RETANNING CHEMISTRY - LEFTOVERS

The fundamental function of retanning has been to impart the final crust leather properties to the leather. The strength, softness, grain break, crust resistance (sweat, fastness, water spotting), fullness, drape, roundness, run, and nap properties; are common properties imparted by a typical retannage.
The traditional leather science textbooks of the last 20 years have taught the options available for tanning are available for pre-tanning and retanning:

• Mineral agents
• Vegetable agents
• Synthetic tanning agents
• Aldehydes
• Polymer (including resins)
• Fillers (including proteins and inorganic salts)

The classifications were confusing and switched between a classification of chemical structure and chemistry function. Going forward the industry should have a discussion about whether a purely structural classification can be used going forward. A possible classification could look as follows:

• Mineral agents (including metal-organic complexes)
• Vegetable extracts (including polyphenols, lignins and iridoids, and their derivatives)
• Aldehydics (not including iridoids and derivatives and polymeric aldehydes)
• Polymers (not including proteins), their monomers; resins and traditional syntans
• Proteins (including hydrolysates and derivatives)
• Inorganic salts (e.g., kaolin and other clays)

The regulation on problematic retanning chemistry started in the 1970s with the concerns raised about formaldehyde. Formaldehyde as a carcinogen, listed by the International Agency on the Research of Cancer (IARC), as Group 2A - probable carcinogen, means that the tanning agent that gave high tear strength, a white tannage, with excellent perspiration resistance is effectively banned.

Figure 1. Dicyandiamide (2-cyanoguanidine)

Since then, formaldehyde was targeted even as a breakdown product of other tannages, such as urea-formaldehyde, phenol-formaldehyde, melamine-formaldehyde, and to a lesser degree aldehydics (like phosphonium). Many countries have restricted formaldehyde levels in leather (virtually banning them by low permitted levels). Formaldehyde is also a known skin and respiratory sensitiser, so modern legislation works at preventing formaldehyde inclusion through that mechanism. Glutaraldehyde was recently added to the EU’s REACH

Authorisation List (Annex XIV) as a respiratory sensitiser and its use has effectively been stopped in the EU. European tanners are no longer being allowed to use pure glutaraldehyde in retanning.

Phenol-formaldehyde syntans, that originated in the early 1900s, known as bakelite; found application as a replacement for vegetable tanning agents.

These synthetic tanning agents offered greater light and heat fastness and could produce full, white, tannages that offered great benefit to leathers. With the restriction on formaldehyde, the manufacture of these syntans meant that the reaction favoured more phenol than formaldehyde to ensure that no loose formaldehyde remained in the product. However, attention was then drawn to the causticity (to skin and eyes) and toxicity (to the central nervous system) of phenol. This meant that phenol-formaldehyde syntans were doomed to survive a regulated market. The replacement was phenol linked by sulfone groups rather than formaldehyde, the so-called dihydroxydiphenyl sulfones. In 2019, Bisphenol S, a reprotoxic agent appeared on the EU’s rolling action plan for chemical restriction. Bisphenol S, BPS, that often occurs in formulations of dihydroxydiphenyl sulfones meant that the days of phenol syntans were limited.

Metals have always been a target for regulators, with many metals known to have toxic or carcinogenic effects. The terms heavy metals have been abandoned by the International Union of Physical and Applied Sciences (IUPAC), with chemists preferring to focus on individual metals and pros and cons. Chromium has been one of the metals that has received the chemist focus for decades, with hexavalent chromium being one of the species of chromium that has been regulated.

Chromium (VI) is a known carcinogen IARC Group 1 - by inhalation and is also a skin sensitiser. Tanners practicing good chromium management know how to control the formation of Cr(VI) from the Cr(III) pool that they use for tanning/retanning. Other metals used in leather have a significantly lower risk profile compared to chromium.

Aluminium, as a metal ion, Al3+, is known to have toxic effects in fish (i.e., in acidic water), however it is very reactive and forms oxides, salts, and organic complexes very easily - so is often not presented to nature as Al3+. Metal-free retannages are however quite popular, mostly as a political and marketing gimmick, rather than a technical consideration. Metals add significantly to the performance of leathers and the industry should look to changing the perception of metals.

Melamine and naphthalene are retanning chemistry that has also received increased awareness of late. Naphthalene is part of the famous polycyclic aromatic hydrocarbons (PAHs) group, derived from petrochemicals and coal. Naphthalene is not used in leather chemicals directly, although it used to be included in safety salt for curing. Naphthalene is reacted with sulfonic acid, to form naphthalene sulfonate (currently unrestricted), however naphthalene can occur in leathers as an unreacted monomer, at low concentrations. Naphthalene is detrimental to red blood cells and has been identified by the IARC as a Group 2B carcinogen.

Melamine is toxic to the kidneys and is progressively being regulated by countries in the Far East, the US FDA and the EU regulatory frameworks. Like naphthalene, melamine is not usually used as is, however the melamine monomer can leach out of leathers.

Remaining options
So, what is left? It is frustrating to have the toolbox shrink, but there should be no sign of panic just yet. Many of the chemicals, especially the natural substances, are represented well within chemical supplier portfolios. Looking at the classification list above, the following groups can be accessed:

• Minerals - titanium, zirconium, aluminium salts, zeolites
• Vegetable extracts - the traditional vegtans, lignins, and new iridoids
• Aldehydics - oxazolidines, and triose
• Polymers - polycarbamoyl sulfonates, epoxies, triazines, acrylates (especially lubricating acrylics), butyl-styrenes, dicyandiamides, styrene-maleics, and polyurethanes
• Proteins
• Inorganic salts (e.g., kaolin and other clays)

It is early days to know whether all the classical crust properties can be replicated with what is left, but it is the synergistic and compound effects of the remaining retanning toolbox that must be understood going forward.

Future directions
There are many new developments in the pipeline. Some of the developments given below will be expanded in future articles, but to give a flavour they are summarised here. The acrylate class is a highly versatile class that has been expanded recently to include polyacrylate ester polycarboxylates, in other words a polymer of acrylate is esterified to produce high volume (filling) materials that can be incorporated as low molecular weight, filling polymers.

Figure 2. Esterified polyacrylate.

Waste carbohydrates like starches have received a lot of attention, with modifications that allow their inclusion in leather making as reactive molecules. Triglycidyl isocyanurate modified starch, dialdehyde polymers, and aldehydic reactive starches have seen a lot of research and increased application.

Hyperbranched, epichlorohydrin reacted polyamide amines are also biopolymers of interest, that together with carboxy-functional epoxy resins look like good candidates for future retanning agents.

In the next issue: Regenerative farming is one of the buzzwords that generates a whole bunch of excitement. Very natural forms of agriculture have been used for millenia, but more research and holistic methods of farming are now of vital interest to users of their end-products and to people looking to make farming more sustainable. Many South African farmers and big companies like Nederberg are leading the way. The next article will examine techniques like biochar, and compost for soil enhancement.