Research Progress in Characterization Methods of Anti-corrosion and Wear-resistant Polyurethane Coatings

: Polyurethane is a coating material widely used in the field of anti-corrosion and wear resistance. At present, a lot of research work has been carried out in the research and development of polyurethane materials and process improvement. However, there are few researches on the characterization methods of polyurethane coatings, and suitable characterization methods are selected. Conducive to the improvement and application of materials. This article summarizes the characterization methods of polyurethane coatings from the aspects of morphology, structure, mechanical properties, electrochemical properties and thermodynamic properties. It is found that there is no standardized and unified evaluation standard for the electrochemical characterization methods of polyurethane; polyurethane as a high molecular polymer has a certain sensitivity to the service environment. Humidity and temperature have a great influence on its physical and chemical properties. Conventional characterization methods cannot perform microstructure characterization under the stress state of the material and the service environment. The establishment of standardized and unified electrochemical evaluation standards for polyurethane; the ability to characterize polyurethane under service conditions is an important research direction for improving the characterization method.


Introduction
Polyurethane [1,2] is made of polyisocyanate and polyether polyol or polyester polyol or small molecular polyol, polyamine or water and other chain extenders or crosslinking agents. The produced polymer has a unique soft and hard segment structure and a microphase separation structure. It is a polymer material with excellent performance. Compared with traditional metal and ceramic-based anti-wear composite materials, polyurethane coatings have better processing properties, wear resistance, erosion resistance [3 ， 4], low temperature flexibility [5], and lower manufacturing costs. It is used in protection fields such as rain [6,7], sand control [8], UV protection [9], and has broad development prospects. At present, domestic scientific researchers and related units have carried out research and improvement on the production and synthesis process of polyurethane, and have achieved obvious development: Zhang Weigang [10] prepared graphene-modified PU/Al composite coating by scratch coating method, which significantly improved the coating. Stability to salt water corrosion; Qiao Longyun [11] used halloysite nanotubes (HNTs) to encapsulate the inorganic corrosion inhibitor Ce3+, and prepared a controlled release nanocontainer, which was doped into a polyurethane base material to enhance the durability of the coating.
Although domestic scientific researchers have done a lot of research and development on the corrosion and wear resistance of polyurethane in recent years, my country's polyurethane industry started relatively late, and some of the core technologies in the polyurethane field are still in the hands of a few large companies [12], Skilled application of various characterization methods to analyze anti-corrosion and wear-resistant polyurethane is an important means to improve the international competitiveness of my country's polyurethane products. Only by fully understanding the properties of materials and establishing the relationship between structure, composition and performance can we better analyze the raw materials and product properties of polyurethane, control the polyurethane production process, explore the failure mechanism of coatings, and complete my country The transformation from a big country of polyurethane to a strong country of polyurethane, therefore, this article will briefly summarize the analysis and characterization methods of anti-corrosion and wear-resistant polyurethane coatings.

Morphology analysis method
In the process of in-depth research on the corrosion and wear resistance of polyurethane, scanning electron microscope (SEM) is the most widely used morphology observation tool. The analysis principle of SEM is mainly to use electron beams to scan the surface of the sample to be tested. The difference in morphology stimulates different morphological information, and the information is studied to obtain the microstructure morphology of the sample [13,14]. Analyzing the morphology of polyurethane before and after erosion by SEM and comparing the erosion resistance of polyurethane coatings with different compositions can help us to screen coating materials with better erosion resistance. Zhong Ping [15] used SEM to compare Observation of different samples before and after the erosion found that the polyurethane/polyurea hybrid system coating itself has a large number of bubbles, resulting in weak erosion resistance, while the pure polyurea coating itself has fewer bubble defects, erosion resistance and anti-erosion performance. The micro-cutting performance is better than that of polyurethane/polyurea hybrid system coatings. Using SEM to observe the morphology before and after the erosion to study the erosion behavior is helpful to explore the erosion mechanism of erosion particles. Gao Jin [16] studied the erosion behavior of sand particles with different sizes through the morphological changes of polyurethane coatings. :Under the same flow velocity conditions, most sand particles with larger diameters will directly impact, and most sand particles with smaller diameters will obliquely rub the coating. Using SEM to observe the corrosion degree of the polyurethane aging process can study the