A Property Of Fluid That Shows Resistance To Flow The Chemistry of Leather Care

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The Chemistry of Leather Care

Leather can be thought of as a non-woven mass of fibrous collagen protein that is derived from animal skin and has been made biorefractive. Animal skin is made biorefractive and thereby transformed into leather through the tanning process. The tanning process involves permanent incorporation of biocide into the collagen protein. Other desired characteristics are imparted to the leather by fatliquing, coloring, dressing, waterproofing, shaping and working.

Faticoloring is the process of incorporating fat, grease, and oil into the body of leather. The presence of fats, greases and oils in leather lubricates the fibers and makes the leather waterproof. Without adequate internal lubrication, friction will damage and break the fibers and can cause the leather to crack.

Leather is given color by dyeing or pigmenting. Depending on the tools used, the color can be incorporated throughout the thickness of the leather, or it can be concentrated on and near the surface.

Dressing is the surface finish that is given to the material. The surface finish is a continuous organic matrix separated from the protein fibers. That organic matrix can range from latex paint to wax. Dressing always refers to the topmost layer of leather organic material. In this way it is possible to “dress up” for dressing as it is possible to apply a wax polish to the surface of the painted leather. Prior to wax polishing, the leather is “dressed” with an organic matrix that forms the outer layer of paint.

Leather working is the repeated bending of leather to reduce stiffness. This is a purely mechanical process.

How to Consider Chemicals for Leather Care

All chemical treatments of leather fall into one or more of these categories: alternative fatliquors, surface dressings, waterproofers, and cleaners.

How Cleaners Work

Cleaners are chemicals or chemical preparations intended to remove foreign matter from leather. The trick for cleaners is that they must be able to remove foreign matter without permanently altering the appearance of the leather being cleaned. Usually, the material to be removed from the leather is solid or semi-solid, and the cleaning chemical is almost always liquid. Cleaning is accomplished by either dissolving the solids or removing the material from contact with the leather by a preferential wetting process. Preferential wetting means that the leather prefers to be in contact with the cleaning liquid over the solid contaminant.

In either case, a cloth or sponge is almost always required to remove the dissolved solids or lifted contaminants from the leather. Foreign contaminants are transferred from the leather to the cleaning cloth.

The cleaner preparation also has to be removed from the leather to fully restore the leather to its former clean state. Cleaners that are not removed by themselves may become foreign contaminants. The cleaner is removed either by rinsing, by transferring it to a cloth or sponge, by evaporation, or by a combination of all three processes.

Rinsing is the process in which excess water is used to dissolve and wash away the leather cleaner. Effectively, rinse water substitutes for chemically clean leather. Water, weakly bound in the fiber mass, then evaporates on its own. This process of removing the cleaner is effective if the leather itself is not adversely affected by wetting.

If the leather to be cleaned is dressed, i.e. a continuous matrix of organic matter above the fibers, other dressing agents can be used to clean the leather product. In reality, it is the surface dressing that needs to be cleaned, not the deeper mass of protein fibers. Water and other leather dressing agents are useful for removing contaminants from dressed leather. Decomposition and preferential wetting processes also occur here, as well as the transfer of solid contaminants from one body to another. Thus, cleaning, waxing and polishing can be done as part of a single process.

Silicones and leather

Silicone is a type of synthetic oil and the term silicone refers to a homogeneous series of organic chemicals based on alternating silicon and oxygen atoms. Organic side chains, often methyl groups, are attached to silicon atoms. The chains are terminated with methyl groups, making them non-reactive. The chains can be terminated with hydroxyl groups or hydrogen, making them reactive.

Silicone oil is a lubricant and acts as an alternative fatlicker when it is able to penetrate the mass of fibers. Silicone is water-resistant, as well as glossy and provides a pleasant “hand” or leather feel. Because silicone soaks into leather fibers so well, a moderate amount of silicone can substitute for fatliquor without hindering the leather’s “breathability”. “Breathing” means that water vapor can penetrate the mass of fibers. Leather can also be “stuffed” with grease or wax that prevents water vapor from escaping through the mass of fibers. The use of silicone enables the leather to be somewhat water repellent without sacrificing its “breathability” quality. Filling leather with grease makes it waterproof, but makes the leather unable to “breathe”.

Silicone is usually an important ingredient in leather polishes, especially those that require buffing to bring out the shine. Silicone helps smooth the wax crystals into a continuous glossy matrix and also contributes to gloss and water repellency.

Silicone is a very useful product for leather care and cleaning. It is used in fatliquors, cleaners and dressings. A drawback to the use of silicone is that silicone has a very low surface tension and may impair the performance of other, water-based leather treatment chemicals because the silicone prevents other products from wetting the leather.

NEATSFOOT OIL

Neatsfoot oil is produced by rendering the feet and shinbones of cattle. It is usually yellow in color and has a low melting point. It is used to keep the leather supple and flexible, meaning it acts as a lubricant for the protein fibers. As such, it acts as an alternative fatlicker.

Clothes and colors

A dressing is a covering that is outside the mass of protein fibers and still attached to it. Typically, dressings consist of a continuous organic matrix that may contain other types of material such as pigments. Because the coating is external, it is possible to dress a leather item by polishing it with wax or by making it water repellent with a silicone-based product. Dressing is applied to the fibrous mass to provide a decorative and often protective coating, and most leather sold commercially is dressed during the manufacturing process. Dressing to dressing is sometimes called top dressing.

The purpose of aftermarket leather dressings is to restore or enhance the dressing applied to the leather during the manufacturing process. Waxes, for example, often contain a dye to match the color of the leather being dressed, and the act of polishing the leather with colored wax applies the dye, a waterproofing agent to the dressing, and adds a pleasant sheen to the leather.

Colorants are dyes and pigments that color leather and are different from dressings. Colorants can be an ingredient in a dressing product. Some leathers, during the manufacturing stage, are treated with coloring solutions that drive the coloring agents deep into the fiber mass so that the color of the leather remains even if it deteriorates. Colorants are added to aftermarket dressings to give a renewed look to leather that has lost some of its color due to wear and weathering.

Waterproofing

Many aftermarket chemical preparations are made to render leather waterproof or water repellent. There is a slight difference in meaning between the two words. Being waterproof means water just won’t seep through the leather. Leather is conditioned by applying wax and/or grease. Leather in this state is often hot to wear, as the leather cannot breathe. It is impermeable because the gaps between the fibers are filled with a large amount of waterproofing agent and water vapor is trapped close to the body. But in driving rainstorms, waterproof leather can be an essential feature.

Being water repellent means that water does not wet the leather, but because the leather is permeable, water can be forced through the leather under pressure. Water repellent leather can breathe because water vapor can escape through the fibers even though liquid water cannot wet them. Water repellency of leather is usually achieved by treatment with silicone. Silicone remains liquid and wets the fibers, forming a film on them that water cannot wet. The film is so thin that the gaps between the fibers are not filled.

Of the two terms, water proof is closer to absolute than water repellent.

A degree of water proofing or water repellency can be obtained from products that apply a surface finish to leather. It is the surface finish that resists liquid water penetration, not the mass of fibers.

Professional products

Leather care has four functions: cleaning, replacing fatliquors, surface dressing and waterproofing. Commercial products for aftermarket leather care offer one, some, or all of these functions.

Waxes, fats, greases and fatty oils all naturally repel water; And most of them are useful lubricants for leather fibers. Thus both mink oil and “dubin” products act as waterproofing agents as well as alternative fatliquor, each better at one function than the other.

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