atmospheric pressure plasma processing of...

ATMOSPHERIC PRESSURE PLASMA PROCESSINGOF POLYPROPYLENE*

Rajesh Dorai 1 and Mark J. Kushner 2

University of Illinois1 Department of Chemical and Biomolecular Engineering, [email protected]

2 Department of Electrical and Computer Engineering, [email protected], IL 61801

http://uigelz.ece.uiuc.edu

November 2002

* Work supported by the 3M Company and the National Science Foundation (CTS99-74962)

University of IllinoisOptical and Discharge Physics

AGENDA

• Introduction to plasma surface modification of polymers

• Description of the model for gas phase and surface kinetics

• Processing of polypropylene in humid air plasmas

• Concluding remarks

RAJESH_AVS_02_02


PLASMA SURFACE MODIFICATION OF POLYMERS

• Atmospheric pressure plasmas (typically coronas) are used for the ease of generation of gas-phase radicals which react with and modify the polymer surface.

RAJESH_AVS_02_03

• Polymers typically require surface activation to improve their wettingand adhesion properties.


SURFACE ENERGY AND WETTABILITY OF POLYMERS

MJK_AUSTIN_RD_01

• Most polymers, due to their low surface energies, are hydrophobic.

• For good adhesion between a liquid and a polymer, the surfaceenergy of the polymer should exceed the surface tension of theliquid by ≈ 2-10 mN m-1.

0

10

20

30

40

50

60

PTFE

Poly

prop

ylen

e

Poly

ethy

lene

Poly

styr

ene

SUR

FAC

E EN

ERG

Y (m

N m

-1)

POLYMER0

10

20

30

40

50

60

Prin

ting

inks

UV

adhe

sive

s

Coa

tings

Wat

erba

sed

adhe

sive

s

UV

inks

Wat

erba

sed

inks

SUR

FAC

E TE

NSI

ON

(mN

m-1

)

LIQUID


COMMERCIAL CORONA PLASMA EQUIPMENT

RAJESH_AVS_02_04

(Tantec Inc.)


POLYMER TREATMENT APPARATUS

RAJESH_AVS_02_04A

HIGH-VOLTAGEPOWER SUPPLY

FEED ROLLGROUNDEDELECTRODE

COLLECTORROLL

PLASMA

~

SHOEELECTRODE

POWERED

TYPICAL PROCESS CONDITIONS:

Gas gap : a few mmApplied voltage : 10-20 kV at a few 10s kHzEnergy deposition : 0.1 - 1.0 J cm-2Residence time : a few sWeb speed : 10 - 200 m/min


REACTION PATHWAY

RAJESH_AVS_02_05

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


POLYPROPYLENE (PP) - STRUCTURE

RAJESH_AVS_02_06

• Polypropylene polymer:

• 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)

C C C C C CH H H H H H

H H HCH3 CH3CH31

2 31 - Primary C2 - Secondary C3 - Tertiary C


FUNCTIONALIZATION OF THE PP SURFACE

RAJESH_AVS_02_07

• Untreated PP is hydrophobic (repels water).

• The increase in surface energy of PP after corona treatment isattributed to the functionalization of the polymer surface withhydrophilic groups (attract water).

• An air-corona-processed PP film contains hydrophilic functionalgroups such as:

• Carbonyl (-C=O) • Alcohols (C-OH)• Peroxy (-C-O-O) • Acids ((OH)C=O)

• The process parameters are energy deposition and relativehumidity (RH).

• At sufficiently high energy deposition, erosion of the polymeroccurs.


DESCRIPTION OF THE MODEL: GLOBAL_KIN

RAJESH_AVS_02_08

• Modules in GLOBAL_KIN:• Circuit model• Homogeneous plasma chemistry• Species transport to PP surface• Heterogeneous surface chemistry

N(t+∆t),V,I

CIRCUITMODULE

GAS-PHASEKINETICS

SURFACEKINETICS

VODE -ODE SOLVER

OFFLINEBOLTZMANN

SOLVER

LOOKUP TABLEOF k vs. Te


REACTION MECHANISM FOR HUMID-AIR

RAJESH_AVS_02_09

• Gas phase products of humid-air corona treatment include O3,N2O, N2O5, 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

HO2

OH

OH

O2

O2(1∆)

HO2

O2HO2H2O2

N2O5

NO3OH


SPECIES TRANSPORT TO THE POLYMER SURFACE

RAJESH_AVS_02_10

• Species in the bulk plasma diffuse to the PP surface through aboundary layer (d ~ a few λmfp ≈ µm).

