June 24, 2007
Study of Wheat-Flour-Based Agropolymers: Influence of Plasticizers on Structure and Aging Behavior
By Saiah, R Sreekumar, P A; Leblanc, N; Castandet, M; Saiter, J-M
ABSTRACT Wheat-flour-based agropolymers are prepared using an extrusion method. The morphology of the native and extruded wheat flours are analyzed by scanning electron micrography (SEM). During plasticization using water (9%, w/w) and glycerol (12.8%, w/w), a change in morphology of native wheat flour occurs. The structure of these materials was investigated by X-ray diffraction (XRD) with special reference to the amount of plasticizers used, such as water and glycerol molecules. The introduction of these plasticizers decreases the crystallinity rate and also increases the average distance between chains in the remaining vitreous phase of the extruded wheat flour. Replacing water molecules with glycerol reduces the crystalline phase and diminishes the average size of crystalline structures. The plasticization effects were confirmed by mechanical investigations. Indeed, increasing the amount of glycerol from 12.8 to 20% decreases the stress at failure and the tensile modulus, while the strain at failure increases. Finally, aging studies show that the percentage of crystallinity increases with time.
During the last few decades, nonbiodegradable petroleum-based polymeric materials have produced some environmental concerns. The use of nonrenewable materials has increased our dependence on crude oil and also created plastic waste, causing environmental pollution that affects the human population as well as wildlife. Biodegradable polymeric materials, especially those prepared from readily available, renewable and inexpensive natural resources such as carbohydrates, starches, oils, lignocellulosic materials, and proteins have thus become increasingly important. The use of natural materials in composites is rising. Initially, these materials were chosen for environmental benefits but later it was found that these materials offer both processing and structural benefits. Investigation of the use of cereal extracts is very important. Cereal extracts are mixtures of starches, natural proteins, and other components. Wheat grain is one of the world's most obtainable crops and it is principally used as flour. Wheat flour is composed of starches and proteins (Ma et al 2005; Tang et al 2005). The other main sources of commercial starch are maize, potatoes, cassava, and waxy maize. As starch is obtainable from many sources (Petersen et al 1999; Copinet et al 2001), it has the advantages of being renewable and available in great quantities and it is cheap and highly biodegradable (Carvalho et al 2003). However, the cost of extracting starch is significant, limiting its profitability. Thus there is an interest in developing materials based on wheat flour starch. Each wheat starch granule consists of >>25% amylose and
The main plasticizers used to prepare starch-based thermoplastic materials are water (Rindlav-Westling et al 1998; Follain et al 2006), glycerol (Poutanen and Forssell 1996; Rindlav-Westling et al 1998; Stepto et al 2003; Follain et al 2006), ethylene glycol (Poutanen and Forssell 1996; Smits et al 2003a), urea (Shogren et al 1992), and sugars (Poutanen and Forssell 1996; Rindlav-Westling et al 1998). Water is an excellent plasticizer but it has some disadvantages because it is very difficult to control the amount used during heat transformation. On the other hand, the hydrophilic nature of the material makes the water molecules dependent on the surrounding atmosphere such as outside humidity (Bangyekan et al 2006). The proportion of plasticizer and its chemical nature strongly influences the physical properties of processed starch, thereby affecting the final properties of the materials (Da Roz et al 2006).
The objective of this work was to produce new, fully biodegradable materials that exhibit physical characteristics comparable to standard thermoplastic materials. In other words, we are trying to produce ecofriendly new plastics from raw materials such as wheat flour. The effects of aging and the role played by plasticizers on the structure of wheat-flour-based extruded new materials was investigated.
