**Ionic Liquids in the Vulcanization of Elastomers**

Magdalena Maciejewska and Filip Walkiewicz

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59064

#### **1. Introduction**

Vulcanization is the basic process used in elastomer technology. The first patent for the sulfur vulcanization of natural rubber was issued to Charles Goodyear in 1839. The most important disadvantage of sulfur vulcanization at its creation was the long process time of several hours. In 1906, Oenslager discovered the accelerating effect of aniline on sulfur vulcanization [1]. This discovery was followed by the introduction of guanidines. Other organic vulcanization accelerators, such as thiazole and sulfenamide, were developed in 1930 [2]. In later years, thiurames and dithiocarbamates were applied. Although vulcaniza‐ tion is one of the oldest and best known processes, thiazoles, sulfenamides and dithiocarba‐ mates were the last significant accelerator classes to be introduced in industry. Soon afterward, it was discovered that zinc oxide could activate the sulfur vulcanization process. Zinc oxide reacts with accelerators to form zinc complexes, which are more reactive than free accelerators. Sulfur is then incorporated into these complexes, and active sulfating agents are formed that react with the allylic hydrogen atoms of unsaturated elastomers to form crosslink precursors. These precursors then react with other rubber chains to gener‐ ate crosslinks that contain large amounts of sulfur atoms in their bridges. In the next step of the vulcanization process, crosslink shortening, usually accompanied by side reactions, produces the final elastomer network [3].

It is commonly postulated that acceleration of sulfur vulcanization results from homogeneous catalysis by zinc complexes or zinc salts [3]. Traditionally, stearic acid is used together with zinc oxide to activate sulfur vulcanization. In this way, a complex composed of zinc, an accelerator, and stearic acid is formed during vulcanization and includes a central zinc cation, two stearyl anions, and accelerator residues. This complex and other components of the crosslinking system are strongly polar and also reveal a tendency for agglomeration in the elastomer, resulting in the reduction of vulcanization efficiency [4]. Therefore, the application

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of novel ionic liquids (ILs)-the derivatives of 2-mercaptobenzothiazole-as accelerators for the vulcanization process seems to be reasonable. Because of their catalytic activity [5], ILs increase the rates of interfacial crosslinking reactions. Moreover, because of their ionic character, ILs should be able to form salts or complexes with zinc ions, similarly to standard accelerators; however, these complexes are more reactive than zinc-accelerator complexes. Additionally, the application of ligands with higher solvation power towards zinc cations than stearyl anions should improve the dispersion degree of the active sulfurating agent as well as of the cross‐ linking system components in the elastomer. As a result, the crosslinking rate and efficiency should increase considerably.

Recently, IL research has been one of the most rapidly growing fields in chemistry and industry, mainly due to the many unique properties of ionic liquids, such as chemical, electrochemical and thermal stability, low vapor pressure and high ionic conductivity [6]. ILs are generally defined as salts with melting temperatures lower than 100 °C and commonly consist of an asymmetric, organic cation and a rather weakly coordinating organic or inorganic anion [6,7]. Most ILs have good miscibility with organic solvents or monomers, are able to solvate a large variety of organic polar and nonpolar compounds, and show potentially "environmentally-friendly" characteristics due to their negligible vapor pressure and flammability [8]. ILs are nonvolatile compounds; thus, the chance for fugitive emissions during their use in technology is limited. Due to their ionic character, ILs are highly polar but non-coordinating, so they may affect the course of ionic reac‐ tions [9]. Therefore, ILs can be used as solvents for electrochemical processes [6,10], and their chemical and physical properties can be tailored by varying the cations and anions for a wide range of potential applications. In recent years, ILs have been widely used in polymer science, mainly as solvents in different types of polymerizations (e.g., free radical polymerization [11], atom transfer radical polymerization [12] or ionic polymerization [13]). Wang et al. used room-temperature ILs such as triethylamine hydrochloride-aluminum chloride (Et3NHCl-AlCl3) as an initiator for the cationic copolymerization of 1,3-penta‐ diene with styrene [14]. The best results were obtained for AlCl3 content of 60 wt%. The reaction produced a high-molecular-weight copolymer with a smaller molecular weight distribution than that produced when using an organic solvent. Increasing the concentra‐ tion of IL led to an increase of copolymerization yield. Moreover, as the concentration of ionic liquid increased, more crosslinked products were achieved as a result of the higher concentration of macromolecular chains in the reaction medium, which favored the intermolecular reaction of carbocations with the copolymer. The cationic ring-opening polymerization of 3,3-bis(chloromethyl)oxacyclobutane was carried out in 1-butyl-3 methylimidazolium tetrafluoroborate and hexafluorophosphate [15]. The polymerization process proceeded to high conversions, although polymer molecular weights were limited, similar to polymerization in organic solvents. ILs were able to be separated from other reagents and reused. The moisture-stable ionic liquid composed of choline chloride and zinc chloride was applied to the radiation-induced copolymerization of vinyl monomers – styrene and methyl methacrylate. The mechanism of copolymerization was the same as in conventional organic solvents at room temperature, and the copolymer was produced with high efficiency [16]. The IL 1-butyl-3-methylimidazolium hexafluorophosphate was also used as a solvent in the radical polymerization of methyl methacrylate [17]. A considera‐ ble increase in the rate constant of propagation was observed that was related to increas‐ ing polarity of the polymerization medium that favored a transition state involving charge transfer or complex formation between IL and either monomer or radical. The rate of polymerization was approximately 10 times higher than in traditionally used benzene. The molecular weights of polymers were also considerably higher, indicating a decrease in the rate of termination. This ionic liquid can be successfully used for synthesis of styrene and methyl methacrylate block copolymers [18]. The IL 1-butylpyridinium tetrafluoroborate was used in the radical polymerization of 2-hydroxyethyl methacrylate [19]. Transparent and highly conductive polymer electrolytes were obtained and characterized with high mechanical strength. ILs can serve as solvents for the organotellurium-mediated living radical polymerization (TERP) of methyl methacrylate (MMA), methyl acrylate (MA), and styrene [20]. The reaction rate of MMA and MA polymerization significantly increased, and the controllability of the polydispersity index (PDI) was also improved. The TERP of MMA yielded poly(methyl methacrylates) with PDI less than 1.1. The improved control of polymerization was ascribed to a faster degenerative chain transfer reaction that is a key parameter in the control of PDI for TERP. Moreover, facile separation of polymers from ILs and the possibility of IL recycling were described.

of novel ionic liquids (ILs)-the derivatives of 2-mercaptobenzothiazole-as accelerators for the vulcanization process seems to be reasonable. Because of their catalytic activity [5], ILs increase the rates of interfacial crosslinking reactions. Moreover, because of their ionic character, ILs should be able to form salts or complexes with zinc ions, similarly to standard accelerators; however, these complexes are more reactive than zinc-accelerator complexes. Additionally, the application of ligands with higher solvation power towards zinc cations than stearyl anions should improve the dispersion degree of the active sulfurating agent as well as of the cross‐ linking system components in the elastomer. As a result, the crosslinking rate and efficiency

Recently, IL research has been one of the most rapidly growing fields in chemistry and industry, mainly due to the many unique properties of ionic liquids, such as chemical, electrochemical and thermal stability, low vapor pressure and high ionic conductivity [6]. ILs are generally defined as salts with melting temperatures lower than 100 °C and commonly consist of an asymmetric, organic cation and a rather weakly coordinating organic or inorganic anion [6,7]. Most ILs have good miscibility with organic solvents or monomers, are able to solvate a large variety of organic polar and nonpolar compounds, and show potentially "environmentally-friendly" characteristics due to their negligible vapor pressure and flammability [8]. ILs are nonvolatile compounds; thus, the chance for fugitive emissions during their use in technology is limited. Due to their ionic character, ILs are highly polar but non-coordinating, so they may affect the course of ionic reac‐ tions [9]. Therefore, ILs can be used as solvents for electrochemical processes [6,10], and their chemical and physical properties can be tailored by varying the cations and anions for a wide range of potential applications. In recent years, ILs have been widely used in polymer science, mainly as solvents in different types of polymerizations (e.g., free radical polymerization [11], atom transfer radical polymerization [12] or ionic polymerization [13]). Wang et al. used room-temperature ILs such as triethylamine hydrochloride-aluminum chloride (Et3NHCl-AlCl3) as an initiator for the cationic copolymerization of 1,3-penta‐ diene with styrene [14]. The best results were obtained for AlCl3 content of 60 wt%. The reaction produced a high-molecular-weight copolymer with a smaller molecular weight distribution than that produced when using an organic solvent. Increasing the concentra‐ tion of IL led to an increase of copolymerization yield. Moreover, as the concentration of ionic liquid increased, more crosslinked products were achieved as a result of the higher concentration of macromolecular chains in the reaction medium, which favored the intermolecular reaction of carbocations with the copolymer. The cationic ring-opening polymerization of 3,3-bis(chloromethyl)oxacyclobutane was carried out in 1-butyl-3 methylimidazolium tetrafluoroborate and hexafluorophosphate [15]. The polymerization process proceeded to high conversions, although polymer molecular weights were limited, similar to polymerization in organic solvents. ILs were able to be separated from other reagents and reused. The moisture-stable ionic liquid composed of choline chloride and zinc chloride was applied to the radiation-induced copolymerization of vinyl monomers – styrene and methyl methacrylate. The mechanism of copolymerization was the same as in conventional organic solvents at room temperature, and the copolymer was produced with

should increase considerably.

