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

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407

Anticorrosion organic coatings for

the automotive industry

M G MEHROTRA, Kansai Nerolac Paints Ltd, India

19.1 Introduction: automotive coatings

A boom in the automotive industry is a boon for the automotive coatings

industry. The automotive coatings industry plays a major role in sustaining

the boom, since many customers give more importance to the color of their

car than the rest of the factors which help in its smooth running. Selecting

the right color that would attract customers is a major challenge for coatings

suppliers and manufacturers, since they have to satisfy a diverse customer

base [1].

There are color watchdogs like the UK-based International Color

Authority and Global Color Research, and the US-based Color Marketing

Group (CMG) which conduct surveys on various car colors. Some compa-

nies like DuPont have their own research survey teams which rate various

colors on the basis of sales and responses from various customers they had

interviewed during their survey. Once the surveying is done, the paint sup-

pliers and manufacturers sit together and decide on the possible colors

based on the sales, chemistries, demography, etc. Chemistry is a very impor-

tant factor here since the paint costs are related to the body of the car. For

suppliers also have to cater to the whims and wishes of their customers. An

executive class color should be exclusively for that class and Ferrari enthu-

siasts would want a bright red car.

If Ferrari’s success was driven by Schumacher, there should be another

Schumacher who must be driving the automotive coatings market. The

main driver for this market currently is technology and innovations brought

about by the paint manufacturers, without which survival becomes a chal-

lenge. Coatings manufacturers have to make sure that their coatings look

great and at the same time perform. A variety of new approaches are being

researched and nano-technology and UV-cured technology have been

the latest solution to the ever-increasing problems. The environmentally

friendly technologies are the ones hunted for nowadays and these technolo-

instance, a pearl finish would be costlier than a metallic/mica finish. Paint

408 High-performance organic coatings

gies are aimed at meeting environmental demands and in addition provid-

ing improved corrosion resistance and protection. Automotive coatings

manufacturers have started shifting to these technologies with environmen-

tal regulations becoming more stringent in Asia. The resin suppliers too are

fueling this advancement with formulator-tailored resins that offer better

adhesion and resistance.

The automotive coatings market is the single largest application for the

radiation cured resins market and nano-technology is fast catching up. The

market is expected to experience double-digit growth rates in the future

with China supporting the cause and India and the rest of Asia helping

it out.

This total systems concept has made the Nippon Paint name synonymous

with premium quality amongst automobile manufacturers worldwide.

Nippon’s comprehensive product range responds to market demands for

beauty and protection, simultaneously setting new industry standards with

specially developed paints and coats that enhance vehicle performance and

ings. Nippon Paint’s end-to-end paint systems provide the long-lasting

durability and hardness that is required of automotive paints, while adding

new dimensions of aesthetics and function to automobiles.

Now it is the turn of the carmaker to sell the color, and companies like

GM display some of the chosen colors to some random customers who are

about the colors they have chosen from the above process and paint sup-

pliers produce the desired color based on the chemistries and budgets. Even

after such a safe process some colors get a lukewarm response and these

colors are shifted to other models which are produced simultaneously.

How often do you see a Mercedes Benz, a Fiat or a GM car on Asian

roads? Even though their sales have increased and many new models have

been unveiled, the Asian automotive manufacturers are giving stiff compe-

tition to their foreign counterparts. The rise of Hyundai, Maruti and other

local manufacturers has certainly made the foreign manufacturers reform

their strategies to gain leadership in the market where they had a monopoly

before.

The automotive coatings manufacturers also want to grab a piece of the

pie when available, and global giants like DuPont and PPG have kicked off

their investments in Asia through joint ventures (JVs). Recently, DuPont

set up a JV with Beijing Red Lion Coatings Ltd, called DuPont Red Lion,

with an annual capacity of 20 000 tons, and to top it all DuPont is planning

to increase its stake. The automotive coatings market looks very bright,

especially in China and India.

UV-cured technology and nano-technology may be catering to the diverse

needs of formulating companies but they also have their own drawbacks.

life span. Vehicles have very specific demands in terms of paints and coat-

brought to the production plants. Now the car manufacturers are confident

Anticorrosion organic coatings for the automotive industry 409

With manufacturers coming up with complex structures for their auto-

mobiles, the onus falls on the coatings companies as they have to meet the

ever-increasing demands.

