Novel Ionic Liquids as Accelerators for the Sulfur Vulcanization of

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Novel ionic liquids as accelerators for the sulphur vulcanisation of butadiene-styrene elastomer composites Magdalena Maciejewska, Filip Walkiewicz, and Marian Zaborski Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie303167z • Publication Date (Web): 17 May 2013 Downloaded from http://pubs.acs.org on June 10, 2013

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Industrial & Engineering Chemistry Research

Novel ionic liquids as accelerators for the sulphur vulcanisation of butadiene-styrene elastomer composites Magdalena Maciejewska*1, Filip Walkiewicz2, Marian Zaborski1 1

Institute of Polymer and Dye Technology, Technical University of Lodz, Stefanowskiego 12/16, Lodz 90-924, Poland

2

Poznan University of Technology, pl. M. Sklodowskiej Curie 2, Poznan 60-965, Poland

*

To whom correspondence should be addressed: Magdalena Maciejewska, Tel.: +48 42 6313213, fax: +48 42 6362543, e-mail: [email protected]

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ABSTRACT. The aim of this work was to study the activity of novel benzalkonium and ammonium ionic liquids with 2-mercaptobenzothiazolate as accelerators in the sulphur vulcanisation of butadiene-styrene elastomer (SBR). In this article, the effect of the ionic liquids on the vulcanisation kinetics of the rubber compounds, the crosslink density and the mechanical properties of the vulcanisates, as well as their resistance to thermal and UV aging, was studied. The application of novel ionic liquids allowed for the elimination of N-cyclohexyl-2benzothiazolesulfenamide from SBR compounds and for the considerable reduction of the amount of 2-mercaptobenzothiazole present in rubber products. Synthesised salts seem to be good substitutes for standard accelerators in the sulphur vulcanisation of SBR rubber, without the observation of any detrimental effects on the vulcanisation process, the physical properties or the thermal stability of the obtained vulcanisates. KEYWORDS. Functional composites, Curing, Mechanical properties, Thermal properties, Dynamic mechanical thermal analysis (DMTA)


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1. Introduction The vulcanisation of elastomers is one of the oldest and best-known processes. The first patent for the vulcanisation of elastomers with sulphur was issued to Charles Goodyear in 1839 and concerned the vulcanisation of natural rubber. Organic accelerators of vulcanisation were not used until 1906, when Oenslager discovered the effect of aniline on sulphur vulcanisation.1 This discovery was followed by the introduction of the guanidine accelerator. In 1930, thiazole and sulphenamide accelerators were developed.2 In later years, other accelerators such as thiurams and dithiocarbamates were applied. Thiazoles, sulphenamides and dithiocarbamates were the last accelerator classes of great commercial significance to be introduced. At about the same time, it was discovered that zinc oxide could activate the vulcanisation process. Zinc oxide reacts with accelerators to form highly active zinc complexes, which are believed to be more reactive than the free accelerators. Sulphur is then incorporated into these complexes, and active sulphating agents are formed. This reactive species reacts with the allylic hydrogen atoms of unsaturated elastomers to form crosslink precursors. The crosslink precursors react with other rubber chains to generate crosslinks, which contain rather large amounts of sulphur atoms in their bridge. Then, crosslink shortening, usually accompanied by side reactions, produces the final elastomer network with a defined distribution and concentration of crosslinks.3 Recently, it was postulated that the acceleration of sulphur vulcanisation resulted from homogeneous catalysis by zinc complexes or zinc salts. The mechanism in which the ZnO surface is both a reactant and the medium of the reaction is also known.3 From this perspective, the most important parameter that affects the process of vulcanisation is the dispersion of inorganic zinc oxide particles in an organic polymer matrix. Because ZnO and vulcanisation accelerators are insoluble in the rubber, it is assumed that the 3 ACS Paragon Plus Environment


