An investigation on modification mechanism of CH₂O-modified low temperature coal tar pitches

Abstract

Rising oil price has brought huge cost pressure for low grade highway construction, and it is urgent to find alternative resources. At the same time, there are nearly 50000–60000 tons of low temperature coal output in inner Mongolia region, China, which has high toxicity and high polluting. To make the low temperature coal be applicable for road constructions, the formaldehyde is used as cross linking agent, the concentrated sulfuric acid is used as catalyst, and the chemical modification of low temperature coal tar pitch in Inner Mongolia region is investigated. The road performance (softening point, penetration and ductility) of modified low temperature coal are tested. Results shown that the road performance of modified low temperature coal is increased significantly. Modification mechanism of low temperature coal is studied by Scanning Electron Microscopy and other analytical tools. Results show that, in the modified low temperature coal, resin content increases and the resin fiber diameter becomes larger with the increasing of formaldehyde content.

Keywords

low temperature / coal tar pitch / modification / road performance

Introduction

In recent years, the contradiction between supply and demand of oil product in China is increasing fast. As one of the petrol product, bitumen is now widely used in pavement engineering area and also face the shortage. Coal tar is the product of coal carbonization in the absence of air condition, and the coal tar pitch is the remaining residue after distillation of coal tar which is extracted from the light oil, oil, heavy oil and anthracene oil. According to different degree of distillation, coal tar pitch is divided into three kinds: low temperature coal tar pitch, medium temperature coal tar pitch (soft coal asphalt) and high temperature coal tar pitch (coal tar pitch) [1]. The main performance of coal tar pitch is visco-elastic property in the usual natural environment and climatic conditions, it can bear the effect of wheel load and restore the deformation itself without any destruction. At higher temperatures, the coal tar pitch shows a significant fluidity and good plasticity [2]. Therefore, coal tar pitch can be used as a road engineering material, and it is widely used in road construction. Coal tar pitch also has some disadvantages, such as easy to age, high sensitivity, narrow viscoelastic temperature range, high toxicity, high pollution and so on, so it cannot be directly used for paving. The research on the modification of the coal tar pitch is mainly focused on three aspects: physical modification, chemical modification, and detoxification mechanism. Because of the different composition structure of coal tar pitch and petroleum asphalt, the pavement performance of coal tar pitch and petroleum asphalt are different. Mixing asphalt into coal tar pitch can improves its temperature stability, low temperature cracking resistance, higher friction coefficient of the pavement and some other properties [3]. Since the 1970s, many European countries such as France, Germany, Switzerland, Poland, etc. have begun to use coal tar pitch as road materials [4,5]. To improve the performance of coal tar pitch, the main method is selecting appropriate additives and cross-linking agent for chemical modification. Additives mainly contain rubber (SBS, SBR, chloride butyl rubber, etc.) and resin (polyethylene, polyvinyl chloride, resin, etc.) [6–9]. Among them, SBS is the best additive to improve the resist permanent deformation of asphalt [10]. The application of cross-linking agent in the modification of coal tar pitch are mainly focused on using resin [11]. The research aim is to better understand the modification mechanism of modified low temperature coal tar pitch although there have been many researches on asphalt-material pavement [13–21].

Based on the above research results, in this paper, the modification mechanism of low temperature coal tar pitch in Inner Mongolia area is discussed. The formaldehyde is used as the cross-linking agent, and concentrated sulfuric acid is used as catalyst. The low temperature coal tar pitch is chemically modified. The modification mechanism of formaldehyde on the low temperature coal tar pitch is studied by conducting Scanning Electron Microscope (SEM) experiments. The research target is to improve three main road performance indicators of the low temperature coal tar pitch (softening point, penetration and ductility), so that the modified asphalt can be used in most low grade highways in Inner Mongolia area.

Experiment

Reagents and instruments

The following regents are used in the experiments. The concentrated sulfuric acid is from Sinopharm Chemical Reagent Co., Ltd. The formaldehyde is from Sinopharm Chemical Reagent Co., Ltd. The low temperature Coal Tar Pitch (CTP) is obtained using the coal tar slag filtration from Inner Mongolia Wuhai Shenhua Mengxi Coking Plant. The chemical compositions of CTP are shown in Table 1.

