Post Curing Epoxy

I'm finishing off an existing composite hull; installing bulkheads, deck, cabin, etc. I am building in an unheated barn in Northern California. Electrical power is via portable generators so any post curing would need to be done with propane or similar.

I came across the following statements on an epoxy manufacturer's website and would appreciate comments from knowledgeable forum members...

Do I need to post-cure a room temperature cure resin?

Room temperature cure resin is the biggest misnomer in epoxies. A thermoset resin is only capable of achieving a Tg of about 30oF above the cure temperature. Therefore, unless the ultimate Tg is within the 100oF range, a post-cure will always be required to complete the cure and increase resin strength. The higher the service temperature, the greater the need for a post-cure, assuming the molecular structure is capable of a Tg greater than the use temperature.

What is Tg and is it the same as HDT?

Glass transition is the temperature a polymer turns from a glassy like plastic to a non-structural rubber. On a molecular level, it is the temperature where the main polymer backbone initiates molecular motion. The Tg is controlled by cross-link density and modulus properties of the structure between cross-links (aromatic, aliphatic, heterocyclic, etc.). All polymers have a Tg and can best be determined using Dynamic Mechanical Analysis (DMA) curves. Tg is an intrinsic bulk polymer property totally determined by molecular structure. HDT is an arbitrary value where a set deflection is achieved under a constant load. As the temperature is raised, the stiffness slowly drops and the specimen deforms until it reaches a pre-set deflection where the temperature is reported. The HDT is always lower (approx. 10oF) that Tg due to the applied load.

I understand that post-cure enhances the strength of the finished laminate, but do satisfactory "room temp cure" epoxies exist, and if so, who makes them?

Regards,

SeaJay

 

post cure

Most epoxy will have a post cure and should be baked in a oven but............ the only person it will bother is you , because Skater has been building boats out of epoxy for years ( shell resin ) and has never used an oven and still does not use one today . i guess the million dollar question is can you live with a little print thru ......... Hell yes ...... no big deal .

 

First lets get rid of the incorrect assumption that the "resin" makes a difference in curing times. It is the hardener!

To the first question, no, you dont need to postcure. If the temperature stays at real "room temp" the matrix will finally cure thoroughly. But raising the temp is never wrong.
As a rule of thumb: the longer the pot life of a resin / hardener combination, the higher the probability of having a hardener that needs postcuring to achieve the final strength. In this case postcuring also adds to temp resistance noticeable. In general these combinations make a stronger matrix than the room temp ones!

All major suppliers provide different hardeners for their formulations so, its your choice.


Regards
Richard

 

Epoxies and polyesters do benefit from post curing and it will typically increase the physical properties of both types of resin. Is it needed for all applications... it must not be, because for the most part these resins are used in ambient cure processes.

For what you're doing there is no need for it to be post cured, the epoxy is far stronger than the surrounding polyester laminate even without it.

 

Thanks for the responses above. They are pretty much in line with my knowledge of the subject, but the statement threw me a bit.

Regards to all,

SeaJay

 

Another way at looking at this is if you can gain 20 -25% increase in strength of your resin even with a low temperature 50 - 60'C post cure then why would you not do it?
This temperature can be achieved in a simple plastic tent and a gas heater.
The starting point in making this decision is to know the mechanical properties for your resin system at ambient cure and various post cure temperatures.
There are a lot of epoxy systems that will not have the properties of PE or VE unless they are post cured, so you need to ask for the data.

Cheers
Andrew

 

You can also hang some heat lamps over the part for post cure. I do this after initial cure, but while still in the bag and under pressure.

 

i'm adding dark blue pigment to the epoxy and exposing to the australian sun for a day. should give 8 hours at 80 degrees.

the amount of strength increase depends on the epoxy chemistry, and there are thousands out there. some chemistries will give near 100% increase.

 

Sam Devlin - Plywood and Epoxy boatbuilder, recommends curing the epoxy, not only for strength, but also to avoid future problems like 'print through',
he likes to leave epoxy for over a month before painting. (and leaving the painted boat in shelter for a month before exposing it to UV)

One of his observations is that Epoxies that use more hardener (say 50-50 ratio instead of 1 in 5) are more 'flexible', and therefore more prone to print-thru.

Bad news sea-jay - epoxying in an unheated barn increases the chances of 'print through' dramatically.

 

Post curing always improves some of the more 'obscure' cured-state properties like elongation and fracture toughness; the post-cured resin is less brittle, even while it is stronger and harder.

The 1:1 mix epoxies typically use a polyamide rather than an amine curing agent. The low AHEW (amine hydrogen equivalent weight, a measure of 'potency') of polyamides accounts for the high catalyst /resin ratio. You can tell if your curing agent is a polyamide as it will be fairly thick (amines are always thin), and faintly smelly, like feet or cheese.


Polyamides always yield cured resin that is somewhat flexible, even a bit rubbery, with only modest strength, barely 'stronger' than cured polyester resin, but with outstanding fracture toughness and elongation (often >100%) and also superlative adhesion, even to oily metal or wet concrete or wet wood. This makes the polyamides a great choice for adhesives.

Curing agents typically cannot be mixed with reactive modifiers as the modifier itself is an epoxidized compound albeit one of very low reactivity and extremely poor cured state properties if cured alone by combining with a curing agent. The modifier is always blended with the resin side of the equation lest the shelf life be measured in mere hours


Jimbo

 

Is there a time limit for when the post curing is still effective?

For example: Can i post cure the boat 1-2 years after I built it???

 

This is a very good question and the answer is mostly 'yes'.

Most room temp curing agents do not have a post cure initiation time limit, but then a few do. Most of the elevated temperature curing agents are time sensitive to post curing, but most of the time this is a non-issue since the full cure will be obtained while the thing is cooking the first time, though there are sometimes processes where this won't work and the part sill has to be post cured.

Most likely the post cure (you're really just driving the system to a higher state of cross-link or cure) can be initiated anytime after initial cure given the most common curing agents encountered. If you must know for sure, seek out the manufacturer's data (MSD sheets are best since they will tell you the actual components) on the resin system in question. Then go to the data sheets for the individual components of the curing agent(s) and see what they have to say. Between the Hexion and Air Products and chemical sites, you can get the story on just about any curing agent made.

Jimbo

 

Epoxy and polyester are thermosetting resins. it needs heat to cure. Heat comes the chemical reaction it generates.

If you look closely on the material data sheet, There is a property "maximum exothermic temperature".

Because we laminate layer by layer, the heat produced tends to be absorved by the mold (in the case of the first layer) or tend to be absorved by the atmoshere. By proportion, there is a large surface area in relation to the thickness of the laminate. Heat dissipate quickly.

If you try to laminate several layers at a time where the laminate gets uncomfortably hot or even smoke, yopu are exceeding the exothermic temperature.

To answer your question, as published in a technical magazine, temperature cured resin will continue to cure for a month untl the polimerization (for polymer) is complete after which the resin is assumed to be completely cured and nothing can be done about it.

As Apex has stated, slow curing resin will be weak. This is because theheat generated is not enough and can be made worse by atmospheric absorption of heat.

Polymer is also Hygroscopic. It absorbs moisture in the air reducing its capability to cure properly. LR requires to stop lamination when the humidity reaches a certain %.

Rx