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PLASMA SURFACE MODIFICATION OF POLYMERS USING ATMOSPHERIC PRESSURE DISCHARGES*
Rajesh Dorai 1 and Mark J. Kushner 2
University of Illinois
1 Department of Chemical Engineering 2 Department of Electrical and Computer Engineering
Urbana, IL 61801
e-mail: dorai@uiuc.edu mjk@uiuc.edu
http://uigelz.ece.uiuc.edu
* Work supported by 3M and NSF (CTS99-74962)
AGENDA
• Introduction
• Plasma processes for polymer surface treatment • Description of the model
• GLOBAL_KIN • Surface site balance model for heterogeneous chemistry • Reactions in air and at the polypropylene surface
• Atmospheric pressure plasma processing of polypropylene (PP)
• Periodic steady states in radical densities • Effect of energy deposition • Effect of relative humidity • Low-Molecular Weight Oxidized Materials (LMWOM)
• Conclusions
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PLASMA SURFACE MODIFICATION OF POLYMERS
• Polymer materials typically require
surface activation to improve their wetting (for dyeing) and adhesion properties.
• Atmospheric pressure plasma
treatment is well suited for this purpose because of the ease of generation of gas-phase radicals which can react with and modify the polymer surface.
• The typical plasma equipment for
treatment are corona discharges. • These devices operate as dielectric
barrier discharges (DBDs) owing to dielectrics on the electrodes and the capacitance of the polymer.
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POLYMER TREATMENT: LOW vs. ATMOSPHERIC PRESSURE
• Low pressure processes
• Advantages
More uniform treatment compared to atmospheric pressure. Less contamination problems (a controlled gas mixture is used). Flexibility of using gases of various types.
• Disadvantages:
Equipment is expensive (vacuum). Problems in using in continuous mode.
• Atmospheric pressure processes
• Equipment is simple, cost effective. • Can be used in continuous operation.
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COMMERCIAL CORONA PLASMA EQUIPMENT
• Commercial atmospheric pressure plasma equipments treat conducting/non-conducting materials at line speeds ~ 400 m/min.
• Suppliers include Enercon Inc., Tri-Star Technologies, Pillar Technologies,
Sherman Treaters.
ENERCON’s PLASMATREAT3 TM
SHERMAN TREATER’s PBS/12Cx1
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THE PLASMA SURFACE MODIFICATION PROCESS
HIGH-VOLTAGEPOWER SUPPLY
FEED ROLL
PROCESSEDPOLYMER FILM
GROUNDEDELECTRODE
COLLECTORROLL
PLASMA
~
SHOEELECTRODE
POWERED
TYPICAL PROCESS CONDITIONS:
Web speed: 10 - 200 m/minResidence time: a few sEnergy deposition: 0.1 - 1.0 J cm-2Applied voltage: 10-20 kV at a few 10s kHzGas gap ~ a few mm
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THE OVERALL PROCESS
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C C C C C C C C C
C C C C C C C C C C
OH
OHO||
OH, O
C C C C C C C C C C C
LAYER 1
LAYER 2
LAYER 3
H
OH, H2O
OH
OHO2
O2
HUMID-AIR PLASMA
BOUNDARY LAYER
POLYPROPYLENE
H2O
e e
H OH
N2
e e
N NO2
O O
e e
O2O3
O2NO
NO
POLYMERS MODIFIED USING PLASMAS
• Plasma surface modification is used on:
• Polyethylene • Polypropylene • Poly (ethylene terephthalate) • Polystyrene
• The extent of modification depends on factors relating to the polymer
structure:
• Unsaturation in the backbone (presence of multiply bonded carbon chains)
• Functional groups attached to the backbone • Orientation of the attached groups with respect to the backbone • Crystalline/amorphous nature of the polymer
• In this study, we address polypropylene (PP).
