Thermoplastic Waxy Starch Films Processed by Extrusion and Pressing: Effect of Glycerol and Water Concentration

Abstract

Thermoplastic starch properties depends of plastification process. The aim of this study was to analyze the effect of concentration of glycerol and water on plastification, glass transition temperature (T_g) and tensile properties of thermoplastic waxy starch (TPWS). Formulations were extruded in the following concentrations of starch/glycerol/water: 70/30/0 (TPWS 0%); 70/25/5 (TPWS 5%); 70/20/10 (TPWS 10%) weight percentage. Crystalline peaks in WAXS diagrams and native grains present in SEM micrographs showed that the TPWS 0% and 5% were not sufficient to promote total plastification of the waxy starch, and TPWS 10% showed the higher starch plastification. T_g measurements by DMTA were 30 ºC to TPWS 0%, 23 ºC to TPWS 5% and 40 ºC to TPWS 10%. These results showed that glycerol and water had effect plasticizer in TPWS 5% and antiplasticizer in TPWS 10%. Mechanical tensile results showed that higher tensile strength was observed in the systems with more effective starch plastification.

Keywords:
waxy starch; plastification process; plasticizer effect; films; extrusion

1. Introduction

Starch is a carbohydrate present in abundance on the environment, which has advantages over others biodegradable polymers such as low cost and biologically absorbable¹. Starch consists of two major components, the linear amylose and the highly branched amylopectin^2,3. There are three types of starch wherein the amylose concentration ranges: native starch (15-30 wt% amylose); waxy starch (0-5 wt% amylose); and high amylose starch (35-70 wt% amylose)^2,3.

Native starch is not processible and this drawback can be resolved by plastification process^4-6. When starch is mixed with plasticizers under shear rate and temperature between 90-180 ºC their amylose/amylopectin chains are interspersed, and the amylopectin structure original is destroyed. This material is called starch thermoplastic (TPS)^7,8.

The type and concentration of plasticizer employed has significant effect on the plastification process, glass transition temperature (T_g) and mechanical properties^6,9-12. Theoretically, plastification process consists essentially in altering the viscosity of the system, in which the material becomes processable. Plasticizers add to the polymer affect all its plastificaton process, physical and mechanical properties. Plasticizer effect is characterized by a drop in T_g and antiplasticizer effect is characterized by a grow in T_g¹³.

The most commons plasticizers utilized for starch are polyols such as glycerol. However, the addition of polyols decreases film mechanical resistance⁴. Recent study has shown it was found that plastification process of waxy starch (~99 wt% amylopectin) were more easily performed than native starch (72% amylopectin and 28% amylose)¹⁴.

In this work, we proposed to investigate the effect of glycerol and water concentration on plastification, glass transition temperature (T_g) and tensile properties of thermoplastic waxy starch (TPWS) films processed by extrusion and pressing. Many works in the literature deal with plasticization of the waxy starch with several polyols, without focusing on the variation of the amount of water together with polyols in the plastification process, mechanical properties and plasticizer effect¹⁵.

2. Experimental

2.1 Materials

The following materials were used: waxy corn starch (Amidex^® 4001 from Ingredion^® with approximately 100 wt% amilopectin ; glycerol from Nitrogenius^®; distilled water; stearic acid (MW = 284.48 g/mol) from Vetec^®; and citric acid (MW = 192.13 g/mol) from Chemco^® 99.5% of purity.

2.2 Preparation of film formulations and film processing

Waxy starch was mixed with different amounts of glycerol and water. The samples compositions are shown in Table 1. It was added citric acid (1 wt%), and stearic acid (1 wt%) in all samples compositions. Citric and stearic acids were used as processing auxiliaries.

The TPWS compositions were plasticized in a co-rotating twin-screw extruder (Coperion 18 mm and L/D = 40). The materials were added into the feed at screw rotation speed of 300 rpm twin screw profile of high shear. The temperature profile of the feed channel to the matrix was 110, 120, 120, 125, 130, 135, and 140 ºC. After the extrusion process pellets were obtained, which were pressed (by initial pressing of 3.0 tons, and final pressing of 5.0 tons per minute each at a temperature of 140 ºC with subsequent cooling under pressing of 5.0 tons). Films obtained with a thickness of about 200 µm are shown in Figure 1.

