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

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166 High-performance organic coatings

range, making the polyesters a diverse group for coating applications.

Synthesis of aliphatic polyesters by polycondensation of diols with dicar-

boxylic acids traces back to Carothers’ work in the 1930s. However, the low

in obtaining high molecular weight polymers, had prevented wide usage of

aliphatic polyesters as polymeric materials for a long time [2, 3]. Later on,

with the advent of new monomers and synthetic protocols, the synthesis of

tailor-made polyester resins and their application span in coatings

increased.

9.3 Raw materials and reactions

The structure of the polyester can be varied by the ratio of di-, tri- and

tetra-functional monomers and degree of conversion. The polyester is

either acid-functional or hydroxy-functional depending on the mole ratio

of the ingredients used. The composition of the di/tri-functional monomers

can be varied perpetually, allowing the structure and properties of the

resulting polyesters to span over a very broad range [1, 4]. Diacids and diols

used for the preparation of polyester resins are far too plentiful to be men-

tioned here. Nevertheless, some structurally relatively simple diacids, diols,

tri- and tetra-functional monomers widely used in the coatings industry are

shown in Fig. 9.2.

Some of the most common glycols or polyols used for polyester prepara-

tion are ethylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,

1,2-propylene glycol, 1,6-hexanediol, 1,5-hexylene glycol, isosorbide, ester-

diol, styrene glycol, bisphenol A, oligo(ethylene glycol)s, neopentyl glycol

OHHO

COOH

OO C

n(diol)

n(diacid)

ester group

polyester

n(H

COOH

C O

O O

terephthalic acid

ethylene glycol

polyethylene terephthalate

Example

9.1 Synthesis of polyester resin.

melting points inherent to aliphatic polyesters, together with the difficulty

Polyester coatings for corrosion protection 167

esterdiol (ED)

trimethylol propane (TMP)

OHHO

glycerol

1,4-cyclohexanedimethylol (CHDM)

n = 2 ethylene glycol

= 3 1,3-propanediol

= 4 1,4-butanediol

= 5 1,5-pentanediol

= 6 1,6-hexanediol

1,2-propanediol

2,2-diethyl-

1,3-propanediol

2-butyl-2-ethyl-

1,3-propanediol

2,2-dimethyl-

1,3-propanediol

(NPG)

2-methyl-

1,3-propanediol

pentaerythritol

isosorbide

n = 1 malonic acid

= 2 succinic acid

= 3 glutaric acid

= 4 adipic acid

= 5 pimelic acid

= 6 suberic acid

COOH

phthalic acid

isophthalic acid

terephthalic acid

malic acid

aspartic acid

COOH

1,3-cyclohexanedi-

carboxylic acid (1,3 CHDA)

1,4-cyclohexanedi-

carboxylic acid (1,4 CHDA)

phthalic anhydride

hexahydrophthalic

anhydride (HHPA)

trimellitic anhydride

HOOC

9.2 Different di-, tri- and tetra-functional monomers widely used in

the preparation of polyester resin for coating applications [1].

168 High-performance organic coatings

(NPG), trimethylol propane (TMP), di-trimethylol propane (diTMP),

mono-pentaerythritol (MPE), di-pentaerythritol (diPE), glycerol, 1,4-

cyclohexanedimethanol (CHDM), etc. Widely used diacids are malonic

acid, succinic acid, glutaric acid, adipic acid, pimetic acid, suberic acid,

azelaic acid, sebacic acid, isophthalic acid (IPA), phthalic acid, terephthalic

acid (TPA), fumaric acid, oxalic acid, aspartic acid, malic acid, etc. Trimellitic

anhydride, phthalic anhydride, hexahydrophthalic anhydride (HHPA),

etc., as dianhydride are widely used to prepare polyester coatings.

The most straightforward route to improve mechanical properties and to

minimize hydrolytic instability is the introduction of aromatic units into the

polyester main chain. The aromatic diacid compounds are used to increase

the glass transition temperature (T

), hardness, and chemical resistance.

