precise and quantitative assessment of automotive coating

© 2019 The Korean Society of Rheology and Springer 89

Korea-Australia Rheology Journal, 31(2), 89-96 (May 2019)DOI: 10.1007/s13367-019-0010-9

www.springer.com/13367

pISSN 1226-119X eISSN 2093-7660

Precise and quantitative assessment of automotive coating adhesion

using new microgap pull-off test

Chi Hyeong Cho1,†

, Intae Son1,†

, Ji Yong Yoo1, Gitae Moon

1, Eunbi Lee

1, Sung Ho Yoon

2,

Jae Sik Seo2, Choon Soo Lee

2 and Jun Hyup Lee

1,*1Department of Chemical Engineering, Myongji University, Yongin 17058, Republic of Korea

2Interior System Plastic Materials Development Team, Material Development Center, Hyundai Motor Company, Hwaseong 18280, Republic of Korea

(Received December 19, 2018; final revision received March 21, 2019; accepted March 27, 2019)

The quantification of coating adhesion on substrates is an important technology that has recently receivedmuch attention in automotive industry because the adhesion characteristics of automotive paints have agreat influence on color and appearance of automobiles. Here, we present a robust and precise method forquantifying the coating adhesion of automotive paints on flat substrates using new microgap pull-off testbased on the application of a micrometer-thick layer of adhesive to the divided compartments. The influenceof water and organic material penetration on the coating adhesion between paint and plastic substrate isinvestigated in order to quantitatively measure the water resistance and organic compound resistance ofautomotive paints. When the paint absorbed moisture and organics, they penetrated through the paint sur-face to interfere with the coating adhesion between the plastic substrate and the paint layer, thereby reducingthe initial coating adhesion. In addition, we investigated the effect of chlorinated polyolefin (CPO) contenton the coating adhesion between nonpolar plastic substrate and polar paint coating. As the CPO content inpolar acrylic paints increased, the coating adhesion of the polar paint to nonpolar plastics was increased dueto the compatibilization effect of CPO resin in the coating interface.

Keywords: automotive paint, coating adhesion, microgap, pull-off test, quantification method

1. Introduction

Paint plays an important role in the purchase of auto-

mobiles because it dictates the color and appearance, but

paint also prevents car body corrosion (Akafuah et al.,

2016; Dosdat et al., 2011; Guo, 2012). Therefore, the

automotive paint characteristics are important, including

color and appearance, corrosion resistance, impact resis-

tance, and coating adhesion. Among these properties, the

adhesion characteristics have a great influence on the

long-term stability of the coating layer, and corrosion

starts when the coating peels off due to low adhesion

strength. Generally, polar plastics, such as polycarbonate

(PC) and acrylonitrile butadiene styrene copolymer

(ABS), and nonpolar plastics, such as polypropylene (PP),

are used for automotive interior materials (Liu and Qiu,

2013). PC and ABS are thermoplastic resins, which are

characterized by high impact resistance and rigidity (Pham

et al., 2000; Zhang et al., 2001). They are also used for

exterior materials of many products, such as mobile

phones and monitors. These polar plastics are commonly

painted using polar paints, such as acryl and urethane

polymers. In contrast, PP is a nonpolar plastic substrate, so

it cannot achieve sufficient adhesion to polar coating resin.

Therefore, it is technically painted with chlorinated poly-

olefin (CPO) mixed with polar acrylic coating resin. CPO

crystals functioned as compatibilizers grow epitaxially on

the PP crystals during baking, which leads to intimate

interactions between PP and acrylic resin, and thus

enhances coating adhesion of polar paints (Schmitz and

Holubka, 1995; Tomasetti et al., 2001). For this reason,

the content of CPO greatly affects the interfacial adhesion

between the nonpolar plastic substrate and the polar paint

coating (Clemens et al., 1994; Ryntz and Buzdon, 1997).

