hard rubber vs caoutchouc

hard rubber and caoutchouc both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard rubber	Yes	No	No	No
caoutchouc	Yes	No	No	No

As nouns, caoutchouc is a hypernym of hard rubber; that is, caoutchouc is a word with a broader meaning than hard rubber:

hard rubber: a hard nonresilient rubber formed by vulcanizing natural rubber
caoutchouc: an elastic material obtained from the latex sap of trees (especially trees of the genera Hevea and Ficus) that can be vulcanized and finished into a variety of products

Other hypernyms of hard rubber include gum elastic, India rubber, natural rubber, rubber.

hard rubber (noun)	caoutchouc (noun)
a hard nonresilient rubber formed by vulcanizing natural rubber	an elastic material obtained from the latex sap of trees (especially trees of the genera Hevea and Ficus) that can be vulcanized and finished into a variety of products