USE OF DECAHYDRODECABORATE AS FLAME RETARDANTS IN COATINGS

Transcription

1 USE OF DECAHYDRODECABORATE AS FLAME RETARDANTS IN COATINGS Austin W. Bailey, Pittsburg State University, Pittsburg, KS Dr. Charles J. Neef and Dr. Timothy Dawsey, Pittsburg State University, Pittsburg, KS Abstract As the need to protect the environment continues to increase, there is a growing demand for non-halogenated flame retardants. Two different decaborate compounds were combined with triphenylphosphine oxide into polyurethane and characterized. The thermal stability and the potential flame retardancy of the new materials were tested via thermogravimetric analysis and cone calorimetry. The cone test provided heat release rates and smoke release rates. Per the results of these tests the combination of the new decaborate, and triphenylphosphine oxide showed potential for flame retardancy at minimal amounts of flame retardant. Introduction Flame Retardants have been used in polymers for years but not without having environmental and toxicology problems. Many of these compounds are halogenated and thus environmentally persistent and can bioaccumulate, causing health problems when sufficient quantities are encountered. Also, upon combustion halogenated materials produce toxic gases and large amounts of smoke which is problematic. To circumvent this problem, non-halogenated materials are needed which are significantly more environmentally friendly [1,2]. With the push for more environmentally friendly fillers and/or additives, there has been considerable research to find new flame retardants. Non-halogenated systems, that are typically used include: phosphorousbased [1,3-5], nitrogen-based [1], silicon-based [1, 6-7], boron-based [1,8-9], intumescent systems [1, 10-12], mineral type of fillers [1], or metal hydroxides [1, 13-15]. The phosphorous-based systems are used in either oxygen or nitrogen containing polymers and promote the formation of a char layer. Also, of note the thermal decomposition products for these materials act in the condensed/vapor phase. The same goes for the products of the nitrogen containing polymers as well. The siliconbased flame retardants are known for substantially improving the thermal stability as well as the heat resistance. An important aspect of these flame retardants is that they produce much less toxic gases as compared to halogenated flame retardants. Intumescent systems grow and increase in volume when heat is applied. This is indicated by the formation of an expanded carbon char layer creating an insulating layer that protects substrates. In addition, mineral fillers that are used as flame retardants are inorganic compounds and functional fillers. Metal hydroxides decompose endothermically releasing non-flammable molecules, and therefore reduce the number of molecules that can ignite [16]. Boron-based flame retardants have a wide variety of uses including the promotion of a char layer, and preventing dripping in some polymers. One major advantage of these flame retardants is that they have shown synergistic effects with nitrogen, phosphorus, and silicon compounds in fire test performances. Some of the boron-based compounds even help to stabilize the polymer package during processing. A boron-based flame retardant that is used in industry today due to its lower environmental effects is boric acid. Even though boric acid provides flame-retardant effects it has several disadvantages. Being that it is an acidic compound; it can corrode metal substrates. Also, boric acid is not recommended for incorporation into non-polar hydrocarbon polymers since it is highly likely to migrate to the polymer surface. It is in this research that a focus was placed on combining triphenylphosphine oxide and decaborate compounds to determine the potential synergistic effects between the two compounds within a polyurethane matrix. These materials were characterized by thermogravimetric analysis and cone calorimetry. Materials A two-part mixture of polyol and isocyanate was obtained from Etco Specialty Products, Inc. in Girard, KS for the creation of the polyurethane material. The polyol is a proprietary mixture of two polyols and other materials blended at Etco. Bis(triethylammonium) decahydrodecaborate was supplied as a powder from 3M. Tetramethyl and tetrabutyl ammonium hydroxide was obtained from Acros Organics in 40 and 25 wt% solutions. Acetonitrile was obtained from Sigma-Aldrich. Triphenylphosphine oxide was obtained from Acros Organics for use as a potential flame retardant. Boric Acid was obtained from the Fisher Scientific Company. SPE ANTEC Anaheim 2017 / 2197

