3) Consumer Considerations

Certain things must be considered when choosing which product is best for your needs.
The three main catagories examined here are:
Effectiveness/Durability
Application
Cost

Effectiveness and Durability

       As discussed earlier in the Science section, previous studies have found that at peak performance PDMS based repellents allow water to slide off at a smaller angle. This means that Rain-x which uses PDMS has a stronger repelling effect on water than Aquapel at peak performance [1]. For uses that require the highest possible performance, Rain-X may be the best option because Aquapel is not as effective. Although Rain-X has a higher peak performance, this comes at a cost. Rain-X also has a short life span and durability. Some users have reported problems in as little as 20 days after application. On the other hand  Aquapel is strongly chemically bonded to the glass it can go even as long as 6 months without severe degradation. Some users report times as long as 8 months without reapplication [2]. The following video compares  Aquapel to Rain-X over time:

Application

To apply Rain-x the user first applies rain-x using small circular motions over the entire windshield. After applying the product must sit for a few minutes to haze over. After this haze appears, it must be buffed out completely using a new dry cloth This buffing stage can be difficult and tedious to complete due to the fact the haze is often hard to buff out. Rain-x also recommends multiple applications for full effect. Even with extensive buffing, some users still angrily report streaks and hazing.

To apply Aquapel the user moves the applicator all over the windshield in both horizontal and vertical motions. This can take significantly less time due to the fact that there is no requirement for small circles. After the initial application stage, the user wipes off the excess with a paper towel. No buffing is required if wiped up immediately. This can save a significant amount of time as rain-x requires a large amount of buffing, following the already lengthy application.  Aquapel also does not require more than one application, which means rain-x application can take over twice the time for a similar effect

Also due to the problems with durability discussed earlier, Rain-x needs to be applied every couple weeks, sometimes as often as every 20 days to retain effect. This can add more time to an already lengthy application. Aquapel on the other the other only needs to be applied every 6 months at the very most.

Cost

The cost for a spray bottle of rain-x Glass cleaner and rain repellent is $9.42 for a 23oz bottle on Amazon [3]. There are cheaper alternatives but these do not include the glass cleaner which is necessary. After factoring that in it comes out to about $10. This bottle is good for a number of uses as it appears the application takes an ounce or less.

Aquapel on the other hand is $11.95 for four single use applicators [4]. Though this is more expensive per application, looking back on the durability, Aquapel lasts significantly longer. In the worst case, Aquapel is a little bit more expensive for a longer lasting repellent. In the best case, its durability allows it to be a cost effective alternative to rain-x. Cost/benefit depends on a number of factors including how effectively and how often either repellent is applied.

One hidden cost that user may not see comes from windshield repair. Because Rain-X is not completely bonded to the windshield, it has the ability to be moved around as discussed earlier. If a crack or chip happens to appear in the windshield, Rain-X can seep into the crack. Due to its hydrophobic nature, Rain-X can actively prevent correct repair of cracks, which may necessitate the complete replacement of a slightly damanged windshield [5]. Aquapel on the other hand does not slowly move around windshields and cannot fill into cracks unless the chemical was applied directly to a crack [6]. In the long term, Aquapel may save tremendous money on windshield replacement.




Conclusion:

Based on all the evidence provided Aquapel seems to be the clear leader in terms of the best water repellent on the market for use by consumers. Aquapel although slightly less effective, has a significantly longer durability. It can last up to 8 times longer than Rain-X. Application is easier quicker and less of a hassle than Rain-X especially due to the fact that no hazing will occur with Aquapel. Lastly, although the cost over time may be hard to estimate, there are significant benifits for windshield repair while using Aquapel over Rain-X.
In conclusion: Aquapel is better choice to use a rain repellent in almost all conditions.
As with any technology, hydrophobic water repellents are constantly going through innovation. In 2003 the Nippon Glass Laboratory [1] was successful in designing a new type of FAS which actually has a comparable effectiveness to PDMS but still retains the advantage of increased durability. Although this new FAS probably has not made it to market yet, these new types of innovations are happening regularly and can easily create a huge impact on the market. Currently though there is a clear leader, Aquapel, in the water repellent debate.

