Editor’s note: This article is based on the work titled “Nanocellulose Hydrophobization,” which garnered the first place Student Poster Award at the 2023 CoatingsTech Conference. CoatingsTech welcomes student submissions year-round. For more information, email Editor-in-Chief Jacqui Barrineau at jbarrineau@paint.org.
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Cellulose, a linear polysaccharide biopolymer, has applications in various industries including food packaging, personal care products, and construction, due to its unique physico-chemical properties, as well as its biocompatibility and biodegradability. This study introduces a methodology to enhance the hydrophobicity of nanocrystalline (CNC) and microfibrillated (MFC) cellulose, using the covalent attachment (grafting) of copolymers based on plant oils.
Current methodologies for cellulose surface modification are limited in terms of hydrophobizing agents due to both their high cost and the fact that most are not renewable. This study focuses on the surface modification of CNC and MFC by graft copolymers from plant oil-based acrylic monomers (POBMs) developed by this research group.
The grafting and the properties of the resulting copolymers were determined using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) to demonstrate the presence of grafted POBMs and their impact on the characteristics of modified CNC and MFC.
Introduction
Cellulose, one of the most abundant natural polymers, boasts remarkable strength, stiffness, and sustainability. However, there are challenges in applying cellulose to materials fabrication which include high hydrophilicity and, therefore, poor compatibility with most polymer matrices.1 Modification of cellulose to improve the material’s water resistance and compatibility with specific polymer matrices remains a challenge among researchers due to cellulose’s complex chemical structure and extensive hydrogen bonding between macromolecules.
The need for cellulose modification arises mainly from the desire to overcome extensive hydrophilicity to unlock the full potential presented in this abundant biopolymer. The modification process needs to tailor cellulose properties to specific applications to make material applicable in diverse industrial applications. For instance, the hydrophobization of cellulose is important when aiming to create water-resistant materials with improved chemical and mechanical properties.2, 3
In this study, emulsion polymerization was utilized as a versatile and highly effective methodology for hydrophobizing cellulose.4 The approach brings several advantages, including minimized environmental impact, scalability, and the utilization of biobased resources.5
Cellulose nanocrystals (CNCs) and microfibrillated cellulose (MFC) already play important roles in biomaterials, providing versatility for sustainable innovations.6 Modifying the CNCs and MFC with plant oil-based polymers can be a significant undertaking, offering opportunities to tailor properties for specific applications. This article explores the importance of hydrophobizing these cellulosic materials and highlighting their promising properties to be applied as sustainable biomaterials.
Methods
To determine the surface energy and the contact angle of modified cellulose, water and diiodomethane contact angle measurements were carried out by a contact angle/surface tension analyzer using a drop shape analyzer (DSA 100, KRÜSS, Hamburg, Germany). Reported values were an average of 5 droplets.
The morphology of modified cellulose was observed on a tungsten filament 100 kV transmission electron microscope (TEM) JEOL JEM-100CX II, (JEOL, Peabody, MA). For the TEM measurements, a small amount of modified cellulose diluted with ethanol was placed onto a copper mesh covered with a thin carbon film. The samples were characterized after drying.
Fourier transform infrared spectra were recorded with a Nicolet 8700 FTIR spectrometer (Thermo Scientific) equipped with a Smart iTR attenuated total reflectance (ATR) sampling accessory. FTIR spectra in the range of 400–4000 cm-1 of modified CNC and MFC samples were recorded in reflectance mode, with 64 scans per sample. The analysis of the obtained FTIR spectra was conducted by identifying the chemical bonds in the molecule’s chemical structure according to standardized absorption peaks of functional groups.
The thermogravimetric analysis (TGA) investigated the thermal degradation of CNC and MFC samples. Employing a Discovery TGA 550 thermogravimetric analyzer (TA Instruments), the samples (ranging from 5 to 15 mg) underwent heating from 20 to 650 °C at a rate of 10 °C min–1 under a nitrogen atmosphere.
The surface elemental composition of modified CNC and MFC materials was analyzed using a Thermo Scientific™ K-Alpha™ X-ray photoelectron spectroscopy (XPS) instrument. The equipment featured a monochromatic Al Kα (1486.68 eV) X-ray source and an Ar+ ion source (up to 4000 eV). To ensure accurate results, all samples went through an argon ion cleaning process to minimize trace contaminants, such as oxygen and carbon, prior to analysis. The argon cleaning was applied to a 2 mm × 2 mm area of each sample by sputtering with an Ar+ ion cluster beam set to 4000 eV for 120 seconds using the MAGCIS® cluster gun. The XPS survey scan focused on the observation of photoemission lines for C1s, O1s, and N1s. The survey scan was an average of 10 scans with a Pass Energy of 200 eV, Dwell Time of 10 ms, and an Energy Step size of 1.0 eV. Spectra were collected at a 90° angle normal to the surface within a 400-μm area. To maintain optimal conditions, the chamber pressure was kept below 1.5 × 10–7 Torr, and the analysis was conducted at ambient temperature. The instrument’s software, Avantage, was employed for the quantification of atomic concentrations.
For elemental analysis, a Leco 932 CHNS combustion analyzer was used to determine percentages of carbon, nitrogen, and hydrogen in all samples. Samples of each material were weighed to a target weight of approximately 2 mg. A series of three calibration standard samples were run in the machine followed by the actual test samples. The actual process was as follows:
- Samples were removed from sample containers and poured into foil sample capsules.
- The capsules were weighed individually, and these weights were programmed into the machine.
- The machine then dropped the appropriate sample into its furnace where the sample combusted.
- After combustion, the percentages of each element were calculated.
Continue reading in the January-February 2024 digital issue of CoatingsTech.