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Multi-layer films laminated and co-extruded with PA
Multilayer films are produced by both lamination and co-extrusion, and both techniques can be used together to produce the material. Currently, the production of high-barrier multilayer films containing polyamide is dominated by the co-extrusion technique. The greatest advantage of multilayer films is economics, resulting from the fact that the final product is obtained directly from granulates.

Multi-layer laminated and co-extruded films with polyamide
Both extruded polyamide films, called "cast", generally produced in the thickness range from 20 to 60 μm, as well as unidirectionally and bidirectionally oriented, with smaller thicknesses, most often 15 and 12 μm, are quite rarely used as ready-made materials. , and are usually a semi-finished product for lamination.

In modern packaging technology, however, PA is now more common in the form of layers with thicknesses ranging from a dozen to even more than 50 μm, in multi-layer co-extruded films.

It should be noted that PA is still one of the basic components of flexible multi-layer materials, either co-extruded or laminated, especially for thermoforming.
It is advisable to distinguish the concept of a multi-layer foil obtained by lamination from the concept of a multi-layer co-extruded foil.
The term laminate is justified in relation to a multi-layer foil, produced by combining, usually gluing, previously made foils. Traditionally, the number of foils or other materials, such as paper or Al foil, included in the laminate is equivalent to the number of its layers. For example, the PA/PE material obtained by gluing foil is a two-layer laminate.
However, a co-extruded foil with the same thickness of PA and PE layers, in which the thickness of the bonding layer does not have to be significantly greater than the thickness of the adhesive layer in the above-mentioned laminate, is considered a three-layer foil.
Taking the above into account, in the case of a multi-layer film produced by co-extrusion, it is better to use the term co-extruded film than to use the term laminate, reserving the term laminate for materials obtained by combining ready-made films [1].

Compatibility and permanent bonding during co-extrusion occur in the case of polymers with a similar structure.
Co-extrusion, e.g. of polyolefins, which are present in most multi-layer materials due to their good weldability, with polar barrier polymers such as EVOH and PA, requires the use of bonding layers.
Standardly used as universal binding polymers are plain or grafted copolymers of polyolefins with maleic anhydride. On the side of the olefin layer, there is mutual diffusion of similar polymers, and on the side of the barrier layer, there is a chemical reaction of the anhydride group with the -OH group in the case of the EVOH copolymer or with the -NH group in the case of PA. There are entire families of binding resins based on various polyolefins and characterized by different flow rates.
Polymers with a high ability to form secondary bonds are also used for specific connections, e.g. zinc ionomers for bonding PA and PE-LD [2].
The appearance of PA/PE laminates suitable for deep forming initiated the implementation of a process, still of dominant importance today, in which packaging, either in vacuum or in inert gas, and later also in a modified atmosphere (MAP), was combined with thermoforming foil constituting the lower part of the packaging.

The earliest PA6 or PA66 used for this purpose are still in use, although in some applications copolyamides and mixtures of PA6 with amorphous PA, which are particularly suitable for deep forming, are also used. Polyamides, belonging to engineering plastics, are characterized by high mechanical strength, and the films made from them are suitable for deep forming. These features also determine the importance of PA as a component of multi-layer films, although, as shown in Table I, which ranks films made of various materials according to oxygen permeability, there are available materials (EVOH, PVDC and PAN) with significantly higher barrier properties compared to gases [3].
TABLE I. Barrier properties of plastic films related to a comparable thickness of 25 μm
Foil type Oxygen permeability at 20°C (65%) Water vapor permeability at 38°C (90% r.v.)
  cm3/ m2.24h 0,1 MPa g/m2.24h
EVOH
PVDC
PAN
OPA
PA6
PET
PVC (unplasticized)
PETG
OPP
PE-HD
PE-LD
PP
PC
PS
EVAC		 0,4
1,2
4,0
35
40
80
150
390
1800
2000
4000
4000
4000
6000
10000		 50
0,5
80
160
40
40
30
40
5
5
20
12
150
100
70

