Barriers and Chemistry in a Bottle: Mechanisms in Today's Oxygen Barriers for Tomorrow's Materials
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applied sciences Review Barriers and Chemistry in a Bottle: Mechanisms in Today’s Oxygen Barriers for Tomorrow’s Materials Youri Michiels 1,* , Peter Van Puyvelde 2 and Bert Sels 1,* 1 Centre for Surface Chemistry and Catalysis, KU Leuven, 3001 Heverlee, Belgium 2 Soft Matter, Rheology and Technology, KU Leuven, 3001 Heverlee, Belgium; [email protected] * Correspondence: [email protected] (Y.M.); [email protected] (B.S.); Tel.: +32-16-377690 (Y.M.); +32-16-321593 (B.S.) Received: 3 June 2017; Accepted: 21 June 2017; Published: 28 June 2017 Abstract: The stability of many organic compounds is challenged by oxidation reactions with molecular oxygen from the air in accordance with thermodynamics. Whereas glass or metal containers may protect such products, these packaging types also offer severe disadvantages over plastics. Large-scale packaging, especially for food and beverage industries, has shifted towards polymeric materials with passive and active oxygen barrier technologies over the last decades. Even though patent literature is flooded with innovative barrier systems, the mechanisms behind them are rarely reported. In a world where packaging requirements regarding recyclability and safety are continuously getting stricter, accompanied by the appearance of emerging applications for plastic oxygen barriers (such as organic semi-conductors), research towards new materials seems inevitable. To this cause, proper in-depth knowledge of the existing solutions is a prerequisite. This review therefore attempts to go deep into the problems at hand and explain the chemistry behind the existing solution strategies and finally discusses perspectives suggesting new applications such as organic light-emitting diodes (OLEDs) and solar cells. Keywords: oxygen barrier; oxygen scavenger; packaging; poly(ethylene terephthalate); organic electronics 1. Introduction In recent years, the world of packaging has been shifting continuously towards the application of more plastic-based materials as replacements for, e.g., aluminum and glass. These traditional materials have a high production cost and high carbon footprint compared to polymeric containers [1–3]. Additionally, plastics are less fragile and extremely lightweight. However, while glass and aluminum are highly impermeable to gases, small molecules may still be able to move in between polymer chains. Therefore, the plastics industry is forced to develop new technologies to meet the packaging requirements of certain products, such as juices, beers and wines. As such, specific barrier solutions are employed in order for these products to retain an appropriate lifetime. Depending on the contents of the packaging, demands can be imposed for retaining light, water, CO2 and/or O2, etc. These demands are quantified in a single parameter, namely the shelf life. The shelf life is defined as the period of time ranging from the packaging of a product to the moment wherein the product is no longer of acceptable quality to be sold. A wide range of solutions has been brought onto the market in order to provide the product’s specific shelf life demands. These can be subcategorized into active and passive barriers, depending on the type of molecular interactions (i.e., chemical reactions, adsorption, desorption, etc.) that occur between the barrier compound and the targeted molecule [4,5]. For many applications, the exclusion of oxygen from the package is critical. The oxidative nature of this molecule can quickly deteriorate the quality of a packaged product [6,7]. Depending on Appl. Sci. 2017, 7, 665; doi:10.3390/app7070665 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 665 2 of 30 Appl. Sci. 2017, 7, 665 2 of 30 For many applications, the exclusion of oxygen from the package is critical. The oxidative nature of this molecule can quickly deteriorate the quality of a packaged product [6,7]. Depending on this product,this product, the requirements the requirements for the for oxygen the oxygen barrier barrier to achieve to achieve a sufficient a sufficient shelf life shelf may life vary. may Fruit vary. preservationFruit preservation thrives thrivesbest under best oxygen under levels oxygen of levels2–5%, ofimposing 2–5%, imposingthe need fo ther very need slow for scavengers very slow andscavengers the creation and the of creationa steady of state a steady between state permea betweention permeation and oxygen and removal oxygen removal[8,9]. In [meats8,9]. In and meats in manyand in beverages many beverages (ex. juice, (ex. beer, juice, wine, beer, etc.) wine, any etc.)oxygen any could oxygen be couldharmful. be Packagers harmful. Packagers should therefore should aimtherefore for the aim exclusion for the