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Effect of ultra-violet cross-linking on the properties of boric acid and glycerol co-plasticized thermoplastic starch films
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Effect of ultra-violet cross-linking on the properties of boric acid and glycerol co-plasticized thermoplastic starch films ⁎
Bahram Khana, Muhammad Bilal Khan Niazia, , Zaib Jahana, Wasif Farooqb, Salman Raza Naqvia, Majid Alic, Israr Ahmeda, Arshad Hussaina a
School of Chemical and Materials Engineering, National University of Sciences and Technology, Islamabad, Pakistan Department of Chemical Engineering, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia c USPCAS-E, National University of Sciences and Technology (NUST), Sector H-12, Islamabad, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Boric acid Glycerol Thermoplastic starch Cross-linking Spray drying UV radiation
Thermoplastic starch (TPS) is a potential alternative to non-degradable plastics. However, the hydrophilic nature and relatively weak mechanical properties is a major hurdle to its commercial application. In this study, Ultraviolet (UV) irradiation based cross-linking was investigated for its effect on the mechanical properties of TPS films plasticized by boric acid, and glycerol. Spray dried amorphous powder was compressed to obtained TPS films. All the samples were amorphous under dry conditions (RH0%) while retro-gradation was resisted at the relative humidity (RH) of50% and 100% for all the films. The concentration of the plasticizers influenced the moisture uptake ability of TPS films. UV irradiation showed no effect on the crystallinity of TPS films. However, it enhanced the mechanical strength and lessened the solubility and degree of swelling. Boric acid plasticized and co-plasticized samples revealed improved mechanical properties before and after UV irradiation compared to glycerol but showed more moisture sensitivity before cross-linking. The UV irradiated starch with 30% boric acid as plasticizer (30BA.UV.PS) showed highest tensile strength (4.28 MPa) among all the samples at 50% relative humidity. Moreover, the strain at break of the same sample decreased from 12.53% to 9.12% at 50% relative humidity.
1. Introduction
Researchers are trying to enhance the mechanical characteristics and shelf-life of TPS and to overcome its hydrophilic nature (Zhou et al., 2008). Recently, spray drying of starch/plasticizers formulated samples were highlighted as a possible technique to produce amorphous thermoplastic materials (Niazi & Broekhuis, 2012; Singh & Van den Mooter, 2016). Spray drying is a fast drying process that inhibits the moisture problem better compared to the extrusion technology and solvent casting. It is believed that amylose to amylopectin ratio had just a minor impact on the characteristics of solution spray dried samples (Thybo, Hovgaard, Lindelov, Brask, & Anderson, 2008). Amorphous thermoplastic films produced by natural plasticizers like glycerol and urea showed that low molecular weight plasticizers exhibit better mixing properties and inhibited retro-gradation. However, resistance to retro-gradation resulted in the loss of mechanical properties such as tensile strength, and more strain with moisture uptake (Ahmed, Niazi, Hussain, & Jahan, 2018; Niazi, 2013). Hypothetically, the properties of TPS films could be enhanced if retro-gradation is inhibited along with controlled moisture uptake. The radiation processing and chemical derivation have been proposed to
In the past few years, research on bio-based plastics has accelerated due to their potential biodegradability for environmental protection as well as a reduction in non-degradable plastic waste generation. The replacement and disposal of petroleum-based plastic materials have triggers the interest of researchers in biodegradable films (ValenciaSullca, Vargas, Atarés, & Chiralt, 2018). Thermoplastic starch (TPS) is a potential candidate to replace the petroleum-based commodity plastics as being biodegradable, renewable, and economical (Mendes et al., 2016; Zhou, Zhang, Ma, & Tong, 2008).TPS based on hydrocolloids can act as a barrier to control the transfer of moisture, CO2, lipids, oxygen and flavor components, leading to increase the shelf life of food products and prevent quality deterioration (Emmambux & Stading, 2007). However, use of TPS as an alternative is limited due to various factors such as its hydrophilic nature and poor mechanical characteristics. Moreover, water sensitivity affects its shelf-life along with its mechanical properties because of retro-gradation of TPS products (Liu, Fan, Mo, Yang, & Pang, 2018; Niazi, 2013).
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Corresponding author. E-mail address: [email protected] (M.B. Khan Niazi).
https://doi.org/10.1016/j.fpsl.2018.05.006 Received 21 November 2017; Received in revised form 16 April 2018; Accepted 26 May 2018 2214-2894/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Khan, B., Food Packaging and Shelf Life (2018), https://doi.org/10.1016/j.fpsl.2018.05.006
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Fig. 1. Plasticizers used for the preparation of TPS powder and films.
2.1. Preparation of TPS powder
generate water-resistant products to avert degradation. Starch formulations prepared with boric acid and glycerol as plasticizer and cross-linked by ultra-violet irradiation were considered as a potential method to enhance the characteristics of the films (Reddy & Yang, 2010; Shah, Naqash, Gani, & Masoodi, 2016). Significant improvement in the mechanical properties of cross-linked starch blends as compared to the non-cross-linked onesare reported in the literature (Guleria, Singha, & Rana, 2018; Niazi & Broekhuis, 2015; Niazi, 2013). Furthermore, the radiation processing generates cross-linked covalent bonding between polymer molecules that enhanced the properties of polymeric items. Boric Acid (BA) and glycerol (G) are non-toxic and inexpensive chemicals (Soltani, Shetab-Boushehri, Mohannadi, & Shetab-Boushehri, 2013; Vieira, Da Silva, Santos, & Beppu, 2011). Both can produce strong hydrogen bonding with starch and improve its water and thermal stability (Fig. 1) (Reddy & Yang, 2010; Zain, Kahar, & Noriman, 2016). Moreover, they act as an inhibitor to starch retro-gradation process (Vieira et al., 2011). This work investigated the effect of ultra-violet irradiations on the physical properties of amorphous thermoplastic starch films (TPS). TPS films were obtained by compression molding of solution spray dried powder. Boric acid and glycerol were used as plasticizer and co-plasticizer. The study mainly focused on the analysis of retro-gradation and mechanical properties of films. Molecular interaction and thermal behavior of TPS films were analyzed by Differential Scanning Calorimetric (DSC) and Thermal Gravimetric Analysis (TGA), respectively. Moisture uptakes at different levels of humidity, retro-gradation and mechanical properties of TPS films were examined.
An aqueous solution of starch was prepared by heating 15% (w/w) of dry oxidized amylopectin from potato starch and 30% (w/w) of plasticizer based on dry starch contents. The concentration of plasticizers (boric acid and glycerol) used in solution alone or in combination was maintained at 30% w/w based on dry starch. The mixtures were heated at 95 °C for 25 min, while solution was continuously stirred at the rate of 600 rpm. The homogenized solutions were fed to Buchi mini spray dryer B-191 equipped with a 0.7 mm nozzle. 2.2. Preparation of TPS films Thermoplastic starch films (∅ 10 mm x 0.5 mm) were obtained by compression molding of TPS powder by using Fontijne Holland Table Press TH400. Each sample was compressed for 5.0 min under 25 bar pressure. The molding temperature was set at 140 °C. 2.3. Surface photo cross-linking Thermoplastic starch films (∅ 10 mm x 0.5 mm) as prepared were conditioned at two different humidi0ty levels (RH50% and RH100%) for 24 h (Zhou et al., 2008). Surface photo cross-linking was performed as mentioned in literature with few alterations (Niazi & Broekhuis, 2015; Niazi, 2013). After conditioning, all the films were exposed to UV irradiations at normal atmospheric conditions using an Intelli-ray 600 W shuttered UV flood light. The irradiation intensity of 100 mW/ cm2 (320–390 nm) was distributed evenly in the chamber. TPS films were placed at 15.5 cm away from the top of the chamber and UV irradiations were applied for 20 min continuously at 100% RH. After ultra-violet exposure, each film was conditioned again for one day at 50% relative humidity. Tensile testing was performed for all the samples after 24 h.
2. Materials and methods Oxidized amylopectin from potato starch (Perfecta-film X-85, moisture content 15% w) was provided by AVEBE (The Netherlands). Analytical grade boric acid and glycerol were acquired from SigmaAldrich. All materials were consumed as provided by the suppliers without any further modifications. The codes symbolized for both the powders and the films are depicted in abbreviations (Table 1). The weight percentage of plasticizer used relative to starch is indicated by the digits in the codes.
