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

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Using organic coatings for repainting and maintenance 437

aliphatic urethane technology was used, obtaining all the same parameters

durability, colour retention and low temperature cure. Some improvement

allow direct application to corroded steel in aggressive environments. As

with the epoxy system above, this material has been marketed worldwide

for around 12 years, again with no recorded problems regarding cracking,

Both of the substrate-tolerant materials described above meet some key

over all aged coatings.

20.8 Epoxy ‘mastics’

Epoxy ‘mastics’ now appears to be the generic term used for maintenance

epoxies in general and can cover a multitude of performance parameters.

In general they are not formulated via the route described above, but rather

they consist of crosslinked liquid epoxy together with hydrocarbon resin or

When subjected to thermal cycling tests on thermoplastic substrates, crack-

ing can occur, and this has been observed in practice on bridges in both

northern Europe and the USA and Canada, which has led these products

now to be treated with some reservations in this type of overcoating situa-

tion. Generally these products are applied to sweep blasted or hydroblasted

steel, and on old coatings which are thermoset types that have been

abraded.

However, this type of coating does have a number of advantages and

taking, for example, the material sold by International Paints in this area,

the following properties can be obtained:

• Excellent water immersion characteristics

• Excellent corrosion resistance on blasted steel

• Suitable for white oil immersion, etc.

of the above areas and the products designed for rusty steel, although

showing advantages in performance in that situation, do not give the same

difference when used on blasted steel.

Table 20.2 illustrates the difference between the truly surface-tolerant

and the commonly used epoxy mastics.

in corrosion resistance was also obtained by this route, but not sufficient to

teristics and coefficient of expansion. This gives the ability to apply them

This is clearly not possible utilizing epoxy technology and thus flexibilized

of high solids, flexibility and total substrate tolerance, along with good

flaking or other detrimental effects on substrates.

parameters required to achieve tolerance, i.e. flexibility, elongation charac-

other external plasticizer to attempt to gain some degree of flexibility.

The flexibilized substrate-tolerant materials above are not suitable for any

flexibilized coatings, which can basically be applied to any aged coating,

438 High-performance organic coatings

20.9 Current development

In both Western Europe and North America most chemical plants operate

to high standards of prevention of environmental pollution. However, we

have found that as we have started to undertake maintenance painting on

plants in Eastern Europe there can be much more severe substrate con-

tamination, especially with sulphates from sulphur dioxide and hydrogen

sulphide. This has led to further investigation and evaluation of materials,

this time on hand-prepared contaminated rusty steel (from site) and then,

as well as normal testing, also subjecting the material to a sulphur dioxide

cyclic test (Kesternich Test). Materials have now been formulated which

give good performance both in accelerated testing as above and on site in

these aggressive contaminated conditions.

20.10 Moisture-cured urethanes (MCUs)

This type of technology has been used for anti-corrosive paint systems for

around 20 years, but has been relatively limited in use until the last few

years, when MCUs have been started to be used on the extensive US Bridge

Painting Program. From a technology viewpoint this chemistry is very

ties and cure rates, even at low ambient temperatures. Limitations are solids

turing due to the inherent water sensitivity of moisture curing systems.

Table 20.2 Comparison between ‘epoxy mastics’ and ‘substrate tolerant’

coatings

Coating Mandrel bend test,

14 days’ cure

(low diameter,

Temperature

cycling over

thermoplastic/alkyd

(number of cycles

before cracking)

Overcoatable

with PU

(5 = excellent,

0 = no adhesion)

Epoxy mastic

– I

13 mm 9

Epoxy mastic

– A

13 mm 9 1

Epoxy mastic

– B

13 mm 13

Flexibilized

epoxy

5 mm >20 4

Flexibilized

urethane

5 mm >20 5

interesting in that it is relatively easy to formulate for different film flexibili-

content, which can limit easily applied film thickness (typical volume solids

50–60%), and cost, but probably most importantly, difficulties in manufac-

more flexible)

Using organic coatings for repainting and maintenance 439

Most currently used systems are based on either MDI based prepoly-

mers or higher molecular weight TDI based prepolymers. In this instance

the priming coat again relies on being of low viscosity to give good pene-

tration into any corrosion on the substrate, but only a limited (30–40

microns) thickness is applied. Generally two subsequent coats are needed

to give a reasonable barrier thickness. The primer used in this situation

is normally pigmented mainly with aluminium; however, it has been

found from recent laboratory testing that surprisingly good results have

been obtained on hand-prepared rusty steel, and hydroblasted rusty steel,

using a primer pigmented with zinc dust and micaceous iron oxide, and

alternative pigmentation can also give good results. Application was to

hand-prepared weathered rusty steel and exposure was for four years

at an industrial site in Germany. It can be clearly seen that where the

moisture-cured urethane system, and that the epoxy largely hides

the corrosion and having poor adhesion is likely to delaminate at some

point.

