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Effects of Environmental Conditions on Degradation of Automotive Coatings

Effects of Environmental Conditions on
Degradation of Automotive Coatings
Mohsen Mohseni, Bahram Ramezanzadeh and Hossain Yari
Department of Polymer Eng. and Color Tech.,
Amirkabir University of Technology
P.O.Box 15875-4413, Tehran,
1. Introduction
Two main goals are expected when coatings are applied to substrates. The main one is
protection of substrate from various aggressive environments such as sunlight and
humidity. The second is to impart color and aesthetic to the substrate to be coated. In some
applications such as automotive coatings, these two are highly important. Exposure for a
long time to different permanent (sunlight, rain & humidity) and occasional (acid rains and
various biological substances) parameters during the service life of these coatings results in
loss of performance. Such phenomena not only render the coating to degrade also lead to
depreciation of appearance attributes of the finished car. Automotive coatings are usually
multi-layered systems in which each layer has its predefined function. These make the
whole system resist to various environmental factors. Figure 1 shows a typical automotive
coating system.
Fig. 1. Specifications of a multilayer automotive system
As figure 1 describes, the substrate is initially coated by a conversion layer such as
phosphate or chromate to enhance the adhesion and corrosion protection of the metallic
substrate. Then, an electro deposition (ED) coating, usually based on epoxy-amine
New Trends and Developments in Automotive Industry
containing anticorrosive pigments and zinc powders, is applied to protect the coating from
corrosion. The primer surfacer which is a polyester melamine coating is then applied. The
main function of this layer is to make the coating system resist against mechanical
deformations such as stone chipping. The color and special effects, such as metallic luster
are obtained using a basecoat layer which is typically an acrylic melamine resin pigmented
with metallic and pearlescent pigments. To protect the basecoat, a non-pigmented acrylic
melamine clear coat is applied over this layer. This latter layer is responsible for the gloss
and smoothness of the coating system. On the other hand, the clear coat, apart from creating
a highly glossy surface, is intended to protect the underneath layers, even the substrate,
against various aggressive weathering (i.e. humidity and sunlight) and mechanical (i.e. mar
and scratch) factors during service life.
It should be noted that all layers are applied when the previous layer has dried, except for
the clear coat that it is applied through a wet-on-wet method in which it is applied on the
wet basecoat layer after a short time for flashing off the solvents. The curing processes of all
layers are presented in figure1.
In order to fulfill the required properties, automotive coating systems are required to remain
intact during their service life, because they are extremely vulnerable to deteriorate (Nguyen et
al., 2002 a; b; 2003; Yari et al., 2009a). There are various environmental factors which can
potentially be fatal for these coatings and may cause loss of appearance and protective aspects
of the system. The consequences of these factors are discoloration, gloss loss, delamination,
crack propagation, corrosion, and gradually building up coating degradation. Acid rain, hot-
cold shocks, UV radiation, stone chips, car washing, fingernail and aggressive chemical
materials are among those parameters rendering the coatings to fail in short and/or long
exposure times to environment. These would lead to dissatisfaction of customers. Therefore, it
is vital to enhance the resistance of the coating against environmental factors.
In the following part of this chapter, different environmental conditions and their effects on
various aspects of coating have been presented. Preventive methods will be given where
necessary. Among the environmental factors, the influence of biological materials will be
explained with more details because their effects have not been discussed elsewhere.
2. Environmental factors
Environmental factors are those substances or conditions imposed by the environment to
which the automotive coatings are exposed. As such, different chemical and/or mechanical
alterations (degradation) may result. Here, they have been divided to three main
subcategories, i.e.; mechanical, weathering, and biological factors.
2.1 Mechanical damages
Automotive coatings can be encountered different outdoor conditions during their service
life. Mechanical objects can put severe effects on these coatings. Depending on the type of
imposed stress to these coatings various kinds of degradation can be observed (Shen et al.,
2004). The most important of these can be seen in Figure 2.
2.1.1 Chipping resistance
The ability of multi-layer automotive coatings to withstand against foreign particles without
being damaged is named stone-chip resistance. It is found that, when stone particles attack a
coating they have velocity near to 40-140 km/h. This can cause coating delimitation from the
Effects of Environmental Conditions on Degradation of Automotive Coatings
Fig. 2. Different type of mechanical damage occurring on automotive coatings (Shen et al., 2004).
paint-substrate interface (Lonyuk et al., 2007; Buter & Wemmenhove, 1993). For multi-layer
system, coating layers interadhesion, coatings mechanical properties and coating interaction to
substrate are the most important factors affecting chip-stone resistance. These can make the
chipping resistance of these systems very complicated. It has been demonstrated that, the
mechanical properties of each layer can affect their chip resistance. In this regard, it has been
found that glass transition temperature of the primer layer is the main factor controlling
coating chipping resistance. The greater glass transition temperature may cause adverse
performance. The temperature at which this measurement is conducted is also very influential.
The failure appeared during chipping in a multi-layer coating system can be both adhesive
and/or cohesive failure. It was found that when the strength between two layers exceeded, the
defect was mainly adhesive failure. As a result of this, delaminating, flaking or peeling will
occur. On the other hand, crack initiation and propagation within a coating layer across the
other layers can cause cohesive failure (Lonyuk et al., 2008) (Figure 3).
2.1.2 Abrasion resistance
Basecoat/clear coat systems create an outstandingly high glossy appearance in comparison to
other automotive paint systems. However, such a high gloss makes mechanical damages more
visible when they appear. Scratch and mar are the most important of these failures. They are
micrometer deep surface damages that may ruin the initial appearance of automotive finishes.
The difference between mar and scratch is mainly in their different sizes and morphologies.
Scratch is a consequence of tribological events encountered by automotive clear coats. The size
for this type of damages is 1-5 μm (Courter, 1997; Tahmassebi et al., 2010). To show how these
types of damages influence coating appearance, the visual performance of coating before and
after scratching are shown in Figure 4.
mar Rough trough Crack
Delamination Crack
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Fig. 3. The SEM micrograph of the chipped surface of coating (Lonyuk et al., 2008).
Fig. 4. Visual differences of automotive coating before and after scratching.
Mechanical damages of these types may be caused by polishing equipments, carwash
bristles, tree branches and sharp objects such as keys (Tahmassebi et al., 2010).
Before scratch After scratch
Before scratch After scratch
Effects of Environmental Conditions on Degradation of Automotive Coatings
2.1.3 Scratch type
The performance of automotive coatings is further complicated by nature of the created
scratches, which in turn is influenced by the viscoelastic properties of the clear coat itself,
and the conditions under which they are created. In this regard, when an external stress is
applied to coating, there would be three different kinds of coating responses: elastic
deformation, plastic deformation and fracture deformation (Tahmassebi et al., 2010; Lin et
al., 2000; Hara et al., 2000). Elastic deformation has limited effect on the appearance of a
coating, therefore determination of plastic and fracture deformation seem more important.
Some scratches are irregular and of a fractured nature (Figure 5-a) and may involve material
loss, while others are smooth (Figure 5-b), regular and involve plastic deformation of clear
coats (Lin et al., 2000; Ramezanzadeh et al., 2010; Jardret & Morel, 2003; Jardret & Ryntz,
2005; Jardret et al., 1998).
Fig. 5. SEM micrographs of two types of (a) fracture and (b) plastic scratches (Tahmassebi et
al., 2010; Ramezanzadeh et al., 2010).
Various parameters such as scratch force, scratch velocity and environmental temperature
would influence the type and form of scratch produced.
There are many differences between these two types of scratches. First, fracture types are
irregular and may involve material loss (Figure 5-a), while others are smooth, regular with
no material loss (Figure 5-b). The visibility of fracture-type scratches is independent on the
direction of incident light and illumination. Conversely, plastic-type scratches are not visible
if the longitudinal direction of the scratch coincides with the direction of the lighting. These
differences are schematically shown in Figure 6-a and b (Lin et al., 2000).
Fracture 1
Material loss
Fracture 2
Irregular shape
Plastic 1 Plastic 2
Without material loss
Smooth surface
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Fig. 6. Schematic illustration of (a) fracture and (b) plastic type’s scratches
Elastic or plastic behaviors of a clear coat result in spontaneous or retarded recovery of the
created scratches, respectively. This is usually named as healing ability of clear coat.
Fracture behavior, on the other hand, arises from tearing apart of polymer chains contained
within the clear coat, therefore recovery or healing of the created scratches would not be
possible. The mechanism by which scratch can be formed by a scratch indenter are shown in
Figure 7 (Hara et al., 2000).
