Fire-Resisting Composites Based on Polymer Matrix

. In the present work we have studied the thermal stability, flammability, fume evolution of epoxy matrix composites with different types and amounts of hardeners, fillers, plasticizers and fire retardants. It is shown that chemical composition of fillers has little effect on the flammability of epoxy composites when the content of mineral fillers is less than 45% by mass. Smoke formation decreases linearly with increasing the degree of filling. It is shown that to obtain low-combustible materials the oxygen index should exceed 31%. It was found that the optimal concentration of industrial brominated fire retardants is 8-10% by mass. To reduce the flammability of epoxy composites, additive brominated fire retardants in the form of a solution in N,N-dimethyl-2,4,6-tribromoaniline have been proposed. The high efficiency of acetyl- and α-hydroxyethyl ferrocene as a smoke suppressor of epoxy composites is shown.


Introduction
Nowadays, epoxy oligomers are widely used as binders in construction industry. Constructional composites with epoxy matricesepoxy matrix composites (EMC)are often characterized by unique combinations of operational properties. Broad range of available epoxy resins and hardeners allows precise control of elastic modulus (in case of continuous reinforcement fibers, the well known application is composite reinforcement), compressive and tensile strength, water, frost and chemical resistance of EMC [1][2][3][4][5]. Epoxy resins are used as binders for production of composite reinforcement (with carbon, basalt and glass fibers), chemically resistant paint coatings, polymer mortars and concretes, foams, adhesives and monolithic floor coverings [6][7][8][9][10]. EMC often outperform even advanced building materials with inorganic matrices [11], especially when the dispersed phases were subjects of special processing [12].
Still, quantitative dependencies between amount, chemical type of admixtures and properties related to fire safety of EMC till now are not revealed in detail. The purpose of this work is to study the dependencies between flammability, combustibility, fume evolution of EMC and amount and chemical type of fillers and admixtures.
Thermal analysis of fillers, flame retardants, ferrocene derivatives and EMC in air and nitrogen flow was carried out using "DuPont-9900" automated modular thermoanalytical system (heating rate was 10 0 C/min). Limiting oxygen index (LOI), ignition (Ti) and autoignition (Tsi) temperature, fume evolution index (Dm) in pyrolysis and flame combustion mode, heat of combustion, critical density of heat flux of ignition (qcr), limiting concentration of oxygen (cl) and velocity of flame propagation (Vpn) on the horizontal surface of epoxy polymers and EMC based on such polymers at different oxygen concentrations in the oxidizer stream were determined according to RU GOST 12.1.044 and methods given in [35][36][37].

Results and Discussion
The presence of epoxy (1-24%) and hydroxyl (0,2-10%) functional groups in diane epoxy resins allows them to be cured by various classes of organic, inorganic and organoelement compounds [3][4][5]. It has been established that the chemical structure of amine curing agents has little effect on the flammability and smoke-forming ability of epoxy composites [41]. Therefore, the choice of amine curing agents should be carried out only taking into account the required technological and operational parameters of epoxy composites.
An effective method of increasing the modulus and strength of EMC is the modification by various compounds [24]. Low-molecular organic compounds increase the combustibility and Dm of EMC. Industrial grades of phosphate plasticizers increase the LOI and Tsi of epoxy binders from 22.1% and 470 0 C up to 23.1-24.5% and 500-510 ° C, respectively, reduce the qcr value from 11.7 to 9.2-10.7 kW/m 2 [20]. The low efficiency of phosphate plasticizers is due to their relatively high combustibility: LOI of plasticizers is 23.1-29.9%, while LOI of ED-20 polymer is 21.6-22.3% [14]. At the same time, chemical type of plasticizers has a significant effect on the smoke-forming ability of EMC. For example, Dm in pyrolysis and flame combustion mode of epoxy polymer plasticized by 8.3-15.4% by diphenyl(2-ethylhexyl)phosphate is reduced from 980 and 990 m 2 /kg down to to 910-640 m 2 /kg and 840-970 m 2 /kg, respectively. Trichloroethyl(propyl)phosphates increase Dm in the pyrolysis mode.
If mass rate of mineral filler is less than 45%, the chemical nature of filler has little effect on the fire hazard properies of epoxy composites: LOI is 19.8-22.3%, Ti is 280-310 0 C, Tsi is 480-515 0 C, qcr is 10.6-16.2 kW/m 2 , Dm in the pyrolysis and combustion mode is 520-840 and 350-

