UV CURING – A Guide Jon Anderson

Outline Background to UV technology UV lamp technology types Temperature effect on UV curing UV curing troubleshooting case studies

Background to UV curing – medium pressure mercury lamps Energy is applied to mercury causing it to vaporise The vapour becomes a plasma and emits UV light Visible wavelengths 200 to 400 nm are the most useful range for curing. Curing is polymerization of monomers and oligomers - polymer cross linking and a phase change from liquid to solid state Fusion UV

Background to UV curing Dose (J/cm2): Total energy delivered Related to total energy emitted time spent underneath UV light i.e. conveyer speed Irradiance (W/cm2): Intensity of light delivered Related to lamp type and geometry of reflector

Bulb types Mercury used in bulbs to produce the UV energy The metal can be doped with another metal (Iron or Gallium)to alter the UV output – used to cure different coating technologies such as thickfilm or pigmented systems Different doping metals produce different spectral outputs Standard H bulb Iron doped bulb H bulb: This is the general purpose UV curing lamp with strong output in the UVC (200-280 nm) and the UVB (280-320 nm). It is typically used for curing litho inks and overvarnishes D bulb: With a much higher percentage of its output in the UVA (320-400 nm), this lamp is used where deeper penetration is required. Applications include thick pigmented coatings and very thick clear coats Courtesy UVDoctors, Inc

Cost – initial and running UV Lamp technology Arc or Microwave? Cost – initial and running Irradiance Lamp length Lamp life

Microwave – electrode-less UV light generated by microwaves irradiating the mercury 300 to 600 watts/in power 10 inch maximum length bulb 10 sec start-up time 3000 hour typical lifetime Consistent doped lamp output In the early 1970s, a lamp that used microwave energy instead of electricity to energize mercury was introduced to the market. A simplified design of this type of lamp is illustrated in Figure 4. Without electrodes, microwave lamps do not suffer some of the problems inherent to arc lamps. There is no aging of the lamp due to decomposition of the electrode, or reduction in output due to blackening of the lamp. Microwave lamps have more consistent UV output and generally last longer than arc lamp counterparts. However, the physics behind the microwave coupling in the lamp restricts the bulb length to certain discrete sizes. Today, microwave lamps that are 3”, 4.5”, 6” and 10” in length are commercially available. Other curing lengths require a number of shorter lamps be placed end-on-end in a modular fashion. This makes microwave lamps ideal for smaller-footprint applications, but a bit more complex and cumbersome for larger footprints, with redundancy in components such as power supplies, or cooling ductwork (Table 2).  Since microwave lamps do not require “restriking” an electric arc within the lamp, they can be refired in a matter of seconds, rather than minutes. Due to the more complex nature of the microwave lamp, its sophistication and various patent related issues, there are fewer (though well qualified) suppliers of microwave UV systems http://www.pcimag.com/articles/96376-some-practical-advice-for-selecting-a-uv-curing-system Figure from http://www.pcimag.com/articles/96376-some-practical-advice-for-selecting-a-uv-curing-system

Arc – with electrode 200 to 750 watt/in power 2 to 3 min start-up time Lengths up to 72 inches 2 to 3 min start-up time 1000 hour typical lifetime Metal halide (MH) lamps consist of an arc tube (also called a discharge tube or "burner") within an outer envelope, or bulb. The arc tube may be made of either quartz or ceramic and contains a starting gas (usually argon), mercury, and MH salts. Traditional quartz MH arc tubes are similar in shape to mercury vapor (MV) arc tubes, but they operate at higher temperatures and pressures. MH lamps start when their ballast supplies a high starting voltage higher than those normally supplied to the lamp electrodes through a gas mixture in the arc tube. The gas in the MH arc tube must be ionized before current can flow and start the lamp. In addition to supplying the correct starting voltage, the ballast also regulates the lamp starting current and lamp operating current. (See "What types of ballasts are available to use with metal halide lamps?") As pressure and temperature increase, the materials within the arc tube vaporize and emit light and ultraviolet (UV) radiation. A bulb (also called "outer jacket" or "outer envelope"), usually made of borosilicate glass, provides a stable thermal environment for the arc tube, contains an inert atmosphere that keeps the components of the arc tube from oxidizing at high temperatures, and reduces the amount of UV radiation that the lamp emits. Some MH lamps have a coated finish on the inside of the bulb that diffuses the light. Often a phosphor coat is used to both diffuse the light and change the lamp's color properties. http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/mwmhl/work.asp# Figure from http://www.pcimag.com/articles/96376-some-practical-advice-for-selecting-a-uv-curing-system

Curing UV product range Cures with arc or microwave technology but….. Sufficient dose and irradiance required Bulb type important – UV range requires UVA, B and C so ‘H’ bulb must be used Correct power but wrong bulb = uncured material UV range = UV40 and 1C63 (UV curable silicone)

Dose and Irradiance values These are the minimum values that will cure UV40 range and result in tack-free surface But….don’t take them as absolute. Always check the cure of the material!!! There may be other factors involved: Temperature from excess IR can lower requirement for dose and irradiance Reflectance – too much/too little can lead to surface defects UV C is hardest to reflect.

Arc vs. Microwave? Irradiance Microwave Arc Lamp technologies deliver UV energy in different way Microwave delivers a higher peak irradiance than arc Microwave Irradiance Arc

Temperature Effect on UV curing Experiment conducted to observe increased temperature effect on the UV curing of UV40-250: 1. After coating application test coupons were heated to a set temperature 2. Material was tested for tack-free surface after UV cure Microwave Arc

Temperature Effect on UV curing Both lamp technologies will generate IR during operation. Increased IR can reduce the threshold required to provide a full cure i.e. tack-free surface Microwave threshold Arc threshold Lower arc threshold with increased IR

Case Study 1: Tacky coating Coating surface sticky after passing under UV light – surface should be tack-free Due to insufficient UV C irradiance - check bulb type – should be mercury ‘H’ bulb - check lamp height. It may be out of focus = insufficient energy reaching surface - check reflector – may be dirty/oxidised so not reflecting effectively

Lamp Focus UV Process Supply Inc.

Lamp Focus Irradiance In focus Out of focus Out of focus lamp does not deliver as high intensity UV energy to the coating In focus Irradiance Out of focus

Case Study 2: Wrinkling Coating surface is not smooth after UV curing

Causes: Excessive ventilation disturbing the coating surface excessive temperatures – not enough cooling Excessive reflection – too much UV light reflected is stressing the coating surface

Case Study 3: Curing 3D device Curing on flat devices (low standoff heights) is standard for most UV curing processes What about sides of components or housings? – these can be difficult to cure fully

Solution – additional reflector Standard ½ ellipse reflector for ‘flat’ surfaces – some energy is lost out the sides of the reflector Additional reflector to add depth to UV cure results in increased energy reflected UV Process Supply Inc.

Summary The lamp technology is irrelevant (UNLESS ITS LED!) What matters is the wavelength, amount and intensity of UV energy reaching the coating Determined by: Bulb type, lamp focus, reflectance, conveyer speed

Useful links for further reading www.fusionuv.com/uvlearning_center.aspx www.uvcuring.com/ www.pcimag.com/articles/96376-some-practical-advice- for-selecting-a-uv-curing-system

Any questions?