According to figure 6, different parameters like indenter tip morphology (tip radiance and
stiffness), tip velocity and coating viscoelastic properties affect the coating response against
applied stress. As shown in this figure, applied force can be divided into tangential and
vertical vectors. Tangential forces cause compression and stretching in the clear coat in front
and behind of such particles, respectively. Tensile stresses produced behind such particles can
cause cracks in the clear coat and/or aid in scratch formation. Consequently, the tensile stress/
strain behavior of clear coats can be used to predict scratch behavior. This phenomenon has
been shown by Jardret and Morel in detail (Jardret et al., 2000; Jardret & Morel, 2003).
Fig. 7. Schematic illustration of how scratch indenters affect coating deformation type (Hara
et al., 2000).
2.1.4 Methods to improve coating scratch resistance
Based on the above explanations, improving scratch resistance and variations in scratch
morphology are of utmost importance in the research and development departments of the
(a) (b)
Tensile zone
Effects of Environmental Conditions on Degradation of Automotive Coatings
automotive finishing industry. Accordingly, researchers have proposed various methods for
improving the scratch resistance of automotive clear coats. The proposed methods include
procedures to increase surface slippage and hardness, as well as enhancing cohesive forces
within clear coats that modify the viscoelastic properties of clear coats as a whole. Increasing
surface slippage and hardness inhibit the penetration of scratching objects into clear coats,
thereby increase the force necessary to create scratches. If forces generated by scratching
objects exceed that of the cohesive forces within a clear coat, then polymer chains of the clear
coat tear apart and show a fracture-type (Hara et al., 2000). There are many methods to
improve coating viscoelastic properties including changing clear coat chemistry and using
different pigments (in both nano and micro size) and additives (like polysiloxane additives).
However, changing the chemical structure of a clear coat would not guarantee modification
of its viscoelastic properties. Furthermore, changing the chemical structure of a clear coat
may incur unwanted adverse effects on other properties of the resultant clear coat and will
in most cases, increase its price. Consequently, attempts have been made in many research
programs to modify viscoelastic properties by physical incorporation of various additives
into a clear coat of known chemical structure. Controlled use of these additives could ensure
minimization of unwanted variations in other properties of the resultant clear coat as well as
being an attractive and economically viable alternative (Tahmassebi et al., 2010;
Ramezanzadeh et al., 2010; Zhou et al., 2002; Ramezanzadeh et al., 2007; Ramezanzadeh et
al., 2007; Jalili et al., 2007).
2.1.5 Methods to evaluate coating scratch resistance
Several methods have been used to evaluate the scratch and mar resistance of clear coats.
Scratch-tabber is one of the most traditional used methods for analyzing coating scratch
resistance. This method can predict coating scratch resistance based on the weight loss of
coating during scratch test (Lin et al., 2000). Laboratory car wash simulator is another
method which has been used in recent years. This is a useful method based on an
appropriate simulation from a real scratching process in an outdoor condition (Tahmassebi
et al., 2010). Nano and micro-indentation are powerful methods to evaluate both scratch
resistance and morphology of coating. In addition, use of these methods could be favorable
for analyzing clear coat scratch resistance, deformation type of the clear coat (plastic or
fracture) and viscoelastic properties (Tahmassebi et al., 2010). Gloss-meter and
goniospectrophotometer have been used to evaluate the effects of scratches produced on the
appearance of clear coat (Tahmassebi et al., 2010). Microscopic techniques including optical,
electron and atomic microscopes have been used to investigate scratch morphology.
2.2 Weathering factors
Weathering factors are those that are applied to the coating by weathering (or climate), and
cause alteration in chemical structure (Nguyen et al., 2002 a; b; 2003, Bauer, 1982), affecting
various aspects of the coating properties such as physical (Osterhold & Patrick, 2001),
mechanical (Tahmassebi & Moradian,2004; Nichols et al., 1999; Gregorovich et al., 2001;
Nichols & Darr, 1998; Nichols,2002; Skaja, 2006) and electromechanical (Tahmassebi
et al., 2005) properties. The severity of degradation caused by weathering factors
depends strongly on climatic condition. Sunlight and humidity are the most important
weathering factors. It is almost impossible to prevent automotive coatings being exposed to
New Trends and Developments in Automotive Industry
2.2.1 Sunlight
Sunlight reaching the earth contains a wide range of wavelengths from 280 to 1400nm
(Valet, 1997). The most harmful part is the uv range (less than 380 nm). Most polymers are
sensitive to this part of the sunlight. For example polyesters and alkyds have absorption
peaks around 315 and 280-310 nm, respectively (Valet, 1997). The absorbed energy can cause
a kind of degradation called "photodegradation", the mechanism of which is known and has
been extensively discussed in litreatures (Pospısil & Nespurek, 2000; Valet, 1997). A brief
description of photodegradation is given here. The absorbed energy by some chromophoric
groups (ch) of the polymer turns it to an excited state (ch
*). This excited state is able to
induce formation of various free radicals. The following equations present different free
radicals produced during photodegradation.
Polymer (p) Free radicals (P•,PO•,HO•,HOO•,…)
A) Initiation
P• + O2 POO•
PO• + HO•
P•+ H
PH + HO•
B) Propagation
P• + P• P-P
P• + PO• POP
C) Termination
As a consequence, chain scission and formation of various stable and unstable spices such as
peroxide, hydroperoxide, hydroxyl and carbonyl groups are the most important reactions
involved in photodegradation. Formation of different polar species leads to an increase in
surface energy of the coating (Tahmassebi & Moradian, 2004). These produce hydrophilic
groups in the coating and increase the susceptibility for water diffusion. Finally, this leads to
greater potential of underneath layer to be corroded.
2.2.2 The effect of basecoat pigmentation
Due to significant role of the clear coat on weathering and mechanical properties of
automotive coatings, most of the previous studies have focused on an isolated clear coat
layer. But there are reasons to believe that the basecoat greatly affects the weathering
performance of its attached clear coat. In order to illustrate how a basecoat could vary the
weathering performance of a clear coat, it is necessary to clarify how a basecoat reacts to
incident light. As stated before, common basecoat contains colored pigments and/or
Effects of Environmental Conditions on Degradation of Automotive Coatings
metallic flakes. Colored pigments absorb and/or scatter incident visible light reaching the
bulk of a basecoat, according to their color, size and refractive index. Metallic flakes, based
on their level of orientation, reflect and/or scatter incident light only at the surface of the
clear coat. In this manner, fractions of returned incident light passing through the clear coat
are decisive in causing chemical changes in the clear coat structure, leading to alterations in
the clear coat properties.
In order to elucidate the influence of basecoat pigmentation on degradation of a typical
automotive clear coat during accelerated weathering tests, using two different basecoats (i.e.
silver and black) can be useful. Amongst common commercial basecoats, silver and black
seem to be two extreme basecoats. In other words, a silver basecoat is characterized by the
presence of high loads of aluminum flakes (acting as a reflective source of visible light), and
a lack of colored pigments, in which the chance of reflecting incident light is high and the
chance of absorbing incident light is minimal. While the black basecoat, is characterized by
the presence of high loads of a black pigment (acting as an absorbent of visible light), and a
lower load of aluminum flakes; this means that the reflection or scattering chances of
incident light are low and its absorption is high. Figure 8 schematically shows how two
different basecoat pigmentations react to incident light.
Silver Basecoat/clear coat
Incident light
Aluminum flake
Black pigment
Polymeric chains
Black Basecoat/clear coat
Incident light
Clear coat
Fig. 8. The reaction of two different basecoat pigmentations to incident light.
Therefore, these two basecoats seems to be two extreme examples in their reaction to
incident light. Other basecoats, depending on their ability to reflect or absorb light could be
ranked to be somewhere between the black and silver.
The rate of variations in carbonyl groups of a coating during weathering can in fact be
considered as the photodegradation rate of that coating (Mielewski et al., 1991). Figure 9
shows normalized absorbances of carbonyl bands of clear coats attached to silver or black
It is clearly obvious that the photodegradation rate of the clear coat having a silver basecoat
is greater than that of the black one during weathering. Such results indicate the higher
ability of silver basecoat to induce photodegradation reactions in the clear coat during
weathering exposure (Yari et al., 2009a).
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0 100 200 300 400 500 600
osure time (hr)
Carbonyl/CH Rati
Black Silver
Fig. 9. Normalized absorbances of carbonyl bands of clear coats attached to silver or black
Various approaches are available for lower photodegradation mechanisms given above. The
first method is to prevent the UV rays from being reached the coating chromophores by
adding substances which are able to strongly absorb and filter the UV wavelengths (Valet,
1997; Bauer, 1994). These materials are called Ultra Violet Absorber (UVA). The
conventional UVAs are benzotriazoles, triazines and bezophenones. Nowadays, by
advances obtained in nanotechnology, new generation of materials have been achieved that
not only are capable to absorb UV rays, but also can improve the mechanical, thermal and
electrochemical performance of the coating (Peng et al, 2008; Dhoke et al., 2009; Xu & Xie,
2003) . The best choices for this purpose are titanium dioxide, zinc oxide, cerium oxide, iron
oxide or even silica nanoparticles. Because of the high surface area of these nanoparticles the
absorption efficiency of these materials has been promoted considerably. Figure10 shows
AFM topographic images of two acrylic melamine clear coats containing 0 and 3.75%
nanosilica after 1000 hours exposure times (Yari, 2008).
