Polysiloxane Topcoats AFA

7/30/2019 Polysiloxane Topcoats AFA

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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)



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


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)



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


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

Polysiloxane Topcoats AFA

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