polysiloxane topcoats afa
TRANSCRIPT
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POLYSILOXANE TOPCOATS PRODUCT CHOICE FOR OPTIMUM PERFORMANCE
Adrian F. Andrews
International Protective Coatings
Akzo Nobel
England
Abstract: Organic coatings degrade as a result of thermal
oxidation, photo-initiated oxidation or by chemical
attack. Silicon based inorganic coatings are much moreresistant to these degradation mechanisms and
Polysiloxane coatings offer significantly enhanced
durability when compared to Polyurethane coatings.
Polysiloxane coatings must however be modified to theright extent with organic resins to enable other coating
performance properties such as flexibility, toughness,adhesion to primers and cost to be obtained while at thesame time not detracting from the Polysiloxane
properties.
This paper discusses Polysiloxane Topcoatstogether with the level and type of organic modification
necessary to optimise coating systems performance in
corrosive environments.
INTRODUCTION
Conventional High Performance Coatings
Systems currently being offered to afford long term
(10 - 15 years) corrosion protection are largely organic-
based and are typically derived from zinc based primers(inorganic zinc silicate or zinc rich epoxies), high build
epoxies and Polyurethane Finishes.
Health, Safety and Environmental (HSE)
legislation is ever increasing and Polyurethane Finishes
generally available have a VOC of 340gl-1 and moretypically 420gl-1 although recently 250gl-1 Polyurethane
Finishes are being trialled in California due to the
impending very strict VOC legislation. The otherconcern with polyurethanes is the pulmonary sensitisation
potential of low mw volatile isocyanates when being
airless sprayed.
Non-isocyanate Finish Technologies developed
to date have too high a VOC content, have reduced
weathering characteristics, poor low temperature cureand reduced mechanical properties when compared to
the versatile polyurethane coatings. The high VOC,
very thin film fluoropolymer based finishes, whilst
offering the potential of superior weatherability, havemet with mixed results in the field and in some cases
offering no better durability than aliphatic acrylicpolyurethanes.
Organic coatings including polyurethanes will
degrade (thermal oxidation, photo-initiated oxidation
and chemical attack) resulting in deterioration of filmproperties leading to gloss loss, discolouration,
embrittlement and adhesion loss.
In contrast Inorganic Coatings based on silicon
are much more resistant to the above degradation
mechanisms. The two main reasons for this are:
1. the higher bond strengths of the Si
O bond(452 kJmol-1) which form the backbone of the
inorganic polymer chain make them more heat
and UV resistant compared to the C C bond
strength (350 kJmol-1) of the organic polymer
chain; and
2. the Si O bonds are already oxidised making
them resistant to atmospheric oxygen and most
oxidising chemicals.
For these reasons many attempts to utilise
silicate technology (hydrolysed tetra ethyl ortho silicate
or alkali metal silicate) have been made to develop highgloss finishes but have been unsuccessful due to the
need to pigment at high PVC to prevent film crackingresulting in matt finishes.
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The use of silicone based materials have
generally been limited by the need to heat to achievecross-linking and in many instances the ability to achieve
films of sufficient toughness and adhesion for general
use.
Hydrolytic polycondensation reactions of alkoxy
silyl functional polyorganosiloxanes (Figure 1) offers the
potential for ambient curing.
Little progress was made in exploiting this
technology until the publication of a patent in the early
1980s claiming Interpenetrating Networks (IPNs)comprising an epoxy amine network and a polysiloxane
network.
Significantly more progress has been made in
the last 5 - 7 years with the publication of a number of
patents covering a range of methods of modifying the
inorganic network by organic polymers together with the
commercialisation of a number of products.
Inorganic-Organic Hybrid Coatings based on
polysiloxane resins with varying organic modifications
have been developed which provide non-isocyanateambient curing viable films offering good HSE
characteristics (low VOC, low toxicity) good appearance
and superior weatherability when compared to aliphaticacrylic polyurethanes.
Extensive corrosion testing has demonstrated
that typical high build epoxy/polyurethane intermediate
systems over zinc primers can have the 200 - 250 micronsof organic coating replaced by 125 microns of the
organically modified polysiloxane. The use of two coat
systems rather than three does however require greatercare during application by applicators to ensure the
correct dry film thickness is achieved.
