Carboxylated Cellulose Nanocrystals Extraction from Kraft Pulp Using Ammonium Persulfate as Low Cost Source & Sustainable Method for High Quality Flexible Packaging Bio-coating

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.

Popular Abstract

PET Coating from Nano-cellulose Makes Plastics More Sustainable

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. But are we ready for a plastic-free world?
Not quite, but we are getting there. Regardless of the ongoing debates on whether we should push for biodegradable plastics, or plastics made from bio-based materials, packaging researchers are also trying to find a new way to start making plastic more sustainable – through bio-coatings.
We may have all seen coatings in packaging in our daily lives, such as the aluminum enclosed by thin plastic film in your bag of chips. Coatings act as barriers to oxygen and water vapor, which can greatly increase the shelf life of food. This substitutes the other option of using very thick plastic – saving material, energy and transportation costs. With the new trend of going nano-scale in the scientific world from electronics to health, nanotechnology is also creating new opportunities to improve our current state of packaging, particularly in the field of bio-coating.
According to latest research performed in University of Milan’s PackLab, it is possible to extract cellulose nanocrystals (CNCs) from one of the ingredients in paper-making, called Kraft pulp, which is very cheap compared to other possible sources. These crystals can then be used as bio-coatings for packaging application.
A fast, low-toxicity, sustainable and low-cost extraction of CNCs was explored in this research by using an oxidizer called ammonium persulfate. 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.
Tests of CNC-coated PET have shown excellent barrier and optical properties, comparable to cotton CNC coating. The results revealed astounding findings – Kraft CNC double coating managed to block oxygen by almost 10 times better than bare film. This result is comparable to Cotton CNC coating, even though the Kraft CNC were more diluted. Furthermore, it does not decrease visibility of the product inside packaging, when it was tested as packaging for breadsticks and readability of small print for as small as font 6.
With the success of this exploratory research on the possibility of utilizing cheaper cellulosic source and its applicability for coating in flexible packaging, more researches are now being lined up. In the far future, bio-coatings can also be applied directly on paper, to make a 100% biodegradable packaging with all the gas and water/ water vapor barrier benefits.
Ready or not ready, nanotechnology is taking us towards that plastic-free world.

• author: Estavillo Marin, Gerald Perry
• supervisor: Erik Andersson
• organization: Packaging Logistics
• course: MTTM01 20151
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: affordable CNC extraction, high quality CNC, sustainable packaging, FT-IR, TEM, XRD, contact angle, transparency, haze, optical properties, food packaging, bio-packaging, bio-coating, plastics, PET film, polyethylene terephthalate, Kraft CNC, Kraft pulp, cotton CNC, oxygen barrier, nanocellulose crystals, ammonium persulfate, CNCs, cellulose nanocrystals
• ISBN: 978-91-7623-392-4
• language: English
• additional info: 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, oxygen permeability, transparence, and haze for comparison. PET film was chosen since it is one of the most common type of plastic being used in food 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 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.; 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 material kappa number α-cellulose % β-cellulose % γ-cellulose % lignin % 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.; 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 O2TR for Kraft 1-layer and 7.78 O2TR for Kraft 2-layer vs 82 O2TR for bare under measurements at 50% RH) (see table 3).; 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.; Ammonium persulfate has proven to be an efficient extracting agent for unbleached Kraft pulp. Kraft pulp preparation, which includes swelling using distilled water at elevated temperature (70°C) for 30 minutes, cooling down, and mixing it with APS at room temperature for 30 minutes, were shown to be important steps to execute before proceeding with APS activation via heating. Kraft CNC was successfully applied on PET, and different parameters related to packaging were performed. Oxygen permeability is an important property particularly for food packaging due to its potential effects on quality and shelf life. Kraft 2-layer showed positive results in terms of oxygen permeability rate at 30% and 40% RH. This is comparable to CNC coatings extracted using acid hydrolysis and acquired from cotton linters, which contains > 99% alpha-cellulose. Optical properties were also tested, given the transparent flexible packaging’s importance to better market the products on the shelves by showing the actual product to consumers via see-through packaging, while keeping its protective barrier properties. Both Kraft 1-layer and 2-layer showed hydrophilic contact angles when in contact with water, denoting a good wettability and anti-fog property. However, more tests must be performed to verify its response when used in actual packaging. It must be highlighted that these results are based on 2.5% CNC re-dilution with distilled water after freeze drying using Kraft CNC extracted from the experiment, compared to 7% re-dilution of cotton CNC from acid hydrolysis. This re-diluted solution was used for coating application on PET film. This shows that Kraft CNC needed a lower amount of CNC concentration to achieve similar improvements observed from cotton CNC.; Utilizing unbleached and semi-processed Kraft pulp as cheaper material to extract CNC, in comparison to the heavily-processed cotton linters, was proven to be possible. Given the results, it can therefore be concluded that CNC bio-coating sourced from unbleached Kraft pulp provided a high quality bio-coating for PET, improving its optical and permeability properties. It has also provided a better alternative for a low cost extracting process at a shorter time (against acid hydrolysis, which includes dialysis step that lasts for 3-4 days). Application of this type of bio-coating can lead to reduction of total packaging weight by utilizing thinner and lighter plastics since its barrier properties were already improved. This can eventually provide economic benefit to producers and environmental benefit such as reduced energy use during transport.
• id: 7442427
• date added to LUP: 2015-06-24 10:44:22
• date last changed: 2015-06-24 10:44:22