Development of Pure Epoxy Rebar Powder Coating

China Coatings Online News Information
(Summary) Using two-step biphenol A-type epoxy resin with phenolic curing agent between 845-900 as a film-forming subject, the effect of curing agent, curing promoter and filler dosage on the overall performance of the coating was studied, optimized the formula, and obtained a high-performance pure epoxy rebar powder coating product to meet the requirements of the current domestic construction industry with epoxy coating.
Reinforced concrete structure has been widely used in modern construction projects, however, in the coastal port terminal, cross-sea bridge, the need to sprinkle salt water anti-freeze airport and highway and other facilities environment, by the dry reinforced concrete structure itself characteristics, inevitably there are gaps or gaps, so that with strong corrosion capacity of seawater, acid rain, salt water and other slowly seep into the structure, further corrosion of steel bars, reduce the force of steel; More seriously, the iron oxide produced by corrosion will produce dozens of times the volume expansion, causing concrete cracking and protective layer peeling off, further shortening the service life of reinforced concrete buildings, and even causing serious catastrophic accidents.
coating protection of rebar in reinforced concrete structures has proven to be the best means of reinforced concrete protection today, and research in North America began as early as the 1950s. At present, the general use of epoxy powder coating rebar coating, epoxy coating rebar has been the United States, Japan, Europe and the Far East, the Middle East and other countries and regions in the harsh environment of the project must use the product. The corrosion problem of construction rebar has also attracted the attention of relevant departments and experts in China, and the industry standard of JG3042-1997 Epoxy Coating Rebar and GB/T18593-2001 (anti-corrosion coating of molten combined epoxy powder coating) have been issued.
domestic research on this aspect more, the product has also formed a series, different manufacturers produce products with their own characteristics, but most of the above two standards are for the performance requirements of the product design.
this study for the above-mentioned standards, combined with rebar coating and epoxy powder coating its own characteristics, design formulation and production process, to meet the requirements of the use of pure epoxy rebar powder coating product formulation.1, test part
1.1 raw materials
epoxy resin GECN704, GESR902, GESR903, GESR904, GESR11907, GESR909, industrial grade, Guangzhou Hongchang Electronic Materials Co., Ltd.; Phenol curing agent KD-410, industrial grade, Shanghai Guodu Chemical Co., Ltd.; Flat agent, sabbath fragrance, breathable agent, methamphetimizole (2MI), etc., are industrial grade, Ningbo Nanhai Chemical Co., Ltd.;
1.2 main equipment
type 30 twin screw extruder, all-round Chinese medicine shredder, ACM grinding powder system, small electrostectrectectrectrosive spraying equipment, impact tester, adhesion tester and other coatings and coating performance testing equipment.
1.3 powder coating and coating preparation
according to the design formula called to take the components of raw materials, after a full premix of the wand Chinese medicine shredder through the twin screw extruder in the set temperature range for melt extrusion, extrusion of the film by cooling, crushing, screening, electrostectrical spraying, thermoset into a film, and finally the coating film performance side test.2, results and discussion
2.1 epoxy resin selection
the main resin of traditional heavy anti-corrosion coatings mostly use epoxy resin, which is determined by the molecular chain structure of epoxy resin, epoxy resin, especially biphenol type A epoxy resin hydroxyl has high polarity, can form chemical bonds with the metal surface, enhance the adhesion of epoxy resin; Propane and benzene rings give epoxy resins heat resistance and mechanical rigidity. Hydroxyl reactions in epoxy-based and curing agents in epoxy resins can be cross-linked into mesh structures, while the reaction activity of epoxy curing agents with a wide selection and rest system is easy to adjust. Based on the above factors, the epoxy resin system is also used in the design and development of epoxy-coated rebar powder coating. The synthesis process of epoxy resin can be divided into one-step and two-step methods. Among them, the epoxy resin obtained by the two-step method has a narrow relative molecular mass distribution, low chlorine content and impurity content, so it is more excellent in corrosion resistance, chemical resistance performance, so this study mainly uses two-step epoxy resin as the main body of the study.
