Ind. Eng. Chem. Res. 2003, 42, 2897-2905
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Polyphosphate Flame Retardants with Increased Heat Resistance Barbara Cichy,*,† Dorota Łuczkowska,† Mariusz Nowak,† and Maria Władyka-Przybylak‡ Institute of Inorganic Chemistry, Sowin´ skiego 11, 44-100 Gliwice, Poland, and Institute of Natural Fibres, Wojska Polskiego 71b, 60-630 Poznan´ , Poland
A review of methods for producing and using condensed melamine phosphates as flame retardants, and analysis of our own investigations on production of melamine polyphosphate, are presented. Physicochemical properties of obtained products are given including thermal properties (DTA, TG, and DSC). Effectiveness of action of melamine polyphosphate as flame retardant for polypropylene was tested by a cone calorimeter according to ISO 5660. Introduction Recent decades brought wider and wider application of plastics in various fields of technology. Their unquestionable merits are low production costs. In many cases substitution of conventional materials by organic polymer-based plastics is possible only when their flammability is limited. In this connection a constant development of market in so-called flame retardant agents (FRs) is observed. Consumption of FRs increases annually at a rate of 3-6% of gain in tonnage depending on type of agent, application range, and origin.1 In recent years, in principle the following three groups of combustibility-reducing agents were of importance: inorganic compounds of mineral origin of the types oxides, hydrated oxides, and hydroxides of metals; halogen derivatives, mostly containing bromine; and organic and inorganic phosphorus compounds.1,2 At present a renewed increase of interest in somewhat less effective, but ecologically safe, inorganic flame retardants can be observed. Within this group mineral FRs dominate to constitute, by tonnage, the largest portion of the market. Their considerable consumption results partly from the fact that fire protection efficiency is assured by their use in a form of relatively large addition to the base plastics. In 2000, bromine-based FRs accounted for 34% of world sale of FRs whereas those phosphorus-containing organic and inorganic retarders made 22%.2 Although the halogen-containing compounds are constantly considered to be very effective, one begins to depart gradually from them, since they are blamed for emission of dense and toxic smoke as well as corrosion gases during combustion.3 Additionally, some of the organic FRs that contain bromine and are used in everyday life are harmful not only to the environment but, first of all, for human life.3 A special position on the market in phosphoric FRs is occupied by polyphosphates, i.e., high-molecularweight phosphates produced by condensation of phosphate salts. Melamine and ammonium polyphosphates are present on the market as components of fire resistant coatings of the “intumescent” type and as individual fillers that decrease the burning rate of
plastics, wood, wood derivatives, and natural fabrics.4 Of them, melamine phosphates are worthy of mention including orthophosphates and condensed forms of phosphates: pyro-, meta-, and polyphosphates. Apart from phosphorus, they contain relatively much nitrogen, which, bonded to the stable ring of melamine, passes to the gaseous phase at comparatively high temperatures. In this connection, for example, melamine polymetaphosphate is an interesting product for plastics that require high-temperature processing or operating, since this compound shows excellent thermal resistance without substantial mass loss up to a temperature of about 350 °C.4,5 Melamine phosphate (orthophosphate (2,4,6-triamino1,3,5-triazine)) as a compound including in its constitution two elementssi.e., nitrogen (37.5% mass fraction) and phosphorus (13.8% mass fraction)swhich are conventionally considered as effectively decreasing combustibility of material can be, and sometimes is, used as a flame retardant agent. However, it is more often used in coatings compared with synthetic polymer plastics, because the processing of almost