Comparative Study of Aluminum Dialkylphosphinates: The Role of Alkyl Groups in Flame‐Retarding Glass‐Reinforced Poly(Butylene Terephthalate)

ABSTRACT This study investigates the effect of alkyl groups in aluminum dialkylphosphinates on the flame retardancy of glass‐fiber‐reinforced poly(butylene terephthalate) (PBT). Aluminum diethylphosphinate (ADP), aluminum dipropylphosphinate (ADPP), and a hybrid phosphinate (AHP, containing 56 mol% ethylpropylphosphinate) are incorporated into PBT and evaluated using various techniques. To elucidate the chemical interactions between the flame retardants and PBT, model reactions with terephthalic acid (TPA) are firstly conducted. The alkyl groups exhibit significant influence on the reactivity of dialkylphosphinates toward TPA. While ADP rapidly loses two‐thirds of its diethylphosphinate moiety, the loss of the second group in ADPP and AHP is slower due to altered coordination. This slow process is accompanied by thermal degradation to phosphates, leading to the accumulation of phosphorus in the residues of TPA/ADPP and TPA/AHP. In PBT composites, both ADPP and AHP outperform ADP in flame retardancy. However, a common gas‐phase mechanism, which involves the in situ release of dialkylphosphinic acids from the reaction with TPA during PBT decomposition, results in a flame inhibition that is relatively insensitive to the alkyl groups. The better flame‐retardant performance of ADPP and AHP stems from a balanced gas‐condensed phase action, with the alkyl‐governed condensed‐phase activity being the decisive factor.