Zinc Dialkyldithiophosphate (ZDDP): Primary vs. Secondary Alkyl Chemistry, Tribofilm Kinetics, and Formulation Limits Under ILSAC GF-6/GF-7

The Tribochemical Mechanism: How Shear Stress Drives Sacrificial Phosphate Glass Formation

ZDDP has been the main additive in oils for boundary lubrication since it was made by Herbert C. Freuler in 1944 under U.S. Patent 2,364,284. Early models assumed that film growth was driven only by heat in the oil. Those early models could not explain why ZDDP is still able to form a durable film on the steel surface when the bulk oil is below 90°C.

Tribochemistry shows that the film reaction is mechanochemical and is driven by stress. The sliding asperities on the steel pass the shear stress to the ZDDP molecules that are adsorbed on the surface, and this happens when the metal parts are in boundary contact. Spikes and Tysoe used the modified Bell-Eyring equation for this reaction rate:

In this equation, the factor A is the pre-exponential constant, k_B is the Boltzmann constant, and T is temperature. The term E_a is the activation energy from heat when there is no stress, and it is 90 to 120 kJ/mol for secondary alkyls or 130 to 160 kJ/mol for primary grades. The stress term has the shear stress at the interface, known as tau, and an activation volume of 0.05 to 0.15 nm³.

The peak shear stress is about 250 MPa when the Hertzian pressure at the contact is above 1.5 GPa, and the activation barrier is up to 50% lower because of the mechanical work from interfacial stress and activation volume. The chemical cleavage is then active at 80 to 110°C on the sliding asperities. The bulk oil in the sump is stable at that level of heat, and the fluid does not break down when it is out of the sliding contact.

The data from TEM and XANES show that the tribofilm has three layers from 20 to 120 nm. The first layer at the reaction interface is 1 to 5 nm thick, where nascent iron on the asperities reacts with the organosulfur fragments to form microcrystalline iron sulfide of FeS. That step also forms iron oxide of Fe3O4, so the steel is passive against wear.

The bulk core in the middle is 20 to 100 nm thick, and it is an amorphous glass of zinc and iron polyphosphate. The orthophosphates and pyrophosphates in this core condense into metaphosphate chains that have more than 20 units, and the glass has a modulus of 20 to 40 GPa when it is under load in the contact. Because of this structure, the polyphosphate layer can shear plastically on the metal surface before the sliding asperities on the steel can weld together.

The top layer is a sacrificial cushion of fluid that is 5 to 20 nm thick. It has oligomers of neutral ZDDP that are adsorbed on the surface, and they are able to take the shock and replenish the glass below when the film is under load. Formulators can choose antiwear additives and ZDDP components so that the blend will match the shear and the heat in the equipment.

Primary vs. Secondary vs. Aryl ZDDP: Chemical Kinetics and Alkyl Cleavage Pathways

The synthesis of ZDDP uses P₂S₅, which is phosphorus pentasulfide, with four equivalents of an ROH alcohol. That reaction is under nitrogen and forms an intermediate of dialkyldithiophosphoric acid, which then has a neutralization step with ZnO, or zinc oxide, to make the salt in the batch. The structure of the alcohol in the feed sets whether the cleavage goes by β-elimination at low energy or by nucleophilic substitution:

Secondary Alkyl ZDDP: β-Elimination and Rapid Low-Temperature Activation

Alcohols like 2-propanol, 2-butanol, or 4-methyl-2-pentanol are used for the secondary grades, which can break down by unimolecular β-elimination because the transition state is stable with alkyl groups on the carbon and E_a is 90 to 120 kJ/mol. The onset is at 130 to 170°C, so the film of polyphosphate can nucleate fast during break-in on a dyno. It is also active in flat-tappet valvetrains, but the stability in heat is lower and the hydrolysis is faster when water or moisture is in the oil:

