TOM 3 styczeń-luty 1999 r. SC<n^to*n&Uf' nr 1
D ariusz M. B ieliński0, Ludom ir Ślusarski0, Jean-Paul C hapel0, Wanda P arasiew icz3)
Constitution and structure of the surface layer and tribological
properties of elastomers*
Effects o f physical and chemical treatments, carried out in the bulk (surface segregation) or on the surface o f rubber respectively are presented.
Special attention is paid to the surface phenomena and associated tribological properties o f the material. Results obtained are interpreted taking into consideration hysteresis and topography o f the surface layer as well as adhesion o f the material to the counterface. The most promising methods showed to be sulfonation in general and iodination specifically with respect to acrylonitrile- butadiene rubber (NBR). Effect o f blending is discussed on the example o f ethylene-propylene-diene rubber (EPDM) modified by low density polyethylene (LDPE). Application o f the mentioned above methods results in significant reduction o f the coefficient o f rubber friction. Additionally, physical modification o f rubber by addition o f low molecular weight substances is discussed from the point o f view o f protection o f the material from ageing.
Some preliminary data, concerning styrene-butadiene rubber (SBR), are presented.
Key words: rubber, modification, surface layer, friction
Budowa i struktura warstw powierzchnio
wych a właściwości trybologiczne elastome-
r k
row
Przedstawiono wpływ modyfikacji fizycznej i chemicznej, prowadzonej odpowiednio w masie (segregacja powierzchniowa) lub na powierzchni gumy.
Specjalną uwagę poświęcono zjawiskom powierzchniowym, odpowiedzialnym za właściwości trybologiczne materiału. Uzyskane wyniki interpretowano biorąc pod uwagę właściwości histerezy i topografię powierzchni, jak również adhezję m ateriału do pow ierzchni nie m odyfikow anej. N ajbardziej obiecującym i metodami m odyfikacji okazały się sulfonowanie w ogólności oraz jodowanie w odniesieniu do kauczuku butadienowo-akrylonitrylowego (NBR). W pływ d o d a tk u p la s to m e r ó w do e la s to m e ró w z o s ta ł p rz e d y s k u to w a n y na p rz y k ła d z ie m ie sz a n in k a u czu k u e ty le n o w o -p ro p y le n o w o -d ie n o w e g o
0 Institute of Polymers, Technical University of Łódź, Żeromskiego 116, 90-538 Łódź, POLAND 2) Laboratoirc d’Etudcs dcs Matcriaux Plastiqucs et dcs Biomateriaux, Univcrsitc „Claude Bernard” Lyon I Bid du 11 Novembre 1918, 69622 Lyon, FRANCE
3) Rubber Research Institute, Harcerska 30, 05-820 Piastów, POLAND
Paper presented at the Int. Conference „ELASTOMERS’98”, Warsaw 13-15.10.1998
nr 1 styczeń - luty 1999 r. TOM 3
(E P D M ) z p o lie ty le n e m m ałej g ęsto ści (L D PE ). Z asto so w an ie w yżej wymienionych metod modyfikacji elastomerów przyniosło znaczące obniżenie wartości współczynników tarcia materiałów. Dodatkowo, modyfikacja fizyczna gum y, p ro w a d z o n a p rz e z d o m ie s z a n ie do e la s to m e ru s u b s ta n c ji małocząsteczkowych, została poddana analizie z punktu widzenia ochrony m ateriału przed starzeniem. Przedstawiono wstępne dane eksperymentalne dotyczące kauczuku butadienowo-styrenowego (SBR).
Słowa kluczowe: guma, modyfikacja, warstwa wierzchnia, tarcie
Introduction
Surface properties o f elastomers play very im
portant role in rubber processing and technology and finally very often decide on the exploitation perfor
mance o f the material. Tack required during proces
sing is influenced by composition and structure o f sub
stances migrating towards the surface o f rubber mix, the so called bloom. The surface com position and structure become different from the bulk o f material.
„New” appearance o f the surface layer can be either the result o f segregation o f low molecular weight com
ponents o f the rubber mix or can be produced by che
mical or physical treatm ent [1-8]. Very often m odifi
cation is associated with changes in the surface geo
metry [9].
