Current Developments in Pulsed Light or "Flash" Curing Technology

Louis R. Panico

XENON Corporation

20 Commerce Way

Woburn, MA 01801

Abstract

I. INTRODUCTION

Flash curing is inherently different from other methods of curing such as mercury vapor or microwave generated ultraviolet sources. Flash curing is particularly useful in applications where other curing methods have proven inadequate. The advantages of flash curing include: faster free radical generation, increased surface penetration, less dependence on photoinitiators, greater safety, lower heat transferred to substrate, curing of opaque systems, and the ease of meeting special lamp configuration requirements. A major obstacle to the growth of this technology was lamp life. In the past, xenon lamps for curing applications have lasted only from 40 to 60 hours. This problem was overcome with a novel (patent pending) approach to long life without sacrificing power density. The new lamps are guaranteed for 1000 hours and prorated for 3000 hours. This new development has opened the door to the commercial exploitation of this exciting technology.

II. BACKGROUND

The relative differences between flash energy and continuous wave (CW) radiation are shown in figure 1 in terms of pulse amplitude and controlled spaced pulses. The peak power levels with flash energy can be as high as 10 x 106 watts with pulsed widths in the microsecond region. The flashlamp spectrum is directly dependent on the characteristics of the electrical operating conditions. The optimum wavelength can be generated by shifting the current density to a reading that will yield the desired spectrum for a particular application be it in the UV, visible, or IR region. The spectral shift in relation to lamp current densities is illustrated in figure 2. The broad spectrum can be "tuned" to peak at different spectral ranges. The basic electronic circuit, as illustrated in figure 3, represents a typical design for flash radiation. The power supply provides the current for charging the main storage capacitor. Then the lamp is triggered, the stored energy is released from the capacitor in the form of a current pulse which is shaped by the pulse forming network. The controlled, short duration current pulses are converted to spaced light by the flashlamp, yielding distinct light pulses of very high peak power. The two key equipment parameters that affect cure are pulsed recurrence frequency and spectrum. These parameters can be "tuned" to a specific chemical system.

III. ADVANTAGES OF FLASH CURING

Why should anyone use this method of curing instead of several of the other alternatives? The most common responses from customers when asked this question are;

A. Low Heat

The lower heat associated with xenon flash curing is a real advantage in curing coatings, inks or adhesives on heat sensitive substrates, especially PVC (polyvinyl chloride) and FE (polyethylene) films. The lower heat is attributed to three characteristics of the xenon flash lamps: 1) pulse separation, 2) conversion to useful radiation without heating to vapor levels as required with mercury lamps, 3) minimum IR generation. In addition, the air around the lamp can be cooled to a lower temperature without effecting radiation efficiency. The maximum temperature of the material being cured is between 100 and 150'F, depending on the formulation.

B. Special Lamp Configuration

The nature of the flashlamp is such that it will work in any configuration a glass blower can shape as long as the operating parameters are properly matched for the efficient generation of useful radiation. Some examples are shown in figure 4.

C. High Radiation Deliverable In A Short Period Of Time

It is clearly demonstrated in figure 1 that a single pulse of light can deliver as much energy as a continuous system in a fraction of the time. A mathematical work problem example is shown in figure 5. The pulsed system can deliver 500 Joules in one millisecond as opposed to the one second required by the continuous system. By simply delivering 10 pulses per second, ten times the energy can be delivered to the chemical system for curing.

D. Less Dependence On Photoinitiatots

The broad band spectral output allows for the consideration of reducing the photoinitiator percentage as well as the use of combinations of photoinitiators and photo accelerators taking advantage of the effective characteristics of each by obtaining synergistic results.

E. High Peak Powers

In addition to delivering more energy faster, the high peak pulses and longer wavelengths have an advantage in penetrating thick and opaque materials. Nigh performance materials are claimed to be obtained with the combination of these curing methods. The production of new photochemical reactions has also been attributed to the high peak powers.

Other useful advantages of pulsed light curing include:

instant on/off operation, low space requirement, and significant output in the visible region of the spectrum. The latter has opened the door to a safer visible light curing technology. Much work remains to be done in the development of visible light curable formulated products.

