Designing Car Floatation Devices: What Alternatives to Sodium Azide?

[Vincent Tang]

I need to make the design of car floatation devices. In my case, the airbag/raft is for floating the car in case of floods. I am considering to use Sodium Azide, just like in car airbags, but it inflates very fast and shrinks in a short time. Is there any other chemical reaction that can produce a large quantity of gas (1000L+) in a relatively quick time? Or is there a way for me to slow down the sodium azide decomposition so it doesn't get too hot and shrink after?

 

[jrmichler]

Have you considered carbon dioxide? It's stored in liquid form, and 1000 liters would be about 4 pounds, with a volume of about a gallon. The CO2 would need only a valve, where chemical gas sources all have more complex triggering systems and significant safety concerns.

 

[ChemAir]

It would be good to look more into how airbags are built and how they work. https://en.wikipedia.org/wiki/Airbag

I am considering to use Sodium Azide, just like in car airbags, but it inflates very fast and shrinks in a short time.

Airbags are not airtight. They are designed to inflate very quickly, and deflate soon after, with holes, or plugs that will let the gas out pretty quickly. They aren't useful for a second impact a few seconds after an initial one. The quick deflation (or vents) provides some 'softening' of the impact from a passenger to the bag, and the bag gets out of the way for accessibility and visibility quickly for the benefit of the driver and rescuer after impact.

If they were airtight, they would still shrink a good bit due to cooling, but not as quickly as the vents let it happen.

Sodium Azide is an acutely toxic explosive. I suspect there are a great deal of trade secrets in 1) the methods of distributing gas in an inflating airbag, and 2) folding/storing procedures so it unfolds rather than destroys itself during deployment.

Is there any other chemical reaction that can produce a large quantity of gas (1000L+) in a relatively quick time?

For this, (as @jrmichler already said) CO2 inflation would be a cheaper, safer option and would be far easier to engineer, since extreme rapid inflation is not as necessary.

 

[Vincent Tang]

Have you considered carbon dioxide? It's stored in liquid form, and 1000 liters would be about 4 pounds, with a volume of about a gallon. The CO2 would need only a valve, where chemical gas sources all have more complex triggering systems and significant safety concerns.

Thank you so much for your response and recommendation. With

It would be good to look more into how airbags are built and how they work. https://en.wikipedia.org/wiki/Airbag
Airbags are not airtight. They are designed to inflate very quickly, and deflate soon after, with holes, or plugs that will let the gas out pretty quickly. They aren't useful for a second impact a few seconds after an initial one. The quick deflation (or vents) provides some 'softening' of the impact from a passenger to the bag, and the bag gets out of the way for accessibility and visibility quickly for the benefit of the driver and rescuer after impact.

If they were airtight, they would still shrink a good bit due to cooling, but not as quickly as the vents let it happen.

Sodium Azide is an acutely toxic explosive. I suspect there are a great deal of trade secrets in 1) the methods of distributing gas in an inflating airbag, and 2) folding/storing procedures so it unfolds rather than destroys itself during deployment.
For this, (as @jrmichler already said) CO2 inflation would be a cheaper, safer option and would be far easier to engineer, since extreme rapid inflation is not as necessary.

Hello! Thank you so much for your explanations and suggestions!
I just have one more question: using CO2 is a great idea, but where should the compressed CO2 cylinder be stored on the car? The floatable airbags are designed to be on the sides of the underbody, but I would assume that it is unsafe to mount it to the underbody because it could get hit by gravel or speed bumps. In this case, where is the best place to store the CO2? One of the requirements for designing this is to make it universally fittable on small cars.

 

[Tom.G]

Please consider:

If the bags are inflated when on the underbody you have the equivalent of either a top-heavy boat or a Hot Air Ballon. The bags will float with the car hanging below them, under water.

 

[CWatters]

As I recall aircraft escape slides use CO2 but also a special venturi like device that drags air in as well to reduce the amount of CO2 needed.

 

[CWatters]

I would put the bags in the cill area beneath the doors. Problem is the cills are box sections for rigidity and adding a door would cause problems.

 

[Nik_2213]

Check out nautical life-rafts. They inflate from pod to raft in a few seconds.

Please consider the stability of your design very, very carefully. Remember the centre of gravity must be sufficiently low to be stable against collision, wave action and possible rapids. Also, your vehicle may have an uneven fore/aft weight distribution. Even if you mount RIB-style flotation tubes to the running-boards and roo-bars of an SUV, you are at high risk of fatal capsize. Worse, they are at risk from collision and floating debris.

