The Synthesis of "Bouncing Putty"
A Cross-Linked Silicone Polymer

Purpose

The goal of this experiment is to synthesize bouncing putty, a cross-linked silicone polymer and study its unique properties.

Background Information

Silicon is the second most abundant element in the earth's crust and is a major component of many minerals (silicates). Silicon is a member of the carbon family and is the family member directly below carbon.

 

Despite their proximity on the periodic table, the chemistry of silicon and carbon are quite different. For example, silica SiO2 (the primary structure of quartz) has a diamond crystalline structure with each silicon bonding to four neighboring oxygen atoms while carbon dioxide CO2 exists as discrete molecules.

Carbon has the ability to form strong pi bonds with other carbons or with oxygen while pi bonds between silicon and other atoms are unstable. Instead silicon forms polymers with very strong single bonds to multiple oxygens rather than discrete molecules possessing multiple bonds with single oxygens.

Polyorganosiloxanes have an empirical formula of R2SiO and might be expected to be similar in structure and chemistry to organic ketones R2CO. These molecules were originally named silicones on the belief that they were silicon analogs of ketones.

However, silicones have little in common with ketones. Ketones exist as discrete molecules possessing a carbon oxygen double bond and silicones consist of polymeric chains with an alternating silicon-oxygen backbone and two alkyl groups bonded to each silicon atom.

Although there is no similarity between the structure of organic ketones and silicones,the name stuck and polyorganosiloxanes are commonly called silicones. Silicones are viscoelastic materials. They behave like viscous liquids at high temperature or when allowed a long period to flow. They behave like elastic solids or elastomers at low temperature. The elastic behavior stems from the very flexible silicon-oxygen bonds whose angle can easily open and close in a scissor-like fashion. When the molecular weight is high, these flexible chains become loosely entangled which imparts a high level of viscoelasticity.

 

Polydimethylsiloxane (PDMS), which has a repeating (CH3)2SiO unit, is the most common of the organosilicones. Altering the number of repeat units (value of n) in the chain and the degree of cross-linking which ties multiple polymer chains together results in polymers possessing different properties. Fluids, emulsions, lubricants, resins, elastomers or rubbers are different classes of commercially important PDMS products. Since silicon-chlorine bonds are very susceptible to cleavage by water, polydimethylsiloxane can be synthesized by hydrolyzing dichlorodimethylsilane.

n [Si(CH3)2Cl2] + n [H2O] ->[Si(CH3)2O]n + 2n HCl

The initial hydrolysis reaction generates the silanol Si(CH3)2(OH)2 which readily condenses through loss of water to form the siloxane polymer. Since dichlorodimethylsilane is bifunctional (has two chlorines), the chain is able to propagate in two directions generating high molecular weight polymers which retain some residual hydroxyl groups. These residual hydroxyl groups can be reacted with boric acid B(OH)3 to form Si-O-B linkages between polysiloxane chains. Since boric acid is trifunctional, a single boron has the ability to join three polysiloxane chains together. This joining of chains is called cross-linking. This cross-linking produces a high molecular weight polymer that is a soft, pliable gum with very interesting chemical properties.

A Little History

During World War II there was a lot of interest in developing an inexpensive substitute for synthetic rubber. James Wright, an engineer at General Electric, who was working on these substitutes, added boric acid to silicone oil (PDMS) to form a product (view Wright's patent) that could be stretched further and bounced 25% higher than rubber. This new substance also transfered ink from newsprint. At about the same time Dr. Earl Warrick of Dow Corning, synthesized the same polymer (view Warrick's patent) although Wright is typically considered the inventor because of the events which followed his discovery.

For a number of years, the polymer termed "nutty putty" at General Electric remained a curiosity. It was played with by the company's chemists but seemed to have no commercial use (read the January 1945 Popular Science article). In 1949, a marketing expert named Peter Hodgson was introduced to the nutty putty at a party and saw the marketing potential it had as a children's toy. As the story goes, Hodgson, who was unemployed at the time borrowed $147 to buy a supply and the marketing rights of the putty. Since it was the Easter season, he packaged it in one ounce lumps in plastic eggs which sold for $1. He needed a name and called it "Silly Putty" for it's principal silicone ingredient. The road to Silly Putty fame was a bumpy one.....Hodgson was told by the experts that his product would never sell; just as he was beginning to find a market, the Korean War spurred the government to place restrictions on raw materials including the silicone needed to make the putty. When the restrictions were lifted, Hodgson, still believing in his putty, made another attempt at marketing it. Although Silly Putty was intended originally to amuse adults, by 1955, it had became popular as a children's toy (view the first Silly Putty commercial). By the sixties, it had become a multi-million dollar seller (view a sixties television commercial , a comic book ad, a timeline of Silly Putty history). At the time of his death in 1976, Hodgson's estate was worth over $140 million - all due to the silicone putty that had no practical use!

In 1977, Binney & Smith Inc., the Crayola Crayon company, acquired the rights to Silly Putty and continues to market it today (see it made). In March 2001, Silly Putty was inducted into the Toy Hall of Fame.

Generic Silly Putty is still produced by Dow Corning as Dow Corning 3179 Dilatant Compound. There is more in that plastic egg the cross-linked polymethylsiloxane. Additives are used to make the gooey polymer into the toy with which we are all familiar (fact sheet, ingredients list). You can buy this generic material in bulk for $400/50 pounds from Dow Corning or $16/pound bulk instead of the $67.75/pound out of the egg. So what do you do with 50 pounds of putty? Watch this video to find out.

