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Effect of soft base layer on scratch properties of acrylic hard coatings.

INTRODUCTION

Improving scratch resistance of transparent polymeric materials without compromising transparency and other desired properties remains a significant challenge for practical applications. To overcome these problems, resins for transparent hard coating such as acrylic resins were developed to protect surface of polymeric substrates [1-3]. However, those coatings were not satisfied for the needs to improve abrasion and scratch resistance, and the mechanisms of the surface damage formation on coating materials have still not fully understood yet.

Considering the scratch behavior of polymer coatings, it is more complicated than that of bulk materials as there are additional factors involved in the systems such as coating thickness, properties of layers and substrate, interfacial adhesion between layers, etc. [4-9] For instance, Browning et al. demonstrated that scratch resistance of acrylic coatings on steel substrate depended on ductility and thickness of the coatings [7]. In addition, not only the material characteristics of the coatings, but also that of the substrates had an impact on scratch resistance [5, 9], Thus, superior scratch resistant coatings can be achieved by integrating the knowledge. Multilayer coatings that can combine the attractive properties of different materials characteristics have been proposed to further enhance the coating performance [10, 11], Each layer in the coatings plays an important role in tailoring the final coating properties. A top layer is designed for improving aesthetic and scratch resistance whereas a base layer intends to enhance adhesion properties between the coating and substrate, and mechanical performance [5]. Considering the previous researches, an effect of soft rubber layer on indentation behavior for multilayer polymeric coating was demonstrated using finite element method [10]. The introduction of soft rubber layer into the coating system [i.e., poly(methyl methacrylate) PMMA/rubber/PMMA and PC/rubber/PC coating with thickness ratio 33/33/33 [micro]m] could minimize the interfacial shear stress at the substrate, thereby preventing coating delamination during indentation. Moreover, by changing the soft rubber layer thickness in PC/rubber/PC coating to be 42/16/42 [micro]m, the tensile stress causing radial crack on the top surface could be eliminated without increasing the interfacial shear stress. Conversely, Roche et al. reported that a hard on soft layer coating on glass substrate was detrimental to scratch performance [5]. The corresponding layer thickness was 15 and 10 [micro]m, respectively. From nanoscratch test, it was found that the presence of soft base layer diminished the scratch performance. However, the effect of soft base layer on scratch resistance of polymeric coating on macroscale is still unclear.

Thus, in this work, single- and bilayer acrylic coatings on PMMA were prepared. Two types of top layer made of trimethylolpropane triacrylate (TMPTA) and pentaerythriol triacrylate (PETA), and the soft base layer with different thickness were introduced into the coating system. To quantitatively investigate the effect of soft base layer on scratch behavior, a progressive load scratch test according ISO 19252 was conducted. This test method can provide the valuable information about scratch damage transition and quantitatively evaluate the scratch properties of coating materials. Moreover, crack formation was carefully observed to understand the scratch mechanism.

EXPERIMENTAL

Coating Preparation

A set of single- and bilayer coatings listed in Table 1 was prepared using UV-curing technique. A solution of top layer, that is, TMPTA and PETA, with 5 wt% photoinitiator was coated on 2 mm thick PMMA substrates with 30 [micro]m thickness and was then cured by ultraviolet light to obtain TMPTA and PETA single-layer coatings (TS and PS), respectively.

For bilayer coatings, the soft base layer was introduced to the coating systems. A solution of soft base layer with 5 wt% photoinitiator was applied on PMMA substrates with different thickness using Mayer rod coating and exposed under ultraviolet light. The solution of top layer (i.e., TMPTA and PETA) was then coated on the soft base layer. TMPTA and PETA bi-layer coatings with 5 [micro]m-thick soft base layer were named as TB5 and PB5, while TMPTA and PETA bilayer coatings with 15 [micro]m-thick soft base layer were named as TB15 and PB15, respectively.

Characterization

Direct imaging indenter (DII, SANKO) combining with a video camera was employed to characterize surface mechanical properties. The soft base layer, TMPTA and PETA, was coated on glass substrates. Coating layer thickness was set at 25 pm. An indentation test was conducted at room temperature with a transparent berkovich tip to directly measure contact area under a load control mode set in the range from 0 to 30 mN (5 s) and from 30 to 0 mN (5 s). The hardness was calculated by the following equation:

[H.sub.M] = P/A (1)

where P is load whereas A is indented area.

Pencil hardness was conducted by Elcometer 501 pencil hardness tester according to ASTM D 3363.

The progressive load scratch test according to ISO 19252 was conducted using a scratch instrument model KK-02 (KATO Tech) at room temperature. A linearly load increasing test of 1-20 N was carried out at scratch velocity of 100 mm/s. The scratch length was set at 70 mm. A diameter of scratch tip was 1 mm. Five tests were performed per sample and the values obtained were averaged for each sample.

After the testing, a laser microscope (Keyence VK-X100 series) was used to observe scratch damage features and obtain the onset point of the damage features. The data obtained were then converted to critical normal load for each damage feature.

To observe crack formation under scratched surface of the acrylic coatings, thin longitudinal section (scratch direction) of scratched samples were prepared for laser microscope. Scratched samples were cut along longitudinal direction to scratch line and embedded in epoxy resin. The samples were carefully polished and then attached onto glass slide. The remaining samples polished to a thickness of ~30 [micro]m. The samples were then polished using [Al.sub.2][O.sub.3] aqueous emulsion. The laser microscope was used to observe crack formation and delamination in subsurface.

RESULTS AND DISCUSSION

Hardness of PMMA substrate and acrylic coatings on glass substrates is shown in Fig. 1. Soft base layer showed 50% lower hardness than top layers and acrylic substrate. For hard coating, it clearly sees that the top layers of TMPTA and PETA exhibited higher hardness than acrylic substrate. Comparing the hardness of top layers, TMPTA revealed 15% higher hardness than PETA.

Pencil hardness was evaluated on the coatings according to ASTM D 3363. All the coatings including single- and bi-layer coatings exhibited the pencil hardness of 7H (Table 1). The results indicated that the introduction of soft base layer into both TMPTA and PETA coating systems did not impact the pencil hardness. Since the pencil hardness test was conducted under constant load, it might hard to differentiate scratch resistance of each coating by using this test method.

Thus, to investigate the influence of introduction of soft base layer on scratch resistance with quantitative method, the progressive load scratch test (ISO 19252) was performed. Scratched surfaces of the coatings were then observed using laser microscope. The representative images of scratched surface for TMPTA and PETA systems are presented in Figs. 2 and 3, respectively. Three damage features including abrasion, crack, and substrate exposure were found as normal load linearly increased from 1 to 20 N. The coatings begun to be abraded at very low load. Minor cracks were then generated on the coating surfaces and propagated through the coating layers. With further increasing normal load, the damage had developed substrate exposure due to the penetration of scratch tip. The coating layers, that is, top and soft base layer, were removed and substrate was exposed and damaged.

The detailed damage features were carefully observed. It is clearly seen that TMPTA and PETA single-layer coating (TS and PS) exhibited more severe damage than bilayer coating systems, especially in abrasion and crack zone. In addition, we found that delamination in TS and PS was occurred simultaneously with abrasion (Fig. 3a) and crack (Figs. 2b and 3b) whereas no delamination was observed in bilayer coating systems with 15 pm-thick soft base layer (TB15 and PB15). These results emphasized the effect of introduction of soft base layer on surface fracture behavior of acrylic coating on PMMA substrate.

The SCOF curves defined as the ratio of the tangential force to the normal force [12] are shown in Fig. 4. Large fluctuation of the SCOF was found at the beginning stage, relating to the inertia effect as the scratch tip experienced sudden change in speed from 0 to 100 mm/s [9]. Considering the behavior at scratch distance about 20 mm for TMPTA system (Fig. 4a) and 30 mm for PETA system (Fig. 4b), the slope of SCOF curves increased suddenly due to the beginning of substrate exposure. After that, the large fluctuation of the SCOF curves was observed, referring to the penetration of scratch tip through the coating layers and substrate [9, 13]. In addition, it obviously sees that the SCOF of TS and TB5 showed the onset of large fluctuation sooner than those of TB15 whereas the onset of large fluctuation of the SCOF for PB5 occurred earlier than those of PS and PB 15, respectively. This result means that the introduction of soft base layer significantly affected the SCOF and the thicker soft base layer could delay the substrate exposure.

To evaluate scratch resistance in quantitative way, a critical normal load at onset of scratch damage transition was calculated. The relationships between critical normal load for the onset of damage features are shown in Fig. 5. The higher critical normal load for the onset of damage features indicated the better resistance to damage. For TMPTA system, the critical load for the onset of crack initiation was not significantly affected by the introduction of soft base layer (Fig. 5a). However, TB15 showed excellent scratch resistance to substrate exposure as indicated by higher critical normal load for onset of substrate exposure. This means that for TMPTA system, the introduction of soft base layer did not affect crack initiation load, but substrate exposure load especially when thick layer was introduced. For PETA systems, PB5 and PB 15, which are bilayer coatings exhibited higher critical normal load for the onset of crack initiation and substrate exposure than PS, and the critical normal load for the damage features increased with increasing the soft base layer thickness (Fig. 5b). These suggested that the introduction of soft base layer significantly influenced scratch resistance of the coating despite of the top layer.

