It is equally critical to have a thorough understanding of the insulating material. The correct insulating material helps to ensure optimal equipment operation and protects personnel and sensitive equipment from dangerously hot surfaces and excessively hot air temperatures.
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At Firwin Corporation, we specialize in finding the best insulation materials and designs for your specialized equipment and unique applications. Our comprehensive selection of insulation materials ensures that we will find the perfect insulation solution for your needs. One of Firwin’s most versatile insulating materials is Aerogel, a porous, solid material with a high percentage of air and a variety of unique properties.
Aerogel is a class of low-density solid gels in which the liquid has been replaced with air or gas. The structural framework of Aerogel is typically composed of silica, so silica aerogel is often referred to as simply “Aerogel.” However, other structural materials have been used to create aerogels, including:
Regardless of the material, the preponderance of air in an Aerogel’s structure gives it a nearly transparent appearance. The high gas content of Aerogels also gives them a variety of unique properties, including extremely low density, very low thermal conductivity, and very high porosity.
In insulation applications, Aerogel easily outperforms traditional fillers such as wool and fiberglass. In fact, Aerogel offers the same quality insulation with 1/3 the thickness of other insulating materials. However, Aerogel is expensive to manufacture and is rigid and brittle in its basic form, so it requires some supporting material. In addition, Aerogel withstands temperatures up to °F (593 °C), but is not suitable for extremely high-temperature applications that operate above that level.
Silica Aerogel is particularly useful for insulating applications and is one of the most effective insulators available. Although it is more expensive than other insulating materials, Aerogel makes up for its cost by the benefits it offers. Some of these include:
Aerogel has been used as an insulating material for decades. Most notably, it has been used in NASA spacesuits for its exceptionally lightweight and durable nature. As manufacturing technology advances, Aerogel has become increasingly desirable in a broad range of insulating applications, including:
Although Aerogel is typically more expensive than other insulating materials, its enhanced thermal insulating properties with thinner layers make it uniquely suited for confined spaces. In addition, its extremely lightweight characteristics make it ideal for use on light and breakable components that could be damaged by the weight of more traditional insulating materials.
It’s true!
There are now many different types of aerogels that are flexible and high-strength, some of which are so mechanically robust they can actually be used for structural applications!
Although it’s true that a typical silica aerogel could hold up to times its weight in applied force, this only holds if the force is gently and uniformly applied. Also, keep in mind that aerogels are also very light, and times the weight of an aerogel still might not be very much. Additionally, most aerogels as-produced are extremely brittle and friable (that is, they tend to fragment and pulverize). As a result, structural applications of aerogels were for a long time totally impractical.
But never fear! There are several ways aerogels can be made strong and even flexible, enough that aerogels can now be used as structural elements.
There are four general ways to enhance the mechanical properties of aerogels:
Particles of oxides, such as silica, are frequently mixed into plastics to make plastics with different properties. This process is called “doping”, in which the oxide particles are called a “filler”.
One day, Prof. Nicholas Leventis at the University of Missouri-Rolla (now the Missouri University of Science and Technology) wondered,
“If you can dope a polymer with a filler, can you dope a filler with a polymer?”
So thinking about cohesive forms of fillers used for doping polymers, he thought of silica aerogels, which are effectively macroscopic assemblies of silica nanoparticles.
Starting with a preformed wet silica gel of the type used for making silica aerogels, Leventis soaked the gel in solutions containing diisocyanates-crosslinking agents used to make polyurethane varnish-and then heated the gels to get the diisocyanates to bond. Upon supercritical drying, a silica aerogel with remarkably improved mechanical properties resulted-an aerogel that can actually bend not unlike stiff rubber! Try doing this with an ordinary silica aerogel and you’ll be left with lots of little broken pieces.
Diisocyanates are linear molecules with two ends that can react with hydroxyl groups to form carbamate bonds. The hydroxyl groups lining the skeleton of silica gels are perfect candidates for reacting with diisocyanates, and since diisocyanates have two reactive ends, they can bond to the aerogel skeleton twice. The result-each diisocyanate molecules acts like a nano-sized piece of Scotch® tape bonded to the surface of the aerogel skeleton, resulting in a conformal polymer skin that ties together the spherical silica nanoparticles that make up the aerogel. This conformal polymer skin makes the resulting aerogel much stronger than a typical silica aerogel and allows the structure to flex without breaking-sort of like the marshmallow coating on a Rice Krispies® treat!
These strong, polymer-crosslinked aerogels are called “x-aerogels”. Importantly, the word x-aerogel (say “ex-air-o-jell”) is not the same thing as the word xerogel (say “zee-ro-jell”), which is a gel that has collapsed into a densified material as a result of evaporative drying.
Pretty much any gel suitable for making aerogels can be modified to produce an x-aerogel. Here’s what has to happen:
Lots of polymers can be used to crosslink aerogels, including:
and lots more. To help these polymers attach to the gel framework, a compound called 3-aminopropyltriethoxysilane, or APTES for short, can be mixed in as the gel sets. This puts amine functional groups (-NH2) over the surface of the gel in addition to the hydroxyl groups that are normally there. These amine groups can be used to bond a wide variety of polymers to the gel framework. Dr. Mary Ann Meador at the NASA Glenn Research Center in Cleveland, Ohio has done extensive work refining this method.
Since the crosslinking takes place separately from the gel formation step, you can crosslink pretty much any gel to make an x-aerogel. X-silica (crosslinked silica aerogel), x-alumina (crosslinked alumina aerogel), and x-RF (crosslinked resorcinol-formaldehyde polymer aerogel) are just a few that have been prepared. In fact, x-aerogels of almost all of the colorful lanthanide oxides have been made as well.
So why isn’t every aerogel made today an x-aerogel? Well there are some trade-offs to consider.
Advantages:
Disadvantages:
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Because of their impressive strength-to-weight ratios, decent insulating ability, and nanostructural properties, there are some interesting applications for x-aerogels:
X-aerogels are still new. Who knows what’s soon to come!
X-aerogel research is currently underway in the Leventis group at the Missouri University of Science and Technology in Rolla, MO and in the x-aerogel research group at NASA Glenn Research Center in Cleveland, OH led by Dr. Mary Ann Meador. Ongoing work to extend x-aerogel compositions to additional polymers, optimize processes for getting desired materials properties, and simplify and reduce cost of manufacture are currently underway.
In addition to strengthening aerogels by crosslinking wet gels prior to supercritical drying, it’s possible to reinforce aerogels after they’ve been supercritically dried as well. This can be done through deposition of a conformal polymer coating throughout the interior porosity of an aerogel by means of chemical vapor deposition (CVD) or atomic layer deposition (ALD).
Boday et al. at Los Alamos National Laboratory demonstrated this principle through ambient-temperature CVD of methyl cyanoacrylate (the main ingredient in superglue) onto silica aerogels and found that the resulting conformal coating increased the strength of the aerogels 30-fold with only a three-fold increase in bulk density!
Based on the data for comparable untreated silica aerogels, this observed increase in strength due to the conformal coating is approximately three times greater than would be achieved by simply preparing a comparably dense aerogel using a more concentrated solution in preparing the precursor gel. Although they are made through a different way, these materials are also considered x-aerogels.
Just like rebar reinforces concrete in bridges, it’s possible to reinforce aerogels with microfibers.
Aspen Systems, a contract research and development company, began experimenting with aerogels in the late ’s. Around , they were experimenting with casting silica gel onto fibrous batting (a porous, flexible fiber mat) and supercritically drying to produce reinforced aerogels. To their surprise, the mat was totally flexible and as co-inventor Dr. George Gould described it, “almost as if it were a totally different material”. The flexible aerogel blanket showed to be almost as insulating as the plain aerogel, except for unlike a typical silica aerogel, it could be rolled up and bent over and over.
In , Aspen Aerogels spun off from Aspen Systems and since then has developed a range of aerogel blankets designed for different applications. Technically their materials are aerogel composites because they combine fibrous battings of inorganic or organic fibers with aerogels and, for high-temperature applications, carbon black as well. Numerous patents from Aspen detail production of mechanically robust, flexible aerogel blankets of both inorganic and organic aerogels supported by meshes of polyimides (i.e., Nylon®), glass fibers, and many other materials. Aspen’s products can be found in subsea oil pipelines, refineries, winter apparel, and even shoe insoles.
The blankets are made first by mixing up a sol as you would a normal silica aerogel. The sol is poured onto a roll of fibrous batting and heated until the gel sets. The mat is rolled up and then placed in a tank under liquid while the gel continues to set and strengthen and is made hydrophobic through chemical reactions. The roll is then moved into a giant supercritical dryer and supercritically dried from CO2. Finally, the roll is heated to drive off excess solvent and can be shipped out.
To the touch, aerogel blankets feel kind of like a soft Brillo Pad® and are a little crunchy in bending. The blankets give off a little bit of powder when handled, but as Aspen reports, the blankets can be flexed over 250,000 and lose only 2% mass. Handling the blankets will make your fingers feel slippery, as the dust from the aerogel is hydrophobic (water-repelling) and will stick to your fingers.