• Flux of the radicals reaching the surface is,

4thnv

=φ , n = density, vth = thermal speed.

• Radicals react on PP based on a site balance model.

POLYPROPYLENE

BOUNDARYLAYER ~ λmfp

BULK PLASMA

DIFFUSION REGIME

O OH CO2

δ


REACTIONS AT PP SURFACE

RAJESH_AVS_02_11

• O and OH abstract H from PP to produce alkyl radicals.

• Reactions of O3 and O2 with alkyl radicals produce peroxyand alkoxy radicals, which further react to form alcohols and carbonyl species.

CH2~O

~CH2 CCH3

+

~CH2 C CH2~

CH3

~CH2 C CH2~

CH3

H O(g)

OH(g), H2O(g)

(ALKYL RADICAL)(POLYPROPYLENE)

O3 (g)

(ALKOXY RADICAL)

O2 (g)

O

~CH2 C CH2~

CH3

~CH2 C CH2~

CH3

OO

(PEROXY RADICAL)

(CARBONYL)

OH

~CH2 C CH2~

CH3

H(s)

(ALCOHOLS)

OH(g)

O(g)CO2 (g)


BASE CASE: ne, Te

RAJESH_AVS_02_12

• Ionization is dominantly of N2 and O2,

e + N2 → N2+ + e + e,

e + O2 → O2+ + e + e.

• Once the gap voltage decreases belowsustaining, electrons decay by attachment(primarily to O2).

• The differences between the 1st and laterpulses are due to the incomplete chargingof the dielectrics on the electrodes.

• N2/O2/H2O = 79/20/1, 300 K, 15 kV at 9.6 kHz.

• Edep = 0.8 J cm-2, Web speed = 250 cm/s.

Time (s)

1

25 - 460

Ele

ctro

n D

ensi

ty (c

m-3

)

1014

1013

1012

1011

1010

10910-610-710-810-910-10

1

25 - 460

10-710-810-910-10Time (s)

Ele

ctro

n Te

mpe

ratu

re (e

V)

6

5

4

3

2

1

0


GAS-PHASE RADICALS: O, OH

RAJESH_AVS_02_13

• Electron impact dissociation of O2 and H2O produces O and OH.

• O is consumed in the gas phase primarily to form O3,

O + O2 + M → O3 + M.

• After 100s of discharge pulses, the radicals attain a periodicsteady state.

O (1

014

cm-3

)

115

30

45

0

25 - 460

10-610-710-810-9 10-5Time (s)

~~

0.0

0.1

0.2

0.3

2

6

10

1OH

(101

3 cm

-3)

25 - 460

10-610-710-810-9 10-5Time (s)

• Surface concentrations of alcohols, peroxy radicals achieve nearsteady state with a few J cm-2.

• Alcohol densities decreased at higher energy deposition due todecomposition by O and OH to regenerate alkoxy radicals.


MODEL VS. EXPERIMENT

RAJESH_AVS_02_14

* L-A. Ohare et al., Surf. Interface Anal. 33, 335 (2002). Air at 300 K, 1 atm, 30% RH

Experiment*Model

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.5 1.51.0

PP

Per

oxy

Rad

ical

s (%

)

0.0Energy Deposition (J cm-2)

3.0

0.0

0.5

1.0

1.5

2.0

2.5

0.5 1.0 1.5Energy Deposition (J cm-2)

PP

Alc

ohol

(%)

0.0


GAS-PHASE PRODUCTS: O3, NXOY, HNOX

RAJESH_AVS_02_15

• O3 is produced by the reaction of O with O2,

O + O2 + M → O3 + M.

• N-containing productsinclude NO, NO2, HNO2 andN2O5,

N2 + O → NO + N,NO + O + M → NO2 + M,

NO + OH + M → HNO2 + M,NO2 + NO3 + M → N2O5 + M.

