Mechanical Performance of Biocomposites Based on Straw Fiber Self-Reinforced Plasticized Flours of Bread Wheat Grown with Different Nitrogen Fertilization Management Strategies

Previous research has demonstrated the possibility to produce wheat flour-based thermoplastics, whose tensile properties depend on flour characteristics that are affected by wheat variety and crop nitrogen (N) fertilization management. This work further investigates the reinforcing effect on thermoplastic composites determined by wheat straw obtained from two wheat varieties (Bologna, BL; Bora, BR) grown under four N fertilization treatments differing in rate and application timing as follows: (1) always well N fed (N300: fertilized with 300 kg N ha−1 and split into five applications of 60 kg N ha−1 each across the growing cycle), (2) N fed only very early (N60+0: fertilized only in one early application of 60 kg N ha−1), (3) N fed only very late (0+120: fertilized only in one application of 120 kg N ha−1 at pollination) and (4) never N fed (N0). The finely cut straw was added by 15% (w:w) to the flour of treatment N300 of each corresponding wheat variety to produce thermoplastic bulk samples. We performed the analysis of straw composition, FESEM imaging of straw stems, X-Ray diffraction analysis of flours and straws, thermal analysis of straw, and tensile tests on bulk samples. The results demonstrate that, for both cultivars, the reinforcing effect of the straw was maximum when the straw came from crops grown with low and early N availability (i.e., N0 and N60+0) and minimum when the straw came from crops grown with high and late N availability (i.e., N300 and N0+120). The greater reinforcing effect of straw from N0 and N60+0 was likely due to greater stem compactness, higher cellulose proportion and higher crystalline fractions. The reinforcing effect decreased for all plasticized composites when they were stabilized for 48 h at higher ambient humidity (53% RH vs. 11% RH) before performing the tensile tests. Overall, our results confirm that plant-based materials engineering needs to carefully consider the variability of source material characteristics as affected by crop growing conditions.