Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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236 High-performance organic coatings

resistance.

12.6.3 High heat resistant coatings

Polysiloxanes themselves can be heat resistant up to 300°C depending on

resistance up to 650°C can be achieved. How does that work? At tempera-

tures above 160–180°C all siloxanes release their organic groups, mainly

methyl or phenyl as we have seen before. Other organic groups will be

released faster and already at lower temperatures. This decomposition of

the Si–C bond is a very slow process at temperatures below 300°C (it may

perature gets into the range of 400°C. At that temperature it is just a

question of minutes or a few hours until all organic groups are released.

Particularly in the presence of oxygen, the Si–C bond will be replaced by

Si–O–Si. Contrary to the above siloxane bond (recall that we speak about

siloxane when we have the Si–O–Si bond and organic groups attached to

the Si), we now get a silicate, similar to the structure of quartz. During the

decomposition of the organic groups, we get very reactive groups that are

able not just to form the silicate but also to a certain extent to react with

ceous iron oxide and mica. This combination of decomposed polysiloxane,

thickness is limited. Depending on the kind of resin, the kind of substrate

in the case of one-layer systems and 80 μm in the case of two-layer

systems.

100

0 6 12 18 24

Exposure time (months)

Gloss retention (%)

at 60° angle

Straight polyester,

crosslinked with iso-

cyanate

Straight polyester,

crosslinked with

melamin

30% silicone

12.4 Weather resistance with outdoor exposure in Florida.

modified polyester

significantly improved gloss retention, which indicates the good weather

the specific composition. With coatings based on polysiloxane resins, heat

last years until completed) and is significantly accelerated when the tem-

inorganic pigments and/or fillers.The best reactions with fillers and the best

i.e. reactive silicate and the inorganic fillers, forms films that are heat resis-

tant to 650°C. Because of differences in the coefficient of temperature

expansion of the metal substrate and the silicate-based coating, the film

and the substrate preparation, the film thickness should not exceed 30 μm

heat resistance will be achieved with aluminium pigments (flakes), mica-

Polysiloxane coatings for corrosion protection 237

Principally, all siloxane resins, particularly those used for high heat resis-

tant coatings, are heat-curing resins, i.e. they cure at a temperature in excess

of 180°C only. Nevertheless, quite often low temperature drying or drying

in environmental conditions is required. This can be achieved by the choice

of a polysiloxane resin that dries at ambient temperature either physically

or if a small amount (3–5% on solid resin) of organic resins, such as ketone

resins or cellulose derivates, e.g. cellulose aceto butyrates (CAB), is added

to the paint formulation. The standard polysiloxane resins are from the

composition T/D = approx. 1 : 1 and phenyl/methyl = 1 : 1 on a molar basis

– see Section 12.2 above.

Tables 12.4 and 12.5 show two guideline formulations with which heat

resistance to 650°C can be achieved. These coatings are resistant not only

shock. Long-term anti-corrosion properties at high heat can be achieved if

a polysiloxane-based zinc dust primer is used too – see Table 12.6.

Table 12.4 Heat resistant polysiloxane coating; top coat micaceous iron oxide;

long-term thermal stability approx. 650°C

% by weight

Polysiloxane resin solution, 50% 26.1

White spirit –

Xylene 7.5

1-Methoxy propyl acetate-2 7.4

Bentone 38, 7.5% 6.6

Micaceous iron oxide 52.4

100.0

Composition: binder 13.1

pigments 51.2

additives 0.5

solvents 35.2

100.0

Application

Application method: spraying

Drying

are only attained after exposure to high temperature, i.e. through normal

operation of plant.

Application viscosity

Flow time, DIN 53 211, 23°C: approx. 20 s (DIN 4 cup)

Dry film thickness: approx. 15 μm

The coating dries at room temperature to form tack-free films resistant to the

knocks of assembly/installation. The final resistance properties and hardness

to such high heat but also against fluctuations of temperature and thermal

238 High-performance organic coatings

Table 12.5 Heat resistant polysiloxane coating; aluminium bronze top coats;

long-term thermal stability approx. 600°C

% by weight

Polysiloxane resin solution, 50% 16.6

White spirit –

Xylene 23.4

1-Methoxy propyl acetate-2 23.4

Bentone 38, 7.5% 3.2

Aluminium bronze 33.4

100.0

Composition: binder 8.3

pigments 21.7

additives 0.2

solvents 69.8

100.0

Application

Application method: spraying

Drying

are only attained after exposure to high temperature, i.e. through normal

operation of plant.

