The manifestation of interlayer adhesives in multilayer films for use in food packaging

The article titled “Properties of Different Types of Polymers at Polymer Films (Part One)” has addressed the significance of polymer films in the packaging business as well as the uses of these films in this sector. The category of polymer films known as multilayer films is among the most significant types of films. The mixing of polymers in multilayer films serves a number of important reasons, including the protection of food products from oxygen and moisture, as well as the preservation of the quality of the materials themselves [1].

Taking into consideration the characteristics that have been discussed in this article, polyolefins are regarded as being of great utility. However, polyolefins do not have an adequate resistance to oxygen diffusion, despite the fact that these materials offer a number of advantages, including the ability to be processed, the absence of moisture diffusion, and excellent thermal and mechanical characteristics. Contrary to polyolefins, a polar polymer such as ethylene vinyl alcohol copolymer (EVOH) has a chemical structure that provides resistance against the passage of oxygen. As a result, this material is being used in place of metalized films and aluminium foils due to its capacity to be recycled and its transparency. According to the information presented in table 1, the addition of ethylene oxide (EVOH) to the middle layer of a multilayer film results in a remarkable resistance against oxygen when compared to a single layer of polyolefins, such as LDPE [1].

Comparative analysis of the oxygen diffusion rate between single-layer and multi-layer films is presented in Table 1 [2].

Analysis of the oxygen diffusion rate comparison

On the other hand, due to the fact that they are incompatible with one another, polar polymers like ethylene oxide (EVOH) and non-polar polymers like polyethylene (PE) are difficult to combine. This is the reason why it is necessary to utilise a new generation of adhesives, which is known as interlayer adhesive or tie layer, in order to create multi-layer films using this combination. The purpose of these adhesives is to generate weak adherence between the layers.

– Characteristics of the films used packaging:
In order to fulfil their roles, packaging films must:

A chemical and thermally stable environment
Resistance to oxygen diffusion and resistance to moisture and moisture
High mechanical qualities, including resistance to abrasion, impact, and variations in volume
optical characteristics that meet certain criteria, such as transparency and brightness
As was noted earlier, the primary objective of combining polymers in multi-layer films is to protect the integrity of food ingredients and to prevent them from being exposed to oxygen and moisture. Polyolefins are a crucial component in the production of multi-layer films because of their suitable characteristics and their resistance to the passage of oxygen. In spite of this, polar polymers like polyamide or ethylene oxide (EVOH) are utilised in the middle layer of these multi-layer films in order to facilitate the creation of resistance to oxygen transport. The regular structure of an ideal film utilised in the packaging film business is seen in Figure 1. The resistance of the film to both oxygen and moisture is achieved by the combination of the polyolefin layer and the polar polymer layer respectively. By inhibiting the flow of oxygen and other gases, a film like this would prevent food from going bad, and it would also be able to maintain the quality of food by preserving its aroma and moisture.

Function of the perfect film with several layers

In the food packaging industry, the function of the ideal multi-layer film is shown in Figure 1. [3]:

By comparing the functionalities of different polymers, it is possible to comprehend that the resistance of EVOH to oxygen diffusivity in 1mm is equivalent to the resistance of 10 m of polyethylene [4]. Figure 2 displays a chart that illustrates the roles of polymers in relation to oxygen diffusion resistance. It can be seen from the picture that the majority of polyolefins, according to the chemical structure of their molecules, have a poor function against oxygen. In contrast to these materials, the majority of polar polymers, such as polyvinyl alcohol (PVA), ethylene vinyl alcohol, polyvinyl chloride (PVDC), and polyamides, have a highly effective function against oxygen transport. After taking into account the circumstances of the process, the cost, and the compatibility with the environment, ethylene oxide (EVOH) is the material that is the most suitable option for use as an oxygen diffusion resistance layer in food packaging.

Oxygen permeability of some polymers is seen in Figure 2 [4].

Polar polymers (PA, EVOH) and non-polar polymers (PE, PP, PS) are nonetheless incompatible with one another due to the fact that their structures are distinct from one another. According to Figure 3, it is possible to observe the compatibility and propensity of various materials for each other.
The ability of certain materials to be combed;

Diagram 3. Differences in the combability of various materials [5].

According to Figure 4, in order to boost the adhesion strength between incompatible layers, it is necessary to apply a thin layer of reaction or non-reaction adhesive that has dual functionality. This is necessary in order to blend incompatible polymers in separate layers. Strength of adhesion between several layers

Figure 4: The strength of the adhesion between the layers [5].

There is a variation in the adhesion process between these adhesives and ordinary adhesives, which helps to explain the discrepancy. Based on the components that make up this adhesive, it would be possible to establish a chemical contact (in the adhesive joint surface or the polar layer of polymer), as shown in Figure 5. The adhesion between the adhesive and the polymer will thus be able to be achieved by the use of chemical surface forces such as covalent and hydrogenic forces.

Figure 5.The interaction between the adhesive and the polar polymer mechanism (covalent bonding) is depicted shown 

The co-extrusion technique is widely regarded as the most effective method for the production of multi-layer films. This technique involves the extrusion of two or more polymers together, which ultimately results in the formation of a composite film. Because of this, it is possible to produce a film that serves a distinct purpose. As shown in Figure 6, there are two procedures that may be utilised in order to create the film through this process: casting and blowing.

Methods for the production of multi-layer films using the co-extrusion process are depicted in Figure 6.

Interlayer adhesion can be affected by a variety of elements which are caused by the method and components that are used in the production of the adhesive. In the manufacturing process, for instance, raising the temperature and the amount of time helps to increase the thickness of the adhesive, and increasing the amount of functionality helps to increase the amount of chemical contact. In addition, the adhesion strength will decrease for every operation that occurs at the production level and results in the orientation of sticky chains inside the product. It is imperative that the components of interlayer adhesives be built in such a way that the reduction caused by orientation is minimised to the greatest extent feasible. The effects of these factors are detailed in Figure 7, which can be found here.

Various stimuli can cause changes in adhesion, as seen in Figure 7 [7].

The adhesives in question are versatile enough to be utilised in a variety of categories and applications. Table 2 provides an example of a list of films that are acceptable for use as packaging for certain types of food materials.

Suitable packaging film for a variety of food grades is listed in Table 2.

In conclusion, in order to choose a good adhesive, we need to take into consideration the layers of a film, the required strength, the physical requirements, the cost, and the processing parameters. Additionally, interlayer adhesive components need to be constructed in such a way that they strike a healthy balance between adhesion, transparency, and flexibility.

The non-flexible films are shown on the left side of Figure 8, while the resistant films against oxygen diffusivity are shown on the right side [8].

Coace Polymer Pishgam was an industry pioneer when it came to the production of interlayer adhesives for a variety of various types of layers. Please contact the specialists at COACE Polymer if you would like to learn more about these adhesives.