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

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226 High-performance organic coatings

contrast, elemental carbon abounds in the earth’s crust in the form of coal,

graphite, and diamonds.

Illustrating the differences in reactivity is the ready hydrolysis of silane

and silicon tetrachloride, as opposed to the complete inertness of carbon

tetrachloride towards water:

SiH

+ 4H

O → Si (OH)

+ 4H

SiCl

+ 4H

O → Si (OH)

+ 4HCl

CCl

+ 4H

O → no reaction

These are only a few of the many chemical similarities and differences

between carbon and silicon, but they are adequate for illustration.

Three different methods are now mainly used by industry to produce the

basic chlorosilane intermediates from which polysiloxanes are made. The

three methods are (1) the direct method, (2) the Grignard method, and (3)

12.2 Direct method for producing polysiloxanes

In the direct method the intermediate chlorosilanes are prepared directly

presence of a metallic copper catalyst.

This reaction can be represented by the following equation, where the

letter R represents a methyl group:

RCl + Si

Cu catalyst

⎯→⎯⎯⎯

mixture of SiCl

, RSiCl

SiCl

, R

SiCl, R

silane can then be hydrolysed and polymerized (see Section 12.5).

By substituting chlorobenzene (phenyl chloride) for methyl chloride and

using a silver catalyst in place of the copper, phenyl chlorosilanes are

produced.

The direct method was discovered by E. G. Rochow in the General

Electric Research Laboratory in 1940 and was patented by General

Electric.

Si SiO

12.1 The siloxane bond.

the olefin addition method.

from finely divided silicon metal by reaction with methyl chloride in the

This mixture of various silanes can be separated by distillation. The specific

Polysiloxane coatings for corrosion protection 227

12.3 Grignard method for producing polysiloxanes

In the Grignard method, organic magnesium chloride is reacted with silicon

tetrachloride to yield chlorosilanes. This reaction can be represented as

follows:

2RMgCl + SiCl

→ R

SiCl

+ 2MgCl

This is the most versatile method but it is expensive and hazardous because

the reaction is carried out in an ether solution.

producing polysiloxanes

the following reaction:

SiHCl + CH = CH CH

322

Peroxide or

platinum catalyst

⎯→⎯⎯⎯⎯⎯ CCH SiCl

rosilanes may be produced, but any further discussion of this phase of the

chemistry is inappropriate here.

12.5 Polymerization

Regardless of the method used to produce the chlorosilanes, the subse-

quent steps are very similar. As indicated, any given reaction produces

many reaction products, but certain controls can be effected to minimize

the yield of the least desired materials. Whatever the mixture, it must be

fractionally distilled to separate, with a high degree of purity, the individual

products. From this point on, it is the ingenuity of the product development

chemist that determines the type and properties of the polysiloxane end

products. From the variety of available chlorosilanes, certain ones are

selected in various proportions, blended, hydrolysed and polymerized to

produce many different types of polysiloxane materials.

As we have already noted, organosilicon compounds are very reactive.

Accordingly, so are the chlorosilanes. The next manufacturing step after

blending the chlorosilanes is the hydrolysis, or reaction with water, to form

the corresponding alcohols, called silanols. In certain instances alcohols

may be used in place of water to form alkoxy intermediates. The silanols

are then bodied by a condensation reaction, with the liberation of water,

several things, one of the most important of which was the original choice

of chlorosilanes.

12.4 Olefin addition method for

The third method is the olefin addition method which, as the name implies,

consists of the addition of an olefin to a chlorosilane. This is illustrated by

Several modifications of this method are possible, whereby different chlo-

to produce a polysiloxane. Whether it be a fluid, gum or resin depends on

228 High-performance organic coatings

anes is the types of polymers that may be produced. We have already noted

that the initial manufacturing reaction produced a mixture of mono-, di-,

and trichlorosilanes. All three could be hydrolysed and polymerized indi-

vidually, but none would have much value as an end product. However,

each contributes to the product into which it is incorporated according to

its individual chemical properties.

The monochlorosilane yields the monosilanol and polymerizes only to

the dimer stage:

Hydrolysation: (CH

)

SiCl + H

O → (CH

)

SiOH + HCl

Polymerization: 2(CH

)

SiOH → (CH

)

Si–O–Si(CH

)

+ H

Since this is the extent of polymerization of the monosilanol, it forms con-

venient terminals for higher polymers and is appropriately named a chain

stopper.

