ISSN 0018-1439, High Energy Chemistry, 2007, Vol. 41, No. 6, pp. 454–459. © Pleiades Publishing, Ltd., 2007. Original Russian Text © A.I. Maksimov, A.Yu. Nikiforov, 2007, published in Khimiya Vysokikh Energii, 2007, Vol. 41, No. 6, pp. 513–519.

PLASMA CHEMISTRY

Comparison of Plasma and Plasma–Solution Modifications of Polymer Materials in the Liquid Phase A. I. Maksimov and A. Yu. Nikiforov Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya ul. 1, Ivanovo, 153045 Russia e-mail: [email protected] Received December 21, 2006

Abstract—The capabilities of atmospheric-pressure gas-discharge plasma and plasma–solution systems as applied to enhancement of industrial processes in solutions, such as flax bleaching and wood delignification, were analyzed. It was shown that the set of active species generated in the plasma systems corresponds to that required for the processes in question. According to estimates, the efficiency of plasma–solution systems is higher than that of cold atmospheric-pressure plasma. A kinetic scheme for the bleaching processes activated in the plasma–solution systems is discussed. DOI: 10.1134/S0018143907060124

Modification of polymeric materials is a change in their surface or bulk properties that improves their processing or end-use characteristics. In the case of synthetic polymers, the surface modification is practiced, which is aimed to impart hydrophilicity, adhesive properties, or some special properties (as in the case biomedical polymers [1, 2]) to the surface. For this kind of modification of the surface properties of synthetic polymers, treatment in cold plasma, most frequently, at a reduced pressure is widely used [2]. This plasma treatment mode is also used in industrial processes for treatment of naturally occurring macromolecular compounds [3, 4]. At the same time, there are problems in modification of the properties of polymer materials that cannot be solved by means of this technique. This primarily concerns the treatment of natural high-molecular-mass materials aimed at the removal of certain components that act as impurities from the viewpoint of practical applications but are, as a rule, chemically bonded to the base component. Such processes include the bleaching of flax (and, to a lesser extent, cotton) in the textile industry and the delignification of wood in the pulp-and-paper industry. In these processes, lignin, pectic acids, and some other components are removed from the starting material, with the lignin elimination being the most topical problem. The existing technologies are based on oxidative processes that can be conducted in aqueous solutions. Since the oxidation of cellulose—the base component of the material—always takes place simultaneously with the desired removal process, the main point of concern in an industrial process (in addition to the rate) is its selectivity. In industrial processes, this task is accomplished through the

selection of the composition of solution and the treatment conditions. Our attempts to run the flax bleaching process by direct treatment in low-pressure oxidizing plasma (air, oxygen, and water vapor as the plasma gas) failed because of the fast oxidation of cellulose. This brings up the question: Is it possible to initiate or just to accelerate the liquid-phase delignification process by plasma? From the practical point of view, atmosphericpressure plasma (not the one at reduced pressure) is to be considered. As applied to the task of enhancement of processes in solution, two different approaches may be considered. One is the dry plasma existing independently of the solution. A gas (air) flow is activated when passing through the plasma zone, the activated gas bubbles through the solution or blows on the material treated in the steaming mode. The other is a version of the plasma–solution system when the solution is one of the gas-discharge electrodes and chemically active species can form both in the plasma zone contacting with solution and in the solution itself. The objective of this work was to determine what chemically reactive species take part in the process that we intend to enhance by plasma treatment. In addition, it was necessary to reveal whether these species are produced in the plasma system designed for practical application and to optimize the yield of generation of the required active species, as well as their transport to the site of interaction with materials subjected to treatment.

454

COMPARISON OF PLASMA

455

Table 1. Reactive forms of reagents involved in cellulose bleaching processes Process (medium)

Oxidation (acidic)

Substrate

Reagent type

Aromatic and unsaturated strucElectrophile tures

Active form

Reacting species +

Cation

H+, OH+, O3, Cl+, NO+, N O 2

Radical

H•, HO•, H O 2 , O2 , Cl•, ClO•,

•

•

•

•

•

Cl O 2 , NO•, N O 2 Reduction (alkaline) Unconjugated and conjugated carbonyl-containing structures

