chitosan nanoparticles nanocomposites films and their use for paper coating

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Novel nanofibrillated cellulose/chitosan nanoparticles nanocomposites films and their use for paper coating Enas A. Hassan a , Mohammad L. Hassan a,b,∗ , Ragab E. Abou-zeid a , Nahla A. El-Wakil a a

Cellulose and Paper Department, National Research Centre, 33 El-Bohouth Street, Dokki, Giza 12622, Egypt Advanced Materials and Nanotechnology Group, Centre of Excellence for Advanced Sciences, National Research Centre, 33 El-Bohouth Street, Dokki, Giza 12622, Egypt b

a r t i c l e

i n f o

Article history: Received 23 September 2015 Received in revised form 22 November 2015 Accepted 6 December 2015 Available online xxx Keywords: Nanofibrillated cellulose Rice straw Chitosan nanoparticles Nanocomposites Paper coating

a b s t r a c t Novel nanocomposites films were prepared from TEMPO-oxidized nanofibrillated cellulose isolated from rice straw, chitosan nanoparticles (CHNP), and glycerol by solution casting. The percentage of chitosan nanoparticles ranged from 2.5 to 20% while a fixed ratio of glycerol (25%) was added. The effect of chitosan nanoparticles on porosity, tensile strength properties, water vapor permeability, grease-proof, and antimicrobial properties was studied. The results showed that addition of chitosan nanoparticles can improve tensile strength properties, reduce porosity, and impart NFC antimicrobial properties against Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli), and yeast (Saccharomyces cervisiae). CHNP did not affect WVP or grease-proof properties of NFC films. The mixture containing CNF and 10% CHNP was used for paper coating and the effect of coating on tensile strength, porosity, water vapor permeability, water absorption, and grease-proof properties of paper sheets was studied. The results showed coating of paper sheets with thin film of NFC or NFC/CHNP can improve tensile strength properties, decrease porosity and water absorption, and increase greaseproof properties of paper sheets but did not affect their WVP. Presence of CHNP in the coating mixture resulted in higher tensile strength properties of coated paper sheets than in case of using NFC alone but no noticeable differences were found regarding porosity, WVP, grease proof, and water absorption of paper sheets coated with NFC or NFC/CHNP under the conditions used. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The development of polymeric materials, based on renewable resources has become increasingly important in recent years due to the inevitable rising prices of petroleum-based materials and environmental concerns. Nowadays, most materials used in the packaging industry are produced from fossil fuels (Vieira et al., 2011). Therefore, the utilization of renewable resources is of great significance for sustainable development. Many biobased polymers from renewable resources such as cellulose, starch, chitin and chitosan have been studied for application in packaging (De Azeredo, 2009). Cellulose and chitosan are the most abundant natural polymers. They have close chemical structure. The former consists of d-anhydroglucose units joined together

∗ Corresponding author at: Cellulose and Paper Department, National Research Centre, 33 El-Bohouth Street, Dokki, Giza 12622, Egypt. Fax: +20 2 33370931. E-mail addresses: [email protected], [email protected] (M.L. Hassan).

by 1,4-␤-glycosidic linkages (Klemm et al., 2005) while the later consists of glucosamine and N-acetyl glucosamine units joint by 1,4-␤-glycosidic linkages (Pillai et al., 2009). Recently, cellulose and chitosan nanomaterials have been in the spot light of research in different areas. Nanofibrillated cellulose (NFC) is an interesting nanomaterial with unique physicomechanical properties such as high tensile strength properties, low density, oxygen barrier properties, transparency, ability for chemical modification, biodegradability and biocompatibility. Different technologies have been used for isolation of NFC from different resources; these technologies are based on mechanical, enzymatic/mechanical, or chemical/mechanical actions on cellulose fibers (Abdul Khalil et al., 2014; Kalia et al., 2014; Zhang et al., 2013). The ability of NFC to form films with good mechanical properties, air and oxygen barrier, and transparency is another advantage since no dissolution and precipitation of cellulose are necessary for making films (Fukuzumi et al., 2009). NFC has been used in different areas such as reinforcing elements in nanocomposites, coating for paper products, tissue engineering, and drug delivery systems (Kolakovic et al., 2012; Abdul Khalil et al., 2014; Kalia et al., 2014; Jorfi and

http://dx.doi.org/10.1016/j.indcrop.2015.12.006 0926-6690/© 2016 Elsevier B.V. All rights reserved.