The dichlorosilanes similarly yield disilanols, but these can be, and are,

polymerized to all types, from short to very long straight chains of linear

Hydrolysation: (CH

)

SiCl

+ 2H

O → (CH

)

Si(OH)

+ 2HCl

Polymerization: 2(CH

)

Si(OH)

→ (CH

)

(OH)Si–O–

Si(OH)(CH

)

+ H

This equation illustrates the polymerization of just two molecules, but any

number could be added onto the chain through the terminal OH groups.

The chain-stopping material which we have just discussed would terminate

the chain.

Finally, we have polymers derived from alkyl and aryl trichlorosilanes.

These are three-dimensional and generally are so hard and brittle that

they are of little value in themselves. The brittleness can be tempered by

copolymerizing some of the intermediates that yield linear polymers. By

judicious blending and copolymerizing we produce three-dimensional

crosslinked polysiloxane resins:

Hydrolysation: CH

SiCl

+ 3H

O → CH

Si(OH)

+ 3HCl

Polymerization: 4CH

Si(OH)

→ CH

(OH)

Si–(O–Si(CH

)

(OH))

–O–Si(OH)

+ 3H

This polymer, derived from only four molecules, the siloxy units, is repre-

sentative of a very large polymer extending in three dimensions. With linear

polysiloxane polymer chains attached, at various points where OH groups

are now shown, this would be representative of a polysiloxane resin.

One final consideration in the fundamental chemistry of the polysilox-

polymers. These are the bases of the fluids and gums, and in turn of the

various other products made from the fluids and gums:

Polysiloxane coatings for corrosion protection 229

As said above, if alcohols are used instead of water in the hydrolysation

step we would get alkoxy functional polymerizates instead of silanol func-

tional ones.

12.5.1 Co-condensation with organic resins

As an interesting corollary, note that the key to this polymerization is the

hydroxyl or alkoxy group. Organic resins such as alkyds, epoxies, polyesters

or acrylics are characterized by hydroxyl groups. It is by means of conden-

sation through these hydroxyls that, for example, alkyd/polysiloxane copo-

lymers are made:

organic polymer−OH + HO–polysiloxane → organic polymer–O–

polysiloxane

of the copolymerization reaction. More details about this process can be

found in Section 12.6.1.

12.5.2 Interjections

Firstly, at this point it is desirable to emphasize a very sharp and very

the resins, characterized by a bulk of trifunctional silanes, are three-

a difference most important to coatings formulators, manufacturers and

ing’ or other surface defects in subsequently applied coatings of any type.

This is not true for polysiloxane resins. Coatings based on them can be

overcoated with impunity with more polysiloxane resin-based coatings or

with conventional, organic resin-based coatings. Equally important is the

fact that the equipment used for processing polysiloxane resin-based prod-

ucts is not contaminated in any way and may be used interchangeably for

see Section 12.8.

Secondly, as said before, all the reactions described before for silanol

(OH) functional polysiloxanes and/or silanes apply also for alkoxy (OCH

or OC

) functional types.

Finally, instead of the mainly mentioned methyl (CH

) group, a phenyl

group plays a major role in polysiloxane resins for protective coatings.

Of course, this is extremely simplified but it is nevertheless characteristic

dimensional. But a more significant difference is found in their behaviour,

of the film in which they are incorporated and/or be responsible for ‘crawl-

all types of coatings manufacture. It is also not true for organic modified

important distinction between polysiloxane resins and polysiloxane fluids,

or oils. Structurally, the fluids are two-dimensional, linear polymers, while

users. Even a slight excess of certain polysiloxane fluid can cause cratering

polysiloxane fluids, which play a major role as, for example, flow promotors:

230 High-performance organic coatings

12.6 Siloxanes for protective coatings

Polysiloxane resins are distinguished from most organic resins by their

use in (protective) coating applications. We focus basically on two main

points: the strong chemical bond and the low surface energy. The following

overview compares the bonding energy of siloxane structures with organic

carbon structures:

• Si–O: 373 kJ mol

−1

• Si–C: 310 kJ mol

−1

• C–O: 293 kJ mol

−1

• C–C: 243 kJ mol

−1

This strong chemical bond leads to interesting properties for coating appli-

cations, i.e. high heat resistance and long-term weather resistance (see next

sections).

