Plastics, rubbers, TPE, composites are not monolithic materials but are made from the association of several entities, one or several polymers, fillers and chemicals. When the polymers are immiscible, one of them forms a continuous phase and the other polymer(s) and ingredients are dispersed into it, as schematized on the following figure 'Good-Blend-Practices'. If there is no compatibility between the continuous phase and the dispersed droplets of the other ingredients, too weak are the mechanical performances and the applications are very reduced. To obtain an industrial material it is necessary to have.
Unfortunately, polymers are often immiscible and incompatible with other polymeric and mineral materials. Consequently it is necessary to compatibilize polymers the ones with the others and with the fillers and other additives. That is made possible by physical and chemical modification thanks to functionalized polymers and chemicals, coupling agents, grafting, reactive extrusion, crosslinking etc.