Replacement of Hazardous Chemicals Used in Engineering Plastics with Safe and Renewable Hydrogen-Bond Donor and Acceptor Solvent-Pair Mixtures

Mixtures of safe and renewable solvents can replace hazardous solvents presently being used in the manufacture of engineering plastics. In this work, a methodology is proposed for identifying solvent-pair mixtures for preparing polymer precursors, with poly(amic acid) (PAA) being studied as an example. The methodology uses a chemical safety index, Hansen solubility parameters and Kamlet–Taft solvatochromic parameters of the pure and solvent-pair mixtures to identify hydrogen bond acceptor (HBA)–hydrogen bond donor (HBD) solvent-pair combinations. Ten replacement solvent-pairs for PAA syntheses identified were cyclohexanone–methanol, cyclohexanone–ethanol, cyclopentanone–methanol, cyclopentanone–ethanol, γ-butyrolactone–methanol, γ-butyrolactone–ethanol, γ-butyrolactone–water, γ-valerolactone–methanol, γ-valerolactone–ethanol, and γ-valerolactone–water. Homogeneous PAA solutions could be obtained from HBA–HBD solvent-pair mixtures when their solubility parameters were within 21–29 MPa0.5 and their Kamlet–Taft solvatochromic parameters were π* (>0.67) and β (>0.67) for nonaqueous solutions and π* (>0.68) and β (>0.59) for aqueous solutions. Replacement solvent-pairs, γ-valerolactone–ethanol, γ-valerolactone–water, and γ-butyrolactone–water gave homogeneous precursor solutions that were comparable with commercial solutions prepared with N-methyl-2-pyrrolidone. The proposed methodology and reported solvatochromic parameters make it is possible to identify other solvent-pair mixtures and new solvent-pairs for preparing polymer precursor solutions used in engineering plastics.