The quality of raw hides has decreased over the course of time and tanners have no influence on this. In addition, they often have very little influence on the way wet-blue and wet-white are produced (sometimes thousands of kilometers away). Therefore, retanning is often the only process step where the leather character can be influenced significantly.
The traditional wet-end process steps are aimed at improving the grain tightness, the fullness in the flanks and the layout of the hides. Generally, the poorer the hide structure, the more chemicals have to be added. This often leads to unwanted side effects such as:
* Higher process costs due to the increased amount of chemicals
* Reduction in physical properties such as tear strength
* Overloading of the grain and grain cracking due to high astringency and poor distribution of retanning agents
* Insufficient fixation leading to high salt freight of the wastewater
* Increasing process time
* Poor dye build up
Is it possible to alter the traditional wet-end process in order to avoid these disadvantages and improve the cutting yield without increasing the costs and the process time? The answer to this question is the development of a new retanning technology, called ‘Acid Retanning’ (AR), which follows a different approach in comparison to the traditional retanning process. While we were working on this project we questioned some of the traditional rules applied in the retanning process:
Why is a neutralisation at pH 5 necessary?
* Is the pH of the leather and the process float the only indication for a sufficient preparation for the retanning, dyeing and fatliquoring?
* What role does adjustment of the isoelectric point (IEP) play against penetration and how can this be measured?
During the development of the acid retanning system it became clear that the pH is not the only and sometimes not even the most important parameter for a sufficiently anionic character of the substrate. Much more important is the change of the IEP during the neutralisation process and the retanning:
* The IEP plays an important role against penetration and distribution of chemicals into the cross section of leather
* Anything added to the drum that does not stay in the leather only changes the pH, but not the IEP
* Anything that stays in the leather will change the IEP (usually to lower as most chemicals used in leather processing are anionic)
The graphs (right) show the development of the IEP of a traditional neutralisation compared to the acid retanning system.
During a traditional neutralisation and retanning process the IEP remains high until the end of the process. Loose and open structured hide material is extremely sensitive to high pH values in the wet-end process. In particular, the neutralisation often leads to additional looseness in the flanks and belly area of the hide.
With the newly developed acid retanning system, the IEP is already lowered at the beginning while the IEP remains high (approximately 6-7) during the neutralisation step of a standard process. Major parameters are:
* Traditional neutralisation allows good penetration of retan/ fat and dye chemicals but gives a higher chance of looser grain/ bellies
* Rule: Lower pH will result in tighter grain but also firmer leather
* Stronger retanning is often required to improve fullness and tightness but decreases physical mechanical properties (eg tear strength)
The basic working principle of the new acid retanning system can be described as follows:
* The acid retanning technology avoids high pH and allows retanning at lower pH
* Neutralisation is replaced by using a new acid retanning agent which provides: pH control; dispersing effect; retanning effect
* Fatliquoring after slight neutralisation but still at lower pH than normal
* Fixation as with a normal process to same final pH
* Dyeing in the retanning float or in a separate bath at lower pH than for a traditional process
The IEP after a traditional neutralisation is significantly higher than the IEP of the leather treated with the acid retanning technology.
Most retanning agents have a relatively low pH IEP. It becomes clear that the substrate treated with the Acid Retanning Technology is closer to the IEP of the chemicals than a traditional neutralised leather. This can explain the fact that anionic products can penetrate an acid retanning treated leather more easily and without overloading the grain throughout the leather cross section.
Sellatan AR is of low astringency with relatively low pH and shows extremely high dispersing power for anionic synthetic and vegetable tanning agents. The product is used in the beginning of the process and prepares the substrate for the subsequent process steps.
Following the pretreatment, other anionic products can be added without depositing on the grain side and without leading to unwanted brittle, empty leather or grain cracking.
During the development of the AR Technology it was soon discovered that this type of processing also has an influence on the physical properties of the leathers. Comparison trials with traditional retanning processes vs AR treated leather showed that tear and tensile strength especially can be improved with this new technology. This fact can be explained by the better distribution and less astringent behaviour of the retanning agents.
Acid Retanning Technology can be used for leathers based on wet-blue as well as on chrome free material. Due to the high dispersing properties of the AR product on synthetic and vegetable tanning agents this technology can also be used as a pre-treatment for the vegetable tanning process as well as for modern FOC technology.