The organically modified Polysiloxane Finish
thus allows reduced application costs (two coats instead
of three) and from performance projections, reduced life
cycle costs due to the increased weatherability and
durability when compared to conventional aliphaticacrylic Polyurethane Finish coating systems.
CURRENT POSITION
Organic Modification
Organic modification is necessary to achieve a balanceof film properties, such as adhesion, flexibility, and
cost. Using various organic modifications it has been
found that around 20% - 30% organic modification gives
the optimum performance in terms of both adhesion anddurability on exterior exposure and accelerated testing.Too low a level of organic modification results in films
which have too high a polysiloxane characteristic i.e.
glass-like and results in films cracking and losingadhesion on prolonged testing. This was shown to be
the case when the level of organic modification of the
polysiloxane was 8% (Figure 2) and applied to a rangeof primers (zinc silicate, zinc phosphate, zinc epoxy) and
subjected to prolonged (>6000 hrs) accelerated testing
(e.g. Salt Spray - ISO 7253, Prohesion - ASTM G85,
Norsok Cycle - Modified NACE TM0184, Cyclic Test -
ASTM D5894, Condensation - ISO 6270, QUV
- ASTM G53).
Interestingly this was only observed when the
films were subjected to a full drying out period duringthe accelerated test (which does not form part of any of
the Accelerated Tests above). Consequently the use of
non-standard test methods e.g. water immersion1000 hrs - ISO 2812 followed by 1 week drying out was
developed to more quickly identify the mode of failure
and to ensure there were no weaknesses in the coating
systems.
Also of interest is that these same films when
exposed externally for a period of 5 years in a C4
environment have shown only minor defects around thescribe.
Too high a level of organic modificationdetracts from the polysiloxane properties and
compatibility may become a problem.
The type of organic modification employed
should detract as little as possible from the polysiloxaneproperties. For example, to avoid poor colour stability,
aromatic epoxies would not be employed as the organic
modification.
The first Polysiloxane Finish to becommercialised was organically modified with a
hydrogenated epoxy. Subsequently second generationpolysiloxane products with acrylated urethane and
acrylic have been developed.
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The use of acrylated urethane and acrylic
modifications should in theory detract less from theproperties of the polysiloxane resin and is more likely to
give improved durability and colour retention whilst not
detracting from overall corrosion resistance whencompared to hydrogenated (or aliphatic) epoxies.
Much work has concentrated on the area of
comparison of acrylated urethane modification (A)compared to the commercially available epoxy modified
product (B). The acrylic (C) is a more speculative
approach and utilises a different process to that of the
commercially available acrylic (D) which is a simpleblend of an acrylic and polysiloxane resin. Consequently
less data is available on the acrylic modified
polysiloxanes than on the epoxy and acrylated urethanemodified polysiloxanes.
CHEMISTRY
The networks produced are complex due to
multi cross-linking reactions. Both the acrylated urethaneand epoxy modified polysiloxanes are cured with an
amino alkoxysilyl functional silane. In addition to these
organic reactions, (amine-epoxy and pseudo Michaeladdition of amine-acrylated urethane) hydrolytic
polycondensation reactions (catalysed by water) occur
between the alkoxysilyl groups of the curing agent andthe polysiloxane resin (Figure 3). The potential for other
reactions to take place exist. The complexity is further
enhanced when the effects of temperature and relative
humidity on both the organic and inorganic reactions
taking place are considered.
GENERAL PROPERTIES
The general properties of the Polysiloxane
Finishes are shown in Table 1. The volume solids of
these materials must be measured due to the significantlevels of alcohols generated during the condensation
reaction of the polysiloxane resin and aminosilane curing
agent. The epoxy modified polysiloxane can be
formulated with no added solvent whereas the acrylatedurethane and acrylic modified polysiloxanes do employ
the addition of a little solvent (VOC still less than
250gl-1) which gives better general application properties.
It should be noted that the adhesion of
Polysiloxane Finishes to organic primers and high buildsis not universal i.e. is more variable and not as consistent
as with Polyurethane Finishes. At the present time this isbest determined by experiment.
DURABILITY
Figure 4 gives an indication from accelerated
weathering using QUV-A of the improvement in
durability achieved over aliphatic acrylic polyurethanes.An approximate view of effectiveness is to consider the
time taken for the finish to lose 50% of its initial gloss.