2.4. X-Ray diffraction (XRD) XRD was used to examine the crystalline structure of the spray-dried powder and film. Bruker D8 equipped with Cu radiation exhibiting a wavelength of 1.5418 Å was used to record diffractograms from 5° 2θ to 40° 2θ. The step size was 0.02° 2θ and scan speed was set at 2 s/step. The XRD was performed at 40 kV and 40 mA. A sample holder was used to analyze the powders having a sample compartment of ∅25 mm x 2 mm. An adjustable sample holder was used to analyze ∅10 mm x 0.5 mm TPS films.
Table 1 Material codes and description. Code
Description
U.S S.D 5B A.25 G.PS 10 BA.20 G.PS 30 BA.PS 30 G.PS UV.PS
Feedstock material Spray dried starch 5% boric acid, 25% glycerol plasticized starch(w/w of starch) 10% boric acid, 20% glycerol plasticized starch(w/w of starch) 30% boric acid plasticized starch(w/w of starch) 30% glycerol plasticized starch(w/w of starch) Ultra violet irradiated plasticized starch
2.5. Thermal gravimetric and differential thermal gravimetric analysis (TGA/DTGA) An open pan TGA was performed in a Perkin-Elmer TGA in nitrogen gas atmosphere to evaluate the thermal properties of TPS samples. The weight of sample ranged from 7.5 to 10 mg for the thermal analysis. The temperature was varied from 25 °C to 900 °C at a heating rate of 2
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Fig. 2. XRD spectra of (a) freshly spray dried powder, (b) freshly prepared films by compression molding.
24 h at room temperature i.e. 25 °C. Moisture remained on the film surface was removed once the equilibrium was attained (24 h). Eq. (2) was used to calculate the swelling behavior in TPS film.
25 °C/min. Software – Origin 8.1 was used to find out the derivative of weight loss curve yielding DTGA curve. The decomposition temperature (Tdec) was determined by the method described by Soliman, Nabila, ElShinnaway, and Mobarak (1997).
Swelling Behaviour(%) = 2.6. Differential scanning calorimetry (DSC)
(We − W0) * 100 W0
(2)
Where, We = weight of the TPS film after 24 h W0 = weight of the TPS films on the first day The swollen TPS films were dried again at 60 °C for 24 h and the solubility was measured by Eq. (3).
DSC-60 Shimadzu Co. was used to determine glass transition temperature (Tg) of the samples by DSC measurements. Before examination, an empty aluminum pan was taken as a reference test. All the samples were scanned at a rate of 10 °C/min from 10 to 200 °C. Open pan DSC was performed to eradicate any remaining moisture from the sample in the first run.
Solubility(%) =
(W0 − Wd) * 100 Wd
(3)
Where, W0 = weight of the TPS films on the first day Wd = dry weight of swollen TPS film
2.7. Moisture uptake Moisture uptake of thermoplastic films was gravimetrically measured at different relative humidity conditions i.e. RH 0% (dried silica), RH 50% (35.64% CaCl2 solution) and RH 100% (distilled water). The measurements were performed on a daily basis for the first week. Later, measurements were taken at day 14, 21, and 28. Eq. (1) was used to determine the moisture uptake of the films (Zhang, Zhang, Wang, & Wang, 2009).
3. Results and discussion
where, Wn = weight at the day of measurement, n, 1,2,3……… W0 = weight directly after film making
Boric acid and glycerol were used as a plasticizer and a co-plasticizer aiming to interact with starch molecules by developing hydrogen bonds. Cross-linking by Ultra-violet irradiation was performed to enhance the mechanical strength of thermoplastic films (TPS). Both plasticizers contain several hydroxyl groups in their structures that tend to show interaction with starch through hydrogen bonding (Amoako & Awika, 2016; Wu, Xu, & Hakkarainen, 2016). Moreover, the amount of each plasticizer, their ratios, and combination in the formulations play a significant role on the properties of plasticized TPS.
2.8. Mechanical testing
3.1. Crystallinity of freshly prepared TPS powder and films
Mechanical behavior of the TPS samples was investigated using Instron 4301 tensile tester. Tensile strength (TS) and percentage elongation at break (%E) were tested according to ASTM D1708. The sample was conditioned at two different valued of relative humidity i.e. RH 50% and RH 100% at 25 °C for 24 h. The crosshead speed was set at 10.0 mm/min and the width and thickness of each sample were measured before testing. UV cross-linked TPS films were also tested using the similar procedure.
The XRD patterns of TPS powder and films formulated with different plasticizers are shown in Fig. 2. All spray dried powders were amorphous (Fig. 2a) and the results were in line with the literature (BeMiller & Whistler, 2009; Lipiainen, Peltoniemi, Raikkonen, & Juppo, 2016). The B and V-type crystalline peaks at 2θ = 15.01°, 17.37°, 20.11°, and 23.64° in sample (U.S) were absent in spray-dried powder samples. Starch chains could not be associated with crystal lattices due to the short drying spans attained in spray drying. XRD analysis of spray dried TPS samples were in line with literature (Niazi & Broekhuis, 2012; Niazi, 2013). Han and Zhang et.al. (Zhang & Han, 2010) prepared solvent cast starch films plasticized with glycerol with various concentrations. Their XRD analysis showed that films possessed B-type crystal that agreed to C-type nature of the pea starch. XRD patterns showed intensity peaks at 16.9° and 22.3°.
Moisture uptake =
Wn − W0 *100% W0
(1)
2.9. Solubility and swelling behavior Solubility (S) and the Swelling Behavior (SB) of films were measured according to the method described by Yun, Wee, Byun, and Yoon (2008). Dried thermoplastic films were immersed in distilled water for 3
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All the freshly prepared spray dried TPS powders and films were completely amorphous irrespective of the amount of plasticizer or coplasticizer used, except S.D sample, which was mat. Freshly prepared TPS samples were transparent. TPS films did not exhibit B-type crystallinity (Fig. 2b). Starch chains were disabled to be associated with crystal lattices due to the short drying time procured in spray drying. The fast drying resisted the starch chains to reallocate into crystal lattices and resulted in producing an amorphous powder and films (Lintingre, Lequeux, Talini, & Tsapis, 2016; vanSoest, Bezemer, deWit, & Vliegenthart, 1996). Moreover, compression molding also had no effect on the amorphous nature of the TPS formulations and retrogradation was resisted. Furthermore, combining boric acid and glycerol in different ratios did not affect the crystallinity. The XRD patterns in Shi et al. (2008) showed that films prepared by solvent casting at 140 °C with glycerol and citric acid used as plasticizers exposed protruding starch and PVA crystalline peaks. Although, the existence of citric acid declined the peak intensity at 19.3° compared to native sample. It’s inhibited crystallinity but could not generate amorphous blends.XRD analysis reported in literature showed the amorphous character of TPS powder and films was attained by solution spray drying technique and it is independent of plasticizer quantity and grouping (Niazi & Broekhuis, 2012; Niazi, 2013; Vialpando et al., 2016).
