Mechanical and Permeability Properties of Thermoplastic Starch Composites Reinforced with Cellulose Nanofiber for Packaging Applications

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Mechanical and Permeability Properties of Thermoplastic Starch Composites Reinforced with Cellulose Nanofiber for Packaging Applications

Preetha Balakrishnan,

Mechanical and Permeability Properties of Thermoplastic Starch Composites Reinforced with Cellulose Nanofiber for Packaging Applications

In this study, a facile, ecofriendly nanocomposite based on glycerol-plasticized thermoplastic potato starch (TPS) containing 0-4 wt.% cellulose nanofiber (CNF) from pineapple leaf was prepared by the solution casting technique. The nanocomposite films showed better mechanical properties compared to neat film. The tensile properties were fit with theoretical predictions. Tensile strength increased with CNF loading and reached up to 4.682 MPa (TPS+3 wt.% CNF) from 1.58 MPa (neat TPS), an improvement of ~196% over the neat polymer. The addition of cellulose nanofiber improved the barrier properties of films by creating a tortuous path for penetrant molecules. Gas transport behavior of CNF-based TPS nanocomposite films was explored using O₂ gas. A 56% decrease (relative to neat TPS) in oxygen permeability of the nanocomposite membranes was achieved by adding only 3 wt.% CNF. The oxygen permeability data were fit with two theoretical models.

Keywords: starch, nanofiber, mechanical, permeability, modeling.

Механические свойства и проницаемость термопластичных крахмальных композитов, армированных нановолокнами целлюлозы, для применения в качестве упаковки

П. Балакришнан, С. Гопи, В.Г. Геетхамма, С. Томас

Описан легкий, экологически чистый нанокомпозит на основе пластифицированного глицерином термопластичного картофельного крахмала (ТКК), содержащий 0-4 мас.% целлюлозного нановолокна (ЦН) из листьев ананаса, полученный методом испарения из раствора. Нанокомпозитные пленки имели улучшенные механические свойства по сравнению с исходной пленкой из крахмала. Свойства на растяжение соответствовали теоретическим предсказаниям. Предел прочности на растяжение композитных пленок повышался при увеличении содержания ЦН с 1,58 МПа (чистый ТКК) до 4,682 МПа (ТКК + 3 мас.% ЦН); это улучшение составило ~ 196 % по сравнению с чистым полимером. Добавление целлюлозного нановолокна улучшало барьерные свойства пленок за счет извилистого пути для проникающих молекул. Газотранспортное поведение нанокомпозитных пленок ТКК-ЦН было исследовано с использованием газообразного кислорода. Ведением всего 3 % ЦН было достигнуто 56%-ное уменьшение кислородной газопроницаемости композитных пленок по сравнению с чистым ТКК. Данные по кислородной проницаемости соответствовали двум теоретичеким моделям.

Ключевые слова: крахмал, нановолокно, механические свойства, проницаемость, моделирование.

Introduction

Biopolymers from renewable resources have recently attracted considerable attention because of their unique properties, including ease in fabrication, low cost and biodegradability (Guohua et al., 2006; Tang and Alavi, 2011). Renewable polymers offer an attractive alternative to maintain sustainable development and an ecofriendly nature (Halley, 2005). The biodegradable nature of these materials will lead to a reduction in carbon dioxide concentration in the environment and they may even replace many synthetic polymers. In recent years, petroleumbased packaging materials lost attention because of increasing environmental pollution. In this scenario, biopolymer composites gain importance. In the family of renewable polymers, starch has been considered to be one of the most promising materials because of its availability, cost effectiveness, and biodegradability.

