Poly(vinyl alcohol)/clay aerogel composites with enhanced flame retardancy

Abstract

The effect of pentaerythritol octahydrogen tetraphosphate (PEPA) on the microstructure and properties of poly(vinyl alcohol) (PVOH)/clay aerogel composites was investigated. The material obtained after the addition of a flame retardant exhibited an improved mechanical performance, in accordance with a decreased porous size of the aerogel and its rheological property. After the addition of PEPA, the thermal stability of the materials was slightly enhanced. The obtained aerogel composites exhibited good flame retardancy, with a significantly increased limiting oxygen index (LOI), and reduced heat and smoke release. The thermal conductivity of the aerogel composites was not significantly affected by the addition of PEPA, with values ranging from 0.029 W m⁻¹ K⁻¹ to 0.039 W m⁻¹ K⁻¹. PVOH/clay aerogel composites with good mechanical properties, thermal conductivity and enhanced flame retardancy have potential applications in the insulation field with requirements for fire safety.

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Introduction

Montmorillonite clay aerogel was first prepared via a freeze-drying method in 1950s.^1,2 This aerogel, due to its excellent thermal insulation and flame-retardant performance, attracts a lot of attention.^3–6 However, pure clay aerogel lacks adequate mechanical properties, thus requiring modifications for practical uses.^7–10

Incorporating polymers is a common method for strengthening clay aerogels. With the introduction of a polymer, the mechanical properties of the clay aerogel are significantly enhanced. By adding ammonium alginate to montmorillonite, Chen et al. prepared a strengthened clay aerogel that had similar mechanical properties to those of commercialized rigid PU foam, with a compressive modulus of up to 97 MPa.¹¹ Arndt et al. fabricated an elastic clay aerogel by the incorporation of a epoxy resin. The obtained aerogel combined the advantages of inorganic clay and organic polymers, showing potential applications as an absorbent and insulation material.¹²

However, organic polymers pose some problems, for example, a decreased thermal stability and increased flammability. Enhancing the mechanical properties of clay aerogels while maintaining its low flammability appears unattainable. Some researchers tried to reduce the polymer content of the aerogel composites, while introducing cross-linkers to balance the mechanical properties. Pojanavaraphan et al. prepared natural rubber/clay aerogel composites by freeze-drying the aerogel precursor suspensions with sulfur monochloride (S₂Cl₂) as the cross-linking agent.¹³ The compressive modulus of the cross-linked aerogel was 26 times higher than that of the control materials. Cross-linked PVOH gels and derivative aerogels have also been reported.¹⁴ The mechanical properties of aerogel materials are significantly enhanced after cross-linking with divinylsulfone as crosslinking agent. Moreover, the irradiated cross-linked PVOH/clay aerogel after an absorbed dose of 30 kGy exhibited a significantly increased compressive modulus.¹⁵ The cation-crosslinked polymer/clay aerogel also showed clearly improved mechanical properties,^16,17 while having a lower polymer content.

Poly(vinyl alcohol) is a widely used, water-soluble and biodegradable polymer material, however its low flame retardancy restricts its wide application. With the introduction of clay particles, such as nanoscale silica, halloysite, montmorillonite (MMT), and LAPONITE®, PVOH showed an improved flame retardancy, as measured by cone calorimetry. The heat release, smoke release, and carbon monoxide production of the aerogel composites are significantly decreased with the addition of nanofillers; the fillers do not lead to decreased limiting oxygen index (LOI) values.¹⁸ Besides reducing polymer content, another way to improve flame retardancy is to incorporate flame retardants.

Wang et al. used ammonium polyphosphates (APP) modified with piperazine (PA-APP) to improve the flame retardancy of poly(vinyl alcohol)/montmorillonite (PVA/MMT) aerogels.¹⁹ In combustion testing, all PVA/MMT/PA-APP aerogels achieved V-0 ratings and had higher LOI values than the unmodified PVA/MMT aerogel. Moreover, the aerogel with 1% PA-APP5 (5% piperazine content in PA-APP), had a lower THR value of 5.71 MJ m⁻². When compared with PVA/MMT/APP, the compressive modulus of PVA/MMT/PA-APP increased by 93.4% due to the enhanced interfacial adhesion between matrix and PA-APP fillers. Wang et al. also fabricated flame retardant PVOH/clay aerogels with the incorporation of flame retardants.²⁰

In this manuscript, we report a facile fabrication for a PVOH/clay aerogel composite with enhanced flame retardancy via adding the flame retardant PEPA. The microstructure, thermal conductivity and combustion behaviors were investigated to confirm that an effective flame retardation was achieved. To the best of our knowledge, the novel polymer/clay aerogel composite herein presented has not been reported previously.

