In this work we have performed a comprehensive study on synthesis, processing, detailed material characterization and preliminary assessment of toxicity of relatively new flame retardant (FR) additives as a key for developing environmentally friendly fire safe polyesters. Two 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide (DOPO) based FR additives were synthesized using principles of green chemistry and incorporated via thermal processing in high temperature polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). The green synthesis strategies included (i) the use of N-chlorosuccinimide as a sustainable chlorinating agent for DOPO and (ii) a solvent and catalyst free microwave-assisted synthesis. Atomistic molecular dynamics (MD) simulations were employed in order to calculate the solubility parameters of these additives so as to estimate their compatibility in the polyesters. Detailed rheological measurements of the polyester/FR blends were carried out and the results indicated a clear difference in all three additives tested. Based on these analyses, 6H-dibenz[c,e][1,2] oxaphosphorin,6-[(1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-yl)methoxy]-, 6-oxide (DOPOPEPA) exhibited the highest compatibility with both polyesters and their blends and showed the highest thermal-oxidative stability guaranteeing stable and steady processing at high temperatures. All polyester/FR formulations exhibited higher flame retardancy compared to the virgin polyesters in the small scale fire tests. The FR additives were evaluated for their potential toxicity using a well-established in vitro platform. Our results indicate no acute cytotoxic potential for all FRs analyzed in two different cell types (the human lung epithelial cell line A549 and macrophages derived from the monocytic cell line THP-1) and under the chosen experimental conditions.

Introduction

There is increasing demand for the development of environmental friendly fire safe materials which fulfill both ecological and commercial requirements. In addition, these materials need to be fully characterized for their physical and chemical properties and their ease of processability for eventual industrial exploitation. Like in case of other additives, it is important that a flame retardant (FR) additive do not interfere with steady melt processing in typical polymer extruders. Therefore, it is required that the melt properties of the polymer/FR blends do not change significantly during the processing. Most common polymers are not suitable for fire safe applications and are rendered flame retardant by incorporating an FR additive in the polymer bulk or via coating on the surface. Incorporating FR additive in the bulk via thermal processing is relatively simple, economical and more durable to environmental influences. The key to reducing waste and improving sustainability of functional materials is to develop additives via green chemistry strategy. There are 12 important principles involved in green chemistry, for example, the chemical should be non-toxic, toxic raw materials and solvents should be avoided, high atom economy and the use of renewable feedstock [1]. Some halogenated flame retardants are prohibited [2] due the concerns regarding their toxicity [3]. In the last decades the impetus has shifted to the development of environmentally friendly flame retardants (FRs). Among the many classes of FR additives, new phosphorus containing FRs [4e7] are considered potential replacements for the toxic FRs [3]. More recently some phosphorus based FRs have also come under scrutiny of various regulatory and environmental agencies. Some phosphorus based FRs have been found to be toxic and are currently being phased out or their usage restricted [8e12].