Paneling occurs when the pressure inside a plastic bottle becomes less than the ambient air pressure outside, causing the walls of the bottle to partially collapse inward. The result is an aesthetically unappealing product package with a distorted shape and badly puckered or wrinkled labels.

Research has shown that a consumer’s first reaction to a bottle affected by paneling is often, “There must be something wrong with it.” potentially leading to a loss of business and damage to the brand. Fortunately, the causes of paneling are easy to identify and just as easy to avoid.

Here are five common causes and their cures:

Cause #1 - Temperature / Pressure Change

When air is heated, it expands. Conversely, when air inside a closed container is cooled, it contracts, reducing the internal pressure and triggering the conditions that can cause paneling.

This is most noticeable in the case of bottles that are hot filled and sealed before cooling. As the product cools, the contents and/or the air in the headspace contract, creating a negative pressure within the bottle. This negative pressure can cause the side panels to suck in to compensate for the loss of product volume.

Five Helpful Solutions

• Allow the hot filled bottle to cool before sealing it.
• Increase the structural integrity of the bottle walls through design mechanics means such as ribbing or vacuum panels - think Gatorade bottle.
• Reduce the headspace in the bottle by increasing the fill level.
• Ensure your bottle is of proper gram weight and consistent wall thickness to combat paneling.
• Design the bottles so that paneling will result in the entire label being intentionally “sucked” into a flat, aesthetically appealing surface.

One of the most effective solutions to combat panel sink is to improve the structural integrity of the bottle by slightly changing the shape, from a cylinder to an oval for example. The design change adds structural integrity by replacing flat areas with a more rigid surface that can resist the pressure.

Cause #2 - Oxygen Absorption / Reaction

It is possible that oxygen in the headspace of the bottle is being absorbed in the bottle’s contents, depending on the nature of the product.

Oxygen may also be reacting with one or more of the product’s ingredients. Since oxygen comprises about 16 percent of the volume of air, reducing its concentration can significantly reduce the bottle's internal pressure, leading to bottle paneling.

Three Helpful Solutions

• Reformulate the product to remove ingredients with which oxygen is reacting.
• Purge the headspace with liquid nitrogen in order to remove oxygen before the bottle is sealed.
• Increase the fill level of the product to reduce the size of the headspace and minimize the effect of oxygen absorption.

Cause #3 - Gas Permeation through Bottle Walls

It is possible that the walls of the bottle may be permeable to the vapor of one or more of the ingredients in the product.

LDPE, HDPE and Polypropylene (PP), for example, are excellent moisture barriers but poor gas barriers, although the higher the density, the better their barrier properties. PET, on the other hand, is an excellent oxygen barrier but not a great water vapor barrier. Any vapor permeation through the bottle walls can result in unequal pressure and paneling.

Four Helpful Solutions

• Reformulate the product to eliminate the problem ingredient(s).
• Change to a resin with barrier properties that match the requirements of the ingredient(s) causing the problem.
• Consider utilizing a barrier, such as fluorination or a multi-layer barrier resin in the design of the bottle. Either will reduce the gas/vapor permeation of the product through the walls of the bottle. Fluorination also reduces the chemical permeation, weight loss, odor emission, and flavor or fragrance loss.
• Explore the use of vented liners to equalize the inside/outside pressure on the container. The liners allow gas and moisture to pass in or out of the container.

Remember, any change in resin would require elevated stability and compatibility studies in order to ensure there will be no adverse reactions between the product and container resin.

Cause #4 - Fill Processing

In some cases, machinery used to package to the product may pinch or compress the bottle before capping, reducing the volume (and the pressure) inside the container.

If the packaging machinery cannot be economically replaced, the solution may lie in redesigning the container itself. Effective design changes could include:

• Fillers should review their manufacturing and production processes for any possible mechanical situations that may be causing panel compression (i.e. areas that rails may be constricting the container or exerting excessive force during packing).

Cause #5 - Environmental Conditions

To avoid paneling issues, environmental conditions, such as altitude, temperature and relative humidity, also need to be taken into account.

When a product is packaged at an elevation of over 5,000 feet, for example, its internal pressure will be equal to the external atmospheric pressure, which is considerably lower than that at sea level. If the product is later sold at sea level, the difference in internal and external pressures can cause paneling – and this just one example of how environmental conditions can affect this issue.

Three Helpful Solutions

• Explore the use of the vented liners recommended above - allowing gas and moisture to pass in or out of the container will equalize the inside/outside pressure. This can be an excellent solution for containers that fill product at high altitude and then ship to distribution at lower geographic elevations.
• Pressurize the bottle slightly during filling to compensate for the drop in atmospheric pressure.
• Consider treating both the bottle and cap with a fluoration process that will serve as a barrier against the problem ingredient(s). (see Cause #3 above).

If your current containers are experiencing paneling or any other difficulties, give the packaging professionals at O. Berk a call. We ready to help identify the causes and find the cures.