Preparation and Properties of Waterborne Acrylic-Modified Epoxy Phosphate Resin and Its Coating

Abstract

An acrylic acid-modified epoxy phosphate resin coating was synthesized by a four-step method marked “A-B-C-D”, and it was used as an efficient protective layer for steel structures. The coating exhibited good properties, mainly including water resistance (≥240 h), salt spray resistance (≥300 h), surface drying time (≤1 h), and adhesion (≥6.5 MPa).

1. Introduction

As environmental awareness rises and environmental protection regulations become more stringent, the development of traditional organic solvent-based coatings is increasingly constrained [1]. Consequently, waterborne coatings have emerged as a popular alternative due to their remarkable environmental friendliness, sparking interest in research and development [2,3,4]. Epoxy coatings show widespread utilization in industrial anticorrosion applications due to their impressive adhesion, corrosion resistance, physico-mechanical properties, etc., making them highly favored by the market [5,6]. However, the hydrophilic groups or surfactants remaining after the curing of waterborne epoxy coatings probably form polar channels, accelerating the corrosion rate of steel structures, and water absorption and seepage make the coating material deteriorate and cause corrosion, thereby losing the corrosion resistance [7,8]. Therefore, waterborne epoxy coatings have some shortcomings such as flash rust and poor adhesion, which limits their application for metal corrosion protection [8]. Moreover, due to their poor weathering resistance, epoxy resin coatings are prone to powdering and fading when exposed outdoors, necessitating urgent research into enhancing their weathering durability [9]. Improving the adhesion and weathering and flash rust resistance of epoxy coating has emerged as the key focus in the modification of these coatings. In addition, the hydrophilic groups (such as hydroxyl and carboxyl groups) can be introduced into the molecular chain of epoxy resin through blocking or grafting processes, such as etherification or esterification [10,11]. The drawback is that the use of these methods may result in the loss of some epoxy active groups, which has a certain negative impact on the performance of epoxy resins [12]. It has become urgent to find a good strategy to solve the problems mentioned above.

The polymer coating on the surface of steel structures after phosphating can effectively improve flash rust, adhesion, and weather resistance [13,14]. However, the toxic effect of the phosphorus reagents on the environment limits their application during coating processing [15]. In addition, the organic hydroxyphosphate monomers (HPs) can be used as fillers in coatings [16]. For example, hydroxy–epoxy phosphate (HEP) monomer, as a corrosion inhibitor, can improve the corrosion resistance of acrylic polyurethane coatings by adding only 0.5% by weight of HEP monomer [17]. A styrene–acrylic emulsion with hydroxyphosphate and hydroxypropyl acrylate are used as a flash rust functional monomer and crosslinked monomer, respectively [18]. These monomers react with the metal substrate surface active groups to form a dense phosphate film, which improves the adhesion and anti-rust properties of the acrylate coating.

In this work, a type of waterborne acrylic-modified epoxy phosphate resin was synthesized by an “A-B-C” three-step method, involving three pathways: esterification, polymerization, and neutralization. Using the obtained waterborne epoxy phosphate resin as the film-forming material, an efficient waterborne epoxy coating can be prepared by screening waterborne additives and flash rust inhibitors. This coating was used for steel structure protection and showed good corrosion resistance (salt spray resistance > 300 h, acid resistance > 96 h), high weather resistance (resistance to artificial weathering ≥ 800 h), and good mechanical properties (impact resistance ≥ 50 cm, bending test ≤ 1 mm, adhesive force ≥ 6.5 Mpa). The research results can provide guidance for the practical development of waterborne epoxy phosphate functional coatings modified by acrylic acid.

