Barrier coatings provide barriers to certain gases, liquids, radiation, or other substances. These may include oxygen, water vapor, aroma, light (including ultraviolet), and, in the case of food packaging applications, products such as oils and fats, and so on. The barrier is usually intended to prevent both penetration and loss from the package, which could otherwise cause advanced spoilage of the packaged product. There is an ongoing industry requirement to improve barrier properties in order to extend shelf life of packaged foods. This section deals only with barrier coatings, applied to flexible plastic packaging films, for the purpose of minimizing oxygen and water vapor transmission. In describing the various coating technologies, comparisons will be drawn with alternative methods of achieving these barriers, in order to provide a balanced view of this important property in relation to food packaging.
The ultimate packaging barriers to oxygen and moisture barrier are tin and glass closely followed by aluminum foils. These materials provide zero gas and vapor transmission and, in the case of glass, also visibility of the product, which is often an important point of sale marketing consideration. While these materials have served the food packaging industry very well for many years and are likely to continue doing so, they have certain drawbacks which leave the door open for new advances in barrier technologies. Glass and tin, while infinitely recyclable, are heavy and therefore expensive to transport, resulting in less environmentally friendly greenhouse gas (CO2) emission compared to transportation of lightweight plastic packaging. Plastic packaging laminates including aluminum foils, while lighter than tin and glass, render the packaging difficult (if not impossible) to recycle, resulting in the need to dispose of it in landfill or by incineration, both of which are negative options in a world where sustainability is of increasing importance.
The gradual replacement of tin, glass, and more recently aluminum foils by plastic packaging materials has been evolving for several decades. The improvements to polymer film technologies, coupled with inorganic and organic surface treatments and coatings, continue to close the gap with tin and glass in terms of barrier performance, while ensuring that due attention is paid to the environmental impact.
There are limited choices for the packaging technologist who seeks the ultimate gas barrier in single or even multilayer plastic package structures. This becomes even more limited when there is a requirement for product visibility, reducing the options to structures that are aluminum foil or (Al) metallized film free. The common polymeric packaging films based upon polyethylene terephthalate (PET), oriented polypropylene (OPP), cellulose in many coated guises (e.g., MXXT/W, MS, etc.), and biaxially oriented nylon (BON/OPA) do not have sufficient oxygen or moisture vapor barrier properties for the many food packaging specifications that call for longer shelf life of foods that spoil if exposed to atmospheres depleted or rich in these gases. It is usual in many packaging situations to replace the atmosphere inside a package with an inert gas substitute. This is known as modified atmosphere packaging (MAP) or sometimes protective atmosphere packaging (PAP). Clearly, it is important that the substitute gas is retained for as long as possible. Typical MAP gases include nitrogen, oxygen, and carbon dioxide which as a rule of thumb permeate through polymeric films in the ratio 1:4:10. Since oxygen is the most reactive of these three, with respect to food degradation, it is seen as the most important, while combinations of nitrogen and carbon dioxide may be selected for reasons of control of microbial growth, ripening, or cost.
The manufacturers of such polymeric films have over the past 60 years developed coatings for their products which, to varying degrees, improve the gas barrier properties as well as other specific performance properties such as heat-seal threshold, product resistance, gloss, and clarity. Such coatings include organic solvent and aqueous applied polyvinylidene chloride (PVdC), aqueous or extrusion- coated ethylene vinyl alcohol (EVOH), inorganic and organic sol–gels, and in a few instances aqueous polyvinyl alcohol (PVOH). Each of these technologies has certain compromises in performance. Certain polymeric films have an intrinsic barrier to moisture or oxygen, but rarely both. Cellulose, being a reasonable barrier to oxygen but poor moisture barrier, is one example.
Alternatives to aqueous or solvent-based coating applications have been available for some time now. These essentially inorganic layers are deposited either as vapor in a vacuum or by various electron-sputtering or plasmaenhanced chemical vapor deposition (PECVD) processes. The most common vacuum-deposited barrier coating is aluminum, but more recent developments include silicon oxide (SiOx) and aluminum oxide (AlOx) where the proportion of oxygen is nonstoichiometric. These coatings have one distinct advantage of being excellent barriers to both oxygen and water vapor. The oxide coatings are also completely transparent and most commonly used on PET and BON. Although improvements to the gas barrier of OPP are also achieved, this is less common due to the expensive nature of the process resulting in mediocre barrier performance. Since the important substrates are not heat-sealable, they are invariably used in multilayer structures, which also serve to protect the oxide coatings which are particularly susceptible to damage through physical abuse, which may result in a loss of barrier properties. Some oxide-coated films are also in-line coated, with sol–gel technologies, to enhance both barrier and abuse resistance properties. Post application of protective coatings is rare due to the risk of damage to the barrier before coating.
