The Effect of Polymer Surface Morphology on Adhesion and Adhesive

11 The Effect of Polymer Surface Morphology on Adhesion and Adhesive Joint Strengths H A R O L D S C H O N H O R N and F R A N K W. R Y A N Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch011

Bell Telephone Laboratories, Inc., Murray Hill, N . J. 07974

The morphological

character of the surface region of poly-

ethylene has been considered with respect to adhesion and adhesive joint strength. high

By melting

energy surface—e.g.,

gold or

polyethylene

onto a

aluminum—extensive

nucleation and the formation of a transcrystalline region in the polymer occurs. Dissolution peeling the

metal from the

region of the polymer intact. amenable to conventional

of the metal rather than

polymer

leaves the

surface

The polymer sheet is

structural adhesive bonding.

parently the weak boundary

layer on polyethylene

now Apis a

consequence of the morphology in the surface region and is influenced

by the method of

preparation.

> " p h e quest for surface treatments of p o l y m e r s w i t h respect to a d h e s i v e A

b o n d i n g has p r o v e n f r u i t f u l i n a p r a c t i c a l sense b u t , u n t i l r e c e n t l y ,

the u n d e r l y i n g p r i n c i p l e s h a v e b e e n s o m e w h a t e l u s i v e ( 5 , 16).

Appar-

ently, w h a t is of p r i m e i m p o r t a n c e i n p r e p a r i n g strong a d h e s i v e joints w i t h p o l y m e r s e x h i b i t i n g w e a k b o u n d a r y l a y e r b e h a v i o r is the n a t u r e of t h e surface m o r p h o l o g y a n d the effect a n a p p l i e d stress has o n i t . I n this p a p e r w e s h a l l demonstrate, u s i n g p o l y e t h y l e n e , that the w e a k b o u n d a r y l a y e r b e h a v i o r n o r m a l l y present i n m a n y m e l t c r y s t a l l i z e d p o l y m e r s is a f u n c t i o n of the surface r e g i o n m o r p h o l o g y of the p o l y m e r a n d is therefore d e p e n d e n t o n h o w the p o l y m e r sheet is p r e p a r e d . I n d e e d , b y c o n t r o l l i n g t h e n u c l e a t i o n a n d subsequent c r y s t a l l i z a t i o n , w e w i l l s h o w that a surface r e g i o n m a y b e g e n e r a t e d , i n c r y s t a l l i z a b l e p o l y m e r s , that is a m e n a b l e to c o n v e n t i o n a l a d h e s i v e b o n d i n g . C o n c o m i t a n t w i t h this c h a n g e i n the m o r p h o l o g y

of the surface

r e g i o n , there is a m a r k e d change i n the w e t t a b i l i t y e v e n t h o u g h the c h e m i c a l c o n s t i t u t i o n is u n c h a n g e d . T h i s c h a n g e i n w e t t a b i l i t y has b e e n 140 In Interaction of Liquids at Solid Substrates; Alexander, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

11.

SCHONHORN

AND

RYAN

Polymer Surface Morphology

141

r e l a t e d to a change i n the surface c r y s t a l l i n i t y , b e i n g s t r o n g l y d e p e n d e n t u p o n the n a t u r e of the substrate u s e d to p r e p a r e the p o l y m e r . Experimental

Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch011

M a t e r i a l s . M a r l e x 5003 p o l y e t h y l e n e ( P E ) w a s s u p p l i e d b y Phillips Petroleum C o m p a n y , Bartlesville, Oklahoma.

