HLB VALUE - A KEY FOR SELECTION OF EMULSIFIER SYSTEM IN METALWORKING FLUIDS IN COLLOID AND INTERFCE SCIENC POINT OF VIEW.

1. WHAT IS HLB?

 

The abbreviation HLB stands for “Hydrophile-Lipophile Balance”, this is a strong value for formulators to select the right HLB system for their basis of MWF design.

 

When I mention HLB value in this topic, I want to talk about the non-ionic surfactant. Anionic surfactant and zwitterionic surfactants can be discussed in the formulation.

 

2. HLB VERSUS SOLUBILITY OF EMULSIFIERS.

 

The HLB value of an emulsifier reflects its solubility. Low HLB value surfactant can be called oil-soluble, and a higher one tends to be water-soluble. Noting that: some emulsifiers may have the same HLB value, but it shows a difference in solubility and its behaviors.

 

When we work on emulsifiers and surfactant systems, we will be soon know the correlation between its solubility and behavior in solution (maybe water or oil).

 

TABLE 1: CORRELATION BETWEEN HLB VALUE AND ITS USE

TABLE 1 has shown the application of emulsifiers based on their HLB value. This is very basic knowledge, but it may go not well when you start to apply them to design the MWF system. 

 

                   

3. REQUIRED HLB (rHLB) VALUE OF OIL

 

Simply said: each type of oil requires a specific HLB value of an emulsifier or emulsifier system to be emulsified. It means the required HLB of the oil needs to be the same as the HLB value of emulsifiers to make a kinetically stable emulsion. (rHLB ~ HLB). Normally the rHLB of mineral oil is from 9 to 11 (+/- 1) for O/W emulsion, but it will become 6 +/- 1 for W/O emulsion. 

 

TABLE 2: rHLB VALUE OF OIL FOR O/W AND W/O EMULSION

 

 

rHLB calculation of oil systems contains many ingredients inside will be the same method as HLB calculation. rHLB of each oil = weight portion of emulsifier x its rHLB. rHLB of system = total rHLB of each composition inside. 

 

For example, if you are making an O/W emulsion textile lubricant. The product might be

30% mineral spirits, 50% cottonseed oil, and 20% chlorinated paraffin to be emulsified in water. The required HLB of the combination can be calculated as follows:

Mineral Spirits ..........30% X Req. HLB 14 = 4.2

Cottonseed Oil ..........50% X Req. HLB 6 = 3.0

Chlorinated Paraffin . . 20% X Req. HLB 14 = 2.8

==> rHLB = 4.2 + 3.0 + 2.8 = 10.0, then we must go with the HLB value 10 +/- 1.

 

However, what if we use some oil that does not have the rHLB value in the above table? Then, we need to run some experiments to determine rHLB of the oil.

 

4. DETERMICATION OF rHLB FOR UNKNOWN OIL

 

No matter if you can find the rHLB of your oil in TABLE 2, it is much better to identify the right rHLB of the oil through experimental determination because oils and waxs from different manufacturers will have different properties and rHLB value. THIS IS ALSO TRUE FOR EMULSIFIERS. DIFFERENT MANUFACTURERS WILL SHOW SLIGHTLY DIFFERENT HLB VALUE.

 

To run the test, we need to select a pair of emulsifiers. It is highly recommended to select the same chemical structure of emulsifiers. For example, “80” SPAN-TWEEN emulsifiers are both oleate esters. It can be used to make a HLB system with a value from 4.3 (Span 80) and 15 (Tween 80). This also my experiments have done before with Base Oil 150N (FORMOSA Taiwan) before, to identify the rHLB value of this 150N Base Oil.

                             TABLE 3: CALCULATION HLB VALUE OF “80” SPAN-TWEEN

 

      

Sample No.	Emulsifiers		Calculated HLB
	4.3	15
	Span 80	Tween 80
1	100	0	4.3
2	90	10	5.37
3	80	20	6.44
4	70	30	7.51
5	60	40	8.58
6	50	50	9.65
7	40	60	10.72
8	30	70	11.79
9	20	80	12.86
10	10	90	13.93
11	0	100	15

For example: Emulsifier systems contain 60 wt.% of Span 80 and 40 wt.% of Tween 80 ==> HLB value of system = (60*4.3 + 40*15.0)/100 = 8.58

 

 

 My experiments 

 

Before running the emulsion test, I ran the solubility test to identify the solubility of 11 emulsifiers system in TABLE 3 with different Calculated HLB values. 11 samples will be added into oil, and the other 11 samples will be added into water. Then, taking note of its solubility. The amount of emulsifier can be equal to 10-20% of the oil. They’re 3 situations may happen:

- HLB < 8 will be oil soluble.

- HLB > 10 will be water-soluble.

- HLB ~ 8-10 can be solubilized in both oil and water. 

 

After that, I ran the emulsion test for 11 samples. Here I chose 1 gram emulsifiers + 4 grams 150N Base Oil + 95 grams water (equal to 5% soluble oil after dilution). The emulsifiers system with HLB less than 10.0, I will mix them with base oil first, and then pour them into water. However, those with HLB value higher than 10.0, I will add them into the water before pouring oil inside. After that, the mixture will be shaken several times, and leave it 24 hours. I found that the emulsifier system has HLB from 8-10 shows very good emulsion stability without creaming or oil separation after 24 hours. This is a very quick method to identify the rHLB value of 150N base oil in my experiment. Then, I go further with the HLB value of 0.5 units apart in the range from 8 to 10. (8.0; 8.5; 9.0; 9.5; 10.0), and observe the emulsion kinetic stability after 10 days. Then, I found that the HLB value of 9.0 is quite stable, it means the rHLB of 150N base oil in my experiment is ~ 9.0.

 

 

However, even though the rHLB value of 150N base oil has already identified, you still face a problem when using the different chemical structures of emulsifiers with the same HLB value to emulsify 150N base oil at the same treat-rate. In some cases, you need to use a higher or lower treat-rate than that of the experimented data when using the different emulsifier systems. If you understand Colloid and Interface Science, you can explain what is going on.

 

5. COLLOID AND INTERFACE SCIENCE IN METALWORKING FLUIDS

5.1. Mole

What is a mole? 1 mole = 6.02214076×10²³ particles, ions, atoms, molecules, electrons…

 

In chemistry, the mole is a very important parameter to calculate the mass transfer in a chemical reaction. When we work with different emulsifiers have the same HLB value with different molecular weight (MW) and both of them do not have any amine/amide or basic functional groups in the structures. At 1 gram of selected emulsifier, the lower MW emulsifier has a bigger number of molecules. Assumption, 2 emulsifiers produce the same mean diameter emulsion, it means lower MW has more chance to reach maximum coverage. It may cause better surface protection and emulsion stability than the higher MW emulsifier.

 

2. pH and Zeta potential

 

pH and Zeta potential are effective factors to identify the kinetic stability of the emulsion. If we choose amide emulsifiers vs. Span-Tween systems with the same HLB value, the amide system may show better emulsion stability due to electrostatic repulsion of the electrical double layer, especially Zeta potential value. 

Zeta potential of colloid (emulsion) behaves as a function of pH. The higher the pH, the better the zeta potential. It means the emulsion will be more stable. In some cases, Zeta potential starts with a positive surface charge at acidic pH and it becomes a negative surface charge at basic pH. There will be an isoelectric point (IEP), where the surface charge is zero. At this IEP, the emulsion will immediately separate into 2 phases: Oil and water. As far as I know, the IEP does not exist in water-based metalworking fluids. This is because the main emulsifiers in this fields are anionic and non-ionic with a few amounts of amide types. Therefore, the MWF emulsions should be negatively charged at all the pH. We can run the zeta potential test to ensure the results if your factory has the zeta sizer machine to run DLS and zeta potential. 

There will be another parameter I have not mentioned yet is the degradation of the non-ionic surfactants which have esters, ether, amide functional groups. These groups may be hydrolyzed in the strong basic pH causing the instability of emulsion.

 

3. Molecular weight of emulsifiers.

 

Steric repulsion beside electrostatic repulsion also plays an important role to check the stability. This may happen when using the big molecular emulsifiers (polymeric emulsifiers), such as PIBSA, in the formula. 

 

To observe this effect, you may find some MWF packages using PIBSA technology from ITALMATCH. However, the combination of PIBSA + Sodium Petroleum Sulfonate + Non-ionic surfactants (Amide types) can bring very good finished emulsion in terms of cost, kinetic stability, corrosion protection. The formulator can protect the emulsion with 2 emulsion stability mechanisms: electrostatic repulsion and steric repulsion.

 

4. Water quality

 

Hard water can destroy the emulsion using anionic surfactants, but it can be handled by using the non-ionic and polymeric surfactants. Anionic surfactants will react with Mg2+ and Ca2+ to form the Lime soap as the picture below. Therefore, by using non-ionic surfactants or the combination of non-ionic and anionic surfactants can make the emulsion with high water hardness tolerance. 

 

Written by Steven Nguyen