iLeatherz – The Chemistry of Leather

The Chemistry of Leather

A Comprehensive Insight into Chemicals in Leather Manufacturing

Suzan Kumar Dass

Leather Chemistry Specialist

Published on May 21, 2025

Introduction

Leather manufacturing is a complex process that transforms raw animal hides into durable, flexible, and aesthetically appealing materials. This transformation is made possible through the application of various chemicals at each processing stage.

These chemicals play a crucial role in cleaning, preserving, softening, coloring, and finishing the leather to meet diverse market requirements. However, their extensive use also raises concerns regarding environmental pollution and health hazards.

This article explores the types of chemicals used in leather manufacturing, their functions, and the emerging shift toward more sustainable alternatives.

Action of Leather Processing

Leather manufacturing process is basically done through mechanical and chemical action. Mechanical action refers the machine actions and other operations without use of chemicals. Chemical action means the treatment of hides and skins with different types of chemicals.

Conventional leather manufacturing process is almost chemical based (almost 70-80)% treatment is done by chemical treatment. These treatment are done by different types of natural, synthetic, semi-synthetic organic and inorganic chemicals.

Steps of Leather Manufacturing Process

Leather manufacturing process conventionally is done by three steps namely pre-tanning, post tanning and finishing. These steps help to turn the raw hides and skins into finished leather.

Pre-Tanning

Initial preparation of hides including soaking, liming, deliming, bating, and pickling to clean and prepare the hide for tanning.

Tanning

The process that stabilizes the collagen fibers of the hide, making it resistant to bacterial attack and more durable.

Finishing

Final processes including dyeing, fatliquoring, and coating that give leather its final appearance and properties.

Conventional Chemicals Used

Pre-Tanning Chemicals

Stages	Chemical	Function
Soaking	Wetting agent	Rehydrate hides and skins Removes dirt, improve water penetration
Linning	Calcium hydroxide (Ca(OH)₂) Sodium sulphide (Na₂S) Sodium hydrosulphide (NaHS)	Swell collagen fiber, loosen hair Breakdown the sulphide bond of Keratin and removes hair Milder alternative to sodium sulphide
Deliming	Ammonium sulphate ((NH₄)₂SO₄) Acetic acid, lactic acid	Remove residual lime, reduce PH Gradually reduce PH
Bating	Proteolytic enzyme (Trypsin, bacterial enzyme)	Remove non-collagenous protein, smooth grain
Pickling	Sodium chloride (NaCl) Sulphuric acid (H₂SO₄) Formic acid (HCOOH)	Prevent swelling during acidification Lower PH to prepare for tanning Mild acid for controlled acidification

Tanning Chemicals

Stages	Chemicals	Function
Tanning	Basic chromium sulphate (Cr₂(SO₄)₃) Vegetable tannin (Mimosa, Quebracho etc) Aldehyde tannin Synthetic tannin agent Al/ Ti/ Zr tannin	Stabilizes collagen, impart thermal stability Natural polyphenols that stabilizes the collagen Chrome free tannin that stabilizes the collagen Enhance specific leather properties Mineral tannin agents used for special types of leather

Post-Tanning Chemicals

Stages	Chemical	Function
Rechroming and Retaining	Syntan Vegetable tannin	Improve fullness, grain tightness and color shade Restore firmness after chrome tanning
Neutralization	Sodium bicarbonate (NaHCO₃) Ammonia	Raise PH to optimum for better dyeing and fatliquoring
Dyeing	Acid dye Basic Dye	Impart color to leather, specially for chrome tanned leather Used for vegetable tanned leather
Fat liquoring	Sulphonated oil Synthetic oil and emulsifiers	Lubricate leather fiber, improve softness and flexibility Enhance penetration and distribution of oils

Finishing Chemicals

Types	Chemical	Function
Binders	Acrylic resin binder Polyurethane Protein based binder	Form film, makes smooth surface, increases durability and flexibility
Dye	Aniline Solvent or water based dye	Penetrative colorant (transparent)
Pigment	Organic and inorganic colorants	Surface colorant (opaque)
Modifier	Silicone emulsion Softener	Adjust touch feel of the finished leather
Crisslinker	Isocyanate Polyfunctional aziridines	Improve film strength and adhesion
Waxes or oil	Natural and synthetic oil or wax	Enhance feel and visual look
Slip additive	Silicone based additives	Improve surface slip and touch

Conclusion

Chemicals play a critical role in every stage of leather manufacturing, from cleaning and preserving raw hides to enhancing their final appearance and performance. Each category of chemicals—such as soaking agents, liming chemicals, tanning agents, fatliquors, dyes, and finishing products—serves a specific function that contributes to the quality, durability, and usability of the leather.

The careful selection and controlled application of these chemicals ensure consistency, safety, and desired properties in the final product. As the industry evolves, there is a growing emphasis on using safer, more sustainable chemical alternatives to meet environmental regulations and consumer demand for eco-friendly leather, making chemical management a central focus of modern leather processing.