i!.BST?:AC'l'

cellulose and sy~thetic ~oly~crs. The ~ond strength incre~scd

2,-[: ~ü.::;hcr tei,:perilcUl'oS of ;~:ccssinG' Co::.'ona treatLent of )olymers

i'" ~,ir )iJcc(')Q the surfe,ce, iH'oduC0d -C=O Groups and -C=C- clouDle

bonds, ~~~ increased t~le bond strG~~th and the surface ener~y.

Coro:1C',c::o'::ltJ.lent iL ~lydl'o.:..;en ;lad no pe:r'ccptiDle efi'ec'c, either

c;leL:icéJ.ll::/ or pl1J'sically on the sUl'f':'lce. The liitro0Cl1 corO!1ét

:18i ther o:.:idi:c;e0" crossliilked 2101' ~üt t ed the )oly;"er SUl' fac es

but 0'-1 ):1.'o:!..O:::,;cd~l'e~,.LlelÜ increased the surfi..-.ce ener.:;y C:lld the

bond s~rerl:;th Ulltil tile -cellsile stl'el1i::;th of the polyethylene

3t1'i) itself .i3.S e~:ceGdcd. ~Io':J8Ver, it ';!as not possi',Jle to

cOl':,:elate '.Jond scre~:ccÏl ',::1::1.1 SUr:LélCe Gnergy i" all cases,

?rolonsod tre~t0ent in arcon or a l-Lin troat~ent in ~icro~on

~roduced sur:~ccs of ~i~h 8urf~cc cllersy -Chat shoved low Dondin~.

SURFACE MODIFICATION OF POLYMERS

IN A CORONA DIS CHARGE

by

Chung Yup Kim, M.Sc. (Seoul National)

A thesis submitted to the Faculty of Graduate

Studies and Research in partial fulfilment of

the requirements for the degree of

Haster of Science

Department of Chemistry

HcGill University

J'.lontreal August 1968

@ Chung Yup Kim 1969

ACKNOWLEDGElvIENT S

The author wishes to express his sincere gratitude

and indebtedness to:

i

Dr. D.A.I. Goring, for his close gUidance, kind und er­

standing and, constant assistance and discussion throughout the

deve10pment of this thesis;

Mr. W.Q. Yean, for genera1 guidance in the 1aboratory;

Mr. G. Suranyi, for use of his equipment and frequent

assistance;

Mr. M. Inoue, for supp1ying the highly purified, dis­

ti11ed Vlater;

Dow Chemical Co., for donation of polymer samp1es;

Union Carbide, for donation of the polyethylene film

used in the infrared study;

The Pulp and Paper Researcb Institute of Canada, for

1ibera1 use of theii faci1ities and services;

The Department of Forestry, for financia1 support

during 1966-1968.

li

FOREWORD

The adhesive properties of cellulose and other polymers

are of fundamental importance to their growing use in composites

or lanunates. The application of a corona discharge on the

polymer surface is a weIl known method for increasing adhesion by

surface modification. However, there is little understanding of

the basic mechanism of the method. This thesis deals with the

effects of a corona discharge on polymers and an attempt is made

to correlate the various physical and chemical properties of the

surface with the changes in adhesion produced by the treatment.

The investigation forms part of a current series of student

projects in the general topic area of the "Surface Chemistry of

Cellulose, Paper and Other Polymers".

The arrangement of material in the the sis is as follows.

Chapter l gives a general introduction to the subject vlith a

literature survey and sorne background material. The main vlork is

presented in Chapt ers II and III which are written in the form

of scientific papers ready to be submitted for publication with

little modification. Chapter II is a study of change in bonding

of synthetic polymers to cellulose after treatment in oXYJen,

air or nitrogen coronas. Chapter III is an investigation of the

effect of different gas coronas on the polyethylene surface.

Sorne concluding remarks and suggestions for further

work are included after Chapter III and the thesis ends with

li

FOREWORD

The adhesive properties of cellulose and other polymers

are of fundamental importance to their growing use in composites

or lanunates. The application of a corona discharge on the

polymer surface is a weIl known method for increasing adhesion by

surface modification. However, there is little understanding of

the basic mechanism of the method. This thesis deals with the

effects of a corona discharge on polymers and an attempt is made

to correlate the various physical and chemical properties of the

surface with the changes in adhesion produced by the treatment.

The investigation forms part of a current series of student

projects in the general topic area of the "Surface Chemistry of

Cellulose, Paper and Other Polymers".

The arrangement of material in the thesis is as follows.

Chapter l gives a general introduction to the subject with a

literature survey and sorne background material. The main work is

presented in Chapt ers II and III which are written in the form

of scientific papers ready to be submitted for publication with

little modification. Chapter II is a study of change in bonding

of synthetic polymers to cellulose after treatment in oxYCert,

air or nitrogen coronas. Chapter III is an investigation of the

effect of different gas coronél.S on the polyethylene surface.

Some concluding remarks and suggestions for further

work are included after Chapter III and the thesis ends with

ti1

cla1ms to original research.

The author would like to continue research on the topic

of the present thesis in order to graduate with a Ph.D. degree

from the Department of Chemistry of McGill University. If

permitted to do this, the author plans to confirm and enlarge on

several of the tentative findings described in Chapter.III.

GENERAL INTRODUCTION

CONTENTS

CHAPl'ER l

Historical Background

Production of a Corona Discharge

Corona Chemistry

Surface Properties

Scope of the Investigation

CHAPTER II

CORONA INDUCED BONDING OF SYNTHETIC POLY~ŒRS Ta

CELLULOSE

ABSTRACT

INTRODUCTION

EXPERIHENTAL

Materials

Corona Treatment

Measurement of Bond Strengths

lticroscopy

RESULTS

Boncling

Hicroscopy of Treated Surfaces

DISCUSSION

. . . .

1

3

8

15

18

23

28

29

30

31

31

31

35

35

37

37

40

46

iv

REFERENCES

ACKNOWLEDGEMENTS

CHAPl'ER III

CORONA TREATMENT OF POLYETHYLENE

ABST RA CT

INTRODUCTION

EXPERHlENTAL

Materials

Gases

Corona Treatment

Bond Strength

Wetting Tension

Water Contact Angle

Microscopy

·49

51

52

5:5

54

56

56

56

56

58

60

61

63

Infrared Spectroscopy 64

RESULTS .... 65

Microscopie Study 65

Infrared Absorption . • • •. 69

Change of Bonding, Wettability and Water

Contact Angle with Time of Treatment 73

DISCUSSION . • • . 82

REFERENCES 91

v

CONCLUDING RE~~RKS AND SUGGESTIONS FOR FUTURE WORK . •

CLAIMS TO ORIGINAL RESEARCH . . . . . . . . . . . . .

vi

Page

93

94

CONCLUDING RE~~RKS AND SUGGESTIONS FOR FUTURE WORK

CLAIMS TO ORIGINAL RESEARCH . . . . . . . . . . . • •

Page

93

94

vi

TABLE

1

2

3

1

2

3

4

LIST OF TABLES

CHAPrER II

Description of Polymer Sheets

Bond Strengths (kg/~2) of Cellulose

to Polymer Sheets oaTreatment in an

Oxygen Corona for 15 min. ••..

Bond Strengths (kg/cm2 ) of Cellulose

to Polymers on Corona Treatment in

Different Gases

CHAPrER III

Gases Used in Corona Treatment. of PE

• • •

Bond Strength of PE Surfa~e Treated in the

Corona Discharge of Oxygen or Oxygen-

Containg Gases . . . . . . . . . . . . . Wett:lng Tension of PE Surfac.e Treated in the

Corona Discharge of Oxygen or Oxygen-

Containing Gases . . . . . . . . . . Water Contact Angle of PE Surface Treated in

the Corona Discharge of Oxygen or Oxygen-

Containing Gases . . . . . . . . . . . .

32

38

41

57

79

80

81

FIGURE

1

2

3

4

5

6

LIST OF FIGURES

CHAPrER l

Effect of humidity on stress cracking of

polyethylene treated in moist air.

l1easurements made with elec.trical stress

of 200 vOlts/mil and mechanical e1ongation

of 50 % • • • • • • • • • • • •• 5

Diagram of corona apparat us

Photograph of various 1ayers of corona

apparatus. From left to right.: Rubber

insulator, brass e1ectrode, diele~tri~,

ruhber chamber, dielectric, brass e1ect­

rode, rubber insulator. The in1et tube

can be seen on the upper right side of

the chamber . . .

9

10

Arrangement of corona apparat us with Oxygen

flowing through and power supply attached • 11

Luminous manifestation (corona discharge)

appearing between e1ectrodes applied

15,000 v in open air. The gap distance

is 0.2" . . . .'. . . . . . . . . . . . . Schematic diagram of corona priming

units on high-speed extrusion coating

13

viii.

7

l

2

3

4

5

6

l

•

line . . . . . . . . . . . . . . . . . . Equi.li.brium of a Vlater drop1et

resting on the PE surface

CHAPTER II

Schematic diagram of nitrogen

. . . . .

corona ceil . . . . . . Clamping 0 f speciman to measure

shear strength of bonding

Effect of pressing temperature on

bond strength between cellulose

and PS • . . . • • •

Surface of cellulose after

treatment for l hr in corona

. . . . .

14

19

33

36

39

discharges of air and nitrogen . . • •• 42

Surface of PE after treatment for l hr

in corona discharges of air and nitrogen. 43

Surface of PVC after treatment for l hr

in corona discharges of air and

nitrogen . . . .. ... 45

CHAPTER III

Measurement of bond strength using a

ix

2

3

4

5

6

7

8

Chatillon Spring Tester . . . . . . Goniometer for contact angle

measurement . . . . . . . . Schematic diagram of Goniometer . . . . PE surface treated for various times

in a corona discharge. A1uminu~ shadowed

and photographed in transmitted

1ight . . . . . . . . . . . . . . . Scanning e1ectron micrograph of PE

surface before and after 1-hr treatment

in the air corona

Decrease in pitting ,produced by increasing

separation of e1ectrodes for 1-hr treatment

in the air corona. A1uminum shadowed and

photographed in transmitted 1ight . . . Surface change of PE before and after

:-hr treatment in corona discharges

of air and nitrogen. A1uminum shadowed

and photographed in transmitted

1ight . . . . . . . . . . . . . . . . . Infrared absorption in the region 1,500-

1,800 cm-lof PE films of untreated

control and after 15-min and 1-hr

59

61

62

66

67

68

70

x

9

10

Il

12

13

14

15

treatment in corona discharges of air

and nitrogen . . . . . . . Transmittancy of infrared in the regio~

800-1200 cm-lof PE films of untreated

control and after l5-min and l-hr treatme~t

71

i~ corona discharges of air and nitrogen 72

Variatio~ of bond strength of PE surfac.es

treated in corona discharges of various

gases with time of treatment . . . . Change of wetting tensio~ of PE surfaces

treated in corona discharges of various

gases with time of treatment • • • •

Water contact angle change of PE surfaces

treated in corona discharges of various

gases with time of treatment

Wetting tension vs. Vlater contact angle

of PE surfaces after treatment in corona

discharges of various gases

Bo~d strength vs. wetting tension of

PE surfaces after treatment i~ corona

discharges of various gases

Bond strength vs. water contact angle

of PE surfaces after treatment in

. . .

74

76

77

83

86

xi

xii

corona discharges of various gases 87

An

An+

An*

d

e

h~

RnH

V

W~L

W,sL

GLOSSARY OF SYMBOLS

Gas Molecules.

Ionized gas Molecules.

Excited gas Molecules.

Distance between two electrodes.

Electrons.

Photons.

Polymer Molecules.

Potential applied between two electrodes.

V/ork of cohesion of a liquide

xiii

Energy required to separate unit area of the liquid­

,solid interface.

Electrical field strength.

