ORIGINAL PAPER Preparation of aluminium ammonium calcium

Chemical Papers 67 (9) 1210–1217 (2013) DOI: 10.2478/s11696-013-0326-8

ORIGINAL PAPER

Preparation of aluminium ammonium calcium phosphates using microwave radiation‡ Kinga L uczka*, Daniel Sibera, Aleksandra Smorowska, Barbara Grzmil Institute of Chemical and Environment Engineering, West Pomeranian University of Technology, Szczecin, ul. Pulaskiego 10, 70-322 Szczecin, Poland Received 15 June 2012; Revised 11 October 2012; Accepted 12 October 2012

Microwave radiation was used in the acquisition of aluminium ammonium calcium phosphates. The substrates such as CaCO3 , H3 PO4 , aqueous ammonia were reagent grade, whereas Al(OH)3 was prepared afresh. The influence of process parameters (pH 6 ± 2, molar ratios of Al3+ : Ca2+ : PO3− 4 in the substrates, respectively 0.31 : 0.62 : 1; 0.5 : 0.5 : 1; 0.72 : 0.36 : 1) on the phase composition and the product properties was determined. Statistical software STATISTICA 10 was used for planning and evaluation of the experiments. The process parameters making it possible to acquire the material with the anticipated physicochemical properties were determined based on statistical evaluation of the planned research by the plan fractional factorial design at three levels 3(k−p) . The phase composition of the samples was studied using XRD analysis. The specific surface area was calculated using the BET method and the particle size was determined by LSM. Materials with a molar ratio of Al3+ : Ca2+ and Al3+ : NH+ 4 in the range of 0.07–0.76 and 0.75–3.4, respectively, with an absorption oil number of 36–56 g per 100 g, SBET within 8.2–73 m2 g−1 , and particle size in the range of 156–252 nm were obtained. c 2013 Institute of Chemistry, Slovak Academy of Sciences Keywords: microwave radiation, anti-corrosive pigments, aluminium ammonium calcium phosphates

Introduction Active passivated pigments constitute an important group of substances exhibiting an inhibitory action in the paint coating in relation to the metal base. A corrosion inhibitor should not only mitigate corrosion but also be compatible with the environment. A corrosion inhibitor can mitigate corrosion in two ways. In some cases, the corrosion inhibitor can transform the corrosive environment into a non-corrosive or less corrosive environment through its interaction with the corrosive species. In other cases, the corrosion inhibitor interacts with the metal surface and provides protection of the metal from corrosion. Thus, depending on the mode of the interactions, there are two broad classes of inhibitors: environment modifiers

and adsorption inhibitors (Sastri, 2011). Environmental protection issues restrict the use of toxic pigments; hence a new generation of pigments, such as phosphates of zinc, molybdenum, calcium, and aluminium, has been developed and widely used as anti-corrosive pigments. However, the environmental regulations dealing with heavy metals are becoming stringent and development of a more efficient and more environmentally friendly pigment is required (Park et al., 2002). An alternative approach involves the use of a protective coating containing inorganic anti-corrosion pigments. With regard to this, recent ecological and toxicological measures have led to the search for potential replacements for lead- and chromiumcontaining pigments (Mošner et al., 2000).

*Corresponding author, e-mail: [email protected] ‡ Presented at the 39th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2012.

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K. Luczka et al./Chemical Papers 67 (9) 1210–1217 (2013)

