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Regenerated cellulose films with chitosan and polyvinyl alcohol: Effect of the moisture content on the barrier, mechanical and optical properties
Journal Pre-proof Regenerated cellulose films with chitosan and polyvinyl alcohol: Effect of the moisture content on the barrier, mechanical and optical properties ´ (Investigation) (Writing - original draft), Manuel Patricia Cazon ´ Vazquez (Conceptualization) (Methodology)Writing- Review and editing), ´ Gonzalo Velazquez (Methodology) (Writing - review and editing)
PII:
S0144-8617(20)30205-8
DOI:
https://doi.org/10.1016/j.carbpol.2020.116031
Reference:
CARP 116031
To appear in:
Carbohydrate Polymers
Received Date:
3 October 2019
Revised Date:
12 February 2020
Accepted Date:
17 February 2020
´ P, Vazquez ´ ´ Please cite this article as: Cazon M, Velazquez G, Regenerated cellulose films with chitosan and polyvinyl alcohol: Effect of the moisture content on the barrier, mechanical and optical properties, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.116031
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Regenerated cellulose films with chitosan and polyvinyl alcohol: Effect of the moisture content on the barrier, mechanical and optical properties
Patricia Cazón1,2, Manuel Vázquez2*, Gonzalo Velázquez1*
1
Instituto Politécnico Nacional. CICATA unidad Querétaro. Cerro Blanco No. 141.
Colinas del Cimatario, Querétaro, 76090, México. Department of Analytical Chemistry, Faculty of Veterinary, University of Santiago de
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2
Compostela, 27002-Lugo, Spain
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*Corresponding author: [email protected]; phone +52 (442)-229-0804 Ext 81058.
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Graphical Abstract
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[email protected].
Highlights
Cellulose is a raw material to develop biodegradable composite films
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The water vapour permeability values increased with the moisture content of the films
Mechanical properties of cellulose-chitosan-PVOH films depended on the water content
Water molecules improve the UV-barrier properties of the films, increasing the film opacity
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Abstract
The aim of this research was to evaluate the effect of moisture content on the mechanical,
barrier and optical properties of films obtained from regenerated cellulose with chitosan
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and polyvinyl alcohol equilibrated at several relative humidity conditions. The experimental moisture adsorption isotherm values were fitted using the Guggenheim-
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Anderson-DeBoer model. The adsorption isotherm showed a typical type II sigmoidal
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shape. The highest moisture content (27.53%) was obtained at a water activity of 0.9. The water vapour permeability values increased up to 6.34·10-11 g/ m s Pa as the moisture
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content of the films increased. Tensile strength, percentage of elongation, Young’s modulus, burst strength and distance to burst showed a significant plasticizing effect of the water molecules. Results suggest that interactions between film components and water molecules decrease the transmittance in the UV region and the transparency.
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Consequently, water molecules improve the UV-barrier properties of the films and increasing the opacity.
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Keywords: adsorption isotherms; plasticization; regenerated cellulose; chitosan; polyvinyl alcohol; water vapour permeability; moisture content
1. Introduction In the last years, the interest on polymers from natural resources has increased due to their potential applications in several areas including food industry (Broek Van Den, Knoop, Kappen, & Boeriu, 2015; Campos, Gerschenson, & Flores, 2011; Cha & Chinnan,
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2004; Pang, Cao, Cao, Sheb, & Wang, 2019; Siracusa, Rocculi, Romani, & Rosa, 2008; Trinetta, 2016), medical field (Kim et al., 2017; Tovar-Carrillo, Tagaya, & Kobayashi, 2013; Yin, Li, Sun, & Yao, 2005) or environmental applications (Foresti, Vázquez, &
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Boury, 2017; Kanmani, Aravind, Kamaraj, Sureshbabu, & Karthikeyan, 2017). Among the raw materials available to develop biodegradable films, polysaccharides such as
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cellulose, chitosan, starch, pectin and alginate are extensively used mainly due to their
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low price and abundance in nature (Da Silva E Silva, Pino Hernández, Da Silva Araújo, Peixoto Joele, & Lourenço, 2018; López-Palestina et al., 2019; Molavi, Behfar, Ali Shariati, Kaviani, & Atarod, 2015; Muangrat & Nuankham, 2018; Nisar et al., 2019).
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Cellulose and chitosan are the first and second most abundant biopolymers on earth. These materials have interesting properties that turn them into biomaterials with several potential applications such as burn wound dressing, scaffold material, bone tissue
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engineering, blood vessel replacement and food packaging (Cazon, Velazquez, Ramírez, & Vázquez, 2017; Devlieghere, Vermeulen, & Debevere, 2004; Wang, Lu, & Zhang, 2016).
Nevertheless, pure polysaccharides-based films present some drawbacks, such as low mechanical and barrier properties (Almeida, Frollini, Castellan, & Coma, 2010; Muthuraj, Misra, & Mohanty, 2018) limiting their applications. To overcome these
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limitations, one of the most commonly used strategies is to composite copolymers. Thus, many research groups have focused their efforts in developing, studying and characterizing blends of biodegradable polymers allowing to obtain modified physicochemical properties and improved functional properties (Cazon et al., 2017; Tharanathan, 2003; Trinetta, 2016). In a previous work, improved regenerated cellulose-based films were developed by the combination of chitosan and polyvinyl alcohol (PVOH) (Cazon, Vázquez, &
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Velazquez, 2018; Cazón, Vázquez, & Velazquez, 2018). One of the most interesting properties of these blended films was the good protective property against UV light. This optical property with adequate transparency values increase the interest in this type of
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film, expanding its potential application as an active food packaging to increase the shelf life of foodstuff.
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However, cellulose and chitosan exhibit a high hydrophilic nature, mainly due to
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the high amount of hydroxyl groups presents on the polymer chain like other polysaccharide-based films (Elsabee & Abdou, 2013; Fotie et al., 2017; Raj, Raj, Madan, & Siddaramaiah, 2003). The water content and the interactions taking place between the
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water and the components of the blend have a significant influence on its final properties. In particular, the way in which water molecules interact with the components and their distribution within the matrix determine their applications. Water is considered as an
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important plasticizer for hydrophilic biopolymers (Fotie et al., 2017). The water content in the film depends on the composition and the hydrophilic degree. The water molecules interact with the biopolymers by hydrogen bonds, increasing the free volume of the polymer matrix (Sothornvit & Krochta, 2005). The result is the modifications in the properties of the films such as the mechanical and gas or vapor permeability. Thus, the properties of these films depend on the moisture content and on the interaction with the
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environmental humidity during storage and application (Bertuzzi, Armada, & Gottifredi, 2007; Srinivasa, Ramesh, Kumar, & Tharanathan, 2003). Moisture sorption isotherms allow studying the relationship between water content and water activity measured at a constant temperature (Lewicki, 1997). The Brunauer, Emmett and Teller (BET) equation was one of the most used models to describe the moisture sorption isotherms. However, although it is useful to estimate the moisture content on the monolayer on the internal surface of the film, this model has some
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limitation for water activities above 0.5 (Lewicki, 1997; Timmermann, Chirife, & Iglesias, 2001). The Guggenheim, Anderson and de Boer (GAB) isotherm equation has
shown a good fit up to water activity values of 0.9 (Blahovec, 2004; Lewicki, 1997). The
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GAB model was recommended by the European Project Group COST 90 on Physical
Properties of Foods to describe moisture sorption isotherms of food matrix (Wolf, Spiess,
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& Jung, 1985).
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Previous works have reported the changes in the film properties of hydrophilic films due to the water molecules depending on the environmental moisture conditions. The different storage conditions resulted in modifications in the moisture content that
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affected the functional properties of hydrophilic films such as pea starch films, cassava starch films, chitosan, fish gelatin and polyvinyl alcohol-chitosan (Aguirre-Loredo, Rodríguez-Hernández, Morales-Sánchez, Gómez-Aldapa, & Velazquez, 2016; Hazaveh,
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Mohammadi Nafchi, & Abbaspour, 2015; Liu et al., 2018; Mali, Sakanaka, Yamashita, &
Grossmann,
2005;
Suppakul,
Chalernsook,
Ratisuthawat,
Prapasitthi,
&
Munchukangwan, 2013; Y. Zhang & Han, 2008). Since the results obtained in previous works showed that it is feasible to obtain regenerated cellulose-chitosan-PVOH films with potential uses for food packaging (Cazon et al., 2018; Cazón et al., 2018), the aim of this research was to evaluate the effect
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of water content on the mechanical, barrier and optical properties of films obtained from these materials. The hypothesis is that the moisture of the environment affects the properties of the films. Regenerated cellulose-chitosan-PVOH samples were exposed to a wide range of relative humidity conditions to study the effect of the moisture content on the analyzed properties. The results are expected to provide valuable information on applying
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biodegradable cellulose-based films, further expanding cellulose applications.
2. Materials and methods
Urea (99.5 %), sodium hydroxide (98 %), microcrystalline cellulose extra pure
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(average particle size 90 μm), extra pure anhydrous sodium bromide (99 %) and acetic
acid (99.5 %) were purchased from Acros organics (Geel, Belgium). Full-hydrolyzed
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(>98%) polyvinyl alcohol with average molecular weight (Mw) of 30,000 g/mol and ester
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value of 12-25 were supplied by Merck (Billerica, MA, US). Chitosan (Mw 100000300000) was purchased from Acros organics (Geel, Belgium). Silica gel 2.5-6 mm, potassium carbonate extra pure, lithium chloride extra pure, barium chloride dihydrate
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extra pure, potassium sulfate extra pure, potassium acetate and sodium chloride reagent grade were supplied by Scharlau Microbiology (Barcelona, Spain).
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2.1. Preparation of pure regenerated cellulose films Films based on a solution of 4% (w/w) of cellulose microcrystalline combined
with 1% (w/w) chitosan and 4% (w/w) PVOH were obtained as described on previous studies (Cazon et al., 2018; Cazón et al., 2018). Briefly, a modified NaOH/urea dissolution method was used to obtain a transparent solution of cellulose (L. Zhang, Mao, Zhou, & Cai, 2005). A certain amount of microcrystalline cellulose (4% w/w) was added
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to an initial NaOH/urea/H2O solution (7:14:79, w/w) at room temperature. The mixture was stirred for 1 h to swell the cellulose and to get a homogenous suspension. The cellulose/NaOH/urea mixture was frozen at -20 ºC for 24 h. At that time, the solution was thawed at room temperature with vigorously agitation for 1 h, obtaining a transparent solution of cellulose. Pure cellulose films were obtained after regenerating the cellulose in an acid coagulation bath. The cellulose solution was cast on Petri dish to a 0.5 mm thick layer,
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and then immersed in acid acetic 4% (v/v). The regenerated films were washed with distilled water until pH = 7. Then, the excessive water of the films was removed using
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filter paper.
2.2. Preparation of regenerated cellulose-chitosan-PVOH films
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Pure cellulose films were dipped in a bath with a solution of chitosan (1% w/w)
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and PVOH (4% w/w) for 1 h. Acetic acid (1% v/v) was used as a solvent for chitosan. Finally, the wet cellulose-chitosan-PVOH films were placed in a Petri dish and dried at room temperature for 2 days. The thickness of each sample was measured at 5 random
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points using a thickness meter ET115S (Etari GmbH, Stuttgart, Germany). The samples were stored in desiccators with the corresponding saturated salt or silica gel for 10 days
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to ensure that the moisture equilibrium was reached.
2.3. Moisture adsorption isotherms The moisture adsorption isotherms of regenerated cellulose-chitosan-PVOH films
were determined following the standard static gravimetric method developed by the European Cooperation Project COST 90 (Wolf et al., 1985) with some modification. Film portions of 3×3 cm2 were stored in preconditioned airtight containers. Each container had
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a specific saturated salt solution to obtain a certain relative humidity of equilibrium (RHeq). The RHeq evaluated were 11.5, 32.7, 43.8, 57.7, 75.4 and 90.6%, using saturated salt solutions of LiCl, MgCl2, K2CO3, NaBr, NaCl and BaCl2, respectively. The containers with the samples were stored in a chamber at 30 ºC for 10 days to reach the moisture equilibrium. The samples were in contact with the water vapour molecules on both sides of the film (Aguirre-Loredo et al., 2016). After equilibrium, the moisture content of the samples was determined
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gravimetrically. The samples were weighed to the nearest 0.0001 g for the initial sample weight. Then, the samples were dried at 105 ˚C in an oven for 24 h until constant weight
(dry weight). The test was carried out by triplicate. Equilibrium moisture content (X) was
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Initial weight−Dry weight Initial weight
× 100
(Eq. 1)
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Equilibrium moisture content (X) =
-p
calculated using the Equation 1:
The GAB model (Guggenheim-Anderson-De Boer) (Equation 2) was used to
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interpolate and plot the moisture isotherm adsorption data (Wolf et al., 1985).
𝑋𝑚·𝐶·𝐾·𝑎𝑤
𝑋 = (1−𝐾·𝑎𝑤)·(1−𝐾·𝑎𝑤+𝐶·𝐾·𝑎𝑤)
(Eq.2)
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where Xm is the moisture content in the monolayer, K and C are the model
parameters corresponding to the heat of sorption of adsorbed layer. C is a constant related to the strength of water bindings to primary adsorption sites, and K is related to the heat of multilayer sorption (Bedane, Eić, Farmahini-Farahani, & Xiao, 2015a). GABconstants (Xm, C and K) were determined by non-linear regression.
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2.4. Water vapour permeability Water vapour permeability (WVP) shows the water that diffuses through the film per unit area and time (g/s·m·Pa). This parameter depends on the pressure and film thickness. The WVP was measured gravimetrically following the ASTM Standard Test Method E96. To evaluate the permeability of the films in a wide range of aw, the method established by (Aguirre-Loredo et al., 2016) was followed in the present work. For this, it was necessary to design cells to recreate environments with different RHeq to generate
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different water vapour pressure gradients. A wide mouth cup (2.206·10-3 m2 area) containing 100 ml of a specific saturated salt solutions or distilled water was used to
obtain a certain RHeq (11.5 - 100%). This cup was sealed with a circular sample of the
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film. The relative humidity recreated inside this cup was named as internal humidity. The second environment was obtained with a double bottom chamber designed to contain
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silica gel (dry chamber) or a saturated salt solution (wet chamber) named external
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humidity. The dry or wet chamber was placed on a precision balance, leaving the balance plate free in the center of the chamber. The cup, sealed with the film sample, was placed on the plate. The chamber remained closed and at a constant temperature of 30 °C during
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the test as shown in Fig. 1. The gradient of partial pressure between both sides of the
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sample produced a driving force for the water vapor flux trough the film.
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Fig. 1. Set-up of system to determine the water vapour permeability of regenerated
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cellulose-chitosan-polyvinyl alcohol films at 30 °C.
The water vapor flux resulted in a decrease or increase of the weight of the cup
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depending on the set up tested. The aw of the samples for each test was assumed as the average of aw values (aw = RHeq/100) at both sides of the film (Aguirre-Loredo et al.,
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2016). Tests were run at least 8 h to reach the equilibrium of the water vapor flux. Weight variation of the cup was recorded to the nearest 1×10−4 g and plotted as a function of time (Gennadios, Weller, & Gooding, 1994). The slope was evaluated by linear regression. WVP was calculated according to the combined Fick and Henry laws for gas diffusion
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through films (Equation 3):
𝑊𝑉𝑃 =
∆𝑤·𝑥 ∆𝑡·𝐴·∆𝑃
(Eq. 3)
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Where Δw/∆t (g/s) is the flux measured as weight loss or gain of the cell per unit of time, x (m) is the film thickness, A (m2) is the exposed area and ΔP (Pa) is the water vapour pressure differential at 30 ºC. Each test was carried out in triplicate.
2.5. Mechanical properties A texturometer (TA-XTplus, Stable Micro System, UK) was used to perform tensile and puncture tests to measure the mechanical properties of the films according to
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the standard method ASTM D-882. Samples were cut at sizes of 15×100 mm or 30x30 mm for the tensile or puncture test, respectively. Ten replicates of each sample were
measured. The replicates were placed into airtight containers with different saturated salt
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solutions to obtain microclimates between 1.5% and 96.6% RHeq . The containers with the samples were equilibrated in a chamber at 30 °C for 10 days. Samples were taken
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immediately before measurement. The climatic conditions for the room was 25ºC and
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60% RH. The tensile test was carried out with an initial separation of 40 mm using a crosshead speed of 0.08 mm/s and the stress-strain data were recorded during the deformation of the sample. Tensile strength (TS), percentage of elongation at break (%E)
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and Young’s modulus (YM) were calculated as shown elsewhere (Cazon et al., 2018; Cazón et al., 2018).
In the puncture test, the samples fixed in a film support rig (HDP/FSR) were
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perforated by a cylindrical probe (d = 3 mm) with a crosshead speed of 1 mm/s until rupture. Curves of force (g) vs deformation (mm) were recorded and burst strength and distance to burst were calculated as described elsewhere (Cazón et al., 2018).
