plasma surface modification of polymers using atmospheric

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: [email protected] [email protected]

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

ICOPS02-02

University of Illinois Optical and Discharge Physics

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.

ICOPS-02-03


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

ICOPS-02-04A


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).

ICOPS-02-06A


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.

ICOPS-02-06B


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

ICOPS-02-07


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

ICOPS-02-09


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

ICOPS-02-10


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

ICOPS-02-13


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.

ICOPS-02-11


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.

ICOPS-02-12


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.

ICOPS-02-12A


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).

ICOPS-02-13A


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


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).

ICOPS-02-14


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.


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


ICOPS-02-15


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.

ICOPS-02-15A


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.


0

5

10

15

20

0.0 0.5 1.0 1.5 2.0D

ENSI

TY (1

015

cm-3

)

N2O

N2O5

ICOPS-02-16


ETCH PRODUCTS AND RATES

ICOPS-02-17


• 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.


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

)


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


O/C

RAT

IO O

N P

P (%

)

(Energy deposition = 0.34 J cm-2)

ICOPS-02-18


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)


0.0

0.5

1.0

1.5

2.0

H2O

2 (1

015

cm-3

)

0

1

2

3

4

5


<HO2>

H2O2

<HO

2> (1

014

cm-3

)

ICOPS-02-19


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


NO

Den

sity

(101

4 cm

-3) NO2

1013

1014

1015

1016


N2O

N2O5

Den

sity

(cm

-3)

ICOPS-02-20


EFFECT OF RH: HNO2, HNO3, CO2

ICOPS-02-21


• 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


Den

sity

(cm

-3)

HNO3

HNO2

0

1

2

3

4


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.

ICOPS-02-22


plasma surface modification of polymers using atmospheric

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