Table 2 Thermal properties of the solution spray dried powder samples. Samples
Onset, Tdec (oC)
Peak, Tdec,b1 (oC)
Peak, Tdec,b2 (oC)
Peak, Tdec,b3 (oC)
Tg (oC)
S.D 5BA.25 G.PS 10BA.20 G.PS 30BA.PS 30 G.PS
293.92 224.54 279.07 312.79 190.51
– 178.05 179 185 179.97
– 250 250.67 – –
334 311.36 312.26 328.70 328.34
No No No No 136.1
2015). The spray dried unmodified sample (S.D) was taken as a reference sample. S.D showed different results as compared to other formulated samples. Only one peak at 293.92 °C was detected for the decomposition of starch. Apart from moisture loss at 100 °C (labeled ‘a’), all plasticized TPS samples showed thermal stability up to 171 °C temperature. Weight loss sections at different temperature ranges were derived depending on each plasticizer used. Sample (30BA.PS) showed a maximum Tdec of 312.79 °C and while 30 G.PS exhibited the lowest Tdec of 190.51 °C. While, decomposition temperature (Tdec) of samples 10BA.20 G.PS and 5BA.25 G.PS was 279.07 °C and 224.54 °C, respectively. Such observations in Tdec are attributed to strong hydrogen bonding capability of boric acid as compared to glycerol plasticized formulation (Reddy & Yang, 2010; Zain et al., 2016). 30BA.PS revealed upgraded thermal stability as compared to 30 G.PS. In co-plasticized samples, an additional peak (section ‘b1’ and ‘b2’) was observed indicating presence of second plasticizer (Shi et al., 2008). Boric acid and glycerol were taken as co-plasticizers. The accumulation of glycerol with boric acid resulted into reduction in the decomposition
3.2. Thermal properties Thermal properties of TPS powders were analyzed by TGA and DTGA (Fig. 3 and Table 2). Results were not simple though, so various sections were allocated on DTGA minima as done in the literature (Niazi & Broekhuis, 2012; Niazi, 2013; Niazi, Zijlstra, & Broekhuis,
Fig. 3. TGA and DTGA curves of solution spray dried powder samples. 4
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temperature of BA.PS samples and an additional peak (section ‘b1’ and ‘b2’) in the DTGA curve appeared (Fig. 3b). Zain et al. (2016) examined the effect of chemical-mechanical hydrolysis on the TPS thermal properties with citric acid and glycerol as plasticizers. A native starch granule was transformed into a continuous phase. It was revealed that citric acid was more thermally stable as compared to glycerol. This was attributed to the volatility of glycerol. Citric acid modified TPS samples presented higher onset temperature and lower mass loss than glycerol modified TPS samples. The addition of boric acid reduced the rate of weight loss and the residual weight was increased. So the higher thermal stability of the TPS films can be elucidated by strong hydrogen bonds generated by boric acid. By increasing glycerol weight, the thermal stability was reduced. By varying the quantity of plasticizers, formulations behave differently regarding stability. The thermal stability increased in the order 30 G.PS < 5BA.25 G.PS < 10BA.20 G.PS < 30BA.PS. In all coplasticized TPS samples, peak‘b1’ was observed at 178 °C representing the decomposition of glycerol in the blend and peak while peak‘b2’ represents the decomposition of boric acid in the blend at 250 °C. It was observed that the rate of weight loss was lower in the higher percentage boric acid plasticized TPS samples because cross-linking occurred to a lesser extent in the presence of glycerol. The addition of boric acid plasticizer enhanced the carbonization. Staroszczyk (2009) investigated the granular potato starch borated with boric acid and sodium tetra borate decahydrate (borax) in a microwave-assisted solid-state reaction. DSC and XRD results indicated that although the boration caused changes in the macrostructure of starch, the thermal stability of borated starch was improved by 20 °C as compared with native starch. It was noticed that the increase in boric acid caused a shift in decomposition temperature of starch from 277 °C to 282 °C suggesting boric acid as a potential candidate for the thermal stability of starch.
increase in mobility results in high crystallization which leads the crosslinking in the crystalline domains and restrains the amorphous amylopectin regions, eventually increasing the Tg. Tg was only observed for 30 G.PS sample, which can be explained by partial miscibility of glycerol and starch, giving rise to a phase separation in terms of Tg. However, Tg was missing for all plasticized and co-plasticized samples. A better performance of plasticizer is expected because of its higher interaction possibility and higher dispersion level with amylopectin chains. The molecular weight of glycerol being more than boric acid results in low efficiency as a plasticizer and increase in the Tg. Mathew et. Al studied the effect of different polyols on waxy maize starch and found out that Tg decreases with the molecular weight of the plasticizer and increasing moisture content. The results were significantly in line with our work. The moisture content of boric acid plasticized and co-plasticized at RH 100% was high than glycerol plasticized sample, resulting in the absence of Tg peak (Mathew & Dufresne, 2002). 3.4. Moisture uptake The moisture uptake data was recorded at RH 0%, RH 50%, and RH 100%for 28 days to assess the particular behavior of TPS plasticized formulations. Fig. 5 showed the moisture uptake data for all the TPS plasticized samples. The rearrangement of amorphous starch chains was influenced by moisture content resulting in weight changes (Reddy, Kimi, & Haripriya, 2016). Both plasticizers are hydrophilic in nature; hence they absorbed water throughout 28 days. The moisture uptake increased in the order RH 0% < RH 50% < RH 100%. All films showed similar behavior at RH 0% (Fig. 5). Retro-gradation was prevented as the water was absent. TPS plasticized samples showed similar behavior at RH 50%. As the quantity of boric acid was increased, the moisture uptake also increased. Initially, amorphous starch matrix absorbed water resulting a gain in weight. Free volume in the polymeric matrix further enlarged as the water contents kept growing. Certain starch to starch hydrogen bonds was being substituted by starch-water interactions, leading to distended structure (Ahammad & Nguyen, 2016). At RH 100%, the moisture absorptions were high for all the formulations. A significant increase in moisture was observed even after the first week. Glycerol resisted the moisture more than boric acid again due to its crystalline structure. Kampangkaew, Thongpin, and Santawtee (2014) studied the synthesis of cellulose nano-fibers from Sesbaniajavanica for filler in thermoplastic starch. He studied the thermal stability of neat thermoplastic starch where Glycerol was used as plasticizers. The results indicated that glycerol resisted the moisture very well due to its crystalline structure. All the samples reached equilibrium moisture (around 20% moisture absorption) within 30 days. In co-plasticized TPS films, similar behavior was observed as expected. Co-plasticized samples could not inhibit retro-gradation due to hydrophilic nature of both plasticizers. Samples (5BA.25 G.PS) and 10BA.20 G.PS showed similar rate of adsorption. All the films maintained their structural integrity for all 28 days and became flexible.
3.3. Glass transition temperature (Tg) DSC analysis was performed to investigate the Tg of all the plasticized and co-plasticized formulated samples (Fig. 4 & Table 2). Sample (S.D) did not show any Tg because of the strong inter- and intra-molecular hydrogen bonding between starch chains as it limited the chain movement (Jin & Torkelson, 2016; Liu, Xie, Yu, Chen, & Li, 2009). Only sample (30 G.PS) showed a Tg around 136 °C.Tg peak was absent from all other samples indicating the disruption of hydrogen bonds between starch molecules (Niazi & Broekhuis, 2015). The molecular weight of plasticizer showed linear relation with Tg. This can be attributed to the effective amylopectin/plasticizer interactions. The Tg is directly proportional to the plasticizer molecular weight. Apparently, the amylopectin chains show more mobility as the Tgof the material decreases. During storage and conditioning, this
3.5. Mechanical properties Mechanical behavior of the formulated TPS films is shown in Fig. 6. Mechanical properties of U.S samples could not be evaluated due to its rigid and brittle nature. Mechanical properties were determined at two different relative humidity values i.e. RH 50% and RH 100%. The samples were divided into two sets; TPS films without UV treatment (PS) as control, and UV treated TPS films (UV.PS). In literature, UV irradiations showed improvement in the strength of TPS films (Niazi & Broekhuis, 2015; Niazi, 2013). Tensile strength was improved after UV treatment of samples (Fig. 6b). All the samples showed similar patterns
Fig. 4. DSC Curves for all the spray-dried formulations. 5
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Fig. 5. Moisture uptake Curves of TPS films at RH 0%, RH 50%, and RH 100%.
both at RH 50% and RH 100%, UV irradiated TPS films showed higher tensile strength than blank TPS films. At 50% and 100%RH, 30BA.PS showed the maximum strength for both UV treated and blank TPS films. All inclusively, boric acid plasticized or co-plasticized TPS films presented higher strengths than glycerol formulated TPS films. The tensile strength of TPS films (both UV treated and blank) was directly proportional to the percentage of boric acid. At RH 50%, the tensile strength was higher as compared to RH 100% for all the TPS samples. However, strain at break revealed different results, control TPS samples showed higher strain at break compared to UV treated TPS samples at both RH 50% and RH 100%. At RH 100%, the strain at break was enhanced as compared to that at RH 50%. Overall, boric acid plasticized TPS samples had lowest strain values at both relative humidity values i.e50% and 100%.In the comparison between RH 50% and RH100%, it was revealed that moisture absorption lowered the tensile strengths but improved the strains at
break. The tensile strength of 30BA.PS enhanced after UV radiations. The possible explanation is the cross-linking after UV treatment than can probably hydrolyze the branched chains of starch molecule. This could lead to the formation high linear structure resulting in hydrogen bonds between the starch chains, which ultimately results in the increase in tensile strength of the films. The results reported by Yin, Li, Liu, and Li (2005) revealed that starch films cross-linked with PVC and boric acid had improved the mechanical properties. Similar results have been reported by Xu, Canisag, Mu, and Yang (2015) where starch films were treated with different cross-linkers such as glycerol, citric acid, sucrose, glutaraldehyde, and boric acid. It was concluded that boric-acid cross-linking improved the tensile strength of starch films.