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2.6. Light barrier properties, color, transparency and opacity A spectrophotometer V-670 (Jasco Inc, Japan) was used to analyze the light barrier, color, transparency and opacity properties of the film as a function of moisture content. UV-VIS spectra (190 nm–800 nm) were obtained in transmittance mode at 2 nm. The test was carried out in duplicate for each sample. The color of the samples was determined using the software Spectra Manager (Jasco Inc, Japan) set at 2º standard observer with light source of D65. The color matching function selected was JIS Z8701-
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1999 with data pitch of 5 nm to obtain the CIE L* a* b* coordinates. The transparency of the films was calculated from the percent of transmittance at 600 nm and the opacity
from de absorbance of light at 500 nm, following the Equation 4 and 5 (Han & Floros,
𝐴𝑏𝑠500 𝑥
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𝑥
(Eq. 4)
(Eq. 5)
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𝑂𝑝𝑎𝑐𝑖𝑡𝑦 =
(log % 𝑇600 )
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𝑇𝑟𝑎𝑛𝑠𝑝𝑎𝑟𝑒𝑛𝑐𝑦 =
-p
1997; Kanatt, Rao, Chawla, & Sharma, 2012).
Where %T600 is the percent transmittance at 600 nm, x is the film thickness (mm) and Abs500 is the absorbance at 500 nm.
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2.7. Scanning electron microscopy (SEM) The films were coated on slides with adhesive carbon tape, metalized with Au and
observed using a high-vacuum microscope (JEOL JSM-6360LV, Jeol Ltd, Tokyo, Japan) at an accelerating voltage of 20kV.
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2.8. Modeling and statistical analysis Non-linear regression using the solver module in Microsoft Excel® 2010 was used to fit the GAB model to the moisture adsorption isotherm. The analysis of variance (ANOVA) was used to assess the effect of the relative humidity condition on the properties of the films. The mean comparison was performed by Tukey’s test at p < 0.05 using Statistica 7.0 (StatSoft Inc.).
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3. Results and Discussion
3.1. Moisture adsorption isotherms
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Experimental moisture adsorption data and the GAB fitting of the isotherm at 30
°C for regenerated cellulose films combined with chitosan and PVOH are shown in Fig.
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2. Moisture adsorption isotherms relate the water activity with moisture content. In many
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cases, the effect of water on the properties of hydrophilic material are better explained by water activity instead of moisture content.
Film samples reached the highest moisture content of 27.53% at aw = 0.9. In
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previous studies, the moisture content of pure regenerated cellulose films ranged from 10 to 16 % at 0.9 aw and 30 ºC, depending on the cellulose source (Bedane et al., 2015a). Pure chitosan films (1% w/w) had moisture content values up to 40 % when conditioned
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at 0.9 aw and 30 °C (Aguirre-Loredo et al., 2016). Blends of chitosan (2%, w/v) and PVOH (2.3%, w/v) reported moisture content values ranging from 25 to 35 % depending on the polymers ratio (Srinivasa et al., 2003). Thus, regenerated cellulose-chitosan-PVOH films showed maximum moisture content values higher than those obtained for pure cellulose, but lower than those observed in chitosan and chitosan-PVOH films at similar conditions. Results suggest that adding chitosan and PVOH increased the moisture content in
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regenerated cellulose-based films due to their hydrophilic nature. This increase was produced by the hydroxyl and amino groups with a high affinity for the water molecules in the environment (Liu et al., 2018; Vargas, Albors, Chiralt, & González-Martínez, 2009). On the other hand, the presence of cellulose limited the amount of water molecules interacting with the matrix due to the cellulose-chitosan-PVOH interactions and the fewer
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sites available for water adsorption.
Fig. 2. Moisture adsorption isotherm of regenerated cellulose-chitosan-polyvinyl alcohol films at 30 °C. The solid line shows the fit obtained with the GAB model. X is the
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equilibrium moisture content of the samples.
Moisture adsorption isotherm showed a typical curve of water sensitive polymers
with sigmoidal shape (Aguirre-Loredo et al., 2016; Bertuzzi, Armada, et al., 2007; Moreira et al., 2011; Rivero, Damonte, Garcia, & Pinotti, 2016; Srinivasa et al., 2003). In literature, three regions are described for this type of isotherms, the first region of the curve (aw < 0.2) corresponding to the monolayer adsorption of water molecules by
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hydrogen bond on polar sites of the film. The second, a linear region (0.2 < aw < 0.6) corresponding to adsorption of multilayers. In the third region (aw > 0.6), there is a drastic increase of the slope due to the condensation of water in the pores of the material resulting in the swelling of hydrophilic polymers (Srinivasa et al., 2003; Su et al., 2010). Experimental adsorption data were well fitted by GAB model (r2 > 0.99). The GAB parameter Xm, C and K obtained were 5.93, 11.00 and 0.88, respectively. Following the Brunauer classifications, regenerated cellulose-chitosan-PVOH films showed a
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sorption isotherm type II (0 ≤ K ≤ 1 and C >2), the most common sorption isotherm of biological and food materials (Blahovec, 2004; Hazaveh et al., 2015).
In previous studies, pure regenerated cellulose showed Xm values of 7.16 and
-p
chitosan-PVOH blend films values ranged from 7.86 to 12.76 (Bedane, Eić, FarmahiniFarahani, & Xiao, 2015b; Srinivasa et al., 2003). A moisture content value in the
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monolayer greater than that reported for pure cellulose films was expected. However,
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regenerated cellulose-chitosan-PVOH showed lower values. Although the presence of chitosan and PVOH provided more active sites, due to the increase of the hydroxyl and amino groups on the surface of the film, the moisture content in the monolayer was lower.
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Probably, the interactions between cellulose-chitosan-PVOH limited the availability of these binding sites. In addition, the presence of chitosan and PVOH modified the regenerated cellulose surface, reducing the empty voids available for water molecules
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retention (Cazón et al., 2018).
3.2. Water vapour permeability (WVP) The WVP of regenerated cellulose-chitosan-PVOH films was analyzed at different moisture contents calculated from the fitted moisture adsorption isotherm at 30 ºC. As shown in Fig. 3, the WVP values of the samples increased as the moisture content
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of the films increased from values from 6.86·10-12 to 6.34·10-11 g/ m s Pa. Thus, the WVP of the films depended on the moisture content. The permeability values remained low until values of aw about 0.3, showing an increase of the slope from that values of aw of
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0.35 (Fig. 3).
Fig. 3. Water vapour permeability of regenerated cellulose-chitosan-polyvinyl alcohol
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films at 30 °C as a function of aw . The solid line shows the isotherm obtained with the GAB model. X is the equilibrium moisture content of the samples.
Usually WVP is independent of the water content of the films in synthetic and
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hydrophobic materials. However, in the case of hydrophilic materials, like cellulose, chitosan and PVOH, the polar groups (amino and hydroxyl groups) interact with permeating water molecules, resulting in important structural changes. These structural changes modify the diffusion of the water molecules through the matrix of the film leading to WVP variations (Bedane et al., 2015b; Chinnan & Park, 1995; Roy, Gennadios, Weller, & Testin, 2000; Su et al., 2010). During the plasticization process, water
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molecules increase the free volume in the polymer due to the swelling of the material, losing the packed structure. The swelling of the films enhances the mobility of the polymeric chains, easing the diffusion of water through the polymer matrix. As a result, an increase in WVP occurs, being more significant at high moisture contents (Bedane et al., 2015b; Chinnan & Park, 1995; Roy et al., 2000; Su et al., 2010; Wiles, Vergano, Barron, Bunn, & Testin, 2000). Similar behavior has been observed in other films based
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on hydrophilic biopolymers like chitosan (Aguirre-Loredo et al., 2016).
3.3. Mechanical properties
In this study, the tensile and puncture properties of the regenerated cellulose-
-p
chitosan-PVOH films were studied in a wide range of RHeq environments to analyze
changes in the mechanical properties depending on the moisture content or water activity.
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The minimum water activity value obtained using silica gel called 0 really could be
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around 0.03 as reported previously (Salgado et al., 2017). As shown in Fig. 4 and 5, the mechanical properties of the developed film were strongly affected by the moisture content or Aw (p < 0.05).
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Regarding the tensile properties, TS values ranged from 8.63 to 72.77 MPa. Two
regions were observed (Fig. 4a). First, up to aw = 0.32, TS increased with the increase in RH and with the moisture content of the film. Second, it was followed by a continuous
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decreasing from aw > 0.32.
The %E values increased slightly from 2.65 to 7.51% when aw increased up to
0.57. From aw > 0.57, the elongation percentage values showed a more significant increase, changing from 5.49 in dry films to 31.43% at the maximum water activity value.
17
90
A)
80 Tensile strength (MPa)
70 60 50 40 30 20 10 0 0.00 40
0.20
0.30
0.40
0.50
0.60
0.70
0.90
1.00
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35 30 25 20 15
-p
10 5
0 0.00 4000
0.80
aw
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.70
0.80
0.90
1.00
aw
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3500 3000 2500 2000
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Young’s modulus (MPa)
C)
0.10
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Percentage of elongation (%)
B)
1500 1000 500
0.10
0.20
0.30
0.40
0.50
0.60
aw
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0 0.00
Fig. 4. Mechanical properties of regenerated cellulose-chitosan-polyvinyl alcohol films at 30 °C as a function of aw. A) Water activity effect on tensile strength. B) Water activity effect on percentage of elongation. C) Water activity effect on Yong’s modulus.
