1. Introduction
The vulcanization of rubber is one of the most important processes in elastomer technology. During this process, crosslinking reactions occur, resulting in useful materials that possess the required physical properties, such as high tensile or tear strengths, a low compression set, recoverable elongation, and improved dynamic performance. The final properties of the crosslinked material depend primarily on the quantity and quality of the crosslinks formed in the resulting elastomer network [
1]. Many types of vulcanization systems have been used industrially for several years. The choice of a specific vulcanization system or crosslinker depends primarily on the type of rubber (saturated, unsaturated, or containing special functional groups), the desired vulcanization conditions (temperature and optimal vulcanization time), and the required physical properties of the final vulcanizates. The most popular crosslinking agents are sulfur in the presence of activators and vulcanization accelerators for unsaturated rubbers [
2,
3], organic peroxides for saturated polymers [
1,
4], and metal oxides for rubbers containing functional groups, such as carboxyl or chlorosulfone [
5,
6,
7]. However, peroxides are capable of vulcanizing both unsaturated and saturated elastomers, as well as fluoroelastomers and silicones [
8].
Peroxide vulcanizates are resistant to thermal aging and ozone. On the other hand, their poor mechanical and dynamic properties compared to sulfur-crosslinked elastomers limit their industrial applications. In order to enhance the mechanical performance, crosslink density, and vulcanization efficiency of peroxide-crosslinked rubber compounds, special auxiliaries called coagents are used [
9]. This role can be played by multifunctional vinyl compounds, which are able to easily and effectively react with free radicals formed in the initial stage of peroxide curing. The presence of double bonds makes the coagents molecules capable of undergoing addition or polymerization reactions during polymer curing. Owing to these reactions, coagents molecules are incorporated into the crosslinked elastomer network. On the other hand, functional groups of coagents can form additional non-covalent linkages, which are ionic or complex crosslinks [
9]. Regarding their influence on the cure rate, coagents are grouped into two categories. The first type includes acrylates, methacrylates, and maleimides. This group of coagents increases the cure rate and may lead to scorching, which is a serious disadvantage because it reduces the safety of the processing of rubber compounds and hinders the formation of the final rubber products. Coagents of the second type improve crosslinking efficiency without affecting the cure rate or adding scorch. This group of coagents includes polybutadienes and allylated compounds like triallyl cyanurate, triallyl isocyanurate, and diallyl phthalate [
10]. Some coagents, such as zinc acrylate or methacrylate, are used to create ionic crosslinks in the crosslinked elastomer network. These are labile ionic links that allow for considerable chain slippage and reformation of bonds, resulting in the increased ability of the material to relax stress. Consequently, its mechanical properties, such as tensile strength and tear resistance, are greatly improved [
11,
12].
Another approach to improve the efficiency of rubber peroxide crosslinking and the mechanical properties of the vulcanizates are hybrid coagents consisting of an inorganic core and an organic shell, which were reported in our previous works [
13,
14,
15]. These coagents are based on nanosized metal oxides (ZnO, CaO, MgO) and layered minerals (hydrotalcite, boehmite) modified with unsaturated carboxylic acids, which possess easily cleavable protons and double bonds, that are readily accessible for reactions with free radicals or polymer chains. Unsaturated acids are grafted onto the inorganic core during the chemical modification process in acetone. Depending on their structure, these hybrid coagents can form ionic crosslinks of a labile nature, which are able to slip on the surfaces of inorganic core particles (metal oxide or layered minerals) under the influence of an external force. As a result, it is possible to increase the material’s susceptibility to stress relaxation. The problem of using coagents based on nanosized metal oxides is the agglomeration of nanoparticles in the elastomer matrix. Agglomerates reduce the surface of contact between the functional groups on the coagent particles and elastomer chains, which is crucial in crosslinking reactions. Therefore, it is necessary to develop substances that improve the dispersion degree of coagent nanoparticles in the elastomer matrix. These potential dispersing agents can be ionic liquids (ILs) that, owing to their catalytic activity in interfacial reactions (including crosslinking reactions), can also increase the efficiency of crosslinking [
16].
Recently, ILs have been widely applied in polymers technology [
17,
18]. The most important advantages of ILs, that determine their use as additives to elastomer composites are high temperature of thermal degradation, non-flammability, and non-volatility [
17]. No less important for some applications is high ionic conductivity resulting from the ionic structure o ILs [
19,
20].