Table 1
Thermoplastic waxy starch films compositions.

Films	Waxy starch [wt%]	Glycerol [wt%]	Water [wt%]
TPWS 0%	70	30	0
TPWS 5%	70	25	5
TPWS 10%	70	20	10

Figure 1
Films of TPWS 0%, TPWS 5%, and TPWS 10%.

2.3 Morphological analysis

The cross-section of the films obtained from cryogenic fracture was analyzed by micrographs obtained from a scanning electron microscope (SEM), model JMS 6510 (JEOL^®), using an accelerating voltage of 10 kV. Samples were mounted on copper stubs, and then coated with gold.

2.4 X-ray diffraction (XRD)

The diffractograms were recorded on a LabX XRD 6000 Shimadzu^® diffractometer operating at 30 kV, 30 mA and CuKα radiation (λ = 1540 Å). The samples were scanned in 2-Theta ranges varying from 10 to 40º and the scan speed employed was 0.5ºmin^-1.

2.5 Mechanical properties

Uniaxial tensile tests of films were performed in a universal testing machine EMIC^® according to ASTM D 882-12¹⁶ at a speed of 0.5 mm/min with a 10 kgf load cell at 25 ºC (± 2 ºC), 50 mm distance between the jaws, and a relative humidity (RH) of 50% (± 5%). The Young's modulus, tensile strength, and strain at break were evaluated.

2.6 Dynamic mechanical thermal analysis (DMTA)

Dynamic mechanical thermal analysis (DMTA) was performed in a model Q800 (TA Instruments equipment^®), using tension-film geometry in the range -50 ºC to 70 ºC, at the scanning rate of 2 ºC/min and constant frequency of 1 Hz. Samples were conditioned at 25 ºC and 30% RH for 48 hours. The glass transition temperature (T_g) was defined on the tan δ broad peak.

3. Results and Discussion

3.1 Morphological analysis

Figure 2 presents cross-section cryogenic fracture by SEM for TPWS 0%, 5% and 10%.

Figure 2
(a) SEM micrographs of TPWS 0%, (b) TPWS 5%, and (c) - TPWS 10%.

SEM micrographs showed that TPWS 0% composition exhibits some native or partly melted starch granules spread over the film indicated by white arrows (Figure 2b). The grains with diameter around one micron show that they were broken in the extrusion process. On the other hand, SEM micrographs also showed that the compositions with the addition of water exhibit less native starch granules, suggesting that water is an excellent agent for plastification (Figure 2c to 2f). Some points observed for TPWS 5% and 10% can be bubbles that caused due to presence of water.

The study about TPS films shows that the number and size of these granules grow up with increasing concentration of plasticizer such as glycerol. The complete plastification of starch under low moisture conditions in the extruder required application of a considerable amount of shear, which was not feasible in the presence of glycerol¹⁷.

3.2 X-ray diffraction (XRD)

Figure 3 has shown X-ray diffraction patterns for waxy starch, TPWS 0%, TPWS 5%, and TPWS 10%. Waxy starch shows a typical A-type pattern, with strong reflections at 2θ about 15º and 23º, and an unresolved doublet at 17º¹⁸. An amorphous state was observed in the thermoplastic waxy starch film with 10% glycerol¹⁹. The single peak at 2θ 21º that occurs for this composition is the retrogradation of waxy starch²⁰. For compositions, TPWS 0% and TPWS 5% are possible to observe peaks characteristic of the waxy starch crystallinity, indicating that not occurred an efficient plastification for these compositions. The peaks characteristic of the waxy starch in the TPWS 0% and 5% compositions are according to Figure 2a and 2b, where it is possible to see the starch in the form of grains. The diffractograms prove the plastification effect of the water in the process of extrusion of the waxy starch to transform it into thermoplastic waxy starch, with less quantity of native starch grains (Figure 2c).

Figure 3
X-ray diffraction patterns for waxy starch, TPWS 0%, TPWS 5%, and TPWS 10%.

3.3 Dynamic mechanical thermal analysis (DMTA)

Figure 4 shows the curves of storage tensile modulus (E′) and damping factor (tan δ) versus temperature, which the decay for E' correspond to tan δ peak. The δ peak corresponded molecular relaxation or glass transition temperature (T_g) of TPWS films that is affected by glycerol and water concentration.

