Feasibility analysis of 3PP anti-corrosion coating for domestic pipeline applications

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　　Since the mid-1990s, 3PE coating has been used for the anti-corrosion protection of most large-scale pipelines in China, and it has undoubtedly become the preferred method for pipeline anti-corrosion in the country. However, in recent years, thanks to the Sudan pipeline project undertaken by our country, people have gradually come to know about the 3PP coating. Both 3PP and 3PE coatings belong to the multi-layer coating system, consisting of an underlying epoxy powder layer, an intermediate adhesive layer, and an outer pP (polypropylene) jacket layer. The processing properties of PP are similar to those of PE; a 3PE production line is fully sufficient for producing 3PP coatings, eliminating the need to establish a separate dedicated 3PP coating production line. However, the differences in the inherent properties of PP and PE materials result in 3PP and 3PE coatings exhibiting varying properties, thereby making them suitable for different environments. The advantages of the 3PP coating in certain performance aspects enable it to be used in areas where the 3PE coating is not suitable; as a result, many pipeline projects abroad utilize the 3PP anti-corrosion coating. This paper explores the feasibility of applying 3PP coating to pipelines in China, for reference by peers. 　　1. Comparison of the main properties of 3PP coating and 3PE coating: In China, no industry standards for 3PP coatings have been established yet; the 3PP-coated anti-corrosion pipes manufactured here generally follow the German DIN30678 standard titled \"Polypropylene Coatings for Steel Pipes\". From the performance criteria specified in the DIN30678 standard, there are certain performances that the 3PE coating cannot match. 　　1.1 Lifespan of the 3PP coating at high temperatures The DIN30678 standard specifies that the service life of the 3PP coating is 30 years at 80°C, 15 years at 90°C, and 8 years at 100°C. The 3PE coating softens when the temperature exceeds 80°C; under the stress exerted by soil, some sand and gravel will inevitably sink into or even penetrate the coating, thereby damaging the anti-corrosion layer. As a result, it is difficult to ensure the service life of the pipeline. 　　1.2 Peeling strength and indentation hardness of 3PP coating at high temperatures: According to DIN30678 standard, at 90°C ± 5°C, the peeling strength of the 3PP coating should be ≥ 80 N/cm and its indentation hardness should be ≤ 0.3 mm; these two parameters are not achievable with 3PE coatings. Regarding the 3PE coating, at 90°C the Vicat softening temperature of the 3PE adhesive specified by relevant standards is reached; in other words, the adhesive has softened, and its peeling strength is very low, almost zero. PE is a crystalline thermoplastic resin that softens as the temperature rises, and this softening becomes more pronounced as it approaches its melting point. Since 90°C is close to its melting point, the indentation hardness of the coating must be very low. 　　1.3 Impact strength of 3PP coating The DIN30678 standard specifies that the impact strength of 3PP coatings should be ≥5 J/mm, while China’s SY/T0413-2002 \"Technical Standards for Polyethylene Coatings on Buried Steel Pipelines\" requires that the impact strength of 3PE coatings be ≥8 J/mm. However, the DIN30678 standard requires an impact test to be conducted at an ambient temperature of 0°C ± 2°C, whereas the SY/T0413-2002 standard does not specify any requirements regarding ambient temperature; tests are generally carried out at room temperature. The impact conditions for the two are different, so it is not possible to make a comparison based solely on numerical values. However, it is certain that ambient temperature has a significant influence on impact strength; the lower the temperature, the lower the impact strength. It can be seen that the impact resistance of the 3PP coating is not inferior to that of the 3PE coating under certain temperature conditions. 　　1.4 Properties of 3PP coating The DIN30678 standard specifies that the resistivity of a 3PP coating should be ≥108 ohms·m; no specification is given for electrical strength. However, the measured volume resistivity of PP material compressed sheets is ≥1×1015 ohms·m, and their electrical strength is ≥25 MV/m. The volume resistivity of these sheets is higher than the value specified in the SY/T0413-2002 standard for PE compressed sheets, which is ≥1×1013 ohms·m; meanwhile, their electrical strength is comparable to the value specified in that standard for PE compressed sheets, which is ≥25 MV/m. Both PP and PE are polyolefin materials with the same electrical insulation mechanism; therefore, the electrical properties of a 3PP coating are similar to those of a 3PE coating. The electrical properties of the coating do not affect the protective capabilities or the setup of cathodic protection. 　　