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Sensitivity of Emulsions
Sunlight, a readily available light source with a high UV component, has even, on occasion, been used to expose screens. But, despite what you hear about the current state of the earth's ozone layer, the sun is hardly a practical UV light source because exposures take forever and the heat alone is enough to cook an emulsion rock solid.

Nevertheless, your light source should be uppermost in your mind when you go shopping for direct emulsions, because emulsions are created far from equal in sensitivity. If you're making exposures with a homemade bank of fluorescent tubes, you need an emulsion that exposes quickly and is fairly forgiving of exposures that don't quite hit the mark. But if your light source is a 6000-watt single-point exposure unit with integrator (a built-in device that measures UV output), the sensitivity of your emulsion will be the last thing on your mind.

While sensitivity is only one of the factors that determine your choice of a direct emulsion, it is a good place to start. So, let's take a look at how emulsions become photoreactive in the first place.

Crosslinking and Sensitizers
When a direct emulsion is exposed to a UV light source a chemical reaction called crosslinking occurs. Crosslinking is a bonding process, an interlocking of molecular chains. It can be described as the ability of a material to form a skin, a quality which chemists have exploited to create stronger materials. To cause a solution to crosslink, you need a catalyst. The catalyst that makes direct emulsions crosslink comes from the energy supplied by UV light. The part of the emulsion that reacts to the UV is the sensitizer.

Emulsions are generally categorized by sensitizers, and only two basic types of photosensitizers are currently used: Diazo, which has been in use for more than twenty years, and the newer SBQ-photopolymers. (SBQ stands for Styryl Basolium Quaternary salt, in case you were wondering.) These sensitizers are added to a resin base of polyvinyl alcohol or polyvinyl acetate.

Most emulsions fall into either the diazo or photopolymer groups. A popular third choice, the diazo-photopolymer or dual-cure emulsion, combines positive features of the other two while compensating for some of their individual weaknesses. Dual-cures, for example, tend to be more light sensitive than diazos and more water-resistance than pure photopolymers.

Tip: In the photoreactive family there is a second cousin with a very bad reputation. You can still find references to bichromate (a.k.a. dichromate) emulsions in old screenprinting books, and you need to be warned to steer clear. Carcinogenic, environmentally harmful, and possessed of extremely short shelf lives, bichromate emulsions should be left strictly alone.

Diazo Emulsions
For most printing, diazo emulsions remain popular. They also happen to be the cheapest emulsions available, which doesn't hurt their approval rating either.

Least light sensitive of the three, diazo emulsions take longer to expose, which makes them a poor choice for someone with a weak light source. But diazos are also fairly forgiving and usually produce good results even when exposures may be far from perfect, which makes them well suited to beginners. They also have an attractive feature for newcomers: they change color during exposure until the sensitizer in the emulsion has reacted completely -- a very handy way to double-check your exposures.

On the other hand, if you're going to be printing fine details, you need a stencil that's as thin as possible. Which means, chances are, you won't be working with a diazo emulsion, because this breed leads the pack in emulsion over mesh (EOM), and a lot of EOM build-up is the last thing you need for fine printing.

Diazo emulsions come in either solvent-resistant or water-resistant types. So, if you use both solvent- and water-based inks, you need two separate emulsions. Diazos don't do well at all around water, and tend to break down in humid environments. They can be made more water-resistant by treating them with a chemical hardener, which is done after exposure, adding yet another step to the pre-press process.

A diazo emulsion comes in separate parts, and before you have a working emulsion, you have to mix the sensitizer and the base together. Getting good results with any two-part emulsion depends a lot on how good of a mixing job you do. Fortunately, you have some help. Most two-part emulsions come with sensitizer and base dyed in two distinct colors. As you mix the two together, a third color appears. When only the third color remains, your mixing job is done.

Tip: To make sure you get every drop of sensitizer out of the bottle, fill it with only half the recommended amount of water. (Use lukewarm water.) Replace the cap, and shake the bottle well. When you've poured the sensitizer solution into the base, refill the bottle with the same amount of water. Shake it up again, and pour that, too, into the base and stir.

