Polymer Wear and Its Control - American Chemical Society

14

Molecular

F e a t u r e s of T r a n s f e r F r a g m e n t s

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High-Density Polyethylene

Rubbed

When

Against Metals

V. A. Belyi and V. V. Nevzorov Institute of Mechanics of Metal-Polymer Systems, Byelorussian S.S.R. Academy of Sciences, Gomel, U.S.S.R. The method of thermal deposition of polymers is suggested for studying molecular weight distribution (MWD) of polyethylene transfer products. The method allows recording variations in the molecular characteristics of P E transfer products and evaluating the amount of the polymer taken from the metallic surface with a solvent using the light—scattering data. By analysing the variations in the integral and differential MWD-curves for the original P E and its transfer products, the peculiarities of wear and transfer for the polymer sliding against steel ( R = 0.04 µm) have been studied as influenced by loads and sliding velocities, chemical nature of the organic additives present in the polymer, and the counterface metal. T h e results obtained helped to substantiate the ways of wear control for polyethylene in terms of transfer. a

R u b b i n g of polymers against metals i s known to c a u s e the c h a n g e s i n molecular weight of m a c r o m o l e c u l e s . In the first place, a n d to a l a r g e r extent, c h a n g e s take p l a c e i n polymer fragments that transfer onto metals. T h o s e fragments a r e the most c h a n g e a b l e portion i n a t r i b o l o g i c a l system ~ Study of c h a n g e s i n molecular features of these fragments, o n the one hand, l e a d s to a better u n d e r s t a n d i n g of the orientation of p h y s i c o - c h e m i c a l p r o c e s s e s within the metal—polymer rubbing z o n e during d r y friction, a n d o n the other hand, it allows the establishment of relations between the transfer p r o c e s s a n d the wear of polymers, which i s of importance today. D e s p i t e numerous investigations i n the field of polymer wear, there i s no unity of opinions o n the wear p r o c e s s a s related to the transfer of polymers onto metals b e c a u s e of inadequate knowledge of the structure a n d p h y s i c a l properties of the transfer fragments. In this p a p e r the structure a n d

0097-6156/85/0287-0205S06.00/0 © 1985 American Chemical Society

Lee; Polymer Wear and Its Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

206

POLYMER

W E A R A N D ITS

CONTROL

p h y s i c a l properties of transfer fragments mean the data a c c u mulated on the distribution of polymers o v e r metallic s u r f a c e s , weight of the t r a n s f e r r e d fragments, molecular weight d i s t r i b u tion (MWD) and a v e r a g e molecular weights. T h e data a v a i l a b l e on the d e g r e e of crystallinity, initiation of orientation in transfer fragments and mode of transfer, be it patch—like or film—like, do not give a proper i d e a of the s t r u c ture and p h y s i c a l properties of the t r a n s f e r r e d particles; b e s i d e s , they do not l e a d to the u n d e r s t a n d i n g of the features and orientation of p h y s i c o - c h e m i c a l p r o c e s s e s resulting from rubbing and o c c u r r i n g at a molecular l e v e l in the transfer fragments. T h e establishment of the orientation in the d i s p e r s i o n p r o c e s s of the transfer fragments m a c r o m o l e c u l e s would p r o v i d e adequate knowledge of the transfer and wear mechanisms, it would also make it p o s s i b l e to find w a y s of improving friction c h a r a c t e r i s t i c s of metal-polymer friction pairs. E x pe rim ental E n g i n e e r i n g thermoplastic polymers are u s u a l l y polydispersed. T h e r e f o r e c h a n g e s in the molecular c h a r a c t e r i s t i c s of the transfer and w e a r products h a v e b e e n studied by u s i n g i n t e g r a l c u r v e s of molecular weight distribution. T h e authors applied the p r o c e d u r e of polymer thermal deposition to studying MWD of H D P E transfer and wear products (4» j?) . MWD characteristics of the transfer and wear products were a n a l y z e d a c c o r d i n g to the r e s u l t s of thermal deposition of highly diluted HDPE solutions ( 1 to 5 • 10—4 g/dl ). D i p h e n y l oxide s e r v e d as the & — solvent. Highly diluted polymer solutions were d e p o sited u s i n g a d e v i c e for polymer thermal oxidation ( T O P - 1 , USSR ) in the automatic regime when the solution was cooled at a rate of 1 deg./ min. T h e intensity of the scattered light was m e a s u r e d at a 2 0 ° — angle with a g r e e n filter adjusted ( ?\ = 0.550 m; absorption coefficient = 40%; and p a s s b a n d A A = 0.013/1 m ). T e m p e r a t u r e c u r v e s of the light s c a t t e r e d by H D P E solutions were replotted to obtain MWD integral c u r v e s by u s i n g F l o r y - H u g g i n s e q u a t i o n ( ^ ) :

