Surface Modification of Carbon Black by Plasma Techniques Influence On

ELASTOMERE UND KUNSTSTOFFE ELASTOMERS AND PLASTICS

Plasma surface modification · Surface Modification of Vulcanization kinetics · Carbon black · CBS, Mechanical properties Although vulcanization has been used Carbon Black by Plasma since a very long time its reaction mechanism and kinetics are still not completely clarified. In this work the Techniques Influence on the

inﬂuence of plasma modiﬁcation (O2,

N2, NH3) of carbon black in the vulcanization reaction and kinetics using CBS Vulcanization Reaction as accelerator were studied both by rheometer and Model Compound vulcanization experiments. The results show that, not only the kinetics but also the reaction mechanism is altered by the introduced oxygen groups. On the other side, changes produced on Since many years it is know that elastomers rried out trying to understand and relate the surface of carbon black by the ammonia plasma only led to faster need to be vulcanized in order to achieve both rheometer and MCV results. reaction kinetics but did not change the good mechanical properties. However, the During this work the effect of CB surface reaction pathway. As a consequence, vulcanization reaction remains still rather modification on the vulcanization reaction final mechanical properties studied by unknown, although many hypothesis have was studied both by means of the rheome- means of tensile-strength curves were been presented a global accepted model ter curves and MCV analysis using squalene also affected and improved for nitro- needs still to be developed. as model. Several plasma treatments were gen containing carbon black surfaces. By studying rheometer curves it has also performed (O2, N2, NH3) and modified CB been evidenced that carbon black (CB) has was afterwards used to study the kinetics, an influence on the vulcanization reaction, mechanism and final properties of the NBR Oberflächenmodifikation von usually faster reaction kinetics has been re- and BR compounds. Ruß durch Plasmaverfahren; ported when this filler is present in the rub- Einfluss auf die ber compound [1-4]. Many authors have Experimental Vernetzungsreaktion stated that the presence of carbon black was not responsible for a change in the vul- CB treatment Oberflächenmodifizierung durch canization reaction mechanism but only For this objective several modifications were Plasma · Vulkanisationskinetik · Ruß · has a catalytic effect. On the other hand, introduced in an atmospheric plasma single CBS · mechanische Eigenschaften chemical composition of carbon black has nozzle connected to a generator at 280 V (Plasma Treat GmbH). Detailed description of Obwohl die Vulkanisation seit langem also been claimed to be involved during the untersucht wird sind der Reaktionsme- the system has been described in a previous cross-linking reaction by changing its cross- work [15]. chanismus und die – kinetik noch nicht linking rate and degree of final vulcaniza- gänzlich aufgeklärt. In dieser Arbeit soll tion [5-9], however no explanation to this The treated CB was N134 (ASTM classifica- der Einfluss der Plasmamodifikation (O , 2 fact has been described. tion) in fluffy state, which helped to have an N2, NH3) von Ruß auf die Schwefelver- netzung mit CBS als Beschleuniger The role of carbon black during vulcaniza- homogeneous modification. No pretreat- anhand von Mischungen im Rheome- tion has also been related to its adsorption ment was done in this case to the filler be- terversuch und durch Modellvulkanisa- capacity as well as the ability of the CB sur- fore its modification. Carbon black has been tion vorgestellt werden. Die Ergebnisse face to react with the acceleration system treated with both air and nitrogen plasma zeigen, dass nicht nur die Kinetik aber and its degradation by-products. Former in a flow of 2000L/h. In both cases CB was auch der Reaktionsmechanismus durch studies by Tof-SIMS have already proved the introduced in the reactor suspended in a sauerstoffhaltige Gruppen auf der presence of the accelerator and its interme- nitrogen flow of 3L/min., which allowed a Rußoberfläche beeinflusst werden. diates on the surface of CB [10]. rate of modification of 1 g/min. Demgegenüber werden Oberflächen- modifikationen die mit Ammoniakplas- Because the complexity to study the vul- ma erzeugt wurden, zu einer schnel- canization reaction kinetics and mechanism Authors leren Kinetik führen ohne den by using only the rehometer tests and poly- N.Tricás, N. Agulló, S. Borrós, Reaktionsablauf zu beeinflussen. mer compounds, model compound vulcani- Barcelona (Spain) R. H.Schuster, Mechanische Untersuchungen des Zug- zation is used to overcome the difficulties. Hannover Dehnungsverhaltens zeigen ebenfalls The substitution of polymer by shorter liq- die Vorteile der stickstoffhaltigen uid molecules allows not only following the Corresponding author: Rußoberfläche. accelerator decomposition but also the evo- Dr. Salvador Borros lution of the intermediates giving the pos- Universitat Ramon Llull sibility to follow them during the whole re- Grup d’Enginyeria de Materials (GEMAT) action [11-13]. By using this method some Institut Quimic de Sarriá new information was brought into the reac- Via Augusta 309 tion pathway of sulphur vulcanization [14] 08017 Barcelona, Spain However, very few studies have been ca- E-mail: [email protected]

