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

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Key issues in applying organic paint coatings 15

1.8 Epoxies

Epoxy is perhaps the most commonly used resin for industrial protective

coatings. It is derived from bisphenol-A (BPA) and epichlorhydrin (ECH).

This combination of raw materials yields a series of related compounds that,

prior to crosslinking, have an epoxide group at each end of the molecule,

and an alcoholic hydroxyl group as a mid-chain pendant. Crosslinking takes

place preferentially through the terminal epoxy groups and to the mid-

chain hydroxyl groups. Figure 1.7 shows the formation of a basic epoxy resin

monomer, diglycidyl ether of BPA, from the starting raw materials, ECH

and BPA, while Fig. 1.8 shows the formation of diglycidyl ether of BPF, from

the starting raw materials of ECH and bisphenol-F (BPF). These two epoxy

monomers are the base for the large number of epoxy-based coatings which

are used for corrosion protection. Figure 1.9 shows the role of various

BPA

NaOH

ECH

Dichlorohydrin of BPA

Diglycidyl ether of BPA (badge)

1.7 Formation of an epoxy (diglycidyl ether of BPA) from epichlorhy-

drin and bisphenol-A.

16 High-performance organic coatings

components of epoxies in providing functional improvement in the proper-

ties. As an example, the adherence of an epoxy coating is provided by polar

from the chain length of the polymer.

The protective properties of epoxy coatings result from reaction with a

very active reagent called a hardener. The role of this reagent is to provide

crosslinking between the polymer chain and the hardener. It is rightly called

becomes solid. The most common hardeners for epoxies are usually a poly-

amine or a polyamide. Figure 1.10 shows a typical crosslinking reaction with

BPA and an amide curing agent.

The crosslinking results in making the coating rigid. Such a coating has

lower permeability and hence better corrosion resistance. The extent of

crosslinking depends upon the number of epoxide groups in the epoxy resin

BPF

NaOH

ECH

Dichlorohydrin of BPF

Diglycidyl ether of BPF (bpfdge)

1.8 Formation of an epoxy (diglycidyl ether of BPF) from epichlorhy-

drin and bisphenol-F.

a hardener since, as the reaction progresses, the coating hardens and finally

groups such as

–

OH. The stretchability and flexibility of a coating comes

Key issues in applying organic paint coatings 17

chain. The greater the number of epoxide groups, the higher will be the

crosslinking. The resultant coating will be rigid, acts as a strong liner to the

substrate and can provide high chemical, erosion and abrasion resistance.

One such epoxy resin is novolac epoxy resin, which is formed by the reac-

tion of formaldehyde with phenol, having three phenolic groups per

monomer [10]. When it reacts with epichlorhydrin, it forms a resin having

three epoxide groups (Fig. 1.11).

This resin on crosslinking forms a hard, dense, rigid coating, which pro-

vides greater hardness as well as resistance to high temperatures. The high

temperature resistance of these coatings is effective up to about 250°C. The

resin also has high chemical resistance but high brittleness and lack of

Polyamide and polyamine hardeners result in different properties. While

the polyamines (diethylene triamine, hydroxy ethyl diethylene triamine,

bishydroxy diethylene triamine) are relatively small molecules with a low

molecular weight compared to the epoxy resin, they lead to tight crosslink-

ing and high chemical and moisture resistance. Polyamide curing agents are

the condensation products of a dimerized fatty acid with the polyamine.

With amine groups at the ends, they allow crosslinking to occur lengthwise

irrespective of the fact that the polyamide molecule is much larger. The

prominent differences in their properties are shown in Table 1.3.

Diglycidyl ether of BPA (badge)

Crosslinking site

Poor UV stability

Adhesion

Flexibility

1.9 Various properties that epoxies gain from structure of polymer.

toughness and flexibility.