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Young’s modulus ranged from 40 to 3224.67 MPa. The decrease with the greatest slope took place between 0.43 < aw < 0.75, diminishing the YM value from 2224.60 to 361.42 MPa. The polar hydrophilic groups in chitosan and PVOH promoted the moisture adsorption in the films (Liu et al., 2018). The hygroscopic nature of chitosan and PVOH allowed incorporating additional water into the matrix of the film. The adsorbed water molecules changed the internal structure of the composite films resulting in soft and elastic polymeric materials (Chang, Abd Karim, & Seow, 2006; Liu et al., 2018). Thus,
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water exerted a plasticizing effect on regenerated cellulose-chitosan-PVOH films, enhancing the chains polymer mobility. The low molecular weight of water molecules
facilitates the molecular mobility of amorphous and partially crystalline polymers (Mali
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et al., 2005). These structural changes in the film as a result of the plasticizing effect of
water were reflected in the mechanical properties, decreasing the TS and YM values and
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a consequent increase of %E (Sothornvit & Krochta, 2005).
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In the puncture test (Fig. 5), BS values ranged from 378.07 to 4370.50 g. The puncture breaking resistance increased when the water activity increased, reaching the maximum value at aw = 0.57. In the range 0.57 < aw < 0.75 the values remained in the
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range from 3940.03 to 4370.50 g, with small variations.
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A) 5000 4500
Burst strength (g)
4000 3500 3000 2500 2000 1500 500 0 0.00
0.20
0.30
0.40
0.50
aw
6.0
0.70
0.80
0.90
1.00
0.90
1.00
-p
5.0
re
4.0 3.0 2.0 1.0
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0.0 0.00
0.60
lP
Distance to burst (mm)
B)
0.10
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1000
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
aw
Fig. 5. Puncture properties of regenerated cellulose-chitosan-polyvinyl alcohol films at
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30 °C as a function of aw. A) Water activity effect on burst strength. B) Water activity effect on distance to burst.
Results indicate that, in general, the moisture content of the film improved the resistance to axial forces properties. The plasticizer effect enhanced the mobility of the polymer chains, as mentioned in the analysis of tensile properties. This effect facilitated
20
the reorganization of the polymer chains during the axial deformation, reducing brittleness and increasing the BS values. This behavior is in accordance with the effect of the water molecules on the DB property increasing the elasticity of the films as the moisture content of the samples increased. DB values increased from 0.50 mm (aw = 0) to a maximum value of 4.18 mm (aw = 0.90). The plasticizing effect of water molecules on hydrophilic materials and its impact on the mechanical properties have also been reported in previous studies (Aguirre-Loredo et al., 2016; Liu et al., 2018; Mali et al.,
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2005; Y. Zhang & Han, 2008).
3.4. Optical properties
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The optical properties like color, transparency and opacity are important
parameters in terms of appearance as the consumers expect the packaged product
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pleasant. In addition, films with good UV-barrier properties allow expanding the shelf
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life in packed products, delaying the degradation of components that diminish their quality. The optical properties may suffer modifications depending on the moisture conditions during the storage due to the water affinity of the film components. Fig. 6
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shows the light barrier properties of the films by the transmittance values of the samples in the UV region from 200 to 400 nm and the VIS region from 400 to 800 nm. Lower wavelengths contain more energy and are potentially more harmful for the lipid
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oxidations. UV-C radiation usually does not cross the atmosphere, but artificial sources of UV-C are used commonly for sterilize surfaces. Table 1 summarizes the results obtained from the analysis of the optical properties
of the films. For the UV-barrier properties, the water activity of the film decreased the transmittance of the samples in the UV region. The presence of water molecules decreased the transmittance from 16.34 to 7.95 %, from 12.06 to 5.03 % and from 5.28
21
to 2.38 % in the UV-A, UV-B and UVC regions, respectively. Hence, data indicated that the presence of water molecules improved the UV- barrier properties of the developed films. This improvement could be the result of multiple factors including the optical properties of the water (Litjens, Quickenden, & Freeman, 1999; Quickenden & Irvin, 1980), modifications in the structure and the increasing of thickness due to the plasticizing effect of the water molecules (Bertuzzi, Castro Vidaurre, Armada, & Gottifredi, 2007). These results are very good comparing with the transmittance of synthetic polymers like
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polyethylene (59% at 240 nm) or poly(ethylene terephthalate) (25% at 310 nm) (Moura et al., 2004). Note that a low transmittance in the UV region is good to avoid lipid
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re
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oxidation of food promote by the UV light
22
Fig. 6. UV-VIS spectra profile of regenerated cellulose-chitosan-polyvinyl alcohol films at 30 °C as a function of aw.
The effects of the moisture content on CIE L*, a* and b* coordinates, transparency and opacity values were assessed. Table 1 and Table 2 summarizes the results obtained from the analysis of the optical properties of the films.
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Table 1. Color parameters (L*, a* and b*) and transmittance values in each region of UVlight radiation UV-A (320−400 nm), UV-B (280−320 nm) and UV-C (190-280 nm) of samples conditioned at different relative humidity. %T is the average percent
a*
(UV-B)
(UV-C)
%T
%T
%T
16.34
12.06
5.28
0
53.55
2.79
0.11
48.31
0.45
4.16
11.00
7.94
4.04
0.32
50.06
0.33
3.27
13.78
10.50
5.18
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0.20
b*
(UV-A)
re
L* Aw
-p
transmittance.
43.70
0.57
4.36
9.20
6.59
3.29
0.57
43.43
0.36
3.07
10.00
7.58
3.87
0.74
43.00
0.16
3.41
8.20
5.11
2.48
0.96
41.93
0.34
3.89
7.95
5.03
2.38
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0.43
23
Table 2. Transparency, Opacity and Thickness of samples preconditioned at different relative humidity.
Thickness Transparency
Opacity
Aw
mm 14.94
7.49
0.0890.008
0.11
13.30
8.48
0.0930.013
0.32
15.69
9.16
0.0820.015
0.43
12.35
9.51
0.0900.009
0.57
12.15
9.45
0.0930.013
0.74
11.19
9.13
0.0990.012
0.96
11.07
9.16
0.1020.009
re
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ro of
0
The main difference was the decreasing of lightness (L* value) of the samples as
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the water activity increased. Moreover, the water molecules produced a slight increase in yellowish color (b* value). The values of b* were similar to those obtained for other films
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with chitosan (Thakhiew, Devahastin, & Soponronnarit, 2010). In the case of redness, the a* values ranged from 0.16 to 0.57 and there was not a clear trend of the variation of the values as a function of the water activity. The increase of the moisture content of the samples produced a decrease of the
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transparency values and consequently, an increase of the opacity (Table 2). Results show that optimal UV-barrier properties of the composite films developed with good visual appearance can be obtained as shown in Fig. 7. These films could have potential applications in packaging of food product.
24
ro of -p
re
Fig. 7. Visual appearance of a sample preconditioned at 43.80 % relative humidity
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conditions and 30 ºC.
Figure 8 shows the top, bottom and section surfaces of the films obtained. Films
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showed a continuous matrix, no pores or cracks were detected, indicating a dense
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structure. Bottom side showed a smoother surface.
25
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re
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Bottom
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Top
Section
Fig. 8. Scanning electron microscopy images of the surfaces of the films.
26
4. Conclusions Results showed that the mechanical, permeability and optical properties of regenerated cellulose-chitosan-PVOH films depended on the moisture content or water activity. The moisture adsorption isotherms data were well fitted by GAB model, showing a sigmoidal curve of type II. Due to the hydrophilic nature of the components of the blend, when the relative humidity of equilibrium increased, the moisture content of the film was
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increased. The water molecules manifested a plasticizing effect on regenerated cellulosechitosan-PVOH films, resulting in an increase of the water vapour permeability when the moisture content increased. The modifications of the mechanical properties of cellulose-
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based films followed the typical behavior of a hydrophilic polymer under the plasticizing
effect of water molecules decreasing the resistance to rupture and increasing the elasticity
re
of the material. Moreover, the water vapour permeability increased when the moisture
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content increased as the swelling of the structure facilitated the diffusion of water molecules through the polymer matrix. UV-VIS spectra showed that the optical barrier properties of regenerated cellulose-based films against UV-radiation improved when the
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moisture content increased. However, the transparency values decreased with the moisture content, increasing the opacity of the samples. Considering the properties evaluated of the films based on cellulosic materials, have potential applications in the
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food industry. This study can help to determine the most suitable application of these composites according to the needs and environmental conditions.
CRediT author statement
27
Patricia Cazón: Investigation, Writing- Original draft preparation.