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1 Rheometer curves obtained for ﬁlled BR (50phr) 2 Rheometer curves obtained for ﬁlled NBR (50phr)

MCV approach Vulcanization system is added and mixed sented in Figures 1 and 2 respectively. Al- The vulcanization reaction was studied by for the last 2 minutes though the figures present only one of the means of Model Compound Vulcanization In order to verify the final content of the curves, three different tests were carried MCV using the composition shown in Table1. filler in the compound a TGA analysis was out in order to obtain a mean value for each As it can be seen N- Cyclobenzothiazole-2- carried out. All compounds were found to parameter. The values are presented in Ta- sulfenamide (CBS) was used as accelerator. differ less than 2 % from the theoretical va- ble3. The mentioned accelerator species together lues. In the case of NBR vulcanization no changes with S8 were followed during the vulcaniza- Rheometric curves were obtained at 160 °C were observed for the t1 which indicates the tion reaction at 140 °C by quenching the in a Montsanto rheometer using a 1.667Hz beginning of the vulcanization reaction. reaction at several reaction times (from 0 to frequency and 0.5° strain. Studied parame- However, it is possible to observe that for

50min.). ters include t1 as scorch time, t90 as ﬁnal vul- the air treated APP CB compound shorter t90

Several vessels containing about 2.5 grams canization time, ML as initial viscosity and were obtained indicating that the vulcani- of the initial mixture were heated in an oil MH-ML. MH-ML was considered the ﬁnal zation reaction is terminated earlier than bath at 140 °C and acetone-CO2 bath was crosslink density as the same amount of CB for the two other CB’s. If the ﬁnal MH-ML used to stop the reaction. The accelerator was included in the formulation for all mix- was the same for all grades a shorter t90 species were extracted by acetonitrile and tures. would indicate that the same crosslinking analyzed by HPLC. Conditions for the HPLC level was achieved in a shorter period of analysis were: Tensile-stregth test time, but looking to the MH-ML levels it is Column: C18, length:12 cm To measure mechanical properties 2 mm possible to observe that air APP treated CB Mobile Phase: Acetonitrile: Water (90:10) plates were vulcanized at 160 °C. Vulcaniza- is lower than the others. Therefore, the

Flow: 1ml/min tion time was considered to be t90+ 1 min. shorter t90 does not indicate probably a

Detector: UV at 254nm (t90 obtained from the rheometric curves). faster reaction but a lower crosslinking le- Mechanical behavior was studied by means vel reached during vulcanization, but also Rheometer studies or the tensile-strength curves, which were could indicate a lower contribution of the In all cases the following procedure was performed in a Zwick Z010 machine at CB network in the measured torque. The used in order to obtain the rubber com- 200 mm/min. toughness measure presented in table 3, pounds. Curing sulfur semi-efficient sys- indicating a lower Shore-A value for the air tems shown in Table2 were incorporated to Results and discussion APP treated CB compound is also an indica- several polymer matrixes. tion that this type of CB influences the vul- Mixtures were prepared in a laboratory Rheometer curves canization reaction. Presentation of the re- scale inner mixer. Temperature was set at The rheometric curves obtained both for sults obtained for the BR rubber will pro- 60°C and rotor speed at 50 rpm. Each com- the BR and NBR filled compounds are pre- bably help to clarify this phenomena. pound was prepared following the next procedure for a total of 12 minutes: 2 Used recipe for the studied compounds amount are given in phr First two minutes polymer mixing CB is added and mixed for next 8 mi- Polymer CB Sulfur CBS ZnO Stearic acid nutes 100 Variable 1.7 2.5 2.5 1