Starch is a semicrystalline polymer composed of two polysaccharides: amylose, a linearmolecule,andhighlybranchedamylopectin. The proportion of amylose and amylopectin varies from plant to plant. Nevertheless, without plasticizers the films made from starch are brittle in nature and not easy to fabricate. Plasticizers are low molecular weight, highly volatile molecules that can enter in between the starch macromolecular chains and thereby reduce the inter- as well as intramolecular interactions in starch (Biduski et al., 2018; Mlalila et al., 2018). nanocomposite thermoplastic glycerin

Sustainable food packaging has a substantial impact on quality, taste, and marketability of products. As per the WHO, approximately 40% of all food products spoil every year in the United States (Wang et al., 2018). Limitations in harvesting, transporting and storage all contribute to this spoilage. The quality of packaging that prevents oxygen and water vapor from coming into contact with food is a major cause of food spoilage.

Shelf life plays an important role in determining the quality of film, and it depends mainly on the humidity and gas permeability through the film. A humid atmosphere with the presence of oxygen favors microbial growth and gradually spoils the quality of food inside (Malmir et al., 2017; Nair et al., 2017). Moreover, the reduced transmission rate favors increasing the degradation rate, since it depends on the transport of water from the surface to the bulk of the material (Park et al., 2003). In that respect, the Water Vapor Transmission Rate (WVTR) and Oxygen Transmission Rate (OTR) measurements of representative films were evaluated as the most important parameters for these properties (Wang et al., 2018).

Cellulose is one of the unique biopolymers that can be used to reinforce starch-based films. Cellulose can be extracted from a wide variety of sources, which include hemp (Girones et al., 2012), wheat straw (Montano-Leyva et al., 2011), pineapple leaf (Arib et al., 2006), cotton (Prachayawarakorn et al., 2010), bacterial cellulose (Grande et al., 2009), etc. One of the important benefits of using cellulose as a reinforcing phase for starch is its similar polysaccharide structure. Starch, which is a hydrophilic polymer, is incompatible with most synthetic polymers.

Low mechanical properties and barrier properties of biopolymers such as starch have limited their large-scale applicability (Di et al., 2006; Muller et al., 2009). Different methods have been proposed to overcome this limitation including blending (Hoyos et al., 2011), cross-linking (Canisag, 2015), UV irradiation (Narayanan et al., 2017), reinforcing (Balakrishnan et al., 2017), etc. These are all common modification techniques to improve the performance of biopolymer composites.

The aim of this paper is to investigate the effect of the filler aspect ratio on mechanical and barrier properties of the polymer and to quantitatively model the results. Starch nanocomposites containing 0-4 wt.% of cellulose nanofiber were prepared by the solution casting technique. The mechanical properties, watervapor and oxygen permeability, and UV characteristics were investigated. The combined effects of filler concentration and aspect ratio on the barrier properties and mechanical performance of thermoplastic starch (TPS) were investigated, and a tortuous path equation was derived to model the results.

Materials and Methods

Materials.

For the fabrication of thermoplastic starch, potato starch (supplied by Lobha chem India

Pvt. Ltd. Mumbai) and glycerol (98% Purity, supplied by Merck India Pvt. Ltd) were used. For nanofiber synthesis, sodium hydroxide, oxalic acid, and hydrogen peroxide were supplied by MERCK India Pvt. Ltd. Cellulose nanofibers were isolated from pineapple leaf with an average diameter of 20-25 nm according to our previous work (Balakrishnan et al., 2017). Briefly, finely chopped pineapple leaves were washed, dried and milled to fine powder. The powder was then subjected to steam explosion using 2% NaOH solution in an autoclave at 138 kPa for 1 hour. Alkali treated fiber was then washed with distilled water until free from alkali, which was confirmed by pH meter measurements. Then, alkali treated fiber was bleached with hydrogen peroxide in an autoclave maintained at 138 kPa. Then, pressure was released abruptly after 30 minutes. This process continued until the residue became white. The bleached fiber was subjected to acid hydrolysis using an 11% oxalic acid solution in an autoclave. Finally, the resultant suspension after acid hydrolysis was centrifuged at 8000 rpm for 6 minutes and dialyzed in tap water for 2 days (Balakrishnan et al., 2017).

