Surface Treatment of Cellulosic Paper with Starch-Based Composites

Aug 26, 2014 - Engineering Research Center for Eco-Dyeing and Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310...
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Surface Treatment of Cellulosic Paper with Starch-Based Composites Reinforced with Nanocrystalline Cellulose Shujie Yang,† Yanjun Tang,*,† Junming Wang,‡ Fangong Kong,§ and Junhua Zhang▽ †

Pulp and Papermaking Center, Zhejiang Sci-Tech University, Hangzhou 310018, China Ningbo Asia Pulp and Paper Company, Ltd., Ningbo 315803, China § Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education, Qilu University of Technology, Jinan 250353, China ▽ Engineering Research Center for Eco-Dyeing and Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China ‡

ABSTRACT: Starch-based composites have become promising materials for eco-friendly packaging applications because of their biodegradability and cost-effectiveness. However, the inherently poor mechanical and barrier properties of starch-based composite films hinder their market potential. Here, nanocrystalline cellulose (NCC) was isolated from cotton cellulose powders using sulfuric acid hydrolysis. Starch-based composite suspensions/films reinforced with 0.1−0.5 wt % of NCC were prepared and characterized. Surface sizing application of the reinforced composite suspensions on cellulosic paper was subsequently studied for improving the mechanical properties and the resistance to air permeability of cellulosic paper. Results revealed that NCC reinforced composite suspensions showed the characteristics of non-Newtonian fluids. The rheological behavior of these suspensions showed strong NCC concentration dependence. NCC addition improved the thermal stability of starch-based composite films. In addition, both the mechanical properties and air permeability of surface-sized paper tended to achieve an optimal state when NCC addition was 0.3 wt %.

1. INTRODUCTION To satisfy the demands of primary packaging application, cellulosic paper is always required to undergo specific treatments because of its poor mechanical and barrier properties.1 Over the past decades, petroleum-based materials including polyethylene (PE) and polyethylene terephthalate (PET) were widely used to enhance the surface properties of cellulosic paper. However, it is becoming more obvious that the ecosystem is considerably damaged as a result of these nondegradable plastic materials used in disposable items. As a result, exploiting biodegradable polymer materials from renewable resources to replace conventional petroleum-based materials is of both scientific interest and practical relevance.2 Among the broad family of biodegradable polymers, starch is one of the most promising candidates due to its low-cost, renewable nature, and thermoplastic behavior.3 In recent years, the development and application of starch-based materials in paper products have received great attention.4,5 Nevertheless, there are still limitations encountered in developing starchbased products because of the inherently poor mechanical and barrier properties.6 To reduce these disadvantages, much effort has been devoted to improving the properties of starch-based composites, such as chemical modification,7 the incorporation of fillers8,9 and melt blending.10 Nanocrystalline cellulose (NCC), derived from naturally occurring cellulose, is an emerging renewable nanomaterial that holds promise in many different application areas, such as textiles,11 enhanced cellulosic papers,12 and improved plastics13 as well as biological and biomedical engineering.14,15 Similarly, the concept of using NCC as a reinforcing phase in composite structure is of great practical interest. In this regard, a © XXXX American Chemical Society

representative example is NCC-reinforced starch-based composites. For instance, Agustin et al.16 obtained NCC from garlic stalks via a combined process, which included alkali delignification, acid hydrolysis, and sonication, and subsequently used it to reinforce the starch-based composites. Zainuddin et al.17 investigated the potential of NCC from kenaf fibers as a reinforcing filler in starch-based biocomposites. It was found that the tensile strength and modulus of the biocomposite films were significantly enhanced as a result of the NCC addition. Johar and Ahmad also reported the role of NCC from rice husks in enhancing the properties of starchbased biocomposite films, and they found that the morphological, thermal, and mechanical properties of the reinforced films were pronouncedly improved.18 Although many studies related to the reinforcing effect of NCC on starch-based composites were conducted, the concept of NCC-reinforced starch-based composite suspensions that were employed as surface sizing agents for enhancing the properties of cellulosic paper has not been reported. In the papermaking industry, surface coating/sizing technology is well established for imparting excellent end-use properties to cellulosic paper.19−21 As a matter of fact, the corresponding composite (also known as surface sizing agent) properties, particularly rheological behavior, played a critical role in enhancing the properties of surface-sized paper. However, there have been limited studies regarding the rheological Received: May 24, 2014 Revised: August 20, 2014 Accepted: August 21, 2014

