| Chrome tanning, with its most reliable high-quality products at thelowest cost, has been the most mature technology in tanning industry.Until now, no substitute for the chrome tanning agent is found to beefficient. But it’s a pity that chromium utilization, in the traditionalchrome tanning process, is only65%to75%. Considering that chrometanning, for a time, is still the main tanning method and that the severeenvironmental pollution caused by Chrome in this industry, it’snecessary to work out a better way to absorb chrome in the tanningprocess so as to achieve the sustainable development in tanningindustry. Therefore, development of an efficient high-exhaustionchrome tanning agent to effectively absorb chrome and reduce thechrome pollution has been an important issue concerned in tanningindustry.The mechanism behind chrome tanning lies in the chemicalprocess of producing a complex consisting of carboxyl groups incollagens in association with chromium (III). Therefore, materialswhich can effectively increase the number of carboxyl groups in leathercollagen can be used to develop a high exhaustion chrome-tanningauxiliary. Polyacrylic acid and its sodium salt has a high content ofcarboxyl groups, some of which can react with amino groups orhydroxyl groups in leather collagens, thus increasing the number ofcarboxyl groups in leather collagen and absorb chrome effectively.Study of the subject in this paper contains four procedures.Procedure1: adopt the dynamic aqueous solution polymerization anduse redox system consisting of sodium bisulfite and oxygen as initiator,to synthesize sodium polyacrylate with various molecular weights atlow temperature. It investigates the impact of the following factors onthe viscosity of the product and conversation rate: the amount of areducing agent, polymerization temperature, monomer concentration,reaction time, the way of neutralization and neutralization value. Thenorthogonal test was adopted to explore the influences of varied experiment conditions on polymerization reaction. After the process ofsynthesis, the polymer is studied by FT-IR and GPC.Procedure2: UV spectrophotometer is used to explore effects ofvaried experiment conditions on the polymer and the complex reaction.Factors under discussion include temperature, pH, reaction time, thepolymer molecular weight, the amount of chromium powder and thepolymer.Procedure3: Conduct experiments with small acid leather piecesto investigate the effects of the following factors on chrome tanningefficiency: joined order of the polymer, the amount of the polymer andmolecular weight of the polymer. Then use chrome tanning leather andchrome-tanning waste liquor to investigate effects of liquor ratio,retanning time, basification pH, reaction time after basification onretanning efficiency. Orthogonal test will be carried out latter to exploreinfluences of different experiment conditions, including reactiontemperature, retannig liquor ratio, reaction time for retanning, theamount of auxiliaries, basification Ph after retanning. This step mainlyconcerns about chrome content in waste liquor and leather pieces.Procedure4: Based on the results optimized from Procedure4, afull range of experiments will be engaged with acid leather pieces andchrome tanning leather pieces. This procedure aims at examining theeffect of the auxiliary’s high exhaustion effect in the process of chrometanning and retanning. Major indicators include chrome content inwaste liquor and leather pieces, TOC in waste liquor and physical andmechanical performance of leather.The optimum chrome tanning conditions: first, add the amount of2%auxiliary with a molecular weight of15K; then add the chromiumpowder. The effect of high exhaustion chrome tanning is obvious. Theoptimum retanning condition is obtained as liquid ratio8, theretanning time180min, post-basification pH5.5, reaction time afterbasification180min, reaction temperature35℃, initial pH2.5, theamount of the auxiliary1.5%, the molecular weight of the auxiliary20K and reaction time after basification180min.The results show that adoption of redox system consisting ofsodium bisulfite and oxygen as the initiator makes it possible tosynthesize a series of low-molecular sodium polyacrylates with various molecular weights at low temperature. This high-exhaustion chrometanning agent works quite well in the tanning and retanning process. Itcontributes to reduce chrome content in waste liquor, thus increasingchrome content in the leather. The finished leather proves to be floppy,elastic and its physical performance parameters are also higher thanconventional chrome tanning. |
