LED the UV Technology for Future Applications

Table of Contents

Introduction
Comparison of Technologies
Inks, Varnishes and Adhesives
Fields of Applications
     UV LEDs for Adhesive Curing
     UV LED Curing in Printing
UV LED Measurement
Conclusion

Introduction

For a number of years, UV technology has been a reliable technique for the curing of photo-reactive chemicals. In response to increasing production speeds and new applications, for instance in the field of 3D, UV lamp technology has also developed. Presently, a significant range of different systems are available, each specific to the particular application.

Users and providers of chemistry are continually developing new applications for UV curing. Their groundbreaking ideas mostly mean increasing demand on UV curing devices – where at times conventional UV technology has touched its technical limits. Therefore, within the recent years, a totally new branch of UV technology has formed: UV LEDs. This article offers the reader with an objective comparison between both technologies, UV and UV LED. It should help the user determine to what degree LEDs can provide a substitute to conventional UV solutions.

Comparison of Technologies

The operating technology of conventional UV lamps is based on plasma physics and optics, while UV LEDs are based on optics and semiconductor technology.

UV technology

UV print unit for web materials

UV print unit for web materials.

A high-voltage arc between two electrodes results in the vaporization of mercury and any optional doping within the lamp. A continuous UV spectrum between 200 nm and 450 nm is discharged.

UV LED technology

PureUV Powerline.

PureUV Powerline.

LEDs are founded on semiconductor technology. Specific wavelengths are directly discharged by the current input. The spectrum is a quasi-monochromatic radiation in distinct wavelengths, for example 365 nm, 385 nm or 405 nm.

spectrum

These technological differences cause the following characteristics:

Medium-pressure lamps

. .
Wavelength [nm] Spectrum between 200 and 400
Ozone generation Yes
Thermal radiation Yes
Effectivity Ca. 30%
Required space Depends on application
Operation Warm-up phase
Standby-mode (15-40%)
Shutter needed
Cooling Air and/or water
Typical lamp life 1.000 - 5.000 h
Maintenance Easy control via UV measurement device. During maintenance only one irradiator needs to be exchanged for one lamp system.
Purchase costs Low

LED

. .
Wavelength [nm] 365, 375, 385, 395, 405
No short wavelengths
Ozone generation No
Thermal radiation No
Effectivity 5 - 20%
Required space Low
Operation No warm-up phase
Instant switch-on and -off
No shutter needed
Cooling Water (rare: air)
Typical lamp life > 10.000 hours*
Maintenance Difficult controlling of the particular dies. In case of a breakdown, the complete module needs to be exchanged.
Purchase costs High

* depending on operating conditions and ambient temperature

LED spectrum

Inks, Varnishes and Adhesives

Along with UV lamp technology, coating formulations have been enhanced and adapted to the needs of conventional UV technology over the past years. This brought about to a wide range of coatings, including low-migration inks and varnishes. Thus far there are only few enhanced systems available for UV LED applications. One reason is because of the lack of suitable raw materials and photo-initiators. Owing to the fact that the central emission wavelength of UV LEDs is in the UVA range, only “long-wavelength sensitive” photo-initiators can be employed. This can result in inadequate surface curing.

To match the curing properties to those of conventional systems, the concentration of photo-initiators may possibly have to be increased. But such systems can have drawbacks, for instance intense odor, yellowing, or lower production speeds. Besides this, UV LED formulations are more costly than conventional UV coatings.

The chemical sector, including the raw-material suppliers, can help to enhance the system; cooperation between raw-material, equipment and coating suppliers is of paramount importance. Suppliers of UV LED curing devices can play an important role in supporting the chemistry in the following areas:

  • Providing adequate curing devices
  • Technological R&D training for employees
  • Support of raw material suppliers
  • Joint projects with end users
  • Continuous further improvements, for example inertization for LEDs

Fields of Applications

LED technology is already well proven in several areas. The major advantages of LEDs in these applications and the possibility to alter the chemistry to the specific emitted wavelengths have resulted in their success in these markets. An example of this can be found in the adhesive sector.

