**4. Results and Discussion**

#### *4.1. Wheat Bran Characterization*

The micrographs of wheat bran fibers (Figure 2a) show that bran particles have various dimension. Mainly the bran powder consists of platelets having low aspect ratio that forms agglomerates having an average size around 250–500 micrometers. However smaller size fraction in large quantity can also be observed. In the SEM micrographs the wheat bran fibers showed a low aspect ratio and their morphology is similar to flakes.

Consequently, for the fibers aspect ratio distribution a flake geometry was assumed where the filler aspect ratio, *ar*, can be defined as the ratio between the average fiber diameter (*d*) (calculated on the larger platelets surface) and the mean thickness of the platelets (*h*) according to Equation (12) [54]

$$a\_{r\prime} \, plate \, lets = \frac{d}{h} \tag{12}$$

In Figure 2b-c a sharp peak can be observed in the range of 0–200 μm and 0–40 μm for the mean fiber diameter and fiber thickness distributions, respectively. Thanks to the fibers distribution the mean weighted fibers lengths for the calculation of the mean fibers aspect ratio was carried out. In particular, a mean weighted fiber diameter of 219.76 μm and a mean weighted fiber thickness of 51.40 μm were obtained that correspond to a mean fiber aspect ratio of 4.27. Consequently, the composites obtained belong to the family of very short fiber composites [27].

**Figure 2.** (**a**) SEM images at 200× of wheat bran fibers (**b**) average diameter distribution and (**c**) average thickness distribution.

In the infrared spectra (reported in Figure 3) the hydrophilicity of the wheat fibers is highlighted by the very sharp –OH groups stretching in the region of 3650–3000 cm−1. The peak at 3294 cm−<sup>1</sup> corresponding to –OH stretching was attributed to specific intramolecular hydrogen bonds of cellulose II [65,66]. The infrared spectra displayed a small adsorption band at 2925 cm−1, typical of the stretching vibrations of the C–H bonds in hemicelluloses and cellulose. The bands at 1639 cm−<sup>1</sup> and 1540 cm<sup>−</sup>1, attributable to amide I and amide II vibrations, can be ascribed at the presence of the protein fraction in bran. Moreover, the shoulder at 1639 cm−<sup>1</sup> is attributable to the acetyl and uronic ester groups of hemicelluloses or to the ester linkage of carboxylic group of the ferulic and p-coumaric acids of lignin [67]. Adsorption bands can also be observed near 1639 cm−<sup>1</sup> and they are attributed to deformation of the C=O groups of xylan [68] that is the main component of hemicellulose.

**Figure 3.** ATR spectrum of wheat bran fibers.
