Dispersion of nano scaled fillers and its impact to elastomer performance

Robert H. Schuster*

Dispersion of nano scaled fillers and its impact to elastomer performance

The objective of the contribution is to emphasize experimental routes to characterize filler dispersion and to describe the influence of the major com­

pound ingredients - the elastomer and the filler - on filler dispersion. Conse­

quences of filler dispersion on processing behavior, the non-linear viscoelastic properties as well as ultimate properties are highlighted.

K e y w o r d s :

elastomers, fillers, reinforcing, filler dispersion, processing

Dyspersja nanonapełniaczy i jej wpływ na właściwości elastomerów

W niniejszym artykule omówiono eksperymentalne metody charakteryzo­

wania dyspersji napełniaczy i określono wpływ oddziaływań głównych składni­

ków mieszanki kauczukowej - kauczuku i napełniacza

na dyspersję napełnia- cza. Szczególną uwagę zwrócono na zależności pomiędzy stopniem zdyspergo- wania napełniacza a właściwościami przetwórczymi, nieliniowymi właściwoś­

ciami lepkoelastycznymi oraz właściwościami produktu finalnego.

S ło w a k lu c z o w e :

elastomery, napełniacze, wzmacnianie, dyspersja napełnia­

czy

przetwórstwo

1. Introduction

R e in fo r c e m e n t o f e la s to m e r s is ty p ica l fo r th e rub­

bery sta te o f c r o s s -lin k e d e la s to m e r s and im p lic a te s d if­

fe r e n t s p e c if ic m e c h a n is m s , a ll o f th em o r ig in a tin g from the e x te n t and th e stren gth o f th e filler-ru b b er in terfa ce c rea ted d u rin g m e c h a n ic a l m ix in g [1, 2 ]. It can b e e a s ily u n d e r sto o d that large filler-ru b b er in ter­

fa c e s can b e a c h ie v e d o n ly b y an e ff e c tiv e s iz e red u c­

tion o f fille r p e lle ts an d fille r a g g lo m e r a te s d u rin g m e ­ c h a n ic a l m ix in g . H o w e v e r , th e d e g r e e o f d isp ersio n rea ch ed d u rin g m ix in g is lim ite d b y th e b a la n ce o f tw o co n tra d icto ry factors: th e fille r -p o ly m e r in tera ctio n and the in tera g g reg a te (f ille r -fille r ) in tera ctio n [3 -5 ].

A p art fro m th e fille r lo a d in g , th e fille r d isp ersio n and its r e in fo r c in g p o te n tia l are g o v e r n e d b y (i) the prim ary p a rticle s iz e and d istrib u tio n , (ii) th e sh a p e and sh a p e d istrib u tio n (stru ctu re) and th e site en erg y d istri­

b u tio n and fu n c tio n a l g ro u p s on th e fille r su rfa ce (su r­

fa c e a c tiv ity ) [5 -7 ]. It w a s r e c o g n iz e d earlier that the f ille r ca n alter th e r h e o lo g ic a l p r o p e r tie s o f ru b b er m ix e s , r e s u ltin g n o t o n ly in h ig h ly n o n -N e w t o n ia n flo w , but a ls o in c o m p a r a tiv e ly h ig h v is c o s ity . T h is can b e attrib u ted to (i) h y d r o d y n a m ic r e in fo r c e m e n t [8 ], (ii) th e o c c lu s io n o f rubber b y a n is o m e tr ic fille r a g g r e g a te s [9] and (iii) th e fo r m a tio n o f a fille r n etw o rk a b o v e a critica l v o lu m e fra ctio n o f th e fille r (j)* [6, 10, 11].

* D eutsches Institut fiir K autschuktechnologie e.V.; Eupener Str. 33; 30519 Hannover, Germany; E-mail: Robert.Schus- ter@ DIKautschuk.de

B e c a u s e fille r n e tw o r k fo rm a tio n req u ires d is p e r sio n at th e m ic r o le v e l th e e le c tr ic a l p erco la tio n th resh o ld is a s e n s itiv e criterio n to d eterm in e carb on b la ck d is p e r sio n [1 2 , 13]. It w a s d em o n stra ted e x p e r im e n ta lly that by stro n g er filler-ru b b er in tera ctio n s th e p erco la tio n lim it is s h ifte d to w a rd s h ig h er fille r v o lu m e fra ctio n s [1 4 , 15].

A b o v e th e p e r c o la tio n th resh o ld the sto ra g e m o d u ­ lu s at v ery sm a ll d efo r m a tio n a m p litu d es

G ’o(y)

n ear v is c o e la s t ic p ro p erties to an y fille d v u lc a n iz a te in th e r e g im e o f sm a ll d e fo r m a tio n s (P a y n e e ffe c t) [1 6 ].

T h e e f f e c t is a ttrib u ted to the g ra d u a lly b rea k d o w n o f fille r n e tw o r k in to su b -n e tw o r k s and clu sters. A q u a n ti­

ta tiv e p ictu re is g iv e n b y th e d iffe r e n c e

G ’

(

G ’

.

G ’

(

in g a sto ra g e p e r io d or a n n ea lin g at h ig h er tem p eratu res (flo c c u la t io n ) [1 5 ,1 7 ] .

T h e d e g r e e o f fille r d isp ersio n and fille r d istrib u ­ tio n are m a in co n tr ib u to r s to th e q u a lity o f ru b b er m ix e s . F ro m th e p ra ctise o f rubber te c h n o lo g y it b e ­ c o m e s e v id e n t that p o o r fille r d isp ersio n and d istrib u ­ tio n le a d to (i) d iffic u lt p r o c e ssa b ility , (ii) q u a lity varia ­ tio n s, (iii) h ig h e r e n e r g y c o n su m p tio n and (iv ) red u ced p ro d u ct p e r fo r m a n c e and life tim e [1 8 ]. E v en i f fille r d is p e r sio n is k n o w n to b e critica l to th e fin a l p erfo r­

m a n c e o f ru b b er parts, th e su b ject has r e c e iv e d m u ch

TOM 10 listopad - grudzień 2006 r. SCa&taM&uf nr 6

m u o m p e ł n i a c z e

in terest durin g the la st d e c a d e in c o n n e c tio n w ith h ig h p erfo rm a n ce e la sto m ers. H o w e v e r , th ere is still to d a y a la ck o f u n d ersta n d in g in th is fie ld .

T h e sc o p e o f th e p resen t co n trib u tio n is to r e v ie w th e ch a r a c te r iz a tio n o f fille r d is p e r s io n on d iffe r e n t len g th s c a le s and to d e sc r ib e in m o re d e ta il, th e e ffe c ts o f p r o c e ss in g and m aterial p aram eters o n fille r d is p e r ­ sio n . It w ill be a ls o s h o w n that r h e o lo g ic a l p ro p erties o f rubber m ix e s as w e ll as d y n a m ic -m e c h a n ic a l and u lti­

m a te p rop erties o f v u lc a n iz a te s are rela ted to th e state o f d isp ersio n o f th e filler. R e su lts fro m resea rch in the la b o ra to ry e m p h a s is e th e m a jo r m a te r ia l d e p e n d e n t c o n tr ib u tio n s to fille r -r u b b e r in te r a c tio n s an d s o m e rou tes to im p ro v e fille r d is p e r sio n and d istrib u tio n by c h a n g in g the su rfa ce a c tiv ity o f th e fille r as w e ll as the c h e m ic a l nature o f th e rubber b y s p e c if ic fu n c tio n a liz a ­ tion . T ak in g in to a c c o u n t that carb on b la c k is to d a y and p rob ab ly in the near fu tu re o n e o f th e m o s t im p ortan t c o m p o n e n ts in a rubber p rod u ct, th e m a jo rity o f the e x a m p le s p resen ted refer to ca rb on b la c k fille d rubber m ix e s and v u lc a n iz a te s.

