Upgrading of leather crusts that have been damaged by hide defects such as tick bites, scars and other imperfections remains a high priority in leather finishing because it enables manufacturers to improve the overall quality of their finished leather.
To address this concern, various upgrading methods have been developed to cover, hide, fill or otherwise reduce the appearance of these defects in the fully processed leather. At the broadest level, upgrading can be defined as any technology that improves the visual and/or performance properties of leather crust. Historically, of course, this has been represented by traditional finishing techniques – be it aniline, semi-aniline, or pigmented multi-coat polymeric coating systems.
In general, these traditional finishes apply a wear and chemical resistant surface layer to the crust and, visually, these systems also have the ability to hide small defects. However, more recent technologies that can be considered ‘surgical pre-bases’ have narrowed the definition of upgrading to mean light polymeric coatings applied to the crust with the specific intention of filling medium-sized defects.
TFL Topcare system leads to more saleable leather
The emphasis on medium-sized defects is significant because the likeliness that a leather crust will benefit from upgrading is largely determined by the size of the defects. Defect sizes may be classified as small, medium or large according to criteria mainly associated with defect depth. Small depth defects consist of normal hair follicle holes and also irregularities in dyes/colour. These tend to be uniformly distributed defects on an essentially flat, well-structured grain surface and these small defects can be covered with standard pigmented base/top coating systems.
Medium depth defects include large hair follicles, tick and insect bites, scars and brands. They are characterised by the fact that the upper part of the structural grain collagen fibre is missing but the underlying looser grain and/or corium is still present. These medium-sized defects are suitable for upgrading techniques.
Lastly, large depth defects include deep puncture wounds or through-thickness cutting or perforations in which the collagen fibres of the grain and corium are completely missing. Large defects generally cannot be repaired with coatings.
* Small-sized defects like stains, small bacteria damage = traditional finishing
* Medium-sized defects like tick and insect bites, small open scars, medium sized bacterial damage, open and scuffed grain = surgical pre-bases
The TFL Topcare system not only treats small to medium sized defects but also covers the entire area with a thin homogenous coat.
This results in a uniform absorption of the following finishing coats with less ‘add on’ and a more even grain break across the hide.
Surgical Pre-bases represent more recent finishing technologies that provide methods of covering crust defects and they are applied by reverse roll-coating (RRC). At the heart of this new finish technology from the chemical side are products based on micro-sphere technology which are suitable for both full grain and buffed leather applications.
Roll coating machines that can apply the finish coat in reverse mode on very soft leathers have changed things, enabling the application of finishing materials based on micro-sphere technology and high performance polymer resins. Advanced helicoidal roller design has complemented the technology, allowing the finishing of large volumes of leather with small to medium-sized defects.
At the heart of this new finish technology from the chemical side are polymer products suited for reverse mode finishing on both full grain or buffed leather.
The combination of finishing chemistry with specific machine settings is the key to successful application. In particular:
* A high speed application roller (up to 60 RPM), has a ‘cleaning’ effect on the surface. Defects and cavities will be filled selectively, leaving only a fine film on the undamaged part of the surface.
* A steep feeding belt angle increases the cleaning effect of the grain
* A tight gap, in combination with the soft conveyor belt, allows selective filling effect
* The conveyor belt speed (8-10 m/min) facilitates selective filling at high throughputs
When compared with hand stucco application, this technique provides an almost continuous thin film on the surface. The differences between these pre-bottom coatings and hand stucco application can be assessed by means of Impulse Thermography images.
This uniformity is also shown in Panels 3 and 4, where surface tension figures and dynamic contact angles were measured across the surface of a coated hide. The pre-bottom coat applied by reverse roller coat shows a more homogenous surface tension pattern. The low figures indicate the surface uniformity with no excessive spill-over outside of the defects as found with hand stucco application. A much more selective filling of small and medium-sized defects is achieved.
The TFL TOPCARE SYSTEM is a pre-base coat especially designed for RRC applications and it shows a very homogenous surface tension pattern. Because of this uniformity of absorbency, the following base coat application can be reduced by up to 25%.
More natural finishes can be achieved, resulting in a finer break for both full and corrected grain articles, hence an increase in cutting value.
Modern technologies allow tanners to better cope with deteriorating leather quality and can help to add value to the final leather article. No single technology will do the job, but attention to a combination of several criteria is essential. The goal is to add maximum value at competitive costs, thus keeping unique characteristics and performance, and prices at viable levels that provide a reasonable profit to the tanner.
The mechanism of the crust upgrading with the Topcare System is the filling of defects plus homogenisation of surface chemical and physical properties. In this regard the TFL finishing technology provides a unique benefit by priming the entire surface for optimal efficiency and uniformity in the subsequent basecoat and topcoat applications.