Upgrading of shoe upper leather in finishing (part 1)

The ‘upgrading of leather’ in simple terms means: to make the best leather possible out of the available crust material. This includes the highest quality as well as the largest cutting yield while keeping the elegance and natural aspect of the final leather. Since the gap between supply and demand of high class raw material is increasing rather than decreasing, upgrading becomes more and more important. It does not seem to be so difficult to cover crust defects by applying a thick and heavy finishing layer, or even to remove the damaged grain by buffing – but that’s only half the solution.
For heavily finished articles such as some automotive or furniture articles, upgrading by using mechanically created foams, special filling components, pre-base coats containing microspheres and/or heavily embossings can be used. For most of the shoe upper leathers these technologies are no alternative, because these leathers do not have thick finishing layers, so upgrading has to be achieved with thin coats. And, of course, there are lots of articles without any finishing coat at all. We’d like to upgrade them as well. Because of the spectrum of shoe upper leathers, it is not possible to deal with them all in the same way. Different defects in different sizes and on different leathers need different upgrading techniques and products/systems. This first part explores upgrading ideas and techniques for shoe upper leather types such as:
– Corrected grain leathers, box types
– Corrected grain leathers, less covered
– Full grain leathers

The second article will concern some more details referring to:
– Pre-base coats
– Base coats
– Effects

Upgrading Techniques
Corrected grain shoe upper leather, box-types
This is the only kind of shoe upper leather, where the upgrading procedures are not so far away from the procedures for furniture/automotive leathers.
To achieve a smooth and even surface and sufficient cutting yield, the crust needs to be well prepared before applying the base coat. Very common today is a pre-treatment of the crust with microspheres-containing pre-base products1 (‘stucco’) by reverse rollercoating. This can be done either on the full grain crust and the subsequent buffing will then remove the surplus and even out the surface or, what’s far more effective in terms of upgrading, after a pre-buffing process.

The procedure could be as set out in table 1:
For a further improvement of upgrading both the products used in the base coat and their application as well as mechanical treatment are of high importance. Currently specially developed compounds3 based on polyurethane3 or acrylic4 dispersions or a mix of both are common. Additives such as fillers,5 wax6 and oil,7 emulsions or proteins8 can be included.
After the first base coat, normally applied by reverse rollercoater, the leather surface needs to be covered. In order to smooth the leather and to seal the surface the base coat is plated. Plating at high temperature is not recommended as the leather could get blotchy due to remaining water that will evaporate if plating is too hot, and the leather could also stick to the plate/roller.
Using a sandblasted roller/ plate instead of a smooth one will increase the uniformity. A fine hair cell embossing pattern is second best and has to be covered completely by the following layers, since these leathers should not show a hair cell at all!

Base coating these leathers should be done as shown in table 2:
The leather can than be top coated for different purposes. It could be an aqueous patent leather top coat,9 a dull one for college types, or any degree of gloss in between. Besides the application of solvent based patent lacquers by curtain coater and nitro cellulose emulsion lacquers10 by spraying, purely aqueous top coats11 are very often applied by rollercoater. The advantage of this rollercoater application is not only the lower product consumption but especially a further increase of evenness and uniformity of the leather.

Box imitation on splits
Almost everything that has been mentioned above for the corrected grain box type leathers is relevant for the box imitation on splits too. As soon as the split has been pretreated to give a surface close to the corrected grain crust leather the following procedures can be used.
– if possible, the split should get an impregnation.2 After this tightening coat the fibres are glued together a little and a smoother surface ensues
– a further increase in evenness will be achieved by applying a special buffing supporting pre-base coat. This pre-base coat is based on the same products1 as for the corrected grain leather pre-coats using the same conditions. The microspheres-containing pre-base should be applied as for the corrected grain leathers. After the application the roller should immediately remove the surplus. After vacuuming / plating and buffing, the surface appearance will be close to that of a corrected grain leather.

The finishing procedure and choice of products are very close to the one for the corrected grain leathers.3,4 When all the pre-work compliments the crust, articles without a resin based finishing layer are possible. For example pull-up or antique effects can be made, and very natural looking leathers do result.

Corrected grain shoe upper leathers
These leathers do not get a deep grain correction. As a general rule the final article and the available crust must be evaluated, as with this kind of shoe upper leather sorting is value-adding!
The intension here is to equalise the leather surface but the final leather may not look corrected. The goal is to achieve a full grain imitation. Therefore the first and most important step is the grain restoration.
Two options are possible: either the leathers will be buffed in the finishing department or they enter the finishing department already buffed.

Corrected Grain Leathers, not yet buffed
– Heavy Covered Leathers
a) Pre-base: Depending to the final article there are at least two options for the finisher.
For the more covered leathers, pre-base coats containing micro- spheres are possible.1 Often some pigment,12 resins or other additives are added. The deeper the defects the more filling must be achieved with this pre-coat. The viscosity should be about 40,000 mPas. For lighter defects the pre-base could be more liquid, but in both cases the application needs to be done by reverse rollercoating. The adjustment and the choice of the roller screen are very important to achieve sufficient results. The roller should apply the pre-base and also remove it again immediately from the non-damaged areas. This ensures an easy buffing action.
b) Buffing and Base coat: Buffing should not be too deep. Thereafter, the base coat is added by spraying or by reverse rollercoating as normal for covered articles. To further improve the upgrading the base coat should be as dull as possible. Approximately 80% of upgrading and covering are achieved by the first base coat application.
The formulation for this base coat contains resins, pigments and additives. More common is the use of suitable compact binders.3,4 In case of high physical demands, a crosslinker13 may be added. Since the first base coat application gets a melting process to seal and smoothen the surface, the crosslinker is used in the second coat only.
A tremendous increase in filling can be achieved by adding some thermo reactive fillers14 into the first base coat application. To gain the maximum upgrading effect plating/sand blasting should be at 130°C or higher.

