Microstructure and micromechanical studies of injection moulded Chemically Modified Wood / Poly(lactic acid) Composites

Abstract: Conventional wood plastic composites (WPCs) in general combine wood
residuals with thermoplastics such as polyethylene and polypropylene. The
basic idea in this work is to replace these olefin plastic matrixes in WPCs
with a so-called biobased plastic, namely poly(lactic acid) (PLA). To
reduce the water sensitivity of such biocomposites, for typical outdoor
use, the idea is also to incorporate a modified wood component. The aim of
this work was to study the microstructure of injection moulded WPCs based
on PLA and modified wood, and to investigate some of their micromechanical
behaviour.
Four different PLA/wood formulations (weight-% ratio 50/50) were studied:
PLA combined with 1) unmodified MDF fibres, 2) acetylated MDF fibers: 3)
acetylated wood particles: and 4) thermally modified wood particles. The
processing effects on the form and shape of the wood component were studied
by a matrix extraction procedure combined with light microscopy. The
microstructure of the WPCs were studied by scanning electron microscopy
(SEM) using a sample preparation technique based on UV laser ablation, i.e.
a surface preparation procedure allowing microscopic observations without
microtoming. Microtensile testing was performed on samples prepared from the
core and skin layers of the WPC samples.
It was observed that the processing of the WPCs resulted in severe damage
and fragmentation of the wood fibres and particles. Especially, the
processing with a thermally modified wood component yielded a lot of wood
cell wall fragments/fines. Good dispersion of the wood reinforcements in
the PLA matrix was observed in all cases, even though the distribution and
orientation of the fibres varied between different regions in the samples.
No principal fibre orientation was outlined in the core, while closer to
the surface regions the fibres tend to orient in the flow direction.
Supposedly a turbulence effect at the end of the moulded samples also
induced disorientation of the fibres.
Long and slender shape, i.e. a higher aspect ratio, of the wood components
were as expected found to be more efficient as a reinforcement than
particulate shaped ones. The composites with acetylated MDF fibers were also
observed to be weaker than composites with unmodified ones. On the other
hand, SEM observations of fracture surfaces exhibited better
reinforcement-matrix interaction for the modified wood than for the
unmodified wood. Although the fracture surfaces observations outlined the
importance of fibre orientation, with a more brittle type of failure in the
core and fibre breakage or debonding closer to the surface, there was no
significant difference found between mechanical properties for the core and
skin layer specimens. Other factors have therefore to be investigated in
order to explain the rather uniform strength measurements within different
regions in the composites.