Cellulose Fiber Reinforced Poly(lactic acid) (PLA) Composites - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 177

40 N/mm². Composites reinforced with lyocell 6.7 and lyocell 15.0 multilayer webs
reached values of 36 N/mm² and 28 N/mm² respectively. For the needle felt reinforced
composites clear reinforcement effects were determined (lyocell 1.3-PLA = 63 N/
mm², lyocell 6.7-PLA = 61 N/mm² and lyocell 15.0-PLA = 57 N/mm²). These results
lead to the assumption that the kind of the semi-finished product has a clear influ-
ence on the mechanical characteristics of the composites produced by the compression
molding technique CP-1. This effect could be attributed to a more homogeneous fiber
distribution in the PLA matrix due to the additional needling process and the lower
thickness of the needle felts compared to the multilayer webs. It is assumed that the
lower thickness lead already in the pre-heating phase to a better heat conductivity and
drying in the press.
Figure 3. Tensile strength of 40 mass% lyocell fiber reinforced PLA in dependence of the fiber
fineness. Composites reinforced with needle felts (left); Composites reinforced with multilayer webs
(right) (mean values, standard deviations are shown as error bars; dots show the dimension of the
fiber diameter).
By a reduction of the reinforcing fibers from 40 to 20 mass% significant better
reinforcement effects were measured (compare Table 1) . This phenomenon is in con-
trast to the composite theory because the higher value should be measured for the
composites with the higher fiber load (Bunsell and Renard, 2005; Vetrotex, 1995). For
the lyocell 1.3-PLA composites the tensile strength could be increased from 42 to 51
N/mm² and for lyocell 15.0-PLA from 28 to 54 N/mm². This effect can be explained
by the worse coating and the bad interfacial interactions between the lyocell fibers
and the PLA matrix due to the lower degree of compaction of the multilayer web
reinforced composites with higher fiber loads. Because of the low degree of compac-
tion the reinforcing fibers can act as weak spots and the reinforcement effect can be
decreased by higher fiber loads. The SEM investigations approve the assumptions that
the 40 mass% multilayer web reinforced composites show a lower degree of compac-
tion which lead to a high number of fiber pull-outs in comparison to the 20 mass%
reinforced composites (Figure 4).