Effect of Silane on the Active Aging Resistance and Anticorrosive Behaviors of Natural Lacquer.

Environmentally friendly and renewable hybrid lacquer coatings with excellent aging resistant and anticorrosion properties were studied. The coatings were prepared using raw lacquer coupled with the silane agent 3-aminopropyltriethoxysilane or N-(2-aminoethyl)-3-aminopropyltrimethoxysilane via an eco-friendly sol-gel preparation process. The physical-mechanical properties, thermal stability, aging resistance, and anticorrosion properties of the as-prepared coatings were analyzed. Additionally, the surface of the coatings before and after an accelerated aging treatment was studied by scanning electron microscopy and X-ray photoelectron spectroscopy. The results revealed that the hybrid lacquer coating A (with a raw lacquer-to-APTES mass ratio of 1.8:1) resulted in films with a significantly enhanced antiaging effect (e.g., six times higher than that of lacquer at a gloss loss rate of 30%). Besides, this film revealed an exceptional anticorrosion performance (with the lowest corrosion current I corr = 2.476 × 10-10 A·cm-2) and a high protection efficiency (99.99 and 94.10%), as demonstrated by its electrochemical characteristics. Furthermore, all films exhibited a good barrier because of their dense structure, which prevents the corrosive medium from penetrating the coating during the salt spray test analysis after 1000 h. And the coating A relatively layered was distributing any significant cancaves, integrity better than all coatings studied, indicating that the based electrolyte was easier to penetrate it after salt spraying 2000 h.

Introduction

Corrosion can seriously damage energy metal structures, which can lead to potential losses of 1/5 of the total energy produced worldwide and 4.2% of the gross national product on average.[1] Thus, the development of effective corrosion inhibition technologies represents a high economic value.[2] Polymer organic coatings have various advantages over their metal counterparts. Thus, polymer organic coatings are widely used in numerous industrial applications owing to their low density, easy processing, and superior anticorrosion and mechanical properties against moisture and gases under rough environments.[3−6] Several methods are available today for preparing anticorrosive coatings including thermal spray coating, chemical vapor deposition, plasma spraying, electroplating, laser cladding, and sol–gel processes, among others.[7,8] As is well-known, raw lacquer is a natural, biodegradable, renewable, and environmentally friendly coating product which is people-friendly, durable, and impermeable and exhibits chemical resistance to corrosive medium.[9,10] Because drying raw lacquer involves an enzyme-catalyzed process, this step is slower than for common organic coatings.[11] Moreover, enzymatic oxidation requires severe and specific conditions (ca. 80–90% relative humidity at 20–30 °C) for the activation of laccase. Thus, there is a growing interest in developing methods that allow rapid drying of raw lacquer under natural environments. In this sense, urushiol–metal polymers were prepared by the reaction between a metal ion and the phenyl hydroxyl groups of urushiol.[12−15] The resultant urushiol–metal polymers can be rapidly dried at room temperature while showing enhanced anticorrosive properties for industrial applications.[16,17] Lacquer materials hybridized with organic silanes have demonstrated rapid drying characteristics under natural environments, and the hybridized coatings have been studied by determining the mechanism of the hybridization reaction.[18] Additionally, basic chemical sol–gel processes have been used to prepare promising protective coatings, allowing further design and control of their compositions and properties.[19,20] Therefore, sol–gel processes can be a potential alternative to prepare corrosion resistant coatings. Considering the specific properties of hybridized lacquer coatings described in the literature, sol–gel methods can be used to expand the scope of application of these materials.

Herein, an eco-friendly sol–gel method was used to conduct the reaction between lacquer and a silane coupling agent. This reaction resulted in fast-drying lacquer hybrid coatings at low cost and with higher mechanical strength and better thermal stability characteristics. Most importantly, the resultant lacquer hybrid coatings showed significantly enhanced aging and corrosion resistance properties characterized by accelerated aging, electrochemical analysis, and salt spray test analyses.

