From the Desk of Dr. Walter Brenner

UV CURING COMPOUNDS FOR COATING,
BONDING, POTTING & ENCAPSULATION

by Walter Brenner, Technical Director, Master Bond Inc.

Commercial organic coatings, adhesive/sealants, pottings, and encapsulants generally contain solvents and various diluents, or they are composed of two components which must be weighed and mixed prior to use. After such compounds are applied, heat is generally used to drive off the solvents and other volatiles or to speed the cure {hardening) — especially for two component systems. UV curing compounds on the other hand are one component materials which remain stable—even after prolonged storage — until activated by UV light. They also do not require any weighing or mixing prior to application. They offer "cure on demand" as a most attractive feature.

In UV curing compositions, radiant energy from a UV light source is absorbed and converted to chemical energy so quickly that cure is practically instantaneous. The formulations are 100 percent reactive with no volatile losses upon curing and therefore are essentially non-polluting. Thermal curing of conventional systems, by contrast, can drive off as many as six gallons of solvent for every gallon of solids deposited. This creates a pollution potential which has been recognized by Environmental Protection Agency pollution control regulations. Mandated pollution control measures, such as solvent recovery systems with afterburners, increase processing costs, and make UV curing more economically attractive.

UV systems cure substrates at lower temperatures than thermal ovens and cure so quickly that the substrates experience only a brief, superficial temperature change. Such "cool curing" makes possible processing of heat-sensitive substrates including plastic films, moldings, and synthetic fibers, as well as elastomers, paper products, etc. It reduces substrate shrinkage and warpage and permits immediate additional on-line processing and off-line handling and stacking. The latter advantage reduces space requirements for storing processed parts while they cool. Additionally, it decreases the labor needed to handle and transport processed parts before and after cooling. These reported benefits are of particular value in the electrical/electronic industries where the electronic properties of delicate components are unfavorably affected by even transient thermal exposures.

UV Curing Systems

A UV curing system employs UV lamps as the radiative energy source. UV lamps are controlled discharge devices that contain mercury and inert gas. At both ends of the tube are electrodes which are joined to metal end caps to form the electrical connections to the lamp. The distance between the electrodes determines the amount of voltage needed to span the gap and strike an arc. The arc generates electromagnetic energy of varying wave lengths, giving off infrared and visible light as well as the desired ultraviolet radiation.

Lamp cooling by air or water is important because electrodes operate most efficiently at around 1500°F. A power source or ballast is required to increase the input voltage and provide constant power to the mercury vapor lamps.

Considerable progress has been and continues to be made in the design and durability of UV lamp equipment. The 200 watt/linear inch, medium pressure mercury vapor lamps have become the industry’s standard. Most 200 watt/inch lamps give trouble free service for more than 2000 hours of operation.

UV-Cure Compounds

Several types of UV-cure compounds are available in 100 percent reactive formulations. One feature of these compounds is that they are not oxygen-inhibited and they exhibit fast curing rates at ambient temperatures and in the presence of air. This eliminates the need for special equipment for atmospheric control— a nitrogen atmosphere, for example—to effect tack-free cures. Equally important for selected formulations, cure continues in the dark after UV exposure until all the UV reacting species have been consumed, thus making the most economical use of UV energy.

Secondly, these compounds can be cured in significant cross-section thicknesses up to ½" and more for specific formulations. In sharp contrast, most older UV curing compounds are limited to depths of less than 10 mils. This "in depth" curing feature greatly extends the range of electrical/electronic applications to potting and encapsulants including sensitive electronic components (IC chips, light emitting diodes, high voltage coils, and optical fibers). Additionally, applications are extended to printed wiring boards, capacitor seals, and electrical connectors. Maximum dimensional accuracy is assured even with "in depth" pottings and encapsulations because the UV-cure compounds cure with minimum shrinkage without the evolution of solvents or other volatiles.

Thirdly, the compounds produce coatings, bonds, pottings, and encapsulations with service capability over a temperature range from -80° to +350°F. Because of their superior chemical resistance, they can function without difficulty for long time periods, even in adverse environmental conditions, such as the presence of moisture and heat.

There are a variety of other reported advantages to using these UV-cure compounds. One-component systems, for example, require no labor-intensive weighing and mixing. Production schedules are simplified due to the elimination of pot life problems and lengthy ambient temperature cures, (often with post-cure requirements), or the use of oven equipment for elevated temperature cures.

