Paint stripping is a crucial part of any coating process, and shops should have schedules, procedures, and policies in place to do it effectively.
Connor CallaisConnor Callais, a Technical Application Specialist at Hubbard-Hall, spoke with Tim Pennington, Editor-in-Chief of FinishingAndCoating.com, on how shops can properly utilize paint stripping to become more efficient and ensure proper coatings.
Tell us why facilities and shops need to get rid of paint? Why do they need to strip parts?
Connor Callais: I’d say the primary cases we see that people have to remove paint or other similar organic coatings would be for the racks. As racks and different masking materials are run through a paint line with parts, they slowly accumulate a really large buildup of paint that has to be removed or removed from some sort of mechanism where it’s just going to be rendered useless. And reclaiming those is very cost-effective because they need to not have to just buy new racks or maskings over and over again. But it’s also an efficiency saver in the actual process because if you don’t have racks, you can’t paint parts. If you can’t mask the materials, you can’t paint the parts. So having those readily available in a clean and effective manner is really important to maintaining the efficiency of all these different painting applications.
I see that a lot, where the biggest problems or errors are that shops are not properly cleaning and stripping their components.
CC: Absolutely. I’ll tell you another aspect of paint stripping that really is a value add for many people is just part reclamation. If you have a paint defect on a part that won’t meet quality standards, well, rather than scrapping that entire piece, you can often remove that painted coating that was applied. And then sometimes there may be a couple of steps in between that have to go before that part can be repainted, but you’re saving on all those steps that have gone into that part prior to and not having to reinvent that also.
I guess a lot of shops do not understand that even a minuscule amount of paint or coating on a part or rack will diminish the amount of current, which will then result in a flaw in the coating.
CC: And even outside of de-coating, even if you’re just doing a powdered spray paint after you build up so much paint on those racks, they begin to not really hold onto the part as well. If you have a very thin metal fixture that you’re hanging your part onto or putting it on a rack, if you build up a certain amount of paint, it’s not going to have the same ability to hold onto that paint in the process, especially if it’s being sprayed with a powder coat or something similar.
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Aside from stripping, what else can they do?
CC: Oftentimes, people will run rack or masking as many times as they can effectively get out of it before it starts to kind of diminish the quality of the coating on the part that you’re working with. And once you kind of reach that point, there are two main methods that are kind of used today. You can use either burn-off oven, which heats them up to the parts and everything, all the coatings on it, heat it up to several hundred degrees, and essentially just burns off any of the organic coatings that were on the rack. Another option that’s sometimes used is using a fluidized sand bed. We have a molten salt bath or sand bath that racks are dipped down into, and it similarly just annihilates the coating off of it.
Why is paint stripping the best route to take with that? Is it the most effective?
CC: Yes, you touched on a great point. The energy costs of running any of those burn-off ovens or sand baths, they do nine times out of 10, have to use natural gas or some sort of combustible source for the heat. With paint stripping, you can use gas heaters, but you also have the option to run electric heaters or use steam-generated heat, which gives you dramatic energy savings because the temperatures at which paint stripping occurs are never over 200 degrees. So, you really just have the solution that you’re warming up somewhere between 140 to 200 degrees depending on what you’re stripping and what sort of application you’re trying to run. And you have multiple cost-effective methods that you can use to heat that source.
Talk a little bit about the difference between organic and acidic applications, and which do you think is the better one?
