Kinetic Model of Melamine Phosphate PrecipitationBarbara Cichy* and Ewa Kuzdzał
Inorganic Chemistry Division “IChN” Gliwice, Fertilizers Research Institute, Sowinskiego 11, 44-101 Gliwice, Poland
ABSTRACT: The kinetics of the melamine phosphate precipitation reaction under stable mixing conditions and at constanttemperature were described along with the morphology of the crystals obtained. The kinetics of the reaction between melamineand phosphoric acid was described using a model of crystallization which included a stage of rapid dissolution of solid melaminedispersed in water in phosphoric acid, followed by nucleation and growth of sparingly soluble crystals of melamine phosphate.Experimental results have confirmed the assumptions made. The size and shape of the obtained melamine phosphate particles isimportant for the size and shape of the product of the second reactionmelamine polyphosphate. This allows for a certain scopeof control over the process of shaping the particles of plastics fillermelamine polyphosphate.
1. INTRODUCTION
Organic synthetic polymers form a basis for many modernmaterials used to replace traditional materials. Syntheticpolymers, being organic materials, are extremely flammable.Whenever this is important for human safety, polymericmaterials are modified with flame retardants (FRs). For along time, the most common FRs used were organic halogenderivatives, particularly brominated cycloalkanes. In recentyears, these compounds were found to be toxic to humans andthe environment, and their use is being reduced. The advantageof haloorganic FRs is, in addition to their high effectiveness asflame retarding agents, miscibility with the polymer matrixresulting from chemical similarity. The alternative halogen freeFR agents group includes mineral and inorganic substances.Properties of polymer composite materials are affected by thesize of interface area of the dispersed phase (filler) and by thenature of interactions between the continuous phase and thedispersed phase. With increasing aspect ratio and decreasingtransverse dimension of the filler, the specific surface area of thefiller increases, as does the total force of interactions betweenthe polymer matrix and filler particles. The mechanicalproperties of the composite usually improve, while thepolymer’s flow properties deteriorate. Many authors point outthe significant role of morphology, that is size and shape, ofFRs, which not only has an effect on the mechanical propertiesof the composite, but also on the effectiveness of flameretardancy of the polymer−FR composite. Small size, lowaspect ratio, and minimum specific surface area are therequirements for fillers for polypropylene and polyamide.1
Filler size affects the mechanical properties of thermoplasticpolyolefin blends. The effects of filler particle size on varioustypes of polypropylene were described by Svehlova andPoloucek.2 Most of the reports concerned mineral fillers, suchas aluminum or magnesium hydroxide. The effect of the size ofmineral FR particles has been studied by Hamed, and variousdegrees of filler dispergation in polymer matrix have beendescribed.3 Huang et al. describe the effect of particle size ofmagnesium hydroxide used as an FR in EVA polymer.4 The sizeof FR grains and its effect on the properties of rubbercomposites was investigated by Zhang et al.5 The small size ofaluminum hydroxide FR particles for EVA ensures its high
dispersion in the plastic and improves the mechanicalproperties of the composite.6
Among the many halogen-free FR fillers, phosphates,diphosphates, and polyphosphates of melamine (2,4,6-triami-no-1,3,5-triazine) play a major role. The effects of melaminepolyphosphate (MPP) use in many polymeric materials hasbeen described, for instance, in polyamides,7 in polypropylene,8
in epoxy compositions,9 and in polyurethane.10 However, thereare no literature reports on the study of the effect of the sizeand shape of particles of polyphosphates, including melaminepolyphosphate, used as FRs in polymers.There are many methods of conceding physicochemical
properties to the grains of inorganic fillers in order to improvethe polymer−filler interaction, for instance controlled commi-nution, surface coating, and use of coupling agents.11 The bestsolution, however, would be to formulate the morphologicalproperties of the filler at the stage of its manufacture.The purpose of the investigations presented herein was to
determine the effect of MPP manufacturing process conditionson the morphological characteristics of the reaction product.The optimum final product (MPP) should be characterized bymonodisperse particle distribution, the highest possible degreeof fineness with low specific surface area and advantageousaspect ratio.MPP is usually obtained in a two-stage process:12,13 first,