Long term stability is possible when stored at ambient temperatures in the absence of UV light. No loss of pertinent application and/or performance properties is apparent after prolonged storage for six months or more in the absence of UV light, and activation occurs only when exposed to UV light of 250-350nm.

The 100 percent reactive polymer systems are said to assure minimal shrinkage upon cure and provide optimum dimensional accuracy and stability due to the absence of solvents and other volatiles. Complete cures are achieved without residual surface tackiness at ambient temperatures and in the presence of air.

Applications

The polymer systems can be cured at ambient temperatures and in the presence of air with commercially available UV lamps in very short time periods (1 minute or less). While 200-300 watt/linear inch, medium pressure mercury vapor lamps are recommended for industrial operations, a variety of other lamp equipment also can be used, providing their output includes UV light of 250-350nm wavelengths. Cures can even be achieved with low intensity UV sources, but cure times will be significantly longer and may require several minutes or more.

No solvents or other volatiles are released during curing. Curing can be accomplished solely at ambient temperatures or can be accelerated by heat. In general, a thicker layer of UV curing material will require somewhat longer exposure to UV light than a thinner one, but the relationship is not directly proportional. Also, the rate of cure increases with the amount of UV intensity deposited on the surface—but again the relationship is not directly proportional. The rate of curing furthermore depends on the distance of the surface of the UV curing polymer system from the UV radiation source.

When cured by UV light, these compounds, used in thermoset coatings, adhesive/sealants, pottings, and encapsulations, reportedly provide durability, strength, hardness, impact resistance, adhesion, electrical insulation characteristics, and inertness to many chemicals, including water and common solvents.

For optimum adhesion, substrates must be carefully cleaned before application, especially because of the possible presence of oils, greases, release agents, dirt, and other contaminants. In many cases, such as with metals and other inorganic substrates, the degree of cleanliness can be ascertained by a simple test which involves spreading a few drops of cool water on the surface. If the water spreads over the area with a continuous film, parts are sufficiently clean for further processing; if the water beads or stays in puddles, EPA acceptable solvents such as IPA or acetone should be used for degreasing. The water test should then be repeated before applying the UV-cure compound.

The various compounds have been successfully applied on many different substrates such as polyester and polyimide films, sputtered metal films, and high purity alumina ceramics. Successful operations are being carried out with high automated, continuous processing equipment featuring line speeds of 80 ft/min and more.

Other common substrates upon which the UV-cure polymer systems have been applied include glasses, silicon and other semiconductors, and optical fibers (including silicas and acrylic plastics). Also included are paper and paperboards, sputtered and ion-plated coatings, elastomers, and printing wiring boards (including epoxy/glass boards).

Polyolefins such as polyethylene and polypropylene, as well as fluorocarbon polymers such as polytetrafluoroethylene and various chlorinated fluorocarbon resins, require special surface treatments to obtain adequate adhesion. The new UV-cure compounds have been formulated to enhance adhesion to even difficult-to-bond substrates. They are said to consistently be able to exceed stringent electrical /electronic requirements in many high volume critical product applications, even upon prolonged exposure in hostile environments, including high energy radiation.

The UV-cure polymer systems are finding wide acceptance not only in the so called "high tech" electrical/electronic industries and fiber optics processing, but also for a variety of other applications with cure-on-demand — a most desirable feature.

Conclusions

These new UV-cure compounds are said to possess some properties which surpass those of conventional thermally cured polymer systems. As previously cited, the compounds adhere to both similar and dissimilar substrates.

Their reportedly high electrical insulation properties include retention of properties at elevated temperatures and high humidities. They are said to feature high-use temperature capability and flexibility/toughness retention up to 350°F (even in hostile environments); and improved appearance, both immediately after cure and after prolonged aging (even after exposure to high temperature/humidity conditions).

Other properties include long term durability with retention of physical strength properties (including outdoor exposure conditions); enhanced abrasion resistance, as well as dimensional stability upon aging (even upon prolonged exposure to hostile environment conditions); long term chemical and solvent-resistance (including water, acids, bases, alcohols, detergents, etc.); and little or no odor in the uncured and cured states.