592 Ionic Liquids - Current State of the Art

ILs can also be polymerized to form ionic gels [21-23]; 1-(8-(acryloyloxy)octyl)-3-methylimi‐ dazolium chloride was reported to photopolymerize and form a hydrogel through selfassembly [24]. This hydrogel was physically crosslinked and possessed an ordered lamellar structure that enabled reversible swelling in water up to 200 times its original volume.

ILs can be used to dissolve polymers such as cellulose [25], starch [26], or silk fibroin [27] and have shown great potential as effective wood preservatives. Hydrophobic ammonium-based ILs, which are air-and moisture-stable, possess anti-bacterial and anti-fungal activity [28]. The ability to dissolve cellulose was described for 1-butyl-3-methylimidazolium chloride, acetate and formate [29]. Pernak et al. reported the possibility of dissolving cellulose using ILs with 4-benzyl-4-methylmorpholinium and alkyl(cyclohexyl)dimethylammonium cations [30,31]. Alkylimidazolium 1,2,4-triazolates and benzotriazolates were tested for fungistatic activity against strains of Aspergillus and Penicillium molds. The highest antifungal activity was demonstrated by 1-dodecyl-methylimidazolium 1,2,4-triazolate, which effectively protected paper against molds [32].

ILs are used for filler modification to increase the interactions between a filler and an elastomer matrix and to improve the dispersion of filler particles in the elastomer, especially in the case of silica, clays, carbon black and carbon nanotubes [33-35]. ILs such as 1-butyl-3-methylimi‐ dazolium chloride, tetrafluoroborate and hexafluorophosphate were used to improve the dispersion degree of magnetic fillers in an ethylene-octene copolymer [36]. ILs increased the magnetorheological effect of composites due to the homogeneous distribution of magnetic filler nanoparticles in the polymer. A significant improvement of the mechanical properties of composites, such as tensile strength, was achieved.

Due to very high ionic conductivity below their decomposition temperature, ILs can have an important role in electrolyte matrices. The application of ILs provides the polymer electrolyte with high tensile strength and elasticity by giving conductivity to elastomers. Conductive butadiene-acrylonitrile elastomer (NBR) composites with an ionic conductivity of 2.54×10-4 S cm-1 have previously been prepared using 1-butyl-3-methylimidazolium bis(trifluoromethyl‐ sulfonyl)imide [37]. The use of ILs resulted in the development of conductive polychloroprene rubber containing a low concentration of multi-walled carbon nanotubes [38]. Stable polymer electrolytes with an ionic conductivity of over 10-4 S cm-1 were obtained from polymerizable ionic liquids with an imidazolium cation [39].

Despite the wide applications of ionic liquids in polymer and elastomer science, their use as accelerators for the sulfur vulcanization of unsaturated elastomers has not yet been reported, with the exception of our previous publications [40,41]. Thus, this chapter represents an innovative and original solution.

For our studies, we believe that the application of ILs will result in the development of new eco-friendly accelerators for elastomer sulfur vulcanization. The most popular accelerator (MBT) is an allergenic agent. Like guanidines, sulfenamides are amine deriva‐ tives with different amine moiety contents. Therefore, their application in technology should be limited because of toxicity. The proposed ILs are derived from conventional accelera‐ tors, but due to their greater activity, the amount of crosslinking system components (sulfur, zinc oxide, accelerator) in rubber products can be reduced. Moreover, because of the application of ILs, conductive elastomer composites should be possible. Preliminary studies have confirmed that mercaptobenzothiazolate or dithiocarbamate anions could be success‐ fully incorporated into IL structures [40]. The proposed solution could make an original contribution to both the synthesis of novel ionic liquids and their application as well as provide for the development of new, environmentally friendly accelerators for elastomer vulcanization. Traditional accelerators have been applied in the rubber industry for years. However, severe European Union directives regarding environmental protection require the reduction of sulfur and zinc oxide contents in rubber products and modification of recipes for producing elastomer composites. Novel vulcanization accelerators based on ILs may allow the requirements of EU legislation to be met and improve the properties of the obtained rubber products.

Because of their catalytic activity, ILs can accelerate the process of elastomer crosslinking and increase the efficiency of sulfur consumption. As a result, reduction in the sulfur content of rubber products should be achieved for ecological reasons. Moreover, ILs may act as disper‐ sants, improving the dispersion of crosslinking system components (sulfur or activator) in elastomers. The dispersion degree of zinc oxide, sulfur, and accelerator is very important for the activation of sulfur vulcanization. Our preliminary studies confirmed the high activity of synthesized ILs with 2-mercaptobenzothiazolate in the vulcanization of elastomers [40,41]. This result was most likely due to the more homogeneous dispersion of crosslinking agents in the elastomer. SEM images showed that, in the case of a standard NBR vulcanizate with 2 mercaptobenzothiazole (MBT), zinc oxide particles were not homogeneously distributed in the elastomer matrix. The agglomeration of zinc oxide particles caused the particle surface area to decrease, which was followed by a reduction in the interfacial area between crosslinking system components. As a result, the efficiency of elastomer crosslinking decreased. In the case of vulcanizates produced with the prepared ILs, more homogeneously dispersed crosslinking agent particles were observed. The application of ILs increased the crosslink density of the vulcanizates. Therefore, the application of appropriately structured ILs as vulcanization accelerators seems to be reasonable.

In these studies, ILs such as alkylimidazolium and alkylammonium salts of 2-mercaptoben‐ zothiazole as well as nanosized zinc oxide were used to successfully form pro-ecological elastomer composites with reduced amounts of vulcanization accelerators and activators. Several aspects were analyzed, including crosslink density, static and dynamic mechanical properties of the vulcanizates, as well as their thermal stability and resistance to UV and thermo-oxidative aging.

Pro-ecological composites based on NBR and SBR elastomers containing ILs as vulcanization accelerators were manufactured and studied. Elastomers were vulcanized with sulfur and nanosized zinc oxide in the presence of ILs as accelerators, alternatively to standard 2 mercaptobenzothiazole (MBT) and N-cyclohexyl-2-benzothiazolesulfenamide (CBS). As accelerators, the salts of 2-mercaptobenzothiazole with the following cations were used: dioctyldimethylammonium (C8DM), didecyldimethylammonium (C10DM), dodecyltrime‐ thylammonium (C12TMA), 1-butyl-3-methylimidazolium (C4mim), 1-methyl-3-octylimida‐ zolium (C8mim), and 1-dodecyl-3-methylimidazolium (C12mim).