Pigments are incorporated in paints in order to enhance the barrier effect, to act as sacrificial protection (zinc-rich primers) or to function as inhibitors, affording protection by a physicochemical mechanism (the former) and by an electrochemical mechanism (the latter). The addition of corrosion inhibitors to the paint film is probably the most common of these mechanisms. These inhibitors, or anti-corrosion pigments, are commonly used in the form of solid particulate materials dispersed throughout the paint film. In this case, the cathodic or anodic reactions, or both, are suppressed for as long as the inhibitor is present (Chico et al., 2008). The majority of coloured and covering pigments as well as the speciality pigments are prepared synthetically in processes such as precipitation, sintering, evaporation, or oxidation (Gluszko, 2008). Depending on the molar ratio of Ca2+ : PO3− 4 , pH, and temperature, the following phosphates can be obtained: hydrated dicalcium phosphate – CaHPO4 · 2H2 O, monetite – CaHPO4 , octacalcium phosphate – Ca4 H(PO4 )3 · 2.5 H2 O, amorphous calcium phosphate – Ca3 (PO4 )2 , hydroxylapatite – Ca5 (PO4 )3 OH, and tricalcium phosphate – Ca3 (PO4 )2 (Valsami–Jones, 2001; Wallton et al., 1967; Van Wazer, 1958). Mastuda et al. (2010) detailed the positive effect of condensed calcium phosphate on the anti-corrosive performance of this product, whereas Park et al. (2002) described the anti-corrosive behaviour of hydroxyapatite as an environmentally friendly pigment. The phosphates and hydroxyphosphates of Zn, Al, Ca, Fe(III), Ba, or Mg belong to type I, while the phosphates and hydroxyphosphates of Zn and Al, Al and Mo as well as phosphomolybdates, phosphovanadates, phosphosilicates form group II. The third type of fillers (composite fillers) is based on an inorganic phosphate matrix containing various organic particles. Zinc phosphate is the anti-corrosive phosphate filler most commonly applied, despite the fact that it exhibits a slightly less effective anti-corrosive activity than zinc chromate. Probably the reduced steelrusting inhibition property is the result of the limited water solubility as well as the coarse grain structure of the former filler (Amirudin et al., 1995). The anti-corrosion properties of the phosphates may result, among others, from their strong complexing interactions (Mošner et al., 2000). There are a number of patents and scientific contributions indicating the relatively high efficiency of aluminium phosphate in anti-corrosive paints based on epoxy, alkyd, or acrylic binders using microsized phosphate fillers (Deyá et al., 2010; Nakano et al., 1987; Takahashi, 1984). Several methods have been developed for the preparation of aluminium phosphate (Kic et al., 2009; Liu et al., 2006; Beppu et al., 1996; Lagno & Demopoulos, 2005) including precipitation methods (Kic et al., 2009; Rosseto et al., 2006; Burrell et al., 2010;

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Rinella et al., 1995). Burell et al. (2011) detailed the difficulties entailed in obtaining aluminium phosphates with similar properties. The main advantage of using microwave heating is that the treatment time can be considerably reduced which, in many cases, represents a reduction in energy consumption (Valente Nabais et al., 2004). The current research sought to develop a process for the preparation of aluminium ammonium calcium phosphates which could be used as pigments in anticorrosive coatings. For this reason, the physicochemical properties of the materials thus obtained, such as chemical composition, oil absorption number and specific surface area, were determined.

Experimental Studies on the synthesis of aluminium ammonium calcium phosphates using microwave radiation were performed on the basis of the experimental planning and analysis in accordance with a three-level 3(k−p) fractional factorial design in 9 experiments. The process-independent variables were: pH of reaction mixture (6 ± 2) (x1 ), molar ratio of Al3+ : Ca2+ in the substrates (0.5 : 1, 1 : 1, and 2 : 1, respectively) (x2 ). Multiple dependent variables defining the physicochemical properties of the products were: molar ratio of Al3+ : Ca2+ (y1 ) and Al3+ : NH+ 4 (y2 ), oil absorption number (y3 ), specific surface area SBET (y4 ), and average particle size (y5 ). The aim of the experiments performed following the this plan was to determine the fractional design at three levels of the important factors affecting the investigated parameters and to find the right input values enabling acquisition of a product with the anticipated properties. The factorial plan and the results obtained from the experiments are given in Table 1. Sample preparation Reagent grade substrates: CaCO3 , (85 %), aqueous ammonia (25 %) (≥ 99 %, ChemPur, Poland) H3 PO4 (≥ 99 %, POCH, Poland) and fresh Al(OH)3 were used. Aluminium hydroxide was precipitated in the reaction of aluminium nitrate with potassium hydroxide at pH 7.5 (≥ 99 %, ChemPur, Poland) (Minczewski and Marczenko, 2005). The molar ratios of Al3+ : Ca2+ : PO3− in the reaction mixture were 4 0.31 : 0.62 : 1; 0.5 : 0.5 : 1; and 0.72 : 0.36 : 1, respectively; the pH of the reaction was equal to (6 ± 2). The total salt concentration amounted to 40 mass %. A suspension of fresh aluminium hydroxide and calcium carbonate was dosed into a phosphoric acid solution while stirring at a constant velocity. The pH of the reaction mixture was adjusted by addition of a 25 mass % aqueous ammonia solution. The suspension of reactants with an appropriate pH was prepared in a glass reactor and the mixture was transferred


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Table 1. Three-level 3(k−p) fractional factorial design Independent variables x1

x2

Molar ratio of 3− Al3+ : Ca2+ : NH+ 4 : PO4 in products

–

–

–

4 4 4 6 6 6 8 8 8

0.5 1 2 0.5 1 2 0.5 1 2

No.

