Pectin Aerogels

Research Article pubs.acs.org/journal/ascecg

Thermally Insulating and Flame-Retardant Polyaniline/Pectin Aerogels Hai-Bo Zhao,*,† Mingjun Chen,‡ and Hong-Bing Chen*,§ †

Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric MaterialsSichuan, Sichuan University, Chengdu 610064, China ‡ School of Science, Xihua University, Chengdu, Sichuan 610039, China § Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621000, China S Supporting Information *

ABSTRACT: Biomass-based thermally insulating and flame-retardant polymer aerogels were fabricated from renewable pectin (PC) and aniline via polymerization−coagulation and a supercritical drying process. A special physical cross-linking action existed between PC and polyaniline (PA). The resultant aerogels showed three-dimensional networks with hierarchical pores and high surface areas (103−205 m2 g−1). With benefits from the cross-linking structure, the pectin-based aerogels exhibited good compressive strengths (4.7−9.2 MPa) and water resistance. The results from thermal conductivity measurements and thermogravimetric analysis revealed that these aerogels also had low thermal conductivity (0.033−0.038 W m−1 K−1) and considerable thermal stability. The limiting oxygen index, vertical burning tests, microscale combustion, and cone calorimetry tests further confirmed that the inherently low flammability of the aerogels could be achieved by the flame retardancy of PA and the cross-linking action between PA and PC. These aerogels with good mechanical properties, water resistance, and low thermal conductivity and flammability show promising prospects in the field of thermal insulation. KEYWORDS: Biomaterials, Aerogel, Mechanical properties, Thermal insulation, Flame retardancy

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INTRODUCTION Aerogels, characterized with low densities, high surface areas, and low thermal conductivities, are novel highly porous materials, which can be used in the fields of thermal insulation, Cherenkov detectors, batteries, supercapacitors, and catalyst supports.1−8 Various aerogels, such as silica, polymer, graphene, and carbon composites, have been fabricated via sol−gel processing and subsequent freeze-drying or supercritical drying. Among them, inorganic aerogels exhibit poor mechanical properties,9−11 while organic polymer aerogels, such as resorcinol−formaldehyde, are sometimes derived from expensive and toxic materials.12−14 Therefore, biopolymer-based aerogels from inexpensive biomaterials have attracted intense attention in recent years.9,15−17 As one of the natural polysaccharides, pectin (PC) is considered to be biodegradable, biocompatible, abundant, and nontoxic.18−20 During the past several years, pectin-based aerogels have been developed and used in the fields of drug delivery and thermal insulation.9,21−24 In particular, it is reported that pectin-based aerogels are superinsulating materials with a low thermal conductivity (about 0.02 W m−1 K−1).9 However, these aerogels tend to be flammable like most polymer foams (such as polyurethane foam) and have poor water resistance, which hinders their many applications.25−28 Unfortunately, very few reports are published on the flame retardancy of pectin aerogels. In our previous study, it was © 2017 American Chemical Society

found that inorganic nanoparticles such as magnesium hydroxide (MH), aluminum hydroxide (AH), and clay can provide polymer-based aerogels with good flame retardancy.29−32 However, these inorganic−polymer hybrid aerogels always exhibit high thermal conductivities with the increase of inorganic nanoparticle concentration.25 Zhang et al. fabricated MH−cellulose aerogels which possess considerable flame retardancy but high thermal conductivities (0.056−0.081 W m−1 K−1).25 A challenge, therefore, for pectin-based aerogels is the improvement of their fire resistance while keeping their low thermal conductivity. If possible, water resistance, low cost, and an easy fabrication process are desirable. In this article, biomass-based polymer aerogels were fabricated from renewable pectin and aniline via polymerization−coagulation and a supercritical drying process. Nonflammable polymer polyaniline (PA) is first used to modify biodegradable pectin-based aerogels. The physical cross-linking structure can be formed between polyaniline and pectin, which plays an important role in flame retardancy and water resistance for aerogels.27,33,34 The obtained aerogels exhibited considerable flame retardancy, water resistance, and low thermal conductivities. In this study, the structure and properties of Received: April 22, 2017 Revised: June 7, 2017 Published: July 14, 2017 7012

DOI: 10.1021/acssuschemeng.7b01247 ACS Sustainable Chem. Eng. 2017, 5, 7012−7019

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ACS Sustainable Chemistry & Engineering

Figure 1. Fabrication process of the PA/PC aerogels.

