Abstract
Intumescent flame retardants are important halogen-free products used in polyethylene. However, their thermal stability and water-resistance are major shortcomings. In this work, a composite charring agent, pentaerythritol (PER) encapsulated by thermoplastic polyurethane (TPU) was used in an intumescent system to improve the flame retardancy of high density polyethylene (HDPE). The encapsulation of macromolecular charring agent TPU can effectively suppress the esterification reaction of PER and acid source in the intumescent system during processing. It can also remarkably decrease the water absorption, thus producing flame retardant HDPE with high performance. The synergistic effects of other common flame retardants including melamine phosphate, melamine polyphosphate and ammonium polyphosphate with TPU-encapsulated PER, as well as the ratio of charring agent to acid source were investigated so as to determine the optimum formula for use in HDPE. The flame retardant HDPE can reach limiting oxygen index of 33 and achieve UL-94 V-0 rating at 3.2 mm thickness when the ratio of MP/composite charring agent is 2:1 w/w.