Research on Preparation and Properties of Solvent-free Internal Friction-reduction Anticorrosive Coatings for Gas Pipelines

ZHENG Jun-sheng, TIAN Xue-hui, WEI Meng-kai, ZHANG Ya, HE Zhi-tong (Shijiazhuang Paint Company, High Solids Coatings Technology Innovation Center of Hebei Province, Shijiazhuang 050051, Hebei, China)

Abstract: According to the requirements of SY/T 6530—2010 Standard, solvent-free epoxy internal friction-reduction anticorrosive coatings were prepared by using the technical basis of solvent-free epoxy anticorrosive coatings and combining the characteristics of natural gas pipeline corrosion. This paper mainly discusses the influence of film-forming resin on coatings performance, the influence of curing agent on coatings performance and the influence of pigment and filler on coatings performance. Finally, a kind of solvent-free internal friction-reduction anticorrosive coatings suitable for gas pipelines was obtained and successfully applied on a large scale.

0 Introduction
With recent years The demand for clean energy has increased sharply, and the country’s ability to extract, transport, and utilize natural gas has also continued to increase. Compared with petroleum, natural gas has the characteristics of low density, good fluidity, and compressibility. Therefore, pipeline transportation is a natural gas transportation method. However, for natural gas pipeline transportation, the pressure loss of natural gas in the pipeline must be considered. During the long-distance pipeline flow of natural gas, due to the surface effect of the inner wall of the pipeline and the internal friction of natural gas, friction resistance will be generated, causing a large number of The energy loss makes the pipeline transportation efficiency decrease; meanwhile, the inner wall of the pipeline is severely worn, which will reduce the service life of the pipeline and pose a serious threat to the reliability and safety of the pipeline transportation system. This problem is particularly important in the long-distance transportation of natural gas.

For gas pipelines, there are many ways to reduce the inner wall roughness. At present, from the perspective of economics and practicability, applying the inner wall drag reduction coating is one of the most effective methods. According to relevant research, with the same natural gas transmission efficiency, gas transmission pipelines coated with internal drag reduction coatings can increase gas transmission by 6% to 30%. The resistance-reduction coating in natural gas pipelines should have high gloss, excellent adhesion, good abrasion resistance, and flexibility. After a lot of practice, epoxy coatings are considered to be the most suitable for natural gas pipelines. Coatings for interior walls. Traditional epoxy internal drag-reducing coatings are solvent-based products and cause certain harm to workers and the environment. Non-solvent epoxy internal drag-reducing paint has a non-volatile content close to 100%, extremely low VOC content, and high coating spraying efficiency, which reduces construction costs. At the same time, its wear resistance and shear resistance are better than solvent-based internal drag-reducing paint Therefore, it is gradually being valued by the market.

This paper has developed a solvent-free epoxy internal drag reduction coating, which has good abrasion resistance, flexibility and corrosion resistance. The solid content of the coating is ≥98%, the coating surface is smooth, and the measured performance meets SY / T 6530— 2010 requirements.

1 Experimental part
1.1 Main raw materials for experiment
DER 331 epoxy resin, Dow Chemical; 828EL epoxy resin, Hansen, Germany; NPEL128 epoxy resin, Kunshan Nanya; SM618, Jiangsu Miki; flexible resin, Zhejiang; dispersant, Beike; defoamer, leveling agent , Huaxia; Teflon wax powder, Sanmei; Iron oxide red, Xinxiang; Reactive diluent, East America; Modified amine curing agent, Cardolite.

1.2 Experimental instrument
electric multi-function mixer, U450 / 80-220, Shanghai Weida Industry and Trade Co., Ltd .; electronic balance, TC3K, Changshu Shuangjie Testing Instrument Factory; sand mill, QSM-Ⅱ type, Tianjin Jingkelian Material Testing Machine Co., Ltd .; Electric constant temperature blast drying oven, TST202A-1B, Chengdu Tesite Instrument Co., Ltd .; Intelligent automatic salt spray testing machine, F-120S, Dongguan Jingzhuo Instrument Equipment Co., Ltd .; Anchor drawing Instrument, SW-300, Beijing Haichuang Hi-Tech Technology Co., Ltd .; contact angle measuring instrument, OCA200, Beijing Aodelinuo Instrument Co., Ltd.

1.3 Basic formulas
Table 1 shows the raw materials and reference formulas for solvent-free epoxy internal drag reduction coatings.

