Introduction: The “Underwater War” on Ship Bottoms and the Billions of Dollars in Annual “Biofouling Removal” Bills
The hull of an ocean-going merchant ship—after being moored at sea for >several hours, larvae of barnacles/oysters/algae/tube worms begin to “settle” on the hull. Once attached—growth rate is astonishing (barnacle diameter >1cm/month)—within several months to a year—the hull is covered by a “biofouling layer” of thickness >several centimeters—hull fluid resistance increases >10-30%—fuel consumption increases >15-40%—global shipping industry consumes >100 million tons more fuel annually due to biofouling (>20 billion USD)—while emitting >300 million tons more CO₂. Anti-fouling Coating is the “first line of defense” for energy saving and emission reduction in the shipping industry. From the first generation TBT (organotin—globally banned in 2008) to the second generation SPC (self-polishing—Cu₂O slow release) to the third generation Foul Release (siloxane elastomer—no biocide)—the evolution of anti-fouling technology is a continuous process of balancing environment and performance.
I. Comparison of Three Generations of Antifouling Technologies
| Generation | Technology | Antifouling Mechanism | Biocide | Duration (years) | Cost (USD/L) |
|---|---|---|---|---|---|
| First Generation | TBT-SPC (Tributyltin Self-Polishing / Globally Banned) | TBT strong biocide / non-selective — all marine organisms are poisoned | TBT (highly toxic / environmentally persistent) | 5-7 | — |
| Second Generation | Cu₂O-SPC (Cuprous Oxide Self-Polishing / Mainstream) | Cu⁺ slow release — poisons larvae — selective (barnacles > oysters > algae) | Cu₂O (>30%) + auxiliary biocide (ZnPT / Zinc Pyrithione) | 5-7 (ocean-going) / 3-5 (coastal) | 30-80 (mainstream) |
| Third Generation | Foul Release (Siloxane Elastomer / Non-biocidal) | Low surface energy (<20mN/m) + low elastic modulus (15 knots) | Zero biocide | 5-10 (ocean-going / high speed) | 80-200 (high-end) |
FAQ
Q1: The “self-polishing” chemistry of SPC coatings — how does Cu₂O “continuously release” in seawater?SPC resin (copper/zinc acrylate copolymer) in seawater — (1) Cu²⁺ in copper acrylate undergoes ion exchange with Na⁺ in seawater, the resin changes from water-insoluble → water-soluble (sodium acrylate) — the surface resin layer slowly dissolves and peels off, exposing the “fresh” underlying Cu₂O-containing layer; (2) Cu₂O dissolves in seawater — releases Cu⁺ (biocidal) — the half-life of Cu⁺ in seawater (> several hours — in nearshore/ports — Cu⁺ accumulates in sediments — long-term toxicity to benthic organisms) — Cu₂O is facing restrictions or bans in multiple regions worldwide (Sweden/Denmark/US West Coast) and is the greatest environmental risk of SPC coatings.
Q2: Why can the Foul Release coating prevent fouling while being “biocide-free”?FR (siloxane elastomer/PDMS-based) surface——(1) Extremely low surface energy (<20mN/m)——the adhesive proteins of barnacles/oysters/algae (mussel byssus/barnacle cement) have extremely weak adhesion on FR surfaces (shear strength 500kPa on SPC); (2) Low elastic modulus (<1MPa)——the "elastic peeling" mechanism of bio-adhesives on FR surfaces——when the FR surface is subjected to force (water shear)——the elastic deformation of FR causes stress concentration at the bio-adhesive interface——the adhesive layer is “torn off” from the FR surface. FR does not kill organisms and is currently the most environmentally friendly antifouling technology.
Q3: The “Achilles’ heel” of Foul Release coating—failure at low speed?FR’s antifouling relies on water flow scouring (>15 knots = approx. 28 km/h). For vessels at low speed (<10 knots) or long-term berthing (>2 weeks)—insufficient water flow—bioadhesives bond to the FR surface; no water flow shear—organisms cannot detach. Low-speed vessels and long-term berthed vessels—FR’s antifouling performance is weaker than SPC—need to use a “hybrid” system of FR + biocide.
Q4: What are the differentiated antifouling mechanisms of Medetomidine (veterinary drug/antifouling) and Selektope (capsaicin analog)?Medetomidine (formerly known as Selektope)activates the α2-adrenergic receptors of barnacle larvaecausing abnormal“swimming behavior”in barnacle larvae, preventing normal attachment—rather than “killing” them—thus effective at pg/L concentrations (extremely low / >1000 times lower than Cu₂O)—with very low toxicity to non-target marine organisms (fish/shrimp/crab). Selektope (capsaicin analog / Capsaicin)activates the TRPV1 ion channel (capsaicin receptor) of barnacle larvaethe larvae perceive “pain” and exhibit a“escape response” (Swimming Escape)actively detaching from the coating surface—also effective at pg/L concentrations—non-toxic, only causing “discomfort”. Both are“behavioral regulation”rather than “killing”, representing a “revolutionary” advancement in antifouling technology.