Experimental

Materials

Poly(vinyl alcohol), (M_w 31 000–50 000, 99% hydrolyzed) was purchased from Sigma-Aldrich. Sodium montmorillonite (Na⁺-MMT; PGW grade with a cation exchange capacity of 145 mequiv./100 g) was purchased from Nanocor Inc. Pentaerythritol octahydrogen tetraphosphate (PEPA, Scheme 1) was purchased from Hunan Fenghua Material Co. Ltd. Deionized (DI) water was prepared using a Purelab flex 3 unit. All chemicals were used as received.

Scheme 1 The chemical structure of PEPA.

Aerogel preparation

Percentages of PVOH, clay and flame retardant are given as a percentage of DI water. For example, a sample containing 5 wt% PVOH, 5 wt% clay and 1 wt% flame retardant is denoted as P5C5F1 (where P stands for PVOH, C stands for clay and F stands for flame retardant). In a typical preparation procedure, 5 g of PVOH solid and 1 g of flame retardant were dissolved in 50 mL of DI water and stirred for 2 hours at 80 °C. Separately, 5 g of Na⁺-MMT were mixed with 50 mL of DI water at a speed of 20 000 rpm to create a clay suspension. Then, the above two suspensions were blended together. The resulting mixtures were poured into plastic molds (diameter: 20 mm, height: 45 mm; 100 mm × 100 mm × 15 mm and 10 mm × 10 mm × 100 mm). Then, the samples were frozen in liquid nitrogen (∼−196 °C), and freeze-dried in a Beijing Sihuan LGJ-25C freeze-dryer under a high vacuum (1–5 Pa). The freeze-dry process typically took about 3 to 7 days depending on the sample size.

Characterization

A SANS CMT7000 testing machine was used to perform compression testing (∼20 mm in diameter and height) with a 500 N load cell, at a crosshead speed of 10 mm min⁻¹. For reproducibility, five samples for each composition were tested. The compressive modulus was calculated from the slope of the initial linear portion of the stress–strain curve.

The rheological properties of the PVOH/clay suspensions with various flame retardant loadings were evaluated on an ARES G2 rheometer (TA) in the oscillatory shear mode with a parallel-plate fixture in atmospheric air. The dynamic storage modulus (G′), dynamic loss modulus (G′′) and complex viscosity (η) were measured as a function of frequency (ranging from 0.01 to 100 s⁻¹) at 25 °C.

A ZEISS EVO 18 special edition scanning electron microscope at an acceleration voltage of 10 kV was used to characterize the morphological structure of the resulting aerogels. Before testing, the samples were sputter-coated with a thin gold layer. After the cone test, the phosphorus content of samples was measured by energy dispersive spectroscopy (EDS) coupled with SEM.

The thermal conductivity of the aerogels (diameter: 20 mm, height: 45 mm) was tested on a hot disk TPS 2500S thermal conductivity instrument with a probe diameter of 3.189 mm and at a power of 0.005 W.

LOI measurements were performed on a sample of 10 mm × 10 mm × 100 mm dimensions with the Oxygen Index Flammability Gauge (SH5706) according to ASTM D 2863-97.

A cone calorimeter (FTT) was used to evaluate the combustion behaviour of the PVOH/clay aerogels with various flame retardant loadings. Specimens (100 mm × 100 mm × 15 mm) were tested under a heat flux of 50 kW m⁻². The corresponding data for heat, smoke and volatile products release was recorded.

Results and discussion

Preparation of PVOH/clay aerogels with various contents of flame retardant

In this study, PVOH/clay aerogel composites with enhanced flame retardancy were fabricated with the introduction of PEPA, a flame retardant. P5C5 and P2C8 were chosen as representatives to confirm the effectiveness of the flame retardant.

The compressive moduli for samples with different PEPA loadings are shown in Table 1. It is proposed that PEPA would form hydrogen bonds with the polymer/clay through its hydroxyl and ester groups, to make the aerogel blends more homogeneous. When increasing the amount of PEPA, the compressive modulus of the samples increased. The compressive modulus increased from 4.3 MPa for P5C5, to 6.8 MPa for P5C5F0.3, and to 8.4 MPa for P5C5F2. When compared with P5C5, the P2C8 series exhibited poorer mechanical properties. For instance, the compressive modulus of P2C8 is only 2.7 MPa. These results indicate that compressive modulus can be improved by increasing the amount of flame retardant as well as the amount of polymer.