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crosslinking reactions occur in a two-phase system and are catalysed by conventional catalysts of interphase reactions.4 Therefore, the application of novel ionic liquids (ILs) - the derivatives of 2-mercaptobenzothiazole - as accelerators of the vulcanisation process seems to be reasonable. First, because of their catalytic activity, ILs should increase the rates of interface crosslinking reactions. Second, because of their ionic character, ILs are believed to be able to form salts or complexes with zinc ions, similar to what has been observed for standard accelerators; however, these salts or complexes are more reactive than the standard zinc-accelerator complex. ILs are room-temperature salts with melting points below 100°C.5,6 They have several important properties such as: “environmentally friendly” characteristics (e.g., negligible vapour pressure, nonﬂammability)6, low volatility, high thermal stability, high ionic conductivity, and a high solvating capability, which allows them to solvate a large variety of both polar and nonpolar organic compounds.7 Because ILs have very good ionic conductivity up to the decomposition temperature, they can play an important role in electrolyte matrixes. Conductive composites with the ionic conductivity of 2.54 × 10-4 S cm-1 have previously been prepared using a acrylonitrilebutadiene

elastomer

(NBR)

containing

1-butyl-3-methyl

imidazolium

bis(trifluoromethyl sulfonyl)imide.8 The dielectric constant of these NBR films was observed to increase with the content of acrylonitrile in the elastomer. Similar results were obtained by Marwanta et al.9. Stable polymer electrolytes were also obtained from polymerisable ILs with an imidazolium cation that showed an ionic conductivity of over 10-4 S cm-1.10,11 In the past few years, ILs have been widely employed as solvents for various types of polymerisations, such as radical or anionic polymerisations.12-15 ILs have been successfully used to prepare porous polymers and polymer gels.16,17 It has also been reported that hydrophilic ILs can dissolve carbohydrate polymers such as


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cellulose effectively due their ability to break the extensive hydrogen bonding network and interact with the hydroxyl groups of polymers.18-20 Despite ILs being widely used in polymerisation processes or as components of polymer matrixes, their application as accelerators for sulphur vulcanisation of elastomers has not yet been reported, with the exception of our previous studies.21,22 Thus, this work represents an original solution, where it is believed that the application of ILs will result in the development of new eco-friendly accelerators for elastomer sulphur vulcanisation. The most popular accelerator (2-mercaptobenzothiazole) is an allergenic agent. Sulphenamides are amine derivatives with different amine moiety contents, just like guanidines. Therefore, their application in technology should be limited because of their harmful properties for human health. The proposed ILs are derived from conventional accelerators, but thanks to their greater activity it is possible to reduce the amount of crosslinking system components (sulphur, zinc oxide, accelerator) in rubber products. In this work, we employed several novel benzalkonium and ammonium ILs with 2-mercaptobenzothiazolate as accelerators for the sulphur vulcanisation of SBR. The influence of ILs on the curing kinetics of rubber compounds and their crosslinking efficiency, vulcanisate crosslink density, tensile strength and resistance to thermal and UV aging is discussed. 2. Material and methods Materials.

Benzalkonium,

didecyldimethylammonium

and

dodecyltrimethylammonium chloride were obtained from Aldrich. N,N,N,-trimethyl –Noctadecylammonium and N,N,N,-trimethyl-N-9-octadecenylammonium chloride were obtained from AkzoNobel. The butadiene-styrene elastomer (KER 1500) containing 2225 wt % styrene was obtained from “Synthos Dwory”, Oswiecim (Poland). The Mooney viscosity was (ML1+4 (100°C):46-54). It was vulcanised with sulphur 5 ACS Paragon Plus Environment


