*4.2. Composites Results*

Tensile tests results reported in Table 2 and Figure 4 evidenced that by increasing the bran content, a decrease in yield stress, stress at break and elongation at break was observed. As it can be expected, the bran addition caused a matrix embrittlement. This behavior is in line with literature papers on similar biocomposites [37,69,70]. The Triacetin addition was revealed a valuable approach to contrast the embrittlement caused by the bran fibers addition. At this purpose, it can be observed that up to 20 wt.% of bran content the elongation at break is still high (about 50%); also with 30 wt.% of Bran, despite the sharp drop down of elongation at break due to the high bran amount, the final elongation at break is still higher (17.3%) if compared to pure PLA (about 4% [6]).


**Table 2.** Tensile data of PLA/PBSA plasticized blends.

The decrease in the stress at break with bran content is ascribable to a low adhesion between the matrix and the fibers that causes an ineffective transmission of the load from the matrix to the fibers.

The application of the analytical models for stress at break (Figure 5a) confirms that low adhesion exists between the wheat bran and the polymeric matrix. The experimental data lies exactly between the lower and upper bound confirming the good applicability of the analytical models to the analyzed system.

(**a**) (**b**)

**Figure 4.** Stress-strain representative curves for plasticized PLA/PBSA composites.

**Figure 5.** (**a**) Comparison between the experimental composites strength and the values predicted according to the analytical models illustrated in Section 2.1. Volumetric fractions are derived from the weight fractions calculated by using the density of each component taken from data sheets; (**b**) Pukánszky's plot for PLA/PBSA plasticized composites.

The poor adhesion is also confirmed by the B value obtained from the Pukánszky's plot (Figure 5b) where a *B* value equals to 2.23 was obtained. Being the B value obtained greater than 2.04, the Equation (5) was used for the *IFSS* estimation and the value, reported in Table 3, was also compared to other similar composites systems containing natural fibers. The IFSS value obtained can be retained acceptable because it is inferior to its maximum theoretical threshold value achievable calculated by the Von Mises relationship [71] (*τ* = *σm*/ √3) equal to 11.78 MPa; furthermore the IFSS value obtained is comparable to similar bio composites described in literature.

**Table 3.** IFSS obtained for the composites system analyzed compared with literature data of similar systems.


The addition of bran fibers causes a decrement of the storage modulus (Figure 6) that is similar to the decrement of the elastic modulus observable in Table 2. This decrement can be ascribed to the slight PLA chain scission induced by the bran addition that, as all natural fibers is sensitive to moisture, and caused the slight elastic modulus decrement and the shift, towards a lower temperature, of the PLA glass transition temperature [75,76]. Independently from the bran content, a shift of the PLA *tanδ* peak (corresponding to the PLA glass transition temperature) of about 5 ◦C is observed; while for PBSA no deviation in its *Tg* (registered at around −44 ◦C) has been detected.

**Figure 6.** Variation of the storage moduli (E') (**left side**) and damping factors (tan δ) (**right side**) with temperature for PLA/PBSA plasticized bran composites.

The height of the tanδ peak is similar for all the composites compositions. However, this height is lower if compared to the *tanδ* height of the matrix. This decrement is due to the introduction of the bran fibers that reduces the molecular mobility of the polymeric matrix [77]. This reduction of the molecular mobility is strictly correlated to the fiber-matrix adhesion: a good adhesion in fact is responsible of a major molecular mobility reduction. The adhesion factor (reported in Table 4) increases with the bran content indicating that the adhesion worsens increasing the fibers content. This result is in accordance to what

was found in literature for other similar systems where a decrement of the interfacial shear stress with the increase of the volume fiber content was registered [27,78,79]. The difference in the high hydrophilic nature of the bran and the mainly hydrophobic nature of the matrix can be the main reason of the observed poor adhesion.