The surface tension of polysiloxanes is approximately 20 mN m

−1

, water

has 70 mN m

−1

and organic resin-based coatings are in the range 30–

40 mN m

−1

. Hydrophobicity, release properties, good substrate wetting and

This is more about this in Sections 12.7. and 12.8.

Concerning the properties of polysiloxane, we also need to consider the

relationship between property and composition. As shown above in Section

step leads to mono-, di-, tri-, and tetrafunctional silanes, which are the basic

units for all polysiloxane syntheses. For resin production the di- and tri-

functional silanes are almost the only units used. Monofunctionals are used

to a certain extent to control the reaction – the molecular weight. We speak

about the T/D ratio. Furthermore, we have heard that R might be alkyl,

phenyl or another organic group. Since this chapter is about polysiloxanes

for protective coatings, particularly about resins which can be used as

binders or to modify organic binders, it is mainly the methyl and phenyl

groups that play a role, and the other groups can be neglected at this point.

We speak here of the M/P ratio.

Based on the above, we now come to the property:composition relations.

An increased amount of phenyl groups increases compatibility with organic

resins, heat resistance and weather resistance; while an increased amount

of methyl groups gives higher hydrophobicity, dirt repellence and release

properties. The more trifunctional units there are, the higher is the hard-

ness, the faster is the drying, but also the more brittle are the respective

coatings. Trifunctional units also improve compatability with organic

properties.

specific properties and performance, which make them highly valuable for

12.2, the most important technical process, the ‘Direct Method’, in the first

improvement of flow and levelling are results of the low surface energy.

polymers. Difunctional units control flexibility but also reduce drying

Polysiloxane coatings for corrosion protection 231

12.6.1 Weather resistant coatings

Organic resin-based coatings are subject to thermal or UV-initiated oxida-

tion, and combined with the impact of chemicals and/or high humidity when

exposed to the environment they will lose gloss, change colour and become

brittle. As said before, the strong chemical Si–O bond of the polysiloxanes

leads to superior weather resistance, i.e. better gloss and colour retention

and less degradation from attack by chemicals or high humidity. This is true

not only for straight polysiloxane resins; they also improve the weather

resistance of organic resins if used in combination with them. Chalking,

polysiloxane resins are used in the said combinations. These combinations

could be blends where the organic resins and the polysiloxane resin are just

added to the formulation during the paint manufacturing process. The reac-

tions as shown in Section 12.5.1 will then take place during the coating

drying process and a while later. Better results can be achieved when the

co-condensation of the organic resin and the polysiloxane resin are done

prior to the coating formulation.

Co-condensation of siloxane–organic resins

Polysiloxane resins can be chemically combined with most of the available

groups to condense with the silanol, which characterizes this resinous inter-

mediate. Processing is in accordance with conventional methods currently

in use in the resin industry.

The one-stage process for a silicone/alkyd resin, using fatty acids, may

are charged to the kettle at the start, and the copolymer is formed directly.

In the event that whole oil is used in place of fatty acids, an alcoholysis of

als, including the silicone intermediate, are added and the one-stage cook

is preformed and cut with some solvent, and then in the second stage of the

cook it is coreacted with the silicone intermediate. Moderate variations in

procedure, to accommodate particular situations, may be made without

altering the net result.

The progress of the cook, whether in one or two stages, can be followed

by making a simple, clear-pill test. A drop of the cooking resin is removed

from the kettle and placed quickly on a glass plate, and then examined for

clarity. This is done periodically during the cook until a clear cold pill is

observed. This indicates complete homogeneity of the kettle charge. This

test is most important in the one-stage cook or in preparation of the organic

portion for a two-stage cook. In the latter, when the silicone intermediate

gloss and colour retention are significantly improved if more than 20% of

film-forming materials so long as they contain sufficient reactive hydroxyl

the triglyceride is accomplished first. Next, the remaining resin raw materi-

proceeds as above. Alternatively, in the first stage an alkyd or other polymer

be either a fusion or solvent-reflux cook. In either case, all of the ingredients

232 High-performance organic coatings

is added, there will be a clear pill at once, in many cases, because of the

compatibility of the silicone portion with the organic (alkyd, in particular)

portion.

