Hygroscopicity

What is Hygroscopicity?

Hygroscopicity is a physicochemical property of certain substances that involves their ability to easily absorb moisture (water vapor) from the surrounding air. Hygroscopic substances can either absorb water directly on their surface or assimilate it into their structure. This phenomenon holds significant importance in the context of many chemical products, including some lubricants and operational fluids, since the presence of water can greatly affect their properties and effectiveness.

Definition of Hygroscopicity

The mechanism of hygroscopicity can vary:

• Adsorption: Water molecules settle on the surface of a substance.
• Absorption: Water molecules penetrate into the interior of a substance’s structure.
• Formation of hydrates: Water binds chemically with the substance, forming new compounds.
• Dissolution: Some solid substances may dissolve in the absorbed water.

The degree of hygroscopicity depends on the chemical nature of the substance, its structure, porosity, and environmental conditions such as relative humidity of the air and temperature.

Popular Hygroscopic Lubricants and Operational Fluids

Many commonly used products exhibit hygroscopic properties. These include:

• Brake fluids (DOT 3, DOT 4, DOT 5.1): They are mainly based on glycols or borate esters, which are highly hygroscopic. Absorption of water by brake fluid is the main cause of reducing its boiling point.
• Synthetic oils based on PAG (Polyalkylene Glycols): Many types of PAG oils, especially those water-soluble, easily absorb moisture.
• Synthetic oils based on POE (Polyol Esters): These are also susceptible to moisture absorption, which may lead to their hydrolysis (chemical breakdown).
• Some transformer and insulating oils: The water content in these oils is a critical parameter affecting their dielectric properties.
• Some greases: Especially those containing soaps or thickeners prone to absorbing water.
• Concentrates of coolant fluids (glycol-based).

Negative Impact of Water Content on Hygroscopic Products

The presence of water in hygroscopic products not designed to operate in its presence can lead to a series of negative effects:

• Lowering of boiling point: Critical in the case of brake fluids. Water lowers the fluid’s boiling point, which during intense braking may lead to vapor bubbles forming in the system (so-called “vapor lock”) and loss of braking effectiveness.
• Corrosion: Water promotes corrosion of metal elements in lubrication or hydraulic systems.
• Chemical degradation of oil/fluid:
  • Ester hydrolysis (e.g., in POE oils): Leads to the formation of acids, which accelerate corrosion and further oil degradation.
  • Accelerated oxidation of oil.
• Change in viscosity: Water content can affect the viscosity of the oil, impairing its ability to form a lubricating film.
• Deterioration of lubricating properties: Water may displace oil from frictional surfaces, leading to increased wear.
• Precipitation of additives: Water may cause the precipitation of some enhancing additives from the oil.
• Microorganism growth: In some cases, the presence of water can foster the growth of bacteria and fungi in the oil.
• Deterioration of dielectric properties (in transformer oils).

Rules for Proper Handling of Hygroscopic Products

To minimize the negative effects of hygroscopicity, the following rules should be observed:

1. Storage: Hygroscopic products should be stored in original, factory-sealed, and airtight packages.
2. Minimize air contact: After opening the package, the product should be used as quickly as possible. The time the package remains open should be minimized.
3. Tight sealing: If the product is not completely used, the package should be immediately sealed tightly.
4. Avoid old, opened packages: It is not recommended to use hygroscopic products (especially brake fluids, POE oils) from packages that have been open for a long time, even if there appears to be a lot of product left.
5. Dry conditions: Store in dry rooms, avoiding areas with high humidity.
6. Clean and dry tools: Use clean and dry tools for transferring and application.
7. Regular inspection and replacement: For brake fluids, it is recommended to regularly check the water content and replace the fluid according to the vehicle manufacturer’s instructions (usually every 2 years). In systems with POE or PAG oils, monitoring water content is also crucial.

Awareness of hygroscopicity and proper handling of such products are key to ensuring their effectiveness, safety, and longevity of lubricated or serviced systems.