Comparison of 40% fiber-filled polypropylene after 31-day immersion in water. (Polybond is the coupling agent)

On a percentage basis: Wood flour absorbs 65% more water than the PlastiCell without the coupling agent. With a coupling agent, the percentage goes to 77% more water.

A research chemist surmised that this is because wood flour is more porous and also because the PlastiCell has natural waxes which repel water.