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46 TECHNOLOGY SEP/OCT 2024 FDM ASIA | www.fdmasia.com

allowed for retaining the lignin content beyond 80 percent
and obtaining a stronger wood template compared to the
delignified counterpart.
Upon infiltration with methyl methacrylate and its subsequent

in situ polymerisation, the transmittance of the resulting TW
was around 83 percent.
After lignin removal, two possible pathways can be followed
to obtain the final TW showing the envisaged properties, namely:
the direct cell wall densification or the infiltration of delignified
wood by a resin and its subsequent polymerisation/curing.
The first approach is suitable for obtaining transparent
wood thin films, which exhibit high transparency and even
flexibility, according to the employed type of pristine wood.

For this purpose, the delignified wood template is simply
compressed using external pressure, aiming at making the
important to prevent the occurrence of dehydration reactions radial thickness of the wood template smaller; as a result, a
of wood polysaccharides, also avoiding the formation of more uniform and compact wood template with a cell wall
undesirable chemical groups: indeed, these phenomena can skeleton is produced.
impact the quality and the overall mechanical behaviour of This dense wood template can be used as it is (i.e.,
the final material. as a thin transparent wood film), or can be further vacuum

The third effective method for removing lignin from wood infiltrated by a suitable polymer resin, thus obtaining transparent
takes advantage of the use of such bleaching agents as compressed wood.
sodium chlorite or hydrogen peroxide, which account for The second approach, which is the most utilised, exploits
almost complete removal of the component (the residual lignin the infiltration of the delignified wood with a suitable resin
is as low as 0.8 and 0.6%, respectively). (i.e., bearing a refractive index that perfectly matches that of
It is worth highlighting that, irrespective of the employed delignified wood, which is often around 1.5) and the successive
method, the thickness of the pristine wood is an important polymerisation/curing.
issue, as, usually, all the described processes are more effective To speed up the process and save time, infiltration may
when carried out on thin wood samples. be carried out under vacuum; both thermal and photoinduced

Aiming to limit both the decrease in bulk wood structural polymerisation strategies can be employed for the successive
features due to the lignin removal and the production of toxic step.
chemical by-products (such as methyl mercaptan, hydrogen The first pioneering works paving the way toward highly
and dimethyl sulphide, and chlorine), Li and co-workers transparent, thermally insulating, and tough infiltrated transparent
demonstrated the possibility of obtaining transparent wood woods come from Hu’s and Berglund’s groups, who respectively
without removing its lignin but changing the structure of the succeeded in obtaining transparent epoxy-backfilled basswood
latter. and poly(methyl methacrylate)-infiltrated balsa wood.

In particular, the use of a bleaching solution (Na2SO3/ Since then, the seeking for other refractive index-matched
NaOH/H2O2) for removing only the chromophore groups polymers, suitable for obtaining TW composites, has grown