melamine orthophosphate (MP) is obtained in a reactionbetween phosphoric acid and melamine at ambient temper-ature. In the second stage, MP undergoes thermal treatmentduring which, often with the aid of catalyzing agents, particlesof MP condense to form an MPP polymer. The second stage isa solid state reaction, reaction 2 is often called calcination; thetemperature required is 300−350 °C.The process of MPP preparation is described by the two
following reactions:
+ → ·C N H H PO C N H H PO3 6 6 3 4 3 6 6 3 4 (1)
Received: August 4, 2012Revised: November 27, 2012Accepted: December 9, 2012Published: December 9, 2012
· → +n nC N H H PO (C N H HPO ) H On3 6 6 3 4 3 6 6 3 2 (2)
The reaction between phosphoric acid and melamine is aheterogeneous reaction in a liquid−solid system. Solid particlesof sparingly water-soluble melamine are dispersed in water and,upon addition of phosphoric acid, react fairly rapidly to form asparingly soluble product (MP) in the form of a solidprecipitate.Model of MP Precipitation: Principles. Two basic
models can be applied to describe a reaction in a liquidphase−solid phase heterogeneous system;14 the sharp interfacemodel and the reactive precipitation model.The authors have adopted the reactive precipitation model as
the one that properly describes nucleation and growth ofprecipitated MP crystals. The adoption of this model meansthat the precipitation reaction starts only after completedissolution of melamine particles in phosphoric acid and thatthe time of dissolution is infinitely short. Initial concentrationof the solid phase is low, and the reactivity of the reagentpresent in the solid phase is high.The authors also assumed that morphological properties of
MP particles precipitated in reaction 1 will have a decisive effecton the morphological properties of MPP formed therefrom inreaction 2, provided that the structure of the basic particles willnot be destroyed in the process of the separation ofagglomerates generated during calcination. Investigationscovered the kinetics of nucleation and crystal growth duringthe reaction of precipitated MP particles formation and theevaluation of morphological structure of MP and of MPPformed therefrom.The effect of solid phase concentration in liquid phase on the
intensity of mixing can be determined with the use ofcharacteristic numbers proportional to the fluid flow velocity,e.g. the Reynolds number. In traditional approach thecharacteristic number for a fluid flow in a pipe is given bythe following formula:15
ρη
=Rew dm 0
s (1)
In physical terms, Re represents the ratio of inertial forces to theforces of internal friction; it also constitutes a criterion thatdescribes the character of the flow. For the flow of a liquid in amixing vessel, it is assumed that wm = πdn and d0 = d (stirrerdiameter). Thus, upon disregarding π = 3.14 as being aconstant value, the equivalent Reynolds number is as follows:
ρη
= =Re Rend
m
2
s (2)
The Reynolds number so defined was a criterion for comparingthe mixing of the reaction mixture under the same conditions atvarious concentrations of the solid phase. The value of theequivalent Reynolds number may be used to determine theintensity of mixing in the mixing vessel operating in the system(for a given liquid/suspension and experiment conditions).Mixing intensity was determined as a function of the Reynoldsnumber.
=Mi CRe vm (3)
The values of constants C and ν take into account the type ofstirrer and geometric characteristics of the mixing vessel. TheReynolds number was determined experimentally for varioussolid phase concentrations, and subsequently, the mixing
intensity of the studied suspensions was determined. Thevalues of constants C and ν were adopted based on literaturefor a double-blade propeller stirrer with blade inclination angleof 22.5° (C = 0.985, ν = −0.15).16The population density of the individual fractions of MP
particles obtained was calculated from the particle sizedistribution; the relationship n = f(L) is described by thefollowing formula:
αρ α=
Δ=
ΔΔ Δn
mL L
VL Li
i
c i i
i
i i3 3
(4)
In order to estimate the rate of crystal nucleation and growth, amodel of a perfect crystallizer with internal suspensioncirculation was applied. This model, despite substantialsimplifications (the crystal nuclei formed are of zero dimension,agglomeration and macroscopic attrition of crystals isdisregarded, and habit of crystals is constant), is the mostwidely used tool for quantitative representation of crystal-lization kinetics.16,17
A few assumptions were made when processing the model ofnucleation and growth:
• perfect mixing throughout the reaction mass;• perfect continuous product removal;• geometric similarity of the particles (aspect ratio equal to
1 (α = 1));• particles are neither comminuted, nor agglomerated;• the number of particles is sufficiently large to assume that
their size distribution is a continuous function;• the values of B (nucleation rate) and G (linear growth
rate) were adopted as average in the entire process.
The simplest crystallizer model was adopted, wherein particlegrowth rate is independent of particle size. With theseassumption taken into account, the overall populationdistribution was determined using the following formula:
τ= −⎜ ⎟⎛
⎝⎞⎠n L n
LG
( ) expo (5)
The value of particle nuclei population density no for L→ 0 wasread for experimental data from a semilog plot of thedistribution function above, and the linear growth rate wasdetermined from the slope of the line and from the knownresidence time. The known values of nuclei population densityand of the linear growth rate enable the calculation ofnucleation rate:
=B n G0 (6)
Coefficients of inhomogeneity and of variation weredetermined analytically for the particle size distributions.These coefficients define the homogeneity of particles. Iftheir values are lower, then the particles are more homogenous.These parameters are sometimes used in exchange for theparticle aspect ratio.The coefficient of inhomogeneity is defined as
=−
CVL L
L( )
284 16
50 (7)
The coefficient of variation is expressed as the ratio of standarddeviation (σ) to mean particle size (L50):
σ=CZL50 (8)
Industrial & Engineering Chemistry Research Article
The above was used to describe the kinetic model of the MPprecipitation reaction.No kinetic model was created for the reaction of MPP
formation from MP; the considerations below are limited to theanalysis of the structure and morphology of particles bycomparing MPP particles formed by calcination of MPobtained at various concentrations of solid phase in liquidphase.
2. EXPERIMENTAL SECTIONMP and MPP were synthesized from technical grade melamine(ZA Puławy) and from technical grade phosphoric acid (ZChAlwernia CIECH SA). The synthesis reaction was conducted ina cylindrical glass laboratory reactor of 2.5 dm3 operatingcapacity and 0.12 dm in diameter, equipped with a propellerstirrer 0.09 dm in diameter. The speed of the stirrer wasconstant during all measurements at 5 revolutions per second,which provided stable and sufficient circulation of thesuspension. The range of melamine concentrations in waterin the study was 1−10% w/w. The concentration of usedphosphoric acid was 55% w/w P2O5. The reactor wascontinuously fed with a flow rate of aqueous solution of themelamine and phosphoric acid and effluent was taken off at thesame flow rate, corresponding to a residence time τ =1800 s.The solid phase content in the product suspension and MPcrystal size distribution were determined. After 4 h (eightresidence times), the steady state distribution particle size inthe product was reached. The measurement were done atconstant temperature T = 40 °C. The precipitated product(MP) was filtered off on a Buchner funnel, the precipitate wasdried in a laboratory drier for 1 h at 105 °C and then calcinedwith 10% urea added at 330 °C within a period of 1 h in anelectric circulation oven on trays, charge height 25 mm. In ourdescription of kinetic model of MP crystallization, we consideronly MP crystal size distribution and population densitydistribution for steady state in continuous mode crystallization.The density of the reaction suspension of reaction 1 of MP
preparation was determined by means of a hydrometer.Viscosities of solutions were measured with a Hoepplerviscometer, while the viscosities of suspensions weredetermined from formulas 9 and 10:15
η η ϕ= +(1 1.5 )s 0 (9)
where
ϕ =VV
s
c (10)
The conversion rate was calculated on the basis of chemicalanalyses as the ratio of the quantity of phosphates introducedinto the process to the quantity thereof in the precipitated solidproduct (MP).
Particle size analysis of MP and MPP samples was performedon a Coulter LS Particle Size Analyzer. Specific surface area ofMP and MPP particles was determined by means of a GeminiVII analyzer from Micromeritics used to determine single- andmultipoint specific surface area by the BET method within therange of 0.001−4000 m2/g. Samples were degassed at 130 °Cfor 1 h and specific surface area (A) and pore volume (Vp) weremeasured. Pore diameter (dp) was calculated from the formula
=dV
A
4p
p
(11)
3. RESULTSThe parameters describing the environment and conditions ofMP precipitation reaction are listed in Table 1.The effect of solid phase concentration in water as the initial
concentration of melamine in water on the nucleation and MPparticle growth rate and on the specific surface area of MPparticles and of MPP particles generated from MP wasdetermined. The n = f(L) curves were plotted for four selectedconcentrations of melamine in water (Figures 1−4). MP and
MPP population densities were calculated from the particle sizedistribution (Figures 5−8). The values of kinetic parameters ofMP particle formation (see Table 2), nucleation rate, and
Table 1. Characteristics of MP Precipitation Process
particle growth rate in the individual tests were determinedfrom the equation of population density distribution.In order to determine the effect of initial solid phase
concentration on the specific surface area of MP and MPPformed from corresponding MP particles, specific surface areaswere measured and appropriate calculations were made(formulas 5, 6, and 9) for MP and the corresponding MPP.Results are shown in Table 3.
