Ji Jungang Penglai Jutao Ocean Engineering Heavy Industry Co., Ltd., Penglai, Shandong 265607, China
Abstract: The composition and properties of polyurea were introduced. Combined with seawater desalination project, the successful experience of coating and coating inspection of polyurea in complex steel structures was obtained.
Key words: polyurea; steel structure; painting construction; coating inspection
CLC number: TQ633 Document code: B Article ID: 1007-9548 (2010) 08-0030-02
Polyurea is a new type of heavy-duty anticorrosive coating, which is divided into A component and B component, A component is a semi-prepolymer of terminal NCO group, and B component is composed of terminal amino polyether, amine chain extender and auxiliary agent. Made. The components A and B are sprayed and mixed by a special equipment for two-component reaction injection molding, and a chemical reaction occurs to rapidly gel into a coating. The polyurea coating has excellent corrosion and aging resistance, and the coating is not affected by ambient temperature and humidity.
The reaction rate of the coating is extremely fast, and it usually gels within a few tens of seconds. Even if it is applied on any curved surface, inclined surface or vertical surface, it will not sag, which not only facilitates the construction but also shortens the construction period. In addition, the polyurea has a solid content of 100%, no solvent, no pollution, and a polyurea coating on the inner wall of the pipeline or the container, which does not cause any pollution to the carried medium after curing, and meets the requirements of health, safety and environmental protection.
1 Use background
Our company undertook the Australian desalination project in the second half of 2009. The project includes important modules such as Intake Caisson and Pretreatment Tank, as most structures are exposed to highly corrosive seawater for extended periods of time, and the coating must be non-contaminating to the treated seawater. Therefore, all the structures of the water caisson and the inner wall of the pretreatment tank are coated with polyurea and have a thickness of 2 mm.
Considering that some modules will be hoisted and loaded after the module is built, and the deformation of the module during sea transportation, the polyurea elastomer coating is finally selected, and the elongation at break reaches 450%, thus avoiding the coating. Damage caused by deformation during lifting and transportation.
In China, polyurea is mainly used in the inner wall of storage tanks, the inner wall of sewage treatment tanks, and various floor-level structures. For the desalination project, there are a large number of T-shaped steels and H-types in addition to the planar structure.
Steel and angle steel, these complex structures are a huge challenge for polyurea spraying.
2 Experience gained from the use of polyurea
For ordinary epoxy coatings, the distance between the scaffold and the structure is about 30mm, which does not affect the normal spraying. The spraying pressure of polyurea is usually 12.4~17.2MPa. The distance from the spray nozzle to the structure is preferably maintained at 40~50cm. Therefore, more pinholes appeared on the first batch of modules that were completed, and these pinholes were mostly visible to the naked eye, mainly concentrated in the lateral scaffolding area. Repairing the pinholes on the coating is time consuming and labor intensive and wastes a lot of material. In the case that the security department allows, the corrective measures taken are that the distance from the scaffolding to the structure is about 45 cm, and the number of pinholes appearing in the subsequent spraying construction is small.
In the polyurea spraying process, it is necessary to strictly control the mixing of component A and component B in a ratio of 1:1, which is the key factor for the success of polyurea spraying, otherwise it can be seen within a few days after the completion of spraying polyurea. Foaming phenomenon.
In northern China, the winter temperature is usually low, which may cause the difference in fluid viscosity and the difference in flow rate due to temperature differences, which ultimately leads to the imbalance of the two components.
[NT:PAGE]In order to avoid the imbalance of the two components, the electric heating belt is used to simultaneously heat the A cylinder and the B cylinder. The heating is started 24 hours before the use, and the stirring is continued, thereby effectively ensuring that the two components are the same. Proportional mixing.
Ordinary coatings are usually sprayed in the order from top to bottom, and for polyureas, if they are also sprayed in the order from top to bottom, due to the extremely high pressure during spraying, a lot of paint mist is formed, and these paint mists will fall under Spherical particles are formed on the side steel structure, which adversely affects the subsequent coating. Therefore, for the spraying of polyurea, the better construction sequence is from bottom to top, and the coating on the lower side is completed and quickly cured, and then covered with a plastic film to prevent the paint mist from falling on the surface of the cured coating and affecting the appearance. If the entire steel structure is very high, it can be divided into small areas along the height direction, and then sprayed one by one in order from bottom to top.
The spray application personnel must check whether the coating has reached the specified film thickness during the spraying process. If the spraying is completed and the coating interval is exceeded, then it is necessary to use a sandpaper to polish the coating, acetone scrubbing and brushing adhesives, etc., which will increase the construction time and increase the amount of polyurea, and Premature failure of the coating is prone to occur.
Since polyurea is a new type of material, there is no uniform construction and inspection standard in the world. The seawater desalination module must be in contact with seawater for a long time, so the protective coating for the immersion zone must be tested with 100% high-voltage spark miss. The detection voltage is a critical factor in the detection of leaky coatings. The commonly used NACERP0188 standard provides a range of voltage selection values, but this standard is for general epoxy coatings. For polyurea, it is recommended to use the formula V=250T/F in Australian Standard AS3894.1, where V is the test voltage (V) to be selected, T is the dry film thickness (μm) of the polyurea coating, and F is the coating. The solid content determines the coating type factor (F = 1, 2, 3, 4). Polyurea is a solventless type of coating, so F = 1, so that a polyurea coating having a dry film thickness of 2 mm can be obtained by selecting 11180 V as the leak point detection voltage. For the adhesion of polyurea, there is also a lack of uniform judgment in the world.
HG/T3831—2006 “spray polyurea protective material†clearly defines the adhesion of polyurea. The adhesion of polyurea elastomer on steel substrate and concrete substrate is 4.5MPa and 2.0MPa respectively, which can be used as judging polyurea. Whether the adhesion meets the requirements.
3 Conclusion
Polyurea is a brand new coating system, and there is still a lack of uniform construction and inspection standards worldwide. However, due to the excellent performance of various aspects of polyurea coating, as long as the successful experience is accumulated in the use, polyurea coating will be more and more widely used in the future.
Concerned about surprises
Label: seawater desalination solvent-free prepolymer aging resistance literature identification code
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