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Foster, 2015; Hassan et al., 2015). But cellulose lacks resistance to attack by some microorganisms due to its polysaccharide nature and absence of functional groups having antimicrobial activity. On the other hand, chitosan nanoparticles have unique physicochemical properties compared to bulk chitosan such as larger surface area, i.e., higher surface charge density (gives more cationic sites) and higher affinity to bacteria cells due to their ability to tightly adsorbed onto their surface to disrupt the membrane and kill them (Avadi et al., 2004; Qi et al., 2004; Zhang et al., 2010; Romainor et al., 2014). Chitosan nanoparticles are prepared by different methods from chitosan solutions such as emulsion-droplet coalescence (Tokumitsu et al., 1999), emulsion solvent diffusion (El-Shabouri, 2002), reverse micellar method (Mitra et al., 2001), ionic gelation, polyelectrolyte complexation (Calvo et al., 1997; Sarmento et al., 2006) and desolvation (Tian and Groves, 1999). Chitsoan nanoparticles have many potential applications in different areas (Singh and Mishra, 2015; Cheaburu-Yilmaz et al., 2015) such as drug delivery systems (Duttagupta et al., 2015), heavy metal ion removal (Trujillo-Reyes et al., 2014) and as antimicrobial and oxygen barrier material in nanocomposites (Radulescu et al., 2015). Due to interesting and complementary properties of both chitosan and cellulose, they have been used together in composites. For preparation of these composites, chitosan is first dissolved in an appropriate solvent such dilute acetic acid then mixed with cellulose or cellulose solution and the mixture is casted and dried. The formed chitosan composites is water sensitive and contain residual acetic acid unless alkali treated to convert the quaternary chitosan to its original non-protonated form. On the other hand, the use of chitosan nanoparticles, which are water insoluble, could have positively charged surface, and highly suspended in aqueous medium with cellulose can impart the later antimicrobial properties without the use of acid for dissolving chitosan. For these reasons, chitosan nanoparticles were used with cellulose to impart it antimicrobial properties. For example, chitosan nanoparticles were coated onto surface of oxidized cellulose fibers to prepare antimicrobial cellulosic materials (Sauperl et al., 2015). The study also proved stronger adhesion of the positively charged chitosan nanoparticles to the surface of oxidized cellulose fibers than non-oxidized fibers. In a similar study, paper sheets made from untreated fibers were impregnated with chitosan nanoparticles and chitosan solution (Fithriyah and Erdawati, 2014). The results showed higher impregnation of paper sheets when chitosan nanoparticles were used than in case of using chitosan solution. The strong adhesion of chitosan nanoparticles was utilized in another study for doping of cellulose film prepared by casting cellulose from NaOH/Urea/thiourea solvent system containing chitosan nanoparticles (Romainor et al., 2014). The doped films showed good antimicrobial properties against Escherichia coli bacteria (∼85% maximum inhibition). Chitosan nanoparticles were also used with some cellulose derivatives to improve mainly their antimicrobial properties. For example, carboxymethyl cellulose films containing chitosan nanoparticles were prepared to prepare edible film with antimicrobial properties (De Moura et al., 2011). Improvement of thermal and mechanical properties was observed in films containing chitosan nanoparticles in addition to the imparted antimicrobial properties. However, the composite films are still highly sensitive to water due to solubility of carboxymethyl cellulose in water. The incorporation of chitosan nanoparticles into hydroxypropyl methyl cellulose to prepare antimicrobial films was studied. Improvement of mechanical properties of cellulose films was also reported (De Moura et al., 2009). Since NFC is not antimicrobial material, the use of nanoparticles to impart it that property is one of the active areas of research to widen the applications of NFC. Different nanoparticles such as silver (Martins et al., 2012; Xiong et al., 2013), zinc oxide (Martins et al., 2013), and titanium dioxide (Missoum et al., 2014) have been

