Hydrophobic polyesters have drawn a great interest in many practical applications such as coatings, packaging and textiles thanks to theirself cleaning, dust removal and anti-icing properties. Researchers have used several approaches to impart hydrophobicity to polymeric materials such as coating with hydrophobic particles, plasma treatments, chemical and physical vapor depositions, and processing in combination with low surface energy additives [1,2]. We demonstrate here the possibility to develop hydrophobic materials without using solvent, directly by melt blending a polybutylene terephtalate matrix and an alkoxysilane reagent. This approach focus on the reaction of alkoxysilane groups (SiOR) from n-octyltriethoxysilane reagent onto hydroxyl end groups from polybutylene terephtalate under molten processing conditions. The effect of the reagent content and the processing temperature on the viscosity and the molar mass of the obtained blends is analyzed in order to evaluate any negative side-reactions. Water contact angle results showed that the introduction of low concentrations of n-octyltriethoxysilane (1 wt.%) allows the formation of hydrophobic material with the water contact angle increased from 77 ± 2° for raw PBT to a maximum of 117 ± 3° (Figure 1). In addition to thermal properties, water sorption and water permeability are also investigated and compared with the reference PBT which proved the improvement of these properties. With the addition of 1 wt.% of n-octyltriethoxysilane, the obtained water diffusion and water permeability coefficients were decreased from 1.47×10-10 to 0.5×10-10 cm2.s-1 and from 6.8 to 1.27 barrer, respectively. REFERENCES 1. S. J. Hardman, N. Muhamad-Sarih, H. J. Riggs, R. L. Thompson, J. Rigby, W. N. A. Bergius, and L. R. Hutchings, Macromolecules 44, 6461 (2011) 2. J. P. Li, P. Cassagnau, F. Da Cruz-Boisson, F. Melis, P. Alcouffe, C. Lucas, and V. Bounor-Legare, Journal of Polymer Science Part a-Polymer Chemistry 55,1855 (2017)