Plasma Modification of Lignin - American Chemical Society

Chapter 19

Plasma Modification of Lignin G. Toriz, F. Denes, and R. A. Young

Downloaded by OHIO STATE UNIV LIBRARIES on June 12, 2012 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch019

Department of Forestry and Engineering Research Center for Plasma-Aided Manufacturing, University of Wisconsin, Madison, WI 53706

Plasma-chemistry technologies offer an alternative and efficient way for functionalization of lignin. Plasma modification approaches have a number of advantages which include environmental benefits because the technique involves dry chemistry, the active species only penetrate about 10 nm deep so it does not alter the base structure and even the most inert surface can be functionalized. In this paper the plasma-enhanced functionalization of lignin is demonstrated with oxygen, argon, and silicon chloride plasma gases using three different cold-plasma installations: A parallel-plate, diode configuration 30 kHz RF-reactor; a rotating 13.56 M H Z RF-plasma reactor and a dense medium (liquid) plasma (DMP) reactor. Silicon chloride plasma functionalized lignin has been successfully grafted with polydimethlsiloxane in a post-plasma procedure.

Lignin is a renewable raw material and it represents the third largest source of organic matter in the plant kingdom. About 95%, of about 60 million tons of annually produced lignin worldwide by processing biomass into cellulose and pulp, is used as an energy source or disposed as waste material (1). It is estimated that only a couple of percents of available lignin of pulping origin are recovered and marketed in the U S A in processes other than fuel technologies. The present scientific and technological efforts for lignin modification are oriented towards converting it into high-performance lignin-based polymeric materials. The successful syntheses of lignin-based polymers, engineering plastics and lignin-based copolymers with special properties were recently reported from various laboratories. Two main directions have been developed in connection with the utilization of lignin

© 2000 American Chemical Society In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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for polymeric materials: a) direct use of unfractionated lignin and lignocellulosic materials for manufacturing of low cost lignin-polymer blends, and b) modification of lower molecular weight degradation products of lignin by graft-copolymerization reactions. Blending various polymers is a less expensive, alternative way for the creation of desired polymeric characteristics. The properties of blends are controlled by compatibility of the components which often have distinctly different character. Glasser and coworkers (2,3) studied extensively lignin-based polymeric blends. Graft copolymerization reactions have also been evaluated to create modified lignin and lignin-based composites (4,5). Some of these approaches have been successful for creation of high-performance novel materials. Despite the fact that lignin is composed of a phenolic-type polymeric network very little of this natural polymer is used in phenolic resins. Competition of aminoplast- and polyphenolic-type adhesives and the moderate prices of crude oil (the source of phenol) represent the main technical and economical limitations for the application of lignin based adhesives. The synthesis of epoxy resins from lignin and lignin-derivatives have been studied as well (6-8). Most of the previous research has not resulted in the development of commercial technologies due to the following reasons: 1. 2.

3. 4.

The lignin modification approaches involve reactions that are based on wet chemistry involving large amounts of organic solvents. The lignin derivatives produced by wet chemistry require sophisticated procedures for separating the desired reactants from a complex reaction mixture. These synthetic- and separation-routes are based on high energy consumption processes and are not cost effective. Some of the larger volume reactions can create serious environmental problems.

Plasma-chemistry technologies offer an alternative and efficient way for functionalization of lignin. The plasma state denotes a partly or completely ionized gas (9). This gaseous complex may be composed of electrons, ions of either polarity, gaseous atoms and molecules in the ground or any higher state, and light quanta. There are two plasma categories: thermal and non-thermal. A cold or non-thermal plasma occurs when gas atoms are at ambient temperatures while electrons are maintained at highly elevated temperatures. Only lO^-SxlQ" percent of the species are ionized. Glow discharge experiments causing surface modifications such as deposition or plasma polymerization require this state because the hot electrons have enough energy to bombard and rupture bonds at the surface, while the cooler ions do not excessively heat the substrate. The cold type of plasma is used for all surface 3

In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

369 modifications of organic polymeric materials. The use of plasmas for the modification of lignin are desirable due to its unique characteristics: 1. 2.