4. DISCUSSIONAs expected, the density and viscosity of the suspensionincreases with the increase of concentration. To achieve highefficiency of the process, MP precipitation requires gooddispersing of melamine particles. At low concentrations, theprecipitation process competes with product dissolution. Withincreasing concentration, the viscosity of the medium increases;excessive viscosity is conducive to the formation of particleagglomerates and affects the efficiency of the process. Theefficiency of the MP precipitation process increases within theconcentration range studied to attain a maximum at about 5−6% melamine concentration in water and gradually decreaseswith increasing viscosity of the reaction suspension. Mixingintensity of the reaction suspension is nearly constant withinthe range studied; however, the Reynolds number decreases.With increasing melamine concentration in water, the particlenucleation rate increases, while the growth rate remains nearlyunchanged. The solid phase concentration, within theconcentration range studied, has no effect on the average sizeof MP particles formed. The maximum efficiency of the MPformation process was achieved at 5% melamine concentrationin water, attaining the highest value of the average MP particlesize and lowest specific surface area of the corresponding MPP.
Figure 3. Population density distribution of MP obtained from 8%aqueous melamine suspension.
Figure 4. Population density distribution of MP obtained from 10%aqueous melamine suspension.
Figure 5. Particle size distribution of MP and MPP obtained from 1%aqueous melamine suspension.
Figure 6. Particle size distribution of MP and MPP obtained from 5%aqueous melamine suspension.
Figure 7. Particle size distribution of MP and MPP obtained from 8%aqueous melamine suspension.
Figure 8. Particle size distribution of MP and MPP obtained from 10%aqueous melamine suspension.
Table 2. Kinetic Parameters of MP Particle Formation atVarious Concentrations
FR particles obtained at this concentration had the lowestcoefficient of inhomogeneity; such parameters are optimal forthe application of this filler in, for instance, polypropylene.The values of specific surface area, volume, and diameter of
pores in MP increases with increasing concentration to reach amaximum at 5% concentration and, then, gradually decreases.During the calcination of MP to MPP, the specific surface areaof particles increases. The lowest value of specific surface areain MP was recorded at 5% melamine concentration in water,while the highest was at 8% concentration. The most uniformparticles (lowest coefficients of variation and inhomogeneity)were attained with 5% initial melamine concentration in water.
5. CONCLUSION
The kinetics of MP particles precipitation were investigated inan aqueous environment resulting from the reaction ofmelamine solid particles dispersed in a water medium withphosphoric acid by determining the linear growth rate of crystaland particle size distribution of the product at variousconcentrations of the solid phase under identical mixingparameters conditions. The process of obtaining MP particlesin an aqueous environment is a precipitation type reactioncrystallization. A relationship between melamine concentrationin water and the density and viscosity of the suspension andnucleation rate has been demonstrated. The kinetic of MPparticles precipitate can be estimated by linear equation ofparticles growth according to model MSMPR (Mixed-Suspension-Mixed-Product-Removal18). Experimental resultshave confirmed this assumption. Reaction efficiency dependsin practice on the concentration of solid phase in water,proportional to initial concentration of melamine in water, andmixing conditions that provide adequate dispersion of the solidphase in liquid phase. The manner of carrying out the reactionof MP precipitation in an aqueous medium has an effect of onthe morphology of MP particles formed and also of MPPparticles obtained therefrom. Calcination caused the specificsurface area of MPP particles to increase 2−3 times ascompared to MP particles, and there was similar increase of theporosity of particles.Morphology of MP and MPP particles was described by
means of coefficients of variation and inhomogeneity ofparticles. Very uniform particles were formed at lowconcentrations of solid phase in the reaction of MPprecipitation, although not lower than corresponding to 5%melamine content in water. At lower concentrations, uniformitywas lower, and the efficiency of MP preparation reaction waslower due to dissolution of a portion of the particles formed. At
10% concentration, the product obtained under the describedconditions was not uniform.The study of the morphology of MP and MPP particles has
shown the feasibility of some scope of control over thesynthesis process effected by controlling solid phase concen-tration in water in order to obtain products of desired particlesize and specific surface area.
