effect of urea-formaldehyde-coated epoxy microcapsule

coatings

Article

Effect of Urea-Formaldehyde-Coated EpoxyMicrocapsule Modification on Gloss, Toughness andChromatic Distortion of Acrylic CopolymersWaterborne Coating

Xiaoxing Yan 1,2,* , Lin Wang 2 and Xingyu Qian 2

1 Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing ForestryUniversity, Nanjing 210037, China

2 College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037, China;[email protected] (L.W.); [email protected] (X.Q.)

* Correspondence: [email protected]

Received: 25 March 2019; Accepted: 7 April 2019; Published: 9 April 2019��

Abstract: The modification experiment of waterborne coating was carried out by addingmicrocapsules. The wall material of the microcapsule was urea-formaldehyde resin and the corematerial of the microcapsule was epoxy resin. Core material can improve the toughness of the coatingand prevent the cracking of the coating. The influences of different contents of microcapsules and theorder of adding microcapsules in the coating process on the properties of gloss, color difference andtoughness were studied. The results showed that the gloss of the waterborne coating decreased withthe increase of microcapsule content. The color difference of coating increased first and then decreased,and when the microcapsule content was 8.0%, the color difference was the largest. The toughnessof the coatings also increased first and then decreased. When the content of the microcapsule was10.0%, the toughness of the coating was significantly enhanced. When the microcapsules with acontent of 10.0% were added to the waterborne coating, under the same process, the coating glossof microcapsules added to the primer was relatively high, and the coating gloss was the highestwhen the coating process was three-layer primer and two-layer topcoat. The microcapsule had littleeffect on the color difference of coating in different coating processes. When the coating process wasthree-layer primer and three-layer topcoat, the coating toughness was the best when microcapsuleswere added to the topcoats. This study provides a basis for industrial application of waterbornecoatings to enhance their toughness.

Keywords: waterborne coating; microcapsule; toughness; gloss; color difference

1. Introduction

In recent years, with the increasing awareness of health and environmental protection, waterbornecoatings have become a new type of environmental protection coating [1] for their high safety,low cost and fast drying speed [2]. The most important feature of waterborne coatings is thatthey do not contain volatile organic compounds [3] and air pollutants, so they are widely usedin various fields, such as woodwork processing, in interior decoration materials, housing construction,automobile manufacturing, in aircraft carriers, etc. [4,5], and have broad market application prospects.However, due to the low hardness, non-wear resistance and insufficient toughness of the waterbornecoating, accidental damage and cracks often occur during its service life, which limits its practicalapplication [6–8]. In view of the above shortcomings, it is necessary to modify the waterborne coatingin order to improve the performance.

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At present, there are many modification methods for waterborne coatings, such as organosiloxanemodification [9], polyurethane modification [10], nanomaterials modification [11] and microcapsulesmodification [12]. In particular, it was found that adding microcapsules to waterborne coatingscan improve the water resistance, wear resistance and the healing properties of coatings [13],which had attracted wide attention [14]. Zhang et al. prepared microcapsule-based self-healingcoatings containing epoxy ester as a healing agent [15]. Mirabedini et al. prepared self-healingacrylic latex coatings using novel oil-filled ethyl cellulose microcapsules with enhanced mechanicalproperties and healing properties of latex coating [16]. Ataei et al. found that the gloss and bondstrength of the coatings decreased and the flexural elongation decreased with the increase of theconcentration of microcapsules in epoxy coating containing microencapsulated alkyd resin based oncoconut oil [17]. These reports focused on improving the water resistance, corrosion resistance andmechanical properties of waterborne coatings, but there are few reports on the toughness enhancementof waterborne coatings [18–22].

In this paper, urea-formaldehyde (the wall material)-coated epoxy (the core material)microcapsules were prepared by two-step in situ polymerization and added to waterborne coatingsto study the effects of microcapsules and coating processes on the gloss, chromatic distortion andtoughness of waterborne coatings. The main purpose of adding microcapsule was to improve thetoughness of the coating and prevent the cracking of the coating. The effects of adding microcapsulesin different coating processes on the performance of coatings were compared, which laid a foundationfor the application of waterborne coatings in engineering.

2. Materials and Methods

2.1. Test Materials

Waterborne wood coating, F. mandshurica veneer (uniform material color, 40 mm × 40 mm × 3 mm),glass substrates (75 mm × 25 mm × 1 mm) and aluminum substrates (50 mm × 40 mm × 1 mm)were supplied by Yihua Lifestyle Technology Co., Ltd., Shantou, China. Waterborne wood coatingconsisted of acrylic copolymers supported by water (the content was 90.0%), dipropylene glycol methylether (the content was 2.0%) and dipropylene glycol butyl ether (the content was 8.0%). The solidcontent of the coating is about 26.5%. Urea (Mw: 60.06 g/mol, CAS No.: 57-13-6), 37% formaldehydesolution (Mw: 30.03 g/mol, CAS No.: 50-00-0), epoxy resin (Mw: 375.86 g/mol, CAS No.: 61788-97-4),sodium dodecyl benzene sulfonate (Mw: 348.48 g/mol, CAS No.: 25155-30-0), citric acid monohydrate(Mw: 210.14 g/mol, CAS No.: 5949-29-1), anhydrous ethanol (Mw: 46.07 g/mol, CAS No.: 64-17-5) andtriethanolamine (Mw: 149.19 g/mol, CAS No.: 102-71-6) were supplied by Xilong Chemical Co., Ltd.,Guangzhou, China.