Critical surface tensioa of a solide

Surface tension of a liquide

Surface energy of a solide

Interfacial energy between a sol1d and a liquide

Contact angle between the solid-liquid phase.

1 ,

CHAPrER l

GENERAL INTRODUCTION

2

GENERAL INTRODUCTION

Modification of the surface of polymers by treatment

in a corona discharge has been used forso~e time in order to

enhance properties such as adhesion and printability. The

technological application has been the subject of several

patentsl - 6) and the method has been described empirically in

a few recent publications7-9). Recently D. BirdlO ) of the

General Electr~c CompaD~ has compiled a bibliography of a num-

~er of references which constitutes a use fuI literature source

on the corona method.

However, in spite of the rapid advances in corona

technology, the underlying principles have not yet been elu-

cidated and remain elusive. Beveral theories have been proposed

but very little has been established. Until recently, it was

generally believed that oxidation of the surface of the polymer

enhanced polarity and wettability, and yielded good bonding.

However, Schonhorn and Hansen in a series of papersll- 13) have

demonstrated that a strongincrease in adhesion is shown by a

surface treated in a microwave dis charge in the absence of

oxygene Apparently surface oxidation is not the only way to

obtain better bonding properties.

The main purpose of the present work was to investi-

gate the corona treatment of cellulose and other polymers under

carefully controlled conditions in order to 9lucidate the basic

mechanism of the process.

3

Historical Background

Rossman14) reported c.orona treatment of polyethylene

for the first time in 1956. He used a Tesla coil over polyethy-

1ene film and found that inkabi1ity of the surface Vias improved

and that the infrared spectrum of treated film had new peaks

at 1709 cm-1 and 1639 cm-l.

Hines15 ) proved that corona treatment chemical1y

modifies the surface of a polymer. Superficial oxidative

degradation occurred and led to the formation of polar groups

such as -COOH. At the same time printabi1ity was imparted to

the surface. However, he claimed that the desired printability

VIas obtained without creation of measurab1e amounts of free

carbonyl groups (ketone or aldehyde) or hydroxyl groups.

Instead, he found the presence of traces of carboxylate salts

or carboxyl groups. Hines pointed out that over-treatment may

cause oxidation to proceed deeper into' the polyethylene films

and result in the formation of both ketone groups and cross-

linking in the polyethylene.

l'1cNahon, l-'Ialoneyand Perkins16 ) investigated the eff-

ects of corona discharges of different gases on strained

polyethylene sheet. They)found that the life of specimens

in the air corona was the short est but the life in the nitrogen

corona was longer than that in the carbon dioxide corona.

When moisture was present in the gases, a semiconducting

surface layer was developed, which diminished the corona

4

intensity. As a result, the corona life of polyethylene was

extended, as shown in Fig. 1. At the sarne time, they found that

moisture caused the production of oxalic acid on the surface

during corona treatment.

Cooper and Prober17 ) subjected polyethylene to an

- oxygen corona and found that the treatment induced carbonyl

group formation on the polyethylene surface, caused loss of

weight and produced oxalic acid on the surface. Grossman and

BeaSley,18) and HOugen,19) who used an air corona discharge,

also reported the formation of oxalic acid on the polyethylene

by the treatment. Spit20 ) while developing new techniques in

electron microscopy made use of gas discharge ,etching for

thinning thick specimens.

Schonhorn and Hansenll- 13 ) used a low radio frequency

coil to generate the excited species of various gases.

Exposing polyethylene to activated gases resulted in the for-

mation of a crosslinked surface layer which had high cohesive

strength and produced strong adhesive joints. Infrared spectra

showed the formation of ethylenic double bonds but they found

no change in adhesion after elimination of double bonds by

bromination. Apparently the unsaturation made no contribution

to the strong bonding. Electron spin resonance showed the

presence of free radicals after the microwave treatment of

polyethylene. The treatment, hovrever, caused no change in

5

Fig, 1

Effect of humidity on stress cr ..lcking of

polyethylene treated in moist air.

Measurements made with electrical stress

of 200 volts/mil and mechanical elongation

of 50 %.

•

200

a, ... ~oo .lIS "'-J ou. Id

; '0 ~ .... ,.

f:! 30

1 20

10 8

10 '0 sa 70 90

•

6

bulk properties, surface properties or dielectric properties of

the polymer.

Schonhorn and Hansen stressed that wettability'or

lower contact angle was not always essential to the formation

oi strong jv~nts. They showed that lowering the critical sur-

face tension of polyethylene by fluorination caused increase

of joint strength. Thus the increase in adhesion was not caused

byincreased surface energy but was due to elimination of

weakness in the surface region. Hansen et al. 2l ) applied an

oxygen glow discharge generated by a radio-frequency coil to a

variety of polymers and found oxidation, increase of wettability

of the surface, and weight loss of the sample.

Glow discharges have been used for polymerization

and graft polymerization on surfaces. Banford and Ward22 )

produced free radicals onpolymer surfaces by application of a

high frequency electric discharge. through hydrogen at low

pressure. Free radicals were detected by electron spin

resonance and were found to initiate graft polymerization.

Williams and Hayes23 ) polymerized styrene in a glow discharge,

and Secrist and Mackenzie24 ) synthesized fluorosiloxane

polymers in a glow discharge.

Noll and McAllister7) were the first to report on the

corona treatment of substrates such as polyethylene, polysty-

rene, nitrocellulose-coated cellophane, saran-coated cello-

7

phane or wet-strength paper, in polyethylene extrusion coatiDg.

They found that there was no adhesion without the corona dis-

charge priming and an· excellent bond was obtained with éorona

discharge priming. Greene8) examined and compared the effect

of flame and corona treatment upon the bonds attained during

the coating of various paper substrates. young25 ) has pub­

lished an excellent review on the adhesion of polyethylene to

paper in extrusion coating.

Most of' the work describing surface modification in

a corona discharge has dealt with adhesion of hot melts,

glassy polymers or pr±ming inks. Recently GOring9) made

the first demonstration of the application of a corona discharge

to improve the strength of the water-induced cellulose bond.

He found that treatment in an oxygen corona produced carqoxyl --groups, caused surface roughening and greatly increased the

strength of the water-induced bond between two cellulose

surfaces. He also noted that oxidation of cellulose Vias not

the sole factor for the production of good bonding. Other

possibilities included physical changes in the surfaces and

removal of debris or adsorbed impurities by the activated

species in the discharge •. Free radicals produced on the

surface in the corona discharge might also be related to

bonding.

8

Produ~tion of a Corona Discharge

As the e1ectrica1 potentia1 between two e1ectrodes

is raised, some sort of a breakdown takes place in the. gap

with 1uminous manifestations appearing at the high1y stressed

e1ectrode. Usua11y currents are relative1y 10w. The 1umin­

ous manifestations have various characteristic shapes, such

as glows, ha10es, cor6nas, brushes, streamers, etc., atid the

genera1 name of corona discharge is given to the se phenomena.

A simple corona apparatus consists of two .c10se1y

spaced insu1ated-p1ate e1ectrodes, between which a samp1e is

inserted. The purpose of the insu1ation is to prevent spark

breakdown across the e1ectrodes. i The gap can contain air or

any other gas. The corona usua11y requires an A.C. potentia1

of 5,000-15,000.v.

For examp1e, a corona apparatus which has been used

in this 1aboratory was bui1t as shown in Figs.2 and 3 and

arranged as shown in Fig. 4. The corona ce11 was formed by

sanc1.wiching a 1/4" neoprene washer between two die1ectric

plates (natu~a1 c1ear mica) and brass e1ectrodes. The samp1es

were he1d mid-way between the die1ectric plates and the desired

gas was passed through the corona cavity by means of a narrow

in1et and out1et. The power suppl y was a 15,000 v neon 1amp

transformer. When 15,000 v is app1ied between the e1ectrodes

in open air, a corona discharge can be seen as shown in Fig. 5.

9

Fig. 2

Diagram of corona apparatus.

e

15000v. -." 60c.

e

CORONA APPARATUS

BRASS PLATES

DIELECTRIC

10

•

Fig. 3

Photograph of vari.oua layera of corona apparat.ua •

. From.left. to right: Rubber inaulator, braaa

eiectrode, clielectric., rubher chamber, dielectric,

brass electrode, rubber insulator. The inlet tuhe

can. be seen on the upper right side 0 f the c.hamber.

·' ','

.Î:.'

'),"

", ~ .

. :

F:lg. 4

Arrangement of corona apparat us with oxygen

flowing through. and power supply at.tached.

12

The gap distance i8 0.2".

~ industrial practice, the L~pel high frequency

generator is often used as a power supply. The schematic

diagram of the Lepel HF SG-lseneJ;'àtor is shawn in Fig. 6.

The generator i8 design~d to control the intensity of corona

treatment by changing. the power imput to the primary circuit .•

Input and output frequencies are 50 or 60 cycles and one

megaclcles, respectively, and the availab~e current for 115

volts is up to 7 amperes.

A corona cell was designed hy Cooper and Prober17)

for treatment of polyethylene in the corona discharge of

oxygene

. ' ..

,; .. ,

fig. 5

Luminous manifestation. (corona discharge)

appearing between. e1ec.trodes applied 15, 000 ~

in. open. air. The gap distance is 0.2".

, ',", - , ~ ~., ,:.'

14

Fig. 6

Schemati~ diagram of corona priming units on

high-speed extrusion c~ating line.

': ::;,:7{:'~~)p::'~;' .• ,~, ,':,~ l :', ~.:"

","',

" -<

", ~ . 1:'

. , ':." ,,' ... ! >/ .... ~ ~

' ... " ..

•

, ,

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', ..

~. ,

,/ '.'

."'~

.',',

. ~ ' .. . ,

',-,

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•• 0 • " ç ..

In the abov.:e manne~,' .~1~·c,tr6·~â' necème 'SutficieIi~ii:;' ' .. ','

energetic to ionize .,or excite' -~~mé Of:,t~'e neut,tal 'specie~, in: " .. ,'. .. 1;"" • '. ::."/~',;. " ' .; .. ,. ., ,.:. ';" .

the gas. Electro%>. prod~ction by .i(#l:t'Z~:tii>If, is, then 'balance'd .;, '." ,,J

by électron loss.by diffusion,attach,ment, and'recombination. ~~ .' '~ : ..

Some of the possible react:i,onsare listed below in sym'bolic .",,;,.

forme If. An and e are gas molecules and, electrons, . respecti- ..

vely, and A! and A: are ionize'd molec:ules .and excited molecules'".

respectively, we can have

Al + e ~ Ar+ 2e ':0 1Qni~ation

Ar, + e + KE .. A! + e .. : exci ta tion

",

i' ~ ,

.. .' ".":' 0

::; :'.0 c ,' •

, ... ,\

.... ;: .. :.:'

... ,-".

,~, .. ' ..

:·,l. ' .. ',".

0"'0'.,0

"

>',.-"

, v "

•

'. '.' " .-.

.<":'.

.;--'

".

.A·'A* "A1 2 --'"""l, ~~. A + A ',+

·,1 .····2

:A+ A ·· .. 1 '+2 -----' •• ' A 1+ 1.,2

.:." .. '

. ....• .; ~'

,,' .' .....• r.,fOmbinâ.tiOri. . ,::. ".:" ,

.. -.

discharge, eleètrons, ionizedmolecule,s,' of 'exci t~d molecules. . . .

can react With the ·surface of the pol~mer.',: If,RnH représents

polymermolècules, we can have

.. RI' T H'

,. Rf + H'

)1 R H+ 1 + A

• Rl·'~ + R2'

+e , 0, .

+ A 1 ", ,

t A, 1. ,

polymer .. radical· f9rmà.tio~ .

. . .polymer radical formation

ionization

degradation

crosslinking by dehydrogenation

"

:J",

l', ~ ":0:

",': ':. : . .1"

. :., -:. ',' .