Again, in the one-stage cook or in the alkyd resin preparation for a two-

periodic acid number determinations. It is most important that these resins

are cooked to low acid numbers if they are intended for use as vehicles for

pigmentation. The low acid number is essential for good pigment compati-

bility and stability.

Progress of the degree of polymerization is determined by periodic

stroke-cure tests on a 200°C cure plate. For short to medium oil length

resins, cure rates of 10–20 seconds by the above test indicate a stage of

polymerization that is safe from gelation and will have at least reasonably

good shelf-life stability. Stroke-cures below 10 seconds point up the need

to check the cook promptly to avoid gelation, and even if this is not immi-

nent, to avoid poor shelf stability. With longer oil length resins, cure rate

is not as critical. Even considerably longer cures than 20 seconds on the

plate are still indicative of good curing resins because of the oxidation

potential of the oil content.

Within the limits of safe cooking as indicated by cure-plate tests, the

resin can be carried to such a degree of polymerization that will yield the

desired solution viscosity. The time to reach this stage will vary depending

on several factors, among which are desired solids concentration and

desired solvent. Periodic withdrawals of resin (or resin solution if a solvent

cook) should be made and the resin cut to desired solids with preselected

solvent. A quick viscosity check can be made on small samples through

the use of Gardner–Holdt viscosity tubes or any other device that one may

choose. The important factor is speed, so that too much polymerization

does not occur between the sampling and the decision that the cook is

completed.

Silicone/alkyd copolymers made with polysiloxane resins may be cut in

a variety of solvents and/or solvent blends, depending on the silicone

content and oil length. Silicone/epoxy resins will always utilize substantial

quantities of polar solvents because of their more demanding solubility

requirements. Short oil silicone/alkyds, regardless of silicone content,

total solvent) n-butanol. The selection of solvent for medium oil length

copolymers will be dependent on such things as silicone content, actual oil

length within the medium range, and desired viscosity. The range of

n-butanol through pure aromatic hydrocarbon to an aromatic/aliphatic

blend. Long oil alkyd/silicone copolymers can be cut in hydrocarbon blends

or, in some cases, in straight aliphatic solvents. Examples of formulations

are shown in Tables 12.1 and 12.2.

stage copolymerization, the progress of the esterification is determined by

should be cut in aromatic solvents, preferably fortified with 20–33% (of

solvents then can vary from aromatic hydrocarbon fortified with up to 20%

Polysiloxane coatings for corrosion protection 233

Table 12.1 Silicone-alkyd: long oil, oxidizing, 25% siloxanes (OH functional)

Formulation – Part I / Alkyd % by weight

Soja fatty acid 58.3

Glycerol (99.5%) 6.4

Pentaerythritol (mono) 13.6

Phthalic anhydride 21.7

Xylol (for azeotropic distillation) 5–10% of kettle charge

Procedure

Charge all ingredients to kettle, heat to 180°C and hold for 30 minutes. Raise

temperature slowly to 230–235°C over a period of one hour and hold at that

temperature for acid number of 5 or less. Cut with mineral spirits to 65–70%

solids. Cooking time: approx. 5 hours.

Formulation – Part II / Silicone-Alkyd % by weight

Part I alkyd (65% solids) 76.1

Polysiloxane resin (69% solids) (silanol functional) 23.9

Procedure

The alkyd portion is in the kettle as a high solids solution, just as prepared in

Part I. Add the polysiloxane intermediate resin to the kettle. With the kettle

equipped for a solvent cook, raise temperature to 165°C and adjust xylol

Towards end of cook, solution will clear and a clear cold pill will be obtained.

mineral spirits to xylol.

Technical data for silicone-alkydresin

Non-volatiles 65%

Acid number 2

Viscosity 400–600 mPa s

Table 12.2 Silicone epoxide: 25% silicone (C

OH functional)

Formulation % by weight

Epikote 1001 X75 63.00

Polysiloxane, ethoxy functional, 100% active 18.01

1-Methoxy propyl acetate-2 (MPA) 17.14

Monomeric butyl titanate 0.064

Isobutanol for cooling 3.29

Procedure

Charge all ingredients in above-mentioned sequence to kettle. Raise

for 2 hours until desired viscosity is achieved. Add the isobutanol for cooling

and cool to 50°C and cut with white spirit to 60% solids.

