The science and performance of Metallocene blends is not as academic as it may sound!
As much as two-thirds of the cost and most of the performance of plastic bags and film is driven by the type of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) with which it’s extruded. Metallocene, named after the catalyst used to make it, is a sort of engineered linear low density polyethylene. It may cost about 50% more than traditional LLDPE but, at levels of 10-20%, it can provide surprising and often cost-effective performance enhancements.
To better understand how the addition of the metallocene can help produce higher performing films, let’s take a deeper look at key product performance drivers and the chemistry of metallocene itself.
THE CHEMISTRY OF METALLOCENE
Polymers aren’t as tough to understand as you might think. The word polymer simply means many (poly) parts (mer). In the case of polyethylene, we’re just talking about many parts of ethylene (the monomer). Like links in a chain, thousands of ethylene pieces form a polymer backbone and then exist as a sort of entangled network.
The best way to understand Metallocene is in the context of its LDPE and LLDPE cousins. Whether you produce low density, linear low density, or Metallocene polymers, one link of ethylene is added to the next and to the next, and so on. Similar in their final sizes, these polymers differ in the number, placement and size of their side chains.
- LDPE has lots of side branches of random placement and length. Picture spaghetti noodles on a place and you get a pretty good visual of the random arrangement of LDPE chains.
- LLDPE has shorter and fewer side branches (about one-fourth that of LDPE). Here and there, these spaghetti boodles group in more uniform patterns.
- Metallocene has 3 to 4 times the side chains as LDPE. More importantly, these side chains are precisely placed throughout the polymer’s backbone.
The arrangement of each polymer’s network of chains and side chains dictates how they fit together. That in turn drives performance, including clarity, strength and sealing.
The degree of polymer crystal (how uniform a polymer can pack together) dictates its clarity. The more crystalline a polymer is, the less transparent it becomes. The more amorphous (random), the clearer. Laboratories often measure this in terms of haze value – which is defined as the percent of light not transmitted. For example, a material with a haze value of 10% allows 90% of light to pass through it.
- LDPE has a very random structure (remember all that spaghetti?). Different length side branches keep it from being crystalline. LDPE, therefore is attractive for its clarity. Common grades of LDPE have a haze value of just 5% or so.
- LLDPE is less bulky because of its fewer and shorter side chains. It forms more uniform patterns which makes it more crystalline. As a result, LLDPE provides the least clarity. Its haze value will range from a low of 8% to as much as 20%.
- Matallocene’s structure allows for the least crystallinity. Its large numbers of side branches make it the most amorphous. Blends containing metallocene regularly exceed LDPE’s haze values of 5%.
For our applications, we speak of strength in terms of resiliency and toughness. We measure success by how much we can stretch a film before it gives way (that’s called tensile strength) and by how much it takes to puncture (that’s called impact strength). A pendulum with increasing weight helps determine tear strength.
LLDPE provides roughly twice the tear and impact strength of LDPE. In a recent impact test, for example, we saw numbers in the 100psi range for LDPE while mid and high graes of LLDPE ranged from 180 to 375psi. But, it is metallocene’s many and uniform side branches that bring the highest numbers. In this test, a metallocene/LLDPE blend exceeded 900psi.
Sealing bars apply pressure and heat to form continuous bands of melted film. Polyethylene materials that melt at lower temperatures tend to flow better during extrusion. They provide better “hot tack,” and they make it easy to form strong, uniform seals.
Today’s seals are rarely an issue, unless you manufacture bags very thin gauges or produce heavier gauges of gusseted bags.
- For thin gauges (think liners), manufacturers star-seal. They increase the amount of plastic at the seal by gusseting them and folding them over before heat sealing.
- For heavier gauges of gusseted bags (typically 6 mils and above), manufacturers are challenged by the variation in gauge. With a 6mil bag, for example, sealing mechanisms see 24 mils at the gussets but only 12 mils ion between.
With melt temperatures of 104C (219F) and 124C (255F) respectively for LDPE and LLDPE, both seal well under standard conditions. Still, metallocene’s well-defined structure melts very uniformly at just 96C (205F), providing a nice solution for tough-to-make seals.
While there is the inherent performance trade-off between LDPE and LLDPE of clarity vs. toughness, Metallocene’s trade-off is with price. Metallocene suppliers typically recommend the use of Metallocene at concentrations of 15-20%. At blends of 15%, the same characteristics of Metallocene that make it so resilient at room temperature provide terrific performance for colder temperature applications (like freezer bags). At levels below 10%, we’ve seen little improvement in overall performance. The cost/benefit of use declines at blends over 20%.
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