Rain-X

[Mark Gordon]

"My American wife once had a lecture at UNC from a very famous German born but naturalised British Nobel Laureate scientist. His first slide was a picture of a BMW motorbike. His opening remark, which got a good laugh, was "This, ladies and gentlemen, is my [*****] cycle." The asterisks represent the letters of his name. Can you guess who he was (think citric acid or energy production)?"

Krebs as in Krebs Citric Acid Cycle which is the biochemical pathway for energy production in the mitochondria. Do I get a prize along with David?

[SEJohnson95]

I think Richard should be the one issuing the prizes here, I'm only an undergraduate

[mooney1el]

Well, I will offer the prize of being treated to a local shrimp dinner for David, Mark (and yes Peter too) whenever any of you can visit beautiful Amelia Island here in NE Florida. Of course, Simon will be included :)

[SEJohnson95]

In all the excitement about the chemistry behind Rain X I forgot to report on the results of my findings. Funnily enough, it works and is incredibly worthwhile. Wiper action is now perfectly smooth, the water now doesn't stick to the windscreen AT ALL (much like the rest of the bodywork, which blows itself dry after a few minutes at motorway speeds) and even the fine rain dries relatively quickly. Even at speeds of 30-40 mph you can notice it working. Waiting for it to properly tip it down to see the full effect but I'm happy with it so far.

[PeterCrespin]

Well, I will offer the prize of being treated to a local shrimp dinner for David, Mark (and yes Peter too) whenever any of you can visit beautiful Amelia Island here in NE Florida. Of course, Simon will be included :)

Be careful what you wish for. With Jaguar being the featured marque there last Nov I was asked to help judge. So instead of being a day late and a dollar short as normal, I was a year early and paid too much!

Yes, Simon, there's a sweet spot between driving too slowly for it to do much, and having to look surprised as you say "Really officer? 90 miles per hour? I was merely keen to get my screen clear for safety reasons."

Pete
Yes, Mark gets the prize: my ticket to Richard's 'all you can eat' Bubba Gump shrimp dinner! :-)

[Mark Gordon]

I'll be in Orlando for a meeting in mid-November. That's a little far from Fernandina to drop by for dinner.

[dal2.0litrefrogeye]

Rain X , great stuff !! one of those products that does what it says on the tin , I also use it on the bathroom mirror , now I can shave after my shower ,

[SEJohnson95]

I showed the article to one of the inorganic lecturers here at the University and the other day and she loved it. We've modified it slightly, we actually took out a bit of the scientific detail that you lot seemed OK with - she writes articles for magazines all the time and is often disappointed about the fact that even at her level, the detail has to be cut down a bit. The essence of the article is the same though. I was assured that what I had written made perfect sense from a chemical point of view however!

Quite happy since Malcolm McKay said it will be published soon. Further, the department will publish it on the website and tweet about it etc, pretty cool! PDMS came up in our lecture the other day, a great explanation of hydrophobicity followed with a lotus leaf and a drop of water, along with applications of silicon based polymers. I will post links when it's live :)

[Tony]

Well not to pour water on rainX but I find it quite disturbing having droplets of rain flying up the screen at high speed. It gives a sensation similar to a car wash, when you think the car is moving forward, when it is not..

[SEJohnson95]

Well not to pour water on rainX but I find it quite disturbing having droplets of rain flying up the screen at high speed. It gives a sensation similar to a car wash, when you think the car is moving forward, when it is not..

No you have a point Tony, it divides opinions whenever I mention it to people, my old man won't go near it for a start, he says the same thing as you.

[Heuer]

Clearly your Dad has not driven an E-Type quickly in the rain! Given the wipers are a tool of last resort, especially with the very poor new blades currently available, Rain-X gives you the best of both worlds because you can still use the wipers if you want to. Would I use Rain-X on a modern car - no.

Actually I managed to find a set of NOS Aeramic wiper blades and they are most effective without having to resort to Rain-X. Not sure what the currently available wiper blades are made of but I don't think it is the original type of rubber; feels like vinyl or plastic.

[Weidi]

Hey there, great post!

Although I am not a Jaguar lover as you, I appreiciate the beauty of car making of Jaguar. I came across your analysis in a purely random search on FAS type chemicals. while I was writing a paper on the superhydrophobic surfaces. Currently I am a sec yr master student in electrical engineering, one of my main research area is superhydrophobic surfaces. I was always wondering what is the content of RainX, and you solved it for me. I'd like to add some of my views and knowledge about the coatings.

