Extrusion Coating Lamination

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.

 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 Matt Tabassi

  

Loss Prevention

 

On the surface

This article originally published on Plastics in Packaging Magazine, June 2013

On the surface

As plastics material science continues to advance retorted packaging technology, Matthew Tabassi* reviews the future role of laminated films

Today’s consumers are seeking flexible packaging formats that protect the flavor at the same time as the quality of the contents.

As a result, retortable and microwaveable flexible packs are top of the list of solutions and these are often produced by bonding two or more flexible materials. Usually each laminate film will have its own specific properties. To achieve the best result, there are some aspects that need to be considered before deciding what type of lamination will suit a particular application.

In conventional laminating processes, aluminum is considered a major component, usually bonded to a coextruded layer of polyamide (PA), PET or polystyrene film. Since aluminum is a perfect barrier but does not seal and has low puncture resistance, it will often be used in combination with another layer that makes up for its weaknesses.

For example, a structure with good sealing might include a PET film laminated to an aluminum foil and polypropylene. PP will provide the heat sealing capability along with strong puncture resistance.

But today’s fast-moving food packaging industry is not looking for a conventional lamination structure. When industries are looking for better barrier properties and, at the same time, cost saving of the final product, innovation of new structures is necessary.

There are no universal packaging solutions on the market, but new laminated structures offer desirable properties for specific types of food packaging.

Retorting is a process that uses heat and pressure to sterilize and cook food in a strong, sealed package. Retort pouches are made of laminated materials such as PET/aluminum foil/mHDPE (sealing medium), or PP/ink/metallized PP/PE (sealing medium). The latter is suitable for snack food or soft drink pouches.

However, by increasing the number of lamination layers, product shelf-life can be increased. A perfect retorted pouch needs to be a barrier to oxygen and moisture, and to protect the food from light, while the outer layer should be tough and printable.

In recent years, metal cans have been augmented by retort pouches because they are lighter and use less storage space. Retort pouches are also easy to open because of their tear notch and can be reheated by placing in boiling water for just a few minutes.

Since only retort pouches not containing aluminum foil could be placed in the microwave for reheating, new retort structure developments were essential.

There are basically two forms of microwaveable package. The first involves transparent materials such as paper, glass and a multitude of plastics materials - polyethylene, polypropylene, polyester, nylon, polystyrene, and polyvinyl chloride.

These transparent materials allow microwaves to pass through the contents of the package and heat the product without interfering with the packaging materials. These packaging materials need to be compatible with elevated temperature and pressure demands.

The second type is called an active microwave package, which involves the use of materials that directly affect the product in the container. Here, susceptors are included into the materials, which absorb microwaves that in turn penetrate the packaging. This process raises the susceptor patch temperature to levels where it may then heat the food by conduction or by infra-red radiation.

The future of laminate applications is almost limitless, as the combination of different materials will meet almost every specific requirement. It does, of course, require state-of-the-art technology and know-how on the implementation, development and production of such packaging formats.

Shelf stability in food packaging and the emergence of newer packaging methods such as flexible pouches and paperboard cartons are key factors currently driving the retort packaging market.

The development and utilisation of retort packaging in North America and Europe has slowed due to competition from well-established frozen and canned product industries. In Japan, however, where these industries were small, the development of retort pouches has been stronger.

One of the materials that researchers have used as a retort substitute for aluminium foil is polyvinylidene chloride (PVdC), which enables the use of thinner films with improved barrier properties.

PVdC offers exceptional barrier resistance to oxygen and carbon dioxide and, unlike PA and EVOH, is not compromised by moisture.

For example, instead of a traditional lamination structure of ink/paper/LDPE/aluminum foil/primer/LDPE, we could use ink/paper/PVdC coating/primer/LDPE.

Further innovation has seen the use of laminating coating technology and done this process at on step as: PP/Tie/PVdC/Tie/PP//PET (or PS or BOPP).

The specific demands of a retort pouch remain consistent - high impact and shelf presence, resistance to high processing temperatures, and strong distribution challenges - but consumer goods companies continue to push the envelope by demanding increasingly convenient ways to build the packaging material.

And resin and machinery companies have risen to this challenge, developing clear packaging with the ability to be microwaved, along with added consumer benefits.

 

 

Extrusion Coating - Lamination

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.

v  Coffee sachets

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

Aluminum foil gives aroma barrier. Ionomer gives good heat seal

 

v  Butter Wrap

Al/LDPE/Parchment

Aluminum gives fold retention.

 

v  Nut Pouch

Coated paper/LDPE/metallized PET/LDPE

Print layer on paper. Barrier from metallized PET.

Sealability from LDPE

 

v  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.

v  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.

 

v  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.

v  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.

v  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.

Flextrusion’ strong commitment to research and development has provided us the tools to develop new solutions in Extrusion Coating & Laminating process to keep up with today's markets demand. In Flextrusion Sdn. Bhd., we are partner with our customer through development to innovation.

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

 

What do you know about Retort packaging?