The Technicals of Twillica
A deep dive into the nitty-gritty science behind how Twillica’s products work.
Written by: Jolie Li, Parmin Sedigh
Edited by: Klara Zietlow, Sora Shirai
Whoever said making revolutionary clothing would be easy? From the physics of carbon nanotubes to the crazy things shape memory polymers can do, Twillica is transforming your clothing. Based on science.
Pssst…. if you haven’t check out our article going over the basics of who we are and what we do, click here. If you’ve already read that article, the next section may seem familiar. Click here to skip to the physics section of this article and dive in deep 🌊
3 Layer Properties: Temperature, Breathability, and Waterproofing
The product uses both carbon nanotubes and shape memory polymers which allows you to manually change the temperature, breathability, and degree of water-resistance of your jacket!
The carbon nanotubes are treated to create a film just like shown below.
The film is used to adjust the temperature of your jacket using currents. Yes, electrical currents. These can be applied both towards the cloth away from it.
So the heat either goes towards you to insulate you or away from you to drag heat into the environment.
Shape memory polymers are used to adjust the clothing’s breathability. This is because they can memorize up to 3 shapes and rapidly alter between different ones based on temperature changes. And since we can alter the temperature of the carbon nanotubes manually, we can also change the shape of these polymers.
When the temperature increases from the nanotubes, the polymers will compact together into their first temporary shape.
This ensures the person stays warm and the heat does not escape, therefore decreasing breathability.
When the temperature of the carbon nanotubes cool, the polymers will loosen and expand, therefore making the material more breathable for you.
The fibers incorporated into the carbon nanotubes and shape memory polymers will include cotton, polyester, and nylon. These will have a hydrophobic coating so don’t absorb water.
The degree to which the clothing is waterproof, though, can be adjusted. When the polymers contract and become smaller, the clothing becomes more waterproof as it allows no water in. When the polymers expand and have spaces between them, the water can more easily penetrate the fabric.
“But how can I control all of these awesome features?” you ask. It’s pretty simple! You can do so using only three buttons that are fashionably hidden inside the pockets.
One of these buttons will direct the current in one direction. Through this awesome physics effect called the Seebeck effect, the carbon nanotubes will heat up, heating you and the jacket up alongside it. As well, the shape memory polymers will contract to become less breathable and more insulating.
When you want to cool down, you press another button that directs the current in the other direction, and that cools down the nanotubes, thereby cooling your body. The polymers, therefore, become more breathable.
The final button offers an extra feature. It activates a circuit breaker for manual control if you don’t want to keep the heat or cold coming. When it’s clicked the current will immediately stop and the polymers will slowly return to their original state of semi-breathability.
The Physics: Carbon Nanotubes & Temperature Changes
Like we’ve mentioned, we’ll be using carbon nanotubes and manipulating them using electric current. Now hold on a second, isn’t that dangerous? Let me explain.
The reason metals are conductive is because of the free electrons they have. And these electrons have thermal energy.
So if our carbon nanotubes are at room temperature but one side is heated up, the electrons on the hot end will move faster and separate from one another. This will cause more electrons to go to the cool end, leaving less on the hot end. This difference in temperature creates a temperature gradient.
This difference in there being more electrons on one side than the other is what creates a voltage difference. This means there’s positive potential on the hot end and negative potential on the cold end. This is called the Peltier effect.
Now the Seebeck effect is just an additional layer to this concept. It adds another dissimilar metal to the mix that is connected to the first metal in a loop, and these two different metals in this loop become known as a thermocouple.
The Seebeck effect states that a current flows in a circuit consisting of two different metals when one junction is heated and the other is kept cool. And the Seebeck coefficient is related to this. It’s the ratio between the voltage produced and the temperature difference. Looking below, you can see how the two alloys come together to form a loop, a circuit.
This finally brings us to ZT, otherwise known as the figure of merit. It helps us determine the efficiency of thermocouples.
