A formulation system contains: (a) a pre-polymer reaction product of a polymeric isocyanate and a polyether polyol, where: (i) the polymeric isocyanate contains five weight-percent or more methylene diphenyl diisocyanate; (ii) the polymeric isocyanate has a functionality of 3.0 or less; (iii) the polyether polyol is present in the pre-polymer at a concentration of one to 25 weight-percent; (iv) the polyol has an equivalent weight of 50 to 500 grams per equivalent; (v) the —NCO concentration of the pre-polymer is 15 to 31 weight-percent; (vi) the pre-polymer is free of isocyanate trimers; (b) a polyester polyol component containing 10 to 25 weight-percent of free glycol; and (c) a blowing agent that contains less than five weight-percent water; where the ratio of pre-polymer and polyester result in foam having a trimer content of 12 to 22 weight-percent, and an —NCO index of more than 300 and less than 700.