2 Experimental Procedures Decaborate Preparation An ion exchange column was prepared using 1M sulfuric acid followed by deionized water until a ph of 7.0 was achieved.bis(triethylammonium) decahydrodecaborate (10g) was dissolved in water (500 ml) and added to the column. Water was then added to the column until a ph of 7.0 was again achieved. The collected solution was titrated with tetramethyl or tetrabutylammonium hydroxide until the solution was at a ph of 7.0. The tetrabutyl derivative was collected using vacuum filtration, while the tetramethyl derivative had the water removed using a rotary evaporator. Both compounds were dried in a vacuum oven at 80 o C for 12 hours. Sample Preparation A design of experiment (DOE) was set up using Minitab software and a central composite design was used. The design dictated the varying amounts of flame retardant 1 (tetramethyl or tetrabutylammonium decaborate) and flame retardant 2 (triphenylphosphine oxide) and is shown below in table 1. Flame retardant 1 was added to the mixing vessel followed by flame retardant 2. The mixture of flame retardants was then dissolved in acetonitrile for ease of mixing in the polyurethane. The dissolved flame retardants were added to the polyol to give a quantity equal to 80% of the remaining mixture needed to get to 80 grams. Following this an addition of isocyanate was made to the mixture in an amount equal to the remaining mixture weight needed to get to 80 grams. The new mixture was stirred by hand for minutes until the mixture began to become viscous. It was at this point that the mixture was poured into a mold designed to provide cone calorimeter samples. Approximate mold dimensions of 100 x 100 x 30mm. The mold was then placed into a 50 C oven for 24 hours to complete curing. Table 1: Amounts of Flame Retardant 1 and 2 in Polyurethane Blends. Sample No. FR1 (wt%) FR2 (wt%) Part A (g) Part B (g) Cone Calorimeter Test Cone calorimetry was performed on a Fire Testing Technology, Limited Cone Calorimeter (United Kingdom). The tests were run per ISO :2002, with a thermal radiation power of 35kW/m 2. The flammability properties measured were heat release rate (HRR), peak heat release rate (PHRR) and smoke release rate (SRR). These values are chosen as they are important in fire safety in today s world [17-19]. Thermogravimetric Testing A sample was taken from each of the design of experiment samples for TGA testing. All samples were tested under nitrogen flow on a TA Instruments Q50 TGA machine (New Castle, DE) with platinum crucible sample holders. The heating rate was 10 C/min with a final temperature of 600 C. Results and Discussion Cone Calorimeter Testing Figure 1 shows the cone calorimetry results for the base material and the samples that had lower average HRR for decaborate compound 1. Table 2 in the appendix contains the HRR, PHRR, and total HR for each of the samples within the DOE. Figure 1 shows that most of the decaborate 1 compounds have a sharp peak as burning begins followed by a strong dip. However, the maximum peak heat release rate was observed by a strong uptick between seconds or between seconds. All the decaborate 1 containing samples except one begin to decrease and then level off around 700 seconds, with the exception not leveling off until about 1000 seconds. Figure 1: HRR of Decaborate 1 and base material SPE ANTEC Anaheim 2017 / 2198

