The purpose of finishing is to improve the use properties of the leather in general and to protect it from wetting and soiling, to level out patches and grain faults and to apply an artificial grain layer to split or corrected grain leather. Furthermore to modify the surface properties (shade, lustre, handle, etc.).
The finish of a leather
can greatly vary mainly depending on the purpose of the leather. Hide stocks
differ in extent of grain damages greatly in the countries. Hides with greater
damages have to be corrected by buffing, which removes more or less much from
the natural grain structure. They are referred to corrected grain. Therewith
the most important element of a natural look get lost.
Furthermore leather vary uneven in grain in respect of structure and coloration
requires more pigments and more pigments generate thicker films and the resulting
leather becomes more plasticlike. A socalled natural feel or look is made
by embossing an artificial grain and by milling and also by application of
a bicolour effect.
4.1.- Finishing Materials
Finishing materials may be classified into two main groups:
4.1.1.- Ad binders
Waxes and coatings made from proteins such as casein and albumen are used mainly on vegetable tanned leather. The material is applied to the surface and is then polished by glazing or burnishing.
Most leather is finished
by the application of solutions or suspensions of synthetic polymers, which
may incorporate dyes or pigments or other additives.
A leather finish will usually consist of 3 layers: The ground coat to seal
the leather or to promote adhesion of the subsequent coats (not always necessary),
the base coat which will carry the dyes or pigments to give the required color
and finally the top coat, which will give the required surface texture and
abrasion resistance.
In general the first two coats will be water-based and contain acrylic or polyurethane polymers or mixtures of both. The top-coat could be polyurethane, nitro cellulose or cellulose acetobutyrate polymers in solution in an organic solvent or organic solvent/water mixtures. Solvent free top coats are now also available.
Acrylic resine are the most commonly used resins in leather finsihing. Nitro cellulose and Cellulose Aceto-Butryrate Polymers are the most widely used solution polymers; they provide extremely flexible films and give high performance topcoats with excellent aesthetic characteristics.
4.1.2.- Ad pigments
Pigments are used to color the finish and may be inorganic or organic. Inorganic types include iron oxides, lead chromates, chrome green, cadmium sulfide, ultramarine, titanium oxide, carbon black and metallic pigments such as aluminum, bronze, copper, plus a sub type which is usually called an extender pigment.
Organic types include mono azo, toners, lakes, phthalocynaine, metal complex and vat pigments.
The characteristic properties of a pigment depend on the type. Inorganic pigments are usually dull in color, have excellent light fastness, thermal stability, high opacity, resistance to bleeding, resistance to chemicals and are easier to disperse. Organic pigments are brightly colored , variable in light fastness and heat stability; have poor opacitiy, variable resistance to bleeding and chemical and are more difficult to disperse.
4.2.- Application Methods
There are four methods used in the leather industry to apply finishes:
4.2.1.- Ad Pad coating
Pad coats are usually applied by means of a plush pad - a wooden board covered with a soft velvet-like cloth. It is done by hand and is, therefore, labor intensive but it does ensure that the finish is worked well into the leather and evenly applied across the hide.
4.2.2.- Ad Spray application
The most commonly used method of application, the finish, is supplied through a fine jet in the atomizing part of the spray gun and applied to the leather in finely dispersed form. Compressed air is normally used to produce the necessary pressure. Spray application is normally carried out on multigun machines, although some small scale work may be applied by a hand-held gun.
Spraying can be wasteful,
with the finish being wasted by overspray at the edges, overspray between
the pieces and finally the spray bouncing back.
4.2.3.- Ad curtain coating
Curtain coating involves applying the finish to leather in the form of a liquid curtain of finish. Gap machines are particularly useful for applying heavy coats of finish, but their use in the industry is limited.
4.2.4.- Ad roller coating
In this application technique
the finish is applied by passing the leather between two cylinders, one of
which is coated with the finish which is then transferred to the leather.
Forward-, and reverse roller coating exist. In reverse coating the application
roller revolves in the opposite direction to the feed roller. This can give
higher levels of finish application and better finish adhesion, but can also
cause feed problems with softer leather.
Today base coats, impregnations, solvent and water based topcoats can be applied by roller coating and because higher concentrations of resin can be used compared to spraying, a 60 - 70 % lower proportion of diluents is achieved
4.3.- Pigment finsihing on full Grain or on corrected Grain
This kind of finishing is based mainly on thermoplastic waterbased binder-emulsions of the polyacrylate, polybutadiene and polyurethane type. But to this kind of pigment finsh are ascribed also organic solvents based full pigmented finishes. It is made up by subsequent application of grounding layer, pigment layers and the season layers. Each of these layers can be consist of more than one coating.
