Insights / Sustainability
The growing use of post-consumer recycled (PCR) content presents a new challenge for plastic containers.
Plastic bottles offer numerous advantages, including lightweight design, durability, recyclability (i.e., PET, HDPE), unique shapes, and various sizes. However, increasing calls for more sustainable packaging solutions — use of PCR content, natural ingredient formulations, hot filling to avoid the use of chemical preservatives, and lighter-weight structures — present new challenges for plastic bottles, particularly in the form of paneling.
Paneling can result in collapsed sidewalls, potential leaking, and loose or wrinkled labels. On the store shelf, these deformed bottles reflect poorly on the brand. Consumers won't put irregular packages in their shopping carts, leading to lost sales for the brand and retailer.
What Causes Paneling?
In a nutshell, the pressure differential between the bottle's external environment and the interior product contents produces paneling. The opposite of paneling is bloating, whereby the pressure inside the package increases beyond the external pressure.
Several factors can lead to sidewall paneling. They include hot filling, product formulation (e.g., raw materials, ingredients), physical forces on the bottling and labeling line, gas and moisture transmission, and changes in product elevation.
- Hot Filling. Brand owners employ hot filling to improve product flowability or control microbial activity. Heating viscous liquids like sauces and syrups enables increased filling speed and bottling line efficiency. The hot temperature of the product not only kills any microbes that might be present on the inner surfaces of the packaging, but the vacuum created in the package when the product cools helps to prevent oxidation and further spoilage. Hot filling negates the use of chemical preservatives and extends product shelf life without the need for refrigeration.
Filling plastic bottles at elevated temperatures (up to 200°F) comes with challenges. As the product cools, the liquid volume contracts, creating a vacuum in the headspace (i.e., the space above the product and below the closure) that can cause the bottle's sidewalls to collapse and buckle inward.
Because HDPE containers can generally only withstand maximum temperatures around 145°F – 165°F, they are typically not recommended for hot-fill applications. Heat-set PET can be hot-filled to temperatures of 185°F – 189° F, while PP can tolerate hot-fill conditions above 200°F.
- Product formulation. Over time, some products' ingredients (e.g., enzymes, ascorbic acid, essential oils, flavor volatiles) may interact and combine with or scavenge oxygen inside the package, reducing the internal pressure. This vacuum draws the sidewalls inward.
- Gas and moisture vapor transmission. Gas and moisture vapor transmission through the bottle's walls (i.e., inside to outside) can reduce the internal pressure, pulling in the sidewalls. Product solvents can permeate through the bottle structure, reducing the volume of the contents and internal pressure.
- Physical compression. Physical forces like rails on the bottling line, before the closure is applied, may compress and pinch the container. If the closure is applied to such a compressed — and therefore distorted container — the paneling will remain. Stacking containers before the product has time to cool properly can also lead to container deformation, especially on the lower levels of a pallet stack.
- Product elevation. A change in altitude between the filling and storage locations can lead to paneling. If a bottle is filled at a high elevation, where atmospheric pressure is lower, and then transported to a lower elevation with higher ambient pressure, the pressure difference can cause the bottle's sidewall panels to collapse inward.
How To Prevent Paneling
Depending on the probable cause of the paneling, brand owners can prevent or mitigate paneling by adjusting the bottle structure, material barrier properties, package components, product formulation, or process parameters.
While paneling affects all packaging, it is generally more visible on cylindrical bottles. Switching to an oblong or oval bottle helps to hide paneling problems as the container tends to flatten out rather than collapse inward.
The container design may provide some remedy. Hourglass-shaped bottles resist distortion more than cylindrical bottles. Incorporating horizontal rings or vertical ribs into bottles increases the containers' structural integrity. Adding flexible sections (e.g., flexible bottom or vertical panels) to the package helps to control the volume reduction.
While adhering to food safety requirements, pre-cooling the product before filling and capping reduces the pressure differential during hot fill and therefore the vacuum forces that lead to paneling.
