In this article, we’ll be discussing a Recycled Fiber Material and Method by SRAM, US publication 20210079191. The publication date is March 18th, 2021 and the filing date is Sept. 14th, 2020. This patent has not been granted yet.
This article won’t be for everyone. It’s wordy and has no pictures. But it’s worth a read to learn something, because we both know you’re not doing shit right now and it’s an important topic.
Brief Summary (tl;dr)
This document is showing that SRAM are attempting to improve their carbon recycling process, which I massively commend them for putting the time and money into this research. In short, they’ve created a process to reuse their carbon scrap and to produce a fiber-containing molding compound, which is a secondary product that can be used in molding parts. Think car bumpers. The unique part of this idea is these compounds have adjustable strength properties based on the chemicals used and the amount of chemicals used to break down the old resin in the scrap carbon. So, SRAM can control the strength/flexural properties of the secondary product they’re making out of the scrap carbon. I love this kind of shit.
A good portion of this section was written based on this fantastic paper on carbon fiber waste, by Gopalraj and Karki. https://link.springer.com/article/10.1007/s42452-020-2195-4
The bike world has a bit of an issue with sustainability, recycling, and overall Captain America mentality – and I support it. I’m a big fan of Earth, it’s where I live. If I can do something to help it last a little longer or to be a decent place for future generations, I’ll do it (to an extent, see image to right).
In the grand scheme of things, the bike industry is not a major player in the carbon use or carbon waste industry – though there’s always room for improvement. It appears that aerospace and wind energy are the major contributors to carbon waste (I can’t find accurate percentage/industry numbers, so I won’t guess). The aerospace statistic didn’t surprise me, but the wind energy did. According to the paper by Gopalraj, wind energy is not quite as green as we’d like to believe. I’m hoping this line opens your mind on the current state of wind energy:
…despite their green credentials, wind turbine blades made from [glass fiber] have a lifespan of approximately 20 years. This means that, by 2030, an estimated 100,000 tonnes/year of wind turbine blades will be cumulated.
That’s a staggering number. To put that in a weight perspective, that’s roughly 41 million 2021 Specialized Stumpjumper frames… every year. But I digress.
There are three primary methods for carbon recycling: mechanical, thermal, and chemical.
Mechanical recycling physically cuts or crushes used carbon material into very small pieces. The processed carbon materials are broken down into two groups: coarse and fine recyclates. Coarse recyclates have high fiber content and fine recyclates have high resin content. Mechanically recycled carbon would typically be used for fillers or reinforcement applications. For example, there are automotive fillers (Bondo) that use crushed carbon as an additional reinforcement to the paste. I can confirm that that shit is strong as fuck — I’ve got it on my Datsun right now. This process would probably be the most cost-effective.
Thermal recycling uses heat to break down waste carbon material. The secondary materials of the carbon (resin, fillers, etc.) are burnt into our atmosphere, and the carbon remains. There’s a delicate balance of temperature vs. material to ensure the carbon isn’t overcooked or undercooked. Additionally, this thermal process can also convert the heat energy to electricity. However, I was not aware that the efficiency of converting heat to electricity is only 35ish%.
Chemical recycling is the process of removing the polymer materials (i.e., resins) from the carbon fibers themselves, using chemicals such as nitric acid or even water (hydrolysis). The waste carbon is treated with chemicals and solvents, leaving the carbon fibers behind. The major benefit to this process is the fiber’s composition is retained, meaning they still have maximum mechanical properties. In a study by Ma et al. (cited by Gopalraj), they achieved 99.18% resin decomposition ratio, an 85% cleanliness rate using nitric acid. The chemical process appears to be the best option, though it may be less cost effective.
I’m not a chemical patent guy, so it’s hard to tell what the explicit novelty in here, so I won’t guess. But SRAM states that current methods are limited by the resin content in the scrap carbon and cannot be adjusted according to the specification requirements of the carbon itself. The resin is set, and they can’t change it. So, they’ve created a process where the resin content of a secondary product, with the reused carbon, is adjustable.
It’s pretty easy to figure out why they’d want to do this. First, they want to reduce their waste product that leaves the facility. Second, they want to provide a secondary product that has more manufacturing flexibility and capabilities. If they can make money with their waste, why not capitalize on what you’re going to throw away anyway.