Results and Discussion

Measurement of the Physical–Mechanical Properties

Raw lacquer is a natural product whose composition and structure depend on the variety, origin, and condition of the lacquer tree. We measured the drying times of various hybrid lacquer coatings containing suitable organic silane ratios using a drying time recorder, and the results are shown in Table .

Table 1

Physical and Mechanical Properties of Film Samplesa,b

entry	organic silane	ratio (wt %)	TF	HD (h)	pencil hardness	gloss
1		L	<24 h	<48	4H	54.4
2	APTES	4:1	<5 h	<36	4H	98.6
3		3:1	<1.5 h	<15	5H	98.3
4		2:1	<1 h	<5	5H	84.5
5		1.8:1	<35 min	<3	5H	99.3
6		1.5:1	<45 min	<2	4H	82.5
7		1.3:1	<30 min	<2	6H	97.7
8	AATMS	4:1	<1.5 h	<20	5H	99.4
9		3:1	<3 h	<28	5H	93.6
10		2:1	<45 min	<9	4H	88.3
12		1.8:1	<25 min	<2.5	4H	99.9
11		1.5:1	<40 min	<9	4H	92.5
13		1.3:1	<35 min	<6	6H	98.0

Ratio between raw lacquer and silane coupling agent.

Drying times were determined by the drying time recorder. TF: touch-free dry, HD: hard dry.

As shown in Table , the cross-link onset times of the hybridized lacquer coatings (entries 2–13) were shortened to less than half of the drying time of L (entry 1). However, when an excess of 3-aminopropyltriethoxysilane (APTES) (entries 2–7) or N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AATMS) (entries 8–13) was added, the cross-link onset time was advanced remarkably. In these cases, the touch-free (TF) and hard-dry (HD) drying times of the hybrid coatings containing high APTES (entries 6 and 7) and AATMS (entries 12 and 13) decreased. The drying time of the hybridized lacquer was shortened via enzymatic oxidation and autoxidation of the lacquer and the sol–gel reaction of silanes. Moreover, alcoholysis between the urushiol hydroxyl groups and the alkoxy group of the organic silanes took place, leading to oxidative cross-linking polymerization. Thus, the autoxidation of the urushiol unsaturated side chain to form a more complex polymer was confirmed, which promoted fast drying of the lacquer material. Because the pencil hardness and gloss properties of the coatings were excellent, all of the films can be selected for various applications. The hybridized lacquer coatings with the shortest drying times (entries 6, 7, 12, and 13) were further studied.

Among the various measurements concerning coatings, gel content plays a vital role in the characterization of relative structures, especially to those coatings that could not be soluble in any organic solvents or water. The measured values corresponded to the different samples (coatings A, B, C, D, and E) are summarized in Table . A gradual decrease in gel content was observed with the increase in exposure time. The main reason is that part of the conjugate and isolated double bond in urushiol long carbon chain did not participate in the polymerization at room temperature and could not completely form the interpenetrating network structure. Obviously, the effect of coating E was the worst, whereas that of A exhibited a better performance, which indicated that coating A relatively completely reacted with a greater degree of gelation.

Table 2

Different Coatings of the Gel Content Value

exposure time/min	A	B	C	D	E
30	100	100	100	100	99.59
60	100	99.29	98.31	99.38	98.52
90	99.46	98.10	97.86	98.26	98.40
120	98.57	97.57	97.56	97.62	97.24
240	98.57	97.40	97.21	97.52	97.01

