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PEER-REVIEWED REVIEW ARTICLE
after reaching the state of its activation energy. The reader is referred to Piringer (2000a,b) and Paine (1992) for more information. Determining Barrier Properties Methods such as water contact angle and the Cobb test (see standard ISO 535, 2014) are useful to assess the surface properties and wettability of the substrate onto which a barrier layer is applied. Most relevant, for quantitatively determining the extent of the four types of barrier properties, standardized and non-standardized methods are available. Relations such as Eqs. 1 to 4, are typically based on phenomena described by permeation or transmission and gravimetrical analyses. Several authors, including (Tunç and Duman 2010; Rastogi and Samyn 2015) have used gravimetric techniques to evaluate the water solubility of films, Eq. 5, Water solubility (%) = 100 ( W 1 - W 2 )/ W 1 (5) where W 1 and W 2 represent the initial and undissolved dry matter weights (g) of films, respectively. For water vapor permeability, there are both standardized and non-standardized test methods. The water vapor transmission rate (WVTR) represents the mass of water per surface area unit of barrier film, which permeates through the surface within a predetermined time interval and pressure difference, within a constant temperature and constant relative humidity. A typical unit of WVTR is g*m 2 *day -1 (Dufresne 2012; Rastogi and Samyn 2015). Han et al. (2010) calculated WVTR by co mbining Henry’s Law and Fick’s first l aw by using Eq. 6, WVTR (g/m 2 *24 h) = w/A (6) where w represents the increase in weight (g) of the film in the measuring unit after 24 h and A is the area (m 2 ) of the exposed film. The unit can be also expressed as cc.m -2 .s -1 (Siracusa et al. 2008). Bilbao-Sainz et al. (2010) calculated water vapor permeability (WVP) by a “gravimetric modified cup method”, following ASTM E96 (ASTM E96 / E96M-16 2016). They also defined adsorption desorption isotherms of water by using a dynamic vapor sorption analyzer (DVS). Being the initiator of life on our planet, oxygen is the most bioactive and reactive gas of all atmospheric, gases. Therefore, it is essential to protect a package contents from oxygen to ensure its high quality and to promote a long-lasting shelf-life. For oxygen, there are a few methods of measurement. The most traditional one is the oxygen transmission rate (OTR) by using the MOCON instrument, according to ASTM D3985- 17 (2017) standard. In this method, the barrier material is placed between two chambers at ambient atmospheric pressure in dry condition (RH < 1%). One chamber contains nitrogen and the other oxygen. The permeated oxygen is detected by a coulometric detector. The unit of OTR is given as cc.m -2 .s -1 (Siracusa et al. 2008). Once the oxygen transmission rate is determined, the transmission rates of other atmospheric gases can be predicted by knowing the relationship between each other. For example, for nitrogen, the transmission rate is one-third of that of oxygen, while carbon dioxide permeates six times as fast as oxygen (Paine and Paine1992). However, an assigned quantity for carbon dioxide also exists, known as carbon dioxide transmission rate (CO 2 TR), expressed as cc.m -2 .s -1 . Similar to WVTR and OTR, one area of importance for CO 2 TR is in food packaging (Siracusa et al. 2008).
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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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