Figure 10 also clearly reveals that the most variations is assigned to neat polymer while
nanocomposite tolerates less variation in surface topology, meaning less weathering
degradation. This indicates that incorporation of nano silica into acrylic melamine not only
has not any effect on weathering durability, it enhances its resistance during weathering.
The better weathering performance of clear coats containing nanosilica is assigned to the
ability of nano silica particles to absorb the ultra violet and visible light, resulting in less
degradation in nano silica-containing clear coats (Jalili, 2007; Zhou, 2002).
Another preventive strategy for improving the resistance of coatings against
photodegradation is the use of quenchers and radical scavengers. Quenchers are materials
that can transfer the excited state of ch* to themselves. They then become excited. Their
excited state is not able to produce free radicals. Radical scavengers convert the active free
radicals to inactive ones and are unable to participate in photodegradation reactions.
Hindered amine light stabilizers (HALS) are the most typical kinds of additives for this
purpose(Bauer et al., 1992; Seubert, 2003; Mielewski et al., 1993). Synergestic effect of HALS
and UVA have made a significant improvement in photostability of the coatings.
Effects of Environmental Conditions on Degradation of Automotive Coatings
Fig. 10. AFM topographic images of different clear coats after various exposure times.
2.2.3 Water and humidity
Raining, car-washing, and dew formation are conditions by which water is in contact with
automotive coatings during its service life. While, most polymers are hydrophobic and are
not affected by water and humidity, some polymers that have water-sensitive linkages in
their structure can be hydrolyzed by water or humidity. Acrylic/melamine as the most
typical structure used in automotive clear coats, is vulnerable to water and well susceptible
to hydrolytically degrade.
Figure 11 depicts different reactions happening in hydrolytic degradation of a typical acrylic
In these hydrolytic degradations, various etheric, esteric and methylene bridges are broken,
creating various OH&NH-containing products, i.e. methylol melamine and primary or
secondary amines (Nguyen et al., 2002 a; b; 2003). Meanwhile, other reactions called self-
condensation reactions occur between methylol melamine groups present either in initial
structure of clear coats or formed during early times of reactions. As a result of self-
condensation reactions, different melamine-melamine linkages i.e. new methylene or etheric
bridges (reactions c and d in figure 11) are formed. These new formed linkages have less
flexibility than the initial linkages. This results in a higher glass transition temperature.
It has been demonstrated that chemical structure (like the ratio of acrylic/melamine or
polyol/isocyanate) and cross-linking density of the clear coat have a significant impact on
the intensity of the hydrolytic degradation (Yari et al., 2009b). The lower the cross-linking
density, the greater is water permeation and blister formation. The assessment of the
resistance of the coating against humidity is carried out by saturated humidity test. The
results of blister formation and the visual appearance of two different coatings (with high
and low cross-linking densities) are shown in Figure 12.
0 % - before exposure 3.75 % - before exposure
0 % - after 1000 hr exposure 3.75 % - after 1000 hr exposure
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a) Hydrolysis of alkoxy melamine
b) Hydrolysis of acrylic/melamine linkages
c) Self-condensation of methylol melamines (forming ether linkages)
d) Self-condensation of methylol melamines and amines (forming methylene bridges)
R''= Acrylic chain
Fig. 11. Degradation and self-condensation reactions for a typical acrylic melamine.
Fig. 12. Results of humidity test for different types of coating.
Coating 1 – Before humidity test Coating 1 – after humidity test
Coating 2 – after humidity test Coating 2 – Before humidity test
Effects of Environmental Conditions on Degradation of Automotive Coatings
Coating 1 is an automotive type with high cross-linking density (νe= 0.002673 mol/cm3) and
coating 2 is the same one with lower cross-linking density(νe= 0.000486 mol/cm3). In
contrary to coating1, which shows no blistering, severe blisters are seen on the surface of
coating2. Blistering is a result of diffusion of water and other soluble materials into coating.
2.2.4 Acid rain
Acid rain which is a very common phenomenon in urban and industrial areas is a catalyzed
type of hydrolytic degradation. Acidic environment catalyzes the hydrolysis reactions.
Various gases like SO2 produced in the polluted areas are converted into sulfuric acids
which makes the precipitates acidic. These acidic rains when fall on the coatings catalyze the
hydrolysis reaction of acrylic melamine clear coat. The acid catalyzed hydrolysis has been
investigated in several works (Mori et al., 1999; Schulz, et al, 2000; Palm& Carlsson, 2002). It
has been found that the acid rain and the acid catalyzed hydrolysis are most likely to occur
at moderate to strong acidic environments. For example, the results reported by Schulz and
co-workers (Schulz, et al, 2000) showed that, the pHs of a real acid rain even at the
aggressive environments (Jacksonville, Florida) lied in the range of 3.5-4.5. Acid rain etches
the acrylic melamine and strongly decreases the coating surface.
Different strategies can be adopted to increase the hydrolytic resistance of an acrylic
melamine coating; decreasing the ratio of melamine, use of hydrophobic chains, decreasing
melamine solubility, decreasing the basic strength of melamine and partially replacing of
melamine with other amino resins.
2.3 Biological materials
Biological materials are those substances produced from bio sources. These are the most
important environmental factors which affect the chemical, mechanical and visual
performance of automotive coatings. These mainly include insect bodies, tree gums and bird
droppings. Whilst, the influence of sunlight, humidity and acid rain on automotive coatings,
especially on clear coat has been studied thoroughly, the effect of biological materials has
not been dealt with in more details. In this regard, an automotive coating is repeatedly
exposed to different biological materials such as bird-droppings, tree gums and insect
bodies. Therefore, the investigation of the influence of such materials and the coating
degradation mechanism seems inevitable. Stevani and co workers (Stevani et al., 2000)
studied the influence of dragonfly eggs, a native insect of north and south America, on an
acrylic melamine automotive clear coat. They found that hydrogen peroxide released during
hardening of eggs, oxidizes the cysteine and cystine residues present in the egg protein,
leading to the formation of sulfinic and sulfonic acids. The acids produced catalyze the
hydrolytic degradation.
2.3.1 Bird droppings
In different papers, the effects of bird droppings on appearance and thermal-mechanical
properties of coating have been investigated (Ramezanzadeh et al., 2009; Ramezanzadeh et
al., 2010 a). Typical defects observed on the clear coats influenced by bird droppings were
investigated by different techniques as shown in figure13 (Ramezanzadeh et al., 2010 a; Yari
et al., 2010).
The optical microscope images of clear coats show that even at a relatively short exposure
time to bird droppings and pancreatin, the clear coat surfaces have been etched severely.
New Trends and Developments in Automotive Industry
Fig. 13. Appearance of defects created after being exposed to bird droppings.
These images may confirm that chemical reactions have occurred at the surface, leading to
dissolved and etched areas. It was found that bird droppings decreased the appearance
parameters of clear coat, i.e. gloss, distinctness of image (DOI) and color values, therefore
affecting the aesthetic properties of the coating system (Ramezanzadeh et al., 2009).
Thermal-mechanical studies also showed that hardness, glass transition temperature and
cross-linking density of degraded clear coats decreased in the presence of bird droppings
(Ramezanzadeh et al., 2010 a). Also, the influence of aging method (pre-aging or post-aging)
and chemical structure of clear coats against such bio attacks, were reported (Ramezanzadeh
et al., 2009; Yari et al., 2009 c) [11,12]. It was observed that post-aging process, which
simultaneously exposes bird droppings and UV radiation to coatings, degraded the clear
coat much more intensively than the pre-aging one, in which only bird droppings on pre-
weathered clear coats was exposed (Ramezanzadeh et al., 2009 ). The investigation of clear
coat chemistry revealed, that incorporating higher ratios of melamine cross-linker, in spite
of resulting a higher cross-linking density, led to an inferior biological resistance (Yari et al.,
2009 c).
Although the main process was a hydrolytic cleavage, it was also a catalyzed hydrolytic
degradation. The mechanism of this bio-attack is shown in figure14.
It has been reported that bird droppings consists of amylase, lipase and protease which are
all hydrolase enzymes and are responsible for cleavage of C-O-C (for example in starches),
COO esteric linkage (for example in glycerin) and CO-NH peptide amide linkages (for
example in proteins), respectively. Enzymes are amino-acid molecules that their function is
to catalyze various chemical reactions in biological environments, e.g. in the human body or
animals. The rate of most enzyme-catalyzed reactions is millions of times faster than those of
comparable un-catalyzed reactions.
