▁which ▁are ▁also ▁known ▁as ▁cur ing ▁agents ▁or ▁hard en ers . ▁As ▁reaction ▁proceed s , ▁larger ▁and ▁larger ▁mole cules ▁are ▁formed ▁and ▁highly ▁branch ed ▁cross link ed ▁structures ▁develop , ▁the ▁rate ▁of ▁c ure ▁being ▁influenced ▁by ▁the ▁physical ▁form ▁and ▁functionality ▁of ▁epo xy ▁res ins ▁and ▁cur ing ▁agents ▁– ▁elev ated ▁temperature ▁post cur ing ▁indu ces ▁secondary ▁cross link ing ▁of ▁back bone ▁hydro x yl ▁functionality ▁which ▁con den se ▁to ▁form ▁ ether ▁b onds ; ▁ ▁Poly ure th anes ▁form ▁when ▁is oc yan ate ▁res ins ▁and ▁pre pol ym ers ▁are ▁combined ▁with ▁low - ▁or ▁high - m ole cular ▁weight ▁poly ols , ▁with ▁strict ▁st och i ometric ▁r ati os ▁being ▁essential ▁to ▁control ▁nucle oph il ic ▁addition ▁polym er isation ▁– ▁the ▁degree ▁of ▁cross link ing ▁and ▁resulting ▁physical ▁type ▁( el ast omer ▁or ▁pl astic ) ▁is ▁adjust ed ▁from ▁the ▁mole cular ▁weight ▁and ▁functionality ▁of ▁is oc yan ate ▁res ins , ▁pre pol ym ers , ▁and ▁the ▁exact ▁combinations ▁of ▁di ols , ▁tri ols ▁and ▁poly ols ▁selected , ▁with ▁the ▁rate ▁of ▁reaction ▁being ▁strongly ▁influenced ▁by ▁catal yst s ▁and ▁in hib itors ; ▁poly ure as ▁form ▁virt ually ▁instant ane ously ▁when ▁is oc yan ate ▁res ins ▁are ▁combined ▁with ▁long - chain ▁am ine ▁functional ▁poly ether ▁or ▁poly ester ▁res ins ▁and ▁short - chain ▁diam