Strange problem with incomplete cure of UV-curing resin (like Bondic)

[peteski]

When I started using Bondic to make model lenses ans such, I noticed that even when cured, the open surface of the resin remained tacky.  Unlike standard 2-part epoxies, which fully cure (including their surface).

For example, if I spread some Bondic on a piece on a smooth surface, then cure it using the UV LED light that was included in it, the resini solidifies and looks glass-smooth, but that shiny surface is actually very thin sticky layer of viscous fluid.  I can wipe it off with a piece of cloth or paoer towel moistened in alcohol.  That gets rid of the sticky film, but now the surface of the resin is dull and not very clear.   But only the surface open to air remains tacky, If I peel the cured resin from the surface on which it cured, the surface that side of the resin does not have sticky film! 

I thought that maybe I had a bad or old batch of Bondic, so I bought a bottle of UV curable resin on eBay (from a dealer in China). It acts just like Bondic - after curing the open surface of that resin also has that sticky film on the surface.  I tried another UV light (a much more powerful UV flashlight) with the same result.

I suppose I can wipe the sticky film off my cured parts, then apply a layer of clear lacquer or even Future to restore the shine and clarity, but I rather not have to do that.  I expected the UV-cured resin to fully harden (even on the open surface).

Can any of the Bondic (or other UV-cured resins) users here try to see if yours also remains sticky on the open side after curing?  Take a piece of aluminum foil and spread a layer of your UV-cured resin (about a size of a Dime) on the foil, then cure it with UV-light.  Then touch the surface.  Is is tacky, and your finger leaves an impression in it?

[ChrisBcritter]

It's happened to me; I got a stronger 10-watt LED UV light to cure it and let it go for several minutes, and it helped. Part of the issue may be the light's NM wavelength rating; 365nm is supposed to be best for the Alumilite AlumiUV resin I have. Higher values like 395-405nm supposedly don't cure as well.

Alumilite recommends a 300 watt 365nm light for the best and fastest cure (about 3-5 seconds); there's one on eBay for $86 I may be picking up soon:

https://www.ebay.com/itm/300W-365NM-395NM-405NM-UV-Curing-Light-UV-Light-Invisible-Inks-Fluorescence-Lamp/254536481014?ssPageName=STRK%3AMEBIDX%3AIT&var=554235880878&_trksid=p2060353.m1438.l2649

I'd like to be able to cure taillight lenses much faster as when you dye the resin red, the longer it sits under the UV light, the more it fades the dye back to clear! 

 

[Curt]

I'd be curious to see (just for grins) if exposure to direct sunlight for some period of time would complete the cure.

[Foxer]

I've used Bondic as a casting material for some time and the sticky surface is typical I always spray a dust coat of DullCoat rid it of that. It also maintains the clear shine.

 

Edited July 15, 2020 by Foxer

[peteski]

So what I'm seeing is not out of the ordinary.  I'll try Sun exposure first (it has a wide range of UV wavelengths).  I hope I can get it to cure on the surface too - otherwise it is not very useful to me. If Sun cures it completely, I'll try another source of UV light.

[mad mike]

This has been described as a general problem with UV curing plastics. The oxygen in air inhibits curing of the surface, one way to avoid it is curing the parts in a UV permeable fluid like glycerine. Putting the parts in direct sunlight for a prolonged time should help too.

[peteski]

11 hours ago, mad mike said:

This has been described as a general problem with UV curing plastics. The oxygen in air inhibits curing of the surface, one way to avoid it is curing the parts in a UV permeable fluid like glycerine. Putting the parts in direct sunlight for a prolonged time should help too.

Interesting information.  That gives me an idea.  I have an aerosol can of "gas blanket" inert gas used to prolong life of uncured urethane resins.  It is heavier than air so it should displace oxygen.  I'll try to put my uncured UV resin item in a small container filled with that gas, and then try to cure it with UV light.

[mad mike]

Good idea! Please tell us if it worked.

