To improve the toughness of polylactic acid, the toughening
agent must be dispersed uniformly as small domains
with good interfacial adhesion to the matrix. The
toughening agent must also be stable under the processing
conditions. Only when most of these criteria are
fulfilled the toughening agent can effectively improve
the overall toughness of the polylactic acid
Table 2 demonstrates
the mechanical properties of the CSR-toughened
polylactic acid. Generally, polylactic acid became less stiff and tougher
when CSR was added. The addition of only 5wt% CSR
increased the impact strength of the PLA by more than
fivefold. The increment indicates that the properties of
polylactic acid have been raised compared with that of polypropylene
(PP)[19,20]. Beyond 5 wt%, the impact strength
still increased but it did at lower rate. These increments
of the impact resistance were believed to have been
induced by the good interfacial adhesion between the
CSR and the polylactic acid coupled with the rather well dispersion
of the CSR particles throughout the polylactic acid matrix. As
a consequence, the rather well-adhered elastic CSR particles
helped absorbing the impact energy and consequently
retarded the polylactic acid rupture. The impact test results
are in good agreement with earlier reports on the rubber-
modified polylactic acid[16–18,21,22]. At identical rubber content,
CSR was found to exhibit a higher impact strength than
those of natural rubber[18], acrylate rubber[16], natural
rubber grafted with poly(butyl acrylate)[21] and
poly(ether)urethane elastomer[22]. It has been well
documented that the addition of an elastic material, such
as rubber, to a polymer matrix generally leads to a drop of
modulus and strength[17]. The tensile modulus and the
strength of the polylactic acid/CSR blends were lower than that of
the neat polylactic acid due to the lower rigidity of CSR. However,
its tensile strength at 45.31 MPa for the blend containing
10 wt% CSR was still larger than those of commodity
polymers such as polyethylene (PE) and PP[23]. The
tensile elongation at break of PLA was improved significantly
with the increase in CSR content, it surged
from 3.08% for neat polylactic acid to 16.1% for the blend containing
5 wt% CSR. Under bending force, the polylactic acid
blended with CSR also softened, as was indicated by the
decline of the flexural modulus and the flexural strength.
The flexural modulus of neat polylactic acid was 3.38 GPa with an
associated flexural strength of 89.48 MPa. With