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Highly efficient flame retardant polyurethane foam with alginate/clay aerogel coating Hong-Bing Chen, Peng Shen, Mingjun Chen, Hai-Bo Zhao, and David A. Schiraldi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11659 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 12, 2016

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ACS Applied Materials & Interfaces

Highly efficient flame retardant polyurethane foam with alginate/clay aerogel coating Hong-Bing Chen,1* Peng Shen, 1 Ming-Jun Chen,2 Hai-Bo Zhao3*, David A. Schiraldi4* 1. Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621000, China 2. School of Science (Sichuan), Xihua University, Chengdu 610039, China 3. Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621000, China 4. Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA Abstract Highly efficient flame retardant polyurethane foams with alginate/clay aerogel coatings were fabricated using a freeze-drying method. The microstructure and the interaction of the samples were characterized with scanning electron and optical microscopy (SEM) and (OM). The results show that PU foam has a porous structure with pore size of several hundred microns, and that of aerogel ranges from 10 to 30 microns. The PU foam matrix and the aerogel coatings have strong interactions, due to the infusion of aerogel into the porous structure of the foam and the tension generating during freeze-drying process. Both the PU foam and the aerogel exhibit good thermal stabilities, with onset decomposition temperatures above 240° C. Combustion parameters, including LOI, TTI, HRR, TSR, FIGRA, CO and CO2, all indicate significantly reduced fire risk. Total heat resease of all but one of the samples

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were maintained, indicating that the flame retardant mechanism is to decrease flame spread rate by forming a heat, oxygen and smoke barrier, rather than by reducing fuel content. This facile and inexpensive post-treatment of PU foam could expand its fire safe applications.

* To whom correspondence should be addressed: [email protected] ; (H.-B Chen), [email protected] ;( H.-B. Zhao), [email protected] ; (D. A. Schiraldi).

Keywords:

alginate; aerogel; polyurethane; flame retardant; coatings


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1. Introduction The development of energy-saving green residential building materials, designs and methods is of strategic importance to the world, given that building-related energy consumption increases annually. For instance, about 95% of the existing 43 billion m2 of buildings in China are high energy-consuming structures.1 Rigid polyurethane (PU) foams have been widely used in green building as thermal insulation materials for their low densities, low thermal conductivities, low water absorption and high compressive strengths.2-8

Neat PU foams are highly flammable,

however, due to the large surface area, good air permeability, and the large aliphatic chain segment content.9,

10

Upon exposure to an ignition source, PU foams are

ignited almost instantaneously and burn rapidly, releasing large amounts of heat and smoke, causing danger of burns and suffocation. Reducing the heat and smoke release and release rate via flame retardation modification, is of importance to the fire safety of PU foams used in society. A number of experimental solutions to flame retardancy of PU foams have been recently reported. Reactive-type flame retardants containing phosphorus, halogens, and nitrogen have been incorporated into the PU chains to modify the foam;11-14 because of the low concentration of flame retardants (to preserve the physical properties of the foams), these copolymers usually possess limited flame retardancy. Chen et al. synthesized a PU copolymer with phosphorus-containing triol, phosphoryltrimethanol (PTMA),12 reporting that the peak of heat release rate decreased only a modest amount, from 381 to 301 kW/m2, and LOI increased from



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17.0 to 23.0 with addition of 15% PTMA per hundred parts of polyol. A more common method to endow PU foams with flame retardancy is to incorporate addition-type additives, such as ammonium polyphosphate (APP),15 hexamethoxycyclotriphosphazene 9,10-dihydro-9-oxa-10-phosphaphen

(HMCPT),10

c-[N=P(OMe)2]3 anthrene-10-oxide

(DOPO

phosphonamidates,16 or expanded expandable graphite (EG)

)

based

8

, among other

additives.17,18 Although this method can give comparatively better flame retardancy, with PHRR decreasing from 304.9 kW/m2 of neat PU to 88.5 kW/m2 of the compound with 10% EG and 15% HPCP,8 poor compatibility, reduced mechanical properties, and easy leaching of FR agents are inherent product defects which cannot be ignored. A novel post-treatment method, layer-by-layer (LbL) assembly has recently attracted a great deal of attention.19-24 This method uses (often) environmentally friendly materials with cationic and anionic charges, such as chitosan (CH) and poly(phosphoric acid) (PPA), carbon nanofibers (CNF) and poly(acrylic acid), titanate nanotubes and alginate, with layer thickness up to 100 nm.

