The Paperboard Coating Renaissance

Although coatings on paperboard have been used for many years, the industry is experiencing a renaissance due to additional coating requirements and corresponding innovative solutions. New requirements are spurring the need for sustainable coating sourcing, enabling the use of more recycled or bio-derived content; disposal solutions, including coatings that are recyclable, repulpable, compostable, and/or biodegradable; and production of coatings that do not contain chemicals of concern that can migrate into food or the environment. Here, we’ll delve into the exciting advancements in different coatings and technologies used for food-contact packaging, offering a glimpse into the promising future of paperboard coatings.

Coating Material Solutions

All paperboard used for food and beverage packaging is coated to enable printing, adhesion, and sealing and to provide adequate barrier properties essential to protect food. The process of applying coatings, whether derived from fossil fuel or biological entities, is a complex one. It occurs after the paperboard is made and calendared, and the coatings are applied as either extrusion laminations or as dispersions. This intricate process involves a wide range of materials, from common fossil-fuel-based coatings like styrene acrylates, synthetic latex, polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET), to numerous bio-derived polymers and starch-, clay-, cellulosic-, and lignin-based coatings.

A select few bio-derived polymers have shown promise (Sand 2023). However, despite being reported in the literature since the late 1980s, bio-derived coatings often exhibit unviable and/or inconsistent barrier properties, require complex adhesion agents and/or multiple-stage coatings, and can come at a higher cost. They represent significant scale-up and sourcing challenges to be suitable for paperboard production. Importantly, coating technology for paperboard is a highly advanced applied science, with a complex array of binding, pigment, and dispersion agents from visible to nanoscale. Differing aspect ratios can alter resultant coating properties.

Coating Material Challenges

The words “mono-material” and “plastic-free” are commonly used to describe coated paperboard that is neither a mono-material nor plastic-free. However, these words define how much and what coating can be applied to paperboard if the paperboard is to be considered recyclable. In the European Union (EU), a mono-material designation is required for the paperboard to be recyclable, and to be considered a mono-material, EU paperboard must have non-cellulose content lower than 5%. As a general guideline, paperboard is considered repulpable or recyclable if non-cellulose content is less than 2% in the United States.

However, test methods for the detection of non-cellulosic packaging vary to the extent that the different methods will reveal different percentages of non-cellulosic material. Further, test methods for recyclability include yield and post-recycled paperboard properties in the United States and not in the EU.

Critically, specialized pretreatment can enable the recycling of paperboard containing non-water-soluble coatings such as PE and PET. After treatment, paper and other separated materials are separately recycled. The Confederation of European Paper Industries has harmonized the EU test method (EN 13430), while the Fibre Box Association and American Forest & Paper Association (AFPA) have published a joint voluntary standard in the United States. In addition, a working group within the Technical Association of the Pulp & Paper Industry is developing a test method slated to be finalized in 2024. Legislation such as Extended Producer Responsibility and the soon-to-be updated Federal Trade Commission guidelines that will define allowed environmental claim labeling will continue to alter requirements and assist in defining recyclability in a more pragmatic manner in the United States. Companies such as Huhtamaki, which makes ICON paperboard, a new water-based barrier coating that is repulpable and recyclable, are placing QR codes on their containers to explain the recyclability of the coated paperboard to consumers.

With regard to plastic-free designations, since most coatings are moldable and polymeric in nature, this wording is problematic. Paperboard coated with fossil fuel–free coatings is readily achievable using bio-based PE (bPE), bPP, bPET, or polyethylene furanoate. The focus on plastic-free applies to many fossil- and bio-derived polymers and addresses the need to ensure that noninherent microplastics do not enter the environment or our food supply via paperboard. If paperboard is used as a solution to address recent regulations prohibiting plastic for produce packaging, paperboard must also be plastic-free.

Achieving 0% of some polymers is challenging because many, such as PE and PP, are made solely of carbons and hydrogen that can be produced as byproducts. Increased attenuation in measuring the presence of these plastics is another significant challenge. Importantly, testing is essential to ensure the veracity of plastic-free claims. And to be viable in different markets, the level for non-cellulosic and plastic-free in the packaging must be low.

A good example is Asia Pulp & Paper’s (APP) Foopak Bio Natura paperboard brand, which is certified by Flustix as plastic-free and is below 0.75% plastic. “This aligns with more stringent EU member states that require non-cellulosic levels below 5% and complies with “plastic-free” requirements, EN 13430 recycling, and EN 13432 compostability,” says Ian Lifshitz, APP’s head, sustainability and public affairs, The Americas. “The value in using an aqueous coating to obtain the lowest level possible in non-cellulosic and ‘plastic-free’ is that the paperboard is suitable for many markets.”

