Fire and Polymers V - American Chemical Society

Chapter 8

Downloaded by STANFORD UNIV GREEN LIBR on June 28, 2012 | http://pubs.acs.org Publication Date: April 27, 2009 | doi: 10.1021/bk-2009-1013.ch008

Development of Polyurea Nanocomposites with Improved Fire Retardancy 1

1

1,3

Walid H . Awad , Calistor Nyambo , Seogjun Kim , Robert J . Dinan , Jeff W. Fisher , and Charles A. Wilkie * 2

2

1,

1

Department of Chemistry and Fire Retardant Research Facility, Marquette University, P.O. Box 1881, Milwaukee, WI 53201 Air Force Research Laboratory, Tyndall Air Force Base, FL 32403 Permanent address: Department of Nano and Chemical Engineering, Kunsan National University, Kunsan, Chonbuk 573 701, South Korea 2

3

The two-component polyurea elastomeric system is a unique technology with a wide range of applications in the coating, lining and caulking industries. Polyureas have shown remarkable properties including, fast cure, abrasion resistance, water repellency, flexibility, exceptional elastomeric qualities, resistance to thermal shock, good adhesion proerties, etc. Despite all these characteristics, one drawback for polyureas is that, like all organic polymers, they have relatively high flammability, which puts restrictions on their broad application. The main objective of this work is to develop a polyurea system with orders of magnitude improvement in fire performance using a nanoadditive approach in combination with conventional flame retardants (FR). The cone calorimeter was used as the tool of evaluation.

102

© 2009 American Chemical Society

In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.


103 Polyurea, as a state of the art coating system, exhibits a variety of remarkable physical and mechanical properties that have a wide range of industrial and commercial applications (1). Pure polyurea is derived from the reaction of an isocyanate and a diamine without a catalyst. The isocyanate can be aromatic or aliphatic in nature. Polyureas are of interest because they are successfully used in protective systems against blast and ballistic effects. The right combination of tensile strength, stiffness and elongation at rupture are critical in this role. One of the challenges is to develop a suitable fire retardant package while maintaining these qualities. In order to accomplish this, the additive package must be limited. Over the last 15 years, a new type of technology capable of reinforcing the polymer on the nanoscale level has drawn considerable attention. This developing technology, termed polymer nanocomposites, uses nanoparticles as the reinforcing fillers. By dispersing the nanometer thick, high aspect ratio, particles in a polymer matrix, a wide array of property enhancements can be achieved, e.g., enhanced mechanical properties (2-4), and superior flame retardancy (5,6). Polymer nanocomposites can be reinforced by iso-dimensional phases, which have three dimensions in the nanometer range such as silica nanoparticles obtained by in-situ sol-gel methods or by polymerization promoted directly from their surface. They can also be reinforced by a phase that has only two dimensions in the nanometer scale, such as carbon nanotubes. A third type of nanocomposite incorporates a reinforcing phase in the form of platelets with only one dimension on the nano-level. Polymer/ layered silicate nanocomposites belong to this class. The utility of nanoparticles as reinforcing additives and potential flame retardants for the polyurea system will be explored. In previous work (7) we have shown that a significant reduction in the peak heat release rate was obtained by blending the polyurea with phosphorus-based flame retardants such as ammonium polyphosphate (APP), melamine polyphosphate or Fyrol P M P (a proprietary phosphonate). As part of our continuing effort to develop polyurea materials with a reduction in peak heat release rate well above 90%, different classes of nanoparticle additives, such as nanoclays, carbon nanotubes and polyhedral oligomeric silsesquioxane (POSS), have been explored for potential synergistic formulations with conventional flame retardants.

Experimental Materials The materials used throughout this work are: Isocynate (Isonate 143L from Dow Chemical Company), Diamine (Versalink PI000 from A i r Products),


104 Ammonium Polyphosphate A P P , (ICL Performance Products Inc), Expandable Graphite (Graftech), Firebrake Zinc Borate (Borax), Cloisite 30B (Southern Clay Products), Carbon nanotubes (Nanocyl) and POSS Poly(vinylsilsesquioxane) (Hybrid Plastics).


