Improving Container Performance

Polyethylene terephthalate (PET) has become a popular material to package food and beverages such as carbonated soft drinks and pasta sauces. Both brand owners and consumers are drawn to the structures made from this material due its clarity, weight, recyclability and shatter-resistant properties. PET use is also pushing boundaries with more research and development (R&D) breakthroughs below:

• Higher hot-fill temperatures make PET containers viable for new applications;

• Improved barrier capabilities extend the shelf life of products;

• New profiles, shapes and geometries enable product differentiation;

• Lighter packaging meets performance goals; and

• The use of up to 100% post-recycled (PCR) PET content minimizes environmental impact.

With more R&D, it is becoming increasingly important for brand owners to understand the impact of structural or production modifications made to packages and products. It is also crucial for them to measure barrier performance and understand its impact on product stability and shelf life.

Using the appropriate instrumentation and testing protocols will therefore enable brand owners to understand, define and measure the effect of oxygen, water vapor and carbon dioxide permeation on their products and packaging structures. This enhances the development of approaches that will help companies meet their goals and brand equities.

Brand owners and consumers are drawn to the structures made from polyethylene terephthalate (PET) due its clarity, weight, recyclability and shatter-resistant properties.

Testing gases

In order to successfully navigate the marketing, technology and material boundaries, brand owners need to understand and measure the performance data of their packaging’s reaction to permeation and/ or transmission of oxygen and carbon dioxide (CO2) – the two gases that have the greatest impact on product degradation and shelf life.

• Oxygen permeation testing

The oxygen transmission rate (OTR) is the constant rate at which oxygen permeates through a film or a package at specified temperatures and relative humidity. Ambient air comprises of about 21% oxygen and 79% nitrogen, with small concentrations of other gases such as carbon dioxide and argon. Oxygen is required in most chemical and biological reactions that create rancid oils, molds and flavor changes. In order to prevent the atmospheric oxygen from compromising on the product, packaging applications need to provide ways to reduce product exposure to the gas and extend the shelf life of oxygen-sensitive products.

The instruments that measure film barrier and the finished package or container OTR are usually designed and operated based on the ASTM D 3985 standard. The most common test conditions are 73°F and 0% relative humidity (RH). Conceptually, dry nitrogen gas enters a chamber that contains a test film, which acts as a membrane that separates this stream from an oxygen stream. The resulting partial pressure difference would cause the oxygen molecules to diffuse through the film to the stream of nitrogen.

The film barrier, package or container (that is mounted on a package/ container fixture) determines the rate of oxygen permeation. This is continuously measured by a coulometric sensor in the outgoing stream of nitrogen. The test is complete when equilibrium, or a steady state, is achieved; indicating a constant amount of oxygen into the stream of nitrogen (see Figure 1).

Figure 1: This diagram shows the testing of a film barrier and the oxygen transmission rate (OTR) of a finished package or container.

• Water vapor permeation degrades active ingredients

Water vapor transmission rate (WVTR) is the steady state rate at which water vapor permeates through a barrier at certain temperatures and relative humidity. PET bottles and containers are designed to keep the active ingredients in food and beverages from degrading. Without protective packaging, products would gain or lose moisture quickly, until they are at equilibrium with the environmental RH. At this point, the active ingredients of the product would be degraded. The instruments used for measuring WVTR in PET containers are designed to operate consistently under the guidelines established by the ASTM F 1249. Dry nitrogen gas is released through a chamber where a test film acts as a membrane that separates the dry gas stream from a ‘wet’ nitrogen stream. The partial pressure difference enables the water vapor to permeate through the fi lm to the low pressure area. The PET barrier will determine how much water vapor is transferred, as it is being continuously measured by an infrared detector in the outgoing dry stream. The test is complete when equilibrium, or steady state, is achieved.

Figure 2: Water vapor transmission rate (WVTR) is the steady state rate at which water vapor permeates through a barrier at certain temperatures and relative humidity (RH).* The percentage of RH is determined by the application needs.

• CO2 permeation testing

Manufacturers find it challenging for PET bottles to contain CO2 as it can escape into the atmosphere through the bottle walls, cap or seal. As carbonation is critical to maintaining flavor and other sensory perceptions, as well as the shelf life of products, the rate of CO2 loss is of great interest to manufacturers and bottlers.

A CO2 analyzer can be used to record the transmission rate of filled bottles, coupled with a non-dispersive infrared sensor that measures the concentration of carbon dioxide in a test chamber. There are various sizes of removable test chambers (capture volume) to accommodate different bottle dimensions. The sample should be ‘conditioned’ before testing. This reduces error as stretching or bottle-creeping, and the absorption of CO2 into the bottle walls typically occur within five days after filling.

To conduct a test, a filled bottle is inserted into the appropriate capture volume, which is mounted and clamped onto the CO2 analyzer. The parameters of the bottle, capture volume and product are used to configure a transmission rate test. The amount of CO2 that permeates out of the bottle is reported as a transmission rate in cubic centimeters per day (cc/day). This rate is quantified using the volume parameters, test time and CO2 concentration.

A projected shelf life can be quantified using the measured transmission rate and user-specifi ed parameters. The shelf life of a product is determined using the measured transmission rate of the bottle, internal volume, initial gas volume and expired gas volume. Shelf life is defined as the amount of time (in days) that is required for the initial gas volume to degrade to the expired gas volume due to CO2 permeation through the bottle walls.

Conclusion

When packaging companies do not use the appropriate testing procedures, they would usually select barriers that exceed cost and environmental objectives. The permeation testing of PET bottles and food containers will enable brand owners to measure the performance of their barrier materials and help them bring to market packages that effectively reduce oxygen, water vapor and CO2 permeation into their products.

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