Material properties and water resistance of inorganic–organic polymer coated cellulose paper and nanopaper

Littering of the environment with plastics is one of the main environmental concerns of the current century. Microplastics formed by wear and fragmentation of plastic-containing materials have been presented as a serious environmental problem, particularly for aquatic ecosystems, and potentially for human health (Guzzetti et al. 2018; Laskar and Kumar 2019). However, owing to the outstanding material properties of plastics, including high flexibility, ductility, and impermeability for water they are ubiquitous and can be found, for example, in packaging materials, shopping bags, toys, medicine, cosmetics, and pharmaceuticals. However, the majority of plastics prepared from fossil resources are not biodegradable, and in addition, only a small fraction of the plastics used today is recycled (Lindström and Österberg 2020). In 2019, only 13% of all plastic waste was recycled, while 40% was dumped in landfills and 19% of the plastic waste ended in unmanaged dumps (Degnan and Shinde 2019). The need for environmentally friendly, sustainable materials that can compete with the outstanding properties of plastics is both clear and pressing.

Bioplastics are more environmentally friendly alternatives to plastics. They can be bio-based, biodegradable, or both (Rujnic-Sokele and Pilipovic 2017). Biodegradable bioplastics such as Polylactic acid (PLA), Polyhydroxyalkanoates (PHA), starch and cellulose are often discussed as potential replacements for plastics from fossil resources (Lörcks 1998; Ahmed and Varshney 2011; Asgher et al. 2020; Qasim et al. 2021). Cellulose, which is mostly found in the form of fibrils in wood and plant cells, is the world most abundant biopolymer. Cellulose is renewable, biodegradable, and non-toxic, and therefore, an excellent alternative to plastics both when considering environmental and health aspects (Alexandrescu et al. 2013; Dufresne 2013; Liu et al. 2016). Owing to the high strength and good paper formation properties of cellulose fibers, cellulose has been extensively used for various papermaking applications (Eriksen et al. 2008; Larsson et al. 2014; Ottesen et al. 2016). Cellulose nanofibrils (CNFs) prepared by mechanical fibrillation methods consist predominately of bundles of a few elementary fibrils. The fibrils are comprised of bundles of cellulose chains bounded together by van der Waals forces, and intra- and intermolecular hydrogen bonds (Jarvis 2003; Purushotham et al. 2020). The elementary fibrils have both crystalline parts and amorphous regions along each fibril (Saïd Azizi Samir et al. 2004). CNFs can be produced by mechanical treatment of the cellulose fibers using a homogenizer or a microfluidizer which results in partial degradation and a significant increase in the specific surface area (Sharma et al. 2019; Turbak et al. 1983). The mechanical treatment can also be combined with chemical or enzymatic pretreatments to tune the fiber dimensions, reduce the amount of energy needed for the mechanical treatment, and change the surface chemistry of the fibrils (Lavoine et al. 2012). Micro- and nanosized cellulose have shown promising potential for bio-composites, reinforcement, and coating applications owing to their abundance, low cost, high strength and stiffness, low weight, and high aspect ratio (Klemm et al. 2011; Lavoine et al. 2012; Djafari Petroudy et al. 2017; Qasim et al. 2021). Furthermore, cellulose-based materials are inherently biodegradable and recyclable, making them an excellent material both for single-use applications, and for applications where controlled biodegradation is wanted.

Despite the potential advantages of cellulose-based materials, they often present challenges when compared to conventional plastics. They generally have poor water resistance properties resulting from the hydrophilicity of cellulose fibers, and low ductility. These inherent characteristics limit the ease of replacing plastics by cellulose-based materials. Different strategies have been used to tune properties such as hydrophobicity, and rate of degradation. These approaches include among others: (1) Chemical modification of the cellulose fibers, such as esterification, etherification, or silylation (Sehaqui et al. 2013; Lu et al. 2015; Deng et al. 2016; Valdebenito et al. 2018; David et al. 2019; Pang et al. 2019; Yu et al. 2019), (2) physical or physiochemical treatment (Rodríguez-Fabià et al. 2022) such as plasma etching (Dimitrakellis et al. 2017), and (3) hydrophobic coating (Wu et al. 2001; Choi et al. 2002; Rastogi and Samyn 2015; Li and Rabnawaz 2018). Notably, chemical modifications only result in a thin layer of modified cellulose on the surface of the fibers, where either small molecules or polymers form an upper layer with thickness in the range of angstroms to a few nanometers, depending on the exact nature of the molecule or polymer grafted. Hence, as compared to surface coating, where an upper layer in the micrometer range can be obtained, only a very thin modified surface layer can be achieved by chemical modifications. Additionally, when considering industrial scaled production, chemical—and physiochemical treatments are often challenging, time-consuming, and expensive. Therefore, surface coating appears to be advantageous, both because it allows for controlling and varying the thickness of the coating layer, and because it appears to be the economically best approach. Different surface coatings available for cellulose are e.g., thin layers of: (1) plastic, for example polyethylene (PE) (Choi et al. 2002), (2) silicone, for example polydimethylsiloxane (PDMS) (Li and Rabnawaz 2018), (3) latex (Wu et al. 2001), or polymer nanoparticles (Samyn et al. 2010). The water barrier properties of the cellulose-based material can be significantly improved with such coatings; however, the biodegradability is lost, and the coated material is often difficult to recycle. Thus, a coating which allows for improved hydrophobicity as well as tunable biodegradability would be ideal. To retain the biodegradability, biopolymers such as PLA, have been tested for paper coating applications (Rastogi and Samyn 2015). However, PLA requires industrial composition conditions for degradation, which also presents challenges for the recycling of PLA coated papers (Li and Rabnawaz 2018). At this time, the development of biodegradable cellulose-based materials with hydrophobic characteristics that can compete with plastics in terms of performance and production costs, remains a challenge.

