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

Abstract

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.

Formaldehyde-Free and Phenol-Free Non-toxic Phenolic Resins with High Thermostability

“…An increase in the amount of NaOH tended to increase the Cannizzaro secondary reaction (without affecting the degradation properties) and increased the crosslinking density of the cured polymer. All other tested bases, except ammonia, showed Td 5% close to 400 °C and char yields of about 60%, which is similar to conventional phenolic resins (IF ≈ 99%, Td 5% ≈ 420 °C, and % C ≈ 65%) . In particular, although, DBU did not yield a sufficiently crosslinked polymer, it nevertheless exhibited thermostability properties similar to NaOH, proving it as an alternative base for the latter.…”

“…In these resins, the final formaldehyde content is zero, and the free phenol content is 0.8%. In previous work, we have also reported a method to trap residual formaldehyde with aminophenol while retaining the thermomechanical properties of the final resin …”

“…In previous work, we have also reported a method to trap residual formaldehyde with aminophenol while retaining the thermomechanical properties of the final resin. 6 Other authors propose 7 to substitute formaldehyde with aromatic aldehydes (benzaldehyde, vanillin, terephthalaldehyde etc.) and to synthesize 2-hydroxymethylphenol (2-HMP) from phenol but without formaldehyde.…”

Phenolic Foams Toughened with Triethylene Glycol by In Situ Polymerization and Prepolymerization Processes

“…7−9 The main chain of the molecular structure of ordinary phenolic resin (O-PR) contains a large number of phenyl groups, which are connected by methylene or methylene ether after being catalyzed by an acidic curing agent. 10 Due to the unstable structure of methylene ether, the stable three-dimensional network structure is mainly formed by a methylene bridge after resin curing. This structure makes the density of the rigid group (benzene ring) too large and the rotational degree of freedom of the chain segment small, which leads to the shortcomings of high brittleness and easy pulverization of ordinary phenolic foam (O-PF).…”

“…Phenolic foam (PF) has been widely used in construction materials, chemical industry materials, transportation, and aerospace applications due to its excellent thermal insulation and flame retardant properties. − PF is usually prepared by mixing phenolic resin (PR), surfactant, foaming agent, and curing agent. , The properties of PR (as the main matrix for the preparation of PF) affect the properties of PF. − The main chain of the molecular structure of ordinary phenolic resin (O-PR) contains a large number of phenyl groups, which are connected by methylene or methylene ether after being catalyzed by an acidic curing agent . Due to the unstable structure of methylene ether, the stable three-dimensional network structure is mainly formed by a methylene bridge after resin curing.…”