2. Experimental

2.1. Materials

Linoleic acid was purchased from Anhui Ruifende Oil Deep Processing Co., Ltd. (Xuancheng, China); crotonic acid was purchased from Suzhou Cheng’en Chemical Co., Ltd. (Kunshan, China); zinc oxide was purchased from Lanzhou Yellow River Institute of Zinc and Magnesium Nanomaterials (Lanzhou, China); dimethylbenzene was purchased from China Petroleum and Chemical Corporation (Lanzhou, China); ethylene glycol monobutyl ether, methyl methacrylate and butyl acrylate were purchased from Jiangsu Sanmu Chemical Co., Ltd. (Wuxi, China); waterborne drier (1101) was purchased from Borchers (Shanghai) Trading Co., Ltd. (Shanghai, China); phosphoric acid was purchased from Shan’xi Beiyuan Group Jinyuan Chemical Co., Ltd. (Taiyuan, China); E-20 epoxy resin and styrene were purchased from Cangzhou Lingang Xingchen Chemical Co., Ltd. (Cangzhou, China); acrylic acid was purchased from BASF; benzoyl peroxide (BPO) and tert-butyl hydroperoxide were purchased from Lanzhou Auxiliary Factory Co., Ltd. (Lanzhou, China); dimethylethanolamine (DMEA) was purchased from TICHEM (Shanghai, China); deionized water came from a water purification system (Medium-1600QE, Hitech Instruments Co., Ltd. (Xi’an, China); triethanolamine was purchased from Gansu Yincheng Petrochemical Products Co., Ltd. (Lanzhou, China); bentonite, dispersant, and thickener (299) were purchased from BYK Chemical Technology Consulting Co., Ltd. (Shanghai, China); defoamer (810) and substrate wetting agent were purchased from Shenzhen TEGO Chemical Co., Ltd. (Shenzhen, China); carbon black was purchased from ORION (Shenzhen, China); zinc phosphate was purchased from Dongying Pioneer Chemical Technology Co., Ltd. (Dongying, China); aluminum tripolyphosphate was purchased from Xinle Yiyang Anticorrosive Material Co., Ltd. (Xinle, China); barium sulfate was purchased from Hejin Hongji Powder Material Co., Ltd. (Hejin, China); mica powder was purchased from Guangdong Grui New Material Co., Ltd. (Dongguan, China); modified fumed nanosilica was purchased from Grace trading Co., Ltd. (Shanghai, China); anti-flash rust agent was purchased from Jiangsu Hemings New Material Technology Co., Ltd. (Changzhou, China); and drier was purchased from OMG Special Chemical Co., Ltd. (Suzhou, China).

An electric heating sleeve and constant temperature oven were purchased from Xi’an Yuhui Equipment Co., Ltd. (Xi’an, China); a three-port flask, condensing tube, thermometer, vacuum pump, and peristaltic pump were purchased from Zhengzhou Dufu Instrument Co., Ltd. (Gongyi, China); a sanding machine, electric mixer, high-speed dispersing machine, and electronic balance were purchased from Shanghai Modern Environmental Technology Co., Ltd. (Shanghai, China); a scraper fineness meter, film flexibility tester, and film impact tester were purchased from Biuged Laboratory Instruments Co., Ltd. (Guangzhou, China); a compounds salt spray test chamber and standard fluorescent UV aging test chamber were purchased from Aisry Instrument Technology Co., Ltd. (Dongguan, China); and a fourier transform infrared spectrometer was purchased from Mettler Toledo (Shanghai, China). A laser particle size analyzer was purchased from Zhuhai OMEC Instruments Co., Ltd. (Zhuhai, China).

2.2. Preparation of the Epoxy Ester Resin A

From Scheme 1, linoleic acid, crotonic acid, epoxy resin, zinc oxide, and xylene were added to a three-neck bottle and slowly heated to 180 ± 2 °C for 2 h. After that, the temperature was continually raised to 200 ± 2 °C for 60 min. The temperature was continually raised to 220 ± 2 °C, maintaining the acid value of 9.0–14.0 mg KOH/g. Later, xylene was removed by vacuum distillation. After the temperature of the reaction bottle dropped to room temperature, a certain amount of ethylene glycol monobutyl ether was added to obtain a target synthetic product, epoxy resin A. From

Table 1. Summary of detection indexes of synthetic epoxy resin.