Figure 2. (a) Complex viscosity plotted against time at room temperature for the pure pectin solution, aniline/pectin solution, and the PA2/PC4 hydrogel. (b) Complex viscosity plotted against time for the aniline/pectin solution with the incorporation of ammonium persulfate (APS). method and Brunauer−Emmett−Teller (BET) theory, respectively, using Quantachrome data software. Rheological tests were carried out using an advanced dynamic rheometric expansion system (TA, ARES G2) at room temperature with a fixed frequency of 1 rad s−1. Water-contact-angle values of samples were measured using a contact-angle system JC2000D4. Compression testing was performed on a SANS CMT7000 machine. The compressive modulus was calculated by the stress−strain curve. Thermal stabilities were collected on a PerkinElmer STA6000 apparatus under nitrogen flow at a heating rate of 10 °C min−1. Vertical burning tests were conducted on the vertical burning test instrument (CZF), using 1 cm × 2 cm × 5 cm aerogel samples. The limiting oxygen index (LOI) tests were performed on an HC-2C oxygen index instrument, according to ASTM D2863. The flammability of the aerogels was tested on the FTT 0001 microscale combustion calorimeter. The combustor temperature was 900 °C. The burning behaviors were measured on a cone calorimeter using 10 cm × 10 cm × 1.5 cm samples at a heat flux of 35 kW m−2.

polyaniline/pectin (PA/PC) aerogels were investigated in detail.

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EXPERIMENTAL SECTION

Materials. Pectin with a methoxyl concentration of 33% was purchased from Yantai Andre Pectin Co., Ltd. Ammonium persulfate (APS), aniline (An), and ethanol were provided by Chengdu Chemical Industries Co. The rigid polyurethane foam (PU) was prepared according to our previous work.29 Fabrication of the PA/PC Aerogel. The PA/PC aerogels were fabricated from pectin and aniline via polymerization−coagulation and a supercritical drying process. Additionally, the detailed fabrication process of the PA/PC aerogels is shown in Figure 1. For preparation of the aerogel containing 1 wt % PA and 4 wt % PC referred to as PA1/PC4 aerogel, for example, 2 g of PC was first dissolved in 47.5 mL of deionized water. Then, 0.5 g of aniline was added into the PC solution with stirring for 4 h. After that, 2.5 g of APS was added into the above mixture with strong stirring at 0 °C. Subsequently, the mixture was kept at 4 °C for complete polyaniline polymerization. Further, the PA/PC gel was coagulated in the bath containing 50 vol % water and 50 vol % ethanol. The gel was immersed for 7 days for the removal of impurities, and the bath was changed every 24 h. Finally, the gel was exchanged with ethanol for 7 days and dried by supercritical CO2 for the acquisition of the PA1/PC4 aerogel. Characterization. Fourier transform infrared spectroscopy (FTIR) was performed using a Nicolet 6700 spectrometer. Densities were calculated from mass and dimension data of the aerogels. Morphological structures were characterized by a scanning electron microscope (SEM, Nova 600i). The porous attributes were evaluated by nitrogen physisorption tests using a Quantachrome Autosorb-1 device. The sample was first degassed for 8 h at 80 °C under vacuum before the adsorption experiment. The pore-size distributions and the surface areas were determined by the Barret−Joyner−Halenda (BJH)

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RESULTS AND DISCUSSION Structure Characterization. Pectin cannot form gel in water or organic solutions by itself, while a small amount of polyaniline (12.5 wt %) can cross-link with pectin to construct gel (PA0.5/PC4) via a coagulation process. The gelation mechanism of the PA/PC aerogel may be attributed to intermolecular hydrogen bonds between pectin and polyaniline, as shown in Figure 1.23,35 This conclusion is supported by the results of FTIR, in which the shift of the carboxyl signal (1724 cm−1) for the aerogel indicates a complex formation between the carboxyl of pectin and the protonated amino of polyaniline. The complex formation also can also be proven by other shifts of peaks for the aerogels from Figure S1 in the Supporting Information. Further, rheological tests were carried out for 7013


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Figure 3. SEM images for PA/PC aerogels: (a) PA0.5/PC4, (b) PA1/PC4, (c) PA1.5/PC4, and (d) PA2/PC4.