The manufacturers are trimming costs to widen their customer base, and

coatings manufacturers also have to come up with cheaper methods and at

the same time should bring about technical innovations. With more tech-

nological innovations the specialty content of the raw material is increasing

and the commodity type is on the downhill. Technological advancements

require huge investments in R&D and coatings suppliers have to keep this

in mind before investing.

To conclude, automotive coatings suppliers have a bright future provided

they constantly improve their technology. Catering to a wide customer base

should not be done at the cost of performance, and coatings manufacturers

have to strike a balance and draw a line somewhere. Kudos to Charles

being the one who comes up with technical innovations at low cost. Thus

optimum surface protection and an attractive appearance are now the most

important requirements of vehicle coatings.

19.2 Types of automotive coating

Despite advances in the technology of surface coatings, metallurgy of steel,

design of motor vehicles, materials of construction and assembly opera-

tions, priming remains a fundamental stage in the defense against corrosion,

whether of cosmetic or perforation type [2]. To the ever-increasing empha-

sis on safety, serviceability and after-sales value of a motor vehicle, protec-

integrity is and will be of paramount importance. Automotive systems of

the past, such as low solids enamels, low and medium solids dispersion lac-

quers and medium solids enamels, required more solvents than today’s

paints. They also were not as resistant to harsh conditions such as acid rain,

bright sun, sand and salt spray. Today’s high solids enamels, high solids and

waterborne basecoat/clearcoats and urethane clearcoat paint systems

greatly reduce the use of solvents while offering improved durability,

19.2.1 Natural

based on table oils and resins, rosin and copals. Copals are semi-fossil rosins

of trees which grew in the tropics. In England, this coating technology,

based on the combination of vegetable oils and copals in a hot blend

process, was developed to a high quality. For the coating process 20 sepa-

Darwin whose ‘Survival of the Fittest’ theory fits in aptly here, the fittest

tion against any form of corrosion and maintenance of the paint film

greater corrosion resistance and better finish than earlier materials.

The coating materials of the first cars consisted of air-drying binder systems

410 High-performance organic coatings

rately painted coat layers are required and up to three weeks’ working time.

In the 1920s the demand for cars increased rapidly and production assembly

lines were invented and developed. Topcoats based on cellulose nitrate as

plasticizers, and the coating systems that consisted of this combination

performed best, drying at ambient conditions and retaining good

appearance.

19.2.2 Synthetic

• Self-crosslinking: two components when mixed result in a chemical reac-

tion and hardening. Mainly consist of unsaturated polyester resins and

polyurethane resins.

• Thermoplastic: softens easily at higher temperatures and also dissolves

in solvents. Mainly consist of vinyl resins and acrylic resins. Examples

are nitrocellulose and cellulose acetate butyrate.

• Thermosetting: heat and chemically reactive resins. These are chemi-

cally resistant. Mainly consist of melamine and epoxy resins.

19.2.3 Alkyds

Alkyds are a form of polyesters containing mainly phthalic esters of differ-

unsaturated fatty acids from vegetable oils (linseed oil, tung oil, soybean

crosslinking with atmospheric oxygen. Alkyds became the most important

group of synthetic resins for many different types of paints and coating

systems over many years until the present time. Today there are different

lines of repair topcoats on the market which are based on fast-drying

alkyds.

19.2.4 Melamine formaldehyde (MF)

Melamine formaldehyde is the current standard crosslinking agent used in

surfacer formulation and most other technologies based on polyesters. The

technology is extremely well established and there are a wide range of

commercially available products. In addition, they are relatively cheap.

Reactivity varies depending on the degree and type of alkylation (normally

butylation). Ratios of resin to crosslinker generally range from 80:20 to

70:30 depending on required properties and the baking schedule available.