Also, the adhesion of these coatings can be greatly

affected by the external environment such as humidity and

organic contaminants. The chemical and physical proper-

ties of automotive paint materials mainly made of poly-

mers can be deteriorated by contact with foreign substances,

such as the water or cosmetics. Therefore, it is necessary

to evaluate the coating adhesion of the paint to confirm

these external effects on the coating layer. Various assess-

ment methods are conducted to evaluate the coating adhe-

sion of paint, such as the cross-cut method (Huang et al.,

2007) and dolly test (Wolkenhauer et al., 2008). In case of

cross-cut method, it is difficult to quantitatively measure

the coating adhesion of paint because the area of coating

detachment is roughly determined by the naked eye after

tape peel-off process. While the dolly test based on the

pull-off adhesion testing can provide the adhesion strength

of coating paints, this method is problematic because the

†These authors are equally contributed to this work.*Corresponding author; E-mail: [email protected]

Chi Hyeong Cho, Intae Son, Ji Yong Yoo, Gitae Moon, Eunbi Lee, Sung Ho Yoon, Jae Sik Seo, Choon Soo Lee and Jun Hyup Lee

90 Korea-Australia Rheology J., 31(2), 2019

accurate and reproducible quantification of coating adhe-

sion is difficult due to the failure of coating layer detach-

ment.

In this study, we established a precise and quantitative

method to reproducibly assess the coating adhesion of

automotive paints using the newly designed microgap

pull-off test. The proposed approach is founded on the

application of a micrometer-thick layer of highly adhesive

material to the divided compartments of coating sub-

strates, leading to the clear detachment of coating layer

from plastic substrate. By using this microgap pull-off

test, we investigated the effect of penetration of external

substances such as moisture and cosmetics on the coating

adhesion of automotive urethane paints. In order to exam-

ine the effect of water penetration on the adhesion between

the polar coating layer and plastic substrate, water was

absorbed into the coating layer through a heating bath

under different time and temperature conditions, and to

investigate the effect of penetration of cosmetics, the coat-

ing surface was treated with commercial cosmetics at dif-

ferent temperatures. Furthermore, in order to examine the

effect of CPO content on the coating adhesion between

nonpolar PP substrate and polar paint layer, the ratios of

acryl and CPO resins were varied and compared. The rela-

tionship between the coating adhesion and various mate-

rial parameters involving water, organic contaminant, and

compatibilizer was demonstrated by using the proposed

quantitative method for automotive coating adhesion.

2. Experimental

2.1. MaterialsEach coated substrate (urethane-coated ABS, CPO-

coated PP) was obtained from Hyundai Motor Company

(Hwaseong, Korea). Urethane-based paint material was

prepared by mixing polyurethane and acryl resins, and

CPO-based paint coatings were manufactured by mixing

CPO and acryl resins in 1:3 (CPO-1), 2:2 (CPO-2), and

3:1 (CPO-3) ratios. The adhesive (Araldite 2014-1) used

to assess the coating adhesion of the automotive paints

was purchased from Huntsman Co., Ltd. (Woodlands,

Texas). The sunscreen (NIVEA Fresh Sun Lotion) used

for evaluation of resistance to organic compound was pur-

chased from NIVEA Co., Ltd. (Hamburg, Germany).

2.2. Preparation of test specimens for water and organic

compound resistanceA urethane-coated substrate prepared for water resis-

tance evaluation was impregnated in a heating bath (Jeio-

tech, BS-11) containing 1000 ml of water. The water-

resistance evaluation was carried out for 7 days at 40°C in

a low temperature environment (Water-L) and for 1 day at

80°C in a high temperature environment (Water-H). Then,

the specimens were dried at 50°C for 2 days in a vacuum

oven (Jeiotech, OV-11) to remove the residual moisture.

The organic resistance evaluation was performed by

applying 0.25 g of sunscreen per 50 mm length and 50

mm width on the surface of the urethane-coated substrate.

