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Hot Melt Coating Laminating

HOT MELT EXTRUSION, COATING AND LAMINATING

 

Modern Hot Melt Application for Coating and Laminating

Regina Reuscher, Andrea Glawe, Dr. Andreas Giessmann

Coatema Coating Machinery GmbH

Introduction

In some lamination fields the Hot melt application is going to increase rapidly. The system in comparable to the wet lamination process is able to run with high speed. An increasing application is realised for the automotive industry, for interlinings, medical textiles and other clothing applications. The coating machinery industry as well as the suppliers of special hot melt adhesives are responsible for developing of new solutions and systems for the required fields.

Important hot melt adhesives

Hot melt adhesives are thermoplastics, based on polymers that become liquid between temperatures of 80 – 220 C and solidify again by cooling down. They consist of 100 % dry substance and are applied in liquid state without using water or solvents. With respective pre-melt systems, pumps and application units (slot die, hot roller and powder scattering) an exact dosage and adjustment is possible. Due to the process only a short binding and setting time is requested in comparison with dispersions or solutions.

Advantages of hot melt coatings:

Disadvantages of hot melt coatings

  • environmentally friendly due to water and solvent-free adhesives
  • low coating weight needed
  • elimination of dryer / low energy requirements
  • not thermal stress of substrate
  • high production speed possible
  • permanent or non-permanent adhesive coatings possible
  • change of adhesive type / cleaning of pre-melt system, hoses, die, etc.
  • limited application due to low softening point of thermoplastics
  • pot life (limited time of coating in case of reactive systems)
  • setting time (time between coating and bonding effect = storage)
  • price

 

 

Hot melts can basically be divided between different adhesives depending on the working temperature and the chemical solution like EVA, PA thermoplastics and PUR. The main differences are regarding properties and handling, which determinate the final application.

The following table shows the main properties of 3 different hot melt types in comparison.

EVA (Ethylene-Vinyl-Acetate)

PA (Polyamide)

PUR (Polyurethane)

e.g. moisture cross linking type

softening and solidification over wide temperature range

softening point higher than EVA

after cross linking no influence on temperature (heat or cold)

heat resistance up to 100 C

heat resistance up to 150 C

damages of adhesive above 150 C

sensitive against steam or water

sensitive against steam or water

boiling resistant

inexpensive

rather expensive

quite expensive

 

The machine layout and process is defined by hot tack ( pot life ), binding / setting time, initial adhesion, production speed, coating weight as well as type of hot melt.

For hot melt coatings it is important to handle the system carefully. The coating head should be positioned near to the substrate to avoid any temperature changes of the hot melt due to cold air flow. Loss of temperature can create interruptions, threads or soiling of the system and the coating effect is not satisfactory.

Hot melt application systems

The hot applicator roller system bases on a standard three roller raster applicator system, which is also used in the printing industry according to deep valley printing. It consists of three roller, one counter roller, one applicator roller and one dipping roller. In case a constant low coating weight is required, the applicator roller can be exchanged to an engraved roller. In comparison to the slot die coating and powder scattering unit, a hot applicator roller system is nearly independent of the to be coated substrate with open or closed surface. In addition a wide range of coating weight can be reached.

The hot melt in the gap between the dipping roller and applicator roller is transported by rotation of the rollers as film onto the applicator roller. The coating amount is determined by the gap between the dipping roller and applicator roller as well as the gap between applicator roller and counter roller = substrate. In addition the speed is variable to influence the coating thickness.

The integration of a lamination station a second substrate can be laminated. Working with thermoplastic hot-melts it is important that the lamination point is very close to the coating point, because a cooling down of the hot melt should be prevented due to viscosity changes in order to ensure a good and sufficient penetration into the fabric.


Figure 1: Roller applicator
   

 

The slot die system is suitable to coat continuously hot melts with a wide range of viscosity. In combination with the right “mask”, which determines the coating weight and width, a closed film > 10 g/m or a porous film < 10 g/m can be coated. The thermoplastic polymer is heated via extruder / tank melting / barrel melting equipment and pumped to the coating head. In the die itself has, depending on the working width, up to four pneumatic operated valves, which supplies the hot melt from the tank. In addition there are distributors and ducts to spread the adhesive to the die outlet equally over the width.

Figure 2: Flat die system Figure 3: Curtain coating system
   

 

The position of the slot die to the counter roller is flexible. A manual/motoric adjustment enables the movement of the die in horizontal and vertical position as well as angle adjustment. The main advantage of this “closed system” is, that the hot melt is protected against any contact with oxygen and air humidity as well as temperature lost from the pre-melt system to the application point. So the hot melt has no chance to crack or to react if they would get in contact with air humidity, which is excluded by using the slot die system.

The coating weight is determined by the pump, web speed and distance of die to substrate or web tension. With this process it is important, that the substrate has a certain stability, because of the shear rate between

Because of the closed system, this coating method via slot die is most suitable to high speed coating of hot melts.

The thermoplastic coating material as powder can be applied with a powder scattering device equally over the working width. After that, the powder needs to be melted, cooled down and/or smoothed. For the application the powder is supplied in a pre-dosing unit and picked up by an engraved dosing roller. In front of the dosing roller there is either an oscillating brush bar or a rotating brush roller, which clear the dosing roller so that the powder falls onto the substrate. The engravure of the dosing roller and its rotation speed determines the amount of powder application. The advantage of this system is the flexible working width up to 5 m.


Figure 4: Powder scattering unit

In addition this coating system is a contact free coating process but the surface of the web should be flat and not too open. Otherwise the powder falls through the substrate and no bonding effect can be given during the lamination. However, after the application of the powder onto the substrate, a heating process via infrared or flat bed lamination must melt the powder to achieve the adhesive effect. During the flat bed lamination a controlled heating and pressure effect can determine the penetration of the adhesive during the melting process.

   

 

The powder scattering system is suitable for applications where a porous and not closed adhesive layer is requested. Most of the interlinings and laminated clothing are prepared with a powder coated material.

Summary

It is possible to laminate every textile material with textiles and/of foil with one side adhesive for bi-laminate or both sides adhesive for tri-laminate in order to give a textile material special properties like stability, tear strength as well as good processing handle quality.

Used textile materials, adhesives and also the coating process are determined by the demands on the final substrate, which will be manufactured in the textile companies or to be used for laminates.

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