FOC leathers do not necessarily require a traditional neutralisation and are, therefore, especially suitable for the application of AR Technology as dispersing of high amounts of tanning agents is an important parameter in the process. The AR process offers cost effective possibilities to optimise the wet-end process by reducing total process time and chemical offer due to high and uniform dispersing power.
Dyeing with the acid retanning technology
Dyeing of wet-blue is considered to be the most cost intensive process step in the wet-end. The cost of the total dyeing process can be split into the following categories: cost of individual dyes for penetration; process time for penetration; cost for dye auxiliaries; time and accuracy of colour matching (reworks); cost of individual dyes for top dyeing; fixation.
The development of the acid retanning technology featuring Sellatan AR liq in combination with selected dyes and auxiliaries, offers the best possible performance regarding dye penetration, dye build up, dye fixation and processing time to optimise and improve dye yield and defect coverage.
Standard dyes used for leather processing (acid dyes, direct dyes, metal complex dyes etc) are usually fixed by acidification, high temperature or cationic auxiliaries. This basic rule remains unchanged and applies also to the AR Technology.
Besides the selection of dyes, the structure of the application recipe is essential to achieve the goal of improved defect coverage.
Comparsion of acid retanning technology versus standard technology:
* faster penetration, uniform cross section
* stronger build up of the shade
* reduction of dyestuff offer and process time
* improved levelness
This picture below shows the dye build up and penetration of a standard formulation compared with the acid retanning technology, (No1 vs No 2). Level-dyed surface and full dye penetration, using 50% less dyestuff can be achieved using acid retanning technology compared to standard formulation, (No 4 vs No1).
Dyes are the most cost intensive chemicals in the wet end process and optimisation of amount, build up and penetration is a challenge for all wet-end specialists. The AR Technology can help to save costs and process time.
Dyeing in a separate bath To achieve superior dye build up and penetration combined with best possible dye levelness, dyeing in a separate bath after acid retanning gives optimum results. This application offers the possibility to achieve full penetration in a fraction of the process time, whilst maintaining excellent dye levelness, dye build up and accurate colour matching. The total dyestuff offer can be reduced when compared to standard recipes.
Dyeing in a separate bath can be considered the best possible application in combination with acid retanning technology.
In order to further enhance penetration, dye levelness and dye build up, two dye auxiliaries proved to be the most effective:
* Invaderm LU added in the first dye bath with the main dye feed, improves levelness and penetration, due to its retarding effect on the individual dyes
* Invaderm AL as an auxiliary for the top dyeing supports dye build up and levelness. due to its balanced retarding and shade intensifying (build up) properties
By the application of the acid retanning technology and using the synergistic effects of Invaderm LU and Invaderm AL the dye yield and leather quality can be considerably improved:
* less dyestuff needs to be offered for full penetration especially with Sella Star dyes
* uniform, level dyeing
* improved coverage of small defects
* reduced dyestuff offer for top dyeing
* reduction in chemical costs and process time
* smaller flanks and tighter grain by using the acid retanning technology
* colour matching is more accurate without wasting production capacities.
Leather character with AR Technology
Leather treated with the AR technology shows good inner softness and a full round handle due to the dispersing effect described above. The positive influence is also visible on the layout of the leather article with small flanks and bellies.
Loose and open structured hide materials are extremely sensitive to high pH values in the wet-end process. In particular the neutralisation often leads to additional looseness in the flanks and belly area of the hide.
AR technology supports the filling and grain tightening even on loose structured materials and helps to disperse retanning and filling agents.
This new approach to the retanning of shoe and handbag leathers is interesting for all those who have to process wet-blue and wet-white material made from open and loose structured hides. It offers the possibilities to reduce overall production costs and process time without negative effects on the leather’s character.
Benefits of the acid retanning technology:
* higher cutting yield due to an increased area with tight grain and firmer bellies
* improvement of tear and tensile strength
* imparts a soft, tight and pleasant handle with a fine and smooth grain
* no significant bleaching effect on dyeing
* good and easy dyestuff penetration, even with pastel shades
* less float changes
* applicable to wet-blue as well as wet-white
* better utilisation of production facilities, raw material and process time
For more information contact Florian Döppert at:
florian.doeppert@tfl.com