all plastics requires that relatively high temperatures be used. On the other hand it is known that heating phosphate salts leads to their gradual condensation, together with evolution of volatile, under these conditions, water and producing polyphosphates.6 Therefore, highly condensed melamine polyphosphate should be characterized by thermal resistance decidedly better than orthophosphate, and that is why polyphosphates should be more suitable for application in the processing of thermoplastics. The condensation of melamine orthophosphate to melamine pyrophosphate (diphosphate) runs according to the reaction equation
* To whom correspondence should be addressed. Tel.: +48 32 2313051. E-mail:
[email protected]. † Institute of Inorganic Chemistry. ‡ Institute of Natural Fibres. 10.1021/ie0208570 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/31/2003
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where M ) melamine:
Table 1. Water Solubility (g/100 g of Water) of Phosphate Derivatives of Melamine13 20 °C 40 °C 60 °C 80 °C 100 °C melamine orthophosphate melamine pyrophosphate
The chemical structure of melamine orthophosphate (FM) is
The chemical structure of melamine polyphosphate (PFM) arising during its thermal condensation is
0.35 0.09
0.63 0.17
1.16 0.24
1.88 0.32
2.94 0.54
Apart from intumescent coatings, melamine phosphate also finds application in protecting films, for example, paints,12 because they do not tend to cause corrosion nor do they influence corrosion resistance of material. In the intumescent systemssin coatings and some plasticssmelamine phosphate is used in binary systems, i.e., together with a multihydroxyl compound and in a suitable mole ratio. For example, this is the case with polyolefins where melamine phosphate is accompanied by pentaerythritol,11 and in numerous paints and intumescent coatings.5,12 Literature Review
where n > 2. When n f ∞ the compound formula can be recorded as (MHPO3)n. This compound is named melamine polymethaphosphate. A number of literature sources suggest that thermal polycondensation of melamine phosphate be conducted in the presence of urea as a so-called “condensing agent”, which is to facilitate the dehydration polycondensation process due to lowering activation energy of the polycondensation reaction and also by impediment of sublimation of melamine from the product.7,8 Under the conditions of the calculation process, urea decomposes to yield gaseous products: NH3 and CO2. Ammonia, water vapor, and carbon dioxide are present in the gaseous phase above the alloy where specific reaction occurs. The presence of ammonia gas in the reaction zone inhibits unprofitable reaction of melamine decomposition also taking place with emission of ammonia gas. The mechanism of action of phosphoric acid derivatives consists, for simplification, of the change of combustion process so as to increase the fraction of solid products of pyrolysis (mainly atomic carbon), and also to reduce the amount of volatile products and decelerate the rate of their formation. A tight layer of carbonized material which is formed on the surface cuts off the oxygen supply to the unburned residue, whereas the present polyphosphates, of which polycondensation degree is elevated with temperature, inhibit glowing of reduced carbon.9 The peculiar character of interacting the phosphates and polyphosphates concerned is used for intumescent coatings, with which steel constructions, electric cables, ceilings, fire doors, etc. are covered. Fire protection utilizing these coatings lies in forming an organic-inorganic foam which by the action of flame is slowly altered into an inorganic one to protect the layers situated below.10,11 In comparison with combustibility-reducing agents of the inorganic salt type, the unquestionable merit of melamine polyphosphates is their exceptionally low solubility in water (Table 1) and in organic solvents, as well as lack of tendency to hydrolysis with formation of electrically conducting particles owing to which dielectric properties of plastics used in electronic and electric industries are not altered.
