How relevant are resin retanning agents in today’s tanning industry?
There is a never-ending demand for higher cutting yield, but the statistics show raw stock quality is only getting worse. Therefore, leather defects, especially looseness, needs to be increasingly targeted. The big advantages of resins are that the filling is selective (targeting primarily the looser areas) and a smooth grain is achieved in spite of the heavy filling action due to low astringency, in comparison with vegetable tannins. Resins are a perfect compliment to our vegetable replacement high molecular weight polycondensated sulphone syntans.
Isn’t there a plethora of resin products on the market?
There is a clear definition of a genuine resin; it must selectively fill the loose and empty parts of the hides or skins so that the structural differences between these areas approach as far as possible the more valuable tighter parts. It must result in a significant and measurable increase in the cutting yield which in turn must more than compensate for the application cost of the resin.
Vague and even misuse of definitions in our industry have lead to many products being incorrectly called and classified as a resin but which are not chemically resin based nor have a selective filling action. These should be reclassified as simple fillers with a price to match.
Apart from selective filling, what other properties do genuine resins impart on leather?
They do not contain phenolic hydroxyl groups so do not change the chrome character as much as syntans or vegetable tannins. Chrome leather possesses high grain extensibility. Even decades ago, in his ground-breaking work, Mauthe showed that grain extensibility with resin retannages was better than with vegetable ones. Resins also give clearly improved buffing results because they increase fibre density, and thus also extensively used for nubuck and splits. Resins primarily work by "coating" the fibres, this increases their resistance to hydrolysis and perspiration too.
Is this "protection" property of genuine resins over vegetable tannins measurable?
It is a well know fact that leather can lose its tensile strength if stored in a moist and warm atmosphere. Nowadays international shipping is common place and there is always the risk that the containers can become both warm and moist. Mauthe also showed that vegetable retanned chrome leather lost 20% strength after 40 days under such conditions, but the leather retanned with resin did not show this reduction.
On what chemical basis are your main resin products?
The industry standard genuine resins Retingan R7 and Retingan R12 are both based on DCD (dicyandiamide). They are alkaline amino resins, which can be readily fixed with the addition of acid. For the vast majority of leather applications Retingan R7 remains today absolutely ideal. For those customers requiring even lower levels of free/hydrolysable formaldehyde then Retingan R12 is the best alternative. For customers that require zero free/hydrolysable formaldehyde then Retingan ZF Plus is the only option.
How important is the formaldehyde content of resins?
In the automotive section in particular, but now also increasingly in footwear etc., extremely low detectable limits for formaldehyde can be found. Here a resin that does not use formaldehyde in its manufacture is the only way to be completely sure of passing such challenging limits. Till recently all genuine resins have used formaldehyde in their manufacture so none can be certified as 100% formaldehyde free.
How did Lanxess achieve a patent for Retingan ZF technology?
Reducing the formaldehyde content of resins resulted in a corresponding reduction in the key property, selective filling. This presented the chemical industry with a considerable challenge. The patent was awarded because we solved this riddle. We removed the formaldehyde from the resin completely and yet maintained the selective filling effect thus fulfilling the demand for zero formaldehyde according to the industry accepted test method.
Lanxess claimed Retingan ZF as the world’s first and only resin without free formaldehyde. What is the key difference between this and the new product Retingan ZF Plus that has replaced it?
The key difference is we have learned to further enhance the property most demanded by our customers, the selective filling property. All other properties, such as low astringency giving smooth tight grain, excellent buffing, high heat and light fastness and uniform dyeing and embossing properties have all been at least maintained. Any product having high selective filling is a challenge for the dyeing. As the resin product is primarily deposited and fixed in the belly and flank areas this means that there is an increased chemical difference between these looser and the more dense areas. Nevertheless we attain excellent uniform dyeing results!
Are resins suitable for today’s high-tech leather like waterproof?
Resins are very good at sealing the grain, this was important for (water based) paste drying in the past, but now appreciated for controlling uniform (aqueous / solvent free) finish penetration today. This negligible water absorption also makes resins particularly suitable for waterproof leather especially when compared with most syntans and vegetable tannins. Furthermore, random voids in leather are commonly blamed for unexpected failures during dynamic waterproof testing. A resin retannage is basically about the deposition of insoluble synthetic organic products inside the leather. This, coupled with the very high particle size of genuine resins helps reduce these voids thus aiding consistency of dynamic Maeser and Bally values. If you are thinking about waterproof, you should also consider our new patented functional silicone Levotan W EXP 4019.
Despite being advantageous for waterproof leather Mauthe also showed that resins do not adversely affect the moisture vapour absorptiveness of chrome tanned leather so that wearer comfort is not reduced.
What problems have you encountered with other so called "counter" products?
Affinity and molecular weight are critical. Too high molecular weight (over polymerisation) and the leather handle can be too hard even resulting in grain cracking. Too small particle size and you get inadequate selective filling. Too high affinity and the grain can become rough. Other problems we have encountered with attempted "copy" products are high dye bleaching, uneven dyeing and low fastness properties, high free/hydrolysable formaldehyde and poor print retention. With decades of successful production of genuine leading resins we are confident to, at the very least, maintain our strong position due to our satisfied and trusted customer base.
