In the pursuit of enhanced sustainability for future generations, developing polymers from renewable sources has become a crucial area of research. This shift is driven by the urgent need to introduce environmentally friendly alternatives that can partially replace non-biodegradable polymers, which, due to their chemical characteristics, pollute our oceans and ecosystems after consumption. Notably, around 40% of today's polymer production is dedicated to packaging, highlighting the challenge. These materials, often used briefly before disposal, significantly contribute to environmental degradation. The solution lies in transitioning to biodegradable materials, a move that could substantially reduce our ecological footprint (Ahmadzadeh and Khaneghah, 2019; Vilarinho et al., 2018)
Despite considerable efforts to introduce biodegradable polymers such as PLA and PCL into the market, their adoption has been hampered by lower competitiveness when compared to traditional materials like PE, PP, PVdC, and EVOH (Wang et al., 2018) This has only intensified the search for innovative materials derived directly from renewable resources that do not compromise on their biodegradable nature (Li et al., 2021).
Cellulose, the most abundant biopolymer on earth, have the potential to replace synthetic polymers in packaging applications (Klemm et al., 2005; Nathalie Lavoine, Isabelle Desloges, Alain Dufresne, 2012) Cellulose fibers, extracted mainly from wood plants by chemical/mechanical process present low cost, low density and good mechanical properties. Some of its byproducts as cardboard paper are already widely applied on tertiary packaging, being responsible for carrying and protection of manufactured products (Ahmadzadeh and Khaneghah, 2019). More recently, cellulose nanofibrils (CNF) started its first steps towards industrial scale production.(Klemm et al., 2018; Shatkin et al., 2014; Shatkin and Kim, 2017) The obtaining process, firstly developed by Turbak et al, 1980 and improved over the last 40 years based on mechanical shearing in combination with an enzymatic and/or chemical pre-treatment produces thin nanoscale elements with high aspect ratio (Gandini and Belgacem, 2016; Rol et al., 2019a).
The drying process of CNF suspensions from water or other solvent leads to highly stiff and compact CNF films, with less porosity than paper, which has opened some opportunities for high valuable application on films for modified atmosphere packaging (Ferrer et al., 2017; Martin A. Hubbe, Ana Ferrer, Preeti Tyagi, Yuanyuan Yin, Carlos Salas, 2017; Pawar and Purwar, 2013; Zhu et al., 2016), flexible films for printed circuits (Agate et al., 2018; Sundararajan et al., 2017), controlled drug releasing media (Chen et al., 2017) and so on (Ferrer et al., 2017; Martin A. Hubbe, Ana Ferrer, Preeti Tyagi, Yuanyuan Yin, Carlos Salas, 2017). Nonetheless, in the presence of water, paper and CNF films present a swelling effect, causing loss of dimension stability and stiffness, limiting their application on packaging.
(Bang et al., 2024) prepared a hydrocolloid suspension of silane modified cellulose nanofibers with silica nanospheres which after dried over a wood surface led to a WCA of 151°. The authors emphasize the role of a rough surface in reaching superhydrophobic surface. (Sadat Fazel et al., 2024) proposed an approach based on freeze dried aerogels followed by surface hydrophobization of CNFs using hexadecyltrimethoxysilane (HDTMS) as the modifier. SEM micrographs showed that increasing the amount of HDTMS modifier in the samples resulted in smaller existing pores, thicker pore walls, and a transition from hydrophilic to hydrophobic aerogels. The contact angle values increased with the increase of modifier concentration, with WCA as high as 140°.for 0.5ml. It is worth to mention that the unmodified aerogels’ wettability could not be determined due to the hydrophilic nature of CNFs.
Surface modification of films can turn CNF more hydrophobic while maintaining the bulk material unaltered. Additionally, the functionalization can enhance its barrier properties to water vapor (de Souza et al., 2021a). Several reports on the literature present success on surface chemical modification of cellulose using a variety of reagents and protocols such as amino propyl trimethoxysilane (APMS) (Reverdy et al., 2018a; Zhang et al., 2014), titanium dioxide (Zhang et al., 2014), esterification with anhydrous moieties(Sehaqui et al., 2014) and plasma etching with fluorinate deposition (Balu et al., 2008a). Yet, the practical application of these methods on a large scale is hindered by its high costs. Consequently, there is a growing demand for cost-effective methods for surface hydrophobization of cellulose films. In this context, the employment of blocked isocyanate chemistry, which can be utilized under non-anhydrous conditions, emerges as a promising alternative (Carvalho et al., 2005; Gironès et al., 2007).
Blocked isocyanates, recognized for their urethane bonds, exhibit a relatively low dissociation temperature ranging from 110°C to 240°C, depending on the blocking agent used (Delebecq et al., 2013; Wicks and Wicks, 2001). Their appeal is further enhanced by their stability and low toxicity levels (Paquet et al., 2010) at room temperature. The effectiveness of combining blocked isocyanates with natural materials has been demonstrated in the treatment of thermoplastic starch (TPS) using phenyl blocked isocyanates through a dipping and heating process, which notably improves moisture resistance (Carvalho et al., 2005).
In contrast to traditional isocyanates that relies on anhydrous conditions for application (Botaro and Gandini, 1998; Carvalho et al., 2005; Rol et al., 2019b; Stenstad et al., 2008), blocked isocyanates offer a versatile alternative for modifying hygroscopic materials like cellulose fibers and nanofibers. Recent successful applications of blocked isocyanates on filter paper, hydroxyethyl cellulose, and cellulose nanocrystals have unveiled their efficacy in enhancing the wet resistance of these materials (Lu et al., 2020; Zhou et al., 2020). Also, (Antonino et al., 2023) prepared lignin polyurethanes from unmodified Kraft lignin or from hydroxypropylated lignin using blocked isocyanate chemistry where the applicability of the prepolymers as adhesives was proven by single lap-shear tests in steel substrates.
In the present study, phenyl/alkyl blocked isocyanates were synthesized by allowing one functional group of 4,4'-methylenebis (phenyl isocyanate) (MDI) to react with phenol for later deblocking, while the other group reacted with a linear chain alcohol, forming a stable urethane bond. The alkylic moiety's chain length was varied, utilizing alcohols with differing carbon numbers—specifically isopropanol, butanol, octanol, dodecanol, and octadecanol—for 3, 4, 8, 12, and 18 carbon chains, respectively. The substrates, including wood pulp papers and CNF films, were fabricated via casting and subsequently underwent a two-step hydrophobization process. This process involved a dipping in the synthesized adduct solution followed by a thermal treatment at 170°C for ten minutes, aiming to deblock the phenol and foster the formation of new urethane bonds with cellulose’s surface. The adducts were full characterized by employing FTIR, Liquid 1H-NMR and TGA. The modified materials (paper and films) were then characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), and water contact angle (WCA) measurements. These analyses provided insights into how surface roughness and the alkylic chain's length influence the hydrophobization efficacy.