In standard PCMO formulations (ACEA A3/B4, API SP, SAE 5W-30/10W-40), Medium TBN Ca Alkyl Salicylate is used as the primary or co-primary detergent alongside Ca sulfonate. The salicylate contributes the detergent's antioxidant function and part of the TBN, while the sulfonate provides the bulk of the high-temperature ring-belt detergency and the primary TBN reserve. A typical ACEA A3/B4 PCMO package might use 1.5–2.5 wt% Medium TBN Ca Salicylate (TBN 100) + 1.0–2.0 wt% Medium TBN Ca Sulfonate (TBN 250) - the salicylate provides 1.5–2.5 mgKOH/g TBN plus phenolic-equivalent antioxidant activity; the sulfonate provides 2.5–5.0 mgKOH/g TBN and the primary detergency for Sequence VH and IIIG performance. The combined package meets ACEA A3/B4 with a balanced sulphur budget (salicylate adds ~0 wt% S; sulfonate adds ~0.02–0.04 wt% S from its sulfonate moiety).

In ACEA E9/API CK-4 HDEO formulations (S/A ≤1.0%, S ≤0.3%), Medium TBN Ca Alkyl Salicylate provides a sulphur-free TBN contribution that frees sulphur budget for ZDDP without consuming any of the finished oil S ≤0.3% limit. At 2 wt% treat rate of a TBN 100 grade, the salicylate contributes 2.0 mgKOH/g TBN and 0.41–0.48 wt% S/A - with zero S impact. This compares favourably to Medium TBN Ca Alkylphenate at similar treat, which would contribute comparable TBN but additionally 0.08–0.12 wt% S from its disulfide bridge. In long-drain HDEO where maintaining finished oil TBN above the condemn limit (typically 50% of new oil TBN) across 60,000–100,000 km intervals is critical, the salicylate's two-stage TBN delivery (chelate + colloidal reserve) provides sustained TBN maintenance through the full drain interval.

Medium TBN Ca Alkyl Salicylate is the most widely used salicylate grade in gas engine oil formulations. Gas engine oils typically target finished oil TBN of 10–20 mgKOH/g (depending on gas quality and H₂S/HCl content) with service intervals of 1,000–2,000 hours. The Medium TBN grade delivers sufficient TBN per treat rate to achieve the finished oil target while the salicylate's six-membered chelate ring provides superior resistance to displacement by NOₓ-derived nitrated compounds - the primary mechanism of detergent degradation in gas engine applications. Zero sulphur from the salicylate also prevents any contribution from the detergent to engine-out SOₓ emissions or exhaust catalyst sulphur poisoning in CHP units with emission abatement.

Medium TBN Ca Alkyl Salicylate is increasingly used in premium marine TPEO (trunk piston engine oil) formulations for medium-speed diesel engines operating on VLSFO/ECA fuels - the salicylate provides TBN and antioxidant function without adding sulphur to the lubricant, which is advantageous for VLSFO-compatibility where sulphur in the fuel is already low and the target BN of the TPEO is consequently lower (BN 25–40). In rotary compressor oils and industrial gear oils requiring 6,000–12,000+ hour service stability, Medium TBN Ca Alkyl Salicylate at 0.5–2.0 wt% provides alkalinity reserve, antioxidant supplementation, and surface-deposit prevention without the sulphur contribution that would occur with phenate alternatives.

Q: Why does Medium TBN Ca Alkyl Salicylate have higher Ca content (4–9%) than Medium TBN Ca Alkylphenate (3–9%) even though the TBN range (80–120) is lower than the phenate's (120–180)?

This reflects differences in the chemistry of overbasing and in how TBN is delivered by each class. In Ca alkylphenate, the phenate anion is highly efficient at stabilising large colloidal CaCO₃ micelles (high alkalinity per Ca²⁺), and the phenate's disulfide bridge also provides secondary AO without needing additional Ca. In Ca alkyl salicylate, the salicylate anion requires a greater molecular volume per TBN unit because the chelate ring ties up two coordination sites on Ca²⁺ more strongly - meaning fewer Ca²⁺ centres are "available" for colloidal CaCO₃ overbasing per unit mass of additive. The net result: Ca salicylate requires somewhat higher Ca% to deliver the same TBN level as Ca alkylphenate, but compensates with superior chelate stability, zero sulphur, and the antioxidant benefit from the ortho –OH group at no additional ash cost.

Q: Is Medium TBN Ca Alkyl Salicylate compatible with Group III and PAO synthetic base stocks?

Yes - Ca alkyl salicylate is fully compatible with Group I, II, III, and PAO base stocks, making it one of the more versatile detergents for synthetic lubricant formulation. The alkyl chain –(CH₂)ₙCH₃ (typically C₉–C₁₂) on the salicylate ring provides sufficient lipophilicity for good solubility in non-polar synthetic bases. In practice, the salicylate is generally more compatible with very non-polar Group III/PAO systems than Ca sulfonate at high treat rates - Ca sulfonate's colloidal CaCO₃ can occasionally cause haze or gel formation in ultra-low-polarity PAO blends, whereas the Ca salicylate chelate micelle tends to remain stably dispersed. Qualification testing (cold soak at −20°C, hot stability at 100°C for 72 hours, and centrifuge haze test) is recommended when using any overbased Ca detergent in Group III+/PAO formulations above 2 wt% treat rate.

Q: When should a formulator choose Medium TBN Ca Salicylate vs Medium TBN Ca Alkylphenate as the primary co-detergent with Ca sulfonate?

Choose Ca alkyl salicylate when: (1) finished oil sulphur S ≤0.3% is a binding constraint and the phenate's sulfur bridges would exceed the limit; (2) the application is a gas engine oil where NOₓ-derived compounds will degrade Ca-phenate coordination faster than Ca-salicylate chelate; (3) using Group III/PAO base stocks where salicylate's colloidal stability is superior; (4) exhaust catalyst compatibility (CHP, TWC, GPF) requires zero sulphur from the detergent. Choose Ca alkylphenate when: (1) sulphur headroom is available and the phenate's –S– or –S₂– secondary antioxidant (hydroperoxide decomposition) provides additional oxidative protection beyond what the salicylate's ortho –OH can deliver; (2) formulation cost is the primary driver (phenates are typically lower cost per unit TBN than salicylates); (3) the application benefits from the phenate's mild sulfur EP contribution (gear oils, industrial oils).

Q: What is the correct sulphated ash calculation for Medium TBN Ca Alkyl Salicylate, and how does it compare to Ca alkylphenate at similar TBN?

For Ca alkyl salicylate, the sulphated ash calculation follows the same formula as all other Ca detergents: Ca wt% × 3.40 ≈ sulphated ash wt% in the finished oil from that component. The distinction vs. Ca alkylphenate is that no sulphur S/A contribution exists from the salicylate (sulphated ash from S compounds = 0), whereas Ca alkylphenate contributes both Ca-derived ash (Ca% × 3.40) and additionally a small amount of sulphated ash from the sulphur bridges (S% × ~3.0 approximately). At equivalent Ca% and treat rate, the salicylate and phenate contribute very similar total sulphated ash (since the phenate's sulfur-derived S/A is small), but the salicylate contributes significantly less finished oil sulphur content - typically 0.05–0.12 wt% less S depending on the phenate grade and treat rate. This S difference, not the ash difference, is the primary reason to choose salicylate over phenate in S-constrained formulations.

Related: Low TBN Ca Alkyl Salicylate · High TBN Ca Alkyl Salicylate · Medium TBN Ca Sulfonate · Primary ZDDP · PIB Bis-Succinimide Dispersant