Adhesion of High Polymers III. - Advances in Chemistry (ACS

7 Adhesion of High Polymers III. Mechanisms of Adhesion at the Rubber-Resin Interface in Hetrophase Systems

Downloaded by COLUMBIA UNIV on March 21, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch007

LIENG-HUANG

LEE

1

The Plastics Laboratory, The Dow Chemical Company, Midland, Mich. 48640

Rubber-resin

heterophase systems are classified as (1) resin

as the disperse phase, (2) rubber as the disperse phase, (3) grafted rubber latex particles as the disperse phase, and (4) filled graft rubber as the

disperse phase.

mechanisms related to these systems are discussed.

Adhesion Special

emphasis is made on the last two systems which involve grafting.

The graft rubber isolated from the fourth system

is characterized.

The graft rubber is shown to function as

a compatibilizer

and as an adhesive or a coupling agent for

the rubber-resin

interface.

" p a r t i c u l a t e o r g a n i c composite m a t e r i a l s c o n t a i n i n g b o t h a r u b b e r a n d -*· a r e s i n c a n b e so f a m i l i a r as to b e u n r e c o g n i z e d as such—e.g., elastom e r i c adhesives (63, 67),

pressure-sensitive tapes (63),

o r g a n i c coatings, or c o m m o n l y r e i n f o r c e d r u b b e r s (10)

recognized

composites

non-pigmented s u c h as r e s i n -

or r u b b e r - r e i n f o r c e d t h e r m o p l a s t i c s (2, 10,

55).

T h o u g h appearances a n d f u n c t i o n s of these m a t e r i a l s differ, the f u n d a m e n t a l p r i n c i p l e s u n d e r l y i n g t h e c h e m i s t r y at the interface a n d m e c h a nisms of r e i n f o r c e m e n t are s i m i l a r . D o b r y a n d B o y e r - K a w e n o k i (14)

concluded that for polymers, com

p a t i b i l i t y is the e x c e p t i o n a n d i n c o m p a t i b i l i t y is the r u l e . F o r a h e t e r o geneous system, i n c o m p a t i b i l i t y is a n a d v a n t a g e for r e i n f o r c e m e n t , p r o v i d e d t h a t the a d h e s i o n at the interface is s t r o n g e n o u g h to w i t h s t a n d the a p p l i e d stresses. interface—e.g.,

T h e n a t u r e of a d h e s i o n depends

o n the t y p e

of

l i q u i d - l i q u i d , l i q u i d - s o l i d , o r s o l i d - s o l i d . I n the case of

t w o p o l y m e r s at the l i q u i d - l i q u i d i n t e r f a c e , a r u b b e r y p o l y m e r - t o - r u b b e r y polymer adhesion ( R - R adhesion) 1

(35, 36)

is l i k e l y to d e t e r m i n e the

Present address: Xerox Research Laboratories, Webster, Ν. Y. 14580. 85 In Interaction of Liquids at Solid Substrates; Alexander, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

86

INTERACTION

O F LIQUIDS A T SOLID

SUBSTRATES

process, w h i l e at the l i q u i d - s o l i d interface, a r u b b e r y p o l y m e r - t o - g l a s s y p o l y m e r a d h e s i o n ( R - G a d h e s i o n ) s h o u l d p r e d o m i n a t e . T h o u g h t h e latter is o u r m a j o r interest i n this s t u d y , w e also b r i e f l y discuss t h e a d h e s i o n at t h e l i q u i d - l i q u i d interface of t r a n s i t o r y r u b b e r - r e s i n systems. W e are specifically i n t e r e s t e d i n t h e system i n w h i c h a l i q u i d - s o l i d interface r e a c t i o n has t a k e n p l a c e . A n e x a m p l e o f this t y p e of r e a c t i o n is t h e c h e m i c a l g r a f t i n g of a r u b b e r w i t h a m o n o m e r at t h e interface. T h e f u n c t i o n of t h e g r a f t e d r u b b e r as a n a d h e s i v e has b e e n p o s t u l a t e d (11,29, 46, 64) b u t has never b e e n p r o v e d .

S i n c e t h e g r a f t e d r u b b e r is t h e k e y


to b r i d g i n g t w o i n c o m p a t i b l e p o l y m e r s together, w e d e v o t e d a m a j o r p o r t i o n of o u r e x p e r i m e n t a l w o r k to t h e c h a r a c t e r i z a t i o n of t h e g r a f t e d p o l y m e r as a n adhesive at t h e interface. C l a s s i f i c a t i o n o f R u b b e r - R e s i n Systems.

R u b b e r - r e s i n heterophase

systems c a n b e classified i n t o f o u r types a c c o r d i n g t o t h e m a i n constituent i n t h e disperse p h a s e : ( 1 ) R e s i n as t h e disperse phase ( 2 ) R u b b e r as t h e disperse phase ( 3 ) G r a f t e d r u b b e r latex p a r t i c l e s as t h e disperse phase ( 4 ) F i l l e d graft r u b b e r as t h e disperse phase RESIN AS T H E DISPERSE P H A S E .