CC: Traditionally, paint stripping, which kind of gives to some of its negative connotations, used to be done with methylene chloride or DCM, which is a very common solvent that was used with somewhat relative frequency in the eighties and nineties, but then kind of started to learn more about the negative health impacts of using that chemistry. So, it really kind of ran out of fashion as a method of the strip. So organic paint strippers today are much safer and user-friendly. They have really high boiling points, so they aren’t evaporating into the air while you’re using them. Just a very admissible amount gets actually released into the environment. That’s kind of where paint stripping started to regenerate its use and effectiveness in the industry. We use a variety of different solvents combined together to remove the paint. Organic strippers are often used in applications where you have multiple metal substrates that you have to strip because of the solvents, they’re effectively inert with all sorts of different metal substrates, but also, they work by de-laminating the paint off of the surface. So, we’ll talk a little bit more about other mechanisms later. But delamination is essentially where the paint goes through swelling, and the solvents are able to get between the paint and the substrate and lift it from its surface. And you typically see these parts strip or racks strip in a sense where the paint kind of peels off the sheets or if you have multiple layers, the paint kind of peels off layer by layer and it sort of swells and then you can filter out those solids from your stripping solution. When we talk about acidics, we classify them as organic strippers and aqueous strippers. So, the aqueous strippers would kind of encompass both our acidic ones and our caustic base strippers as well. Acidic strippers are really effective with cleaning materials such as aluminum, where you couldn’t use something with a high pH on aluminum; otherwise, it would destroy the part. With acidic strippers, we’re able to remove a variety of coatings off of most nonferrous metals, including CARC coatings, which are chemical-resistant coatings that are often used in military applications and sometimes in marine applications; those coatings are very difficult to remove from parts because they are chemically resistant. We use acidic strippers, we use an alkali sulfonic acid base, so it’s not a mineral acid, not a super strong aggressive acid. So, it’s not going to necessarily attack the metal, but it does work in softening up that paint, getting it to swell, and helping aid the delamination off of the material that you’re using. We use acidic strippers quite frequently in applications that have those CARC coatings that they have to remove from different military equipment whenever they go to refurbish or manufacture those. And we see that as a very effective method for removing those types of coatings.
I’m seeing more shops that are actually masking, and masking is expensive. Are you seeing more of them try to strip their masks and get as much out of them?
CC: Absolutely. In cases that we’ve kind of seen and solved over the past few years, we work with a metal finishing shop that they have several different types of maskings that they use with great frequency. I mean, these maskings on a single part will sometimes run two or three times a day, four to five days a week. And being able to continue to reuse those maskings and not have to deal with reordering new ones or degrading the material that the maskings are made of is really important for them to be able to maintain that level of production and also kind of minimize their operational cost. So, we use an acidic product, Aquastrip ACB, and they run it at probably around 140 to 150 degrees, and they can just put their maskings in a barrel and tumble them in that solution for 15-20 minutes, and the mask materials come out good as new. One question people often have when they’re looking at stripping maskings is whether this chemistry is going to affect the integrity of that masking itself. Especially when you have supercritical components that you can’t have little bits of silicon kind of coming off and replacing out of the part and being silicon at these relatively lower temperatures, they’re able to withstand that several times over. We can often see people able to get 10 stripping uses out of single masking.
You talked about organic, and you talked about acidic. Give us a little insight into the caustic solutions that are out there for that.
CC: Caustic paint strippers are really kind of taking off, especially with the rack stripping of steel racks and even steel components as well, compared to organic strippers, particularly the cost of the chemistry is significantly less. So, you have a much lower regular operational cost with using those processes. But they were very much; I would kind of classify them as a workhorse product. You can have steel components that have many, many layers of paint that are on them, and you can heat those solutions well up to 190 or 200 degrees, and they really can work to really destroy any of that coating in a pretty timely manner. When we say a caustic stripper where you have a base builder of sodium hydroxide or potassium hydroxide, and then you also have the organic additives that are kind of added in. So it kind of works as more of a biphasic solution. You have that aqueous component with a smaller organic component, and that kind of works together in unison to really not delate the paint that you’re trying to remove but really dissolve it. It breaks the bonds between the individual paint molecules, typically polyester or an epoxy-based paint type; those are going to be nine times out of 10, and we’re going to use a caustic product for that process. We touched on it a little bit before, but you really want to make sure you have that impingement and overall agitation of the bath itself because it will split out into your aqueous and organic layers. So being able to make sure that solution is well mixed really is going to render you some effective results.
You mentioned with the delamination, is there a cycle time of how many coatings are on the rack or anything like that, that they should be really pulling it and getting it stripped off?
CC: The actual strip time it’ll take for, say, one application or one layer of paint to be removed does have some variability. Kind of depends on the porosity or the structure of the surface structure of the substrate that you’re tripping off of. And then also what sort of coating and that particular coating respective thickness, how thick that coating is on the part. But generally, we’re able to strip coatings in about; I want to say, maybe 10 to 20 minutes per layer is our average or standard. Sometimes we can get them stripped quicker; sometimes, it takes a little bit longer. And then we really see that strip rate tends to be relatively linear with the number of coatings that you applied. So, if you have a part that takes, let’s say, 15 minutes for a single layer to strip, well, if you have 20 layers on there, you’re looking at 300 minutes or close to five hours to strip. And it really kind of continues on that linear rate for stripping.
