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Preparation and properties of thermoplastic pea starch using N,N-bis(2-hydroxyethyl)formamide as the plasticizer.

INTRODUCTION

The improper disposal of the enormous volume of petroleum-derived plastics leads to environment pollution and raises the question how to replace them with nature polymers, hiodegradable and renewable resources. Currently, much research is devoted to starch, because it is inexpensive and abundant (1). Starch commonly exists in the form of granules with about 15-45% crystallinity. To produce biodegradable thermoplastic starch (TPS) without granular structure, plasticizers have to be incorporated, because they can form hydrogen bonds with starch, replacing the strong intra- and intermolecular hydrogen bonds in starch, resulting in plasticization. Generally, pol-yols are used as plasticizers for TPS, e.g., glycerol (2), glycol (3), xylitol (4), sorbitol (5), and sugars (6). Amides such as urea (7), formamide (1), and ethylenebisforma-mide (8) promote the plasticization of starch too. The type of plasticizer influences the properties of TPS. It is very important to prepare a low-cost and nontoxic plasticizer, which is used to produce TPS with desirable properties. In basic research, the preparation of new plasticizers is also necessary to study the relationship of plasticizers structure and TPS properties.

N,N-bis(2-hydroxyethyl)formamide (BHF) belongs to the hydroxyalkylformamides that serve as physiologically harmless humidifiers for cosmetics (9) or are used for the impregnation of tire-cord made from nylon (10). Many synthetic methods of BHF have been reported. However, some of these methods have drawbacks such as high pressure (11) and use of much methanol as solvent (12). In this paper, an efficient and practical method is described to synthesize BHF without any solvent.

Pea starch is mainly available as a by-product of protein extraction. Therefore, it is considered to be a relatively cheap source of starch compared to corn, wheat and potato starches (13). BHF is used to prepare BHF-plasticized thermoplastic pea starch (BTPS) and the characteristics of BTPS are compared with those of glycerol-plasticized thermoplastic pea starch (GTPS).

This work is focused on processing and characterization of BTPS in terms of FT-IR, morphology, XRD, water vapor absorption, rheological properties, and mechanical properties.

EXPERIMENTAL

Materials

Pea starch (12.0% moisture) composed of 35% amylose and 65% amylopectin, and with average particle size of about 30 [micro]m, was supplied by Nutri-Pea Limited Canada (Portage la Prairie, Canada). Glycerol, diethanol-amine, and ethyl formate (analytical grade) were purchased from Tianjin Chemical Reagent Ltd. (Tianjin, China).

Synthesis of N,N-bis(2-hydroxyethyl)formamide

For the complete reaction of diethanolamine, ethyl formate was added step by step. Diethanolamine (1 mol) was introduced into a 500 mL flask equipped with a stirrer, a reflux condenser, a dropping funnel, and a thermometer; it was cooled to 27[degrees]C in an ice bath, and then 1 mol ethyl formate was added during 30 min. The temperature in the flask rose to 30[degrees]C. The reaction mixture was stirred for 1 h at 60[degrees]C by monitoring with thin layer chromatography (TLC). Then the byproduct ethanol evaporated under reduced pressure at 60[degrees]C until ethanol was not distilled from the mixture again. Subsequently the mixture was cooled to 27[degrees]C in an ice bath, followed by addition of 0.05 mol ethyl formate. The reaction mixture was stirred for 0.5 h at 60[degrees]C, and then the byproduct ethanol evaporated under reduced pressure at 60[degrees]C until ethanol was not distilled from the mixture again. The residue was cooled to 27[degrees]C in an ice bath, followed by addition of 0.05 mol ethyl formate. The reaction mixture was stirred for another 0.5 h at 60[degrees]C, and heated under reduced pressure at 110[degrees]C until the mass of the liquid was constant. A viscous liquid was obtained. The purity was 97% according to the column chromatography on neutral aluminum oxide (200 ~ 300 mesh) eluted with mixture of dichloromethane and methanol. The yield of BHF was 97%. The product was confirmed by [.sup.1]H N[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], IR, and MS.

[.sup.1]H N[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] spectra were measured on a VARIAN INOVA 500 MHz spectrometer using TMS as internal standard and [CD.sub.3][SOCD.sub.3] as solvent. The IR spectra were recorded with a BIO-RAD FTS3000 IR spectrometer (KBr). Mass spectra were determined on an LCQ Advantage MAX spectrometer (ESI).

