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Biobased Poly(furfuryl alcohol)/clay Aerogel Composite Prepared by a Freeze Drying Process Tianwei Wang, Hua Sun, Jiawei Long, Yu-Zhong Wang, and David A. Schiraldi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00089 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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ACS Sustainable Chemistry & Engineering

Biobased Poly(furfuryl alcohol)/clay Aerogel Composite Prepared by a Freeze Drying Process Tianwei Wanga, Hua Suna, Jiawei Longb, Yu-Zhong Wangb, David Schiraldia a Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Rd, Cleveland, OH 44106-7202, USA b Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China

Abstract Low density poly(furfuryl alcohol)/clay “PFA/clay” composite aerogels were fabricated by in-situ polymerization of aqueous solutions of furfuryl alcohol monomer in the presence of hydrophilic sodium montmorillonite, followed by freeze drying. Uncured PFA/clay composite aerogels exhibited poor mechanical properties and low water resistance. Oven curing of these materials increased their compressive modului, water resistance and thermal stabilities. The structures of these PFA/aerogels before and after curing were studied by SEM; their flammabilities were studied by a horizontal burning test and cone calorimetry.

Cured aerogels exhibited low

flammability, withstanding a gas flame for over 20 sec without noticeable combustion.

a.

Correspondance to David A. Schiraldi; [email protected]

Keywords: biomaterials; furfuryl alcohol; clay; aerogel; flammability; mechanical properties;

Introduction Montmorillonite clay aerogels were first described by Mackenzie1 and Call2 who reported a freeze drying method in the 1950s. Building on this early work, we have reported a robust, environmental-friendly method to synthesize polymer/clay aerogel materials using a similar freeze-drying process.3-7 These polymer/clay composite materials typically rely upon exfoliation of layered, smectite clays, such that

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individual clay platelets are dispersed within a polymeric phase, generating a network which can transfer physical loads, and can serve to create both polymer support and confinement.8

Polymer/clay aerogels composed primarily of air, with densities

typically in the 0.05 – 0.15 g/cm3 lend themselves to applications ranging from catalyst supports, packaging, thermal insulation, absorption and structural applications.9-11 A major defect of this environmentally friendly (typically, water vapor is the only process effluent) freeze-drying process is that the final products are water sensitive, limiting their uses. Since these aerogels are based upon polymeric matrixes, they can also be flammabile, again a potential commercial limitation. Poly(furfuryl alcohol) is a biopolymer of potential interest due to the commercial availability of its monomer, furfuryl alcohol, which can be converted from renewable biomasses such as corn, bagasse, wood and wood products.12

Furfuryl alcohol (FA)

is water soluble and compatible with hydroxylated surface of clays, such as montmorillonite (MMT).13 Polymerization of FA can be catalyzed by acid and it can be carried out in water or pure FA monomer.14,15 The early stage of FA polycondensation involves reactions between the OH group on one monomer and a hydrogen at the C5 position of the furan ring in head to tail style (Scheme 1). Head to head linkage occur less frequently and the moieties will return to the head to tail structure upon loss of formaldehyde.14,16 The linear oligomer then further polymerizes into highly branched or crosslinked resins, typically exhibiting dark colors and complex structures. head to tail H+ OH

n

OH O

O

O

furfuryl alcohol

n

-CH2 O H+ O O

H

n head to head

Scheme 1

In the present work, we examine fabrication of PFA/clay aerogels by in situ polymerization of FA in water, in the presence of MMT clay, followed by an


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environmentally friendly freeze drying process, in hopes that the resultant PFA/clay aerogels would be hydrophobic and insensitive to water. This would be one of the few cases of in situ polymerization to polymer/clay aerogels, whereas most of these materials incorporate fully polymerized feedstocks at the onset.

We postulate that

the MMT nanoparticles would act as matrix modifiers to increase the thermal stability of the polymeric system.

Experimental Materials Sodium montmorillonite (Na-MMT; PGW grade, cation exchange capacity (CEC) 145 meq/100 g; Nanocor Inc., Arlington Heights IL USA), furfuryl alcohol (assay≥ 98% Sigma-Aldrich, St louis, MO, USA), and sulfuric acid (Sigma-Aldrich, St louis, MO, USA) were used as received.

Deionized water (DI) was obtained using a

Barnstead ROpure low-pressure, reverse-osmosis system.

Fabrication of PFA/Clay aerogels 5.0 g Na-MMT clay was combined with 50 mL DI water in a Waring (Chula Vista CA USA) model MC2 mini laboratory blender operating at 22,000 rpm for approximately 1 min to prepare a clay suspension. In a 100 ml flask, 50.0 mL of deionized water and furfuryl alcohol were added. The amount of furfuryl alcohol used ranged between 1020g, the resulting aerogels are noted as FA10C5 and FA20C5. The clay suspensions and furfuryl alcohol solutions were combined and kept stirred to make the solutions homogenous. The solutions were poured into a three-necked flask, 1 ml of 0.1 M sulfuric acid was added to the solution slowly as a catalyst. Polymerizations were carried out in the flask with mechanical stirring and heating in a 100° C silicone oil bath. After 19h reaction, the resulting products were poured into 7 cylindrical, 1-dram polystyrene vials and immediately frozen within a -72° C solid carbon dioxide/ethanol bath. The frozen samples were dried in a VirTis AdVantange® freeze dryer (Warminster, PA USA) for 3-4 days to completely remove the ice. Samples of FA10C5 and FA20C5 were placed in a 148° C oven to allow thermal curing.



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Characterization Densities were calculated by mass dimensional data obtained using an analytical balance and a digital caliper. Compression testing was conducted on 2 cm diameter/2 cm high cylindrical test pieces using an Instron model 5500 universal testing machine, fitted with a 1 kN load cell. These tests were performed at a constant strain rate of 10.00 mm/min. Water resistance was tested by soaking aerogels in DI water. IR spectrum was collected by Cary 600 series FTIR spectrometer. SEM images were taken by JEOL JSM-6510LV; the samples were prepared by fracturing in liquid nitrogen, and then coated with gold before testing. TGA data were collected on a TA Instruments Q500, at a heating rate of 10° C/min. Samples were heated up to 800° C under a flow of 40 mL· min-1 nitrogen. Flammability was testing using a method similar to that of UL94, but using 2 cm x 2cm cylindrical aerogel samples. The burning behavior of PFA/clay aerogels were also tested by cone calorimetry; specimens with a size of 100 mm×100 mm×2.5 mm, wrapped in aluminum foil, were tested under a heat flux of 50 kW/m2 according to the ISO 5660 standard with a FTT (UK) cone calorimeter.

Results given in the Figures which follow are

exemplary curves, typical of the ±10% reproducibility typically found in such tests; values reported are averages of three measurements.

Results and discussion Chemical Characterization

The IR spectrum of pure poly(furfuryl) alcohol is shown in in Figure 1(a). The presence of signals at 735, 1016 cm-1 were assigned to the furan rings. Furfuryl alcohol’s polymerization leads to an increased peak at 1562 cm-1 due to the skeletal vibration of 2,5-disubstituted furan rings. Due to ring-opening of furan unit, γ-diketone structures are formed in PFA system, leading to a peak at 1712 cm-1.15,17 The incorporation of clay platelets contribues a peak at 480 cm-1 which does not exist in the virgin PFA IR spectrum (Figures 1(b)-(f)).18


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Figure 1.

(a) IR spectrum for PFA; (b) IR spectrum for FA10C5; (c) FA15C5; (d) FA20C5; (e) FA10C5-2d; (f) FA20C5-2d

Mechanical Properties

Aerogel samples were cut into cylindrical specimens, and tested under compression. Table 1 lists the measured compressive moduli and calculated specific moduli of the aerogels produced in this study. While the compressive modulus of FA20C5 was the highest of uncured aerogels, its value of 0.94±0.15 MPa is relatively low. The specific modulus of FA20C5 is just 5.0±0.8 MPa-cm3g-1as well; the uncured aerogels’ mechanical properties are low relative to previously reported polymer aerogels and commercial foams. FA10C5 and FA20C5 were oven cured for 1-2 days; these samples are coded FA10C5-1d, FA10C5-2d, FA20C5-1d and FA20C5-2d, respectively. After curing for 1day, the compressive moduli of FA10C5-1d and FA20C5-1d increased 8x and 5x compared to their uncured analogs, respectively, with values of 1.7±0.1 MPa and 4.8±0.7 MPa. Cured for a second day, no additional increase in compressive modului

were observed.



