Condition of the raw material (how
greasy?)Swystem Logic Services:
Control Points for Consistent Quality in Leather Making
E. Retanning - Syntans & Resins
Condition of the raw material (how
greasy?)
Temperature and time - maximum liming
temperature 86°F
- maximum bating temperature 97°F
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 (80°F max.) to delime the
grain before proceeding to warm washing and warm bating at 97°F
End point of bating: - mobility and
removal of natural pigment
- thumb print impression
- air permeability.
Consistency of preceding processes
Salt concentration and Specific
Gravity in
solution (Bé)
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
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.
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
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
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.
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.
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
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
Your feedback, comments and questions to the Author are welcome!