LbL assembly is a type

of surface treatment, which would not influence the structure and properties of PU foam. This method requires complicated procedures with deposition of each layer (sometimes up to 30 layers), and thus may be difficult to scale up. The flame retardancy performance of LbL-treated PU foams show 33% improvements in key fire safety parameters, but further improvements would be required to adequately safeguard public safety in building products.19 In our previous studies, a series of polymer/clay aerogel materials were fabricated,


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which possess mechanical properties similar to those of rigid PU foams.

25-29

These

aerogel composites possess very low flammabilities, due to their high concentration of montmorillonite clay. The freeze drying process utilized to produce these aerogels is unique in its ability to produce polymer/clay composites containing 25-75 wt% clay, whereas the traditional melt compounded clay composites rarely exceed 5% clay content due to viscosity limitations.

Among the materials previously reported,

alginate/clay aerogels exhibited the lowest flammabilities, with a PHRR of 32 kW/m2 for an aerogel produced by freeze drying a aqueous solution of 5 wt% ammonium alginate and 5 wt% clay.26 In the present study, we report a facile post-treatment of PU foam with an alginate/clay aerogel coating. The microstructure and combustion behaviors of the such materials were investigated for the first time, to be best of our knowledge. 2. Experimental 2.1. Materials Polyether polyols (4110, average functionality 3.0, OH content 440 mg KOH/g), polymeric methane diphenyl diisocyanate (PMDI, average functionality 2.7, NCO content 30%), surfactant, catalyst (A33 and DMP30) were supplied by Maoteng Polyurethane Technology Co., Ltd., China. Polyethylene glycol (PEG, molecular weight 400, functionality 2.0) and n-hexane were purchased from Chengdu Kelong Chemical Regent Factory, China. Sodium alginate was purchased from Chengdu Chemical Institute. Sodium Montmorillonite (Na+-MMT; PGW grade with a cation exchange capacity of 145 mequiv/100 g) was purchased from Nanocor Inc. Deionized



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(DI) water was prepared using a Purelab flex 3 unit. All reagents were used without further purification. 2.2. Preparation of neat PU foam. The neat PU foam was prepared by one-pot and free-rise method. Polyols (4110 and PEG), DI water, catalysts (A33 and DMP30), surfactant were firstly added in a 1 L plastic beaker with stirring for 15 min. Secondly, n-hexane was well mixed in the beaker within 2 min with stirring. Then, PMDI was added into the beaker with vigorous stirring for 10 s. The mixture was immediately poured into an open metal mold to produce free-rise foam. The foam was first cured for 4 h at 80° C in oven, then, it was completely cured under ambient conditions for 24 h. The molar ratio of NCO (from PMDI) to OH (from 4110, PEG, and water) was 1.1:1. The formulation of neat PU foam is shown in Table 1. 2.3. Preparation of modified PU foam with aerogel coatings Alginate/clay suspensions were prepared according to our previous work.26 To produce a suspension containing 7.5 wt% sodium alginate and 7.5 wt% clay for instance, 7.5 g of Na+-MMT was blended with 100 mL of DI water to obtain clay aqueous suspension. 7.5 g of sodium alginate powder was then slowly add into the clay suspension with constant stirring. The predetermined amount of suspension mixture (eg, a thickness of 0.2 mm) was then cast onto rigid PU foams. The resulting mixture was frozen in an ethanol/liquid nitrogen bath (~-114° C). The frozen samples were dried in a Beijing Sihuan LGJ-25C freeze-dryer with a shelf temperature of 25° C with a vacuum of 5 Pa. The final resulting sample is identified as A7.5C7.5-0.2mm, where A stands for alginate, C denotes clay, the number after A and C refers to their