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Fig. 6. Tensile properties of TPS films, (a) at RH 50%, (b) at RH 100%.
Hence, the movement of starch molecules was restricted in the amorphous region that limited the water solubility and absorption into the cross-linked starch (Mirmoghtadaie et al., 2009). The solubility and the degree of swelling of cross-linked films at RH50% were higher than those at RH100%; 5BA.25 G.UV.PS.RH50 > 5BA.25 G.UV.PS. RH100, 10BA.20 G.UV.PS.RH50 > 10BA.20 G.UV.PS.RH100, and 30BA.UV.PS.RH50 > 30BA.UV.PS.RH100. At both relative humidity i.e. RH50% and RH100%, plasticized films having a higher percentage of boric acid showed a lower value of solubility and degree of swelling: 5BA.25 G.UV.PS > 10BA.20 G.UV.PS > 30BA.UV.PS.
3.6. Crystallinity of ultra-violet irradiation treated samples Effect of UV cross-linking on starch crystallinity was investigated for, two formulated samples i.e. 5BA.25 G.PS and 10BA.20 G.PS at different relative humidity by XRD. The XRD patterns of UV crosslinked and non-cross-linked TPS films are shown in Fig. 7 for day 0, day 7, and day 14 for non-cross-linked (as blank) and day 7 and day 14 for cross-linked. The X-ray diffraction patterns revealed that the crystallinity of TPS films was not affected by UV irradiation at any of the three relative humidity values (RH 0%, RH50%, and RH 100%). The degree of crystallinity after cross-linking remained same. Hence, it was concluded that UV irradiation had no effect on recrystallization. Results reported by Nawapat and Thawien (2013) that UV irradiation of cross-linked rice starch prepared by solution casting did not affect the crystalline structure of starch.
4. Conclusion Boric acid and glycerol plasticized and co-plasticized TPS films were treated with UV irradiation. Boric acid plasticized TPS samples exhibited better thermal stability and hydrogen bonding interactions with starch in comparison with co-plasticized and glycerol plasticized formulations. DSC analysis revealed that all the plasticized and co-plasticized blends except sample 30 G.PS showed no Tg peak that can be attributed to cross-linking. Ultra-violet cross-linking enhanced the strength of TPS films and reduced the degree of swelling and solubility of TPS films. UV treated samples conditioned at RH 50% showed improved strength and a higher degree of swelling compared to those conditioned at RH 100%. The higher percentage of boric acid plasticized films had higher strength and lower swelling as compared to the higher percentage of glycerol plasticized samples. The decrement in solubility and degree of
3.7. Solubility and swelling behavior Solubility and degree of swelling of cross-linked TPS films were determined as shown in Fig. 8. The findings were in line with the literature (Majzoobi et al., 2011; Mirmoghtadaie, Kadivar, & Shahedi, 2009). Both solubility and the degree of swelling were shortened by cross-linking of TPS films. Cross-linking made the bonding between starch chains stronger resulting in an increase in the resistance of the TPS films to swell. Solubility and degree of swelling of cross-linked films at RH 50% were higher than at RH 100%. The structure of the starch films was reinforced by cross-linking. 7
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Fig. 7. XRD spectra of blank and UV treated 5BA.25 G.PS & 10BA.20 G.PS samples (a) at RH 0%, (b) at RH 50%, and (c) at RH 100%.
significantly. Nevertheless, low retro-gradation can be associated with higher moisture uptake. Hence, other pathways are needed to be developed to minimize the effect of moisture so that far better mechanical
swelling was in order PS > UV.PS. Boric acid behaved differently than glycerol as plasticizer. It clearly showed more intensive interactions and reduced retro-gradation
Fig. 8. Solubility and degree of swelling of TPS cross-linked and non-cross-linked formulations at RH 50% and RH 100%. 8
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characteristics can be achieved.
Niazi, M. B. K., & Broekhuis, A. A. (2012). Production of amorphous starch powders by solution spray drying. Journal of Applied Polymer Science, 123, E143–153. Niazi, M. B. K., & Broekhuis, A. A. (2015). Surface photo-crosslinking of plasticized thermoplastic starch films. European Polymer Journal, 64, 229–243. Niazi, M. B. K., Zijlstra, M., & Broekhuis, A. A. (2015). Influence of plasticizer with different functional groups on thermoplastic starch. Journal of Applied Polymer Science, 132, 42012–42023. Reddy, C. K., Kimi, L., & Haripriya, S. (2016). Variety difference in molecular structure, physico-chemical and thermal properties of starches from pigmented rice. International Journal of Food Engineering, 12, 557–565. Reddy, N., & Yang, Y. (2010). Citric acid cross-linking of starch films. Food Chemistry, 118, 702–7011. Shah, U., Naqash, F., Gani, A., & Masoodi, F. A. (2016). Art and science behind modified starch edible films and coatings: A review. Comprehensive Reviews in Food Science and Food Safety, 15, 568–580. Shi, R., Bi, J., Zhang, Z., Zhu, A., Chen, D., Zhou, X., et al. (2008). The effect of citric acid on the structural properties and cytotoxicity of the polyvinyl alcohol/starch films when molding at high temperature. Carbohydrate Polymers, 74, 763–770. Singh, A., & Van den Mooter, G. (2016). Spray drying formulation of amorphous solid dispersions. Advanced Drug Delivery Reviews, 100, 27–50. Soliman, A. A. A., Nabila, A., El-Shinnaway, & Mobarak, F. (1997). Thermal behavior of starch and oxidized starch. Thermochimica Acta, 296, 149–157. Soltani, M., Shetab-Boushehri, S. F., Mohannadi, H., & Shetab-Boushehri, S. V. (2013). Proposing boric acid as an antidote for aluminium phosphide poisoning by investigation of the chemical reaction between boric acid and phosphine. Journal Medical Hypotheses Ideas, 7, 21–24. Staroszczyk, H. (2009). Microwave-assisted boration of potato starch. Polimery, 54, 31–41. Thybo, P., Hovgaard, L., Lindelov, J. S., Brask, A., & Anderson, S. K. (2008). Scaling up the spray drying process from pilot to production scale using an atomized droplet size criterion. Pharmaceutical Research, 25, 1610–1620. Valencia-Sullca, C., Vargas, M., Atarés, L., & Chiralt, A. (2018). Thermoplastic cassava starch-chitosan bilayer films containing essential oils. Food Hydrocolloids, 75, 107–115. vanSoest, J. J. G., Bezemer, R. C., deWit, D., & Vliegenthart, J. F. G. (1996). Influence of glycerol on the melting of potato starch. Industrial Crops and Products, 5, 1–9. Vialpando, M., Smulder, S., Bone, S., Jager, C., Vodak, D., Van Speybroeck, M., et al. (2016). Evaluation of three amorphous drug delivery technologies to improve oral absorption of flubendazole. Journal of Pharmaceutical Sciences, 105, 2782–2793. Vieira, M. G. A., Da Silva, M. A., Santos, L. O., & Beppu, M. M. (2011). Natural-based plasticizers and biopolymer films: A review. European Polymer Journal, 47, 254–263. Wu, D., Xu, H., & Hakkarainen, M. (2016). From starch to polylactide and nano-graphene oxide: Fully starch derived high performance composites. RSC Advances, 6, 54336–54345. Xu, H., Canisag, H., Mu, B., & Yang, Y. (2015). Robust and flexible films from 100% starch cross-linked by biobased disaccharide derivative. ACS Sustainable Chemistry & Engineering, 3, 2631–2639. Yin, Y., Li, J., Liu, Y., & Li, Z. (2005). Starch crosslinked with poly(vinyl alcohol) by boric acid. Applied Polymer Science, 96, 1394–1397. Yun, Y., Wee, Y., Byun, H., & Yoon, S. (2008). Biodegradability of chemically modified stacrg (RS4)/PVA blend films: Part 2. Journal of Polymers and the Environment, 16, 12–18. Zain, A. H. M., Kahar, A. W. M., & Noriman, N. Z. (2016). Chemical-mechanical hydrolysis technique of modified thermoplastic starch for better mechanical performance. Procedia Chemistry, 19, 638–645. Zhang, S., Zhang, Y., Wang, X., & Wang, Y. Z. (2009). High carbonyl content oxidized starch prepared by hydrogen peroxide and its thermoplastic application. Stach-Starke, 61, 646–655. Zhang, Y., & Han, J. H. (2010). Crystallization of high-amylose starch by the addition of plasticizers at low and intermediate concentrations. Journal of Food Science, 75, N8–N16. Zhou, J., Zhang, J., Ma, Y., & Tong, J. (2008). Surface photo-crosslinking of corn starch sheets. Carbohydrate Polymers, 74, 405–410.
