**3. Results and discussion**

wrapping 50 g of biomaterials (50 mm major and 10 mm minor axial dimensions) using aluminium foil and the layout was traced on the graph paper. The size of the plastic mould

where *N* is the number of equivalent biomaterials (50 g), *a* = is the surface area of the mould

Based on the expression in Eq. (1), the total surface area of the mould measured was 350 × 180 mm and this was used to cast the thermoplastic solutions into films. The thickness of the dried film, whose moisture content was 4% (db), was determined at the four edges of the films and the average taken. The plastic mould with 8 mm depth gave an average dried thickness of 15.14 ± 0.22 µm, whereas those with 10 and 12 mm depths were 16.21 ± 0.36 and 17.38 ± 0.13 µm, respectively. Twenty-seven samples of the cassava starch-zinc nanocomposite film, obtained

and thickness), were prepared and stored in separate polyethylene bags to avoid subsequent

The nanoindenter was used to determine the rheological properties of the nanocomposite films. A typical profile of the load-displacement curve of the film, obtained from the nanoindenter, is shown in **Figure 1**. The profile shows loading, unloading and holding stages from which other rheological behaviours such as the hardness, Young's modulus and creep were computed (Eqs. 2 and 3). Also, the strain rate sensitivity, which corresponds to creep response of the films, was determined at the holding stage of the profile (stage 2). The elastic and plastic works correspond to the areas under the loading and the unloading stages of the hysteresis

> *max c c*

*<sup>P</sup> <sup>H</sup>*

2 2 11 1 *<sup>i</sup> r i*

*EE E*

*v v*

where *Er* is the reduced modulus (MPa), is the Poison's ratio of the nanocomposite film, which was obtained by assuming that the material is isotropic in nature with the elastic modulus

full factorial experiments (three levels from each of the zinc nanoparticles, glycerol

*TSA Na e* = + (1)

) and *TSA* is the total surface area of the mould

*A h* <sup>=</sup> (2)


), *hc* is the contact depth (nm) and

was computed from the empirical relationship expressed in Eq. (1).

, *e* is the allowance (assumed 600 mm2

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**2.4. Determination of rheological properties of the film**

where *P*max is the maximum load, *Ac* is the contact area (nm2

*H* is the hardness of the nanocomposite film.

mm2

(mm2 ).

from the 33

hydration.

loop [7, 11]:

#### **3.1. Response of cassava starch-zinc nanocomposite film to dynamic loading**

The responses of the thickness and the various concentrations of glycerol and zinc nanoparticles on the rheological behaviour of the cassava starch-zinc nanocomposite films to dynamic loading are shown in **Figures 2**–**10**. It can be seen that the nanocomposite films behave differently under the various glycerol concentration and zinc nanoparticles at 15, 16 and 17 µm. The holding rate sensitivity of the nanocomposite film, which corresponds to the creep behaviour, was more pronounced in the 16 and 17 µm of the 45% glycerol film than the 15 µm counterpart, as shown in **Figure 2**. This may suggests that the strain rate sensitivity of the 15 µm nanocomposite film, which is a quantitative representation of a film's ability to stretch and the amount of energy absorbed when stretched [12], was lower than the other two nanocomposite films with 45% glycerol and 0% zinc nanoparticles formulation. The addition of 1% zinc nanoparticles in the formulation abruptly increased the strain rate sensitivity of the 15 µm film, as can be seen in **Figure 3**. The presence of the nanoparticles in the 15 µm film formulation might have caused an improvement in the ability of the material to withstand dead load, as exhibited by its high holding rate sensitivity in comparison to the other two thicknesses. The responses of the nanocomposite films to dynamic loading were somewhat conspicuous with a further increase in the concentration of the zinc nanoparticles to the matrix of the 45% glycerol films (**Figure 4**). It can be seen that the 15 µm film responded to dynamic loading and strain rate sensitivity than both the 16 and 17 µm films due probably to its lower brittle nature. Sanyang et al. [13] observed similar decrease in the strain energy of biodegradable films based on sugar palm at 45% glycerol concentration. It is possible that the high amount of the dry matter in the matrix of the films, which may be associated with the increased thickness [9], might have been responsible for the higher brittle nature and hence the poor responses to dynamic loading of the 16 and 17 µm films. Interestingly, the responses of the nanocomposite films containing 50% glycerol at the various concentrations of the zinc nanoparticles and thicknesses were slightly different from those containing 45% glycerol with respect to the strain rate sensitivity (**Figures 5**–**7**). For instance, the nanocomposite films responded to dynamic loading almost in the same manner irrespective of their distinct thickness, as shown in **Figure 5**. It is likely that the 10% increment in the glycerol concentration might have led to the improvement in the strain rate sensitivities of the nanocomposite films.

**Figure 2.** Response of the 45% glycerol–0% zinc nanocomposite films of different thickness to dynamic loading causing wear.

**Figure 3.** Response of the 45% glycerol–1% zinc nanocomposite films of different thickness to dynamic loading causing wear.

