EFFECT

OF MECHANICAL

F A C T O R S ON R U B B E R W E A R IN A G G R E S S I V E M E D I A

Yu. S. Z u e v a n d A. D. C h e l m o d e e v Mekhanika Polimerov,

V o l . 4, No. 3, p p . 4 9 9 - 5 0 3 ,

1968

UDC 6 7 8 : 6 2 0 . 1 9 3 + 6 2 0 . 1 7 8 . 1 6 The effect of mechanical factors on rubber wear in aggressive slurries has been investigated. As the mechanical action becomes more intense, the effect of the aggressive medium increases. Rubber wear in an abrasive flow is a two-stage process: the aggressive medium modifies the surface layer, which is then worn away by the abrasive. A p a r t f r o m t h e a g g r e s s i v i t y of t h e m e d i u m a n d t e m p e r a t u r e [1], w e a r i n a n a b r a s i v e f l o w i s s t r o n g l y affected by such basic mechanical factors at the solidp h a s e c o n c e n t r a t i o n of t h e s l u r r y , t h e v e l o c i t y of t h e slurry solids relative to the rubber, the granulometric c o m p o s i t i o n of t h e a b r a s i v e , a n d t h e i n i t i a l s t r e t c h of the rubber. T h e e f f e c t of c e r t a i n m e c h a n i c a l f a c t o r s o n r u b b e r wear has been investigated only in relation to sandw a t e r s I u r r i e s o n a p p a r a t u s of t h e i m p a c t i n g - j e t t y p e [2] a n d , t o s o m e e x t e n t , o n a p p a r a t u s w i t h r o t a t i n g s p e c i m e n s [3]. t t h a s b e e n f o u n d t h a t r u b b e r w e a r i n e r e a s e s w i t h i n c r e a s e i n i m p a c t v e l o c i t y [2] a n d p a t t i c l e s i z e [2, 3]. T h e d a t a o n t h e e f f e c t of t h e s o l i d s c o n c e n t r a t i o n of t h e s l u r r y r e p o r t e d i n t h e s e t w o s t u d i e s a r e n o t e o m p a r a b I e . T h e r e a r e no d a t a o n t h e e f f e c t of mechanical factors on rubber wear in aggressive slurries. For purposes of investigation we used slurries composed of 1KO315 molding sand with a mean particle size 0. 315 mm and synthetic white corundum No. 6, 16, 20, 32, 40 and 80 with mean particle size 0.06, 0,16, 0.20, 0.32, 0.40, and 0.80 mm in water, 30% nitric and 20°]o acetic acid. The following rubbers were selected: chemical-resistant butyl rubber (13), neoprene rubbers extended with carbon black (Nb) and carbon white (Nw) commercial butadiene-styrene rubbers with furnace black (Sb) and with carbon with (Sw) , as well as unfilled natural rubber vulcanizate (NR). The rubbers were investigated by tile method previously described in [4], that is, specimens in the form of short tubes were fitted over a spindle rotating at high speed in a vessel containing the aggressive slurry, in which the abrasive was uniformly distributed. The rate of wear Vis under steady-state conditions vis =

i: r

%

,

(1)

Train

where isr is the wear (loss of weight relative to the originaI weight of the specimen) in time r with allowance for swelling of the rubber in the aggressive medium. The reproducibility of the results was determined on 18, 6, and 3 specimens. The coefficients of variation for 18, 6, and 3 specimens were ±9.1, i11.8 and ±13.0%, respectively. Accordingly, in subsequent experiments we generally tested three specimens or, in the presence of a nonmonotonic dependence of the wear on various factors, six. The experiments were conducted in a thermostated apparatus at 20~ C, The prestretehed rubbers were tested at the same end thickness. The deformation was calculated from the outside diameter. T h e e f f e c t of t h e c o n c e n t r a t i o n of s a n d i n t h e s l u r r y i s s h o w n i n F i g . 1. T h e w e a r r a t e i n c r e a s e s u p to a s o l i d s c o n c e n t r a t i o n of a p p r o x i m a t e l y 30-35% b y v o l u m e and then remains constant both in water and in nitric