2.2. Preparation of Microcapsules

Urea-formaldehyde-coated epoxy microcapsules were prepared by in situ polymerization.A mixture of 20.0 g urea and 34.0 g 37% formaldehyde solution was added into the beaker. The systemwas mixed evenly. The triethanolamine was added to the mixture slowly, and the pH value of thesolution was adjusted to 8.0–9.0 by triethanolamine. The mixture was stirred continuously for 1 h ina constant temperature water bath at 70 ◦C to prepare the wall material solution and was cooled atroom temperature. 1.95 g sodium dodecylbenzene sulfonate was added in 193.05 g deionized water todissolve completely, and a 1.0% sodium dodecylbenzene sulfonate aqueous solution was obtained.Twenty-five g epoxy resin was added to the sodium dodecylbenzene sulfonate aqueous solution.A stable core material emulsion was obtained by stirring at a speed of 1200 r/min for 30 min in a 60 ◦Cwater bath. Then the wall material solution was added into the core material emulsion and stirred tomake it evenly mixed. The citric acid monohydrate was added to the above mixture, and the pH valuewas adjusted to 2.5–3.0. The system was reacted at 70 ◦C for 3 h. After several rinses with deionized

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water and absolute ethanol, the product was put into the oven and dried at 80 ◦C for 4 h. The finalwhite powder was the required urea-formaldehyde-coated epoxy microcapsules.

2.3. Preparation of Coatings

The coatings were sprayed on F. mandshurica veneer, glass substrates and aluminum substrates.The incorporation of the microcapsules into the coating was done at room temperature. The specificsteps were as follows: The microcapsules were added into the waterborne wood coating accordingto the mass fraction of microcapsule mass to total mass of liquid coatings (the sum of microcapsulemass and waterborne coatings mass) of 0%, 1.0%, 3.0%, 5.0%, 8.0%, 10.0% and 12.0%. The waterbornecoatings were sprayed onto the F. mandshurica veneer, glass substrates and aluminum substrates by anairbrush (Guangzhou Zhongtian Electrical Equipment Co., Ltd., Guangzhou, China). The coating wasnaturally dried for 3 h and then the waterborne coating was sanded using 1000 grit sandpaper, and adry cloth was used to wipe off the dust. The spray process needed to be repeated twice. The thicknessof the waterborne coating was about 40 µm. After the coatings on the glass substrates were dried,they were immersed in water. After the coatings were whitened, the coatings were removed from theglass substrates with an artistic knife, and then dried in an oven at 40 ◦C for 30 min for elongationat break testing. The coatings sprayed on the aluminum substrates were for the bend testing. Thecoatings sprayed on the F. mandshurica veneer were for the gloss and chromatic distortion testing.

In the process of waterborne wood coating, in order to fill wood holes, smooth the surface of thewhole coating and increase the sealing property of the coating, the layer number of primer and topcoatwas set to be 2 and 3, respectively, and the order of adding microcapsules in different coating processeswas changed, and all-factor experiments were carried out. The layer number of primer and topcoatapplied during the experiment is shown in Table 1. Taking the coating process of two-layer primer andtwo-layer topcoat when the microcapsules were added to the primers as an example:

Table 1. Number of primer and topcoat coatings.

Samples Layers of Primer Layers of Topcoat

1 2 22 2 33 3 24 3 3

Firstly, the microcapsules were added into the waterborne coatings according to the mass fractionof 10.0%. They were evenly dispersed and sprayed once as primer. The coatings were dried naturallyfor 3 h. Then the waterborne coatings were polished with 1000 grit sandpaper and the dust was wipedoff with a dry cloth. Then the above process was repeated once to obtain the two-layer primer.

Secondly, after the two-layer primer was dried, the waterborne coatings were sprayed as thetopcoat, and the coatings were dried naturally for 3 h. Then the waterborne coatings were polishedwith 1000 grit sandpaper and the dust was wiped off with a dry cloth. Then the above process wasrepeated once to obtain the two-layer topcoat.

2.4. Performance Test

The microstructure of the microcapsules was analyzed using a Quanta 200 environment scanningelectron microscope (SEM), FEI Company (Hillsboro, Oregon, USA), and L2800 Biomicroscope,Guangzhou Liss Optical Instrument Co., Ltd., (Guangzhou, China). The dimension of themicrocapsules was measured with a L2800 Biomicroscope. The components of the microcapsules wereanalyzed using a VERTEX 80V infrared spectrum analyzer, Germany BRUKER Co., Ltd., (Karlsruhe,Germany). The HP-2136 chromatic distortion meter (Zhuhai Tianchuang Instrument Co. Ltd., Zhuhai,China) was used to directly measure the lab value of the specimen. A HG268 gloss meter, produced by3NH Technology Co., Ltd., Shenzhen, China, was used to measure the gloss of the waterborne coatings

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on the F. mandshurica veneer. The coating flexibility of microcapsule aluminum-based coatings withdifferent mass fractions and aluminum-based coatings with different coating processes containingmicrocapsules were tested by a coating flexibility tester. Tensile fracture tests of coatings containingmicrocapsules with different mass fractions were carried out using the Model AG-IC100KN precisionelectronic universal capability experiment machine, SHIMADZU Co., Ltd., Kyoto, Japan. All theexperiments were repeated four times with an error of less than 5.0%.

3. Results and Discussion

3.1. Effect of Microcapsule on the Properties of Waterborne Coatings

3.1.1. Analysis of Gloss of Coating

The prepared microcapsules are shown in Figure 1. The particle size distribution is shown inFigure 2. The prepared microcapsules are spherical, with a particle size of about 3–5 µm and lowbreakage rate.

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with different coating processes containing microcapsules were tested by a coating flexibility tester.

