# Development and Characterization of New Coating Material of Blended Epoxy-Lacquer with Aluminum

Authors: Xiaoxing Yan; Yunting Cai; Rong Lu; Tetsuo Miyakoshi
Journal: International Journal of Polymer Science
 (2017)

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## Abstract

This study investigated the effects of aluminum (Al) mass fraction on the performance of an epoxy-lacquer based coating. The conductivity increased and IR emissivity and gloss decreased with increased Al mass fraction. When the Al mass fraction was decreased from 40% to 30%, the adhesion and impact strength of epoxy-lacquer based coatings increased and were grade 1 and 50 Kg·cm, respectively. When the Al mass fraction was decreased to 10%, the mechanical properties of epoxy-lacquer based coatings decreased. Potentiodynamic polarization and electrochemical impedance spectroscopy of coatings showed that epoxy-lacquer based coating with 30% Al mass fraction has the best corrosion resistance and the lowest porosity. The epoxy-lacquer based coating with the best stability was that of 30% Al mass fraction. Comparing epoxy-lacquer based coating with polyurethane based coating, the epoxy-lacquer based coating presents not only lower emissivity and gloss but also better mechanical properties and corrosion resistance.


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## Body

## 1. Introduction

Applications of low infrared (IR) emissivity coatings (LIREC) [1] in the civil, aero, and military fields have received much attention [2]. In recent years, researchers have conducted extensive work on several LIREC, such as La0.67Ca0.33-xSrxMnO3 compounds [3], ceramic composite [4], indium tin oxide composites [5], organic/inorganic composite coatings [6], and silver and aluminum (Al) [7]. In particular, the application of Al filler to LIREC has practical application potential [8].Gloss is one of the important criteria for evaluating the quality of coatings. In fact, gloss is a complex phenomenon caused by the interaction between the light and the coating surface [9]. High gloss of polyurethane (PU)/Al leads to incompatibility between visible light and near infrared light. Tan et al. [10] and Wu and Yu [11] studied the effects of modified Al on the gloss and IR emissivity. However, these methods mainly concentrated on the effects of modified filler on LIREC. An organic adhesive is an important component of organic/inorganic composite coatings, but little work has been published which quantifies the effect of an organic adhesive on the gloss of LIREC.Not only should LIREC have low IR emissivity, but also they should have good durability and mechanical properties suitable for practical application [12]. It is still a challenge to prepare low IR emissivity coatings with high corrosion resistance and mechanical properties today [11]. Although many synthetic polymers such as PU have been widely used as organic adhesives of low IR emissivity coatings, they show low mechanical properties and low corrosion resistance; therefore, developing natural products is still a very attractive solution for environmental protection purposes [13]. Raw lacquer [14] is a natural coating material that has excellent properties such as environmental friendliness, high durability, and corrosion resistance, and it has been widely used in areas such as chemical, textile, industrial, shipbuilding, mechanical, and electrical products [15]. However, the high price of raw lacquer and poor adhesion of lacquer to metal substrates have limited its application in low IR emissivity coatings. Modification of lacquer with epoxy (epoxy-lacquer) not only reduces the cost of raw lacquer but also improves the adhesion of the lacquer coating [16].In this study, LIREC were obtained by adding the epoxy-lacquer as an organic adhesive. The effects of Al mass fraction on the performance of the epoxy-lacquer based coatings were investigated, and the epoxy-lacquer based coating was compared to PU based coatings.


## 2. Experimental

### 2.1. Materials

Al powder flakes with a particle size ~10μm and a silane coupling agent (KH560) were supplied by Beijing Fengean Science Ltd. Co. Epoxy-lacquer (T09-17, epoxy-lacquer = 3 : 4) was supplied by the Xi’an Raw Lacquer and Coating Research Institute of China.


### 2.2. Preparation of Epoxy-Lacquer Based Coating

Firstly, sandpaper was used to polish the tinplate substrate, oil was removed using gasoline, and then the substrate was dried in an oven. The epoxy-lacquer and Al powder were mixed, and the Al mass fraction was controlled at 40%, 30%, 20%, or 10%. Then KH560 (2.0%) was added to the mixture as an organic surface treatment to improve the dispersion for the Al powder in the epoxy-lacquer [17]. Second, the mixture was blended uniformly by an ultrasonic mixer for 10 min and then sprayed on the tinplate substrate. Finally, the epoxy-lacquer based coating was gotten after 24 h placed in room temperature. The thickness of the epoxy-lacquer based coating was about 40 μm after drying.


