Why is compressed wood weaker than natural wood in presence of moisture?

Recently, compression of wood has emerged as an attractive processing route to transform natural wood into high performance structural materials. While the wood strength is increased substantially by the compression, we show for the first time here, that, concomitantly, diffusion of water is enhanced by the compression. This discovery has come from studies with Balsa and Japanese Hinoki wood which contain controlled porosity. Because of the enhanced diffusion, densified compressed wood is weaker than natural wood in presence of moisture. The diffusion characteristics are related to the wood microstructure. In particular, we find that water diffuses mostly through the solid cell wall and very little via the pores. We obtain the diffusion coefficient of the cell wall material and find it to be an intrinsic property of the wood. The increased effective surface area for diffusion (cell wall) due to eliminating pores in compressed wood is what results in the higher diffusion, with adverse consequences for structural integrity. The directional effect of diffusion in wood is also controlled by the effective surface area distribution. Incidentally, the diffusion study has also provided insights into why nature has engineered specific porous structures in wood.