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Old 24th August 2011, 02:06 AM   #277
Ivan Kminek
Join Date: Jun 2011
Posts: 906
Burning of Laclede primer paint imitation and other epoxide samples under air – still macroscopic approach

I found yesterday (see post #256) that chip of the cured epoxy resin “Pattex Repair Universal Epoxy” filled with 20 wt% of aluminum oxide can be ignited very easily and burns with a bright flame for some seconds.

Then, I decided to prepare some imitation of Laclede primer paint used for the corrosion protection of WTC1 and WTC2 floor joists. Such imitation can serve not only for simple macroscopic burning tests, but also for: a) measurements of its TGA, DSC properties; b) microscopic study of its appearance during/after heating/burning; c) as a sample of typical epoxy resin suitable for the determination of a typical carbon, nitrogen and hydrogen content in epoxides by elemental analysis. Other uses of this paint imitation might be found later.
All these experiments could serve as a supporting info for the hypothesis that chips (a) to (d) in the paper of Harrit et al. were particles of Laclede primer.

In this contribution, my intention was not only to prepare such “Laclede imitation”, but also to compare its (macroscopically observed) burning with burning of previously prepared epoxy resins filled with aluminum oxide (“Al2O3 resin”) and with the pristine (non-filled, “pristine resin”) epoxy as well.

Declared composition of Laclede primer paint was (NCSTAR1-6b report of NIST, Appendix B):

Pigment: (21 % of total weight of dry/cured paint)
Iron oxide 55 % (wt%)
Aluminum Silicate 41 %
Strontium Chromate 4 %
Total Pigment 100 %

Unmodified Epoxy Amine 45 %
Deionized Water and Amine 55 %
Total Vehicle 100 %

I gathered powdered iron oxide and a form of aluminosilicate called Nanoclay. Since I was not able to get strontium chromate, I decided to replace it with potassium chromate. Although there was not very much of chromate in the Laclede paint, the paint imitation should also contain some chromate, since chromates are generally strong oxidizing agents and can influence thermal/oxidative/burning behavior of the original paint as well as of its imitation.

Preparation of “Laclede imitation” layer:
Used chemicals
- Epoxy: Pattex Repair Universal Epoxy (Henkel, 5 min hardening time), 2.45 g
- Iron oxide, particle size between 1 and 3 μm; dark red-brownish powder (Lachema), 0.55 g
- Nanoclay, an aluminosilicate (hydrophilic bentonite) in the very fine platelet form (Sigma Aldrich), whitish powder, 0.41 g.
- Potassium chromate, yellow powder (Lachema), 0.04 g (molar masses of strontium chromate and potassium chromate are similar – 203.6 vs 194.2, so this amount of potassium chromate is adequate).

The sum of component weights in this epoxy composite is 2.45 + 0.55 + 0.41 + 0.04 g = 3.45 g and its composition should be in a good accordance with the composition of the real Laclede paint. The wt. ratio between epoxy resin and inorganic fillers is 2.45 : 1.

You can see the collection of starting chemicals on this Fig. 1: .

Potassium chromate was first thoroughly grinded in a grinding mortar in order to achieve fine particles of this chemical.

Then, all chemicals were thoroughly mixed with a spatula (as usual in the preparation of epoxy adhesives/sealants) and the resulting viscous dark red matter was casted by spatula on polyethylene substrate. After curing (1 h) the tough (but flexible) dark red layer of the “Laclede imitation” was stripped from the substrate.

Preparation of “Al203 resin” layer
It was described in the post #256, but new layer (thicker) was prepared for better comparison here.

Preparation of “pristine resin” layer
It was prepared like in other cases but no filler was added.

All layers were about 0.5-0.8 mm thick.

Macroscopic burning experiments:
The epoxy “macrochips” (irregular shapes, ca 6-8x10-12 mm), were placed on the microscopic glass slides and ignited with an ordinary lighter. Some typical burning experiments were filmed and links to videos are given in the next section.
After burning, chips were again filmed and video is available (see later).

Results and discussion:
You can see typical burning of epoxy chips under study here: (pristine resin); (Al2O3 resin); (Laclede imitation).

I can judge from this:
All epoxy samples are easily ignited with an ordinary lighter and burn with the bright flame for ca 30 s to 90 s.
The most flammable is “pristine resin” – the flame was the brightest and lasted for the longest time.
“Laclede imitation” burns better, longer and with the brighter flame than “Al203 resin”. This could be caused (at least partially) by the presence of chromate as oxidizing agent. Also iron oxide might play some role (?).

Chips after burning can be seen here:
From the left to right:
“Pristine resin”: molten dark matter as a result, almost whole chip was burned.
“Al2O3 resin” some minor part was burned, this part was dark and brittle.
“Laclede imitation”: about half of the chip was burned, dark brittle matter resulted.

Generally, fillers in epoxy resins seem to “quench” the burning to some extent, but even both filled resins ignite easily and burn with the bright flame.
In the case of Laclede imitation, it can serve as some hint that the real Laclede primer paint can be easily oxidized, e.g., during DSC measurements under air (as performed by Harrit et al.). This oxidation can be a source of exotherms observed by Harrit et al. for chips (a) to (d).
Further measurements on Laclede imitation (TGA, DSC, some microscopy on burned/heated resin, elemental analysis) will follow soon (I hope).

Thank you for your kind attention)

Last edited by Ivan Kminek; 24th August 2011 at 03:25 AM.
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