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ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
UDC 661.728; 661.634
ACID-BASE PROPERTIES OF PHOSPHORYLATED CRUMB RUBBER
N-A.Efendiyeva1, LA.Buniyat-zadeh1, N.M.Binnetova2, N.M.Guliyeva3, J.A.Naghiyev4, R-M-Alosmanov1
!Baku State University 2Azerbaijan University of Architecture and Construction 3Western Caspian University 4Ministry of Digital Development and Transport of the Republic of Azerbaijan
r_alosmanov@rambler.ru
Received 22.12.2022 Accepted 15.02.2023
Phosphorylation of crumb rubber was carried out by the reaction of oxidative chlorophosphorylation and hydrolysis of the obtained modifier. The modified crumb rubber was studied by potentiometric titration using the method of various samples. The ionization constants of phosphorylated crumb rubber were calculated using the Henderson-Hasselbach equation.
Keywords: crumb rubber, oxidative chlorophosphorylation reaction, potentiometric titration, ionization constants.
doi.org/10.32737/0005-2531-2023-3-18-25
Introduction
The rapid growth of the population and the development of transport in our days leads to the fact that the production of car tires is growing exponentially inexorably. The annual production of tires in the world in 2017 already exceeded 2.9 billion, which most eloquently speaks of the growth of rubber waste corresponding to this indicator [1]. End-of-life tires make up the bulk of rubber waste and, being non-biodegradable, are taking up ever-larger areas, impacting the environment. Toxic gases released when tires are burned or used as fuel harm the environment and cause devastating air pollution [2, 3]. Tire rubber contains highly toxic styrene [4], which makes crumb rubber (CR) dumps very hazardous to human health
In recent years, researchers have been proposed various ways to recycling old tires, since any form of waste recycling is always benefit. A significant part of the proposed ways for tire disposal relates to heat treatment and mechanical processes. For example, disordered carbon can be obtained by pyrolysis of waste tires [5, 6]. Waste tires powder was chemically activated using sulfuric acid and/or alkali to recover pyrolytic carbon black with different characteristics was suggested in [7].
Yanushevich et al. presented the technologies of limonene production from waste tire pyrolysis [8]. In addition, a number of review articles are devoted to the recycling of tires and contain detailed information on the production of valuable products [9], the synthesis of pyrol-ysis oil as an energy source [10], the preparation of concrete [11], the use of inexpensive and environmentally friendly modifiers for thermo-setting polymers [12]. The review [13] provides information on the preparation and study of the properties of activated carbon obtained from waste tires, with the potential for its application as an adsorbent.
More than a decade of research our group has been synthesized various functional polymers with phosphorus- [14], phosphorus/nitrogen [15, 16], and phosphorus/nitrogen/sulfur-containing groups [17] using the reaction of oxidative chlorophosphorylation (OxCh). A positive particularity of the reaction is the opportunity to carried out using easily available commercial reagents under soft conditions on the main laboratory equipment. In previously works we reported the results of studies, where both individual (butadiene rubber [14-17], sty-rene-butadiene rubber [18, 19], nitrile- butadiene rubber [20]) polymers and polymer
mixtures [21] were used as the initial matrix. The obtained products were successfully applied for the preparation of nanocomposites [21, 22], adsorbents [23, 24], and extractants for the concentration of metal ions by the solid phase extraction method [15, 16, 22].
In this paper the results of investigation of the modification of waste tires by the reaction of OxCh have been discussed. The nature of reaction and functional groups introduced to the reaction product allowed to propose its potential use as an adsorbent. Therefore, the acid-base properties of phosphorylated waste tires were studied in detail, because of important such kind of analysis for various materials using in adsorption processes. It should also be noted that the main component for tire rubber is sty-rene-butadiene rubber [25]. Based on this, comparative studies of the data obtained in this research with the corresponding works published by us earlier will be carried out and discussed. The conclusions that we will come to in this comparative study will help evaluate the usefulness of this approach for the disposal of various polymeric wastes.