Nano-Rheological Behaviour of Cassava Starch-Zinc Nanocomposite Film under Dynamic Loading for High Speed... http://dx.doi.org/10.5772/64984 247

loading and strain rate sensitivity than both the 16 and 17 µm films due probably to its lower brittle nature. Sanyang et al. [13] observed similar decrease in the strain energy of biodegradable films based on sugar palm at 45% glycerol concentration. It is possible that the high amount of the dry matter in the matrix of the films, which may be associated with the increased thickness [9], might have been responsible for the higher brittle nature and hence the poor responses to dynamic loading of the 16 and 17 µm films. Interestingly, the responses of the nanocomposite films containing 50% glycerol at the various concentrations of the zinc nanoparticles and thicknesses were slightly different from those containing 45% glycerol with respect to the strain rate sensitivity (**Figures 5**–**7**). For instance, the nanocomposite films responded to dynamic loading almost in the same manner irrespective of their distinct thickness, as shown in **Figure 5**. It is likely that the 10% increment in the glycerol concentration might have led to the improvement in the strain rate sensitivities of the nanocomposite films.

246 Composites from Renewable and Sustainable Materials

**Figure 2.** Response of the 45% glycerol–0% zinc nanocomposite films of different thickness to dynamic loading causing

**Figure 3.** Response of the 45% glycerol–1% zinc nanocomposite films of different thickness to dynamic loading causing

wear.

wear.

**Figure 4.** Response of the 45% glycerol–2% zinc nanocomposite films of different thickness to dynamic loading causing wear.

**Figure 5.** Response of the 50% glycerol–0% zinc nanocomposite films of different thickness to dynamic loading causing wear.

**Figure 6.** Response of the 50% glycerol–1% zinc nanocomposite films of different thickness to dynamic loading causing wear.

**Figure 7.** Response of the 50% glycerol–2% zinc nanocomposite films of different thickness to dynamic loading causing wear.

**Figure 8.** Response of the 55% glycerol–0% zinc nanocomposite films of different thickness to dynamic loading causing wear.

**Figure 9.** Response of the 55% glycerol–1% zinc nanocomposite films of different thickness to dynamic loading causing wear.

**Figure 10.** Response of the 55% glycerol–2% zinc nanocomposite films of different thickness to dynamic loading causing wear.

**Figure 7.** Response of the 50% glycerol–2% zinc nanocomposite films of different thickness to dynamic loading causing

**Figure 8.** Response of the 55% glycerol–0% zinc nanocomposite films of different thickness to dynamic loading causing

**Figure 9.** Response of the 55% glycerol–1% zinc nanocomposite films of different thickness to dynamic loading causing

wear.

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wear.

wear.

The addition of 1% zinc nanoparticles might have caused the nanocomposite films to behave differently (**Figure 6**), with the 16 µm film responding inconsistently to dynamic loading with a further increase in the concentration of the nanoparticles (**Figure 7**). The addition of 1% zinc nanoparticles to the nanocomposite films containing 55% glycerol does not show any improvement from those with 0% of the nanoparticles, as shown in **Figures 8** and **9**. An improvement was noticed when 2% zinc nanoparticle was used in the film formulation; where the strain rate sensitivity was considerably lower (**Figure 10**). As pointed out previously, here again the 15 µm film responded considerably to dynamic loading than the other two counterpart films. Tall et al. [11], in a similar research noted the response of Ni-Ti thin films to dynamic loading and unloading with distinct strain rate sensitivity. Additionally, the strain energy released during the loading part of the hysteresis loop is in the range of 40–190 pNm, and this varies with thickness and glycerol concentration (**Figures 2**–**10**). In all cases, the energy dissipated was recovered into the system during the unloading part of the loop. Also, the energy dissipated by the nanocomposite films was lower than those with 0% zinc nanoparticles, irrespective of the concentration of the glycerol but decreases with the thickness of the films. This corroborates the works of Tall et al. [11] who observed regions of loading and unloading of the hysteresis loop of the Ni-Ti thin films. Also, regions of loading and unloading were observed in Cu2O thin films [7]. This behaviour might be associated with the increased displacement of the film with a slight change in the applied load as the concentration of the glycerol and zinc nanoparticles increased. The rheological properties are important parameters for the formation, application and quality of the films. The measure of wear and tear of the films, and its knowledge informs the engineer on the choice of appropriate films to withstand high mechanical load at nanoscale, especially during transportation on rough roads.

