Nonflammable Alginate Nanocomposite Aerogels Prepared by a

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Nonflammable Alginate Nanocomposite Aerogels Prepared by a Simple Freeze-Drying and Post-Cross-Linking Method Ke Shang,† Wang Liao,*,† Juan Wang,† Yu-Tao Wang,† Yu-Zhong Wang,*,† and David A. Schiraldi‡ †

Center for Degradable and Flame-Retardant Polymeric Materials, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, College of Chemistry, Sichuan University, Chengdu 610064, China ‡ Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States ABSTRACT: Nonflammable materials based on renewable ammonium alginate and nano fillers (nanoscale magnesium hydroxide, nanoscale aluminum hydroxide, layered double hydroxide, sodium montmorillonite, and Kaolin) were fabricated through a simple, environmentally friendly freezedrying process, in which water was used as a solvent. A simple and economic post-cross-linking method was used to obtain homogeneous samples. The microstructure of the cross-linked alginate aerogels show three-dimensional networks. These materials exhibit low densities (0.064−0.116 g cm−3), low thermal conductivities (0.024−0.046 W/m K), and useful mechanical strengths (0.7−3.5 MPa). The aerogels also exhibit high thermal stabilities and achieve inherent nonflammability with limiting oxygen indexes (LOI) higher than 60. Related properties were conducted and analyzed by cone calorimeter (CC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). These results combine to suggest promising prospects for use of these aerogel nanocomposites in a range of applications. KEYWORDS: aerogel, alginate, post-cross-linking, freeze-drying, flame retardant, thermal insulation

1. INTRODUCTION Foams are high-volume products used worldwide for packaging, insulation, cushioning, and floating applications based on their low densities and high porosities.1,2 Foam materials are almost always derived from unsustainable petroleum resources and pose high fire risks.3−6 The invention of aerogels7 has greatly widened the path for developing low-density foam-like products with novel and versatile functions.8−13 Among these novel materials, silica aerogels are prepared via a sol−gel hydrolysis of silicon alkoxides following with a supercritical drying step which avoids cell collapse due to high capillary pressures.14,15 Although the resultant aerogels possess extraordinarily low densities and are generally nonflammable, they exhibit poor mechanical properties and high production costs. On the other hand, organic aerogels are sometimes derived from toxic and expensive materials, such as melamine16 and resorcinol,17 which are not environmentally friendly and may not be economical for consumer applications. Aerogels derived from biodegradable and renewable resources are therefore highly desired.18−26 Extracted from abundant seaweeds, alginate has many merits such as nontoxicity, biocompatibility, and biodegradability. It is widely used in the biomedical materials as cell scaffords and carriers for controlled drug delivery.27−32 This oceanic polysaccharide is composed of randomly arranged mannuronic acid (M block) and guluronic acid (G block).33 Because of the © XXXX American Chemical Society

anionic and structural characteristics, G−G blocks between adjacent alginate chains can be cross-linked by various di- or trivalent cations, especially Ca2+, to form a hydrogel.34 Moreover, alginate is known as an inherently flame-retardant material with a limiting oxygen index (LOI) value of 48.0 and a peak heat release rate (PHRR) of 4.99 kW/m2 comparing favorably with the corresponding values of 20.0 and 168.75 kW/m2 for viscose fiber.35 This polymer, therefore, is highly promising for fabrication into low flammability aerogels.36 Inorganic nano/microparticles have attracted intense attention for efficiently enhancing the flame retardancy, thermal stability, and physical properties of the polymer matrixes.37−39 Layer nanoparticles have been demonstrated to be a catalyst while promoting a char-forming reaction. The superheated microenvironment in layered silicates traps decomposition products and provides additional carbonization pathways.40−42 Furthermore, because of the lower surface free energies, these nanoparticles migrate faster and accumulate easier at the surface of the combustion area than those behaviors of the polymer degradation products,43,44 thus acting as thermal insulators and Received: October 14, 2015 Accepted: December 16, 2015