 

IV. EQUIPMENT

There are two standard xenon flash curing units currently available: a portable 120 watt unit for curing areas up to 1" x 1", and a 5 kilowatt production unit for larger areas. A 1 kilowatt unit is also under development.

A. 120 Watt Portable Xenon Flash Curing Unit

As can be seen in figure 6, this unit consists of a desk top cabinet with a hand held lamp connected to the cabinet by a flexible hose. It was designed for high speed curing of coatings and adhesives In small areas, that is less than or equal to 1" x

1". The fast cures are achieved with a high frequency pulse train delivered In-a burst mode. This unit may be run continuously if desired.

B. 5 Kilowatt Xenon Flash Curing Unit

This unit is illustrated in figures 7 and S. It contains a novel lamp design (patents pending) that finally makes it possible to obtain long lamp life without sacrificing power density. This is accomplished with an alternating lamp. The linear section of

the IS" conveyor received 280 watts/inch but the lamp itself is operated at 50 watts/inch because it is actually 95" long coiled into five parallel 15" sections. Many attempts were made in the past to duplicate the mercury vapor lamp power/inch rating with xenon linear flashlamps but at 200-300 watts/inch, the flashlamp life was extremely short. Other advantages of this design include the capability of varying lamp configuration to suit particular applications and the ability to, operate at lower temperatures than is possible with other lamps. As discussed earlier in this paper, alternative lamp configurations are illustrated in figure 4.

V. APPLICATIONS

The applications have been categorized into two groups based on the standard xenon flash curing equipment available. The first group is called small area and refers to curing areas less than 1" x 1'~, using the 120 watt unit. The second group, large area, refers to any area or geometry larger than 1" x 1", using the 5 kilowatt unit.

A. Small Area Applications

A common small area application is retouching or repairing of UV cured coatings. The portable unit with the curing head connected by a flexible hose makes a handy touch up tool. Original curing of small areas include: fiber optic connections, lens alignment, wire tacking, coil termination, and general purpose connectors and switches. The key advantages of the xenon flash curing in these applications are speed of cure and delivery of light from a localized source.

B. Large Area Applications

The large area applications include: fast cure of clear and pigmented coatings, curing laminating adhesives through one transparent substrate, curing composites, and catalysis of photochemical reactions. Several other current applications are confidential.

1. Coatings

Due to the broad spectrum available, both radiation curable and heat curable formulations have been cured by the flash method. The xenon source is an efficient free radical generator. Depending on the formulation, systems containing photoinitiators or heat activated peroxides, and some containing neither have been cured faster with xenon flash than by alternative methods. One of the first costing applications was the rapid curing of unsaturated polyester by xenon flash for a motor overcoating. Cure time was reduced from the conventional heat curing for 4-S hours to a 2 minute flash curing.

Another coating application for flash curing is on heat sensitive substrates. As more products are designed with plastics of low heat tolerance, conventional heat curing methods can no longer be utilized. The xenon source has been found to generate much less heat than the mercury vapor DV sources.

2. Laminating Adhesives

Flash curing is especially attractive in curing laminating adhesives through a transparent substrate because of its deep penetration characteristic. Neat sensitive substrates such as PVC (polyvinyl chloride) and PR (polyethylene) are laminated to film, foil or paper and the adhesive is rapidly cured with the xenon flash.

3. Composites

Glass epoxy prepregs of 1/16 and 1/5 inch thickness have been cured by xenon flash. The ANMRC (Army Materials & Mechanics Research Center) in Watertown, Massachusetts, has an ongoing project evaluating flash cured prepregs in comparison to conventional autoclaved prepregs. Military and commercial use of glass reinforced plastics has been extensive because of the strength to weight ratio and cost advantages offered by these materials over other structural materials.

4. Photochemical Reactions

There are numerous projects underway to initiate chemical reactions with the xenon flash source. There is a great potential for increasing chemical reaction rate, thereby reducing processing costs.

 

VI. CONCLUSION

Flash curing is a practical method for the delivery of energy to certain chemical systems. Lower heat, greater penetration, increased cure speed and high performance products are the main incentives for using this curing method. It can be used for DV, visible or IR curing by appropriately adjusting the electro-magnetic energy.

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