Tensile fracture tests of coatings containing microcapsules with different mass fractions were carried

out using the Model AG-IC100KN precision electronic universal capability experiment machine,

SHIMADZU Co., Ltd., Kyoto, Japan. All the experiments were repeated four times with an error of

less than 5.0%.




The prepared microcapsules are shown in Figure 1. The particle size distribution is shown in

Figure 2. The prepared microcapsules are spherical, with a particle size of about 3–5 μm and low

breakage rate.

Figure 3 is the infrared spectrum of microcapsules. It can be seen from Figure 3 that there are

prominent characteristic absorption peaks of N–H near 3360 cm−1, which indicates that there are

many amino groups in the system. The characteristic absorption peaks of the C–H bond at 2966 cm−1,

C=O at 1645 cm−1 and C–N at 1556 cm−1 correspond to the chemical bonds in urea-formaldehyde

resin, indicating that urea-formaldehyde resin has been synthesized during the preparation of

microcapsule powder. The symmetrical vibration absorption peak of the epoxy matrix is 1247 cm−1,

which indicates that epoxy resin exists in the prepared microcapsule powder.

Figure 1. Scanning electron microscope (SEM) of microcapsules: (A) large magnification; (B) small

magnification.

Figure 1. Scanning electron microscope (SEM) of microcapsules: (A) large magnification; (B)small magnification.


with different coating processes containing microcapsules were tested by a coating flexibility tester.

Tensile fracture tests of coatings containing microcapsules with different mass fractions were carried

out using the Model AG-IC100KN precision electronic universal capability experiment machine,

SHIMADZU Co., Ltd., Kyoto, Japan. All the experiments were repeated four times with an error of

less than 5.0%.




The prepared microcapsules are shown in Figure 1. The particle size distribution is shown in

Figure 2. The prepared microcapsules are spherical, with a particle size of about 3–5 μm and low

breakage rate.

Figure 3 is the infrared spectrum of microcapsules. It can be seen from Figure 3 that there are

prominent characteristic absorption peaks of N–H near 3360 cm−1, which indicates that there are

many amino groups in the system. The characteristic absorption peaks of the C–H bond at 2966 cm−1,

C=O at 1645 cm−1 and C–N at 1556 cm−1 correspond to the chemical bonds in urea-formaldehyde

resin, indicating that urea-formaldehyde resin has been synthesized during the preparation of

microcapsule powder. The symmetrical vibration absorption peak of the epoxy matrix is 1247 cm−1,

which indicates that epoxy resin exists in the prepared microcapsule powder.

Figure 1. Scanning electron microscope (SEM) of microcapsules: (A) large magnification; (B) small

magnification.

Figure 2. Particle size distribution of microcapsules.

Figure 3 is the infrared spectrum of microcapsules. It can be seen from Figure 3 that there areprominent characteristic absorption peaks of N–H near 3360 cm−1, which indicates that there are manyamino groups in the system. The characteristic absorption peaks of the C–H bond at 2966 cm−1, C=Oat 1645 cm−1 and C–N at 1556 cm−1 correspond to the chemical bonds in urea-formaldehyde resin,indicating that urea-formaldehyde resin has been synthesized during the preparation of microcapsulepowder. The symmetrical vibration absorption peak of the epoxy matrix is 1247 cm−1, which indicatesthat epoxy resin exists in the prepared microcapsule powder.

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Figure 3. Infrared spectrogram of microcapsules.

The effect of the change of the mass fraction of microcapsules on the gloss of the coatings was

shown in Figure 4. When the visible light was incident at different angles of 20°, 60° and 85°, with the

increase of the contents of microcapsules from 0% to 12.0%, the gloss of the coating decreased from

23.4%, 68.3% and 50.3% to 1.8%, 5.7% and 0.9%, respectively. This is because the waterborne coating

used in the experiment was a colorless varnish; without adding other substances, the surface is

smooth, and the gloss is higher. With the addition of the white powder of microcapsules, the original

gloss of the coating was changed so that the surface was not as smooth as before. At the same time,

due to the addition of microcapsules, the proportion of waterborne solvents in waterborne coating

decreased, the drying time of the coating decreased, and the drying speed became faster, resulting in

higher surface roughness and lower gloss of coating. Therefore, the more microcapsules added, the

higher the surface roughness and the lower the gloss of the coating.

Figure 4. The relationship between the contents of microcapsules and the gloss of coating.

3.1.2. Color Difference Analysis of Coating

The color difference of coating refers to the difference in color when the light source is

polychromatic light. The results are shown in Table 2. L, a* and b* respectively represent the

black-white, red-green and yellow-blue values of a point in the coating. L’, a*’ and b*’ respectively

represent the black-white, red-green and yellow-blue values of the rest of the coating [23]. After


The effect of the change of the mass fraction of microcapsules on the gloss of the coatings wasshown in Figure 4. When the visible light was incident at different angles of 20◦, 60◦ and 85◦, with theincrease of the contents of microcapsules from 0% to 12.0%, the gloss of the coating decreased from23.4%, 68.3% and 50.3% to 1.8%, 5.7% and 0.9%, respectively. This is because the waterborne coatingused in the experiment was a colorless varnish; without adding other substances, the surface is smooth,and the gloss is higher. With the addition of the white powder of microcapsules, the original gloss ofthe coating was changed so that the surface was not as smooth as before. At the same time, due to theaddition of microcapsules, the proportion of waterborne solvents in waterborne coating decreased,the drying time of the coating decreased, and the drying speed became faster, resulting in highersurface roughness and lower gloss of coating. Therefore, the more microcapsules added, the higherthe surface roughness and the lower the gloss of the coating.




