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PEER-REVIEWED REVIEW ARTICLE
swelling tendency of cellulose, because of its hydrophilic and hygroscopic nature, limits its film-forming capability while it requires an energy-consuming drying process. The motivation to produce cellulose derivatives is to respond to the limitations of native cellulose (Rastogi and Samyn 2015), while maintaining the bio-based origin and characteristic biodegradability. Cellulose derivatives include cellulose ethers and cellulose esters (Brydson 1999; Kuusipalo et al. 2008; Granström 2009). Their preparation can be done by heterogeneous or homogeneous modification (Granström 2009), the former of which generally maintains the fibrous structure while the latter applies to organic solvents for derivatizing the cellulose backbone. There are several variables that affect the properties of cellulose derivatives, out of which the degree of substitution (DS), and the chain length, described by the degree of polymerization (DP) of the anhydroglucose monomers, are quite relevant. The DS relates to the average number of hydroxyl groups substituted per anhydroglucose units. Thus, the higher the DS, the more complete the reaction (Reese 1957). There are several properties that are dependent on the DS. Whereas viscosity indicates the average DP, associated with mechanical and rheological properties, the softness, hardness, and moisture absorption of the material depend strongly on the DS (Gilbert 2017). Moreover, the DS affects the biodegradability of cellulose ethers; a higher DS means a larger number of ether linkages and therefore lower biodegradability (Andersson 2008). Another important parameter is the purity of the cellulose used as raw material, as indicated by the relative mass fraction of pure cellulose (alpha-cellulose) (Burton and Rasch 1931). The more amorphous and the less crystalline the structure, the higher the rate of diffusion, being specific for different reagents (Gilbert 2017). In heterogeneous modification, the DS is significantly dependent on the location of the anhydroglucose unit in the fibrous structure (Thielking and Schmidt 2006; Andersson 2008). For improved results in cellulose modification, both the position of the modified anhydroglucose unit in the chain and in the fibrous structure as well as the positions of the substituents around the anhydroglucose ring and along the chain should be controlled (Fox et al. 2011; Gilbert 2017). Uniform distribution of reaction during cellulose structural modification is required for targeting advanced end- use properties. Regioselectivity is facilitated by homogeneous modification and associated solvents, while today’s heterogeneous modification relies on analytical methods and controlling regioselectivity (Fox et al. 2011). Cellulose derivatives have been reported to act as suitable matrix materials for cellulose nanocrystals, which in turn improve the water vapor barrier of cellulose derivatives, while acting together as a filler composite (Paunonen 2013). Cellulose Ethers The preparation sequence of cellulose ethers consists of an alkalization and an etherification step. The purpose of alkalization is to activate the hydroxyl groups of the cellulose polysaccharides, which yields alkali cellulose. In the subsequent etherification, the alkali concentration of the preceding alkalization step defines the final amount of substituents that shall replace the hydroxyl groups in the backbone of alkali cellulose. Therefore, the alkaline concentration regulates the DS of the polymer (Brydson 1999; Thielking and Schmidt 2006). The cellulose ethers presented in the following section can all be used as barrier materials (Rastogi and Samyn 2015). Except for hydroxypropyl cellulose (HPC), cellulose ethers are non-thermoplastic, which means that they do not provide heat-sealable coatings, although they can be cast as films and are water- and ethanol-soluble. Moreover, they inherently display modest moisture barrier and slow
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Helanto et al. (2019). “ Bio-based barriers ,” B io R esources 14(2), Pg #s to be added.
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