Starch destructurization

Here we will briefly discuss two major topics related to thermoplastic starch physical chemistry the starch granule destructuration or gelatinization during TPS production and the macromolecular properties of the semicrystalline plasticized starch which include indistinctly thermoplastic starch, destructurized starch and plasticized starch. 

In the following paragraphs, a resume of the state of the art related to starch destructurization with synthetic polymers will be made. 

Usually, starch destructurization in presence of synthetic polymers, gives rise to Vh and Eh structures (always a V-type one). The latter is especially obtained with high mechanical stress and low moisture content [129]. 

Referring to starch destructurized in the presence of EVOH copolymer, this sort of composition leads to the Vei structure (WAXD patterns are reported in Figure 2.8). 

Figure 2.8 WAXD patterns of corn starch Native (a), destructurized with EVOH (b). The solid line pattern corresponds to the starch destructurized with EVOH, while the dashed line corresponds to the EVOH pattern.

The drive to use starch at higher addition levels requires it to contribute to the expected strength properties. For this to happen, the starch must be disrupted or destructured so that it can form a continuous phase in an extruded matrix. This can be done by extrusion of starch under low moisture conditions, which effects granular fragmentation, melting of hydrogen-bonded crystallites and partial depolymerization. Thermoplastic blends of up to 50% starch and poly(ethylene-co-acrylic acid) (EAA) were produced in the presence of aqueous base, which solubilized EAA and increased its compatibility with starch and urea, which aids in starch gelatinization.147,148... 

Another proposed process employed injection molding in which starch and limited amounts of plasticizing water are heated under pressure to temperatures above the Tg and Tm to transform the native starch into a homogenous, destructured, thermoplastic melt. The process melt is then cooled to below the Tg of the system before pressure release to maintain the moisture content. Additives include natural and synthetic polymers, plasticizers and lubricants.136-139 159 160 The technology has been used to prepare pharmaceutical capsules and shaped objects, such as disposable cutlery, straws and pens. 

These discoveries provided the technology base for commercialization efforts of starch-based plastics by the Novon division of the Warner-Lambert Company.157,158 The thermoplastic compositions were based on blends of destructurized starch and various hydrophilic polymers, such as ethylene-vinyl alcohol copolymers (EVOH). Hydrophobic polymers and additives, such as plasticizers and lubricants, could also... 

Starch can be destructured in the presence of more hydrophobic polymers such as aliphatic polyesters. Aliphatic polyesters with low melting points are difficult to process by conventional techniques such as film blowing and blow moulding. Films such as polycaprolactones (PCL) are tacky as extruded and have a low melt strength (over 130 °C). Also, the slow crystallisation of the polymer causes the properties to change with time. Blending starch with aliphatic polyesters improves processability and biodegradability. 

In polymeric terms, a main distinction between starch and cellulose is that the former contains highly branched molecules whereas the latter contains linear molecules. The branching means that crystalline sequences are shorter in starch and fibres do not form. Accordingly, native starch is more readily destructuned than native cellulose. Such destructuring is, of course, the basis of much food preparation and, hence, the processing of starch dates back several millennia into human history. In addition, starch adhesives were known by 3500 BC. 

Destructurized and plasticized starch used as a thermoplasticlike material. 

The literature concerning starch is vast and its chemistry and technology have been comprehensively reviewed [3, 4, 7, 8]. Renewed interest has arisen in the last decade, more specifically in converting starch into a plastic material capable of replacing petroleum-based counterparts, which is the subject of this chapter. More specifically we will focus on the utilization of starch after destructuration by extrusion processing in the presence of plasticizer and discuss the drawbacks and new perspectives of this emerging technology. 

A different approach was used by the Ferruzy Company, the main difference being the use of high boiling-point plasticizer instead of water for the destructuration of starch. In this technology, starch was plasticized together with polymers such as polyethylene-vinyl alcohol (EVOH), EAA, poly-e-caprolactone, with small amounts of moisture, in a twin-screw extruder [49], to produce an intimate mixture between starch and the synthetic polymer. The commercial trade name of this product family is Mater-Bi . 

L. Gustav, J. Rehm, R.F. Stepto, R. Thoma, J.P. Sachetto, D.J. Lentz and J.Silbiger, Polymer composition containing destructurized starch, U.S. Patent 5095054, assigned to Warner-Lambert Company, March, 10,1992. 

DIV 04] Divers T, PillinL, Feller J.F., efa/., Starch controlled destructuration in diluted formic acid solutions followed by iheometry . Starch, vol. 56, no. 9, pp. 389-398,2004. 

Sachetto, J.-P., Stepto, R., Zeller, H. Destructurized starch essentially containing no bridged phosphate groups and process for making same. UK Patent... 

Starch is one of these renewable resources. Starch is widely available and is especially suited for the production of foamed thermoplastic materials due to the intrinsic presence of a blowing agent. Destructurization of starch by means of extrusion compounding will facilitate the formation of expandable beads or foams of thermoplastic starch. 

To determine the degree of destructurization X-ray diffraction measurements were performed on the extruded beads. Fig 2 shows the results of these measurements. In the figine the X-ray diagram of native potato starch is added as a reference. The curves clearly show that the initial peaks that can be attributed to native starch disappear upon processing. 

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, gelatinized, retrograded, destructurized, complex) giving more details for those which are related to starch-based polymers. 

In the patent literature, the terminology destructurized starch [46-65] refers to a form of thermoplastic starch described as molecularly dispersed [56]. Destracturization 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, at the same time, the native crystallinity of starch... 

Starch can be destructurized using extrusion technologies in specific conditions. Sufficient work, heat and time have to be applied to a cereal-based starch product in the presence of plasticizers to destructurize it. The best plasticizer for starch is water in quantities lower than 45%. Other plasticizers are glycols such as glycerol and sorbitol. Whereas thermoplastic starch can contain a certain amount of granular residue and a few Maltese crosses can be detected in polarized light microscopy, destructurized starch is substantially free from those features. 

After having undergone destructurization, starch does not show its native crystallinity any more and loses its structure characterized by left-handed double helixes, as reported above. 

Figure 2.4 Phase contrast microscopy of native (a), gelatinized (c) and destwcturized potato starch (e). Polarized optical microscopy of native (b), gelatinized (d) and destructurized (0 potato starch.

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

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

Starch can undergo a thermoplastic transformation up to destructurization in the presence of different synthetic polymers to satisfy a broad spectrum of market needs. Destructurized starch composites can reach starch contents higher than 50%. 

In 1989, studies on EAA-thermoplastic starch films containing 40% by weight of EAA (acrylic acid content 20% by weight), processed at water content lower than 2%, led to destructurization of starch, improved processability and film properties with elongation at break up to 200% without the need for ammonia or sodium hydroxide [86], Observing these materials... 

Starch can also be destructurized in presence of more hydrophobic polymers totally incompatible with starch, such as aliphatic polyesters [111]. 

Other APE having a higher melting point that can be used in presence of destructurized starch are those formed by the reaction of glycols such as 1,4-butandiol with succinic acid, sebacic acid, adipic acid, azelaic acid, dodecanedioic acid, or brassylic acid. 

For example, destructurized 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 constituted by microsphere aggregates is produced, whose individual particle diameter is lower than 1 pm (Figure 2.5). 

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