[peteski]

I tried using sunlight to harden the resin and it went better than I hoped for.  I put a puddle of the resin about a size of a quarter on a piece of aluminum foil and left it out on a hot sunny day.  I forgot about it, so it was exposed for few hours.  Not only the resin fully hardened on the surface (with a glass-smooth finish), it also set much harder than when using the UV LED.  It is is quite stiff and cant be dented or gouged with a fingernail (like the UV-LED-cured resin).  It is probably harder than polystyrene.

I'll have to see if the gas blanket works too, but now that I know that sun sets the resin hard, I think I'll be using it (or find another source of UV light, more compatible with the resin than the UV LED that comes with Bondic, or the UV flashlight I found.  The 365nm light source sounds like it might work as well as sunlight.

Edited July 19, 2020 by peteski

[keyser]

Price is right too. I had this issue and figured I'd screwed up. Lots of good ideas thanks all.

[ChrisBcritter]

The 300-watt UV light arrived today. Put it to the test with AlumiUV on a headlight lens mold and, while it wasn't the 3-5 seconds Alumilite said, in one minute it was fully cured. Next I molded a set of '63 Plymouth Fury turn signals and got the same result; it was a clear rubber mold so I bet that helped. Later I'll try tinting the stuff red and see if I can get some good '65 Belvedere lenses (since Moebius didn't make them that way), and I'll see how Bondic reacts as well.

[ChrisBcritter]

Update: Looks like the taillight experiment worked! Practiced on a few smaller lenses in the clear mold; 1:15 on top, then turned it over for another :30 and they cured with little to no dye fade (if anyone needs a set of '41 Plymouth or '52 Chevy clear red lenses, first PM gets 'em).

[TarheelRick]

On 7/19/2020 at 12:40 PM, peteski said:

I tried using sunlight to harden the resin and it went better than I hoped for.  I put a puddle of the resin about a size of a quarter on a piece of aluminum foil and left it out on a hot sunny day.  I forgot about it, so it was exposed for few hours.  Not only the resin fully hardened on the surface (with a glass-smooth finish), it also set much harder than when using the UV LED.  It is is quite stiff and cant be dented or gouged with a fingernail (like the UV-LED-cured resin).  It is probably harder than polystyrene.

I'll have to see if the gas blanket works too, but now that I know that sun sets the resin hard, I think I'll be using it (or find another source of UV light, more compatible with the resin than the UV LED that comes with Bondic, or the UV flashlight I found.  The 365nm light source sounds like it might work as well as sunlight.

The only issue I see with this process is what will the direct sunlight and its associated heat do to a piece of styrene, i.e. a grille with Bondic clear headlight lenses?

[gman]

11 hours ago, ChrisBcritter said:

Update: Looks like the taillight experiment worked! Practiced on a few smaller lenses in the clear mold; 1:15 on top, then turned it over for another :30 and they cured with little to no dye fade (if anyone needs a set of '41 Plymouth or '52 Chevy clear red lenses, first PM gets 'em).

You should do a tutorial of the process with photos- it sounds interesting!

[peteski]

14 hours ago, TarheelRick said:

The only issue I see with this process is what will the direct sunlight and its associated heat do to a piece of styrene, i.e. a grille with Bondic clear headlight lenses?

MY test was some resin on a piece of aluminum foil. I also left it out for quite some time.  So, I don't have an answer for you.

Just guessing, if Sun completely sets the resin in less than a minute, there should be no danger to the styrene.  Plus grills are usually metalized ("chromed"), so they should reflect Sunlight (including the heat, or IR radiation), leaving the plastic cool.

[Phirewriter]

This is helpful information, I purchased some Bondic and have yet to use it for anything. What are you tinting or coloring it with? I'd assume it's a resin specific dye. My plan was to make lenses for headlights and taillights.

[ChrisBcritter]

I'm using the transparent red dye Alumilite makes for their two-part clear resin. It doesn't take very much - a small drop will tint 1/4 teaspoon of resin very dark. They also make an orange dye you can use for turn signals. 

Edited August 16, 2020 by ChrisBcritter

[peteski]

Smooth-On also has a line of resin dyes and tints.  That's what I use.  Then of course one can paint transparent colors over a clear-cast lens (like we often do when building model cars which include clear directional and taillight lenses).