Figure 4
(a) Tensile storage modulus E′ versus temperature, and (b) Damping factor tan δ versus temperature for TPWS 0%, TPWS 5%, and TPWS 10%.

T_g measurements by DMTA were 30 ºC to TPWS 0%, 23 ºC to TPWS 5% and 40 ºC to TPWS 10%. These results showed that glycerol and water had effect plasticizer in TPWS 5% and antiplasticizer in TPWS 10%.

The addition of glycerol in starch films provides more active sites by exposing its hydrophilic hydroxyl groups in which the water molecules could be adsorbed that exerted a plasticizing effect^21,22. On the other hand, the addition of more water and removal of more glycerol causes the opposite effect, in which there is an increase in the interaction of the polymer chains, causing the antiplasticizer effect¹³.

The mechanical tensile test was performed at a temperature close to the T_g of TPWS 5%, in which it is not possible to observe the mechanical tensile plasticizer and antiplasticizer effect for this composition. Black line at -10 °C on E' diagrams, we observed the smallest Young's modulus for TPWS 5%, showing the mechanical plasticizing effect for this composition¹³.

3.4 Mechanical properties

Table 2 shows the results of mechanical tensile properties. The tensile strength and Young's modulus grow, while strain at rupture diminishes when add water and decrease the amount of glycerol. These results prove that the effective plastification effect with water improved the mechanical properties, as seen in SEM micrographs and WAXS diagrams.

Table 2
Young's modulus, tensile strength, and strain at rupture of the films.

Films	Tensile strength (MPa)	Young's modulus (MPa)	Strain at rupture (%)
TPWS 0%	5.0 (± 0.7)	218.9 (± 27.4)	0.062 (± 0.018)
TPWS 5%	7.5 (± 0.5)	360.4 (± 49.6)	0.051 (± 0.012)
TPWS 10%	9.2 (± 0.2)	704.4 (± 80.8)	0.024 (± 0.001)

As the mechanical test was performed below the T_g of TPWS 10% it is possible to observe the antiplasticizer effect on the mechanical properties for this composition. The phenomenon antiplasticization occurs for TPWS when the Young's modulus and tensile stress grow, while strain at rupture diminishes in relation to others compositions¹³. When there is an increase in the amount of water into the starch network, direct interactions and the proximity between starch chains were increased, thus under tensile forces, movements of starch chains were reduced, increasing T_g and Young's modulus^21-23.

4. Conclusions

We conclude that only the addition of glycerol was not effective for the total deconstruction of native starch grain and semicrystalline amylopectin structure as seen in SEM images and crystallinity peaks on WAXS diagrams. The water helped the destruction of native starch grains and semicrystalline amylopectin structure, showing an effective plastification process. Mechanical tensile results showed that higher tensile strength and Young's modulus were observed in the systems with more effective starch plastification. Glycerol and water had effect plasticizer in TPWS 5% and antiplasticizer in TPWS 10%, proving the effect of the amount of glycerol and water in T_g.

5. Acknowledgements

The authors are grateful for the support provided by CAPES, PPG-NMA/UFABC, PPGCEM/UFSCar and EMBRAPA Instumentation. This work is within the scope of the scientific-technological agreement between Federal University of ABC (UFABC) and Brazilian Agricultural Research Corporation (EMBRAPA), according to agreement number 23006.001234/2014-23.

