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Application of Finishes
- 1. Application of Finishes Name: Tanvir Siddike Moin University: University of Dhaka Department: Leather Engineering
- 2. Application of finishes: General Introduction: The finishes are applied on leather generally by brushes and pads and finally by hand spray and this practices is still very widespread in major tanneries. Although automatic spraying machines are becoming popular in large tanneries as large quantities of leather can be produced by automatic conveyer belt spraying machine.
- 3. Application of finishes Padding: Method to achieve high gloss or a mostly dark contrast colour on raised sections of embossed grain leathers. This is done manually with a sponge, a folded cloth, cleaning wool or cotton wool covered with a permeable cloth. The pad is first dipped into a lacquer or dye solution and then lightly wiped over the raised parts. This serves to achieve two-colour effects and high-gloss or matt effects. A former variant consisted in applying wax coats onto the raised parts of embossed grain leathers The recessed (attach) parts were dyed in dark shades by spraying with black dye solutions. After removing the wax coat with a cloth soaked in benzene a reversed two-colour effect had been achieved (was called antique finish)
- 4. Application of finishes Printing: Several methods are possible. One method is performed with negative stencils (cut design thin sheet), i.e. the sections to be printed remain free on the stencil. The stencil is placed onto the leather and a finish paste is pressed through the opening by means of a doctor blade. After removing the screen box the patent is visible on the leather. Multicolour printing is possible by placing several stencils on top of each other. Instead of the finish paste, colourless binders can be applied by means of the doctor blade and then sprinkled (small quantity) with textile flakes. This produces velvety, raised patterns.
- 5. Application of finishes Printing: To ensure a perfect printing process the leather should not be too thin and of uniform thickness so that it is fed into the printing machine without creases. There are two printing methods: in the direct method the printing colour is carried in the recessed, engraved printing surface of the print roll and printed onto the leather. In the offset or high-pressure method the colour is carried on the tips of the engravings and then applied. With the indirect method the colour is transferred from the screen roll to a rubber roll and form there onto the leather. This method is very suitable for uniform printing, that is, for top coats, lacquer emulsions or pigment finishes. The printing method results in a reduced loss of colour compared to spraying.
- 6. Application of finishes Laminating A ready-made finish film which is on a carrier foil is applied to the leather surface. For this purpose an adhesive, mostly a polymer binder, is sprayed onto the foil and this side of the foil is placed smoothly onto the leather. It is then plated at about 80 °C, after which the carrier foil is drawn off. Uni-colour laminating or any effect is possible by this method. Firm polyvinyl chloride foils can also be applied. As the foil is about 0.2 mm thick this method is classified as coating
- 7. Application of finishes Coating: Two-component polyurethane starting material is applied to split or buffed grain leathers by means of special coating machines. The crosslinking agent and the polymer are mixed together directly before use and applied onto the leather on stencils or foils. The films obtained are highly resistant and very suitable for football leather, for example. The use of expanding agents is also very common. Due to the formation of polyurethane foam they provide particularly good films which have high flexing endurance and bending elasticity. The firmness properties can be improved by inserting fleece or textiles between the polyurethane coat and the leather.