the

E p o x y resin D E R 3 3 2 L C ( D o w C h e m i c a l Co., M i d l a n d , M i c h i g a n ) is a d i g l y c i d y l ether of b i s p h e n o l A , h a v i n g a n e p o x y e q u i v a l e n t w e i g h t of 179 m a x i m u m ( t h e p u r e m a t e r i a l w o u l d h a v e a n e p o x y e q u i v a l e n t w e i g h t of 1 7 0 ) , a t o t a l c h l o r i d e content less t h a n 0 . 1 % b y w e i g h t , a n d a viscosity of 6400 centipoises m a x i m u m at 25 ° C . D i e t h y l a m i n o p r o p y l a m i n e ( M i l l e r - S t e p h e n s o n C h e m i c a l C o . , Inc., Philadelphia) was distilled under nitrogen through a 6-inch Vigreux c o l u m n a n d the first f r a c t i o n d i s c a r d e d . T h e p r o d u c t d i s t i l l i n g at 68 ° C . a n d 2 6 - m m . pressure w a s stored i n the d a r k i n t i g h t l y s t o p p e r e d glass containers p r i o r to use. T h e e p o x y adhesive consisted of 100 parts b y w e i g h t of the a b o v e r e s i n a n d seven parts b y w e i g h t of the d i e t h y l a m i n o p r o p y l a m i n e , t h o r oughly mixed and used immediately. T h e m e t a l tensile-shear a d h e r e n d s w e r e of 2 0 2 4 - T 3 a l u m i n u m ( A l u m i n u m C o . of A m e r i c a ) . T h e i r d i m e n s i o n s w e r e 5 b y 1 b y 1/16 i n c h . T h e surface of the a l u m i n u m w a s p r e p a r e d b y first v a p o r - d e g r e a s i n g i n t r i c h l o r o e t h y l e n e a n d t h e n e t c h i n g for seven m i n u t e s at 6 5 ° C . i n t h e following solution: Sodium dichromate ( N a C r 0 · 2 H 0 ) Water Sulfuric acid ( 9 5 % ) 2

2

7

2

1 part by weight 30 parts by weight 10 parts by weight

A f t e r e t c h i n g , the specimens w e r e r i n s e d for five m i n u t e s i n r u n n i n g t a p w a t e r a n d for one m i n u t e i n r u n n i n g d i s t i l l e d w a t e r , a n d t h e n d r i e d i n a f o r c e d a i r o v e n at 60 ° C . S p e c i m e n s w e r e stored i n desiccators o v e r A s c a r i t e a n d r e m o v e d just p r i o r to use. F i l m P r e p a r a t i o n . P o l y m e r films (10 m i l t h i c k ) w e r e p r e p a r e d b y c o m p r e s s i o n m o l d i n g for one h a l f h o u r at 175 ° C . b e t w e e n e v a p o r a t e d g o l d films o n glass m i c r o s c o p e slides or sheets of 0.7 m i l a l u m i n u m f o i l ( R e y n o l d s A l u m i n u m W r a p ) w h i c h h a d b e e n c h e m i c a l l y e t c h e d as a b o v e . A f t e r m o l d i n g , to a v o i d d a m a g i n g the surface r e g i o n of the p o l y m e r b y p e e l i n g f r o m the a l u m i n u m , the f o i l w a s d i s s o l v e d b y i m m e r s i o n i n a s o d i u m h y d r o x i d e s o l u t i o n ( f r o m 1 0 % to c o n c e n t r a t e d ) w h i c h w a s s u r r o u n d e d b y a n ice b a t h to a v o i d o v e r h e a t i n g of the specimens. T h e r e s i d u a l b l a c k o x i d e ( t r a c e m e t a l c o n t a m i n a n t s ) was r e m o v e d b y r i n s i n g i n c o n c e n t r a t e d h y d r o c h l o r i c a c i d . N e i t h e r the h y d r o c h l o r i c a c i d n o r the s o d i u m h y d r o x i d e treatment affects the surface r e g i o n of the p o l y e t h y l e n e w i t h respect to w e t t a b i l i t y ( y m e a s u r e d b e f o r e a n d after exposure is u n c h a n g e d ) a n d joint strengths. N o r e s i d u a l a l u m i n u m or m e t a l c o n t a m i n a n t s w e r e d e t e c t e d u s i n g e l e c t r o n m i c r o p r o b e t e c h n i q u e s or a t o m i c a b s o r p t i o n spectroscopy ( A A S ) . c

In Interaction of Liquids at Solid Substrates; Alexander, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


