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[ATM] composite mirrors and epoxy handling hazards
The risks associated with developing hyper-sensitivity to epoxies
can be minimized through the religious use of vinyl or latex
gloves (Some are allergic to Latex). That will minimize direct
contact with the skin. Some who were less than careful, developed
a hypersensitivity that was so severe that years later, they only
needed to be within a few feet of a composite part and they would
break out in a severe rash.
One man I know, from Orange New Jersey, got it so bad that his
hands looked like he had leprosy! Once so affected, the hyper-
reaction stays with the individual for the rest of their lives,
or at least so I've been told.
Chemistry has changed significantly in the years that have past
since those occurrences and those who anticipate developing
composite parts can relax some (some! Don't get stupid like I
did).
There exists a brand of epoxy marketed under the name of "Safety-
Epoxy" that is popular amongst aircraft builders and whose
development did not compromise the structural or bonding
properties. I've not heard of any recent cases of hyper-
sensitivity to such systems in a long time but that does not mean
one can get reckless with it.
In the 30 years I have been involved in the use composites, the
only time I was at serious risk was when I ignored the warnings
associated with the use of Vinylester based system and
intermittently breathed the fumes of it's catalizer for two days.
The regular epoxy I was so familiar with could not be used
because of part specs and client demands. Since the epoxy I
normally use is relatively benign, I foolishly used the same
safety methods I normally depended on. That was a stupid thing to
do and it could have killed me.
The vinylester system I used had an MEKP based catilyzer. It was
my first long-term exposure to it and I must have been asleep or
on stupid pills. It was immediately apparent from the smell that
the stuff had a bad kick to it yet I continued. (God takes care
of children, drunks and apparently some stupid fabricators too!)
Three days AFTER I ceased working with the material, my breath
STILL smelled of MEKP, a sure sign that I had inhaled substantial
quantities! That I hadn't killed myself inhaling those fumes can
only be interpreted as blind luck or Divine Intervention.
More recently, I had no choice but to use a clear casting resin
that uses MEKP. Now I knew better so I purposefully cast the
parts (colored lenses) on a Friday afternoon, knowing the
building would be empty for the weekend; surely enough time for
the fumes to be exhausted by the ventilation system.
Wrong! On Monday afternoon we started experiencing headaches even
though the smell of the MEKP was no longer apparent!
Unfortunately, MEKP based hardeners are still the most popular
amongst part time users and ATM's. It is the only type available
in cans from places like Home Depot, Loews, etc. You can identify
them right away: the hardener comes in a small tube, usually
clear and it's contents look like water. Read the label and
you'll likely see that the content is MEKP. The P stands for
Peroxide. One drop of that in your eyes and you'll be walking
with a white cane for the rest of your life! One only uses a few
drops of the stuff to harden the part, that's how reactive it is.
On the other hand, safety epoxies and related but equally benign
adhesives can often be recognized by the fact that they are two
part systems in which the mixing ratios are much lower: One part
Part B (hardener) to four, five or six parts (by weight) of Part
A. The viscosity of BOTH is also much higher.
Their use requires that a proportional measuring device be built
(a simple balance scale) but that's an easy, one evening project.
There are epoxy pumps that can be had whose ratios can be
adjusted but it's difficult justifying the expense of such a
dispenser for a one time project as simple as a telescope. I've
built several very large projects and mixed hundreds of gallons
of epoxy (less than one cup at a time to prevent exotherm) using
such a balance scale and have had no regrets. It also helps
prevent excessive use of epoxy and needlessly heavy parts.
There are barrier creams that can be had from composite suppliers
like Fiber Glast, Inc (http://www.fibreglast.com/) I urge you to
browse that web site, if for no other reason than then education
it provides some of which is safety oriented.
The most convenient skin barrier for those handling epoxy is
ordinary soap. Squirt some on your hands with just a little water
and continue to lather both hands, covering all surface as far up
as your short sleeve, until it dries completely and forms a
"skin".
If you have a bar of regular soap, drag your finger nails through
the bar so as to drive the soap up and under the nails. This will
help prevent epoxy from getting under the nails and speed up
clean up dramatically. These are both old soap tricks taught to
me by an 80 year old retired house painter who used to compete
with the neighborhood kids to help me in my shop. Just one of the
amazing tricks I've learned from such people.
Any paste wax that contains Carnauba is desirable as a mold
release. Avoid any spray release that contains silicone as it is
very hard to remove and can frustrate your efforts to paint a
part later. Aside from contamination by oil, the "best" fisheyes
in freshly applied paint are caused by silicone.
If I were gluing two piecees of glass together, for whatever
purpose, I'd use a vacuum bagging process as it is the only means
I know of whereby clamping pressure can be safely assumed to be
evenly distributed throughout the part. In this case 14.7 PSIG if
you are at sea level.
Any mechanical clamping is sure to cause unequal distribution of
pressures during setup, and if those forces are maintained after
the epoxy cures, it is destined to negatively affect the mirror
later. I'd also suggest that you not get to frenetic when mixing
epoxy as that generates air bubbles which can become trapped in a
woven matrix or between the glass parts.
I wish I could offer you a composite sandwich and resin system
with a zero C of E, or failing that, a C of E equal to Pyrex. I
am still looking but I've yet to uncover one.
Only once have I ever had a composite part stick to the mold
aggressively. I was forced (I thought) to build yet a new mold
and attempt the part again, only this time with a better mold
release.
