I've been working with pure HDPE recycled lumber for a while, using it for various non-blade parts of a turbine under development for the commercial market. The bottom line is that it's not suitable for the type of loading for blades. A 2" x 4" x 6' piece of the lumber is very heavy and floppy like a noodle, and doesn't do well with tensile loading. The lumber that has HDPE mixed with wood chips is stiffer, but even worse tensile performance which would be a problem for blades.
One tip-off is the charts the RPL companies publish for building decking. They specify the maximum beam spacing to support a load safely, and for RPL the spacing is closer together than wood. So a blade out of RPL would need to be thicker than wood to get the same load carrying capacity. Fatigue performance over time for tension is a big scary unknown for blades, but compression loading is fine as they use for decks.
There are many advantages of the material (especially pure HDPE lumber) including superior UV and weather endurance with no painting, and excellent impact resistance. It machines like butter, but it cannot be sanded to get a nice finish. A metal blade must be used at high speed to get the surface to take the smooth, slick sheen that resists dirt and stains. Care must be taken when machining the molded lumber, as the material will warp like many other plastics that have surface stresses. Great material for certain uses, but not blades.