### **2. Experimental section**

#### **2.1. Materials**

Due to very high ionic conductivity below their decomposition temperature, ILs can have an important role in electrolyte matrices. The application of ILs provides the polymer electrolyte with high tensile strength and elasticity by giving conductivity to elastomers. Conductive butadiene-acrylonitrile elastomer (NBR) composites with an ionic conductivity of 2.54×10-4 S cm-1 have previously been prepared using 1-butyl-3-methylimidazolium bis(trifluoromethyl‐ sulfonyl)imide [37]. The use of ILs resulted in the development of conductive polychloroprene rubber containing a low concentration of multi-walled carbon nanotubes [38]. Stable polymer electrolytes with an ionic conductivity of over 10-4 S cm-1 were obtained from polymerizable

Despite the wide applications of ionic liquids in polymer and elastomer science, their use as accelerators for the sulfur vulcanization of unsaturated elastomers has not yet been reported, with the exception of our previous publications [40,41]. Thus, this chapter represents an

For our studies, we believe that the application of ILs will result in the development of new eco-friendly accelerators for elastomer sulfur vulcanization. The most popular accelerator (MBT) is an allergenic agent. Like guanidines, sulfenamides are amine deriva‐ tives with different amine moiety contents. Therefore, their application in technology should be limited because of toxicity. The proposed ILs are derived from conventional accelera‐ tors, but due to their greater activity, the amount of crosslinking system components (sulfur, zinc oxide, accelerator) in rubber products can be reduced. Moreover, because of the application of ILs, conductive elastomer composites should be possible. Preliminary studies have confirmed that mercaptobenzothiazolate or dithiocarbamate anions could be success‐ fully incorporated into IL structures [40]. The proposed solution could make an original contribution to both the synthesis of novel ionic liquids and their application as well as provide for the development of new, environmentally friendly accelerators for elastomer vulcanization. Traditional accelerators have been applied in the rubber industry for years. However, severe European Union directives regarding environmental protection require the reduction of sulfur and zinc oxide contents in rubber products and modification of recipes for producing elastomer composites. Novel vulcanization accelerators based on ILs may allow the requirements of EU legislation to be met and improve the properties of the

Because of their catalytic activity, ILs can accelerate the process of elastomer crosslinking and increase the efficiency of sulfur consumption. As a result, reduction in the sulfur content of rubber products should be achieved for ecological reasons. Moreover, ILs may act as disper‐ sants, improving the dispersion of crosslinking system components (sulfur or activator) in elastomers. The dispersion degree of zinc oxide, sulfur, and accelerator is very important for the activation of sulfur vulcanization. Our preliminary studies confirmed the high activity of synthesized ILs with 2-mercaptobenzothiazolate in the vulcanization of elastomers [40,41]. This result was most likely due to the more homogeneous dispersion of crosslinking agents in the elastomer. SEM images showed that, in the case of a standard NBR vulcanizate with 2 mercaptobenzothiazole (MBT), zinc oxide particles were not homogeneously distributed in the elastomer matrix. The agglomeration of zinc oxide particles caused the particle surface area

ionic liquids with an imidazolium cation [39].

innovative and original solution.

594 Ionic Liquids - Current State of the Art

obtained rubber products.

The acrylonitrile-butadiene elastomer (EUROPREN N3960) containing 39 wt % acrylonitrile was obtained from Polimeri Europa (Rome, Italy). The Mooney viscosity was (ML1+4 (100°C): 60). This elastomer was vulcanized with sulfur (Siarkopol, Poland) with microsized zinc oxide as the activator (ZnO, Aldrich, Germany).

The butadiene-styrene elastomer (KER 1500) containing 22-25 wt % styrene was obtained from Synthos Dwory, Oswiecim (Poland). The Mooney viscosity was (ML1+4 (100°C):46-54). This elastomer was vulcanized with sulfur (Siarkopol Tarnobrzeg, Poland) with microsized zinc oxide as the standard activator (ZnO, Aldrich, Germany). Accelerators included 2-mercapto‐ benzothiazole (MBT, Aldrich, Germany) and N-cyclohexyl-2-benzothiazolesulfenamide (CBS, Aldrich, Germany).

To reduce the number of zinc ions in rubber compounds, nanosized zinc oxide (nZnO, Nanostructured & Amorphous Materials, Inc., USA) was used as an alternative to microsized ZnO. Silica with a specific surface area of 380 m2 /g (Aerosil 380, Evonic Industries, Germany) was used as a filler. The ionic liquids listed in Table 1 were synthesized at the Poznan Uni‐ versity of Technology, according to a procedure described elsewhere [40,41].


**Table 1.** Ionic liquids used in this study

#### **2.2. Preparation and characterization of rubber compounds**

Rubber compounds with the formulations given in Table 2 were prepared using a laboratory two-roll mill. The samples were cured at 160 °C until they developed a 90% increase in torque, as measured by an oscillating disc rheometer (Monsanto).


**Table 2.** Composition of SBR-and NBR-based rubber compounds (\*2 phr of nanosized zinc oxide was used alternatively with 5 phr of standard activator)

The kinetics of rubber compound vulcanization was studied using a DSC1 (Mettler Toledo) analyzer by decreasing the temperature from 25 to-100 °C at a rate of 10 °C/min and then heating to 250 °C with the same heating rate.

The crosslink densities (νT) of the vulcanizates were determined by their equilibrium swelling in toluene, based on the Flory-Rehner equation [42]. The Huggins parameter of the SBR-solvent interaction (χ) was calculated from the equation χ=0.3700+0.5600Vr (Equation 1), where Vr is the volume fraction of elastomer in the swollen gel, and χ=0.3809+0.6707V<sup>r</sup> for NBR-solvent interaction (Equation 2).

The tensile properties of the vulcanizates were measured according to ISO-37 standard procedures using a ZWICK 1435 universal machine.

#### **2.3. Dynamic-mechanical analysis**

**Ionic Liquid Symbol**

Dioctyldimethylammonium 2-mercaptobenzothiazolate C8DMA Didecyldimethylammonium 2-mercaptobenzothiazolate C10DMA Dodecyltrimethylammonium 2-mercaptobenzothiazolate C12TMA

1-Butyl-3-methylimidazolium 2-mercaptobenzothiazolate C4mim 1-Methyl-3-octylimidazolium 2-mercaptobenzothiazolate C8mim 1-Dodecyl-3-methylimidazolium 2-mercaptobenzothiazolate C12mim

Rubber compounds with the formulations given in Table 2 were prepared using a laboratory two-roll mill. The samples were cured at 160 °C until they developed a 90% increase in torque,

**SBR (phr) NBR (phr)**

**Without IL (reference sample)**

Ionic liquid - 2.5

**With IL**

**With IL**

Elastomer 100 100 Elastomer 100 100 Sulfur 2 2 Sulfur 2 2 \*ZnO 5 2 \*ZnO 5 2 MBT 1 - MBT 2 - CBS 1 - Silica 30 30

**Table 2.** Composition of SBR-and NBR-based rubber compounds (\*2 phr of nanosized zinc oxide was used

The kinetics of rubber compound vulcanization was studied using a DSC1 (Mettler Toledo) analyzer by decreasing the temperature from 25 to-100 °C at a rate of 10 °C/min and then

The crosslink densities (νT) of the vulcanizates were determined by their equilibrium swelling in toluene, based on the Flory-Rehner equation [42]. The Huggins parameter of the SBR-solvent

**2.2. Preparation and characterization of rubber compounds**

as measured by an oscillating disc rheometer (Monsanto).

**Table 1.** Ionic liquids used in this study

596 Ionic Liquids - Current State of the Art

**Without IL (reference sample)**

Silica 30 30

Ionic liquid - 3

heating to 250 °C with the same heating rate.

alternatively with 5 phr of standard activator)

Dynamic-mechanical measurements were carried out in tension mode using a DMA/ SDTA861<sup>e</sup> analyzer (Mettler Toledo). Measurements of the dynamic moduli were performed between-80 and 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 tan δ=f(T), where tan δ is the loss factor and T is the measurement temperature.

#### **2.4. Scanning Electron Microscopy (SEM)**

The dispersion degree of zinc oxide and filler nanoparticles in the elastomer matrix was estimated using scanning electron microscopy with an LEO 1530 SEM. The vulcanizates were broken down in liquid nitrogen, and the surfaces of the vulcanizate fractures were examined. Prior to measurements, the samples were coated with carbon.

#### **2.5. Thermogravimetry analysis**

The thermal stability of the vulcanizates was studied using a TGA/DSC1 (Mettler Toledo) analyzer. Samples were heated from 25 °C to 700 °C in an argon atmosphere (60 ml/min) with a heating rate of 10 °C/min. Decomposition temperatures at weight losses of 2% (T02), 5% (T05), 50% (T50) and total weight loss during decomposition of vulcanizates were determined.

#### **2.6. Thermo-oxidative and UV aging**

The thermo-oxidative degradation of the vulcanizates was performed at a temperature of 100 °C for 240 h. The UV degradation of the vulcanizates was carried out for 120 h using a UV 2000 (Atlas) machine in two alternating segments: a day segment (irradiation 0.7 W/m<sup>2</sup> , 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 crosslink densities after aging were determined and compared with the values obtained for vulcanizates before the aging process. The aging factor (S) was calculated as the numerical change in the mechanical properties of the samples upon aging (Equation 3) [43], where TS is the tensile strength of the vulcanizate, and EB is the elongation at break:

$$\mathcal{S} = \left(\text{TS} \cdot \text{EB}\right)\_{\text{after\\_aging}} / \left(\text{TS} \cdot \text{EB}\right)\_{\text{before\\_aging}} \tag{1}$$

### **3. Results and discussion**

#### **3.1. SBR composites containing ILs**

#### *3.1.1. Curing characteristics and crosslink densities of SBR vulcanizates*

The activity of novel ILs as accelerators in the sulfur vulcanization of SBR was estimated based on rheometry measurements. The curing characteristics of the SBR compounds and the crosslink densities of the vulcanizates are given in Table 3. To reduce the amount of vulcani‐ zation activator, nanosized zinc oxide was applied (2 phr) alternatively with the normally used microsized ZnO (5 phr).