Many methods of producing melamine polyphosphate have been described in the literature. In the 1940s Russians were engaged in this problem. Vol’fkovich, Zusser, and Remen described two methods for the production of melamine pyrophosphate.13 The first method consists of producing melamine orthophosphate from orthophosphoric acid and melamine in suspension, and then its calcination at temperatures of 250-270 °C. In the second method melamine reacts with sodium pyrophosphate in a solution, while using hydrochloric or nitric acid. Modern methods mainly consist of reacting melamine, sometimes in the presence of water, with a suitable phosphoric acid. Muszko et al.14 patented a method for producing melamine polyphosphate from melamine and polyphosphoric acid. And so the reaction product is maintained at a temperature of 200-450 °C till the content of P2O5 in the ortho form is reduced to less than 2%, resulting in product contents above 90% water insoluble compounds at room temperature. In ref 15, besides polyphosphoric acid, the substrate is a melamine-melam-melem mixture. The process consists of two stages. In the first stage, reactive product is obtained as a result of mixing the mentioned compounds at a temperature of 0-330 °C. In the second stage, the product is calcinated in a kiln at 340-450 °C. Tomko and Aaronson patented the invention16 which is grounded on direct reaction of pyrophosphoric acid with an aqueous solution of melamine that proceeds at a temperature below ambient temperature owing to which the hydrolysis rate of pyrophosphoric acid to orthophosphoric acid is minimized. In the ref 7, Suzuki describes a method for producing melamine polymetaphosphate in two stages. In the first stage, melamine, urea, and an aqueous solution of orthophosphoric acid are mixed. The mixing process proceeds at a temperature of 0-40 °C till water removal. As a consequence an intermediate product, being the double salt of orthophosphoric acid with melamine and urea, is obtained. The second stage consists of calcination of this intermediate product at a temperature of 240-340 °C to prevent formation of agglomerates. The obtained product the authors called melamine polymetaphosphate. According to them the chemical compound produced by the process concerned shows excellent thermal resistance. During the thermogravimetric
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analysis no mass loss is observed up to 350 °C, and diffractometry does not show any presence of either orthophosphates, melamine pyrophosphate, or the unreacted melamine itself. The subject matter of consecutive invention8 is a process for producing melamine pyrophosphate accomplished by contacting an aqueous suspension of melamine with pyrophosphoric acid obtained directly via replacement of cations of pyrophosphoric acid salts by hydrogen cation using the method of column ion exchange wherein an ion-exchange resin is applied. The research work conducted by the authors of this paper have resulted in a consecutive patent application.17 The gist of the suggested solution is a direct use of highly condensed polyphosphoric acid for the reaction carried out in aqueous suspension at low temperatures and, if need be, an additional calcination at a milder time-temperature rigor for a version of the product finding special applications to the plastics that require processing at temperatures of about 300 °C. The product obtained as a result of calcination is practically insoluble in water, it contains less than 1% mass fraction of orthophosphate forms, and its thermal resistance exceeds 350 °C depending on the calcination conditions. Also the running of the calcination process has appeared to be advantageous in the presence of urea, i.e., the component assisting the condensation process. Experiments and Methods At the Institute of Inorganic Chemistry in Gliwice research was carried out on producing one of the more interesting phosphoric flame retardants: melamine polyphosphate. As it was mentioned previously this high-molecular-weight compound is a product of high thermal resistance, and it is in demand on the market of the FRs used for plastics that require high-temperature processing or operation in a particular rigor. Production of melamine polyphosphate was performed in two stages. In the first stage, melamine and thermal orthophosphoric acid having a concentration of 53% P2O5 were mixed in the molar ratio 1:1, and after evaporation of physical water at 130 °C in a dryer. Melamine orthophosphate was obtained according to the reaction
C3N6H6 + H3PO4 f C3N6H6‚H3PO4
(2)
The second stage comprised preparation of melamine polyphosphates having a different degree of condensation as a result of calcination of melamine orthophosphate. Calcination was conducted in a muffle kiln under various time-temperature conditions (time 0.5-4 h; temperature 260-360 °C). The relationship of physicochemical properties of the preparation obtained was examined against time and calcination temperature. It has been assumed that the principal required product obtained as a result of thermal reaction of melamine orthophosphate polycondensation is melamine polymetaphosphate (C3N6H6‚HPO3)n. The thermal polycondensation is described by eq 3.
