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

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206 High-performance organic coatings

rarely used, except for hand-applied materials. One particular area where

Flake size and thickness are only two of the issues involved in obtaining

means that it is impossible to state simply the requirement for an amount

concentration to resin ratio (CPVC). If the same quantity by weight of a

be at least two and a half times that of the thicker one. There may now be

exceeded. In any case the viscosity may become so high when changing

impossible.

has been chosen it is important to optimise the addition level. That level

will depend upon the type of resin being used and what other pigments or

A further consideration is whether or not coupling or bonding agents are

coating to the substrate. Adhesion plays a substantial role in the perfor-

into the resin is also a very important facet in obtaining performance, both

from a corrosion resistance point of view and in mechanical performance.

There are a number of alternative materials which claim to improve

adhesion within coating systems and these should be evaluated for the resin

system in use. Silane adhesion promoters have been used for many years

consequent performance. This improvement in performance is seen as

both an increase in some mechanical properties and a decrease in moisture

vapour transmission. In thermoset resins it is possible to get substantial

improvements in performance, simply by adding the silane chosen to the

thermoplastic materials, however, addition during processing is generally

coatings and fillers, generally used as high build repair materials.

insufficient resin to wet out this increased area and the PVC level may be

fillers are being used in conjunction with it.

mance of organic materials in corrosion protection. The bonding of fillers

in the glass fibre industry to improve bonding within GRP laminates and

blockages. Large flakes also tend to produce rougher surface finishes. It is

therefore common that flakes of around 500 μm and below are used for

spray application, while flakes above this size, i.e. as large as 1500 μm are

such flake is used is in the manufacture of highly viscous trowel-applied

performance. The quantity of glassflake added and particle distribution are

also critical. It is obvious that if thin flakes of glass are used there are many

more flakes than if thick ones are used for the same weight, and therefore

the surface area to be wetted with the thin flakes is vastly greater. This

of glassflake, e.g. 20% by weight. It may be possible to add 20% by weight

of flake at a thickness of 5 μm and not exceed the critical pigment volume

flake at 2 um thickness were added, then the surface area of this flake will

from the thick flake to the thin flake that addition at the same level becomes

It is obvious from the preceding statements that once a thickness of flake

used to provide better adhesion of the glassflake to the resin and of the

resin component either just before or just after the glassflake is added. With

not possible and it is the glassflake which has to be pre-treated with silane

during manufacture. It is noticeable that pre-treated flake will often improve

207

the bonding performance not only in thermoplastics but also in thermosets,

to a higher level than that achievable by adding the silane indirectly via the

resin. Where the silane is added to the resin, it has been shown that there

is a critical level and the optimisation peak is often very steep. This value

applies for each particular resin, glass thickness, particle distribution and

thixotroping agent will also affect the optimisation level).

There are number of manufacturers of silane around the world, all of

whom produce silane of various types and functionalities. Using vinyl func-

tional silane for unsaturated esters, epoxy functional silane for epoxy based,

amine functional silane for epoxy hardeners, etc., would appear obvious.

These will bond the adhesion promoter into the relevant polymer matrix.

However, you will need to perform rigorous evaluation work on the fully

formulated systems. It has been found, for example, that some silane types

can adversely affect other properties such as thixotropy, and that such

effects can be time dependent. Best results may also be obtained by using

a blend of silane types, rather than just one material.

Where silane is added by pre-treating the glass, the level of silane used

is not achieved. It is also observed that with pre-treated glass a much higher

without exceeding the CPVC level. One downside of using pre-treated

It seems fairly obvious from the previous paragraphs that barrier pig-

added at a nominal value if good results are going to be obtained. However,

what knowledge do they base this information on?