where ff/ is the molecular weight of an i—polymer fraction, T7 and Q are the deposition temperatures of an i - p o l y — mer molecular weight and of an infinitely large polymer m o l e c u lar weight, and i s the constant for H D P E - d i p h e n y l oxide system. T o estimate the amount of the polymer t r a n s f e r r e d onto the metal a master c u r v e was drawn b a s e d on the light scattering r e s u l t s obtained for solutions of given HDPE concentrations. F r i c t i o n tests were performed on H D P E film s p e c i m e n s a c c o r d i n g to the s c h e m a t i c "shaft - partial b e a r i n g " under


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207

Polyethylene Transfer Products

BELYI A N D N E V Z O R O V

the l o a d s of up to 0.5 M P a at s l i d i n g v e l o c i t i e s of up to 0.5 m/s. S t e e l r o l l e r s ( 40 mm in diameter ) s e r v e d a s a model shaft. T h e roller s u r f a c e s were p o l i s h e d with diamond paste to R = 0.04yUm and r i n s e d in b e n z e n e and acetone before tests. a

R e s u l t s and D i s c u s s i o n Effect of R u b b i n g Conditions. S t u d y of the molecular c h a r a c t e r i s t i c s of the fragments t r a n s f e r r e d at v a r i o u s l o a d s and s p e e d s s h o w e d c o n s i d e r a b l e c h a n g e s in H D P E molecular weight even d u r i n g r u n n i n g - i n ( test time was 1 h ) ( F i g . 1, T a b l e I ). It was found out that at the experimental conditions lower s l i d i n g v e l o c i t i e s and higher l o a d s c a u s e d greater shifts in the transfer product MWD c u r v e s to the lower molecular weight region ( F i g . 1 a, curves 1 to 3 ). A n a l y s i s of the s h a p e of q -MWD differential curves ( Fig. l b ) g a v e interesting results. q Curves were obtained by g r a p h i c differentiation of MWD integral c u r v e s . q C u r v e s for the o r i g i n a l HDPE c l o s e l y resembled unimodal ones ( F i g . l b , 1 ), while for the transfer products they may a p p e a r bimodal or trimodal d e p e n d i n g on test c o n d i t i o n s ( F i g . 1 b, 2 and 3 ). C h a n g e s in modality of q -curves are c a u s e d b y d i s p e r s i o n , b r a n c h i n g and i n c r e a s e i n molecular weight of m a c r o m o l e c u l e s in the t r a n s f e r r e d particles defined by the c h a r a c t e r i s t i c s of m e c h a n i c a l and c h e m i c a l p r o c e s s e s in the c o u r s e of rubbing. w

w

w

w

T a b l e I. Effect of R u b b i n g C o n d i t i o n s on M o l e c u l a r C h a r a c t e r i s tics of H D P E Transfer Fragments Parameters

S l i d i n g Velocity, m/s

Original HDPE

0.5

0.25

0.1

Load,

MPa

0.1

0.5

0.1

0.5

0.1

0.5

32

28

28

38

42

35

54

27

18

18

27

27

22

28

1.2

1.6

1.6

1.5

1.5

1.6

1.9

Weight of transfer fragments, ^ mg • 10 2.9

4.8

6

9.7

Coefficient of friction

0.35

3

*M -

10

*M .

