A review of coextrusion for flexible packaging

Modern, high-performance flexible packaging applications involve multi-layer films or coatings in order to match the desired performance and cost requirements for each specific application. Coextrusion is one of the most efficient methods to produce multi-layer films. In this presentation, we will review different coextrusion 
methods.

 

Coextrusion is a continuous processing method where two or more streams of different polymeric melts are merged together to create a multi-layered product in a single operation. The main reason to combine different polymers is that a single material cannot usually achieve all of the requirements. 
Cost is also another common driver to optimize a film or coating structure. Today this method is commonly used in many manufacturing operations and in this paper we will review some of the methods used in the flexible packaging area, for flat film, extrusion coating and laminating. In addition, a review of the principles of coextrusion will be covered, together with known issues and solutions for coextrusion. Finally, we will review an example of flexible packaging products: barrier cast film for food packaging. This application clearly shows how more elaborate coextrusion structures are used to create unique, high performance packaging. Coextrusion methods for this specific application will be reviewed as well. 

Processing Methods 
 
Extrusion coating and laminating 
 
Extrusion coating and laminating is widely used in the flexible packaging industry. A substrate like a metallic foil, metallized film, paper, or polymeric film is unwound and possibly surface treated (e.g. to improve adhesion). The substrate is coated in a nip by a polymeric melt curtain which is extruded by a die. The curtain can be either a single-layer or multi-layer coextrusion and can also be treated for adhesion if necessary before the nip. Typically, specific resins with relatively high melt-flow index (MFI) – when compared to cast film resins - are used. Highly branched polymers having good melt strength like LDPE are very common. Figure 1 is an illustration of the extrusion coating process. 
In this case it shows a coextruded curtain with a feedblock and extrusion coating specific die. The extruders are represented in the machine direction for the illustration, but they would typically be placed perpendicular to the main machine direction. 

Extrusion coating dies are very specific in the design of the internal geometry to allow the use of a complex internal deckle system. These systems are designed to adjust the coated web width precisely and to allow for potential manipulation of the flow near the edges so to minimize the inherent edge bead. 
Extrusion lamination is a process variation where two substrates are combined with the melt curtain simultaneously in the nip. There are many types of products that involve extrusion coating as one of the fabrication steps. 
Food packaging is an area where this process is very involved. For example, potato chip bags are typically made by extrusion-lamination of a print film and a high barrier film. 
Other examples of flexible packaging applications involving extrusion coating are shown in Figure 4. 

Regarding the extrusion coating / lamination process current capacity, it can be summarized as follows to the best of our knowledge: 
 From 2 to 100 layers 
 Typical width: 1 m to 4.6 m 
 Line speed from 100 to 600 m/min 
 Typical thickness: from 5 to 80 μm. 

Cast film 
 
Cast film is another process that is widely used for the production of flexible packaging where coextrusion is a critical technology. In this process, a film is coextruded from a feedblock and die combination or a multi-manifold die. The web is cast onto a chill roll with the help of electro static and air pinning for the edges and a vacuum box to aid in uniform quench across the full film width. A schematic of such a process is shown in Figure 5 and an actual picture of a cast stretch film line is shown in Figure 6.A few examples of flexible packaging products made from the coextrusion cast film process are shown in Figure 7. 
The typical capabilities of coextrusion cast film process 
can be summarized as follows: 
 From 2 to 35 layers 
 Typical width from 1 m to 6.5 m 
 Line speed from 100 to 1000 m /min 
 Thickness from 5 to 200 μm 

Coextrusion technology for flat products 
 
Coextrusion by feedblock 
 
Feedblocks are used to combine layers and create a multi-layer structure that is fed into a single manifold extrusion die (Figure 8). The function of the die is to spread this composite structure evenly across the desired width. When properly designed, the die is optimized specifically for coextrusion and will minimize distortion of the layers. Today, feedblocks and single manifold dies are used to produce from 2 to over 1,000 layers in some applications. More details on feedblock technology will follow. 

