Wednesday, November 10th, 2010

How Do You Know if a Crack or Defect Has Compromised Safety? Plastics to the Rescue!

Photo: Felizitas Gemetz, © Fraunhofer IWM

Recently around the Washington, DC area, bike sharing programs have begun to pop up as the “green” transportation method. Riding a bike is an easy way to get around town and helps you get some exercise too. As part of city bike riding, bike helmets are a must. Helmets are often made of polycarbonate, polyethylene terephthalate (PET) or ABS (acrylonitrile butadiene styrene) on the outer “shell” combined with an inside liner made from expanded polystyrene (EPS) foam. Helmet straps are generally made of nylon or polypropylene.

We know that cycling helmets have one main purpose: protect the cyclist‘s head. But only completely damage-free helmets do the job properly. So how do you know when it is time to get a new helmet so that you don’t throw away something that is still perfectly usable?

A new process developed by research scientists at the Fraunhofer Institute for Mechanics of Materials IWM in Germany in cooperation with the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT will help cyclists determine when it is time to get a new helmet. Based on their work, the helmets will start to smell distinctively if they need to be replaced. A new process causes odoriferous oils to exude from plastic materials if they are cracked.

The plastics produced by the process start to smell if they develop small cracks, and large cracks really cause a stink. The smell comes from oils enclosed in microcapsules. Since cyclists often replace their helmets unnecessarily after dropping them on the ground (because they can‘t tell with certainty whether they are damaged or not), the capsules eliminate this problem. If cracks form, smelly substances are released. The capsules are added to a polypropylene mass which is injection-molded to form the final component. In the case of the bicycle helmet, the microcapsules are inserted in a thick foil made of polypropylene, which is fastened to the head gear.

A layer of melamine formaldehyde resin encloses the capsules so that they are completely airtight and mechanically sealed. Inside the capsule there is a porous, hardly deformable silicon oxide core which absorbs the odoriferous substance. The process could be used for all products which are difficult to test for defects, such as cycle, motorbike and construction helmets. It also can be used to check pressure hoses (in washing machines for example), which are difficult to access. Smell sensors could also monitor plastic water and gas supply pipes to detect any cracks, because the odoriferous substances emitted are noticeable over long distances.

The project is another example of plastics to the rescue (again).

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