The Future of Reversible Glue

GEORGE IMAFIDON

If you think about the materials that our modern world is built from, what springs to mind? Concrete, steel, plastic, wood? You'll be right. But what about the thing that holds all that stuff together? Glue. Adhesives are everywhere. From the glues in your shoes, your furniture, and your house. To every phone, car, or aeroplane we use, they literally stop our world from falling apart, and yet we hardly ever think about them. There's some pretty clever chemistry behind the way modern adhesives work, but many of the glues we use today suffer from a big problem. They're all incredibly strong, and that means we can't easily unstick things. When it comes to taking products apart to fix them or recycle them at their end of use, glues put us in a bit of a sticky situation. As we move towards a more sustainable world, with circular design and manufacturing, what we need are reversible glues that can be turned on or off when we decide.

I'm George Imafidon and you're listening to Create The Future from the Queen Elizabeth Prize for Engineering. And today, we're looking at the science of unsticking and how reversible glues can change the way we recycle and how we design our world. I'll be speaking to Professor Mark Geoghegan…

MARK GEOGHEGAN

I kind of got interested in the idea of reversible adhesion at the turn of the millennium, but it was only after arriving in Newcastle that I realised how to do it.

GEORGE IMAFIDON

And Dr Adriana Sierra Romero.

ADRIANA SIERRA ROMERO

I quite enjoy doing basic science but I also wanted to do something that felt more real.

GEORGE IMAFIDON

From the University of Newcastle about their work in this space.

Today we're speaking about reversible glues, so it would be great to give our listeners as much context as possible. People might have not thought about adhesives, but we've been using them for thousands of years. So Mark, maybe you can give us a sense of how important glues have been for us as, you know, as humans since we started making things and doing engineering up until today.

MARK GEOGHEGAN

Yes, very important. So we know that our species is about 300,000 years old. And we know there's records of us using some form of glue for about 200,000 years. So for virtually all of our existence, we've been using glue. But in those prehistoric times, it was basically taking tree bark and heating it and that was used to put, you know, handles on axes and the like. Later on glues became, you know, developed from animals and they were used to do things like sticking tiles on in early houses and bathrooms and so on. Not much really changed until about the middle of the 19th century when we're able to manufacture glues at scale and the first superglue just after the Second World War. So since then we've been using largely synthetic glues rather than the bio-based one that we were using, you know, hundreds of years before and thousands of years before. So that's where we are now. It's a massive industry. Glues and adhesive are worth about 50 billion pounds a year for the worldwide economy. So it's a very, very big industry. Basically about 10 pounds per person in the world, really. Something like that. But, you know, it's a big deal.

GEORGE IMAFIDON

I was looking and supposedly superglue, which you know, almost everyone has in their house at some point in order to fix something was actually invented by accident.

MARK GEOGHEGAN

Yeah that's right. But those glues are really typically very, very strong and often much stronger than the materials that they're bonding. You know, so typically an epoxy type glue, it will connect two surfaces due to mechanical interconnects. So as it hardens, it cures around the imperfections on the surface. In some cases, you can get some chemical reactions between them and the surface, which also adds to strength. So there's a variety of mechanisms that makes these adhesives work very effectively. They form very, very strong bonds. In fact, you've seen these videos of blue companies that are showing off the strength of their adhesives by lifting, you know, like 15 ton lorries with their glue.

GEORGE IMAFIDON

Adriana, maybe, onto you. You've got these amazing strengths, but, you know, alongside that, it also causes a lot of environmental issues as well when it's actually that strong. So how much of a problem is this?

ADRIANA SIERRA ROMERO

The problem with modern glues is that almost all of them are made with petroleum derivatives. So this is an issue not only because we are relying on these non-renewable resources, but because they can be toxic themselves or they need toxic substances during their manufacture. Most of these glues, as Mark mentioned, are designed to be very strong. So even at their end of life, the materials in the products that we use, they remain stuck together or they break before we can even attempt to remove the glue. So as you can imagine, now we are unable to recycle many materials that go to waste. It goes to landfills and then all this plastic pollution, for example, that we are seeing nowadays is partially due to the use of these structural adhesives. So we need to do something about it. And our approach was to develop these reversible glues.

GEORGE IMAFIDON

So we need something that is strong and moisture resistant, but can also be reversed when we actually want to eventually take it apart. So Mark, I know you've been working in this field for a very long time and probably the best person to answer in terms of what actually makes the reversible glue so challenging. Why haven't we had it just yet?

