Thermoplasticized starch modified by reactive blending with epoxidized soybean oil

Industrial Crops and Products 53 (2014) 261–267

Contents lists available at ScienceDirect

Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop

Thermoplasticized starch modified by reactive blending with epoxidized soybean oil Ramzi Belhassen a , Fabiola Vilaseca b , Pere Mutjé b , Sami Boufi a,∗ a b

University of Sfax-Chemistry department, LMSE, BP 802-3018 Sfax, Tunisia LEPAMAP Group, University of Girona, Campus Montilivi, 17071 Girona, Spain

a r t i c l e

i n f o

Article history: Received 17 August 2013 Received in revised form 23 December 2013 Accepted 24 December 2013 Keywords: Plasticized starch Epoxidized soybean oil Strength

a b s t r a c t Thermoplastic starch (TPS) was modified in situ with epoxidized soybean oil (ESO) using melt reactive blending in an internal mixer, in order to improve the stiffness and the mechanical strength of the TPS. Evidence of the condensation reaction between the epoxide ring of ESO and the hydroxyl groups was supported by FTIR. The evolution of the properties of the modified TPS, in terms of mechanical properties, microstructure and water absorption, was investigated using tensile mechanical, dynamic mechanical analysis (DMA), X-ray diffraction (XRD) and water uptake. The addition of ESO results in a huge enhancement in both Young’s modulus and tensile strength. The hydrophobicity of the TPS was also increased upon the addition of ESO. © 2013 Elsevier B.V. All rights reserved.

1. Introduction In recent years, more and more research has been carried out on the substitution of petro-based plastic materials by biodegradable ones in order to solve the problems generated by plastic waste. Thus, there is increasing interest in biodegradable biopolymers made from renewable and natural materials such as starch. Due to its relatively low cost and renewability, starch is a very attractive source for the development of sustainable biodegradable plastics. The price of starch in 2012 was about $ 0.55 kg−1 (http://www.ncga.com) which accounts for the lower cost of starch-based biopolymers compared to the other biopolymers such as PLA, PHA or polyester. However, starch based biopolymers have several shortcomings, such as brittleness, poor mechanical properties, which limit their uses or applications and water sensitivity (Liu et al., 2009). The main method of decreasing the brittleness of starch biopolymers is to add plasticizers able to reduce the intermolecular forces by increasing the chain mobility and improving the flexibility and extensibility of the biopolymer (Parra et al., 2004). Another approach to overcome this aspect is to blend starch with other polymers or additives such as PCL or PLA (Bourtoom and Chinnan, 2008; Ghanmbarzadeh et al., 2010; Walia et al., 2000; Wang et al., 2004). Moreover, the application of hydrophilic films, such as starch-based films, is limited by the poor water vapour barrier properties and water solubility of these films. Surface chemical modification is

∗ Corresponding author. Tel.: +216 74 274 400; fax: +216 74274 437. E-mail address: sami.boufi@fss.rnu.tn (S. Boufi). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.12.039

a promising method to reduce the surface hydrophilic character without changing the bulk composition and properties of TPS products. The hydroxyl groups of TPS can be partially substituted with hydrophobic groups or react with cross-linking agents to form starch molecule networks so that the surfaces of TPS products become less sensitive to moisture. It was reported that surface hydrophobicity of TPS samples was improved greatly when the TPS surface has been treated with prepolymers having -NCO groups (Yu and Liu, 2002; Belhassen et al., 2011). Bengtsson et al. (2003) reported a decrease in the water vapor transmission and water absorption after surface esterification with octanoyl chloride. By the surface reaction of thermoplastic starch film with several reagents bearing different reactive function such as isocyanate, phenol blocked isocyanate, epoxy or acid chloride, Carvalho et al. (2005) succeeded to change the superficial properties of TPS and to effectively reduce the hydrophilic character of the films. Surface of corn starch sheets was modified by UV cross-linking using sodium benzoate as photosensitizer. It was shown that surface photo-cross-linking modification significantly reduced the hydrophilic character of the starch sheet surface and enhanced water resistance of starch sheets (Zhou et al., 2008). Epoxidized oils constitute an important sustainable chemical and proved to be useful in many applications. They are biodegradable, environmentally friendly, and easily available raw materials. Currently, the epoxidized soybean oil (ESO) is a low cost epoxy compound derived from a renewable resource, and one of the most utilized epoxidized oil mainly used as a polyvinyl chloride additive but also as a starting material to produce polyol and polyurethane foam (Lin et al., 2008) and thermosetting materials with improved