Bird - (Optical)
10 μm
Bird - (AFM)
Bird - (Digital camera)
Bird - (SEM)
Effects of Environmental Conditions on Degradation of Automotive Coatings
Acrylic melamine chain
or bird
Broken catalyzed
by amylase
Broken catalyzed
by Lipase
Broken catalyzed
by Lipase
Broken catalyzed
by amylase
Susceptible to be
broken catalyzed
by amylase
Susceptible to be broken
catalyzed by Lipase
Broken catalyzed
by amylase
Self-condensation reactions: ( Tr = Triazine ring )
2 Tr-NH-CH
-O- CH
( formation new etheric linkages)
-OH → Tr-NH
-OH + Tr-NH
formation new methylene bridges)
Fig. 14. Degradation Mechanism of a typical acrylic melamine caused by bird droppings.
New Trends and Developments in Automotive Industry
After pancreatin or bird-droppings deposition on clear coat surface, the hydrolysis reaction
can take place. The enzymes present in these materials catalyze the hydrolysis reaction.
Among these enzymes, protease due to the absence of amide linkages (-CONH-) is relatively
inactive on acrylic melamine. However, amylase and lipase enzymes act on etheric and
esteric linkages respectively, accelerating the cleavage of these bonds. Due to the presence of
high active sites (etheric and esteric linkages) in acrylic melamine, the cross-linked network
is cleaved. This leads to formation of soluble products and releasing from the coating,
leaving etched area on the surface. The clear coat consists of high cross-linking and low
cross-linking regions. The latter are more vulnerable against hydrolytic degradations
(Sangaj & Malshe, 2004) and are more affected.
As seen in SEM images of figure13, there are some micro cracks at degraded areas. This may
be attributed to an ion-induced oxidation due to the presence of metal ions (Ratner et al.,
Moreover, extensive studies on the similarity of bird droppings and pancreatin using X-ray
fluorescence and Fourier Transform Infrared Analyses (Yari et al., 2010) showed that the
chemical structures are generally similar. So same effects are created on coating after being
exposed to bird droppings and pancreatin. Therefore, the use of pancreatin instead of
natural bird dropping seems an alternative. The effect of clear coat chemistry
The monomer types of acrylic resin, the functional groups of melamine cross-linker and the
acrylic/melamine ratio, are the main factors which affect the curing (and inevitably its
performance) in the resultant coating. However, due to the presence of esteric and etheric
linkages in the structure of these resins, the occurrence of hydrolytic reaction seems
probable, leading to inferior chemical and weathering resistance. It has been found that the
chemistry of clear coat affects the coating performance against bird-dropping. It was shown
that two acrylic melamine clear coats differing in melamine ratio had different resistance
against bird dropping. Figure 15 shows the optical Images of two different partially
methylated acrylic/melamine clear coat (Cl-1 and Cl-2) which only differ in
acrylic:melamine ratios. Cl-1 has more melamine portion in its formulation.
The comparison of optical images of both clear coats shows that the Cl-1 undergoes more
catastrophic etching compared to Cl-2. It may be attributed to higher portion of melamine
component of Cl-1 which is more susceptible to hydrolysis reaction and therefore, a higher
etching. whereas Cl-2 sample, with less amount of melamine, experiences lower etching
(Yari et al., 2009 c). The effect of basecoat pigmentation.
It has been demonstrated that basecoat pigmentation via affecting the efficiency of curing
process of its attached clear coat influences the biological resistance of automotive coating
system. In seeking the reason why the degrees of cure are different, the effect of pigmentation
on heat transfer should be considered. In Figure 16 various mechanisms of heat transfer
during the curing process are schematically shown (Ramezanzadeh et al., 2010 b).
The typical ovens used for curing of the coatings utilize hot air conditioning as well as IR
lamps. It may also be expected that convection and radiation heat transfer are more
important during such curing processes. The difference in curing behavior of clear coats
attached to black and silver basecoats (two extreme basecoats) can be explained by
emissivity factor of these basecoats. Emissivity factor of a material is the relative ability of its
surface to emit energy by radiation. It is defined by the ratio of energy radiated by a
Effects of Environmental Conditions on Degradation of Automotive Coatings
Fig. 15. Optical microscope micrographs of different samples differing in melamine ratio
(Cl-1 has more melamine portion) degraded by pancreatin (or bird droppings) .
Aluminum flacks
Incident radiation energy Reflected radiation energy
heat transferring
Black basecoat
Clear coat
Clear coat
Silver basecoat
heat transferring
Fig. 16. Schematic representation of heat transfer during the curing process
particular material to energy radiated by a black body. According to Thomas (Thomas, 2005)
the infrared emissivity factor of basecoats containing a typical carbon black pigment or a
typical non-leafing aluminum pigment are 0.88-0.9 and 0.29-0.33, respectively. The greater
the emissivity factor of a coating the lower is its infrared reflection. Additionally, it is highly
likely that a silver basecoat would contain larger loads of an aluminum pigment compared
to a black basecoat. Therefore, it is probable that the clear coat attached to a silver basecoat
would be exposed to extra infrared radiation than that attached to a black basecoat. This
extra energy may in turn induce a more complete degree of cure in the clear coat attached to
a silver basecoat than that attached to a black basecoat.
Cl-1 After Pancreatin Cl-2 After Pancreatin
New Trends and Developments in Automotive Industry
Effects of basecoat pigmentation on visual performance of clear coats experiencing bird
dropping attack can clearly be observed in Figure17.
Fig. 17. Optical micrographs of the clear coat samples on silver and black basecoats exposed
to biological materials
It is seen that the more efficient curing on clear coat having silver basecoat results in better
performance compared to that of having a black basecoat. The effect of aging process
As the biological materials affect the coating both in aged samples (exposed to environment)
and its freshly prepared form, the aging conditions used to study the effect of these
materials included a pre-aging and post-aging. Pre-aging means that before the exposure of
clear coat to biological attack a four-stage aging process is performed. This multi-stage aging
is conducted according to PSA D27 5415 standard. The details of stages have been explained
elsewhere (Ramezanzadeh et al., 2009). In summary, these stages are schematically
presented in figure 18. In post-aging method, the coating is subjected to both aging and
biological attacks simultaneously.
Steel substrate
Automotive coating system 5 cycles of climatical
situation in high temp.
and humidity
105 hours weathering exposure (in accordance
to the Peugeot D27 1389-95 standard,
continuous irradiance of 0.55 w/m2
at 340 nm,
relative humidity 50%, dry temperature 54°C.
Covering samples by a strip of an absorbent cotton
wool, adding 10 times of fresh water as of cotton
weight, keeping samples in polyethylene bag at 60° C
for 48 hrs ± 30 min in an oven.
5 cycles of
climatical situation
in high temp. and
Deposition of Arabic
gum solution, keeping
samples at 60° C for
24 hrs ± 30 min
Thermal shocking at -19°C
for 3 hrs
Removing of biological
Tape adhesion test for
Fig. 18. A brief description of test method PSA Peugeot – Citroen D27 5415.
It was found that clear coats which have experienced simultaneous weathering and
biological materials(post-aging) was more degraded than those being initially experienced
Bird drop-pre aged
Black basecoat
Bird drop-pre aged
Silver basecoat
10 μm 10 μm
Effects of Environmental Conditions on Degradation of Automotive Coatings
weathering condition followed by exposure to biological materials. It is due to the
intensifying effect of UV radiation as well as sunlight.
Different methods can be pursued to prevent from degradation caused by bird droppings.
Making the surface more hydrophobic, using clear coat with fewer esteric or etheric
reactions can also be useful.
2.3.2 Natural tree gum
It is a general belief, that cars should be better kept under the shadow of trees in order to
prevent them from a direct sunlight exposure. However, in this case the effects of gums
extracted from the tree may be simply neglected. This can be seen from Figure 19.
Fig. 19. Effect of Arabic gum on car body.
The visual effect which is produced under tree gum attack can be shown in Figure 20.
According to Figure 20, the clear coats exposed to natural tree gums show considerable
surface cracks indicating a severe physical effect. In addition, etching behavior of both
materials, shown itself as numerous holes on the surface, can be also observed in the case of
this kind of degradation. According to the above explanation, the general effects of gums
can be appeared in both physical and chemical on the surface of coating. In addition, SEM
micrographs of this kind of degradation can reveal sub-cracks inside of macro cracks shown
in optical images. It can be also found that, the affected area (inside the crack) is lighter than
the unaffected parts of coating. This can illustrate different elemental composition inside
and outside the cracks. This can, similar to bird droppings, reveal the presence of metal
compounds in gums structure. This phenomenon will be discussed later. The increased
roughness in nano-scale of degraded parts of coatings (exposed to Arabic gum) can be
obtained from the AFM micrograph (Ramezanzadeh et al., 2010b).
