Novel Polymer Aerogel toward High Dimensional Stability, Mechanical

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Novel Polymer Aerogel toward High Dimensional Stability, Mechanical Property, and Fire Safety Ke Shang,† Jun-Chi Yang,† Zhi-Jie Cao,† Wang Liao,*,† Yu-Zhong Wang,*,† and David A. Schiraldi‡ †

ACS Appl. Mater. Interfaces 2017.9:22985-22993. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 11/03/18. For personal use only.

Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China ‡ Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States S Supporting Information *

ABSTRACT: Inorganc silica-based aerogels, the earliest and widely used aerogels, have poorer mechanical properties than their organic substitutes, which are flammable. In this study, a novel polymeric aerogel with high strength, inherent flame retardancy, and cost-effectiveness, which is based on poly(vinyl alcohol) (PVA) cross-linked with melamine−formaldehyde (MF), was prepared under aqueous condition with an ecofriendly freeze-drying and postcuring process. Combined with the additional rigid MF network and benifited from the resulting unique infrastructure of inter-cross-linked flexible PVA segments and rigid MF segments, PVA-based aerogels exibited a significantly decreased degradation rate and sharply decreased peak heat release rate (PHRR) in cone calorimeter tests (by as much as 83%) compared with neat PVA. The polymer aerogels have a limiting oxygen index (LOI) as high as 36.5% and V-0 rating in UL-94 test. Furthermore, the aerogel samples exposured to harsh temperatures maintain their dimensions ( 32%. Figure 7 shows P5M5 samples with high LOI values in the range of 36−38%, but these values decrease with cross-linking time. Two reasons could be responsible for this phenomenon. First, melamine itself is well-known for the gas-phase flameretardant effect. Second, the cross-linking and formation of melamine network could continue at a high temperature, during which the heat was taken away by the reaction and evaporation of water molecules. The first factor is believed to contribute more to the high LOI values, for the amount the melamine used; once a full cross-linking was achieved (≥24 h), the decreasing trend for LOI slowed. The combustion behaviors of the aerogels were further investigated by cone calorimetry. The 24 h cross-linked PVA aerogels with different MF contents were measured under a heat flux of 50 kW/m2. The evolution of heat release rate and total heat release are exhibited in Figure 8, and corresponding key parameters are listed in Table 4, including time to ignition (TTI), peak of heat release rate (PHRR), total heat release (THR), time to PHRR (TTPHRR), fire growth rate (FIGRA), and residue. For the neat PVA aerogel, the TTI is only 3 s, whereas, for cross-linked PVA aerogels, i.e., P5M1, P5M2.5, and P5M5, TTI values increase to 11, 25, and 60 s, respectively, indicating that the decomposition of MF segments hindered the spread of flame. The increase of MF segments also decreased the PHRR and FIGRA values, especially for the P5M5 aerogel, where the PHRR is only 60.9 kW/m2, 84% lower than that of the neat PVA aerogel. The FIGRA of P5M5 decreased to 0.4 W/s in dramatic contrast to that of the neat PVA aerogel (18.3 W/s). However, as the flame retardant of the aerogels is mainly due to the gas-phase flame-retardant effect of melamine, all the materials almost burned completely and therefore the THR values were not significantly reduced. To better understand the combustion behaviors of the aerogels, determination of the residue microstructures of P5M5 and P5M1A1 was conducted by SEM (Figure 9). The residue of P5M5 adopts a char structure with bubble-like pores, which are attributed to the gas released by MF and hence supported by the primary gas-phase flame-retardant mechanism. Based on the above-mentioned results, the fire safety of a material is indicated by two primary indexes, i.e., LOI and PHRR, and put

Figure 10. Summary of aerogels with different strengths and different fire safety (data from refs 30−32, 35−37, 39, and 48 and this work).

