Sophorolipid-derived unsaturated and epoxy fatty acid estolides as plasticizers for poly(3-hydroxybutyrate)

Authors: Ashby, Richard - Rick; Solaiman, Daniel; Liu, Cheng Kung; Strahan, Gary; Latona, Nicholas - Nick

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2015
Publication Date: 12/31/2015
Publication URL: http://handle.nal.usda.gov/10113/62637
Citation: Ashby, R.D., Solaiman, D., Liu, C., Strahan, G.D., Latona, N.P. 2015. Sophorolipid-derived unsaturated and epoxy fatty acid estolides as plasticizers for poly(3-hydroxybutyrate). Journal of the American Oil Chemists' Society. 93:347-358. doi: 10.1007/s11746-015-2772-7.

Interpretive Summary: In an ongoing effort to maintain a healthy environment, many biodegradable plastics (bioplastics) such as polylactic acid (PLA) and polyhydroxyalkanoates (PHAs) have been developed for potential widespread use. Regrettably, some of the more well-known of these bioplastics suffer from property limitations which restricts their use. Poly(3-hydroxybutyrate) (PHB) is the most abundant of the PHA bioplastics, but is known to be rigid and brittle in its natural state. One method to advance the properties of PHB is to add diverse molecules (plasticizers) in different concentrations. These plasticizers may then aid to improve the flexibility and durability of the bioplastics. Unfortunately, many of the more commonly-used plasticizers have been deemed unsafe due to concerns over human toxicity. In this paper we have chemically-synthesized two structurally-distinct molecules termed ‘estolides’ from the components of a separate yeast-based biomolecule (sophorolipid) and demonstrated their effectiveness as plasticizing-additives in PHB films. The successful demonstration of the plasticizing effects of these estolides on PHB may provide an alternative means of improving PHB properties without the use of potentially harmful materials and could improve the application potential of these bioplastics.

Technical Abstract: Unsaturated and epoxy fatty acid estolides were synthesized from the omega and omega-1 hydroxy fatty acids derived from sophorolipids (SLs) prepared by fermentation from glucose:soybean oil and glucose:oleic acid, respectively. These estolides were utilized as additives in solution-cast poly(3-hydroxybutyrate) (PHB) films and their plasticizing effects reported. The presence of estolides in the PHB film matrix resulted in small reductions in melting temperatures (Tm) and glass transition temperatures (Tg) while the %-crystallinities remained relatively constant (+/-2.0%). Scanning electron microscopy (SEM) revealed irregular film surfaces in the presence of both estolides and the formation of pores within the PHB film matrix at 50 wt% estolide concentration. These physical irregularities influenced the tensile properties of the films by causing a decrease in the tensile strength and moduli of the PHB films and increasing their elongation. Curing the films for 3 months at room temperature triggered an enhanced tensile strength when compared to the initial films but the aged films also became more rigid and brittle than the initial films at the same estolide concentration. Irradiation (25 kGy) was implemented in an attempt to crosslink the estolides to form a ‘netting’ affect and improve the film properties. Results showed that radiation did not result in crosslinking but rather caused chain scission and reduced molecular weights which further reduced the tensile strength, elongation and modulus of the PHB films. These tensile property variations showed that while the strength of the PHB films declined in the presence of both estolides, the plasticity and elongation improved validating that these unique SL-derived estolides can be effectively used as plasticizers in PHB films.