Sample preparation The polyurea samples were prepared in a beaker by mixing the additives in the diamine for several minutes. The dispersion of the additives was achieved using mechanical mixing followed by ultrasonication. Then, the isocyanate was added to the mixture with continuous stirring for one minute, and the contents of the beaker were poured into a mold. The samples were cured at room temperature for 12 hours, and then were placed in a vacuum oven at 70 °C for additional 24 Hours.

Instrumentation Cone calorimetry was performed using an Atlas Cone 2 instrument according to A S T M E 1354 at an incident flux of 50 K W / m using a cone shaped heater. Exhaust flow was set at 24 L/s and the spark was continuous until the sample ignited. Typical results from cone calorimetry are reproducible to within about ± 10%; these uncertainties are based on many runs in which thousands of samples have been combusted. X-ray diffraction ( X R D ) data was obtained using a Rigaku MiniflexII diffractometer equipped with C u - K source (X = 1.5404 A). It was operated at 50 k V and 20 mA and scans were obtained from 20 = 1 to 10 °, at 0.1 step size. 2

a

0

Results and Discussion Polyurea, as a state of the art coating system, exhibits extraordinary performance characteristics such as: fast cure (gel time < 5 seconds), abrasion resistance, water repellency, flexibility, exceptional elastomeric quality, resistance to thermal shock and good adhesion properties. The use of nano materials can result in enhanced modulus, strength, elongation at break, toughness, glass transition temperature, reduced thermal shrinkage as well as reduced flammability. The nano-additives-based polyurea materials hold the potential of providing a new generation of materials to withstand blast and ballistic effects. Two approaches have been employed during the course of this investigation to reduce the flammability of a polyurea system. The first is to use nanoparticles with different dimensionality and aspect ratios, such as organo clay, carbon nano-


105 tubes and POSS materials. The second approach is to use conventional flame retardants, such as expandable graphite, ammonium polyphosphate and zinc borate.


Polyurea nanocomposite X-ray patterns for the pristine organoclay (Cloisite 30B) reveals a broad intense peak around 20 = 5° corresponding to a basal spacing of 1.85 nm. This peak disappears in a polyurea nanocomposite having an approximate clay concentration of 3 wt.% . The disappearance of the diffraction peak for the clay after compounding with polyurea indicates good dispersion of the clay in the polymer matrix and the formation of a disordered nanostructure. A graphite-based nanocomposite was obtained by dispersing expandable graphite particles in the polyurea matrix using ultrasonification. Expandable graphite is a sulfuric acid intercalated graphite. X R D for the pure expandable graphite gave a very strong peak at 20 = 26.6° corresponding to the typical dspacing between the carbon layers in graphite (0.33 nm). The huge reduction in the intensity of the diffraction peak for the expandable graphite after compounding with polyurea provides evidence for the dispersion of the graphite particles in the polyurea matrix.

Flammability Measurement We have characterized the flammability properties of polyurea formulations, under fire like conditions, using the cone calorimeter. A set of fire-relevant properties can be obtained from the cone calorimeter, such as peak heat release rate (PHRR), mass loss rate (MLR), specific extinction area (SEA), ignition time (t ), and specific heat of combustion. These data will give indications about the fire performance of the polyurea materials. ign

Nano-additive formulations The effect of nanoclays, carbon nanotubes and POSS on the flammability properties of polyurea have been investigated. Ultrasonication was employed to achieve good dispersion of these nanoparticles in the polyurea matrix. A n improvement in the flammability properties of the base polyurea has been obtained with nanoscale additives and these filled systems provide an alternative to conventional flame retardants. The cone calorimeter data for polyurea-Cloisite 30B nanocomposite showed that the peak heat release rate (PHRR) of polyurea is reduced (33%) upon the addition of the nanoclay. For carbon nanotubes and POSS nanoparticles the results are more significant. A substantial reduction (45%) in the peak heat