Technical data

Viscosity: 1200–2000 mPa s

Solid content: 61%

Acid value: ≤10 ppm HCl

Reduce with xylol and mineral spirits to yield a final solvent blend of 2 : 1

content, if necessary, to hold reflux at that temperature. Cook for 5 to 7 hours.

temperature to 150°C by complete distillation of the overflowing solvent. Cook

234 High-performance organic coatings

Test results

Figures 12.2 and 12.3 show the gloss retention of a long oil alkyd after 3000

hours of accelerated weathering (Weather-o-Meter) and after two years of

exterior weathering in a tropical climate. In both cases we can clearly see

that the silicones improve the gloss retention (weather resistance), and

Straight silicone resins might be an even better option. However, straight

silicone resins are somewhat weak in mechanical properties, while the

or curing conditions. Finally, also the cost/performance calculation speaks

12.6.2 Coil coatings

iron roofs, aluminium window blinds or wall claddings. Beside the use of

play a prominent role. Processes that are similar to those described before

are used to formulate the polyester–polysiloxane copolymer – see Table

12.3. Figure 12.4 shows the Florida test results. Again we see the

100

0 1000 2000 3000

Exposure time (hours)

Gloss retention (%)

Straight silicone resin

Straight alkyd resin

20% silicone resin

50% silicone resin

12.2 Weather resistance with Weather-o-Meter.

Straight silicone resin

Straight alkyd resin

20% silicone resin

50% silicone resin

Gloss retention (%)

100

Exposure time (years)

0 0.5

1.5 2

modified alkyd resin

12.3 Weather resistance with outdoor exposure in Pacific Region.

already with a moderate modification (20% polysiloxane), particularly in

the exterior test, a more than significant improvement can be achieved.

modified resins are close to the organic ones in these properties and drying

more for the modification.

A specific topic is coil coating for exterior applications, such as corrugated

straight polyesters, PVDF or polyurethanes, siloxane-modified polyesters

Polysiloxane coatings for corrosion protection 235

Table 12.3 Weather-resistant silicone-polyester resin: 27.5% siloxane, C

functional

Formulation – Part I / Polyester % by weight

Trimethylol propane 51.00

Glycerol 1.80

Isophthalic acid 32.70

Adipic acid 14.36

Polymeric butyltitanate, 10% in xylene 0.14

Xylene (see formulation)

Procedure

Gradually heat all components together with a small proportion of xylene to

220°C in an agitator and control to an acid value of 9–12 (OH value: 360–400).

Reaction time: 5–6 hours. Continually remove the resultant water together with

the xylene proportion.

Formulation – Part II / Co-condensate =

silicone-polyester

% by weight

Polyester resin, according to Part I 43.4

Polysiloxane, ethoxy functional, 100% 18.6

1-Methoxy propyl acetate-2 (MPA) 18.3

Solvesso 150 10.7

Xylene (see formulation)

After co-condensation, added for controlling the solid content:

Solvesso 150/butanol 8 : 5 9.0

100.0

Procedure

quantity of 1-methoxy propyl acetate-2, together with a small proportion of

xylene to 140–150°C in an agitator to remove residual amounts of water. After

cooling to approx. 90–100°C, add polysiloxane intermediate and possibly 1-

methoxy propyl acetate-2 as well as Solvesso 150. Subsequently stir the

Continually remove the methanol resulting during the reaction. Then cool

down to 90–100°C and control to a solids content of approx. 60% using a

mixture of Solvesso 150 and butanol.

Technical data for silicone polyester co-condensate

Silicon content based on solid resin approx. 27.5%

Solids content approx. 60%

Viscosity, DIN 53 015, 23°C approx. 2.5 Pa.s

Density at 20°C approx. 1.1 g/ml

Acid value 4–6

OH value 130–160

Briefly preheat the polyester resin, possibly predissolved in the indicated

reaction mix under reflux at approx. 165–170°C. Time: approx. 2 hours.