Application viscosity

Flow time, DIN 53 211, 23°C: approx. 20 s (DIN 4 cup)

12.7 Concrete protection

Concrete is perhaps one of the most extensively used construction materi-

skyscrapers of every kind are made from concrete. Its importance is steadily

increasing and it seems to be an almost unlimited construction material.

steel bars is incorporated into concrete, making a reinforced concrete

or direct tension than a plain concrete section with the same dimensions

would be. The alkaline chemical environment provided by portland cement

more resistant to corrosion than it would be in neutral or acidic

conditions.

Although well known for its long durability, concrete is subject to erosion,

with constant attack from humidity and chloride ion penetration. Not only

Dry film thickness: approx. 15 μm

The primer dries at room temperature to form tack-free films resistant to the

knocks of assembly/installation. The final resistance properties and hardness

als. Highways, tunnels, bridges, airport runways, hotels, office buildings and

To get sufficient compressive and tensile strength, reinforcement such as

section. This is much more efficient in carrying tensile forces due to bending

causes a passivating film to form on the surface of steel, making it much

Polysiloxane coatings for corrosion protection 239

does this have a destructive impact on the concrete itself, but even more

important is the effect it has on the reinforcement steel used to improve

the tension of the concrete structure. When rebar corrodes, the rust expands,

cracking the concrete and unbonding the rebar from the concrete.

The water in the pores of the cement is normally alkaline; this alkaline

environment is one in which the steel is passive and does not corrode.

Carbon dioxide from the air reacts with the alkali in the cement and makes

the pore water more acidic. This acidity, in combination with humidity, will

lead to corrosion of the rebar. Chloride ions will further promote the cor-

rosion of steel rebar.

All these destructive reactions require water. Keeping the concrete dry

is an important goal of concrete protection and maintenance. Water-

repellent protection that leaves the concrete surface breathable (vapour

permeable) and reduces (or suppresses) the chloride ion penetration is

required. Broadly used for that purpose are bitumen coatings. However,

bitumen coatings are reported to have limited durability and after a while

Table 12.6 Heat resistant polysiloxane primer; zinc dust primer; long-term

thermal stability approx. 500°C

% by weight

Polysiloxane solution, 50% 18.6

Xylene 1.4

Bentone 38, 7.5% 11.0

Zinc dust 69.0

100.0

Composition: binder 9.3

additives 0.8

pigments 69.0

solvent 20.9

100.0

Technical data

Pigmentation level, calculated on solid binder: 740%

Storage stability: >3 months

Application

Application method: usually by spraying, though also by brushing or rolling,

max. 30 μm per coat (two-coat system)

Drying

are only attained after exposure to high temperature, i.e. through normal

operation of plant.

providing the maximum film thickness is not exceeded.

Dry film thickness: max. 50 μm (single-coat system)

The primer dries at room temperature to form tack-free films resistant to the

knocks of assembly/installation. The final resistance properties and hardness

240 High-performance organic coatings

the above-mentioned destructive impact will start. Modern alternatives are

siloxanes and the most modern are water-based siloxane emulsions. Tests

for roadwork applications (such as tunnels, bridges, etc.) carried out by

independent and authorized institutes in Germany and Sweden have proven

ing to the National Cooperative Highway Research Program Report 244-II

The main subjects of the tests have been:

• Water absorption

• Drying (vapour permeability)

• Chloride ion absorption.

For this test the concrete formulation used was:

• Type I Portland Cement 439 parts

• Eau Claire Sand 1500 parts

• Eau Claire Coarse Aggregates 1758 parts

• Water 225 parts

A siloxane emulsion which is supplied with 60% content of active ingredi-

ent was diluted with water in the ratio 1 : 3, i.e. the content of active ingredi-

ent of the applied material was 15%. The application rate was 400 square

feet per gallon = approximately 5 m

/litre.

The diluted siloxane emulsion was applied on the concrete after one day

of drying of the concrete as well as after 5 days and 21 days of the concrete

drying (curing). After 7 days of the curing of the impregnant, the prepared

specimen was exposed to aqueous 15% NaCl solution and the water intake

was measured frequently up to 21 days. Then the specimens was stored in

dry conditions and the water evaporation (drying) was also measured fre-

quently up to 21 days. Parallel to the water absorption, the intake of chlo-

ride ions after 21 days of exposure to the salt bath was analysed. Of course,

the test was done in comparison with an untreated reference. Details of the

test report are shown in Figs 12.5 and 12.6.

