Kuiper Belt Objects and cometary nuclei are considered to be among the most primitive bodies of the outer Solar system. However, their composition may not reflect that of the primordial planetesimals from which they formed since these latter might have experienced some physico-chemical differentiation, due to the heating of impacts. Here, we examine the implications of collisional effects on the physical and chemical differentiation of the planetesimals located in the primitive Edgeworth-Kuiper Belt by using a cometary nucleus model that ensures conservation of mass and energy during and after the impact. We then discuss the influence of the composition (dust fraction and CO/H_2O ratio) and of the adopted values for heat capacity and conductivity within the matrix. We observe that deep modifications of the physical composition in the planetesimals are only possible in the case of a poorly dusty planetesimal without CO inside the body. In all other cases, variations of the parameters show that collisions induce modifications on the initial physical and chemical composition only in the subsurface layers.