Wet Stage Leather Processing

Module 4: Wet Stage Leather Processing

• The following notes cover leather processing in the wet stage, from the beamhouse to the tanyard.

Leather Processing Stages

• The typical stages include raw hides, selection/trimming, soaking, green fleshing, unhairing/liming, lime fleshing, lime split/trimming, deliming/bating, degreasing, pickling/tanning, leather creation sammying/set-out/sort, and leather storage.

Leather Processing Vessels

• Main types are used to transform hide/skin into leather.
• Vessels incude pits, vats, rocking frames, paddles, processors, and drums.

Pits and Vats

• These are containers or tanks used for soaking, liming, tanning, and dyeing.
• Pits and vats facilitate controlled chemical reactions.
• These improve leather's durability, flexibility, and quality.

Rocking Frames

• Rocking frames are mechanical devices, used in the industry.
• Rocking frames uniformly treat hides during tanning, dyeing, or fat liquoring.
• Frames gently rock back and forth to ensure chemical penetration and prevent skins from sticking.

Paddles

• Rotating blades made of wood or metal.
• Paddles agitate hides and solutions during soaking, liming, tanning, and dyeing in large drums or tanks.

Processors

• Machines automate and enhance stages like soaking, liming, tanning, dyeing, and finishing.
• Processors ensure consistency, improves efficiency, and reduces manual labor, while maintaining desired leather quality

Drums

• Large cylindrical containers for treatments like soaking, liming, tanning, dyeing, and fat liquoring.
• Drums ensure uniform chemical penetration and mechanical action, improving softness, flexibility, and overall leather quality.
• Drums can be made of wood or stainless steel.

Reception/Selection

• At reception, preservation quality (bacteria, stains, hair loss), flaying pattern, and trimming are controlled on skins.
• Preservation type, skin size/weight, hair type/color key aspects considered on skin selection

Soaking Objectives

• Rehydrating fibers restores skin hydration after flaying, providing flexibility.
• This facilitates penetration of subsequent chemicals.
• Soaking separates fibers and eliminates non-proteic components like dirt, blood, salt, and some non-fibrous protein.

Influential Soaking Factors

• Factors include skin type/origin, mechanical effect, preservation type, water composition/quality/density, temperature, pH, auxiliary products, and time.
• Auxiliary products are detergents, enzymes, or surfactants that enhance dirt removal and fiber opening.

Soaking Parameters

• Temperature is preferably around 23°C (20°C - 26°C).
• Time usually ranges from 6 to 20 hours.
• Density is measured in °Baumé, indicating salinity between 2.0 and 5.0.

°Baumé Scale

• A unit used to measure the density of liquids.
• Formula for liquid heavier than water: SG= 145 / (145 - Bé)
• Formula for liquid lighter than water: SG = 140 / (130 + Bé)

Soaking Problems

• Improper soaking can lead to loose leather structure.
• Improper soaking can result in difficulties in subsequent operations, hard touch, grain pipiness, or wrinkled grain and grain burst.

Unhairing/Liming Definition

• Treats animal hides with alkalis (lime [Ca(OH)2] and sodium sulfide [Na2S]).
• This process achieves saponification of natural fats & prepares hides for increased flexibility and ease of handling.

Unhairing/Liming Objectives

• Removes hair/wool and the epidermis.
• Eliminates non-collagenic fibers and promote partial saponification: Fat/Oil+NaOH→Soap+Glycerol.
• Chemically attacks collagen to create reactive points causing skin swelling and weakening the reticular structure, allowing mechanical operations, if necessary.

Influential Unhairing/Liming Factors

• Include skin type/origin, time, mechanical effect, temperature, and auxiliary products (type and quantity).

Unhairing/Liming Parameters

• Temperature should be preferably around 23°C (20°C – 26°C).
• Time should be normally from 14 to 24 hours.
• pH of around 12.5.

Unhairing/Liming Problems

• Improper liming can lead to loose leather structure, difficulties, hard touch, pipiness, wrinkled grain, grain eroded effect, remnants of hair roots, and marked veins.

Liming Mechanical Operations

• Mechanical operations are fleshing and lime splitting.

Fleshing Objectives

• Cleans skin by removing subcutaneous and adipose tissue to facilitate chemical penetration in subsequent stages and regularize skin thickness.