New Trends and Developments in Automotive Industry
Fig. 20. Visual performance of coating after the tree gum attack, (a) visual performance, (b)
optical image, (c) SEM micrograph and (d) AFM micrograph (Ramezanzadeh et al., 2010c). Tree gum characteristics
Tree gums are completely soluble in water and have a sticky behavior in this state. The pH
of this material is about 4.5 in a slurry state. Arabic gum has been used as a synthetic
equivalent for tree gums. This was due to the similar acidic nature, physical state in water
and their similar chemical structures (shown in Figure 21). However, many parameters i.e.
soil nature, climatic condition (which trees grow there) and the type of tree can influence
these characteristics (Ramezanzadeh et al., 2010b; Ramezanzadeh et al., 2010c).
The solubility of these gums can be explained by the presence of high amount of –OH
groups (as shown in Figure 21) making them soluble in water. Due to these OH groups,
Arabic gum has a sticky behavior in the slurry state. Therefore, when gum is exposed to the
clear coat surface, based on the surface chemistry (hydrophobicity or hydrophilicity) a good
adhesion can be obtained between the polar groups of coating and gum. When this system
is exposed to higher temperatures, water is gradually vaporized. During the gum drying
process, a significant stress can be imposed by the gum to the surface. To have a more
understanding on how these applied stresses act, the visual performance of gum exposed
clear coats are given in Figure 22 (Ramezanzadeh et al., 2010c).
According to the observations made in Figure 22, two different phenomena can be observed
on the free films and the full system on metal plates. In the latter, a severe crack formation
can be seen for the dried gum exposed samples, causing similar cracks on the clear coat
layer. On the other hand, gum applied on free films has made the films to shrink. For the
(a) (b)
(c) (d)
Effects of Environmental Conditions on Degradation of Automotive Coatings
Wavenumber (cm
Transmitance (%)
Fig. 21. FTIR spectra for natural and synthetic (Arabic) tree gum (Ramezanzadeh et al., 2010c).
Fig. 22. The visual effect of gum attacked to (a) fully coated system on metal substrate and
(b) free film of basecoat/clear coat (Ramezanzadeh et al., 2010c).
clear coat applied on the full automotive system, due to the great adhesion of clear coat to
basecoat layer, and basecoat layer to other layers which are in contact with metal substrate,
the stress cannot overcome the adhesion force and therefore, it causes surface cracks to
propagate. However, for the free films, due to the lack of adhesion to the substrate, the
greater cohesion force, in comparison to its adhesion, would turn the film to shrink.
Different factors including aging condition, clear coat surface chemistry and basecoat
(a) (b)
New Trends and Developments in Automotive Industry
pigmentation can influence this kind of degradation which will be briefly discussed later.
Regarding the above explanations, the main source of producing this kind of degradation is
the stress formation during gum drying. The stress can overcome adhesion force (between
coating and substrate or clear coat and the other coating layers in a multi layer system)
and/or cohesion of the clear coat and/or basecoat layers. The ability to store such stress and
dissipate it can be depended on many factors, mainly coating viscoelastic properties and
temperature. These will be explained later. Physical attack by tree gum Effect of aging condition on gum attack
In a real outdoor condition, coatings properties are continually affected by aging conditions.
These effects could irretrievably change the chemical and mechanical properties of coatings.
Aging process has been shown as an important factor which significantly influences the
clear coat properties before and after the biological attack by bird droppings (Yari et al.,
2010c; Ramezanzadeh et al., 2010). As previously discussed, two different ideas of the effect
of aging on gums attack can be available. Aging condition can influence the degradation
occurring during gum attack by affecting coating properties before the test. The second idea
is the effect of weathering condition imposed to clear coat in contact with gum. The
mechanisms by which these two aging conditions affect gum attack severity are completely
different. To show how aging, before or after gum attack, can influence coatings properties,
the visual performances of samples experienced these are given in Figure 23.
Fig. 23. Samples attacked by gum in A1 (optical micrograph) and B1 (SEM micrograph)
post-aging and A2 (optical micrograph) and B2 (SEM micrograph) pre-aging processes
(Ramezanzadeh et al., 2010c).
Plastic crack
(B1) (B2)
Plastic crack
Effects of Environmental Conditions on Degradation of Automotive Coatings
According to Figure 23, a greater surface crack has been produced on the samples exposed
to gum at the post-aging process, in comparison to the pre-aging one. These results can
reveal that, aging is an important parameter influencing the clear coat biological behavior.
This explains that the effects of gum to give rise in surface attack may not only show the
crack density or size differences but also reveals that the cracks produced on the samples
experienced pre-aging condition have a plastic morphology, whilst cracks created under
post-aging show a fracture nature. Highly fractured cracks observed on the clear coats
exposed to gum under post-aging process, in comparison to the plastically deformed ones
shown in the samples exposed to pre-aging, clearly reveal the importance of aging process
on the crack morphology evolution (Ramezanzadeh et al., 2010c).
A significant increase of Δ[NH/NH2 and OH]/ [CH] after pre-aging process (before
biological test) has occurred. This indicates that aging may significantly affect the clear coat
by a chemical degradation mechanism. The increase in surface OH groups in the pre-aging
condition, leads to significant increase of clear coat hydrophilicity (Ramezanzadeh et al.,
2010c), and therefore, stronger interaction with gum. In addition, decreased Tg and
crosslinking density of clear coat were obtained at this condition. These changes can
negatively influence coating properties against the stress performed by gum. On the other
hand, during the biological test in the post-aging, using water sprayed to the clear coat
surface (during 300 h of xenon test), a greater interaction of gum and clear coat surface can
be created. This can cause a more severe crack on the samples experienced post-aging. A
decrease in drying process of gum can result in a greater interaction to clear coat, and
therefore enhanced surface attack (Ramezanzadeh et al., 2010c). Effect of coating chemistry on gum attack
As it was previously shown (Ramezanzadeh et al., 2010b; Ramezanzadeh et al., 2010c), due
to the sticky behavior of gums in the slurry state, it makes a good adhesion to clear coat
surface. Many researchers have tried to distinguish the main source of this adhesion. In fact,
the tendency of Arabic or natural tree gum to adhere to the surface, results from the polar
groups existed in this material. Therefore, the adhesion of Arabic gum to coating can
directly depend on the clear coat surface energy (balance of hydrophilicity and
hydrophobicity). According to the above explanations, the effect of gums on the clear coat
can be directly corresponded to the strong adhesion before the experiment, as well as to the
weak attachment after the drying. This behavior causes a great stress to the clear coat, which
in turn is responsible for the physical degradation of coating, as shown in Figures 22 and 23.
The failure which this stress can perform to clear coats can depend on both clear coats
compositions and the undercoat layers mechanical and viscoelastic properties. In addition,
the effects of this stress on coating performance can be discussed by two different
phenomena as (i) stress restoring and (ii) crack propagation. Based on coating viscoelastic
properties, different behaviors of the coating against the inserted stress is predictable. The
greater toughness and elastic properties of a coating, the higher the ability is for stress
restoring, leading to relaxation during a period of time. In this case, stress can not affect the
coating properties. However, most coatings have a viscoelastic properties rather than elastic.
The viscose part of the coating does not have restoring and relaxing behavior against
applied stress. So, the stress causes a failure on the coating. When the applied stress is not
able to overcome the adhesion forces between coating layers, it can affect the cohesion.
Different factors may affect the coating cohesion properties, especially the cross-linking
density. A lower cross-linking density can cause a lower cohesion. In a real condition,
New Trends and Developments in Automotive Industry
coating surface contains different areas having different cross-linking densities. Theses parts
of coatings have a lower elastic behavior and, therefore are able to restore the stress. In
addition, the lower cross-linking density of some parts of clear coat may be attributed to a
lower curing degree. Therefore, it may be expected that, these parts of coating have a more
polarity than other parts due to the presence of unreacted functional groups. This may cause
a stronger interaction of gums polar groups to clear coat surface at these areas. Based upon
the above explanations, the stress inserted to the clear coat can affect some parts of the
coating more intensively than the other parts. Therefore, stress can be propagated from the
weak points. In this way, the applied stress may be dissipated by the crack formation
(Ramezanzadeh et al., 2010c). Effect of basecoat pigmentation on gum attack
It was previously demonstrated that (Ramezanzadeh et al., 2010c; Yari et al., 2009a),
basecoat pigmentation can considerably influence the mechanical properties of an
automotive clear coat. This behavior was attributed to the curing degree and post reactions
occurred. Therefore, according to these results, outdoor weathering conditions may well
affect the clear coat properties based on the type of basecoat. Biological resistance of an
automotive clear coat can also be expected to show different behavior depending on the
basecoat pigmentation type. To show how basecoat pigmentation affect coating behavior
against gum attack, the effect of gum on fully coated and free films of the clear coats applied
over silver and black basecoat are shown in Figure 24 (Ramezanzadeh et al., 2010c).