Q5: IMO AFS Convention (2008) — Historical Lessons from the Global Ban on TBT?TBT (Tributyltin) — (1) Extremely high toxicity (>1ng/L causes sexual deformity in oysters / female individuals develop male characteristics — population collapse); (2) Extremely long environmental persistence (half-life in sediments > decades) — TBT caused “total crop failure” of oysters/shellfish in coastal areas worldwide in the 1980s. The 2008 IMO AFS Convention’s global comprehensive ban on TBT coatings is one of the largest environmental treaties for the global shipping industry. The historical lesson of TBT: the environmental persistence of antifouling coatings matters more than toxicity — a “low-toxicity but persistent” biocide is more harmful to the environment than a “high-toxicity but rapidly degrading” biocide.
Q6: Cu₂O—Is it “repeating the TBT story”?Cu₂O releases Cu⁺ in seawater—in nearshore and port areas—sediment Cu concentration (>50-500mg/kg) > background value (<10mg/kg)—causing chronic Cu poisoning in benthic organisms (Cu is an essential trace element—but toxicity begins at >50μg/L). Sweden (2018) and California, USA (2025) have begun restricting the use of Cu₂O antifouling coatings on recreational boats. Restrictions on Cu₂O for commercial ships are still under discussion—Cu₂O is expected to be gradually replaced within the next 10-20 years—FR and biological antifoulants are the main alternative directions.
Q7: “Whole-life economic efficiency” of marine antifouling coatings—SPC vs FR?SPC—lower initial cost (30-80 USD/L)—but requires dry-docking for recoating every 2.5-5 years + treatment of waste coating removal (contains Cu₂O—cannot be discharged directly—requires professional treatment—extra cost). FR—higher initial cost (80-200 USD/L)—but longer service life (5-10 years) + zero waste coating treatment (no biocide). On a whole-life “cost per vessel·year” basis, FR is slightly higher than SPC (>10-20%)—but as Cu₂O restrictions tighten—SPC’s compliance costs will rise—and FR’s whole-life economic efficiency will gradually overtake it.
Q8: Impact of “polishing rate control” of antifouling coating being too fast/too slow?SPC’s polishing rate (Polishing Rate)——the coating thickness polished per year (μm/year). Too fast (>30μm/year)——SPC coating is depleted prematurely (depleted within >5 years)——requires early dry-docking for recoating. Too slow (<10μm/year)——insufficient surface resin erosion rate——slow renewal of Cu₂O's "fresh" release surface——reduced antifouling efficacy. Optimal polishing rate 10-20μm/year (>5-7 year service life + good antifouling efficacy)——regulated through the acrylic copper/zinc ratio of the resin and hydrophilic/hydrophobic balance (HLB) control.
Q9: Wear resistance challenges of Foul Release coating in “ice-zone navigation”?Polar/ice-zone navigation——friction and impact between floating ice and the ship bottomFR (siloxane elastomer) has low mechanical strength (tensile strength 15MPa) and is highly prone to scratching / peeling and failure in ice zones. Antifouling coatings for ice-zone vessels——require the use of high-strength SPC (higher crosslinking + more wear-resistant) or the addition of a “wear-resistant protective layer” (glass flakes + epoxy) on the outermost layer.
Q10: The future of antifouling coatings—the ultimate form of “completely non-toxic + completely maintenance-free”?(1)Shark-skin mimic (Sharklet/Riblet)—micrometer-scale grooves (>3μm)—inhibits barnacle larvae’s antenna detection and attachment purely physical antifouling; (2)Self-healing FR—siloxane elastomer’s self-healing microcapsules repair themselves after abrasion in ice zones—no need for dry-dock repainting; (3)Underwater cleaning robot (Hull Cleaning Robot/ROV) replacing the “coating antifouling” with a “mechanical cleaning” solution robots periodically clean fouling organisms from the hull bottom—hull requires no antifouling coating—achieving “zero biocide + zero coating”. The ultimate competition in antifouling coatings: “chemical antifouling” vs “mechanical cleaning”—the latter is growing rapidly.
Related Reading
Summary
Three generations of marine antifouling coating technologies — SPC (Cu₂O controlled release — 5–7 year cycle) / FR (siloxane elastomer — zero biocide — water flow shear) / biological antifoulants (Medetomidine/Selektope — behavioral modulation — pg/L level). Cu₂O faces global restrictions — FR and biological antifoulants are the future direction. Kexin New Materials focuses on the frontier of antifouling coating technology — providing clients with eco-friendly antifouling solution consulting and product support.