Primary Alkyl ZDDP: Nucleophilic Alkyl Transfer and High-Temperature Durability

Primary alkyl ZDDP is made from 1-butanol, 1-octanol, or 2-ethylhexanol, and it has no easy path for β-elimination, so the cleavage is active at 180 to 220°C where E_a is 130 to 160 kJ/mol. This step is an intermolecular transfer by an SN2 path that moves the alkyl group from oxygen to sulfur. The products are O,S,S-trialkyl tetrathiophosphates and condensed pyrophosphates of zinc, and the primary grade is stable against heat and water in heavy-duty diesel engines and in hydraulics at high pressure above 90°C so that no sludge will drop out:

Aryl ZDDP: Thermal Extremes and Film-Forming Limitations

Aryl ZDDP is made from alkylated phenols, and it has a direct bond from the aromatic ring to the oxygen. The resonance in the ring is strong enough that there is no SN2 displacement and no cracking of the olefin, so the onset of cleavage is above 230°C and the film kinetics are very slow when the oil is below 200°C. This aryl grade is used only in niche aviation and turbine oils.

The Multi-Functional Coupling: Peroxide Decomposition and Cam Lobe Wear Protection

An ashless agent for antiwear cannot replace ZDDP on its own, because the protection on the steel is coupled to the breakdown of hydroperoxides in the oil. The base stocks of hydrocarbon in an engine with internal combustion will go through auto-oxidation and form ROOH, or alkyl hydroperoxides. Those peroxides are active against the fresh iron that is bare during rubbing at the boundary, and this oxidizes the elemental iron to Fe³⁺ so the grain boundaries are degraded and the cam lobes and tappets can spall or scuff.

The corrosive cycle is stopped by ZDDP when it is in the blend as a secondary antioxidant. The neutral ZDDP has a stoichiometric reaction with the hydroperoxides to form a basic salt of zinc, a disulfide, and an alcohol, and then trace moisture can hydrolyze that basic salt. That step releases an intermediate of dialkyldithiophosphoric acid, which is able to catalytically reduce the remaining hydroperoxides into alcohols that are inert:

Hindered phenols and diphenylamines can scavenge only the ROO• radicals of propagation. The hydroperoxides are still intact when the oil in the sump is at 80 to 110°C, and they will corrode the metal if they are not broken down. ZDDP is able to neutralize these corrosive oxidants and put down a protective film of polyphosphate at the same time.

Navigating the Regulatory Phosphorus Cap: ILSAC GF-6, GF-7, and Low-SAPS Formulations

The Catalyst Deactivation Mechanism and ILSAC GF-6/GF-7 Mandates

The use of ZDDP in oil is limited by mandates on exhaust emissions. Volatile species of phosphorus from the ZDDP can enter the chamber of combustion through blow-by in the crankcase when the engine has direct injection of gasoline. That phosphorus will condense on the catalytic converter in the exhaust stream and will react with the washcoat to form a glassy pyrophosphate of metal, so the active sites of platinum, palladium, and rhodium are blinded.

The 23rd Edition of API 1509 has strict caps on the phosphorus in an oil. When a blend is under ILSAC GF-6A or GF-6B from May 2020, the finished oil has 600 to 800 ppm of phosphorus, which is 0.06 to 0.08 mass %, and it has at least 81% retention against volatility in Sequence IIIH under ASTM D8111. Those same limits of 0.08 mass % and 81% retention are still in the GF-7A and GF-7B rules from March 2025.

Those GF-7 rules have tighter limits on the wear of the timing chain in Sequence X and on the retention of fuel economy in Sequence VIE and VIF. Oils for diesel engines in heavy-duty service are under API CK-4 or FA-4 instead. The phosphorus in those blends can be up to 0.12 mass %, or 1,200 ppm, when the sulfated ash in ASTM D874 is at or below 1.0 mass %.