1 2 3 4 5 6 7 8 9

0.08 0.27 0.53 0.12 0.30 0.48 0.10 0.32 0.51

: : : : : : : : :

1.13 0.95 0.70 1.31 1.10 0.72 1.50 1.06 0.67

: : : : : : : : :

0.04 0.10 0.16 0.13 0.32 0.50 0.14 0.41 0.64

: : : : : : : : :

1 1 1 1 1 1 1 1 1

Dependent variables y1

y2

y3

y4

y5

–

–

–

m2 g−1

nm

0.07 0.29 0.76 0.09 0.27 0.67 0.07 0.30 0.76

1.85 2.80 3.40 0.94 0.95 0.96 0.75 0.79 0.80

43 45 36 48 47 41 55 56 50

8.20 41.7 27.4 44.4 43.8 38.0 73.0 64.2 41.4

252 243 252 222 248 177 184 229 156

Table 2. Chemical and phase composition of products obtained Content/% Phase composition

No.

1 2 3 4 5 6 7 8 9

Al

Ca

NH3

P2 O5

1.36 4.33 6.03 1.76 4.63 7.43 1.48 5.92 7.83

27.24 22.49 11.71 29.4 24.64 16.81 28.85 29.13 15.16

0.46 0.97 1.1 1.26 3.04 4.98 1.13 4.74 6.15

42.84 41.97 29.6 40.02 40.13 41.49 34.19 48.84 40.31

CaHPO4 ; Al2 (NH4 )(OH)(PO4 )2 · 2H2 O; (NH4 )3 Al2 (PO4 )3 CaHPO4 ; AlH6 (PO4 )3 · 2H2 O; (NH4 )3 Al2 (PO4 )3 CaHPO4 ; CaHPO4 · 2H2 O; AlH6 (PO4 )3 · 2H2 O; (NH4 )3 Al2 (PO4 )3 ; AlPO4 (NH4 )3 Al2 (PO4 )3 ; Ca5 (PO4 )3 (OH); CaHPO4 ; AlH6 (PO4 )3 · 2H2 O (NH4 )3 Al2 (PO4 )3 ; Ca5 (PO4 )3 (OH); CaHPO4 ; AlH6 (PO4 )3 · 2H2 O (NH4 )3 Al2 (PO4 )3 ; CaHPO4 ; Ca5 (PO4 )3 (OH); AlH6 (PO4 )3 · 2H2 O Ca5 (PO4 )3 (OH); (NH4 )3 Al2 (PO4 )3 (NH4 )3 Al2 (PO4 )3 ; Ca5 (PO4 )3 (OH); AlH6 (PO4 )3 · 2H2 O; CaHPO4 (NH4 )3 Al2 (PO4 )3 ; Ca5 (PO4 )3 (OH); AlH6 (PO4 )3 · 2H2 O; AlPO4

into the microwave reactor and treated at a pressure of 2.5 MPa (microwave reactor type ERTEC Magnum, output 750 W at frequency of 2.45 GHz (Ertec Poland), for 15 min. The precipitate thus obtained was separated from the mother liquor using a vacuum filter, followed by triple washing with water (mass ratio of liquid to the solid phase was 3 : 1). Finally, the product was allowed to dry at 70 ◦C for 3 h. Sample characterisation The aluminium and calcium contents in the products were determined by ICP-AES analysis (Optima 5300 DV, Perkin–Elmer, USA). The phosphates and ammonium contents were determined using the spectrophotometric method and ion selective electrode Orion 11-35 type (Thermo Electron, USA), respectively (Minczewski and Marczenko (2005); also see Thermo Electron Corporation instruction material (2003)). The phase composition of the products was studied by X-ray diffraction analysis (X Pert PRO Philips diffractometer, CuKα radiation, The Netherlands). The FTIR studies were performed using IR spectroscopy: Nexus, provided with snap-ATR (Golden Gate Corp., Thermo Nicolet, USA). The oil absorption number (grams of oil required

to form a homogeneous paste with 100 g of tested dry pigment) was determined to PN-EN ISO 7875 (Polish Comittee for Standardization, 1999). The measurements of the Brunauer–Emmett–Teller surface area (SBET ) of phosphates were performed using a Micrometrics Quadrasorb SI Quantachrome Instrument (ASAP 2010 M instrument, USA). N2 adsorption/desorption measurements were carried out at liquid N2 temperature. The average particle size of the materials was determined by laser optic microscopy (Keyence VK-9710k Colour 3D Laser scanning Microscope, Canada). The surface morphology was examined by 5 kV scanning electron microscopy (Hitachi SU-70, Germany) operating under different magnifications of 30 × and 40 ×.

