Reducing the Carbon Footprint of String and Glue (Fiber and Resin) Manufacture

#composites, #sustainability, #carbonfootprint, #renewableenergy, #fibers, #resins

Last week I wrote about the raw materials for composites and how high their carbon footprint is, primarily because at present most if not all raw materials for advanced composites are based in petroleum – big oil.  This week I want to follow the roadmap outlined in my new book to the next step.  That step is the production process for the fibers and resins that make up modern composites, that is – how to increase composite materials sustainability by focusing on reduction of the carbon footprint of the production and manufacturing processes for both the strings (fibers – mostly carbon fiber) and the glues (resins – predominantly epoxies).

From Research Gate - https://www.researchgate.net/figure/GHG-emissions-from-each-stage-of-the-production-process-in-the-manufacture-of-an-upper_fig1_354788267

I thought I would start this discussion with a pic (above) that shows where the major pieces of the greenhouse gas emissions come from in making a carbon fiber / epoxy composite part.  As you can see from the pie chart here more than half of the greenhouse gas emissions (CO₂) come just from the production of the fiber itself.  Another 8% comes from the production of the epoxy resin.   So, in total as much as 60% of the CO₂ emissions in the carbon fiber / epoxy part manufacturing process comes just from the production of the fiber and resin.  It is these two processes that constitute the second step in the roadmap to circularity and that I want to focus on in this post. 

Carbon Fiber Production

Let’s start with carbon fiber production.  To understand where the CO₂ emissions come from just getting to a carbon fiber, we need to look at the entire process from the raw materials that I talked about last week to a carbon fiber ready to use to make a part.  This has become a fairly standard process which I have described previously, but need to repeat here just so that everyone knows what it takes to make the carbon fiber that makes up your golf club shaft, ski pole, snowboard, bike frame, skis, tennis racket……... 

From “Carbon Fiber from Biomass” DOI:10.13140/RG.2.2.33590.80963

I like this pic because it tells a story about how the process works to get a usable carbon fiber from traditional precursors (blue-green boxes across the top) as well as some of the precursors that I wrote about last week (green boxes down the left side).  What I want to focus on in this post is everything from the box that says “Precursor” down to the winding of the carbon fiber on spools to either get high modulus (left side) or high strength (right side) fibers. 

Before we get to these orange boxes in this figure I want to make note of the greenhouse gas evolution that happens between taking the Crude Oil out of the well to where we get acrylonitrile.  As you can see from the process flow above, first the crude has to be refined which is commonly a distillation process that separates the heavier hydrocarbons from the lighter hydrocarbons like the naphtha that is used as the starting point for acrylonitrile.  If you see an oil refinery, typically there are at least two tall cylindrical towers, one for the distillation of the crude and one that is called a cracking tower where the distillation products are further broken down into the products that come from the refinery.  Typical refineries use some of the crude itself to provide process heat to the distillation and cracking processes.  And the cracking process itself also evolves quite a bit of waste CO₂.  So just getting to the propylene and ammonia that is used to make acrylonitrile is a fairly greenhouse gas intensive process. 

This process diagram, since it is from a paper about making carbon fiber from biomass, also includes the lignocellulosic sugars and glycerol that some have used to make acrylonitrile from plant based sources.  But what I want to move to next is taking the acrylonitrile and polymerizing it to polyacrylonitrile or PAN as it is called in the industry and the process energy that is required.  This requires heating the acrylonitrile in a solution that will allow it to start the polymerization reaction.  Typically this involves dissolving acrylonitrile in a solvent and heating it to about the boiling point of water to initiate the reaction.  Unfortunately, the reaction once it is initiated is highly exothermic, so this needs to be controlled by cooling the reactants and allowing the reaction to go to completion.  The cooling of the reactants also requires process energy. 

