The Dyeing of Polyolefin Fibers in Supercritical Carbon Dioxide. Part I: Thermo-mechanical Properties of Polyolefin Fibers after Treatment in CO2 under Dyeing Conditions

The viscoelastic properties of gel-spun polyethylene (PE) and polypropylene (PP) fibers after treatment in supercritical CO₂, at 280 bar were examined to determine the optimum conditions for dyeing without causing fiber damage. Above 100°C, a greater amount of shrinkage, mainly of PP fiber, is observed in CO₂, than in air at 1 bar. Under isothermal conditions at 100°C, pressure-dependent on-line measurements of a PP-fiber monofli up to 280 bar show that shrinkage becomes very considerable at low pressures of only 10 bar. In comparison with isobaric conditions at 280 bar, shrinkage starts at temperatures above 60°C and is greatest between 90 and 100°C. In both cases, the degree of shrinkage is 11–12%. The value of F._max on the stress—strain diagrams of all fibers tested is not influenced by the treatment in CO₂, up to 120°C. However, the elongation increases only for polyolefins with a lower degree of crystallinity to the extent of about 15% for the PP monofil and 13% for the LM-PE fiber. Furthermore, CO₂, is able to penetrate into the hydrophobic PE and PP fibers and acts as a quasi-impurity, reducing the melting point (mp) of LM-PE fibers by between 5 and 7^°C and that of PP monofli by 9 and 22°C at 50 and 280 bar, respectively. This was observed by differential-dynamic-heat-flow calorimetric measurements in CO₂. The decrease in mp of the highly crystalline HM-PE fiber is only 2°C because the diffusion of CO₂ into the fiber is extremely slow.