Summary
To determine how natural fat varies between different species and how pretreatments affect grease removal, an analysis was developed by the Cognis leather department. This test screened the effectiveness and performance of widely used emulsifying agents in relation to extracted natural fats from skins. After numerous analyses, it was observed that the performance of surfactants varies substantially from skin to skin, depending on the difference of the natural fat composition of the skin. This approach allows the appropriate type of emulsifying agent to be used in the most effective way for the degreasing process. Using this technique, sheepskins from different sources were screened to consider the effects of pretreatment, since this may influence the effectiveness of the degreasing. The structural changes in the fat composition as influenced by the chemicals and process parameters such as soaking, liming and pickling were also investigated.
The results of the small scales tests were then verified on degreasing processes in laboratory drums and subsequently in tannery trials. Through this work, a totally new approach to the degreasing process was built up.
Introduction
Degreasing is one of the most important processing steps since the consequences of inadequate degreasing can result in irreversible damage to the finished leather. This process step is usually carried out using an appropriate emulsifier with some degree of biodegradability, after bating or in some cases after pickling or depickling, in a short float and under mild temperature conditions.
Different aspects of the process have been widely investigated, taking into account the efficiency of the degreasing in terms of remaining natural fat, its distribution, concerning chemistry and process parameters1,2,3,4,5, including the use of enzymatic degreasing6,7.
The ecological aspects of degreasing have also been investigated6,7,8,9,10,11,13,14, and with the necessity for more ecofriendly degreasing agents, alkylpolyglycosides (APG) were presented in London at the IULTCS 1997 centenary congress8. In this work, surfactants with different degrees of polymerisation and alkyl chain lengths were used to degrease English domestic and New Zealand sheepskins.
Natural fat
Given that the natural fat from one skin source varies to another (Table 1), both by its composition and structure, it would seem reasonable to assume that varying the surfactant to suit the skins would be a suitable way to degrease properly. The emulsifying capacity of emulsifiers varies with respect to the fat composition, which itself is a function of the carbon chain length, amount and nature of triglycerides, phospholipid content, the existence, or absence, of waxes and their nature.
The differences in the chemical composition of fat from Turkish sheep and English domestic sheep were also investigated using TLC techniques. The Turkish fat contains less triglyceride and phospholipids; is lower in cholesterol and exempt of monoglycerides. However, it is more viscous, while the English origin skin’s fat has more triglycerides, cholesterol and monoglycerides.
The premise that degreasing agents act differently depending on the fat composition was first tested in 1998 on four pigskins from different sources by using the same degreasing agent and increasing the concentration. The results of this work are shown in Tables 2 and 3, where the related carbon chain distribution of concerning extracted pig fats is given without considering eventual waxes and phospholipids, which may surely differ as well. These results encouraged us to review different chemical structures, which are or probably will be used in degreasing through the newly-developed simulation test method.
This method consists firstly of the extracting the skin’s natural fat, then the standard mixing in a different ratio of fat/emulsifying chemical, using a micromixer and finally defining and ranking the type and the stability of the fat/emulsifying chemical-water system emulsion for predefined periods of time.
Simulation tests showed that regarding skin origins, different surfactants/degreasing agents act differently as a function of origin. For instance, one degreasing agent proved very efficient on English domestic skin but performed poorly on Entrefino. These results demonstrate clearly a justification of the above-mentioned premise and create a basic conceptual change for the traditional approach to degreasing. This allows an a priori identification of the most suitable degreasing agent for each skin/hide origin and, hence, the most efficient and economic way of degreasing.
For the simulation tests various skins and hides, both raw and pretreated were selected.
For the sheepskin degreasing assessment, English Domestic, American, Norwegian, South African, Azerbaijani, Spanish Entrefino and Merinos, Australian, Libyan, Turkish and French Lechal raw skins were investigated in their raw and pickled forms considering, in some cases, the time left in pickle as well.