Gonzalo Velazquez: Methodology, Writing- Reviewing and Editing,
Manuel Vazquez: Conceptualization, Methodology, Writing- Reviewing
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and Editing.
Acknowledgements
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A grant from CONACYT (Mexico) to author Patricia Cazón (#435948) is
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re
gratefully acknowledged.
28
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PII:
S0144-8617(20)30205-8
DOI:
https://doi.org/10.1016/j.carbpol.2020.116031
Reference:
CARP 116031
To appear in:
Carbohydrate Polymers
Received Date:
3 October 2019
Revised Date:
12 February 2020
Accepted Date:
17 February 2020
´ P, Vazquez ´ ´ Please cite this article as: Cazon M, Velazquez G, Regenerated cellulose films with chitosan and polyvinyl alcohol: Effect of the moisture content on the barrier, mechanical and optical properties, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.116031
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Regenerated cellulose films with chitosan and polyvinyl alcohol: Effect of the moisture content on the barrier, mechanical and optical properties
Patricia Cazón1,2, Manuel Vázquez2*, Gonzalo Velázquez1*
1
Instituto Politécnico Nacional. CICATA unidad Querétaro. Cerro Blanco No. 141.
Colinas del Cimatario, Querétaro, 76090, México. Department of Analytical Chemistry, Faculty of Veterinary, University of Santiago de
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2
Compostela, 27002-Lugo, Spain
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*Corresponding author: [email protected]; phone +52 (442)-229-0804 Ext 81058.
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Graphical Abstract
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[email protected].
Highlights
Cellulose is a raw material to develop biodegradable composite films
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The water vapour permeability values increased with the moisture content of the films
Mechanical properties of cellulose-chitosan-PVOH films depended on the water content
Water molecules improve the UV-barrier properties of the films, increasing the film opacity
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Abstract
The aim of this research was to evaluate the effect of moisture content on the mechanical,
barrier and optical properties of films obtained from regenerated cellulose with chitosan
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and polyvinyl alcohol equilibrated at several relative humidity conditions. The experimental moisture adsorption isotherm values were fitted using the Guggenheim-
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Anderson-DeBoer model. The adsorption isotherm showed a typical type II sigmoidal
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shape. The highest moisture content (27.53%) was obtained at a water activity of 0.9. The water vapour permeability values increased up to 6.34·10-11 g/ m s Pa as the moisture
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content of the films increased. Tensile strength, percentage of elongation, Young’s modulus, burst strength and distance to burst showed a significant plasticizing effect of the water molecules. Results suggest that interactions between film components and water molecules decrease the transmittance in the UV region and the transparency.
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Consequently, water molecules improve the UV-barrier properties of the films and increasing the opacity.
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Keywords: adsorption isotherms; plasticization; regenerated cellulose; chitosan; polyvinyl alcohol; water vapour permeability; moisture content
1. Introduction In the last years, the interest on polymers from natural resources has increased due to their potential applications in several areas including food industry (Broek Van Den, Knoop, Kappen, & Boeriu, 2015; Campos, Gerschenson, & Flores, 2011; Cha & Chinnan,
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2004; Pang, Cao, Cao, Sheb, & Wang, 2019; Siracusa, Rocculi, Romani, & Rosa, 2008; Trinetta, 2016), medical field (Kim et al., 2017; Tovar-Carrillo, Tagaya, & Kobayashi, 2013; Yin, Li, Sun, & Yao, 2005) or environmental applications (Foresti, Vázquez, &
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Boury, 2017; Kanmani, Aravind, Kamaraj, Sureshbabu, & Karthikeyan, 2017). Among the raw materials available to develop biodegradable films, polysaccharides such as
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cellulose, chitosan, starch, pectin and alginate are extensively used mainly due to their
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low price and abundance in nature (Da Silva E Silva, Pino Hernández, Da Silva Araújo, Peixoto Joele, & Lourenço, 2018; López-Palestina et al., 2019; Molavi, Behfar, Ali Shariati, Kaviani, & Atarod, 2015; Muangrat & Nuankham, 2018; Nisar et al., 2019).
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Cellulose and chitosan are the first and second most abundant biopolymers on earth. These materials have interesting properties that turn them into biomaterials with several potential applications such as burn wound dressing, scaffold material, bone tissue
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engineering, blood vessel replacement and food packaging (Cazon, Velazquez, Ramírez, & Vázquez, 2017; Devlieghere, Vermeulen, & Debevere, 2004; Wang, Lu, & Zhang, 2016).
Nevertheless, pure polysaccharides-based films present some drawbacks, such as low mechanical and barrier properties (Almeida, Frollini, Castellan, & Coma, 2010; Muthuraj, Misra, & Mohanty, 2018) limiting their applications. To overcome these
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limitations, one of the most commonly used strategies is to composite copolymers. Thus, many research groups have focused their efforts in developing, studying and characterizing blends of biodegradable polymers allowing to obtain modified physicochemical properties and improved functional properties (Cazon et al., 2017; Tharanathan, 2003; Trinetta, 2016). In a previous work, improved regenerated cellulose-based films were developed by the combination of chitosan and polyvinyl alcohol (PVOH) (Cazon, Vázquez, &
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Velazquez, 2018; Cazón, Vázquez, & Velazquez, 2018). One of the most interesting properties of these blended films was the good protective property against UV light. This optical property with adequate transparency values increase the interest in this type of
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film, expanding its potential application as an active food packaging to increase the shelf life of foodstuff.
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However, cellulose and chitosan exhibit a high hydrophilic nature, mainly due to
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the high amount of hydroxyl groups presents on the polymer chain like other polysaccharide-based films (Elsabee & Abdou, 2013; Fotie et al., 2017; Raj, Raj, Madan, & Siddaramaiah, 2003). The water content and the interactions taking place between the
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water and the components of the blend have a significant influence on its final properties. In particular, the way in which water molecules interact with the components and their distribution within the matrix determine their applications. Water is considered as an
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important plasticizer for hydrophilic biopolymers (Fotie et al., 2017). The water content in the film depends on the composition and the hydrophilic degree. The water molecules interact with the biopolymers by hydrogen bonds, increasing the free volume of the polymer matrix (Sothornvit & Krochta, 2005). The result is the modifications in the properties of the films such as the mechanical and gas or vapor permeability. Thus, the properties of these films depend on the moisture content and on the interaction with the
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environmental humidity during storage and application (Bertuzzi, Armada, & Gottifredi, 2007; Srinivasa, Ramesh, Kumar, & Tharanathan, 2003). Moisture sorption isotherms allow studying the relationship between water content and water activity measured at a constant temperature (Lewicki, 1997). The Brunauer, Emmett and Teller (BET) equation was one of the most used models to describe the moisture sorption isotherms. However, although it is useful to estimate the moisture content on the monolayer on the internal surface of the film, this model has some
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limitation for water activities above 0.5 (Lewicki, 1997; Timmermann, Chirife, & Iglesias, 2001). The Guggenheim, Anderson and de Boer (GAB) isotherm equation has
shown a good fit up to water activity values of 0.9 (Blahovec, 2004; Lewicki, 1997). The
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GAB model was recommended by the European Project Group COST 90 on Physical
Properties of Foods to describe moisture sorption isotherms of food matrix (Wolf, Spiess,
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& Jung, 1985).
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Previous works have reported the changes in the film properties of hydrophilic films due to the water molecules depending on the environmental moisture conditions. The different storage conditions resulted in modifications in the moisture content that
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affected the functional properties of hydrophilic films such as pea starch films, cassava starch films, chitosan, fish gelatin and polyvinyl alcohol-chitosan (Aguirre-Loredo, Rodríguez-Hernández, Morales-Sánchez, Gómez-Aldapa, & Velazquez, 2016; Hazaveh,
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Mohammadi Nafchi, & Abbaspour, 2015; Liu et al., 2018; Mali, Sakanaka, Yamashita, &
Grossmann,
2005;
Suppakul,
Chalernsook,
Ratisuthawat,
Prapasitthi,
&
Munchukangwan, 2013; Y. Zhang & Han, 2008). Since the results obtained in previous works showed that it is feasible to obtain regenerated cellulose-chitosan-PVOH films with potential uses for food packaging (Cazon et al., 2018; Cazón et al., 2018), the aim of this research was to evaluate the effect
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of water content on the mechanical, barrier and optical properties of films obtained from these materials. The hypothesis is that the moisture of the environment affects the properties of the films. Regenerated cellulose-chitosan-PVOH samples were exposed to a wide range of relative humidity conditions to study the effect of the moisture content on the analyzed properties. The results are expected to provide valuable information on applying
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biodegradable cellulose-based films, further expanding cellulose applications.
