How Suitable Is UV LED Exposure for Your Stencil System?
Understanding the photochemistry of direct stencil systems and the output characteristics of screen-exposure units can help you determine which combinations will work for you.
I don’t often laugh out loud while reading a technical article, but I did when I read Steve Duccilli’s “An Update on UV LED Screen Exposure” (August/September 2014). This isn’t an insult to Steve; it was the tale he told about the screen maker who calculated his exposure times by how long it took him to smoke a cigarette. Sadly, this is a true story! This article stayed with me and, in combination with a number of phone calls I have received over the past few months from companies asking whether their stencil system will work on a UV LED exposure unit, motivated me to understand more fully what may be occurring when direct stencil systems are exposed and processed.
My answer to “Will it work?” is a favorite one I have been using for over 30 years: “It depends!” Many variables must be considered to make a suitable determination. To name just a few of the critical ones I ask customers:
• What resolution and edge quality does the print require?
• How long is the run?
• What inks and cleaning chemicals will be used, and is the stencil system resistant to them?
• Do you plan to reclaim the screen?
But the UV LED question is relatively new. I cannot recall reading any recent articles that have addressed the photochemistry of direct stencil systems and suitability of various exposure lamps, so I thought this technical subject was worthy of our time and understanding. I will endeavor to keep the topic interesting for readers who don’t have chemistry degrees and ask for forgiveness from those who do for whatever liberties I may take.
It is great to see an established industry such as ours incorporate new technology. At the 2014 SGIA Expo, numerous LED exposure units were on display (at least six companies come to mind), ranging from conventional vacuum-enclosed units to the most advanced computer-to-screen direct imaging systems. I do not believe that this means the total demise of traditional exposure lamps; the market is simply driving change and incorporating innovation, and the success of these new units will depend on how well they perform.
Exposure Lamp Output
UV LED technology offers some enticing benefits, including potentially faster exposure times, energy efficiency, and consistent output without degradation of intensity over the life of the bulb. The primary question is whether the relatively narrow wavelength of energy (in the form of UV light) from these units can be suitably harvested by the sensitizers in the stencil system to generate the required reaction.
Before we look at the spectral output of UV LED units, let us first briefly compare the more common traditional exposure lamps. Over the past 20 years, metal halide lamps have become the dominant light source for producing high-quality, durable stencils. I also see mercury vapor bulbs and fluorescent tube units in the industry. I have not seen a carbon arc unit for many years and quartz bulb units are also uncommon. With the exception of fluorescent tube units, all of these systems usually have just one light source, which helps to obtain the maximum resolution and edge-quality capabilities of direct stencil systems. (With very large frames, a second lamp may be required.)
Mercury vapor bulbs give off a limited spectrum with a high output at 365 nm and smaller outputs between 400-420 nm (Figure 1). UV fluorescent bulbs are relatively weak UV emitters, though they can have a broad spectral output that peaks between 350-360 nm as well as a narrower spike around 420 nm.
Metal halide units come in a variety of spectral outputs depending on which metal is used to dope them. The typical metal halide bulb, sometimes called a diazo or diazo/photopolymer bulb, is doped with Gallium Iodide and has a similar spectral output to the mercury vapor bulb with greater efficiency above 400 nm (Figure 2). Other choices in metal halide include what are sometimes called multispectrum bulbs, so named because they enhance the spectral output at additional wavelengths. The primary dopant metal in multispectrum bulbs is Iron Iodide, which is a broad emitter of UV light and enhances the spectral output in the 380-nm region, though it is weaker above 400 nm (Figure 3).
Unlike conventional light-emitting sources, LEDs aren’t bulbs; they are semiconductors that emit specific wavelengths of light. By adjusting the chemical composition of the semiconductor, manufacturers can change the wavelength emitted by the LED, and in fact they have done so for many different purposes across an extremely wide range of the light spectrum. The key point is that each LED is built to peak at a specific wavelength, with very little energy above or below that value (at left). The LEDs employed in most screen-exposure units today peak at either 385 or 395 nm; some also have a secondary source at 405 nm.