So a material that is thermoelectrically efficient can heat and cool the wearer well. It would have a higher ZT value, a larger Seebeck coefficient, high electrical conductivity, and low thermal conductivity (in this case, it’s 8.1 ± 1.2 W/m/K for the 5-MWCNT/PEI film).
But wait a minute isn’t low thermal conductivity bad? Not at all; it means that when it’s hot and you want to be cooled, the heat wouldn’t easily travel back to your body.
Another important factor is the power factor! It’s another ratio just like ZT and it’s between true power and apparent power. An awesome analogy for this is through drinks and whipped cream. If you’re a big fan of whipped cream you might want to plug your ears but the analogy still stands.
When you’re buying a frappuccino from Starbucks, you want the good stuff, the drink itself. The whipped cream is just the stuff at the top that takes up space and makes it look like the drink is full.
The entire thing, both the whipped cream and frappuccino are the apparent power. The frappucino itself is the true power; it’s the one that does the work and is useful. The whipped cream is the reactive power (which is equal to the apparent power minus the true power). It’s essentially useless for our purposes.
So this means that we don’t want our reactive power, the whipped cream, to be too high. And the challenge with our carbon nanotubes comes from ensuring that both the power factor and the ZT stay high.
That’s why carbon nanotubes or CNTs for short are awesome. They’re flexible, have high electrical conductivities, low density (~ 1 g/cm compared to 7.86 g/cm for similar popular material called Bi2Te3), and they can act as semiconductors in ambient conditions.
There are two main categories of CNTs, single-walled ones or SWCNTs and multi-walled ones or MWCNTs.
We’ll be using MWCNTs for several reasons: SWCNTs are relatively more expensive to produce and worse for large-scale manufacturing and they can have issues when it comes to purity.
In one study using MWCNTs for thermoelectric purposes, the purity was only 77%; however, our material will have 87.7% purity.
More specifically, we’ll be using either p-type MWCNTs or n-type MWCNT/PEI composite films with high power factors (~ 340 and ~ 520 μW/mK , respectively) and relatively high ZT values (0.019 and 0.015, respectively). Their Seebeck coefficients are about 56.19 μV/K.
This is all great and all but how do you, wearing the jacket, become cooler or warmer? To cool you, the CNTs have properties that would allow heat to be drawn away from the body when an external source of current is applied. “Think of it like a film, with cooling properties on one side of it and heating on the other” is how Tushar Ghosh, co-corresponding author of a study done at North Carolina State University explained it.
When current is going one way, the CNTs warm up thereby warming you up. When the current is going the other way, the CNTs cool down thereby cooling you down.
But how exactly do we plan to create current? Using piezoelectricity of course!
The Physics: Piezoelectric Current Generation
Let’s be realistic; batteries are clunky, annoying, and don’t belong on clothing. Many clothing brands of the “future” have batteries but Twillica is different. Instead, we utilize the power of human movement to create currents.
We’ll do this using the concept of piezoelectricity and piezoelectric crystals. Piezoelectricity relies on the creation of a voltage across a piezoelectric crystal when the crystal is hit.
Piezoelectricity depends on two principles. One is the electronegativity of certain elements, or how badly they want to pull the electrons towards themselves, creating net positive and net negative charges. Think of this as a greedy baby who wants all of the toys. In this case, we celebrate the greedy baby because they make piezoelectricity possible!
The other is a lack of symmetry that allows for the imbalance of charges when the crystal is hit.
As for what type of piezoelectric crystal we’ll be using, it’s potassium bismuth titanate, Bi0.5K0.5TiO3 (or BKT for short). It really rolls off the tongue, we know.
A large majority of the piezoelectric materials commonly in use today contain lead, which is certainly not good for the environment or for human health. BKT entirely bypasses this without sacrificing the quality.
With the simple push of one of three buttons every three hours, you can heat and cool your clothing. This number comes from careful calculation using how much voltage is needed for the jacket and the safe amount of electricity for a human to be in contact with, which is about 50V.
In this case, the Seebeck coefficient can be as high as 56 for our 5-MWCNT/PEI, making the voltage 56 as well. However, we can decrease the electrical conductivity to ensure safety of our users.