142

INTERACTION

OF

LIQUIDS A T

SOLID

SUBSTRATES

T h e g o l d films w e r e p r e p a r e d b y v a p o r d e p o s i t i n g a b o u t 3 0 0 0 A . of g o l d o n m i c r o s c o p e slides ( 3 b y 1 b y 1 / 1 6 i n c h ) . T h e thickness of t h e g o l d film is n o t c r u c i a l p r o v i d e d that a c o n t i n u o u s film is generated. Sections of p o l y e t h y l e n e film w e r e p l a c e d b e t w e e n g o l d - c o a t e d m i c r o scope slides t o f o r m s a n d w i c h e s . T h e s e s a n d w i c h e s w e r e m o l d e d u n d e r s i m i l a r c o n d i t i o n s to the a l u m i n u m composites d e s c r i b e d earlier. A f t e r c o o l i n g t h e composites, t h e g o l d w a s d i s s o l v e d either b y a m a l g a m a t i o n w i t h m e r c u r y o r b y e x p o s i n g t h e composites to a c o n c e n t r a t e d aqueous s o l u t i o n of s o d i u m c y a n i d e . A f t e r a t y p i c a l c y a n i d e treatment, a final rinse i n c o n c e n t r a t e d h y d r o c h l o r i c a c i d r e m o v e d a n y trace o f surface residue—i.e., carbonates. N o r e s i d u e of g o l d o n t h e p o l y m e r w a s d e t e c t e d i n either case u s i n g electron m i c r o p r o b e t e c h n i q u e s . A t o m i c a b s o r p t i o n spectroscopy ( A A S ) i n d i c a t e s that f r o m zero to 1 / 1 0 0 of a n e q u i v a l e n t m o n o l a y e r of g o l d r e m a i n s after t h e r e m o v a l treatments. S m a l l flecks of g o l d m a y h a v e been e n c a p s u l a t e d o n c o o l i n g t h e p o l y m e r . P o o r r e p r o d u c i b i l i t y o f t h e A A S results i n d i c a t e s this. I f t h e r e a l surface area w e r e greater t h a n t h e g e o m e t r i c area, w h i c h is l i k e l y , t h e r e s i d u e w o u l d b e f a r less t h a n t h e e s t i m a t e d 1 / 1 0 0 of a n e q u i v a l e n t m o n o l a y e r . N e i t h e r aqueous c y a n i d e s o l u t i o n n o r m e r c u r y affects t h e w e t t a b i l i t y o f polyethylene. T w o other substrates, M y l a r a n d p o l y t e t r a f l u o r o e t h y l e n e s e p a r a t e d easily f r o m t h e p o l y e t h y l e n e u p o n c o o l i n g .

(PTFE)

A T R i n f r a r e d t e c h n i q u e s u s i n g K R S - 5 crystals a n d t r a n s m i s s i o n i n f r a r e d s h o w e d n o e v i d e n c e of o x i d a t i o n after d i s s o l u t i o n of t h e a l u m i n u m o r g o l d . N o e v i d e n c e of a c h a n g e i n t h e surface c o n s t i t u t i o n of polyethylene was observed using similar infrared techniques. Thermal History. T h e p o l y e t h y l e n e w a s m o l d e d at 1 7 5 ° C . f o r v a r y i n g lengths of t i m e , t h e n c o o l e d b y c i r c u l a t i n g c o l d w a t e r t h r o u g h t h e press platens. T h e rate of c o o l i n g h a d n o a p p a r e n t effect o n either t h e m e c h a n i c a l s t r e n g t h of t h e surface r e g i o n o r t h e w e t t a b i l i t y . Density. T h e b u l k d e n s i t y of t h e p o l y m e r films before a n d after m o l d i n g w e r e m e a s u r e d i n a d e n s i t y g r a d i e n t c o l u m n at 23 ° C . A l l t h e p o l y e t h y l e n e samples h a d t h e same d e n s i t y of 0.955 g r a m / c c . Adhesive Joints. F o r t h e m e a s u r e m e n t of tensile-shear strengths, s t a n d a r d c o m p o s i t e test pieces c o n s i s t i n g of a l u m i n u m - e p o x y a d h e s i v e p o l y e t h y l e n e film-epoxy a d h e s i v e - a l u m i n u m w e r e p r e p a r e d f o r b o n d i n g i n a s p e c i a l d e v i c e d e s i g n e d to m a i n t a i n a h a l f - i n c h o v e r l a p . T h e t h i c k ness of t h e e p o x y a d h e s i v e w a s m a i n t a i n e d constant b y i n s e r t i o n of a p i e c e of 0.003-inch-diameter g o l d w i r e i n e a c h g l u e l i n e b e t w e e n t h e a l u m i n u m a n d t h e p o l y e t h y l e n e . C l e a n gloves a n d tweezers w e r e u s e d i n a l l specimen preparations to a v o i d possible contamination. B o n d i n g w a s a c c o m p l i s h e d , at a p p r o x i m a t e l y 20 p.s.i. pressure, b y p l a c i n g w e i g h t e d stacks of composites i n f o r c e d a i r ovens at specified temperatures f o r 16 hours. T h e b o n d e d specimens w e r e tested i n tensile shear i n a c c o r d a n c e w i t h A S T M D 1 0 0 2 - 6 4 , except that t h e j a w separation rate w a s 0.1 i n c h per minute. Wettability. A d e s c r i p t i o n of the contact angle g o n i o m e t e r a n d the scheme u s e d i n d e t e r m i n i n g t h e c r i t i c a l surface t e n s i o n of w e t t i n g ( y ) are d e s c r i b e d elsewhere (15, 18). c