.' . .,.' '.. . . ,~: .

. .... ::'.

", ) . ~' .. ~ ','"

",i

; :

. '. ~ .. y: ..-' : . .t..~ _. . , \

'.'" '\;:·~!/.",.~i·f:<:)"

:':~"':~F!",':- '~.~C:6~~~~a~,~~n."

'." . 1.,',".

,RI·· '"" .,

'/. .. '':, . " .. :,' ~,~ ,. ./'~.';

R :tf.r:r .·rM. "., .+' R • . . ':':::::;,'.".,. ,·.'.R:',O':'H'· -_." C .•. '~.'~,~'. ':";"~~<'H<" '{'d:'O':'U"~~"'e'{ :bo: .... ;nd!~:·:/ .. ]. V~ VA2' " , '" '.2" ' 7' I ~;:"'T ""2 "'~')' ;,~ .!': ' ;,'. ':. ." : .. ;:' 2,'.,,' ",: ;' .. '';'' . , "." .... ,:., .. ·i, ... , ·'<·'f.,:"·.',, .. :. ,:,~,:~, "\'''' .. 'formation·':",:

" " ':,"':j' . ," {'", ' .~ , .' . " , "" :,',: :.~,,\!~. ',' (': . .':."~ .

It is qUir~';{~~:;~;OIi1~~~:~i>.j* ·ih~éà+!~tY.,C)i;C9;,.a~i changes can .b~' ei~e~1;èd' 6n.,~):p~lynier sur:t'a~~:tré~te'9:/:ln a~

, .' . ,< ;" ,::';:"".\., ".' '.".~" . ",' ..:;'>;":' '.' ""': :.: .... :,,·'f;. , ..... ' ., .

coronadi'scharge,:~'i;:::t:e, for,: ;~~ple," the.:~ga'$ molecûï~"c,oli~Eiins:: ... , . i '.,,~ • '"', .... ,' ,- .,'" "'''',,,,:.~ ,~ .. '.'" ':"""1"::' ,,:., .'. ~'" r •. ••· , ),,:~:.,'

oJGygen, we .can ha'lle .. '

0* 2

RIB t, 0,

Ri' + 0'

RIO t o .' 2

R 00' . I t R4B

RIOO'

R CHO '+0' 2 '. '

--+~ 'R~ . + OB-

----:~~ R O· + OR' , I •

or

. "·'l,::.:i\' '. .",

'. \ .. '

,.

,," '

,.-;

" ':,,"

. . ,i~;;(t~~~ f."'·

. ' . "';:'~.

, :" '- ,~::' ·f. :. .. :~~ ... ,,,'. :

,.>J. :;.-

.• " . \;,. '. "', ~:':r,:!:::~,:~ .... -, '. ~ ,:/r;.':;:'~~~:-;.

",'('::,i'1:l:;~:~ . :.-, ~, ~:::~,)

,',;

: .. . :, ," .. ~~ ,

. ~ ,

18

R· + R • 1 2

~le can expect formation of free radicals, carboxylation, car-

oonyl groups, cros61inkinc, double bonds and other functional

groups. It i6 cenerally believe{ that the life of carbonyl

radical is qui te lone ai.ld the decomposition of hydropel~oxides

is known to be a re1ativ81y slow process, lasting several days

or weeks at room temperature. Thus a surface primed in an

oxygen corona is likely to rehlain chemically active for some

tLJle.

Surface Properties

~hen a liquid is placed on a solid, there are mole-

cular forces of attraction between the surfaces of the two

bodies in contact. At the surface, there exist unbalanced

forces TIhich giv8 rise to a surface free enerGY. In the case

of incowplet0 vettinG of a surface by a liquid, there is ~

definitc an~le of contact, e, betTIeen the liquid-solid phase.

As shc~n iL Fi~. 7, for equilibriuD of forces i~ air

(1)

'.lhere YSA i8 the surface Gnercy of the GOlid, YLA is the surface

tension of the liquid and ~L is the interfacial energy corres-

ponCinG ~o the equilibriuill co~tact anGle.

In order to pull apart a liquid-solid joint of unit

1

1.9

Fig. 7

Equilibrium of a vlater droplet resting on the

PE surface.

•

l . , •

.....

'.

•• ,1

"

, .

•

';!

•

2.0

area, work must be done. The Vlork is called work of adhesion,

Ws\., 'which is defined by the relationship

WSL = Y!:.A + YLA - '( SL (2)

In equation (2), W&~ represents the energy required to separate

unit area of the liquid/solid interface. Thus WSL is a measure

of the adhesion of the liquid to the solid surface.

Combining equations (1) and (2), we have

VlSL :: YI.A ( l + cos e )

The surface tension of the liquid, YLA is readily determined,

so that if e is known, WSL will be obtained.

Consider the case where the angle of contact is zero,

and

Equation (4) means that the liquid spreads on the solid com­

pletely. The quantity 2YLA is equal to the work of cohesion,

·Ull., of the liquid i tself. Thus, if e = 0, the work done in

separating the liquid froD the solid is exactly equal to that

done in separating the liquid from itself. Correspondingly,

if e is greater than zero, the solid surface is energetically

different from the liquid surface. Thus, the value of e can

be a gUide to the differenbe in the surface energies of the

2l

two phases in contact.

The observation of liquid contact angle~ forms the

basis of several methods of estimating the surface energy of

a solide In the simplest method, one liquid only is used.

A series of surfaces can be compared with each other by mea-

surement of the contact angle they each make with the single

liquide Lower surface energies will give higher values of a and thus the materials can be ranked with respect to their

surface energies. Zisman27 ) has extended this somewhat expe-

rimental method by uSing a series of liquids of varying surface

tension (preferably a homologous series of organic liquids).

A plot of cos9 against the liquid surface tension, r~A , usually

gives a straight line. If extrapolated to cosS = l, this graph

will yield the critical surface tension, te, as the value of

liquid surface tension at the point where the plot crosses the

line cosS =1. The critical surface tension is then a measure

of the surface energy of the solide A disadvantage of the

method is that it is often necessary to use liquids which are

not chemically inerte Then, when the surface of the polymer

is also active, chemical combination may occur and produce

very loVi contact angles and erratic results in the Zisman plot. 27 )

A convenient modification of the Zisman method is

the AST~1 D 2578-67 Test for measurement of the wetting tension

of polyethylene and polypropylene films. Drops of a series

22

of mixtures of formamide and ethyl cellosolve of gradually

increasing surface tension are applied to the surface of the

polymer until a mixture is found that just wets the film sur-

face. The wetting tension of the polymer surface is then

given approximately by the surface tension of this mixture.

The method is reproducible to ;ti dynes/cm but unfortunately

cannot be used outside the range 30-58 dynes/cm. Possibly

the ASTM method could be used over a wider range if suitable

test liquids could be found.

Zisman and his sCh00128 ,29) have maintained that

wettability is an important prerequisite to good adhesion and

several other workers30-36 ) have supported this view. However,

the roughness of the surface is also important. Huntsberger37)

reported new concepts:

(1) For most practical adhesive systems, thermodynamic

equilibrium will correspond to a completely wetted

state.

(2) Maximum interfacial contact or complete wetting is

possible for systems exhibiting finite c.ontact angles

even greater than 900 •

(3) Contact angles are not directly related to the interfa-

cial contact between a liquid and solid, nor adhesion.

(4) Best adhesive performance may frequently be obtained

with adhesives exhibiting equilibrium contact angles

23

on the order of 250 or higher.

Scope of the Investigation

The first part of the investigation is concerned

with the effect of corona treatment on. the adhesion. of syn­

thetic polymers to cellulose. Both cellulose and polymers

were treated in a corona discharge, and the bond strength

developed when the cellulose and the polymers were pressed

together Vias measured. The effect of increasing the pressing

tempera;ture was examined and physical changes in the polymer

surfaces were recorded microscopically. In an attempt to

elucidate the chemistry of the process, an apparatus was

built which permitted the treatment to be carried out in an

atmosphere other than oxygen or air. The effect of treatment

in oxygen could then be compared with that produced by treat­

ment in pure nitrogen.

In the second part of the investigation, the experi­

mental system was somewhat simplified by use of a single, sim­

ple substrate-polyethylene. Physical effects ,produced on

polyethylene surfaces in oxygen and nitrogen coronas were exa­

mined ~icroscopically and chemical chahges studied,by infrared

spectroscopy. The varying phenomena obtained by corona treat­

ment in different gases viere characterized by measurement of

bond strength, wetting tension and water contact angle. An

24

at tempt vias made to establish whether or net i t was possible

to correlate bonding with wettability, water contact angle,

or other physical and chemical changes on the surface.

From the above results it vias possible to draw some tentative

conclusions concerning the basic mechanism involved in corona

treatment.

25

REFERENCES

1. B.P. 715914 (Sept. 1954), Visking Corp.

2. USP 3081214 (Har. 1963), T.H. Strome.

3. B.P. 920860 (1963), E.r. du Pont de Nemours.

4. B.P. 923846 (1963), E.r. du Pont de Nemours.

5. B.P. 921276 (1963), I.C.I. Ltd.

6. B.P. 852982 (Nov. 1960), Siemens Schuckentwerke A.G.

7. P.B. No11 and J.E. McA1lister, Paper, Film and Foi1

Converter, Oct. 1963, p. 46.

8. R.E. Greene, TAPPI, ~ No. 9, 80A (1965).

9. D.A.I. Goring, Pu1p Paper Mag. Can., 68, T-372 (1967).

10. D. Bird, The Chemistry of Low Current-Density E1ectrica1

Discharge, a Bib1iography, G.E. Report No. 67-C-233.

Il. H. Schonhorn and R.H. Hansen, J. App1. Po1y. Sei., 12,

1231 (1968).

12. H. Schonhorn and R.H. Hansen, J. App1. Po1y. Sei., Il,

1461 (1967).

13. R.H. Hansen and H. Schonhorn, Po1y. Letters, ~, 203 (1966).

14. K. Rossman, J. POly. Sei., 12, 141 (1956).

15. R.A. Hines, Paper Presented at the 132nd Heeting of

the American Chemical Society, New York, Sept. 1957.

16. E.J. McNahon, D.E. Eia1oney, and J.R. Perkins, Trans.

Am. Inst. E1ect. Engrs., A.I.E.E., Nov. 1959, p. 654.

17. G.D. Cooper and M. Prober, J. Po1y. Sei., ~, 397 (1960).

26

18. R.F. Grossman and W.A. Beas1ey, J. App1. Po1y. Sei.,

~, 163 (1959).

19. L.R. Hougen, Nature, 188 No .. 4750, 577 (1960).

20. B.J. Spit, Po1ymer,:2, 109 (1·962).

21. R.H. Hansen, J.V. Pascale, T.· De Benedictis, and P.M.

Rentzepis, J. Poly. Sci., A-3, 2205-2214 (1965).

22. C.H. Bandford and J.C. Ward, Po1ymer, ~, 277 (1961).

23. T. Williams and M.W. Hayes, Nature, 209 No. 5025,

769 (1966).

24. D.R. Secrist and J.D. Hackenzie, Po1y. Letters,!!;,

537 (1966).

25. J.R. Young, Paper Packs, Dec. 1966, p. 23.

26. J.A. Coffman and W.R. Brovme, Scientific American

212, No. 6:90 (1965).

27. W.A. Zisman, Ind. Eng. Chem., 21,18 (1963).

28. Vi.A. Zisman, Aià. Chem. Soc., Advances in. Chem. No. 43,

1964. p. 1.

29. W.A. Zisman, Encyclopedia of Po1ymer Science and

Techno1ogy, Vol. l, 1964, ~. 445.

30. H.A. Arbit, E.E. Griesser, and \V.A. Haine, TAPPI,

!tQ, 161 (1957).