Figure 4. (a) Nitrogen adsorption and desorption isotherms and (b) Brunauer−Emmett−Teller (BJH) pore-size distribution for PA/PC aerogels.

investigation of the effect of polyaniline on the gelation of the aerogel. Figure 2a shows the complex viscosity (η) plots against time at room temperature for a pure pectin solution (P4), aniline/pectin solution (A2P4), and the PA2/PC4 hydrogel. It can be observed that the pure pectin solution and aniline/ pectin solution both exhibited very low values of 50 nm) were also found by this approach, in accordance with the results of the SEM observation. Combined with nitrogen adsorption−desorption and SEM analyses, the PA/PC aerogels contained mainly micropores, mesopores, and some macropores. This hierarchical pore feature may be important for low thermal conductivity and other potential applications. Meanwhile, the PA/PC aerogels also showed large Brunauer−Emmett−Teller (BET) surface areas (103−205 m2 g−1) and considerable total pore volumes (0.7−1.5 cm3 g−1). From Table 1, the surface area values for the PA0.5/PC4, PA1/PC4, PA1.5/PC4, and PA2/PC4 aerogels were 205, 146, 115, and 103 m2 g−1, respectively. The values of surface area and total pore volume decreased with the increase of PA content, probably because of the aggregation of PA. The high porosity with high surface area would play an important role in low thermal conductivity. Mechanical Properties and Water Resistance. The mechanical properties of the PA/PC aerogels were investigated by a uniaxial compression test. The compressive stress−strain dependence for the PA/PC aerogels is shown in Figure 5, and the detailed compressive-modulus data were summarized in Table 1. From Figure 5, under small compressive stress, the PA/PC aerogels exhibited linear elastic deformations, which determined the compressive moduli of the materials. These aerogels were not fragile like inorganic aerogels and exhibited a substantial toughness property. From Table 1, the compressive 7015


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thermal stability of aerogels. More importantly, the char residue from the thermal decomposition of aerogels increased significantly with increasing PA content, which was ascribed to the high charring ability of PA and the cross-linking structure between PA and PC.37 This carbonaceous residue would have a positive effect on the flame retardancy of the PA/PC aerogels. Flammability. The vertical burning test and limiting oxygen index (LOI) are classical methods for the evaluation of flammability of materials. Petroleum-based polymer insulation foams, such as polyurethane foam, are easily ignited, producing a large flame with a high fire hazard. In contrast, all PA/PC aerogels displayed a similar and low flammability. As shown in Figure 7, the PA2/PC4 aerogel generated a small flame when the igniter flame was in contact. No flame can be observed, and the aerogel exhibited only smoldering after the removal of the igniter. Also, all PA/PC aerogels displayed similar and high LOI values. When the oxygen concentration was as high as 28%, the aerogel could not be ignited. Additionally, the LOI value of polyurethane foam was only 22%. It was clear that PA/PC aerogels showed a high flame-retardant property in the small fire test. Microscale combustion calorimetry (MCC) is a new method for the investigation of the flammability of materials.38 During MCC testing, volatile products of materials are burned, and several parameters, such as heat release rate (HRR), heat release rate capacity (HRC), and total heat release (THR), are recorded. MCC results of all aerogels are presented in Table 3,

The thermal stabilities of the PA/PC aerogels were investigated by thermogravimetric analysis (TGA), as shown in Figure 6. The data, such as the onset decomposition

Figure 6. TGA thermograms for PA/PC aerogels in N2 atmosphere.

temperature and percentage of char residue, were listed in the Table 2. As mentioned above, pectin-based materials always Table 2. Thermal Properties of PA/PC Aerogels

a

sample (aerogel)

thermal conductivity (W m−1 K−1)

Tda (°C)

char residue (wt %)

PA0.5/PC4 PA1/PC4 PA1.5/PC4 PA2/PC4

0.035 0.034 0.033 0.038

217 220 210 217

25.3 26.8 28.1 32.0

Table 3. Flammability Data of PA/PC Aerogels from MCC Tests

Td is the temperature of 5 wt % loss from 150 °C.