11.

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143


Results and Discussion T h e tensile shear strengths of joints p r e p a r e d u s i n g p o l y e t h y l e n e w h i c h h a d b e e n m o l d e d at 175 ° C . for v a r y i n g lengths of t i m e are s h o w n i n F i g u r e 1. P r e p a r a t i o n of t h e joints is c o n f i n e d to temperatures b e l o w the m e l t i n g p o i n t of the p o l y e t h y l e n e . It is i m p o r t a n t to note that i f t h e p o l y m e r is p e e l e d or r e m o v e d f r o m the m e t a l other t h a n b y d i s s o l u t i o n of the m e t a l , the resultant joint strengths are c o m p a r a b l e to those o b t a i n e d f o r films generated against b o t h P T F E a n d M y l a r ( b o t t o m l i n e , Figure 1). Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch011

3500

-

—

— —

•

3000

CO 2500 ι i2 2000

QC (/>

jLV >

SUBSTRATES

W e t t a b i l i t y of

ytiv /yLv>

V

d

d

Liquid

dynes/cm,

dynes/cm,

l. /(dynes/cm.)

Water Glycerol Formamide a-Bromonaphthalene

72.8 63.4 58.2 44.6

21.8 37.0 39.5 44.6

0.0641 0.0959 0.1080 0.1497

generated

against g o l d , P T F E ,

11/2

a n d the single c r y s t a l aggregates are

i l l u s t r a t e d i n T a b l e I. T h e w e t t a b i l i t y of the a l u m i n u m - g e n e r a t e d surface is essentially that of the single c r y s t a l aggregate. A d h e s i v e J o i n t S t r e n g t h . G e n e r a l l y , to f a c i l i t a t e r e m o v a l f r o m m o l d s or substrate surfaces, c r y s t a l l i z a b l e p o l y m e r s

have been prepared i n

contact w i t h l o w energy surfaces—i.e., P T F E , M y l a r — m o l d release agents o r h i g h energy surfaces—i.e., metals, m e t a l o x i d e s — f o r

short times at

l o w temperatures—i.e., just a b o v e the m e l t i n g p o i n t of the p o l y m e r . T h i s p r o c e d u r e g e n e r a l l y results i n the f o r m a t i o n of c o n s i d e r a b l e i n t e r f a c i a l v o i d s a n d a s m a l l r e a l area of contact b e t w e e n the p o l y m e r m e l t a n d t h e n u c l e a t i n g phase. T h e rate of w e t t i n g has b e e n s h o w n to b e p r o p o r t i o n a l to t h e surface tension of the p o l y m e r m e l t ( 7 L V ) a n d i n v e r s e l y p r o p o r t i o n a l to t h e m e l t viscosity (η)

(15).

S i n c e the m e l t v i s c o s i t y varies s t r o n g l y w i t h t e m p e r a

ture, to p r e c l u d e i n t e r f a c i a l v o i d s at l o w temperatures, it is i m p o r t a n t t o a l l o w for sufficient t i m e to i n s u r e extensive i n t e r f a c i a l contact

between

the p o l y m e r m e l t a n d the adjacent substrate. F i g u r e 2a illustrates a p o o r l y w e t t e d substrate. A l t h o u g h the t h e r m o d y n a m i c r e q u i r e m e n t s for s p r e a d i n g are fulfilled—i.e., y

s v

^ ?SL + 7LV—

s p r e a d i n g m a y not take p l a c e because of the k i n e t i c r e q u i r e m e n t s

(15).