6. References

• ¹
  Famá LM, Goyanes S, Pettarin V, Bernal CR. Mechanical Behavior of Starch-Carbon Nanotubes Composites. In: Kar KK, Pandey JK, Rana SK, eds. Handbook of Polymer Nanocomposites Processing, Performance and Application. Volume B: Carbon Nanotube Based Polymer Composites New York: Springer; 2015. p. 141-171.
• ²
  Moad G. Chemical modification of starch by reactive extrusion. Progress in Polymer Science 2011;36(2):218-237.
• ³
  Gao J, Vasanthan T, Hoover R, Li JH. Structural modification of waxy, regular, and high-amylose maize and hulless barley starches on partial acid hydrolysis and their impact on physicochemical properties and chemical modification. Starch 2012;64(4):313-325.
• ⁴
  Talja RA, Helén H, Roos YH, Jouppila K. Effect of various polyols and polyol contents on physical and mechanical properties of potato starch-based films. Carbohydrate Polymers 2007;67(3):288-295.
• ⁵
  Vieira MGA, da Silva MA, dos Santos LO, Beppu MM. Natural-based plasticizers and biopolymer films: A review. European Polymer Journal 2011;47(3):254-263.
• ⁶
  Sanyang ML, Sapuan SM, Jawaid M, Ishak MR, Sahari J. Effect of Plasticizer Type and Concentration on Tensile, Thermal and Barrier Properties of Biodegradable Films Based on Sugar Palm (Arenga pinnata) Starch. Polymers 2015;7(6):1106-1124.
• ⁷
  Avérous L. Biodegradable Multiphase Systems Based on Plasticized Starch: A Review. Journal of Macromolecular Science, Part C: Polymer Reviews 2004;44(3):231-274.
• ⁸
  Corradini E, Teixeira EdM, Agnelli JAM, Mattoso LHC. Amido termoplástico São Carlos: Embrapa Instrumentação Agropecuária; 2007.
• ⁹
  Zhong Y, Li Y. Effects of glycerol and storage relative humidity on the properties of kudzu starch-based edible films. Starch 2014;66(5-6):524-532.
• ¹⁰
  Aguirre A, Borneo R, León AE. Properties of triticale protein films and their relation to plasticizing-antiplasticizing effects of glycerol and sorbitol. Industrial Crops and Products 2013;50:297-303.
• ¹¹
  Razavi SMA, Amini AM, Zahedi Y. Characterisation of a new biodegradable edible film based on sage seed gum: Influence of plasticiser type and concentration. Food Hydrocolloids 2015;43:290-298.
• ¹²
  Smits ALM, Kruiskamp PH, Van Soest JJG, Vliegenthart JFG. Interaction between dry starch and plasticisers glycerol or ethylene glycol, measured by differential scanning calorimetry and solid state NMR spectroscopy. Carbohydrate Polymers 2003;53(4):409-416.
• ¹³
  Tager A. Physical Chemistry of Polymers Moscow: Mir Publishers; 1978.
• ¹⁴
  Li J, Luo X, Lin X, Zhou Y. Comparative study on the blends of PBS/thermoplastic starch prepared from waxy and normal corn starches. Starch 2013;65(9-10):831-839.
• ¹⁵
  Mathew AP, Dufresne A. Plasticized Waxy Maize Starch: Effect of Polyols and Relative Humidity on Material Properties. Biomacromolecules 2002;3(5):1101-1108.
• ¹⁶
  ASTM International. ASTM D882-12 - Standard Test Method for Tensile Properties of Thin Plastic Sheeting West Conshohocken: ASTM International; 2012.
• ¹⁷
  Pushpadass HA, Marx DB, Hanna MA. Effects of Extrusion Temperature and Plasticizers on the Physical and Functional Properties of Starch Films. Starch 2008;60(10):527-538.
• ¹⁸
  Cheetham NWH, Tao L. Variation in crystalline type with amylose content in maize starch granules: an X-ray powder diffraction study. Carbohydrate Polymers 1998;36(4):277-284.
• ¹⁹
  Xu YX, Kim KM, Hanna MA, Nag D. Chitosan-starch composite film: preparation and characterization. Industrial Crops and Products 2005;21(2):185-192.
• ²⁰
  Kim JO, Kim WS, Shin MS. A Comparative Study on Retrogradation of Rice Starch Gels by DSC, X-Ray and ?-Amylase Methods. Starch 1997;49(2):71-75.
• ²¹
  García MA, Martino MN, Zaritzky NE. Edible starch films and coatings characterization: scanning electron microscopy, water vapor, and gas permeabilities. Scanning 1999;21(5):348-353.
• ²²
  García MA, Martino MN, Zaritzky NE. Microstructural Characterization of Plasticized Starch-Based Films. Starch 2000;52(4):118-124.
• ²³
  Mali S, Grossmann MVE, García MA, Martino MN, Zaritzky NE. Effects of controlled storage on thermal, mechanical and barrier properties of plasticized films from different starch sources. Journal of Food Engineering 2006;75(4):453-460.

Publication Dates

• Publication in this collection
  31 Aug 2017
• Date of issue
  2017

History

• Received
  26 Nov 2016
• Reviewed
  11 July 2017
• Accepted
  27 July 2017