#composites, #sustainability, #bamboo, #bamboocomposites, #biobased, #naturalfiber, #cellulose, #lignin, #plantbased
The answer to this question is yes, bamboo is a very usable plant-based source of raw material for ligno-cellulose based composites. And the fact that it is a woody and perennial grass makes it potentially a very good, sustainable source for raw material for composites.
Bamboo is of course cellulose and lignin, with a few other polysaccharides in the mix. And it has a unique structure where most of the cellulose fibers are oriented in the growth direction of the bamboo stalk. Interestingly, the cellulose framework of bamboo is rather complex and three dimensional, which is what gives bamboo its strength. That framework is made up of cellulose, hemicellulose, and lignin all connected together in a complex network. This three dimensional network has its strength axis in the long direction of the bamboo, allowing the bamboo to grow very tall very fast. And the tubular structure is hollow and in sections separated by a solid web which is where the leaves of the bamboo stalk grow.
People where bamboo grows have been using it as a structural material for centuries. India and China have the largest plantations, with smaller plantations in the Americas, mostly in the jungles in South America. So, there is a lot of tribal knowledge about how to process green bamboo into a structural material primarily for habitable structures – people actually build houses out of this stuff. In modern times, bamboo flooring in contemporary houses has become quite popular because it looks good and wears well. And there are nearly 1700 species of bamboo, so it is a very diverse group of plants.
Since this is a newsletter about composites, I need to talk a bit about the potential for using the ligno-cellulose biomass from bamboo as precursors for both fiber and resin in traditional composites. To start with, bamboo has a fairly high percentage of cellulose – as much as 50% in some species. The remainder is hemicellulose and lignin each at around 25% more or less depending on the species of bamboo. And the structure of this material has been described as a “sophisticated three dimensional composite frame” structure (1). In other words, bamboo biomass is ripe for using for precursors for composites because it is abundant, grows very rapidly, and is a sustainable material.
One interesting article I ran across from some researchers in China (2) demonstrates that with fairly minimal processing the cellulose in bamboo can be made into a very effective composite for use as a structural material, primarily in buildings. What these researchers did was to remove the lignin in the bamboo through a “steam delignification” process and then convert the remaining cellulose acetate into cellulose. This left this tangled three-dimensional structure into which they infused raw cellulose acetate as a binder, and compressed it at an elevated temperature to enhance the cross-linking and hydrogen bonding between the cellulose fibers. What they ended up with was an all cellulose composite that had nearly 7 times as strong and 4.5 times as stiff as natural bamboo. And they could make common sized structural members (like 2x4’s here in the US) to use in basic engineering structural applications.
And, bamboo fiber itself makes a better fiber than most natural fibers – even better than flax fiber – because of their three dimensional complex structure. In one study researchers at Michigan State University were working on a plant-based epoxy using an epoxidized softwood lignin. They used this plant-based epoxy with unidirectional bamboo fibers to create a composite material that was entirely plant based. When they originally made the epoxy, they compared it to a pure bisphenol-A based epoxy (one of the EPONs originated by Shell Chemical before they sold off all of their chemical business) the plant-based epoxy had approximately 36% lower stiffness than the standard petrochemical based EPON resin. However, when they married it with unidirectional bamboo, their composite resulted in an 11% increase in strength and stiffness over the same composite made from EPON resin and the same bamboo fibers. This is a very good result and can possibly be explained by the potential hydrogen bonding sites on the epoxidized lignin versus what’s available in traditional bisphenol-A epoxies. And, of course lignin is the glue that holds plants together so it is no wonder that it holds the bamboo fibers together better than the traditional epoxy which is really just a synthetic glue.
And, at the University of Maryland some researchers in the Forest Products Laboratory developed a technique for using the lignin naturally occurring gin the bamboo to make a completely consolidated composite material that has better mechanical properties than just the bamboo itself. What they did was to chemically treat the bamboo in sodium hydroxide (NaOH) to remove some of the lignin and hemicellulose which softened the bamboo and left a porous structure. Then they used high pressure and an elevated temperature to compress this softened bamboo and got the remaining lignin to act like a very strong adhesive, effectively making a high fiber content, completely consolidated composite material.
Interestingly enough, the researchers in their paper pointed directly at the NaOH softening process opening up more sites for hydrogen bonding between the cellulose fibers themselves as well as between the cellulose fibers and the lignin glue. And, the mechanical properties of this new material – which is a much higher strength version of glubam – are far better than what you can get with other common structural materials used for housing construction. As an example, the glulam beams that are holding up the roofs of some modern houses have a tensile strength on the order of 5 ksi whereas the glubam came in over 40 ksi. This is better than most bamboo composites using more traditional approaches. The problem with bamboo composites in the past has been that bamboo fibers do not typically stick all that well to traditional resins. It doesn’t form cross-linked bonds and there are not that many hydrogen bonding sites on the fibers. By using the naturally occurring lignin in the NaOH softened bamboo and consolidating what’s left of the bamboo at high temperatures allowed many more hydrogen bonding sites to open up on the cellulose fibers as well as the lignin, so not only did they get cellulose-lignin bonding, they also got cellulose-cellulose bonding. This made for a very strong material that can be used in the very same way that you would use a traditional glulam beam made from wood.
So, bottom line, the answer to whether or not you can use bamboo to make a composite material is absolutely yes. And in fact, this could be the sustainable building material of the future because bamboo grows to maturity in the space of 3 to 4 months – a typical growing season, whereas trees take decades to grow the same quantity of useful biomass for composites.
That’s about it for this week. I hope everyone that reads these posts enjoys them as much as I enjoy writing them. As usual I will post this first on my website – www.nedpatton.com – as well as on LinkedIn. And if anyone wants to provide comments to this, I welcome them with open arms. Comments, criticisms, etc. are all quite welcome. I really do want to engage in a conversation with all of you about composites because we can learn so much from each other as long as we share our own perspectives.
I also wanted to remind everyone that I will be speaking at the SAMPE conference in Long Beach in May. I’m going to be talking about the subject that I have a passion for – composites sustainability. Maybe I can help the industry a bit again, maybe even rattle a few cages like what happened at the Carbon Fiber Conference in Salt Lake. One can only hope. Anyway, for anyone that is interested in materials and process engineering, SAMPE will be a great conference. And they will have a really great exhibit as well.
And, for my readers that are located in the Bay Area, I am doing a book signing at the Stanford bookstore on May 10 from 2 to 5 PM. I am really excited to do this and I really hope that I see as many of you as can come to this. I would love to meet you all and have conversations about composites, and life in general to be honest.
And, finally, I always need to plug my book, so here’s the plug. The book pretty much covers the watershed in composites, starting with a brief history of composites, then introducing the Periodic Table and why Carbon is such an important and interesting element. The book was published and made available last August, and is available both on Amazon and from McFarland Books – my publisher. However, the best place to get one is to go to my website and buy one. I will send you a signed copy for the same price you would get charged on Amazon, except that I charge $8 shipping. Anyway, here’s the link to get your signed copy: https://www.nedpatton.com/product-page/the-string-and-glue-of-our-world-signed-copy. And as usual, here’s a picture of the book, for those of you just tuning in.
