Composite Materials Journal of Thermoplastic - LSU School of ...

218 S.-Y. LEE ET AL.
This result could be due to an improved interfacial adhesion between the
polymer matrix and fillers.
Neat PLA had the highest H c (34.1 J/g) and X c (44.5%) values
compared to PLA/WF composites. The H c decreased as the loading
level of WF increased (Figure 4(a)). Obviously, this result could be
attributed to the amount of PLA in the composites as the WF content
increased. The application of 10 wt% talc to PLA/WF composites further
decreased the H c . The large decrease in the X c of the composites with the
addition of WF, talc and silane was observed in Figure 4(b). This may be
caused by the partial inhibition effect of WF, talc and silane on polymer
crystal formation.
Mechanical Properties
The tensile modulus of neat PLA was 1,346 MPa (Figure 5(a)). As WF
levels increased, the tensile modulus was enhanced. With the WF loading
levels increased from 10 to 40 wt%, the tensile modulus increased from 62.5 to
169.5%. The tensile strength of neat PLA was 63 MPa (Figure 5(b)). For
PLA/WF composites, the tensile strength increased 13.7, and 16.7, and 13.9%
at the loading levels of 10, 20, and 30 wt% WF, respectively. At 40 wt% WF
loading, PLA/WF composite had a similar tensile strength as the neat PLA.
As shown in Figure 5(a), the addition of talc (10 wt%) to PLA/WF
composites improved the tensile modulus by about 18.3 to 50.2%.
In addition, the use of 1 and 3 wt% silane led to the higher tensile modulus
of composites, compared to neat PLA and PLA/WF composites without
silane treatment. As shown in Figure 5(b), the addition of talc (10 wt%) to
PLA/WF composites resulted in a slightly lower tensile strength than neat
PLA. At 10 wt% talc loading, the tensile strength slightly decreased as the
WF level increased. The PLA50%/WF40%/talc10% composite showed an
8.6% lower tensile strength than the PLA80%/WF10%/talc10% composite.
In addition, PLA50%/WF40%/talc10% composite had a 10.7% lower
tensile strength than neat PLA. The addition of 1 wt% silane to PLA/WF/
talc composites improved the tensile strength considerably, which indicates
that silane facilitates the chemical bonding between WF and PLA, and
between inorganic filler and PLA. The tensile strength of PLA/WF/talc
composites with 1 wt% silane was significantly greater than that of PLA/
WF/talc composites with 3 wt% silane. As discussed earlier, the addition of
1 wt% silane was more effective to improve interfacial adhesion than the
addition of 3 wt% silane. An improved interfacial adhesion could be
explained by a mechanism shown in the following three steps;
1st step: the formation of silanol (Si-OH) by hydrolysis, or R-Si-
(OH) 3 þ MeOH.
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