Esters cellulose

Cellulose dinitrate is obtained by treating cotton linters with a mixture of nitric acid, sulphuric acid and water (25 55 20), and treating the product with alcohol. Camphor, a favoured plasticizer, is mixed into the alcohol-wet cake, leading to the product celluloid , for which the most important application is the manufacture of table-tennis balls. 

Cellulose propionate (CP) and cellulose acetobutyrate (CAB) are more expensive than the acetate, but offer better dimensional stability (having lower water absorption) and improved toughness. For each ester class, a variety of grades is available, as with CA. Typical applications of CA, CP and CAB include transparent packaging, lamp shrouds and table lamps, control buttons, telephone dials, toys, tabular keys, spectacle frames and sunglasses, decorative trim for cars, and domestic appliances. Handles, from toothbrushes to cutlery and tools, and hammer heads, provide significant outlets for these plastics. 

An important characteristic of cellulose acetate is its transparency compared with other transparent materials it has much better impact properties 

Cellulose esters are usually classified in organic and inorganic depending on the acid that is used to esterify the hydroxyl groups of cellulose. 

Nevertheless, the use of fatty acids ( C6) is still possible, but only with the use of a coreagent that forms new stronger entities in situ. Among them we can cite triflu-oroacetic anhydride (Hamalainen et al, 1957) and A7,A7-dicyclohexylcarbodiimide (Samaranayake and Glasser, 1993). 

Cellulose acetates are by far the most important organic esters. The diacetate has DS = 2.4 and is fabricated either in filament form for fibres or in powder form to melt. Diacetate filaments are obtained by dissolution in acetone, extrusion through a spin and then evaporation of the solvent. The obtained fibres are used in textiles (called simply acetate ) and in cigarette filters (tow). The triacetate (DS = 2.9) finds application in brilliant textiles easy to dye. 

The industrial preparation of cellulose diacetate employs acetic anhydride with sulphuric acid as catalyst. The reaction is conducted at low temperature and cellulose starts to dissolve in the acetylation bath as the reaction progresses. The reaction is conducted until practically full acetylation. The homogeneous solution obtained is then hydrolysed to reduce the DS to 2.4. Precipitation in dilute acetic acid, then washing with water and finally drying produce cellulose acetate flakes. 

The dramatic reduction of hydrogen bonding renders cellulose acetates thermoplastic. However, their softening points are very close to their decomposition temperatures (around 300 °C). The use of external plasticizers (e.g. phtalates) is often used to process cellulose diacetate in plastic applications (eyewear, screwdriver handles, etc.). 

The predominant cellulose ester fiber is cellulose acetate, a partially acetylated cellulose, also called acetate or secondary acetate. It is widely used in textiles because of its attractive economics, bright color, styling versatiUty, and other favorable aesthetic properties. However, its largest commercial appHcation is as the fibrous material in cigarette filters, where its smoke removal properties and contribution to taste make it the standard for the cigarette industry. Cellulose triacetate fiber, also known as primary cellulose acetate, is an almost completely acetylated cellulose. Although it has fiber properties that are different, and in many ways better than cellulose acetate, it is of lower commercial significance primarily because of environmental considerations in fiber preparation. 

Cellulose triacetate is obtained by the esterification of cellulose (qv) with acetic anhydride (see Cellulose esters). Commercial triacetate is not quite the precise chemical entity depicted as (1) because acetylation does not quite reach the maximum 3.0 acetyl groups per glucose unit. Secondary cellulose acetate is obtained by hydrolysis of the triacetate to an average degree of substitution (DS) of 2.4 acetyl groups per glucose unit. There is no satisfactory commercial means to acetylate direcdy to the 2.4 acetyl level and obtain a secondary acetate that has the desired solubiUty needed for fiber preparation. 

The performance of a textile fabric is characterized by terms such as strength, hand, drape, flexibiUty, moisture transport, and wrinkle resistance. Although the interactions among fibers in a fabric array are complex, its properties reflect in part the inherent properties of the fiber as well as how the fibers are assembled. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

The cellulose esters are useful polymers for the manufacture of plastics. Until about 1950 they did in fact form the most important group of thermoplastics materials. The historical importance and significance of these materials have been discussed more fully in the first chapter of this book. 

The most important of the esters is cellulose acetate. This material has been extensively used in the manufacture of films, moulding and extrusion compounds, fibres and lacquers. As with all the other cellulose polymers it has, however, become of small importance to the plastics industry compared with the polyolefins, PVC and polystyrene. In spite of their higher cost cellulose acetate-butyrate and cellulose propionate appear to have retained their smaller market because of their excellent appearance and toughness. 

The early 1990s have also seen much interest in biodegradable cellulose acetate compounds. 

Cellulose can be reacted with lactones to result in cellulose hydroxy carboxylic acid esters (36). Cellulose can be activated with lye and subsequently reacted with lactones in suspension. There is no need to use cellulose specific solvents. In this way, polymeric cellulose esters with side chains are obtained. The cellulose esters are thermoplastic and biodegradable. 

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These cellulose esters form tough, strong, stiff, hard plastics with almost unlimited color possibilities. Articles made from these plastics have a high gloss and are suitable for use in contact with food. 

Although not as resistant as cellulose esters to acids, it is much more resistant to bases. An outstanding feature is its toughness at low temperatures. 

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