Abstract: A process for the production of polyester resins having an intrinsic viscosity higher than 0.57 dl/g from polyester resins having intrinsic viscosities of lower than 0.57 dl/g in which 1) a melt of the polyester resin having an intrinsic viscosity lower than 0.57 dl/g is mixed with a polyaddition additive containing at least two groups which react with the terminal OH and COOH groups of the resin, 2) the melted mixture is converted into solid particles which are subsequently crystallized at temperatures higher than the Tg of the polyester resin and lower than its melting point, and 3) the particles are heated at temperatures higher than 150.degree. C. but lower than the melting point of the resin to increase the intrinsic viscosity.

Abstract: A composition of polyester resins comprising diblock copolymers formed of two different polyester blocks joined by a bifunctional radical deriving from a dianhydride of a tetracarboxylic acid. The compositions are prepared by a reaction in the solid state of two different polyesters in the presence of a dianhydride of a tetracarboxylic acid.

Abstract: A foamed cellular material from polyester resins is obtained by extrusion foaming of polyester resins having melt strength higher than 8 centinewton, intrinsic viscosity greater than 0.8 dl/g and complex melt viscosity higher than 25,000 poises.

Abstract: A process for the solid state upgrading of a polyester resin by polyaddition reaction in which the resin is upgraded in the presence of an upgrading additive which is a compound different from a dianhydride of an aromatic tetracarboxylic acid, containing two groups capable of addition reactions with the terminal groups of the polyester, selected for example in the group of the dianhydrides of aliphatic and cycloaliphatic tetracarboxylic acids.

Abstract: A process for the solid state polyaddition of a polyester resin in which the resin, blended in a molten state with a dianhydride of an aromatic tetracarboxylic acid is granulated, crystallized and then upgraded, characterized in that the crystallization and upgrading temperature is comprised in the range between the TG and 170.degree. C., preferably between 130.degree. and 170.degree. C.