Data for N,N-bis(2-hydroxyethyl )formamide: [v.sub.max] (KBr): 3461-3332 (OH), 2943 (-[CH.sub.2]-), 2886 (-[CH.sub.2]-), 1659 (C=O) [cm.sup.-1]; [.sup.1]H N[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] ([CD.sub.3][SOCD.sub.3], [delta] ppm): 7.950 (s, 1H, HCON), 4.738 (t, 1H, J = 5.5 Hz, -OH), 4.701 (t, 1H, J = 5.5 Hz, -OH), 3.475-3.439 (m, 4H, -[CH.sub.2]O-), 3.305-3.264 (m, 4H, -[CH.sub.2]N); m/z (%): 134 ([MH.sup.+], 100), 156 (M[Na.sup.+], 8.1).

Preparation of Glycerol-Plasticized Thermoplastic Pea Starch (GTPS) and BHF-Plasticized Thermoplastic Pea Starch (BTPS)

The plasticizer was blended (3000 rpm, 2 min) with pea starch in the high-speed mixer GH-100Y (Beijing Plastic Machinery Factory, Beijing, China), and then stored overnight. The mass ratios of plasticizer to pea starch were 30:100, 35:100, and 40:100, respectively. GTPS and BTPS were prepared as follows: The mixtures were manually fed into the single-screw plastic extruder SJ-25(s) (screw ratio L/D = 25:1, Beijing Plastic Industry Combine Corporation, Beijing, China) with a screw speed of 30 rpm The temperature profile along the extruder barrel was 130[degrees]C, 140[degrees]C, 145[degrees]C, and 140[degrees]C (from feed zone to die). The die was a round sheet with holes 3 mm in diameter. The extruded samples were obtained.

Fourier-Transform Infrared (FT-IR) Spectroscopy

The IR spectra were recorded with a BIO-RAD FTS3000 IR Spectrum Scanner (Hercules, CA). The GTPS and BTPS samples were pressured to transparent slices at 10 MPa and 100[degrees]C using the Flat Sulfuration Machine (a compression molder). The samples were measured in reflection mode after they were exposed to the air for 7 days.

Scanning Electron Microscopy (SEM)

Pea starch and the fracture surfaces of the BTPS samples were investigated with the scanning electron microscope Phillips XL-3 (FEI Company, Hillsboro, OR), operating at an acceleration voltage of 20 kV.

Pea starch powders were suspended in acetone. The suspension drops were drawn on the glass slide, dried for removing the acetone, and then vacuum-coated with gold for SEM. The conditioned BTPS samples were cryofrac-tured in liquid nitrogen. The fracture faces were vacuum-coated with gold for SEM.

X-Ray Diffraction (XRD)

The extruded BTPS strips were pressed to slices with a flat sulfuration machine (Beijing Plastic Machinery Factory) and the slices were placed in a sample holder for X-ray diffractometry. The pea starch powders were packed tightly in the sample holder. X-ray diffraction patterns were recorded in the reflection mode at an angular range of 10-30[degrees] (20) at the ambient temperature by a BDX3300 diffractometer (Peking University Instrument Factory, Beijing, China) operated with the Cu/Ka radiation.

Measurement of Water Contents of New GTPS, New BTPS, and Plasticizers

The bars of samples were cut into small pieces, and the pieces weighed immediately. They were then dried in an oven at 105[degrees]C overnight. The original water content (k) of GTPS or BTPS was calculated as follows:

k = [[w.sub.1] - [w.sub.2]/[w.sub.1]] x 100% (1)

Here [w.sub.2] was the mass of the dried sample and [w.sub.1] was the mass of the sample before drying.

Glycerol and BHF were stored in a closed container in the presence of distilled water [providing relative humidity (RH) 100%] at 20[degrees]C and weighed. According to Eq. 1 above, the water content of plasticizer was calculated. [w.sub.1] and [w.sub.2] were the masses of plasticizer containing water and pure plasticizer, respectively. The data were averages of three specimens.

