Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
10% active dilution we did not see any sign of precipitation.
Boric acid salts are typically prepared by addition of boric acid to alkanolamines in water at temperatures too low for the condensation reaction to take place. This observation is taken to be a result of the borate condensates being more water soluble and containing a negligible level of boric acid.
The next phase of additive evaluation assessed laboratory prepared materials based on commercial additives. The reactions were controlled to minimise by-product formation and thereby simplify the analysis. The condensation reaction is carried out at a temperature sufficient to ensure water of reaction is evolved, with a subsequent measurable mass loss in the reaction product. The condensation product is a complex mixture of polyborate amides, polyborate esters, ionic polyborate species, monoborate amides and monoborate esters and unreacted MEA. The lab products were based on boric acid and monoethanolamine at 2% molar excess MEA, to give nominal elemental boron content of approxi- mately 6%. We selected this material as it is at the upper limit of elemental boron found in most metalworking fluid formulations. As it has a relatively high initial boric acid charge, this material should have the greatest level of residual boric acid remaining after the condensation reaction.
B NMR gives strong peaks at -6, -10 and -13.5 ppm, a weak peak at -18.3 ppm and a broad tail of unresolved species between -13 to -20 ppm. No peak is visible at the reference 0 ppm for the materials at 100%, 50% or 10% active in D2
Analysis of the lab samples at 50% active by 11
O. This suggested
that either the sample did not contain boric acid or that boric acid was not detectable in the complex product. To validate the analysis and confirm whether the interpretation of ‘no detectable boric acid’ was genuine or an artefact of the analysis, boric acid was spiked into the samples. A 10% addition of boric acid spike to the sample gives very strong peaks at 0 ppm and -18.3 ppm chemical shifts, indicating the presence of free boric
acid and the simple borate anion.
As the analytical methodology was found to be suitable to detect boric acid spiked in the additive, the next stage was to evaluate a wider range of additives. Lab prepared additives as controlled references and plant prepared products were assessed. Products prepared using a range of alkanolamines and boric acid expressed as elemental boron from 2% to 5.5% were analysed under the same conditions at 50% active in D2
O. These
samples covered the full range of Afton metalworking fluid borate additives.
In each case key peaks were identified at -6.3, -10, -13.5, -15.4 and -18.3 ppm, with the relative peak heights varying dependent on the nature of the raw materials and the products formed. In each case no evidence of any free boric acid at 0 ppm was observed.
As the analysis of lab prepared samples all indicated no boric acid present, this work was repeated on freshly prepared plant products. Samples of the various borate additives were taken from recent plant retained production samples, all less than four weeks old. The analysis of these plant retained samples was indistinguishable from the lab prepared samples, indicating a high level of consistency between the lab and plant prepared materials. Again the analysis indicated no boric acid present. Further this analysis demonstrated that the lab samples were truly representative of the plant materials.
The logical question was asked; “How do these additives behave in real formulations?” Metalworking fluid formulations are complex products with between 10 to 20 individual components, of which borate corrosion inhibitor is only one component. Several industry typical formulations were prepared using a selection of borate condensate additives. Typical metalworking fluid concentrates have between 10 to 40% active borate additives. The fluid concentrates had between 0 to 50% oil content to cover the range of synthetic oil-free to medium oil semi-synthetic working fluids. We were concerned that incorpo- rating the borate additives into these
complex formulations may case any number of potential interactions to take place which could potentially liberate boric acid.
As anticipated when analysed, the same range of key NMR peaks at -6.3, -10, - 13.5, -15.4 and -18.3 ppm were seen, with peak heights varying dependent on the additive used. We did observe in some formulations a small shift in the relative peak heights of these major peaks, which we attribute to interaction with the various acid and alkali components and possible complexation with hydroxyl species. However, despite the complex nature of the formulated fluids, no evidence of free boric acid at 0 ppm was seen.
The final evaluation was to assess the long term stability of the borate condensates. Two representative borate condensate products at 4.8% & 5.6% elemental boron were selected. Plant retained samples spanning 18 months, stored under typical warehouse conditions were obtained. The warehouse, located in Manchester UK, is typical in as much as it does not have active climate control. This means that the samples depending, on age have been exposed to cold and warm extremes of normal storage conditions, with outside air temperatures of between -12 o the time period.
C to +25 o
No.83 page 3
C recorded over
Fig 4a Nov 2010
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