Synthesis and Investigation of Heterocyclic Organophosphorus Compounds as Flame Retardants for Epoxy Resins

Abstract

In the course of this doctoral work, a series of organophosphorus derivatives were synthesised in order to deepen our understanding of the influence of the environment around the phosphorus on the flame retardant mechanism. The Friedel-Crafts reaction of PCl3 with 2-hydroxybiphenyl, 2,2-dimethylpropane-1,3-diol and 2-aminobiphenyl yielded the phosphorus chloride precursor of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), 5,5-dimethyl-[1,3,2]-dioxaphosphorinane-2-oxide (DDPO) and 9,10-dihydro-9-aza-10-phosphaphenanthrene 10-oxide(DAPO) respectively. The phosphorus chloride derivatives were then either hydrolised, oxidised, or thionised to yield a series of molecules where the environment around the phosphorus atom varied. The synthesized molecules were chemically incorporated(preformulation) in phenol epoxy novolac (DEN 438) with different phosphorus concentrations. According to standard EE processes, the modified resins were then cured using DICY as hardener and Fenuron as accelerator. The influence of the chemical modification of the epoxy phenol novolac resin, on the glass transition temperature (Tg) of the cured material, was investigated using DSC. As a result of the decreasing number of epoxy functional group available for cross-linking, the Tg decreased with increasing phosphorus loading. This effect was enhanced when sulphur containing molecules were used. However, chemical modification with DAPO only resulted in a negligible impact on Tg as the amine functionality could contribute to the cross-linking. The flame retardant efficiency of the modified resin with different phosphorus loading was evualted using the UL-94 test, in accordance with the DIN IEC 60695-11-10. All phosphorus derivatives improved the flame retardancy of phenol epoxy novolac resin with comparable phosphorus loading to DOPO. However, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-sulfide (DOPS) imparted UL-94 V0 rating with only 1.0 % phosphorus loading. In order to further understand the role of the PO and PS radicals in the mode of flame retardancy of DOPS, Limiting Oxygen Test (LOI) and thermal desorption mass spectrometry (TD-MS) were carried out. TD-MS experiments revealed that PS and PO radicals are released sequentially in the gas which can rationalise the better efficiency of DOPS in comparison to DOPO. LOI experiments showed that the intumescent mechanism of DDPO was reduced when sulphur replaced oxygen in 5,5-dimethyl-[1,3,2]-dioxaphosphorinane-2-sulfide (DDPS). Finally, a part of a cooperation with Prof. G. Camino (Politecnico di Torino, Italy), a DOPO modified resin was cured in the presence of an organically modified montmorrillonite nanoclay (Cloisite 30B), in an effort to understand the interactions between DOPO and nanoclays and their impact on the flame retardancy of epoxy phenol novolac. The flammability of cured materials was tested using UL-94 test, LOI and cone calorimetry. The cone calorimeter enables to measure the heat released by the polymeric material under controlled conditions, as well as the gases evolving from the combustion process.