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Table 1 Mechanical property data (5 samples were tested for uncured compositions; 2 samples were tested for cured compositions)

Sample

Density/(g/cm )

Compressive Modulus (MPa)

FA10C5 FA20C5 FA10C5-1d FA20C5-1d FA10C5-2d FA20C5-2d

0.117±0.004 0.188±0.015 0.119±0.004 0.177±0.001 0.121±0.002 0.179±0.003

0.22±0.06 0.94±0.15 1.7±0.06 4.8±0.7 2.2±0.6 5.8±1.8

3

Specific Modulus (MPA-cm3g-1) 1.8±0.6 5.0±0.8 14±1 27±4 18±6 32±10

Aerogel samples were soaked in deionized water to test their water resistance. The uncured FA20C5 sample dissolved rapidly in water as Figures 2(a) and (b) show. Oven curing created a more crosslinked polymeric component in the composite materials; cured sample FA10C5-1d and FA20C5-1d both exhibited good water resistance with only extraction of a light yellow solution after soaking in DI water for 1 day. From Figure 2(c), some decomposition can be observed dispersed in solution but the major structure remains the same.


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Figure 2.

(a) FA20C5 at t=0 (b) FA20C5 soaked in deionized water for 1 h. (c) FA10C5-1d and FA20C5-1d soaked in DI water for 1 day.

A morphological study of PFA/clay aerogel composite was conducted by SEM and is given in Figures 3 and 4. FA10C5’s SEM images from Figure 3(a) shows a typical lamellar morphology generated by the growth of ice crystals, similar to that reported in prior work.7,19-22 The viscosity of solution increased with increasing furfuryl alcohol concentration, leading to the denser lamellar morphology observed for FA20C5 (Figure 3(c)). The regular layered structure transitioned into a more co-continuous network after curing. At higher magnification (Figure 4), FA10C5 exhibits a relatively smooth surface, which gains a grainy polymer texture upon curing; a similar phenomenon is observed FA20C5, though that system exhibits more initial polymer structure as produced. Co-continuous network together with higher apparent PFA coverage could explain why FA10C5-1d exhibits a higher compressive modulus than FA20C5 and why FA20C5-1d shows the highest mechanical properties among these four samples.



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Figure 3 SEM Images at 170x (a) FA10C5 (b) FA10C5-1d (c) FA20C5 (d) FA20C5-1d

Figure 4 SEM Images at 700x (a) FA10C5 (b) FA10C5-1d (c) FA20C5 (d) FA20C5-1d

Thermal Stability Table 2. Onset degradation temperature and residual weight for PFA/clay aerogel

Onset of degradation (5% weight loss; °C) Residual weight after Degradation (%)

FA10C5 FA10C5-2d FA20C5 FA20C5-2d 148 249 143 261 58

63

50


58

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Figure 5.

TGA data for (a) FA10C5 (b) FA10C5-2d (c) FA20C5 (d) FA20C5-2d

0.16

d 0.14 0.12

c

Weight loss rate(%)

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0.10

b 0.08 0.06

a

0.04 0.02 0.00 0

100

200

300

400

500

600

700

800

Temperature(° C)

Figure 6. Derivative (d(weight %)/dT) of (a) FA10C5 (b) FA10C5-2d (c) FA20C5 (d) FA20C5-2d

Three steps are proposed for the decomposition of poly (furfuryl alcohol).23 Above 200° C, random chain scission of the weaker chemical bonds will occur in the system. The second and third decomposition steps are associated with bond breaking at the 2 and 5 positions of the furan ring about 320 and 400° C, respectively. A delay of the second decomposition is observed for the cured aerogels, as shown in Figure 6, which likely results from the higher degree of crosslinking and/or a better interaction between clay platelets and the PFA matrix. Since the aerogels already underwent a chemical reaction at 148° C during curing process, it is not surprising that they would not undergo decomposition in that temperature range during



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TGA testing; hence the 100° C temperature difference of the onset 5% weight loss between uncured and cured aerogels shown in Table 2. Flammability FA20C5 and FA20C5-2d samples were cut into 20mm cylinders, hung by a copper wire, and exposed to a natural gas flame for 10s. The uncured aerogels ignited easily, with self-extinguishing of the samples 30s later. A dark char layer formed as a result of combustion, Figure 7.

