Thermoplastic starch matrices

Yimlaz, G. (2003) Thermoplastic starch matrices for encapsulation and controlled release of volatile compounds. PhD Thesis. Utrecht University, the Netherlands. 

Cobut, A., Sehaqui, H., Beiglund, L. A. (2014). Cellulose nanocomposites by melt compounding of TEMPO-treated wood fibers in thermoplastic starch matrix. 9(2), 3276-3289. 

The bond strength between fibers and the thermoplastic starch matrix can be determined as the shear stress (r) at the interface multiplied by the surface upon which it acts ... 

In the matrix of PLA/ polycaprilactone (PCL)/OMMT nano-composites, the silicate layers of the organoclay were intercalated and randomly distributed (Zhenyang et at, 2007). The PLA/PCL blend significantly improved the tensile and other mechanical properties by addition of OMMT. Thermal stability of PLA/PCL blends was also explicitly improved when the OMMT content is less than 5%wt. Preparation of PLA/thermoplastic starch/MMT nano-composites have been investigated and the products have been characterized using X-Ray diffraction, transmission electron microscopy and tensile measurements. The results show improvement in the tensile and modulus, and reduction in fracture toughness (Arroyo et ah, 2010). 

In terms of nanocomposite reinforcement of thermoplastic starch polymers there has been many exciting new developments. Dufresne [62] and Angles [63] highlight work on the use of microcrystalline whiskers of starch and cellulose as reinforcement in thermoplastic starch polymer and synthetic polymer nanocomposites. They find excellent enhancement of properties, probably due to transcrystallisation processes at the matrix/fibre interface. McGlashan [64] examine the use of nanoscale montmorillonite into thermoplastic starch/polyester blends and find excellent improvements in film blowability and tensile properties. Perhaps surprisingly McGlashan [64] also found an improvement in the clarity of the thermoplastic starch based blown films with nanocomposite addition which was attributed to disruption of large crystals. 

There are many patents in the literature, mostly on static devices for melt-spinning multiple fibers with the islands in a sea configuration. This is usually done by a multiple series of flow-divider plates that take the initial side-side polymer flow (as in a heterofil spinner) and subdivide it and cross over the flow many times before the spinneret plate, so that each spun filament emerges with the desired structure. Some examples are further ideas of Okamoto [78] and Dugan [79]. More recently, the dissolvable matrix has been made of such materials as polylactic acid (already mentioned), thermoplastic starches, or water-soluble copolyesters. Good review articles on microfibers have been written by Robeson [80], Murata [81], and Isaacs [82]. 

Montrorillonite (MMT) is the most popular filler used for developing thermoplastic starch (TPS)/clay nanocomposites. Nanocomposites showed a significant improvement in tensile properties compared to the pure matrix [231]. 

Natural polymers such as starch and protein are potential alternatives to petroleum-based polymers for a number of applications. Unfortunately, their high solubility in water limit their use for water sensitive applications. To solve this problem thermoplastic starches have been laminated using water-resistant, biodegradable polymers. For example, polylactic acid and P(3HB-co-3HV) were utilised as the outer layers of the stratified polyester/PWS (plasticized wheat starch)/polyester film strucmre in order to improve the mechanical properties and water resistance of PWS which made it useful for food packaging and disposable articles [65]. Moreover, improved physic-chemical interactions between P(3HB-CO-3HV) and wheat straw fibres were achieved with high temperature treatment. It resulted in increased P(3HB-co-3HV) crystallization, increased Young s moduli and lowered values of stress and strain to break than the neat matrix of P(3HB-co-3HV). There was no difference in the biodegradation rate of the polymer [66]. 

Different compositions of wheat thermoplastic starch (TPS) and polycaprolactone (PCL) were melt blended by extrasion and injected [104]. It was noticed that the addition of PCL to the TPS matrix allowed the weakness of pure TPS to be overcome low resilience, high moisture sensitivity and high shrinkage, even at low PCL concentrations, e.g. 10 wt%. However, a fairly low compatibility between both polymeric systems was reported. For PCL-based blends, mechanical properties depend both on plasticization level and PCL content (Table 3.22). 

The early research stage of thermoplastic starch-based composites was focused on the use of plasticized starch as matrix for EPN. The preparation of thermoplastic starch for EPN by melt intercalation in twin screw extrader was first reported by de Carvalho et al. (2001). The composites were prepared with regular com starch plasticized with glycerin and followed by reinforced with hydrated kaolin. The research result showed a significant incensement in the tensile strength from 5 to 7.5 MPa for the composite from matrix only up to 50 % clay composition. The result also indicates the maximum value of elasticity modulus and tensile strength incorporated in the matrix. 

However, increasing cellulose fiber content and time of photo-irradiation led to decreasing elongation (%) values. Other research on the use of thermoplastic starch without further modification (i.e., changes in experimental conditions) include the work of Lu et al. (2006), Ma et al. (2007), Fama et al. (2009), Kaushik et al.(2010), Liu et al. (2010), Guimaraes et al. (2010), and Kaith et al. (2010). These studies show a significant increase in tensile and thermal properties of thermoplastic starch when the matrix reinforced with nanofibers. 

Sago starch with ZnO-NR films exhibited excellent antimicrobial activity against 5. aureus, suggesting these nanocomposites have potential applications as active packaging materials in the pharmaceutical and food industries. Rahman et al. (2013) used concentration of ZnO-NR between zero and 10 wt% to incorporate into a thermoplastic sago starch matrix. Initially, the authors showed no filler particles agglomeration in all film samples indicating an uniform distribution of the nanorods into the starch film. Unusually, FTIR analysis revealed that there was no presence... 

Thermal properties of thermoplastic starch composites reinforced with pehuen husk showed the potential of this bioliber as an excellent reinforcement for composite materials. TPS composites showed a good interaction between the fibers and the plasticized starch matrix due to the natural affinity between husk and starch in the pehuen seed. TPS/PLA/PV A blend showed partial miscibility or co-continuous phase and TPS/PLA/PV A composites presented also discontinuities at the biofiber-polymeric matrix interface. The incorporation of biofiber improved the thermal stability of the composites, increasing the initial decomposition temperature. The biofiber hinders the out-dififusion of the volatile molecules (e.g., glycerol), retarding the decomposition process of starch composites. On the other hand, the degree of crystallinity of composites decreases when pehuen husk content increases (Castano et al. 2012). 

Chen et al. (2008) studied the mechanical properties of composite films from the suspension of hemp cellulose nanocrystals (HCNs) and thermoplastic starch. The films exhibited significant increase in the tensile strength and Young s modulus, with increasing HCN content from 0 to 30 wt% of HCNs. In addition to the improvement in mechanical properties, the incorporation of HCNs into the PS matrix also led to a decrease in the water sensitivity of the final composite materials. Therefore, the CNs played an important role in improving the mechanical properties and water resistance of the starch-based material. 

---