The Mathematical Treat Rate Ceiling and Element Balancing

Formulators can find the treat rate for the ZDDP when they have the target balance of phosphorus in the blend. The ceiling on the treat rate is about 1.08 wt% under the 800 ppm, or 0.080 wt%, GF-6 limit when a primary grade like CheMost SPZS-S1 has 7.4 wt% of phosphorus. The treat rate is then about 0.88 wt% and the budget for the ZDDP is 650 ppm, or 0.065 wt%, when a secondary additive like an organomolybdenum that is MoDTP puts in 150 ppm, or 0.015 wt%, of phosphorus:

A blend that must balance the wear on the valvetrain within the 800 ppm limit can use a mixed grade like primary-secondary alkyl ZDDP SPZS-M1. It has fast activation on the steel. This grade also has low volatility and can stay in the sump when it is in the engine.

Formulation Antagonism and Synergy: Interfacial Interactions

Antagonism with Overbased Detergents and Ashless Dispersants

An additive package in an oil can have up to 25 wt% of total chemistry, so there is competition at the metal interface for the ZDDP. The polar micelles of the detergent can push the neutral ZDDP off the asperities when overbased sulfonates of calcium or magnesium are in the blend, and the colloidal cores of CaCO₃, or calcium carbonate, will neutralize the acidic precursors of dialkyldithiophosphoric acid too early. The nucleation of the polyphosphate glass is then up to 300% slower, so formulators will balance the basicity of the detergent with neutral carboxylates or with fractions of secondary ZDDP.

Dispersants of polyisobutylene succinimide, which is PIBSI, have basic atoms of amine nitrogen in their structure. Those nitrogen atoms can coordinate with the cation of Zn²⁺ in the ZDDP because that ion is a Lewis acid, and this keeps the ZDDP in the hydrocarbon phase so the adsorption on the surface is slower. Formulators will control the ratio of nitrogen to phosphorus, or N to P, or they can use dispersants with borated heads where the amines are passive.

Synergy with Organomolybdenum Complexes (MoDTC / MoDTP)

A blend that has ZDDP with an organomolybdenum like MoDTC has a strong synergy in tribochemistry, because the MoDTC breaks down under sliding shear and forms sheets of lamellar MoS₂, which is molybdenum disulfide, in the growing glass of zinc and iron polyphosphate. A standard film of polyphosphate from ZDDP has a coefficient of friction, or μ, that is 0.10 to 0.12. The friction at the boundary is lower at about 0.04 to 0.06 when 0.5 to 0.9 wt% of MoDTC and 0.65 wt% of ZDDP are in the oil.

That drop in μ is from shearing between the lamellar sheets in the film. Formulators can then keep the phosphorus dose at 600 to 700 ppm in the finished blend, and the oil will still pass the wear limits in Sequence IVB. It can also meet the rules on fuel economy in Sequence VIE.

Industrial Application Selection: Hydraulic Systems, Gear Oils, and Greases

Mobile & Industrial Hydraulics: Managing Thermal Varnish and Hydrolytic Stress

Industrial and mobile hydraulics are under DIN 51524 Part 2 and Part 3 for HLP and HVLP oils, or they are under ISO 11158 for HM and HV grades. Those circuits also have to meet the rules in Denison HF-0 from Parker Hannifin and in Eaton-Vickers M-2950-S when the fluid in the compact system is above 90°C, and a grade of ZDDP with secondary alkyls has a severe risk of failure there. The secondary ZDDP can dissociate when the oil is above 90°C and will drop out the cations of zinc as an insoluble sludge of zinc pyrophosphate.

That sludge will coat the coolers for the oil, and it can stick the proportional valves of the servo that have tight clearances of 2 to 5 μm when it is in the circuit. Any water that gets into the system will make the hydrolysis faster. That step will form acidic species in the fluid, and they can attack the slippers of bronze and the plates of the valves in the axial pumps that have pistons.

Blenders can stop the varnish and the corrosion on yellow metals when they use pure primary-alkyl ZDDP grades like CheMost SPZS-S1. That primary structure is stable against cleavage from heat up to 210°C. It will also pass the test for hydrolytic stability in ASTM D2619 where the loss of copper is below 0.2 mg/cm², and it can meet the rules of RDE 90235 from Bosch Rexroth without sediments that will clog the filters.