Even if the characteristics of a particular resin and formulation are

of addition could in many formulations exceed the CPVC level and the

is more sensible if performance criteria, rather than formulation criteria,

are used to specify coatings.

consistent thickness, which may be varied for different purposes from

around 10 μm thickness to as low as 100 nanometres, almost limitless

addition level (it should also be noted that other fillers or additives such as

known, product formulation rather than performance specification can be

a dangerous method of assessment. For example, a specification could state

tion is specified. It is also possible that a coating with very high performance

could be precluded from being used under such a specification regime. It

Glassflake coatings for corrosion protection

is not so critical, provided that saturation of the flake causing agglomeration

level of glassflake can be added to the resin and particularly for thermosets,

flake, however, is the cost and change in safety hazard classification.

ments with high aspect ratios and in particular glassflake cannot simply be

some specifiers of coatings containing glassflake often state the minimum

loading of glassflake and a thickness for the product to be applied at, but

‘Epoxy with a minimum glass flake loading of 20% by weight.’ This level

coating would therefore give better performance at lower glass flake load-

ings. In addition, neither the flake thickness nor the particle size distribu-

With modern production methods glassflake can now be produced at a

208 High-performance organic coatings

particle size distributions being possible. The effects of thickness, particle

size, volume concentration, etc., were formerly little understood. A sub-

with differing particle distributions. Some of the results were surprising,

others expected. Because testing was carried out over a wide range of

properties and resin systems and looked not just at diffusion and corrosion

resistance, some interesting parameters were discovered.

were those used within the GRP industry. However, before commencing

work on these resin systems, bear in mind that these resins were generally

prior to service. Performance values and physical properties are often

quoted based on post-cure schedules which are wholly unrealistic, espe-

order to optimise the performance.

10.4

• Moisture vapour transmission (MVT)

• Water absorption

• Atlas cold wall (osmotic blister) testing

• Cathodic disbondment

• Glass transition (T

) (DSC and DMTA)

• Abrasion resistance

• Chemical resistance

• Mechanical properties.

wall testing and cathodic disbondment testing. Other properties will gener-

ally fall into place, once these key factors have been optimized. When for-

As can be seen from the results shown in Table 10.1 using an unsaturated

ester resin, the quantity versus permeation curve is very steep, with a 1%

change in the addition level changing the permeation rate from 10.61 to

3.46. A further addition of glass changes the permeation rate for the worse,

developed for use in thick film systems, which are then generally post-cured

thinner films than GRP and are often not post-cured prior to service. It

may therefore be necessary to significantly modify the resin chemistry in

is fully quantified. The following tests are considered to be of particular

changes in addition levels may have significant effects on performance.

stantial amount of work has been carried out, initially evaluating glassflake

coating formulations, using flakes of differing thickness and diameters, and

The resin systems initially used as carrier resins for glassflake systems

cially for field-applied coatings. Glassflake coatings are applied at relatively

Testing the performance of glassflake coatings

It is crucial that whatever glassflake is used in the coating, its performance

benefit when comparing the glassflake and coating performance:

Tests of particular relevance to glassflake coatings are MVT testing, cold

mulating glassflake coatings it should be borne in mind at all times that the

high surface area to volume ratio of glassflake particles means that modest

209

but only marginally so; further additions (not shown here) showed a pro-

gressive worsening. As would be expected, subsequent additions show a

rapid increase in MVT. These results are typical as the CPVC level is

approached and exceeded, and this effect is also shown in Fig. 10.5.

The steepness of the MVT performance curve is not always so high. In

This type of addition/performance curve is preferred, as it not only allows

some deviation for production tolerances but also allows the formulator to

promising the other aspects.

Figure 10.5 shows two different curves. The top curve represents glass at

a thickness of approximately 5 mm, whereas the bottom curve shows glass

at a thickness of approximately 2.2 mm. There are also some differences in

curve on the thicker glass is much steeper and the best loading value is at

Glassflake addition % by weight

7.66

g/m

/24 h

Thickness 5 μm ± 2

1.3 g/m

/24 h

Thickness 2.2

μm ± 0.3

Test method: ASTM D1653

Test temperature: 25°C

Film thickness: ±1mm

Perm. inches×10

–4

Perm. inches×10

–5

12 14 16 18 20

21 22

10.5 Water vapour transmissions for milled glass.

tested in accordance with ASTM D1653

Average result of 5 samples (perm. inches ×10

−5

)

14% 10.61

15% 3.46

16% 3.64

look at other properties, i.e. mechanical or fire retardancy, without com-

Glassflake concentration

Glassflake coatings for corrosion protection

Table 10.1 Moisture vapour permeation at various glassflake addition levels,

some instances the curve is a gradual slope at both ends with a flat bottom.

particle distribution, with the thicker glassflake having a wider range of

nominal diameters than that of the thinner glassflake. The difference in

performance with the change in glassflake type can clearly be seen. The

210 High-performance organic coatings

approximately 23.5% by weight. A move either side of this addition level

the optimum level of addition being at approximately 18%.