To clarify how the introduction of soft base layer influence scratch resistance of acrylic coating, crack formation on the coating surface was carefully observed. Figure 6 demonstrates the comparison of crack formation at 12 N between single- and bi-layer coatings in TMPTA and PETA system. As mentioned above, in TS, the delamination was generated simultaneously with crack (Fig. 6a), while the introduction of soft base layer can prohibit the delamination of the coatings. In addition, crack extension along the coating surface was reduced by increasing soft base layer thickness (Fig. 6c). The similar results are found in PETA systems. The crack extension was restricted by introducing the soft base layer (Fig. 6d-f). Moreover, the inhibition of crack extension is much more effective when the thickness of soft base layer increased. The reason behind these phenomena might be explained by stress distribution. The stress generated by scratch tip was easily distributed and absorbed through the soft base layer [10, 14, 15]. Thus, the introduction of soft base layer could prevent cracks occurred on the coating surface. However, the difference in scratch behavior of TMPTA and PETA was found. This difference could be explained by hardness of top layer and interfacial adhesion between top and soft base layer in both systems. Since PETA had hydroxyl group in theirs molecules that could form H-bonding with other molecules, PETA coating system might show higher interfacial adhesion than TMPTA.

To verify the above explanation, the subsurface observation was performed. The onset for crack initiation underneath the coating surface was observed as shown in Fig. 7. For TMPTA system having higher surface hardness, the onset of crack initiation for TB5 and TB15 was observed at only the top layer (Fig. 7a and b), while PB5 and PB 15 displayed the onset of crack initiation at both top and soft base layer (Fig. 7d and e). For the onset of substrate exposure in TMPTA and PETA system, the top layer was removed simultaneously with soft base layer (Fig. 7c and f). Considering crack extension on the scratched surface, TB15 and PB 15 showed shorter crack length than TB5 and PB5 (Fig. 8). Although the different top layers are used, the similar relationship of crack length as a function of normal load in TMPTA and PETA coating systems were found as shown in Fig. 8a and b. The results clearly showed that the critical normal load for generating the same crack length as TB15 and PB 15 was higher than the other cases. This implied that the thicker soft base layer could delay the crack propagation on the coating surface. In addition, preliminary result of FEM supported the above explanation. The introduction of soft base layer could help to delay crack deformation in our coating systems. The thicker soft base layer was introduced, the less localized stress was concentrated at the top surface due to the stress distribution. The results will be reported elsewhere.

Thus, the present study shows how the soft base layer affects the scratch properties of polymer coatings. All the results indicated that the soft base layer significantly influenced the scratch properties of polymer coating. The introduction of soft base layer resulted in the excellent scratch properties and can retard the scratch damage generated on the coating surface.

CONCLUSIONS

The influence of soft base layer on scratch properties for acrylic hard coatings was investigated using a progressive load scratch test. The excellent resistance to crack initiation and substrate exposure under scratch loading were found in bilayer coatings with 5- and 15 pm-thick soft base layer as indicated by higher critical normal load for the onset of scratch damages. The introduction of soft base layer was able to retard the cracks generated on the coating surface. Furthermore, the introduction of thick soft base layer (15 pm) caused significant improvement in scratch resistance when compared with the introduction of 5 pm-thick soft base layer. This study suggested that the scratch properties of acrylic coatings were significantly improved by introducing the soft base layer.

NOMENCLATURE

DII     Direct imaging indenter
PETA    Pentaerythriol triacrylate
PMMA    Poly(methyl methacrylate)
SCOF    Scratch coefficient of friction
TMPTA   Trimethylolpropane triacrylate


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Patcharida Chouwatat, (1) Masaya Kotaki, (2,3) Masahiro Miyamoto, (2) Riichi Nishimura, (4) Atsushi Yokohama (1)

(1) Department of Advanced Fibro-Science, Kyoto Institute of Technology, Sakyoku, Kyoto 606-8585, Japan

(2) Kaneka US Material Research Center, Kaneka Americas Holding Inc, College Station, Texas, 77843

(3) Center for Fiber and Textile Science, Kyoto Institute of Technology, Sakyoku, Kyoto 606-8585, Japan

(4) Frontier Materials Development Laboratories, Kaneka Corporation, Settsu, Osaka 566-0072, Japan

Correspondence to: K. Kotaki; e-mail: masaya.kotaki@kaneka.com

DOI 10.1002/pen.24276

TABLE 1. Formulations and pencil hardness of coatings.

            Top layer        Base layer         Pencil
Sample ID   materials   thickness ([micro]m)   hardness

TS                               --               7H
TB5           TMPTA               5               7H
TB15                             15               7H
PS                               --               7H
PB5           PETA                5               7H
PB15                             15               7H
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Author:Chouwatat, Patcharida; Kotaki, Masaya; Miyamoto, Masahiro; Nishimura, Riichi; Yokohama, Atsushi
Publication:Polymer Engineering and Science
Article Type:Report
Date:May 1, 2016
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