A

DOI: 10.1021/acsami.5b09768 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Preparation of Alginate/Nanoparticle Composite Aerogel via a Freeze-Drying and Post-Crosslinking Methoda

a

Magnesium hydroxide (MH) is used as an example. magnesium hydroxide was first dispersed in 100 mL of deionized water at 14 000 rpm (A-555, INAYOU, China). Five grams of ammonium alginate was then added slowly with continuous stirring to produce a homogeneous gel. The gel produced from 5% ammonium alginate and 5% magnesium hydroxide solutions was poured into a mold and rapidly frozen using liquid nitrogen. The frozen samples were then freeze-dried using a VFD-1000 lyophilizer (Boyikang Co. Ltd., China) at −20 °C and less than 10 Pa. After 72 h, the samples were moved from the freeze-dryer to a vacuum oven with further drying to produce the final product. The resulting sample was named A5MH5, where A and the number 5 refer to alginate and percentage of these materials in water. The other samples are named A5ATH5, A5MMT5, A5Kaolin5, and A5LDH5, respectively. Preparation of Cross-Linked Alginate Aerogel. The A5MH5 aerogel, for example, was immersed in a saturated calcium chloride/ ethanol solution at RT for 6 h to ensure it could be cross-linked completely. It was then dried in a vacuum oven at RT overnight to eliminate ethanol. The resulted sample is identified as A5MH5Ca, in which “Ca” refers to the post-cross-linking process. Other post-crosslinking samples are named as A5ATH5Ca, A5MMT5Ca, A5Kaolin5Ca, and A5LDH5Ca, respectively. 2.3. Characterization. The limiting oxygen index (LOI) values were tested on an HC-2C oxygen index meter (Jiangning, China) according to ASTM D 2863-2009, and the dimension of all samples is 120 mm × 10 mm × 10 mm. The vertical burning test (UL-94) was performed on a CZF-2 instument (Jiangning, China) according to GT/T 8333-2008, and the dimension of all samples is 125 mm × 10 mm × 10 mm. The combustion behaviors were measured by a cone calorimeter device (Fire Testing Technology). Samples with a size of 100 mm × 100 mm × 10 mm were tested under a heat flux of 50 kW m−2. Thermogravimetric analysis was carried on a TG 209F1 (NETZSCH, Germany) thermogravimetric analyzer at a heating rate of 10 °C min−1 under N2. The densities were calculated from the mass and dimension measurements using mass measurements and digital calipers. Morphological microstructure of the samples was characterized with scanning electron microscopy (SEM, JEOL JSM 5900LV) at an acceleration voltage of 20 kV. The observed cross-section was prepared by fracturing in liquid nitrogen. Compression testing was conducted using an electronic universal testing machine (CMT4104, SANS Co. Ltd., China), fitted with a 10 KN load cell, at a crosshead of 15 mm min−1. The dimension of all samples is about 20 mm in diameter and height. The compression test samples were prepared in the columniform PP mold which is 20 mm in diameter and height. Thermal conductivity was measured by Hot Disk 2500-OT (Hot Disk, Sweden) at room temperature. The optimized parameters for measuring were: time of 80 s, heating power of 5 mW and sensor of radius 2.001 mm. X-ray photoelectron spectroscopy (XPS) were recorded by a XSAM80 (Kratos Co., U.K.), using Al Kα excitation radiation (hν1486.6 eV).

