Preparation and Characterization of Clay Nanocomposites of Plasticized Starch and Polypropylene Polymer Blends

Abstract

Plasticized starch (PLS) is a renewable, degradable, and inexpensive polymer, but it suffers from poor mechanical properties. The mechanical properties can be improved by blending PLS with polyolefins, nonetheless, at high PLS content, the mechanical properties remain poor. Here we show that addition of clay can greatly improve the mechanical properties of PLS/polypropylene blends at high starch content. Unmodified and organically modified montmorillonite clays, MMT and Cloisite 30B respectively, were added to blends of glycerol-plasticized starch and polypropylene, compatibilized using maleated polypropylene. TEM indicates that MMT is well dispersed in the PLS phase of the blends, while Cloisite 30B is located both within the PLS phase as well as at the interface between PLS and PP. At high PLS content, the addition of clay increased the tensile strength and tensile modulus by an order of magnitude, while reducing the ultimate elongation only slightly. Such improvements are attributable to both the addition of clay as a reinforcing component, as well as to the change in the two phase morphology due to addition of clay.

Appendix

The XRD patterns (Fig. 5) showed the scattering peaks for dried MMT (left) and 30B (right) were 2θ = 8.2 and 4.8, respectively. These values correspond to d₀₀₁ values of 1.07 nm and 1.83 nm, respectively and agree well with the literature [35]. The larger d₀₀₁ value for 30B arises from the organic modification. Upon swelling in glycerol, the interlayer distance increased for the both MMT (d₀₀₁ = 1.81 nm) and 30B (d₀₀₁ = 2.32 nm). This suggests that glycerol intercalates into the galleries of both clays. Some level of exfoliation cannot be ruled out. Upon extrusion of the glycerol-swollen clays with starch to obtain PLS100-MMT and PLS100-30B the peak position remains essentially unchanged. In summary, from the XRD experiments we can conclude that glycerol intercalates both MMT and 30B clays, and melt extrusion with PLS does not exfoliate the clay any further than glycerol.

Fig. 5

The XRD patterns showed the scattering peaks for dried MMT, MMT-glycerol and PLS100-MMT (left) and dried 30B, 30B-glycerol and PLS100-30B (right)

Figure 6 shows the dynamic mechanical properties of selected blends. Strongly polar polymers, especially with hydrogen bonding have slightly higher glassy moduli [36], and accordingly, the samples containing higher concentrations of PLS, which is more polar than PP, show the highest moduli in the glassy region (−100 °C). Moreover, the effect of clay was negligible in the glassy region. The T_g of all samples (as judged by the peak in tan δ), was near −25 °C and was not significantly affected by the addition of clay. The composition of the samples (i.e. the fraction of the PP phase) also had negligible effect on T_g which likely reflects the complete immiscibility of the phases. In all samples, the modulus increased with increasing PP content in the temperature range from the glass transition temperature to 100 °C. In the PLS70-MMT sample, the addition of clay resulted in a large increase in the storage modulus as temperature is increased, presumably due to the stiffening of the material due to the restriction of chain mobility by the nanoclays. The same was observed for the PLS80-MMT sample. In contrast, the addition of 30B does not increase in the plateau modulus at high temperature significantly.

Fig. 6

DMA of (a) plasticized starch and polypropylene blends, (b) PLS/PP blends with MMT and (c) PLS/PP blends with 30B