1.5 Tensile properties of the 3PP coating The DlN30678 standard specifies that the elongation at break for the 3PP coating should be 300, while no specification has been set for its tensile strength. Standard SY/TI0413-2002 specifies that the tensile strength of the 3PE coating shall be 200 MPa, and the elongation at break shall be @600. Based on the technical specifications set by standards, the elongation at break required for 3PP coatings is lower than that for 3PE coatings. However, tests on the tensile properties of PP material show that its tensile strength is 300 MPa and its elongation at break is 600%. According to DIN 30678 standards, the elongation at break required for 3PP coatings is 300; this means that a coating material with an elongation at break of 300 can meet the requirements for anti-corrosion pipes under relevant conditions. 　　1.6 Thermal aging properties of the 3PP coating The DIN30678 standard specifies that the 3PP coating shall not become brittle after being exposed to a heat treatment of 100°C for 100 hours. Standard SY/T0413-2002 specifies that after undergoing a heat treatment of 100°C for 10 hours, the rate of change of the melt flow rate for PE particles shall be ≤35. Although the performance parameters used to describe aging are different for the two, they undergo the same thermal history; this indicates that both PP and PE retain their usability after undergoing such identical thermal treatment, thereby showing that under conditions of 100°C x 1000 hours, the thermal aging properties of both PP and PE meet the required standards. 　　2. Application fields of 3PP anti-corrosion pipes: The technical characteristics of the 3PP coating determine its scope of application, which in turn defines the fields where 3PP anti-corrosion pipes can be used. Apart from the areas where it can be used in ordinary 3PE anticorrosion pipes (use with caution at temperatures below -20°C), the following environments are more suitable for the use of 3PE coatings. 　　(1) Used as anti-corrosion pipes for transporting media at higher temperatures. The 3PP coating can operate at 80 degrees Celsius for 30 years, and can even withstand temperatures up to 110 degrees Celsius; whereas the maximum operating temperature for the 3PE coating is 84 degrees Celsius. Therefore, the 3PP coating should be chosen when temperatures exceed 84 degrees Celsius. 　　(2) Used as an anti-corrosion coating for pipelines in desert areas with high surface temperatures and long daylight hours. In the hot summer months, the surface temperature in desert areas is high, often reaching 60–70 degrees Celsius. This poses a challenge to the high-temperature resistance of 3PE coatings; therefore, 3PP anti-corrosion pipes are generally used in desert areas with high surface temperatures. 　　(3) Used as anti-corrosion pipes near the pressure station. Currently, pressurized transportation is widely used to improve the efficiency of pipeline transport, which raises the temperature of the fluid inside the pipes. As a result, pipeline anti-corrosion coatings capable of withstanding temperatures above 80 degrees Celsius are required; therefore, 3PP anti-corrosion treatment needs to be applied near the pressure stations and even along the entire high-pressure transmission pipeline. 　　(4) Anti-corrosion coating for underground crossing pipelines. The high hardness of the PP material itself enhances the scratch resistance of the 3PP coating. Western Europe has achieved significant results in using PP protective layers for pipelines subjected to directional drilling. 1. When the pipeline passing through such areas is located in rivers, railways, or other locations where excavation is not possible, it is difficult to repair any damage to the anti-corrosion coating; using a 3PP coating can undoubtedly reduce damage to this coating during the passage process. 　　3. Production status of 3PP anti-corrosion pipes: There are yet few engineering examples in China where the 3PP coating is used on a large scale, but abroad, there are many successful projects that have utilized this coating. Hi, Nont’s Italian branch has been using 3PP anti-corrosion technology in various projects since the mid-1980s; to date, many 3PP-coated pipelines have been installed. According to the French company ATO, its PP materials have also been utilized on multiple occasions for 3PP anti-corrosion applications. Although there are no large-scale projects in our country that use 3PP-coated pipes at present, the pipe anti-corrosion manufacturers here do have experience in producing 3PP coatings, and have already manufactured a large quantity of such pipes for overseas markets. According to incomplete statistics, since Yadong Anti-corrosion Company began producing 3PP anti-corrosion pipes for Sudan in 1998, China has produced nearly one million meters of such pipes for foreign markets in total. Alone in 2004, approximately 500 km of 3PP anti-corrosion pipes were produced for Sudan. Furthermore, given the excellent performance of 3PP coatings in certain aspects, China is also gradually adopting them; to date, they have been used to a limited extent on the natural gas transmission pipelines of the Kela-2 gas field in Xinjiang. Additionally, 3PP coating is also used on individual sections of the finished sleeve conveyance pipeline in the western area. 　　4. Progress of 3PP anti-corrosion materials in China 　　4.1 Improvement of PP material properties 　　PP is a thermoplastic material with excellent overall properties; compared to other common plastics, it boasts advantages such as low density, ease of processing, and