31. F.J. Bockhoff, E.T. r·1cDone1, and J.E. Rutzler, Ind.

Eng. Chem., 22, 904 (1958).

32. P.B. Noll and J.E. McA11ister, Paper, Film and Foi1

27

Convarter, Oct. 1963, p. 46.

33. A.J.G. Allan, J. Po1y. Sei., ~, 297 (1959).

34. J.W. Swanson and J.J. Becker, TAPPI, ~ No. 5, 198

(1966).

35. N. Glossman, TAPRI, 2QNo. 5, 224 (1967).

36. H.J. Barbarisi, Nature, 215 No. 5099, 383 (1967).

37. J.B. Huntsberger, Cham. Eng. News, Nov. 2, 1964, p. 82.

28

CHAPT ER II

CORONA INDUCED BONDnm OF SYNTHETIC

POLYl1ERS TO CELLULOSE

29

ABSTRACT

Corona treatment improved bonding between sheets of

cellulose and synthetic polymers. The bond strength increased

at higher temperatures of pressing. Physical changes in the

surface were detected microscopically after corona treatment

in air. Sheets treated in pure nitrogen made strong bonds

although the surface treated in nitrogen was indistinguishable

from the untreated surface.

30

INTRODUCTION

It has been known for some time that treatment of

polymer surfaces in a corona discharge produces marked incr­

ease in the adhesive properties of the surface. l ) The print­

ability of polyethylene is improved by such treatment. 2 ,3)

Paper passed through an electrical discharge, shows better

adhesion to melt coatings,4) and, corona treatment enhances

water-induced bonding between cellulose surfaces. 5)

The present paper deals specifically with the bond­

ing properties between cellulose and polymer sheets after ~

treatment in a corona discharge. The effect of the treat-

ment of each componen.t alone Vias compared vlith the adhesion

produced when both the cellulose and the plastics viere

treated before bonding. The surfaces of samples were examined

microscopically in order to detect any physical changes

produced by the corona. Samples were treated in air, oxygen

and nitrogen at atmospheric pressure, and changes in bond.

strength and surface roughness produced by corona treatment

in the different gases Viere examined.

31

EXPERIMENTAL

Materials

One foot square sheets of commercial secondary cellu­

lose acetate film of 0.005" thickness, were de~cetYlated,5)

framed and dried for three days at room temperature.

Four of the polymer samples Viere donated by the Dow

Chemical Co.* while the remainder were purchased locally.

A description of the polymer samples is given in Table I.

Corona Treatment

Corona treatment in oxygen or air was carried out

by the method described in a previous report. 5 )

For treatment in nitrogen, a corona discharge cell

was set up in a vacuum system. The cell is shawn in Fig. 1.

Four 24/40 outer joints were attached to a 2 1. reaction

kettle, two joints for electric leads and two for inlet and

outlet of a gas. The electrodes were cylinders of stainless

steel, 3.5" in diameter and 0.5" in height, and \'lere houscd

in truncated glass bottles sealed with glass plates, 0.030"

in thickness. Small pi3ces of glass plate with thickness of

0.12" \'lere used for the gap between electrodes. The sample

* The authors are indebted to Dr. B.B. Hillary of the Dow

Chemical Co. for supplying these samples.

32

TABLE 1

Description of Po1ymer Sheets

Po] ymer Type Thickness Descripti.on

Polyethylene Dow C.I.L. 220G 1/32" low" densi.ty (PE) no additives

n COIDlllercia1 1/32"

Po1yvinyl- Dow 1/1611 unp1asticized ch~oride(PVC)

" COIDlllercia1 1/1611

Po1yviny1:Ldene- Dow Saran B-115 1/32" chloride(PVDC)

Po1yst.yrene Dow Styron 456 1/32" high impact (PS)

n Commercial 1/3211

33

Fig. 1

Schematic diagram of nitroge~ corona celle

e-

8

GAS OUTlET- -GAS INLET

A Pol ymer Sheet 15000 v 8 Reoction Kellie 60- C Truncoted Giou Boille

0 Sloinle" Steel Electrodes

E Gloss Spacer

F 30 Mil Giou Plates

G Glass Cylinder

34

was put on the bottom electrode.

The nitrogen used was stated to be 99.998% pure.

For complete removal of oxygen, the nitrogen was passed

through a COPlJer colur:m, 45 cm high and 4 cm. in diameter,

heated to 4500 C. Then the nitro6on was dried in a liquid

nitrogen trap.

The system containing the sample was evacuated to

10-3 mm lIg and then filled to atmospheric pressure with the

purified nitrogen. This was repeated three times before the

corona treatment was oeGun.

The corona discharge '.'las generated between the elec­

trodes, using the 15,000 v, 60 cycle power supply descrioed

previously. 5) Gas flo\'! ';IaS maintained at the rate of 5 llll/uin

through the cell during the corona treatment.

An approxir:lé:.tel:leaSUre of the energy dissipated i11

the discharge Vias obtained by connecting an Anrprobe AC \'Jatt-

neter to the input of the ~ower supply. Jith its high-tension

output lcads disconnected, the power supply drew 45 watts

which increased to 50 watts when the leads were connected to

the electrodes of the corona cell with air and no sheet in the

Gap. The difference of 5 watts corresponded to 0.5 watts pel'

sq. in. of electrode surface and was a measure of the energy

of the discharge. An al~.lost identical power factor was

obtained by similar 1:1easurel~lents on the largel~ corona cell

35

descrioed yreViouSly.5)

~easurehlent of Bond StrenBths

The polyner and cellulose sheots were cut into strips,

20 r.lEi ;~ 5 l::m, Lli:lcdiately after the corona treatlllent. The

strips ~ere overlapped by 5 mm and pressed for tuo minutes

2 at a pressure of 5.7 kg/cm and at the required telilpe::.~ature.

The bonded specimens were then clamped as shown in Fig. 2 and

the strength at rupture measured with a Chatillon Spring

Tester as previously dcscribed. 5)

iÜcroscopy

The photocraphs of the yOlYlller surfaces coated with

alu~inuill were taken, using a Reichert Zetopan Research ~Ucro-

scope. Samples TIere photosraphed both untreated and after

one ho ur tre.s.tment in air and nitrogen. The shadoVling angle

. -00 \'las;; •

36

Fig. 2

Clamping of specimen to measure shear

strength of bonding.

37

RESULTS

Bonding

The bond strengths between cellulose and plastic

strips, treated for 15 min in the corona àischarge in oxygen,

are shown in Table 2. In all cases the pattern is the same.

Considerable increase in bond strength was found uhen the

treated polymer sheet was pressed to the untreated cellulose.

A smaller, but ill lilost cases, significant il:lprovement \'las noted

when the cellulose alone was treated. However the results

show the largest increase in bond strength when both cellulose

and plastics aretreated in the oxygen corona.

The effect of pressing temperature on the bond stren­

gths betv:een cellulose and PS is shown in Fig. 3. Salilples

':rere treated for 15 min in an oxygen corona. Treat:nent . not

only 10V/ers the temperature at vrhich bonding begins but produces

stronger bonds at a given temperature. As in Table 2, the

Greatest increase '.'las found ... .'11en both samples 1;iere treated

but significant enhancenent of bonding also occurred when either

the plastic or the cellulose was treated alone. Similar results,

but over slightly different teL'.perature ranses were found for

the other polymers. In all cases, the temperature at which

adhesion became pronounced TIas a little below the softening

telilperature of the polymer.

38

TABLE 2

Bond Strengths (kgjcm2 ) of Cellulose to Polymer Sheets

on Treatment in an Oxygen Corona for 15 min.

Polymer PE PS PVC PVDC

Temp. of Pressing 90°C 110°C 110°C 110°C

Source Dow COmIn Dow Comm Dow Comm Dow

Untreated Control 1.3 0.2 0.0 3.3 0.0 3.6 0.5

Cellulose Treated 1.3 0.9 1.3 7.3 0.5 5.4 2.0

POlymer Treated 4.4 12.2 6.7 10.2 3.3 5.8 2.2

Both Treated 6.5 13.8 15.2 21.8 5.3 6.9 3.8

39

Fig. 3

Effect of pressing temperat~re on bond strength

between cellulose and PB.

e

_20 N

Ë Co,)

ci> ..:.0:

:J:

~ :z ~ 10 l­V')

01 :z o al

o

80 90 100 no PRESSING TEMPERATURE (OC)

Both Treated

PS Alone Treated

Ce" ulose Alone Treated

U ntreated Control

120

e

40

The bond strengths produced by use of different gases

.i11 the corona discharce are shown in Table 3. The bond strent;th

b etv!een cellulose and PE or PVC, both treated in oX;YGen, is not

much differel1t froï:.1 bond strengths produced by ti1e air corona

discharce. Houever, the bonding produced by treatment in pure

nitrogen is so high that the cellulose strips failed before the

bond released. High bonding of the sal:lples treatcd in nitrogen

vas not expected because previous uork showed negligible increase

in the water-induced bond if cellulose was treated in nitrOgen. 5 )

In the earlier experiments, rather impure nitrogen was used.

It is possible that traces of o:i:ygen in the ni trogen lJ.arkedly

decreased the concentration of active nitrocen atoms. 6,7) Also,

ni tric o::ide produced by the corona dischal'ge in the ni trogen-

l'ich hlixture could quench adhesion-?roducing free radie aIs on

the polymer surface. S)

~icroscony of Treated Surfaces

As shown in Fig. 4, the cellulose surface vThen treated

in air appears to increase in roushness. 5 ) However in nitrogen

the treated surface is almost the same as the surface of the

untreated material.

Surface effects were more evident with the polymers.

Ï!'i:.;. 5 shows tha·~ the surface of PE treated in air has many

pits. In contrast, the surface treated in nitrogen is indistin-

L5ui.:;hable from the untreated surfa.ce of ?E. Th9 surface of pve

41

TABLE 3

2· . Bond Strengths (kg/cm ) of Cellulose to Polymers on

Corona Treatment in Different Gases

Untreated Control Oxygen Air Nitrogen

Dow PE 1.3 9.5 19.1*

Dow PVC 0.0 22.3*

*Cellulose strip failed before bond released.

42

Fig. 4

Surface of cellulose after treatment for l hr

in corona discharges of air and nitrogen.

c W N I-Z ~Z ~ -1-

Cl LLI~ ~.-

~~ Wz ~-1-

c w

~ w ~ 1-Z :)

~

0 ....

1

1

l \ )

~:---~-. ~ .. , ."'" "

()

i

J ;1 l , 1 1

1 .! , , 'j

1 1

J .1 ! i j

1 '1 j 1 1 1

1 ,

1

1

( \

o W N I-Z « wZ ~ -1-

c w~ 1--«« wz ~-l-

0 w 1-« w ~ l-z ::>

.::t 0 r-

43

Fig. 5

Surface of PE after treatme~t for 1 hr i~ ~rona

discharges of air and nitrogen.

\

a ,ad: w· ... : ....... . ~ .-« . .. ~ z" w.··· .~ -.....

(

~~ 1---~~ wz ~ 1-

Cl W 1--~ W ~ 1--Z ::::>

~

0 .--

44

is also rough after treatment in an air corona but the surface

treated in a nitrogen corona looks almost the same ~s the sur­

face of the untreated material as shown in Fig. 6.

45

Fig. 6

Surface of PVC after treatment for 1 hr in

corona discharges of air and nitrogen.

1 ,. " ,

e 1 1

r-

1

i

1

1

L

o N

~Z ~

.wZ ~-1-

c~ w .... I-~

~z ~-1-

Cl w

~ w ~ 1-·Z­:=)

. . . . .

- . ~ ..

:t 0 ...

--

-.. .. ,

. ,. "

---:..:.:..-.... ~ . .. -.... -. ' ..

.. ".

1 { ! ·l \

1

i

1 , , \

J,

! ! j 1

1 \

! ,[ 1 ! i

'1 !