The effect of the change of the mass fraction of microcapsules on the gloss of the coatings was

shown in Figure 4. When the visible light was incident at different angles of 20°, 60° and 85°, with the

increase of the contents of microcapsules from 0% to 12.0%, the gloss of the coating decreased from

23.4%, 68.3% and 50.3% to 1.8%, 5.7% and 0.9%, respectively. This is because the waterborne coating

used in the experiment was a colorless varnish; without adding other substances, the surface is

smooth, and the gloss is higher. With the addition of the white powder of microcapsules, the original

gloss of the coating was changed so that the surface was not as smooth as before. At the same time,

due to the addition of microcapsules, the proportion of waterborne solvents in waterborne coating

decreased, the drying time of the coating decreased, and the drying speed became faster, resulting in

higher surface roughness and lower gloss of coating. Therefore, the more microcapsules added, the

higher the surface roughness and the lower the gloss of the coating.



The color difference of coating refers to the difference in color when the light source is

polychromatic light. The results are shown in Table 2. L, a* and b* respectively represent the

black-white, red-green and yellow-blue values of a point in the coating. L’, a*’ and b*’ respectively

represent the black-white, red-green and yellow-blue values of the rest of the coating [23]. After



The color difference of coating refers to the difference in color when the light source ispolychromatic light. The results are shown in Table 2. L, a* and b* respectively represent the black-white,red-green and yellow-blue values of a point in the coating. L’, a*’ and b*’ respectively represent theblack-white, red-green and yellow-blue values of the rest of the coating [23]. After subtraction,the difference values ∆L, ∆a* and ∆b*, respectively, are expressed as illumination difference, red-greenindex difference and yellow-blue index difference. Thus, the color difference ∆E can be obtainedaccording to the formula:

∆E =

√(∆L)2 + (∆a∗)2 + (∆b∗)2 (1)

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Table 2. Effect of microcapsules on color difference of waterborne coating.

MicrocapsuleContent/% L a* b* L’ a*’ b*’ ∆L ∆a* ∆b* ∆E

0 59.80 ± 0 13.00 ± 0.02 21.20 ± 0.02 59.3 ± 0.01 11.8 ± 0.01 21.20 ± 0.22 0.50 ± 0.01 1.20 ± 0.02 0 ± 0.21 1.3 ± 0.05

1.0 60.60 ± 0 12.40 ± 0 20.30 ± 0.07 60.20 ± 0.02 10.90 ± 0.01 21.10 ± 0.01 0.40 ± 0.02 1.50 ± 0.01 −0.80 ± 0.07 1.70 ± 0.05

3.0 61.50 ± 0 11.80 ± 0.08 20.20 ± 0.01 61.30 ± 0.02 10.10 ± 0.07 19.00 ± 0.01 0.20 ± 0.01 1.70 ± 0.04 1.20 ± 0.01 2.10 ± 0.05

5.0 62.40 ± 0.02 11.00 ± 0.22 15.80 ± 0.06 62.40 ± 0 9.00 ± 0.23 18.00 ± 0.14 0 ± 0.02 2.00 ± 0.04 −2.20 ± 0.09 3.00 ± 0.05

8.0 62.4 0 ± 0.02 11.00 ± 0.09 22.60 ± 0.02 61.90 ± 0.01 10.30 ± 0.08 15.90 ± 0.02 0.50 ± 0.02 0.70 ± 0.01 6.70 ± 0.04 6.80 ± 0.06

10.0 65.20 ± 0.09 8.00 ± 0.01 15.60 ± 0.04 62.60 ± 0.02 9.20 ± 0.04 14.40 ± 0.02 2.60 ± 0.07 −1.20 ± 0.05 1.20 ± 0.04 3.10 ± 0.05

12.0 62.30 ± 0.09 9.30 ± 0.04 15.20 ± 0.08 62.50 ± 0.09 10.80 ± 0.02 13.90 ± 0.08 −0.20 ± 0.01 −1.50 ± 0.04 1.30 ± 0.08 2.0 ± 0.05

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Table 2 shows the effect of mass fraction of microcapsules on the color difference of the coating.Combined with Figure 5, it can be seen that the variation of the color difference of the coating wasas follows: When the microcapsules were added to waterborne wood coating, the color difference ofcoating also changed with the increase of microcapsule mass fraction. When microcapsule contentincreased from 0% to 8.0%, the color difference of coating increased from 1.3 to 6.8. When microcapsulecontent increased from 8.0% to 12.0%, the color difference of coating decreased from 6.8 to 2.0. When themass fraction of microcapsules was 8.0%, the color difference of coating reached the maximum.Therefore, the addition of microcapsules had a certain effect on the color difference of coatings, and thecolor difference of the coating increased as compared with that without microcapsules.


subtraction, the difference values ΔL, Δa* and Δb*, respectively, are expressed as illumination

difference, red-green index difference and yellow-blue index difference. Thus, the color difference

ΔE can be obtained according to the formula:

2*2*2 )()()( baLE (1)

Table 2. Effect of microcapsules on color difference of waterborne coating.