Coextrusion by multi-manifold die 
 
Multi-manifold dies are another alternative method that creates multi-layer structures. Each material is spread separately using an optimized flow channel. The different materials are then merged together slightly before the die exit and spend a relatively short amount of time together. Figure 9 represents a triple manifold die from Cloeren. 
 
Choosing between feedblock and multi-manifold die 
 
Both multi-manifold and feedblock technologies are suitable for coextrusion, however, one technology may be more suitable to a particular application. Generally speaking, feedblock are suitable for:  Processes requiring flexibility in films / coating structures: materials, layer ratios etc… 
 Structures with a number of layers greater than three: although multi-manifold dies for 5 and more are possible, they are often not practical 
 Heat or residence time sensitive polymers. 
Multi-manifold dies are generally more optimized for a given structure and can in theory achieve better individual layer distribution performance. They can be required for the following: 
 Structures involving large viscosity differences 
 Structures involving large thickness differences 
 Special structures like BOPP for example (with naked edges). 
 
It should also be noted that in some cases there is a need to utilize both a feedblock and multi-manifold die. These cases are typically very product and application specific. 
Coextrusion technology for flat product 
 
Coextrusion principles 
 
Coextrusion is possible because of the laminar nature of polymer melt flows. Owing to (i) the high viscosity of polymer melts, (ii) their relatively low density, and (iii) the relatively low flow rates polymer processing situations do not experience any turbulence 2. The Reynolds dimensionless number (R e ), which balances the inertia and viscous forces, is used to determine the transition between laminar flow and turbulent flow. For instance, with pipe flow, the critical Reynolds number is about 2,200. 

The following is the determination of R e for the practical example of extrusion through a flat die, assuming the following conditions: 

 Polymer melt viscosity = 100 Pa.s 
 Density = 103  kg/m3 
 Output = 103/kg/h 
 Die width = 1 m 
 
The Reynolds number is calculated by: 
Where p is the melt density; V is a characteristic velocity – for example the average flow velocity; D is a characterisitic dimension of the flow - for instance, the diameter in case of a Poiseuille pipe flow; and n is the melt viscosity. 

The flat die in this example is described in Figure 11 and the table below shows the calculated Reynolds numbers at different location in the die. 

The calculated Reynolds number values anywhere in the extrusion die are orders of magnitude below the critical value of 2,200. This means that the extrusion or coextrusion process is always under the laminar flow regime. Even in the case of faster polymer processing situations such as micro-injection molding, or fiber spinning, R e can reach a value of about 1, which remains orders of magnitude lower than the critical value where the regime transitions from laminar to turbulent. 

Potential coextrusion defects 
 
There are two main categories of coextrusion defects: 
 Steady state defects 
 Transient defects 
The steady state defects affect the layer distributions and are not time dependent. The transient effects are by definition affecting the coextruded structure differently at various times. 
The steady state coextrusion issues are known as: 
 Viscous encapsulation 
 Viscoelastic layer rearrangement 
 
The time-dependent coextrusion issues are also known as instabilities and there are two kinds of known coextrusion related instabilities, which affect the layer interfaces: 
 Chevron type instabilities 
 Zig-zag type instabilities 
 
Steady-state coextrusion defects 
 
Viscous encapsulation: 
 
Viscous encapsulation is a well-known phenomenon occurring in coextrusion of polymers with different viscosities 3. As illustrated by Figure 12, the polymer of lower viscosity will tend to encapsulate the higher viscosity polymer as the coextrusion flow develops. While this phenomenon is well-known and empirically understood, as of today there are no predictive models for the occurrence of this defect. 

Viscoelastic layer rearrangement
 
Another known steady-state coextrusion defect is the layer deformation under viscoelastic forces. Polymer melts having strong elastic behavior and flowing in channel sections with no axi-symmetry can result in non-negligible normal forces. These forces create secondary flows perpendicular to the main flow direction. J. Dooley has studied the secondary flow pattern in various flow channel geometries using viscoelastic constitutive equations (Giesekus)4

This author also demonstrated how colored layers of same coextruded elastic polymer (PS) can be affected by the elastic effects, which validates the existence of the calculated secondary flow patterns due to the first normal stress difference. 

