Science on surfaces - Tips, Tricks and Tools
Science on surfaces - Tips, Tricks and Tools
13. Adhesion – relevant for so much more than just glue
How come paint sticks to the wall? And what makes the ink, used to print logos and text on the milk carton, stay in place?
In this episode of Season 2 of Science on surfaces we talk to Dr. Susanna Lauren at Biolin Scientific about adhesion. Susanna did her Ph.D. on microfluidics and superhydrophobic surfaces and is an expert on surface-related phenomena, such as surface tension, surface free energy, wettability, and adhesion.
Susanna describes how adhesion is defined and talks about the three different components that, in combination, give rise to this phenomenon. We then move on to talk about in what areas adhesion is important and what different factors affect it. We also get to learn more about what possibilities there are to predict the quality of the adhesion and how it can be tested.
Thanks for listening! If you are interested in surface and interface science and related topics, you should check out our blog - the Surface Science blog
Surfaces
Speaker 2:Welcome to this podcast today, we will talk about adhesion. We will talk about what elevation is when and why it is important and what different factors that affect the quality of the addiction. Here with me in the studio, I have Susana[inaudible], who is an expert on surface related phenomenon, such as
Speaker 1:With debility surface tension, surface free energy,
Speaker 2:Also adhesion. So sun has been working at violin scientific around these topics for over eight years. And before that she did her PhD on microfluidics and superhydrophobic surfaces. Welcome. Thank you. So just let's dig into it and start with the basics. What is adhesion?
Speaker 3:So a adhesion is attraction between two dissimilar faces, so American society for testing and materials. So ASD act. So they defines a adhesion as the state in which two surfaces are held together by interface or forces, which may consist of valance forces or interlocking forces or boat. So, uh, this basically means that, uh, I mean, there is no actual, um, good defense definition for adhesion as such, but it's, uh, it's divided typically two into mechanical interlocking, uh, physical and chemical bonding. So we've met mechanical interlocking. I, I really mean that the adhesive material will feel all the way for you to send pores of, of the surface. And the two surfaces are really mechanistically mechanically hold together.
Speaker 2:Okay. So, so you have the, the material fills the void and then when it's sort of it's, um, how do you say goes from a solid to a or to[inaudible] then there is no way back, so say exactly kind of stuck in. Yeah.
Speaker 3:So this is basically how the, how many of the clues work, right? So you have a clue that is kind of flowy in the beginning and then it dries out, or you might have like two component, uh, clues that you need to mix together to do, to get the curing to, to, to, uh, in place. So that's a, that's probably one of the most important mechanisms of, of adhesion, uh, such. Uh, then we have a physical adhesion, which is always present. I mean, it's, uh, based on the, uh, Wando war one, the world's forces in between different, uh, materials, but these forces are very weak. So as such, they are not really, they, they are not able to do form a very strong adhesion between two, two materials. Um, then we have chemical bonding. So this includes, uh, all kinds of cava, lint, ionic, metallic bonds that we have. I mean, for example, in a meth piece of metal, you have metabolic bonds between the metal atoms, uh, and they are, they are actually usually quite strong bonds, but, um, uh, and, and if you would get this chemical ponding, that would mean that the interface almost disappears between the two, uh, two, um, two different materials, but this is very difficult to achieve with two dissimilar surfaces. So you can, of course be matching that is quite difficult to get to, uh, different materials to bond sole that you don't even see the interface.
Speaker 2:Right. So there are these three, three sort of possible components that could be present in the adhesion. Yes,
Speaker 3:Exactly. So it's of course, usually a mix of, especially the, uh, the mechanical interlocking and the physical adhesion, and then maybe in some cases also chemical,
Speaker 2:Not yet. Okay. So why is addition important? I mean, with the glue situation, that's kind of an obvious one. Yeah. But other situations.