have some water associated with them, and thus all PA/PC aerogels show a decomposition process with two weight-loss steps. From Figure 6, the first weight-loss step (∼10 wt % loss) could be observed up to about 100 °C, which was related to the loss of adsorbed water. The second weight-loss process beginning at about 200 °C was attributed to the thermal decomposition of the PA/PC aerogels. With the first loss of water being neglected, the onset decomposition temperature of T5% (defined as the temperature of 5 wt % loss) was used for the investigation of the thermal stability of materials. From Table 2, all PA/PC aerogels exhibited a similar thermal stability: T5% values of aerogels were in the 210−220 °C range, indicating that the PA content had little influence in the

sample (aerogel)

HRC (J g−1 K)

PHRR (W g−1)

THR (kJ g−1)

PA0.5/PC4 PA1/PC4 PA1.5/PC4 PA2/PC4

100 40 33 34

92 37.3 30 30

4.8 3.7 3.5 3.7

and the HRR curves are shown in Figure 8. The HRC value is the peak of HRR (PHRR) normalized to the heat rate, and lower values of PHRR and HRC mean lower flammability and fire hazard.38 From the HRR curves (Figure 8), the PA0.5/PC4 aerogel exhibited a sharp peak with a high HRC value of 100 J g−1 K. With the increase of PA content, the HRR peaks of the PA/PC aerogels became lower and broader, and the corresponding HRC values also significantly decreased: the HRC values for the PA1/PC4 aerogel, PA1.5/PC4 aerogel, and PA2/PC4 aerogel were 40, 33, and 34 J g−1 K, indicating lower flammability. Meanwhile, the incorporation of PA reduced both

Figure 7. Combustion process for PA2/PC4 aerogel during the vertical burning test: images during and after the 1st and 2nd ignitions. 7016


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the PA0.5/PC4 aerogel and PA2/PC4 aerogel were very similar, which may be caused by different fire testing conditions. Compared with the PA0.5/PC4 aerogel, the PA2/PC4 aerogel is more difficult to ignite in the cone test. However, heat from burning pectin will be sharply released in a short time, as shown in Figure 9a, when it is totally ignited in the macroscale fire condition. As a result, the PA0.5/PC4 aerogel and PA2/PC4 aerogel show similar PHRR values. The fire growth rate (FIGRA), which is determined by the ratio PHRR to the time to PHRR, is also considered an important parameter for the estimation of the fire spreading rate and fire risk. Lower FIGRA values mean a slower flame spread and longer time for people to escape a fire. From Table 4, the FIGRA value of PU was 12.9 W s−1, while the values for the PA0.5/PC4 aerogel and PA2/ PC4 aerogel were only 0.22 and 0.18 W s−1, respectively. The extremely low FIGRA values further prove that the PA/PC aerogel shows a very low flammability and fire hazard compared with organic PU foam. As for THR values, some interesting results can be observed. From Figure 9b, PU still gave a high THR value of 25 MJ m−2, while those of the PA/PC aerogels were much lower. Compared with the PA2/PC4 aerogel, the PA0.5/PC4 aerogel exhibited a rather higher total heat release, although they had similar and low PHRR and FIGRA values. This phenomenon may be attributed to a high THR value from burning pectin. A small amount of PA can slow the heat release rate during burning, leading to low PHRR and FIGRA values, but cannot effectively reduce the total heat release for composites. It was noted that the PA2/PC4 aerogel showed a very low THR value of 3.1 MJ m−2. This means that a large amount of PA incorporated in composites can effectively reduce both the HRR and THR. With the increase of PA content, the crosslinking action between PA and PC becomes stronger, bringing out higher residues. Thus, less fuel generation during combustion contributes to lower THR values, which is consistent with the TGA results. The cone test also provides some smoke parameters of samples. From Figure 9c, the total smoke release curves for the PA/PC aerogels were similar to the THR values: the value for the PA0.5/PC4 aerogel was as high as 45.1 m2 m−2, while that for PA2/PC4 was only 4.0 m2 m−2. The incorporation of PA can significantly reduce the TSR values of materials and decrease the harm from smoke produced in fire. The low smoke emission for the PA/PC aerogel should also be ascribed to the barrier effect of charring caused by the cross-linking structure between PC and PA. Consequently, the vertical burning, MCC, and cone tests strongly revealed that the PA/PC aerogels had a rather low flammability and that PA can effectively improve the flame retardancy of composites. On the basis of these data, the flameretardant mechanism of the PA/PC aerogels can be postulated. First, the nonflammability of PA and cross-linking structure between PA and PC make composites difficult to ignite in microscale (vertical burning and LOI) and macroscale (cone calorimeter) tests. Second, the char caused by the cross-linking action between PA and PC can prevent the transfer of heat and

Figure 8. Heat release rate of PA/PC aerogels in MCC testing.

