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UV LED Curing InsightsRegarding Surface Hard Coats
Jennifer Heathcote
Technical & Commercial Consulting Advisor
The Surface Summit
November 5, 2019
Ultraviolet Spectrum
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Ultraviolet Bands
Numerical wavelength ranges for UV bands vary
slightly by reference source and meter.
Ultraviolet Curing Process
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Additives
Photoinitiators
Oligomers Resins
Monomers Water
Additives
Photoinitiators
Monomers
Oligomers
Characterizing UV Output
Spectral Output (nm) – combination of emitted wavelengths defined by the distancebetween corresponding points of a wave
Irradiance (Watts/cm2) – radiant power arriving at a surface per unit area. Decreases with distance traveled. Independent of exposure time.
Energy Density (Joules/cm2) – cumulative radiant energy (potential to do work) arriving at a surface per unit area. Increases with greater lamp output and more exposure time.
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IRRADIANCE
Power at a Point in Time / Unit Area
ENERGY DENSITY
Power over Exposure Time / Unit Area
Total Energy / Unit Area
Functional UV curable surface coatings offer
Instant cure
Primers, hardcoats, sealants, and varnishes
Mar, abrasion, scratch, and chemical resistance
Gloss, matte, and textured visual effects
Ingress sealing against moisture and foreign matter
Light fastness and weatherability
Protection for layers beneath surface coating
Surface energy adjustment
Less heat transfer and reduced floor space compared to thermal dryers
No volatile organic compound (VOC) emissionsNovember 5, 2019www.eminenceuv.com 5
UV LED functional coating challenges
LEDs currently limited to longer UVA and UVV wavelengths
surface cure photoinitiators require shorter UVC wavelengths
photoinitiators that react to longer UVA and UVV wavelengths yellow and cloud during curing
Oxygen inhibition - exposure to air during cure can result in sticky, tacky, and greasy surfaces (unreacted chemistry)
Curing shapely profiles - UV LED lamp heads must deliver irradiance to cure surface across a greater working distance
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Wall Plug Efficient (WPE) is the percentage of electrical energy converted into optical power
Material science and manufacturing processes drive what is possible
Market economics drive what is produced
Production yields and market economics drive price
Application viability drives what is adopted over the long term
LED Development
Source: Lawal, O., Pagan, J. Hansen, M. 2017. When Will UV-C LEDs be Suitable for Municipal Treatment? Conference Presentation. IUVA World Congress. 18 September 2017.
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Source: Kopp Glass analyzed 702 products from 29 UV LED chip suppliers in 2019. Sample included chip, chip on board (COB), surface mounted diode (SMD), and T-Type.
LED Development
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LED Roadmap
Source: AMS Spectral compiled irradiance estimates provided by leading UVC LED suppliers.
Irradiance (Watts/cm2) – radiant power arriving at a surface per unit area
Irradiance measured at the module
Reality may prove to be exponentially more positive, less steep, or non-linear
Real output power for UVA LEDs was exponential compared to original estimates
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UV CURING SOURCE SPECTRAL OUTPUT
| | 200 Wavelength (nm) 700
Rela
tive
Irrad
ianc
e
UVC 200-280
nm
UVB
UVA 315-400
nm
Visible 400 - 700 nm
365 nm
385 nm
395 nm
405 nm
UV LED
275 nm
Mercury Lamp Mercury lamps are broad spectrum
UV LEDs limited to 275, 365, 385, 395, and 405 nm
365 nm LEDs are 20% less powerful at maximum output than 385, 395, and 405 nm
275 nm LEDs are more than 95% less powerful at maximum output than 385, 395, and 405 nm
Mercury lamps ≤ 5 Watts/cm2 UVA
LEDs available in a wide range of irradiances up to 50 Watts/cm2 UVA
Increasingly greater irradiance not always beneficial
Spectral Output
Status of UVC LEDs today compared to UVA and UVV LEDs
Lower power
Lower yields
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3.5 mm (0.138”) square module
4, 6, 8, 10” wafers
Higher price
Less efficient
Shorter life
Inconsistent quality
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Surface cure photoinitiators react to UVC wavelengths
Source: IGM Resins
UVC: 200 – 280 nmUVB: 280 – 315 nmUVA: 315 – 400 nmUVV: 400 – 445 nm
275 – 280 nmUVC LEDs
Oxygen inhibition impedes surface cure
Occurs when oxygen molecules present in air combine with intermediary photopolymer chains
Reduces number of free radicals and inhibits or weakens further crosslinking
Full polymerization near top surface is incomplete
Resulting shorter chains are not as hard and may feel sticky, tacky, or greasy
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Ways to counter oxygen inhibition
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Corrective Action Applied To Result Trade-off
Nitrogen Inertion Environment Eliminates oxygen above chemistry by flooding surface with non-reactive gas Increases cost
Ways to counter oxygen inhibition
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Corrective Action Applied To Result Trade-off
Nitrogen Inertion Environment Eliminates oxygen above chemistry by flooding surface with non-reactive gas Increases cost