This is why TPA is such a tremendously important building block in com-

mercial thermoplastic polyesters. In conventional systems, TPA usually

replacement of TPA by IPA, an isomer of TPA in the polyester backbone,

aromatic monomers have the disadvantage that they are more susceptible

to photo-oxidation, leading to yellowing of the coating with time [6–8]. In

the early 1990s, cyclohexyl dibasic acids were proposed as replacements for

the aromatic dibasic acids. The cyclohexane isomeric diacid monomers

which are typically used in the preparation of polyesters are hexahydroph-

thalic anhydride (HHPA), 1,3-cyclohexanedicarboxylic acid (1,3-CHDA),

and 1,4-cyclohexanedicarboxylic acid (1,4-CHDA). The cycloaliphatic

structure gives the intermediate physical properties between that of aro-

matic and linear aliphatic polyesters except the yellowing resistance, which

is better with linear diacids. Relative to aromatic polyesters, T

of cycloali-

phatic polyester is lower, but is higher compared to linear aliphatic polyes-

the aromatic and linear aliphatic polyesters due to the presence of cyclic

structure in the monomer that can absorb energy through the interconver-

sion of chair and boat conformations [9, 10].

9.3.1 Reaction setup

Usually two processes are used in the preparation of saturated polyesters,

i.e., the melt process and the solvent process. In the solvent process, an

azeotropic solvent such as toluene or xylene is used to take out the water

formed during the polycondensation reaction. The basic raw materials are

charged in the reactor and reaction is carried out in general at around 200–

240°C. The reaction is monitored by following the acid value periodically

[11] and stops when the acid value comes down below 5. A laboratory-scale

reaction setup for polyester preparation is shown in Fig. 9.3.

provides chain rigidity and thus sufficiently high T . On the other hand,

significantly increases the weathering resistance of coatings [5]. However,

ters. The flexibility of cycloaliphatic polyester is also intermediate between

Polyester coatings for corrosion protection 169

9.4 Polymerization methods

polyols. The mixture of monomers should be charged to the reaction kettle

and the reaction takes place after melting of the reactants with the elimina-

tion of water molecules (Fig. 9.4). When dianhydrides are used instead of

diacids, no water removal takes place from the system.

di-/polyols. During this reaction, methanol or ethanol (depending on the

acid ester used) is generally collected into a graduated receiver as a

Condenser

Stirrer motor

inlet

Glass reactor

Heating

mantle

Dean Stark

apparatus

Stirrer

blade

9.3 Laboratory-scale reaction setup for polyester preparation [12].

9.4.1 Direct esterification

The monomers for the polyesterification reaction are di-/polyacids and di-/

9.4.2 Transesterification

In transesterification, the methyl or ethyl ester of the diacid is reacted with

170 High-performance organic coatings

by-product to allow estimation of the extent of conversion. The electro-

shown in Fig. 9.5 [13, 14].

9.4.3 Effect of catalysts

Many researchers have studied the effects of various catalysts for both the

catalysts such as dibutyltin oxide, dibutyltin dilaurate, titanium tetrabutox-

ide, titanium (IV) n-butoxide, p-toluene sulfonic acid (p-TSA), etc., are

based catalyst is more active than antimony- and tin-based catalysts. The

activity of the polycondensation catalysts follows the order Ti > Sn > Sb >

Mn > Pb [14]. However, titanium-based catalysts have the disadvantage of

imparting a yellow color to the polyester [15].

9.5 Determination of hydroxyl value and acid value

hydroxide (KOH) equivalent to the hydroxyl content of one gram of the

sample. Hydroxyl value is estimated by following the method set out in

grams of potassium hydroxide required to neutralize the free carboxylic

acid present in one gram of the polyester. The acid value of the resin is

R OH

R O

(+)

Transition state

()

R OR

cat

R OR

cat

R OR

cat

R O

cat

9.4 Mechanism of direct esterification reaction.

9.5 Mechanism of transesterification reaction.

philic mechanism for metallic catalysis of the transesterification reaction is

transesterification and polycondensation reactions. Tin-based esterification

widely used as esterification catalyst. Different results show that titanium-

The hydroxyl value is defined as the number of milligrams of potassium

the ASTM standard [16]. The acid value is defined as the number of milli-

Polyester coatings for corrosion protection 171

estimated as in the ASTM D 1639–70 method [17] by titration of the sample

with standardized KOH or NaOH solution. The solvent in the sample must

be removed and the sample in solid basis must be weighed. Then the sample

is dissolved in isopropyl alcohol and toluene. Phenolphthalein is used as

indicator and a pink coloration indicates the end point. The acid number

is calculated from the following equation:

Acid number

KOH KOH

sample

××VNMw

KOH

where V is the volume of potassium hydroxide solution consumed in mL,

N is the normality of the solution, and m is the weight of the sample in

grams (solid basis).

9.5.1 Effect of hydroxyl and acid number

In general, the synthesized hydroxyl or carboxyl terminated polyester resins

are crosslinked with different crosslinkers in thermoset coatings to obtain

desired properties. Therefore an increase in hydroxyl or acid number in the

polyester backbone results in an increase in the crosslinking density of the

high solid coatings

In general an increase in the hydroxyl number of polyester resin produces

thermoset coatings with better chemical resistance and mechanical proper-

ties. This is due to the increase in the crosslink density of the coatings.