1 Formulation for MCV experiments 3 Results for CB treated by Atmospheric Pressure Plasma Chemical Amount (phr) Polymer NBR BR N 134 CB grade Air APP N APP Untreated Air APP N APP Untreated Squalene 100 2 2

CB 10 MH-ML 25.36 28.12 29.45 18.43 25.27 24.5

CBS 1.6 ML 1.83 2.22 2.41 4.99 3.39 2.9

S8 2 T1(min) 0.61 0.63 0.64 0.65 0.87 1.05

ZnO 5 T90 (min) 3.34 4.78 4.59 5.93 3.67 4.32 Stearic Acid 2 Shore-A 72 77.6 77.1 66.6 69 68.3

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possible to observe a final slightly lower M - 3 3 Stress-Strain curve H M , which could be related to a CBS perma- fort he BR filled com- L pounds nent adsorption on CB. On the other hand, in the case of BR, the higher apolarity of this polymer promotes the migration of the CBS to the CB surface. The CBS-CB initial contact might be beneficial to start CBS decomposi-

tion and therefore to decrease t1 as observed. However, if the adsorption becomes too strong the effect explained above will take

place obtaining longer t90 and lower MH-ML values. As a conclusion it could be said that, adjusting CB polarity to the polymer and the accelerant molecules nature (CBS in the present case), vulcanization rate and final 4 crosslink could be optimized as is the case for nitrogen atmospheric plasma treated CB in BR compounds. The effect of atmospheric plasma modification was confirmed by looking at the stress- strain curves for the BR vulcanized compounds which confirmed that better properties were obtained after nitrogen atmospheric plasma (Fig. 3). Air treated CB formulation presented a curve which correspond to a lower crosslink level as it has been al- 4 Studied accelerator species and intermediates during the MCV reaction ready seen in the vulcanization curve. There- fore, the influence of CB in the vulcanization 5 5 does not only affect the curing time but also showing the the final properties of the compound which separation of CBS, shows the importance to know more about CBS compounds the CB surface properties which may influ-

and S8 ence in the vulcanization reaction. The above presented results by means of the rheometric curves were studied also using the MCV approach in order to understand in more detail the molecular process which leads to such macromolecular responses.

MCV results As it has been stated before, the objective of this study is to find a possible explana- The rheometric curves obtained for BR do teract with the vulcanization chemicals such tion for the changes that have been already show an evident change in t1. Much shorter as the accelerating system, including sulfur observed in the vulcanization reaction due values are observed for air APP treated CB and accelerators (CBS) [16-18]. It has been to modification of CB surface nature. This compound followed by nitrogen APP treat- proposed that the interaction of CB with technique allows following the accelerator ment and finally the unmodified N134. On such chemicals may be the reason for a decomposition and its intermediates for- the other hand, t90 differ also between the faster vulcanization kinetics when this filler mation along the vulcanization reaction three vulcanized samples. On one hand, air is present. However, when the filler presents and therefore, it can provide very interest-

APP treated CB presents the longer t90 while high polarity (as it is the case of the air at- ing information about potential mechanism the nitrogen APP CB formulation is the fast- mospheric plasma treated CB) this species changes due to the presence of each CB. er achieving 90% of the vulcanization level. will be highly adsorbed on the CB surface The species that were monitored during the