The following chemical moieties were used:
1. Non-ionic surfactants based on fatty alcohol alkoxylates with various fatty alcohol chain lengths and degrees of alkoxylation; fatty acid ethoxylates; various fatty alcohol chain lengths and degrees of ethoxylation; APGs with various alkyl chain/polymerisation degree ratios
2. Anionic surfactants based on fatty alcohol sulfates; fatty alcohol ethersulfates; sulfurised fatty alcohol esters and hemiesters and their ethoxylates
3. Pseudocationic nitrogenated surfactants
Simulation test
A simulation method for investigating the efficiency of different chemical structures and their combination for revealing probable synergetic effects on the degreasing capability on different natural fats was developed. This method consisted of Soxhlet dichloromethane/hexane extraction of the natural fat from the skin, hide or pelt powdered and then dried, taking care of the denaturation of the material.
After evaporating the solvent, the fat was separated into two parts and the first part analysed for physical and chemical characterisation, while the second part was examined according to the simulation test. Different fat/emulsifier ratios were first prepared and analysed fixing all other parameters. A micromixer has been used to carry out the simulation test in order to create a water in oil (W/O) system of the natural fat first, excluding the influence of emulsifying style on the test results. W/O emulsions were then inverted to an oil in water emulsion using distilled water to a fixed volume and the emulsion was observed for fat separation at various times.
Salt water, simulating pickle media, was also used for some of the trials in a comparative manner. These observations were ranked from 0-10, 0 corresponding to the non-emulsifying ability and 10, a 24-hour stable emulsion. All emulsions were evaluated according to this ranking.
Results
As a first step, the degreasing effect of selected surfactants was investigated, firstly on American, English domestic and Turkish origins. These trials showed that with the varying chemical structures the emulsion ability of the natural fat varied according to the differences in their composition.
For example, surfactant K2 shows very high efficiency on Turkish skins but was less effective on American, while K12 is efficient on American but not to the same degree on Turkish. Surprisingly K10 was effective to some extent on American and English domestic but showed no effectiveness at all on Turkish origin natural fat.
Following this preliminary study, reselected surfactants were investigated using various skin origins including from raw and pickle sources. It became apparent that changes in structure greatly affected the emulsifying ability of the surfactant.
This was especially apparent in the case of linear long-chain medium alkoxylate and long-chain non-linear ethoxylate, while medium chain ethoxylate surfactant shows more regular efficiency, although its efficiency differed only slightly from one origin to another.
On the other hand, Turkish, South African, Australian and Libyan sources presented more variability with regards to emulsion stability.
The small-scale trial results suggest that the chemistry of the surfactant is important in determining the release of fat from skins, and that this is also dependent on the source of the skin. These findings form the basis for a new approach in defining the most suitable degreasing agent for a given tannery, which aims to produce a given article from a defined raw material. The simulation test results were verified on lab scale degreasing trials using specially designed products. Trials were carried out using the result of the simulation tests where first single products were applied and then mixtures of products added to determine probable synergy in order to economise the degreasing process.
Tannery trials
This new degreasing concept was successfully applied in some Turkish tanneries. Raw material of the participating tannery was first tested through the simulation test using the Cognis product portfolio. Defined best performing product(s) were then applied to the production of the tannery. Tannery application results are shown in Table 4. In this step of the study, at least ten samples were tested according to IUC4 and results are shown as median of random selected samples. In the table, standard deviations are also given.
Conclusion
This study has demonstrated that each skin/hide origin comprising particular pretreatment applied has to be degreased precisely according to the analysis by the newly developed simulation technique which can indicate the most effective degreasing agent and/or suitable mixture of them.
By using this study, it has been revealed certain degreasing agents are not to be used for some skin/hide sources.
Using this approach, it is possible to signify an a priori degreasing agent and reduce the risk of leaving excess natural fat in the skins/hides.
This study also showed it is possible to find a degreasing agent less dependent on origins. Hence the most appropriate, more economic and safer degreasing agents using the New Degreasing Concept are offered to the tanners as a beneficial tool.
Acknowledgments
The authors thank Prof Meral Birbir, Faculty of Art and Sciences, microbiology division and Prof Ayse Ogan, Faculty of Art and Sciences, chemistry division, for their support on differentiation of natural fats using TLC techniques.
