|
 |
Cure for rear window fogging (real science). (archive) |  |
Posted by Fred Kern on October 14, 1998 at 22:47:00:
In Reply to: Cure for rear window fogging. posted by stephen on October 14, 1998 at 14:47:55:
There's plastic, then there's plastic.
Seals around the windows, especially of older cars like the 2002 were rubber. Rubber is susceptibl;e to attack by ammonia, and is porous, so ammonia solutions soak in and can't be easily removed.
The Z3 window is vinyl - plasticized PVC. Ammonia can react with plasticized PVC by emulisfying the plasticizer. This can lead to clouding and eventually cracking.
First, the PVC in your back window is exactly the same polymer as the PVC pipe in your household drain lines ("vinyl" is clear naturally, pigments are added to color/make it opaque) . A plasticizer is addded to make it flexible. Plastricizers are high molecular weight solvents (non-volatile) that are added to a polymer formulation to increase flexibility by causing polymer molecules to form bonds with the plasticizer rahter than with other polymer molecules. In doing so, the plasticizer also increases the specific volume and changes the index of refraction of the resulting engineering plastic. Typically they are phthalic acid esters and are recognized by that "Samsonite stored in a hot attic" smell. Don't ever drink large amounts of water from a plastic garden hose as that lovely taste is the plasticizer. It is probably carcinogenic, and of a more immediate concern, is one hell of a laxative.
Car interiors get hot. This heat can cause plasticizers to evaporate from interior vinyls. The blue haze you see on windows is known as "vinyl sweat" and is the plasticizer evaporating from interior vinyl and recondensing on other surfaces. Vinyl will eventually crack because its specific volume decreases as the plasticizers evaporate. This decrease occurs in three dimensions, so the effect is compounded by lateral shrinkage increasing the tensile stress on the vinyl, while the decrease in thickness increases unit loading. Products such as Armor-All poreserve vinyl by coating it with an aqueous emulsion of plasticizer. This is why leaving your top down after using vinyl restoration products is a good idea. On "tin tops" open the windows.
Ammonia cleans vinyl sweat because it emulsifies most organics, including plasticizers. If it cleans the film off your window, then it can also extract some plasticizer. Ammonia accelerates the clouding process. Clouding is caused because the plasticizer does not evaporate uniformly but starts evaporating from the surface and depletion works it way into the material. Clouding occurs when sufficient plasticizer has evaporated to shift the index of refraction of the surface material relative to the bulk, so light is scattered internally. Further clouding can result from microcracks in the surface of the plastic due to specific volume change.
Ammonia can lead to clouding as it enhances extraction of plasticizer by emulisifying it. The good news is that ammonia needs time to accomplish this. If you clean the plastic rear window with ammonia, get it off fast. Ammonia can also cause black vinyl to develop a haze. If you clean windows, watch the overspray.
Wax-like products for the rear window work by replenishing plasticizers and sealing the surface with a higher molecular weight (less volatile) species.
In case you are wondering, I am a Chemical Engineer trained in polymer processing. I now work in semiconductor manufacturing where I evaluate high purity polymers for processing equipment.
© 1997 -
2010
Jump Internet Inc. All rights reserved.