Stoving temperatures are recommended to ensure a complete reaction at

a typical stoving time of 20 minutes. Technologies of two or more MFs

blended with differing reactivities are often used to achieve crosslinking

a film building vehicle were used. Cellulose nitrate was combined with

ent polyalcohols and are modified with monocarboxylic acid, normally with

oil), which gives this resin group the opportunity to dry and build films by

Anticorrosion organic coatings for the automotive industry 411

over a range of temperatures to help anti-sagging properties of the partially

cured coating.

19.2.5 Thermosetting acrylics

In the 1950s acrylic resins started to be used in automotive topcoats.

Copolymers of different esters of methacrylic acid with rather high molecu-

lar weights are one class of acrylic resins, which dry physically. They are

still used in repaint topcoats. In the 1960s coating systems were introduced

based on the combination of –OH-functional copolymer esters of acrylic

and methacrylic acids with amino resins for automotive topcoats.

19.2.6 Epoxies

Epoxy resins have been used in surfacers, in one form or another, for a

number of years because of their inherent strengths of adhesion, toughness

and corrosion resistance. They are still present in current polyester prod-

ucts to improve adhesion to bare steel and to provide a degree of corrosion

protection. In some cases stone-chip performance is upgraded.

19.2.7 Polyurethanes [3]

Two-component topcoats came on the market, based on –OH-functional

polymers (polyesters or acrylic resins) and crosslinked by polyfunctional

isocyanates. In this case the content of the paint is delivered in two separate

parts, so that the curing reaction can take place chemically when they are

mixed and be applied to the substrate to be painted.

19.2.8 Hardeners for two-component systems

Paints use various types of isocyanates in different types of coatings. Primers

mainly use highly reactive toluene diisocyanate. Being aromatic in nature,

the yellowing tendency will not affect the appearance. Topcoats mainly use

hexamethylene diisocyanate and isophorone diisocyanate, which do not

yellow but have a lower reactivity rate.

19.3 Sequence for coating automobile body shells

19.3.1 Pretreatments of metallic substrate

Pretreatment chemicals used in the automotive industry are essential to

prepare substrates prior to topcoat application and to provide corrosion

resistance, promote good paint adhesion, and bond/seal automotive bodies.

412 High-performance organic coatings

In a typical automotive assembly process, a vehicle body is pretreated to

prepare the substrate surface for paint adhesion and corrosion resistance.

The body is then electrocoated with cationic primer to provide primary

corrosion resistance, especially on interior or recessed areas.

Primer surfacer may also be applied using the electrocoat process.

Structural adhesives and sealants are applied at various stages of the assem-

bly process to bond and reinforce components during vehicle fabrication,

value through improving appearance, paint adhesion, and the overall cor-

rosion protection of the vehicle.

• Superior corrosion protection

• Improved metal cleanliness

• Excellent paint adhesion

• Increased uniform metal appearance.

19.3.2 Electrodeposition [4]

The metal frame of a car or light truck has an extremely complex shape.

Many areas are hidden from a direct line of sight. It is virtually impossible

to coat such a complex shape with conventional spray-applied paint. In

order to achieve maximum coverage and therefore maximum corrosion

deposition. The metal body is electrically charged, then immersed in a bath

containing oppositely charged paint particles. These particles are attracted

to the metal surface, are neutralized, and can then be baked into a coherent,

this process (this minimizes corrosion for steel). These products are called

cathodic electrocoats.

The requirements are as follows. Electrocoat products must exhibit high

throw power (the ability to penetrate deeply into crevices) as well as excel-

lent corrosion resistance. The electrocoat products need to be applicable

on all substrates used in the automotive industry (steel, zinc-coated steel,

aluminum alloy, etc.). The baking requirements must be compatible with

the different customer needs, and the products must provide a smooth

appearance. A new environmental requirement requires that the electro-

coats must function without the addition of heavy metals such as lead and

in some areas in Europe also without tin.

The coating industry has been providing electrocoat products to the

automotive industry for over 40 years, protecting over 300 million vehicles

in this time. A full line of cathodic dip primers used by all major automobile

manufacturers worldwide would include the following steps:

to provide sound deadening, and to seal seams and joints. This finish adds

Automotive pretreatment exhibits a wide array of benefits, including:

resistance, this first coating layer is applied in a process known as electro-

tough film. Current practice favors making the vehicle body the cathode in

Anticorrosion organic coatings for the automotive industry 413

• Degreasing

• Acid treatment

• Phosphating

• Water rinsing

• Passivation.