2. Experimental

C o m m ercia l a v a ila b le natural (N R ), b u ta d ie n e -s ty ­ ren e (S B R ) and e th y le n e -p r o p y le n e -d ie n e (E P D M ) and a v ariety o f fille r s (carb on b la c k and s ilic a ) w ith d if fe ­ rent p rim ary p a rticle s iz e and stru ctu re (D B P -n u m b e r , sorp tio n o f d ib u ty l p h th a la te) w e r e u se d fo r th is study.

In a d d ition ru b b er/filler c o m p o s ite s (R F C ) b a se d on e m u ls io n and s o lu tio n S B R (E -S B R , S -S B R ) and N R w ere u sed as d e liv e r e d b y D e g u s s a A G (G erm a n y ).

R u b b er m ix e s w e r e p rep ared fro m b a le rubber and fille r p e lle ts b y d is c o n tin u o u s m ix in g in a 1.5 L lab o ra ­ tory internal m ix e r (W ern er und P fle id e r e r ) e q u ip p e d w ith in term esh in g rotors and a lte r n a tiv e ly fro m R F C b y c o n tin u o u s m ix in g on tw in sc r e w ex tru d ers (T S E , Far­

rei F T X 8 0 and B e r sto r ff Z E 2 5 x 5 4 0 ). F o r b o th p r o c e ­ d u res the p r o c e s s in g p a ram eters w e r e v a r ie d s y s te m a ti­

ca lly . To in v e s tig a te th e fille r d is p e r sio n , d y n a m ic -m e ­ ch a n ic a l p rop erties as w e ll as u ltim a te p ro p erties the sa m p le w ere cu red w ith a s e m i-e f fic ie n t s u lfu r /a c c e le - rator s y s te m to the rh eo m eter o p tim u m .

T h e m a c r o -d isp e r s io n o f th e fille r w a s d eterm in ed by the r e fle c ta n c e o f a v ertica l lig h t b ea m o n h o rizo n ta l razor b la d e cu ts o f rubber c o m p o u n d or v u lc a n iz a te s [1 9 ]. T h e r e fle c te d lig h t w a s m o n ito r e d w ith an o p tic a l m ic r o s c o p e eq u ip p ed w ith a C C D -c a m e r a . T h e im a g e is transferred to a c o m p u ter and can b e p r o c e s s e d fu r­

ther. F iller a g g lo m e r a te s are reco rd ed a b o v e o f a lim it­

in g v a lu e o f 3 pm . T h e d e g r e e o f d is p e r s io n w a s d eter­

m in ed a cco rd in g to A S T M D 2 6 6 3 - 6 9 . In a d d itio n the a g g lo m e r a te s iz e and s iz e d istrib u tio n w e r e d eterm in ed . T h e e le c tr ic a l p erco la tio n th resh o ld w a s d e te r m in e d b y e le c tr ic a l c o n d u c tiv ity m e a s u r e m e n ts w ith d irect cu r­

ren t (D C ) and a lte r n a tiv e ly b y d ie le c tr ic r e la x a tio n sp ectro m etry (N o v o c o n tr o l, A L P H A -S ) .

T r a n s m is s io n e le c tr o n m ic r o s c o p y (T E M Z e is s 9 0 2 ) w a s p e r fo r m e d o n u ltr a -th in m ic r o t o m e c u ts

3. Results and discussion

3.1. Characterization of filler macro- and micro-dispersion

F ille r d is p e r sio n w a s in v e s tig a te d for d is c o n tin u - o u s ly as w e ll as c o n tin u o u s ly m ix e d s a m p le s as a fu n c ­ tion o f p r o c e ss p aram eters and th e state o f th e raw m a ­ terials. A s it can b e s e e n fro m F ig u re 1, d u rin g m ix in g th e sh ear fo r c e s tear apart th e p e lle t fra g m en ts and ero d e th em b y th e w e ll k n o w n “ o n io n s k in n in g ” m e c h a ­ n ism . T h e rou n d ed areas rep resen t the “hard” frag m en ts that w ill p o o r ly d is p e r se further. A s a resu lt th e n u m b er and th e area o f th e fra g m en ts o f fille r p e lle ts are re­

d u ced b y in c r e a s in g m ix in g tim e.

T h e A F M m icro g ra p h s s h o w n in F ig u re lb d e m o n ­ strate n ot o n ly th e d iffe r e n c e s b e tw e e n a p o o r and a

Sta& twt& ity nr 6 listopad - grudzień 2006 r. TOM 10

(5 0 - 1 0 0 n m ) to ch a ra cterize th e fille r m ic r o -d is p e r sio n . A ltern a tiv ely , th e p o ly m e r w a s q u a n tita tiv e ly extra cted fro m rubber m ix e s and th e rem a in in g fille r a g g lo m e ­ rates w e r e in v e s tig a te d w ith r e sp e c t to th eir s iz e and sh ap e.

T h e a b ility o f th e A F M te c h n iq u e for m a p p in g a su rfa ce b y rep ea ted p a ra llel traces w a s u sed to ob tain c o m p le m e n ta r y m o r p h o lo g ic a l data on fille r d isp ersio n a cco rd in g to [2 0 ]. T h e m a in a d v a n ta g e o f A F M is that the su rfa ce r o u g h n e ss o f m ic r o to m e cu ts can b e re­

co rd ed a b o v e and b e lo w th e p m sc a le . H o w e v e r , it is d iffic u lt to q u a n tify th e d e g r e e o f d is p e r sio n w ith the A F M te c h n iq u e u n le s s a p o w e r fu l im a g in g so ftw a r e is e m p lo y e d .

M o o n e y v is c o s it y w a s d e te r m in e d a c c o r d in g to A S T M 1 6 4 6 . T h e v is c o e la s t ic b e h a v io u r o f rubber m ix e s w a s m ea su red w ith th e R u b b er P r o c e ss A n a ly z e r 2 0 0 0 (R P A , A lp h a T e c h n o lo g ie s ). Strain s w e e p s w ere carried ou t at 1.0 H z fro m 0 ,2 8 % to 100% strain at co n sta n t ex tern a l tem p eratu re.

D y n a m i c - m e c h a n i c a l m e a s u r e m e n t s ( D M T A ) w h ere m a d e w ith an A R E S 3 A 5 (R h e o m e tr ix S y ste m ) w ith a p r e c is io n o f ± 5% in a d o u b le s a n d w ic h arrange­

m en t.

S tress-stra in e x p e r im e n ts w e r e carried o u t u sin g a Z w ic k 1 4 4 5 /0 3 d y n a m o m e te r eq u ip p e d w ith an o p tica l e x ten so m eter. T h e tests w e r e p erfo rm ed on d u m b b ell s p e c im e n s (D I N ) at .strain rates o f 2 0 0 m m /m in at room tem p eratu re.

