Viscose rayon aging

Acetate showed a decline in aging rate from a value nearly equal to unity at 25 °C to 0.21 at 75 °C. This finding suggests that acetate has a Tg near but greater than 75 °C. The Tg of acetate has been reported to be 84 °C when conditioned at 65 rh and 70 °F (23). However, the broadness of the aging rate decline is inconsistent with expectations associated with Figure 1 as in the case of viscose rayon. 

A follow-up study of 343 Finnish viscose rayon workers was performed to examine the incidence of cardiovascular mortality from 1967 to 1982 (Nurminen and Hemberg 1985). Exposure to carbon disulfide varied greatly (approximately 22 ppm to <10 ppm), with a decrease in exposures after 1972. Within the first 5 years of follow-up (1967-1972), there was a 4.7-fold increase in ischemic and heart disease mortality compared with a cohort of paper mill workers. In the period of 1972-1974, the relative risk ratio was 3. 2. After all workers with high coronary risk factors were removed from exposure (19% of the cohort was exposed in 1977 compared to 53% in 1972), the risk of cardiovascular death was reduced to a ratio of 1.0 in the years 1974-1982. This study indicates that the cardiotoxic effects of carbon disulfide may be reversible with removal of individuals from the toxic environment. Caution must be used in interpreting these data because of the increase in the incidence of cardiovascular events in the aging cohort population and the possibility that carbon disulfide accelerates death in high-risk individuals. 

However, there are potential risks associated with using bamboo as a polymer source for rayon since there is currently a lack of transparency in the supply chain. It is not always clear which type of bamboo is used for fibre, where it is grown, how it is cultivated, how it is harvested, and so forth. To date, there is no known organic certification of bamboo. The process to make bamboo viscose rayon fibre is the same process that is used to produce viscose/rayon from any other plant source. The cellulose is extracted from the bamboo and then mixed with chemicals to convert the plant pulp into textile quality fibre. This process can be very polluting unless it is carefully controlled, and can be influenced by the age and condition of the equipment as well as by whether there is any by-product recycling or effluent treatment. Note that in most countries the fibre cannot be called bamboo, only rayon or viscose from bamboo (textileexchange.org). 

Viscose is a solution of cellulose xanthate in caustic soda. This solution can be processed into rayon or cellophane. The production of viscose consists of two steps. First, certain forms of cellulose, mainly from sulfite pulp and cotton linters, are reacted with pure caustic soda. This reaction converts the -OH groups on the cellulose unit to -ONa. The caustic soda must be pure and is obtained from mercury cells. Caustic soda from diaphragm cells contains sodium chloride which makes it unusable without purification. This solution must be aged for 2-3 days. Aging reduces the length of the cellulose chain which makes it easier to dissolve into caustic soda. The second step involves the reaction of the alkali cellulose and carbon disulfide in excess caustic soda. The net reaction of viscose is ... 

The first synthetic fiber for tires was rayon. Cellulose is initially treated with sodium hydroxide to form an alkali cellulose. It is then shredded and allowed to age in air, where it is oxidized and undergoes molecular weight reduction to enable subsequent spinning operations. Treatment with carbon disulfide produces cellulose xanthate, which is then dissolved in sodium hydroxide to form viscose. The material undergoes further hydrolysis and is then fed into spinnerets to produce the fiber. This fiber is passed through a bath of sulfuric acid and sodium sulfate, where the viscose fibers are further coagulated. 

The viscose obtained from Cross and Bevan was so successful for production of lamp filaments that Steam asked Topham to try to spin it for use in textiles. The first experiments failed dismally. After several years of painstaking work, Topham made several discoveries essential to the spinning of yam from viscose aging (ripening) of the solution, filtration to remove particles, multiple-hole platinum spinnerettes, and a circular, centrifugally operated yarn collecting device that twisted the yam and packaged it in convenient cake form [117]. The Topham box, as it is still called, or variations of it are still on many of the continuous-filament rayon machines today. 

The viscosity of the viscose, an important processing parameter, and the final rayon properties are dependent on the average chain length or DP of the cellulose. Control of this variable is achieved by aging the alkali cellulose crumb under conditions yielding the appropriate extent of depolymerization for the type of rayon produced. 

Another way to achieve rapid depolymerization is to irradiate the cellulose with a beam of accelerated electrons [146-148]. In this case, the time required for the depolymerization is of the order of seconds. This technology has been investigated as a means to eliminate the need to age alkali cellulose in the viscose process, and rayon has been successfully made from irradiated pulp [149,150]. The process, however, has not been adopted by the industry because, like double steeping, material handling poses special requirements. 

After shredding, the alkali cellulose crumb is aged to obtain the DP for the type of rayon to be produced. Not only is the DP important to fiber properties, but it also affects the processing characteristics of the viscose. Solution viscosity is proportional to the DP and polymer concentration. Consequently, those fibers requiring a relatively high DP for reasons of strength must be made from viscose containing less cellulose [236]. 

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