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percentages, and 0.2 mm means its thickness on PU foams is 0.2 mm (Figure 1). 2.4 Characterization Morphological microstructure of the materials was characterized with ZEISS EVO 18 special edition scanning electron microscope at acceleration voltage of 10 kV. The samples were fractured in liquid nitrogen, and then sputter-coated with a thin gold layer before testing. The thermal stabilities (by thermogravimetric analysis, TGA) were measured on a Perkin-Elmer STA6000 apparatus under a nitrogen flow (40 mL/min). About 4 mg of samples were placed in a platinum pan and heated from ambient temperature to 700° C at a rate of 10° C/min. Limiting oxygen index (LOI) tests were carried out at room temperature according to standard method ISO 4589-1:1996 using HC-2C oxygen index instrument. The size of the specimens were 150 ×10 ×10 mm3. The combustion behaviors of coated PU foams and the controls were tested with FTT cone calorimeter. Specimens with a size of 100 mm × 100 mm were tested under a heat flux of 50 kW/m2, with thickness ranging from 25mm to 30 mm (for the thickness of different coatings). The heat, smoke and volatile products release information were recorded. 3. Results and discussion

3.1 Fabrication of PU foam with alginate/clay aerogel coatings. Post-treatment of PU foam is a potentially workable solution to endow the material with good flame retardancy while maintaining its intrinsic merits. The LbL assembly



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method offers a potentially good option, but requires complicated procedures. In this study, a facile coating of aerogel onto PU foams is reported; the precursor of alginate/clay aerogel was cast onto rigid PU foam with rough surfaces (because of the cell structure), then freeze-dried to obtain the coated PU foams. Figure 2 shows SEM micrographs of a PU foam matrix, alginate/clay aerogel coatings, and the coated samples: the PU foam present closed cellular structures with pore size of several hundred microns, while the pore size of alginate/clay aerogel samples range from 10 to 30 microns. Since the aerogel was prepared via freeze-drying method, the pores of aerogel microstructure are open. In coated PU foams, both foam matrix and aerogel coating maintain their independent structure, suggesting mainly a physical coating. The compressive modulus and density values for the subject samples are listed in Table 2; it can be seen that the foam matrix and aerogel coated samples have similar mechanical properties and densities.

The interaction between foam matrix and aerogel coating was of interest. We prepared a series of specimens (10mm×10mm×100mm) with half PU foam and half aerogel by volume (Figure 3). It was speculated that the joining point would break off during tensile testing, thus the tensile strength could be used to characterize the interaction (Figure 3A). The joining point turned out not to be the weakest point of the material during tensile test, indicating a strong interaction between the two different materials.

Figures 3B and C show the cross section of specimens obtained from

tensile tests; the PU foam and the aerogel both break, but not specifically at their


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jointing points. The magnification of the interface is shown in Figure 3D; the alginate/clay show particles with the size of PU foam cells on the interface. It is proposed that the aerogel precursor was filled into the cells of the foam, then templated when freeze dried (as shown in Figure 1). The volume of the precursor would expand during freezing, causing tension because of the stretched foam cells. The SEM micrographs also show intercalation of aerogel coating into the foam matrix, thus leading to strong interaction even with physical coating. Thermal stability. The thermal stability of PU foam matrix and alginate/clay aerogel coatings were investigated by TGA; the corresponding curves are given in Figure 4, and the data are summarized in Table 3. As previously mentioned, bio-based polymer easily absorbs water because of the high concentration of hydroxyl and carboxyl groups in its structure. Sodium alginate therefore absorbs considerable water, with first weight loss stage, Td 5%, which is seen as low as 58.4° C. There exists two decomposition three stages of sodium alginate in air, 50-80 °C for the desorption of water, 230-270° C for the decomposition of alginate, 420-470° C for the decomposition of the alginate residue formed at lower temperature, respectively. The addition of clay slightly increased the onset decomposition temperature, Td 15%, from 235° C of A5C5 to 241° C of A5C10; the residues closely corresponded to the clay loading in the materials. The PU matrix has a slightly higher thermal stability than the alginate/clay aerogel, with Td 5% of 262° C, which is about 20 degrees higher than that of the aerogel coatings, leaving almost no residue at the end of the TGA test. The PU foam matrix and alginate/clay aerogel coatings have comparable thermal stabilities.



