Cellulose, Starch, and Chitin Reinforcement

As reported by Habibi et al., CNCs have a theoretical elastic modulus higher than that presented by steel and similar to that of Kevlar [119]. Besides the mechanical behavior, the addition of CNC increases other interesting properties. For instance, Zhang et al. reported the use of modified CNC to avoid the yellowing of PUs [120]. 

Zhu et al. studied polyurethane foams from soy reinforced with cellulose microfibers. They found an increase on the onset degradation temperature of the thermal degradation of polyurethane with the addition of 2 wt % cellulose fibers. They attributed this fact to the insulator effect of cellulose fibers [52]. Navarro-Baena et al. studied shape memory PU based on PLA-PCL-PLA block copolymer and reinforced with both CNCs and PLA grafted CNCs [72]. Aside the increment on the shape memory behavior of the polyurethane-based nanocomposites, they reported an increase on the thermal stability of the PU matrix in particular, they reported that, although CNCs improved the thermal stability of both PCL and PLEA blocks, in particular the thermal stability of the PCL block was improved in the nanocomposites increasing the maximum degradation temperature of about 40 with respect to the PCL block of the neat PU-matrix [72]. 

Gavgani et al. [127] synthesized new intumescent flame-retardant polyurethane/ starch composites by melt blending which show three stages on the thermal degradation. Moreover, the temperature of maximum rate of mass loss for the composite systems is higher than that of pure PU, indicating that incorporation of starch has effectively improved the thermal stability of the composites. Moreover, they indicated that 

On the other hand, chitin and chitosan are employed as reinforcement for polymers and in particular for PUs. Saad et al. synthesized PUs with block copolymers of PHB and they reinforced them with chitin nanocrystals. They reported an improvement on the thermal stability due to the addition of nanofillers [128]. Barikani et al. developed hybrids of PU and chitosan and found that the chitosan increases the thermal stability of the PU matrix [129]. Similarly, Zia et al. added chitin to the polyurethane backbone, reporting an enhancement on their thermal properties [130]. 

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