Water repellence, breathability and reduction of chloride ions could be

proved impressively, thus the rebar will be protected from corrosion, and

The above discussion of siloxane emulsions as a more modern solution

serves to introduce some ideas about the durability of a siloxane impregna-

tion. One can subdivide the silicones or silanes used in the hydrophobation

of constructions or construction materials into three classes (Fig. 12.7),

essentially:

• Monomeric alkyl alkoxy silanes

• Oligomeric alkyl alkoxy siloxanes

• Polymeric siloxanes (methyl silicone resins).

their efficiency. The most recent tests for US highway applications accord-

(Report 244) confirmed the European test results.

freeze–thaw resistance will also be improved significantly.

Polysiloxane coatings for corrosion protection 241

3.00

2.50

2.00

1.50

1.00

0.50

0.00

0 5 10 15 21 5 10 15 21

Water intake phase

(days of water exposure)

Drying phase

(days of drying)

day

days

Control

Application rate:

200

sq ft/gallon

Days of concrete drying

before application of the

impregnant

Weight difference (%)

12.5 Siloxane emulsion, 15% active ingredients, water intake and

evaporation (test according to US National Cooperative Highway

Research Program report 244, performed by CTL, Chicago).

100

Days of concrete drying

before application of

the impregnant

day

days

Control

Application rate:

200

sq ft/gallon

Control1521

Days of drying before coating

Ion content

(%)

16 % 11 %

15 %

100 %

12.6 Siloxane emulsion, 15% active ingredients; reduction in chloride

ion content (%) after 21 days’ exposure to salt solution (test according

to US National Cooperative Highway Research Program report 244,

performed by CTL, Chicago).

Polysiloxane

(silicone resin)

Siloxane, oligomer

Silane, monomer

R, R*, R** = alky1

Si SiO

ORO

Si SiO

Si OO

Si RSi OO

R* R*

SiSi O R

n = 3–5

RR** OSi

12.7 Structure principles of silanes and siloxanes.

242 High-performance organic coatings

It is common for all three types that they build in the end use after curing

a stable, three-dimensional, resin-like network. The chemical similarity to

the silicate components of building materials leads to stable and durable

bonds with most mineral substrates. At the same time, the alkyl groups

orient to the exterior (air) side and protect the surface like little umbrellas

against penetration of humidity (see Fig.12.8).

Depending on their chemical structure (see Table 12.7) one must distin-

guish the mode of action or the site of attachment of the siloxanes. While

molecular weight silanes and oligomeric siloxanes penetrate into the avail-

able pores, i.e. the low molecular weight products are subject to capillary

absorption. Embedded in the pores and chemically reacted to the building

material, they reduce the capillary absorption of water, the well-known

sucking effect of all capillaries. Therefore, low molecular weight silanes and

the appearance of the building material remains visually unchanged. The

The siloxane backbone reacts with silicate from the substrate

and forms a durable connection, while the alkyl groups

protect the substrate like umbrellas

CH3

3HC

CH3

12.8 Principle of water repellency.

Table 12.7 Properties of silane, siloxane and polysiloxane

Silane Siloxane Polysiloxane

Structure monomeric oligomeric polymeric

Penetration; capillar activity very deep deep poor

Application form (% active

ingredient)

20–100 1–10 3–8

approx. 40% approx.

80%

100%

Optical effect to the

substrate

none none to

some

darkening,

‘wet-look’

effect

Water vapour permeability high high medium

Beading poor poor

Efficiency

significant

polysiloxane creates a film particularly at the substrate surface, the low

oligomer silanes have almost no influence upon the substrate surface, and

Polysiloxane coatings for corrosion protection 243

a visual change, i.e. a dark, wet-looking surface.

Oligomeric siloxane emulsions quite effectively penetrate into the pores

of the concrete and do not change the surface appearance. Because they

do not remain on the surface of the substrates, the emulsions are recoatable

and can be used as a primer, e.g. for coatings and plaster. The strong chemi-

12.8 Siloxanes as paint additives

As said above (Section 12.6), siloxanes have low surface energy, i.e. are

surface active materials. Particularly the linear structures (D based only),

attached, are very surface active. In Section 12.5.2. we learned that just this

class of products can cause cratering and recoatability problems. These

issues are closely related to the molecular weight or the chain length of the

siloxanes – the higher the molecular weight the more severe the problem.