Among many parameters describing surface pro
perties, there are some especially important from tri
bological point o f view i.e. adhesion (surface energy and its components), hysteresis o f the surface layer (stiffness and stress relaxation), surface geometry and perfection o f the segregated or modified layer (age
ing, especially ozone resistance).
Phenomena accompanying friction o f elastomers have to be considered assuming friction force being composed o f two components: adhesional (FA) and hysteretical one (FH) [10],
F = Fa + Fh (1)
where FH is additionally recognized to be dependent on the degree o f FA „developm ent”. Starting from this assumption it should be possible to obtain rubber of desired performance, the so called „tailored material” . M odification o f rubber restricted to the surface layer preserves the bulk elasticity o f the material, the most important feature o f rubber, deciding its engineering usefulness.
Experimental
Materials
Chemical treatm ent was applied towards diffe
rent synthetic elastomers: IR - isoprene rubber (Cari- flex IR 305, Shell Int., UK), BR - butadiene rubber (Nipol BR 1221) and NBR - acrylonitrile-butadiene copolymer (both Nippon Zeon Co., Japan), SBR - sty
rene-butadiene copolymer (Ker 1502, Oświęcim S.A., Poland), EPM - ethylene-propylene copolymer (Du- tral CO 054) and EPDM (Dutral TER 054/E, both DSM, Holland). Samples were vulcanized using DCP - dicumyl peroxide (92wt.% o f purity, Merck-Schu- hardt, Germany). The amount o f curing agent was in
dividually adjusted to obtain similar crosslink density o f v — 8 ± 0.5 ' 10'5 mol/cm3. Degree o f crosslinking was calculated from the values o f equilibrium swelling in toluene, according to Flory and Rehner [11-12].
Surface effect o f physical m odification was stu
died towards polyolefine blends made o f EPDM and LDPE - low density polyethylene (Malen E) or iPP - isotatic propylene (Malen P, both Petrochemia Płock S.A., Poland). Plastom er content in the range o f 0-50 phr was analyzed. Samples were peroxide cured using DCP in the amount o f 0.6 phr.
Phenom ena o f the surface segregation o f low m olecular substances or polymer fractions were inve
stigated for another SBR (Cariflex 1502, Shell Int., UK). The following chemicals, added in the amount o f 5-25 phr, were used: hexadecane, octadecane, do- decane, palmitinic acid, stearic acid, paraffin wax and LDPE o f different molecular weight. Polyethylenes of m olecular weight, MW: 4k LDPE - 4,000, 15k LDPE - 15,000, 35k LDPE - 35,000 and 60k LDPE - 60,000 were applied.
Sample preparation/modification
Rubber m asterbatches were prepared in a David
TOM 3 styczeń - luty 1999 r. nr 1
Bridge laboratory two-rolls mill during time assuring homogeneity o f the mixtures, usually about 15 min.
M ixing was carried out at temperature o f 40 °C. Only blending o f elastomer with plastom er demanded ele
vated temperature to m elt the crystalline phase o f the latter. In the case o f LDPE the process was perform ed at 130 °C whereas for iPP tem perature o f the rolls was maintained^at 160 °C.
Rubber samples were steel-m oulded in an elec
trically heated press at 160 °C, under optimal time determined rheom etrically according to ISO 3417. To rem ove products arising from degradation o f DCP during the vulcanization, the samples were extracted with accetone (excluding NBR, extracted with etha
nol), in a Soxhlet apparatus for 72 hrs in the dark. Prior to the further treatm ent the extracted samples were dried in a vacuum cham ber at 60 °C until constant weight was achieved. The samples were then surface modified by dipping in m odifying solutions, descri
bed elsewhere in details [13-16]. The time o f treat
ment usually did not exceed 1 min. Longer action in
fluenced the surface morphology too strongly, causing shrinkage o f the top layer and crack formation, that adversely affected tribological properties o f rubber.
Only in the case o f iodination the process was carried out for a longer time, up to 30 min. After rem oving from the solvent bath, samples were extensively w a
shed with a stream o f distilled water and dried in a vacuum chamber at 60 °C to the constant weight.