Characterization of the Synthesized Hybridized Coatings

Fourier transform infrared (FT-IR) measurements were carried out to investigate and confirm the structures of the hybrid lacquer films and lacquer coatings, and the spectra are shown in Figure . The spectra of APTES and AATMS are shown in Figure a. The obvious absorption peaks at 1050 and 690 cm–1 were associated with Si–O–C and RC=CRH cis-structure extending vibrations, respectively. In Figure b, the spectra of the hybrid lacquers (A, B, C, and D) showed the presence of a stretching vibration peak at 3400 cm–1, which was ascribed to the phenolic hydroxyl group (−OH), whereas absorption bands ascribed to −CH2 and −CH groups were observed at 2925 and 2854 cm–1, respectively. The peak at 1710 cm–1 indicated that more urushiol quinones appeared. The phenolic hydroxyl group in urushiol was replaced by a Si atom, thereby reducing the strength of the absorption peak as compared to lacquer (E). However, the absorption band assigned to −CH3 at 1460 cm–1 was relatively intense. New absorption peaks at 1120 and 1050 cm–1 were associated with Si–O–Si and Si–O–C extending vibrations, respectively. Moreover, the intensity of the peaks at 1060 and 690 cm–1 (ascribed to trienes) and the peak of the RC=CRH cis-structure increased, suggesting that laccase polymerization took place.

Figure 1

FT-IR spectra of silane (a) and different coatings (b).

Thermogravimetric Analysis of Different Coatings

The thermogravimetric (TG) analysis of the hybrid lacquer films (A, B, C, and D) and lacquer (E) is shown in Figure and Table . The lacquer films (A, B, C, D, and E) underwent two-stage major weight-loss processes. The weight loss of the A, B, C, and D films started at 319, 327, 305, and 304 °C, respectively, whereas that of lacquer E started at 335 °C. This weight loss resulted from the degradation of the uncompleted cross-linked urushiol oligomers and other constituents such as glycoproteins and polysaccharides. The maximum temperature of the weight loss associated with the decomposition of the polymer for the hybrid films was 10 °C higher than that of raw lacquer. This weight loss was produced by the decomposition of the basic structural framework of Si–O–Si formed by the reaction of lacquer and the silane coupling agent. These Si–O–Si groups have superior chemical stability and heat resistance. In addition, as a result of the electronegativity difference between Si and C atoms, the Si–O bond is more polar than the C–O group. Thus, the Si–O bonds can effectively shield the connected hydrocarbon groups and improve the activation energy for the main chain fracture.[21,22] It is not easy to break the resultant chemical bond between the atoms, thereby improving the thermal stability of the hybrid lacquer films (A, B, C, and D), which showed higher char yields as compared to lacquer (E, 19.19%). In addition, the A films showed good thermal properties.

Figure 2

TG analysis of different lacquer coatings.

Table 3

Thermal Decomposition Parameters of Different Coatings

samplea	Tb (°C)	Tc (°C)	Td (°C)	char yield/%
A	319	458	464	27.30
B	327	468	467	28.33
C	305	459	469	26.64
D	304	461	465	27.80
E	335	441	451	19.19

(A) L/APTES = 1.8:1, (B) L/APTES = 1.3:1, (C) L/AATMS = 1.8:1, (D) L/AATMS = 1.3:1, and (E) L.

Onset of weight loss.

Weight loss to 50%.

Weight loss to the maximum.

Aging Resistance Analysis of the Hybrid Lacquer Coatings

After accelerated aging, all coatings were not obviously damaged. On the basis of the surface gloss, films that underwent accelerated aging treatments were analyzed. Gloss was used to illustrate the surface of the film. The film surface gloss decreased with time and under the impact of external conditions.[23] In the experiments, the films were placed in a xenon lamp aging box and the gloss of the films was reduced for a long time. The gloss loss rate was calculated using the following equationwhere P0 and P are the gloss before and after aging, respectively.

