Versatile Method To Reduce the Free Formaldehyde Content in Phenolic Resins for High-Temperature Applications

Discovered by Baekeland a century ago, phenolic resins are still widely used today with a global production of about 12 million tons/year. Typically, these resins contain residual free formaldehyde, which is highly toxic and carcinogenic and will soon be banned by the European REACh regulation. Consequently, phenolic resins must be substituted. These resins are extensively used as a char precursor for composite-based thermal protection systems. Commonly, urea is a widely used formaldehyde scavenger and can be added to phenolic resins to decrease their free formaldehyde content, but urea is not suitable for high-temperature applications. Thus, an easy-to-implement method was developed with an aminophenol, giving access to a noncarcinogenic resin while maintaining the thermomechanical properties. The chemistry and the reaction mechanism of aminophenols with formaldehyde were also studied. The interest of this additive is multiple: it allows trapping of formaldehyde, to comply with the European REACh regulation (wt % free formaldehyde <0.1%), while the modifying agent is integrated into the polymer network, maintaining the thermostability (Td5% = 395 °C and char yield = 63%). This method is also applicable on an industrial scale. In addition, this method uses mild reaction conditions and limits volatile organic compound emissions by trapping free formaldehyde, which are two ways to improve durability and reduce toxicity of the resin.