failure mechanism of the coating in the corresponding environment, and then predict the service life of the coating. Liu Jinhe [17] simulated the desert atmospheric environment on the acrylic polyurethane coating The layer is aged, and the microscopic morphology before and after the aging is observed by SEM. Before aging, the coating has no obvious defects under low magnification, and there are microscopic defects such as pores under high magnification. The anti-wind sand erosion performance of the coating is reduced, which weakens the coating of the pigment, causing a large number of white pigment particles to precipitate on the surface of the polyurethane and more erosion pits; Hao Yunhong [18] used SEM to observe the microscopic damage morphology of polyurethane coatings with different aging times, as shown in Figure 1. After multiple cycles of high and low temperature cyclic aging, cracks, peeling, and a large number of polyurethane coatings appeared. The temperature aging seriously reduces the smoothness of the polyurethane coating surface, which reduces the erosion resistance of the coating and shortens the service life of the coating. SEM has high magnification and spatial resolution, and the imaging is full of stereoscopic effect, and can be used in conjunction with X-ray energy spectrometer devices [19]. However, SEM also has many problems when processing polyurethane samples. The electron beam of SEM will damage the polyurethane samples. Polyurethane is a non-conductive material. In order to make the polyurethane surface emit secondary electrons, it is necessary to spray the surface of the polyurethane scanning electron microscope sample. A layer of precious metal particles, which will increase the difficulty of the operation process. If you do not need to study the failure mechanism of the coating, only observe the flatness of the coating surface and judge whether the coating fails, you can choose to use an Optical Microscope, Li Xu [20] use OM to observe different PTMGs PUA coating prepared with a molar ratio of N-330, it is found that polyurethane prepolymer with high N-330 content has a high degree of branching, which will lead to poor compatibility between the prepolymer and acrylic resin, which is not conducive to the formation of a good surface condition. coating. Cui Xiaofei [21] used OM to observe the aging morphology of the coating as shown in Figure 2. As the aging cycle increases, the smoothness of the coating surface decreases severely. During the 8-9 cycle of aging, the bubbles are serious and corrosives are precipitated, which can be preliminarily judged. Coating failure.

Structural analysis method
1) Fourier infrared absorption spectrum Fourier Transform Infrared Spectroscopy can qualitatively identify and quantitatively analyze the molecular structure according to the position and intensity of the infrared absorption peak [22,23,24,25]. Fourier infrared spectroscopy can be used to analyze the changes of polyurethane molecular structure and functional groups, which is an effective method for the modification of polyurethane coatings. The mechanism of the polyurethane modification process is more complicated. According to the corresponding relationship between the absorption spectrum and the substances in each stage of the reaction, the group changes in the substance can be inferred. According to the position and relative intensity of the infrared characteristic absorption peak, each group in the sample can be analyzed. The content of points. Liu Gang [26] In the study of porous nano SiO2/graphene oxide modified polyurethane coating materials, the analysis of graphene oxide modified polyurethane by Fourier infrared spectroscopy showed that: phthalic anhydride polyester polyol and polyphenyl polyisocyanate The high-viscosity polyurethane prepared as a raw material was successfully synthesized and there are obvious characteristic peaks in it, making the product molecular structure mostly benzene rings to form a higher-strength modified polyurethane; compared to the simple graphene oxide infrared spectrum curve, graphene oxide The hydroxyl group, carboxyl group and epoxy group are obviously present, which can effectively complete the modification of polyphenyl polyurethane by graphene oxide. In determining the degree of crosslinking and curing of polyurethane, infrared spectroscopy is also an important analysis tool [27]. -NCO group has a sharp absorption peak at 2300cm-1, and the absorption peak of -NCO group will inevitably decrease with the curing process of polyurethane. In the case of a known control sample, infrared spectroscopy can be used to make a preliminary judgment of the curing speed. Fourier infrared absorption spectroscopy is often used to analyze the reasons for the failure of polyurethane coatings. The construction of polyurethane on the contaminated substrate is an important reason for the failure of coatings. In general, as long as we use microscopy to observe the surface of the coating delamination Contaminants can be found, but some contaminants, such as lubricating oil, small molecule curing agents, etc., are difficult to observe with the naked eye and need to be analyzed by infrared spectroscopy. In addition, infrared spectroscopy can also be used to determine the mixing ratio of polyurethane materials. Polyurethane is a two-component coating material. Only by strictly controlling the mixing ratio can the polyurethane coating material with the best performance be obtained. The variety of cyanate resins and the multiplicity of -NCO group reactions greatly increase the difficulty of infrared spectroscopy to determine the mixing ratio.