Nucleophile

–

2–

S2 O 4

–

H•, eaq, S O 2

Radical

CHEMICALLY ACTIVE SPECIES INVOLVED IN DELIGNIFICATION AND BLEACHING PROCESSES OF CELLULOSE MATERIALS The reactive forms of reagents involved in the processes of liquid-phase bleaching of cellulose are given in Table 1 [5]. The most widespread environmentally friendly methods for bleaching of cellulose materials are the oxygen and peroxide processes. At the same time, the presence in the liquid phase of H2O2 resulting from a number of transformations dependent on the pH of the medium leads to the formation of a set of species that is practically identical to the group produced in a solution containing molecular oxygen [5]. Indeed, hydrogen peroxide in an alkaline medium undergoes degradation via the radical mechanism yielding dioxygen: 2–

H2O2 + 2OH– 2–

O 2 + H2O2 •OH

O 2 + 2H2O,

(1)

O 2 + •OH + OH–,

(2)

–•

•

+ H2O2

H O 2 + H2O,

O 2 + H2O

H O 2 + OH–,

–•

•

2H O 2

(3)

•

(4)

H2O2 + O2.

(5)

The oxidation of lignin by oxygen in an alkali medium is accompanied by the generation of radical –• anions O 2 , hydroxyl radicals, and singlet oxygen (1O2). The pathways of formation of the active forms of oxygen during delignification and the interrelations between their transformations are described by the equations: •

O2 + e HIGH ENERGY CHEMISTRY

–•

O2 , Vol. 41

(6) No. 6

–

H–, HO–, H O 2 , ClO–, Cl O 2 ,

Anion

2007

–•

O 2 + H+ –•

•

•

H O2 + H O2 –

H O 2 + H2O2 2(1O2)

(7)

H O 2 + 1O2,

(8)

H2O2 + 1O2,

(9)

–

O2 + H O2 •

•

H O2 ,

–•

•

O 2 + H O + H2O, 2O2 + hν.

(10) (11)

A comparison of these equations with Eqs. (1)–(5) shows that, regardless of the oxidant used (molecular oxygen or H2O2), the system contains all of the above species. GENERATION OF CHEMICALLY ACTIVE SPECIES IN ATMOSPHERIC-PRESSURE GLOW DISCHARGE IN DRY AND MOIST AIR As a practically important example of nonsolution plasma, we consider the plasma of an atmosphericpressure glow-discharge stabilized by a fast gas flow. The scheme of excitation of such a discharge is reported in [8]. On the basis of solution of the Boltzmann kinetic equation, the properties of the discharge in dry and moist air were analyzed and the rates of generation of the main chemically active species were calculated. In the calculations, 230 processes occurring in plasma were taken into account. According to the reported data, the discharge current was 50 mA at a total voltage drop of about 30 kV and the linear air velocity was 123 cm/s, which corresponds to the volumetric flow rate of ~100 cm3/s at a contact time of the gas with plasma of 0.04 s. In accordance with the results of kinetic calculations, this contact time of the gas corresponds to establishment of steady-state concentrations of all main products. Using the kinetic curves reported in [8], we estimated the steady-state

456

MAKSIMOV, NIKIFOROV 1

1 (a)

(b)

2

2

3

4

5

4

3

Fig. 1. Type of plasma–solution systems. (a) Electrolytic-cathode glow discharge: (1) electrodes, (2) plasma zoned, (3) electrolyte solution, and (4) stirrer. (b) Diaphragm discharge: (1) electrodes, (2) quartz cell with the diaphragm, (3) plasma zone, (4) electrolyte solution, and (5) stirrer.

concentrations of neutral active species carried over from the plasma zone by dry or moist air. According to these data, the maximum number density of ions in the plasma zone is 1.5 × 1011cm–3 and the maximum number density of neutral active species (ozone) is 2 × 1016 cm–3. These values give the fluxes of ions and neutrals as 1.5 × 1013 and 2 × 1018 c–1 s–1, or ~0.25 × 10-10 and ~3 × 10-6 mol/s, respectively. It is easy to see that many of the active species involved in the cellulose delignifi-

