SHR HOUT RESEARCH

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MEASUREMENT OF HYGROTHERMAL STRESS IN EXTERIOR
WOOD COATINGS
Mari de Meijer - Drywood Coatings Johan Nienhuis
- SHR Timber Research
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Content of presentation
Origin of thermal and hygroscopic
stress Wateruptake of free paint films swelling,
E-modulus and elongation Internal stress
measurement method Effect of temperature and R.H.
changes on internal stress and stress
relaxation Unanswered questions and future work
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How internal stress can occur
Difference in thermal or hygroscopic expansion
between coating and wood
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Quantification of IS
Calculate maximum stress from difference in
expansion or contraction between coating and wood
due to temperature or moisture change Measure
stress in coating directly, possibility to assess
relaxation of IS Determination of E-modulus
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Measurement of Internal Stress
Coating is applied on one side of very thin metal
strip Coated strips are exposed to change in T or
RH Expansion or contraction of the coating cause
strip to deflect Deflection is measured as a
function of time
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Measurement of Internal Stress
Internal stress is calculated as
follows ISDEd3/(3?L2(d?)(1-?)) IS internal
stress D deflection E modulus of elasticity of
paint d thickness of substrate (metal strip)
L length of unclamped substrate ? Poisson
ratio of substrate ? thickness of paint
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Measurement of Internal Stress
Application of paint film (80 ?m dft) on stress
free metal strip (155 ?m) Drying of paint for 2
weeks _at_ 20 C and 65 R.H. Place metal strip in
clamps Measure deflection at different
temperature and relative humidity change climate
in the morning measure deflection during the day
and overnight
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Apparatus for measuring deflection
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E-modulus measurement
Tensile test of free paint films (_at_ Zwick,
loadcell 500 N, 50 mm/min, preload 0.1 N) Free
paint films (70 mm length, 15 mm width, 40-60 µm
dft) Additional pretreatments immersion in water
for 24 h - 28 days Leaching according to EN
927-5 Ageing _at_ 105 C for 4 h Volume changes also
recorded
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Paint systems tested
4 white pigmented paints solventborne alkyd
primer / topcoat (SB) waterborne primer / topcoat
(WB) waterborne primer with good
durability(WP1) waterborne primer with restricted
durability (WP2 ) Various types of unpigmented
binders
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Change in volume of free paint films
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Change in E-modulus of free paint films
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Swelling of wood (width) and volume change of
free paint films
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Results E-modulus / volume change
Volume free coating film increases up to 20
during immersion in water Leaching followed by
drying and 2nd immersion reduces swelling Heating
reduces volume E-modulus changes due to moisture,
leaching or heating
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Internal stress in paint filmschanges in R.H. _at_
constant temperature and changes in T _at_ constant
R.H.
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Overview of IS results
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Overview of IS results
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Increase of RH _at_ low T
65
40
99
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Increase of RH _at_ low T
65
40
99
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Increase in temperature at high RH
40 degrC
1 degrC
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Increase in temperature at high RH
40 degrC
1 degrC
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Increase in temperature at low RH
50 degrC
20 degrC
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Increase in temperature at low RH
20 degrC
50 degrC
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Comparison various unpigmented films
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Conclusions
Swelling due to wateruptake of free paint film
(volume) more than coated wood (width) This will
cause IS in coating E-modulus changes for
immersed, leached or aged coatings increase in
modulus and thereby internal stress
ISDEd3/(3?L2(d?)(1-?))
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Conclusions
Increase of stress at temperature or RH
changes Increase of stress (built up of stress)
due to changes in climate (R.H. and temperature)
no complete stress relaxation
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Future work
Tg and E-modulus in relation to stress
relaxation Impact of E-modulus changes on
stress Stress measurements while
immersed Calculation of coating stress on
wood Difference expansion coating-wood Anisotropy
in dimensional changes
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Relation IS -durability
What levels of stress cause failure? Depending on
type of coating both high and low T or RH can be
critical (what does this mean for ICP in EN
927-3?) Time between T / RH changes to allow
relaxation is also important for durability