Composites – fiber reinforced polymers – offer numerous advantages to construction, including light weight, high strength, good thermal insulating properties, great durability, and long service life. Sustainability, however, has been a sore spot. The polymers – resins such as epoxy resin – used to bind composite materials are derived from petroleum, a dwindling resource. Carbon fiber is also petroleum based. Glass fiber, even though its source is abundant, can hardly be considered a renewable resource, either. The process to turn these raw materials into epoxy resin, glass fiber, or carbon fiber all have relatively high carbon footprints, too.
But work is being done to solve that problem. Bio-composites – composites made from biologically-sourced raw materials – are being developed, and some are available already. We have reported previously on wall cladding and other products made of Nabasco, a combination of soy-based resin and flax or hemp fiber reinforcement, created in the Netherlands. This week, and in the near future, we’ll be looking at biocomposites being made in the US, some under development and some already commercially available.
At North Dakota State University (NDSU), a team of researchers are pursuing an even more ambitious bio-composites goal than cladding: structural materials. Bio-fiber research is being led by Chad Ulven, PhD, Associate Professor in Mechanical Engineering, College of Engineering. Bio-resin research is being spearheaded by Dean Webster, PhD, Professor and Department Chair, Coatings & Polymeric Materials. The bio-fiber and bio-resin have successfully been combined into a composite that has real structural potential.
(Another research effort at NDSU, towards an even more sustainable bio-resin, will be featured here soon.)
Ulven and his team are working with flax fiber. North Dakota is the source of 90% of the flax grown in the US, so the university has an interest in developing its uses. When Ulven was hired as faculty there, he came from a background in composites for the defense and mass transportation industries. “I wanted to develop technologies that had benefit for the rural economy, and that were more sustainable,” he recalls.
Developing flax fiber for an industrial use like construction requires striking a delicate balance. US flax production is almost entirely for the seeds. In other countries, the fibrous stalks are harvested for making linen. US farmers usually just leave them in the fields. After harvesting the seeds, our farmers often they burn the field to get rid of the stalks. For those stalks to become an industrial commodity in the US, the price of the fiber has to be high enough to make it worth the farmer’s while to bring it in from the field. At the same time, it has to be low enough (or have some other compensation) to make it competitive with mineral fibers in the composites market.
Since flax is a natural material that’s been used for textile fiber for thousands of years, the casual observer might assume that there’s not much left to develop about it. But when you’re putting it to structural uses that have to be engineered, consistent and reliable physical properties are essential. That level of consistency is something natural materials often lack.
Ulven is trying to find the optimum level of processing to produce fibers that are usable for structural composites.
It turns out that the strength and other properties of flax fiber can vary considerably depending on weather and other environmental factors affecting growth and harvest. “There’s a tremendous amount of variability in natural fibers. That’s challenging and scary for engineers trying to design structural components. If strength modulus can vary 15-20% based on growing conditions, harvest conditions, and storage conditions, engineers are reluctant to rely on it.”
The fiber can be left in a relatively rough condition, or separated and broken down into thinner strands, even down to the nano scale. This is another balancing act. “You can mechanically damage the fiber if you process it too much,” Ulven explains. On the other hand, “the more you separate, the more surface area there is for resin to bond. There’s this happy medium between separation and damage.” However, there are very few decorticators – people who separate the natural fibers – in the US, because of the lack of a flax textile industry. Ulven has had to do considerable testing in this area.
Ulven is also invovled in developing standards that will allow natural fibers to be tested and rated for construction purposes, so they can be trusted when desgining applications. He is a member of ASTM International’s committee D13.17 dealing with flax and linen.)
Another major fiber issue is compatibility with resins. “Natural fibers, being hydrophilic in nature, don’t tend to mix well with resins, petro or bio-based, which are usually hydrophobic. To get good structural reinforcement, the fiber has to adhere well to the resin. We looked at one approach where we modify the fiber’s surface chemically – treatment with various chemicals, plasma treatments, hot water – to try to expose the fiber and make it chemically so its polarity is similar to the resin. We’ve also looked at manipulating the resins, adding things to existing resins or making new resins that favor bonding to the fiber.