**Table 3.** Curing characteristics and crosslink densities of SBR vulcanizates (ΔG-increment of torque in the rubber compound during vulcanization; t90-optimal vulcanization time; tp-scorch time, νT – crosslink density of vulcanizates).

Nanosized zinc oxide seems to be very active in SBR vulcanization. Despite the reduced amount of vulcanization activator, an increase in the crosslink density of vulcanizates is observed, and, as a consequence, the torque increment during vulcanization is comparable to that of the reference rubber compound with microsized ZnO. Moreover, a reduction of vulcanization time by 10 min was achieved. The nanosized zinc oxide does not influence the scorch time of rubber compounds. Applying ILs as vulcanization accelerators results in a further increase in torque increment during the vulcanization process because of the increase in vulcanizate crosslink densities. Alkylammonium and alkylimidazolium salts of MBT seem to catalyze the interface crosslinking reactions, which is evident from the considerable reduction of vulcanization time (approximately 25-30 min) and scorch time (approximately 1.5 min) compared to conventionally crosslinked SBR compounds. Alkylammonium and alkylimidazoilum ILs have comparable activity in the vulcanization of SBR. However, the crosslink density of vulcanizates decreases slightly with the increase of alkyl chain length in the ammonium or imidazolium cation of ILs.

The most important parameters of the vulcanization process are temperature and energetic effect. The influence of nanosized zinc oxide and novel ILs on these parameters is given in Table 4.


**Table 4.** Temperatures and energetic effects of SBR vulcanization measured by DSC

**3. Results and discussion**

598 Ionic Liquids - Current State of the Art

microsized ZnO (5 phr).

**3.1. SBR composites containing ILs**

the ammonium or imidazolium cation of ILs.

*3.1.1. Curing characteristics and crosslink densities of SBR vulcanizates*

The activity of novel ILs as accelerators in the sulfur vulcanization of SBR was estimated based on rheometry measurements. The curing characteristics of the SBR compounds and the crosslink densities of the vulcanizates are given in Table 3. To reduce the amount of vulcani‐ zation activator, nanosized zinc oxide was applied (2 phr) alternatively with the normally used

**SBR ΔG (dNm) t90 (min) tp (min) νT \* 104 (mol/cm<sup>3</sup>**

ZnO 53.3 60 3.7 15.4 nZnO 59.3 50 3.3 16.5 C8DMA 72.3 30 2.1 19.9 C10DMA 64.1 30 2.5 18.3 C12TMA 60.5 30 2.0 17.5 C4mim 72.9 35 2.6 20.8 C8mim 69.2 35 2.8 18.3 C12mim 67.8 35 2.5 17.3

**Table 3.** Curing characteristics and crosslink densities of SBR vulcanizates (ΔG-increment of torque in the rubber compound during vulcanization; t90-optimal vulcanization time; tp-scorch time, νT – crosslink density of vulcanizates).

Nanosized zinc oxide seems to be very active in SBR vulcanization. Despite the reduced amount of vulcanization activator, an increase in the crosslink density of vulcanizates is observed, and, as a consequence, the torque increment during vulcanization is comparable to that of the reference rubber compound with microsized ZnO. Moreover, a reduction of vulcanization time by 10 min was achieved. The nanosized zinc oxide does not influence the scorch time of rubber compounds. Applying ILs as vulcanization accelerators results in a further increase in torque increment during the vulcanization process because of the increase in vulcanizate crosslink densities. Alkylammonium and alkylimidazolium salts of MBT seem to catalyze the interface crosslinking reactions, which is evident from the considerable reduction of vulcanization time (approximately 25-30 min) and scorch time (approximately 1.5 min) compared to conventionally crosslinked SBR compounds. Alkylammonium and alkylimidazoilum ILs have comparable activity in the vulcanization of SBR. However, the crosslink density of vulcanizates decreases slightly with the increase of alkyl chain length in

**)**

Nanosized zinc oxide has no significant influence on the temperature and heat of vulcanization in comparison with SBR compounds containing a microsized activator. Vulcanization is an exothermic process that occurs in a temperature range of 173-226 °C, with an energetic effect of 9 J/g. 2-Mercaptobenzothiazolate ILs, especially alkylimidazolium salts, decrease consider‐ ably the onset temperature of vulcanization. The vulcanization process begins in the temper‐ ature range of 150-156 °C for SBR compounds containing alkylammonium ILs and 147-149 °C in the case of alkylimidazolium salts. This result confirms the catalytic effect of ILs on interfacial crosslinking reactions. The energetic effect of vulcanization ranges from 8.0 J/g to 9.8 J/g and decreases with alkyl chain length for rubber compounds with alkylimidazolium salts. A considerable reduction in the heat of vulcanization is achieved for alkyldimethylammonium ILs, indicating a decrease in the efficiency of the vulcanization process and confirming that the structure of the accelerator (ILs) affects vulcanization efficiency.

#### *3.1.2. Dispersion degree of crosslinking system particles in SBR*

Having concluded that the novel ILs catalyze interfacial crosslinking reactions and influence the efficiency of vulcanization, we then examined their effect on the dispersion degree of crosslinking system components in the SBR elastomer. The homogeneous distribution of ZnO, sulfur and accelerator particles is very important for the activation of sulfur vulcanization. Sulfur and accelerator diffuse inside the elastomer matrix and are adsorbed on the zinc oxide surface during vulcanization.

For the vulcanizate with MBT, crosslinking system particles are not homogeneously distrib‐ uted in the elastomer matrix (Figure 1). Microsized agglomerates (several micrometers in size) are created. The agglomeration of particles causes their surface area to decrease, followed by a reduction of the interface between zinc oxide, sulfur, and the accelerator. As a result, the efficiency of elastomer crosslinking decreases.

**Figure 1.** SEM images of SBR vulcanizates containing nZnO without IL

**Figure 2.** SEM images of SBR vulcanizates containing nZnO with C8DMA

**Figure 3.** SEM images of SBR vulcanizates containing nZnO with C10DMA

a reduction of the interface between zinc oxide, sulfur, and the accelerator. As a result, the

efficiency of elastomer crosslinking decreases.

600 Ionic Liquids - Current State of the Art

**Figure 1.** SEM images of SBR vulcanizates containing nZnO without IL

**Figure 2.** SEM images of SBR vulcanizates containing nZnO with C8DMA

**Figure 4.** SEM images of SBR vulcanizates containing nZnO with C12TMA

In the case of vulcanizates with novel ILs, a more homogeneous distribution of crosslinking agent particles in SBR is observed (Figures 2-6). The dispersion degree depends on the IL structure and improves with the decrease in the ammonium or imidazolium cation alkyl chain length. In the case of ILs with C8 chains in the cation, particles of the crosslinking system are homogeneously distributed in the elastomer, and no agglomeration is observed. For ILs with C12 chains, some agglomerates can be observed in the SEM images but with a smaller size than conventionally crosslinked SBR vulcanizates. Therefore, it can be concluded that, apart from accelerating action, ILs act as dispersing agents that prevent crosslinking system particles from agglomeration. The homogeneous dispersion of particles provides better contact between the activator and other components of the crosslinking system, resulting in higher vulcaniza‐ tion efficiency.

**Figure 5.** SEM images of SBR vulcanizates containing nZnO with C8mim

**Figure 6.** SEM images of SBR vulcanizates containing nZnO with C12mim

#### *3.1.3. Mechanical properties of SBR vulcanizates*

than conventionally crosslinked SBR vulcanizates. Therefore, it can be concluded that, apart from accelerating action, ILs act as dispersing agents that prevent crosslinking system particles from agglomeration. The homogeneous dispersion of particles provides better contact between the activator and other components of the crosslinking system, resulting in higher vulcaniza‐

**Figure 5.** SEM images of SBR vulcanizates containing nZnO with C8mim

**Figure 6.** SEM images of SBR vulcanizates containing nZnO with C12mim

tion efficiency.