Practically, viscoelastic layer rearrangement can occur for wide dies with highly elastic polymer melts. Adapted die flow channel design can minimize these effects. 

Time-dependent coextrusion defects

Interfacial instabilities
 
We are discussing here only coextrusion related defects. There are many other extrusion related instabilities, such as shark skin, gross melt fracture, draw resonance5 etc… that can also affect the quality of coextruded products. However, the intent is to discuss the flow instabilities originating from the coextrusion process. These occur at the interface and there are reportedly two kinds. 
 
The zig-zag instability
 
This type of instability has been described by W.J. Schrenk et al.6 and was found to occur when excessive stress occurs on the interface between the coextruded layers. As the output is increased, interfacial stresses increase and the aspect of the defect worsens from an “apple sauce” effect to distinct small scale waves. 

This type of instability has been the subject of more recent studies, such as the one by M. Zatloukal et al.7

Wave instabilities 
 
“Wave” or “chevron” type instabilities are another type of interfacial instability that affects coextruded products. The defect is a repetitive pattern as shown in Figure 17 and is on a much longer scale length compared to the zig-zag defect. It originates at the merging point (feedblock) between the layers and will grow as the coextrusion structure flows through the die flow channel. 
This defect is related to the stretching of the layer at the merge point and more specifically at the elongational stresses at this point. When elongational stresses are too high for a given material, then the defect occurs. 
 
Solutions for coextrusion defects 
 
Most of the coextrusion defects discussed earlier can be addressed by proper extrusion equipment design, material selection and coextrusion structure design. When defects do occur, it is important first to identify the defect carefully so the right decision is made to resolve the issue. First, it is important to have a good control of the melt quality for each polymer melt stream. By melt quality, we mean good control of melt temperature (no gradient and no variation over time) and good control of flow rates (no surging). Then if the defect happens only in coextrusion situation, it is important to identify if it is time-dependent (interfacial instability) or if it is steady over time (layer rearrangement). Once the defect is properly identified, we can find the appropriate remedy. 

Actions for viscous encapsulation
 
Viscous encapsulation is driven by “viscosity mismatch”. So an obvious solution is to try to match the rheological behavior of the processed polymers by picking the best grades. This is however not always a trivial task as the coextrusion structures can be complex. Another complexity is the rheological behavior of polymers themselves. Polymer shear viscosity is highly temperature and shear rate dependent and each polymer behaves differently. Relying on melt flow index (MFI) is clearly not sufficient. Below is an example of polymers used for a barrier film structure: 
 
PET//Tie/LDPE/Tie/PA/EVOH/PA/Tie/LDPE/LDPE/Seal
ant. 
 
In this particular case, there is a relatively good matching 
of the shear rheology for the polymers, except for the LDPE grade, which exhibits higher zero-shear viscosity and stronger shear thinning behavior than the other polymers. This particular LDPE grade could be a problem and could result in layer thickness uniformity problem. A metallocene polyethylene grade could be a better candidate in this structure for viscosity matching purpose. 

Action for steady-state layer re-arrangements (viscous encapsulation or elastic effects)
 
When the thickness uniformity of individual layers of a coextrusion structure is compromised in the coextrusion die, it is usually possible to facilitate a change in the feedblock flow passage to correct the layer deformations. This can be done whether the origin of layer thickness non-uniformity is from viscous or from elastic effects. 
Modern feedblock technology allows the manipulation of the flow for each layer. This technique is often referred to as “profiling” as it involves changing the profile of some flow passages (see Figure 19). 
 
Depending on the type of feedblock, profiling can be done by interchangeable inserts, which requires shutting down the extrusion line (Figure 20). Alternatively, with “variable geometry” feedblocks, it can be done online while running, by changing the position of a pre-profiled moveable insert (Figure 21 and Figure 22). 

Today “profiling” remains an empirical approach, based on experience. Figure 23 is an actual example of coextrusion optimization by profiling for the following structure: 
PE 1 / tie / PA / EVOH / PA / tie / PE 2 
 
The initial layer distribution was not good as the barrier polymer (EVOH) was not reaching the edges. The final layer distributions obtained by profiling feedblock inserts shows a much improved barrier structure. 