D y n a m ic cu t g ro w th w a s in v e s tig a te d w ith a “Fa- tiq u e T ear A n a ly z e r ” (F T R ) s y s t e m , d e v e lo p e d b y B a y e r /C o e s fe ld [2 1 ]. T h e s y s te m h as ten lo a d units, w h ic h in d iv id u a lly co n tro l th e m in im u m stress o f the s p e c im e n s . S in g le e d g e n o te d s p e c im e n s o f 6 5 x 15 x 1.5 m m w ith a p rim ary in c is io n o f 1 m m w ere u sed under strain co n tro l. H y s te r e sis as w e ll as cra ck co n to u r len g th w ere a u to m a tic a lly reco rd ed d u rin g c y c lin g . T h e ex p erim en ta l param eters are: p u ls e d e x c ita tio n s w ith a fr eq u en cy o f 10 H z and 5 0 m s p u ls e len g th , v ariab le strain s, a m b ien t tem p eratu re.

n a n o n a p d n i a c z e

nnnonnpełniane

F ig . 1. I n f l u e n c e o f m i x i n g t i m e o n f i l l e r d i s p e r s i o n : ( A ) o p t i c a l r o u g h n e s s m e a ­

s u r e m e n t , ( B ) A F M n o n - c o n t a c t m o d e

i m a g e

R y s. 1. W p ł y w c z a s u m i e s z a n i a n a d y s ­ p e r s j ę n a p e ł n i a c z y : ( A) o p t y c z n y p o m i a r

s z o r s t k o ś c i , ( B) o b r a z z m i k r o s k o p u A FM

( b e z k o n t a k t o wy s p o s ó b o b r a z o w a n i a )

Increased mixing time przedłużony czas mieszania

F ig . 2. D e c r e a s e o f t h e t o t a l a r e a of u n d i s p e r s e d p e l l e t f r a g m e n t s a s a f u n c t i o n o f m i x i n g t i me ( A) a n d C B

l o a d i n g ( B)

R y s. 2. Z m n i e j s z e n i e c a ł k o wi t e j p o w i e r z c h n i n i e z d y s - p e r g o w a n y c h f r a g m e n t ó w g r a n u l a t u

w

g o o d d is p e r sio n , but r e v e a l a ls o a h ig h r e so lu tio n from th e a g g lo m e r a te le v e l d o w n to that o f p rim ary p a rticles [2 0 ]. T y p ica l d ata u sin g th e im a g e a n a ly s is p rogram are

p r e se n te d in F ig u r e 2. A s a resu lt the n u m b er o f p e lle t fra g m en ts, th eir area and p erim eter d ec r e a se w ith in ­ c r e a sin g m ix in g tim e. H o w e v e r , th e e ff e c tiv e n e s s o f the sh ea r fo r c e s r e s p o n s ib le fo r the fille r d isp ersio n is lo a d ­ in g d e p e n d e n t. It s e e m s that fille r d is p e r sio n is m o re e ff e c t iv e a b o v e th e p e r c o la tio n th resh o ld w h en the a g ­ g r e g a te s are in c o n ta c t w ith e a ch other.

T h e in v e s t ig a t io n o f fille r m ic r o -d is p e r s io n b y T E M is h in d ered b y th e o v e r la p p in g o f the fille r a g g r e ­ g a te s and th e fa ct that o n ly 2 D -p r o je c tio n s can b e re­

co rd ed . I f th e fille r a g g lo m e r a te s are sep arated from the rubber m ix b y ca refu l ex tra ctio n , th e a g g lo m e r a te s iz e d istrib u tio n ca n b e d eterm in ed . A s it can b e se e n from F ig u re 3 th e a g g lo m e r a te s iz e d istrib u tion is sh ifte d to w a rd s s m a lle r v a lu e s w ith in c r e a s in g m ix in g tim e.

T h e fille r n e tw o r k fo r m a tio n w a s e x a m in e d by e le c tr ic a l m e a s u r e m e n ts and d ie le c tr ic rela x a tio n s p e c ­ tro sco p y . B a s e d on th e fa ct that C B acts as a c o n d u c tiv e

F ig . 3. A g g l o m e r a t e s i z e d i s t r i b u t i o n a s a f u n c t i o n o f m i x i n g t i m e ( s y s t e m : E - S B R / N 3 3 0 ; 5 0 p h r )

R y s. 3. Ro z k ł a d w i e l k o ś c i a g l o m e r a t ó w w f u n k c j i c z a s u m i e s z a n i a ( u k ł a d : E - S B R / N 3 3 0 ; 5 0 p h r )

TOM 10 listopad - grudzień 2006 r. SiaafotK & iy nr 6

F ig . 4.

Conductivity of a rubber vulcanizate (A) with different CB loadings (<&) and permittivity of the same mixtures (B) as function of frequency (system: E-SBR/N 550)

R y s. 4.

Przewodność wulkanizatów (A) z różną zawartością sadzy (O) i stała dielektryczna tych samych mieszanek (B) w funkcji częstotliwości (układ: E-SBR/N 550)

F ig . 5.

Shift of the percolation threshold during vulcanization as indicated from dielectric measurements as function of volume filler volume fraction

(system: S-SBR/N330)

R y s. 5.

Zmiany wartości granicznych przenikalności dielektrycznej podczas wulkanizacji

funkcji ilości cząstek napełniacza

(układ: S-SBR/N330)

m aterial at th e critica l v o lu m e fra ctio n 0 * w h e r e the p erco la tio n th resh o ld is r e a c h e d th e ru b b er c o m p o u n d b e c o m e s s ig n if ic a n t ly le s s r e s is t iv e . T h r e e d is tin c t z o n e s ch a ra cterize th e co n c e n tr a tio n ran ge: th e in s u la t­

in g z o n e , th e p e r c o la tio n z o n e and c o n d u c tiv e z o n e . B y u sin g d ie le c tr ic re la x a tio n s p e c tr o s c o p y th e p e r c o la tio n th resh o ld can b e d e r iv e d eith e r fro m th e s ig n ific a n t s te p w is e in c r e a s e o f th e c o n d u c tiv ity in th e lo w fre­

q u e n c y r e g io n (up to 1 0 ,0 0 0 H z ) or fr o m th e in c r e a s e in th e p erm ittiv ity v a lu e s in th e s a m e r a n g e o f fr e q u e n c ie s . T h is is s h o w n in F ig u re 4 A an d 4 B .

T h e s lo p e o f the p lo ts lo g c o n d u c tiv ity v s . lo g fre­

q u e n c y eq u a ls 1 fo r C B c o n c e n tr a tio n s b e lo w th e p er­

c o la tio n th resh o ld . A b o v e th e p e r c o la tio n th r e s h o ld th e s lo p e o f th e c u r v e s is lo w e r ( 0 .6 5 ) in d ic a tin g an a n o m a ­ lo u s d iffu sio n o f th e ch a rg e carriers, w h ic h ty p ic a lly o ccu rs for h e te r o g e n e o u s m a teria ls w ith fractal clu sters.

W ith in c r e a s in g lo a d in g th e c o n d u c t iv e p la te a u b e ­ c o m e s larger and th e c r o ss o v e r in to th e a n o m a lo u s ch arge d iffu s io n r e g im e is sh ifte d to w a r d s h ig h e r fre­

q u en cy . T h is resu lt in d ic a te s that th e p o ly m e r gap b e ­ tw e e n th e fille r clu s te r s in th e n e tw o r k b e c o m e s sm a ller and th e s iz e o f th e c lu sters b e c o m e s a ls o sm aller.

B e c a u s e th e p r e c is e d e te c tio n o f th e p erco la tio n th resh o ld d ifferen t e ffe c ts o n th e fille r n etw o rk fo rm a ­ tion in sp e c ia l and m ic r o -d is p e r s io n in g en era l (d u e to c h a n g e s in th e p o ly m e r n ature, fille r a ctiv ity , sto ra g e tim e ) can b e m o n ito red . F ig u r e 5 s h o w s tw o d ifferen t d ia g ra m s, w h ic h in d ic a te th e sh ift o f th e p erco la tio n th resh o ld d u rin g v u lc a n iz a tio n o f a E -S B R co m p o u n d .