### 2.3. Characterization

The IR emissivity in the range of 8–14μm of epoxy-lacquer based coating was tested in an IR-2 double wave IR radiometer (Shanghai Institute of Technological Physics of the Chinese Academy of Sciences). The conductivity of the epoxy-lacquer based coating was tested on a Keithley 6514 four-point probe conductivity device. The microstructure of epoxy-lacquer based coatings was observed by scanning electron microscope (SEM) (SX-40). Fourier transform infrared (FT-IR) spectrometry of epoxy-lacquer based coatings was performed by a VERTEX 80V infrared spectrometer. A WGG gloss meter (Tianjin Test Equipment, China) was used to observe the gloss of epoxy-lacquer based coatings. The adhesion of the epoxy-lacquer based coatings was tested by a QFZ-II circular cutting instrument (Tianjin Test Equipment) according to the Standardization Administration of China GB 1720-79 (89), and the impact resistance of the epoxy-lacquer based coatings was measured with an impactor based on the Standardization Administration of China GB/T 1732-93, respectively. The roughness of the epoxy-lacquer based coatings was measured by a JB-4C roughness meter (Shanghai Taiming Optical Instrument, China).The coatings were placed in a deionized NaCl water solution (3.5% NaCl, pH 7) that had been purified by argon for 1 h in an electrochemical workstation CHI750C to carry out a polarization test and electrochemical impedance spectroscopy (EIS) measurements under normal temperature conditions [18]. A saturated calomel electrode and a platinum plate were as the reference electrode and counter electrode, respectively, which were exposed to a corrosive environment. The rosin to paraffin ratio was 1 : 1, and they were mixed and heated to 60°C, coated on the edge of the epoxy-lacquer based coating. The exposed area was ~1 cm2. Before the test, the coatings were soaked in 3.5% NaCl solution for 30 min, and the open circuit potential was determined until a constant value appeared. EIS of epoxy-lacquer based coating was performed in 3.5% NaCl solution with the frequency range of 0.1 Hz to 100000 Hz using the amplitude of 5 mV. The Z-view software was used to analyze the EIS and the relative error of each element in the equivalent circuit was less than 5%.


## 2.1. Materials

Al powder flakes with a particle size ~10μm and a silane coupling agent (KH560) were supplied by Beijing Fengean Science Ltd. Co. Epoxy-lacquer (T09-17, epoxy-lacquer = 3 : 4) was supplied by the Xi’an Raw Lacquer and Coating Research Institute of China.


## 2.2. Preparation of Epoxy-Lacquer Based Coating

Firstly, sandpaper was used to polish the tinplate substrate, oil was removed using gasoline, and then the substrate was dried in an oven. The epoxy-lacquer and Al powder were mixed, and the Al mass fraction was controlled at 40%, 30%, 20%, or 10%. Then KH560 (2.0%) was added to the mixture as an organic surface treatment to improve the dispersion for the Al powder in the epoxy-lacquer [17]. Second, the mixture was blended uniformly by an ultrasonic mixer for 10 min and then sprayed on the tinplate substrate. Finally, the epoxy-lacquer based coating was gotten after 24 h placed in room temperature. The thickness of the epoxy-lacquer based coating was about 40 μm after drying.


## 2.3. Characterization

The IR emissivity in the range of 8–14μm of epoxy-lacquer based coating was tested in an IR-2 double wave IR radiometer (Shanghai Institute of Technological Physics of the Chinese Academy of Sciences). The conductivity of the epoxy-lacquer based coating was tested on a Keithley 6514 four-point probe conductivity device. The microstructure of epoxy-lacquer based coatings was observed by scanning electron microscope (SEM) (SX-40). Fourier transform infrared (FT-IR) spectrometry of epoxy-lacquer based coatings was performed by a VERTEX 80V infrared spectrometer. A WGG gloss meter (Tianjin Test Equipment, China) was used to observe the gloss of epoxy-lacquer based coatings. The adhesion of the epoxy-lacquer based coatings was tested by a QFZ-II circular cutting instrument (Tianjin Test Equipment) according to the Standardization Administration of China GB 1720-79 (89), and the impact resistance of the epoxy-lacquer based coatings was measured with an impactor based on the Standardization Administration of China GB/T 1732-93, respectively. The roughness of the epoxy-lacquer based coatings was measured by a JB-4C roughness meter (Shanghai Taiming Optical Instrument, China).The coatings were placed in a deionized NaCl water solution (3.5% NaCl, pH 7) that had been purified by argon for 1 h in an electrochemical workstation CHI750C to carry out a polarization test and electrochemical impedance spectroscopy (EIS) measurements under normal temperature conditions [18]. A saturated calomel electrode and a platinum plate were as the reference electrode and counter electrode, respectively, which were exposed to a corrosive environment. The rosin to paraffin ratio was 1 : 1, and they were mixed and heated to 60°C, coated on the edge of the epoxy-lacquer based coating. The exposed area was ~1 cm2. Before the test, the coatings were soaked in 3.5% NaCl solution for 30 min, and the open circuit potential was determined until a constant value appeared. EIS of epoxy-lacquer based coating was performed in 3.5% NaCl solution with the frequency range of 0.1 Hz to 100000 Hz using the amplitude of 5 mV. The Z-view software was used to analyze the EIS and the relative error of each element in the equivalent circuit was less than 5%.