When exposed to the xenon lamp, the films underwent further cross-linking to form a more dense and smooth structure with the increasing ambient temperature. Therefore, before exposure, the gloss of the coatings declined more slowly or even increased, exhibiting lower and even negative gloss loss performance, such as for coating C. The gloss loss of coatings increased because the coatings may have holes or even cracks to form the rough surface in a relatively higher temperature when continuely exposed under the xenon lamp. As shown in Figure and Table , the gloss loss rates of the hybrid lacquer films (A, B, C, and D) were less than 40% when exposed to UV irradiation for 192, 288, and 336 h (A and C showed values below 21.67 and 9.79%, respectively). The loss rates of the E film were 61.28 and 86.64% after 288 and 336 h, respectively, indicating that the surface of the film may be significantly damaged or peeled off. After exposure for 576 h, the loss rates of the A, B, C, and D films reached 27.63, 39.49, 81.48, and 89.32%, respectively, with the surfaces of the C and D films being seriously damaged. After irradiating for 720 h, the loss rates of the A and B films were 80.86 and 86.65%, respectively, and the films showed no aging resistance. Hence, the data above demonstrated that the A film possessed good antiaging effects (six times higher than those of the E film) at a gloss loss rate of 30%.

Figure 3

Gloss loss analysis of different coatings.

Table 4

Effect of Exposure Time on Different Lacquer Coatings

T/h	GA (%)	GB (%)	GC (%)	GD (%)	GE (%)
0	0	0	0	0	0
48	7.74	4.30	–0.11	4.37	16.20
96	9.78	15.38	–2.40	21.51	27.98
144	13.44	23.14	0.53	31.44	25.49
192	14.37	28.89	–3.26	37.42	19.33
240	16.00	32.26	–0.22	40.04	41.68
288	21.37	34.52	–0.22	35.40	61.28
336	21.67	35.34	9.79	36.04	86.64
384	17.22	33.67	7.23	25.24
432	16.44	39.90	9.15	30.37
480	22.03	38.01	61.57	43.78
528	23.08	40.71	72.59	62.15
576	27.63	39.49	81.48	89.32
624	33.56	58.81
672	41.86	80.65
720	80.86	86.65

Higher film surface UV exposure time and temperatures favored the polymerization of lacquer with APTES or AATMS to form a denser structure which provided the film with improved aging resistance. As a result, the loss rate of the films was relatively low after 240 h of UV exposure. In addition, the good chemical and heat resistance properties of those Si–O–Si and Si–O–C groups resulted in hybrid lacquer films (A, B, C, and D) with superior thermostability as compared to lacquer (E). The phenol hydroxyl (−OH) groups in lacquer favored ionization (oxidation) of H atoms and oxygen anion, with the film surface becoming rough and the gloss significantly changing as a result. Besides, upon increasing the illumination time, the Si–O–CH3 groups in the structure of the A, B, C, and D films readily ionized −OCH3 radicals, destroying the molecular branch structure and gradually decreasing antiaging as a result. As shown in Figure , the A and B films showed better aging resistances as compared to C and D. These films were not densified because of the higher stereo-hindrance effect of AATMS as compared to that of APTES.[24,25] The A, B, C, D, and E films showed fracture surfaces before aging, with a, b, c, d, and e images showing diverse morphologies after artificial aging. As shown in Figure , before the aging treatment, the surface of the films was relatively smooth and with no cracks, whereas cracks of varying degrees were observed after the artificial aging treatment. The surface of the B and D films also revealed the existence of holes. In addition, the C, D, and E films showed deeper cracks than A and B, indicating that the former were more significantly affected by the UV exposure (a gloss rate of 80%). As a whole, the A film showed the best aging resistance (six times higher than that of the E film with a gloss loss rate of 30%) among the films tested herein.

Figure 4

SEM analysis of different films before and after antiaging analysis.