PHRR and THR values of the aerogels: THR of the PA0.5/ PC4 aerogel was about 4.8 kJ g−1, while, for the PA2/PC4 aerogel, THR was only 3.7 kJ g−1. It is clear that PA can effectively reduce the HRC, PHRR, and THR of the PA/PC aerogels and decrease the flammability of materials in the MCC tests. The cone calorimeter test, as a classical fire testing technique that simulates real-scale fire conditions, was used for the characterization of the combustion performance of the PA/PC aerogels in a macroscale fire. The corresponding data, including time to ignition (TTI), total heat release (THR), peak of heat release rate (PHRR), fire growth rate (FIGRA), and total smoke release (TSR), are summarized in Table 4. The flammability data for rigid polyurethane (PU) were also listed as a comparison. TTI is used for the evaluation of the flame retardancy of materials, and a long TTI indicates that the testing sample is difficult to ignite.39 Most polymer foams have a short TTI during the cone test. As listed in Table 4, PU foam gave a very low TTI value of 3 s, showing a highly flammable nature. In contrast, the TTI of the PA0.5/PC4 aerogel was 41 s, and that of the PA2/PC4 aerogel was as long as 186 s. That means that the PA/PC aerogel is difficult to ignite during the cone test under 35 kW m−2 of heat flux, which is in accordance with the results of the vertical burning test. It was noteworthy that the PA/PC aerogel with higher PA content exhibited a longer TTI, indicating higher flame retardancy. The nonflammability of PA and strong cross-linking structure between PA and PC may be the key to the improvement of the TTI of materials. The peak of heat release rate (PHRR) is an important parameter which determines the flame spread rate for testing materials.40−42 As can be seen, PU foam had a very high PHRR value of 193 kW m−2, while those of the PA0.5/PC4 aerogel and PA2/PC4 aerogel were only 35 and 38 kW m−2, respectively. From Figure 9a, the PA/PC aerogels burned slowly with very low HRR values, indicating a low flame spread and fire hazard. Unlike the results of MCC, the PHRR values of

Table 4. Burning Parameters of PA/PC Aerogels from Cone Tests samples

TTI (s)

PHRR (kW m−2)

THR (MJ m−2)

FIGRA (W s−1)

TSR (m2 m−2)

PU foam control PA0.5/PC4 aerogel PA2/PC4 aerogel

3±1 41 ± 2 186 ± 5

193 ± 19 35 ± 3 38 ± 3

25 ± 2 14.8 ± 1 3.1 ± 0.3

12.9 ± 1 0.22 ± 0.01 0.18 ± 0.01

866 ± 50 45.1 ± 4 4.0 ± 0.5

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Figure 9. (a) HRR, (b) THR, and (c) TSR of PA/PC aerogels as a function of burning time in cone testing.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (51503191 and 51403192).

combustible volatiles, leading to low HRC, PHRR, and FIGRA values for MCC and cone calorimeter tests. Third, this char reduces the fuel and smoke generation during combustion and contributes to the low THR and TSR values.

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CONCLUSION We have successfully fabricated biomass-based thermally insulating and flame-retardant polymer aerogels from renewable pectin (PC) and aniline via polymerization−coagulation and a supercritical drying process. The resultant aerogels exhibited three-dimensional networks, hierarchical pores, and high surface areas (103−205 m2 g−1). Likely with benefits from the cross-linking structure between PA and PC, the aerogels showed good compressive strengths (4.7−9.2 MPa), water resistance, and considerable thermal stability. Because of a homogeneous organic component and high porosity, these aerogels also had low thermal conductivity (0.033−0.038 W m−1 K−1) and considerable thermal stability. Furthermore, LOI, vertical burning tests, microscale combustion, and cone calorimetry tests confirmed that the inherently low flammability of the aerogels could be achieved by the flame retardancy of PA and the cross-linking action between PA and PC. These aerogels with good mechanical properties and water resistance, and low thermal conductivity and flammability, show promising prospects in a range of applications, such as heavy metal ion adsorption, thermal insulation, and sensors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01247. FTIR spectra and water contact angles (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hong-Bing Chen: 0000-0001-8580-0231 Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest. 7018


Research Article

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Pectin Aerogels

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