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Combustion Behavior. LOI values of PVOH/clay aerogels have previously been reported, ranging from 22.0 to 25.0. The addition of different clays just slightly increased the LOI values, indicating that LOI was mainly determined by polymer. In this study, alginate with high LOI value (up to 48.0) was chosen as coating polymer30. The corresponding values are summarized in Table 4. Neat PU foam has low LOI value of 17.0, which corresponds to a highly flammable material. The alginate/clay aerogel, A7.5C7.5, would self-extinguish even in pure oxygen as in LOI testing. It is believed that LOI testing fails evaluating this material. The LOI of this situation is noted as F (means fail). All the coated samples tested have high LOI values, from 32.5 of A5C5-1.5mm to F of A7.5C7.5-1.5mm, generally increasing with increasing coating thickness and clay content. As the aerogel coatings were burned, they maintained their shapes (as shown in Figure 5, which is the SEM micrographs of the LOI residues), then the burned integrated aerogel “shell” protects the inside PU foam from further burning, thus leading to a very high LOI value. Since LOI testing fails to evaluate the flammability of several highly flame retardant samples, cone calorimetry was also utilized in this study. The corresponding parameters, including the time to ignition (TTI), heat release rate (HRR), peak of heat release (PHRR), time to peak of heat release (TTPHR), total heat release (THR), specific extinction area (SEA), total smoke release (TSR), carbon monoxide (CO) and carbon dioxide (CO2) production, were obtained to evaluate their flammability and toxicity. The relevant date of neat PU foam and alginate/clay aerogel are also included as controls and summarized in Table 4.


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Alginate/clay aerogels show no open fires during the entire testing process, indicating a low concentration of flammable gas products during decomposition. On the contrast, neat PU foam has a very short TTI of 1s, which immediately ignites once exposed to heat flux (50 kW/m2); the flammable decomposition products of the coated samples appear to come primarily from inside the foam matrix. Thus, TTI of the coated samples generally increase with clay content and coating thickness, from 1 s of neat PU foam to 10 s of A5C10-1.5mm and A7.5C7.5-1.5mm. Figure 6 shows the HRR of coated PU foams and the controls. Neat PU foam has a sharp HRR curve, with PHRR of 323 kW/m2. In contrast, A7.5C7.5 has a broad HRR curve with PHRR of only 20 kW/m2. The coated PU foams generally have a combined shape of neat PU and aerogel, depending on the thickness and the clay content of the coating. A5C10-1.5mm has the lowest PHRR of 110 kW/m2 among all the coated samples. The shapes of the HRR curves generally accord well with weight loss as a function of burning time (Figure 7). The fire growth rate index (FIGRA), which is defined as the ratio of PHRR to TTPHRR, is utilized to evaluate the fire spread rate. The FIGRA of neat PU foam, measured at 12.9 W/s, decrease to 6.4 W/s of A7.5C7.5-0.2mm, and then to 2.1 W/s of A7.5C7.5-1.5mm, indicating that all the coated PU foams have significantly decreased fire risk even with very thin aerogel coatings. THR is used to measure the total heat release of material during combustion, which is determined by the flammable decomposition products. Figure 8 illustrates the THR curves of coated PU foams and the controls. Neat PU foam has a THR of 20 MJ/m2,



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while the corresponding value for A7.5C7.5 is only 4 MJ/m2. Most of the coated foams have THR values similar to those as neat PU foam, indicating complete combustion of the foam matrixes.

A5C10-1.5mm has a THR of 16 MJ/m2, 80% of

that of neat PU, probably due to the incomplete decomposition of PU foam due to a higher concentration of clay.

As is shown in Figure 9, neat PU foam almost burned

completely, whereas the shape of alginate/clay aerogel remains unchanged after burning. For the coated PU foams, the coating itself remains in large part, but the foam inside almost completely burned out. Interestingly, A5C10-1.5mm has an intact shape after test (43.7% residue left, as listed in Table 4), probably preserving foam inside from completely burning, and corresponding to a slightly reduced THR. These results suggest that aerogel coatings do not significantly reduce the total amount of the decomposition. Figure 10 presents the curves of TSR as a function of the burning time. It can be seen that the smoke release was significantly decreased with aerogel coating. A5C10-1.5mm has the lowest TSR of 581 m2/m2, compared with 1527 m2/m2 for the neat PU foam, probably attributable to the denser microstructure resulting from the high clay concentration.26 Carbon monoxide production was also obtained to study the toxicity of combustion products (Figure 11). Both neat PU foam and alginate/clay aerogel have comparatively high CO production, while those of PU foams with aerogel coating are much lower.