Results and discussion The phase composition of the products was studied using X-ray diffraction analysis. Table 2 summarises the crystalline phases depending on the process parameters. It was found that if the pH of the reaction mixture (irrespective of the molar ratio of reactants) was higher, the products contained more Ca5 (PO4 )3 (OH) and (NH4 )3 Al2 (PO4 )3 but less CaHPO4 and CaHPO4 · 2H2 O. It was also observed



Fig. 1. X-ray diffraction patterns of products obtained in experiment no. 2: CaHPO4 (), (NH4 )3 Al2 (PO4 )3 ( ); AlH6 (PO4 )3 · 2H2 O ( ); Ca5 (PO4 )3 (OH) ( ).

◦

Fig. 2. X-ray diffraction patterns of products obtained in experiment no. 8: (NH4 )3 Al2 (PO4 )3 ( ); Ca5 (PO4 )3 (OH) ( ); AlH6 (PO4 )3 · 2H2 O ( ).

◦

that the greater molar ratio of Al : Ca in the starting materials (irrespective of the pH) resulted in a higher fraction of (NH4 )3 Al2 (PO4 )3 in the reaction product (Figs. 1 and 2). In recent decades, considerable advances have been made towards understanding the phase transitions of sparingly soluble calcium phosphates (House, 1999). It is well known (ValsamiJones, 2001; Wallton et al., 1967; Van Wazer, 1958) that the precipitation of calcium phosphates proceeds at a lower pH, whereas hydroxyapatite precipitates at a higher pH. Moreover, the reaction mixture at the higher pH contained more NH+ 4 ions than that at the lower pH. On account of this, the mutual fraction of such phases as AlH6 (PO4 )3 · 2H2 O and (NH4 )3 Al2 (PO4 )3 was varied in the reaction products. This change was reflected in the molar ratio of Al3+ : NH+ 4 in the material obtained, which decreased with the increase in pH value. In order to demonstrate the presence of hydrogen-

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Fig. 3. FTIR absorption spectra of aluminium ammonium calcium phosphates obtained in experiments: no. 2 (a); no. 5 (b); no. 8 (c).

and hydroxyphosphates in the products obtained, FTIR measurements were performed. The spectra of three selected materials (Table 2, experiments 2, 5, and 8) are compared in Fig. 3. On the basis of the available literature (Burrell et al., 2000; Socrates, 1980; M¨ uller et al., 1997; Ciba et al., 1998), the band at wavenumber of approximately 870 cm−1 is ascribed to the vibrations of the group. This band occurs prominently in the FTIR spectrum of the product precipitated from the reaction mixture with pH 4 and gradually disappears in the precipitation reaction above this pH value. All three spectra display a poorly developed band at a wavenumber of 3100 cm−1 corresponding to the vibrations of the OH− group. The remaining bands with the consecutive wavenumber were attributed to the vibrations: 1100–950 cm−1 the PO3− 4 group, 1500–1300 cm−1 and 3340–3030 cm−1 the NH+ 4 group, and 1670–1606 cm−1 and 3600–3000 cm−1 to H2 O. The chemical composition of materials depended on the process parameters (Table 2). The contents of the individual components were in the range: 1.48– 7.83 mass % of Al, 11.7–29.1 mass % of Ca, 0.46–6.15 mass % of NH3 , and 29.6–48.84 mass % of P2 O5 . The studies were carried out for two levels of variance of the input factors; this statistical analysis predicts the approximation of the interactions of the process operating variables based on the models with a linear character (L) as well as with a quadratic one (Q). This increases the reliability of the evaluation and makes possible an evaluation of the interactions between the operating parameters (Box et al., 2005). Statistical analysis was directed towards determination of the influence of the process parameters having a significant impact on the properties of the phosphates obtained. The effects of the impact of the extreme values of input factors x1 , x2 (independent variables) on the changes in the values of output factors y1 –y5 (dependent variables) were evaluated. The eval-


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Fig. 4. Response surface for molar ratio of Al3+ : Ca2+ (y1 ) and Al3+ : NH+ 4 (y2 ) in the product: x1 – pH of the reaction mixture (6 ± 2), and x2 – molar ratio of Al3+ : Ca2+ in the substrates (0.5 : 1; 1 : 1; and 2 : 1) respectively.