Measurement of Water Contents of GTPS and BTPS Stored at Different Relative Humidities (RHs) for a Period

The bars of GTPS and BTPS were stored in closed containers in the presence of different salt solutions, i.e., saturated [MgCl.sub.2], [K.sub.2][CO.sub.3], and [CuCl.sub.2] solutions (providing RHs of 33, 44, and 68%, respectively) (14) at 20[degrees]C. The water contents of GTPS and BTPS at different RHs were calculated on the basis of masses of GTPS and BTPS absorbing water. The data were averages of three specimens.

Rheology

TPS was cut into small pieces, which were tested by XLY-II rheometer (Jilin University Instrument Factory, Jilin, China). The capillary viscometer was used to determine the viscosity of the samples. The capillary viscometer consisted of barrel into which material was loaded before being pushed by a plunger through a capillary was controlled by a surrounding heating unit. The diameter of capillary was 1 mm and L/D was 40. The small pieces were placed into the barrel through a funnel and then packed down with the plunger until the first extrudate appeared at the capillary exit. The sample was allowed to come to temperature (10-15 min), and then forced through the capillary by the plunger at preselected velocities. The load on the plunger and plunger speed provide the total pressure drop through the barrel and capillary and the volume flow rate. Shear rate ([gamma]) and shear stress ([tau]) were calculated by stand methods. In order to understand the processing properties and fluidity of TPS, the rheology experiments were earned at 150[degrees]C, 160[degrees]C, and 170[degrees]C.

The melting volumetric flow rate through the capillary was given

Q = [[pi][R.sup.3]/4] [gamma] [4n/3n + 1], (2)

where R = capillary radius, [gamma] = shear rate at the capillary wall, and n = flow index depending on the temperature. The term [4n/3n+1] was the Rabinowitsch correction factor.

Pressures were monitored, and shear stress values were calculated using the following equation:

[tau] = [[DELTA]PR/2L], (3)

where [DELTA]P = pressure at the capillary entrance, L = capillary length, and R = capillary radius.

According to Onteniente et al. (15), TPS exhibited power-law behavior:

[tau] = K[[gamma].sup.n](4)

The apparent viscosity [eta] was defined by

[eta] = [[tau]/[gamma]], (5)

where [tau] = shear stress, [gamma] = shear rate at the capillary wall, and K = consistency of the materials depending on the temperature, the structure, and the formulation of the polymer.

Substituting Eq. 5 for [tau] in the relationship 4 between [tau] and [gamma] yielded:

[eta] = K[[gamma].sup.[n-1]], (6)

log [eta] = log K - (1 - n)log [gamma]. (7)

According to the Arrehnius equation,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)

log [eta] = logA + [[DELTA][E.sub.[eta]]/(RT 1n 10)], (9)

where [DELTA][E.sub.[eta]] = the viscous flow activation energy, A = the consistency related to structure and formulation, and R = the gas constant 8.314 J * [mol.sup.-1] [K.sup.-1] (16).

Mechanical Testing

Mechanical properties (National Standard of China GB1040-79) of samples were determined in the AX M350-10KN Materials Testing Machine (Testometric, Rochdate, UK) at a crosshead speed of 10 mm/min. The data were averages of five specimens.

RESULTS AND DISCUSSION

FT-IR Analysis

The IR spectra of pea starch and BTPS were shown in Fig. 1. The peak wavenumber 1153 [cm.sup.-1] was ascribed to the C-O bond stretching of the C-O-H group in starch. The characteristic peaks at 1095 and 1028 [cm.sup.-1] were attributed to C-O bond stretching of the C-O-C group in the anhydroglucose ring of starch (17). The peak wavenumber 3464 [cm.sup.-1] was attributed to starch O-H bond stretching. In BTPS, all the peaks shifted to lower wave numbers, for example, the peaks 1153 and 1095 [cm.sup.-1] in starch shifted to 1149 [cm.sup.-1] and around 1076 [cm.sup.-1] in BTPS, respectively.

[FIGURE 1 OMITTED]

The analysis of FT-IR spectra of the blends enabled the hydrogen bond interactions to be identified (18). Hydrogen bonds between plasticizer and starch were directly related to the wavenumber shift of the stretching bands of functional groups in starch. The lower the peak wavenumber was, the stronger the interaction was (19). All these results indicated that stronger hydrogen bonds were formed between BHF and pea starch compared with intra- and intermolecular hydrogen bonds in starch. FT-IR spectra had been analyzed; because of the groups -O-H and -CO-N in BHF, stable hydrogen bonds were formed between BHF and starch, which could weaken the intra- and intermolecular hydrogen bonds in starch; starch was plasticized.