S e v e r a l k i n d s o f resins ( J O ) h a v e b e e n

u s e d to r e i n f o r c e rubbers—e.g., p h e n o l i c o r c o u m a r o n e resins f o r n a t u r a l r u b b e r , s t y r e n e - b u t a d i e n e r e s i n f o r s t y r e n e - b u t a d i e n e r u b b e r , etc. O n e other i m p o r t a n t system, pressure-sensitive adhesive, also belongs t o this class.

T h e s e adhesives g e n e r a l l y c o n t a i n a l o w m o l e c u l a r w e i g h t r e s i n

f u n c t i o n i n g as a tackifier. I n 1957, W e t z e l (68) a n d H o c k (19)

found

that these adhesives w e r e a c t u a l l y t w o - p h a s e systems ( F i g u r e 1 ) . U n d e r

Figure 1. Electron-micrograph of twophase pressure-sensitive adhesive (Magnification, 11,000 X), dark phase = resin, 3.2 pentalyn H and natural rubber by Hock (17) t h e n o r m a l c o n d i t i o n , t h e d i s p e r s e d phase is t h e r e s i n p l u s l o w m o l e c u l a r w e i g h t r u b b e r a n d t h e c o n t i n u o u s phase is the r u b b e r s a t u r a t e d w i t h resin.

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

7.

LEE

Rubber-Resin Interface

87

T h e y also r e p o r t e d a phase i n v e r s i o n t a k i n g p l a c e at h i g h r e s i n c o n c e n trations ( F i g u r e 2 ) . A f t e r t h e phase i n v e r s i o n , t h e tack v a l u e of t h e


adhesive d r o p p e d t o zero.

Figure 2. Phase inversion of rubber-resin phases in pressure-sensitive adhesives discovered by Wetzel and Hock in 1957 P r i o r t o this d i s c o v e r y , i n 1 9 5 4 S i l b e r b e r g a n d K u h n ( 6 2 ) w e r e first to s t u d y t h e p o l y m e r - i n - p o l y m e r e m u l s i o n c o n t a i n i n g e t h y l c e l l u l o s e a n d polystyrene polymer

i n a nonaqueous

solvent,

benzene.

e m u l s i f i c a t i o n , d e m i x i n g , a n d phase

T h e mechanisms reversal were

of

studied.

W e t z e l a n d H o c k ' s d i s c o v e r y w o u l d t h e n equate t h e pressure-sensitive adhesive t o a p o l y m e r - p o l y m e r e m u l s i o n i n s t e a d of a p o l y m e r - p o l y m e r suspension.

S i n c e t h e interface is l i q u i d - l i q u i d , t h e a d h e s i o n t h e n b e -

comes one t y p e of R - R a d h e s i o n ( 3 5 , 3 6 ) . A c c o r d i n g to o u r p r e v i o u s d i s c u s s i o n , d i f f u s i o n is not o p e r a t i v e unless b o t h r e s i n a n d r u b b e r h a v e a n i d e n t i c a l s o l u b i l i t y parameter.

T h e m a j o r i n t e r f a c i a l i n t e r a c t i o n is

p h y s i c a l a d s o r p t i o n , w h i c h , i n t u r n , determines adhesion.

O u r previous

w o r k o n t h e w e t t a b i l i t y of elastomers ( 3 7 , 38) c a n h e l p p r e d i c t a d h e s i o n results. D e t a i l e d studies o n t h e f u n c t i o n of tackifiers h a v e b e e n m a d e b y W e t z e l a n d A l e x a n d e r ( 6 9 ) , a n d b y H o c k ( 2 0 , 21), a n d therefore t h e subject r e q u i r e s n o f u r t h e r e l a b o r a t i o n . RUBBER

AS T H E DISPERSE

PHASE.

I n p o l y b l e n d systems, a r u b b e r is

masticated mechanically w i t h a polymer or dissolved i n a polymer solut i o n . A t t h e c o n c l u s i o n of b l e n d i n g , a r u b b e r is d i s p e r s e d i n a r e s i n as particles of s p h e r i c a l or i r r e g u l a r shape. W e c a n f u r t h e r s u b d i v i d e this system i n t o three classes a c c o r d i n g to t h e m a j o r i n t e r m o l e c u l a r forces g o v e r n i n g a d h e s i o n : ( a ) b y d i s p e r s i o n forces—e.g., t h e p o l y b l e n d of t w o i n c o m p a t i b l e p o l y m e r s , ( b ) b y d i p o l e interaction—e.g., t h e p o l y b l e n d of p o l y v i n y l chloride a n d a n acrylonitrile rubber ( 5 6 ) , a n d ( c ) b y covalent bond—e.g., a n e p o x y resin r e i n f o r c e d w i t h a n a c i d - c o n t a i n i n g elastomer r e p o r t e d b y M c G a r r y (43). I n general, r u b b e r particles i n a l l these classes are n o n - p o r o u s a n d c o m p a c t . A n e l e c t r o n m i c r o g r a p h of a p o l y s t y r e n e - r u b b e r b l e n d ( F i g u r e 3 ) c a n i l l u s t r a t e the g e n e r a l feature of t h e disperse phase. T h e a d h e s i o n



88

INTERACTION

OF

LIQUIDS A T

SOLID

SUBSTRATES

Figure 3. Electron-micrograph of polystyrene-rubber blend by the solvent-etch-double replica method (Magnification, 8670 X), dark phase = shape rubber particles

odd-

b e t w e e n these t w o i n c o m p a t i b l e phases c a n b e i n c r e a s e d b y so d e s i g n i n g t h e c o m p o n e n t s i n the r u b b e r c o p o l y m e r or b y the a d d i t i o n of a t h i r d c o m p o n e n t to act as a n a d h e s i v e or a c o m p a t i b i l i z e r . A recent r e v i e w i n this r e g a r d w r i t t e n b y B o h m ( 8 ) s h o u l d b e c o n s u l t e d for the m e c h a n i s m s of c o m p a t i b i l i z a t i o n . GRAFTED

RUBBER

LATEX

PARTICLES

AS

THE

DISPERSE

PHASE.

ABS

p o l y m e r s or a c r y l o n i t r i l e - b u t a d i e n e - s t y r e n e p o l y m e r s , c a n b e g e n e r a l l y m a d e b y p i g g y - b a c k g r a f t i n g of a p o l y b u t a d i e n e latex w i t h styrene a n d

Figure 4. Electron-micrograph of grafted latex ABS polymer by the osmium tetroxide technique (Magnification, 12,530 X), dark phase = grafted latex particles


7.

89


LEE

a c r y l o n i t r i l e m o n o m e r s (4,13).

G r a f t i n g p r e d o m i n a n t l y takes p l a c e o n the

surface of these latex p a r t i c l e s . U n d o u b t e d l y a c e r t a i n f r a c t i o n of m o n o mers c a n diffuse i n t o the latex p a r t i c l e s a n d w i l l p o l y m e r i z e a n d / o r graft onto r u b b e r w i t h i n the particles. A recent e l e c t r o n m i c r o g r a p h of a t y p i c a l A B S p o l y m e r ( F i g u r e 4 ) m a d e w i t h the o s m i u m tetroxide t e c h n i q u e

(27)

shows c o n v i n c i n g l y t h a t these p a r t i c l e s are u n i f o r m a n d r e l a t i v e l y n o n porous. I n F i g u r e 5* w e i l l u s t r a t e the c h a n g e of the p a r t i c l e surface p o l a r ity b y the p i g g y - b a c k g r a f t i n g . T h e s e o u t e r shells ( 17) w e r e a c t u a l l y t h e interface responsible for b o n d i n g b e t w e e n the t w o phases. A s a result of Downloaded by COLUMBIA UNIV on March 21, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch007

g r a f t i n g , besides the surface p o l a r i t y , m o l e c u l a r c o n f i g u r a t i o n , i n t e r m o l e c u l a r forces a n d r h e o l o g i c a l properties c h a n g e a c c o r d i n g l y . O n the basis of these properties, i n 1955, H a y e s ( 18 ) p a t e n t e d the use of the graft c o p o l y m e r to c o m p a t i b i l i z e styrene c o p o l y m e r s a n d p o l y b u t a d i e n e . T h e c o n f i g u r a t i o n of the m e t h y l m e t h a c r y l a t e - n a t u r a l r u b b e r graft c o p o l y m e r s t u d i e d b y M e r r e t t (45).

was

H e i n t r o d u c e d the c o l l o i d c h e m i s t r y t e r m i n o l o g y

for the graft copolymer—e.g., m i c e l l e s a n d s t a b i l i z a t i o n w h i c h w e r e n o t c o m m o n l y u s e d i n the p o l y m e r l i t e r a t u r e at t h a t t i m e . H o w e v e r , i t w o u l d p r o b a b l y b e m o r e a p p r o p r i a t e to use the t e r m c o m p a t i b i l i z a t i o n for l i q u i d s o l i d a n d s o l i d - s o l i d interfaces. D e s p i t e the c o n f u s i o n i n t e r m i n o l o g y the graft c o p o l y m e r c a n b e also c o n s i d e r e d as a n adhesive

(Figure 6)

for

these interfaces i n a d d i t i o n to b e i n g a c o m p a t i b i l i z e r . FILLED

GRAFT

RUBBER

AS

THE

DISPERSE

PHASE.

Rubber-modified

p o l y s t y r e n e is g e n e r a l l y o b t a i n e d b y p o l y m e r i z a t i o n g r a f t i n g of a r u b b e r i n the presence of styrene m o n o m e r .

