The primary difficulty in adjusting the stability of emulsified metalworking fluid formulations lies in the hard-to-control emulsification balance. Soluble oil concentrates are W/O (Water-in-Oil) types, allowing the addition of more oil without becoming turbid. Micro-emulsion concentrates are O/W (Oil-in-Water) types, allowing the addition of more water without affecting their appearance and stability.
Emulsified metalworking fluids generally contain the following components:
Every functional block offers a wide variety of raw materials to choose from. The approximate content of some raw materials can be easily determined, such as base oils, lubricants, biocides, and alkanolamines. However, other raw materials are difficult to predict accurately at the beginning of the formulation design. These include surfactant combinations and their dosages, fatty acid ratios, and coupling agent dosages. Some additive ratios can have significant flexibility. Meanwhile, a difference of just a fraction of a percent in others can severely impact product stability. This requires engineers to repeatedly adjust and blend to find the optimal ratio.
When other components are fixed and the system is balanced, if we solely consider the content of “Emulsifier A”, its impact on formulation stability falls within a proportional range—for instance, from 6.3% to 9.1%. This can be viewed as an interval on a single line. We can call this interval the “content window.”
If we simplify the formulation components into two dimensions—the emulsifier combination and fatty acids (or all other components combined)—its stable range becomes a 2D plane, rather than a single point or line.
If we increase the formulation to three dimensions—emulsifier combination, fatty acids, and coupling agents (or integrate other components as one factor)—its stable range becomes a 3D spatial cube. It is not necessarily a regular shape, as interactions between additives must be considered.
If expanded to 20 dimensions (representing 20 different raw materials), the stable window range becomes a highly irregular, indescribable multi-dimensional spatial surface body. This is theoretically very difficult to predict. This is exactly why formulation balance is so complex, and why changing a single raw material can drastically affect overall stability.
To ensure the “proportional composition” of the formulation falls within this stable window, the system requires simplified inductive design. Through extensive experimental work and repeated adjustments, the balance point finally lands inside the “stable window.” This results in a visually transparent and stable concentrate. At this point, you have a good start. The formulation development is 10% complete and on the right track.
The remaining work includes evaluating long-term storage appearance stability, performance indicators, and quality consistency. It also involves testing the fluid at customer sites to identify potential issues. Based on field feedback, the formulation must be continuously optimized—much like the PDCA (Plan-Do-Check-Act) cycle—until the final product is established. During this period, a massive amount of time, money, manpower, and opportunity costs will be spent. Leveraging expert experience can significantly reduce these costs.
Here are some experiential recommendations from Ruqinba for all formulators and R&D technicians to reference:
(1) The surfactants in the formulation must include both hydrophilic and lipophilic properties simultaneously.
(2) Whether using hydrophilic or lipophilic surfactants, compounding several surfactants together yields better results than using just one.
(3) Amines and fatty acids (or amides) must be sufficient in quantity, and the amines should be in excess. Unless the formulator has unique insights into the design, it is generally necessary to add a sufficient amount of fatty acids (or amides).
(4) Too much water content in soluble oils or too little water content in micro-emulsions is detrimental.
(5) Pay close attention to the acid-base balance, the hydrophilic-lipophilic balance (HLB), and the oil-water two-phase balance.
(6) It is recommended to simultaneously test sample stability at 55°C, room temperature, and -5°C. The final tests must concurrently evaluate samples both without and with defoamers.
(7) If you have blended dozens of sample groups and only obtained samples that are transparent in appearance but poor in stability, it might be time to consider changing the base oil.
(8) In formulation development, regarding raw material content and varieties, generally start with “addition” (adding components). When this is ineffective, be adept at doing “subtraction” (removing components).
(9) Simple formulation design has its rationale, and complex design has its rationale too. Do not blindly copy others’ routines.
(10) Finally, do not be easily “fooled” by suppliers into adding every suggested ingredient into your formulation. Doing so results in increasingly complex formulas, soaring costs, and harder-to-control stability. Always rely on actual experimental results.
