Crystallization, flame-retardant, and mechanical behaviors of poly(lactic acid)\9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide–calcium montmorillonite nanocomposite

Poly(lactic acid) (PLA) nanocomposite with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) intercalated calcium montmorillonite (Ca-MMT) nanocompound was prepared, and the properties were compared with those of PLA/Ca-MMT, PLA/DOPO, and PLA/DOPO+Ca-MMT nanocomposites (where DOPO+Ca-MMT indicates a physical mixture of DOPO and Ca-MMT). The structures and properties of the four PLA nanocomposites were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry, thermogravimetric analysis, limiting oxygen index (LOI) testing, UL-94 vertical testing, and cone calorimetry measurements. The XRD and TEM tests showed that the DOPO–Ca-MMT nanocompound existed in both intercalated and exfoliated montmorillonite morphologies in the PLA matrix, a better dispersion state than the Ca-MMT added alone and DOPO+Ca-MMT in PLA; this benefited improvements in the crystallinity, thermal stability, and flame retardancy for the PLA/DOPO–Ca-MMT nanocomposite, as seen in the higher degree of crystallinity, higher LOI values, and lower peak heat-release rate. A loading of only 5 wt % DOPO–Ca-MMT increased the LOI value of PLA from 20.0 to 28.3% and made the PLA pass the UL-94 vertical test V-0 rating at a 3.2 mm thickness. The nanocomposites based on DOPO–Ca-MMT could be used as very efficient systems for flame-retardant PLA. The PLA/DOPO–Ca-MMT also had better tensile mechanical properties than the pure PLA and PLA/DOPO+Ca-MMT. The annealing specimens of the four nanocomposites were also characterized to study the relationship between the PLA crystallization and flame retardancy or mechanical properties.