Understanding Emulsion

The first and foremost important thing to remember with emulsions is that they react to ultraviolet light. This reaction causes it to “cure” or become hard. Normal ambient white light has enough UV to expose emulsions over longer periods of time. This is why emulsions are worked with under yellow or red safelight conditions.

 

That means your exposure source has to output a good amount of UV in order to expose your screens within a reasonable time. The less UV your light source outputs, the longer your exposure times will be.

 

Because emulsions can also react to excessive heat, prolonged exposure times to high intensity light sources with low UV output can be problematic. An example of such a light source would be the sun. Light sources like the sun can literally cook the emulsion with heat alone causing it to solidify.

 

Different emulsions vary in their photo sensitivity. Some will expose faster than others depending on light sources. This is why it is necessary to consider your light source when choosing an emulsion.

 

Today’s emulsions use only two types of photo sensitizers. Diazo, which has been around for some time and Styryl Basolium Quaternary photopolymers called SBQ photopolymers which are relatively newer. The sensitizers are added to a resin base of polyvinyl alcohol or polyvinyl acetate to make the emulsion.

 

This means that emulsions can be categorized into three groups based on the two photo sensitizers; diazo, photopolymer, and dual cure emulsions. Dual cure emulsions combine the best features of the other two. At the same time they compensate for some of their individual flaws.

 

The basic three types of direct emulsions: Diazo, Photopolymer, and Dual Cure.

 

Diazo emulsions are the least expensive and have the lowest light sensitivity of the three. If your light source is weak, a diazo emulsion can cause you difficulty. However, they also have the widest exposure latitude and can be forgiving. Diazo emulsions are not well suited for fine detail because they tend to make a thicker stencil. You will want a thinner stencil for the reproduction of fine details and halftones.

 

Another thing to note about diazo emulsions is that they come in either solvent-resistant or water-resistant types. That means if you use both solvent and water based inks, you would need to stock two types of diazo emulsions. They also have to be mixed with the sensitizer correctly to achieve optimum results. Once mixed, they have a shelf life of about two to three months depending on conditions.

 

Photopolymers or SBQ photopolymers are also called one-pot emulsions. These are the most light sensitive and therefore fastest emulsions. They also have excellent shelf lives in excess of one year but they tend to cost a lot more too. Because the sensitizer is mixed with the polyvinyl base by the manufacturer, SBQ photopolymer emulsions are excellent at the reproduction of fine detail.

 

Although SBQ photopolymer emulsions are faster, they tend to perform best with good light sources because their exposure latitude is small. If you work with water based inks, you can find some SBQ emulsions formulated to be specifically water resistant. They are also less affected by ambient humidity most of the time.

 

Dual cure emulsions have a combination of the qualities of both diazo and SBQ emulsions. As a result their exposure times fall in the middle range of diazo and SBQ emulsions. The exposure latitude is wider than diazo and they can reproduce fine detail better as well. Dual cure emulsions are less expensive than SBQ photopolymers and work well in high humidity. The shelf life of a dual cure is less than that of a SBQ photopolymer emulsion.