T h e p o l y m e r i z a t i o n is c a r r i e d out

t o t a l l y or p a r t i a l l y i n mass w i t h the a i d of s h e a r i n g a g i t a t i o n , as p a t e n t e d b y A m o s et al. (1).

T h e s t u d y o n the i n i t i a l stage of this t y p e of p o l y m e r i -

z a t i o n w a s first p u b l i s h e d b y B e n d e r ( 5 ) , a n d phase i n v e r s i o n s i m i l a r to that d i s c o v e r e d for the t w o - p h a s e pressure-sensitive adhesives w a s served. T h e m e c h a n i s m of p a r t i c l e f o r m a t i o n has also b e e n r e v i e w e d NON-POLAR SURFACE

POLAR

SURFACE

GRAFTED WITH STYRENE S ACRYLONITRILE

Figure 5. Polybutadiene latex particles are grafted with relatively polar monomers predominantly on the surface In Interaction of Liquids at Solid Substrates; Alexander, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

ob(47).

90

INTERACTION

O F LIQUIDS

A T SOLID SUBSTRATES


GRAFT RUBBER

Figure 6. Adhesion at the rubber-resin interface in the rubber-reinforced thermoplastics, type III (assuming that most of the grafting takes place on the surface of ht ex particles) H o w e v e r , n o reports of surface energetics at t h e interface h a v e y e t b e e n published. T h e r u b b e r particles w e r e e x a m i n e d w i t h a n e l e c t r o n

microscope

after t h e s a m p l e w a s t r e a t e d w i t h o s m i u m tetroxide (27). T h e m i c r o g r a p h ( F i g u r e 7 ) c l e a r l y indicates t h e porous n a t u r e of t h e r u b b e r phase a n d the o c c l u s i o n of p o l y s t y r e n e .

W e therefore classify this t y p e of r u b b e r

phase as filled graft r u b b e r . S i n c e g r a f t i n g takes' p l a c e before a n d after the r u b b e r c h a i n is c o i l e d , therefore, f o r this case, t h e m o n o m e r is g r a f t e d o n t o t h e r u b b e r b o t h w i t h i n a n d w i t h o u t t h e r u b b e r phase.

Polybutadiene

is thus m a d e m o r e c o m p a t i b l e to t h e p o l y m e r m a t r i x s u r r o u n d i n g t h e r u b b e r phase a n d t h e p o l y m e r filling t h e r u b b e r phase. H e r e w e h a v e a n

Figure 7. Electron-micrograph of rubberreinforced polystyrene by graft polymerization The sample was prepared by osmium tetroxide technique (magnification, 8670 X), dark phase = rubber particles, white spots in the particles are occluded polystyrene


7.

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91


interface o r t w o interfaces of r e s i n - r u b b e r - r e s i n , a n d t h e graft r u b b e r acts as a n adhesive f o r t h e s o l i d - s o l i d interface ( F i g u r e 8 ).

IN THE MATRIX Α

θ

Α θ

Α

I

I

I

I


θ

Α

θ

I

Α

θ

β

I

A

RUBBER a GRAFT RUBBER WITHIN THE RUBBER PHASE

Figure 8. Adhesion at the resin-rubber-resin interface in the rubber-reinforced thermoplastics type IV, A and Β are comonomers S i n c e these r u b b e r particles a r e h i g h l y filled w i t h a h o m o p o l y m e r o r a c o p o l y m e r , t h e r u b b e r is a l r e a d y r e i n f o r c e d w i t h a r e s i n to give a h i g h e r m o d u l u s p a r t i c l e t h a n t h e g r a f t e d r u b b e r latex. O n t h e basis of the uniqueness of these r u b b e r p a r t i c l e s , this process is also m o r e a p p r o priate i n manufacturing high-strength medium-impact A B S polymer or r u b b e r - r e i n f o r c e d s t y r e n e - m e t h y l m e t h a c r y l a t e c o p o l y m e r

(31),

(32). T h e

Figure 9. Electron-micrograph of an ABS polymer by graft polymerization The sample was prepared by osmium tetroxide technique (magnification, 4130 X) p h y s i c a l features of t h e r u b b e r phase i n a n A B S p o l y m e r p r e p a r e d b y a p o l y m e r i z a t i o n g r a f t i n g process a r e s h o w n i n a n electron

micrograph

( F i g u r e 9 ) p r e p a r e d b y t h e o s m i u m tetroxide t e c h n i q u e . In Interaction of Liquids at Solid Substrates; Alexander, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

92

INTERACTION

Mechanisms of Reinforcement.