As previously noted, PA-based multilayer films are produced by both lamination and coextrusion, and both techniques can be used together to produce the final material.
Currently, the production of high-barrier multilayer films containing polyamide is dominated by the co-extrusion technique. The most important advantage of multi-layer films produced by co-extrusion, compared to laminates, is the economy resulting from the fact that the final product is obtained directly from granulates in one production process. Both economic and ecological benefits include the possibility of using very thin layers in the co-extrusion technique, unavailable in the previously implemented lamination technique.
Even before the appearance of co-extruded films, the general rule when using PA/PE laminates was that the maximum forming depth in millimeters should not exceed the thickness of the PA film in micrometers.
Currently, both as a result of the use of copolyamides and mixed polyamides and the use of structures in which PA layers constitute the inner layer, as well as the frequent separation of the PA layer in the same material, the forming depths are much greater, compared to the previously adopted general rule.

Although the barrier properties of PA towards gases are not the highest, they are sufficient in many applications, both in the case of vacuum packaging and in modified atmosphere, and this material is much cheaper compared to the EVOH copolymer, which has the highest barrier properties.
The analysis of the barrier properties of plastic films listed in Table I indicates the importance that the EVOH copolymer can play in multilayer materials.

The importance of the EVOH copolymer is even greater because the VC/VDC copolymer used previously as a barrier layer against gases is controversial and is not accepted in some countries. Although the cost of EVOH copolymer is high, a layer of 5-8 μm is sufficient to achieve the required barrier properties. The possibility of using such thin layers determines the importance of high-barrier multilayer structures obtained by semi-extrusion.
Compared to PA, the barrier properties of the EVOH copolymer towards oxygen are approximately 100 times higher. However, to achieve such a high barrier, it is necessary to cut off moisture access to the EVOH copolymer, to which it is not resistant and loses its barrier properties. This effect is achieved by using external layers that adsorb moisture, e.g. PA or water vapor barrier layers, which may be PE layers.
One of the significant advantages of producing multi-layer co-extruded films, from the point of view of environmental protection, is the possibility of obtaining high barrier properties at significantly lower thicknesses, compared to laminated multi-layer films, which reduces material consumption. PA is often irreplaceable in multilayer structures.
This is due to the exceptionally good adaptability of the PA layer to deep forming and the strength of the formed shapes, even in places of maximum thinning.
Where necessary, an increase in the barrier properties of PA towards gases is achieved by increasing the thickness of this layer.
Water vapor permeability in multilayer materials is usually determined by the polyolefin layer.
The standard material for the sealing layer, both in laminates and in multilayer films co-extruded with PA, is most often PE-LD.

 For special requirements regarding:
• higher strength of welded joints,
• higher resistance of joints to elevated temperatures,
• higher weld strength in the molten state ("hot tack"),
• reduced lower welding limit, ethylene copolymers (PE-LLD, PE-ULD, EVAC), ionomers, as well as metallocene polyethylenes and polyolefin mixtures are used respectively.
In order to obtain resistance to elevated temperatures, polypropylene is used for the weldable layer.
Multi-layer barrier co-extruded flexible films are most often produced in the form of three-, five- and seven-layer structures.

The structures of co-extruded barrier films, as well as laminates containing co-extruded films, are illustrated below:
Examples of three-layer structures
• PA/adh/ PE-LD
• PA/adh/PP
• PA/adh/Ionomer

Examples of five-layer structures
• PE/adh/PA/adh/PE-LD+EVAC
• PP/adh/PA/adh/kop. PP
• PE/adh/EVOH/adh/mPE
• PA/adh/PA/adh/PE-LD+EVAC
• PA/EVOH /PA/adh/Ionomer
• PET/glue/PA/adh/PE

Examples of seven-layer structures
• PE-LD/adh/PA/EVOH/PA/adh/PE-LD+mPE
• PET/glue/PE/adh/EVOH/adh/PE-LD


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