• Apparel (Active wear or sportswear) • Home furnishings (indoor and outdoor carpets, upholstery) • Packaging
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POLYPROPYLENE (PP) - STRUCTURE
• Polypropylene polymer:
C C C C C CH H H H H H
H H HCH3 CH3CH31
2 31 - Primary C2 - Secondary C3 - Tertiary C
• Three types of carbon atoms in a PP chain:
• Primary C – attached to only one another carbon; • Secondary C – attached to two carbon atoms; and • Tertiary C – attached to three carbon atoms.
• The reactivity of an H-atom depends on the type of C bonding. • Reactivity scales as: HT > HS > HP (HT=tertiary H; HS=secondary H;
HP=primary H).
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FUNCTIONALIZATION OF THE PP SURFACE
• Untreated PP is a saturated hydrocarbon chain which is hydrophobic
(repels water). • The increase in surface energy of PP after corona treatment is attributed to
the functionalization of the polymer surface with hydrophilic groups (attracts water).
• An air-corona-processed PP film contains hydrophilic functional groups
such as: • Aldehydes (-CHO) • Ketones (-C=O) • Alcohols (-C-OH) • Hydroperoxides (-COOH)
• The hydroperoxides photolytically degrade to produce alkoxy radicals (-C-
O) and OH. • Energy deposition and relative humidity (RH) of air plasmas significantly
affect this functionalization.
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LOW-MOLECULAR WEIGHT OXIDIZED MATERIALS (LMWOM)
• Corona-treatment also produces cross-linking and degradation. • Smaller chain-length oxidized compounds soluble in polar solvents (e.g.,
H2O, ethanol) are formed. These are called LMWOM. • The role of LMWOM in improving ink adhesion is not well understood. • Strobel et al. suggest that LMWOM may be beneficial to the adhesion of
polyamide inks on corona-treated PP. 1 • Briggs et al. observed poor ink adhesion (nitrocellulose-based ink) in the
presence of LMWOM and attributed it to the weak bonding of the LMWOM to the polymer. 2
1 Strobel et al. J. Adhesion Sci. Technol. 3, 321 (1989). 2 Briggs et al. Polymer 24, 47 (1983).
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GOALS OF THIS INVESTIGATION
• Oxidized functional groups incorporated onto the surface are responsible
for increased adhesion. • The reaction mechanism and processes leading to the formation of
LMWOM are still not well understood. • Based on experimental data (O/C ratios on surfaces, surface densities of
functional groups), reaction mechanisms are constructed for heterogeneous chemistry at the PP surface.
• With the help of a global kinetics model validated against experiments,
parameterizations are performed over energy depositions and relative humidities to study their effect on surface properties.
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DESCRIPTION OF THE MODEL – GLOBAL_KIN
Modules in GLOBAL_KIN: • Homogeneous plasma chemistry • Transport to surface through a boundary layer • Heterogeneous surface chemistry • Circuit model
N(t+∆t),V,I
CIRCUITMODULE
GAS-PHASEKINETICS
SURFACEKINETICS
VODE -ODE SOLVER
OFFLINEBOLTZMANN
SOLVER
LOOKUP TABLEOF k vs. Te
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HUMID-AIR REACTION MECHANISM
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• Gas-phase products of humid-air corona treatment include O3, NO, NO2, HNO2, HNO3.
N2
O2
H2O
N N2(A)
e
e O
H OH
e
O2
NO
O2,OH
HNO2OH
NO2O3, HO2
OHNO3
OHN2
N
O3 HO2OH
OH
O2
O2(1∆)
HO2
O2
SPECIES TRANSPORT TO THE POLYMER SURFACE
• Bulk plasma species diffuse to the PP surface through a boundary layer (d~ a few λmfp; λmfp~µm at 1 atm).
• Flux of the radicals reaching the surface is,
4th=φ vn , n = density, vth = thermal speed.
• Radicals react on the PP based on a user-defined mechanism.