Speaker 3:Yeah. So, I mean, yes, glowing is of course, something that you do on your, I mean, even in, in everyday life, you, you might want to clue things together. And I mean, the other very obvious example is, is painting, for example. So if you, if you paint a wall that is basically adhesion between the wall and the paint that you are, you are applying to, and then, but then we have, for example, in packaging, uh, adhesion is very important because there you have, uh, if you think about a meal cartoon for carton, for example, um, it's, it's actually composed of several different layers of materials. So you have a polo[inaudible] layer, which is basically a plastic or a polymer layer. Then you have, uh, Alamina foil, then you have the, the paper port and all of those layers need to stick together. Right. So that, uh, they form a, uh, a product or a package that actually works right. How, how is how it is supposed to work. And then on, I mean, if you think about it still, the, the, the packaging you have, you usually want to print something on that package. So you want to highlight the name of the product, tell the ingredients of the product. So then you need to print something on it. And that's, again, an adhesion, uh, issue between the material that you are printing and the ink that you're using to, to print. And then, um, adhesion is also present in completely different areas, like in, uh, biomaterials where you have adhesion between cells and pyrometry are surface. Right. And I mean, that's their resolve of course, other, uh, other things also affecting on that. So you can't really say that it's, it's only like mechanical interlocking or a physical chemical. And so there are other things that affect in that situation, but the mechanisms are such are kind of similar.
Speaker 2:So would this then be basically any situation where you have two materials that should be in contact and stay in contact without any sort of external force.
Speaker 3:Yeah, exactly. Yeah. Yeah. And then the, basically how good is the adhesion? I mean, how, what kind of, uh, um, forces, uh, are there after the interface define the, the strength of the acuity situations we basically have everywhere. Yeah, exactly. I mean, yeah, absolutely.
Speaker 2:Um, okay. So then what factors affect the quality of the addition?
Speaker 3:I would say probably the most important one is a clean surface. So, I mean, if you think about the, um, I mean the, how, um, if you have any kind of contamination on the surface that usually, uh, means that you have a lower surface, be energy on the surface, and that will then make it more difficult for anything that you want to spread on it to, to, to, to, to spread and then stick on the surface. So if you think about, uh, something that you would, I mean, if you would touch some surface with your fingers, you would add grease on it and that would be contamination. So everything usually starts with clean surface. That's probably the most, most important one, uh, then, uh, that is kind of related to the wettability of the surface. So I mentioned the surface, we energy.
Speaker 2:So, uh,
Speaker 3:I mean, if you're applying, coating on the surface, you want it to, to spread on it. And as I mentioned, if you have very low, I mean, if you have high surface being a two on the surface, it's more likely that whatever you are applying on it will spread on it. So, uh, then, uh, this in, in, uh, the good wet abilities is always important when you want to do half, uh, especially for example, in paints, if you want to have to paint to stick, okay.
Speaker 2:Was thinking about this. Sometimes you, you want to write something with a pen on certain materials and the inklings just like, yeah,
Speaker 3:Yeah, exactly. Yeah,
Speaker 2:Exactly. And you're like, you kind of have an idea of what kind of pen will work in, which will not work.
Speaker 3:Yeah. That's a, that's quite a kind of practical example on that. And then, I mean, um, then vulnerability, um, and, and adhesion as well then is it's also affected by roughness. So, um, if you think about, I mean, usually, uh, I mean, mechanical interlocking, you need some kind of roughness so that you can get that. Um, cool. For example,
Speaker 2:I am, I'm even picturing that you need some sort of, almost like an entry hole
Speaker 3:In a sense yeah. Then like a big void behind. Exactly. But then if you would have those and you would have poor wettability so that whatever you are applying doesn't want to go in the dose holes that would actually make the adhesion really, really bad, because then you would basically have like air pockets in between the substrate and whatever you are applying on top. So that's why you, I mean, roughness can be good for adhesion, but it can also be very, very bad if the vet the ability of the site.
