Degradation
Mechanisms of
TPB Coatings
Ben Jones, MIT
TPB Workshop, October 2012
MicroBooNE
Optical System
Cryogenic photomultiplier
tube (PMT) assemblies
LBNE Optical
System
Degrada on of Standard and Scin lla on Grade TPB Plates
1.1
Rela ve WLS Performance
1
0.9
Scin lla on Grade Plate
0.8
0.7
Reference (Dark)
0.6
0.5
Standard Grade Plate
0.4
0.3
0.2
0.1
-10
10
30
50
70
90
Exposure Time (Minutes)
110
130
150
The Next Step
 What
 Can
is the mechanism for this degradation?
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.
WHITE TPB
Plot shows diffusion speed through the
GC column
Each bin then has a mass spec taken.
TPB Mass Spec,
NIST
TPB Mass Spec,
measured
WHITE TPB
Lots of subtleties to getting down to ppm
Concentration measurements
Credit to Jennifer VanGemert (Thompson),
who worked very, very hard on this
YELLOW TPB
PPO (first
column only)
Benzophenone
Unidentifiable,
degradation
dependent,
vary between
solvents
Powder BP Concentrations
TPB
Benzophenone
Benzophenone
One of the only sensible oxidation products
of TPB
Wikipedia says:
A 30ppm solution
apparently has a
100% absorbance
to UV below
300nm
Also supported by NMR data
benzophenone
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
Scin lla on Grade TPB Performance vs BP
Content
Benzophenone Content (%)
0.5
0.05
0.005
0
0.2
0.4
0.6
Performance
0.8
1
1.2
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)
BHT
If they slow down this reaction, confirmation that we
see a radical mediated photo-oxidation.
BC
20% BC
No 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% BC
No BC
Scint grade 1H NMR
TMS
water
Yellow TPB
Compare NMR
benzophenone
Connectivities
Overlaid Average Absorption
Spectra
Overlaid 1H Spectra
Yellowing in DHCL3 Solution
Scint
White Standard
YellowStandard
Yellowing in DHCL3 Solution
Yellow
herring?
Scint
White Standard
YellowStandard