Ron Magen wrote:
Jay,
Let me just point out something . . .
Because I want to do something - NOT 'boatbuilding critical' - with a small
MASS of epoxy, I contacted the 'repackager' I use{Larry at RAKA Epoxy}. He
confirmed, & refined, my thinking. Due to the chemistry involved, as little
as 5 PERCENT 'hardener' will cause properly mixed epoxy to cure. That 'cure
time' will simply increase {probably geometrically or exponentially}.
Bearing in mind that the stuff is 'time & temperature dependent' - in this
instance lets agree to an approx 70 degree constant.
The other point is that the 'higher' the ratio { I favor the 5 to 1,
resin-to-hardener, formulation for most of my use}the HARDER {and more
brittle} the cure. Everything being relative, if you are NOT making a
'block' of the stuff, it shouldn't make any difference in 'our' type of
projects. What 'additives' & 'fillers' we use has more of an effect. Talc
vs. 'Agricultural Limestone', for instance.
What Larry and I were discussing was using a 2 to 1 'formula' at 3 to 1. The
idea being to slow down the exothermic reaction. It would do exactly that,
the cure time would lengthen very much, and the finished 'slug' would be
'less brittle, and a bit less hard' - EXACTLY what I wanted !! {In a
'boatbuilding scenarios that would equate to 'more flexible' - again a 'good
thing'.}
Regards & Good Luck,
Ron Magen
Backyard Boatshop
------------------------------------------
NO NO NO! ALL WRONG!
With epoxy resins the mix ratio is exactly the amount of part a needed
to react with part b. Shift that ratio and you have excess part a or
part b remaining. Note that one can usually be off about 15-20% and
things will still cure up OK. With polyester resins the part b is a
catalyst not a reactant so you can adjust the part b to control the
reaction. Some epoxies blush more than others, that should a real
concern to everyone - especially if they use excess part B.
The mix ratio tells you nothing about the properties of the epoxy - it
just is the way the chemistry worked out between the resin and the
curing agent(s) used.
Some epoxies are more brittle than others. Flexing agents can be added,
also.
Unless the vendor is a simple repackager - the vendor/formulator can
'create' the epoxy properties they want (hardness, brittleness, potlife
etc. - I'm currently working on a very thin epoxy with long potlife for
vacuum bagging). The 'creation' process also affects epoxy paints more
than the 'marine resins' which are typically just less complex recipes.
Some epoxies also offer slow (warm weather) curing agents but you can
control pot life and cure by temperature and mass. The chemical
reaction, like all chemical reactions, doubles or halves with every 18
degree F change in temp. The amount of product mixed and the shape of
the container also control the 'heat' and potlife (like critical mass in
a nuclear reaction).
I find it interesting that none of the other epoxy vendors/formulators
bother to monitor/lurk/respond to any of the boating newsgroups.
paul oman
progressive epoxy polymers, inc.
www.epoxyproducts.com/marine.html
"Jay Chan" wrote in message
oups.com...
I have decided to totally remove the epoxy that had taken many weeks to
cure.
I thought the reason why the epoxy took so long to cure had to do not
spending enough time to mix it with hardener. Then I caught myself
adding only half of the required hardener into the mixture when I was
working on a different area of the deck. The manufacturer's 2-to-1
ratio is only applicable if I measure the ingredients by volume. When
I am using the pump, I should stick to the "one-pump-each" ratio. I
might have pumped two times for the resin, and pumped one time for the
hardener; and this resulted with very slow curing.
I have done some limited tests on various hardeners and various way of
mixing them. And I come to this conclusion regarding the use of MAS
brand of epoxy and hardener:
1. Spent half of the required mixing time will result with slightly
slower
curing time than normal if I use _slow_ hardener. But the
difference
is small.
2. Spent half of the required mixing time will not increase the curing
time if I use _medium_ hardener.
3. Added half of the required hardener will result with very long
curing
time if I use _slow_ hardener. And the "cured" epoxy is not
strong.
4. Added half of the required hardener will result with long curing
time
if I use _medium_ hardener. And the "cured" epoxy doesn't
seem to be strong (not sure).
My observations from the test result a
o Because I used slow hardener when I was preparing the area of the
deck that cured very slowly, I am sure that I must have added too
little hardener (that matches the test case # 3 above). This
means
all the works that I have done on the deck so far is questionable.
I should remove them and start over.
o Manufacturer's suggested mixing time must have added enough
margin of error in case people doesn't spend enough time mixing.
That doesn't mean I should cut back on mixing time though.
o MAS brand of slow hardener seems to be less forgiving than
their medium hardener. I think I should stick to medium
hardener from now on as much as possible.
Jim Conlin said:
Unless you have done some tensile tests on your epoxy, and
have found that its strength equals the manufacturer's published
figures, your statement that it has cured "completely" is
unfounded. Further, it is the height of irresponsibility to suggest
that others listening here might rely on the approach you've taken.
In my view, you've rolled the dice and whether your boat holds
together remains to be seen. Please don't suggest that others
take the same risk. Lives could be at stake.
Thanks for pointing this out. That got me thinking along that
direction, and found out what went wrong.
Jay Chan