Microcapsule

Content/% L a* b* L’ a*’ b*’ ΔL Δa* Δb* ΔE

0 59.80 ± 0 13.00

± 0.02

21.20

± 0.02

59.3 ±

0.01

11.8 ±

0.01

21.20

± 0.22

0.50

±

0.01

1.20

±

0.02

0 ± 0.21 1.3 ±

0.05

1.0 60.60 ± 0 12.40

± 0

20.30

± 0.07

60.20±

0.02

10.90±

0.01

21.10

± 0.01

0.40

±

0.02

1.50

±

0.01

−0.80 ± 0.07 1.70±

0.05

3.0 61.50 ± 0 11.80

± 0.08

20.20

± 0.01

61.30±

0.02

10.10±

0.07

19.00

±0.01

0.20

±

0.01

1.70

±

0.04

1.20 ± 0.01 2.10±

0.05

5.0 62.40 ±

0.02

11.00

± 0.22

15.80

± 0.06

62.40 ±

0

9.00 ±

0.23

18.00

± 0.14

0 ±

0.02

2.00

±

0.04

−2.20 ± 0.09 3.00 ±

0.05

8.0 62.4 0 ±

0.02

11.00

± 0.09

22.60

± 0.02

61.90±

0.01

10.30±

0.08

15.90

± 0.02

0.50

±

0.02

0.70

±

0.01

6.70 ± 0.04 6.80±

0.06

10.0 65.20±0.09 8.00 ±

0.01

15.60

± 0.04

62.60 ±

0.02

9.20 ±

0.04

14.40

± 0.02

2.60

±

0.07

−1.20

±

0.05

1.20 ± 0.04 3.10 ±

0.05

12.0 62.30 ±

0.09

9.30 ±

0.04

15.20

± 0.08

62.50±

0.09

10.80±

0.02

13.90

± 0.08

−0.20

±

0.01

−1.50

±

0.04

1.30 ± 0.08 2.0 ±

0.05

Table 2 shows the effect of mass fraction of microcapsules on the color difference of the coating.

Combined with Figure 5, it can be seen that the variation of the color difference of the coating was as

follows: When the microcapsules were added to waterborne wood coating, the color difference of

coating also changed with the increase of microcapsule mass fraction. When microcapsule content

increased from 0% to 8.0%, the color difference of coating increased from 1.3 to 6.8. When

microcapsule content increased from 8.0% to 12.0%, the color difference of coating decreased from

6.8 to 2.0. When the mass fraction of microcapsules was 8.0%, the color difference of coating reached

the maximum. Therefore, the addition of microcapsules had a certain effect on the color difference

of coatings, and the color difference of the coating increased as compared with that without

microcapsules.

Figure 5. Effect of microcapsule content on color difference of coating. Figure 5. Effect of microcapsule content on color difference of coating.

3.1.3. Flexibility Analysis of Coating

The waterborne wood coating with microcapsules of different mass fractions on the surface of thealuminum sheet was placed on the coating flexibility tester. The coating was upward, and the basematerial adhered to the steel axis [24]. The coatings were bent in turn according to the diameter ofthe steel shaft from large to small. The minimum diameter of the steel shaft through which the basematerial can pass represents the flexibility of the coating. The smaller the diameter of the steel shaft,the higher the flexibility of the coating. After the bending test, it was found that, when bending with aminimum diameter of 0.5 mm, no cracks were found in all the coatings, which proved that the coatingswere relatively soft. Through the test of coating flexibility, it was found that the waterborne coatingitself was relatively soft. It is difficult to reach a conclusion with the flexibility test method on the effectof adding microcapsules on the flexibility of waterborne coating. It is necessary to further test thetensile fracture of waterborne wood coating modified by microcapsules with different mass fractions.

The coatings were stretched on a precision electronic universal capability experiment machine.The elongation at break of the coating was calculated according to the displacement length of thecoating at break and the original length of the coating before stretching. The elongation at break curveof the coating was obtained as shown in Figure 6. With the increase of the content of microcapsules,the elongation at break of the coating first increased and then decreased, that is, the flexibility of thecoating first increased and then decreased. When the amounts of microcapsules increased from 0% to10.0%, the elongation at break increased from 2.67% to 4.91%. When the amounts of microcapsulesincreased from 10.0% to 12.0%, the elongation at break decreased from 4.91% to 4.11%. The resultsshow that the toughness of the coating was the highest when the amount of microcapsule was 10.0%.

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The waterborne wood coating with microcapsules of different mass fractions on the surface of

the aluminum sheet was placed on the coating flexibility tester. The coating was upward, and the

base material adhered to the steel axis [24]. The coatings were bent in turn according to the diameter

of the steel shaft from large to small. The minimum diameter of the steel shaft through which the

base material can pass represents the flexibility of the coating. The smaller the diameter of the steel

shaft, the higher the flexibility of the coating. After the bending test, it was found that, when bending

with a minimum diameter of 0.5 mm, no cracks were found in all the coatings, which proved that the

coatings were relatively soft. Through the test of coating flexibility, it was found that the waterborne

coating itself was relatively soft. It is difficult to reach a conclusion with the flexibility test method on

the effect of adding microcapsules on the flexibility of waterborne coating. It is necessary to further

test the tensile fracture of waterborne wood coating modified by microcapsules with different mass

fractions.

The coatings were stretched on a precision electronic universal capability experiment machine.

The elongation at break of the coating was calculated according to the displacement length of the

coating at break and the original length of the coating before stretching. The elongation at break

curve of the coating was obtained as shown in Figure 6. With the increase of the content of

microcapsules, the elongation at break of the coating first increased and then decreased, that is, the

flexibility of the coating first increased and then decreased. When the amounts of microcapsules

increased from 0% to 10.0%, the elongation at break increased from 2.67% to 4.91%. When the

amounts of microcapsules increased from 10.0% to 12.0%, the elongation at break decreased from

4.91% to 4.11%. The results show that the toughness of the coating was the highest when the amount

of microcapsule was 10.0%.

Figure 6. Effect of amounts of microcapsules added on elongation at break of coating.

