Upgrading of shoe upper leather in finishing (part 2)

The first part of this paper reviewed upgrading ideas and techniques for several shoe upper leathers (see Leather International, October 2009, page 34). The second part will provide more details on upgrading.

Pre-Base Coats
Until now finishers have focused on the different ways of using pre-base coats with micro spheres as filling agents, but there are still lots of possibilities for alternative upgrading techniques.
In general the most influence is achieved with the first coat. In many cases this is any kind of pre-base coat, either for sealing corrected grain leathers, or for full grain leathers, where it is most often a polishing coat.

Polishing
Polishing is used in order to close the surface and to equalize the absorption of the crust leather. Some crusts are very sensitive to polishing and tend to become loose. The application of an oil or an oil-emulsion4 helps here in two ways:
– the oil keeps the grain softer
– due to the penetration of the oil the shade of the crust intensifies
This characteristic is very helpful for the finishing of dark or even black articles. However, pre-trials are recommended to ensure that the oil does not highlight any defects.

Oil-coats
If the absorption of the crust is very uneven, an oil-coat helps to level out the penetration of the following applications. Where even this is not enough some hydrophobing additives13 can be added into the pre-base coat mix, but don’t make the pre-coat too water repellent as insufficient adhesion or flow problems within the next coat could occur.

Cationic pre-bases
Lots of pre-base coats are based on cationic products.14 Their advantage on cationic crusts is that with only a thin layer a very good sealing is achieved. Since the cationic products do not highlight open defects of the crust as much, a very equal surface is achieved while still keeping the natural aspect of the leather. A few years ago it was not easy to achieve sufficient adhesion with those pre-bases, but today these problems have been largely solved.
However, more and more leathers are now chrome-free leathers, and the advantages of cationic pre-base coats for upgrading are not so obvious. Secondly, today’s anionic pre-base coat products16 are at least as good at covering and sealing as the cationic ones. And those products that are slightly cationic15 are mixable with anionic products and so overall the mix often is not cationic at all.
The main disadvantage of pure cationic products is that in production either one spraying line has to be kept for cationic applications or spray finishing machines have to be cleaned very, very carefully.

Sealing pre-coats
Sealing pre-coats for corrected grain leathers follow another philosophy. Here the damaged or probably snuffed grain is restored using soft resins, which remain on the surface, and additives ensure both sealing and rewet ability.15 A solid film should not be created on the leather! This would lead to a double-skinny finish.
After plating the base coats should neither build up too strongly nor penetrate into the leather too much. This means that the adjustment of the pre-coat for corrected grain shoe upper leathers is very important and also very much dependant on the crust leather.

Base Coats
For leathers not heavily base coated some details have to be given.

Composition
Generally speaking these base coats should be thin coats only. Therefore, the viscosity should be high and the solid content of the mix low. This leads to a reduced penetration on the one side and a thin layer on the other. The choice of binders and additives are important and, in order to achieve a fast covering, filling additives have to be used. Since the final articles normally have at least a medium gloss, too high amounts of silica-based fillers can’t be used. Some silica fillers2 or dullers have to be used since the lower the gloss level in the base, the better the upgrading. About 50% of the filling effect can be achieved by silica based products; the remaining 50% must be achieved by using waxes3 and/or oil emulsions.4 Microspheres containing filling agents1,9,10 can be used as well, but their usage is limited due to their strong dulling effect.

Thermo-active fillers
Another option to increase the filling and upgrading capacity of the base coat is the use of un-expanded microspheres.8 These products will expand during hot plating and so they improve the filling. But in thin coats the microspheres can’t be used to their maximum effectiveness.

Finishing Effects
Aniline look
In order to achieve high aesthetics a natural aniline look is of great importance. After the leather is base coated and ready for the final top coat or even before the last base coat application, the time is right for an effect coat. It can be a base coat mixed with some aniline dye, a separate coat based on aqueous emulsion lacquers6 or aqueous top coats7 with an aniline dye.17 This coat will intensify the shade and also improve the transparent look of the finish. In fact, the leather is covered but the aniline dye makes the leather look more natural.
Effects are limitless! This effect coat can be applied unevenly to make the surface equally uneven, which can improve the upgrading, because while grain defects are obvious on an even surface, they can be hidden by applying clouds or by a design roller for example. An effect on leather can be done from the crust up to the top coat, and applying two-tone-effects can be done on the crust leather as well as on the base coated leathers.

Waxes and Oils
On crust leathers effects such as burnishing,18 pull-ups,19 crunch or antique effects are possible. All of them have in common the creation of a uniform unevenness. Grain irregularities are not covered by these effects but very effectively hidden.
For all the wax and oil related effects on crust leathers, a very even uptake is important. Depending on the crust and its tanning/re-tanning, different products may show different behaviour. Pre-trials are necessary to find out the most suitable products.