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(TGL-16, Wei Jia Instrument Manufacturing Co., Ltd., China) to separate the nanocellulose, which was washed with distilled water, and the centrifugation/wash were repeatedly conducted five times. The nanocellulose colloidal suspension was filtered with distilled water in a dialysis bag for 2 days to a constant pH of 7.0. Eventually, the resulting NCC suspension was evaporated at 38 °C and 50 mbar in a rotary evaporator (Buchi Rotavapor R-210, Switzerland) for the purpose of increasing the NCC concentration to 20 wt %. 2.3. Preparation of Starch-Based Composite Suspensions and Films. Starch/CMC/NCC composite suspensions were prepared by a physical blending process. Initially, 40 g (dry weight) of cationic starch was gradually added into 20 mL of distilled water in a round-bottom flask under stirring by an electrical agitator (IKA RW 20) at 600 rpm for 5 min to form a suspension. Second, 0.8 g of CMC was dissolved in 100 mL of distilled water and then was totally transferred to stabilize the starch-based composite suspension. The mixed suspension was continuously stirred at 700 rpm for 45 min. To evaluate the effect of NCC addition on the rheological behavior, air permeability, and mechanical properties of cellulosic paper, various dosage levels (based on dry weight of starch) of NCC at 0.1, 0.2, 0.3, 0.4, and 0.5 wt % were added into the above suspension. Eventually, the total weight of each composite suspension sample reached 200 g with a supply of distilled water. For morphology characterization, a small portion of each composite suspension was further poured in a polytek mold and dried in an oven at 50 °C for 24 h to form composite films. 2.4. Paper Surface Sizing. The starch-based composites were sized on the surface of base paper using the Meyer rod coating setup23 (K303 multicoater, RK Print Coat Instruments Ltd., UK) at a constant sizing speed of 15 m/min. The sized paper was initially dried at room temperature and then dried at 105 °C for 30 s. The sizing weight of the resulting paper samples was about 2 g/m2. To deepen the understanding within the present project, the industrial surface sizing application configuration of starch-based composites reinforced with NCC on cellulosic paper was also demonstrated in Figure 1. 2.5. Rheological Measurement. The rheological behavior of starch-based composite suspensions was determined by using a Physica MCR301 advanced cylinder rotary rheometer (Anton Paar, Austria) at 25 °C. The steady shear rheological curves of the composite suspensions were realized in a range of shear rates from 10 to 3000 s−1. The oscillatory shear measurement was performed at a given strain level of 1.0%, which was within the linear viscoelastic region as determined by dynamic strain sweep experiments. The diameters of the top and the bottom plates were 50 and 60 mm, respectively. The gap height was maintained constant at 0.5 mm. The storage modulus (G′) representing the elastic properties, and the loss modulus (G″) standing for the viscous properties, were defined on the basis of the computer software Tool-master (automatic measuring and accessory detection system) connected to the rheometer. 2.6. Field Emission Scanning Electron Microscopic (FE-SEM) Observations. A FE-SEM (ULTRA-55, JEOL, Japan) with an accelerating voltage of 1.00 kV was employed to observe the morphology of cotton cellulose powders and NCC. All samples were coated with gold to avoid charging. 2.7. Transmission Electron Microscopic (TEM) Observations. The morphology of NCC, starch-based composites without and with NCC was characterized by TEM (JSM-2100, JEOL, Japan), using an accelerating voltage of 80 kV.

behavior of NCC-reinforced starch-based composite suspensions in the literature. Moreover, the potential application of NCC-reinforced starch-based composite suspensions in the surface sizing process of cellulosic paper has never been identified. For this purpose, the scope of the study includes the following: (1) starch-based composite suspensions/films reinforced with various dosage levels of NCC were designed and prepared; (2) the effect of NCC addition on the steady and dynamic shear rheological behavior of starch-based composite suspensions (i.e., surface sizing agents) was highlighted; (3) a surface sizing application of starch-based composite suspensions reinforced with different dosage levels of NCC on cellulosic paper was achieved to improve the mechanical properties and the resistance to air permeability, and the optimal process was obtained.

