If I was to ask you to name an oil additive, if you can name any, the first one would most likely be ZDDP. Now that isn’t just because you read the title of this article, but because it is one of the most widely used oil additives in the world. It in particular stands out for its critical role in extending the life of machinery. Its full name is a mouthful; Zinc Dialkyl Dithiophosphate, and why we shorten to ZDDP. This chemical has been the unsung hero in many lubricating oils, acting as a potent anti-wear agent and antioxidant for decades. But what exactly makes ZDDP so effective? Let’s dive into the chemistry behind its action, a topic that never fails to fascinate.
The Role of ZDDP in Lubrication
ZDDP serves a quadruple purpose in lubricants: it’s an anti-wear agent as you probably know, but it’s also a metal deactivator, an ash valve cushion and an antioxidant. Its mechanism of action is a fine ballet at the molecular level, where it prevents metal-to-metal contact, reactions on metal surfaces and oxidative degradation of the oil. Understanding this requires a foray into its structure and how it reacts under the stressful conditions of an machinery operation.
Molecular Structure and Reactive Nature
Now this is often where people get scared as I’m going to give a little explanation of the chemistry of the molecule. I am going to keep it very simple I promise. So let’s start with the hardest bit, the name. ZDDP is a compound consisting of zinc ion coordinated to two alkyl dithiophosphate molecules. Now they are some great scrabble scores but what does it actually mean. So let’s break up the words. I also have a simplistic image of the molecule above so you can picture it as we describe it.
- Zinc (Zn): A metallic element that serves as the central component of this molecule. Zinc acts as the backbone to which the other components are attached. It’s crucial for the anti-wear and anti-corrosion properties of the compound.
- Di – instead of us saying one or two we use terms like Mono, Di, Tri or Tetra to mean there are 1,2,3 or 4 or something. For instance carbon dioxide which you hear about on the news in relation to global warming is carbon with two oxygens (di-oxide).
- Dialkyl: This term refers to two (Di-) alkyl groups. Alkyl groups are carbon and hydrogen chains on the molecule. These alkyl groups can vary in size and structure, affecting the physical and chemical properties of the compound. The alkyl groups are responsible for making the compound oil-soluble, allowing it to blend well with lubricants. I have used a simple CH3, but they can be much longer. Now for those clever clogs amongst you will notice there are actually four CH3 (or alkyl groups), not 2 in the molecule, so why is it not called Tetra-alkyl? Well the molecule is actually just a mirror copy on both sides if you put a line down the zinc so on the left there are only two alkyl groups. The doubling this up comes in the DithioPhosphate, which the Di at the beginning doubles everything up.
- Thio: Thio means two sulphurs, not to be confused with Di which also means two. This Thio prefix suggests the presence of two sulfur atoms which if you take the left hand side of the zinc it has two sulphurs. These sulfur atoms are part of the phosphate group and are essential for the compound’s anti-wear properties. The sulfur forms strong bonds with metal surfaces, creating a protective layer that reduces wear.
- Phosphate: A chemical group consisting of phosphorus surrounded by oxygen atoms. The phosph comes from phosphorus and “Ate” means an oxyanion is present, which means oxygen with a spare electron, which in this case bonds onto the alkyl groups. The phosphate part of the molecule plays a critical role in its thermal stability and anti-oxidation properties, helping to protect the oil from breaking down under high temperatures.
Finally the fact there is a Di- in front of the thiophosphate explains it’s got two of these in the overall molecule. So hopefully you understand a little about the Zinc dialkyl dithiophosphate as it has a zinc in the middle, dithiophosphates (two sulphur phosphate compounds either side) all branched on the end with varying length carbon hydrogen chains as alkyl groups.
Why am I telling you all this? To show off? To scare you? Neither, it’s because this structure is pivotal to how ZDDP works because it’s both thermally stable and reactive when it needs to be. Under normal operating temperatures, ZDDP is a benign passenger in the lubricant, coexisting without reacting. However, as temperatures rise and pressure increases, ZDDP springs into action. We will soon cover these points in turn, but let’s first explain how ZDDP is made.