Test Item	Indicator Standards	Detection Results
Character	1. transparent viscous liquid 2. no mechanical impurities	1. transparent viscous liquid 2. no mechanical impurities
Color (Fe-Co), ≤	12#	12#
Acid value/mg KOH/g	9.0–14.0	9.5
Solid/%	64.0 ± 2	64.5
Fineness/μm, ≤	20	20

, all detection results met the indicator standards. Among them, the solid of the prepared resin A reached 64.5%. The mass ratio of linoleic acid/E-20/crotonic acid was controlled to be 3:23:1.

2.3. Preparation of the Epoxy Phosphate Resin B

Ethylene glycol monobutyl ether, E-20 epoxy resin, and phosphoric acid were added to the reactor, as displayed in Scheme 1. The reaction temperature was raised to 120 ± 2 °C and maintained for 7 h. After the reaction temperature dropped to room temperature, the acid value (10.6 mg KOH/g) and solid content (60.3%) were detected, and finally filtered and stored. The detection results of the epoxophosphates are shown in

Table 2. Summary of detection indexes of synthetic epoxophosphates.

Test Item	Indicator Standards	Detection Results
Character	1. transparent viscous liquid 2. no mechanical impurities	1. transparent viscous liquid 2. no mechanical impurities
Color (Fe-Co), ≤	9#	9#
Acid value/mg KOH/g	9.0–12.0	10.6
Solid/%	60.0 ± 2	60.3
Fineness/μm, ≤	20	20

. All detection results met the indicator standards. The mass ratio of phosphoric acid/E-20 was controlled to be 1:20.5.

2.4. Preparation of the Acrylic-Modified Epoxy Phosphate Resin C

As shown in Scheme 1, a certain proportion of epoxy resin and epoxy phosphate were added to the three-neck bottle, and the reaction temperature was raised to 115 ± 2 °C and maintained for 20 min. After that, a certain amount of mixture (including styrene, methyl methacrylate, butyl acrylate, acrylic acid, and BPO) was dropped into the reaction bottle at a constant stirring rate for about 3.5 to 4 h. Then, the reaction temperature naturally cooled down to 90 °C. The synthetic products were collected by vacuum filtration for about 30 min. When the solid fraction reached 70 ± 2%, the reaction temperature dropped to 50~70 °C. Then, dimethylethanolamine (DMEA) was added to the reaction system and stirred to make the pH of the reaction system neutral. Finally, waterborne acrylic-modified epoxy phosphate resin was prepared by dispersing deionized water into the reaction system three times. The detection results are listed in

Table 3. Test results of acrylic-modified epoxy phosphate resin.

Test Item	Indicator Standards	Detection Results
Character	1. transparent viscous liquid 2. permissive light fluorescence	1. transparent viscous liquid 2. permissive light fluorescence
Color (Fe-Co), ≤	12#	12#
Solid/%	50 ± 2	50.1
Fineness/μm, ≤	20	20
pH value	8.0–9.0	8.5

. All detection results met the indicator standards. The mass ratio of styrene/methyl methacrylate/butyl acrylate/acrylic acid was controlled to be 1.0:1.0:1.3:0.174~1.414.

2.5. Preparation of Waterborne Epoxy Coating D

Dimethylethanolamine (DMEA) and triethanolamine (TEA) were added to the waterborne acrylic-modified epoxy phosphate resin, and the pH value of the reaction system was adjusted to 8.0–9.0. Then, dispersant, defoamer (50 wt.%), and distilled water were added to the reaction system. In the state of agitation, the modified fumed nanosilica, carbon black, zinc phosphate, and other pigments were added to the reaction system in turn, and the mixture was ground and dispersed by adding glass beads (grinding fineness ≤ 30 μm). Finally, drying agent, wetting agent, anti-flash rust agent, thickening agent, deionized water, and the remaining defoamer were added to the reaction system, stirred evenly, and filtered to obtain waterborne epoxy anticorrosive coating. The specific formula of the waterborne epoxy anticorrosive coating is listed in

Table 4. Formulation of waterborne epoxy coating.

