Property of Polyester Resin

[scrimshaw]

My question concerns that very useful property that unwaxed polyester resin has of not curing in air so you can get a good chemical bond. Is it just a coincidence of it's chemical composition or is it engineered in?

If it is manufactured that way on purpose why can't it be made to cure in air? (without adding wax to it)

Thanks for any information.

 

[Compositepro]

Air inhibition of the cure of polyester resin is not designed-in but wax is used to design it out. Polyester resin is a free-radical cure polymer. The peroxide catalyst in it breaks down into free-radicals and causes the resin to react and harden. Oxygen reacts very rapidly with fre-radical which then prevents them from hardening the resin.

 

[berkshire]

scrimshaw (Mechanical)
The air inhibition is an unfortunate side effect of the peroxide curing mechanism.
The discovery that wax could be added to the uncured resin and would separate during the cure and seal the surface, was a step forward in improving the handling qualities of the resin.
B.E.

 

[scrimshaw]

Thanks for the replies
Berkshire -- I noticed you called it an 'unfortunate side effect'. I would of thought it was an advantage. Having seen the effects of delamination many times and I am sure you have too, don't you think the better chemical adhesion is an advantage with polyester.

(Of course I do not know for sure what caused the delam. but I suspect it was the poor mechanical bond of polyester and/or difficulty in cleaning wax off.)

Any thoughts.

 

[berkshire]

scrimshaw (Mechanical)
If you are working in a controlled environment it can be an advantage. In that the uncured resin will cure when sealed over with new resin with active peroxide in it. It is often used on large part layups to allow an exothermic reaction to subside before adding more resin and reinforcements.
The basis of my remark comes from old history of working on fiberglass boats in the 70s (Bass Boats). They were typically built from orthophalic resin, air inhibited cure applied with a chopper gun. Secondary bonding was very often done weeks after the initial layup by wiping the area to be bonded with a little acetone on a rag and gluing in the part.
Testing on bond lines later revealed this to be hit or miss at best. We evolved a policy, that if the part was cured more than one week you sanded the bond line after rinsing it off with acetone, before gluing on a part.
Relying on a mechanical bond more so than a chemical one.
B.E.

 

[Oceania]

The use of waxed resins has very particular problems and from my experience of filament winding pipes does make a difference when laying up fittings etc. My preference is not to construct the main laminate with wax added, but to finish off the last layer with wax. This ensures that air inhibition does not take place, but reduces somewhat the volitiles, but also means that if you need to bond to the part, you know that you can remove the waxed resin, and the underlying laminate is going to be sound. Some ancedotal evidence would suggest providing the working environment is warm and dry and some element of post curing ie radiant heaters are used then AI is not a problem, as I have also heard that by opening the workshop door and letting an icy blast of winter air in can give significant AI problems.

 

[scrimshaw]

What temps do you consider to be 'an icy blast'?