Modified lyocell process for producing biopolymer fiber

Abstract

Cellulose is one of the most abundantly available biopolymer in nature. This semi crystalline cellulose cannot be melted directly to form fiber, as it degrades before melting. Hence processing of cellulosic fiber requires dissolution process. There are many solvents available for direct dissolution of cellulose to form polymer, N-Methyl Morpholine Oxide (NMMO) is one of the solvents, which has commercial feasibility to dissolve the cellulose to form spinnable polymer matrix. The process of direct dissolution of cellulose using NMMO is conventionally termed as Lyocell process. It is a multiple step process that involves swelling and dissolution of cellulose in N-Methyl Morpholine Oxide (NMMO) solvent and spinning of solvated cellulose solution to produce fiber by dry jet wet spinning. The conventional process of lyocell manufacturing consists of single stage of slurry (cellulose suspension in NMMO) preparation in which solvent gets penetrated in cellulose fibers (Swelling). It is necessary at slurry stage to have optimum pulp swelling and higher NMMO to cellulose ratio to achieve better uniformity of slurry for fast dissolution and homogeneous solvated cellulose polymer formation. To achieve this, pulp sheets are shredded to an optimum size before addition to aqueous solution of NMMO. The shredding of pulp sheet requires intensive energy and raises safety concerns due to the static charge generation which can lead to dust explosion. This makes conventional processes of dissolved cellulose preparation commercially and operationally unviable. In the present work a new method is developed to overcome the safety concern. In this new method, slurry preparation is carried out in two stages. In first stage dilute solvent is used to prepare slurry with low (4-6%) solid, slurry is later pressed to upto 45-50% solid percentage, the pressed solvent removes metal ion impurities and short chain hemicellulose by 10-20% from pulp. The pressed cake of cellulose is again mixed with high concentration of NMMO to form the slurry mixture for further processing. The dissolution of cellulose starts when monohydrate state of NMMO is reached by removing excess water from cellulose NMMO slurry to get Lyocell dope at 90-100 °C. This polymer solution is then spun through spinnerets by jet-wet spinning to get the Lyocell fiber. This method of cellulose-NMMO slurry preparation for the manufacturing lyocell fiber has shown reduction in undissolved particles of cellulose and metal ions impurities in polymer matrix of cellulose with no change in the fiber properties compared to the conventional process. The advantage of this process over conventional process is that high impurity pulp can be easily blended with good quality pulp in first stage of slurry-making, while the excess impurity is removed before second stage to get better quality of solvated cellulosic polymer network.