Feng Shaoguang 1, Wu Fangxuan 2
1. China Petroleum Pipeline Science and Technology Research Center
2. Zhejiang University School of Construction Engineering
Abstract: In view of the long evaluation period of anti-corrosion coating performance of steel structure across the sea bridge, a rapid test technology based on differential scanning heat method (DSC), thermal weight method (TGA) and infrared spectral analysis (FTIR) is proposed. Using these three techniques, 5 batches of samples were randomly selected from 5 epoxy powder products for comparative testing, and the data such as reaction heat release, glass temperature, decomposition mode, filler content and infrared spectroscopy were obtained and analyzed. The results show that the DSC analysis should be used to determine the mass consistency of each batch of epoxy powder, the thermal change of epoxy powder should not exceed ±5J/g and the temperature change of glassing should not exceed
degrees C; Zhoushan Port main channel project using the recommended DSC and FTIR testing technology, has completed 11 batches of epoxy powder raw material quality rapid determination, for anti-corrosion coating quality to provide a guarantee.
key words: cross-sea bridge; steel structure; epoxy powder; corrosion-resistant coating; differential scanning heat method; thermal heavy method
infrared spectral analysis; Quality control
Cross-sea bridge is generally near the mouth of the sea, the atmosphere is humid, chloride ion content is high, the environment is very harsh, the bridge steel pipe piles and steel guards and other steel structures caused a serious corrosion threat. In addition, the corrosion risk of early damage to reinforced concrete due to corrosion of rebar, and the resulting economic and safety issues require special attention to be paid to concrete structures exposed to corrosive environments. Cross-sea bridge steel structure corrosion protection is usually used to increase the metal protective layer, apply epoxy resin coating, apply cathode protection and so on. With the continuous development of steel structure corrosion protection technology, as an organic coating corrosion prevention technology to prevent metal corrosion, has been gradually developed from the beginning of the paint performance is more excellent, more cost-effective and automated spraying of fused epoxy powder. At present, the fused epoxy powder coating has been used in many petroleum, water transmission and cross-sea bridge projects at home and abroad, and has achieved very good application results, greatly improving the corrosion-proof performance of steel structures.
present, steel pipe piles, steel guards and epoxy-coated rebar and other steel structures generally use the "Steel pipe melting epoxy powder external coating specifications" (SY/T0315-2013), "melting combined epoxy" Standards or specifications such as corrosion-resistant coating of powder coatings (GB/T18593-2010) and epoxy-coated rebar (JG/T502-2016) provide quality control of raw materials and coatings. Before the project is officially started, the performance of raw materials, coating process and anti-corrosion coating is often systematically tested and evaluated through process evaluation tests. However, when the project started, the contradiction between tight duration and long anti-corrosion detection cycle will be highlighted, there is an urgent need for rapid and accurate testing technology to determine the consistency of the quality of each batch of raw materials, in order to effectively prevent the occurrence of "false truth, second-best" problem.
in view of this, considering the actual needs in the construction of the project, this paper proposes a rapid detection technology based on differential scanning amount heat method (DSC), thermal heavy method (TGA) and infrared spectral analysis (FTIR), through 5 batches of samples from 5 products The comparison of data such as reaction heat release, glass temperature, decomposition mode, filler content and infrared spectra obtains the basis for determining the consistency of mass of each batch, and applies the research results to the main channel project of Zhoushan Port in Ningbo to confirm the feasibility of the rapid determination of epoxy powder quality.
1 Epoxy Powder Batch Quality Rapid Detection Technology
Curing reaction is the key to the formation of coating by coating of epoxy powder, heat release during curing can be indicated in epoxy powder can reflect whether the number of reactive groups meet the requirements, the ratio is correct and so on. Resin type and filler content are the basis of epoxy powder coating, the specific structure of epoxy resin and the appropriate proportion of filler addition to achieve the best coating performance, the resin decomposition curve obtained during heat reheating process can obtain accurate information about the resin structure and filler content. Infrared spectroscopy is the most commonly used fingerprint recognition technology for organic matter, based on the absorption of each base group with a specific wave number range, and the existence of the characteristic cluster is inferred by comparing the absorption spectrum of the measurement with the re-combination of the specific absorption. The consistency of the powder is judged by the evaluation of the similarity between different batch samples and the original epoxy powder base map. Based on the above characteristics of epoxy powder, a rapid detection technique based on differential scanning heat method (DSC), thermal heavy method (TGA) and infrared spectral analysis (FTIR) is proposed, which can be used separately or in combination.
selected the domestic cross-sea bridge project in use and representative of 5 kinds of epoxy powder samples (A-E), each sample randomly selected 5 batches, respectively, DSC, TGA and FTIR testing.