262

R. Belhassen et al. / Industrial Crops and Products 53 (2014) 261–267

Native starch

Water

Scheme 1. Structure of the epoxidized soybean oil (ESO).

toughness (Gupta et al., 2010; Liu et al., 2004a,b; Wool et al., 1998). ESO was also used as an additive to PLA and PET in order to improve their processing and rheological properties (Japon et al., 2000; Ali et al., 2009; Zhan et al., 2008). However, the use of ESO as additive or modifier for TPS based biopolymer was not investigated. Referring to the literature data, only two recent relevant works have reported the use of ESO with starch. Xia et al. (2012) reported the use of ESO as compatibilizing agent to improve the dispersion of starch granule within plasticized cellulose diacetate. Xiong et al. (2013) prepared a composite of polylactide (PLA) and starch grafted with maleic anhydride by melt compounding in presence of epoxidized soybean oil (ESO). The reactive composite blending resulted in an improved elongation at breaks and impact strength compared to the virgin composites of PLA/starch blends. However, in both of these recent paper, the ESO was used mainly as a reactive compatibilizer to improve the dispersion of starch granules within the host matrix, and no clear support regarding the involvement of the epoxyde ring in the reaction with hydroxyl, and/or carboxyl groups of the two phases was provided. In the present work, epoxidized soybean oil (ESO) was added to TPS up to a content of 4% by melt reactive blending in order to ensure the chemical coupling between starch, the plasticizer and ESO. The effect of the ESO addition on the mechanical properties of the TPS was analyzed using dynamic mechanical analysis (DMA) and tensile test measurement. The evolution of the polymer hydrophobicity and the water absorption were also studied. In fact, given the high reactivity of oxirane ring with wide range of active hydrogen containing compounds such as alcohol, amines, carboxylic acid, oxirane-containing compounds, such as ESO would be expected to undergo coupling with starch but also the polyol plasticizer by reaction of the oxirane with the hydroxyl function. Because this reaction is a ring opening condensation reaction, it does not produce any reactive volatiles and the chemical linkage formed are stable with excellent chemical resistance properties. 2. Materials and methods 2.1. Materials ˜ S.A. Native corn starch, provided by Roquette Laisa Espana (Barcelona, Spain), was used as polymer base material. This material is a non-modified starch typically used as an additive in the paper industry. Glycerol (Gly) provided by Quimivita, S.A. (Sant Adrià de Besòs, Spain) was used as a plasticizer without any prior purification. Commercial epoxidized soybean oil (ESO) with epoxide content of 7% and soybean oil (SO) were kindly supplied by Traquisa S.L. (Barcelona Spain). Triethylamine was purchased from Aldrich. The illustrative chemical structure of ESO is given in Scheme 1. 2.2. Methods 2.2.1. TPS processing and reactive blending with ESO A pre-blending process of native starch, water, glycerol and triethylamine at a weigh proportion of (58/19/23/0.1) was carried out

PRE-BLEND ING PRE-BLENDING

Glycerol

INT ERNAL INTER NAL MIXING MIXING ESO addition

INT ERNAL INTER NAL MIXING MIXING

GRINDI GRINDING NG

COMPRES COMPRESSION SION G MOULDIN MOULDING

CHARACTERI ZATION CHARACTERIZAT ION − Water uptake − Contact angle − Tensile test − DMTA − X-Ray diffraction Scheme 2. Flow chart for the processing of TPS modified with ESO.