Preparation of thermoplastic potato starch (TPS)/cellulose nanofiber (CNF) composites

Thermoplastic starch cellulose nanocomposites were prepared by the solution casting technique. Calculated amounts of cellulose nanofibers were dispersed in distilled water and bath sonicated for 30 minutes. Preweighed potato starch was mixed well with 30% glycerol (on a dry starch basis) using a mechanical stirrer by adding 100 ml distilled water. The suspension of cellulose nanofiber was added to a starch-glycerol water mixture and mixed in a mechanical stirrer (700 rpm) for 30 minutes at a temperature of 90 °C. After the solution became viscous, it was poured on to leveled borosilicate glass Petri dishes and kept at 50°C for 12 h until it was completely dry. Solution cast films of TPS cellulose nanofiber composites were made at 1, 2, 3, 4 wt.% nanofiber concentrations (as per the dry weight of the matrix). Films of approximately 80 gm thickness were obtained by this technique.

Transmission electron microscopy

The morphology of prepared nanofibers was evaluated using high-resolution transmission electron microscopy (JEM-2100 HRTEM, JEOL, Japan). A drop of well sonicated dilute suspension was drop-cast on a copper grid and dried under an IR lamp.

Mechanical properties

Tensile properties of the films were evaluated by a Smitweld tensile testing unit in accordance with the ASTM 638 Standard. The tensile test was carried out at room temperature with a crosshead speed of 10 mm/min and load cell of 5 kN. Five dumbbell shaped samples of each system were used to determine Young's modulus, tensile strength and elongation at break. The tensile modulus was taken as the slope of the initial linear region of the stressstrain curve.

Water vapor transmission rate

The WVTR of the films was determined according to the ASTM E96-95 Standard (desiccant method). The films were cut into circles with a diameter of 6.3 cm and sealed onto an aluminum permeation cup containing 45 g of dry calcium chloride with silicone grease, and a ring was used to hold the films in place. The whole device was weighed and then placed in a climate-controlled chamber (32°±2°C and 50±5% RH). The permeability cups with the films were weighed at regular time intervals, and a linear relationship was obtained between the quantity of water transferred per unit of air and time.

Oxygen transmission rate

The samples were conditioned at a temperature of 23±2°C and 50±5% RH 24 h ^before iiicasuiing tte ота. ота was detefmmed at tte tun cottons usin^{g a} Finite °X2/2³° (Labthink 'ns,",s '"" ''d Filmthick"ss was measured with an electronic digital caliper before testing and input to the computer program WinPerme OX2-230 W3-330(En). The OTR was measured after the film had been placed in a cell, and oxygen flow was introduced on one side of the film. The OTR (ml m'² day¹) was calculated from the mean OTR multiplied by the film thickness ^{(mm) and d}i^vided ttythcostygMi ^gra^di^ent widkta the^ ft^n"^ (We"). Thre^e independent determinations were carried out for each film sample, and the mean of those three values is given as thefinal result.

Stress-strain behavior characterizes the reinforcing effect of filler onto the matrix. Typical stress-strain curves of thermoplastic starch and thermoplastic starch reinforced with CNF isolated from pineapple leaf fiber are shown in Fig. 2. Tensile strength, modulus and elongation increase as filler concentration increases (Table 1). The maximum improvement in tensile strength was observed for 3 wt.% CNF. This is because the thermoplastic starch transmits and distributes stress to the nanofibers, resulting in higher tensile strength of the composites. Thus, the composite can withstand a higher load before failure compared to neat TPS. Figure 2 also shows an incremental increase in moduli (curved slopes) and strength with an increase in the reinforcing phase. The improvement in t^his ^mee^han,e"l propert^{y may} I--' attributed to the following reasons: (1) higher mechanical strength of CNF, (2) better reinforcing efficiency of CNF to the TPS matrix due to the same polysaccharide functionality, and (3) effective dispersion of CNF in the thermoplastic starch (up to 3 wt.% CNF, and beyond that, agglomeration occurs).

