combining the components in the right way allows the achievement of appealing coatings designs on various metal substrates, heat sensitive metal alloys and sandwich elements.
Take a look at the section on Additives and Speciality Resins below for UVECOAT® 9010 and UVECOAT® 9146 sample requests.
- Outdoor durable coatings
- Application on metal substrates, heat sensitive metal alloys
- Powder applications on pre-assembled metal parts
- Application on heavy metal pieces
UV curing powder coating resins for applications on wood
UVECOAT® 3002 has been developed for high performance wood coatings; it has special suitability to formulate textured finishes and clear coats for indoor applications; correspondent coatings have good yellowing resistance.
UVECOAT® 3005 is another grade for the wood coatings segment has excellent pigment wetting and is preferred choice for pigmented coatings but can be used for clears too. Coatings based on UVECOAT® 3005 show excellent scratch resistance.
- Wood coatings
- Scratch and yellowing resistance
- Pigmented coatings and clears
UV curing powder coating resins for applications on PVC tiles
Preferred indoor application of UVECOAT® 3003 based powders are for coatings of PVC flooring tiles; coatings based on UVECOAT® 3003 are characterized by improved chemical- and abrasion resistance with good flexibility, they can be applied as single coat in one operation in high film thicknesses without formation of any unpleasant odors.
Application on PVC flooring tiles
Good chemical and abrasion resistance
Additives and Specialty Resins for UV curing powder coatings
Several additives and specialty resins were developed for blending with base binder resins. Like in a tool-box, they enable formulators to optimize film characteristics to achieve all specific requirements of intended applications.
UVECOAT® 9010 is a semi-crystalline co-resin for UV curable powder coating formulations; at the molten stage it significantly reduces the viscosity of the molten powder, enhances the substrate wetting and provides improved mechanical performance, flexibility and smoothness.
UVECOAT® 9146 is a unsaturated urethane acrylate with high functionality, introduced in UV curing powders it increases reactivity, cross-link density and hence chemical resistance and surface hardness of coatings.
Focal point for the development of UVECOAT® 9539 was the improvement of adhesion, especially for application on metal substrates in higher film thicknesses than usual. It can be blended with UVECOAT® 2100 and UVECOAT® 2200 in any ratio.
Relative reactivity of UV curing powder resins
The reactivity of UV curing powder formulations can have an impact on adhesion of the cured films. Since the reactivity of individual carbon-carbon double bonds depends on the molecular structure of the monomers, an indication of a carbon-carbon double bond equivalent weight can be misleading. The product mapping shown below can guide formulators to select the right resin for the desired reactivity.
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