Properties

Bouncing putty has interesting properties. If you pull it slowly, it stretches; itcan be molded into all sorts of shapes like clay but over time will flow (a video example); if you pull it quickly it breaks; it bounces but under very high impact it shatters. The secret to the behavior of bouncing putty lies in the cross-linking. In the polymer the boron is a bridge that links polysilicone chains together. However, this linkage is dynamic. Thermal motion in the molecule can break the bond allowing it to slide to a new position along the chain where it re-attaches. A pair of chains can move past each other when the cross-linking bonds are temporarily broken. The putty is able to be stretched when the deformation occurs on a time scale that is slower than the cross-link bond break-and-reconnect rate. If the material is deformed too rapidly it increases in rigidity because there is not time for the cross-links to be broken and reconnected. If the deformation is only somewhat faster than the breaking-reconnecting rate, the chains move some and then snap back elastically resulting in bouncing behavior. If the deformation is greatly more rapid, the cross-links break and do not reconnect resulting in the chains separating, snapping the sample. Hitting a sample with a hammer causes it to shatter. If you warm a sample of putty, the disconnection-reconnection rate increases and if you freeze a sample the rate decreases.

Experimental Procedure

This part of the procedure must be carried out in the fume hood because the hydrolysis releases toxic hydrogen chloride gas. Place 20 mL of dichlorodimethylsilane (into a dry 250 mL Erlenmeyer flask fitted with a rubber stopper and a stirring bar. The dichlorodimethysilane is very sensitive to moisture in the atmosphere so make your transfer quickly. Add 40 mL of anhydrous diethyl ether and hydrolyze the silane by dropwise addition of 40 mL of water with stirring. The water must be added slowly or the evolution of the HCl will be too vigorous. Have a an ice water bath available to slow your reaction if the hydrolysis becomes too vigorous. After adding about 10 mL of water, you should be able to increase the rate of addition. When all the water has been added, let the mixture stir for a few minutes and then transfer it to a 250 mL separatory funnel. Separate the layers discarding the aqueous layer. Wash the ether layer with three 100 mL portions of 1 M sodium carbonate solution to neutralize any residual acid. Remember that neutralization of an acid by carbonate solution evolves carbon dioxide gas. After washing with the first portion of carbonate solution, add 10 mL of diethyl ether to replace any solvent that evaporated during the exothermic neutralization reaction. Wash the ether layer with 100 mL of water and dry over anhydrous magnesium sulfate in a stoppered Erlenmeyer flask.

Decant the ether solution into a tared flask and evaporate the ether in a warm water bath. Record the yield of your dimethylsilicone oil.

With continuous stirring, add 5% by weight of boric acid. Your oil should become very viscous so mix it well. Heat the mixture using a 180 C oil bath for 2 hours (longer will give a better product). At the end of the cross-linking reaction, allow the flask to cool and collect the product by scrapping it from the flask with a spatula. Kneed the gummy substance to get a putty like consistency.

Perform the following tests on samples of your putty:

  • Roll it into a ball and test its bouncing properties
  • Test its stretching properties by first pulling it slowly and then by pulling it sharply
  • Place a sample on a hard surface and apply a constant pressure with your thumb
  • Place a small sample on a hard, non-breakable surface and hit it with a hammer
  • If you press a sample on a newspaper, does ink transfer to the sample?
  • Place a small sample on the blackboard and mark its location, come back in a day or two and compare the location of the sample
  • Do all the same tests on a commercial sample of Silly Putty
In your notebook compare your sample with the commercial sample and provide an explanation for each the behavior you observe in each test.


 

Dropping 50 lbs of silly puttyfrom 7th floor

Earlier I referred to these cross-links as bridges between the long polymer molecules. That isn't completely accurate because while they do indeed connect the side of one long molecule to another long molecule, they aren't fixed in place like a bridge. Rather, they are dynamic bonds, moving from one connection point to another along the polymer as the thermal motion of their component atoms periodically shake them loose. At a given temperature this migration of cross-link bonds takes place at a fixed rate. At lower temperatures it's slower, at a higher it's faster. A pair of the long polymer molecules can only slide past each other when the cross-linking bonds have temporarily broken free. Similarly, a large mass of such cross-linked material can only flow or be stretched at time scales that are slower than the time scale of the cross-link bond break-and-reconnect rate. If you try to stretch or squash the material faster than this, the cross-links slow the ability of the long molecules to slide over one another and the material gets harder. The faster the material is deformed the fewer cross-links are disconnected and the more rigid the material is. At very high deformation rates, snapping it or throwing it at the floor, none of the cross-link bonds have time to disconnect and the material is locked up solid: it snaps in two or bounces. Taken to the extreme, smashing it with a hammer, causes the material to shatter. (Its behavior of mechanical properties changing in response to deformation rates places Silly Putty in the family of non-Newtonian fluids or dilatant compounds.)

Silly putty is made from mixtures of the two principle compounds at concentrations, mixing rates and mixing durations so that at room temperature, when manipulated at normal human-hand articulation speeds (1-inch per second) it feels soft and pliable. This corresponds to a slow squeeze of the hand. At higher speeds, suddenly squashing it between two fingers as fast and hard as you can (around 3-inches per second) the texture feels more like that of a rubber ball. Faster still, place it on a table and pound it with a fist (12-inches per second) and it feels like hard rubber and doesn't yield at all. Place it in a freezer and it gets stiffer. But note: it doesn't freeze because there is no phase change to a rigid solid. It just gets harder to deform because the lower thermal energy in it means the atoms of the cross-link bonds aren't as active and the cross-links don't migrate as fast. Warm it and the reverse happens: it flows easily at higher deformation rates and may even "splat" instead of bounce if dropped from a short distance.