Preparation and characterization of low-temperature coal tar toughened phenolic foams

CHENG Jin-yuan ，LI Zhan-ku ，YAN Hong-lei ，LEI Zhi-ping ，YAN Jing-chong ，REN Shi-biao ，WANG Zhi-cai ，KANG Shi-gang ，SHUI Heng-fu

（School of Chemistry & Chemical Engineering, Anhui Key Laboratory of Coal Clean Conversion and High Valued Utilization,Anhui University of Technology, Ma'anshan 243002, China）

Abstract: In this study, coal tar-based phenolic foam (CPF) was prepared using low-temperature coal tar as raw material to partially replace phenol. The chemical structure, apparent morphology, compressive strength, thermal stability, flame retardancy and thermal insulation properties of CPFs were characterized. The results show that CPFs have similar chemical structures to conventional phenolic foam. Comparing with conventional phenolic foam, the compressive strength of 30%CPF and 40%CPF increases by 18.3% and 55.9%, and the pulverization rate decreases by 22.9% and 50.8%, respectively. The results indicated that toughness was significantly strengthened due to the incorporation of aliphatic structures such as alkylphenols. In addition, the thermal stability of CPFs in the low temperature stage also improves. Although the limited oxygen index of CPFs decreases and thermal conductivity of CPFs increases, they still maintain good flame retardancy and thermal insulation properties. The obtained results prove that low-temperature coal tar can significantly replace phenol to prepare phenolic foam with good performance,which provides a new idea for the high-value utilization of low-temperature coal tar.

Key words: low-temperature coal tar；phenolic foams；toughness；characterization

A huge amount of low-temperature coal tar (more than 3.5 Mt/a) is produced by coking, liquefaction, and pyrolysis of low-rank coals in China[1,2], especially in Yulin[3]. With the fast development of advanced coal chemical industry, an increase in the yield of lowtemperature coal tar is expected continually. Due to the existence of massive compounds containing aromatic rings and heteroatoms in low-temperature coal tar[4,5],the high-value utilization of low-temperature coal tar into fuels by catalytic hydrogenation[6,7], chemicals such as phenols by separation[8-10], and carbon materials such as needle coke by carbonization[11,12]has attracted much attention. However, some problems such as coking,poisoning, and deactivation of catalysts exist during the catalytic hydrogenation of low-temperature coal tar. On the other hand, the conversion of low-temperature coal tar to fuels may not meet the goal of carbon neutrality in China. In addition, only heavy components in lowtemperature coal tar can be utilized by producing needle coke. By contrast, separating phenols as feedstocks seems to be more promising. However,conventional acid-alkali washing method for separating phenols consumes huge acids and alkalis and produce massive waste water. Although novel separation approaches using ionic liquids or deep-eutectic solvents have been developed, the efficiency and recovery of solvents still faces challenge. Producing phenolic resin[13]or carbon materials[14]without purification are the alternative ways for utilizing low-temperature coal tar. For example, Pyshyev et al.[13]prepared coal tarbased phenolic resin and used to modify the road bitumen. However, most researches on coal tar-based phenolic resin focus on binders rather than phenolic foam.

Phenolic foam is a closed-cell rigid polymeric foam prepared from phenolic resin. Due to the excellent properties of flame retardance, thermal stability, and thermal insulation of phenolic foam,the phenolic foam is considered to be a leading insulation material in the construction industry[15-17]. At present, the output of phenol resin in China exceeds 1 Mt/a[16]. Until now, about 95% of phenol for phenolic resin/foam comes from petroleum via the conventional cumene process, resulting in environmental pollution and high cost[18,19]. Therefore, it is necessary to develop a more environmentally friendly and cost-effective substitute to the petroleum-based phenol.