In this case MH-ML values are much lower for and they are prevented from taking part in reaction are presented in Figure 4 where the air APP CB formulation. Once again because the vulcanization reaction. That should also their evolution during the reaction is also the CB level is the same for all three compounds, explain why such a difference is observed shown. It is known that in order to start vul- the most probable reason for such difference between NBR and BR compounds. The high- canization the N-cyclobenzothiazole-2-sul- is a much lower crosslink density which is er NBR polarity due to its nitrile groups, pre- fenamide (CBS) molecule (Initial accelerator) also indicated by a lower Shore A hardness. vents the CBS molecule from being highly must be decomposed to 2-mercapto-ben- A possible hypothesis which will be further absorbed on the CB surface as compatibility zotiazolsulfenamide (MBTS) (accelerator discussed by means of the MCV approach is of this molecule with the polymer matrix is intermediate) which will further react and here presented: It is known that CB may in- enhanced. However, even in this case it is give 2-mercaptobenzothiazole (MBT) as ﬁnal

84 KGK · März 2009 vulcanization specie. It has also been shown 6 that MBT may act as a catalyst for CBS decomposition; therefore, once the reaction starts reaction velocity increases [14]. Results are given as percentage of the total area detected in the chromatogram which can be observed in Figure5. Reproducibility of the sample preparation and species de- tection was already studied in previous works of our research group [12-14].

Influence of CB structure on the vulcanization reaction As first result, the effect of CB N134 on the decomposition of CBs is presented in front of a formulation containing no CB. It was also thought that it would be very interes- 6 CBS, MBTS; MBT and Sulphur decomposition for unfilled, N134 and N 134g formulation ting to observe the effect of N134g, in order to study whether all type of C surface had the same effect or if other parameters such 7 7 as C structure were also relevant. As it can 40min heating up be seen in Figure 6, the presence of CB does CBS/squalene (80 °C) increase the velocity of CBS decomposition being completely depleted after 15minutes reaction time. However, this is not the case either for the non-containing CB formulation or for the N134g. The results show that not only the presence of CB is important for the vulcanization kinetics but also the nature of such carbon surface. As it can be observed in the figure 6, CBS 8 8 decomposes faster in the presence of N 134 Chromatogram after 40min heating up than without CB or N134g. Consequently, CBS/squalene/CB MBTS intermediates are formed earlier. (80°C) However, MBT is not the only final specie for this reaction, as is the case for N 134g and without CB, were relative area becomes equal to the MBT area. This might be an in- dication that other species are formed during the reaction. These phenomena will be studied in more detail. In order to confirm this catalytic role of carbon black, next two samples were analyzed. 9 On one hand CBS in squalene was left al 80°C showing no decomposition was seen after 40minutes. On the contrary when CB was introduced in the mixture after 40minutes not only is possible to observe CBS on the chromatogram but also other decomposition species such as MBTS and MBT. Chroma- tograms are presented in Figures 7 and 8, respectively.

Influence of APP treated CB in the vulcanization reaction Because filled rubber with APP modified CB’s vulcanization was studied by means of the rheometric curves and some interesting changes were found as presented, the same CB’s were also studied by means of the MCV 9 CBS, MBTS; MBT and Sulphur decomposition for unfilled and atmospheric plasma approach in order to try to find more infor- treated CBs

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10 10 MTBT evolution 11 during MCV experiments

