TEN LAYER DIE TECHNOLOGY
Brampton Engineering Inc.
ABSTRACT
Today, more than ever, the packaging sector is facing increasing demands from
consumers for improvements in packaging materials including advances in barrier
properties (including gas, aroma, etc.), mechanical requirements and chemical
resistance; puncture resistance; optical clarity; hot tack strength and sealing range; and
thermoforming properties.
In the past decade, a number of trends have emerged in blown film technology in
order to meet the aforementioned performance goals-none more influential than the shift
to multilayer co-extrusion. This paper will look at the some of the factors driving the shift
to multilayer co-extrusion from five-layer structures into seven, nine and even 10 layers,
as an alternative to lamination or extrusion coating. The ability to achieve unusual
properties or resin cost savings, as well as to meet customer demands for new film
applications, has driven blow film processors beyond the five layer structures. The
continuous improvement of the modular (stackable) streamlined die technology has
contributed to the accomplishment of multi-layer blown film structures.
INTRODUCTION
The 1960s saw the development of processes for making multilayer blown films in
one operation and their application in production of LDPE double-layer films.
Concurrently, heavy-duty bag film with improved properties was also introduced into the
market in North America. However, as moisture and gas barrier requirements became
increasingly more demanding, particularly in food packaging applications, and could no
longer be met by a single raw material, the 1970s, 80s and 90s saw further
developments of co-extrusion considering other materials such as PA, EVOH, PVDC,
PP, etc. Not too long ago, only a few companies around the world made five-layer blown
film; and very few made six layers and beyond. Presently hundreds of processors make
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film in five layers and a good number are making six, seven, eight or even nine- layer
films.
Ongoing developments in die and blown film technology have led to a dramatic
increase in blown film co-extrusion, and paved the way for the introduction of beyond
five-layer structures, which I will refer to as “multi-layer co-extrusion”. The process of
multilayer co-extrusion enables the film producer to combine the properties of different
polymers, cost effectively, into a single structure, without the associated costs for
processes such as coating or laminating.
Multilayer co-extrusion offers a number of benefits including the ability to use
different polymers and polymer combinations to produce an improved product; increased
versatility in being able to run a variety of film structures; and the ability to split layers
and use less expensive materials resulting in cost savings.
FIVE-LAYER STRUCTURES
The development of high barrier film using EVOH, or Nylon-EVOH required fivelayer structures. For example, a five-layer blown film structure typically being used for
meat or cheese packaging comprises 35% Ionomer, 10% tie, 10% EVOH, 10% tie, and
35% LDPE. The structure can be made with four or five extruders in a five-layer die. The
Ionomer serves as the heat seal layer of the structure while the vinyl alcohol provides
the barrier layer and the LDPE gives the structure bulk and integrity. An adhesive is fed
in two thin layers to hold the incompatible skin and core layers together.
In this case, the moisture-sensitive EVOH is protected by the outer polyolefin
layers when dry, EVOH can be an excellent oxygen barrier, but when exposed to
humidity, the polymer's oxygen permeability increases. The oxygen barrier layer's
effectiveness would be decreased if it was not encapsulated by a water vapor barrier
layer. The Ionomer layer is used to balance the structure to add strength and primarily to
provide a good heat seal. This five-layer structure can be made more economically as a
six-component film with the same barrier properties and heat seal ability. This is
accomplished by dividing the Ionomer skin layer into two components-one less
expensive than the other-while still maintaining the strength of the film as well as
reducing the cost. Comparing the cost per pound of film for the two structures, the five-
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layer film has a material cost that is 15% to 20% higher than the six-layer film which
amounts to approximately $50,000 a month on a typical 40 in. wide blown film line.
MODULAR DIE DESIGN
Co-extrusion was limited to five layers until 1991, when Brampton Engineering
Inc. introduced their Streamlined Co-extrusion Die (SCD) or modular (stackable) die at
the NPE >91 show in Chicago, and promptly revolutionized the process of co-extrusion.
In the SCD, all layers have approximately the same wetted surface area because
polymer distribution takes place on the face of the disk rather than on the outside of a
cylinder. The entire melt flow passage from the extruder adapter to the exit of the die is
highly streamlined with no sharp bends, which cause dead spots. Consequently, the
primary advantages of the modular die are its' low residence time, streamlining and the
small volume of molten polymer inside the die.
The design of the spiral distribution system is one of the most critical parts of die
design. Co-extrusion dies are often required to process several different resins through
the same layer, which makes de design of the distribution system more difficult. The
design of the spiral distribution system is generally done with the assistance of computer
simulation programs. The design of modular dies is a result of basically re-orienting the
distribution system. Rather than distributing the polymer melt in the typical annular flow
area, which is concentric with the axis of the die (Figure 2), the distribution is performed
in a plane which is perpendicular to the die axis. This type of design makes each
distribution system, in terms of surface area, independent of its position. Thus this
system can substantially reduce the surface area to which the polymer is exposed in
each layer. The wetted surface areas of two five-layer dies (conventional and modular)
are compared in Figure 3. Both dies were designed to produce the same five-layer
structure. The modular design has a lower wetted surface area, which means a lower
residence time, lower pressure and lower degradation potential.