The LD162 asphalt ductility testing machine (Beijing Chaoyang Luda Instrument) is used to obtain the ductility indicator. The LD164 asphalt penetrometer (Beijing Chaoyang Luda Instrument) is used to obtain the asphalt penetration indicator. The QUANTA FEG450 environmental scanning electron microscope by FEI Company (USA) is used for microscopic experiments.

Modified test and road performance test

In our research, the formaldehyde is adopted as the cross-linking agent to modify the low temperature CTP. The B-stage COPNA resin is generated under the catalysis of concentrated sulfuric acid (H₂SO₄) to improve the softening point of low temperature CTP. The morphology of low temperature coal tar pitch will therefore change from liquid state to solid state at room temperature. In our experiments, the low temperature CTP is used as raw material, formaldehyde is used as cross-linking agent, and concentrated sulfuric acid is used as catalyst. The testing materials are mixed according to a certain proportion. The mixture will be stirred, heated and cooled under the protection of nitrogen at different temperature. The testing methods of softening point, penetration and ductility is based on the standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) in China. Finally, the scanning electron microscope (SEM) experiments are conducted to observe the distribution of synthetic resin of crosslinking agent in CTP. The nuclear magnetic resonance (NMR) is used to analyze the chemical shifts in coal tar pitch. The two test results are combined to study the modification mechanism of the modified asphalt.

A three-mouth flask is used as the reactor, as shown in Fig. 1. Different raw materials ratio and reaction conditions of the modified experiments are as follows:

(1) Cross-linking agent dosage effect test

With low temperature CTP as matrix and formaldehyde as cross-linking agent, formaldehyde is added to the coal tar pitch matrix in accordance with different mass percentage (0, 0.025%, 0.05%, 0.075%, 0.10%, 0.15%, 0.20%, and 0.30%). 5% concentrated sulfuric acid is added as catalyst. The mixtures are placed on the heating plate under the protection of nitrogen until the temperature reaches 130 °C. The high-speed stirring is then conducted for 3 hours. The road performance of the modified coal tar pitch is tested after the mixture is cooled to room temperature.

(2) Reaction temperature effect test

With low temperature CTP as matrix and formaldehyde as cross-linking agent, 0.075% formaldehyde is added to coal tar pitch matrix. 5% concentrated sulfuric acid is added as catalyst. The mixtures are placed on the heating plate and heated to different temperatures (25 °C, 110 °C, 120 °C, 130 °C, 140 °C, 150 °C, 170 °C) under the protection of nitrogen. The high-speed stirring is conducted for 3h. The road performance of the modified coal tar pitch is tested after cooling to room temperature (25 °C).

(3) Reaction time effect test

With low temperature CTP as matrix and formaldehyde as cross-linking agent, 0.075% formaldehyde is added to coal tar pitch matrix. 5% concentrated sulfuric acid is added as catalyst. The mixtures are placed on the heating plate under the protection of nitrogen until the temperature reaches 130 °C. High-speed stirrings are conducted for 0, 1 h, 3 h, 4 h, 5 h, 6 h. The road performance of the modified coal tar pitch is tested after it is cooled to room temperature (25 °C).

Results and discussion

The effect of formaldehyde dosage on the performance of CTP is shown in Fig. 1. It is observed that with the increasing of formaldehyde dosage, softening point, penetration and ductility all change significantly. When CH₂O/CTP ratio is in the range of 0–0.075, softening point and ductility are increasing with the increasing dosage of formaldehyde. The penetration grade decreases with the increasing of the dosage of formaldehyde. When CH₂O/CTP ratio is 0.075, the softening point, penetration and ductility reaches maximum values, where the values of softening point, penetration and ductility are 54.6 °C, 60 °C, 600 °C, respectively. When CH₂O/CTP ratio is in the range of 0.075–0.3, softening point and ductility decrease with increasing dosage of formaldehyde. The penetration grade increases with the increase of the dosage of formaldehyde. The reason is that formaldehyde has the substitution reaction with the coal tar pitch, which links the active small molecules in the coal tar pitch to form a larger molecule. When the dosage of formaldehyde and coal tar pitch reaches a certain proportion, softening point, penetration and ductility of the modified coal tar pitch reach maximum value. When the dosages of formaldehyde and coal tar pitch reach over a certain proportion, the system contains excess cross-linking agent formaldehyde. Due to the plasticizing effect of cross-linking agent, softening point of the modified coal tar pitch decreases. Penetration grade can reflect whether the structure of the material is tight or not. A lower penetration grade means that coal tar pitch system reaches a more compact structure. So the best dosage ratio of formaldehyde is 0.075 (CH₂O/CTP).

