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

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176 High-performance organic coatings

OCH

COH

OCH

OBu

OBuBuOH

BuOH

OBu

HMMM BMF

9.9 Structure of fully substituted methylated (HMMM) and butylated

melamine (BMF) resin.

polyeste r-melamine coatings

OCH

Polyester

OCH

Polyester

hydroxylated polyester

( )

HMMM

9.10 Crosslinking reaction of hydroxyl-terminated polyester with

HMMM.

proposed by Blank [44] is described in Fig. 9.11. Besides protonation of the

methylated methoxy group, also protonation of already formed crosslinks

may take place and results in cleavage of the bond formed between poly-

ester and melamine resin. Evaporation of the alcohol results in shifting the

equilibrium towards the right-hand side and thereby a crosslinked network

forms.

OCH

(+)

9.11 Catalyzing mechanism of the reaction of HMMM with polyols.

Polyester coatings for corrosion protection 177

Previous studies have shown that, under acidic conditions, the primary

groups on the polyester. Additionally, there can be a number of self-

condensation reactions between reactive groups on two melamine rings

[45–50]. The cure rate and reaction pathways are important variables and

changes of these parameters result in coatings with alteration in properties.

A number of reactions during the baking condition of polyester-melamine

peting reactions that take place during cure.

9.7.3 Evaluation of curing condition

As already discussed, the cure reaction between HMMM and the hydroxy-

polyester hydroxyl groups and the crosslinker methoxy groups to create an

process in which a liquid material transforms into a solid layer with simul-

taneous solvent evaporation and crosslinking. The speed of solvent evapo-

ration and crosslinking depends on the baking conditions applied to such

the system under investigation has taken place, otherwise premature failure

of the coating may take place during exposure. In general, the cure condi-

tion can be evaluated using Dynamic Mechanical Analysis (DMA) or

Differential Scanning Calorimeter (DSC) instruments, applying isothermal

or dynamic heating rate and different evaluation time [22]. The mechanical

by the degree of crosslinking during the curing process. It is demonstrated

on the relative rates of co-condensation (between polyester and HMMM)

and self-condensation (of HMMM) reactions, forming coating matrices

with different chemical structures [51].

9.7.4 Properties of the cured coatings

Polyester-melamine cured systems can exhibit moisture resistance and

good mechanical properties and can be extremely hard. The moisture resis-

tance feature of melamine-based coatings, combined with their durability,

may provide advantages for exterior applications. Crosslinked polyester-

melamine coatings are colorless, chemically resistant and resilient. They

to harsh environments. There is a great deal of empirical knowledge relat-

ing crosslinking mechanisms and the effects of conditions on basic physical

reaction is the transetherification of the crosslinker with the hydroxyl

coatings make it difficult to sort out the pathways between different com-

functional polyester involves acid-catalyzed transetherification between the

ether bridge. Therefore, the film formation in such coatings is a complex

a system. It is therefore necessary to ensure that sufficient crosslinking of

the film surface, and chemical resistance of the cured film, may be affected

that the changes in cure condition and composition have a significant impact

provide a tough and durable finish to items that will be repeatedly exposed

properties, such as hardness, flexibility, impact resistance, appearance of

178 High-performance organic coatings

and mechanical properties of isothermally cured polyester-melamine coat-

ings. The ultimate service properties of thermoset coatings are determined

in part by the structure of the matrix and in part by the severity of the

environmental factors to which the coating is exposed. Variations in the

of the coating by affecting the extent of crosslinking and, therefore, the

structure of the matrix. Researchers have described the thermal character-

ization of polyester-melamine crosslinked matrices of various compositions

and cure conditions [51], the effect of composition and cure conditions on

the distribution of melamine during cure [52], etc.

Kojima et al. [53] studied the surface properties of melamine-polyester

amount of the acid catalyst used because of the increasing self-condensation

nature of melamine with increasing catalyst amount. Kanai et al. [54] showed

that amine-blocked acid catalyst enhances surface melamine content,

whereas a non-blocked acid catalyst did not. Hirayama et al. [55] used

content was low and the cure temperatures were relatively high. Researchers

[54, 55] also reported that surface hardness and stain resistance could be

towards the polymer/air interface.

