Up until the early part of the twentieth century adhesives used in plywood manufacture were available only from natural materials such as hide, bone, casein, soybean, and other vegetables.  These natural adhesives were not very moisture resistant and while wonderful for indoor uses as one can imagine that use of these materials outdoors resulted in  outright failure.

    In the 1930's the intoduction of synthetic resins such as urea-formaldehyde (while urea-formaldehydes see primarily indoor use) and phenol-formaldehyde allowed greater applications in outdoor environments due to their greater water resistance.

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Phenol and formaldehyde are typically formulated in a ratio of 1:1.2-3 and heated in the presence of catalyst to form a low molecular weight polymer.
 

The polymer precursor is then applied to the substrate and then heated to approximately 180^oC and a crosslinked network is formed with methylene and ether linkages.

    Similar chemistries are used with urea, melamine, other phenols, formaldehydes and other aldehyde derivatives under a variety of conditions.

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