Degradation Mechanisms of TPB CoatingsBen Jones, MITTPB Workshop, October 2012

MicroBooNE Optical System

Cryogenic photomultiplier tube (PMT) assemblies

LBNE Optical System

The Next Step What is the mechanism for this

degradation?

Can the effect be controlled and / or prevented?

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.

Plot shows diffusion speed through the GC column

Each bin then has a mass spec taken.

WHITE TPB

TPB Mass Spec, measured

TPB Mass Spec, NIST

Lots of subtleties to getting down to ppmConcentration measurements

Credit to Jennifer VanGemert (Thompson), who worked very, very hard on this

WHITE TPB

Benzophenone

Unidentifiable, degradation dependent, vary between solvents

PPO (first column only)

YELLOW TPB

Powder BP Concentrations

TPB Benzophenone

BenzophenoneOne of the only sensible oxidation products of TPB

Wikipedia says:

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

benzophenone

Also supported by NMR data

NMR also has hints of other impurities which we are still disentangling (not appearing in this paper)

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

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

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…

Impact of Added BP on Yellowing

TPB + benzophenone TPB only

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

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

20% BCNo BC

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!

BACKUP SLIDES

20% BCNo BC

water

TMS

Scint grade 1H NMR

Yellow TPB

benzophenone

Compare NMR

Connectivities

Overlaid Average Absorption Spectra

Overlaid 1H Spectra

Yellowing in DHCL3 Solution

Scint YellowStandardWhite Standard

Yellowing in DHCL3 Solution

Scint YellowStandardWhite Standard

Yellow herring?