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| June 2005 issue of IMPRESSIONS |
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How
to Choose the Right Emulsion
Understanding how emulsions work is critical when preparing screens
for exposure
by Douglas Grigar, Master
Screen Printer
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(A version of this article originally appeared
in the June 2005 issue of IMPRESSIONS.)
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The average consumer owns dozens of t-shirts,
but most would be surprised to know just what it takes to get ink onto
those shirts. Screen printers confront hundreds of variables when creating
basic designs. Once you take a close look at the individual procedures,
you’ll find that each is based on a fairly simple premise.
Take emulsion for example. As part of the photostencil phase of the
production cycle, emulsions are those light-sensitive liquids or capillary-direct
films that you coat or adhere to the degreased screen fabric prior to
taping your film positives in place.
Emulsions come in a variety of formulations, and your selection should
be based on the features needed for the final application. To decide
which emulsion to choose for your job, first learn the properties of
each, then determine what you want to do for a particular job.
The Science
Most emulsions contain two chemical polymers: polyvinyl acetate, a water-resistant
polymer often referred to as PVAC; and polyvinyl alcohol (PVOH), a solvent-resistant
polymer. Although some emulsions have both solvent- and water-resistant
features and can offer a more universal approach to customers’
needs, long print runs call for ink that demands an emulsion formulation
based on the ink manufacturer’s recommendation.
All emulsions shrink when dry (see Figure 1) and will follow the threads
of the mesh. This creates divots in the surface that can break and cause
pinholes if the dried emulsion is too thin to bridge the mesh holes.
Low-solids-content emulsions shrink excessively and are sometimes prone
to this problem. Therefore, look for emulsions containing a high solids
content - more than 35% or 40%.
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[Figure 1: When applied to screen mesh,
emulsion shrinks as it dries. This creates divots where the emulsion
shrank the most.]
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Be aware that some lower-quality emulsions use inert filters
to boost the solids content ratings. However, introducing these inert
solids into an emulsion is akin to adding contamination to it. The additives
also lengthen the screen’s exposure time and cause interference
with the chemical linking that takes place during exposure.
The Big Three
The three basic emulsion types represent
two basic chemical technologies: the older light-reactive benzene
diazonium (shortened to diazo), and/or the newer Styryl
Basolium Quaternary salt (or SBQ-based emulsions).
Here’s a quick lesson in chemical linking to help you make an informed
emulsion choice: The oldest of the choices, diazo-sensitized
emulsions, are mixed with raw polymers. When dried on the
screen mesh, they make a photo-reactive product capable of reproducing
a mid-range level of stencil definition. When exposed to ultra-violet
light, diazo links with the polymers to make a solid and strong stencil
block (see Figure 2). Diazo emulsions typically have the slowest exposure
times.
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[Figure
2: Diazo-sensitized emulsions make chemical links from the diazo to the
polymers when exposed to UV light. These emulsions are typically slower
to dry.]
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In the ‘80s, when pre-sensitized
emulsions using SBQ chemicals entered the industrial market, a new screen
printing technology was introduced. Often called pure photopolymers or
single pot emulsions, SBQ-sensitized emulsions
use photo-reactive molecules and are pre-bonded to the polymer solids
that link to each other when exposed to UV light (see Figure 3).
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[Figure
3: Pure photopolymer emulsions have SBQ sensitizers pre-bonded to polymers.
They can quickly bond with each other when exposed to UV light.] |
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quick to expose. The bonds formed in SBQ emulsions are not quite as strong
as the bonds made with diazo, but they are much faster to form. This high
solids-content formulation creates good bridging capabilities.
The newest choice in emulsions are hybrid
mixtures of diazo and SBQ-sensitized emulsions. Dual-care
emulsions are popular because many of the best features
of both types of emulsions are available in these types of mixtures. While
partially pre-sensitized, dual-care emulsions need to have the diazo component
mixed before use, in the same way as diazo-only emulsions.
As expected, the linking of polymers in dual-care
formulations shares characteristics of both SBQ and diazo emulsions. They
offer pre-bonded linkable molecules, and free-floating diazo photo-sensitive
properties together in one product (see Figure 4). Dual-care emulsions
often have excellent bridging and detail resolution. They are slower to
expose than a pure photopolymer, but are considerably faster than a diazo-only
mixture.
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[Figure 4: Dual-care emulsions are a hybrid of SBQ
and diazo emulsions, and share linking characteristics of both types.]
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Film School
Screen printers sometimes assume that capillary
film is different form the other three emulsions (mentioned
above). Actually, capillary film is any of the three choices of emulsions
pre-sensitized, and dried onto a film base that is ready for application
by the end user. Any of the three types of emulsions are available in
capillary film form and continue to have the same exposure characteristics.
While more expensive, capillary film is an
excellent choice for new shop owners, special applications, or jobs needing
the most accurate thickness of emulsion. Special applications such as
super-thick stencils almost demand a capillary film for its exacting thickness.
Many printers find that the ease of application (no mess!) and the time
saved using capillary film more than makes up for the higher-per-inch
coverage cost.
The main difference between emulsion applied
in liquid form and as capillary film is that liquid emulsions form a total
encapsulation of the mesh threads. Capillary films, however, only draw
into the mesh about half way up the thread diameter.
In the past, capillary films suffered from
a bad reputation because they were assumed to have a short print life.
But with proper preparation of the mesh and a good application procedure,
they can easily make thousands of impressions without stencil breakdown.
To benefit from the best features of capillary
film and the encapsulation-holding-strength of liquid emulsion, combine
liquid emulsion (as a kind of adhesive) with capillary film. Attaching
this combination to your mesh creates a combination stencil. It is the
most expensive stencil option but holds the features of both types of
emulsion products.
The best choice in emulsions is based on
specific shop needs. To determine the best match, list the desired features
of each emulsion for your print jobs.
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