Actions for coextrusion instabilities
 
Wave-like coextrusion instabilities
 
Wave-like interfacial instabilities are understood to be related to the layer merging section and the excessive stretching of one of the layers. The following are a few considerations that can help with the issue: 
 Lowering overall output (will lower elongational stresses at the combining merging section) 
 Select polymers with lower elongational viscosity (will lower elongational stresses) – for example, minimal long-chain branching, narrower molecular weight distribution. 
 Matching velocities at the merging by changing layer ratios or the channel gaps at the merging section 

The layer stretching phenomenon is more likely to occur when the gaps of the channel prior to merging are not optimal with regards to the layer flow rates. The variable geometry feedblock was designed to allow the on-the-fly adjustment of merging gaps and hence avoid or minimize situations with coextrusion instability (Figure 24). 
 
Zig-zag interfacial instabilities
 
This type of instability is understood to be related to the interfacial stress. Interfacial stresses are usually high at the die lip land, which is usually the tightest gap within the process. To minimize interfacial stresses, the following actions can be implemented: 
 Lower the overall output 
 Increase the lip gap 
 Increase the skin layer thickness (will move the 
interface away from the higher stress region) 
 Lower the viscosity of the skin material (higher 
melt temperature, substitute material). 
 
An example of flexible packaging coextrusion: cast barrier film 
 
Over the years, the barrier film structures used in the food industry have become more complex. EVOH is one of the materials of choice when oxygen barrier is necessary. Polyolefin skins have also been standard materials as they can bring specific properties (e.g. lower cost, flexibility, printability, sealing etc…). Because EVOH and polyolefin have poor adhesion, a compatible tie layer is necessary to get a good bond between the two. For this reason, the most simple barrier structures are 5-layer structures. 

Cloeren has been heavily involved in the coextrusion technology for high performance, cast barrier films. The main coextrusion considerations for tooling that makes for a successful process are: 
 Optimized single manifold coextrusion die flow channel. 
 Die equipped with edge encapsulation option to avoid EVOH in trim material (Figure 28) 
 Feedblock with variable geometry function (Figure 29) 
 One extruder per layer approach with smaller extruders that are optimized for the corresponding layer (optimal size for output, screw design for specific material). Despite the higher capital costs, the operating costs are lower because of faster recipe change-overs / purge times, higher performance, better layer thickness control etc… 
 Adapter that optimizes extruder layout and minimizes residence time. 

Conclusion 
 
Coextrusion has been a very important technology for flexible packaging, regarding extrusion coating and lamination or even cast film. Because of the complex nature of polymer flow, many issues can occur when trying to coextrude various complex multi-layer polymer structures. Over the years, improved understanding of the phenomena have resulted in technological advances on tooling design. These solutions have allowed for improved coextrusion and greatly increased structure complexity, layer thickness control, and processing rates. 

References

[1] K.S. Laverdure, TAPPI PLACE (2009) 
[2] J.F. Agassant, J.A. Covas, “An Overview of Polymer Processing Modelling”, Advances in material forming, 37-59 (2007) 
[3] J. Dooley, “Viscoelastic Flow Effects in Multilayer Polymer Coextrusion”, Ph.D. Thesis Technische Universiteit Eindhoven, 29-35 (2002) 
[4] J. Dooley, “Viscoelastic Flow Effects in Multilayer Polymer Coextrusion”, Ph.D. Thesis Technische Universiteit Eindhoven, 89-110 (2002) 
[5] S. G. Hatzikiriakos and K.B. Migler, “Polymer Processing Instabilities – Control and Understanding”, Marcel Deker (2005) 
[6] W.J. Schrenk, N.L. Bradley and T. Alfrey, Polym. Eng. Sci., 18, 620 (1978) 
[7]M. Zatloukal, W. Kopytko, J. Vlcek and P. Sáha, ANTEC Tech. Papers, 101 (2005)