19.3.3 Primers [5]

The primer layer is the coating layer that joins a corrosion-protected sub-

strate to the topcoat. It functions to smooth out surface irregularities and

improve stone-chip performance, and helps to protect the substrate from

visible and UV light. The primer must be capable of reliable and uniform

application, and should contribute a minimal environment footprint.

Today’s primers include solventborne, waterborne and powder forms.

ness, to optimize the painting process and to minimize defects related to

stone-chip damage.

19.3.4 Basecoats [5]

Basecoat is the coating layer that provides color and aesthetic effects.

Basecoat must provide uniform appearance initially and for many years

without fading. Modern requirements for appealing vehicles often require

porated in one basecoat layer or in a separate layer (for example in two-

tones). If two basecoat layers are used, the sandwich with the clearcoat is

called a tricoat topcoat.

19.3.5 Clearcoats [5]

The clearcoat is the coating layer that forms the last interface to the envi-

ronment. It carries the biggest part of the technological performance and

must be able to resist environmental etch, bird droppings, car wash machines

clearcoat systems. The clearcoat in combination with the basecoat forms

the automotive topcoat, which gives the vehicle its appearance. One- and

two-component (known as 1K and 2K) clearcoats are applied over both

solventborne and waterborne basecoats.

The driving forces for clearcoat development are cost of material and

process, appearance, etch and scratch resistance and the environmental

• Ultrafiltration of paint to increase efficiency

Color-keyed primers are often used to allow minimum basecoat film thick-

footprint. Environmental pressures have led to intensified development of

micas, aluminum flakes and other pigmentation effects. These can be incor-

and other outside influences. To improve performance against all these

influences, OEM coating systems move from colored topcoats to basecoat/

414 High-performance organic coatings

super high solids, waterborne and powder clearcoats. Worldwide, approxi-

mately 80% of all passenger cars are coated with 1K clearcoats, and the

remainder with 2K clearcoats. The 1K clearcoats are subdivided into sol-

ventborne acrylic melamine systems, clearcoats with enhanced etch resis-

tance, high-solid systems, waterborne systems and powder versions. The

2K clearcoats include solventborne, super-high-solids and waterborne

versions.

19.4 Detailed methodology of primers, basecoats and

19.4.1 Introduction

Solventborne 2K epoxy/amine chemistry has been used in the anticorrosion

primer market for general industry coatings, particularly when ambient

temperature cure is needed. These systems are chosen for many reasons,

including the excellent chemical resistance achieved from a crosslinked

due to low volume shrinkage during cure. The key to the performance of

2K epoxy systems is in the epoxy/amine crosslinking reactions.

A primary polyamine can react with epoxy. The resulting secondary

amine can react with the epoxy ring. The key to achieving low volume

shrinkage are these ring-opening reactions (unlike many other crosslinking

for epoxy reaction(s) between the –OH groups to further contribute to the

development of a highly crosslinked coating.

Formulation of anticorrosion primers

Waterborne products (mainly those based on solid resin dispersions) have

an even better adhesion level compared to solventborne products [6]. A

critical method to verify anticorrosion performance is to employ a cross-

hatch tape test after the salt fog or humidity chamber exposure. With this

test method it was found that on critical substrates, like galvanized steel or

to be the fact that crosslinking reactions between epoxy groups of the disper-

sion and the active hydrogens of the amine hardener in waterborne systems

are not reaching the same level as in solventborne or solvent-free systems.

