Analysis of the wood coating ageing and
prediction of the durability through
calorimetric investigations
Laurence Podgorski
CTBA Technical Centre for Wood and Furniture
Bordeaux, France
Background
• Performances of wood coatings evaluated
through artificial (prEN927-6) and natural
weathering test (EN927-3)
• Visual assessments (cracking, flaking,
blistering…) rated from 0 to 5.
• To avoid subjective interpretations in the
degradations the wood coating ageing has
been analysed thanks to an analytical method
Objective
• To study the coating ageing through the
changes in its glass transition temperature
(Tg) for different types of weathering (artificial
and natural)
• To use Tg as means of prediction of the
durability
The glass transition temperature Tg
• Tg is the temperature at which an amorphous polymer
changes from the glassy state to the rubbery state
• Tg is a useful parameter since it changes when the
structural parameters of the polymer change
Effect on Tg
Tg ↑
Molecular weight
X
branching degree
X
Structural
cross-linking degree
X
parameters
bulking of substituents
X
plasticization
polarity of constituents
Tg ↓
X
X
X
Changes in Tg for an alkyd stain
Materials and methods
•
•
•
•
3 coats of an alkyd stain applied on oak, meranti and moabi
Weathering
– Natural weathering during 12 months near Paris
– Artificial weathering
Wheel 1 cycle =
• 12 min water
• 27 min ambient atm
• 24 min UV lamps
• 27 min ambient atm
Total = 1300h
•
•
QUV 1 cycle =
• 5 hours UVB-313nm
• 1h water spray
Total = 500h
Changes in Tg for an alkyd stain
Materials and methods
• Calorimetric analysis
– 10 mg of coating cut off with a scalpel
– Analysed by Differential Scanning Calorimetry
(heating rate 5°C/min)
• Tg measurements at different times of
weathering
Changes in Tg for an alkyd stain
Results
• Wheel
Changes in Tg for an alkyd stain
Results-Analysis
• For the three types of weathering and the three wood
species, same Tg variations
Tg
∞
Tg(t)
Tg
Tg (t) = Tg0 + (Tg∞ - Tg0)(1-exp(-t/τ ))
Tg = Tg0 when t=0
Tg → Tg∞ = Tg0+Tg1 when t→∞
τ is a time constant.
Tg0
τ
Exposure time t
Changes in Tg for an alkyd stain
Result-Analysis
Tg (t) = Tg0 + (Tg∞ - Tg0)(1-exp(-t/τ ))
This equation describes a first order kinetic model when
(Tg∞ - Tg (t)) is taken as a parameter:
d [Tg∞ −Tg(t)]
= −k [Tg∞ −Tg(t)]
dt
Integrating this equation leads to:
Tg∞ −Tg(t)
= exp(−kt) with k = 1
Tg∞ −Tg0
τ
Changes in Tg for an alkyd stain
Results-Analysis
The equation of Dibenedetto links Tg to
the conversion rate x
Tg0
Tg(t)−Tg0
λ
x
λ=
=
Tg∞
Tg∞ −Tg0 1−(1−λ)x with
Tg0 corresponds to x=0
Tg∞ corresponds to x=1 (polymer completely cured)
With our model and the Dibenedetto equation it is
possible to express the cure rate of the coating as
follows :
dx = 1 (1− x)²+ 1 x(1− x)
dt τλ
τ
Changes in Tg for an alkyd stain
Results-Analysis
Comparison of Tg0, Tg∞ and τ for the different types of
weathering (meranti)
Weathering
Natural
Artificial / wheel
Artificial / QUV
Tg0 (°C)
3.8
3.8
6.2
Tg∞ (°C)
27.4
24.8
16.5
τ (h)
714
60
50
• The wheel reproduces Tg variations observed in
natural weathering. The cycle used in the QUV does
not reproduce these variations
• The phenomena observed in natural weathering are
accelerated by about 12 times with the wheel and 14
times with the QUV
Changes in Tg for an alkyd stain
Influence of a UV absorber
•
Alkyd stain modified with 3% of UV absorber (hydroxyphenylbenzotriazole)
30
Tg in °C
25
without UV absorber
20
15
with UV absorber
10
5
exposure time in hours