Very recently, there has been a resurgence in the interest shown by converters to apply barrier coatings in-line with the printing process for high-barrier packaging laminates. New organic polymer technologies combined with nano-particulate materials applied from aqueous or solvent-based systems that may not require extended drying conditions are at an early stage of commercialization and demonstrating certain advantages over existing technologies.
The gas permeation process for any packaging film or coating is specific to the chemical structure and morphology of that material (1). The transmission of gases across a barrier usually involves several processes. After colliding with the barrier surface, the gas molecule must be adsorbed and subsequently dissolved in the voids of the barrier polymer matrix. Diffusion of the gas molecules then takes over and is governed by the kinetic energy of the molecules directly influenced by temperature. Diffusion of the gas generally proceeds through a barrier layer from a more to a less concentrated atmosphere or partial pressure. In all cases, permeation is controlled by the solution and diffusion steps, according to Henry’s Law of Solubility and Fick’s Law of Diffusion, respectively. This is why metals and the previously mentioned metal oxides exhibit such high barrier properties in exceptionally thin films, to both moisture and oxygen. Crystallinity in organic polymer barriers plays a less important role, although in coatings based upon PVdC copolymers, where the drying temperature is critical to imparting crystallinity, variation in oxygen and water vapor transmission has been reported previously by the author (2). An indepth consideration of the theories governing gas transmission are dealt with elsewhere in this publication.
PVdC was developed by Ralph Wiley and co-workers at the Dow Chemical Company during the period 1932–1939 (3). It was initially commercialized under the trade name of ‘‘Saran’’ in 1939. PVdC is still used today by both packaging film manufacturers and converters. Although it can be applied from solution, it is far more popular to use the aqueous dispersion form which enables much higher solids application at the required viscosities. One reason for the longevity of this technology is its undoubtedly unique properties that distinguish it from all other polymer dispersions, even today. It is fair to say that PVdC coatings are still the only organic polymers capable of providing both moisture and oxygen barrier. The fact that they also provide heat-sealability, high gloss, transparency, and flexibility are further reasons for its continued use. However, its use has gradually declined in various regions of the world as increased importance is placed on environmental factors. Chlorinated polymers are now excluded in many packaging structures due to the problems related to incineration and the difficulties associated with handling the toxic by products including dioxin. Furthermore, as higher barrier requirements have been introduced, partly driven by the need for longer shelf life, the need for alternative polymer technologies and inorganic barrier solutions has resulted in superior barrier technologies, albeit with their own set of compromises.
Ethylene vinyl alcohol (EVOH) copolymers are hydrolyzed copolymers of ethylene and vinyl alcohol. Polyvinyl alcohol (PVOH), made by the hydrolysis of polyvinyl acetate (PVAc), provides increasing oxygen barrier with the percentage conversion of PVAc, but this is accompanied by increasing water sensitivity. Consequently, the oxygen barrier of PVOH, while superior to polymers such as PVdC at low relative humidity, is drastically compromised under most ambient conditions that packaging must resist. Occasionally, PVOH coatings are used where the coating is protected by an efficient moisture vapor barrier such as that conferred by polyethylene or polypropylene films in multi-ply laminates. However, it is not suited, even in these structures, for high oxygen barrier specifications where relative humidity is above 50% or the processing of the packaged product involves pasteurization or autoclave steam cooking.
EVOH copolymers became commercially available in 1972 in Japan, although their use was very limited until some 10 years later. By copolymerizing vinyl alcohol with ethylene, the hydrophobic properties of the latter improve the resistance to humidity, while preserving much of the oxygen barrier. Copolymers with the highest oxygen barrier contain in the region of 27–32% ethylene and provide an environmentally acceptable alternative to PVdC under similar conditions of use. Again, the oxygen barrier properties are still sensitive to increasing levels of humidity, which limits this barrier in certain (e.g., tropical) specifications. The solubility of EVOH is troublesome; although some lower ethylene content copolymers are soluble in alcohol/water solutions, they have a tendency to gel upon storage, making them less than convenient to use. In fact, storage of the solutions at elevated temperatures is the only way to avoid gelation. Therefore, the use of EVOH in barrier packaging tends to be more commonly achieved by extrusion coating which is an expensive converting exercise.