appropriate addition level will vary from resin to resin. In some viscous

epoxy systems, addition values of only 8–12% may be required to optimise

release of solvent in a similar way to that by which it reduces vapour trans-

mission. As can be seen from Fig. 10.6, the addition of small percentages

coating. Similar results have also been found in water-borne coatings.

formulated polyester, vinyl-ester and epoxy systems. These include:

• Very low permeation values

• High temperature resistance

• Excellent chemical resistance

• Excellent adhesion (including immersed adhesive strength)

• Resistance to undercutting at damaged sites (essential to prevent the

coating peeling back and disbonding from localised damaged sites. This

Tested according to BS2782

Part 5: Method 513A

Film thickness: 175 microns

Granular filled

sample RC4

Glass flake filled

sample RC3

010

50 60 70 80 90

100

Time (hours)

Moisture vapour transmission

(g/m

/24 h)

changes the MVT performance quite significantly. Conversely the thinner

It must be noted that this graph is specific to this resin system and the

used as discussed earlier in the text. There are many benefits to correctly

10.6 Moisture vapour transmission reduction using 2% glassflake.

glassflake only requires an addition of between 14% and 18% to obtain an

MVT which is almost a magnitude better. The curve is much flatter with

the coating. It must also be stressed that such high levels of glassflake are

only suitable for 100% polymeric resin binders. Glassflake will prevent the

of glassflake to solvent-borne coatings can be advantageous, in this case 2%

of glassflake being substituted for a granular filler on a solvent-borne epoxy

10.5 Applications of glassflake coatings

The largest use of glassflake within the coatings industry is within heavy-

duty polymeric lining, where much higher percentages of glassflake are

211

is not uncommon when damage occurs on solvent-based epoxies, ure-

• High abrasion resistance

• High impact resistance

• High tensile strength

• Machinability

• Dimensional stability during immersed service

• Repairability

• High resistance to cathodic disbondment

• Ability to be used over a wide range of substrates

• Durability, offering very long service lives

• Cost effectiveness.

These polymeric coatings are often used instead of expensive metallurgical

solutions. They also allow for refurbishment or protection of capital equip-

ment at a fraction of their cost. They can be used not only for corrosion

protection but also to execute repairs to existing equipment. Such repairs

can be carried out even on seriously corroded substrates, due to the high

tensile strength and composite stability. This work is often combined with

The coatings are often used as composite repair materials which can be

machined back to the original dimensions of the component on the protec-

tion and refurbishment of engineering components.

The ranges under which these materials have been used around the world

make all their applications too diverse to list in this text. But taking just

offshore and onshore applications within the petrochemical industry. On

offshore platforms areas protected by vinyl ester and polyester systems

have included rig jackets, cellar decks, riser pipes, mud tanks, de-aerators,

superstructure steelwork, pumps, valves, heat exchangers, tube faces, water

boxes and potable water makers, to list but a few.

including seawater, where service lives of correctly formulated materials in

excess of 25 years are not uncommon. Developments in technology are

30 years or longer in aggressive service applications.

thanes, rubbers and glass fibre reinforced systems)

engineering modifications, to improve the design and functionality of plant.

separators, filter vessels, production pipework, wellhead manifolds, ballast

tanks, chemical storage tanks, cooling water pipework, fire water mains,

allowing these coatings to be specified now with life expectancies of 20 or

Glassflake coatings for corrosion protection

• Good flexibility

one area of work, glassflake materials have a proven track record of use in

tanks, effluent tanks, pre-load tanks, helicopter decks, drill decks, effluent

Glassflake coatings have a proven track record in many environments,

212

Fluoropolymer coatings for corrosion protection in

highly aggressive environments

A S HAMDY, Central Metallurgical Research and

Development Institute (CMRDI), Egypt

11.1 Introduction

Organic coatings are used widely to improve the corrosion protection per-

formance of ferrous and non-ferrous metals against corrosive environments.

Fluoropolymers are considered to be one of the most popular high perfor-

mance organic coatings for their exceptional resistance to solvents, acids,

and bases.

rine atom, combined with the unique physical and chemical properties that

force for increasing the research interest in this area [1]. Many kinds

aggressive environments [2].

