The barrier properties of sustainable multiphase and multicomponent packaging materials: A review

Abstract

The current packaging materials market is dominated by petrochemical-derived, single-use, and inexpensive plastics that are incredibly stable in the environment. After their sole use, only a small fraction is recycled, with the remaining ending up in landfills, incineration facilities, or littered in the environment threatening the health of our environment. Nature-derived, compostable, and biodegradable polymers are appealing alternatives to replace single-use plastics in packaging and coating applications. However, most sustainable polymer alternatives cannot compete with traditional plastics in key packaging property attributes, such as gas and moisture barrier properties. Thus, there is a dire need to design chemistries and processing technology to enhance the barrier properties of sustainable polymer systems. In this review, we conducted a critical and comprehensive summary of the important and impactful contributions on improving the gas and moisture barrier properties of sustainable polymer systems for packaging applications. The work highlights signature accomplishments on the effect of tuning the microstructure, surface energy, morphology, and self-assembly of polymers on the gas and moisture barrier properties of the resulting films and coatings. The recent developments in designing multiphase and multicomponent systems and the impact of nanotechnology on barrier properties of sustainable packaging plastics are emphasized.

Introduction

Polymer coatings and films, which act as stable decorative or protective layers of consumer goods, have been widely utilized in various engineering applications and rapidly growing over the past few decades [1], [2]. Due to the rising popularity of take-out food in recent years, it is anticipated that polymer packaging and coating in the food industry will continue to play an important role. Polymer films and coatings in food packaging applications can be divided into three main categories, as shown in Fig. 1. These are enhanced barrier, active, and intelligent/smart packaging materials, each with its unique functionalities. Barrier packaging’s primary functionality is to provide moisture, gas, and aroma protection of packaged foods during production, transportation, storing, and selling.

On the contrary, active packaging materials provide functional attributes in addition to the barrier and are utilized to contain fresh and sensitive foods [3], [4]. Furthermore, active packaging materials with antimicrobial and antioxidant properties can prevent the growth and contamination of microbes, such as fungi and bacteria, in food [5]. Thus, the freshness and quality of the food can be prolonged, further providing functional characteristics to enhance the protective properties of the packaging material. The third type of packaging material is intelligent/smart packaging. Such packaging materials are embedded with state-of-the-art sensor technology to monitor food conditions and display its information. These include smart features in the packaging that can measure the freshness of the foods and provide users with up-to-date information about products near-field communication (NFC) and quick response (QR) barcode scanning [3]. Unfortunately, the uses of these intelligent packaging films are still not widespread and only present in a few niche products.

Most of the commercial polymer packaging is derived from petrochemicals. The most common examples are poly(ethylene) (PE), poly(propylene) (PP), poly(amide) (PA), poly(vinyl chloride) (PVC), poly(ethylene terephthalate) (PET), and ethylene vinyl acetate (EVA) [6]. Although most of these synthetic polymer coatings and films possess many convenient and beneficial properties, they also impose major sustainability issues that jeopardize the environment and cause accumulating risks to consumers’ health. The exceptional barrier properties of synthetic polymers such as gas, chemical, biological, or microbial resistance can be a double-edged sword as those resistance properties can make synthetic polymers extremely difficult to degrade at the final disposal step of the product life cycle [7]. Thus, most synthetic polymer-based packaging and coating materials end up in landfills, waste-to-energy facilities, or the general water and land environment after only a single-use [8], [9]. It is essential to highlight that despite the ongoing COVID-19 pandemic, the production of traditional synthetic polymers or plastics in 2020 was estimated to maintain at the colossal number of 367 million, in which food service and packaging industries still constitute more than one-third of the total plastic usage [10], [11].

For instance, a 2019 report from Deloitte Canada based on the data in 2016 indicated that only 9% of plastic wastes were recycled in Canada, while the remaining 86% ended up in landfills and 5% incinerated or littered in the environment [12]. Unfortunately, this is not so different in other jurisdictions. If the trend continues, the quantity of plastic waste accumulated in the environment could increase by up to one-third of the current waste by 2030. Moreover, some plastics can release harmful chemicals (e.g., Bisphenol A) and partially break down into microplastic, posing health and safety risks to humans and other animals. Thus, there is a desperate need to accelerate the design and development of sustainable alternatives to resolve plastic waste accumulation, the generation of microplastics, in conjuncture with ecological challenges associated with the production of plastics [13], [14], [15].

