Turning recovered plastic mulch into opportunities for asphalt

Podcast Transcript

[00:00:00] Nataliya Shcherbatyuk: Hello and welcome to the Mulch Matters podcast where we will explore the intriguing world of mulch and its impact on agriculture and the environment, as well as update you on the latest research about soil-biodegradable mulch and recycling options for plastic mulch. I am your host, Dr. Nataliya Shcherbatyuk, and I am a Research Associate working on the project, “Improving end-of-life management of plastic mulch in strawberry system”. In each episode, we’ll dive into the latest research, trends, news, and insights on why mulch matters and how we can improve plastic mulch end-of-life options. We’ll also branch out and discuss other plastics as well as talk to researchers, experts, and practitioners in the field who will share their insights and experiences on how to use mulch effectively in different settings.

[00:01:02] Nataliya Shcherbatyuk: Welcome back to a new episode of Mulch Matters podcast. And today we have two guests, Karl Englund and Kun Zhang. Hello and welcome, Karl and Kun. How are you today?

[00:01:17] Karl Englund: I’m doing well. How about yourself?

[00:01:19] Nataliya Shcherbatyuk: Pretty good. So, jumping straight to the conversation with both of you, can you start by sharing a little bit about yourself, your background, and how you actually got involved in our current project? And Karl, how about you go first?

[00:01:35] Karl Englund: Sure, so I most of my research, I’m a faculty here at Washington State University since 2003, I think it was, something like that. And most of the research that I’ve done has really been focused on the built environment and putting products into buildings and, and, and bridges and those kinds of things that, that we use on an everyday basis kind of thing, and the way I have done a lot of my research has really been in the wood sector, and so a lot of forest product sector, so lumber and, and engineered wood composites and those kind of things. And I started branching out, getting more into the plastics and other types of composites and other types of low-value feedstocks, as I like to classify them, and a lot of the work that I had done both with, you know, government and sponsored research groups and also in industry has really been focused on recycled plastics. And, and with that being said, you know, Lisa DeVetter, the PI of this project, reached out to me back when we were putting this thing together and said, “Hey, would you be interested in this?” And I said, “Yeah, that falls right into my, my realm of world of, that I like to work in,” kind of thing. So that, that kind of is what got me started.

[00:02:48] Nataliya Shcherbatyuk: Kun, how about you?

[00:02:49] Kun Zhang: Hello, everyone, greetings from California. My name is Kun Zhang. I’m an Associate Professor, uh, from the Department of Civil Engineering at California State University, Chico. So, my research mainly focuses on the solid waste recycling in asphalt pavement materials, such as reclaimed asphalt pavement, recycled asphalt shingles, agricultural byproducts, including the grape pomace and olive pomace as antioxidant additives. One of the projects is to reuse the carbon fiber composite to reinforce the porous asphalt, which I worked with Dr. Karl Englund between 2016 and 2018. So since then, we have teamed up to develop and assess the use of asphalt pavement as a value-added solution to recycle solid waste, including the plastic in this project.

[00:03:48] Nataliya Shcherbatyuk: This is great, and both of you have wonderful experiences and quite different. So based on your experience, how has your perspective on plastic recycling in agriculture been influenced by your perspective? And when we particularly talking with plastic mulch. Kun, how about you?

[00:04:08] Kun Zhang: Yeah. I’ve been paying close attention to the issue of plastic pollution and recycling since early 2010s after I watched the documentary film Addicted to Plastic. So actually, my first academic publication was related to plastic recycling in construction materials. In that project, I recycled PET from plastic bottles and mixed with fine aggregate to develop plastic mortar and assess the mechanical property of this newly designed mix. For the asphalt industry, it has a long history of responsible and successful recycling or reusing solid waste in pavement, such as crumb rubber tires. Since 2018, lots of national and local efforts have been made to evaluate plastic recycling in asphalt pavement, including the laboratory studies as well as a field pilot project. However, fewer studies focus on the recycling of the ag plastic mulch with a higher contamination level in this work. So, this project really provides a great opportunity to explore, uh, these issues.