3. 4.

The particle-energies of plasma states are high enough (e.g. average electron energy: 0.2-5 eV) to alter all chemical bonds of organic structures. The active species of the discharge interact only with the very top layers of the exposed substrates (around 10 nm deep surface chemistry). Most of the plasma technologies involve dry chemistry. Even the most inert polymeric surfaces (e.g. polyolefins, Teflon) can be efficiently functionalized under proper plasma environments (10-12).

In this paper the plasma-enhanced functionalization of lignin will be demonstrated with 02, Ar, and SiC14-plasma gases using three different cold-plasma installations: A parallel-plate, diode configuration 30 kHz RF-reactor; a rotating 13.56 M H Z RF-plasma installation and a Dense Medium Plasma (DMP) reactor. Experimental. Powdery kraft lignin (Indulin AT) from Westvaco Corporation, Charleston, SC, was dried in oven at 60°C for at least one week prior to use. The following reactants were purchased from Aldrich Chemical Co.: silicon tetrachloride, dichlorodimethylsilane (DDS), polydimethylsiloxane (PDMS) (viscosity 100,000 centistokes), sulfuric acids (99%, A.C.S. regent), HPLC grade ethyl ether, acetone and absolute ethanol. Both survey and high resolution (HR) ESCA multiplex spectra were taken with a Perkin Elmer PHI 5400 Spectrometer (Mg source; 15 k V and 300 W; angle: 45 degrees). A Mattson Galaxy Series 7000 FT-IR Spectrometer was used for collecting KBr-IR signatures in the 700-4000 cm-1 wavenumber region (resolution 4 cm ), of the lignin samples. The solid state S i N M R spectra were obtained on a Varian Unity 300 Fourier transform N M R spectrometer. 1

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Plasma Reactors. The Parallel Plate Reactor has a stainless steel diode configuration reaction chamber, provided with two disc-shaped electrodes, and it is operated by using a 70 kHz (0-1000 W) RF power supply. This reactor has been described in our earlier work (13). In a typical experiment a thin layer of vacuum-dried powdery lignin is placed on the lower, grounded electrode, the reactor is closed and the base-pressure is established. Then the preselected reaction pressure and flow rate of the plasma gas or vapor is created and the plasma is ignited and sustained under the desired R F power and treatment time conditions. At the end of the plasma exposure, the base pressure was


370 established in the reactor, then the system is re-pressurized with argon. The lignin powder is then removed from the reactor and stored under vacuum for later analysis. The following experimental conditions were employed for surface functionalization of lignin in the parallel-plate reactor: Plasma gases, vapors: S i C l , 0 , and Ar; RF-power: 100-250 W; Base pressure: 30-50 mT; Pressure in the absence of plasma: 200 mT; Gas flow rate: 5-7 standard cubic cm (oxygen units); Treatment time: 0.5,1,3,5,7, and 10 minutes.


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The Rotating Plasma Reactor can be used to create novel, advanced composites from dissimilar polymeric materials (e.g. renewable, natural polymers and synthetic polymers) through surface functionalization. Cold plasma environments are the best approaches for creating compatible polymeric surfaces. High efficiency can only be achieved if the composite components have high specific surface areas (e.g. powders). To plasma-process powdery materials an original 13.56 MHZ-RF-plasma reactor was designed and developed (Figure 1). The reactor is composed of a Pyrex glass chamber [11] provided with connecting rubber and stainless steel rings [9, 13] on both ends. The vacuum-tight connection of the reactor to the monomer- and gas-supply system and to the vacuum line is assured with the aid of two ferrofluidic feed-throughs [8,14]. The hollow shaft of the stainless steel chambers [6,15] are made of special magnetic material and they are a part of the ferrofluidic sealing system. The R F power is transferred to the reactor through two semi-cylindrical copper electrodes [10] located outside from a 1000 W, 13.56 M H Z R F power supply and matching network assembly [12]. A large cross- section gate valve [17] separates the reactor from the vacuum line and allows the control of the out-flow of the plasma gases. The vapor and gas flow into the reactor is controlled through individual flow controllers [4, 5]. The rotation of the reactor at various angular velocities is assured by a digitally speed-controlled electric engine-transmission system [18,19]. In a typical experiment vacuum-dried lignin powder is introduced into the reactor, the system is closed and the base pressure is created. The rotation of the reactor is started at the selected speed and the system is kept under these conditions for 30 minutes in order to complete the gas- and moisture-desorption from the extremely large powder surface. In the next step the selected gas-flow and pressure conditions are established, the plasma is ignited and sustained for the desired treatment time. At the end of the reaction the system is vacuumed to base pressure level, re-pressurized with argon, and the sample is removed and stored in vacuum desiccator until analytical procedures and/or second stage reactions are initiated.



