The Panelling of Plastic Bottles and Jars

The panelling of capped plastic bottles and jars is caused by the creation of a pressure differential between the inside sealed environment of the pack and the external ambient air pressure.

The internal pressure of the bottle or jar is lowered which results in the external pressure exerting a force which causes partial inward collapse of the container wall. The resulting distorted appearance of the product pack can create the impression that the product is defective, can deter consumers and ultimately damage the brand.

Different causes and suggested remedies for panelling

Temperature effect

Filling and capping products such as sauces at an elevated temperature (such as during hot filling of certain products) causes the air inside the container to expand. The subsequent cooling of the air to ambient temperature in the sealed pack environment causes the air to contract and reduces the internal pressure inside the container, causing the walls of the pack to sink inwards.

Remedies include:

  • Allowing the product to cool to ambient temperature prior to applying a closure.
  • Using a specially designed bottle or jar with structural indented panels or ribbing that limit the sinking of the container walls.
  • Creating a heavier bottle or jar with thicker more robust walls that are more resistant to sinkage. However, excessively thick walls can reduce the ability to squeeze the container to dispense products such as sauces and ketchups.
  • Increasing the fill content of the container to limit the airspace above the product. This means there is less air to contract when the temperature is lowered and so minimises the pressure reduction inside the container.
  • Changing the shape of the bottle. Oval shaped bottles with large flat surfaces can often withstand panelling more effectively than cylindrical bottles. The visual appearance of panelling is also less obvious on flat surfaces compared to cylindrical surfaces.
  • Applying a ‘breathable’ vented liner inside the closure on the bottle or jar. This has the effect of enabling air and pressure equalisation between the inside and outside of the container.

Product interaction effect:

Certain products, over time, can interact with the oxygen in the air inside the sealed container.This can have the effect of reducing the amount of oxygen inside the pack, reducing the internal pressure and causing subsequent panelling.

The same remedies as previously explained can be considered, alongside some specific suggestions for this phenomenon that include:

  • Purging the headspace in the bottle or jar with nitrogen to eliminate the oxygen prior to sealing the container.
  • Consider reformulating the product to remove the raw materials that are responsible for the oxygen depletion.

Gas transmission effect:

The walls of plastic bottles and jars are permeable to gasses that are enclosed within the product and container. These gases, over time, can pass through the plastic walls from the inside to the outside of the containers. This process can reduce the internal pressure within the bottle or jar and induce the panelling effect. Different plastic materials possess different gas transmission characteristics, with PET providing the most effective barrier to gas permeability.

The same remedies as previously explained can be considered, alongside some specific suggestions for this phenomenon that include:

  • Consider using a multi-layer, co-extruded bottle or jar that provides a greater barrier to gas permeation and also provides enhanced product shelf life, whilst enabling hot-filling up to 90 degrees Celsius.
  • Change the resin used to manufacture the container to reduce gas permeability.
  • Fluorination of the container can also improve the barrier properties of the bottle or jar. This process involves the surface treatment of the container with fluorine gas.