Effects of Gamma Irradiation on Clay Membrane with Poly(vinyl

Oct 9, 2015 - (39) A flexible clay membrane (RP1C9) with large area and acceptable transparency was obtained in Figure 1, indicating the compatibility...
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Effects of Gamma Irradiation on Clay Membrane with Poly(vinyl alcohol) for Fire Retardancy Hong-Bing Chen,*,† Hai-Bo Zhao,‡ Wei Huang,† and Peng Shen† †

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621000, China Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621000, China

‡

ABSTRACT: Clay membrane hybrid composites were prepared by facile solution-casting of aqueous precursor suspensions of poly(vinyl alcohol) (PVOH) and montmorillonite clay (MMT), then cross-linked by gamma irradiation. The influences of absorbed dose and polymer loading on composite structure and properties were investigated. A moderate amount of PVOH (P4C6) is found to be optimum for fabricating mechanically strong PVOH composites; however, gamma irradiation has a positive influence on strengthening composites only with low PVOH content. Scanning electron microscopy (SEM) observation shows a layered structure from the cross-section of a cryo-fractured surface for all composites, and a comparatively smooth surface. The wide-angle X-ray diffraction (WAXD) characterization shows an intercalated MMT structure with incorporation of PVOH. Cross-linking and the increase of clay content lead to a decreased onset decomposition temperature but an increase in the temperature at the maximum decomposition rate and enhancement of the residue of the resulting materials. The clay membranes possess very low flammability, which is not significantly influenced by gamma irradiation.

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INTRODUCTION Polymeric−inorganic hybrid composites have drawn more and more attention both industrially and academically,1−13 related to the possibility of achieving significantly increased material properties with a low price. The first known example is polyamide/clay nanocomposites developed by Toyota,14,15 with improved mechanical, thermal, and gas barrier properties and flame retardancy.16 Then, much research followed. Pinnavaia et al.17 developed an epoxy−clay fabric film composite by impregnation of clay films in an epoxy monomer and a curing agent. The oxygen permeabilities of the obtained epoxy−clay fabric film composites were lower by 2−3 orders of magnitude in comparison to that of the pristine polymer and by 3−4 orders of magnitude in comparison to that of the pristine clay film. Manias and Strawhecker18 prepared poly(vinyl alcohol)/sodium montmorillonite nanocomposites of various compositions by casting them from a polymer/ silicate water suspension. The resulting composites have both exfoliated and intercalated MMT layers, with increased thermal and water vapor transmission properties. The tensile testing results show that 4 wt % of clay is optimum from the viewpoint of stress at break. The well-known reason for the improved properties is that montmorillonite clay used possesses high intrinsic platelet modulus and strength and a large aspect ratio, and the nanoscale distribution of clay and the interaction with the polymer matrix further enhance material properties. The obtained polymeric−inorganic hybrid composites are widely used as engineering materials,19,20 membranes,10,21 in medical apparatuses,22−26 and so on. Each field has many branches; say, biomimetic organic−inorganic composites with high mechanical functionality can be used as bone, antler, enamel, dentin, nacre, sea shells, and egg shells, which are of great interest for promising medical applications.27−30 Other than blending and in situ polymerization, clay-based nanocomposites with a layered structure can be fabricated © XXXX American Chemical Society

through the bottom-up colloidal assembly of submicrometerthick ceramic platelets within a ductile polymer matrix, which possess high stiffness, strength, and also ductility.31−35 The stiffness and tensile strength of these materials are 1 order of magnitude greater than those of analogous nanocomposites with a so-called brick and mortar structure.36 Based on these, Liu et al.37 fabricated clay nanopaper with tough cellulose nanofibers. In this method, the authors first made nanofibrillated cellulose (NFC) and MMT dispersion, respectively. Then, they blended the two together to create an NFC/clay colloidal mixture, filtrated it with a 0.65 μm filter membrane under a vacuum, and finally dried it in an oven to obtain freestanding clay nanopaper. The resulting nanopaper has an ultimate strength up to 124 MPa (50 N/50 M) with a layered microstructure. In addition, this clay nanopaper is a selfextinguishing composite due to high clay content. In the present study, we report another clay membrane with a facile fabrication procedure. γ-Radiation-induced cross-linking was also used to enhance the mechanical properties. The objectives are to investigate processing feasibility and discuss structure−property relationships of the resulting membrane with various compositions.