E n h a n c e m e n t of w e t t i n g m a y b e a c c o m p l i s h e d b y e m p l o y i n g h i g h e r t e m peratures o r l o n g e r times, o r b o t h i f c o n v e n i e n t , b e a r i n g i n m i n d t h a t d e g r a d a t i o n of the p o l y m e r is to b e a v o i d e d .

S i n c e η decreases q u i t e

d r a s t i c a l l y w i t h t e m p e r a t u r e , r e l a t i v e l y short times at e l e v a t e d t e m p e r a tures are r e q u i r e d to a c h i e v e extensive i n t e r f a c i a l contact b e t w e e n

the

p o l y m e r m e l t a n d the substrate ( F i g u r e 2 b ) . S i n c e w e c a n f o r m s t r o n g a d h e s i v e joints b y m e l t i n g onto a h i g h energy surface w e c a n i n q u i r e w h e t h e r the surface generated at the h i g h e n e r g y s o l i d - p o l y m e r m e l t interface is a m e n a b l e to adhesive w h e n the m e t a l is r e m o v e d .

bonding

A s s h o w n i n F i g u r e 1 i t is i m p o r t a n t to

r e m o v e the m e t a l b y d i s s o l u t i o n r a t h e r t h a n b y p e e l i n g . P e e l i n g removes t h e surface r e g i o n of interest. T h i s c a n b e seen b y e x a m i n i n g t h e b o n d a b i l i t y of the d a m a g e d p o l y m e r a n d f o i l surfaces after p e e l i n g t h e f o i l


11.

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145


Polyethylene at 2 0 ° C . Nucleated


Single Crystal Aggregate

Gold

Teflon

Θ, deg.

Cos θ

A deg.

Cos θ

0, deg.

Cos6

93 67 55 Spreads

-0.052 0.391 0.574 1.000

94 79 77 35

-0.070 0.191 0.225 0.818

84 53 41 Spreads

0.105 .602 .755 1.000

f r o m the p o l y m e r . I n b o t h cases the joint strengths are l o w ( b o t t o m l i n e , Figure 1).

W h e n the f o i l is p e e l e d , f a i l u r e occurs i n the surface r e g i o n

of t h e p o l y m e r e x p o s i n g t w o n e w surfaces w h i c h are not a m e n a b l e to adhesive bonding.

TRAPPED AIR (a)

NOTE LOW REAL AREA OF INTERFACIAL CONTACT

(b)

NOTE LACK OF VOIDS FROM TRAPPED AIR IN PORES AND CREVICES

Figure 2. (a) Poorly wetted inter face and (b) extensive intermolecular contact between liquid and solid


146

INTERACTION

O F LIQUIDS A T SOLID

SUBSTRATES

T h e residue o n a peeled a l u m i n u m foil w h i c h h a d been

molded

against p o l y e t h y l e n e at 1 7 5 ° C . for one h o u r w a s e x a m i n e d b y G e l P e r m e a t i o n C h r o m a t o g r a p h y ( G P C ) to d e t e r m i n e t h e n a t u r e of the m o l e c u l a r w e i g h t d i s t r i b u t i o n a n d to d e t e r m i n e w h e t h e r a n y f r a c t i o n a t i o n oc c u r r e d at the m e t a l - p o l y m e r m e l t interface. T h e p o l y m e r w a s d i s s o l v e d off t h e f o i l a n d e x a m i n e d . T h e M w a n d M N values for the surface r e g i o n f o r m e d at the s o l i d - l i q u i d interface w e r e 4 7 , 6 0 0 a n d 8,180, r e s p e c t i v e l y . T h e c o r r e s p o n d i n g values f o r the b u l k p o l y m e r w e r e 4 5 , 1 0 0 a n d 9,300, r e s p e c t i v e l y . A p p a r e n t l y , there is n o f r a c t i o n a t i o n t a k i n g p l a c e as a result Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch011

of t h e extensive n u c l e a t i o n a n d g r o w t h of the t r a n s c r y s t a l l i n e r e g i o n ( F i g u r e 3 ) . W h a t e v e r species are present i n the p o l y m e r are a c c o m m o d a t e d i n t o the c r y s t a l structure. H o w e v e r , w h i l e the m o l e c u l a r w e i g h t d i s t r i b u t i o n s m a y b e s i m i l a r , because of m o r p h o l o g i c a l differences t h e m e c h a n i c a l strengths o f t h e surface regions are different.