If this accelerated testing could be translated into real
life then the epoxy modified polysiloxane would give3 - 4 times the durability of polyurethane and theacrylated urethane modified polysiloxane 4 - 5 times.
Table 2 shows exterior exposure data after4 years in C4 and C5 (ISO 12944 classification)
environments for the acrylated urethane and epoxy
modified polysiloxanes. This data supports the rankingshown in QUV-A data although the result in Singapore
for the epoxy modified polysiloxane is lower than
expected!
Figure 5 shows the colour change arising after
12 months external exposure at a coastal UK site andgives an indication of the improvement in colour
stability of the Polysiloxane Finishes achieved over
aliphatic acrylic polyurethane.
CORROSION RESISTANCE
Tables 3, 4 and 5 show the accelerated test
results for the Polysiloxane Finishes applied as a single
coat (125 microns) over zinc rich epoxy, zinc silicate
and zinc phosphate epoxy primers (75 microns)
respectively. Generally, the only breakdown isunderfilm creep from the scribe with no face breakdown
in terms of cracking, flaking or blistering. In all the
tables, the data refers to underfilm corrosion in mmsfrom the scribe. No creep is 0mm and an excellent
result, 3 - 4mms are generally considered good with up
to 6mms being acceptable and above 6mms the result isconsidered to be poor.
It is immediately apparent that over the range
of anti-corrosive tests the two coat 200 micron dft
polysiloxane systems give similar if not betterperformance than the three coat 325 micron dft
conventional high performance system. This
performance has been verified by an external
independent testing laboratory for the two coat
polysiloxane (acrylated urethane-A and acrylic-Cmodifications) systems over zinc rich epoxy primer.
Similar conclusions can be drawn from Tables
4 and 5.
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Table 6 shows the exterior exposure results after
periods of up to 4 years in coastal marine environments(C5 - ISO 12944). These clearly show that zinc primers
give significantly better performance than non-zinc
primers in terms of underfilm creep from the scribe. Themagnitude of the difference between the zinc primed
systems and the non-zinc primed systems is significantly
greater than that shown by any of the accelerated tests.
Comparison with conventional three coat zincprimed and non-zinc primed (barrier and zinc phosphate)
systems shows these to give similar results i.e. negligible
scribe creep with zinc rich primer and typically5 10mms with non-zinc primers.
This suggests that for offshore, coastal andmarine corrosive environments zinc rich primer should
always be used and other primers limited to less
aggressive environments.
MECHANICAL PROPERTIES
The mechanical properties of the organically
modified polysiloxanes were monitored with time (up to
12 months) to ensure long term film integrity. There wasa concern that any unreacted alkoxy silyl groups in the
early stages of cure would result in further cross-linking
on long term exposure which in turn may result in stressgeneration and embrittlement of the films. The properties
monitored were amongst others fracture toughness
(Figure 6), tensile strength (Figure 7), elongation at
break, (Figure 8) and flexibility.
Flexibility was determined in two ways:
1) using a cylindrical mandrel (Table 7) and;
2) by controlled blending of a 5mm steel panel
(zinc rich epoxy primed) to three displacementvalues (15mms, 20mms and 25mms) and
monitor for signs of cracking (Table 8 and
Figures 9 and 10).
From these results it can be seen that theacrylated urethane modified polysiloxane (A) is tougher
and more robust than the epoxy modified polysiloxane
(B) or the acrylic modified polysiloxane (C) and would
be expected to give the best long term performance in
terms of film integrity. This is clearly demonstrated inFigure 9 where cracking occurs across the panel with the
epoxy modified polysiloxane (B) whereas the acrylatedurethane modified polysiloxane (A) shows no cracking
even at a greater displacement value.
The acrylated urethane modified polysiloxane
(A) develops its coatings properties over1 month compared to 1 week for the epoxy (B) and
acrylic (C) modified polysiloxanes. This may be
explained in terms of the reaction kinetics of the organicreactions taking place to form a network. The rate
constants k1 and k2 for the epoxy-amine and acrylated
urethane-amine reactions are outlined below.