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(“Siarkopol” Tarnobrzeg, Poland) with microsized zinc oxide as the activator (ZnO). 2mercaptobenzothiazole (MBT, Aldrich) and N-cyclohexyl-2-benzothiazolesulfenamide (CBS, Aldrich) were applied as standard accelerators. Synthesised ionic liquids, such as: benzalkonium 2-mercaptobenzothiazolate (IL1), didecyldimethylammonium 2mercaptobenzothiazolate (IL2), dodecyltrimethylammonium 2-mercaptobenzothiazolate (IL3), N,N,N,-trimethyl –N-octadecylammonium 2-mercaptobenzothiazolate (IL4) and N,N,N,-trimethyl-N-9-octadecenylammonium 2-mercaptobenzothiazolate (IL5) were used as vulcanisation accelerator alternatives to the standard MBT and CBS. Hydrophilic fumed silica with a specific surface area of 380 m2/g (Aerosil 380, Evonic Industries) was applied as a filler. Synthesis of 2-Mercaptobenzothiazolate ILs. A 0.1 mol portion of 2mercaptobenzothiazole and a stoichiometric amount of potassium hydroxide were dissolved in methanol. Afterward, 0.1 mol of tetralkylammonium (respectively didecyldimethyl, benzalkonium, dodecyltrimethylammonium, N,N,N,-trimethyl –Noctadecylammonium or N,N,N,-trimethyl-N-9-octadecenylammonium) chloride in methanol were added. The mixtures were stirred at room temperature for 1 h, and the inorganic solids were then filtered off. After the evaporation of methanol, the product was extracted with anhydrous acetone. Potassium chloride was filtered, and the acetone was evaporated. The obtained product was dried in vacuum at 80 °C overnight. The structure of synthesized ILs was confirmed using 1H NMR,

13

C NMR and elementary

analysis. Results are presented in Table S1 of the Supporting Information. Preparation

and

Characterisation

of

Rubber Compounds.

Rubber

compounds with formulations given in Table 1 were prepared using a laboratory tworoll mill. The samples were cured at 160 °C until they developed a 90% increase in torque, which was measured by an oscillating disc rheometer (Monsanto). The kinetics


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of rubber compound vulcanisation was studied using a DSC1 (Mettler Toledo) analyser by decreasing the temperature from 25 to -60 °C at a rate of 10 °C/min and then heating to 250 °C with the same heating rate. In Table S2 of the Supporting Information the content of MBT used in the accelerators is given. Table 1. Composition of the SBR-Based Rubber Compounds [phr] Ingredient

Without IL

With IL1

SBR 100 100 100 100 MBT 1 1 1 CBS 1 1 1 Sulphur 2 2 2 2 ZnO 5 5 5 5 Silica 30 30 30 Ionic 1 1 liquid The crosslink density (νT) of the vulcanisates

100 2 5 30 2

100 2 5 30 3

With IL2-IL5 100 2 5 30 3

was determined by their

equilibrium swelling in toluene based on the Flory-Rehner equation.23 The Huggins parameter of the elastomer-solvent interaction (χ) was calculated from the equation χ=0.37+0.56Vr [eq 1], where Vr is the volume fraction of the elastomer in the swollen gel. The tensile properties of the vulcanisates were measured according to ISO-37 standard procedures using a ZWICK 1435 universal machine. Dynamic - mechanical measurements were carried out in the tension mode using a DMA/SDTA861e analyser (Mettler Toledo). Measurements of the dynamic moduli were performed over the temperature range of -80 to 100 °C with a heating rate of 2 °C/min, a frequency of 1 Hz and a strain amplitude of 4 µm. The temperature of the elastomer glass transition was determined from the maximum of tgδ = f(T), where tgδ is the loss factor and T is the measurement temperature.



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The thermal stability of the vulcanisates was studied using a TGA/DSC1 (Mettler Toledo) analyser. Samples were heated from 25 °C to 700 °C in an argon atmosphere (60 ml/min) with a heating rate of 10 °C/min. The thermal degradation of vulcanisates was performed at a temperature of 100 °C for 168 h. The UV degradation of vulcanisates was carried out for 120 h using a UV 2000 (Atlas) machine in two alternating segments: a day segment (irradiation 0.7 W/m2, temperature 60 °C, time 8 h) and a night segment (without UV radiation, temperature 50 °C, time 4 h). To estimate the resistance of the samples to aging, their mechanical properties and crosslinking densities after aging were determined and compared with the values obtained for the vulcanisates before the aging process. The aging factor (S) was calculated as the numerical change in the mechanical properties of the samples upon aging, where TS – vulcanisate tensile strength, EB – elongation at break [eq 2]:24 S = (TS · EB)after aging /(TS · EB)before aging