18 High-performance organic coatings

1.10 Mechanism of crosslinking.

Table 1.3 Differences in the coating properties achieved by different hardeners

With polyamide as hardener With polyamine as hardener

Larger molecule with respect to

epoxy

Smaller molecule with respect to

epoxy resin

Tight crosslinking, making the

coating rigid

High crosslinking, still retains good

Lower water resistance Excellent water resistance

Excellent chemical resistance Good chemical resistance

High solvent and alkali resistance Lower solvent and alkali resistance

Low UV light resistance Better UV light resistance

flexibility and flow

Key issues in applying organic paint coatings 19

HCHO

NaOH

n = 1.6

n= 1.6

EPH

EPN resin

Formaldehyde

Novolac resin

1.11 Formation of Novolac epoxy resin which has three epoxide

groups per monomer.

1.9 Coal-tar epoxies

Coal-tar enamel coatings are cheaper coatings which have excellent barrier

properties. However, these coatings have poor dielectric properties and

higher permeability. They also have poor mechanical properties such as

at higher temperatures limits their applicability to a limited temperature

range. In order to improve the properties of coal-tar coatings, an epoxy is

added to produce a coal-tar epoxy coating. The level of epoxy varies from

formulation to formulation, with a range from 25% to 40% [11]. The epoxy

resin is packed separately in one pack, while the second pack consists of

mixed just before use. The crosslinking reaction is the same as that described

earlier for epoxy resins. Active hydrogens from the amine crosslink with

the epoxide groups and, in some conditions, the hydroxyl groups of the

well-refined coal-tar pitch with a hardener polyamide. The two packs are

epoxy resin. The coal-tar acts as a filler within the crosslinked epoxy matrix.

brittleness, and their tendency to go hard at lower temperatures and to flow

This gives better flexibility and good dielectric properties to the coal-tar.

20 High-performance organic coatings

The coating thickness, which can range from 125 to 300 µm, has better

toughness, adhesion, UV resistance and thermal stability and extremely

high moisture resistance. The polyamide-cured coal-tar epoxies have higher

commonly used in walls immersed under water such as sea walls and bar-

riers, as well as on piping above ground level.

1.10 Polyurethane coatings

The urethane monomer is formed by the reaction of an isocyanate

(

–

O) with an alcoholic group (

–

OH) as shown in Fig. 1.12. A

cyanate group for the active hydrogen of the polyolhydroxyl. The extent of

tion of both the isocyanate and polyol materials, temperature, etc. The

reaction also requires a catalyst.

The important constituents of a urethane coating are isocyanates. The

properties of a polyurethane coating depend upon the type of isocyanate

as well as on the type of polyol. One of the greatest advantages of polyure-

thanes is their resistance to UV. For this reason, they are used as top coats

on various epoxy and other intermediate coats. Two main categories of the

isocyanates are:

• Aromatic isocyanates

• Aliphatic isocyanates.

As can be seen in Fig. 1.13, an aromatic isocyanate is one which has an

unsaturated hydrocarbon or benzene or phenolic group. All isocyanates

Polyol Isocyanate

Polyurethane

1.12 Formation of urethane monomer from the reaction of an isocya-

nate with a polyol.

quick reaction occurs because of the high reactivity and affinity of the iso-

crosslinking depends on a number of factors, such as the type and configura-

water resistance, are more flexible and are easier to topcoat. They are very

Key issues in applying organic paint coatings 21

except the second in Fig. 1.13 are aliphatic. The main difference in the two

types is in their UV resistance. While polyurethanes formed using aliphatic

isocyanates are resistant to UV light, aromatic polyurethanes are severely

affected in UV light. They chalk and lose gloss and color when exposed to

UV light. The aromatic isocyanates have properties very similar to that of

epoxy with the exception of poorer chemical resistance.

