Biomedical Engineering Reference
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based on the energy absorption by using fibers with high elongation, like lyocell fibers,
and fiber pull-outs from the matrix (Gassan and Bledzki, 1999; Mieck et al., 2000).
The lignin leads to better interfacial interactions between fibers and matrix and stronger
bonding, so less energy can be absorbed by fiber pull-outs and debonding, and impact
strength values were decreased. Due to this, the lignin treatment leads to a decline
of the impact strength. However, the lignin treated composites showed still a clear
reinforcement effect compared to the pure PLA sample with an impact value 16 kJ/
m². The impact strength of the 40 mass% reinforced lyocell 1.3-PLA composites was
reduced from 33 to 32 kJ/m², of lyocell 6.7-PLA from 39 to 28 kJ/m² and of lyocell
15.0-PLA from 31 to 24 kJ/m² (Table 1). Similar to the untreated PLA composites,
the better impact strength values were measured for the composites with higher fiber
loads (Figure 8).
Figure 8. Unnotched Charpy impact strength of the untreated (left) and lignin treated (right) lyocell/
PLA composites with fiber loads of 20 and 40 mass% (mean values, standard deviations are shown
as error bars; dots show the dimension of the fiber diameter).
Due to the low elongation at break of the PLA matrix (2.2%) and the high elonga-
tion of the Lyocell fibers (10-15%), a direct investigation of the fiber matrix interac-
tions by using the single fiber fragmentation test could not been carried out. But a
better adhesion between cotton fibers and a PP matrix by the addition of lignin could
be proved through a single fiber fragmentation test according to Huber and Müssig
(2008). For the lignin treatment the cotton fibers were stored for 30 min in a lignin so-
lution (1 g lignin in 100 ml ethanol). Figure 8 shows the results of the fragment lengths
of raw cotton in the PP matrix and lignin treated cotton in the PP matrix. It can be seen
that the fragment length of raw cotton of up to 10 mm is significantly longer than the
fragment length of lignin treated cotton fibers which is of approximate 7 mm (proved
by the t-test with α = 0.05). This proves a better adhesion of the lignin treated cotton
fibers in the PP matrix compared to the raw cotton fibers. Due to the great structural
and chemical differences of PLA and PP it cannot be proved that the same effect exists
for PLA but the higher mechanical values of the lignin treated composites and lower
fiber pull-outs allow us to assume that there is an influence of the lignin on the fiber/
matrix interactions.
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