Scanning Electron Microscopy (SEM)

The morphology of starch and the extruded BTPS was shown in Fig. 2. The pea starch granules are oval, and the size distribution of pea starch granules are two distinct populations (large granules and small granules). Compared with starch granules, BTPS formed a homogeneous phase. Due to the action of plasticizer under the condition of high pressure, shear stress, and heat treatment initiated by the single-screw plastic extruder, starch granules were molten or physically broken up into small fragments; BHF penetrated into the starch granules and formed hydrogen bonds with starch molecules, which weakened the strong action of starch intermolecular and intramolecular hydrogen bonds. The starch was plasticized.

[FIGURE 2 OMITTED]

X-Ray Diffraction Analysis (XRD)

The X-ray diffraction patterns of pea starch and BTPS exposed to air (RH around 50%) for 7 days were shown in Fig. 3. Pea starch exhibited a "C" type diffraction pattern, which was intermediate between the A type (cereal) and B type (tuber) (13). [E.sub.H]-type crystalline was formed in both GTPS and BTPS. Compared with pea starch, the crystal structure of GTPS and BTPS changed. During processing, plasticizer molecules entered into starch particles, replaced starch intermolecular and intramolecular hydrogen bonds, and destroyed the crystallinity of pea starch. C-type crystallinity of pea starch disappeared and [E.sub.H]-type crystallinity was formed (20).

[FIGURE 3 OMITTED]

Water Vapor Absorption

TPS was sensitive to humidity, the influence of which on the properties of TPS was important. The water contents of TPS stored at 33%, 44%, 68% RHs were examined.

As shown in Fig. 4a, 4b, and 4c, at the same RH, the balance water content of GTPS was higher than that of BTPS. The highest balance water contents of GTPS and BTPS stored at RH 68% for 30 days were 19.8%, 22.3%, 23.1% and 16.0%, 17.1%, 17.3% (corresponding to the plasticizer contents 25%, 30%, 35%), respectively. The water resistance of BTPS was better than that of GTPS. With increasing BHF content, the water resistance of BTPS decreased at high RH.

[FIGURE 4 OMITTED]

In our opinion, the hydrophilicity of plasticizer was related to water resistance of TPS. As shown in Fig. 4d, under the same conditions, water vapor absorption of glycerol was higher than that of BHF, so glycerol was more hydrophilic than BHF. The water resistance of BTPS was better than that of GTPS.

Rheology

As shown in Fig. 5, the [eta]-[gamma] curves were plotted using a double logarithmic. With increasing shear stress, the viscosity of each TPS decreased. Such flow behavior was called shear thinning, which was mainly ascribed to the gradual demolishment of starch intermolecular action. The effect of temperature on the rheological behavior of TPS was much. According to the listed linear fit equations in Fig. 5, the apparent viscosity [eta] was decreasing with the increasing of temperature at the same plasticizer content.

[FIGURE 5 OMITTED]

The flow indexes n of TPS were listed in Table 1. The flow index n between 0.0656 and 0.27204 meant that they were non-Newtonian fluid. The slope of the log [eta] - log [gamma] curves were above 0.72796. It meant that TPS was sensitive to the shear rate. Therefore, during the thermoplastic processing, the extruder screw speed could effectively adjust the flow behavior of TPS.
TABLE 1. The effect of plasticizer weight contents on the viscous flow
activation energy [DELTA][E.sub.[eta]] (log [gamma] = 1) and flow index
n of TPS.

Samples                       40% BHF  35% BHF  30% BHF

150[degrees]C           n     0.0656   0.11507  0.07919

160[degrees]C           n     0.08661  0.11867  0.17496

170[degrees]C           n     0.15499  0.10777  0.24617

[DELTA][E.sub.[eta]]  kJ/mol  3.728    4.559    11.014
(log [gamma] = 1)

Samples                       40% glycerol  35% glycerol  30% glycerol

150[degrees]C           n       0.0948         0.06745       0.09885

160[degrees]C           n       0.07395        0.08116       0.27204

170[degrees]C           n       0.09373        0.15506       0.10618

[DELTA][E.sub.[eta]]  kJ/mol    1.250          3.730         3.066
(log [gamma] = 1)


