FATLIQUORING OF LEATHER
Leather, at the time of completion of the tannage does not contain sufficiant lubricants to prevent it from drying into a hard mass.Almost all light leathers need a greater softness and flexibility than is imparted by tannage. This is attained in the fatliquoring process by introducing oil into the leather, so that the individual fibres are uniformly coated. The percentage of oil on the weight of leather is quite small, from 3-10 %.
The precise manner in which this small quantity of oil is distributed throughout the leather materially affects the subsequent finishing operations and the character of the leather. Proper lubrication or fatliquoring greatly affects the physical properties of break,stretch, stitch tear, tensile strength, and comfort of leather. Over lubrication will result in excessive softness and raggy leather in the bellies and flanks. Under lubrication, or improper penetration, results in hard bony leather that may crack in use.
To allow a small amount of oil to be spread uniformly over a very large surface of the leather fibres it is necessary to dilute the oil. Although this could be done with a true solvent such as benzene, it is cheaper, safer and more convenient to use the method of emulsification.
In an emulsion with water, the oil is dispersed in microscopically small droplets, giving it a white, milky appearance. It is important that the oil drops in water should remain as an emulsion until they penetrate the leather, and should not separate out as large drops or as a layer of oil, which could not penetrate the leather fibre and would only give a greasy surface layer.
The properties of the finished leather can be varied by controlling the degree to which the emulsion penetrates the leather before it “breaks” depositing the oil on the fibres. By such a technique, in the case of chrome-tanned leather, it is possible to concentrate the bulk of the fatliquor in the surface layers, leaving the middle containing relatively little oil. This yields a leather which is soft but resilient, with a tight break. In contrast if the fatliquor is allowed to penetrate uniformly, the leather will be soft and stretchy, with any natural grain looseness accentuated.
The commonest material used as a surfactant is soap. However in the presence of hard water, calcium or mineral salts or acid, the hydrophilic nature of he soap is reduced and it loses its surfactant powers.Most leather is acid; sulphated or sulphited, alcohols or oils, have much better resistance to these conditions and thus much better wetting action, and emulsions formed with their aid are much more stable to these conditions.
They are all classed as colloids, anionic or cationic surfactans, depending on the charge of the ionic group carried.Anionic surfactants are more effective at high pHs and on anionic materials, eg.vegetable tanned leather.Cationic surfactants are more effective at lower pHs and on cationic materials,eg. Chrome leather. Non Ionic surfactants in which the hydrophilic group does not ionize(consists of several hydroxyl groups) are used as auxiliaries in parafin degreasing, as wetting agents, and to stabilize fatliquors to obtain emulsion penetration into the leather.
Location of the oil: If we consider a cross section of the hide upon bending, we see that on the outside of the bend the fibers must stretch, and on the inside of the bend must compress.In the center of the skin there is very little motion of the fibers over one another during bending. Therefore both the grain and the flesh surfaces must be lubricated(to prevent break or grain wrinkle), but less lubrication is necessary in the center.
OILS, FATS and WAXES
Mineral Oils and Waxes:
Simplest type is mineral oil, obtained from crude oil from oil wells. They are mixtures of many substances which are separated by distillation. They are relatively cheap and chemically stable and are not affected by mould or bacteria.Can be obtained in pale color. Mineral oils do not mix with water therefore give waterproof properties and can be obtained at any viscosity. Despite the advantages they have only limited use in leather manufacture. Relative to other oils:
a) they are more difficult to incorporate thoroughly without giving a slightly oily or waterproof surface, which is a disadvantage for many leathers which are to be dyed or finisked
b) they have a poor “feeding action”, and used alone they give leathers which feel thin or empty but may be quite flexible
c) if the resultant leather is heated, the oil may migrate to the surface, which becomes oily or discoloured.
These oils do not appear to be as firmly held by the leather fibres as other oils; they are saturated hydrocarbons (unsaponifiable).