also mass-transport barriers to oxygen and flammable volatiles.45−47 As mentioned above, the combustion behaviors of polymer/ nanofillers aerogels have been studied; however, the mechanism is not fully understood. Cross-linking is another effective way to increase aerogel’s mechanical properties and thermal stability, helping it reach required fire safety standards and mechanical properties with a limited amount of matter. Pojanavaraphan et al.48 cross-linked natural rubber/clay aerogels with sulfur monochloride; the mechanical strength increased 26-fold compared to the control sample. Chen et al. reinforced poly(vinyl alcohol)/clay aerogels by cross-linking with divinylsulfone49 or γ-ray irradiation11 under an aqueous condition. The compressive moduli of aerogels increased 10− 29-fold with these cross-linking methods. However, crosslinking with a chemical reagent has inherent drawbacks, such as difficulty in obtaining a homogeneous suspension for high viscosity and leaving unreacted agents behind. Cross-linking by irradiation requires specific equipment to apply and to protect personnel from the high energy radiation, which is a restriction to preparation in an ordinary chemical lab and further scale production of the materials. For an alginate-based material, direct addition of Ca2+ leads to a fast process during the formation of a hydrogel and also an uncontrollable, heterogeneous infrastructure. The CaCO3-GDL system (D-glucono-δ-lactone), which was prepared by Kuo et al.,50 could slowly release Ca2+ to produce a structurally uniform gel. In this work, a more facile post-cross-linking method was applied to obtain homogeneous aerogels. Different kinds of inorganic nanoparticles, including hydroxides and clays, were introduced into these samples. The improved mechanical properties and thermal stabilities of the aerogels were studied, and the mechanism of flame retardancy was investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Ammonium alginate and calcium chloride were purchased from Bright Moon Seaweed Group (Qingdao, China) and Kermel, respectively. Magnesium hydroxide (MH) and aluminum hydroxide (ATH) were supplied by Albemarle corporation. Sodium montmorillonite (Na+-MMT; PGW grade, cation exchange capacity (CEC) 145 mequiv/100 g) was purchased from Nanocor Inc. Kaolin was received from Fengcheng, Shanghai. Layered double hydroxides (LDH) were synthesized by the coprecipitation method, following a procedure similar to that reported by Yun and Pinnavaia.51 All ingredients were used without further purification. 2.2. Aerogel Preparation. Preparation of Alginate Hydrogel and Aerogel. The preparation process of the aerogels is shown schematically in Scheme 1. To produce these aerogels, such as an alginate/magnesium hydroxide aerogel, for example, 5 g of nanoscale B


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Figure 1. Compressive stress−strain curves of the alginate/inorganic nanoparticle aerogels.

3. RESULTS AND DISCUSSION 3.1. Mechanical Properties of Aerogels and Their Morphology. Alginate aerogels were prepared with different nanofillers. Two different kinds of inorganic fillers were utilized in this study: hydroxides (MH, ATH and LDH) and clays (MMT and Kaolin). The performances of the aerogels with the addition of clays and hydroxides were compared. Density values of the aerogel products were calculated after measurements by a digital balance and a digital caliper. Neat alginate aerogels (A5) without cross-linking exhibit extremely low density values of 0.066 g/cm3. The samples gain some densities but still persist at very low levels, upon cross-linking. The increase of compressive moduli were more significant than those of the densities after being cross-linked. Compression tests were conducted to investigate the mechanical behaviors of the aerogels. Each sample was compressed to 75% of its height. As shown in Figure 1, the compressive stresses of the aerogels increase slowly below 60% strain. When the strain increased to more than a critical value, e.g. 60%, the stresses increased steeply. This behavior is consistent with a classic foam-like material that when applied stress induced large strain, the cellular structure of a aerogel would be broken, resulting in a closely contacted network skeleton. Table 1 summarizes the physical properties

significant physical reinforcement and that the reinforcing nanofillers are relatively immobile; little change could therefore be expected upon ionic cross-linking of such systems. Due to a major increase of strength in contrast to a minor gain of density, the specific densities were enhanced significantly by ionic cross-linking, demonstrating the effectiveness of the simple post-cross-linking method. To determine the relationship between structure and properties, the morphologies of the alginate/composite aerogels were examined via SEM (Figure 2). All of the aerogels

Table 1. Densities and Mechanical Properties for the Resulting Aerogels sample A5 A5Ca A5MH5 A5MH5Ca A5ATH5Ca A5LDH5Ca A5MMT5Ca A5Kaolin5Ca

density (g/cm3) 0.066 0.069 0.104 0.110 0.115 0.102 0.0931 0.109

± ± ± ± ± ± ± ±

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

modulus (MPa) 0.94 3.45 4.92 7.07 8.63 8.42 10.28 10.04

± ± ± ± ± ± ± ±

0.12 0.53 0.75 1.12 0.97 1.23 1.12 1.54

specific modulus (m2/s2) 14.2 50.0 47.3 64.2 75.1 82.5 110.4 92.1

± ± ± ± ± ± ± ±

2.3 6.5 5.7 8.7 6.3 7.8 9.8 8.6

Figure 2. SEM images of the alginate/inorganic nano particles aerogels.

of the aerogels. After being post-cross-linked by Ca2+, the compressive modulus of neat alginate aerogel increases 3.7-fold, from the 0.94 MPa value of A5 to 3.45 MPa of A5Ca. Comparing with A5, the compressive modulus of alginate/MH nanocomposite aerogel (A5MH5) increased 5.2-fold. The strength of the composite aerogel could be further enhanced by post-cross-linking, i.e. from 4.92 MPa of A5MH5 to 7.07 MPa of A5MH5Ca (1.4-fold). In those systems wherein ionic cross-linking brings about only small increases in mechanical properties, this could suggest that those systems already possess

examined possessed cellular microstructures with an obvious orientation that followed the direction of ice crystal growth.52 Inorganic nanoparticle fillers strengthened the aerogel by different ways. The hydroxides were found to embed in the polysaccharide network, likely stabilized by multihydrogen bonding, which could be responsible for the higher strength of A5ATH5Ca than that of A5MH5Ca. For the layered silicate, MMT, however, the nanoparticles could combine with alginate with multihydrogen bonding and result in a cocontinuous network structure, which is responsible for their higher C


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Figure 3. TGA weight loss and DTG curves of the alginate aerogels with different flame retardant agents.

of A5 to 9.76%/min of A5Ca and from 6.76%/min of A5MH5 to 4.73%/min of A5MH5Ca. 3.3. Thermal Conductivity. Low thermal conductivity is critical to most applications of foam-like materials. The measured thermal conductivity value for A5 was 0.025 W m−1 K−1, close to that of cellulose aerogels (0.029 W m−1 K−1 with density of 0.25 g cm−3)53 or polyurethane (0.017 W m−1 K−1 with density of 0.26 g cm−3).54 Addition of inorganic nanoparticles reduced the porosity of the aerogels and hence resulted in higher thermal conductivities. Measured values for A5MH5Ca, A5MMT5Ca, and A5Kaolin5Ca were 0.042, 0.039, and 0.037 W m−1 K−1, respectively. 3.4. Combustion Behavior. The LOI tests and UL-94 tests were carried out to investigate the flame-retardant behavior of the samples; results are listed in Table 3. Neat

strength. Viscous solutions may interfere with the cross-linking process and lead to an uneven structure. The reason to that end in an alginate gel may be caused by a fast gelation with direct addition of calcium ions. Therefore, a CaCO3-GDL system was developed to slowly release Ca2+ into the polysaccharide matrix.50 Compared with the aerogel structure produced by the sustained release system,36 alginate/MMT aerogel post-crosslinked by Ca2+ was more uniform and its reinforcement more efficient. 3.2. Thermal Stability. Figure 3 shows thermogravimetric analysis results of alginate aerogels, and the related thermal data are summarized in Table 2. As a hydrophilic polymer rich in Table 2. TGA Data of the Alginate/Flame Retardant Aerogels under N2 Atmosphere samples

Td 5% (oC)

Td max (oC)

dW/dT (%/min)

residue (%)

