I have a beautiful, 19 year old daughter that has cerebral palsy and is bound to a wheelchair most of the time. I've done several modifications to that wheelchair to better accommodate her personal needs, and among them is a tray that helps her eat, interact with things during therapy, and play with her iPad.

The tray's design is fine and working just as expected. But I've fabricated it in the past using several different materials, to see which one works best. I've made it out of wood, but it gets easily scratched. I've also covered it with formica, but the edges are a little sharp (even after thoroughly sanding them) and can be easily chipped.

The latest material I've tested is solid polyethylene (1/2" thick), and so far I'm more or less pleased with it. It's easy to clean, and its edges can be rounded to minimize accidental cuts or scratches. But the thing that bothers me it's its appearance.

When I bought the sheet, it's surface was too shiny and looked greasy and oily, even after being thoroughly cleaned and de-greased using windex, for instance. So I decided to sand it using fine (400 grit) sand paper, to give it a more matte appearance ... it didn't work as expected. The surface now looked dusty and dirty.

I had no more time to waste, so I installed the thing on my daughter's wheelchair, and I didn't take it out of the house out of embarrassment because of how dirty it looked, even though it had been thoroughly cleaned and disinfected.

Lo and behold! ... after about two or three weeks of use, the sanded surface began to look more or less "normal" and now it looks just fine. Not shiny and greasy looking, and not dusty and dirty... My guess is that somehow "normal dirt" like human skin oil and other stuff have somehow "cured" the surface. The other side of the tray, the one that's normally not used, still looks as dirty and dusty after being sanded.

Question, is there some sort of treatment that could be applied to the polyethylene surface to make it look more or less normal?

@jpanhalt, if I remember correctly, chemistry is your thing. Any suggestions?

 

You may be able to achieve something like a pebbly texture by coarse sanding followed by gentle heating with a heat gun or heat lamp to fuse the surface abrasions.

You can get HDPE "cutting boards" in various colors, thicknesses and sizes. They have a matte pebble texture and can have custom routing of grooves and holes.

https://www.cuttingboard.com/custom-plastic/

 

I also use HDPE for a cutting board. It has a frosted or slightly pebbled surface that I assume is made when it is molded.

In thinking about your experience, it seems likely the sanded surface simply wore down to a more pleasant looking surface. Maybe exposure to bright sunlight helped that process. From a chemical standpoint, the only surface treatment I have used was flame/plasma treatment to enhance bonding. That technique goes way back (http://www.dtic.mil/dtic/tr/fulltext/u2/660276.pdf ). This is a bit more recent and explanatory (https://en.wikipedia.org/wiki/Flame_treatment ). However, that treatment does not change the surface appearance much if any.

Other chemical surface treatments have been developed, but because PE is so non-reactive, the treatments use quite aggressive chemicals. I would worry about using them as a surface for eating.

 

You may be able to achieve something like a pebbly texture by coarse sanding followed by gentle heating with a heat gun or heat lamp to fuse the surface abrasions.

Maybe there were very fine hairs that were left by the sanding. Polyethylene is somewhat soft so the material may have stretched before breaking creating the hairs.

If that is what happened then the heating sounds like a good solution.

 

A melamine "sponge" might work reasonably well to take the surface gloss off of polyethylene.

Damp sodium bicarbonate (baking soda) is a reasonably good scouring agent for some things. It is widely touted as an environmentally friendly cleaner, but it really isn't much good for anything once in solution (unless you boil the solution, which will decompose it to sodium carbonate which has high enough pH to be more effective). Its solubility is quite low and the crystals are fairly sharp so it behaves as a very mild abrasive.

Abrasives have a tendency to embed in the surface of soft materials like PE and removing the particles can be quite difficult.