Why does my steering wheel weep and what can be done about it?
There were and are many different chemical compounds used by various companies through the years to form steering wheels. This article discusses only the black hard plastic steering wheels. Soft or firm grip plastic used in newer cars and translucent or colored wheels are made of different compounds. I will limit this explanation to 1949-1995 (roughly) black hard plastic wheels made by GM.
Steering wheels have been made of various cellulous based plastics for at least 100 years. The most common and longest in use chemical basis for the plastic was cellulose acetate-butyrate. Sometime in the late 1950s, GM started using cellulose acetate-propionate. GM used the -propionate version for their hard rubber wheels up until airbags were added to the hub. Then they moved to a firm grip wheel that has different chemicals and therefore different reactions to the environment.
I am not a chemist. Most of the publications I studied in preparation for this article were incomprehensible to me. I trudged through the articles, books, and patents so I could glean at least some understanding of the why my 1975 C10 steering wheel weeps a sticky and oily goo that turns my hands black. My reading lead me to two conclusions.
First, you can know which of the two most common cellulose acetate plastic materials your steering wheel is made from by the symptoms of decompositon. Cellulose acetate-butyrate will powder, crack, craze, and basically disintegrate when exposed to the right conditons. (More on the right conditions in a bit.) Cellulose acetate-propionate weeps as it begins to decompose. Basically, the plastic is in a process of a slow melt down. Both conditions will cause the plastic to shrink and eventually crack. Left to the natural process the plastic will eventually break away from the rim.
Second, the decomposition cannot be reversed. Anything you have read about dish liquid, brake cleaner, polishes, waxes, etc., are only cleaning the residue of the decay; the decay continues and it is only a matter of time until you will have the same problem again. Attempts to seal the surface to prevent further degradation might buy some time. But the time bought is an illusion. The decay is still in process under the sealing epoxy or acrylic used to seal the plastic. This is because once the decay process starts, it leads to a chemical reaction that is self-perpetuating -- and accelerating. The amount of time before the sealer begins to lift-off the plastic depends on the stage of decomposition of the plastic, among other things.
What causes the plastic to decay? Just about everything. High humidity, exposure to sunlight, contamination by chemcials, temperature extremes, and just plain old time. After 40+ years in use, plastic steering wheels are about at the end of their useful life.
Accepting that the original plastic cannot be saved is the first step to fixing the problem. I spent hours on the internet reading home remedies, professional interventions, commercial product testimonials, and so forth. Almost everyone has a fix that they swear by. Most involve hours of labor to repair cracks and then sealing the repair. But the chemistry does what it must. There is no intervention that will save the original plastic. The least expensive thing to do immmediately is to clean the steering wheel and wrap it in a cheap lace-up cover while you decide how to proceed next. Keep a pair of gloves in the car because eventually you will need them to prevent the black goo from getting on your hands.
There are three remedies. The first is to find a steering wheel that has not yet begun to get sticky to the touch and install it in place of the original. Don't bother trying to find an exact replacement from the same model; it is just as old as the one you are replacing. If the decay process is not evident when you install it, it soon will be. Get one that looks like it belongs in the vehicle from one that was built as recently as you can. Unfortunately with the advent of airbags, the mounting method changed. I suppose you could find a way to mount an airbag equipped steering wheel on an older car or truck but it does add to the cost and complexity.
The second course of action is to buy an aftermarket wheel. This costs about ten times as much as a junkyard wheel but it has the advantage of being of newer and, hopefully, made of more stable materials. This is probably the best way to go for a daily driver.
The third fix is the only one open to those building a concourse restoration. Have the original plastic replaced. There are specialty companies that do this. The cost runs anywhere from $600 to over $1000.
References -- All URLs were active in April 2017.
https://www.madisongroup.com/publications/Jansen_ASM_Chapte_TMG2011.pdf
Characterization of Plastics
in Failure Analysis
Jeffrey A. Jansen
https://www.madisongroup.com/publications/Degradation Article - Plastics Eng Mag.pdf
Plastic Failure Through Molecular
Degradation
Jeffrey A. Jansen
https://mmics.files.wordpress.com/2008/04/problem-plastics-check-list.pdf
Problem plastics, types of deterioration & where you find it
Fran David
https://books.google.com/books?id=B...ical breakdown plastic steering wheel&f=false
Page 519, Handbook of Plastics Joining: A Practical Guide
PDL Staff
https://link.springer.com/article/10.1007/s10924-010-0258-0
Degradation of Cellulose Acetate-Based Materials: A Review
Juergen Puls, Steven A. Wilson, Dirk Hölter
https://www.google.com/patents/US3020661
Method of polishing and deflashing molded plastic articles
US 3020661 A
https://www.google.com/patents/US2651811
Process of solvent polishing-injection molded articles of cellulose acetate-butyrate
US 2651811 A