Table 1 Raw Materials and Reference Formula

1.4 Coating preparation process
1.4.1 Coating preparation process
According to the requirements in Table 1, the epoxy resin, flexible resin, reactive diluent and auxiliary agent are added to the batching tank in order, and dispersed at a high speed of 600 r / min. During the process, pigments and fillers were added in order according to the proportion, and dispersed at a speed of about 1 200 r / min until no large materials were dispersed at high speed. After grinding by a sand mill, the fineness was less than 50 μm, and filtered with 60 mesh silk. After packaging.

1.4.2 Preparation process of
the test sample Mix the A component and the B component according to the calculated ratio, spray the test plate, and dry it at room temperature for 10 d according to the requirements of SY / T 6530, and then bake in a 50 ℃ circulating air oven for 24 h. , And then cooled to room temperature for testing. The test board is a low-carbon steel plate that meets the requirements of SY / T 6530, with a size of 75 mm × 150 mm × 0.81 mm, which meets the surface treatment requirements specified in this standard.

1.5 Testing of coating and film performance

Table 2 Performance Indicators of Coatings and Testing Methods

1.6 Experimental ideas
Low surface treatment anti-corrosion and drag reduction coatings for oil and gas pipelines should have the following characteristics:
(1) the coating film is dense and the chemical structure is stable;
(2) the mixed viscosity is low, and the coating is convenient;
(3) the adhesion is strong, and the corrosion resistance is excellent ;
(4) good wear resistance, having a certain hardness, resistance is small and smooth;
(5) having a certain flexibility.
Therefore, when designing the experimental formula, it is necessary to consider making the coating have the above properties, and achieve a balanced relationship between salt spray resistance, wear resistance, flexibility and workability.

2 Experimental results and discussion
2.1 Effect of epoxy resin on coating performance
2.1.1 Selection of epoxy resin model
Common solvent-free epoxy resins are mainly E44 and E51, compared with E44 epoxy, E51 epoxy The molecular weight is smaller, the viscosity of the prepared coating is lower, the applied coating is denser, the hardness is higher, and the corrosion resistance is more excellent. Therefore, E51 was selected as the main film-forming substance of the solvent-free epoxy internal resistance reduction anticorrosive coating in this experiment.
The commonly used E51 epoxy resins are Dow’s DER331, Hansen’s 828EL, South Asia’s NPEL128, and Miki’s SM618 resin. In the case where the resin content (40%) is the same as the curing agent, this experiment prepared these four resins. Testing of coating viscosity, adhesion, salt spray resistance and bending. The specific test results are shown in Table 3.

Table 3 Test Results of the Influence of Four Resins on the Properties of Coatings

Analyzing the test results in Table 3, it can be seen that the coating viscosity prepared by Nanya NPEL128 resin and Miki SM618 resin is the lowest; while Hansen’s 828EL resin and Dow’s DER331 resin have higher adhesion and salt spray resistance than the remaining two Resin, but the improvement is not obvious than the South Asian NPEL128 resin, which is in the acceptable range. In terms of overall cost, Nanya NPEL128 resin was selected as the solvent-free epoxy resin for this experiment.

2.1.2 Amount of epoxy resin
The higher the amount of epoxy resin, the higher the crosslinking density of the resin in the coating, and the denser the coating will be. The different resin content will have a significant effect on the gloss, adhesion and corrosion resistance of the coating. An experimental scheme of different resin contents was used to test the prepared coatings for gloss, adhesion, and salt spray resistance. The specific test results are shown in Table 4.

Table 4 Test Results of Different Resin Contents

From the experimental results in Table 4, it can be seen that with the increase of the content of NPEL128 resin, the gloss, adhesion and salt spray resistance of the prepared coatings also increase, but the flexibility is reduced to an amount of 40 At about%, the extent of these performance enhancement tends to be gentle. In addition, the increase in resin content will cause the hardness of the coating to increase, the overall cost of the coating and the cracking phenomenon in the bending test. Considering this, the total amount selected in this experiment is 40 % (Mass fraction) of NPEL128 resin is used as the main resin of the coating.

2.2 Selection of Flexible Resin The
NPEL128 resin coating alone has greater hardness and poor flexibility. In order to meet the bending test requirements in the SY / T 6530-2010 standard, it is necessary to use toughening materials to improve the toughness of coating films. However, in order to meet the abrasion resistance requirements of the coating, the selected toughening material should not reduce the hardness of the coating. In this experiment, three kinds of toughening agents were selected for the experiment. The specific test results are shown in Table 5.

