▁applications ▁up ▁to ▁the ▁decomposition ▁temperature ▁since ▁they ▁keep ▁their ▁shape ▁as ▁strong ▁c oval ent ▁b onds ▁between ▁polym er ▁ch ains ▁cannot ▁be ▁broken ▁easily . ▁The ▁higher ▁the ▁cross link ▁density ▁and ▁a rom atic ▁content ▁of ▁a ▁therm os et ▁polym er , ▁the ▁higher ▁the ▁resistance ▁to ▁heat ▁de grad ation ▁and ▁chemical ▁attack . ▁Mechan ical ▁strength ▁and ▁hard ness ▁also ▁improve ▁with ▁cross link ▁density , ▁although ▁at ▁the ▁exp ense ▁of ▁brit t len ess . ▁They ▁normally ▁de compose ▁before ▁mel ting . ▁ ▁Hard , ▁pl astic ▁therm os ets ▁may ▁under go ▁permanent ▁or ▁pl astic ▁de formation ▁under ▁load . ▁El ast om ers , ▁which ▁are ▁soft ▁and ▁spring y ▁or ▁rub ber y ▁and ▁can ▁be ▁de formed ▁and ▁revert ▁to ▁their ▁original ▁shape ▁on ▁loading ▁release . ▁ ▁Con vent ional ▁therm os et ▁pl ast ics ▁or ▁el ast om ers ▁cannot ▁be ▁m elt ed ▁and ▁re - sh aped ▁after ▁they ▁are ▁c ured . ▁ ▁This ▁usually ▁prevents ▁rec y cling ▁for ▁the ▁same ▁purpose , ▁except ▁as ▁fill er ▁material . ▁New ▁develop ments ▁involving ▁therm os et ▁epo xy ▁res ins ▁which ▁on ▁controlled ▁and ▁contained ▁he ating ▁form ▁cross link ed ▁networks ▁permit ▁repeatedly ▁res h aping , ▁like ▁sil ica ▁glass ▁by ▁revers ible ▁c oval ent ▁bond ▁exchange ▁re actions ▁on ▁re he ating ▁above ▁the ▁glass ▁transition ▁temperature . ▁There ▁are ▁also ▁therm os et ▁poly ure th