'Self-healing' Polymer

'Self-healing' Polymer May Facilitate Recycling Of Hard-to-dispose Plastic

Researchers in The Netherlands are reporting development of a new plastic with potential for use in the first easy-to-recycle computer circuit boards, electrical insulation, and other electronics products that now wind up on society's growing heaps of electronic waste.

Antonius Broekhuis and colleagues note in the new study that so-called thermoset plastics are widely used in consumer electronics due to their hardness and heat resistance. These plastics, however, contain additives and reinforcement materials that make them almost impossible to recycle. So-called thermoplastics, in contrast, are softer and can be remelted easily. As a result, thermoset plastics often end up in landfills or incinerators, where they can contribute to pollution. Scientists have long-sought a simple, inexpensive process to make these plastics recyclable, but they have been largely unsuccessful until now.

Broekhuis and colleagues describe development of a new type of thermosetting plastic that can be melted and remolded without losing its original heat-resistance and strength. The scientists showed in laboratory tests that they could melt granules of what they term a "self-healing" polymer and reform them into uniform, rigid plastic bars. They also showed that the plastic could be remolded multiple times, setting the stage for a new generation of recyclable plastics.

A self-healing polymer was developed that improves the reliability of thermosetting polymers by initiating a healing

process in response to damage. Fatigue-life extension was achieved by a combination of crack-tip shielding mechanisms induced by self-healing functionality. Viscous flow of the healing agent in the crack plane initially

retarded crack growth.

Polymermization of the healing agent, resulted in a long-term crack closure effect, which prevented unloading of the crack tip, reduced the crack length and retarded additional crack growth. A material able to autonomically respond to fatigue crack growth represents a milestone in the development of safer, longer-lasting material.

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. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised.Experiments exploring the concept of self-repair have been previously reported, but the only successful crack-healing methods that have been reported so far require some form of manual intervention. Here we report a structural polymeric material with the ability to autonomically heal cracks.The material incorporates a microencapsulated healing agent that is released upon crack intrusion.

SELF-HEALING POLYMER COMPOSITES FOR EXTENDED FATIGUE LIFE

An extensive experimental investigation is carried out to assess the fatigue response of a self-healing polymer composite. Mode-I fatigue crack propagation is measured for a range of material parameters and loading conditions. Significant fatigue life extension and permanent fatigue crack arrest are achieved at moderate crack growth rates.A novel approach is explored for improving the fatigue life of thermosetting polymers through the addition of self-healing functionality. Thermosetting polymers are used in a wide variety of applications ranging from composite structures to adhesive joints to microelectronic packaging. Due to their low strain-to-failure these polymers are highly susceptible to damage in the form of cracks. Fatigue loading is particularly problematic, giving rise to the initiation and propagation of small cracks deep within the structure where detection is difficult and repair is virtually impossible.

These cracks often lead to catastrophic failure of the material. We utilize a strategy based on recent developments in self-healing technology to autonomically repair fatigue cracks and extend the service-life of many polymeric components. The material under investigation is an epoxy matrix composite which utilizes embedded microcapsules to store a healing agent and embedded Grubbs catalyst. A propagating crack exposes particles of catalyst and ruptures the microcapsules, which release healing agent into the crack plane.

Polymerization of the healing agent is triggered by contact with the catalyst, reestablishing structural integrity acrossthe crack plane. The fatigue-crack propagation behavior is investigatedusing the tapered double-cantilever beam

(TDCB) specimen with constant range of applied stress intensity factor delta K. All measurements are made with a loading frequency of 5 Hz and load ratio R!=!Kmin/Kmax of 0.1. Samples are cast in a silicon mold, cured for 24 hrs at room temperature followed by 24 hrs at 30degreeC, and then precracked with a razor blade.

FATIGUE CRACK GROWTH PRECLUDING SELF-HEALING

Fatigue crack propagation in neat epoxy, epoxy with embedded microcapsules, and epoxy with embedded catalyist is accurately captured by the Paris power law. The effect of embedded microcapsule size and concentration on fatigue crack growth is that the addition of microcapsules significantly reduces the crack growth rate above a transition delta KT. Above the transition, the Paris law exponent n is strongly dependent on the content of microcapsules,

varying from 9.7 for neat epoxy to approximately 4.5 above 10 wt% microcapsules. Similar retardation behavior has been reported for epoxy with embedded rubber particles.