(1) test the reaction of epoxy powders with DSC analysis techniques to release heat (H) and glassing temperature (Tg). The test instrument is tested using U.S. TA company DSCQ2000, in accordance with Appendix B of the Technical Specification for The External Coating of Steel Pipe Melting Epoxy Powder. The test procedure is: (1) N2 atmosphere, heating rate of 20 degrees C/min, from (25±5) to (70±5) degrees C, and then cold to (25±5) degrees C±; The temperature of 5 degrees C warms up to (285±10 degrees C, then cools sharply to (25±5) degrees C, and (3) N2 atmosphere, the rate of warming is 20 degrees C/min, from (25±5 degrees C to (150±10 degrees C).
(2) test material decomposition mode, decomposition temperature, filler content and weight loss curve using TGA thermal decomposition technology. The test instruments are tested using THE company Q50 and are tested in accordance with Part 1: General Rules of the Plastic Polymer Thermal WeightIng Method (TG) (GB/T33047.1-2016). The test program is: N2 atmosphere, the heating rate is 10 degrees C/min, the test temperature range is room temperature to 1000 degrees C.
(3) uses an infrared feature map of the FTIRT scanning test material. The test instrument is tested using the Nicolet 6700 infrared spectrometer of the AMERICAN company Nicolet, and is tested according to the General Rules of Infrared Spectroscopy (GB/T6040-2019), with a scanning range of 4000 to 400 cm-1.
2 Test Results Analysis
2.1 DSC Test
Through DSC testing under specific curing conditions, heat release and glassing temperature of the coating when the epoxy group is mixed with the curing agent reaction can be obtained to determine whether the composition of epoxy powder and the structure of the raw material have changed. Since the cross-linking reaction of the thermosolytic resin is irreversible, the heat release during the chemical bond reaction can be recorded by the DSC. DSC testing of epoxy powder samples, typical epoxy powder and coating thermal properties curve as shown in Figure 1. As can be found from Figure 1: The peak of the epoxy powder curve is the heating peak when the epoxy powder cross-linked, and the peak area is 35.22J/g. The coating curve is the coating thermal characteristic curve obtained from the second scan after powder curing, from which the glassed temperature Tg of the coating after the epoxy powder curing can be determined to be 101.17 degrees C.
In order to study the DSC test to determine the thermal properties of epoxy powder and the repeatability and reproducibility of the glassed temperature of the cured coating, 5 batches of DSC test were randomly selected for each of the 5 epoxy powder samples selected. The heating test results of different batches of epoxy powder are shown in Table 1, and the glassed temperature test results of coating after curing of different batches of epoxy powder are shown in Table 2.
can be determined by Table 1: The average heating of samples A to E is 43.02, 35.08, 36.16, 35.57, 40.76J/g, respectively. The difference between the maximum and minimum heat discharges in different batches of samples A-E was 1.70, 0.84, 1.90, 1.66, and 1.04J/g, respectively.
can be found in Table 2: Sample A-E cured coating glassed temperature Tg average of 100.09, 100.56, 100.92, 100.57, 9.28 degrees C, respectively. The difference between the maximum and minimum glasses temperatures of the different batches of samples A-E was 0.43, 1.45, 1.16, 0.82, and 0.26 degrees C, respectively. Test results from 5 epoxy powder samples show that the repetition and reproducibility of heat release and glucification temperature between different batches of epoxy powders in the same sample are good. According to the above test results, taking into account the differences in sampling operation, different equipment, different operators, different quality control and evaluation levels in the actual engineering, the change in the heat of epoxy powder can not exceed ±5J/g and the change of glass temperature does not exceed ±5 degrees C, as the DSC quality control requirements of different batches of the same epoxy powder.
2.2 TGA test
tests the content of fillers in powders and the decomposition properties of resins to accurately control the amount of filler additions and resin properties of epoxy powders. The TGA curve of the 5 epoxy powder samples is shown in Figure 2. As can be found from Figure 2, the TGA curve difference of 5 epoxy powder samples is obvious. The decomposition mode of A-D4 samples is single-order loss, but sample E is double-order loss, indicating that the resin used in A-D4 samples is very different from sample E. The decomposition temperature of phase 1 of each sample shows that the structure of the A and B resins is similar, and the structure of the C and D resins is similar, but there are also significant differences between the two categories. Overall, the resin systems of the five epoxy powder samples are different.
