Direct calcination 64

The content of glass fiber reinforcement in the PP 20GF samples was carried out by the determination of ash through the direct calcination method, following the guidelines set by the ISO 3451-1 standard [195]. Different portions (around 1 g) from the cylindrical bars and square plates (Figure 3.5) were pre-dried at 80ºC and vacuum conditions for 1 hour in a desiccator Selecta (JP Selecta SA, Spain). The sample in the crucible was weighed and then heated on a burner slowly until volatile products were driven off. After that, the crucible was introduced in a muffle furnace Selecta 367 PE model preheated at 600 ºC, and it was calcined until reaching constant mass. Finally, the crucible was placed in the desiccator and cooled until reaching room temperature, and weighed again in an analytical balance with an accuracy of ± 1 mg. The resulting ash, corresponding to fiber content, is given by the Equation (3.5):

% 𝐹𝑖𝑏𝑒𝑟𝑠 = 𝑚1

𝑚₀ (3.5)

Where m0 is the mass, in grams, of the dried test portion and m1 the mass of the ash

3.4. Mechanical characterization

The mechanical properties of solid and foamed samples were assessed through tensile, flexural and impact tests. For all materials and types of tests, the specimens were machined out of the cylindrical bars or plates according to the schemes shown in Figure 3.8, ensuring the correspondence between the tested section and the morphology previously analyzed. At least five samples of solid and foamed materials were tested under room temperature for each position and direction.

3.4.1. Tensile properties

Tensile tests were made on samples of 4 and 5 mm in diameter and a length of 77 and 110 mm extracted from the cylindrical bars (Figure 3.8a)). The specimens were tested in a universal testing machine Zwick/Roell Amsler HC25/2008 (Zwick GmbH & Co. KG, Germany) equipped with a 5 kN load cell, at a crosshead speed of 50 mm min-1 and an initial distance between clamps of 40 mm and 50 mm, respectively.

Additionally, type 5A specimens indicated in the ISO 527-2 standard [196] (Figure 3.8c)) and tooled from the rectangular plates were employed to compare the properties of MuCell® and IQ Foam® foams. Tests were carried out on a universal testing machine Zwick/Roell Z010 (Zwick GmbH & Co. KG, Germany) using a 10 kN load cell, at a crosshead speed of 50 mm min-1 and an initial distance between clamps of 72 mm.

Following the recommendations given by the ISO 527-1 standard [197], stresses (yield strength σy and break stress σu, MPa) were calculated according to Equation (3.6):

𝜎 = 𝐹

𝐴 (3.6)

Where F is the measured force (N) and A is the nominal cross-sectional area of the samples (mm2).

Strain values (yield strain εy and elongation at break εu, %) were determined by means

of the initial gauge length L0 (mm) and the change in the gauge length ΔL0 (mm):

𝜀 = 𝛥𝐿0 𝐿0

Figure 3.8. Schematic representation of samples extracted for mechanical characterization and HDT tests from a) cylindrical bars; b) square plates; c) rectangular plates.

Finally, the elastic modulus (Et, MPa) was obtained as follows:

𝐸𝑡=

𝜎_𝑡2− 𝜎_𝑡1

𝜀_𝑡2− 𝜀_𝑡1 (3.8)

Where σt1 is the stress measured at a strain of εt1 = 0.0005 and σt2 is the stress measured

at a strain of εt2 = 0.0025. When this method did not work accurately, the elastic modulus was

calculated by linear regression applied to the elastic portion of the stress - strain curve, dividing the difference in stress (dσ) corresponding to any segment of section on this straight line by the corresponding difference in strain (dε):

𝐸_𝑡= 𝑑𝜎

𝑑𝜀 (3.9)

3.4.2. Flexural properties

Flexural tests were carried out in 100x10 mm2 _{(length x width) specimens machined out}

of the square and rectangular plates (Figure 3.8). Experiments were done following the ISO 178 standard [198], in a Galdabini Sun 2500 (Galdabini SPA, Italy) testing machine equipped with a 5 kN load cell, at a crosshead speed of 10 mm min-1. According to the aforementioned standard, the span length (S) was (16±1)xh, where h is the thickness of the samples.

Flexural strength (σf, MPa) and flexural strain (εf, %) at failure were calculated as

follows: 𝜎_𝑓= 3𝐹𝑆 2𝑏ℎ2 (3.10) 𝜀𝑓 = 6𝑠ℎ 𝑆2 x 100% (3.11)

Where F (N) and s (mm) are the measured force and deflection, S (mm) is the span length, and b and h (mm) are the width and thickness of the specimen, respectively.

The flexural modulus (Ef, MPa) was determined by means of σf1, which is the stress

measured at a strain of εf1 = 0.0005, and σf2 as the stress measured at a strain of εf2 = 0.0025:

𝐸_𝑓= 𝜎𝑓2− 𝜎𝑓1

The linear regression method (Equation (3.9)) was also employed to calculate Ef when

the application of this procedure was not possible. 3.4.3. Impact properties

Charpy impact tests were made on 80x10 mm2 (length x width) unnotched samples in flatwise configuration (Figure 3.8). The impact tests were carried out using an instrumented Ceast Resil impactor (Instron Ltd., UK), equipped with a 15J hammer. The pendulum had a length of 0.374 m and a reduced mass of 3.654 kg. It was impacted at an angle of 99º, resulting in an impact rate of 2.91 m s-1. The span length was 62 mm. Force, displacement, energy and time values were recorded by a data acquisition system DAS-1600 from Ceast. According to ISO 179-2 standard [199], the impact resistance (acU, kJ/m2) was calculated

following Equation (3.13):

𝑎_𝑐𝑈 = 𝐸𝑐 ℎ𝑏 x 10

3 _(3.13)

Where Ec (J) is the energy absorbed during the test, and b and h (mm) are the width and

thickness of the sample, respectively. 3.4.4. Thermal aging

Polymer degradation can occur due to different causes, such as atmospheric agents, heat, ultraviolet light or radiant energy absorption. Automotive parts must keep their properties beyond certain limits over the service life of the vehicle. The aging effect was performed by exposing PP 20GF solid and foamed square plates at 150 ºC for periods of 200, 300, 400 and 500 hours in a TU 60/60 heating and drying oven (Vötsch Industrietechnik GmbH, Germany). The change in properties was assessed by flexural and impact tests, as described in the respective sections above.

3.5. Fracture characterization

The fracture behavior of the solid and foamed square plates was characterized at low loading speed by the Crack Tip Opening Displacement (CTOD), as well as at high testing rate by the fracture toughness (KIc). In order to relate the fracture properties with the cell structure

determined by morphology analysis, the same distances from the injection gate and orientations were employed. All fracture tests were performed at room temperature and the

notch was sharpened by sliding a razor blade. At least five specimens were tested for each material, orientation and loading speed.