Now that we have expended the energy to make PAN and we have gone through the spinning step to give us PAN fiber, that fiber needs to be stabilized so that it doesn’t react with anything else and lose its strength.  This is typically done by stretching and slightly heating the fiber and running it through an almost inert atmosphere (has just a little oxygen in it) to slightly oxidize the surface of the fiber itself.  This does not require much process energy, so we don’t need to talk more about this part of the process.

It is the next step in the process where most of the process heat is required.  That step is carbonization or graphitization of the PAN fiber into raw carbon fiber.  It is in this step of the process where most of the CO₂ evolution happens.  That is because what has to happen to get from PAN to carbon fiber is that you have to heat the stabilized PAN fiber in an inert atmosphere to a high enough temperature to drive off everything that isn’t carbon.  The atmosphere is typically nitrogen, and the temperature range is from about 1000°C to 3000°C (1800° to 5400° F).  The carbonization furnaces in most if not all current commercial carbon fiber manufacturing plants are run either by natural gas or fuel oil.  Both of these produce copious amounts of CO₂ as well as other noxious combustion products just to get the carbonization temperature high enough.  So this is the major source of greenhouse gas emissions from the carbon fiber manufacturing process. 

It has been estimated that the total amount of CO₂ equivalent that is released per kilogram of carbon fiber can range from 20 to more than 70 kilograms.  The carbon lost from the PAN fibers in the conversion from PAN to carbon fiber is only about 50% of what was in the PAN fiber to begin with, so the rest of the CO₂ equivalent is entirely the natural gas or fuel oil combustion for generation of the process heat to drive off all the stuff that isn’t carbon in the PAN fiber. 

Courtesy of Microwave Chemical Co. Ltd.

Fortunately there are a couple of ways around this problem.  One that I want to highlight is from Microwave Chemical Co. Ltd. of Osaka, Japan.  I wrote about this company, an offshoot startup from the Graduate School of Engineering of Osaka University, a while back after I learned about them at the 2023 Carbon Fiber Conference.  I also wrote about them in my new book.  What these folks were able to demonstrate is the carbonization of carbon fiber through the use of microwave energy rather than the high temperature pyrolysis ovens of the current carbon fiber producers.  What Microwave Chemical Co. were able to do was to tune the frequency of the intense microwave energy that they bombard PAN fiber with to the correct frequency range to drive off what isn’t carbon.  Just to give you some reference, the microwave oven in your kitchen is tuned to a natural frequency of the water molecules in your food.  The difference here is that the microwaves are not only more intense, they are tuned to natural frequencies of the stuff that isn’t carbon in PAN fiber. 

There are really two differences in what their process does versus the traditional pyrolysis ovens used to carbonize the PAN fiber.  First, instead of heating from the outside in, microwaves heat from the inside out, especially if you have the frequency tuned to the molecules within the center of the fiber that you want to get rid of.  This is basically the same process as your kitchen microwave in that the heat in your leftovers is created by the microwaves vibrating all of the water in that leftover slice of pizza you put in your microwave which heats up the water and makes your pizza slice hot.  Secondly, the microwaves in this Japanese process are quite a bit more intense than your 1200 watt kitchen microwave. 

This company has patented this process and is working on commercializing it right now.  They have partnered with Mitsui Chemicals and Asahi Kasei separately in agreements to scale their microwave process up to something that can be commercialized. 

CARBOWAVE Project (https://carbowave.uv.es/)

In Europe there is another consortium called CARBOWAVE headed up by the University of Limerick that is also using microwave heating of the PAN fiber in a somewhat different approach from the Microwave Chemical process.  The CARBOWAVE project is first developing a microwave susceptible coating for PAN fiber to direct microwave energy (as heat) into the fiber and focus it so that they can first stabilize the fiber and then heat it from the inside.   The project is scaling this entire process up through a number of European organizations and companies, including members from Germany (Fraunhofer, DITF, Muegge), Italy (Stellantis which owns the Fiat brand), France (Microwave Technologies Consulting), and Spain (Universitat de Valencia), as well as two other companies in Ireland, Eire Composites and Juno Composites.  As of the writing of this post, the project has successfully met its first year milestones and has planned the next steps in the industrialization of this process to produce “green” carbon fiber through the use of microwave heating for process energy.  And while this process appears to be a bit more expensive than the Microwave Chemical process and involves the additional step of coating the carbon fiber with this microwave susceptible coating, it is nonetheless eventually going to win out over the use of petroleum for process heat to make carbon fiber.  Especially in today’s ever increasing prices for both natural gas and fuel oil.  Electricity after all is increasingly being generated using renewable sources. 

