I. Introduction
At present, the common external anticorrosion coatings for underground pipelines at home and abroad can be divided into six categories, namely petroleum asphalt coating, polyethylene tape anticorrosion coating, extruded polyethylene anticorrosion coating, molten epoxy powder coating, coal tar enamel coating and epoxy coal tar asphalt anticorrosion coating. The three-layer PE anticorrosive coating is a composite anticorrosive coating made of sintered epoxy powder and extruded polyethylene. These six types of anti-corrosion coatings are used in different scales in China and have corresponding technical standards or specifications.
These six types of anticorrosive coatings have their own advantages and disadvantages. Petroleum asphalt anticorrosive coatings were first used after the founding of the People's Republic of China. Due to the limitation of industrial technology at that time, there was no special anticorrosive asphalt, that is, asphalt was not modified by adding fillers such as talcum powder and rubber powder, but was replaced by building asphalt. Petroleum asphalt layer has low mechanical strength, brittle cracking at low temperature, softening and flowing at high temperature, low cohesive force, high water absorption, short service life, corrosion and perforation of pipelines, which causes accidents and losses. Thermal spraying during construction has caused great environmental pollution and bad working conditions, and has been gradually eliminated in China.
Coal tar enamel is a kind of thermal spraying material. Heat it to about 240℃ and coat it on the outside of the steel pipe. Excellent corrosion resistance and long service life. But it needs to be prefabricated in the factory, which causes great environmental pollution. The pipeline bureau in the northwest Gobi has used it, but it is difficult to use it in other places.
Polyethylene tape, also known as cold-rolled tape, is convenient to use, clean and quick to construct. It is a good anticorrosive material. However, its disadvantage is that the plastic belt has low mechanical strength and is vulnerable to mechanical damage, and it cannot be constructed when it is windy, rainy, dewy and frosty, otherwise it will affect the adhesion and cause delamination, which is a potential corrosion hazard. When used in spiral welded pipe with high seam, the seam is overhead, which will lead to water channeling and pipeline corrosion when wet with water.
Extruded polyethylene anticorrosive coating is a very good anticorrosive coating, which is actually two layers of PE, the outer layer is polyethylene and the inner layer is rubber asphalt glue or polymer glue. Polyethylene anticorrosive coating can only be prefabricated in the factory, and can't be anticorrosive on site. The most important thing is the quality of polyethylene. In terms of technical indicators, the environmental stress cracking resistance (F50) must be greater than 1000 hours to prevent the use of waste plastics and low-density polyethylene to reduce costs. It is difficult and expensive to repair the joint of polyethylene anticorrosive coating. When using heat-shrinkable sleeve or heat-shrinkable tape for construction, the joint is prone to deformation, warping and poor sealing, which has potential corrosion hazards.
Fused epoxy powder coating is also a good coating with smooth and hard surface and high mechanical strength. Its disadvantage is that it must be prefabricated in the anti-corrosion factory, and the pipeline needs to be heated to 230 ~ 270℃, which consumes a lot of energy. Because the film coating is not resistant to collision during construction and transportation, careful and thoughtful protection measures should be taken to reduce the damage. It is difficult to repair the damage and weld the pipeline by on-site spraying with epoxy powder, and the construction process is complicated.
The three-layer PE anticorrosive coating is a combination of molten epoxy powder coating and extruded polyethylene anticorrosive coating, and the epoxy powder layer plays the role of primer. Three-layer PE is a double-insurance anticorrosive coating, which should be the highest and best coating. At present, the anti-corrosion building market is fiercely competitive, and the price is very low, even lower than the cost price of anti-corrosion coatings. Some manufacturers have to make a fuss about reducing the material and coating thickness, which has a bad influence on the three-layer PE.