XPS Analysis of the Hybrid Lacquer Coatings

To sufficiently understand the chemical reaction taking place on the coating surfaces during the aging process, the surface of hybrid coatings was analyzed by X-ray photoelectron spectroscopy (XPS). The C 1s, O 1s, N 1s, and Si 2p spectra of coatings A, C, and E before and after the accelerated aging treatment are shown in Figure , and the results are summarized in Tables –10. The C 1s spectra consisted of various components, which were fitted into three Gaussian–Lorentzian peaks corresponding to C–C or C=C (284 eV), C–O–C (286 eV), and O–C=O (288 eV).[26−29] The N1 and O1 bands were centered at 400 and 532 eV, characteristic of C–N and Si–O–Si species, respectively.[30] The C 1s, O 1s, and N 1s peaks of coating A before the accelerated aging treatment were observed at 298, 543, and 413 eV, respectively (Figure ). The C, O, and N elemental contents of coating A before the accelerated aging treatment were 74.86, 19.60, and 0.42%, respectively (Table ). After the accelerated aging treatments, these values were 67.99, 23.26, and 2.43%, respectively (Table ). The oxygen-to-carbon (O/C) ratio increased from 0.262 to 0.342 after the aging treatment, indicating that the UV radiation can promote molecular chain scission and degrade the polymer. This loss of carbon may be ascribed to the migration and volatility of the oxidation products obtained via degradation of the carbonaceous compounds.

Figure 5

XPS analysis of coatings A, C, and E before and after accelerated aging.

Table 5

Coating A before Accelerated Aging

element	start BE	peak BE	end BE	height Counts	area (N)	at. (%)	O/C
C 1s(1)	298.3	284.6	279.3	38623.2	0.66	54.20
C 2s(2)	298.3	286.4	279.3	13549.5	0.23	19.15
C 3s(3)	298.3	288.8	279.3	3027.7	0.02	1.51	0.262
O 1s	543.3	532.9	528.3	25251.8	0.24	19.60
Si 2p	113.3	102.1	98.3	3422.0	0.06	5.12
N 1s	413.3	400.1	395.3	4881.6	0.01	0.42

Table 10

Coating E after Accelerated Aging

element	start BE	peak BE	end BE	height counts	area (N)	at. (%)	O/C
C 1s(1)	297.9	284.6	278.9	17494.4	0.33	23.34
C 2s(2)	297.9	286.2	278.9	7729.9	0.16	11.10
C 3s(3)	297.9	288.4	278.9	5550.1	0.09	6.42	0.974
O 1s	542.9	532.5	527.9	47778.7	0.56	39.79
Si 2p	112.9	102.6	97.9	8359.9	0.24	17.37
N 1s	412.9	400.2	394.9	4992.9	0.02	1.33
N 1s	412.9	402.1	394.9	4182.3	0.01	0.65

Table 6

Coating A after Accelerated Aging

element	start BE	peak BE	end BE	height counts	area (N)	at. (%)	O/C
C 1s(1)	298.0	284.7	279.0	28379.5	0.53	47.78
C 2s(2)	298.0	286.4	279.0	8260.3	0.15	12.98
C 3s(3)	298.0	288.2	279.0	5034.5	0.08	7.23	0.342
O 1s	543.0	532.2	528.3	24711.0	0.26	23.26
Si 2p	113.0	101.9	98.3	3550.6	0.07	6.32
N 1s	413.0	400.2	395.0	6051.0	0.03	2.43

As shown in Tables and 8, the C and O elemental contents of coating C before the accelerated aging treatment were 76.03 and 16.90%, respectively, whereas these values changed to 51.14 and 30.96% after the aging treatment, respectively. The O/C ratio of coating C increased from 0.233 to 0.605 after the aging treatment. Compared to that of coating C, the lower O/C ratio of coating A after aging revealed good radiation damage resistance.

Table 7

Coating C before Accelerated Aging

element	start BE	peak BE	end BE	height counts	area (N)	at. (%)	O/C
C 1s(1)	298.3	284.5	279.3	46713.4	0.83	59.08
C 2s(2)	298.3	286.1	279.3	11500.3	0.19	13.38
C 3s(3)	298.3	287.2	279.3	3571.1	0.05	3.57	0.233
O 1s	543.3	532.7	528.3	25493.7	0.24	16.90
Si 2p	113.3	101.9	98.3	3622.0	0.07	5.24
N 1s(1)	413.3	400.1	395.3	6275.7	0.02	1.25
N 1s(2)	413.3	402.3	395.3	5560.0	0.01	0.59

