Coextruding Blown Film Needs Deeper Understanding of Resin Rheology & Die Design
More film processors are investing in lines with nine or more layers. With more resin possibilities and combinations coming into play, it’s important to learn more about material rheology and compatibility, as well as die design.
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Technology advances in blown film equipment and resins have made coextrusion a “must have” in many applications. But how many layers are enough? In a world where energy prices keep rising, the biggest challenge facing processors is to produce a structure economically with all the properties the application requires. This requires film processors to have a good understanding of the material properties and a good grasp of polymer rheology to achieve the best formulation. Processors also need a better understanding of what happens to these materials inside the various styles of dies on the market. This article is intended as an overview of topics that bear investigation.
Coextrusion has opened the door for processors to incorporate resins that otherwise would not have been possible in blown film. Low-melt-strength materials like homopolymer polyester or nylon 66, which are difficult to run on monolayer lines, can now be incorporated in coextruded structures to provide differentiated products.
In die and screw designs, the rheological properties of the materials must be taken into account as well. In coextrusion, material compatibility is absolutely critical to providing the highest quality films.
Versatility has become increasingly important in today’s ever-changing markets. The equipment you buy today should not only be intended for your current needs, but with an eye on what requirements you may face in the future. More often than not, savvy processors who invest in nine or 10-layer lines don’t necessarily need to produce nine- or 10-layer structures right away. But they recognize the advantages afforded by higher numbers of layers:
• More versatility: More flexible equipment can accommodate more structural changes, thus allowing processors to respond better to market needs.
• Better film quality: It is often noted that a higher number of layers gives a flatter film with more uniform properties.
• Lower material costs: With more layers, it becomes possible to embed lower-cost materials in the structures.
The accompanying table shows two typical nine-layer structures. The polyolefins can be polyethylenes or polypropylenes. The PE can be one or a combination of LLDPE, LDPE, mLLDPE, HDPE, EVA, or plastomers. The nylon could be a homopolymer or a copolymer. Polypropylene could be PP homopolymer, copolymer, or elastomer. With the incorporation of more engineering resins, other materials that may need to be taken into consideration are polyester, styrene-butadiene copolymers (SBC), and PS, among others. As one can imagine, the biggest challenge for the film processor is to ensure its die design engineer comes up with an optimal system to process all these materials.
POLYMER PHYSICS & DIE DESIGN
To meet this challenge, processors need, at the very least, to work with their die design engineer to make sure the structure-processing-property relationship of the polymers are considered. The most important rheological parameter to consider is shear viscosity. Polymers exhibit shear-thinning behavior. This means that the shear viscosity of a polymer decreases with increasing shear rate. The most important factors that will affect polymer viscosity are molecular-weight distribution (MWD), molecular weight (MW), and long-chain branching (LCB) architecture. LCB structure can be different between mLLDPE, an autoclave-reactor LDPE, and a tubular-reactor LDPE. As a result, it is crucial to obtain specific shear viscosity curves for each resin.
Figure 1 illustrates the relationship between MWD and shear viscosity. Generally, higher MW gives higher shear viscosity; more LCB gives a more shear-thinning behavior. Also, the narrower the MWD, the less shear thinning the polymer exhibits. For example, since mLLDPE has a narrower MWD than conventional LLDPE, the shear viscosity does not decrease as quickly with shear rate. A similar comparison exists between LLDPE and LDPE.
As a result, a 1-MI mLLDPE will experience higher pressures in the die than a 1-MI LLDPE. This is one major indication why categorizing materials by MI is a very poor way of measuring viscosity. Since MI is only one point on the shear-viscosity curve, it is possible for different materials with very different processing behavior to have the same MI.
You’ll want your design engineer to consider three important parameters in the die design process: Pressures, shear rates (or velocity of the material in the die), and shear stresses. Too high a pressure limits the operating window in terms of output, whereas too low a shear rate or shear stress increases the residence time of a polymer, which contributes to premature polymer degradation.
Shear-viscosity curves of the different polymers are very useful in helping the design engineers to factor in pressures and shear rates in the extrusion process so that the best design can be achieved. There are many equations that can be used to describe shear viscosity. Two of the most common ones are the power-law and the Carreau models.
In the power-law model:
ή = Kγ (n-1)
where ŋ is the viscosity; K is the consistency index; n is a constant; and γ is the shear rate.
In the Carreau model: