Microfibrous cyclodextrin boosts flame retardancy of poly(lactic acid)

Abstract 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) microfibres with diameters ranging between 3 and 7 µm were prepared by aqueous solution based high-speed electrospinning (HSES) technique and then used as carbonising agent at 3 wt% loading besides 15 wt% ammonium polyphosphate (APP) to obtain flame-retarded poly(lactic acid) (PLA) composites. The high specific surface area of the microfibrous HP-β-CD was found to have a crucial role in its flame retardant efficiency. Compared to the effect of the same amount of conventional HP-β-CD powder additive, microfibrous HP-β-CD resulted in higher char yields both during thermogravimetric analysis and cone calorimeter test accompanied with significantly increased mechanical resistance and consequently with improved flame retarding efficacy. The Limiting Oxygen Index (LOI) of the intumescent flame-retarded PLA composite increased noticeably, from 29.0% to 32.5%, while the Flame Retardancy Index (FRI), determined from cone calorimetry data, increased from 1.9 (with powder HP-β-CD) to 2.6 (with microfibrous HP-β-CD) only by changing the physical form i.e. the specific surface area of the used HP-β-CD. The advantage of the special microfibrous structure of the oligosaccharide type charring agent lies in the efficient interaction with APP and the attribution of the intumescent char layer with improved thermal and mechanical resistance without compromising its swelling ability. Besides, the microfibrous structure of the HP-β-CD also contributed to the improvement of the mechanical performance of the flame retarded PLA composites.