A while ago, I wrote an editorial titled, "No More New Polymers" that expressed dismay that the most exciting periods of new polymer development may be in the past. This arguably may be true for the emergence of new polymers, but it certainly is not for the emergence of new materials ? many of which are innovative composites, blends, or amalgamation of existing technologies.

This editorial looks at several of these newly developing materials. Even though many of these materials have existed for some time, they have only recently been commercialized and given special attention. They may be worth watching for how they progress in the future. Some of these applications may be in the field of adhesives and/or sealants. At the very least, they are fun to think about as a diversion, and they may just embolden some creative thinking of your own.

Auxetics

Auxetics are materials that have a negative Poissons ratio. Most materials have a positive Poissons ratio (i.e., when strengthened they become thinner and longer). Auxetics behave just the opposite. When stretched they thicken. This occurs due to their hinge like structures (Figure 1). Typically auxetic materials have low density.

Figure 1: Auxetic material design

Auxetic materials are expected to have mechanical properties such as high energy absorption and fracture resistance. As a result, they may be useful in developing body armor, packaging materials, protective human body pads, shock absorbing material and absorbent materials. Another potential application of auxetics is as a substitute for conventional polymers in arterial replacement ? think of arteries expanding due to stress rather than narrowing.

Auxetic materials can be single molecules or a particular structure of macroscopic matter. Examples of auxetic materials include: certain rocks and minerals, variants of polytetrafluoroethylene polymers, and paper of all types.

Aerogels

Aerogels have sometimes been referred to as "solid smoke". It is a manufactured material with the lowest bulk density of any known porous solid. It is derived from a gel in which the liquid component of the gel has been replaced with a gas. The result is an extremely low density solid with a notable effectiveness as a thermal insulator. Aerogels nullify all three methods of heat transfer (convection, conduction, and radiation).

There are several main types of aerogels: silica, carbon, and alumina. Silica is the most common. Silica aerogels strongly absorb infrared radiation. Therefore, they allow the construction of materials that let light into buildings but trap heat for solar heating. Carbon aerogels have been used to create supercapacitors, and alumina aerogels are used as catalysts since they are easily "doped" with another metal. Certain aerogels have also shown promise in absorbing the heavy metal pollutants (e.g., mercury, lead, and cadmium) from water.

Aerogels are also extending even into consumer products. Dunlop has recently incorporated aerogel into the mold of its new series of tennis rackets and has previously used it in squash rackets. Shiver Shield, a brand of cold weather gear is insulated with aerogel.

Cabot Corporation has developed a line of aerogel materials specifically for adhesive and sealant applications. They can be used in highly filled, high viscosity formulation to provide benefits such as: stronger adhesion and less sag, better viscosity stability, less bleeding, higher transparency, faster recovery time, and decreased tack after curing.

Magneto-Rheological Fluids

Magneto-rheological (MR) fluid is a carrier fluid (generally oil but could be an organic resin) that is filled with magnetic particles. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity to the point of becoming a viscoelastic solid. The fluids ability to transmit force can be controlled with an electromagnet, which gives rise to many possible control-based applications.

The main use of MR fluids to date is as a shock absorber with the magnetic field dynamically controlling the amount of damping in the material. In this application the MR fluid acts in a flow mode to control the force required to flow the fluid through channels. It can also be used in a shear mode for clutches and brakes in places where rotational motion must be controlled. MR dampers are also utilized in semi-active human prosthetic legs. This decreases the shock delivered to the patients when jumping for example.