In recent years, biomass-derived phenol is widely used to substitute petroleum-based phenol for preparing phenolic resin/foam[20-23]. Although bio-based phenolic resin/foam has received a certain success, it is difficult to produce in large scale for the seasonality and diversity of biomass. In our recent work, the hightemperature coal tar was successfully used to prepare phenolic foam, which presented with good performance[24]. However, due to the low content of phenols in high-temperature coal tar, the substitution rate of phenol is only limited to 20%. Based on the characteristics of low-temperature coal tar rich in phenols, it is expected to be more promising for replacing phenol to prepare phenolic foam. In addition,low-temperature coal tar contains many alkyl substituted compounds, which can toughen the phenolic foam[24,25].

In this study, the foamable phenolic resin was synthesized from low-temperature coal tar, phenol and formaldehyde, and coal tar toughened phenolic foam(CPFs) with different substitution rates were prepared by foaming process. The components of lowtemperature coal tar were analyzed by gas chromatography-mass spectrometry (GC/MS), and the chemical structure of phenolic foam was characterized by Fourier transform infrared spectroscopy (FT-IR).The apparent morphology, compressive strength,pulverization rate, thermal stability, flame retardancy and thermal insulation of CPFs were studied.

1 Materials and methods

1.1 Materials

Low-temperature coal tar was collected from a coking plant of Shenmu, Yulin City, Shanxi Province,China. Phenol, formaldehyde (37%), NaOH,isopropanol, acetic acid, Tween-80,n-pentane, andptoluenesulfonic acid were purchased from Sinopharm Chemical Reagent Co., Ltd. All commercial chemicals with analytical reagents were used directly.

1.2 Pretreatment of low-temperature coal tar

The mixture of low-temperature coal tar and isopropanol (1∶20) was sonicated for 10 min, before being centrifuged at 10000 r/min. Then the upper liquid was taken out and some impurities were removed by filtration, and the solvent was removed by a rotary evaporator under reduced pressure at 55 °C. The yield of pretreated low-temperature coal tar is approximately 70%.

1.3 Synthesis of low-temperature coal tar phenolic resins (CPRs)

Low-temperature coal tar has a lower reactivity than phenol, therefore the amount of formaldehyde used in the preparation of CPRs was reduced, which can also decrease free formaldehyde in the CPFs[26].The formulation of CPRs is shown in Table 1. Phenol,low-temperature coal tar and NaOH aqueous solution were sequentially added to a three-necked flask, which was heated to 85 °C and held for 0.5 h. Formaldehyde was added in 2 steps. The reaction mixture was cooled to 65 °C, and 70% formaldehyde was added dropwise with a constant pressure dropping funnel followed by reaction for 1 h. Then the temperature was raised to 85 °C and the remaining 30% formaldehyde was added dropwise followed by reactions for 2 h. Following that the temperature was lowered to 60 °C and the pH was adjusted to neutrality with acetic acid. Finally, water was distilled off under reduced pressure with a rotary evaporator at 55 °C to obtain CPRs with foamable viscosity. To facilitate elaboration, CPRs prepared with different substitution rates of phenol were denoted as 10%CPR, 20%CPR, 30%CPR, and 40%CPR. The fundamental characteristics of PR and CPRs are shown in Table 2.

Table 1 Formulations of PR and CPRs with different substitution rates

Table 2 Basic properties of PR and CPRs

1.4 Preparation of CPFs

In total, 50 g of resin, 2.5 g of Tween-80(surfactant), 5 g ofn-pentane (foaming agent), and 12.5 g ofp-toluenesulfonic acid (curing agent) were put into a paper cup and stirred quickly at room temperature. Then the mixture was moved into a preheated oven at 70 °C for foaming and curing for 40 min to obtain CPF. The CPFs prepared with corresponding CPRs were denoted as 10%CPF,20%CPF, 30%CPF, and 40%CPF.

1.5 Characterization

Low-temperature coal tar was analyzed by a Shimadzu QP2010 plus gas chromatograph/mass spectrometer (GC/MS), equipped with a HP-5ms capillary column and quadrupole analyzer with am/zrange from 33 to 550. The column was heated from 60 to 300 °C at 5 °C/min and kept for 10 min. The analytical method was set according to our previous work[27].

The viscosity of PR and CPRs was measured by a Thermo Fisher Scientific HAAKE Mars III rotary rheometer. The solid contents of the resins were determined according to GB/T 14732—2006.

The functional groups of PF and CPFs were analyzed by a Thermo Fisher Scientific NICOLET 6700 Fourier transform infrared spectroscope (FT-IR).The apparent density was determined according to the weight of the foams and the dimensions based on GB/T 6343—2009. The cell morphology of the foams was observed by an Olympus BX 41 optical microscope and a JVC charge-coupled device image sensor with 6.1 million pixels. A Hot Disk TPS 2500S thermal conductivity analyzer was used to measure the thermal conductivity of the foams. A NETZSCH STA449 F3 thermogravimetric analyzer was used to determine the thermal stability of the foams. In each run, about 10 mg of sample was put into a crucible and heated from 35 to 700 °C at a constant heating rate of 10 °C/min with an argon flow rate of 80 mL/min. The pulverization rate was obtained by pulling the foam with a known volume(～5 cm × 5 cm × 2 cm) 30 times on a 200-mesh sandpaper according to a previous paper[25]. The compressive strength was measured by a universal testing machine based on GB/T 8813 —2008. The limited oxygen index (LOI) was determined by a JF-5 oxygen index meter according to GB/T 2406.2—2009.

2 Results and discussion

2.1 Composition and polymerization of lowtemperature coal tar

The detected compounds in low-temperature coal tar with GC/MS can be classified into alkanes, alkenes,monocyclic arenes, dicyclic arenes, polycyclic arenes,phenols, other oxygenates, nitrogen- and sulfurcontaining compounds. As shown in Figure 1, the content of phenols in low-temperature coal tar is 20.39%, which is much higher than that in hightemperature coal tar[24]and its distillates such as washing oil and naphthalene oil. Different from hightemperature coal tar rich in other polymerizable species(e.g. indenes, styrenes, and benzofurans), the content of such compounds in low-temperature coal tar (Table 3 and Table 4) is neglectable. However, the maximum substitution rate of low-temperature coal tar is up to 40%, which is significantly higher than that of hightemperature coal tar[24]and its distillates. It is worth noting that when 20% naphthalene oil is used instead of phenol, it is difficult to foam due to the low viscosity of the resulting resin. Therefore, low-temperature coal tar is more suitable for replacing phenol to prepare phenolic foam.

Figure 1 Distributions of group components in lowtemperature coal tar

Table 3 Arenes with RC ＞ 0.2% detected in low-temperature coal tar by GC/MS

Table 4 Other oxygenates with RC ＞ 0.2% detected in low-temperature coal tar by GC/MS

As Table 5 demonstrates, most of the phenols have one or more active sites, indicating that many compounds in the coal tar have good reactivity with formaldehyde, such as 6,7-dimethylnaphthalen-1-ol, 3-cresol, and 3-ethylphenol. As Figure 2 displays, these phenols with different side chains can react with formaldehyde or PR to add flexible groups to the rigid structure of the phenolic resin, thereby improving the toughness of the foam. At the same time, other compounds with side chains such as ketones, esters,and alcohols can also improve the toughness of the foam. In addition, like formaldehyde, aldehydes could also react with unreacted phenol reducing the content of free phenol. Although most arenes and other oxygenates in low-temperature coal tar cannot polymerize under the reaction conditions, they can be encapsulated by “cage ” of phenolic resinviaπ-π interaction and hydrogen bonds[28]. Moreover, low boiling points compounds can be used as the blowing agent, further reducing the cost of the foam. The results imply that low-temperature coal tar has enormous potential for partially substituting phenol to prepare toughened phenolic foam.

