The modal manufacturing course of is the place each the sustainability story and the toxicity questions start. Sounds good and pure to date, proper? Beechwood bushes, regenerative forestry, delicate material? Nicely, the uncooked wooden fibers don’t keep pure for lengthy, as a result of turning stiff wooden chips into silky-soft underwear requires some critical chemistry.
Modal makes use of a modified viscose course of, which suggests it nonetheless includes among the identical chemistry as viscose, however with essential enhancements.
Right here’s the way it works, step-by-step.
Step 1: Wooden pulp preparation
Beech bushes are harvested and chipped into small items. The chips are cooked and chemically handled to separate cellulose from lignin and different compounds, then bleached and pressed into dissolving pulp sheets. At Lenzing’s built-in facility in Austria, pulp and fiber production happen at the same site, which saves vitality and supplies.
Step 2: Dissolving the cellulose
The dissolving pulp is steeped in sodium hydroxide (caustic soda) answer to create alkali cellulose. After growing older, this materials reacts with carbon disulfide (CS₂) in a course of known as xanthation. The ensuing substance dissolves in additional sodium hydroxide to create a deep orange, extremely viscous answer, which is the place viscose will get its identify.
That is the step that raises probably the most questions, as a result of carbon disulfide is a well-documented poisonous chemical. We cowl the toxicity query intimately within the subsequent part.
Step 3: Fiber formation
The viscose answer undergoes ripening and deaeration, then will get compelled by way of tiny holes known as spinnerets right into a sulfuric acid tub. This solidifies the filaments into lengthy, steady fibers. The fibers are then stretched, washed, and completed.
Modal fibers comply with the same path to viscose, however with particular modifications to the spinning answer and course of settings that end in fibers with characteristically excessive moist energy. That is what makes modal stronger and extra sturdy than customary viscose, and what separates it as a definite fiber sort.
Step 4: Chemical restoration (the closed loop)
That is the place the standard of manufacturing issues enormously.
In older, open-loop viscose manufacturing, the chemical substances used (notably carbon disulfide and hydrogen sulfide gasoline) had been launched into the setting, inflicting critical hurt to staff and surrounding communities. In response to the ATSDR Toxicological Profile for Carbon Disulfide, the most typical explanation for mortality related to uncovered viscose rayon staff in historic research was heart problems.
Trendy closed-loop methods are designed to seize these gases and get well them for reuse relatively than releasing them. In response to Lenzing’s chemicals page, of their viscose and modal manufacturing, carbon disulfide and hydrogen sulfide gases “are usually not launched into the setting however are recovered, transformed and returned to the manufacturing course of for use as uncooked supplies.”
How a lot will get recovered relies on the producer. Newer closed-loop applied sciences can get well 90-95% of the carbon disulfide used. Some sources point out Lenzing’s TENCEL™ Modal course of achieves up to 95% solvent recovery however we will’t discover a clearly disclosed quantity immediately revealed by Lenzing. It is a significant enchancment over older open-loop processes, nevertheless it’s additionally price noting that modal’s chemical restoration charge is decrease than lyocell’s 99.8% NMMO restoration charge, as a result of modal and lyocell use basically completely different chemistry.
So what occurs to the opposite 5-10%?
The unrecovered portion of carbon disulfide (and a associated byproduct, hydrogen sulfide) is primarily launched as air emissions. That is the primary environmental output of viscose and modal manufacturing, and it’s important sufficient that the ZDHC MMCF Guidelines particularly set limits on sulphur-to-air emissions and require services to report their restoration charges. The rules observe this utilizing a sulphur mass stability strategy. The CS₂ added to the method is measured in opposition to the CS₂ (and different sulphur compounds) recovered, and the distinction is what escapes to the environment.
As soon as within the environment, carbon disulfide has a comparatively quick lifespan. In response to peer-reviewed research cited in the ATSDR Toxicological Profile, CS₂ has a 12 day lifetime within the environment earlier than breaking down. It doesn’t accumulate persistently in the way in which that greenhouse gases, endlessly chemical, or microplastics do.
Nevertheless, the native impression might be significant. A 2022 study published in Atmospheric Pollution Research estimated world CS₂ emissions from the viscose fiber business at roughly 577.9 gigagrams in 2017, with China (65.7%) and India (11.1%) accounting for the most important shares. The ATSDR has documented elevated carbon disulfide ranges in communities close to viscose manufacturing services.
For context, Lenzing’s built-in Austrian facility operates below EU environmental rules, that are among the many strictest on the earth for industrial air emissions. The corporate has achieved ZDHC’s Progressive stage for chemical administration throughout its viscose and modal services, which means it meets the very best tier of the business’s chemical restoration, wastewater, and air emission requirements. However not all modal producers function below the identical regulatory framework or spend money on the identical stage of emission controls.
The underside line is that even at 90-95% restoration, the unrecovered CS₂ from modal manufacturing is launched into the air. At well-managed services in tightly regulated jurisdictions, these emissions are managed and monitored. At services with weaker oversight, they’ll pose actual dangers to staff and surrounding communities.
So how does this evaluate to lyocell?
Modal makes use of a modified viscose course of (sodium hydroxide + carbon disulfide). Lyocell makes use of a completely completely different chemistry (NMMO, a non-toxic natural solvent) in a closed-loop course of that recovers more than 99.8% of the solvent.
In plain language, modal is the improved second era of the viscose household, whereas lyocell is the third era constructed on utterly completely different know-how. Modal is healthier than customary viscose, nevertheless it nonetheless makes use of carbon disulfide, a chemical that lyocell avoids totally.
So lyocell is extra sustainable and fewer poisonous than modal.
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