Effect of fillers on the foamcoke structure based on ammonium polyphosphate

S. A.Nenakhov, V. P. Pimenov, L. I. Nateykina. "Fire and explosion safety" Magazine № 7, 2009

Electronic microscopy research of the foamcoke structure on the basis of organic ammonium phosphate compounds with fillers of different nature (oxides, borates, hydroxides, graphite, etc.)  has been performed. It was shown that oxides and borates and hydroxides have the nucleating effect on coke with a high temperature of expansion. Intumescinggraphite in the composition of organo-ammonium phosphate compound imposes its own structure to coke. Analysed silicates (zeolite, ethyl silicate) demonstrated the destructive effect on the foam.

Key words: fire-resistant coating, electron microscopy, foaming, ammonium polyphosphate, film former, coke forming phase, fillers, nucleating, gasifier, oxides, borates, hydroxides, graphite, sodium tripolyphosphate, zeolite, foamcoke structure, polyhedral cells, spherical cells.

1. INTRODUCTION

Foaming fire-retardant materials consist of components of the actual foaming coke forming phase and additional functional components - foaming agent and filler, also involved in the formation of foamcoke.

Fillers can perform three different functions in the flame retardant foaming compositions: of nucleating vapor-gas dispersed phase (foam), gasifying agent and a stiffener.  In 1951, H. Scholze and E. Savile said about improving the quality of fireproof materials by the introduction of inert fillers such as borates, gypsum, zinc sulfide, titanium dioxide, clay, silica, mica, etc., based on the assumption that these inert materials should act as nucleating agents for the foam. Gas-forming properties are achieved using non-combustible inorganic fillers which decompose to form non-flammable gases (eg CO2) and bereaving of the heat from a system. A typical representative of this class of inhibitors is alumina trihydrate. About active participation of the chemical structure of the fillers in foamcoke structure reported, for example, Levchik G. F. et al, showing that at temperatures up to 600 0C ammonium polyphosphate interacts with talcum powder to form a crystalline product of types Mg2P4O12, MgP4O11, SiP2O7, Si5O (PO4) 6, and at a temperature close to 1000 0 C - to form a glassy phosphate of magnesium and silicon. The aim of this work is to study the effect of fillers of various natures on the foamcoke structure, formed during hyperthermal impact on the composition based on ammonium polyphosphate.

2. OBJECTS AND METHODS

Structural studies were performed on model compounds, including foam-carbonization stage, foaming agent and filler at a constant ratio of components. Also the structure of the composition containing no filler was studied. Foam-carbonization stage phase consisted of the three traditional components: ammonium polyphosphate (EXOLIT AP422, Clariant, Germany), polyol (Charmor PM 1940, Perstorp, Sweden) and melamine (Melafine, DSM, The Netherlands) in the traditional ratio of 3/1/1. Foaming agent - aqueous dispersion of a copolymer of vinyl acetate and vinyl ester versatic acid (Mowilith DM230, Celanese), polymer content in the dried composition was 18 wt%. As a fillers were used: titanium dioxide (Kemira 660, Kemira), barium borate (TU 113-07-014-91, FGUP UNIHIM with OZ, Russia), zinc borate (TU 113-07-015, Grade B, Federal State Unitary Enterprise "UNIHIM with OZ, Russia), magnesium hydroxide (HIMEKS, Russia), aluminum hydroxide (APYRAL 60CD, NABALtec, Germany), graphite (JLS-GR-803 and JLS-GR-1002, JLS, China), zeolite (TU 38.102168-95, NaA-II, Type "A" Company "Nizhny Novgorod sorbents, Russia), sodium tripolyphosphate (GOST 13493, Class I, Kazphosphat, Russia), ethyl silicate (GOST 26371, ethyl silicate-40, highest grade, Ltd. Penta -91 ", Russia). Filler content in the dry residue was 9.0 wt%.

Aqueous dispersion material was prepared by mixing fillers with other pre-dispersed and mixed units in dissolver (room temperature, the speed of cutters 600 rpm) for 15 minutes up to degree of grinding (GOST 6589-74) of about 30 microns (compositions without filler, with titanium dioxide with sodium tripolyphosphate) and 100 microns (formulations with aluminum hydroxide, with graphite JLS-GR-1002). Then formed a coating on a steel plate so that the thickness of the dry coating was about 1 mm. Coatings were dried at 20 0C for 6 hours, then at 60 0C for 2 hours, and again at 20 0 C for 24 hours. Dried coatings were exposed to a one-dimensional heat flow under the so-called standard curve of fire until the steel substrate temperature of 500 0C (time to reach maximum temperature was 18 ± 4 min), and the foaming ratio of coke reached 15-20 units.