POLYPROPYLENE
BOUNDARYLAYER ~ λmfp
BULK PLASMA
DIFFUSION REGIME
O OH CO2
d
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HETEROGENEOUS PROCESSES AT THE PP SURFACE
• Radicals at the polymer surface react with the surface by addition or abstraction.
• After surface reactions, desorbed products diffuse through the boundary
layer into the bulk plasma. • Inter-surface-species reactions are also included.
C C C C C C C C C LAYER 1
C C C C C C C C C C LAYER 2
OH
OHO||
O, OH
C C C C C C C C C C C LAYER 3
H
OH, H2O
OH
OHO2
O2
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SURFACE SITE BALANCE MODEL
• The total number of sites allowed for reaction is variable. • When a “hole” is made in the PP chain, radicals are allowed to diffuse
through it to the layers beneath and react.
C C C C C C C C C LAYER 1
C C C C C C C C C C LAYER 2
C C C C C C C C C C C LAYER 3
AVAILABLE FORCARBON SITES
REACTION
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REACTIONS AT PP SURFACE
• O and OH abstract H from the PP chain to produce alkyl radicals. • Further reaction of O atoms with alkyl radicals produce alkoxy radicals
which undergo scission reactions to form aldehydes and ketones.
(ALKOXY RADICAL)
O
~CH2 C CH2~
CH3
O
~CH2 C CH2~ + CH3
CH2~O
~CH2 CCH3
+
a
b
~CH2 C CH2~
CH3
~CH2 C CH2~
CH3
HO, OH
OH, H2O
(ALKYL RADICAL)(POLYPROPYLENE)
O
(ALKOXY RADICAL)
O2
O
~CH2 C CH2~
CH3
~CH2 C CH2~
CH3
OO
(PEROXY RADICAL)
NO2NO
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BASE CASE: ne, Te
• As the voltage across the gap increases, electrons gain energy and by ionizing the background gases, produce an electron avalanche,
e + N2 → N2
+ + e + e, e + O2 → O2+ + e + e, e + H2O → H2O+ + e + e.
• Once the gap-voltage decreases below sustaining, electrons decay by
attachment (primarily to O2).
109
1010
1011
1012
1013
1014
n e (c
m-3
)
10-10 10-9 10-8 10-710-11Time (s)
1st of 921
51, 102, ... , 921
51, 102, ... , 921
1st of 921T e (e
V)
0
21
345678
10-10 10-9 10-810-1110-12Time (s)
• N2/O2/H2O=79/20/1, 300 K, 1 atm, 10 kV at 9.6 kHz. • Web speed=250 cm/s, gas gap=2.5 mm.
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GAS-PHASE RADICALS: O, OH, N
• Electron impact dissociation of O2, H2O and N2 produces O, OH and N,
e + N2 → N + N + e, e + O2 → O + O + e, e + H2O → H + OH + e. • O consumption in the gas-
phase occurs primarily by ozone (O3) formation,
O + O2 + M → O3 + M. • Although large densities of N
atoms are produced, they are 0
1
2
3
4
5
6
7
OH
, O, N
(101
4 cm
-3)
Time (s)10-9 10-8 10-7 10-6 10-5
N
O
OH
1st O N, OH
51
921
51
921
921
51
relatively unreactive with PP compared to O and OH. • After 100s of discharge pulses, the radicals attain a periodic steady state.
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GAS HEATING
• Typical energy deposition for PP treatment is a few J cm-2
= a few 10s of J cm-3 (few mms gas gap). • Back-of-the-envelope calculation
N CP ∆TGAS = Energy deposition At atmospheric pressure, N ≈ 3×1019 cm-3, CP, AIR ≈ 4×10-23 J molecule-1 K-1.
∴ for edep=1 J cm-3,
FEED FILM
PROCESSEDPOLYMER FILM
GROUNDEDELECTRODE
PLASMA
SHOEELECTRODE
POWERED
AIRFLOW500 cm s-1
K∆TGAS 1000
1041031
2319 ≈×⋅×
= − !
