Coconut Husk Fiber for Composites? And some Other Stuff

#coir, #composites, #fiber, #sustainability, #coconut_husk, #coir_fiber, #coconut_husk_fiber, #natural_fiber, #green_composites, #natural_composites

Article reposted from www.nedpatton.com

As a matter of fact – yes.  Coconut husk fiber, or coir fiber can be made into a rather good natural fiber for composites.  As it turns out, coconut husk has a higher percentage of lignin than do a lot of other ligno-cellulose plant fibers (1), which makes it more workable and also more damage tolerant which is great for making a tough composite material.  And, since coconuts evolved in the tropics, the husk fiber itself is rot resistant, so coir fiber is resistant to biological attack.  In fact, intact coconuts can float on the ocean for weeks to months without degradation.  Obviously that’s why there are so many coconut palms near the water in the tropics. 

So, my post this week is going to be focused on a natural plant fiber that largely goes to waste in the developing world – coir or coconut husk fiber.

I have a person to thank for bringing my attention to this.  A gentleman from a region of the world that grows a lot of coconut contacted me through my website.  I don’t want to provide his name of course without his permission, but he is an entrepreneur who lives where they grow copious quantities of coconuts, and has recently started a small coir fiber processing mill.  He is trying to find another vertical market for this fiber other than sending all of the raw fiber to China for processing into ropes and mats. 

After taking a look at this, I came to the conclusion that this is a nearly untapped market for a renewable fiber that – with some additional processing – would be a very good natural fiber for use in composites.  Currently, at least according to the paper in reference 1 to this newsletter, the fiber tends to be a little inconsistent in raw form, which is to be expected from a natural fiber.  But with some surface treatments as well as some additional processing it appears that Coir fiber has potential as a natural fiber reinforcement for composites.  This will be important in those parts of the developing world that have ready access to this fiber – as in where coconuts are grown – for them to develop their own processes for making a composite fiber out of what is currently organic waste. 

It is estimated that only about 15% of the coconut that comes off the palm tree is actual edible fruit.  The rest of the nut, which includes the hard shell around the coconut meat as well as the husk is about 85% of the coconut by weight.  That is a lot of green waste from one coconut.  And coconut palms can produce as many as 100 coconuts a year per tree.  So, there’s lots of this stuff being turned into compost or being land-filled. 

All of the fiber that you see in the husk sitting on top of this pile is usable fiber, once it is reprocessed into a form more suitable for using in a composite.  Doing this starts with a process that is ancient, called “retting.”  I had to look this up to understand it, but basically the husk pieces like what you see above are placed in a slow moving stream or creek bed, covered with dirt, and left for a few months.  The water softens up the husk and the biological action eats away at everything that isn’t a usable fiber, leaving behind just the fibers that can be made into things like coconut mats, rope, etc.  And, if the fibers are then given a few more treatments they can be readied for use in a composite.

One of the properties of coir fiber that make it as tough and rot resistant is the fact that, unlike most other plant fibers that are mostly cellulose with just a small amount of lignin, coir fiber is composed a little less than half cellulose, roughly half lignin, and with 1-20% hemicellulose thrown in the mix depending on the species and growing region.  Cellulose and hemicellulose are both polysaccharides whereas lignin is an organic alcohol (polyphenol).  So, while cellulose being sugar-like is crystalline, lignin being alcohol-like is more amorphous and slightly rubbery.  This makes coir fiber very tough, harder than other plant fibers, abrasion resistant, and resistant to biological attack.  They also have high stiffness, tensile strength, and elongation at break.  Coir can have as much as 40% elongation at break depending on the quality of the original fiber and how it is treated.  This is much higher than most other plant fibers that have been proposed for use in composites. 

The most common fiber treatment used with coir fiber is to put it in a solution of sodium carbonate or soda ash which is an alkali treatment that breaks down the oils, waxes, and fats on the surface of the fibers and leaves some disrupted hydrogen bonds that attach themselves nicely to resins of all types, including thermosets and thermoplastics.  Dilute solutions of sodium hydroxide also work in the same manner – effectively stripping all of the oily hydrocarbons off the surface of the fibers and activating some hydrogen bonding to the surface of the lignin.  Fortunately for coir fiber, the lignin is primarily on the surface of the fibers and the cellulose is mostly a backbone in the center, so the composite resins stick mostly to the lignin. 

There are of course other treatments to the fiber that are use, like silane treatment, treatment with maleic acid based coupling agents, adding acetylene groups to the cellulosic parts of the fibers, and benzoylation treatments that put a phenol functional group on the surface of the fiber for the plastic resin to bond to.  Most of these treatments start with an alkali soak of either sodium carbonate or sodium hydroxide to remove the waxy and oily residue from the surface of the fiber. 

Once the fibers have been treated to enhance their surface properties, they will stick to almost any plastic resin.  This includes nearly all of the common thermoplastic composite resins like PLA, Polypropylene, Polyethylene and HDPE, and some others.  In addition, nearly all thermosets are very compatible with the surface treated coir fiber.

One more interesting use is the incorporation of coir fiber into a cement product to produce high performance building materials.  Since the fiber is rot resistant and sticks well to almost anything, as well as being very inexpensive, it could easily be used to make a very reasonable filled cement board for construction of living spaces.  In addition to its strength, stiffness, and toughness, a high fiber loading in a cement board will also provide some insulative properties to the structure, especially if there is the incorporation of air in the cement board.  This would also make the board quite a bit lighter weight and easier to handle. 

So, as you can see, there is a tremendous potential for this natural fiber for making “green” or sustainable composites.  And, on that note, I need to let everyone know that I am going to start focusing more on sustainability in composites as time goes on in this newsletter.  Sustainability of composites has become one of the fastest growing areas of research and new product development in the composites business.  In fact, Composites World has recently announced a new microsite dedicated to sustainability of composite materials (https://www.compositesworld.com/kc/sustainability). 

That’s about enough for this week.  I am going to be reminding everyone right here each week that I will be presenting two papers toward the end of this year.  Both of them are about sustainability of composites – a subject that all of you that have read my posts know is a passion of mine.  The first one is a paper about sustainability efforts for composites in general, with a focus on what to do with wind turbine blades.  That one is going to be at the International Mechanical Engineering Congress and Exhibition in New Orleans next week (https://event.asme.org/IMECE).  Since I will be in New Orleans all week, I’m going to skip next week and I will be back the second week in November with a new post.

The second paper will be at the Carbon Fiber Conference in Salt Lake City being put on by Composites World (https://www.carbonfiberevent.com/).  In that presentation I will focus on current work in sustainability of carbon fiber in particular.  I’m of course going to talk about new fibers and fiber precursors made from plants, so again, focusing on closing the circle.

And, finally, for those of you that have not heard, my book has been published and is for sale.  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.