3.2. Influence of mixing on filler dispersion

K e e p in g th e fill fa cto r o f C B fille d c o m p o u n d s ( e s ­ ta b lish ed in p relim in a ry e x p e r im e n ts ) as co n sta n t the in flu e n c in g fa cto rs w h ic h d e te r m in e th e s p e c ific en er­

gy, th e m ix in g tim e , th e rotor s p e e d and tem p eratu re o f th e c o o lin g w ater w e r e v a ried s y s te m a tic a lly . T h e c o o l-

SćaAtotK&ity nr 6 listopad - grudzień 2006 r. TOM 10

mnompełninae

F ig . 6. ^E f f e c t o f c o o l i n g o n i n c o r p o r a t i o n t i m e a n d C B m a c r o - d i s p e r s i o n ( s y s t e m : E - S B R / N 3 3 0 ; 5 0 p h r )

R y s. 6. ^{W p ł y w} c h ł o d z e n i a n a c z a s w r a b i a n i a i m a k r o - d y s p e r s j ę s a d z y ( u k ł a d : E - S B R / N 3 3 0 ; 5 0 p h r ) .

in g e ff e c t on m a c r o -d isp e r s io n is d e p ic te d in F ig u re 6.

T h e sh o rtest C B in co rp o ra tio n is o b se r v e d for the h ig h ­ e st tem p eratu re o f th e c o o lin g w ater. O n the o n e hand the lo w e r v is c o s it y e n a b le s the rubber m elt to fill in th e a g g lo m e r a te v o id s in a sh orter tim e but on the oth er h a n d e x a c t ly th e lo w e r v is c o s it y is r e s p o n s ib le fo r s m a lle r sh ea r fo r c e s and fin a lly a red u ced d eg ree o f d is p e r sio n . B e s id e s th e larger in co rp o ra tio n tim e th e b e n e fit o f c o o lin g is d em o n stra ted b y th e h ig h er sp e e d o f d is p e r sio n an d , the m o s t im p ortan t, by a h ig h er d e ­ g ree o f d is p e r s io n and a narrow a g g lo m e r a te s iz e d istri­

b u tion .

B y in c r e a s in g th e m ix in g tim e and the rotor sp e e d d u rin g m ix in g th e m a c r o -d isp e r s io n is in crea sed s ig ­ n ific a n tly in th e p e r io d o f tim e and than ste a d ily a p ­ p r o a c h e s a sta tio n a ry fin a l v a lu e (F ig . 7 ). H o w e v e r , e le c tr ic a l D C m e a s u r e m e n ts in d ic a te that m ic r o -d is p e r ­ sio n is further im p r o v e d u n d er th e s e c o n d itio n s. T ak in g in to a c c o u n t that d u rin g m ix in g th e c o m p o u n d tem p era ­ ture in c r e a s e s it ca n b e e a s ily u n d ersto o d that th is le v e l o f m a c r o -d isp e r s io n ca n b e o v e r c o m e after c o o lin g in a s e c o n d a r y m ix in g step . A s th e p erco la tio n th resh o ld a ls o th e lim itin g le v e l o f d is p e r sio n is in f lu e n c e d in the

F ig . 7. I n f l u e n c e of m i x i n g t i m e a n d r o t o r s p e e d o n c a r b o n b l a c k d i s p e r s i o n ( s y s t e m : E - S B R / N 3 3 0 ; 5 0 p h r )

R y s. 7. ^{W p ł y w} c z a s u m i e s z a n i a i s z y b k o ś c i r o t o r a n a d y s p e r s j ę s a d z y ( u k ł a d : E - S B R / N 3 3 0 ; 5 0 p h r )

s a m e w a y as th e p e r c o la tio n th resh o ld by the c h e m ic a l nature o f th e p o ly m e r and the structure and su rfa ce a c tiv ity o f th e filler.

F o r a c o m p a r a tiv e stu d y a s in g le sta g e d is c o n tin u ­ o u s m ix in g s e q u e n c e w a s c h o s e n starting fro m b a le E -S B R an d C B N 2 3 4 (s a m p le B -M 5 ) as w e ll as a R F C (s a m p le P -M 1 2 ) w ith th e id e n tic a l p o ly m e r /fille r ratio.

T h e m ix in g tim e w a s k ep t co n sta n t in th e p re-sta g e and v a ried s y s te m a tic a lly in th e fin is h in g sta g e o f th e p ro ­ c e s s . T h e so fte n e r w a s in co rp o ra ted during th e fin is h ­ in g sta g e, w h e n C B d is p e r sio n w a s n early co m p le te d . T h e sa m e c o n d it io n s w e r e m a in ta in ed for d is c o n tin u ­ o u s m ix in g o f R F C . D u e to th e fa ct that R F C alread y c o n ta in e d fille r th e p r e -s ta g e o f m ix in g w a s red u ced to p la s tic iz e th e fr e e f lo w in g m aterial. C h aracteristic d ia ­ gra m s w e r e r eco rd ed fo r the p o w e r co n su m p tio n (F ig . 8 ) and th e s to c k tem p eratu re.

T h e m a in a d v a n ta g e o f th e R F C is th e r e la tiv e ly rap id in c r e a s e in th e p o w e r c o n s u m p tio n fo r R F C to a fir st p ea k . S u b s e q u e n tly a s lig h t d e c r e a s e in the p o w e r

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u a n o n a p e ł n i a c z e

F ig . 8. P o we r c o n s u m p ­ t i o n d i a g r a m f o r b a l e

E - S B R r ub b e r m i x e s

wi t h C B N 2 3 4 ( B- M 5 )

a n d R F C ( P- M J 2 )

R y s. 8. Di a g r a m p o b o ­ r u m o c y d l a m i e s z a n k i

k a u c z u k u E - S B R z s a ­

d z ą N 2 3 4 ( B - M 5 ) i R F C

( P - M 1 2 )

nsmompetniane

F ig . 9 . S c r e w c o n f i g u r a t i o n s u s e d ( A ) a n d t h e f u n c t i o n o f t h e s p e c i f i c e n e r g y i n p u t ( B )

R ys. 9. K o n f i g u r a c j a ś l i m a k a wy t ł a c z a r k i ( A ) i wa r t o ś c i e n e r g i i p o b r a n e j p r z y s p o r z ą d z a n i u m i e s z a n k i ( B )

c o n s u m p tio n is attrib u ted to an in c o r p o r a tio n o f filler.

T h e d is p e r s iv e m i x in g is in d ic a t e d b y th e s e c o n d p o w e r p ea k f o llo w e d b y a d e c a y w h e r e th e fin a l d e ­ g r e e o f d is p e r s io n is o b ta in e d . C o m p a r e d to th e R F C th e p o w e r c o n s u m p tio n d ia g r a m d e m o n s tr a te s a c o n ­ sid e r a b le d e la y fo r th e in c o r p o r a tio n an d d is p e r s io n as w e ll.

A t a m ix in g tim e o f 1 5 0 s (c o r r e s p o n d in g to the first p o w e r c o n su m p tio n p ea k ) th e b a le rubber m ix e s co n ta in h ig h a m o u n t o f la rg e a g g lo m e r a te s and p e lle t fra g m en ts w h erea s th e R F C m ix d e m o n s tr a te s a d isp er­

sio n in d e x o f 9 9 .2 % . P r o lo n g e d m ix in g le a d s to a s iz e red u ction o f th e u n d isp ersed C B and b ro k en p e lle ts . A t 3 6 0 s the d e g r e e o f d is p e r sio n fo r th e b a le m ix e s reach 97% and that o n e o f th e R F C m ix e s ten d s to 9 9 .4 % . T h is e v o lu tio n is p a r a lle le d b y th e a m o u n t o f b o u n d rubber. T h e larger p o ly m e r -fille r in te r fa c e p r o v id e d b y better d is p e r sio n is d o c u m e n te d at a m ix in g tim e o f 3 6 0 s by a b o u n d rubber v a lu e o f 23% fo r th e b a le m ix e s and o f 3 1 .2 % for the R F C m ix e s .

