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  • 03 Feb 2012

Understanding barrier films for flexible packaging

Tarquin Crouch (pictured) and Rob Carter of Alliance Packaging help to guide narrow web converters in ensuring the films they purchase meet the requirements of the products they are there to package

Good design is critical in persuading consumers to purchase, but the repeat purchase will only be made when the promise of the design is matched by the quality of the product inside the packaging. The quality of the product inside the pack can only be guaranteed over the period of the shelf life by selecting a good-quality packaging film and matching the capability of the film used to the protection requirements of the product. Get that step right, and the product will reach the consumer in the condition intended by the manufacturer.
So, what general information does the printer need to understand? Shelf life is the period in which consumer acceptability is maintained. The quality of most products changes over a period of time through changes to color, texture and flavor. Shelf life is shortened by a number of factors, including: moisture, where a gain or loss can affect the texture and make a product go stale or go soft, or it can act as a catalyst to degradation in products containing fat; oxygen, which causes oxidation of products that contain fat or oil, and can assist in color changes and the onset of mould; light, another catalyst for oxidation that causes rancidity and oils and fats to break down, causing odors; aroma/odor, with aromas desirable smells and odors unwanted contaminating smells, and highly flavored foods likely to lose aroma compared to bland food that are likely to absorb odors; and the environment, where secondary packaging, warehousing, transportation, distribution, temperature and in-store situations can all affect the shelf life of a product.
Products requiring protection
The list below is not a complete list of food types, and products within these categories can vary in terms of their needs, but as a general guide:
• Bakery – Degraded by loss of humidity, however water retention can also cause a loss of crispiness, so perforated films are used for crusty products
• Biscuits – Generally degraded by humidity uptake, leading to loss of crispiness. Complex products with chocolate and cream are degraded by oxidation, odor loss or uptake
• Chocolate confectionary – Can be degraded by: moisture/humidity, which causes sugar bloom; odor, often coming from the inks or the use of recycled board; insects, if poorly sealed; and light/oxygen, which causes rancidity
• Dehydrated food and beverage – Generally, these products have a very long shelf life and therefore require a very high barrier to water, aroma and oxygen (when gas flushed)
• Pet food – When wet, pet food is degraded by oxygen, light and loss of aroma and/or contamination by odor
• Sugar confectionary – When uncrystallized, products tend to absorb water. When crystallized, they tend to lose moisture
• Chips and snacks – Degrade through rancidity, which necessitates a barrier to oxygen and light, and loss of crispiness, which is solved by providing a moisture barrier
Barrier measurement
Unless there is good seal integrity there is no point in spending money on a barrier film. Whether the sealant is a coextruded heat seal layer, a coating, cold seal, CPP or PE, it must hermetically seal the inside of the pack from the outside.
As narrow web printers move into flexible packaging they must understand sealant layers and make sure they understand how the film will be sealed, and which side seals to which side, so that they can purchase the appropriate films. Most flexible packaging films are heat sealed, but not all sealant layers are compatible. Generally sealant layers will always be compatible with themselves.
But, do not assume that side A will seal to side B. Ensure you understand how the film will form into the package, and where there is an A to B seal make sure the two sides will actually seal, and there is no ink or lacquer preventing this.
Moisture is shown as WVTR (water vapor transmission rate) on most data sheets and is the steady state rate at which water vapor permeates through a film at specified conditions. Most plastic films have low WVTR. Although, for instance, OPP is significantly better than BOPET, the difference to food degradation is small. The relevant measurements are normally expressed in gm/m2/24hrs, and conditions of 37.8 degrees C and 90 percent relative humidity. Increase WVTR with humidity and also temperature or pressure rises.
Oxygen is shown as OTR (oxygen transmission rate) on most data sheets and is the steady state rate at which oxygen permeates through a film at specified conditions. There are very significant differences between the OTR of different films.
OTR is normally expressed in cc/m2/24hrs and conditions of 23 degrees C and 0 percent relative humidity. OTR can be higher in a humid environment, and becomes faster as temperature or pressure increases.
The rate of aroma or odor transmission cannot be directly measured as it varies for different contaminants, but it can be proven. Only coatings or certain coextruded barrier films (generally those with an EVOH layer) provide odor and aroma protection. Metalized films alone do not provide an aroma or odor barrier.
Light transmission is the percentage of incident light that passes through a film. Opaque and metalized films provide a good light barrier.
When combining two films to make a laminate, the actual barrier created will be a combination of the barrier of the two films. This is a complex issue as the converter actually needs to have the equipment to measure the above properties. Ideally, you should try to work with one film that provides the barrier, with the other being used just to protect the inks. If you do not have the equipment to measure the barrier, you should claim only the barrier performance of the web that has the best barrier for that property. However, with caution and experience, the WVTR of a laminate can sometimes be calculated, providing the WVTR of each substrate is known. This is done by using the ‘reciprocal of the reciprocals’ formula.
Barrier film ready reckoner
There are many variations in the properties of films from different manufacturers, but it is a useful starting point. Films with an improved barrier are generally more expensive and more difficult to obtain. So, it is important not to over-specify the barrier required, just to play safe.
As shown, a substantial barrier can be achieved through coatings or metalization. Film manufacturers have been applying these for years. However, converters do have the opportunity to apply their own barrier to film. Originally, these coatings were the same as those applied by the main suppliers of coated film, Shiner, ExxonMobil, Innovia, Treofan and Toray, and were PVOH or PVdC-based. Now however, a new type of coating is available which allows gravure and standard flexo converters to achieve the highest levels of barrier in a flexible coating that is not subject to flex cracking. These are based on mica or silica particles that provide a difficult path for the water vapor or oxygen particles.
These are currently all water-based, so are not suitable for application on UV flexo presses. This will change in time. The only word of caution is that the converter then becomes responsible for the barrier and the protection of the food in the package, and there is a risk of very high claim levels if the film underperforms.
Generally, brand owners should know the barrier properties required to protect their products. They have food scientists working for them, so if you are intending to make a speculative approach, or propose a new structure, you will need to do your research carefully, and have an idea of what you are proposing and why it will work.
Look at the structures already being used in the market place; generally they are used for good reason. If the product is successful it means that the packaging is probably fulfilling its purpose and preserving the product, allowing it to be enjoyed with the texture, flavor and aroma with which it left the factory.
As UV flexo printers move into food packaging, we can expect film manufacturers to offer new products. In the near future, you can expect to see films that provide a barrier to keep the migratory additives in the inks away from the food. This will suppress all concerns from the brand owners, and allow printers to use the full range of flexible packaging applications.