Ingredients	Percentage/wt.%
acrylic-modified epoxy phosphate resin	45.0–55.0
dimethylethanolamine	1.0–1.3
triethanolamine	0.5–0.8
distilled water	21.7–25.7
dispersant	3.0–3.5
defoamer	0.2–0.3
carbon black	3.0–3.5
zinc phosphate	7.0–8.0
aluminum tripolyphosphate	3.5–4.0
barium sulfate	10.0–15.0
mica powder	3.5–4.0
waterborne bentonite	0.2–0.3
modified fumed nanosilica	1.5–2.0
triethanolamine	0.3–0.5
substrate wetting agent	0.1–0.2
waterborne anti-flash rust agent	0.3–0.4
waterborne drier	1.3–1.6
waterborne thickener	0.1–0.2
deionized water	2.6–3.6

.

2.6. Performance Test of Waterborne Epoxy Coating

Performance testing of waterborne acrylic-modified epoxy phosphate coating was carried out by referring to the “waterborne epoxy resin anticorrosive coating” Chinese standard (HG/T4759-2014), as shown in

Table 5. Indicator standards and detection results of waterborne epoxy acrylic-modified epoxy phosphate coating.

Test Item	Indicator Standards	Detection Results
state in container	normal	normal
film appearance	normal	normal
no volatile matter content/%, ≥	40	48
drying time/h, ≤	-	-
surface drying time	4	1
actual drying	24	12
bending test/mm, ≤	3	1
impact resistance/cm, ≥	40	50
grid test/grade, ≤	1	1
storage stability, (50 ± 2°C, 14 d)	normal	normal
volatile organic compound content, (g/L), ≤	200	134
resistance to flash rust	normal	normal
water resistance (240 h)	no foaming, no peeling, no rust, no cracking	correspondency
salt spray resistance (300 h)	no foaming, no peeling, no rust, no cracking	correspondency

. All the test results were up to the indicator standard [5,9]. Moreover, it should be noted that the surface drying time can decrease to 1 h, which has a positive impact on the improvement of construction efficiency and coating performance. The mechanical properties (bending test ≤ 1 mm, impact resistance ≥ 50 cm) and corrosion resistance (water resistance ≥ 240 h, salt spray resistance ≥ 300 h) also significantly improved [6]. In addition, the performance test of the composite coating was carried out according to the “waterborne anticorrosive coatings for steel structures” Chinese standard (HG/T5176-2017, C4 corrosion environment grade (M) standard) [5,9,15], as shown in

Table 6. Indicator standards and detection results of composite coating.

Test Item	Indicator Standards	Detection Results
adhesive force (pulling method)/MPa, ≥	3	6.5
water resistance (120 h)	no foaming, no peeling, no rust, no cracking	correspondency
acid resistance (50 g/L H2SO4, 96 h)	no foaming, no peeling, no rust, no cracking	correspondency
alkali resistance (50 g/L NaOH, 96 h)	no foaming, no peeling, no rust, no cracking	correspondency
oil resistance (3# regular paint, 96 h)	no foaming, no peeling, no rust, no cracking	correspondency
continuous condensation experiment (240 h)	no foaming, no peeling, no rust, no cracking	correspondency
resistant to neutral salt spray (480 h)	no foaming, no peeling, no rust, no cracking	correspondency
resistance to artificial weathering (800 h)	discoloration ≤ 3, pulverization ≤ 1, cracking ≤ 1, foaming ≤ 1, rust = 1	correspondency
adhesive force (pulling method, after salt spray test)/Mpa, ≥	2	4.2

. The composite coating showed good properties, including mechanical properties (adhesion ≥ 6.5 Mpa, more than double the standard value (3 Mpa)), anticorrosive properties (water resistance > 120 h, acid/alkali resistance > 96 h, resistance to neutral salt spray > 480 h), and weather resistance (resistance to artificial weathering > 800 h), indicating stronger stability, a longer service life, and better integrity and aesthetics [2,9]. Furthermore, even after 480 h of the salt spray resistance test, the adhesion of the coating can still reach 4 Mpa. This strong adhesion ensured that the coating was firmly attached to the substrate, enhancing the durability and protection of the coating.