T h e b eh a v io u r o f R F C in th e c o n tin u o u s m ix in g p r o c e ss is a ffe c te d b y th e d e s ig n o f th e m ix in g e le m e n ts and the sc r e w c o n fig u r a tio n . B o th p erm it a v a ria tio n o f

the m ix in g e f f ic ie n c y fro m the fe e d in g z o n e to the tip o f th e screw . A s a g en era l resu lt th e p o w e r c o n su m p tio n in the c o n tin u o u s m ix in g p r o c e ss is r o u g h ly 1/3 o f that in the d is c o n tin u o u s m ix in g p r o c e s s . B y in c r e a s in g the sc r e w s p e e d the p o w e r c o n s u m p tio n rises b y le s s than 20% as th e e x it tem p eratu re a ls o d o e s . A n o th er feature o f th e c o n tin u o u s p r o c e ss is th e s ig n ific a n t sh orter resi­

d e n c e tim e. T ak in g in to a c c o u n t that c o n v e y in g e le ­ m en ts d o n o t co n trib u te to m ix in g , the actu al m ix in g tim e for R F C ca n b e estim a te d to le s s than 4 0 s.

D iffe r e n t sc r e w c o n fig u r a tio n s h a v e b een u sed for stu d y in g th e m ix in g b e h a v io u r (F ig . 9 A ). K e e p in g in all the e x p e r im e n ts the th rou gh p u t and th e torq u e as c o n ­ stant, it w a s s h o w n that b y in c r e a s in g th e d is p e r siv e and d istrib u tiv e m ix in g e le m e n ts th e o u tp u t tem p eratu re in ­ c rea ses. T h e fille r d is p e r sio n and the M o o n e y v is c o s ity o b ta in ed u n d er th e s e c o n d itio n s w ith th e s e v e n screw c o n fig u r a tio n are s h o w n in F ig u re 9 B . W ith the h ig h er s p e c ific e n erg y in p u t the d is p e r sio n in c r e a s e s and the M o o n e y v is c o s it y d e c r e a se s .

D u rin g th is stu d y w e d id n o t in v e s tig a te w h eth er the share o f n o n -d isp e r se d C B is r e d u ced b y a s e c o n d run.

F ig . 10. I n f l u e n c e of t h e m o l e c u l a r we i g h t o f t h e r u b b e r ( A) a n d e f f e c t o f s e l e c t i v e e l a s t o m e r - f i l l e r i n t e r a c t i o n s ( B ) o n C B d i s p e r s i o n i n d e x ( D I )

R ys. 10. ^{W p ł y w} c i ę ż a r u c z ą s t e c z k o w e g o k a u c z u k u ( A) i e f e k t u s e l e k t y w n y c h o d d z i a ł y w a ń e l a s t o m e r - n a p e ł n i a c z ( B ) n a w s k a ź n i k d y s p e r s j i s a d z y ( D I )

SCtui&uttentf, nr 6 listopad - grudzień 2006 r. TOM 10

3.3. Influence of materials on filler dispersion

A ll th e m o le c u la r ch a ra cteristics o f the p o ly m e r that su p p ort th e d is in te g r a tio n o f the fille r a g g lo m e r a te s in a sh ea r fie ld b e c o m e im p ortan t for d isp ersio n . S in c e th e sh ea r fo r c e s crea ted in th e intern al m ix e r or T S E are tra n sm itted to th e e x tern a l su rfa ce o f fille r a g g lo m e ­ rates th ro u g h th e p o ly m e r c h a in s a ttach ed on th e s o lid su rfa ce th e o c c u r r e n c e and th e stren gth o f filler-ru b b er c o n ta c ts p la y a d e c is iv e ro le in th is p h e n o m e n o n .

T h e im p o r ta n c e o f th e m o le c u la r w e ig h t (or v is c o ­ sity ) o f th e p o ly m e r b e c o m e e v id e n t b y c o m p a rin g sa m ­ p le s o b ta in e d u n d er th e sa m e m ix in g c o n d itio n s fro m N R w ith d if f e r e n t m o le c u la r w e ig h t ( 5 9 0 ,0 0 0 and 2 3 0 ,0 0 0 g /m o le ) and C B N 3 3 0 . T h e d eg ree o f m a cro ­ d is p e r sio n w a s s y s te m a tic a lly su p erior fo r th e m ix e s w ith h ig h m o le c u la r w e ig h t N R (F ig . 1 0 A ). I f the lo w m o le c u la r w e ig h t N R is r e p la c e d b y E - S B R , w ith a sim ila r m o le c u la r w e ig h t, the d eg ree o f d is p e r sio n is h ig h er in th e E -S B R m ix than in th e h ig h m o le c u la r N R m ix (F ig . 1 0 B ). T h is d e m o n stra tes that in p r e se n c e o f m o re fa v o u r a b le p o ly m e r -fille r in te r a c tio n s th e m o ­ lecu la r w e ig h t o f th e rubber p la y s a se co n d a ry role.

T h e in flu e n c e o f th e c h e m ic a l nature o f th e p o ly ­ m er on th e filler-ru b b er in tera ctio n can b e clea r ly se e n w h en th e e le c tr ic a l p e r c o la tio n th resh o ld is c o n sid ered . B y in v e s tig a tio n o f th e e le c tr ic a l p erco la tio n th resh o ld for C B N 3 3 0 un d er s im ila r m ix in g c o n d itio n s in a s e ­ q u e n c e o f rep r e se n ta tiv e rubbers w e o b se r v e d that the c o n cen tra tio n at w h ic h th e fille r n etw o rk is fo rm ed co r ­ relates a lm o st p e r fe c tly w ith th e so lu b ility p aram eter o f the p o ly m e r as d e te r m in e d b y IG C (in v e r se g a s -c h r o ­ m a to g ra p h y ) [2 1 ]. R u b b ers w ith a saturated b a c k b o n e lik e E P D M are n o t in tera ctin g as e ff ic ie n tly w ith C B and fo rm a fille r n e tw o r k at lo w fille r c o n cen tra tio n s, as rubbers w ith u n satu rated b a c k -b o n e s and w ith s p e c if i­

c a lly in tera ctin g g ro u p s (i.e . p h e n y l) d o (F ig . 11).

P o ly m e r s w h ic h are fu n c tio n a liz e d for the p u rp o se o f a b etter in tera ctio n w ith a particular fille r sh o u ld d em o n stra te a s y s te m a tic a lly h ig h er p erco la tio n th re­

sh o ld and b etter d is p e r sio n . It w a s a lso o b se r v e d for a lm o s t a ll c a s e s that th e d is p e r sio n k in e tic s are im -

F ig . 11.

Electrical percolation threshold

solubi­

lity parameter of the elastomer

R y s. 11.

Wartości graniczne przenikalności dielektrycz­

nej w funkcji parametru rozpuszczalności (Hilde- branda) elastomerów

p r o v e d i f th e in tera ctio n p o ten tia l o f the p o ly m e r in ­ c r e a se s .

C o n c e r n in g th e in flu e n c e o f the fille r on the d isp er­

s io n p r o c e s s it w a s fo u n d that the a g g reg a te sh a p e or stru ctu re and th e su rfa ce a c tiv ity p la y s a p red o m in a n t ro le. I r r e s p e c tiv e o f th e su rfa ce s p e c ific area o f the C B th e d e g r e e o f m a c r o - d is p e r s io n in c r e a s e s w h e n th e stru ctu re o f C B in c r e a s e s. T h is b e c o m e s m o re e v id e n t fro m th e in v e s tig a tio n o f the d isp ersio n k in etics. T h e rate co n sta n t fo r d is p e r sio n d eterm in ed from th e n o n - d is p e r se d C B as a fu n c tio n o f tim e in c r e a s e s w ith the D B P -n u m b e r and d e c r e a se s w ith th e su rfa ce s p e c ific area o f th e C B (F ig . 12).