Considering applications in polymer technology, ILs are commonly used as “green” solvents for various types of polymerizations, including free radical polymerization and living cationic polymerization [
21,
22,
23]. They have also been used as very effective solvents of polymers, such as cellulose [
24] or starch [
25]. Due to the high ionic conductivity and good electrochemical stability, ILs have been successfully applied as an alternative to commonly used lithium salts in solid polymer electrolytes (SPEs) based on elastomers. Alkylimidazolium salts of tetrafluoroborate or bis(trifluoromethylsulfonyl)imide were used as ion sources for SPEs based on acrylonitrile–butadiene elastomer (NBR) to produce flexible and mechanically stable SPEs [
26,
27]. Alkylimidazolium and pyrrolidinium ILs were applied by Likozar [
28] to produce SPEs based on hydrogenated acrylonitrile–butadiene elastomer (HNBR) filled with hydroxy-functionalized multi-walled carbon nanotubes. SPEs were prepared by melt compounding of the nanotubes in the elastomer, curing of the rubber compounds and immersion of the elastomer composites in the ionic liquid.
Recently, ILs have been applied to improve the distribution of particles in rubber composites [
29,
30], especially for silica, clays, and carbon fillers, especially carbon nanotubes [
30,
31,
32]. Filler modification with ILs was reported to strengthen the interactions between a filler and an elastomer matrix and consequently promote the uniform distribution of filler particles in the elastomer. As a result, the mechanical performance of various elastomer composites was effectively improved [
30,
33,
34]. ILs have also been used to increase the dispersion degree of vulcanization activator such as nanosized zinc oxide in styrene–butadiene (SBR) rubber. This resulted in increasing the rate of vulcanization, lowering the temperature of vulcanization and increasing the concentration of crosslinks in the crosslinked elastomer network [
35].
Catalytic activity in interfacial reactions and dispersing action of ILs have been used successfully in the crosslinking of elastomers. For example, a novel ionic liquid-crosslinked flexible polyurethane elastomer was fabricated using tris(2-hydroxyethyl)methylammonium methylsulfate as crosslinker [
36]. Polyurethane elastomers crosslinked with ionic liquid demonstrated significantly higher tensile strengths and elongation at break compared with conventional thermoplastic polyurethane elastomers, as well as improved high-temperature oil resistance. The accelerating action of alkylimidazolium ILs with various anions on the crosslinking process was reported for carboxylated acrylonitrile–butadiene elastomer (XNBR) filled with hydrotalcite [
37] and NBR filled with silica [
38]. The accelerating effect of ILs resulted in the reduction in the scorch time and the curing time of rubber compounds. Additionally, vulcanizates with ILs exhibited higher crosslink density compared to those without ionic liquid.
In this work, nanosized CaO with its surface grafted with an unsaturated acid, such as allylmalonic acid (ALA), was applied as a coagent for the peroxide crosslinking of an ethylene–propylene copolymer (EPM). A hybrid coagent consisting of an inorganic core and an organic shell was achieved in this way. A thermal analysis and mechanical methods were employed to determine the effects of CaO/ALA and ILs on the crosslinking characteristics and performance of EPM composites. ILs, such as 1-butyl-3-methylimidazolium bromide, chloride, tetrafluoroborate, and hexafluorophosphate, were applied to improve the dispersion of CaO/ALA and nanosized silica particles in the elastomer matrix. Additionally, ILs were reported to catalyze interfacial reactions [
35,
39,
40]. Thus, they can be assumed to play the same role in peroxide crosslinking.
ILs with 1-butyl-3-methylimidazolium (Bmim) cation were chosen due to their positive influence on the vulcanization, crosslink density and performance of other elastomers, such as acrylonitrile–butadiene elastomer (NBR) and hydrogenated acrylonitrile–butadiene elastomer (HNBR), as was confirmed by our previous studies [
39,
41]. Furthermore, ILs with Bmim cation and different anions were reported to promote the dispersion of various fillers in the elastomer composites [
34,
42,
43]. On the other hand, ILs with Bmim cation were observed to be more easily introduced into the rubber during the preparation of rubber compounds compared to ILs with shorter alkyl chains, which is important for technological reasons.
