Better Oxygen and Grease/Oil Barriers, Longer Life

Packaging extends the shelf life of food. Oxygen and grease/oil barriers extend the shelf life of foods with high fat and high unsaturated fat content. And because foods with a high unsaturated fat content require both an oxygen barrier and grease resistance, oxygen, grease, and oil barriers frequently are used in conjunction with each other. Grease and oil barriers also preserve flavors, texture, and mouthfeel. When oxygen, grease, and oil barriers are more efficient, less packaging material is required to achieve the same amount of protection. Producing, processing, and disposing of less packaging material reduces the cost and environmental impact.

Material science is used to improve existing packaging materials’ oxygen, grease, and oil barrier properties and to define the coatings applied to these materials. To provide superior grease and oil barriers, recent developments provide alternative oxygen barriers while avoiding chemicals of concern, such as per- and polyfluoroalkyl substances (PFAS).

Oxygen Barriers 

To completely understand oxygen barrier improvement strategies, permeation dynamics such as sorption and diffusion must be considered. Oxygen absorbs onto materials to varying degrees. Less oxygen is available to diffuse through the packaging when the sorption rate is low. For example, metals such as steel and aluminum, and glass have a low oxygen sorption, whereas polymers have a higher and variable oxygen sorption. Materials that allow lower amounts of oxygen sorption are more effective oxygen barriers.

Fick’s law governs oxygen diffusion through materials, which occurs after sorption as the next phase of permeation. The inherent properties of packaging materials define diffusion. The degree of crystallinity correlates with diffusion; amorphous polymers have more diffusion than crystalline polymers. When packaging materials are exposed to high food processing or storage temperatures, they can undergo a glass transition, which causes them to change from a crystalline state with low permeability to an amorphous polymer with significantly higher permeability. Polypropylene packaging, for example, has lower oxygen permeability when frozen than when stored at room temperature.

On the other hand, aluminum foil packaging oxygen permeability properties do not change between freezing and refrigerating temperatures since no phase transition occurs. Generally, materials with a glass transition temperature (Tg) higher than the intended use temperature and crystalline polymers provide superior oxygen barriers. As a result, the Arrhenius relationship between Tg, oxygen permeation, and temperature must be understood to evaluate oxygen permeability.

Given the significance of the relationship between temperature, humidity, and oxygen permeation, it is worth noting that ASTM D3985, ASTM F1927, ASTM F1307, and other oxygen transmission rate standards are conducted at humidity and temperature conditions that do not reflect frozen or refrigerated storage temperatures or humidity levels. ASTM D3985 standards call for 73.4°F and 0% relative humidity, while ASTM F1927 standards call for 100.4°F and 90% relative humidity. Notably, moisture has less effect on the oxygen permeability of low-density polyethylene (LDPE) and polyethylene terephthalate (PET) than on ethylene vinyl alcohol (EVOH) and nylon. As a result, the oxygen permeability of prospective packaging must be determined at three to five temperatures within the intended storage temperature range. This ensures that the Arrhenius relationship is known and can be used to predict shelf life. Furthermore, the entire package can be measured at different temperatures and humidities using modified methods and instruments such as OxySense, a noninvasive oxygen measurement system.

It is possible to create a barrier with superior properties by using material chemistry to reduce the amount and rates of sorption and diffusion. The primary alternatives are material science and tortuosity, followed by active packaging solutions.

In terms of chemistry, functional chemical groups within polymers modify oxygen permeability. The oxygen barrier is tripled when a large methyl group is substituted for a hydrogen atom to form polypropylene instead of polyethylene. Furthermore, substituting fluorine, chlorine, or nitrogen groups for hydrogen improves oxygen permeability by 30, 60, or 12,000 times, respectively.

Layers and coatings with naturally high oxygen absorption resistance effectively reduce oxygen solubility within packaging materials. Aluminum metallization via plasma-enhanced chemical vapor deposition, for example, and glass coating with silicon dioxide reduce oxygen sorption. This reduces the amount of oxygen available for diffusion through a structure. Metallization improves oxygen permeability by 45 and 85 times in polypropylene and polyethylene terephthalate, respectively. Similarly, lowering the ethylene content of EVOH from 44% to 32% increases the oxygen barrier by a factor of 4, whereas the presence of naphthalate rather than terephthalate in polyethylene increases the barrier by a factor of 10. By increasing the crystallinity of PET to 40%, the oxygen barrier is doubled. Increasing the orientation of polypropylene and polyethylene by 300% doubles the oxygen barrier properties of film and blow molding containers due to the increase in crystallinity.