Table 5 Coatings Film Bending Test Results of Different Toughened Materials

It can be seen from Table 5 that the use of macromolecular hydrocarbon hydrocarbon resins, E12 resins and modified epoxy resins with flexible functional groups has improved the bendability of the coating, but it will appear when the amount is too large. The phenomenon of reduced hardness, which does not meet the screening requirements. In order to ensure that the coating is a solvent-free system, the E12 resin needs to be diluted with a reactive diluent, and the amount added is 10%, which is discarded first. Although the modified epoxy resin is used less, its viscosity is larger than that of hydrocarbon petroleum resin, which is not conducive to construction, and the price and cost are higher than that of hydrocarbon petroleum resin. The non-volatile content of hydrocarbon petroleum resin is 100%, and the viscosity is 3 000 ~ 3 500 mPa · s. Therefore, 8% hydrocarbon petroleum resin is used as the toughening material in this experiment.

2.3 Effect of curing agent on coating performance
Studies have shown that epoxy curing agents have a significant effect on the gel time, dryness, adhesion, flexibility and anticorrosive properties of coatings. The above properties are tested by using different types of curing agents to screen the best type of curing agent . Table 6 shows the test results of the effects of different types of curing agents on the above properties of coatings, where: # 1 curing agent is a modified polyetheramine adduct; 2 # curing agent is a modified alicyclic amine adduct; 3 # curing agent It is a modified fatty amine adduct; 4 # curing agent is a novolac adduct.

Table 6 The Effects of Different Curing Agents on Coatings' Properties

From the test results in Table 6, it can be seen that the gel time and drying property of 1 # curing agent are poor, the low temperature applicability is poor, and the corrosion resistance is poor; the comprehensive performance of 2 # curing agent and 3 # curing agent is not much different. Although its applicability and drying properties are good, its adhesion and anticorrosive properties are not as good as 4 # curing agent. 4 # curing agent is a solvent-free, low-viscosity phenol-formamide curing agent, which provides coatings with the advantages of polyamide and phenalkamine curing agents. It has excellent low-temperature applicability, drying properties, flexibility, adhesion and Anticorrosive. Therefore, in this experiment, novolac curing agent was selected as the curing agent of solvent-free epoxy internal drag reduction anticorrosive coating.

2.4 The influence of pigments and fillers on the performance of coatings
After the film-forming substance is determined, the choice of pigments and fillers and its amount become one of the key factors that determine the performance of the coatings. The action mechanism of anticorrosive coating is: (1) the isolation and shielding effect of the coating; (2) the passivation and corrosion inhibition effect of the coating film; (3) the electrochemical protection effect. The mechanism of drag reduction coating is: (1) improve the smoothness of the coating film, reduce the roughness and reduce the friction coefficient; (2) the surface energy of the coating film is low, reduce the precipitation and adhesion of impurities in the fluid, and keep the surface smooth. At present, non-solvent internal drag reduction coatings are usually iron red, and have higher requirements on the corrosion resistance of the coatings. Therefore, the antirust pigment of this experiment is iron oxide red. Based on the above requirements, phosphoric acid is initially selected in this experiment. Zinc, aluminum tripolyphosphate, precipitated barium sulfate, wollastonite, polytetrafluoroethylene wax powder.
Design experiments of 4 kinds of fillers with different dosages, and test the prepared coatings for salt spray resistance, abrasion resistance, gloss, and hydrophobic angle. The specific experimental design scheme is shown in Table 7.

Table 7 Four Experimental Schemes

It can be seen from Table 8 that when the total amount of zinc phosphate and aluminum tripolyphosphate is large, the combination of the two can provide better salt spray resistance for the coating, but these two fillers play a certain matting effect on the coating. ; The addition of precipitated barium sulfate has a significant improvement on the abrasion resistance and hardness of the coating, and has almost no effect on the gloss of the coating; the addition of polytetrafluoroethylene wax powder can effectively increase the hydrophobic angle of the coating, thereby giving The coating has hydrophobicity, stain resistance, and anti-adhesion properties, which improves the gas transmission efficiency of natural gas, and it is more suitable to add 6% to 8%.

Table 8 Performance Test Results of Four Experimental Schemes

3 Conclusion
using E51 and the macromolecular chain hydrocarbon petroleum resins as a film forming coating composition, a curing agent with phenolic amide and a polyamide having phenolic amine dual advantage dried and solidified, while selecting good rust resistance and drag-reducing properties Pigment filler, solvent-free epoxy internal drag reduction anticorrosive coating was prepared. The coating has good comprehensive performance, meets the market and environmental protection requirements, and provides a certain promotion for the application and promotion of solvent-free epoxy technology in the field of drag reduction in natural gas pipelines.

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