Regenerated Cellulose Fabrics: Facts Behind the Fibers

 This is the first in a series of posts on regenerated fibers and their fabrics. Many of the sustainably fashionable new fabrics that are giving eco-designers and even conventional runway designers hot eco-flashes are from the family of regenerated fibers – corn fibers such as Ingeo and Sorona, soy fabrics such as SoySilk® from Soy Protein Fiber (SPF – a byproduct of soybean manufacturing), bamboo, rayon, lyocell / Tencel®, Modal®, and Viscose®. 

A Short Overview of Textile Fibers.Fibers are the basic component of fabrics. Fibers from natural or manufactured sources are twisted together to form yarn and threads that are then woven or knit into fabrics and garments. Natural fibers come from plants (such as cotton, hemp, kenaf and flax), or from animals (such as wool, hair and fur), or insects (such as silk).

Manufactured fibers come in two flavors: synthetic fibers and regenerated fibers. Synthetic fibers are cooked up in large vats and are made entirely from chemicals. Some of the most common synthetic fibers are the thermoplastic, petroleum-based synthetic fibers such as polyester and nylon. Synthetic fibers also include the “green” PET fabrics (PolyEthylene Terephthalate) such as EcoSpun from Wellman Inc. which is made from recycled plastic soda bottles. EcoSpun lined coats and jackets are sold by several environmentally credentialed companies such as Sierra Club and Patagonia.

Manufactured regenerated fibers are made from the chemical-induced transformation of natural polymers and basically fall into two categories: protein origin and cellulose origin. Regenerated fibers of protein origin come from plant protein (such as corn, soy, alginate, and peanut), or from animal protein (such as casein from milk). Many of the new, hot eco-friendly fabrics – like Ingeo from corn and soy from soybeans – are manufactured from proteins found in plants.

Regenerated fibers of cellulose origin – bamboo, rayon, lyocell / TENCEL®, Modal® and Viscose® – are made of cellulose from tree wood and inner pith and leaves from bamboo plants using differing fiber manufacturing processes with common roots going back to France in the 1890s to produce a textile that was then called “artificial silk” or “art silk”. The textile industry adopted the term “rayon” in 1924. This family of regenerated cellulose fibers for textiles and fabrics has also been called reconstructed fibers or natural synthetic fibers. This post will be an overview of regenerated cellulose fibers and fabrics.

A Bit of Botany and Chemistry. Cellulose has been used to make fabric and clothing for millennium. Cellulose, the structural component of cell walls in green plants, is the most common organic compound on earth. Cotton is 90% cellulose and measurements of bamboo vary from 50% to 60% cellulose. Wood is composed of fibers that are 40% to 50% cellulose, 15% to 25% hemicellulose and fortified with 15% to 30% lignin. The most common organic compound on Earth, cellulose is the structural component of cell walls in all green plants. Like cellulose, hemicellulose is also a polysaccharide but hemicellulose is composed of short, weak sugar chains that can be easily hydrolyzed and decomposed by dilute acids or alkalis in water and by some enzymes. Lignin is the glue which fills the spaces in plant cell walls between the cellulose, hemicellulose and other compounds found in the cell walls. Lignin locks and sequesters atmospheric carbon into green plants and the decomposition of lignin in plants releases the trapped carbon back into the atmosphere. Generally, the higher the lignin content the harder the wood. The essential production processes for chemically manufacturing regenerated cellulose from bamboo and wood are:

1. Preprocessing of Wood Chips and Bamboo Pith. Imagine just for a moment about what kinds of processes and chemicals it must take to “cook” hard wood chips into a soft, pliable cellulose pulp that can be transformed into a softly, flowing dress.
   
   To transform hard wood into silky fabric, the cellulose must be separated from the hemicellulose, lignin, and all the other sugars, starches and other compounds found in plant cells and then formed into a cellulose wood pulp. Trees from tree farms are logged, debarked, and hacked into one-inch square wood chips. Strong bases such as sodium hydroxide (caustic soda) and sodium sulfide - called white liquor - are commonly used in a process called the kraft/soda process to digest the wood chips and produce cellulose wood pulp. The resulting waste products containing the chemical wastes, lignins, hemicellulose, and other non-cellulose compounds are called the black liquor. If the color of the wood pulp requires lightening, the wood pulp can be bleached using enzymes such as xylanase enzymes from bacterial isolates or ligninolytic enzymes, or with a hydrogen peroxide solution or sodium hydrosulfate solution, or with a dilute acids such as trifluoroacetic acid, or with elemental chlorine or other chlorinated substances.
   
