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Multilayer Milk & Beverage Films

 

 


 

When dealing with food packaging, the film used must be designed to satisfy a number of criteria at all stages of the film's useful life.  During production, the film processor must ensure the film stands up to the mechanical stresses imparted on the film, such as temperature stresses during extrusion and physical stresses when winding.  For the food processor, the film must meet government food safety approval standards for its barrier and compositional properties.  In addition, the film must also have acceptable seal-ability, it must be suitable for sterilization, and must be processed at a packaging speed fast enough as to not hinder production.  Once the product is packaged and leaves the food processor's plant, the packaging must stand up to the abuse of transport, including effective impact and puncture resistance, and adequate barrier properties to prevent contamination.  Finally, a long shelf life for the product when at the retail store is a must.  All of these challenges require careful consideration when designing an effective packaging film, especially to keep costs low.


Coextrusion

 

The processor's best tool for designing such a film without losing its competitive edge is coextrusion.  Coextrusion is the process where two or more polymers are extruded together to make a composite film.  The result is a layered structure that reaps the cumulative functional benefits specific to each of its layers.  In other words, this process allows for polymers with distinct functional properties such as abuse resistance, gas and moisture barrier, bonding, and seal-ability to be combined to form a single multi-functional film.

 

With the price of resins constantly on the rise, it is in the processors best interest to produce films with as little disposable materials as possible, and like so many other types of packaging, as thin as possible.  Through well planned coextrusion, high-performance barrier films can be produced in a cost-effective manner by optimizing (minimizing) the amount of costly barrier and tie resins used and using less expensive PE resins to make up the bulk of the film.
 

Milk and Beverage Packaging

 

A typical application of Macro's 5-layer and 7-layer BXL blown film coextrusion lines is the production of multilayer barrier films used for milk and beverage packaging. Not unlike the packaging requirements for many other types of foods, to be successful these films must meet a variety of uncompromising criteria to protect the quality of the product.  For milk and beverage applications, these criteria usually always include the need to provide a barrier to oxygen, aromas and moisture, and facilitate a long shelf life, particularly without the need for freezing.


Milk packaging machinery

 

To process film on the processor’s converting equipment/filling machinery, several factors are vital including film quality, film gauge  and the film’s dimensions.


Film Quality

 

Leakers, those tiny pinholes that slowly let the package’s contents leak out, commonly originate from flex cracking and edge splitting at the folds and seals of the package.  Careful resin selection, particularly with the high-barrier resins that are less flexible, is very important to avoid these problems.


Film Gauge Control

 

An additional cause of leakers that originate from the film used in beverage packaging is deficiencies in gauge control.

 

To convert and fill the film with a minimal amount of leakers the film must be created with excellent gauge.  Common practice is the use of automatic dies and/or air rings that perform frequent, small adjustments to maintain consistent gauge across the width of the film during production.

 

Specific Width Calculations for Milk Packaging Machines

 

Milk packaging machines typically require a film width of 900mm.  With different die sizes and varying blow-up ratios (BUR), the optimal film width can be achieved.

 

With a 350mm (14”) blown film die and a BUR of 1.64:1, the layflat width of the film (flattened bubble) will be 900mm.  The flattened film may then be slit at the edges to produce two 900 mm wide films, which can be wound into rolls on two separate winders.

 

If the die diameter is 550mm (22”), a BUR of 2.1:1 is required to produce a film width equaling 1800 mm, which means two rolls of 900 mm wide film can be produced per winder.

 

Typical Milk Film Structures

 

This section provides example structures for milk film produced on 5- or 7-layer blown film lines.

 

The first example is for 5-layer film produced on a 350mm (14”) diameter die, with a total thickness of 90 microns, a BUR of 1.64:1, and film width of 900mm.

 

Layer

Composition


Thickness
(microns)

1

PE Blend

36

2

Tie

6

3

EVOH-38

7

4

Tie

6

5

PE Blend

36

 

 

 

 

 

 

 

 

 

 

 

 

The second example is for a 7-layer film structure, produced on a 350mm (14”) diameter die, with a total thickness of 90 microns, a BUR of 1.64:1, and a film width equaling 900mm.

 

Layer

Composition


Thickness
(microns)

1

LLDPE

25

2

PE + White MB

11

3

Tie

6

4

EVOH-38

7

5

Tie

6

6

PE + White MB

11

7

LLDPE

25

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Layer Configurations for 7-Layer Films

 

For the production of 7-layer milk packaging films on Macro's BXL 7-layer lines there are many layer combinations that can be used to make up the film, some of which are indicated in the table below.

 

Layer

1

2

3

4

5

6

7

Structure 1

PA 6

Tie

PA 6

EVOH

PA 6

Tie

LLDPE

Structure 2

PA 6/66

Tie

PA 6/66

EVOH

PA 6/66

Tie

LLDPE

Structure 3

LDPE

Tie

PA 6

EVOH

PA 6

Tie

LLDPE

Structure 4

LLDPE

LLDPE

Tie

EVOH

Tie

LLDPE

VLDPE

Structure 5

LLDPE

LLDPE

PP (random
copolymer)

PP (random
copolymer)

PP (random
copolymer)

LLDPE

mLLDPE

Structure 6

PP (random
copolymer)

LDPE

LDPE/HDPE

LDPE/HDPE

LDPE/HDPE

LDPE

LLDPE

 


Looking for More?

 

Macro offers blown film technology (5 – 9 layers) to process resins such as LLDPE, mLLDPE, plastomers, LDPE, tie, cPP, EVOH and Nylon to make barrier films for milk and beverage packaging.  For more information regarding the production of these and other types of barrier films, please contact us.  Macro helps its customers achieve their processing goals with a complete technology transfer, such as technical know-how to process resins, film property testing and new formulation development.

 

 

 

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