flexible performance is an extremely important performance parameter of rebar powder coating, so this study first compared the difference in flexibility of the coating film made from different epoxy-like resins, with KD410 as an epoxy curing agent, according to epoxy: hydroxyl 1:1 formulation ratio design formula; 20g of precipitated radon is added to every 200g resin formulation as a filler, and the remaining general additives such as leveling agents, sabbaths and desostageants are added in proportion to conventional powder coating formulations, with carbon black as the pigment test results listed in Table 1.
as can be seen from Table 1, the epoxy equivalent increases and the flexibility of the coating film increases. This is due to the increase in epoxy equivalent, the number of epoxy-based per unit of epoxy resin can participate in the reaction is relatively reduced, the curing reaction after the completion of the formation of the coating film crosslink density is also correspondingly reduced, toughness increased. Taking GECN704 and GESR903 as an example, the epoxy equivalent of 704 is about 1/4 of the general E-12 epoxy resin GESR903 in powder coating, so theoretically the cross-link density of the coating film formed after the complete reaction of 704 can reach 4 times that of the 903 system, which shows that the brittleness of the coating film is very strong and the softness is very poor; With the increase of epoxy equivalent after the coating cross-link density also decreased accordingly, toughness increased, but the formula epoxy resin epoxy equivalent should not be too high otherwise the degree of density of the coating is too low, corrosion resistance is insufficient, from Table 1 can also be seen that epoxy equivalent is too high or too low, the appearance of the coating film is not good, because epoxy equivalent is too low, epoxy resin viscosity and softening point is also low, but the reaction of epoxy resin is also low, but the reaction of epoxy resin is also low. The viscosity of the whole system changes sharply in a short period of time and does not fully level, resulting in poor appearance of the coating film, the surface appears a lot of hemp points and shrink holes; Based on the above analysis, this study selected GESR904H as the formulation resin of reinforced powder coating.
2.2 Curing System Selection
On the basis of this study, GESR904H was selected as the main resin, the curing agent was added according to the ratio of epoxy: hydroxyl: 1:1, and 20g of precipitated radon was added to every 200g resin formulation as a filler. The remaining leveling agents, sabbaths, desostage agents and other conventional additives were added in proportion to conventional powder coating formulations, with carbon black as pigment, comparing the effects of different types of curing agents on coating performance, the results of which can be seen in Table 2.The test results of
show that the acid anhydride system is low in activity, the resulting coating membrane is not good, and at the same time in the extrusion and baking process there are small molecules released, on the one hand, leading to the film to produce pinholes, on the other hand will have an impact on the human body and the environment, so it is not suitable as a formulation of rebar powder coating. The coating obtained by dicyanide, phenol curing agent and argon as curing components can meet the requirements, and in theory can be used as curing components of rebar powder coating formulation. However, reinforced powder coatings usually require coatings with high temperature rapid curing characteristics, which is conducive to improving production efficiency and saving coating costs, so for double cyanide curing agents, although micro-powdering can be achieved without the addition of curing promoter 180-200 degrees C completely curing, but usually its curing time is longer generally In more than 5min, therefore, if you want to meet the construction requirements, you need to add a large number of 2-methyl mimi (2MI) and other curing promoter to improve the system's reaction activity, and the common mimi curing promoter on the system's activity has a great impact, once dispersed evenly, will lead to product reaction activity in a large range of fluctuations, to the production inconvenience. The curing agent has a very high reaction activity, without the need to add curing promoter conditions can be achieved 150 degrees C fast curing, coating film yellowing degeneration and comprehensive performance is very ideal, the United States and China are not enough is the existence of a certain toxicity, limiting its wide application. While phenolic curing agents and epoxy resin compatible well, the reaction activity is relatively high, the addition of a small amount of 2MI and other curing promoters can greatly improve the reaction activity of the rest system, and even in the synthesis process to pre-add curing promoter to facilitate dispersion, the resulting coating film comprehensive performance is excellent, especially chemical resistance performance is excellent. Based on the above factors, the phenolic curing agent is intended to be used as the formula curing part of rebar powder coating.
