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Assessing Coatings Durability Assessing Coatings Durability via COvia CO22 Generation RatesGeneration RatesAn Alternative Rapid Test of Paint Durability

DuPont Titanium TechnologiesS.Daptardar, Michael P. Diebold30 January, 2011

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AbstractAbstract

�Coating durability is of high importance in most outdoor applications.

�Durability must, therefore, be assessed and accurately characterized

�Existing Test Methods are either time consuming or having poor correlation to actual

outdoor results.

�A new accelerated test that relies on UV exposure conditions typical of exterior

exposures.

�With a focus on the effect of TiO2 pigment grade on paint durability.

�With better correlation to exterior results than longer term accelerated tests.

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Agenda

• Effect of TiO2 on paint durability• Paint durability test strategies• Using [CO2] to monitor paint degradation

– Development of the technique

– Effect of exposure conditions

• Conclusions

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Binder Degradation MechanismsPhotocatalytic:

TiO2 + UV light mostly heat, some radicals

radicals + binder degradation products

Direct:

UV light + binder degradation products

Thermal degradation also occurs

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The TiO2 Durability Balance

Protection fromDirect Degradation

Destruction fromPhotocatalytic Degradation

The balance depends on the grade of TiO2 and the resin chemistry

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Existing Test Methodologies

Conditions

Real

Artificial

Actual Degradation

Secondary Reaction

Response

Outdoor Exposures

Accelerated Exposures

Lab Tests

AppearanceChange

SecondaryEffect

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Outdoor ExposureDescription:

– Expose paints under natural conditions

Advantages:

– Measures the changes of interest (appearance) under the conditions of interest (ambient)

Disadvantages:

– Very slow (years to decades)

– Slowest for paints of the most interest (superdurable)

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Accelerated ExposureDescription:

– Expose paints under high UV light conditions

Advantages:– Measures the changes of interest (appearance)

– Quicker than traditional exterior exposure

Disadvantages:– Relatively slow (weeks to months)

– Does not measure reaction under conditions of interest

• Degradation occurs via complex reaction chains, and increased light intensity affects each reaction rate differently

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Lab TestsDescription:

– Measure non-appearance (physiochemical) changes to paint on exposure

– Often under intense (accelerated) conditions

Advantages:– Fast (hours to days)

Disadvantages:– High intensity (accelerated) conditions

– Measures a property that is of no interest to the paint consumer

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Existing Test Methodologies

Conditions

Real

Artificial

Actual Degradation

Secondary Reaction

Response

Outdoor Exposures

Accelerated Exposures

Lab Tests

New Approach

AppearanceChange

SecondaryEffect

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Monitor the Formation of Carbon Dioxide

– Flexibility – CO2 is the eventual degradation product for all organic resins

– Unambiguous - Does not further react• High concentrations can only mean fast degradation

– Manifests quickly (hours to days)• Even under ambient conditions – no need for high

intensities• Therefore maintains proper balance of different

degradation reactions.

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Experimental Set-up - Lab

“Full Spectrum” fluorescent lighting

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Effect of Liquid Water

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1 2 3 4 5

ml water added

CO

2 A

mo

un

t (A

rbit

rary

Un

its)

0.25 0.5 0.75 1.00.0

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Effect of Pigment Durability

R2 = 0.9288

4000

4500

5000

5500

6000

6500

7000

7500

8000

0 5 10 15 20 25 30 35 40

Chalk/Fade Value

[CO

2] p

pm

One week exposure to Summer Sunlight

Approximately 1.5x rate seen in lightbox

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7 Day Comparison of 3 Paints

0

50000

100000

150000

200000

250000

A B C A B C A B C

Co

un

ts

Paint A Paint B Paint CPigment

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Effect of Light Intensity

0

500

1000

1500

2000

2500

3000

3500

4000

0 1 2 3 4 5 6 7 8

Day

[CO

2] (

ppm

)low durability pigment 0.5 intensity

low durability pigment 1.0 intensity

low durability pigment 2.0 intensity

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Conclusions• CO2 evolution rate from paint film

degradation, under ambient conditions, is a simple, useful predictor of TiO2 grade durability performance.– Method appears general since most paints

ultimately degrade to CO2 and H2O– A number of options for exposure and CO2

detection

• This technique can also be used to probe the degradation mechanism

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Thank You