Zhu Baoying1, Xu Fei1,2, Zhang Hanqing1, Liu Ming1, Hu Zhong1, Zhuang Zhenyu1,2,
Wang Yanyan1, Liu Hangong1, Zhu Ke2
(1.CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd., Changzhou, Jiangsu
213016, China; 2.CNOOC Changzhou EP Paint Co., Ltd., Changzhou, Jiangsu 213012, China)
Abstract : Nonionic curing agents contain nonionic polyether segments introduced therein via block process, which might result in high viscosity of the curing agent and consequently more solvents should be applied to reduce the viscosity. To solve this problem, a side chain modified waterborne nonionic epoxy curing agent is prepared through the incorporation of nonionic segment of hydrophilic polyethylene glycol into the curing agent. The process of isocyanate half adduct, the amount of functional monomer polyethylene glycol and blocking agent have been discussed. The prepared curing agent is applied in pigments and fillers, which were further mixed with epoxy emulsion to form a 2K waterborne epoxy coating. The coating is characterized in terms of its properties.
Epoxy resins have unique chemical structure and excellent mechanics Performance, more and more widely used in the field of adhesives and flooring, anticorrosive coatings, especially in the field of coatings has received widespread attention and application. With the emphasis on the concept of environmental protection, environmental regulations have become more stringent, waterborne epoxy coatings have emerged at the historic moment, and users have become increasingly strict with their solvent content limits, and have higher and higher requirements for coating film performance. Waterborne epoxy coating system consists of waterborne epoxy resin and waterborne epoxy curing agent. Its main film-forming substances, structural characteristics, solvent content of waterborne epoxy curing agent, coating film performance and total volatile organic compounds (TVOC). Plays a key role.
The existing water-based epoxy curing agents are generally divided into ionic and non-ionic, and a few curing agents are cationic-nonionic. Ionic curing agents are used to form an aqueous system by neutralizing tertiary amines with organic acids in the resin. Related data have proven that organic acids can inhibit the curing agent’s appearance, filler dispersion stability, coating corrosion resistance, and flash rust suppression. Has a negative effect; non-ionic curing agents are non-ionic polyether segments introduced into the curing agent to make it water-based. Generally, non-ionic segments are linked into the curing agent through blocks and extensions to make the curing agent aqueous, but it is easy to cause the viscosity of the curing agent to increase, requiring more solvent dilution to reduce the viscosity.
There are also some curing agents that use excessive polyamines and epoxy resins or epoxy resins containing polyether segments to extend the chain, and then reduce the solvent and excess amine by distillation under reduced pressure to ensure the low solvent content and regular structural characteristics of the curing agent. However, the preparation process is complicated, and it is difficult to separate and recycle the distilled solvent and excess amine, resulting in waste of raw materials and environmental pollution.
In this study, isophorone diisocyanate and polyethylene glycol monomethyl ether were first reacted to obtain a semi-addition product of isophorone diisocyanate. The semi-addition product containing nonionic segments was connected to an aqueous ring. The end group of the oxygen curing agent is used to prepare an aqueous epoxy curing agent with a side chain containing a nonionic segment. In this study, the preparation of the waterborne epoxy curing agent was discussed, and the prepared curing agent was mixed with the epoxy emulsion to prepare a two-component waterborne epoxy coating, and its performance was tested.
1.1 Experimental materials
Isophorone diisocyanate (IPDI): industrial grade, Covestro; polyethylene glycol monomethyl ether (MPEG-600): industrial grade, Sundar Chemical (Nantong) Co., Ltd., Laurel Dibutyltin acid (DBTDL): analytical grade, Sinopharm; bisphenol A epoxy resin E-44: industrial grade, Hunan Yueyang Petrochemical General Plant; tetraethylenepentamine: chemical grade, Jiangsu Yongfeng Chemical Reagent Factory; Ding Glycidyl ether: Industrial grade, Anhui Hengyuan New Materials Co., Ltd .; Epoxy emulsion (Araldite PZ 3961-1): Industrial grade, Huntsman; Dispersant (AFCONA-4560): Industrial grade, Evcona; Defoamer (BYK-025): industrial grade, BYK; propylene glycol butyl ether (PnB): industrial grade, Dow, USA; fumed silica Aerosil R812s: industrial grade, Evonik Degussa; iron red Bayferrox 4140 : Industrial grade, LANXESS Chemical; precipitated barium sulfate: industrial grade, Nanfeng Group; mica powder: industrial grade, Anhui GREE; molybdenum zinc phosphate ZMP: industrial grade, Kaibai pigment.