(2)

3. Results and Discussion Cure Characteristics and the Crosslink Density of Vulcanisates. The influence of novel ILs on the vulcanisation process was estimated based on rheometer measurements. The cure characteristics of the SBR compounds and the crosslink densities of the vulcanisates are given in Table 2. Table 2. Cure Characteristics, Crosslink Densities and Mechanical Properties of SBR Vulcanisates Containing ILs Accelerator MBT/CBSh MBT/CBSi MBT/IL1 CBS/IL1

Amount of ΔGa t90b tp c Td * 104 IL [phr] [dNm] [min] [min] [mol/cm3] 0 40.4 40 2.2 40.5 0 62.6 60 3.0 38.4 1 3.8 57.8 60 43.8 1 3.7 69.5 55 41.6

SE300e [MPa] 1.15 5.02 5.06 5.29

TSf [MPa] 3.5 20.9 13.3 12.9

EBg [%] 550 657 546 571 8

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IL1 IL1 IL2 IL3 IL4 IL5 a

2 3 3 3 3 3

63.9 82.5 76.4 71.5 72.3 70.0

50 40 38 32 37 40

3.7 2.9 2.1 2.0 3.1 2.9

46.8 55.1 49.3 47.3 46.0 44.2

5.49 5.94 4.71 4.87 4.52 3.97

12.9 22.6 22.7 16.3 17.7 18.2

Increment of torque in the rubber compound during vulcanisation.

vulcanisation time.

c

Scorch time.

d

Crosslink density of vulcanisates.

300% vulcanisate relative elongation.

f

e

b

559 619 640 606 684 717

Optimal

Modulus at

Tensile strength, g Elongation at break,

h

For

unfilled vulcanisate, i For vulcanisate filled with silica. For curing, the standard SBR compound conventional system consisting of two accelerators (MBT and CBS) was used. Applying of silica decreased the crosslink density of SBR vulcanisate. It could be supposed that accelerators adsorb on the silica surface decreasing the vulcanisation efficiency. Sulphur vulcanisation prefers alkaline conditions, whereas silica can absorb some water and form silanol-groups on its surface in the acidic way. This also may be a reason for reduction of vulcanisate crosslink density. It is possible that ionic liquids could play a role of both accelerators and shielding agents, increasing the efficiency of vulcanisation. The first stage of the study was to verify whether it is possible to replace one or both standard accelerators with ILs without a detrimental effect on the cure characteristics and

crosslink

density

of

the

vulcanisate

formed.

Benzalkonium

2-

mercaptobenzothiazolate (IL1) was applied as a novel accelerator. The application of IL1 as one of the accelerators had no considerable effect on the torque increment compared with that of the reference rubber compound containing MBT and CBS. Similar results were obtained for optimal vulcanisation time, whereas the scorch time increased by about 30%. Additionally, no significant influence on the torque increment was observed when both standard accelerators were replaced with the IL1. The