MARK GEOGHEGAN

Primarily because of exactly what we've been talking about is when you stick something together, you tend to want to stick it so it stays stuck. And glues are typically just chemical, right? They form chemical bonds and chemical bonds tend not to be reversible. And that's the challenge is what sort of bond can you have that's actually reversible? And the one that we were working on was the electrostatic bond.

ADRIANA SIERRA ROMERO

So our reversible glue works through what we call electrostatic addition, which is based on the interaction between negative and positive charges. So our glue consists of two different formulations. They are both water-based emulsions, but one contains a polymer with positive charges and the other a polymer with negative charges. So what happens when we coat one surface with a positively charged formulation, another surface with a negatively charged one, and we bring them together? Well, those charges create a bond. Now that we have this bond, how can we reverse the addition? What we need to do in our glue is to expose it to either acidic or basic conditions. These are more or less the equivalent to lemon juice or soapy water. So when we expose this adhesive to an acidic environment, our negatively charged polymer becomes neutral. The bond breaks because we don't have this interaction between charges anymore.

GEORGE IMAFIDON

Yeah, that is very useful context. You know, we've spoken about polymers, we've spoken about, you know, making an emulsion. Maybe you can explain some of these terms, so we're all on the same page.

ADRIANA SIERRA ROMERO

Yes. So first, a polymer is simply a very long chain of atoms joined together. So it's a very long molecule that can have different properties depending on how you prepare them. So polymers that are prepared through emulsion polymerisation, they tend to be particles. So an easy way to understand what an emulsion is, is if we think about water and oil. We know that under normal circumstances, water and oil do not mix. Even if you shake them very strongly, they will eventually separate. However, there are some molecules that are called surfactants. And these surfactants are molecules that have two phases. One phase likes water and the other phase likes oil. So they are quite useful to help us stabilise an emulsion. So when we mix these surfactants with water and oil, what we get are very small particles of oil that are entrapped by these surfactant molecules with water all around them. So these emulsions are widely used in paint industry or in coatings, and they have the advantage that they are quite easy to manufacture at large scales. So our glue is made of oily polymers that are entrapped by a surfactant in water and all these charged polymers that we have talked about are surrounding these particles and they are more sustainable than other glue formulations because they are water-based. There is quite a lot of water in our formulations.

GEORGE IMAFIDON

Now we've heard of these reversible glues. I'm curious to know how well they perform, you know when you tested it, Adriana, and how does it compare to existing glues? What are some of advantages slash disadvantages?

ADRIANA SIERRA ROMERO

Yes, so our glue has a strength between the kind of adhesives that we find in tape, for example, and structural adhesives, for example superglues. So it makes it good enough for medium load applications. For example, we were thinking that we could use this glue to bond plastic panels in car interiors or maybe some pieces of furniture that they don't require a high load. We also found that our glue is quite good to glue plastics that are generally difficult to stick to each other.

MARK GEOGHEGAN

The technology, as we've discussed, it's got its limits on its strength. The best we've got is about a tenth of what you'd get from a structural adhesive in terms of how powerful it is. At the moment, the nanoparticles in the emulsion are petroleum based and we'd like to improve the technology so that we had some more bio-based components instead. One of the problems that we need to solve is at the moment we've got quite a high water content in the materials and water is quite slow to dry and so when you put two surfaces together you get a lot of water trapped inside and so that can make the formation of a good bond to be quite slow or quite difficult. So that's something that needs to be addressed. Because it's an emulsion it can be sprayed. We think it's quite multipurpose. It's quite versatile. It can stick all sorts of different surfaces like woods and metals and silicon and plastics. We've done some work on additives, so we know how to make it conducting. But if we can do that, then we should be able to make something that will join two components together and allow electricity to move between those components. So yes, we think there's quite a lot that we can do with this, but ultimately, we need to make a prototype at scale and we haven't done that yet.

GEORGE IMAFIDON

And that was actually going to be my next question. When do you think I could, you know, buy this from my local B&Q or Screwfix? So you know, wherever it may be.

MARK GEOGHEGAN

Other hardware stores are available, yeah! I'd love to be able to answer that because there’s always, you know you always achieve something, and then the achievement of a particular goal just shows that there's another problem ahead of it.

GEORGE IMAFIDON

Yeah, every time.

MARK GEOGHEGAN

So there is of course the technological valley of death. So I really couldn't put a date on it, but if it were within five years, I'd be well happy.

