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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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PVOH, the alcohol, gives an emulsion its solvent resistance. It is also the component that reacts with the diazo sensitizer to "harden" the emulsion, giving it the ability to form a film on the mesh. Finally, it is the PVOH that makes an emulsion reclaimable.

PVA, the acetate, provides water resistance. It is a very dense material that is a major contributor to the solids content of an emulsion. On its own, PVA takes no part in the reaction with the diazo sensitizer. You might say it lets the PVOH do all the work while it goes along for the ride. PVA molecules are actually coated with a microscopic layer of PVOH. This keeps them from finding each other and forming unmanageable lumps, like the kind you find on a beach after an oil spill.

Table 3 Properties of One-Part Photopolymer Garment Emulsions
Feature1 One-part photopolymer garment emulsion
Solvent resistance
Water resistance
Humidity resistance
Fast exposure XX
Wide exposure latitude
Image resolution X
Print definition X
Reclaimability X
Drying time XX
Stencil build XX
XX Outstanding feature
X Secondary feature
1Relative to dual-cure and diazo-sensitized emulsions

The water-resistant PVA tries to resist reclaiming solutions, but the PVOH coating makes it possible to remove the PVA in a normal reclaiming procedure. If the stencil has been underexposed, however, acetate removal cannot be aided by the PVOH, and it stays in the mesh. This explains why that stubborn, nonreclaimable emulsion stain can only be removed with--you guessed it--solvent. Acetate staining is a common but preventable problem, and every screen printer has experienced it.

Figure 1 shows a diagram of a solvent-resistant diazo emulsion. As you can see, the level of PVOH is high to give the emulsion its solvent resistance. While the proportion of PVA is low, it is a necessary ingredient to give the emulsion its solids content and a degree of water resistance. Figure 2 is a water-resistant diazo emulsion. In this case, the proportion of PVA is higher. No matter what type it is, an emulsion requires both resins, or a reasonable substitute for the PVA, in order to perform properly. The levels in which they are used will determine the characteristics of the emulsion.

Plasticizers are commonly used to give an emulsion flexibility and mechanical resistance. A wide variety of plasticizers are used in emulsions, depending on the performance characteristics required by a specific formulation. The more plasticizer in the formulation, the more flexible the emulsion.

Diazo sensitizer is supplied as a powder or syrup. Syrups are easier to dilute, but powders are more stable. Both perform equally well when mixed into the emulsion. Diazo is an excitable, rather unstable substance. It is most reactive with UV light, but heat and water will also set it in motion.

Table 4: Summary of Direct-Emulsion Features
Feature Benefits and/or drawbacks Found in
Presensitized No mixing; saves time; extended shelf life One-part photopolymer
Excellent solvent resistance Not required for garment inks, but needed for harsh washup solvents Solvent-resistant diazo; solvent-resistant dual-cure; water/solvent-resistant dual-cure
Excellent water resistance Required for water-based inks; can make reclaiming more difficult Water-resistant diazo; many dual-cure formulations
Excellent humidity resistance Required in areas of high humidity Water-resistant diazo; many dual-cure formulations
Best mechanical resistance Good for very long runs on abusive equipment; e.g. belt presses Many dual-cure formulations
Fast exposure Saves time; can compromise print quality in some cases One-part photopolymer (fastest); fast-exposing dual-cure; fast-exposing solvent-resistant diazo
High solids Reduces number of coats needed for optimum stencil thickness; bridges coarse meshes easier; in some cases, enhances print definition; can inhibit reclaiming One-part photopolymer, as high as 50%; many dual-cures (range from 35-50%)
High resolution (100-line halftone or 6- to 10-mil line) Enables reproduction of fine lines and halftone dots without resorting to underexposure Many dual-cure formulations
High definition Prints a clean line; reduces incidences of printed edge faults Many dual-cure formulations
Easiest reclaimability No pressure washer required, as long as the stencil is not underexposed All solvent-resistant diazo emulsions, particularly acetate-free formulations

Store unused diazo in cool, dry conditions for optimum performance, and by all means, keep it away from the beach! Diazo will begin to degrade from the moment it is mixed with water, so it should be added to the emulsion all at once. Sensitized emulsion will last on the shelf for about three months, but most printers usually use up a gallon long before that. Emulsion-coated screens are another story. They really hate the heat. One hot summer weekend without air conditioning will render your stock of coated screens useless. All you can do is try to reclaim them and start over.

What happens when the lights go on?

The sensitizer has been mixed into the emulsion. The screen is coated, dried, and ready to be exposed (not burned). When you turn on the exposure unit and leave the room for a cup of coffee, guess what happens?

There's a party going on in there. The diazo has a very interesting personality. It is excitable and outgoing, a "life-of-the-party" type. It is wreaking all sorts of photochemical havoc in the emulsion. Call it a chain reaction, polymerization, or crosslinking: The diazo is transforming the emulsion into a stencil. Here's a palatable, if not terribly technical, explanation of what happens during emulsion crosslinking.

UV light energizes the diazo sensitizer to the point of instability. Each diazo molecule, which has six to eight "arms," throws off free radicals that crosslink or bond the PVOH chains together. To illustrate how this process works, imagine that you have a pile of strings. Try to pick up the pile and it will fall apart. Now, drizzle glue over the pile, let it dry, and pick it up. The glue has bonded many pieces of string into a single structure.

The diazo in a direct emulsion works in the same fashion. If the glue isn't completely dry, the pile of strings will fall apart. If the diazo isn't thoroughly crosslinked with the PVOH, the same thing will happen to your stencil.

What happens to the PVA while all of this is going on? Remember that it is surrounded by a thin layer of PVOH, which at optimum exposure, will be crosslinked. In this way, the PVA is able to become part of the chain without actually doing anything. It's the passenger, and the PVOH is the driver.

The process of crosslinking is critical to the performance of the stencil, and in both types of diazo emulsions, the process works in the same manner (Figure 3). The structure that is created is greater than the sum of its individual parts. In the case of a water-resistant emulsion, the water-soluble PVOH becomes fairly water resistant, so the stencil can be processed with water or printed with water-based ink, and it can resist shop humidity. The solvent-soluble PVA receives a degree of solvent resistance, allowing the emulsion to be cleaned with solvent-based screen washes. To prevent any weak links in the chain, all of the diazo must be spent. Underexposure creates weak links that lead to pinholing, screen breakdown, and difficult reclaiming.

If the emulsion was not thoroughly dried before exposing, the diazo will crosslink with the water as well as the PVOH, and the emulsion will not be sufficiently hardened. In fact, the resulting stencil may contain as many weak links as one that was simply underexposed. Heat from exposure units will not dry the emulsion during exposure, as was theorized a few years ago. Stencil problems caused by moisture in the emulsion are the same as those caused by underexposure.

Like their advantages, the inherent limitations of diazo-sensitized direct emulsions come from their basic chemical makeup. Certain feature combinations are not feasible. Diazo emulsions cannot be made resistant to both water and solvents. Also, you cannot make a diazo-sensitized direct emulsion with fast exposure and high solids content, or easy reclaimability and water resistance. This is where the magic component in dual-cure emulsions becomes significant.

Dual-cure emulsions

Dual-cure (diazo-photopolymer) formulations are similar to diazo emulsions. They require the addition of a diazo sensitizer and contain both PVOH and PVA, but all types of dual-cure emulsions contain a similar proportion of both resins. are PVOH-rich--even the water-resistant versions. This is an important factor in their ability to reclaim easier than their diazo sensitized counterparts. But if they are so rich in a water-soluble resin, how can they be water-resistant?

The dual in "dual-cure" refers to an acrylate (acrylic monomer) that is "sensitized" with a photoinitiator before it is added to the emulsion by the manufacturer. The acrylate is oil-based, but highly resistant to both water and solvent. It is a great contributor of high-quality solids, and it is the reason that dual-cure emulsions can produce better printed edge definition than conventional diazo emulsions. The acrylate has an affinity for everything in the emulsion. It brings out the best in the PVOH and PVA by minimizing their weaknesses. Figure 4 shows a diagram of a dual-cure emulsion before exposure.

Remember that the PVA doesn't like to be reclaimed. It can fuse with inks and solvents when they penetrate its PVOH shell. The acrylate mediates the negative influences of the PVA. It permeates the PVA, and whatever is not absorbed remains dispersed in the PVOH. Before the addition of the diazo, a dual-cure emulsion is only mildly sensitive to light. You cannot make an acceptable stencil without the addition of the diazo sensitizer. (In dual-cure emulsions, the acrylate is sometimes referred to as the "photopolymer" and is not to be confused with the photopolymer chromophore in one-part emulsions.)

When light hits a screen coated with a dual-cure emulsion, the diazo again becomes very excited and begins crosslinking the PVOH, just as in diazo emulsions. That's one chain reaction. The second reaction starts with the acrylate.

UV light also causes the acrylate to react, setting off a series of internal crosslinks within the PVA and to some extent, the PVOH (Figure 5). Two separate chain reactions occur, and the crosslink density (number of links in the chain) becomes very great. The PVA gets a boost from the acrylate in the form of internal crosslinks and is given a much higher degree of solvent resistance, while the PVOH gains much more water resistance. The acrylate boosts the solids content so that more PVA doesn't have to be used. Because of its effect on the PVA, dual-cure emulsions are less prone to acetate staining on the mesh during reclaiming and make it easier to reclaim the acetate.

One-part photopolymer emulsions

One-part photopolymer emulsions are formulated for either garment or graphics printing. The graphics-quality emulsions are different enough in makeup and function that we can leave them out of this discussion, so we'll focus only on formulations for garment printing.