Table 8

Coating C after Accelerated Aging

element	start BE	peak BE	end BE	height counts	area (N)	at. (%)	O/C
C 1s(1)	298	284.6	279	23516.0	0.45	37.39
C 2s(2)	298	286.2	279	6922.5	0.11	9.19
C 3s(3)	298	288.2	279	4403.7	0.05	4.56	0.605
O 1s	543	532.5	528	33615.2	0.37	30.96
Si 2p	113	102.6	98	6468.0	0.18	15.35
N 1s	413	400.4	395	5393.1	0.03	2.55

Tables and 10 summarize the XPS spectra and elemental content data of coating E before and after accelerated aging treatment. The C, O, and N elemental contents of coating E before the accelerated aging treatment were 76.54, 15.17, and 3.89%, respectively, whereas these values changed to 40.86, 39.79, and 1.98% after aging exposure, respectively. The O/C ratio of coating C before the aging treatment was 0.198, and this value increased to 0.974 after the aging process. The O/C reduction percentages of coatings A, C, and E after the accelerated aging treatment were 0.342, 0.605, and 0.974%, respectively. Therefore, coating A showed the best antiaging properties among the three coatings, whereas coating E showed the poorest properties.

Table 9

Coating E before Accelerated Aging

element	start BE	peak BE	end BE	height counts	area (N)	at. (%)	O/C
C 1s(1)	298.4	284.6	279.4	43038.1	0.71	52.87
C 2s(2)	298.4	286.0	279.4	14107.9	0.30	22.16
C 3s(3)	298.4	288.2	279.4	3503.6	0.02	1.51	0.198
O 1s	543.4	532.5	528.4	22441.0	0.20	15.17
Si 2p	113.4	102.2	98.4	3092.0	0.06	4.40
N 1s	413.4	400.2	395.4	7862.6	0.05	3.89

Corrosive Resistance Analysis of the Hybrid Lacquer Coatings

The corrosion resistance ability of bare steel coated with A, B, C, D, and E films was investigated by studying the potentiodynamic polarization curves under a 3.5 wt % NaCl aqueous solution, and the results are shown in Figure . The coatings showed a marked shift in both the cathodic and anodic branches of the polarization curves toward lower current densities.[31] The corrosion-related parameters were calculated on the basis of the Tafel potential curves, and the results are shown in Table . The polarization resistance (Rp) values were calculated from Tafel plots, in eq .[32−34] In addition, the corrosion rate (CR) and protection efficiency (PE) were calculated following eqs and 4.where ba and bc are the anodic and cathodic slopes, respectively.[35]where M is the molecular weight, V is the valence, and D is the density.[36]where Icorr,bare and Icorr,coated stand for the corrosion current density for bare steel and coating, respectively.

Figure 6

Potentiodynamic polarization curves for bare metal and coated by different films.

Table 11

Corrosion Parameters Determined from Tafel Curves for Bare Metal and Filmsa

	electrochemical corrosion measurements				PE/%
sample	E0/V	I0/(A·cm–2)	Rp (Ω)	CR (mm/year)
bare metal (F)	–0.3013	8.310 × 10–6	1.456 × 105	8.726 × 10–2	0a	_a
L (E)	–0.4430	4.200 × 10–9	7.975 × 107	4.411 × 10–5	99.94	0
A	–0.5852	2.476 × 10–10	2.328 × 109	2.650 × 10–6	99.99	94.10
B	–0.5769	2.796 × 10–10	3.106 × 109	2.936 × 10–6	99.99	93.34
C	–0.2828	1.257 × 10–9	3.454 × 108	1.320 × 10–5	99.98	70.07
D	–0.2919	1.438 × 10–9	2.500 × 108	1.510 × 10–5	99.98	65.76

0—based on this sample; _—excluding this sample.

