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

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Causes and assessment of failures in organic paint coatings 105

shrinkage stress at higher thickness (above 75 microns) due to rapid surface

drying leaving behind a mud-cracked surface (see Fig. 6.9 on page 110).

Unless formulated otherwise, the thickness of organic or inorganic zinc-rich

epoxy coatings also show failure in the form of alligatoring on ageing when

applied at higher thickness, due to faster drying of the top layer leaving

behind a soft underlayer below it.

6.5 Improper system selection

Apart from those caused by inappropriate formulation, many coating fail-

ures can occur due to improper coating selection. Coating consultants and

and properties, exposure environments, surface preparation standards and

performance expectations before recommending a coating system for a

particular end use. Coatings designed for steel structures may fail rapidly

if applied over concrete or wood surfaces. Similarly, coatings meant for

exclusively interior use should never be recommended for exterior applica-

tion. Coatings suitable for atmospheric exposure may fail completely if used

for immersion services. There are plenty of examples where failure can be

ascribed to wrong coating selection. Alkyd-based coatings are susceptible

6.5 Network formation during crosslinking.

coatings should be restricted to around 75 microns. Highly filled coal-tar

specifiers must have a thorough understanding of coating characteristics

106 High-performance organic coatings

to alkaline hydrolysis, therefore such coatings should not be recommended

for underwater application, buried pipeline protection or direct application

on highly alkaline surfaces, e.g. cement, galvanized iron, concrete or even

organic or inorganic zinc-rich coatings (Fig. 6.6). Coaltar and bituminous

coatings, being thermoplastic in nature, are susceptible to bleeding during

overcoating with solvent-based top coat, as depicted in Fig. 6.7. Thermoset

coatings like 2K epoxy or 2K PU, if used for overcoating thermoplastic

coatings like chlorinated rubber, vinyls, coal tar based coatings, etc., result

in lifting due to swelling of the undercoat by the strong solvents of the top

coat (Fig. 6.8).

Another example of coating failure due to wrong product selection is in

the area of internal lining of tank cars meant for road transportation or

tanker ship linings for chemical cargo transportation. Linings applied inside

tank cars and tanker ships are frequently exposed to different cargo chemi-

cals as well as hot water and steam used for cleaning after unloading or

before reloading. In such situations the lining must be resistant to a broad

spectrum of chemicals that may come into contact with it, otherwise a lining

6.6 Wrong recommendation – alkyd top coat over inorganic Zn silicate

primer.

6.7 Bleeding during overcoating coal-tar based coating with

solvent-based top coat.

Causes and assessment of failures in organic paint coatings 107

suitable for a particular chemical may fail miserably when it comes into

contact with different chemicals. Very often an epoxy lining that shows

excellent resistance to crude petroleum, fuel or lubricating oil develops

hydrocarbons and ketonic solvents. Therefore it is critical that any coating

used for cargo ship lining must be tested in advance for resistance to all

possible chemicals, including washing chemicals.

6.6 Stress-related failure

A coating system is subjected to multiple types of stresses during its envi-

ronmental excursion emanating from different sources during its lifetime.

Stress, indeed, is a source of external energy which can cause some strain or

deformation to the object to which it is applied. Applied coating systems that

under stress and resist failure. Conversely, weaker coatings break in the form

of either cracking or delamination from the substrate when subjected to

stress. Failure is the ultimate means for stress dissipation to relieve the excess

energy. Stress-related failures are more prevalent in high-build multicoat

systems where the area of the weakest link acts as a stress relief centre and

is subsequently manifested as the failure point. Analysis of failure modes

and its interaction with different types of stress-related variables, their origin

factors that can induce coating failure, and these will now be discussed.

6.6.1 Mechanical stress

Several types of mechanical or physical stresses are imparted on the coating

system from the environment in which it is exposed. These include tempera-

6.8 Failure due to swelling of thermoplastic base coat by thermoset-

ting top coat (wrong recommendation).

severe blistering if comes into contact with refined aromatic petroleum

are strong both cohesively and adhesively can retain their film integrity

requires a clear understanding of the specific nature of the coating system

and mode of action. Broadly, there are five different types of stress-related

108 High-performance organic coatings

vibration coming from adjoining areas, impact of falling objects, absorption

or desorption of water from a coated wooden surface, movement of people

ing expansion and contraction. Stress-induced failure always occurs at the

cohesive or adhesive weakest link of the coatings. Coatings that are cohe-

sively strong and also provide high adherence to the substrate can resist

failure, since they can relieve the stress through internal molecular orienta-

tion. Usually epoxy primers are cohesively strong materials and also provide

strong adhesion with steel and therefore withstand stress-related failure

it will show adhesion failure from the base metal under external stress, since

epoxy primer is cohesively strong. On the other hand, cohesively weak

materials like zinc-rich primer, coal-tar epoxy, etc., will fail through internal

splitting within the coat to relieve the stress. Porous substrates like wood

tuation, water absorption and desorption phenomena. Stress due to exces-

sive water absorption causes swelling of the coating, leading to volume

expansion with the resultant effect of delamination. Therefore coatings

stress.