3 The contour plot of the peak heat release rates for the DOE with decaborate compound 1 is shown in Figure 2. Figure 2 shows that combinations with lower amounts of decaborate compound 1 and phosphine oxide compound provide the lowest PHRR. If greater than 8% of either is in the sample, then the PHRR increases. The sample with the lowest PHRR was at 5% of both materials. The contour plot also shows that the highest PHRR occurs at two formulations, when there is 5% decaborate 1 and 12% phosphine oxide compound, and when there is 12% decaborate 1 and 12% phosphine oxide compound. This shows that adding more flame retardant does not cause a decrease in the PHRR. The base material and the decaborate 2 compound, curves from figure 3, very closely resemble each other except for the decaborate compound staying somewhat high for heat release rate until about 200 seconds. Both materials begin to decrease and then level off around 650 seconds. Figure 4 below shows a contour plot of the peak heat release rates based upon the varying weight percent of decaborate 2 and phosphine oxide compound. As observed for decaborate compound 1, the lowest PHRR was achieved at lower amounts of flame retardants. The lowest PHRR for decaborate 2 and phosphine oxide compound was achieved at 5% of each in the compound. Interestingly there were two areas on the contour plot where the highest PHRR was observed. One was at 5% phosphine oxide compound and 9-12 wt% decaborate 2 and the other area was at 12% of phosphine oxide compound, and 8-11 wt% decaborate 2. Figure 2: Contour Plot of Decaborate 1 PHRR Data Figure 3 shows the HRR differences between the base material and the sample that showed the lowest PHRR for decaborate compound 2. There was only one sample for decaborate compound 2 that has a lower PHRR as compared to the base sample. That was the sample with just 5% of each flame retardant mixed into the compound. Figure 4: Contour Plot of Decaborate 2 PHRR Data Figure 5 below shows the peak heat release rates for boric acid as compared to the trials of the decaborate compounds with the lowest heat release rates. It can be seen for 5% boric acid that the peak heat release rate was kw/m 2, average HRR was kw/m 2, and the time when PHRR was achieved was 85s, in addition the total HR was Mj/m 2. For 10% boric acid sample the peak heat release rate was kw/m 2 at 165s, an average HRR of kw/m 2, and a total HR of Mj/m 2. There is a PHRR difference of kw/m 2 between decaborate compound 1 run 2 and the 5% boric acid sample, showing the lowest PHRR of the decaborate samples shown in Figure 5, and the boric acid for peak heat release rate. This indicates that the decaborate 1 has potential for flame retardance applications. Figure 3: HRR of Decaborate 2 and base material SPE ANTEC Anaheim 2017 / 2199

4 24.72% char. The 8.75/8.75% sample began decomposing at 250 C with a char yield of 23.96% and the 8.75/8.75% sample decomposed at 253 C and yielded 21.42% char. The sample of 5/5% decomposed at 264 C and gave 23.11% char yield. The 5/12.5% sample began decomposing at 255 C and yielded 21.28% char. All samples containing decaborate compound 1 showed similar thermal stability and higher char yields compared to the base material. Compare to the samples with boric acid, most of the decaborate compound 1 samples also exhibited higher char yields. Figure 5: HRR of Boric Acid, Base and Representative Runs Figure 6 shows the lowest smoke release rates from the two decaborate compounds and the boric acid. The peak smoke release rate of decaborate compound 1 is m 2 /s and the time that it reaches it is 30 seconds. For decaborate compound 2 a peak smoke release rate of m 2 /s at time 50 seconds was observed and the base material showed a peak smoke release rate is m 2 /s at 245 seconds. For the 5% boric acid sample the peak smoke release rate was m 2 /s at 60 seconds. Comparing the smoke production rates of each material showed that boric acid provides the lowest amount of smoke production. However, decaborate compound 1 has a small difference compared to boric acid of m 2 /s. A small difference in smoke production supports the potential use of this material as a flame retardant. Figure 6: Smoke Production Rate for Representative Runs Thermogravimetric Testing by TGA Figure 7 below shows six TGA curves indicative of the four samples for decaborate compound 1 with the lowest degradation temperatures, at 5% degradation, and highest char yields, along with the base sample and the representative boric acid sample. The neat resin started to degrade at 284 C and has a char yield of 16.76%. The sample with 10% boric acid degrades at 268 C and yields Figure 7: TGA Curves for Decaborate Compound 1 An increase in char yield suggest synergistic effects between the decaborate and phosphine oxide compounds which improved char formation and thus protect material under the char layer. In addition, the decaborate compound 1 samples showed similar curves with a gradual decrease at 255 C followed by a second loss point at approximately 400 C, suggesting a change in the mode of degradation. This was different from the boric acid and base urethane curves that showed a gradual decrease in weight. The decaborate 1 compounds overall had a range of degradation temperatures from C and a range of char yield of 16.28% %. Figure 8 below shows the three TGA curves for decaborate compound 2 with the highest thermal stability, along with the base sample and a representative boric acid sample. From the TGA curve, it can be seen that the decaborate 2 sample began to decompose at a similar temperature as the boric acid sample. Each sample exhibited similar curves with an initial decomposition at 270 o C and then gradually decreased in weight until reaching the char point. The decaborate 2 sample showed a char yield of 19.59% which was less than the other samples shown or previously mentioned. Also, the decaborate 2 sample decomposed at C, which is only a difference of 6 C when compared to the boric acid sample. The decaborate 2 samples had an overall SPE ANTEC Anaheim 2017 / 2200