Experimental part
Reagents and methods
CR was used as waste tire and obtained by grinding the chamber part of old tire. PCl3, CCl4, CHCl3, H2SO4, NaOH, NaCl, HCl were pushed from Sigma (Germany) and used without previously purification.
Phosphorylated CR was prepared by the reaction of OxCh, with further hydrolysis of the obtained product [14]. Thus, CR (10 g) was placed in a reaction flask, filled with chloroform (100 ml) and kept for 24 hours. A preliminary check of the swelling degree of CR in various solvents showed that the swelling degree reaches of the highest value in chloroform and equal 200%. After 24 hours, the suspension was stirred for 3 hours at 600C temperature, then cooled and100 ml of CCl4 was added to reaction mixture for best solubility of oxygen that has been provided the oxidation phosphorylation. Then the reaction medium was bubbled with oxygen during an hour with rate of 7 l/hour af-
ter that the necessary amount of PCl3 was introduced to reaction medium by portions. The reaction was carried out during 20 hours and the ratio of PCl3 : CR equal to 6:1.
After modification the solid phase (modified CR) was separated from liquid phase (CCl4, PCl3, POCl3). The obtained product was hydrolyzed using distilled water at 500C during 4 hours, washed with water and ethanol and dried for the first time in the air and then in vacuum oven at 400C during two hours.
Determination of the static exchange capacity for sodium ions (SECNa+)
The analytical weighed sample (0.1000 g) was placed in a titration flask, and 20 ml of 0.1 N sodium hydroxide solution was added into it and bubbled with nitrogen for 1 hour. Then, tightly closed flask was kept for 24 hours. One day later, the solution was filtered, and 2.5 ml of the filtrate was taking for testing. 2-3 drops of 1% solution of phenolphthalein in an alcohol medium were added to testing sample as an indicator and titration with a 0.1 N HCl solution was carried out [26].
The value of static sorption capacity (mg-eq/g) is calculated in according with the equation:
where, F - correlation coefficient for 0,1 N NaOH; V - amount of 0.1 N HCl, used for titration, ml; F1 - correlation coefficient for 0.1 N HCl; 0.004 - titer of 0.1 N NaOH; 40 - molecular weight of NaOH; g - mass of sample, g.
The obtained results were used for determination of ionization constants of functional analytical groups.
Potentiometric titration
Potentiometric titration was carried out using the method of various samples. For carry out the experiments 0.005 g of each of the samples has been weighted on an analytical balance and placed into the bottles. As known the value of pKion essentially depends on the ionic
strength of solution. An increase in the value of ionic strength of solution provides the increasing of the ionization constant too. Therefore, it was necessary to determine the value of the ionization constant at a regular ionic strength of solution. For maintaining of regular ionic strength of solution, 20 ml of 2M NaCl solution was added to each bottle. Then, various amounts of 0.025 M NaOH solution were added to the contents of the bottles, varying its amount in such a way that the value of degree of complete neutralization of functional groups (Q) changed from 0 up to SECNa+ value (where Q value is the ratio of the amount of added alkali solution (in mmol) to the total number of functional-analytical groups of the sample, taken as found). After stirring the solutions with nitrogen, they were kept in a desiccator filled with nitrogen at room temperature for 24 hours to create an ionic balance. Then pH of the solution was measured.
Results and discussion
Modification of CR has been proceeded with the release of heat (exothermic reaction), that is characteristic for OxCh. The reaction product is black fine-grained particles of irregular shape. Usually, the modification of industrial polymers by OxCh is accompanied by an increase in the mass of the reaction product, which can be explained by the introduction of various functional groups into the polymer matrix. But, in the case of CRs, on the contrary, a decrease in the mass of the reaction product has been observed. Why? It is known that, in addition to rubbers, tire rubbers contain ingredients such as sulfur, zinc oxide, various oils, and etc. [25]. Taking into consideration the nature of the reaction, it can be assumed that, along with the rubber component, modification is also carried out on its other components, which are converted into a water-soluble form and removed from the final product decreasing yield of it.
