One of the reasons composites perform so well is that they combine the physical properties of two materials that have different, so that they work together in a complementary manner. In order to get consistent, reliable performance, the two materials have to be combined intimately, mixed together thoroughly, so that they are still distinct substances, but essentially inextricable. As a result, fiber reinforced polymers are notoriously tough and durable.
For recycling to be effective, you would have to un-composite the composite. That has been the main thing that composites have always promised NOT to do. It also makes them virtually impossible to recycle. FRP is made with thermoset resins that are hardened by a chemical reaction. The reaction is irreversible, and the resins have been developed to resist degradation by light, chemicals or other agents. If the resin and reinforcement have been thoroughly intermixed, once the resin hardens, they become truly inextricable. They can’t really be reused in any other way. They can’t even be repurposed except in exactly the same shape.
So recycling has always been considered impossible, and it has been a black mark on the sustainability resume of FRP.
Now, recyclable composites are arriving. Research at North Dakota State University, and a startup in the San Francisco Bay area are both coming up with resins designed to be un-polymerized on command, and perform with full composite strength until then.
Jayaraman Sivaguru, the James A. Meier Jr. Professor in the Department of Chemistry and Biochemistry at North Dakota State University is a photochemist by training. He is working on thermoset resins that can be degraded using light. (He is doing his research in parallel with NDSU colleagues who are researching bio-composites, profiled here a few weeks ago.)
“Start with a monomer bio-mass,” explains Sivaguru, “and make a polymer out of it. After its lifetime, you degrade it back to the monomer. We want them to degrade when you want them to. You program the degradation. We don’t want it to degrade in just any way, we want it to degrade into the original material.”
Sivaguru has been using plant-derived fructose, which can come from a number of different plant sources (he mentions beet sugar as one of them).
His method is to degrade the polymer with light. You could use 350 nm wavelength – ultraviolet (UV), for example, but that would make it susceptible to degradation by UV in sunlight. “You can customize it for which valent you want,” Sivaguru points out. “You can make it for 300 nm, which is absent in sunlight.” 300 nm is the far UV range, and is filtered out of sunlight by the small molecules of oxygen and nitrogen in the upper atmosphere.
“You don’t want it to degrade into something more toxic than the original. That’s also being studied right now, whether it’s degradable by microbes, etc. So we’re doing a life-cycle analysis.”
His goal is that the material must be comparable to petro-based resins in cost and performance. “We are not there yet in terms of performance,” Sivaguru states. “We are just starting. Petro-chemicals have a head start of 80 or 90 years. We are nowhere close to the performance of fossil fuel-derived materials. We have shown it as a proof of principal. We have shown that you can complete the cycle: make the polymer, degrade it to monomer, make it back into a polymer. A cradle to cradle approach.”
Sivaguru comments, “Basic science is slow. We are taking baby steps with it. I don’t know when we will be able to run with it. My goal is in five years to have a completely workable system. Let’s see how far we can go with that goal. This is a problem that cannot be solved by one person. It is a problem that is too complex. It requires multiple disciplines, multiple solutions. This is a problem that really requires teamwork.
Connora Technologies of Hayward, CA is developing recyclable thermoset resins. (Connora is the sister company of Entropy Resins, makers of plant-based resins for composite manufacture, profiled here a few weeks ago.) Their motto is “Performance Without Permanence.” They explain the program succinctly on their website:
Connora’s revolutionary epoxy resin system, Recyclamine®, enables the next generation of performance composites and adhesives to be recyclable for the first time. The Recyclamine® epoxy system delivers chemical resistance and mechanical properties similar to conventional, non-recyclable amine-cured epoxies. Flexibility, toughness, and low temperature properties are also equal or superior to conventional epoxies.
Using a low energy recycling process, high value materials (e.g., carbon fiber, fiberglass, kevlar) can be recovered and repurposed from composite waste. Today’s high performance thermoset composite products can now be designed for recyclability, while maintaining performance characteristics.
“When you recycle our epoxy thermoset,” explains Connorra CEO Rey Banatao “you actually get back an epoxy thermoplastic.” Thermoplastics are polymers that melt and go into a liquid state at a certain temperature, and return to a solid state when cooled. They are not as high-performance as thermosets, and are not used for composites, but they have a very broad range of uses, and they are recyclable. (This transformation, from thermoset to thermoplastic, is an important difference between Connora and the work being done at NDSU, where the aim is to revert the polymer back to its original monomer and be able to make the same polymer from it again.)
Connorra’s approach is based on a their special Recyclamine curing agent. “Reclyamine is applicable to any di-epoxy. The most basic one, di-epoxy, is what we’ve been working with. It would work with bio-based epoxy. It’s very complementary.”
“The main value we’re finding is in manufacturing waste. Due to unrecyclability, there’s a lot of waste. The American Composites Manufacturer’s Association (ACMA) says that 10-40% of raw materials become a waste product that goes to landfill. Our initial customers, mostly in sporting goods, are finding they have a waste problem that this solves for them. For companies who are really trying to walk the talk around sustainability, there aren’t a lot of products out there. Companies like that who have a focus on moving the needle on sustainability, we have a lot of success helping them.”
“We use pH as the main recycling mechanism of action. The way the chemistry works, we build in a bond in the middle of the polymer, a cleavable bond that will cleave at an acidic pH. When you cut that bond, you basically get back a long chain polymer. There is also a required solubility. To make the pH trigger happen, we have to make sure the polymer can dissolve into the solution as we go.
“We put it in a bath. Then neutralize the acid. When you neutralize that acid, the plastic precipitates out. You can filter it and dry it, comes back as a whitish powder. The solution can be disposed of or recycled as water, with a byproduct of salt. Powder can be compounded and extruded, sometimes you fiber fill it to give it strength, pelletize it and it becomes injection moldable, or film, hot melt adhesive film. There are very few highly adhesive thermoplastics. Thius is good anywhere where you need high strength adhesion and fast application.”
As far as the costs of bio-based materials and recyclables, Banatao observes, “Most customers don’t want to pay more than 10% more for anything, no matter what your value proposition is. Our job as a technology company is to find ways to make the chemicals less exotic and more affordable. We quickly kill projects where the chemicals get to be too exotic. If you need exotic products or equipment, it kills the viability. We look for processes you can do with more affordable chemistries. Connorra’s validation point ended up getting us a development and license agreement with Aditya Birla Group, one of the largest conglomerates in India. They found value in our technology because we have an industrial approach. Our approach has always been on cost-effective chemistries.”
The architectural community has been a leader in addressing sustainability issues. If the non-recyclability of composites has been a sticking point for architects, take notice: that’s about to change.