Another approach for hydrophobic surface coatings are inorganic–organic hybrid polymers, such as organically modified ceramics (ORMOCER®s, registered trademark of Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e. V., Munich, Germany), which are a combination of ceramics/glass and organic polymers. Organic–inorganic hybrid polymers are a promising group of materials for coating applications due to their good adhesion properties, transparency, chemical and mechanical stability, and good processability. Additionally, a large variety of the physical properties, such as mechanical and barrier properties can be tailored by changing the ratio of organic to inorganic constituents or by integrating additional functional groups (Haas and Wolter 1999). ORMOCER® coatings have previously been used to coat polymer films and glasses (Amberg-Schwab et al. 1998; Charton et al. 2006; Iotti et al. 2009), and it was shown that the barrier properties of polymer films could be significantly improved by the coatings (Amberg-Schwab et al. 1998; Iotti et al. 2009). Based on these promising findings, ORMOCER® was suggested as an excellent alternative for the coating of paper and nanopaper to obtain biodegradable materials with desirable properties such as improved hydrophobicity. ORMOCER® coated paper and nanopaper represent a more sustainable alternative plastics for the preparation of functional materials. Moreover, the composition of the ORMOCER® can be varied allowing for optimizing the coating composition depending on the substate and the specific application. For applications where the material would be used in the nature, biodegradability is important. In the case of ORMOCER®s, the degradation products would predominately be CO₂, H₂O, and SiO₂ in addition to minor products sometimes containing nitrogen and sulfur or oxides, which depend on the exact composition of the ORMOCER®.

In this paper we present the preparation and material properties of paper and nanopaper with and without ORMOCER® coating. The coated papers and nanopapers were prepared aiming to improve their water-repellent properties as a first step towards making cellulose-based materials that could potentially replace plastics for some applications. Paper and nanopaper were coated with two ORMOCER® coatings having different chemical formulations. Paper and nanopaper with and without coating with ORMOCER® were characterized by mechanical tests, contact angle—and Cobb analyses, and by physical and chemical characterization methods. Surface properties were characterized by Fourier-transform infrared spectroscopy (FT-IR), as well as with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Finally, the biodegradability of coated nanopaper was studied, and compared to the degradation of uncoated nanopaper. The degradation was monitored over a period of 3 months under controlled conditions. We show that the rate of degradation depends both on the temperature—and pressure conditions used during the pressing of the nanopaper, and on the composition of the coating, and that for certain combinations, the coated nanopaper remains intact for up to three months.

Never dried pulp from Södra was used as a raw material and processed using two different methods for paper production. The pulp was grinded in a PFI mill (10% suspension, 3000 revolutions), and used to produce cellulose paper. CNFs were produced by disintegration and homogenization of never dried pulp from Södra using a Masuko grinder (Supermasscolloider MKCA6-2, 5 passes at 2500 rpm) and a homogenizer (1 pass at 600 bar and 1 pass at 1000 bar). The dry content of the resulting CNF suspension was 0.93%. The CNFs were used to produce the cellulose nanopaper.

ORMOCER® CBS 010/30 (hereby referred to as ORMOCER® I) and ORMOCER® CBS 121/30 (referred to as ORMOCER® II) were prepared as previously and exemplarily described (Vartiainen et al. 2019). Essential components of these types of materials are the inorganic Si–O–Si-based networks which can be combined with ceramic (such as Al–O–) components as in ORMOCER® I and specific hydrophobic organic containing functional groups as in ORMOCER® II. The former will contribute to the high density of the resulting material exhibiting especially the barrier properties, and the latter will be responsible for specific hydrophobic surface properties.

The chemical compositions of the selected coatings described in the present study are summarized in Table 1. The components are presented after hydrolysis and condensation reactions as they are incorporated in the inorganic basic network. Wet coatings were applied as described later.

ORMOCER®-coated paper and nanopaper with improved hydrophobicity have been prepared and characterized. The nanopaper is less porous, mechanically more stable, but also more brittle as compared to the paper. The denser structure of the nanopaper is a good substrate for the coating, and coating results in a material with a distinct two-layered structure. Furthermore, nanopapers prepared at elevated temperature and pressure prior to coating remained intact for two months in a biodegradation study. Taken together, the properties of the coated paper and nanopaper make them interesting for future studies aimed at finding good alternatives to plastics for applications such as agricultural mulch films and food packaging.