And then the coated surface was covered with a cotton

cloth, pressed with an acrylic plate, and treated at 80°C for

1 h (Sun-L) in a convection oven (Jeiotech, OV-11E). In

order to examine the effect of thermal treatment tempera-

ture on the coating adhesion strength under the above con-

ditions, we treated sunscreen-covered substrates at a

relatively higher temperature of 100°C (Sun-H). After the

treated test specimens were cleaned with a neutral deter-

gent, the surface of specimens was wiped with a tissue

paper, and they were dried at room temperature.

2.3. Preparation of test specimens for the microgappull-off testing

As shown in Fig. 1a, the test specimen consists of a top

plate and a bottom plate. The top plate is a substrate made

of aluminum, and the bottom plate is made of a plastic

substrate coated with an automotive paint. The overall size

of the specimen is 50 mm in length and 25 mm in width

for both substrates. To measure the adhesion strength of

the coating paint in a certain area, the center of the lower

plastic plate is divided into a section with 3 mm length

and 3 mm width, as shown in Fig. 1b. In order to apply a

micrometer-thick layer of highly adhesive material to the

divided compartments, a 5.75 μm ball spacer dispersed in

ethanol was sprayed on the surface of the lower plastic

plate, and then dried to adhere to the plate for 10 min at

room temperature. After applying adhesive to the inside of

the compartment, the upper plate and lower plate are

bonded to each other and heat-treated at 70°C for 2 h in

a convection oven for curing the adhesive while pressing

it in a clamp with a pressure of about 5 kgf/cm2.

2.4. Coating adhesion measuring principleThe adhesive strength of the coating paints was mea-

sured by pull-off test using a universal testing machine

(UTM; Lloyd Instruments, LR-5K), and the adhesive

strength was recorded by quantifying the required force to

separate the upper and lower plates of the test specimen.

As shown in Fig. 1c, a UTM machine is equipped with the

test specimens prepared by connecting a jig made for the

pull-off test. The mounted specimens were tested in a pull-

off test mode of UTM. The jig connected to the bottom

plate is fixed to the bottom of the UTM, and the jig con-

nected to the top plate rises vertically at a rate of 1000

mm/min. As the upper jig rises, the divided portion

adhered to the coating paint falls off and the force at that

time is measured by the UTM. The measured force is cal-

culated into stress using

(1)N

cm2

--------- =

Fmax

Ad

----------

Precise and quantitative assessment of automotive coating adhesion using new microgap pull-off test

Korea-Australia Rheology J., 31(2), 2019 91

where is the adhesive strength at failure, Fmax is the

maximum force, Ad is the detached area by adhesive, as

shown in Fig. 1d. In order to measure the precise detach-

ment force, the area of the coating layer desorbed on the

actual aluminum plate was calculated as the actual detached

area (Ad). The adhesive strength was determined using the

highest force (Fmax) during the microgap pull-off test.

2.5. CharacterizationThe structures of the pure coating paints, the detached or

residual parts of the coating paints, and the adhesives were

compared by using Fourier-transform infrared spectros-

copy (FTIR; Jasco, FT/IR-460 plus). The surface mor-

phology of the test specimens was examined by optical

microscopy (OM; Olympus, BX51). The surface penetra-

tion depth of the coating layers was determined by using

a nanoscratch tester (Anton Paar, NST3). The condition for

the scratch test was set with the speed of 2 mm/min, force

of 79.8 mN/min, and total length of 1 mm. In addition, the

hardness of the coating layer was measured by using

nanoindentation (Anton Paar, NHT3) at loading and un-

loading rates of 20 mN/min.

3. Results and Discussion

3.1. Effect of water absorption on the coating adhe-

sion of automotive paint

Figure 2a shows the FTIR spectra of the adhesive, pure

paint layer of coated substrate, and desorption regions of

the upper and lower plates from the pull-off test specimen.

