How extrusion coating laminating works?

Extrusion coating is the coating of a synthetic resin molten strip and extruding it through a flat die on a substrate. Combining two inexpensive materials to make a higher performance product adds value and utility. It is a flexible coating technique used for applying various economic plastics, including polyethylene, on cardboard , corrugated cardboard , paper, aluminum foil, cellulose, nonwoven fabrics, or plastic films.

The objective of extrusion coating is to combine the best properties of each material into a third product that can perform a function neither of the individual products can do on their own.

Potential value-added functionality might be;

• Heat sealability for packaging applications

• Improved tear or crease resistance

• Better barrier properties to water or oxygen and other gases

• Improved appearance

• Additional chemical resistance

• Improved printing or decorating ability

In extrusion coating operations often use high melt temperatures to lower the melt viscosity. This improves coating thickness uniformity and adhesion to substrate. Adhesion depends on;

• Resin melt temperature

• Resin viscosity (the reason high temperatures are used)

• Film/substrate compatibility

• Coating speed

• Coating thickness

Typically an adhesive is a material that will chemically bond to both surfaces. The adhesive could be the top layer in coextruded film that is compatible with the substrate, producing good adhesion. A water or solvent based adhesive can be applied to the substrate prior to the preheat drum. The substrate can pass through a corona or plasma treatment to allow improved wettability and adhesion of inks, coatings and adhesives.

As a result, the materials treated will demonstrate improved printing and coating quality, and stronger lamination strength.

Defects in the coating that can render the coated product useless are;

• Voids

• Pinholes

• Thick or thin coating in the machine direction

• Orange peel

• Contamination due to gels or foreign material

Voids are caused by poor adhesion between the coating and substrate, where the two materials are not properly bound together.

Pinholes are tiny holes in the coating. Pinholes may be caused by excessively high coating speeds that are drawing the polymer melt too much.

The other defects such as gels, oxidized or burnt particles and orange peel are the result of poor extrusion conditions or raw material supply.

Lamination is similar to extrusion coating with the exception that two substrates are added to each side of the extruded film.

This can be considered a three-ply process, with two substrates and a molten film. If the film is produced on a coextrusion line, it can have multiple layers.

It is essential to consider maximum winding tension for laminated structures. For that simply add the tensions for the different webs that have been laminated together and usually disregarding any coating or adhesives between the webs and apply the sum of these tensions as the winding tension for the laminate.

The individual webs need to be tensioned before they are laminated so that the elongation of the web due to web tension will be approximately equal for each web. If one web is strained significantly more than the other web, then, when they are laminated, curl problems or delamination wrinkling known as "tunneling" can occur in the laminated webs. The amount of tension should be a ratio of the modulus and the web thickness to prevent curl and/or tunneling after lamination process.

The futures of laminates applications are virtually limitless. The combination of different materials and their positive features will meet almost every specific requirement.

Applications for extrusion coating and lamination include the following:

• Film lamination

• Heat seal layer used in general packaging

• Dairy packaging

• Juice and folding cartons

• Cups

• Paper

• Foil

• Carpet coating and backing

• Food pouches

• Cheese bags

• Can linings

• Photographic paper

• Potting soil bags

• Release paper

• Frozen food containers

• Paperboard trays

• Oven-safe paperboard trays

Solid food packaging

There are many quality requirement to prevent migration of oil, grease and flavor/aroma when the coating is applied to an aluminum foil substrate. On aluminum substrates the coating also provides a heat sealable surface. When applied to reverse printed films the coating protects the printed surface.

Coating can be on one or both sides of the substrate though aluminum foils require 2 side coating. Two side coatings can be applied either by passing the web through the coating line twice or by one pass through a line fitted with twin coating systems.

Extrusion lamination involves casting the extrudate between two substrates. The extrudate then bonds the substrates together. Such laminates are often then coated on one surface either in-line or via a second pass through the line.

Aluminum based structures normally use monolayer coatings to give heat sealability. Coextrusion is reserved for barrier structures based on EVOH with no aluminum layer or combined bulk/tie layers or bulk/heat seal layers.

The major resin used is LDPE. Ionomers are used to give improved heat seal through contamination and higher bond strengths to aluminum. PP coatings give improved heat stability over LDPE. In recent years we have seen increase in use of structures based on barrier layers of EVOH or PA rather aluminum foil in order to reduce recycling problems.

* Coffee sachets

High gloss lacquer/Paper/LDPE/Al/LDPE/Ionomer

Aluminum foil gives aroma barrier. Ionomer gives good heat seal

* Butter Wrap

Al/LDPE/Parchment

Aluminum gives fold retention.

* Nut Pouch

Coated paper/LDPE/metallized PET/LDPE

Print layer on paper. Barrier from metallized PET.

Sealability from LDPE

* Powdered soup

PET/EAA/LDPE/Al/LDPE

Print layer on PET. EAA layers give good bond to PET and Al. LDPE gives sealability.

Aseptic Packaging

Aseptic packaging is defined as the filling of a commercially sterile product into a sterile container under aseptic conditions and hermetically sealing the containers so that reinfection is prevented.

Similar to any other process, final product should meet few quality criteria, such as acts as barrier to moisture and oxygen and prevents contamination with micro-organisms. Also prevents loss of flavor compounds from juices.

* Juice, Milk Cartons

LDPE/Al/LDPE/Waxed Board

LDPE gives sealability. Aluminum gives barrier to oxygen and in fruit juices prevents loss of flavor compounds.

* Juice, Milk Pouches

Oriented PA/LDPE

PA/EVOH/PA/LDPE

Polyamide gives toughness and puncture resistance with moderate oxygen barrier. EVOH gives high oxygen barrier. LDPE gives sealability.

Liquid/Paste Packaging

Acts as barrier to moisture and prevents loss of flavor compounds. Also needs to be tough and puncture resistant.

* Juice, Milk Cartons

LDPE/Al/LDPE/Waxed Board

LDPE gives sealability. Aluminum gives barrier to oxygen and in fruit juices prevents loss of flavor compounds.

* Juice, Milk Pouches

Oriented PA/LDPE

PA/EVOH/PA/LDPE

Polyamide gives toughness and puncture resistance with moderate oxygen barrier. EVOH gives high oxygen barrier. LDPE gives sealability.

In current era, the film industry continues to expand its frontier by facing challenges requiring further development and innovation in both resin and machinery design advances. This includes improvements in both monolayer and co-extrusion blown and cast film, as well as downstream conversion operations. Finally, increasingly complex structures combining lamination and co-extrusion are possibilities further extending the possibilities of plastics of packaging.

Only companies with strong commitment to research and development can provide developers with the tools to innovate new solutions in Extrusion Coating & Laminating process to keep up with today's markets demand.

References

1- Film Extrusion Manual - Second Edition, PROCESS, MATERIALS, PROPERTIES, TAPPI PRESS,

2- Future direction of lamination in retort packaging, Packaging Films issue 4-2013, By Matthew Tabassi