in a low density polyethylene bag. The starch was used as received without drying. The triethylamine was added as a catalyst for the condensation reaction of hydroxyl groups with oxirane rings. The components were mixed for 45 min by hand until a homogeneous high viscosity mass was obtained. Afterwards, the pre-blend was added in internal mixer (Brabender PlastographTM , Duisburg, Germany) torque rheometer equipped with two counter-rotating screws and a capacity of 50 ml working at 140 ◦ C and 80 rpm. Subsequently, and after complete water evaporation, ESO (1, 3 and 4%) was added and blended at 140 ◦ C for at least 20 min until the torque reached a stable plateau. The removal of water was confirmed with FTIR, by the total vanishing of the peak at 1645 cm−1 associated with water. 2.2.2. Compression moulding processing Each formulation obtained after mixing in the internal mixer was pelletized to obtain a particle size able to be processed by compression moulding. The materials were grinded using a mill (AgrimsaTM, Sant Adria’ de Besos, Spain) equipped with a set of knifes and sieves. Once the material was milled, it was introduced in a stainless mould to obtain specimens whose mechanical characterization corresponds to ASTM D638 standard specifications with specimen corresponding to type I for tensile strength. The mould filled with the material was introduced in a hydraulically operated laboratory press LabEcon300 (Fontijne GrotnesTM, Vlaardingen, The Netherlands). The first stage in the compression moulding was carried out by approaching the warm plates of the press to force the softening of the material before compression, then compression at 180 ◦ C and 30 kg cm−2 was implemented. The different steps in the processing of modified TPS are depicted in Scheme 2.

R. Belhassen et al. / Industrial Crops and Products 53 (2014) 261–267

2.2.3. Melt flow index evaluation (MFI) Measurement was performed in a melt flow quick index apparatus (CeastTM , Torino, Italy). The measurements were carried out at 180◦ C with a load of 2.16 kgf. The dried TPS samples were obtained were pelletized into pieces of about 2 mm using a blade cutting mill, and then immediately introduced on the capillary and extruded through a die of length 8 mm and diameter 2.096 mm. The melt mass flow rate (MFR) during 1 min is determined and expressed in g/10 min as MFI value. 2.2.4. Fourier transform infrared spectroscopy (FTIR) The reaction between the epoxidized soybean oil and TPS was monitored with a FTIR Perkin–Elmer instrument model BXII equipped with a temperature-controlled sample holder that allowed the in situ analysis of the condensation reaction between epoxidized soybean oil and TPS. Each sample in the form of a thin film was prepared by pressing a sample of the TPS from internal mixer between two Teflon sheets at 100 ◦ C to obtain a thin film of about 100 ␮m thicknesses. In order to make easier the thin-film formation by mild pressing, the TPS sample was cute into small pieces and mixed with 20–30% of water to further plasticizes the sample and succeed obtaining extremely thin TPS film. Above 100 ␮m thickness, the absorption by the sample is too much and a good quality FTIR spectrum is no-longer possible. Then the film was introduced in the measuring cell purged with nitrogen and kept at 90 ◦ C for 5 min until the peak indicating 1645 cm−1 and associated with water completely vanishes. Then the film was coated with the appropriate amount of ESO and pressed in a sandwich one more time to ensure homogenous distribution within the TPS film, and the FTIR spectrum was recorder at regular intervals at 140 ◦ C under nitrogen atmosphere by means of 10 scans with a resolution of 2 cm−1 . 2.2.5. Water uptake A sample with 2 mm thickness obtained by compression moulding was submitted to a controlled environment (temperature: 23◦ C; moisture: 40%) by using a Dycometal climate chamber (Sant Boi de Llobregat, Spain). Each material was studied by triplicate, evaluating the water uptake by weighting the samples after different periods of time. The water uptake was determined by using Eq. (1): W.U. =

(W2 − W1 ) × 100 (W1 )

(1)

where Mt is the value of water uptake at time t and M∞ is the value of water uptake once the water equilibrium is reached; k and n are constants; L is the thickness of the sample and D is the diffusion coefficient. Eq. (1) can be applied only for materials with Fickian behaviour (n = 0.5) and for values of (Mt /M∞ ) ≤ 0.5. With Fickian behaviour (n = 0.5) and for values of (Mt /M∞ ) ≤ 0.5. 2.2.6. Contact angle Contact angle measurements were carried out by deposing a calibrated liquid drop on the surface of TPS films. The contact angle apparatus used was an OCA 15 from Dataphysics, equipped with a CCD camera, with a resolution of 752 × 582 square pixels, working at an acquisition speed of 50 frames per second. The processing of the collected data was achieved using OCA software. 2.2.7. Mechanical properties Tensile test was carried out by means of a Universal testing machine (Instron 1122, Zamudio, Spain), equipped with a load of 5 kN, conforming to ASTM D638M-90 standard specifications using Type I specimen form. The specimens were tested after 40 days of conditioning at 23◦ C and 40% relative humidity (RH) in the climate chamber to reach the water content equilibrium (M∞ ).

