CALCULATION OF KINETIC PARAMETERS OF AMINO-FORMALDEHYDE POLYMERS FORMATION IN THE PRESENCE OF CALCIUM IONS

В.В. Архипова, Р.В. Смотраєв, О.В. Груздєва. Розрахунок кінетичних параметрів утворення карбамідоформальдегідних полімерів в присутності йонів кальцію. Карбонат кальцію ⎯ один із широко використовуваних наповнювачів композиційних матеріалів. Сфери його застосування залежать від дисперсного складу, форми частинок та ін. Модифікація карбонату кальцію високомолекулярними полімерами дозволяє в широкому діапазоні змінювати його характеристики і властивості поверхні. Модифікація карбонату кальцію часто проходить з використанням карбамідоформальдегідних полімерів (КФП). Мета: Метою дослідження є визначення кінетичних характеристик процесу поліконденсації карбамідоформальдегідних полімерів в присутності йонів кальцію. Матеріали і методи: Механізм поліконденсації КФП складний і залежить від багатьох чинників. Поліконденсація КФП проходила за таких умов: температура ⎯ 20, 30, 60 °С; мольне співвідношення карбаміду до формальдегіду ⎯ 1:1,25; тривалість поліконденсації ⎯ 2 години; масове співвідношення СаСО3:КФП = 1:1. Процес поліконденсації проводився в розчині хлориду кальцію з рН = 2...5,5. У процесі експерименту визначали концентрацію формальдегіду і метилольних груп хімічним титриметричним методом. Результати: Показано, що проведення процесу поліконденсації КФП в присутності йонів Са при їх концентрації від 0 до 2,25 моль/л (0...90 г/л) приводить до прискорення процесу при температурі 20 °С більш, ніж в 1,8 разу. Подальше збільшення концентрації Са призводить до зниження швидкості процесу. При температурі 30 °С швидкість практично не змінюється у діапазоні концентрацій Са від 0 до 2,25 моль/л і далі несуттєво знижується. При температурі 60 °С для всього діапазону концентрацій Са спостерігається зменшення швидкості процесу. Вплив Са на процес поліконденсації підтверджує раніше висунуте припущення про утворення слабких зв’язків між КФП і йонами кальцію, які при низьких температурах перешкоджають гідролізу метиленсечовини і руйнуються при підвищенні температури процесу. Ключові слова: карбонат кальцію, карбамідоформальдегідні полімери, константа швидкості.

Introduction.Calcium carbonate is on of widely used fillers of composite materials.The main characteristics defining possibility of its use as a filler are dispersion and particle shape.Calcium carbonate modification by high-molecular polymers allows changing its characteristics (dispersion, phase structure, particle shape) and properties of surface with the broad range.The widespread aminoformaldehyde polymers (AFP) have been used for modification.Carrying out of polycondensation process and hardening of AFP under certain conditions leads to sludge formation with the developed surface and reactionary and active functional groups on it.Such structure of AFP allows applying ISSN 2223-3814 (online) ХІМІЯ.ХІМТЕХНОЛОГІЯ 90 them for coprecipitation with non-organic substances with further use of precipitations in various composition structures.
The polycondensation mechanism of AFP is complex and depends on many factors (рН of environment, temperature, concentration of reagents, availability of catalysts and ions of metals or inhibitors) [1…4].The equation of carbamide reaction with formaldehyde at equimolecular ratio of components, leading to formation of linear structure prepolymers (monomethylol ureas) has the following form: Н 2 N-CO-NH 2 + СН 2 O → Н 2 N-CO-NHCH 2 OH.Further polycondensation of monomethylol urea with formation of polimetilenmochevina courses in the following way: . Small excess of formaldehyde (25 % of stoichiometry) leads to formation of dimethylol ureas which are also pass polycondensation process: Further increase in amount of formaldehyde leads to formation of three-and tetramethylol ureas, but the speed of these products formation is significantly lower than the formation speed of the first two polymers, therefore their influence on polycondensation process is insignificant.The formed polycondensates either turn into carbamide-formaldehyde resins at рН ≥ 5, or, at рН < 5, are emitted from solutions in the form of insoluble high-disperse powders (hardening).
The рН value of the reactionary environment significantly influences not only on the type of the formed product, but also influences on the speed of formation and polycondensation of polymers, which has the minimum values in the рН range from 4 to 8 [1].Considerable decrease of рН leads to sharp reduction of the metilol groups, that are promote binding of inorganic precipitation due to formation of donor-acceptor connection of polymer with cations (calcium ions) or due to physical interaction, and at рН > 5 the hard products are not formed.Therefore, the рН values during the experiments were used in the range of 2…5, that have been recommended for AFP coprecipitations processes with metal hydroxides [5…8].
As it has been established earlier [9,10], the presence of amino-formaldehyde polymers at solution allows increasing of transformation degree of calcium ions that can demonstrate formation of the weak bonds arising between calcium ions and amino-formaldehyde polymers by electrostatic attraction or on the donor-acceptor mechanism.Availability of such bonds also influences on polycondensation process of AFP, therefore it was necessary to establish the influence of calcium ions on the speed of this process.
The aim of the research is to determine the kinetic characteristics of polycondensation process of amino-formaldehyde polymers in the presence of calcium ions.
Results.During the polycondensation process rather fast formation of white precipitation is observed (during 2…5 min.)that demonstrates that process of AFP polycondensation and their hardening proceeds in parallel.During the polycondensation process the concentration of free formaldehyde were defined.It characterizing the course of reaction completeness of the methylol ureas and metilol groups formation, characterizing stitching degree at polycondensation in the solution containing Са 2+ and in control solution without Са 2+ .Dependences of the free formaldehyde and metilol groups concentration on polycondensation time at different рН values are presented on Fig. 1.In the presence of Са 2 + cations the acceleration of the first two stages is observed: the maximum concentration of metilol groups (the minimum concentration of free formaldehyde) at the first stage is reached in 40 minutes at presence of Са 2 + and in 55 minutes without Са 2 +; duration of the second stage -15 minutes at presence of Са 2 + and 20 minutes without Са 2 +.The similar accelerating influence of cations on polycondensation process has been described by other researchers [4], however the data about the influence of calcium ions and its mechanism are absent in literature.
Reduction of the solution рН from 5.5 to 2 leads to reduction of free formaldehyde concentration more than by 1.5 times that confirms expediency of the рН value equal 2 accepted earlier for initial stage of AFP obtaining.So, the further calculations were carried out for рН = 2.
The quantitative measure characterizing completeness and speed of all stages of AFP polycondensation (methylol ureas formation, their polycondensation and hardening) is the concentration of the general formaldehyde in solution.Therefore, kinetic parameters of polycondensation process are determined by processing of the general formaldehyde concentration dependences in solution on polycondensation time.
The preliminary analysis of experimental data has shown that the kinetics of the general formaldehyde expenditure in the reactionary environment is described by the kinetic equation of direct reaction of the second order with reverse reaction of the first order [11]: where k 1 and k 2 -constants of direct and return reactions speeds; τ -reaction time; ω -reaction speed; а 0 , b 0 , c 0 -initial concentration of carbamide, formaldehyde and derivatives of methyleneurea, mol/l; 0

