Tikalon Blog by Dev Gualtieri

Graphene Putty

January 16, 2017

There are many examples of materials research producing a different product than what was intended. One example is WD-40, an oil originally designed as a corrosion-barrier coating for metal, especially the stainless steel fuel tanks of the Atlas missile.[1] WD-40 is now an ubiquitous household penetrating lubricant used for extracting rusted bolts and many other applications. My favorite "off-label" use is the removal of adhesive residue, especially from old duct tape, on compatible materials. The WD-40 Web Site lists many more uses.[2]

The composition of WD-40 is a trade secret, but its ingredients, as disclosed on Wikipedia, are as follow:

• 50% Aliphatic hydrocarbons.
• 25%, or less, mineral oil or light lubricating oil.
• 12-18% low vapor pressure aliphatic hydrocarbons. These reduce the low-viscosity to allow for spray application, and they evaporate quickly.
• 2-3% carbon dioxide, used as a flammability-retarding propellant.
• 10%, or less, inert ingredients.

Dupont chemist, Stephanie Kwolek, was looking for a lightweight substitute for steel wire in radial tires when she discovered the ultra-strong polymer, Poly-paraphenylene terephthalamide, known as Kevlar, which has a tensile strength five times that of steel.

Belgian-American chemist, Leo Baekeland (1863-1944), was searching for a synthetic replacement for the natural product, shellac, when he discovered Bakelite, the first synthetic thermosetting plastic. His "insoluble product of phenol and formaldehyde" was commonly used as an electrical insulator.[3]

Modern chemists are safety conscious, and they wear protective gloves when working with chemicals. This wasn't the case in 1879 when Constantin Fahlburg noticed a sweet taste on his hand after working with coal tar reactions. He had discovered saccharin, the first artificial sweetener. Saccharin became important when sugar supply became difficult during World War I, and it was important for sugar-free diets in later years.

The Eastman Kodak company, the giant of photography before its chemical phase was superseded by digital photography, did more than adhere silver halide to film. Its Kodak Research Laboratories did research in many chemical areas, and it was there that Kodak engineer, Harry Coover discovered cyanoacrylate "super-glue", a material originally tried as a clear plastic for gun sights. This product was originally sold as "Eastman 910."

As I wrote in an earlier article (Silly Putty, August 6, 2014), research on a natural rubber substitute during World War II led to development of Silly Putty® based on a mixture of polydimethylsiloxane, [C₂H₆OSi]_n, and silicone oil reacted with boric acid.[4-5] Silly Putty® itself is a trademarked name of a material sold by Crayola, but similar materials sold under different names.[6] More than four thousand tons of Silly Putty have been sold since 1950.[7]

The structure of polydimethylsiloxane.

(Via Wikimedia Commons.)

Silly Putty has the unusual mechanical characteristic that it can be slowly worked, like a clay, but it behaves as an elastic solid when the rate of applied force is large. This characteristic is a result of the viscoelasticity of the polydimethylsiloxane. According to Wikipedia, this is the composition of Silly Putty:

Dimethyl siloxane polymer, terminated with boric acid, 65%
Silica (crystalline quartz), 17%
Thixatrol ST (castor oil derivative), 9%
Polydimethylsiloxane, 4%
Decamethyl cyclopentasiloxane, 1%
Glycerine, 1%
Titanium dioxide, 1%

Silly putty is the basis of a novel sensor material created by physicists at the School of Physics of Trinity College Dublin (Dublin, Ireland) and the University of Manchester (Manchester, UK). Silly putty mixed with graphene exhibits an electrical conductivity that varies with applied force.[8-9] This research was enabled by a the €1 billion Graphene Flagship initiative.[9] This research team has just published their results in Science.[8]

While graphene has been used as an additive in nanocomposites, its behavior in highly viscoelastic polymer matrices has not been well characterized.[8] When graphene is added to silly putty, forming a material the research team calls "G-putty," the lightly cross-linked polysilicone, its electromechanical properties are changed substantially.[8-9] There's a post-deformation temporal relaxation of electrical resistance and a nonmonotonic change in resistivity with strain.[8]

The gauge factor of the material, the ratio of the resistance change to the strain, is greater than 500. This compares with about 2-5 for metals, 30 for polysilicon, and up to 100 for thick film resistors of special compositions. The sensitivity is such that a force as small as a spider's footstep can be detected.[9] While spider detection would be a niche application, the material could prove useful as a sensor for respiration, pulse rate, and blood pressure.[9]

Graphene-filled silly putty (G-putty) is soft and flexible. It also exhibits a large resistance change with applied pressure. (Still images from a Trinity College Dublin video.)

While conductivity changes like this usually arise from percolation, the electrical properties of G-putty appear to be associated with nanosheets of graphene moving in the low-viscosity polymer matrix.[8] The research team was able to devise a mathematical model that describes the electromechanical properties of G-putty.[8]

Says Jonathan Coleman, a professor at Trinity College Dublin,

"When we added the graphene to the silly putty, it caused it to conduct electricity, but in a very unusual way. The electrical resistance of the G-putty was very sensitive to deformation with the resistance increasing sharply on even the slightest strain or impact. Unusually, the resistance slowly returned close to its original value as the putty self-healed over time."[9]

Study co-author, Jonathan Coleman, and son, Oisin, playing with types of silly putty.

In Coleman's case, it's graphene-filled silly putty, G-putty.

(Trinity College Dublin image.)

Graphene Putty

References: