<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=iso-8859-1">
<META content="MSHTML 6.00.2800.1458" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>
<DIV><FONT face=Arial size=2>Brian, I look forward to Gray's answer, but I can
share my experience with West Systems epoxy.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>West Systems epoxy is not a 1:1 mixture like many
of the hobby epoxies. As I understand it the 1:1 epoxies have extra fillers
added to the hardeners to provide the 1:1 ratio to allow for easier to mixing.
West Systems uses five parts resin to one part hardener. When I first started
using the West epoxy I tried measuring the volume either in a small mixing cup
or sometimes counting drops (when using very small amounts). This yielded very
unreliable results, sometimes it hardened, sometimes I just made rubber. Then I
broke down and bought a triple beam balance that measures to the 10th of a gram
so I can mix by weight. Using this method I'm able to mix very small quantities
and it always hardens properly.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>My "guess" is that the hobby epoxies add fillers
such that the volume of the hardener is equal to the volume of the
resin.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>BTW, for those using the epoxies from the hobby
shop, I highly recommend trying the West Systems epoxy. This stuff is MUCH
cheaper by volume and seems to be a much higher quality epoxy. You purchase one
resin and different hardeners for different drying times. Also, they have a
larger variety of fillers for various purposes, I personally like the Colloidal
Silica (406) which will thicken up the epoxy to keep it from running, but unlike
micro balloons it is extremely strong. (This is not my own discovery, Lance Van
Nostrand <FONT size=1></FONT>sold me on the West Systems epoxy, thanks
Lance!)</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Keith Black</FONT> </DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=brianyemail-nsrca@yahoo.com
href="mailto:brianyemail-nsrca@yahoo.com">brianyemail-nsrca@yahoo.com</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=discussion@nsrca.org
href="mailto:discussion@nsrca.org">discussion@nsrca.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Tuesday, January 18, 2005 8:58
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Re: epoxy joint</DIV>
<DIV><BR></DIV>
<DIV>Gray, since it is epoxy school time. I was curious what is the best way
to measure parts a and b for the epoxy mix. My method is to squeeze out a
couple of lines of equal length on top of my mixing pad. I
usually only have a tablespoon leftover of one or the other parts when I
run out.</DIV>
<DIV> </DIV>
<DIV>Ive tried my gram scale as well, but its not sensitive enough to measure
the small amounts Im mixing. So then I wonder is it best to mix by weight or
volume or does it matter. </DIV>
<DIV> </DIV>
<DIV>Ive always been curious how sensitive the ratio is to get the optimal
properties.</DIV>
<DIV> </DIV>
<DIV>Thanks for the information.</DIV>
<DIV> </DIV>
<DIV>Brian<BR><BR><B><I>Gray E Fowler <gfowler@raytheon.com></I></B>
wrote:</DIV>
<BLOCKQUOTE class=replbq
style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px solid"><BR><FONT
face="Courier New" size=2>"Gray<BR>What are the negatives to thinning with
alcohol. Suppose you add fiber filler the epoxy gets thicker.Suppose you
thin to a just a little to work well.Have you hurt the joints strength.I do
it and it never has failed. This way I use much less.<BR>Lotta talk here
about epoxy and none about thinning.<BR><BR>Jim Ivey</FONT><FONT
face=sans-serif size=2>"</FONT> <BR><BR><BR><FONT face=sans-serif
size=2>Jim</FONT> <BR><BR><FONT face=sans-serif size=2>So many
negatives......where do I start.</FONT> <BR><FONT face=sans-serif
size=2>Epoxy does not "dry" it cures. The epoxy resin reacts with a hardener
which for our applications is usually a primary amine. The hardener must be
added in a stoichiometric amount. Any excess hardener means it does not
react in as all the epoxy reaction sites are used up. Too little hardener
and then you have unreacted epoxy. Both of these situations cause a loss of
properties. The most dramatic will be a reduction in the glass transistion
temperature (Tg) which is the temperature when the epoxy softens and
no longer transfers stress because it is too soft. Considering that the room
temp epoxies that we use have a max Tg of about 128F, a bad mix ratio could
have a Tg of 100F which is a temperature that many people fly at in the
summer.</FONT> <BR><FONT face=sans-serif size=2>Adding solvents means you
just cured the epoxy with trapped unreacted stuff, which will do the same
thing as a bad mix ratio. Alcohol can really be bad, if it has water in it.
You have not had any failures because once again, it is still strong
enough for our applications. Trapped solvent will reduce the Tg and
rubberize the epoxy, which will change over time as the solvent slowly
escapes, but the epoxy will never reach full properties.</FONT> <BR><FONT
face=sans-serif size=2>Using a "thinned" epoxy as a coating such as on a
wing is different because in the very thin coat, the solvent can evaporate
before the epoxy cures. <BR><BR><BR><BR>Gray Fowler<BR>Principal
Chemical Engineer<BR>Composites
Engineering</FONT></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>