## 3. Results and Discussion

### 3.1. IR Emissivity

The relationship between IR emissivity of epoxy-lacquer based coatings and Al mass fraction is shown in Table1. IR emissivity of epoxy-lacquer based coatings increased with the decrease of Al mass fraction. When Al mass fraction ranged between 30 and 40%, IR emissivity of epoxy-lacquer based coatings was in the range of 0.032 to 0.065. However, with decrease in the mass fraction of Al from 30% to 20%, the IR emissivity of epoxy-lacquer based coatings proliferated from 0.065 to 0.479. As can be seen from Table 1, lower Al mass fraction led to higher IR emissivity, and the reason will be analyzed below.Table 1
IR emissivity of different Al mass fraction.
Al mass fraction (%)
IR emissivity
40
0.032
30
0.065
20
0.479
10
0.796Under different Al mass fraction, the microstructure of the epoxy-lacquer based coatings was shown in Figure1. Figure 1 shows that when the Al mass fraction was low (10%), Al particles were distributed discretely in the epoxy-lacquer (Figure 1(a)). However, when the Al mass fraction was high (30%), close contact between Al particles in epoxy-lacquer based coatings was observed (Figure 1(b)).Figure 1
SEM of blended coating with (a) 10% and (b) 30% of Al.With a further increase of Al mass fraction in the coating, the roughness of epoxy-lacquer based coating increased (Figure2), many agglomeration particles can be seen, and a smooth coating could not be obtained when the Al mass fraction was higher than 40%; therefore, the Al mass fraction was increased no further for the preparation of coatings.Figure 2
Roughness of coating with different Al mass fraction.Basic principle of optics implies the relationship between the refractive index (n) and conductivity (σ) by the following formula [7]:(1)n2=μ2ε2+4σ2ν2+ε,where μ is magnetic conductivity, ε is the dielectric constant, and ν is the Planck constant. According to formula (1), it can be inferred that high σ can cause an increase of n. Reflectivity (ρe) can be obtained through Kirchhoff’s law by using the following formula [7]:(2)ρe=n-1n+12=1-2n+12.According to formula (2), it can be seen that high n leads the increase of ρe. On the basis of the law of Kirchhoff and the law of conservation of energy, the relationship of IR emissivity and ρe was the following formula [7, 19]:(3)IRemissivity=1-ρe.Formulas (1)–(3) indicate that the higher conductivity can decrease IR emissivity. With an increase of Al mass fraction, the Al particles can come into closer contact and the coating’s conductivity improved (Figure 3); therefore, the IR emissivity is reduced.Figure 3
IR emissivity and conductivity changes with Al mass fraction.As Figure4 shows, FT-IR of epoxy-lacquer based coating was related to Al mass fraction. The assignment of the peaks in spectra of epoxy-lacquer based coatings was shown in Table 2 [20, 21].Table 2
The assignment of peaks.
Peak (cm−1)
Assignment
2920, 2856
Stretching vibration of C-H bond
1746
Aldehyde groups
1620
C=C bond vibration
1056
C-O bond stretching vibration of chain ether
1102
C-O-C bond stretching vibration of the open-chain ether
813, 1258
C-O-C stretching vibration of the epoxide ringFigure 4
FT-IR of epoxy-lacquer based coating with different Al mass fraction.Figure4 shows that the pure epoxy-lacquer coating had strong infrared absorption peaks; and increasing the mass fraction of Al from 0 to 40%, the infrared peaks were weakened and even disappeared. Kirchhoff’s law shows that, in the infrared equilibrium state, the infrared energy absorbed by the object is equal to the infrared energy emitted by the object. Therefore, according to Figure 4, with the increase of Al mass fraction, the IR emissivity of epoxy-lacquer based composite coating was reduced.


### 3.2. Gloss

Figure5 showed the changes in the gloss of the epoxy-lacquer based coating with Al mass fraction. The gloss of the epoxy-lacquer based coatings increased rapidly from 4.8% to 51.7%, while the mass fraction of Al was decreased from 40% to 10%. The gloss of the epoxy-lacquer based coating was affected by the formulation, surface smoothness, refractive index, and other factors. With the decrease in Al mass fraction, the roughness of the coating decreased and the intensity of the reflected visible light increased, so that gloss increased. According to the specification for infrared camouflage coatings (GJB 1888–1994), when the detection angle is 60°, the gloss of the coating should be no more than 6% [10]. Figure 5 shows that when the Al mass fraction was more than 30%, the gloss of coating was less than 6%.Figure 5
Gloss changes with different Al mass fraction.


