Carboxylated Cellulose Nanocrystals Extraction from Kraft Pulp Using Ammonium Persulfate as Low Cost Source & Sustainable Method for High Quality Flexible Packaging Bio-coating GERALD PERRY E. MARIN MASTER´S THESIS Packaging Logistics Lund University 1 This Master’s thesis has been done within the Erasmus Mundus Master Course FIPDes, Food Innovation and Product Design. www.fipdes.eu FIPDes has been funded with support from the European Commission. This publication reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein. MASTER´S THESIS Packaging Logistics Lund University 2 Carboxylated Cellulose Nanocrystals Extraction from Kraft Pulp Using Ammonium Persulfate as Low Cost Source & Sustainable Method for High Quality Flexible Packaging Bio-coating Gerald Perry E. Marin 3 Carboxylated Cellulose Nanocrystals Extraction from Kraft Pulp Using Ammonium Persulfate as Low Cost Source & Sustainable Method for High Quality Flexible Packaging Bio-coating Production Copyright © Gerald Perry E. Marin Published by Division of Packaging Logistics Department of Design Sciences Faculty of Engineering, Lund University P.O. Box 118, SE-221 00 Lund, Sweden This Master´s thesis was done in PackLab, Università degli studi di Milano within the Erasmus Mundus Master Course FIPDes, Food Innovation and Product Design. www.fipdes.eu ISRN LUTMDN/TMFL-15/5139 ISBN 978-91-7623-392-4 4 Abstract One of the leading challenges presented in 21st century for packaging industry is to address the growing environmental problems related to non-renewable flexible packaging. This leads to new growing interest in bio-based materials, among them cellulose nanocrystals (CNCs), which have already shown good performance in improving anti-fog and oxygen & water vapor barrier properties when applied to flexible film. A fast and low-cost CNC extraction was explored in this research by using unbleached Kraft pulp as the cellulosic source and treatment with ammonium persulfate as sustainable method for extraction. Presence of CNCs and its properties were verified and investigated using fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and x-ray diffraction (XRD). CNCs were then used to coat PET plastic film and were subjected to contact angle measurement, oxygen permeability, transparence, and haze for comparison. Tests have shown excellent barrier and optical properties, comparable to cotton linter CNC coating extracted using acid hydrolysis, even with lower amount of CNC and thinner coating used by Kraft pulp. Making CNC bio-coating more affordable can reduce the amount of plastic usage in production leading to reduction of total weight, which can provide economic benefit to producers and environmental benefit through reduced energy use during transport. 5 6 Executive Summary Introduction One of the leading challenges presented in 21st century for packaging industry is to address the growing environmental problems related to non-renewable flexible packaging. This leads to new growing interest in bio-based materials, particularly cellulose nanocrystals (CNCs) (see figure 1), which have already shown good performance in improving anti-fog and oxygen & water vapor barrier properties when applied to flexible film. Current existing extraction methods include use of acids, enzymes and oxidizers, by mechanical means, or combinations of these to isolate CNCs from the cellulosic material. A fast and low-cost CNC extraction was explored in this research by using unbleached Kraft pulp as the cellulosic source and treatment with ammonium persulfate (APS) as sustainable method for extraction. APS extraction has recently been attracting attention due to its properties being ideal for CNC extraction, such as low long-term toxicity, high water solubility, and low cost compared to its sodium and potassium counterparts, as well as to other previous harsh extractive agents. Presence of CNCs and its properties were verified and investigated using fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and x-ray diffraction (XRD). CNCs were then used to coat PET plastic film and were subjected to contact angle measurement, Figure 1. Simplified illustration of (a) cellulose molecule repeat unit, showing the 1- oxygen permeability, transparence, and haze for 4 linkage & intramolecular hydrogen bonds (dotted line), (b) cellulose microfibril showing comparison. PET film was chosen since it is one of the crystalline & disordered regions, and (c) cellulose nanocrystals after extraction, due to most common type of plastic being used in food acid hydrolysis of the amorphous (disordered) region (Moon et al., 2011) packaging. Materials and Methods Characterization of kraft pulp material was performed together with InnovHub – Paper Division, who assisted in performing the experiments according to standards being used in the paper industry. For 7 the CNC extraction, swelling preparation was initially performed, where 10g dry weight of Kraft pulp was placed in 1000 ml beaker, diluted in 1-layer of distilled water, and stirred using magnetic stirrer. Heater temperature was increased to 70°C for 30 minutes, then cooled down in a cold water bath to reach the room temperature of 25°C. 340.5 g of ammonium persulfate (APS) was added to the cooled Kraft pulp solution to reach 1.5M APS, and was then stirred for another 30 minutes to allow the powder to dissolve completely. APS Extraction of Kraft Pulp This extraction method was based on patent with publication number EP 2513149 A1 filed by Leung, et al. It made use of 1.5M APS and was heated for 16 h at 70°C with high stirring speed for the cellulose nanocrystals extraction to occur completely. The sample was then removed from the heater, and was centrifuged using deionized water at 15,000 RPM for 20 minutes to concentrate the cellulose. It was centrifuged several