Swystem Logic Services:

Control Points for Consistent Quality in Leather Making


 

A. Beamhouse

B. Tanning

C. Washing

D. Retanning - General

E. Retanning - Syntans & Resins

F. Fatliquoring & Softening

G. Dyeing

H. Finishing

 

 

A. Beamhouse

Condition of the raw material (how greasy?)

Temperature and time - maximum liming temperature 86F

                                     - maximum bating temperature 97F

Addition of surfactants

Drum speeds

Drum dimensions

Water quality (dissolved CO2 in wash water)

  Air quality (proximity of solid fuel or oil fired boilers producing CO2)

Ageing of limed hides, before and after fleshing

 Covering of limed hides to prevent lime blast

Handling of limed hides should avoid dragging grain down over floor or other surfaces

Cold water (80F max.) to delime the grain before proceeding to warm washing and warm bating at 97F

  End point of bating: - mobility and removal of natural pigment

                                   - thumb print impression

                                   - air permeability.

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B. Tanning

Consistency of preceding processes

Salt concentration and Specific Gravity in solution (B)

Temperature and time

pH/time profile

Drum speeds

Drum dimensions

Use of amphoteric speciality auxiliary

Consistent accurate basification

Shrinkage temperature

% Cr2O3 in leather

% Cr2O3 in exhaust float

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C. Washing

  Temperature

  Time

  Closed door or lattice door (float fixed or running water)

Complete draining of drum, before and after washing

  Two shorter float washes with closed door are more efficient than one long-float wash.

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D. Retanning General

Consistency of preceding processes

Age of the wet-blue

Temperature and time

pH/time profile

Drum speeds

Drum dimensions

Use of amphoteric speciality auxiliary / fat-disperser / chrome-soap disperser

Consistent accurate basification

% mineral oxide in leather

% mineral oxide in exhaust float

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E. Retanning - Syntans & Resins

  Neutralize grain to pH xx before retannage (check on optimum for retan)

  Check cut edge with bromophenol blue - green/blue color penetrates only to hair roots, i.e. through grain enamel depending on grain tightening type.

  Adjust with neutralizing agents

  pH of system at start, during & end of application

  Uniform period  of horsing up before drying (8 - 10hours - to allow the system to react check supplier recommendation).

Uniform neutralization - maximum pH 5.0

Dilute before addition 1:4 with water

Add resins 30 minutes before other anionic filling system

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F. Fatliquoring & Softening

The softening of leather depends on the following factors:

  preparation of the separated fibers in the collagen fiber weave

  fatliquor lubrication, lubricant distribution and concentration

  type of lubricant

  uniformity of IEP or zones of varying IEP through the leather

  degree and uniformity of neutralization before fatliquoring

  temperature, pH/time profile of fatliquoring

  drum dimensions and rotation speed

  drying conditions

  moisture conditioning and relaxing time (5 - 8 hours)

  staking and dry milling.

 

There are nine factors needing control to make soft leather:

correct preparation of well separated fibers

careful preparation of fatliquor emulsions

correct selection of lubricating oils

exact manipulation of emulsion chemistry

control of fatliquor penetration and deposition

as gentle, slow drying as productivity permits

suitable conditioning and relaxing

adequate staking and/or dry milling.

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G. Dyeing

Washing and complete draining

Standard, unchanged pretreatments and prior processes

Uniform penetration of tans, retans and auxiliaries influencing IEP

Standard age of wet-blue

Correct preparation of dye solution

Quality of water

Temperature of dyeing

Time/pH profile

Compatible dye components (affinity and migration)

Final wash duration and temperature

Horsing up period

Drying conditions.

 

Other important points which should be considered as Control Points are:

 

  Constant concentration of dyes and chemicals use liquids where possible

  Constant moisture content of hygroscopic dyes and chemicals

  Constant moisture content of wet-blue and crust leather

  Uniformity of pack weight

  Accurate measurement of float ratio

  Accurate weighing of dyes and chemicals - especially shading elements

  Uniform dimensions and rotation speeds of all drums in the dye-yard.

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H. Finishing - Empirical Controls to assist in development of consistent quality finishes.

Leather's end-use application: softness, colour, "effect", physical fastness specifications, cost/value of finished leather

Crust Leather to be finished:

            i.    grain coarseness/fine-ness; defects; veins - suitability for transparent finsh

            ii.   compactability of surface (for polishing/glazing)

            iii.  grain tightness/looseness

            iv.  grain's water absorbency/repellency & tendency to swell on wetting

            v.   pH of grain surface

  'Handle' of the Crust to be finished compared with the handle oh desired end-result:

            i.   elasticity/plasticity (stretchiness)

            ii.   tendency to crease on 'crumpling'

            iii.  tendency to absorb resins

  Mechanical surface treatments available/needed (buffing/polishing/glazing/dry milling/staking - N.B. Moisture Content)

  Resins and Binders individually checked on actual crust to be finished, and evaluate the potential of the combined resins in a mixture:

            i.    adhesion

            ii.   penetration

            iii.  film-forming

            iv.  gloss/matt

  Evaporate the resins / binders to dryness in a suitable vessel: evaluate the resins for:

            i,    % total solids

            ii.   tackiness/stickiness

            iii.   elasticity

            iv.   tensile strength

            v.    transparency

            vi.   gloss/matt

            vii.  re-wetting/swelling

  Evaluation of colours:

            i.    pigments - % total solids, particle size & covering power, fastness properties, water-resistance, heat resistance.

            ii.   dyestuffs - % total solids (active dye), fastness properties, migration into PVC, water-resistance, heat resistance.

  Evaluation of Top Dressings:

            i.    handle required

            ii.   fastness properties specified (especially wet & dry rub, resistance to perspiration)

            iii.  influence of cross-linkers to a. finish performance, fastnesses and elasticity; b. pot-life

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Note

Experience in controlling all the above potential sources of error, by manually or automatically monitoring the Control Points,  will give information on the limits between which a deviation can be permitted without practical noticeable effects. These limits will then correspond to the maximum and minimum 'set-points' for any automated process control system installed to ensure consistency of process conditions.

Author's Copyright: John C. Crowther

Swystem Logic GmbH 1995 -2003

email: John C. Crowther - Swystem Logic GmbH

Your feedback, comments and questions to the Author are welcome!