*2.4. Mechanical Characterization*

Both tensile and dilatometry tests were carried out on ISO 527-1A dog-bone specimens using an MTS Criterion model 43 universal tensile (MTS Systems Corporation, Eden Prairie, MN, USA) testing machine, at a crosshead speed of 10 mm/min, equipped with a 10 kN load cell and interfaced with a computer running MTS Elite Software. Tests were conducted 3 days after the injection molding process and during this time the specimens were stored in a dry keeper (SANPLATEC Corp., Osaka, Japan) at controlled atmosphere (room temperature and 50% humidity).

For tensile tests at least ten specimens were tested and for each blend composition the average values of the main mechanical properties were reported.

Regarding dilatometry, because of the large number of formulated blends, the tests were carried out only for two selected compositions. At least five specimens for each selected material were tested at room temperature and also in this case the tests were carried out after 3 days from the injection molding process. Transversal and axial specimen elongations were recorded, during the tensile test, using a video extensometer (GenieHM1024 Teledyne DALSA camera) interfaced with a computer running ProVis software (Fundamental Video Extensometer). Furthermore, the data in real-time were transferred to MTS Elite software in order to measure not only the axial and transversal strains but also the load value.

The volume strain was calculated, assuming equal the two lateral strain components, according to the following Equation [60–62]:

$$\frac{\Delta V}{V\_0} = (1 + \varepsilon\_1)(1 + \varepsilon\_2)^2 - 1 \tag{1}$$

where the volume variation is Δ*V*, the starting volume is *V*0, *ε*<sup>1</sup> is the axial (or longitudinal) strain and *ε*<sup>2</sup> is the lateral strain.

The impact tests were performed using V-notched ISO 179 parallelepiped specimens on a Instron CEAST 9050 machine (INSTRON, Canton, MA, USA) equipped with a 15 J Charpy pendulum. At least ten specimens for each blend were tested at room temperature. The impact tests were also carried out 3 days after the injection molding process.

To evaluate the energy stored by the sample before the fracture, three-point bending tests were also carried out. In this case, the tests were performed only on the most significant formulations. The already cited MTS universal testing machine was used. The methodology adopted to calculate the fracture energy at the starting point of crack propagation (*JIlim*)

follows the ESIS TC4 load separation protocol [63,64]. According to this protocol, the tests must be carried out at 1 mm/min crosshead speed on 80 mm × 10 mm × 4 mm SENB specimens (that is the same parallelepiped specimen typology adopted for Charpy impact test) cut in two different ways: "sharp" (half notched samples) and "blunt" (drilled in the center with a 2 mm diameter hole and then cut for half width). To obtain the sharp notch (5 mm), during the cutting process, compressed air was used in order to avoid the "notch closing" phenomenon caused by excessive overheating generated by the cutter. A "sacrificial specimen" placed under the "good one" was used to guarantee a correct notch of the sample without closure (qualitatively evaluated with a "passing" paper) and to avoid plastic deformation around it. At least five specimens were tested for each selected blend. Thus the *Jlim* was calculated following the Load Separation Criterion procedure [65–69] for which it is necessary to construct a load separation parameter curve, obtained from the load (named *P*) vs. displacement (named *u*) during the three-point bending tests. The load vs. displacement curves must be obtained for both types of specimens used (sharp and blunt). In fact, in the sharp specimens the fracture is able to propagate, while in the blunt specimens crack growth cannot occur. At this point it must be defined the *Ssb* curve that represents the variation of the load separation parameter and is defined as follows:

$$S\_{sb} = \frac{P\_s}{P\_b} \Big| u\_{pl} \tag{2}$$

where the subscripts *s* and *b* indicate the sharp and the blunt notched specimens, respectively. The plastic displacement is denoted as *upl* and it is expressed as:

$$
\mu\_{pl} = \mu - P \cdot \mathbb{C}\_0 \tag{3}
$$

in which *u* is the total displacement and *C*<sup>0</sup> is the initial elastic specimen compliance. It must be pointed out that for ductile polymers (like those investigated in this paper), fracture initiation is a complex and progressive process that is characterized by the slow development of the crack front across the thickness of fracture transition [20,67,69]. This limit point is the pseudo-initiation of fracture. Thus, once that limit point was defined, the corresponding *Jlim* can be calculated as:

$$J\_{lim} = \frac{\mathbf{2} \cdot \mathbf{U}\_{lim}}{b \cdot (w - a\_0)} \tag{4}$$

where *Ulim* is the elastic behavior limit point, *b* is the sample thickness, *w* is the sample width and *a*<sup>0</sup> is the initial crack length.