The effect of polyester molecular weight on the surface properties of

investigated by Yoshida et al. [56]. Their results showed that surface

as water contact angle and adhesion. Kawanishi et al. [57] showed that the

ment of melamine in polyester-melamine coatings was attributed to the

difference in free energy between polyester and melamine components [24,

54]. For example, butylated melamine resin possesses lower surface free

energy than polyester and hence a surface enrichment of BFM may take

melamines, because of their higher surface free energy, segregate towards

a moderate surface free energy, showed an intermediate result of surface

segregation behavior. The effect of the polyester and HMMM mixing ratio

below a certain value of this ratio there was no melamine enrichment at

the surface, but a higher melamine concentration was observed in the bulk

[58]. Gamage et al. [52] investigated the effects of the polyester-HMMM

films using X-ray photoelectron spectroscopy (XPS) and showed that the

nitrogen content at the air–film interface increased with an increasing

at the surface of polyester-HMMM films was uniform unless the hydroxy

improved in polyester-melamine films by allowing segregation of melamine

polyester-melamine films cured for a short time at high temperature was

place in the cured film. On the other hand, methylol- and imino-substituted

the bulk of the cured film and hardly exist on the surface. HMMA, having

cure conditions and crosslinker concentrations can influence the properties

attenuated total reflectance (ATR) and showed that melamine distribution

melamine content influences the surface properties of the cured resin such

surface melamine had a negative influence on the adhesive properties and

a positive influence on the stain resistance of coatings. The surface enrich-

also has a favorable influence on the surface segregation behavior, and

Polyester coatings for corrosion protection 179

binder composition and cure conditions on the melamine distribution in

model clear coats during a dynamic rapid cure experiment.

performance

performance:

• Type of acid and di-/polyols selected for making polyester resin

• Hydroxyl number of the polyester backbone

• Melamine concentration and type of melamine crosslinker, e.g. HMMM

or BMMM

• Concentration and type of catalyst

• Curing conditions such as baking temperature and time

• Environment to which coating is exposed.

9.8 Polyester-urethane coatings

Demands for improved interior and exterior quietness as well as good

weathering stability have risen over the years until it has now become one

of the fundamental performance requirements of coatings. Polyester-

urethane (PU) is a well-known coating material that produces good protec-

tion of the substrate on demand. The basic chemistry involved in its synthesis

is the addition reaction of a polyester-polyol with a di-/polyisocyanate,

where the hydroxyl group of the polyester acts as a neucleophile and iso-

cyanate carbon (—N

O) acts as an electrophile (Fig. 9.12).

Different secondary reactions such as the formation of allophanates,

biurets, dimerization (uretidione ring formation) or isocyanurate (trimer)

may also take place depending on the reaction condition. Traditional

approaches in enhancing performance are to select appropriate raw materi-

als to produce polyester-urethane coatings, achieve a good trade-off

in NCO/OH ratio, and control the side reactions. In order to prevent

OCN

NCO

OCN R'

O R

HN R'

NCO

hydroxylated polyester

diisocyanate

(excess)

urethane

group

NCO-capped polyester-urethane

9.12 Basic reaction scheme for polyester-urethane formation.

9.7.5 Parameters influencing polyester-melamine coating

The following parameters influence the polyester-melamine coating

180 High-performance organic coatings

premature gelation of the coating composition, either the polyisocyanate

must be kept separate from the resin in what is known as a two-package or

two-component coating system, or the highly reactive isocyanate groups on

the curing agent must be blocked (e.g., with an oxime or alcohol). Blocked

polyisocyanates, however, require higher temperatures (e.g., 150°C or

more) to unblock and begin the curing reaction. The volatile blocking

agents released during cure can possibly adversely affect the coating prop-

erties, as well as increase the volatile organic content for the composition.

In this part of the chapter, we will discuss fundamental aspects of polyester-

urethane coatings [59].

9.8.1 Structure of isocyanates

Isocyanates are highly reactive chemicals of low molecular weight contain-

the number of —N

O groups in the molecule into monoisocyanates

(one NCOs), diisocyanate monomers (two NCOs), or polyisocyanates

(multiple NCOs). Diisocyanate monomers are important because the two

NCO groups allow them to undergo direct polymerization reactions with

alcohols to form PUs. The self-condensation reaction of diisocyanates

as aromatic (one or more aromatic rings) or saturated (aliphatic and

alicyclic), the term aliphatic isocyanate referring to saturated isocyanates.

Toluene diisocyanate (2,4- and 2,6-TDI), methylene diphenyl diisocyanate

(4,4′-MDI), and their higher polymers are commercially the most important

aromatic isocyanates, whereas hexamethylene diisocyanate (1,6-HDI),

isophorone diisocyanate (IPDI), methylene dicyclohexyl diisocyanate

MDI) and their higher polymers are the most important aliphatic iso-

cyanates [59, 60].

Figure 9.13 lists the chemical structures of some major isocyanates of

commercial importance. Aliphatic isocyanates such as those based on HDI

are used mostly in external paints and coatings due to their excellent resis-

tance to chemicals, good abrasion resistance, and superior weathering char-

acteristics such as gloss and color retention. Aromatic isocyanates such as

TDI and MDI are used where a fast curing rate is essential.