Increased tortuosity is another way to change the chemistry of a material. Using tortuosity to extend the diffusion path in polymers such as Nylon-MXD6 increases the oxygen barrier by a factor of 10. The aspect ratio and nanoparticle orientation determine a packaging material’s oxygen diffusion rate. Fillers, such as calcium carbonate or nanoparticles of the same composition as the film base or impermeable nanoscale glass or metal within a core material, are other methods for improving the oxygen permeability of packaging (Schuchardt et al. 2023).

Grease and Oil Resistance

Similarly, grease- and oil-resistant packaging is a crucial development area to the paperboard and plastics industries. Grease is typically associated with animal fat cooking byproducts, whereas oils are derived from plants. Grease- and oil-resistant packaging is required for products such as cooked or fried meats, fish, and poultry, French fries, chips, cookies, crackers, salad dressing, and certain fried foods. To determine grease resistance, ASTM D7334-08 is modified to determine grease-substrate contact angle measurements. Material science and coating chemistry govern the strategies used to improve grease and oil resistance in packaging.

A viable solution to obtain more grease/oil resistance is to switch to a material with a better inherent grease/oil resistance. Surface hydroxyl groups on paperboard fibers “soak” in grease and oil during direct contact, and common polyolefins such as polyethylene, polypropylene, and polystyrene are softened or plasticized by grease and oil, which can result in delamination or loss of packaging integrity. It is possible to transition from polyolefin to metal, glass, or a more chemically resistant polymer such as PET.

Another way to achieve grease/oil resistance is to increase the density of paperboard and strengthen the internal bonds by controlling the length and refinement of paper fibers. The increased fiber content accounts for the higher cost of higher-density paperboard. However, since tensile and burst strength are improved, a thinner, less expensive paperboard is often viable.

Paper chemistry approaches such as the use of strong acids to break existing bonds within the cellulose with wet paper fibers is also used. This results in a cellulose gel, which forms a water- and grease-resistant barrier when dried and shaped. Wetting agents and chemical compounds that bond with the hydroxyl groups of cellulose can also be used to modify wet fibers. This produces paperboard that retains a high grease resistance even when the surface is punctured. The extended drying time required for these wet fiber–altering processes lengthens the preparation time and increases energy costs.

Notably, before coating and calendaring operations that eliminate air pockets, most paperboard is composed of 50% air. As a result, because the inherent air pockets of paperboard are filled during the drying process, applying grease and oil resistance additives to dry paperboard rather than during the wet pulp stage of paper production is often more cost-effective.

The application of grease- and oil-resistant coatings composed of long- and short-chain compounds, such as PFAS, which bioaccumulate and are regarded as “forever” chemicals, is problematic (Sapozhnikova et al. 2023). The U.S. military prohibited the use of PFAS in packaging in 2020, and PFAS producers have voluntarily agreed not to sell PFAS to food packaging suppliers. While there is a lack of consistency in legislation regarding the maximum threshold for PFAS and the Intentionally Added Substance (IAS) and Non-Intentionally Added Substances (NIAS) status, the use of PFAS in food packaging is declining, and detection methods have been established (Ignacio et al. 2023, Schultes 2019).

Notably, there are other sources of PFAS in packaging other than paperboard coatings. Fortunately, grease and oil resistance can be achieved without the use of PFAS. Among the grease-resistant fillers are dispersions and clays, carboxymethyl and hydroxy- ethyl celluloses, starch, talc-polyacrylate blends, montmorillonite-polyethylene, and wheat proteins. Furthermore, coatings such as styrene-butadiene, chitosan, wax, polybutylene adipate terephthalate, polybutylene succinate, alkyl ketene dimer, and polyhydroxyalkanoates provide varying degrees of grease and oil resistance. Significantly, food-contact coatings must be regulated and recyclable.

Alternatives to PFAS often exhibit cracking of coatings. This cracking occurs during long-term storage, high temperatures, and cutting and scoring operations, which can puncture the coating and thus the grease resistance layer. Bonding coatings to cellulose reduces cracking, and compounds like alkenyl succinic anhydride form ester bonds with the cellulose hydroxyl groups. Because of the high calcium carbonate filler content, alkaline pH coatings are required. However, more effective grease-resistant coatings can be developed by considering coatings that are compatible with acidic pHs. Another way to address cracking is to apply coatings after paperboard containers have been assembled and are ready to fill. In the fast food industry, for example, paper-based sleeves can be opened and sprayed with coatings before adding French fries, allowing the coatings to fill in gaps and creases without being damaged during shipping and handling.

No Regrets

Grease- and oil-resistant food packaging solutions should not be regrettable substitutes and should be consistent with current toxicity knowledge and projected restrictions on food packaging as an indirect food additive. This is because removing hazardous chemicals from packaging, such as PFAS, allows for the safe and cost-effective reuse of recovered, recycled, or repurposed packaging.ft