Technology Based Coolants
Engine coolants perform several functions in addition to providing freeze and boil protection. Coolants must also contain additives that inhibit corrosion and scale formation in the engine and radiator. There are very many different types of corrosion inhibitor compounds in the market, which are used in the formulation of engine coolants. Many of these are selected specifically for the purpose of inhibition of certain types of metal surfaces found in engine systems. Many modern engines use a variety of metals – from lightweight aluminium and aluminium alloys, through to heavy duty mild steel and heavier alloys. Each of these metal surfaces, and coolant design systems, present different corrosion inhibition problems which are addressed by different inhibitor compounds.
Generally, coolant corrosion inhibitors can be categorised into three distinct types:
1. Conventional Inorganic corrosion inhibitor compounds (sometimes called “traditional” coolants).
2. Extended Life organic-acid inhibitor compounds (sometimes called “OAT” coolants.
3. Combinations of OAT and Traditional inhibitors (sometimes called “Hybrid” coolants.
Conventional (Traditional) Inhibitors
Conventional inorganic (traditional) inhibitors have been known in the marketplace for over 60 years, and were first known to exhibit corrosion prevention features early in the previous century. These compounds are soluble inorganic molecules, which react (passivate) with the metal surfaces to produce an impervious physical layer which prevents any further corrosion reactions from taking place. These compounds are therefore very effective at producing a “barrier” against corrosion. They are typically readily available and relatively cost effective.
Such types of compounds include the following: Silicates, Nitrites, Nitrates, Molybdates, Phosphates and Borates. In the past, Amines were also used, but these have since become unfavourable due to the formation of carcinogenic nitrosamines. Recochem has never used amines in any coolant formulations.
The disadvantage of the traditional inhibitor type is that over time, these impervious barriers can break away from the metal surface, exposing fresh metal underneath to further corrosion, and also creating insoluble particles in the bulk fluid which can be further abrasive. Therefore, such coolants have limited life, generally in the order of 50,000km maximum (32,000 miles).
The life of a traditional coolant can sometimes be boosted by the incorporation of SCAs (supplemental coolant additives) which effectively are boosters containing superconcentrated mixtures of the same traditional inorganic inhibitors.
Extended Life OAT Inhibitors
Since the late 1980s considerable research has been undertaken on the development of genuine long-life coolants which form a molecular layer/barrier. Unlike the physical barrier (formed in the use of traditional inorganic inhibitors), these molecular layers are formed by the OAT molecules lining up on the metal surface due to very finely tuned behaviour in their polarity. These molecular layers exhibit the same effect, preventing the metal surfaces from coming into contact with the corrosive base fluid. When the molecular layer is periodically interrupted (say from particles of debris in the cooling system) it automatically reforms from other temporarily unattached OAT molecules in the cooling system. In this way, the layer continually reforms and repairs itself, and therefore the fluid’s life is considerably extended.
OAT molecules are typically weak, buffered carboxylic acids with long (C8-C10) carbon chain length “tails” and polar heads. There are a wide number of different OAT molecules which are now used; some more widely than others, some with unique preference than others. Different OAT molecules exhibit different corrosion behaviour in different metal surfaces, and therefore many modern OEMs can be quite specific about the types of OAT molecules that the formulation must (or must not) contain.
Typically, such fluids will last considerably longer than the traditional inorganic chemistry coolants, and commonly will allow users to have drain intervals exceeding 250,000km (135,000 miles).
Due to the rapid development of different engine componentry and metallurgy since the 1990s, much effort has gone into the development of enhanced OAT coolants. Many engine types have indeed been shown to exhibit improved corrosion resistance, over longer life, with a combination of both OAT molecules (providing an extended life molecular barrier) with some specific inorganic compounds which enhance very specific performance features of the coolants.
This is particularly the case in some Heavy Duty on and off road engine types which may specify the use of OAT type coolants fortified with Nitrites and/or Nitrites and Molybdates, but also some passenger car engine types which specify OAT inhibitors fortified with Silicates and others, or Phosphates.
Such engine coolants are generally referred to as “Hybrids” and typically offer similar life and performance to OAT type coolants, specifically tailored to certain engine types.
The Future of Coolant
Currently the majority of engine coolants are ethylene glycol based. There is a small percentage of propylene glycol based coolant on the market. Propylene glycol is less toxic than ethylene glycol, but is less biodegradable than MEG. PG is also more expensive.
Despite is higher levels of toxicity, MEG based coolants still dominate the market, and many countries have enforced the use of bitterants (denaturants) which are added to the coolant formulation to reduce the likelihood of ingestion by humans or animals.
Recochem adds Bitterants to it's Reco-Cool branded coolants.
More recently, interest has risen in glycerin and 1, 3 propanediol (PDO) as coolant base fluids. Glycerin is a by-product from the production of biodiesel, thus an increase in supply is expected. Glycerin is much more viscous than ethylene glycol thus handling problems will probably limit its use to blend with ethylene glycol. PDO is an isomer of propylene glycol and similar in cost. Since it can be made from natural products it may have some environmental attraction. In engine coolant formulations, PDO demonstrates improved heat stability, less corrosion especially to lead solder, and lower toxicity than ethylene glycol coolants, all important considerations for today’s highly engineered autos and trucks.
In addition to new technologies influencing base fluid choice, continuing research moves at a fast pace in the development of new and innovative inhibitor types. Different OAT molecules are constantly being evaluated for inhibition performance, and Recochem has one of the world’s leading coolant research laboratories for the testing of new innovative inhibitor types.
Recochem is constantly engaged in modifying and optimising our formulations to meet the ever-changing needs of engine manufacturers.