Chemistry Amine-Epoxy
Amine-
Acrylated
Urethane
k1 (1mol-1g-1)
k2 (1mol-1g-1)
5.05 x 10-5
2.98 x 10-5
2.88 x 10-4
7.06 x 10-6
From the reaction kinetics it can be determined
that the epoxy-amine reaction achieves 80% conversion
in 24 hours at 20C whereas the acrylated urethane-
amine reaction takes 100 hours at 20C to achieve thesame extent of reaction.
Once the coatings properties have been
developed there is little change over a 12 month period
suggesting that little if any further cross-linking is taking
place with any unreacted alkoxysilyl functionality.
FUTURE POSITION
The superior performance and beneficial HSEcharacteristics exhibited by organically modified
polysiloxanes ensures their continued development.
It should be recognised that Inorganic Organic
Hybrid Technology is a new technology and there is no
real knowledge database to fall back on as is the case
with organic (epoxy, polyurethane) technologies. It is
critical to understand the network characteristics andstructure-property relationships and the extent to which
these can be carried through coatings design if new
coatings with well defined, reproducible performance
characteristics are to be developed. In the absence ofthis information new coatings development utilising this
technology will be reliant on testing a wide range offormulations potentially increasing both the product
development time and the uncertainty regarding longterm coating performance.
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SUMMARY
For optimum performance it is important to
choose the right organically modified polysiloxane. The
acrylated urethane modified polysiloxane showsimproved long term performance benefits in terms of
durability (gloss and colour retention), flexibility, film
toughness and edge protection when compared to the
epoxy modified polysiloxane. On the other hand theepoxy modified polysiloxane shows improved earlyhardness as a result of faster development of film
properties. All of the organically modified polysiloxanes
discussed from a series of accelerated tests and exteriorexposure would be expected, when part of a two coat zinc
rich primed system, to have significantly better long term
performance than a conventional three coat system.
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APPENDIX
FIGURE 1
Si OR + H2O Si OH + ROH
Si OH + RO Si Si O Si + ROH
Si OH + HO Si Si O Si + H2O
Figure 1. Hydrolytic Polycondensation Reactions
of Alkoxysilyl Functional Polysiloxanes
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FIGURE 2
Figure 2. Cracking and adhesion loss of Polysiloxane Films modifiedwith 8% Organic when applied over a range of Primers
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FIGURE 3
1. R'NHR'' +
O
R'R''N
OH
Epoxy
R'NHR'' + R'NR''COO COO
2. (RO)3-n(R'')nSi
Hydrolytic
Polycondensation
(Ref Fig. 1.)
OSi(R'')2 OSi(OR)3-n(R)n
Si SiO
3. Hydrolytic Silanol condensation reactions between 1. and 2.
= e.g. (RO)3-nR''nSi(CH2)m
= e.g. Alkyl, Aryl, Hydrogen
= Alkyl
= 1, 2
= Integers
R'
R''
R
n
a, b, m
OSiR''(OR)a b
Acrylated Urethane
Figure 3. Multi Cross-linking Reactions producing Complex Networks
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TABLE 1
Property
A
(Acrylic
Urethane)
B
(Epoxy)
C
(Acrylic)
D
(Acrylic)
Volume Solids (%)
ISO 3233