Aluminum-metallized substrates such as PET, OPP, and cellulose are manufactured by the vacuum metallization process. These films have been available for around 30 years and provide high levels of barrier to oxygen and moisture vapor and have undergone an evolutionary process that has seen improvements to surface stability, film adhesion, and printing ink adhesion. The presence of aluminum (foil or metallized film) in food packaging is gradually being discriminated as environmentally unacceptable and will ultimately decline. The use of other vacuum metal oxide deposition processes such as plasma-enhanced chemical vapor deposition (PECVD) to lay down ultrathin layers of aluminium oxide or silicon oxide is also well-advanced, producing transparent films with equivalent barrier to the more common aluminum metallization or in some cases even better. The major disadvantage with these ceramic-type layers is their brittleness, which makes them prone to damage either during the converting process or in the final laminate structure during the packaging operation.
Over the last five years, there has been considerable worldwide investment in nanotechnology. Some of this investment has been focused on the development of new barrier coating technology. In an effort to take cost out of the process whilst achieving the highest possible barrier properties combined with sustainability (low carbon footprint, compostability or recyclability) coatings applied via the conventional coating processes (gravure, reverse roll, flexo, blade coat, etc) from water or organic solvent are ready for commercialization. These coatings exhibit a balance of properties which makes them suitable for both film manufacturer and converter applied applications. Their performance in packaging will now be discussed with reference to the graphs referenced.
Compared to other wellknown organic barrier polymers, this new generation of barrier coatings offers a superior oxygen barrier that is less susceptible to high levels of relative humidity. Compared to inorganic barriers the oxygen barrier is very similar, but inorganics still offer slightly better stability to high relative humidity. Owing to the combination of inert particle technologies and ‘‘green’’ polymer selection, the new-generation coatings are generally more sustainable and provide better abuse resistance as is demonstrated in Figure 2. Indeed the latest coating technology can be used to reinforce the performance of aluminium-metallized and oxide-coated films, achieving barriers of less than 0.1 cm3/ m2/day as well as improving the flex resistance of these substrates. In Figure 3 it can be seen that significant improvements to the oxygen barrier of common packaging substrates is possible without the need for complex application/ vapor deposition processes, opening up the potential for conversion of high barrier packaging at various points along the value chain (e.g., film manufacturer or converter). In many respects these coatings may be seen as a means to produce cheaper laminate high-barrier structures (e.g., using barrier-coated OPP) with equivalent barriers to those currently used (e.g., PET).
Compostable and sustainable substrates such as those derived from cellulose or poly lactic acid (PLA) are being specified more and more by the large retail outlets. These substrates do not have the appropriate barrier properties required for many perishable food types. Improvements through inorganic and new-generation-type coatings are presently a high priority. In Figure 4 the improvement to oxygen barrier on PLA is very significant. Cellulose, however, which has even higher sensitivity to moisture, shows less improvement.
Combining oxygen and moisture vapor barrier in organic polymer-based coatings remains a target. Although nano particulate materials are capable of improving both barrier properties it is still necessary to apply separate coatings to achieve the highest barriers. In Figure 5, improvements to moisture vapor barrier on PET, PLA, and paper are compared with a commercial barrier board. With this type of coating technology, it is possible to confer moisture vapor barrier performance on substrates such as polylactic acid (PLA) and cellophane approaching that of polyethylene or OPP.
Present technologies include both water- and solventbased products permitting applications from gravure, flexographic, and lithographic (tower) coaters.
The new-generation barrier coating technologies are set to evolve rapidly in-line with the requirement for a single coating combining both oxygen and moisture barriers, sustainability and high abuse resistance at low applied film weights. Heat-sealability and surface print performance will follow. The use of barrier coatings, especially oxygen barrier types, will also allow consideration for simplifying or lightweighting complex laminates, resulting in waste reduction and a lower carbon footprint than is currently the case (4). Furthermore, these highly transparent and flexible coatings will allow new packaging design innovations where product visibility is an advantage. Combining these gas barrier coating technologies with UV light-curing barrier coatings will result in clear plastic packaging with the potential to supplant glass, cans, and conventional laminates in a wider variety of packaged produce.