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

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266 High-performance organic coatings

13.8.1 Co-solvents in waterborne coatings

Although water is an environmentally friendly solvent for the coating

industry, it requires some additives to make it suitable for waterborne coat-

ings owing to its unique chemical and physical nature. In most waterborne

systems, the inclusion of some co-solvent is necessary to improve coales-

cence, freeze–thaw stability, levelling and wetting. Solvents speed up water

release, adjust the drying time, assist pigment dispersion and control

foaming. Some of these co-solvents and their functions in waterborne

coating formulations are listed in Table 13.6.

13.8.2 Surfactants

The constituent particles of liquid and solid, which may be ions, atoms or

molecules, in bulk are attracted to their neighbours on all sides, whereas

those in the surface layer are attracted on only three sides. This results in

an unbalanced force, known as the surface tension. The establishment of

this surface tension in water causes the adoption of the minimum possible

area (a spherical shape) by the water droplets, which results in poor wetting

of the substrate by waterborne coating formulations. Therefore, a water-

borne coating requires some surfactants to reduce the surface tension of

the coating, which improves the wetting of the substrate. Surfactants may

Table 13.6 Function of co-solvents in waterborne coatings

Solvent properties Function Example

Coalescing agent for

latex

Reduces glass transition

temperature (T

) of

resins

Hexylene glycol,

n-methyl-2-pyrrolidone

Freeze/thaw stabilizer Depresses the freezing

point of water

Alcohols, non-ionic

surfactants

Levelling aid

mobility

Propylene glycol

Wet edge extender Reduces viscosity of

paint and evaporation

rate of solvent

Propylene glycol,

diethylene glycol

monoethylether

Defoamer Imparts low surface

tension to foam cell

wall

Octanol, pine oil,

mineral spirits

Dispersant Provides good wetting

and dispersion of

pigments

n-Methyl-2-pyrrolidone

Prolongs wet film

Waterborne coatings for corrosion protection 267

also be added to aid substrate wetting, to assist stabilization of the latex or

to maintain dispersion of pigments. There are three major functions of

surfactants:

• To improve substrate wetting

• Stabilization of latex

• Proper dispersion of pigment.

Surfactants consist of polar (hydrophilic) and non-polar (hydrophobic)

moieties. The non-polar moiety experiences a repulsive force by water

molecules, which leads to the relative depletion of surfactants from the bulk

of the liquid water and accumulation at the surface where they have to

reduce surface tension. Thus, in the presence of surfactants, the surface

energy of the interface will have a lower energy state than in the bulk.

Surfactants are divided into three groups: anionic, cationic and non-ionic

(Fig. 13.15). Anionic surfactants possess a negative charge in solution and

agents, whereas cationic surfactants possess a positive charge in solution

and are typically used as antibacterial agents, fabric softeners, corrosion

is quaternary ammonium compounds, R

, though in coatings their use is

limited.

However, these surfactants are present in the paint formulation but are

not reactive (no covalent bonding with polymeric particles) in the polym-

erization process. They result in the destabilization of the paint formulation

under high ionic strength, freezing and high shearing. Poor adhesion, water

sensitivity and low-dimensional stability have also been observed when the

meric surfactants. Reactive surfactants are able to participate in one of the

chemical reactions involved in the polymerization process. The other way

to avoid the desorption of the surfactants from the particle surface is to use

Guyot [11] reviewed the performance of reactive surfactants and their

applications in the synthesis of latexes for waterborne coatings. A large

number of reactive surfactants, non-ionic, anionic, cationic or even

zwitterionic (Fig. 13.16), have been prepared and tested in emulsion, mini-

emulsion, micro-emulsion or dispersion polymerizations, of styrene and

acrylic monomer(s), in batch or semi-batch processes, to prepare both

homopolymers and core-shell copolymers. A novel wetting agent, sodium

are typically used as soaps, detergents, emulsifiers, dispersants and wetting

film was exposed to water or high conditions of humidity. The bad perfor-

mance of film is due to desorption of surfactant from the particle surface

or migration towards the film surface, and formulation of hydrophilic

domains within the film upon phase separation. There are two possible

ways to avoid these difficulties: by using either reactive surfactants or poly-

polymeric surfactants that are very difficult to desorb.

inhibitors, ore flotation additives, emulsifiers and dispersants. One example

268 High-performance organic coatings

salt of N,N-dipalmitoyl-ethylenediamine-diacetic acid (Fig. 13.17), was syn-

thesized by Bi et al. [12].

ogy is essential for the stability of the paint, the manufacturing process

(pigment dispersion) and transport through pipelines as well as for its

application characteristics. Thixotropes or rheological control agents for

waterborne coatings are as important as those for solventborne coatings.