D u e to th e in c r e a s in g irregularity o f the a g g reg a tes w h ic h in c r e a s e s w ith th e D B P -n u m b e r the fre q u e n c y o f in te r -a g g r e g a te co n ta c ts b e c o m e s sm aller. I f th e a v er­

a g e e n e r g y fo r in te r -a g g r e g a te in tera ctio n s is c o n s i­

d ered as c o n sta n t, th e in tegral in teraction en ergy, w h ic h k e e p s th e a g g lo m e r a te as su ch , is further red u ced for h ig h ly b ra n ch ed a g g r e g a te s .

F ig . 12.

Dispersion kine­

tics for CB with different DBP number; UCB

un­

dispersed carbon black

R y s. 12.

Kinetyka dysper­

sji sadzy o różnej liczbie ftalowej (DBP number);

UCB

sadza niezdysper- gowana

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n a n o n a p e ł n i a c z e

It w as sh o w n b y electrica l resistiv ity m ea su rem en ts that the electrical p erco la tio n th resh old o f C B fille d c o m ­ p ou n d s is sh ifted tow ard s h ig h er filler co n cen tra tio n s w h en the prim ary p article s iz e in crea ses or th e su rface sp e c ific area d ecrea ses [2 3 ]. T h e resu lt in d ica tes that the m icro-d isp ersio n o f C B w ith sm a ll prim ary p articles is im p aired b y ch a n g es in su rface activity.

B y in v e s tig a tin g th e en e r g y site d istrib u tio n o f a large variety o f C B w ith p r e c is e sta tic g a s a d so rp tio n m ea su rem en ts it w a s r e c o g n iz e d that a ll fu rn a ce b la ck s s h o w q u a lita tiv e ly the sa m e e n e r g y d istrib u tio n on the unit su rface. F ou r d is c r e te ty p e s o f e n e r g e tic site s are p resen t in all g rad es. W h ile the p rim ary p a rticle s iz e d e c r e a se s the su rfa ce fra ctio n o f th e h ig h en e r g y sites in c r e a s e s [7]. T h is lea d s to s ig n ific a n t d iffe r e n c e s fo r the a v era g e stren gth o f in te r -a g g r e g a te in te r a c tio n s for fin e and c o a rse C B . F rom th is it b e c o m e s c le a r w h y th e p erco la tio n th resh o ld o f fille r s w ith h ig h su rfa ce s p e ­ c if ic area o ccu rs at lo w c o n c e n tr a tio n an d o v e r a ll d e ­ g ree o f d is p e r sio n is rather p o o r o n e.

3.4. Influence of filler dipersion on rheological properties and processability

I f the M o o n e y -v is c o s ity o f sa m p le s w a s m e a su red 2 4 h after m ix in g an in v e r s e p r o p o r tio n a lity b e tw e e n d isp e r sio n and v is c o s it y is o b s e r v e d (F ig . 13). T h e e f ­ fe c t can b e e x p la in e d b y th e s iz e red u ctio n o f large fra g m en ts o f p e lle ts and a g g lo m e r a te s w h ic h are c o ­ v ered during m ix in g b y rubber, w h ic h r e d u c e s th e h y ­ d ro d y n a m ic flo w barriers.

T h e p r o c e s s in g b e h a v io r o f u n cu red m ix e s w a s in ­ v e stig a te d b y R PA m e a s u r e m e n ts. B y in c r e a s in g the

F ig . 13.

Evolution of Mooney viscosity as a function of mixing time and rotor speed (system: E-SBR/N330; 50 phr)

R y s. 13.

Zmiany lepkości Mooney'a w funkcji czasu mieszania i prędkości rotora (układ: E-SBR/N330; 50 phr)

m ix in g tim e th e sto ra g e m o d u lu s in th e r e g im e o f sm a ll d efo rm a tio n s is in an y c a s e red u ced . T h is in d ica tes a better m ic r o -d is p e r sio n . I f b a le rubber m ix in g is c o m ­ pared w ith R F C m ix in g u n d er th e sa m e c o n d itio n s a s ig n ific a n t drop o f

G '

G'

w o rk is further red u ced .

In rubber p r o c e s s in g th e e la s tic r e sp o n s e o f fille d c o m p o u n d s is r e fle c te d in term s o f d ie s w e ll and the

F ig . 14.

Comparison of RPA-strain-amplitude sweeps of G ’for bale rubber mixes (B-M5 and B-M7) and RFC mixes

(P-M10 and P-M12); system: E-SBR/N234, 50 phr, t

80 ° C ,f = 0,33 Hz; P-M10 (RFC) - 3 min mixing, P-M12 (RFC) - 5 min mixing; B-M5 - bale mix, 3 min mixing, B-M7 - bale mix, 5 min mixing

F ig . 14.

Porównanie odchylenia G ’ w funkcji naprężę nie-amplituda dla mieszanek kauczukowych (B-M5 i B-M7) i przedmieszek kauczukowo sadzowych RFC (P-M10 i P-M12); Oznaczenia za pomocą aparatu RPA. Układ:

E-SBR/N234, 50 phr, t

80 °C, f

0,33 Hz; P-M10 (RFC) - 3 min mieszania; P-M12 (RFC) - 5 min mieszania;

B-M5 - mieszanka kauczukowa: 3 min mieszania, B-M7- mieszanka kauczukowa: 5 min mieszania

nr 6 listopad - grudzień 2006 r. TOM 10

n a n m a p e ł n i a c z e

a p p ea ra n ce o f rubber ex tru d a tes. T h is p h e n o m e n o n is a s s o c ia te d w ith th e e la s tic re c o v e r y c a u se d b y th e in ­ c o m p le te r e le a s e o f lo n g -c h a in m o le c u le s o rien ted in the sh ea r fie ld o f th e extru d er d ie. A c tiv e fille r s red u ce the e la s tic c o n tr ib u tio n and th e d e c r e a se the e ff e c tiv e rela x a tio n tim e o f th e m ix [2 4 ]. T h u s, d ie s w e ll is g e n e ­ rally r e d u c e d b y fille r lo a d in g and b e c o m e le s s p ro­

n o u n c e d w h e n th e structure o f th e fille r in c r e a s e s and d is p e r sio n r e a c h e s a h ig h e r le v e l.

F ro m F ig u r e 14 it ca n b e o b se r v e d that at h ig h strain a m p litu d e s G ’ is h ig h e r for b a le rubber m ix e s than fo r R F C m ix e s . T h is in d ic a te s a lso that s w e ll for R F C is le s s p r o n o u n c e d (d u e to better m ic r o -d is p e r ­ sio n ).

A n o th e r e ff e c t w h ic h a ffe c ts p r o c e ss in g is the reor­

g a n iz a tio n o f th e fille r n etw o rk durin g sto ra g e or d u rin g th e v u lc a n iz a tio n p r o c e s s . T h e e ffe c t is k n o w n as “f l o c ­ c u la tio n ” and le a d s to an in c r e a s e o f the fille r -fille r c o n ta c ts and p ro b a b ly to a s iz e red u ctio n o f the p o ly m e r gap b e tw e e n fille r c lu sters. T h e resu lt is an in c r e a s e o f the m e lt v is c o s it y . T h e reo rg a n iz a tio n o f th e fille r n et­

w o rk is fa c ilita te d b y a lo w m o le c u la r w e ig h t fraction o f th e p o ly m e r or a r e d u c e d v is c o s it y o f the m atrix and by fille r s w ith stro n g in ter-a g g reg a te in tera ctio n s [2 5 ].

