Water-based UV coatings cured with LED systems offer several advantages over mercury lamp systems, including being more environmentally friendly and allowing curing of heat-sensitive substrates. However, oxygen inhibition can reduce curing depth and conversion rates for some systems. Using phosphine oxide photoinitiators and minimizing oxygen penetration through physical drying of dispersions can help overcome this. New thiol-modified chemistries also show promise for oxygen inhibition-free curing and shadow area cure. Proper selection of photoinitiator and coating type are important to realize the full benefits of LED curing for water-based UV systems.

1. 1
Water-based UV-
systems with the focus
on LED-curing
Décio Lima, Klaus Menzel
BASF SA
S-ED/SET
decio.lima@basf.com
Tel.: +55 12 3955-1549

2. UV-LED-Lamps:
What advantages are discussed
 “Cold” curing
 No IR-radiation to the substrate but the diode needs to be cooled
– Perfect for heat sensitive substrates (e.g. plastic foil)
 Standby time extremely short
 Perfect for non continuous application processes
– Helps to save energy
 Environment-friendly
 No mercury is needed / used
 No UV-B and UV-C  ozone free process
– No need to connect the UV-unit to the waste air system

3. UV-LED-Lamps:
What advantages are discussed
 Safety
 Since no UV-B and UV-C wavelengths will be emitted – less risk for
human skin and eye
– Less protection needs to be designed at the UV-unit
 Small and flexible lamp design
 LED-UV-lamps can be easily carried by an robot- since they are light
and small (ink-jet application)
 Different diode arrays can be combined to adjust the lamp to the
substrate
 Reduced power consumption compared to an mercury lamp
 Seams to be right but needs an individual case study

4. LED-UV versus Mercury vapor Lamp
230 255 280 305 330 355 380 405 430 455
Wavelength (nm)
Intensity
LED: 395 nm ± 20 nm
The different wavelengths distribution affects heat-, ozone-development
and the selection of raw materials (photoinitiators, binders, …)
Source: Phoseon
Mercury lamp

5. Wavelength versus Irradiance
Choose the right LED-lamp
High peak irradiance helps to reduce oxygen inhibition and allows a
larger lamp / substrate distance
Source: Phoseon
All tests where done with an
395nm; 8 W/cm² LED-UV-lamp

6. LED-UV lamp – photoinitiators
what fits best …
The absorption characteristic of the initiator has to fit into the narrow
emission spectrum of the LED-UV lamp
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
320 330 340 350 360 370 380 390 400 410 420
Wavelength [nm]
Extinction
BAPO
MBF
MAPO
HCPK/BP
LED 395 nm
BAPO – bis(2,6-trimethylbenzoyl)-phenylphosphineoxide
MAPO – 2,4,6-trimethylbenzoyl-diphenylphosphine oxide
MBF – phenyl glyoxylic acid methyl ester
HCPK – 1-hydroxy-cyclohexyl-phenyl-ketone
BP – benzophenone

7. Photoinitiator / UV-lamp relation
100% UV resin and a-Hydroxy-ketone PI
Coating:
Resin: 100% PE (Polyester acrylate)
Initiator: HPCK/BP
UV-source: mercury / LED
Atmosphere: air / N2
UV-source: Mercury lamp
atmosphere: air
UV-source: LED-UV
atmosphere: air
UV-source: LED-UV
atmosphere : N2
 a-Hydroxy-ketone PI does not work with an LED-UV-lamp
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 5 10 15 20 25 30
depth [µm]
conversionrate[%]

8. Coating:
Resin: 100% PE (Polyester acrylate)
Initiator: BAPO
UV-source: mercury / LED
Ambiance: air / N2
UV-source: LED-UV
atmosphere: N2
UV-source: mercury
atmosphere: air
UV-source: LED-UV
atmosphere: air
 Phosphine oxides do work with an LED-UV lamp but need N2 atmosphere to
overcome oxygen inhibition (under air: surface still tacky)
0%
20%
40%
60%
80%
100%
0 5 10 15 20 25 30
depth [µm]
conversionrate[%]
Photoinitiator / UV-lamp relation
100% UV resin and Phosphine oxide PI

9. Coating:
Resin: PUD (UV-Polyurethane
acrylate dispersion)
Initiator: BAPO (emulsified version)
UV-source: LED-UV
Ambiance: air / N2
UV-source: LED-UV
atmosphere: N2
Physical dried (15 min 60°C)
atmosphere: air
UV-source: LED-UV
atmosphere: air
 Conversion rate almost independent on curing atmosphere (air or N2);
surface always tack-free, minimized oxygen inhibition
20%
30%
40%
50%
60%
70%
80%
90%
0 5 10 15 20 25 30
depth [µm]
conversionrate[%]
Photoinitiator / UV-lamp relation
Waterbased resin and Phosphine oxide PI