Conflict of interest There is no conflict of interest statement in the manuscript. Acknowledgment This study was supported by National University of Sciences and Technology (NUST), Islamabad, Pakistan. References Ahammad, M., & Nguyen, T. T. M. (2016). Influence of the presence of chemical additives on the thermal properties of starch. Food and Nutrition Sciences, 7, 782–796. Ahmed, I., Niazi, M. B. K., Hussain, A., & Jahan, Z. (2018). Influence of amphiphilic plasticizer on properties of thermoplastic starch films. Polymer - Plastics Technology and Engineering, 57, 17–27. Amoako, D., & Awika, J. M. (2016). Polyphenol interaction with food carbohydrates and consequences on availability of dietary glucose. Current Opinion in Food Science, 8, 14–18. BeMiller, J. N., & Whistler, R. L. (2009). Starch chemistry and technology (3rd/ed). London: Academic. Emmambux, M. N., & Stading, M. (2007). In situ tensile deformation of zein films with plasticizers and filler materials. Food Hydrocolloids, 21, 1245–1255. Guleria, A., Singha, A. S., & Rana, R. K. (2018). Mechanical, thermal, morphological, and biodegradable studies of Okra cellulosic fiber reinforced starch‐based biocomposites. Advanced Powder Technology, 37, 104–112. Jin, K., & Torkelson, M. (2016). Enhanced Tg-confinement effect in cross-linked polystyrene compared to its linear precursor: Roles of fragility and chain architecture. Macromolecules, 49, 5092–5103. Kampangkaew, S., Thongpin, C., & Santawtee, O. (2014). The synthesis of cellulose nanofibers from sesbania javanica for the filler in thermoplastic starch. Energy Procedia, 56, 318–325. Lintingre, E., Lequeux, F., Talini, L., & Tsapis, N. (2016). Control of particle morphology in the spray drying of colloidal suspensions. Soft Matter, 12, 7435–7444. Lipiainen, T., Peltoniemi, M., Raikkonen, H., & Juppo, A. (2016). Spray-dried amorphous isomalt and melibiose, two potential protein-stabilizing excipients. International Journal of Pharmaceutics, 510, 311–322. Liu, H., Xie, F., Yu, L., Chen, L., & Li, L. (2009). Thermal processing of starch-based polymers. Progress in Polymer Science, 34, 1348–1368. Liu, Y., Fan, L., Mo, X., Yang, F., & Pang, J. (2018). Effects of nanosilica on retrogradation properties and structures of thermoplastic cassava starch. Journal of Applied Polymer Science, 135. Majzoobi, M., Radi, M., Farahnaky, A., Jamalian, J., Tongdang, T., & Mesbahi, G. (2011). Physicochemical properties of pre-gelatinized wheat starch produced by a twin drum dryer. Journal of Agricultural Science and Technology, 13, 193–202. Mathew, A. P., & Dufresne, A. (2002). Plasticized waxy maize starch: Effect of polyols and relative humidity on material properties. Biomacromolecules, 3, 1101–1108. Mendes, J. F., Paschoalin, R. T., Carmona, V. B., Neto, A. R. S., Marques, A. C. P., Marconcini, J. M., et al. (2016). Biodegradable polymer blends based on corn starch and thermoplastic chitosan processed by extrusion. Carbohydrate Polymers, 137, 452–458. Mirmoghtadaie, L., Kadivar, M., & Shahedi, M. (2009). Effects of cross-linking and acetylation on oat starch porperties. Food Chemistry, 116, 709–713. Nawapat, D., & Thawien, W. (2013). Effect of UV-treatment on porperties of biodegradable film from rice starch. International Food Research Journal, 20, 1313–1322. Niazi, M. B. K. (2013). Production of plasticized thermoplastic starch by spray drying. Doctoral thesis. Univeristy of Groningen.
9
Contents lists available at ScienceDirect
Food Packaging and Shelf Life journal homepage: www.elsevier.com/locate/fpsl
Effect of ultra-violet cross-linking on the properties of boric acid and glycerol co-plasticized thermoplastic starch films ⁎
Bahram Khana, Muhammad Bilal Khan Niazia, , Zaib Jahana, Wasif Farooqb, Salman Raza Naqvia, Majid Alic, Israr Ahmeda, Arshad Hussaina a
School of Chemical and Materials Engineering, National University of Sciences and Technology, Islamabad, Pakistan Department of Chemical Engineering, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia c USPCAS-E, National University of Sciences and Technology (NUST), Sector H-12, Islamabad, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Boric acid Glycerol Thermoplastic starch Cross-linking Spray drying UV radiation
Thermoplastic starch (TPS) is a potential alternative to non-degradable plastics. However, the hydrophilic nature and relatively weak mechanical properties is a major hurdle to its commercial application. In this study, Ultraviolet (UV) irradiation based cross-linking was investigated for its effect on the mechanical properties of TPS films plasticized by boric acid, and glycerol. Spray dried amorphous powder was compressed to obtained TPS films. All the samples were amorphous under dry conditions (RH0%) while retro-gradation was resisted at the relative humidity (RH) of50% and 100% for all the films. The concentration of the plasticizers influenced the moisture uptake ability of TPS films. UV irradiation showed no effect on the crystallinity of TPS films. However, it enhanced the mechanical strength and lessened the solubility and degree of swelling. Boric acid plasticized and co-plasticized samples revealed improved mechanical properties before and after UV irradiation compared to glycerol but showed more moisture sensitivity before cross-linking. The UV irradiated starch with 30% boric acid as plasticizer (30BA.UV.PS) showed highest tensile strength (4.28 MPa) among all the samples at 50% relative humidity. Moreover, the strain at break of the same sample decreased from 12.53% to 9.12% at 50% relative humidity.
1. Introduction
Researchers are trying to enhance the mechanical characteristics and shelf-life of TPS and to overcome its hydrophilic nature (Zhou et al., 2008). Recently, spray drying of starch/plasticizers formulated samples were highlighted as a possible technique to produce amorphous thermoplastic materials (Niazi & Broekhuis, 2012; Singh & Van den Mooter, 2016). Spray drying is a fast drying process that inhibits the moisture problem better compared to the extrusion technology and solvent casting. It is believed that amylose to amylopectin ratio had just a minor impact on the characteristics of solution spray dried samples (Thybo, Hovgaard, Lindelov, Brask, & Anderson, 2008). Amorphous thermoplastic films produced by natural plasticizers like glycerol and urea showed that low molecular weight plasticizers exhibit better mixing properties and inhibited retro-gradation. However, resistance to retro-gradation resulted in the loss of mechanical properties such as tensile strength, and more strain with moisture uptake (Ahmed, Niazi, Hussain, & Jahan, 2018; Niazi, 2013). Hypothetically, the properties of TPS films could be enhanced if retro-gradation is inhibited along with controlled moisture uptake. The radiation processing and chemical derivation have been proposed to
In the past few years, research on bio-based plastics has accelerated due to their potential biodegradability for environmental protection as well as a reduction in non-degradable plastic waste generation. The replacement and disposal of petroleum-based plastic materials have triggers the interest of researchers in biodegradable films (ValenciaSullca, Vargas, Atarés, & Chiralt, 2018). Thermoplastic starch (TPS) is a potential candidate to replace the petroleum-based commodity plastics as being biodegradable, renewable, and economical (Mendes et al., 2016; Zhou, Zhang, Ma, & Tong, 2008).TPS based on hydrocolloids can act as a barrier to control the transfer of moisture, CO2, lipids, oxygen and flavor components, leading to increase the shelf life of food products and prevent quality deterioration (Emmambux & Stading, 2007). However, use of TPS as an alternative is limited due to various factors such as its hydrophilic nature and poor mechanical characteristics. Moreover, water sensitivity affects its shelf-life along with its mechanical properties because of retro-gradation of TPS products (Liu, Fan, Mo, Yang, & Pang, 2018; Niazi, 2013).