References
[1] Development of Water-Repellent Glass Improved Water-Sliding Property and Durability http://www.springerlink.com/content/x72674302445h130/fulltext.pdf

[2] Rain-X vs Aquapel http://www.jwardell.com/mini/2006/10/20/reviews-in-review/

[3] Rain-X cost on Amazon http://www.amazon.com/Rain-X-5071268-2-In-1-Cleaner-Repellent/dp/B000NWJJLG/ref=sr_1_5?ie=UTF8&qid=1330677276&sr=8-5

[4] Aquapel cost on Amazon http://www.amazon.com/Aquapel-Glass-Treatment-Single-Applicators/dp/B004NFU300/ref=sr_1_1?ie=UTF8&qid=1330680570&sr=8-1

[5]  More Rain-x Havoc Coming for Windshield Repair, via Car Washes and Windshield Washer Fluids.

http://www.windshieldrepairtools.com/More%20Rain%20Havoc.htm

[6] Aquapel Frequently Asked Questions http://www.aquapel.com/FAQ.php

2) Science Of Rain-x

Hydrophobicity:

       Glass is essentially a network of bonded Silicon and Oxygen atoms. Every once in a while there is an exposed OH or hydroxyl group [1]. This hydroxyl group has a strong polarity just like water due to the fact that the oxygen is electronegative. Because the exposed hydroxyl groups are polar just like water, water is actually attracted to these groups. This attraction causes water to stick to glass and spread out instead of rolling off. Rain-X and Aquapel prevent this interaction allowing the water to roll right off. To fully understand how the works though the chemical structures must be examined.

 Rain-x is made of mostly Polydimethylsiloxane (PDMS) [2],[3] as seen below:

[4]

      PDMS is a simple polymer made up of the silicon, oxygen, and carbon containing “methyl” groups. These carbon containing methyl groups make the polymer similar to oils due to the fact that these carbons are non-polar just like the carbons in oil.

       Aquapel, one of the main competitors of rain-x is made of different compound call a fluoroalkylsilane (FAS). The most common FAS is known as FAS17 for the 17 fluorine atoms that it has on it [5]:

       The long chain of Carbon and fluorine’s create a similar non-polar chemical as Rain-X

       The polarity of water causes it to repel substances like oil and Rain-X. When water (like a raindrop) lands on a hydrophobic surface (like a rain-x treated windshield), the water balls up because it repels the hydrophobic surface and collects into a sphere. This sphere is then touching very little of the windshield. This allows the bead of water to run off very easily as compared to normal glass which attracts the water instead.

       To truly measure hydrophobicity a measurement called the contact angle is used. Physical chemistry is able to predict specific quantities [6]. Knowing the contact angle allows physical chemists to predict the “surface tension” of the glass which is the same thing as its hydrophobicity. The contact angle also allows physical chemistry to calculate how easily the drop will slide off of the glass.

[7]

       The more hydrophobic the material, the greater the contact angle, 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.

       According to a study by Kamitani [8], PDMS has a smaller sliding angle (which means it slides off easier) but a smaller angle of contact, while current FAS chemicals have a larger contact angle but a larger sliding angle. In this case the discrepancy is due to physical nature of the chemicals. The perfluorocarbon chains interact with the water molecule more causing to stay in place [9]. The lab was also able to design a more effective FAS chemical that matches PDMS. This could possibly be used in the future to create a better windshield repellent.

Adhesion to Glass

       To be able to bind to glass chemicals like FAS must be slightly modified. Oxygen containing group are excellent at binding to glass but only if the oxygen’s are exposed. In the case of FAS, all of the oxygen’s are bound up with carbons and not exposed. To expose these oxygens, the FAS17 monomer is exposed to specific amounts of nitric acid. This nitric acid cleaves the bond between oxygen and carbon allowing the oxygen groups to be exposed [10]. The exposed oxygen then easily binds to the exposed hydroxyl groups on glass. A byproduct of this addition of acid is that the FAS monomers polymerize. Previous research has shown that dimers (two monomers put together) and trimers (three monomers put together) were most effective in bonding to the glass. With more polymerization, the polymer became too bulky to bond to the glass. The scheme below shows the process of activating the FAS chemicals [11]:

       Because FAS is chemically bonded to the glass it is very hard to remove. This allows the compound to stay active on the windshield over a long period of time [8].