The reaction nature allows us to conclude that the modified CRs will contain phosphonic acid groups, and it is therefore advisable to determine the SECNa+. The SECNa+ value determined by the method described above equal to
3.13 mg-eqv/g, which, indirectly, confirms the fact of CR modification.
Although thermodynamic constants provide more complete information about the dissociation of acid groups, their calculation is, in most cases, very difficult. Therefore, for practical purposes, the usual dissociation constants can be used. In this work, based on the analysis of the results of potentiometric titration, using the modified Henderson-Hasselbach equation, the ionization constants of functional-analytical groups (pKion) were calculated:
PKion = pH-mlg-
(2)
where: a - is the degree of neutralization of the acid groups of the sample, calculated from the results of potentiometric titration, m - is a constant value characterizing the polymeric state of the substance.
Based on the results obtained, potentiom-etric titration curves Q (amount of titrant in mmol/g) versus pH were plotted (Figure 1).
By graphical differentiation of the integral titration curve, differential titration curves dpH/dQ versus Q (Figure 2) to precise determination of equivalence point have been plotted.
As can be seen, the titration curve has a step character, which indicates the presence of two ionogenic groups in the studied sample. The a value for each step was calculated from the static sorption capacity of the functional group in solid phase, as the ratio of amount of NaOH (in mmol) added to amount (in mmol) of each active group in grams. Titration curves characterizing the dependence of the pH of solution on the added alkali, allow to determine the maximum capacity of the sample.
The degree of ionization calculated in according to the equation (3) allowed to discuss the results of potentiometric titration:
(3)
where: N - normality of NaOH solution; V -volume of added NaOH solution, ml; g - mass of example, g; SECNa+ - static sorption capacity of each group in the solid phase.
Fig. 1. Integral curve of Potentiometrie titration of phosphorylated CR.
Fig. 2. Differential curves of Potentiometrie titration of phosphorylated CR.
follows (Figure 3).
As can be seen from the Table 2, the ionization constants of functional groups of phosphorus-containing crumb rubber are lower in compared with those of phosphorus-containing rubbers. It can be explained by the fact that crumb rubber samples have a higher dense network structure. As a rule, the acidic properties of polyelectrolytes decrease with increasing of network density. This regularity also holds for the parameter m in pK2. Thus, in the crumb rubber sample, the parameter m is bigger than in case of rubber (for pK2). But for pKi, this explanation is unsuitable. As can be seen from the table, the parameter m in the crumb rubber sample is smaller in compared to rubbers. This can be explained by the fact that the acidity of the ionogenic groups of polyelectrolytes depends not only on the network density, but also on the local density and microscopy of the iono-genic group.