O F LIQUIDS

A T SOLID

SUBSTRATES

B o t h rubber reinforced with a

filler

(49, 50) a n d t h e r m o p l a s t i c s r e i n f o r c e d w i t h a r u b b e r ( 2 9 , 5 5 ) h a v e b e e n r e v i e w e d e s p e c i a l l y w i t h respect to the m e c h a n i s m of A c c o r d i n g to N i e l s e n ( 5 0 ) , K e r n e r ' s e q u a t i o n (28)

reinforcement.

is a p p r o p r i a t e to

d e s c r i b e t h e m o d u l i o f b o t h of these systems p r o v i d e d perfect a d h e s i o n exists a t t h e i n t e r f a c e s : Ε (filled)

_

G (filled)

_

Ε (unfiled) ~~ G (unfilled) ~~ G V / [ ( 7 - 5 y ) G + (8-10y)G ] + V /[15(1 Downloaded by COLUMBIA UNIV on March 21, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch007

F

F

p

F

G V /[(7-5v)G + (8-10v)G ] P

F

p

Where G and G p

F

F

p

,)]

+V /[15(l-v)] p

are t h e shear m o d u l i of t h e p l a s t i c m a t r i x a n d t h e filler

r e s p e c t i v e l y , ν is Poisson's r a t i o of t h e m a t r i x , a n d V is t h e v o l u m e f r a c t i o n . E's are Y o u n g s m o d u l i . R e c e n t l y this r e l a t i o n w a s v e r i f i e d f o r the r u b b e r - r e i n f o r c e d t h e r m o p l a s t i c s (29).

T h e effect of a d h e s i o n at t h e

r u b b e r - r e s i n ( o r r u b b e r - f i l l e r ) interface has b e e n s t u d i e d ( 4 9 , 5 7 , 58) w i t h mathematical models. T h e o r i e s r e g a r d i n g f u n c t i o n s of r u b b e r i n t h e r u b b e r - r e i n f o r c e d t h e r m o p l a s t i c s are also d e r i v e d o n t h e basis of t h e same p r e m i s e of a perfect adhesion.

I n 1965, N e w m a n a n d S t r e l l a (48) p o i n t e d o u t that

both the original micro-crack theory proposed b y M e r z , Claver, a n d Baer (46) a n d t h e m y r i a d - c r a c k hypothesis suggested b y S c h m i t t a n d K e s k kula

(59)

f a i l e d to e x p l a i n t h e r e i n f o r c e m e n t m e c h a n i s m .

T h e y sug

gested, i n t u r n , t h a t t h e c o l d - d r a w of t h e glassy m a t r i x is t h e k e y to the a c h i e v e m e n t of h i g h e l o n g a t i o n . A most significant a d v a n c e i n recent years w a s m a d e b y B u c k n a l l and Smith ( 9 ) w h o established the reinforcement mechanism o n the c r a z e t h e o r y ( 2 5 , 26). A c c o r d i n g to t h e i r theory, t h e differences b e t w e e n r e i n f o r c e d a n d u n - r e i n f o r c e d p o l y s t y r e n e s i m p l y l i e i n t h e m a x i m u m size a n d c o n c e n t r a t i o n of t h e c r a z e - b a n d s . T h e r u b b e r d i s p e r s e d i n t h e m a t r i x serves t o increase t h e n u m b e r s of c r a z e - b a n d s .

W i t h o u t good adhesion

a c h i e v e d b y g r a f t i n g , t h e r u b b e r fails to sustain tensile stresses at some stage i n t h e c r a z i n g process.

A f t e r t h e b r e a k d o w n at t h e r u b b e r - r e s i n

interface, l a r g e v o i d s a r e generated to w e a k e n t h e composite.

W e dem

onstrate this t y p e o f b r e a k w i t h t h e m o d e l s h o w n i n F i g u r e 10. B u c k n a l l a n d Smith's t h e o r y has b e e n f u r t h e r c o n f i r m e d b y recent work.

Matsuo

(42)

published electron micrographs

of

stress-crazed

r u b b e r - r e i n f o r c e d p o l y m e r s a n d f o u n d his results to b e i n g o o d agreement w i t h those of B u c k n a l l a n d S m i t h . R e c e n t l y , A r e n d s (3) r e l a t e d t h e c o l d flow of t h e r m o p l a s t i c s to E y r i n g ' s t h e o r y of viscous flow a n d e n l a r g e d t h e scope o f t h e i r theory. Grafting and Adhesion. F r o m t h e discussion i n t h e p r e c e d i n g p a r a g r a p h , i t c a n b e c o n c l u d e d that t h e r e i n f o r c e m e n t i n r u b b e r - r e s i n systems


7.

LEE

93


UNSTRETCHED MODEL

ι— STRETCHED MODEL


(No

I

^

adhesion)

STRETCHED MODEL

^CAVITY

*j I

(Perfect adhesion)

Figure 10. Adhesion and rubber-resin in terface in rubber-reinforced thermoplastics φ Rubber phase c a n o n l y b e a c h i e v e d w h e r e a g o o d a d h e s i o n is o b t a i n e d at the interface. T h e a d h e s i o n m e c h a n i s m (24, 51, 52, 53, 54 66) r e l a t e d to the first t w o systems d e s c r i b e d earlier are better u n d e r s t o o d a n d n e e d not b e d i s cussed f u r t h e r . H o w e v e r , the a d h e s i o n m e c h a n i s m s r e l a t e d to the last t w o systems, w h i c h i n v o l v e g r a f t i n g , deserve a d e t a i l e d study. 70).