10~

W

n

M

*

/

w

M

w

M

3

n

and

0.4 M

n

0.4

0.4

are the weight-average molecular weights

9.6

0.4 and

49.5

0.4 number-average


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P O L Y M E R W E A R A N D ITS

W,

CONTROL

%

I

0

I

I

I

I

0

MWD

I

I

I

i

50

F i g u r e 1. M o l e c u l a r fragments transferred 0.1 m/s and l o a d s a)

I

I 100

M.10

3

c h a r a c t e r i s t i c s of H D P E ( l ) and onto s t e e l at a s l i d i n g v e l o c i t y of of 0.1 M P a (2) and 0.5 M P a ( 3 ) .

integral c u r v e s , b)

q w

MWD

differential c u r v e s .


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209

q - Curves allow a qualitative estimation of the v a r i a t i o n s in weight fractions of m a c r o m o l e c u l e s of v a r i o u s molecular weights with r e g a r d to r u b b i n g conditions. T h e t r a n s f e r r e d p a r ticles demonstrated a n i n c r e a s e in the weight fraction of macromolecules of 1 x 1 0 to 1.2 x 10 molecular weights a s c o m p a r e d to the o r i g i n a l H D P E ( F i g a . l b, 2 and 3 ). T h e higher the load within the rubbing z o n e the greater the weight fraction ( peak intensity i n the q - c u r v e ). New p e a k s i n q - c u r v e s for molecular weights of 3 x 1 0 and 5.5 x 1 0 indicate that b e s i d e s macromolecular d i s p e r s i o n b r a n c h i n g a n d molecular weight i n c r e a s e may take p l a c e on the rubbing s u r face resulting from m a c r o r a d i c a l recombination and c h a i n breaking. Variations i n the H D P E m o l e c u l a r features s u c h a s M and M a n d MWD integral c u r v e s d e p e n d mainly o n the mode of transfer. A t a s l i d i n g v e l o c i t y of o . l m/s p a t c h - l i k e transfer w a s o b s e r v e d that r o u g h e n e d the metallic s u r f a c e s . T h e s i z e of the polymer p a t c h e s t r a n s f e r r e d onto metal i n c r e a s e d with l o a d which intensified macromolecular d i s p e r s i o n ( F i g s . 1 to 3, T a b l e I ). M a c r o m o l e c u l a r d i s p e r s i o n in H D P E with patch-like transfer i s defined b y polymer-metal a n d polymer-polymer a d h e s i v e interactions. T h e major contribution to m a c r o m o l e c u l a r d i s p e r s i o n i s from the alternating a r e a s of polymer-polymer and metal-polymer contacts. M a c r o r a d i c a l s g e n e r a t e d within polymer-polymer contact may recombine o n the metallic s u r f a c e to form chemisorption a n d coordination complexes with a n o x i d e film . U n d e r the dynamic contact this p r o c e s s may increase the effect of m e c h a n i c a l actions on the macromolecular d i s p e r s i o n of polyolefine. At a sliding velocity of 0.5 m/s under the e n v i r o n m e n tal conditions film-like transfer took p l a c e w h i c h c a u s e d a d e c r e a s e in m a c r o m o l e c u l a r d i s p e r s i o n a s c o m p a r e d to the p a t c h - l i k e mode of transfer ( Table I ). A t a load of 0.5 MPa M and M dropped b y 48 % a n d 37 % for patch-like transfer at a s l i d i n g velocity of 0.1 m/s, a n d by 35 % a n d 23 % for film-like transfer at a s l i d i n g velocity of 0.5 m/s. It s h o u l d be noted that the p o l y d i s p e r s i o n ratio d o e s not, i n fact, d e p e n d upon rubbing conditions a n d i n most c a s e s it maintains the v a l u e s of 1.5 to 1.6 ( T a b l e I ). T h e data g i v e n i n the paper on H D P E macromolecular d i s p e r s i o n b a s e d o n rubbing conditions allow a c o n c l u s i o n that the d e g r e e of d i s p e r s i o n d e p e n d s mainly o n load. Effect of v e l o c i t y o n this p r o c e s s i s indirect and related to the transfer mode o r to the ratio of the metal—polymer a n d polymerpolymer contact a r e a s . If the metal-polymer contact a r e a i s predominant this l e a d s to intensification of macromolecular dispersion. T h e m i c r o s c o p y r e v e a l e d large films of polymer particles attached to the t r a n s f e r r e d film s u r f a c e s . It was found out that these particles might h a v e b e e n r e s u l t e d from the a d h e s i v e sticking between the t r a n s f e r r e d films a n d the polymer. In terms w