Pigment finishing is applied often after buffing the grain. Buffing has to be done as careful and as few as possible but also as far as necessary to equalize the aspect of the grain surface structure. Never the buffing is made so deep, that no more hair pores are visible. Buffing starts with coarse papers and ends with very fine grained paper (400 grains). Removal of dust has to be done as most complete as possible.
Groundings sometimes consist of a pregrounding with few binder and many oil. After drying the second ground is applied. This main groundcoating is usually applied very wet with a finish containing 15 to 20 % solid float and a mass on dry basis between 15 and 20 g/m². After drying the leather is often dried.
The pigment layers sometimes
consist of three coatings, they contain the full load of pigments, which can
be incorporated in the binder film system, which is at most about one part
pigment on 2 parts binder. Binders are polymer dispersions, which form films
on drying. Each coating has to be dried before the next is coated. The concentration
is about 20 % solid content. The leather takes up about 7 g solids per m²
per coat.
The season layer is classical made by nitrocellulose solutions. The applied
mass vary between 5 and 7 g/m².
4.4.- Splitfinishing
A finished split is just
a painted piece of leather, which without grain in any case is from inferior
value. Chromesplits but also vegetable tanned splits are sometimes finished.
In principle they can be finished in the same way as strongly corrected grain
leather.
4.4.- Drying
The two aspects of leather drying are the different designs for apparatus and processes in which streaming air of different humidity and temperature is used as carrier for water during drying and the behavior of leather during drying and its resulting properties.
The natural state of hides and skins is the fully hydrated fiber network with a water content of about 60 to 70 %. Fresh hides, when soaked, take up additional amounts of water and swell.
During tanning, retanning, dyeing and fatliquoring additional substances are taken up by the fiber network in the amount equal to about half the weight of the dry protein present. But the water may be assumed to be unaffected by the presence of these substances, because the nature of the fiber - moisture bond remains essentially unchanged.
When tanned or untanned material is exposed to dry air, water is evaporated from its surface. According to figure 8, in the first phase of drying (a), evaporation occurs from the surface film of water at a constant surface area of the leather. In the second, slower phase (b) the plane of evaporation retreats into the interior and evaporation proceeds from within the leather. This unsaturated surface stage of drying reflects the migration and evaporation of inner, hygroscopic water which is weakly bound to the surface of the fiber bundles, to the fibers, to the elemental fibers and, finally, to the surface of the fibrils. This kind of water migrates easier than the structural water which, being chemically bound, behaves like crystalline water and is removed in the last and slowest phase (c) of drying.
1. Air-drying without supply of energy (hang drying) 2. Air-drying - Air circulation method (hang-drying) - Drying in channel, tunnel, chamber (hang-drying) - Wet-toggled drying - Paste drying
3. Hot water drying
-Secotherm process (paste drying)
4. Infrared drying
5. Vacuum drying
6. High-frequency drying
Sammying and setting out are two operations which always are conducted before drying. Mechanical water removal is cheaper than removal by drying. If setting out is performed before sammying, it reverses the shrinkage of the leather, caused by drumming and leads to a smoother grain.
With the exceptions of drying by freely hanging of the leathers, all the drying methods hold the shape of the leather constant during drying in an expended state. This is the way leather cannot shrink and acquires a smooth and wrinkle-free surface.
After chrome tanning, dying and fatliquoring the leather is toggled, using special clamps and screens. This technique permits leather to be dried from both sides although drying from the side screen is somewhat slower than from the exposed grain side. Garment and upholstery leather is toggled and dried from the wet to the completely dry condition in one step which lasts several hours.
For shoe upper leather the vacuum drying technique is preferred. The wet leather is spread out by hand and put, grain down, on a polished, heated metal plate. Leather must be well wet to get a smooth grain. An airtight hood is placed over the leather before vacuum is applied. Inside, the hood is equipped with a felt and a wire net to press the leather down on the plate and to prevent loosening and shrinkage. Drying is interrupted after several minutes (to prevent overdrying), when the water content is reduced to a level lower than 30 % but not under 20 %. Drying of this leather is completed in a tunnel drier at relatively low temperature over a period of several hours.
The vacuum drying has
to be interrupted at a moisture content at which leather is not yet dry but
otherwise not too wet. Leather shrinks more the higher its initial water content.
Therefore it is often toggled during the final drying to get a better area
yield. Due to the vacuum the grain surface of the leather is flat and free
of wrinkles.
