Thermoplastic Starch Polymer Blends and Nanocomposites

Chapter 19

Downloaded by STANFORD UNIV GREEN LIBR on April 24, 2013 | http://pubs.acs.org Publication Date (Web): August 16, 2012 | doi: 10.1021/bk-2012-1105.ch019

Thermoplastic Starch Polymer Blends and Nanocomposites C. M. Chaleat,1 M. Nikolic,2 R. W. Truss,3 I. Tan,3 S. A. McGlashan,3 and P. J. Halley*,1,3 1AIBN,

UQ QLD 4072, Australia Discipline, QUT QLD 4000, Australia 3School of Chemical Engineering, UQ QLD 4072, Australia *E-mail: [email protected] 2Chemistry

This paper reviews the development of bio-based thermoplastic starch polymer blends and nanocomposites at the University of Queensland. Starch-based thermoplastics are relatively cheap and more importantly manufactured using a renewable biomaterial. However traditionally most thermoplastic starch polymers suffer from low water resistance and loss of mechanical properties when in contact with water. This paper will highlight the development of thermoplastic starch polymer blends and nanocomposite materials to overcome some of the challenges of thermoplastic starch. Specifically we will highlight understandings in the genetics-structure-property-biodegradation relationships for thermoplastic starch polymers, developments in blends, nanocomposite materials and reactive extrusion and highlights of scale-up of film blowing and injection molding grades.

Thermoplastic Starch Polymers Starch Polymers Starch-based biodegradable plastics were first developed in the 1980s (1, 2) where thermoplastic starch (TPS) polymers were produced from starch and selected plasticisers and additives (eg. glycerol, water, urea, salts). Unlike traditional thermoplastics starch has a complex multiscale synthesis and subsequent structure as shown in Figure 1. © 2012 American Chemical Society In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Note also this structure itself undergoes changes (gelatinization (breakdown of granular structure), plasticization and recrystallisation during and after addition of plasticisers and polymer processing. Thermoplastic starch polymers low cost base and hence cost-competitiveness compared to conventional plastics makes them an attractive choice platform material for biodegradable and renewable resource strategies. However due to poor properties and inherent water susceptibility, these materials have tended to focus on niche markets for low performance applications. Specifically some of these challenges include

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starch requires relatively high temperatures to be melt processed and at these temperatures starch may degrade by fragmentation (depolymerisation (3)) or lose volatile plasticisers even at these processing temperatures, gelatinisation (breakdown of starch granular structures), breakdown of residual crystallinity and melt mixing is difficult (4, 5) starch has a relatively high processing viscosity (which increases the energy required for processing). It is rheologically equivalent to a power-law fluid (6) with physical entanglements and macromolecular interactions leading to a high dependence of viscosity on shear rate traditionally used plasticisers are hydroscopic and increase the water sensitivity of TPS properties (7)

Figure 1. Synthesis and structural hierarchy of starch polymers.

With this level of complexity and challenges, any research on developing new thermoplastic starch polymers requires a good grasp of the fundamental macromolecular science and engineering behavior.

324 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Genetics-Structure-Property-Biodegradation Relationships


As seen from Figure 1, knowledge of the relationship between geneticsstructure-processing-biodegradation for thermoplastic starch polymers is key to understanding their development. In previous work (8) we examined the effects of manipulating the starch genetic pathway (as shown in Figure 2 below) on structure and properties of model thermoplastic starch polymers. Specifically we examined turning branching and debranching enzymes on or off, and the subsequent effects on macromolecular, granular and thermophysical properties.

Figure 2. Schematic representation of starch biosynthetic pathway in plants.


As an example for a range of Maize starch variants described in Table 1, the change in amylose content (Table 1) and a relative change in amylopectin molecular weight distribution (Figure 3; where the normalised distributions were analysed by subtracting the normal maize starch (W64A) distribution from those of others) can be seen.


