Preparation and coating process of powder coating for steel bars

Abstract:
a hot-melted epoxy powder coating on the reinforced surface can delay the corrosion of the rebar and extend the service life of the concrete structure.
this paper introduces the technical requirements, research and development process and coating process of epoxy powder coating for rebar coating, and shows that the use of epoxy coating is a practical measure to protect rebar against corrosion.
introduction,
is an indispensable building material for all kinds of construction

. In the coastal port terminals, cross-sea bridges, buildings with reinforced concrete and facilities such as airports and highways that require salt water to be sprinkled with ice and snow, there are inevitably gaps or voids in reinforced concrete that can cause corrosion of reinforced concrete when eroded by erosion such as salt water or acid rain.
iron oxide produced by corrosion expands dozens of times, causing the concrete to crack and the protective layer to peel off, so that the rebar is directly exposed to the atmosphere, water and its corrosive medium, and the corrosion speed is accelerated.
buildings are often destroyed early before they reach the design life, resulting in catastrophic engineering accidents. Therefore, the stability of rebar performance has been directly related to the final service life of the building.
the requirements of the Ministry of Construction, the China Academy of Architectural Sciences, together with the relevant units, has completed the construction industry's coating rebar product standard "epoxy resin coated rebar" JG3042-1997.
the standard refers to the United States ASTM A775M-95a and the National Organization for Standardization ISO 14654 (1995 draft work) and other relevant content of the standard, based on the preparation of China's national conditions.
the standard specifies the technical requirements of the coated rebar product, the test method of quality inspection, the inspection rules of the product and the performance of the coating material.
In addition, in view of China's current concrete structure design, construction norms have not yet made provisions on the application of coating rebar requirements, as an industry standard, the standard also put forward the design and construction methods of coated rebar principle requirements, the standard has been implemented since December 1, 1997.
1, the experimental
1.1 experimental raw materials
epoxy resin A: Guodu chemical;
epoxy resin B: Hengyuan chemical;
Epoxy resin C: Guodu Chemical;
Phenol curing agent A: Daqing Qinglu;
Phenol curing agent B: Daqing Qinglu;
Phenol curing agent C: Guodu Chemical;
Phenol curing agent D: South China Sea flattery;
phenol curing agent E: South China Sea flattery;
promoter: Six Antonda.
preparation of 1.2 powder coatings . According to the formula accurately named the ingredients of raw materials, put into the mixer high-speed mixing, and then through the twin screw extruder extruder extruded cooling, the cooled pieces of material by coffee grinding and sieve through the screen to obtain the finished powder coating.
the powder coating with an electrostation gun butcher's knife on the blasted rebar substrate, cured and cooled to get the sample rebar.
1.3 test and . Use a rebar bending machine to bend the sample rebar according to 4D requirements and observe cracking when bending the surface coating.
1.4 reaction principle . (1) Bisphenol A epoxy resin structure introduction
Epoxy resin curing reaction mainly occurs on the epoxy base, because the epoxy base by the ether bond and benzene ring on the impact of large π bonds, the distribution of the electron cloud occurred a large offset, resulting in even moment, resulting in more negative charge of oxygen atoms on the epoxy base.
the carbon atoms at their ends retain more positive charges, so both the electro-electric reagent and the pro-nuclear reagent are eicoxygenated in the form of a addition reaction, resulting in a mesh structure polymer.
(2) Phenol curing agent structure introduction
phenol curing agent is made by the chain expansion process using biphenol A closed epoxy resin. It is more complex with the curing process of epoxy resin, it mainly contains: phenolic hydroxyl and epoxy-based etherization reaction, a mid-hydroxyl and epoxy-based etheric reaction, serotonin catalytic epoxy self-polymerization reaction and 2-methyl mide to make epoxy open ring reaction.
(3) Description of the structure of the double cyanide curing agent
Because the double cyanide has a high melting point (207-209 degrees C) and contains a compound of passivation group CN, its reaction with epoxy resin must occur at least >150 degrees C.
in order to improve its low reaction properties, catalysts or promoters such as metazole or add metazole are needed to speed up the reaction.
the mixture obtained by adding these promoters directly to the group is called accelerated double cyanide, and the double cyanide obtained by derivative introduction to replace the base group is replaced by double cyanide.
