this solubility is characterized by what is known as the 34 solubility parameter 34 this parameter describes how much a material will swell in the presence of different solvents this swelling will directly impact the characteristics of the reaction potential within the capsule as well as making the capsule more permeable both of which adversely affect the material solvents must therefore be chosen with great care the goal is to minimize the interactions between the dye and its surroundings the capsule is the major part of this the capsule is hard non polar thermally very stable it won 39 t melt and relatively impermeable the infiltration of compounds through the capsule must be stopped or slowed to the point that the characteristics of the dye are not effected shelf life is an issue and must be considered in the context of the above principles almost any solvent will penetrate the capsule given enough time the question that each person using this technology will need to ask themselves is what is an acceptable shelf life for the product we are developing and will our formulation give us the desired result we already know that the capsules and the dye they contain will not react with water why not it is a highly polar small molecule why doesn 39 t it destroy the dye 39 s properties the fact that it does not react with the capsule or the dye gives us several hints about the material the most important principle to learn from this is that there must be certain combinations of characteristics present for a substance to effect the dye melamine formaldehyde is not soluble in water the water is not absorbed by it so the capsules is impermeable to water water.