formaldehyde and derivatives of methyleneurea;
х -amount of the reacted substance, mol/l.Equilibrium constant of the process (K = k 1 /k 2 ) is: where x ⎯ equilibrium value х.
The x value is defined by solution of quadratic equation: Taking into account that 0 0 a b < , the gain cannot be more than a 0 .Thus, only those values of x are physically sensible which are in the range from a 0 to -c 0 .The values of the trinomial standing, put in the left side of the equation ( 4), at the ends of this interval, i.e. in points .
If to designate the second root of trinomial as the x′ , then it is possible to write down: At the same time x and x′ are also connected among themselves by: Then the equation ( 1) takes the following form: where x ⎯ equilibrium value of x, defined from experimental data, at known composition of original and equilibrium stock; x′ and k 1 ⎯ the sizes, determined of the solution of the differential equation using the formulas given below.
The solution of the differential equation ( 5) gives the straight line equation in coordinates: The ( ) The examples result for the calculation of the kinetic constants and balance of polycondensation process are presented in table 1.The calculation results of effective kinetic constants of direct and reverse reaction of AFP polycondensation (k 1ef , k 2ef ) for different conditions (initial ion concentrations of Са 2+ are 0; 0.565; 1.13; 1.695 respectively; 2.26 mol/l, formaldehyde ⎯ 1.0 mol/l, carbamide ⎯ 0.8 mol/l; temperature 20, 30, 60 °C; рН=2) are presented in Table 2.

Table 1
Example of calculation of the AFP formation process kinetic parameters t, °С С 0 (Са), mol/l τ, min K х′ Based on the obtained data, the activation energy and preexponential multipliers for direct (E 1 ) and the reverse (E 2 ) reaction in solution without presence of Са 2+ has been defined: Е 1 = 30.2kJ/mol; В 1 = 4.13 l/(mol•s); Е 2 = 4.3 kJ/mol; В 2 = 5.55•10 -4 s -1 .The Arrhenius equations for process of AFP formation in solution without presence of Са 2+ takes the following form: ⎯ for direct reaction According to the obtained data, the addition to the reactionary Са 2+ environment leads to sluggishness of both direct and inverse reactions.The dependences of efficient kinetic constants of a direct and reverse reaction on concentration of Са 2+ (Fig. 3) in a general by the following equations: where k 1 ′ and k 2 ′ ⎯ coefficients of proportionalities for a direct and reverse reaction, that are temperature-dependent.
Processing of the dependences presented in Fig. 3 results in the following dependences for coefficients of proportionalities for direct and inverse reactions calculation: Thus, the dependences of efficient kinetic constants on concentration of Са 2+ and on temperature can be written down in the following form: The common speed of AFP formation process in the presence of calcium ions is described by the following equation ( ) The relative deviation of the experimental and calculated kinetic constants of reactions does not exceed 5.0 % for a forward reaction, and 11 % for inverse one.I.e. the received results are agree within an experiment error ⎯ 3…10 %.
Conclusions.The analysis of equation (6) shows that polycondensation process of AFP in the presence of Сa 2+ ions at their concentration from 0 to 2.25 mol/l (0…90 g/l) leads to acceleration of process more than by 1.8 times at temperature of 20 °C.Further increase of Сa 2+ concentration leads to reduction of process speed.At temperature of 30 °C the speed of process almost does not change in the range of Сa 2+ concentration from 0 to 2.25 mol/l and further decreases slightly.For all range of Сa 2+ concentration at temperature of 60 °C the reduction of process speed is observed.Influence of Сa 2+ on process of polycondensation confirms assumption made earlier of formation of weak bonds between AFP and calcium ions which at low temperatures interfere with hydrolysis of methyleneurea and collapse at increasing of process temperature.
Therefore, it is expedient to conduct the process of AFP obtaining at a temperature of 20 °C in solutions with Сa 2+ concentration within 1…2.25 mol/l (40…90 g/l).
The question of plastics properties studying that use the amino-formaldehyde polymers modified by lime carbonate as an excipient can be considered as a subject of further researches.