nC3N6H6‚H3PO4 f (C3N6H6‚HPO3)n + nH2O (3) Within the framework of these studies calcination of orthophosphate was also carried out in the presence of urea, which, according to literature sources,7 is believed
to facilitate dehydration condensation of orthophosphates due to lowering activation energy of polycondensation and decrease a tendency of melamine to sublimation and decomposition under the reaction conditions.15 The tests were repeated to produce polyphosphate in the presence of a stoichiometric excess of melamine. Experiments were performed as follows: preliminary mixing (about 1/3 of the total amount of melamine with phosphoric acid) was conducted in a laboratory assembly composed of a beaker and a mechanical turbine mixer driven by an electric motor with infinitely variable adjustment of revolutions. Further mixing of the intermediate product with the residual amount of melamine was carried out using a Fritsch planetary-ball mill. The resultant intermediate product was dried to constant weight in a laboratory dryer. Calcination was conducted in a ceramic heat-resisting vessel heated by silite elements. Considering that using flame retardants in plastics and coatings requires adequate size reduction, the sintered product was ground by means of a Fritsch disintegrating laboratory mill, while employing simultaneous separation through a 0.08 µm sieve. In samples, size composition obtained was determined using a laser analyzer Coulter LS 230. During the experimental preparation of melamine polymetaphosphate some assaying was necessary as well as the measurement of physicochemical properties. Determination of the total content of phosphates in sample was made after its previous mineralization with dipping acid by the method of titration of phosphates using a titrant of magnesium chloride in alkaline aqueous-alcohol medium as against thymoloftalexon and phenolphthalein. In preparations, nitrogen concentration was determined after conversion using dipping acid in the presence of catalyst of the urea and organic form of nitrogen to ammonia. Then ammonia was distilled off from the alkaline solution and absorption conducted in the standard solution of sulfuric acid. The excess acid was back-titrated by a standard solution of sodium hydroxide. Analysis of thermal decomposition of the obtained FR specimens was accomplished by thermogravimetric analyzer, type TGA/SDTA 851 Mettler Toledo Star System, at a heating rate of 10 °C/min. By means of the scanning calorimetry method, TG, DTG, and DSC curves were determined within the range of temperatures 20-1000 °C. The preparation of FRs, particularly those used for external protective coatings, but also in electroinsulating plastics, should be characterized by minimum water solubility. In this respect, phosphates and polyphosphates of melamine behave better than, for example, ammonium polyphosphate, an agent often used in intumescent coatings, and the more condensed the polyphosphate, the less the water solubility should be. Determination of FR preparation solubility in water was the essential part of these investigations. Precisely weighed out samples (about 0.1 g) of the preparation were dissolved in 100 g of distilled water at an assumed temperature (40 and 60 °C), these temperatures being maintained by means of a thermostat and a digital thermometer to an accuracy of 0.2 °C. The dissolution process was carried out in a closed glass vessel with a water jacket. Dissolvent was heated to a required temperature by mixing with a magnetic mixer, whereupon an accurately weighed out preparation sample was
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poured and mixed for 3 h at a constant temperature. Suspension was filtered off through a funnel G4, and the dry residue after drying at 105 °C was precisely weighed. Effectiveness of action of manufactured melamine polyphosphate as the FR agent was verified in polypropylene. Moulded pieces designed for combustibility testing were made of polypropylene with 20% addition of selected FR specimens made during the present investigations. The transfer moulding method was used for their fabrication. Also polypropylene moulded pieces were made comprising a binary system for inhibition of combustibility: melamine polymetaphosphate-pentaerythritol. 7.5% pentaerythritol, 22.5% melamine polyphosphate, and 70% granular polypropylene were used. The components were mixed in an extruding press mixer at a normal temperature of plastic processing, and moulded pieces were executed in two thicknesses: 2 and 4 mm. Flammability tests of polypropylene moulded pieces were carried out at the Institute of Natural Fibres in Poznan by means of a cone calorimeter according to ISO 5660.18 Horizontally positioned samples were subjected to controlled thermal radiation with intensity of 35 kW/m2. Heat release rate HRR in the cone calorimeter is determined on the basis of measurement of oxygen content loss during combustion according to the theory that 13.1 MJ of energy is released for each kilogram of oxygen consumed by burning organic material.19 Furthermore, the rate of mass loss MLR, time to ignition TTI, and effective heat of combustion HOC were determined. Comparable studies were made for PP moulded pieces with and without flame retardant.