263

2.2.8. Dynamic mechanical analysis (DMA) Dynamic mechanical analysis was conducted in a tension mode using a Diamond (Perkin–Elmer). Temperature scans were run from −80 ◦ C to 100 ◦ C at a heating rate of 2 ◦ C/min, frequency of 1 Hz, and amplitude deformation of 10 ␮m. The storage (E ) and the loss (E ) modulus of the sample and the loss factor tan ı (E /E ) were measured as a function of temperature. TPS (with and without ESO) samples with 20 mm length, 5 mm width and 2 mm thickness were obtained by hot-pressing in moulded plates. Analyses were performed after conditioning the samples for one month at 23 ◦ C and 40% RH. The main relaxation temperature T is defined as the temperature where the maximum of tan ı is reached. 2.2.9. X-Ray diffraction measurement Diffractograms were recorded using a Bruker powder diffractometer (Model D8 Advance) operating at the CuKa wavelength ˚ Measurements of diffracted intensities were made over of 1.542 A. the angular range of 7–30◦ (2) at ambient temperature, with an increment step of 0.1◦ and a rate of 1 step per 10 s. Unless otherwise specified, all the samples were previously conditioned at 40% relative humidity for 40 days. The compression-moulded samples were cut and milled during 5 s in order to obtain a powder. 3. Results and discussion ESO is a low cost epoxy compound derived from a renewable resource which might be used as a crosslinking agent for starch through the capacity of the epoxide ring to react with hydroxyl groups by condensation. Because this reaction is a ring opening condensation reaction, it does not produce any reactive volatiles and the crosslinks formed are stable linkages with excellent chemical resistance properties. For this reason, ESO have been chosen in order to bring about in-situ a partial crosslinking of the starch by reactive blending of the TPS with ESO. This is expected to occur through the condensation of the epoxide ring with the starch hydroxyl groups. 3.1. FTIR probe of the reaction between starch and ESO To select the appropriate reaction conditions in terms of the reaction temperature and the residence time of the ESO during the melt processing with TPS, FTIR analysis was used for the in situ monitoring of the reactive blending process by following the disappearance of the epoxide ring. Several precautions were taken in order to simulate the condition during the blending process as reported in the experimental section. Given the low content of ESO and the interference of the ESO bands with those of starch, only the typical band of the epoxide ring at 824 cm−1 assigned the stretching C O C of epoxide group is still detected after mixing the TPS with the ESO. Using this band, it was possible to monitor the condensation between the hydroxyl groups (of starch but also glycerol) and the oxirane rings of ESO. An example of FTIR spectra, after the background recording, is reported in Fig. 1. It clearly showed the progressive vanishing of the oxirane band at 824 cm−1 . 3.2. X-ray diffraction The crystallinity degree of TPS is an important parameter that determines to what extend the material is useful as it affects considerably the mechanical and barrier properties of the material. During processing, plasticizer molecules enter into the starch granules, replacing starch intermolecular and intramolecular hydrogen bonds with starch plasticizer-hydrogen bonds and destroying the lattices of crystalline domains. However, during the ageing of the plasticized starch, a time-induced

264

R. Belhassen et al. / Industrial Crops and Products 53 (2014) 261–267

0 min 1 min 2 min 5 min 15 min

10 min

20 min Epoxide ring at 824 cm -1

900

850

800

750

Wave leng th (cm-1) Fig. 1. FTIR spectra of TPS film during reaction with ESO at 140 ◦ C recorded in situ at different time after the background recording. Arrows indicate the time evolution after the addition of ESO.

recrystallisation or retrogradation, involving amylose reorganisation in helical structure occurs. This crystallinity is important for the stiffness of the TPS. Unlike native starches which exhibit A, B or C-type crystal lattices with a double helical structure, the most common crystalline arrangement in plasticized starch is E and V-type structure, a single-helical structure ‘Inclusion Complex’ made up of amylose and glycerol (Souza and Andrade, 2002). X-ray diffraction has been used for investigating changes in crystallinity after melt processing and reactive blending. The Xray diffraction patterns of starch powder, TPS and TPS-ESO are given in Fig. 2. As expected, the native starch exhibited the A-type diffraction pattern typical of cereal starch, with the peaks at Bragg