Fleshing Procedures

• Procedure may be manual, in reptile and fish skins, or automated within fleshing machines.

Fleshing Parameters

• The machine must be fine-tuned to the skins being processed.
• Adjustments must account for the distance between the blade roller and the supporting roller, in addtion to accounting for the sharpening of the blades.

Fleshing Problems

• Improper fleshing machine operation can cause wrinkles.
• Other potential problems can include transport cylinder marks, very pronounced veins, cuts, bellies with very weak structure, or insufficient fleshing.

Lime Splitting Objectives

• Is a mechanical process -to section the skin into two distinct layers.
• Layers are Skin itself (Grain SIde) & Split (Flesh Side)

Grain Side

• Outer layer commonly used for high-quality leather products
• The grain side provides a smooth and durable surface.

Flesh Side

• The inner layer can be used for different leather products, according to thickness.

Lime Splitting Procedures

• Operation passes the skin through machine between two cylinders, with a moving blade.

Lime Splitting Parameters

• The lime splitting machine should be adjusted depending on the skins to be processed
• Thickness should be higher (2 to 3 tenths of a millimeter) in the head and bellies.

Lime Splitting Periodicity

• It is convenient to periodically check the thickness throughout the work.

Lime Splitting Problems

• An Improper operation can cause problems: Transport cylinder marks, cuts, skins with irregular thickness.
• Other problems include bellies with thinner thickness than the rest.

Deliming

• A leather manufacturing process that removes excess lime from the leather, this lime was introduced during the liming process.
• Deliming restores pH to a level suitable for further processing.

Deliming Objectives

• Eliminates lime (calcium hydroxide) from the liming step.
• Reducing alkalinity of the skin by lowering the pH level of the leather.
• Is a crucial step to prepare the skin for bating.

Deliming Chemical use

• Chemicals are commonly used to neutralize alkaline condition and remove lime.
• Chemicals include acid (sulfuric or hydrochloric acid), ammonium salts (ammonium chloride or sulfate), and organic acids (formic acid).

Deliming Influential Factors

• Factors are skin type, time, mechanical effect, temperature.
• Additional parameters include auxiliary products, thickness, and final pH value.

Deliming Parameters

• Tempereature should be preferably around 32°C (30°C – 35°C).
• Time should be normally from 20 to 120 minutes.
• The desirable PH is around 8.0.
• Cut section should be colorless to phenolphthalein (or at most 1/3 pink).

Deliming Stage Control

• Stage control during deliming involves section cut control using pH indicators.

Deliming - Problems

• Improper deliming causes structure loss, hard touch, grain peppiness, grain erosion, operational difficulties, or excessive hardness/softness.

Bating

• Definition is a crucial leather manufacturing process.
• Bating of hide through treating with enzymes further removes proteins, leaving leather softer, smoother, and more flexible.

Bating Objectives

• Eliminates Non-Collagenous Proteins to enhance leather quality by removes unwanted proteins like elastin and albumins.
• Improves Softness and Flexibility & Enhances Grain Smoothness
• Facilitates Chemical Penetratio, bettering the preparation to absorb tanning agents in the next stage.

Bating - Parameters

• Temperature should be preferably around 32 °C (30 °C – 35 °C).
• Time should be normally from 30 to 120 minutes
• pH value: normally around 8.0
• Type and concentration control of enzymatic product used

Bating - Enzymatic products

• Natural (excrement): Dog & Pigeon
• Artificial
  • Animal origin (pH 8.0 – 9.0)
  • Vegetal origin (pH 3.5 – 5.0)
  • Bacterial origin (pH 6.5 – 8.0)

Bating - Problems

• An Improper action can lead to loose leather structure, wrinkled grain, grain erosion, high hardness/softness and grain burst.

Pickling

• Prepares the skin for tanning, lowering pH value and adjusting ionic charge.
• Stops enzymatic action, complements deliming, and preserves skin.

Pickling Influential Factors

• Includes deliming degree, mechanical effect, acid type/quantity, temperature, thickness, time, and salts type/quantity.