Table 2. Transformations of primary particles in acidic and alkaline medi Medium Primary particle alkaline eaq

eaq

OH

OH

H

H +

acidic eaq

O–

OH

eaq

H +

H2O

H aq –

O H aq

H2O2

H aq

–

H2O

O H aq H2O2 –

H O2

–

H O2 –

O2

H2O2

H

cation processes are generated under discharge conditions in moist air. GENERATION OF CHEMICALLY ACTIVE SPECIES IN PLASMA–SOLUTION SYSTEMS The two main types of low-voltage plasma–solution systems are sketched in Fig. 1. A substantial difference between these systems is that the plasma zone occurs outside the solution in the first system and inside the solution in the second system. It is undoubted that chemically active species are generated in the plasma zone of both of these plasma– solution systems. It should be expected that their set is close to the one generated under the moist-air glow discharge conditions. However, in the case of discharge with electrolytic electrodes, the main source of chemically active species is a thin surface layer of the solution. Under these conditions, aqueous solutions that serve as gas-discharge electrodes undergo bombardment with positive and negative ions injected from the plasma zone. The energy of these ions can be as high as a few hundreds of electronvolts. The interaction of ionizing radiation with solution is supposed to occur in three stages: physical, physicochemical, and chemical [6]. By the end of the physicochemical stage (about 10–12 s), e aq , H, OH, +

H aq , O, and H2 exist in water. At the chemical stage, these species diffuse from their generation sites and react with one another and with solutes. The set of primary species generated as a result of ion bombardment is given in the first column of Table 2. Table 2 also presents the results of transformations of these species in acidic and alkaline media. The subsequent main transformations of the primary active species are presented below as given in [7]. HIGH ENERGY CHEMISTRY

Vol. 41

No. 6 2007

COMPARISON OF PLASMA

H2 + OH–

eaq + eaq + 2H2O eaq + O2 eaq + H eaq + H2O H2O2 + eaq

–

–

O2 , eaq + Haq –

OH radicals and O–

ç atoms

Solvated electrons

–

H , H + H2O

H+H

H2 + OH

–

H2

H + OH

H –

–

H + O2

H + OHaq

OH + OH

H2O

H + OHaq

457

eaq + H2O HO2

O + O2 OH + H − OH +OHaq

H2O2 –

O3 H2O –

O +H2O

–

OH + OHaq

It is easy to see that this set of reactions of the primary active species creates a medium in which all processes (1)–(11) that accompany the delignification of cellulose-containing materials can occur. COMPARISON OF THE EFFICIENCIES OF GENERATION OF CHEMICALLY ACTIVE SPECIES IN THE DRY ATMOSPHERICPRESSURE PLASMA AND PLASMA–SOLUTION SYSTEMS Dry Plasma As has been shown above, the maximal flux of active species is observed for O3, which is about 3 × 10−6 mol/s at a power dissipated in the discharge of 1500 W. Correspondingly, the power yield of ozone is about 2 × 10–9 mol/J. The yield of other active species is substantially lower. Plasma–Solution Systems In this case, the approach to the estimation of the efficiency of chemical activation should be different. First of all, the generation of chemically active species both in the plasma zone and in the surface layer of the solution should be taken into consideration. Decreasing the size of the gas-discharge region, we may keep only the cathode region. The contribution to chemical activation will be made in this case only by processes in the surface layer of the solution. The anticipated benefit of this activation channel is wide opportunities for the active species to initiate processes in the solution itself. Evaluating the efficiency of this channel, one should take into account the following circumstance. The integral efficiency of activation processes in solution can be represented as the sum of the yields of the main primary active species H, OH, and esolv. It is convenient to measure the yields of the primary active species in plasma– solution systems as the number of particles produced per ion that entered the solution. This yield, of course, depends on the value of the potential drop, which accelerates the ions that move toward the solution. To determine the efficiency, we take into account that the total yield of these active species is about 10 [6]. This means that, at a discharge current of 50 mA, the overall rate of HIGH ENERGY CHEMISTRY

Vol. 41

No. 6

2007

generation of active species in the solution will be ~3 × 1018 particle/s or ~5 × 10–6 mol/s. We obtained almost the same value, as in the case of dry plasma. However, at a discharge current of 50 mA, the cathode fall of the potential is less than 1 kV. This value gives the yield of primary active species as ~10–7 mol/J. Thus, in the case of the plasma–solution system, the efficiency of chemical activation turns out to be at least an order of magnitude above that of the dry plasma. THE EFFICIENCY OF USE OF PRIMARY ACTIVE SPECIES IN ATMOSPHERIC-PRESSURE PLASMA AND PLASMA–SOLUTION SYSTEMS Dry Plasma In this case, we deal with the following main type of active species generated in atmospheric-pressure plasma: ions, free atoms and radicals, and metastable (excited) molecules. Let us estimate the characteristic values of their relaxation times and lengths. Simple estimates with allowance for the ion composition of both dry-air and moist-air plasmas show that the lifetime of ions is controlled by three-body ion–ion recombination [9] and amounts to a few microseconds under the given conditions, a value that corresponds to the relaxation length of the order of micrometers. The main neutral active species in the case of dry air are oxygen and nitrogen atoms having number concentrations of 1.5 × 1015 and 3 × 1014 cm–3, respectively. In the case of moist air, the set of species is broader: O(3 × 1014 cm–3), N (2 × 1013 cm–3), H (1013 cm–3), OH (8 ×1014 cm–3), and HO2 (4 × 101)4cm–3). The rate constants of the fastest gas-phase reactions involving these particles are given in [2, 10]. An analysis shows that bimolecular reactions between radicals are so fast that they will decay immediately in the plasma zone at a linear gas velocity of about 100 cm/s. This implies that the major oxidant entering into the solution from dry- and moist-air plasmas will be ozone. Plasma–Solution Systems In the case of plasma–solution systems, the radial OH produced in the plasma zone should reach the surface of the solution. To evaluate this ability, we will use