However, an increase in the hydroxyl number in the resin produces increased

intermolecular hydrogen bonding, which increases the viscosity of the

system. Therefore it is essential to modify the resin structure to give low

viscosity and good crosslink density with the crosslinker. This can be

achieved by partial replacement of the available hydroxyl groups in hydroxy-

lated polyester with the less polar acetoacetate groups, which leads to an

increase in solid content at the application viscosity as well as increase in

adhesion due to the chelate effect (Fig. 9.6). Acetoacetylated or β-ketoester

incorporated polymers offer a versatile crosslinking mechanism. This ver-

satility results from the presence of two sites available for crosslinking

reactions. These sites are the active methylene group and the ketone

carbonyl group. β-ketoester groups are amphoteric and can participate in

a variety of chemical transformations, which might be used to modify or

crosslink polymers. There are several routes by which acetoacetylated

ferred for coating applications [18–21]. The crosslinking reaction of the

final thermoset coatings.

9.6 Backbone modification of polyester resin for

materials can be prepared, of which the transesterification route is pre-

172 High-performance organic coatings

active methylene group in acetoacetylated polyols with diisocyanate pro-

duces additional crosslink density with superior properties and weathering

stability [22–25].

9.6.1 Functional branched polyesters for

coating application

The ability to synthesize new polymers with predetermined and controlled

structures is important when new materials are to be created with desired

physical and chemical properties. Hyperbranched polymers (HBPs) have

been extensively studied as part of recent research efforts aiming at tailor-

ing the properties of polymeric materials by variation in the molecular

architecture. HBPs are highly branched structures based on AB

or A

monomers (x ≥ 2 and A and B are hydroxyl and carboxylic acid moieties,

respectively), which introduces potential branching points in every repeat-

ing unit as well as at the end-groups. The large number of end-groups and

branching points gives HBPs different properties compared to their linear

analogs. For instance, HBPs have low melt and solution viscosity and high

solubility, and are easy to modify to obtain tailored properties [26]. The

lower solution viscosity of HBPs compared with linear polymers is due to

the lack of entanglements, which result from their packed structure. They

also often possess very high solubility compared to those of linear polymers,

as a result of the large number of peripheral terminal functional groups

available per macromolecule. Therefore, the use of HB polyesters as spe-

sizing new three-dimensional networks with controlled architecture [27].

Recently, different methodologies for constructing HB polyester polyols

from AB

monomers have been explored. The routes involve thermally

driven homo-polycondensation or activation of either A or B functional-

O O

keto form

O OH

OH O

enol form

active methylene group

ethyl acetoacetate

- a β keto ester

hydroxylated polyester

1,4-addition

addition reaction.

9.6 Backbone modification of hydroxylated polyester by Michael

cific crosslinking agents for coatings seems a very promising way of synthe-

Polyester coatings for corrosion protection 173

ities. Thermally driven polycondensation of AB

monomers uses p-TSA or

an organometallic catalyst. Huybrechts and Dusek [28] reported that the

properties of low volatile organic component (VOC) coating formulations

containing HB polyol are superior to those of linear polyols. In Sweden, a

systematic investigation of HB polyesters as curing agents was developed

[29, 30]. At present, several dendritic/HB polyester polyols are commer-

cially available from Perstorp Polyols, Inc., Perstorp AB, Sweden, with the

trade name ‘Boltorn’, and are easy to synthesize [31]. For instance, HB ali-

phatic polyester prepared from 2, 2-bis(methylol) propionic acid (DMPA)

and 2-ethyl-2-(hydroxymethyl) 1,3-propanediol was used as crosslinking

agent in coating formulation. Ziemer et al. [32] synthesized HBP using 4,4-

bis(4-hydroxyphenyl) valeric acid as an A

B type raw material (Fig. 9.7(a)).

Researchers also proposed the polycondensation reaction of AB

monomer

in the presence of a B

core molecule as an improved method to control the

condensation reaction [33]. Examples of such an AB

-monomer/B

-core

approach include the preparation of HBPs from DMPA (an A

B monomer)

and pentaerythritol [34] (Fig. 9.7(b)), triethanol amine (Fig. 9.7(c)) and

trimethylolpropane (TMP) [35, 36] as core molecules. Pavlova et al. [37]

used HB polyester prepared from 4,4-bis-(4′-hydroxyphenyl)pentanoic acid

and TMP as core molecule for coatings preparation. The commercially

available (Perstorp Polyols, Inc.) hydroxyl-functional HB polyester of

the third generation (G3) prepared from DMPA and ethoxylated

COOH

Boltorn

H20

pentaerythritol

DMPA

p-TSA catalyst

HO OH

4,4-bis (4-

hydroxyphenyl) valeric

acid

DBTL catalyst

B approach

COOH

B + a core molecule

(a)

(b)

(c)

DMPA

triethanolamine

9.7 Different synthetic approaches for the preparation of hyper-

branched polyols [59].