3. Results and Discussion

3.1. FT-IR Analysis of Epoxy Ester Resin and Acrylic-Modified Epoxy Phosphate Resin

From Figure 1, the peaks at 915 cm⁻¹, 1181 cm⁻¹, and 1245 cm⁻¹ in the curve of the epoxy ester resin and acrylic-modified epoxy phosphate resin were the bending vibration signal and stretching vibration signal of the C-O bond in the epoxy group [19,20]. It should be noted that the peak intensity at 915 cm⁻¹ in the curve of the epoxy ester resin obviously decreased after esterification with phosphoric acid, indicating that the C-O bond in the epoxy group was opened [17,21]. The peaks at 829 cm⁻¹, 1606 cm⁻¹, 1510 cm⁻¹, 1454 cm⁻¹, and 2920 cm⁻¹ belonged to the bending vibration of the para-substituted benzene ring, the skeleton vibration of the benzene ring, and the C-H bond stretching vibration of the benzene ring, respectively, which may be ascribed to styrene monomer or benzene residue in the solvent [21,22]. Compared with epoxy ester resin, the extra peaks at 698 cm⁻¹ and 967 cm⁻¹ in the curve of the acrylic-modified epoxy phosphate resin belonged to the stretching vibration signals of the P-OH bond and P-O bond in the phosphate ester group, respectively [22,23]. A new peak appeared at 1731 cm⁻¹ in the curve of the acrylic-modified epoxy phosphate resin, which belonged to the stretching vibration signal of the C=O bond in the carboxyl group [24]. In addition, after the introduction of acrylic monomer, the hydroxyl group (C-OH) near 3416 cm⁻¹ in the curve of the epoxy ester resin shifted towards a higher wave number [25]. These results mean that the acrylic monomer can be successfully grafted onto the epoxy phosphate resin.

3.2. Selection of Epoxy Resin Type

In the synthesis of fatty acid epoxy ester, the carboxyl group of fatty acid was usually used to open the loop of the epoxy group of the epoxy resin [18]. This modification method fully combined the essences of fatty acid and epoxy resin, which was conducive to the preparation of epoxy ester resin with good dryness and corrosion resistance. The effects of different types of epoxy resins on fatty acid/epoxy ester hybrids are shown in

Table 7. Effects of different kinds of epoxy resins on fatty acid/epoxy ester hybrids (rotary viscometer method, finger-touch method, and color standard method were used in test; test condition: 180–220 °C, acid value at 9.0–14.0 mg KOH/g).

Epoxy Resin Type	Reaction Time	Epoxy Ester Viscosity, Appearance, Dryness
E-12	2~3 h	large viscosity, light brown, poor drying
E-20	2~3 h	large viscosity, light brown, good drying
E-51	≤1 h	large viscosity, light brown, good drying

.

Generally, the better the performance of epoxy resin, the greater its relative molecular weight [24]. From Table 7, it can be seen that E-12 epoxy resin had a higher molecular weight and less fatty acid content in the molecular chain, and the dry property of the fatty acid/epoxy resin was poor. The fatty acid/epoxy ester synthesized from E-51 epoxy resin with relatively small molecular weight had a fast reaction rate (≤1 h), which made it difficult to control the reaction process and easy to form gel. In contrast, the fatty acid/epoxy ester synthesized from E-20 epoxy resin with the medium molecular weight had better drying properties and a more stable reaction process. Therefore, E-20 epoxy resin was used as the base resin in this work.

3.3. Selection of Fatty Acid Type

Eleostearic acid, dehydrated ricinoleic acid, and linoleic acid are common dry fatty acids used in resin synthesis. Compared to ricinoleic acid and linoleic acid, the price and reactivity of eleostearic acid were higher due to its special chemical structure (three unsaturated double bonds). Dehydrated ricinoleic acid has a darker color and is mainly used in dark coatings. To avoid the issues with the price, activity, and color mentioned above, linoleic acid was selected as the fatty acid for this work.