4

4

w

4

w

w

n

w

n


4

210

POLYMER

W E A R A N D ITS C O N T R O L

of wear this p r o c e s s may i n c r e a s e HDPE wear rate when the attachment of p a r t i c l e s i s slight, otherwise wear rate d e c r e a s e s due to the retention of particles within the rubbing z o n e and i n c r e a s e i n the transfer film t h i c k n e s s . T h e s t e e l s u r f a c e s with transfer fragments r i n s e d in b o i l ing solvent (toluene and x y l e n e ) exhibited stable brown p a t c h e s (up to 5 jura i n s i z e ) of the polymer. Strong attachment of the retained transfer fragments to the p o l i s h e d s t e e l may be due to m e c h a n i c a l interaction and c h e m i c a l bonding. T h e incomplete solution of the transfer fragments i s a l s o indicative of the p o s s ible c r o s s l i n k a g e of H D P E m a c r o m o l e c u l e s at the initial stage of transfer. Active

Additives

M e c h a n o - c h e m i c a l p r o c e s s e s that a r e e s s e n t i a l for H D P E macromolecular d i s p e r s i o n were defined b y a n a l y z i n g MWD of the transfer fragments of p o l y o l e f i n e — b a s e d composites d o p e d with active additives (5 wt.%), s u c h a s antioxidant ( N e o z o n e D ) , aromatic compound ( a n t h r a c e n e ) a n d metal salt ( z i n c s t e a r a t e ) . Effect of the active additives o n molecular features a n d c e r t a i n friction c h a r a c t e r i s t i c s of H D P E ( l o a d = 0.5 MPa; sliding v e l o city = 0.25 m/s) i s s h o w n i n T a b l e II. T a b l e II. Effect of A c t i v e A d d i t i v e s C h a r a c t e r i s t i c s of H D P E Original HDPE

H D P E +Zinc Stearate R

M w

10

3

W e a r rate, mg/h Coefficient of friction

on M o l e c u l a r and F r i c t i o n Transfer Fragments

u

b

HDPE + Neozone b

i

n

D

HDPE + Anthracene g

Before After

Before After

Before After

Before After

54

62

76

56

41

0.06

0.4

35

0.02

0.2

68

1.22

0.4

45

0.3

0.4

H D P E s p e c i m e n s d o p e d with N e o z o n e a n d anthracene exhibited higher wear rates ( b y 20 a n d 5 times, r e s p e c t i v e l y ) than the original polyolefine ( T a b l e II ). T h e microscopy found a great difference i n the mode of transfer. With Neozone p r e s e n t within the rubbing zone, oriented polymeric film b a n d s had b e e n p r o d u c e d o v e r the s t e e l surface, while anthracene had g i v e n unoriented i s l a n d s of large p a r t i c l e s . In both c a s e s the t r a n s f e r r e d p a r t i c l e s were slightly attached to the metallic s u r f a c e s , w h i c h c a u s e d the high wear rates ( Table II ).


14.



