Degradation Mechanisms of TPB Coatings

1. Degradation Mechanisms of TPB Coatings Ben Jones, MIT TPB Workshop, October 2012

2. MicroBooNEOptical System Cryogenic photomultiplier tube (PMT) assemblies

3. LBNE Optical System

7. The Next Step • What is the mechanism for this degradation? • Can the effect be controlled and / or prevented?

8. 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.

9. WHITE TPB Plot shows diffusion speed through the GC column Each bin then has a mass spec taken.

10. TPB Mass Spec, NIST TPB Mass Spec, measured

11. WHITE TPB Lots of subtleties to getting down to ppm Concentration measurements Credit to Jennifer VanGemert (Thompson), who worked very, very hard on this

12. YELLOW TPB Unidentifiable, degradation dependent, vary between solvents PPO (first column only) Benzophenone

13. Powder BP Concentrations

14. TPB Benzophenone

15. Benzophenone One of the only sensible oxidation products of TPB Wikipedia says:

16. A 30ppm solution apparently has a 100% absorbance to UV below 300nm

17. Also supported by NMR data benzophenone NMR also has hints of other impurities which we are still disentangling (not appearing in this paper)

18. 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

21. Log scale here – initial ~exponential rise suggests radical mediated reaction

22. 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…

23. Impact of Added BP on Yellowing TPB + benzophenone TPB only

24. 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

25. 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. BC BHT

27. 20% BC No BC

28. 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!

29. BACKUP SLIDES

30. 20% BC No BC

31. Scint grade 1H NMR TMS water

32. Yellow TPB

33. Compare NMR benzophenone

34. Connectivities

41. Overlaid Average Absorption Spectra

42. Overlaid 1H Spectra

43. Yellowing in DHCL3 Solution White Standard YellowStandard Scint

44. Yellowing in DHCL3 Solution Yellow herring? White Standard YellowStandard Scint