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<DIV>Thanks, Keith. I, like most of us, was not able to make my own epoxy formulations and wanted to find the best product. Even now though Gray has shown me how its done, I am not an expert by any means. Therefore, a few years ago I bought EZLam, Anchor Bond, and West epoxies and did experiments. There are other brands out there but these are the ones I had heard mentioned in various modeling forums as "the best". By far the West epoxy was the most predictable, very hard, and flexible system that is widely available (West marine is a national chain). Even with this, the mix ratios are probably still jacked with using chemical mixtures. But the results are very good and being able to use the same resin with different hardeners is much easier. Most pattern plane applications require very small amounts to be mixed at a time. On my triple beam balance I can mix up 4 grams properly so a gallon of resin and a quart of hardener lasts for years.</DIV>
<DIV>--Lance</DIV>
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<DIV class=signature id=signature>--<BR>District 6 AVP <BR>www.aeroslave.com</DIV>
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<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>
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<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>
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<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>
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<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>
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<DIV><FONT face=Arial size=2>Keith Black</FONT> </DIV>
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<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>
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<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>
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<DIV>Ive always been curious how sensitive the ratio is to get the optimal properties.</DIV>
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<DIV>Thanks for the information.</DIV>
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<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 c
oat, the solvent can evaporate before the epoxy cures. <BR><BR><BR><BR>Gray Fowler<BR>Principal Chemical Engineer<BR>Composites Engineering</FONT></BLOCKQUOTE></BLOCKQUOTE></BLOCKQUOTE></body></html>