1.2 Preparation of
Waterborne Epoxy Curing Agent Waterborne epoxy curing agent is synthesized in 3 steps. The specific operation is as follows.
(1) Preparation of isocyanate semi-closed material A.
Add isophorone diisocyanate, polyethylene glycol monomethyl ether and DBTDL in a reaction ratio in a reaction vessel, and keep different reaction temperatures (30 ℃ and 60 ℃), every 1 h Determine the content of -NCO until the content of -NCO is about 5.1% and it will not decrease. That is, isocyanate semi-blocking A (MPEG-IPDI semi-blocking) is obtained. The above operations are all performed under stirring;
(2) Preparation of intermediate products B
Add bisphenol A epoxy resin E-44 to the reaction vessel, raise the temperature to (60 ± 2) ° C, and heat the isocyanate semi-blocking A obtained in step (1) at (80 ± 2) ° C for 4 h. Increase the temperature to (100 ± 2) ℃ and keep it for 2h. Measure the -NCO content. When the -NCO content is ％ 0.3%, the reaction is terminated to produce intermediate product B. The above operations are all performed with stirring;
(3) Preparation of non-ionic water-based epoxy curing agent. Add tetraethylene to the reaction container. Pentaamine, heated to (50 ± 2) ° C. At this temperature, butyl glycidyl ether was added dropwise, and the addition was completed in 2 hours. The temperature was raised to (70 ± 2) ℃, and the temperature was maintained for 4 h. Thereafter, the temperature was lowered to (50 ± 2) ° C, and the intermediate product B prepared in step (2) was added dropwise within 2 to 3 hours, and the dropping temperature was maintained at (50 ± 2) ° C until the intermediate product B was completely added. Incubate at (50 ± 2) ℃ for 2 h. After that, the temperature was raised to (70 ± 2) ° C for 2 h. After heating up to (80 ± 2) ° C for 2 hours, deionized water was added to prepare a non-ionic water-based epoxy curing agent. The above operations were all carried out with stirring.
This curing agent had a solids content of 60.3% and an amine hydrogen equivalent weight = 228 ± 10 (as supplied).
1.3 Preparation and construction of
two-component water-based epoxy coatings The formula of two-component water-based epoxy coatings is shown in Table 1. Put the raw materials 1 ~ 10 into the drawing cylinder in turn, disperse them at high speed (300 ~ 500 r / min) for about 5 minutes, grind through the machine, and control the fineness to ≤40 μm. The solid content is adjusted, and the material is filtered to form a water-based epoxy curing agent iron red slurry, which is used as component A of the two-component water-based epoxy coating. Mix component A and component B evenly according to the mixing ratio, spray on the workpiece, and dry it at 60 ~ 70 ℃ for 30 minutes to solidify into a film.
1.4 Analysis and testing
(1) Isocyanate content test: Di-n-amine-hydrochloric acid titration method is used for testing. In the measurement step, weigh about 3 g of the sample into a clean conical flask, add anhydrous toluene to dissolve the sample, add di-n-butylamine-toluene solution with a pipette, shake to make the liquid in the bottle mix well, and leave it at room temperature for 20 ~ 30 minutes, add isopropanol, add a few drops of bromocresol green as an indicator, and titrate with HCl standard solution. When the solution changes from blue to yellow as the end point, do a blank experiment.
(2) Coatings and coating properties: Tested in accordance with corresponding national standards.
2 Results and discussion
2.1 Determination of the process conditions for the preparation of the semi-blocks
Firstly, the isocyanate semi-blocks containing non-ionic segments were prepared, and the non-ionic segments were inserted into the side chains of the aqueous epoxy curing agent through the semi-blocks. Using isophorone diisocyanate and polyethylene glycol monomethyl ether to prepare the isocyanate semi-blocking, the reaction conditions for this reaction need to be determined.
Since the functionality of IPDI is 2, this reaction only expects one of the isocyanates to react. IPDI is an alicyclic diisocyanate containing two NCO groups with different reactivity. Because the primary NCO group in the IPDI molecule is sterically hindered by the α-substituted methyl group and the cyclohexane ring, the secondary NCO group attached to the cyclohexane ring is more active than the primary NCO. Studies have shown that in the case of dibutyltin dilaurate as a catalyst, the secondary NCO group is more than 10 times more reactive than the primary NCO. Therefore, the secondary NCO attached to the alicyclic ring will react preferentially . During the synthesis of isocyanate-containing semi-blockers, the NCO content in the system will gradually decrease with the progress of the reaction. Therefore, the progress of the reaction can be monitored by monitoring the NCO content in the system. The results are shown in Figure 1.
It can be seen from Figure 1 that during the synthesis of the isocyanate semi-blocker, the NCO content in the reactant continuously decreases. In the catalyst-containing system, the NCO content decreased sharply 120 minutes before the reaction. This is because the reaction group (including NCO and OH groups) had a higher concentration and the reaction speed was faster at the beginning of the reaction. Later, due to the NCO concentration and As the OH concentration decreases, the reaction rate decreases and the NCO content decreases slowly. In the system with DBTDL (30 ℃), the NCO content reached the theoretical value after 240 minutes of reaction. In the system without catalyst (60 ℃), it took about 720 minutes for the NCO content to reach the theoretical value. Considering the reactivity and efficiency of the two NCOs of IPDI, the isocyanate semi-blocked reaction conditions were determined as follows: DBTDL was the catalyst, the reaction temperature was 30 ° C, and the reaction time was 4 h.
2.2 Polyethylene glycol monomethyl ether The
introduction of flexible segments in waterborne epoxy curing agents can significantly improve the flexibility of epoxy systems and improve the overall performance of curing agents. The polyethylene glycol segment has a lower Tg and is a non-ionic flexible segment, which is of great help to improve the flexibility of the epoxy resin. At the same time, the polyethylene glycol segment is hydrophilic, which can improve the hydrophilic property of the curing agent. During the preparation of non-ionic water-based epoxy curing agents on the market, the above functions are realized by selecting polyethylene glycols as the hydrophilic segments embedded in the long chain. However, the introduction of such hydrophilic substances in the curing agent molecular chain in a block manner will increase the relative molecular mass of the resin, resulting in a higher viscosity of the curing agent, and its content is usually higher (about 25% or more) in order to achieve water-based. ), Is not conducive to coating corrosion. In this study, polyethylene glycol monomethyl ether was introduced into the molecular chain of the aqueous epoxy curing agent as a non-ionic segment, and grafted on the side chain of the curing agent to make the viscosity of the curing agent relatively low, avoiding block The disadvantage of the higher viscosity of the curing agent caused by the introduction of non-ionic hydrophilic segments.
In this study, polyethylene glycol monomethyl ether (MPEG-600) with a number-average molecular mass of 600 was used to react with isophorone diisocyanate, and its semi-adducts were incorporated into the molecular chain of the curing agent. The effect of the loading amount on the performance of the curing agent, the experimental results are shown in Table 2.
Studies have shown that in a system using polyethylene glycol diglycidyl ether, the amount of polyethylene glycol diglycidyl ether reaches more than 25%, and the prepared curing agent has good water solubility. However, there are many hydrophilic polyether segments, which results in a decrease in the water resistance and solvent resistance of the coating film after curing, and the hardness of the coating film also decreases due to the increase of ether bonds. It can be obtained that, under the premise of ensuring good water content, the content of polyethylene glycol segments should be reduced as much as possible, which is conducive to improving the hardness, water resistance, and chemical resistance of the coating film after curing of the two-component waterborne epoxy system. . When the content of polyethylene glycol monomethyl ether used in this study is controlled above 15%, a water-based epoxy curing agent with good stability can be obtained, and the amount of non-ionic hydrophilic segments is significantly reduced. Considering the other properties of the water-based epoxy curing agent, such as water resistance, the amount of polyethylene glycol monomethyl ether is preferably 15%.