~ . . ,-.ge-- ,in O°t°i qoo~

"' , , ~

. . . . ×. . . . . ×-- Ic

30

50

F i g . 1. E f f e c t of s a n d c o n c e n t r a t i o n ( p a r t i c l e s i z e 0. 315 r a m ) o n t h e w e a r of r u b b e r s i n w a t e r (1, 2, 3) a n d 3 0 % HNO~ ( l a , 2a, 3a) a t 20 ° C; n = 8 9 0 0 r p m ( l a ) , n = 9700 r p m (lb)~ n = 8 1 0 0 r p m ( l c ) . 1, l a , l b , l c ) B ;

2, 2a) Sb; 3, 3a)Sw. Cso!-solids concentration, % by volume.

acid. Calculations show that at any monodisperse partic l e s i z e s a s o l i d s c o n c e n t r a t i o n of a b o u t 30% b y v o l u m e corresponds to a distance between particles equal to their size Obviously, the increase in wear rate with increase i n a b r a s i v e c o n c e n t r a t i o n s h o u l d b e p r o p o r t i o n a l to t h e number of particles striking the specimen in unit time. T h i s i s , i n f a c t , o b s e r v e d u p to a c o n c e n t r a t i o n o f a b o u t 30% b y v o l u m e ° W i t h f u r t h e r i n c r e a s e i n c o n c e n t r a t i o n the abrasive part[cies Iose their freedom of movement, and begin to interfere with each other, as a result of which the number of particles striking the surface of the specimen increases much more siowly and the rate of wear increases at a sharply reduced rate° The published data are also consistent with the results obtained. T h u s , i n [3] i t w a s s h o w n t h a t a t a s a n d c o n c e n t r a t i o n a b o v e 38 a n d u p to 84% b y v o l u m e t h e w e a r i s a l m o s t i n d e p e n d e n t o f t h e c o n c e n t r a t i o n . A n a n a l y s i s of t h e d a t a o f [2] s h o w s t h a t t h e w e a r b e c o m e s i n d e p e n d e n t of the concentration of sand in the slurry at a volume concentration of 16-19%. However, when the particul a r c o n d i t i o n s of t h e s e e x p e r i m e n t s a r e t a k e n i n t o a c c o u n t ( s t r e a m o f s l u r r y , a n g l e o f a t t a c k 30°), i t i s f o u n d t h a t t h e a c t u a l c o n c e n t r a t i o n of s a n d i n t h e s l u r r y - r u b ber contact zone was approximately twice as great, o w i n g to t h e f a c t t h a t p a r t i c l e s r e f l e c t e d f r o m t h e specimens were atso present in that zone. It s h o u l d also be noted that the increase in wear rate with increase in solids concentration is greater for an 395

