Whats latest in Debonding?

Why is it so difficult to design an adhesive to debond? After all, if we are smart enough to glue something together at the molecular level, shouldnt we also be smart enough to get it apart?

Several researchers actually have discovered ways of creating molecules that unravel on-demand.

These newly developed technologies have many positive environmental implications. For example, they could be used for the purpose of recycling and reusing plastic parts. When an automobile is scrapped, the plastics and metals that are bonded together can be separated for easier disposal. They can even be reused if they are kept intact. The European Commissions End of Life Vehicles directive has set a reuse and recovery target of 95% of materials from scrapped vehicles by 2016.

Several examples of the development activity in this area are summarized below. For a more detailed introduction to this topic and a more comprehensive understanding of the mechanisms and types of adhesive being developed, the reader may be interested in attending a SpecialChem webinar, "Debonding on Demand Structural Adhesives: Strategies & Future Opportunities", to be given on October 24.

Get Expert Help to Implement the Right Debonding Strategy Depending on Your Applications: Debonding on Demand Structural Adhesives: Strategies & Future Opportunities Online short course on Wed. October 24, 2012

One of the more interesting "debonding" concepts is a reversible adhesive that can be undone by placing it in a magnetic field. Such an adhesive has been developed by the Fraunhofer Institute and Degussa in Germany. The adhesive uses nanoscale magnetic particles to provide this function. The addition of the particles produces an adhesive that bonds and debonds without having to heat the material to achieve cure.

The additive consists of special iron oxide particles called Magsilica which are produced by Degussa. The particles are super-paramagnetic, meaning that they behave like tiny magnets when an external magnetic field is applied. They become magnetic only when exposed to an external magnetic field. This causes the particles to oscillate rapidly and, in turn, heat the adhesive. This is somewhat similar to the oscillation of dipoles when exposed to a electromagnetic field in a microwave oven. The equipment needed to bond and debond materials consists of magnetic coil and a high frequency transmitter, with frequencies in the range of several hundred kilohertz.

There is one catch to this technology. The filler can be used for any application where at least one nonmetal part is bonded. Preferably, both parts should be non-metallic. Metal by itself is strongly heated in the magnetic filled and shield the field.

There have recently been several commercial developments in the field of "reversible adhesion" that lend themselves to products that must be disassembled for reuse.

EIC Laboratories (Norwood, MA, USA) has developed an electrically debonding adhesive mechanism to simplify the disassembly process. ElectRelease includes a high strength epoxy sandwiched between metal substrates. The epoxy can support more than 2,000 psi. Applying between 10 and 50 volts of electricity to the joint causes the epoxy to debond, cleaving the "sandwich" into two pieces (one clean substrate and one substrate with the epoxy attached). Applying a voltage polarizes the adhesive surface and chemically releases it from the substrate, which must be conductive. The expected first use of this adhesive is as a delatching or deployment mechanism.

Researchers at Elf Atochem and the National Center for Scientific Research in Paris created a new adhesive that can switch from "sticky" to "not-sticky" with just a slight temperature change. They created this adhesive by combining two types of polymer molecules. Both have long carbon backbones with side groups. The side groups of one molecule contain hydrogen atoms while the other contains mostly fluorine atoms. At room temperature, the two molecules organize themselves in neat, alternating layers to form a hard material. Raising the temperature to 35°C causes the adhesive to lose its tacky nature. The change is very sudden (within 2°C). The researchers can control the temperature at which the transition occurs by either altering the ratio of polymers in the adhesive or controlling the composition and length of the side changes.

Researchers at Sandia National Laboratories in the U.S. have developed a removable structural epoxy adhesive that also reacts to temperature change. The approach relies on the use of a reversible chemistry that breaks apart the adhesive at elevated temperatures, resulting in a low adhesive molecular weight and low bond strength. Diels-Alder bonds, which are common chemical linkages that break at high temperatures, are built into the epoxy material. No other adhesive is claimed to have the strong bonding characteristics of this epoxy with the capability of melting and losing its bonding capability at high temperatures and then rebonding when the temperature is lowered. The bond in the Sandia adhesive breaks at 90°-130°C depending on the formulation. Minimal force is then required to separate the substrates. The adhesive rebonds between 20° and 60°C.

Yet, another mechanism, feasible on thermosetting resins, involves implanting small microcapsules in