MATERIALS AND METHODS
The wheat flour was provided by Grands Moulins de Paris (France). After a dry division of cereal, the flour was separated into two categories, one rich in protein that is used by the food industry and a second one with low protein content (
The wheat flour (68.2%) and the additives, except water and glycerol, were placed into a thermo-regulated turbo mixer (Kaiser, Germany) and mixed with a rotating speed of 750 rpm over 3 min. A mixture of water and glycerol (21.8%) was introduced slowly through a valve fixed on the lid to a single-screw extruder (Scamex, S0262, France) that exhibits two heating zones. The extrusion conditions were heating up to 12O0C (same temperature for each heating zone) and a screw rotating speed of 40 rpm. The glycerol and water allows the rupture of starch granules and plasticizes the materials. Silicium dioxide allows good dispersion of plasticizers in the formulation and facilitates passage of the formulation in the extruder. Magnesium stearate is used as a lubricant and sorbitol is the solid plasticizer. This method produces pellets. After that, stress relaxation and stabilization were performed by maintaining the pellets at room temperature for several hours. A second extrusion using the same temperature and screw speed was performed to get the final film, which was immediately put into desiccators and kept in controlled atmospheric conditions (75% relative humidity and room temperature). The additive chosen has been patented (Leblanc and Dubois 2001). This choice produces film of good quality. The wheat-flour-based material was used to prepare the samples and to analyze their characteristics. The thickness of extruded film was >>400 urn and the width was
Scanning electron microscopy (SEM) was performed (LEO 1530 FEG). Each sample was cryogenically fractured in liquid nitrogen and then coated with gold palladium. The thickness of the coating was
The structures of the samples were tested with X-ray diffraction experiments using a Rigaku miniflex wide-angle X-ray diffractometer. The X-ray diffraction patterns were performed with the Cu Ka radiations (e = 1.54 A) scanning 2E angles from 5[degrees] to 35[degrees] with a step of 0.02[degrees] and a counting time of 5 sec.
Tensile tests of the films were conducted on a universal testing machine (Instron 4301). The tests were performed using a load cell of 1 kN (cell reference: 2518-806 Instron) at a crosshead speed of 2 mm/min. The samples were taken from the center of the film in the extrusion direction. The sample geometry is 100 ? 10 ? 0.4 mm. Determination of tensile modulus (E) is a tangent at the origin of the stress-stain curve. For each sample, the values of the mechanical characteristics were the arithmetic mean of at least five different specimens.
RESULTS AND DISCUSSION
Morphology of Native and Plasticized Wheat Flour
SEM showed the different structures for native and thermoplastic wheat flour. The native wheat flour has a granular structure. These granules are spherical or oval and have different domain sizes. The surfaces of these granules are rough due to the existence of protein, lipid, and pentosan molecules that were not eliminated during the flour-milling extraction. For wheat flour there is a wide distribution of granular size, which is a common feature of cereal starches (Buleon et al 1998; Charles et al 2003). During plasticization (the transformation of granular morphology into a homogeneous polymeric film), the destruction of hydrogen bonds between the starch molecules occurs synchronously with the formation of the hydrogen bonds between the plasticizer and starch molecules (Yang et al 2006). The extrusion method is a combination of thermal and mechanical input. During this process, wheat flour was plasticized and a homogeneous molten phase characteristic of thermoplastic polymeric material was obtained.
Structure of Native and Plasticized Wheat Flour
The molecular organization of the wheat granules was investigated by XRD. This powerful tool reveals the existence and quantity of crystalline and amorphous phases. The X-ray diffraction patterns for native and plasticized wheat flour (with 9% water and 12.8% glycerol) are displayed in Fig. IA and B, respectively. The signal obtained for native wheat flour presents diffraction rays superimposed on a diffusion halo, proving the semicrystalline nature of this material. The diffraction peaks obtained for 2E values are equal to 11.3, 15.2, 17.3, 18.1, 20.1, 23.3, and 26.7[degrees], leading to the conclusion that these raw materials present A-type crystalline structure. This general characteristic of cereal starches was already observed in many other works (Katz and Van Italie 1930; Le Bail et al 1993; Krogars et al 2003). This double helix structure is organized in a monoclinic unit cell (a = 2.124 nm, b = 1.172 nm, c = 1.069 nm, a = 123.5[degrees], space group B2) where eight water molecules are located between the double helices (Katz and Van Italie 1930; Imberty et al 1988, 1991). Different peaks can be observed for the extruded material. Extrusion seems to change the initial crystalline structure of native wheat flour. After extrusion, the diffraction peaks appear at 2E = 7.2[degrees], 12.9[degrees], 19.8[degrees], and 22.6[degrees] (Fig. IB); they are characteristics of a Vh-type structure (Le Bail 1995; Fanta et al 2002). This structure was obtained by complexation of amylose with lipids. The models suggest that the chain conformation consists of six left-handed residues per turn helix with a rise per monomer between 0.132 and 0.136 nm. This structure is characterized by an orthorhombic unit cell (a = 1.37 nm, b = 2.37 nm, c = 0.805 nm) with the space group P212121 and 16 water molecules within the unit cell (Winter and Sarko 1974). However, another peak at 2E = 17.3[degrees] was observed. This peak corresponded to an A-type structure. As a consequence, we find two types of crystalline plasticized structures for wheat flour material: Vh and A-type. In the plasticized material, the A-type residual crystallinity is due to incomplete destructuration and fusion during the transformation (Van Soest et al 1996; Willett and Doane 2002).
