Conclusions
The objective of the work was to investigate the effects of
lignin in cotton/PLA composites, its influence on
mechanical properties and its potential as a natural adhesion
promoter in natural fibre-reinforced composites. The
assumption was that lignin improves the fibre matrix
adhesion in cotton fibre-reinforced PLA composites. For
this purpose, cotton was well suitable because the fibres do
not contain any lignin. Thus, cotton/PLA fibre webs with a
cotton mass proportion of 40% were manufactured by
carding. Some of the fibre multilayer webs were treated
with powdery lignin. As reference samples, bast fibrereinforced
kenaf/PLA composites were produced. Kenaf
contains natural lignin. The fibre mass content was also
40%. The boards were manufactured by compression
moulding. Pure PLA boards without reinforcement acted as
further reference samples.
The composites and the pure PLA sample were examined
in terms of their tensile characteristics (tensile
strength, Young’s modulus and elongation at break) as well
as their impact characteristics by the Charpy impact test.
Optical investigations of the fibre/matrix interface were
carried out by SEM. The results of the SEM analysis showed that adhesion
between fibre and matrix as well as between individual
layers of the multilayer webs could be improved by the
presence of lignin. While the untreated cotton/PLA composites
showed clear delaminations of the individual layers
of the multilayer web, less delaminations were found in the
lignin-treated cotton/PLA. No delaminations were found in
the kenaf/PLA composites.
Compared to the untreated cotton/PLA composites, lignin
treatment increased tensile strength and Young’s modulus by
9% and 19%. On the other hand, the impact strength was
decreased by 17%. However, it was still higher than that of
the pure matrix. In contrast, kenaf/PLA composites had
barely half the impact strength of the pure matrix.
The measured results prove the assumption that the
presence of lignin improves fibre/matrix adhesion. However,
the impact strength is reduced by the embrittlement of
the composites.
Thus, after further optimisations, lignin could be used in
natural adhesion promoters for lignin-free fibres in a PLA
matrix if an increase of tensile strength and stiffness is
required. The moisture and odour development during
processing needs to be kept under control.