2. Materials and Methods
2.1. Materials
An EPM copolymer (Dutral CO 034) containing 28 wt % of propylene was obtained from Versalis (San Donato Milanese, Italy). Its Mooney viscosity was ML1+4 (125 °C):44. It was cured with dicumyl peroxide (DCP). Nanosized calcium oxide (CaO) with an average particle size <160 nm grafted with allylmalonic acid (ALA) was applied as a crosslinking coagent, except for the benchmark rubber compound in which triallyl cyanurate (TAC) was used as a commercial coagent. All these reagents were provided by Sigma-Aldrich, Darmstadt, Germany and used without purification. The structure and molar mass of the ALA are shown in
Table 1. The ILs presented in
Table 2 were manufactured by Ionic Liquids Technologies GmbH, Heilbronn, Germany. Silica Aerosil 380 with a specific surface area of 380 m
2/g (Evonic Industries, Essen, Germany) was used as a filler.
2.2. Preparation of the CaO/ALA Coagent
ALA was grafted onto the nanosized CaO surface in the process of chemical modification using acetone as a solvent. CaO nanopowder was mixed with the solution of ALA in acetone for 30 min during ultrasonic treatment (Bandelin Sonorex Digitec DT 255, Berlin, Germany) with a frequency of 35 kHz. Then, the mixture was left for 24 hours. Acetone was evaporated the next day using a vacuum evaporator (BUCHI Labortechnik AG, Flawil, Switzerland) at 50 °C. The contents of the flask were mixed for 10 min before the evaporation process. The CaO/ALA obtained as a beige powder was dried in a vacuum drier (Memmert, Schwabach, Germany) at 60 °C for 96 hours. The quantity of ALA used for grafting was 8 g/100 g of CaO.
2.3. Characterization of CaO/ALA Coagent
Raw and ALA-grafted CaO was characterized using thermogravimetry (TG) with a TGA/DSC1 (Mettler Toledo, Greifensee, Switzerland) analyzer. Analyses were conducted in an inert gas atmosphere (argon, 50 mL/min) by heating the samples from 25 to 600 °C, with a heating rate of 10 °C/min. The mass losses obtained for raw and ALA-grafted CaO, were applied to calculate the content of ALA on the CaO surface, and consequently the efficiency of CaO modification. Analysis of evolved gas was performed using Setsys TG-DTA 16/18 analyzer (SETARAM Instrumentation, Caluire-et-Cuire, France) coupled to a Balzers (Pfeiffer, Aßlar, Germany) mass spectrometer.
Fourier transform infrared (FTIR) spectroscopy was used to identify impurities present in raw CaO. Measurements were carried out using a FTIR Nicolet 6700 (ThermoFisher Scientific, Waltham, MA, USA) spectrophotometer. FTIR spectrum was obtained in the range of wavenumber from 4000 to 400 cm-1 during 128 scans. Attenuated Total Reflectance (ATR) technique equipped with a single reflection diamond ATR crystal on ZnSe plate was used for all measurements.
2.4. Preparation and Characterization of EPM Compounds
EPM compounds with their compositions presented in
Table 3 were prepared using a laboratory two-roll mill. After 48 hours of storage, the rheometric measurements were performed at 160 °C using an oscillating disc rheometer WG-02 (ZACH Metalchem, Gliwice, Poland). The optimal vulcanization time (t
90%) and scorch time (t
20%) were determined according to the standard PN-ISO 3417:1994.
The temperature and enthalpy of the EPM compound’s vulcanization were examined with a DSC1 differential scanning calorimeter (Mettler Toledo, Greifensee, Switzerland). Prior to the measurements, the samples were cooled to −60 °C using liquid nitrogen as the cooling agent. Next, the frozen samples were heated to 250 °C (heating rate of 10 °C /min) in an inert atmosphere. The mass of the test samples was approximately 10 mg.
The crosslink density (
νT) of the EPM vulcanizates was determined based on the results of equilibrium swelling in toluene. The Flory–Rehner equation [
44] and the Huggins parameter of elastomer–toluen interactions given by Equation (1) [
15] were used to calculate the crosslink density, where
Vr is the volume fraction of the elastomer in the swollen gel.