Va

Eh

Taking account of the high functionality of the ESO and its ability to react with starch and glycerol, we should expect an increase in the melt viscosity of the TPS following the reactive blending of TPS with ESO. To confirm this hypothesis, MFI measurement at 180◦ C on TPS with different content of ESO was carried out. The results in Fig. 3 show a huge drop in the MFI with the addition of ESO, which is indicative of an important grow in the melt viscosity. To further confirm the involvement of the epoxide groups in the crosslinking process of starch, non-epoxydized soybean oil (SO) were mixed with starch and glycerol following the same processing condition, and the MFI was measured. As shown in Fig. 4, no change in the MFI value is observed under such condition, which confirmed that the rose in the melt viscosity after the reactive blending of TPS with ESO is imparted by the epoxide ring.

Eh Vh

TPS-3% ESO TPS-1% ESO

TPS

3.4. Surface hydrophobicity by contact angle analysis Native starch

5

angles (2) 15, 17, 18 and 23◦ (Zobel, 1964). Furthermore, for all the studied compositions, the residual crystalline peaks for the native starch crystals at ∼17◦ were absent, suggesting that the inherent crystalline structure of the native starch was completely disrupted during the melt processing condition. In the absence of ESO, TPS exhibited Vh -type crystalline structure characterized by the diffraction peaks at 2 equal to 13.7◦ and 19.3◦ . However, in TPS processed in presence of ESO, the crystallisation mode evolved from Vh -type to a mixture of Eh and Va -type crystallisation mode with typical diffraction peaks at 2 equal to 12.2.7◦ and 18.4◦ (for Eh ) and 13.6◦ and 21.1◦ (for Va ). Although all of the Va , Vh and Eh involved a single-helical amylose crystals, their occurrence is related to the differences in hydration of the unit cell (Van Soest et al., 1996). The Va lattice has more contracted amylose helices and contains less water than the Vh lattice. The prevalence of Va and Eh crystalline form might be due to the lower water content during the crystallisation process of TPS when ESO is added. This will be confirmed later in the Section 3.6. 3.3. Changes in the MFI

Va

TPS-4% ESO

Fig. 3. Change in the MFI value at 180 ◦ C according to the ESO content.

10

15 2θ (°)

20

25

30

Fig. 2. X-ray diffraction patterns of TPS samples with different amount of ESO after storing for 40 days under 40% RH and 23 ◦ C.

To assess how the addition of ESO might affect the film hydrophilicity, contact angle measurements on TPS modified with different content of ESO was carried out using water as a probe (Fig. 5). In the absence of ESO, the water contact angle reached about 65◦ which is indicative of a hydrophilic surface. The hydrophilic character is expected because of the presence of high density of hydroxyl groups within the starch backbone but also due to the

R. Belhassen et al. / Industrial Crops and Products 53 (2014) 261–267

Fig. 4. MFI value of TPS at 180 ◦ C; in the absence of ESO (0% ESO), and in presence of 4% ESO and 4% SO.

incorporation of glycerol. The reactive blending with ESO led to an increase in the contact angle which attained about 77◦ , 80◦ , and 88◦ in presence of 1%, 3% and 4% ESO, respectively. Thus although the amount of added ESO is quite small, a meaningfully enhancement in the hydrophobic character of the TPS was observed. This effect is likely due to the ability of long hydrocarbon chains of the soybean oil to mask the surface hydroxyl groups of TPS. 3.5. Mechanical properties in the linear and non-linear domain To evaluate the mechanical properties of the TPS and TPS-ESO materials, the tensile strength ( T ), Young’s modulus (E) and elongation at break (ε) were determined from stress–strain curves at room temperature after 40 days ageing at 40% RH in order to reach the water adsorption equilibrium. Actually, just after processing the water content is close to zero, and conditioning at constant relative humidity was necessary in order to reach the water equilibrium adsorption and avoid errors arising from the plasticizing effect of water effect. It is worth noting that, based on our previous work; the crystalline degree remained unchanged during the storage period (Belhassen et al., 2011). As shown in Fig. 6, the reactive blending of ESO with TPS led to a huge evolution in the mechanical properties of the material. Both of the Young’s modulus and tensile strength increase sharply up to 3% of added ESO, and then slightly decrease when the content of ESO reaches 4%. The modulus E grows from 15.54 MPa for 100

