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A Functional Family: Personalities of Direct Emulsions

(December 1999) posted on Tue Dec 14, 1999

A comparison of three direct emulsions


By Tina Scarpelli

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How do they do that?

Like mother, like daughter; like father, like son. Like people, direct emulsions possess the personalities of their raw materials. But unlike people, emulsions don't have the ability to reason their way out of difficult situations. They don't sense a rise in humidity and decide to resist it. They cannot recognize fine detail and pull out all the stops to resolve it. The reasons are found in the chemistry that makes them tick. To understand how all emulsions work, we need to start with the basics: diazo emulsions.

Diazo-sensitized emulsions

The composition of these emulsions forms the basis on which other types of emulsion are designed. Diazo emulsions contain two vinyl resins, a sensitizer, pigments, plasticizers, surfactants, and a smattering of other ingredients that give individual products their character and personalities.

The two vinyl resins in diazo emulsions are usually polyvinyl alcohol (PVOH) and polyvinyl acetate (PVA). (Some emulsions substitute other components for the PVA, but let's not complicate a complicated subject.) PVOH and PVA are suitable for use in direct emulsions because they act like the oil and vinegar in a salad dressing. The PVOH is water soluble and highly solvent resistant. The PVA is just the opposite--solvent soluble and highly water resistant. Each of these resins adds important characteristics to the emulsion.

Table 2 Properties of Diazo-Photopolymer (dual-cure) Emulsions
Feature1
Solvent-resistant dual-cure emulsion
Water/solvent-resistant dual-cure emulsion
Water-resistant dual-cure emulsion
Solvent resistance
XX
XX
Water resistance
XX
XX
Humidity resistance
XX
XX
XX
Fast expsoure2
Possible
Possible
Possible
Wide exposure latitude
XX
XX
XX
Image resolution
XX
XX
XX
Print definition
XX
XX
XX
Reclaimability
X
X
Drying time
X
Stencil build
X
XX
XX
XX Outstanding feature
X Secondary feature
1Relative to ratings for diazo emulsions in Table 1
2Can be formulated 25-50% faster than conventional diazo-sensitized emulsions

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