Figure 1. Schematic diagram of rotating RF-plasma reactor: 1,22. Metallic rigid support; 2. Vent valve; 3. Monomer and gas reservoirs; 4. Needle valves; 5. Gas and vapor flow controllers; 6,15. Stainless steel gasmixing chambers; 7. Pressure gauge-MKS Baratron; 8,14. Ferrofluidic sealings; 9,13. Stainless steel connecting rings; 10. Semi cylindrical copper electrodes; 11.1m long and 0.1 m ID Pyrex glass reaction chamber; 12.13.56 M H z RF power supply; 16,23. Additional gas connecting lines; 17; Gate valve; 18. Angular speed digital controller; 19. Electric engine; 20. Ground connection; 21. Faraday cage; 24. Vacuum pump; 25. Large cross section valve; 26. Liquid nitrogen trap; 27.1 inch diameter stainless steel connection.


372 The following experimental conditions were used for the plasma treatments in the rotary reactor: Substrates: lignin (Indulin AT) Grafting monomer : dimethyldichlorosilane (DDS); Plasma gases: Ar, S i C l ; R F power dissipated to the electrodes: 300 W Base pressure: 30-40 mT; Pressure in the absence of plasma: 250 mT; Gas flow rate: 7 seem (oxygen units); Treatment time: 5 and 10 minutes; Angular speed: 30 rpm;


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The post plasma grafting reactions were carried out with DDS under both acid and base catalyst conditions according to the following procedure: Equal amounts (e.g. 20/20 or 250/250 g) of silicon tetrachloride-plasma functionalized lignin and DDS were mixed under in-situ or ex-situ conditions and then the polycondensation catalysts were added (0.5 ml 99% H2S04 for 20/20 mixture and 20 ml, 1 Ν, K O H for the 250/250 mixture). The graft-polycondensation reactions were developed under continuous stirring for 1 hour. At the end of the grafting processes the raw reaction products were washed with distilled water (pH; 6.5-7), extracted with ether (to remove the non-grafted PDMS homopolymer), then dried and analyzed. A l l grafted lignin samples were washed with water in order to remove the catalyst and extracted with ether (a good solvent for PDMS). The Dense Medium Plasma (DMP) reactor was especially designed to plasma-process liquids and suspensions of materials at atmospheric pressure. Figures 2 show the schematic diagrams of the D M P system. The D M P reactor is composed of tubular shaped Pyrex glass (length-150 mm; BD-60 mm), double walled (Pyrex glass jacket for thermostating purposes), a reaction vessel, with removable Teflon top and bottom caps. The caps are tightly fastened to the reactor by means of silicon rubber O-ring. The electrical discharge is sustained between the two vertically positioned electrodes. Both electrodes have a hollow-cylindrical form, with disc-shaped smooth surfaces, and a triangular vertical plane cross section. The upper electrode is connected through an elastic coupling [3] to a rotating system, which allows the controlled rotation of the upper electrode between the limits of 0-5,000 rpm. Three symmetrically positioned (120 degree) holes located in the middle range of the upper electrode and the axial inner channel assure the centripetal-force-driven intense recirculation of the reaction media. The lower electrode has a fixed position during the discharge, however, it can be vertically translated by a metric thread system for selecting the desired distance between the electrodes. The inner channel permits the feeding of inert and reactive gases before and/or during the plasma processes, both for degassing and gas-mediated reaction purposes. In and out thermostat connections allow the re-circulation of cooling agents (e.g. alcohol, water, etc.). Teflon-tubing connections [5] (ID-5 mm), mediate



