Improved Eco-Friendliness of a Common Flame Retardant through Inclusion Complexation with Cyclodextrins

Triphenyl phosphate (TPP) is used as a plasticizer and a flame retardant worldwide. However, in recent years, TPP has been detected in indoor/outdoor air and biota at high concentrations, and exposure to TPP has been indicated as possibly leading to obesity and osteoporosis in humans. Cyclodextrins (CDs) are known to form inclusion complexes (ICs) with a wide variety of guests due to their ring/cavity structure. The capability of β-CD to form an IC with TPP was recently reported by us, and those studies also revealed that poly­(ethylene terephthalate) surfaces treated with TPP−β-CD ICs performed effectively as a flame retardant while also reducing the amount of TPP needed for flame retardancy by at least a factor of 10. Thus, the focus here is whether TPP can form stable ICs with other CDs. Quantum chemistry calculations reveal that IC formation with both α-CD and γ-CD is feasible. However, results from a series of characterization methods indicate that γ-CD forms stable ICs with TPP, but α-CD does not. In terms of the stability of these ICs, for γ-CD, differential scanning calorimetry reveals that the thermal stability of TPP increases via IC formation, and ¹H NMR data indicate that the molar ratio of TPP to γ-CD is 1:2, which coincides with our previous work for IC formation with β-CD. In contrast, another previous study from our laboratories indicates that CD ICs in aqueous solution possess a 1:1 stoichiometry for both β-CD and γ-CD. Quantum chemistry calculations suggest that different ratios were observed due to one of the CDs being more weakly bound and thus able to disassociate under certain conditions. Molecular dynamics simulations indicate that TPP is only released from the ICs with both stoichiometries at temperatures above the degradation temperature of CDs. Thus, these studies suggest that ICs with the common flame retardant TPP and both β-CD and γ-CD are stable under normal conditions and that IC formation enables the unnecessary release of the flame retardant during use to be avoided. Therefore, forming ICs prior to treatment on polymer substrates with flame retardants like TPP that are known to have health and environmental risks is an eco-friendly alternative to current treatment practices.