Antifouling

Recent scientific papers have focused on EALs’ possible roles as antifouling agents. Marine biofouling refers to the adhesion of organisms onto ship surfaces, particularly the hull. Of chief concern are the larvae of mussels, barnacles and other bivalves, which proliferate on the bottom of boats and turn streamlined surfaces into rocky colonies. The consequent increased weight and hydrodynamic drag increases fuel consumption by up to 41%, equating to about 20 million tons per year in greenhouse gas emissions. 

Historically, ships have been coated with antifouling agents containing such biocides as copper, arsenic and tributyltin (TBT). These compounds are toxic to macrofouling bivalve larvae and act by disrupting the mitochondria of their liver cells. Antifouling systems containing TBT coatings have proven the most successful and account for 70% of the world’s ships today. 

However, TBT is indiscriminately toxic to not just mollusks but also to marine life in general. Due to the chemically and physically turbulent conditions of the ocean, even water-insoluble biocide coatings would crumble and eventually contaminate the environment. Huang Guolan and Wang Yong found significant bioaccumulation of TBT throughout the food chain via absorption by phytoplankton. Moreover, Derek Ellis and Agan Pattisina reported genetic defects in the genitalia of mussels around the globe, indicating widespread TBT contamination. Prompted by the risk to sea life and human consumers, the International Marine Organization (IMO) banned the use of TBT coatings in 2008. 

Shahrouz Amini and team’s work was the first paper to discuss the use of slippery liquid-infused porous surfaces (SLIPs) to deter marine biofouling. 

At the time, there were two prevailing fabrications of liquid infused surfaces. The first, dubbed by the authors as the “two-dimensional” option, involved creating a microscopically rough surface, which evenly distributes and retains lubricants via capillary forces. The second involved permeating a three-dimensional polymer gel network with a lubricant. 

To create their 2-D surface, Amini’s team deposited layers of silicon nanoparticles onto glass, layer by layer (LBL). For the 3-D gel, the authors used polydimethylsiloxane (PDMS). They infused both surfaces with silicone oil, then compared the rates of mussel adhesion to the commercially popular antifouling coatings Intersleek (IS) 700 and 900. 

The authors performed a preliminary assay, testing the mussels’ ability to attach to each surface in insolation. While a small number of plaques formed on the oil-infused LBL and IS-treated surfaces, PDMS exhibited no biofouling at all. In a multiple-choice assay, in which mussels were given the choice between all surfaces, IS-700 was observed to have the greatest frequency of plaque formation (75 ±10), followed by LBL and IS-900 (30 ±10). The authors reported only five mussel attachments to infused PDMS, which they attribute more so to coating defects. 

Amini’s team performed further field tests, which paralleled their laboratory results. They also analyzed the adhesion strength of byssal threads and the mussels’ behavior when contacting each surface. The measured adhesion strength to PDMS was smaller by a factor of two and five compared to IS-700 and IS-900, respectively. Moreover, it was observed that gel networks—owing to their low elastic modulus—will bend in reaction to a probing mussel. Not only did the mussels have to bypass the silicone oil lubricant, but their feet also had to dig deeper to detect a stiff surface to attach on. When touching a treated PDMS surface, the mussels often immediately retracted their tendrils, opting to attach to their own shells or nearby substrates instead.

Amini’s work represented a breakthrough moment of SLIPs as a biofouling deterrent. However, more research was needed to improve the biocompatibility of PDMS networks, as well as their long-term stability and producibility on an industrial scale.  

Recent studies have built upon SLIPs technology, aiming to replace the silicone lubricant with even more eco-friendly alternatives. 

Snehasish Basu and team opted for non-toxic biolubricants fabricated with fatty oleic acid (OA) and its ester derivative methyl oleate (MO). The authors base this choice on Ji-Young Kang and team’s report that oleamide is the active antifouling agent on mussel shells themselves, inhibiting algae growth. Moreover, oleic acid is abundant in nature, existing in many vegetable fats and agricultural products, making it more industrially feasible. It was demonstrated that MO had a surface tension closer to that of PDSM than OA, demonstrated in its stronger capillary bonds to the gel network. The authors further improved the infusion of MO lubricant by first UV-treating the PDSM to create a more uniform surface. Mussel assays performed under laboratory conditions demonstrated similar antifouling performance to synthetic fluorinated lubricants. Basu’s team’s results revealed fatty acid biolubricants’ promise in SLIP systems to combat marine biofouling. 