Table 1 Mechanical properties, density, thermal conductivity and LOI of PVOH/clay aerogels

Sample	Moduli (MPa)	Density (g cm^−3)	LOI	Thermal conductivity (W m^−1 K^−1)
P5C5	4.3 ± 0.6	0.102 ± 0.003	23.0 ± 0.2	0.030 ± 0.005
P5C5F0.3	6.8 ± 0.7	0.102 ± 0.002	30.0 ± 0.3	0.031 ± 0.006
P5C5F1	7.2 ± 0.6	0.116 ± 0.003	36.0 ± 0.2	0.031 ± 0.005
P5C5F2	8.4 ± 0.8	0.120 ± 0.004	39.0 ± 0.4	0.029 ± 0.004
P2C8	2.7 ± 0.5	0.100 ± 0.003	—	0.034 ± 0.006
P2C8F2	3.9 ± 0.7	0.120 ± 0.003	—	0.039 ± 0.005

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SEM micrographs of PVOH/clay aerogels containing flame retardant are shown in Fig. 1. With the incorporation of a flame retardant, the aerogels showed a denser microstructure. P5C5 shows a network microstructure with a pore size of about 8–10 μm, which was only about 1–2 μm for P5C5F2. P2C8 series, with a higher clay content, also showed a network microstructure. Generally, increasing the suspension viscosity would generate a denser structure. Both increasing the concentration of polymer or flame retardant loading are effective ways to increase the suspension viscosity. Moreover, a denser structure shows better mechanical properties.¹⁵

Fig. 1 SEM micrographs of PVOH/clay aerogels with various flame retardant loadings.

Rheological test

The complex viscosity (η) of the sample solutions with various flame retardant loadings versus angular frequency is shown in Fig. 2. All the samples exhibit a shear thinning phenomenon when increasing the frequency. Compared with P5C5, P2C8 had a higher complex viscosity, due to the higher clay loading. The sample P2C8 with a higher PEPA content had a decreased complex viscosity; on the contrary, the sample P5C5 with higher flame retardant content had an increased complex viscosity.

Fig. 2 Dependence of complex viscosity with angular frequency for PVOH/clay suspension with various flame retardant loadings.

Generally, higher solution viscosity generates an aerogel with a denser microstructure, thus leading to better mechanical properties.¹⁵ This is probably the reason why P5C5s with flame retardants possess higher compressive modulus. Besides solution viscosity, PEPA may interact with the polymer/clay system, which would improve mechanical properties. This may be the explanation for the increased compressive moduli even with decreased solution viscosity in the P2C8 series. The dependences of storage modulus (G′) and loss modulus (G′′) on angular frequency for PVOH/clay suspensions with various flame retardant loadings is shown in Fig. 3. The G′ and G′′ of the samples show similar trends with increasing frequency, which increase with increasing flame retardant.

Fig. 3 (a) G′ and (b) G′′ of PVOH/clay suspensions with various flame retardant loadings as a function of angular frequency.

Thermal insulation performance

The thermal conductivity of the PVOH/clay aerogels with various flame retardant loadings are listed in Table 1. Neat PVOH/clay aerogel has a low thermal conductivity, being 0.030 W (m⁻¹ K⁻¹) for P5C5. All the aerogel composites tested possess very low thermal conductivities, lower than 0.05 W (m⁻¹ K⁻¹), and are clearly not affected by PEPA loading. It should be noted that thermal conductivity can be significantly affected by many factors, such as temperature, bulk density, moisture and sample surface, which may be the reason for the slightly changed thermal conductivity of the samples.²¹

Combustion behaviour

The flammability of PVOH/clay aerogels with various flame retardant loadings was tested with an LOI instrument and a cone calorimeter. The corresponding data, such as LOI, time to ignition (TTI), peak heat release rate (PHRR), mean PHRR, total heat release (THR), time to peak heat release rate (TTPHRR), total smoke release (TSR), and fire growth rate (FIGRA), are summarized in Tables 1 and 2.

Table 2 Burning parameters for different PVOH/clay aerogels

Sample	TTI (s)	PHRR (kW m^−2)	Mean HRR (kW m^−2)	TTPHRR (s)	THR (MJ m^−2)	FIGRA (W s^−1)	TSR (m^2 m^−2)	Residue (%)
P2C8	3	62	16.4	15	5.9	5.3	199	79.1
P2C8F2	17	26	13.4	50	4.4	0.76	31	80.0
P5C5	2	126	33.7	15	19.7	10.8	140	52.4
P5C5F2	5	102	38.0	15	20.7	6.7	756	52.5

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Due to the high concentration of clay, P2C8 cannot be ignited, not even in pure oxygen, so the LOI could not be measured. The LOI value of P5C5 is 23, which belongs to the category of flammable materials (LOI ≤ 26). The LOI of the samples increased with increasing flame retardant loadings. The LOI of P5C5 F0.3 increased to 30, which means that it has a high flame retardancy. By further increasing the PEPA loading of the aerogel materials, LOI values of up to 36 and 39, for 1 wt% (P5C5F1) and 2 wt% (P5C5F2) PEPA loadings, respectively, could be achieved. This confirms the higher flame retardancy of the PVOH/clay aerogels containing PEPA.

