NEW AND EFFICIENT PROCESSES FOR MAKING QUALITY LEATHER
Location: Eastern Regional Research Center
Title: FIBROUS COLLAGEN MATERIAL: LEATHER TREATED WITH TOCOPHEROL
Submitted to: Fiber Society Meeting
Publication Type: Proceedings
Publication Acceptance Date: September 1, 2005
Publication Date: October 17, 2005
Citation: Liu, C. 2005. Fibrous collagen material: leather treated with tocopherol. In: The Fiber Society, Fall 2005 Annual Meeting and Technical Conference, Fiber Science - The Next Generation. October 17-19, 2005, Newark, New Jersey. p. 65-66.
As a collagen fibrous material, leather is economically significant because it is a major coproduct derived from the meat industry. Although the majority of leather is tanned using Cr-III salts, environmental concerns over the use and disposal of chrome-tanned leather encourage the use of chrome-free leather, particularly in the European automotive leather markets. The objective of the current study is to improve UV and heat resistance of non-chrome-tanned leather made with a glutaraldehyde tanning process. Glutaraldehyde tanning was developed by Filachione et al. in the Eastern Regional Research Center (ARS, ERRC) in the early 1960’s. It has become the most common alternative tanning agent to chrome salts, because it is less expensive, is readily available, and is highly soluble in aqueous solution. In recent years, it has been the dominant tanning agent for the preparation of chrome-free leather. The chemistry of fixation of glutaraldehyde to collagen is not fully understood. Presumably, it involves crossslinking by the reaction of an aldehyde group with an amino group of lysine or hydroxylysine or an aldol condensation between two adjacent aldehydes. Presently there is an increasing demand for the domestic production of automotive leather. The quality of chrome-free leather, such as glutaraldehyde-tanned leather, in some respects is inferior to that of chrome-tanned leather, for example in colorfastness and thermal stability. Leather for car interiors is required to meet exceptionally high quality standards. Consumers expect leather to be able to withstand exposure to extreme and varying temperatures, light, moisture and mechanical loading conditions over time. Most automobile leather is colored to improve its appearance and aesthetic value. Sunlight on the other hand is a powerful form of energy that can break up the colored molecules into smaller pieces and can cause a yellowing or bleaching effect. If chrome-free leather is used for instrument panels and consoles, the problem of shrinkage due to poor heat resistance is especially acute, as temperatures well over 100°C can be reached. The poor lightfastness of crust leathers prevents the use of light pigments or dyes and, therefore, more natural looking or aniline finishes for leather are difficult to achieve. When using such leather in automotive applications, these inadequacies are of paramount importance, and they need to be studied to find ways of producing lightfast leather with improved thermostability. Antioxidants are often included in sunscreens as free-radical scavengers. Tocopherol (vitamin E) is a potent free radical scavenger and a highly protective agent against UV skin damage. It is a light yellow-colored oil and a fat-soluble vitamin. The principal role of tocopherol as an antioxidant must be to neutralize free radicals that could initiate a chain reaction, resulting in the formation of peroxides and products of their subsequent degradation. Thermal stability of leather may be improved by using antioxidants such as tocopherols to protect against thermal oxidation, thereby improving the stability of the triple helical structure of collagen molecules. We recently experimented with the application of alpha-tocopherol to the grain layer of chrome-tanned leather. Results showed that chrome-tanned leather treated with alpha-tocopherol yielded a significant improvement in mechanical strength and softness and, more importantly, increased strength retention and color fading resistance against UV radiation and heat. However, how the alpha-tocopherol treatments affect the fine structure of collagen fibers is unknown. Collagen fibers are the building blocks of leather. The need to study the fine structure of collagen fibers has led us to employ a polarizing microscope to measure the birefringence of fibers and thereby gain insights on the effects of UV and heat on the fine structure of fibers. Non-chrome-tanned (chrome-free) leather has gradually gained commercial importance, particularly for automobile upholstery applications. The current study focuses on the non-chrome-tanned leather made with a glutaraldehyde tanning process. We applied alpha-tocopherol to the grain layer of leather and also studied the addition of tocopherol to the fatliquoring process. Following exposure in a Fade-Ometer, the treated samples were evaluated by colorimetry and mechanical testing for the efficacy of UV- and heat resistance. This on-going research aims to develop a process treatment that is environmentally friendly and yet significantly increases the UV and heat resistance of chrome-free leather. A polarizing microscope equipped with a Berek compensator was employed to determine the birefringence of the untreated and treated leather collagen fibers to determine the treatment effects on the degree of orientation. Data showed that coating leather with alpha-tocopherol significantly improves the color fading resistance and strength retention against UV radiation and high temperature. We also studied the addition of tocopherol to the fatliquoring drums, but this treated leather did not show a similar improvement.