Cellulosics are synthetic plastics, but they are not synthetic polymers; they are made from a naturally occurring polymer, cellulose, which is obtained from wood pulp and cotton linters. Cellulose can be made into a film (cellophane) or into a fiber (rayon), but is must be chemically modified to produce a thermoplastic material.
The resins used for plastics production are cellulose acetate, cellulose acetate butyrate, and cellulose propionate, all of which are cellulose esters, and ethyl cellulose, which is a cellulose ether. The plastics produced from them are commonly referred to as "acetate," "butyrate," "propionate," and "ethyl cellulose." They are processed by conventional thermoplastic processes.
The cellulosic plastics are supplied in a wide range of transparent, translucent, and opaque colors, including pearlescents and mottles. Clear resins are available in all except ethyl cellulose, which is light amber in uncolored formulations.
Properties: Because the cellulosics can be compounded with many different plasticizers in widely varying concentrations, property ranges are broad. These materials are normally specified by flow (defined in ASTM D569), which is controlled by plasticizer content. Hard flows of (low plasticizer content) are relatively hard, rigid, and strong. Soft flows (higher plasticizer content) are tough, but less hard, less rigid, and less strong. They also process at lower temperatures. Thus, within available property ranges listed, no one formulation can provide all properties to the maximum degree. Most commonly used formulations are in the middle flow ranges.
Molded cellulosic parts can be used in service over broad temperature ranges and are particularly tough at very low temperatures. Ethyl cellulose is outstanding in this respect. These materials have low specific heat and low thermal conductivity -- characteristics that give them a pleasant feel.
Dimensional stability of butyrate, propionate, and ethyl cellulose is excellent. Plasticizers used in these materials do not evaporate significantly and are virtually immune to extraction by water. Water absorption (which causes dimensional change) is also low, that of ethyl cellulose being lowest. The plasticizers in acetate are not as permanent as those in other plastics, however, and water absorption of this material is slightly higher.
Butyrate and propionate are highly resistant to water and most aqueous solutions except strong acids and strong bases. They resist nonpolar materials such as aliphatic hydrocarbons and ethers, but they swell or dissolve in low-molecular-weight polar compounds such as alcohols, esters, and ketones, as well as in aromatic and chlorinated hydrocarbons. Acetate is slightly less resistant than butyrate and propionate to water and aqueous solutions, and slightly more resistant to organic materials. Ethyl cellulose dissolves in all the common solvents for this polymer, as well as in such solvents as cyclohexane and diethyl ether. Like the cellulose esters, ethyl cellulose is highly resistant to water.
Although unprotected cellulosics are generally not suitable for continuous outdoor use, special formulations of butyrate and propionate are available for such service. Acetate and ethyl cellulose are not recommended for outdoor use. Most cellulosic formulations are available in 94HB flammability classification. Also available are formulations that can be used in contact with food.
Applications: Acetate applications include extruded and cast film and sheet for packaging and thermoforming, and extruded rod for tool and appliance handles and machine parts, eyeglass frames, pen barrels and caps, and quality toys.
Propionate molding applications include automotive and marine steering wheels, fuel-filter bowls, toothbrush handles, containers for cosmetics, face shields, safety goggles, and machine components that require transparency and toughness.
Transparent sheets are used in heavy-duty blister packaging and transparent packaging for food contact (FDA-accepted formulations) applications.
Butyrate is used for data-processor and cash-register keys, transparent dial covers, tool handles, and covers for instrument-panel lights. Opaque formulations are used for pens, switch covers, and knobs.
Typical applications for weathering-grade formulations are outdoor signs, window-well covers, skylights, and sprinklers. Extruded sheet is used for windshields, industrial shields, and thermoformed signs. Extruded-profile applications include table edging, tool handles, and trim strips.
Ethyl cellulose is used in helmets, gears, rollers, slides, flashlight housings, and tool handles. Military applications utilize its toughness and its resistance to nitroglycerine and black powder.