### 3.3. Mechanical Properties

The adhesion of the coating is the degree of coating attached to the substrate, and it is associated with the load that separates the coating and substrate completely. Figure6 shows the adhesion curve of the epoxy-lacquer based coating with different Al mass fraction. According to Figure 6, the adhesion of epoxy-lacquer based coating changed obviously with Al mass fraction. As shown in Figure 6, with increasing the Al mass fraction, the adhesion change presents a “U” type. When the Al mass fraction was 40%, the epoxy-lacquer based composite coating had poor adhesion of grade 3 due to the accumulation of filler which was difficult to disperse in the epoxy-lacquer. When the Al mass fraction was decreased from 40% to 30%, the epoxy-lacquer based composite coating had good adhesion, mainly grade 1 due to the good dispersion of the Al filler particles (Figure 1(b)). However, when the Al mass fraction decreased from 30% to 10%, the adhesion of the epoxy-lacquer based coating decreased from grade 1 to grade 4. This is because the distribution of Al particles in the epoxy-lacquer changed from a continuous state to a discrete state with the increase of Al mass fraction (Figure 1), which led to a decrease in the cohesive strength and the mechanical interlock, thereby making the adhesion of the coating worse.Figure 6
Adhesion with different Al mass fraction.The change in impact resistance of the coating with different Al mass fraction was also studied in order to investigate the dynamic loading. Figure7 illustrates the impact strength of the epoxy-lacquer based coating at different Al mass fraction. With the decrease of the Al mass fraction from 40% to 30%, the impact strength of the epoxy-lacquer based coating rose from 40 Kg·cm to 50 Kg·cm. However, the impact strength of the coating decreased with the Al mass fraction decreasing from 30% to 10%. A weak point of epoxy-lacquer is its high brittleness, which has poor ability to resist impact. The impact strength of the epoxy-lacquer based coating was affected by the Al mass fraction obviously: when the Al mass fraction was too high or too low, it cannot improve the impact strength performance. When Al particles are uniformly dispersed in the epoxy-lacquer, the impact strength of the epoxy-lacquer based coating is improved. The change of impact strength and adhesion of epoxy-lacquer based coating showed the same trend, implying that good adhesion could strengthen the impact resistance of the epoxy-lacquer based coating [22].Figure 7
Impact strength changes with different Al mass fraction.


