Synthesis and Application of a Novel Polyamide Charring Agent for Halogen-Free Flame Retardant Polypropylene

A novel charring agent, poly(p-ethylene terephthalamide) (PETA), for halogen-free flame retardant polypropylene was synthesized by using p-phthaloyl chloride (TPC) and ethylenediamine through solution polycondensation at low temperature, and the effects of PETA on flame retardance of polypropylene (PP)/IFR systems were studied. The experimental results showed that PETA could considerably enhance the fire retardant performance as proved by evidence of the increase of limiting oxygen index (LOI) values, the results of UL-94 tests, and cone calorimeter tests (CCT). Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) demonstrated that an appropriate amount of PETA could react with PP/IFR system to form cross-link network; a more compact char layer could be formed which was responsible for the improved thermal and flame retardant properties of PP/IFR systems. However, the superfluous amount of PETA would play the negative role. 1. Introduction Polypropylene (PP) is widely used in many fields, due to its easy processing property, low density, excellent mechanical properties, good performance to cost ratio, and so on. However, the use of PP in electric and electronic, building, or transport aspects is strongly limited because of their easy flammability with low LOI [1]. Thus, flame retardant PP is urgently demanded. In all plastic flame retardant technologies, the halogen-free intumescent flame retardants are promising and effective to be used in PP [2–4], due to their characteristics of low smoke, low toxicity, and no harmful gas produced during burning. Intumescent flame retardant (IFR) system is usually composed of three components: acid catalyst, carbon source, and the gas former [5]. As their name describes, intumescent flame retardants rise up in response to heat, they create a protective carbon foam under fire conditions. This class of flame retardants is a strictly condensed phase in its activity, and either provides its own carbon char or uses the polymer as a carbon char source [6]. The conventional IFRs are the phosphorus-nitrogen containing compounds; they cause a cross-linked and a thermally stable form of swollen multicellular char. Generally, the acid catalyst and gas former are ammonium polyphosphate (APP) [7] or phosphate and melamine, respectively. The choice of carbon source usually focuses on the polyol and its modified products, for example, pentaerythritol (PER) [8] or 1-oxo-4-hydroxymethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane (PEPA) [9]. Much of them are low molecular weight compounds; they have