• In experiments, this heating is avoided by cooling the electrodes using air. • An overall heat transfer coefficient has been incorporated into the model
for this purpose.
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REACTION PROBABILITIES ON PP
• Probabilities for surface reactions on PP were estimated based on rate constants for similar gas-phase reactions with long-chain saturated hydrocarbons.
• A gas-kinetic rate constant (~ 10-10 cm3 s-1) ≈ unit reaction probability at PP. • Results from the model are then compared with experiments (for example,
O/C ratios). • To “better” the agreement between model and experiments, probabilities
for key reaction pathways are adjusted. • For example, most of the O impregnated on the PP surface was by,
NO + PP-O2 → NO2 + PP-O (alkoxy radical)
• For a reaction probability of 0.02, “agreement” was observed between
model and experiments (of O/C ratios on PP).
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MODEL VALIDATION: O/C RATIOS ON PP, LMWOM
• O/C ratios on PP as a function of energy deposition were compared to experiments*.
• Most of the O on the surface came from aldehydes and ketones (LMWOM).
At larger energy depositions (> 1 J cm-2), the aldehydes are converted to CO2.
ENERGY DEPOSITION (J cm-2)
0
5
10
15
20
25
0.0 0.5 1.0 1.5 2.0
O/C
(%)
MODEL
EXPERIMENT
ENERGY DEPOSITION (J cm-2)
DEN
SITY
(101
4 cm
-2)
0.0
0.5
1.0
1.5
2.0
2.5
0.0 0.5 1.0 1.5 2.0
ALDEHYDES
KETONES
Air at 300 K, 1 atm, 55% RH
* Zenkiewicz, M., J. Adhesion Sci. Technol., 15 63 (2001).
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GAS-PHASE PRODUCTS: O3, H2O2
• O3 is produced by the reaction of O with O2,
O + O2 + M → O3 + M. • At higher energy depositions,
more O is produced resulting in increased O3 formation.
ENERGY DEPOSITION (J cm-2)
O3
(101
7 cm
-3)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0
• H2O2 is produced by,
H + O2 + M → HO2 + M, HO2 + HO2 + M → H2O2 + O2 + M.
• At high energy depositions (lower
film speeds), H2O2 density decreases due to decrease in <HO2>.
0.0
0.5
1.0
1.5
2.0
2.5
H2O
2 (1
015
cm-3
)
0
1
2
3
4
0.0 0.5 1.0 1.5 2.0
<HO
2> (1
014
cm-3
)
<HO2>
H2O2
ENERGY DEPOSITION (J cm-2)
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GAS-PHASE PRODUCTS: NO, NO2
• NO and NO2 (combinedly called NOx) are produced by, N2 + O → NO + N, NO + O + M → NO2 + M. • NO is also converted to NO2 by
reaction with peroxy radicals at the PP surface,
NO + R-OO → NO2 + R-O. • At higher energy depositions, most of
the NOX is converted to N2O, N2O5, HNOX,
0
2
4
6
8
10
12
14
0.0 0.5 1.0 1.5 2.0ENERGY DEPOSITION (J cm-2)
NO
NO2
Den
sity
(101
3 cm
-3)
NO2 + N → N2O + O, NO2 + NO3 + M → N2O5 + M, NO + OH + M → HNO2 + M, NO2 + OH + M →HNO3 + M.
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GAS-PHASE PRODUCTS: N2O, N2O5
• Nitrous oxide (N2O) and di-nitrogen pentoxide (N2O5) are gases of interest from an environmental perspective.
• Increasing amounts of these are generated at higher energy depositions. • N2O is generated by the reaction
of NO2 with N, NO2 + N → N2O + O. • N2O5 is formed by the reaction of
NO2 with NO3, O + NO2 + M → NO3 + M, NO2 + NO3 + M → N2O5 + M.
