Polyhydroxyl buteric acid (better poly-3-hydroxybutyrate), is a polyhydroxyalkanoate (PHA), a polymer belonging to the polyesters class. PHB is produced by micro-organisms apparently in response to conditions of physiological stress. The polymer is primarily a product of carbon assimilation (from glucose or starch) and is employed by micro-organisms as a form of energy storage molecule to be metabolized when other common energy sources are not available. PHB has properties similar to those of PP, however it is stiffer and more brittle.