Speaker 2:Right. Is that so with a good wettability, you will, you will get a bigger contact area with the roughness and with bad what's ability would have a lower
Speaker 3:Yeah, exactly, exactly. And also, I mean, the roughness also add the, the possibilities for physical, uh, bonding, right. Because I mean, of course I said that they are weak, but if you have, I mean, we forces, but if you have a lot of them, then they start to, to, to make an effect as well. So, and then maybe the last thing is, uh, also, I mean, we've been talking a lot about how the surface has to be in order to get good adhesion, but then there are, of course, some requirements for the, whatever you are applying on the surface as well, because, uh, if you think about paints, the viscosity of the paint suit be low enough, so that there is some kind of a flow, because I mean, of course you need that flow to, to be able to, uh, for, uh, for the paint to penetrate into those, those forms. Right. I mean, if you would have like a paste and you can imagine that it could be quite difficult to, to, to cover the whole wall with some kind of a base like yeah,
Speaker 2:Yeah, yeah, yeah. It would be. Um, so you mentioned cleanliness is important roughness, and then the thing you want to coat with yes. They are all affecting the quality of the adhesion. Is there any of these factors there is more important than the other, or
Speaker 3:I think wettability is probably one of the most important ones, uh, that, that is really kind of what defines or dictates how good to adhesion will be. So that, um, and I mean, gleaning, you can't really say that. I mean, cleaning is of course important, but that's also in a, in a sense related to two bedrooms
Speaker 2:Stability, so, yeah. Uh, okay. So what can you do to improve that Haitian them clean
Speaker 3:The surface? Of course. Yeah. So, I mean, that's, I mean, that's kind of given, so, I mean, if you, if you paint something at your house, you are always at Weiss to, to clean the, the wall first. I mean, I don't know if everybody does that. I don't[inaudible], but you should do it. Um, but luckily the paint manufacturers maybe know that because they know that people are lazy and maybe don't do it. And usually the paint sticks quite well anyway. Uh, but, uh, you are really, you should do it in order to improve, uh, the, uh, the adhesion. And usually that is enough, but then, uh, in some cases you might need some undercoat layers, for example. So it's a sort of primers, exactly different types of primers are basically used to, to improve the adhesion. So improve adhesion between your substrate and whatever you are then, uh, like the, the outer most layer that you are applying to. Uh, and then, so you're basically, um, before coating, you are making the surface reenergize the S as high as possible, so that the, what you are putting on top with spread on it, and in, uh, if you think about industrial processes, then, uh, it also starts with, uh, with cleaning typically. Uh, I mean, because if you have any type of contaminants, they are quite likely to decrease the surface we intercede if your surface, uh, but if we think about this packaging example that I discussed there, you usually want to, to put together some type of a polymer surfaces, and those surface typically have very low surface energy values. So it's, it's quite difficult to stick anything on towels. I mean, if you want to try to glue plastic together, it's, it's quite difficult.
Speaker 2:Yeah. So,
Speaker 3:Uh, you usually need to do some type of a surface treatment on that plastic to increase the surface[inaudible] of the plastic before you apply to coating. So, uh, so plasma treatment is, is, uh, is very often used in impact.
Speaker 2:So that's a known method for increasing the surface.
Speaker 3:Exactly. So you, can you plus my treat or flame treat your, uh, your polymer surface, which then increases the surface energy of, of, uh, the polymer.
Speaker 2:So what is flame treatment? Is that with fire?
Speaker 3:Well, not me. It's, uh, so basically, uh, all these treatments are, uh, kind of like high, energetic apartments off of the surface with some ions we tie in her teas. Okay. So, um, but those are basically used to, to, uh, to get to, to the situation where you have higher surface be energy, and then it's easier to, to apply your adhesive.
Speaker 2:Right. Right. So that will mean that will basically help in two ways then I guess, because you have, um, no, no contaminants in the way. Absolutely.
Speaker 3:Yeah. It's can also be kind of a, like a cleaning procedure, because I mean, you bombarding your, your surface with ions will also remove what ever contaminants you might have,
Speaker 2:And it helps also with interaction or whatever you put on top. Yeah.
Speaker 3:And, and also what good B this is, I mean, it, it depends a little bit on the material, but this could also, uh, add little bit of roughness on your surface. So that is also something that is, is, uh, kind of always present. When you think about your adhesion problems, basically you need to kind of take into account what is the variability of the surface, and then what is the roughness of your surface because they boat matter in terms of, of adhesions.
Speaker 2:Right, right. Um, so can you measure the adhesion somehow?
Speaker 3:Um, yeah, so, I mean, there are, uh, basically you need to little bit separated to, to, um, measure adhesion. Yes. You can measure in some ways after you have applied, uh, to coating, but then you can also, uh, before you even apply to culting, you can try to do basically optimize.
Speaker 2:Right. So we can predict how well a material.