G r a f t i n g of m o n o m e r s onto r u b b e r has b e e n r e v i e w e d , (2, 13,

30,

A recent p a p e r r e p o r t e d the c a t i o n i c g r a f t i n g of r u b b e r ( 6 5 ) .

We

d o not i n t e n d to elaborate f u r t h e r o n this subject. T h e g e n e r a l agreement is t h a t g r a f t i n g takes p l a c e r i g h t after i n i t i a t i o n .

D u r i n g o u r s t u d y of

the r u b b e r - r e i n f o r c e d p o l y s t y r e n e w e f o u n d that g r a f t i n g took t h r o u g h o u t the process.

place

D i f f i c u l t i e s arose w h e n c r o s s l i n k i n g i n t e r v e n e d ,

r e n d e r i n g the graft r u b b e r i n s o l u b l e . I n the c o m i n g sections, w e d e s c r i b e the s e p a r a t i o n , the c h a r a c t e r i z a t i o n a n d the d e t e r m i n a t i o n of Zisman's c r i t i c a l surface t e n s i o n (71, 72)

of the graft r u b b e r phase before i t is

h e a v i l y c r o s s l i n k e d . M e c h a n i s m s of a d h e s i o n are discussed o n the basis of these

findings.

Experimental Preparation of G r a f t Copolymer. T h e graft c o p o l y m e r s u s e d for this s t u d y w e r e p r e p a r e d w i t h a g i t a t i o n i n a 2-liter r e s i n flask e q u i p p e d w i t h a stainless steel stir-tube. T h e f e e d m i x t u r e for t h e p o l y b u t a d i e n e g r a f t e d w i t h styrene c o n t a i n e d 200 grams of " D i e n e " r u b b e r ( F i r e s t o n e ) a n d 1,800 grams of styrene m o n o m e r , w h i l e the one for the p o l y b u t a d i e n e g r a f t e d w i t h b o t h styrene a n d a c r y l o n i t r i l e c o n t a i n e d 200 grams o f " D i e n e " r u b b e r , 500 grams of a c r y l o n i t r i l e a n d 1,300 grams of styrene. T h e p r e p o l y m e r i z a t i o n w a s c a r r i e d out i n the absence of a catalyst at 105 ° C . f o r the f o r m e r a n d at 100 ° C . for the latter, to a s o l i d content In Interaction of Liquids at Solid Substrates; Alexander, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