( .~.

o~ w_ 1-< ~z ~ -1-

, ., : ' ,

~

0 ....

i. i· 1

l-I

[,

46

DISCUSSION

The results show that the plastics stuck to the cellu-

lose when the plastics began to soften at higher temperatures.

Note that the temperatures of pressing were more than 100 Oc

below the softening temperature of the cellulose. 9) Thus the

adhesion process in the present experiments may be considered

to be liquid (rubbery plastic) to solid (cellulose).lO)

The importance of the wettability of the solid surface

by the liquid in such a system has been stressed by a number of

authors. 11- 14 ) For complete wettability, the solid should

have a critical surface tension greater than that of the liquide

Swanson and Becher12 ) have found increased polyethylene adhesion

with increases in the critical surface tension of paper.

Similar results have been obtained by Glossmanl3 ) for the ad-

hesion of wax/polymer blends on paperboard. If corona treatment

is assumed to erulance the critical'surface tension of the

cellulose sheet used in the present work, then the increase in

adhesion on treatment only of the cellulose surface supports

the above concept (compare the first and second lines of

Table 2).

However, an important contradiction is introduced

by the marked increase in adhesion produced by corona treatment

of the plastic alone (compare lines land 3 in Table 2). As

reported in detail in a later paper,15) the critical surface

47

tension of polyethylene, treated in a nitrogen corona under

the conditions used in the present work, increases from 31

dynes/cm to over 58 dynes/cm. It is likely that similar incl'e­

ments in surface ellergy vlill occur Vii th the other polymers.

Sandell16 ) has found that the critical surface tensions of

a variety of celluloses are in the range of 36 to 42 dynes/cm.

Thus the corona treated polymer should wet the cellulose less

effectively than the untreated polymer, in spite of v/hich the

treated polyrner sticks more strongly to the cellulose. Hence,

it seems likely that the increased adhesion is not due solely

to increased wettability of the cellulose by the polymer.

Surface roughening is sometimes advanced as a possi­

ble cause of increased adhesion. ll ) Physical changes in the

surface produced by corona treatment in oxygen or air vere

expected from the results of previous work. 5 ,17) 'IIith the

plastics (particularly PEl pits might be formed when the

surface is heated locally in the corona discharge18 ) although

heat alone is not sufficient as ~as shown by the absence of

pits on a surface exposed to heated but electrically neutral

~lates. The high bond strength and unchanged appearance of

the surfaces treated in the nitrogen corona suggests that

surface roughness (at least at the microscopic level) was not

necessary for strong bondingll ) and may, indeed, be deleterious

to the adhesive properties of the sheet. 17 )

48

We must, aIso, consider the possibility that chemical

modification of the surface is responsible for the increase in

adhesion. Rossmann2 ) and several later authors5 ,18,19) have

demonstrated oxidation of a polymer surface in an air or oxygen

corona. Our results suggest that enhancement of adhesion can

be produced without marked oxidation of the surface. It may

be that a monolayer of adsorbed oxygen molecules or moisture

is the active agent in the nitrogen experiments. However,

from infrared analysis (to be reported in more detail in a

Iater paper)15) it was found that the strong increase of the

C=O bond produced by corona treatment of PE in air did not

occur when the polymer was treated in nitrogen gas. This'

finding supports the views of Hansen and SchOnhorn20 ,21) but

does not necessariIy prove that the surface crosslinking mech­

anism proposed by these authors is the correct one for the

conditions of surface treatment used in the presentinvestiga­

tion. Further work is planned to elucidate the cause of the

observed effects.

48

We must, also, consider the possibility that chemical

modification of the surface is responsible for the increase in

adhesion. Rossmann2 ) and several later authors5 ,18,19) have

demonstrated oxidation of a polymer surface in an air or oxygen

corona. Our results suggest that enhancement of adhesion can

be produced without marked oxidation of the surface. It may

be that a monolayer of adsorbed oxygen molecules or moisture

is the active agent in the nitrogen experiments. However,

fr~- infrared analysis (to be reported in more detail in a

Iater paper)15) it was found that the strong increase of the

C=O bond produced by corona treatment of PE in air did not

occur when the polymer \Vas treated in nitrogen gas. This

finding supports the views of Hansen and SChOnhorn20 ,21) but

does not necessariIy prove that the surface crosslinking mech-

an1àm proposed by these authors is the correct one for the

conditions of surface treatment used in the present investiga­

tion. Further work 1s planned to elucidate the cause of the

observed effects.

49

REFERENCES

1. J.R. Young, Paper Packs, Dec. 1966, p. 23.

2. K. Rossmann, J. Polymer Sei., 12.,141 (1956).

3. R.A. Hines, Paper presented at the 132nd Heeting of

the American Chemical Society, New York, Sept. 1957.

4. R.E. Greene, TAPPI,~ No. 9, BOA (1965).

5. D.A. I. Goring, Pulp Paper ~lag. Can., 68, T-372 (1967).

6. A.S. Vlastaras and C.A. Winkler, Cano J. Chem., ~,

2837 (1967).

7. L.F. Phillips and H.I. Schiff, J. Chem. Phys., ~,

1509 (1962).

8. A.F. Trotman-Dickenson, Gas Kinetics, p. 153, Buttervrorths

Scientific Publications, London, 1955.

9. D.A.!. Goring, Pulp Paper Bag. Can.,~, T-517 (1963).

10. L.H. Lee, J. Polymer Sei. A-2, 2, 751 (1967).

Il. A.J.G. Allan, J. Polymer Sei., ~, 297 (1959).

12. J.W. Swanson. and J.J. Becker, TAPPI, !±.2. No. 5, 198 (1966).

13. H. Glossman, TAPPI, 2Q No. 5, 224 (1967).

14. U.J. Barbarisi, Nature, 215 No. 5099, 383 (1967).

15. C.Y. Kim and D.A.I. Goring, to be published.

16. H.A. Sandell, "Wetting of Wood Polysaccharides", H.Sc.

Thesis, State University College of Forestry, Syracuse,

N.Y.

17. G. VI. Bailey, Norelco Reporter, ~ No. l, Jan. -Har., 1967.

50

18. G.D. Cooper and H. Prober, J. Polymer SeL, !Jlt, 397 (1960).

19. R.F. Grossman and W.A. Beasley, J. Appl. Polymer Sei.,

,g, 163 (1959).

20. R.H. Hansen and H. Sehonhorn, Polymer Letters, ~, 203

(1966).

21. H. Schonhorn and R.H. Hansen, J. Appl. Polymer Sei., Il,

1461 (1967).

•

51

ACKNOWLEDGEl1ENTS

This paper is based on resu1ts of research supported

in- part by a grant from the Department of Forestry

of Canada •

52

CHAPTER III

CORONA TREATl-iENT OF POLYETYLENE

53

ABSTRACT

Corona treatment of polyethylene in air pitted the

surface, produced -CaO groups and -C=C- double bonds,and in­

creased the bond strength and the surface energy. Corona

treatruent in hydrogen had no perceptible effect, either che­

mically or physically on the surface. The nitrogen corona

neither oxidized, crosslinked nor pitted the polyethylene sur­

face but on prolonged treatment increased the surface energy

and the bond stre~gth until the tensile strength of the

polyethylene strip itself was exceeded. However, it was not

possible to correlate bond strength with surface energy in

ail cases. Prolonged treatment in arGon or al-min treatment

in nitrogen produced surfaces of high surface energy that

showed low bonding.

53

ABSTRACT

Corona treatment of polyethylene in air pitted the

surface, produced -CaO groups and -C=C- double bonds,and iLl­

creased the bond strength and the surface energy. Corona

treatruent in hydrogen had no perceptible effect, either che­

mically or physically on the surface. The nitrogen corona

neither oXidized, crosslinked nor pitted the polyethylene sur­

face but on prolonged treatment increased the surface energy

and the bond strength until the tensile strength of the

polyethylene strip itself was exceeded. However, it was not

possible to correlate bond strength with surface energy in

aIl cases. Prolonged treatment in argon or al-min treatment

in nitrogen produced surfaces of high surface energy that

showed low bonding.

54

IN'TRODU CTION

When polymers are subjected to a corona discharge

in air or oxygen, the surface changes cheffiically and physi­

cally. Corona treatment of polyethylene (FE) in air or oxygen

produces -C=O bondsl ,2) and forms pits.3 ,4) When placed in

an electrical discharge of oxygen, cellulose shows surface

roughness and -COOH groups are produced. 5)

As described in the preceding chapter,6) corona

treatment considerably enhanced the adhesion of several

synthetic polymers to cellulose. However, the effect was

found v/hen the treatment Vl8S carried out not only in air or

oxygen but also in pure nitrogen, even though treatment in

nitrogen produced no perceptible surface roughening.

The purpose of the present study was to extend the

previous investigation in an attempt to discover the basic

factors which contribute to corona-induced adhesion. Treat-

ment in several other gases was included. As before, adhesive

properties were measured and the surfaces were examined micro-

scopically. Surface energy and contact angle observations

were also made in an attempt to correlate the adhesion behavior

with these important surface properties. In addition, infrared

analysis r;as used to detect any changes produced in the polymer

by the corona troatment. PE was chosen as the p~lymer for the

investigation because of the siJ:lplicity of its chemical struct-

55

ure, and because corona treatment of PE is important commer­

cially.

56

EXPERIHENTAL

Naterials

PE sheet vrith a thickness of 1/32" was donated by

the Dow Chemical Co. For infrared analysis, use Vias made of

PE film, 0.002" in thickness, kincily donated by Union Carbide.

Both samples yrere low density PE and Viere used without further

treatment.

Gases

A description of gases used is given in Table 1.

Nitrogen, argon and hydrogen were purified by passage over a

heated copper column as described in a previous chapter. 6 )

Carbon dioxide, oxygen and air were deœoisturized using two

dry ice-acetone traps.

For sorne experiments air ";.;as not dried and nitrogen

was passed through saturated CaC12 solution in order to obtain

31 % relative hur:ûdity in the nitrogen corona cell.

Corona Treatment

Corona treatment was carried out in the cell used

for nitrogen described in a previous chapter. 6) For most

experiments, the electrodes were 0.1211 apart and the app1ied

voltage was 15, 000 v. In the case of argon, potential cou1d

be applied only up to 5,000 v because e1ectrical breakdown

56

EXPERIHENTAL

Haterials

PE sheet with a thickness of 1/3211 Vias donated by

the Dow Chemical Co. For infrared analysis, use Vias made of

PE film, 0.002" in thickness, kindly donated by Union Carbide.

Both samples TIere low density PE and were used without further

treatment.

Gases

A description of gases used is given in Table 1.

Nitrogen, argon and hydrogen were purified by passage over a

heated copper COlUlùl1 as described in a previous chapter. 6 )

Carbon dioxide, oxygen and air were demoisturized using two

dry ice-acetone traps.

For sorne experiments air '.'las not dried and nitrogen

was passed through saturated CaC12 solution in order to obtain

31 % relative hUl:üdi ty in the ni trogen corona cell.

Corona Treatment

Corona treatment TIas carried out in the cell used

for nitrogen described in a previous chapter. 6 ) For most

experiments, the electrodes were 0.12" apart and the applied

voltage was 15, 000 v. In the case of argon, potential could

be aplüied only up to 5,000 v because electrical breakdown

57

TABLE l

Gases Used in Corona Treatment of PE

Gas Supplier Purity (%)

°2 Liquid Carbonie 99.5 Canadian Corp.

N2 Liquid Air 99.998

CO2 l>1atheson 99.99

A Hatheson 99.998

H2 Hatheson 99.95

58

occurred. In one series of experiments, the e1ectrode separa-

tion was increased in order to study the effect of surface

roughening Vii th a v/ider gap.