2. EXPERIMENTAL SECTION 2.1. Materials. A commercially available cotton cellulose powder (Tonnor EM100) was provided by Shanghai Tonnor Material Science Co., Ltd., China. Sulfuric acid (95 wt % H2SO4) and dialysis bags (molecular weight cutoff of 8000− 14000) were purchased from Hangzhou Mike Chemical Instrument Co., Ltd., China. Sodium carboxymethyl cellulose (CMC) (molecular weight of 250000 g mol−1 and DS of 0.90) was obtained from the National Medicine Group Chemical Reagent Co., Ltd., China. Commercial cationic starches (YZ121) with a solid content of 60 wt % were supplied by Hangzhou Paper Technology Co., Ltd., China. Cellulosic paper with a basic weight of 228.0 g/m2 used as substrate for surface sizing was supplied by Ningbo Asia Pulp and Paper Co., Ltd., China. 2.2. Preparation of NCC. The preparation of NCC followed a similar procedure as reported by our earlier work,22 which is schematically presented in Figure 1. In this process, 6.25 g of cotton cellulose powder was slowly poured into 50 mL of 65% sulfuric acid aqueous solution. The hydrolysis reaction was conducted at 50 °C for 60 min under continuous stirring. Subsequently, the hydrolysis was terminated by adding 500 mL of distilled water. Afterward, the resulting mixture was centrifuged at 11000 rpm for 10 min

Figure 1. Process configuration of surface treatment of cellulosic paper with starch-based composites reinforced with nanocrystalline cellulose. B

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2.8. Thermogravimetric Analysis. The thermal decomposition behavior of composite film samples was performed from room temperature to 800 °C at a heating rate of 20 °C/ min using Pyrisl TGA (PerkinElmer, USA) under nitrogen atmosphere. 2.9. Particle Size Distribution Analysis. All samples were diluted with distilled water to 1.0 wt % concentration and then sonicated for 5 min prior to the determination of particle size distribution by dynamic light scattering (DLS) (Horiba LB550). This instrument can provide accurate results for average particle size and distribution shape based on the Fouriertransform/iterative deconvolution technique. 2.10. Determination of Mechanical Properties and Air Permeability of Surface-Sized Paper. The base paper and surface-sized paper samples were treated for 24 h at 23 ± 1 °C and 50 ± 2% relative humidity prior to mechanical measurement. The mechanical measurements including tensile index (SE062, L&W), tear index (Protear-60-2600, Thwing-Albert, UK), folding endurance (31-23-00, Tiniuso, UK), and burst index (SE180, L&W) of base paper and surface-sized paper followed the relevant TAPPI test methods. The air permeability was determined (SE166, L&W) in accordance with ISO 56362:1984.