Paraffin wax(mp 35-36 C), Montan wax(mp 76-84 C), Ceresine wax(mp 60-85 C).
Natural oils and fats: Most of the oils and fats in animals, fish and plants are fatty acid glycerides. When boiled with caustic soda, they decompose to give soap and glycerine (saponification). By adding acid to the soap the free acid is formed. These fatty acids are water insoluble and range from very fluid oily liquids to greasy pastes and hard waxy materials. The property of the natural oil is largely governed by which of these fatty acids are combined with the glycerine.
All these glycerides can be split into glycerine and free fatty acid (rancidity) by acids and by action of enzymes(produced by moulds). It may happen to the oil or fat in the leather and if the solid type fatty acids are liberated they may crystallize on the surface of the leather spoiling the appearance of the leather giving a whitish dusty appearance known as “fat spue”. Another trouble due to rancidity of the oil is that free fatty acids form compounds with chromium, alum or zirconium salts used in tanning, which make the leather water-repellent and difficult to wet back uniformly for dyeing or finishing purposes.
Fatty acids may be classified according to their chemical reactivity that is their degree of unsaturation. Saturated fatty acids are usually more viscous or solid, do not darken with sunlight, unaffected by damp,warm air, do not combine with sulphur or iodine, difficult to sulphate.
Unsaturated fatty acids are morefluid, darken with sunlight, become sticky or gummy on oxidation by air, readily combine with sulphur or iodine, easily sulphated. Thus highly unsaturated oils may cause trouble on aging of the leather. In the paint trade they are classified as semi-drying (ie. castor oil) because they become gummy on exposure to air, and drying oils( ie. linseed oil) which on exposure “dry” to a hard, non-oily or non-tacky varnish.
Practically all naturally occurring fatty acids have an even number of C atoms. Shorter chain saturated fatty acids C-6,C-8, and C-10 are found in coconut and palm oils, milk fat and other softer oils. C-12, lauric acid, is found in sperm oil. Saturated fatty acids of C-16 and C-18 are common to animal fats and many vegetable oils.
The C-24 and C-25 category are found in waxes.ie.carnauba wax and bees wax.
The unsaturated fatty acids, primarily of C-18 type are quite common in animal and vegetable oils. Fatty acids with more than 1 double bond are classified as drying oils such as linseed, cottonseed oils. Some contain OH groups such as lanopalmic (C-16 hydroxy, saturated) found in wool fat and ricinoleic (C-18 hydroxy, unsaturated) found in castor oil. Both wool fat (lanolin) or wool grease and castor oil are common fatliquoring materials when sulfated.
Typical natural oils used:
1) Animal oils and fats:
Beef tallow (mp 35-38 C)
Mutton tallow (mp 40-45 C)
Wool fat and grease
Stearine (mp 49-55 C)
Stearic acid(mp 71 C)
Neatsfoot oil (I value=85)
2) Vegetable oils:
Coconut oil. (I value=10)
Olive oil, palm oil, palm kernel oil (I value=53)
Castor oil- contains large quantity of C-18 ricinoleic acid, has OH groups that render it water Soluble and is easily sulfonated.
Linseed oil
Soybean oil (I value=135)
3)Fish oils:
cod oil-(I value=150)high degree of unsaturation, drying properties, may be sulfated
Newfoundland Cod Liver Oil
Coast Cod, British Cod, etc.
Degras or Moellon- oxidized raw cod-liver oil
Herring oil, Salmon oil,Sardine oil, jap fish oil, menhaden oil
Whale oil
Sperm oil- rich in fatty alcohols and upon sulfonation becomes a very strong emulsifier.
Fatliquors:
Waxes:
Carnauba wax (mp 78-81 C)
Candelilla wax (mp 68 C)
Beeswax (mp 60-63 C)
Spermaceti-sperm oil (mp 42-49 C)
Wool fat and grease (Yorkshire grease) ( mp 30-40 C)
