# Epoxy vs polyester for thermal conductivity.



## picclock (Jul 10, 2011)

I'm hoping someone here is an expert on adhesives and their properties. I'm fixing some fins to a heat exchanger core using an adhesive. I've made a test rig and have found that the best type of resin for thermal conductivity appears to be a polyester based one, Plastic Padding chemical metal. 

In my tests I've found that its thermal conductivity is 3-4 times as good as various epoxy types. I have a couple of issues with it though. Firstly, I don't place a great deal of trust in my test apparatus, although it does seem to produce consistent results. Secondly, the cure time is too fast for me to fill a slot and insert an aluminium fin. 

Is it likely that polyester resins have a higher thermal conductivity than epoxy ones ?

If I try to reduce the hardener/resin ratio this should reduce the setting time. Is this a marginal road to go down ? I know I can cool the parts and adhesive to try to slow things down but I don't want condensation rearing it's head.

Does anyone know of a slower setting polyester resin that I could use. I live in the UK.

Free karma point to anyone who can read the above without falling asleep .. .

Many thanks

picclock


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## Stan (Jul 10, 2011)

It is a lot of years since I worked with these products, but do a search for 'heat sink epoxy'. There are many on the market that are either electrically conductive or electrically insulating and all have high thermal conductivity. They are used for mounting high power electronic devices where optimum heat dissipation is required. You should be able to find one that meets all your requirements.


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## Tin Falcon (Jul 10, 2011)

First of all I am no expert and do not have and engineers license. 
I have had a bit of experience and have done some reading on polyester resins. putty etc. I worked in a boat yard for 6 years r &D and had some training and experience with epoxy based composite repairs in the USAF. 
Somewhere around here I have a book on making and blending polyester resin compounds. so will do my best to answer. 



> Is it likely that polyester resins have a higher thermal conductivity than epoxy ones ?


I would research published info to give an answer I do not know. 



> If I try to reduce the hardener/resin ratio this should reduce the setting time. Is this a marginal road to go down ?



This is an acceptable way to control cure time with polyesters within reason. 



> I know I can cool the parts and adhesive to try to slow things down but I don't want condensation rearing it's head.


this is the only way to increase pot time with epoxy. 

there are lots of trade trick to control the properties of polyesters. I will try to hit some basics here 

polyester putty contains 
1)Resin
  a) layup resin
   b) casing resin
   c) styrene monomer
2) filler just about anything can be used here
  a) fumed silica 
   b) ground walnut shells 
   c) talc
   d gravel 
3) hardener.

 you should have plenty of cure time . a putty with aluminum powder should increase thermal conductivity. 
Tin


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## Jasonb (Jul 10, 2011)

I think the reason the Chemical Metal conducts the heat well is that it uses iron powder as a filler. 

So in theory you could use another resin with the required setting time and add iron powder to obtain your conductivity.

Jason


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## MachineTom (Jul 10, 2011)

DevCon epoxy is iron powder mixed in filer. Directions caution against thin film application. .005" as I recall.


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## Noitoen (Jul 10, 2011)

One thing I've heard about epoxy curing is that it should be cured with a temperature little higher than the working temperature.


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## RonGinger (Jul 10, 2011)

Do not alter the mix ratio for epoxy. Proper cure depends on enough of each material- if the mix is wrong the mess will never cure completely. The only way to alter rate of cure is with temperature- colder slows the cur.

Polyester uses a catalyst to cause the cure, so its OK to alter the amount a bit. Again, temperature makes the biggest difference in cur rate.

I know in boat building with wood polyester is a MUCH poorer material. It is a lousy glue. I just today launched my latest boat, after a couple months work and 3 gallons of epoxy.


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## picclock (Jul 11, 2011)

@RonGinger
I'm aware of the mix issues with epoxy and polyester. Fortunately, for this application the strength required is minimal. 

I've done some further research on the internet and the difference in thermal conductivity for these resin types seems to be dominated by the filler material. Boron Nitride seems to be the best filler, yielding some of the highest thermal conductivity values. 

I have come across a UK manufacturer, Robnor, which produces thermally conductive epoxies (thanks Stan) for encapsulation of electronic assemblies. So I think I'll go with their PX439N/GY resin. It only appears to be available as a twinpack in small volumes, but as it takes a long time to cure I'm hoping to finish the assembly in one manic hit ;D.

Many Thanks 

picclock


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## Ken I (Jul 11, 2011)

I think there is little difference - other than determined by the filler powder used.

Try adding aluminium powder 50:50 to the epoxy - or as much as you can and maintain workability - I do this all the time with polyurethanes.

I once worked in an RF electronics environment where we used a silver filled conductive epoxy - but it cost an arm and a leg as I recall.


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