The most fundamental question to be answered before you clean or care for leather is to establish the type of leather finish used in the vehicle as the methodologies are very specific for each type
Always remember you are dealing with the finished coating on the leather and not with the leather hide itself
Automotive leather upholstery is a multi-strata urethane coating; consisting of the actual hide, colour pigmentation and the surface finish. An acrylic and polyurethane resin binder system is used to improve flexibility, fastness and adhesion to the leather, then two or three water-based pigmented base coat applications a clear water-based top coat is then applied as the final stage of the finishing process. Urethane has micro-pores that allow evaporation and hydration (the passage of water vapour) they are not sealed per se as some liquids penetrate it easily; others stay on top dependent upon their molecule size.
The urethane (Ethyl carbamate) used for protecting automobile upholstery is classified as a semi-solid permeable membrane, being a thermosetting polymer (elastomers) it remains flexible while retaining its tensile strength, to enable it to expand and contract, following the temperature fluctuations (elasticity) of the substrate.
The urethane although very resilient to abrasion wear from entering and exiting the vehicle, still maintains its physical properties like flexibility, tactile hand and its patina
A urethanes fibre structure will stretch in all directions with no particular grain or stress pattern. The urethane surface coating will not withstand multi directional stress, however, and when it’s flexed or stretched continuously in the same place the surface coating develops minute cracks. It also has micro-pores that allow evaporation and hydration (the passage of water vapour through a membrane or pore) they are not sealed per se as some liquids penetrate it easily; others stay on top dependent upon their molecule size.
Leather's greatest enemies are; sun, heat, body oils, perspiration (that contains urea as well as organic salts and acids) and ultra violet radiation (UVR), which dries the hide, fades the colour by bleaching, and can cause the leather to fail by drying out the fibres causing the urethane and / or the hide to crack.
Since body dirt and oil are a big stain factor in leather, be cognizant of bare skin when you are in your vehicle. If you use suntan oil or spray tan lotion, be sure to use a towel when you get back in your vehicle so that the oil does not get onto your leather. You can also use a towel when leaving the gym as body oils/ perspiration contain organic acids that will stain.
Water-based products are able to permeate deep into the hide, unlike oil, as water molecules are smaller than the molecules of urethane, which enables water-based products to permeate and provide hydration, which is essential for suppleness recovery.
Leather is very dynamic with respect to its moisture content; the leather hides needs to be kept supple. The purpose of rehydration is to restore moisture lost through evaporation, so whatever the surface finish, it has to allow the movement of moisture back and forth (evaporation and hydration). The liquoring (fats and oils) that are put into the leather during the tanning process do not dry out of the leather so therefore do not need replacing.
A water-based product accomplishes this with an emulsion of micro droplets of oil (for surface lubrication) and water that permeate the leather finish and / or the polyurethane as the emulsions molecular structure is smaller than the molecules of the finished leather or the polyurethane covering.
Particulate size - you can tell how small the emulsion droplets are and in some cases how concentrated an emulsion is by its colour. Opaque white emulsions typically have a large particle size, while faintly opaque or pearlescent emulsions typically have a small particle size approaching 1µ or less.
Water - unlike other organic or hydrocarbon-based solvents, is non-flammable, odourless, non-toxic and non-sensitizing to the skin and it doesn’t impart a greasy or tacky feel to the surface of the leather
Beeswax (or any other organic or inorganic wax), even the so-called Banana Oil (it is impossible to get any oil derived from a banana, its real name; Isoamyl acetate is a chemical additive) Mink Oil, Neat’s-foot, Lanolin (Latin lāna, "wool", and oleum, "oil"), Mink Oil, Silicone oil, Tea Tree Oil and oil based conditioners. These are all damaging to urethane coated leather, as they block the movement of moisture back and forth (evaporation and hydration) regardless of what they say on the products label.
Simple chemistry; urethane is hygroscopic, meaning it can absorb moisture. However a cross-linked polymer is damaged by oils which cannot penetrate the leather finish due to its particle size, which is larger than that of water (and no heat does not expand the micro pores of a urethane) oils attract dust and grime, trapping it on the surface, where it is easily transferred to any material (clothing) it comes in contact with, which along with abrasion from entering and exiting the vehicle helps to further damage the coating.
In summary, an aqueous emulsion is readily absorbed into the fibres and provides lasting and effective lubrication without migration, while re-hydration leaves leather feeling silky soft and pliable. Modern automotive leather upholstery use a completely different tanning processes and finishing system, utilizing advanced polymers and chemicals and as a consequence they do not need to be treated with aftercare products containing oils.
Oil vs. Water
• A water-based product accomplishes this with an emulsion of micro droplets of oil (for surface lubrication) and water that permeate the leather finish and / or the polyurethane as the emulsions molecular structure is smaller than the molecules of the finished leather or the polyurethane covering.
• Particulate size - you can tell how small the emulsion droplets are and in some cases how concentrated an emulsion is by its colour. Opaque white emulsions typically have a large particle size, while faintly opaque or pearlescent emulsions typically have a small particle size approaching 1µ or less.
Water unlike other organic or hydrocarbon-based solvents, is non-flammable, odourless, non-toxic and non-sensitizing to the skin and it doesn’t impart a greasy or tacky feel to the surface of the leather.
If oil is allowed to permeate any micro fissures or the stitching in the urethane it will delaminate from the hide releasing its adhesive bond, and it will be able to move in a different direction from the hide, which will result in surface fissures and cracking , further compounding the problem resulting in the need to replace the protective coating
In summary, an aqueous emulsion is readily absorbed into the fibres and provides lasting and effective lubrication without migration, while re-hydration leaves leather feeling silky soft and pliable.
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