6.6.2 Chemical stress

Chemical stress results from the reactivity of various functional groups

present in the polymers with the external chemicals, leading to breakdown

mers as the binder component, which contains a large number of functional

groups with varying degrees of reactivity with different types of chemicals.

Binders having a polymeric backbone comprising carbon–carbon single

bonds, ethers (

–

), amides or urethane linkages are relatively

more stable towards chemical attack than those containing esters, carboxyl,

hydroxyl, carbon–carbon double bonds or destabilized aromatic rings. This

is why high-performance industrial coatings based on epoxy, phenolics,

vinyls and polyurethane exhibit far superior chemical resistance to conven-

tional oleoresinous coatings. This does not mean that these binders are

permanently indestructible but certainly their degradation process due to

resistant to chemical attack than common organic polymers due to higher

bond dissociation energy of

–

(128 kcal/mole) and

–

(185 kcal/mole) bonds in comparison to

–

(85 kcal/mole) or

–

(86 kcal/mole) bonds in esters.

efficiently. But if the primer is highly rigid due to excessive crosslinking then

of film integrity. Most surface coatings are formulated with organic poly-

ture variation during day and night time, humidity fluctuation, machinery

on painted floors or roof decking, etc. In all such cases the coating system

will first absorb the stress energy and then relieve it by flexing or undergo-

undergo expansion and contraction due to humidity and temperature fluc-

designed for wooden surfaces must be flexible enough to withstand such

chemical attack is much slower. Silicones and fluoropolymers are more

Causes and assessment of failures in organic paint coatings 109

Alkyd and polyester-based coatings are highly susceptible to undergoing

hydrolytic cleavage in the presence of nucleophiles like water, amines and

hydroxyl ions, and therefore such coatings, if applied directly on alkaline

surfaces like concrete, aged galvanized structures, exposed zinc rich coatings

or underwater immersion services or for storage of alkaline chemicals, are

bound to fail (Fig. 6.6). Similarly, oleoresinous binders with oxidative

carbon–carbon double bond (

–

) in the backbone are prone to

progressive embrittlement on exposure, leading to paint cracking and

not be allowed to come into direct contact with highly acidic or highly

alkaline chemicals, due to the amphoteric nature of zinc and aluminium,

unless they are overcoated with a layer of polyurethane top coat.

Many micro-organisms use coatings as a source of nutrients and food and

derive energy from the organic matter under favourable conditions by the

process of biochemical degradation of polymers with the help of enzymes

leached by these organisms. The effect of biodegradation is more pro-

nounced in highly humid environments and diffused north light which

favours damp retention for a longer period. Bio-deterioration of coatings

is manifested initially in the form of ugly dark blackish patches of fungus

and mould, and later affects corrosion resistance and adhesion due to

Poor crosslink density of the paint is the major reason for the poor chemi-

cal resistance. While formulating the coating, the binder and hardener

should be selected and taken in the correct quantity so that no reaction site

would be left after complete curing, otherwise attack by chemicals over the

perature of the binder matrix play a major part in determining the solvent

resistance. If the T

of the binder is below atmospheric temperature, the

free volume of the system is already on the higher side. In such conditions

attack by solvents becomes easier. The polarity of the coating system and

the solvent also affects the solvent resistance.

6.6.3 Internal stress

solvent evaporation the molecules are drawn closer to each other and the

crosslinking process starts. During crosslinking the mobility of polymer

chains reduces considerably, due to the forced internal reorganization and

breakdown of film integrity.

paint film becomes severe. The free volume and the glass transition tem-

Internal stresses build up within the paint film during the process of drying

and curing. Immediately after application the wet film is more fluid and

freely mobile due to its lower viscosity. As the film viscosity increases during

orientation of molecules resulting in film shrinkage. At the beginning the

stress produced due to reorganization of molecules is relieved by film

shrinkage. However, as crosslinking proceeds the film will not be able to

flaking. Coatings such as zinc-rich epoxies and aluminium mastics should

110 High-performance organic coatings

continue to shrink, due to the absence of free volume space, as a result of

which internal stress starts to build up. The fate of the coating will depend

or on what route it takes to dissipate the excess energy coming from the

internal stress. It should be remembered that shrinkage does not create

internal stress; rather, shrinkage is the means or path to relieve the gener-

ated internal stress. But very often the term internal stress is also referred

to as shrinkage stress.