### 3.4. Corrosion Resistance

Potentiodynamic polarization is a method widely used in the laboratory to evaluate the durability of a coating. The durability of epoxy-lacquer based coatings was studied in 3.5% aqueous NaCl by corrosion. The better corrosion resistance, the higher durability. Figure8 shows the potentiodynamic polarization curves of epoxy-lacquer based coatings with different Al mass fraction. Electrochemical parameters were calculated using the software of the electrochemical workstation CHI750C. The calculated results were summarized in Table 3. Ecorr is the corrosion potential, Icorr is the corrosion current densities, RP is the polarization resistance, and βa and βc are anode Tafel constant and cathode Tafel constant, respectively.Table 3
Electrochemical corrosion results with different Al mass fraction.
Mass fraction
E
c
o
r
r (V)
R
p (Ω/cm2)
I
c
o
r
r (A/cm2)
β
a (mV/dec)
β
c (mV/dec)
P
Tinplate
-
0.626
4.101 × 103
5.251 × 10−6
191
67
—
40%
-
0.368
3.070 × 107
1.635 × 10−9
226
236
5.9 × 10−6
30%
-
0.189
7.729 × 107
6.899 × 10−10
236
255
2.7 × 10−7
20%
-
0.292
3.476 × 107
1.486 × 10−9
231
244
2.1 × 10−6
10%
-
0.617
1.416 × 107
3.780 × 10−9
273
224
2.6 × 10−4Figure 8
Polarization curves changes with different Al mass fraction.In general, when the corrosion potential is more positive, the polarization resistance is larger, and the corrosion current is lower, the coating has better anticorrosion properties. As shown in Table3, when the Al mass fraction decreased from 40% to 30%, Ecorr of the epoxy-lacquer based coating became more positive, RP became higher, and Icorr became lower, which indicated that epoxy-lacquer based coatings with Al mass fraction of 30% had better corrosion resistance than that of 40% Al mass fraction. However, when the Al mass fraction decreased from 30% to 10%, Ecorr of the epoxy-lacquer based coating became more negative, RP became lower, and Icorr became higher, which implied that the corrosion resistance properties of the epoxy-lacquer based coatings decreased with decreasing Al mass fraction from 30% to 10%.When the Al mass fraction is high, the coating becomes uneven, resulting in a lot of pores. The porosity of the coating has an important effect on corrosion resistance. The porosity (P) of the epoxy-lacquer based coating can be calculated from the following formula [23]:(4)P=RpsRpc×10-ΔEcorr/βat.In formula (4), Rps and Rpc are the polarization resistance of the substrate and the coating coated substrate, respectively, ΔEcorr is corrosion potential difference between the epoxy-lacquer based coating and tinplate, and βat is the anode Tafel constant of the tinplate. The porosity of epoxy-lacquer based coatings with different Al mass fractions was calculated, as is shown Table 3. When Al mass fraction decreased from 40% to 30%, the porosity of epoxy-lacquer based coatings decreased. The porosity value first decreased to the minimum of 2.7 × 10−7 and then began to increase with the further decrease of Al mass fraction. With the decrease of the Al mass fraction from 40% to 30%, the state of Al particles changed from aggregation to uniform distribution, which may lead to a denser coating structure and decrease the porosity, thereby decreasing the NaCl solution penetration and increasing the corrosion resistance [24]. When the Al mass fraction was decreased from 30% to 10%, the morphology of the coating changed from the continuous particle contact state to the independent particle state, and thus the mixture was more loose (Figure 1). This made the epoxy-lacquer based coating more inhomogeneous, and thus the porosity was increased. The experimental results showed that when the Al mass fraction was controlled at 30%, the epoxy-lacquer based coating had good corrosion resistance and durability, which had more positive Ecorr, higher RP, lower Icorr, and lower porosity.Figure9 showed the EIS of epoxy-lacquer based coatings with different Al mass fraction and the equivalent circuit. Rer represents the resistance of electrolyte, CPE represents the phase element related to the capacitance of epoxy-lacquer based coatings, and Rcp represents the pore resistance of epoxy-lacquer based coatings. The parameters of EIS were presented in Table 4.Table 4
Impedance parameter values with different Al mass fraction.
Mass fraction
R
e
r  (Ω/cm2)
R
c
p  (Ω/cm2)
C
c  (F/m)
n
40%
3.520
1.514 × 1010
6.428 × 10−3
0.30
30%
2.643
3.225 × 1011
3.001 × 10−3
0.32
20%
3.296
1.183 × 1011
3.052 × 10−3
0.33
10%
2.204
7.274 × 109
7.412 × 10−3
0.27Figure 9
Nyquist plot of epoxy-lacquer based coatings with different Al mass fraction. Internal illustration was the equivalent circuit.As can be seen from Table4, Rer has always been a small value for a given electrolyte. Rcp of epoxy-lacquer based coatings with Al mass fraction of 30% is 3.225 × 1011, much higher than that of Al mass fraction of 10%. The higher value of Rcp and lower value of Cc showed good corrosion resistance of epoxy-lacquer based coatings with Al mass fraction of 30% [25].The change trend of the IR emissivity of the epoxy-lacquer based coatings with different Al mass fraction in the corrosive medium was used for the evaluation of stability. The smaller the variation of IR emissivity is, the better corrosion resistance and the stability of the coating are [26]. The change trend of IR emissivity of the epoxy-lacquer based coatings with different Al mass fraction in 3.5% NaCl solution was shown in Figure 10.Figure 10
The change of IR emissivity of epoxy-lacquer based coatings with different Al mass fraction with corrosion time.It can be seen that the IR emissivity of epoxy-lacquer based coatings with Al mass fraction of 30% was almost unchanged with the corrosion time, and the IR emissivity of the epoxy-lacquer based coatings with Al mass fraction of 20% and 40% increased slightly with increasing corrosion time. However, the IR emissivity of the epoxy-lacquer based coatings with Al mass fraction of 10% increased significantly from 0.796 to 0.951 in the corrosion time of 0–700 h. It was apparent that the stability of epoxy-lacquer based coatings with Al mass fraction of 30% was the best.


### 3.5. Comparison of Epoxy-Lacquer Based Coating with PU Based Coating

The performance of epoxy-lacquer based coating was compared with that of the PU based coating [7, 23, 27], as shown in Table 5. The low IR emissivity of these coatings (~0.100) can be obtained when the mass fraction of filler is appropriate. The gloss of the epoxy-lacquer based coating was much lower than that of PU based coatings. The epoxy-lacquer based coating had better durability, lower porosity, and better mechanical properties compared with PU based coatings.Table 5
Performance comparison of epoxy-lacquer with PU based coatings.
Coating
Filler mass fraction (%)
Emissivity
Gloss (%)
Adhesion (grade)
Impact resistance (kg⋅cm)
P
Epoxy-lacquer/Al
30
0.065
5.7
1
50
2.7 × 10−7
PU/Cu
50
0.100
45.2
3
20
—
PU/(ball-milled Ag-Cu)
50
0.129
35.6
3
20
3.2 × 10−3
Epoxy-PU/bronze
40
0.069
35.4
1
50
1.0 × 10−4
PU/Al
40
0.068
38.3
1
40
8.6 × 10−6