Production of ZDDP
Don’t worry to much if you don’t understand this section fully. The main thing to know is we make the dialkylthiophosphoric acid first with an alcohol and phosphorus pentasulphide. Then this reacts with zinc oxide to neutralise the acid. This is important as these reactions are reversible under the right conditions. Other than that, the rest of this production section is purely if you are interested, which I’m sure you are.
The synthesis of Zinc Dialkyldithiophosphate (ZDDP) typically involves the reaction of an alcohol with phosphorus pentasulfide to form Dialkyldithiophosphoric acid, which is then neutralized with a zinc salt such as zinc oxide to form ZDDP. Here’s a simplified version of the chemical equation for this process, using methanol as the alcohol for illustrative purposes:
Here, methanol (CH₃OH) reacts with phosphorus pentasulfide (P₂S₅) to yield Dialkyldithiophosphoric acid ((CH₃O)₂PS₂H) and hydrogen sulfide (H₂S) as a byproduct.
The Dialkyldithiophosphoric acid then reacts with zinc oxide (ZnO) to form Zinc Dialkyldithiophosphate ((CH₃O)₂P(S)S₂Zn) and water (H₂O).
The actual alkyl groups (R) used in commercial ZDDP are typically longer than the methyl groups (CH₃) used in this example, and the process may involve additional steps to purify the product or remove byproducts. The overall stoichiometry and substances involved can vary depending on the specific alkyl groups and the industrial process employed.
So why have I explained this to you, well it’s interesting first to understand dithiophosphoric acid is present and converted to ZDDP and water. In organic chemistry the reactions are often reversible and this means addition of water to the oil degrade the additive (known as additive wash out). It also explains why new oils with ZDDP, which although the molecule is neutral have an acid number, as the titration conditions release dithiophosphoric acid which is what is measured. It is this compound that is important also in an oils antiwear properties.
Anti-Wear Action
The anti-wear mechanism kicks off when the lubricant film between metal surfaces thins to the point of near metal-to-metal contact. At these hotspots, ZDDP decomposes, and this decomposition plays a crucial role. The breakdown of ZDDP releases phosphate, sulphide, and zinc compounds that react with the metal surface to form a protective layer, often referred to as a tribofilm. This film is remarkably resilient and can self-heal, ensuring that the metal surfaces are shielded from wear and tear. It’s like having a microscopic repair crew on standby, ready to fix any damage that occurs during operation.
So how does this tribofilm work? It’s two fold. Sulphur and glassy protection. We will explain these now.
Sulphur
Everyone assumes it’s the zinc and phosphorus that are the primary actors in ZDDPs mechanism of action. So much so, a well respected lubricant company in the UK sent me a message after watching my YouTube channel telling me they had not heard of the sulphur theory and where had I heard this and sent some papers on how phosphorus works in ZDDP to me they had contributed to. However, although a lot of research is specifically on phosphorus actions in modern research, if you do some good academic searches you will find sulphur action was worked out decades ago, but everyone seems to forget sulphur is part of ZDDP nowadays. So let’s correct that injustice and start with the Thio part of the molecule, aka the sulphur part. Sulphur is a good antiwear agent and the ORIGINAL antiwear agent. It is in crude and was the compound additive manufacturers tried to put back into the oil when they removed the sulphur during refining. In fact many gear oils today you can smell the sulphur in the oil. So why is sulphur so good? Well sulphides are often much harder (measured on the Mohs hardness scale) than their base metal. For instance iron is approximately 4 Mohs whereas iron sulphides are about 50% higher at about 6 to 6.5 Mohs. This reaction makes a toughened coating on surfaces called the boundary lubrication layer.
Zinc sulphide also forms with any remaining sulphur which has an hardness of about 3.5, giving some softer cushioning and reducing abrasives from free iron sulphide material that breaks off the surface. Why is that relevant? You will soon find out.
When we have something very hard that also means it’s less malleable and more prone to work hardening, friability (crumbling) and this can be an abrasive problem especially when the protective surface is only about 0.1 microns in size (100nm).
Also although iron is harder, copper sulphide is actually softer than copper and so quickly leaches from surfaces. This can cause drastic increases in copper during running in periods of engines and the higher sulphur EP extreme pressure antiwear additives can eat away entire copper systems over time. Hence ZDDP has to have a way of stabilising this formation and this is where the dithiophosphoric acid part of the molecule comes in as a glassy protector.