very small for the other reactions and not taken into account, increased when using the air APP treated carbon during the MCV. The new peak appeared at a retention time mation about the changes produced in the position and vulcanization reaction, the level of 1,544 min. instead of 1,304 min. corre- vulcanization reaction. The evolution of the for the air APP modified CB is much lower. A sponding to MBT. In this case we suppose accelerator species is presented here. possible explanation for this effect is that the that the low levels of MBT are due to the As it can be seen in Figure 9, the CBS decom- reaction path for the vulcanization reaction is presence of this new final product. The evo- position, which has to take place before start- altered by the presence of such filler. For nitro- lution of this peak is shown at Figure 10 for ing the vulcanization reaction and which is gen APP treated CB the MBT is formed faster the different MCV’s reactions. A previous responsible for the scorch time, changes for than for the other two CB grades. As a matter study of our research group helped to iden- the plasma modified carbons. Both oxygen of fact the MBT curves are also consistent with tify this compound as 2-mercaptotiobenzo- and nitrogen APP treated carbons reach the the observed results for the BR vulcanization. tiazole (MTBT) (Fig. 11) [14]. total decomposition of CBS before the un- In those curves air and nitrogen APP CB pre- In order to corroborate that the oxygen spe- modified CB. It can be concluded then, that sented also shorter t1 but while N2 APP com- cies created during air atmospheric plasma the plasma modification helps to decompose pound increased fast and steeply the torque, are the ones responsible for the mechanism the accelerator, and which is even more im- air APP’s increase was slow and lower. change during the CBS decomposition reac- portant that there is an interaction between The hypothesis of a change of mechanism tion, a last experiment was performed. APP the CB surface and the accelerator. Whether during crosslinking is also confirmed when air treated N-134 was treated with NH3 it is just a physical adsorption or if it chemi- following the intermediate sulfur level dur- plasma in order to make the acidic groups cally reacts is more difficult to state, how- ing the reaction, which also indicates the react with the reactive basic species present ever the figures corresponding to the other absence of crosslinking reaction. Even when in the plasma. The MCV reaction was carried species can give us important information. the reaction is considered to be finished and out using such modified CB. Results are shown It is worth to remember that the same effect all the accelerant (CBS) is decomposed (aff- in Figure12. It can be seen that even after was observed when vulcanizing BR com- ter 30min. of reaction) there is still a high the ammonia treatment the CB still decom- pounds which is coherent as squalene can level of sulfur that has not been introduced poses CBS faster than regular unmodified also be considered an apolar media more in the chains for the formulation containing N134. However, in this case of sulfur it can similar to BR than to NBR. APP air treated N-134. be observed how already after 18minutes As it can be observed from Figure10 showing It was also observed that during the reac- there is no more sulphur left in the formula- MBT levels, the final product after CBS decom- tion another unidentified peak which was tion which indicates that have been used to cross-link the molecules. It seems therefore that the presence of the oxygen groups is 12 needed in order to catalyze the MTBT formation which prevents vulcanization. This is confirmed when looking at the MBT formation. It is possible to observe that higher MBT levels than for APP air treated N-134 are ob-

tained for APP air treated N-134+NH3 indicating that although some changes are still present the reaction path may be more similar to that of the original N 134. To conclude this experiment the level of the new ﬁnal product (MTBT) was also observed

to be lower after the NH3 plasma modiﬁca- tion (Fig. 13). This fact leads to the conclusion that the oxidized groups formed on the CB surface during the air_APP treatment are the ones responsible for changing the CBS decomposition path leading to unreac- tive cross-linking specie (MTBT). On the 12 CBS, MBTS; MBT and Sulphur decomposition for unﬁlled, N134 and APP air treated other hand when polarity of the CB is in- N-134 and APP air treated N-134 + NH 3 creased without the presence of such