IN SITU SELF-HEALING

Fatigue crack retardation and arrest from self-healing functionality result from crack-tip shielding mechanisms.

Hydrodynamic pressure generated initially by uncured, liquid healing agent reduces both the loading and unloading of

the crack tip. After polymerization, the fully cured healing agent forms a polymer wedge at the crack tip, which inhibits

loading of the crack tip through adhesive mechanisms and unloading the crack tip through artificial-crack closure.

Healing efficiency of the self-healing epoxy under cyclic loading is characterized with a fatigue-life-extension protocol

where Nhealed is the total number of cycles to failure for the self-healing sample and Ncontrol number of cycles to failure for a similar sample without healing. Under high-cycle fatigue (moderate deltaKI), and low-cycle fatigue (high delta KI)

when a rest period was employed, in!situ healing extended fatigue life though temporary crack arrest and retardation.

In!situ self-healing permanently arrested crack growth under low-cycle fatigue conditions at low delta KI, and at moderate delta KI when a rest period was employed.

Self-healing polymer advance could mean scratch-free iPhones

Material scientists from the University of Southern Mississippi have created a new polymer that can fix its own scratches under regular sunlight, a feat that has no end of practical applications.

When we get scratched, our skin can repair itself. Nonliving coatings can't currently do the same thing, so we cannot put self-repairing surfaces on cars, cell phones, laptops, and many other items. As the commercial applications are numerous and the financial payoffs are potentially huge, material scientists have been actively developing polymers that can self-heal. Everything from nanoparticles to expandable gels have been tried. While some of the developments are certainly promising, nothing is quite at the stage where it's ready to be commercialized.

In today’s issue of Science, Biswajit Ghosh and Marek Urban from the University of Southern Mississippi present a new polymer design that can employ UV light from the Sun to activate a latent self-pair capacity. Their strategy involves using the combined functions of three chemical components.

At the core of their design is polyurethane, which is an elastic polymer that already has decent scratch resistance. To enhance its ability to withstand mechanical damage, Ghosh and Urban added two more components, OXE and CHI.

OXE has an unstable chemical structure (a four-membered ring containing three carbons and one oxygen) that makes it prone to being split open. CHI is UV sensitive.

The idea is that, if the polyurethane gets damaged by a scratch, the unstable ring structure of OXE will open to create two reactive ends. Then, UV light can trigger CHI to form new links with the reactive ends of OXE and thereby fix the break in the polymer.

Scientists have purposefully created scratches in films of their polyurethane-CHI-OXE material and tested to see if it mended itself under UV light. When they placed the damaged film under a 120 W fluorescent UV lamp, the scratches became negligible within half an hour. This repair reaction can work under a variety of conditions, ranging from dry air to high humidity.

Self-Healing Polymer Autonomous Material System

Everywhere you look, exposed surfaces are cracking. Asphalt streets are cracked, building facades are cracked, the paint on your house is cracked and flaking - the list is endless.

What if you could have a surface that repaired itself?

After all, you have a surface that heals itself - your skin. When you get a cut or a scratch, living cells deep in the living layers of your skin replace the old ones on the surface.

A team of researchers at the University of Illinois have used a technique create a polymer-based system that heals itself. An epoxy-resin base is infused with a network of interconnected channels about 200 microns in diameter. The channels are filled with low viscosity monomeric dicyclopentadiene - the healing agent. This "vascularized" substrate has a solid epoxy layer deposited on top of it. A catalyst is incorporated in this solid coating.

When the coating layer is damaged, healing agent wicks from the channels through capillary action.

"Once in the crack plane the healing agent interacts with the catalyst particles in the coating to initiate polymerisation, rebonding the crack faces autonomically. After a sufficient time period the cracks are healed and the structural integrity of the coating restored. As cracks reopen under subsequent loading the healing cycle is repeated."

The idea of a "self-healing" surface is part of a larger set of ideas called "autonomous materials systems." The intent is to develop materials that can respond to their environment without additional attention from human beings.