Pull-up leathers
In order to adjust the shade of pull-up leathers, tinting is common before applying the pull-up oil/wax. For the oil pull-up leathers with their typical soft pull-up effect, a high upgrading is hard to achieve. Most important is to avoid uneven penetration of the oil and to avoid highlighting defects, such as open scratches, tick marks or pin holes. It is helpful to apply a barrier based on an oil emulsion7 before the pull-up oil or to apply a strong penetrating oil from the flesh side beforehand. Never the less it is of major importance to find out the right oil suitable for the crust and the demanded effect. If a strong effect is demanded, a strong darkening oil is necessary and ensure the defects are not highlighted significantly. For the lighter pull-ups the idea of the barrier is very helpful.
Aqueous wax emulsions20 are a highly effective barrier. This procedure is important for wax pull-ups, where a sharp and pronounced effect is demanded.
Very often the water-based wax emulsion is applied at first followed by a pull-up wax. Depending on the final article this wax coat is either plated, polished or followed with an adhesion coat.21 The adhesion coat is needed for waxy leathers that have to be finished, in order to guarantee sufficient adhesion for the subsequent finishing coat. A lot of these leathers are made with a gloss or even high gloss top coat. The finishing coat should not be too thick as that would decrease the pull-up effect and its re-polishability, nor should the plating be too hot.
Even for the high glossy leathers plating too hot is not advisable, as the pull-up products could migrate into the top coat, which could lead to a greyish tinge undermining the gloss.
Overall, waxy crunch effects or antique pull-up effects, have a big upgrading capacity. The leathers usually get a final milling until the desired effect is reached. Sometimes this can be only five drum rotations!

Burnishing effect
Also based on special wax-based products18 are the so called burnish or dark polish effects. Some articles are delivered without the final polishing so that after manufacturing the shoe is given a hand polish to pronounce the shape of the shoe or just to upgrade it with a fashionable look.
Some leathers get their final polishing before the shoes have been made. This polishing does really upgrade the leather! A very attractive optical effect together with an intensive shade and a natural gloss will be achieved by this finishing type.
For more upgrading and in order to match the colour some pigments or dyestuffs can be added to the burnish wax coat. But this will not increase the wet rubs!

Pull-up Imitation
To upgrade a heavily covered shoe upper leather, it doesn’t matter whether the leather is full, corrected grain or a finished split, an artificial pull-up not only gives a higher sales appeal to the leather due to a fashionable look, it also imparts an appearance of naturalness that the fully covered leather normally cannot generate.
The principle is as follows: the leather receives a base coat as normal and than a special additive mixture,22 tinted with aniline dyes. The important point to consider is the shade of both the base and the pull-up coat: in ideal cases the base coat is black, or at least very dark, while the additive mixture dries colourless. When the leather is pulled this coat turns white. This effect can be tinted or matched with the aniline dyes. The nicest effects are obtained when this shade matches with the shade of the crust. Since the aniline dyed pull-up coat does not cover the black or dark base coat beneath it is invisible until the leather is pulled. This pull-up effect is easily re-polishable as long as the polymeric top coat is not too thick. The thicker the top coat the weaker the re-polishability.

Antique effects
Antique or distressed effects can be made on both full grain and corrected grain leathers and also on splits. For splits a short-fibred crust is easier to finish and the effect will also be more elegant.
The upgrading potential with such an effect is pretty high. Due to the regular uneven­-
ness of the two-tone effect most of the grain defects can be hidden on the full grain leathers and both the corrected grain leathers and the splits get a high fashionable appearance. Different options are possible:
• a thin and hard dark pigmented coat can be applied and partly removed either by milling or any other mechanical action5, 23
• a hard, dark pigmented, resin is applied to the leather, which creates a cracking effect during drying or when milling23
• a light base coat is applied and afterwards a darker very lightly sprayed, non-stretchy coat is applied. The darker coat will crack when the leather is stretched and the lighter shade appears.
After the desired effect is achieved it has to be fixed and protected by a clear or slightly coloured binder coat. Any top coat can be used in order to finish the article.
Very closely related to these antique effects are the high glossy crystal effects, mostly used on splits and corrected grain leathers. The important point here is the very high gloss of the remaining particles on the leather and these particles should be very small. The result is a suede or nubuck leather with a glossy shimmering just like a crystal.
To achieve this effect the coat must not penetrate too much, must have enough adhesion to ensure the effect does not disappear over time. Therefore, it is helpful to apply a hydrophobing coat first13 and to balance the hard resin23 (for the effect) with the softer ones11,12 (for the adhesion). The application is done by spraying, better one or two light coats than one heavy coat. If the crystal effect is sprayed in one thick layer it’s hard to get a suitable effect afterwards.
To increase the gloss and to smooth the surface this crystal coat needs a hot plating. Careful milling will reveal the high glossy coat, but it can feel rough, so a feel coat24 has to be sprayed followed by a light and hot final plating.
An overview of the various processes and procedures is shown in table 1, and for more information contact your local Clariant representative.