Foam coke was cut on samples with size 3h3h3 mm, which were studied by scanning electron-probe microscopy and electron probe X-ray microanalysis using a scanning electron microscope JSM-U3 firm «JEOL» (Japan), and using an attachment in the form of digital scanning and X-ray spectrometer with energy dispersion of firm «Getac» (Germany). The samples were mounted on a graphite holder with the two sides of the conducting adhesive tape, then blew a weak flow of air and deposited them in a vacuum layer of carbon with a thickness about 100-50?. Viewing and photographing of the objects was carried out at an accelerating voltage of 15 kV electron beam current of 10 - ¬ 9A.

3. RESULTS

3.1. Morphology of the foamcoke

Composition without filler (k = 14)
Foamed coke (Img. 1), forming during the annealing of the composition containing no filler, has a gradually-abdominal structure, ie in the investigated volume, there are regions consisting of foam cells, and extended cavities (in fact, defects in the foam), surpassing the size of foam cells. Foam cells are spherical and ellipsoidal formations from 10 to 100 microns; they are grouped into associates of various forms. Sheath of cells have different thicknesses: from very small for electrons and transparent (less than 0.1 microns), up to  high (1 micron or bigger).  Cavities in foamcoke remain from the original state material (cavities always appear in the water-dispersed materials). Dimensions of cavities reach 300 microns. On the Img. 1 can be also seen the wreckage of the supramolecular structure in the form of the lamellae and whiskers. Fragments of the shell of cells - lamellas have different length and thickness of about 1 micron or less. Whiskers (cylindrical formations) have the length to 150 microns and a diameter of 1 to 10 microns. The formation of debris occurs when preparation of the samples is the presence of a large number of fragments suggests that this foamcoke at the time of preparation had low mechanical properties.

       
                                                  a                                                                                                        b                                                                                  c
IMG. 1. Foamcoke based on the composition containing no filler; A, B, C: micrographs of a section at different magnifications.

Titanium dioxide (k = 15)
Foamed coke on the basis of a composition containing titanium dioxide (Img. 2) has a foamy structure, but with a different character of the foam cells. In this case, a continuous framework occur, and the cells clearly tend to polyhedral form. Cell sizes range from 10 to 400 microns. Edges of cells (the edges Platt Gibbs) for the most part have a linear shape, but there are edges to form circles. The diameter of the edges is around 1 micron. The edges of the cells (lamellae) have different shapes: flat, arched, cylindrical, and different surface thickness: from electron-transparent to thick, approaching 1 micron. Cavities in this foamcoke also present, their sizes are commensurate with the size of large cells, and the number of cavities per unit volume is less than in the composition without filler.

          
                                                 a                                                                                                      b                                                                                                   c
IMG. 2. Foamcoke based on the composition of titanium dioxide;
A, B, C: micrograph of a section at different magnifications.

Barium and zinc borates (k1 = 14 and k2 = 26, respectively).
Foamed coke, formed during the annealing of the composites containing borates, has gradually-abdominal structure (Img. 3). Cell foams have a spherical and ellipsoidal shape up to 100 microns, and fill almost the entire visible volume. Ie it can be said about the supramolecular structure that it is presented both in the form of individual bubbles, and multi-cells associates (clusters). Cavities also present in small numbers, having dimensions of 300 microns.

        
                                                 a                                                                                                        b
IMG.3. Foamcoke based on the composition of barium borate (a) and zinc borate (b).

Metal hydroxides (k1 = 10; k2 = 25)
Studied magnesium hydroxide and aluminum have different effects on the foamcoke structure. In the case of magnesium hydroxide (Img. 4a) foamcoke forms to a gradually-abdominal structure in which the foam bubbles create a sort of continuous matrix with a fairly narrow size distribution, not exceeding 40 microns. In the case of aluminum hydroxide (Img. 4b) foam bubbles as such do not exist, so we can speak of a continuous low-density matrix, consisting of fragments of the lamellae and mustache, and containing a cavity size of about 100 microns.

       
                                                a                                                                                                        b
IMG.4. Foamcoke based on the composition of magnesium hydroxide (a)
and aluminum hydroxide (b).

Graphite (k1 = 14; k2 = 22)
In compositions with graphite (Img. 5) the expanding graphite defines the structure. Here, basically, is expanded graphite in different forms, which can be roughly described as "acorns", "fir cones", "tracks". These formations are sized in diameter to 300 microns, and a "long" part - up to 1000 microns. These formations themselves are sufficient loose, separated by large cavities (length up to 300 microns). Img. 5 shows that the components of the foaming-carbonization phase do not form their own areas of foaming; and apparently polyaromatic hydrocarbons produced during the reactions are fully condensing in the expanding graphite.

       
                                                    a                                                                                                   b
IMG. 5. Foamcoke based on the composition of graphite JLS-GR-1002 (a)
and graphite JLS-GR-803 (b).