[00:05:28] Nataliya Shcherbatyuk: Karl, what about you?

[00:05:31] Karl Englund: Yeah, yeah, I agree with Kun on that one big time. And, and, and, you know, when you look at how people have recycled these plastics in the past, you know, we’ve, we’ve dealt with, you know, the bottles, the recycled post-consumer goods, and you see a lot of that. But one of the things that has been missing is, is the recycled plastics from the ag industry and it has, you know, it definitely has some negative connotations out there, especially when you look at and I’m looking mainly in the lens of the manufacturers and the, the polymer processors, of it just being dirty, um, nasty material that, that just doesn’t, is going to cause more issues than anything else and that I like a good challenge, and that’s definitely one of them is to kind of get that way. And now you’re starting to see more and more folks looking at this and saying, “Well, there, there’s some, there’s some definite positive attributes.” Um, the biggie, of course, is always going to be the contamination of soil and plant debris that you’re gonna see on some of these things. And so, we can work with growers and farmers and, and people that are producing these things and keeping the soil and debris and all that in the fields and, and making sure the plastic is nice and clean, then we have an option to work with.

[00:06:47] Nataliya Shcherbatyuk: Yeah. And, and Karl, speaking about the contaminations, what are the most common? And we’re talking about the soil, but are there any different types of contamination found that it’s been used, I mean recycling, trying to recycle mulch?

[00:07:03] Karl Englund: Yeah. You know, there’s, the contamination of the soil is probably the biggie and any type of plant debris or organic debris that you see on there. But then, you know, the other thing that people don’t quite understand is a lot of the fumigants and the pesticides and herbicides and everything else that they apply on here, and then what’s left in the soil that does get bound to the plastic. And, and some of those things cannot, you know, maybe they can process all right and all that kind of stuff, but it becomes, you know, as these things heat up, they off gas, and most polymer processing, both asphalt and, and in any type of composite production or densifying these plastics into pellets and stuff, is gonna require heat. And, anytime you put heat on something that’s volatile like that, you get some gases that come out there. And so, it’s not exactly if we can get those things out of there and the soil out of there and get nice clean plastic, we’re good to go.

[00:07:56] Nataliya Shcherbatyuk: Yeah, that sounds easier than it is.

[00:07:59] Karl Englund: Yeah, it is.

[00:08:00] Nataliya Shcherbatyuk: Karl, while we’re talking about, uh, while we’re talking about recycling plastic, can you, um, tell us a little bit more in details actually about the process of recycling plastic mulch, and how do recycling facilities handle the contamination and breakdown of these materials?

[00:08:18] Karl Englund: Yeah, that’s a good question, and that’s where a lot of ours; you know, what’s great about this grant that we have is we have some really good collaborators. You can see on our list on our website there all the different folks that are there, that are actually out in the field and doing these things. And most of them, one of the things that you’re gonna be concerned about is if you’re too small of a, a farm and, or a grower area kind of thing, you, you probably don’t wanna invest a lot of money in, in trying to get it recycled and go through the logistics of everything that occurs out there. And so, you know, I think bigger farms are where we see more people look at recycling these materials, and it’s, and it’s great. And so, they’re using some techniques that will help clean it in the field and create a roll or a wrap that’s got a lot lower contaminant level in there. And then depending upon where it is and what’s close by, and who wants to deal with it, then they can send it on towards that type of recycling operation, whether it be into consumer goods or, or if it goes back into, uh, some type of agricultural product or something of that nature. It there, there are success stories out there, but there still needs quite a bit of knowledge about it and, and some more information built up about it.

[00:09:34] Nataliya Shcherbatyuk: Well, and you mentioned that there are some success stories. Can both of you share any success stories or examples where plastic mulch recycling has been particularly effective? Who goes first? Kun, how about you?

[00:09:53] Kun Zhang: Yeah. So, uh, when we use plastic mulch, especially the plastic mulch is the low-density polyethylene, the LDPE. So, the PE can increase the high-performance grade of the asphalt binder and benefit the rutting resistance. This is especially promising as climate change, temperature rise, and the more frequent heat waves over the summertime, which could cause more rutting issues. While the recycling of plastic might be a promising and a sustainable solution to tackle the impact of climate change on asphalt pavement, which is the most important transportation infrastructure. So that is the most promising or the successful thing I can tell when we use plastic to modify the asphalt pavement.