You're probably safer with the flotation devices at window-sill height, but you'll have to sit on the roof.
Why not put a yachty life-raft pod up there, and modify your sun-roof so it may be opened wide for egress in extremis ??

 

[ChemAir]

In this case, where is the best place to store the CO2?

This is difficult to say, without more specific knowledge of the cars you would put it in. I would have a couple of considerations- It shouldn't share the airspace of the operating compartment, nor should it's relief system. It should be mounted rigidly, with reasonable protection around the valve. I suspect, like most things, the location will be a compromise.

And pay attention to the comments above discussing how this thing will float.

 

[Baluncore]

Following deployment the vehicle would be a write-off due to water damage. If that was not the case, the vehicle when deployed would be unstable and the order of inflation would be critical. This brings up the question of why such a system is needed to save a vehicle. It might be more economic to supply the occupants with standard CO2 inflated personal life jackets.

A standard low-profile module could be designed to attach to the sides of the vehicle, just below the window level. That would place the buoyancy in the right place. When deployed the windows would remain unobstructed and above water. The modules could be glued to the external body surface and triggered by wire from inside the vehicle, integrated through the CAN bus. When not in use, the module would appear to be a flat bumper strip, with a protective UV resistant plastic carapace. Maybe the packed dimensions would be 500 mm long, by 200mm high and 20mm thick which could fit on most vehicles.

The injection of a fixed volume of water through a one way valve, into a dry mix of citric acid and baking soda, sherbet, could provide the flotation gas. Maybe a less edible and faster reacting combination could be selected. Also, the injection of water could be driven by something like the opening of a can of carbonated soft drink. We have that technology.

 

[AlexCaledin]

In the WWII, the English Swordfish planes had rubber boats which get inflated with CO2 in seconds automatically, on contact with the sea water starting the chemical reaction, when the plane got into the sea.

 

[Mark Harder]

I need to make the design of car floatation devices. In my case, the airbag/raft is for floating the car in case of floods. I am considering to use Sodium Azide, just like in car airbags, but it inflates very fast and shrinks in a short time. Is there any other chemical reaction that can produce a large quantity of gas (1000L+) in a relatively quick time? Or is there a way for me to slow down the sodium azide decomposition so it doesn't get too hot and shrink after?

I found this on Wikipedia: << moderator added the link: https://en.wikipedia.org/wiki/Airbag >>
" In a patent containing another plausible alternative to NaN3 driven airbags, the gas generating materials involved the use of guanidine nitrate, 5-aminotetrazole, bitetrazole dehydrate, nitroimidazole, and basic copper nitrate." ... "It was found that these non-azide reagents allowed for a less toxic, lower combustion temperature reaction and more easily disposable air bag inflation system.[citation needed]" I seem to remember these same propellants mentioned in connection with airbag inflation during some searches for rocket propellants.

 

[Nik_2213]

Remember a car's airbags must inflate in the moments of a collision.
Flotation devices are allowed a few seconds to deploy, so spectacular and potentially dangerous pyrotechnics are not required.
In fact, for 'Health & Safety' considerations, pyros should be avoided to mitigate the inspections, documentation, licensing, lawyering, insuring etc etc.

 

[anorlunda]

Please consider the stability of your design very, very carefully. Remember the centre of gravity must be sufficiently low to be stable against collision, wave action and possible rapids. Also, your vehicle may have an uneven fore/aft weight distribution. Even if you mount RIB-style flotation tubes to the running-boards and roo-bars of an SUV, you are at high risk of fatal capsize. Worse, they are at risk from collision and floating debris.

I agree.

Even worse. In fast flowing water, the bottom of the car can get stuck on the bottom, and the surge of water can capsize it completely. I'm afraid you need to make is completely self-righting able to capsize 180 degrees, then turn itself upright again.

It is somewhat analogous to self-driving cars. If you save lives in 9 consecutive cases, but in the 10th case, your invention actually kills the people, you will be hated rather than loved.

 

[LURCH]

I would add that most cars “turn turtle” when driven into water. The car may already be upside down when inflation begins. I think this is easily rectified by placing at least one inflatable device on the roof. If the car is inverted or becomes inverted, it will self-rite.