What are the advantages of a pure photopolymer emulsion?

What is a “dual-cure”?

There are many types of stencils that I can buy. What are the different types?

What are stencils made of? What is the chemistry of a stencil?

 


What are the advantages of a pure photopolymer emulsion?
Photopolymer emulsions have three main advantages

1. The require no mixing of added sensitizer. They are used “straight from the can”.
2. They have a shelf life measure in years
3. They expose in a fraction of the time required by either dual-cure or diazo emulsions

For large volume shops, the fast exposure times (typically less than one quarter_ mean that hundreds of screens can easily be produced every day. This attribute also keeps exposure times reasonable when exposing large format screens with a single lamp, or when only low power exposure lamps are available. For the small shop, the long shelf life means that the emulsion should be good to the last drop. Other notable benefits include reduced sensitivity to environmental moisture, since, unlike emulsions that contain diazo and must be bone dry before exposure, photopolymer emulsions are unaffected by humidity.

What is a “dual-cure”?
Stencil advancements created dual-cure emulsions that already contain a light-sensitive photopolymer system (added during manufacture), in addition to the normal diazo sensitizer added by the user.

Having two curing systems results in more effective stencil hardening. This leads to superior mesh bridging and resolution, and permits higher solids contents for reduced shrinkage and improved print definition. With dual-cure technology, it is even possible to create an emulsion that’s both solvent and water resistant, yet easy to reclaim.

Dual-cure polymer photoemulsions are ideally suited to the demands of a screen-printing industry that has become more quality conscious, sophisticated and competitive.

There are many types of stencils that I can buy. What are the different types?

Commercial available photostencils fall into four main categories. The first is known as Indirect Film, where the stencil imaging and developing process is carried out independently of the screen mesh. The finished stencil is applied to the mesh with gentle pressure, blotted with newsprint, and dried prior to removal of the backing film. Although capable of the highest quality reproductions, the thin edge of the finished stencil is very fragile and easily damaged, and therefore unsuitable for long print runs or for printing on difficult substrates. Indirect film is only suitable for use on finer mesh counts that are capable of supporting the fragile stencil.
The second type of stencil is known as Direct Film or Capillary Film. In this case, a much thicker layer of pre-coated photographic emulsion, which has been manufactured to a precise thickness, is adhered to a wet screen mesh through capillary action. After drying and removal of the backing film, exposure and development produces a much stronger and more firmly adhered stencil than in the previous case, but still with the image quality associated with a film-based product.
With the third type of stencil, known as Direct/Indirect, the film is laminated to the mesh with a layer of photographic emulsion instead of water. Once this sandwich has dried, processing is carried out the same as for capillary film, but with the advantage that an even more firmly adhered and durable stencil is produced. The downside is that the stencil making process is more complicated and messy, particularly in larger formats, and is also more costly sing both film and emulsion are required.
That brings us to the last, and most commonly used, type of stencil, which is known as Direct Emulsion. In this case the mesh is coated with a light-sensitive emulsion, which when dry is imaged and then developed in the same fashion as capillary film. This is by far the least expensive method, in terms of material cost, and results in the most durable stencils. However, it is also capable of producing poorer print quality than any of the film-based systems, unless the correct choices are made in terms of emulsion type and methods of processing and bringing several variables under control.

What are stencils made of? What is the chemistry of a stencil?
With the exception of indirect stencil films, which generally are thin coatings of gelatin containing an iron salt sensitizer, the other types of photostencil system, mainly direct emulsion and capillary film, which is real pre-coated emulsion, are based upon a resin known as polyvinylalcohol. Polyvinylalcohol possesses an unusual combination of three properties that make it uniquely suited to be used as the basis of most stencil materials. First, it is a water-soluble polymer, which means that stencil processing and developing can be carried out with water, rather than organic solvents. Second, it is highly solvent resistant, unlike most other water-soluble polymers that tend to dissolve even more readily in solvents, and therefore stencils are able to stand up to a wide variety of different ink types. Third, polyvinylalcohol contains a link in its polymer chain that is easily broken by the application of dilute aqueous solutions of sodium metaperiodate (AKA emulsion remover). This means that after printing, the mesh can be recovered and reused by stripping the stencil without harsh chemicals.

In order to make capillary films and direct emulsion light sensitive, there is a choice of three basic types of technology, Diazo, Dual-Cure, or Photopolymer. In addition, other ingredients such as fillers or bulking agents are added to increase the solid content and improve wet strength of the stencil during processing. The choice of sensitizers, and the type or combination of fillers used will determine the properties of the end product. Ancillary ingredients include pigments, surfactants that improve coating quality, and defoamers to kill bubbles during processing.

The simplest technology employs a diazo sensitizer, which is actually a polymeric yellow dye that is unstable and decomposes when exposed to actinic blue and UV light. When exposed, the diazo reacts with the polyvinylalcohol cross-linking the polymer chains and decreasing its solubility in water. This enables the stencil to form on the mesh during developing. The other ingredients that are added during manufacturing of the emulsion determine what its final properties will be. With diazo emulsions and films, the other main ingredient is known as polyvinylacetate. Polyvinylacetate is used to add bulk, increase solids content, and due to its water repellent nature is also effective in increasing the wet strength of the stencil during processing by preventing over-swelling of the cross-linked polyvinylalcohol, and loss of detail. If enough polyvinylacetate is used then the final stencil can become water resistant enough to be used for printing water-based inks. The problem with polyvinylacetate however is that it is very sensitive to organic solvents. If a high level is sued, then the excellent solvent resistance and easy reclaiming conferred on the stencil by the use of the polyvinylalcohol component is compromised. For this reason diazo emulsions tend to fall into one of two categories, solvent resistant or water resistant.

  With dual-cure emulsion and film, the diazo sensitizer, which is still used, is fortified by including an additional cross-linking system at the time of manufacture. This additional cross-linking system is used to reinforce, or in certain cases even replace, the polyvinylacetate component of the stencil. By combining these two separate cross-linking systems, each separately for the two main components of the emulsion, it is possible to engineer properties into the stencil that were mutually exclusive with diazo-sensitized products. For instance water and solvent resistance, or high solids and easy reclaiming. For this reason, most manufacturers of stencil materials now offer a universal type of dual-cure direct emulsion that combines most of the properties of the “ideal” stencil.

Photopolymer stencil products do not contain diazo, since they are manufactured with a light-sensitive polymer. Emulsions are supplied presensitized and ready-to-use with no mixing required, and both photopolymer emulsion and film have a shelf life that is measured in years, and not weeks or months. (Diazo is affected not only by light, but also by heat and humidity.) The other distinguishing feature of photopolymer is that exposure times are a fraction of what would be used for either diazo or dual-cure products. This is due to the very high sensitivity of the polymer that is sued. The resistance properties of photopolymer fall into the same categories as those for diazo sensitized material, either solvent or water resistant. Having said that however, the water resistance of commercially available photopolymer emulsions does not yet rival that of diazo. Products designed for garment printing are really more suited for use only with plastisol inks, unless a hardener is used to reinforce the screen. The very fast exposure times achievable with photopolymer has also enabled the development of products that are suitable for use with extremely weak light sources, such as projection exposure.