Once the two parts are mixed together, the clock starts ticking. Diazo emulsions have the shortest shelf life of the three types, about two-to-three months depending on environmental conditions. If you make only a few screens at a time, buy your emulsion in smaller quantities so you use it up before it goes bad.

Tip: You can add months to an emulsion's shelf life by storing unused portions in a refrigerator. Keep containers sealed and make sure the temperature never drops below freezing. Once ice crystals form in an emulsion, it's cooked!

SBQ-Photopolymer Emulsions
Also known as one-pot emulsions, photopolymers are almost the exact opposite of the diazo group. Of the three emulsion types, they are the most sensitive, most expensive, and have the longest shelf lives. Some manufacturers cite a life span of over a year; others claim two years, or more.

Unlike two-part emulsions, photopolymers are pre-mixed from the factory. The sensitizer, a photoreactive salt (Styryl Basolium Quaternary), is bonded to the polyvinyl base during the manufacturing process. SBQ photopolymers beat all other emulsions at reproducing the fine detail, but can be finicky about exposure. So, while the super-sensitivity of photopolymers might seem to recommend them to someone using a weak light source, they are mostly found in shops with better exposure units. Beginners would be well advised to approach this group of emulsions with a bit of caution.

While photopolymers do demand a really good exposure system, their sharp edge-definition, low EOM, and quick exposure times have won over many high-production screenprinters. If they have a weakness, it's that, like the diazo group, unless specially formulated to be water resistant, they don't do well with water-based inks. They do, however, tend to be less affected by humidity.

Diazo Photopolymer/Dual-Cures
Emulsion manufacturers have attempted to create the best of all possible worlds for screenprinters by combining the best qualities of diazo and photopolymer emulsions in a third class of emulsions known as dual-cures.

With improved water resistance and exposure times that fall somewhere between the lengthy burns of the diazo group and the lightning fast ones of the photopolymers, dual-cures have become an instant hit. Far from finicky about exposure, dual-cure emulsions reproduce detail better than diazos, sport a lower price tag than the pure photopolymers, and are unfazed by humid working environments. If they have a weakness, it is that they are not nearly as long-lived as the pure photopolymers.

Within the dual-cure family you can find many water-resistant varieties that make dual-cures the emulsion of choice for printers using water-based inks. Like their diazo parent, dual-cure emulsions come in two parts. You have to mix them just like the diazos -- in fact, it's the diazo part that you add to the photopolymer base.

Viscosity and Solids Content
You have two other crucial factors to consider when choosing an emulsion: viscosity and solids content. Among other things, they can tell you the range of mesh counts the emulsion has been designed to work with. If you want to waste an afternoon, try coating a fine mesh with a thick emulsion, or applying a thin one to a coarse mesh.

Viscosity is measured in miliPascals (mPas), and the higher the number, the thicker the liquid. Basically, it tells you how easily a liquid flows. Most emulsion viscosities range between 3,000 mPas and 12,000 mPas, or more. Printers who use coarser meshes will stick to the higher viscosities, because thicker emulsions do a better job of clinging to open meshes. Thinner emulsions, on the other hand, easily penetrate the denser weaves of finer meshes.

Solids Content
Most emulsions have solids contents of between 30% and 50%. Generally, higher viscosity emulsions have a higher percentage of solids. If you have to choose between emulsions equal in other respects, choose the one with the higher solids content because that one will do a better job of reproducing fine details. Solids content can be even more important when using coarser meshes, because solids help to fill those wide-open spaces.

But don't rely on the numbers alone to tell you whole story. The best guarantee of performance is to select a good quality emulsion from a reliable manufacturer, and always buy the best emulsion you can afford. And even then other factors have to be taken into account. Dave Dennings of Kiwo cautions, "High quality emulsion can perform poorly if coated poorly."

Now all you have to do is keep all of this in mind when choosing an emulsion.