94

INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES

b e t w e e n 3 5 - 4 0 % b e f o r e t h e r u b b e r phase b e c a m e h i g h l y c r o s s - l i n k e d a n d i n s o l u b l e . T h e resultant p r e p o l y m e r c o n t a i n e d t h e h o m o p o l y m e r o r t h e copolymer, the ungrafted a n d grafted polybutadiene. Separation of Polybutadiene. T h e u n g r a f t e d a n d g r a f t e d p o l y b u t a d i e n e w e r e separated f r o m t h e h o m o p o l y m e r o r t h e c o p o l y m e r b y t h e following precipitation technique: P r e p o l y m e r ( 4 0 g r a m s ) w a s d i s p e r s e d i n m e t h y l e t h y l ketone (1,000 m l . ). T h e d i s p e r s i o n w a s d i v i d e d i n t o several c e n t r i f u g e tubes a n d c e n t r i f u g e a at 9,500 r . p . m . T h e supernate c o n t a i n i n g t h e m o n o m e r a n d t h e soluble polymer was decanted, the precipitate containing the rubber phase w a s r e - w a s h e d w i t h m e t h y l e t h y l ketone. W e u s e d a set of t h e singly-precipitated polybutadiene for the study a n d f o u n d that they c o n t a i n e d a g o o d p o r t i o n of o c c l u d e d p o l y s t y r e n e . W e t h e n u s e d a d o u b l e precipitation technique to remove the occluded polystyrene ( o r p o l y ( s t y r e n e - c o - a c r y l o n i t r i l e ) ). T h e first p r e c i p i t a t e w a s r e d i s s o l v e d i n b e n z e n e a n d a n e q u i v a l e n t of 0 . 1 % of I o n o l w a s a d d e d to t h e b e n z e n e s o l u t i o n ( 1 0 6 m l . ) . ( N o t e : I o n o l is 2 , 6 - d i - t e r t - b u t y l - p - c r e s o l . ) T o t h e clear s o l u t i o n , m e t h y l e t h y l ketone ( 9 0 0 m l . ) w a s g r a d u a l l y a d d e d . T h e doubly-precipitated polybutadiene contained both the grafted a n d the u n g r a f t e d portions a n d u n d o u b t e d l y a r e m a i n i n g t r a c e of p o l y s t y r e n e . W e f o u n d t h a t t h e d o u b l e p r e c i p i t a t i o n w a s necessary b u t t h e r u b b e r w a s v e r y sensitive to o x i d a t i o n a t r o o m t e m p e r a t u r e , e v e n after a n a d d i t i o n of I o n o l p r i o r to t h e p r e c i p i t a t i o n . Determination of the Degree of Grafting. F o r a d e t a i l e d q u a n t i t a t i v e s t u d y , w e g e n e r a l l y separated t h e g r a f t e d a n d t h e u n g r a f t e d p o l y b u t a d i e n e . H o w e v e r , f o r this s t u d y , w e w e r e o n l y interested i n t h e extent of g r a f t i n g i n t h e p o l y b u t a d i e n e d u r i n g t h e i n i t i a l p o l y m e r i z a t i o n . T h e r e fore, t h e t w o portions of p o l y b u t a d i e n e w e r e n o t separated, a n d t h e degree of g r a f t i n g w a s d e t e r m i n e d b y i n f r a r e d o n t h e basis of b o t h t r a n s - p o l y ( l , 3 - b u t a d i e n e ) ( ~ 9 6 4 cm." ) a n d poly (1,2-butadiene) (909 c m . " ) . T h e a m o u n t of styrene g r a f t e d w a s d e t e r m i n e d o n t h e basis of p h e n y l - r i n g m o d e at 1493 c m . " ( F i g u r e 11) w h i l e t h e c y a n o g r o u p a b s o r p t i o n w a s m e a s u r e d at 2245 c m . " . T h e a m o u n t of I o n o l w a s n o t e n o u g h to affect t h e p h e n y l - r i n g m o d e . Determination of Critical Surface Tension. T h e m e t h o d f o r t h e d e t e r m i n a t i o n of contact angles w a s d e s c r i b e d i n p r e v i o u s papers ( 3 7 , 3 8 ) . F o r this s t u d y , w e u s e d o n l y o n e goniometer m a n u f a c t u r e d b y Râme-Hart, I n c . T h e t e m p e r a t u r e w a s c o n t r o l l e d at 20 ° C . w i t h a n e n v i r o n m e n t a l c h a m b e r . T h e p r e c i s i o n w a s z±2° f o r t h e contact a n g l e m e a s u r e m e n t . T h e l i q u i d s u s e d w e r e alcohols, P o l y g l y c o l s P-1200, 15-200, a n d E - 2 0 0 , ethylene glycol, formamide, glycerol, a n d water. T h e s t y r e n e - a c r y l o n i t r i l e c o p o l y m e r s w e r e p r e p a r e d i n t h e f o r m of a t h i n film. T h e graft p o l y b u t a d i e n e s o l u t i o n w a s c o a t e d o n a glass slide. B u t , f o r t h e graft p o l y m e r c o n t a i n i n g a c r y l o n i t r i l e , i t w a s u n d e s i r a b l e to use t h e glass slide b e c a u s e of t h e i n d u c e d o r i e n t a t i o n , therefore, w e u s e d a M y l a r film t o s u p p o r t a t h i c k s m o o t h film of t h e graft r u b b e r . Preparation of Electron-micrographs. T h e p o l y s t y r e n e - r u b b e r p o l y b l e n d s a m p l e w a s e t c h e d b y solvent a c c o r d i n g to t h e t e c h n i q u e d e v e l o p e d b y T r a y l o r . A d o u b l e r e p l i c a t e c h n i q u e w a s u s e d t o p r e p a r e t h e sample. T h e first r e p l i c a w a s m e t h y l c e l l u l o s e , t h e s e c o n d p l a t i n u m a n d c a r b o n , 800 A . t h i c k . 1

1

1

1


7.

LEE


95

WAVE LENGTH, MICRON 6.0

6.5


~τ

1

1700

1600

7.0

1—ι

7.5

1 ι

1500

1400

8.0

8.5

1—ι—ι—π

1300

1200

9.0 Π—

9.5

1100

WAVE NUMBER. CM*

Figure 11.

10

II

12

\

Π

1000

900

1

1

13

—Γ

1

800

14

15 16

Π— — 1

Γ

700

1

Infrared spectra of polybutadiene before and after moderately grafting with styrene, doubly precipitated

O t h e r e l e c t r o n - m i c r o g r a p h s w e r e p r e p a r e d a c c o r d i n g to K a t o ' s t e c h n i q u e (27). O s m i u m tetroxide m a d e the r u b b e r particles d a r k a n d d i s tinct. T h e thickness of the u l t r a t h i n s p e c i m e n w a s 1000 A . Results and

Discussion

Wettability

of

Elastomers

and Copolymers.

The

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

elastomers (37, 38) i n terms of c r i t i c a l surface tension w a s r e p o r t e d p r e v i ously.

