About Surface Filling of Leather

Bao Guanghua

Author Profile: Bao Guanghua, Senior Economist, Chairman of Wenzhou Fennis Leather Chemical Co., Ltd., Chairman of Shanghai Aio Fine Chemical Co., Ltd., Chairman of Wenzhou Fennichi Leather Decoration Technology Development Co., Ltd., Chairman of Rui'an Fennis Hotel Management Co., Ltd., Chairman of Wenzhou Lucheng Binjiang Holiday Hotel, Vice Chairman of the Editorial Board of the Party School of the CPC Central Committee, Researcher of the Academic Committee of the China Academy of Management Science, Deputy Director of the Leather Care Professional Office of the China Commercial Federation's Laundry Professional Committee, Vice President of the Shanghai Leather Technology Association, Vice President of the Wenzhou Industrial and Commercial Direct Bureau Private Association, Vice President and Party Branch Secretary of the Wenzhou Leather Chemical Industry Chamber of Commerce, Vice President of the Wenzhou Shoe Material Merchants Association, Expert in the Chinese Laundry Industry. He has published many papers in national and provincial journals such as "China Leather", "Theoretical Frontiers", and "Zhejiang Senior Economist". He has been awarded honors including the "China Science and Technology Progress Third Prize", "China's Most Influential Wealth Figure", "China Private Entrepreneur Social Contribution Award", and "Outstanding Chinese Entrepreneur".

      
Producing smooth leather, such as edge bead, wiping leather, and patent leather, it is very important to fully fill the shaved leather base for finishing. Napa leather or those dried by dry beating to produce irregular grain patterns do not require filling.
Surface filling of leather generally aims to enhance the leather's aesthetics. First, by eliminating the loose grain surface to obtain a uniform grain surface; secondly, to impart fullness, wet rub resistance, flex resistance, and wash resistance to the leather without affecting its softness.

Under a microscope, fresh hide has a layer of tightly packed epithelial cells. The growth and renewal of animal skin start from the inner layer. Hair passes through hair follicles in the dermis, and sebaceous glands located in the middle of the follicles produce secretions. The epidermis is useless to tanners and is removed along with hair during the liming process. Adjacent to the dermis is a very thin, cell-free basal membrane preserved by tanning. This smooth and compact membrane forms the "grain surface" of the leather, which should be preserved as much as possible in full-grain leather.
The basal membrane is usually tightly woven with the inner layer and is the most important part for tanners, as this part transforms into the "dermis."
For ease of analysis, we focus on the folds of the leather. It is important to consider the different internal structures of the dermis. Elastic fibers and collagen fibers are perpendicular to the leather's cross-section, with nerve endings, blood vessels, hair follicles, sweat glands, and sebaceous glands located in this layer. This part of the dermis accounts for about one-third of the total thickness and is called the "grain layer," "papillary layer," or "secondary dermis." The lower part of the dermis forms the reticular layer, which has very firm fiber weaving with dense grain patterns, accounting for half of the total thickness, called the "primary dermis."
Unwanted subcutaneous tissue, mainly coarse horizontal fibers and many fat cells, is removed during the physical and chemical processes of tanning. Since they have no practical use, the epidermis and subcutaneous tissue are removed during tanning, preserving the natural structure of the rawhide. When bending the grain surface inward by hand, no wrinkles, loose surfaces, or any degree of stretching and compression occur. This perfect and regular structure of the leather is called "grain surface compactness."
When folding leather, it exhibits another excellent property, the fine regularity of the structure. This proves that leather is a uniform material. Tanners strive to preserve this especially valuable property. If the two layers of leather differ significantly, loose surfaces will occur between the "secondary dermis" and "primary dermis." This looseness may also be caused by improper processing. Depending on the initial state of the leather, the looseness may become more or less apparent. The solution depends on the leather's specific condition and initial state.
Therefore, the condition of the grain surface depends on many factors. The smaller the animal, the finer and more elastic the secondary reticular layer and fibers of its skin, resulting in more delicate grain patterns, such as calf leather. However, heavy leather like cowhide, due to different breeding methods under various climatic conditions, usually has larger defects.
The grain surface filling operation is specifically designed to fill the grain surface with appropriate elastic substances to enhance the leather's resistance to stretching and compression when folded, producing delicate grain patterns. It also needs to tightly bind the two internal fiber layers. Besides the smoothness of the leather surface, we must consider the quality of individual animal skins and their different behaviors when folded. If highly discontinuous fibers form different layers, the grain pattern becomes apparent during mechanical pressing due to fibers yielding or resisting pressure differently, causing looseness. In some cases, internal fiber layers can produce true looseness due to misaligned, loose, overly folded, or poorly woven fiber areas, such as the belly area. In surface filling operations, improper use of overly hard polymers or excessive resin penetration into the leather worsens looseness.
Of course, the fibers of the primary dermis are more open, looser, and more elastic when folded, producing no visible defects and reducing looseness.