Figure 3. The transcrystalline region generated at a high energy solid-polymer melt interface after solidification. The depth of the transcrystalline region is estimated to he about 25μ S t r o n g joints w i t h e p o x y adhesives c a n b e m a d e to p o l y e t h y l e n e surfaces w h i c h h a v e b e e n o x i d i z e d b y a v a r i e t y of t e c h n i q u e s

(4. 5 ) .

T h e g e n e r a l b e l i e f has b e e n that t h e presence of p o l a r groups o n the p o l y m e r surface creates a n affinity for the p o l a r e p o x y a d h e s i v e w h i c h i m p r o v e s w e t t a b i l i t y a n d results i n a strong adhesive joint. W e t t a b i l i t y has l o n g b e e n c o n s i d e r e d to b e of p r i m a r y c o n c e r n i n b e i n g a b l e to f o r m strong a d h e s i v e joints w i t h the l o w e n e r g y c r y s t a l l i n e p o l y m e r s . H a n s e n a n d S c h o n h o r n ( 5 ) h a v e d e m o n s t r a t e d that i f the weakness i n t h e surface r e g i o n of a p o l y m e r is r e m o v e d , w i t h o u t c h a n g i n g


11.

SCHONHORN

AND

its w e t t a b i l i t y , strong joints c a n b e p r e p a r e d . H a n s e n (17)

147


RYAN

I n fact, S c h o n h o r n a n d

h a v e s h o w n that l o w e r i n g the w e t t a b i l i t y b y , for e x a m p l e ,

e x p o s i n g p o l y e t h y l e n e to

fluorine

gas a n d c r e a t i n g a p o l y t e t r a f l u o r o -

e t h y l e n e - l i k e surface is not d e t r i m e n t a l , p r o v i d e d the w e a k l a y e r has b e e n e l i m i n a t e d i n the process. w e t t i n g (y ) c

of the

fluorinated

boundary

T h e c r i t i c a l surface t e n s i o n of

surface is a b o u t 20 d y n e s / c m . , s i m i l a r to

polytetrafluoroethylene. S u r f a c e treatments ( o x i d a t i o n ) are p r o b a b l y effective because t h e y , l i k e C A S I N G ( c r o s s l i n k i n g b y a c t i v a t e d species of i n e r t gases) ( 5 , 1 6 ) , Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch011

e l i m i n a t e the w e a k b o u n d a r y l a y e r n o r m a l l y present o n the surface of polyethylene.

D u r i n g C A S I N G , the p o l y m e r m o l e c u l e s at a n d near the

surface are k n i t t e d together to f o r m a c r o s s - l i n k e d m a t r i x h a v i n g h i g h m e c h a n i c a l strength. A c r o s s - l i n k e d s k i n o n p o l y e t h y l e n e has b e e n

ob-

s e r v e d after surface treatment b y c o r o n a d i s c h a r g e a n d c h e m i c a l e t c h i n g (2, I I , 21).

T h i s p r o b a b l y accounts for the strong joints f o r m e d b y t h e

use of these t e c h n i q u e s ( F i g u r e 4 ) .

T h e m e r e presence of p o l a r groups

w o u l d not b e sufficient i f t h e y w e r e o r g a n i z e d i n a l a y e r h a v i n g l o w m e c h a n i c a l strength. T o demonstrate the presence of the w e a k b o u n d a r y l a y e r i n p o l y e t h y l e n e f o r m e d at a l o w e n e r g y surface, w e a t t e m p t e d to e l i m i n a t e i t b y several t e c h n i q u e s a n d to observe the resultant joint strengths o b t a i n e d w i t h a c o n v e n t i o n a l e p o x y a d h e s i v e i n the c o m p o s i t e : adhesive-polyethylene-epoxy

aluminum-epoxy

adhesive-aluminum. T h e bottom curve i n

F i g u r e 4 is b a s e d o n joint s t r e n g t h d a t a o b t a i n e d f r o m u n t r e a t e d p o l y e t h y l e n e film ( m o l d e d against P T F E ) .

