Polyurethane hardness

[2626716]

Dear sirs,
We are trying to develope a polyurethane foam for potting electronic printed circuit boards.But i am not getting proper hardness for the material.I would like to know the chemical used to increase the hardness of polyurethane foam.
Your prompt attention to this matter would be appreciated.
Thanks & Regards
Mehaboob.P.K

 

[edrush]

I am not an expert on this subject, but I will tell you what little I know (or think I know). There are many ways to change the hardness of the base urethane. Some urethanes have plasticizer for softening a harder version. Another way is to modify the chain constituents. And modifying the crosslink density should also do the trick (for urethanes this may require modifiying the chain structure).

Since you have a foam, reducing the amount of air in the foam will increase the density and hardness. Most foams are created by adding very small amounts of water. The water alters the stochiometry of the mixture and must be accounted for in the mix. So there are a variety of ways to change the hardness.

Foams can be tricky to process. I assume you have an automated foam dispenser. I suggest you get some tech help from the dispenser manufacturer. I don't think it is possible to succeed with foam mixing manually.

 

[Rogerio1]

Your question is not an easy one. Polyurethane is the denomination for a whole class of polymers - typical foam formulations have 10 or more individual compounds. There isn't a "chemical to increase the hardness" of every foam formulation without knowing its composition, you have to adapt the whole system. This usually involves switching polyols, cross-linkers or isocyanates in your mixture.

But if you are looking for a quick fix then try each of the following:
1- Increase the amount of Isocyanate in the mixing ratio (start with 10%)
2- Add 2-8 % Monoethylene Glycol to your polyol component and adjust the amount of Isocyanate correspondingly
3- Reduce the amount of water, as edrush suggested

If you're in Europe you might want to check out

http://www.rampf-resins.com,

they do a lot of potting of electronic parts with rigid foams.

Good Luck!
Giacaglia

 

[BLN]

We used polyurethane potting on a couple of our electronic modules, and I'm curious as to why you are using a foam? If we have any voiding, moisture makes its way to the board and shorts it out.

 

[Rogerio1]

Hello BLN,

It is all a matter of weight and costs saving: with foams you use less polyurethane resin and the parts weigh less.

The foam is usually formed in place, not die cut, so it has a hydrophobic compact integral skin on all sides. Furthermore, if the foam is mostly composed of closed cells and if the cell walls are thick enough, there won't be any relevant water absorption.

There are however drawbacks to using a foam, specially regarding electrical properties.

Regards
Giacaglia