## 3.1. IR Emissivity

The relationship between IR emissivity of epoxy-lacquer based coatings and Al mass fraction is shown in Table1. IR emissivity of epoxy-lacquer based coatings increased with the decrease of Al mass fraction. When Al mass fraction ranged between 30 and 40%, IR emissivity of epoxy-lacquer based coatings was in the range of 0.032 to 0.065. However, with decrease in the mass fraction of Al from 30% to 20%, the IR emissivity of epoxy-lacquer based coatings proliferated from 0.065 to 0.479. As can be seen from Table 1, lower Al mass fraction led to higher IR emissivity, and the reason will be analyzed below.Table 1
IR emissivity of different Al mass fraction.
Al mass fraction (%)
IR emissivity
40
0.032
30
0.065
20
0.479
10
0.796Under different Al mass fraction, the microstructure of the epoxy-lacquer based coatings was shown in Figure1. Figure 1 shows that when the Al mass fraction was low (10%), Al particles were distributed discretely in the epoxy-lacquer (Figure 1(a)). However, when the Al mass fraction was high (30%), close contact between Al particles in epoxy-lacquer based coatings was observed (Figure 1(b)).Figure 1
SEM of blended coating with (a) 10% and (b) 30% of Al.With a further increase of Al mass fraction in the coating, the roughness of epoxy-lacquer based coating increased (Figure2), many agglomeration particles can be seen, and a smooth coating could not be obtained when the Al mass fraction was higher than 40%; therefore, the Al mass fraction was increased no further for the preparation of coatings.Figure 2
Roughness of coating with different Al mass fraction.Basic principle of optics implies the relationship between the refractive index (n) and conductivity (σ) by the following formula [7]:(1)n2=μ2ε2+4σ2ν2+ε,where μ is magnetic conductivity, ε is the dielectric constant, and ν is the Planck constant. According to formula (1), it can be inferred that high σ can cause an increase of n. Reflectivity (ρe) can be obtained through Kirchhoff’s law by using the following formula [7]:(2)ρe=n-1n+12=1-2n+12.According to formula (2), it can be seen that high n leads the increase of ρe. On the basis of the law of Kirchhoff and the law of conservation of energy, the relationship of IR emissivity and ρe was the following formula [7, 19]:(3)IRemissivity=1-ρe.Formulas (1)–(3) indicate that the higher conductivity can decrease IR emissivity. With an increase of Al mass fraction, the Al particles can come into closer contact and the coating’s conductivity improved (Figure 3); therefore, the IR emissivity is reduced.Figure 3
IR emissivity and conductivity changes with Al mass fraction.As Figure4 shows, FT-IR of epoxy-lacquer based coating was related to Al mass fraction. The assignment of the peaks in spectra of epoxy-lacquer based coatings was shown in Table 2 [20, 21].Table 2
The assignment of peaks.
Peak (cm−1)
Assignment
2920, 2856
Stretching vibration of C-H bond
1746
Aldehyde groups
1620
C=C bond vibration
1056
C-O bond stretching vibration of chain ether
1102
C-O-C bond stretching vibration of the open-chain ether
813, 1258
C-O-C stretching vibration of the epoxide ringFigure 4
FT-IR of epoxy-lacquer based coating with different Al mass fraction.Figure4 shows that the pure epoxy-lacquer coating had strong infrared absorption peaks; and increasing the mass fraction of Al from 0 to 40%, the infrared peaks were weakened and even disappeared. Kirchhoff’s law shows that, in the infrared equilibrium state, the infrared energy absorbed by the object is equal to the infrared energy emitted by the object. Therefore, according to Figure 4, with the increase of Al mass fraction, the IR emissivity of epoxy-lacquer based composite coating was reduced.


## 3.2. Gloss

Figure5 showed the changes in the gloss of the epoxy-lacquer based coating with Al mass fraction. The gloss of the epoxy-lacquer based coatings increased rapidly from 4.8% to 51.7%, while the mass fraction of Al was decreased from 40% to 10%. The gloss of the epoxy-lacquer based coating was affected by the formulation, surface smoothness, refractive index, and other factors. With the decrease in Al mass fraction, the roughness of the coating decreased and the intensity of the reflected visible light increased, so that gloss increased. According to the specification for infrared camouflage coatings (GJB 1888–1994), when the detection angle is 60°, the gloss of the coating should be no more than 6% [10]. Figure 5 shows that when the Al mass fraction was more than 30%, the gloss of coating was less than 6%.Figure 5
Gloss changes with different Al mass fraction.


