Application of surfactants in leather nitrocellulose brightener

1 Neutralization of surfactant in nitrocellulose brightener

Nitrocellulose brighteners occupy an important and even dominant position in the top coating of leather, although there are many finishing materials such as proteins, polyacrylates, and polyurethanes competing with them. Surfactants play a decisive role in the development of nitrocellulose brighteners. Nitrocellulose brightener has been used in solvent form for top coating since its appearance in 1920, but after the 1970s, many countries have formulated environmental regulations to control the use of organic solvents, so as to avoid harming health, polluting the environment and endangering safety, and it is imperative to change the solvent type to water and milk type. Relying on the solubilization, emulsification, wetting and penetration of surfactant, the continuous medium of nitrocellulean brightener was changed from solvent to water, and the water-emulsion type brightener was developed, which greatly reduced its dependence on solvent and basically met the requirements of environmental protection laws and regulations. The surfactant is also used as a leveling aid to reduce the surface tension of the nitrocellulose brightener, so that the coating has a smooth and high bright feel and effect.

2 Main types of nitrocellulose brighteners

Nitrocotton brightener can be divided into solvent type, water-emulsion type and oil-water amphoteric type.

2.1 Solvent-based

Solvent-based brightener is a plasticizer or (and) resin modified nitrocellulose, with solvent, diluent, additives and prepared. The additives include matting agents to adjust gloss, additives to improve leveling and feel, mainly silicon surfactants such as polyether modified silicone, so that the bright coating can obtain smooth feel and high gloss effect. At present, the application range of solvent-based brighteners is getting smaller and smaller, and it is only necessary in the finishing of some effect leather.

2.2 Water and milk type

The successful development of water-emulsion brightener makes the nitrocelluene brightener widely used. It is a solvent emulsion obtained by emulsifying solvent-based brightener (oil phase) with water as medium and containing solvent. During the emulsification process, the oil phase is dispersed into emulsion by means of the mechanical or physical energy provided by emulsifying equipment such as high pressure homogenizer, high shear mixing emulsifier, ultrasonic dispersing machine, etc., and the surfactant is mainly emulsifier to reduce the surface tension of the oil phase and increase the thermodynamic stability of the emulsion. The solid content of the water-emulsion brightener is 10-20%, the pH value is 4-6, the average particle size of the emulsion is 05-2.0 microns, and the storage period of the emulsion is about half a year. How to compound emulsifier to improve emulsion stability has been a hot topic in the research of nitrocellulose brightener.

2.3 Oil water amphoteric type

Also called adjustable type. Its appearance is solvent-based, solid content of 25-30%, in use, can be diluted directly with solvent into solvent-based brightener, can also be added to water, so that the reverse emulsification, become water-emulsion brightener. Due to the current use, it has passed the problem of poor storage stability of water-emulsion brightener, at the same time, in the preparation, there is no need for emulsifying equipment, only manual mixing, saving energy, saving equipment, and easy to use, so it has become the mainstream of leather light high agent.

Theoretically speaking, emulsification is not miscible two phases, one phase with particles dispersed in the other phase to form the emulsion process, this process needs to do work, this work (W) is equal to the dispersed phase surface area increase value (ΔA) multiplied by the surface tension, that is, W=ΔA×γ, this process also adds surfactants to reduce the surface tension, increase the stability of the emulsion. If this work is the mechanical energy or physical energy applied by the emulsifying equipment, the emulsification process is the formation process of the water-emulsion brightener. If the surfactant is added, the surface tension of the dispersed phase is greatly reduced, which is the emulsification process of oil-water amphotic brightener. This process requires a balance between emulsification and wetting of the surfactant. In general, resin modified nitrocellulose brightener, to enhance the wear resistance of the coating, emulsification must be with the emulsification equipment, into the form of water emulsion, plasticizing type or low molecular weight resin modified brightener, can be made into the form of oil water amphoteric.

Increasing the amount of surfactant can simplify the emulsification process and improve the stability of the emulsion, but the amount of surfactant is subject to the finishing effect of the brightener, such as brightness, dry and wet rub resistance, folding fastness and other properties, and is characterized by wet rub resistance. Dress is the difficulty of emulsifying nitrocellulose brightener. Leather is hydrophilic, pH 4-6, if the conditions are right, the bright coating can wet the following coating and leather, and penetrate into it to increase the bright coating adhesion, to reduce the bright layer surfactant content. Through the clever combination of emulsifier, wetting agent and penetrant, we can find the balance and unity of the contradiction between emulsification and finishing properties. This is also the secret of oil-water amphoteric brightener which is convenient for emulsification and excellent performance.

3 Surfactants for nitrocellufos brightener and application examples

In order to effectively wet the leather and penetrate the leather, the pH value of the nitrocellulose brightener is usually adjusted to 4-6 close to the leather, so the surfactant is required to be stable in the weak acid medium, the nitrocellulose brightener is often not used alone, but mixed with a variety of additives to improve the coating, these additives are mainly non-ionic and actinic. The surfactant of the nitrocellulocene brightener should be mainly non-ionic, supplemented by a small amount of anionic type.以上翻译结果来自有道神经网络翻译（YNMT）· 通用场景