Studies On The Influences Of Ultrasound On Leather-making Processes And The Removal Of Chrome In Leather Shavings By Oxidation

As a dominant traditional manufacture, leather-making industry in China is faced with counteraction of environment pollution, and low efficiency of processing and source utilization. Therefore, it is imminent to adopt advanced technologies to rebuild leather production and make it environment-friendly and sustainable development.The basic functions of ultrasonic waves on leather-making were systematically investigated in this thesis, which include the effect of ultrasound on the removal of soluble matters in pelt and opening-up of collagen fibers, and the influence of cavition on the dispersion, penetration and adsorption rate of leather chemicals (enzymes, tanning agents, dyestuffs and fatliqours). The effect of ultrasound on enhancing oxidative chrome removal of leather shavings was also studied.It was found that ultrasound is helpful to remove soluble proteins of pelt without damage of collagen fibers. Meanwhile, the break of fat cellular wall was observed under ultrasound, which accelerates the degreasing process.The effect of ultrasonic cavition on enzyme activity was inconsiderable, but the riseof solution temperature due to ultrasound could influence the activity of enzymes. So a proper control of temperature is important if ultrasound is used in the processes of enzyme treatment.The experiments showed that ultrasound can promote the penetration of chrome tanning agent, vegetable tannins and titanium tanning agent in pelt at the beginning of tanning, but at the end of tanning, the difference of shrinkage temperature of leathers with and without ultrasound is not remarkable. A further investigation indicated that the molecular structures of the tanning agents had not been changed under ultrasound and therefore, their combination modes with collagen are not altered. These proved that the only function of ultrasound in tanning processes is to increase the dispersion degree of tanning agents and accelerate their penetration rate in pelt.The effect of ultrasound at different frequencies on leather dyeing was studied. The factors influencing dyeing rate and dyestuff adsorption rate, such as sonication time and dyeing temperature, were investigated. The results showed that, for acid dyestuff, power ultrasound (23.7kHz) can considerably increase dyeing rate and the adsorption rate of dyestuff, but that of 17kHz showed no remarkable influence. It was found that the adsorption rate of dyestuff at 45 癈 was increased to 99.24%-99.75% after treating the dyestuff solution with power ultrasound for 20-30min. For direct dyestuff, the power ultrasound (23.7kHz or 17kHz) is not able to increase dyeing rate and absorption rate of dyestuff.The influence of ultrasound on fatliquoring was also studied. Considerable change was observed in the early stage of fatliquoring process when ultrasound was used,showing faster penetration of fatliquor. As a result, ultrasound remarkably accelerates fatliquoring process.The oxidation dechroming of leather shavings using hydrogen peroxide under ultrasound was studied. I was showed that the chrome removal rate of the shavings was influenced by reaction time, temperature, medium and intensity of ultrasound. When 20kHz and 40kHz ultrasounds were employed, the intensity of ultrasound should be regulated to 7-8W/cm2. The proper circumstance for ultrasonic oxidation dechroming was 20癈 and pH=9.0-10.0 adjusted by CaO. In these conditions, dechromed with 10% hydrogen peroxide (cone. 30%) under ultrasound for 8 times, a chrome removal rate of 99.46% was achieved, which is remarkably higher than 89.6%, the chrome removal rate using conventional method of oxidation dechroming.