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The chemical and molecular structures of new enhanced polyethylene (EPE) products are opening up new application opportunities in extrusion coating. To make the most of this new category of products, says Bernard Fehr of Dow Europe GmbH, it's crucial that converters and brand owners increase their functional understanding of resin performance attributes.
Brand owners may be overlooking a very small detail in their drive to develop differentiated packaging solutions. With so much focus on improvements in packaging aesthetics to satisfy consumer demand for convenience, quality and safety, some material science advances at the molecular architecture level have gone almost unnoticed.
This is particularly true in the case of polyolefin products for extrusion coating and lamination, with the emergence of a new generation of enhanced polyethylenes that offer greater conversion flexibility than traditional low-density polyethylenes (LDPEs). With no pun intended, the expression "good things come in small packages" might never have been truer for brand owners.
Beyond butcher paperExtrusion coating is a continuous process for applying a thin film of polyethylene to a non-plastic packaging substrate such as paper, paperboard, metal foil, polymeric film substrate or even a non-woven or fabric.
There are typically three events in the extrusion coating process. First, polyethylene pellets are made molten by the blending action of an extrusion screw inside an extruder. Then the flux of molten polyethylene is forced through a flat die to extrude a film that is only a few microns thick. Finally, the film is laid on a substrate and cooled until adhesion is tight.
One of the first flexible packaging applications for extrusion coating was the use of LDPE on butcher paper in the 1950s. Because of its processability, adequate moisture barrier, heat-sealing properties, and relatively low cost, LDPE soon became the dominant material used in extrusion coating applications.
As recently as 2003, LDPE was still by far the most commonly used polymer in European extrusion coating, accounting for 90 per cent of the 600,000 million tonne market (according to Dow estimates). However, new packaging performance requirements and higher capacity demands, coupled with environmental and economic incentives to down-gauge materials and continuously reduce costs, are now challenging converters to extrude a wider portfolio of polymers.
New polyethylenesThe good news for brand owners and converters alike is that material science advances have enabled the creation of enhanced polyethylenes that can deliver unique combinations of performance attributes and be extruded at high speed on systems designed for LDPE. These products have been designed to meet the differentiated conversion and application requirements of today's leading converters. While extruding closest to LDPE, they offer improved environmental stress crack resistance (ESCR) and water vapour transmission rates (WVTR), an enlarged hot-tack window, tailored barrier properties and high-performance sealability.
Dow has made significant progress with enhanced polyethylenes in recent years, principally by combining the high-performance attributes of its resins with INSITE technology, the company's proprietary approach for linking the science of catalyst, process and materials to address market needs.
This approach has allowed Dow to manipulate the molecular structure and viscosity of these products to achieve previously unattainable performance attribute combinations, such as low-temperature sealability and stiffness, high modulus and high puncture-resistance, as well as LDPE-like processability on high-speed lines.
Dow has discovered that the ability to adjust such performance variables can make an enormous difference to the manufacture and end-use performance of a packaging structure. These products are also potentially viable for use in different forms of conversion.
Unlike traditional autoclave LDPE extrusion coating products, which have been increasingly fine-tuned for processing on larger capacity lines, the excellent processability of Dow's enhanced polyethylenes might actually turn the tables on extrusion equipment. Whereas line speed is often limited by the processability of the material, these enhanced polyethylenes will allow conversion at the highest line speeds and with the thinnest possible web gauges, while maintaining normally experienced web stabilities. Without compromising performance, these products are designed to offer significant down-gauging of packaging structures, helping brand owners meet new sustainability targets (reduced raw materials in packages) and cut logistics and handling costs (reduced warehouse space needs and reduced weight of shipped packages).
The development of these new products recognises that resins cannot succeed solely on the basis of their molecular architecture. In equal measure they must be able to take into account various delivery options, allow for ease of use on highly efficient and often existing conversion equipment, stay well balanced at lower gauges, and, of course, most importantly be optimised to meet end-use application requirements. A key characteristic of this new wave of enhanced polyethylene products is that they are being developed to exhibit controllable performance on high-capacity conversion equipment. This is a departure from what converters might normally expect, since hardware design in extrusion coating predominantly accommodates high-pressure LDPE polyethylene. As an impressive demonstration of specific macro molecular architecture, the design of enhanced polyethylene products takes into account how high-capacity conversion equipment absorbs the visco-elastic behaviour of the converted polymer.
Because of their molecular architecture, the nature of the new products doesn't further heat up through shear heating in the extrusion system, exhibits stable edges even with controllable and relatively low neck-in, and provides extraordinary stable thin web curtains at line speeds up to 700m per minute. Surprisingly enough, the products don't exhibit more than about 15 per cent higher machine power use at slightly elevated pressures compared to autoclave high-pressure LDPE extrusion coating products.
In summary, and while recognising the need for some high shear in melting, the enhanced polyethylenes exhibit a fairly constant rise in temperature over a broad extrusion operating window. Their ramping rate (as judged by average melt temperature over a screw speed range experienced during production start-up) has little impact on extrusion control parameters or web curtain geometry (as judged by neck-in).
Another high-performance attribute of these enhanced polyethylenes is their capability for outstanding draw down without web defects (i.e. draw resonance or edge tear). Draw down is assessed by a specific method developed by Dow (see Figure 1).
