2.4. Mechanical and Thermal Characterization

Tensile, flexural, hardness and impact testz were carried out in order to analyze the mechanical properties. Tensile and flexural testz were carried out with an Ibertest ELIB 30 universal testing machine from S.A.E. Ibertest (Madrid, Spain) at room temperature. A load cell of 5 kN and crosshead speed of 10 mm·min⁻¹ were used. Sample sizes were 150 mm length, 4 mm thickness and 10 mm wide for tensile test as was indicated by ISO 527. Moreover, an axial extensometer from S.A.E Ibertest was employed to measure the tensile modulus with high accuracy. Regarding the flexural and impact tests, the samples sizes were 80 × 10 × 4 mm³. The impact tests were performed using a 1 J Charpy pendulum from Metrotec S.A (San Sebastián, Spain) according to ISO 179. Samples were notched with a “V” at 45° and radius of 0.25 mm. Shore D hardness mesurments were carried out with a model 673-D durometer from Instrumentos J. Bot S.A. (Barcelona, Spain) according to ISO 868. At least five samples were tested to calculate the average values and deviations.

Regarding thermo-mechanical properties, evolution in storage modulus (G’) were assessed by dynamic mechanical thermal analysis (DMTA) in an oscillatory rheometer AR G2 from TA Instruments (New Castle, DE, USA) equipped with a torsion clamp for solid samples. Rectangular sample sizing 40 × 10 × 4 mm³ were tested to a ramp temperature from −50 °C to 100 °C at a constant heating rate of 2 °C·min⁻¹. Frequency tested was 1 Hz and maximum percentage of deformation 0.1%.

Thermal properties of samples were evaluated using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). DSC tests were performed in a mod. 821 DSC from Mettler Toledo Inc. (Schwerzenbach, Switzerland). Samples with a weight of 5–10 mg were evaluated using a heating program from 30 °C to 300 °C at heating rate of 10 °C·min⁻¹ with a constant flow rate of 66 mL·min⁻¹ in nitrogen atmosphere. The percentage crystallinity of Bio-HDPE compositions with CSF was determined by Equation (1):

where ΔHcc and ΔHm represent the crystallization and cold enthalpies, respectively. ΔHm(100%) was the melt enthalpy of theoretically 100% crystalline of Bio-HDPE structure, which value reported in bibliography was 293 J·g⁻¹ [53] and ws was the weight proportion of green composites.

Thermal decomposition studies were carried out in a TGA/SDTA 851 from Mettler Toledo Inc. with a heating program from 30 °C up to 700 °C at 20 °C·min⁻¹ as heating rate in a nitrogen atmosphere with a constant flow rate of 66 mL·min⁻¹. Samples with an average of 10 mg were employed in the TGA evaluations. Both 5% weight loss and maximum degradation temperatures were measured in order to assess the thermal stability of Bio-HDPE with different lignocellulosic filler compositions.