602 Ionic Liquids - Current State of the Art

Having established the influence of ILs on the vulcanization process and on the crosslink density of vulcanizates, we then examined their mechanical properties. The results are given in Table 5.


**Table 5.** Tensile strength (TS) and elongation at break (EB) of SBR vulcanizates

Despite the reduced amount of zinc oxide, vulcanizates containing nanosized ZnO exhibit approximately 4 MPa higher tensile strength than the reference sample. The elongation at break is similar in both vulcanizates. Applying ILs increases the tensile strength of SBR by 10 MPa in the case of C8DMA as a result of the improved dispersion degree of crosslinking system and filler particles in the SBR. The tensile strength decreases with the increase in IL alkyl chain length, as the dispersion degree of crosslinking system particles deteriorates. The elongation at break is reduced by 100 – 150% due to the increase in the crosslink density of the vulcanizates.

For technological applications of SBR, dynamic mechanical properties are very important. Rubber products must meet requirements of rigidity and strength so that their stability during use is sufficient. Even more important is their ability to dampen vibrations. Using DMA, the influence of ILs on the loss factor (tan δ) was determined. The loss factor, tan δ, as a function of temperature for the vulcanizates containing ILs is presented in Figure 7 and Table 6.

The existence of one transition can be observed (Figure 7), the glass transition of SBR, with a maximum that represents Tg. Applying nanosized zinc oxide alternatively with microsized activator does not influence the glass transition temperature of SBR, which is approximate‐ ly-41.0 °C. Vulcanizates containing ILs exhibit higher glass transition temperatures than the reference samples because of their higher crosslink density. A greater number of crosslinks in the elastomer network results in greater restriction of mobility of the elastomer chains and, as a consequence, higher Tg. As expected, the highest Tg values occur for vulcanizates with the highest crosslink densities.

Nanosized zinc oxide or ILs have no significant effect on the tan δ values of SBR vulcanizates (Table 6). Only a slight increase in the tan δ at glass transition temperature is observed for IL- containing vulcanizates. The loss factor is practically unchanged in the temperature range of 25-100 °C. Therefore, it could be concluded that SBR vulcanizates exhibit stable dynamic mechanical properties at application temperatures. It is worth noting that ILs applied as novel vulcanization activators alternatively to standard MBT and CBS do not deteriorate the dynamic mechanical properties of SBR and its ability for vibration damping, which is very important for technological reasons.


**Table 6.** Glass transition temperature (Tg) and loss factor (tan δ) of SBR vulcanizates

**Figure 7.** Loss factor (tan δ) versus temperature for SBR vulcanizates

#### *3.1.4. Thermal stability and aging resistance of SBR vulcanizates*

containing vulcanizates. The loss factor is practically unchanged in the temperature range of 25-100 °C. Therefore, it could be concluded that SBR vulcanizates exhibit stable dynamic mechanical properties at application temperatures. It is worth noting that ILs applied as novel vulcanization activators alternatively to standard MBT and CBS do not deteriorate the dynamic mechanical properties of SBR and its ability for vibration damping, which is very important

**SBR Tg (°C) tan δ at Tg (-) tan δ at 25 °C (-) tan δ at 100 °C (-)**

ZnO -40.9 0.58 0.13 0.09

nZnO -40.1 0.63 0.13 0.09

C8DMA -33.4 0.71 0.09 0.07

C10DMA -36.1 0.73 0.12 0.11

C12TMA -38.0 0.69 0.12 0.11

C4mim -36.8 0.68 0.12 0.09

C8mim -38.8 0.71 0.13 0.12

C12mim -38.9 0.68 0.11 0.09

**Table 6.** Glass transition temperature (Tg) and loss factor (tan δ) of SBR vulcanizates

**Figure 7.** Loss factor (tan δ) versus temperature for SBR vulcanizates

for technological reasons.

604 Ionic Liquids - Current State of the Art

Rubber products often work at elevated temperatures. Therefore, novel vulcanization accel‐ erators must not reduce the thermal stability of vulcanizates. Using TGA analysis, the effect of ILs on the thermal stability of SBR vulcanizates was determined (Table 7).

Nanosized zinc oxide increases the onset decomposition temperature of SBR (T02) by 15 °C. Most likely, the network created by the zinc oxide and filler nanoparticles is a barrier for the transport of gases and volatile pyrolysis products during the decomposition process, thus increasing the thermal stability of the elastomer. ILs result in a further increase in the onset decomposition temperature of SBR, likely due to the improved dispersion degree of the nanoparticles in the elastomer matrix. The T02 for IL-containing vulcanizates is in the range of 363-365 °C. No influence of nanosized ZnO and ILs is achieved for T05 and T50 temperatures as well as for the total weight loss during decomposition (approximately 76%).


**Table 7.** Decomposition temperatures at weight losses of 2% (T02), 5% (T05), 50% (T50) and total weight loss during the decomposition of SBR vulcanizates

Having established the effects of nanosized zinc oxide and ILs on thermal stability, we then examined SBR resistance to thermo-oxidative and UV aging. The influence of ILs on aging resistance was studied through changes in mechanical properties and the crosslink density of the vulcanizates (Figures 8-10).

Thermo-oxidative and UV aging significantly increase the crosslink densities of SBR vulcani‐ zates (Figure 8). This change is most evident in the case of thermo-oxidative degradation, where an almost threefold increase in the number of crosslinks is determined. Prolonged exposure to elevated temperatures results in further crosslinking of SBR. However, the increase in crosslink density is slightly smaller in the case of vulcanizates containing ILs. The elongation at break is reduced by 300-400% in comparison with vulcanizates before the aging process. This change is due to the large increase in the crosslink density of vulcanizates (Figure 9). In most cases, thermo-oxidative and especially UV aging deteriorate the tensile strength of vulcanizates with ILs, as in the case of vulcanizates with standard accelerators (Figure 10).

**Figure 8.** Crosslink density of SBR vulcanizates after aging

**Figure 9.** Elongation at break of SBR vulcanizates after aging

**Figure 10.** Tensile strength of SBR vulcanizates after aging

The aging factor S was calculated to quantitatively estimate the change in mechanical prop‐ erties of vulcanizates. This factor is the change in the sample deformation energy under the aging process (Table 8).

Nanosized zinc oxide used as an alternative to the microsized ZnO does not influence the resistance of vulcanizates to aging. ILs, mainly alkylammonium salts, improve the resistance of SBR vulcanizates to thermo-oxidative and UV aging, most likely due to the limitation of the increase in vulcanizate crosslink density. An increase in the ST and SUV values of 0.2 is achieved for alkylammonium salts.


**Table 8.** Thermo-oxidative and UV aging factors for SBR vulcanizates

#### **3.2. NBR composites containing ILs**

**Figure 8.** Crosslink density of SBR vulcanizates after aging

606 Ionic Liquids - Current State of the Art

**Figure 9.** Elongation at break of SBR vulcanizates after aging

**Figure 10.** Tensile strength of SBR vulcanizates after aging

Acrylonitrile-butadiene elastomer (NBR) is characterized by high resistance to oil, petrol and aliphatic hydrocarbons. As a result, NBR is widely applied to the manufacture of seals, inner tubes, hoses for liquid fuels and oils as well as elements used for damping vibration. Therefore, the use of novel ILs as accelerators for sulfur vulcanization of NBR was studied. To reduce the amount of zinc oxide in NBR compounds, nanosized ZnO was applied (2 phr) alternatively with traditionally used microsized activator (5 phr).

#### *3.2.1. Curing characteristics and crosslink densities of NBR vulcanizates*

Based on rheometric measurements, the effect of nanosized zinc oxide and ILs on the curing characteristics of NBR compounds was examined. Results are given in Table 9.

Nanosized zinc oxide increases the torque increment during vulcanization by approximately 20 dNm compared with the reference NBR compound containing microsized ZnO due to the increase in crosslink density of the vulcanizate. The amount of nanosized ZnO is 60% lower than for the microsized activator. Therefore, nanosized zinc oxide effectively activates the sulfur vulcanization of NBR. Zinc oxide particle size does not appear to affect vulcanization time or scorch time. Applying ILs causes a threefold increase in the torque increment during vulcanization compared to the rubber compound with microsized ZnO and a twofold increase in ΔG in comparison with nanosized ZnO-containing NBR. These results occur because of the considerable increase in the crosslink density of vulcanizates. Moreover, ILs reduce the vulcanization time of NBR compounds by 10-20 min, whereas no influence is observed for scorch time. Higher activity in the vulcanization process is observed for alkylimidazolium salts of MBT. The length of alkyl chains in the imidazolium or ammonium cations of ILs does not seem to affect their activity in vulcanization.


