2. Experimental

Hydrolyzed corn starch–gelatin pellets, with or without 5% cellulose (Whatman®) were processed using a single screw extruder. The cellulose content was limited at 5% to avoid agglomeration and to obtain good tensile characteristics  according to previous results reported for the work group for similar materials and same polymer matrix [44,45]. After some prelim- inary trials, the extruder (L/D ratio of 20:1) was operated at a temperature profile of 35–40–50 °C and 30 rpm to produce the compounds before being pelletized (die diameter = 3 mm). The samples for characterization were produced by ompression molding (Carver Mini C press, Wabash, USA), with 2 tons of force for 10 min at 100 °C. The samples were then cut form the plates (115 × 115 × 3 mm) for analysis.

Moisture content was determined using a thermobalance (MX-50) operated at 100 °C to record mass as a function of time.

Dynamic mechanical analysis (DMA) was performed on a TA Instruments RSA3 (New Castle, USA). A three-point bending geometry was used with rectangular samples having dimensions of 40 mm × 25 mm × 1.35 mm. Temperature ramps were performed between 35 and 120 °C at a rate of 2 °C/min. From the data obtained, the storage modulus (E0 ) associated with the elastic properties (stiffness) and the loss modulus (E00 ) associated with the viscous properties (energy dissipation) were determined [46]. Also, the loss factor or Tand, which represents the ratio E00 /E0 , was used (especially the presence of a peak) to  characterize  the samples.

Thermogravimetric analysis (TGA) was made on a TA instruments Q5000 (New Castle, USA) between 50 and 800 °C at a heating rate of 10 °C/min using platinum crucibles. All the measurements were done in a nitrogen environment.

Extrusion blow molding was carried out on a Vulcano machine (Mexico), with L/D ratio of 20 and a 30 mm vertical die. Screw compression was 17° as the thread angle and the mold shape was a bottle of 500 mL volume. Mold closing and opening, as well as its horizontal displacement, were performed automatically through a control panel.

Scanning electron microscope (Philips X130 ESEM, Eindhoven, Holland) was used to examine blown materials, by an ESEM XL-30 software and micrographs were taken  at   1000×.

Finally, the extrusion blow molded specimens were cut to get samples to perform tensile mechanical testing. Measure- ments were performed following ASTM D638 [47] for films with 10 repetitions using a Texture Analyzer TA-XT2® (Texture

Technologies Corp., Scarsdale, NY/Stable MicroSystems, Haslemere, Surrey, UK) with 1 mm/s as the strain rate.

3. Results and discussion

3.1. Thermogravimetric analysis

From the TGA analysis, the derivative of the curves were calculated and presented in Fig. 1. For the hydrolyzed corn starch, degradation mostly occurs around 287 °C, while for cellulose the peak is close to 345 °C. For gelatin, several transi- tions are observed. The first at 95 °C is associated with water content. The second transition at 285 °C corresponds to long chains breaking, while at 590 °C denaturation occurs. For the starch–gelatin polymer matrix reinforced with cellulose, a peak at 125 °C was associated to the glycerol content in the matrix, while the peaks at 220 °C, 233 °C, and 281 °C were associated to the degradation of, starch, and cellulose, respectively. Finally, gelatin degradation continued up to around 600 °C. The addition of cellulose in a starch–gelatin polymer matrix exhibited a thermal behavior that was previously reported in the literature [12,48]. From these curves, it can be concluded that the most suitable temperature to process the starch–gelatin polymer matrix reinforced with cellulose is less than 120 °C to avoid thermal degradation.