Polymer/clay aerogels have a very short TTI because of the flammable polymer and the porous structure of the aerogel.²² The TTI of P2C8 is only 3 s, which increases to 17 s for P2C8F2. It is possible that PEPA can form non-combustible gas or solid products, thus decreasing the concentration of flammable gas, which prevents the material from burning. The flame retardant agent is less effective in enhancing TTI for P5C5 than for P2C8. P5C5 has a higher concentration of PVOH, which is more flammable. We speculate that due to this, the material would decompose and be ignited in a shorter time. Thus, the flame retardant would have less time to take part in the corresponding reactions before burning.

Fig. 4 shows the HRR of PVOH/clay aerogels with various flame retardant loadings. All the samples burned quickly after being ignited. The PHRR of P2C8 is about 60 kW m⁻², which for P2C8F2 decreases to 26 kW m⁻². It shows a similar tendency for P5C5; PHRR decreases from 126 kW m⁻² for neat P5C5 to 102 kW m⁻² for P5C5F2. It clearly indicates that PEPA reduced the heat release rate of the aerogel materials.

Fig. 4 Heat release rates of PVOH/clay aerogels as a function of burning time.

The THR of P2C8, P2C8F2, P5C5 and P5C5F2 are 5.9, 2.6, 28 and 11.4 MJ m⁻², respectively (Fig. 5). The heat release of aerogels is attributed to the PVOH. The sample possesses more fuel, thus it can release more heat. Compared with P5C5, P2C8 contains less combustible mass, so it releases less heat. P5C5 and P5C5F2 have the same amount of combustible mass, therefore adding flame retardant does not clearly affect the THR.

Fig. 5 Total heat release of PVOH/clay aerogels as a function of burning time.

Fig. 6 shows the photos of PVOH/clay aerogels after burning in a cone calorimeter. After burning, the overall sample shape remains unchanged, only showing disconnected cracks.

Fig. 6 Photos of PVOH/clay aerogels after burning in a cone calorimeter.

Fig. 7 shows the TSR of the samples as a function of burning time. It can be seen that P2C8 has a significantly lower smoke release when compared to P5C5. The addition of a flame retardant clearly reduced the smoke release. This is probably attributed to the hindrance effect of clay and the decreased flammability conferred by the flame retardant.

Fig. 7 Total smoke release of PVOH/clay aerogels as a function of burning time.

The CO and CO₂ release as a function of burning time are shown in Fig. 8. The CO release seems irrelevant to the material composition, however, the CO₂ release curve is consistent with the HRR curve. Considering the release of CO, it can be speculated that the samples were not completely combusted.

Fig. 8 Release rates of main volatile products from PVOH/clay aerogels as a function of burning time: (a) carbon monoxide, (b) carbon dioxide.

Flame retardant mechanisms

Phosphorus-containing PEPA is used as flame-retardant in this study. It is believed that phosphorus takes part in the flame retardant mechanism. Thus we have analysed the phosphorus content in the char residue by EDS, to understand the flame retardant mechanism.

The theoretical phosphorous content of P5C5F2, and P2C8F2 are 2.87% and 2.87%, respectively. The remnant phosphorous in the P5C5F2 and P2C8F2 residues are 1.16% and 1.33%, respectively, indicating the mixed flame retardant mechanisms both in the condensed phase and the gaseous phase.

Besides the phosphorus content, the intrinsic features of the polymer/clay aerogels significantly contribute to the flame retardation of the material as reported before,^18,23,24 such as high content of non-combustible clay, porous structure and low thermal conductivity. Possibly, phosphorus has a synergistic effect with clay in the flame retardancy mechanism of polymer/clay aerogels.

Conclusion

We used an environmentally friendly freeze-drying process to fabricate aerogel composites with enhanced flame retardancy. The compressive moduli were clearly improved when increasing the flame retardant and clay content. The LOI test showed that the flame retardant had clearly increased the LOI. The thermal conductivity of the obtained aerogels is very low, and is not significantly affected by the flame retardant, indicating a good thermal insulation of the materials. The cone calorimetry test showed that the aerogels have a very low flammability, which is further decreased with the incorporation of PEPA. Phosphorus may play an important role in both the condensed phase and the gaseous phase when burning, thus obviously decreasing the flammability. Due to their facile fabrication and enhanced properties, PVOH/clay aerogels are promising as insulation materials, building insulation materials, for instance.

Acknowledgements

This study was supported by the NSAF (Grant No. U1530259) and the National Science Foundation of China (Grant No. 51403192, 51503191). The authors appreciate their support.

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