BEYOND FIVE-LAYERS
The move to multilayer co-extrusion beyond five layers has had a dramatic impact
on various market sectors such as food packaging where barrier film can be tailor-made
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using different combinations of polymers in order to provide more shelf life for a product,
improve barrier properties, and reduce costs. For example, Holmes Packaging in
Rotorua, New Zealand reports that being able to tailor-make film has enabled the firm to
prolong the shelf life of packaged cheese products by three years. Prior to 1991, the
major problem with the design of conventional blown film co-extrusion dies was the
material stagnation region, which caused polymer degradation. In addition, because the
outer layer in a cylindrical seven-layer die, for example, had a polymer/metal interface
six to 10 times larger than the corresponding surface area of the inner layer, the number
of layers in co-extrusion was effectively limited to five. With new multi-layer die
technology Holmes Packaging that had first considered five-layer for their barrier films
started successfully a seven-layer line, but expanded soon to nine layers. With the type
of films they have developed they are entering markets as far away from their home
base as Argentina and Chile.
In addition temperature isolation between layers permits to process materials at
the best processing temperature. For example, adjacent materials could run at
temperatures up to 120 F (59 C) - or more; so EVOH with a processing temperature of
440 F (227 C) can be sandwiched between two layers of polyamide at 500 F (260 C)
with reduced risk of degradation. Also consider that another very important achievement
is that a structure containing PA/EVOH/PA, with nylon only 5 to 7% of the total film
thickness, we create a “super-barrier” film. No tie is required. And this sandwich reduces
oxygen transmission 95% compared with a five-layer film with a 20% nylon layer.
Figures 4 to 6 show how the structures of multi-layer films can be modified to save
money on materials and improve the properties of the film. This leads to the 10-layer
die technology, which can be appreciated looking at Figure 7. The following are some of
the improved features of this 10-layer film:
Three layers of nylon for less stiffness - more flexibility
$ Super barrier including the PA/EVOH/PA sandwich - splitting nylon layers
$ Ionomer combined with EVA for improved sealing
$ Tie layer for proper adhesion
$ PA for improved seal bar release
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$ LLDPE for bulk
$ Nylon for formability and to avoid flex crack
$ Nylon for moisture barrier
$ EVA to save cost (vs. ionomer)
I would like to point out also that in a ten-layer die a processor could also run
structures with less layers: from three to nine layers, with improved flexibility.
As a result of the introduction of the SCD, processors have been able to move
beyond the previous limit of five-layers and have acquired six, seven, eight, and nine
layer structures. The next step is 10 layers. Introducing additional layers to a blown film
operation can provide versatility in a number of ways. For one thing, having many layers
allows the blown film line to produce a number of different film structures. As a result,
one blown film line can be used to make a number of specialty films. Running a variety
of structures on one line also becomes more practical with product changeovers
becoming faster as the number of layers is increased. This is due to the flow passage
size decreasing in individual layer modules as more modules are added while the total
flow is kept the same.
Another advantage of introducing additional layers is that it provides the processor
with increased adaptability to a changing market. Consumer demand for packaging is
constantly evolving, and having a large number of layers gives the film producer the
capability to adapt to future demands and eliminates the need to order new capital
equipment to meet changing market requirements.
Clearly, Brampton Engineering has proven the advantages of multilayer co-extrusion.
MATERIALS
The performance of a multilayer barrier co-extrusion is dependent on the barrier
characteristics of the individual materials and where they are placed. A multilayer barrier
structure generally consists of a relatively expensive barrier resin co-extruded between a
less expensive structural resin. In most cases, the barrier resin restricts the transmission
of oxygen into the package to prevent the food from spoiling. The structural resin is
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generally a polyolefin and is primary used to add strength to the package and protect the
barrier layer in the presence of moisture. In cases where the barrier resin does not
adhere to the structural resin an additional adhesive layer is used.
Gas transmission, which is the rate at which a substance will pass through a film,
is a critical consideration when designing barrier film structures. The rate of gas
transmission is a function of flux (rate of diffusion of a substance); diffusivity of a
substance through a solid; concentration of gas on either side of a solid; and width of the
solid.
A multilayer die is designed with the emphasis placed on the most hard-toprocess material. Heat sensitive materials, such as EVOH and PVDC that require short
residence times demand short passages with small cross-sectional areas in order that
the velocities remain relatively high.
On the other hand, polyolefins are generally more viscous and more stable
allowing for the more open flow channels that are commonly used to keep operating
pressures low. A flow passage designed for a high viscosity polyolefin will, most likely,
have too high a residence time for a heat sensitive polymer. Conversely, processing a
high viscosity polyolefin through a flow passage designed for a heat sensitive material
will cause very high backpressures and restrict the output. A compromise will result in a
die that will process the polyolefin at below its maximum output and will require more
frequent cleaning when processing heat sensitive materials.
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CONCLUSIONS
Looking ahead, film structures will continue to become more complex in response
to ongoing packaging demands and market applications. In addition to providing an
improved product, increasing the number of layers in a structure can provide enhanced
versatility and cost savings.
To be successful in the future, the packaging supplier will have to have versatile
equipment that can accommodate a multitude of potential structures at low production
costs. Multilayer co-extrusion could be the answer.
REFERENCES
1.
Wybenga, W.J., Brampton Engineering Inc., Why Produce Five-Layer Film on a
Seven-Layer Die, 1996
2.
Perdikoulias, J., Wybenga, W.J., Brampton Engineering Inc., Designing Barrier
Film Structures, 1989
3.
MacEwen, S., Waslowski, J., Brampton Engineering Inc., An Eleven-Layer Blown
Film Line: Outlining the Advantages of Multilayer Co-extrusion, 1995
4.
Perdikoulias, J., Brampton Engineering Inc., Annular Co-extrusion Die Design,
Tappi Journal, August 1992
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