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Solvent spun cellulose fibers: an environmental perspective.

Solvent Spun Cellulose Fibers: An Environmental Perspective

the environment is affecting every segment of the nonwovens industry, including fiber production; the author explains the Mobile, AL solvent spun cellulosic installation from an environmental viewpoint Life on this planet can be said to depend on the carbon cycle in general and the processes of respiration and photosynthesis in particular. Animal respiration converts oxygen to carbon dioxide in the process of extracting energy from carbon compounds created by green plants. Green plants in their turn convert carbon dioxide into oxygen needed by animals and in the process make the carbon compounds (mainly cellulose) required to enable both plants and animals to grow.

In the last 200 years industrialization and the accompanying rapid growth of population, fossil fuel burning and deforestation have begun to affect the balance between the plant and animal kingdoms. Our survival as a species could be at stake and as this becomes increasingly clear the pressure to modify "industry" to increase our chances of survival will be enormous. We are stuck with a large and growing population and must therefore turn to science to help us move towards a balanced environment maintained by a sustainable industrial system.

The nonwovens industry, defined in cradle-to-grave terms to include such sectors as fiber and binder production and disposal route methods, produces some of the basic necessities of 20th century life. Our responsibility is to try and minimize the environmental impact of the processes by which such products are manufactured.

To do this we need to make comparisons, using the best available science, of the environmental impacts of the routes to nonwoven products. We must also remember that all products will ultimately require disposal and that it is our duty to ensure that the disposal methods are environmentally sound.

The Solvent Spun Cellulose Process

The new solvent spun cellulose process has one major raw material--the cellulose polymer--and one minor raw material--the amine oxide solvent. It also requires energy. If markets evolve along traditional lines, then some versions of the fiber will contain titanium dioxide dulling agents and some will be bleached. All are likely to be finished with the surfactants needed by the subsequent conversion processes. The non-cellulosic components and bleaching options arise because the market demands them, not because they are an essential part of the process.

Cellulose is the natural polymer that makes up the living cells of all vegetation. It is the material at the center of the carbon cycle and the most abundant and renewable biopolymer on the planet. Rayon fiber producers have converted it from the fine short fibers that come from trees into the fine long fibers used by textiles and nonwovens for almost a century. Rayon nevertheless remains unique among mass produced man-made fibers because it is the only one to use the natural polymer directly.

The manufacture of viscose rayon consumes 2.6 million tons of cellulose annually, with two million tons going into the staple fiber process. The Mobile, AL rayon plant of Courtaulds currently uses about 100,000 tons a year. When the first solvent spun plant is on-stream an additional 18,000 tons a year will be required.

Cellulose Extraction (Pulping)

The 2.6 million tons of dissolving grade pulp currently manufactured to feed the rayon fiber industry represent 0.01% of the annual production of cellulose (on land, in nature) and about 0.7% of the cellulose in wood used by industry.

The solvent spun process as currently designed will use the same sources of pulp (at slightly higher levels of efficiency) as the viscose process, but concerns related to pulp mill effluents are still with us. However, concerns over dioxins in the pulp itself and products made from the pulp are now behind us. The raw material and final products, both viscose and solvent process, have been shown by independent analysis to be free of such compounds at a detection limit of 0.5 parts per trillion.

Amine Oxide Solvent

N-methylmorpholine N-oxide (NMMO) is the solvent used. It is manufactured by methylation of morpholine, which comes from a reaction between diethylene glycol and ammonia.

While this is the only other major chemical used in the solvent spun process, its consumption is reduced to the absolute minimum by the recycling that is made possible by solvent recovery. In Courtaulds' Grimsby, U.K. plant, which has been operating semi-commercially for the last two years, techniques have been developed that now recycle virtually all of the solvent used to dissolve the pulp.

The Energy Factor

The usage of energy and the means by which it is obtained contributes a major component of the environmental impact of most complex industrial process sequences.

The methodology of assessing the energy usage of products and processes is currently the subject of much debate and a standardized approach has yet to emerge. Not surprisingly, most of the published work on fibers was carried out during the last energy crisis in the 1973-81 period; nothing could be found in the public domain concerning more recent studies.

There have been the following attempts [1-5] to assess the total energy required to make baled staple fiber from naturally occurring raw materials, wood in the case of cellulosics and oil in the case of synthetics. In general they break the fiber production sequence into monomer making, polymer making and fiber production and, while a variety of fibers are covered, only viscose rayon and polyester are mentioned in all of them. Tons of fuel oil equivalent per ton of fiber were the most popular units (TFOE/T) and Table 1 gives the values.

In addition, unpublished data gathered in 1982 by the International Committee for Rayon and Synthetic Fibers (CIRFS) puts polyester at 4.17 TFOE/T and viscose rayon at 1.69. A paper by Van Winkle (1978) compared cotton and polyester in shirts on a cradle-to-grave basis. He concluded that the natural fiber route was far more energy efficient up to the first use of the manufactured shirt, but in the lifetime of the shirt the costs of cleaning and drying the cotton product allowed polyester shirts to appear the most energy efficient overall.