     Rain-X contains mostly PDMS [2],[3]. From the example above, PDMS has no possible ways to bind with glass. As with FAS, PDMS can also be activated by acid, which breaks down the polymer. In the case of FAS, dimers and trimers were produced. For PDMS though the polymer is broken down. This still leaves very large bulky groups which do not bind effectively to the glass [5]. Rain-X also has chloroalkylsilanes left over from production. These chemicals have been reported to add to existing polymers to create difference types of polymers some of which are unwanted [12]. This is important because larger polymers make the binding ability even lower [9]. This means that wiper blades can remove PDMS over time, which causes it to lose its effectiveness. This is similar waxes that are applied to cars. As time goes on the waxes are removed by external forces, though this effect is more pronounced with Rain-X due to wiper blades being an almost constant force in rain.

Conclusion

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

References:

[1] Quantitative infrared spectroscopic measurement of hydroxyl concentrations in silica glass http://www.sciencedirect.com/science/article/pii/0022309396003304

[2] Rain-X MSDS https://www2.itap.purdue.edu/msds/docs/7756.pdf

[3] What’s Inside: Rain-X. Better Driving Through Alcohol
http://www.wired.com/magazine/2010/06/st_whatsinside_rainx/

[4] PDMS Repeating Unit http://en.wikipedia.org/wiki/File:PDMS_repeat_unit.png

[5] Water-repellent coating films on glass prepared from hydrolysis and polycondensation reactions of fluoroalkyltrialkoxylsilane http://www.sciencedirect.com/science/article/pii/S0040609001009014

[6] Improvement on Hydrophilic and Hydrophobic Properties of Glass Surface Treated by Nonthermal Plasma Induced by Silent Corona Discharge http://www.springerlink.com/content/w020187756124126/fulltext.pdf

[7] Behaviour of water drops on different surfaces
http://www.nature.com/nmat/journal/v1/n1/fig_tab/nmat715_F1.html

[8] Development of Water-Repellent Glass Improved Water-Sliding Property and Durability http://www.springerlink.com/content/x72674302445h130/fulltext.pdf

[9] Coatings made by sol–gel and chemical nanotechnology http://www.springerlink.com/content/rq2l3h621r111275/fulltext.pdf 

[10] Improvement on Hydrophilic and Hydrophobic Properties of Glass Surface Treated by Nonthermal Plasma Induced by Silent Corona Discharge http://www.sciencedirect.com/science/article/pii/S0040609001009014

[11] Easy-to-clean properties–Just a temporary appearance http://www.sciencedirect.com/science/article/pii/S0040609005011612

[12] Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces:  Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon http://pubs.acs.org/doi/abs/10.1021/la000471z

1) Introduction

This blog will be dedicated to discussion hydrophobic rain repellents like Rain-x and the effectiveness of each.
Things this blog will cover:
An Introduction into Rain-x and other hydrophobic compounds
Science behind rain-x and other compounds
Comparison behind the different compounds.
Best choice for consumer use.

The target audience of this blog will be college students and graduates who have at least a minimal knowledge of chemistry.

Hydrophobic Self Assembling Monolayers:

For those who have not heard of it, Rain-X is an especially helpful tool for your windshield. Rain-X and its competitors coat the surface of glass and allow water to effectively bead up and slip away. A quick demonstration shows how effective Rain-X can be on a windshield (Ignore the science of ‘pores’ in this video, the real science behind rain-x will be discussed later):

As you can see Rain-X allows for a much better water repellent effect than normal glass. This is especially helpful for cars that have broken or ineffective windshield wipers. Although not recommended, some people have even used rain-x without windshield wipers. For many consumers Rain-x is a great addition to the car. Some studies have even examined the response time of drivers using these windshield treatments. In a study by the University of Michigan, response times were found to decrease by a full second while using a treated windshield as compared to an untreated one [1]. A extra second of response time can be the difference between a crash and no crash in some situations. For many consumers Rain-x is a great addition to the car for safety and convenience.

As there are many different brands of Rain-x like products on the market today, it can be hard to choose the correct product.


References
[1] The Influence of Hydrophobic Windshield Coating on Driver Visual Performance: http://www.aquapel.com/PDF/UniversityofMichiganStudy.pdf