The viscosity of latex systems is generally very low and in consequence

such systems require viscosity builders. Viscosity builders include water-

Anionic surfactant

Non-ionic surfactants

Hydrophobic surfactants

Linear and branched aliphatic hydrocarbons

Alkylbenzene

Alkylnaphthalenes

Alkylphenols

Polyoxypropylenes

Silicones

Fluorocarbons

Acetylene glycol Alkanolamide

Carboxylate Sulphonate SulphatePhosphate

–

13.15 Common surfactants used in waterborne coatings.

13.8.3 Rheology modifier

Rheology (flow properties) is the study of the deformation and flow of

matter under the influence of an applied stress. The control of paint rheol-

Waterborne coatings for corrosion protection 269

O C

COONa

Hemiester (sodium salt)

Alkyl maleic sulphonate

Methacrylol sulphate

O COONa

Undecanylol sulphate

Crotonoylol

Hemiesterethoxylate methyl ester (sodium salt)

13.16 Reactive surfactants.

Na O C

Sodium salt of N,N′-dipalmitoyl-ethylenediamine-diacetic acid

13.17 A novel wetting agent.

soluble high molecular weight polymers such as cellulosic, alkali-activated

polymers, such as those based on acrylic acid, ethoxylated urethane-based

associative thickeners, and clays such as attapulgites. A list of some com-

monly used thixotropes and their functions is given in Table 13.7.

13.9 Film formation

13.9.1 Film formation by water-soluble polymer

Film formation involves the deposition of a polymeric solution onto a sub-

strate and its transformation into an adherent solid coating as the water

dries out. During the application process, polymer molecules tend to

become stretched, owing to the following:

270 High-performance organic coatings

• The energetically favourable interaction between the polymer segments

and the water molecule, which may be maximized by uncoiling of the

polymer.

• The presence of mutual forces of repulsion in polymer.

Once the coating process is complete, the polymer molecules become

state of high shear exists within the coating solution during the coating

application.

13.9.2 Film formation by latex paint

Fig. 13.18.

1. Evaporation of water by leaving behind the polymer, pigment and other

constituents of paint on the substrate.

formance of the resulting coating.

Thixotrope Example Properties

Cellulosic polymer Methyl cellulose, hydroxyethyl

cellulose, hydroxy propyl

cellulose

Increases viscosity of

latex

Polyacrylate Polyacrylic acid, polymeth-

acrylic acid and their salts

Not prone to microbial

attack

Anionic thickener Anionic polycarboxylic acid No microbial attack

Non-ionic

thickeners

Ethoxylated polyether-based

urethane block co-polymer

Thicken by means of

secondary valency

association

Clay-type

thickeners

Appapulgite clays (anti-

microbial attack)