#### **3.2. Effects of the processing parameters on rheological properties of the films**

The effects of the processing parameters (thickness, zinc nanoparticles and glycerol concentration) on the rheological properties of the cassava starch nanocomposite films are shown in **Figures 11** and **12**. It can be seen that the hardness and Young's modulus varied inconsistently with the concentrations of glycerol and zinc nanoparticles at 15, 16 and 17 µm thickness (**Figure 11**). This implies that an increase in the applied load does not necessarily reduce the area of cross-section due to the influence exerted by the zinc nanoparticles and the glycerol on the crystalline lattice of the films. The elastic and plastic works, which determines the plasticity indices of the nanocomposite films, decreased generally with the thickness, zinc nanoparticles and glycerol concentration (**Figure 12**). Higher plasticity index was obtained for 15 µm film than 16 and 17 µm films irrespective of the concentrations of glycerol and zinc nanoparticles. It is likely that the high amount of the dry matter in the matrix of the films, which is associated with the increased thickness [9], might have been responsible for the decreased elastic and plastic works of the 16 and 17 µm films. Jorge et al. [14], who studied the mechanical properties of gelatine nanocomposite films and investigated the effect of montmorillonite concentration on the properties, corroborated our findings. The authors revealed that the hardness and Young's modulus of the films were inconsistent with montmorillonite concentration, thus indicating the reinforcement of the film matrix by the nanoparticle. The lower values of plasticity indices of the films might also be associated with the plasticising effect of the absorbed glycerol during formulation. Thus, the zinc nanoparticles can be said to enhance the rheological properties, particularly stress and Young's modulus for high speed packaging application [15].

**Figure 11.** Effect of experimental variables on creep and Young's modulus of the films.

Nano-Rheological Behaviour of Cassava Starch-Zinc Nanocomposite Film under Dynamic Loading for High Speed... http://dx.doi.org/10.5772/64984 251

**Figure 12.** Effect of experimental variables on the elastic and plastic works of the films.

There is a correlation between the elastic work done during dynamic loading and the viscoelasticity of the nanocomposite films [16–18]. Thus, since the elastic work is affected by the concentration of glycerol and thickness of the film formulation, the viscoelastic behaviour of the nanocomposite film might as well be influenced. It is possible that the increased thickness of the nanocomposite films, which can affect their flexibility, might be associated with their poor viscoelasticity. Therefore, in order to adapt to the possible deformation or wear, occurring especially on rough roads [19, 20], the viscoelasticity of the nanocomposite films must be high enough so that the elastic work done during the dynamic loading is subsequently recovered in the unloading part of the cycle.

#### **4. Conclusions**

**Figures 11** and **12**. It can be seen that the hardness and Young's modulus varied inconsistently with the concentrations of glycerol and zinc nanoparticles at 15, 16 and 17 µm thickness (**Figure 11**). This implies that an increase in the applied load does not necessarily reduce the area of cross-section due to the influence exerted by the zinc nanoparticles and the glycerol on the crystalline lattice of the films. The elastic and plastic works, which determines the plasticity indices of the nanocomposite films, decreased generally with the thickness, zinc nanoparticles and glycerol concentration (**Figure 12**). Higher plasticity index was obtained for 15 µm film than 16 and 17 µm films irrespective of the concentrations of glycerol and zinc nanoparticles. It is likely that the high amount of the dry matter in the matrix of the films, which is associated with the increased thickness [9], might have been responsible for the decreased elastic and plastic works of the 16 and 17 µm films. Jorge et al. [14], who studied the mechanical properties of gelatine nanocomposite films and investigated the effect of montmorillonite concentration on the properties, corroborated our findings. The authors revealed that the hardness and Young's modulus of the films were inconsistent with montmorillonite concentration, thus indicating the reinforcement of the film matrix by the nanoparticle. The lower values of plasticity indices of the films might also be associated with the plasticising effect of the absorbed glycerol during formulation. Thus, the zinc nanoparticles can be said to enhance the rheological properties, particularly stress and Young's modulus for high speed packaging

**Figure 11.** Effect of experimental variables on creep and Young's modulus of the films.

application [15].

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The dynamic behaviour of nanocomposite film, which is associated with viscoelastic behaviour, has not being well understood and characterised. This has often caused wears and consequent damages of the packaging materials, particularly during the handling and transportation of food. Nanoindentation provides a convenient tool for probing the basic rheological properties and the extent of stress-induced phase transition in small volumes of nanocomposite materials. Thus, this research was carried out to determine the nano-rheological behaviour of cassava starch-zinc nanocomposite films under dynamic loading for high speed transportation of packaged food. The films, whose dried thicknesses were in the range of 15–16 µm, were prepared by casting mixtures of 24 g of cassava starch, 45–55% w/w of glycerol and 0–2% w/w of zinc nanoparticles. The responses of cassava starch-zinc nanocomposite films to dynamic loading were determined, thus assessing their suitability for high speed packaging application. The increase in the concentrations of glycerol (>45%) and zinc nanoparticles (>1%) in the formulation might be responsible for the increased strain rate sensitivity of the 15 µm film. The hardness and Young's modulus of the films varied inconsistently with the concentrations of glycerol and zinc nanoparticles at 15, 16 and 17 µm thickness, because of the increased displacement with a slight change in the applied load. The strain energy absorbed was lower for the 15 µm film, which absorbed 40 pNm during the loading part of the hysteresis loop, than for the 16 and 17 µm films. Also, only 0.5 pNm of the strain energy was finally dissipated during the unloading part of the loop. The creep response of the 15 µm film makes it viscoelastic enough to withstand death loads or wear at higher loads. Consequently, the film can be used for the high speed packaging of food and food products.