MAKSIMOV, NIKIFOROV

0.05

2 (a)

× 10–4 1.0 0.8

0.03

0.6

0.02

0.4

0.01

0.2

[H2O2], mol/l

0.04

1

0 × 10–4

0

2 (b)

5

× 10–5

[HO2], mol/l

4

2.0 1.5

3

1.0

2 0.5 1

[OH], mol/l

[H2O2]0 = 0.05 mol/l

1

d[HO2]/dt, mol(/l s)

458

0

0 0

20

40

60 80 Time, s

100

120

Fig. 2. Kinetic calculation of the chemical composition for the fabric-bleaching working solution: (a) change in the concentration of H2O2 and OH radicals and (b) change in the concentration ofHO2 radicals and in their generation rate d[HO2]/dt.

as a reference value the self-diffusion coefficient of water molecules. Estimates with allowance for the data reported in [11] give the self-diffusion coefficient of water molecules as about 0.93 cm2/s at a temperature of [HO2], 10-4 mol/l 1.00

7 6

5

0.75 4

0.50

3 1, 2 0.25

0

50

100 Time, s

150

200

Fig. 3. Rate curves for the buildup of HO2 radicals in solution during plasma activation at d[OH]/dt of (1) 0, (2) 10–9, (3) 10−8, (4) 10–7, (5) 10–6, (6) 5 × 10–6, and (7) 105 mol/(l s).

1500 K (the gas temperature under the conditions of atmospheric-pressure glow discharge with electrolytic cathode [2]). For the recombination lifetime of hydroxyl radicals estimated above at 1.4 × 10–4 s, this value of the coefficient results in a diffusion length of about 0.01 cm. This means that hydroxyl radicals will predominantly react in the plasma zone, not managing to reach the solution, even in the case of electrolytic-cathode glow discharge whose plasma zone occurs immediately over the solution surface and is a few millimeters in length. Even in a diaphragm discharge when the longitudinal size of the plasma zone also does not exceed a few millimeters and its lifetime is about one millisecond, hydroxyl radicals in most part will not enter into solution. Thus, plasma–solution systems are to be considered, first and foremost, a generator of oxidizing active species, of which only secondary active species— hydrogen peroxide—accumulate in the bulk of the solution. FEASIBILITY OF INITIATION AND ENHANCEMENT OF CONVENTIONAL OXIDATIVE PROCESSES IN ELECTROLYTE SOLUTIONS BY PLASMA ACTIVATION In calculations of the kinetics of reactions occurring in solutions, processes (1)–(5) were considered. The effect of discharge on the solution was defined by setting a value for the generation rate of OH radicals or hydrogen peroxide. The rate constants of reactions (1)−(5) were borrowed from [5, 7, 12, 13]. In the choice of the properties of solutions, actual compositions used in the industrial practice were taken into account. Figures 2a and 2b present the results of calculation for one of the working solutions: c H2 O2 = 0.05 mol/l, pH 10. As a criterion for the efficiency of initiation of the industrial process, the generation rate of HO2 radicals responsible for bleaching was taken. The plasma activation of the solution can be performed in two modes. In one mode, an alkaline solution that initially has not contained H2O2 is subjected to treatment. In the second mode, the solution contains a certain amount of H2O2. The role of discharge reduces to the acceleration of the desired process via a more intense buildup of HO2 and OH radicals in the liquid phase. Calculation showed that activation of the solution in the second mode when the system initially contains an insignificant amount of H2O2 is more effective. The calculation results for HO2 radicals, pH 10, and [H2O2]0 = 10–3 mol/l are given in Fig. 3 and Table 3. A comparison of these results with the data presented in Fig. 2 shows that plasma activation becomes effective at an OH radical generation rate of the order of 10–5 mol/(l s), a value that can be reached at a discharge current of about 100 mA per liter of solution subjected to treatment. HIGH ENERGY CHEMISTRY