Bond Strength

Bond strength '.'las measured on lapped joints of PE

to PE as described in a previous ehapter. 6) The 1/32" PE

sheet was eut into strips, 20 mm x 5 mm, immediately after the

corona treatment. The strips Viere over1apped by 5 mm and

pressed for 2 minutes at a pressure of 5.7 kg/cm2 and at 45 oc.

The 1aminate ';las then clamped as shown in Fig. 2 of Chapter

lland the bond broken with a Chatillon Spring Tester as shown

in Fig. 1.

The number of measurements for any one set of condi­

tions \Jas a-c 1east six. If PE Vias treated for less than 5 min

in the corona discharges of oxygen-containing gases or nitrogen,

the mean deviation between the six or more readings was +5 %.

But if FE V/as treated for times longer than 5 min, the mean

deviation increased to ±10 %. When PE was treated in the

corona discharges of hydrogen or argon the bond strength Vias

very loV! and the me an deviation ..... las +15 ~~.

59

Fig. 1

Heasurement of bond strength uSing a Chatillon

Spring Tester.

60

Wetting Tension

For the measurement of wetting tension, the ASTM D

2578-67 procedure was used. Dr.ops of a series of mixtures

of formamide and e"thyl cellosolve were applied to the PE

surface with cotton swabs until a mixture Vias found, which

just wetted the film surface. The surface tension of such

a mixture then corresponded to the wetting tension of the poly­

mer surface. Wetting tensions of up to 58 dynes/cm could be

measured by this method. The method was reproducible + 1

dynes/cm.

Water Contact Angle

A telescopic goniometer Vias used for the measurement

of the contact angle of water with the PE surface. A photo-

graph of the experimental arrangement is shown in Fig. 2 and

a diagram of the apparatus is given in Fig. 3. The PE sheet

was mounted on the adjustable platform, which could be raised,

lowered, moved from side to side, or rotated. A drop of water,

-2 1.5 x 10 ml, Vias ejected from the tip of the micropipette

and the platform with the FE was raised slowly to reach the

drop and then lowered slowly. The platform vias then adjusted

to give the alignment as shown in Fig. 7 of Chapter l.

The drop VIas observed thro~gh a horizontal gonio-

meter which had a protractor divided in degrees on a rotating

61

Fig. 2

Goniometer for contact angle measurement.

1 1

62

Fig. 3

Schematic diagram of Goniometer.

e.

•

1&.1 CL o u (1)

~ 1&.1 t-

.... 1&.1 1&.1 % (1),

"

63

scale. There were independently rotating cross-hairs in the

eyepiece of the goniometer. The edge of the drop was aligned

with the center of the eyepiece and one of the cross-hairs

was adjusted to give a tangent to the drop image by rotating

the protractor while the other cross-hair remained parallel

to the surface of the SOlid. 7)

The precision of the goniometer was about ± 0.1

degree. The number of drops used for the measurement of a ~ater

contact angle was more than three and observations were made

on both sides of each drop. The mean deviation of water con-

tact angle \'las ±3 % if the PE surface was treated in oxygen-

containing sases but the deviation increased to ±10 % for

PE treated in the corona discharges of nitrogen, hyàrogen and

argon.

The water used was distilled, passed through ion-

exchange resin and th en a.fter treatment with a solution of

KMn04

and NaOH,was obtained by reflux distillation. 7)

Surface tension of the water measured vlith a Wilhelmy type

glass Plate8 ) was 72.2 dynes/cm at 24 oC.

:rÜcroscopy

HicroscolÜC examination and microphotography were

carried out by the methods described in a previolls chapter. 6)

In addition, a few specü:ens were studicd vIi th the Scanning

64

Electron Hicroscope. 5)

Infrared Spectroscopy

The treated film, 0.002" thick, was folded to give

a stack of 10 sheets. It was found that if the sheets were

pressed bet\'reen KBr discs, light transmission \'las improved

and a considerable amount of light scattering was avoided.

The instrument used \Vas a Unicam SP 100G infrared spectro­

meter.

65

RESULTS

Hicroscopic Study

Fig. 4 shows the surface change of PE as a function

of tille of treatment in the air corona.. Some roughening could

be detected after 5 min V/hile 2-hr treatment produced gross

pitting on the surface. Scanning electron micrographs of

the heavily pitted surface indicated that a film had r.isen

from the surface to form a partial canopy over each pit as

shovm in Fig. 5.

Increased separation of the electrodes caused a

decrease in the degree of pitting as shown in Fig. 6. To the

first approximation, the electrical field strength, X is given

by

x = V/d (1)

where V is potential applied and d is distance between the

electrodes. The results in Fig. 6 suggest that the strength

of the electrical field in the discharge might affect the

size of the pits produced.

The pitting may be due to localized pyrolysis of

the polymer. The electrodes were found to be warm after pro­

lonsed running of the corona. If the effect were due to incre­

ased temperature in the corona, the trends with time of

•

66

Fig. 4

PE surface treated for various times in a corona

discharge. Aluminum shadowed and photographed

in transmitted light .

• 1 ..

UNTREATED

i

;. '. ~ ;

! "

.;;-J TREATED FOR 30 MIN.

1 /

.. ., 1 1 j ~ : " . 1"1 : '

.. 1 1 l

. . ,

TREATED FOR 5 MIN. '

TREATED FOR 1 HOUR 10p .....

.: .... _-'"

. ' ~ .. ~'. ' ,:~

;.A "

',1

" , ' .. )

TREATED FOR 15 MIN.

TREATED FOR 2 HOURS

.~. '~:;'> .. ?-~." -:' ...... ;.::-.....

: "' .....

. .... ';'< ....

...... ~.:: , 1

/ ~

,1 .. '; '~" .....

;,;.:

1':

i'

~ ,

1

i L' :

i 1 ;

~. ':;'/,.. "'e'" i~.

, ~'--

,:.c-~"_;;"" ..

-'"----.~ '-:t"1;:" --...14.

._~ ...... .;..:...- ... ,_ .. _-'"

,"~ .

. '~.

.. ... '... ... , ..

1:'

'~r~

'~- ,

.. / :/-_'." ; .

., ·,....,..,.r.:=-. __ ;

. " :' c,,-

'~ !', ~ '.'~ i:',

.:. .;.~.

. /' ~'.'

.1;1 .. ; ~ ~ ,.;

< ".;:',

. '

-7i:;~j

U NTREATED

TREATED FOR 30MIN.

TREATED FOR 5 MIN.

TREATED FOR 1 HOUR 10fJ ~

TREATED FOR 15 MIN.

TREATED FOR 2 HOURS

67

Fig. 5

Scanning electron micrograph of PE surface

before and after I-hr treatment in the air

corona.

.,

.--

( .1

68

Fig. 6

Decrease in pitting produced by increasing

separation of electrodes for I-hr treatment

in the air corona. Aluminum shadowed and

photographed in transmitted light.

.. "J :';".'

'. ;"-"'~.:"'." ... '; .. '.:,',',~":"/~.'~';':";/ '.

-;i;~j·'l\~·.~~>' . ~

. : . ~ .:. ":,' '-'~" •. _., :;><.'-

\,.' ,._'''-''-'._,~~'' ::·~ .. ~.:.~ .. _._ . .' .. _~~_...:-...:.-":'::,.2;~"-·.~-~_·_~,_~L::":è;'~~~'~~di~l~1tJI

'" ~·~'~~·.:···~·~·~·~·,~:ffi:-~~_ ;.; ~~;~;~ ·;·:~:~f:. :~~ç,j~}~L1j~

• :~;:~I~';:;~i:~\;I~~~~l:JBr~ 1,,"

.. '

. : ' .~ .~ ( ..

'.Yi:F~:H~jJ~R0l:~l~~ '":.! ~ ~~i? ~~ ~·2-~,:·>:~~:~~X~k~-:~.r;~

i;~f.i:~!~~fff~ ·',i .... \ .. "" '. \ 'l

:! "'" \>:1,fj.~~~.: ,,/.",:' 'I- :l'<:....j:\:~. >:", ';.~.:.:.' .. '.:,::\_.':;[.' , ;. '?' ,f' . - l'~ ::,' -l' •. ~r

". ':'?':;'/:~:?f.\~:~:f;

:;', Il ~ ... :::.~ ", ~~:~'i'

', .. :"".1'

.',"

-"l"';""';' ••

", ;, " ~

~;; ':~' ", ~ ~:... . ~';-: .

. il·

, ....

()

()

~1'.'r'I:' \' .. 1 ,,~. ';' :\1 ", '*: .•• , '."\; ,. t,' ,",'" .• , ~. ft. ,

.. '" ~" •• ' ',f, ",:' ., ,. l " . .' .. , , • ., ,~ l ' ,. " ~, .. ' \ . " .. ' " ~

.. ,,' ~ •• (~ \, l ' , f 'I ·.f

f,f," ·"C·~·' ','r ~ .. ',. • ~~". (, i \" "~"f, ' , ': ' 4 ' •• i' I",.~J f" t. • '~; . ," f fi'.'!.

,'" , •. , ,.,., f • f.'III '/,:.1,-,·' " '.' /f' " ,' .. ,{ ... r ' •

,." " , ' ") 'f' .,. , l' " . !,' 'f 'l ," .. ,. t \., .\ •• " ., ,

'i '" .' •• ' . .' ;',,' 4

..

" ... , .' ,. i Il '., •• f "', ,.,." .. '. "W , • • ~ , ~ (, .f,' • ,. '. , '". fi ,'. f' , " • 1 f', .' , , .,.. """.,' f' .,' r' ." .f, •• f

, ""," " ,,',f', ; ." ....• , .. ,";" " " 1 f 'f f ..... ' .,..f .f .. l' ' •. : - .~ ,f . f ~ ! " L. , , '« " " .' .,,, '., ," f' • '!.'r.~ .' • 1,6 " f' , . .,' , .• ~- '.-- , If ""1. 1\ ,f' 0'

f W , " ~ ,f ,', ,f_ 1 .,. f f 'I ~ .1 ,1 f f' .. ,. 'f.'. or .~ f

~., " f ~ . • , '! . , 1 ..••• ' 1 .' 'r •.• ~ "

l,· ., '.

~I

69

treatment and electrode separation shown in Figs. 5 and 6,

respectively, might be expected. However, when heated elect-

rodes Viere held near the FE sheet in the absence of an elec-

trical discharge, the polymer softened but Vias not pitted.

Also, as shown in Fig. 7, the surface treated in the nitrogen

corona appears to be unchanged when compared with the heavy

pitting produced in the air corona. Apparently, localized

pyrolysis only occurs when the corona cell contains a certain

amount of oxygene

Infrared Absorption

The infrared absorption of FE treated in air and in

nitrogen is comparcd with that of an untreated control in Figs.

8 and 9.

As shown in Fig. 8, there are quite marked peaks at

about 1720 cm-l and 1640 cm-l after treatment of the FE film

in, the air corona for 15 min. These peru~s increase in size

when the time of treatment is extended to one hour. Absorp-

-1 1" -1 tions at 1720 cm and 1040 cm may be due to oxidation of

FE to give the -C=O group and -C=C- double bond, respectively.

In contrast to the effect of the treatment in air,

the spectrum of the surface subjected to the nitrogen corona

was indistinguishable from that of the untreated film. Similar

but less detailed experiments showed that PE corona-treated in

70

Fig. 7

Surface change of PE before and after I-hr

treatment in corona discharges of air and

nitrogen. Aluminum shadowed and photographed

in transmitted light.

, ~ .

ç,

:;.,', .

,':,

,., :;.

--------_._----;----.-.

•••• ' .. . '.

,,'"

.,. z z -

-~ ~

~ z -Q w

~ ~ ....

l "

2"1

/ -.

-----, -----.--_._--:--

--,

"., 1 •. 1::.:~ ,;/.:·~·':·~~·~'A~~fl~~!:\~:-t~{.~f~

~: :;"{ ..... -. ~:c;,,:~_ ... "I\:a:.;;.