A l l experiments w e r e p e r f o r m e d

b e l o w the m e l t i n g p o i n t of p o l y e t h y l e n e since a b o v e the m e l t i n g p o i n t it w i l l s p r e a d o n the c u r e d e p o x y a d h e s i v e surface a n d i n this process w i l l p r e c l u d e the f o r m a t i o n of the w e a k b o u n d a r y l a y e r a n d result i n r e l a t i v e l y strong joints ( 1 9 ) .

W h e n p o l y e t h y l e n e is n u c l e a t e d i n the

presence of a h i g h energy surface, a t r a n s c r y s t a l l i n e r e g i o n ( 1 , 3, 6, 12) is f o r m e d at the s o l i d - l i q u i d interface o n c o o l i n g ( F i g u r e 3 ) . A p p a r e n t l y , this results i n g e n e r a t i o n of a r e g i o n at the interface w h o s e c h a r a c t e r i s t i c strength is s i m i l a r to i f not greater t h a n t h a t of the b u l k p o l y m e r It w a s felt that the w e a k b o u n d a r y l a y e r w a s c o m p r i s e d of

(10). low

m o l e c u l a r w e i g h t p o l y m e r m o l e c u l e s w h i c h w e r e f o r c e d to the surface d u r i n g r e c r y s t a l l i z a t i o n of the m e l t (7,8)

i n contact w i t h a l o w energy

surface, so w e h o p e d to m i n i m i z e t h e i r presence b y m o l d i n g w e l l c h a r a c t e r i z e d single c r y s t a l aggregates of p o l y e t h e y l e n e (18).

However, on

m o l d i n g against a l o w energy surface, w e a k b o u n d a r y layers w e r e g e n e r a t e d a n d o n l y s l i g h t increases i n joint s t r e n g t h w e r e o b s e r v e d ( F i g u r e 4).

W e t t a b i l i t y of the m o l d e d " s i n g l e c r y s t a l aggregate" w a s essentially

the same as a n u n t r e a t e d film b u t c o n s i d e r a b l y different f r o m the freshly p r e c i p i t a t e d single c r y s t a l aggregate

(18).


INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES


3500

01

I

20

I

40

I

60

1

80

L_

100

T E M P E R A T U R E OF JOINT FORMATION ( ° C )

Figure 4. The tensile ear strength of the com posite aluminum-epox> Ahesive-polyethylene-epoxy adhesive-aluminum Ated as a function of the temperatui of the joint formation •—Untreated polyethylene (molded against PTFE) (no surface treatment) •—Marlex 5003 polyethylene crystallized from 0.04% solution in xylene at 85°C. then molded into 10 mil. sheets at 160° C. No surface treatment Δ—Polyethylene film (untreated) exposed to vapors of a boiling 1.1 hexane, heptane mixture for 5 minutes A—Polyethylene film (untreated) irradiated with a Van de Graaff generator to a dose of 10 Mrads Ο—Polyethylene film exposed to glass cleaning solution at 80° C. for 4 minutes φ—CASED polyethylene, exposed to activated helium at 1 mm. pressure and high power for 5 seconds ^-^Polyethylenefilmgenerated against vapor degreased aluminum foil (subsequently removed by dissolution) φ—Polyethylene film generated against vapor degreased and chemically etched aluminum foil (subsequently removed by dissolution) Φ—Polyethylene film generated against gold


11.

SCHONHORN

AND

149


RYAN

W e resorted to other t e c h n i q u e s s u c h as solvent e x t r a c t i o n

(16).

S p e c i m e n s s u i t a b l e for joint strength m e a s u r e m e n t w e r e p r e p a r e d a n d t h e n extracted i n a b o i l i n g 1:1 hexane-heptane s o l u t i o n for several m i n utes.