174 High-performance organic coatings

pentaerythritol (a core molecule) is shown in Fig. 9.8(a). An A

+ B

approach produces an AB

molecule as an intermediate during the conden-

sation reaction and is important due to the commercial availability of A

and B

type monomers. As an example, the synthesis of aliphatic HBP

polyol from adipic acid and glycerol can be mentioned [38]. Recently, Frey

and coworkers [39, 40] and Xinling and coworkers [41, 42] developed diffe-

rent synthetic routes that emerge from the combination of the multi-branch-

ing polymerization of glycidol with well-established epoxide polymerization

techniques, leading to unprecedented polymer architectures. The synthesis

of hyperbranched polyglycidol (HPG) involves the cationic ring-opening

diethyl etherate catalyst in chloroform. The reaction was carried out at 25°C

for more than 3 h and in a nitrogen atmosphere (Fig. 9.8(b)).

9.7 Polyester-melamine coatings

Melamine-formaldehyde (MF) resins are important components of ther-

mosetting resin products. They are widely used in the coatings industry as

binders and crosslinkers to produce highly durable coatings. Thermoset

coatings based on the reaction of MF resins with hydroxyl functional

OHHO

O(C

)

OHHO

Hyperbranched polyglycidol

base catalyst or

Hydroxy functional HBP of the third generation (G3)

(a)

(b)

9.8 A third-generation hyperbranched polyol (a) and hyperbranched

polyglycidol (b).

polymerization of glycidol, by glycerol in the presence of boron trifluoride

Polyester coatings for corrosion protection 175

polyester have long been used in the coatings industry. For instance, poly-

ester resin crosslinked with hexamethoxymethyl melamine (HMMM) were

developed in 1972. MF crosslinkers have been shown to give improved

chemical, heat, wear and scratch resistance, improved hardness, and

exterior durability to coatings. Typical applications include coatings use in

household appliances, automotive OEM coatings, industrial coatings for

coils and cans, agricultural and construction, etc. Because of their value,

MF resins have been extensively studied in order to optimize the properties

temperatures above 100°C for 10 to 40 min in the presence of p-TSA cata-

lyst. The wide acceptance of p-TSA is due to the compatibility of its molec-

ular structure and hydrophobic nature with the paint components.

9.7.1 Melamine-formaldehyde resins

MF resins are mixtures of compounds with different degrees of substitu-

temperature, pH of the medium, reactant ratio, and the degree of polym-

erization. HMMM resins are the reaction products of melamine (2,4,6-

triamino-1,3,5-triazine) with formaldehyde to form methylol groups and

converting these methylols to methoxymethyl groups using methanol. Up

to six moles of formaldehyde can be combined with one mole of melamine

to produce hexamethylol melamine. In the preparation of coating resins,

usually 5–6 moles of formaldehyde are reacted with one mole of melamine.

The degrees of methylation and methoxy-methylation are varied according

to the intended end-use applications. HMMM resin may contain, besides

pure HMMM, also dimers and trimers, and other end-groups than

2 3

nism and rate of reaction. Furthermore, during the crosslinking reaction

self-condensation of HMMM can occur between the various functional

groups. HMMA is a versatile crosslinking agent for a wide range of poly-

meric materials, both for solvent-borne high solids and for water-borne

coatings. Butylated melamine resins (BMF) are widely used in industrial

cured coatings. They are widely used for interior applications, interior

9.9 shows the structure of fully substituted methylated MF and butylated

MF resin.

9.7.2 Reaction of polyester with melamine resin

Hydroxylated polyester resins are generally crosslinked with BMF and

HMMM in a ratio of 90/10 to 70/30. The curing reaction (Fig. 9.10) is

catalyzed by a strong acid such as p-TSA. The catalyzing mechanism as

of the final coating. Such formulations are commonly cured by baking at

tions [43]. The final form of the MF resin produced depends on the reaction

stoving primers and enamels, in acid-catalysed wood finishes and in force-

container coatings, and two-component solvent-based wood finishes. Figure

–CH OCH . The various functional groups influence the reaction mecha-