This method has been proposed, used and publicised in various previous papers and consists of accelerating line speed from a coating base of 15g per m2 at 100m per minute at set extrusion conditions. The total throughput and die settings are fixed and the line speed is accelerated in steps of 50m per minute until the first web defect occurs. The line speed or draw down value of 700m per minute for the new enhanced polyethylenes reflects a stable web with minimal coating weights of about 2.2g per m2.  Figure 2. Enhanced polyethylene: hot tack curve (coex coating adhesive sealant 1:3 on foil at 25g/m2 - 280° set T per 100m per minute.
While the neck-in behaviour of enhanced polyethylenes versus LDPE has even been found to be 15-25 per cent higher, the linear structure of the new products does not excessively push the limits and allows most current equipment and settings to be used. More important, the total neck-in for enhanced polyethylene stays very stable during any ramp-up.
Application performance benefitsCurrent linear polyethylene coating products don't have a considerable market share in their respective applications. The major obstacle to growth has been the limitations of converting LLDPE products on high-pressure polyethylene hardware. This began to change with the introduction of polyolefins based on constrained geometry catalyst technology.
Products began to emerge with properties designed to match new conversion and end application requirements. These products, together with the more recent success of polyolefin plastomers (such as AFFINITY PT 1451G1 polyolefin plastomers) in extrusion coating applications, have demonstrated significant improvements in sealant, mechanical and sensory properties. For brand owners and end-users this translates into higher package integrity, extended shelf life and improved appearance over longer periods of time, as well as low-flavour ab- or adsorption of contact layers to the filling product.
 Figure 3. Enhanced polyethylene: environmental stress crack-resistance.
Linear products (and specifically polyolefin plastomers) allow low heat-seal initiation temperatures, exhibit high seal strength forces, and show broader sealing windows (which is important for package structure flexibility). Samples of the new enhanced polyethylene exhibit these properties with product densities similar to LDPE or VLDPE. It is interesting to observe that some traditional linear property relationships to densities have been broken up and almost disconnected. A differential scanning calorimetry (DSC) melting point can also be widespread and offer excellent low hot tack and seal-initiation temperatures, even at high values.
The options in molecular architecture also allow property combinations such as a higher water vapour barrier at the same densities as current high-pressure polyethylene. Up to a 20 per cent reduction in water vapour transmission at similar densities has been measured. This translates to either thinner total gauges for the same performance or in reality higher transmission resistance at same thickness. Another interesting observation is the excellent stress crack-resistance properties of basically linear products. In the usual test, even with higher nominal densities, the behaviour of pure high-pressure polyethylene is outperformed by a factor of seven or more.
An aspect of permanent interest to brand owners and converters is the possible interaction of the polymeric product layer with the packed goods. This is where the benefits of products based on constrained catalyst technology come to the fore. Its homogeneous characters allow the creation of neutral and stable products with lower interaction behaviour. The metrics used to express such values are evaluated by taste and odour panels.
 Figure 4. Enhanced polyethylenes for extrusion coating and lamination.
Depending on the targeted end-use, enhanced polyethylenes can be designed for use either as a core layer or as the outside skin layer of laminates. Accordingly, their functionality can be tailored to enhance particularly desired properties (i.e. mechanical, barrier or sealing properties) and optimise their use in the intended application.
Apart from primary flexible packaging for food articles, the property balance is very interesting for industrial and consumer paper wrap applications too, where moisture barrier as a single property and highly efficient conversion create an excellent marriage. A thin layer of such products like ELITE 5810G will allow excellent product protection of various printer or photographic base paper packages. Summary
The chemical and molecular structures of new enhanced polyethylene (EPE) products are opening up new application opportunities in extrusion coating.  Figure 5. European extrusion coating market by material.
Dow does see interesting benefits for brand owners and end-users based on the improved and therefore extended integrity of package laminate solutions. To capitalise on this new category of products, it is more important than ever for converters and brand owners to increase their functional understanding of resin performance attributes. Takeaways
- Some materials and science advances at molecular architecture level have gone unnoticed, while the focus has been on packaging aesthetics to satisfy consumer demand for convenience.
- This lack of attention has been especially true for polyolefin products for extrusion coating and lamination.
- One of the first applications for extrusion coating in flexible packaging was the use of LDPE on butcher paper in the 1950s.
- New packaging performance demands, allied to environmental and economic incentives, are now challenging converters to extrude a wider portfolio of polymers.
- Dow has recently made significant progress with enhanced polyethylenes, mainly by combining the high-performance attributes of its resins with INSITE technology.
- Unlike traditional autoclave LDPE extrusion coating products, which have been improved for processing on larger capacity lines, the processability of enhanced polyethylenes might actually turn the tables on extrusion equipment.
- The linear structure of the new enhanced polyethylenes does not excessively push the limits and allows most current equipment and settings to be used.
- The major obstacle to growth has been the limitations of converting LLDPE products on high-pressure polyethylene hardware.
- The options in molecular architecture allow property combinations such as a higher water vapour barrier at the same densities as current high-pressure polyethylene.
- Of real interest to brand owners and converters is the possible interaction of the polymeric product layer with the packed goods.
- The ultimate tensile strength as well as the high ultimate elongation values of the new enhanced polymers will contribute to excellent puncture- and abuse-resistance.
- The chemical and molecular structures of new enhanced polyethylene (EPE) products are opening up new application opportunities in extrusion coating.
Bernard Fehr is application technology leader for food and specialty packaging at Dow Europe GmbH. |
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