Catalytic Effects of Nickel (Cobalt or Zinc) Acetates on Thermal and Flammability Properties of Polypropylene-Modified Lignin Composites

Abstract

Previous work has demonstrated that functionalized lignin (PN-lignin) was shown to enhance thermal stability and flame retardancy of polypropylene (PP). To further strength these effects, nickel (cobalt or zinc) acetates were introduced into PP/PN-lignin systems in the present work. Thermogravimetric analysis demonstrates that adding 2 wt % nickel acetate (Ni(Ac) 2) catalytically degrades PP/PN-lignin but significantly increases the char residues in both N 2 and air conditions. For instance, the presence of Ni 2+ reduced the initial degradation temperature (Ti) by ~13 °C but doubled the char residue in the N 2 condition relative to those of PP/PN-lignin. In comparison, the addition of cobalt or zinc acetate has little effect on thermal properties and char residues of PP/PN-lignin. Cone calorimeter results indicate that although the presence of Ni(Ac) 2 shortens slightly the time to ignition (t ign) from ~38 s to ~31 s, it reduces the peak heat release rate (PHRR) from 380 kW/m 2 to 330 kW/m 2, suggesting a further improved flame retardancy of PP/PN-lignin. Moreover, the addition of Ni(Ac) 2 significantly increases char residue by ~44%. Limited oxygen index (LOI) measurements show that adding Ni(Ac) 2 increases the LOI value from 22 for PP/PN-lignin (17.5 for pure PP) up to 26, also indicating a better flame retardancy. Unlike Ni(Ac) 2, cobalt or zinc acetate still has no obvious effect on flame retardancy. Char residue analysis shows that not only does Ni(Ac) 2 participate in the char-forming process of PN-lignin by itself but its degradation products, NiO and Ni(0), also promote the carbonization through the catalytic action of the PP matrix, both of which are primarily responsible for the enhanced flame retardancy of the PP/PNlignin system.