Finish coatings

Among the entire cycle of finishing processes, topical dyeing of the skin occupies a central place. Leather with a natural front surface, a very limited coating thickness (without pigments) and a natural measure are in greatest demand. To obtain such leathers, the semi-finished product must be of high quality with a minimum of defects. However, most of the raw materials have numerous facial defects and, therefore, a significant part of it is subjected to refinement, that is, either masking defects and defects on the facial skin, or sanding the skin, followed by the creation of an artificial front surface made of polymeric materials.

Requirements for coating quality

Coating dyeing must meet a range of requirements. First of all, it gives the skin high aesthetic properties: uniform color over the entire area, elegant appearance, gloss or dullness of the coating, and for skin with a natural front surface – emphasizing the skin’s natural measure (aniline finish).
In addition, coatings must satisfy a set of properties dictated by the operating conditions of products from it and the technology of their production. This includes: high adhesion, water, light, heat and frost resistance, resistance to solvents, friction, impacts and scratches, to repeated bending, stretching and compression. Considerable attention is paid to the hygienic properties of the skin, which, unfortunately, are sharply reduced as a result of coating dyeing. But even with all these technical requirements, the skin should have a beautiful appearance. Also, one of the main requirements is the ease and simplicity of skin care in the product.
Satisfaction of all these requirements is possible only with the correct selection and combination of finishing materials and the proper preparation of the skin itself for covering dyeing.

All coatings can be divided according to several criteria.
By the nature of the film former:

Polymerization, also called emulsion or latex:
a) acrylate;
b) based on copolymers of acrylates with diene and vinyl derivatives.

Nitrocellulose:
a) nitro enamels in organic solvents;
b) nitride emulsions.

Polyurethane:
a) pure polyurethanes obtained by the method of stepwise polymerization of isocyanates with alcohols or other substances containing active hydrogen in their molecules;
b) polyurethanes in combination with acrylates (in organic solvents);
c) polyurethanes with the addition of nitrocellulose or polyvinyl chloride as one of the components of the varnish (in organic solvents;
d) aqueous dispersions of polyurethanes.

Protein:
a) casein;
b) based on casein modification products;
c) based on collagen dissolution products.

Regardless of the type of film former, 2 types of coatings are distinguished by opacity:
Opaque, that is, consisting of several layers of a film former containing mineral or organic pigments that cover the skin with a natural measure, and in the case of polished skin imitate its front surface. Used in semi-aniline and pigment finishes.
Coatings, consisting of a translucent film of the minimum thickness with “transparent” dyes, is the so-called “aniline” finish, which preserves and even emphasizes the specific natural extent of natural skin.

None of the film-forming agents are used independently, but, as a rule, their combinations are used — a combination of several film-forming agents directly in the coating paint itself or applying different film-forming agents in layers: primers based on finely dispersed polyurethanes and acrylates, middle layers based on polyacrylates, upper ones based on nitrocellulose or polyurethane. (see photo)

General skin finishing scheme
Impregnation – seals the structure of the skin, reduces the minkiness of the front surface.
Soil (pre base coat) – levels the absorbing ability of the skin and is responsible for adhesion. There are several varieties of soil:

  • adhesive ground (adhesion ground) – provides reliable adhesion of the main coating to the crust;
  • soil with subsequent polishing (polising ground) – levels the surface of the crust;
  • soil with subsequent grinding (buffing ground) – compacts the surface of the crust for further smoothing grinding;
    Base coat (base / main coat) – gives color and improves physical properties.
    Protective coating (top coat) – protects previous coatings and gives the skin a finished look. Also gives shine and touch. Responsible for good physical properties.

Coating compositions for finishing leather consist of the following components:
film-forming agents – are the basis of the coating film;
pigments (in the form of pastes) – give the film the desired color;
plasticizers – to give the film softness, elasticity, etc .;
solvents – for ease of application
and a number of different excipients – to give the film specific properties.