Effect of compatibilization on polyethylene/thermoplastic starch blends

29 August 2013
Saeed Mortazavi, Ismail Ghasemi, and Abdulrasoul Oromiehie
Reactive mixing of low-density polyethylene and thermoplastic starch increases the viscosity of the resulting material owing to formation of a network of dispersed phase droplets.

One of the most important issues to emerge in recent years is how to ensure a sustainable world. Polymeric wastes pose a major threat to the environment because of their non-degradability and resistance to micro-organisms. The time it takes many synthetic polymers to fully decompose is estimated to be around 500 years, and during the long interim natural phenomena may be influenced by the presence of these materials. Consequently, numerous attempts have been made to devise new ways of producing green polymers, especially from natural sources such as starch. An additional driver is the high price of the oil used as raw material in making conventional plastics.

Starch is a renewable, inexpensive, and biodegradable polymer. Production of thermoplastic starch (TPS) usually employs gelatinization, which causes the structure of starch granules to destruct when subjected to shear and high temperature in the presence of plasticizers such as water and glycerol. However, TPS is very moisture sensitive and has poor mechanical properties. These drawbacks can be mitigated by blending it with other polymers, such as polyolefins. For example, polyethylene (PE) enjoys widespread use in nonbiodegradable polymers—in particular in packaging and agricultural films—and many attempts have focused on blending PE and starch to enhance the biodegradability of the polymer. PE-grafted-maleic anhydride (PE-g-MA) is the most common compatibilizer for low-density polyethylene (LDPE)/TPS blends. The maleic anhydride groups of PE-g-MA react with the hydroxyl groups of starch, while the PE chains interact with the PE matrix.

Compatibilization is performed in the melt state, which may affect the rheological (flow) properties of the blends. Although many studies report the effect of compatibilizers on the morphology and mechanical properties of blends, less attention has been paid to their effect on rheological properties. Yet these are important owing to the low processability of thermoplastic starches and their blends. In addition, most previous studies were done under oscillating conditions and small-amplitude deformation, whereas in the real world these blends are subject to large deformations.1 Here, we describe investigations into the morphological and rheological properties of LDPE/TPS/LDPE-g-MA blends with differing composition.2

We dried commercially obtained wheat starch, LDPE, and LDPE-g-MA in an oven at 60°C for 24h. We prepared TPS by gelatinizing starch granules with glycerol, and then blending the result with LDPE and LDPE-g-MA. We readied samples for rheological and morphological tests by compression-molding them at a temperature of 160°C and pressure of 25MPa. For morphological analysis, the samples were 1mm-thick rectangular bars, and for rheological tests, they were 2mm-thick cylindrical pieces of 25mm diameter.

Prior to morphological analysis, we cryogenically fractured the samples in liquid nitrogen. We then etched them at room temperature with 6N HCl (hydrogen chloride) solution for 12h to extract the TPS phase. We washed the etched samples with water, oven-dried them for 24h, and coated them with gold before investigating them by scanning electron microscopy (SEM) using a Vega Tescan model microscope. We used ImageJ software to analyze the size of the particles. We characterized the morphological parameters of each sample based on examination of at least 200 particles.

All rheological tests were carried out with an Anton Paar Physica shear rheometer, using parallel plate geometry with a plate diameter of 25mm. We set the gap between the plates to 1mm. We performed strain sweep tests at a constant frequency of 1rad/s to ensure that the dynamic tests were done in a linear viscoelastic region (i.e., where the structure of the sample does not change due to small deformations). We set the shear strain amplitude to 0.3% for frequency sweep tests. The angular frequency range was 0.02–600rad/s. Tests of startup (i.e., transient) shear flow were performed at 130°C, the shear rate was set at 0.2s−1, and the gap between the plates was 1.2mm.

Under oscillating conditions, we observed the complex viscosity of TPS, LDPE, and their blends at a range of angular frequencies (see Figure 1). Reactive blending of TPS and LDPE with LDPE-g-MA enhances viscosity, especially at low frequencies.3, 4 This behavior implies the existence of an elastic network structure originating from clustering of droplets of TPS. In contrast, in the uncompatibilized blend, the interface of TPS and LDPE is very weak. Consequently, interlayer slip can occur, which considerably reduces viscosity.5


The complex viscosity of low-density polyethylene/thermoplastic starch (LDPE/TPS) blends. The starred number relates to the blend without the TPS compatibilizer.

We performed scanning electron microscopy to capture images of the droplet-matrix morphology of LDPE/TPS blends for TPS content of 20, 35, and 60wt%, where LDPE is the matrix (see Figure 2). TPS is highly elastic, which at higher concentrations (75wt%) prevents droplets from breaking and forming smaller droplets, and the structure becomes co-continuous. This elasticity is the result of a crystalline network that forms through complexation of amylose and lipid molecules and the physical entanglement of long chains of starch.6–8


Scanning electron microscopy images of the LDPE/TPS blends. The numbers indicate the weight percentage of TPS.


The transient shear stress of (top) LDPE and TPS individually, and (bottom) LDPE/TPS blends. The starred number relates to the blend without the compatibilizer.

Figure 3 shows the transient shear flow stress ratio of the neat components and their blends. The transient shear stress of LDPE alone (top) shows no overshoot—i.e., peaking before plateauing—because of its short relaxation time. On the other hand, TPS does show a strong stress overshoot at the beginning of the test that may be due to its elastic structure. The compatibilized blends (bottom), too, show significant overshoot in transient experiments due to strong networking, whereas uncompatibilized blends exhibit no overshoot and reach steady-state rapidly. Overall, these results indicate that formation of a strong network between TPS droplets increases the viscosity of the system.

In summary, reactive compatibilization with PE-g-MA results in a networked structure between TPS droplets that increases the viscosity and elasticity of LDPE/TPS blends. Moreover, rheology is a satisfactory method of investigating blend structure. These findings have implications for the development of biodegradable polymers. In a next step, we plan to study the effect of nanoclays on the rheological and morphological properties of LDPE/TPS blends.


Authors

Saeed Mortazavi
Plastic Department Iran Polymer and Petrochemical Institute

Ismail Ghasemi
Plastic Department Iran Polymer and Petrochemical Institute

Abdulrasoul Oromiehie
Plastic Department Iran Polymer and Petrochemical Institute


References

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DOI:  10.2417/spepro.004914