Fig. 5. Response surface for dependent variables: oil absorption number (y3 ) and specific surface area SBET (y4 ) of the product: x1 – pH of the reaction mixture (6 ± 2), and x2 – molar ratio of Al3+ : Ca2+ in the substrates (0.5 : 1; 1 : 1; and 2 : 1), respectively.

uation of the results by the probability test was performed at a significance level equal to 5 %. Figs. 4–6 present the effect of pH and the molar ratios of Al3+ : Ca2+ : PO3− 4 on the reaction mixture dependent variables y1 –y5 . In order to confirm the significance of the evaluation results, ANOVA variance analysis was performed (Table 3). The molar ratio of Al3+ : Ca2+ had the most statistically significant impact on the value of the molar ratio of Al3+ : Ca2+ (y1 ) in the substrates (x2 ). Taking into account the positive sign of this effect (x2 (L), effect 0.65) (Table 3), it was demonstrated that the increase in the x2 value caused the increase in the value of the resultant factor y1 . Aluminium ammonium calcium phosphates with a higher content of aluminium (high molar ra-

tio of Al3+ : Ca2+ ) were precipitated from the reaction mixture with the pH 4–8, when the molar ratio of Al3+ : Ca2+ : PO3− 4 in the reactants amounted to 0.72 : 0.36 : 1 (Table 1). The molar ratio of Al3+ : Ca2+ in the products varied within a range from 0.07 to 0.76. It was found that the pH of the reaction mixture (x1 ) had the most statistically significant impact on the value of the molar ratio of Al3+ : NH+ 4 (y2 ). Taking into account a negative sign of this effect (pH(L), effect –1.99) (Table 3), it was demonstrated that the increase in the x1 value caused the decrease in the value of the resultant factor y2 . In the materials obtained, this molar ratio of Al3+ : NH+ 4 was within the range from 0.75 to 3.40. Phosphates with a small content of ammonium in relation to aluminium were


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Fig. 6. Response surface for average particle size (y5 ) of the product: x1 – pH of the reaction mixture (6 ± 2), and x2 – molar ratio of Al3+ : Ca2+ in the substrates (0.5 : 1; 1 : 1; and 2 : 1), respectively.

mainly precipitated from the reaction mixture with pH 4 (Table 1). By evaluating the impact of independent variables on the value of oil number (y3 ) of the materials obtained, it was found that the increase in the reaction pH would cause the increase in the value of the resultant factor (x1 (Q), effect 12.44) (Table 3). The oil number of the phosphates was within the range of 36–56 g per 100 g. The increase in the oil number along with the increase in the pH value of the reaction mixture could result from a change in the phase composition of the products and their surface area. It is possible that such compounds as Ca5 (PO4 )3 (OH) and (NH4 )3 Al2 (PO4 )3 , the fraction of which in the products ions increases along with the increase in pH, are characterised by a higher oil number and higher surface area than, for example, CaHPO4 or CaHPO4 · 2H2 O. The fraction of the latter phases in the materials obtained was higher at a lower pH value.

Table 3. ANOVA table analysis for dependent variables y1 –y5 Input

Effect

Factor

Input

y1 0.40 0.00 –0.04 0.65 –0.01 –0.00 0.01 –0.06 –0.00

Mean/constant 0.00 –0.02 0.33 –0.00 –0.00 0.00 –0.03 –0.00

–1.99 –0.83 0.54 0.15 –0.75 –0.21 –0.39 –0.11

–0.99 –0.41 0.27 0.08 –0.38 –0.10 –0.19 –0.06

y4

Mean/constant

46.43 12.44 –2.22 –6.33 2.78 1.00 –0.83 –0.50 –1.08

Mean/constant 6.22 –1.11 –3.17 1.39 0.50 –0.42 –0.25 –0.54

x1 (L) x1 (Q) x2 (L) x2 (Q) 1L vs. 2L 1L vs. 2Q 1Q vs. 2L 1Q vs. 2Q

y5 Mean/constant x1 (L) x1 (Q) x2 (L) x2 (Q) 1L vs. 2L 1L vs. 2Q 1Q vs. 2L 1Q vs. 2Q