⁎
Corresponding author. E-mail address: [email protected] (M.B. Khan Niazi).
https://doi.org/10.1016/j.fpsl.2018.05.006 Received 21 November 2017; Received in revised form 16 April 2018; Accepted 26 May 2018 2214-2894/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Khan, B., Food Packaging and Shelf Life (2018), https://doi.org/10.1016/j.fpsl.2018.05.006
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Fig. 1. Plasticizers used for the preparation of TPS powder and films.
2.1. Preparation of TPS powder
generate water-resistant products to avert degradation. Starch formulations prepared with boric acid and glycerol as plasticizer and cross-linked by ultra-violet irradiation were considered as a potential method to enhance the characteristics of the films (Reddy & Yang, 2010; Shah, Naqash, Gani, & Masoodi, 2016). Significant improvement in the mechanical properties of cross-linked starch blends as compared to the non-cross-linked onesare reported in the literature (Guleria, Singha, & Rana, 2018; Niazi & Broekhuis, 2015; Niazi, 2013). Furthermore, the radiation processing generates cross-linked covalent bonding between polymer molecules that enhanced the properties of polymeric items. Boric Acid (BA) and glycerol (G) are non-toxic and inexpensive chemicals (Soltani, Shetab-Boushehri, Mohannadi, & Shetab-Boushehri, 2013; Vieira, Da Silva, Santos, & Beppu, 2011). Both can produce strong hydrogen bonding with starch and improve its water and thermal stability (Fig. 1) (Reddy & Yang, 2010; Zain, Kahar, & Noriman, 2016). Moreover, they act as an inhibitor to starch retro-gradation process (Vieira et al., 2011). This work investigated the effect of ultra-violet irradiations on the physical properties of amorphous thermoplastic starch films (TPS). TPS films were obtained by compression molding of solution spray dried powder. Boric acid and glycerol were used as plasticizer and co-plasticizer. The study mainly focused on the analysis of retro-gradation and mechanical properties of films. Molecular interaction and thermal behavior of TPS films were analyzed by Differential Scanning Calorimetric (DSC) and Thermal Gravimetric Analysis (TGA), respectively. Moisture uptakes at different levels of humidity, retro-gradation and mechanical properties of TPS films were examined.
An aqueous solution of starch was prepared by heating 15% (w/w) of dry oxidized amylopectin from potato starch and 30% (w/w) of plasticizer based on dry starch contents. The concentration of plasticizers (boric acid and glycerol) used in solution alone or in combination was maintained at 30% w/w based on dry starch. The mixtures were heated at 95 °C for 25 min, while solution was continuously stirred at the rate of 600 rpm. The homogenized solutions were fed to Buchi mini spray dryer B-191 equipped with a 0.7 mm nozzle. 2.2. Preparation of TPS films Thermoplastic starch films (∅ 10 mm x 0.5 mm) were obtained by compression molding of TPS powder by using Fontijne Holland Table Press TH400. Each sample was compressed for 5.0 min under 25 bar pressure. The molding temperature was set at 140 °C. 2.3. Surface photo cross-linking Thermoplastic starch films (∅ 10 mm x 0.5 mm) as prepared were conditioned at two different humidi0ty levels (RH50% and RH100%) for 24 h (Zhou et al., 2008). Surface photo cross-linking was performed as mentioned in literature with few alterations (Niazi & Broekhuis, 2015; Niazi, 2013). After conditioning, all the films were exposed to UV irradiations at normal atmospheric conditions using an Intelli-ray 600 W shuttered UV flood light. The irradiation intensity of 100 mW/ cm2 (320–390 nm) was distributed evenly in the chamber. TPS films were placed at 15.5 cm away from the top of the chamber and UV irradiations were applied for 20 min continuously at 100% RH. After ultra-violet exposure, each film was conditioned again for one day at 50% relative humidity. Tensile testing was performed for all the samples after 24 h.
2. Materials and methods Oxidized amylopectin from potato starch (Perfecta-film X-85, moisture content 15% w) was provided by AVEBE (The Netherlands). Analytical grade boric acid and glycerol were acquired from SigmaAldrich. All materials were consumed as provided by the suppliers without any further modifications. The codes symbolized for both the powders and the films are depicted in abbreviations (Table 1). The weight percentage of plasticizer used relative to starch is indicated by the digits in the codes.
2.4. X-Ray diffraction (XRD) XRD was used to examine the crystalline structure of the spray-dried powder and film. Bruker D8 equipped with Cu radiation exhibiting a wavelength of 1.5418 Å was used to record diffractograms from 5° 2θ to 40° 2θ. The step size was 0.02° 2θ and scan speed was set at 2 s/step. The XRD was performed at 40 kV and 40 mA. A sample holder was used to analyze the powders having a sample compartment of ∅25 mm x 2 mm. An adjustable sample holder was used to analyze ∅10 mm x 0.5 mm TPS films.
Table 1 Material codes and description. Code
Description
U.S S.D 5B A.25 G.PS 10 BA.20 G.PS 30 BA.PS 30 G.PS UV.PS
Feedstock material Spray dried starch 5% boric acid, 25% glycerol plasticized starch(w/w of starch) 10% boric acid, 20% glycerol plasticized starch(w/w of starch) 30% boric acid plasticized starch(w/w of starch) 30% glycerol plasticized starch(w/w of starch) Ultra violet irradiated plasticized starch
2.5. Thermal gravimetric and differential thermal gravimetric analysis (TGA/DTGA) An open pan TGA was performed in a Perkin-Elmer TGA in nitrogen gas atmosphere to evaluate the thermal properties of TPS samples. The weight of sample ranged from 7.5 to 10 mg for the thermal analysis. The temperature was varied from 25 °C to 900 °C at a heating rate of 2
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Fig. 2. XRD spectra of (a) freshly spray dried powder, (b) freshly prepared films by compression molding.
24 h at room temperature i.e. 25 °C. Moisture remained on the film surface was removed once the equilibrium was attained (24 h). Eq. (2) was used to calculate the swelling behavior in TPS film.
25 °C/min. Software – Origin 8.1 was used to find out the derivative of weight loss curve yielding DTGA curve. The decomposition temperature (Tdec) was determined by the method described by Soliman, Nabila, ElShinnaway, and Mobarak (1997).
Swelling Behaviour(%) = 2.6. Differential scanning calorimetry (DSC)
(We − W0) * 100 W0
(2)
Where, We = weight of the TPS film after 24 h W0 = weight of the TPS films on the first day The swollen TPS films were dried again at 60 °C for 24 h and the solubility was measured by Eq. (3).
DSC-60 Shimadzu Co. was used to determine glass transition temperature (Tg) of the samples by DSC measurements. Before examination, an empty aluminum pan was taken as a reference test. All the samples were scanned at a rate of 10 °C/min from 10 to 200 °C. Open pan DSC was performed to eradicate any remaining moisture from the sample in the first run.
Solubility(%) =
(W0 − Wd) * 100 Wd
(3)
Where, W0 = weight of the TPS films on the first day Wd = dry weight of swollen TPS film
2.7. Moisture uptake Moisture uptake of thermoplastic films was gravimetrically measured at different relative humidity conditions i.e. RH 0% (dried silica), RH 50% (35.64% CaCl2 solution) and RH 100% (distilled water). The measurements were performed on a daily basis for the first week. Later, measurements were taken at day 14, 21, and 28. Eq. (1) was used to determine the moisture uptake of the films (Zhang, Zhang, Wang, & Wang, 2009).
3. Results and discussion
where, Wn = weight at the day of measurement, n, 1,2,3……… W0 = weight directly after film making
Boric acid and glycerol were used as a plasticizer and a co-plasticizer aiming to interact with starch molecules by developing hydrogen bonds. Cross-linking by Ultra-violet irradiation was performed to enhance the mechanical strength of thermoplastic films (TPS). Both plasticizers contain several hydroxyl groups in their structures that tend to show interaction with starch through hydrogen bonding (Amoako & Awika, 2016; Wu, Xu, & Hakkarainen, 2016). Moreover, the amount of each plasticizer, their ratios, and combination in the formulations play a significant role on the properties of plasticized TPS.
