Effect of Ammonium Pentaborate on Curing of Aqueous Phenol Formaldehyde Resin

Ammonium pentaborate (APB) is used as fire retardant in polymers or additive inproducing wood-based composites. In order to understand the reactionsbetween APB and aqueous phenol formaldehyde (PF) resin to provide areasonable explanation of the properties of wood composites made with APB modifiedPF resin, the effect of APB on curing behaviour of aqueous PF resin was investigated,by using dynamic differential scanning calorimetry (DSC) and Fourier transforminfrared spectrometry (FTIR) studies. Activation energies at each conversion point ofPF resin reference and its mixtures were calculated by Kissinger-Akahira-Sunose(KAS) algorithm according to isoconversional kinetic method. The enthalpy change ofreaction, the conversion progress with temperature, and the chemical groups in PFresin formed by APB were analyzed at the same time. The results showed that additionof APB at about 3.20 wt% brought about a minute distinction between addition andcondensation reactions. However, as the loading of APB increased to 8.88 wt%,another exothermal peak was observed at temperature around 172°C. The consistentdata of activation energy and enthalpy suggested that this peak was caused by thesecond curing process in APB modified PF resin systems, which helped the completionof cure reaction but delayed the curing time. This second curing process wasconsidered to include the following features: the formation of more stable methylenebridges, addition reactions between eliminated formaldehyde and active groups on PFresin and the formation of hexamine which were consistent with FTIR resultscharacterized by the altered network.