O3

N O52

HNO2

NONO2

0.0

0.5

1.0

1.5

2.0

2.5

0.0

2.5

5.0

7.5

10.0

12.5

0.0 0.5 1.0 1.5Energy Deposition (J cm-2)

O3

(101

7 cm

-3)

NO

, NO

2, H

NO

2, N

2O5

(101

4 cm

-3)


EFFECT OF RH: PP FUNCTIONALIZATION

RAJESH_AVS_02_16

• With increasing RH, more OH is produced.• Due to the high reactivity of OH, more PP alkyl radicals are generated.• As a result, the densities of peroxy radicals increase,

PP-H + OH(g) → PP• + H2O(g) PP• + O2(g) → PP-O2• .

• Alcohol and carbonyl densities decrease at higher RH due to increased consumption by OH to form alkoxy radicals and acids.

0

1

2

3

1 10 100RH (%)

Peroxy

Alcohol

CarbonylAcidS

urfa

ce C

once

ntra

tion

(%)

PP-OH+ OH(g)→ PP-O• + H2O(g)

PP=O• + OH(g)→ (OH)PP=O

0

5

10

15

1 10 100RH (%)

Den

sity

(101

3 cm

-3)

<OH>

<O>


EFFECT OF RH: GAS-PHASE PRODUCTS

RAJESH_AVS_02_17

• Higher RH results in decreasing O atom densities and so theproduction of O3 decreases.

• Due to the increased production of OH with RH, larger densities ofHNO2 and HNO3 are produced.

N + OH → NO + H, NO + OH → HNO2, NO2 + OH → HNO3.

0

4

6

8

10

HNO3 (x 0.1)

1 10 100RH (%)

Den

sity

(101

4 cm

-3)

2

CO2

HNO2

O3

N O52

NO (x 10)

NyO

x (1

015

cm-3

)

0

1

2

3

O3

(101

7 cm

-3)

NO2 (x 10)

N O2

0

1

2

3

4

1 10 100RH (%)


EFFECT OF TEMPERATURE: GAS-PHASE PRODUCTS

RAJESH_AVS_02_18

• With increasing gas temperature, consumption of O3 increases.• Most of the NO is lost by reduction to N2 and oxidation to NO2,

NO + N → N2 + O, NO + O3 → NO2 + O2.

• N2O5 is a maximum at intermediate temperatures,

NO2 + NO3 + M → N2O5 + M, N2O5 → NO2 + NO3.

NyO

x (1

014

cm-3

)

O3

(101

7 cm

-3)

0.0

0.5

1.0

2.0

1.5

0

1

2

3

42.5

300 310 320 330 340 350Gas Temperature (K)

N O52

N O (x 0.1)2

O3

NO2

NO

Den

sity

(101

4 cm

-3)

HNO3 (x 0.1)

15

10

5


HNO2

CO2


EFFECT OF TGAS: PP FUNCTIONALIZATION

RAJESH_AVS_02_19

• With increasing gas temperature,the production of O3 decreases leading to lower alkoxyproduction,

PP• + O3(g) → PP-O• + O2(g).

• … and decreased production ofalcohols, carbonyl, and acids,

PP-O• + PP-H → PP-OH + PP•

PP-O• → PP=OPP=O → PP=O•

PP=O• + OH → (OH)PP=O•

• Decreased consumption of alkyl radicals by O3 enables increasedconsumption by O2 increasing the density of peroxy radicals.

Peroxy

Alcohol

Carbonyl

Acid

0.0

1.0

3.0

4.0


2.0

Sur

face

Con

cent

ratio

n (%

)



SUMMARY

RAJESH_AVS_02_20

• A surface reaction mechanism for PP has been developed andvalidated against experiments.

• With increasing energy deposition the surface concentrations ofalcohol, acid, carbonyl, and peroxy groups increase.

• However, significant densities of environmentally sensitive gasessuch as O3 (1017 cm-3) and HNO3 (1016 cm-3) are generated.

• Increasing RH resulted in increased surface concentrations ofperoxy and acid groups and decreased alcohols and carbonyls.

• Operating at larger RH resulted in reduced production of O3.

• Surface concentrations of alcohol, carbonyl, and acid groupsdecreased with temperature while those of peroxy groupsincreased.

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