**Table 9.** Curing characteristics and crosslink densities of NBR vulcanizates (ΔG-increment of torque in the rubber compound during vulcanization; t90-optimal vulcanization time; tp-scorch time, νT – crosslink density of vulcanizates)

Having established the effect of nanosized zinc oxide and ILs on the vulcanization time and crosslink density of vulcanizates, we then examined their influence on vulcanization temper‐ ature and energetic effect (Table 10).


**Table 10.** Temperatures and energetic effects of NBR vulcanization measured by DSC

Despite the reduced amount of ZnO, nanosized zinc oxide has no significant influence on vulcanization temperature. Moreover, the heat of vulcanization is higher in comparison with the NBR compound containing microsized ZnO, so the vulcanization is most likely more effective. Vulcanization is an exothermic process that occurs in a temperature range of 165-231 °C, with an energetic effect of 14.6 J/g. Therefore, nanosized ZnO can be applied as an activator for NBR without detrimental effects on vulcanization characteristics. ILs decrease the vulcan‐ ization temperature by 17-24 °C. Alkylammonium ILs, especially dioctyl-and didecyldime‐ thylammonium salts decrease the energetic effect of vulcanization. Alkylimidazolium salts also reduce the amount of heat generated during vulcanization but to a smaller degree than in the case of alkylammonium ILs.

The reduction of vulcanization time and temperature as well as the increase in the crosslink densities of vulcanizates could be due to the catalytic action of ILs during NBR crosslinking, similar to the SBR vulcanization process. However, the dispersion degree of crosslinking system components in the elastomer should be taken into account.

#### *3.2.2. Dispersion degree of crosslinking system particles in NBR*

in ΔG in comparison with nanosized ZnO-containing NBR. These results occur because of the considerable increase in the crosslink density of vulcanizates. Moreover, ILs reduce the vulcanization time of NBR compounds by 10-20 min, whereas no influence is observed for scorch time. Higher activity in the vulcanization process is observed for alkylimidazolium salts of MBT. The length of alkyl chains in the imidazolium or ammonium cations of ILs does not

**NBR ΔG (dNm) t90 (min) tp (s) νT \* 105 (mol/cm<sup>3</sup>**

ZnO 44.5 50 2.2 19.8 nZnO 67.8 50 2.9 20.4 C8DMA 113.1 40 2.9 25.4 C10DMA 122.2 40 3.0 24.8 C12TMA 122.9 35 2.8 24.5 C4mim 127.6 30 2.6 26.3 C8mim 127.0 35 2.5 26.0 C12mim 127.2 35 2.4 25.1

**Table 9.** Curing characteristics and crosslink densities of NBR vulcanizates (ΔG-increment of torque in the rubber compound during vulcanization; t90-optimal vulcanization time; tp-scorch time, νT – crosslink density of vulcanizates)

Having established the effect of nanosized zinc oxide and ILs on the vulcanization time and crosslink density of vulcanizates, we then examined their influence on vulcanization temper‐

**NBR Vulcanization temperature range (°C ) Energetic effect of vulcanization (J/g)**

Despite the reduced amount of ZnO, nanosized zinc oxide has no significant influence on vulcanization temperature. Moreover, the heat of vulcanization is higher in comparison with

ZnO 163-240 10.1 nZnO 165-231 14.6 C8DMA 144-232 6.1 C10DMA 148-231 5.5 C12TMA 143-220 13.5 C4mim 141-220 12.8 C8mim 142-220 9.7 C12mim 143-220 10.4

**Table 10.** Temperatures and energetic effects of NBR vulcanization measured by DSC

**)**

seem to affect their activity in vulcanization.

608 Ionic Liquids - Current State of the Art

ature and energetic effect (Table 10).

Assuming that particles of accelerators, sulfur, and fatty acids diffuse through the polymer matrix and are adsorbed on the surface of zinc oxide to form intermediate complexes, the dispersion of zinc oxide nanoparticles in the elastomer matrix has great importance to the activation of sulfur vulcanization. SEM images of the vulcanizate surfaces were collected to estimate the dispersion degree of activator particles and other components of crosslinking system in the NBR elastomer (Figures 11-16).

**Figure 11.** SEM images of NBR vulcanizates containing nZnO without IL

ZnO nanoparticles are poorly dispersed in the elastomer matrix (Figure 11), creating micro‐ sized clusters of particles with complex structures. The agglomeration of zinc oxide nanopar‐ ticles decreases the efficiency of vulcanization. However, in IL-containing vulcanizates, particles are homogenously distributed in the elastomer (Figures 12-16). The estimated size of crosslinking system particles seems to be in the nanometer range, which makes their surfaces available for interactions. Thus, the interactions of these particles with sulfur and accelerators are more efficient. Furthermore, filler nanoparticles are also homogeneously distributed in the elastomer.

**Figure 12.** SEM images of NBR vulcanizates containing nZnO with C8DMA

**Figure 13.** SEM images of NBR vulcanizates containing nZnO with C10DMA

**Figure 14.** SEM images of NBR vulcanizates containing nZnO with C12TMA

particles are homogenously distributed in the elastomer (Figures 12-16). The estimated size of crosslinking system particles seems to be in the nanometer range, which makes their surfaces available for interactions. Thus, the interactions of these particles with sulfur and accelerators are more efficient. Furthermore, filler nanoparticles are also homogeneously distributed in the

**Figure 12.** SEM images of NBR vulcanizates containing nZnO with C8DMA

**Figure 13.** SEM images of NBR vulcanizates containing nZnO with C10DMA

elastomer.

610 Ionic Liquids - Current State of the Art

**Figure 15.** SEM images of NBR vulcanizates containing nZnO with C8mim

**Figure 16.** SEM images of NBR vulcanizates containing nZnO with C12mim

#### *3.2.3. Mechanical properties of NBR vulcanizates*

Due to the industrial applications of novel ILs as accelerators for the sulfur vulcanization of NBR, their effect on the mechanical properties of vulcanizates should be studied.


**Table 11.** Tensile strength (TS) and elongation at break (EB) of NBR vulcanizates

From the data compiled in Table 11, the application of nanosized zinc oxide increases by 2 MPa the tensile strength of vulcanizate compared to the use of conventional ZnO. Therefore, it is possible to reduce the amount of ZnO by 60% without a detrimental effect on the tensile parameters. In addition, ILs used an alternative to standard accelerators considerably increase the tensile strength of NBR vulcanizates, which is very important technologically. The highest tensile strength is exhibited by vulcanizates with dioctyl-and didecyldimethylammonium salts (approximately 33 MPa). The lowest TS value of the C12TMA-containing vulcanizate is the result of an inhomogeneous distribution of nanoparticles in the elastomer matrix. In the case of alkylimidazolium ILs, the tensile strength of vulcanizates is between 28 MPa (C12mim) and 31 MPa (C8mim). Vulcanizates containing ILs exhibit approximately 100% lower elongation at break compared to reference vulcanizates, which results from their higher crosslink densities.

The effect of nanosized zinc oxide and ILs on the dynamic mechanical properties was examined using DMA and is presented in Table 12 and Figure 17.


**Table 12.** Glass transition temperatures (Tg) and loss factors (tan δ) of NBR vulcanizates

**Figure 16.** SEM images of NBR vulcanizates containing nZnO with C12mim

**Table 11.** Tensile strength (TS) and elongation at break (EB) of NBR vulcanizates

Due to the industrial applications of novel ILs as accelerators for the sulfur vulcanization of

**NBR TS (MPa) EB (%)** ZnO 20.4 531 nZnO 22.6 526 C8DMA 33.4 418 C10DMA 32.1 433 C12TMA 26.5 398 C4mim 29.4 411 C8mim 30.7 415 C12mim 28.3 420

From the data compiled in Table 11, the application of nanosized zinc oxide increases by 2 MPa the tensile strength of vulcanizate compared to the use of conventional ZnO. Therefore, it is possible to reduce the amount of ZnO by 60% without a detrimental effect on the tensile parameters. In addition, ILs used an alternative to standard accelerators considerably increase the tensile strength of NBR vulcanizates, which is very important technologically. The highest

NBR, their effect on the mechanical properties of vulcanizates should be studied.