Zeolite (k = 10 with the destruction of the layer)
Foamcoke composition with zeolite (Img. 6a) consists of extended fields (500 microns or more), including fragments of flat and arched slats extending up to 90 microns and cane-like formations up to 30 microns long  and a with a diameter of about 1.5 microns, and cavities with a size of up to 100 um.

Tripolyphosphate Na (k = 18)
Foamcoke composition with sodium tripolyphosphat (Img. 6b) is a rather loose mass with cavities up to 40 microns. In this mass can be seen the solid formation of irregular shape with a size along the major axis up to 30 microns, these formations are packed loosely and randomly. Lamellae and no whiskers.

Ethyl silicate (k = 14)
Foamecok composition with ethyl silicates (Fig. 6c) consists of loose mass sections and chain-like structures formed from elements of irregular shape. Lamellae and no whiskers. In this mass there are empty cavities up to 100 microns.

        
                                         a                                                                                                           b                                                                                                      c
IMG. 6. Foamcoke based on the composition of zeolite (a)
sodium tripolyphosphate (b) and ethyl silicates (c).

2.2. The elemental composition of foamcoke

The result of elemental analysis, performed by X-ray electron-probe microanalysis, is presented in the table. These data allow us to ascertain changes in the concentration of any element in foamcoke under the influence of filler. All studied fillers can be divided into two groups according to the degree of influence: (a) the concentration of elements remains practically unchanged, and (b) the concentration of the elements change (increase or decrease).

According to the table not all fillers change the concentration of major elements with respect to the original system. Thus, ethyl silicate ETS-40 and Celite Na, aluminum hydroxide, and even titanium dioxide (!) practically do not change the ratio of the elements.

Table. The concentration of major elements of foamcoke for different fillers

The remaining fillers on the degree of "concentration swapping" can be summarized as follows:

a) increase the concentration of carbon and thus reduce the concentration of phosphorus and oxygen - titanium dioxide, barium borate, and of course, graphite;

b) increase the concentration of oxygen and thus lower the concentration of carbon and phosphorus - zinc borate and sodium tripolyphosphate;

c) significantly increases the concentration of phosphorus and thus lower the concentration of oxygen and carbon atoms - magnesium hydroxide;

d) zinc borateand sodium tripolyphosphate increase the concentration of phosphorus and oxygen, and reduce the concentration of carbon.

Comparative structural studies of two different stages of transformation - the stage of carbonized foamcoke and stage of white foam ashes - made on the basis of the composition of titanium dioxide. It may be noted that in the ash, as compared with coke, the concentration of phosphorus decreases more than half (60%), slightly decreases an oxygen concentration (? 13%) and significantly (? 30%) increases the concentration of carbon. Such structural elements as whiskers (edges) preserve, and slats virtually disappear. Since, during the annealing of coke? Ash, transverse dimensions of edges increase (diameter increases from 1 to 5 microns), it can be assumed that the material of lamellas at annealing rebuilding of coke in the ash almost all moved to the edge. Whiskers in coke and ash are straight-line segments and triple branching, as well as other more subtle forms: rings and arcs of different diameters. Strictly it was not possible to observe the supramolecular structure of ash - during the transportation and preparation one, being extremely fragile, spontaneously disintegrates.

3. Discussion of results

Consider the obtained results in terms of performance by flame retardant fillers in foaming compositions of the two above functions: of gasifier and nucleating agent of foaming. Addressing the issue of participation of the compounds studied as a physical or chemical elements in coke framework – the third possible function of the fillers - was not the subject of present study.

In organic-ammonium phosphate compositions, in which foaming-carbonization phase traditionally includes ammonium polyphosphate, polyol and melamine – an aerogenous agent, there is no sense to use an additional blowing agent in the form of filler. Most likely, it will contribute to the destruction of the foam structure. Indeed, the composition with aluminum hydroxide, which is extensively dehydrated at a temperature of about 260 0C, coke does not show the foam structure, but has a view of loose swell mass. The composition of magnesium hydroxide, having a maximum speed of dehydration at 360 0 C, close to the decomposition temperature of melamine, which melts, decomposes and sublimates at T? 340-370 0 C, shows a typical foam bubble structure. It can be assumed that, the difference between the aluminum hydroxide and magnesium hydroxide in the composition is that the first for the second decomposes at a lower temperature, contains and releases more water, and finally exceeds the valence. Ethyl silicate zeolite shows similar effects to that of aluminum hydroxide. Apparently, the zeolite and aluminum hydroxide as blowing agents may be useful in the formulations based on sodium silicate compositions.

From the perspective of the studied fillers as nucleating foam structure of coke, studied compounds can be divided into two groups:

a) promoting the formation of the foam structure of coke – they are titanium dioxide, borates, barium and zinc and magnesium hydroxide;
b) suppressing foam formation in the coke – they are aluminum hydroxide, sloughing, graphite, zeolite, sodium tripolyphosphate, and ethyl silicate.