Table 1. The novel starch varieties utilised as model materials Plant origin

Starch type

Maize

Description

Amylose content

W64A

Normal (non-mutant) maize

13.6 ± 0.98

Wx1M

Waxy maize - zero amylose

0.4 ± 0.03

Du1R

dull1 mutant

25.4 ± 1.83

Su1R

sugary1 mutant

49.9 ± 3.59

High amylose

amylose extender mutant

75.3 ± 5.42

Figure 3. Comparison of the chain length distributions of isoamylase debranched amylopectin from various maize starches, relative to maize starch. 326 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Here amongst all maize starch varieties, the high amylose maize starch exhibits the most substantial differences in the amylopectin chain length profiles. The proportions of short chains with DP 6-16 are significantly reduced with the maximum reduction is detected at DP 10, while a considerable increase in the fractions of amylopectin chain lengths with DP > 16 is observed, which leads to the noticeable shift in the HAM amylopectin chain length profiles to longer chain lengths. Further correlations of effects of genetics on crystallinity, granular structure and thermal properties have also been made in this work (8–10), and lead to a wealth of understanding for the effects of genetic varieties on macromolecular, granular, thermophysical and function starch polymer properties.

Thermoplastic Starch Polymer Blends Further research in our laboratories (7, 11, 12) has extended the capabilities of low cost TPS by developing TPS-based polymer blends with improved performance. However once again a fundamental understanding of basic structure-function relationships needed to be developed, For example we investigated the fundamental mechanical properties and fracture behaviour of plasticised thermoplastic starch/high molecular weight polyol blends (11). In this work the material was stored at various relative humidities to simulate a large range of storage conditions, and then the influence of the equilibrium moisture content on tensile and fracture properties was investigated. Results from effects of equilibrium content on thermal transitions (Figure 4), tensile properties (Figure 5) and fracture properties (Figure 6) are given here.

Figure 4. Thermal transitions of TPS blends as a function of moisture content. 327 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 5. Stress-strain curves for the thermoplastic starch blend equilibrated at different moisture contents.

Figure 6. Variations of the strain energy release rate, JQ for plane-stress and mixed-mode conditions with the equilibrium moisture content.



Clearly the moisture content was shown to strongly affect the tensile and ultimate properties of a plasticised starch/high molecular weight polyol blend due to changes in the glass transition temperature. As the moisture content increased, there was a reduction in the Young’s modulus and yield stress. This was further supported by the effect of equilibrium content on fracture surfaces by SEM as shown below in Figure 7, where a more brittle surface is seen at low moisture.

Figure 7. SEM fracture surfaces of thermoplastic starch blends as a function of moisture content.

Further work on dynamic mechanical thermal analysis (13) revealed the presence of two relaxations associated with the glass transition of each phase of the starch-high molecular weight polyol blend. The effect of moisture could be explained by a ‘time-moisture’ superposition demonstrating that the effect of the water content was analogous to the effect of temperature. The variation in the yield stress with strain rate and temperature was described accurately by a two-process Eyring’s model. The process, which dominates the deformation behaviour at low strain rates and/or high temperatures correlated well with the relaxation of the plasticised starch-rich phase, while the second process involved at higher strain rates and/or lower temperatures could be associated with the relaxation of the polyol-rich phase Very interesting results (7) on the effects of equilibrium moisture content on water diffusion in thermoplastic starch / high molecular weight polyol blends were observed by characterization of the water diffusion into these materials using MRI. Table 2 shows the values of diffusion (Do), and Fickian model paramenters (A and n) for thermoplastic starch / high molecular weight polyol blends stored at various conditions, and tested with water at different temperatures. In this work water diffusion was significantly affected by the temperature during the sorption study and by the relative humidity storage conditions of the material. Diffusion was faster into samples studied at higher water temperatures and those stored at higher relative humidity environments, due to an increase in free volume and plasticization within polymer. 329 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Table 2. Summary of values of D0, A and n determined from the analysis of the MRI images at (a) 25 °C, (b) 37 °C and (c) 45 °C


(a) A

n

0.6 ± 0.6

2.2 ± 0.1

0.50

23%

1.3 ± 0.7

2.3 ± 0.1

0.53

43%

1.6 ± 0.1

2.1 ± 0.3

0.51

54%

1.8 ± 0.6

2.3 ± 0.2

0.50

81%

2.4 ± 0.1

2.2 ± 0.2

0.47

A

n

Condition

D0 × 10-7 (cm²/s)

vac dried

(b) Condition

D0 × 10-7 (cm²/s)

vac dried

1.2 ± 0.4

2.3 ± 0.1

0.50

23%

1.8 ± 0.5

2.3 ± 0.1

0.57

43%

2.0 ± 0.2

2.2 ± 0.1

0.52

54%

2.7 ± 0.6

2.1 ± 0.5

0.51

81%

3.3 ± 0.4

2.2 ± 0.2

0.50

A

n

(c) 10-7

Condition

D0 ×

(cm²/s)