2.3 Choice of curing agent and curing promoter dosage
The effect of different curing agent dosages (hydroxy/epoxy group substance ratio, same below) on coating performance is seen in Table 3.
Table 3 data show that when the curing agent is excessive, the coating film crosslink density after curing is too high, the flexibility performance is reduced, the curing agent dosage is too low, the coating film crosslink density after curing is not enough, the overall performance of the coating is reduced, and the suitable curing agent and epoxy resin ratio is between 0.9-1.1. From the comprehensive cost consideration, the curing agent dosage of 0.9 is more suitable. In addition, by the test data of Table 3, it is not difficult to find that the curing agent ratio in the range of 0.7-1.2, the effect of curing agent dosage on powder coating reaction activity (gel time) and horizontal flow can be almost ignored, under this formulation condition, the reaction activity of the coating film is too small to meet the index requirements of epoxy resin coating rebar, so a certain amount of 2MI must be added as a curing promoter to regulate the system's reactive.
effects of different promoter dosages on the reaction activity of powder coatings are shown in Figure 1.
As can be seen from Figure 1, after the addition of curing promoter, the gel time of the system began to shorten sharply, with the increase of the curing promoter dosage of the system gel time continued to decrease indicating increasing activity, but the overall flattening.
For rebar powder coating, its spraying method is generally: the surface finished rebar after 230-250 degrees C preheated 30-45min while hot spray, sprayed powder coating using the residual heat of the rebar itself to complete the final cross-curing, cured rebar by cold water immersion cooling can be sent for inspection.
the powder coating formulation of 7 different promoter dosages in Figure 1 is applied and cured according to the above coating method. The effects of different curing promoter dosages on coating performance were compared, and the results were shown in Table 4.
table 4 shows that with the increase of curing promoter dosage, the flow of powder coating gradually becomes smaller; At the same time, the flexible performance of the coating is increased and then reduced. This is because in the case of less curing promoter dosage, the system reaction activity is not enough, in the standard rebar powder coating conditions can not be fully cured, so the comprehensive performance of the coating can not achieve the best; With the increase of the amount of promoter, the activity of the system is increasing, the specified time basically reached full curing, when the amount of promoter exceeds 2.0g (per 200g epoxy resin, the same between 200g), the system has excessive curing (2MI itself can also participate in cross-linking reaction), but reduces the overall performance of the coating. Through the series of tests in Table 4, this study found that the amount of promoter between 0.5-1.0g is more suitable.
2.4 Filler Dosing Determination
In powder coating formulations, inorder fillers, while generally not involved in reactions, can play a critical role in reducing formulation costs. Filler content should not be too high, otherwise it is easy to reduce the overall performance of the coating, especially the flexibility of the coating for reinforced powder coatings, the flexibility of the coating is a critical factor, so this study needs to determine the best filler dosing in the formulation. The effect of different filler dosing on the coating performance of powder coatings can be found in Table 5.
As can be known from Table 5, with the increase of filler content, the flexible performance of the coating correspondingly decreased, filler addition does not exceed 40g, the resulting coating film can meet the requirements of flexible performance, and in the case of filler content of no more than 20g, the resulting coating flexibility performance is more excellent, can be tested with a larger bending angle of flexibility performance.
2.5 Determination of the main formula of rebar powder coating
In summary, the main formula of pure epoxy powder coating applied to epoxy-coated rebar is listed in Table 6.
the effect of applying powder coating products made from Table 6 to rebar is shown in Figure 2.
formulation products by the Pipeline Science and Technology Research Center of PetroChina Pipeline Branch test technology test to meet the JG3042-1997 on epoxy powder coating related indicators requirements. The technical indicators of the coating are listed in Table 7. 3, conclusion
This paper by using a higher epoxy equivalent of two-step epoxy resin with phenolic curing agent, through 2 methyl mide to adjust the reaction rate, control inorganic filler addition, to obtain a pure epoxy-type reinforced powder coating products. Can meet the current domestic construction epoxy coating rebar performance requirements, while high epoxy equivalent epoxy resin, to a certain extent, can extend the high reaction activity of pure epoxy powder coating products storage cycle, so it has greater practical value and promotion significance.