A5 A5Ca A5MH5 A5MH5Ca A5ATH5Ca A5LDH5Ca A5MMT5Ca A5Kaolin5Ca

229.8 220.9 263.0 255.6 233.6 177.7 140.1 249.2

286.3 292.5 287.4 281.1 276.5 292.3 266.2 278.1

11.1 9.76 6.76 4.73 7.54 5.75 3.64 6.80

20.26 18.42 45.41 48.18 43.30 32.49 54.86 42.40

Table 3. LOI Test and UL-94 Test Results of the Aerogels

hydroxyl and carboxyl groups, alginate aerogels easily absorb moisture even after carefully drying. So the slight initial weight loss may attribute to the loss of water. The onset decomposition temperature is defined at 5% of mass loss (Td 5%), which gives a measurement to the thermal stability. MH and ATH increase the thermal stability of aerogel for their higher decomposition temperatures than that of alginate. LDH and MMT appear to have lost their crystalline water before the decomposition of the materials, so these aerogels have lower Td 5%. For the sample of A5LDH5Ca, the peak at approximately 200 °C corresponds to the removal of unreacting interlayer water molecules and through reaction with interlayer carbonate anions in LDH. The char yield is also different between the samples. Table 2 shows that the A5MMT5Ca sample has the highest level of carbon residue in contrast to that of A5LDH5Ca with the lowest value. This difference could be attributed to a series of changes in LDH during the heating process, releasing significant amounts of water and carbon dioxide. MMT, by contrast, would only lose waters of crystallization without any oxidization or combustion in this process. The increase of thermal stability indicated by the decomposition rates change, which decrease from 11.1%/min

sample

LOI (%)

UL-94

A5 A5MH5Ca A5ATH5Ca A5LDH5Ca A5MMT5Ca A5Kaolin5Ca

47.7 >60 >60 >60 >60 >60

V-0 V-0 V-0 V-0 V-0 V-0

alginate aerogel (A5) is a nonflammable material with a high LOI value of 47.7%, which is only exhibited smoldering during combustion testing, and is assigned a UL-94 V-0 class. Flammability can be further suppressed by addition of the inorganic nanoparticles and cross-linking, and the measured LOI values are surprisingly higher than 60 for all nanocomposite aerogels; no flame appeared when lighting. The extremely high LOI values may be attributed to two reasons. First, decomposition of inorganic nanoparticles releases gas to dilute the concentration of flammable gas and inhibit heat transformations. The residue of the nanoparticles can also form a compact layer to prevent flame propagation. Second, Ca2+ could contribute to the flame retardant. Zhang et al.55 revealed that CaO or CaCO3 was generated during combustion of the samples, which has a similar effect with inorganic nanoparticles. Furthermore, Ca2+ provided an alkaline environment for the decarboxylation of alginate and promoted formation of inert CO2. Therefore, both the inorganic nanofillers and Ca2+ act as flame retardants for aerogels to impart notable flame retardancy upon the materials. The combustion behaviors of the aerogels were further investigated by cone calorimetry. The evolution of the heat D


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Figure 4. Heat release rate (HRR, a), total heat release (THR, b), rate of smoke release (RSR, c) and total smoke release (TSR, d) plots of alginate/ nanocomposite aerogels under a heat flux of 50 kW/m2.

Table 4. Burning Parameters of the Alginate/Flame Retardant Aerogels sample A5 A5MH5 A5ATH5 A5LDH5 A5MMT5 A5Kaolin5

TTI (s) no no no no no

49 flame flame flame flame flame

PHRR (kW/m2)

THR (MJ/m2)

TTPHRR (s)

FIGRA (W/s)

residue (%)

71.27 24.63 20.36 22.28 22.68 20.61

8.11 1.79 1.98 1.67 2.14 1.79

75 75 110 110 45 55

0.95 0.33 0.19 0.20 0.50 0.37

0.95 66 52 58 71 72

release and smoke release with time were exhibited in Figure 4, with corresponding key parameters, including the time to ignition (TTI), the peak of heat release rate (PHRR), the total heat release (THR), the time to PHRR (TTPHRR), fire growth rate (FIGRA), and carbon residue listed in Table 4. All of these indexes decreased significantly when nanofillers were added. For instance, the PHRRs of the alginate aerogel decreased by ca. 70% when combining with the current nanofillers, indicating a significant reduction in flammability (no flame could be observed). All samples were ignited for 2 min and then the tests were ceased. The data from cone calorimetry further proved the nonflammability advantage of the nanocomposite aerogels over most polymeric foams.6,56 Figure 5 presents the digital photos of residues of two representative samples (A5MH5 and A5MMT5) after cone calorimetric tests. For A5MH5, powdered carbon residue forms. And for A5MMT5, it seems the whole material survives just with a tanned surface, in which base materials were protected by the carbon layer. The carbon residue of A5MMT5 and A5Kaolin5 are even higher than 70%.

Figure 5. Digital photographs of the aerogels after cone calorimeter test.

To better understand the non-burning behaviors, the residue microstructure of A5MH5 was compared with A5MMT5 by SEM (Figure 6). For A5MH5, powdered carbon residue indicates a large amount of MgO particles left, because MH decomposed into water and magnesium oxide during combustion. These products flame retard materials by absorbing heat and smoke. For A5MMT5, compact carbon layers could be observed, compared to the behavior when the nano clay, MMT, was incorporated into the aerogels, with only loss of their crystal structure waters during combustion tests, E


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Figure 6. SEM images of carbon residue of A5MH5 and A5MMT5 samples after cone calorimeter tests.

Figure 7. (a) XPS C1s narrow scan spectrum of the carbon residue of A5MH5. (b) XPS C1s narrow scan spectrum of the carbon residue of A5MMT5. (c) XPS Si2p narrow scan spectrum of A5MMT5. (d) XPS Si2p narrow scan spectrum of the carbon residue of A5MMT5.

284.6 eV, was observed to be formed. Figure 7c,d present the XPS Si2p narrow scan of the A5MMT5 before and after combustion. In contrast to carbon, the state of silicon was almost unchanged, which demonstrates the structural integrity of montmorillonite.

and retention of structural integrity. As polymers were removed by combustion, the MMT network remaining exhibited increasingly large porous structures. The residues were also studied by X-ray photoelectron spectroscopy (XPS). This method reveals elements present, their elemental concentrations, and the change of the element valence state. Figure 7a,b compare the C1s intensities of carbon residues in A5MH5 and A5MMT5, respectively. The carbon element existed as C− C bond and C−O bond in alginate before burning test. After burning, a portion of the C atoms remained in the residue, consistent with incomplete combustion, while the carbon lost in the system exited as CO or CO2. In addition, graphite, a newly formed C structure, which corresponds to the peaks at

4. CONCLUSION The preparation of aerogels based on ammonium alginate and nanoparticles by an environmentally green and simple freezedrying process, combined with a post-cross-linking method was demonstrated. The densities of resultant aerogels are very low. After a facile post-cross-linking process, the mechanical strength F


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of the aerogels was enhanced. Combustion tests show that ammonium alginate aerogel itself is a low flammability material. Inorganic nanoparticles further suppress its flammability. In one aspect of a possible flame retardant mechanism, metal hydroxides dehydrated to reduce the temperature and density of combustible gas, comparing to the protection carbon layer formed by the layered nanoclays. The aerogels prepared herein have excellent thermal insulation and flame retardant properties, and the process is simple, green, and economic. They hence have enormous potential in the building insulation materials.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.-Z.W.: [email protected]. Tel./Fax: +86-2885410755. *E-mail for W.L.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 51320105011 and 51421061) and Program for Changjiang Scholars and Innovative Research Team in University (IRT. 1026).

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REFERENCES

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