Fluoropolymers are high performance polymers with diverse applica-

ropolymers have replaced many metals and alloys for maintaining the

purity of processing streams in the chemical processing, food, pharmaceuti-

cal, petroleum, polymer, semiconductor, and pulp and paper industries.

tance to galling, non-stick, non-wetting, electrical resistance and abrasion

resistance.

bases, inorganic oxidizing agents and salt solutions. They are also inert to

organic compounds such as organic acids, anhydrides, aromatics, aliphatic

carbons, and mixtures of these organic compounds. However, in order to

(1) good wet adhesion and (2) low aggressive solution/oxygen permeability.

has played a distinctive role in many significant and highly diverse techno-

extensive application in various fields as highly protective coatings against

A fluoropolymer is a polymer that contains fluorine. Fluorine chemistry

logical developments over the last decades. The singular nature of the fluo-

the fluorine imparts to any fluoride-containing compounds, are the driving

of fluoropolymer coatings such as PVF, PVDF, PEVE, etc., have found

tions. By virtue of their outstanding chemical resistance properties, fluo-

Other benefits of fluoropolymer coatings include reduced friction, resis-

Generally, fluoropolymers are inert to strong mineral acids, inorganic

hydrocarbons, alcohols, aldehydes, ketones, esters, chlorocarbons, fluoro-

be effective against corrosion, such fluoropolymer coatings need to exhibit

Fluoropolymer coatings for corrosion protection 213

strate in the presence of aggressive solution. By low aggressive solution and

oxygen permeability is meant the ability to retard aggressive solution and

plays an important role in improving the adhesion and the corrosion protec-

This chapter provides an extensive study on the effect of surface treat-

aluminium composites in marine environments.

11.2 Corrosion protection of aluminium

metal matrix composites

The material under investigation was aluminium metal matrix composite,

AA6061 T6–10% Al

(v/v). Aluminium and its alloys are used in many

industrial applications such as in the automotive and aerospace industries

[11]. The demand for materials of superior mechanical, thermal and electri-

cal properties has focused attention on aluminium metal matrix composites

(AMMCs) [2]. AMMCs usually consist of an aluminium alloy metal matrix

with reinforced non-metallic phases such as alumina, silicon carbide, graph-

ite, etc. However, AMMCs have a less homogeneous structure than alloys

due to the presence of reinforcing particles. Such an inhomogeneous struc-

ture can enhance the pitting corrosion susceptibility, as well as cause a

preferential dissolution of the interface between the matrix and the rein-

forcing particles [12, 13].

In this chapter, the corrosion protection performance of AA6061 T6–

2 3

coat, was investigated in 3.5% NaCl solutions. A comparison between com-

system and novel surface treatments based on vanadia, ceria or molybdate

coating conditions.

systems, were designed to overcome the effect of intermetallic particles on

the aluminium surface and to offer a thick layer of aluminium oxide enriched

with vanadia, ceria or molybdenum oxide. These pretreatments are char-

acterized by superior corrosion resistance even after 30 days of immersion

in 3.5% NaCl aqueous solutions [14–19], which enables them to be used as

alternatives to epoxy.

By good wet adhesion is meant the adhesion of the polymer film to a sub-

oxygen permeation through the polymer film to the underlying substrate

A modified version of such coatings was prepared by adding iron oxide

[3]. Therefore, the surface pretreatment prior to fluoropolymer coating

tion performance of any fluoropolymer coating system [4–10].

ment prior to fluoropolymer top coatings on the corrosion resistance of

10% Al O composite, coated with a commercially available fluoropolymer

coating based on epoxy primer and clear or pigmented fluoropolymer top

mercial coating systems consisting of epoxy primer + clear fluoropolymer

salts + clear fluoropolymer system will be carried out under scratched

The novel surface treatments, based on clear fluoropolymer coating

and titanium oxide as pigment to the fluoropolymer top coat. Pigments such

214 High-performance organic coatings

as aluminium, titanium oxide, iron oxide and bronze pigments are solid

materials that are used to impart colour, control gloss, improve perfor-

such as titanium oxide and iron oxide can impart special effects, chemical

resistance and/or colour to a coating and are also used to decrease the

porosity of the coatings [20, 21].

When selecting paint pigments, it is important to understand the condi-

tions under which the coating will be used. The pigment must maintain its

original colour under the conditions to which the coating will be subjected.

