Coating Matters | Advances in Slot Die Technology for Hot Melt Adhesives, Part 1

 

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What differentiates extrusion, hot melt, and liquid coatings? 

When it comes to fluid coating onto substrates, the equipment required falls into three areas:

  • Extrusion coating
  • Hot melt coating
  • Liquid coating

What makes these three coating methods the same and what makes them different?

Extrusion coating is typically non-tacky molten polymers being cast onto substrates with a nipped roll and a large air gap. In extrusion coating, the coating head is directly fed from an extruder. Hot melt coating is typically tackified polymer adhesives that behave similar to molten polymers but engage with the substrate in a proximity arrangement. Hot melt coating heads are fed by a precision metering pump being fed by an adhesive melter. Liquid coating resembles the proximity arrangement of hot melt coating, but the fluid is flowing at room temperature. Unlike extrusion and hot melt coating, liquid coating requires some form of curing to solidify the liquid on the substrate. Extrusion coating utilizes cooling to develop a solid film coating, while hot melt adhesive coatings solidify from cooling or curing.

In pre-metered coating of extrusion, hot melt and liquid coating, the slot die design is very similar. If the rheological characteristics of the polymer, adhesive or liquid is identical, then the internal manifold cavity of the slot die would be identical also. So why does an extrusion coating head, a hot melt coating head and a liquid coating head look so different.

An extrusion coating head has adjustable internal deckle flags that allow the operator to tune the polymer melt curtain flowing out of the slot die over the large gap between the slot die exit and the roll nip. Without this tuning capability, the extruded polymer melt curtain would neck in and have heavy edge beads. This tuning deckle reduces, but does not eliminate, these edge beads. Because polymer melts need to exit the slot die and not hang up on the lip faces, sharp edges to the lip face geometry is helpful.

A hot melt slot die operates without the gap and nip arrangement of extrusion coating. This proximity coating arrangement, where the slot die lip faces are 1-2 times the coating thickness from the substrate, allow the slot die to operate without forming the edge bead and neck-in associated with extrusion coating. However, with the narrow gaps associated with proximity coating, different lip geometries may be needed for different coating thicknesses.

Liquid coating slot dies are operated at room temperature. So while the external design of the slot die is very different from a hot melt and extrusion coating die, the internal functionality is very similar. As the fluid enters the slot die, it is controlled by the manifold cavity to exit the slot die at the same pressure, velocity and volumetric flow rate across the entire opening. Because temperature does not influence the slot die operation, the equipment can be manufactured and maintained to set gaps and tolerances. So for an ambient temperature application a fixed design for the slot die bodies and utilizing a shim creates the best operating condition.

What happens if you add heat to the fluid? At some point, the metal starts to move. With the slot die body moving, an operator needs the ability to adjust the slot die back to the precise position that is required for precision coating operation. That is why the hot melt slot die and extrusion coating die are designed with a flexible lip design. Utilizing a flexible lip design for an ambient temperature coating adds variability that will make coating less precise. However, in a heated application, the lack of adjustability will not allow the slot die to provide the precision required. So, for hot melt adhesive coating, you need to understand the proximity coating arrangement of liquid coating while adjusting the slot die similar to an extrusion coating application. With this added complication, why would a company opt for hot melt coating over liquid coating? Well, while hot melt coating requires additional process knowledge to operate effectively, liquid coating requires additional curing that can create a rate limiting step in the process. If the liquid is solvent based, there is the additional environmental concern and cost. In order to understand the details of implementing hot melt coating, more background is required.

ADHESIVE RHEOLOGY (VISCOSITY AND ELASTICITY)

In hot melt adhesive coating, we are fundamentally coating rubber bands. Unlike many liquid coatings, hot melt adhesives have both viscous and elastic characteristics. This means that the adhesive polymer melt doesn’t just change the viscous flow characteristics with shear, but the adhesive may snap back after being stressed. This added effect can create unique coating defects not seen in liquid coating.

The adhesive may act one way when being stressed and another upon relaxation. This difference, or hysteresis, of the polymer melt can create die swell, edge beads or film split upon coating. The more rheological knowledge you can have regarding the adhesive the better. And I’m not just talking about simple, single data point viscosity. A complex polymer melt requires a more sophisticated study of the flow effects. Develop testing protocols for complex rheology that show viscous and elastic behavior of the polymer under stress and shear will help an operator understand the variation in coating that may occur as lot-to-lot variation of the adhesive is presented to the coating head. Remember, the adhesive takes a tortuous path to the substrate. We need to understand the effects of the stress and design for both the viscous and elastic nature of the polymer for both the equipment and the process settings required for an economically viable coated product.

Hot melt adhesives cover a wide range of applications, but the fundamental chemistries associated with EVAs, PURs and PSAs are all based off tackified polymers applied to substrates at elevated temperatures. Hot melt adhesives provide a functional character to the substrate for temporary or permanent bonding to a future surface after converting. These adhesives can be purchased or compounded in-house. These on-site compounds should be done with a word of caution. A slot die requires a consistent feed of adhesive with minimal lot-to-lot variability. If the adhesive and feed system is not consistent, the coating operator will chase the variation for the entire time the coating system is in operation. Until the volumetric flow and pressure of the adhesive delivery system is stable and the lot-to-lot adhesive chemistry is stable, the coating will vary. In complex rheological studies, we gain an understanding of whether the viscous or elastic characteristics dominate. If there is a time dependence on the loss or storage modulus in the complex rheological study, then elasticity overpowers viscosity. With increased influence from the elastic nature of the polymer, more coating defects can occur including wrinkling, curling or voiding. If the adhesive has met the level of voiding, then the molecular structure of the adhesive has failed and the adhesive cannot be coated thinner. In other words, you have developed Swiss cheese and there is no going back.

Part 2 of this series will cover equipment design and operation.

Mark D. Miller, author of PFFC's Coating Matters column, is a fluid coating expert with experience and knowledge in the converting industry accumulated since 1996. Mark holds a Bachelor's degree in Chemical Engineering from the Univ. of Wisconsin-Madison and a Master's degree in Polymer Science & Engineering from Lehigh Univ. and a Juris Doctor from Hamline Univ. Mark is a technical consultant and CEO of Coating Tech Service LLC. He has worked in web coating technologies and chemical manufacturing operations and is a certified Six Sigma Black Belt trained in both DMAIC and DFSS disciplines. Coating Tech Service provides process troubleshooting and project management for precision coated products. Mark has extensive process knowledge in high precision coating applications including thin film photo voltaic, Li-Ion battery, and optical systems technology. Mark has been integral to new developments and technology that minimize product waste and improve process scalability.

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