10. Curing Conditions versus Conversion Rate
12
22
424
50
60
70
80
90
1 2 3 4 5 6
conversion[%]
conversion [%] difference to LED/ N2
PUD (water-based)
Curing after drying
PE (100%)
UV-lamp:
Atmosphere:
Initiator:
Mercury . .
Air .
HPCK / BP .
Mercury .
Air .
BAPO .
LED .
Air .
BAPO .
LED .
N2 .
BAPO .
LED .
Air .
BAPO*.
LED .
N2 .
BAPO* .
* Emulsified version

11. Chemical resistance versus
curing conditions
Residence
time
Detergent
PE (100%)
LED / air
PE (100%)
LED / N2
PUD (water)
phys. dried
PUD (water)
LED / air
PUD (water)
LED / N2
16 h coffee 1* 3-4 1 3-4 4
6 h Mustard 1* 3 1 3 3-4
6 h Red wine 1* 4 1 4 4-5
1 h Ethanol 1* 4 1 5 5
1 h Detergent 1* 5 1 5 5
2 min Ammonia 1* 5 1 5 5
10 sec Acetone 1* 5 1 4 4
5 = excellent 1 = bad 1* = not suitable due to tacky surface
 for water-based dispersions the curing conditions do effect the chemical
resistance just marginal

12. Types of water based UV-products
Dispersions:
• Polyurethane 40% solid content
• best hardness / flexibility relation
• lowest viscosity of the liquid binder
• physical drying independent of UV exposure
Emulsions:
• 100% products (polyester acrylates)
modified with a protective colloid
• 50% solid content
• no physical drying
Water soluble:
• 100% solid content
• Polyether- or Epoxy acrylate
• highly hydrophilic backbone
• soluble with up to 25% of water
• no physical drying
 different types show different characteristics

13. Does this excellent behaviour apply for all waterbased UV coatings?
Hardness after drying resp. curing
0
20
40
60
80
100
120
140
160
UV-water soluble
(polyether acrylate)
UV-emulsion
(polyester acrylate)
UV-dispersion
(polyurethane acrylate)
pendulumhardness[osc.]
water evaporated / uncured (physical dried) LED-air LED-N2
solid
 Physical drying stops the oxygen inhibition
= PD Test not possible
tacky
Liquid(0,5Pas)
Liquid(6,0Pa*s)
tacky

14. Why does physical drying affect oxygen inhibition ?
Polymerization rate versus viscosity
h (Pa.s)
Rp
[M]0
( )max
0
1
2
3
4
0.1 1 10 100 1000 10000
CO
2
-19°C
6°C
6°C
RT
RT50°C
50°C
80°C
80°C air
-19°C
 curing the same coating at higher film-viscosity (before curing) under air
increases the polymerization rate due to reduced oxygen inhibition
Resin: Polyurethane acrylate
UV-lamp: Mercury
Irradiance: 15 mW/cm²
Rp - rate of polymerization
Source: R. Schwalm, UV Coatings (Elsevier, 2006), p. 243

15. Perspective: oxygen inhibition free chemistry
Thiole modification of a Polyurethane dispersion
0
20
40
60
80
100
120
140
LED / air LED / N2 1 day 3 day 5 day
pendulumhardness[osc.]
PUD PUD thiole modified
UV-cured UV-uncured,
storage at room temperature
 thiole modification helps to overcome the oxygen inhibition and
allows a satisfying curing in shadow areas

16. Crosslinking of thiole modified
acrylate dispersions
network
UV/PI
O
O
OS
RT
Michael addition
Michael addition
Reaction in shadow areas
R SH
SH
SH
MF Thiol
O O
O
PI *
MFA O
O
O
+ MFA (multifunctional acrylate) under inert conditions
R S*
SH
SH MFA
O O
O
PI
OOH
MF Thiol
+
O2
O O
O
PI
OO*

17. R
SSH
SH
OOH
R
SSH
SH
H
R S*
X
SH
without O2
with O2
+
network
*
R
S
S
O2** +Thiol
+O2Poly-ene
(acrylate)
R S*
SH
SH
+
+
X may be SH or ene residue
The Thiol – Ene reaction:
Why is it not oxygen inhibited ?
R SH
SH
SH
R* +
- RH
..because the hydroperoxy radical can abstract a „labile“ hydrogen radical from a thiol,
and the thiol radical does add to an acrylate monomer,
whereas the hydroperoxy radical does not initiate the acrylate polymerization
Monomer

18. Water-based UV-systems LED-cured
 Conclusion:
 The choice of the right photoinitiator is mandatory
(Acylphophine oxides are preferred)
 Oxygen penetration into the uncured film needs to be minimized
 UV-curable dispersions (physical drying) do work even at
ambient atmosphere (air)
 100% UV-resins (liquid) do need inert (O2-reduced) atmosphere
 Outlook:
 Thiole modified products can overcome the oxygen-inhibition …
 and help to cure shadow areas
 Smell and speed of curing needs to be optimized

19. Thanks for your attention …
… any questions ?