Usually, the introduction of double-bond functional groups can be achieved by adding maleic anhydride. However, this method mainly introduces conjugated double bonds. When the use of maleic anhydride is small, it cannot improve the double bond. Moreover, when the amount of maleic anhydride added is high, it can cause uncontrollable phenomena in the later stages of the reaction, and it is easy for gel to appear. The unsaturated fatty acid selected in this work was linoleic acid, which is a monadic acid with no conjugated double bonds in its molecular structure, and it is easy to terminate during the reaction without producing an implosion phenomenon. The linoleic acid molecule contains double bonds and a carboxyl group, which have strong reactivity. On these grounds, more double bonds can be introduced into the epoxy ester system, and the reaction rate and conversion rate of late grafting can also be improved [25]. At the same time, the overall viscosity of epoxy resin and acrylic-modified epoxy phosphate resin can be reduced to meet the technical requirements of high solid content and low viscosity of acrylic-modified epoxy phosphate resin, improving the uniformity and integrity of the coating.

3.4. Influence of Ratio and Dosage of Base Resin

There were many active groups in the molecular structure of the epoxy phosphate ester. In the modification process, when the amount of epoxy phosphate ester was too great, the viscosity of the modified resin increased too fast, affecting the water dispersion and storage stability of the modified resin. Secondly, too much epoxy phosphate also reduced the dryness of the modified resin. Under the premise of improving the anticorrosion performance of the modified resin without affecting its drying property and storage stability, this work determined the grafting copolymerization of m(epoxy ester resin)/m(epoxy phosphate ester) = 9:1 with the acrylic monomer reaction. Figure 2 and

Table 8. Effects of different ratios of epoxy ester resin, epoxylated phosphate ester, and acrylic acid monomers on the properties of the modified resin (format tube, 25 °C, rotary viscometer as a test method for sample viscosity; reaction condition: 120 °C, acid value at 10.6 mg KOH/g).

M(Epoxy Ester Resin/Epoxy Phosphate Ester = 9:1)/M(Acrylic Monomer)	Water Dispersibility	Storage Stability	Viscosity (Mean Value, Format Tube, 25 °C)/mPa·s
5:5	normal	no separation transparency	40.2
6:4	good	no separation transparency	25.9
7:3	good	no separation translucency	10.0
8:2	normal	no separation translucency	6.6

show the effects of different ratios of epoxy ester resin, epoxy phosphate ester, and acrylic monomer on the properties of the modified resin.

As can be seen from Table 8, when m(epoxy ester resin)/m(epoxy phosphate ester) = 9:1, whether the mass ratio of resin to monomer was too much (8:2) or too little (5:5), the water dispersion and storage stability of the product system became worse. In addition, when the amount of acrylic monomer was too small, the viscosity value of the modified resin was very low (6.6 mPa·s), the modification effect was not obvious, and the film performance could not reach its best level. Taking all factors (viscosity, water dispersibility, storage stability) into consideration, when the m(epoxy ester resin)/m(epoxy phosphate ester) = 9:1, the optimal ratio between the mixed resin and acrylic monomer was controlled at 6:4.

3.5. Effect of the Amount of Acrylic Monomer on the Properties of Resin C

The effects of different proportions of acrylic monomers on the properties of the resin were studied with the total dosage of styrene, methyl methacrylate, butyl acrylate, and acrylic acid unchanged. The results are shown in Figure 3 and

Table 9. Effect of the amount of acrylic monomers on the resin properties (format tube, 25 °C, rotary viscometer as a test method for sample viscosity; reaction condition: 120 °C, acid value at 10.6 mg KOH/g).

wacrylic monomer/%	Water Dispersibility	Viscosity (Mean Value, Format Tube, 25 °C)/mPa·s
5	no transparency no separation	4.7
10	translucency no separation	6.2
15	transparency no separation	9.3
20	transparency no separation	18.9
25	transparency slight separation	23.2
30	translucency slight separation	30.2

.

In general, the experimental results shown in Table 9 showed that the water dispersibility of the modified resin increased with the increase in the amount of acrylic monomer. However, when the amount of acrylic monomer exceeded 25%, the water dispersion of the modified resin began to deteriorate (slight separation). That is, when the self-polymerization rate exceeded the grafting rate of the modified resin, the transparency of the modified resin gradually decreased. When the amount of acrylic monomer accounted for 15% and 20% of the total monomer, the water dispersion of the modified resin was the best. However, when the amount of acrylic monomer was too small, the viscosity of the modified resin was also low (<5 mPa·s), the modification effect was not obvious, and the coating performance did not reach its best level. In total, when the amount of acrylic monomer accounted for 20% of the total monomer, the water dispersion and modification effect (appropriate viscosity) of the modified resin was the best.