added with NFC to impart it antimicrobial properties. However, for the best of our knowledge, the use of chitosan nanoparticles with nanofibrillated cellulose to prepare films with antimicrobial properties has not been studied so far. The current work is focused on the development of new and natural nanostructured films of NFC and chitosan nanoparticles. The properties of glycerol-plasticized NFC/chitosan nanoparicles films regarding their tensile strength, water vapor permeability, porosity, grease proof, and antimicrobial properties were investigated. In addition, use of NFC/chitosan nanoparticles mixture for coating of paper sheets and effect of coating on tensile strength, water vapor permeability, porosity, and grease proof properties were briefly studied. 2. Experimental 2.1. Materials Rice straw pulp was obtained by pulping of rice straw with 15% aqueous sodium hydroxide solution at 150 ◦ C for two hours. After washing the pulp to remove excess chemicals, it was bleached using sodium chlorite/acetic acid mixture (Wise et al., 1946). Chemical composition of the bleached pulp was determined according to the standard methods (Browning, 1967) and was: Klason lignin 1.46%, alpha-holocellulose 69.7%, hemicelluloses 19.7%, and ash content 10.6%. Chitosan was a low molecular weight grade and used as received (75–85% deacetylated; Brookfield viscosity 20–300 cP, 1 wt.% in 1% acetic acid at 25 ◦ C). Analytical grade sodium tripolyphosphate, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), sodium hypochlorite, ethanol, and glacial acetic acid were used as received. 2.2. Preparation and characterization of chitosan nanoparticles Chitosan nanoparticles (CHNP) were prepared according to the previously published method via ionic gelation where chitosan dissolved in acetic acid (2% chitosan solution in 2% acetic acid) was precipitated by slow addition of 0.01 wt.% sodium tripolyphosphate solution (Dounighi et al., 2012). The obtained nanoparticles were characterized using high-resolution transmission electron microscopy (JEM-2100 transmission electron microscope, JEOL, Japan). Particle size distribution and the zeta potential of chitosan nanoparticles were determined using Zetasizer Nano-ZS90 (Malvern Instruments). The analysis was performed at a scattering angle of 90 at a temperature of 25 ◦ C using samples diluted with de-ionized distilled water. 2.3. Preparation and characterization of nanofibrillated cellulose (NFC) Nanofibrillated cellulose was prepared from bleached rice straw pulp according to the previously published methods (Saito et al., 2007). Rice straw pulp (3 g) was dispersed in distilled water (400 ml) with TEMPO (0.048 g, 0.3 mmol) and sodium bromide (0.48 g, 4.8 mmol). Then 30 ml of sodium hypochlorite solution was then added with stirring and the pH was adjusted to 10. At the end of reaction the pH is adjusted to 7 and the product was centrifuged at 5000 rpm. The product was further purified by repeated adding water, dispersion, and centrifugation. Finally the product was purified by dialysis against de-ionized water. To obtain nanofibrillated cellulose (NFC), the oxidized pulp was disintegrated by a high-shear homogenizer (CAT Unidrive 1000) at 10000 rpm using pulp suspensions of 2%. The prepared nanofibrillated cellulose was characterized regarding its carboxylic groups’ content of oxidized fibers according to TAPPI Test Method T237cm-98 and found to be 0.31 mmol/g.

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The isolated NFC was also characterized using high-resolution transmission electron microscopy (JEM-2100 transmission electron microscope, JEOL, Japan).