times until it increased the pH level from around 0.2 to 3 (approximately 6 times). pH correction was then performed to the Kraft CNC solution, increasing it to pH 8 to avoid aggregation of the crystals in acidic environment. It was then subjected to ultrasonicator (UP400S 400W, hielscher Co., Germany) at 0.7 cycles of 20 minutes at 70% output to distribute CNCs evenly in the suspension. The solution was vacuum filtered using Whatman glass microfiber filter (grade GF/F, 0.7 µm) to remove fibers that did not react fully with APS treatment, and other big cellulose agglomerates and large contaminants that might have been introduced during the process. The Kraft CNC suspension was subjected to lyophilization by using a freeze drying machine (LIO-10P) for 3 days to get white Kraft CNC powder. The powder was rediluted using deionized water (18MΩ cm, Millipore Milli-Q Purification System) to reach 2.5% Kraft CNC solution, ultrasonicated for 5 minutes (0.7 cycles at 70% output), and applied to corona-treated PET film (25x20 cm2) for 20 rounds of rolling on one side of the plastic for approximately 3 minutes, improving adhesion of the nanocrystals on the surface of PET film. Automatic film applicator (ref 1137, Sheen Instruments, Kingston, UK) was used to apply the Kraft CNC solution evenly on top of the PET. Two samples were created: sample 1 having applied only 1-layer of Kraft CNC, while sample 2 was made by directly applying another round of coating using the automatic film applicator immediately after drying the first layer. It was then dried using the blower and air-dried for 24 hours. Results & Discussion TEM was used to identify physical properties of Kraft CNCs extracted in nanoscale level. 8 Figure 2. TEM image of Kraft CNCs (left) and cotton CNC (right) at 92,000 x magnification Upon observing the image, it must be noted that the CNCs obtained have two distinct shapes: spherical and rod-like (see figure 2). APS concentration can influence the shape of CNC, as shown by the experiment involving different concentrations applied to a lyocell fiber matrix. It yielded a mixture of rod-like and spherical CNCs for 0.5M APS, but produced 100% spherical CNCs at 1M APS. On the other hand, acid hydrolysis extraction of Kraft pulp have yielded only rod-like crystal structure. Table 1. Characterization of Kraft pulp raw material Raw kappa β-cellulose γ-cellulose lignin material number α-cellulose % % % % Ash Kraft pulp 35.48 86.8 0.37 13.57 7.87 0 The Kraft pulp sample obtained was carefully characterized to identify its kappa number, lignin, and α, β, γ cellulose contents (see table 1). Having a kappa number of 35.48 for the sample acquired is around the kappa number range of 30-35 for Kraft pulp that underwent conventional cooking. Having the lignin content of 7.87% shows that the Kraft pulp is subjected to an alkalinity of 20-25% in a span of 60-90 min. The high α cellulose in the resulting characterization experiment shows that previous processes have caused low degradation to the cellulose. Table 2. Coating thickness comparison of kraft 1-layer, kraft 2-layer and cotton Sample Thickness (nm) Uncoated PET 0 Kraft 1-layer coating 132.90 Kraft 2-layer coating 411.39 Cotton layer coating 660.00 The thickness values in Table 2 show that the cotton CNC coated PET film has the highest thickness, which can be explained by the total amount of CNC used in the solution. 9 The wettability of different samples shows that PET coated with cotton CNC has the best anti-fog property due to its very low contact angle measurement, allowing the water to spread to the solid surface. It is closely followed by Kraft 2-layers, with Kraft 1-layer exhibiting the lowest wettability. The increased amount of carboxylated CNCs in 2-layer compared to 1-layer have improved its hydrophilic interaction with polar water, thereby lowering the contact angle. Cotton APS managed to have a high transparency, with its transparency value being close to the bare film. Kraft 1-layer and 2-layer have lower transparency values, even though both of them are thinner than cotton APS (see table 2), and lower percentage of CNCs applied in the coating (2.5% vs 7%). This can be due to the fact that in comparison to cotton linters, which has been bleached and contains >99% cellulose, the unbleached Kraft pulp as source is relatively unpure, hence ion impurities might have influenced the slight decrease in transparency. To further verify the optical property of the samples, a sample logo with a subtitle of font 6, and website URL with font 11, were used. PET coated films still do have the same level of readability for both font sizes as compared to the bare film. This shows that in application to production, using Kraft CNC coating (both 1-layer and 2-layer at 2.5%) has almost negligible influence to transparency. Table 3. Oxygen transmission rate values of Kraft 1-layer, Kraft 2-layer, cotton and bare PET Oxygen Transmission Rate (O2TR (cc m-2 24h-1) 23°C) %RH Kraft 1-layer Kraft 2-layer Cotton Bare* 0 74.95 30 0.10 0.10 0.10 40 6.72 3.94 4.2 50 15.428 7.78 8.30 82 Given the relatively thinner size of both Kraft single and double layers as compared to cotton as seen on table 2 (132.90 nm and 411.39 nm vs 660.00 nm, respectively), and using less amount of CNC in the solution (2.5% vs 7%), the result has shown that Kraft CNC has exhibited a good potential as bio- coating source to improve oxygen barrier properties for PET film (15.43 O TR for Kraft 1-layer and 2 7.78 O TR for Kraft 2-layer vs 82 O TR for bare under measurements at 50% RH) (see table 3). 2 2 Conclusions The experiment has exhibited that high quality CNCs can be extracted from unbleached Kraft pulp, an unpure cellulose material source, and can be utilized as a high quality bio-coating for PET to improve its packaging properties. 10