106 release rate (PHRR) has been obtained by the dispersion of multiwall carbon nanotubes in the polyurea matrix at 0.5 wt.% loading, as shown in Figure 1. Increasing the loading of carbon nanotubes above this limit has no significant effect on the flammability properties of polyurea. A more significant reduction in P H R R (73%) has been achieved by dispersing POSS nanoparticles in the polyurea matrix (Figure 2). This improvement in flammability properties have been obtained at loading of 5 wt.%. Again, increasing the loading of POSS above this limit has little effect on PHRR value. Two mechanisms have been reported in the literature to describe the improvement in flame retardancy of nanocomposites. The first mechanism proposed by Gilman et. al (8) states that during the burning process, a carbonaceous silicate multilayer structure is formed. This layer acts as a barrier to protect the underlying polymer from further degradation. Recently, a nanoconfinement mechanism has been proposed to describe the improvement in the flame retardancy of nanocomposites (9). According to this mechanism, the radicals produced from the polymer degradation are nanoconfined permitting a variety of bimolecular reactions to occur. On the other hand, Kashiwagi et. al (10) have indicated that the enhancement in the flammability properties of nanocomposites is due to the formation of a network structure protective layer, and that the aspect ratio of the nanoparticles has an impact on the flammability properties. The particles with higher aspect ratio tend to form a more protective network structure that is critical to the reduction of the heat release rate of the nanocomposites (11).

Conventional flame retardants formulations As a flame retardant, the important property of expandable graphite is the onset of expansion temperature and the expansion volume. Therefore, using expandable graphite, the temperature should be controlled to avoid its expansion during the processing. The d-spacing in expandable graphite increases from 0.335 nm to approximately 0.789 nm, and upon heating one obtains expanded graphite, in which an increase in the d-spacing of several hundreds times along the c axis is observed. Our results indicated that expandable graphite (EG) is effective in reducing the flammability of polyurea. Figure 3 compares the heat release rate curves for different loadings of expandable graphite. The peak heat release rate (PHRR) was reduced by up to 85% compared to the pure resin at 3% loading content. Increasing the loading of expandable graphite made a little improvement in P H R R value. As shown in Figure 3, the reduction obtained in PHRR values were 86% and 88% at 5 wt.% and 10 wt. % respectively.



Figure 1. heat release rate curves for polyurea/CNT

nanocomposite.


©


Ο

100

150

250

Time (Seconds)

200

Figure 2. heat release rate curves for polyurea/POSS

50

nanocomposites.

300


350

400

© 00


100

200

300

500 Time (seconds)

400

600

700

800

900

Figure 3. Heat release curves for polyurea with different loading contents of expandable graphite

Ο


ν©

©


110 Expandable graphite acts as a blowing agent as well as a carbonization agent. The intumescent shield formed after burning is composed of expanded graphite 'worms' embedded in the degraded matrix of P U . Its structure consists of a carbonaceous and polyaromatic structure. This expanded carbon layer works as an insulating layer to reduce the heat transfer (12-14). The influence of ammonium polyphosphate (APP) on the flammability of polyurea has also been examined. The chain length (n) of this polymeric compound is both variable and branched, and can be greater than 1000. Short and linear chain APPs (n < 100) are more water sensitive (hydrolysis) and less thermally stable than longer chain APPs (n >1000), which show a very low water solubility. Cone calorimetry at an external heat flux of 50 K W / m for polyurea compounded with varying amounts of A P P reveals significant reduction in PHRR values. There is a 67% reduction in PHRR value at 3 wt. % loading content. Increasing the A P P content to 5 wt.% , 10 wt.% and 15 wt. %, the reduction in P H R R values were 68%, 75% and 79% respectively (plots are not shown). Ammonium polyphosphate is known to have an intumescent effect, when exposed to heat. When exposed to fire, A P P starts to decompose, commonly into polymeric phosphoric acid and ammonia. The polyphosphoric acid then converts into a non-stable phosphate ester. The dehydration of the phosphate ester follows and carbon foam is formed on the surface of the sample preventing further decomposition of the polymer (15,16). Zinc borate (ZB) also proved to be effective in imparting flame retardancy to the polyurea. A 36 % reduction in PHRR was obtained at 3 wt.% loading content (not shown). Increasing the zinc borate content to 5 wt.% only gave a slight improvement in the flammability properties of polyurea (41% reduction in PHRR value). The principle mechanism of zinc borate is the formation of a glassy char in the condensed phase. This acts as a fire resistant coating that protects unburned polymer from contact with heat and further combustion. Approximately 14% of the zinc borate crystal is composed of water, which contributes to flame retardation by reducing the energy in a burning system when it is released at 290 °C (an endothermic reaction). 2