Films from NFC and NFC/CHNP were casted from aqueous suspensions of CHNP and NFC in Petri dishes and dried at 40 ◦ C in an oven with circulating air for 12 h. Glycerol was added as plasticizer to the films at a constant weight ratio of 25%. The ratios of CHNP in the mixture ranged from 2.5–20 wt.% (wt.% based on dry weight of NFC and glycerol). The films were then conditioned at 50% relative humidity for 48 h before testing. Tensile tests were carried out with a Lloyd instrument (Lloyd Instruments, West Sussex, United Kingdom) with a 100-N load cell. The measurements were performed on strips with 1-cm width and 8 cm length at a crosshead speed of 2 mm/min at 25 ◦ C. Five replicates of each samples were measured and the results averaged. Porosity of films was measured according to TAPPI T460-06 using a Gurley porosity tester 4110 (W. & L.E. Gurley Troy, NY. Static WVP of films was determined according to the ASTM standard E96-95 (ASTM Standards, 1995). Grease-proof testing of NFC/CHNP films was carried out using turpentine oil (TAPPI standard T454). The antimicrobial activity of NFC/CHNP films was evaluated using Gram-positive bacteria (Staphylococcus aureus), Gramnegative bacteria (E. coli), as well as yeast (Saccharomyces cervisiae) according to the previously published method (Shanmugam and Gopal, 2014). The test organisms were grown in sterile Tryptic Soy Broth medium with 0.6% yeast extract overnight at 37 ◦ C or 30 ◦ C for bacteria and yeast, respectively. NFC/CHNP films were transferred aseptically in sterilized 10-cm Petri dishes and one ml of each inoculums was distributed all over the surface of each test piece; the dish was covered with a lid then incubated for 24 h at 37 ◦ C and 30 ◦ C for bacteria and yeast, respectively. After the end of the incubation period, 9 ml of sterile distilled water was added to each sample and shaken for 10 min. One milliliter of solution was withdrawn at each sampling and diluted to 10−6 and 10−4 for bacteria and yeast, respectively, with sterile distilled water. Finally, 0.1 ml of the diluted solution was plated onto Tryptic Soy agar medium and incubated for 24 h at 37 and 30 ◦ C, for the bacteria and yeast, respectively and the number of colony forming units (CFU) were counted. The number of colony forming units represents the survival number of microorganisms. The antimicrobial activity was expressed as a reduction of the bacterial colonies after contact with the test specimen and compared to the number of bacterial colonies from the control. The percentage reduction (inhibition) was calculated using the following equation: %Reduction = [(B – A)/B] × 100 where A are the surviving cells (CFU—colony forming units) for the plates containing the treated substrate and B are the surviving cells from the control. 2.5. Coating of bagasse paper sheets with NFC and NFC/CHNP films Paper hand sheets from bagasse pulp produced by Qena Company for Pulp and Paper, Qena, Egypt, were made using laboratory sheet former (Frank apparatus according to the Rapid–Kothen method—ISO Standard 5269). After forming, the sheets were pressed for 5 min at 420 kPa at 80 ◦ C. Basis weight of the produced sheets was 70 g/m2 . Paper sheets were coated manually with NFC or NFC/CHNP aqueous suspension (solid content ∼2%) using coating bar having gap clearance of 120 ␮m. Surface of coated paper sheets was examined by FEI Quanta 200 scanning electron micro-

50 40 Number (%)

2.4. Preparation and characterization of NFC and NFC/CHNP films

3

30 20 10 0 0.1

1

10 Size (d. nm)

100

1000

Fig. 1. Size distribution by number of CHNP determined by Zetasizer.

scope (FEI Company BV, Netherlands) with an acceleration voltage of 20 kV. Tensile strength testing of coated paper sheets was carried out according to TAPPI T494-06 standard method using a universal testing machine (LR10K; Lloyd Instruments, Fareham, UK) with a 100-N load cell at a constant crosshead speed of 2 mm/min. Strips of 20cm long and 15-mm width were used in the test and the span was 10 cm. Porosity was measured according to TAPPI T460-06 using Gurley air permeability tester 4110 (W. & L.E. Gurley Troy, NY). Water absorption of paper sheets was carried according to ISO 535 method (known as Cobb method). The test determines the quantity of water that can be absorbed by the coated surface of paper sheets in a given time. Static water vapor permeability (WVP) test was carried out according to the ASTM standard (ASTM E96). Grease-proof testing of paper sheets was carried out using turpentine oil (TAPPI standard T454).