2. Materials and methods
Urea (99.5 %), sodium hydroxide (98 %), microcrystalline cellulose extra pure
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(average particle size 90 μm), extra pure anhydrous sodium bromide (99 %) and acetic
acid (99.5 %) were purchased from Acros organics (Geel, Belgium). Full-hydrolyzed
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(>98%) polyvinyl alcohol with average molecular weight (Mw) of 30,000 g/mol and ester
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value of 12-25 were supplied by Merck (Billerica, MA, US). Chitosan (Mw 100000300000) was purchased from Acros organics (Geel, Belgium). Silica gel 2.5-6 mm, potassium carbonate extra pure, lithium chloride extra pure, barium chloride dihydrate
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extra pure, potassium sulfate extra pure, potassium acetate and sodium chloride reagent grade were supplied by Scharlau Microbiology (Barcelona, Spain).
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2.1. Preparation of pure regenerated cellulose films Films based on a solution of 4% (w/w) of cellulose microcrystalline combined
with 1% (w/w) chitosan and 4% (w/w) PVOH were obtained as described on previous studies (Cazon et al., 2018; Cazón et al., 2018). Briefly, a modified NaOH/urea dissolution method was used to obtain a transparent solution of cellulose (L. Zhang, Mao, Zhou, & Cai, 2005). A certain amount of microcrystalline cellulose (4% w/w) was added
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to an initial NaOH/urea/H2O solution (7:14:79, w/w) at room temperature. The mixture was stirred for 1 h to swell the cellulose and to get a homogenous suspension. The cellulose/NaOH/urea mixture was frozen at -20 ºC for 24 h. At that time, the solution was thawed at room temperature with vigorously agitation for 1 h, obtaining a transparent solution of cellulose. Pure cellulose films were obtained after regenerating the cellulose in an acid coagulation bath. The cellulose solution was cast on Petri dish to a 0.5 mm thick layer,
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and then immersed in acid acetic 4% (v/v). The regenerated films were washed with distilled water until pH = 7. Then, the excessive water of the films was removed using
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filter paper.
2.2. Preparation of regenerated cellulose-chitosan-PVOH films
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Pure cellulose films were dipped in a bath with a solution of chitosan (1% w/w)
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and PVOH (4% w/w) for 1 h. Acetic acid (1% v/v) was used as a solvent for chitosan. Finally, the wet cellulose-chitosan-PVOH films were placed in a Petri dish and dried at room temperature for 2 days. The thickness of each sample was measured at 5 random
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points using a thickness meter ET115S (Etari GmbH, Stuttgart, Germany). The samples were stored in desiccators with the corresponding saturated salt or silica gel for 10 days
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to ensure that the moisture equilibrium was reached.
2.3. Moisture adsorption isotherms The moisture adsorption isotherms of regenerated cellulose-chitosan-PVOH films
were determined following the standard static gravimetric method developed by the European Cooperation Project COST 90 (Wolf et al., 1985) with some modification. Film portions of 3×3 cm2 were stored in preconditioned airtight containers. Each container had
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a specific saturated salt solution to obtain a certain relative humidity of equilibrium (RHeq). The RHeq evaluated were 11.5, 32.7, 43.8, 57.7, 75.4 and 90.6%, using saturated salt solutions of LiCl, MgCl2, K2CO3, NaBr, NaCl and BaCl2, respectively. The containers with the samples were stored in a chamber at 30 ºC for 10 days to reach the moisture equilibrium. The samples were in contact with the water vapour molecules on both sides of the film (Aguirre-Loredo et al., 2016). After equilibrium, the moisture content of the samples was determined
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gravimetrically. The samples were weighed to the nearest 0.0001 g for the initial sample weight. Then, the samples were dried at 105 ˚C in an oven for 24 h until constant weight
(dry weight). The test was carried out by triplicate. Equilibrium moisture content (X) was
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Initial weight−Dry weight Initial weight
× 100
(Eq. 1)
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Equilibrium moisture content (X) =
-p
calculated using the Equation 1:
The GAB model (Guggenheim-Anderson-De Boer) (Equation 2) was used to
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interpolate and plot the moisture isotherm adsorption data (Wolf et al., 1985).
𝑋𝑚·𝐶·𝐾·𝑎𝑤
𝑋 = (1−𝐾·𝑎𝑤)·(1−𝐾·𝑎𝑤+𝐶·𝐾·𝑎𝑤)
(Eq.2)
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where Xm is the moisture content in the monolayer, K and C are the model
parameters corresponding to the heat of sorption of adsorbed layer. C is a constant related to the strength of water bindings to primary adsorption sites, and K is related to the heat of multilayer sorption (Bedane, Eić, Farmahini-Farahani, & Xiao, 2015a). GABconstants (Xm, C and K) were determined by non-linear regression.
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2.4. Water vapour permeability Water vapour permeability (WVP) shows the water that diffuses through the film per unit area and time (g/s·m·Pa). This parameter depends on the pressure and film thickness. The WVP was measured gravimetrically following the ASTM Standard Test Method E96. To evaluate the permeability of the films in a wide range of aw, the method established by (Aguirre-Loredo et al., 2016) was followed in the present work. For this, it was necessary to design cells to recreate environments with different RHeq to generate
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different water vapour pressure gradients. A wide mouth cup (2.206·10-3 m2 area) containing 100 ml of a specific saturated salt solutions or distilled water was used to
obtain a certain RHeq (11.5 - 100%). This cup was sealed with a circular sample of the
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film. The relative humidity recreated inside this cup was named as internal humidity. The second environment was obtained with a double bottom chamber designed to contain
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silica gel (dry chamber) or a saturated salt solution (wet chamber) named external
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humidity. The dry or wet chamber was placed on a precision balance, leaving the balance plate free in the center of the chamber. The cup, sealed with the film sample, was placed on the plate. The chamber remained closed and at a constant temperature of 30 °C during
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the test as shown in Fig. 1. The gradient of partial pressure between both sides of the
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sample produced a driving force for the water vapor flux trough the film.
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Fig. 1. Set-up of system to determine the water vapour permeability of regenerated
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cellulose-chitosan-polyvinyl alcohol films at 30 °C.
The water vapor flux resulted in a decrease or increase of the weight of the cup
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depending on the set up tested. The aw of the samples for each test was assumed as the average of aw values (aw = RHeq/100) at both sides of the film (Aguirre-Loredo et al.,
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2016). Tests were run at least 8 h to reach the equilibrium of the water vapor flux. Weight variation of the cup was recorded to the nearest 1×10−4 g and plotted as a function of time (Gennadios, Weller, & Gooding, 1994). The slope was evaluated by linear regression. WVP was calculated according to the combined Fick and Henry laws for gas diffusion
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through films (Equation 3):
𝑊𝑉𝑃 =
∆𝑤·𝑥 ∆𝑡·𝐴·∆𝑃
(Eq. 3)
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Where Δw/∆t (g/s) is the flux measured as weight loss or gain of the cell per unit of time, x (m) is the film thickness, A (m2) is the exposed area and ΔP (Pa) is the water vapour pressure differential at 30 ºC. Each test was carried out in triplicate.
2.5. Mechanical properties A texturometer (TA-XTplus, Stable Micro System, UK) was used to perform tensile and puncture tests to measure the mechanical properties of the films according to
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the standard method ASTM D-882. Samples were cut at sizes of 15×100 mm or 30x30 mm for the tensile or puncture test, respectively. Ten replicates of each sample were
measured. The replicates were placed into airtight containers with different saturated salt
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solutions to obtain microclimates between 1.5% and 96.6% RHeq . The containers with the samples were equilibrated in a chamber at 30 °C for 10 days. Samples were taken
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immediately before measurement. The climatic conditions for the room was 25ºC and
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60% RH. The tensile test was carried out with an initial separation of 40 mm using a crosshead speed of 0.08 mm/s and the stress-strain data were recorded during the deformation of the sample. Tensile strength (TS), percentage of elongation at break (%E)
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and Young’s modulus (YM) were calculated as shown elsewhere (Cazon et al., 2018; Cazón et al., 2018).
In the puncture test, the samples fixed in a film support rig (HDP/FSR) were
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perforated by a cylindrical probe (d = 3 mm) with a crosshead speed of 1 mm/s until rupture. Curves of force (g) vs deformation (mm) were recorded and burst strength and distance to burst were calculated as described elsewhere (Cazón et al., 2018).
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2.6. Light barrier properties, color, transparency and opacity A spectrophotometer V-670 (Jasco Inc, Japan) was used to analyze the light barrier, color, transparency and opacity properties of the film as a function of moisture content. UV-VIS spectra (190 nm–800 nm) were obtained in transmittance mode at 2 nm. The test was carried out in duplicate for each sample. The color of the samples was determined using the software Spectra Manager (Jasco Inc, Japan) set at 2º standard observer with light source of D65. The color matching function selected was JIS Z8701-
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1999 with data pitch of 5 nm to obtain the CIE L* a* b* coordinates. The transparency of the films was calculated from the percent of transmittance at 600 nm and the opacity
from de absorbance of light at 500 nm, following the Equation 4 and 5 (Han & Floros,
𝐴𝑏𝑠500 𝑥
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𝑥
(Eq. 4)
(Eq. 5)
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𝑂𝑝𝑎𝑐𝑖𝑡𝑦 =
(log % 𝑇600 )
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𝑇𝑟𝑎𝑛𝑠𝑝𝑎𝑟𝑒𝑛𝑐𝑦 =
-p
1997; Kanatt, Rao, Chawla, & Sharma, 2012).
