Degradation of cellulose triacetate

Derham, M., Edge, M., WTUiams, D. A. R., WTUiamson, D. M. (1992). The Degradation of cellulose triacetate studied hy nuclear resonance spectroscopy and molecular modeling. In Postprints of Polymers in Conservation conference Manchester, 17-19 July 1991 (N.S. AUen, M. Edge and C.V. Horie, eds.) pp. 125-137 Royal Society of Chemistry. 

Deters, and Huang (129) describe the formation of graft copolymers of cellulose triacetate and vinyl chloride in vibratory mill treatments. Through hydrolytic degradation of the triacetate backbone, they isolated the polyvinyl chloride side chains and characterized them by infrared spectroscopy and cryoscopic molecular weight determination. The length of the side chains has been found to be between 15 and 30 vinyl chloride units. 

The majority of investigations on the degradation of cellulose acetate have been conducted on photographic film (cellulose triacetate) rather than moulded material. like cellulose nitrate, cellulose acetate (CA) is deteriorated by both physical and chemical factors and the physical cause of degradation is plasticizer loss. Three-dimensional objects moulded from cellulose acetate comprise 20-40 per cent by weight plasticizer. Typical plasticizers include triphenyl... 

A series of simultaneous, complex reactions occurs during acetylation of cellulose and during hydrolysis of cellulose triacetate. Reactions include acetylation of the cellulose, some sulfation of the cellulose, degradation of the cellulose chain and reaction to chain-ends, acetylation of the hemicellulose components, and then random hydrolysis of the acetylated or sulfated product [6-12],... 

Many cellulose derivatives form lyotropic liquid crystals in suitable solvents and several thermotropic cellulose derivatives have been reported (1-3) Cellulosic liquid crystalline systems reported prior to early 1982 have been tabulated (1). Since then, some new substituted cellulosic derivatives which form thermotropic cholesteric phases have been prepared (4), and much effort has been devoted to investigating the previously-reported systems. Anisotropic solutions of cellulose acetate and triacetate in tri-fluoroacetic acid have attracted the attention of several groups. Chiroptical properties (5,6), refractive index (7), phase boundaries (8), nuclear magnetic resonance spectra (9,10) and differential scanning calorimetry (11,12) have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers this calls into question some of the physical measurements on these mesophases, because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem (13), and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined (14,15). The mesophase formed by (hydroxypropyl)cellulose (HPC) in water (16) is stable and easy to handle, and has thus attracted further attention (10,11,17-19), as has the thermotropic mesophase of HPC (20). Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl acetamide (21) and for the benzoic acid ester of HPC in acetone and benzene (22) have been published. Anisotropic solutions of methylol cellulose in dimethyl sulfoxide (23) and of cellulose in dimethyl acetamide/ LiCl (24) were reported. Cellulose tricarbanilate in methyl ethyl ketone forms a liquid crystalline solution (25) with optical properties which are quite distinct from those of previously reported cholesteric cellulosic mesophases (26). 

Allen, N. S., Edge, M., Jewitt, T. S., Horie, C. V. (1992). Degradation and stabilization of cellulose triacetate base motion picture film. Journal of Imaging Science and Technology. 36(1), 4-12. 

Allen, N. S., Edge, M., Jewitt, T. S., Horie, C. V. (1992). Degradation and stabilization of cellulose triacetate base motion picture film. Journal of Imaging Science and Technology, 36(1), 4—12. Ankersmit, H. A., van Langh, R. (2002). The removal of lacquers from silver by steam. In 1. A. Mosk, N. H. Tennent (Eds.), Contributions to conservation Research in conservation at the Netherlands Institute for Cultural Heritage (ICN) (pp. 1-9). lames lames. 

Solution Process. With the exception of fibrous triacetate, practically all cellulose acetate is manufactured by a solution process using sulfuric acid catalyst with acetic anhydride in an acetic acid solvent. An excellent description of this process is given (85). In the process (Fig. 8), cellulose (ca 400 kg) is treated with ca 1200 kg acetic anhydride in 1600 kg acetic acid solvent and 28—40 kg sulfuric acid (7—10% based on cellulose) as catalyst. During the exothermic reaction, the temperature is controlled at 40—45°C to minimize cellulose degradation. After the reaction solution becomes clear and fiber-free and the desired viscosity has been achieved, sufficient aqueous acetic acid (60—70% acid) is added to destroy the excess anhydride and provide 10—15% free water for hydrolysis. At this point, the sulfuric acid catalyst may be partially neutralized with calcium, magnesium, or sodium salts for better control of product molecular weight. 

Excess acetic anhydride is then killed by adding stop acid (aqueous acetic acid). It is important to note that the addition of the stop acid has three purposes it kills excess anhydride it helps to desulfate the residual sulfate linkages, especially when added slowly [6] and it provides some water in the reaction mixture so that the latter is no longer anhydrous. This last item is important because chain degradation of the cellulose triacetate is much slower in an aqueous acetic acid system than it is in an anhydrous acetic acid system, especially at elevated temperatures. Therefore, stop acid helps to maintain the target viscosity because in many processes not all of the sulfuric acid catalyst is neutralized at this point. 

Neither acetate or triacetate has good stability in the melt. Severe discoloration and considerable decomposition of the melt occur, especially if held long as a melt. Therefore, meltspinning of cellulose acetate or triacetate is not considered to be an attractive method for producing fibers. However, a relatively small amount of triacetate yarn has been made by melt-spinning. Special techniques are necessary to prevent degradation at high temperatures at which triacetate melts (>300°C). Therefore, a primary requirement is to hold the polymer in the molten state for only a short time [36]. 

To illustrate, chloroparalEns are compatible with cellulose triacetate up to 50% but do not render any plastieiziug actioa On heating these plasticized films become fragile. On immersion in water, the ehloroparafBns are washed away completely. Chloroparaffins are extracted from the other derivatives of eellnlose in a similar way. The increase in the chloroparaffin content in PVC has httle effect on Tg. Low temperature performance is degraded as the chloroparafBn fraction increases in a mixture with phthalate plasticizers. 

Cellulose triacetate can be spun to fibers from methylene chloride solution. The triacetate fiber is very resistant to weathering and has good crease resistance. Part of the triacetate is oxidatively degraded and then spun from methylene chloride or chloroform to provide fibers for cable coverings. 

Synthetic fibers have been characterized by a resistance to degradation over forensically relevant timescales (Table 7.4). Nylon (polyamide), polyester, and acrylic fibers show considerable resistance to soil burial. Regenerated cellulose fibers (rayon viscose), however, share the vulnerability of natural cellulose to decomposition (Rowe 1997). However, they do show a higher degree of resistance to biodegradation compared with natural fibers or regenerated cellulose, with the exception of triacetate. 

In the presence of acetic anhydride, acetic acid, and a little sulfuric acid, cellulose is converted into the triacetate. Partial hydrolysis removes some of the acetate groups, degrades the chains to smaller fragments (of 200-300 units each), and yields the vastly important commercial cellulose acetate (roughly a //acetate). 

Cellulose ester membranes (diacetate or triacetate) these membranes are highly permeable, so they have a good filtration capacity. They are inexpensive and easy to implement. However, they have a few drawbacks sensitivity to temperature aud pH, and risk of degradation by microorganisms. Cellulose acetate and nitrate... 

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