Denis Lourdin

Influence of equilibrium relative humidity and plasticizer concentration on the water content and glass transition of starch materials

Abstract Sorption behaviour and calorimetric glass transition were measured on cast starch films plasticized with varying concentrations of different components (glycerol, sorbitol, lactic acid sodium, urea, ethylene glycol, diethylene glycol, PEG 200, glycerol diacetate). Precision analysis showed that the water level in samples conditioned at 57% relative humidity reached a minimum for a plasticizer content of 10–20% (dry basis). Starting from 14.8% of water (dry basis) as measured in the amorphous starch-water system, a minimum of 12.7 to 14.4% can be attained according to the type of plasticizer added. Glass transition, as determined on samples conditioned at a constant relative humidity (57%), depended on the type of plasticizer used, although general behaviour was broadly similar for all components except glycerol diacetate. Casting trials performed with this substance revealed an evident phase separation and Tg of starch did not appear really modified by its presence. For glycerol, sorbitol and lactic acid sodium, glass transition was measured with respect to plasticizer and water content. Couchmans relation was applied, which describes the Tg variation of the monophasic polymer—diluant system. The likelihood of phase separation for a high plasticizer level is considered.

Antiplasticization in starch-glycerol films?

Calorimetric, mechanical, and thermomechanical properties have been measured on starch films with various glycerol/water contents. For calorimetric measurements, a continuous decrease in Tg was observed as glycerol increases from 0 to 25%. Mechanical properties exhibit a minimum of elongation at break for glycerol content ∼ 12%. In slightly hydrated starch films not containing another plasticizer, a β relaxation is detected by DMTA around −68°C. This relaxation is modified by the presence of glycerol, the α relaxation of which appears in the same temperature range. Results are discussed comparing with the well-known antiplasticization effect in synthetic polymers such as PVC and PC. The combination of the plasticizer α mode and the polymer β mode is considered.

Influence of amylose content on starch films and foams

Abstract After extraction of smooth pea starch and waxy maize starch from pure amylose and amylopectin fractions, films with various amylose contents were prepared by casting in the presence of water or water with glycerol. For unplasticized films, a continuous increase in tensile strength (40–70 MPa) and elongation (4–6%) was observed as amylose increased from 0 to 100%. Discrepancies with values obtained for native starches with variable amylose content and different botanical origins were attributable to variations in the molecular weights of components. Taking cell wall properties into account, the values obtained in the laboratory were used to improve the relation between the flexural behavior of extruded foams and the model of cellular solids with open cavities. The properties of plasticized films were not improved by the presence of glycerol and remained constant when amylose content was higher than 40%. Results are interpreted on the basis of topological differences between amylose and amylopectin.

Study of plasticizer–oligomer and plasticizer–polymer interactions by dielectric analysis: maltose–glycerol and amylose–glycerol–water systems

Abstract Dielectric analysis was performed on amorphous maltose–glycerol and amylose–glycerol–water systems. A primary relaxation, appearing in the range of the calorimetric glass transition temperature, was observed in the maltose-glycerol system. From 0% (w/w) to approximately 30% (w/w) glycerol, a secondary relaxation was observed at lower temperature in both systems. In this concentration range, the addition of glycerol shifted the primary relaxation to lower temperature and the secondary relaxation to higher temperature. For mixtures containing around 30% (w/w) glycerol, the relaxation map revealed a merging of the primary and secondary relaxations or a disappearance of the secondary relaxation. Above 30% (w/w) glycerol in the amylose–glycerol–water system, indication of a phase separation was obtained. This behaviour is compared with the relaxation behaviour of some plasticized synthetic polymers (polyvinylchloride and polyisoprene).

Thermal transitions of cassava starch at intermediate water contents

Order-disorder transitions were investigated in native cassava starch at intermediate moisture contents (35 to 60% wt. water), using Differential Scanning Calorimetry (DSC) and dynamic Wide Angle X-ray Diffractometry (WAXS) with a synchrotron radiation source.The gelatinization of granules occurs as a cooperative process, due to constraints induced in crystallites by the amorphous areas. Variations of water content (water volume fraction from 0.28 to 0.86) and heating rate (0.2–10‡C min−1) allowed access to equilibrium melting conditions. Cassava starch exhibits a higher melting temperature of the undiluted starch (Tmo) and an equivalent melting enthalpy of the repeating glucosyl unit (δHu), compared to other A-type starches. At intermediate water content (45% wt. water), a two-stage melting process is evidenced, with different kinetic rates below and above 75 ‡C.

Compatibilization of starch-zein melt processed blends by an ionic liquid used as plasticizer.