Table 2. Statement of Calcination Conditions of Melamine Orthophosphate to Polyphosphate and the Obtained Properties calcination conditions
product properties
no.
time, h
temp, °C
content P2O5 total, % mass
content N, % mass
1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 0.5 0.5 0.5 0.5 2 2 2 2 4 4 4 4 3
267 281 318 357 262 283 318 357 258 283 318 357 330
32.1 31.8 32.0 33.0 34.6 32.9 33.8 35.6 36.9 33.6 34.2 35.6 37.6 34.2
37.5 37.6 38.2 37.5 37.9 39.2 37.5 38.8 39.5 37.7 37.6 38.6 40.0 39.8
Results and Discussion The subject of the presented studies was to determine conditions for the process of calcination of melamine orthophosphate to melamine polyphosphate, so that thermally stable product at least to a temperature of 300 °C could be obtained, and the thermogravimetric analysis of orthophosphate as well as obtained polyphosphate preparations with determination of characteristic thermal properties. Other substantial physicochemical properties of manufactured FR preparations were also examined. Melamine and thermal orthophosphoric acid were mixed in the molar ratio 1:1 to obtain melamine orthophosphate, which after drying at 130 °C and dividing into 13 batches was calcined in muffle kiln for various times and different temperature ranges. Conditions under which particular tests were performed, and also analyses of prepared melamine polyphosphate samples, are shown in Table 2. In the course of a process conventionally called “calcination” a process of thermal polycondensation of melamine orthophosphates to the more and more concentrated polyphosphates takes place which, in conformity with reaction 3, is accompanied by water evolution. During calcination, polycondensation leads to a progressive increase in the fraction of phosphorus in the product. Theoretical mass loss as calculated from reaction 3 is 8.03%. In practice greater mass loss can be observed (Figure 1) which results from melamine decomposition. Under conditions of running the process, reactions of decomposition of condensed and uncondensed melamine phosphates are observed according to the scheme of reaction briefly presented by way of
Figure 1. Mass loss in samples during calcination of melamine orthophosphate to polyphosphate against time and temperature.
example of melamine orthophosphate (reaction 4):
C3N6H6‚H3PO4(s) f 1/2P2O5(s) + 3/2H2O(g) + C3N6H6(g) (4) In melamine phosphates the gram-atom ratio N:P is 6. At elevated temperatures and longer periods of time it is observed in calcinated samples that this proportion becomes lower, which manifests melamine decomposition in the course of prolonged annealing necessary for the described manufacturing process (Figure 2). Samples received under extremely drastic temperature-time conditions, apart from lower molar ratio N:P, are characterized by a high content of phosphates, even higher than that calculated for the compound of formula (C3N6H6HPO3)n, which theoretically contains 34.4% P2O5 and 40.7% N. Excessive acidity of the preparation is not advantageous for cooperation of the agent with basic material. Samples calcined for 3 h at 330 °C and calcined for 2 h at 360 °C show chemical composition mostly approximate to those calculated from the theoretical formula.
Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003 2901 Table 3. Characteristic Physicochemical Properties of Melamine Polyphosphates product properties
no. 1 2 3 4
Figure 2. Change in the ratio of N:P gram-atoms for the products of condensation of melamine phosphate depending on the mass loss observed during the calcination process.