M ixing o f SBR with low m olecular weight sub
stances and polyethylenes was carried out in a Bra- bender internal m ixer (V = 80 cm 3, two screws sys
tem), operating with n = 30 rps during 5 min. at eleva
ted temperature o f about 60 °C. To m elt the crystalline phase o f the plastomers used, tem perature o f the pro
cess was m aintained at about 120 °C.
Techniques
Chemical microanalysis
Samples o f the area o f 1 cm2, cut from the thin films (50±10 mm thick), were analyzed for halogen (chlorine, bromine or iodine) or sulphur content.
A tom ic fo rc e m icroscopy (AFM)
The surface topography of the samples was cha
racterized by atomic force microscopy ARIS - 3300 (Bur
leigh Co., USA). The area scanned was 5'5 mm. The cantilever had a spring constant o f ca. 0.05 N n r1 and surfaces were imaged at a constant force, ca. 8.4 nN.
Contact angle
Surface energy, gs o f the samples and its com po
nents: dispersive, gsd and polar gsp, were determined by means o f the contact angle measurements. Experimental procedure, utilizing a wide range o f liquids, of different polarity, proposed by Kuczyński [17] was applied. The value of contact angle was calculated as an average of six measurements. The accuracy o f a contact angle esti
mation was ±2°. Experiments were carried out at am
bient conditions. The polar component of the surface energy was calculated from the value of the polar inte
raction parameter, Islp. Following Owens [18] and Ka- eble [19], Islp was considered as a geometric mean of polar components of the solid and the liquid.
Fourier Transformed Infra-R ed spectroscopy (FTIR)
Samples were studied with a Bio-Rad 175C FTIR spectrometer equipped with an attenuated total reflec
tance (ATR) attachment, over a wavelength range 600- 4000 c n r1 at the following experimental conditions:
64 scans/resolution o f 2 c n r 1. Utilization o f an ATR technique make gradient phenomenon to be analyzed.
M aterials were probing using Thallium bromide/Thal- lium iodide (KRS-5, nr = 2.4) as well as Germanium crystal (Ge, nr = 4.0). The depth o f penetration, was calculated from H arrick’s equation [20]. Thin films o f about 50± 10 mm were used to determine the bulk cry
stallinity or composition.
X -rays photoelectron spectroscopy (XPS) The X-ray source (Vacuum Science Workshop) was operated at 110-130 W and generated Mg Ka pho
tons (1253.6 eV). A 100 mm concentric hemispheric analyzer (CHA) was operated in the fixed analyzer transmission (FAT) mode, with a 50 eV pass energy.
The electron take off angle was normal to the surface.
The relative atomic concentration o f elements present on the surface was calculated using W agner’s sensiti
vity factors [21], modified for the instrument. High resolution XPS spectra o f the materials were deconvo
lv e d using the method o f Evans [22]. Shape param e
ters o f the peaks: C (Is), O (Is), N (Is), S (2p) and I (3d), applied in the calculations were taken from known compounds. Spectra were corrected for charging by assigning the principal C ls signal a binding energy of 284.6 eV.
M echanical properties
M echanical properties in extension were studied with a mechanical testing machine Zwick 1435 (Zwick GmbH, Germany), according to ISO 37. Dumbell spe
cimens were used.
Network structure parameters o f the samples were also obtained from equilibrium moduli in extension, deter-
S ta te 'o m e n y nr 1 styczeń - luty 1999 r. TOM 3
mined cathetometrically. Stripes o f the same width and length were cut from the thin rubber film (50±10 mm thick). The elasticity constants, C 1 and C2 were calcula
ted from the Mooney-Rivlin equation [23-24].
Tribological properties
Experiments were carried out with the “ring-on-disc”
apparatus described earlier [25]. Basically it is a “pin-on- disc” machine, modified for testing elastic materials.
Kinetic coefficient o f friction was investigated at room tem perature, over the sliding speed range, v = 0.05-1.0 m/s and normal pressure, p = 5-200 kPa. Fric
tion without lubrication was studied.
