Civil Engineering Reference
In-Depth Information
With the purpose of enhancing the interfacial adhesion between cellulose nanowhis-
kers and castor oil biobased polyurethane (BPU), Park
et al.
[5] modii ed the nanopar-
ticles by reacting the hydroxyl groups of the nanowhiskers with TDI isocyanate groups
(m-CNW), leading to the formation of urethane bonds between them. h e raw CNWs
were prepared using an isolation of microcrystalline cellulose (MCC) by an ultra-
sonication treatment followed by modii cation of the particles by acid hydrolysis to
incorporate sulfate groups on the surface of the particles. In order to prepare the nano-
composites, the (m-CNW)-reinforced BPU prepolymer was synthesized by reacting a
castor oil-based polyol with TDI in the presence of m-CNW dispersed in DMF for 3
hrs at 90ºC, and then 1,4-butanediol, as the chain extender, was incorporated into the
prepolymer for 30 min. at 30ºC. Subsequently, the solution was cast on a Tel on mold
and cured at 60ºC for 24 hrs. h e residual DMF in the elastomer i lms was removed at
60ºC under vacuum for 48 hrs. Consequently, biobased PU elastomer i lms containing
0.5, 1, and 5 wt% m-CNWs were obtained. h e tensile strength and modulus of the
m-CNW-reinforced biobased PU composites were signii cantly improved, as compared
with the neat rubbery matrix. h e elongation at break of the nanocomposites decreased
with increasing m-CNW content, indicating that m-CNW turned the biobased PU stif
and rigid. Dynamic mechanical analysis showed that the storage modulus increased
and the loss tangent peak shit ed toward higher temperatures by the incorporation of
m-CNW, due to the increased crosslink density of the rubber network resulting from
the strong interaction between m-CNW and the biobased PU matrix, with restriction
of the molecular mobility. h e biobased PU/m-CNW composites showed a two-stage
thermal decomposition behavior, which was attributed to the degradation of urethane
bonds and castor oil-based polyols. h e obtained activation energies for the thermal
decomposition of the biobased PU/m-CNW composites indicated that the incorpora-
tion of m-CNW into the BPU matrix improved the thermal stability of both stages.
Highly crystalline castor oil-based polyols and corn-sugar-based chain extenders,
were used by Saralegi
et al.
[12] to prepare biobased polyurethanes with 17 and 46
wt% of hard segment content. Dif erent amounts of cellulose nanocrystal (CNC) con-
tent were added by a solvent casting procedure to the highly crystalline biobased poly-
urethanes, and thus, bionanocomposites containing 1, 3, 5 and 10 wt% of CNC were
obtained. CNCs were isolated by sulphuric acid hydrolysis in order to obtain rod-like
crystalline structures. A good dispersion of CNCs was obtained for all the bionano-
composites since they interacted with the polyurethane segment that was not asso-
ciated in ordered domains. h e improvement observed in the mechanical properties
when adding CNCs, especially in the tensile strength and strain at break, was not as
high as the improvement observed on amorphous systems due to the highly crystal-
line nature of both sot and hard segments; in addition, they also served as reinforcing
agents. A comparison between the experimental values obtained for the storage modu-
lus at er the melting temperature of sot segments (SS) (about 60ºC) and calculations
based on dif erent theoretical models (i.e., Halpin-Kardos, Pan, and Percolation mod-
els) were performed using a CNC aspect ratio and storage modulus of 18 and 12 GPa,
respectively. Details and equations about these models are provided elsewhere [36, 61,
62]. As can be noticed from Figure 3.9, the magnitude of the reinforcement followed
the Halpin-Kardos model for the bionanocomposites based on the lower hard segments
(HS) content matrix, while for the higher HS content, the Pan model was the one that
