The decay resistance of fully bio-derived wood plastic composites, WPCs, was tested in both laboratory and field tests. The laboratory tests were performed according to modified versions of AWPA E10 (soil-block test) and ENV 807 (tests in three un-sterile soils) and the field tests according to EN 252 (stakes in ground) and EN 275 (resistance to marine borers). The WPC materials for laboratory tests were injection molded test specimens with 50% modified wood particles and 50% cellulose ester (CAP) or poly-lactic acid (PLA) content. The field test specimens were taken from larger extruded decking board profiles with 60% wood content and 40% CAP. 60/40-mix (wt/wt) for CAP corresponds to the same volumetric composition as 70/30-mix (wt/wt) with polypropylene as matrix that was presented in Part 1 and 2. In all laboratory tests the control WPCs performed much better than the pine sapwood control blocks. The WPCs from modified wood performed better than the control WPC and WPCs from acetylated wood performed best with no detectable decay whatsoever. In the field stake test, the WPC from unmodified wood were slightly decayed whereas the WPCs from modified wood were sound. In the marine field test the WPC from unmodified wood were severely attacked by shipworm (Teredo navalis), whereas the WPCs from modified wood were sound.

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