Friday, September 21, 2012

UV Exposure Meter in a Wiper Blade. Not a Great Idea

A few years ago, the replacement wiper blades that I bought for a car had a feature that I had never seen before: small UV-sensitive pieces that were suppose to change color to indicate that the wiper blades needed to be replaced. They didn't work very well as the blades needed replacing long before the color change occurred, but I kept the blades around (outside) to see what would happen. The dot that was originally black [1] has in the course of a few weeks become yellow [2].

Making an appropriate UV-sensitive indicator would be extremely challenging. The amount 0f UV light that the wiper blades receive is all over the map - literally. It is well known and documented that UV exposure varies with latitude, but also with longitude. For instance, Georgia and Arizona share many of the same degrees of latitude, but the weather in Georgia is much cloudier and rainier than in Arizona. Other factors that influence UV exposure would be how long the car was parked inside or outside during the day and the direction the car faced when parked outdoors.
Parking the car in the shade of say a tree for example, is not as effective at reducing UV exposure as you might think. Rayleigh scattering intensity goes with the inverse 4th power of the wavelength, so UV light is the most highly scattered light from the sun. If the wiper blades can see blue sky at all, they are getting some UV exposure.

But all these factors could be swept aside and ignored if you make the assumption that UV exposure leads to degradation of the rubber and that the degradation of the rubber is the reason that wiper blades need to be replaced.

That unfortunately is false. Wiper blades are made with 2 well-defined 90 angles. I learned from past clients that this sharp angle is essential for wiper performance, but friction and erosion from dirt lead to this angle becoming rounded off over time. This is why wipers need to be replaced. This means that you cannot correlate UV exposure to wiper wear. As an extreme example consider a very rainy climate where the amount of UV exposure will be lower, but the demand and wear on the wiper blades will be very high.



[1] It could also have been clear too and just looked black because of the black background.

[2] I will have the lab here look into the chemistry of this color change and post what I find. My initial guess: it was a polyacetylene or polydiacetylene polymerization. As the polymerization proceeds, the conjugated backbone develops which then absorbs UV light. The color would first be blue but that wouldn't be visible on the black background....

4 comments:

Bend said...

Why would someone need a spot on the blade to tell you that it needs replacing? When it stops working, shouldn't that be the first clue?

John said...

Personally, I thought that they would have changed in 6 months or so - in order to try and get you to replace the blades prematurely. (All in interest of protecting the consumer, of course!)

wheel alignment said...

Replacing and even upgrading wiper blades takes minutes and can provide improved visibility even in the worst of weather conditions. This is a task best performed on a balmy day with a cold drink in wait, instead of during a freezing rainstorm at the superhypermart parking lot.

Ricardo Spencer said...

Interesting post, I am going to spend more time learning about this subject. ok

Wiper Blade Replacement