Finishing materials:
In leather finishes the finishing materials are suspended, dispersed, emulsified or dissolved in water or solvents. The important finishing materials are generally the followings:
1. Finely ground pigments with or without dyes of same shade to intensify, brighten & level up the shade.
2. Film forming materials having binding power.
3. Preservatives to prevent putrefaction.
4. Plasticizers to make the film soft, flexible & stretchy.
5. Gloss giving materials.
★Pigments:
A pigment is any particulate matter that is insoluble in & essentially physically & chemically unaffected by the media which it is dispersed. The particles of pigment are too large to penetrate into the substrate. They are usually present on the substrate surface. So the pigment is easily removed unless fixed with an adhesive.
Source of pigment powders:
1. White pigment:
Titanium di oxide, Zinc Oxide, Zinc sulphide, Lithopone, Alumina, China clay.etc.
2. Black pigments:
Carbon black or gas black, Lamp black or vegetable black, Bone black, Graphite, Black iron oxide, Manganese di-oxide, Aniline black, Black lakes, Cobalt oxide etc.
3. Yellow pigments:
Lead chromes, Barium chromate, Zinc chrome, Molybdenum chrome, Cadmium yellow, Cadmium lithopone, Hansa yellow, Napthol yellow, Chrome orange. etc.
4. Red pigments:
Iron oxide, Cadmium reds, Para reds, Toluidine reds, Lithol reds, Permanent reds, Lakes of phosphotungstic acid. etc.
5. Blue pigments:
Prussian blue, Cobalt blue, Ultramarine, Phthalocyanine blues. etc.
6. Green pigments:
Chromic oxide, Pigment green-B, Phthalocyanin green. etc.
★ Binder:
Pigment are insoluble materials & having no affinity for leather must be bonded onto leather surfaces. For this a wide range of binder is used in finishing.
1. Casein
2. Shellac
3. Egg albumin
4. Blood albumin
5. Glues
6. Isinglass
7. Methyl-cellulose
8. Nitro-cellulose emulsions
9. Nitro-cellulose lacquer
10. Linseed
11. Locust bean
12. Resin emulsion
★ Diluents:
For better application on leather the viscosity of lacquer should be reduced considerably & that is done by dilution. Diluents are liquid which don't themselves dissolve cellulose esters but can be blended with out interfering with film forminf properties of the lacquer, eg: alcohol - methyl, ethyl, propyl, butyl alcohol. etc.
★ Plasticizers:
Plasticizers are usually heavy, non-volatile liquids with high boiling point compound (ester). Primary function is softening the film. Other functions are ease of glazing or provide the film additional water resistance. For instance- Castor oil, Dibutyl-phthalate, butyl stearate, olive oil. etc.
★ Waxes:
Waxes are generally used to provide the finish film polishing, which gibe very good shine, also provide very good feel, water proof-ness & good-gloss on brushing to the finished leather surfaces. Commonly used waxes are- carnuba wax, bees wax, paraffin wax, montan wax, synthetic wax, etc.
★ Antibacteriacides:
Mercuric chloride, Sodium fluoride, thymol, phenol or carbolic acid or beta napthol, tri-chloro-phenol, etc are used as antibacteriacides in finishing.
★ Wetting agents:
Soap, Sulphated oil etc are used.
★ Anti-foams:
Wax emulsion is used.
★ Leveling agents:
These are largely surfactants which enable the finish to have good flow out over the tight finishes.
★ Fillers:
They are used to fill any open or loose grain & prevent the finish sinking into the grain of the leather.
★ Thickener:
Used on heavily buffed or split leathers in order to get a good thick film or to achieve a reasonably good film very quickly in one coat. Sodium or ammonium poly-acrylates are chiefly used.
★ Matting agents or dullers:
silica.
★ Fluorescent materials:
Zinc sulphide.
★ Fixing agents:
Formaldehyde, acetic acid, Chromium salts are used as fixing agent in finishing.