Roksol ST7 is an anionic lubricating and hydrophobizing agent used for lubricating of chrome and combination tanned leather i.e. chrome tanned leather filled and retanned with synthetic, vegetable, resin or aldehyde tanning agents.
The product is a mixture of compounds containing sulfosuccinate groups, aliphatic hydrocarbons and salts of fatty acids. It is a viscous liquid with a beige colour and a freezing point of -10°C, which means that it does not require special storage and transport conditions. It forms milky emulsions with water.
Roksol ST7 is miscible with all anionic and non-ionic lubricants. It is soluble in alcohols, hydrocarbons and tetrachloroethylene. The product is resistant to alkalis and salts of monovalent metals, e.g. NaHCO3, NaCl, Na2S04, as well as to hard water within the hardness limit of ordinary industrial waters. The commercial product is an emulsion with 40-50% active substance content.
The product has limited miscibility with cationic fats and in each case requires preliminary verification. The resistance of Roksol ST7 emulsions to dye and anionic tannin solutions depends on the pH value, exhibiting sufficient resistance to compositions above pH 5. Strong cationic products (aluminium and zirconium salts, cationic resins, calcium and magnesium salts and cationic tanning liquor) cause the precipitation of hydrophobic sediments from Roksol ST7 emulsions. Anionic chromium tannins, such as cold-solubilized, sulphated chromium extracts or masked chromium tanning liquids, have a slightly weaker precipitating effect.
The ability to precipitate hydrophobic deposits by strong cationic agents is used to increase the lubricating and hydrophobic effect, however, in practice it is mainly the salts of chromium and aluminium and sulphates or alkaline aluminium chloride that are used for this purpose. For emulsification, it is recommended to use water at a temperature of 40-60°C.
Roksol ST7 is a preparation with high resistance to sunlight and moderate equalising effect, which does not cause excessive brightening of the colour of leather dyed with anionic dyes. An important parameter for the correct course of the lubricating and hydrophobizing process using Roksol ST7 is the appropriate leather neutralization – at least green or preferably blue-green cross-section of leather is required (when tested with bromocresol green). Bivalent cations such as calcium formate should not be used for leather neutralization.
The process of hydrophobic lubrication is always carried out after the filling (re-tanning) and dyeing processes, and in case of dyeing it is possible to add Roksol ST7 to the dyeing bath that had been exhausted of the dye. The use of emulsifiers or strong hydrophilic lubricants in wet finishing processes reduces the hydrophobic effect of Roksol ST7. Chromium or aluminium salts are required for the full hydrophobic effect and its stabilisation. The use of formic acid alone enhances the greasing (but only moderately hydrophobic) qualities of the product. Where hydrophobisation problems do not play a major role, simple fixation with formic acid is fully sufficient.
Cationic fixatives are used on the skived leather in the amount of 2-3% depending on the amount of Roksol ST7 used. Leather poorly filled with anionic tannins can be fixed with smaller amounts of fixative additives. An appropriate amount of fixative is added to the exhausted lubricating bath and homogenized for 30-60 minutes. After this time, the complete exhaustion of the lubricating emulsion is usually achieved. Fixing and rinsing are the last wet finishing processes.
Leather greased with Roksol ST7 in combination with cationic fixation process using aluminium or chromium salts is characterized by low static and dynamic absorption as well as low wettability from the grain and flesh layer. High softness and bulkiness as well as preservation of natural grain pattern with good resistance to weather conditions enable the use of such leathers in processing without the need of coating for production of sports, tourist, work, winter, military footwear. Dry finishing of this type of leather with covering pastes is not difficult. However, it requires the use of appropriate technology and binding agents, suitable for leathers with very low wettability of the grain.