*3.2.3. Mechanical properties of NBR vulcanizates*

612 Ionic Liquids - Current State of the Art

**Figure 17.** Loss factor (tan δ) versus temperature for NBR vulcanizates

The glass transition temperature for conventionally crosslinked NBR vulcanizates is-7.3 °C. Nanosized zinc oxide slightly increases the Tg of NBR. An increase is observed for loss factor values at Tg and temperatures of 25 °C and 100 °C. ILs cause a further increase in glass transition temperature due to the higher crosslink density of vulcanizates, which reduces the mobility of elastomer chains. An increase in the loss factor is also achieved, so vulcanizates containing ILs should exhibit a higher ability for vibration damping compared with the reference sample. These properties are important for the industrial application of rubber products based on NBR elastomers.

#### *3.2.4. Thermal stability and aging resistance of NBR vulcanizates*

In Table 13, the effect of nanosized zinc oxide and ILs on decomposition temperature and weight loss for NBR vulcanizates is given.


**Table 13.** Decomposition temperatures at weight losses of 2% (T02), 5% (T05), 50% (T50) and total weight loss during the decomposition of NBR vulcanizates

Replacing microsized zinc oxide with a nanosized activator increases the onset decomposition temperature of NBR by 20 °C. Applying ILs as vulcanization accelerators does not considerably influence the thermal stability compared with a vulcanizate containing nanosized ZnO. The thermal decomposition of vulcanizates begins at a temperature range of 323-329 °C. A weight loss of 5% is achieved in the temperature range of 364-374 °C, whereas 50% weight loss occurs at 446-448 °C. ILs increase the total weight loss during NBR decomposition, which is approx‐ imately 77%. The positive effect of nanosized ZnO and ILs on the thermal stability of NBR is, as in the case of SBR, caused by the network formed by nanoparticles in the elastomer, which restrains the diffusion of gases and volatile decomposition products in the elastomer matrix.

NBR rubber products often work at elevated temperatures and are exposed to factors that cause aging (e.g., temperature and UV radiation). If ILs are industrially used as accelerators instead of MBT, they should not deteriorate the aging resistance of vulcanizates.

Prolonged exposure to UV radiation and especially elevated temperatures results in further crosslinking of NBR. The increase in crosslink density is comparable for all NBR vulcanizates (Figure 18). Thermo-oxidative aging, despite the increase in crosslink density, has no consid‐ erable influence on the tensile strength of vulcanizates (Figure 19). UV degradation deterio‐ rates the tensile strength of vulcanizates with ILs, as it does to vulcanizates with standard accelerator and nanosized zinc oxide. There is no influence of IL structure on the change in crosslink density or tensile strength of vulcanizates. In the case of vulcanizates without ILs, the aging process causes a reduction in elongation at break of approximately 100% for UV radiation and approximately 300% for elevated temperatures (Figure 20). These results arise from the increase in crosslink density of vulcanizates as well as their stiffness and fragility. ILs decrease the changes in elongation at break, especially under thermo-oxidative aging. This result is most evident for vulcanizates containing alkylimidazolium salts. It is possible that these ILs limit the loss of vulcanizate elasticity during aging. Vulcanizates containing these ILs are less rigid, which could confirm this assumption.

**Figure 18.** Crosslink density of NBR vulcanizates after aging

The glass transition temperature for conventionally crosslinked NBR vulcanizates is-7.3 °C. Nanosized zinc oxide slightly increases the Tg of NBR. An increase is observed for loss factor values at Tg and temperatures of 25 °C and 100 °C. ILs cause a further increase in glass transition temperature due to the higher crosslink density of vulcanizates, which reduces the mobility of elastomer chains. An increase in the loss factor is also achieved, so vulcanizates containing ILs should exhibit a higher ability for vibration damping compared with the reference sample. These properties are important for the industrial application of rubber products based on NBR

In Table 13, the effect of nanosized zinc oxide and ILs on decomposition temperature and

**NBR T02 (°C) T05 (°C) T50 (°C) Total weight loss (%)**

**Table 13.** Decomposition temperatures at weight losses of 2% (T02), 5% (T05), 50% (T50) and total weight loss during the

Replacing microsized zinc oxide with a nanosized activator increases the onset decomposition temperature of NBR by 20 °C. Applying ILs as vulcanization accelerators does not considerably influence the thermal stability compared with a vulcanizate containing nanosized ZnO. The thermal decomposition of vulcanizates begins at a temperature range of 323-329 °C. A weight loss of 5% is achieved in the temperature range of 364-374 °C, whereas 50% weight loss occurs at 446-448 °C. ILs increase the total weight loss during NBR decomposition, which is approx‐ imately 77%. The positive effect of nanosized ZnO and ILs on the thermal stability of NBR is, as in the case of SBR, caused by the network formed by nanoparticles in the elastomer, which restrains the diffusion of gases and volatile decomposition products in the elastomer matrix.

NBR rubber products often work at elevated temperatures and are exposed to factors that cause aging (e.g., temperature and UV radiation). If ILs are industrially used as accelerators

instead of MBT, they should not deteriorate the aging resistance of vulcanizates.

ZnO 300 368 446 68.5 nZnO 320 367 448 72.6 C8DMA 323 364 447 76.1 C10DMA 324 367 447 77.3 C12TMA 324 369 448 77.3 C4mim 323 371 446 77.8 C8mim 324 370 446 76.1 C12mim 329 374 446 76.6

*3.2.4. Thermal stability and aging resistance of NBR vulcanizates*

weight loss for NBR vulcanizates is given.

decomposition of NBR vulcanizates

elastomers.

614 Ionic Liquids - Current State of the Art

**Figure 19.** Tensile strength of NBR vulcanizates after aging

**Figure 20.** Elongation at break of NBR vulcanizates after aging

Based on the above results, it is difficult to estimate the resistance of NBR vulcanizates to the aging process considering the changes in tensile strength and elongation at break separately. Therefore, UV and thermo-oxidative aging factors were calculated to quantitatively estimate changes in mechanical properties of NBR vulcanizates (Table 14).


**Table 14.** UV and thermo-oxidative aging factors for NBR vulcanizates

In terms of SUV factor, nanosized zinc oxide and ILs do not affect the resistance of NBR to UV degradation. However, ILs increase the resistance of vulcanizates to thermo-oxidative aging. In the case of vulcanizates containing alkylammonium ILs, an increase in ST values from approximately 0.45 to 0.72 is achieved. A considerable improvement of resistance to thermooxidative degradation is observed for NBR containing alkylimidazolium salts. The values of ST are close to 1, so the mechanical properties of vulcanizates with alkylimidazolium ILs do not change under the influence of thermo-oxidative aging.

#### **4. Conclusions**

Applying ILs allowed for the replacement of standard accelerators in sulfur vulcanization of SBR and NBR elastomers, resulted in the shortening of the optimal vulcanization time and decreased the temperature of vulcanization by approximately 20 °C. These results are very important for technological and economical reasons. NBR and SBR composites containing ILs exhibited tensile strengths and crosslink densities higher than those of standard vulcanizates crosslinked with MBT and CBS. These results were due to the considerable improvement of crosslinking system nanoparticle distribution in the elastomer. ILs restrained the tendency of nanoparticles to agglomerate in the elastomer; as a result, the surface of contact between nanosized zinc oxide and other components of crosslinking system was maximized and the efficiency of vulcanization increased. Moreover, ILs could catalyze the interface crosslinking reaction that improves the efficiency of vulcanization.

Nanosized zinc oxide and ILs increased the thermal stability of elastomers. The onset decom‐ position temperature increased by approximately 20 °C. Most likely, the network created by zinc oxide and filler nanoparticles homogeneously distributed in the elastomer may be a barrier for the transport of gases and volatile pyrolysis products during the decomposition process, thus increasing the thermal stability of the elastomer.

NBR and SBR composites containing ILs exhibited stable dynamic properties at the tempera‐ ture of use. Moreover, NBR vulcanizates could be expected to demonstrate better damping properties than conventionally crosslinked samples. ILs, especially alkylimidazolium salts, increased the resistance of NBR vulcanizates to thermo-oxidative aging. In the case of SBR, improved resistance to UV and thermo-oxidative degradation was achieved, mainly for alkylammonium ILs. Further crosslinking of elastomers during aging was greatly reduced in comparison with vulcanizates containing standard accelerators.

Most important, applying nanosized zinc oxide and ILs allowed for the reduction of zinc oxide amount by 60% as well as the elimination of CBS from SBR composites and, in the case of NBR, reduction in the amount of MBT, an allergic agent, by 30-50% in comparison with conventional rubber compounds (Table 15). This result is very important ecologically.


