Can Copper Antifouling Paint Be Kind?
Posted by Darrell Nicholson at 01:37PM - Comments: (0)
September 25, 2012
As we point out in the October 2012 of Practical Sailor, cuprous oxide (copper) still rules the roost when it comes to long-term antifouling protection, with hard paints and ablative paints fairly evenly matched for durability. For those who care about reducing their impact on the ocean, this raises a question: If we want to stick with copper (as opposed to an eco-friendly, copper-free antifouling), which type of paint—hard or ablative—is easier on the environment?
With respect to bottom paints, marine biologists are most concerned about the effects of copper leaching, so it would seem a simple matter to discern which paints have the most copper in them. However, when comparing copper paints, it is important to note that the copper percentages listed on the can are usually calculated by weight. Which means that a paint that has heavier solids and resin may actually have more copper than a “thinner” paint that boasts a higher copper percentage. While some low-copper freshwater antifouling paints clearly have less copper than saltwater blends boasting 65-percent copper, there may only be a small difference in the volume of cuprous oxide in a paint that advertises 39-percent copper versus one that shows 45 percent.
To see whether we could nail down a more definitive answer this question, we turned to Interlux’s Jim Seidel (who, as far as I can tell, talks about bottom paint in his sleep). While he wasn’t able to provide a clear-cut answer to our question, Seidel did provide some extremely useful insight into the way bottom paints work, which will help you make an informed decision when choosing a bottom paint—whether or not eco-friendliness is your top concern.
When an antifouling paint goes through the registration process, the maker has to prove to the EPA that it is effective. One of the ways paint makers prove this is by indicating what registered biocides are in the paint. Then, the paint maker has to provide a leach rate, which measures how much biocide leaches out of the paint. This is expressed in micrograms per square centimeter per day. The scientists at EPA have enough data on some biocides that leach-rate testing is no longer required for certain biocides like cuprous oxide when submitting new registrations. In Canada, however, it is required for every paint in every color.
So the key is to establish a level at which the biocide coming out of the paint film will control fouling. Cuprous oxide is an excellent biocide for all types of fouling, but it requires a much higher leach rate to control slime and algae than it does to control barnacles. For instance, to control so-called “hard” macro-fouling—barnacles, mussels, worms and the like—it might take 3 micrograms per square centimeter per day of leaching copper; but for micro-fouling—slime and algae—it might take 12 micrograms.
To control both macro and micro fouling, you would make a fairly soft paint with a high load of cuprous oxide. With cuprous oxide currently selling at somewhere between $4 and $4.25 per pound, this paint would be expensive and not last very long.
The ideal paint will release the correct amount of biocide at the right time to last a long time. The way to do this is to formulate a paint that will control the leach rate of the biocide plus be somewhat self-cleaning so that the biocide is released at the correct rate.
This leads to a discussion of ablative paints. Ablative means "to wear away," and there are several ways paint wears away. The most basic ablatives are soft sloughing paints that wear very quickly, with the wear-rate varying with the temperature of the water. The warmer the water, the faster it wears away.
Other paints use more complex formulas of controlled depletion polymers or controlled solubility copolymers. Generally speaking, the maker blends rosin (solids) and resin (gummy liquid) to arrive at an acceptable wear rate. Rosin is less expensive than resin, so the less expensive “ablative” paints will probably contain more rosin than the more expensive self-polishing copolymers that wear by means of a chemical reaction that prolongs the paint's effectiveness. What made the now-banned TBT paints so effective was not the TBT but the tin acrylic resin (tributyltin methacrylate) that controlled the wear rate. The same is true of the copper acrylics used today.
Hard paints work by leaching biocide out of the paint film and leaving the paint film behind, which creates a honey-combed matrix surface. The biocide that is supposed to leach out is trapped in the paint film below until either the honey-combing becomes brittle enough to break off, or someone scrubs it to remove the layer of paint and exposes fresh biocide. Scrubbing of course, causes biocide to be leached into the water at a higher than usual rate, which is why some ports like the Port of San Diego have adopted new bottom cleaning regulations.
A little about hard paints: Not all hard paints are rock hard. One quick way Seidel tests hardness is to rub his thumb on the paint film as the boat is pulled from the water. If there is no paint on his thumb, it scores a 10, and if his thumb is colored with paint and he can see through to gelcoat or primer, he gives it a 1. On Seidel's scale, Interlux’s VC Offshore racing paint is a 10, which means that there is certainly enough biocide being released to control barnacles until it is overwhelmed by the slime, which lets shell and barnacles gain a foothold. Interlux’s hard paint, Ultra, he says, would probably be an 8. “Ablative” paints run the gamut from 1 to 8.
Some hard paints actually have higher leach rates than some “ablative” paints, but because of the paint film, you may never get all of the biocide into the environment. So if you got 80 percent of the cuprous oxide released from a paint that has 15-18 pounds of cuprous oxide in it, you will release 12-14 pounds of biocide. And if you get 90 percent of the cuprous oxide released from a paint that has 6-8 pounds of cuprous oxide in it—as most “ablatives” do—you will release only 5-7 pounds of biocide.
Comparing copper content alone is also problematic because not all of the copper leaches out of some paints. This residue copper often enters the environment as it is sanded or rinsed from your hull ashore. This brings up the issue of boatyards, a common non-point source of copper and other pollutants entering the environment. In some instances, where and how you carry out your bottom maintenance can have more impact on the environment than which copper paint you choose. Most states have clean marina programs that certify boatyards at marinas that carry out best environmental practices.
We also spoke with Frank Winkelman, manager of product development, at Kop-Coat Marine Group, maker of Pettit antifouling paints. According to Winkleman, if you go by the ISO standards for measuring leach rates over the life of the paint, in general, ablative antifouling paints tend to leach out less copper over the lifetime of the coating. Winkleman said this is "mostly due to the fact that these ablatives contain less copper than their hard counterparts and also deliver that copper more efficiently over their lifetime." He added that there probably isn't a significant difference between the softer and harder ablatives when you take into account the life of the paint. "The harder ablative products, which have lower wear or erosion rates, will leach copper at a rate similar to the soft ablatives products when calculated over the longer lifetime of these harder ablatives, even though they may contain a higher percentage of copper," he said.
Bottom line: Because of the number of variables involved, it is very difficult to make any definitive conclusion as to whether hard paints or ablative paints are the “friendliest” for every sailor. Total copper (cuprous oxide) content does seem to offer a good starting point, but there are many other factors to consider. If you are not interested in copper-free alternatives, but want to minimize your impact, the best approach, in our view, is to buy a paint with the least amount of cuprous oxide that meets your needs (in other words, don’t buy a two-year paint if you are going to repaint every spring). Advanced ablatives with relatively low volume of copper that releases at a controlled rate will typically fit the bill. You also want to keep hard scrubbing to a minimum, and look for a boatyard that meets your state’s clean marina criteria. We would advise sailors in sensitive freshwater areas to avoid soft-sloughing paints, or paints high copper content and to consider the low- or no-copper paints designated for these environments.