The Grime Suspects: Migrating Auto Chemicals Love to Leave a Film on Windshields

I’ve heard from professors; current and retired workers in the auto, glass and vinyl industries; a couple of retired auto company researchers, and a spokesman for General Motors Corp. It’s unanimous: The main culprit is chemicals used to make the plastics and vinyls in our cars more flexible and durable.

It’s also almost impossible to remove. This milky, somewhat sticky film defies cleaning agents. You think you’ve got it wiped away — till sunlight or headlights hit your windshield and you see streak after streak.

There is a reason for it. Our dashboards and other interior components are supple and long lasting for a reason — there are chemical compounds in them to make them that way. Without those ingredients, they’d get brittle and be prone to cracks.

Problem is, those chemicals don’t stay stuck to the vinyl or plastic forever. They gas out, especially in heat, and stick to nearby surfaces such as our windshields.

“The process that you are observing is called desorption,” writes David Felbeck, a University of Michigan professor emeritus of mechanical engineering. “This is the process where individual molecules that are weakly attached to a surface randomly get kicked off into space and then are deposited on another surface.

“The problem is worse with new cars because there is so much junk put into upholstery to make it look good. If you have ever kept a car for many years (as I did during my student years), you would notice that the problem diminishes with time. I currently have a 2-year-old car and I still have the desorption problem.”

Another reader was more specific.

“The nonsmoking fog noticed on vehicle interiors is due to plasticizer migration,” writes Paul Breakey of Livonia, a retired Ford Motor Co. steering wheel engineer. “Plasticizers are various chemicals blended into plastic material to improve molding processes and to make molded components more flexible. Vinyl floor mats, seating material, dash panel and instrument panel molded plastic components as well as interior plastic trim all have various plasticizers in varying amounts.

“Over time and with sun load,” or ultraviolet ray exposure, “the plasticizers migrate to the plastic surface, enter the atmosphere and most noticeably collect on the inside of the windshield forming the cloudy deposit noted by your readers,” Breakey adds. “Older readers will remember cracked steering wheels and cracks in the dashboards and armrests on cars from the 1950s through about 1975. The car companies have been addressing the problem over the years and certainly things are better now but still not perfect.”

Certainly not. GM, for one, says it’s continually working on the problem.

“It is an annoyance, so absolutely we’re looking at ways to eliminate or reduce it,” GM spokesman Dave Hederich told me Monday.

“Interestingly enough, that’s part of the new-car smell that people like so much,” Hederich says. “Out-gassing is more pronounced when a vehicle is new — that’s why it contributes to that new-car smell. A lot of it is due to the sun hitting it, so you don’t have it as much if your car is garaged.”

But why is it so difficult to clean? I couldn’t nail that answer down, in large part because I’m not a chemist and there are a variety of chemicals and plastics used in vehicles. I suspect the explanation is something only an engineer or researcher could love.