Industrial Gear Lubricants and Extreme-Pressure Greases

Drives with enclosed gears under DIN 51517-3 for CLP oils and NLGI Grade 2 greases have high stresses at the contact from shock loads. When the oil is for hypoid gears under API GL-4 or GL-5, a secondary or mixed ZDDP is in the blend at 1.5 to 3.5 wt% with SIB, which is sulfurized isobutylene. That gear blend can pass a weld load above 2450 N in the EP test of ASTM D2783 with Four-Ball rigs, and it can meet stage 11 or higher for FZG scuffing in DIN under ISO 14635-1 when it is under load.

The ZDDP in an NLGI 2 grease is an antioxidant for antiwear. It is in synergy with fatty esters that are sulfurized in the thickener, so the blend can replace the older additives of lead and antimony. The matrix below has the data for the primary grade of CheMost SPZS-S1 and the primary-secondary grade of SPZS-M1, and they are compared against Lubrizol 1395 for primary ZDDP and Afton HiTEC 7169 for secondary ZDDP.

Physical-Chemical Property Matrix & International Benchmark Comparison

The primary grade of CheMost SPZS-S1 is equal to the benchmark of Lubrizol 1395 in its thermal endurance. Its decomposition onset is at 195 to 210°C, and it is passive on copper with a 1a rating in the test of ASTM D130. This primary grade has 7.4 wt% of phosphorus in the fluid where Lubrizol 1395 has 9.5 wt%, so blenders will have more room for the treat rate under the rules of GF-6 and GF-7.

The mixed grade of CheMost SPZS-M1 has a high capacity for antiwear and extreme pressure on steel. It has a wear scar of 0.38 to 0.42 mm in ASTM D4172 and a weld load at or above 2450 N in ASTM D2783, which is equal to what is found for Afton HiTEC 7169. The flash point of SPZS-M1 is at or above 180°C, so it is safer in operations than HiTEC 7169 where the flash point is at 140°C.

Formulator Troubleshooting & Frequently Asked Questions (FAQ)

Can modern low-zinc GF-6 engine oils be safely used for flat-tappet camshaft break-in?

They are not safe for the break-in of flat-tappet camshafts in an engine. Contacts at the boundary on those lobes have stresses above 1.5 GPa, and modern GF-6 oils cap the phosphorus at 800 ppm and use a primary ZDDP that is slow to cleave under the volatility limits in ASTM D8111. That film is too slow to grow on the steel, so formulators will specify and will use break-in oils with a secondary ZDDP that has 1,200 to 1,600 ppm of phosphorus.

What is the root cause of hydraulic filter clogging in zinc-based hydraulic fluids?

The clogging of filters in hydraulic systems is from heat and from water that is in the fluid. When the oil is above 90°C, secondary ZDDP will dissociate and will drop out cations of zinc as an insoluble sludge of zinc pyrophosphate, but any water in the circuit will make basic detergents react with the zinc and will form sediments. Blenders can stop that plugging of the filters when they use a pure grade of primary ZDDP and succinimide dispersants that do not react with the zinc.

How does ILSAC GF-7 impact ZDDP treat rates compared to GF-6?

The ceilings on the treat rate of ZDDP are unchanged when an oil is under the rules of ILSAC GF-7. This standard keeps the phosphorus limits of 600 to 800 ppm from GF-6, but it has tighter rules for the wear of the timing chain in Sequence X and for the retention of fuel economy in Sequence VIE and VIF. Formulators cannot add more phosphorus under the cap of 800 ppm, so they will have to balance the ratio of primary to secondary alkyls and will use MoDTC for synergy in friction.

Why does secondary ZDDP provide superior wear protection on cam lobes despite lower thermal stability?

The protection against wear on a cam lobe is based on film growth, and it is vital for engine life. Secondary ZDDP cleaves by beta-elimination where the activation energy is 90 to 120 kJ/mol at 130 to 170°C, and primary ZDDP is in an SN2 pathway where the barrier is 130 to 160 kJ/mol at 180 to 220°C. The secondary grade will nucleate a polyphosphate glass within seconds on the sliding steel, so it can arrest adhesive wear before damage spreads on the cam lobe.

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