## 3.3. Mechanical Properties

The adhesion of the coating is the degree of coating attached to the substrate, and it is associated with the load that separates the coating and substrate completely. Figure6 shows the adhesion curve of the epoxy-lacquer based coating with different Al mass fraction. According to Figure 6, the adhesion of epoxy-lacquer based coating changed obviously with Al mass fraction. As shown in Figure 6, with increasing the Al mass fraction, the adhesion change presents a “U” type. When the Al mass fraction was 40%, the epoxy-lacquer based composite coating had poor adhesion of grade 3 due to the accumulation of filler which was difficult to disperse in the epoxy-lacquer. When the Al mass fraction was decreased from 40% to 30%, the epoxy-lacquer based composite coating had good adhesion, mainly grade 1 due to the good dispersion of the Al filler particles (Figure 1(b)). However, when the Al mass fraction decreased from 30% to 10%, the adhesion of the epoxy-lacquer based coating decreased from grade 1 to grade 4. This is because the distribution of Al particles in the epoxy-lacquer changed from a continuous state to a discrete state with the increase of Al mass fraction (Figure 1), which led to a decrease in the cohesive strength and the mechanical interlock, thereby making the adhesion of the coating worse.Figure 6
Adhesion with different Al mass fraction.The change in impact resistance of the coating with different Al mass fraction was also studied in order to investigate the dynamic loading. Figure7 illustrates the impact strength of the epoxy-lacquer based coating at different Al mass fraction. With the decrease of the Al mass fraction from 40% to 30%, the impact strength of the epoxy-lacquer based coating rose from 40 Kg·cm to 50 Kg·cm. However, the impact strength of the coating decreased with the Al mass fraction decreasing from 30% to 10%. A weak point of epoxy-lacquer is its high brittleness, which has poor ability to resist impact. The impact strength of the epoxy-lacquer based coating was affected by the Al mass fraction obviously: when the Al mass fraction was too high or too low, it cannot improve the impact strength performance. When Al particles are uniformly dispersed in the epoxy-lacquer, the impact strength of the epoxy-lacquer based coating is improved. The change of impact strength and adhesion of epoxy-lacquer based coating showed the same trend, implying that good adhesion could strengthen the impact resistance of the epoxy-lacquer based coating [22].Figure 7
Impact strength changes with different Al mass fraction.


## 3.4. Corrosion Resistance

Potentiodynamic polarization is a method widely used in the laboratory to evaluate the durability of a coating. The durability of epoxy-lacquer based coatings was studied in 3.5% aqueous NaCl by corrosion. The better corrosion resistance, the higher durability. Figure8 shows the potentiodynamic polarization curves of epoxy-lacquer based coatings with different Al mass fraction. Electrochemical parameters were calculated using the software of the electrochemical workstation CHI750C. The calculated results were summarized in Table 3. Ecorr is the corrosion potential, Icorr is the corrosion current densities, RP is the polarization resistance, and βa and βc are anode Tafel constant and cathode Tafel constant, respectively.Table 3
Electrochemical corrosion results with different Al mass fraction.
Mass fraction
E
c
o
r
r (V)
R
p (Ω/cm2)
I
c
o
r
r (A/cm2)
β
a (mV/dec)
β
c (mV/dec)
P
Tinplate
-
0.626
4.101 × 103
5.251 × 10−6
191
67
—
40%
-
0.368
3.070 × 107
1.635 × 10−9
226
236
5.9 × 10−6
30%
-
0.189
7.729 × 107
6.899 × 10−10
236
255
2.7 × 10−7
20%
-
0.292
3.476 × 107
1.486 × 10−9
231
244
2.1 × 10−6
10%
-
0.617
1.416 × 107
3.780 × 10−9
273
224
2.6 × 10−4Figure 8
Polarization curves changes with different Al mass fraction.In general, when the corrosion potential is more positive, the polarization resistance is larger, and the corrosion current is lower, the coating has better anticorrosion properties. As shown in Table3, when the Al mass fraction decreased from 40% to 30%, Ecorr of the epoxy-lacquer based coating became more positive, RP became higher, and Icorr became lower, which indicated that epoxy-lacquer based coatings with Al mass fraction of 30% had better corrosion resistance than that of 40% Al mass fraction. However, when the Al mass fraction decreased from 30% to 10%, Ecorr of the epoxy-lacquer based coating became more negative, RP became lower, and Icorr became higher, which implied that the corrosion resistance properties of the epoxy-lacquer based coatings decreased with decreasing Al mass fraction from 30% to 10%.When the Al mass fraction is high, the coating becomes uneven, resulting in a lot of pores. The porosity of the coating has an important effect on corrosion resistance. The porosity (P) of the epoxy-lacquer based coating can be calculated from the following formula [23]:(4)P=RpsRpc×10-ΔEcorr/βat.In formula (4), Rps and Rpc are the polarization resistance of the substrate and the coating coated substrate, respectively, ΔEcorr is corrosion potential difference between the epoxy-lacquer based coating and tinplate, and βat is the anode Tafel constant of the tinplate. The porosity of epoxy-lacquer based coatings with different Al mass fractions was calculated, as is shown Table 3. When Al mass fraction decreased from 40% to 30%, the porosity of epoxy-lacquer based coatings decreased. The porosity value first decreased to the minimum of 2.7 × 10−7 and then began to increase with the further decrease of Al mass fraction. With the decrease of the Al mass fraction from 40% to 30%, the state of Al particles changed from aggregation to uniform distribution, which may lead to a denser coating structure and decrease the porosity, thereby decreasing the NaCl solution penetration and increasing the corrosion resistance [24]. When the Al mass fraction was decreased from 30% to 10%, the morphology of the coating changed from the continuous particle contact state to the independent particle state, and thus the mixture was more loose (Figure 1). This made the epoxy-lacquer based coating more inhomogeneous, and thus the porosity was increased. The experimental results showed that when the Al mass fraction was controlled at 30%, the epoxy-lacquer based coating had good corrosion resistance and durability, which had more positive Ecorr, higher RP, lower Icorr, and lower porosity.Figure9 showed the EIS of epoxy-lacquer based coatings with different Al mass fraction and the equivalent circuit. Rer represents the resistance of electrolyte, CPE represents the phase element related to the capacitance of epoxy-lacquer based coatings, and Rcp represents the pore resistance of epoxy-lacquer based coatings. The parameters of EIS were presented in Table 4.Table 4
Impedance parameter values with different Al mass fraction.
Mass fraction
R
e
r  (Ω/cm2)
R
c
p  (Ω/cm2)
C
c  (F/m)
n
40%
3.520
1.514 × 1010
6.428 × 10−3
0.30
30%
2.643
3.225 × 1011
3.001 × 10−3
0.32
20%
3.296
1.183 × 1011
3.052 × 10−3
0.33
10%
2.204
7.274 × 109
7.412 × 10−3
0.27Figure 9
Nyquist plot of epoxy-lacquer based coatings with different Al mass fraction. Internal illustration was the equivalent circuit.As can be seen from Table4, Rer has always been a small value for a given electrolyte. Rcp of epoxy-lacquer based coatings with Al mass fraction of 30% is 3.225 × 1011, much higher than that of Al mass fraction of 10%. The higher value of Rcp and lower value of Cc showed good corrosion resistance of epoxy-lacquer based coatings with Al mass fraction of 30% [25].The change trend of the IR emissivity of the epoxy-lacquer based coatings with different Al mass fraction in the corrosive medium was used for the evaluation of stability. The smaller the variation of IR emissivity is, the better corrosion resistance and the stability of the coating are [26]. The change trend of IR emissivity of the epoxy-lacquer based coatings with different Al mass fraction in 3.5% NaCl solution was shown in Figure 10.Figure 10
The change of IR emissivity of epoxy-lacquer based coatings with different Al mass fraction with corrosion time.It can be seen that the IR emissivity of epoxy-lacquer based coatings with Al mass fraction of 30% was almost unchanged with the corrosion time, and the IR emissivity of the epoxy-lacquer based coatings with Al mass fraction of 20% and 40% increased slightly with increasing corrosion time. However, the IR emissivity of the epoxy-lacquer based coatings with Al mass fraction of 10% increased significantly from 0.796 to 0.951 in the corrosion time of 0–700 h. It was apparent that the stability of epoxy-lacquer based coatings with Al mass fraction of 30% was the best.