The pure coating paint fabricated on the basis of urethane

resin showed N-H stretching and C=O stretching peaks at

3323 cm1 and 1734 cm1, respectively. The FTIR spectra

of the detached regions of the upper and lower plates

showed similar spectral features to that of pure paint layer,

which indicates that the coated paint material is present on

both substrates due to the breakdown in the bulk layer of

the coating paint after pull-off test. In order to further ver-

ify the interlayer separation of the coating paint, the sur-

face morphology of the upper and lower plates of the

specimen was examined through the OM experiment, as

shown in Fig. 2b. The OM image was measured at 100×

magnification and it was confirmed that the optical texture

of the upper plate was almost identical to that of the lower

plate, which suggests that the delamination occurred in the

bulk layer of the coating paint. These results confirmed

that the proposed microgap pull-off test provides the suc-

cessful detachment of coating paint from plastic substrate.

Figure 3 shows the result of the coating adhesive strength

for the pristine urethane-coated substrate and water-treated

specimens with different treatment temperatures. While

the coating adhesion strength of a pure specimen before

water absorption was 405.6 N/cm2, the coating adhesion

of Water-L specimen with a long-term water absorption at

Fig. 1. (Color online) Photographic images of (a) the upper and lower plates of the specimen, (b) the divided sections of the coating

layer, and (c) the pull-off test jig. (d) Basic principle of the coating adhesion testing mechanism.



low temperature of 40°C decreased to 105.6 N/cm2. Since

the water can penetrate into the bulk layer inside the coat-

ing film to reduce the cohesive strength between the res-

ins, the coating adhesive strength is greatly reduced after

the water absorption (Arslanov and Funke, 1988; Kim and

Kim, 2015). Comparing Water-L and Water-H specimens

under different treatment conditions, the coating adhesion

of Water-H specimen decreases even more when the water

absorption is performed at a high temperature of 80°C for

a short period of time. Therefore, it is inferred that the

coating adhesion of automotive paint may be greatly influ-

enced by the water absorption temperature rather than the

absorption time.

3.2. Effect of organic compound absorption on thecoating adhesion of automotive paint

Figure 4a shows the FTIR spectra of the adhesive,

untreated paint layer, and the separated regions of the

upper and lower plates from the sunscreen-treated speci-

men. Similar FTIR spectra to those of water absorption

evaluation were observed for the organic compound resis-

tance assessment due to the use of the same urethane-

coated substrate. The separated regions of the upper and

lower plates showed similar spectra, which suggests that

the delamination occurs in the bulk layer of the coating

paint after pull-off experiment. For further verification, the

OM images of the upper and lower plates had similar opti-

cal textures, which is analogous to that of water absorp-

tion test, as shown in Fig. 4b. Figure 5 shows the results

of coating adhesion strength of the urethane-coated spec-

imen after absorbing the sunscreen. Compared to the

unprocessed substrate, the Sun-L specimen treated at a rel-

atively low temperature of 80°C exhibited a reduced adhe-

sion strength of 76.3 N/cm2. Since the desorption of the

sunscreen-treated specimen occurs inside the bulk layer of

the coating paint, the organic compound of the sunscreen

transfers from the surface of the coated substrate to the

bulk layer, thereby reducing the cohesion between the res-

ins in the coated paint. The Sun-H specimen treated at a

higher temperature of 100°C had a coating adhesion of

231.6 N/cm2, which is an increased strength compared to

that of the Sun-L specimen. Previous studies have shown

that some of the penetrated organic compounds can improve

the cohesion of the resins by inducing a curing reaction

with the resin in the polymer film at high temperatures

(Daniels and Klein, 1991). It is inferred that the organic

compound such as sunscreen weakens the coating adhe-

sion of the paint layer when it is absorbed into the interior

of the coating layer like water absorption, but unlike

water, the organic compound may increase the coating

adhesion by bonding with the resin in the paint layer.

Since the nanoscratch and indentation tests are often

used to analyze the viscoelastic-plastic characteristics of

the coating film (Pelletier et al., 2008), the influence of the

Fig. 2. (Color online) (a) FTIR spectra of the adhesive, pure paint layer, and water-desorption regions of the upper and lower plates

from the pull-off test specimen. (b) OM images of the surface of the upper and lower plates.

Fig. 3. Coating adhesive strength of the pristine urethane-coated

substrate and water-treated specimens with different treatment

temperatures.



water and organic compounds on the viscoelastic-plastic

properties of the coating films was analyzed. Figure 6a

shows the penetration depth results according to scratch

length. During the scratch test, the variation in penetration

depth of Water-L and Sun-L specimens was significantly

higher than that of the pure coating layer. The final pen-

etration depths at the endpoint of Water-L and Sun-L

specimens were 10163.3 nm and 11019.8 nm, respec-

tively. This result is ascribed to the increased viscoelastic

properties stemming from the absorption of water and

organic compounds in the coating layer. Similarly, nanoin-

dentation measurements demonstrated that the Water-L

and Sun-L specimens exhibited a lower indentation hard-

ness of 222.9 MPa and 208.4 MPa than pure coating layer,

as shown in Fig 6b.

3.3. Effect of chlorinated polyolefin content on the

coating adhesion of automotive paintFigure 7 presents the FTIR spectra of CPO-1 on the

nonpolar PP substrate before and after the evaluation of

coating adhesion. While the unvalued CPO-1 paint layer

exhibited the characteristic stretching vibrations of CPO

and acryl resins, the totally different spectra were found

for desorption regions of the upper and lower plates of the

CPO-1 specimen. These spectra were very similar to that

of the pure PP substrate, which indicates that the failure

occurred at the side of PP substrate near the interface

between paint layer and substrate. Since the polar acryl

resin-rich CPO-1 layer and nonpolar PP substrate are

incompatible, the low interfacial adhesion between the

nonpolar substrate and the polar paint coating is expected,

leading to the delamination of paint coating layer on PP

substrate. To verify this failure of CPO-1 coating paint,

photographic and OM images were additionally measured

and shown in Fig. 8. After pull-off test of CPO-1, the dark

coating paint layer was confirmed to be completely

attached to the upper aluminum substrate through the pho-

tographic image. For further confirmation, the OM images

of CPO-1 showed that the surface microscopic image of

the unvalued CPO-1 paint layer was different from those

of the detached upper and lower plates of the specimen.

These results indicate that the delamination of CPO-1

coating paint occurs in the middle layer of PP substrate

rather than the bulk layer of the coating paint. The CPO-

1 coating layer is inferred to have a higher cohesive

strength between the polar resins than the interfacial adhe-

sion between the nonpolar PP and the polar coating paint,

resulting in complete desorption of the coating paint.

To examine the effect of CPO content on the coating

adhesion of paint, the photographic and OM images of the

CPO layer on the PP substrate after the pull-off test were

observed according to the CPO content, as shown in Fig.

8. In case of CPO-2 specimen, the dark desorbed paint

area and the bright adhesive area were found for the upper

Fig. 4. (Color online) (a) FTIR spectra of the adhesive, pure paint layer, and the separated regions of the upper and lower plates after

the sunscreen treatment. (b) OM images of the upper and lower plates.

Fig. 5. Coating adhesion strength of the urethane-coated speci-

men before and after absorbing the sunscreen.