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vulcanisation time was reduced by 10 minutes. Application of the IL1 increased the crosslink density of the vulcanisate in comparison to the reference, especially when both MBT and CBS were eliminated from the rubber compound, indicating that IL1 may have acted as a catalyst of the interface crosslinking reactions. A similar effect was observed for commercially available ILs (alkylimidazolium salts) applied to sulphurvulcanised nitrile rubber.25 In the next step of the studies, the amount of IL1 in the rubber compound was increased from 2 phr to 3 phr. This increased the torque increment during vulcanisation by 20 dNm compared to the previously described rubber compounds. Moreover, a significant increase in the crosslink density was achieved, especially in comparison to the SBR compound containing MBT and CBS. Shortening of the optimal vulcanisation time to 40 min. was also observed. Based on the results obtained for the vulcanisate with 3 phr of IL1, the same amount of the rest of the synthesised ILs was used to prepare rubber compounds. It was shown that other ILs also accelerated the SBR vulcanisation, however, they were less active than IL1. Application of IL2-IL5 significantly increased the torque increment, as well as the vulcanisates crosslink densities as compared to SBR-containing standard accelerators. Shortening of the optimal vulcanisation time by 20 minutes and of the scorch time by approximately 1 minute was also observed, which confirms the catalytic effect of ILs on the efficiency of the interface crosslinking reactions. Moreover, ILs may act as shielding agents, which are adsorbed on the silica surface and reduce the possibility of silanol-groups formation. As a result, sulphur vulcanisation can be performed in the more alkaline conditions, that increases its efficiency. Having studied the effect of ILs on the cure characteristics of SBR compounds, we then examined their influence on the temperature and energetic effects of


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vulcanisation using DSC analysis. The results are given in Table S3 of the Supporting Information. Novel 2-mercaptobenzothiazolate ILs can be applied as accelerators for SBR without

detrimental

effects

on

the

vulcanisation

characteristics.

ILs

with

alkylammonium cations had no significant influence on the temperature and heat of vulcanisation in comparison with the SBR compounds containing MBT and CBS. The vulcanisation is an exothermic process that took place in a temperature range of 158242 °C, with an energetic effect of 10 J/g. The reduction of vulcanisation energetic effect to 6.4 J/g was observed only for the rubber compound containing IL1, which is benzalkonium salt. Mechanical Properties of Vulcanisates. To use synthesised ILs in SBR vulcanisation as alternatives to commercially used MBT and CBS, it is technically important that they should not deteriorate the mechanical properties of vulcanisates such as tensile strength. The mechanical properties of SBR vulcanisates were studied under static and dynamic conditions. The results of the tensile tests are presented in Table 2. Generally, silane coupling agent is used to increase the interaction between nonpolar elastomer and highly polar silica. In this case, the improvement in the tensile strength of vulcanisates, from 3.5 MPa for unfilled system to 20.9 MPa for vulcanisate containing silica, was observed without the addition of coupling agent. In terms of the ILs influence on the mechanical properties of SBR vulcanisates, it was noted that applying IL1 together with MBT or CBS decreased the tensile strength by 7 MPa. The elongation at break was reduced by approximately 100%, most likely due to the increase in the vulcanisates crosslink density. A similar effect was achieved when 2 phr of IL1 was used instead of the standard accelerators. Increasing the amount of IL1 to 3 phr resulted in the improvement of the tensile strength (22.6 MPa) in comparison with the



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conventionally crosslinked vulcanisate. Similar results were obtained when IL2 was used as accelerator. It is possible that these ILs were adsorbed on the silica surface acting partially as the coupling agents that improved the interactions between the filler and elastomer. Other ILs decreased the vulcanisate tensile strength by 2-4 MPa. However, the tensile strength of these vulcanisates is still acceptable from a technological point of view. The IL1 increased the vulcanisate modulus at 300% relative elongation by 20%, other ILs decreased the SE300, in the case of IL5 even by 20%. IL5 also increased the elongation at break of vulcanisate to 717%. It was most likely a result of the lower crosslink density of the IL5-containing vulcanisate as compared to the other vulcanisates (see Table 2). The dynamic mechanical properties of rubber products are very important for technological applications. Vulcanisates should meet requirements of rigidity and strength so that their stability during use is sufficient. Even more important is their ability to dampen vibration. The influence of ILs on the loss factor (tgδ) was determined with DMA. The values of the glass transition temperature and the loss factor at Tg, at room and elevated temperatures (100 °C) are compiled in Table 3. The loss factor tgδ as a function of temperature for the vulcanisates containing ILs is presented in Figure S1 of the Supporting Information. The glass transition temperature of SBR containing standard accelerators was (-40.9 °C). Applying ILs increased the glass transition of elastomer due to higher crosslink density of these vulcanisates. It is shown that Tg increased with the vulcanisates crosslink density. The crosslinked elastomer network that was formed during vulcanisation restricted the elastomer chains mobility increasing its Tg. ILs increased the value of loss factor at Tg but had no considerable influence on tgδ values at room or elevated temperature. Vulcanisates exhibited stable dynamic properties in the


