bioresources. com
PEER-REVIEWED REVIEW ARTICLE
postulates that HPC is the only both edible and film-forming thermoplastic cellulose derivative, which makes it especially interesting for multi-component dispersion coatings. HPC formulates readily as gel because of its low DS and abundant hydroxyl groups that form strong hydrogen bonds. The film-forming properties of HPC are relatively good, yet the resulting oil barrier is sufficient only for fast food packaging. HPC can be used in coatings as a film-forming material, thickener, binder, and as a suspending agent (Andersson 2008). Johnson et al. (2008) mixed fillers of cellulose nanocrystals (CNC) and microfibrillated cellulose (MFC) into HPC matrix material. Hydroxypropyl methyl cellulose As with most cellulose ethers, hydroxypropyl methyl cellulose (HPMC) is non- thermoplastic, and therefore it cannot form heat-sealable coatings (Paunonen 2013). However, it has been commonly used as barrier material in coatings (Rastogi and Samyn 2015). It displays moderate resistance to fat and oil, yet one of its drawbacks is its poor mechanical film integrity (Bilbao-Sainz et al. 2011). Its moisture barrier properties have been reported to improve considerably when introducing fatty acids (Andersson 2008). Coma et al. (2001) improved the moisture barrier with fatty acids as well as reduced the resistance of HPMC films against the bacteria Listeria innocua and Staphylococcus aureus . The etherification step of HPMC consists of methylation, in which pure MC is produced by reacting alkali cellulose with methyl chloride, either in its liquid or gaseous forms. The grades produced in this preparation display a DS between 1.7 and 2.3. Among the several applications, the most popular use of HPMC in industry is as a protective colloid used in the production of vinyl chloride (Thielking and Schmidt 2006). HPMC films are reported for their potential applications in the food industry due to environmental appeals, low price, as well as their flexibility and transparency (Bilbao- Sainz et al. 2011). Mahadevajah et al. (2016) studied the mechanical and barrier properties of HPMC films by applying various combinations of plasticizers. Larsson et al. (2017) applied HPMC at different concentrations in films of cellulose nanofibrils and nanocrystals. However, increasing the content of HPMC did not improve water vapor barrier. Bilbao-Sainz et al. (2011) used HPMC as a film matrix for microcrystalline cellulose (MCC), in which MCC improved the moisture barrier compared to a pure HPMC film. Cellulose Esters From the advent of petroleum-based plastics around 1950 and until today, the cellulose esters are some of the most applied thermoplastics. In the 1990s there was a large interest in biodegradable cellulose esters (Gilbert 2017). High oxygen barrier properties while applying cellulose ester to fillers have been reported (Dou et al. 2013; Uddin et al. 2016), yet water vapor barrier properties are limited. Cellulose esters are produced by reacting an organic or inorganic acid substituent with the three hydroxyls of an anhydroglucose unit (Kuusipalo et al. 2008). The preparation sequence of cellulose esters is in principle the same as that of cellulose ethers, yet instead of alkalization and etherification it involves acidification and esterification. Similar to alkalization, the purpose of acidification is to activate the hydroxyl groups of the anhydroglucose units. The extent of activation depends on the acid concentration (Granström 2009). In the subsequent esterification step, the substituent is an acid that corresponds to the final cellulose ester (Brydson 1999).
13
Helanto et al. (2019). “ Bio-based barriers ,” B io R esources 14(2), Pg #s to be added.
Made with FlippingBook - Online catalogs