Fig. 24. Effect of basecoat pigmentation on their biological performance in the case of
gum attack on (A1) and (A2) full coated and (B1) and (B2) free films (Ramezanzadeh et al.,
Effects of Environmental Conditions on Degradation of Automotive Coatings
It can be seen that the surface cracks produced by gums over the clear/black system are
smaller in size. However, fewer cracks being greater in size for the clear coat on the silver
basecoat can be observed. Smaller cracks appeared in the black coating system revealed the
greater ability to restore and relax the stress. To show how gum can differently affect clear
coats applied over different basecoats, the drying process of gum on these two different
samples are shown in Figure 25 (Ramezanzadeh et al., 2010c).
Fig. 25. Effect of gum on (A1) silver and (B1) black (before exposure) samples, and (A2)
silver and (B2) black samples after exposure (Ramezanzadeh et al., 2010c).
According to Figure 25, greater shrinkage of black sample can be obtained. These differences
can be resulted from the many different factors mainly the difference between the chemical
structures of the clear coats applied over these basecoats and the mechanical properties of
basecoat layer. In a silver system, due to the lower emissivity factor of basecoat, a greater
curing can be obtained resulting in a higher cross-linking density and toughness. In
addition, as a result of this better curing, less hydrophilicity and therefore adhesion of gum
to clear coat surface can be obtained. The higher Tg of the clear coat on the silver system, as
well as its greater cross-linking density, would result in different mechanical properties
(Ramezanzadeh et al., 2010c). Moreover, a greater clear coat storage modulus of the silver
system (at the temperature of biological test), can reveal different mechanical properties of
this clear coat, which can effectively influence the biological attack to tackle the stress. The
result is a higher capability of this coating to restore and further dissipate the stress
performed by Arabic gum. This means that the greater cross-linking density of this sample
(silver one) causes a higher cohesion. Therefore, the ability of the clear coat to distribute the
stress on the entire film and other layers is prevention of the stress concentration and
formation of cracks, the consequence of which is that the mechanical properties of basecoat
(A1) (A2)
(B1) (B2)
New Trends and Developments in Automotive Industry
layer are affected. Hence, as the mechanical properties of basecoat layers are different, the
higher vulnerability of the black system in biological attack is probable. The greater
aluminum flakes presented in the silver basecoat, due to the formation of a stronger physical
network, causes a higher toughness, in comparison to the black basecoat, causing greater
resistance of coating against the applied stress for this system. In addition, the presence of
aluminum flakes in the basecoat can cause a greater damping behavior of this layer by
preventing the stress concentration on coating (Ramezanzadeh et al., 2010c). Chemical attack by tree gum
As shown in Figure 20, several etched areas can be observed on the clear coats which only
experienced 300 h exposure to simultaneous weathering and gums. The presence of these
etched areas in a relatively short exposure time indicates that, gums can accelerate the
hydrolytic degradation of acrylic melamine, leading to extensive formation of soluble
products which are easily released from the coating, leaving spotted etched areas. As stated
before, the pHs of Arabic and natural gums are acidic (4.7 and 4.28, respectively). The acidic
environment created by gums may also account for the occurrence of such accelerated
etching phenomenon. It has been found that, acidic solutions can affect and catalyze the
hydrolysis reactions in the same way (Zhou et al., 2002; Schulz et al., 2000). Several
researchers have studied this condition for acrylic melamine, in terms of degradation caused
by ‘‘acid rain’’, which is also a very common phenomenon. The pH of acid rain is around
3.5–4.5. A stronger acidic environment causes more catastrophic degradations. Therefore,
the grater variations in FTIR spectra of clear coats exposed to natural gum, compared to that
of samples in contact with gum, can be explained by the more acidic nature of this material.
These observations can illustrate that, gums can influence coating properties both in
chemical and physical ways but mainly in physical direction (Ramezanzadeh et al., 2010c).
3. Concluding remarks
The properties and characteristics of automotive coatings have been discussed. The
complicated conditions imposed to these systems need to be well understood in order to
enhance their resistance against environment. Photo and hydrolytic degradations are the
two common phenomena occurring under external conditions. In addition, the viscoelastic
behavior of coatings is also detrimental for a proper mechanical performance. Above all,
biological degradation is as important as the other types of failures.
To highlight this kind of degradation the effects of bird droppings and tree gums on an
automotive clear coat have been studied. Results showed an irretrievable effect of these
biologicals on the visual performance, mechanical and chemical properties of clear coat.
Effect of clear coat chemistry on its biological performance, exposed to natural and synthetic
biological materials, has been studied. The effects of basecoat pigmentation and aging
condition on the biological performance of the clear coat have been also investigated. The
general conclusions obtained are shown below:
1. It has been found that the catalytic hydrolysis of etheric and esteric bonds are the
reasons for coating degradation when exposed to bird droppings. It was found that
natural bird droppings, due to containing some digestive hydrolyse enzymes such as
amylase and lipase, are able to catalyze the hydrolytic cleavage of etheric and esteric
linkages of acrylic melamine clear coat. The consequence of these cleavages is the
release of water soluble products from the coating, leaving etched areas and local
Effects of Environmental Conditions on Degradation of Automotive Coatings
defects as well as decreased appearance on clear coat surface. Results clearly revealed
that bird droppings considerably affect the clear coat mechanical properties. According
to these results, Tg and elastic modulus were negatively decreased. In addition, the
decreased micro hardnesses of clear coats exposed to these biological materials was a
further observation indicating the severe effects of biological materials on the
mechanical properties of clear coats.
2. The pronounced effect of natural tree gum was a severe crack formation and shrinkage
on fully coated systems and free film samples, respectively. It was also shown that, gum
could strongly attach to clear coat surface before a drying process commenced. During
gum drying, significant stress can be applied on the coating layers, especially the clear
coat. Based on the coating properties, i.e. viscoelastic and toughness, different behaviors
of coatings against applied stress, such as stress relaxation and/or coating failure were
3. It has been demonstrated that many parameters mainly surface chemistry and
viscoelastic properties of clear coat (the balance of surface hydrophobicity/
hydrophilicity), aging condition (post or pre aging) and basecoat pigmentation (metallic
or non-metallic) can influence coatings biological performance.
4. Future trends
It would be interesting to further study the effects of surface chemistry
(hydrophilicity/hydrophobicity balance) on the biological resistance of automotive coatings.
Also investigating the influences of viscoelastic properties of coating systems need more
attention. Use of nano-based materials such as additives and pigments seem to be effective.
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... Primarily, the basic objectives of the painting process are to protect and decorate functions of the motor vehicle body, in which its purpose is to enhance or change an object's appearance, to change its colour or level of gloss or to draw attention to a particular region on the object [1] [3]. However, just as important, there is a need for protection from any environmental conditions (e.g., mechanical, weathering, biological) in terms of degradation of automotive coatings or to lengthen their lifetime [4]. Since the introduction of automotive coatings systems to luxury car lines in the early 1980s, their use has increased dramatically to the point where they are used on all cars nowadays, as protective prescriptions for items as diverse as airplanes, automotives, bridges, houses and machinery [5]. ...
... Moreover, there are different parameters which may affect the clear-coat surface chemistry and can directly influence the anti-corrosive performance of this layer during their exposure to outdoor factors [8]. These mainly include weathering environments (e.g., UV radiation from sunlight, water and humidity, acid rain, hot-cold shocks) [9] [10] [11] and biological materials (e.g., bird droppings, raw eggs, tree gum, insect bodies) [2] [4] [12] [13] [14] [15] [16] [17]. In response to this, a variety of exposure methods and test protocols have been developed to anticipate the long-term weathering behavior of coating failure and predict performance [10] [11] [12] [15] [18] [19] [20] [21] [22] [23]. ...
... Automotive Layers Clearcoat 50-60 µm Dry Film Thickness Approximately Basecoat 10-30 µm Primer Surface 25-35 µm Cathodic Electro-deposition 20 µm Metal/Plastic Bodywork and Pre-treatment (a) (b) Figure 1. (a) a typical cross-section through a painted panel and (b) paint layers on vehicles with approximated dry film thickness, adapted from [4] ...