For dosage ratio of formaldehyde 0.075 (CH₂O/CTP), the dosage of catalyst 5%, reaction time 3 h, the influences of different reaction temperature on the soften point, penetration and ductility of the modified coal tar pitch are shown in Fig. 3. It is observed that, softening point and ductility are increasing with increasing of reaction temperature. Penetration grade decreases with the increasing of reaction temperature. Consider that the functional groups during electrophilic substitution between formaldehyde and coal tar pitch is the carbonyl, where carbon atom in carbonyl has a positive charge, and oxygen atoms has a negatively charge, if the functional group linked with carbonyl is electron withdrawing group, the substitution reaction is easily occurring at this time. Otherwise, the electrophilic substitution reaction is not easy to occur. Due to the push electronic effect of methyl and phenyl, the carbonyl carbon atoms will be less electropositive the reaction activity of the carbonyl group is decreased, and the reaction speed becomes slowly. According to Arrhenius formula k=Ae^−Ea/(RT), the reaction rate increases with the increasing of temperature. In order to ensure that the reaction time is not too long, the reaction system must reach a high temperature in a certain degree. However, due to the aging characteristics of the coal tar pitch, the reaction temperature cannot be too high. Under the same reaction time (3 h), when the reaction temperature is 130 °C, the synthetic resin wrapped around the stirring rod, which indicates that electrophilic substitution reaction is almost finished.

For dosage ratio of formaldehyde 0.075 (CH₂O/CTP), the dosage of catalyst 5%, reaction temperature 130 °C, the effects of different reaction time on the soften point, penetration and ductility of modified coal tar pitch are shown in Fig. 4. It is observed that with the increasing of reaction time, softening point, penetration and ductility increase with the basically same trends. Due to the high activity of formaldehyde, under certain reaction temperature, the longer the reaction time, the more the electrophilic reaction will occur, resulting in high values of softening point, penetration and ductility. But beyond a certain time (3 h), electrophilic reaction has reached equilibrium reaction, and thus the prolonged time will not accelerate the reaction process. When the reaction time is 3h, the synthetic resin is wrapped around the stirring rod, which indicates that electrophilic substitution reaction is almost finished.

The above single factor test results show that the influences of modifier dosage, reaction temperature and reaction time on the performance of modified coal tar pitch. The orthogonal test design method is conducted to obtain the optimum content of modified coal tar pitch with best performance. The factors and levels of orthogonal experiment are shown in Table 2.

Since our main research target is to obtain the solid state CTP at room temperature, the softening point is selected as the research indicator. The orthogonal test scheme is shown in Table 3.

It can be observed from Table 3 that, the best program is A₂B₃C₁, that is,

A: dosage of formaldehyde, second level 0.075;

B: reaction time, third level 4 h;

C: reaction temperature, third level 140 °C.

It can be seen that the best design does not appear in the nine tests that have been done. The sixth test result is the closest to it, where the softening point is 54.9 °C. In the sixth test, only the reaction temperature is not at the best level, and the effect of the reaction temperature on the softening point is the smallest of the three factors. Meanwhile, consider that the higher reaction temperature coal tar pitch toxic will have a higher carcinogens volatilization, we finally choose that A₂B₃C₁ as the optimal solution, where the modification results is that the softening point is 54.9 °C, penetration grade 56 (0.1mm), ductility at 25 °C is 565 mm.

Modification mechanism analysis

The experimental results show that when the dosage ratio of formaldehyde is 0~0.075 (CH₂O/CTP), the performance of the modified coal tar pitch significantly increases, and the peak value reaches at 0.075. The scanning electron microscopy (SEM) experiments are conducted to observe the distribution of synthetic resin of cross-linking agent in asphalt.

