Chrome tannage still remains the most important tanning system in the leather industry. In the past 20 years, different systems have been developed to replace chrome tanning, some with more success than others, eg wet-white. The most common alternatives at the moment are based on glutaraldehyde or modified glutaraldehyde.

Before continuing, we need to distinguish between the terms, wet-white and organic tanning.

Wet-white is used to describe leathers that are free of chrome tanning salts. However, it may not mean that it is free of all metal ions, eg it can be used to describe leathers which contain aluminium, titanium or zirconium salts – all of which give a white appearance.

Organic tanning is a recent term used to describe leathers that are tanned with syntans, polymers, vegetable tannins and aldehydes. No metal tanning salts are used at all.

We would like to refer to our tanning system as an organic tannage, because it is based on phosphonium compounds.

One of the drivers behind the growth of interest in chrome free leathers is the possible oxidation of Cr (III) to Cr (VI) – which is a known carcinogen and causes allergic reactions. Also in 2005, new legislation takes effect which will force car manufacturers to produce cars that are fully recyclable.

This will increase demand for mineral free leathers as they are easier to recycle/dispose of and they can be incinerated without the risk of generating Cr (VI). This forecasted increase in demand has opened up new opportunities for producing leathers tanned without chrome but retaining the characteristic feel and handle.

One of these alternatives, originally identified at the beginning of the 1980s, is THPS (tetrakis hydroxymethylphosphonium sulfate). Applications developed at the beginning of 1990 confirmed THPS as a very promising organic tanning agent for the production of chrome and metal free leathers.

Our research and development work has focused on further developing products that produce leathers that satisfy all physical and environmental requirements, both present and future, without compromising the typical characteristics which make leather a natural, attractive and uniquely desirable product.

The points that have been taken into consideration are the production of leathers, free of chrome and prohibited materials, that have a low free formaldehyde content and are completely recyclable by using an application process that is practical, easy-to-use and one that does not require any changes within the tannery.

The results of this research and development programme are two new, perfectly compatible products that are applied in a system that is logical, practical and reproducible, resulting in leathers with excellent aesthetic properties and a high physical strength.

System basis

The system uses two products. The first product is based on a masked multifunctional phosphonium compound, giving the following characteristics:

* completely metal free

* good tanning effect

* easy to handle and apply in the drum

* very good penetration into the skin

* white appearance

* excellent light fastness

* possible to produce leathers in a wide range of colours, including light shades and white

* low toxicity

The second product is a special balanced mixture of a phenol condensation product with colloidal silicates, working as a supporting auxiliary in the system and giving the following characteristics:

* completely metal free

* ensures good water release during sammying

* imparts good structural stability to ensure accurate shaving

* easy to handle and apply in the drum

* very low astringency and makes possible the production of very flat leathers with excellent yield

* fully compatible with the phosphonium compound resulting in a perfect balance between tanning agent and auxiliary for the subsequent mechanical operations

* improves the feel and softness of the retanned leather

Mechanism and tanning process

Starting with a limed pelt that has been fleshed and split:

* deliming and bating

* pickling

* tanning

* basification

* fixation

* oxidation

Deliming and bating

The process must be free of ammonium salts because of the possible interference with the penetration of the phosphonium compound and the possibility of developing a characteristic odour. Products or systems containing significant quantities of free sulfite and bisulfite should also be avoided for the same reason. Deliming should be completely through the cross section as any residual lime will effect the tanning process. Running time will vary with the thickness of the pelt.


The final pickle pH should be 3.5-3.6 to ensure perfect penetration and optimum function of the tanning agent. It is not recommended that the pH be lower than 3 to avoid any undesirable reaction of the tanning agent. Running time will depend upon the thickness of the pelt and the acids applied. We recommend using a combination of sulfuric and formic acids.


