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Consultant's Corner: Extrusion Coating in an Adhesive Lamination World

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Extrusion Coating in an Adhesive Lamination World

By Lou Piffer, Senior Sales Engineer, Davis-Standard, LLC

 

Extrusion coating and Adhesive lamination have coexisted amicably for many years. The decision on what process to use is often made based more upon availability of equipment and output requirements, and less on process requirements. With the increased globalization of the converting industry, there has been increased interest in the viability of one process versus another. This paper will attempt to go beyond these points by investigating the opportunities and challenges a converter encounters as they move from the adhesive lamination world to extrusion coating and visa versa. These differences are highlighted in the equipment, the process, and finally, how the end product handles on a form, fill and seal machine (FFS).

 

Equipment Modification

There are many similar components shared between the adhesive lamination line and the extrusion coating system. This sharing of components permits a converter to run an adhesive lamination structure on an extrusion coating line as well as the reverse.

 

Adding Adhesive Coating Capability

You can convert an adhesive lamination line to run extrusion coating by including this capability when the line is initially purchased or upgrading the equipment at a later time.

To add an adhesive lamination capability when an extrusion coating line is initially designed, you would:

  • Increase the size of the dryer
  • Provide a cartridge coater to permit changing the coating methods
  • Include an adhesive laminator
  • Provide a separate auxiliary unwind for the adhesive laminating station (see figures 1 and 2)

 

The advantage of adding the adhesive laminating capability initially is that you have increased the flexibility of the line and maximized the speeds for both the extrusion coating and adhesive lamination process. The disadvantage is, of course, the added costs and the fact that you are tying up a relatively more expensive extrusion coating line to run an adhesive laminating structure that could be run on a less expensive piece of equipment. In addition, the hybrid line will result in an extended web length from the adhesive laminator to the winder. The extended length could cause problems if you do not have sufficient green strength in the laminate.

 

Instead of including an adhesive laminating feature initially, you could add it to an existing extrusion coating line (see figure 3). This line has been modified by replacing the dryer pull roll with a dry bond laminator, modifying the coater as needed to handle the adhesive, and providing a bridge between the single auxiliary unwind and the adhesive laminator. By modifying the line in this way you create a similar flexibility to the previous example at a lower cost. The disadvantage is that with this lower cost comes less functionality. Production speeds when running the adhesive laminate are reduced since you will have a dryer sized for a simple primer trying to dry the much greater water levels coming off an adhesive. 

 

Another disadvantage is that you have only one auxiliary unwind doing double duty. The increased web path from the unwind to the adhesive laminator may preclude running thinner foils or at least require closer idler spacing and better alignment.

 

Adding Extrusion Coating Capability

Like the previous examples, it may be possible to modify an existing line and add the functionality of the other process later. First, we look at modifying a standard solventless coating line (Fig 4). The modified line includes an extrusion station as shown in Fig 5. In this new arrangement, the secondary unwind is moved to make room for the extrusion station and pull rolls are replaced with treaters. Note that this is just one possible way to rearrange the equipment that requires less movement of the existing equipment. Since the solventless laminator has no coater/dryer, all functionality must be provided by the treaters and/or the resin coming from the extruder.

 

We next look at a traditional solvent/water based adhesive lamination line. Again, adding an extrusion coating station requires moving an auxiliary unwind and replacing pull rolls with treaters. The existing coater/dryer should be more than sufficient to handle the traditional PEI primer used in extrusion lamination structures. The revised equipment arrangement will still permit running either adhesive or extrusion laminations.

 

Running adhesive lamination structure by extrusion coating

Most flexible packaging structures can be created through extrusion coating or adhesive lamination.  Often, which method is used depends more on equipment availability than any true economic analysis between the two processes. For the balance of this paper, we will look at the opportunities and pitfalls one encounters when trying to run an adhesive lamination structure by extrusion coating.

 

Weight

When making a structure through adhesive lamination, the adhesive adds little to the overall thickness of the structure, which creates the rigidity or “stand-up ability”. That characteristic must be created by the substrate used in the structure. One advantage of extrusion coating is you can affect the thickness by increasing or decreasing the amount of resin used. By using more extrudate, one can down gauge the substrates and maintain the product thickness.