For the dosage of formaldehyde 0, 0.025, 0.05, 0.075 (CH₂O/CTP), the morphology of modified coal tar pitch is observed by using SEM, as shown in Fig. 5. It is observed that the microstructures of modified coal tar pitch vary hugely under different dosage of cross-linking agent.

It can be seen from Fig. 5 that the non-modified low temperature coal tar pitch has discontinuity and delamination, while the modified coal tar pitch is not stratified with uniform structure and smooth section. The microstructure of non-modified coal tar pitch is shown in Fig. 5(a). Fig. 5(b) shows the morphology of modified coal tar pitch when the dosage ratio of CH₂O/CTP is 0.025. At this time, there exists fibril with about 10-2mm diameter. Fig. 5(c) shows the morphology of modified coal tar pitch when the dosage ratio of CH₂O/CTP is 0.050. At this time, there exists fine fiber with 10-1mm diameter. Figure 5d shows the morphology of modified coal tar pitch when the dosage ratio of CH₂O/CTP is 0.075. At this time, there exists micro fiber with about 1mm diameter. Due to the active small molecules in coal tar pitch, long chain fiber generates through the carbonyl crosslinking in formaldehyde. Obviously, when the dosage ratio of CH₂O/CTP is less than 0.075, the optical structure of the modified coal tar pitch is better; when the dosage ratio of CH₂O/CTP is greater than 0.075, the optical structure of the modified coal tar pitch becomes poor. The coal tar pitch after CH₂O modification has a relative high molecular weight and provides an appropriate environment for molecules to do migration and rearrangement.

In order to analyze the chemical reaction of the modified coal tar pitch with CH₂O (the dosage ratio is 0.075 CH₂O /CTP), the modified coal tar pitch is analyzed and compared with non-modified coal tar pitch by 1H-NMR Spectrum analysis (as shown in Fig. 6(b). Fig. 6(a) shows the FT-IR spectrum of the non-modified coal tar pitch, and Fig. 6(b) shows the FT-IR spectrum of the modified coal tar pitch. 2790–3000 cm⁻¹ is the stretching vibration absorption peak of the aliphatic C-H, and 3000–3130 cm⁻¹ is the stretching vibration absorption peak of the aromatic C-H. The vibration absorption peak of the aromatic ring skeleton C= C is around 1618.6 cm⁻¹, and there exists deformation vibrations of saturated C-H near 1 398 cm⁻¹. There are two and three adjacent aromatic C-H absorption peaks which are not too obvious in the fingerprint region of 870 cm⁻¹ and 810 cm⁻¹; there is obvious four adjacent aromatic C-H absorption peaks around 750 cm⁻¹, indicating that there are different aromatic substitution types existing in the coal tar pitch. The absorption peak intensity is enhanced in the curve near 2922 cm⁻¹. The methylene C-H stretching vibration peak is appeared around 2900 cm⁻¹. The characteristic absorption peak of the benzene ring occurs around 1262 cm⁻¹ which illustrates that there are cross-linking reactions between the formaldehyde and small molecular in CTP. It can be seen that the absorption peak is enhanced near 1620 cm⁻¹ and benzene fingerprint region, which indicates that the substitution type of aromatic ring has changed. In addition, the absorption peaks of the sulfuric acid are shown near 1226 cm⁻¹, 1180 cm⁻¹, 1126 cm⁻¹, 1035 cm⁻¹, which indicates that there are some sulfuric acid retain in the modified coal tar pitch after modification.

Conclusions

In this paper, the low temperature coal tar pitch (CTP) in Inner Mongolia Region (China) is modified with CH₂O. The optimum modified low temperature coal tar pitch is obtained by orthogonal experiment. The morphology of modified coal tar pitch was analyzed by conducting SEM experiments and FT-IR experiments scovered that when the dosage ratio of CH₂O/CTP is 0.075, the formaldehyde in system occurs crosslinking reaction with the aromatic ring and formaldehyde in coal tar pitch, forming a high molecular by thermal polymerization crosslinking without residual formaldehyde, so the performance of modified coal tar pitch at that ratio is best.

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