How can we overcome this issue? On the one hand we have a surface active

and levelling, while on the other hand it causes craters and adhesion prob-

lems due to extreme incompatibility. The solution is to organically modify

Various properties can be controlled by the siloxane : polyether ratio, but

also by the type of polyether – ethylene oxide and/or propylene oxide,

or on the end, plays an important role in achieving the intended properties,

the chain lengths of the siloxane and the polyether. The fact that this strat-

egy is very successful shows that there is no automotive top coat that does

not contain a small amount (approximately 0.1% of the total paint formula-

There are two main ways to modify the siloxane with polyether, the fol-

lowing showing them schematically:

1. condensation of an OH-functional siloxane with a butanol-initiated

polyether:

−−− − ⎯ →⎯⎯⎯

−−−

O Si OH + C H O (EO) (PO)

OSiO

catalyst

e.g. amine

−−(EO) (PO) +C H OH

49XY

2. Addition of allyl-initiated polyether to a hydrogen-functional siloxane:

−−− − − ⎯ →⎯⎯⎯

−−−

O Si OH + C H=CH CH (EO) (PO)

OSi(CH

Pt catalyst

223

)(EO)(PO) −

polymeric siloxanes, on the other hand, form a surface film which leads to

cal bond of the product leads to a very efficient long-term durability.

the polydimethyl siloxane. The modifiers most often used are polyethers.

random or block structures. The place of the modification, on the chain side

too. Finally, of course, a very significant factor for the overall behaviour is

tion) of this kind of modified siloxane.

which we call polysiloxane fluids, with just methyl groups as organic groups

material which might be able to improve substrate wetting as well as flow

244 High-performance organic coatings

Both reactions can be done on the side or on the end of the siloxane chain,

depending on where you functionalize the siloxane.

As said, these materials are surface active products and interact with the

various surfaces:

• coating surface ⇔ air

• substrate ⇔ coating

These surfaces are obvious, but within the paint formulation there are also

solvents (or water) and various other additives, one can easily imagine that

if a surface active material is added. If the uncountable number of paint

formulations are considered, it will become impossible to predict the behav-

iour. So the right choice of siloxane paint additive has to be related to a

some general rules that might be considered:

• Generally, siloxane-polyethers with Si–O–C bonds are not stable against

hydrolysis, and not recommended for water-based systems (reaction 1

above).

• Those with Si–C bond are stable against hydrolysis (reaction 2 above).

• Linear siloxane-polyethers mainly result in increased slip and mar resis-

tance, as well as defoaming.

• Low molecular weight grades have more surfactant properties; they

improve substrate and pigment wetting.

Figure 12.9 shows a water-based polyester polyurethane coating applied

on uncleaned steel. The left panel lacks a siloxane paint additive, while the

right panel contains 0.1% of a siloxane-polyether of structure 2 with low

doubt that substrate wetting is improved.

12.9 Water-based polyester polyurethane coating applied on

uncleaned steel.

various surfaces such as pigment ⇔ binder, and filler ⇔ binder; adding

there so many surfaces that it is difficult to foresee what would happen

specific test. Independently of the above-mentioned difficulties, there are

molecular weight, where the siloxane chain is side-modified. There is no

• Side-modified types improve flow and levelling.

Polysiloxane coatings for corrosion protection 245

12.9 How do siloxane paint additives work?

There are mainly three effects resulting from the use of the siloxane-

polyether paint additives:

1. Reduction of surface tension

3. Concentration of the products on top of the coating.

Before we come to discuss these effects in more detail, recall that the silox-

anes are surface active materials. Siloxanes have a surface tension of

approximately 20 mN m

−1

with surface tensions of approximately 21–25 mN m

−1

12.9.1 Reduction of surface tension

The addition of approximately 0.1% of these products to paint formulations

reduces the surface tension of wet paints from approximately 30–35 mN m

−1

to a level of 25–28 mN m

−1

. The exact level depends on the structure and

molecular weight of the siloxane-polyether and the total paint formulation.

It is well known that the wetting of surfaces with liquids depends on the

surface tensions – the surface tension of the liquid must be lower than that

of the substrate. The lower the surface tension of the liquid (paint), the

better the wetting.

12.9.2 Surfactant structure of the organically

where A represents the siloxane and B the EO-polyether, we get a product

(siloxane). These hydrophilic and oleophilic sides interact respectively with

the corresponding hydrophilic or oleophilic parts in the paint formulation

and/or between paint and substrate. Thus substrate and/or pigment wetting

is improved.

12.9.3 Concentration of the products on top of the coating

Another well-known surface tension-related effect is that in liquids the

parts with lower surface tension tend to migrate to the top of the liquid

(liquid/air surface) and to spread there. During drying of liquid paints

(water or solvent based) the diluent evaporates from the top, increasing the

Surfactant structure of the organically modified siloxanes

. The polyether modification results in products

modified siloxanes

Particularly if the polyether in the modified siloxane is ethylene oxide based

and if there is a linear AB structure, i.e. an end-modified siloxane-polyether

with a significantly hydrophilic side (polyether) and an oleophilic side

surface tension there. The lower parts of the wet film now have lower