T h e elastomers c o m m o n l y u s e d for the r e i n f o r c e m e n t of b r i t t l e

p o l y m e r s are p o l y b u t a d i e n e , s t y r e n e - b u t a d i e n e r a n d o m a n d b l o c k polymers, and butadiene-acrylonitrile rubber.

co

C r i t i c a l surface tensions

for several t y p i c a l elastomers are 31 d y n e / c m . for " D i e n e " r u b b e r , 33 d y n e / c m . for b o t h G R - S 1 0 0 6 r u b b e r a n d s t y r e n e - b u t a d i e n e b l o c k polymer

co

( 2 5 : 7 5 ) a n d 37 d y n e / c m . for b u t a d i e n e - a c r y l o n i t r i l e r u b b e r ,

( " P a r a c r i l " B J L T n i t r i l e r u b b e r ) . T h e c o p o l y m e r i z a t i o n of b u t a d i e n e w i t h a r e l a t i v e l y p o l a r monomer—e.g., styrene or a c r y l o n i t r i l e — g e n e r a l l y r e sults i n a n increase i n c r i t i c a l surface tension. T h e increase i n p o l a r i t y is also reflected i n the increase i n the s o l u b i l i t y p a r a m e t e r (34, 39, 40) a n d i n the increase of glass t e m p e r a t u r e (40).

W e also n o t e d a s i m i l a r increase

i n c r i t i c a l surface tensions of s t y r e n e - a c r y l o n i t r i l e c o p o l y m e r s w i t h the


96

INTERACTION

O F LIQUIDS

AT

SOLID

SUBSTRATES

increase i n a c r y l o n i t r i l e content ( F i g u r e 12 ). T h e c r i t i c a l surface t e n s i o n of p o l y s t y r e n e d e t e r m i n e d b y o u r m e t h o d is 36 d y n e / c m . , a n d values f o r these c o p o l y m e r s v a r y b e t w e e n 37 a n d 43 d y n e / c m .

P a r a l l e l increases

i n b o t h t h e s o l u b i l i t y p a r a m e t e r a n d glass t e m p e r a t u r e h a v e also b e e n


noted for the copolymers.

-0.1

I

20

1 1 1 I I 25 30 35 40 45 SURFACE TENSION

Figure 12.

Δ A Ο •

I 50

I 55

I I 1 60 65 70 Dynes/cm.

1 75

Wettability of styrene copolymers

( Dynes/cm. ) Acrylonitrile 32% 43 Acrylonitrile 25% 42 Acrylonitrile (exptl) 16% 40 Acrylonitrile (exptl.) 6% 37

C o m p a t i b i l i t y of p o l y m e r s i m p l i e s a s e m i - q u a n t i t a t i v e m e a s u r e c a n b e u s e d to p r e d i c t w h e t h e r t w o o r m o r e p o l y m e r s are c o m p a t i b l e . T h e use of one of t h e s e m i - q u a n t i t a t i v e approaches, s o l u b i l i t y p a r a m e t e r , w a s d e m o n s t r a t e d b y H u g h e s a n d B r i t t (22).

I t w a s c o n c l u d e d (8) t h a t one

p a r a m e t e r w a s insufficient to p r e d i c t t h e c o m p a t i b i l i t y . I n this p a p e r , w e n o w i n t r o d u c e c r i t i c a l surface t e n s i o n w h i c h is d e t e r m i n e d f r o m t h e surface properties of a p o l y m e r . T h o u g h b o t h of these parameters h a v e b e e n r e l a t e d b y G a r d o n ( 1 5 ) , w e are i n c l i n e d to use t h e latter because w e c a n further describe the wettability between t w o polymers.

For in

stance, b y t h e use of y , w e c a n p r e d i c t e q u a l l y w e l l that c o m p a t i b i l i t y c

b e t w e e n p o l y s t y r e n e a n d p o l y b u t a d i e n e c a n b e i m p r o v e d i f b u t a d i e n e is


7.

LEE

97


1

30 X •

25

20

1

1

BASED ON

1

1

P0LY(I,2-BUTADIENE)

1

BASED ON TRANS-POLY ( 1,3-BUTADIENE ) (DETERMINED ANALYSIS)

BY INFRARED

-

/

X

X

X

1 DEGREE OF GRAFTING, % |

y/*

/

•-

/


15

-

10

-

5

0

/y /

χ

x

I 5

0

Figure 13.

/

—

χ

-

χ

I I I ! 10 15 20 25 CONVERSION OF MONOMER , %

I 30

35

Degree of grafting of polybutadiene vs. conversion of styrene monomer, singly precipitated 1

4 5

χ/

-

1

1

1

l

1

C R I T I C A L S U R F A C E T E N S I O N OF GRAFTED POLYBUTADIENE

-

40 Te AT 2 0

e

C„

DYNES/CM.

•

35

|

^

_ _ _ _ _ — « - — · " "

# *"

30

Adhesion of High Polymers III. - Advances in Chemistry (ACS

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