Where %T600 is the percent transmittance at 600 nm, x is the film thickness (mm) and Abs500 is the absorbance at 500 nm.
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2.7. Scanning electron microscopy (SEM) The films were coated on slides with adhesive carbon tape, metalized with Au and
observed using a high-vacuum microscope (JEOL JSM-6360LV, Jeol Ltd, Tokyo, Japan) at an accelerating voltage of 20kV.
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2.8. Modeling and statistical analysis Non-linear regression using the solver module in Microsoft Excel® 2010 was used to fit the GAB model to the moisture adsorption isotherm. The analysis of variance (ANOVA) was used to assess the effect of the relative humidity condition on the properties of the films. The mean comparison was performed by Tukey’s test at p < 0.05 using Statistica 7.0 (StatSoft Inc.).
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3. Results and Discussion
3.1. Moisture adsorption isotherms
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Experimental moisture adsorption data and the GAB fitting of the isotherm at 30
°C for regenerated cellulose films combined with chitosan and PVOH are shown in Fig.
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2. Moisture adsorption isotherms relate the water activity with moisture content. In many
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cases, the effect of water on the properties of hydrophilic material are better explained by water activity instead of moisture content.
Film samples reached the highest moisture content of 27.53% at aw = 0.9. In
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previous studies, the moisture content of pure regenerated cellulose films ranged from 10 to 16 % at 0.9 aw and 30 ºC, depending on the cellulose source (Bedane et al., 2015a). Pure chitosan films (1% w/w) had moisture content values up to 40 % when conditioned
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at 0.9 aw and 30 °C (Aguirre-Loredo et al., 2016). Blends of chitosan (2%, w/v) and PVOH (2.3%, w/v) reported moisture content values ranging from 25 to 35 % depending on the polymers ratio (Srinivasa et al., 2003). Thus, regenerated cellulose-chitosan-PVOH films showed maximum moisture content values higher than those obtained for pure cellulose, but lower than those observed in chitosan and chitosan-PVOH films at similar conditions. Results suggest that adding chitosan and PVOH increased the moisture content in
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regenerated cellulose-based films due to their hydrophilic nature. This increase was produced by the hydroxyl and amino groups with a high affinity for the water molecules in the environment (Liu et al., 2018; Vargas, Albors, Chiralt, & González-Martínez, 2009). On the other hand, the presence of cellulose limited the amount of water molecules interacting with the matrix due to the cellulose-chitosan-PVOH interactions and the fewer
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sites available for water adsorption.
Fig. 2. Moisture adsorption isotherm of regenerated cellulose-chitosan-polyvinyl alcohol films at 30 °C. The solid line shows the fit obtained with the GAB model. X is the
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equilibrium moisture content of the samples.
Moisture adsorption isotherm showed a typical curve of water sensitive polymers
with sigmoidal shape (Aguirre-Loredo et al., 2016; Bertuzzi, Armada, et al., 2007; Moreira et al., 2011; Rivero, Damonte, Garcia, & Pinotti, 2016; Srinivasa et al., 2003). In literature, three regions are described for this type of isotherms, the first region of the curve (aw < 0.2) corresponding to the monolayer adsorption of water molecules by
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hydrogen bond on polar sites of the film. The second, a linear region (0.2 < aw < 0.6) corresponding to adsorption of multilayers. In the third region (aw > 0.6), there is a drastic increase of the slope due to the condensation of water in the pores of the material resulting in the swelling of hydrophilic polymers (Srinivasa et al., 2003; Su et al., 2010). Experimental adsorption data were well fitted by GAB model (r2 > 0.99). The GAB parameter Xm, C and K obtained were 5.93, 11.00 and 0.88, respectively. Following the Brunauer classifications, regenerated cellulose-chitosan-PVOH films showed a
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sorption isotherm type II (0 ≤ K ≤ 1 and C >2), the most common sorption isotherm of biological and food materials (Blahovec, 2004; Hazaveh et al., 2015).
In previous studies, pure regenerated cellulose showed Xm values of 7.16 and
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chitosan-PVOH blend films values ranged from 7.86 to 12.76 (Bedane, Eić, FarmahiniFarahani, & Xiao, 2015b; Srinivasa et al., 2003). A moisture content value in the
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monolayer greater than that reported for pure cellulose films was expected. However,
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regenerated cellulose-chitosan-PVOH showed lower values. Although the presence of chitosan and PVOH provided more active sites, due to the increase of the hydroxyl and amino groups on the surface of the film, the moisture content in the monolayer was lower.
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Probably, the interactions between cellulose-chitosan-PVOH limited the availability of these binding sites. In addition, the presence of chitosan and PVOH modified the regenerated cellulose surface, reducing the empty voids available for water molecules
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retention (Cazón et al., 2018).
3.2. Water vapour permeability (WVP) The WVP of regenerated cellulose-chitosan-PVOH films was analyzed at different moisture contents calculated from the fitted moisture adsorption isotherm at 30 ºC. As shown in Fig. 3, the WVP values of the samples increased as the moisture content
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of the films increased from values from 6.86·10-12 to 6.34·10-11 g/ m s Pa. Thus, the WVP of the films depended on the moisture content. The permeability values remained low until values of aw about 0.3, showing an increase of the slope from that values of aw of
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0.35 (Fig. 3).
Fig. 3. Water vapour permeability of regenerated cellulose-chitosan-polyvinyl alcohol
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films at 30 °C as a function of aw . The solid line shows the isotherm obtained with the GAB model. X is the equilibrium moisture content of the samples.
Usually WVP is independent of the water content of the films in synthetic and
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hydrophobic materials. However, in the case of hydrophilic materials, like cellulose, chitosan and PVOH, the polar groups (amino and hydroxyl groups) interact with permeating water molecules, resulting in important structural changes. These structural changes modify the diffusion of the water molecules through the matrix of the film leading to WVP variations (Bedane et al., 2015b; Chinnan & Park, 1995; Roy, Gennadios, Weller, & Testin, 2000; Su et al., 2010). During the plasticization process, water
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molecules increase the free volume in the polymer due to the swelling of the material, losing the packed structure. The swelling of the films enhances the mobility of the polymeric chains, easing the diffusion of water through the polymer matrix. As a result, an increase in WVP occurs, being more significant at high moisture contents (Bedane et al., 2015b; Chinnan & Park, 1995; Roy et al., 2000; Su et al., 2010; Wiles, Vergano, Barron, Bunn, & Testin, 2000). Similar behavior has been observed in other films based
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on hydrophilic biopolymers like chitosan (Aguirre-Loredo et al., 2016).
3.3. Mechanical properties
In this study, the tensile and puncture properties of the regenerated cellulose-
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chitosan-PVOH films were studied in a wide range of RHeq environments to analyze
changes in the mechanical properties depending on the moisture content or water activity.
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The minimum water activity value obtained using silica gel called 0 really could be
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around 0.03 as reported previously (Salgado et al., 2017). As shown in Fig. 4 and 5, the mechanical properties of the developed film were strongly affected by the moisture content or Aw (p < 0.05).
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Regarding the tensile properties, TS values ranged from 8.63 to 72.77 MPa. Two
regions were observed (Fig. 4a). First, up to aw = 0.32, TS increased with the increase in RH and with the moisture content of the film. Second, it was followed by a continuous
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decreasing from aw > 0.32.
The %E values increased slightly from 2.65 to 7.51% when aw increased up to
0.57. From aw > 0.57, the elongation percentage values showed a more significant increase, changing from 5.49 in dry films to 31.43% at the maximum water activity value.
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90
A)
80 Tensile strength (MPa)
70 60 50 40 30 20 10 0 0.00 40
0.20
0.30
0.40
0.50
0.60
0.70
0.90
1.00
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35 30 25 20 15
-p
10 5
0 0.00 4000
0.80
aw
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.70
0.80
0.90
1.00
aw
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3500 3000 2500 2000
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Young’s modulus (MPa)
C)
0.10
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Percentage of elongation (%)
B)
1500 1000 500
0.10
0.20
0.30
0.40
0.50
0.60
aw
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0 0.00
Fig. 4. Mechanical properties of regenerated cellulose-chitosan-polyvinyl alcohol films at 30 °C as a function of aw. A) Water activity effect on tensile strength. B) Water activity effect on percentage of elongation. C) Water activity effect on Yong’s modulus.