The pigment must be heat resistant if the coating will be subjected to heat.

The pigment must be capable of withstanding UV light and humidity if the

coating will be used in an open atmosphere. The pigment must also have

corrosion and chemical resistance to the environment to which it will be

exposed. Other important properties, such as colour, opacity, tint strength,

brightness, toxicity, and oil absorption are important factors that must be

considered in selecting a pigment. The choice of pigment depends on the

colour desired and the durability and performance needed.

resistant coatings

ings with new coatings systems based on treating the aluminium composite

substrate with novel corrosion resistant surface treatments prior to applying

ponents is as follows:

butyl acetate and consists of a proper combination of PFPE, HDI and

IPDI trimer macromers. FLBZ 1074 is the trade name of a commercial

• The second novel surface treatment component is a vanadia, ceria or

molybdate-treated specimen. Vanadate-treated samples (Vanadia)

dilute potassium hydroxide solution, followed by oxide thickening in

treated samples (Ceria) were prepared by immersion in boiling water

followed by immersion in a cerate salt solution. Molybdate-treated

samples were prepared by pickling the aluminium composite samples

in dilute potassium hydroxide solution followed by oxide thickening in

• The first component is a coating system consisting of solvent based

were prepared by first pickling the aluminium composite samples in

boiling water, and finally by immersion in a vanadia solution. Cerate-

mance, such as infra-red reflectance or corrosion resistance, or simply

occupy space in a paint film.In general, pigments in fluoropolymer coatings

11.3 Application of fluoropolymer as corrosion

The present study compares commercial epoxy-based fluoropolymer coat-

the fluoropolymer top coat. The composition of the coating systems com-

epoxy primer (80 μm) + clear fluoropolymer top coat of FLBZ 1074

(40 μm); 40.5% fluorine. FLBZ 1074 was obtained as a clear solution in

fluoropolymer top coat produced at ausimont S.p.A, Milan, Italy.

Fluoropolymer coatings for corrosion protection 215

boiling water followed by immersion in a molybdate salt solution for

1 h at open circuit potential (Molybdate 1) or at −500 mV (Molybdate

2). After the sample treatments, a commercially available top coat of

To investigate the effect of pigment addition on the corrosion resistance

was prepared by adding iron oxide and titanium oxide as pigment to the

used to evaluate the coating performance after immersion in 3.5% NaCl

solution. The salt spray chamber test was used to measure the durability of

such new coatings. The adhesion performance was also measured. The

mechanism of protection was investigated.

11.4 Evaluating coating performance using

electrochemical impedance spectroscopy

EIS has been successfully applied to the study of corrosion systems for 30

years and has proved to be a powerful and accurate method for measuring

tage of EIS over other laboratory techniques is the possibility of using very

being measured. To make an EIS measurement, a small amplitude signal

is applied to a specimen over a range of frequencies.

The expression for impedance is composed of a real and an imaginary

part. If the real part is plotted on the z-axis and the imaginary part on the

y-axis of a chart, we get a ‘Nyquist plot’. However, Nyquist plots have one

major shortcoming: frequencies are not designated on the plots. Therefore

other impedance plots such as Bode plots are important to make a correct

interpretation. In Bode plots, the impedance is plotted with log frequency

on the x-axis and both the absolute value of the impedance (|z| = z

) and

phase-shift on the y-axis. Unlike the Nyquist plot, the Bode plot explicitly

shows frequency information.

In this work we used both Nyquist and Bode plots to evaluate the coating

performance after immersion in 3.5% NaCl solution. According to the EIS

coated samples increased dramatically after less than 30 days of immersion

in NaCl under scratched conditions (after drilling a 1 mm hole at the coated

Conversely, the samples pretreated with vanadia, ceria or molybdate salts

of the above coatings systems, a modified version of such coating systems

corrosion rates, especially for coatings and thin films. An important advan-

small amplitude signals without significantly disturbing the properties

surface). Severe filiform corrosion was observed around the scratched area.

fluoropolymer clear FLBZ 1074 was applied to all samples.

fluoropolymer top coat. Electrochemical impedance spectroscopy, EIS, was

11.4.1 Clear fluoropolymer top coat

results, the corrosion rates of the epoxy primer-based, clear fluoropolymer-

prior to applying a clear fluoropolymer top coat showed outstanding