3. Results and discussion 3.1. Preparation and characterization of CHNP CHNP were prepared based on ionic gelation interaction between the positively charged quaternary amine groups of chitosan dissolved in acetic acid and the negatively charged tripolyphosphate anion. The mean size, size distribution, and zeta potential of the prepared nanoparticles suspension were analyzed using the Zetasizer analysis (Fig. 1). The mean hydrodynamic diameter of the prepared nanoparticles was 164 ± 25 nm (polydispersity index 0.316) and zeta potential was about +39 ± 3.71 mV. The positive charge of chitosan nanopartiles impart them good dispersion in water as well as expected antimicrobial properties. The produced nanoparticles were also characterized by TEM (Fig. 2). The image showed that the nanoparticles were generally polyhedron/round in shape with a diameter from about 10–65 nm. The difference between diameter of using Zetasizer and TEM is expected since the former gives hydrodynamic diameter of the particles suspended in water while the later gives the diameter of nanoparticles dried on the grid used. On the hand, the prepared NFC had very uniform width of about 3–4 nm and several microns in length, and form a web-like structure as shown in Fig. 2. The exact length of the isolated NFC could not be determined due to the intrinsic alignment and formation of continuous network of the fibrils. These dimensions means the isolated NFC has high aspect ratio (length/width), which is the main reason for high strength properties of NFC and also the reason for improving tensile strength properties and toughness of nanocomposites containing nanocellulose. High aspect ratio means high surface area and consequently higher interaction between NFC fibrils (hydrogen

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Fig. 3. SEM of NFC/10%CHNP film at 30000× magnification.

Fig. 4. Porosity of NFC containing different ratios of chitosan nanoparticles.

Fig. 2. TEM of (a) the prepared CHNP and (b) NFC used in film preparation.

bonding and van der Waal forces) as well as between NFC and other added ingredients compatible with it. The prepared NFC has negative charge due to the presence of carboxylate groups at the surface; the estimated total carboxylic groups’ content was about 0.3 mmol/g.

thickness of about 0.06 mm whereas films containing 2.5, 5, 10, and 20% of CHNP had thickness of about 0.08, 0.09, 0.13, and 0.16 mm, respectively. It is also noted that the film containing 20% of CHNP suffered from remarkable shrinkage in diameter upon drying. SEM image of the prepared NFC/10%CHNP nanocomposite film (Fig. 3) shows nanoporous structure with a wide pore size distribution at the surface. CHNP were shown in the images with good distribution and without agglomeration. No noticeable difference between the pore size at the surface of the NFC and NFC/CHNP samples was noticed.

3.2. Preparation and characterization of NFC/CHNP nanocomposites films

3.2.1. Effect of CHNP on porosity of NFC The porosity of the prepared films was measured and the effect of presence of CHNP on the porosity is presented in Fig. 4. As shown in the figure, presence of chitosan nanoparticles in the NFC film resulted in reduction of porosity probably by blocking the pores of the film. The effect was noticeable even at the lowest concentration of CHNP used (2.5 wt.%). The decrease in porosity ranged from 32 to 44% and the maximum decrease was recorded on using 5% CHNP.

The idea of the current work was to use the negatively charged TEMPO-oxidized cellulose with the positively charged chitosan nanoparticles to ensure high interaction between the components and get improvement in the properties of the nanocomposites. Films from NFC or NFC/CHNP plasticized by glycerol were casted from their aqueous suspension mixtures. The weight ratios of CHNP to NFC ranged from 2.5 to 20%; the films produced were homogeneous and did not show signs of chitosan nanoparticles aggregation as seen by naked eye. The films had good flexibility and their thickness was varied as a result of presence of CHNP. Neat NFC films had

3.2.2. Effect of CHNP on tensile strength properties of NFC Tensile strength properties (Tensile strength, Young’s modulus, Strain at break) of the plasticized NFC and NFC/CHNP nanocomposites films were measured and the results are presented in Fig. 5. As shown in the figures, presence of CHNP in the NFC films resulted in increasing their tensile strength; the increase ranged from 6% to 40% with maximum at 10% CHNP loading. Increasing CHNP in the NFC film resulted in increasing Young’s modulus; the increase ranged from 8 to 42% on increasing CHNP loading from 2.5 to 20%. The increase in tensile strength means that posi-