Glassy protection
So we have made this sulphide hardened layer. Think of it a newly concreted garage floor. It’s very hard and the car isn’t going to sink through it compared to the soil and earth beneath, but we have the problem now that it’s very dusty. I write this soon after building a second lab and I can tell you the dust gets everywhere. In a lubricated system these particles can be abrasive if they get somewhere not coated and cause damage. So what we really need is a way to paint the floor and this is where the phosphate comes in. These react with any uncovered metal to form metal polyphosphates. These literally paint the surface and sulphide layers to keep the sulphide in place. They are often called a glass like layer as they are amorphous and not crystalline and look glassy under electron microscope studies. However, I think of glass less as a protective layer and I prefer to use the term paint as it makes more sense in my analogy especially as we come to the second function of ZDDP. But first let’s picture how the layer looks.
Below is a simple schematic of how the tribofilm works which is only around 100nm in size.
Metal deactivator
If you have made it this far through the article you probably have at least some basic chemistry knowledge. Oxidation is a key degradation mechanism of a lubricant and metals such as iron and copper are excellent catalysts for this process to occur. Now one way of reducing oxidation is to remove the things that catalyse it such as metal. However metal is not usually an optional thing in mechanical components, so to have an engine with no metal would be pretty difficult. However, you don’t have to remove the metal. You just have to stop the metal coming into contact with the oil. In this case the glassy layer or as I call it painted layer of the metal polyphosphate with sulphides underneath effectively prevents oil contacting the underlying metal catalyst. We have basically painted the metal with additive to both stop it rusting and wearing, but, also to stop it being in contact with the oil and being a catalyst.
Cheating the catalyst testing
You may be familiar with the RPVOT test. It’s a way of artificially ageing an oil to see how it copes under high oxygen, high temperature, high pressure, water contamination and a huge copper catalyst to finish it all off. The test can turn an oil that would ordinarily last decades into an oil that’s 20 years older in a matter of days. The test is measured in minutes and is a regular test we do in our lab for industrial hydraulic, compressor and turbine oils to see how much life is left. In the early years the RPVOT was a number to compete on and often stated on product data sheets. However in recent years, some oil companies have been overdosing oils with metal deactivators to remove the copper from the equation so inferior base oils with low amounts of antioxidants can apparently have an RPVOT that is much better than an ordinarily better quality base oil and antioxidants purely to cheat the test. This does not reflect better in lubricant life but looks good on the test. There has been some controversy over this practice as the deactivator additives are usually skin irritants too so there are health and safety aspects to using such high concentrations purely to cheat a test. Hence part of a laboratories job is when unrealistically good RPVOT values of several days are seen is to look if there is a high concentration of these deactivators to identify those cheating the system.
So we now understand the antiwear and metal deactivator properties but we need to understand another property and that is as an antioxidant.
Antioxidant Properties
When we talk about antioxidants in oil they come in 3 groups; amines, phenols and ZDDP. Amines and phenols use nitrogen or benzene rings and alcohol groups to soak up free radicals. I have covered these in great details elsewhere on the learn oil analysis portal. So let’s now cover ZDDP.
As an antioxidant, ZDDP’s action is equally fascinating. Oxidation of lubricants is a common problem, leading to the formation of acids and sludge that can degrade the oil’s performance and damage engine components. ZDDP intervenes by decomposing to form protective layers on metal surfaces, which also happen to inhibit the oxidation process by its metal deactivator properties outlined above. Moreover, the sulphide produced during its breakdown has antioxidant properties, neutralizing radicals that would otherwise accelerate the oxidation of the oil.
Sulphated ash cushion
Now sulphated ash is often seen as the enemy in lubricants and you may have heard of low ash or low saps and zinc has earned a bad reputation in engine oils. Well this is because Zinc when the oil burns in the combustion chamber – as oil is part of the sealing process – over time you are left with ash at the end. Ash includes additive metals such as zinc. Now these ordinarily coat the surface of the engine valves so when they slam shut during combustion they are cushioned by a thin layer of ash. So these are really beneficial. The problem comes when too much ash forms it can cause the valve to not shut properly and eventual tunnelling though the layer of ash by the combustion gases. Hence there is a balance to ash content. Although you may prefer low ash to prevent engine coking, no ash is actually bad for engine valves so you do need a little ash.
