(181j) Next Generation Food Packaging Materials Using Cellulose Biopolymer Substrates Supercritically Impregnated with Alkyl Ketene Dimer (AKD) Wax

Problem Statement: Cellulose, i.e. paper, is the world’s most abundant biopolymer^1–4, and has been a predominant material in food packaging for more than a century⁵. On its own however, its barrier properties are undesirable as a protective packaging material, and modifications are typically required to improve its functionality. A common method is to surface-coat the paper substrate with inorganic pigments (e.g. clay, calcium carbonate), along with polymeric binders and several additives which promote suitable barriers to better protect the encapsulated foodstuffs. While these methods work well, difficulties arise in recycling operations, where energy-intensive processes are used to separate the coating materials from the paper fibers. In the past five decades or so, the explosion of plastics into the packaging industry has resulted in materials with considerably better barrier properties against light, oxygen, microbes and moisture⁶. Despite these advances, plastic packaging waste is exceedingly difficult to decompose or recycle, and can take tens of years to decompose compared with paper that decomposes in weeks^7,8. In 2018, the recycle rate of waste paper/board was 68% while only 9% of plastics waste was recycled⁹.

With the advent of nanoparticles (NP) and nanoscale developments at the turn of the century, a new generation of food packaging development and innovation was instigated, incorporating NP as well as exploiting the many enhanced properties at the nanoscale. One such innovation is to impregnate cellulose biopolymers with various functional chemicals to produce a renewable, sustainable, and highly functional, material. Supercritical impregnation (SCI) is a relatively mature process being rediscovered as an environmentally benign method of introducing functionality into porous substrates, and works by first dissolving the chosen solute into supercritical carbon dioxide (scCO₂), often with small quantities of a cosolvent. Once dissolved, the supercritical fluid (SCF) laden with solute is passed over and through the substrate of interest – in this case cellulose – and deposits the solute into the pores of the substrate. Upon depressurization of the SCF, the solute remains embedded deep within the cellulose matrix while the now gaseous CO₂ exits from the system. Rapid expansion of the supercritical solvent (RESS) can be exploited to generate surfaces on the cellulose substrate that have both micro and nano scales, which promote additional functional and barrier properties¹⁰. The reverse process, supercritical extraction (SCE), can be used to extract the solute back into the SCF, thus purifying the cellulose biopolymer for recycling.

This work summarizes several developments of AKD-functionalized cellulose studies using SCI, which have the potential to contribute substantially to ongoing improvements in food packaging technology.

Methods: In many studies conducted by this research group, improvements to the hydrophobicity of paper is a key property investigated. Alkyl ketene dimer (AKD, Aquapel^TM 364 K) is a common sizing agent in papermaking processes. Small amounts of n-heptane cosolvent (Sigma-Aldrich) are used to help dissolve the wax into the scCO₂ (supercritical fluid grade, Airgas USA, LLC). An in-house developed SCI rig^11,12 is used where the high-pressure CO₂ is raised to the experimental pressure (100 – 250 bar) via an ISCO 260D syringe pump, and is then sent through a small vessel that contains the wax/n-heptane solution where it is circulated for 15 min to fully dissolve the material. This vessel is contained within a water bath to control the temperature (20 – 60^oC). Afterwards, the scCO₂-laden solvent is pumped to another vessel containing Whatman 1 filter paper (pure cellulose) where the impregnation process takes place. After a further 10 minutes, the system is depressurized and the paper samples are taken from the vessel for analysis.

Several analyses have been conducted on the treated paper: contact angle (CA) using a Biolin Scientific OneAttension CA analyzer; Fourier Transform Infrared (FTIR, Cary 630 Agilent Technologies) measurements to test for new bond formation between the wax sample and the cellulose; scanning electron microscope (SEM, JSM-7200 FLV FE-SEM) imaging. Thermodynamics modeling of the solute solubility in scCO₂ using PR-EOS equations has also been performed, as have experimental solubility studies where wax quantities at the solubility limit were impregnated, and amounts related to the degree of hydrophobicity observed. Experiments with a Quartz Crystal Microbalance (QCM-D, NanoScience) have been conducted to better understand the interfacial relationships between the solutes and cellulose.

Results: New learning is presented with regard to the resulting behavior and properties of paper supercritically impregnated with AKD. This learning covers both macroscopic properties of the improved paper as well as interfacial phenomena between the functional chemicals and cellulose molecules.

Paper was typically impregnated at 20^oC and 200 bar, using a solution of 1.8 g/L AKD/n-heptane. SEM images showed a progressive reduction in porosity between the fibers with time, over a 2-3 week period. This confirms earlier observations that AKD ‘spreads’ across the cellulose fibers taking at least two weeks to fully develop and adopt its stand-up orientation where the hydrophobic tails from the AKD molecule are pointing outwards. CA measurements show a similar rate of development where constant CA were not achieved until approximately 2-3 weeks later, remaining uniform for up to 3 months of testing. The average CA at steady-state was around 141^o, indicating a surface known as ‘sticky hydrophobicity’. Droplets stick to the surface even when it is upside down, due to retentive forces being greater than adhesive lateral forces. FTIR traces indicate the presence of hydrogen bonding between the AKD molecules and -OH groups of the cellulose^11,12.

Thermodynamic modeling of AKD solubility data in scCO₂ reported by Rodriguez-Meizoso et al.¹³ was based on PR-EOS with VDW mixing rules, and the model data was confirmed with Chrastil’s approximation (straight line relationship between ln solubility vs ln scCO₂ density). This work also revealed that the critical density of scCO₂ was important, below which the parallel straight-line relationships at different temperatures tended to collapse onto a single line¹⁴. Cloud point solubility data of AKD/n-heptane in scCO­₂ was obtained by this research group, values of which were subsequently used in SCI studies that showed direct correlation between the quantity of solute solubilized the SCF and CA development of the resulting treated paper¹⁵.

Recent investigations (data not yet published) are focusing on determining the molecular interactions between AKD and cellulose fibers at the interface, using a QCM-D. Initial results have shown that AKD/n-heptane is strongly attached to cellulose molecules, as judged by a reduction in frequency vibration of the quartz crystal. Since this value only marginally increases again during the n-heptane flush, this indicates that the AKD is firmly attached to the cellulose molecules. Although these experiments are not performed at high pressure, the results indicate that upon SCI, the solute will adhere sufficiently to the cellulose fibers to enact the desired functional and long-term improvements.

Implications / Conclusions to date: Several studies with AKD wax and SCI methodology have shown the ease with which paper substrates can be functionalized with small amounts of solute, using an environmentally responsible method. Impregnation of wax-based chemicals have been shown to not only improve hydrophobicity because of their long-chain hydrocarbon content, but also the development of micro/nano roughness of the surface. The hydrophobic development of the paper is a result of both chemical interactions between the solute and substrate, as well as the morphology of the substrate itself (other similar studies with different waxes have also shown the roughness development, especially after annealing of the treated paper¹⁶). Further, it has been shown that solubility of a solute in the SCF directly impacts the quantity of solute that can be impregnated, which directly impacts the development of the macroscopic property, in this case, hydrophobicity. These studies highlight the importance of quantifying solubility and recognizing the influence at the micro and nanoscale on bulk properties. Finally, studies that directly investigate the various interfaces within the SCI setup can provide important understanding on adhesion behavior and kinetics, which can be further exploited in the design of composite materials.