Toughening of phenolic resins

Blending of rubber has been tried out for toughening phenolic resin. Phenolic-nitrile rubber (NBR) was developed in the early 1950s, which has found wide applications [242] such as structural adhesives, O-rings, and gaskets. However, it was reported that when NBR is mixed with resole and cnred, they phase separate into undesirably large domains with low adhesion between the phases [243]. A discussed earlier, for an effective toughening effect the domain size and interfacial adhesion are important. 

Addition of p-cresol formaldehyde (PCF) into phenolic/NBR blends resulted in rednction in the domain size of the dispersed phase and improvement in mechanical properties [244]. PCF resin has an intermediate polarity compared with NBR and resole and can react faster with NBR. Therefore, PCF molecules are likely to be concentrated at the phenolic/NBR interface and act as an external compatibilising agents [245]. Thus compatibility and chemical bonding between NBR and phenolic resin is improved, leading to the enhancement in properties. The other materials used as toughening agents of phenolic resin include elastomers such as natural rubber and nitrile rubber [246, 247], reactive liquid polymers [248] and thermoplastics such as polysulfone, polyamide, polyethylene oxide [249, 250]. 

Gietl and co-workers [251] recently investigated the efficiency of various types of toughening agents such as CTBN, ATBN and core-shell rubber particles. They found the best performance using ATBN rubber. 

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