One day, with nothing to be risked but a few ounces of acetone, I
submerged the married parts in a shallow tub of acetone, covered
it tightly (acetone evaporates like crazy!) and forgot about it.
A week later, I saw the tub, got curious and when I opened it,
found that the resin system had all but dissolved and the glass
plies could be lifted out like dish rags from a sink. I see no
reason why two glass pieces can't be separated without harm to
either one, provided one is patient.
Testing the cure: from time to time, a poorly catalized mix will
occur. Perhaps the room or the chemistry was too cold or the
mixture not properly stirred. Whatever the cause, I think its a
worthwhile idea to pour a thin portion of unused resin or epoxy
into a small plastic or cardboard cup. The ones they use at
McDonalds or Burger King for dispensing catsup are perfect
provided that the hardener isn't one that uses MEKP. It dissolves
plastic and styrofoam on contact!
When you think the epoxy has cured , take an ice pick, a scribe
or some other equally sharp object and scratch the sample you
saved. You should hear a distinct scratching sound and there
should be powdered epoxy dust generated. If instead the sharp
object simply gauges out soft mounds of epoxy on either side of
the channel, the part is not cured. You can try heating it
carefully. Sometimes that will save a bad part, but not always.
One last note about clean up: acetone is often the prescribed
chemistry for cleaning parts that have been used in a composite
building session. It devolves resin quickly. However, because
it's so effective at cleanup, some are tempted to use it to clean
their skin. DON'T!!. The skin absorbs acetone at a frightfully
rate and brings any disolved chemistry right along with it.
Working with resin systems is safe and fun, provided one
exercises sound judgment and a little care when choosing and
using chemistry. The results can be so remarkable that you'll
wonder why anyone would consider a cardboard tube!
Clear Skies!
Art Bianconi
-----------------------------
From: "Peter C Chen" <peterchenmd@comcast.net>
Subject: Re: [ATM] To: "Michael Lindner" <mikell@optonline.net>
Mr. Lindner:
The type of mold release depends on the specifc epoxy
system used, and everyone I know has his/her own favorite
formula. I have heard of hair spray, WD-40, candle wax, mineral
oil, KY Jelly, etc., being used. If you use the Star Technology
ER2109 epoxy sytstem http://www.star-technology.com/table1.html
that I suggested in my Stellafane talk, the appropriate mold
release is Johnson's Furniture Paste Wax. The idea is not
original. I owe it to Donn Starkey who owns the company and is
also an avid atm. I have also used Freeman wax with some
success.
http://www.freemansupply.com/FreemanWaxRelease.htm
Be sure you have done some experiments and have gained
some experience on disposable molds before you do the serious
casting.
Also, be aware that a new mold will have a tendency to stick. Two
pieces of 16" glass blanks epoxied together, as once has happened
to me, is not a pretty sight. (Reminds me of a memorable episode
in Jerome K. Jerome's book
'Three Men in a Boat'. Does anyone still read that?). One
solution to the
problem is to condition the mold with polyvinyl alcohol (PVA).
The
procedure is described in
http://winshipmodels.tripod.com/pva_mold_release.htm
For vacuum bagging procedure, see
http://composite.about.com/library/weekly/aa000109.htm
Finally, the key to getting a good optical
figure, as
you have astutely observed, is to have an exactly even coating
of mold
release. This can be done by careful polishing and watching the
interference
fringes between the glass and the wax. It is a tricky business
which I
cannot claim to have completely mastered. On the other hand, I
am sure
there are many atms who are true craftsmen (craftsperson?
craftspeople?
craftshumankind?) for whom it will be a snap.
Have fun.
P. C. Chen
----- Original Message -----
From: "Michael Lindner" <mikell@optonline.net>
To: "Peter C Chen" <peterchenmd@comcast.net>; <atm@atmlist.net>
Sent: Friday, September 24, 2004 10:04 PM
Subject: Re: [ATM] composite mirrors and epoxy handling hazards
> Mr. Chen,
>
> Thanks for sharing your expertise with our list. I wish I had
been at
> Stellafane this year to hear your talk. Regarding mold
releases, can you
> recommend any which you have successfully used to make a
replica mirror? I
> would think that's the most difficult part, as an uneven
release layer
would
> destroy the figure.
>
> Clear skies.
> --
> Michael Lindner
> http://www.starastronomy.org *** http://home.att.net/~mikel
> http://www.atmsite.org *** http://www.atmlist.net
>
------------------------------
Message: 19
Date: Sat, 25 Sep 2004 14:43:43 -0400
From: "Ellen Mackenzie" <mackenzieconsulting@sympatico.ca>
Subject: [ATM] re: Peter Chen
To: "atm" <atm@atmlist.net>
Message-ID: <003801c4a32f$983f9270$2002a8c0@mackenzio2mm5r>
Content-Type: text/plain; charset="us-ascii"
I think kudos are needed in appreciation to Dr Chen for his
sharing so much
of this valuable information with the ATM community, there are
others too
whose contribution to the list do not go unrecognized. I am in
great
appreciation of those who have endeavoured to keep this list
alive and
current. I had seen a great decline in amateur interest in
astronomy partly
the result of the efforts of NASA. I do however recognize those
like myself
who drifted away to family responsibilities coming back with the
advent of
new approaches and the old desire seething to be satisfied.
Thank you Peter
------------------------------
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