Figure 2 Possible ways of some components in coal tar participating in polymerization of phenolic resins

Table 5 Phenols detected in low-temperature coal tar by GC/MS

2.2 Morphology

The photographs and micrographs of CPFs with different substitution rates are displayed in Figure 3.Obviously, conventional phenolic foam is pink, while with the increase of the substitution rate, the foam color begins to deepen to brown, which should be attributed to the existence of polycyclic aromatics in coal tar. The structure and morphology of the cells have a significant impact on the mechanical and thermal insulation properties of the foam[16]. As Figure 3 demonstrates, all the CPFs have closed cell structures, which can effectively reduce hot air convection, delivering good thermal insulation performance of phenolic foams. It can also improve the mechanical properties of the foams to a certain extent. However, with the increase of substitution rate, the size of foam cells varies and the uniformity decreases, which has a great influence on the mechanical properties of the foams. The reason may be that the low reactivity of coal tar reduces the curing rate of phenolic resin, and the unreacted organic compounds in coal tar also have a certain influence on the formation of cells.

Figure 3 Micrographs (up, 50 × ) and photographs (down) of PF and CPFs

2.3 FT-IR analysis

As shown in Figure 4, 3448 cm-1is -OH stretching vibration peak, and 2923 and 2855 cm-1are the C-H antisymmetric and symmetric stretching vibration absorption peak of methylene, respectively.The absorption bands around 1642 and 1483 cm-1are considered to be aromatic ring skeleton vibration.1037 cm-1is C-O stretching vibration peak in coal tar.In addition, 814 and 689 cm-1are aromatic C-H stretching vibration absorption peaks[29].

Figure 4 FT-IR spectra of PF and CPFs

As Figure 4 exhibits, PF and CPFs have similar absorption peaks, indicating that CPFs prepared by partially replacing phenol with coal tar have similar chemical structure with PF and further confirming that phenols are the major compounds in coal tar participated in the polymerization. However, CPFs and PF have some differences in absorption peak intensity.For example, the peak intensity of aromatic ring skeleton vibration and aromatic C-H stretching vibration increases with the increase of substitution rate. The result shows that some other aromatics in coal tar also exist in CPFs. The enhancement of C-H stretching vibration absorption peaks of CPFs indicates the introduction of flexible long-chains into the skeleton of CPFs. The results imply that coal tar can not only replace phenol in the synthesis of phenolic resin, but also toughen the resulting foams.

2.4 Compressive strength and pulverization rate

As Figure 5 demonstrates, all the foams exhibit two stages of deformation under different stresses. The initial stage is a higher slope curve, which is mainly from the elastic compression of the closed bubble in the foams and the stretching of the bubble wall. In contrast, the slope of the second stage curve is much lower, resulting from the rupture and compression of the closed bubble in the foams under high stress[30]. The maximum compressive strength of CPFs and PF at 10% strain was summarized in Table 6.

Figure 5 Stress-strain curves of PF and CPFs

Table 6 Basic characteristics of PF and CPFs

With the increase of substitution rate, the maximum compressive strength of CPFs also increases. The maximum compressive strength of 30%CPF (0.268 MPa) and 40%CPF (0.203 MPa) is much greater than that of PF (0.172 MPa). The one possible reason is the increase of foam density, which is generally proportional to the compressive strength[16]. With the raise of substitution rate, the viscosity of phenolic resin increases (Table 2),resulting in the gradual increase of foam density(Table 6). Notably, the maximum compressive strength of CPF from low-temperature coal tar is much higher than that from high-temperature coal tar[24], which should be ascribed to the worse reactivity of high-temperature coal tar than that of lowtemperature coal tar. The alternative reason may be that the compounds with long side chains in coal tar can enhance the toughness of CPFs, leading to more difficult for the foams to be broken under the elastic pressure in the first stage and thus improving the compressive strength. In addition, with the increase of substitution rate, although the uniformity of CPFs decreases (Figure 3) to a certain extent, the bubble wall increases gradually, which may also elevate the compressive strength of the foams. In a word, the compressive strength of CPFs meets the requirement of GB/T 20974—2014.

Compared with compressive strength, the pulverization rate is a more important index for PFs.The pulverization rates of CPFs and PF were measured as shown in Table 6. The pulverization rates of CPFs is lower than that of PF, and significantly decreases with substitution rate more than 10%. The result indicates that the introduction of coal tar reduces the rigid structure and improve the toughness of PF, which is mainly related to the compounds with long side chains in coal tar[25].