Conventional flame retardants with nano-additives The question of synergy between the traditional flame retardants and nanoparticles has been addressed during this investigation. Adding carbon nanotubes at 0.5 wt% to 3 wt% expandable graphite yieded a slight decrease in PHRR value from 85% to 88%. Adding zinc borate to the previous formulation had no impact on the PHRR value. The cone calorimeter residue indicated that both the carbon nanotubes and zinc borate enhanced the integrity of the char structure.



Ill Figure 4 shows that combining the intumescent additives, expandable graphite and ammonium polyphosphate in polyurea at 10 wt.% loading from each, gave a 86% reduction in the PHRR value. Adding 0.2 wt% carbon nanotubes to this intumescent formulation delivered 91% reduction in PHRR value . Another remarkable improvement in the flammability properties of polyurea was achieved by adding either zinc borate or POSS at 3 wt.% to the following formulation: PU/10% EG/10% APP. The PHRR was reduced 92% and 93%, respectively (Figure 5). The char residue indicates that zinc borate imparts integrity to the char structure while the POSS nanoparticles made the char structure more brittle.

Mechanical properties Polyureas are characterized by high elongation, good tear strength, and a moderate combination of tensile strength and modulus of elasticity. The extensive intermolecular hydrogen bonding of polyureas leads to "tough" mechanical properties. Polyureas are used as energy-absorbing material in systems that provide protection against ballistic and blast effects (17, 18). Such applications motivate the mechanical response study of the material under high strain rates (200-800 in/in per second). The P U molecular mechanism during blast and ballistic response is not entirely understood. A contributing factor is the substantial energy dissipation within due to a strain-induced transition from the rubbery to the glassy state (19). The glass transition zone of polymers is the region of greatest energy dissipation and this transition can be induced by fast deformation rates. Our goal is to develop polyurea nanocomposites that are flame retardant yet provide the needed mechanical resistance. Figures 6-8 show the engineering tensile stress-strain curves (340/s) for polyurea with different additives (APP, Cloisite 30B, and EG). The data indicated that the addition of 5% A P P does not have a significant effect on the mechanical behavior but 10% A P P does cause a signficant reduction in tensile strength, effective modulus of elasticity and elongation at rupture.(Figure 6). Likewise, an organically-modified montmorillonite at 1% loading does not have as great effect as 3% clay (Figure 7). Finally, expandable graphite does cause a significant loss in all three key properties at either 5% or 10% (Figure 8). Based upon this testing, the total additive package must be kept as low as possible to achieve fire retardancy with minimal effect on the mechanical material properties.

Conclusions The main objective was to develop a polyurea system with orders of magnitude improvement in fire performance. The impact on tensile strength,



Figure 4. HRR curves offormulations ofpolyurea with expandable graphite, ammonium polyphosphate and carbon nanotubes.



Figure 5. HRR curves ofpolyurea formulations with EG, APP, ZB and POSS.