Figure 12 shows the CO2 production of the

materials as a function of burning time. The shape of the curves is almost the same as the HRR curves, indicating the complete combustion of the most materials under heat


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flux. Figure 10, Figure 11, and Figure 12 indicate the decreased toxic combustion products with aerogel coatings, thus decreasing the risk of suffocating in a real fire. The possible flame retardant mechanism for the aerogel-coated materials can be proposed considering all the combustion parameters above: the coated PU foams have similar THR as the controls, suggesting that the aerogel coating does not reduce the foam fuel content of the sample in a fire. The flame spread rate (decreased TTI, HRR and FIGRA) and toxic decomposition products (smoke and CO) significantly decreased with treatment; this may be attributed to the barrier effect of aerogel coating. The clay-containing aerogel maintains a stable porous structure with low thermal conductivity after being burned (Figure 5), which protects the polymer underneath from decomposing (decreasing the burning rate), and may also act as filter of incompletely combusted particles (decreasing smoke release; Figure 13). Conclusions Highly efficient flame retardant polyurethane foams with alginate/clay aerogel coatings were fabricated using a freeze drying method. SEM characterization revealed porous structure of both foam and aerogel, however with different pore size. The aerogel coating has strong interaction with PU foam matrix, because the infusion of aerogel into the porous structure of the foam matrix and tension generating during freeze-drying process. TGA tests show that both foam matrix and aerogel coating have good thermal stability. Combustion testing showed significantly increased LOI values and TTI with aerogel coatings. HRR, TSR, FIGRA, CO and CO2 production all decreased significantly, much lower than those of neat PU foams, translating to



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significantly reduced fire danger. The THR of the samples, however, were unchanged by coating, except for the A5C10-1.5mm composition, indicating the mechanism of flame retardancy is likely due to decrease in the flame spread rate by forming heat, oxygen and smoke particle barriers (the burned aerogel coating), but not by reducing fuel content. This facile and inexpensive post-treatment of PU foam make it promising in insulation materials with requirement of fire safety.

Acknowledgements Support by NSAF (Grant No. U1530259) and National Science Foundation of China (Grant No. 51403192, 51503191), are gratefully acknowledged.


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24. Laufer, G.; Kirkland, C.; Cain, A. A.; Grunlan, J. C. Clay–chitosan Nanobrick Walls: Completely Renewable Gas Barrier and Flame-retardant Nanocoatings, ACS Appl. Mater. Inter. 2012, 4, 1643-1649. 25. Chen, H. B.; Wang, Y. Z.; Schiraldi, D. A. Preparation and Flammability of Poly (vinyl alcohol) Composite Aerogels, ACS Appl. Mater. Inter. 2014, 6, 6790-6796. 26. Chen, H. B.; Wang, Y. Z.; Sanchez-Soto, M.; Schiraldi, D. A. Low Flammability, Foam-like Materials Based on Ammonium Alginate and Sodium Montmorillonite Clay, Polymer 2012, 53, 5825-5831. 27. Chen, H. B.; Liu, B.; Huang, W.; Wang, J. S.; Zeng, G.; Wu, W. H.; Schiraldi, D. A. Fabrication and Properties of Irradiation-cross-linked Poly(vinyl alcohol)/clay Aerogel Composites, ACS Appl. Mater. Inter. 2014, 6, 16227-16236. 28. Chen, H. B.; Hollinger, E.; Wang, Y. Z.; Schiraldi, D. A. Facile Fabrication of Poly(vinyl alcohol) Gels and Derivative Aerogels, Polymer 2014, 55, 380-384. 29. Chen, H. B.; Chiou, B. S.; Wang, Y. Z.; Schiraldi, D. A. Biodegradable Pectin/clay Aerogels, ACS Appl. Mater. Inter. 2013, 5, 1715-1721. 30. Zhang, J.; Ji, Q.; Shen, X.; Xia, Y.; Tan, L.; Kong, Q. Pyrolysis Products and Thermal Degradation Mechanism of Intrinsically Flame-retardant Calcium Alginate Fibre, Polym. Degrad. Stab. 2011, 96, 936-942.