/

t Vis%/rain

/la

O.OfO

/

6~008 1f

0.088

S-I

Ib

)I

I

ff

dot6

l

00¢2 C002[ ~0~

I O

v

ft./03

rpm

Fig. 2. Effect of s p e c i m e n s p e e d n, r p m , on the w e a r of rubbers in a sand s l u r r y in w a t e r ( 1 , 2 , 3) and 30% nitric acid ( l a , 2a, 3a) at 20* C and a s o l i d s concentration of 35% by v o l u m e witiiout a l l o w a n c e f o r the w e a r path and reduced to the s a m e w e a r path ( l b , 3b). ] , l a , l b ) B ; 2, 2 a ) S b ; 3, 3a, 3b) Sw. a g g r e s s i v e m e d i u m than for w a t e r . A s i m i t a r dependence ( i n c r e a s e in w e a r r a t e only up to a concentration of 3 0 - 3 5 % ) is o b s e r v e d w h e n corundum No. 20 is u s e d as the a b r a s i v e , and also at different s p e e d s (9700, 8100, and 8900 rpm). It is c l e a r f r o m Fig. 2, which s h o w s the effect of the speed of the s p e c i m e n s on w e a r in a sand s l u r r y , that on the n a r r o w range investigated ( 7 0 0 0 - 1 0 000 rpm) the dependence of the r a t e of w e a r of v a r i o u s r u b b e r s on the spindIe s p e e d i s c l o s e l y a p p r o x i m a t e d by a straight line. This holds t r u e both for the s a m e w e a r t i m e (1, l a , 2a, and 3, 3a) and for the s a m e path t r a v e r s e d by the s p e c i m e n s (lb and 3b), both in w a t e r and in nitric acid. H o w e v e r , if w e start f r o m the fact that the impact v e l o c i t y of the a b r a s i v e is proportional to t h e speed of the s p e c i m e n , s i n c e the a b r a s i v e i s s p e c i c i a l l y retarded and its v e l o c i t y can be n e g l e c t e d , the v s = = f ( v c ) c u r v e s should p a s s through t h e coordinate o r i g i n (since at v c = 0 m / s e c , Vis -- 0), and in m o r e g e n e r a l f o r m the dependence of the rubber w e a r on spindle s p e e d is n e a r l y quadratic in a c c o r d a n c e with the fact that the k i n e t i c e n e r g y p o s s e s s e d by two bodies on i m p a c t i s r e l a t e d with their v e l o c i t y a c c o r d i n g to a quadratic law. The m o s t rapid i n c r e a s e in w e a r r a t e is o b s e r v e d in the p r e s e n c e of an a g g r e s s i v e m e d i u m in the s l u r r y and a l s o in the c a s e of l e a s t w e a r - r e s i s tant r u b b e r s . A s i m i l a r relationship b e t w e e n rubber w e a r and the s p e e d of the s p e c i m e n i s a l s o o b s e r v e d in a corundum No. 20 s l u r r y . In studying the effect of p a r t i c l e s i z e on rubber w e a r it should be taken into account that the m a s s of the p a r t i c l e m , which d e t e r m i n e s the amount of e n e r g y r e l e a s e d when a s i n g l e p a r t i c l e of a b r a s i v e s t r i k e s the s u r f a c e of the rubber, i n c r e a s e s as the cube of the linear d i m e n s i o n s . H o w e v e r , the total m a s s of the p a r t i c l e s M = m m (where n is the number of p a r t i c l e s ) s t r i k i n g the s p e c i m e n d o e s not depend (at a constant s o l i d s concentration) on their d i m e n s i o n s . AceordingIy, one would expect the ratio A = ~ i s / M not to depend on the s i z e of the a b r a s i v e p a r t i c l e s . H o w e v e r , as m a y b e s e e n f r o m Table 1, the v a l u e s of 396

0002

6mm 8o~

~I,5

a3z

0.~o

850

880

0,7o

0.80

F i g . 3. Effect of a b r a s i v e p a r t i c l e s i z e 5 on the w e a r of rubbers in w a t e r ( 1 , 2 ) , 20% a c e t i c acid ( l a , 2a), and 30% nitric acid ( l b , 2 b ) at 20" C, s o l i d s concentration 35% by v o l u m e and n = 8900 r p m . 1, l a , lb) Sw; 2, 2a, 2b) Sb.

Table 1 The Ratio v i s / M as a Function of A b r a s i v e ]?article S i z e ~ C nthetic } orun- /

i /~ i 6 1 h ~

Nb Nw

132 300 300 180

Sb Sw

~o3z

32

162

360

380 430

620

~ 40

460 700 560 390

495 340

240

Vis %1 rain

• / lb /

~828 /

f

/

/

/f 0,02~ / f

J

/J

q828 /

~8¢6

/ z /

z/

f

~008

p

I,

la

I

//

~Q~2

/

Zm.-.-*Z~

•

~V f

~00~ Q002

0

0.86

Oj~

88z 8~0

6ram 0,88

F i g . 4. Effect of a b r a s i v e p a r t i c l e s i z e 5 on the w e a r of rubbers in w a t e r ( 1 , 2 ) , 20% a c e t i c acid ( l a , 2a), and 30% nitric acid (lb) at 20 ° C, s o l i d s concentration 35~oby v o l u m e and n -- 8900 r p m . 1, l a , lb) Nw; 2, 2a) Nb

Table 2 E f f e c t of P r e s t r e t c h i n g E x t r u d e d R u b b e r s on W e a r in S l u r r i e s Wear rate o f rubbers Vis in % rain at a prestreteh (%) o f Rubber

Medium 0

B Sw Sb NR Nb

H20 30% HNOa H20 30% HNO3 H20 30% HNOa H~O H20

li

I!

0,0110 i 0.01t0 0-0122 E 0,0t20 0.0047 i 0.0049 0.0058 ] 0.0065 0.0045 ] 0.0051 0.0065 ! 0.0070 0.0048 i 0,0049 0.0066 I --

A increase, i. e., the wear rate increases (Figs. 3 and 4), with increase in particle size. This is evidently because the energy release is more concentrated in the case of a larger number of small particles (calculations show that the kinetic energy of a particle of corundum No, 40 is 50 times greater than that of a particle of corundum No, 6). Therefore the rubber is worn faster by a coarse than by a fine abrasive, This dependence can also be extended in the direction of much larger p a r t i c l e s ( 1 - 2 0 m m [2, 3]), a s h a s b e e n shown in c o n n e c t i o n with s a n d - w a t e r s l u r r i e s . The e f f e c t of p r e s t r e t c h i n g the r u b b e r s i s shown in T a b l e 2, f r o m w h i c h it i s c l e a r t h a t t h e w e a r r a t e i n c r e a s e s with p r e s t r e t c h both in w a t e r and in n i t r i c a c i d . S t r e t c h i n g t h e r u b b e r , which is a c c o m p a n i e d by o r i e n t a t i o n o f t h e p o l y m e r m o l e c u l e s , m a k e s it s t r o n g e r . In t h e p r e s e n c e of a g g r e s s i v e m e d i a t h i s is e x p r e s s e d a s an e x t r e m a l d e p e n d e n c e of l i f e on d e f o r m a t i o n with a m i n i m u m l i f e in t h e r e g i o n of r e l a t i v e l y s m a l l m e a n s t r a i n s ( 1 0 - 4 0 % ) . A c c o r d i n g l y , in t h e c a s e of r u b b e r w e a r in a flow of a b r a s i v e in an a g g r e s s i v e m e d i u m the p r e s t r e t e h m i g h t b e e x p e c t e d to h a v e m o n o t o n i c effect on w e a r r a t e . The a b s e n c e of such an effect ( T a b l e 2) m a y b e a s s o c i a t e d e i t h e r with the f a c t that w e a r t a k e s p l a c e without a p p e a r a n c e of s t r e s s r a i s e r s in t h e r u b b e r , though t h i s is u n l i k e l y in the c a s e of a b r a s i v e p a r t i c l e s , o r w i t h the f a c t t h a t the a c t i o n of t h e a g g r e s s i v e m e d i u m and s t r e s s i s s e q u e n t i a l , not s i m u l t a n e o u s a s in s t a t i c f a t i g u e . A c o n s i d e r a t i o n of t h e r e s u l t s a s a w h o l e l e a d s to the s a m e c o n c l u s i o n . In f a c t , f o r s t a t i c a l l y d e f o r m e d r u b b e r s in a g g r e s s i v e m e d i a it h a s b e e n found t h a t a s t h e s t r e s s i n c r e a s e s t h e r e l a t i v e r o l e of t h e m e d i u m d e c r e a s e s [4], i . e . , in t h i s c a s e t h e r e i s no " a c t i v a t i o n " of c o r r o s i v e d e s t r u c tion by the m e c h a n i c a l s t r e s s , which m a y be a s s o c i a t e d with s t r e t c h - h a r d e n i n g of t h e p o l y m e r (i. e . , a s t h e s t r e s s i n c r e a s e s ) , which m a s k s t h e a c t i v a t i n g effect of the s t r e s s . Under d y n a m i c l o a d i n g the r o l e of the c h e m i c a l r e a c t i o n s i s a l s o the m o r e i m p o r t a n t , the l e s s the mechanical fatigue regime is forced. Conversely, w h e r e the m e c h a n i c a l a c t i o n i s i n t e n s e , t h e r o l e of c h e m i c a l c h a n g e s i s l e s s i m p o r t a n t , e s p e c i a l l y if t h e t e m p e r a t u r e i s not high [5]. In s l u r r i e s , h o w e v e r , a s the mechanical action (solids concentration, velocity, particle size, prestretch)becomes more intense, the o p p o s i t e effect i s o b s e r v e d ( F i g s . 1-4 and T a b l e 2): a s the m e c h a n i c a l a c t i o n b e c o m e s m o r e i n t e n s e , the effect of the a g g r e s s i v e m e d i u m on r u b b e r w e a r i n c r e a s e s .