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EXPERIMENTAL SECTION Materials. Sodium montmorillonite clay (Cloisite Na+) was purchased from Southern Clay company with a cation-exchange capacity (CEC) of 145 mequiv/100 g. Poly(vinyl alcohol) (PVOH, Mw 31 000−50 000, 99% hydrolyzed) was purchased from Sigma-Aldrich. Deionized (DI) water was prepared using Received: July 23, 2015 Revised: September 27, 2015 Accepted: October 9, 2015

A

DOI: 10.1021/acs.iecr.5b02703 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research a Purelab flex 3 unit. All reagents were used without further purification. Preparation of Clay Membrane. Percentages of PVOH and clay structural components are given as percentages of DI water. To produce a clay membrane containing 50 wt % PVOH and 50 wt % clay, for example (noted as P5C5, where P stands for PVOH, C stands for clay), 5 g of PVOH solid was dissolved in 50 mL of DI water at 80 °C overnight by stirring. A total of 5 g of Na+-MMT was blended with 50 mL of DI water at high speed (20 000 rpm) to create a clay suspension. The resulting clay suspension and PVOH solution were mixed together and stirred overnight and then further dispersed for 30 min by ultrasonic equipment. Then, the mixture was casted onto an aluminum plate and irradiated with γ rays (using a 60Co source at the Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, China). The samples were irradiated at about 150 Gy min−1, for times necessary to expose the samples to 30 kGy (noted as RP5C5). After being irradiated, the wet films were dried at room temperature. Finally, clay membranes with a thickness in the range of 40−80 μm were obtained. Characterization. The tensile tests of the films were performed with a SANS CMT7000 testing machine, fitted with a 100 N load cell. Specimens of 40 mm length and 40−80 μm thickness and 4 mm width were tested with a strain rate of 4 mm/min. Five samples for each composition were tested for reproducibility. Morphological microstructures of the clay membranes were examined with a ZEISS EVO 18 special edition scanning electron microscope at an acceleration voltage of 10 kV. The samples were prepared by fracturing in liquid nitrogen and then sputter-coated with a thin gold layer prior to imaging. The thermal stabilities (by thermogravimetric analysis, TGA) were measured on a STA449C apparatus under a nitrogen flow (40 mL/min). Then, 5 mg samples were placed in a platinum pan and heated from room temperature to 700 °C at a rate of 10 °C/min. Wide-angle X-ray diffraction (WAXD) measurements (transmission) of MMT (powder) and the clay membrane (film) were performed using X-ray diffraction (Philips X’Pert X-ray diffractometer), with Cu Kα radiation in a 2θ range from 2° to 45°. The combustion behaviors of the clay membrane and the reference neat PVOH were evaluated using an FTT cone calorimeter. Specimens with a size of 100 mm × 100 mm × 1 mm (about 20 sheets, 11 g) were tested under a heat flux of 50 kW/m2. The heat and smoke release information were recorded.

Figure 1. Photos of obtained clay membrane (RP1C9).

