Metalworking fluids: Could low viscosity oil be used?
To investigate whether low viscosity oil might be suitable for use in water-based metalworking fluids, Nynas’ researchers started by testing the stability of emulsions in different formulations.
Thomas Norrby
Technical Manager Lubricants and Electrical industry, Nynas AB.
Affiliate Professor, KTH, Stockholm.
“At Nynas, we have access to plenty of low viscosity oils such as NYNAS® NS 3, and this led us to want to investigate the possibility of using such products in water-based metalworking fluids. As an initial step in this investigation, we decided to first study how low viscosity NYNAS NS 3 performs in emulsions,” explains Professor Thomas Norrby, Nynas Technical Manager.
Water-based metalworking fluids are sold in the form of concentrates. End users will dilute these concentrates with water, the final mixture containing 5-10% of concentrate. Three different types of concentrates are available on the market – one based on conventional soluble oils, one called semi-synthetic as it is a mixture of water and oil, and one going under the slightly misleading name synthetic, although it contains only water and additives, and no oil of any kind.
“In our study, we compared the performance of our NYNAS NS 3 oil with several different emulsifier blends and in one model semi-synthetic formulation,” says Thomas Norrby.
Two different emulsifiers were used with NYNAS NS 3: Span 80 (Sorbitan monooleate) with a hydrophilic-lipophilic balance (HLB) value of 4.3, making it highly lipophilic, and Tween 80 (Polyethylene glycol sorbitan monooleate) with a HLB value of 15, making it highly hydrophilic. A small amount of butyldiglycol was also added as a solubiliser or coupling agent.
“By blending Span 80 and Tween 80, we were able to produce concentrates of varying HLB values, ranging from 11 to 15. And our results show that the NYNAS NS 3 formulation with the HBL value of 13 was the most stable. NYNAS NS 3 also performed well in the semi-synthetic model that we used,” says Thomas Norrby.
“These are initial results, but they do confirm that stable emulsions can be achieved with NYNAS NS 3 in both conventional soluble oil and semi-synthetic formulations. However, further investigation and testing will be required before any proper formulation advice can be provided,” concludes Professor Norrby.
Small droplets – superior stability
- The stability of the emulsions was studied using Droplet Size Distribution (DSD). This was determined by light scattering at high dilution in a Malvern MasterSizer instrument. DSD changes were monitored over time, enabling coalescence and ripening to be detected.
- As shown in Figure 1, the median droplet size Dx (50) in the emulsions varied widely between the samples of HLB 11 and HLB 15. At HLB 13, the minimal droplet size was achieved.
Figure 1. Median droplet size, Dx (50), for soluble oil Span/Tween formulations and Model A, a semi-synthetic formulation. At the optimal HLB of 13, the droplet size is the smallest, as in Model A.
Figure 2. The superior stability of the HLB 13 emulsion was clearly visible in samples after one day.
