Inhibited polyester

[Quetzalsailor]

I usually use epoxy in my boat repair efforts but for economy and 'plenty good enough' I use polyester. I have made through-hull backup blocks and similar small layups. I normally use Ikea shopping bags for mould release and cast against glass for flatness, bag on top, plywood and ballast. Gives me great glass/resin ratio. The Ikea bags are/were not polyethylene. Last time, I used 6 mil polyethylene.

I laid up cradle components for my Flying Dutchman using the boat as the plug. A couple strips of carpet pad, and some polyethylene laundry bag for release, and polyethylene tarp for protecting the boat hull between the strips.

In all cases, I get a good, rock hard cure. But the resin spilled on or against the polyethylene does not cure; it remains sticky.

Does polyethylene inhibit the cure? It seems pretty obvious that the polyethylene absorbs something of the resin as it swells and ripples at the contact places.

[Quetzalsailor]

I asked my question elsewhere and got the following useful reply:

Standard orthopthalic polyester laminating resin (often referred to as
GP for general purpose) is "air-inhibited." Polyethylene is inert to
the resin and has no chemical nor physical affect on cure. Of course
polythene can be contaminated.

Drips etc are exposed to air, and have a high surface area to volume
ratio and therefore do not achieve proper exothermic temperatures, as
well as having a non-cured air-inhibited surface layer.

Air inhibition is a function of the chemistry used in these resins, and
the chemistry of the air.

"Finishing resin" has a styrene-soluble wax additive that precipitates
to the surface during the crosslinking stage, and this wax seals the air
interface--therefore allowing full cure right to the surface. The
drawback is that the wax must be removed and the surface roughened if
any further bonding is to be done.

We often refer to the MEKP as "catalyst" but that is not actually how it
works. Most resins are pre-promoted, inhibited resins. The MEKP is an
oxidant that runs around eating up the inhibitor, which allows the
promoter to drive the chain reaction polymerisation forward. This is
why epoxy has a longer shelf-life than polyester. The latter, if
pre-promoted, will go off all by itself in due course. Even without
promoter, it will polymerize, just more slowly. Polyester resin is
already pretty close to being a solid when you buy it in the can. The
styrene monomer reduces the average molecular weight to an acceptable
level, and it will polymerize with the unsaturated polyester that is in
solution. (This should be obvious by the ratios of "catalyst" to resin.
How could such a little bit of something do anything but start something
off?) On the other hand, Epoxy is a step-wise polymerisaation that
takes place in situ when you mix a and b together.

Polyester is promoted with a cobalt or other heavy metal soap. I forget
what the inhibitor is. I also forget the specifics of the reaction at
the open air surface that leads to air inhibition. Oh well.



>Then how come the majority of the surface of the layup, that part well
>away from the PE, does not feel sticky?
>
>
Sticky is a relative term.

If it has thickness, it will have good cure through the thickness and
only a thin layer of "sticky."

If you mix a batch of resin in a cup and let it go off, the meniscus
will be sticky for a long time compared to the surface, which if hot
enough, won't even be sticky at all.

[westsail42]

One advantage of the inhibited resins is that you can control working time by controlling the amount of MEKP used to "catalyze" the resin. With epoxy, you cannot adjust the amount of catalyst, to do so would risk an incomplete cure. The best you can do is use cataysts specially formulated for slow or fast cure.