Figure 7 was the effect of different mass fractions on infrared spectra of waterborne coatings. It

can be seen from the graph that there were absorption peaks representing C=C–H near 810 cm−1,

absorption characteristic peaks of C–O–C near 1150 cm−1, absorption peaks representing C=O at 1724

cm−1 and stretching vibration peaks representing CH2 and CH3 near 2950 cm−1. The characteristic

peaks of urea-formaldehyde resin and epoxy resin appeared simultaneously in the infrared spectra

of waterborne coatings with microcapsules, which indicated that the components of microcapsules

existed in the waterborne coatings and were not destroyed.

Figure 6. Effect of amounts of microcapsules added on elongation at break of coating.

Figure 7 was the effect of different mass fractions on infrared spectra of waterborne coatings.It can be seen from the graph that there were absorption peaks representing C=C–H near 810 cm−1,absorption characteristic peaks of C–O–C near 1150 cm−1, absorption peaks representing C=O at1724 cm−1 and stretching vibration peaks representing CH2 and CH3 near 2950 cm−1. The characteristicpeaks of urea-formaldehyde resin and epoxy resin appeared simultaneously in the infrared spectraof waterborne coatings with microcapsules, which indicated that the components of microcapsulesexisted in the waterborne coatings and were not destroyed.Coatings 2018, 8, x FOR PEER REVIEW 8 of 13

Figure 7. Influences of microcapsules on infrared spectrum of coatings.

SEM images of waterborne coatings with different mass fractions of microcapsules are shown in

Figure 8. It can be seen that the coating surface was very smooth without microcapsules (Figure 8A).

When the microcapsule content was 10.0%, it could be evenly distributed (Figure 8B). But when the

microcapsule content increased to 12.0%, there was reunion (Figure 8C). After adding

microcapsules, the gloss decreased and the color difference increased due to the unevenness of the

coatings. Microcapsule is a core-shell structure of epoxy resin as the core and urea-formaldehyde

resin as the shell, which has good elasticity, therefore, the elongation at break and toughness of the

coating increases. However, when the content of microcapsules was too high, the agglomeration was

more serious in the coating (Figure 8C), which reduced the elongation at break of the coating.

Figure 8. SEM of waterborne coatings with different contents of microcapsules: (A) 0; (B) 10.0% and

(C) 12.0%.

3.2. Effect of Different Coating Processes


Because the color difference of the coating changed little when the mass fraction of

microcapsule was 10.0%, and it had low color difference and good toughness and mechanical

properties, the effect of coating process on the performance of the coating was further considered by

fixing the mass fraction of microcapsule at 10.0%. The influence of the coating process on the gloss of

coating is shown in Figures 9–11. It can be seen from the figure that the gloss of the coatings with

microcapsules added to primer was higher than that with microcapsules added to topcoat under the

same coating process at different incident angles of 20, 60 and 85. When the microcapsules were

added to the primer and the layer number of topcoat was two, the gloss of coating increased with

the increase of the layer number of primer. When the microcapsules were added to the primer and

the layer number of topcoat was three, the gloss of coating decreased with the increase of the layer


SEM images of waterborne coatings with different mass fractions of microcapsules are shown inFigure 8. It can be seen that the coating surface was very smooth without microcapsules (Figure 8A).When the microcapsule content was 10.0%, it could be evenly distributed (Figure 8B). But when themicrocapsule content increased to 12.0%, there was reunion (Figure 8C). After adding microcapsules,the gloss decreased and the color difference increased due to the unevenness of the coatings.Microcapsule is a core-shell structure of epoxy resin as the core and urea-formaldehyde resin asthe shell, which has good elasticity, therefore, the elongation at break and toughness of the coatingincreases. However, when the content of microcapsules was too high, the agglomeration was moreserious in the coating (Figure 8C), which reduced the elongation at break of the coating.

Coatings 2019, 9, 239 9 of 14



SEM images of waterborne coatings with different mass fractions of microcapsules are shown in

Figure 8. It can be seen that the coating surface was very smooth without microcapsules (Figure 8A).

When the microcapsule content was 10.0%, it could be evenly distributed (Figure 8B). But when the

microcapsule content increased to 12.0%, there was reunion (Figure 8C). After adding

microcapsules, the gloss decreased and the color difference increased due to the unevenness of the

coatings. Microcapsule is a core-shell structure of epoxy resin as the core and urea-formaldehyde

resin as the shell, which has good elasticity, therefore, the elongation at break and toughness of the

coating increases. However, when the content of microcapsules was too high, the agglomeration was

more serious in the coating (Figure 8C), which reduced the elongation at break of the coating.

Figure 8. SEM of waterborne coatings with different contents of microcapsules: (A) 0; (B) 10.0% and

(C) 12.0%.



Because the color difference of the coating changed little when the mass fraction of

microcapsule was 10.0%, and it had low color difference and good toughness and mechanical

properties, the effect of coating process on the performance of the coating was further considered by

fixing the mass fraction of microcapsule at 10.0%. The influence of the coating process on the gloss of

coating is shown in Figures 9–11. It can be seen from the figure that the gloss of the coatings with

microcapsules added to primer was higher than that with microcapsules added to topcoat under the

same coating process at different incident angles of 20, 60 and 85. When the microcapsules were

added to the primer and the layer number of topcoat was two, the gloss of coating increased with

the increase of the layer number of primer. When the microcapsules were added to the primer and

the layer number of topcoat was three, the gloss of coating decreased with the increase of the layer

Figure 8. SEM of waterborne coatings with different contents of microcapsules: (A) 0; (B) 10.0% and(C) 12.0%.