The literature sources cited earlier recommend to use a small stoichiometric excess of melamine. So at the next examination stage tests were repeated to produce melamine polyphosphate by using determined calcination parameters, and in successive tests: (1) addition of urea (about 10% mass fraction); (2) stoichiometric excess (1.1 N:P) of melamine. The obtained results are presented in Table 3. The addition of urea into the calcination process positively influenced the physiochemical properties of the product: pH, water solubility; however, a surplus of melamine allows the pH to be kept relatively high (Table 3). Melamine phosphates condensed at the selected parameters were characterized by a very small water solubility in comparison with both literature data (Table 1) and the values determined in our own investigations on melamine orthophosphate. For this compound which
calcination conditions 330 °C, 3 h 330 °C, 3 h (+urea) 360 °C, 2 h (+urea) 330 °C, 3 h (stoichiometric excess of melamine)
water solubility, content content g/100 g of water P2O5, N, pH 10% % mass % mass suspension at 40°C at 60°C 34.2 33.2 35.6 32.7
39.8 37.5 37.2 37.4
4.1 4.9 3.1 4.3
0.0153 0.0148 0.0101 0.0099 0.0721 0.0363
was obtained as a result of the first stage of manufacturing process previously described, the following solubilities were determined: 0.692 g/100 g of water at 40 °C and 0.787 g/100 g of water at 60 °C. Particularly good thermal resistance of FRs of condensed melamine phosphate type was confirmed by thermogravimetric tests. Examinations were carried out on the basis of the thermogravimetric analysis (TGA) method and the method of scanning calorimetry (DSC) as well. Measurements of thermal decomposition of melamine orthophosphate (FM) and polyphosphates that differed in calcination conditions were made. A statement of analyzed samples, along with calcination conditions, is presented in Table 3. Thermogravimetric analysis of melamine orthophosphate is indicative of a several stage process of its decomposition as temperature increases (Figure 3). The stages of polycondensation and decomposition are as follows: (A) condensations formation of melamine pyrophosphate; (B) condensations formation of melamine polyphosphate; (C) polycondensation and partial decomposition; (D) total decomposition and mineralization The thermogravimetric curves of polyphosphate are presented in Figures 4 and 5 and the curves of examination of scanning calorimetry in Figure 6. Characteristic data for particular stages of decomposition are demonstrated in Table 4. A considerable effect of the temperature and calcination time on the degree of thermal stability of melamine phosphates was observed. Almost whole melamine
Figure 3. TG and DTG curves of thermogravimetric analysis of melamine orthophosphate.
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Figure 4. Thermogravimetric analysis of melamine polyphosphates (depending on calcination conditions).
Figure 5. Thermographic analysis of melamine polyphosphates (effect of urea).
orthophosphate was converted into a polyphosphate structure when the calcination process was conducted at 360 °C for 2 h, which ensures its thermal stability up to a temperature of a minimum of 300 °C (peak onset at 358 °C). Conducting the calcination process in the presence of urea also has an advantageous effect on the melamine phosphate properties.
for PP with melamine polyphosphate (PFM) is in the range 365-458 kW/m2, and for polypropylene with melamine polyphosphate and pentaerythritol (PP + PFM + PT) it amounts to 274-365 kW/m2. So the 20% addition of PFM to PP decidedly decelerates maximum heat release rate, and this effect is still intensified by 7.5% addition of pentaerythritol (PT).
Evaluation of the Action of Melamine Polyphosphates as Flame Retardants in Polypropylene
It can also be seen in Figure 7 (right) that PP samples protected by PFM with PT addition, especially those of larger thickness, burn less intensively; the HRR curve is shifted in the direction of prolonged period of time due to formation of an intumescent protective coating.
When analyzing data contained in Table 5 and Figure 7 it can be stated that pure polypropylene (PP) emits a very large amount of heat: an average value of maximum heat release rate HRRmax is, depending of the thickness, 925-1111 kW/m2, whereas this parameter
Addition of PFM to PP also affects decreasing average HRR, ignition time TTI, and effective heat of combustion HOC. Effectiveness of PFM action increases after PT addition.
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Figure 6. Analysis of scanning calorimetry (DSC) of melamine phosphates. Table 4. Test Results of Thermal Decomposition of Melamine Phosphates calcination conditions temp, °C
time, h
267
0.5
330
3
360
330
type of conversion
additives
type of anal.
conversion param
A
B
C
D
FM: Melamine Orthophosphate TGA onset [°C] DSC peak [°C] mass loss [%] heat of transition [J/g]
230 261 4 -157
285 308 4 -174
362 393 59 -1182
579 634 21
PFM 1: Melamine Polyphosphate DTG onset [°C] peak [°C] mass loss [%]
203 263 4
295 307 5
364 390 57
614 648 22