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0

2.5

10

20

0

1E-09

25

2.5

5

10

20

8E-10

20 15 10 5 0 0

1000 Young's modulus (MPa)

5

WVP (gm-1s-1pa-1)

Tensile strength (MPa)

30

5

2.5

0

2.5

15 % CHNP 5

10

10

20

6E-10 4E-10 2E-10 0 0

2.5

51

10

20

% CHNP

20

Fig. 6. Effect of CHNP loading on water vapor permeability of NFC film.

800

strain at break of 12.86 MPa, 0.56 GPa, and 11.1% respectively. The neat NFC film is more brittle and less flexible than the plasticized one.

600 400

3.3. Water vapor permeability of NFC/CHNP nanocomposites films

200 0 0

51

2.5

10

20

10

20

% CHNP

Strain at break (%)

25

0

2.5

5

10

20

20 15 10 5 0 0

2.5

51 % CHNP

Fig. 5. Effect of CHNP loading on tensile strength properties of NFC film.

tively charged CHNP could act as crosslinks between the negatively charged NFC by the electrostatic attraction. But as can be seen from the results, the increase in tensile strength properties was moderate (6–40% and 8–42% increase in tensile strength and Young’s modulus, respectively). This could be attributed to that presence of CHNP between NFC fibrils may reduce the bonding between NFC fibrils in the areas close to the CHNP due to more spacing between the fibrils. As mentioned above, the presence of CHNP in NFC films resulted in increasing their thickness. The expected decrease in tensile strength properties as a result of more spacing between NFC due to presence of CHNP is compensated by the presence of CHNP as crosslinks between the fibrils and therefore, the net increase in tensile strength properties was moderate. Regarding strain at break, presence of CHNP in NFC film resulted in its decrease by about 12–38% with maximum at 10% CHNP loading. It should be pointed out that the tensile strength properties of the plasticized NFC film mentioned in Fig. 5 is noticeably lower than that of neat NFC film. This could be due to the presence of glycerol in the NFC film. Neat NFC film showed tensile strength, Young’s modulus, and strain at break of 47.8 MPa, 2.07 GPa, and 6.2% respectively, while plasticized NFC film tensile strength, Young’s modulus, and

Fig. 6 shows the effect of CHNP on WVP of NFC films. In spite of the tight surface structure of NFC, it is permeable to water vapor. This could be due to the porosity of the films and the strong hydrophilic nature of NFC. Transport of water vapor through cellulose is strongly affected by adsorption of moisture by hydroxyl groups and carboxylic groups of cellulosic fibrils. In addition, the film formed by casting was dried by air and no pressure was applied during drying. Finally, presence of glycerol as plasticizer could also affect moisture sorption. Presence of CHNP in NFC film did not cause noticeable effect on WVP in spite of the lower hydrophilic nature of CHNP and the lower porosity of NFC/CHNP films than that of neat NFC. In fact, WVP through a hydrophilic film like NFC depends on both diffusivity and solubility of water molecules in the film matrix (Gontard et al., 1992). Inclusion of molecules or particles between the fibrils increases the spacing between the fibrils and thus may promote higher water vapor diffusivity through the NFC film. That can be the reason for not improving WVP of NFC films by addition of CHNP. As mentioned above in Section 3.2, the thickness of NFC films increased with the addition of the CHNP, i.e., there is an increase in the spaces between the NFC fibrils. Therefore, the expected decrease in WVP due to lower porosity of NFC containing CHNP in the pores of the films was counterbalanced by increasing the spacing between NFC fibrils and finally no improvement in WVP was found. In addition, presence of amino and hydroxyl groups at the surface of chitosan nanoparticles which can still absorb water vapor molecules can also promote water vapor sorption and diffusion. 3.4. Grease-proof property of NFC/CHNP films Grease-proof commercial paper is manufactured by different methods such as high degree beating of pulp, super-calendaring of paper sheets, or chemical treatment of fibers. Other types of grease proof paper are made by applying coating layer of synthetic polymers with high hydrophobicity (Houde et al., 2006). Grease-proof paper has high density and closed surface structure, which impart it with grease-proof property (Stolpe, 1996). Grease proof properties of NFC are rarely studied. For the best of our knowledge, only Aulin et al. studied the use of cellulose nanofibrils produced by passing carboxymethylated softwood pulp through high-pressure fluidizer, as coating for paper to improve oxygen barrier and grease-barrier properties of paper sheets (Aulin

