Polycotton, poly-viscose, polyester -lycra are the most common blends. Usually, the formula for deciding which dye to use with your blended fabric is this - look at the care label. If the care label says 35% or more polyester fibers in the fabric content, use the dye for synthetic fabrics. If the polyester fiber is marked Less than 35%, you can go for the dye suitable for the other major fiber. For, eg. you will have to dye polycotton with cotton fibers 60% with fabric dye suitable for cotton. The result will be a lighter shade than if the whole fabric is made of cotton. Because the polyester fibers will remain undyed. There is another method, though it can go wrong and get a botched look if you are not an expert; Blends are dyed like - first, the fabric is dyed with reactive dyes. This will dye the natural fibers but leave the polyester fibers undyed; This can then be dyed with dispersed dyes. Do not use heat with polyester-lycra fabrics - When your dyeing involves heating, you need a fabric that can withstand heat. Polyester-lycra blended fabrics with elastane fibers are not the best for this. You cannot apply more than 40 degrees celsius heat on any lycra fiber fabric. So you cannot use dispersed dyes with these fabrics. In short, if you want to do a good job, do not dye polyester-lycra fabrics. In the textile manufacturing process, polycotton blended fabrics are dyed with a process called Alkali shock process dyeing. In this, the reactive dye is used in fixed on the material using concentrated caustic soda. Cotton-polyester blends can be dyed with vat dyes.