Destructurised

Films may be cast or extruded from preparations of destructurised starch. The ratio of the two starch constituents (amylose and amylopectin) characterise materials with different properties. A preponderance of amylose (>70%) in starches gives stronger, more flexible films. The branched structure of amylopectin generally leads to films with poor mechanical properties (decreased tensile strength and elongation) [24]. 

The reference commercial starch-based biodegradable polymers are marketed by Novamont in Italy imder the name Mater-Bi . This starch-based technology permits going beyond conventional compounding. In the presence of different synthetic polymers, starch can undergo a thermoplastic transformation up to destructurisation the destructurised starch composites can reach starch contents higher than 50% [83]. 

Starch-based polymers have been intensively studied for their extrusion characteristics, since extrusion processing plays a primary role in establishing the polymer s properties. Starch can be made thermoplastic using technology very similar to extrusion cooking [13]. Granular beads of starch are approximately 15-100 pm in diameter and can be blended as a filler with other polymeric materials [91]. Under special heat and shear conditions during extrusion, starch can be transformed into an amorphous thermoplastic material by a process known as destructurisation. 

Starch can also be destructurised in the presence of polymers such as aliphatic polyesters, which are hydrophobic materials [92]. It is difficult to process aliphatic polyesters with low melting points by conventional techniques such as film blowing and blow moulding. For example, extruded PCL films are tacky, rigid and have a... 

Besides PCT and its copolymers, other examples of biodegradable polyesters which can be used in the presence of destructurised starch are those obtained by the polycondensation of glycols such as 1,4-BDO with succinic acid, sebacic acid, adipic acid, azelaic acid, dodecandioic acid, or brassylic acid and PBTA [85, 86]. 

Novamont has recently patented polymeric compositions comprising biodegradable polyalkylenefuranoates, aliphatic/aromatic polyesters, and starch in destructurised, gelatinised or filler form [102]. 

This chapter reviews the main topics related to starch in polymer technology taking into account all the different forms in which starch can appear (native, gelatinised, retrograded, destructurised, complex) giving more details for those which are related to starch-based polymers. 

In the patent literature, the terminology destructurised starch [46-65] refers to a form of TPS described as molecularly dispersed [57]. The destructurisation of starch is defined as melting and disordering of the molecular structure of the starch granules as a molecular dispersion [55, 56]. It means that, simultaneously, the native crystallinity of starch and its granular structure disappear. 

Following destructurisation, starch does not subsequently show its native crystallinity and loses its structure characterised by left-handed double helices, as reported above. 

Another term that can be found in the literature is thermoplastically processable starch (TPPS), defined as a TPS that is substantially water free. TPPS is a form of destructurised starch that is obtained without water, because instead of it, other high boiling point plasticisers additives are used [68-73]. 

This aspect makes the transformation of destructurised starch in finished prodncts possible throngh traditional manufacturing technologies for plastics in the absence of water. Their sensitivity to humidity, however, makes TPS or TPPS alone unsnitable for most applications [76]. 

Figure 8.4 Phase contrast microscopy of native (a), gelatinised (c) and destructurised potato starch (e). Polarised optical microscopy of native (b), gelatinised (d) and destructurised (f) potato starch...

The complexation of starch by macromolecules can give rise to even more stable complexes which play an important role on the final properties of the starch-based polymer. Unlike amylose, the majority of amylopectin does not interact with the complexing agent and remains in its amorphous state. In the following paragraphs, a summary of the state of the art related to starch destructurisation with synthetic polymers will be made. 

Starch can also be destructurised in the presence of more hydrophobic polymers totally incom- patible with starch, such as aliphatic polyesters (APE) [112]. 

For example, destructurised starch and vinyl alcohol copolymer composites, materials containing starch with an amylose/amylopectin ratio above 20/80 w/w, do not dissolve even under stirring in boiling water. Under these conditions a microdispersion comprised of microsphere aggregates is produced, where the individual particle diameter is lower than 1 pm (Figure 8.5). 

This peak is attributed to starch ring vibrations [92, 121] and does not correspond to crystalline or gelatinised amylose but to a complexed one, which results when amylose assumes V-type structural conformations after destructurisation. 

Figure 8.7 FTIR second-derivative spectrum of corn starch (dashed line) and destructurised with EVOH (solid line)...

As far as WAXD analysis is concerned, characteristic peaks of different V-type structures can be detected in destructurised starch in the presence of either molecular or macromolecular complexing agents. 

Usually, starch destructurisation in the presence of synthetic polymers, gives rise to Vh and Eh structures (always a V-type one). The latter, in particular, is obtained with high mechanical stress and low moisture content [130]. 

Referring to starch destructurised in the presence of the EVOH copolymer, this sort of composition leads to the Va structure ( V XD patterns are shown in Figure 8.8). 

Figure 8.8 WAXD patterns of corn starch Native (a) and destructurised with EVOH (b). The solid line pattern corresponds to the starch destructurised with EVOH, while the dashed line corresponds to the EVOH pattern...

This chapter reviews the main results obtained in the fields of starch-filled plastics and thermoplastic starch with particular attention to the concept of gelatinisation, destructurisation, extrusion cooking, and the complexation of amylose by means of polymeric complexing agents with the formation of specific supra-molecular structures. The behaviours of products now in the market are considered in terms of processability, physical-chemical and physical-mechanical properties and biodegradation rates. 

If starch is heated above the Tg and T in the presence of plasticisers the endothermic transition can be replaced by an exothermic transition. Destructurised starch, in simple... 

In 2003 the market of destructurised and complexed starch-based bioplastics accounted for about 30000 tons/year, 75% of which was for packaging applications and included soluble foams for industrial packaging and films for bags and sacks. The market share of these products accounted for about 70% of the global market of bioplastics [174]. 

The wide patent portfolio of Novamont covers the technologies of complexed starch developed by Novamont and of destructurised starch developed by Warner-Lambert and acquired by Novamont in 1997 after the exit of Warner-Lambert from the market in 1993. Moreover, in August 2001, Novamont acquired the film technology of Biotec which included an exclusive license of the Biotec s patents on TPPS in the sector of film [174]. 

All studies discussed in this chapter deal with thermoplastic starch (TPS) which is manufactured through destructurisation in presence of specific amounts of plasticisers and under certain extrusion conditions. Depending on the type of application either pure starch polymers or various types of blends with different ratios of petrochemical copolymers are used. 

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