ENERGY DEPOSITION (J cm-2)
0
5
10
15
20
0.0 0.5 1.0 1.5 2.0D
ENSI
TY (1
015
cm-3
)
N2O
N2O5
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ETCH PRODUCTS AND RATES
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• The primary etch product is CO2,
~CH2 C C=O
CH3
H H
+ O ~CH2 C C=O
CH3
H
OH + ~CH2 C
CH3
H
+ CO2+ O
• Higher energy depositions result in increased flux of O and OH to the PP
surface which increases the etch rate.
ENERGY DEPOSITION (J cm-2)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0
CO
2 (1
014
cm-3
)
ENERGY DEPOSITION (J cm-2)
0
10
20
30
40
50
60
0.0 0.5 1.0 1.5 2.0
ETC
HR
ATE
(mon
olay
ers/
min
)
EFFECT OF RH: O/C RATIO ON PP
• At higher RH, more fraction of input energy is channeled into OH production and hence <OH> increases and <O> decreases.
• With higher <OH>, more alkyl radicals (through OH abstraction) are
produced which leads to increased O impregnation on the PP surface.
0.0
0.3
0.6
0.9
1.2
1.5
1 10 100RELATIVE HUMIDITY (%)
<O>
<OH>
Den
sity
(101
4 cm
-3)
0
5
10
15
20
1 10 100RELATIVE HUMIDITY (%)
O/C
RAT
IO O
N P
P (%
)
(Energy deposition = 0.34 J cm-2)
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EFFECT OF RH: O3, H2O2
• Higher RHs result in decreasing <O> and as a result O3 production decreases.
• Larger amount of HO2 is produced at higher RH and this leads to increased
H2O2 production,
HO2 + HO2 + M → H2O2 + O2 + M.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
O3
(101
7 cm
-3)
1 10 100RELATIVE HUMIDITY (%)
0.0
0.5
1.0
1.5
2.0
H2O
2 (1
015
cm-3
)
0
1
2
3
4
5
1 10 100RELATIVE HUMIDITY (%)
<HO2>
H2O2
<HO
2> (1
014
cm-3
)
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EFFECT OF RH: NXOY
• NO and NO2 densities increase with RH because of the increased rate of the reactions,
N + OH → NO + H, NO + HO2 → NO2 + OH.
• Larger NO2 leads to increased formation of N2O and N2O5,
NO2 + N → N2O + O, NO2 + NO3 + M → N2O5 + M.
0.0
0.4
0.8
1.2
1 10 100RELATIVE HUMIDITY (%)
NO
Den
sity
(101
4 cm
-3) NO2
1013
1014
1015
1016
1 10 100RELATIVE HUMIDITY (%)
N2O
N2O5
Den
sity
(cm
-3)
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EFFECT OF RH: HNO2, HNO3, CO2
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• Increasing RH leads to increased HNOX byproduct formation,
NO + OH + M → HNO2 + M, NO2 + OH + M → HNO3 + M.
• Due to the increased rate of abstraction by OH at higher RH, the etch rate increases.
OH + PP-H → PP• + H2O, PP• + …. → aldehydes, ….+ O → CO2 + ….
1013
1014
1015
1016
1 10 100RELATIVE HUMIDITY (%)
Den
sity
(cm
-3)
HNO3
HNO2
0
1
2
3
4
1 10 100RELATIVE HUMIDITY (%)
CO
2 (1
013
cm-3
)
SUMMARY
• A global kinetics model has been used to study the gas-phase and surface
chemistries during the air-corona discharge treatment of PP. • O/C ratio on PP increased with increasing energy deposition and relative
humidity. • Continued increase in energy deposition however lead to PP
decomposition (to CO2). • Gas-phase products produced in significant amounts (>1015 cm-3) include
O3, H2O2, HNO3, N2O, N2O5. • Although increased energy deposition results in more hydrophilic
surfaces, the production of environmentally sensitive gases could be an issue.
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