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usage temperature. The loss factor practically did not change in the temperature range of 25-100 °C. Table 3. Glass Transition Temperature and Loss Factor of SBR Vulcanisates Containing 3 phr of ILs Accelerator MBT/CBS IL1 IL2 IL3 IL4 IL5 a

Tga [°C] -40.9 -37.3 -37.7 -38.5 -38.0 -39.6

tgδb at Tg[-] 0.58 0.77 0.76 0.65 0.70 0.77

tgδ at 25 °C [-] 0.13 0.12 0.11 0.13 0.13 0.13

tgδ at 100 °C [-] 0.09 0.10 0.10 0.12 0.10 0.11

Glass transition temperature. b Loss factor.

Thermal Stability and Aging Resistance of Vulcanisates. Rubber products often work at elevated temperature and are exposed to the factors that cause aging (e.g., temperature, UV radiation). If ILs are used as accelerators alternatively to MBT or CBS, they should not deteriorate the thermal stability and aging resistance of vulcanisates. In Table S3 of the Supporting Information the effect of ILs on decomposition temperature and weight loss for SBR vulcanisates is given. Applying ILs as vulcanisation accelerators did not influence the thermal stability of SBR. The thermal decomposition of vulcanisates began at temperatures above 295 °C. The weight loss of 50% was achieved in the temperature range of 440-445 °C. The total weight loss during decomposition was similar for all vulcanisates and was in the range of 73.3-75.1%. Having established the effect of ILs on the thermal stability, we then examined the influence of vulcanisate resistance on thermal aging. The effect of ILs on resistance to thermal aging was examined through the changes in the mechanical properties and the crosslink density of the vulcanisates.



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In Figures S2 and S3 of the Supporting Information, the change in modulus at 300% relative elongation (SE300) and elongation at break upon thermal aging is given for the vulcanisates containing ILs. Thermal aging caused the SE300 to increase considerably. The elongation at break was reduced by 200-400% in comparison with the vulcanisates before the aging process. This change was due to the large increase in the crosslink density of vulcanisates (Figure S4 of the Supporting Information). It is shown that prolonged exposure to elevated temperatures resulted in the further crosslinking of the SBR. However, the increase in the crosslink density was considerably smaller in the case of the vulcanisates containing ILs. In most cases thermal aging deteriorated the tensile strength of vulcanisates with ILs, as it did to the vulcanisate with standard accelerators (Figure S5 of the Supporting Information). The highest decrease in TS was achieved for vulcanisates containing the benzalkonium salt (IL1) and the MBT/CBS system, whereas no considerable effect of thermal aging was observed for the vulcanisates with IL2 and IL3. It should be noted, that for these vulcanisates the smallest increase in the crosslink density during the aging process was observed. This is most likely the reason for the small effect of aging on the vulcanisates tensile strength. It is difficult to estimate the resistance of vulcanisates to the aging process considering the changes in tensile strength and the elongation at break separately. Therefore, the thermal aging factor ST was calculated to quantitatively estimate the change in the mechanical properties of vulcanisates (Table 4). Table 4. Thermal and UV Aging Factor for SBR Vulcanisates Containing 3 phr of ILs Accelerator STa [-]