This would mean that a temperature increase or decrease of 1℃. While this may not seem like a great amount, a one-degree difference in body temperature is the difference between normal body temperature and fever.
The Physics: Shape Memory Polymers & Breathability + Water Resistance
Now we get to our shape memory polymers (SMPs) and how they contribute to both breathability and provide additional water resistance. Let’s get to know them on a more personal level now.
At their core, SMPs are polymers that can change from one or two temporary states to their original state without any mechanical force. They can do this through temperature changes, pH changes, humidity changes, chemical stimuli, and more. In our case, we’ll be using temperature-sensitive SMPs to work in sync with our CNTs.
More specifically, we’ll be using MDI/EDA-PTMG. Another awesome name, eh?
The hard segments in this substance are methylene bis/ethylenediamine (MDI/EDA) and the soft segment is polytetra-methylene glycol (PTMG). We’ve chosen this material for two reasons. Firstly, it works and transitions well at around room temperature. Secondly, it can be produced rather easily in bulk.
The molecular free volume–that’s the unoccupied space between molecules– increases when the temperature is above what’s called the glass transition temperature or Tg. And when the temperature around the MDI/EDA-PTMG is below the Tg , the free volume decreases, closing up the space between the “pores” of the fabric.
But let’s back up a second. What are these hard and soft segments? They might sound pretty complicated, but it’s actually quite simple. The soft segment is what helps the SMP transition between states while the hard segment is the permanent one, the one that doesn’t change with temperature.
The MDI/EDA-PTMG will be incorporated into the fabric through the use of shape memory polymer fibers that are then woven together to create a fabric that can change its breathability. MDI/EDA-PTMG has a great recovery percentage as well at 95%.
Shape memory polymer fibers in general (or SMPFs for short) can be woven together on their own or together with fiber and yarn such as cotton, polyester, and nylon, which is what we’ll be doing to ensure extreme comfort for you!
How Will It Be Made
We will be using the floating catalyst chemical vapor deposition (FCCVD) method for fabricating the CNT film.
First off, ethanol, ferrocene, and thiophene, three chemical compounds that must be handled with care, are injected into the heated reactor, at a temperature of 1300 °C, at a feeding rate of 0.15 ml min-1.
Next, a mixture of Ar-H2 is injected into the reactor at a rate of 4000 sccm. This causes the carbon nanotubes to grow while in the reactor and they are then blown out as the Ar-H2 continues to create a movement almost like wind. They then wound up on a roller as shown in the below diagram and they all align in one direction automatically.
After the films are created, a certain amount of polyethyleneimine (PEI) is added to a fixed amount of ethanol.
The varying amounts of PEI (ranging from 5% of the weight of the solution to 20%) create varying types of films: 5-MWCNT/PEI, 10-MWCNT/PEI, 15-MWCNT/PEI, 20-MWCNT/PEI. These films are left in the PEI and ethanol solution for 60 minutes at room temperature.
These treated films are called doped films. After this process, they are given a heat treatment for 2 hours at 120℃ to ensure there was no excess ethanol on the films. This leaves the films with the following thicknesses: 10.2 ± 1.5 μm, 10.7 ± 1.3 μm, 9.9 ± 0.7 μm, 9.1 ± 1.7 μm.
To ensure the utmost safety for all Twillica customers, there will be an automatic system that will cut off the circuit in case water is to come in contact with the carbon nanotubes. Our SMPs will prevent this in nearly all situations, but this is simply for maximum safety.
As well, to prevent the user from pressing the button too many times and causing too much current to run throughout the CNT system, the system will lock the button once the button has been pressed once until the three-hour time period is over.
Let’s take a more in-depth look at the mechanism. We’ll be using automatic circuit breakers; when there is just one volt higher than 50V running through the circuit, the circuit breaker would open the circuit, stopping the flow of electrons.
As for the water contact scenario, the same mechanism will be at work. Water decreases resistance in a circuit, thereby increasing current. So when it surpasses the 50V, the circuit breaker will stop the flow.