86 KGK · März 2009 oxidized groups the reaction is faster with- 13 13 Evolution of the new out involving such mechanistic changes. final accelerant product MTBT Discussion of CB role on vulcanization reaction Both rheometric studies and MCV have con- tributed to study the effect of CB surface during the vulcanization reaction. Both techniques confirmed that the presence of amorphous carbon on the surface of CB leads to a faster reaction rate without severe modifications on the final cross-link level. A faster decomposition of the accelerant (CBS) on the surface of CB containing such amorphous structure is a very likely reason 14 14 Possible change in to be responsible for such changes. the vulcanization mechanism for APP On the other hand changes produced during treated CB APP treatment have helped to understand already described effects such as lower crosslink densities and lower vulcanization rated when vulcanizing NR in the presence of oxidized CB.2 Although faster CBS decomposition was observed in the presence of both air and nitrogen APP treated CB’s the vulcanization mechanism has been shown to be altered by the APP air treated N-134 CB. However, in order for this change to take place, CBS must be adsorbed on the surface of CB which only happens when the polarity difference between the filler and the rubber is high enough. This is the reason why such shown. The results have demonstrated that, References effects were not observed during NBR (high not only the kinetics but also the reaction [1] R.Cotten, Rubber Chem. Technol. 45 (1972) 129. nitrile content) vulcanization. mechanism is altered by the introduced [2] R. Cotten, B. B. Boonstra, D.Rivin and F.R.Wil- Adsorption and presence of high oxidized oxygen groups on Carbon black surface. The liams, Rubber Chem. Technol. 9 (1979) 477. groups on the CB surface is necessary in or- interaccion of oxygen-modified carbon black [3] J. J. Brennan and D. H.Lambert, Rubber Chem. der to obtain MTBT during the vulcanization seems to produce an intermediate molecule Technol. 45 (1972) 94. reaction which prevents sulfur to go into the MTBT which leads to lower cross-link den- [4] M.Porter, T.D. Skinner, M.A.Wheelans, J. App. Poly. Sci. 11 (1967) 2271. crosslink reaction staying in the formulation. sity. However, in order for this change to take [5] M.Gessler, Rubber Chem. Technol, 42 (1969) 850. However, this specie was also shown to ap- place, CBS must be adsorbed on the surface [6] S.Bandyopadhyay, P.P. De, D. K. Tripathy and pear at long reaction times for N134. The of CB which only happens when the polarity S.E.De, J. App. Poly. Sci. 58 (1995) 719. appearance of such molecule could be due difference between the filler and the rubber [7] S.-J.Park, K.-S.Cho S.K.Ryu, Carbon 41 (2003) to a final reaction of MBT with some lousy is high enough. This is the reason why such 1437. bounded Sulfur of a polysulfuric bound be- effects were not observed during NBR (high [8] S.J.Park, M. K.Seo and Ch. Nah, J. of Colloid Int. tween two squalene or polymer chains which nitrile content) vulcanization. On the other Sci 291 (2005) 229. [9] F.Cataldo, Angew. Chem. 270 (1999) 81. could help to provoke reversion. On the side, changes produced on the surface of [10] S.Borrós, E. Vidal, N.Agulló, W. Van Ooij, Kaut- other hand if such reaction takes place be- carbon black by the ammonia plasma only sch. Gummi Kunstst. 53 (2000) 711. fore the sulfur forms part of sulfur bounds led to faster reaction kinetics but did not [11] J.H.M.Van den Berg, J.Beulen, J.M.H.Hacking. between the chains it will never get there as change the reaction pathway. F. J.Duynstee, Rubber Chem. Technol. 57 (1984) the case of vulcanized BR in the presence of As a consequence, final mechanical proper- 725. APP air treated N-134. A scheme of the proc- ties studied by means of tensile-strength [12] E.Garreta, N. Agulló, S.Borrós, Kautsch. Gummi ess taking into account MTBT formation is curves were also affected and improved for Kunstst. 55 (2002) 711. [13] S.Rodríguez, N.Agulló, F. Broto, L. Comellas presented in the Figure14. nitrogen containing carbon black surfaces. N.Agulló, S.Borrós, Kautsch. Gummi Kunstst. 52 Direct reaction of MBTS to form (MTBT) pre- (1999) 438. vents the formation of vulcanization inter- Acknowledgements [14] S.Borrós, N.Agulló, Kautsch. Gummi Kunstst. 53 mediates and thus the sulfur transfer to the Tha authors would like to thank the Gener- (2000) 131. chain to form the cross-link bridges. alitat de Catalunya for the pre-doctoral fel- [15] N. Tricás, S. Borrós and R. H.Schuster, Kautsch. lowship 2003 FI 00864 to N.T. and the Re- Gummi Kunstst., 10 (2005) 511. Conclusions search Consolidated Group grant: SGR-2005 [16] P.D’Silva, Carbon, 36 (1998) 1317. [17] J.M.Peña, N. S.Allen, M.Edge and CM. Liauw, The influence of the plasma surface modifi- to Grup d’Enginyeria de Materials (GEMAT) J.Mat. Sci. 36 (2001) 4419. cation (O2, N2, NH3) of carbon black in the for providing funds to support this re- [18] J. M. Peña, N.S.Allen, M.Edge and CM.Liauw, vulcanization reaction and kinetics has been search. J.Mat. Sci 36 (2001) 4433.

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