(4) phenolic curing reaction characteristics
This curing process is free of small molecules and does not have defects such as pinholes in the coating film. The presence of large amounts of hydroxyl has further promoted the attachment to the substrate.
And the two have similar chemical structures, have better mutual solubility, do not produce structural damage caused by curing contraction, together with epoxy resin coating film for better compaction, heat resistance, solvent resistance and corrosion resistance.
at the same time the curing agent itself is resin state, the structure on the main chain can also be changed in demand, thus ensuring the flexibility of the coating film.
(5) Dual cyanide curing reaction characteristics
The coating film of this curing process has better tightness, heat resistance, solvent resistance and corrosion resistance, but the cross-link density of the coating film is large, flexibility is poor for phenol curing.
the above characteristics, the formula selects phenolic curing agent curing agent.
2, results and discussion
2.1 epoxy resin selection taking into account the requirements of rebar toughness, the choice of strong rigid phenolic resin is excluded. The experiment selected Bisphenol A epoxy resin A, epoxy resin B, epoxy resin C, the specific indicators can be seen in Table 1.
From Table 1, when using epoxy resin A or epoxy resin B alone, the molecular chain is too long, the viscosity is too large, the level difference and the substrate wetting is poor, bending cracking;
, the next experiment was to choose the combination of epoxy resins.
the ratio of 2.2 epoxy resins . Epoxy A molecular chain is longer, not conducive to melt extrusion, and the substrate wetting is poor, the experimental choice of epoxy resin B and epoxy resin C matching, the specific experimental design see Table 2.
From Table 2, it can be seen that when the amount of epoxy resin B: epoxy resin C is 4:1, it can not only meet the requirements of the flexibility of the coating, but also ensure the good wetting of the coating film and substrate, and achieve the best flexibility of the coating.
2.3 phenol curing agents are . The experimental epoxy resin B and epoxy resin C were used in combination with 4:1, and the phenolic curing agent A curing agent B, curing agent C curing agent D, curing agent E were selected for the experiment, the results are seen in Table 3.
From Table 3, it can be seen that if the chain segment of phenolic curing agent is too short, flexibility is not enough, bending and cracking, if the chain segment of phenolic curing agent is too long, then the wetting of the coating and substrate is poor, adhesion problems. Selecting curing agent C and curing agent D can make the bending performance meet the requirements.
2.4 promoter dosage is determined
in order to adapt to the characteristics of rapid curing on the rebar spray line, the addition of the promoter is particularly critical. In order to improve the dispersion performance of the promoter and ensure the full mixing of the promoter and the base material, the experimental selection of epoxy plus methazole K7101 as a curing promoter, the amount of the promoter added in each Kg material on the collation time is shown in Figure 1.
from Figure 1: with the addition of the promoter, the reaction activity increased, the gel time was shortened. However, the addition of 22g k7101 is already a catalytic limit.
2.5 Final experimental formula
3, coating process
coating epoxy-coated rebar production process, see Figure 2.
3.1 Construction Guide . Sand blasting treatment: to GB / T8923 -1988 provisions of the visual assessment of rust removal grade Sa21/2;
medium frequency heating: heated to the surface temperature of the rebar between 220 to 230 degrees C;
powder room: powder room atomization should be equal, spray gun arrangement should not be too tight to avoid the return electric field increase spider web;
quenching time: quenching time should be adjusted accordingly with the warm-up temperature and rebar model, but the quenching time should not be too long, to avoid excessive bending.
3.2 Reference curing window
warm-up temperature in Figure 3 is the actual temperature of the reinforced surface. The actual post-curing time is adjusted accordingly with the ambient temperature and rebar model.
4, Conclusion
(1) Compared with phenolic epoxy and dicyanide curing agents, the choice of bisphenol A epoxy and phenolic curing agent reaction after the coating film has good denseness, heat resistance, solvent resistance and corrosion resistance, but also to ensure good flexibility of the coating film;
(2) Bisphenol A epoxy resin selection, if the chain segment is too long, and the substrate wetting is not good, affecting the coating and substrate adhesion needs, but also to
meet the needs of coating flexibility;
(4) This reinforced epoxy powder coating passed the national building materials testing center testing, in line with GB/T25826-2010 "reinforced concrete epoxy coating rebar" requirements.
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