Lightly covered leathers
The more natural, less covered leathers don’t get a high viscous pre-base coat. More common is the more liquid pre-base coat with microspheres-containing products.15 Buffing is done as for the more covered leathers above.
These leathers get a careful sealing first instead of a heavily covering base coat. This sealing coat contains aqueous oil emulsions, selected resins and additives. The intention is to re-close the surface after buffing and creating an artificial grain. Therefore, this pre-base coat has to be treated by any melting process, commonly by hair cell embossing. For smooth leathers plating is recommended.
After this initial step a further base coating is necessary. Since the leather has a new (but artificial) grain, the final base coating can be done as for full grain leathers.

Corrected grain shoe upper
– Corrected grain leathers, already buffed
These leathers can also have heavily or lightly covered articles made out them.

Heavily covered Leathers
a) Pre-base: The most effective upgrading can be achieved by a pre-base coat with microspheres-containing products, applied by reverse rollercoater.1 The viscosity should be at the higher end, approx. 30-40,000 mPas at an application of 20-25 g/m2. The adjustment of the reverse rollercoater follows the same rules as for the full grain leathers with the deep defects. Some resins can be added to achieve a better adhesion and a little impregnation effect2 too. Also beneficial is the addition of some pigment. This coat does not have to be buffed. Depending to the final article and the used pre-base mix plating or embossing could be sufficient.16 If re-buffing is necessary it should be done very lightly and the addition of resins into the pre-base would probably have a negative influence to the buffing behaviour.
b) Base coat: Again, a subsequent base coat should cover the leather and lead to a final upgrading. The formulation is similar to those already described.

Lightly Covered Leathers
Away from the heavily finished articles is a method for lightly covered, almost semi-aniline leathers.
The first coat is related to the above mentioned one for lightly covered leathers. The mix also contains oil-emulsions,7 resins18,19 and additives, but the concept is different. This mix should penetrate and seal the leather.

Soft and elegant upgrading with a soft pre-coat
A fine balanced mix of resins and additives needs to be used to ensure minimum hardness is introduced to the leather. Besides this penetration, some of the components should remain on the surface to seal it by the subsequent sand blasting or embossing. The following effects should result:
– the surface is sealed, an artificial grain is created;
– the leather does not get harder, does not loose its natural feel;
– no looseness or double-grain effect;
– print retention is improved;
– good adhesion properties
The subsequent base coat can be done as for full grain leathers.

Full Grain Shoe Upper Leathers
For upgrading full grain leathers special products and systems have been developed based on the experience made on the corrected leathers.
Upgrading corrected grain leathers by using microspheres in the pre-base is a very effective state-of-the-art technology. The use of microspheres in the pre-base coat for full grain leathers seemed to be no way to go, but today it’s found to be a really helpful upgrading tool. Those pre-base coats are applied either by spraying or reverse rollercoater.16 Unlike corrected grain leathers these pre-base coats do not get a subsequent buffing. Polishing is more suitable, or alternatively plating could be done.
For full grain leathers this kind of application is used as a very strong covering and polishing coat. The idea is to apply only a little surplus on the leather surface that will be removed from the undamaged parts and pushed into the smaller remaining defects such as pin holes. It’s obvious that a very well balanced mix is necessary to:
– cover the defects;
– adhere to the leather;
– be easily polishable and to;
– ensure good adhesion to the subsequent finishing layer

After the application of such a pre-base coat the filling components will remain on the leather surface, so this layer is touch-sensitive. Polishing will remove the filling components from the surface and will simultaneously move the components into the small defects. If no polishing follows this kind of pre-base makes the handling of these leathers difficult. In order to avoid scratches or finger prints some resin17 should be added into the pre-base coat mix.
End of Part 1. Part 2 will be published in the November / December edition of Leather International. For more information contact your local Clariant representative.

Appendix

Product names
01 Melio Aquabase RN-03, Melio Aquabase RN-04
02 Melio Resin A-719, Melio Resin A-776
03 Melio Promul C-81
04 Melio Resin A-821, Melio Resin A-822, Melio Resin A-824
05 Melio Filler D, Melio Mattpaste EC
06 Melio Wax 180, Melio Wax 178
07 Melio Oil PW-110, Melio Ground BG
08 Relcawaxtop B
09 Aqualen Top LN.A
10 Melio EW-Series
11 Aqualen Top-Series
12 Neosan 2000, PPE Range
13 Aqualen IW-80.A, Aqualen AKU, Aqualen EPC
14 Melio Filler TF-50
15 Melio Prograde Series
16 Melio Aquabase FPR-01/FPS-01
17 Melio Resin A-Range
18 Melio Promul K-27.A, Melio Top K-201.A
19 Melio Ground UP