Crosslinking and The Phenol/formaldehyde Ratio
When the molar ratio of formaldehyde:phenol reaches one, in theory every phenol is linked together via methylene bridges, generating one single molecule, and the system is entirely crosslinked. This is why novolacs (F:P <1) don't harden without the addition of a crosslinking agent, and why resoles with the formula F:P >1 will.
Read more about this topic: Phenol Formaldehyde Resin
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