The lyocell manufacturing course of is the place the material earns most of its sustainability credentials, and the place the toxicity story begins.
Step 1: Wooden pulp preparation
Wooden is debarked, chipped, and cooked to separate cellulose from lignin and different compounds. It’s then bleached (within the case of Lenzing, utilizing a completely chlorine-free course of) and pressed into dissolving pulp sheets.
Step 2: Dissolving the cellulose in NMMO
The dissolving pulp is combined with a solvent referred to as N-methylmorpholine N-oxide (NMMO, generally written as NMO), a cyclic amine oxide that has the bizarre capacity to dissolve cellulose instantly with out chemically altering it.
Within the older viscose course of, cellulose is dissolved utilizing sodium hydroxide and carbon disulfide (CS₂), a extremely poisonous fuel linked to neurological and reproductive hurt in manufacturing unit staff. In response to peer-reviewed analysis published in the journal Fibers, the viscose course of produces 200-300 tonnes of wastewater per tonne of fiber, and 25-35% of the carbon disulfide used is rarely recovered.
Lyocell skips that chemistry fully as a result of NMMO breaks the hydrogen bonds in cellulose by way of purely bodily dissolution.
Step 3: Extrusion and fiber formation
The dissolved cellulose (“dope”) is filtered to take away impurities, then pressured by way of tiny holes referred to as spinnerets. The filaments drop right into a water tub, the place the cellulose precipitates out as lengthy, skinny fibers. These are then washed, dried, crimped, and lower into staple fibers.
Step 4: Solvent restoration (the closed loop)
As an alternative of dumping the spent NMMO and water into the setting, the system captures and recovers them. In response to Lenzing’s official technical documentation, greater than 99.8% of NMMO is recovered and fed again into the manufacturing loop of their TENCEL™ Lyocell course of.
A 2024 pilot recovery study published in Fibers confirmed that NMMO may be efficiently recovered from course of wastewater utilizing evaporation, with no degradation of the solvent.
So what occurs to the opposite 0.2%?
The small fraction of NMMO that isn’t recovered results in the method wastewater in very dilute type. In response to a ScienceDirect engineering reference on the lyocell process, these very small portions are “readily degraded in organic wastewater remedy vegetation.”
It’s value including a little bit of nuance right here, although. NMMO truly does not meet the threshold for “readily biodegradable”, which sounds worse than it’s. What a 1998 study in the journal Biodegradation confirmed is that activated sludge may be tailored to interrupt down NMMO and its metabolites to beneath detection limits, and that the difference takes about 15–20 days. In follow, this implies wastewater remedy vegetation at lyocell manufacturing websites develop the proper microbial populations to deal with the residual NMMO, and the substance is finally biodegradable with no persistent breakdown merchandise shaped.
There’s a second, smaller loss pathway value understanding about. A fraction of NMMO can thermally degrade in the course of the restoration step itself, breaking down into byproducts that embrace N-methylmorpholine (NMM) and morpholine, as documented in peer-reviewed reviews of the lyocell process. To forestall this, producers add stabilizers (generally propyl gallate, a phenolic antioxidant additionally extensively used as a meals preservative) which entice reactive intermediates earlier than they will trigger issues, and so they then regenerate degraded NMM again into usable NMMO utilizing hydrogen peroxide. So most of what would in any other case be “misplaced” is chemically reconstituted and put again into the loop. None of those course of intermediates find yourself in Lenzing’s completed fiber, which is repeatedly washed and examined towards OEKO-TEX® STANDARD 100 limits for dangerous substances earlier than it leaves the manufacturing unit.
That’s what “closed-loop” means in follow. It isn’t zero chemical substances however rater near-total restoration and reuse, with the small unrecovered fraction both chemically regenerated contained in the plant or biologically damaged down at commonplace wastewater remedy vegetation earlier than it will probably attain the broader setting.
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