What advice would you give about how to set up and maintain a really good paint stripping operation?
CC: I’ll kind of answer it in this regard: when we see issues with paint stripping it nine times out of 10 is not the product that’s the problem. We either have an issue with dwell time, which is a simple fix, just leave it in there a little bit longer, or we have an issue with agitation. Agitation is just as important as the product that you’re putting in there and the temperature that you’re running at. If you don’t have that agitation in the stripping solution, you’re not able to remove those coatings. You’ll be able to swell them and degrade them to an extent. But if you don’t have the impingement of solution kind of brushing in and hitting that part, you’re not going to be able to remove those coatings effectively. We typically will run agitation with a system called an induction system whereby your remove solution from the tank, it’s being pumped through a high-pressure filter, and then it is sprayed out of nozzles back into that solution underwater or under the liquid layer. It’s not spraying outside of the tank, but it’s spraying that solution directly into the part that you’re stripping. And that usually occurs at about 40 to 50 PSI. And that impingement action is what really does the lifting work of removing those coatings from the part. Kind of circling back, I said we had the two main mechanisms that paint are going to be stripped, it’s going to be delamination or dissolution. Dissolution kind of occurs more with caustic paint strippers, but once you break down that bond either between the paint molecules themselves or the adherence of a paint layer to the substrate, that’s kind of your first stage. Either the delamination or the dissolution, and then it comes to the physical removal, and that’s where those adapters come in.
I guess with any operation, especially with any chemistry, there are some safety protocols to follow. What are some regulations that shops really need to follow to make sure that it’s safe for their employees and also for the parts as well?
CC: When it comes to using the paint strippers, as with any wet application you’re going to be using in the field or in the industry, you’re going to want to make sure you’re using your proper PPE. I mean, this is a heated solution of chemistry, so you’re going to make sure that you’re wearing your eye protection and any respective gloves or aprons that your operators may need to use. And you also want to make sure that everyone’s thoroughly trained on what exactly they’re dealing with. If you’re dealing with an acidic solution, you are heating up acid, and you want to make sure that you’re minimizing any potential for contact between the operator and that solution. And then, similarly, you’re going to want to make sure that wherever you’re performing this application, you’re able to adhere to the appropriate discharge regulations. So, we’re really not having to dump this material often if it’s a well-maintained bath; the solution itself should theoretically never have to be completely disposed of. So you never have to dump a full bath into wastewater treatment, but you are going to have some of that residual solution that is brought up from the part of the rack into your rinse area, and you want to make sure that you appropriately treat that rinse water.
Let’s recap: you’ve talked a lot about what you’ve seen and some of the solutions to solve part of it. What are some things that finishers and coaters should always keep in mind with these operations?s
CC: You really see the most success when they don’t treat this sort of application as an afterthought or something they just have to do occasionally. If it’s an application where they take the time and the resources to maintain this bath, maintain the operation and the use and the bath life, they really can get a considerable return on that investment itself.
Give readers a few bullet points that they need to think about with the paint stripping process. They need to have in place schedules, procedures, and policies of how they’re going to be doing this.
CC: And one thing we talked about as well is just the general bath maintenance. I know we’ve kind of said that term in the past, but what it really comes down to is, are you removing those paint solids from the system? Those paint solids are what’s going to really blind up the product from performing effectively. Those are typically removed either through a bag filtration where you have a predetermined micron size for the type of paint that you’re stripping, and it removes those from the solution and returns the product back to the tank that is solid-free and able to continue to perform. But you can also filter with centrifuge filtration as well. Both of those methods are effective in removing the solids from the bath. And then this is more common with our aqueous or acidic and caustic components. But when you’re operating those products at 150 to 200 degrees, the main loss you’re going to see is just water from evaporation. So, when we start up an application, we try to get a feel for what is our rate of evaporation, whether it be a few gallons an hour depending on the size of the tank. And then we want to make sure that we’re routinely adding back fresh water to replace that that’s lost from evaporation. So that will maintain the concentration and the relative performance of the bath as well.