0
0
•
100
200
300
400
500
600
700
800
900
Model still valid but lower Tg
• Plasticization effect possible due to the additive
• The additive absorbs the UV radiations which are not available
anymore to decompose the hydroperoxides and polyperoxides into
free radicals necessary to the curing of alkyd resins
•
No cracking with the UV absorber because of the low Tg (better
flexibility of the coating)
Changes in Tg for an alkyd stain
Natural weathering: influence of the period of exposure
Exposure from April to April
Exposure from January to December
Suitability of the model for different kinds
of coating
• Material and methods
– Five coating systems applied on pine
System
Resin
SB1
SB2
alkyd
alkyd
modified
acrylicalkyd
acrylic
acrylic
WB1
WB2
WB3
Primer
Solid
content
36%
69%
Topcoat
Solid
content
50%
76%
30%
Nb of Resin
coats
1
alkyd
1
alkyd
modified
1
acrylic
43%
2
37%
29%
1
1
2
2
acrylic
acrylic
42%
31%
Nb of
coats
2
1
Suitability of the model for different kinds
of coating
• Material and methods
– Weathering
• Natural weathering during 13 months near Paris
• Artificial weathering
•
•
Wheel 1 cycle =
• 12 min water
• 27 min ambient atm
• 24 min UV lamps
• 27 min ambient atm
Total = 1200h
•
•
QUV 1 cycle =
• 48 h freezing(-20°C)
• 24h condensation
• (3h UVA-340nm then 1h
spray) 96h
Total = 2016h
Suitability of the model for different kinds
of coating
• Coating system SB1
35
35
Tg (°C)
Tg (°C)
30
30
25
25
20
15
experim
model
20
experim
model
15
Coating system SB1
10
Coating system SB1
10
5
5
0
0
0
500
1000
1500
2000
2500
Exposure time in the QUV (hours)
QUV
3000
0
500
1000
Exposure time on the wheel (hours)
Wheel
1500
Suitability of the model for different kinds
of coating
• Coating system WB3
30
30
Tg (°C)
Tg (°C)
25
25
20
20
15
experim
model
15
experim
model
10
10
Coating system WB3
5
Coating system WB3
5
0
0
0
500
1000
1500
2000
2500
exposure time in the QUV (hours)
QUV
3000
0
500
1000
Exposure time on the wheel (hours)
Wheel
1500
Suitability of the model for different kinds of
coating
Comparison of Tg0, Tg∞ and τ for the different types of
weathering
weathering
system
SB1
SB2
Wheel
WB1
WB2
WB3
SB1
SB2
QUV-340nm WB1
WB2
WB3
SB1
SB2
Natural
WB1
WB2
WB3
Tg0 (°C)
8.3
-9.1
-1.7
8.6
15.3
6.3
-8.3
1.9
8.2
15.4
7.0
-8.6
1.7
5.8
16.2
Tg∞ (°C)
29.1
17.2
6.0
11.2
21.4
26.7
17.2
8.3
10.0
24.4
26.8
17.2
6.3
11.2
24.8
τ (h)
151
187
2
35
30
602
607
163
10
164
7225
5932
780
7129
2831
r²
0.96
0.98
0.60
0.33
0.71
0.87
0.88
0.90
ns*
0.93
0.98
0.97
0.95
0.55
0.87
Suitability of the model for different kinds of
coating
• The model is suitable for all coatings
• The second QUV cycle tested is adapted to
reproduce Tg variations observed in natural
weathering
Cycle 1
•
•
QUV 1 cycle =
• 5 hours UVB-313nm
• 1h water spray
Total = 500h
Cycle 2
•
•
QUV 1 cycle =
• 48 h freezing(-20°C)
• 24h condensation
• (3h UVA-340nm then 1h
spray) 96h
Total = 2016h
Relation Tg - Durability
• 2 years of natural weathering
• Tg = 15°C
Relation Tg - Durability
• 2 years of natural weathering
• Tg = 36°C
Relation Tg - Durability
• 2 years of natural weathering
• Tg = 9°C
Conclusions
• The DSC is a useful analytical method to access to a
quantified parameter: Tg
• The changes in Tg during different types of
weathering has been studied for an alkyd stain
• A model has been developed
• This model is suitable to describe Tg variations of
most coatings
• The method is useful to validate artificial weathering
cycles
• The durability of the coating is higher if Tg < 25-30°C