In 2021, Eunseok Seo and team proposed erucamide as a further improvement over oleates in SLIPs systems. Erucamide is a fatty amide naturally present in many animals. Most relevantly, it exists in the mucosal films on fish skin, where it helps reduce drag and prevent biofouling by algae. The authors noted that erucamide is more hydrophobic than oleamide, which improves its stability in water. Erucamide’s intensely nonpolar character also means it mixes well with the similarly hydrophobic PDMS network. Seo’s team confirmed its hypothesis with a 5.5-month field test in real marine environments, which revealed both improved drag resistance and antifouling performance. 

EAL applications in SLIPs systems are novel. Instead of merely reducing the friction of contact-surfaces in mechanical systems, lubricants can find a whole new economic niche as biological deterrents in the boating industry. 

Stern Tubes

Perhaps the greatest challenge to EALs’ performance lies in the stern tube of boats. The stern tube is a hollow tube which connects the engine of a ship to its propellers. Relevantly, it houses the bearings that support the propeller shaft, which require copious amounts of lubrication. The stern tube is also an extreme environment with high frictive, shear and oxidative stress, and thus provides a large proof of efficacy for EALs. 

The conventional lubricants in the stern tube are mineral oils. Though cost-effective and functional, constant contact between the stern tube and the ocean means that mineral oils readily leak into the environment, posing a risk to both human and marine life, according to Jiaojun Deng and team. As such, regulatory bodies like the IMO have advocated the use of eco-friendly alternatives. 

In 2021, Jerzy Kowalski and team assessed EALs in stern tube bearings, comparing their performance to mineral oils under a wide range of load conditions and degrees of wear. Previous studies either examined only the theoretical tribological characteristics of EALs, or they observed EAL performance on bearings too narrow for actual stern tube usage. The authors tested two EALs, with viscosity grades of 100 or 150. Operating loads of 0.5 to 1 MPa were tested, along with speeds of 1 to 11 rev/s. In addition, the authors simulated bearing wear by increasing the clearance circle of the bearing by increments of 0.1 mm. They report no statistically significant difference in the performance of EALs compared to mineral oils across a myriad of standards—pressure distribution of the oil, carrying capacity and friction coefficient. In short, EALs appear to function as a direct substitute for mineral oil in the stern tube bearing of ships. 

However, Kowalski’s team noted that the most determining factor of EAL performance in the stern tube is its viscosity. Compared to mineral oils, the density of EALs tend to be much greater at the same viscosity. The scope of the paper does not include how this increased density may affect the smoothness of operations nor the degradability of EALs in field environments. 

Sam Davidson corroborated Kowalski’s findings. This paper compared the performance of four EALs to a mineral oil using a bearing test rig closely mimicking the extreme conditions of the stern tube. The test rig accounted for bearing load, speed and operating temperature. Ultimately, no difference between the tribological performance of EALs and mineral oils was reported. The effect of metal catalysts on the oxidation of EALs was also studied. Biolubricants were found to be susceptible to corrosion by tin. The author concluded that biolubricants are a feasible replacement for mineral oils in the stern tube, though more must be done to ensure long-term stability under oxidative stress. 

Marek Vecer and team formulated a custom EAL with a saturated fatty ester as a base (92 wt%). A complex mix of polymers was added, each known to increase the stability of lubricants under environmental stress—for instance, inactive linear sulfur polymers to increase extreme pressure resistance, phosphorus and nitrogen for anti-wear, and corrosion inhibitors, amongst other additives. The new lubricant, dubbed N-EAL-A1X, possessed a higher viscosity index than both mineral oil and commercial biolubricants, retaining high viscosity even across a broad range of temperatures. N-EAL-A1X also sustained good performance under high shear stress and possessed similar wear to mineral oils. Overall, Vecer’s team’s findings represent a step in tailoring EAL properties for stern tube environments. 

Interest in EAL applications in stern tubes has swelled significantly in the past few years. Though the literature has proven that biolubricants can match the tribological properties of mineral oils, researchers are just beginning to formulate EALs that can withstand the extreme environments of stern tubes. 

In conclusion, the application of EALs in ships is a budding field of scientific interest. Biolubricants and other eco-friendly alternatives may find a niche in anti-fouling systems as well as in the bearings of stern tubes. These innovations promise to open new doors in the lubricants industry while helping to preserve our planet.   

Raj Shah is a director at Koehler Instrument Company in New York, where he has worked for more than 25 years. He is an elected Fellow by his peers at IChemE, AOCS, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. Shah is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, U.K. He can be contacted at rshah@koehlerinstrument.com

Bob Fang is part of a thriving internship program at Koehler Instrument company in Holtsville and is a student of Engineering Chemistry at Stony Brook University, Stony Brook, New York.