0.7 Polymers  (Page 2/5)

Synthetic celluloid derives from natural cellulose and stems from an accident that Christian Schoenbein, a chemistry professor had in 1846. The age of plastic had begun, although the interest in cellulose nitrate was initially more for their explosive properties. When cellulose (from wood chips or fiber) is treated with a mixture of nitric acid, camphor, and alcohol, the resultant product is called Celluloid ${}^{\text{TM}}$ and bears very little resemblance to the starting material. Celluloid ${}^{\text{TM}}$ possess the ability to be molded into hard, smooth billiard balls (replacing the original, very expensive ivory balls) and into thin sheets for making movie pictures. Celluloid ${}^{\text{TM}}$ is highly flammable and today has been replaced by greatly improved synthetic polymers such as bakelite discovered in 1907 by the Belgian-American Chemist, Leo H. Baekeland.

When cellulose is treated with sodium hydroxide and carbon disulfide $({\text{CS}}_2)$ , cellulose xanthate is formed. A viscous (thick) solution of cellulose xanthate, forced through fine holes into dilute sulfuric acid, regenerates the cellulose as fine, continuous, cylindrical threads called rayon. If the solution is forced instead through a narrow slit, a thin transparent film or sheet is obtained called cellophane.

Classification of Polymers: Thermoplasts versus Thermosetting

Polymers generally are classified into two broad groups in accordance with their behavior upon heating. Polymers that can be repeatedly melted and solidified (without damage) are said to be thermoplasts; those that solidify once but will not melt again without damage are said to be thermosets. Technically, only thermoplasts are true plastics, even though the term "plastic" is commonly applied to all synthetic polymers.

The thermoplastic substance contains long, thin molecules which form tangled chains and is rigid at lower temperatures but gradually softens upon the application of heat, after it passes a characteristic temperature known as its glass transition temperature (Tg). Below Tg, the substance is brittle, having the characteristic properties of a glass; above Tg, the substance becomes flexible and soft. Chewing gum is a thermoplast that becomes extremely brittle when the outside temperatures drop below its glass transition temperature - this is a useful property to use in order to remove chewing gum from your clothes. Once warmed above Tg, however, the gum quickly softens and regains its flexibility. Some thermoplasts, such as polystyrene, melt before reaching their glass transition temperatures and remain rigid materials up to their melting points. Thermoplastic polymers are used frequently for injection molding of such items as food storage containers and toys that are not exposed to high temperatures. Additionally, thermoplastic polymers can be molded, pressed and extruded.

 Thermosetting substances contain large, cross-linked molecules that are rigid at lower temperatures, and undergo irreversible chemical and physical changes (including decomposition) upon heating. Such substances remain solids at higher temperatures than thermoplastic materials, and they do not melt. Thermosets are often employed in high-temperature environments, such as for electrical insulation in electric motors and gasoline engines.