DOI: 10.1007/s13367-019-0010-9
Precise and quantitative assessment of automotive coating adhesion
using new microgap pull-off test
Chi Hyeong Cho1,†, Intae Son1,†, Ji Yong Yoo1, Gitae Moon1, Eunbi Lee1, Sung Ho Yoon2,
Jae Sik Seo2, Choon Soo Lee2 and Jun Hyup Lee1,*
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 received much attention in automotive industry because the adhesion characteristics of automotive paints have a great influence on color and appearance of automobiles. Here, we present a robust and precise method for quantifying the coating adhesion of automotive paints on flat substrates using new microgap pull-off test based on the application of a micrometer-thick layer of adhesive to the divided compartments. The influence of water and organic material penetration on the coating adhesion between paint and plastic substrate is investigated in order to quantitatively measure the water resistance and organic compound resistance of automotive 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 reducing the initial coating adhesion. In addition, we investigated the effect of chlorinated polyolefin (CPO) content on the coating adhesion between nonpolar plastic substrate and polar paint coating. As the CPO content in polar acrylic paints increased, the coating adhesion of the polar paint to nonpolar plastics was increased due to 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.
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 resistance
A 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
2.3. Preparation of test specimens for the microgap pull-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 principle
The 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 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
---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. Characterization
The 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
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 the coating 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.
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 paint
Figure 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.
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, resultcur-ing 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.
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.
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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