adjective vs adjectival

adjective is a noun but adjectival is not a noun.

Word	Noun	Adjective	Verb	Adverb
adjective	Yes	Yes	No	No
adjectival	No	Yes	No	No

As adjectives, adjective and adjectival are synonyms defined as:

adjective and adjectival: of or relating to or functioning as an adjective

adjective (noun)	adjectival (noun)
a word that expresses an attribute of something
the word class that qualifies nouns

adjective (adjective)	adjectival (adjective)
relating to court practice and procedure as opposed to the principles of law	of or relating to or functioning as an adjective
of or relating to or functioning as an adjective