Thursday, April 30, 2015

Living Hinges

One mechanical entity that is almost exclusively limited to being constructed from polymers is a "living hinge". Trying to describe a living hinge can be difficult, so let me provide a few pictures of living hinges you might have around the house:
Living Hinge - Ketchup bottle
What American home doesn't have a ketchup bottle or two?

Fish sauce bottle

The common feature in all cases is that there is a thin strip of the polymer that provides a flexible, continuous (usually) hinge between two other more substantial parts. Trying to make such a hinge from metal would be pretty challenging (let alone ceramics or other rigid materials).

Living hinges are most commonly produced from polypropylene (PP) and polypropylene copolymers, not only because that material is plenty cheap, but also because when properly made, the hinges can last for years. I have a large polypropylene tub that originally was used for dog food some 12 years ago. It has since been repurposed for storing bird seed in an icy-cold/sultry-hot Minnesota garage. It has been flexed two or three times a week for its entire life, and other than where mice tried chewing through it last winter looking for some free seeds, it still works beautifully.

Last week when helping out with the laundry, I ran into a living hinge that I thought was a little unusual. It's a container for Tide Pods:
Living Hinge - Tide Pods
Tide Pods Container

I say it is unusual because the container is polyester. While polyester (PET) is used for living hinges, it's usually for food packaging such as this:
Living Hinge - Fresh Herb Container
Fresh Mint
or other items at the grocery store. PET is much more expensive that PP, but it is used in food packaging because its clarity shows off the food so well. But that argument doesn't hold up with the PET used for the Tide Pods as it is not clear at all. Additionally, the hinge will be flexed quite a bit - the container holds 72 pods so that is at least 72 cycles of opening and shutting. So why does P & G use PET instead of PP? I have no idea. Perhaps some other aspect of the molding (cycle time for instance) dominates and allows for the use of a more expensive resin.

The term "living" hinge has always bothered me as there is nothing alive about the hinge. It's a hinge. A thin plastic hinge. But misuse of the word "living" in polymer science and engineering is nothing new. Look at "living" polymerizations. In a living polymerization, the polymerization reaction runs until the monomer is depleted, but unlike with more common reactions, the termination reaction never kicks in to permanently halt the polymerization. If more monomer is added to a depleted living polymerization, the reaction can pick up right where it left off, even if the newly added monomer is different that what was used previously. But is the living polymerization still "alive" while it is in that paused, deplete state, or is it more like a dormant state? (Or maybe even a "zombie" state as was discussed yesterday?) I guess polymer scientists and engineers have a hard time distinguishing between "live" and "dead". But then again, I have wondered that about my colleagues themselves at times too, so perhaps the misuse is understandable.

Previous Years

April 30, 2014 - Dow Chemical Keeps Plodding Along and Silencing Critics

April 30, 2012 - Another Set of Rants about a Rejected SBIR Grant

April 30, 2010 - Natureworks

April 30, 2010 - This Changes Everything!

April 30, 2009 - Skip the ER for potential heart attacks and find a spectroscopist instead

April 30, 2009 - This isn't even peer-reviewed

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