Furthermore, to investigate the content of ionic crosslinks (Δ
ν) in the crosslinked elastomer network, the samples were inserted in glass weighing bottles with toluene and placed in a desiccator with saturated ammonia vapor (25% aqueous solution). Equation (2) was applied to calculate the content of ionic crosslinks, where
νA is the crosslink density determined for the vulcanizates treated with saturated ammonia vapor:
The tensile properties of the vulcanizates were determined for dumbbell-shaped samples according to ISO-37. A Zwick Roell 1435 (Zwick Roell, Ulm, Germany) universal machine was employed to perform tensile measurements.
A TGA/DSC1 (Mettler Toledo, Greifensee, Switzerland) analyzer was employed to study the thermal decomposition of the vulcanizates. Small pieces of the vulcanizates with a mass of approximately 10 mg were placed in open ceramic crucibles (polycrystalline alumina) and were heated from 25 to 700 °C in an inert atmosphere (argon, 50 mL/min) at a heating rate of 10 °C/min.
The weather aging of the vulcanizates was conducted for 100 hours using a CI 4000 “Xenon Arc Weather-Ometer” (Atlas, Mount Prospect, IL, USA) aging machine. During the aging process, day–night segments were repeated with the following conditions: day (duration 102 min, irradiation of 60 W/m
2, 367 kJ, black panel temperature 80 °C, panel chamber temperature 38 °C, humidity 50%, spray); night (duration 18 min, temperature of black panel 80 °C, panel chamber temperature 38 °C, irradiation 60 W/m
2, 64 kJ, humidity 5%, no spray). The resistance of the EPM vulcanizates to weather aging was determined following the procedure described in [
27].
The dispersion of the CaO/ALA and silica particles in the EPM matrix was examined using a LEO 1530 SEM (Zeiss, Oberkochen, Germany) scanning electron microscope. Prior to the measurements, the vulcanizates were immersed in liquid nitrogen for 5 min and broken down. The surfaces of their fractures were coated with carbon and were then examined.
4. Conclusions
A new coagent for EPM peroxide crosslinking was studied. This hybrid coagent based on nanosized CaO modified with unsaturated carboxylic acid (i.e., allylmalonic acid) was developed to provide non-covalent crosslinks in the elastomer network, in the form of labile ionic aggregates that are capable of sliding on the surfaces of solid CaO particles. ILs with 1-butyl-3-methylimidazolium cation were applied to improve the dispersion degree of the filler and CaO/ALA nanoparticles in the crosslinked elastomer matrix.
Application of the CaO/ALA slightly increased the optimal vulcanization time compared to the conventional TAC coagent. Most importantly, the reaction of the CaO/ALA coagent functional groups with the rubber chains considerably increased the crosslink density of the vulcanizates due to the formation of ionic crosslinks. ILs did not affect the optimal vulcanization time but significantly improved the crosslink density of the vulcanizates. This may result from the better dispersion of the CaO/ALA particles in the elastomer, which increased the contact between the functional groups of the coagent and the EPM rubber chains, consequently improving the activity of the coagent in the crosslinking reactions. The highest crosslink density demonstrated the vulcanizates containing BmimBF4, for which the most uniform dispersion of the coagent and filler particles in the EPM matrix was observed. CaO/ALA increased the tensile strength and considerably reduced the elongation at break in comparison with the vulcanizate containing TAC. ILs resulted in further improvement in the TS of the EPM vulcanizates without a significant influence on their elongation at break. The highest TS showed the vulcanizates with BmimBF4. We concluded, that two main factors contributed to improving the tensile strength of the vulcanizates. The first is the increase in crosslink density due to the action of the CaO/ALA coagent, while the second is the formation of ionic crosslinks in the elastomer network and their labile nature.
CaO/ALA slightly reduced the resistance of the EPM to weather aging, whereas ILs did not affect this property compared to the CaO/ALA-containing vulcanizate. On the other hand, CaO/ALA had no influence on the thermal stability of the EPM vulcanizates. Applying ILs reduced the onset temperature of thermal degradation by approximately 18 °C compared to the conventional vulcanizate with TAC. However, taking into account the range of changes in the vulcanizates aging resistance, and thermal stability, it can be concluded that CaO/ALA and ILs will not reduce the potential applications of EPM composites. Most importantly, CaO/ALA and ILs allow one to improve crosslink density and, consequently, the mechanical properties of the EPM vulcanizates obtained by peroxide crosslinking.