## 3.5. Comparison of Epoxy-Lacquer Based Coating with PU Based Coating

The performance of epoxy-lacquer based coating was compared with that of the PU based coating [7, 23, 27], as shown in Table 5. The low IR emissivity of these coatings (~0.100) can be obtained when the mass fraction of filler is appropriate. The gloss of the epoxy-lacquer based coating was much lower than that of PU based coatings. The epoxy-lacquer based coating had better durability, lower porosity, and better mechanical properties compared with PU based coatings.Table 5
Performance comparison of epoxy-lacquer with PU based coatings.
Coating
Filler mass fraction (%)
Emissivity
Gloss (%)
Adhesion (grade)
Impact resistance (kg⋅cm)
P
Epoxy-lacquer/Al
30
0.065
5.7
1
50
2.7 × 10−7
PU/Cu
50
0.100
45.2
3
20
—
PU/(ball-milled Ag-Cu)
50
0.129
35.6
3
20
3.2 × 10−3
Epoxy-PU/bronze
40
0.069
35.4
1
50
1.0 × 10−4
PU/Al
40
0.068
38.3
1
40
8.6 × 10−6


## 4. Conclusions

With Al mass fraction decreasing from 40% to 10%, IR emissivity of the epoxy-lacquer based coating was raised from 0.032 to 0.796, and the gloss of the epoxy-lacquer based coating increased from 4.8% to 51.7%. The adhesion and impact resistance first increased and then decreased with decreasing Al mass fraction. When the Al mass fraction is 30%, the coating had the best adhesion and impact strength. They were grade 1 and 50 Kg·cm, respectively. In addition, potentiodynamic polarization and EIS showed that the corrosion resistance of the epoxy-lacquer based composite coating increased when the Al mass fraction was decreased from 40% to 30%. When the Al mass fraction was less than 30%, the corrosion resistance of the coating decreased. The stability of epoxy-lacquer based coatings with Al mass fraction of 30% was the best. Investigation results showed that when the Al mass fraction was 30%, the epoxy-lacquer based coating had lower gloss and IR emissivity, more excellent mechanical properties, and higher durability compared to PU based coatings (PU/Al coating). Using epoxy-lacquer and Al to produce coatings will open up a new method for novel functional coatings.



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*Source: 5017356-2017-05-31.xml*