3.5. Dynamic properties

T h e b e n e fic ia l e ff e c ts o f th e im p ro v ed filler-ru b b er in te r a c tio n s can b e s e e n d ir e c tly fro m d y n a m ic -m e ­ ch a n ic a l p ro p erties. A lm o s t all s y s te m s in v e s tig a te d in our stu d y d em o n stra te a stea d y d e c r e a se o f ^{G ’ 0 ( y )} and the d iffe r e n c e ^{G ’ J y ) - G ’ ^ y)} w h e n the m ix in g tim e or the rotor s p e e d are in c r e a s e d (F ig . 15).

P a ra llel to th e s e c h a n g e s the m a x im u m o f the lo s s m o d u lu s ^{G ”} is re d u c e d and sh ifte d tow a rd s lo w e r strain a m p litu d es. A t th e sa m e tim e th e e le c tr ic a l p erco la tio n th resh o ld o f th e s y s te m u n d er co n sid e r a tio n is sh ifte d to w a rd s h ig h e r C B lo a d in g s .

A sim ila r e ff e c t ca n b e o b ta in ed i f th e fille r su rfa ce a c tiv ity is c h a n g e d d u e to a c h e m ic a l or p h y sic a l treat-

F ig . 15. D e c r e a s e o f t h e P a y n e e f f e c t wi t h t h e m i x i n g t i me

R ys. 15. Z m n i e j s z e n i e e f e k t u P a y n e ’ a z p r z e d ł u ż a n i e m c z a s u m i e s z a n i a

m en t. S u c h a trea tm en t w e a k e n s th e in ter-a g g reg a te in ­ tera ctio n s and p r o m o te s stro n g er fille r -p o ly m e r in tera c­

tio n s and b etter m ic r o -d is p e r sio n . A s can b e s e e n from a so c a lle d G -p lo t th e treatm en t lea d s to a red u ctio n o f

G ” . F lo w e v e r , th e red u ctio n in h y ste r e sis is m o re p ro ­ n o u n c e d (F ig . 16). R e c e n tly , sim ila r resu lts w ere o b ­ ta in ed b y trea tm en t o f C B w ith a tm o sp h eric n itro g en p la s m a [2 6 ].

C o m p a r e d to ca rb o n b la ck , s ilic a can form stron ger and m o re d e v e lo p e d a g g lo m e r a te s. T h is is m a c r o s c o p i- c a lly r e fle c te d in h ig h er sto ra g e m o d u lu s at lo w strain a m p litu d e and larger P a y n e e ff e c t at lo w and h ig h te m ­ p eratu res. In a h y d ro ca rb o n p o ly m e r the p o ly m e r -s ilic a in tera ctio n is lo w e r than the in ter-p article in tera ctio n su p p o rted b y th e p o la r sila n o l g ro u p s. C o n se q u e n tly the p o o r c o m p a tib ility w ith the e la s to m e r and the stron g fille r -fille r in tera ctio n d u e to th e h ig h su rfa ce a ctiv ity

F ig . 17. P a y n e e f f e c t f o r f i l l ed B R - c o m p o u n d s w i t h s u r ­ f a c e t r e a t e d s i l i c a

R y s. 17. E f e k t P a y n e ’ a d l a m i e s z a n e k B R n a p e ł n i o n y c h k r z e m i o n k ą o m o d y f i k o w a n e j p o w i e r z c h n i

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F ig . 16. ^G - p l o t f o r S B R vu l e a n i z a t e s f i l l e d w i t h u n ­ t r e a t e d C B a n d c h e m i c a l l y t r e a t e d C B

R y s. 16. Wy k r e s p r z e d s t a w i a j ą c y z m i a n y m o d u ł u G ’ i G ” d l a wu l k a n i z a t ó w S B R n a p e ł n i o n y c h s a d z ą n i e m o -

d y f i k o w a n ą i s a d z ą m o d y f i k o wa n ą c h e m i c z n i e

lead to a m ore d e v e lo p e d fille r n etw o rk . C h a n g in g the su rfa ce a c tiv ity o f s ilic a b y trea tm en t w ith m o n o - or b ifu n ctio n a l sila n e s (a b b rev ia ted as S i- C X ), th e fille r d is p e r sio n in p o ly d ie n e s can b e s ig n ific a n tly im p r o v e d w h ile

G ’o

G ”mwc

tio n s are d ra m a tica lly r ed u ced and th e fille r d is p e r sio n im p ro v ed (F ig . 17).

S im ila r b e n e fic ia l e ff e c ts are o b s e r v e d in th e g la s s tem p eratu re

(Tg)

”max.

T h e in co rp o ra tio n o f p recip ita te d s ilic a in to S B R c o n ta in in g 7 m o l % e p o x y -g r o u p s lea d s to a s ig n ific a n t h ig h er in crea se o f

G ”max

Tg

3.6. Effect on ultimate proper­

ties

A lth o u g h th e m e c h a n is m o f te n s ile fa ilu r e o f e la s ­ tom ers has n ot b een fu lly u n d ersto o d , it ca n b e reg ard ed as cata stro p h ic tearin g b y g r o w th o f cra ck s in itia te d b y a ccid en ta l fla w s su c h as la rg e fra g m en ts o f p e lle ts , m i­

c r o -d e w e ttin g or c a v itie s on th e fille r su r fa c e [2, 2 8 ]. A t the sa m e lo a d in g the s m a ll p a rticle fille r s , h a v in g b o th the sm a ll a g g reg a te s iz e and h ig h su r fa c e a ctiv ity , c o n ­ fer a greater re sista n c e a g a in s t crack p ro p a g a tio n . T h u s,

F ig . 18 . I n f l u e n c e of f i l l er d i s p e r s i o n o n t e n s i l e s t r e n g t h o f S B R / C B N 3 3 0 ( 5 0 p h r ) c o mp o u n d s

R y s. 18. W p ł y w d y s p e r s j i n a p e ł n i a c z a n a w y t r z y m a ł o ś ć n a r o z c i ą g a n i e m i e s z a n e k S B R / C B N 3 3 0 ( 5 0 p h r ) .

it is n o t su rp risin g that th e te n s ile stren gth in crea ses w ith in c r e a s in g su rfa ce area o f th e carb on b la ck s. T h e te n s ile stren gth in c r e a s e s further w ith fille r lo a d in g up to a certain le v e l is rea ch ed . T h is o b se r v a tio n w a s c o n ­ n ected w ith th e fille r d is p e r sio n , b a se d on the a ss u m p ­ tion that large a g g lo m e r a te s m a y fu rn ish fla w s that can crea te cra ck s in th e m aterial.

U s in g th e s a m p le s o b ta in ed w ith in th e sta tistica l d e sig n m ix in g e x p e r im e n t (F ig u re 1 8) it w a s o b se r v e d

F ig . 19. C r a c k p r o p a g a t i o n a s a f u n c t i o n o f t h e n u m b e r of c y c l e s ; s y s t e m s : E - S BR b a l e m i x e s ( B) a n d R F C m i x e s ( P ) m i x e d i n t h e i n t e r n a l mi x e r . M i x i n g t i m e B- M 3 a n d P - M 1 0 : 3 m i n , B - M 5 , P - M 1 4 - a d d i t i o n a l t w o r o l l m i l l i n g

R ys. 19. P r o p a g a c j a p ę k n i ę c i a j a k o f u n k c j a l i c z b y c y k l i ; m i e s z a n k a E - S B R ( B ) i m i e s z a n k a R F C ( P) s p o r z ą d z a n e w mi k s e r z e w e w n ę t r z n y m . C z a s m i e s z a n i a B - M 3 i P - M 1 0 : 3 mi n , B - M 5 , P - M 1 4 - d o d a t k o wo w a l c o w a n e n a

w a l c a c h l a b o r a t o r y j n y c h

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that te n s ile stren g th is rela ted to th e d eg r e e o f m a cro ­ d is p e r sio n . In an in itia l sta g e o f th e m ix in g p r o c e ss any in c r e a s e in m ix in g tim e and rotor sp e e d lea d s to an im p r o v e m e n t o f th e te n s ile stren gth w h ic h p a ra llels th e e v o lu tio n o f th e m a c r o -d isp e r s io n in the rubber m ix . A t p r o lo n g e d m ix in g or at h ig h rotor sp e e d th e te n s ile stren gth le v e ls up at a g iv e n lim it in th e sa m e w a y as the m a c r o -d isp e r s io n d o e s (F ig . 1 8 ).