Fig. 1. X-ray diffraction (XRD) pattern of native (A) and extruded (B) wheat flour showing different contributions to rebuilding experimental signal.
Results of Nested Chi-Square Tests for Latent Mean-Level Differences
As a consequence, it is expected that the amount of free volume linked to existence of an amorphous or a vitreous phase increases in the extruded materials.
Next we studied the relative effect of each plasticizer by varying the percentages of glycerol and water but keeping the sum of the plasticizers constant at 21.8%. The XRD patterns for these materials are shown in Fig. 5. For materials with different water contents (O, 5, and 9%), the characteristic 2E values observed on the XRD pattern were the same. A Vh structure was observed for all compositions. But for material with no water (0%), the peak at 2E = 17.3[degrees] does not appear. It is notable that when water content decreases, the intensity of all peaks decreases and peak broadening occurs as well. As water content decreases from 9 to 0%, the peak intensity at 2E = 17.3[degrees] also decreases, and this peak vanishes when the water content is 0%. The lowering of crystallinity was in the order: 9% water and 12.8% glycerol (14%) > 5% water and 16.8% glycerol (11%) > 0% water and 21.8% glycerol (8%). The disappearance of the peak at 2E = 17.3[degrees] in the material with 0% water and 21.8% glycerol was due to the complete granule destructuration and to a crystalline phase fusion.
Values Characterizing Amorphous Halo Observed by X-ray Diffraction (XRD)"
Fig. 3. X-ray diffraction (XRD) pattern showing variations in the amorphous halo when the glycerol content changes.
Both phenomena occurred during the sample preparation. This means that the glycerol contributes more to the destructuration of crystals than the water molecules. The replacement of water by glycerol before extrusion led to the reduction of percentage of crystallinity and crystal size.
Effect of Plasticizers (Glycerol and Water) on Mechanical Properties
Mechanical testing measurements were performed to analyze these behaviors. Figure 6 shows the stress-strain average curves for samples with various glycerol contents (12.8, 16.5, and 20%). From these curves we determined the values of stress at failure, strain at failure, and tensile modulus. These data are regrouped in Table II. Adding glycerol decreases the stress at failure from 3.2 to 2.1 MPa and the tensile modulus from 125 to 57 MPa, while an increase from 17 to 22.9% in strain at failure is observed. These results indicate that the ductility of material increases and, as expected, a plasticization effect is obtained by introducing glycerol in the sample composition. These plasticization effects due to glycerol have been observed in materials made of starch (Mali et al 2006). In this latter system, introducing plasticizer reduces direct interaction between starch chains, thus facilitating movement of starch chains under tensile forces (Garcia et al 1999; Mali et al 2002). The same scenario occurs for wheat-flour-based materials.
Another material was prepared by replacing 9% of the water with glycerol. The average stress-strain curves for the materials (21.8% glycerol and 0% water) and (21.8% glycerol and 9% water) are presented in Fig. 7. A small increase in the strain at failure from 17 to 19.7% was observed. On the other hand, stress at failure decreases from 3.2 to 2.4 MPa (which means a variation of 25%). The tensile modulus of material also decreases from 125 to 82 MPa (variation of 35%). This is consistent with the previous XRD data showing that incorporation of plasticizer increases the intermolecular distance between chains belonging to the vitreous phase. These results also show that a better plasticizing effect is reached when glycerol molecules are used.
