TECHNICAL EXCELLENCE

Vinyl windows (technical, long)
Posted by John Schaefer on Sunday, 12 July 1998, at 6:47 p.m.

The back window is made of vinyl. I'm Chief Engineer for a medical manufacturer; we buy flexible vinyl in multi-ton lots as raw materials for our fabricated products. We don't make car tops, but we do use material that would work fine for back windows.

Vinyl is inherently soft-surfaced, so it scratches readily. It easily absorbs a wide range of chemistries from water to acids to all manner of hydrocarbons, so it tends to cloud slightly when humidified and it can be colored by industrial fallout or cleaning agents. It is relatively optically transparent to UV, and while it is relatively stable against UV-crosslinking itself, it can absorb stuff that is not.

Last and worst, it is made flexible during original formulation by inclusion of plasticizers, which are oil-like complex hydrocarbons. Vinyl, in a sense, is a sparse matrix of plastic sponge which is saturated with liquid plasticizer. It is this high percentage of plasticizer that makes gradual contamination by airborne junk so problematic, because the plasticizer rapidly dissolves the junk from the surface, and it spreads throughout the vinyl.

Plasticizers are meta-stable liquids under the best of conditions; for instance, in a new car, the film that forms on the inside of the glass consists of condensed plasticizers that heat-evaporated from the back window, the other vinyl in the car, and perhaps from the leather seats. (Plasticizers of similar chemistries are used to add softness and flexibility to materials other than vinyl, as well.)

Automotive vinyls are made with premium plasticizers to minimize evaporative loss, but nonetheless there are limits to how much stability can be achieved. Evaporative stability and good low-temperature flexibility are difficult to achieve together.

Plasticizers are readily stripped from the surface of vinyl by anything that can bind and lift oils, including detergents and soaps, and certainly including ammonia such as is found in some glass cleaners. More plasticizer will leach to the surface to replace what is stripped, but when cleaning loss is combined with gradual evaporative loss, eventually the vinyl will become less well plasticized, and thus stiffer. Decreased plasticizer content, cold temperatures, and stiffening due to UV-crosslinking of various absorbed contaminants can all lead to flex cracking.

There is no good fix for this. Vinyl is an imperfect material, used only because nothing else is better.

The best options for auto maintenance are to scrupulously avoid exposing the vinyl to even trace amounts of anything not watersoluble; wash the window with water and as little cleaning agent as possible; and regard the back window as a disposable/replaceable item. ($$ Ouch.)

I'm surprised that Porsche and other soft-top car makers don't offer zip-in windows that can be made and sold as reasonably-priced, owner-installable replacement parts. Probably when my back window wears out, I'll do this to my top. But I have resources available to me that probably are similar to those of Car Tops in Germany. Most owners probably don't have that option without some manufacturer's support.

Any auto-parts manufacturers out there?

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