1. Curing Behavior of UV-LED Curable Inks
Dr. Michael Peil, Susanne Hertsch, Harald Maiweg
Heraeus Noblelight GmbH, Germany.
Dr. Andreas Langenscheidt, Dr. Jens Simon
Tritron GmbH, Germany.

2. Outline
Perspectives and challenges of using UV-LEDs for digital inkjet printing
Experimental setup for studying the influence of dose, irradiance, and wavelength on the curing of UV-LED inks
Relevance of dose, irradiance, and wavelength on surface and deep cure
Conclusions and optimization possibilities of UV-LED systems for digital inkjet printing
Dr. Michael Peil,

3. LEDs: a Future Alternative Technology
UV gas discharge lamps are used in many industrial curing processes:
Drying of inks
Adhesives
Coatings
Exposure
Increasing relevance of energy and cost
UV-LED technology establishes for UV curing
Fast switching
Lack of infrared emission
Long life-time
Environmentally friendly
Dr. Michael Peil,

4. Peculiarity of UV-LEDs for Curing Applications
Conventional UV lamps emit in broad spectral range
LEDs emit monochromatic radiation
Dr. Michael Peil,

5. Using UV-LEDs for Curing Applications
UV-C
UV-B
UV-A
VIS
absorptivity
of typ. PIs
Only UV-A LEDs commercially available
Suitable curing materials required
Matching of UV-LED system and curing material required
Dr. Michael Peil,

6. Influence of Lamp Parameters on Curing of Inks
LED curable inks available for digital inkjet printing
Influence of relevant UV-LED lamp parameters on cure
Optimization potential of lamp parameters for UV-LED curing
Deeper understanding of the influence of dose, wavelength, and irradiance on surface and deep cure quality necessary
wavelength
dose
irradiance
Influence of wavelength on deep cure: R. Karsten, et. al.: Characterizing the Efficiency of UV-LED Curing, Radtech EU 2009.
Dr. Michael Peil,

7. Experimental Setup
LED unit
conveyor belt
card with
test ink
UV-LED systems with different wavelengths, power and irradiances mounted 10 mm above conveyor belt
Commercially available industrial UV-LED inks for testing
12 µm thick layer of ink applied on PVC cards
Experiments always without inertization
Dr. Michael Peil,

8. Variation of Lamp Parameters
NobleCure® LED units with peak wavelengths of 385, 395, nm and emission windows of 88x44 mm²
Variation of UV-LED lamp parameters:
Dose via speed of conveyor (5-150 m/min), channel selection, and dimming
Irradiance via channel selection (1-8, 1-4) and dimming ( %)
Wavelength by choosing LED unit
Dr. Michael Peil,

9. Optical Characteristics of LED Units
Wavelength intrinsically fixed in semiconductors (LEDs)
Different maximum powers depending on wavelength
Units have been measured at Heraeus measurement labs using calibrated measurement equipment
Dr. Michael Peil,

10. Properties of Inks
Tritron UV-LED inks of series V-Photon Wet - designed for 395 nm
Cyan (C) pigmentation
Black (K) pigmentation
C and K because of their maximum difference of absorption in the UV-A
Thickness of ink: 12 µm
Ink applied on PVC cards
Transmittance of C and K pigments diluded in ethanol
(without PI, same concentration)
Dr. Michael Peil,

11. Analysis Procedure
Evaluation of surface and deep cure quality with fingernail scratch testing
Individual analysis by two test experts leads to consistent results
Transfer of results to mark system  graphical presentation
Good cure = mark 2
Mark
Surface cure
Deep cure
1.0
absolutely scratch resistant, no damage possible
resistance against x double scratches x ³ 50
2.0
good scratch resistance, surface hardly damaged
3.0
moderately scratch resistant, surface slightly damaged
4.7
moderately scratch resistant, surface can be scratched
50 > x ³ 40
5.0
not scratch resistant, surface scratched, but not smeared
40 > x ³ 30
5.3
not sratch resistant surface scratched, slightly smeared
30 > x ³ 20
5.7
not scratch resistant surface scratched and smeared
20 > x ³ 10
6.0
surface smeared and wet
10 > x
Dr. Michael Peil,

12. Surface Cure Results K C C requires lower doses than K
Higher doses are required for longer wavelengths
Strong increase of dose from nm to 405 nm
Influence of irradiance on required cure dose rather small
385 nm
395 nm
405 nm
Dr. Michael Peil,

13. Deep Cure Results K C C requires significant lower doses than K
Similar doses required for and 395 nm
Strong increase of dose from to 405 nm
Only small dependence on irradiance
385 nm
395 nm
405 nm
Dr. Michael Peil,

14. Influence of Wavelength on Cure
Analysis of minimum doses required for cure (mark 2) at constant wavelength averaging over varying irradiances
Surface cure: minimum doses increase with wavelength
For C: strong increase of dose between 395 and 405 nm  sensitivity of PI
C requires lower dose at 395 nm than K  transmittance of C pigmentation
Deep cure: small increase of doses with wavelength
Smaller doses than for surface cure  absence of consumption of oxygen
K requires higher doses than C  higher absorbance of K pigments
Dr. Michael Peil,

15. Influence of Irradiance on Cure
surface cure
deep cure
Surface cure: minimum doses increase with irradiance
could be related to startup phase or saturation effect
For 385 nm for small irradiances similar doses required for C and K
Deep cure: trend for increase of doses with irradiance*
Significantly lower doses for C than for K  transmittance of C pigmentation
* Also observed for black ink by R. Karsten, et. al. Characterizing the Efficiency of UV-LED Curing, Radtech Europe 2009.
Dr. Michael Peil,

16. Summary
Full cure can be achieved with modern UV-LED systems without inertization
Cyan inks cure faster than black inks due to the higher transmittance of the pigmentation
Surface cure tends to require higher doses than deep cure; for shorter wavelength this difference may level
Better cure is achieved for shorter wavelengths than the design wavelength of the ink of 395 nm
Due to the spectral sensitivity of the photoinitiators (PI) curing degrades for longer wavelengths of >400 nm
Dr. Michael Peil,

17. Conclusions
UV-LEDs are suitable light sources for curing of industrial inkjet inks even without inertization.
The peak wavelength of an LED system is a relevant parameter for minimizing the required dose for full cure and thus process efficiency.
Cost effectiveness could be improved by realizing more uniform ink sets by reduction of the PI content in the more reactive ink thus levelling the different reactivity of black and cyan ink.
Dr. Michael Peil,

18. This presentation including all its parts (e. g
This presentation including all its parts (e.g. photographs, diagrams, drawings etc.) is protected by copyright. Any exploitation outside the close limits of the Copyright Act is inadmissible and punishable, unless pre-approved by Heraeus. This applies in particular to photocopies, publications, translations as well as storage and processing in electronic systems.
All data in this presentation were thoroughly ascertained by Heraeus. However, Heraeus does not assume any liability for their correctness or completeness.
The data are based on conditions of use and environmental influences assumed by Heraeus. They cannot be adopted indiscriminately, but require prior verification for the respective use of the customer.
Exclusively Heraeus’ General Terms of Delivery shall apply to all deliveries of Heraeus for commercial transactions with business enterprises.
Dr. Michael Peil,