DOPO-based phosphorus and nitrogen rich flame retardant additives

Abstract

One of the most destructive natural events on Earth is fire. In additon to injuring and killing thousands of people, this disaster also poses a threat to other living things. Furthermore, fires cause serious material damage and economic loss. In this content, the synthesis of new and effective flame retardant materials is an important research area. Simply described, flame retardants (FRs) are chemicals that are incorporated or added to materials to provide varying degrees of flammability protection. There exist numerous flame retardant additives and they can be classified different ways. Halogenated, organophosphorus-based, silicon-containing, nitrogen-containing, nanocomposites, and metal-containing compounds are examples of common flame retardants. One of the most preffered FR was the halogen-based ones. However, harmful products formed as a result of decomposition of halogen-containing FRs at high temperatures. Forming contaminants are persistent in the environment and have the capacity to migrate over long distances. Moreover, they cause significant biological harm and tend to accumulate organisms. Some of the halogen-based FRs have been forbidden due to environmental and biological concerns which resulted in development of halogen-free FRs in recent years. Phosphorus (P)-containing and nitrogen (N)-containing FRs are the most promising of all the halogen-free FRs. Their high efficiency, molecular diversity, multiple flame retardant processes and low toxicity have made them compatible to plenty different polymers including epoxy resins (EPs). Flame retardant additives interrupt the combustion process via various mechanisms. The basic mechanisms are such as gas phase mechanism, endothermic mechanism and char-layer mechanism. Different additives retard the fire via different mechanisms. To increase the efficiency of flame retardancy, groups have several flame retardant effects can be used together. The most famous and influential example is the combination of phosphorus and nitrogen. During combustion, P and N can formed synergy to enhanced the flame retardancy. Organophosphorus structures generate phosphorus radicals during combustion which quench the flammable radicals. Moreover, they produce a protective char layer to block reaching oxygen to material. Nitrogen-containing structures dilute the density of combustible gasses by producing inert gases. Furthermore, as a result of the gases generated by the synergistic effect of P and N-containing structures, intumescent forms a barrier against oxygen which is called intumescent effect. Flame retardant additives are utilized in numerous industries such as electrical and electronics, construction, transportation, textile and furniture industries. FRs are generally used to improve the non-flammability of polymers used in these areas. Among these polymers, epoxy resins are the most preffered ones due to excellent properties such as high tensile strength and modulus, strong adhesion to substrates, excellent chemical, corrosion, moisture and solvent resistances, good dimensional stability, effective electrical properties, low shrinkage on cure, thougness, high versatility. Epoxy resins are used so extensively that it is critical to enhance their non-flammability. For this purpose, it is known that FRs containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and triazole units are particularly effective. In this thesis, two different novel FR containing phosphorus and triazole units were synthesized by using activated alkynes and metal-free azide-alkyne click chemistry. The FRs were designed to have different N/P ratios. Nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopies were used to characterized the synthesized FRs. The FRs were mixed into an epoxy resin in different ratios, and the mixture was thermally cured. The thermal, mechanical, and the flame retardancy properties of the thermosets were characterized. The thermal degradation temperatures of the epoxy thermosets was reduced to lower temperatures but the char yields increased. The combination of the triazole and DOPO units led to superior flame retardancy. The limiting oxygen index (LOI) value of the thermosets was found to be as high as 47.4%. All the FR-containing thermosets were rated as V-0 according to the UL-94 test. Furthermore, owing to the presence of N and P, intumescent behavior was observed. Finally, according to the WCA test, it was observed that the contact angle of thermosets with the surface decreases with increasing additive content. This result was attributed to the presence of polar triazole and phosphorus groups leading to more hydrophilic surfaces. As a result, it can be said that EPs containing FRs become much more polar, which is beneficial for composite and adhesive preparation.