A process for securely binding the adjoining fibers of a fibrous material in the absence of the use of a phenol-formaldehyde reaction product is provided. A curable binding composition comprising a water-soluble intermediate reaction product having hydroxyl and carboxylic acid end groups that is capable of undergoing an esterification crosslinking reaction is provided. The intermediate reaction product is formed by the reaction of (i) a polyanhydride, and (ii) certain polyol crosslinkers other than an alkanolamine. Such crosslinkers have been found to yield a bound product that displays improved color (i.e., lighter color). Particularly preferred crosslinkers are triethylene glycol and tripropylene glycol. When applied to a fibrous material, the intermediate reaction product is reacted with crosslinking to form a cured reaction product. In a preferred embodiment the securely bound fibrous product is building insulation of good color that well resists water.