mechanical strengths observed. Because the cross-linking reaction occurred gradually with time, the resulting microstructure became more uniform. The strength of PVA aerogels was therefore significantly elevated after the combination and cross-linking of MF. Comparing with high-fire-safety PVA/ MMT aerogel,30 the specific modulus increased 4.7-fold from 32.6 m2/s2 for the PVA/MMT sample to 187.4 m2/s2 for the PVA-MF aerogel. 3.4. Thermal Stability. Figures 5 and 6 show thermogravimetric analysis (TGA) results of PVA-MF aerogels; resultant characteristic thermal degradation data are summarized in Table 2. A small peak at the beginning of all the curves is assigned to the loss of water absorbed in the air. For the samples with cross-linking times of less than 24 h, this process could further develop with increased temperature. The Td 5%, which indicates the onset of decomposition, decreased, therefore, in contrast to that of PVA aerogel. The increase of Td max and the remarkably reduced degradation rate at that temperature indicate improved thermal stability of the intercross-linked networks. To clarify the effect of cross-linking time, the TGA and corresponding DTG curves of P5M5 aerogels with different cross-linking time are summarized in Figure 6. For P5M5-0h and P5M5-4h, there is an obvious degradation at ca. 130 °C, which does not exist for the P5M5-24h and P5M57d samples. This phenomenon strongly suggests an uncompleted cross-linking reaction at a comparatively short reaction time, and release of water at ignition is hence possible.

Figure 11. Dimensional stability and property stability of the P5M5 aerogels. The volume variation (a) are the sizes of aerogels before and after an exposure in −20 or 100 °C and the values of modulus (black) and LOI (red) (b) before and after the same conditions. 22991

DOI: 10.1021/acsami.7b06096 ACS Appl. Mater. Interfaces 2017, 9, 22985−22993

ACS Applied Materials & Interfaces

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in the figure with its mechanical strength (Figure 10 and Table S1). In this figure, a double y-axis was designed for LOI (in sequence) and PHRR (in inverted sequence) to describe the fire safety of a material, and specific modulus was along the xaxis to indicate the strength. A material with points close to the upper right corner is stronger and consistent with higher fire safety. By comparing with the fire-safety polymeric aerogels reported in recently years, the PVA-MF aerogels, which have a special soft/hard segments interconnected network, are obviously robust with remarkable fire safety. 3.6. Dimensional and Property Stability. Dimensional and property stability are very important for the devices used for aerospace exploration. Silicon aerogels are inherently nonflammable but fragile. Newly developed, strong polymeric aerogels, as above-mentioned, still underwent some shrinkage (from about 10% to 30%) and gave little consideration to their fire safety.6,7 In this study, P5M5 aerogels were exposed in harsh conditions of −20 and 100 °C for a week; as Figure 11 and Table S2 show, the sizes of the aerogels were fairly stable with shrinkages ca. 10%. For the strength and LOI, the later a key parameter for fire safety, were almost unchanged.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06096. Summary of aerogels with different strengths and different fire safety and change of dimensional, strength and fire-safety stability of P5M5-24h aerogels (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel. and Fax: +86-28-85410755. E-mail: [email protected] (W.L.). *Tel. and Fax: +86-28-85410755. E-mail: [email protected] (Y.-Z.W.) ORCID

Wang Liao: 0000-0002-8588-3492 David A. Schiraldi: 0000-0001-5111-0558 Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS The preparation of inherently flame-retardant aerogels based on poly(vinyl alcohol) and melamine−formaldehyde by a simple freeze-drying and routine curing process was demonstrated. The resulting aerogels adopted inter-cross-linked rigid and flexible segments infrastructure. The flame retardancy was improved by this structure and based on this novel structure, and the mechanical strength was significantly enhanced. In particular, the aerogels had excellent dimensional stability, which were confirmed by the tests in dissolution and hot/cold temperature conditions.

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Research Article

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (Grants 51320105011, 51121001, and 51603130) and Program for Changjiang Scholars and Innovative Research Team in University (IRT 1026). 22992

DOI: 10.1021/acsami.7b06096 ACS Appl. Mater. Interfaces 2017, 9, 22985−22993

Research Article

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DOI: 10.1021/acsami.7b06096 ACS Appl. Mater. Interfaces 2017, 9, 22985−22993