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There are different aggressive biological materials which may potentially deposit on a painted automotive body surface during its service life, causing possible local damage, loss of appearance and loss of protective aspects of the system. In this study, the effect of two types of aggressive biological materials on a painted automotive body surface, i.e., natural bird droppings and raw eggs were studied and subsequently explained in more detail. Furthermore, two different testing conditions approaches including indoor and outdoor were utilized in order to investigate the surface roughness, R a , and also to study the behavior of biologically degraded automotive body surface at nano-level scale. The effects of these biological materials on a painted automotive body surface and its appearance were investigated by Atomic Force Microscopy (AFM) and a stylus-based inductive gauge (Taly-surf®, from Taylor Hobson, Inc.), having electromagnetic control of the contact force. Engaged vertically on the top of the specimens, the force could be set much lower than the weight. Results showed that natural bird droppings and raw eggs have a dramatic effect on the appearance and surface roughness of a painted automotive surface body. It was also found that the degradation which occurred due to the natural bird droppings was more severe than that of the samples exposed to raw eggs.
... The exposure to UV light can cause alteration in chemical structure of coating which leads to the reduction of molecular weight to the deterioration of mechanical properties and physical properties of coating paint film (Yousif and Haddad, 2013;Nguyen et al., 2002). Exposure to UV-A may cause loss of appearance such as discolouration, gloss loss, higher surface roughness and stone chips (Mohseni et al., 2011). These would lead to dissatisfaction to the customer and therefore it is crucial to enhance the resistance of the coating against UV-A light. ...
... The material must have sufficient initial gloss and must adequately retain the gloss during long term exposure to UV light. This test is was carried out to find determine out whether the exposure of the coating paint film towards UV light will cause alteration of the surface roughness or not (Mohseni et al., 2011;Ecco et al., 2017). It appears that a the factor that contributes to the decrease of gloss of the coating paint coating film upon exposure is the due to the generation of free radicals caused by the UV light through photooxidation. ...
This work investigates the visible optical stability of coating paint film consisting organic synthetic dye, 4-hydroxycoumarin derivatives to the exposure to ultraviolet A (UV-A) light. An azo dye was synthesized by coupling diazonium salt of aniline derivative (obtained by diazonation of 4-chloroaniline in presence of sodium nitrite and hydrochloric acid) with 4-hydroxycoumarin in the presence of sodium hydroxide. The azo dye was mixed with xylene before using it as a pigment in the coating composition. The mixture of poly(methyl methacrylate) (PMMA) and acrylic polyol was used as the coating binder. The synthesized dye and binder were then mixed at specific ratio to form a complete coating solution. Two coating solutions with PMMA having different molecular weights (Mw: 350,000 gmol<sup>-1</sup> and Mw: 996,000 gmol<sup>-1</sup>) were used in this study. Each of them was labelled as 350K and 996K indicating its molecular weight. Both mixtures were applied as coating paint films on glass substrates were exposed to UV-A for fast photo-degradation process. The visible optical stability of the coating paint films was measured and recorded at an interval of eight-hours exposure for 35 days using the Commission Internationale de l'Eclairage (CIE) L*a*b* colour coordinate system. The obtained datas were analysed using standard deviation (STD). In this study, both coating samples showed low standard deviation for hue angle, namely 0.206 for 996K and 0.258 for 350K which indicates a high colour stability. However, the 350K possesses a smaller colour difference (ΔE) of 0.798 compared to 1.418 for 996K.
... However, some products are often marketed without proper evaluation of the durability and resistance of these materials. Various composite materials may show wear in color and physical resistance after long periods of utilization by the user (Mohseni et al. 2011). ...
Bio-composites are promising materials to be used as an alternative to composites. Weathering studies can evaluate the effects of the degradation on bio-composites properties. By accelerated weathering testing, it is possible to predict the durability of materials in a short time. Exposure to UV radiation, moisture, and temperature can cause leading to loss of color and brightness, roughness, and aging of bio-composites. The chemical characteristics, crystallinity, and molecular weight of bio-composites are also affected, with the possibility of randomly break down the polymer chains. The reduction in tensile and flexural strength and thermal properties are also consequences of the degradation process. In this way, to solve this problem, the reinforcement of bio-composites has been investigated. Therefore, this chapter addresses the main environmental factors involved in accelerated weathering and the degradation effects on morphology, molecular characteristics, and thermal and mechanical properties of bio-composites caused by this testing. The influence of nanofillers on the degradation rate of composites are also mentioned.
... However, some products are often marketed without proper evaluation of the durability and resistance of these materials. Various composite materials may show wear in color and physical resistance after long periods of utilization by the user (Mohseni et al. 2011). ...
Bio-composite materials, which are a serious alternative to synthetic-based fibre and matrix materials due to their high characteristics and biodegradability, cause difficulties and uncertainties for usage conditions due to their high sensitivity to climatic conditions. Scientific studies have shown that climatic factors such as temperature, humidity, radiation, UV rays, and acid rain that act synergistically in natural weathering conditions, cause degradation and changes in the bio-composite material's characteristics. Examining the material's behaviour under natural weathering conditions provides the most realistic and reliable results in terms of determining the shelf life of the material and knowing its behaviour in the usage environment. In this study, changes in thermal, mechanical, and aesthetic properties of bio-composite materials exposed to natural ventilation conditions were investigated. It has been observed that natural weathering induces dramatic decreases in thermal and mechanical properties of bio-composite materials, especially with the effect of prolonged exposure times, and causes changes in colour, surface deterioration and changes in shape.
... Although most polymer materials are hydrophobic and thus not affected by water and relative humidity, some that have pH-sensitive chemical linkages in their structure can be hydrolysed by relative humidity or water. Acrylic/Melamine as the typical structure used in automotive clearcoats is vulnerable to water and highly susceptible to degrade hydrolytically and yet another is the diverse class of organic coatings[40]. InFigure 10, for the house-use detergent, the surface roughness measurements obtained before the exposure (mean = 0.075 µm, ±SD = 0.011 µm) were lower than those which were obtained after exposure (mean = 0.089 µm, ±SD = 0.009 µm). On the other hand, for the car wash detergent, the surface roughness measurements obtained before the exposure (mean = 0.079 µm, ±SD = 0.007 µm) were very close to those which were obtained after exposure (mean = 0.086 µm, ±SD = 0.007 µm). ...
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The ability to predict the weathering performance of the clearcoat system over a short period of time is essential for the design and development of coating production. Thus, the primary objective of the present study is to investigate whether it is possible to predict the weathering performance of an automotive paint system through determination of surface roughness, R a , and micro-hardness before and after various weathering exposure times (0, 24, 168, 336, 504, 672 hours) and when employing two different detergent materials (house-use detergent and car wash detergent). The data were analysed using a pair-sample t-Test, with 0.05 level of significance. It was found that the total net of degradation in the clearcoat level during the first 24-hours was R a ≈ 30.3 nm (for surface roughness) and 1.358 HV (for the µ-hardness) when using the house-use detergent. In contrast, it was found to be R a ≈ 4.6 nm (for surface roughness) and 1.133 HV (for µ-hardness) when using the car wash detergent. Also, increased time of weathering (up to 672 hours) increases the R a and µ-hardness values. It can therefore be concluded that the effect of house-use detergent was more severe than that of car wash detergent on the clearcoat system.
... The field of plasmonic and metasurface structural color is still in its infancy. Study and analysis of the mechanical (abrasion, wear resistance), electrochemical (corrosion and moisture resistance), and flexibility (bending and formability) properties are required and probably have to be improved before the concept step into the industry and real product, in particular in automotive industry where abrasion and scratch resistance are essential [176][177][178]. ...
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The environmental concerns in the current century is not only limited to the polluting effect of the fossil fuel consumption but also the recycling challenges of waste turns to be a substantial challenges of the industry. Recycling of colored discarded materials is very difficult because of the problems in relation to the dissociation of diverse chemical compounds present in the colorant agents. Single or double component materials which could create various colors by geometrical changes can be a great solution to the mentioned limitations. Metasurfaces’ and metamaterials’ structural color therefore draws attention as they enable generation of vivid colors only by geometrical arrangement of metals which not only ease the recycling but at the same time enhance the mechanical stability of the colors. In this review, the progress in the field of plasmonic metasurface- and metamaterial-based structural colors is reviewed.
A series of clearcoats separately loaded with different concentrations of functional silicon–polyacrylate and polydimethyl siloxane additives were prepared. Optical performances of the cured films were studied by gonio-spectrophotometry. Dynamic mechanical thermal analysis was used in order to investigate viscoelastic properties of the additive-containing films. Contact angle measurements, (FTIR) spectroscopy, scanning electron microscopy equipped with energy-dispersive analysis of X-ray, and atomic force microscopy techniques were utilized to evaluate surface properties of the clearcoats. It was shown that clearcoat surface free energy decreased and its crosslinking density increased in the presence of the additives. Results revealed that addition of both additives to the clearcoat enhanced its resistance against pancreatin (simulated bird droppings). A decrease in surface degradation was observed in the presence of the additives. Results also showed that functional polydimethyl siloxane influenced coating viscoelastic, surface chemistry, and biological resistance more effectively compared to that of the functional silicon–polyacrylate one.