18

25

0.0112

0.0t26 0,0050 0,0090

m

31

35

43

0.0140

0.0t58

0,0057 0.0135

0,0048

0,0065 0,0092

0.0072 0.0050 0,0072

0.0053 0.0078

0.0120

This may be as sociated with the fact that in this case the s t r e s s is a p p l i e d in i n d i v i d u a l p u l s e s , while the a c t i o n of the m e d i u m i s continuous, a s a r e s u l t of which the d e s t r u c t i o n of t h e r u b b e r t a k e s p l a c e in two s t a g e s : 1) the c h e m i c a l a c t i o n of the a g g r e s s i v e m e d i u m , w h i c h m a y be m e c h a n i c a l l y a c t i v a t e d by the i m p a c t of abrasive particles; 2) m e c h a n i c a l r e m o v a l of t h e p a r t i c l e s of the d e graded layer. An i n c r e a s e in t h e i n t e n s i t y of m e c h a n i c a l a c t i o n l e a d s to the a c c e l e r a t i o n of t h e s e c o n d s t a g e , and in the p r e s e n c e of c h e m i c a l a c t i v a t i o n , of the f i r s t s t a g e a l s o , so that the r o l e of a g g r e s s i v e m e d i u m b e c o m e s m o r e

i m p o r t a n t . Thus, the m o r e i n t e n s e the c h e m i c a l action, t h e m o r e r a p i d the w e a r and, on the o t h e r hand, the m o r e i n t e n s e the a c t i o n of the a b r a s i v e , the m o r e r a p i d l y t h e d e g r a d e d s u r f a c e l a y e r of r u b b e r is r e moved; a c c o r d i n g l y , t h e i n t e r a c t i o n of the r u b b e r and t h e a g g r e s s i v e m e d i u m is a c c e l e r a t e d and the w e a r rate increases.

CONCLUSIONS 1. The w e a r r a t e i n c r e a s e s with i n c r e a s e in s o l i d s c o n c e n t r a t i o n to 3 0 - 3 5 % b y v o l u m e and then r e m a i n s practically constant. 2. The w e a r r a t e i n c r e a s e s l i n e a r l y with p a r t i c l e s i z e and i t s d e p e n d e n c e on i m p a c t v e l o c i t y (in the r a n g e 7 - 1 0 m / s e c ) is c l o s e l y a p p r o x i m a t e d by a straight line. 3. A s t h e p r e s t r e t c h i n c r e a s e s the w e a r r a t e i n creases monotonically. 4. As the m e c h a n i c a l a c t i o n ( s o l i d s c o n c e n t r a t i o n , specimen velocity, particle size, prestretch) becomes m o r e i n t e n s e , t h e effect of t h e a g g r e s s i v e m e d i u m on w e a r in s l u r r i e s i n c r e a s e s , t h e m o r e so t h e m o r e i n t e n s e t h e c h e m i c a l a c t i o n of the m e d i u m on the r u b b e r . 5. R u b b e r w e a r in an a b r a s i v e flow i s e v i d e n t l y a t w o - s t a g e p r o c e s s : f i r s t , t h e s u r f a c e of t h e r u b b e r is m o d i f i e d by t h e a c t i o n of t h e a g g r e s s i v e m e d i u m , which m a y be a c t i v a t e d by m e c h a n i c a l s t r e s s ; then it i s b r o k e n up and c a r r i e d away by the a b r a s i v e p a r t i c l e s . REF

ERENC

ES

1. Yu. S. Z u e v a n d A . D. C h e l m o d e e v , in: C h e m i c a l S t a b i l i t y of R u b b e r s and E b o n i t e s in A g g r e s s i v e M e d i a [in R u s s i a n ] , M o s c o w , 1 9 6 7 . . 397

2. N. S. Penkin, Tr. LIVT, 79, Moscow-Leningrad, 1965. 3. K. Wellinger and H. Uetz, VDI-Forschungsch., Ausg. B, 21, 449, 1955. 4. G. M. Bartenev and Yu. S. Zuev, Strength and Fracture of High-Elastic Materials [in Russian], Moscow, 1964.

398

5. M. M. Reznikovskii and A. I. Lukomskaya, Mechanical Testing of Natural and Synthetic Rubbers [in Russian], Moscow-Leningrad, 331, 334, 1964.

13 June 1967

Moscow