German research scientists have devised a method of embedding capsules of odoriferous oil into plastics, a development that could lead to the improved integrity of safety-critical components.
The process has been developed at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg in cooperation with the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen.
The polymer materials produced by the process start to smell if they develop small cracks. One application could be in cycle helmets as it is difficult to ascertain if the protective headwear is damaged after an impact.
Duncan Vernon, road safety manager at the Royal Society for the Prevention of Accidents (RoSPA) told The Engineer that it is important to replace helmets involved in an impact because they may not give the user the level of protection that they would expect in subsequent impacts.
Vernon added that there is often uncertainly over how severe an impact needs to be before a helmet needs to be replaced.
‘Therefore, designing in an indicator will hopefully help cyclists to make an informed judgements about when to replace a helmet,’ he said.
Dr Christof Koplin, a research scientist at the IWM said: ‘Cyclists often replace their helmets unnecessarily after dropping them on the ground, because they cannot tell 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 airtight and mechanically sealed. It also protects the tiny pods, which are subjected to temperatures of 200 to 300 degrees during injection molding as well as static pressures of up to 100 bar.
‘Melamine formaldehyde resin proved to be the most suitable encapsulation material in the comparison we conducted of the material systems,’ said Koplin. ‘Inside the capsule there is a porous, hardly deformable silicon oxide core which absorbs the odoriferous substance. This core produced the best results,’ he added.
To determine the loads at which the miniscule capsules measuring just 1 to 50 micrometers break open, the scientists test them at the IWM with a Vickers indenter, which measures hardness.
The engineers calculate the number of capsules required by using numerical computer simulation. The finished component is then subjected to bending and drawing tests, which are only considered to be successfully completed if the capsules open and exude the odoriferous substances just before the component fails.
‘Clearly, the difficulty will be in ensuring that the indicator accurately reflects the level of damage caused to a helmet, no matter the mechanism by which the helmet has been damaged,’ said Vernon.
Koplin told The Engineer that the bicycle helmet was employed as a ‘demonstrative object for the possibilities of the plastics.’
‘The intent of Fraunhofer in this case is to inspire and accompany companies to undertake the necessary steps to a production relevant level,’ he said.
He added that manufacturing costs for the new process were yet to be calculated and that relevant life time ‘smell controls’ have not been undertaken.