**Table 15.** MBT content in accelerators applied in NBR compounds

**Figure 20.** Elongation at break of NBR vulcanizates after aging

616 Ionic Liquids - Current State of the Art

changes in mechanical properties of NBR vulcanizates (Table 14).

**Table 14.** UV and thermo-oxidative aging factors for NBR vulcanizates

not change under the influence of thermo-oxidative aging.

**4. Conclusions**

Based on the above results, it is difficult to estimate the resistance of NBR vulcanizates to the aging process considering the changes in tensile strength and elongation at break separately. Therefore, UV and thermo-oxidative aging factors were calculated to quantitatively estimate

> **NBR SUV (-) ST (-)** ZnO 0.68 0.43 nZnO 0.70 0.45 C8DMA 0.63 0.65 C10DMA 0.62 0.64 C12TMA 0.70 0.72 C4mim 0.63 0.96 C8mim 0.60 0.96 C12mim 0.66 0.92

In terms of SUV factor, nanosized zinc oxide and ILs do not affect the resistance of NBR to UV degradation. However, ILs increase the resistance of vulcanizates to thermo-oxidative aging. In the case of vulcanizates containing alkylammonium ILs, an increase in ST values from approximately 0.45 to 0.72 is achieved. A considerable improvement of resistance to thermooxidative degradation is observed for NBR containing alkylimidazolium salts. The values of ST are close to 1, so the mechanical properties of vulcanizates with alkylimidazolium ILs do

Applying ILs allowed for the replacement of standard accelerators in sulfur vulcanization of SBR and NBR elastomers, resulted in the shortening of the optimal vulcanization time and

#### **Acknowledgements**

The authors wish to acknowledge the National Centre for Research and Development (Poland) for supporting this research.

### **Author details**

Magdalena Maciejewska1\* and Filip Walkiewicz2

\*Address all correspondence to: magdalena.maciejewska@p.lodz.pl

1 Technical University of Lodz; Institute of Polymer and Dye Technology, Lodz, Poland

2 Poznan Technical University; Institute of Technology and Chemical Engineering, Poznan, Poland

#### **References**


[8] Kubisa P. Application of Ionic Liquids as Solvents for Polymerization Processes. Progress in Polymer Science 2004;29(1): 3-12. http://www.sciencedirect.com/science/ article/pii/S0079670003001199 (accessed January 2004).

**Acknowledgements**

618 Ionic Liquids - Current State of the Art

for supporting this research.

Magdalena Maciejewska1\* and Filip Walkiewicz2

[2] Harman M.W., U.S. Patent 2,100,692 (1937).

\*Address all correspondence to: magdalena.maciejewska@p.lodz.pl

**Author details**

Poland

**References**

232-7.

2000).

May 2005).

2003).

VCH; 2008.

The authors wish to acknowledge the National Centre for Research and Development (Poland)

1 Technical University of Lodz; Institute of Polymer and Dye Technology, Lodz, Poland

2 Poznan Technical University; Institute of Technology and Chemical Engineering, Poznan,

[1] Oenslager G. Organic Accelerators. Industrial & Engineering Chemistry 1933;25(2):

[3] Nieuwenhuizen PJ. Zinc Accelerator Complexes. Versatile Homogeneous Catalysts in Sulfur Vulcanization. Applied Catalysis A: General 2001;207(1): 55-68. http:// www.sciencedirect.com/science/article/pii/S0926860X0000613X (accessed 8 May

[4] Heideman G, Noordermeer JWM, Datta RN. Effect of Zinc Complexes as Activator for Sulfur Vulcanization in Various Rubbers. Rubber Chemistry and Technology 2005;78(2): 245-57. http://rubberchemtechnol.org/doi/abs/10.5254/1.3547881 (accessed

[5] Lu J, Yan F, Texter J.Advanced Applications of Ionic Liquids in Polymer Science. Progress in Polymer Science 2009; 34(5): 431-48. http://www.sciencedirect.com/

[6] Rogers RD, Seddon KR. Ionic Liquids-Solvents of the Future? Science 2003;302(5646): 792-3. http://www.sciencemag.org/content/302/5646/792.full.pdf (accessed 31 October

[7] Wasserscheid P, Welton T., editor. Ionic Liquids in Synthesis. New York: Wiley-

science/article/pii/S0079670008001226 (accessed 13 January 2009).


Ionic Liquids. Macromolecules 2002;35(15): 5738-41. http://pubs.acs.org/doi/full/ 10.1021/ma025518x (accessed 17 June 2002).


pubs.rsc.org/en/content/articlehtml/2006/gc/b604353d?page=search (accessed 13 July 2006).

[29] Swatlowski R, Spear SK, Holbrey JD, Rogers RD. Dissolution of Cellose with Ionic Liquids. Journal of American Chemical Society 2002;124(18): 4974-75. http:// pubs.acs.org/doi/full/10.1021/ja025790m (accessed 17 April 2002).

Ionic Liquids. Macromolecules 2002;35(15): 5738-41. http://pubs.acs.org/doi/full/

[19] Noda A, Watanabe M. Highly Conductive Polymer Electrolytes Prepared by In Situ Polymerization of Vinyl Monomers in Room Temperature Molten Salts. Electrochi‐ mica Acta 2000;45(8-9): 1265-70. http://www.sciencedirect.com/science/journal/

[20] Feng S, Xu W, Nakanishi K, Yamago S. Highly Controlled Organotellurium-Mediat‐ ed Living Radical Polymerization (TERP) in Ionic Liquids (ILs). The New Role of ILs in Radical Reactions. ACS Macro Letters 2012;1(1): 146−9. http://pubs.acs.org/doi/full/

[21] Lu J, Yan F, Texter J. Advanced Applications of Ionic Liquids in Polymer Science. Progress in Polymer Science 2009; 34(5): 431-48. http://www.sciencedirect.com/

[22] Yoshizawa M, Ogihara W, Ohno H. Novel Polymer Electrolytes Prepared by Copoly‐ merization of Ionic Liquid Monomers. Polymers for Advanced Technology 2002;13(8): 589-94. http://onlinelibrary.wiley.com/doi/10.1002/pat.261/pdf (accessed

[23] Ohno H. Design of Ion Conductive Polymers Based on Ionic liquids. Macromolecular Symposia 2007;249(1): 551-6. http://onlinelibrary.wiley.com/doi/10.1002/masy.

[24] Batra D, Hay DNT, Firestone MA. Formation of Biomimetic Liquid-Crystalline Hy‐ drogel by Self-Assembly and Polymerization of an Ionic Liquid. Chemistry of Materi‐ als 2007;19(18): 4423-31. http://pubs.acs.org/doi/full/10.1021/cm062992z (accessed 7

[25] Sun N, Rahman M, Qin Y, Maxim ML, Rodriguez H, Rogers RD. Complete Dissolu‐ tion and Partial Delignification of Wood in the Ionic Liquid 1-Ethyl-3-Methylimida‐ zolium Acetate. Green Chemistry 2009;11(5): 646-55. http://pubs.rsc.org/en/content/

[26] Biswas A, Shogren RL, Stevenson DG, Willett JL, Bhownik PK. Ionic Liquids as Sol‐ vents for Biopolymers: Acylation of Starch and Zein Protein. Carbohydrate Polymers 2006;66(4): 546-50. http://www.sciencedirect.com/science/article/pii/

[27] Phillips DM, Drummy LF, Conrady DG, Fox DM, Naik RR, Stone MO, Trulove PC, De Long HC, Mantz RA. Dissolution and Regeneration of Bombyx mori Silk Fibroin Using Ionic Liquids. Journal of American Chemical Society 2004; 126(44): 14350-1.

[28] Pernak J, Smiglak M, Griffin ST, Hough WL, Wilson TB, Pernak A, Zabielska-Matejuk J, Fojutowski J, Kita K, Rogers RD. Long Alkyl Chain Quaternary Ammonium-Based Ionic Liquids and Potential Applications. Green Chemistry 2006;8(9): 798-806. http://

http://pubs.acs.org/doi/full/10.1021/ja046079f (accessed 19 October 2004).

articlehtml/2009/gc/b822702k?page=search (accessed 13 March 2009).

science/article/pii/S0079670008001226 (accessed 13 January 2009).

10.1021/ma025518x (accessed 17 June 2002).

00134686/45/8-9 (accessed 3 January 2000).

200750435/pdf (accessed 28 March 2007).

S0144861706001834 (accessed November 2006).

15 August 2002).

620 Ionic Liquids - Current State of the Art

August 2007).

10.1021/mz200133d (accessed 8 December 2011).