All hybrid films (A, B, C, and D) showed significantly reduced corrosion currents (Icorr) and increased potentials (Ecorr), indicative of a superior corrosion protection. Compared to bare metal or lacquer (E), all of the hybrid films (A, B, C, and D) showed enhanced Rp and lower CR, both indicative of superior corrosion protection. Higher PE is indicative of good and efficient coating protection.[37−39]

Therefore, by comparing these values, the hybrid lacquer coatings apparently demonstrated efficient corrosion resistant properties under harsh chemical conditions, preventing oxidation and corrosion. Nevertheless, the A film showed superior protective properties versus the B, C, and D films under similar conditions. Thus, the A film showed superior PE and the lowest Icorr (2.476 × 10–10 A/cm2) and the highest PE (99.99 and 94.10%) values among the studied coatings, suggesting that a dense film was formed. Generally, a dense protective coating prevents the corrosive medium from penetrating the film, avoiding electrochemical reactions at the coating/substrate interface. Therefore, the electrochemical results obtained revealed that the A film possessed superior corrosion resistance under a 3.5 wt % NaCl solution, indicating that the cross-linking density film possesses a good shielding effect that prevented the corrosion medium from reaching the metal surface.

The samples were studied by different coatings on the mild steel substrate surface. The visual microcosmic observation of the coatings is shown in Figure after a 1000 h exposure in a 5.0 wt % NaCl solution in the salt spray test. In addition, this test can be used as an indication of barrier properties in the hybrid matrix. Macroscopic results of anticorrosion coatings before and after the 2000 h salt spray test are shown in Figure and Table . They did not obviously change before and after the spray test, in that the coatings show no color change, bubbles, cracks, and rust, as well as no obscission in macroscopic observation. All coatings show no obvious damage on the whole after exposure, as shown in Figure , and the corrosion manifest as wide relatively shallow areas, with no deep pitting of localized corrosion.[40] The dense surface of samples A and B was microscopic, integrity better than C, D, and E films. The films were showing the corrosion products, and distribution of numerous pores and pits, bad overall, especially E film. That is the presence of pores and pits based electrolyte is easier to penetrate the corrosive medium of the coatings, accelerating corrosion. In contrast to the entire process, the coating microstructure E determines more rapid corrosion than A, B, C, and D coatings because it loses its electrochemical protection in a relatively weak somewhere, such as pores and pits. Sectional surface of sample E became larger than those of other films after 1000 h of corrosion immersion period. This is attributed to the inner corrosion and/or corrosion product wedging, a process that might be similar to the exfoliation corrosion of metal steel. After spraying for 2000 h, the coatings fell off with different degrees of damage. coatings D and E formed holes which seemed a bit deep. Particularly, the damaged area of coating E was relatively large during complete exposure to the salt spray environment. After the shielding effect of protective coatings during increasing exposure time, the metal steel matrix is completely exposed to the corrosive environment.[41] Thus, the apparent resistance to the corrosion of coating A exhibited excellent barrier to the corrosive atmosphere.

Figure 7

Macroscopic results of anticorrosion coatings before (a) and after (b) the 2000 h salt spray test.

Table 12

Macroscopic Results of Anticorrosion Coatings before and after a 2000 h Salt Spray Testa

	sample
degree	A	B	C	D	E
color	+	+	+	+	+
pulverization	+	+	+	+	+
crack	+	+	+	+	+
blistering	+	+	+	+	+
rust	+	+	+	+	+
obscission	+	+	+	+	+

“+”—no change in degree.

Figure 8

Results of anticorrosion coatings after the 1000 (a) and 2000 h (b) salt spray test.

Conclusions

In this work, renewable hybrid lacquer coatings were prepared. These coatings showed good aging resistance and anticorrosion properties. These coatings were prepared by an eco-friendly process, and no volatile organic compounds were employed, reducing the material costs. The hybrid lacquer coatings with a raw lacquer-to-APTES mass ratio of 1.8:1 showed significantly enhanced antiaging characteristics (six times than that of regular lacquer with a gloss loss rate of 30%). This coating showed the lowest Icorr value (2.476 × 10–10 A/cm2) and highest PE (99.99 and 94.10%), and no obvious damage was observed in the salt spray test when compared to all films studied, significantly protecting the metal from the corrosive medium and avoiding penetration issues through surface cracks or pores and pits after 2000 h. Therefore, the hybrid lacquer coatings represent an environmentally friendly coating technology with promising applications in the aging resistance and corrosion protection industries.