M e a su r e m e n ts o f sta tic cu t g ro w th re sista n c e s h o w that v u lc a n iz a te s w ith b etter d is p e r sio n h a v e an in ferio r cu t g r o w th r e sista n c e . T h is m is le a d in g resu lt is a c o n s e ­ q u e n c e o f th e fa c t that th e sto ra g e m o d u lu s o f th e s a m ­ p le s w ith th e b etter d is p e r sio n is lo w e r and th e e lo n g a ­ tion d u rin g th e fo r c e -c o n tr o lle d test is higher.

I f d y n a m ic cra ck p ro p a g a tio n is c o n s id e r e d th e a b o v e m e n tio n e d p a r a lle lism to m a c r o -d isp e r s io n is n o m o re o b s e r v e d . D y n a m ic c ra ck p ro p a g a tio n e x p e r i­

m en ts p e r fo r m e d o n a “Tear A n a ly z e r ” under rep ea ted p u ls e d stra in in g d em o n stra te th e w id e sp an o f p rop er­

ties b ro u g h t a b o u t b y im p r o v in g th e state o f fille r d is ­ p ersio n . A n e v a lu a tio n o f th e crack p rop a g a tio n rate w ith in th e in itia l stea d y state s e c tio n o f th e cu rv es lea d s to a q u a n tita tiv e p ictu re and p r o v id e s in p u t data for fracture m e c h a n ic s c a lc u la tio n s . F ig u re 19 illu stra tes the e ff e c t o f th e state o f d is p e r sio n on th e e v o lu tio n o f th e cra ck co n to u r le n g th in rela tio n to th e n u m b er o f c y c le s fo r b a le rubber m ix e s and R F C m ix e s .

T h e crack p ro p a g a tio n rates o f S B R -v u lc a n iz a te s o b ta in ed b y c o n v e n tio n a l b atch m ix in g and o f S B R - -v u lc a n iz a te s o b ta in e d fro m R F C w ith a h ig h er d eg ree o f m ic r o -d is p e r s io n d em o n stra te s ig n ific a n t d iffe r e n ­ c e s. A ro u g h e s tim a tio n o f th e crack p ro p a g a tio n until fa ilu re s h o w s that th e p e r io d o f the lin ea r cu t p ro p a g a ­ tion and th e sh a p e o f th e cra ck are sim ila r fo r th e sa m e m ix in g tim e. A n a d d itio n a l m ix in g step on th e tw o roll

F ig . 2 0. I n f l u e n c e of f i l l e r d i s p e r s i o n o n d y n a m i c c u t g r o w t h r e s i s t a n c e ( D C G ) o f S B R / C B ( N3 3 0 ) c o m ­

p o u n d s

R y s. 20. W p ł y w d y s p e r s j i n a p e l n i a c z a n a o d p o r n o ś ć n a w z r o s t r y s y p r z y w i e l o k r o t n y m z g i n a n i u m i e s z a n e k

S B R / C B ( N 3 3 0 )

m ill o f th e R F C fo r th e sa m p le P -M 1 4 r e v e a ls th e e n o r ­ m o u s r o le p la y e d b y an im p r o v e d m ic r o -d is p e r sio n b y d o u b lin g th e lin ea r cra ck g ro w th reg im e.

C o m p a r e d to th e c o n v e n tio n a l m ix e d m aterials, th e s a m p le s o b ta in e d fro m R F C resu lt in a m aterial w ith a fo u r f o ld (3 0 % strain ) to te n fo ld (20% strain ) lo w e r c r a c k p r o p a g a t io n rate. T h e e f f e c t s are m o r e p r o ­ n o u n c e d at lo w e r strain s [2 9 ]. T h e im p o rta n ce o f the fille r m ic r o -d is p e r s io n is u n d erlin ed i f the e v o lu tio n o f th e d y n a m ic cra ck g ro w th r e sista n c e is rep resen ted as a fu n c tio n o f th e m ix in g param eters. T h e m a c r o -d isp e r ­ s io n and te n s ile stren g th s h o w a p lateau v a lu e th e d y ­ n a m ic cu t g r o w th r e sista n c e d em o n stra tes an e x p o n e n ­ tial in crea se.

W h e n d u rin g th e m ix in g p r o c e ss the co n cen tra tio n o f p e lle t fr a g m e n ts is red u ced and th e co n cen tra tio n o f s m a ll a g g lo m e r a te s and a g g r e g a te s in crea ses, the p ro ­ b a b ility fo r e n e r g y d is sip a tio n and e ff e c tiv e d e v ia tio n o f th e c u t p ath in c r e a s e s a lso .

4. Conclusion

F ille r d is p e r s io n and the fo rm a tio n o f the rubber- -fille r in te r fa c e s ap p ear as d eterm in a n t p r o c e s s e s for h ig h p e r fo r m a n c e e la s to m e r s . A lm o s t all m o le c u la r c h a r a c te r istic s o f th e p o ly m e r and the m o r p h o lo g ic a l p a ram eters o f th e fille r s, e s p e c ia lly the su rfa ce a c tiv ity ca n b e e x p lo ite d to en h a n c e th e d e g r e e o f d isp ersio n in d is c o n tin u o u s (b a tch ) m ix in g p r o c e s s e s . A v e r ita b le p r o g r e ss is m a d e b y u sin g R F C ’s w ith a m u ch better d is p e r sio n in th e in itia l state. T h is m aterial is su ita b le fo r c o n tin u o u s m ix in g on T S E or a sim ila r eq u ip m en t w h ile th e e n e r g y c o n su m p tio n is o n ly 1/3 from that in an in tern al m ix er. T h e d eg ree o f fille r d isp ersio n and th e p e r fo r m a n c e o f th e m aterial is su p erio r to the c o n v e n ­ tio n a l b a tc h m ix in g . It w a s s h o w n to w h a t e x te n t r h e o lo g ic a l p ro p erties as w e ll as d y n a m ic -m e c h a n ic a l p ro p erties are a ffe c te d b y th e state o f fille r -d is p e r sio n . T h e m o s t s p e c if ic r e sp o n s e o f m ic r o -d is p e r sio n w a s se e n in th e d y n a m ic crack g ro w th resista n ce. T h erefo re, life tim e an d p erfo r m a n c e are p rim arily g o v e r n e d by th e fille r m ic r o -d is p e r s io n . Im p ro v em en ts o f filler-ru b ­ ber in te r a c tio n s b y fu n c tio n a liz a tio n o f the p o ly m e r or s u r fa c e m o d if ic a t io n o f th e f ille r o p e n e c o n o m ic a l rou tes fo r m ix in g .

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t e r s c i e n c e , N e w Y o r k ( 1 9 6 5 )

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( 1 9 6 3 ) 8 9 2

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T e c h n o l . 6 6 . ( 1 9 9 3 ) 3 2 9

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