M arek M ikrut*,
J.W .M . Noordermeer*, G. Verbeek**
Low surface energy rubber m aterials: influence of the netw ork structure of silicone ru bb er on tack
T h e r e l a ti o n s h i p b e t w e e n s t r u c t u r e o f p o l y m e r n e t w o r k s a n d t h e i r t a c k i n e s s h a s b e e n s t u d i e d . T h e a i m o f th i s r e s e a r c h w a s to c h a r a c t e r i z e th e in f lu e n c e o f t y p e a n d i n i t i a l m o l e c u l a r w e i g h t o f th e u n c r o s s l i n k e d r u b b e r , r e s p e c t i v e l y a m o u n t o f c r o s s - li n k i n g a g e n t o n th e c r o s s l i n k d e n s i t y a n d o n th e r e s u lt i n g r u b b e r - r u b b e r t a c k i n e s s . T h e p o l y m e r u s e d w a s p o l y d i m e t h y l s i l o x a n e ( P D M S ) , c r o s s l i n k e d w ith tr i- , t e t r a - , a n d m u l t i f u n c t i o n a l s i l a n e s . T h e ta c k e x p e r i m e n t s w e r e p e r f o r m e d w i th a t a c k t e s t i n g d e v i c e , d e v e l o p e d s p e c i a l l y f o r th is p u r p o s e . T h e n e t w o r k s w e r e c h a r a c t e r i z e d w i th N M R s p e c t r o s c o p y , s w e l l i n g a n d m e c h a n i c a l p r o p e r t i e s m e a s u r e m e n ts . H i g h m o l e c u l a r w e i g h t P D M S s h o w s m u c h m o r e t a c k c o m p a r e d to l o w m o l e c u l a r w e i g h t s p e c i e s c r o s s l i n k e d to th e s a m e l e v e l w i th s i l a n e o f th e s a m e f u n c t i o n a l i t y . I n c r e a s in g th e c r o s s - li n k i n g a g e n t f u n c t i o n a l i t y f r o m th r e e t o f o u r s i g n i f i c a n t l y r e d u c e s th e P D M S ta c k ; f u r t h e r i n c r e a s e o f f u n c t i o n a l i t y c a u s e s o n l y s m a l l c h a n g e s . T h e s u r f a c e h y d r o p h o b i- c i t y m e a s u r e d b y c o n t a c t a n g l e h y s t e r e s i s c h a n g e s o n l y s lig h tly . T h e o b s e r v e d d i f f e r e n c e s in t a c k i n e s s c a n b e e x p l a i n e d b y a n i n c r e a s e d m o b i l i t y o f e l a s t o m e r c h a in s e g m e n ts . T h e p r e s e n c e o f n u m e r o u s f r e e o r p e n d a n t c h a in s f a v o r s th e t a c k b y i n c r e a s i n g i n t e r p e n e t r a t i o n o f th e c h a i n s a n d s t r e n g t h e n in g th e r u b b e r - - r u b b e r in te r f a c e . I n c r e a s e in th e c r o s s - l i n k i n g a g e n t f u n c t i o n a l i t y r e s u lt s in a m o r e c o n s t r a i n e d n e t w o r k , w h i c h s t r o n g l y r e d u c e s m o b i l i t y a n d c o n s e q u e n t l y th e ta c k .
K e y w o r d s : r u b b e r , P D M S , t a c k
* Dutch Polymer Institute, P.O Box 902, 5600A X Eindhoven, The Netherlands, University of Twente, Fac. of Science and Technology, D ept, o f Rubber T echnology, P.O.Box 217, 7500AE Enschede, The Netherlands