Results & discusion
Chemical modification
Chem ical treatm ent affects the surface o f the vulcanizates from 0.01 mm (sulfonation) up to seve
ral microns (brom ination or chlorination) deep, pro
ducing a modified outer layer that has a different phy
sical and chemical structure from those o f the original material. The depth o f m odification depends on the time o f treatm ent and type o f chemistry taking place.
Reactions accom panying the m odification are very complex, involving substitution, replacement and cyc- lization. Their mechanism is additionally interfered by oxidation, taking place sim ultaneously [26].
From tribological point o f view, the most effective in terms of lowering o f the coefficient of friction showed to be sulfonation in general and iodination in particular if applied to butadiene acrylonitrile rubber, Fig. 1.
tion o f rubber. Friction conditions: p - 100 kPa, v=0.1 m/s, T=23 +5 °C, „dry friction”
Rys. 1. Wpływ modyfikacji chemicznej na właściwo
ści trybołogiczne gumy. Warunki tarcia: p=100 kPa, v=0,l m/s, T=23 +5 °C, tarcie „na sucho”
Contrary to sulfonation, for which the low fric
tion effect is definitely attributed to the stiffening of the top surface layer („skin”) o f rubber produced by cyclization, the m echanism o f iodination still remains unknown. It will be the subject o f further studies. The treatment also produces stiffening effect, but the m o
dification definitely reaches deeper layers o f the m a
terial [8]. Due to the discontinuity of the modified layer, the surfaces o f iodinated NBR resembles rigid stiff pieces anchored in the elastic rubber matrix. Signifi
cant development o f the surface geometry is clearly visible, Fig. 2. Such m orphology makes the modified
HanoScope Tapping AFM
Fig. 2. Surface effect o f iodination carried out in Lu- golś solution. Time o f treatment 15 min.
a) before treatment b) after treatment
Rys. 2. Powierzchniowy efekt jodowania prowadzo
nego poprzez zanurzanie w płynie Lugola. Czas trwa
nia modyfikacji 15 min a) przed modyfikacją b) po modyfikacji
TOM 3 styczeń-luty 1999 r. S toM foift& iy nr 1
material wear resistant, unlike in the cose o f bromina- tion or chlorination, despite a thick and stiff layer be
ing produced.
From the data presented it seems likely that ne
ither the surface m icroroughness nor adhesion plays the dominant role in the friction o f rubber. Lowering o f the coefficient o f friction was obtained for the sm o
oth surface produced by sulfonation as well as for the rough one being the result o f iodination. Both treat
ments lead to increase o f the surface energy o f rubber.
It can be concluded that hysteresis o f the material plays a predom inant role in friction performance o f rubber.
Stiffening o f the surface layer prevails over an incre
ase o f adhesion or changes in the surface topography, although the real contact area plays also some role in
fluencing value o f the friction force.
Physical modification
Polymer blends, due to lim ited m iscibility o f components, are o f heterogeneous structure. This cre
ates possibility for low molecular weight materials to segregate towards the surface. Compared to the bulk, there is less steric hindrance on the surface o f a blend for crystallization o f a stereoregular component. This phenomenon could be utilized to get gradient poly
mers o f defined properties, e.g. low friction elasto
mers. By blending o f semicrystalline olefmic plasto- mers with an amorphous elastomeric one, an im pro
vement of mechanical properties o f the elastomer were expected [27]. Together with better mechanical cha
racteristic, tribological properties o f the material also improved, what is especially visible for blends o f low density polyethylene (LDPE) with ethylene propyle
ne diene rubber (EPDM), Fig. 3 [28].
ug. 3 .Influence o f plastomer addition on tribological properties o f EPDM. Friction conditions: p= 100-400
kPa, v=0.1 m/s, T=23 +5 °C, „dry friction”
Rys. 3. Wpływ dodatku plastomeru na właściwości try
bo logiczne EPDM. Warunki tarcia: p= 100-400 kPa, v=0,l m/s, T=23 +5 °C, tarcie „na sucho”
D espite better m echanical prop erties o f the blends containing isotactic polypropylene (iPP), the co efficient o f friction is low er for LD PE/EPD M blends. In our opinion this is the result o f low m o
lecular w eight polyethylene fraction segregation to the surface w here it acts as a some kind o f lu b ri
cant. Such interpretation follow s from ATR FTIR studies, Fig. 4 [29].