GEORGE IMAFIDON

How practical is the reversing stage at the moment? Do you basically just dunk it in the bath of lemon juice? How long does that actually take?

ADRIANA SIERRA ROMERO

So currently we need at least a day to achieve this reversibility, but we know that we can improve.

GEORGE IMAFIDON

As we kind of wrap up, I've got a few more questions. The first question being, where is the most promising application, you're hoping that we could potentially use this in. We've kind of mentioned a few different places, but if you were to pick one to two areas where you think, you know, this glue could really fill a gap, where would that be for you?

MARK GEOGHEGAN

Well, we've been working with Biffa. So Biffa are the company that often empties many of our bins and they are involved in recycling and they do a lot of work in bottle recycling, for example. Our technology will take the labels off bottles in the standard wash. And labels are around 2 to 3% of bottle mass very often on average. So that's an important area. But what we'd have to overcome there is not so much getting the labels off the bottles, but getting the labels on the bottles, because they work at, you know, thousands of bottles an hour. And that's something that we find quite a challenge, not insurmountable, but quite a challenge. But we also need to coat both surfaces to make this work, whereas typically that's not the case for other, the technologies that they currently use. So that's one area. Adriana's already mentioned the sort of plastic car interiors. And the advantage of our glue is that if you separate two components, you've got glue left on the two components. And we can just remove it with paint thinner because it's a paint. So actually we can clean the surface and you can end up with a pristine surface at the end. So areas like that are quite good. But again, if we can make a really good conducting glue, then we could look at replacing solder in many applications. And that in terms of e-waste has got big benefits too. So there's lots of different areas where we can take this.

GEORGE IMAFIDON

Amazing. Thank you. And Adriana, I'll start with you, you know, more broadly, looking into the future, how do you think reversible glues will change the way we engineer, you know, build and design things?

ADRIANA SIERRA ROMERO

I think that the role of reversible adhesives will play a significant role in what we are seeing now in circular economy and zero waste industries. So this means that all the products that we use have to fulfil a cycle that starts with obtaining raw materials, transforming them into something that is useful to us, and then trying to incorporate these materials again into this production cycle. To keep us away from single-use plastics, for example, that end up in a landfill that creates all of these environmental difficulties that we are facing. So yes, basically reversible adhesives will help us to be more environmentally friendly.

GEORGE IMAFIDON

And yourself, Mark, as you look to the future of reversible glues, what do you hope for? You know, you're pioneering in this sector right now and you know, there's a lot to be hopeful for. But yeah, what is the North Star for you?

MARK GEOGHEGAN

So the North Star, I actually wouldn't put things in that way, I would say that one of the things that's very surprising about this is the other, is the ways that the technology can be developed. So you can imagine that if we had, say, stronger glues, that you might want to take a hotel room that would be capable of coping with people who are in wheelchairs, for example, and needed support bars and so on, and then that could be removed for other guests, and so on. You could think about wound dressings because they're quite painful to remove. Now, if we could reduce the pH at which the switch was changed so it wasn't quite so, you know, so aggressive, removing bandages and replacing them is something that people, you know, companies are very, very interested in and there's a high premium on that sort of technology. So there's certain areas where this can go in and there's areas that just might be completely unexpected. And also from my own personal perspective, it started out as blue sky research. So to take it from basically the soft nanotechnology to a chemical problem, to making a glue, to becoming an engineering challenge, and to actually finally seeing it as a product would be just such a, such a thrill. It's been a long, long journey starting in about 1999 when I first had the idea to where we are now. It would be absolutely fantastic.

GEORGE IMAFIDON

Yeah, it's been 25 years and I think that product will be well deserved and will make a huge impact on people's lives. You know, in the healthcare context but also just in the everyday context you know as we said with a piece of furniture maybe we might just need it, it might become very very useful so no thank you both Adriana, Mark for your time. I've been fascinated by this conversation and just learning more about glues. I did not know this is a 50 billion you know pound industry and yeah that is just yeah amazing to see the impact that you can make with this substance, you know, this one substance alone. So thank you so much for your time today.

MARK GEOGHEGAN

Thank you.

GEORGE IMAFIDON

You've been listening to Create The Future, a podcast from the Queen Elizabeth Prize for Engineering and Peanut and Crumb. This episode was presented by me, George Imafidon, and was produced by Anand Jagatia. To find out more about the podcast and the work of the Queen Elizabeth Prize for Engineering, follow QEPrize on Twitter, Instagram, and Facebook.