T h e specimens w e r e not s w e l l e d a p p r e c i a b l y , a n d resultant joint

strengths w e r e c o n s i d e r a b l y greater t h a n those o b t a i n e d w i t h u n t r e a t e d film.

Again y

c

w a s a b o u t 35 d y n e s / c m .

W h e n b o i l i n g octane w a s u s e d

to extract the l o w m o l e c u l a r w e i g h t p o l y m e r f r o m p o l y e t h y l e n e considerable swelling occurred.

film,

U p o n r e m o v a l of the solvent, a d d i t i o n a l

l o w e r m o l e c u l a r w e i g h t m a t e r i a l w a s t r a n s p o r t e d to the s o l i d - a i r interface Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch011

a n d o n l y w e a k joints c o u l d b e o b t a i n e d . T h e weak boundary layer was partially eliminated b y of the p o l y m e r w i t h h i g h energy electrons.

bombardment

T h e results i n F i g u r e 4 s h o w

that stronger joints w e r e o b t a i n e d — b u t not s h o w n is the fact t h a t b u l k properties of the p o l y m e r w e r e seriously affected.

A l t h o u g h the y

was

c

r a i s e d , since i r r a d i a t i o n w a s d o n e i n a i r , joint strengths w e r e a b o u t t h e same as those o b t a i n e d b y the solvent e x t r a c t i o n t e c h n i q u e . T h e u p p e r m o s t curves i n F i g u r e 4 are results o b t a i n e d w i t h surface t r e a t m e n t t e c h n i q u e s w h i c h result i n b o t h c r o s s - l i n k i n g a n d s t r e n g t h e n i n g of the w e a k b o u n d a r y l a y e r w i t h o u t affecting b u l k properties of polymer.

H i g h joint strengths w e r e o b t a i n e d after b o t h C A S I N G

the (five

seconds i n e x c i t e d h e l i u m ) a n d after e t c h i n g w i t h glass c l e a n i n g s o l u t i o n (four

m i n u t e s at 8 0 ° C ) .

G l a s s c l e a n i n g s o l u t i o n treatment of

poly-

e t h y l e n e l i k e a l l o x i d a t i v e surface treatments causes a b l a t i o n of some of the p o l y m e r at the surface a n d changes w e t t a b i l i t y . T h e results for t h e films generated against the g o l d are c o m p a r a b l e to those of the C A S I N G technique. It is i n t e r e s t i n g to note t h a t the joint strengths for

films

generated

against g o l d are i n excess of those p r e p a r e d w i t h films generated against e t c h e d a l u m i n u m . V a p o r d e g r e a s i n g the a l u m i n u m f o i l w i t h o u t a c h e m i c a l e t c h is o n l y as effective

as h i g h e n e r g y e l e c t r o n b o m b a r d m e n t

in

e l i m i n a t i n g the w e a k b o u n d a r y layer. T h e i m p o r t a n c e of the c h e m i c a l e t c h o n t h e a l u m i n u m cannot over-emphasized.

S u r f a c e layers f r o m r o l l i n g oils or w e a k oxides

be may

o b v i a t e a n y d e s i r e d effects. B y e x a m i n i n g the strength of joints p r e p a r e d over

an

t e m p e r a t u r e range w h i c h i n c l u d e s the m o l t e n r e g i o n , w e s t r i k i n g features ( F i g u r e 5 ) .

extensive

note

some

T h e d a t a for the m e l t c r y s t a l l i z e d p o l y m e r

sheet that w a s p e e l e d f r o m the m o l d h a v e b e e n r e p o r t e d p r e v i o u s l y

(19).

W e c o m p a r e these d a t a w i t h the d a t a of this r e p o r t to p o i n t out t h e significance of the u n d i s t u r b e d t r a n s c r y s t a l l i n e r e g i o n . T h e joint strengths are c o n s i d e r a b l y h i g h e r t h a n the same m a t e r i a l s w h i c h h a v e a d a m a g e d t r a n s c r y s t a l l i n e région.

F r o m the a b o v e analysis w e c a n c o n c l u d e t h a t


150

INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES

3000

2500

ι ho

2 0 0 0

ζ

a: hco


UJ

The Effect of Polymer Surface Morphology on Adhesion and Adhesive

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