exclusion of as much of asoxygen much as oxygen possib asle possible until other until contaminations other contaminations will limit will the limit shelf the life.shelf For life. example, For example, in solid in solid products products such such as meats, as meats, microbial microbial threats threats limit limit the the shelf shelf life life to to 8–9 8–9 weeks weeks anyway,anyway, soso oxygenoxygen barriers barriers that that provide provide protection protection for for at least at least this this long long are sufficient are sufficient [10]. Aseptic[10]. Aseptic filling fillingof liquid of liquid products products provides provides a much a better much protection better protection from microbial from microbial contaminants, contaminants, so a longer so potentiala longer potentialshelf life canshelf be life achieved can be inachieved comparison in comparison to the modified to the atmosphericmodified atmospheric packaging (MAP)packaging of solid (MAP) foods. of solidTherefore, foods. the Therefore, most potent the oxygen most barrierspotent oxygen in the state-of-the-art barriers in the are state-of-the-art preferred for beverages are preferred to achieve for beveragesmaximum to durability achieve maximum [11]. Aside durability from foodstuffs, [11]. Aside other from technical foodstuffs, applications other technical for polymeric applications oxygen forbarriers polymeric can comprise oxygen thebarriers protection can comprise of organic the electronics protection (e.g., of organic organic electron light-emittingics (e.g., diodes organic (OLEDs) light- emittingand solar diodes cells) [12(OLEDs)–14]. OLED-structures and solar cells) comprise[12–14]. OLED-structures molecules with high comprise conjugation molecules that maywith excitehigh conjugationand subsequently that may luminesce excite and when subsequently an electric luminesc currente is when applied. an electric At these current excited is applied. energy At levels, these it excitedmay readily energy interact levels, withit may water, readily but interact also oxygen, with wa aster, will but be also further oxygen, explained as will below.be further The explained substrate below.on which The the substrate electrodes on andwhich the the organic electrodes semi-conductor and the organic phase ofsemi-conduct OLEDs areor grown phase should of OLEDs therefore are grownprovide should long-lasting therefore protection provide fromlong-lasting oxygen protec and water.tion from The encapsulationsoxygen and water. in the The traditional encapsulations set-up inare the glass traditional plates and set-up metal are lids, glass which plates achieve and me excellenttal lids, which performances. achieve excellent Now the performances. industry is focusing Now theon producingindustry is itsfocusing next generation, on producing flexible its next OLEDs. generation, The targeted flexible flexibility OLEDs. The can targeted only be achievedflexibility with can onlythe exclusionbe achieved of with the rigid the exclusion glass plates. of the Transparent rigid glass plates. polymers Transparent with high polymers barrier propertieswith high barrier would propertiesoffer an appropriate would offer alternative, an appropriate but the alternative, current solutions but the are current still expensive solutionsand are insufficientstill expensive for longand insufficientlifetimes. Applications for long lifetimes. encompass, Applications amongst encomp others,ass, the amongst incorporation others, ofthe flexible incorporation computer of flexible screens computerin clothing. screens A scheme in clothing. listing the A relativescheme oxygen listing barrierthe relative desires oxygen for above barrier applications desires isfor given above in applicationsFigure1. is given in Figure 1. FigureFigure 1. 1. ApproximateApproximate values values for for the the pe permeationrmeation limitations limitations that that oxyge oxygenn barriers barriers should should provide provide in in differentdifferent applications. applications. The The values values are are expressed expressed as the as themaximum maximum volume volume of oxygen of oxygen that may that mayenter enter1 m2 of1 mbarrier2 of barrier surface surface area areaeach each day dayat 1 atbar 1 bar of ofexte externalrnal oxygen oxygen pressure, pressure, aver averagedaged over over the the relevant relevant lifetimelifetime [8,10,14]. [8,10,14]. ManyMany inventions inventions in in the the field field of of oxygen-resistan oxygen-resistantt packaging packaging materials materials have have been been investigated investigated in in confidentialconfidential industrial industrial environments, environments, while while little little fundamental fundamental research research is is published published on on the the science science behindbehind the the oxygen oxygen barrier. barrier. This This is is especially