2.5 Thermal stability

Figure 6 is the TG and DTG curves of CPFs and PF in the temperature range of 35-700 °C. Obviously,the thermal decomposition of the foams could be divided into three stages (Table 7). The initial stage occurred at 40 to 200 °C with the weight loss rate of 5%. The main reason for this stage was the volatilization of residual moisture, foaming agent,free formaldehyde, and free phenol in the foams. The temperature for the second stage was mainly at 200 to 350 °C, and the weight loss rate increases due to the decomposition of foam surfactants and hardener and the release of water from ether bonds. The third stage is majorly at 350 to 700 °C, in which the foam structure was destroyed and phenolic hydroxyl groups are decomposed to produce volatile gases such as H2O, CH4, CO, and CO2. This stage is the main degradation stage of the foam[31]. As Table 7 exhibits,t-5%of CPFs increased with the increase of the substitution rate. When the substitution rate is 40%,t-5%of the phenolic foam is much higher than that of the conventional phenolic foam, indicating that the high substitution rate of coal tar can enhance the thermal stability of the phenolic foam at low temperature. In the second and third stages, the peak temperature of CPFs slightly decreases. In addition,the residual mass of CPFs at 700 °C is similar to that of PF, suggesting that CPFs still maintain good thermal stability.

Figure 6 TG (a) and DTG (b) curves of PF and CPFs

Table 7 TG and DTG analysis of PF and CPFs

2.6 LOI and thermal conductivity

For phenolic foam, flame retardation and thermal insulation are two very important properties for application in architectures. Thus, the LOI and thermal conductivity of PF and CPFs were measured. As exhibited in Figure 7, the LOI value of PF is 33.7%,which is higher than those of CPFs, indicating the decline of flame retardancy after introducing coal tar.The possible reason is the decrease of benzene rings on the skeleton chains by using coal tar to replace phenol.However, when the substitution rate is more than 20%,the LOI value of CPF increases due to the existence of anti-flaming elements in coal tar. Noteworthily, the LOI value of 40%CPF is higher than 30% (B1 standard according to GB 8624—2012), indicating the good flame retardation property of 40%CPF.

Figure 7 LOI and thermal conductivity of PF and CPFs

Low thermal conductivity can not only prevent heat transfer, but also reduce the thickness of the foam sheet and save production cost. There are three ways of heat transfer of adiabatic foam materials: gas passing through the pores, thermal radiation through the pores,and thermal convection of the gas in the cell[32]. As Figure 7 displays, the thermal conductivity of CPFs shows an upward trend with the increase of replacement rate, implying that low-temperature coal tar has a negative effect on the heat insulation performance of the foams. The possible reason is that with the increase of the substitution rate, the bubble rupture also increases (Figure 3), leading to the increase of thermal convection in the bubble and leading the rise of thermal conductivity of CPFs.Although the thermal conductivity of CPFs increases to some degree, they also have good thermal insulation performance.

3 Conclusions

CPFs with good performance were successfully prepared using low-temperature coal tar to replace phenol (up to 40%). Compared with PF, 30%CPF and 40%CPF have higher compressive strength and lower pulverization rate, indicating that low-temperature coal tar can improve the toughness and strength of phenolic foam. According to TG-DTG analysis, CPFs have better thermal stability than PF in the low temperature stage. Although the LOI values of CPFs are lower than that of PF, CPFs have good flame retardancy,especially 40%CPF. In addition, the thermal conductivity of CPFs increased slightly, indicating that low-temperature coal tar reduces thermal insulation performance of phenolic foam. Fortunately, CPFs still have good thermal insulation performance. In summary, low-temperature coal tar can be used as a substitution of phenol for producing phenolic foams satisfying Chinese national standards, which not only provides a route for preparing phenolic foams with low cost, but also provides an alternative way for the highvalue and low-carbon utilization of coal tar.

Acknowledgments

Authors are also appreciative for the financial support from the Provincial Innovative Group for Processing & Clean Utilization of Coal Resource.