At a conference on fibre composites recently I heard one speaker repeatedly assert that natural fibres were better than timber, on sustainability grounds. Next he showed many graphs comparing the strength properties of long natural fibre composites (NFCs) with particleboard and MDF.
The whole exercise appeared to be a farce, starting from the picture of a man felling an oak tree with an axe through to the conclusion that bioresins and bast fibres were superior. It was also surprising to find a fierce criticism from such a closely connected discipline – we are accustomed to fending off challenges from other materials, but usually expect it to be perhaps those further away like metals, plastics and ceramics.
Writing today, it seemed timely to remind ourselves of the enormous benefits of using wood in such an efficient manner as we do in wood-based panels. Start with plywood – several millennia before long fibre composites for aerospace and motorsport were invented, the ancient Egyptians were already using the longitudinal strength of wood in plywood-type laminates. The use of cross-laid lamellae giving a material which was strong in both axes of the plane, and also mouldable. Now, just to show I have no hard feelings for those working with natural fibres I will point out that the Egyptians also invented papyrus on similar principles – using the length of the papyrus stems to assist in assembly of this wonderful technological advancement. This was four millennia BCE.
When we started working properly with particleboard in the 1940s and 50s (a product called ‘artificial wood’ had already been developed in 1887 in Germany using albumin as the binder) this capitalised on the lack of large timber and availability of synthetic resins. Large pieces of usable material could be formed from sawdust, offcuts, floor sweepings, wood chips, etc. Yet the properties of this new material were excellent for the many applications needing timber – furniture in particular, freeing up the sawn timber for structural applications.
MDF was a similar leap forward in technology, allowing even better use of the fibres, by generating a much more uniform feedstock, and using new blowline technology to obtain maximum resin dispersal. As a result, this technology uses substantially less adhesive (4-6% perhaps) than the natural fibre composites mentioned in the opening paragraph (where 40-70% binder is not unusual). In terms of responsible resource use I think this is a tale worth telling. Again the panels were well suited to many applications where strength in the plane, and ability to cover a wide area, are sought. Of course, MDF makes superior mouldings and has a fine finish, making it well suited for decorative purposes, and gives design freedom to the customer.
Coming to OSB in the 1970s we see the idea of strength and orientation taken to a new level. I noticed that the presenter in question did not choose to compare OSB with their NFCs. By using precision engineered strands of wood (very thin, but broad and 10-15cm long), and devising a system to cross lay these to give a three layer construction of the panel (like plywood but with strands instead of veneers), a new wonder material was developed. This reduced our reliance on plywood (and trees of peelable quality) while providing excellent performance for structural applications. It is now the staple of timber frame housing systems across Europe and North America. What about resin use? Again, this is incredibly low, compared to NFCs, near 4%, and a small amount of wax to provide some moisture resistance for the typical locations where it is used.
In terms of density, all of these wood-based panels offer far superior options, at approximately 550-600 kg/m3, so for specific strength we expect great results. NFCs are usually higher than 1,000kg/m3, as the resin and the fibres are dense, and air is excluded as far as possible.
Looking at the data presented by this speaker for tensile strength, MDF was 0.6 MPa, which sounds like an internal bond strength value to me – whereas the tensile properties of long fibre composites were tested in axial mode. So I offer this short piece as a reminder to look at the details when you hear claims of great performance from new materials. Often there is more sleight of hand occurring than actual substance. I too believe NFCs have a role to play, but I’d prefer to argue their case on the grounds of real comparisons.
Meanwhile, let’s be proud of the great things that can be achieved with a brilliant starting material – wood – and its excellent and sustainable supply chain, plus a little engineering, ingenuity and clever design.