Diagram of the main fiber direction of leather

Therefore, several factors affect obtaining a smooth and delicate grain pattern. In some cases, the filling system is insufficient to solve all problems under different conditions. Fennis Company has thoroughly studied this issue and launched a series of professional products that are effective in most cases or slightly adjusted according to different leathers.
Among the many causes of looseness, the most common is the weak spot between the grain surface and dermis, where the two layers loosen when folded. Polymers distributed at the interface of the two layers can strengthen their bonding strength, eliminate or significantly reduce the possibility of looseness, and improve the leather's wear resistance.
The filling resin is enriched beneath the grain surface, so it does not hinder the sanding process and can act as a protective layer during mechanical sanding, making the leather surface smoother and more uniform. Because the polymer exists in the gaps between fibers, it enhances the compactness of the leather surface, benefiting subsequent finishing. By covering the leather surface, it allows uniform absorption, making subsequent finishing lighter and faster.
After the filling polymer penetrates the leather surface, its affinity and good elasticity ensure excellent adhesion of the subsequent coating. This undoubtedly improves the grade of the leather. Proper leather surface filling can give the leather excellent mechanical properties and solve the problem of loose surfaces. However, if the filling is too shallow, or the resin content is insufficient, or it is incompatible with the subsequent finishing materials, or unsuitable for the selected finishing system (such as polyurethane, or only butadiene resin instead of acrylic), it may cause poor coverage. Finess's professional filling products, AR 334 and AR 370, can be used in various situations.
The following table is an example of roll coating filling:
Recommended formula:

Mixing steps:
Dilute PE 104 with water, add other resin emulsions while stirring. When adding softening oils like DC 223, mix PE 104 and DC 223 evenly, then add to water while stirring continuously until the resin solution is added.
       