The compounds of the first group act as a fairly active nucleating agents of the foam structure of coke, contributing to a more regular and stable foam compared with foamcoke based on the composition, not containing nucleating fillers. In the presence of titanium dioxide polyhedral foam is formins, cells of which are composed of facets and edges. Apparently, some very fine lines (lamellae) - they have the form of planes, spheres, cylinders - can be attributed to the so-called graphenes. Coke, foamed in the presence of borates (borates contain crystallization water) detects spherical and polyhedral bubbles. Coke in the presence of magnesium hydroxide has spherical foam cells.

For the first group of compounds it should be added that the most effective nucleating foam include titanium dioxide and magnesium hydroxide. Titanium dioxide promotes the formation of polyhedral foam, with achieving of enough large foam cells - up to 400 microns and characterized by a broad distribution of foam cells in size. Magnesium hydroxide promotes the formation of polyhedral foam with sizes up to 40 microns and with a narrow size distribution. It can be assumed that the difference in format between the foaming compositions with titanium dioxide and magnesium hydroxide is as follows. Titanium dioxide acts as a nucleating agent in the form of particles with size of about 1 micron due to specific acid-base nature of the surface. Magnesium hydroxide, which is the white fibers, which consist of colorless crystals with a layered trigonal lattice, dehydrates in the temperature range 340-410 0C with the formation of magnesium oxide having a kind of grayish-green transparent octahedral crystals. During dehydration layered hydroxide particles are destroying, ie, a kind of self dispersion of filler occurs. And finely divided magnesium oxide appears as nucleating agent of foam. It can be also suggested that magnesium can be built in condensing coke structure through the formation of products with ammonium polyphosphate.

But it should be noted that titanium dioxide and other compounds from the first group of composition act in particular as structuring coke foam agents without increasing the coefficient of expansion. It is obvious that than more dispersed filler in the composition and than more evenly it is distributed over the volume of the melt, than more "comfortable" conditions will be presented for nucleation and, hence, the foam will be more regular and homogeneous. It can be also supposed with some degree of probability that these fillers can also serve as a new phase (structuring agent) in the transition of the liquid phase (ash) of coke into a gel.

Speaking about the second group of compounds that suppress the formation of foam in the coke, we must say a few words about the bloating graphite. Electron microscopic studies showed that the bloating graphite imposes its own structure to coke, thus suppressing the foaming proper organic phosphate composition, as was mentioned above, due to condensation of decomposition products forming on graphite at high temperature and reaction foam-carbonization phase. This applies to graphite JLS-GR-1002, with a temperature of the expansion of 190 0C and graphite JLS-GR-803, a beginner increase at 300 0C. Having high coefficients of expansion (the first - 35 and over, the second - more than 70) they work before the enlargement processes of organic phosphate composition. Composition of graphite JLS-GR-1002 showed the same foaming rate (k = 14) as the not filled composition, the composite with graphite JLS-GR-803 showed the increase in the foaming up to 22 units. Apparently, in the paint and varnish compositions based on intumescing graphite it is enough of foaming agent, such as polyvinyl acetate, so foaming carbonization phase in this case is unnecessary. The work reports of successful use of intumescing graphite in the polyisocyanate-polyurethane foams. Rather unexpected should be recognized the result of the foaming of compositions with sodium tripolyphosphate. On the basis of general considerations, it should act as a surfactant on the wall of foam cells, ie, stabilizes them. This could be because the concentration is too high for this product. Since forming in its presence foamcoke has a very high turnover, in this concentration, it probably just plasticizes coke. Besides, if the assumption that the stabilizing role of sodium tripolyphosphate is true, then it is possible, that it could carry out its mission of stabilizing, but it does not possess at the same time any nucleating effect. Zeolites increase the balance of the annealing of tracks with ammonium polyphosphate pentaerythritol. The authors of present work suppose, that zeolites can catalyze the esterification reaction between ammonium polyphosphate and pentaerythritol at a temperature below 250 0C and enhance cross-linking and carbonization through SiO2 and Al2O3, obtained by the decomposition of the zeolites at a temperature of 250 0C. But, as shown by structural studies, zeolite dehydration, occurring at a temperature below the melt formation, does not contribute to the formation of the foam structure.

Thus, the compounds of the second group direct "building" a completely different way, to architecture, far from the foam structures. Supramolecular structure of coke on the basis of these compounds can be attributed to the granular-porous. It is unlikely that such a coke possesses ideal mechanical properties.

***

The authors express their deep appreciation to the staff of the Institute of Physical Chemistry A. J. Aliyev and A. E Chalykh, who had fulfilled electron microscopic study.

REFERENCES