Performance Testing Methods for Hard Water Resistance Additives in Metalworking Fluids

In the metalworking fluid industry, the initial motivation for studying the "calcium/magnesium soap dispersing power" of surfactants comes from a common pain point in field applications. The fatty acid components in metalworking fluids easily react with calcium, magnesium, and other hard water ions. This reaction generates water-insoluble metal soaps.

When boiling water at home, these calcium and magnesium ions form limescale. In the circulation system of metalworking fluids, they appear as fatty acid calcium/magnesium soaps. These insoluble substances act like "curd-like scum." They float and adhere to any solid surface they touch, including machine tools, workpieces, and filters, causing severe contamination and blockages.

Since they are insoluble in water, we only have two choices in formulation design: let them agglomerate, or disperse them. Our expectation is to break these calcium and magnesium soaps down into sufficiently fine particles and keep them evenly and stably suspended throughout the system.

To achieve this goal, formulators must deeply study the stability of formula additives in hard water environments. Solid particles immersed in liquid easily clump together and either sink or float. However, excellent surfactants can disperse these clumped solid particles into tiny spots, preventing them from re-aggregating. This ability is called dispersing power.

Within the industry, this is typically evaluated by measuring the dispersing power against "calcium soap." It is generally believed that substances with excellent calcium soap dispersing power will also exhibit outstanding dispersion effects on other solid particles.

How can we scientifically evaluate this ability? Currently, the industry primarily uses the following two mainstream testing methods:

Method 1: Determination of Power to Disperse Calcium Soap (Acidimetric Titration Method)

This method primarily refers to the Chinese national standard GB/T 7463-2008: Surface active agents - Determination of power to disperse calcium soap - Acidimetric titration method (Modified Schoenfeldt method).

• Definition: The power to disperse calcium soap refers to the mass (in grams) of soap that can be completely dispersed by 1g of additive. The larger this value, the stronger the dispersing power of the product.

• Method Summary:
  Prepare a 0.5% (mass fraction) soap aqueous solution. Let it stand at the test temperature for 24 hours, then take an aliquot of this solution. Mix this solution with a dilute solution containing a specific amount of dispersant. Next, add a specified volume of standard hard water with a known calcium hardness. Let the mixture stand at the test temperature for 1 hour (allowing the undispersed calcium soap to flocculate and float to the surface). Finally, using bromocresol green as an indicator, titrate the dispersed calcium soap in the lower clear liquid layer using a standard hydrochloric acid titration solution.

Method 2: Determination of Lime Soap Dispersing Power (LSDP Method)

• Method Overview:
  LSDP (Lime Soap Dispersing Power) refers to the minimum amount of dispersant required to completely disperse insoluble metal soaps (calcium and magnesium soaps) under test conditions. It is usually expressed as the mass fraction of the dispersant (surfactant) required for sodium oleate in a specific hard water. The lower this value, the less chemical is needed for dispersion, indicating a stronger dispersing power.

• Reagents and Equipment:

  1. 0.5% Sodium Oleate Solution (5g/L): You can prepare a 5g/L aqueous solution using pure sodium oleate. Alternatively, weigh 2 to 3g of chemically pure oleic acid, dissolve it in 500ml of distilled water by heating, and add 0.5g of anhydrous sodium carbonate in batches to adjust the final solution's pH to 8–9.

  2. 1000ppm Standard Hard Water: Weigh 0.665g of anhydrous calcium chloride and 0.986g of magnesium sulfate heptahydrate. Dissolve them in distilled water and dilute to 1000ml (this is equivalent to a hardness of 1g CaCO3/L).

  3. Dispersant Solution: The concentration should be 2.5g/L (i.e., 0.25%).

  4. Equipment: A 100ml glass-stoppered measuring cylinder (or a 30ml glass-stoppered colorimetric tube).

• Operational Procedure:
  At room temperature (generally kept at 25°C), use a pipette to transfer 5ml of the 5g/L sodium oleate solution into the 100ml glass-stoppered measuring cylinder. Add an appropriate amount of the dispersant test solution (5ml is recommended for the initial test). Then, add 10ml of standard hard water, and fill with distilled water to the 30ml mark. Insert the stopper and invert the cylinder 20 times, returning to the starting position each time. Let it stand for 30 seconds and observe the state of the liquid. If there is cohesive precipitation within the transparent solution, it indicates that the amount of dispersant is insufficient. You need to increase the dispersant amount and retest. The titration endpoint is reached when the liquid in the cylinder appears as a semi-transparent emulsion with no large agglomerates present.

• Calculation Formula:
  LSDP (%) = [ (V1 × 0.25%) / (V2 × 0.5%) ] × 100
  (Where V1 is the volume of the dispersant solution required to reach the endpoint; V2 is the 5ml volume of the sodium oleate solution.)

Summary and Industry Application

The two testing methods mentioned above characterize the calcium soap dispersing power of additives from different angles, but essentially, they lead to the same result. Generally, additive manufacturers use these tests to quantify monomer metrics, while terminal metalworking fluid blending plants prefer to directly test the hard water tolerance of the finished products. Both approaches are highly effective.

As synthesis experts focusing on high-end additives for metalworking fluids, Ruqinba has conducted long-term research on hard water resistance technologies. Our core Alcohol Ether Carboxylates (AEC) products—9H-LF and M10—were specifically created to solve the challenge of calcium soap dispersion in harsh water conditions.

After rigorous laboratory testing in accordance with GB/T 7463-2008 (Method 1), Ruqinba 9H-LF's calcium soap dispersing power reached a remarkable 80.5 (meaning 1g of 9H-LF can powerfully disperse 80.5g of calcium soap). This exceptionally outstanding data performance makes it the top choice for many R&D technicians and formulators when dealing with high-hardness water and high-load machining conditions, earning widespread acclaim within the industry.