An ionic liquid (1-butyl-3-methyl imidazolium chloride [BMIM]Cl) was used as a plasticizer in starch, zein and their blends; and compared to glycerol, a classical plasticizer of starch. Thermoplastic plasticized biopolymer materials were obtained by melt processing using a twin screw microcompounder. Such a device allows simulating a twin screw extrusion process on small batches of a few grams, and to evaluate the necessary specific mechanical energy input for native starch destructurization; and the final apparent melt viscosity. Both were shown to be significantly reduced for starch in presence of [BMIM]Cl (compared to glycerol), while zein processing behavior was less sensitive to the plasticizer used. This induces significant starch/zein viscosity ratio differences, which affect melt mixing of the starch zein blends. In starch rich blends, this results in smaller zein aggregates in the case of [BMIM]Cl. The characterization of the materials indicates that, compared to glycerol, the use of [BMIM]Cl leads to less hygroscopicity, a more efficient plasticization of both starch and zein phases and a compatibilization of starch/zein blends.

Structural relaxation and physical ageing of starchy materials

The structural relaxation during the ageing of an amorphous maltose and a starch-sorbitol mixture was examined using a range of physical techniques. Heat capacity, measured by differential scanning calorimetry, showed an overshoot in the glass-transition region, the size of which was temperature and time dependent. Volume relaxation measurements were made at different ageing temperatures in the range T(g) -15 to -30 K. The volume decreased with increasing ageing time, in an essentially linear fashion with log time. The mechanical behaviour of the materials showed a progressive embrittlement on ageing. For both materials, the mechanical relaxation time increased with ageing, and the material became stiffer. Investigation of the effect of physical ageing on transport properties was also performed using conductivity measurements on a maltose-water-KC1 mixture. A decay in conductivity, which was almost linear with log time, was observed. The structural relaxation was modelled using the Tool-Narayanaswamy approach to describe the calorimetric data.

Viscous Properties of Thermoplastic Starches from Different Botanical Origin

Abstract Starches from various botanical origins were plasticized in a twin screw extruder with different glycerol contents (from 23 to 32%, total wet basis) in order to prepare pellets of amorphous thermoplastic starches (TPS) that can further be processed for manufacturing purposes. The viscous behaviour of these starches was studied with a capillary pre-shearing rheometer, which achieved an homogeneous molten phase under controlled thermomechanical conditions. After performing Bagley corrections and Rabinowitsch analysis, it was found that all samples exhibited a non-Newtonian shear-thinning behaviour, which could be modeled in the range 1 to 103 s−1 either by a simple power law or by a Herschel-Bulkley relationship. For the consistency index, the influence of temperature and plasticizer content was accounted for by Arrhenius-like laws, applying time-temperature and time-plasticizer superposition. For the same plasticizer content and temperature, comparison of starches from different botanical origins showed that shear viscosity values usually increased with the starch amylose content. This trend was partly confirmed for the elongational behaviour, estimated by the Cogswells analysis of entrance effects. These results can be interpreted by the greater ability of linear molecules of amylose to entangle, compared to the more compact amylopectin molecules.

Relationship between thermomechanical properties and baking expansion of sour cassava starch (Polvilho azedo)

The factors involved in the baking expansion of native and sour cassava starch doughs were compared with those of native corn starch. Unlike corn starch dough, native and sour cassava starch doughs showed expansion properties during baking. The storage modulus E ′ decreased for cassava starch doughs before baking expansion, but remained unchanged for corn starch dough. Expansion during the baking of sour cassava starch was attributed to water vaporisation and the fluidity of starch paste. The fact that temperature and weight loss variations at adequate water contents were significantly greater for cassava than corn starch dough is indicative of the important role played by starch melting in expansion. Expansion ability could be correlated with changes in dough–crumb thermomechanical properties when close to the starch melting temperature. © 2001 Society of Chemical Industry

Shape memory starch-clay bionanocomposites.

1-10% starch/clay bionanocomposites with shape memory properties were obtained by melt processing. X-ray diffraction (XRD) and TEM evidenced the presence of a major fraction of clay tactoids, consisting of 4-5 stacked crystalline layers, with a thickness of 6.8 nm. A significant orientation of the nanoparticles induced by extrusion was also observed. Tensile tests performed above the glass transition of the materials showed that the presence of clay nanoparticles leads to higher elastic modulus and maximum stress, without significant loss in elongation at break which typically reached 100%. Samples submitted to a 50% elongation and cooled below the glass transition showed shape memory behavior. Like unreinforced starch, the bionanocomposites showed complete shape recovery in unconstrained conditions. In mechanically constrained conditions, the maximum recovered stress was significantly improved for the bionanocomposites compared to unreinforced starch, opening promising perspectives for the design of sensors and actuators.