# Semi-synthetic machining coolants

Semi-synthetic metalworking-fluid identity, product-specific composition disclosures, measurement bases, and limits of compatibility and performance claims.

Reviewed: 2026-10-30

Tags: chemicals, machining, metalworking-fluids, emulsions

## Identity and neighboring classes

DCT Unicool 427’s SDS reports **15–20% mineral oil**, with no explicit percentage denominator. [DCT SDS, section 3][dct-sds] Rustlick **ULTRACUT 375R** identifies chlorinated extreme-pressure additives and excludes titanium from its recommendations. [ITW TDS][itw] OSHA’s example for the semi-synthetic class is 5–30% base oil by volume in the concentrate. It is a class example, separate from the Unicool SDS disclosure. [OSHA, section B and appendix 1][osha]

Emulsifiers disperse oil in water and improve wetting. OSHA lists optional functions such as reserve alkalinity, corrosion inhibition, foam reduction and microbial control. [OSHA, appendix 2, table 3][osha]

The supplied concentrate, a laboratory dilution, and a used circulating fluid are different samples. The workpiece metal is another material, not an ingredient of the original concentrate. As explanatory synthesis, the class name identifies neither a full formulation nor interchangeable performance. A separately supplied defoamer or biocide is an additive product, not a complete semi-synthetic coolant.

## Product variants and applicability

These examples establish differences within the class. They do not rank products or establish customer approval.

| Exact product and primary source | Supplier description and boundary |
| --- | --- |
| DCT Unicool 427, product ID DCT201810 | Supplier describes machining and grinding of steel, cast aluminum, and aluminum-magnesium alloys, with lubricity and extreme-pressure additives. It claims low foam and absence of chlorine, triazine, and secondary amines. No comparative test conditions accompany these claims. [DCT product page][dct-page]. |
| Rustlick ULTRACUT 375R, US part numbers 74905 and 74955 | ITW describes chlorinated extreme-pressure additives. Its application list limits use on aluminum, brass, bronze, and copper and states that the product is not recommended for titanium. These restrictions remain relevant beside the broader multi-material marketing claim. [ITW TDS][itw]. |
| Mobilcut 250, Philippines Mobilcut Series PDS, marked 04-2026 | ExxonMobil identifies a semi-synthetic fluid primarily for aluminum and aluminum alloys, with possible ferrous-metal use. The multi-grade property table leaves the numerical Mobilcut 250 cells blank. Values for Mobilcut 231 or 251 do not fill those gaps. [Mobil PDS][mobil]. |

As synthesis, "semi-synthetic" does not mean chlorine-free. Nor does a claim about secondary amines mean absence of every amine. Supplier free-of language is a formulation claim with a source and date. It is not a current-lot assay, an analytical detection limit, or proof of absolute absence. The reviewed sources do not establish boron-free composition for the class.

## Partial composition disclosures

Unicool 427 SDS version 00, dated December 16, 2019, discloses these ingredients. Its percentage heading does not specify mass or volume basis. [DCT SDS, section 3][dct-sds].

| Disclosed ingredient | CAS number | Reported percentage range |
| --- | --- | --- |
| Mineral oil, petroleum distillates | 64742-52-5 | 15–20 |
| Triethanolamine | 102-71-6 | 10–15 |
| Diethylene glycol monobutyl ether | 112-34-5 | 1–3 |

The footnote withholds other identities or exact concentrations as trade secrets. This is a partial hazard disclosure, not a complete formulation, active-content specification, or manufacturing recipe. [DCT SDS, section 3][dct-sds].

As synthesis, an SDS range cannot establish the undisclosed water fraction by subtraction. The remainder can include multiple ingredients. Even an explicitly stated ingredient wt% would mean ingredient mass per total product mass, not the oil fraction of a used fluid or the concentration of the whole coolant product in water. No such conversion is justified by this table.

