# Barriers and Chemistry in a Bottle: Mechanisms in Today's Oxygen Barriers for Tomorrow's Materials
## 4 Active Barriers  
For many applications, any contact with oxygen, even small concentrations, is undesirable during the shelf life of a packaged product. Passive barriers often do not comply with these specifications. Incorporating highly oxygen-sensitive compounds to the packaging might provide a solution by chemically reacting with oxygen, removing its oxidizing nature and providing an \"active oxygen barrier\". Such a substance can be implemented into the packaging by several methods, shown in Figure 8. However, not every active barrier can be incorporated by each method. Solutions such as multilayer or blending may provide both a passive and an active barrier. However, the passive barrier usually can be neglected in comparison to the active scavenging. This is especially the case in an active blend, where the concentration of barrier polymer is usually much lower to limit hazing due to immiscibility. Usually, the oxygen flux from the outside of the bottle wall is zero or even negative in these systems. A reaction rate needs to be added to Equation (6), leading to Equation (10) [130]. \[\mathbf{Q_{package}}=\int_{\mathbf{A}}\frac{\mathbf{P}}{\mathbf{L}}\boldsymbol{ \sigma}\left(\mathbf{po_{2,outside}}-\mathbf{po_{2,inside}}\ \right)-\mathbf{r_{OS}}\mathbf{d}\boldsymbol{\sigma} \tag{10}\]  
With \(\mathbf{r_{OS}}\) the reaction rate of the oxygen scavenger within a surface area of the package (cm\({}^{3}\) oxygen scavenged m\({}^{-2}\) day\({}^{-1}\)). For highly active scavengers, the reaction rate is often limited by its access to oxygen in the wall. The rate can therefore be indirectly related to permeability, but permeability is unaffected by the activity of the scavenger. Expressing the active barrier performance in terms of measured permeability or OTR data is therefore incorrect. However, this is a very common mistake. Another approach to incorporating active barriers to packaging is capturing the oxygen scavenger in a pouch or sticker, usually consisting of a silica gel matrix that can be glued to the inner wall or the cap. The pouch is most often placed in the headspace, as [PERSON]'s law states that oxygen is preferably in the gas phase. While the oxygen content in the headspace is lowered, the concentration in the product will follow this trend through [PERSON]'s law. Oxygen flux through the bottle wall is now completely separated from reaction rate so the volume of oxygen in the contents of the bottle, \(\mathbf{V_{in}}\) (cm\({}^{3}\)), changes in time according to Equation (11). \[\frac{\mathbf{d}\mathbf{V_{in}}}{\mathbf{d}\mathbf{t}}=\mathbf{Q_{package}}- \mathbf{r_{OS,\ total}}=\int_{\mathbf{A}}\frac{\mathbf{P}}{\mathbf{L}} \boldsymbol{\sigma}\left(\mathbf{po_{2,outside}}-\mathbf{po_{2,inside}}\ \right)\mathbf{d}\boldsymbol{\sigma}\ - \mathbf{r_{OS,total}} \tag{11}\]  
The scavenger reaction rate in the equation now covers the total scavenging rate (cm\({}^{3}\) oxygen scavenged day\({}^{-1}\)) and is dependent on the oxygen content in the product itself instead of the oxygen content in the wall. The applications with pouches are very popular in Asia, but are still limited in Western countries due to the food safety regulations that prohibit their use in beverages. Aside from regulatory aspects, the visual appearance of the pouch is often unattractive towards the consumer [131]. Given how the reaction rate is usually dependent on the bottle constitution, it is very difficult to compare different literature values as quantification for the active barrier efficiency. Many active barriers are activated by water, so only analysis techniques that allow contact between water and the barrier material can provide sufficient information. Many techniques however do not or only partly offer humidification (see Section 2.2).