Alexei Volkov
Plastic packaging is used to preserve a wide range of products, from food to medicines. However, not all plastics are created equal when it comes to their ability to act as a barrier against oxygen. Oxygen permeability can cause significant issues for certain products, such as caking in powders or changing the composition of fluids. This is especially important in the food industry, where oxygen can cause nutritional and organoleptic degradation. So, which plastics are the worst offenders when it comes to oxygen permeability?
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What You'll Learn
Plastic packaging and polymer permeability
Plastic packaging is commonly used to provide a barrier to oxygen, moisture, and other gases. However, the effectiveness of this barrier varies depending on the type of plastic and the specific gases involved.
Plastics are formed by long polymer chains that create a "molecular ball" structure with voids that allow small gas molecules, such as oxygen and water vapour, to pass through. This is why plastics are not completely "waterproof", despite the common perception. The gas permeability of polymers can impact the performance of plastic packaging and the shelf life of the products inside.
The barrier properties of polymers used in plastic packaging can vary depending on factors such as the type of polymer, temperature, and humidity. Different polymers have different cohesive energy densities, free volume, and degrees of crystallinity, which affect their oxygen permeability. For example, methylcellulose- and hydroxypropyl methylcellulose-based films have lower oxygen permeability than low-density polyethylene film.
To improve the barrier properties of plastic packaging, multi-layered structures or laminates are often used. These can include materials such as aluminium foil, which provides an effective barrier to moisture, gases, and light, or barrier resins like ethylene/vinyl-alcohol (EVAL) or polyvinylidene chloride (PVDC). Biaxial orientation processes can also enhance the barrier properties of polymers, increasing their toughness, stiffness, and resistance to water vapour and oxygen.
Overall, the optimisation of packaging permeability is crucial to extending the shelf life of food products and ensuring their quality preservation. By studying the permeability characteristics of different polymers under different environmental conditions, manufacturers can select the most suitable materials for effective oxygen and moisture barriers.
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Plastic oxygen barriers in construction
Plastic oxygen barriers are an important consideration in the construction industry, particularly in maintaining the integrity of structures and extending the shelf life of products. While plastic packaging provides a degree of protection against oxygen ingress, it is not always impermeable, and the choice of polymer significantly impacts its effectiveness.
In construction, plastic oxygen barriers are used in various applications, including packaging, piping systems, and agricultural storage. Plastic packaging, such as bottles and laminates, offers varying degrees of oxygen protection depending on the polymers used. Biaxially oriented polymers like polypropylene (PP-BO) and polyethylene terephthalate (PET-BO) exhibit improved oxygen barrier properties through increased stiffness and toughness. However, achieving all desirable barrier properties in a single polymeric film can be challenging, necessitating the use of multi-layered structures.
In piping systems, polypropylene pipes are commonly used for water disposal, sewerage, and water supply. While these pipes offer advantages such as sustainability and efficiency, they possess a level of permeability to gases, including oxygen. This oxygen permeability can lead to issues like oxidation of components, sludge formation, and biofilm development in wastewater disposal systems. To mitigate this, modern systems incorporate antioxidant barrier layers composed of materials like ethylene-vinyl-alcohol (EVOH) or aluminium, enhancing oxygen barrier properties. However, these additions may compromise the recyclability of the pipes.
In agriculture, the use of plastic oxygen barriers is prevalent, especially with the transition from upright silos to bunker silos and piles. Polyethylene, a common plastic in agriculture, is not an effective oxygen barrier due to its microscopic holes. To address this, multi-layer oxygen barrier plastics have been introduced, incorporating polymers like nylon (polyamide) to improve oxygen barrier properties. However, these plastics may face structural issues, become brittle or rubbery, and have limited recyclability.
The effectiveness of plastic oxygen barriers is quantified using oxygen permeability coefficients (OPCs), which indicate the oxygen transmission rate per unit surface area of the material. Different polymers exhibit varying oxygen permeability due to factors such as cohesive energy densities, free volume, and crystallinity. While some plastics offer superior oxygen barrier properties, it is important to consider their potential impact on the environment, especially when introducing non-recyclable materials into barrier layers. Overall, the selection of appropriate plastic oxygen barriers in construction requires a careful balance between performance and sustainability.
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Plastic oxygen barriers in agriculture
Plastic oxygen barriers are crucial in agriculture, particularly for silage, to reduce spoilage and extend the shelf life of produce. The effectiveness of these barriers depends on the type of plastic used and its oxygen transmission rate (OTR).
Polyethylene (PE), the most widely used polymer in packaging, has a high oxygen permeability, making it a poor oxygen barrier. However, it can be combined with other materials to improve its barrier properties. For example, multi-layer films with a thin layer of ethylene vinyl alcohol (EVOH) or nylon (polyamide) core surrounded by polyethylene can significantly enhance oxygen barrier capabilities. These films utilise a "layer-by-layer" technology, creating a tortuous path for oxygen molecules to pass through.
The number of layers in these multi-layer films can range from five to nine, with additional "glue" polymers required to bind certain materials like EVOH and nylon to polyethylene. While these complex structures increase manufacturing costs, they provide superior oxygen barrier performance. The optimal films balance high resistance to oxygen transmission with long-term flexibility to prevent the film from becoming brittle over time.