Table 1. Calculation of pKion values of phosphorylated crumb rubber
pH a 1-a a/(1-a) lg(a/(1-a))
4.23 pKj 0.100 0.900 0.111 -0.95
4.52 0.200 0.800 0.250 -0.60
4.92 0.300 0.700 0.429 -0.37
5.32 0.400 0.600 0.667 -0.18
6.48 0.500 0.500 1.000 0.00
6.78 0.600 0.400 1.500 0.18
7.63 0.700 0.300 2.333 0.37
8.65 0.800 0.200 4.000 0.60
8.98 0.900 0.100 9.000 0.95
9.23 pK2 0.100 0.900 0.111 -0.95
10.89 0.200 0.800 0.250 -0.60
10.90 0.300 0.700 0.429 -0.37
10.95 0.400 0.600 0.667 -0.18
11.36 0.500 0.500 1.000 0.00
11.39 0.600 0.400 1.500 0.18
11.45 0.700 0.300 2.333 0.37
11.65 0.800 0.200 4.000 0.60
11.72 0.900 0.100 9.000 0.95
Table 2. Ionization constants of of phosphorylated samples
Samples pKj m pK2 m References
Phosphorylated butadiene rubber 4.40 3.23 8.64 0.84 [27]
Phosphorylated styrene-butadiene rubber 4.92 2.32 9.43 1.96 [19]
Phosphorylated CR 6.40 1.08 11.06 2.88 This work
The results of calculating pKion value for the half-neutralization point a=0.5 of the phosphorylated sample, in according to the Hender-son-Hasselbach equation, are shown in the Table i. In according to the modified Henderson-Hasselbach equation, the pH of the solution is a
linear function of fi}s~~) and the slope of
the straight line (m value) must be equal to i. Actually, there is a deviation from this value. The m value was used to calculate the conditional pKion constants of active groups of the sample: pK1= 6.4 (titratable group OH (1)); pK2 = 11.06 (titratable group OH(2)). Then, the obtained results were compared with the ionization constants of butadiene and butadiene-styrene rubber samples modified according to the same methodology (Table 2). The dependence of
based on found a values and the correspond-ding pH values can be presented graphically as
pH
12 • |iK:
• 10 Q V 1.084X+ 11.06 Ftz = 0.7395
•
y Q • pKi
O 7 u - 7 ÖÖQ1 V 4- t M
V ¿ = 0.9528
• 5 -4—
•
-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1
lg(ctf<l-a))
Fig. 3. Graphical determination of ionization constants (pKion) of phosphorylated CR.
Conclusions
In the present work, the reaction of OxCh of CR obtained from used car tires in a heterogeneous medium, was carried out, and a sample from the product of hydrolysis of obtained modified crumb rubber was prepared. The static exchange capacity of the sample for sodium ions was calculated: 3.13 mg-eqv/g. Ionization constants of phosphorus-containing rubber were determined by potentiometric titration: pKi=6.4; pK2=11.06. In according to comparison of the obtained results with literature data, the acidity of the ionogenic groups of polyelectrolytes depends on the network density, local density, and micro-coating of the ionogenic group.
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FOSFORLA§DIRILMI§ REZiN TULLANTILARININ TUR§U-ЭSAS XASSЭLЭRi
N.A.Эfэndiyeva, LA.Bйшyatzadэ, ^М.Бтпэ1оуа, ^М.ОиИуеуа, С.Э^а§1уеу, К.М.А1озшапоу
Oksidlэ§dirici xlorfosforla§ma reaksiyasl vэ а1тап modifikatln hidrolizi Пэ rezin tullantlsmln fosforla§dlrIlmasl hэyata кедт1т1§&1\ Modifikasiya о1тти§ tullantl muxtэ1if штит^ usulunun tэtbiqi ilэ potensiometrik titrlэmэ metodu ilэ tэdqiq е&1т1§&1\ Ш^эЬГ эsasmda Henderson-Hasselbax tэnliyinэ иу§ип o1araq rezin tu11antl1anшn ion1a§ma sabit1эri hesab1anml§dIr.
Адаг sдzlэr: ге2т ШПсШйсп, oksidlэ§dmci xlorfosforlс§mс reсksiyсsl, potensiometrik titrlэmэ, юп1щтс sсbitlэri.
КИСЛОТНО-ОСНОВНЫЕ СВОЙСТВА ФОСФОРИЛИРОВАННОЙ РЕЗИНОВОЙ КРОШКИ
Н.А.Эфендиева, И.А.Буният-заде, Н.М.Биннатова, Н.М.Гулиева, Дж.А.Нагиев, Р.М.Алосманов
Проведено фосфорилирование резиновых отходов реакцией окислительного хлорфосфорирования, с последующим гидролизом полученного модификата. Модифицированные отходы исследованы методом потенциометрического титрования с использованием метода различных образцов. На основании полученных результатов рассчитаны константы ионизации резиновых отходов по уравнению Гендерсона-Хассельбаха.
Ключевые слова: резиновые отходы, реакция окислительного хлорфосфорилирования, потенциометрическое титрование, константы ионизации.