Pickling - Problems

• Improper execution can result in loose leather structure, loss of mechanical resistances (weak leather structure), Grain erosion, difficulties in the tanning stage and grain burst,

Pickling Parameters

• Bath density (°Baumé) levels btween 6.5 - 7.5 before adding acid.
• Time lasts normally from 2 to 12 hours
• Final pH value: normally around 3.0 (chrome tanning).
• Cut section; yellow with the use of bromocresol green.
• Temperature of 25 °C (23 °C – 27 °C).

Tanning Objectives

• Convert unstable skin (putrescible) into leather.
• Give the skin imputresivability, dimensional stability at high temperatures, mechanical resistance
• The final leather product should present resistance to hydrolysis

Classification of Tanning Products

• Organic vegetable tanning products: They are environmentally friendly.
• Often used for high-quality, durable leather.
  • Tannins from bark (e.g., oak, chestnut, hemlock)
  • Tannins from leaves (e.g., sumac)
  • Tannins from fruits (e.g., myrobalan, tara, valonea)
  • Tannins from wood (e.g., quebracho, mangrove) Organic non-vegetable tanning products: Synthetic or oil-based agents.

Organic Non-Vegetable Tanning Products

• Used as a alternative to traditional vegetable tannins.
• Aldehyde tanning agents (e.g., glutaraldehyde, formaldehyde-based compounds)
• Synthetic tannins (Syntans) - chemically modified organic compounds that improve softness and colorfastness
• Oil tanning agents - primarily in chamois leather, made from fish or animal oils Inorganic tanning products are from metallic salts/inorganic compounds.

Inorganic Tanning Products (Mineral)

• Often used to produce soft, water-resistant, and durable leather.
• Chromium tanning agents are the most common method for modern leather.
• Aluminum tanning agents produce white leather.
• Zirconium tanning agents are known for heat resistance and are often used for specialty leathers.
• Iron or titanium tanning agents are used for specific applications/ are less common.

Transferred Properties

• Color
• Resistance to temperature (TC)
• Flexibility (Hardness / Softness)

Tanning Step Summary

• Collagen, if untreated, will result in Colagen without treatment.
• After tanning treatment, results in Tanned Colagen
• The final results involves transversal bounds between proteic chains

Process Type

• Types include mineral, organic, chromium, vegetable, aluminum, aldehides, etc

Chemical Bonds

• Established in collagen as a result of different types
• minerals: coordination bonds
• Mechanism: Metal ions form coordination complexes w/ carboxyl (-COO) & hydroxyl (-OH) groups in the collagen structure.
• Effect on Leather: Provides high stability, flexibility, and resistance to heat and water.
• Tanning with aldehydes: covalent bonds
• Mechanism: Aldehydes react with amino (-NH2) & hydroxyl (-OH) groups in collagen, forming strong covalent cross-links.
• Effect on Leather: Enhances stability, softness, and resistance to hydrolysis, commonly used for chrome-free tanning.
• Tanning with vegetable tannins: hydrogen bonds and dipole bonds
• Mechanism: Polyphenolic tannins interact with collagen through hydrogen bonding and dipole interactions with peptide groups.
• Effect on Leather: Produces firm, durable leather with a characteristic reddish-brown color.
• Tanning with Synthetic Tannins (Syntans)
• Mechanism: Synthetic tannins (often phenol-formaldehyde derivatives) bind to collagen similarly to vegetable tannins via hydrogen bonding and dipole interactions.
• Effect on Leather: Improves softness, lightfastness, and color stability, often used in combination with other tanning methods.

Mineral Tanning Contraction Temperature

• Aluminium sulphate masked with citrate 69 °C
• Aluminium sulphate masked with formiate 73 °C
• Zirconium sulphate masked with acetate 95 °C
• Ammonium titanyl sulphate masked with citrate 99 °C
• Chromium sulphate 107.5 °C

Organic Tanning Contraction Temperature

• Vegetable Tanning:

  • Quebracho (after 40h / pH=6) 86 °C

• Chestnut (after 50h / pH=6) 78 °C

• Aldehyde Tanning:

  • Glutaraldehyde 80-85 °C

Tanning Two Fundamental Steps

• Involves the penetration of the tanning product into the fibrous structure of the skin.
• Ensures uniform tanning & prevents surface fixation, which can lead to poor leather.
• Effectiveness depends on pH, temperature, and the tanning agent type.