(s/Qp£)

ΖΙΌΛ

uwajs

USJJJ

ç

dunSi^


Bjoudsoud/Cjod tumuouiuiv φίΜ uBunrfjodfo



115

Figure 7. High strain rate(340/s) measurement of polyurea with cloisite 30B

o

I

2

^ 4 ( a tenia teil Engineering Strain

5

4

Figure 8. High strain rate(340/s) of polyurea with expandable graphite


t>

116 effective modulus of elasticity and elongation at rupture was evaluated during the process. Several combinations of nanoadditives with conventional flame retardants were evaluated for this purpose. The following can be concluded: •


•

•

The nano particles, clay, carbon nanotubes and POSS, showed a substantial improvement in flammability of polyurea. Expandable graphite and ammonium polyphosphate as intumescent materials proved to be very effective in improving the flammability of polyurea. Combining the nano-additives with conventional flame retardants provided slight improvement in the flammability performance of polyurea.

Acknowledgment Work carried out at Marquette University has been supported by the U S A i r Force under award number FA8650-07-1-5901 and this support is gratefully acknowledged.

References 1. 2. 3. 4.

http://www.pda-online.org/ Wang, Z; Pinnavaia, TJ. Chem Mater 1998; 10(12): 3769. Fornes, T D ; Yoon, PJ; Keskkula, H ; Paul, DR. Polymer 2001; 42(25): 9929. Fornes, T D ; Yoon, PJ; Hunter, D L ; Keskkula, H; Paul, DR. Polymer 2002; 43(22): 5915. 5. Zhu J; Uhl FM; Morgan A B ; Wilkie C A . Chem. Mater. 2001; 13: 4649. 6. Zhu J; Morgan A B ; Lamelas FJ; Wilkie C A . Chem. Mater. 2001; 13: 3774. 7. Costache, M . C . ; Kanugh, E . M . ; Sorathia, U . ; Wilkie, C.A. J. Fire Sci., 2006, 24, 433. 8. Gilman, J.W.; Jackson, C.L.; Morgan, A . B . ; Harris, R. Jr., Manias, E.; Giannelis, E.P. Wuthenow, M.; Hilton, D. and Phillips, S.H. Chem Mater, 2000, 12, 1866. 9. Chen, K . ; Wilkie, C.A. and Vyazovkin, S. J. Phys. Chem B, 2007, 111, 12685. 10. Kashiwagi, T; Du, F; Douglas, J.F.; Winey, K.I.; Harris, R . H ; Shields, J.R. Nat Mater 2005, 4, 928. 11. Cipiriano, B . H . ; Kashiwagi, T.; Raghavan, S. R.; Yang, Y . ; Grulke, E . A . ; Yamamoto, K.; Shields, J. R.; Douglas, J. F. Polymer, 2007, 48, 6086 12. Uhl, F . M . ; Yao, Q.; Wilkie, C.A. Polym. Adv. Tech. 2005, 16, 533



117 13. Uhl, F . M . ; Yao, Q; Nakajima, H ; Manias, E. Wilkie, C.A. Polym. Degrad. Stab. 2002, 76, 111. 14. Duquesne,S.; Le Bras, M.; Bourbigot, S.; Delobel, R.; Vezin, H . ; Camino, G.; Berend, E.; Lindsay, C. and Roels, T. Fire Mater. 2003; 27:103. 15. Duquesne, S.; Le Bras, M.; Bourbigot, S.; Delobel, R.; Camino, G.; Eling, B.; Lindsay, C.; Roels, T.; Vezin, H . J. Appl. Polym. Sci, 2001, 82, 3262. 16. http://www.specialchem4polymers.com/ 17. Davidson, J.S.; Porter, J.R.; Dinan, R.J.; Hammons, M.I. and Connell, J.D. J. Perform. Constr. Facil., 2004, 18(2), 100. 18. Davidson, J.S.; Fisher, J.W.; Hammons, M.I.; Porter, J.R. and Dinan, R.J. J. Struct. Eng. 2005, 131, 1194. 19. Roland, C.M. ; Twigg, J.N. ; V u , Y. ; Mott, P.N. Polymer, 2007, 48, 574.


Fire and Polymers V - American Chemical Society

Recommend Documents