** Oce Technologies B. V.; P.O.
Box 101, NL-5900MA, Venlo, The Netherlands
Materiały elastomerowe o niskiej energii powierzchniowej: wpływ struktury sieci elastomeru silikonowego na kleistość
B a d a n o z a l e ż n o ś ć p o m i ę d z y s t r u k t u r ą s i e c i e l a s t o m e r o w y c h a k l e i s t o ś c i ą . C e le m p r a c y b y ł o z b a d a n i e w p ł y w u c i ę ż a r u c z ą s t e c z k o w e g o e l a s t o m e r u o r a z r o d z a ju i i l o ś c i s u b s t a n c j i s i e c i u j ą c e j n a g ę s t o ś ć u s i e c i o w a n i a i n a k l e i s t o ś ć e la s to m e r - e l a s to m e r . Z a s t o s o w a n o k a u c z u k s i l i k o n o w y ( P D M S ) u s i e c i o w a n y z a p o m o c ą tr ó j- , c z t e r o - o r a z p o l i f u n k c y j n y c h s ila n ó w . P o m i a r y k l e i s t o ś c i p r z e p r o w a d z o n o z a p o m o c ą u r z ą d z e n i a s k o n s t r u o w a n e g o s p e c j a l n i e w ty m c e lu . P a r a m e t r y s i e c i s c h a r a k t e r y z o w a n o z a p o m o c ą b a d a ń m e t o d ą N M R , p ę c z n i e n i a r ó w n o w a g o w e g o o r a z w ł a ś c i w o ś c i m e c h a n i c z n y c h . W ł a ś c i w o ś c i p o w i e r z c h n i o w e g u m y s i l i k o n o w e j b a d a n o p o p r z e z p o m i a r y h i s t e r e z y k ą t a z w i l ż a n ia . P D M S o d u ż y m c i ę ż a r z e c z ą s t e c z k o w y m w y k a z u j e z n a c z n i e w i ę k s z ą k l e i s t o ś ć w p o r ó w n a n i u d o p o l i m e r ó w o m a ł y m c i ę ż a r z e c z ą s t e c z k o w y m u s i e c i o w a n y c h d o t e g o s a m e g o s t o p n i a z a p o m o c ą s i l a n ó w o t a k ie j s a m e j f u n k c y j n o ś c i . Z w i ę k s z e n i e f u n k c y j n o ś c i z t r z e c h d o c z t e r e c h p o w a ż n i e z m n i e j s z a k l e i s t o ś ć P D M S ; d a l s z e z w i ę k s z a n i e f u n k c y j n o ś c i p o w o d u j e t y l k o n i e w ie l k ie z m ia n y . H y d r o f o b o w o ś ć p o w i e r z c h n i , m i e r z o n a z a p o m o c ą h i s t e r e z y k ą ta z w i l ż a n ia , z m i e n ia s i ę t y l k o n i e z n a c z n i e . Z a o b s e r w o w a n e r ó ż n i c e w k l e i s t o ś c i m o g ą b y ć s p o w o d o w a n e z w i ę k s z o n ą r u c h l i w o ś c i ą s e g m e n t ó w ła ń c u c h ó w p o l i m e r o w y c h . D o d a t k o w o o b e c n o ś ć l ic z n y c h s w o b o d n y c h k o ń c ó w ła ń c u c h ó w s p r z y j a z w i ę k s z e n i u k l e i s t o ś c i p o p r z e z z w i ę k s z o n e w z a j e m n e p r z e n i k a n i e ł a ń c u c h ó w i w z m o c n i e n i e p o ł ą c z e n i a e l a s t o m e r - e l a s t o m e r . U ż y c i e ś r o d k a s i e c i u j ą c e g o o w i ę k s z e j f u n k c y j n o ś c i p r o w a d z i d o s i e c i o m n i e j s w o b o d n y c h ł a ń c u c h a c h i w r e z u l t a c i e d o z m n i e j s z e n i a k l e i s t o ś c i .
S ł o w a k l u c z o w e : g u m a , P D M S , k l e i s t o ś ć
TOM 10 lipiec - sierpień 2006 r. S ia A tM ten y nr 4
ki eistość silikonów
1. Introduction
Tack of materials is the ability to resist separation after bringing their surfaces into contact for a short time under pressure. Two types of tack can be defined: auto- hesive, when the materials in contact have the same chemical composition; and adhesive, when both mate
rials have different compositions [1]. Adhesion of rub
ber is important in many industrial processes. Good adhesion is required for tire building, in adhesive labels and tapes (so called pressure-sensitive adhesives), etc.
Conversely, elastomeric materials that exhibit low tack are also important in industry.
Lots of work has been done in the field of adhesion [2]. However, the problem of contact between two vis
coelastic bodies is generally poorly understood [3,4]. It is already well known, that tackiness is a result of for
mation of intermolecular forces at the interface: chemi
cal, hydrogen and van der Waals bonds [5]. The pre
sence of numerous interfacially anchored chains is an
other factor strongly promoting the adhesion of elas
tomers [6].
