Visem Technodag 18.06.2013

Radiation Curing of Coatings:

From Traditional UV Curing to Prospective

Coating Applications with NIR-

Photopolymers

Christian Schmitz

visible

wavelength

ultraviolet UV-C / UV-B / UV-A

(near)infrared (N)IR

Radiation Curing

380nm 780nm

coated substrate

light source: mercury arc lamp

- fast curing by high-intensive, UV emitting light sources

- reduce of organic solvents

environmentally-friendly technology

UV-Curing vs. Thermal Curing

- solvent

- resin

- crosslinking of resin and reactive solvents

- no evaporating of reactive solvents

UV

heat

- solvents reduce viscosity

- thermal-induced crosslinking of resin

- simultaneous evaporating of the solvents

UV-Curing via Radical and/or Cationic

Polymerization I - Initiator R - reactive resin/solvent

I UV-radiation

generate high-reactive species

I R

growing by chain reaction

I R R R ...

- UV-radiation absorption of initiator and photochemistry generates radicals/cations

- increase of network density by chain reaction

- solidifies coating resulting in film-formation --> fast proceeding in every step --> short curing period

Applications of UV-Curing

Advantages specify the field of application - printing-ink - wood coating - powder coating --> economic aspect

--> heat sensitve substrate; high output

*www.kem.de/konstruktion/-/article/30599594/31436464/maximized/

More and more applications

are going to switch to these

energy-saving technologies!

Limitations of UV-Curing-Systems

- curing is influenced by thickness of coating --> loss of adhesion

- critical for depth-curing particular if system contains UV-absorbing components

- optimized OD for a system with no bleaching: 0.43

- inhibition of curing process by atmospheric oxygen

- need of safety installations --> risk for operator (eyes, skin) caused by UV-radiation

radiation

coating layer

Selection of Light Sources

- emission must fit the absorbance of the initiator

- energy-efficiency by maximal overlapping

Research HS-Niederrhein: NIR-

Photopolymerization and new Light Sources

visible

wavelength

ultraviolet UV-C / UV-B / UV-A

(near)infrared (N)IR

380nm

- (n)ir-radiation well-known as thermal-energy source

- shift of the absorption --> solve problems of competing absorption in the UV-range

- initiatorsystem consists of two components

780nm

sensitizer absorbing component

coinitiator generates radicals

UV-curing vs. NIR-Curing

UV NIR

- high sensitivity

- established curing system

- yellow, red and blue pigmented coatings

- curing of layers upto several mm in just one step

- NIR-light uncritical in handling

- problematic with pigments or light stabilizers

- harmful UV-light

- requires safety light shielding

- unknown to coating-technology

- less sensitive than UV-systems

Research HS-Niederrhein: NIR-

Photopolymerization and New Light Sources

NIR-LED arrays

- small range of emission suitable to the sensitizer

- low intensity

- requires development of more intensive LEDs in different wavelengths including UV, visible and NIR

IR-radiant heater

- wide range of emission

- small amount of usable emission

Diode laser

- laser spot emission needs modification for exposure of large areas

- x,y-modulation of the emitter

- homogenous laserlines *Beier; Laser Technik Journal Vol 8, No.2 (2011) *Homburg et al.; Proc. of SPIE Vol 6216 (2006)

Laser technology as NIR Light Sources

x,y modulated laser or objekt

diode laser bars

line generator

NIR-Photopolymerization in Powder Coating

Technology

- first experiments with NIR-polymerizable powder coatings (at 110C)

- state of the arts: two-step curing process

- diode laser for dual process technology --> melting by energy absorption and curing process simultaneously

melting process

< 120C application radiation curing

Thank you for your attention!

Acknowledgement

- BMWi (ZIM cooperation project, grant # KF2914003BN2)

- University of Applied Sciences (start up funding)

- County of NRW (scientific equipment program)

- FEW Chemicals GmbH (supplying of NIR sensitizers)