The bad side of ZDDP – Ash
ZDDP is a remarkable molecule and I believe its inventor’s deserve the highest accolades for developing such an ingenious multi-faceted oil additive. Soon after its invention it became the most popular oil additive in the world. It’s cheap, can be mass produced and seems to cover all the bases. So what’s not to like? Well a few things.
First is the fact it forms ash.
Here’s a breakdown of why ash from zinc and other metals can be problematic in engines:
Ash Accumulation and Valve Deposits
Ash can accumulate on valve seats and other engine surfaces, leading to deposits that interfere with the engine’s efficiency and performance. These deposits can reduce the engine’s ability to “breathe” properly by obstructing air and exhaust flow, potentially leading to a reduction in power output and fuel efficiency.
Catalyst and Particulate Filter Blockage
In engines equipped with exhaust after-treatment systems, such as diesel particulate filters (DPFs) and catalytic converters, ash can lead to blockages. Unlike soot, which can be burned off during regeneration processes, ash remains in the filter, gradually accumulating over time. This can reduce the effectiveness of these components, increase backpressure, and eventually require costly maintenance or replacement.
Sensor Malfunction
Ash particles can coat and contaminate sensors within the engine and exhaust system, leading to incorrect readings and potential malfunction of engine management systems. This can affect everything from fuel efficiency to emission control systems, leading to increased emissions and potential failure to meet regulatory standards.
Oil Deterioration and Wear
Though zinc serves as an anti-wear additive, the resultant ash from its combustion can contribute to oil degradation over time. Increased ash content in oil can reduce its lubricity and overall effectiveness, leading to accelerated wear of engine components.
The bad side of ZDDP – sludge and varnish
It’s not just ash that’s a problem, ZDDP can contribute to the formation of sludge in hydraulic systems, sometimes called varnish. Understanding this phenomenon involves delving into the chemical reactions and operational conditions that may lead to sludge formation.
Conditions Leading to Sludge Formation
- Oxidation: Hydraulic systems, especially those operating under high pressures and temperatures, can accelerate the oxidation process of the oil. ZDDP is designed to be a sacrificial agent that reacts with oxidation products to protect the machinery. However, when it reacts with oxygen and other contaminants under these conditions, it can form by-products that contribute to sludge.
- Thermal Degradation: At high temperatures over about 120 Celsius, ZDDP can thermally decompose, leading to the formation of sulfides, phosphorus oxides, and other decomposition products. These compounds can react with the oil, additives, and contamination in the system to form sludge, which is a mixture of oil degradation products, spent additive and contaminants. It’s worth noting the whole system doesn’t need to be this high a temperature but just one local hotspot can cause this.
- Water Contamination: Water is a common contaminant in hydraulic systems and can have a significant impact on the stability of ZDDP. The presence of water can catalyze the hydrolysis of ZDDP, breaking it down into phosphoric acids, sulfides, and zinc salts. These breakdown products can interact with other elements in the oil to form sludge. Remember we discussed the reaction is reversible well this acid formation can propagate further oil degradation.
- Incompatibility with Other Additives: The interaction between ZDDP and other additives within the hydraulic oil can also lead to sludge formation. Additive incompatibility can result in chemical reactions that produce undesirable by-products, contributing to the overall sludge content in the system.
Implications of Sludge Formation
Sludge can have several negative implications for hydraulic systems:
- Reduced Efficiency: Sludge can restrict the flow of oil through the hydraulic system, reducing its efficiency and responsiveness.
- Component Wear: Particles in the sludge can act as abrasives, contributing to increased wear of hydraulic components.
- Blocked Filters: Sludge can clog filters and narrow passages, leading to increased maintenance requirements and potential system failures.
- Corrosion: Some sludge components can be corrosive, leading to increased corrosion of metal surfaces within the hydraulic system.