Measured
78
Measured 90 Measured 82 Measured
76
Datasheet90
VOC (g/litre)
EPA Method 24172 120 150 170
Hard Dry (hrs)
ISO 9117
50% RH 10C
20C
40C
12
84
11
63
12
64
10
6-
Initial Gloss (%)
ASTM D52370 - 80 80 - 90 80 - 90 70 - 80
Heat Resistance
1 month at 130C
Very slight
yellowingYellowing
Very slight
yellowing
Very slight
yellowing
Adhesion (P.A.T.)ISO 4624
over Zinc Rich Epoxy Primer
13 MPa 13Mpa 11 MPa -
Table 1. General Properties of Polysiloxane Finishes
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FIGURE 4
C
C
A, D
BPU
Polysiloxane Quv A
0
20
40
60
80
100
120
0 2000 4000 6000 8000 10000 12000
Hours
GlossReten
tion%
A (Acrylic)
PolyurethaneB (Epoxy)
C (Acrylic)
D (Acrylic)
PU
B
C
A
D
Figure 4. QUV-A, Polysiloxane Finishes
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TABLE 2
Gloss Retention (60) A B
Blyth, UK - C5M/I
Houston, USA - C4
Brisbane, Australia - C4
Singapore - C5M/I
90 95
90 95
85 90
85 90
80 85
85 90
80 85
55 60
Table 2. Exterior Exposure of Polysiloxane Finishes
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FIGURE 5
CA, D
BPU
Months
Polyurethane
A (AcrylatedUrethane)Pol siloxane
B (Epoxy)Polysiloxane
Delta E
14121086420
3
2.5
2
1.5
1
0.5
0
External Exposure of Polysiloxanes
Figure 5. Colour Change of Polysiloxane Finishes on External Exposure
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TABLE 3
SYSTEM ZINC RICH EPOXY (75m)
6000 hrs Acc. Testing
Corrosion Creep (mms)
A(Acrylated
Urethane)
- 125m
B(Epoxy)
- 125m
C(Acrylic)
- 125m
D(Acrylic)
- 125m
HP Epoxy (200 m)
PU (50m)
ISO 7253Salt Spray
0.2 1.5 1.0(4000 hrs)
1.0(4000 hrs)
1.8
ASTM G85
Prohesion
0.5 0.5 0
(4000 hrs)
0
(4000 hrs)
3.3
BS 3900Cold Salt Spray
0 0 1.5
ISO 2812
40C Immersion0* 0 -
Norsok Cyclic M501
(4200 hrs)2.5 0.5 4.6
ASTM D5894 Cyclic
(4032 hrs)0.9 1.6 2.2
* No blistering at 4000 hrs but thereafter small blisters develop.Blisters do not occur if the film is fully dried out every 1000 hrs.
Table 3. Corrosion Resistance of Polysiloxane Finishes over Zinc Rich Epoxy Primer
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TABLE 4
SYSTEM INORGANIC ZINC SILICATE PRIMER (50 - 75m)
6000 hrs Acc. Testing
Corrosion Creep (mms)
A(Acrylated
Urethane)
- 125m
B(Epoxy)
- 125m
HP Epoxy (200 m)
PU (50m)
ISO 7253Salt Spray
0.9 0.2 1.8
ASTM G85
Prohesion0.7 0 3.3
BS 3900Cold Salt Spray
0 0 1.5
ISO 2812
40C Immersion0* 0 -
Norsok Cyclic M501
(4200 hrs)1.0 1.3 4.6
ASTM D5894 Cyclic
(4032 hrs)1.9 2.1 2.2
* No blistering at 4000 hrs but thereafter small blisters develop.Blisters do not occur if the film is fully dried out every 1000 hrs.
Table 4. Corrosion Resistance of Polysiloxane Finishes over Zinc Silicate Primer
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TABLE 5
SYSTEM HIGH SOLIDS ZINC PHOSPHATE PRIMER (75m)
6000 hrs Acc. Testing
Corrosion Creep (mms)
A(Acrylated
Urethane)
- 125m
B(Epoxy)
- 125m
HP Epoxy (200 m)
PU (50m)
ISO 7253Salt Spray
5.0 2.8 3.2
ASTM G85
Prohesion2.8 2.8 3.5
BS 3900Cold Salt Spray
0.5 0.4 2.5
ISO 2812
40C Immersion0 0 0
Norsok Cyclic M501
(4200 hrs)3.1 2.7 5.5
ASTM D5894 Cyclic
(4032 hrs)7.9 8.0 4.1
Table 5. Corrosion Resistance of Polysiloxane Finishes over Zinc Phosphate Epoxy Primer
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TABLE 6
Urethane Polysiloxane (A)
(mm creep)
