Wednesday, November 12, 2014

First reports on solvent spinning (1976-77)

In the mid-seventies the rising influence of the US anti-pollution lobby motivated several US cellulose research groups to renew efforts to find alternatives to the xanthate dissolution route.  Chief among these was ITT Rayonier and it was their publications which stimulated a review of the new possibilities.  This was done in Courtaulds Development Fibres and Viscose Laboratory (DFVL) - the new name for what had been the Viscose Research Laboratory (VRL) in Coventry.

Brian Gardner's August 1976 File Note entitled "Solvents for Cellulose and their Application to Film and Fibre Production - A Literature and Patent Survey." reviewed the available routes but made no attempt to evaluate their suitability as a commercial alternative to xanthation.

Mike Summers-Smith extended Brian's survey in early 1977 and added conclusions on how useful these processes might be.  He listed the disadvantages of the viscose route as identified by ITT Rayonier as:
  • High levels of air and water pollution.  Viscose uses strong acids, alkalis, CS2 and its regeneration liberates H2S.  (Mike noted that none of these pollutants need be emitted from the plant and all chemicals could be efficiently recycled given a high enough capital expenditure and energy outlay.  However this would render the fibres uneconomic compared with cotton and the new synthetics.)
  • High capital costs.  The lengthy process with critical ageing times for alk-cell and viscose, coupled with dilute polymer solutions and slow wet-spinning processes made viscose plants fundamentally expensive.
  • High energy consumption.  Major advances in energy conservation were unlikely.
  • High labour costs.  (However the new automated viscose plants with slurry steeping, belt mercerising, wet-xanthation, backwash filters and continuous deaeration were bringing this under control).
  • Increasing raw material costs, but these could be limited by increased recovery.
The new processes were divided into 4 classes according to the chemistry involved i.e. cellulose behaving as a complex or as a derivative, as a base, or as an acid.

Copper, Cadmium or Nickel complexes in aqueous solution were well known to dissolve cellulose as a complex and the Cu version - Cuprammonium rayon - was a long-established commercial process which had been eclipsed by the viscose route in Europe but was still important in Japan for making special yarns and nonwovens. Obtaining complete recovery of the expensive and polluting heavy metals was the key problem, and the Japanese process was known to need cotton rather than woodpulp as a raw material.

New Cellulose Derivative processes were receiving much attention in the USA.

8% Cellulose, 15% Dinitrogen tetroxide with 77% dimethyl formamide dope had been prepared in Hammer and Turbak's group in ITT Rayonier although for best results the cellulose had to be Cell II from steeping in caustic soda.  The DMF could be replaced by dimethyl sulphoxide or acetonitrile.  Fibres, including inflated fibres, could be spun into water or alcohol, but their properties while polynosic in character (only 7% to 10% extensibility dry) were inferior to viscose fibres.  Furthermore solvent and spinbath recovery appeared complicated and energy intensive, and the solvents were toxic, corrosive, suspected carcinogens and potentially explosive on heating.

6% Cellulose, 6% paraformaldehyde with 88% dimethylsulphoxide dope had also been tested but this too gave inferior polynosic fibres, and unlike the N2O4/DMF route, ITT Rayonier had no credible recovery scheme.  Furthermore the toxicology of the solvents made their handling hazardous.

Sulphur dioxide with amine cosolvents had been used to make 3% solutions of cellulose but here again resulting fibres were poor, and the difficulties of handling and recovering the SO2 were daunting.

The "base" processes involved strong acids both (protic and Lewis), were good dissolvers of cellulose but degradation occured and regeneration was expensive.

The "acid" processes using both inorganic and organic bases would dissolve cellulose with DP > 1000.   Complete dissolution had been obtained in hydrazine and the tertiary amine-N-oxides, and spinning into water or alcohol had produced fibres.  However "solvent recovery would be very difficult, as the compounds are unstable to heat.  Their toxicology is not well documented." 

This was thought to be the only likely route to a commercial solvent process.

Was there any hope for the viscose process, or would the costs of complying with the ever-tightening environmental regulations be its downfall?  Could the concentrations of CS2 required for xanthate production be much reduced?  Could the liquid effluent problems be abated?

One possibility, floated by Leslie Rose was recorded by Mike Summers-Smith.

This involved a review of the "Lanusa" and "Telusa" processes commercialised in Germany by IG Farben and by Thuringische Zellwolle respectively  during WW2 to save chemicals.  Here "any" viscose was spun into a slightly alkaline zinc-free spin-bath to coagulate and partially desulphurize it.  Regeneration occurred in the subsequent hot-stretch bath.  Tensile properties were similar to regular rayon but with higher modulus and more equal wet (16%) and dry (14%) extensions; rather polynosic - or Tencel-like in fact.  

In addition to chemical savings this process would allow the spent spin-bath to be digested by bacteria thereby oxidising the sulphur compounds to sulphates to give a zinc-free, low BOD effluent. 

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