5 degradation range of C. These samples also showed a char yield ranging, from 19.50% to 29.85%. Figure 8: TGA Curves for Decaborate Compound 2 Conclusions Two decaborate compounds were synthesized using an ion exchange column and incorporated with a phosphine oxide compound into a polyurethane matrix by hand mixing. Cone calorimeter effects showed the potential of these decaborate compounds as flame retardants with heat release rates similar or less than that of the boric acid samples used for comparison. The decaborate compound with the lowest PHRR gave a value of kw/m 2. While the decaborate sample with the lowest smoke released was slightly greater for HRR but had a lower smoke release rate with a value of m 2 /s. Further research will investigate the material s affinity to char for application as thermal barrier coatings. 4. D. Chen, Q. Zheng, F. Liu, K. Xu, M. Chen, J. Thermoplas. Comp. Mater., 23, 175 (2010). 5. H. Singh, A. K. Jain, T.P. Sharma, J. Appl. Polym. Sci, 109, 2718 (2008). 6. S. Hamdani, C. Longuet, D. Perrin. J.-M Lopez- Cuesta, F. Ganachaug, Polym. Deg. Stab., 94, 465 (2006). 7. Q. Li, H. Zhong, P. Wei, P. Jian, J. Appl. Polym. Sci., 98, 2487 (2005). 8. M. Doga, A. Yilmaz, E. Bayramli, Polym. Deg. Stab., 95, 2584 (2010). 9. K.K. Shen, S. Kochesfahani, F. Jouffret, Polym. Adv. Techol., 19, 469, (2008). 10. B. Du, H. Ma, Z. Fang, Polym. Adv. Technol., 22, 1139 (2011). 11. C. Jiao, J. Zhang, F. Zhang, J. Fire Sci., 26, 455 (2008). 12. B. Li, M. Xu, Polym. Deg. Stab., 91, 1380 (2006). 13. M. Sain, S.H. Park, F. Suhara, S. Law, Polym. Deg. Stab., 83, 363 (2004). 14. S.M. Lomakin, G.E. Zaikov, E.V., Koverzanova, Oxid. Commun., 28, 451 (2005). 15. M. Antunes, J.I. Velasco, L. Haurie, J. Cell. Plast., 47, 17 (2011). 16. V. Realinho, M. Antunes, O. Santana, J. I. Velasco, SPE Foams Event, (2012) 17. M. Zanetti, G. Camino, R. Thomann, R. Mulhaupt, Polym. Papers, 42, (2001). 18. M. Zanetti, T.Kashiwagi, L. Falqui, G. Camino, Chem Mater. Papers, 14, (2002). 19. J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, J. E. Mania, E.P. Giannelis, M. Wuthenow, D. Hilton, S.H. Phillips, Chem Mater. Papers, 12, (2000). Acknowledgements The authors would like to acknowledge and thank Pittsburg State University for financial support of this project and Etco Specialty Products, Inc. for donation of materials to support the project. The Kansas Polymer Research Center should also be thanked for the use of lab space and test equipment. References 1. A. B. Morgan, C. A. Wilkie, Non-Halogenated Flame Retardant Handbook, Scrivener Publishing, Massachusetts, pp. 2, 17, 75, 143, 169, 201, 293 (2014). 2. A. Hermansson, T. Hjertberg, B. Sultan, Fire Mater., 27, 51, (2003). 3. J. Alongi, A. Frache, Polym. Deg. Stab., 95, 1928 (2010). SPE ANTEC Anaheim 2017 / 2201

6 Appendix Table 2: Cone Calorimetry Data for Base Material and Decaborate Compounds 1 &2. Sample Peak HRR (kw/m 2 ) Time to Peak (s) Average HRR (kw/m 2 ) Total HR (Mj/m 2 ) Base DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB Table 3: SRR results for All Samples Sample PSRR (m 2 /s) Time (s) Base Boric Acid % Boric Acid % DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB DB SPE ANTEC Anaheim 2017 / 2202