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Figure 1. Procedure for preparation of PU foam with alginate/clay coatings.



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Figure 2. SEM micrographs of coated PU foams and the controls: A and B, foam matrix/aerogel interfaces; C. lower magnification of PU foam; D. higher magnification of alginate/clay aerogel.


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Figure 3. Photos of coated PU foams and the controls: A. Specimen with half PU foam and half aerogel; B and C are the cross sections of fractured sample during tensile testing; D. magnification of the interface between foam and aerogel.



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Figure 4. TGA weight loss and DTG curves of neat PU foam and alginate/clay aerogels.


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Figure 5. Char residue of PU foam and aerogel after LOI test: A. PU foam residue; B. Aerogel residue.



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Figure 6. HRRs of coated PU foams and the controls as a function of the burning time.


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Figure 7. Weight loss of coated PU foams and the controls as a function of the burning time.



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Figure 8. THRs of coated PU foams and the controls as a function of the burning time.


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Figure 9. Photos of coated PU foams and the controls as well as their residues after burning in a cone calorimeter: A. PU foam; B. A7.5C7.5; C. A5C5-1.5mm; D. A5C10-1.5mm; E. A7.5C7.5-0.2mm; F. A7.5C7.5-0.7mm; G. A7.5C7.5-1.5mm.



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Figure 10. TSRs of coated PU foams and the controls as a function of the burning time.


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Figure 11. CO production of coated PU foams and the controls as a function of the burning time.



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Figure 12. CO2 production of coated PU foams and the controls as a function of the burning time.


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Figure 13. Flame retardant mechanism of coated PU with alginate/clay aerogel.



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Table 1. Formulations of neat PU foam 4110

PEG

H2O

A33

DMP30 Surfactant n-hexane

PMDI

(php)

(php)

(php)

(php)

(php)

(php)

(php)

(php)

70

30

2.0

2.5

0.5

2.0

15

143

Sample

Neat PU php: parts per hundred of polyol (4110 and PEG) by weight.


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Table 2. Mechanical property of PU foam and alginate/clay aerogels Property Neat PU A5C5 A7.5C7.5 A5C10 Modulus 3.6±0.5 5.8±0. 7 21.2±3.0 16.5±4.5 Density 0.04±0.03 0.09±0 0.11±0 0.13±0.01 Specific modulus

87.6±9.5

68.2±7.9

196.3±27.2


126.9±36.22


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Table 3.TGA data of neat PU foam and alginate/clay aerogels Samples

Td 5%(°C)

Td 15% (°C)

Td max(°C)

dW/dT (%/°C)

PU

262.1

289.1

307.5

8.28

0.6

A5C5

58.4

235.4

243.4

7.89

52.6

A5C10

64.6

240.6

242.6

4.92

62.7

A7.5C7.5

60.6

234.6

243.6

8.48

50.6


Residue (%)

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Table 4. Burning parameters of PU/aerogel composites

Sample

PU A7.5C7.5

Mass LOI Mean PHRR Mean of PU TTI HRR THR FIGRA TSR Residue 2 TTPHRR(s) SEA 2 matrix (%) (s) (kW/m ) (MJ/m2) (W/s) (m2/m2) 2 (m /kg) ) (kW/m (g) 7.5 17.0 1 0

F

No flame

323

54

25

20

12.9

1527

1616

0.1

20

10

35

4

0.6

19.3

0

65.1

A5C5-1.5mm

8.2 32.5 7

144

52

80

21

1.8

929

695

14.8

A5C10-1.5mm

7.5 36.5 10

110

41

35

16

3.1

581

624

43.7

A7.5C7.5-0.2mm 7.7

38

4

223

49

35

19

6.4

817

819

9.4

A7.5C7.5-0.7mm 7.6

60

3

220

61

35

19

6.3

964

960

19.7

A7.5C7.5-1.5mm 7.8

F

10

128

54

60

19

2.1

820

751

34.0



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Procedure for preparation of PU foam with alginate/clay coatings 338x190mm (96 x 96 DPI)


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Highly Efficient Flame Retardant Polyurethane ... - ACS Publications

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