Phenol–formaldehyde (PF) resin continues to dominate the resin industry more than 100 years after its first synthesis. Its versatile properties such as thermal stability, chemical resistance, fire resistance, and dimensional stability make it a suitable material for a wide range of applications. PF resins have been used in the wood industry as adhesives, in paints and coatings, and in the aerospace, construction, and building industries as composites and foams. Currently, petroleum is the key source of raw materials used in manufacturing PF resin. However, increasing environmental pollution and fossil fuel depletion have driven industries to seek sustainable alternatives to petroleum based raw materials. Over the past decade, researchers have replaced phenol and formaldehyde with sustainable materials such as lignin, tannin, cardanol, hydroxymethylfurfural, and glyoxal to produce bio-based PF resin. Several synthesis modifications are currently under investigation towards improving the properties of bio-based phenolic resin. This review discusses recent developments in the synthesis of PF resins, particularly those created from sustainable raw material substitutes, and modifications applied to the synthetic route in order to improve the mechanical properties.

Phenolic foams are widely used as insulation materials in construction, transportation, and spacecraft industries due to their flame retardancy, thermal stability, low toxicity, and low smoke generation. However, these foams are typically produced from non-renewable resources. With increasing environmental pollution, volatility in petrochemical prices, and depletion of petroleum resources, the demand for polymeric materials produced from renewable resources is ever-increasing. Numerous recent studies have introduced biomass as an alternative resource for petroleum-based raw materials for pro-ducing phenolic foams. In addition to their environmental benefits, bio-based phenolic foams exhibit properties similar or even superior to traditional phenolic foams. In this review, traditional phenolic foams are discussed. Recent progresses on synthesis of foams from bio-based resources, modification of the bio-alternatives, performance enhancements, and comparison with traditional phenolic foams are outlined. Challenges and methods to further improve performance characteristics of bio-based phenolic foams and their expanding range of applications are highlighted.