vac dried

2.3 ± 0.3

2.2 ± 0.2

0.53

23%

2.4 ± 0.5

2.3 ± 0.1

0.50

43%

2.5 ± 0.7

2.2 ± 0.2

0.50

54%

2.9 ± 0.1

1.6 ± 0.2

0.52

81%

3.8 ± 0.4

2.2 ± 0.1

0.50

Of course water diffusion is a vital first step in understanding biodegradation of biodegradable polymers and subsequent to these tests, the effects of equilibrium moisture and plasticisers were further examined on lab scale enzymatic degradation (14) and composting (15) behavior. Interesting observations were found with influence of high molecular weight polyol and low molecular weight plasticisers on lab enzyme degradation rates (14), and on substrate solubilisation on a COD basis in anaerobic digestion (15). This extensive fundamental work has lead to understanding of performance of thermoplastic starch blends in various environments and subsequently has led to the development of sheet, thermoforming and injection molding grades (shown in Figure 8) that were developed by our CRC Food Packaging and Plantic Technologies Ltd. 330 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 8. Examples of commercial uses of thermoplastic starch polymers.

Starch Polymer Nanocomposites Work in our labs (16, 17), developed starch polymer layered silicate nanocomposites systems that allowed processibility of TPS at higher temperatures enabling better melt mixing, increased clarity and antiblocking of blown films and improved tensile properties via an intercalated nanocomposite formation (16). For example Figure 9 shows the improvement in clarity in thermoplastic starch/Enpol™ polyester blend films with increasing level of cloisite 30B nanoclay (Southern Clay products).

Figure 9. Thermoplastic starch/polyester blend films with (a) 0, (b) 2.5wt% and (c) 5wt% cloisite 30B nanoclay.

Further development of this nanocomposites work included the investigation of compatibilisation of the thermoplastic starch and Enpol™ polyester phases by using malaeic anhydride with dicumyl peroxide initiator (18). Figure 10 shows the improvement in compatibilisation of thermoplastic starch/ Enpol™ polyester / cloisite 30B nanocomposite materials, when extruded in a reactive extruder (18). 331 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 10. Thermoplastic starch/polyester (50:50 wt) blend with 2.5% Cloiste 30B nanocomposite with 1wt% malaeic anhydride compatibiliser and (a) 0.3, (b) 0.5, (c) 0.8 and (d) 1.2 wt% dicumyl peroxide initiator.

Averous (19) extended this work by incorporating a cationic starch nanomaterial masterbatch producing better exfoliation and dispersion of the nanoparticles in starch polymers resulting in superior properties. All these efforts highlight the importance of tailoring the nanomaterial-interface to produce superior processing and performance properties. However, these starch-nanocomposites have been formed via high temperature melt extrusion where, as reported by Dennis (20), some of the key parameters necessary for success are high shear rate (for exfoliation of the nanoparticles) and long residence times (for dispersion and distribution of the nanoparticles) in the extruder. Also as mentioned above, high temperature and high shear processing may also cause the starch to fragment or depolymerise, reducing the effectiveness of the nanocomposite enhancement. Thus clearly a major issue for processing of starch nanocomposites is to maximise exfoliation and dispersion to maximise interfacial surface area (and thus subsequent material and performance properties), whilst being able to reduce depolymerisation. Thus we believe there is an important 332 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

role for rheology, polymer processing optimization and modeling in the further development of thermoplastic starch bionanocomposites. In this way starch nanocomposites may be able to achieve the remarkable property enhancements seen in conventional polymer systems (21–23)and be able to be used in more high performance and responsive polymer applications (such as for controlled drug release devices, smart packaging for release of antibacterial or anti microbial actives to foods or other controlled barrier property applications).


Conclusions In this short review we have attempted to highlight the importance of conducting fundamental research work in parallel commercial development work for thermoplastic starch polymers. Specifically we have highlighted the importance of structure-processing-function relationships in developing thermoplastic starch polymers, blends and nanocomposites. There are abundant opportunities in the further development of thermoplastics starch polymers, especially in the areas of modified starch systems, novel plasticisers, novel processing (such as reactive extrusion, coextrusion and supercritical CO2 processing) and new blends that will further expand the range of applications for thermoplastic starch polymers especially in the biomedical, pharmaceutical and higher value added agricultural and packaging applications.

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Thermoplastic Starch Polymer Blends and Nanocomposites

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