An active programme is now underway following a re-evaluation of our

old coatings than the ‘epoxy mastics’ described earlier, despite their higher

solvent content. This is almost certainly due to the ability to ‘build in’ stable

to become more brittle with age due to migration of low molecular weight

plasticizing species.

This technology gives potentially a single-pack high performance solvent-

tolerant system, which is especially useful in low temperature, high humid-

ity environments. It is interesting to note that, in general, moisture-cured

urethanes appear to outperform epoxies on a thickness for thickness basis,

are used.

20.11 Rust converters

This type of material is often promoted heavily in the literature and appears

to be the solution to many of the problems of painting rusty steel:

• minimizes effort required in surface preparation;

• improves performance;

• ‘chemically converts’ an inherently unstable substrate into a stable

one.

There are basically two types of these materials, one based on tannic acid

as the active component, which can sometimes act simply as an active

full system is present, significantly better adhesion is obtained from the

laboratory results with moisture-cured urethanes and field evaluations

where they have been shown to have significantly less detrimental effect on

70–80% of epoxy dry film thickness giving equal performance when MCUs

flexibility into urethane systems, whereas in many instances epoxies tend

440 High-performance organic coatings

stable PVDC resins), and the other on phosphoric acid chemistry.

Experiments have shown that the claim of tannic acid reacting with rust

is correct; however, it does not stabilize voluminous corrosion products or

scale but rather tightly bound corrosion adjacent to the steel surface. To

give longer-term protection, both variants need to be overcoated with at

least some type of barrier coating system. Comparison of this on the same

substrate as for the rust converter shows little advantage to using the rust

converter.

The other approach of using phosphoric acid type materials can give very

good substrates for painting, but is more suitable for dipping treatment of

exposure time and temperature, uniformly grey surfaces of phosphate can

be produced by this process that are ideal for painting and give excellent

environmental and health and safety issues associated with the used of acid,

limit this approach to small areas and articles.

20.12 Conclusions

From all of the above review, it can be seen that the basic requirements for

successful surface-tolerant coatings are:

•

• Adhesion

• High solids

• High barrier effect.

However, it must be realized that even when an excellently formulated and

developed coating is applied to a rusty surface, this surface is not only

potentially extremely variable in character but it is also thermodynamically

unstable, with the corrosion process already underway. Given this scenario,

we can slow down this process by using the correct coatings, but we cannot

stop it. It is thus unlikely that without thorough blast cleaning (or preferably

ultra high pressure hydroblasting) a suitable surface, free of contamination,

will be achieved. Thus it is unlikely that, even with all care and precautions

taken, maintenance painting will give the same coating lifetime as that

obtained from new construction systems. With hand or power tool cleaning

only, the expected time before repaint is likely to be only 30–50% of the

time for a suitable high quality new construction system in the same envi-

ings are used.

This is found to be especially the case in severe offshore/marine environ-

ments where correctly applied high quality coatings on clean grit-blasted

chemical and sometimes in conjunction with a film-forming resin (e.g. acid-

small articles rather than using on large areas in the field. Given the correct

performance. The practicality of achieving this reaction in the field, and the

ronment, and significantly less than these times (10–20%) if incorrect coat-

Long term coating flexibility – no embrittlement on ageing

Using organic coatings for repainting and maintenance 441

steel can be expected to last 12–15 years or more, whereas experience has

shown that maintenance coatings applied onto power tool cleaned, and

possibly contaminated, surfaces often need further repair within 3–5 years.

This difference tends to be less obvious in milder environments where there

is less electrolyte, much less surface contamination, and considerably less

nance systems can easily last in excess of 10 years, even when applied to

power tool cleaned steel.

The old adage that the performance and lifetime of a coating system is

directly related to the degree of surface preparation holds as well today as

ever, and the ideal of a coating directly over rust with long life has still not

been achieved, and questionably never will be.

stress on the film due to continuous wetting and drying out. Here mainte-