Other methods to improve the oxygen barrier properties of plastics include biaxial orientation processes, which increase the toughness, stiffness, and barrier protection of polymers like polypropylene (PP-BO), polyethylene terephthalate (PET-BO), and polyamide (PA-BO). Additionally, aluminium foil laminates are commonly used in food packaging to protect oxygen-sensitive products. These laminates provide a barrier to moisture, gases, and light while maintaining the desired shape and stiffness.
The oxygen transmission rate (OTR) is a critical factor in selecting plastic oxygen barriers for agriculture. A lower OTR indicates superior oxygen barrier performance, with values under 15 cc/m2/24 hr considered effective in blocking oxygen ingress. This standard test can help purchasers make informed decisions about the quality of oxygen barrier plastics.
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Plastic food packaging and oxygen barriers
Plastic food packaging is essential for extending the shelf life of products and protecting the health and safety of consumers. Oxygen barrier packaging is particularly important for preventing food spoilage. The oxygen transmission rate (OTR) is a key metric used to measure the effectiveness of different packaging materials in preventing oxygen permeation.
Traditional materials such as glass, metal, and plastic bottles provide varying degrees of protection against oxygen. Metal packaging, for instance, offers superior safety and the longest shelf life by preventing the ingress of microbes, light, and oxygen. However, these traditional materials may not always meet market demands for product visibility and protection.
To address this, advancements have been made in plastic packaging to create transparent high-barrier films. This involves using multilayer nanolayer technology, where films with alternating EVOH and adhesive nanolayers provide oxygen resistance, and polyethylene nanolayers offer moisture protection. The manufacturing process includes rapid water quenching to maintain transparency while preserving barrier properties.
Additionally, improvements in the barrier properties of plastic can be achieved through biaxial orientation processes, resulting in enhanced toughness, stiffness, clarity, and barrier properties to water vapour and oxygen. Multi-layered, laminated, coextruded, coated, and metallised films are often used to meet the diverse barrier requirements of food packaging.
There are also specific plastic materials designed for high oxygen barrier capabilities. For example, PET HB PE is a high-oxygen barrier material suitable for liquids, sliced cheese, and cold meats. PET HB PP is another option that provides excellent oxygen barrier properties, along with resistance to aroma, liquid, and water vapour.
Overall, advancements in plastic packaging technology have led to the development of effective oxygen barrier solutions, ensuring food preservation and extended shelf life while also meeting market demands for product visibility and sustainability.
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Improving the oxygen barrier properties of plastic
The gas permeability of polymers is a critical factor in the performance of plastic packaging. Small gas molecules like oxygen can easily diffuse through polymers, causing issues for products. This is where oxygen barriers come in.
Oxygen barrier properties of plastic can be improved through a variety of methods, including:
- Biaxial Orientation: This process increases the toughness, stiffness, and barrier properties of polymers such as polypropylene (PP-BO), polyethylene terephthalate (PET-BO), and polyamide (PA-BO). Biaxial orientation enhances the clarity of the plastic and improves its resistance to oil and grease, in addition to providing better protection against oxygen and water vapour.
- Multi-Layered Laminates: Using multiple layers of different materials, such as foil, paper, and plastics, can improve the oxygen barrier properties of packaging. For example, a multi-layer container with an EVOH barrier layer between polypropylene co-polymer skin layers can prevent oxygen ingress while maintaining structural integrity.
- Nanomaterial Applications: New composite membranes utilising nanomaterials have shown superior stability, plasticity, and barrier performance. These membranes effectively block gas permeation, including oxygen.
- Ultrasonic Seals: Ultrasonic seals, such as those used in potato chip bags, allow less oxygen permeation than traditional heat seals.
- Strategic Material Selection: Innovations in paper-based barrier packaging have shown that strategic choices in materials can transform porous substrates into high-performance oxygen barriers, providing effective protection for oxygen-sensitive liquids.
- Active Oxygen Barriers: A multilayer film with an active oxygen-scavenging layer between passive barrier layers can effectively remove oxygen and extend the lifetime of the scavenging component. This design improves mechanical properties and transparency while reducing contamination risks.
- Coextrusion: For plastics like PET, coextrusion can improve processability and enhance oxygen barrier properties.
The Worst Oxygen Barriers in Plastic
Plastics with poor oxygen barrier properties are typically those with larger pore sizes, allowing for easier oxygen permeation. Low-density polyethylene film, for example, has higher oxygen permeability than other plastics. Additionally, traditional polypropylene containers may struggle with maintaining oxygen barrier properties while preserving structural integrity.
Overall, the key to improving oxygen barrier properties lies in the selection of appropriate polymers, utilisation of multi-layered designs, implementation of advanced sealing techniques, and exploration of emerging technologies like nanomaterials and active oxygen-scavenging systems.
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Frequently asked questions
Polyethylene, the type of plastic found in everything from grocery bags to garbage cans, is not a good oxygen barrier. It has a high oxygen permeability, meaning oxygen molecules can easily pass through it.
Oxygen permeability is quantified by oxygen permeability coefficients (OPCs), which indicate the amount of oxygen that permeates the unit surface area of packaging material per unit of time.
Traditional packaging materials such as glass containers, metal cans, and laminates (e.g. paper laminated with aluminium foil) provide a proper barrier to oxygen. In the case of metal cans and glass containers, they can be regarded as almost completely impermeable to gases.