the pristine TPS up to 156 MPA, 232 MPa and 187 MPa in presence of 1, 3 and 4% of ESO, respectively, indicating a huge increase in the stiffness of the material. This effect is likely to be the consequence of the partial crosslinking of starch induced by the reaction of epoxide ring of the ESO with the starch hydroxyl groups during the reactive melting processing of TPS. However, a decrease in both a modulus and the strength is noted up to 3% ESO. A possible origin of such unexpected behaviour might arise from a chain breakage by thermal degradation of the TPS due to the excessive rise in the melt viscosity. The elongation at break of the modified TPS (Fig. 7) decreased with the addition of ESO which is quite expected given the strong stiffening effect imparted by the addition of ESO. 3.6. Water absorption The effect of the ESO addition on the water absorption of TPS was analysed by following the water uptake of TPS sample during storage around 40% relative humidity. The results shown in Fig. 8 revealed that in the absence of ESO, the water absorption increases progressively until reaching a plateau at about 5 wt% with respect to TPS after 8 days conditioning at 40◦ RH. In presence of 120

Elongation at break (%)

90 Contact angle(°)

Fig. 6. Evolution of the tensile modulus and tensile strength according to the ESO content: (samples were stored for 40 days at 23 ◦ C and 40% relative humidity to reach the water equilibrium adsorption).

100

TPS TPS-1%ESO TPS-3%ESO TPS-4%ESO

95

265

85 80 75

80

60

40

20

70 65

0 0

60 0

2

4 Time(s)

6

8

Fig. 5. Evolution of water contact angle versus time of TPS samples with different amount in ESO.

1

2

3

4

5

ESO content (%) Fig. 7. Evolution of the elongation at break according to the ESO content: (samples were stored for 40 days at 23 ◦ C and 40% relative humidity to reach the water equilibrium adsorption).

266

R. Belhassen et al. / Industrial Crops and Products 53 (2014) 261–267

1.5 × 10−8 cm2 s−1 in presence of 4% ESO. This evolution might be due to the increase in the hydrophibicity following the reactive melting of TPS with ESO. The increase in the stiffness of the material might be also likely to account for the decrease in the water diffusion.

6

Water uptake (%)

5 4 TPS TPS-3%ESO

3

3.7. Dynamic mechanical analysis (DMA)

TPS-4%ESO

2 1 0 0

4

8

12 Time (days)

16

20

Fig. 8. Evolution of the water uptake with time, at 23 ◦ C and 40% relative humidity, for TPS with different content in ESO.

4% ESO, the adsorption plateau reaches about 5 wt% after 18 days. Therefore, the addition of ESO retards the diffusion kinetic of water, without alerting the extent of the adsorption maximum. This is confirmed by the decrease in the diffusion coefficient of TPS-4%ESO which passes from 3.4 × 10−8 cm2 s−1 for the initial TPS to about 1.0E+10

A

1.0E+09 E' (Pa)

TPS-3% ESO TPS-1% ESO

DMA analysis was also carried out to investigate the mechanical behaviours of materials and to obtain information about the relaxation mechanisms that may be correlated with the dynamic and the microstructure of the material. The evolution of the storage modulus (E ) and loss factor (tan ı) of TPS samples (Fig. 9) containing 24% glycerol displayed a typical behaviour of a partially miscible system with two main transitions. The first relaxation at around −60 ◦ C (labelled ␤) is attributed to the relaxation of the phase rich in glycerol (Averous and Boquillon, 2004; Forssella et al., 1997). The second one (labelled ␣), in the 10–50 ◦ C range, corresponds to the glass transition temperature (Tg ) of a plasticized starch phase (Forssella et al., 1997). With the addition of ESO, the modulus remained almost constant below the glass transition and increased above Tg . For instance, at 80 ◦ C and in presence of 4% ESO, the storage modulus attained about 5 MPa, which represents about 2.5-fold enhancement with respect to the pristine TPS. The increase in the E above Tg is the consequence of the partial crosslinking of starch following the reaction with ESO. The position of ␣ relaxation underwent also an upward shift to higher temperature with the addition of ESO along with a decrease in its magnitude. This shift is indicative of a reduction in the mobility of the plasticized starch macromolecule upon the reactive blending with ESO. This phenomenon further supported the hypothesis of the partial crosslinkling of starch macromolecules resulting from the condensation between the starch hydroxyl groups and the epoxide rings of ESO.

