Urea-formaldehyde resin binders containing acrylic bi-modal molecular weight solution polymer

A method for making a fiber mat, comprising:

• dispersing fibers in an aqueous medium to form a slurry;
• forming the slurry into a wet fiber mat;
• applying an aqueous binder composition to the wet fiber mat, wherein the aqueous binder composition comprises as a major component a thermosetting urea-formaldehyde resin and as a minor component a water-soluble additive containing at least one styrene/acrylate solution bi-modal polymer comprising a first molecular weight species having a molecular weight of 1,000 daltons to 100,000 daltons and a second molecular weight species having a molecular weight greater than 100,000 daltons, wherein the styrene/acrylate solution bimodal polymer is present in the aqueous binder composition in an amount of about 0.1 wt % to about 10 wt %, based on total binder solids, and
• curing the binder composition at an elevated temperature of about 135° C. to about 300° C. to provide a fiber mat comprising fibers bound together with the cured binder composition.