Failure Analysis And Study Of XLPE On Passenger Vehicle Oil Pump

Wire and cable are more and more important in our lives, wide variety industries are using it directly or indirectly. With the improvement of the technologies, the requirement for wire and cable are increasing. Due to XLPE has a very wide using temperature range, good solvent resistance, insulating properties and mechanical properties, so it can be used in variety environments. Therefore, more and more people using XLPE as insulation material on wire and cable.Currently, there are two major cross-linking methods, physical cross-linking method and chemical cross-linking method. Among them, physical cross-linking method includes radiation and UV cross-linking. Chemical cross-linking method includes organic peroxide cross-linking, silane cross-linking and salt cross-linking. Different cross-linking method has different requirement for the formulation of polyethylene, cross-linking process has its own consideration also. But most of them have common additives, such as antioxidants, flame retardants, anti-copper agent and some processing additives.Because UV cross-linking method has high efficiency, low cost, easy operation, it has been recognized and applied by many manufacturers. Because XLPE cable has these advantages as above, it has been applied on car oil pump widely. The pump was put and working in fuel tank, the environment in the fuel tank changed with season, driving or parking time and region changing. Meanwhile, cars have very strict safety requirement, so auto industry has special requirement for the XLPE which used on oil pump. Auto industry association and many automakers have their own specifications for the cable. At present, SAE J1128 is the famous one which issued by SAE International (The Engineering Society For Advancing Mobility Land Sea and Air Space).In SAE J1128 standard, the cable used on car oil pump was defined as thin wall cross-linked polyethylene insulated. And there are many basic requirements for the cable, majors are:appearance and properties of insulation material, test method for external diameter, mechanical properties of insulation material, properties which under cold bend, dielectric property of insulation material, flame resistance, fluid compatibility, ozone resistance, pinch resistance, sandpaper abrasion resistance, cross-linking performance, strip force, hot water resistance, insulation volume resistivity and resistance for environment cycling. Normally, besides the above requirements, many automakers have their special requirements and will make some real sample to do design verification and product validation, the cable must pass these tests.In 2006, there has a famous automaker recall more than 50 thousand cars in China, this was caused by the pump can not work well due to the cable insulation peeling, and the probability of occurrence is high. According to the regulation of the automotive industry, this automaker taken recall action after the failure reported. This automaker is a global company, their North America branch and Europe branch using their local cable supplier. Their North America and Europe supplier have been recognized widely and did not find any issue in the same application. In China, they were using a local supplier too, but the cable failed on the pump.In order to find out the root cause of this failure, we compared the import cables and local cable, major tests and analyses are:1) Simulation test in the fuel tank 500 hours, we used the three kinds of cable working 500 hours in the same condition. After this test completed, we found local cable peeling, and local cable has a noticeable fading. Meanwhile, the import two kinds of cable are well after this test.2) According to the standard of SAE J1128, we do a series tests. Found even local cable can meet some requirements but still have gap compared with import cables, and high temperature resistance & solvent resistance can not pass the test, but import cable can.3) FT-IR test for the three insulation materials. Found the main component of Germany sample and Chinese sample are cross-linked polyethylene, both of them contained some EVA. The main component of U.S sample is cross-linked polyethylene, no EVA found.4) SEM analysis for the three insulation materials. Inorganic component observed in all the three insulation materials, the content of inorganic component by percentage are:Chinese sample> U.S sample> Germany sample. At same time, we found the interface between polymer and inorganic filler of Chinese sample is clearly visible. While the polymer and inorganic filler of U.S sample has better integration. And the inorganic filler of Germany sample was completely embedded in the polymer matrix.5) TGA analysis for the three insulation materials. Test results show that all the three samples have residues after temperature reach 600℃, the residues of Chinese sample, U.S sample and Germany sample respectively are 37.4%,10.5% and 2.1%. These results matched with SEM observation. Additionally, from the results, we found the initial decomposition temperature of the three samples is also different.6) We have done EDX and XRD analysis for the three samples, we can get conclusion as below:the flame retardant of Chinese sample could be Mg(OH)2 and Al(OH)3, flame retardant of U.S sample could be SB-halogen system, flame retardant of Germany sample could be St2O3.7) DSC analysis for the three insulation materials. Melting temperatures of the three materials respectively are:100,122 and 127℃. And also, we can get below conclusion:the crystallinity of U.S sample and Germany sample respectively are 25% and 36%, while crystallinity of Chinese sample is very low.8) Test of cross-linked density for the three insulation materials. Solvent extraction method has been used; the results of Chinese sample, U.S sample and Germany sample respectively are 53.0%,61.4% and 77.3%.From these analyses as above, basically, we found the difference of the three samples. And also, we finally got the root cause of this failure.