Post Curing of Epoxy & Polyester Resins

[Robin Colbourne]

At several places I've worked, incorrect or omitted post-curing has been a source of problems with epoxy bonds.

Post-curing is the warming of the joint in an unstressed condition for a period of time a higher temperature than the initial setting temperature to increase the cross-linking in the resin, increasing its strength and resistance to softening in a warm or hot enviromnment.

The strange thing is that from what I've seen modellers almost never do post-curing, so I just wondered if anyone here does do post-curing and if not, why not?

[Biggles' Elder Brother - Moderator]

We probably don't do it because a lot of us don't know about it - including me!

When you say "higher temperature" how high are we talking about and how would we do it? We don't have ovens for this sort of thing! For example could we do this by wafting a hot air gun around the joint? And for how long?

BEB

[Former Member]

[This posting has been removed]

[Dickw]

I think this is a typical example of post curing, and probably explains why I have never done it with any of my mouldings - I don't have an oven large enough :-

"Cold/Hot Curing
These resins integrate at room temperature so that components can be easily removed and processed. In "normal" applications the strength provided is often sufficient. However, further strength can be attained with post-cure heat treatment. Normally a temperature of 50 - 60 °C is sufficient for approximately 10 hours."

Dick

Edited By Dickw on 04/08/2018 12:43:00

[alex nicol]

sounds great, any ideas where I can get an oven big enough for wing joining ;o)

[Piers Bowlan]

You could ask Boeing if you could borrow their autoclave!

Edited By Piers Bowlan on 04/08/2018 17:13:58

[Martin Dilly 1]

Those of us involved in competition free-flight who build our own aircraft usually post cure epoxy laminates, or I certainly do. Building an oven isn't difficult and there was a paper by Martin Gregorie and Chris Edge on that in the 2001 BMFA Free Flight Forum Report. Details here: http://freeflight.bmfa.org/forum-reports-contents. A copy will set you back £7.00 and there's a lot more stuff there that you'll probably find interesting, specially if you've only flown RC so far. Basically you need an insulated box with a heat source (a 100 watt bulb works), a fan to get the heat moving, and a thermostat. Mine is lined with that aluminium foil lined foam used in the building trade.

While I'm no polymer chemist, I think the general idea is to heat the laminate to a considerably higher temperature than it's ever likely to meet in use.

[Levanter]

Robin mentions Epoxy and Polyester resins which have very different compositions and curing mechanisms. Without going into the chemistry epoxies use curing agents based on amines and polyesters use catalysts usually based on peroxides.

Like many things, the answer depends on a number of factors and there are a huge range of specifications available depending on the application.

Taking epoxies first, some are designed to cure at room temperature and gain little benefit from post curing. Once the resin mix has passed from the liquid stage, heating will reduce the cure times but if over-done could be at the cost of strength. Other epoxies have to be heated to cure in the first place such as "pre-pregs" and these are often stored in refrigerators to maintain then in a usable condition for building up laminates. These are mainly used in vacuum processes. There are other epoxies that are used conventionally, cured conventionally and then post-cured to achieve their design properties. Depending on the formulation then, heating requirement could range from 0% to 100%.
In modelling terms I think most of our laminating resins will benefit from post cure but I have not seen specifications. Martin has the right idea in that if you post cure at a higher temperature than you would expect in use then you will not get any distortion that sometimes associates with post curing. Worst case you put your model out in the sun and the wings start twisting so 60 degrees would be a reasonable temperature (could get hotter in a car however)

Polyesters are generally self curing once the accelerator (normally a cobalt compound and normally pre-mixed) and the catalyst are added. If the mixes, temperatures an cure times are correct theoretically you should get a 95% to 98% cure. In the real world however the cure will be variable and post curing is sometimes done expressly to stop further curing if the moulding is exposed to heat in service. This is less to do with increasing strength but more to do with avoiding distortion and shrinkage that can look horrible. Over catalysing polyester will make a very rapid cure but have a serious effect on the mechanical properties. Catalysing is typically done between 1 and 1.5% by volume so small amounts of polyester require tiny amounts of catalyst. It is very easy to over catalyse and a common problem.

Both processes are exothermic with the cure producing heat. If too much resin is applied in one go or a slower cure is not selected the exotherm can destroy resin and extreme cases can catch fire.

Epoxies can be subject to post cure as high as 200 degrees Celsius. These sort of temperatures would make polyester very unhappy.

If strength is critical then product selection is important and then following the instructions to the letter. As a general principle, the higher the specification the less tolerant it is to application errors.

Levanter

[alex nicol]

I appreciate all the technical detail and get the fact if you cook it, it becomes stronger meaning you can use less epoxy to get the same stregnth and its lighter..........my question to the floor is how many general sports fliers out there have ever had a good wood to wood uncooked epoxy join on fire walls or wing joints fail.?

That said the only failures I've seen (and none of them mine I hasten to add) have been down to poor workmanship, badly fitting wing braces, re glued fuel soaked firewalls etc

[onetenor]

If you register an interest with companies like 3M or sites like Master Bond they will send you info on new products and provide on request tech details on application and use. Really useful to know. They also have question sections where technical details like post curing etc can be sorted.

Re the statement that epoxies "go runny when heated" Yes they do while curing but not once solid or low heat is used. In fact Araldite instructions recommended in an oven to hasten the cure Another heat source is for cold weather use only. Radiators but care as water temperatures bout 80 degrees.I used Araldite original ensure an airtight seal on the broken off crankcase nose on an Allbon Javelin which I alloy welded back together..I then smeared the epoxy all round the repair. Still going strong since '58.

Apologies for missing words as I get blue patches round some letters or words sometimes whole messages which cause them to disappear if I try to correct the word etc. Anyone know the cure for this? It seems to be worst on this site. Another annoyance is the panel scrolling or jumping up.

[Denis Watkins]

It is your dictionary OT, trying to add new words to your prescriptive text.

Your machine likes to learn your frequently used words to help your typing ! ! !

It is a pain sometimes,

Maybe better to switch it off, in preferences or settings

[Manish Chandrayan]

When making repairs on cowls or other glass bits, once I have applied the epoxy on the glass laid down, I do generally use a good waft from my heat gun so that the epoxy gets more runny and self levels better. After about two hours I give it another shot from the heat gun. This accelerates the curing. For all such repairs I use Araldite standard cure that requires 24 hours curing

Edited By Manish Chandrayan on 05/08/2018 08:56:09