Fig. 2 presents the thermal behavior of the starch–gelatin polymer matrix for the extruded pellets and the parison form. After extrusion, the peak around 233 °C decreased and the thermal stability of the material decreased by about 10 °C for the

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Fig. 1. Derivative of the TGA curves for the raw materials and the matrix with cellulose.

Fig. 2. Derivative of the TGA curves for the starch–gelatin polymer matrix after extrusion (pellet) and after blow molding (parison) with and without cellulose.

polymer matrix without reinforcement. This could happen due the fragmentation that occurred for shear and stress conditions of extrusion process in starch-based materials [6]. On the other hand, the peak around 360–400 °C disappeared after extrusion, as well as the transition between 500 and 600 °C. In all cases, the material coming from the parisons has a

similar behavior than the pellets indicating that no further degradation occurred in the starch–gelatin polymer matrix in the extrusion step and it should be possible to blow the material.

A widely used materials in the packaging industry are poly ethylene terephthalate (PET), high density polyethylene (HDPE) and polypropylene (PP). Authors have reported decomposition onset temperature of PET at 377 °C, and the beginning of volatile emissions at 300 °C [49]. In the same way decomposition peak of HDPE has been reported around 471.1 °C [50] and  for PP over  448 °C  [51].  All these  polymers presented  a  higher thermal stability than starch–gelatin polymer    matrix,

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hence processing natural polymers in equipment designed for conventional plastics, shear and temperature control in important to avoid thermal damage into the    material.

3.2. Humidity

Humidity of the samples increased with cellulose addition, taking longer time as shown in Fig. 3. Starch–gelatin polymer matrix reached an equilibrium at 33 min, while the starch–gelatin with cellulose took up to 50 min, meaning that this moisture is bound water in the material.

Increase in moisture due to cellulose has been previously reported in lignocellulosic materials reinforced with fibers [38,52]. Gilfillan et al. [12] reported that higher relative humidity lowered the storage modulus of starch films; i.e. moisture content is  an important parameter to control for dynamic  mechanical   studies.

Rodríguez et al. [52] also reported a reduction in the glass transition temperature of polylactic acid composites with sisal fiber increasing moisture. Overall, moisture content in the composites can be reduced chemically by treating the fibers to decrease their hygroscopicity because moisture absorption directly affects the mechanical properties of the material [53].

3.3. Dynamic mechanical analysis

Both temperature and deformation (stresses) are factors that can degrade the molecular chains of natural polymers. So these  parameters  must  be  known  to  obtain  a  parison  suitable  for  blow  molding.  Processing  starch-based  materials  at temperatures close to 100 °C causes some problems due to viscosity increase related to gelatinization. Also, the addition of lignocellulosic materials in starch-based materials decreases thermal stability [12,48]. For these reasons, dynamic mechanical analysis was used to determine the viscoelastic behavior of natural starch–gelatin blends as a function of temperature and determine some possible processing condition related to the viscoelastic properties of the materials.