When a two-component thermosetting top coat such as 2K PU is applied

over an improperly formulated primer, the shrinkage stress developed

during the drying of the top coat can result in cracking or adhesion-related

failure. If the primer coat is highly rigid with low extensibility, as in aged

two-component epoxy, then the shrinkage stress cannot be relieved through

(above CPVC) and cohesively weak then stress release will cause failure in

the form of intralayer splitting of primer, which is often witnessed while

overcoating under a cured inorganic zinc-rich coating with two component

epoxy or PU material.

When a coating is applied at very high thickness, excessive shrinkage

stress develops within the coating due to the rapid drying of the top layer

in preference to the underlayer. When such a hard top layer sits over a soft

underlayer, the shrinkage stress generated during the gradual drying of the

defects, namely checking, cracking, alligatoring, mud cracking or even wrin-

kling. Coal-tar based coatings, if applied at higher thickness, tend to alligato r

when exposed to sunlight, since the top layer of coal-tar dries faster than

the bottom layer (Fig. 6.9). Such failures are best prevented by applying

6.9 Alligatoring due to high internal stress; hard top coat over soft

undercoat.

on how efficiently it can relax or withstand the effect of this internal stress

underlayer is passed on to the top layer, resulting in various types of film

the inflexible primer layer and the resultant effect is intercoat failure

between the primer and the top coat. If the primer is a highly filled material

Causes and assessment of failures in organic paint coatings 111

coal-tar coatings coat by coat according to the recommended thickness and

ensuring thorough drying between the coats. Similarly, oxidizing top coats

such as alkyd coatings, if applied over asphaltic material, show crack failure

due to internal stress.

6.6.4 Weathering stress

Weather-induced coating degradation is caused principally by three factors,

namely heat, sunlight and moisture. Sunlight comprises ultraviolet (UV),

visible and infrared radiation, of which UV light has the highest potential

for damage due to its higher radiant energy. The Earth’s atmosphere receives

4–6% of UV radiation, depending on location, which contains mostly UV

A (320–400 nm) and a small percentage of UV B (280–320 nm). The most

destructive short-wave UV radiation (UV C), with a wavelength less than

280 nm, does not reach the Earth’s surface due to absorption in the strato-

sphere. The UV component of sunlight can trigger the photodegradation

reaction of organic coatings, leading to bond breakdown by two processes,

namely direct absorption by UV-active functional groups present in the

polymer backbone and attack by the free radicals produced by the action

of UV with moisture, oxygen and heat. Such light-induced degradation

causes several types of irreversible chemical changes such as depolymeriza-

tion, hydrolytic breakdown, elimination or even embrittlement due to auto-

oxidation, cyclization, etc.

The effect of light-induced degradation of organic coatings is manifested

in the form of surface chalking, gloss dulling, yellowing, colour fading and

sometimes embrittlement due to auto-oxidation. Not all organic binders

and pigments are equally active towards their response to UV radiation.

Epoxy resins are highly susceptible to chalking in the presence of light, due

to their aromatic backbone, in spite of their excellent corrosion and chemi-

cal resistance Therefore for exterior application epoxies are necessarily

overcoated with a UV resistant top coat. Aliphatic polyurethanes are

exceptionally UV stable and as a result show a high degree of gloss and

colour retention in outdoor exposure. Oleoresinous coatings containing

carbon–carbon double bonds are more susceptible to photo-oxidation than

silicone based coatings. In general, rutile-based coatings show much better

resistance against yellowing and chalking than do anatase-based coatings.

Similarly, colour organic pigments belonging to the azo family display far

inferior light fastness compared to those in the quinacridone, benzimida-

zole, arylamine, dioxazine, isoindoline family. Heat-induced stress can

stand this stress.

cause rapid expansion and contraction of coatings, which can finally erupt

as multiple surface cracking if the coating is not flexible enough to with-

112 High-performance organic coatings

6.6.5 Osmotic stress

When two liquids of different concentration are separated from each other

by a semipermeable membrane, more liquid from the diluted side perme-

ates towards the concentrated side till the concentration on both sides

becomes equal. This process is known as osmosis. Osmotic action sets in

due to either concentration gradient or pressure gradient between two

liquids or gases separated from each other by a semipermeable membrane,

and the process continues till the chemical potentials on both sides are

equalized. An idealized high-build, high-performance coating provides an

effective physical barrier to moisture and oxygen due to its very low perme-

is lower in thickness, if it is thinner, it can act as a semipermeable membrane

soluble or water-miscible species between the substrate–coating interface

can dramatically increase the rate of coating failure. Sources of such water-

soluble species can be many, e.g. soluble electrolytes from pigments and

the steel, hand touch, entrapped water-miscible solvents, etc. When a small

quantity of water permeates through the coating and dissolves these species

at the coating–substrate interface, an osmotic cell is created due to variation

in ionic strength between the outer side and back surface of the coating

impurities, and the process continues till the potentials on both sides are

equilibrated. Accumulation of more water at the interface leads to water-

Therefore the formation of an osmotic blister are the resultant effect of

neutralizing the osmotic stress developed due to the creation of a concen-

tration gradient.