2) Raman spectroscopy The theoretical basis for the qualitative analysis of the molecular structure of Raman spectra is that the Raman shift depends on the change of the molecular vibrational energy level. Different chemical bonds or functional groups have different vibration modes, which determine the energy change between their energy levels [28,29,30]. Fourier infrared absorption spectroscopy has the advantages of high light flux, low noise, fast measurement speed, high resolution, high wavenumber accuracy, and wide spectral range. It mainly reflects the functional groups of molecules. Raman spectroscopy mainly reflects the skeleton of molecules. Complement the results. Like infrared absorption spectroscopy, Raman spectroscopy can also be used in the modification of polyurethane. Hao Weihong [31] used Raman spectroscopy to study the resistance of modified graphene oxide/polyurethane coatings to wind and sand erosion. The ID/IG of GO was about 1.02, while the ID/IG of FGO was about 1.18. Functional groups will be grafted on the GO surface, which will increase GO defects and disorder, and the ratio of ID/IG will increase, indicating that the modification will graft functional groups on the surface of graphene and increase defects, and GO modification will be successful. Raman spectroscopy can also detect the progress of the polyurethane production reaction. In the process of industrial production of polyurethane prepolymers, the offline detection of the content of -NCO to judge the reaction process has huge safety hazards, and the specific situation cannot be grasped in time. Lu Jinxing [32] Using Raman spectroscopy and multivariate analysis methods based on angle measurement, a quantitative analysis model of -NCO groups in the synthesis process of polyurethane prepolymers was established, and a method for online monitoring of -NCO content was provided.
3) XRD phase analysis XRD (X-ray diffraction) phase analysis is a method for analyzing and determining the existence of each component based on the diffraction effect of polycrystalline samples on X-rays. Raman spectroscopy and Fourier infrared spectroscopy are important research methods to study polyurethane materials from a molecular perspective, but the factors that determine the properties of polyurethane materials are not only the chemical composition of the molecules, but also the way related atoms combine into molecules in space. In the research of anti-corrosion and wear-resistant polyurethane, XRD is mainly used to determine the existence of modified substances in polyurethane. Hao Weihong. [31] used XRD to analyze the crystal structure of FGO before and after modification and found that the order of the GO lattice structure was destroyed after modification, and the Si-O-Si bond grafted on the GO surface and increased The functional group increases the interlayer spacing, which is beneficial to improve the dispersibility of FGO. Ye Yinping [33] used XRD diffractometer to characterize nano-Ni powders when preparing Ni/polyurethane nanocomposite coatings. From the diffraction peaks, it can be determined that the Ni nanocrystals have a facecentered cubic structure. Wide and 2θ angles can calculate that the initial average grain size of Ni nanoparticles is 10nm, and the final Ni nanoparticles prepared are uniformly distributed in the polyurethane, and the particle size is 50nm, indicating that the magnetic and magnetic properties of the Ni nanoparticles are Surface activity produces agglomeration.