Speaker 3:Yeah, exactly. So if we think about this, uh, so you typically, you predict the true variability measurements, so to conduct ankle measurements. And, um, as I was talking about the, the plasma treatment, uh, one quite common way. I mean, because it's, it's very known that plasma treatment can be used to increase the surface, we energy off of, uh, polymers. So, uh, quite often, what is really actually done is that, uh, I mean, in the beginning, you optimize the plasma treatment time by measuring conduct ankle, because depending how long you plus material, the surface we enter energy can either first maybe increase, but then if you do it too long, it starts to decrease again. So it's not, I mean, you have to find that optimum time that, uh, that you should reach material each metric. Yes, yes, exactly. So it can be different depending on the polymers. So this something that you can find in a chart or table, or what do you have, I mean, I would say probably you can, you can, I mean, rely on previous knowledge or you can, I mean, of course, just by testing different plasma treatment times, I mean, there are other factors like bossman treatment power of what type of, uh, Cassius you are using and things like that that will affect. Um, but, um, so you can, I mean, if you have like your own polymer, you can, of course optimize it for your own polymer by, uh, by, uh, measuring contact ankles or surface V energies and look where the, the optimum values are. Uh, but then when you know that, okay, I know that, uh, three seconds with this plasma, this power is, is good for my material. So what people are then commonly doing is that they just check the quality of the plasma treatment, kind of like a quality control of the plasma treatment. Just look at the conduct ankle after the plasma treatment to check that it's, it has worked as you has, have, has predicted. And then that could be kind of on your quality control process as part of it. So those are, uh, those are of course, quite, quite two people situations where you, how you optimize the adhesion, and then you can optimize also your, the surface tension of your coating formulation. Uh, so yeah, it's, uh, matches their surface. We not value so Fure, and then after it, can you test it? Well, it works. Yeah. That's actually quite tricky. So, I mean, you can, uh, I mean, if you have already applied the coating on the substrate, then it's quite difficult to, I mean, there are methods to do it, but it's not very straight forward. I mean, for example, because the failure mechanisms, like how, from what, uh, point the adhesion failure, I mean, it's con it can be adhesion failure when it's like in between the substrate and the adhesive material, but it could actually also be within the, uh, the adhesive so that it's not really related to your due to the adhesion itself. It's not just the interface, it's not interfacial failure. It's more like in the Polk you'll, you can even have to stop straight a failure. So it's, it's kind of hard to, to, uh, to say where the, the failure has actually occurred and then how to even do that. I mean, there are, I mean, believe it or not, but the tapes they test, like just putting a tape on, uh, on the coating and then pulling it out, rightly yeah. That's kind of a quality control method. So if the coating, so if it sticks on the tape on the substrate, but I mean, you can imagine that there you, then the kind of force that you can measure is of course dependent on how well the tape actually attaches on, uh, on your, uh, coating. So then, I mean, it's it, it's not going to be very, you can't measure very high. I mean, you've got heat on it's good, then nothing should happen. I mean, you can't get, hopefully the paint out of the wall by putting, I mean, the, the, the paint out of the world, just putting date there. No.
Speaker 2:And essentially you can only say it's that Haitian is stronger than the tape.
Speaker 3:Yeah, exactly. Yeah, exactly. And you don't, usually, you don't usually say any numbers, you usually just say pass or fail because it's a quality control method basically. Then you can use different type of scratch tests. So you have like a stylist that you pull across the, uh, the surface and then put a load on stylist and that way get some kind of an idea what he's doing at fish have control of the forest. Yeah, exactly. Yeah, that's true. But yeah, I mean then, and people are of course looking, using like, uh, electron microscopes and things like that to just to see, uh, various, the adhesion failure happening, as I said, it can happen in various different points. So there are methods, but it's not, it's not as straight forward as you would think, because I mean, you would think that there are lots of really good methods to do that because it's something that is, everybody are painting the walls or coating stuff.
Speaker 2:Yeah. That's so interesting. I mean, addition is, it's like a concept that we all have a relation to it. I think it's like a very straightforward yeah. Constantly, but it's more complicated. It is. It is. Yeah. Okay. So I think that's all we had for this episode. So thank you for listening to this episode with me, Marlene Raton, and to summit Lorianne from violin scientific. Um, and I would also like to take the opportunity to mention to those of you who are listening or watching that if you're interested in surface science and related phenomena, you should check out our blog, the surface science blog. Thank you.