MBT/CBS 0.32

IL1 0.32

IL2 0.74

IL3 0.66

IL4 0.33

IL5 0.43 14

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SUVb [-] a

0.36

0.47

0.65

0.65

0.48

0.51

Thermal aging factor. b UV aging factor As could be expected, the highest ST values were achieved for the vulcanisates

containing IL2 and IL3. Therefore, these ILs improve the resistance of SBR vulcanisates to thermal aging, most likely due to the limitation of the increase in the vulcanisate crosslink density. A small improvement in the aging resistance was also observed for IL5. Other ILs did not influence the vulcanisates’ resistance to thermal aging. The same measurements were performed to study the resistance of SBR vulcanisates to UV radiation. The results are presented in Figures S2-S5 of the Supporting Information. UV aging caused the crosslink density of the vulcanisate with MBT and CBS to increase considerably. ILs reduced the increment of the crosslink density upon UV radiation. The smallest changes in crosslink density were achieved for vulcanisates containing IL2 and IL3. The increase in the crosslink density resulted in an increase in the SE300 values and reduced the vulcanisates elongation at break. However, the changes were considerably smaller than in the case of the thermal aging process. In most cases, UV aging deteriorated the tensile strength of vulcanisates with ILs and standard accelerators. The smallest reduction in the TS was observed for vulcanisates with IL2 and IL3, similarly to the thermal aging. Considering the values of the aging factor SUV (Table 4) and the results described above, it can be concluded that all synthesised ILs increased the resistance of SBR vulcanisates to UV aging. The highest activity, from this point of view, was exhibited by IL2 and IL3, similar to the results seen in thermal aging. The protecting action of the ILs



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resulted mainly from the incremental limitation of the vulcanisate crosslink density as a function of the aging factors (UV radiation, increased temperature). 4. Conclusions Ionic liquids (2-mercaptobenzothiazolate salts with benzalkonium and ammonium cations) were used as accelerators for the sulphur vulcanisation of butadiene-styrene elastomer, alternatively to MBT and CBS. Applying ILs allowed for the replacement of both standard accelerators and resulted in the shortening of the optimal vulcanisation time by 20 min. No influence of the ILs on the vulcanisation temperature and energetic effects was observed. The ILs increased the vulcanisates crosslink density considerably. Probably, ILs could act not only as the accelerators but also as shielding agents increasing the interactions between non-polar elastomer and polar filler (silica) and decreasing the possibility of acidic reactions of silanol-groups formation on the silica surface. As a result, sulphur vulcanisation can take place in the more alkaline conditions and its efficiency is higher in comparison with the rubber compound not containing ILs. Benzalkonium IL improved the tensile strength of the vulcanisate, whereas other ILs slightly reduced this parameter. However, the tensile strength of these vulcanisates is still acceptable for technological applications. ILs, especially IL2 and IL3, increased the resistance of SBR to thermal and UV aging. Further crosslinking of the elastomer-containing ILs upon aging was greatly reduced in comparison with the vulcanisate-containing standard accelerators. Applying ILs allowed for the elimination of CBS from SBR compounds and a reduction in the amount of MBT by 30-40% in comparison with conventional rubber compounds containing 2 phr of this accelerator, alternatively to 1 phr of CBS and 1 phr of MBT. The rubber composites based on the SBR elastomer crosslinked with ILs as accelerators


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could be friendlier to the environment, as well as to human health, than the traditional CBS amine derivative or MBT allergenic agent. ACKNOWLEDGMENT The authors wish to acknowledge the Polish Ministry of Science and Higher Education and the National Center for Research and Development for supporting this research.

Supporting Information Available Figures presenting the loss factor (tgδ) versus temperature for vulcanisates and their mechanical properties and crosslinks density after UV and thermal aging as well as tables that present MBT content in accelerators in SBR compounds and experimental data concerning the NMR analysis of ILs, energetic effect of vulcanisation and thermal stability of vulcanisates. This material is available free of charge via the Internet at http://pubs.acs.org.

List of Figures Figure 1. Loss factor (tg δ) versus temperature for SBR vulcanisates containing ILs. Figure 2. Vulcanisates modulus at 300% relative elongation after thermal and UV aging. Figure 3. Vulcanisates elongation at break after thermal and UV aging. Figure 4. Vulcanisates crosslink density after thermal and UV aging. Figure 5. Vulcanisates tensile strength after thermal and UV aging.



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Novel Ionic Liquids as Accelerators for the Sulfur Vulcanization of

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