Details
Material hierarchy: Polymer - Thermoplastic
Cellulose (C6H10O5) is a carbohydrate polymer commonly occurring in nature. Wood and cotton fibers, the chief industrial sources of cellulose, contain about 50% and 95% of the polymer, respectively. When cellulose is dissolved and reprecipitated during chemical processing, the resulting polymer is called regenerated cellulose. When this is produced as a fiber for apparel it is known as rayon (of course, cotton itself is a widely used fiber for apparel). When it is produced as a thin film, it is cellophane, a common packaging material. Cellulose itself cannot be used as a thermoplastic because it decomposes before melting when its temperature is increased. However, it can be combined with various compounds to form several plastics of commercial importance; examples are cellulose acetate (CA) and cellulose acetate-butyrate (CAB). CA is produced in the form of sheets (for wrapping), film (for photography), and molded parts. CAB is a better molding material than CA and has greater impact strength, lower moisture absorption, and better compatibility with plasticizers. The cellulosic thermoplastics share about 1% of the market.
The family of cellulosics includes cellulose acetate, cellulose acetate butyrate, cellulose propionate, and ethyl cellulose. There are other cellulosics as well, but these are the most widely used. Cellulosics are characterized by good strength, toughness, and transparency and a high surface gloss. In addition, they have good chemical resistance. Generally, these thermoplastics should not be used at temperatures much above 170 to 220°F.
Cellulose acetate, which is the lowest-cost cellulosic material, has good toughness and rigidity. This easily molded material is available in a variety of grades ranging from "soft" to "hard."
Cellulose acetate butyrate, although a little more expensive than the straight acetate, is somewhat tougher, with a hornlike quality, and has lower moisture absorption. It has relatively good weatherability and excellent transparency.
Cellulose propionate is similar to cellulose acetate butyrate in both cost and properties, but it has somewhat higher tensile strength, modulus, and impact strength than the latter material.
Ethyl cellulose is what might be called the impact grade of the cellulosics. The excellent toughness of this material (Izod impact strenths range from 1.7 to 8.5 ft-lb/in. of notch, depending on the specific resin) is maintained over a wide temperature range. Ethyl cellulose also has moderately low moisture absorption. In addition, this cellulosic is available in self-lubricating grades.
Nearly all cellulosics are noted for their toughness, but none of these materials is generally recommended for applications involving anything more than relatively low loads. The main feature of cellulosics is their excellent moldability, which results in a brilliant, high-gloss finish.
Cellulose acetate is subject to dimensional changes due to cold flow, extreme heat, and moisture absorption. Cellulose acetate butyrate is a slightly more stable material, but still not outstanding in comparison to some other thermoplastics. This material, however, is one of the few thermoplastics that resist weathering. This property, combined with its good optical properties, makes cellulose acetate butyrate an excellent material for outdoor signs. Cellulose propionate is similar in properties to the butyrate but has higher hardness and stiffness. Ethyl cellulose has several outstanding properties. It withstands heavy abuse and has good environmental resistance, and it retains toughness at low temperatures. Like other cellulosics, however, it is not primarily a load-bearing material.
Cellulosic compounds are available for extrusion, injection molding, blow molding, and rotational molding. Cellulosics also are widely used in the form of film and sheet.
Typical applications of the material include: cellulose acetate - knobs, tool handles, and face shields; cellulose acetate butyrate - tool handles, knobs, tubular packaging, toys, and signs; cellulose propionate - automobile steering wheels, pen and pencil barrels, packaging sheet, and toys; ethyl cellulose - flashlight housings, tool handles, and roller wheels.