Because the color difference of the coating changed little when the mass fraction of microcapsulewas 10.0%, and it had low color difference and good toughness and mechanical properties, the effect ofcoating process on the performance of the coating was further considered by fixing the mass fractionof microcapsule at 10.0%. The influence of the coating process on the gloss of coating is shown inFigures 9–11. It can be seen from the figure that the gloss of the coatings with microcapsules added toprimer was higher than that with microcapsules added to topcoat under the same coating process atdifferent incident angles of 20◦, 60◦ and 85◦. When the microcapsules were added to the primer and thelayer number of topcoat was two, the gloss of coating increased with the increase of the layer number ofprimer. When the microcapsules were added to the primer and the layer number of topcoat was three,the gloss of coating decreased with the increase of the layer number of primer. When the microcapsuleswere added in topcoat, the gloss of the coating remained unchanged with the increase of the layernumber of topcoat when the layer numbers of primers was 2 and 3. Therefore, adding microcapsules toprimers had better gloss; in particular, when the coating process was three-layer primer and two-layertopcoat, the gloss of the coating was the highest.


number of primer. When the microcapsules were added in topcoat, the gloss of the coating remained

unchanged with the increase of the layer number of topcoat when the layer numbers of primers was

2 and 3. Therefore, adding microcapsules to primers had better gloss; in particular, when the coating

process was three-layer primer and two-layer topcoat, the gloss of the coating was the highest.

Figure 9. Gloss variation of coating at 20 incident angle (samples 1–4 from Table 1).



Figure 9. Gloss variation of coating at 20◦ incident angle (samples 1–4 from Table 1).

Coatings 2019, 9, 239 10 of 14




















The chromatic distortions of the coatings after adding microcapsules under different coatingprocesses are shown in Tables 3 and 4. From Table 3, the color difference decreased from 2.4 to 0.9with the increase in the layer number of primer added with microcapsules when the layer number oftopcoat was two. When the layer number of topcoat was three, the color difference increased from1.1 to 3.7 with the increase of the layer number of primer. From Table 4, it can be observed that thecolor difference decreased from 3.1 to 1.1 with the increase in the layer number of topcoat with addedmicrocapsules when the layer number of primer was two. When the layer number of primer was three,the color difference increased from 1.1 to 1.5 when the layer number of topcoat increased. In general,adding microcapsules to primer and topcoat had little effect on the color difference of waterbornecoating and low fluctuation, respectively. When the microcapsules were added to primer and thecoating process was three-layer primer and two-layer topcoat, the color difference of the coating wasthe smallest, at 0.9. This is because the microcapsule particles are small, and when embedded in theprimer, they have little effect on the topcoat.

Coatings 2019, 9, 239 11 of 14

Table 3. Effect of microcapsules in primer on color difference of waterborne coating.

Coating Process L a* b* L’ a*’ b*’ ∆L ∆a* ∆b* ∆E

Two-layer primer andtwo-layer topcoat 58.90 ± 0.02 10.40 ± 0.02 15.60 ± 0.04 58.90 ± 0.02 9.60 ± 0.02 17.90 ± 0.08 0 ± 0.01 0.80 ± 0.01 −2.30 ± 0.07 2.40 ± 0.05

Two-layer primer andthree-layer topcoat 60.40 ± 0.02 9.50 ± 0.02 17.90 ± 0.09 60.40 ± 0.02 10.50 ± 0.07 17.40±0.08 0 ± 0.01 −1.00 ± 0.06 0.50 ± 0.01 1.10 ± 0.05

Three-layer primer andtwo-layer topcoat 63.90 ± 0.05 6.50 ± 0.02 13.70 ± 0.02 63.20 ± 0.02 6.00 ± 0.01 13.40 ± 0.02 0.70 ± 0.03 0.50 ± 0.01 0.30 ± 0.01 0.90 ± 0.05

Three-layer primer andthree-layer topcoat 65.20± 0.13 7.40 ± 0.02 14.80 ± 0.01 61.80 ± 0.06 7.70 ± 0.02 16.20 ± 0.02 3.40 ± 0.07 −0.30 ± 0.01 1.40 ± 0.02 3.70 ± 0.08

Table 4. Effect of microcapsules in topcoat on color difference of waterborne coating.

Coating Process L a* b* L’ a*’ b*’ ∆L ∆a* ∆b* ∆E

Two-layer primer andtwo-layer topcoat 65.90± 0.02 5.60 ± 0.02 14.30 ± 0.02 64.10 ± 0.02 6.30 ± 0.02 11.90 ± 0.02 1.80 ± 0.01 −0.70 ± 0.01 2.40 ± 0.03 3.10 ± 0.05

Two-layer primer andthree-layer topcoat 64.00± 0.29 9.30 ± 0.02 14.10 ± 0.01 62.90 ± 0.27 9.10 ± 0.02 14.20 ± 0.01 1.10 ± 0.02 0.20 ± 0.01 −0.10 ± 0 1.10 ± 0.05

Three-layer primer andtwo-layer topcoat 61.80± 0.09 8.10 ± 0 15.10 ± 0.01 62.90 ± 0.05 8.00 ± 0 14.90 ± 0.01 −1.10 ± 0.04 0.10 ± 0 0.20 ± 0 1.10 ± 0.05

Three-layer primer andthree-layer topcoat 64.80 ± 0 7.90 ± 0.09 14.70 ± 0.02 64.70 ± 0.02 6.70 ± 0 13.80 ± 0.02 0.10 ± 0.02 1.20 ± 0.06 0.90 ± 0.01 1.50 ± 0.05

Coatings 2019, 9, 239 12 of 14


When the microcapsules were added to the primer and topcoat, the flexibility tester was used totest the coating prepared by different coating processes [25]. After bending, it was found that whenbending with the smallest diameter of the steel shaft of 0.5 mm, no cracks appeared in all the coating,which proved that the coating had good flexibility.

The results of the elongation at break of the coating obtained by placing the coating in theuniversal capability experiment machine are shown in Figure 12. Except for the coating methods ofthree-layer primer and three-layer topcoat, the elongation at break of the coatings with microcapsulesin the primer was better than that with microcapsules in the topcoat under the same coating process.When the layer number of primer was two, the elongation at break of coating increased from 19.6%to 25.7% with the increase of the layer number of primer added with microcapsules. When the layernumber of topcoat was three, the elongation at break of coating decreased from 24.7% to 23.0% withthe increase of layer number of primer added with microcapsules. When the microcapsules wereadded in the topcoat, the elongation at break of the coating did not change significantly when the layernumber of primers was two, but when the layer number of primers was three, the toughness of thecoating increased with the increase of the layer number of topcoat. When the coating process wasthree-layer primer and three-layer topcoat, the elongation at break of the coating was the highest whenthe microcapsules were added to the topcoat.


bending with the smallest diameter of the steel shaft of 0.5 mm, no cracks appeared in all the coating,

which proved that the coating had good flexibility.

The results of the elongation at break of the coating obtained by placing the coating in the

universal capability experiment machine are shown in Figure 12. Except for the coating methods of

three-layer primer and three-layer topcoat, the elongation at break of the coatings with

microcapsules in the primer was better than that with microcapsules in the topcoat under the same

coating process. When the layer number of primer was two, the elongation at break of coating

increased from 19.6% to 25.7% with the increase of the layer number of primer added with

microcapsules. When the layer number of topcoat was three, the elongation at break of coating

decreased from 24.7% to 23.0% with the increase of layer number of primer added with

microcapsules. When the microcapsules were added in the topcoat, the elongation at break of the

coating did not change significantly when the layer number of primers was two, but when the layer

number of primers was three, the toughness of the coating increased with the increase of the layer

number of topcoat. When the coating process was three-layer primer and three-layer topcoat, the

elongation at break of the coating was the highest when the microcapsules were added to the

topcoat.

Figure 12. Effect of different coating processes on elongation at break of the coating (samples 1–4

from Table 1).

4. Conclusion

The effects of the mass fractions of microcapsules and the sequence of microcapsules on the

properties of waterborne coating were studied. The gloss, color difference and flexibility of the

modified coating were studied. The results showed that the gloss of the coating decreased with the

increase of the mass fractions of microcapsules, and the color difference increased first and then

decreased. When the mass fraction of microcapsules was 8.0%, the color difference was the highest,

at 6.8. The elongation at break of the coating first increased and then decreased with the mass

fractions of microcapsules. When the mass fractions of microcapsules were 10.0%, the toughness of

the coating was the best. At the same time, adding 10.0% microcapsule in waterborne coating, under

the same coating process, the gloss of the coating with microcapsule in primer was higher, and the

gloss of the coating was the highest when the coating process was three-layer primer and two-layer

topcoat. Microcapsule had little effect on the chromatic distortion of coating in different coating

processes. When adding microcapsule to primer with three-layer primer and two-layer topcoat, the

color difference was the lowest of 0.9. When the coating process was three-layer primer and

three-layer topcoat and microcapsules were added to the topcoat, the elongation at break and

toughness of the coating were the highest. In the coating process of three-layer primer and two-layer

topcoat, when the microcapsule content was 10.0% in the primer, the gloss of the waterborne coating

was the highest, the color difference was the lowest, the toughness was better and the

comprehensive performance was better.

Figure 12. Effect of different coating processes on elongation at break of the coating (samples 1–4 fromTable 1).

4. Conclusions

The effects of the mass fractions of microcapsules and the sequence of microcapsules on theproperties of waterborne coating were studied. The gloss, color difference and flexibility of themodified coating were studied. The results showed that the gloss of the coating decreased withthe increase of the mass fractions of microcapsules, and the color difference increased first and thendecreased. When the mass fraction of microcapsules was 8.0%, the color difference was the highest, at6.8. The elongation at break of the coating first increased and then decreased with the mass fractions ofmicrocapsules. When the mass fractions of microcapsules were 10.0%, the toughness of the coatingwas the best. At the same time, adding 10.0% microcapsule in waterborne coating, under the samecoating process, the gloss of the coating with microcapsule in primer was higher, and the gloss ofthe coating was the highest when the coating process was three-layer primer and two-layer topcoat.Microcapsule had little effect on the chromatic distortion of coating in different coating processes. Whenadding microcapsule to primer with three-layer primer and two-layer topcoat, the color differencewas the lowest of 0.9. When the coating process was three-layer primer and three-layer topcoat andmicrocapsules were added to the topcoat, the elongation at break and toughness of the coating were

Coatings 2019, 9, 239 13 of 14

the highest. In the coating process of three-layer primer and two-layer topcoat, when the microcapsulecontent was 10.0% in the primer, the gloss of the waterborne coating was the highest, the colordifference was the lowest, the toughness was better and the comprehensive performance was better.

Author Contributions: Formal Analysis, X.Q.; Investigation, X.Y. and L.W.; Data Curation, X.Y.; Writing—OriginalDraft Preparation, X.Y.

Funding: This research was funded by Natural Science Foundation of Jiangsu Province, grant number BK20150887and Youth Science and Technology Innovation Fund of Nanjing Forestry University, grant number CX2016018.

Conflicts of Interest: The authors declare no conflict of interest.

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© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).


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http://dx.doi.org/10.1134/S1560090417030149


http://dx.doi.org/10.1016/j.compscitech.2017.02.018

http://creativecommons.org/

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effect of urea-formaldehyde-coated epoxy microcapsule

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