   The cellulose wood pulp is dried to produce hard sheets of purified cellulose, also known as “dissolving pulp” or “dissolving cellulose” from selected wood chips or bamboo stalks. The purified cellulose sheets are sometimes bleached with sodium hypochlorite (NaOCl) to remove remaining color. The preprocessing removes most of the lignin, hemicelluloses, free sugars, mineral salts, and starches found in plant cell walls along with cellulose. The resulting purified cellulose sheets contain 87% to 98% long-chain cellulose molecules. Wood chips that are commonly used include spruce, pine, hemlock, beech, and the leaves and inner pith of bamboo. The preprocessing chemicals and amounts used will vary according to the different types of wood.


2. Processing of Purified Cellulose. The dissolving cellulose pulp sheets are soaked in a caustic alkali solution of 15% to 20% sodium hydroxide, also known as caustic soda, to produce sheets of alkali cellulose. The alkali cellulose sheets are shredded, aged for a few days under closely controlled temperature and humidity, and then bathed in liquid carbon disulfide which transforms the cellulose into cellulose xanthate. Excess carbon disulfide is removed to produce cellulose sodium xanthogenate which is then dissolved in a solution of sodium hydroxide creating a viscous solution.
3. Regenerating Cellulose into Fibers. The viscose cellulose solution is aged to allow xanthate groups to revert back to cellulosic hydroxyls and free carbon disulfide, filtered to remove undissolved materials, vacuum treated to remove tiny air bubbles which could weaken the fiber strands, and then forced through spinneret heads (similar to a shower head) to create fine streams of viscose threads in a sulfuric acid bath. The sulfuric acid causes the cellulose xanthogenate to coagulate and bond into filaments of pure regenerated cellulose fibers.


4. Drawing, Bonding, and Cleaning. The newly regenerated fibers are washed in a weak solution of sodium sulfide to remove sulfur impurities and sometimes bleached again to remove discolorations. The fibers are stretched which causes the cellulose chains to reorient along the fiber axis and allows the cellulose chains to also cross-bond as they become parallel to each other. The cellulose fibers are given a final bath to wash away lingering chemicals from the manufacturing process. The fibers are dried, and rolled onto spools for new yarn and threads.

Environmental Hazards & Health Problems. The preprocessing of wood chips into a cellulose wood pulp for conventional rayon fibers can be environmentally messy. Factories for manufacturing wood pulp are often not located at the same facility which later takes the dissolving cellulose pulp sheets and transforms them into rayon fibers for textile. Because the pulp manufacturing process requires large amounts of water, they are often located near large rivers. The inorganic chemicals are recovered for reuse in other pulping processes.  For more about about the recover process for pulp manufacturing, check this out.  Removing the lignins and other contaminants from wood releases large amounts of organic materials, high biological orxygen demand (BOD), dissolved organic carbon, and a variety of alcohols and heavy metals into the waste waters and into rivers if they are not properly treated.

The early manufacturing of regenerated cellulose into rayon created worker safety hazards from chemical fumes escaping during the processing and environmental hazards from harsh and toxic chemicals escaping in wash waters and waste byproducts. Strengthened environmental protection standards and worker health regulations have lead to improved manufacturing processes but most pulp producing and rayon fiber manufacturing factories are still a long way from being sustainable.

Sodium hydroxide in strong solutions used during the pulping process can be very caustic and can burn skin. Sodium sulfide can react to produce hydrogen sulfide which is a toxic gas. The bleaching process to lighten pulp color is generally the most environmentally problematic, especially if it uses elemental chlorine, chlorine dioxide, or hypochlorous acid in aqueous solution. The use of chlorine in bleaching wood pulp can result in chlorinated byproducts that are toxic and difficult to eliminate with conventional waste treatment. Bleaching processes that use hydrogen peroxide are safer for the environment and for human health.

Toxic chemicals used to manufacture cellulose wood pulp into rayon fibers must also be reclaimed or neutralized from all waste waters and a considerable amount of solid waste byproducts from the non-cellulose components in the wood. Carbon disulfide, lignin and xanthates in the waste solutions are environmental hazards and must be removed from the waste waters. Depending upon the pulping and bleaching processes, contaminants from the pulping process can span a wide range of toxicity from suspended waste solids to carcinogens like dioxins and polychlorinated biphenyls (PCBs).

