Study on Manufacturing Technology of Plastic-Coated Steel Pipe at Normal Temperature

when steel pipes are transporting media or placed in natural environments such as the atmosphere and soil, the pipe walls will be subject to varying degrees of corrosion, which directly affects the service life of the pipes. food, drinking water pipelines, petrochemical pipelines, and many other media are not allowed to corrode the pipe wall and affect the contamination of the transmission medium, so the anti-corrosion protection of the steel pipe wall has become a key technical issue. plastic-coated steel pipes emerged as the times require. at present, domestic steel-plastic pipes have moved from plastic-lined to plastic-coated. most of the plastic-coated materials are made of plastic powder materials, among which polyethylene (pe) and epoxy (ep) have better performance. however, their construction requirements are high, the process is difficult, the quality control is not easy, and the equipment investment is large, which seriously affects the promotion and application of plastic-coated steel pipes. at present, only plastic-coated steel pipes with a diameter of less than ø250mm can be produced in china. in the conventional production process, the sandblasting process produces great pollution and harms the human body, while the heating process consumes a lot of energy, which makes it impossible to produce large-diameter plastic-coated steel pipes. to effectively solve the problem of difficult production of large-diameter plastic-coated steel pipes, the production process expert group first started with the coating materials, developed and applied elastomer anti-rust coatings, and then introduced protective coatings with better anti-corrosion and hygienic performance. the method of direct spraying at room temperature is adopted, which solves the problem of the difficult production of large-diameter steel pipes with plastic coating.

2. preparation of plastic-coated (elastomer) steel pipe at room temperature
it systematically introduces how to break through the original technical difficulties and manufacturing process of plastic-coated (elastomer) steel pipes at room temperature. since the spraying process of the outer tube is relatively conventional, this article focuses on the process of processing and spraying inside the tube.

2.1 breakthroughs in technical difficulties
in the conventional method of manufacturing plastic-coated steel pipes, firstly, the steel pipes need to be subjected to sandblasting and de-rusting treatment, which causes large pollution and complicated process. the room-temperature plastic-coated (elastomer) steel pipe adopts the most cutting-edge technology in metal anti-corrosion, and the anti-rust paint with rust is used as the primer. anti-rust coatings with rust need to meet special properties: anti-rust with rust, strong adhesion, non-toxic, sprayable, good anti-corrosion performance, etc.
in addition, conventional plastic-coated steel pipes are fired at high temperatures, and electrostatic powder is not sprayed to achieve good adhesion of paint to the inner wall of the steel pipe. therefore, when choosing internal spraying materials, they must be considered: strong adhesion, non-toxic, wear-resistant, impact-resistant, resistant to various chemical corrosion, and can be sprayed at room temperature.
the selected materials are all polyurethane coatings, which are non-toxic and have a good cleaning effect.

2.2 preparation process of plastic-coated steel pipe

2.2.1 pretreatment of the steel pipe surface
for new and old steel pipes, generally, no special rust removal process is required. it is best to use newly-made steel pipes without galvanizing, and a small number of rust spots in the pipes can also meet the requirements, as long as the loose floating rust is removed and the oil stains are cleared. that is to say, it can meet the requirements of manual de-rusting st2 or electric de-rusting sa1. if there is oil on the surface of the steel pipe, clean the surface with acetone.

2.2.2 spraying of paint
spraying equipment includes an air compressor, airless spraying machine, in-pipe traction mechanism, etc. the power device is an air compressor, and an airless sprayer and a traction mechanism inside the pipe are used to spray the rust-resistant and anti-rust primer on the inner wall of the steel pipe with a thickness of about 30 microns. after the primer is cured, spray the polyurethane topcoat twice, each 80 microns. the total thickness of the entire coating film is about 200 microns. the cost of the entire spraying equipment can meet the spraying construction requirements in tens of thousands of yuan.

2.2.3 detection and processing
sampling inspection of the coating quality of plastic-coated steel pipes, the surface of the coating should be uniform, the minimum thickness of the coating should reach 150 microns, and the average thickness should be above 180 microns.

2.3 comparison of different plastic-coated steel pipe manufacturing processes
compared with polyethylene and epoxy plastic-coated steel pipes, because we use polyurethane paint as the primer, the requirements for surface treatment of steel pipes are reduced, and the work can be done without sandblasting and de-rusting to remove floating rust and grease; in addition, there is no need to heat the steel pipes, can be sprayed at room temperature. it achieves convenient construction, reduces pollution, saves energy, can be constructed on-site, has low cost, and has excellent performance, which fully meets the needs of the project.