Increase Viscosity Formulation Decreases oxygen diffusion Affects material handling & ability to apply coating
Increase Thickness Formulation Decreases oxygen diffusion Increases cost
Increase Photoinitiators Formulation Increases free radical generation Increases cost
Ways to counter oxygen inhibition
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Corrective Action Applied To Result Trade-off
Nitrogen Inertion Environment Eliminates oxygen above chemistry by flooding surface with non-reactive gas Increases cost
Increase Viscosity Formulation Decreases oxygen diffusion Affects material handling & ability to apply coating
Increase Thickness Formulation Decreases oxygen diffusion Increases cost
Increase Photoinitiators Formulation Increases free radical generation Increases cost
Increase Irradiance Lamp Head Increases free radical generation Increases lamp cost & power consumption
Increase Energy Density* Lamp Head Increases free radical generationIncreases lamp cost & power consumption
Add UVC Lamp Head Increases free radical generation at surface
Increases lamp cost today
UVC and greater irradiance in UVB, UVA, and UVV counter O2 inhibition
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UVB UVC UVA UVV
substrate, construction, part, or media
ink, coating, or adhesive
Irradiance and the Inverse Square Law
Irradiance is inversely proportional to the square of the distance from the emitting source
As UV rays travel further from their effective origin, they spread or diverge from one another which results in reduced concentration over a defined area at greater distances
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Irradiance1 = (Distance2)2
Irradiance2 (Distance1)2
Irradiance2 = Irradiance1 ⋅ (Distance1)2
(Distance2)2
Irradiance and the Inverse Square Law
A matrix of UV LEDs does not closely follow the inverse square law due to blending of neighboring diodes
Effective origin(s) of LED sources…
each discrete raw diode
point at which the collimated light (when optics or reflectors are used) diverges from its optical containment
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UV LED
UV LED output concentration and uniformity
Maximum concentration of UV output
beam angle
emitting angle
concentration angle
cone of concentration
Recommended working distance
Actual working distance
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UV LED
120° Beam Angle Stray UV Light Stray UV Light Working
Distance
Matrix of Diodes
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Curing coatings on sheets and webs with UV LED
Peak irradiance decreases rapidly with increasing working distance.
UV LED
Web Path
Working Distance
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Curing coatings on 3D part profiles with UV LED
Working Distance
Peak irradiance decreases rapidly with increasing working distance.
Working Distance
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Working distance for flat matrix UV LED systems
UV LED
Recommended Working Distance 10 – 12 mm (0.4 to 0.5”)
So how do we cure surfaces of parts with shapely 3D profiles that require UV LED lamp heads to be mounted outside the
recommended working distance?
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Adjusting beam angle through optics, reflectors, and collars
LED
CollimatingRod Lens Optic
CollimatingReflector
UV LED
External Containment Collar
Phoseon UVC LED development
278 nm
4 Watts/cm2 peak irradiance
Lengths of 125 to 300 mm
7.9 W/cm (20 W/inch)
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Available for testing
LEDs arranged in flat matrix configuration
20 mm wide emitting window
Air-cooled and liquid-cooled variations
AMS Spectral UVC LED development
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Liquid-cooled
280 nm
3.8 Watts/cm2 peak irradiance
Lamp heads built to custom lengths
7.5 Watts/cm (19 Watts/inch)
New technology adoption
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Disruptive Stress & Opportunity
exponential outpaces linear
linear leads & innovation disappoints
Time (years)
Pace
& V
alue
of I
nnov
atio
n
Exponential Growth
Linear Growth
Initially, new technology is never a direct and
seamless replacement for mature
technology…
…but eventually disruptive stresses
increase the value of innovation opportunities
which accelerates adoption.
Conclusions and final comments
Over Print Varnishes (OPVs) being cured in the graphics industry with UVA LED today (negligible yellowing)
Industrial surface cure photoinitiators require UVC to achieve desired functional properties and no yellowing
UVC LED systems available as prototypes for formulators to evaluate
UVC LED systems currently not economical and not reliable enough for industrial integration
UVC LED systems roughly 3 to 5 years (or more) away from commercial viability
Oxygen inhibition can be addressed with greater irradiance in UVA and/or the use of UVC
Greater working distances for shapely profiles require 1) higher power lamps, 2) optics, reflectors, or collars, or 3) advancements in formulations
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Thank You!
Jennifer Heathcote
Technical & Commercial Advisor, UV Curing
[email protected] +1 (312) 550-5828
47 W Division Street, #509, Chicago, IL 60610
www.eminenceuv.com
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