TPS-4% ESO

1.0E+08

4. Conclusions TPS-0% ESO

1.0E+07 -80

-60

-40

-20

0

20

40

60

80

100

120

Temperature(°C)

0.35

B 0.3

TPS-% ESO

0.25 Tang δ

TPS-1 %ESO

0.2 TPS-3 %ESO

0.15 TPS-4 %ESO

0.1

0.05 -80

-60

-40

-20

20 40 0 Temperature(°C)

60

80

100

120

Fig. 9. Evolution of the (A) the storage tensile modulus E , and (B) loss angle tan ı vs. Temperature at 1 Hz for TPS with different amount of ESO. (samples were stored for 40 days at 23 ◦ C and 40% relative humidity)

Reactive blending of ESO with TPS, plasticized with glycerol, was successfully carried out using internal mixer. Evidence of the ring opening condensation reaction between the epoxide of the ESO and the hydroxyl groups of starch and glycerol was confirmed using FTIR. The addition of ESO to TPS led to a huge enhancement in both Young’s modulus and tensile strength. The glass transition of the plasticized starch phased underwent a shift to higher temperature upon the addition of ESO and significant grew in the hydrophobicity of the TPS was also observed. However, due to the high functionality of the ESO (about 5), the addition of even small amount of ESO led to a meaningful evolution of the mechanical properties and resulted in a huge increase in the melt viscosity, which adversely affect the processing behaviour of TPS. An enhancement in the hydrophobic character of the film was noted, namely as the content of ESO exceeded 3%. Although the equilibrium water uptake did not show meaningful reduction, the adsorption kinetic demonstrated a delay compared to the pristine TPS. To alleviate the shortcoming of ESO, namely in term of the rigidification effect and in order to increase the amount of modifying oil, epoxydized fatty acid with lower functionality might be added instead of ESO. Accordingly higher content of reactive epoxy modifier might be added, without impairing the processing behaviour. This is expected to further enhance the hydrophobic character and reduce the water uptake of the TPS. This work is under investigation.

R. Belhassen et al. / Industrial Crops and Products 53 (2014) 261–267

References Ali, F., Young, W., Chang, Y.W., Kang, S.C., Yoon, J.Y., 2009. Thermal, mechanical and theological properties of poly (lactic acid)/epoxidized soybean oil blends. Polym. Bull. 62, 91–98. Averous, L., Boquillon, N., 2004. Biocomposites based on plasticized starch: thermal and mechanical behaviours. Carbohydr. Polym. 56, 111. Belhassen, R., Fabiola, V., Pere, M., Boufi, S., 2011. Preparation and properties of starch-based biopolymers modified with difunctional isocyanates. Bioresources 6 (1), 81–102. Bengtsson, M., Koch, K., Gatenholm, P., 2003. Surface octanoylation of highamylose potato starch films. Carbohydr. Polym. 54, 1–11. Bourtoom, T., Chinnan, M.S., 2008. Preparation and properties of rice starch chitosan blend biodegradable film. LWT – Food. Sci. Technol. 41 (9), 1633–1641. Carvalho, A.J.F., Curvelo, A.A.S., Gandini, A., 2005. Surface chemical modification of thermoplastic starch: reactions with isocyanates, epoxy functions and stearoyl chloride. Ind. Crop. Prod. 21, 331–336. Forssella, P.M., Mikkilti, J.M., Moates, G.K., Parker, R., 1997. Phase and glass transition behaviour, of concentrated barley starch–glycerol–water mixtures, a model for thermoplastic starch. Carbohydr. Polym. 34, 275–282. Ghanmbarzadeh, B., Almasi, H., Entezami, A.A., 2010. Physical properties of edible modified starch/carboxymethyl cellulose films. Innov. Food. Sci. Emerg. Technol. 11 (4), 697–702. Gupta, A.P., Ahmad, S., Dev, A., 2010. Development of novel bio-based soybean oil epoxy resins as a function of hardener stoichiometry. Polym. Plast. Technol. Eng. 49, 657–661. Japon, S., Boogh, L., Leterrier, Y., Manson, J.A.E., 2000. Reactive processing of poly (ethylene terephthalate) modified with multifunctional epoxy-based additives. Polymer 41, 5809–5818. Lin, B., Yang, L.T., Dai, H.H., Yi, A.H., 2008. Kinetic studies on oxirane cleavage of epoxidized soybean oil by methanol and characterization of polyols. J. Am. Oil Chem. Soc. 85, 113–117. Liu, Z.S., Erhan, S.Z., Xu, J., Calvert, P.D., 2004a. Solid freeform fabrication of epoxidized soybean oil/epoxy composites with di-tri- and polyethylene amine curing agents. J. Appl. Polym. Sci. 93, 356–363.