9.8.2 Reaction of polyester polyol with isocyanate

The discovery of the excellent catalytic effect of organometallic compounds

as well as of the metal carboxylates on the reaction of hydroxyl compounds

the reaction of an isocyanate with an alcohol, many effective urethane cata-

lysts are available commercially. Those used most often are tertiary amines

ing the functional group —N

O. Isocyanates are classified based on

results in the formation of polyisocyanates. Isocyanates are also classified

with isocyanates has been of great significance for the synthesis of PUs. For

Polyester coatings for corrosion protection 181

and organotin compounds. Dibutyltin dilaurate, dibutyltin diacetate, dibu-

tyltin dichloride, tributyltin chloride, butyltin trichloride, stannous octoate,

stannic chloride, triethyltin chloride, trimethyltin bromide, tetraethyltin,

otin compounds are widely used as catalysts. Of the many metals available,

tin compounds like stannous octoate (SnOct) and dibutyltin dilaurate

(DBTL) are the most popular. Organometallic catalysts act as Lewis acids,

and are generally thought to function by interacting with basic sites in the

isocyanate and polyol compounds. They form an intermediate complex

with an isocyanate group and a hydroxyl group of the polyol. Researchers

[61, 62] have suggested that the metal carboxylates as well as organometal-

lic species catalyze the isocyanate–alcohol reaction by formation of a

ternary complex. Robins [63] has elaborated this concept and suggested

that the metal acts both to increase the electrophilic character of the iso-

cyanate carbon through coordination with the isocyanate group and to

OCN NCO

NCO

2,4-TDI

OCN

NCO

OCN

NCO

4,4'-dibenzyl diisocyanate

4,4'-methylene bis(phenyl isocyanate)

(CH

)

NCO

IPDI

IPCI

TMXDI

NCO

OCN

DIMP

IMCI

NCO

HDI

OCN

NCO

polymeric-MDI

HDI-trimer

(isocyanurate)

O O NCO

NCO

OCN

Desmodur-RFE

(CH

)

(CH

)

NH HN

(CH

)

(CH

)

HDI-biuret

(CH

)

NCO

OCN

Asymmetric-trimer

(CH

)

NCO

9.13 Structure of different isocyanates widely used in the coatings

industry.

dibutyltin difluoride, triethyltin fluoride, cobalt naphthenate, etc., as organ-

182 High-performance organic coatings

bring the reactants into close proximity through the formation of a ternary

complex (Fig. 9.14(1,2,3,4)) [64, 65].

Some of the more commonly used tertiary amines are N,N-dimethyletha-

nolamine (DMEA), diaminobicyclooctane (DABCO), triethyl amine

(TEA), triethylene diamine (TEDA), bis(2-dimethyla minoethyl) ether

(BDMAEE), N-ethylmorpholine, N′N′-dimethylpiperazine, N,N,N′,N″,

N″-pentamethyl-diethylene-triamine (PMDETA), N,N-dimethylcyclohex-

ylamine (DMCHA), N,N-dimethylbenzylamine (DMBA), N,N-dimethyle-

thylamine, N,N,N′,N″,N″-pentamethyl dipropylene triamine (PMDPTA),

tions depends upon their chemical structure. The basicity and steric hin-

the rate of isocyanate reactions. The catalytic activity of tertiary amines

generally increases as the basicity of the amine increases and the steric

shielding of the amine nitrogen decreases. In addition to these factors, the

structure of the tertiary amine also has a profound effect on the promotion

of different reactions [66–69].

The formation of biurets from the reaction of an isocyanate and an aro-

matic urea requires vigorous conditions in the absence of a catalyst. Under

these conditions, where the temperature may be in the range of 100–150°C,

side product formation also takes place [70].

M M R'NHCOR

R'NCO

slow

–

ROH

fast

(1)

(2)

(3)

(4)

fast

C δ

–

9.14 Mechanism of metal catalysis in urethane formation.

etc. The efficiency of tertiary amine catalysts (Fig. 9.15) in isocyanate reac-

drance of tertiary amine are considered the two main factors influencing

Polyester coatings for corrosion protection 183

9.8.3 High-solids coatings

Solvent-borne one- and two-component coating materials, for the most part

thermally curable, play an important role in industrial applications.

However, the reduction or elimination of VOCs has been a main task of

paint research for many years due to environmental requirements and

regulatory pressure. Thus, the percentage of common solvents containing

coating materials has been reduced despite their well-known and excellent

properties [71]. However, in these systems, the molecular weight of the

polymer is less and reactive functionality has been incorporated therein so

that further polymerization and crosslinking can take place after the coating

has been applied on the material. High-solids coatings (HSCs) currently

used in different parts of the coatings industry are typically thermoset

systems which derive most of their physical properties from the formation

of highly crosslinked networks.

