Effects of heat during cure and elongation?

[George77577]

All things equal with a BPO/DMA cured vinyl ester laminate (.150" thick)60" diameter cylinder with a HDT of 220F:

1. Speed of reaction and ultimate exotherm produced will effect degree of initial cure but also can induce MORE stress into the laminate than a slower cure due to higher shrinkage?

2. Post cure ramped up to 260F - 280F within 60 minutes.
Could this level of heat beyond the HDT create micro cracking and/or reduce the elongation of the resin to the point that could later cause "in service" cracking? In other words, embrittlement?

If so, the advantages of a high elongation resin could be negated by too aggressive post cure and the result might be similar to what you would expect from a more rigid resin system.

 

[Demon3]

Your Tg and therefore your HDT is defined by the cure temperature. As you cure, reaction proceeds until Tg hits Tcure. At that point the resin is no longer mobile enough for significant further cure.

Too high a temperature will induce chemical degradation and embrittlement.

The built in stress part of the question is a little more tricky to explain but is understood. A great treatment can be found in my friend Jakob Lange's PhD thesis from KTH in Stockholm.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Demon3]

P.S. What I wrote about Tg, HDT and cure temperature applies to condensation polymers like epoxies. It may not apply to polymers made by free radical polymerization because they need less mobility to form polymer chains.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[George77577]

I think you meant that your cure temperature is defined by your Tg and HDT?

The problem highlights issues that can arise when publishing generalized data on a highly focused subject such as ultimate cure and there is not enough consideration for the possible deleterious effects.

We performed a simple experiment today and it was quite clear that heat above the HDT creates significant embrittlement/cracking with only slight bending. A similar sample subjected to heat below the HDT would not crack even when bent in half but rather snapped at the breaking point.

 

[Demon3]

No, I mean that the Tg depends on the cure temperature. This is well established. If you take an epoxy resin and heat it, the Tg progressively increases because the cross-link density increases. Eventually the Tg reaches Tcure and then cure essentially stops because the polymer is now immobile and can't react further. After all, that is the definition of Tg, the temperature at which the chains become immobile.

If you then heat to a new temperature, higher than the original cure temperature, then cure will resume until Tg reaches the new Tcure.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry