U-V Protection for Plastic Knobs

[Jon37]

I have an open car. Years ago I installed a NOS plastic knob and within a few years it crumbed away! I'm pretty sure that's because the sun's U-V rays degraded the plastic. Does anyone know of any sort of clear U-V-filtering coating one can apply to dashboard plastic to protect it from the sun's rays? I just heard about some sort of "U-V Polyurethane" designed for wood, and wondered if this might be a possibility.

[Dave@Moon]

Even if you parked your car top down every day outdoors, unless these knobs are on top of the dash I'd be surprised if UV light was the major cause of their degradation. If they are exposed to intense sunlight on a constant basis (i.e. stored next to a window, etc.) you could simply cover them with baby socks, rags, plastic bags, etc. If this is your problem you'll likely notice discoloration/fading long before there is physical damage.

I think it may be more likely that a chemical exposure did this. If the car is stored in an area where there are a lot of volatile organic fumes some of the airborne solvents will penetrate plastics, swelling them. This can cause cracking in hard plastics. Do you store the car in an area where other cars are being painted? Do you frequently drive the car with solvent residue on your hands?

Two other common exposures that can do this are ozone and chlorine. If the car's stored with swimming pool treatment chemicals or near large amounts of high voltage equipment this may be the case. If the car's stored where there are a lot of bleach fumes that might be the cause. High oxygen environments can do this as well, but you'd discover that problem sooner the minute you started the car and burst into flame!

For hard plastic knobs to disintegrate quickly like this it's probably a number of factors. Check with other people that have your car to see if there's a specific problem to it. (You might post another thread here in the appropriate marque section.)

Good luck!

[Jon37]

Thanks for your thoughts, Dave. Actually, I did notice a greying effect on the black knob for a number of years before it crumbled away to dust. As to the other possibilities, I don't park the car near any chemical dumps, swimming pools or high tension lines, so I'm still considering UV rays as the culprit. I now have an original knob to replace the crumbled one, and don't want to lose that, too!

[Shop Rat]

Do you by any chance know the specific type of plastic the knob is made of? My husband Bill worked for many years with FMC and Carbide/Dow working with plastics. If you can find out the type of plastic it is he might be able to help you with what would cause the problem you had.

[Guest Curt Fouse]

I would suspect an inferior grade of plastic. I restored a 1941 Buick a few years ago and machined just about all the plastic knobs and other plastic parts from DuPont Delrin. It has held up well. Delrin is nice to work with. If you could make the part out of wood, you could make it out of Delrin. The current project is a 1948 lincoln. It has the original plastic in it that has held up well. The worse part is the steering wheel, and I will probably help it all I can, and use it as it is.

Curt

[Jon37]

Thanks for your additional thoughts.

Susan, I don't know the type of plastic but I suspect it is some early version of "Tenite", since the manufacturer refers to the steering wheel material by that name. The car is a 1937 and I'm sure plastic (used as knobs) was in its infancy then. Curiously, the window crank knobs have held up just fine, as have the other dashboard knobs and buttons. And actually the knob that crumbled was on a NOS replacement windshield wiper switch, located on the top of the dash (where it receives more sun than the other knobs). Also, the replacement knob was not of the original type (brown marbellized plastic), but solid black. Possibly this made it absorb more heat in the sun and aided in its degradation. Quite possibly, were I to replace that knob with the original one (marbellized) I found later, that knob would <span style="font-style: italic">not</span> degrade. But if I were to cover it with an ultraviolet-filtering coating, I would feel safer!

The marbellized plastic makes it hard to replicate in a modern solid-color material.

[Guest De Soto Frank]

Most pre-1950 "plastic" is "inferior" in terms of resistance to heat and sunlight (UV)...

This is true for dash knobs, trim pieces (hood ornaments, fins, figureheads), lenses...

Also delicate were the "cheap" Catalin plastic radios of the '30s & '40s (often marbelized)... nice unwarped, un-crumbling originals are no longer "cheap"

My Grandad's '54 Chey pick-up has black knobs on the dash. The ones that always seem to crumble are the wiper control knob: which sits on top of the dash, in the full sun. I 've never seen a '54 with an intact original wiper knob. I think it may have been moulded from a different formula than the headlight/choke/throttle knobs.

My long-lost 1950 Ford Tudor had ivory/butterscotch colored knobs... they were cracked and crumbling by the time I got the car (c. 1980).

The only 1940-'42 Chryslers with decent plastic dashes are those cars that have been garaged their entire lives.

The only "early plastic" I know of that is "stable", is black bakelite or "phenolic".

Does your Buick wiper knob fit on a split-shaft (so that it is easily removed) or is it part of an assembly?

[Dave@Moon]

Have you tried using an original to cast new pieces as needed? This can be done with any number of kits, like this one from Eastwood.

[Jon37]

Thanks for your additional thoughts.

Frank, my knob (actually, Hudson) is force-fit on a splined shaft and should be easily replaceable.

Dave, thanks for the reference to that plastic moulding kit...looks interesting! Getting a marbellized effect may be difficult but at least it's a start!