3.6. Effect of BPO Dosage on the Properties of Resin C

The addition ratio of BPO to the total amount of the acrylic monomers was adjusted to study the effect on the properties of resin C, and the results are shown in Figure 4 and

Table 10. Effect of BPO dosage on resin C properties (format tube, 25 °C, rotary viscometer as a test method for sample viscosity; reaction temperature: 115 ± 2 °C).

WBPO/%	Water Dispersibility	Stability	Viscosity (Mean Value, Format Tube, 25 °C)/mPa·s
3	normal	no separation translucency	7.1
5	normal	no separation transparency	10.9
7	good	no separation transparency	18.7
9	good	no separation translucency	30.5

.

It can be seen from Figure 4 and Table 10 that with the increase in the BPO addition, the water dispersion of the modified resin gradually became better. However, when the amount of BPO was more than 9%, although the viscosity became higher, the water dispersion of the modified resin became worse, and the reaction was difficult to control. When the amount of BPO was small, the modified resin had poor water dispersion, low viscosity, and an unobvious triggering effect. In short, when the amount of BPO accounted for 7% of the total amount of acrylic monomers, the water dispersion and modification effect of the modified resin was the best.

3.7. The Effect of Reaction Temperature of Acrylic Acid Monomer on Resin Properties

The influence of the drop in the adding temperature of the acrylic monomer on the properties of the modified resin is shown in Figure 5 and

Table 11. Effect of drop in addition temperature of acrylic monomer on properties of modified resin (format tube, 25 °C, rotary viscometer as a test method for sample viscosity; reaction time: 3.5–4 h).

Reaction Temperature/°C	Water Dispersibility	Stability	Viscosity (Mean Value, Format Tube, 25 °C)/mPa·s
75	normal	no separation translucency	7.7
95	good	no separation transparency	10.1
115	good	no separation transparency	18.1
135	normal	no separation translucency	32.3

. When the reaction temperature in the system was too low, the water dispersion of the modified resin was poor, and the viscosity value was also low (7.7 mPa·s). This may be due to the fact that the activity of BPO cannot be maximized at low temperatures, resulting in a lower degree of polymerization of acrylic monomers [26]. When the reaction temperature was too high (135 °C), the decomposition rate of BPO was accelerated, and the system became unstable, resulting in excessive viscosity of the modified resin (32.3 mPa·s). Therefore, the modified resin had the best water dispersion effect at 115 °C, and the modification effect was the best.

3.8. Effect of Reaction Time of Acrylic Acid Monomer on Resin Properties

The effect of the reaction time of the acrylic monomer on the properties of modified resin is shown in Figure 6 and

Table 12. Influence of the drip time of acrylic monomers on the properties of modified resins (format tube, 25 °C, rotary viscometer as a test method for sample viscosity; reaction temperature: 115 °C).

Reaction Time/h	Water Dispersibility	Stability	Viscosity (Mean Value, Format Tube, 25 °C)/mPa·s
3.0	normal	no separation translucency	7.6
3.5	good	no separation translucency	10.9
4.0	good	no separation transparency	18.8
4.5	good	no separation translucency	27.3

. The experimental results showed that the reaction time of the acrylic monomer was short (<3.5 h), the epoxy resin, epoxy phosphate, and acrylic monomer could not be fully grafted, and the water dispersion and viscosity of the modified monomer were low (7.6 mPa·s). However, when the crosslinking density of the epoxy resin, epoxy phosphate ester, and acrylic monomer increased (>4 h), the viscosity of the modified resin also increased with low stability, and the reaction was not easy to control. In summary, the optimal water dispersion and modification effect of the modified resin were controlled at 4 h.