In this scenario, there would remain a level of non-reacted (hydrophilic)

system in salt spray or humidity chamber exposure are obtained with a lower

level of amine hardener than is suggested by stoichiometry alone.

topcoats and metallic finishes

coating film and excellent adhesion to various substrates (especially metals)

reactions). The final reaction illustrates that there still remains the potential

hardener used in the formulation. The main reason for this finding is likely

amine hardener left in the film, resulting in increased polarity of the coating

film. These polar sites would allow interaction with ions and water. It seems

reasonable that the best results with a specific waterborne epoxy/amine

aluminum, the adhesion level is significantly influenced by the amount of

Anticorrosion organic coatings for the automotive industry 415

19.4.2 Basecoat technology

Applying a basecoat is a process for producing color in a paint or creating

a special effect for two-layer coatings on substrates. The process mainly

comprises successive steps in the following sequence:

1. Applying a special effect basecoat layer (comprising metallic pigments)

dispersed in a liquid onto the substrates in one or more successive spray

passes

2. Drying or curing of the special effect basecoat layer

3. Applying a clearcoat layer

4. Curing the clearcoat layer.

Basecoats mainly consist of

• Metallic/pearlescent pigments

• Resin medium

• Fast-evaporating solvents

• Flow additives and setting agents.

The main effects in a basecoat/color layer arise from the effect of pig-

ments used in this suspension. Aluminum is subject to various manufactur-

color that arise at different angles of a car.

Some of the salient features of aluminum pigments are:

• High area to weight ratio

• Inherent opacity and spectral response of metallic aluminum.

composition:

• Thickness 0.1–0.2 μm, diameter 1–200 μm

• Metal content: 95–98.8%

• Oxide content: 1–3%

• Lubricant content: 0.2–2%.

Al pigments are generally sold in paste form. Most products contain 65%

Pearlescent pigments offer luster, sparkle, depth and metallic-looking shine,

and enable Multiple Color Play.

ing processes to produce Al pastes which could be leafing or non- leafing

film

pigments, ‘ cornflake’ or silver dollar pigments. The basic nature of the pig-

ments gives rise to flip- flop effects determining the different gradations of

• Highly preferred flat, parallel orientation of the flakes in the paint

Industrial grade ( leafing or non- leafing) aluminum flakes have the following

flake metal and 35% volatile hydrocarbons (solvent naphtha/mineral spirit).

416 High-performance organic coatings

19.4.3 Basecoat application process

When producing single-tone basecoat/clearcoat two-layer coatings, the

single-tone basecoat is conventionally applied by means of electrostatically

assisted high-speed rotary application [7]. This is not conventional when

producing special effect basecoat/clearcoat two-layer coatings, the special

effect basecoat instead generally being applied in two spray passes. In this

cally assisted high-speed rotary application (approximately 60–70% of the

special effect basecoat layer), while in the second pass spray application

generally proceeds by pneumatic spraying without electrostatic assistance

pneumatic spray application does indeed have the disadvantage of lower

clearcoat two-layer coatings with regard to pronounced and uniform devel-

opment of the special effect, avoidance of clouding and overall appearance.

If the subsequent pneumatic spray application is replaced by electrostati-

cally assisted high-speed rotary application, the optical results achieved are

generally less good, in particular in the case of special effect basecoats in

19.4.4 Clearcoat technology

Modern automotive original lacquer coatings generally consist of a base

lacquer/clear lacquer topcoat, which is applied onto a body which has been

electrophoretically primed and coated with surfacer [8]. In such coating

processes, the colored and/or effect base lacquer and the clear lacquer are

preferably applied wet-on-wet, i.e. the base lacquer and subsequently

applied clear lacquer are jointly stoved.

The optical/aesthetic qualities of such base lacquer/clear lacquer two-

clear lacquer layer. Ideally, the texture of the clear lacquer layer is identical

on the horizontal and vertical surfaces of a three-dimensional substrate,

for example an automotive body, which may be ascertained, for example,

by determining the long- and short-wave components of the surface texture

of the clear lacquer surface.

This ideal situation is, however, not straightforward to achieve in prac-

tice. For example, differences in surface texture or leveling of the outer

clear lacquer layer may occur as, once applied and in particular during the

heating phase of the stoving process, the clear lacquers have a tendency to

run or sag on non-horizontal surfaces due to a temperature-determined

reduction in viscosity. There are various ways of counteracting unwanted

case, spray application proceeds in the first pass by means of electrostati-

(approximately 30–40% of the special effect basecoat layer). While the final

application efficiency associated with increased losses due to overspray, it

guarantees excellent optical quality of the finished special effect basecoat/

very light metallic shades or with a strong color flop effect.

layer lacquer coatings are substantially influenced by the quality of the