What are the requirements for selecting brake fluid?

Key properties of brake fluid: 1) Good viscosity-temperature performance and low-temperature performance. During braking, frictional heat can raise the temperature of the brake pads to as high as 250°C. Part of its heat is transferred to the brake fluid, raising its operating temperature to 70–90°C. In situations such as driving down long slopes where frequent braking is required, this operating temperature can reach 110°C. In large freight trucks, the temperature of the brake fluid can sometimes go as high as 150°C. In winter, in certain regions, the temperature of the brake fluid can drop below –40°C; therefore, the brake fluid needs to have good viscosity-temperature properties and flow characteristics at low temperatures. Appropriate high-temperature viscosity, a low freezing point, and low-temperature viscosity. 2) Appropriate lubricity: To ensure that the brake cylinder and rubber bowl can slide smoothly, the brake fluid must have appropriate lubricity, which can be determined ultimately through bench tests based on the friction conditions between the piston and the cylinder. 3) Ensure that braking is safe and reliable, with no air resistance. In modern high-speed vehicles, frequent braking during driving generates a large amount of frictional heat, raising the temperature of the braking system. If a brake fluid with a low boiling point and high evaporation tendency is used, its evaporation at high temperatures can cause vapor to fill the pipes within the braking system, resulting in air resistance and brake failure. Therefore, modern vehicles often require brake fluid with a high boiling point and low volatility in order to minimize the formation of air resistance. 4) Good corrosion resistance: Braking fluid should provide good corrosion resistance for various metal components of the brakes. 5) Good chemical stability: Braking fluid is used under high humidity conditions for extended periods of time; therefore, it is required not to undergo thermal decomposition or polymerization, which could cause the fluid to become more viscous, nor should sludge deposits form. It is also required to have good miscibility; when mixed with another brake fluid, it should not cause stratification or precipitation, which could affect its use. 6) Good compatibility with rubber: There are many rubber seals and gaskets in braking systems to ensure that these systems remain completely airtight; therefore, the brake fluid should have good compatibility with rubber seals, in order to prevent them from expanding due to the presence of the fluid and resulting in a decrease in their mechanical strength. 2. Classification of brake fluid: Automotive brake fluids are generally divided into the following 3 categories: alcohol-based, mineral oil-based, and synthetic. 1) Alcohol-based brake fluid: The basic composition of alcohol-based brake fluid is castor oil accounting for 45％–55％ and alcohol accounting for 55％–45％ (the percentages refer to mass fractions). This type of fluid has good lubricating properties, readily available raw materials, high viscosity at low temperatures, and a simple production process. However, it has poor performance at low temperatures, a low boiling point under equilibrium conditions, is prone to air lock, has poor solubility with water, and tends to oxidize and deteriorate over time, thus failing to ensure safe driving. 2) Mineral oil-based brake fluid: Mineral oil brake fluid is made using refined diesel fractions that have been thoroughly dewaxed as the base oil, to which thickeners, antioxidants, rust inhibitors, colorants, and other additives are added. This type of brake fluid has a wide temperature range of operation, good performance at low temperatures, and does not cause corrosion to metals. However, it is not miscible with water and synthetic brake fluids; the presence of even a small amount of water causes it to vaporize at high temperatures, resulting in air resistance that affects braking performance. It also causes swelling in natural rubber, so oil-resistant rubber seals must be used. 3) Synthetic brake fluid: Synthetic brake fluid is the most commonly used type of brake fluid at present, and it can be divided into 3 categories: alcohol-ether type, ester type, and silicone type. (1) Alcohol-ether type brake fluid is composed of lubricants, diluents, and additives. Common lubricants include ethylene glycol, polypropylene glycol, ethylene oxide adducts, and polymers of propylene oxide, while common diluents include diglyme ethers, triglyme ethers, tetraglyme ethers, etc. Common additives include antioxidants, corrosion inhibitors, rust preventives, anti-wear agents, pH regulators, etc. The product has relatively stable performance, low cost, and is the most widely used. Its disadvantages are that the boiling point for balanced reflux is not very high, it has strong wetness, poor performance at low temperatures, and the brake components are prone to rusting when used in humid and hot climates. (2) Ester-type brake fluid: Its base fluid consists of carboxylate and borate esters, with their addition amount (by mass fraction) being approximately 20% to 50% of the total volume. Common buffering agents include monoalkyl ethers of polyethylene glycol, while typical additives are antioxidants, corrosion inhibitors, and pH regulators. The performance has improved significantly compared to the former. (3) Silica-based brake fluids are generally alkylene polyether silicates such as polyalkylene glycol silicates, along with rubber antiswelling agents and other additives. This type of brake fluid has good performance, but it is expensive. Today, we generally use synthetic brake fluid; the specific model is indicated in the vehicle’s user manual, as well as clearly marked on or next to the vehicle’s brake fluid filler cap. It is essential to purchase and use the model specified; the standards must not be increased or decreased arbitrarily, as this relates to the compatibility with the performance of the piston seals. Brake fluid types are classified as DOT3, DOT4, DOT5, DOT6, and so on. It depends on the vehicle you plan to use; generally, domestic vehicles use DOT3 or DOT4. Imported cars use DOT5 or DOT6 fluids, and it’s best not to use them indiscriminately. If your car is domestic, using DOT6 fluid may accelerate the aging and corrosion of the rubber seals and gaskets in your braking system. There is a huge gap between imported and domestic technologies; don’t think that using good brake fluid is sufficient. Braking and steering are the lifeblood of a vehicle, so use them with caution! ! ! .