The poly(dimethylsiloxane) is well known for its surface properties. PDMS has a very surface active pen
dant group - the methyl group: see Figure 1.
CHs CH3 CH
-|—S i - O —S i- O - S i- j- n
c h
3
c h3
c h3
Fig. 1. S t r u c t u r e o f P D M S m o l e c u l e
Rys. 1. S c h e m a t c z ą s t e c z k i P D M S
The methyl groups are arranged along the flexible backbone: the siloxane chain. Because of this, PDMS has a very low surface energy [7, 5]. This makes PDMS a very good substrate for low-tackiness applications.
lii eistość silikonów
internal NMR standard without further purification.
The solvents used were all of pro analysis quality.
Samples preparation
For every batch of polymer the exact amount of vinyl groups was determined using NMR measure
ments (Bruker 300 MHz apparatus); pyrazine as an in
ternal standard. From those results and the molecular structure of the cross-linking agent, the hydrogen-to-vi- nyl ratio (H/V) was calculated. For both types of PDMS samples and the different cross-linking agent function
alities, samples were prepared using the H/V ratios: 1.0, 1.2, 1.4 and 1.7. The curatives were mixed together with the polymer, then the mixture was degassed and cured in a com pression molding machine (WLP 1600/5x4/3 Wickert laboratory press) at 120 °C for 30 min. Clean Teflon foil was placed between the cured mixture and the mold plates to avoid surface contami
nation and sticking of the material to the mold. The 90x90x2 mm sheets were post-cured in an oven at 120 °C for 48 hours.
Mechanical testing
Shore A hardness was measured using Zwick hard
ness tester according to the ISO R868 standard.
Contact angle hysteresis
Contact angle hysteresis measurements were per
formed using the Dataphysics OCA 15 plus apparatus using deionized water as a liquid probe.
Crosslinking density
All crosslinking density measurements were made by swelling rubber samples in toluene for 48 hours;
calculations were performed using the Flory-Rehner equation.
2. Experimental
Materials
Vinyl-terminated PDMS of molecular weight of 17 000 and 50 000 g/mol (MQ 17 and MQ 50 respec
tively) were used for the study. As cross-linking agent tris(dim ethylsiloxy)ethoxysilane (trifunctional), tetrakis(dimethylsiloxy)silane (tetrafunctional) and a multifunctional silane were used. The platinum-cy- clovinylmethylsiloxane complex was used as cure reac
tion catalyst and 1-ethynylcyclohexanol (99%) was used as a temporary reaction inhibitor. All the above materials were obtained from ABCR, with the excep
tion of the multifunctional silane, which was provided by a proprietary source. The inhibitor was obtained from Aldrich. Pyrazine (Aldrich, 99%) was used as an
Tack measurements
Tack measurements were performed using a cus
tom-made device based on the Tel-Tak principle [8].
Fig. 2. T h e p r i n c i p l e o f t a c k - t e s t i n g d e v i c e ; 1 - r u b b e r s a m p l e s , 2 - c la m p s , 3 - s e p a r a t o r
Rys. 2. Z a s a d a d z i a ł a n i a u r z ą d z e n i a d o p o m i a r u k l e i s to ś c i; 1 - t e s t o w a n e p r ó b k i , 2 - z a c i s k i , 3 - s e p a r a t o r
SiaAtotn&Uf, nr 4 lipiec - sierpień 2006 r. TOM 10
Pieces of rubber 20x20x2 mm were used as test sam
ples. Pairs of samples were pressed against orifice disks to generate a curved contact surface, Figure 2. The cur
vatures were compressed under 2.5 N load for 10 mi
nutes, and then separated; the maximum separation force was recorded. The contact area was calculated from the radius of curvature.
IR spectra were recorded using a Bio-Rad FTS60 spectrometer. 256 scans were collected. The mixtures were analyzed as films cast on KBr dishes. The spectra taken before and after crosslinking were normalized using the PDMS methyl peak at 2900 cm"1 [9].