1.50


y3


Factor

y2

Mean/constant x1 (L) x1 (Q) x2 (L) x2 (Q) 1L vs. 2L 1L vs. 2Q 1Q vs. 2L 1Q vs. 2Q

Effect

216.76 –60.89 –5.39 –24.33 28.78 –14.00 31.67 –15.50 9.17

–30.44 –2.69 –12.17 14.39 –7.00 15.83 –7.75 4.58


42.11 30.94 –0.59 –6.27 10.12 –25.40 –12.69 –0.10 –6.44

15.47 –0.30 –3.13 5.06 –12.70 –6.34 –0.05 –3.22

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Fig. 7. SEM images of aluminium ammonium calcium phosphates obtained in experiments no. 8 (a), no. 5 (b), and no. 2 (c).

In Fig. 7, SEM images of three chosen products precipitated from the reaction mixture at a molar ratio of Al3+ : Ca2+ equal to 1 : 1 at pH 4, 6, and 8, respectively, were compared. It was observed that the shape of the grains of the analysed materials was dependent on the pH of the reaction. The grains with a rod-like shape, besides a few grains with ellipsoidal and spherical shapes, predominated in the product obtained at the lowest pH (Fig. 7a) whereas the grains precipitated from the reaction mixture at pH of 6 and 8 had a spherical shape and, in the latter case, the grains were smaller (Figs. 7b and 7c). This relationship correlates with the physical properties determined. The product obtained at the highest pH and at the molar ratio of Al3+ : Ca2+ equal to 1 : 1 had the highest oil number and specific surface area (Table 1). The differences observed in grain morphology resulted from the different phase composition of the materials examined. The main phase in the product obtained at pH 4 was CaHPO4 , while for the two remaining products it was (NH4 )3 Al2 (PO4 )3 , and Ca5 (PO4 )3 (OH). Hence, it can be assumed that the grains with a rod-like shape originated from CaHPO4 , whereas the grains with spherical and ellipsoidal shapes originated from (NH4 )3 Al2 (PO4 )3 and Ca5 (PO4 )3 (OH), respectively. It was demonstrated that, statistically, the magnitude of the surface area SBET (y4 ) of aluminium ammonium calcium phosphates was affected the most significantly by the interaction of the pH reaction mixture (x1 (L), effect 30.94) (Table 3). The surface area of the products obtained was within the range from 8.2 to 73.0. Aluminium ammonium calcium phosphates with the highest surface area were precipitated from the reaction mixture with pH 8, when the molar ratio of Al3+ : Ca2+ : PO3− 4 in the reactants amounted to 0.31 : 0.62 : 1 (Table 1). Evaluation of the impact of independent variables on the value of particle size (y5 ) of the materials revealed that the decrease in the reaction pH would cause the increase in the value of the resultant factor (x1 (Q), effect –60.89) (Table 3).

Conclusions Depending on the process parameters, products with different contents of nitrogen, aluminium, calcium, and phosphate and with different crystalline phases were obtained. It was demonstrated that the molar ratio of Al3+ to NH+ 4 in the phosphates and the remaining dependent variables y3 –y5 were dependent on the pH of the reaction mixture. In the statistical evaluation of the model examined, the molar ratios (independent variable x2 ) of Al3+ : Ca2+ : PO3− 4 in the reaction mixture did not have a significant influence on the dependent variables y2 –y5 . Nevertheless, this factor can modify the strength of the effect caused by the pH of the reaction mixture on the molar ratio of Al3+ : NH+ 4 , oil number, specific surface area, and particle size. Statistical evaluation of the experimental design, as performed, made it possible to select the process parameters in order to obtain aluminium ammonium calcium phosphates with the predicted values of the input magnitudes examined, i.e., the designed physicochemical properties. In order to obtain phosphates with a high content of aluminium in relation to ammonium and calcium, they should be precipitated from the reaction mixture with a low pH. On the other hand, materials with a large specific surface area and small particle size should be synthesised from the reaction mixture with a pH value as high as 8. The authors’ further studies will be devoted to applications of the selected phosphates discussed in this paper in the two types of paints: epoxy paints and polyurethane paints. Acknowledgements. This work was financially supported by the National Centre for Science under project no 7596/B/H03/ 2011/40.

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