The present invention provides modified urea formaldehyde (UF) resin binder compositions and mineral fiber mats having an emulsion polymer modifier comprising, in copolymerized form, from 5 to 25 wt. % of co polymerized carboxylic acid functional monomers. The emulsion polymers have a measured glass transition temperature (DSC) ranging from −40° C. to 70° C. and a large average particle size. Further, the emulsion polymers comprise less than 30% of the copolymerized acid in neutralized form and do not excessively thicken aqueous compositions containing them. The modified UF resin binder compositions enable good dilution stability in use and high tensile strength in products containing them.