high temperature resistance. As a result, it is widely used in industries such as daily necessities, packaging materials, home appliances, the automotive industry, and construction. However, PP also has some disadvantages; when it comes to its use in pipeline anti-corrosion, its main drawback compared to PE is low-temperature brittleness. Due to its high degree of crystallinity and large crystal nuclei, PP has poor toughness. In particular, their low-temperature toughness is even worse; the toughness decreases significantly below 0 degrees, and some varieties lose almost all of their utility. However, with technological advancements and the development of copolymerization processes and copolymer monomers, the impact strength of copolymerized PP has improved significantly. The PP materials produced in China now approach the international standards: an impact strength of 5 J/mm at room temperature and 50.5 J/mm at -20°C. With appropriate improvements, it is entirely possible to create materials that meet the requirements of 3PP coating standards. 　　Furthermore, although the thermal aging resistance of PP materials is inferior to that of PE materials due to the presence of a higher number of tertiary carbon atoms in their molecular structure, it can be improved by adding appropriate additives, without affecting the performance of the anti-corrosion coating on buried pipelines. The anti-aging system commonly used for PP anticorrosion materials both domestically and internationally consists of hindered phenol-based primary antioxidants combined with thio-based or phosphite-based secondary antioxidants, along with the use of rutile-type titanium oxide. With the aforementioned anti-aging formulation, PP anti-corrosion materials can fully meet the requirements of DIN30678 standard regarding the anti-aging properties of 3PP coatings. 　　4.2 Production status of PP materials As early as 1998, when Yadong Anti-Corrosion Company was producing 3PP anti-corrosion pipes for Sudan, some domestic manufacturers of anti-corrosion materials began researching 3PP anti-corrosion materials. However, due to the limitations in the properties of PP materials at that time, many modifications were required, and ideal results were not achieved. However, in recent years, with the development of PP technology, various PP materials suitable for use in pipes have emerged, whose properties are closer to those required by 3PP coatings. Therefore, by modifying appropriate types of PP materials, it is entirely possible to create materials suitable for 3PP anti-corrosion applications, and the resulting 3PP coatings meet the requirements of standards such as German DlN30678. Several domestic companies have now developed PP materials suitable for 3PP anti-corrosion applications, including adhesives for 3PP and jacketing materials for 3PP, whose performance parameters are similar to those of products from foreign petrochemical companies. Through on-site testing and performance evaluations, it fully meets the requirements of the 3PP coating. 　　5. Comparison of costs for 3PP and 3PE coating materials The selection of a coating depends mainly on factors such as the coating’s performance, its workability, the cost of the coating materials, and the manufacturing costs. Compared to 3PE coatings, the production cost of 3PP coatings is roughly the same, but the cost of the materials used is lower than that of 3PE coatings. PP is the lightest among common plastics; its density is lower than that of PE. With the same coating thickness, less material is required for PP coatings, resulting in lower costs compared to 3PE coatings. Based on foreign project examples, for anti-corrosion coatings on pipes of the same diameter, the thickness of the PP coating is generally smaller than that of the PE coating. Studies from the former Soviet Union have shown that PP tape has a higher tensile strength than dish tape, and its thickness can be reduced by 30%. 1. Using a 3PP coating in this way will result in lower costs compared to a 3PE coating. For the anti-corrosion of pipes with a diameter of D6OOmm or more, 3PP anti-corrosion coating with a thickness of 2.5 mm is sufficient to meet the protection requirements. In contrast, when 3PE anti-corrosion is used, its coating hardness and mechanical strength are lower than those of 3PP coatings; taking into account factors such as the weight of the pipe that may cause damage to the coating, a reinforced version is often chosen, with a coating thickness greater than 3 mm. It can be seen that to achieve the same level of protection, 3PP coatings require less material compared to 3PE coatings. 　　6. Conclusions 　　(1) Under certain operating conditions, the corrosion protection coating of 3PP pipes performs better than that of 3PE coatings. In high-temperature environments where the 3PE coating is not suitable, the 3PP coating can be used as a substitute. The scratch resistance of the 3PP coating gives it advantages for pipe penetration, allowing it to replace other coating types for such applications. 　　(2) China has production experience in 3PP anti-corrosion pipes, and the anti-corrosion materials used for 3PP can already meet the requirements of the 3PP coating. 　　(3) The cost of the 3PP coating is lower than that of the 3PE coating.