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540 m 2 /kg, respectively (Table 1; amount of fillers is 43.5% by mass, amount of plasticizer is 8.7% by mass). Significant reduction of the fire hazard of EMC is observed when mass rate of mineral filler exceeds 50%: for 61% LOI grows up to 30.8%, Tiup to 290-320 0 C, Tsiup to 490-520 0 C, qcr is linearly increased from 10.3 to 12.2-18.5 kW/m 2 . If mass rate of mineral filler is more than 20%, than fume evolution index in the pyrolysis and flame combustion mode linearly decreases from 1470 and 800 m 2 /kg up to 460-550 and 190-250 m 2 /kg, respectively. This index only slightly depends on the chemical nature of the fillers [20]. For all the studied composites, a linear dependence between 100/LOI and relative content of mineral fillers (k) is observed. The tangent of the slope of the lines to the abscissa axis (tgα) is proportional to the specific heat absorption of the filler (Figure 1). The value of tgα can be used to evaluate the efficiency of fillers as fire retardants [16]. The values of tgα for the plasticized ED-20 polymer are given below: An effective method of reducing the flammability of epoxy composites is the use of additive brominated fire retardants. As a rule, LOI of such retardants exceeds 90% (for N(2,4,6tribromophenyl) maleimide LOI is 65%). Their heat of combustion is 9.4-10.8 kJ/kg. The influence of the chemical structure of brominated fire retardants (amount is 5.7% by mass) on the fire hazard of epoxy composites modified with SKN-26-1A synthetic rubber and filled with grinded quartz sand (amount is 41% by mass) is presented in Table 2. As it follows from Table 2, aromatic brominated fire retardants reduce the combustibility and flammability of EMC: LOI and cl of composites increase from 21.6 and 34.0% up to 27.2-28.8 and 36.1-39.6%, respectively; VFS decreases from 0.41 down to 0.23-0.31 mm/s; Ti decreases by ~20 0 C, and Tsi is 460-480 0 С. Dm of epoxy composites in the pyrolysis mode slightly increases from 410 to 440-490 m 2 /kg, and in the combustion mode it increases from 570 up to 890-990 m 2 /kg. At the same time, the chemical structure of industrial grades of additive aromatic brominated fire retardants has a negligible effect on the combustibility of epoxy composites. It should be noted that purified hexachlorobenzene, while been less efficient than hexabromobenchole, practically does not increase the smoke forming ability of EMC. The mechanism of their action is