Once again, we have PVOH and PVA. They are each resistant to solvent or water and require the crosslinking of a sensitizer to harden the emulsion into a stencil. The sensitizer, in this case, bears no resemblance to diazo or the acrylate that is found in dual-cure emulsions. This sensitizer is a photopolymer chromophore, which we'll refer to as "the photopolymer." (Technically, all photostencils are photopolymers, but that's a different article.)

Let's review the sensitizers we have seen so far. Diazo is gregarious; it grabs the hand of everything within its reach. It reaches out for all the PVOH it can link up with, as long as the lights are still shining.

Acrylate is laid back, peaceful, and confident. It is not easily agitated, and it doesn't grab the spotlight from the diazo. It is simply there to help the PVA be a little more friendly, and give some strength to the PVOH. It is another set of hands in the chain reaction.

The photopolymer has a completely different personality. It is introduced to the emulsion during manufacturing, where it is attached first to the PVOH. When exposed to UV light, the "hands" on the photopolymer chains begin to seek each other out. They are very fast-acting, but short-lived. If they do not find each other and form a bond very quickly, they will become completely ineffective. They act fast or don't act at all.

The photopolymer chains are also a bit lazy. In their resting state (before the lights go on), they align themselves within reaching distance of each other (Figure 6). Then, when they are called upon to crosslink, they can virtually lean over and shake hands with their neighbor (Figure 7). It's like being able to reach the remote control, the popcorn, and your drink without having to move from the sofa!

Remember that in a diazo emulsion, the sensitizer uses its six to eight "arms" to crosslink with the PVOH. In the case of a photopolymer emulsion, only one or two chains of the sensitizer are grafted onto the PVOH. There are fewer "hands" to form a chain. With any diazo-sensitized emulsion, including dual-cures, UV light will keep the diazo at work until it is completely used up. In a one-part photopolymer emulsion, the sensitizer has only a limited amount of time to crosslink before it becomes useless.

Because of their narrower exposure latitude, the crosslink density of one-part photopolymer emulsions is relatively low. The emulsions are not the most resistant to water or harsh solvents and do not offer the ultimate in printed edge definition or image resolution. Like other emulsion types, if they are not thoroughly exposed, the PVA can fuse with washup solvents and make reclaiming difficult. They also happen to be the most costly to manufacture, and you know what that does to selling prices. However, they are the fastest growing group of stencil materials. So what's the big attraction?

Speed. One-part photopolymer emulsions are presensitized and save time from the outset by eliminating mixing of the sensitizer. They coat quickly and dry fast. Exposure times are lightning-fast compared to diazo and dual-cure emulsions. Many garment printers love these features and tend not to notice, or be concerned with, the system's limitations.

Stencil selection

Now that you know what's available and how each stencil system works, let's look at what features you should consider when selecting a direct emulsion. First, stop and ask yourself how you chose your current emulsion. Here are some common reasons that are often cited for emulsion selection: "We've always used it." "They were using it when I got here." "It's fast." "It's cheap." "It's worked okay so far." "The screenmakers like the color."

While nothing is wrong with any of these reasons, none of them is as right as it could be. The following is a list of characteristics you should consider when choosing a stencil system. Some are essential, and others fall into the "would be nice" category.

Resistance/durability

Resistant to what? Durable for how many impressions? One of the first things you must consider is what kind of inks and solvents will come in contact with the stencil. Fortunately for garment printers, plastisols are friendly to every emulsion. Be careful with those washup solvents, though. An underexposed stencil might react with them and give you those infuriating acetate stains. Any direct-emulsion stencil that is correctly processed should print well over 25,000 impressions. (Put away your calculator--that's almost 2100 dozen.) That's durable enough for almost everyone. Water-based inks will require a water-resistant emulsion. Belt printing presses are murder on most emulsions. They require a product that is resistant to water-based inks, mechanical wear-and-tear, and the adhesive used on the belt.

Print quality

Fine detail and good edge definition are relative terms. Different types of screen printers have individual standards for these terms. Again, you are fortunate. What a garment printer might deem a "fine line" could be considered medium by printers in the electronics sector. You only need to select an emulsion that will, at optimum exposure, reproduce the detail in your artwork. Unless you are printing four-color process, vignettes, or complicated patterns, almost any emulsion will do this (yes, it's true) unless you have a really inefficient light source.

There is more to image reproduction than resolution (the ability of the emulsion to reproduce the artwork). Edge definition refers to the actual quality of the printed line or dot. You strive for a clean, sharp edge and very often you end up with sawtoothing or wavy lines. Today's menu of direct emulsions contains many products that offer print quality that is far superior to the diazo emulsions of ten years ago.

Mesh selection

This plays a key role in emulsion selection, and no screen printer uses as many off-the-wall mesh counts as a garment printer. You print glitter with a 45-thread/in. white polyester mesh and then turn around and print four-color process through a 420. Obviously, it may not be practical to use the same emulsion for both jobs. Very coarse meshes, not to mention your screenmakers, deserv


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