Coating Performance

Characteristics to assess in coatings include mechanical properties relating to forming, such as stiffness and heat-sealing water vapor and oxygen barrier properties, and water adsorption or other substances. Modified Cobb, wet rigidity, and seam integrity testing is commonly tailored for actual applications. For example, if a coated paperboard is to be used with hot chocolate, testing will be performed using hot chocolate for the length of time that the hot chocolate is intended for use with the cup. For direct food contact, coated virgin and recycled paperboards need to comply with 21 CFR 176.170 and 21 CFR 176.80, and recycled paperboards are specifically regulated in 21 CFR 176.260.

Foopak extrusion PE lamination operation in Indonesia. Photo courtesy of Asia Pulp & Paper

Application of Coatings

Extrusion laminations and dispersions are used to apply coatings to paperboard. The traditional extrusion lamination method provides a distinct layer on the paperboard substrate and excellent heat sealing, water vapor, and oxygen barriers than do alternative methods. Coatings composed of PE or PET are commonly used on paperboard to package products that require a high moisture barrier, such as frozen foods or produce. However, when the coating is applied to 16- to 20-point paperboard, the non-cellulosic layer within the composite structure is generally higher than 5% and unviable in most recycling operations. Further, extrusion lamination is often associated with more so-called “stickies” (i.e., adhesives or coatings from recycled paper) that are not recycling-compatible because they become lodged in papermaking equipment after they fragment into small non-water-soluble particles. This dictates the composition of coatings for paperboard destined for recycling.

Dispersion of binders and pigments within an aqueous matrix allows for much thinner coating layers. Many paperboard suppliers and converters, including APP, Huhtamaki, and Stora Enso, employ aqueous dispersion to ensure that non-cellulosic compounds are below the 5% threshold. Dispersions are aqueous coatings and essentially pH-balanced oil-in-water emulsions with kaolin clay, platy clay, calcium carbonate, and binders/pigments such as styrene-acrylate, styrene-butadiene, ethylene acrylic acid, polyvinyl acetate, or polyesters. These components with the aqueous dispersion can be nano-sized and have defined aspect ratios to create specific properties. The recent WestRock 20200232164A1 patent highlights the use of commercial clays and binders to formulate innovative coatings for beverage cups. Aqueous dispersion typically provides lower heat seal strength than extrusion emulsions, but they can be heat sealed at much lower temperatures due to the lower glass transition temperature (T_g) of the dispersant.  Further, dispersed coatings rarely delaminate but are more susceptible to abrasion than extrusion lamination coatings.

Dispersions can also be applied just before food contact. By coating paperboard just prior to use with food, more coatings are viable, can be selected for each specific application, and can be thinner, allowing for ease of recycling and plastic-free compliance. This essentially decouples the coating requirements in the paperboard supply chain with the need to protect food.  Regarding the supply chain requirement, coated paperboard can develop pinhole cracks when exposed to creasing, scoring, cutting, high temperature and humidity storage environments, multiphase distribution systems, and printing operations. If a coating on a paperboard exhibits pinholes or cracks, then it does not protect the food or inhibit transfer/migration of oil from French fries in a paper carton or water from juice in a paper cup, for example.

Many coatings cannot withstand this process, and thus, the paperboard can be coated for a specific food just before filling. This has the advantage of eliminating damage to the coating and sealing interfaces between carton side walls and end flaps and sealing the base of paper cups or trays. In this process, coatings are atomized into erected containers to meet specific needs and time of use. This also allows for more coating options. For example, aqueous dispersion has a high moisture content that prevents its use in some tray manufacturing operations. However, aqueous dispersion can be applied post-tray forming, before the actual contact with food.

Likewise, the clay within dispersions can flake off during carton forming and distribution. However, a thin dispersion can be applied once cartons are formed to eliminate the flaking. For example, French fry cartons need only provide an oil-and-water vapor barrier for 1 hr, while coatings for cereal boxes (without the inner PP/PE bag) must inhibit lipid oxidation and moisture loss/gain for 18 months in North America and less in the EU. Because the base paperboard structure can be the same, this offers opportunities for paperboard to gain greater economies of scale in producing fewer varieties of paperboard and allows for more specific coatings to be applied based on the food being protected, and regional recyclability or compostability requirements.ft