1) Macroscopic mechanics
When testing the macro-mechanical properties of polyurethane, the universal testing machine is an indispensable testing tool. The universal testing machine is a macro-mechanics testing instrument with high precision and stable performance that integrates stretching and shearing functions. It is mainly used to test the tensile strength and tear strength of polyurethane. Tensile strength is an important parameter affecting the wear resistance of polyurethane coatings [34]. Polyurethane is a typical soft-hard segment block polymer. Capacitiveness is closely related. Li Shuai [35] studied the relationship between the content of soft and hard segments and the tensile strength and found that: when the content of hard segments is low, as the content of hard segments increases, the physical cross-linking points inside the material gradually increase, and orderly hydrogen bonding With the increase of the degree, the tensile strength and elongation at break show an increasing trend; when the content of the hard segment is higher, the rigidity of the molecular chain becomes larger and the overall hydrogen bonding degree decreases significantly, which makes the orientation of the molecular chain difficult during the stretching process. Decrease in strength and elongation at break; Yang Yilin [36] and others used the tensile strength to characterize the self-healing ability of polyurethane. The crosslink density increased with the increase of the amount of HDI trimer, which increased the hard segment content of the prepared polyurethane material and improved the tensile strength performance. Wang Xinggang [37] conducted a tensile experiment on butyl polyurethane, and the results showed that polyisobutylene is a highly saturated molecular chain structure and cannot form intermolecular forces such as hydrogen bonds, which leads to an increase in the content of soft segments, which makes butyl polyurethane The tensile strength decreases. At present, there are many testing methods for the tear strength of polyurethane, but there are few reports on the tear strength of polyurethane alone. Tear strength and tensile strength are commonly used to characterize the mechanical properties of polyurethane together, and then characterize the ratio of soft and hard segments, crystalline properties and properties of polyurethane. compatibility. In the process of tearing experiment, the selected test standard is different, the shape of the sample and the tensile speed will also be different. The shape of the sample has a particularly significant effect on the tear strength. For a right-angled shape, the tear strength with cuts is lower than without cuts. The stretching speed also has a certain influence on the tearing behavior. It shows brittle failure at a higher stretching speed, and shows elastic failure at a lower stretching speed. It is generally concluded that the tearing strength will vary with The stretching speed increases and decreases. 2) Micromechanics Nanoindentation technology makes the load change continuously through the control of the program, and measures the depth of the indentation at the same time. It has the characteristics of simple operation, low applied load and high displacement resolution of the monitoring sensor. Therefore, it is in the measurement of the mechanics of polyurethane film materials. It has unique advantages in performance and can test the mechanical properties of polyurethane on the nanometer scale. Carroll [38] et al. used the nanoindentation method to show that reducing the indentation storage modulus of the polyurethane coating is beneficial to increase the shock wave dissipation. The literature on using nanoindentation technology to characterize polyurethane coatings is relatively rare at present, but this technology can be used as a reference for the research of polyurethane in the testing of other materials. Yan Xiaoxin [39] used the nanoindentation test technology to study and showed that with the increase of CB content, the elastic modulus and hardness of CB/PP composites increase. Among them, the elastic modulus increases significantly, and the hardness increase has obvious three-stage characteristics.

Electrochemical analysis method
1) Electrochemical impedance spectroscopy Electrochemical Impedance Spectroscopy (EIS) is an electrochemical testing technique with a wide frequency range and little disturbance to the system. The structure and performance of the sample are analyzed by measuring the impedance shape of the sample under different frequency sinusoidal AC voltages. It is one of the important technical means to evaluate whether the polyurethane coating fails [40]. It is believed that 10Ωꞏcm2 is the critical value of whether the organic coating has protective performance. Cao Chunan and Zhang Jianqing [41,42] established 6 different equivalent circuit models to analyze the coating system, and realized EIS data analysis by nonlinear least squares fitting. Liu Jinhe [17] and others used acrylic polyurethane coating to simulate the accelerated test of the desert atmosphere and used EIS technology to find that after the 8-cycle accelerated test, the low-frequency impedance modulus is close to the critical value, and the coating basically loses its protective performance. ,As shown in Figure 4. Song et al. [43] found that multi-walled carbon nanotubes (MWCNTs) can enhance the adhesion of acrylic polyurethane coatings on the surface of carbon steel through EIS technology and delay the peeling rate of the coatings. Zhu et al. [44] used EIS to find that during the aging process of acrylic polyurethane coatings in 3.5% NaCl solution and ultraviolet radiation, the rate of decrease in coating resistance and the rate of increase in coating capacitance and porosity were lower than that of aliphatic polyurethane coatings. 2) Electrochemical noise Electrochemical Noise Method (ENM) is a technology that uses the phenomenon of random and spontaneous fluctuations of potential or current on the electrode surface to reflect the information of the system from quantitative change to qualitative change. Iverson [45,46] discovered electrochemical noise for the first time, and it has made great progress in coating protection in recent years. Coating microcracks, coating blistering and changes in ambient temperature are the main factors that cause electrochemical noise. Jamali et al. [47] used the same working electrode to evaluate the anti-corrosion performance of different organic coatings by electrochemical noise and electrochemical impedance spectroscopy, and the results of the two test methods were consistent. Although electrochemical noise technology is relatively rare in the research reports of corrosionresistant polyurethane coatings, ENM, as an in-situ, nondestructive testing technology, has huge application potential in the research of polyurethane coatings.

