Canadian Plastics

Understanding High-End Polyolefins

During the post World-War II rush to embrace new technology, early injection-grade polyethylenes (PE) and polypropylenes (PP) were inconsistent, often impure and typically misapplied. But they were ch...

October 1, 2006   By Jim Anderton, technical editor



During the post World-War II rush to embrace new technology, early injection-grade polyethylenes (PE) and polypropylenes (PP) were inconsistent, often impure and typically misapplied. But they were cheap, which was all that mattered for mass-produced consumer goods.

Today, however, consumer expectations are higher, and this has resulted in hundreds of new resin species. In high-performance applications, engineering resins offer many advantages, but the bulk of plastic components in modern vehicles and major appliances are still PP or PE-based resins.

GOOD CHEMISTRY

The reason polyolefins are still widely used is because of their chemistry, specifically the method used to polymerize the feedstock — typically ethylene or propylene gas — into the solid resin form.

Catalysts, the platforms that polymers are built on, are the key and are hugely important to the resin’s performance, providing a site where carbon atoms are linked to each other, growing upward like hair from a follicle.

As the chains grow, they eventually beak off, and the varying chain lengths form a distribution described as molecular weight, important because it determines the resin’s physical properties like melting point, tensile strength, and density.

CARBON BACKBONE

Polymerized from ethylene gas, PE is a collection of long chains of carbon atoms, tangled or partially ordered to create variable crystallinity. PE is popular because, by simply altering the catalysts used to synthesize it, its properties can easily be altered.

Molecular weight is a major part of the story, but so is the amount of branching of the carbon chains grown from the catalyst surface. Short chain segments branch off the main carbon chain and are difficult to compact into a neat bundle. This problem makes low-density polyethylene (LDPE) — which melts in the 120-degrees Celsius (C) range — too soft and weak for most under-the-hood automotive and appliance applications.

STRONGER THAN STEEL

But using an advanced catalyst like a metallocene, which allows the main chains to lie closer together, can reduce the amount of branching, resulting in medium- and high-density PE. At its furthest point, the result is ultra high molecular weight polyethylene (UHMWPE), which has fibres that are packed closely together to produce a level of crystallinity approaching 85 per cent. The result is a polyolefin that melts in the range of 140 to 150C, and is 15 times stronger than steel. For this reason, UHMWPE is beginning to appear in demanding applications such as body armour and automotive fuel tanks.

HDPE’S MULTI-LAYER APPROACH

Metal replacement has always been a major driver of plastic technology, but the steel industry is maneuvering to reclaim ground lost to PE.

Dismissed until recently as a dinosaur, steel fuel tanks, for example, have been given new life because tighter vapour emissions standards initiated by California’s Air Resources Board — which pose a challenge for the HDPE tank — have been adopted continent-wide.

The resin community responded with multi-layer technology. For example a combination of EVOH as a barrier layer sandwiched between HDPE layers, binding the materials together. This approach provides high performance with minimal use of expensive engineering resins, thus lowering the cost while providing enough strength to allow wider spec and regrind HDPE in the process.

STEEL COMPETITOR

The steel industry has also developed new plating technologies and tighter control of the welding process.

Steel, although currently expensive, is also less volatile price-wise compared to polymers, a disadvantage for blow molders who must price-in the risk of raw materials cost escalation, or buy materials forward, requiring larger production commitments from their customers.

Under the chassis, however, steel is saddled with a major disadvantage. The flange needed to weld the tank halves together reduces the usable volume of fuel in the tight confines of many smaller vehicle chassis. Resin tanks, with their flangeless one-piece design, avoid this problem.

For vehicle designers, plastics mean more interior space in an environment where every centimetre counts. In the long term, however, recylability will remain an issue for resin technologies, as will the long-term effects of contact with new fuels like ethanol and biodiesel.

A MIXED BAG

Every recipe has ingredients, and resins are no exception.

The majority of resin grades are made up of more than one polymer. Copolymers use different segments of carbon chains forming the polymer at the molecular level.

Block copolymers, for example, have segments appearing on its main carbon chain, while graft copolymers splice the second type to a consistent backbone chain.

In general, copolymers behave like simple resins, but with tuned properties matched to the application.

Alloys and blends, however, are combinations of finished polymers. These combinations perform better than any of these polymers do individually.

A common technique to reduce brittleness and boost impact strength, for example, is to add soft regions to a stiffer base. Unfortunately, this blending process is an extra step, and it’s reflected in the resin cost.

In the end, there are many variables and issues in polyolefin technology and no one article can address them all.

But the key points for designers and processors are simple: Copolymers and blends can produce resins whose properties are better than could be expected from any of the individual components.

Additionally advanced polyolefins can replace or minimize the content of expensive engineering resins while being recyclable with current technology and infrastructures, making them environmentally friendly and cost-effective in the long term.


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