The morphological microstructure of the membrane is characterized with SEM (Figure 2). In the published papers, a cellulose-based clay membrane has a smooth surface and layered structure as nacre.37 That phenomenon was also reported in biomimetic composites made from PVA/clay by LbL or MTM/polymer self-assembly,31,33,35 with a layered thickness below 100 nm. Here, in this study, a layered structure is also observed with a similar layer thickness as reported,37 which is more obvious with increasing clay content. From Figure 2, a solution-casting method generates a rougher surface than with vacuum filtration, but still comparatively smooth. After irradiation, the microstructure of both samples seems not to be influenced, probably because an irradiation-induced reaction only occurs on the molecular scale, while not influencing the microstructure. Compared with the aforementioned self-assembly methods, solution-casting is more facile and can be easily scaled up. Neat clay products or a product with high clay content are very brittle, which cannot satisfy application requirements.40,41 In contrast, this clay membrane may have the potential to be ductile material even with a high concentration of clay (Figure 1). It is reported that cellulose-based clay nanopaper is a very strong film material with a tensile strength ranging from 30 MPa (11N/89M) to 130 MPa (50N/50M). The reason sources from both strong cellulose and the specific organic−inorganic structure. The mechanical properties of the PVOH-based clay membrane and the irradiation-cross-linked ones are summarized in Table 1. Incorporation of PVOH into the clay membrane first increases the strength; then a large amount of PVOH slightly decreases the strength, from 18.9 ± 1.7 MPa for P1C9 to 52.1 ± 5.2 MPa for P4C6 then to 48.7 ± 5.3 MPa for P5C5, probably due to the low modulus of PVOH compared to MMT clay. It might also be because of the hindered selfassembly of the system for superfluous PVOH.33 The irradiation induced samples have obviously increased tensile strength at high clay content, from 18.9 ± 1.7 MPa to 26.0 ± 2.1 MPa for P1C9; then it decreases with increasing PVOH content, say, from 48.7 ± 5.3 MPa to 39.4 ± 1.7 MPa for P5C5. This is probably because that radiolysis and cross-linking plays an important role in high polymer-containing systems. Although a PVOH-based clay membrane generally has much lesser mechanical properties, the tensile strength at high clay content is still comparable with that of reported organic− inorganic hybrid material, yet with a facile preparation process. The sample RP1C9 has a tensile strength of 26.0 MPa, showing remarkable flexibility in bending as illustrated in Figure 1. The

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RESULTS AND DISCUSSION The mechanism of irradiation-induced cross-linking of PVOH is generally the coupling of free radicals caused by gamma irradiation.38 In this study, 30 kGy is adapted because it was regarded as the optimal dose in our previous study.39 A flexible clay membrane (RP1C9) with large area and acceptable transparency was obtained in Figure 1, indicating the compatibility of the clay and PVOH. It also demonstrates the feasible, large-scale procedure of making such material, considering the nanostructured characteristics of the material obtained. The transparency of the material decreases with increasing clay content, probably attributed to the opaque clay in a large amount. B

DOI: 10.1021/acs.iecr.5b02703 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. SEM images of surfaces (A) and cross-section of cryo-fractured surfaces (B) of a PVOH-based clay membrane.

Table 1. Mechanical Properties of PVOH-based Clay Membrane (Tensile Strength in MPa, Elongation at Break in %) sample neat irradiation-induced

