Flame retardant novolac-bisphthalonitrile structural thermosets

Void-free, thermoset networks prepared from phenolic novolacs cured with bisphthalonitrile (BPh) reagents display attractive mechanical, thermal, and fire characteristics. The network properties depend strongly on the novolac-BPh composition. Networks containing 15 wt% BPh or greater had T(g)s above 180 degreesC and good fracture toughness (K-1c similar to 0.8 MPa m(1/2)). Cone calorimetry results for an 80:20 (wt/wt) novolac/BPh network, at an incident heat flux of 50 kW/m(2), demonstrated a peak heat release rate of 137 kW/m(2) and char yields > 50%. These excellent flame properties are similar to those of phenolic resole networks, which are brittle but known to be flame retardant. The cure reaction between the novolac and BPh was monitored by the disappearance of the nitrile peak at 2200 cm(-1) in the FTIR. The nitrile was approximate to 90% consumed after 1 h at 200 degreesC and similar to 20 min at 220 degreesC. The molecular structure of a novolac-BPh crosslink was studied in model reactions using monofunctional phenols. 2-Hydroxydiphenyl methane was reacted with BPh in the melt at 200degreesC for 3 h using a 25:1 molar ratio (6.1:1 eq. ratio) of 2-hydroxydiphenylmethane/BPh. FTIR and H-1 NMR suggested that the product generated from the model melt reaction contained a diiminoisoindoline structure. (C) 2002 Elsevier Science Ltd. All rights reserved.