NOVELTY - A polyurethane composition, comprises: a polyisocyanate; and a branched polyol that reacts with the polyisocyanate at 20-90 degrees C for 20-40 minutes to form a cured polyurethane having a glass transition temperature of at least 70 degrees C.    USE - As polyurethane composition used for preparing molded composite structure i.e. blades for wind generators (claimed).    ADVANTAGE - The polyurethane composition is able to cure at temperatures as low as room temperature (20-25 degrees C) to provide a cured polyurethane having a glass transition temperature (Tg) of at least 70 degrees C without the need of a post-curing process. The polyurethane composition has a first viscosity of 1000 mPas measured at 40 degrees C after 10-70 minutes of the reaction time and a second viscosity of at least 5000 mPas measured at 40 degrees C after 20-120 minutes of the reaction time. The suitability of the polyurethane composition for use in a composite structure is due at least in part to its viscosity at the time of achieving a mix of the branched polyols and polyisocyanates (i.e. a uniform dispersion). The viscosity of the polyurethane composition at the time of achieving the mix is sufficiently low enough to allow the polyurethane composition to be infused into a composite structure that is very large, such as a blade of a wind generator. In addition, the cure rate of the polyurethane composition is sufficiently fast and has a cure temperature that is sufficiently low that the amount of heat generated by the curing polyurethane composition is kept to a level that is suitable for use with a wide variety of materials. This is advantageous when it is desired to use a material in the composite structure that has a melting temperature that is less than or equal to 90 degrees C.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a composite structure, comprising: the polyurethane composition; and a reinforcement material, where the cured polyurethane has a glass transition temperature of at least 70 degrees C after reacting at a cure temperature of 20-90 degrees C for 20-40 minutes;    (2) preparing a composite structure, involving: impregnating a reinforcement material with the polyurethane composition; and reacting the polyisocyanate and the branched polyol of the polyurethane composition at 20-90 degrees C for 20-40 minutes to form the cured polyurethane having a glass transition temperature of at least 70 degrees C.