[00:10:44] Nataliya Shcherbatyuk: Karl, what stories do you have?

[00:10:45] Karl Englund: Yeah. You know, we have a three-prong approach on, on the way we’re looking at these recycled plastic streams and, and recycled plastic mulches from ag, and one of them being asphalt like Kun is, is mentioning. The other thing that we do here at Washington State quite a bit is looking at composites and, and different applications there. And then we also have PDO Technologies that’s our pyrolysis person that turns these plastics back into a naphtha or a monomeric state, somewhere in that range. And so, you know, when you look at the plastics that are consumed quite a bit in the composite world, a lot of the decking, the Trex decking, the Fiberon or TimberTech kind of decking and other brands that are out there on, in your Lowes and Home Depot kind of places. You’ll see a lot of them starting to branch out. They’re usually some of the early adopters on different plastic streams that come out there. And they’ve been doing this for quite a few years. We’ve seen some success with folks that can handle like Kun was mentioning, is the linear low-density or a li- a low-density polyethylene, which is not used for too much in the post-consumer or, or in the durable goods sector, but it can be used. And so, there, you’re starting to see more people actually use that type of material. Which is good. It’s still got a long way to go, but we definitely are showing some, some progress.

[00:12:04] Nataliya Shcherbatyuk: And Kun, I have a question for you. So, you’ve mentioned a few times about asphalt recycling and seems like you have great experience of it. And we know that asphalt is often recycled in construction. So, can you explain why asphalt is a good candidate for recycling and how its recycling process actually differs from or shares something similar when we’re talking about or comparing it with plastic recycling?

[00:12:33] Kun Zhang: Thank you. Good question. So, I’m glad to take this opportunity and mention a little bit of, of the Asphalt 101. Yes, I’m like, a, competent in asphalt. So, um, yeah, for this project we use a, a plastic mulch, which, again, is made from the low-density polyethylene, LDPE, so we can consider as a polymer to modify the asphalt. And actually, the LDPE is the most compatible with asphalt production than other types of plastic because the melting point of the LDPE is around 110 to 130 degrees Celsius, around like 250 Fahrenheit. So, which is below or within the production temperature of the asphalt mixture, which is between 130 and 170 degrees Celsius. Roughly speaking, around like 3-, 300 Fahrenheit. So, plastic mulch recycling in the asphalt is a technical solution, but still with a higher contamination level the design and the performance of the asphalt mixture should be evaluated as a responsible, recycling solution.

Another thing is comparing the recycling of plastic in asphalt versus other mechanical recycling. So, asphalt roads may have a higher tolerance for the contamination in the plastic mulch. So, if we just do a quick mathematical calculation, so roughly speaking, in the asphalt mixture, it consists of 95% aggregate or rocks, and 5% aggregate binders to glue the aggregate together. Based on our preliminary results from this project, we have found adding 3% of plastic pellets based on the weight of the binder, showing a very promising result. That means we will add about 0.15% mulch pellet by weight of the total mix. And considering the contamination is around like 60 or 70%, so we will introduce only around 0.1% soil by weight of the total mix. So, the soil may also work as additional fillers in asphalt mix. So, this is the justification why asphalt is a good candidate for recycling. But again, we have to conduct extensive assessments in the lab, and ideally conduct the field test for the performance evaluation before we conclude the tolerance of the contamination in the asphalt applications

[00:15:17] Nataliya Shcherbatyuk: Wow, that’s fascinating. Super interesting.

