I need some small/cheap waterproof junction boxes for some 12v garden lighting. I am considering using little (25mm x 25mm x 15mm) potting boxes and after soldering the wires, then filling the potting box with hot melt glue.

However I am not sure how good hot melt glue is at protecting against water? I understand glue sticks are typically a thermoplastic - so should be waterproof?

The best information I can find in a datasheet is "Water Resistance: Good": http://www.farnell.com/datasheets/1504158.pdf

Thanks!

up vote 4 down vote accepted

Potting compounds are a science.

If you are just protecting some spliced wires, then hot glue may work if you do a good job. The worst situation is if the junction box is completely filled with water. If you do a complete job around each splice, then you might succeed. If there are holes, however, water will get in and be trapped.

If there is a PCB in there, then be careful. A hard and rigid potting compound will expand and contract with temperature, and often the thermal coefficients between the compound and the PCBA are different. The result is parts being snapped off the PCB.

The softest potting compound for a PCB is silicon. It is gentle on the components and does very well at temperature extremes. However, silicon doesn't adhere very well and will pull away from the surfaces it is stuck to, which will invite water ingress.

Typical potting compounds nowadays are urethane-based. They are a balance between the adherence ("stickiness") of epoxies, but has a bit of softness to be nice to the PCB components.

  • Thanks for your advice, I am not using a PCB, just splicing wires. One issue I wasn't expecting, the large volume of hot melt glue caused the potting box to deform. However I guess that means it is more likely to adhere well. – njh Nov 5 '16 at 19:20

The "glue" plastic itself is pretty much watertight – the question is whether water will be able to creep along the interfaces of glue and cable etc, and then corrode/shorten things.

In my experience, hot glue sticks terribly bad to rubber cables, so it's not really an option if you use those.

  • Probably using silicon sealant instead of hot glue would be an option to overcome this. Don't know if it will have bad side effects, though. – dim Oct 27 '16 at 20:58
  • 3
    Regular silicone (the type that smells like vinegar as it cures) is horrible as a potting compound. The vinegar smell is caused by acetic acid being evolved, and this causes severe corrosion to the potted electronics. What is needed (unless you go with @Smith answer, is electronic silicone - which is very expensive. – WhatRoughBeast Oct 27 '16 at 21:15
  • @WhatRoughBeast Indeed, you're totally right. It has even been covered in this question. – dim Oct 28 '16 at 9:35
  • Thanks for the warning about rubber cables and silicone sealant. – njh Nov 5 '16 at 19:17

I have used direct-burial low-voltage splice kits consisting of a plastic tube filled with some kind of grease. They were closed on one end like a chemistry test tube. The other end had some kind of snap closure that held the wires from pulling out. I saw an ad for one brand that advertised that silicone grease was the filling. These should be available where outdoor lighting or automatic lawn watering components are sold. The wire connection was done with a twist-on connector that fit into the tube. The grease remains soft.

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