Friday, November 21, 2014

The Cuprammonium route re-evaluated (1980)

The April 1980 report on the possible adoption of the cuprammonium hydroxide solvent route to cellulosic fibres considered it's pros and cons c.f. viscose at that time.  Reconsideration of this old route was based on new patents, optimistic assessments by Russian workers and the view, expressed by Dr Hergert of ITT Rayonier during a visit to DF&VL, that it merited reinvestigation.

The disadvantages of the cupro route were mainly the result of tricky dope-making requiring a high quality pulp or cotton linters which had to be further purified by treatment with caustic soda.  Cost of the cellulose for viscose was put at £1.69/kg compared and compared with £2.75/kg for cupro and this dominated the economic comparison.  Both dopes contained 9-10% cellulose.  Cupro caustic costs were double those of viscose, but copper and ammonium costs were insignificant in the cupro total. 

Spinning into fine continuous filament yarns involved tube-spinning using a sulphuric acid spinbath. This and the need to recover copper as the sulphate produced sodium sulphate as a by-product just like viscose.  The fact that cupro had never really been produced as staple fibre was a serious disadvantage, but the vertically-downwards tube-spinning system and conveyor washing used to make filament yarns did lend itself to spunlaid nonwoven production.

Despite the patented advances in copper recovery and spinning speeds (much faster than ever achieved with viscose), the process was judged to be only suitable for speciality yarns and nonwovens and not a contender for viscose replacement.  The fact that the process, still known as Bemberg rayon after the German company that commercialised it in 1897, has remained a small-volume speciality yarn and nonwoven process (while the market demanded massive viscose and NMMO process expansion) would appear to confirm this judgement.

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