Effects of thermal annealing and solvent-induced crystallization on the structure and properties of poly(lactic acid) microfibres produced by high-speed electrospinning

This research concentrates on the marked discrepancies in the crystalline structure of poly(lactic acid) (PLA) nano- and microfibres, achieved by different annealing strategies. PLA nonwoven mats were produced by high-speed electrospinning (HSES). The high-speed production technique allowed the manufacturing of PLA microfibres with diameters of 0.25–8.50 µm with a relatively high yield of 40 g h-1. The crystalline content of the inherently highly amorphous microfibres was increased by two methods, thermal annealing in an oven at 85°C was compared to immersion in absolute ethanol at 40°C. The morphology of the fibres was examined by scanning electron microscopy (SEM), crystalline forms and thermal properties were assessed using X-ray diffractometry (XRD), Raman spectrometry, differential scanning calorimetry (DSC) as well as modulated differential scanning calorimetry (MDSC). As a consequence of 45 min heat treatment, the crystalline fraction increased up to 26%, while solution treatment resulted in 33% crystallinity. It was found that only disordered α’ crystals are formed during the conventional heat treatment, however, the ethanol-induced crystallization favours the formation of the ordered α polymorph. In connection with the different crystalline structures, noticeable changes in the macroscopic properties such as heat resistance and mechanical properties were evinced by localised thermomechanical analysis (LTMA) and static tensile test, respectively

Application of Melt-Blown Poly(lactic acid) Fibres in Self-Reinforced Composites

The aim of our research was to produce poly(lactic acid) (PLA) fibres with diameters in the micrometer size range, serving as the reinforcing phase in self-reinforced (SR) PLA composites. Nonwoven PLA mats were manufactured by solvent- free melt-blowing technology. Three types of PLA differing in d-lactide content were processed with a productivity as high as 36 g/h. The crystallinity of the PLA microfibres was enhanced by thermal annealing. A 2–3-fold increase in the degree of crystallinity was obtained, as measured by differential scanning calorimetry (DSC). Fibre diameters between 2–14 µm were revealed by scanning electron microscopy (SEM). Static tensile tests were performed on the nonwoven mats, showing the reduced moduli of the annealed fibres due the amorphous relaxation. The PLA mats were processed via the hot compaction technique and formed into SR–PLA composites. The morphological and mechanical properties of the obtained microstructural composites were comprehensively studied. Composites prepared from annealed, thermally more stable PLA nonwoven mats showed superior mechanical properties; the tensile strength improved by 47% due to the higher residual fibre content

Development of Intumescent Flame Retardant for Polypropylene: Bio-epoxy Resin Microencapsulated Ammonium-polyphosphate

As polypropylene (PP) has no charring ability on its own due to the lack of hydroxyl functional groups, the flame retardant system needs the addition of carbonizing agent in a relatively great amount. Ammonium-polyphosphate (APP), a conventional flame retardant additive was modified by microencapsulation with a sorbitol-based bioepoxy resin shell to create an intumescent flame retardant system with enhanced charring ability for PP. The flame retardant efficiency of the microencapsulated additive, which contains all the components needed in an effective intumescent flame retardant system, was evaluated in PP matrix at different loadings.When compared to the physical mixture of the component, the microencapsuated form of APP (MCAPP) was found to have improved flame retardant efficiency in PP. The LOI values of the MCAPP containing PP samples increased by 8–11 V/V% besides achieved V-0 classification according to the UL94 test. During cone calorimeter tests, the burning intensity was reduced (peak of heat release rate decreased by 20–35% and shifted in time), increased amount of charred residue was obtained, and based on the calculated Flame Retardancy Index (FRI) “Excellent” fire performance was achieved when MCAPP was used. The improved flame retardant performance is attributed to the effective interaction between the APP core and the readily available carbonizing shell, which promoted the formation of increased amount of char accompanied with improved heat protecting and barrier efficiency