Recrystallization of Starch (Retrogradation)
Aging studies were performed on a material with 9% water and 12.8% glycerol. Materials were kept at room temperature under a surrounding atmosphere with 75% relative humidity for different time periods (1 week, 1 month, 6 months, and 12 months). The XRD patterns for this material are shown in Fig. 8. It appears that aging for a long period (12 months) increases the intensity of the peak located at 2E = 17.3[degrees]. This peak corresponds to the A-type crystalline structure. Thus, after a long period of storage, the starch macromolecules of wheat flour film can be reorganized and undergo the phenomenon of retrogradation (recrystallization). During this period of storage, the Vh structure is not modified. Therefore, the percentage of crystallinity is modified during aging, increasing from 14 to 18% (Fig. 9). This retrogradation phenomenon depends on the nature of plasticizer used because smaller plasticizers favor crystallization by mobilizing the starch molecules, while larger particles prevent crystal propagation (Smits et al 2003b). It is also clear from Fig. 9 that the kinetics of crystallization observed during aging were not linear. Assuming in a first approximation, a value close to 18% crystallinity for an infinite aging duration, we find that 87.5% of possible recrystallization was done during the first six months.
Fig. 4. Variations in crystalline fraction of samples and modification of average intermolecular distance in amorphous domains when glycerol content changes.
Fig. 5. X-ray diffraction (XRD) patterns obtained for extruded wheat flour with varying amounts of plasticizers: a) 12.8% glycerol and 9% water; b) 16.8% glycerol and 5% water; c) 21.8% glycerol and 0% water.
Values of Mechanical Characteristics of Wheat Flour Materials with Different Glycerol Content
Fig. 6. Stress-strain curves calculated for wheat flour materials: a) 12.8% glycerol and 9% water; b) 16.8% glycerol and 5% water; c) 20% glycerol and 0% water. Sample (a) shows the full set of measurements where the data dispersion is relatively small.
Fig. 7. Stress-strain curves calculated for wheat flour materials: a) 12.8% glycerol and 9% water; b) 21.8% glycerol and 0% water.
Fig. 8. X-ray diffraction (XRD) pattern of extruded wheat flour with a composition of a) 12.8% glycerol and 9% water for one week; b) one month; c) six months; and d) 12 months. Zoom of the domain 2theta = 17.3[degrees] shows the existence of residual A-type crystalline structure.
Fig. 9. Variations of crystallinity with duration of aging. Samples stored at room temperature under a controlled humidity atmosphere (75% rh).
The present study investigated the morphology of new agropolymeric films obtained by extrusion of wheat flour. Scanning electron micrography (SEM) shows that the films were homogeneous. X- ray diffraction (XRD) shows that the crystalline structure is very sensitive to the nature and amount of plasticizers. Indeed the XRD analysis for extruded wheat flour exhibits two types of crystallinity: Vh and A-type. The degree of crystallinity reaches 14% while the native material was at 30% crystallinity. When the percentage of glycerol was increased while keeping percentage of water constant, the crystallinity of all materials decreased from 14 to 11%. We also observed modification in the amorphous phase. Indeed, the average intermolecular distance (dm) increased as the glycerol content increased. When the relative percentage of glycerol and water were varied but the total content of plasticizers was kept constant, the crystallinity increased when the water content increased. The plasticizer molecules modify the mechanical behavior by decreasing stress at failure (variation of 34.4%) and tensile modulus (variation of 54.5%), while increasing strain at failure (variation of 26%) when the glycerol content varied from 12.8 to 20%. Finally, aging also leads to structural modifications by increasing the percentage of crystallinity. ACKNOWLEDGMENTS
We thank Malandain (UMR 6634 University of Rouen-CNRS) for microscopy and the Region Haute Normandie for financial support.
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[Received September 27, 2006. Accepted January 12, 2007.]
R. Saiah,1 P. A. Sreekumar,1,2 N. Leblanc,1,3 M. Castandet,3 and J.-M. Saiter1,4
1 Laboratoire Polymeres, Biopolymeres et Membranes, Unite CNRS 6522, equipe LECAP, Institut des Materiaux Rouen, Universite de Rouen, Faculte des Sciences, Avenue de L'universite BP 12, 76801 Saint Etienne du Rouvray, France.
2 National Institute of Technology Calicut, NtTC P O, Calicut, Kerala-673601, India.
3 Laboratoire de Genie des Materiaux de (LGMA), Esitpa rue grande, BP 607, 27106 Val de Reuil Cedex, France.
4 Corresponding author. Phone: 33 (0)2 32955085. Fax: 33 (0)2 32955082. E-mail: [email protected]
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