While maintaining the optimum pH, full penetration of the tanning agent takes approximately two hours, depending upon the thickness of the pelt, temperature and concentration of the tanning bath. The phosphonium salt is less reactive than glutaraldehyde and will start to fix at around pH4. However, the tannage can be started at this pH due to the masking of the tanning agent. Penetration of the tanning agent can be verified by using a selenium solution (see Appendix). This should be checked before basification begins.


The tanning agent reacts to form coordinate bonds with the amino groups of the collagen). This reaction commences at about pH4 and increases in reactivity up to pH7. We recommend carrying out basification with sodium bicarbonate, using small quantities to avoid rapid/abrupt increases in pH. As the phosphonium compound is less reactive than glutaraldehyde, the risk of drawn grain and wrinkle is reduced, giving an improvement in yield. We recommend a final basification pH of 5, where approximately 95% of the tanning agent will be fixed.


The incorporation of the syntan based on phenol condensates with colloidal silicates produces a reaction with the tanning agent that stabilises the structure of the pelt and improves the physical properties. This results in a fuller leather with better water release, accurate and uniform shaving properties, and avoids uneven increases in substance during the retanning process.


To finalise the tannage and give optimum stability to the crosslinks, we recommend oxidising them. This has the additional effect of oxidising any residual tanning agent, rendering it inert and avoiding interference with the retanning chemicals. Oxidation is carried out using either sodium perborate or hydrogen peroxide (see Table 1).

By the end of the tannage following these guidelines, it is possible to obtain leathers with a shrinkage temperature of around 80ºC and excellent physical properties (see Table 2).

Minimising free formaldehyde

Consideration must be given to the method that is going to be used to measure free formaldehyde. In our experience, errors can occur that should be clarified and justified accordingly.

Wrong and misleading results can be obtained from immersion detection systems and systems that use acetylacetone, as this reacts with THPS giving a false reading. Generally, the automotive industry has a limit of less than 10ppm free formaldehyde, therefore a difference of 1-2ppm will give an error margin of up to 20%.

Methods that may cause problems

– JIS 1041

– Japanese low 112

– IUC commission 94.5000/a

– IUC 19

– DIN 53315

Methods that do not cause problems

– AATCC test method 112

– BSENISO 14184-2

– PV3925

We recommend using chromatographic techniques rather than immersion detection systems to differentiate between THPS and free formaldehyde.

Fastness properties

Numerous trials have been carried out in the laboratory as well as in production, with results that are more than satisfactory, exceeding the high standards already required in this industry (see Table 3).

Characteristics and properties of the leather

The resultant leather has an almost white appearance with high light fastness, making it suitable to produce leathers in a wide range of colours including pastel shades and, of course, white. Mechanical operations such as sammying and shaving are easily carried out, with excellent, consistent results. Shrinkage temperatures around 80ºC are normally achieved.

Very good hydrothermal stability, excellent water vapour permeability and inherent flame retardancy are typical of the system. The final leather has excellent resistance to dry heat with good dimensional stability, and performs well on most weathering tests.

The leather is comparable with normal production in appearance, texture and softness, with very bright, level dyeings.


Our work over the past two years has resulted in a safe, simple, reliable tanning process that can be easily modified to suit a variety of raw materials and tannery conditions. We have been able to optimise the process, removing the earlier difficulties associated with phosphonium salt tannages, eg odour development.

As the main demand for this leather type is automotive leather, we have focused our trials on this sector, developing a reliable and consistent process that achieves the aesthetic, physical and environmental requirements of today and the near future.

We recognise the increasing demand of chrome free leathers in other areas and we will continue our research and development programme to produce a full range of other leather types, eg garment, gloving, shoe upper etc.


Claus Reineking

Ian Hill

Mark Bartle

Dr Matt Walker

Dr Song Ma

Clariant UK

Appendix: Preparation of the selenium indicator

Dissolve 10g of selenium powder in 90g of concentrated nitric acid. Slowly add the solution into 900cm³ of distilled water.