Impart viscosity by

creating a network of

Fibrous

thixotropes

Cellulosic and polyaremid

Table 13.7 Rheology modifiers in waterborne coatings

fibres

free to return from a stressed state to their original configuration, as long

as they have not reached their elastic limit, and form an even film, since a

The mechanism of film formation in latex paint is more complex than for

paints derived from true polymeric solutions. Once the film has been spread

on the substrate, drying of the film starts. It involves two steps as shown in

particles take place for film formation. In this step, full incorporation of

the pigment particles into the film is crucial for assuring excellent per-

flocculated needles

2. Once the water evaporates, flattening and coalescing of the polymer

Waterborne coatings for corrosion protection 271

a number of factors such as the natural hardness of the polymer, the pres-

ence of coalescing solvent in the latex, and electrostatic repulsive interac-

tion at the surface of the polymeric molecules which themselves depend on

while others form powdery layers. Commonly, loss of water by evaporation

transition temperature and the particle size of the polymer components of

13.10 Application methods

The application technology for waterborne coatings is comparable to that

of conventional solventborne coatings. If a facility is using a water wash

booth, overspray is easily recovered and reused if colours are appropriately

segregated. Uncured waterborne coatings can be cleaned from equipment

Latex

Substrate

Polymer particle

Water

Application of latex

Evaporation of water and approach of

polymeric particles towards each other

Generation of small capillaries of

water between polymeric molecules

13.18 Film formation in latex paint.

Polymeric film on substrate

the temperature of film formation. The temperature at which latexes cast

continuous and clear film is called the minimum film formation tempera-

ture. When drying, some latex paints produce continuous and strong film

results in a powdery finish to the coating, which has no cohesion. The glass

the latex decide the minimum film formation temperature [1].

The rate of flattening and coalescing of polymer particles is influenced by

272 High-performance organic coatings

with water. Electrostatic spray can be used if the electrically conductive

waterborne paint is isolated from the electrostatic system. However, some

formulations or substrates might require special pumps and piping to

prevent corrosion from water in the formulation. In addition, for product

complete cure in a reasonable period of time.

13.11 Performance evaluation of waterborne coatings

Selection of coating recipes depends on several factors such as the desired

life of the coating, the nature of the substrate, and climatic conditions.

There are several laboratory tests in use to demonstrate the protective

behaviour of coatings and to give reliable predictions of their lifetime. It is

generally expected that the test environment should be the same as or

similar to that of the actual application. However, such a natural exposure

test requires too long a time. For reducing the test time, accelerated natural

exposures and laboratory tests have been developed. Commonly used

accelerated tests such as salt spray, accelerated weathering, humidity, tem-

perature and different contamination conditions, are traditionally used to

assess the anticorrosive behaviour of the paint coatings.

Like solventborne coatings, waterborne coatings are also available for

high-performance applications such as corrosion protection, abrasion resis-

tance, UV stability, thermal stability etc. Various waterborne coatings

derived from conventional polymers have been well discussed in various

texts [1, 2]. A number of review articles are available on sol-gel based coat-

ings [1, 14] but only a few address waterborne sol-gel products [15, 16].

Today sol-gel coating is one of the most promising high-performance water-

borne coatings. In this section, we will discuss the performance of water-

borne sol-gel coatings specially developed by our group.

13.11.1 Corrosion resistance

Excellent corrosion resistance is one of the most desirable properties of

high-performance coatings. Potentiodynamic polarization, electrochemical

impedance spectroscopy and salt spray tests are traditionally used to

assess the anticorrosive behaviour of paint coatings. Waterborne sol-gel

coatings based on methyltrimethoxysilane (MTMS) and 3-glycidoxyprop-

yltrimethoxysilane (GPTMS) were developed [13]. Two amino-silanes,

namely γ-aminopropyltriethoxysilane (1N), N-(β-amino ethyl)-γ-aminopro-

pyltrimethoxysilane (2N) and hexakis(methoxymethyl)melamine (HMMM),

were used as crosslinking agents. Aluminium alloy AA6011 was dip coated,

dried at room temperature and placed in a furnace at 120°C for 30 min for

finishing, coatings need to dry or cure at elevated temperatures to ensure

final curing. The anticorrosive behaviour of the coatings in 3.5% NaCl solu-

Waterborne coatings for corrosion protection 273

tion was found to be excellent. The corrosion currents of the coated alu-

minium alloy decreased by two orders of magnitude compared to bare

aluminium alloy, which implied that the coating forms a dense barrier

against penetration of water and chloride ions (Fig. 13.19).

It was found that the anticorrosion ability of various crosslinkers is in the

order HMMM > 2N > 1N. Addition of p-TSA as catalyst in the 1N coating

tance. The shifting of open circuit potential towards the noble direction, as

compared with the uncatalysed 1N coating, and the reduction in the corro-

sion current from 0.67 × 10

−6

A to 1.26 × 10

−7

of the addition of the p-TSA as catalyst (Fig. 13.20).

Although organosilanes exhibit excellent corrosion resistance, their use

reduction and easy handling of sol-gel materials have become important

issues for manufacturers of paint raw materials, paint users and researchers.

One solution to this problem is the incorporation of conventional organic

polymers in silanol sol.