In the same papers, nylon and acrylic fibers, where included, require more energy than polyester (about five times the rayon value), and polypropylene requires less (about 1.7 times the rayon value). Cotton requires less energy than viscose up to bales of raw fiber, but data for the bleached and cleaned versions generally needed in nonwovens is not presented.

The overall picture that emerges from these early studies was that while the wet spun cellulosic fibers required more energy than melt spun polyester for the fiber making step, they had no monomer energy requirement and the "polymerization" requirement was minimal. In the case of the very low values for rayon emerging from Lenzing, we think full credit was being given for the fact that the pulp mills' energy needs were in fact renewable and not dependent on fossil fuels and that pulp is fed directly into the viscose process without incurring any transport or drying costs. In other words, the pulp mill could be driven entirely by energy obtained from burning the parts of the tree that were not needed in the final product and this "free" and renewable energy was not counted.

From an energy viewpoint, the solvent route to cellulosic fibers is identical to the viscose route up to the point where the cellulose enters the solvent. The energy requirement for the non-cellulosic raw materials is significantly lower in the case of the solvent route, but the solvent route will require similar energy levels in dope handling, spinning, washing and recycling. The lower water imbibition of the solvent fiber (65% versus 95%) will yield savings in fiber drying and, of course, in any subsequent washing and drying operations.

Overall, the solvent route will show a useful economy in this important resource when compared with viscose production on the same scale.

Fossil Reserves

Renewable resources will become increasingly important as the planet's stocks of fossilized reserves are depleted. The viscose route currently needs fossil reserves for energy generation but for little else. At the Mobile plant, the vast majority of energy requirements come from locally available natural gas.

The solvent used in the new process is made from ethylene glycol, which currently comes via ethylene from oil refineries. However, as indicated above, the recycling rate is so high that solvent usage is kept down to a few kilos per ton of fiber.

Gaseous, Liquid Effluents

The process involves direct dissolution of cellulose in a liquid, which is recycled very efficiently. There are no chemical reactions and no by-products of the sort that are unavoidable in the regeneration of cellulose from the viscose route.

In the viscose process, gaseous effluent control and treatment is a fundamentally important part of the overall process and is continuously improving as the technology of the "closed box" process evolves. The air handling and cleaning systems employed are costly and most of the emissions to atmosphere are collected and discharged through tall stacks.

The solvent process produces very little atmospheric emission. There are traces of volatile organic compounds associated with the solvent and the soft finish that will leave the plant in the normal course of ventilation. There is no need for any central air handling or emissions stack.

The spinning and washing liquors from the viscose process are recycled to allow reuse of the sulfuric acid and zinc sulphate components wherever this is feasible from economic and environmental standpoints. Nevertheless, in common with most industrial washing and bleaching systems, large volumes of process water have to be cleaned on-site before discharge into the river.

The solvent route uses much less water overall and the process effluent needs significantly less treatment.


Cellulosic fibers are, as pointed out at the start of this article, simply a tiny subset of the most abundant bio-polymer on the planet. Like natural vegetation, they can become food for microorganisms and higher life forms (they biodegrade) and they will burn with a rather greater yield of energy than natural vegetation.

In complete biodegradation or incineration, the final breakdown products are carbon dioxide and water and in the overall sense these disposal methods simply recycle the cellulose to the atmospheric components from which it was made.

It is also possible to liberate and use some of the "free" solar energy that powered the polymerization step. In the case of incineration this is straightforward in that the free-burning cellulose can be used to generate steam and other energy.

In the case of landfill disposal, it is now well known that slow anaerobic biodegradation occurs in all municipal solid waste landfill sites. This process generates methane from cellulose, which can, and increasingly is, being used to drive gas turbines directly. Admittedly, this process makes only a small contribution to reducing the volume of waste in the landfill, but as fuel costs rise this "free" and renewable energy source will become more important.

Cellulosic fibers, made for the past century by the direct conversion of abundant vegetable matter, have always had much to recommend them in nonwovens compared with synthetics made from fossil fuels.

The renewability of their main raw material, their overall energy efficiency, their lack of dependence on fossil fuels, their long history of safe use in hygiene applications and their ease of disposal and natural recyclability make them strong contenders for tomorrow's nonwovens industry as well.

The new solvent route to cellulosics reinforces these inherent strengths by using a modern fiber production system that, being physical rather than chemical, reduces environmental impacts to a minimum. [Tabular Data Omitted]


[1]Woodhead; ICI; International TNO Conference, 1976 [2]Lane and McCombes; Courtaulds; Textile Manufacturer, 1:1979. [3] Kogler; Lenzing; EDANA Annual General Meeting, Munich 1980.6. Armstrong; consultant; EDANA Annual General Meeting, Munich 1980. [4] Armstrong, consultant; EDANA General Meeting, Munich 1980 [5]Van Winkle; "Cotton Versus Polyester;" American Scientists, Vol. 66, May/June 1978.
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Title Annotation:the effect of environmental concerns on the nonwovens industry
Author:Woodings, C.R.
Publication:Nonwovens Industry
Date:Jun 1, 1991
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