Flexible packaging is exactly as it sounds; packaging that is flexible to the end-user, allowing them to bend, fold and shape it without compromising or breaking the packaging.

Multi-layer structures need more than one layer of material, or substrate, to accomplish the desired properties and are developed in two ways:
1. Mono-material. Mono-material multi-layer structures include 2 or 3 layers that are from the same family matrix. They do not need an adhesive to bond them and can be recycled because they are made from the same plastics.

2. Multi-material. Multi-material multi-layer structures have 3 or more layers (3, 5, 7, 9, 11) that are from different polarities or a mix of different substrates. They need an adhesive to bond that is determined by the processing method, but can be a tie layer, lamination adhesive, heat seal coating, for example. Recycling multi-material structures is a bit more challenging as many polymers are bonded in a same structure.

When designing multi-layer structures for flexible packaging, the entire process needs to be considered from the get-go to ensure the right balance of packaging properties and the appropriate processing and testing methods. Let’s walk through each step to uncover the key elements at each stage with flexible packaging and multi-layer structures.

The processing method for flexible packaging is determined by the machines available to produce the products. With the machines in mind, the next step is to consider the processing method. To do this, its critical to look at the end-product and think about the packaging properties at hand and how the processing will affect the product when produced.

For flexible packaging, there are five primary processes used: extrusion, co-extrusion, lamination, extrusion coating, and extrusion lamination. Often, these methods are combined in different steps to meet the unique needs of the project. Explore the table below to understand each processing method, the initial form, structures and adhesives needed.

With multi-layer structures, each layer has its own function. And when determining the required packaging properties for flexible packaging each technical requirement needs to be considered on every layer: the outer, functional and sealing layers.

Let’s discuss the key questions you should consider with each layer:

Outer layers

The outer layer is giving printability and mechanical stability to the structure. When defining your outer layers, it’s important to answer:

(1) Does it need to resist a specific temperature?
With flexible packaging, it’s important to consider what temperature the multi-layer structures can resist to avoid deformation and loss of shape. If it’s going in the oven, for example, you need to ensure the materials you are using are not going to blend or melt the polymers. If high temperatures do need to be considered, a high Tg (glass transition temperature) is needed on the outer layers that resist the external conditions.

(2) Is it going to be printed?
If the structure will be printed to display what consumers see in stores, you may consider the use of a primer or adding an extra outer layer to protect the inks.

Functional layers

Functional layers are those placed in the middle of the structure. They can give barrier properties to many different things: oxygen, moisture or light. They can also reinforce mechanical properties if needed. When planning your functional layers, consider:

(1) Is an oxygen barrier required?
Oxygen barriers are highly used properties for the inner layers of multi-layer structures, especially in the food industry. This barrier layer will prevent oxygen from entering and oxidizing food, making it last longer on the shelves and in homes.

(2) What mechanical properties does it need? 
It’s important to consider which mechanical properties are needed to resist scratching, withstand stretching, resist puncture, etc.

Sealing layers

The sealing film is usually the film that covers, protects and seals another part. It is the responsibility of giving resealable and/or reclosable properties to the package. When planning your sealing layer, consider:

(1) Will it need a sealing parameter?
Unlike the temperature resistance where high Tg (glass transition temperature) are ideal, with sealing, the lower the Tg, the less energy needed to seal it and the more money saved. This comes back again to the balancing act of finding the right Tg to increase machine efficiency, but one that still can resist the temperature needed.

Across all layers

Across each layer of your multi-layer structures, you must consider:

(1) Will it be transparent?
When you’re adding multi-materials that are not naturally compatible, transparency can be affected. Each layer needs to maintain its transparency, for example if this structure need to go throughout a thermoforming phase.

(2) Which are the final polymers to bond?
Once you have defined the polymers that will be needed in each layer, the next step is finding the proper adhesive. The selection of the adhesive is key to obtaining good performance and avoiding delamination problems of the structure.

Consider factors such as the substrates and polymers' nature, processing methods and dosing equipment to make a proper choice.

Testing methods for multi-layer structures in flexible packaging vary based on the final application. Some products only need one test, where others, with more complex structures might need to undergo several types of testing to assess their impact resistance, tensile strength, elongation ability and haze.

Impact resistance test

A common way to assess impact resistance is with the “Dart Drop” method, testing the impact resistance and strength of films by dropping specified weighted balls into the end-product. The test is commonly used for quality monitoring and comparison of films and often finds quality imperfections on recycled compound blending, formulation inconsistencies, oxidative degradation and strain.

Tensile strength test

Tensile strength measures the force that a film can withstand from external forces such as poking or tearing. This is measured in two directions, Machine Direction (MD) and Transverse Direction (TD), by pounds per square inch or PSI.

Elongation test

Elongation at Break is measured in percentages: the percent of elongation compared to the initial size when break occurs. The maximum elongation (emax), or point at break, is also called “strain to failure”. Elongation values of several hundred percent are common for elastomers and film and packaging polyolefins.

Haze

A haze test assesses the optical properties and is especially relevant for flexible packaging in food. Haze is measured as the percentage of incident light scattered by more than 2.5° through the plastic, and a haze value greater than 30% implies the material is diffusing. Therefore, the lower the haze value, the higher the clarity.

With all flexible packaging products, the products must adhere to FDA and EU regulations. But with multi-layer structures involve, it’s a bit more complex. Not only do you need to get each layer certified, you must also get the entire structure certified as well. The amount of certifications required will vary based on the products number of layers, thickness and more factors.

Consumer behaviour & end-of-life

When considering consumers in the design of multi-layer flexible packaging, it’s important to remember that most consumers do not know what multi-layer means. Many companies are already starting to mark their packaging to share information about the structure of recycled content, but this is still confusing to consumers.

For this reason, end-of-life and recyclability for this type of film is a bit more challenging. Currently, a lot of work is being done by the industry to find innovative solutions that will turn this waste stream into a fully circular model.

Today, multi-layer flexible packaging has been mostly created without considering recyclability because there aren’t many sustainable options. As a result, 2.6 million tons of multilayers films waste are incinerated or landfilled every year and more than €3 billion in value is lost annually.

Why is this?

Because multi-layer films provide unique barrier properties that are difficult to recreate with sustainable solutions. In fact, it’s very difficult to develop multi-layer structures with mono-materials because often the sustainable alternative in a mono-material structure doesn’t provide the necessary properties on its own. At this point, a multi-material structure must be considered, increasing the total number materials, layers and properties to maintain.

The question to consider in design is how can we the multi-layer flexible packaging structures back into the circular economy? That will help in the designation of packaging properties to determine what changes can be made without sacrificing on the quality of the product.