267

Liu, H., Xie, F., Yu, L., Chen, L., Li, L., 2009. Thermal processing of starch-based polymer. Prog. Polym. Sci. 34 (12), 1348–1368. Liu, Z.S., Erhan, S.Z., Xu, J., Calvert, P.D., 2004b. Solid freeform fabrication of soybeanoil composites reinforced with clay and fibers. J. Am. Oil Chem. Soc. 81, 605–610. Parra, D.F., Tadini, C.C., Ponce, P., Lugão, A.B., 2004. Mechanical properties and water vapour transmission in some blends of cassava starch edible films. Carbohydr. Polym. 58 (4), 475–481. Souza, R.C.R., Andrade, C.T., 2002. Advantages of low energy adhesion pp for ballistics. Adv. Polym. Technol. 21, 17–24, ISSN 1098-2329. Van Soest, J.J.G., Hulleman, S.H.D., de Wit, D., Vliegenthart, J.F.G., 1996. Crystallinity in starch bioplastics. Ind. Crop. Prod. 5 (1), 1–122. Walia, P.S., Lawton, J.W., Shogren, R.L., 2000. Mechanical properties of thermoplastic starch/poly (hydroxy ester ether) blends: effect of moisture during and after processing. J. Appl. Polym. Sci. 84 (1), 121–131. Wang, X.L., Yang, K.K., Wang, Y.Z., 2004. Crystallization and morphology of a novel biodegradable polymer system: poly (1,4-dioxan-2-one)/starch blends. Acta. Mater. 52 (15), 4899–4905. Wool, R.P., Kusefoglu, S.K., Khot, S.N., Zhao, R., Palmese, G., Boyd, A., Fisher, C., Bandypadhyay, S., Paesano, A., Ranade, S., Dhurjati, P., Khot, S., LaScala, J., Williams, G., Ligon, M., Gibbons, K., Wang, C., Soultoukis, C., Bryner, M., Rhinehart, J., Robinson, A., 1998. Affordable composites from renewable sources (ACRES). Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.), 39–90. Xia, F.-X., Wang, X.-L., Song, F., Wang, Y.-Z., 2012. A Biobased blend of cellulose diacetate with starch. J. Polym. Environ. 20, 1103–1111. Xiong, Z., Yang, Y., Feng, J., Zhang, X., Zhang, C., Tang, Z., Zhu, J., 2013. Preparation and characterization of poly (lactic acid)/starch composites toughened with epoxidized soybean oil. Carbohydr. Polym. 92, 810–816. Yu, J.G., Liu, Z.H., 2002. Properties of the surface hydrophobicated thermoplastic starch. China Plastics 16, 35–38 [Chinese]. Zhan, G.Z., Zhao, L., Hu, S., Gan, W.J., Yu, Y.F., Tang, X.L., 2008. A novel biobased resin epoxidized soybean oil modified cyanate ester. Polym. Eng. Sci. 48, 1322–1328. Zhou, J., Zhang, J., Ma, Y.H., Tong, J., 2008. Surface photo-crosslinking of corn starch sheets. Carbohydr. Polym. 74, 405–410. Zobel, H.F., 1964. X-ray analysis of starch granules. In: Whistler, L.R., Smith, R.J. (Eds.), Methods in Carbohydrate Chemistry IV. Academic press, New York, pp. 109–113.