The most important monomeric diisocyanates used for coatings are toluene

diisocyanate (TDI), 4,4-diphenylmethane diisocyanate (MDI) and 1,6-hexa-

methylene diisocyanate (HDI). They are normally converted into isocya-

nate-terminated polymers or adducts of polyols such as hydroxyl-terminated

polyesters and polyethers. The molecular weight of these isocyanates can be

increased by self-reaction in the presence of catalysts to form dimers and/or

trimers. Aliphatic isocyanates are more expensive and react more slowly but

allow the formulation of non-yellowing, light-stable coats with a high gloss

sists of large molecules (commonly acrylics, polyesters, polyethers, epoxies,

vinyls and alkyds) that have been reacted with di- or polyfunctional alcohols

such as propylene glycol, trimethylolpropane, pentaerythritol and others.

The most common industrial polyurethane coatings are two-component

component is the hydroxyl-terminated polyol. Just before application the

isocyanate and polyol constituents are mixed, and crosslinking proceeds via

the isocyanate-hydroxyl functions and liberates carbon dioxide gas.

In general, the characteristics and properties of the urethane coating

depend predominantly on the properties of the polyhydroxylated coreac-

tant. Tremendous improvement in the properties of the polyurethane

coating takes place when polyol has various prepolymers such as acrylic,

polyester, polyethers, epoxies or vinyls. Acrylic urethanes are perhaps the

most widely used urethanes for corrosion protection and atmospheric

service. They have excellent weatherability, gloss and color retention and

good chemical and moisture resistance. Polyester urethanes are tightly

crosslinked and have great chemical and moisture resistance, but they are

chemical environments in which acid fumes or highly corrosive conditions

NCO

OCN

N (CH

)

NCO

OCN

NCO

1.13 Structures of various isocyanate molecules.

finish. One of the most important aliphatic isocyanates is HDI [9].

Polyols react with isocyanates to form a polyurethane film. A polyol con-

coatings. The first component is the isocynate prepolymer while the second

not as flexible and tough as the acrylic urethanes. They are used in exterior

22 High-performance organic coatings

are encountered. They also have high gloss, are light fast and have a good

appearance.

1.11 Moisture-cured polyurethanes

One of the limitations of polyurethane coatings is that they are very sensi-

tive to moisture. Therefore, for application of polyurethane coatings, the

relative humidity must be low (preferably lower than 60). This makes them

offshore. A moisture-cured polyurethane is based upon the reaction of iso-

cyanates with the hydroxyl group in water (H–OH) to form a unique class

of coatings known as moisture-cured urethanes. These are single-pack coat-

ings. They use an isocyanate prepolymer that, when applied, reacts with

rapid rate of reaction, aromatic isocyanates are used almost exclusively in

moisture-cured urethanes. When applied by air spray, they form a tough,

moisture-cured polyurethane is used as a primer. One of the best applica-

tions of moisture-cured polyurethane is as a maintenance coating, because

moisture-cured polyurethanes can be applied on partially cleaned surfaces,

followed by an intermediate epoxy or aromatic polyurethane coating and

an aliphatic polyurethane as top coat [9].

There are several other resins which are used sparingly in several differ-

ent applications. The most common are polyesters and vinyls. Polyester

coatings include oil-free, acid or hydroxyl functional polyester resins. They

are mainly composed of coreacted di- or polyhydric alcohols and dibasic or

tribasic acid or anhydride, and are thinned by normal solvents. They are

relatively expensive resins and are used for long-lasting applications such

as automotive clear coats and steel coil coatings.

The term vinyl resin commonly refers to polymers and copolymers of

vinyl chloride, which is a cheap monomer, whose polymer possess good

and extrusion properties are required. They are also used in heavy duty and

marine coatings where toughness, elasticity and water resistance are the

main requirements. They also have application as thick coatings on coil strip

highest durability coil coating on aluminum.

