faster the fact of having all the components in a powder form allows to obtain very good mixing between the components without having melting of the polyester and pmda it is thus possible to intimately contacting pet and pmda avoiding reaction between them since the processing temperature is much lower than their melting temperature uniform pmda distribution in the polyolefin matrix is also obtained thus avoiding any branching and gelyfication effects and no sublimation of pmda during the masterbatch preparation in addition no polyolefin degradation occurs thanks to the low extrusion temperature used still another important advantage is due to the presence of pmda in the masterbatch as 100 unreacted form and therefore available in that form in the subsequent foaming step as further advantage the masterbatch can be dried at a higher temperature than that used for the masterbatch described in u s pat no 5 810 206 thanks to the presence of a large amount of crystallized pet finally the thermomechanical properties of the obtained foam are greater than those of the masterbatch of u s pat no 5 810 206 due to the lower amount of polyolefin present last but not least the uniform distribution of pmda in the polyolefin matrix assures constant reological properties of the polyester during the foaming process with consequent improvement of the properties of the foamed product the presence of powdered pet in the masterbatch assures uniform distribution of pmda during the foaming process this because pmda is already in close contact with the pet powder thus facilitating the solubility of pmda into pet during the.