Vol. 41

No. 6 2007

COMPARISON OF PLASMA Table 3. Results of calculation of the kinetic scheme of plasma-enhanced reactions d[OH]/dt

d[HO2]/dt

HO2

mol/(l s)

OH

0

5.2 × 10–7

7.5 × 10–5

1.18 × 10–6

10–9

5.2 × 10–7

7.6 × 10–5

1.2 × 10–6

10–8

5.5 × 10–7

7.9 × 10–5

1.33 × 10–6

10–7

7.9 × 10–7

9.4 × 10–5

2.85 × 10–6

10–6

9.9 × 10–6

9.9 × 10–5

2.0 × 10–5

5 × 10–6

1.0 × 10–5

9.97 × 10–5

1.02 × 10–4

10–5

1.03 × 10–5

1 × 10–4

2.1 × 10–4

CONCLUSIONS The feasibility of enhancement of the bleaching and delignification processes in natural polymer materials by application of plasma–solution systems and a fastflow, high-voltage discharge in air was analyzed. It was shown that the compositions of discharge-induced active species and species that determine the occurrence of the processes in the conventional technology of bleaching and delignification of textile materials are close to one another. The major active species generated in the discharge zone are ozone in the case of the dry plasma and OH radicals and hydrogen peroxide in plasma–solution systems, with the efficiency of activation being higher in the latter case. The calculation of the kinetic scheme of reactions showed that the industrial process of activation of solutions can be realized in plasma–solution systems at a high rate. The plasma activation becomes effective at a generation rate of OH radicals of the order of 10–5 mol/(l s), which is attain-

Vol. 41

No. 6

able at a discharge current of about 100 mA per liter of treated solution. REFERENCES

mol/l

HIGH ENERGY CHEMISTRY

459

2007

1. Surface Characteristics of Fibers and Textiles, Pastore, C.M. and Kiekens, P., Eds., New York: Marcel Dekker, 2001. 2. Kutepov, A.M., Zakharov, A.G., and Maksimov, A.I., Vakuumno-plazmennoe i plazmenno-rastvornoe modifitsirovanie polimernykh materialov (Vacuum Plasma and Plasma–Solution Modification of Polymer Materials), Moscow: Nauka, 2004. 3. Pavlath, A.E., Techniques and Applications of Plasma Chemistry, Hollahan, J.R. and Bell, A.T., Eds., New York: Wiley, 1974, p. 280. 4. Klapperich, C., Pruitt, L., and Komvopoloulos, K., J. Mater. Sci.: Mater. Med., 2001, vol. 12, p. 549. 5. Demin, V.A., Shereshovets, V.V., and Monakov, Yu.B., Usp. Khim., 1999, vol. 68, no. 11, p. 1029. 6. Bugaenko, L.T., Kuzmin, M.G., and Polak, L.S., HighEnergy Chemistry, New York: Ellis Horwood and Prentice Hall, 1993. 7. Pikaev, A.K., Kabakchi, S.A., and Makarov, I.E., Vysokotemperaturnyi radioliz vody i vodnykh rastvorov (High-Temperature Radiolysis of Water and Aqueous Solutions), Moscow: Energoatomizdat, 1988. 8. Akishev, Yu.S., Deryugin, A.A., Karal’nik, V.B., Kochetov, I.V., Napartovich, A.P., and Trushkin, N.I., Fiz. Plazmy, 1994, vol. 20, no. 6, p. 571. 9. Massey, H., Negative Ions, London: Cambridge Univ. Press, 1976. 10. Kondrat’ev, V.N., Konstanty skorosti gazofaznykh reaktsii (Rate Constants of Gas-Phase Reactions), Moscow: Nauka, 1970. 11. McDaniel, E.W., Collision Phenomena in Ionized Gases, New York: Wiley, 1964. 12. Kazarnovskii, I.A., Dokl. Akad. Nauk SSSR, 1975, vol. 221, no. 2, p. 353. 13. Pikaev, A.K. and Kabakchi, S.A., Reaktsionnaya sposobnost’ pervichnykh produktov radioliza vody (Reactivity of Primary Products of Water Radiolysis), Moscow: Energoizdat, 1982.