1.12 High-performance coatings

A high-performance coating can have one or more of the following

characteristics:

usually difficult to apply in highly humid environments such as coastal and

moisture in the air to form a tough, hard, resinous film. Because of their

glossy, highly protective chemical and solvent-resistant film. These days

color, flexibility and chemical resistance. Vinyl resins are used in coatings

for strips in venetian blinds and for bottle tops where extreme flexibility

building facings. Polyvinylidene fluoride dispersion is now used as the

Key issues in applying organic paint coatings 23

• High corrosion resistance in highly aggressive environments

• Can be applied in high relative humidity (more than 90%)

• High thickness (DFT) can be achieved per coat

• Provides resistance to weathering, especially UV light

• Good mechanical properties such as high impact strength, abrasion

resistance and hardness.

Some situations in which high-performance coatings can be applied are

in the splash zone of offshore structures, underwater in sea and rivers, on

damp surfaces, in highly acidic and alkaline environments such as pickling

units, in the battery chamber of a ship or submarine, in sewage disposal

units, seawater transport through jetties, etc. The requirements of such paint

coatings are:

• High chemical resistance (as in epoxies)

• Solventless so that a large thickness can be obtained per coat

• Long life and structural stability that can be obtained by addition of

A good example of a high-performance coating is a solventless epoxy

system. Solventless resins are those which do not use any solvent in paint

formulation. There can be several advantages of using such coatings:

• These coatings are eco-friendly as they do not contain any solvent and

are thus free of volatile organic content (VOC).

• They can be applied to give a higher coating thickness per coat.

A comparison of solventless and solvent-borne coatings is summarized in

Table 1.4.

The most common solventless coatings are epoxy-based. These are used

as linings in highly corrosive environments, such as for seawater jetty pipes

for condenser cooling, tank linings and offshore structures. These epoxies

are usually low molecular weight polyfunctional liquid epoxy resins, and

the hardeners are polyamines which provide crosslinking, giving highly

tough, corrosion- and erosion-resistant linings. Presence of a tertiary amino

phenolic catalyst, such as tri(dimethylaminomethyl) phenol can produce

polymerization of the epoxy resin with itself. Silicone resins may be added

Further improvement in the coating life can be achieved by adding glass-

increasing the tortuous path as discussed earlier, and thus enhance the life

erosion resistance. They are used in highly corrosive and erosive applica-

tions such as in the splash zone on offshore structures. Their short curing

time coupled with high thickness per coat make them the right choice for

pigments such as glassflakes and fibers.

as flow agents, and dibutyl phthalate can be added as a plasticizer [9].

flakes, which not only improve the strength of the coating but also help in

of the coating [12]. Such glassflake epoxy coatings have high corrosion and

24 High-performance organic coatings

Table 1.4 Comparison of solventless and solvent-borne coatings

Property Solventless epoxies Solvent-borne epoxies

State Mostly liquid epoxy resin Solid or semisolid

epoxy resins

Hardener Low-viscosity amines Medium/high-

viscosity amides

Pigments Yes – depending upon

color and other

required properties,

e.g. color pigments are

very common,

added for high

strength

Pigments and

additives essential

to obtain useful

properties such as

color, UV absorp-

tion, etc.

Solvents Usually no solvent is

required, but some-

times thinners are

added to adjust the

viscosity for smooth

application

Solvent essential to

dissolve resin,

pigments and

additives

Application Usually by airless spray Using brush, roller

and airspray

Plural

components

Always Can be single-

component or

two-component

Curing Chemical reaction of two

components

By solvent evapora-

tion as well as

chemical reaction

Pot life Usually small Relatively larger

Edge protection Solventless coatings

because of low

viscosity as well as

due to application by

high-pressure airless

technique protect the

edges very well and

the paint reaches

down into the pits,

even after coating on a

highly pitted surface

Edge protection is

very poor and

uneven surface is

seen after paint

coating on a pitted

surface

Safety and

health hazard

No risk of explosion,

good skin protection

as there is no or only a

little VOC.

Forced ventilation

necessary to reduce

explosion risk due

to VOC, and skin

protection needed

and filling of pits

giving a smooth finish

glassflakes are usually