Canadian Plastics

Giving Polyolefins the Slip

By Jim Anderton,technical editor   



I don't write much about the film community, and I'll be the first to admit that it's a weak spot for me. It's one of those technologies that, if I hadn't initially seen it in action, I wouldn't have ...

I don’t write much about the film community, and I’ll be the first to admit that it’s a weak spot for me. It’s one of those technologies that, if I hadn’t initially seen it in action, I wouldn’t have believed that it could work. How in the world can they maintain that outrageous bubble?

What’s in the resin is one reason, which brings me to the subject of this month’s column: slip additives. Why slip additives? Even you I/M people have used Saran Wrap at some point in your lives, and the properties that create that gummy ball are bad news for anyone attempting to process film. The problem is friction, or more specifically, a high coefficient of friction. Nip rolls and collapsing frames are sources, and their necessity in handling film makes friction control essential for decent line speeds. It’s tough to reduce the forces applied by haul off, collapsing frame, etc., but the right additives can reduce “” (pronounced “myou”) the coefficient of friction.

So how does it work? The trick is to use an additive that’s not perfectly compatible with the carrier resin, allowing it to migrate to the surface and isolate the bulk resin from rollers, die lips, or the previous layer of film on the roll. Normally we struggle to keep additives in solution, and when they don’t we call it “bloom”, but the key here is controlled migration.

Control is achieved by adjusting the compatibility of the agent to the resin, and since one way to measure polymers is by the average length of their carbon atom “backbones” the length of the chains in the additive have a major impact on their compatibility. Longer additive chains migrate more slowly, while shorter ones move faster. And other groups attached to the additive carbon chain define their slip properties too.

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A popular family of slip additives is “primary amides” which is organic chemicalese for oxygen and nitrogen hanging off the end of the carbon chain. These “functional groups” are at the heart of the additive’s performance, but surprisingly, a small variation in the number of carbon atoms in their relatively short backbones (compared to the resins they are blended into) makes a large difference in their performance.

Take two popular families of slip agent, erucamides and oleamides. If you could lay them both down on your desk, the difference would appear to be that oleamides have 18 carbon atoms in their main chain, while erucamides have 22. So what? It turns out that the smaller-chained oleamides migrate faster than the bulkier erucamides, are less heat stable and create more volatiles during processing

That’s why there are so many different formulas, and why a seemingly small change in your parameters can bite, especially where your additive loadings are marginal.

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