Epoxy coal tar pitch anticorrosive coating is composed of liquid epoxy resin and coal tar pitch. It not only has the advantages of high mechanical strength, strong adhesion, stable chemical properties and corrosion resistance of chemical media of epoxy resin, but also has the characteristics of low water absorption, anti-microbial and plant root thorn resistance of coal tar pitch. It is an excellent waterproof and anticorrosive material with low cost and high efficiency. Its cold coating construction and construction technology are simple and feasible, and it can be processed in the prefabrication plant and also used for on-site anti-corrosion construction. Domestic epoxy coal tar pitch coating was successfully developed by the Pipeline Bureau of the Ministry of Petroleum in the 1970s. At that time, it was a solvent-based product with a solvent content close to 30%. The coating is not dense, and there are many pinholes, which are potential corrosion hazards. During construction, a large amount of solvents are discharged into the air, which pollutes the environment. In 1980s, Donghua Anticorrosion Company improved it into a thick slurry product, and the solvent content decreased to about 10%, which improved the coating quality and reduced the pollution. In 1990s, Donghua Anticorrosion Company improved it into a solvent-free product, which was used for epoxy coal tar pitch cold winding products. The primer of epoxy coal tar pitch cold winding tape is solvent-free epoxy coal tar pitch paint, and the base tape is polypropylene non-woven fabric impregnated with epoxy glue. The coating thickness is controlled by the thickness of non-woven fabric, and the corresponding standard requirements of construction, petroleum, petrochemical and other departments can be met by winding the adhesive tape once. Compared with the old process of multi-layer winding painting of glass cloth, it saves labor and materials, and the coating is dense and pinhole-free, which has been favored by users for more than ten years.
Solvent-free epoxy coal tar pitch cold rolled strip is a convenient, simple and stable anticorrosive material. However, after the sizing glue is finally coated, it should be left standing until the anticorrosive coating is at least dry, and then transported, stacked and backfilled. It takes about 8 hours in summer and a few days in winter, which brings various inconveniences to the construction arrangement. In order to understand this problem, Donghua Anticorrosion Company further developed RPC cold winding tape which can be constructed quickly.
RPC cold winding tape is the abbreviation of rubber-plastic epoxy coal tar pitch cold winding tape, which consists of rubber-plastic anticorrosive tape and solvent-free epoxy coal tar pitch primer. Rubber and plastic have the advantages of both rubber and synthetic plastics, such as good toughness, good insulation, acid, alkali and salt corrosion resistance, water resistance and sunlight aging resistance. The primer is solvent-free epoxy coal tar pitch, which has strong adhesion, high mechanical strength, compactness and no pinholes. RPC cold winding tape is the latest generation of epoxy coal tar pitch anticorrosive coating, which has the characteristics of the lowest cost, the most convenient construction technology and the best anticorrosive quality.
Second, formulate research requirements
1, development of winding tape
Usually, the epoxy coal tar pitch cold-rolled strip is hard and brittle after curing, so it is impossible to make it into a coil with good plasticity and toughness. Donghua Company adopts high-tech means to mix, calender and vulcanize epoxy coal tar pitch, rubber and plastic to make strip coil. This kind of adhesive tape is soft, tough and easy to construct. Because it has the same composition as the solvent-free primer (epoxy coal tar pitch), they can be well bonded together to form an inseparable anti-corrosion layer.
2. Development of primer
Primer is solvent-free epoxy coal tar pitch coating, which has the following advantages:
(1) Improve coating quality. During the construction process, there is no solvent volatilization, and the coating is dense and pinhole-free, which improves the anticorrosion performance.
(2) Simplify the construction procedure. In the construction of solvent-based coatings, pinhole is produced due to solvent evaporation coating, which forms hidden dangers. In order to reduce pinholes, multi-layer coating construction must be adopted, and pinholes can be eliminated at least 3 ~ 4 times.
(3) Improve work efficiency, speed up the construction period, reduce the number of construction workers and save labor costs.
(4) There are no volatile substances (such as toluene and xylene) in the production and use of solvent-free coatings, which reduces the environmental problems in production workshops and construction sites and improves the working environment.
(5) Save energy. Toluene, xylene and other substances in solvent are energy products, and solvent-free products save a lot of energy.
Third, technical performance.
Monomer technical index method of serial number project
1 Baseband has a black appearance, a flat and elastic surface, and a non-woven fabric layer on the back, which is convenient for visual inspection.
2 The measured baseband width is mm 250±5.0.
3 baseband thickness ≥0.75㎜.
4 baseband tensile strength n/cm ≥ 30 GB/t 528
5 dry primer (A: B = 1: 1)
(25℃, hard work) h ≤ 16 GB/t 1728.