3. Results
Tack measurements
Figure 3 presents the shape of a sample separation curve. A force less then zero means, that the sample is still in compression. At zero force separation starts, but the samples are still held together by the interfacial tack forces.
Fig. 3. Example o f sample separation curve
Rys. 3. Przykład krzywej uzyskanej podczas separacji próbek
Fig. 4. Tackiness in relation to crosslink density fo r MQ 17. Trifunctional silane used as crosslinker; IS tacki
ness, ♦ crosslink density
Rys. 4. Zależność kleistości od gęstości usieciowania MQ 17 usieciowanego za pomocą trójfunkcyjnego sila
nu; ^ kleistość, ♦ gęstość usieciowania
Fig. 5. Tackiness in relation to crosslink density fo r MQ 17. Tetrafunctional silane used as crosslinker; & tacki
ness, ♦ crosslink density
Rys. 5. Zależność kleistości od gęstości usieciowania MQ 17 usieciowanego za pomocą czterofunkcyjnego silanu; ^ kleistość, ♦ gęstość usieciowania
At the maximum force the contact between the two sample halves breaks. That value was taken for further calculations of rubber-rubber tackiness.
Figures 4 to 6 present the results of tackiness mea
surements for MQ 17 vs. H/V ratio in correlation with crosslink density of the samples, and the cross-linking agent functionality varied from 3, 4 to multifunctional.
In all cases, the crosslink density increases almost linearly with increasing amount of silane. For the stoichiometric amount of cross-linking agent, the sam
ple is still clearly undercrosslinked and exhibits a high level of tackiness. The rubber-rubber tack decreases quickly with increasing crosslink density, finally falling below the detection level of the apparatus.
From Figure 5 it can be seen, that for the tetrafunc
tional crosslinker, the trend in tackiness change is simi
lar to the trifunctional crosslinker. However, the values of recorded tack are greater that those seen in Figure 4.
The crosslink density also follows a slightly different trend. There is a large increase between rubbers with hydrogen to vinyl ratios 1.2 and 1.4.
Fig. 6. Tackiness in relation to crosslink density fo r MQ 17. Multifunctional silane used as crosslinker; II tacki
ness, ♦ crosslink density
Rys 6. Zależność kleistości od gęstości usieciowania MQ 17 usieciowanego za pomocą wielofunkcyjnego si
lanu; ^ kleistość, ♦ gęstość usieciowania
TOM 10 lipiec - sierpień 2006 r. StaAtatH&ity nr 4
\ kleistość silikonów
Fig. 7. Tackiness in relation to crosslink density fo r MQ 50. Trifunctional silane used as crosslinker; ^ tacki
ness, ♦ crosslink density
Rys. 7. Zależność kleistości od gęstości usieciowania MQ 50 usieciowanego za pomocą trójfunkcyjnego si
lanu; li kleistość, ♦ gęstość usieciowania
Figure 6 presents the relation between tackiness and crosslink density for MQ 17 crosslinked with the multifunctional silane. The values of tackiness for the lowest crosslinked samples are similar to the analogous samples crosslinked with the tetrafunctional silane, but the rubber-rubber tackiness falls below the detection level already for the sample with 1.2 Fl/V ratio of cross- linking agent added. The crosslink density follows a similar trend, the large increase between Fl/V ratios 1.2 and 1.4 is again clearly visible.
Figures 7 to 9 show the relation between rubber- -rubber tackiness for the high molecular weight PDMS MQ50. It exhibits significantly higher levels of tack, than its low molecular weight counterpart. The decreas
ing trend in tackiness is even more pronounced, a very high tack for H/V 1.0 decreases much after a slight in
crease in crosslinking density.