SRAM have a little example scenario of where this process starts. They do machining work on set carbon material, which is basically a block of carbon. The byproduct of any machining process is tiny particulate. This particulate is largely useless without a recycling process and can’t be re-resined since it’s already full of resin. In the end, SRAM want that sweet, sweet full strength carbon.
The final product is a fiber-containing molding compound with adjustable strength characteristics. A ‘fiber-containing molding compound’ is:
…[a] molding compound which is composed of a resin matrix (for example, an epoxy resin matrix or the like) and short fibers (for example, short carbon fibers, short glass fibers, or the like) impregnated with the resin matrix, and is known as a bulk molding compound (BMC), a sheet molding compound (SMC), or the like.
An example use of an SMC are ready-to-mold sheets to create automotive body panels using a molding process. So, it’s possible SRAM will use this process to create less-stressed parts of a bike. Or sell it to other companies for them to use it.
SRAM start with the carbon scrap. This carbon scrap is full of small fibers and cured resin, which is effectively useless in its current state. SRAM want to change that. SRAM say this process is not exclusive to carbon, but can be used with other fibers such as:
…carbon fibers, glass fibers, Kevlar fibers, basalt fibers, and aromatic polyamide fibers.
The carbon scrap is mixed with a release-paper removing agent (Epocide, for example). Release paper is present in used carbon, and this chemical will separate the resin/carbon from the release paper. The agent and the paper are poured out, and a viscous blend of resin and carbon remains.
Next, the viscous resin and carbon are combined with another resin and a diluent. A reactive and non-reactive diluent can be used. A reactive diluent will reduce the viscosity of the fluids during the process, as well as performing a cross-linking reaction with a resin hardener. Cross-linking is the process of chemically bonding polymer chains. The non-reactive dilutant will also reduce viscosity, but won’t perform cross-linking, so it’s not used with a resin hardener. This creates a viscous mixture of carbon, old resin, and new resin.
As said above, in another process, a resin hardener can be used with the reactive diluent, where the resin hardener and another resin are added to the viscous mixture. It’s important to note here that the other resin is miscible with the resin already in the carbon, meaning it can be combined with the already-present resin. The resins can be just about any currently available resin, but I would assume they’re going to be using epoxy resin:
…[this process can use] epoxy-based resin, phenolic-based resin, unsaturated polyester-based resin, furan-based resin, vinyl ester-based resin, polyurethane-based resin, and combinations thereof.
This viscous mixture is heated for a certain amount of time to evaporate the diluent. This final product is then fed into a pair of rollers to create a flat sheet of molding compound. And that’s it, SRAM have another usable carbon product from all the scraps they’ve produced.
There are quite a few examples in this document, but I’ll just go through the first one. Remember, this is in a lab setting, so the real-world process will probably be on an industrial scale.
SRAM provide a bunch of carbon fragments from a carbon machining process to a beaker. They then pour release-paper removing agent into the beaker. They stir this mixture around to remove the release paper from the carbon scrap. They pour all of this mixture out to remove the release agent and paper. They then put the vicious carbon/resin mixture into another beaker. A second epoxy resin and methyl ethyl ketone (non-reactive diluent) is then added to the beaker. This is heated to 70°C for 30 minutes to evaporate the diluent. The final product is fed through a pair of rollers to create a useable fiber-containing molding compound.
They did this a bunch of times with varying fiber content, resin content, diluent content, etc. to create this table. The table shows the different examples and comparative examples of the same process with varying levels of additional chemicals. The right-hand column shows the variation in flexural strength of the different fiber-containing molding compounds that have been created. Looks like example 1 and 9 turned out to be the strongest. Remember, strength may not be desirable in another product using this fiber-containing molding compound, so the varying strength characteristics provide a wide range of options with the same process. Pretty nifty!
This is the kind of thing that’ll keep me coming back to SRAM for my componentry. Just the fact that they’re trying to improve their process is enough for me to support them. I love my SRAM stuff anyway, but this is just a cherry. Even if this doesn’t even up working, it’s still an attempt.
I’m happy to see this kind of work being done in this industry. It shows a particular level of care these companies are putting into their own processes to maintain a level of sustainability in their operations. The reality is, is they’re not making that much waste (compared to other industries), but SRAM are taking it upon themselves be better. And if they can make a little money at it, more power to them!