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Young’s modulus ranged from 40 to 3224.67 MPa. The decrease with the greatest slope took place between 0.43 < aw < 0.75, diminishing the YM value from 2224.60 to 361.42 MPa. The polar hydrophilic groups in chitosan and PVOH promoted the moisture adsorption in the films (Liu et al., 2018). The hygroscopic nature of chitosan and PVOH allowed incorporating additional water into the matrix of the film. The adsorbed water molecules changed the internal structure of the composite films resulting in soft and elastic polymeric materials (Chang, Abd Karim, & Seow, 2006; Liu et al., 2018). Thus,
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water exerted a plasticizing effect on regenerated cellulose-chitosan-PVOH films, enhancing the chains polymer mobility. The low molecular weight of water molecules
facilitates the molecular mobility of amorphous and partially crystalline polymers (Mali
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et al., 2005). These structural changes in the film as a result of the plasticizing effect of
water were reflected in the mechanical properties, decreasing the TS and YM values and
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a consequent increase of %E (Sothornvit & Krochta, 2005).
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In the puncture test (Fig. 5), BS values ranged from 378.07 to 4370.50 g. The puncture breaking resistance increased when the water activity increased, reaching the maximum value at aw = 0.57. In the range 0.57 < aw < 0.75 the values remained in the
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range from 3940.03 to 4370.50 g, with small variations.
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A) 5000 4500
Burst strength (g)
4000 3500 3000 2500 2000 1500 500 0 0.00
0.20
0.30
0.40
0.50
aw
6.0
0.70
0.80
0.90
1.00
0.90
1.00
-p
5.0
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4.0 3.0 2.0 1.0
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0.0 0.00
0.60
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Distance to burst (mm)
B)
0.10
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1000
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
aw
Fig. 5. Puncture properties of regenerated cellulose-chitosan-polyvinyl alcohol films at
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30 °C as a function of aw. A) Water activity effect on burst strength. B) Water activity effect on distance to burst.
Results indicate that, in general, the moisture content of the film improved the resistance to axial forces properties. The plasticizer effect enhanced the mobility of the polymer chains, as mentioned in the analysis of tensile properties. This effect facilitated
20
the reorganization of the polymer chains during the axial deformation, reducing brittleness and increasing the BS values. This behavior is in accordance with the effect of the water molecules on the DB property increasing the elasticity of the films as the moisture content of the samples increased. DB values increased from 0.50 mm (aw = 0) to a maximum value of 4.18 mm (aw = 0.90). The plasticizing effect of water molecules on hydrophilic materials and its impact on the mechanical properties have also been reported in previous studies (Aguirre-Loredo et al., 2016; Liu et al., 2018; Mali et al.,
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2005; Y. Zhang & Han, 2008).
3.4. Optical properties
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The optical properties like color, transparency and opacity are important
parameters in terms of appearance as the consumers expect the packaged product
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pleasant. In addition, films with good UV-barrier properties allow expanding the shelf
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life in packed products, delaying the degradation of components that diminish their quality. The optical properties may suffer modifications depending on the moisture conditions during the storage due to the water affinity of the film components. Fig. 6
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shows the light barrier properties of the films by the transmittance values of the samples in the UV region from 200 to 400 nm and the VIS region from 400 to 800 nm. Lower wavelengths contain more energy and are potentially more harmful for the lipid
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oxidations. UV-C radiation usually does not cross the atmosphere, but artificial sources of UV-C are used commonly for sterilize surfaces. Table 1 summarizes the results obtained from the analysis of the optical properties
of the films. For the UV-barrier properties, the water activity of the film decreased the transmittance of the samples in the UV region. The presence of water molecules decreased the transmittance from 16.34 to 7.95 %, from 12.06 to 5.03 % and from 5.28
21
to 2.38 % in the UV-A, UV-B and UVC regions, respectively. Hence, data indicated that the presence of water molecules improved the UV- barrier properties of the developed films. This improvement could be the result of multiple factors including the optical properties of the water (Litjens, Quickenden, & Freeman, 1999; Quickenden & Irvin, 1980), modifications in the structure and the increasing of thickness due to the plasticizing effect of the water molecules (Bertuzzi, Castro Vidaurre, Armada, & Gottifredi, 2007). These results are very good comparing with the transmittance of synthetic polymers like
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polyethylene (59% at 240 nm) or poly(ethylene terephthalate) (25% at 310 nm) (Moura et al., 2004). Note that a low transmittance in the UV region is good to avoid lipid
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oxidation of food promote by the UV light
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Fig. 6. UV-VIS spectra profile of regenerated cellulose-chitosan-polyvinyl alcohol films at 30 °C as a function of aw.
The effects of the moisture content on CIE L*, a* and b* coordinates, transparency and opacity values were assessed. Table 1 and Table 2 summarizes the results obtained from the analysis of the optical properties of the films.
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Table 1. Color parameters (L*, a* and b*) and transmittance values in each region of UVlight radiation UV-A (320−400 nm), UV-B (280−320 nm) and UV-C (190-280 nm) of samples conditioned at different relative humidity. %T is the average percent
a*
(UV-B)
(UV-C)
%T
%T
%T
16.34
12.06
5.28
0
53.55
2.79
0.11
48.31
0.45
4.16
11.00
7.94
4.04
0.32
50.06
0.33
3.27
13.78
10.50
5.18
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0.20
b*
(UV-A)
re
L* Aw
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transmittance.
43.70
0.57
4.36
9.20
6.59
3.29
0.57
43.43
0.36
3.07
10.00
7.58
3.87
0.74
43.00
0.16
3.41
8.20
5.11
2.48
0.96
41.93
0.34
3.89
7.95
5.03
2.38
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0.43
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Table 2. Transparency, Opacity and Thickness of samples preconditioned at different relative humidity.
Thickness Transparency
Opacity
Aw
mm 14.94
7.49
0.0890.008
0.11
13.30
8.48
0.0930.013
0.32
15.69
9.16
0.0820.015
0.43
12.35
9.51
0.0900.009
0.57
12.15
9.45
0.0930.013
0.74
11.19
9.13
0.0990.012
0.96
11.07
9.16
0.1020.009
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0
The main difference was the decreasing of lightness (L* value) of the samples as
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the water activity increased. Moreover, the water molecules produced a slight increase in yellowish color (b* value). The values of b* were similar to those obtained for other films
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with chitosan (Thakhiew, Devahastin, & Soponronnarit, 2010). In the case of redness, the a* values ranged from 0.16 to 0.57 and there was not a clear trend of the variation of the values as a function of the water activity. The increase of the moisture content of the samples produced a decrease of the
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transparency values and consequently, an increase of the opacity (Table 2). Results show that optimal UV-barrier properties of the composite films developed with good visual appearance can be obtained as shown in Fig. 7. These films could have potential applications in packaging of food product.
24
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Fig. 7. Visual appearance of a sample preconditioned at 43.80 % relative humidity
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conditions and 30 ºC.
Figure 8 shows the top, bottom and section surfaces of the films obtained. Films
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showed a continuous matrix, no pores or cracks were detected, indicating a dense
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structure. Bottom side showed a smoother surface.
25
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Bottom
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Top
Section
Fig. 8. Scanning electron microscopy images of the surfaces of the films.
26
4. Conclusions Results showed that the mechanical, permeability and optical properties of regenerated cellulose-chitosan-PVOH films depended on the moisture content or water activity. The moisture adsorption isotherms data were well fitted by GAB model, showing a sigmoidal curve of type II. Due to the hydrophilic nature of the components of the blend, when the relative humidity of equilibrium increased, the moisture content of the film was
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increased. The water molecules manifested a plasticizing effect on regenerated cellulosechitosan-PVOH films, resulting in an increase of the water vapour permeability when the moisture content increased. The modifications of the mechanical properties of cellulose-
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based films followed the typical behavior of a hydrophilic polymer under the plasticizing
effect of water molecules decreasing the resistance to rupture and increasing the elasticity
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of the material. Moreover, the water vapour permeability increased when the moisture
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content increased as the swelling of the structure facilitated the diffusion of water molecules through the polymer matrix. UV-VIS spectra showed that the optical barrier properties of regenerated cellulose-based films against UV-radiation improved when the
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moisture content increased. However, the transparency values decreased with the moisture content, increasing the opacity of the samples. Considering the properties evaluated of the films based on cellulosic materials, have potential applications in the
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food industry. This study can help to determine the most suitable application of these composites according to the needs and environmental conditions.
CRediT author statement
27
Patricia Cazón: Investigation, Writing- Original draft preparation.
Gonzalo Velazquez: Methodology, Writing- Reviewing and Editing,
Manuel Vazquez: Conceptualization, Methodology, Writing- Reviewing
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and Editing.
Acknowledgements
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A grant from CONACYT (Mexico) to author Patricia Cazón (#435948) is
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re
gratefully acknowledged.
28
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