Experimental Section

Materials

Raw lacquer (L) was collected from the Hubei Province. APTES and AATMS were purchased from Aladdin Industrial Corporation. A cardanol phenolic resin diluent was purchased from the Biological Materials Technology Co. Ltd (Jiangsu, China). Crystalline NaCl (99.2%) was used to prepare a 3.5 wt % NaCl aqueous solution. All chemicals were used as received.

Preparation of the Hybridized Corrosion Coatings

Raw lacquer (25 g) was stirred in an open glass container at 25–40 °C for 2–4 h until the water concentration decreased to 3–5%.[21] The hybrid coatings were subsequently prepared by adding APTES or AATMS to the raw lacquer at different raw lacquer-to-APTES or raw lacquer-to-AATMS mass ratios (i.e., 4:1, 3:1, 2:1, 1.8:1, 3:2, and 1.3:1). A 20 wt % cardanol phenolic resin diluent was subsequently added to the above mixture. After stirring at room temperature for 0.5 h, the mixture was coated on the surface of dry tinplate and copper pieces previously treated with sandpaper and cleaned with ethanol. The specimens were dried at room temperature. A silane coupling agent-free lacquer coating (denoted as L) was prepared under similar conditions for comparison. In addition, the hybrid coatings of lacquer to APTES or AATMS in different mass ratios (1.8:1 and 1:3:1) formed the films, including L/APTES = 1.8:1, L/APTES = 1.3:1, L/AATMS = 1.8:1, and L/AATMS = 1.3:1, which are designated as coatings A, B, C, and D, respectively.

Characterization Experiments

Drying properties of hybrid lacquer coatings were evaluated by a drying time recorder (TaiYu Equipment Co. Ltd., Japan), according to the Chinese standards GB/T1728-1979 and GB/T6739-2006. The gel content method was performed using the following procedure. Gel content is expressed aswhere m0 is the initial weight of the film and mt is the final weight after extraction with xylene.

FT-IR spectroscopy measurements (KBr) were conducted on an American Nicolet 5700 FT-IR spectrometer. TG and derivative TG analyses were carried out on a DSC822e-type TG analyzer (Mettler-Toledo) at a rate of 10 °C/min within a temperature range of 30–600 °C under a nitrogen atmosphere. Following the Chinese standard GB/T 1865-1997, the aging resistance of all films was tested on an LXD-080 xenon lamp aging box (Shanghai Huitai Instrument Manufacturing Co. Ltd), with wavelengths ranging from 290 to 800 nm and with an irradiance of 0.51 W/m2. The spraying rainfall cycle lasted 24 min and was repeated in intervals of 120 min, with a distance between the sample and the lamp of 30 cm. Scanning electron microscopy (SEM) images were obtained on a field-emission scanning electron microscope (JSM-7500) under vacuum with an accelerating voltage of 20 kV. XPS analysis was performed on a Thermo VG ESCALAB250Xi spectrometer at a residual pressure of 10–9 mbar. An Al Kα monochromatized radiation (photon energy = 1486.6 eV) was employed as the X-ray source. The corrosion resistance of the coatings was investigated via electrochemical measurements on a CHI660E electrochemical workstation with a 3.5 wt % NaCl standard aqueous solution and with a platinum auxiliary electrode and saturated calomel as reference electrodes. Anticorrosion measurements were performed in a salt-spray testing chamber (FQY015 Cyclic Corrosion Tester) with a 5.0 wt % aqueous NaCl, according to the Chinese standard GB/T 1771-91.