## Property values need their sample basis

DCT's December 18, 2019 product information sheet labels the following table as typical concentrate characteristics. It gives no measurement temperature for density or pH and no pH sample-preparation detail beyond the concentrate heading. [DCT TDS, page 1][dct-tds].

| Unicool 427 typical concentrate characteristic | Value | Reported method or qualification |
| --- | --- | --- |
| Appearance | Clear golden liquid | Visual |
| Specific gravity | 1.01 | Calculated from density |
| Density | 8.43 lb/gal | ASTM D1475 |
| Water solubility | Soluble | Visual |
| pH | 9.81 | ASTM E70-97 |
| Refractometer factor | 1.31 | Calculated |
| Product identification | DCT201810 | Identifier, not a measured property |

The SDS also reports specific gravity 1.01 relative to water, pH 9.81, and viscosity as not determined. No viscosity value or measurement temperature follows. [DCT SDS, section 9][dct-sds].

ITW's undated ULTRACUT 375R TDS reports pH 9.6, specific gravity 1.01, density 8.3 lb/gal, flash point above 200 °F with 93 °C alongside, total chlorine 4%, and mineral oil 30%. It does not state pH preparation, density temperature, specific-gravity reference temperatures, flash-point method, or percentage basis. [ITW TDS, properties][itw].

The lb/gal entries do not explicitly define the gallon basis. As explanatory synthesis, density has mass-per-volume units, while specific gravity is a ratio to a reference density. Equal rounded specific gravities do not establish identical formulations. A pH value without comparable preparation and temperature does not support a controlled product ranking. A missing viscosity is unknown, not zero. A concentrate property is not an operating target for a diluted fluid.

## Refractometry and conditional interpretation

DCT's second TDS page distinguishes refractometer reading from product concentration. It gives the product-specific multiplier 1.31 and a rounded conversion table. A factor is not itself a concentration or a raw refractive index. [DCT TDS, page 2][dct-tds].

As synthesis, the factor's meaning depends on the supplier's stated scale and calibration. A numerical reading cannot establish ingredient wt%, oil content, or microbial condition without additional evidence. Two fluids can give the same reading but have different compositions. Neither the DCT sheet nor the ITW sheet provides uncertainty or validation of its refractometer relationship for contaminated used fluids. This reference supplies no working concentration or dilution recommendation.

## Performance and evidence limits

DCT markets rust protection, resistance to aluminum staining, tramp-oil rejection, and low foam, but the reviewed product page gives no named corrosion, foam, lubricity, or biostability test with numerical results. Its statement about typical biological control does not establish sterility or a universal bacterial-control result. [DCT product page][dct-page].

The Mobil PDS marks properties as typical rather than specifications and allows manufacturing and blending-location variation. Its series-level claims about biological resistance and corrosion protection do not establish a measured result for every alloy or every grade. [Mobil PDS, features and closing qualification][mobil].

HSE distinguishes exposure to fluid from exposure to machining mist and identifies skin and lung risks. Product performance language therefore does not establish universally safe handling or exposure. [HSE, updated December 4, 2024][hse]. DCT's historical SDS itself identifies irritation and sensitization hazards and states that whole-product hazard classification was not experimentally determined. [DCT SDS, sections 2 and 11][dct-sds].

The reviewed evidence includes all six DCT SDS pages, its ingredient footnote, both DCT TDS pages, and the complete ITW TDS. The SDS references US OSHA and WHMIS 2015. The ITW sheet lists US package codes but no technical revision date. These documents do not establish current composition in every sales region. No ITW or Mobil exact-product SDS, lot certificate, independent performance trial, or current customer qualification was evaluated.

The complete Philippines Mobil PDS and relevant OSHA classification and additive sections were also read. OSHA's manual is advisory and based on historical 1998–1999 source work. It is not a product specification or a new regulation. [OSHA introduction][osha]. No universal tool-life gain, surface finish, material compatibility, machine compatibility, or service life follows from this evidence.

[osha]: https://www.osha.gov/metalworking-fluids/manual
[dct-page]: https://www.dchem.com/products/unicool-427-premium-semi-synthetic-coolant/
[dct-sds]: https://www.dchem.com/wp-content/uploads/2021/06/DCT201810-Unicool-427-Dec16-2019.pdf
[dct-tds]: https://www.dchem.com/wp-content/uploads/2021/06/DCT201810-Unicool-427-Product-Info-Sheet-2019-12-18.pdf
[itw]: https://www.itwprobrands.com/files/techdatasheets/ULTRACUT375R-Semi-SyntheticMetalworkingCoolant.pdf
[mobil]: https://www.mobil.com/en-ph/industrial/pds/ap-xx-mobilcut-series
[hse]: https://www.hse.gov.uk/metalworking/about.htm