The increase in crosslinker functionality from three to four results in different behavior from MQ17. As can
kleistość silikonów
Fig. 9. Tackiness in relation to crosslink density fo r MQ 50. Multifunctional silane used as cross-linking agent;
^ tackiness, ♦ crosslink density
Rys. 9. Zależność kleistości od gęstości usieciowania MQ 50 usieciowanego za pomocą wielofunkcyjnego si
lanu; ii kleistość, ♦ gęstość usieciowania
Fig. 10. Tackiness in relation to functionality of cross- linking agent fo r MQ50 ofH /V = 1.0
Rys. 10. Zależność kleistości od liczby grup silanowych w cząsteczce substancji sieciującej dla MQ 50 i H/V - 1.0
Fig. 8. Tackiness in relation to crosslink density fo r MQ 50. Tetrafunctional silane used as cross-linking agent;
^ tackiness, ♦ crosslink density
Rys. 8. Zależność kleistości od gęstości usieciowania MQ 50 usieciowanego za pomocą czterofunkcyjnego sianu; M kleistość, ♦ gęstość usieciowania
be seen in Figures 8 and 9, MQ50 crosslinked with tetra and multifunctional silanes show very low levels of tack, which in addition decreases very fast with increas
ing H/V ratio. The crosslink density behaves in a diffe
rent way as well: it is much higher at low H/V ratios compared to the trifuntional cross-linking agent, and exhibits only a slightly increasing trend.
Figure 10 shows how rubber-rubber tackiness changes for MQ50 with increase in cross-linking agent functionality for H/V ratio 1.0. The level of tackiness decreases by orders of magnitude with increase of si
lane functionality from three to four. Further increase in functionality does not cause significant changes any
more.
Figure 11 shows the results of surface characteriza
tion done by the contact angle hysteresis measure
ments.
The general trend for MQ17 is, that the contact angle hysteresis decreases with increasing excess of cross-linking agent. This is less pronounced in the case
SCcbtftMt&ity nr 4 lipiec - sierpień 2006 r. TOM 10
Fig. 11. Contact angle hysteresis measurements fo r MQ 17 (a) and MQ 50 (b); trifunctional; (M) tetrafunc- tional; ( • ) multifunctional silane as crosslinking agent Rys. 11. Histereza kąta zwilżania dla (a) MQ 17 i (b) MQ 50 usieciowanych za pomocą (+) trój funkcyjne go;
(M) czterofunkcyjnego; (* ) wielofunkcyjnego silanu.
of the high molecular weight MQ50, which exhibits similar values of hysteresis like MQ17, but it tends to vary much less with degree of crosslinking.
Hardness
The hardness of MQ17 increases with increasing H/V ratio. It follows the trend exhibited by crosslink density, however the gaps visible for higher functiona
lity crosslinkers are not so pronounced in the hardness.
It is interesting, that the hardness of the MQ50 samples does not really change with crosslinker amount, but only with the functionality.
Spectral analysis
FT-IR spectra were collected to gather information about changes in the amount of vinyl bonds during the cure reaction and to explain the role of different mecha
nisms responsible for tack. Figure 13 shows a sample spectrum. The peaks at 1599 and 2140 cm"1 are as
signed to the vinyl double bonds and Si-H vibration, respectively [10]. Table 1 shows the intensities of both peaks relative to the C-H vibration peak of the methyl group in PDMS, which served as an internal standard.
The values are shown for mixtures of MQ17, cured with trifunctional crosslinker and for H/V ratios of 1.0 and 1.7.
It can be seen from Table 1, that for H/V = 1.0 the level of silane functional end-groups, as represented by
Table 1. Changes in reactive groups concentration for MQ 17
Tabela 1. Zmiany stężenia grup reagujących na przy
kładzie MQ 17
Time [min]
. ..
H/V = 1.0 ; H/V := 1.7 Ratio C=C Ratio Si-H Ratio C=C Ratio Si-H
0 0.29 0.28 0.12 0.16
H I 0.11 0.15 0.10 0.03
5 0.06 0.00 0.12 0.02
10 0.06 0.00 0.12 0.00
the level of IR absorbtion of C=C-bonds, decreases quicker than for H/V = 1.7. There are still some unre
acted double bonds present, most probably in a form of pendant chains. For the H/V = 1.7, the silane reacts slightly slower. It is surprising to see, that the amount of double bonds seems to stay at the same level during the
Fig. 12. Hardness measurements fo r MQ17 (a) and MQ50 (b). For some samples, hardness at H/V 1.0 was too low to measure using the Shore A scale; (♦ ) trifunc
tional; /■ ) tetrafunctional; multifunctional silane as crosslinking agent
Rys. 12. Pomiary twardości (a) MQ 17 i (b) MQ 50.