Epoxy Polysiloxane (B)
(mm creep)
PRIMER4 years
Blyth
2 years
Bohus Malmon
4 years
Blyth
2 years
Bohus Malmon
Zinc Silicate
(75m)0 0 2 1
Zinc Rich Epoxy
(75m)0 0 0.5 0
HS Zinc Rich Epoxy
(75m)0 0 1 0
Zinc Phosphate Epoxy
(75m)13 12 9 10
HS Surface Tolerant Epoxy(150m)
15 13 18 12
HS Epoxy Mastic Aluminium(150 m)
5 6 7 7
HS Glass Flake Epoxy
(200m)12.5 10 12 10
Table 6. Exterior Exposure Corrosion Resistance of Polysiloxane Finishes
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FIGURE 6
0
0.3
0.6
0.9
1.2
1.5
Time (Months)
0 1 2 3 4 5 6 7 8 9 10 11 12
Kic(MNm-3/2)
A (Acrylated Urethane
Modification)
B (Epoxy Modification)
C (Acrylic Modification)
Fracture Toughness (MNm-3/2)
CoatingBefore
Exposure
1 Month
( std error)
3 Months
( std error)
6 Months
( std error)
12 Months
( std error)
A
(AcrylatedUrethane Modification)
0.611.36
( 0.07)
1.43
( 0.08)
1.43
( 0.08)
1.48
( 0.09)
B
(Epoxy Modification)0.87
0.75
( 0.02)
0.77
( 0.04)1.14 ( 0.16)
0.80
( 0.03)
C
(Acrylic Modification)0.59
0.69
( 0.04)
0.78
( 0.05)
0.80
( 0.04)
0.73
( 0.02)
Figure 6. Fracture Toughness of Polysiloxane Finishes
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FIGURE 7
0
5
10
15
20
25
30
0 1 2 3 4 5 6 7 8 9 10 11 12
Time (Months)
TensileStrength(MPa)
A (Acrylated UrethaneModifciation)
B (EpoxyModification)
C (AcrylicModification)
Tensile Tests
Before Exposure 1 Month 6 Months 12 MonthsCoating
UTS eb M UTS eb M UTS eb M UTS eb M
A
(Acrylated Urethane
Modification)
13.0 10.2 0.45 22.9 5.2 1.1 26.8 3.3 1.32 24.5 3.8 1.2
B(Epoxy Modification)
22.2 2.8 1.1 24.3 2.8 1.3 25.3 2.1 1.4 18.5 1.5 1.4
C(Acrylic Modification)
9.3 4.4 0.39 10.9 2.0 0.73 13.0 1.2 0.97 10.9 1.2 1.1
UTS = Ultimate Tensile Strength
eb = Elongation at break
M = Modulus
Figure 7. Tensile Strength of Polysiloxane Finishes
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FIGURE 8
ElongationatBreak(%
0
2
4
6
8
10
12
0 1 2 3 4 5 6 7 8 9 10 11 12
Time (Months)
A (Acrylated UrethaneModification)
B (Epoxy Modification)
C (Acrylic Modification)
Tensile Tests
Before Exposure 1 Month 6 Months 12 MonthsCoating
UTS eb M UTS eb M UTS eb M UTS eb M
A(Acrylated Urethane
Modification)
13.0 10.2 0.45 22.9 5.2 1.1 26.8 3.3 1.32 24.5 3.8 1.2
B
(Epoxy Modification)22.2 2.8 1.1 24.3 2.8 1.3 25.3 2.1 1.4 18.5 1.5 1.4
C
(Acrylic Modification)9.3 4.4 0.39 10.9 2.0 0.73 13.0 1.2 0.97 10.9 1.2 1.1
UTS = Ultimate Tensile Strength
eb = Elongation at breakM = Modulus
Figure 8. Elongation at Break of Polysiloxane Finishes
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TABLE 7
Minimum Diameter (inches) that Coating PassedPolysiloxane
(Organic Mod)1 Week 1 Month 3 Months 6 Months 12 Months
A
(Acrylated Urethane)3/16 1/4 3/8 3/8 3/8
B
(Epoxy)3/8 1 >1 >1 >1
C
(Acrylic)3/16 1 >1 >1 >1
Table 7. Cylindrical Mandrel Flexibility of Polysiloxane Finishes
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TABLE 8
MAXIUMUM DISPLACEMENT WITHOUT FAILURE
Exterior Exposure
1 Week 1 Month 3 Months 6 Months 12 Months
A
(Acrylated Urethane)15mm 25mm 25mm 25mm 25mm
B
(Epoxy)20mm 15mm
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FIGURE 9
A (Acrylated Urethane)
B (Epoxy)
Figure 9. Panel Bend Flexibility of Polysiloxane Finishes
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FIGURE 10
Figure 10. Equipment used to Bend Panels