aluminum substrate under photographic observation. In

addition, a comparison of the surface of the unvalued

CPO-2 with that of the detached lower plate through the

OM image revealed that the coating paint was partially

peeled off from the PP substrate. Since CPO-2 has a

higher CPO content than CPO-1, the interfacial coating

adhesion between the polar paint and the nonpolar sub-

strate is expected to be strengthened, leading to the partial

desorption of CPO paint layer on PP substrate. In case of

CPO-3 with the highest CPO content, the photograph

showed that only the bright adhesive area was present on

the upper aluminum substrate, indicating that the coating

paint was not desorbed from substrate. The OM observa-

tion also revealed that the surface of the unvalued CPO-3

was similar to that of the lower PP substrate. As a result,

since the content of CPO in paint layer is very high, CPO-

3 has a very strong coating adhesion to nonpolar PP sub-

strate, resulting in the failure at interface between the

adhesive and the coating paint. The cause of the increased

coating adhesion according to the CPO content was found

in the literature (Aoki, 1968; Bonnerup and Gatenholm,

1993; Tomasetti et al., 2000). Previous study showed that

the Cl content controls properties such as melting point,

glass transition temperature, solubility, and polarity of the

material, and that the CPO layer improves the coating

adhesion of polar paint to nonpolar PP substrate by com-

patibilization effect of chlorinated polyolefin resin in the

coating interface. The CPO crystals in paint layer grow

epitaxially on the PP crystals of substrate during paint cur-

ing process, resulting in the increased interactions between

PP substrate and polar paint resin (Schmitz and Holubka,

1995; Tomasetti et al., 2001). Therefore, as the CPO con-

tent increases, the coating adhesion of the polar paint on

nonpolar substrate increases.

Based on the above results, the coating adhesion strength

of CPO paint layer according to the CPO content is shown

in Fig. 9. While the CPO-1 specimen showed the lowest

coating adhesion strength of 843 N/cm2, the coating adhe-

sion of CPO-2 and CPO-3 gradually increased to 971 N/

cm2 and 1085 N/cm2, respectively. Therefore, as the CPO

content in the polar paint layer increases, the coating adhe-

sion with the nonpolar PP substrate gradually increases.

Since the coating layer of CPO-3 specimen was not detached

from the PP substrate, the coating adhesion strength of

Fig. 6. (Color online) (a) Scratch penetration depth and (b) indentation hardness of the urethane-coated specimen before and after

absorbing the water and sunscreen.

Fig. 7. (Color online) FTIR spectra of CPO-1 specimens before

and after the evaluation of coating adhesion.



CPO-3 is expected to be higher than the measured value.

As a result, it is confirmed that the chlorinated polyolefin

content in polar acrylic paints played a crucial role in

improving the coating adhesion on nonpolar PP substrate

through the proposed new microgap pull-off test.

In order to investigate the effect of chlorinated polyole-

fin on the viscoelastic characteristics of the coating layer,

the nanoscratch and indentation experiments were con-

ducted. As shown in Fig. 10a, the penetration depth of the

CPO-3 specimen was 14099.8 nm, which was lower than

that of the CPO-1 (20821.8 nm). This result suggests that

the plastic tendency of the coating layer becomes more

pronounced as the content of the polar CPO increases. In

addition, indentation hardness was measured for the CPO-

Fig. 8. (Color online) The photographic and OM images of the CPO layers according to the CPO content after the pull-off test.

Fig. 9. Coating adhesion strength of CPO paint layer according

to the CPO content.

Fig. 10. (a) Scratch penetration depth and (b) indentation hardness of the CPO paint layer according to the CPO content.



treated coating layers, as shown in Fig. 10b. Similar to the

surface scratch test results, it is confirmed that CPO-3

specimen with high CPO ratio exhibited higher indenta-

tion hardness of 192.6 MPa than CPO-1 (178.3 MPa).

4. Conclusions

A quantitative method to measure the coating adhesion

of automotive paints has been presented. The proposed

microgap pull-off approach applied a micrometer-thick

layer of adhesive material to the divided compartments of

coating substrate, leading to the reproducible detachment

of coating layer from plastic substrate. By using this

microgap pull-off test, the relationship between the coat-

ing adhesion and various material parameters such as

water, organic contaminant, and CPO compatibilizer was

confirmed. The absorption of water and organic com-

pound reduced the cohesive strength between the paint

resins, resulting in the decrease in the coating adhesion.

The increased CPO content in polar paints improved the

coating adhesion of the automotive paint on the nonpolar

plastics due to the increased compatibilization between

nonpolar PP and polar acrylic resin. This study provides a

method for quantifying the coating adhesion of automo-

tive paint and defines a relationship between coating adhe-

sion and various parameters involving materials and

environment. These results will help facilitate the appli-

cation of functional paints to automotive exterior and inte-

rior materials.

Acknowledgments

This work was supported by Hyundai NGV and the

National Research Foundation of Korea (NRF) grant

funded by the Korea government (MSIT) (No. NRF-

2018R1A5A1024127).

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