211

Slight attachment of the t r a n s f e r r e d particles to the metal allows a c o n c l u s i o n that inhibition of oxidation ( N e o z o n e ) a n d relaxation ( a n t h r a c e n e ) of the H D P E transfer fragments m a c r o molecules d e c r e a s e the number of m a c r o r a d i c a l s c a p a b l e of strong attachment to the s t e e l surface. T r a n s f e r fragments u n dergo fewer deformation c y c l e s and faster become wear d e b r i s . T h e molecular features of the a b o v e fragments h a v e b e e n found to c o r r e l a t e or to be s u p e r i o r to those of the o r i g i n a l H D P E ( P i g . 2, c u r v e s 1, 3, and 4 ). T h e incorporation of z i n c stearate ( up to 5 wt.% ) into H D P E promoted the formation of the t r a n s f e r r e d films strongly attached to the s t e e l surface, w e a k e n e d the friction f o r c e s i n the contact and l e d to a three-fold d e c r e a s e i n wear rates a s c o m p a r e d to the original HDPE. M of the transfer fragments d r o p p e d b y 4 5 % ( T a b l e II). Low molecular weight fractions and z i n c stearate of the r u b b e d s u r f a c e c a u s e d w e a k e n i n g of the friction f o r c e s a n d d e c r e a s e i n the wear rate. T h e experimental r e s u l t s obtained with the active additives promoted the following recommendations b e d e r i v e d about the s u b s t a n c e s that control the p r o c e s s of polymer transfer onto metals for optimizing unlubricated s l i d i n g regimes: - T h e s u b s t a n c e s h o u l d not eliminate the a d h e s i v e i n t e r action between the transfer fragments and the metal. - T h e s u b s t a n c e must be a lubricant for polymer—polymer contacts. - T h e s u b s t a n c e s h o u l d provide intensive m a c r o m o l e c u l a r d i s p e r s i o n i n the transfer and wear products. T h e molecular c h a r a c t e r i s t i c s of the H D P E wear particles may be s u p e r i o r to or comparable with those of the transfer fragments. However, the test time ( from 2 to 8 h ) did not affect the v a l u e s of M and M a n d MWD of the H D P E wear p a r t i c l e s . T h e origin of the metal influences undoubtedly the H D P E macromolecular d i s p e r s i o n . A t a s l i d i n g v e l o c i t y of 0.25 m/s and load of 0.5 M P a M of the t r a n s f e r r e d p r o d u c t s d e c r e a s e d a s c o m p a r e d to the original H D P E i n the following way: i n contact with s t e e l - b y 10%; with c o p p e r - b y 12%; with aluminum a l l o y — b y 30%. In contact with titanium a r e v e r s e p r o c e s s of the titanium oxide transfer onto the polymer was o b s e r v e d . w

w

n

w

Conclusions T h u s , the new method for studying the molecular c h a r a c t e r i s t i c s of the thermoplastic polymer transfer and wear p r o d u c t s m a k e s it p o s s i b l e to e s t a b l i s h the orientation of m e c h a n o - c h e m i c a l p r o c e s s e s i n the rubbing z o n e and to find w a y s of d i s p e r s i o n and wear control. T h e method a l s o e n a b l e s the wear m e c h a n i s m s of antifriction polymers to b e studied i n terms of the transfer phenomena.


212

P O L Y M E R W E A R A N D ITS

0

40

80

120

CONTROL

M-10

F i g u r e 2. M o l e c u l a r c h a r a c t e r i s t i c s of the transfer products of the o r i g i n a l HDPE (l) and composites thereof d o p e d with : 5%-wt. z i n c stearate ( 2 ) ; 5%-wt. anthracene ( 3 ) ; 5%-wt. Neozone D ( 4 ) .

Literature Cited 1. Balakhanov, T. S., Babaev, A. G.; Guseinov, A. B.; Mustafaev, V. A.," On Solids Friction"; Nauka i Tekhnika: Minsk, 1971; pp. 407-414. 2. Kragelskii, I. V., 809.

Mekhanika Polimerov, 1972, No.5,

pp.793-

3. Sviridyonok, A. I. ; Savkin V. G.,Friction and Wear (Trenie i Iznos), 1981, 1 , No. 1, pp. 150 - 167. 4. Belgovskii, I. M., G-oldberg, V. M., ; Krasotkina, I. N.; Toptygin, D. Ya., Doklady AN S S S R (Reports of USSR Academy of Sciences), 1970, 192, No. 1, pp. 121 - 122. 5. Goldberg, V. M., Belgovskii, I. M.; Izyumnikov, A. L. , Vysokomolekul. Soedinenia (High Molecular Weight Compounds), 1971, 13A, No. 4, pp. 977 - 981. 6. Krylov, O. V.; Kiselev, V. F.,"Adsorption and Catalysis on Transition Metals and Oxides Thereof"; Khimia: Moscow, 1981. RECEIVED January 23, 1985


Polymer Wear and Its Control - American Chemical Society

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