6 The combined thickness of anticorrosive coating (adhesive tape) is mm ≥1.0sy/t0447.
7. Leakage detection of anticorrosion coating by electric spark kV 10 No leakage point SY/T0447
8 Adhesion of anticorrosive coating (180 peeling) N/cm 20 No peeling GB/T 2790
9. The impact resistance of anticorrosive coating J 4.9 passed GB/T 1732.
Note: 6 ~ 9 are the comprehensive properties of the "one-adhesive-one-belt" structure, which were measured after the laboratory specimens were baked at 60℃ for 6 hours.
Fourth, the method of use
1, surface treatment. Remove the oil stain on the steel surface, and carry out sand blasting and rust removal to reach the Sa 2 level of GB/T 8923, preferably Sa 2 1/2 level. When the construction conditions are limited, tools can also be used to remove rust to St 3 level. After the steel surface treatment is qualified, anti-corrosion construction must be carried out as soon as possible before rust return. Concrete should be thoroughly dried, and loose materials on the surface should be removed to present a flat and solid surface.
2, with primer. Component a: component B = 1: 1, fully and evenly mixed, and let stand for 5 ~ 10 min before use. Primer adopts quick-drying curing agent, which has fast curing speed. It should be used up within 2 hours after preparation, and should be used immediately during construction to avoid gel loss. Diluent can't be added when preparing primer, so as not to be too thin and the thickness of anticorrosive coating can't meet the requirements.
3. Welding putty. Putty should be used to form a smooth transition surface on both sides of the weld and the lap joint of two steel plates with the steel pipe surface higher than 1mm, and the pits on the concrete surface should also be smoothed. Putty is a mixture of primer and talcum powder, which should be used up within 4h after mixing.
4. Wrap with primer and tape. Before painting the primer, the surface of steel or concrete must be dry, and the primer should be painted evenly without leakage. Immediately after painting the primer, wrap (or paste) the baseband, and lap it for 20 ~ 25mm. Must be tightened or flattened, so that the surface is smooth, close contact, no bubbles. Before winding, it is best to dip one end of the baseband into the primer for about 20mm, and then wrap it with the primer, or brush the primer at the lap joint while winding, which can effectively increase the tightness of the lap joint. Wrap polyethylene tape at both ends of the wrapped anticorrosive coating to fix it.
5. Quality inspection. After winding the anticorrosive coating, let it stand and dry. After practical work, the appearance, thickness, electric spark inspection and adhesion inspection shall be carried out according to the conditions and methods specified in the GB 50268-97 standard of the Ministry of Construction (or other industry standards).
6. In case of urgent need for backfilling in the ditch, after winding the rubber belt, transport, dig the ditch and backfill under the condition of paying attention to protection and avoiding collision, so that the anti-corrosion layer can be completely cured under the ditch.
Verb (abbreviation of verb) economic indicator
1, cost per square meter comparison
(1) epoxy coal tar pitch glass cloth anticorrosive coating, thickness ≥0.6 ㎜, detection voltage 3000 volts, excluding rust removal fee.
From the comparison between them, it can be seen that the construction method of RPC cold winding belt is simple and the labor cost is much lower than that of epoxy coal tar pitch coating.
Conclusion of intransitive verbs
PRC cold-rolled strip is a new type of external anticorrosion coating for pipelines, which combines the advantages of various anticorrosion coatings:
(1) has the advantages of high mechanical strength, strong adhesion, water resistance, microorganism resistance and plant root resistance.
(2) Because the main components in the cold-rolled strip of PRC are synthetic rubber and synthetic plastic, it has the toughness and aging resistance of extruded polyethylene anticorrosive coating.
(3) The construction technology of tape winding is simple and flexible, which has the advantage of polyethylene cold winding. It can be used in prefabrication factory to process anticorrosion coating and conduct anticorrosion construction on site, but it is better than polyethylene tape winding, because the primer is solvent-free epoxy coal tar pitch with high viscosity and thickness, which can be used in windy and sandy weather and has no influence on adhesion.
To sum up, RPC cold-rolled strip has the advantages of simple construction method, uniform thickness, stable quality, water resistance, microorganism resistance, plant root resistance, corrosion resistance of various chemical media, high mechanical strength, collision resistance and convenient repair. It is a new type of anticorrosive coating with low cost and high efficiency.