**Table 4.** Bran composites adhesion factor calculated according Equation (8) at room temperature (25 ◦C).

It is also worthy to notice, from the adhesion factor versus temperature illustrated in Figure 7, that the adhesion factor seems to be very sensitive to *Tg*. Throughout the temperature range analyzed, the adhesion factor increases with the bran amount. This trend is a further confirmation that, at 30 wt.% of bran content, a net decrement of the mechanical properties of the composite occurs due to the slows down of the fiber-matrix interfacial adhesion. Moreover, the adhesion factor in correspondence of the PLA glass transition temperature suddenly increases reaching its maximum value. A similar behavior can be found in literature for other composite systems [57,80] and is ascribed to the higher mobility of the polymeric chains. Above the glass transition the difference in A value between the composites with different bran content are less evident than below the glass transition. Another increment of the adhesion factor, observable by another slight peak, is registered in correspondence of the PLA cold crystallization temperature (at about 90–110 ◦C [81]) for which the reorganization of the PLA polymeric chains caused another change in polymeric chains mobility, that is decreased due to the occurrence of crystals formation.

**Figure 7.** Adhesion factor vs. temperature for plasticized PLA/PBSA bran composites.

The influence of the presence of the wheat bran filler on the fracture of the material was also investigated with the aim of integrating it with the studies of interactions and adhesion.

The load displacement curves Figure 8a–d show a common behavior in the range of the ligament lengths examined (from 1.5 to 3.5 mm). As ligament lengths decreases, the maximum load decreases. Once the maximum has been reached, the materials undergo to a drop down of the load. By increasing the bran content, the load dropping is more marked and smaller displacement values were recorded. The fracture occurs suddenly after that the maximum load is reached and the introduction of the bran particles decreased the material ductility in accordance to the tensile test results.

**Figure 8.** Load displacement curves of plasticized PLA/PBSA blend with and without bran at different ligament lengths.

The plot of the specific total work of fracture versus ligament length for the different blend compositions is shown in Figure 9 while the results of the EWF parameters are reported in Table 5. A good linear relationship between *wf* and *l* can be observed from the regression coefficients that lies in the range of 0.81 and 0.97 allowing the use of the calculated *we* and *βwp* parameters. Comparing the data of fracture parameters emerges that both the essential work of fracture and non-essential work of fracture decreases with the bran amount. Since *we* involves both the plastic deformation process of the necking ligament section and the work needed for the cracks to start growing [82], the maximum *we* value will be reach for the material having the highest yield stress. This trend is confirmed, in fact increasing the bran content, a decrease of the yield stress was registered at which corresponds also a decrease of the essential work of fracture. The *βwp* increases with the material ductility and it decreases with the yield stress similarly to what was found by Arkhireyaya et al. [83] and in accordance to tensile test results. The lowest value of the essential work of fracture is registered for PLA\_PBSA\_TA\_30 composite where the poor adhesion and the probable presence of agglomerates lowers abruptly the fracture energy.

**Figure 9.** Specific total work of fracture wf versus ligament length l for plasticized PLA/PBSA blend with and without bran.


**Table 5.** EWF fracture parameters for PLA/PBSA blends with and without bran.

The bran particles act as a stress intensification factor, reducing the cross section of the material and leading to a decrease of the tensile properties and fracture resistance. Reasonably because of the high dimension and low aspect ratio, the mechanism of pull out (that generally leads to a toughness improvement even in the case of natural fibers reinforcement [84]) is not beneficially impacting the fracture resistance of these composites. Similar results were observed by Anuar et al. for composites with Kenaf fibers [85]. Thus, data about essential work of fracture are not largely available in literature, a systematic comparison with different natural fibers is currently difficult.

SEM images (Figure 10) confirm the results. At 30 wt.% of bran (Figure 10C) the detachment from the matrix around the bran fibers becomes more marked. There is some fiber/matrix adhesion (although is not optimal) as evidenced by the magnification at 10 000X, for the composites containing 10 wt.% of bran (Figure 10A1). At 20 wt.% of bran the adhesion starts to worsening (Figure 10B1) the detachment of the bran fiber from the matrix, is present only in some regions of the fibers contours while in another there is continuity between the fiber contour and the matrix. At 30 wt.% of bran (Figure 10C1) an almost complete fiber detachment can be observed. Increasing the fiber content also increases the likelihood of finding fibers agglomerates that contribute to the decrement of the stress at break and to the essential work of fracture.

**Figure 10.** SEM micrographs of cryogenic fractured cross-section for: (**A**,**A1**) PLA\_PBSA\_TA\_10; (**B**,**B1**) PLA\_PBSA\_TA\_20; (**C**,**C1**) PLA\_PBSA\_TA\_30.