P1C9 strength elongation strength elongation

18.9 2.8 26.0 2.1

± ± ± ±

1.7 0.5 2.1 0.4

P2C8 21.4 2.7 31.2 2.7

± ± ± ±

P3C7

1.4 0.6 1.1 0.2

39.1 3.1 31.9 2.9

± ± ± ±

3.4 0.5 2.6 0.2

P4C6 52.1 3.6 42.7 3.5

± ± ± ±

5.2 0. 6 3.1 0.1

P5C5 48.7 4.4 39.4 4.0

± ± ± ±

5.3 0.5 1.7 0.3

PVOH 44.2 180.2 50.5 120.6

± ± ± ±

3.6 10.3 2.5 20.4

investigated by TGA (Figure 4 and Table 2). From Figure 4a, it can be seen that all the samples have three main weight loss stages, which is attributed to the desorption of water and a different decomposition section of PVOH, respectively. Both PVOH and clay easily absorb water because of the existence of abundant hydroxyl groups and ionic character.42−46 Thus, there exists a tiny weight loss stage other than the two main weight loss stages. The onset decomposition temperature, Td5%, decreases with increasing clay content, for the desorption of water. The Td10% decreases with increasing PVOH content, due to the degradation of polymer component. The onset decomposition temperatures of the cross-linked membrane also decreased with increasing absorbed gamma irradiation doses, probably the result of PVOH radiolysis. The composites after gamma irradiation exhibited higher thermal stability in the 350−500 °C temperature range, which is probably attributed to the irradiation-induced network structure.47,48 From Figure 4b, it is interesting to note that the maximum weight loss rate decreases with increasing clay content and with treatment of irradiation. This is most likely due to the decreased organic content and chain mobility with irradiation. The final residue weights are almost proportional to the clay content, which is not influenced by the irradiation. In general, the content of clay does increase the thermal stability of the clay membrane, whereas the gamma irradiation has a minor side effect on the thermal stability. Combustion Behavior. The thermal stability tests show that the PVOH-based clay membrane would not decompose until 240 °C (Td10%). It is reported that incorporation of a clay nanofiller into the polymer matrix would increase the flame retardance. The flammability of a material is an important parameter in applications with a requirement of fire safety, such as automobiles, trains, and building decorations. In this study, PVOH, P5C5, P2C8, P1C9, and RP2C8 were chosen as

elongation at break generally slightly increases with increasing PVOH content and decreases after irradiation. WAXD. Structural characteristics of clay may explain the mechanical properties. The WAXD scans for the samples with different concentrations of clay are illustrated in Figure 3. For

Figure 3. WAXD patterns of MMT clay and a PVOH-based clay membrane.

all the samples, the d-spacing of MMT is higher than the control, suggesting the existence of intercalated inorganic layers throughout the polymer matrix.18,20 But the corresponding dspacing further decreases with decreasing PVOH concentration, from 17.35 Å of P5C5 to 14.04 Å of MMT, indicating that the d-spacings of intercalated MMT for the respective hybrids is determined by PVOH concentration. This is consistent with previous publications.18 Thermal Stability. The thermal stability of a PVOH-based clay membrane and the irradiation strengthened ones were C

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samples P2C8, RP2C8, and P1C9 immediately self-extinguish upon removal of the flame. A similar phenomenon was also observed for a cellulose-based clay membrane, self-assembled MMT-polymer nanocomposites, and so on.35,37 Cone calorimetry was utilized to investigate the combustion behaviors of the materials. The relevant combustion data, such as time to ignition (TTI), peak of heat release rate (PHRR), mean PHRR, total heat release (THR), time to peak of heat release rate (TTPHRR), total smoke relase (TSR), and fire growth rate (FIGRA), are summarized in Table 3. In this study, neat PVOH exhibits a very short TTI value (27 s). The addition of MMT first increases TTI values (108 s of P5C5), then decreases them with a further increase in clay content (94 s of P2C8 and 27 s of P1C9), probably due to the rapid temperature increase on the surface of nanocomposites.49 Cross-linking slightly increases TTI from 94 s of P2C8 to 100 s of RP2C8. Figure 5 shows the HRR of clay membranes and the

Figure 4. TGA weight loss and DTG curves of PVOH-based clay membrane.

Table 2. TGA Data of PVOH-based Clay Membranes samples

Td5%(°C)

Td10% (°C)

P5C5 P4C6 P3C7 P2C8 P1C9 RP5C5 RP4C6 RP3C7 RP2C8 RP1C9

205 199 195 176 172 162 208 147 191 136

267 276 279 304 313 241 268 241 295 279

Tdmax(°C)

dW/dT (%/°C)

residue (%)

292 292 297 313 300 296 301 310 307 278

0.71 0.53 0.32 0.19 0.15 0.62 0.36 0.25 0.12 0.11

48 60 66 78 85 48 60 65 79 85

Figure 5. Heat release rate of PVOH-based clay membranes and the control as a function of burning time.

