uv led curing insights regarding surface hard coats

UV LED Curing InsightsRegarding Surface Hard Coats

Jennifer Heathcote

Technical & Commercial Consulting Advisor

The Surface Summit

November 5, 2019

Ultraviolet Spectrum

November 5, 2019www.eminenceuv.com 2

Ultraviolet Bands

Numerical wavelength ranges for UV bands vary

slightly by reference source and meter.

Ultraviolet Curing Process


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.


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



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.


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


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


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


3.5 mm (0.138”) square module

4, 6, 8, 10” wafers

Higher price

Less efficient

Shorter life

Inconsistent quality


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


Ways to counter oxygen inhibition


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 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


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


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


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


UV LED

120° Beam Angle Stray UV Light Stray UV Light Working

Distance

Matrix of Diodes


Curing coatings on sheets and webs with UV LED

Peak irradiance decreases rapidly with increasing working distance.

UV LED

Web Path

Working Distance


Curing coatings on 3D part profiles with UV LED

Working Distance

Peak irradiance decreases rapidly with increasing working distance.

Working Distance


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?


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)


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


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


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


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


uv led curing insights regarding surface hard coats

Documents