Fixation

• The fixation of the tanning product on the collagen must chemically bind to collagen fibers, stabilizing skin
• This makes the skin resistant to heat, decomposition, and microbial attack.
• Effectiveness depends on pH adjustments (often through basification) & reaction time.

Common Tanning Products

• Organic Vegetable and Inorganic Tanning Products

Organic Vegetable Tanning Products Classification

• Divided into two main types depending on their chemical structure, including Hydrolyzable Tannins (Pyrogallol ) & Condensed Tannins (Catechol),,.

Hydrolyzable Tannins (Pyrogallol Tannins)

• Tannins are ester-linked polyphenols that can be hydrolyzed into simpler phenolic acids.
• Key compounds: Gallic Acid & Ellagic Acid
• Properties: Produce lighter-colored and make good tanning for soft and flexible results in leather.

Condensed Tannins (Catechol Tannins)

• Tannins are polymerized flavonoids that do not hydrolyze easily.
• Key compound: Catechin.
• Properties: Produce firmer and darker leathers, can be used in heavy-duty leather applications.

Inorganic Tanning Products - Chromium

• Widely used in the leather industry du to efficiency and properties.
• Trivalent chromium (Cr³+) has a strong tendency to form coordination complexes due to its electronic configuration and empty orbitals available for bonding.
• The atomic number of chromium is 24.

Electronic Configuration of Chromium (Cr)

• Ground state electronic configuration: [Ar] 3d5 4s¹
• Electronic Configuration of Cr³+ is [Ar] 3d³ due to losing 3 electrons.
• Chromium (III) has six empty orbitals that can be used to form coordinate bonds with ligands (water, sulfate, or collagen in tanning).

Coordination Tendency and Hybridization

• Chromium (III) has six empty orbitals to form coordinate bonds with ligands.
• Possible hybridization for six coordination sites is d²sp³, resulting in octahedral complex geometry.
• Six ligand molecules (e.g., H₂O donate lone pairs to these six empty orbitals, forming stable complex.

Hydration and Complex Formation

• In aqueous solutions, Cr³+ readily hydrates.
• Forms the complex [Cr(H₂O)₆]³+, where six water molecules are coordinated around the chromium center.
• Important to hydrating because it allows chromium ions to penetrate the leather and bind to collagen fibers.
• Cr³+ has octahedral coordination due do its d²sp³ hybridization that forms stable complexes.
• This complexation is fundamental to leather tanning.

Tanning Power and Penetration Power

• Basicity is the concentration of basic (alkaline) substances used in tanning, affecting power.
• In the basicity of a tanning solution is increased, it affects both the tanning power and the penetration power differently

Tanning Power in Detail

• Ability of tanning agent to form stable complexes with collagen, leather durable and resistant to decomposition.
• Increase improves the tanning power, with higher pH levels facilitating the bonding between tanning agents and collagen fibers, strengthening the leather

Penetration Power in Detail

• Ability of tanning agent to diffuse deeply into leather fibers and into electrostatic repulsion and fiber compactness.
• Higher pH causes collagen fibers to be negatively charged, which leads to fiber swelling and more water absorption.
• Even with fiber swelling, spaces between the can close up, making a "gel-like barrier".
• Can be found greatly reduced and affected

Basicity and The Tanning Process

• Swelling of the skin reduces capillary channels, slowing down or reduces tannin absorbtion.
• Power may incrrease, but penetration is reduced as fibers become compact due to tanning agents

Leather Treatment types

• Vegetable leather characteristics: Organic, hydrofilic, plastic, anionic, inferior Apparent humidity, superior Apparent temperature, inferior acidity, superior affinity to body and inferior Resistance to temperature.

• Chromium leather characteristics: Inorganic, hydrophobe, elastic, cationic, superior Apparent humidity, inferior Apparent temperature, superior acidity, inferior affinity to body and Maior Resistance to temperature.

Characteristic Qualities

• Vegetable leather has inferior color clearness/general fastness/capacity to hydrofuge/colour versatility and superior to water absorption, shape retention, polishing, buffing, biodegradability.

• Chromium leather has superior color clearness/general fastness/capacity to hydrofuge/colour versatility and inferior to water absorption, shape retention, polishing, buffing, biodegradability.