control. Neat PVOH burns quickly after being ignited, with a PHRR of 627.7 kW/m2. HRR decreases to extremely low values with increasing clay content, with a PHRR of 7 kW/m2 for P1C9, which is almost noncombustible. The THR values of PVOH, P5C5, P2C8, RP2C8, and P1C9 are 21.9, 88.0, 2., 2.4, and 1.0 MJ/m2, respectively (Figure 6). The heat release of the clay membranes is attributed to the combustible PVOH. In fact, the THR is not proportional to the PVOH content, indicating the incomplete combustion of the samples during testing. FIGRA is defined by the ratio of PHRR to TTPHRR. This parameter contains information on both heat release and time and is thus important when evaluating the flame spreading rate. The FIGRA of PVOH was measured at 8.9 W/s, which decreases to 1.2 W/s of P5C5, then to 0.4 W/s of P2C8 (the same as RP2C8), and finally to 0.01 W/s (almost zero),

representatives to investigate the influence of clay content and irradiation on the flame retardancy. The self-extinguishing characteristics of the clay membrane were first measured by using a 45° flammability test. P5C5 has high flammability and burns quickly once ignited. The flammability decreases with increasing clay content. The

Table 3. Burning Parameters of PVOH-based Clay Membranes and the Control sample

TTI (s)

PHRR (kW/m2)

mean HRR (kW/m2)

TTPHRR(s)

THR (MJ/m2)

FIGRA (W/s)

TSR (m2/m2)

residue (%)

PVOH P5C5 P2C8 RP2C8 P1C9

22 108 94 100 27

627.7 150.7 51.1 44.6 7.0

76.2 40.7 9.7 12.2 1.2

70 125 115 120 560

21.9 8.0 2.0 2.4 1.0

8.9 1.2 0.4 0.4 0.01

2145 720 141 117 94

4.1 52.2 69.9 65.3 80.0

D

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decreased with increasing clay content, from 2145 for PVOH to 720 for P5C5 and then to 94 m2/m2 for P1C9, indicating that clay membranes possess greater fire safety characteristics. The SEM images of residual char of samples burned in the air are illustrated in Figure 8. It shows that all the residues show similar morphological structures, full of bubbles on the surface and with a layered structure maintained inside the residues. The bubbles are generated by the decomposition gas during burning; thus the sample with more polymer seems to possess bigger bubbles. The integrity of the bubbles and the maintained layered structure demonstrate that clay can act as the barrier of heat and oxygen during burning. Furthermore, the low PVOH content (fuel content) and the low thermal conductivity of composites are also the reason for the low flammability of the clay membrane.39,50 Figure 6. Total heat release of PVOH-based clay membranes and the control as a function of burning time.

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CONCLUSION

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AUTHOR INFORMATION

Facile fabrication of strengthened and ductile poly(vinyl alcohol) (PVOH)/clay membrane composites with low polymer content was demonstrated, using gamma irradiation. The irradiated membrane composites exhibit obviously increased tensile strength at low PVOH loading, whereas it is decreased at high PVOH loading. The obtained clay membrane shows a layered microstructure from the cross-section of a cryofractured surface, while WAXD characterization shows an intercalated MMT structure in the composites. TGA testing surprisingly shows slightly decreased thermal stability with increasing clay content and irradiation-induced cross-linking. The direct ignition of clay membrane composites in air and cone calorimetry both show that the composites possess extremely low flammability, due to the low PVOH content (fuel content) and barriers of clay coupled with low thermal conductivity of clay. Irradiation-induced cross-linking does not significantly influence the combustion behaviors of clay membranes.

indicating that clay membranes have an extremely low tendency to burn with high clay content. Figure 7 shows TSR as a function of burning time. It shows that the smoke emitted during combustion significantly

Corresponding Author

*Tel.: +86 816 2480412. Fax: +86 816 2480412. E-mail: [email protected].

Figure 7. Total smoke release of PVOH-based clay membranes and the control as a function of burning time.

Notes

The authors declare no competing financial interest.

Figure 8. SEM of clay membrane after burning in air: surfaces (A) and cross-section of cryo-fractured surfaces (B). E

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ACKNOWLEDGMENTS This paper was supported by NSAF (Grant No. U1530259), National Science Foundation of China (Grant No. 51403192, 51503191) and Innovation Foundation of Institute of Nuclear Physics and Chemistry for financial support (Grant No. 2013CX04). The authors appreciate that.

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DOI: 10.1021/acs.iecr.5b02703 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.5b02703 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX