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Associate

Join Date: Dec 2007
Location: Penang, Malaysia.
Posts: 48

CTE of the epoxy

02/28/2008 9:34 PM

I got one question, I already did CTE measurement and the results show the changes of the specimen length (dL) value is negative. It is possible the samples shrink during the test operation or it might be caused by the rheology of the epoxy itself.

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Active Contributor

Join Date: Jul 2007
Posts: 11
#1

Re: CTE of the epoxy

02/29/2008 8:36 AM

Your specimen may not have been fully cured before you tested it. Further cross linking will cause shrinkage. Try post curing your specimens before testing.

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Commentator

Join Date: Feb 2008
Location: Somewhere south of Canada
Posts: 68
Good Answers: 1
#2
In reply to #1

Re: CTE of the epoxy

02/29/2008 11:35 AM

Exactly! Usual ASTM time for post-curing plastics is >48 hours. Good show, thixotrope, and good science. There are a few negative CTE resins, but if you get something screwy it is just good practice to double check your experiment.

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Guru

Join Date: Dec 2006
Location: Germany 49° 26' N, 7° 46' O
Posts: 1950
Good Answers: 109
#3

Re: CTE of the epoxy

03/01/2008 4:35 AM

Hi,

this problem is much more complicated!

As stated by others: curing may not be complete.

And: mixing may be not good enough causing uncured and incurable regions to slowly evaporate part of the mix.

And: mixing may be slightly deviating from ideal ratio: resulting in the same problem of evaporation and bleeding.

And: volatile content to be removed by post-cure is not totally removed (else the epoxi will show sign of beginning damage)

And: water content is in an equilibrium (longterm) with your environment or lab, slowly diffusing out or in depending on outside and inside conditions. Rising water content will expand the specimen.

This problem caused some delay in the Hubble Space Telescope as the fabrication of the bars that fix the distance between primary and secondary mirror/camera had to be corrected. At fabrication the carbon-fiber-epoxi contained water as usual, at measurement a different amount in use all diffusing out! So an outgassing procedure (mostly water going out but some other too) was designed and a shielding of the outgassed bars to prevent water going in again.

There are a lot of data compiled by NASA and ESA "Outgassing data for spacecraft materials" that show how different the amount of volatile material can be.

So coming back to your problem:

I would do the test with different post-curing time/temperature data, with very thorough mixing of very precise ratio of the two epoxi parts,

if this is still not ok then I would deviate slightly from the recommended mixing ratio to see if this has an influence.

I would measure the weight loss at post curing as an indicator of material diffusing out and I would measure in a controlled humidity after minimum 24hours of accommodation in this atmosphere (if the strip is 1 mm thick).

RHABE

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Anonymous Poster
#4

Re: CTE of the epoxy

03/18/2008 7:30 AM

hye all..thanks for the comment.. a lot of appreciate..and that comment is usefull for me..thanks again for your help

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Anonymous Poster (1); Ben Bonsens (1); RHABE (1); thixotrope (1)

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