“I think from a sustainability standpoint,” Ulven concludes, “it’s much better to do a minimal cleaning of the fiber, and utilize the right resin or additive for your resins, rather than treating the fibers – sometimes with some pretty harsh chemicals – to make them more compatible with existing resins.”
The reinforcement fiber he’s deriving has a modulus similar to glass fiber, but about 40% lower tensile strength. However, it is also less dense than glass fiber. If you make two composite parts of the same weight, glass fiber vs. flax fiber, they have roughly equivalent tensile strength, but the flax-based composite has about 40% more volume.
To bind the bio-fibers, Dean Webster is making epoxy out of soy bean oil and sucrose, epoxy sucrose soyate (ESS). “We start off with soybean oil, and react it with sucrose (sugar),” Webster explains. “Sucrose has eight different sites where you can attach a fatty acid from soybean oil. You can convert double bonds into epoxy groups. It has a lot of epoxy groups.
“When we cure it, it makes a very dense, highly cross-linked material that gets very stiff and has very good strength. We’re getting thermosets that have mechanical properties in the ballpark of petro-based resins. We have a lot of variations we can do with the chemistry, but we generally, have similar modulus, strength, flexibility and things. It’s durable to UV light, more than petro-based resins, although we still need to do more work.
“We’re really excited about the technology, proving that it’s feasible. Compatibility is very important. Bio-based and glass fibers seem to interact well with the resins. We’re able to get good performance, even in the structural composite regime, which I don’t think either of us expected.”
Unlike petro-based resins, Webster’s soy-based resins contain no bisphenol-A (BPA). BPA is a controversial synthetic compound that is used in many plastics such as water bottles, and has been banned for use in baby bottles in Europe.
Moreover, Webster believes that this use of soybean oil does not compete for food resources. “The United Soybean Board did a study looking at total amount of soy avail worldwide, and demand for food uses is well below total available supply,” he notes.
The composite they produce has been tested not only for strength, but also for durability. The tested samples were laminate panels made with ESS and flax fiber, compared with control samples made of Aradite 8601 epoxy resin with flax fiber, or Aradite 8601 with glass fiber. They were produced via vacuum infusion with 4.5 tons of pressure added.
The bio-composites displayed similar flexural strength, tensile strength, and modulus when the samples were new. After accelerated weathering tests, the bio-composite held up as well or better than mineral-based composites. (Bio-resin and flax fiber actually did better than bio-resin and glass fiber.) The ESS sample exhibited less loss of physical properties than the petro-based composite in all areas except flexural strength.
They have also made a variety of objects as proof-of-concept, including components for farm implements, busses, and sporting goods. Ulven describes the performance of their bio-composite bicycle frame as “really quite amazing. It provides a lot more dampening to vibration because of the natural fiber. It’s lighter-weight than a lot of its counterparts. And it’s stiff out of plane, but provides more give in-plane.”
Ulven’s goals in the construction world are somewhat unusual for composites. Many of the folks currently designing and fabricating architectural composites come from a background in boat-building, and think very much in terms of self-supporting shells. Ulven is looking at something a bit more like traditional construction, a frame-and-cladding scheme. He envisions a structural frame made of pultruded bio-composites. “If we can produce things like I-beams, C-channels, things of that nature – and we’ve shone that we can – then sheeting would be added to that. The structural component is a panel system with a pultruded frame and [laminate] skins. We‘ve done some preliminary work with a pultrusion company, and I’m aware of some other people who have done some experimentation.”
“We’re able to achieve properties that are considered structural,” Ulven says of the composite, “with renewable content up to 85%.” (The resin is about 66% renewable content.)
Next week: bio-based resins that are commercially available right now!