[Shop Rat]

Jon, I copied and pasted your message and sent it to our e-mail address for Bill to have a look. I will let you know what he says. Thanks.

[Guest De Soto Frank]

Jon,

Sorry about that... I don't know why I thought you had a Buick...

My '41 De Soto originally had a tan/brown tortoise-shell wiper knob, which was missing when I bought the car. It pushes onto a split-shaft, just like the radio knobs. So I used an extra radio knob for quite some time. Eventually, I found a metal version of the wiper knob from a Chrysler, so I'm using that.

Last fall, I took the heater out of the car for cleaning & lube of the fan motor... when I took the heater cabinet apart, I found the desicated remains of the orignal wiper knob in the bottom: it kind of looked like one of those African "shrunken-heads"... I'm still trying to figure-out just how it got to the bottom of the heater cabinet...

At any rate, it's another victim of "early plastic disease"...

Good luck with your efforts to make a new one !

[Guest bkazmer]

Delrin is a trade name - the material is polyacetal. It machines nicely, but is typically not all that good for weathering. Most of the early plastics are thermoset phenolics, and they have both poor control of residual materials and not very effective UV and heat stabilizers. A lot of shrinking and cracking is typical by now.

[Peter J.Heizmann]

Excellent response...

Much appreciated.

Speaking from the sand-cast foundry business, we found "urethane" coreboxes & pattern impressions to be the "cure-all" for cost effective tooling over the past 18-20 years. Urethane remains the best for short run, cost effective casting quantities, however, pattern shelf-life "dimensionally" is an average of 10-12 years.

We have now shifted to the "only cure-all": CNC, tool steel, machined patterns for the same reasons you mentioned...urethanes/plastics shrink and crack over time. We still pour urethane tooling, however, only for limited life and use. Though urethane tooling is stored in dry, dimly lit rooms, it still shrinks, becomes brittle, and, cracks with no UV interference. We are very careful pulling out an aged pattern that may cause a dimensionally unstable casting.

Thanks, again, bkazmer for great input.

Peter J...

<img src="http://forums.aaca.org/images/graemlins/grin.gif" alt="" />

[Guest Ken G]

I had need to make a duplicate radiator cap, the original being some sort of black material (ebonite?). (I wanted to install a Motometer, which involved a slightly larger central hole, and I didn't want to drill out the original). I made a mold using the flexible rubber stuff (I cannot remember names), and then made the duplicate using fiber-glass resin with a little black pigment. It is difficult to distinguish the new one from the original, apart from a tiny bit of shrinkage. The result was not a polished surface, because the new one reproduced the matt finish of the original, but I suspect the resin would polish quite well with some effort. Anyway, this is really to say that reproducing small parts is really quite easy.

Ken G, 1925 Rover 16/50 (San Francisco)

[Jon37]

This just in -- Plasti Dip (they make the rubbery dip which some people apply to tool handles) comes in colors as well as clear. According to the manufacturers, the clear coating has UV-screening properties when it is sold in the spray can or in 1 gallon or larger cans (in liquid form). In black, this material is also used to re-coat runningboards. It's available at such chains as Ace Hardware, Home Depot and Lowe's. It might be possible to spray the clear version onto a plasic knob (or other dashboard plastic) to protect it from the sun's rays.

[Shop Rat]

Could covers be made to protect the knobs and handles when the car is not being shown or driven? A friend of ours had flannel lined, light canvas bags made to cover the brass items on his Model T, even the brass valve stems on the tires. They had drawstrings also to hold them in place. He polished the brass each time he brought it home and put the covers on. No scratches, no damage and less cleaning.

[Guest bkazmer]

you might be OK on urethane molds if you specify aliphatic, not aromatic, polyurethane. It's more weatherable. But I agree CNC makes hard tooling the way to go for commercial use. I'm encouraged by this discussion to make some silicone molds and try a few pieces myself. Model/hobby shops have the supplies.

[Peter J.Heizmann]

That's the spirit! Molding parts is a lot of fun.

(Not urethane related, however...) Years ago, we heard wind of the lost foam process of molding cast iron parts. One day, I took an empty styrofoam coffee cup to a very experienced manual molder. During his lunch break, he packed sand around and in the cup, cut a sprue, used wire to add vents. (I told him to add quite a few vents to allow the more-than-usual gasses to vent.) Well, we poured the mold. Looked like a broken gas main when it vented. We wound up with a perfect cast iron coffee cup. (As the iron flowed, it melted the syrofoam, naturally, and wella.

Regards,

Peter J... <img src="http://forums.aaca.org/images/graemlins/grin.gif" alt="" />

[Joe Block]

Early Plastic like that founds on 40-42 packards were made from a soybean products amd even new start plastic showed UV deteration with a couple of years. Luckily I reproduce the adjustable radio buttons, so that is solved.

[Joe Block]

IN my college foundry class this one of the process we did, I made a nice bookend with my intails "JB: cas in. My use as door stop now some 35 years later.