Increasing the wall thickness is often not a viable option because it negates the weight and cost advantage of plastic versus glass. Even thick-walled plastic containers are susceptible to the forces of the vacuum.
Adding barrier additives, such as EVOH, as a layer or dispersed throughout the plastic material can minimize gas transmission rates. Properly applied labels (e.g., shrink sleeve, in-mold) can further enhance the barrier properties of the packaging.
Fluorination provides a permanent chemical barrier to PE and PP bottles, preventing vapor from escaping or oxygen from entering the container. In-line fluorination during the blow mold process can help mitigate the negative side effects of batch fluorination.
Other methods to protect against paneling include:
- Adding a vented liner inside the closure allows the package to "breathe" and equalize the pressure without introducing contaminants or leakage.
- Purging the headspace with (gaseous) nitrogen and reducing the headspace can minimize the oxygen available for the product to scavenge and combine with.
- Purging the headspace with liquid nitrogen (LN) before capping removes the atmospheric oxygen. LN dosing also pressurizes the bottle during filling when the liquid nitrogen warms and evaporates.
- Reformulating the product to remove the raw materials responsible for oxygen depletion.
- Eliminating excessive physical forces and heat on the filling and labeling line.
PCR Content
Incorporating PCR content in plastic packaging extends material life cycles, saves energy, lowers greenhouse gas emissions, reduces the carbon footprint, and contributes to the circular economy, according to the U.S. Plastics Pact. While PCR content in plastic packaging offers numerous environmental benefits, it can present challenges in terms of material properties.
Three major factors are driving the demand for PCR resin in plastic packaging:
- Consumer preferences for sustainable packaging.
- Consumer packaged goods (CPG) companies' commitments to increase the recyclability and recycled content of their packaging.
- State legislation requiring increasing amounts of recycled content in plastic packaging. Five U.S. states mandate minimum PCR content requirements in plastic packaging.
It's important to note that while PCR may contribute to paneling, the recycled content is not inherent to the deformity. As described above, multiple factors can cause paneling of the sidewalls.
However, PCR resin is not identical to virgin plastic resin. While food-grade PCR is the most uniform, recycled plastic generally comprises several polymer grades. As a result, PCR possesses slightly different properties (e.g. melt flow, intrinsic viscosity, etc.) than virgin plastic, requiring more control by the molder.
In some cases, PCR may exhibit reduced tensile strength or reduced thermal stability. When exposed to heat, PCR may shrink slightly more than virgin material. These variances may make PCR more prone to paneling during hot-fill and ambient-temperature bottling.
Plastic blow molders understand these material differences and can control their processing parameters and optimize the blend of PCR and virgin materials. Additionally, hot-filling PET containers require specific heat-set PET preforms. Preform vendors offer bottle preforms with varying percentages (e.g., 10%, 20%, and up to 100%) of recycled PET (rPET) suitable for hot-fill products.
Even regulators understand the challenges of using PCR plastic in hot-fill applications. For example, New Jersey currently requires that single-use plastic beverage bottles sold in the state contain, on average, at least 15% PCR content. In 2027 and every subsequent three years, the percentage of PCR content in beverage containers will increase by 5% until reaching a maximum threshold of 50%. Hot-filled plastic beverage bottles are capped at 30% PCR content.
We Help Unpack Your Sustainability Potential
At Berlin Packaging, we partner with our customers to unpack their full sustainability potential by creating packaging solutions that optimize sustainability, functionality, performance, brand impact, cost, and material availability.
Collaborating with qualified suppliers, we source a wide range of PCR materials, some with no upcharge, and work with our customers to seamlessly integrate PCR into their product portfolios and advance their sustainable packaging goals. In 2023, we sold more than 900 metric tons of PCR plastic, reducing the amount of virgin material in our customers' packaging.
We team up with our customers to ensure their packaging is in the right format, material, size, etc., and meets the needs of their target consumers. For hot-fill products, our comprehensive review process covers packaging materials, bottle design, filling and capping operations, cooling, quality control procedures, product formulation and pH, shelf life, downstream secondary packaging, and finished product testing.
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