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Self healing polymers


A self healing polymer based on Supra molecular technology has been developed by Arkema. This new elastomeric material can be cut and rejoined at the same spot simply by pressing the broken ends together for a few minutes. This self-healing rubber retains its stretchability even after being severed five or six times, or cut and left alone overnight. The chemical manufacturer is now working to create batches of the material for applications such as sealants. The secret of the material is a molecular structure that resembles a plate of spaghetti. The self-mending occurs because each strand consists of molecules of vegetable fat linked to one other via relatively weak hydrogen bonds. These are the same chemical bonds that give water molecules their cohesiveness. The resulting rubber can stretch to six times its resting length. The National Center for Scientific Research (CNRS) in Paris has developed the material. A full repair requires up to 6 hours of bonding. A ripped sample could be left overnight before being repaired, although it would not stretch as far, because some of the severed bonds had linked to their neighbors. Recycling a sample into a new shape is easy, as it only requires heating, so that the bonds break and reform.
Chemical maker Arkema, Inc. is working on scaling up the synthesis process. This substrate will find application in self-healing toys, pipe seals and pavement as well as plastic medical pouches that can be punctured and reused. NASA is interested in the substrate as it may help it pursue its interest in printed electronics because its wide area can confer fault tolerance, like from particle damage.
Prior self-healing materials relied on embedded capsules of sealant that opened during a break and then had to be replenished, or polymers that required high heat to rebond. Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation of fiber-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. Another structural polymeric material with the ability to autonomically heal cracks has been developed.
Engineering this self-healing composite involves the challenge of combining polymer science, experimental and analytical mechanics, and composites processing principles. Autonomic healing is accomplished by incorporating a microencapsulated healing agent and a catalytic chemical trigger within an epoxy matrix. An approaching crack ruptures embedded microcapsules, releasing healing agent into the crack plane through capillary action. Polymerization of the healing agent is triggered by contact with the embedded catalyst, bonding the crack faces. The damage-induced triggering mechanism provides site-specific autonomic control of repair. An additional unique feature of the healing concept is the utilization of living polymerization (that is, having unterminated chain-ends) catalysts, thus enabling multiple healing events. The fracture experiments yield more than 90% recovery in toughness, and the developers expect that the approach will be applicable to other brittle materials systems (including ceramics and glasses).

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