Many beginners start with the diazo type of emulsion because it is thick and durable. It is easy to see when the emulsion on the screen is exposed because the color changes. It is the least expensive type of emulsion, available in art supply stores for $25 a quart (as of April 2010). A quart will cover between 20 to 25 screens, if they are the standard 18- by 24-inch size. It can be messy however, and needs to be mixed with a sensitizer or activator before it becomes light sensitive. This emulsion will keep for six to eight months before being mixed with activator, but after you mix it the shelf life is only three months maximum.
This is a professional quality emulsion that has much quicker and more stable exposure times. Diazo emulsion can take 15 minutes to expose in the sun, and the times change depending on the humidity. Photopolymer can take the image in seconds and dries fast after being coated. You can apply several coats quickly, creating a thick layer of emulsion that will work well if you are printing on a rougher textile, like sports clothes or denim. Pure photopolymer is often sold "pre-sensitized" so it does not need to be mixed and generally costs around $10 more than diazo for a quart. Some brands, like Ulano, can be used with plastisol inks, the durable kind for t-shirts, and water-based inks.
This emulsion combines qualities of diazo and photopolymer together. It has a fast exposure time and changes color when it's finished exposing. Dual cure also gives the screen a heavy coating and holds the image well. It needs to be mixed with an activator chemical and has the same shelf life as the diazo. Inexperienced printers can also use it successfully, but the fast exposure time makes it a little more challenging for beginners. Dual cure is sold in two varieties, one for use exclusively with plastisol inks and another only for water-based inks.