How Green is Your Plasticizer?

Abstract

Plasticizers are additives that are used to impart flexibility to polymer blends and improve their processability. Plasticizers are typically not covalently bound to the polymers, allowing them to leach out over time, which results in human exposure and environmental contamination. Phthalates, in particular, have been the subject of increasing concern due to their established ubiquity in the environment and their suspected negative health effects, including endocrine disrupting and anti-androgenic effects. As there is mounting pressure to find safe replacement compounds, this review addresses the design and experimental elements that should be considered in order for a new or existing plasticizer to be considered green. Specifically, a multi-disciplinary and holistic approach should be taken which includes toxicity testing (both in vitro and in vivo), biodegradation testing (with attention to metabolites), as well as leaching studies. Special consideration should also be given to the design stages of producing a new molecule and the synthetic and scale-up processes should also be optimized. Only by taking a multi-faceted approach can a plasticizer be considered truly green.

Keywords: additive; biodegradation; leaching; metabolites; phthalate; plasticizer; toxicity.

Figure 1

Green plasticizer design considerations. Adapted from R.F. Boyer, 1951 [22].

Figure 2

The 12 Principles of Green Chemistry. Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice, Oxford University Press: New York, NY, USA, 1998; p. 30. By permission of Oxford University Press [23].

Figure 3

Chemical structures of four commercial phthalate plasticizers and two structurally similar compounds [39].

Figure 4

Biodegradation of di(2-ethylhexyl) phthalate (DEHP) through the action of esterases in microbes. Reprinted by permission from Springer Nature: Springer. Applied Microbiology and Biotechnology. Leaching of the plasticizer di(2-ethylhexyl)phthalate (DEHP) from plastic containers and the question of human exposure, Erythropel et. al., 2014 [4].

Figure 5

Candidate green plasticizer families: succinates, maleates, fumarates (all with unbranched side chains), and linear alkyl dibenzoates. The only difference between the dibenzoate 1,5-PDB and the commercial diethylene glycol dibenzoate (DEGDB) is the molecule in the center of the diol linker: carbon (in the case of 1,5-PDB) or oxygen (in the case of DEGDB). 1,5-PDB is 1,5-pentanediol dibenzoate, DEGDB is diethylene glycol dibenzoate.