From the last remaining mass percentage of epoxy powder (see Table 3), it can be seen that the amount of filler addition varies greatly from sample to sample C, with the largest amount of filler addition of sample C, about 46.13wt., and the least addition of sample A, about 16.61 wt.. Sample B and sample E have filler ratios of 39.39wt.% and 40.72 wt., respectively, and the data are very similar, but sample B has only 1 decomposition (the temperature at the fastest weight loss rate is 417.50 degrees C), while sample E has 2 decompositions (404.08 degrees C and 698.60 degrees C, respectively), indicating a significant difference between the two samples. It can be seen that the TGA test not only pay attention to the amount of filler addition, but also need to compare the decomposition mode of the resin (single-order loss or multi-order loss) and the maximum weight loss rate when the temperature.
2.3 FTIR test
DSC and TGA analyze the reaction properties and composition characteristics of epoxy powders, which can be achieved through FTIR test analysis if differences between powders need to be distinguished from the microscopic function groups. According to the General Rules of Infrared Spectroscopic Qualitative Analysis (GB/T32199-2012), the attribution of each sample peak is determined by comparing the relative strength of the spectral bands. If the spectra of the sample to be tested are consistent with the control spectra of the powder at the time of the initial process assessment, two compounds can usually be determined to be the same substance, and if the two spectra are different, two compounds can be determined to be different. The infrared spectral contrast curve of the five epoxy powder samples is shown in Figure 3. From Figure 3, it can be found that there are more obvious differences between the sample spectra. Therefore, the earliest confirmed by process assessment of epoxy powder scanning infrared spectra, and as the basis of this sample map, in order to facilitate the comparison of subsequent samples, at the same time, the location and strength of the characteristic peak and the basic map in line with the criteria for evaluation.
3 Engineering Application
Ningbo Zhoushan Port Zhu Channel Road Project connects Zhoushan Island to Lushan, the main bridge of the sea area is 16.347km long, of which the main bridge of the non-navigation hole bridge is prefabribriated with 70m full hole, the whole hole is set up, and the non-navigation hole guide bridge is made of 62.5m pre-stressed concrete pre-made box beams. The non-navigation hole main bridge and the non-navigation hole guide bridge foundation are based on large diameter, ultra-long steel pipe piles. The steel pipe piles, steel guards and rebar used in this project use epoxy powder extensively as an anti-corrosion coating. Epoxy powder batch quality control, you can increase the detection frequency to improve the corrosion quality control of steel structure, but the detection cycle is long, if the performance of each batch of powder will be all tested before put into use, will greatly affect the progress of the project. Therefore, the rapid confirmation of the quality of raw materials in each batch is the key to ensure the progress of the project and ensure the quality of the project. The rapid detection technology proposed in this paper is applied to the quality control of the bridge epoxy powder batch to verify its effectiveness and feasibility.
Epoxy powder batch quality control, select a representative sample for analysis and testing, the results as a sample base map, each batch of entering materials and sample base map comparison, where the basic map results of the master sample is not consistent, as non-conforming materials. The three analytical techniques can be used separately or in combination. When using DSC test analysis, it should meet the ±etonic oxygen powder heat change does not exceed ±5J/g and the glass temperature change does not exceed ±5 degrees C";
the bridge uses the recommended DSC and FFTIR test analysis techniques to further improve the accuracy of the analysis results by adding TGA tests when test results are disputed. The testing and analysis of DSC and FTHR were carried out on 11 batches of powder samples randomly sampled, which effectively solved the possibility of raw materials being "charged in second place", and achieved the goal of rapid quality testing of epoxy powder batches, which provided a guarantee for the quality of corrosion protection.
Quality control of epoxy powder batches in steel structures across the sea bridge is the key to ensuring long-term safe service of the bridge, and a DSC, TGA and FTIR analysis techniques for determining the consistency of the quality of epoxy powder batches are proposed, which can be used separately. Combined use, and related testing and application research, the following conclusions were drawn:
(1) when using DSC testing technology, the thermal change does not exceed ±5J/g and the glass temperature change does not exceed ±5 degrees C, the quality of epoxy powder is determined to be consistent.
(2) when using TGA testing technology, the resin decomposition mode is the same and the filler content should not change by more than 5%, and the epoxy powder quality is determined to be consistent.
(3) the position and strength of the feature peaks are in line with the basic map when FTIR testing techniques are used