Enrichment of the top surface layer of the material in CH2 groups, comparing to the bulk is apparent. Even
"ig. 4. Surface composition o f LDPE/EPDM blends Rys. 4. Skład warstwy wierzchniej mieszanin LDPE/EPDM small addition of the plastomer results in significant chan
ge to the surface morphology, what can be seen in Fig. 5.
AFM pictures reveal considerable flattening of microroughness present on the rubber surface by the
nr 1 styczeń-luty 1999 r. TOM 3
Fig. 5. Surface segregation o f plastomer fraction in LDPE/EPDM blends.
a) EPDM
b) 5 phr LDPE/100 EPDM c) 25 phr LDPE/100 EPDM
Rys. 5. Segregacja powierzchniowa frakcji plastome
ru w mieszaninach LDPE/EPDM.
a) EPDM
b) 5 cz. wag. LDPE/100 cz. wag. EPDM c) 25 cz. wag. LDPE/100 cz. wag. EPDM
plastomer layer o f an amorphous structure. The surfa
ce o f LDPE sample presents supramolecular structure o f clear spherulitic nature, Fig. 6. State o f the surface is not only important from tribological point o f view, but brings ageing protection to the elastomer as well.
Fig. 6. Supramolecular structure (spherulites) visible on the surface o f polyethylene.
Rys. 6. Struktura nadcząsteczkowa (sferulityczna) wi
doczna na powierzchni polietylenu
The last aspect was analyzed on examples of low molecular weight substances added to styrene-buta
diene rubber (SBR), Fig. 7.
All the chemicals used show a protection beha
viour towards the elastomer, although perfection of
TOM 3 styczeń-luty 1999 r. £Ła&&y*KCUf- nr 1
Fig. 7. Ageing protection o f SBR realized by addition o f selected chemicals (5 phr)
a) SBR b) octadecane c) stearic acid d) paraffin wax e) LDPE
Rys. 7. Ochrona przed starzeniem SBR realizowana przez dodatek wybranych związków chemicznych (5
cz. wag./100 cz. wag. kauczuku) a) SBR
b) oktadekan c) kwas stearynowy d) wosk parafinowy e) LDPE
the surface covering differs from sample to sample.
The AFM pictures brings an explanation why e.g. pa
raffin wax is more efficient than stearic acid from the point o f view o f ageing protection. The data presented are just the result o f prelim inary study. To understand completely influence o f chemical structure and m ole
cular weight o f the additives on tribological proper
ties and ageing resistance o f rubber demand further, more deep studies on the rubber surface. In our opi
nion, geometrical orientation o f functional groups, i.e.
nr 1 styczeń-luty 1999 r. TOM 3
m icroseparation o f polar groups towards phase boun
daries, is o f great importance.
Conclusions
Rubber can be effectively m odified to obtain de
dicated tribological properties enabling its more effi
cient engineering application. The surface o f material is of the greatest importance from this point o f view.
Its geometry, adhesional and to the highest extent hy- steretical properties, influence value o f the coefficient o f friction. Effective modification o f the material can be carried out in a chemical or physical (blending with the surface segregation) way.
Chemical m odification to be effective, in terms o f low friction, should produce either a very thin, flat but stiff surface - so called „skin” (sulfonation), or develop on the surface an „island” structure (iodina- tion). Only such structures o f the surface layer are able to cooperate well with an elastic material o f the bulk under dynamic conditions. Range o f deform ations possible to undergo by a continuous, stiff layer does not correspond with a response o f the elastic bulk, le
ading finally to the surface cracking and the intensive wear o f rubber. Physical m odification has to produce the surface layer enriched with low molecular weight substances, on the one hand protecting the material from ageing and on the other hand acting as some kind o f lubricants what leads to low friction. Low adhesion in the area o f friction makes also dynamic deform a
tion accompanying friction significantly lower.
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