Silly String

Silly String (generically known as aerosol string) is a toy of flexible, sometimes brightly colored, plastic propelled as a stream of liquid from an aerosol can. The solvent in the string quickly evaporates in mid-air, creating a continuous strand. Silly String is often used during weddings, birthday parties, carnivals and other festive occasions, but it has also proven useful militarily to detect tripwires.

Silly String is made of a mixture of components dispersed throughout a liquid solvent in the product’s aerosol can. These substances include a polymer resin that provides the string’s structure, a plasticizer to tune the physical properties of the string, and a surfactant that promotes foaming of the product. Other ingredients include silicon fluid (to make the strands easier to clean up), flame retardant, and a pigment for color.

A key component in Silly String is its and the propellant that ejects the product mixture from the can. The product originally used chlorofluorocarbon propellant mixed with Freon 11, both part of a group of compounds believed to damage the ozone layer. In 1978, the United States banned the use of CFCs like Freon 11 and 12 in aerosol cans. The manufacturers then changed the formulation to use permitted propellants. Aerosol propellants are liquids with very low boiling points. When under pressure inside the can, the propellant is in liquid form, but when the nozzle is opened, it rapidly escapes – along with the compounds mixed in it – and evaporates as it enters the air. The string takes shape as the propellant evaporates.

The inventors sought to control the ratio of compounds in the product to form a string that holds itself together while remaining slightly sticky to the touch. This allows the product to weakly adhere to people and windows, for instance, but easily be cleaned up without the string falling apart or staining inert surfaces.

The current formulation is not published, but one of the primary recipes in the original patent calls for 12.2% of the synthetic resin poly(isobutyl methacrylate) by weight. It additionally calls for 0.5% of the selected plasticizer, dibutyl phthalate, 2.5% of sorbitan trioleate surfactant, 0.35% silicon fluid such as dimethyl siloxane or methyl phenyl siloxane, 5.6% of flame retardant hexabromobenzene, and 2-3% pigment (all percentages by weight). The aerosol propellant represents the bulk of the product. Solubility of the resin and other materials in the product is enhanced by addition of another solvent, originally Freon 11, in 6.6% by weight.

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