I agree that it is PDMS which makes the glass hydrophobic, however, I don't think the contact area is minimized by the filling of PDMS, but rather through the PDMS coating itself. When coating the PDMS, the structure of the polymer layer is quite rough, by the nature of spary coating technique. And the roughness of the coating creates a micro-structure that minimize the contact area of water. The theory behind this phenomenal is called Cassie-Baxter wetting model, where they took roughness of surface into account.

Coating FAS-17 is another story, where ideally only one layer of FAS-17 is on the surface, formed up something as chemists called "Self Assembled Monolayers", or SAMs. The ideas is to bond the "head group", this case the Silane group, to the surface and expose the long tail and as you stated, what makes the molecule hydrophobic. One of my research goal is to make transparent superhydrophobic coating or surfaces where the water contact angle on it has to be higher than 150 degree. So besides coating the surface by FAS type molecules, I had to drastically increase the surface roughness to boost the contact angle.

Just some info for you and everyone, as it is my study.
I do appreciate your explaination of the coaitng! I would never know this much when I was first yr. Comments are very welcome!

Simon (SEJohnson95) - As one chemist to another, I think a good challenge for you would be to investigate: What is Rain-X? How does it work? Are there pitfalls in the application procedure? Could the product be improved?

Hint; think surface tension/ surface energy and "molecular bonding".



Richard Gray

OK then, here goes (I hope this makes sense from a chemical point of view and I'm not about to ridicule myself) Take a deep breath everyone!

Background

Glass is composed of a network of silicon and oxygen atoms bonded together, with hydroxyl (OH) groups present at various places, these are arranged in a tetrahedral manner around a central silicon atom to create a large 3D structure. (Glass is amorphous, not crystalline) Image shown below of a representation of the structure after adding Sodium Carbonate which makes the glass less brittle, hence the Na+ counter ions(this is the sort of glass used in windows.)



Hydrogen Bonding

Water is a very polar compound owing to the large difference in electronegativity between its two constituent elements, the oxygen operating what is known as a -I inductive effect on the hydrogen, pulling the electron density through the sigma bonded framework of the molecule. This causes the hydrogen atom to become deshielded , hence it carries a delta+ (slight positive charge) because the electron density is not shared equally between the two atoms in the bond.

Water has some unusual properties for a molecule of its size anyway (high boiling point, its solid form is also less sense than its liquid form, etc.) because of hydrogen bonding. This is the attraction between a de-shielded/electron deficient hydrogen on one water molecule and a lone pair of electrons on another oxygen in a neighbouring molecule, i.e. an intermolecular interaction.




Since glass has hydroxyl groups present, Hydrogen bonding between the water on the windscreen and the surface of the glass can occur, causing a greater interaction between the two, which is why water tends to stick to windscreens.

Polysiloxanates

Rain-X's primary active ingredients are known as polysiloxanates, more specifically Polydimethylsiloxane (PDMS), which is a Silicon based polymer with two methyl groups attached. I.e. two organic functional groups attached to a inorganic chain. This makes the compound very non-polar, so it will be repelled by the water droplets. The monomer is shown below: (one uses many monomer to make a polymer, e.g. styrene and poly(styrene) )



PDMS binds to the hydroxyl group of the glass surface and thus inhibits the interaction of water molecules with the surface of the glass. The two methyl groups provide low surface tension, and this serves to repel high-tension fluids such as water. (Think about what happens when you skim a stone across water, or do a belly flop in a pool, it's the surface tension caused by Hydrogen bonding across molecules on the surface which allows this to happen.)

Glass is also very porous owing to its slightly random 3D structure and the Rain X coating also serves to fill in these pores to to minimise the surface area that the water can make contact with. It increases what is known as the contact angle, and the greater the contact angle,the easier the water will slide off. This makes sense because when the water has a large angle, there is less surface ?holding? onto the water droplet.



This forces the water to bead and roll off the windscreen. Rain X is a type of superhydrophobic coating (water hating), it works at the molecular level to inhibit the interaction.

Adhesion to Glass

Oxygen groups are good at bonding to glass (see above explanation) but only when they are exposed. In a polymerised chain there is a lot of steric bulk (the large amount of space taken up by the rest of the molecule) which reduces their ability to bond to the glass.