Epoxy Resin

Looking at the original pie chart at the outset of this post shows that making the carbon fiber is by far the biggest piece of the carbon footprint in the production of carbon fiber composite parts.  However, the epoxy resin also has a fairly significant footprint, and from the perspective of the composites fabricator is the second largest in carbon footprint of the things that need to be purchased to make a composite part. 

Epoxy Production Process - Courtesy Anshula Technological Engineering Consultants (ATEC) Pvt. Ltd.

Using some of the same metrics that we used for carbon fiber production, it has been estimated that epoxy resin production evolves about 4 to more than 8 kg of CO₂ equivalent per kg of resin produced.  This energy is first in the epoxy manufacturing plant energy requirements – process heat to generate the required steam, provision of nitrogen for the polymerization reaction, and circulation and cooling of the process water.  The next and possibly largest use of process heat is the heating and pressurization of the reactants to make bisphenol-A, epoxidizing the BPA to its Diglycydil ether and polymerizing the DGEBA into a liquid epoxy resin. 

All of this chemistry is endothermic (requires heat to maintain the reaction) and requires both steam to heat the reactants to make the reaction proceed and cooling water to cool the products and condense out the liquid resin from the gaseous reaction products.  As you can see from the cartoon above, this is a somewhat involved process and requires quite a bit of process control to make it run efficiently.  And of course, what you also see in the cartoon above is a very typical petrochemical production process, so there are quite a few smart petrochemical engineers that have not only worked this out but continue to tweak not only this process but also the stuff that goes into the cauldron to begin with.  That is why there are so many different epoxies on the market today.  And unfortunately all of them that start with crude oil have about the same level of carbon footprint as what was stated above. 

So, that’s it for this week’s post.  And that is the second step in the roadmap that is laid out in my new book, if you’re interested.  Next week I will move on to the next step which is the manufacture of the composite part itself, and the energy as well as waste material that is involved in that process.  So, stay tuned since I will be providing the entire roadmap to circularity in composite materials in this series of newsletters. 

As always, I hope everyone that reads these posts enjoys them as much as I enjoy writing them.  And I hope people who are interested find something they can use in their lives or at least some ideas that they might be able to put into practice.  At least I hope that these make people think a bit about sustainability and some of the major issues looming before us. 

I will post this first on my updated website – www.nedpatton.com – and then 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.  And that is especially true of the companies and research institutions that I mention in these posts.  The more we communicate the message the better we will be able to effect the changes in the industry that are needed. 

My second book, which will be officially released on April 6, is a roadmap to a circular and sustainable business model for the industry which I hope that at least at some level the industry will follow.  Only time will tell.  Maybe it will get noticed – as always that is just a crap shoot.  As far as schedule is concerned, I received my author copies last week, so the second pic at the end of this post is the cover of the actual book.  Let me know whether or not you like the cover.  Hopefully people will like it enough and will be interested enough in composites sustainability that they will buy it.  And of course I hope that they read it and get engaged.  We need all the help we can get. 

Last but not least, I still need to plug my first book.  “The String and Glue of our World” 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 August of 2023 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. 

So, I will send you a signed copy of either or both books for the same price you would get charged on Amazon for an unsigned one, except that I have to charge for shipping.  Just go to the link to the product page on my website (https://www.nedpatton.com/product-page) and order either book.  And as usual, here are pictures of the covers of both books.