9.8.4 Waterborne polyester-urethane coatings

Waterborne polyester-urethane coatings (WBPUs), as a non-toxic, non-

attention in the coatings sector. An aqueous polyurethane dispersion is a

binary colloidal system in which the particles of PU are dispersed in a con-

tinuous phase, the water. The particle size tends to be of the order of 20–

200 nm, and the particles have a high surface energy. This results in a strong

usually contain a minor component carrying pedant acid (carboxylic acid

RNCO

+ R

NRNCO

RN C O

HOR

RHN C O

RNHCOR

RN C O

9.15 Mechanism of tertiary amine catalysis in urethane formation [59,

61].

driving force for film formation with the evaporation of water. PU ionomers

flammable and environment-friendly material, have gained considerable

184 High-performance organic coatings

or sulfonic acid) or tertiary nitrogen groups, i.e., tertiary amine groups

(<10 mol%) that are completely or partially neutralized or quaternized, to

form salts. Aqueous PU dispersions are of three types: (a) non-ionic: sta-

bilized by hydrophilic polyether chains, (b) cationic: stabilized by tertiary

nitrogen groups, and (c) anionic: stabilized by carboxylic or sulfonate

groups depending upon the type of hydrophilic segments present in the PU

backbone [72].

The preparation and description of aqueous PUDs is well known and

described in a number of reviews [73, 74]. Routes available to prepare

aqueous PUDs are the prepolymer mixing process [75, 76], the acetone or

solution process [77], the ketamine/ketazine process [78] and the hot-melt

process [79].

Crosslinking of PUDs can be used to improve physical properties such

as tensile strength, toughness, thermal resistance, solvent resistance, etc.

Different methods used for the crosslinking reaction of PUDs are as

follows:

• Addition of aziridines or melamines to carboxylated PUDs [59, 80,

81]

• Introducing UV curable groups [82]

• Use of blocked isocyanates [81]

• Addition of functionalized silanes, etc.

• Degree of neutralization [83, 84]

• Type of ionic moiety present [85]

• Neutralization agents [86]

• Counter ion used [59], etc.

The disadvantage of water-borne coatings is their application problems

in the presence of high relative humidity in the atmosphere, which produces

defects in the coating structure. These defects result from the fact that

under conditions of high humidity, water evaporates more slowly than the

mity and pinholes unfortunately often result. Additionally, water-borne

coatings are not as resistant to corrosive environments, as we observe for

conventional solvent-borne coatings [87].

9.8.5 Photo- and thermal cure polyester-urethane-acrylate

coatings

Polyester-urethane-acrylate represents a new class of PU coatings, which

contains polyester soft segments as well as acrylic structural units. The

organic co-solvents of the coalescing aid, which reduces the film formation

capability as well as the film’s physical properties. Poor gloss, poor unifor-

Factors influencing properties of PUDs are as follows:

Polyester coatings for corrosion protection 185

relatively easy polymerization of liquid acrylic polyester-urethane oligo-

mers by UV light represents another convincing proof of their technical

attractive alternative to conventional curing processes because it offers

several distinct advantages over solvent-based processes or thermal-curing

processes, such as high speed, operation at room temperature, lower energy

consumption, and environmental compatibility. The liquid resin is trans-

formed in a very short time into a solid polymer that is absolutely insoluble

in organic solvents and has a high level of thermal and mechanical proper-

ties. UV light initiates free-radical polymerization with very low activation

energy. This allows high polymerization rates at room temperature

[88–90].

Polyester-urethane-acrylates are usually synthesized in a three-step

reacts with a polyester diol to form an isocyanate-terminated prepolymer

in bulk or in the solvent. The prepolymer is then reacted with hydroxyethyl

acrylate (HEMA) or methacrylate (HEA) to form unsaturated terminated

polymer in the second step (Fig. 9.16), which later is self UV/thermal cured

or UV/thermal cured in the presence of other unsaturated monomers or

polymers (step III). Acrylated resins are preferred over methacrylated ones

because of their higher cure rates at room temperature and their lower

oxygen inhibition [91]. Parameters such as functionality of urethane acylate,

type of isocyanate used, polyester polyol components and molecular weight

ality in urethane acylate produces lower reactivity and viscosity but higher

urethane acrylate results in improved reactivity, chemical resistance, hard-

ness and scratch resistance. However, a higher functionality in the polymer

Polyester

NCOOCN

NCO

OCN

diisocyanate

NCO terminated polymer

NHO

(CH

)

Acrylic terminated PU

(hydroxyethyl acrylate)

Polyester

9.16 Preparation of acrylic terminated PU prepolymer for UV cure

coatings [59].

and scientific interest in coating science. The UV-curing process is a very

process. In the first step, an excess molar concentration of a diisocyanate

influence the properties of the cured coatings. For instance, lower function-

flexibility in the final cured coatings. On the other hand, a higher functional