In order to make these sort of polymer adhere to glass, the oxygen groups need to be exposed. Consider a similar product to Rain X, known as FAS17. This a fluoroalkylsilane (long groups of singly bonded C-C atoms with fluoro substituents instead of the typical hydrogen substituent as in a simple alkane) with three silyl ether functional groups attached to the Silicon.





FAS17 can be made to adhere to glass better by breaking down the compound with a catalyst (in this case acid catalysed hydrolysis -the breaking down of a substance using water , an organic mechanism that I learned this year), to produce a Silanol (a silicon with an alcohol functional group) followed by a step known as condensation, in which effectively water is to bridge the Silanol and the OH group on the inorganic substrate, in this case the surface of the glass. You then apply a large amount of heat (indicated by the delta H) and polymerise the Silanols, which gives you something like the starting product but now with a more permanent adhesion to the glass surface instead of a coating.

Because the FAS is chemically bonded to the glass it is very hard to remove. This allows the compound to stay active on the windscreen over a long period of time as opposed to the Rain X (PDMS) which can be removed over time by external factors such as wiper blades which is the reduction in effectiveness that you all observe (hence the need for a washer additive to bridge the gap in between applications) however since the compound has been altered chemically through various syntheses, its effectiveness is reduced.




Conclusion

In conclusion, Rain-X and PDMS based repellents have a greater water repellent effect but a much lower binding ability to glass and so need to be replenished. Rival FAS based repellents have a greater lifespan but come with sacrificed effectiveness.

I hope I'm on the right lines with that lot!

Oh and on a final note if I may mention my first-year results briefly (which I received in June but kept quiet about) 87.3% overall, with 89,90 and 86% in Inorganic, Organic and Physical chemistry respectively. The cherry on top was receiving the Harding Prize for the top undergraduate year 1 laboratory marks out of a 120 strong cohort. I'm still a little in shock to be fair! I was never that good at A level

[Polse7317]

congratulations WEIDI Now i don't know what i must exactly do when i drive Under a rainy day...!
( i am just kidding )

[SEJohnson95]

Hi Weidi

Thanks for your reply, it sounds like you are indeed much better informed about the subject as you are studying the subject in much greater detail than I ever have or likely ever will. (I am a synthetic organic/medicinal chemist in training!) We are looking at contact angles, wetting, surface tension and the like in this year's physical chemistry module, but I certainly hadn't touched upon this at the time of this article, which was between my 1st and 2nd years.

I'm glad that you liked the article, and that you came across the slightly more scientific (as opposed to the one published in the club magazine) version, but the differences are minor. Please keep us all updated on your work (if you are allowed to) or feel free to drop me an email/PM.

Cheers

Simon

[JagWaugh]

In any discussion about Jaguar windscreen wipers it is important to understand the original design philosophy.

Many people labour under the false impression that the design was meant to push any water off the surface of the glass. This is mistaken, the way they are designed to work is to merely annoy the water sufficiently that it leaves the windscreen of it's own volition.

Once you've understood this basic truth it becomes obvious that products like Rain-X cannot help but improve matters.

[Tony]

Ok, seeing as the experts are present. I race sailing dingies, so of I sprayed the hull of my boat with rain- x would it go through the water faster and if so how long would it last?

[malcolm]

I suspect it would Tony, although I am definitely NOT an expert! I say this on the basis that surfers wax the underside of their boards to make them faster by being more "slippery" to water. I imagine that a planing hull on a racing dinghy would similarly benefit, but that a displacement hull wouldn't. I also imagine that rainX wouldn't last long, and that waxing like the surfers may be better.
What sort of dinghies? I used to sail a Fleetwind (quick) and an enterprise (not so quick), and then a hybrid surfing dinghy/sailboard (fun but the worst of both systems!)

[mgcjag]

Im having visions of Tony polishing his bottom over xmas

[Tony]

Malcolm, was Lasers but I now sail a Solo. Need a 18 to 20 knot wind to get them on a plane. But a good competitive class.

Steve I was hoping you would come and do it, but perhaps not "Darling"

[Hugo]

I suspect it would Tony, although I am definitely NOT an expert! I say this on the basis that surfers wax the underside of their boards to make them faster by being more "slippery" to water.

Sharks, which have had hundreds of millions of years to practise this, have 'riblets' I think they're called, on the surface of their skin - little irregularities rather than being smooth and 'slippery'. Apparently that helps them slip through the water in ways that neither I nor the sharks fully understand.
I wonder if orange-peel on an E Type will help it go faster than a perfect paint job?