3. RESULTS AND DISCUSSION 3.1. Effect of NCC Dosage on Steady Shear Rheological Behavior of Starch-Based Composite Suspensions. Rheological measurement is known to reflect the interactions between polymers and other ingredients; therefore, it has the potential to predict the performance of a starch-based composite formulation in the surface-sizing process.24 Actually, the information derived from rheological measurement is vital in guiding the end-use application of the composites. The steady shear rheological behavior of starchbased composite suspensions as a function of various dosage levels of NCC was performed, and the results are shown in Figure 2. In the absence of NCC, the electrostatic interaction between anionic CMC and cationic starch acted to form a composite system with strong flocculation, which may be responsible for the observation that the viscosity of the starch-based composite suspensions showed a progressive decline as the shear rate varied from 10 to 3000 s−1, as shown in Figure 2a. However, in the presence of NCC, all composite suspension samples appeared to exhibit a stronger shear-thinning behavior. This observation is in good agreement with the results reported in the literature.25 Basically, due to its large surface area and high aspect ratio, NCC addition strengthened the network structure of starch-based composites; therefore, the interparticle interactions of the NCC crystals could generate a high resistance to flow (a higher viscosity) at low shear rate. Afterward, the network structure would break down into individual crystals or small fragments under the high shear rate field,25,26 which may be responsible for the occurrence of the obvious shear-thinning behavior. It should be noted that in industrial paper coating/sizing process, dilatant behavior should usually be avoided. Shear-thinning behavior is good for highspeed runnability on film-splitting devices. In this sense, the presence of NCC offers advantages for the potential application of starch-based composite suspensions in the surface-sizing process. Furthermore, Figure 2a shows that NCC addition led to increased viscosity of starch-based composite suspensions. At a

Figure 2. Effect of NCC dosage on (a) viscosity and (b) shear stress of starch-based composite suspensions at various shear rates.

given shear rate of 100 s−1, the viscosity of the composite suspensions without NCC was 7.06 mPa·s, which increased to 8.12 mPa·s at 0.1 wt % of NCC and further to the maximum of 12.79 mPa·s at 0.3 wt % of NCC. In particular, the starch-based composite suspensions with 0.3 wt % of NCC exhibited a viscosity of 10.14 mPa·s at a shear rate of 1000 s−1, which greatly satisfies the demands in high-shear viscosity of industrial sizing process.27 The significant increase in viscosity of starchbased composite suspensions can be mainly ascribed to two reasons: (1) NCC interacts strongly with water due to its inherent high polarity and large surface area, acting to form a highly entangled network displaying its well-known gel-like behavior,28 thus finally resulting in the increased resistance to flow; (2) the abundance of hydroxyl groups on the surface of starch and NCC provides opportunities for the formation of hydrogen bonds between their interfaces, thereby leading to improved interfacial adhesion. It can also be observed that the viscosity progressively increased with increasing the NCC dosage from 0.1 to 0.2, 0.4, and 0.5 wt %. Noticeably, 0.3 wt % NCC addition led to a dramatic increase in the viscosity of starch-based composite suspensions, which may be related to the relatively low zeta-potential of the dispersion system at this condition, implying a high friction between the polymer− polymer interfaces.29 On the other hand, Figure 2b also reveals that the shear stress increased with increasing shear rate from 10 to 3000 s−1. Similarly, the addition of NCC exerted an important effect on the shear stress. More addition of the NCC resulted in the higher shear stress of the starch-based composite suspensions at the same shear rate. C

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Table 1. Rheological Parameters of the Bingham Model Fitted to Flow Curves of Starch-Based Composite Suspensions with Various Dosage Levels of NCC NCC dosage (%) 0.1 0.2 0.3 0.4 0.5

yield stress (Pa) 0.0445 0.0654 0.1421 0.0643 0.0995

plastic viscosity (Pa·s) −3

7.506 × 10 8.345 × 10−3 10.31 × 10−3 8.024 × 10−3 9.015 × 10−3

Bingham model τ τ τ τ τ

= = = = =

0.0445 0.0654 0.1421 0.0643 0.0995

+ + + + +

7.506 8.345 10.31 8.024 9.015

× × × × ×

standard error −3

10 γ̇ 10−3γ̇ 10−3γ̇ 10−3γ̇ 10−3γ̇

5.39 7.50 14.03 7.86 9.91

Various classical models are often employed to describe the rheological behavior of composite suspensions.30,31 The choice of the rheological model that characterizes the network structure information on composite suspensions is of great importance, which mainly depends on the standard error and the parameters used to describe the flow state of various composites. In this work, the Bingham model was introduced to simulate the rheological data of starch-based composite suspensions as

τ = τy + ηγ ̇

(1)

where τ and η are the shear stress and plastic viscosity, respectively, τy represents yield stress, and γ̇ is the shear rate. Once the applied stress exceeds the yield stress, flow initiates.27 The Bingham model is commonly recognized as an idealized type of flow with a combination of yield stress and Newtonian or constant viscosity flow. The fitted rheological parameters of starch-based composite suspensions with various dosage levels of NCC are illustrated in Table 1. As shown in Table 1, eq 1 can describe the flow trend of the obtained composite suspensions and shows a good fit result, with a standard error of around 10, implying that these composite suspensions are characteristic of Bingham-like fluids. Moreover, the composite suspensions with 0.1, 0.2, 0.3, 0.4, and 0.5 wt % NCC possessed a yield stress at 0.0445, 0.0654, 0.1421, 0.0643, and 0.0995 Pa, respectively. Furthermore, the composite suspensions with 0.3 wt % NCC had the highest η value, in good agreement with the steady shear rheological curves (Figure 2b). 3.2. Effect of NCC Dosage on Viscoelasticity of StarchBased Composite Suspensions. Viscoelasticity is the property of materials that exhibit both viscous (liquid-like) and elastic (solid-like) characteristics when undergoing deformation.32 In general, starch-based composite suspensions exhibit a pronounced viscoelastic character, and the degree of viscoelasticity is strongly associated with the dissolved polymer in the aqueous phase. There has been significant work on the viscoelastic properties of starch-based composite suspensions in the past years.33,34 Despite these intensive efforts, the prediction of sizing handling and sizing runnability from viscoelasticity measurements remains an elusive goal. In this work, to understand the role of NCC in starch-based composite suspensions better, a set of oscillatory shear measurements of various starch-based composite suspensions was conducted. The storage modulus (G′) and loss modulus (G″) as a function of angular velocity (ω) are shown in Figure 3. As can be seen, for all starch-based composite suspensions, G′ is basically lower than G″ within the ω range of 0.1−10 rad/ s, providing direct evidence that the composite suspensions exhibited a typical viscoelastic liquid-like behavior at a low ω field. In addition, as expected, the viscoelastic properties of the starch-based composite suspensions showed strong dependence on NCC concentration, as revealed in Figure 3. Increasing the

Figure 3. Effect of NCC dosage on (a) storage modulus G′ and (b) loss modulus G″ of starch-based composite suspensions at various angular velocities ω.

NCC dosage from 0 to 0.3 wt % considerably increased the values of G′ and G″ at increased ω. The significant increase in viscoelasticity as a result of increased nanocellulose concentration was also noted in pigment-coating formulations,35 highconsistency nanofibrillated cellulose furnishes,36 and hydroxyethyl cellulose/carboxymethyl cellulose solutions.37 Besides, there seems to be a drop of G′ at a critical ω (which also increases with increasing the NCC content of up to 0.3 wt %). This might be the effect of the network not being able to reform quickly enough to store more energy and without having intact bonds between the entities. The suspension is not able to store energy anymore and, therefore, starts to behave again like a purely viscous “liquid” (that is, possessing no G′). With further increase in the NCC dosage levels up to 0.4 and 0.5 wt %, G′ and G″ experienced a pronounced decreasing trend. The structural inhomogeneity at high NCC dosage levels might be responsible for the decreased viscoelasticity of starch-based composite suspensions. 3.3. Morphology of NCC and Starch-Based Composites. SEM and TEM were employed to investigate the NCC microstructure. The SEM images give a clear indication of the D

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Figure 4. SEM images of (A) raw material and (B) NCC and TEM images of (C) NCC, (D) starch/CMC composites, and (E) starch/CMC/NCC composites.

change in microstructure from cotton cellulose materials to NCC. It is evident that the raw material had a uniform structure with a diameter in the range of 15−30 μm (Figure 4A). NCC, extracted from this raw material by sulfuric acid hydrolysis, exhibited rod-like structures with widths of