3.9. Effect of the Neutralizing Agent on the Modified Resin

When preparing waterborne resin by the salting method, researchers generally use triethylamine (TEA) or N,N-dimethylethanolamine (DMEA) to neutralize the carboxyl group to improve the water dispersity of the modified resin [27]. However, compared with DMEA, the carboxyl group free in the system was easily produced in the neutralization process of TEA, which affected the water dispersion and storage stability of the modified resin. When DMEA was used as a neutralizer, the hydroxyl group in the molecule not only increased the water dispersion and storage stability of the modified resin, but also reduced the number of hydrophilic functional monomers in the acrylic acid. In short, DMEA was selected as the neutralizing agent for aqueous acrylic-modified epoxy phosphate resin in this work. In addition, the effect of DMEA neutralization on the modified resin is shown in Figure 7 and

Table 13. Influence of N,N-dimethylethanolamine (DMEA) neutralization degree on modified resin (laser particle size analyzer as test method; reaction temperature: 50–70 °C).

Neutralization/%	Mean Particle Size/nm	Dispersion State	Stability/d
60	120.5	no transparency, having particulate matter	/
70	107.7	no transparency, having particulate matter	/
80	101.2	no transparency, having particulate matter	≤18
90	56.1	transparent, no particulate matter	≤24
100	34.3	transparent, no particulate matter	≥36
110	34.6	transparent, no particulate matter	≥36
120	33.5	transparent, no particulate matter, strong smell, yellowish appearance	≤30

.

From Table 13, it can be concluded that with the increase in the neutralization amount, the hydrophilicity of the modified resin began to become better, the appearance became gradually transparent, and the particle size became smaller and smaller (from 120.5 nm to 33.5 nm). This was due to the existence of more residual carboxyl groups in the reaction, resulting in the agglomeration of a large number of unsalted particles, poor water dispersion, and poor storage stability. When the neutralization was too high, although the particle size of the modified resin was better, too much DMEA made the coating yellow and brittle, and it had a strong smell, which would be difficult for customers to accept. In short, when the neutral degree was 100%−110%, the water dispersion and modification effect of the modified resin was the best. In addition, when the resin reaction reached the destination, due to the large viscosity of the modified resin itself, if the reaction temperature was directly lowered to room temperature or below for neutralization, the DMEA could crosslink with a small amount of residual epoxy groups in the system, resulting in gelation. Therefore, a certain reaction temperature should be maintained during neutralization.

The effect of neutralization temperature on the dispersion of resin C is shown in

Table 14. Influence of neutralization temperature on dispersibility of modified resin.

Neutralization temperature/°C	35–45	50–70	70–90
Particle size distribution index	1.036	0.932	0.987
Dispersibility of resin	poor dispersibility	good dispersibility	general dispersion

. The excessive reaction temperature caused the part of DMEA to volatilize, and, thus, the dispersion became worse. A reaction temperature that was too low made the modified resin difficult to disperse. The result showed that the water dispersion and modification effect of the modified resin can be optimized when the neutral temperature is 50–70 °C.

4. Conclusions

In conclusion, through the implementation of an innovative “A-B-C” three-phase strategy, an efficient waterborne epoxy resin was prepared via the dual modification process utilizing acrylic acid and phosphoric acid to solve the problems of low adhesion, poor weather resistance, and limited corrosion resistance currently faced by waterborne epoxy coatings on the market. Upon optimizing various parameters, including reaction time, temperature, acrylic monomer dosage, resin type, resin feed ratio, fatty acid type, BPO dosage, and neutralizer dosage, the prepared waterborne acrylic-modified epoxy phosphate resin showed high water dispersion and stability for a period of at least 36 days, and a small particle size between 100 and 110 nm, alongside a desirable viscosity range of 18–25 mPa·s. Moreover, the coatings formulated by using the obtained waterborne acrylic acid-modified epoxy phosphate resin as a film former showed good mechanical properties (adhesive force ≥ 6.5 Mpa, impact resistance ≥ 50 mm), high anticorrosion properties (water resistance ≥ 120 h, salt spray resistance > 300 h, acid and alkali resistance > 96 h), and outstanding weather resistance (resistance to artificial weathering ≥ 800 h). The results of this work can be used for the synthesis of high-quality water-armored multifunctional epoxy resins. In this case, the characteristics of epoxy coatings will be significantly improved.