Due to the increasing societal and environmental needs active and sustainable packaging material trends, natural macromolecules (or biopolymers), such as proteins, carbohydrates, polysaccharides, and lipids obtained from plant resources, agriculture and forestry residues, or marine sources have become popular for the fabrication of sustainable food packaging films. Unfortunately, such sustainable materials’ success in packaging applications is curtailed by insufficient thermomechanical properties, high cost, and most notably inferior moisture and oxygen gas barrier properties [16]. Compared to traditional plastics, some biodegradable polymers (including polymers derived from natural resources or synthetic bioplastics designed to be biodegradable) demonstrate remarkable degradation in a short period into CO₂, H₂O, and benign residue in various environments via the action of microorganisms. Furthermore, unlike non-biodegradable synthetic polymers, which can sometimes release toxic residues causing health concerns for consumers, biopolymers generated from natural macromolecules are safe and can satisfy both health regulations and customer demands[17], [18]. It should be noted that not all bio-based plastic or polymers are not necessarily biodegradable or sustainable, as many other factors, such as chemical structures, synthesis processes (e.g., chemical crosslinking), additives, etc., can compromise their biodegradability, safety or sustainability [19]. Some examples of bio-based polymers that are not biodegradable includes bio-poly(ethylene terephthalate) (bio-PET), bio-poly(ethylene) (PE), or bio-poly(amide) (bio-PA) due to their carbon-packed structure preventing them from being biodegradable [20]. Thus, to highlight on the renewable aspect of high barrier multicomponent systems, this review will focus only on sustainable plastics that are either biodegradable, compostable or both.

Bioplastics used as a replacement for traditional plastics are classified into bio-based (e.g., poly(lactic acid) (PLA), poly(glycolic acid) (PGA)); petroleum-based polyesters (e.g., (poly(butylene succinate) (PBS), poly(caprolactone) (PCL), poly(butylene adipate terephthalate) (PBAT)); and microorganism-derived (e.g., poly(hydroxyalkanoates) (PHAs), bacterial nanocellulose) (Fig. 2) [21], [22], [23]. Most of these bioplastics are compostable polyesters in various environmental conditions, which can also be easily processed with the existing polymer processing equipment like melt extrusion and injection molding, giving them advantages in terms of scalability. Besides these polyester bioplastics, the direct use of natural macromolecules with minimal modification demonstrated appealing effectiveness in food packaging and coating applications[24]. Nature derived macromolecules, such as polysaccharides (e.g., starch, cellulose, alginate, pectin, chitin, lignin, and chitosan)[25], [26], [27], lipids(waxes, oils, fatty acids, and glycerides)[28], [29], [30], proteins (e.g., soy, zein, canola, whey protein, gluten, keratin, and gelatin) [31], [32], [33], [34], [35] are some examples of highly biodegradable sustainable material options that attracted interest in packaging applications [33], [36], [37], [38]. Nevertheless, unlike bio-polyesters, natural macromolecules are not easily processable, and scale-up operations are complex, limiting their potential usages in many food packaging applications. In general, bioplastics and biopolymers still have a few obstacles that need to be overcome, including high cost, unsatisfactory mechanical, thermal, and barrier properties to compete and displace the incumbent packaging plastics [39].

The strong drive for biodegradable polymers that provide good barrier properties against moisture and oxygen while simultaneously displaying good mechanical properties without compromising their biodegradability is ever-increasing, mainly for food packaging and coating applications [40], [41]. Coatings and films need to have excellent barrier properties to promote appropriate isolation of food and minimize contact with oxygen (oxidation reaction, aerobic microorganisms’ growth) or water (microbial growth, water activities, flavor change), as well as to avoid dehydration of moist foods, such as fresh meat, fruits, or vegetables [42], [43]. Many strategies can be employed to improve the barrier properties of polymeric coatings. Most common approaches involve fabricating multiphase and multilayer film and coating materials through blending, surface coating, controlled assembling, layering, etc.

Thus far, there are only few comprehensive reviews on the moisture and oxygen barrier properties of packaging and coating materials. A recent review by Yu et al. reported the progress in the barrier, biodegradability, and recyclability requirement for natural chitin and cellulose-based packaging materials [44]. Another recent review by Wu et al., showed more general insights on the overall moisture/oxygen barrier properties of stand-alone films and summarized strategies to improve the permeation of biodegradable polymers, including chain architecture tailoring, crystallinity control, melt blending/multilayer co-extrusion, nanotechnology, and surface coating [45]. This review aims to present researchers with an in-depth understanding of moisture and oxygen gas barrier properties in renewable multiphase and multicomponent systems by providing the summary of the mathematical models of barrier properties and the factors that affect the permeability of polymer systems; as well as to report the most updated contributions on the improvement of the barrier performances of films and coatings strictly in the context of food packaging applications, such as blends, multilayer assembly, nanocomposites, emulsion food coating, and paper coating. Furthermore, challenges associated with renewable food packaging materials are also emphasized, as well as the potential of research that focuses on improving barrier properties is also discussed.