A tuneable switch for controlling environmental degradation of bioplastics: addition of isothiazolinone to polyhydroxyalkanoates

Abstract

Controlling the environmental degradation of polyhydroxybutyrate (PHB) and polyhydroxyvalerate (P(HB-co-HV)) bioplastics would expand the range of their potential applications. Combining PHB and P(HB-co-HV) films with the anti-fouling agent 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOI, <10% w/w) restricted microbial colonisation in soil, but did not significantly affect melting temperature or the tensile strength of films. DCOI films showed reduced biofouling and postponed the onset of weight loss by up to 100 days, a 10-fold increase compared to unmodified films where the microbial coverage was significant. In addition, the rate of PHA-DCOI weight loss, post-onset, reduced by about 150%; in contrast a recorded weight loss of only 0.05% per day for P(HB-co-HV) with a 10% DCOI loading was observed. This is in stark contrast to the unmodified PHB film, where a recorded weight loss of only 0.75% per day was made. The 'switch' that initiates film weight loss, and its subsequent reduced rate, depended on the DCOI loading to control biofouling. The control of biofouling and environmental degradation for these DCOI modified bioplastics increases their potential use in biodegradable applications.

Figure 1. Chemical structure of the antifouling agent 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one, (DCOI).

Figure 2. Differential Scanning Calorimetry curves of virgin PHB films and P(HB-co-8HV) films (black line) and films containing 10% DCOI (grey line) in the second post annealed heating (a,c) and cooling (b,d) runs respectively.

Figure 3. Residual weight of PHB-DCOI (a) and P(HB-co-8HV)-DCOI films buried in soil; () 0% w/w, (▪) 2.5%, (○) 5% and (□) 10% (w/w) initial DCOI loadings.

Figure 4. DCOI content in PHB () and P(HB-co-8HV) (○ ) films with burial time, (a) 2.5%, (b) 5% and (c) 10% (w/w) initial DCOI loadings.

Figure 5. Weight loss rates after initiation of weight loss during burial in mature soil for PHB () and P(HB-co-8HV) (○) films as a consequence of initial DCOI loadings.

Figure 6. Fluorescent micrographs illustrating comparatively greater microbial colonisation of scl-PHA films compared to their DCOI loaded counterparts (2.5% w/w) after burial in mature soil.

Figure 7. Microbial percent surface area of PHB (a) and P(HB-co-8HV) (b) films, with burial time; () 0% w/w, (▪) 2.5%, (○) 5% and (□) 10% (w/w) DCOI loadings, ‘X’ material weight loss too great to accurately measure biofilm coverage, arrows indicate onset of material weight loss for scl-PHA-DCOI films.

Figure 8. Confocal Laser Scanning Microscopy microtopography and SEM images of undegraded PHB (a, b) and P(HB-co-8HV) films (c, d) illustrating surface roughness.

Figure 9. Average Surface Roughness (R _a) for PHB () and P(HB-co-8HV) (○) films with DCOI loadings.