The great unknown is how willing and capable is each individual fiber manufacturing facility at removing the toxicity of waste products before discharging them into community rivers and streams or dumping them into landfills. It all depends upon the host countries environmental protection laws, worker safety laws and the willingness of local government to enforce any laws that might exist.

Improved Rayon Processing. Technical advancements in rayon processing have lead to improved rayon fabrics such as high wet modulus (HWM) rayon, also known as polynosic rayon and better known by its trade name of MODAL®. Another advanced rayon is lyocel, which is better known by the Lenzing Group trade name for their highly popular TENCEL®. These technical advancements have created a rayon that is not only less prone to stretching when wet but, more importantly, they have also created a closed-loop processing that allows 99.5% of the chemical solvents to be recycled and reused and any remaining emissions and pollutants can be decomposed in waste treatment plants.

The manufacturing processes for lyocell and modal differ significantly from those commonly used to manufacture other varieties of rayon. An informed article by Angela Woodward outlines the generic lyocell / TENCEL® processes for closed-loop manufacturing as follows:

1. Preprocessing of Wood Chips. Select hardwood logs are chipped into small pieces and feed into large metal digester tanks to be cooked at high temperatures under steam in a strong alkali chemical solution which reduces the wood chips to a pulp. The pulp is washed with water to remove the chemicals and then bleached to lighten and create a uniform color. The wood pulp is dried into sheets that are rolled into large spools which are functionally similar to the purified cellulose sheets in the generic rayon process.
2. Processing of Purified Cellulose. The cellulose sheets are crumbled into small pieces, and cooked again in large, enclosed tanks under high pressure and temperatures in a solution of N-methylmorpholine-N-oxide or some other member of the amine oxide family to dissolve the crumbed cellulose pulp sheets into a liquid solution. The liquid solution is then filtered to remove any undissolved pulp chips.
3. Regenerating Cellulose into Fibers. The filtered cellulose solution is forced through spinneret heads into a diluted amine oxide solution to cause the cellulose strands to set and align. The cellulose fibers are then washed in a de-mineralized water solution.
4. Finishing. The filaments are dried and then coated with a lubricant, such as soap or silicone, so that their filaments untangle easily. This is something like applying conditioner to hair after washing. The filaments are carded so they all lay in the same direction, crimped to give the fibers body (again, think of crimping hair), the crimped and carded filaments are cut to a uniform length and baled together. The baled filaments can then be sent to fiber mills where they can be twisted into yarns and woven or knit into fabric. The whole process from processing the purified cellulose sheets to baling the regenerated cellulose fibers is supposed to take only 2 hours.
5. Recovery of Solvents. The amine oxide is recovered from the processing solutions and reused. The recovery process is supposed to reclaim more than 99% of the amine oxide and any unrecovered amine oxide is decomposed in the waste treatment processing.

The basic manufacturing process for regenerating bamboo leaves and pith into bamboo fibers for clothing is covered in our post “Bamboo: Facts Behind the Fiber”.

The preprocessing of wood chips into pulp for the advanced cellulose fibers of lyocell, Modal and Viscose is essentially the same as for conventional rayon. The processing of the wood cellulose pulp into fiber is more sustainable than the processing used for conventional rayon because the closed-loop process is supposed to capture and reclaim almost all the chemical solvents used in the manufacturing. Also, the solvents such as N-methylmorpholine-N-oxide (NMMO) used to dissolve the bamboo or wood chip cellulose into a viscose solution are from the chemical family of amine oxides which are supposed to be environmentally less harmful.

The important point in understanding the nature of regenerated cellulose fibers is that the underlying process of extracting and purifying the cellulose cells, reducing them to a viscose solution, and then regenerating them into manufactured fibers is essentially the same regardless of whether the source is wood from trees or from bamboo. The differences in fabric properties such as texture, hand, pilling, fibrillation, and dye acceptance generally result from different chemicals and enzymes and their dilution strengths and from processing techniques used during the fiber and finishing.

To be an environmentally and socially ethical fashionista, know how your fabrics are made and then make responsible decisions.

Enjoy.

-Michael