Thermodynamic analysis method
Thermodynamic analysis is an analytical method for testing the relationship between the properties of sample materials and temperature under program-controlled temperature. In recent years, thermodynamic analysis technology and its development have greatly promoted the in-depth study of polymer coating materials. As the temperature changes, the structure and properties of polyurethane will also change. Therefore, the use of thermal analysis technology to study the evolution of the physical and chemical properties of polyurethane with temperature changes has unique technical advantages. The thermodynamic analysis methods commonly used in the research of polyurethane mainly include differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). 1) Differential scanning calorimetry Differential scanning calorimetry is a technology that measures the heat flow of the test volume relative to the reference material with temperature under the control of the program. DSC is an important tool for studying the polymerization, curing, cross-linking and decomposition reactions of anti-corrosion and wear-resistant polyurethane coatings. It is mainly used to analyze the change of the glass transition temperature Tg of polyurethane, When Dong [48] studied CB/TPU nanosynthetic coatings, they used DSC observation and found that the glass transition temperature of the coating increased with the increase of carbon black content. Li Tingting [49] found through DSC that the increase of the hard segment content in the polycarbonate-type highhardness polyurethane elastomer increases the degree of cross-linking of the polyurethane, increases the rigidity of the molecular backbone and affects the molecular motion of the soft polyurethane segment, which significantly weakens the soft and hard segments. The force between the two degrees increases the energy required for the soft segment to reach the activated state, which in turn increases the glass transition temperature of the polyurethane.
2) Thermogravimetric analysis Thermogravimetric analysis can measure the relationship between the quality of polyurethane and temperature under program control temperature. This method is highly quantitative and can accurately measure the change and rate of polyurethane quality, and understand the thermal stability of polyurethane by observing the weight loss of polyurethane. When Dong [48] studied CB/TPU nano-synthetic coatings, compared with pure TPU using TGA analysis, when the mass loss of TPU-12CB during thermal decomposition is 5% and 10%, the corresponding temperature increases respectively. Observing the DTG curve at 16.65°C and 13.56°C, it is concluded that the maximum thermal decomposition temperature of TPU-cycle 12CB is 36.32°C higher than that of pure TPU, and the thermal stability of the coating is improved. Guo Mingjie [50] and others have shown that when the content of MWCNTs is about 20wt%, the temperature at which the weight loss of mMMCs/WPU composite reaches 10% of its final value increases by 20℃, and the weight loss reaches half of its final value. The temperature at that time increased by 29°C. After being modified by mMMCs nanocomposite, the thermal stability of WPU has been significantly improved.

Outlook
Polyurethane as an excellent anti-corrosion and wearresistant polymer coating material has important research significance in various industrial fields. This article reviews some of the commonly used characterization methods in the research of anti-corrosion and wearresistant polyurethane coatings, such as the use of SEM to observe the micro-morphology of polyurethane, analysis of erosion and aging mechanisms; use of Fourier infrared absorption spectroscopy, Raman spectroscopy, and XRD Phase analysis is used to study the reaction process and material modification of polyurethane; the universal testing machine is used to measure the tensile strength and tear strength of the polyurethane material, and then the content of the soft and hard segments of the polyurethane is characterized, etc., the conventional polyurethane characterization method has been developed relatively mature, Its future research work should focus on the following two aspects: (1) Standardization of electrochemical evaluation standards. As an efficient characterization method for analyzing the structure and performance of polyurethane, electrochemical characterization technology has not yet established a standardized and unified evaluation standard, which is not conducive to the exchange and comparison of scientific research and production. Therefore, the establishment of standardization and uniformity for different purposes of polyurethane The standard of electrochemical evaluation is of great significance.
(2) Characterization of polyurethane in service state. Polyurethane as a high molecular polymer has a certain sensitivity to the service environment. Humidity and temperature have a great influence on its physical and chemical properties. Conventional characterization methods cannot perform microstructure characterization under the stress state of the material and the service environment. Therefore, it can Characterization in the service state of polyurethane is an important research direction to improve the characterization method.