THESIS - ROC CH ... - FINAL - resubmission.pdf - University of Guelph

4.4.3 MECHANICAL PROPERTIES
Mechanical properties of SPI films were analyzed in different relative humidity as shown
in Table 4.4. With the increase in humidity, the tensile strength and elastic modulus drastically
decreased whereas elongation increased (Figure 4.9, and Figure 4.11). Films exhibited highest
tensile strength and elastic modulus at 11% RH but were extremely brittle. Specimens tested in
the MD had overall higher tensile strength than specimens tested in the TD, as expected. This
suggests similar characteristics to synthetic resins whereby alignment of polymer chains in the
MD increases tensile strength in comparison to TD. The increase of RH from 11% to 43% RH,
resulted with an increase in elongation (Figure 4.10), but further increase in RH did not
significantly change with specimens tested in the MD. In the TD, a similar positive correlation
could be observed with greater elongation values seen at 84% RH. However, increased
variation of results caused a higher degree of error. This error can be attributed to the alignment
of microscopic holes in the MD, where premature failure is more prevalent for films under
tension in the TD. These results indicate that SPI films are very sensitive to RH leaving it
undesirable for use in water sensitive applications. These trends are comparable to the SEM
findings whereby defects in the film were more pronounced for films exposed to increased RH.
With the combination of defects and increased chain mobility from water plasticization
(Kokoszka et al. 2010), mechanical strength should decrease. With the increase in RH, SPI
films exhibited behaviour ranging from rigid, semi rigid to soft plastics. Similar trends were also
found with Zhang et al. (2001) who extruded thick SPI sheets (0.35 to 1.5 mm).
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