BY: MATTHEW TABASSI
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.
• 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 Matt Tabassi