This work aims at improving the surface chemistry and the mechanical properties of a commercial acrylic–melamine clear coat using a functional siliconized additive. The resistance of films against biological degradation was then investigated using pancreatin (simulated bird droppings) and Arabic gum (simulated tree gum). Variations in the surface and bulk chemical structures, as well as the thermomechanical characteristics of the clear coats at different concentrations of the additive, were investigated by a wide range of techniques inclusive of contact angle measurement, gonio‐spectrophotometery, dynamic mechanical thermal analysis (DMTA), energy‐dispersive spectroscopy, atomic force microscope, optical microscope, and attenuated totalreflectance Fourier transform infrared (ATR‐FTIR) spectroscopy. Negligible effect of additive on color change was revealed. It was shown that even at low loadings of additive it could migrate to the surface, producing hydrophobic films with very low surface free energies with water contact angle exceeding 100°. In addition, it was found by DMTA and ATR‐FTIR studies that the functional additive was covalently attached to the acrylic–melamine chains through its hydroxyl groups. However, phase separation was observed at high concentrations of additive, leading to reduced crosslinking density. The clear coat resistance against pancreatin and Arabic gum was improved using optimum concentrations of the additive. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013
The aim of this study is enhancing an automotive clearcoat easy-to-clean property against simulated tree gum (Arabic gum) using hydroxyl-functional silicone polyacrylate additives having different hydroxyl contents. The clearcoat surface, mechanical and chemical properties were studied using a contact angle measuring device, dynamic mechanical thermal analysis (DMTA) and Fourier Transform Infrared spectroscopy (FT-IR) respectively. It was found that additive with lower hydroxyl content gave rise to better easy-to-clean properties of the clearcoat against Arabic gum. This additive also resulted in lower contact angles and higher cross-linking density, tensile stress and work of break of the clearcoat.
Abstract The present study aims at elucidating the effect of basecoat pigmentation on the chemical structure and surface topology of its attached clearcoat during weathering exposure. Two extremes of differently behaviored basecoat pigmentations (i.e. silver and black) were chosen. Different analyses such as FTIR, ATR, surface energy measurements and AFM were carried out on such coatings after they were subjected to accelerated weathering conditions. It was found that the black basecoat procured more post-curing reactions to the attached clearcoat at initial weathering times, while the silver basecoat induced higher degrees of photodegradations during the whole time of weathering. Such inductions were attributed to the inherent absorptive or reflective behavior of the black or the silver basecoats toward incident radiation.
Scratch resistance of polymers has been the subject of numerous studies, which have lead to specific definitions for plastic characteristic and fracture phenomena during scratch behavior. Viscoelastic and viscoplastic behavior during a scratch process has been related to dynamic mechanical properties that can be measured via dynamic nano-indentation testing. Yet, the understanding of the origin of the fracture process of a polymer during scratch remains approximate. Parameters like tip geometry and size, scratch velocity and loading rate, applied strain and strain rates, have been considered critical parameters for the fracture process, but no correlation has been clearly established. The goal of this work was to analyze the scratch resistance and indentation properties of PMMA as a function of temperature. Compression and tensile properties from literature have also been compared to the scratch results for more complete understanding of the material's behavior. The analysis of the evolution of the properties of PMMA at various temperatures, along with the evolution of the fracture toughness during scratch with temperature and scratch velocity has helped in identifying a correlation between the tensile stress-strain behavior and scratch fracture toughness for PMMA. This correlation brings a new understanding of the origin of the fracture mechanisms during a scratch process, and the strain effect on the fracture strength of PMMA.
Literature discussing coating mechanical properties and mar resistance is reviewed. Commercial car washing is the "standard" environmental source of damage. Most lab tests involve rubbing the coating with an abrasive and measuring the level of damage. The types of deformation that result are elastic, plastic, and fracture. For glassy coatings mar resistance is optimized at an intermediate toughness. The author proposes that optimization occurs due to the minimization of the number of both plastic- and fracture-type scratches. In addition, minimization occurs because the yield stress and toughness are in balance. Elastomeric coatings appear to have a minimum in mar resistance at intermediate crosslink density.
Modern automotive basecoat/clearcoat enamels are expected to maintain their attractive appearance over the life of the vehicle with no catastrophic failures such as cracks or delaminations. Ensuring the durability of automotive topcoat requires an understanding of the relationships between exposure conditions, the chemical changes that can occur in the various coating layers, and potential failure modes. This paper reviews those relationships with particular emphasis on the chemistry and mechanisms of coating degradation and stabilization. The key factors that affect topcoat appearance durability are as follows: resistance of clearcoat to acid attack and hydrolysis; intrinsic clearcoat photostability (free radical formation rates); nature of photooxidative pathway in the clearcoat (e.g., chain scission versus crosslink formation); effectiveness of hindered amine light stabilizer; permanence of ultraviolet light absorbers; and photostability of basecoat and primer. These factors are discussed in terms of specific coating chemistries. The effects of application variables as well as protocols for evaluating performance are discussed.
The lifetime of organic coatings is reduced in outdoor applications by attacks of solar radiation, oxygen and atmospheric pollutants. Undesirable mechanical, physical and chemical consequences of the resulting degradation can be substantially restricted by properly selected photostabilizers. Structures of absorbers of UV radiation (UVA) and photoantioxidants (hindered amine stabilizers, HAS), mechanisms of their activity, processes responsible for loss of their durability, interferences with other coating components (catalysts, pigments, extenders) and application in various coatings are outlined.
Properties of materials -- Classes of materials used in medicine -- Some background concepts -- Host reactions to biomaterials and their evaluation -- Biological testing of biomaterials -- Degradation of materials in the biological environment -- Application of materials in medicine, biology, and artificial organs -- Tissue engineering -- Implants, devices, and biomaterials: issues unique to this field -- New products and standards -- Perspectives and possibilities in biomaterials science
Dragonflies are attracted by the reflection of sunlight on car surfaces and lay their eggs on the clearcoat resin. Considering that the surface can reach up to 93°C and that during the egg hardening process (sclerotization) H2O2 is released, cysteine and cystine residues present in the egg protein can be oxidized to sulfinic and sulfonic acids. These are strong acids which, like acid rain, can hydrolyze the acrylic/melamine resin causing damage where the eggs were laid. Confocal Raman spectroscopy revealed that the spectra obtained from damaged and intact portions of the clearcoat were similar, in agreement with infrared absorption spectroscopy data. These data demonstrate that the attack by eggs, H2SO4 and cysteine/H2O2 only promotes solubilization of resin through acid hydrolysis of the resin ester and amide moieties. Furthermore, surface enhanced Raman scattering (SERS) spectra obtained from dragonfly eggs and cysteine/H2O2 reaction products treated with a silver colloid were very similar, thus confirming the presence of sulfinic and sulfonic acids.
Iodometric titration has been used to measure hydroperoxide concentrations in a series of unstabilized acrylic melamine and acrylic urethane coatings as a function of laboratory weathering time. Hydroperoxide concentrations are found to be a function of the initial free radical formation rates, the exposure conditions used, and the crosslinker type. For coatings with high initial free radical formation rates, the hydroperoxide level rises rapidly to a maximum and then decreases. For a given copolymer, the hydroperoxide level in the melamine crosslinked coating is always significantly lower than that in the urethane crosslinked coating. For coatings with low initial radical formation rates, the initial rise in hydroperoxide concentration is small. The hydroperoxide concentration in the melamine coating remains small, while that in the urethane coating slowly increases suggesting autocatalytic oxidation. Hydroperoxides contribute significantly to free radical formation at long exposure times, especially in the urethane coatings. The lower level of hydroperoxide in the melamine coatings is attributed to the ability of the melamine crosslinker to decompose hydroperoxides.
There are various biological materials which may repetitively deposit on a painted automotive body during its service life, causing possible local defects. This study is an attempt to reveal the mechanism of degradation caused by bird-droppings and to compare the performance of clearcoats having various resin/hardener ratios. Two different testing methods varying in aging conditions, of the effect of natural bird-droppings, were applied to two types of clearcoats. Variations in chemical structure were characterized by the aid of FTIR spectroscopy and DMTA analysis. Also, in order to establish an experimentally viable procedure to assess such an effect, synthetic countertype of bird-droppings (pancreatin) was used to simulate this natural phenomenon.The results revealed that a digestive enzyme (lipase) present in bird-droppings, can induce the hydrolysis reaction of coating polymer leading to a locally distributed etched surface. It was also found that the clearcoat containing higher ratios of melamine cross-linker had poorer performance against bird-droppings in spite of having a greater cross-linking density.