Cellulose acetate high acetyl

Cellulose Acetates (CA). Acetylation of cellulose is obtained by reaction of the natural polymer with acetic anhydride. The reaction is catalyzed by sulfuric acid. However, to obtain derivatives of high D.S. (>92%), it is advisable to operate in the presence of a diluent. When the diluent is a solvent of cellulose acetate—for instance, acetic acid—the cellulose is gradually swollen by the solvent as substitution proceeds, the latter being catalyzed by mineral acids (Lewis or Brpnsted acid). This process is called acetylation in the homogeneous phase. 

Most cellulose acetate is manufactured by a solution process, ie, the cellulose acetate dissolves as it is produced. The cellulose is acetylated with acetic anhydride acetic acid is the solvent and sulfuric acid the catalyst. The latter can be present at 10—15 wt % based on cellulose (high catalyst process) or at ca 7 wt % (low catalyst process). In the second most common process, the solvent process, methylene chloride replaces the acetic acid as solvent, and perchloric acid is frequentiy the catalyst. There is also a seldom used heterogeneous process that employs an organic solvent as the medium, and the cellulose acetate produced never dissolves. More detailed information on these processes can be found in Reference 28. 

High temperature acetylation of cellulose above 50°C produces cellulose acetate from low purity wood pulp cellulose in shorter reaction times. In a high temperature method recently disclosed (102), cellulose reacts with 200—400% acetic anhydride in the presence of <5% acid catalyst at 68—85°C for 3—20 min. After the acid catalyst is neutralized with magnesium acetate, the cellulose acetate is hydrolyzed at 120°C for two hours (103). Several modified catalyst systems have been developed for acetylation of cellulose above 90°C (89,90). 

As previously discussed, solvents that dissolve cellulose by derivatization may be employed for further functionahzation, e.g., esterification. Thus, cellulose has been dissolved in paraformaldehyde/DMSO and esterified, e.g., by acetic, butyric, and phthalic anhydride, as well as by unsaturated methacrylic and maleic anhydride, in the presence of pyridine, or an acetate catalyst. DS values from 0.2 to 2.0 were obtained, being higher, 2.5 for cellulose acetate. H and NMR spectroscopy have indicated that the hydroxyl group of the methy-lol chains are preferably esterified with the anhydrides. Treatment of celliflose with this solvent system, at 90 °C, with methylene diacetate or ethylene diacetate, in the presence of potassium acetate, led to cellulose acetate with a DS of 1.5. Interestingly, the reaction with acetyl chloride or activated acid is less convenient DMAc or DMF can be substituted for DMSO [215-219]. In another set of experiments, polymer with high o -celliflose content was esterified with trimethylacetic anhydride, 1,2,4-benzenetricarboylic anhydride, trimellitic anhydride, phthalic anhydride, and a pyridine catalyst. The esters were isolated after 8h of reaction at 80-100°C, or Ih at room temperature (trimellitic anhydride). These are versatile compounds with interesting elastomeric and thermoplastic properties, and can be cast as films and membranes [220]. 

Overall the results led to the conclusion that acetylated nanoparticles of both starch and cellulose offer potential eco-friendly substitutes for the conventional filler carbon black upto 40 phr. They imparted high mechanical strength and elasticity with minimum compromise in themal stability and moisture absorption of the resulting bionanocomposites. Cellulose acetate nanoparticles afforded effective reinforcement even upto loadings as high as 50 phr. 

Nitrocellulose Cellulose Acetate Cellulose Acetate Butyrate High Acetyl High Butyl ... 

Whistler has reviewed the behavior of xylan on acetylation, and notes that dry xylan is acetylated with difficulty. Under reaction conditions which avoid drying, xylan is acetylated readily, to yield a diacetate which is insoluble in most solvents. Because of its insolubility, xylan diacetate, especially if the D. P. is relatively high, can cause filtration difficulties and haziness in commercial cellulose acetates. 

Cellulose acetate is an ester of cellulose and acetic acid. Hence, hydrolysis takes place when the pH of the solution with which a cellulose acetate membrane is in contact is too high or too low, lowering the degree of acetylation, defined as the number of hydroxyl groups (total of three in one D-glucopyranose unit) that can be acetylated. Because a degree of acetylation above 2.5 is required for satisfactory salt rejection in seawater desalination, excessive hydrolysis results in poor membrane performance. The pH values between 5 and 7 should be maintained when cellulose acetate membranes are used. ... 

The commercial products can be broadly distinguished as cellulose acetate (37-40% acetyl), high-acetyl cellulose acetate (40-42% acetyl), and cellulose triacetate (43.7-44.8% acetyl). 

High-acetyl cellulose acetate (40-42% acetyl) has found occasional use in injection-molding compounds where greater dimensional stability is required. However, processing is more difficult. 

There is no satisfactory commercial means to directly acetylate to the 2.4 acetyl level and obtain a secondary acetate that has the necessary solubility for fiber preparation. Since cellulose is highly crystalline and its polymer chains are held tightly together in an ordered manner through extensive hydrogen bonding, it is insoluble in the reaction medium until almost complete acetylation is achieved. Thus, commercially, cellulose is fully acetylated to triacetate and then hydrolyzed back to secondary acetate of 2.4 DS. Careful hydrolysis is nearly random yielding uniform polymer. 

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