2.8. Mechanical testing
3.1. Crystallinity of freshly prepared TPS powder and films
Mechanical behavior of the TPS samples was investigated using Instron 4301 tensile tester. Tensile strength (TS) and percentage elongation at break (%E) were tested according to ASTM D1708. The sample was conditioned at two different valued of relative humidity i.e. RH 50% and RH 100% at 25 °C for 24 h. The crosshead speed was set at 10.0 mm/min and the width and thickness of each sample were measured before testing. UV cross-linked TPS films were also tested using the similar procedure.
The XRD patterns of TPS powder and films formulated with different plasticizers are shown in Fig. 2. All spray dried powders were amorphous (Fig. 2a) and the results were in line with the literature (BeMiller & Whistler, 2009; Lipiainen, Peltoniemi, Raikkonen, & Juppo, 2016). The B and V-type crystalline peaks at 2θ = 15.01°, 17.37°, 20.11°, and 23.64° in sample (U.S) were absent in spray-dried powder samples. Starch chains could not be associated with crystal lattices due to the short drying spans attained in spray drying. XRD analysis of spray dried TPS samples were in line with literature (Niazi & Broekhuis, 2012; Niazi, 2013). Han and Zhang et.al. (Zhang & Han, 2010) prepared solvent cast starch films plasticized with glycerol with various concentrations. Their XRD analysis showed that films possessed B-type crystal that agreed to C-type nature of the pea starch. XRD patterns showed intensity peaks at 16.9° and 22.3°.
Moisture uptake =
Wn − W0 *100% W0
(1)
2.9. Solubility and swelling behavior Solubility (S) and the Swelling Behavior (SB) of films were measured according to the method described by Yun, Wee, Byun, and Yoon (2008). Dried thermoplastic films were immersed in distilled water for 3
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All the freshly prepared spray dried TPS powders and films were completely amorphous irrespective of the amount of plasticizer or coplasticizer used, except S.D sample, which was mat. Freshly prepared TPS samples were transparent. TPS films did not exhibit B-type crystallinity (Fig. 2b). Starch chains were disabled to be associated with crystal lattices due to the short drying time procured in spray drying. The fast drying resisted the starch chains to reallocate into crystal lattices and resulted in producing an amorphous powder and films (Lintingre, Lequeux, Talini, & Tsapis, 2016; vanSoest, Bezemer, deWit, & Vliegenthart, 1996). Moreover, compression molding also had no effect on the amorphous nature of the TPS formulations and retrogradation was resisted. Furthermore, combining boric acid and glycerol in different ratios did not affect the crystallinity. The XRD patterns in Shi et al. (2008) showed that films prepared by solvent casting at 140 °C with glycerol and citric acid used as plasticizers exposed protruding starch and PVA crystalline peaks. Although, the existence of citric acid declined the peak intensity at 19.3° compared to native sample. It’s inhibited crystallinity but could not generate amorphous blends.XRD analysis reported in literature showed the amorphous character of TPS powder and films was attained by solution spray drying technique and it is independent of plasticizer quantity and grouping (Niazi & Broekhuis, 2012; Niazi, 2013; Vialpando et al., 2016).
Table 2 Thermal properties of the solution spray dried powder samples. Samples
Onset, Tdec (oC)
Peak, Tdec,b1 (oC)
Peak, Tdec,b2 (oC)
Peak, Tdec,b3 (oC)
Tg (oC)
S.D 5BA.25 G.PS 10BA.20 G.PS 30BA.PS 30 G.PS
293.92 224.54 279.07 312.79 190.51
– 178.05 179 185 179.97
– 250 250.67 – –
334 311.36 312.26 328.70 328.34
No No No No 136.1
2015). The spray dried unmodified sample (S.D) was taken as a reference sample. S.D showed different results as compared to other formulated samples. Only one peak at 293.92 °C was detected for the decomposition of starch. Apart from moisture loss at 100 °C (labeled ‘a’), all plasticized TPS samples showed thermal stability up to 171 °C temperature. Weight loss sections at different temperature ranges were derived depending on each plasticizer used. Sample (30BA.PS) showed a maximum Tdec of 312.79 °C and while 30 G.PS exhibited the lowest Tdec of 190.51 °C. While, decomposition temperature (Tdec) of samples 10BA.20 G.PS and 5BA.25 G.PS was 279.07 °C and 224.54 °C, respectively. Such observations in Tdec are attributed to strong hydrogen bonding capability of boric acid as compared to glycerol plasticized formulation (Reddy & Yang, 2010; Zain et al., 2016). 30BA.PS revealed upgraded thermal stability as compared to 30 G.PS. In co-plasticized samples, an additional peak (section ‘b1’ and ‘b2’) was observed indicating presence of second plasticizer (Shi et al., 2008). Boric acid and glycerol were taken as co-plasticizers. The accumulation of glycerol with boric acid resulted into reduction in the decomposition
3.2. Thermal properties Thermal properties of TPS powders were analyzed by TGA and DTGA (Fig. 3 and Table 2). Results were not simple though, so various sections were allocated on DTGA minima as done in the literature (Niazi & Broekhuis, 2012; Niazi, 2013; Niazi, Zijlstra, & Broekhuis,
Fig. 3. TGA and DTGA curves of solution spray dried powder samples. 4
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temperature of BA.PS samples and an additional peak (section ‘b1’ and ‘b2’) in the DTGA curve appeared (Fig. 3b). Zain et al. (2016) examined the effect of chemical-mechanical hydrolysis on the TPS thermal properties with citric acid and glycerol as plasticizers. A native starch granule was transformed into a continuous phase. It was revealed that citric acid was more thermally stable as compared to glycerol. This was attributed to the volatility of glycerol. Citric acid modified TPS samples presented higher onset temperature and lower mass loss than glycerol modified TPS samples. The addition of boric acid reduced the rate of weight loss and the residual weight was increased. So the higher thermal stability of the TPS films can be elucidated by strong hydrogen bonds generated by boric acid. By increasing glycerol weight, the thermal stability was reduced. By varying the quantity of plasticizers, formulations behave differently regarding stability. The thermal stability increased in the order 30 G.PS < 5BA.25 G.PS < 10BA.20 G.PS < 30BA.PS. In all coplasticized TPS samples, peak‘b1’ was observed at 178 °C representing the decomposition of glycerol in the blend and peak while peak‘b2’ represents the decomposition of boric acid in the blend at 250 °C. It was observed that the rate of weight loss was lower in the higher percentage boric acid plasticized TPS samples because cross-linking occurred to a lesser extent in the presence of glycerol. The addition of boric acid plasticizer enhanced the carbonization. Staroszczyk (2009) investigated the granular potato starch borated with boric acid and sodium tetra borate decahydrate (borax) in a microwave-assisted solid-state reaction. DSC and XRD results indicated that although the boration caused changes in the macrostructure of starch, the thermal stability of borated starch was improved by 20 °C as compared with native starch. It was noticed that the increase in boric acid caused a shift in decomposition temperature of starch from 277 °C to 282 °C suggesting boric acid as a potential candidate for the thermal stability of starch.
increase in mobility results in high crystallization which leads the crosslinking in the crystalline domains and restrains the amorphous amylopectin regions, eventually increasing the Tg. Tg was only observed for 30 G.PS sample, which can be explained by partial miscibility of glycerol and starch, giving rise to a phase separation in terms of Tg. However, Tg was missing for all plasticized and co-plasticized samples. A better performance of plasticizer is expected because of its higher interaction possibility and higher dispersion level with amylopectin chains. The molecular weight of glycerol being more than boric acid results in low efficiency as a plasticizer and increase in the Tg. Mathew et. Al studied the effect of different polyols on waxy maize starch and found out that Tg decreases with the molecular weight of the plasticizer and increasing moisture content. The results were significantly in line with our work. The moisture content of boric acid plasticized and co-plasticized at RH 100% was high than glycerol plasticized sample, resulting in the absence of Tg peak (Mathew & Dufresne, 2002). 3.4. Moisture uptake The moisture uptake data was recorded at RH 0%, RH 50%, and RH 100%for 28 days to assess the particular behavior of TPS plasticized formulations. Fig. 5 showed the moisture uptake data for all the TPS plasticized samples. The rearrangement of amorphous starch chains was influenced by moisture content resulting in weight changes (Reddy, Kimi, & Haripriya, 2016). Both plasticizers are hydrophilic in nature; hence they absorbed water throughout 28 days. The moisture uptake increased in the order RH 0% < RH 50% < RH 100%. All films showed similar behavior at RH 0% (Fig. 5). Retro-gradation was prevented as the water was absent. TPS plasticized samples showed similar behavior at RH 50%. As the quantity of boric acid was increased, the moisture uptake also increased. Initially, amorphous starch matrix absorbed water resulting a gain in weight. Free volume in the polymeric matrix further enlarged as the water contents kept growing. Certain starch to starch hydrogen bonds was being substituted by starch-water interactions, leading to distended structure (Ahammad & Nguyen, 2016). At RH 100%, the moisture absorptions were high for all the formulations. A significant increase in moisture was observed even after the first week. Glycerol resisted the moisture more than boric acid again due to its crystalline structure. Kampangkaew, Thongpin, and Santawtee (2014) studied the synthesis of cellulose nano-fibers from Sesbaniajavanica for filler in thermoplastic starch. He studied the thermal stability of neat thermoplastic starch where Glycerol was used as plasticizers. The results indicated that glycerol resisted the moisture very well due to its crystalline structure. All the samples reached equilibrium moisture (around 20% moisture absorption) within 30 days. In co-plasticized TPS films, similar behavior was observed as expected. Co-plasticized samples could not inhibit retro-gradation due to hydrophilic nature of both plasticizers. Samples (5BA.25 G.PS) and 10BA.20 G.PS showed similar rate of adsorption. All the films maintained their structural integrity for all 28 days and became flexible.