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caused both by inhibition of radical chains of processes in the flame, and by the phlegmatization of the flame by products of decomposition of brominated fire retardants. It should be noted that the LOI of glass-reinforced bromine-containing epoxy resins is well correlated with the flammability index when tested by the ceramic pipe method ( Figure 2). As it can be seen from Figure 2, the flame-retardant (low-combustible) composites can be obtained when LOI is more than 31%, and incombustible (moderately combustible)when LOI is more than 27%. With the increase of the content of additive brominated fire retardants, the flammability and combustibility of epoxy composites decrease. For example, with an increase of the content of tetrabromodian to 9.8% the Ti of EMC decreases from 300 to 280 0 С, Tsi increases from 460 to 490 0 С, and LOI increases from 21.6 to 29.2%. Dm in pyrolysis mode is practically independent of the content of fire retardants and its value is 420-440 m 2 /kg. In the combustion mode Dm increases from 570 to 990 m 2 /kg. The dependence between LOI of ECM and amount of bromine in the composite is shown in Figure 3.
Influence of Redant bromine-organic fire retardants on the heat resistance and fire hazard of EMC is shown in Table 3 (amount of fire retardants is 8.6 % by mass, amount of marshalite is 47.2 % by mass). EMC modified with Redant 1-C have a higher thermal conductivity (the specific heat of combustion of the composition containing 20 wt. parts of Redant 1 is 31570 kJ/kg, while for materials modified with Redant 2 and Redant 1-2 these values are 29030 and 29900 kJ/kg, respectively) and also characterized by reduced heat resistance. The mass burnup rate of composites modified with 20 wt. parts of Redant 1 and Redant 2-1 are 32, 17 and 30.59 g/(m 2 ·s), respectively, with a heat flux density of 10.58 kW/m 2 .
Considering that additive brominated fire retardants are crystalline or amphoric powders, is was suggested to use them in form of solution in N,N-dimethyl-2,4,6-tribromoaniline. It was revealed that as the concentration of Redant 1 in N,N-dimethyl-2,4,6-tribromoaniline increases up to 50%, the LOI of composites increase from 25.8% to 30.1%, mass burnup rate at a warm flux density of 10.58 kW/m 2 is reduced from 29.1 to 23.   Thus, the use of industrial grades of additive brominated fire retardants allows the production of EMC with LOI of 31-33% for bromine concentration of 6-8% by mass. The optimal amount of such fire retardants is 8-10% by mass. For such concentation, the strength of EMC is only slightly reduced. The mechanism of action of bromine-containing fire retardants was considered in detail in [23][24][25].
The effect of the content of reactive bromine-containing epoxy compounds on the combustibility of cured binders is shown in Figure 6. As it follows from Figure 6, bromine-containing epoxy oligomer UP-631 is more effective than aniline-modified diglycidyl ether of tetrabromodiamine and brominated ED-22 epoxy oligomer. This is due to different concentration of bromine in these epoxy compounds. This conclusion is confirmed by the experimentally observed linear dependence between the LOI epoxy polymers and bromine concentration (Figure 7). It should be noted that reactive bromine-containing compounds are significantly less efficient than additive fire retardants. For example, to obtain composites with LOI equal to 27%, the concentration of bromine in the composition with pentabromophenol has to be 8.3%, while concentation of bromine-containing UP-631 oligomer has to be 20%. This is due to higher concentration of bromine in additive brominated fore retardants (58-86%) compared to brominecontaining epoxy compounds (bromine concentration is 25.0-48.8 The promising direction of reducing the combustibility of EMC is the use of microencapsulated fire retardants. Microcapsules containing fire retardants explosively break down when exposed to elevated temperatures or flames, throwing the vaporized flame-extinguishing agent into the flame zone. As an example, Figure 8 illustrates the effect of the content of microencapsulated fire retardants on the combustibility of EMC. In this case, the microencapsulated DBDFO is inferior to the ordinary DBDFO. The efficiency of microencapsulated fire retardants depends mainly on the diameter of the microspheres, chemical nature of the fire retardant used and, to a lesser extent, on the chemical nature of the microcapsule shell. The effects of microencapsulated DBDFO (5.85% by mass) on the combustibility of epoxy composites filled with marshallite (41.8% by mass) are summarized in Table 4.  Type of decabromodiphenyl oxide LOI, % DBDPO with a particle diameter of 240 μm 36.9 DBDPO with a shell of epoxy resin with a particle diameter of 150-400 μm 32.1 DBDPO with a shell of epoxy resin with a particle diameter of at least 150 μm 33.5 DBDPO with a shell of a copolymer of styrene and N(2.4.6-tribromophenyl)-maleimide with a particle diameter of at least 150 μm 32.1 DBDPO with a shell of a copolymer of styrene and N(2.4.6-tribromophenyl)-maleimide with a particle diameter of not more than 150 μm 32.8 DBDPO with a shell of an aromatic polyamide with a particle diameter of 150-400 μm 32.1 DBDPO with a shell of an aromatic polyamide with a particle diameter of at least 150 μm 32.1 Chladon-II4B2 with a shell of polyvinyl alcohol with a particle diameter of 125-250 μm 28.6 The effective smoke suppressor of polymeric materials is ferrocene (Figure 9). It has been established that the cyclopentadienyl sandwich-like derivatives of iron do not affect the ignition temperature (220-230 0 C) and decomposition of epoxy compositions in the low-temperature region ( Table 5), but significantly affect pyrolysis at temperatures above 300 0 C. In later case, the higher the efficiency of the ferrocene derivatives, the higher the decomposition rate of the epoxy composites in the second stage. The LOI of EMC with ferrocene derivatives can also be increased ( Figure 10).
The high efficiency of acetyl-and α-hydroxyethylferrocene in comparison with ordinary ferrocene is due to the ease of formation of ultradispersed catalytically active iron oxides (Fe2O3, Fe3O4) during pyrolysis of derivatives. This affects the mechanism of pyrolysis of carbonizing polymers, increases the probability of formation of carbonized layer on the surface of the polymer, supresses formation of benzene and smoke. Optimal concentration of ferrocene derivatives in EMC is 0.3-0.5% by mass.

Conclusion
In the present work, we summarized and discussed the results of several laboratory tests. The tests were carried out to answer the question concerning the suitability and effectiveness of different measures that allow to reduce fire hazards of EMC. Considerable amount of experimental data concerning properties of EMC was obtained. Influence of the type and content of different fillers and fire retardants to the heat resistance, ignitability, flammability and smoke-forming ability of EMC was examined. The compositions of EMC that are characterized by elevated performance properties, including low combustibility, are determined. The developed composites successfully can be used in the construction industry.