Przy H/V 1.0 twardość niektórych próbek była zbyt nis
ka na pomiar metoda Shore A. Próbki usieciowano za pomocą /♦ ) trójfunkcyjnego; czterofunkcyjnego;
( • ) wielofunkcyjnego silanu
TOM 10 lipiec - sierpień 2006 r. S fa d & w tw / nr 4
kleisto,st silikonów
Fig. 13. Sample FT-IR spectrum o f the uncured mixture Rys. 13. Przykładowe widmo FT-IR nieusieciowanej mieszanki
reaction. The same phenomenon was observed for MQ17 cured with the tetrafunctional cross-linking agent.
The spectra for longer curing times have been made, but obviously the crosslinking reaction is fin
ished after ten minutes. There were no significant changes observed anymore.
4. Discussion
As expected, rubber-rubber tackiness decreses with increase in crosslink density for all combinations inves
tigated. Lower crosslink density results in larger amounts of unattached chains, pendant network chains, as well as in a less constrained network. All these phe
nomena are known to promote tack by partial migration or penetration of polymer entities across the contact interface. During the contact, polymers can also ex
change other interactions, like van der Waals or hydro
gen bonding. It was already proven by others, that for PDMS-PDMS contact the forces are dispersive: van der Waals in nature, due to the presence of numerous methyl groups on the interface [11]. Thus the tack in
crease must mainly be due to the presence of pendant chains and increased chain mobility. The FT-IR spectra show, that there is always some amount of pendant PDMS chains (Table 1), irrespective of the FI/V-ratio and the cure time, due to network imperfections. The contact angle hysteresis gives some additional informa
tion. The difference between advancing and receding contact angle is related to the mobility of macromolecu- lar segments at the surface [12, 13]. It can be seen from Figure 11, that hysteresis decreases with increase in crosslink density, which indicates, that at least partiali chain mobility plays a role as well.
The increased molecular weight of the polymer re
sults in a large increase in tackiness values for the sam
ples crosslinked with the trifunctional silane: Figures 4 and 7. The contact angle hysteresis does show, that molecular mobility stays similar within the measure
ment error: Figure 11. However, during separation the chains are extended before full release. The extensions and disentanglements cost more energy for the longer, higher molecular weight chains [13]. Thus, the increase in tack value can be explained by an increased disentan
glement energy and possibly a higher amount of entan
glements formed by the large macromolecules.
Interesting phenomenon is the significant tack de
crease by change from trifunctional to tetrafunctional silane. This is most pronounced for the high molecular weight PDMS: Figure 10. The big decrease in tack is combined with increase in crosslinking density: Figures 7-9, which seems to be the main reason for this effect.
5. Conclusion
The experiments indicate that the cross-linking agent amount, its functionality and molecular weight of polymer have a large influence on rubber-rubber tacki
ness. It decreases with increasing crosslink density and crosslinker functionality. However, the latter causes a very significant drop of tackiness.
There are several mechanisms involved; the sur
face analysis and a spectral investigation of the cure reaction help to understand their role. The reduction in chain mobility combined with a decrease in amount of pendant chains seems to be the main reasons for the tackiness drop. However, the molecular weight effect is mostly caused by an increase in amount of entangle
ments of molecules and a higher energy needed to un
ravel the entangled chains. The drop of tackiness with increased crosslinker functionality seems to be the combined effect of decrease in amount of pendant chains and increase in crosslink density.
S ia A tM t& iy nr 4 lipiec - sierpień 2006 r. TOM 10
kleistość silikonów
Acknowledgment
The Dutch Polymer Institute (DPI) is gratefully acknowledged fo r financial support o f this research in the project #317.
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13. Hillborg H., Gedde U. W: Polymer, 1998,1 2 1991
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• kleje do łączenia gum z metalami, zapewniające optymalne połączenia dla różnych zastosowań:
MEGUM™ i THIXON™
• kleje do tłokowania typu POLYFLOCK™
zapraszamy
Dystrybucja w Polsce:
P rzed sięb io rstw o P rodukcyjne H. Leszczyński ul. K om ornicka 2, 62-052 Głuchow o
te l. 061 8990600 lub 061 8990603, fax 061 8997217 e-m ail: leszczynski@ hl.biz.pl
TOM 10 lipiec - sierpień 2006 r. SOz&totH&iy nr 4
k l e i s t o ś ć s i l i k o n ó w
