Wha Seop Lee

Physical Properties of Lyocell Fibers Spun from Isotropic Cellulose Dope in NMMO Monohydrate

This article describes the physical properties of lyocell fibers spun from an isotropic cellulose spinning dope in N-methyl morpholine N-oxide (NMMO) monohydrate (IPS lyocell fibers). Wide angle x-ray diffraction (wAxD) experiments on the crimped fibers exhibit little difference in the crystal structure of IPS and Tencel lyocell fibers: both fibers reveal a cellulose II structure. However, the IPS lyocell fibers have a lower tensile modulus than Tencel by about 12.5%, although they have a similar tensile strength. Wetting and subsequent drying shift the loss tangent values higher, which is more remarkable for the IPS lyocell fibers dried under tension. Further, the IPS lyocell fibers dried under tension have a sharper α-transition peak than the fibers dried without tension. The IPS lyocell fibers exhibit excellent wet tensile strength: 90% of tensile strength is retained after wetting twice in distilled water. In addition, the IPS lyocell fibers show good chemical stability to acids and alkalis, except for an extremely strong alkali—pH 14. Mercerization of the IPS lyocell fibers in the buffer solution of pH 14 notably decreases the tensile modulus but increases elongation.

Dry Jet-Wet Spinning of Cellulose/N-Methylmorpholine N-oxide Hydrate Solutions and Physical Properties of Lyocell Fibers

In dry jet-wet spinning of a cellulose/N-methylmorpholine N-oxide hydrate solution, the effects of the hydration number n in NMMO hydrates and the concentration and molecular weight of cellulose are investigated in terms of the physical properties of the fibers. Dry jet-wet spinning of lyocell fibers is also investigated using three different set-ups; a piston type, an N2 gas pressure type, and spinning equipment with an extruder. The effects of spinning conditions such as the spin draw ratio, air gap distance, and composition of the coagulation bath are investigated. The physical properties of the fibers such as birefringence, initial modulus, and tensile strength increase with a decrease in n and an increase in the air gap distance and spin draw ratio. The relationship between the physical properties and the fiber denier is newly suggested in this spinning system. The tensile fracture morphology reveals that fibers from the NMMO hydrate containing less water have more fibrils due to their higher molecular orientation. Further, the orientation structure of the cellulose becomes more noticeable with the decreased hydration levels of the solvent because it produces thicker and longer fibrils when the cellulose fibers are treated with an ultrasonic generator. The crystallite size of the cellulose depends on the composition of NMMO in the coagulation bath. The crystallite size also decreases with the increased air gap distance.

Effect of heat treatment on structural changes and dynamic mechanical relaxation of polyacrylonitrile fibers

SummaryThe structural changes of polyacrylonitrile fibers have been studied through the effect of heat treatment on dynamic mechanical relaxation of undrawn and drawn fibers. It is generally accepted that polyacrylonitrile has two transition temperatures. This study showed that the activation energies of the low temperature transition and the high temperature transition were about 53 and 155kcal/mole, respectively. The latter was related to the intermolecular diploe-dipole dissociation of the nitrile groups in the amorphous region and the former was related to the molecular motion in the paracrystalline region.

Preparation of Regenerated Cellulose Fiber via Carbonation. I. Carbonation and Dissolution in an Aqueous NaOH Solution

Cellulose carbonate was prepared by the reaction of cellulose pulp and CO2 with treatment reagents, such as aqueous ZnCl2 (20–40 wt%) solution, acetone or ethyl acetate, at −5–0°C and 30–40 bar (CO2) for 2 hr. Among the treatment reagents, ethyl acetate was the most effective. Cellulose carbonate was dissolved in 10% sodium hydroxide solution containing zinc oxide up to 3 wt% at −5–0°C. Intrinsic viscosities of raw cellulose and cellulose carbonate were measured with an Ubbelohde viscometer using 0.5 M cupriethylenediamine hydroxide (cuen) as a solvent at 20°C according to ASTM D1795 method. The molecular weight of cellulose was rarely changed by carbonation. Solubility of cellulose carbonate was tested by optical microscopic observation, UV absorbance and viscosity measurement. Phase diagram of cellulose carbonate was obtained by combining the results of solubility evaluation. Maximum concentration of cellulose carbonate for soluble zone was increased with increasing zinc oxide content. Cellulose carbonate solution in good soluble zone was transparent and showed the lowest absorbance and the highest viscosity. The cellulose carbonate and its solution were stable in refrigerator (−5°C and atmospheric pressure).

Double crystallization behavior in dry-jet wet spinning of cellulose/N-methylmorpholine-N-oxide hydrate solutions

The structure and resultant mechanical properties of fibers in the dry-jet wet spinning process of cellulose solutions in N-methylmorpholine-N-oxide (NMMO) hydrates were investigated in terms of molecular weight of cellulose, concentration, and hydration number (n) of NMMO hydrate. The value of n had an effect on the crystallization behavior of the cellulose solution system, which influenced the resultant fiber structure. Increasing cellulose concentration and decreasing the value of n retarded crystallization because of the increased interactions between cellulose and NMMO hydrate. Reducing the value of n from 1 to 0.72 produced more highly oriented cellulose fibers. However, incorporating n-propyl gallate, an antioxidant, had little effect on the fiber structure. When n=0.72 a double crystallization behavior was observed in the fiber spinning process irrespective of molecular weight of cellulose and concentration over the experimental ranges examined. It should be noted that such a double crystallization took place in the absence of foreign additives. The double crystallization behavior was more noticeable when the aspect ratio of spinning nozzle was greater. The double layer structure had a positive effect on the mechanical strength.