3.1.3. Mechanical properties
The tensile properties of the composites before and after
weathering are displayed in Table 4. As it shown, incorporation
of FRs increased strength of the composite, whereas stiffness of
all composites, excluding graphite, was decreased. The decreased
modulus can be explained by the fact that filler can disturb matrix
continuity and induce microcracks due to stress concentration near
filler particles [53]. There are few factors, such as high stiffness
[54], layered structure and high aspect ratio (>1000), are responsible
for the reinforcing properties of the graphite; degree of dispersion,
however, can play significant role. Positive influence of ZB on
the flexural strength of PE-based WPC was reported previously
[55]. Melamine molecule, having three very reactive amine groups,
can react with cellulose, whereas, aromatic s-triazine ring might
have affinity to PP through hydrophobic interaction. Thus, melamine
reactivity can improve interfacial interaction in the composite
and strength. The fine particles of TiO2 [56] and ATH at high
degree of dispersion have potential as reinforcing filler for WPC.
According to weathering test result, mechanical properties of
the composite showed tendency to decrease during weathering.
However, statistical significance test indicated that properties failure
was insignificant in many cases, especially after 500 h weathering.
Generally, the reduction of the tensile strength and
modulus of the weathered composites is associated with fiber/
matrix interfacial bonding degradation and leaching of
water-soluble material from the composite [51]. Also, increased
embrittlement of the matrix due to PP molecules scission influences
the mechanical properties [31]. It has been shown that water
facilitates composite degradation [7]. Improved water resistance of
the WPC, e.g. interfacial interaction, also decreased mechanical
properties loss after weathering [57]. Fig. 5 shows the SEM micrographs
of composite fractures before and after weathering. There
are not big difference is observed in the samples; some cellulose
fibers are pulled out, but most of cellulose fibers are embedded
into matrix. After weathering, however, pulled-out cellulose fibers
amount is slightly more due to possible influence of wet conditions
onto interfacial cellulose/matrix interaction. Shell/core interphase,
which is displayed in Fig. 5(c) and (d) is not deteriorated during
weathering. The top polymer layer degradation can be observed
in the FTIR analysis (see in FTIR spectral analysis section). The
inspection of the surface with SEM technique also revealed significant
changes on the composite surfaces (Fig. 6). Prior to weathering
(Fig. 6a–f) the surfaces are smooth; the cellulose particles are
covered with the PP matrix. After weathering (Figs. 6a⁄–f⁄), cracks
can be found on all the surfaces of the studied composites. The
cracks can be induced by the expansion/contraction of cellulosic
particles due to water absorption/desorption. In addition, UV radiation
could induce changes in the crystallinity of the PP matrix,
which results in cracks. Not only UV radiation and presence of
water have a damaging effect on the mechanical performance of
the composite. Another important reason for the decay of mechanical
properties may be the enhanced temperature, 38 C, in the
weathering chamber during the experiment. The combination of
UV radiation and thermal energy result in a higher degradation
rate than that due to thermo-oxidation only [10]. The photo- and
thermo-oxidation of the PP film increase remarkably with the
increased temperature [58]. The lifetime of the polymer material
can be decreased by 40% when the temperature of the accelerated
weathering chamber rises from 30 to 40 C [59]. The thickness of
the oxidized layer increases with the temperature increase