UV LEDs for Adhesive Curing

Adhesives are frequently applied at higher coating weights, or the irradiation has to happen through an absorbing substrate. Thus the reactivity of these systems traditionally falls in the UVA and UV visible region. Additionally, the adhesives do not contain pigments that may impact the curing process. These are the key reasons why the modification of the chemistry to the narrow, longer-wavelength, emission band of LEDs has proved successful in this area. Another benefit that makes it ideal for LED curing, is the comparatively slow process speed (compared to print applications). The system technology also offers a number of benefits compared to conventional UV curing:

  • Compact assembly size
  • Easy handling in cyclical operation
  • No thermal load of substrates (can have a negative impact on chemistry)
  • Effective light shielding to avoid premature curing in the application system
  • Best effective production owing to life-time of LED devices (long life-time)

The combination of chemistry and UV LED curing devices is so successful that this application can be viewed as a real driver behind the growth of LED curing technology as a whole.

Based on the positive experiences attained from this application, LED technology has been endlessly enhanced, developed and transferred into other areas. LED point and spot curing devices are primarily used for conformal coatings or adhesives.

UV LED Curing in Printing

A few of the inkjet applications profit from the benefits offered by LEDs. There are already applications operating in the small format area, which gain from the advantages of the cyclic operation mode and smaller dimensions of LEDs. The PureUV Powerline was created for wide format and web printing applications. This LED line can be stretched in 40 mm steps. Also, it is possible to vary the output in 1% steps between 10% and 100%. Another benefit: Full irradiation cycles can be programmed.

1.000 mm long LED array for printing applications.

1.000 mm long LED array for printing applications.

After earlier determination of the reactivity of printing ink or coating, the PureUV Powerline can be fitted with the suitable LED wavelength.

UV LED Measurement

UV LED Measurement

UV measurement assures manufacture process security and for R&D, reliable and repeatable test results in the laboratory. The market offers a choice of devices for measuring the intensity and/or the dose with various sensor geometries which can be effortlessly matched to the specific application. The physical classification of the UV spectrum in the UVB region is from 320-380 nm, UVA from 400-320 nm and UVC from 280- 200 nm, in a majority of cases the spectral sensitivity of the sensor is adapted accordingly.

The specific features of a wide UV spectrum can thus be examined in detail. However, LED irradiation units do not create a wide UV spectrum, but release narrow bandwidths at specific wavelengths. Thus, any intensity measurement of these bandwidths with conventional sensors is incorrect.

To enable the measurement of LED UV units, Dr. Hönle AG has built a broad band sensor which can measure the dose and the intensity across the bandwidth of all LED wavelengths from 365 to 405 nm - with just one sensor. The UV LED measuring sensor is attached to a basic UV meter which automatically identifies the LED sensor when connected. The measured value for the intensity is shown in W/cm2 or mW/cm2 with a maximum intensity of 20 W/cm2, for the dose it is displayed in J/cm2 or mJ/cm2.

UV LED Measurement

Conclusion

In contrast to conventional UV irradiators, it is clear that LEDs can offer multiple advantages in a range of coating applications. However, there are some limitations, governing or preventing the use of LEDs. The advantages of LED curing should be carefully considered, case by case.

Dr. Hönle AG can provide information, advice and technology to assist users in making these decisions. As an expert in industrial UV technology, Dr. Hönle AG has been successful in the advancement and production of UV LED curing devices for numerous years. The company has now utilized their significant experience of UV LED curing and applied it to the coating sector, through competent training courses, discussions with the equipment suppliers by supporting the chemical sector, and end users.

Techsil

This information has been sourced, reviewed and adapted from materials provided by Techsil.

For more information on this source, please visit Techsil.

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