[00:15:19] Karl Englund: And Kun kind of brings up some really good points in that in, in the sense that, you know, we targeted these different recycling technologies to be able to be a little bit more robust. You know, we’re not… one of the, the difficult things to do is to take this agricultural film, clean it up, and then turn it back into an agricultural film again, just because the processing parameters are pretty tight to make a film. And so we wanted to look at other technologies that would not necessarily close the loop, but continue on its life cycle in things like asphalt. And everybody has asphalt in their backyard or, or in the front yard or, or on the roads that they go to work and the parking lots that you, you’re in. And that’s really the key is to be able to have something that’s robust, that can handle this kind of contaminant loads that are gonna inevitably be on there. Even if you wash them really, really well, you’re still gonna end up with some contaminant load on there. And, and that having technologies like asphalt, like, like the composites that we’re producing really helps out as far as giving them a value-added pathway to go.

[00:16:29] Nataliya Shcherbatyuk: And on the topic of technologies, Karl, could you expand a little bit more on what innovations are being developed to improve the efficiency of mechanical recycling of plastic mulch?

[00:16:43] Karl Englund: Yeah, I mean, there’s, there’s quite a few. And our collaborators are probably the best places to go to see exactly how they do a lot of these things. But we do have some bailing operations that will scrape and clean the soil off of the plastic mulch as it comes off. And a lot of times they’ll take the drip tape too, which is a similar plastic but a little bit different than you’ll see on there. The key really in a lot of this whatever application you, you’re looking a is to keep the soil and the debris out in the farmland and not bring it back into right recycling facility, and then you have all this contamination that there you have to get rid of. You know, the soil and the plant debris is what makes the soil strong and good for growing again the next year. So, whatever the technology is, it’s really crucial that it keeps a lot of that, gets as much of it off in the field as you can.

And, and we purposely did not look at trying to research different technologies because we’re looking at it and saying, “All right. Whatever technology you decide to use, let’s look at the contaminant load.” If you get a lower…if you’re, if you’re at, like, 60%, 70%, like Kun was mentioning, you’re not doing much to clean that off. And, if you get down to 30% or, or 20% load, then, you know, you’re starting to clean it off, and these are the products lines that you’re gonna get to. And you look at it purely economically, say, “Well, you know, at 60%, 70%, you’re gonna get this kind of product yield and people are gonna pay this much for it. If you get it nice and clean, people are gonna pay a little bit more for it, and you’re gonna be able to have different options to do it.” And so, we wanted to kind of put forward the research that really evaluated the different product lines and different areas that it can go into, whereas, you know, you as a grower or a farmer can really kind of dictate how you get it cleaned up.

[00:18:31] Nataliya Shcherbatyuk: And now I’m curious, so both of you are looking at the plastic waste and its recycling. But how do you both approach the characterization of plastic waste to determine its recyclability? And what specific challenges arise in the context of plastic mulch additionally to what we already spoke about?

[00:18:55] Karl Englund: Yeah, I, I can start off with that one there, Kun.

Okay. So, you know, one of the things that really influences or changes the behavior of these plastics is not necessarily their final product performance. So that’s always a big issue, kind of how does it perform in the field, and how does it perform after you’ve turned it into that secondary product? It’s, it’s a lot of that processing issues. It’s how this material flows. And when you start putting contaminants there and you get soil mixed in with the plastic and stuff, you really start to disrupt the flow of it. And you start breaking rollers and screws a little bit quicker, kind of thing. They start wearing ’cause a lot of these soils are high in silica contents that wear and tear on metals. And so, a lot of what we’re looking at—both Kun and I, and especially here—is looking at what we classify as the rheology or how this material will flow. And the different types of soil that we have on and so has a really big impact on how these materials are processed, and that, that is one of the limitations of it going back into a film is how this material flows. And so that, that’s one of the things that we’re gonna look at really carefully and evaluate and, and then with that knowledge, then a lot of these polymer processors can make some more decisions about, “Yeah, we’re gonna take this stuff in. We can tolerate that,” or “We can’t tolerate that.” Kind of make a little bit better decision matrix that they’ll have and a little bit more power in the research behind it, so.

[00:20:34] Kun Zhang: Yes, exactly. As Karl mentioned that the rheology of the plastics, that will affect the performance of the asphalt as well. So, uh, as I mentioned, plastic can be a good polymer to modify the asphalt. So right now we see some, like, a positive impact, like when we add the plastic into the mix. So, the mix, the stiffness of the mix will increase, as I mentioned which will be the good news for the rutting resistance, especially when we use the plastic modified asphalt in the warm, uh, climatic regions. Well, like the asphalt mixture with the plastic mulch also will have slightly reduced cracking resistance. So that is the most challenging part when we recycle the plastic into the asphalt, especially if we use the plastic modified asphalt in the cold climatic regions. So that might be the issue.

Another issue, uh, we may have, or that requires further study, is the microplastic leaching from the pavement. So right now, we add the plastic pellets into the pavement, but, uh, because of the wearing from the tire, from the, uh, weather, like rainfall, there might be some like potential for the microplastic, uh, to leach out. So that requires further study on this topic.

[00:21:54] Nataliya Shcherbatyuk: That’s interesting. And for both of you, what are the most interesting and even surprising things you’ve learned about plastic recycling throughout your work?

[00:22:09] Karl Englund: That’s a good question. You know, one of the things that we, we, you know, in this study, you know, we look at different sites throughout the U.S. And, and not being a soils guy, I don’t really know much about soils, but it’s really kind of interesting how Washington soil compares to Nebraska soil, to California, to Florida, our different sites, on how those materials will actually stick to the plastic, and how hard it is to actually get some of those materials off the plastic. They kinda act like a clay-cement kind of material. Some will bond or bind a little bit harder to the plastic and make it a little bit harder to get off. And so, our levels change and, and they become a little different. But they also affect the rheology different. And, and so those things have been really kind of, I wouldn’t say pleasant, but they’ve been interesting things to study and evaluate, and I think it really shows the value of why we’re doing what we’re doing.

[00:23:04] Nataliya Shcherbatyuk: Kun, what about you?

[00:23:07] Kun Zhang: Yeah, for the asphalt study, so generally speaking, there are two recycling methods to include incorporating the plastic into the asphalt pavement, and we tried both. So, the first one is called the ‘wet mixing’ method which will blend the plastic with the asphalt binder first, and then the plastic modified asphalt binder is used to mix with the aggregate. So, in this way we consider the 100% blending, uh, between the plastic and the asphalt binder, if the temperature is high enough and the mixing time is long enough.

So, the second method is called ‘dry mixing’ method. The plastic pellets are blended with the aggregate first as a mixture additive and then mixed with the binder. So, I know there will be partial blending between the plastic and the asphalt using the dry mixing method, but surprisingly from this project, we see, comparing the both methods, we found the degree of blending might be even higher than 50% when we use the dry mixing method. So, this is probably due to the formation of plastic. It’s a thin film versus the solid pellets. So, in this way, the thin film can interact or react with the asphalt binder more effectively.

[00:24:30] Nataliya Shcherbatyuk: That’s pretty cool. Yeah.

[00:24:31] Karl Englund: Yeah, doing some cool things. That kind of goes back into that mixing. How do these things flow? How do these things behave? And, and, and what Kun is talking about is exactly that, you know, is how these materials are gonna flow and behave. And, and those are the really interesting things that come out of these things on a, on a research side, so.

[00:24:49] Nataliya Shcherbatyuk: That it is pretty interesting, ’cause, you know, if you think about pulling mulch, for example, from the field, one would think, “Well, it’s not a big deal. You just pull it off, you’re rolling it, and that’s it, it’s out.” But we don’t really think how much contamination of soil and the rest of contaminants are sticking to that mulch. Like you’re saying, they’re, they’re like glue, they’re just right there. So, it’s not as easy as we might think it. It’s just clean it off, shake it off.

[00:25:16] Karl Englund: Yeah. And I think you really ki- you hit a point there when you first were discussing this, is that, you know, there one of the, one of the biggest problems with plastic recycling is the logistics of getting that plastic to some place where they can modify it or they can, they can change it back into something that’s usable. And so, plastics kind of offer in some cases, it offers a unique attribute in the sense that you can get. So, you get these large farms in California or Florida or wherever that we can get a boatload of plastic, and that that pencils out very well in your logistics. If you get a lot of these smaller farms and, and people don’t wanna get rent or, or there’s no access to baling machines that will run these plastics up, or you have to do it by hand and everything like that, it becomes a little bit more of a logistic nightmare. Not only just getting it off the soil but also getting it to a site. And so that logistics and that supply chain logistics is, is huge in this and we recognize that big time as being one of the hurdles that we’re gonna see.

[00:26:26] Nataliya Shcherbatyuk: Yeah. Well, you know, looking a little bit into the future what are both of you most excited about the future of plastic mulch recycling, and what steps do you think need to be taken to improve current system?

[00:26:44] Karl Englund: I’ll take the lead on that one. I am excited about, you know, a lot of the research that we’re all doing within this recycling world and also some of the work that’s been done on the soil and microplastics that are in the soil and all that kind of stuff. And, and, and there’s lots of really cool, fundamental, geeky research stuff that goes on. But in the big picture being able to go to somebody like a, a wood plastic guy or, or an asphalt, hot mix asphalt guys and say, “Hey, we’ve done the research. We know what the contaminant loads are gonna do. We know, if you can get the contaminant load down to this level, it’s gonna behave like this.” And you can get different classifications like, like Kun has looked at for temperature zones that you can say, “Hey, you know, this will work out great for you if you, if you can get this stuff.” And so, you know, our goal really is to provide those pathways, to be able to say to the end user, to, to lower that risk, to put the investors out there and say, “Hey, this, if you get it cleaned, you have a pathway for it. And you can, you can get money out of this, and this can be a sustainable operation, not only environmentally, but also economically.” And that to me is, you know, that’s our goal really, to look at how we can do this thing, and keep it going on its own.

[00:28:04] Kun Zhang: Yeah. So, I’m pretty excited about this project because of the interdisciplinary work involved. So, actually look at specifically for the asphalt, as I mentioned that when we add the plastic into the asphalt mixture, it may compromise the cracking resistance a little bit. But luckily, we have another technology, like the pyrolysis of plastic. So right now, we are trying to use some, like, a byproduct from pyrolysis, so it’s a, it’s a wax-based product. So, we will try it and incorporate into the plastic recycling into the asphalt and improve the cracking resistance. So, in this way, so we have the very, like, a, uh, intensive collaboration among the teams, so then in this way, we can deliver the better solution for plastic recycling.

[00:28:57] Nataliya Shcherbatyuk: Wow. It’s definitely very interesting, and it’s going to be very interesting to follow it up and watch it. You know, a few years from now we might be looking at a totally different recycling system. Kun exciting. Yeah.

[00:29:10] Karl Englund: That’s what we’re hoping for.

[00:29:11] Nataliya Shcherbatyuk: Yes. That’s what you’re working for too.

[00:29:15] Karl Englund: Yep. Yeah. That’s correct.

[00:29:17] Nataliya Shcherbatyuk: Thank you so much, both of you. I think that was great conversation. Lot of information, thank you so much for making your time.

[00:29:26] Karl Englund: Yeah. Thank you, Nataliya. It’s been great talking with you and, and Kun and hopefully we helped out and pushed the needle a little forward on this, so.

[00:29:36] Nataliya Shcherbatyuk: I’ll definitely will be reaching out at some point to see if there is any updates that we can share with our audience.

[00:29:40] Karl Englund: Yeah. That’s the other thing is, you know, we are kind of just getting started in this in some regards, and so there’s gonna be a lot of really cool stuff coming out of this research and, and information coming out of this pretty soon. And so, tune back in.

[00:29:53] Nataliya Shcherbatyuk: Thank you, Karl. Thank you, Kun.

[00:29:55] Kun Zhang: Thank you.

[00:29:55] Karl Englund: All right. Thanks a lot.

[00:29:59] Nataliya Shcherbatyuk: That’s it for today and until the next episode. You can find more information by following us on Instagram and LinkedIn by @mulch_matters and going to our websites (www.smallfruits.wsu.edu) and choose ‘Mulch Technologies’. This work is supported by Specialty Crops Research Initiative Award 2022-51181-38325 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed on this podcast are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.