Mechanism of collagen processed with urea determined by thermal degradation analysis

Authors: TANG, KEYONG - Zhengzhou University; LI, WEILIN - Zhengzhou University; LIU, JIE - Zhengzhou University; Liu, Cheng Kung; PAN, HONGBO - Zhengzhou University

Submitted to: Journal of American Leather Chemists Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2020
Publication Date: 10/1/2020
Citation: Tang, K., Li, W., Liu, J., Liu, C., Pan, H. 2020. Mechanism of collagen processed with urea determined by thermal degradation analysis. Journal of American Leather Chemists Association. 115(10):380-391.

Interpretive Summary: Collagen is an abundant natural polymer in animal skins and bones, which has been widely used in food, medicine, bioengineering and the cosmetic industry. The thermal stability is an important criterion for collagen fibrous materials such as leather in processing and application. Urea is commonly used in the liming process of leathermaking. It is important to gain the knowledge on the influence of urea on the structure, shrinkage, thermal degradation, and aggregation state structure of collagen. In this research, collagen fibers were soaked in various concentrations of urea and then washed with distilled water. The influence of urea on the thermal degradation activation energy and aggregation state structure of the collagen fibers were analyzed. The interaction mechanism between urea and collagen fibers was also investigated. Observation showed that the influence of urea processing on the thermal degradation activation energy of collagen fibers is temporary and reversible. When the urea-processed collagen fibers were fully washed in distilled water, the thermal degradation activation energy would recover to the level of unprocessed collagen fibers. The research results help scientists to understand that the influence of urea processing on collagen fibers is governed by hydrogen bonding interactions, not by chemical reactions.

Technical Abstract: During the beamhouse process for nappa leather, pelts are usually limed with amino compounds such as urea, ethylenediamine, and triethanolamine. However, the interaction between amino compounds and collagen is not well known. In this work, collagen fibers were soaked in various concentrations of urea and the thermal degradation of collagen fibers were studied by the methods Horowitz-Metzger and Coats-Redfern. The mechanism of the reaction between urea and collagen fibers is discussed, wherein the thermal degradation activation energy first decreased and then increased. The lowest thermal degradation activation energy of urea processed collagen appears at 2-3 mol/L urea, suggesting that the stability of collagen is the poorest when the pelt is processed in the urea solution. When the urea concentration is above 6 mol/L, the thermal degradation activation energy of the sample is similar to samples without urea processing and the higher concentrations does not have the same effect as lower concentrations of urea. The collagen fibers with a urea processing history were washed to remove the urea in them, and the samples were studied again for their thermal degradation behavior. The results indicated that the thermal degradation activation energy of the collagen fibers might recover to the unprocessed level. Therefore it was suggested that the reaction between urea molecules and collagen fibers is reversible. Urea molecules might help to destroy some of the hydrogen bonds between collagen peptides in the urea solution. After the urea is washed out, the structure of the collagen will return to its original state, because the hydrogen bonds might be reconstructed.