Despite the seemingly obvious potential of aerogels for all sorts of applications, it actually took quite a while for Aspen to figure out how to make a business of aerogel blankets. After exploring a number of markets and developing technologies for a number of applications with government funding, the killer app appeared-sub-sea oil pipelines. Slaggy oil carried in sub-sea pipelines needs to be kept warm to keep flowing, otherwise the cold temperatures of the surrounding water will freeze up the slag. To keep the oil warm, a “pipe-in-pipe” configuration is used, that is, an inner pipe surrounded by insulation in a larger outer pipe. Because of the thickness of polyurethane insulation required to keep the oil sufficiently warm, the outer pipe had to be large. And because of the size of the outer pipe required, only three ships in the world were capable of laying that kind of pipe. Enter flexible aerogel blankets. Because the thermal conductivity of aerogel blanket is about twice as low as polyurethane per unit thickness, a much thinner aerogel blanket can do the job of much thicker polyurethane, meaning the diameter of the outer pipe can be that much smaller. Suddenly, over 250 ships around the world could lay that diameter pipe, saving the oil industries billions of dollars!
With that market in place, Aspen has started to address insulating needs in refineries and is working to replace mineral wool. Although today aerogel blankets are more expensive than other forms of insulation, they are not significantly more expensive and in the future with increased scale, aerogel will displace many types of insulation.
Aspen Aerogels aerogel blankets boast:
Different aerogel blanket formulations are designed for different applications. Cryogel® is designed for cryogenic applications, Spaceloft® is designed for clothing and apparel, and Pyrogel® is designed for high-temperature applications-six times better than mineral wool at temperatures of 350°C!
Aerogel blankets have many potential applications. One recent product is called Toasty Feet® and contains aerogel blanket in an insole for shoes to keep your feet warm in the winter and cool in the summer. Another product is aerogel blanket strips and aerogel wall wraps as home insulation that can help reduce heat loss through studs in the walls of a house.
For more information about fiber-reinforced silica aerogel blankets, see US patents G. Gould, et al., United States Patent Application , , and J. Ryu, United States Patent 6,068,882, .
Prof. A. Venkateswara Rao (say “ven-cat-a-swar-a”) at Shivaji University in Kolhapur, India invented a really interesting way to make flexible silica aerogels that also strongly repel water-by reducing the amount of bonding in the aerogel!
Normally a silica aerogel is made with tetrafunctional silicon compounds such as TMOS (tetramethoxysilane), that is, compounds that will result in a silicon atom with four oxygen bridges connecting it to four other silicon atoms, with each of those silicon atoms having four oxygen bridges connecting them to four other silicon atoms, and so on and so forth. This makes a rigid structure in which each silicon atom is highly mechanically constrained.
But if you use a trifunctional silicon compound such as MTMS (methyltrimethoxysilane) instead, you get a structure in which each silicon atom only has three oxygen bridges to other silicon atoms and on the fourth bond (because silicon tends to form four bonds to stuff), one terminating methyl group that doesn’t connect to anything else. This makes a structure that has reduced overall bonding, with longer internal struts that are less mechanically constrained, which makes the overall aerogel flexible!
The methyl groups attached to each silicon atom in a flexible aerogel made with MTMS are highly hydrophobic, that is, repel water, and so where normal silica aerogels would have sticky, water-attracting hydroxyl groups on their surface, MTMS-based silica aerogels are highly water-repellant.
Mixing water with powdered silica aerogel made from MTMS and pouring the mixture out will result in an aerogel-coated liquid “marble”. Kind of like mixing oil and vinegar, the aerogel powder (which repels water like oil) separates out from the water, but because the droplet is small and has surface tension, the aerogel moves to the surface of the droplet and creates a shell around the water.
Here’s the wacky part-an aerogel-coated water droplet is itself water-repellant because of its aerogel shell, meaning it will float on water! That’s right, a water droplet that floats on water!
Combining both MTMS and TMOS, you can make silica aerogels with varying degrees of flexibility, optical transparency, and hydrophobicity.
This technique is called “reduced bonding” since compared to the typical way to make silica aerogel, each silicon atom makes fewer network bonds.
One exciting application for reduced bonding aerogels is absorption of toxic chemicals. The same thing that makes MTMS-derived silica aerogels water-repellant makes them good at absorbing non-polar organic compounds. MTMS-derived silica aerogels can absorb 13 times their weight in gasoline and 20 times their weight in toxic benzene!
For more information about reduced bonding, see: