In recent years, with the rapid development of my country's economic construction, the demand for epoxy resins and curing agents for heavy-duty anticorrosive powder coatings for oil and gas buried pipelines is increasing, and the quality requirements are becoming higher and higher. The requirements in this regard are mainly reflected in the requirements: good corrosion resistance powder coatings with good workability (high temperature rapid curing), wear resistance, corrosion resistance, epoxy phenolic resin resistance to cold and hot shocks, long-term stability, etc. With the launch of my country's "West-East Gas Pipeline" project, the demand for high-performance heavy-duty anticorrosive powder coatings will increase rapidly. In order to break the monopoly of foreign epoxy powder coatings in this field, we have carried out research on epoxy resins and supporting curing agents for heavy-duty anticorrosive powder coatings, and developed special epoxy resins and curing agents with properties close to those of similar foreign products.
According to the infrared and liquid chromatogram analysis of foreign epoxy resin samples, the basic component is determined as phenol epoxy resin. The molecular structure of the resin has both bisphenol A phenolic components and phenolic phenolic components, and is a composite phenolic epoxy resin. This resin has the characteristics of strong corrosion resistance, high hardness and good wear resistance. The synthesis method is divided into two steps: the step is the synthesis of novolak phenolic resin, the second step is the synthesis of phenolic epoxy resin.
Put phenol, bisphenol A, and formaldehyde into the reaction kettle, stir and heat to dissolve, add catalyst to reflux at 90℃ for 3 to 5 hours; neutralize with dilute alkali to terminate the reaction, suck out the upper layer water; wash repeatedly with hot water to neutral Then, after dehydration at elevated temperature, after vacuuming to remove a small amount of free phenol above 150℃/0.095MPa, filtering and discharging, the epoxy phenolic resin can obtain novolak phenolic resin. The synthetic flow chart of novolak resin is as follows:
Put the novolak resin synthesized in the step and the measured epichlorohydrin into the reaction kettle, dissolve at elevated temperature, add the catalyst to the etherification reaction at 80～90℃ for 5～6h; after the reaction is completed, lower the temperature, add part of the solvent to dissolve, and gradually add the hydroxide Aqueous sodium solution for ring-closing reaction; after the ring-closing reaction is completed, add remaining solvent, stir to dissolve, wash repeatedly with hot water to neutrality, separate the lower layer of water; remove the solvent and low-boiling substances at elevated temperature, and keep at 140～160℃ ～2h, cool down after no liquid comes out; filter, tablet crush and pack, then get phenolic epoxy resin. The synthesis flow chart of phenolic epoxy resin is as follows:
Previous anti-corrosion epoxy powder coatings mostly used dicyandiamide-imidazole curing system. Although the system has a faster reaction speed, the curing reaction exotherms violently, which easily generates internal stress and makes the coating film brittle. Analysis of foreign samples of curing agent, the main component of which is a compound of hydroxyl-terminated high molecular polymer and anionic catalytic curing agent. Through the synergistic effect of the two, the powder coating has good flexibility and fast reaction. speed. The polymer hydroxyl-terminated compound may be a hydroxyl-terminated polyester or a hydroxyl-terminated polyether resin, or may be a hydroxyl-terminated phenolic resin. The reaction activity of the latter is higher than the former, while the flexibility of the former is better than the latter. Based on the above analysis, combined with the results of the sample analysis, a composite curing agent system with a combination of hydroxyl-terminated polyether, hydroxyl-terminated phenolic resin and anionic catalytic curing agent was selected.
Put the metered glycol (neopentyl glycol, hexylene glycol, polypropylene glycol, etc.), dihydric phenol (hydroquinone, resorcinol, bisphenol A, etc.) and mono- and bi-epoxides into the reactor, Dissolve at elevated temperature and perform addition polymerization at 90-130°C; when the softening point is satisfactory, cool down, filter, and discharge to obtain hydroxyl-terminated polyether resin. The synthesis flow chart is as follows:
Factors affecting the synthesis of phenolic epoxy resins include the phenol/aldehyde molar ratio, reaction temperature and time, phenol composition in the synthesis of novolac resins, and epichlorohydrin, alkali, catalysts and phenolic resins in the synthesis of phenolic epoxy resins Molar ratio etc.
In the synthesis of novolac resin, as the ratio of bisphenol A and phenol increases, the softening point of the resin increases, and the hardness and temperature resistance of the coating film increase, but the softening point of the resin is too high, which will make the powder processing difficult . The molar ratio of bisphenol A/phenol is preferably from 1/2 to 1/4. As the molar ratio of aldehyde/phenol decreases, the amount of aldehyde decreases, the reaction rate of phenol is low, the softening point of the resin is low, and the softening point of the resin during epoxidation is also low, which is easy to agglomerate. However, the softening point of the resin is too high and it is not suitable for processing, and the solubility is reduced, and the epoxidation is difficult. The aldehyde/phenol molar ratio is preferably from 3/4 to 4/5. The condensation reaction temperature of phenolic resin is generally reflux temperature (85～95℃), and the time is 4～6h. If the reaction time is too short, the reaction is insufficient; if the reaction time is too long, it will have little effect on the improvement of the reaction yield.
In the epoxidation of phenolic resin, as the amount of epichlorohydrin increases, the epoxy value of the resin increases, the softening point decreases, and the brittleness also increases. Taking into account the comprehensive performance of the coating film and the processing performance of the powder, the epoxy value of the resin is controlled at 0.11～0.13eq/100g, and the corresponding amount of epichlorohydrin is 1/1.0～1/1.2 (mol/mol hydroxyl) The amount of alkali is generally controlled to be equimolar or slightly excessive with epichlorohydrin.
The factors that affect the performance of the curing agent are the ratio of the hydroxyl-terminated polymer compound to the anionic catalytic curing agent, followed by the molecular weight of the hydroxyl-terminated polymer compound and the molar ratio of phenolic to polyether. In this compound type curing agent, the curing reaction mechanism of the hydroxyl-terminated polymer compound and the anion-catalyzed curing agent is different: the hydroxyl-terminated polymer compound mainly undergoes addition polymerization with the hydroxyl group and the epoxy group to make the molecular chain It grows gradually; while the anionic catalytic curing agent acts on the epoxy group to ring open the epoxy group to form a new anion. This new anion then reacts with the epoxy group to make the molecular chain grow. The influence of the amount of anionic catalytic curing agent is shown in Table 1. Epoxy phenolic resin
According to resin: curing agent=100:25～26, plus other pigments, fillers, additives, etc. to make a standard model, the mechanical properties, chemical resistance and electrical properties of the coating film are tested. The results are shown in Table 3.
(2) The mixed phenolic epoxy resin prepared by the two-step method, the epoxy resin curing agent system composed of a high molecular terminal hydroxyl compound and an anionic catalytic curing agent, can meet the processing requirements of heavy anticorrosive powder coatings and the coating machinery, Electrical and chemical resistance performance requirements.
(3) The factors that affect the synthesis of epoxy resin are: novolak resin composition, raw material ratio, reaction temperature and time; the factors that affect the performance of the curing agent are: the composition and molecular weight of the polymer terminal hydroxyl compound, and the amount of anionic catalytic curing agent.