Fig. 1 TEM image of cellulose nanofiber

Values of tensile strength, percentage of elongation, and tensile modulus of the composite are shown in Table 1. These values were obtained as averages of five measurements per set of composites. Both the tensile strength and modulus increased with increasing nanofiller content. The enhanced mechanical performance of the composites revealed better reinforcing efficiency of cellulose nanofibers to the thermoplastic starch matrix. The tensile strength of neat TPS is 1.58 MPa, reaching a value of 4.68 MPa upon the addition of 3 wt.% nanofiller: an improvement of almost ~198% compared to the virgin polymer. A fine dispersion of cellulose nanofibers over the starch matrix results in an effective load transfer from matrix to filler, which results in uniform stress distribution and minimizes the stress concentrated centers. Another possible reason for the reinforcement is the high surface to volume ratio interacting with the matrix. Upon the addition of filler, the modulus of the composite was also found to increase without reducing the tensile strength, and this dynamic is due to appropriate impregnation of the filler in the matrix and good interfacial adhesion by hydrogen bonding interactions.

Fig. 2. Stress-Strain curves of TPS-CNF nanocomposites

Table 1. Tensile strength, elongation at break and tensile modulus values of nanocomposites at various filler loadings

Modeling of tensile results

Two of the most widely used models (the Halpin-Tsai Model and Kraus Model) were selected from literature. The main purpose was to predict the modulus of the starch/cellulose system. Typical parameters in such models include the moduli of the matrix and filler, the adhesion between fillers and the host polymer matrix, the volume fraction of the filler and the filler shape factor (e.g., aspect ratio).

Halpin-Tsai Model

To date, many mechanical models have been proposed to explain the mechanical properties of polymer nanocomposites. Among these models, the Halpin-Tsai model is widely used to predict the modulus of fiber-filled nanocomposites as a function of filler content from the moduli of the pure matrix and filler material and the aspect ratio of the filler.

Halpin-Tsai equation was originally used for composites with unidirectional reinforcement, but later it was modified and improved to account for the randomness of discontinuous fibers by introducing an orientation factor (в). If the fiber length is greater than the thickness of the specimen, the fibers are assumed to be randomly oriented in two dimensions, and the orientation factor (в) is 1/3. If the fiber length is smaller than the thickness of the specimen, the fibers are assumed to be randomly oriented in three dimensions, and the orientation factor (в) is 1/6.

Water vapor transmission rate (WVTR)

Water vapor permeability results can be useful to understand possible mass transfer

Fig. 3 Theoretical modeling of Young's modulus . different aspect ratios for CNF - co^mparison w^|th experimental data

Fig. 4. Knnis pl^ot for ire/CNF ^nanocomp^osi^tes

Fig. 5. WVTR ofcomposite filmswith varying percentages of cellulosenanofibers mechanisms and solute and polymer interactions in edible films.

According to the thermodynamics of irreversible processes, the water chemical potential difference is the driving force of the water transfer through a film (Chivrac et al., 2010; Tan and Thomas, 2016). When the process occurs at constant temperature and p^ressure, ^{the water chemi}cЈd potential diffe re nee is proportional to water vapor concentration difference between the two faces (Bertuzzi et al., 2007). The permeability of unmodified thermoplastic film is 0.073±0.004 g miff'm'² (Fig. 5). However, the WVP values decrease after each filler reinforcement, possibly due to their nanoscopic size and better adhesion to the matrix phase and due to the fine dispersion of nanofibers over the starch matrix producing strong hydrogen bonding, which hinders molecules entering into the nanocomposites. Lower WVP was recorded for TPS modified with 3 wt.% CNF 0.0568±0.002 g min⁴m'² compared to unmodified starch films. However, at 4 wt.% loading, the permeability increases slightly because of the chance of agglomeration at higher filler loading.

