Phenolic Resins

Phenolic Resins Phenolic resins are polycondensation products resulting from the reaction between phenol and formaldehyde. Phenolic resins were developed commercially as early as 1908 and stand out as the first fully synthetic resins ever developed. Although they have broad and diverse industrial applications today and significant advances have been made in understanding their structure, technology and applications, many aspects of the chemical properties of phenolic resins remain unresolved. Since polymers derived from the reaction of phenol with formaldehyde differ significantly from other polycondensation products, their structure is still not fully understood. Polyfunctional phenols can react with formaldehyde at positions in the hydroxyl group designated as both ortho and para. This means that condensation products exist at any chain length as numerous positional isomers. Therefore, resolving the organic chemistry of the reaction occurring between phenol and formaldehyde is quite difficult.

Types of Phenolic Resins

Phenolic resins are divided into two different groups: novolacs and resols. Both exhibit high temperature stability up to 300–350°C, high water stability and chemical stability. Phenolic resins are generally dark in colour, ranging from yellow to dark red, and have a fairly good price/performance profile.

The main differences between resol and novolac phenolic resins are as follows:

1. Resol phenolic resins are produced using alkaline catalysts, while novolac resins are produced using acidic catalysts. 2. Resol phenolic resins are methylol-bearing resins, while novolacs are resins that do not contain methylol. 3. Resol phenolic resins are produced through B-staging, while novolac resins are produced through prepolymerization. 4. Resols typically have shelf lives of less than 1 year (less than 60 days), while novolacs have unlimited shelf life. 5. Resols release water when cured, while novolacs release ammonia when cured. 6. Novolac resins have twice the dimensional chemical stability compared to resols. 7. Resols are typically casting and adhesive resins, while molding compounds are made from novolacs. However, some molding compounds can also be made from resols. 8. Resols are generally liquid, while novolacs are solid.

Applications

Phenolic resins are also frequently used as binders in exterior-grade plywood and particleboard that require superior water resistance. In plywood manufacture, phenolic resin adhesives are typically applied to wood veneers by roller or extrusion coating. The coated veneer is then cross-grained, stacked and hardened in a high-pressure press at 120 to 130°C and 11 to 16 kg/cm² for 5–10 minutes. In particleboard manufacture, phenolic resin adhesives are sprayed onto particles and distributed with continuous mixers. Phenolic resins are widely used as adhesives and coatings in the production of many industrial molded products, including billiard balls and laboratory benches. For example, phenolic laminates are produced by impregnating one or more layers of base materials such as paper, glass fibre and carbon with phenolic resin and laminating the resin-saturated base material under heat and pressure. The resin fully polymerizes during this process. Selection of the base material is made based on the intended application of the product. Prepared by: Murat Soygür Sources https://www.sciencedirect.com/topics/engineering/novalac-phenolic-resin https://thechemco.com/chemical/phenolic-resin/ https://polymerinnovationblog.com/wp-content/uploads/2015/02/handbook-of-adhesivetechnology.pdf35