·("~~~~,;~~:~:· .. :,:\;~éD~i:6p~!/j·,i.~ .':"'::'.;< :.:. ~,~., ,~.,,);., ':~ '': ~,~j::.?~),'. ,~~' ~~; ;.~::~~;

':/~ I;_~~!.

:;~?:;,:':;,:D~"::;4::;~:;;;',:~,U::t};; ~~. '::>:(.,~' ,(?,~< ~~. _'.r~~ .. \'.~, .:';'."

::~?~;~>,~:j:?;,~;~;,j~:<;;.Z,~;:!i :, ~',.~,:;.r:~~';,~ ':,;~~>:~': ;":J:'",'::'

'::;;~:\\'":} ,)~~~

l'

. " ~ : ;-' , .

" . :~ .,(. ,.' 'f,

,<

tT) '(.\.

-0::: o u..

C'4

Z Z o w ~ W 0::: .... ~ J: -0::: 0 u.. 0:::

« z 0 w ~ W 0::: ....

o w ~ W 0::: .... Z ::J

~I

•

71

Fig. 8

Infrared absorption in the region 1500-1800 -1 cm

of PE films of untreated control and after 15-min

and 1-hr treatment in corona discharges of air

and nitrogen •

: ,

. J J

. .~ . I~

. J

. l .,

\,....

~ ° -'"'-1 U Z

~ -:JE (J)

.~. h-

..

·A-IOO/. TRANSMITTANÇE

1800 . 1700 1600 . . FREQUEN,CY (cm"")

r---.J..-... _ ... ~ .......... a_""""",, __ - • ' • h, ;'

1500

72

Fig. 9

Transmittancy of infrared in the region 800-

1200 cm-lof PE films of untreated control and

aîter 15-min and I-hr treatment in corona dis­

charges of air and nitrogen.

, ,

~ ,j

. . , . , , .

LaJ (J 2:

~ .... -:E (/) z ct et:

',.1-

" l '

:1 ",',' . l

..

1200

. .

4 -.10 °/0 TRANSMITTANCE • 0, •

•

1 Hr. IN AIR

o. ••

IS MIN. IN AIR

1 Hr. IN Nz

15 MIN. IN Na

.' .'

. .

1100 1000 .900 FREQUENCY (cm-'l

~-----.._---- ...... _ .. _ ....... _ ......... --_ .... --_ .. _ ... ~ ..... . 1ft' . ft _1 • • ~ ..... --.I_ •.•.. ..••.•• ,_., .... '

.Â

80

73

hydrogen gave an infrared spectrum identical to that of the

untreated control.

Fig. 9 shows that no new peaks in the region 850 -

1200 cm-l are produced by corona treatment in.air or nitrogen.

However, in the case of FE treated in air, the peaks are blunt­

ed due to poor transmittancy \'lhile the peaks of the film treated

in the nitrogen corona are almost as sharp as those in the spec­

trum of the untreated film. Treatment of FE in a corona dis-

charge of oxygen or oxygen-containing gases is known to produce

oxalic acid. 2 ,3,9,lO) Possibly, a layer of oxalic acid crystals

on the surface of the sheet may considerably enhance the scatt- '.

ering of infrared radiation by the film.

Change of Bbnding, o\'lettability and Water Contact Angle Vlith

Time of Treatment

As shown in Fig. 10, the bonding of the FE surface

treated in the corona discharge of oxygen or oxygen-containing

gases reached a maximum in less than 5 seconds and longer times

of treatment did not change bond strength significantly. The

nitrogen corona also enhanced tne bond strength in the first

5 seconds. Further treatment, hOViever, caused bonding to decre-

ase to a minimum after one minute, after Vlhich it increased

again until the bond produced exceeded the tensile strength

of the PE strip itself. Corona treatment in hydrogen or argon

74

Fig. 10

Variation of bond strength of FE surfaces treated

in corona discharges of various gases with time

of treatment.

-, •... ~~.: .•. ~ ''',.r. : ''1'.

. f. ;'.

'f (, '," ..

.... " ."

".\

_'.J . "'

.. ',.:

"o'

"

., .. '

.,' -'!. ' ',f"

.. '~.' . C -, .

"",:

.:

•• • ",,f

'~, ..

.,\ ' .

. N ':.0' ·';0:

''-''.'i'G . ·0'

, ... -"a:' . -<[

'<, ... Q. ·,.;i

'.'

" .

. ,\': .~. <.,

"

.. ,:: .'

"

.'"

'",-. '.;

o

.1, . ,

"

'.\, ".,' .'. ':',.

.~ ..

00 :,i

. ,

' ....

..

"

.... ' ...... : . ~ "::-. ~

:.,. l'"

,!.:"

'" .~ . , .,' )~,;

:':;' ,.' . .. : ......

....

. , . , "',:

1./, '.

, ... cg ...... ,: .... ,.N "CD '" ...... ,'. " , '" ,." -ta~~/~".f H19N3YlS

'.

:.' )it.2<t ON.oa

.. , .

...

... : .~

"

""-...

.~

o 'L&J :1.'. -~

...... . ~.".~ ... ~".'

'. '-4-.'

,".; .~~~'"

O::~':', " .- 1

:,\'

--..,;.,.-. -. , ... ---.. ...:: ... ~:..

.,

. ~ '.

75

did not enllance the bond strength noticeably.

Fig. 11 shows that the corona discharge in oxygen

or oxygen-containing sases increased the wetting tension con­

siderably in the first few seconds, after which the wetting

tension shO\"red little further change. The increase with time

was more gradual at first for PE treated in the nitrogen corona

but for times of treatment longer tllan 3 min, the wetting ten­

sion Vias more than 58 dynes/cm and this \'las too high to be

measured by the AST~ test. The corona discharge in argon

changes the wetting tension only slightly at first but substan­

tial increases are found after 5 min treatment. The wetting

tension of PE is not changed by treatment in the hydrogen

corona.

The chanGe of water contact angle with time of treat­

ü'lent corl'elated \~lell with the i'retting tension, as shown in Fig.

12. The water contact angle decreased rapidly in the first few

seconds of treatment in oxygen or oxygen-containing gases, but

re:L,ained almost constant on further tl~eatment. The nitrogen

corona caused the \'later contact angle to decrease more gradually

with time of treatment. The \'fater contact angle of the PE

sheot was not changed markedly with time of treatment in hydro­

gen. However, prolonged corona application in argon did produce

a fairly substantial decrease in the water contact angle.

III Figs. 10, Il and 12, the data points for corona

76

Fig. Il

Change of \'letting tension of PE surfaces treated

in corona discharges of various gases with time

of treatment.

e. Il

N

8 .. N o .. a::

<t .---.. 1 •

l,'"

.,'

o 0 II') q-

('~8S/8UAp) NOISN31 ~N1113M

c

N X

o o o

,.... . U CD CI)

0 .... 1 Oz -LaJ

':E

~ LaJ a::

~I 0 1

w.1 :E

0 .... -

1

1 :

1 i

0 1 ft)

,',1

, 1.

1

77

Fig. 12

Water contact angle change of PE surfaces

treated in corona discharges of various

gases with time of treatment.

~I

., ",-,,'1 '." \\,t,,··

, , . ~

.. .. , ..... '

., -,lI -

.. ... , .\. ~:

. " :f~

.~ ., 1 ~

.' 1 J '.

:,";

~. .

1. J , ,.

D

. .

':'"

".'.

, '

•

"".' ).'

. ,.,:,/j

,'f,.

"'.0'

0' f-

t .8 .. D.ep l ']',9NV

-.'.:.-.

00

',:,:

........ '­........ ~.~. :J.~

o 00

o

o

.'

. . . . . .

l~Y.L,NOO· H31YM

"'.

..... ,

,:

~.

' .. , ' ..

:;~.~~ .. ,~ .

. "'--..--:

•

... ~ ':. ,; . -J" l1li'--:"'"'

•

•

. "

i i 1 t t

".'. ,,-.

. ;:: 1

·1

: ~ '- .

\ .

78

treatment in air, oxygen or carbon dioxide are not differentj.­

ated. The reason for this i8 that the effects produced by these

gases Viere found to be quantitatively silllilar as shown by the

more detailed results in 'rables 2, 3 and 4. Apparently, gases

which contain a certain amount of oxygen always oxidize FE

surfaces in a corona discharge and thus produce similar effects.

Interestingly, corona treatment in nitrogen-containing water

vapor at 31 % relative humidity produced the same results as

the o:{ygen-containing gases. Also, as discovered in an ear1ier

investigation, 11) drying the gas before corona treatment l:'lade

no difference in the case of treatment in air.

79

TABLE 2

Bond Strengths (kg/cm2 ) of PE Surfaces Treated in the Corona

Discharge of Oxygen or Oxygen-Containing Gases

Time of Treatment °2 Air CO2 N2 + 31% H20

Control 1.5 1.5 1.5 1.5

2 sec 8.7 11.6 13.3

5 sec 12.9 13.3 15.0 12.3

15 sec 12.5 13.6 14.1

30 sec 12.7 12.2 15.3

1 min 11.4 13.1 16.1 18.0

5 rJin 14.9 13.4 13.1 16.5

15 min 14.9 12.5 10.5

80

TABLE 3

',Vetting Tensions (dynes/cm) of FE Surfaces Treated in the Corona

Discharge of Oxygen or Oxygen-Ccntaining Gases

Time of Treatment °2 Air CO2 N2 + 31% H20

Control 31 31 31 31

2 sec 37 39 39

5 sec 39 43 45 38

15 sec 41 44 43

30 sec 42 43 44

1 min ~-2 40 45 46

5 iJin 48 LI-3 45 46

15 min 44 44 46

81

TABLE 4

Water Contact Angles (0) of PE Surfaces Treated in the Corona

Discharge of Oxygen or Oxygen-Containing Gases

Time of Treatment °2 Air CO2 N2 + 31% H20

Control 95 95 95 95

2 sec 60 58 64

5 sec 62 56 62 60

15 sec 54 49 57

30 sec 51 50 56

1 11in 50 51 56 61

5 min 53 53 50 45

15 min 47 48 43

82

DISCUSSION

For aIl the measurements made, the wetting tension

of PE surface measured by the ASTH test correlated reasonably

weIl 1:'/ith v/ater contact angle as shoVln in Fig. 13. This indi-

cated that either the uetting tension or the water contact

angle may be used as a relative measure of the surface energy,

and thus may be correlated v/ith other properties such as bond-

ing behavior. For corona treated surfaces, the water contact

angle may be hlore useful because it has a wider range of appli-

cation than the ASTM test which is restricted to 30 - 58 dynes-

lem.

Atternpts were made to measure surface energy by

Zisman's procedure,12) and this method i'lorked weIl on untreated

PE sheet. But the procedure shov/ed irregular results 'l'vith

corona treated PE, perhaps because of chemical interaction

between some of the liquids used, and the chemically activated

PE surface.

oxygen or

(1)

(2)

(3)

(4)

(5)

Treatment of PE surface in the corona discharge of

air produced the following expected results:

Surface oxidation. l ,2)

Carbon-carbon double bond formation. l )

Surface Pittin~.3,4)

Increase of surface el1crgy.13)

Increase of bond strength. 5 ,6)

83

Fig. 13

i1etting tension vs. v:ater contact angle of

PE surfaces after treatment in corona dis­

charges of various gases.

':. ' . ""If: '/1:> . ~'. ;:

• ~,1 '.'

!' :~ 'ï,

1 1

'.'

,'.

.,,\

-'

" . , "':'

.. ,.,

"

.. ) , .. \

", ,

,~',. ~ , ;

~'. \ '

",

.:~ .

;'·;?:·~)~~i?i~;~y:j";;~;Ji'0':·:f.)t· . " ~: ": :': .. _ ..... "

'.' . ~.: "

. '.'

