wood and pp

I'm assuming that you want information about mixing WOOD and PP (as in the subject) and not PP and PP (as in the post).
WPC's (wood plastic composites) are a very large business that continues to grow. There are annual conferences on the subject and numerous papers are published each year. A Google search for "Wood plastic composites" will give you lots of good hits.
PP/wood composites can be improved by using some maleated PP, although there are a number of patents in this area that you will need to know about and work around.
John

Wood fiber is typically acts as a reinforcing agent up to about 50-60 wt.%, beyond that you no longer have enough polymer to act as an effective binding matrix. Your modulus (both tensile and flexural) should increase dramatically and you should see improvements in yield stress but probaly not the stress or strain at failure. Likewise, your impact properties will drop off as you add wood fiber to the PP. As John suggested you can boost some of these properties by using 1-5% of PP grafted with maelic anhydride (MAH), with typical graft levels of 0.5-1.0% MAH. However, your processing typically suffers - the MAH grafted PP typically increases extrusion torques and increase injection molding cycle times.
Hope this helps
Larry Effler