According to Eq. 9, log [eta] (log [gamma] = 1) ~ 1/(R * T * In 10) curves were linearized, and then [DELTA][E.sub.[eta]] was the slope. The viscous flow activation energy [DELTA][E.sub.[eta]] represented the effect of the temperature on the behavior of blends (16). The larger [DELTA][E.sub.[eta]] was, the more sensitivity of the blend was to the temperature. As shown in Table 1, [DELTA][E.sub.[eta]] (log [gamma] = 1) of BTPS was larger than that of GTPS for TPS containing the same plasticizer content. For example, [DELTA][E.sub.[eta]] (log [gamma] = 1) was 11.014 kJ/mol for BTPS containing 30% BHF, whereas [DELTA][E.sub.[eta]] (log [gamma]=1) was only 3.066 kJ/mol for GTPS containing 30% glycerol. So BTPS was more sensitive to processing temperature than GTPS.

Mechanical Properties

Tensile strength and elongation at break of TPS stored at 33%, 44%, and 68% RHs, respectively, for 28 days were examined in comparison to those of GTPS. For TPS containing 30% plasticizer stored at low RH, the tensile strength of BTPS was higher than that of GTPS, while the elongation at break of BTPS was similar to that of GTPS. As shown in Fig. 6, when TPS containing 30% plasticizer was stored at low RH (33%) for 28 days, the tensile strength and the elongation at break were 9.2 MPa and 15.2% for GTPS and 15.2 MPa and 14.1% for BTPS. At high RH, the mechanical properties of BTPS were similar to those of GTPS. When TPS containing 30% plasticizer was stored at high RH (68%) for 28 days, the tensile strength and the elongation at break were 2.2 MPa and 21.5% for GTPS and 2.2 MPa and 19.1% for BTPS.

[FIGURE 6 OMITTED]

With increasing BHF content, the tensile strength of BTPS decreased, whereas the elongation at break increased. For example, after BTPS was stored at RH 33% for 28 days, the tensile strengths of BTPS containing 30%, 35%, and 40% of plasticizer were 15.2, 7.2, and 5.7 MPa, respectively. The elongations at break of BTPS containing 30%, 35%, and 40% plasticizer were 14.0%, 24.6%, and 64.9%, respectively. BHF acted as a dilutor and lowered the interaction of the molecules; thus the tensile strength decreased. At the same time, it also acted as a plasticizer that improved the movement of the segments and macromolecules, which led to the increase of the elongation at break.

When BTPS was stored at RH 68%, the mechanical properties became poor. At the high RH, BTPS absorbed too much water. As a plasticizer of BTPS, water weakened the effect of BHF contents on the mechanical properties of BTPS.

CONCLUSION

BHF was synthesized by an efficient and practical method and proven to be effective as a novel plasticizer for pea starch. BHF formed strong and stable hydrogen bonds with starch, as shown by the analysis of the FT-IR spectra. From the analysis of SEM, starch granules were completely disrupted and a homogeneous phase was obtained. X-ray diffraction analysis indicated that crystal-Unity of pea starch disappeared and starch was plasticized by BHF. BTPS exhibited better water resistance than GTPS. BTPS was non-Newtonian fluid, and BTPS was more sensitive to processing temperature than GTPS. For TPS containing 30% plasticizer stored at low RH, the tensile strength of BTPS was higher than that of GTPS, while the elongation at break of BTPS was similar to that of GTPS. At high RH, the mechanical properties of BTPS were similar to those of GTPS.

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Hongguang Dai, (1) Peter R. Chang, (2) Jiugao Yu, (1) Xiaofei Ma, (1) Peng Zhou (1)

(1) School of Science, Tianjin University, Tianjin 300072, People's Republic of China

(2) Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, Saskatchewan, S7N 0X2, Canada

Correspondence to: J.Yu; e-mail: hongguang2007@yahoo.com.cn

DOI 10.1002/pen.21615

Published online in Wiley InterScience (www.interscience.wiley.com).

[C] 2009 Society of Plastics Engineers
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Author:Dai, Hongguang; Chang, Peter R.; Yu, Jiugao; Ma, Xiaofei; Zhou, Peng
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1CANA
Date:May 1, 2010
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