When defibrating chips in an atmosphere of steam, a portion of the wood substance is dissolved by hydrolysis.
https://patents.justia.com/patent/4012279

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The defibrator is a thermo mechanical pulping refiner in which the pulp material, such as wood chips, is ground in an environment of steam between a rotating grinding disc (rotor) and a stationary disc (stator) each with radial grooves that provides the grinding surface. Wood chips are fed into the centre and are broken down as the centrifugal force pushes them towards the circumference of the discs where the grooves are finer to produce wood fibre. The size of the refined fibres can to some extent be controlled by altering the distance between the discs where a closer distance produces finer fibres but also requires higher grinding force. The capacity per machine is largely determined by the size of the machine, as well as the motor speed 1500 rpm or 1800 rpm.
https://en.wikipedia.org/wiki/Defibrator