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Fig. 7. SEM images of (a) paper sheet, (b) edge of paper sheet coated with NFC, (c) paper sheet coated with NFC/10%CHNP coating mixture, and (d) edge of paper sheet coated with NFC/10%CHNP coating mixture.

et al., 2010). The results showed that carboxymethylated nanofibers have good grease proof properties when turpentine and castor oil were tested. In the current work, NFC films and NFC/CHNP nanocomposites films were tested for their grease-resistance properties using turpentine oil test, where the time required for penetration of turpentine oil through the films is measured. The results of the test showed that films made from NFC or the different NFC/CHNP films showed excellent grease proof property since the time needed for penetration of turpentine oil was >1800 s, which can be classified as high grease-proof materials. The grease-proof property of NFC film could be attributed to the very tight and closed surface structure with nano-size pores as shown in the SEM image in Fig. 3. The presence of CHNP in the NFC film did not deteriorate its grease-proof property. CHNP can block the pores of the film. 3.5. Antimicrobial properties of NFC/CHNP films NFC/CHNP nanocomposites films were evaluated to estimate the amount of CHNP necessary to impart NFC film antimicrobial activity. Three different microorganisms were used: S. aureus (gram positive), E. coli (gram negative), and S. cervisiae (yeast). As shown in Table 1, addition of CHNP to NFC imparted the later antimicrobial activiy but the ratio of CHNP to achieve maximum inhibition of microorganisms growth was different from a microorganism to another. CHNP ratio of 5% can completely inhibit growth of Saccharomyces cervisiae yeast and 94% of S. aureus but cannot inhibit growth of E. coli. At 10% of CHNP, the maximum inhibition achieved was 100%, 75%, and 94% in case of S. cervisiae, E. coli, and S. aureus, respectively. The results are in accordance with previous results

which showed that chitosan exhibits higher antibacterial activity against Gram-positive bacteria than Gram-negative bacteria (Zhishen et al., 2001; No et al., 2002). 4. Use of NFC/CHNP for coating of bagasse paper sheets The use of nanocelluloses for coating of paper sheets has been recently studied with the aim to explore use of nanocelluoses as alternatives to the petroleum-based ones or other biopolymers (Syverud and Stenius, 2009; Lavoine et al., 2012; Hult et al., 2010; Aulin et al., 2010; Lavoine et al., 2014). In all of these studies it was proved that microfibrillated cellulose or nanofibrillated cellulose prepared from TEMPO oxidized fibers can improve air barrier properties of paper sheets. Some of these studies found that MFC or NFC improve tensile strength properties of coated paper (Syverud and Stenius, 2009; Aulin et al., 2010) while others found no improvement in tensile strength properties of coated paper sheets. Those who did not find improvement in tensile strength of coated paper, ascribed that to the lack of homogeneity of nanocellulose coating (Hult et al., 2010) or to the effect of wetting and drying on paper sheets during coating with multilayers of microfibrillated cellulose (Lavoine et al., 2014). In the current work, the aqueous NFC/CHNP mixture containing 10% CHNP was used for coating bagasse paper sheets and compared to that of NFC. The mixture was manually applied onto bagasse paper sheets using coating rod with gap clearance of 120 microns. The surface of paper sheet was examined using SEM to get information about the homogeneity and thickness of the film formed. As shown in Fig. 7, SEM images show good film formation as a result

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Table 1 Antimicrobial properties of NFC/CHNP nanocomposites films. Samples

Microorganism Staphylococcus aureus CFU ml

NFC NFC + 2.5%CHNP NFC + 5%CHNP NFC + 10%CHNP NFC + 20%CHNP

−1

16.0E+04 16.0E+04 1.0E+04 1.0E+04 1.0E+04

Escherichia coli −1

R (%)

CFU ml

– 0 94 94 94

8.0E+05 8.0E+05 6.0E+05 2.0E+05 2.0E+05

Saccharomyces cervisiae R (%)

CFU ml−1

R (%)

– 0 25 75 75

35.0E+04 3.0E+04 0 0 0

– 91 100 100 100

CFU: number of colony forming units; R%: % reduction.

Table 2 Properties of paper sheets coated with NFC/CHNP mixture.

Paper sheet (P) P + NFC P + NFC + 10%CHNP

Tensile strength (MPa)

Tensile modulus (GPa)

Strain at break (%)

Porosity (s/100 ml)

WVP (gm−1 s−1 Pa−1 )

Oil resistance (s)

Water sorption (g/m2 )

48.34 (5.45) 55.2 (4.8) 60.8 (5.17)

2.27 (0.11) 2.75 (0.16) 3.12 (0.14)

2.99 (0.35) 3.89 (0.47) 3.85 (0.38)

381 (25) 447 (37) 452 (34)

7.49E-010 (1.17E-11) 8.72E-10 (3.12E-11) 8.591E-10 (2.1E-11)

6 (0.67) 73 (4.9) 78 (5.1)

117 (8.6) 78 (5.9) 81 (6.5)

of coating of paper sheets with NFC/10%CHNP composition. The thickness of film of NFC/CHNP formed was about 2 microns while thickness was in case of using NFC was about1 micron. The effect of coating of bagasse paper sheets on their tensile strength properties, porosity, water vapor permeability, grease proof, and water absorption was studied and the results are listed in Table 2. In spite of the small thickness of NFC/CHNP film formed on surface of paper sheets, the effect on the different properties was clear. As shown in the table, coating of paper sheets with neat NFC resulted in increasing tensile strength, tensile modulus, and strain at break of paper sheets. The increase in tensile strength, tensile modulus, and strain at break was about 14%, 21%, and 30%, respectively. On the other hand, coating of paper sheets with NFC/CHNP resulted in increasing tensile strength, tensile modulus, and strain at break by about 26%, 37%, and 29%, respectively. The higher tensile properties of paper sheets coated with NFC/CHNP than those coated with NFC could be due to the higher tensile strength of NFC/CHNP film than that of NFC as discussed before. Coating of bagasse paper sheet using NFC or NFC/CHNP resulted in decrease of porosity of paper sheets. The decrease in porosity was about 17% and 19% in case of NFC and NFC/CHNP, respectively. The thin film of coating could be the reason of close porosity of NFC and NFC/CHNP films. Regarding the effect of coating on WVP of bagasse paper sheets, NFC coating resulted in ∼16% increase in WVP while in case of using NFC/CHNP the increase was ∼15%. The increase in WVP could be due to the high hydrophilic nature of NFC as well as the nanoporous structure of the NFC films. Presence of chitosan nanoparticles in the coated film did not significantly affect WVP of paper sheets. Water absorption using Cobb test method on the coated side of paper sheets showed that the coating reduced water absorption of paper sheets by about 33%. This could be due to the much less porosity of NFC layer than that of paper sheet and therefore penetration of water is slower. Presence of chitosan nanoparticles in NFC coating mixture did not noticeably affect the water absorption values of paper sheets and the decrease in water absorption than blank paper sheet was about 31%. Regarding the effect of coating on grease-proof properties, paper coated with NFC showed remarkable improvement in oil resistance of paper sheets in spite of the very thin thickness of the film formed by coating. Paper sheets coated with NFC/CHNP had close greaseproof property to that coated with NFC.

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Please cite this article in press as: Hassan, E.A., et al., Novel nanofibrillated cellulose/chitosan nanoparticles nanocomposites films and their use for paper coating. Ind. Crops Prod. (2016), http://dx.doi.org/10.1016/j.indcrop.2015.12.006