Oxygen permeability

Relative oxygen permeability of nanocomposites with different filler loading is shown in Fig. 6. From the figure, it can be seen that the relative permeability of the composite decreases after each nanofiller inclusion. This finding is consistent with the abovementioned results. The decrease in oxygen permeability can be directly correlated to the filler dispersion and better matrix filler adhesion reducing the free volume within the matrix as well as a decrease in the diffusivity created by the incorporated nanofillers, which creates a tortuous pathway for the penetrant gas molecules. The permeation of gas molecules through a polymeric system is primarily due to two main reasons: diffusivity of the penetrant molecule and penetrant _s"^lub,lit_y The .e..,. inthese twofae^affeots the penetration of solvent molecule. Zeppa et al. studiedthe oxygen barrier properties of potato starch reinforced with closite 30 B, and upon the addition of 6.25 wt.% nanofiller, they reported a relative permeability of 0.87 (Zeppa et al., 2009). Later, Sadegh-Hassani and Mohammadi Nafchi (2014) studied the permeability of potato starch reinforced with Halloysite clay and reported an average permeability of 0.59 upon the addition of 3 wt.% nanoclay. In our present work, with nanofibers isolated form pineapple, we obtained a good permeability resistance as per the earlier reported data. Upon the addition of 3 wt.% CNF, the relative permeability obtained was 0.43, which is lower than the data reported in the literature.

Fig. 6. Oxygen permeability of TPS/CNF nanocomposites

To predict the permeability of gases through the nanocomposite membrane, several models have beenproposed. Relative oxygen permeability of the composite to the neat polymer matrix can be modeled by the geometric path developed by Bharatwaj, which suggests the orientation of impermeable disk plate-like particles dispersed within the matrix.

Table2. Oxygen permeability values of nanocomposites

The average values of aspect ratio were deduced from Nielsen's and Bharadwaj model and were estimated to be approximately 179.2 and 222.55, respectively (Fig. 7). The calculated aspect ratio w111 have disparities because of orientation,entanglement ofnanofibers, flexibility and network structure of CNF. Another possible reason for the inconsistent values could be explained by the limitation of theoretical models, which neglect the physical interaction at the filler polymer interface and the chain confinement in this area (Aulin et al., 2012; Dash and Swain, 2013).

Fig. 7. Experimental values fitted withBharatwaj andNielsen models with different weight percentages ofCNF

Table 3. Light transmittance of nanocomposites at different wavelengths

UV-Vis spectroscopy

The packaging was able to absorb light in the UV region to avoid the oxidative rancidity (Castillo et al., 2013; Chen et al., 2013). In case the films were not UV responsive, the packaging films must be irradiated with UV light to destroy the microbial charge (Narayanan et al., 2017). Table 3 shows the transmission values for UV (200-400 nm) and visible (500-700 nm) light. Nanocomposite films have higher transmittance values in the range of visible light compared with ultraviolet light. There is a reduction in the UV absorbance of the nanocomposites after the addition of cellulose nanofibers. This is due to the blocking effect induced by nanoreinforcement; similar findings were also reported by Hietala et al. (2013). This reduction in the light transmittance in the UV region after successive filler incorporations confirms the UV absorption potential of incorporated nanofillers in the matrix. This will certainly help protect food items from UV degradation.

Conclusions

In this study, thermoplastic starch nanocomposites were prepared using a solution casting technique by incorporating cellulose nanofiber from pineapple leaf. Better adhesion and dispersion of cellulose nanomaterials in the starch matrix could lead to better nanocomposite properties compared to the pristine sample. However, at higher filler loading the mechanical and barrier properties were reduced, possibly due to agglomeration. The composites fit well with the Halpin-Tsai model at higher aspect ratios, thereby confirming the long fibrous nature of the filler, which helps to effectively transfer the load. Both water vapor permeability and oxygen permeability decreased with increasing nanofiber content. The tortuosity factor of the nanocomposite increased because of the formation of a more difficult path for the movement of oxygen gas. UV-Vis spectral data confirmed the UV protective properties of the films. The results of the present study could prove that TPS/CNF nanocomposites possess the ability to be used as food packaging materials in the future.

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