Transparent resin to fill small acrylic plastic container?

Question

I have a small clear acrylic plastic container, roughly spherical and about 1.5 cubic inches in volume, with a 1/4-inch wall and thinning to about 2mm at the top edge (near the 1-inch diameter lip). I want to put some objects in it and fill it with clear resin and a small amount of tinting (glow-in-the-dark powder) so that the objects and glow can be seen through the clear container. What resin or other substance (silicone, etc.) can I use for this?

Some concerns, in rough order of importance:

1) Shrinkage. I want the resin to entirely fill the container once it has cured, with no gaps. If it shrinks and pulls away from the edges this would be bad.

2) Crazing or other chemical reactions with acrylic. I have heard that polyester resin (or maybe just the hardening catalyst?) reacts with acrylic plastic to cause crazing (tiny cracks) on the acrylic surface. This would be bad in my application. I am not sure if epoxy or other resins have this problem.

3) Ultraviolet transparency. Ideally I would like the resin to be completely transparent to ultraviolet, although some translucency (browning?) may be OK. (The glow-in-the-dark powder needs to charge.)

4) Transparency. I would like it to be as clear as practical. It is going to become somewhat translucent when I add the glow-in-the dark powder, so I would prefer to at least start fairly clear.

I would appreciate suggestions as to a resin or process that could satisfy these criteria. Thanks!

Answer 1

I’m answering my own question since I seem to have found a solution (and nobody else has replied). If anyone has other suggestions I still encourage them to add additional answers.

My first attempt was to use JB Weld brand ClearWeld two-part epoxy. This didn’t go well: As the epoxy cured it shrank, just a tiny amount, but enough to pull away from the acrylic plastic that surrounded it in most places, leading to a thin partially visible mirrorlike bubble boundary between the two plastics. Not what I wanted.

What actually did work was MG Chemicals 832WC Water Clear Epoxy Potting compound. This is designed for encasing electronic components, and did not appear to shrink at all. It does not adhere strongly to the acrylic (I can scrape it off with my fingernail in places where I had drips), and it is a pain to work with due to the long curing cycle (72 hours at room temperature, or one hour under 176-degree temperature). It is also fairly expensive (about $40 for 375 mL). These caveats aside, it does seem to have done the job, and its index of refraction is such that there is no visible boundary between the acrylic plastic and the cured epoxy: the entire project looks like a single piece of clear plastic, which is what I wanted.

Since I did not want to wait 72 hours, and had no oven that I wanted to use for epoxy, I wrapped my project in watertight plastic wrap (basically a Ziploc bag) and heated a pot of water on the stove to 176 degrees (checking temperature periodically with an infrared thermometer), and submersed the wrapped project in the water for an hour, using clamps to hold it in place.

Answer 2

If you need to do this again, you might want to look into "bar top epoxy". This is usually clear and is a 2 part epoxy. Some are polyester resins, but not all. Some of these are expected to be applied in thin layers, such as 1/4", instead of a cylinder or bowl, so dry/cure times may differ. However, some are expected to be applied at over an inch in depth at one shot, so you'd just need to read the directions before purchase. Also, no telling if there's shrinkage at these thicknesses.

Some of these specifically say they are UV resistant. I'm not sure if that means it block it or lets it pass through. This could mean some testing.

I used a fair amount of this for some furniture I've made. I purchased from Menards, but the below link is the same stuff.

https://www.amazon.com/Environmental-Technology-32-Ounce-Pour-Finish/dp/B000BZYYQ0/

Loctite seems to make a wide variety of clear epoxies, although maybe not in the quantities you need.

You might have some luck asking at a local hardware or craft store. Not all employees will be able to answer your questions, but they might be able to help find someone who can, or at least point you in the direction of the epoxies they do have so you can look on the box or bottles.

I'm sorry I don't have all your answers, but hopefully I at least provided some alternative suggestions to the expensive potting epoxy.

Edit:

I've just recently gotten into UV resins for 3D printing and many of them are clear. I'm wondering if you can use that to fill your mold, using a high intensity UV light to cure the resin. Depending on the brand, UV resin isn't exactly cheap, but it's not horribly expensive either. I haven't tried to cure something this thick, though.

UV lights that match the wavelength of UV resins are easy to come by on online stores. I have 3x lights that are 60 watts each from Amazon, and I've seen up to 100 watts, maybe more, for not too expensive.

Another thought is to 3D print the mold with the UV resin, then fill it with more liquid resin. This would make it so that everything could shrink or expand at the same rate, so it wouldn't be an issue.

I think these two ideas are something I need to try.