Civil Engineering Reference
In-Depth Information
Shielded PCN
Net-point
S
w-s
PCL crystals
S
T
S
w-n
Naked
PCN
Figure 3.7
Schematic illustration of the triple switches in CNW-SMPU nanocomposites.
Reproduced with permission from [42] .
3.3
Waterborne Polyurethanes Reinforced with Nanocellulose
Fibers
Waterborne polyurethanes are fully reacted urethane polymers dispersed in water, and
they show many excellent features compared to conventional organic solvent-based
polyurethane, including higher rates of biodegradation [8]. However, they show low
mechanical strength and thermal stability, restricting some of their potential applica-
tions. In this way, reinforcements from renewable biomass are viable supplements for
these polymers, as was demonstrated by Liu and co-workers [3] in their work by using
cellulose nanocrystals as nanoi llers to improve properties of rosin-based waterborne
polyurethanes. h e research group coni rmed that cellulose nanocrystals were compat-
ible with the waterborne polyurethane, and thus, no cellulose crystal sedimentation or
l occulation occurred during the i lm forming process. Moreover, the surface of the
composite i lms were smooth, had no cracks, and showed a homogeneous dispersion
of the crystals in the polymer matrix, as presented in Figure 3.8. Tensile strength of
the composite i lms increased from 28.2 to 52.3 MPa while the tensile modulus was
improved from 316.2 to 1045.4 MPa with increasing cellulose amount from 0 to 20
wt%, respectively. However, elongation at break was sharply reduced from 267% to
22% for the same i ller concentration range. Moreover, the thermal stability of water-
borne polyurethane was also improved by the addition of cellulose crystals while the
glass transition temperature of the composites decreased in comparison with the neat
polymer.
Santiago de Oliveira Patricio
et al.
[8] prepared nanocomposites based on cellu-
lose nanocrystals and synthesized waterborne polyurethane using a two-step method
commonly referred to as a pre-polymer method. h ey developed nanocomposites
with dif erent properties by altering the mode and step in which the nanoi llers were
incorporated during the polyurethane formation. h e process utilized is as follows:
WPU/CN nanocomposites were obtained by physical mixtures of waterborne polyure-
thane and cellulose nanocrystals in an aqueous suspension, WPU/CN-P samples were
obtained by adding the cellulose nanocrystal suspension during the dispersion step of
the prepolymer (before adding the hydrazine hydrate used to react the remaining free
-NCO groups), and WPU/CN-PP composites were produced by adding the cellulose
nanocrystals, dispersed in the polyols, at the beginning of the pre-polymer synthesis.