To mitigate the risk of sludge formation due to ZDDP and other factors, several strategies can be employed:
- Regular Oil Analysis: Monitoring the condition of the hydraulic oil can help identify early signs of oxidation, contamination, and additive degradation.
- Maintaining Optimal Operating Conditions: Keeping hydraulic systems operating within their recommended temperature and pressure ranges can reduce the rate of oil degradation.
- Proper Filtration and Water Removal: Effective filtration systems and water separators can minimize the contamination that contributes to sludge formation.
- Using High-Quality Hydraulic Fluids: Selecting hydraulic fluids specifically formulated to resist oxidation and thermal degradation can help reduce the likelihood of sludge formation.
In essence, while ZDDP plays a crucial role in protecting hydraulic systems from wear, its interaction with the operating environment requires careful management to prevent sludge formation. Through proactive maintenance and strategic formulation of hydraulic fluids, the negative effects of sludge can be minimized, ensuring the longevity and efficiency of hydraulic systems.
The bad side of ZDDP – zinc free requirements
There are specific scenarios where zinc or ZDDP might be detrimental, leading to the need for zinc-free lubricants. Let’s explore these scenarios.
Silver Components
Zinc additives can lead to corrosion and wear in components made of or containing silver. This is because zinc and its compounds can react chemically with silver under certain conditions, causing the silver to deteriorate. This is particularly concerning in high-performance applications where silver is used for its superior thermal and electrical conductivity, such as in bearings, bushings, and electrical contacts found in various machinery systems. The use of zinc-free lubricants in these applications helps preserve the integrity and functionality of the silver components, extending their service life and maintaining the machinery’s performance.
Food-Grade Applications
In industries where lubricants can potentially come into contact with food products, such as in the manufacturing, processing, and packaging sectors, there is a stringent requirement for food-grade lubricants. These lubricants must meet specific safety standards and regulations, such as those set by the U.S. Food and Drug Administration (FDA) or the NSF International. Zinc and other metals present a contamination risk in these settings, as they are not considered safe for ingestion in the quantities that might be found in conventional lubricants. Thus, zinc-free lubricants are required to minimize health risks and comply with safety standards, ensuring that the food products remain safe for consumption.
While zinc is a beneficial additive in many lubricating oils, its use is not universal. In applications involving silver components, zinc can cause damage through chemical reactions, necessitating the use of zinc-free lubricants to protect these parts. Similarly, in food-grade applications, safety standards restrict the use of zinc to prevent contamination, again making zinc-free lubricants the preferred choice. Understanding these requirements is crucial for selecting the appropriate lubricant for each specific application, ensuring both the longevity of machinery components and compliance with health and safety regulations.
The Balance of Concentration
The issues of ash, sludge, and zinc free requirements can make ZDDP a Jekyll and Hyde of additives. Hence, oil formulations are carefully balanced to provide necessary protection while minimizing adverse effects. For instance, Low-ash oils have been developed specifically for engines with sensitive exhaust after-treatment systems, reducing the risk of ash-related problems while still protecting engine parts from wear.
It’s worth noting that the effectiveness of ZDDP is not just about its presence in the oil but also its concentration. Too little, and its protective actions are insufficient; too much, and it can lead to undesirable deposits and affect the performance of catalytic converters in automotive exhaust systems. Thus, oil formulations with ZDDP are carefully balanced to provide optimal protection without side effects.
Conclusion: The Critical Role of ZDDP
ZDDP’s functionality as an anti-wear agent, ash cushion, metal deactivator and antioxidant makes it indispensable in lubrication. Its molecular structure and reactive nature underpin a mechanism of action that protects critical engine components from wear and oxidation, ensuring longevity and reliability. In the lab, we see evidence of its protective actions in the form of reduced wear particles and lower oxidation levels in used oil samples. It’s a reminder of the importance of chemical innovation in everyday technologies.
However, ZDDP although great does not have enough antiwear properties for high load, which introduces extreme pressure additives, but that’s a topic for another day. If you’re intrigued by how such chemical innovations can benefit your machinery or have further questions about lubricant chemistry, don’t hesitate to press the ‘Contact Us’ button. Our lab is always ready to delve deeper into the science that keeps the world moving smoothly.
