Bisphenols and the relation to leather

Bisphenols have become an important topic of discussion in the leather industry, especially since the restriction proposal of The European Chemicals Agency on bisphenols. The potential restrictions could impact the leather making process, so it is imperative to deepen your knowledge about bisphenols.

In this article we explain the basics for a better understanding on bisphenols, focusing on the leather industry.

What are bisphenols?

Bisphenols are a family of very similar synthetic organic chemicals with two hydroxyphenyl functional groups which allow polymerization reactions to form larger molecules. Each type of Bisphenol is identified by a letter. Most commonly known bisphenols are:

• Bisphenol-A (BPA)

  

  Bisphenol-A is a precursor to important plastics, primarily certain polycarbonates and epoxy resins. BPA-based plastic is clear and tough and is made into a variety of common consumer goods, such as plastic bottles (including water bottles), food storage containers, baby bottles, sports equipment, CDs, and DVDs.

• Bisphenol-B (BPB)

  

  Bisphenol B (BPB) is very similar to BPA and is found as a replacement for this substance in some products. It has the same endocrine disrupting properties as BPA. It is not used as a raw material in the EU and it is in the process to be identified as Substance of Very High Concern.

• Bisphenol-F (BPF)

  

  Bisphenol-F can potentially be used in the manufacturing of plastics and epoxy resins. However, it is mainly found in chemicals as by-product from the initial polymeric reaction. BPF is not used as a raw material in the EU.

• Bisphenol-S (BPS)

  
  Bisphenol-S is used in curing fast-drying epoxy glues and as a corrosion inhibitor. It is also commonly used as a reactant in polymer reactions. In the leather industry its main use is in the manufacturing of (di)sulphonic condensates.

Bisphenols & Leather – FAQ’s

How are bisphenols used to make retanning agents?
Does SMIT use bisphenols to make retanning agents?
What are the effects of disulphonic/phenolic condensates in the making of leather?

Will leather without disulphonic/phenolic condensates give the same quality of leather?
What is SMIT’s position on bisphenols?

How are bisphenols used to make retanning agents?

Depending on the type of Bisphenol it can be used as a starting material (raw material) as is done for Disulphonic condensate. Here the BPS-monomer is used to form the polymer. In the leather industry, due to its applications, disulphonic condensates are predominantly called: sulphone-based syntan.

Bisphenols can also be formed as side reaction during the production process where phenol is used as raw material.  When phenol is reacted with formaldehyde, BPF can form as an unwanted side-reaction. When phenol is used as a monomer the polymer is called a phenolic condensate. In the leather industry, due to its applications, phenolic condensates are predominantly called: phenol-based syntans.

What are the effects of disulphonic/phenolic condensates in the making of leather?

Disulphonic/phenolic condensates are polymer produced out of disulphonic and/or phenolic monomers and can therefore contain bisphenol. In the leather industry these condensates are mainly used in the pre-tanning, retanning, fatliquoring and dyeing. There are many different disulphonic/phenolic condensates on the market, each tailored for a specific touch to the leather and are therefore very versatile in their usage.  Their performance has been clearly reported in different articles (J. Ammen et al. (JALCA 2015), G. Reich (Das Leder, 1996), and others).

The main effects of disulphonic/phenolic condensates on the substrates are:

• Tightness
• Softness
• Increased shrinkage temperature
• Increased tear strength
• Whitening effect

Additional benefits of disulphonic/phenolic condensates in comparison to other retanning agents (e.g. acrylic polymers, resins, vegetable tannins, etc).

• Good lightfastness
• Good heat yellowing
• No iron stain sensitivity (seen when using vegetable tannins)

Will leather without disulphonic/phenolic condensates give the same quality of leather?

Based on the unique combination of leather performance and due to the high usage of disulphonic/phenolic condensates in leather article recipes, it will be hard to maintain the same quality on:

• Lightfastness
• Heat yellowing
• Softness
• Light/pastel shade colored leather
• Iron staining

Other quality aspects of the leather can be maintained by other available retanning agents. However, a one-to-one exchange between disulphonic/phenolic condensates and other available retanning agents, whilst maintaining the same leather quality, is highly unlikely to be achievable and leather recipes have to be closely reviewed.

SMIT is offering our full support to the industry to make leather fulfill new regulations. For support or additional information, please contact your sales representative.

What is SMIT’s position on bisphenols?

Holding on to our mission statement “To create a socially and environmentally sustainable leather value chain”, SMIT has analysed its core-range on bisphenols. We’ll communicate openly our bisphenols values in our products to the industry, so our partners can make the responsible choice.

In the meanwhile, the Research and Development-team is redeveloping products to lower the BPS and BPF content by checking critical parameters like impurities in raw materials, eliminating substances, evaluating manufacturing processes extending reaction times, eliminate exceeding substances and increasing our quality measures.

For the long run, we are looking into bio-based alternatives to (partially) replace petrochemicals (incl. Disulphone and phenol) from our products to fulfil even the lowest requirements of Bisphenol content in leather possible.

For more information about our developmental products, please contact Product Management.