 

Since the substrates come from either a blown or cast process, they have their own converting cost. By replacing their thickness with an extrudate, more of the structure comes from a pellet as opposed to a substrate that has already been converted. This can help reduce the cost of the structure.

 

The above example assumes that a thinner substrate is available. If not, you will have a product that can be as much as 8#/rm thicker (1 – 2 #/rm adhesive versus a 7 – 10#/rm extrudate).  As noted before, this increased thickness can affect the rigidity, stand-up ability, and general “feel” of the structure. This may not be acceptable to the end user. Also, if the converter is selling by weight instead of by the impression, the end product will be more expensive to the consumer. This is why it’s important to check the availability of thinner substrates before assuming replacing an adhesive with an extrudate will improve the cost of your structure.

 

Consistency

Many adhesive converters need to run similar structures with slight variations in the weight of the substrate.  As noted before, the only way to “dial in” a specific thickness is by the thickness of the individual substrates. Therefore to supply five different thicknesses of laminates, you would have to stock five different substrate thicknesses. If you make that product range via extrusion coating, you could stock one substrate thickness and make up the difference with increases or decreases in the extrudate. This is assuming the end product in extrusion coating will have the needed barrier and mechanical properties. You also have to look at the differences in the mechanical properties of the substrate and compare it to the extrudate. To maintain properties, eliminating ½ mil of substrate may not translate directly into adding ½ mil of substrate.

 

Haze

A question posed by many adhesive manufacturers is, “How will my product look?”  This concern stems from the fact that, technically, an adhesive has no haze when properly applied. This is as opposed to a PE extrudate that will have some amount of crystallinity and, therefore haze.

 

Looking at a standard flexible packaging structure, the eye will go through an OPP film, the print, then either an adhesive or resin. Consensus among many converters (adhesive and extrusion coating) is that any haze difference would be very small and really only detectable by an expert, not by the general consumer.

 

If instead of the film/metallized film laminate described above, a see-through product is required, extrusion coating may have the upper hand.  That is because the adhesive laminate will need to use a sealant film (probably blown) that will have a much larger haze level when compared to a possible extrudate sealant.

 

Crazing

Crazing occurs on a metallized product when there is a loss in the bond between the metallizing material and the base film due to a difference in expansion. This can affect both the optics and barrier properties of the structure. Differential thermal expansion is often the culprit with cross machine, while tension control can cause the problem with machine direction differential expansion.

 

If an OPP or OPET is used on a machine with reasonably good tension control, machine direction elongation should not occur.  To reduce the potential for this type of crazing, unwind the metallized substrate from the auxiliary unwind for a shorter web path and less chance for tension related issues. 

 

Cross machine elongation is more of an issue in extrusion coating since the substrate needs to handle the temperature difference when the hot extrudate hits the ambient temperature metallized film. It is important to run the metallized film off the auxiliary unwind to have it hit the chill roll as soon as possible. Also, as long as it will not adversely affect bonds, you can move the extrudate more to the primary unwind side.

 

A non-oriented film may work fine with an adhesive lamination, but could craze when subjected to the temperatures associated with extrusion coating. 

 

Even when running an oriented metallized film such as OPP, you need to make certain the structure is designed to work with extrusion coating. This normally requires an OPP with a stable core and good skins to bond to the metallizing. If the OPP grows more than 4 to 5 percent in the cross machine direction when hit with an extrudate, crazing can be an issue.

 

Crazing and thermal stability is not an issue when working with OPET.

 

Pigmented background

When an adhesive laminator needs to provide a colored background on their product, a “flood coat” is run on one of the decks on the printer or a pigmented film is purchased. An extrusion coating converter can include the pigment directly into their extrudate.

 

Like the weight example mentioned previously, any time you can provide the feature you want at the pellet level, you are saving the conversion cost of buying the feature from somebody else  (pigmented blown film versus pigmented extrudate). Compared to a “flood coat”, printing with an extrusion coating structure with a pigmented resin can be faster and easier since you are not putting down as much ink.