:..: .... . ," , .!

':;

,',

, ... : ~ "

c,z' .

'; '.'-

,', '. ,'.' ..

0.'0,'

• ••

, j,

\ ~ .. '., . ~.

" ...

!: '

,,:' ,.'.

',,;

'.:

o ~'.,

o.

'\ .:"

" ; l' "

.~ ,

, ','.'

., .~ .

/, . ',! . .':'

:/,' .'.,

,\ \

'l'

.. ' .:

;.,

.. '~.

., .... ':,',

"',' , ...•. ,.

~'

1, "

", :.\,t '\~

"':.'

,~

"',:

,~

o ",. "

........ -

'-

(W~8U"P) NOISN3.L" SNI.L.L]M'

"oJ

i r .. " "o~o' l '

>'"

.' '

'"

-----~:::-----:-~-:'''.: ... , ..

.' .,

, .

84

Only a very short time (less than 5 seconds) Vias required for

nearly r.1aximUlil offect under the experimental conditions used

and longer treatment did not produce much further change,

except in the pitting of the surface. Bailey4) has also found

pitting on over-treatment of polymers in the corona discharge

and states that this can produce a decrease in the adhesion

behavior of the surface.

It is interesting to note that free oxygen in the

discharge was not necessary to produce sorne of the effects

listed above. Similar increases in surface energy and bonding

'\'lere obtained by treatment of FE sheet in dry carbon dioxide

or nitrogen with 31 % moisture. Apparently, mole~ules con-

taining oxygen \'lere rapidly broken down in the corona discharge

to produce the active species. It will be interesting to

confirm this by infrarea and microscopie examination of surfaces

treated in carbon dioxide or inert gases containing water vapor

but no free oxygen.

The results with the hydrogen corona provide a direct

ccntrast to the effects produced with the oxygen-containing

gases. Treatment in hydrogen produced no oxidation (by infrared

test), no enhanced bonding and no change in Vletting tension

or ITater contact angle.

From the results of the experililents with oxygen and

hydrogen, it is reasonable to assume that enhanced adhesion of

85

PE is associated with oxidation of the surface and an increase

in the surface energy. A similar explanation has been given

by a number of authors. 14- 20 ) However, the basic cause of the

increased bonding has not yet been elucidated. It has been

suggested that increased dipole-dipole attraction between the

oxidized surfaces could cause increased adhesion. 13- 15 ,17)

The formation of free radicals on the surface by the electrical

discharge has also been cited as a cause of the increased bond­

ing5) but no clear demonstration of this effect has been pub­

lished.

A significant result in the present investigation

is that the behavior observed wit,h argon and nitrogen did not

fit into the above pattern obtained ~ith the oxygen-containing

gases. Figs. 14 and 15 show no general correlation between

bond strength and wetting tension or water contact angle.

The PE surface treated in the argon corona for a longer time

than 5 min showed high wetting tension and water contact angle

but low bonding. PE treated in the nitrogen corona for l min

also displayed high wetting tension and ~ater contact angle

but low bonding. However, the highest bonding of aIl was ob­

tained by prolonged treatment of the PE surface in the nitrogen

corona, and this treatment produced no oxidation or carbon­

carbon double bond formation.

In a recent series of papers, Schonhorn and Hansen21- 24 )

86

Fig. 14

Bond strength vs. wetting tensioL of PE

surfaces after treatment in corona discharges

of various gases.

.'

.\:. 1 :

,\.'

,'-

'. ',:

.-

'\

""...

t\I

E u " '" .:JI! --...-:t: 1-e!) Z I.LI

~ Q Z 0 CD

, -;

:

"

T:

15

"

l. ''-1. , \.

:'j' . ,~·ll'· l' , , .. ':,

Il, ' • '.

. "f .

10 .....

i.'

"

• J •

'.,

<--.,

, ' , '

5

L----.,---------~~ .... --' A

.-...... -- .. " ... ""-.".

a

o----~--~----~------~~------~~~ .t~;· 30 , 40 . 50 60

WETTING, TENSION: ( dyne/cm) " - .~. ~ ;', .' , .': ""1:

.:.;' .', . '. . . . . ----""'"--.-.'_ .. ~~ .. ---.. _-.~._ .. :-" ....... -""c / ,

'. " .. :: , . ,1 -" :

" ,

, . · .'f • l: ':1

· . ~.

'.:

.~

1 !"

\

l >. , .' ,

87

Fig. 15

Bond strength vs. water contact angle of

PE surfaces after treatment in corona discharges

of various g&ses.

ç.: ",:, .... :. '~~','" ~ Il'''

" ,

,',

':,

• ",C •• ', ";:'

'.': .... -r' •• '

-\ .. " " ,

. ~:

, " ~ .

>" •

;. o. ',' , , 'r ," ,-.. '. ,,' , "

.. ' 1",. .' "

~ "', .,' };.

::";.1. ,"

'.j"', "

88

have described extensive studies on the effects of activated

gases on polymer surfaces. Inert gases such as helium, neon,

argon, krypton and xenon, and hydrogen Viere activated by a

radio frequency coil at reduced pressure. The activated gas

was found to abstract hydrogen atoms from the polymer surface

and produced radicals, which eventually formed crosslinks and

increased cohesive strength in the polymer surface region.

They found no weight loss and no change in wettability, color,

tensile strength, elongation or conductivity. But electron

spin resonance and infrared spectroscopy show the existance of

free radicals and transethylenic unsaturation, respectively.

Schonhorn and Hansen maintained that neither produc­

tion of carbon-carbon double bonds in the surface region nor

increase in vvettability of the surface contributed to adhesive

strength. They proposed, instead, that crosslinking occurred

in the surface layer during exposure to the activated gases.

Such crosslinking enhanced the cohesive strength of the surface

region and eliminated the weak boundary layer thus giving in­

creased adhesion.

The present \'Tork also confirmed that the correlation

between wettability and bonding i'las not always positive. Corona

treatment for one minute in nitrogen and for longer times in

argon produced high wettability but virtually no bonding.

This supports the General concepts of Schonhorn and Hansen. 21- 23 )

89

However, when attempts were made to measure the extent of cross­

linking, sorne divergence from the proposaIs of these authors

Vias found. Preliminary Vlork (not to be described in detail

here) showed that treatment of a PE surface in the oxygen corona

produced a significant amount of crosslinked material. In

contrast, a one hour treatment of a PE surface in the nitrogen

corona produced no detectable crosslinking but, as shown in

Fig. 10, much shorter times of treatment gave high bond strength.

Thus it seems that in the case of the nitrogen corona, increased

bonding can occur without crosslinking. Further work is planned

to establisb this trend.

In sorne other prelil'ilinary work, it was found that if

the sample Y/as allowed to stand after the corona treatment in

nitrogen, the bonding decreased fairly quickly without a corres­

ponding change in vlettability or water contact angle. This

suggests the possibility of free radical decay but further work

will be required here also.

In conclusion it must be admitted that the cause of

the effect on bonding of a PE surface treated in a corona dis­

charge is not certain. One possibility, not yet studied in

detail, is that the increase in bonding may be related to the

subsequent chemical instability of the PE molecules. In other

\'lords, free radicals on the PE surface might cause high bonding

rather than crosslinY~ng of the molecules in the surface layer

e·

90

as suggested by Schonhorn and Hansen. It is planned to examine

this possibility on continuation of the investigation.

91

REFERENCES

1. K. Rossmann, J. Poly. Sei., 12, 141 (1956).

2. G.D. Cooper and H. Prober, J. Poly. SCi.,!±!I:, 397 (1960).

3. E. J. McMahon, D. E. I>1aloney, and J. R. Perkins, Trans.'

Am. Inst. Elect. Engrs., A.I.E.E., Nov. 1959, p. 654.

4. G.VI. Bailey, Norelco Reporter, 1!t No.l, Jan-Mar., 1967.

5. D.A.I. Goring, Pu1p Paper Mag. Can., 68, T-372 (1967).

6. Chapter II of this thesis.

7. loi.A. Sandell, \'Ietting of Wood Polysaccharides, Thesis,

State Univ. Coll. of Forestry, Syracuse, N.Y.

8. A.W. Adamson, Physical Chemistry of Surfaces,

Interscience Publishers, New York, 1960, p. 27-28.

9. R.F. Grossman and N.A. Beasley, J. Appl. Poly. Sei., g,

163 (1959).

10. L.R. Hougen, Nature, 188 No. 4750, 577 (1960).

Il. D.A.I. Gori~g, ta be pub1ished.

12. W.A. Zisman, Ind. Eng. Chem., 22,18 (1963).

13. R.A. Hines, Paper Presented at the l32nd Meeting of

the American Chemical Society, New York, Sept. 1957.

14. H.A. Arbit, E.E. Griesser, and VI.A. Haine, TAPPI, !±Q,

161 (1957).

15. F.J. Bockhoff, E.T. McDonel, and J.E. Rutzler, Ind.

Eng. Chem., 2Q, 904 (1958).

16. P.B. Noll and J.E. McAl1ister, Paper, Film and Foil

92

Converter, Oct. 1963, p. 46.

17. A.J.G. Allan, J. Po1y. Sei., ~, 297 (1959).

18. J.W. Swanson and J.J. Becker, TAPPI, ~ No. 5, 198 (1966).

19. N. G1ossman, TAPPI, 2Q No. 5, 224 (1967).

20. N.J. Barbarisi, Nature, 215 No. 5099, 383 (1967).

21. H. Sehonhorn and R.H. Hansen, J. App1. Po1y. Sei., 12,

1231 (1968).

22. H. Sehonhorn ~nd R.H. Hansen, J. App1. Po1y. Sei., 11,

1461 (1967).

23. R.H. Hansen and H. Schonhorn, Po1y. Letters, !i, 203 (1966).

24. R.H. Hansen, J.V. Pascale, T.De Benedietis, and P.M.

Rentzepis, J. Po1y. Sei., A-3, 2205-2214 (1965).

93

CONCLUDING REI·iARKS AND SUGGESTIONS FOR FUTURE V/ORI<:

It is quite clear that the understanding of the basic

mechanism of surface modification of polymers in a corona dis­

charge is in a fairly primitive state. However, it is equally

evident that corona treatment can greatly improve ·the adhesive

properties of cellulose or other polymers. Several leads in the

present W'ork seem promising and, if followed, may elucidate

further the physical and chemical process occurring in the corona

discharge. The EiOSt immediate experiments :.ire:

(1) measurement of crosslinking by the production of gel

(2) measurement of crosslinking by changes in s'Nelling rate

(3) change in bonding, wettability or water contact angle

with time of standing

(4) identification of free radicals by electron spin reson­

ance

(5) more detailed infrared study of polyethylene treated in

the corona discharges of inert Gases or hydrogen.

Somewhat broader fields for future study include the

ap?lication of the corona discharge to

(1) surface pnlymerization

(2) coating

(3) Graft polymerization

(4) introduction of reinforcing material into a fiber web.

94

CLAIMS TO ORIGINAL RESEARCH

(1) A corona discharge system for use with pure gases was

designed and built.

(2)

(a)

It was shown that:

The bond strength between.cellulose and several

synthetic polymers \'las enhanced sj,gnificantly after

corona treatment in oxygen, air or nitrogen. Surface

roughness increased after treatment in oxygen but the

surface Vias unchanged after treatment in nitrogen.

(b) Polyethylene treated in a hydrogen corona gave no

change in bonding or surface energy.

(c) Treatment of polyethylene in an argon corona enhanced

surface energy but had little effect on bonding.

(d) Treatment of polyethylene in a nitrogen corona

increased the surface energy and gave strong bonding

with no oxidation of the surface.

(e) Corona treatment of polyethylene in oxygen-containing

gases gave the same results as treatment in oxygen,

increasing surface energy, bonding and surfice

roughness and producing -C=O groups on the surface.