2.3 Use of capping agent
Epoxy polyamine curing agent contains two kinds of active hydrogen: primary amine hydrogen and secondary amine hydrogen, in which the reactivity of primary amine hydrogen is significantly higher than that of secondary amine hydrogen. In general, in order to improve the construction performance of water-based epoxy curing agents, the pot life needs to be extended. This must reduce the reactivity of the curing agent. Usually, a capping agent is used to block the primary amine hydrogen to reduce the activity of the polyamine curing agent. . The capping agent usually uses a compound containing a single epoxy functional group. Commonly used monoepoxides include glycidyl tert-carbonate, butyl glycidyl ether, phenyl glycidyl ether, and the like.
In this study, butyl glycidyl ether was used as a capping agent. After the capping, a flexible group can be introduced into the curing agent to prevent the film-forming material from being hard and brittle. In addition, when using butyl glycidyl ether as a capping agent, The steric factor in the terminating agent causes the remaining secondary amine groups in the curing agent to react with epoxy groups on the epoxy emulsion to become less active, extending the pot life and making the pot life of water-based epoxy coatings more than 4 h. . Change the capping ratio (calculated based on the molar ratio of capping agent to the remaining primary amine hydrogen), and examine the effect of the capping ratio on the performance of the aqueous epoxy curing agent. The results are shown in Table 3.
As can be seen from Table 3, when the capping ratio is low, more primary amine hydrogen is stored in the curing agent, which will increase the curing agent activity and shorten the activation period. The corresponding aqueous epoxy emulsion will increase the crosslinking density when reacting with the curing agent. , Causing the coating volume to shrink, eventually leading to coating hardness, chemical resistance and other properties are affected; when the amount of capping agent is higher, the molar amount of amine hydrogen in the curing agent will be reduced. In addition, the capping agent is a hydrophobic monomer It will reduce the water solubility of the curing agent. According to the data in Table 3, when the final capping ratio is selected to be 110%, the water solubility of the curing agent and the comprehensive properties of the cured coating film can achieve a good balance.
2.4 Performance of water-based epoxy coatings
Mix A component prepared with water-based epoxy curing agent and epoxy emulsion B component, add deionized water to adjust the viscosity to 25 ～ 30 s (coated-4 cups), which is double. Component water-based epoxy coating. The above coating was spray-coated on the polished cold-rolled steel sheet, and dried at 60 ° C. for 30 minutes, and the dry film thickness was controlled to 40-50 μm. The coating properties are shown in Table 4.
From the data in Table 4, it can be known that the coating is smooth and smooth, has excellent adhesion and flexibility, high hardness, excellent resistance to chemical media, resistance to neutral salt spray for more than 500 hours, and excellent corrosion resistance.
In order to reduce the performance defects of non-ionic segments of non-ionic aqueous epoxy curing agents that are brought to the curing agent products, this study uses a non-ionic segment of isocyanate The ethylene glycol nonionic segment was inserted into the aqueous epoxy curing agent to obtain a side chain nonionic modified aqueous epoxy curing agent. The following conclusions were obtained:
(1) Preparation of the semi-blocked isocyanate intermediate product containing nonionic segments The reaction conditions are: DBTDL is the catalyst, the reaction temperature is 30 ℃, and the reaction time is 4 hours;
(2) Considering the hydrophilicity and chemical resistance of the aqueous epoxy curing agent, the amount of polyethylene glycol monomethyl ether is selected at 15% It is suitable;
(3) butyl glycidyl ether is selected as the capping agent of the aqueous epoxy curing agent, and the capping ratio is 110%;
(4) Use the prepared new type of water-based epoxy curing agent to grind pigment and filler paste, and cure it with epoxy emulsion to obtain a two-component water-based epoxy coating. The coating has excellent comprehensive performance and has a wide application prospect in the field of metal anticorrosion. .