Figure 7. Burning of uncured FA20C5

Curing brought about a different burning behavior than was seen with the starting aerogels. The air-cured FA20C5-2d aerogels did not exhibit a noticeable flame when exposed to a natural gas flame. Vacuum-cured FA20C5-2d did show some flaming, but to a much lower extent than was observed with the uncured aerogels, Figure 8. It is likely that the air curing process would produce carbonyl and carboxylate groups in the polymer,17 which when combined with the co-continue network of FA20C5-2d, formed a protective layer to limit flammability.

Figure 8 (a) (b) (c) burning of ACFA20C5-2d (d) (e) (f) burning of VCFA20C5-2d


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To quantitatively characterize flammability of both cured and uncured aerogels, cone calorimetry tests were conducted, Table 3, which lists relevant data such as time to ignition (TTI), peak of heat release rate (PHRR), total heat release (THR), total smoke released (TSR) and ﬁre growth rate (FIGRA). All aerogels show a relatively short TTI values, from 2.3-6 sec. Uncured aerogels, with their lower levels of crosslinking, ignited in the shortest time; oxidation during air-curing of PFA aerogels likely led to their TTI values, compared to vacuum-cured samples. Table 3. Cone calorimetry data

Samples EPS Foam control24 Uncured FA20C5 Vacuum cured FA20C5-2d Air cured FA20C5-2d

TTI(s) 9 2.3±0.4 3.6±0.4 6±~0

THR(MJ/m2) PHRR(kW/m2) 9.1 256 7.5±0.5 213±13 7.2±0.4 6.6±0.2

189±18 195±9

FIGRA(W/s) 5.7 10±1.5

TSR(m2/m2) n/a 128±7

10±1.7 9.7±0.5

81±29 45±19

Figure 9 illustrates heat release rate results from cone calorimetry. All three samples burned very quickly but with low levels heat release. With the addition of clay, the peak of heat released rate decreased from 682 kW/m2 to uncured aerogels’ 213 kW/m2. 25 The THR of FA20C5, VCFA20C5-2d and ACFA20C5-2d were 7.5±0.5, 7.2±0.4 and 6.6±0.2 MJ/m2, respectively (AC=air cured; VC=vacuum cured), likely reflecting the degrees of crosslinking. The FIGRA values indicate the scale of fire and the flammability of materials, which is defined by the ratio of PHRR to time to the PHRR; curing did not appreciable change the FIGRA values of the three materials. Total smoke released did decrease from 128±7 to 45±19 m2/m2, probably due to lower volatility of the crosslinked polymer, increasing its tendancy to char. The cone calorimetry results are consistent with TGA and open burning data, showing that curing does decrease the tendency of PFA aerogels to burn, and that air-curing especially pre-oxidizes the material, reducing its fire potential.



FA20C5 VCFA20C5-2d ACFA20C5-2d

200

2

Heat Release Rate/(KW/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

150

100

50

0

0

50

100

150

200

time/s

Fig. 9 Heat release rate of PFA/clay aerogel as a function of burning time

Conclusions PFA/clay aerogel nanocomposites were produced by an in situ polymerization with dispersed Na-MMT in a PFA matrix, one of very few such examples of polymerization during a freeze drying aerogel production process. An oven curing process of the resultant aerogels did increase the degree of polymer crosslinking, leading to enhanced mechanical properties, water resistance and thermal stability. Both vacuum-cured and uncured PFA/clay aerogel exhibit self-extinguishing behavior, while air cured PFA aerogels do not combust with a noticeable flame. The overall flammabilities of the aerogels were similar, exposed to the significant combustion source in a cone calorimeter.

Funding: The authors thank the Case School of Engineering for funding.


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2011; 11(10):2640-6. 21 Somlai LS, Bandi SA, Schiraldi DA. AIChE J 2006; 52(3):1162e8. 22 Chen, Hong-Bing, et al. Polymer 53.25 (2012): 5825-5831. 23 Guigo, Nathanael, et al. Polymer Degradation and Stability 94.6 (2009): 908-913. 24 Chen, H-B, Wang, Y-Z, Schiraldi, D.A., ACS Appl. Mater. Interfaces, 2014, 14, 6790-6. 25 Monti, Marco, et al. European Polymer Journal 67 (2015): 561-569.



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Clay Aerogel Composite Prepared by

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