Acetylated rayon fiber-cellulose acetate

Fig. 53. Scanning electron micrograph of the composite acetylated rayon fiber-cellulose acetate matrix.

Fig. 55. Scanning electron micrograph of a cellulose-cellulose con osite. (A) and (B) Rayon fiber-cellulose acetate composites, after regeneration of cellulose acetate in aUcahne conditions, showing the poor quality of the interface. (C) and (D) Acetylated rayon fiber-ceUulose acetate composites after coregeneration in alkaline conditions, showing disappearance of the interfece.

The basic cellulose unit contains three hydroxyl groups. The triester cellulose triacetate forms when cellulose is reacted with glacial acetic acid. Hydrolysis removes some of the acetate groups to form a secondary ester, which averages about 2.4 acetyl groups per unit rather than three. The secondary ester is then dissolved in acetone and the solution ejected through a spinneret to form fibers. Cellulose acetate processed in this manner is referred to as acetate rayon, but it may be more commonly known by its trade name Celanese. 

One common feature of the viscose, euprammonium, and carbamate processes is the chemically modified cellulose is regenerated into cellulose after the extrusion. Fibers made from these processes often are called rayon fibers. However, useful fibers can be produced by derivative methods without the regeneration of cellulose. Two important examples are cellulose acetate and cellulose triacetate fibers. Cellulose acetate can be obtained by aeetylation of cellulose with acetic acid and acetic anhydride with sulfuric acid as a catalyst. Cellulose triacetate, which is partly saponified to get the desired degree of substitution, is produced by a similar process. Both cellulose acetate and cellulose triacetate keep their derivative structure in final fibers. The major difference between these two fibers is that in cellulose acetate fibers, less than 92% but at least 74% of the -OH groups are acetylated, but in triacetate fibers, at least 92% of the hydroxyl groups are acetylated. 

Uses Manufacture of acetate rayon, acetic anhydride, acetone, acetyl compounds, cellulose acetates, chloroacetic acid, ethyl alcohol, ketene, methyl ethyl ketone, vinyl acetate, plastics and rubbers in tanning laundry sour acidulate and preservative in foods printing calico and dyeing silk solvent for gums, resins, volatile oils and other substances manufacture of nylon and fiber, vitamins, antibiotics and hormones production of insecticides, dyes, photographic chemicals, stain removers latex coagulant textile printing. 

Acetate rayon is cellulose (from any source) in which about two of the hydroxyl groups in each unit have been acetylated. This renders the polymer soluble in acetone from which it can be spun into fiber. The smaller number of hydroxyl groups in acetate rayon compared to cotton makes direct dyeing of rayon more difficult than of cotton. 

Cellulose acetate was first prepared in 1865, and was a suitable candidate to replace nitrocellulose. Cellulose triacetate, however, was less soluble in common solvents than nitrocellulose and also was difficult to dye. Lower levels of acetylation gave more tractable, fiber-formable products, which could be more easily dyed, and could be formed into film suitable for photographic use. These were the so-called acetate rayon fibers and plastics, which were also considerably safer to use than nitrocellulose. 

These results show that the fibers of native rayon are not at all dispersed in a cellulose acetate matrix, but appear there as tangled bundles. On the other hand, acetylated fibers are able to disperse fiilly into tire cellulose acetate matrix, into... 

Fig. 52. Optical microscopy of (A) cellulose acetate matrix and rayon fibers (B) cellulose acetate and partially acetylated (38%) rayon fibers and (C) partially acetylated (38%) rayon fibers after partial removal of the cellulose acetate matrix. (See Color Plate 23.)...

A blank experiment has used this approach to produce a cellulose-eellulose eom-posite, using cellulose acetate to effect aggregation of rayon fibers. Analysis of the product obtained upon alkaline hydrolysis showed that the cellulose acetate had been fully regenerated as cellulose, but with no cohesion with preexisting cellulose at the interface level. The poor quality of the interface is clearly observable at the surface of the thin sections (Fig. 55). The same experiment was performed using the partially acetylated cellulose fibers that had been previously synthesized for the production of... 

Acetylated cellulose is then dissolved in a suitable solvent, precipitated, and drawn into fibers known as acetate rayon. Today acetate rayon fibers rank fourth in production in the United States, surpassed only by Dacron polyester, nylon, and rayon. 

Amongst the important chemical conversions of macromolecular substances are the various reactions of cellulose. The three hydroxy groups per CRU can be partially or completely esterified or etherified. The number of hydroxy groups acetylated per CRU are indicated by the names, i.e., cellulose triacetate, cellulose 2-acetate, etc. Another commercially important reaction of cellulose is its conversion to dithiocarboxylic acid derivatives (xanthates). Aqueous solutions of the sodium salt are known as viscose they are spun into baths containing mineral acid, thereby regenerating the cellulose in the form of an insoluble fiber known as viscose rayon. 

Because acetylation of cellulose proceeds in a heterogeneous system, the reaction rate is controlled by the diffusion of the reagents into the fiber structure. The quality of the cellulose raw material used for acetate rayon is... 

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