In order to reduce the cost, conventional waterborne alkyd and polyester

resins were incorporated in the MTMS–GPTMS sol. These conventional

organic polymer-incorporated sol-gel coatings have shown excellent

corrosion resistance on aluminium and magnesium alloys. The impedance

spectra of bare and polyester-incorporated MTMS–GPTMS coated

0.0000000001

–2

–1

HMMM

AA6011

AA6011_3.5% NaCl_5mV

(Ovl) AA6011_1N_3.5% NaCl_5mV_1

(Ovl) AA6011_2N_3.5% NaCl_5mV

(Ovl) AA6011_Si/HMMM_3.5% NaCl

0.00000001 0.000001 0.0001

Current (A)

Potential (V)

0.01 1

13.19 Potentiodynamic polarization curves (E vs log I) of bare and

coated AA6011 in 3.5% NaCl at scan rate of 5 mV/s.

further improved the rate of film formation as well as the corrosion resis-

A, confirm the positive role

is limited since they are costly and difficult to apply. In recent years, cost

274 High-performance organic coatings

Along with their corrosion resistance, waterborne coatings also impart

improved hardness and hydrophobicity and enhanced UV resistance, as

discussed below.

–2

–1

AA6011_3.5% NaCl_5mV

(Ovl) AA6011_1N_3.5% NaCl_5mV_1

(Ovl) AA6011_1N/PTSA_3.5% NaCl

0.00000001 0.000001 0.0001

Current (A)

Potential (V)

0.01 1

13.20 Potentiodynamic polarization curves (E vs log I) of bare, Si/1N

and Si/1N-PTSA coated AA6011 in 3.5% NaCl at scan rate of 5 mV/s.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.5

4.0

450035002500

Bare AI

Coated AI

2000

Z′ (ohm)

–Z″ (kΩ)

15005000 1000 40003000

13.21 Nyquist plot of bare and coated (conventional polyester–MTMS–

GPTMS) aluminium in 3.5% NaCl at scan rate of 5 mV/s.

aluminium confirm the superiority of waterborne coatings (Fig. 13.21).

Waterborne coatings for corrosion protection 275

13.11.2 Mechanical properties

Coatings, generally, are subjected to stresses when fabricated into products

by rolling, bending, or other deformation processes. These stresses can

ture of the coating, exposing the substrate, scratch, or loss of adhesion to

the substrate. Therefore, to withstand the stress of fabrication, coatings

must adhere to the substrates on which they are applied, and be hard

enough to resist scratching during fabrication and during service life. A

variety of recognized methods can be used to determine how well a coating

is bonded to the substrate. Commonly used measuring techniques are the

cross-hatch test, the bend test, the pencil hardness test and the impact

test.

Bend and impact tests suggested that all MTMS–GPTMS coatings were

well adhered to the surface and were able to withstand the stresses during

service. The pencil hardness of MTMS–GPTMS coatings was found to be

above 6H. The hardness of 2N crosslinked coating was found to be better

than that of 1N crosslinked coating. Incorporation of organic polymers to

sol provides a coating of intermediate hardness between pure MTMS–

GPTMS coating and pure organic polymer-derived coating. However, the

hardness can be controlled by the MTMS–GPTMS and organic polymer

MTMS–GPTMS coating decreases with increasing polyester/alkyd content.

Figure 13.22 shows that the addition of conventional polyester in MTMS–

GPTMS sol reduces the hardness of the coating from 6H to 3H, depending

upon the polyester–sol weight ratios.

13.11.3 Hydrophobicity

The water contact angle is a quantitative measure of the wetting of the

coated substrate by water. The total surface free energy of a sol-gel coating

and its components can also be determined using Young’s equation, which

relates the surface tension of water in equilibrium with its vapour, with the

contact angle of the water drop on the surface of a sol-gel coating:

= γ

− γ

cosθ

where

= interfacial free energy between the bare/sol-gel coated substrate and

the water

= surface free energy of the bare/sol-gel coated Al in equilibrium with

water vapour

= surface tension of the water in equilibrium with its own vapour

θ = contact angle.

ratios. We observed that the hardness of the polyester and alkyd modified

exceed the flexibility or adhesive strength of the coating, resulting in frac-