Virtually all direct emulsions and capillary films today use one of three photochemical systems: diazo, dual cure (also referred to as diazo/photopolymer), and SBQ photopolymer (also referred to as one pot). The goal is to change the water-soluble polyvinyl alcohol resin in the stencil matrix to an insoluble polymer upon exposure to UV light. The photochemical reactions that take place in the emulsion during screen exposure are complex and dependent on the chemicals in the stencil system and the wavelengths of the UV light to which the materials are exposed.
Diazo-sensitized emulsions and capillary films are the oldest of these systems. Numerous diazo sensitizers are used in other graphic reproduction methods, but the most common one in screen printing comes from a complex reaction that generates a multichain diazonium salt that you may see described in an SDS as Benzenediazonium -4 – phenylamine sulphate formaldehyde condensate. (Don’t worry: No formaldehyde is present.) The general chemical structure and the molecular chain that forms when the structures repeat are represented below.
Diazo: Benzenediazonium -4 – phenylamine sulphate formaldehyde condensate
Diazo photochemistry is complex and can undergo many different reactions that have been studied for over 100 years. Chemists believe that when the diazo used in direct emulsions is subjected to suitable energy, the diazonium N2 is substituted with the polyvinyl alcohol hydroxyl (Figure 6), reactions that ultimately form a crosslinked polymeric network. The key questions are: What spectral range does the diazo absorb, and where is it most sensitive? Diazo has a peak absorption between 370-375 nm, with sensitivity continuing into the low 400-nm area. It is therefore not surprising to find that the diazo bulb (Gallium Iodide) is very suitable for diazo emulsions, because it has a high spectral output in the 403- and 417-nm wavelengths that is sufficient to activate the diazo.
The diazo reaction is relatively slow, so naturally, manufacturers sought alternatives. The other familiar sensitizer, which emerged about 25 years ago, is called SBQ and features a photopolymer that has been grafted onto the polyvinyl alcohol chain (Figure 7). The full chemical name – polyvinyl alcohol acetalized with N-methyl-4 (p-formyl styryl) pyridinium methosulfate – is quite a mouthful, so we’ll continue to abbreviate it to SBQ.
The reactive component of SBQ is the ethylene group on the side chain, which combines with another ethylene group to crosslink the chains (show in Figures 7 and 8). This reaction takes place significantly faster than with diazo, one of the popular features of these products. The peak absorption of SBQ (about 340 nm) is slightly lower than diazo, but as with diazo, there is a broad range of absorption wavelengths. Because Iron Iodide (multi-spectrum) metal halide bulbs have a strong wavelength emission from 370-390 nm, they would appear to cause a faster reaction than Gallium Iodide bulbs, but both work with SBQ.
The reactions of either the diazo or SBQ polymers cause the polyvinyl alcohol to crosslink, which imparts water insolubility. But a third reaction is often incorporated into a diazo-sensitized system known as dual cure. Available in direct emulsions and capillary films, dual-cure technology has significantly improved the performance of stencil systems over the last 20 years by providing greater durability to both solvent- and water-based inks, as well as improved stencil hardness, flexibility, and reclaimability. Additional benefits include resistance to underexposure and faster (though not as fast as SBQ) exposure times compared to traditional diazo systems. As new ink formulations are developed that require more resistant and tougher stencils, the advantages of dual-cure technology become more significant.
The chemistry behind dual-cure technology is based upon photoinitiated radical polymerization. The reaction does not crosslink the polyvinyl alcohol, but rather converts small molecules (monomers and oligomers) into much larger, multibranched polymers. These additives can confer additional moisture durability and solvent resistance to the stencil system. You may have heard of photosensitizers, photoinitiators, and acrylates, which are all part of this complex system. The main aim is to break the ethylene linkage in the acrylate monomer, which then allows crosslinking to another ethylene linkage from another acrylate monomer – not that dissimilar to the reaction that takes place with SBQ.
Dual-cure products use what are known as Type II photoinitiators, which, when in excited states, react with a hydrogen donor, thereby producing an initiating radical that has sufficient energy to enable crosslinking. The main spectral absorption peak is at 380 nm.
Keep reading with Getting the Best Results from Your LED Exposure System.