Researching synonyms for the English translations of this word, I came up against words in the line of "Recognition" "appreciation" "tribute". (I swear by thesaurus.com)
TO push this idea even further, I even found "celebration" (a pure celebration of..") which, although quite removed literally from the word is the way I interpret the flow of the text anyway.
As I often find with a difficult Italian type word like this, it sometimes helps to restructure the whole sentence. Maybe something like:

"....appreciating the use of plastic in design." (and ignoring "a favore di" as I think this is just saying the same thing twice).
OR
"designed exclusively for practicality and beauty,
celebrating/recognizing the use of plastic in modern design"

These ideas may be reaching the limits of the boundaries, but I don't think its a misinterpretation.

This might work.

Piet Mondrian's “The Determinate and the Indeterminate (Supplement ...
- [ Traduci questa pagina ]
17 Oct 2010 ... [107] Art had to free its plastic expression of the indeterminate (the natural) in order to achieve pure plastic expression of the ...
modernistarchitecture.wordpress.com/.../piet-mondrian’s-“the-determinate-and-the-indeterminate-supplement”-1918/

Manuel Carbonell - Bandi Trazos Gallery - Korean and Latin ...
- [ Traduci questa pagina ]
Removed from the figurative conventions, Carbonell creates a pure plastic expression whereby the human anatomy is brought to abstraction. ...
english.banditrazos.com/.../artistas_masinfo.php?...

Urban design reader - Risultati da Google Libri
Matthew Carmona, Steven Tiesdell - 2007 - Architecture - 375 pagine
If, therefore, we design our towns from the point of view of the moving ... city becomes a plastic experience, a journey through pressures and vacuums, ...
books.google.it/books?isbn=0750665319...

"in favour of their pure plastic form" ie: the appeal of their shape and contours as given by the material used (plastic)