Degradation Mechanisms of TPB Coatings
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Degradation Mechanisms of TPB CoatingsBen Jones, MITTPB Workshop, October 2012
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MicroBooNE Optical System
Cryogenic photomultiplier tube (PMT) assemblies
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LBNE Optical System
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The Next Step What is the mechanism for this
degradation?
Can the effect be controlled and / or prevented?
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GCMS Studies Gas chromatography to separate ions of
different mobility, followed by mass spectrometry
In all cases, TPB peak is by far the dominant GC peak
Impurities are visible in the GCMS at ppm levels.
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Plot shows diffusion speed through the GC column
Each bin then has a mass spec taken.
WHITE TPB
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TPB Mass Spec, measured
TPB Mass Spec, NIST
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Lots of subtleties to getting down to ppmConcentration measurements
Credit to Jennifer VanGemert (Thompson), who worked very, very hard on this
WHITE TPB
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Benzophenone
Unidentifiable, degradation dependent, vary between solvents
PPO (first column only)
YELLOW TPB
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Powder BP Concentrations
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TPB Benzophenone
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BenzophenoneOne of the only sensible oxidation products of TPB
Wikipedia says:
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A 30ppm solution apparently has a 100% absorbance to UV below 300nm
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benzophenone
Also supported by NMR data
NMR also has hints of other impurities which we are still disentangling (not appearing in this paper)
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Controlled Study: Produce ~30 similar plates Measure WLS performance of each at t0 Degrade under a UV lamp (300nm) After some exposure time, measure WLS
performance again Dissolve off coating with toluene bath with
sonnicator Quantify benzophenone using prescribed GCMS
sequence (takes ~2 hours) Fresh solvent run between each sample to
minimize cross contamination
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Log scale here – initial ~exponential rise suggests radical mediated reaction
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Yellowing? Sadly, BP not yellow.
Somewhat questionable whether BP is powerful enough to block this much UV and kill the TPB on its own at these concentrations (our spiking studies are inconclusive on this so far)
Many BP derivatives are excellent UV blockers - much, much better than BP itself. Many are also yellow.
Both of these very yellow and very UV blocking…
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Impact of Added BP on Yellowing
TPB + benzophenone TPB only
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Hypothesis Photo-initiated radial mediated reaction turns
TPB into benzophenone
Second stage reaction turns benzophenone into various derivatives.
Some of them are yellow (shown – spiking study)
Some of them probably highly UV blocking
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Terminating Radical Reactions Can we show more conclusively that this is a radical
reaction? And can we stop it?
Radical mediated decomposition is common in UV polymer degradation.
Chain terminators sometimes added to end the radical chain reaction prematurely and protect polymers.
We tried two common radical eaters: Butylhydroxytoluene (BHT) 4-tert Butylcatechol (BC)
If they slow down this reaction, confirmation that we see a radical mediated photo-oxidation.
BHTBC
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20% BCNo BC
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Conclusions We have identified an impurity, benzophenone, which is a
known UV blocker and photo-initiator, which builds up with UV exposure in TPB
BP seems to be intimately related to yellowing and performance loss. This makes sense - we know many BP derivatives are yellow and great UV blockers
Rate of buildup indicates radical mediated photo-oxidation, this was confirmed by stabilizer tests
We can improve coatings by 20% and significantly stabilize them with 4-tert butylcatechol.
Investigations of other radical removing stabilizers are ongoing – maybe we can find an even better one!
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BACKUP SLIDES
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20% BCNo BC
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water
TMS
Scint grade 1H NMR
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Yellow TPB
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benzophenone
Compare NMR
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Connectivities
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Overlaid Average Absorption Spectra
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Overlaid 1H Spectra
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Yellowing in DHCL3 Solution
Scint YellowStandardWhite Standard
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Yellowing in DHCL3 Solution
Scint YellowStandardWhite Standard
Yellow herring?