 

For many years, converters were concerned that if they put pigment into their extrudate, the pigment will build up onto their die lips, necessitating line stoppages and cleaning. A solution to this issue was to run a coex structure with pigmented resin in the center and unpigmented resins on the skins.

 

This solution may be needed if the pigment can adversely affect your bond, but it is not needed in most instances if plate out is your concern. Today the vast majority of TiO2 (or other pigments) in master batches have coating to reduce agglomerates as well as moisture infiltrating into the pigment (both are sources of die lip build up).

 

Form Fill and Seal Machines – Adhesive Lamination vs. Extrusion Coating

After the product is made, the responsibility of the converter does not end after a finished roll is taken off the winder. Any structure you make has to effectively run through a form, fill, and seal machine (FFS). For this article, the author contacted many FFS manufacturers to ascertain what differences they see when running an extrusion coated versus adhesive laminating structure. Unfortunately, there appears to have been little work done in this area. Most of the FFS manufacturers say they have very little info on the structure they have to seal and they simply receive sample roll stock that they run through their lab machines to develop a list of operating parameters. 

 

So we must carry on without information from FFS manufactures. A simple review indicates a few important parameters when comparing adhesive lamination to extrusion coating:

 

COF

When running a vertical FFS machine (see figure 7), the COF of the inside layer (sealing) must be lower than the outside layer. This is so that the belts on the outside can pull the material forward, having it slide on the forming cone. For adhesive laminating converters this should not be a problem since the sealant film is most often a blown film that is made with slip agents, lowering the COF. If a converter attempts to run the product with an extrudate as the sealing film, sufficient slip levels must be included to create the desired lower COF levels on the inside layer. The amount of slip needed to accomplish this can be significantly reduced if the extrusion coating structure is made with a matte chill roll, resulting in a rougher surface finish where less of the extrudate is in contact with the forming cone.

 

Another way to play with the COF levels is not to change the slip levels or surface finish, but to change the FFS machine. Some machines have replaceable cones that are more slippery and melts that create greater friction.

 

“Triangle of Death”

This humorous term, used by a brand owner, describes an area in flexible packaging that can result in potential leakage. This potential is minimized through extrusion coating as compared to adhesive lamination.

 

When a fin seal is created and flattened out during clamping, stresses occur as material that normally do not dead fold are forced to fold. After the clamp is released, the material will attempt to normalize those folding stresses by reverting back to its pre-clamped form. This action can create a void in the package (see figure 8).

 

The adhesive laminating process deposits a very thin layer of adhesive; this thin layer cannot effectively relieve those forces. Alternatively, extrusion coating gives a thicker and softer layer. This layer can help reduce the stress relieving “rebound” effect that can lead to the triangle of death.

 

Bleed Out

Compared to an adhesive, an extrudate can help fill in the gaps between substrates, acting as a caulk as it flows when heated. This flowability can help bond, but if the clamps are not aligned properly or have too much compression, the resin can “bleed out”, causing a potential loss of bond.

 

Heat Transfer

Heat needed to clamp a structure is generated from the outside. This temperature must permeate through the structure to get to the sealing point. Plastic acts as a great insulator and can resist this heat transfer. It is possible that as one changes from a thin adhesive to a thicker plastic extrudate, the heat transfer resistance caused by the extrudate can adversely affect the clamping process by preventing the clamp heat from getting to the sealing surface.

 

Summary

Adhesive lamination and extrusion coating can coexist peacefully. It is possible to run both processes on a single piece of equipment through equipment modifications. It is important to understand that changing a structure from adhesive lamination to extrusion coating requires more than simply replacing the adhesive with an extrudate.

 

The author would like to thank the following people for their contribution to this paper.

Larry Jopko – Dow

Scott Marks - DuPont

Allen Smart – Printpack

Rob Cotton – PepsiCo

Jason Barber – PepsiCo

David Kempk – Ampacet

Michael Gensheimer - Bosch

 

Table #1

 Feature

Adhesive Lamination

Extrusion Coating

Equipment Cost

+

-

Long Runs

-

+

Short Runs

+

-

Bond Strength

+

+

Flatness

+

-

Green strength

-

+

For more information, please contact Lou Piffer at lpiffer@davis-standard.com

 
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