3.3. Glass transition temperature (Tg) DSC analysis was performed to investigate the Tg of all the plasticized and co-plasticized formulated samples (Fig. 4 & Table 2). Sample (S.D) did not show any Tg because of the strong inter- and intra-molecular hydrogen bonding between starch chains as it limited the chain movement (Jin & Torkelson, 2016; Liu, Xie, Yu, Chen, & Li, 2009). Only sample (30 G.PS) showed a Tg around 136 °C.Tg peak was absent from all other samples indicating the disruption of hydrogen bonds between starch molecules (Niazi & Broekhuis, 2015). The molecular weight of plasticizer showed linear relation with Tg. This can be attributed to the effective amylopectin/plasticizer interactions. The Tg is directly proportional to the plasticizer molecular weight. Apparently, the amylopectin chains show more mobility as the Tgof the material decreases. During storage and conditioning, this
3.5. Mechanical properties Mechanical behavior of the formulated TPS films is shown in Fig. 6. Mechanical properties of U.S samples could not be evaluated due to its rigid and brittle nature. Mechanical properties were determined at two different relative humidity values i.e. RH 50% and RH 100%. The samples were divided into two sets; TPS films without UV treatment (PS) as control, and UV treated TPS films (UV.PS). In literature, UV irradiations showed improvement in the strength of TPS films (Niazi & Broekhuis, 2015; Niazi, 2013). Tensile strength was improved after UV treatment of samples (Fig. 6b). All the samples showed similar patterns
Fig. 4. DSC Curves for all the spray-dried formulations. 5
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Fig. 5. Moisture uptake Curves of TPS films at RH 0%, RH 50%, and RH 100%.
both at RH 50% and RH 100%, UV irradiated TPS films showed higher tensile strength than blank TPS films. At 50% and 100%RH, 30BA.PS showed the maximum strength for both UV treated and blank TPS films. All inclusively, boric acid plasticized or co-plasticized TPS films presented higher strengths than glycerol formulated TPS films. The tensile strength of TPS films (both UV treated and blank) was directly proportional to the percentage of boric acid. At RH 50%, the tensile strength was higher as compared to RH 100% for all the TPS samples. However, strain at break revealed different results, control TPS samples showed higher strain at break compared to UV treated TPS samples at both RH 50% and RH 100%. At RH 100%, the strain at break was enhanced as compared to that at RH 50%. Overall, boric acid plasticized TPS samples had lowest strain values at both relative humidity values i.e50% and 100%.In the comparison between RH 50% and RH100%, it was revealed that moisture absorption lowered the tensile strengths but improved the strains at
break. The tensile strength of 30BA.PS enhanced after UV radiations. The possible explanation is the cross-linking after UV treatment than can probably hydrolyze the branched chains of starch molecule. This could lead to the formation high linear structure resulting in hydrogen bonds between the starch chains, which ultimately results in the increase in tensile strength of the films. The results reported by Yin, Li, Liu, and Li (2005) revealed that starch films cross-linked with PVC and boric acid had improved the mechanical properties. Similar results have been reported by Xu, Canisag, Mu, and Yang (2015) where starch films were treated with different cross-linkers such as glycerol, citric acid, sucrose, glutaraldehyde, and boric acid. It was concluded that boric-acid cross-linking improved the tensile strength of starch films.
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Fig. 6. Tensile properties of TPS films, (a) at RH 50%, (b) at RH 100%.
Hence, the movement of starch molecules was restricted in the amorphous region that limited the water solubility and absorption into the cross-linked starch (Mirmoghtadaie et al., 2009). The solubility and the degree of swelling of cross-linked films at RH50% were higher than those at RH100%; 5BA.25 G.UV.PS.RH50 > 5BA.25 G.UV.PS. RH100, 10BA.20 G.UV.PS.RH50 > 10BA.20 G.UV.PS.RH100, and 30BA.UV.PS.RH50 > 30BA.UV.PS.RH100. At both relative humidity i.e. RH50% and RH100%, plasticized films having a higher percentage of boric acid showed a lower value of solubility and degree of swelling: 5BA.25 G.UV.PS > 10BA.20 G.UV.PS > 30BA.UV.PS.
3.6. Crystallinity of ultra-violet irradiation treated samples Effect of UV cross-linking on starch crystallinity was investigated for, two formulated samples i.e. 5BA.25 G.PS and 10BA.20 G.PS at different relative humidity by XRD. The XRD patterns of UV crosslinked and non-cross-linked TPS films are shown in Fig. 7 for day 0, day 7, and day 14 for non-cross-linked (as blank) and day 7 and day 14 for cross-linked. The X-ray diffraction patterns revealed that the crystallinity of TPS films was not affected by UV irradiation at any of the three relative humidity values (RH 0%, RH50%, and RH 100%). The degree of crystallinity after cross-linking remained same. Hence, it was concluded that UV irradiation had no effect on recrystallization. Results reported by Nawapat and Thawien (2013) that UV irradiation of cross-linked rice starch prepared by solution casting did not affect the crystalline structure of starch.
4. Conclusion Boric acid and glycerol plasticized and co-plasticized TPS films were treated with UV irradiation. Boric acid plasticized TPS samples exhibited better thermal stability and hydrogen bonding interactions with starch in comparison with co-plasticized and glycerol plasticized formulations. DSC analysis revealed that all the plasticized and co-plasticized blends except sample 30 G.PS showed no Tg peak that can be attributed to cross-linking. Ultra-violet cross-linking enhanced the strength of TPS films and reduced the degree of swelling and solubility of TPS films. UV treated samples conditioned at RH 50% showed improved strength and a higher degree of swelling compared to those conditioned at RH 100%. The higher percentage of boric acid plasticized films had higher strength and lower swelling as compared to the higher percentage of glycerol plasticized samples. The decrement in solubility and degree of
3.7. Solubility and swelling behavior Solubility and degree of swelling of cross-linked TPS films were determined as shown in Fig. 8. The findings were in line with the literature (Majzoobi et al., 2011; Mirmoghtadaie, Kadivar, & Shahedi, 2009). Both solubility and the degree of swelling were shortened by cross-linking of TPS films. Cross-linking made the bonding between starch chains stronger resulting in an increase in the resistance of the TPS films to swell. Solubility and degree of swelling of cross-linked films at RH 50% were higher than at RH 100%. The structure of the starch films was reinforced by cross-linking. 7
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Fig. 7. XRD spectra of blank and UV treated 5BA.25 G.PS & 10BA.20 G.PS samples (a) at RH 0%, (b) at RH 50%, and (c) at RH 100%.
significantly. Nevertheless, low retro-gradation can be associated with higher moisture uptake. Hence, other pathways are needed to be developed to minimize the effect of moisture so that far better mechanical
swelling was in order PS > UV.PS. Boric acid behaved differently than glycerol as plasticizer. It clearly showed more intensive interactions and reduced retro-gradation
Fig. 8. Solubility and degree of swelling of TPS cross-linked and non-cross-linked formulations at RH 50% and RH 100%. 8
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characteristics can be achieved.
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