time for complete fabrication on site. During this interval a lot of salty

6.10 Osmotic blisters due to presence of water-soluble salts below the

coating.

ability. But if the coating film contains physical defects such as pinholes or

and allow permeation of water through the film. The presence of water-

fillers, hydrophilic additives used in paint, residual salt contamination on

film. This osmotic stress forces more water from the outside to push through

the film and accumulate at the interface to dilute the concentration of

filled blisters, which are often referred to as osmotic blisters (Fig. 6.10).

Shop primers are generally overcoated after a long gap to allow sufficient

Causes and assessment of failures in organic paint coatings 113

contaminant can be deposited on the primer. This must be removed by

secondary surface preparation followed by fresh water washing prior to

overcoating to avoid failure due to blistering. Entrapment of water-miscible

polar solvents like alcohols, cellosolve, etc., in the epoxy-amine based lining

of potable water tanks or ballast tanks can create an ideal environment for

osmotic cell setup when put back into service (Fig. 6.11). Therefore appro-

priate care should be taken to ensure that such water-miscible solvents are

avoided as far as possible during formulation of development or thinning

during application to prevent blister-related failure.

6.7 Application-related failure

In spite of the best material selection and satisfactory surface preparation,

a carefully chosen coating system may fail if adequate care is not taken to

ensure correct application. A number of application-related coating failures

are the direct outcome of carelessness, lack of proper machinery and facili-

ties, poor workmanship, inadequate training and understanding of coating

fundamentals, and absence of structured post-painting inspection processes.

The common defects usually encountered due to poor application practices

are non-uniform thickness, low DFT, poor coverage at edges, runs and sags,

pinholes, craters, dry spray, brush marks, poor gloss, etc. There are several

instances where the best two-component high-performance coatings

undergo failure due to various types of application-related problems, a few

of which are described below:

• Wrong mixing ratio

• Non-adherence to recommended induction time

• Non-compliance to prescribed overcoating interval

• Improper coating thickness

• Use of wrong thinner

• Overbaking/underbaking of stoving paints

6.11 Blistering in ballast tank coating.

114 High-performance organic coatings

• Poor control of spray process

• Inappropriate control of application environment.

6.7.1 Wrong mixing ratio

All two-component thermoset materials are required to be mixed in a

mixing ratio, as stipulated by the paint supplier, can produce various types

cation and a few of them appear afterwards during top coating or subse-

quent exposure. Mixing-related error occurs due to carelessness of the paint

applicator or to unavailability of a suitable measuring device at the site. In

the case of two-component epoxy, if the base quantity is higher than the

hardener, the mixed coating after application will not achieve the desired

crosslink density and consequently solvent and other chemical resistance

will be severely affected. At the same time the applied coating will remain

propensity to develop rundown and sag marks. When such partially cured

soft epoxy undercoat is overcoated with a solvent-borne 2K PU top coat,

the base coat swells due to undercutting by the solvent coming from the

On the other hand, if the hardener component is added to excess in the

case of two-component epoxy-amine based products, then the excess amine

hardener, being a low molecular weight oligomer, exudes to the surface and

reacts with atmospheric moisture and carbon dioxide to form amine carba-

mates, resulting in what is popularly known as ‘amine blush’ in the industry.

Amine blush appears on the top surface in the form of haze, thereby reduc-

ing the original gloss of the coating to a considerable extent. Amine blush

is a loose whitish powdery material on the surface which is not part of the

the shrinkage stress generated during top coat drying will be passed on to

this layer, leading to intercoat delamination. Similarly, if considerable excess

of isocyanate hardener is used during application of 2K polyurethane top

coat, the mixed coating will cure in a different chemical pathway from the

properties.

6.7.2 Non-adherence to recommended induction time

Induction time is more relevant with two-component epoxy materials where

ratio and then the mixed material is allowed to stand for 15–20 minutes

before the start of application. Technically, induction time relates to the time

defined ratio prior to application. Any deviation from the recommended

of paint film defects. Some of these defects surface immediately after appli-

soft due to uncured base component and the film will be tacky with increased

top coat, resulting in film lifting.

integrated film. If a top coat is applied over such loosely attached material,

regular one, resulting in a cured film with substantially different

the base and hardener components are first mixed in the recommended