Application steps:
All formulas are suitable for both roll coating and wiping operations. AR 334 is recommended for curtain coating (less foam, stable mechanical properties). Observations: AR 334 is very soft. AR 370 is a harder type suitable for leather surfaces where grain fixing is difficult, generally not used. Leathers requiring filling have special fiber structures; surfaces that are too closed or excessively chrome-re-tanned or with surface oils and excessive fillers are unfavorable or unsuitable for surface filling. Therefore, proper re-tanning must be done to ensure the filling operation.
Of course, proper shaving must also be done, and the shaving dust must be carefully cleaned. To allow the polymer to fully penetrate the entire cross-section of the leather, a special penetrant must be used. The penetrant for this operation is a surfactant, which, according to the principle of reactive activity, binds with the leather fibers after completing its function. Therefore, the waterproofing after filling will not be affected by the surfactant participating between the leather fibers. Sometimes a small amount of defoamer is added to the finishing mixture. It is best to use water-based fillers rather than solvent-based ones. They can improve softness, dry at low temperatures (>65°C) (>149°F), penetrate quickly, and tighten the grain surface. Because AR 334 penetrates quickly, it is widely used. If the effect is not good, use pure AR 370 and special penetrant PE 104 according to the recommended formula and usage method.
However, it is necessary to adjust the process according to the type of leather being processed to ensure the polymer fully binds. The leather should be adjusted according to fullness, softness, and grain tightness. The penetrant specially used for leather surface filling is PE 104, which can also be used in other finishing stages. It can reduce surface strength, improve penetration, or adjust absorption. PE 104 is efficient and reactive (it can complete penetration within 5-8 seconds). In most cases, such as when shoe upper leather is treated with steam in shoe factories, films containing PE 104 do not swell or become cloudy. PE 104 must be diluted with water before being added to the resin or added as a final additive to the finishing mixture. Because it has significant swelling properties, it can increase particle size, which hinders rather than helps penetration.
PE 104 is a swelling agent for the leather industry, so do not exceed the recommended dosage during use. Otherwise, it will excessively swell the fibers, reducing the gaps between fibers and hindering polymer penetration. In finishing pure chrome-tanned or lightly vegetable-tanned leather, using a small amount of alcohol ester (or ethanol) and the recommended amount of penetrant can effectively promote penetration.
However, these polar solvents cause the shoe upper leather to swell when treated with steam. To prevent or remove foam generated during the operation, a small amount of defoamer can be added. If the results are unsatisfactory, the operation can be repeated. It is best to use less silicone-containing additives because they can cause "fish eyes" or other defects. Penetrants that produce excessive foam will affect the amount of polymer deposited.
To ensure the success of the first trial, the polymer deposition amount and penetration intensity are evaluated as follows: for most processes, depositing 8-10% solid polymer is sufficient. Sometimes, to achieve special effects or when the leather is severely crusting, this operation needs to be repeated.
If penetration or repeated filling is done in two stages, it is best to use low solid content emulsions (6-8%), noting that the total polymer amount is the sum of both times. This operation is usually used when the leather has poor absorption performance, while for leather with high absorption performance, even the first trial can use high solid content emulsions (12-15%), which will reduce its absorption rate.
When the leather reacts well to folding, using 6-8% polymer emulsion can produce soft leather. After determining the solid content of the acrylic resin emulsion, 200-300g/㎡ of solution can be roll coated, curtain coated, or wiped. For thicker heavy leather, it can be increased to 400g/㎡. Amounts below 200g/㎡ cannot guarantee sufficient penetration.
Generally, rather than reducing the amount of solution used, it is better to use solutions with lower solid content. The penetration effect can be observed and tested by checking the longitudinal cross-section of the leather. The polymer must pass through the grain layer to reach the dermis. The best finishing techniques are those that ensure stable solution quality. Therefore, spraying should be avoided as much as possible to prevent water atomization during compound composition adjustment. Except for the requirement to coat 200g/㎡, all difficulties are concentrated on the flank areas on both sides of the backbone line.
The next step is to hang or stack grain face to grain face overnight. To allow the polymer to distribute evenly through capillary and penetration effects, at least several hours are needed. Before drying, the initial bonding of the polymer with the leather has been completed. At this time, the possibility of adjusting the softness of the leather is small. Vacuum drying is usually performed, which can make the leather surface smooth and uniform, flatten the leather, and prepare it for shaving.
The abrasive paper used is 320-600#. Using abrasive paper that is too fine will reduce the absorption of the grain layer, preventing the leather surface from being wetted and sealed, which causes difficulties for subsequent finishing. However, if the paper is too coarse, it will cause marks or cuts on the leather surface. Although these defects can be covered during finishing, they can reappear during the stretching and lasting of the upper leather, and at that time they are difficult to cover up.
After the above operations are completed, the leather generally undergoes softening by vibration. Another method is to move the leather fibers through wetting and swelling, then fix them by drying. The final mechanical operation restores the leather's original softness.

(Author's affiliation Wenzhou Finnis Leather Chemical Co., Ltd.)