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1 Motor speed controller; 2 Electric motor; 3 Elastic coupling; 4 Thermostat; 5 Teflon tubing outer loop for recirculation of reaction media; 6 Pyrex glass vessel for reaction product; 7 Gas cylinder.


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the centripetal-force driven re-circulation of the reaction media from the bottom to the upper part of the reactor. Stainless steel tubing (BD-5 mm), assures the removal of the final reaction products and the evacuation and trapping of plasma generated gaseous components. A typical reaction is performed in the following steps: 3.0g dried lignin was added under an argon-purged-glove-box environment, and under vigorous stirring, to 250 ml silicon tetrachloride to form the lignin/SiC14 suspension. After selecting the required distance between the electrodes, the suspension was transferred through a stainless steel connection into the reactor. It is noteworthy that the level of the liquid suspension should cover the re-circulating holes of the shaft in order to assure vigorous stirring of the system. During these operations argon is passed continuously through the reaction medium, and the temperature of reaction system is maintained at the pre-selected value by recycling the cooling agent through the thermostat [4]. The stirring of the reaction mixture is initiated in the next step by starting the speed controlled electric engine [1,2] and selected angular velocity established. The D C discharge is then ignited and sustained for the desired time interval. The high speed rotation of the upper electrode is maintained during the entire length of the plasma reaction to sustain an intense re-circulation of the reaction mixture through both the inner and outer loop of the reactor. At the end of the reaction the DC-power supply is disconnected and the final reaction mixture is transferred into a Pyrex glass vessel [6] under an argon blanket, and under the continuous rotation of the upper electrode. The reaction mixture is filtered and the solid phase (powder) is washed with absolute ethanol in an argon-protected glove box. The dried reaction product is stored in vacuum dessicator until analytical evaluations are started. Plasma Reactions with Lignin. S i C l (ST)-plasma treatment of lignin was performed to implant extremely reactive SiClx groups onto the lignin particle surfaces in the parallel plate reactor. These functionalities can initiate graft-polycondensation reactions, for instance, in the presence specific bifunctional compounds (e.g. dimethyldichlorosilane, diacids, diamines, etc.) and can lead, in the presence of moisture, to very hydrophilic Si(OH)x groups. 4

Comparative survey and high resolution (HR) Electron Spectroscopy for Chemical Analysis (ESCA) data collected from unmodified and ST-plasma treated lignin indicated that Si is present in the plasma exposed samples. Besides the characteristic C l s binding energy peaks of lignin (C-C, C=C 284.7 eV; C-O 286.3 eV; CO=0 288.6 eV), C-Si (284.2 eV) and a high binding energy area O-CO-0 (291 eV) peaks can also be identified in the H R E S C A spectrum of plasma exposed lignin (Figure 3). The relative surface atomic composition of untreated and ST-plasma processed samples at various treatment times, show that the higher the plasma exposure period the higher the Si and oxygen relative surface concentrations, and that long treatment times and



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Binding energy (eV) Figure 3. High resolution ESCA C l s spectrum of (a) untreated Kraft lignin (b) SiC14-plasma treated Kraft lignin (100W, 10 min, 200 mT, 7sccm)


376 higher RF-power conditions result in lower Si atom concentrations (Table 1). This phenomenon might be explained by the presence of a more intense fragmentation of ST at higher R F powers resulting in significantly increased amounts of very high volatility SiCl species (x

Plasma Modification of Lignin - American Chemical Society

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