The influence of new pro-ecological curing agents on the crosslinking process of chloroprene rubber (CR) was examined. The proposed curing system used a simpler recipe (no need to apply harmful products such as zinc oxide and ethylene thiourea) and cost less than standard metal oxides. It was expected that the mechanism of crosslinking would be similar to that of Heck-type reactions. Heck-type reactions are powerful tools for the creation of new C=C bonds. They provide the simplest and most efficient way to synthesize a variety of important compounds used in many areas, such as pharmaceuticals, antioxidants, ultraviolet absorbers, and industrial applications. However, despite their wide application, Heck-type reactions have not been used in the rubber industry so far. Rubber blends containing acetylacetonates with different transition metals as new crosslinking agents were filled with fumed silica Aerosil 380 or carbon black Corax N-550. It was found that metal complexes are active crosslinking agents of the CR composites. The obtained vulcanizates were characterized by a high degree of crosslinking and good mechanical properties. Considering the high tensile strength and degree of crosslinking, iron acetylacetonate was the most effective curing agent of the used metal complexes. Compared with the reference sample cured with metal oxides, the CR samples crosslinked using metal acetylacetonates had a higher activity.
Vulcanization is a process that allows the conversion of raw material into final rubber products. Additional ingredients are used to design the proper curing system and to give the vulcanizates specific characteristics. The main problem of different crosslinking systems is that not all properties of the vulcanizates reach an optimal level at the same time. It is important to achieve a balance of the requirements of the most important properties by developing the curing system and temperature-time cycle.1,2
Chloroprene rubber (CR) was first vulcanized in 1930 after a polymer was compounded with natural rubber using a curing recipe (zinc oxide, sulfur, stearic acid, piperidinium pentamethylene–dithiocarbamate, and antioxidant). Later, in 1931, a recipe using benzidine in place of sulfur as the crosslinking agent and accelerator was investigated, and it was demonstrated that the vulcanization chemistry of CR differed from that of other diene polymers.3 As curing knowledge matured, it became apparent that electronegative chlorine interferes with both the double bond and the α-methylenic group of CR by deactivation. Thus, direct vulcanization with sulfur is limited because sulfur is ineffective and/or slow in the case of CR.3–6
The most frequently used crosslinking agents for CR are metal oxides. A CR rubber can be crosslinked with iron (III) oxide,7 zinc oxide, magnesium oxide, lead oxides,8 or tin (II) oxide.9 The curing methods for CR also involve a combination of zinc oxide and magnesium oxide and, in most cases, the addition of ethylene thiourea (ETU).3,4,10 This combination causes the formation of both C–C and C–S bonds,11 which results in a significant improvement in the mechanical properties of the vulcanizates. However, the addition of this kind of organic accelerator is limited because of the carcinogenic properties of ETU. Although alternatives to ETU have been actively pursued, no comparable accelerator has been developed to date.4
It should be noted that peroxide crosslinking, instead of metal oxide curing, is also possible.12,13 However, the lower heat resistance of the vulcanizates in comparison with those achieved with a metal oxide/ETU system causes this application to be limited.14
Because of all these disadvantages, it is crucial to develop new methods for crosslinking CR that do not involve the formation of harmful products, retain a suitable rate of the vulcanization process, cause no problems with scorching, and produce vulcanizates with good mechanical properties.
Recent studies have presented key results on the application of metal coordination compounds as novel curing agents for CR crosslinking. The aim of these studies was to verify if the Heck reaction also works for halogenated diene rubber.
Mizoroki–Heck reactions are the most common method to achieve carbon–carbon (C=C) bond formation between aryl/vinyl halides and olefins.15–17 These reactions are typically catalyzed by precious palladium catalysts and proceed in the presence of a base15,17–20 (Scheme 1). Pd-catalysts exhibit excellent performance, but they are very expensive. For this reason, scientists have put much effort into investigating low-cost Heck-type reactions. Wang and Yang18 studied different low-cost transition metal-catalyzed Heck-type reactions and obtained many positive results. High chemo- and stereoselectivity associated with mild reaction conditions result in many applications of this type of reaction,18 which provides the simplest and most efficient way to synthesize a variety of important compounds used in many areas, such as pharmaceuticals, antioxidants, drug intermediates, ultraviolet absorbers, and industrial applications.15,16 Despite being used in many applications, Heck-type reactions have not been applied to the rubber industry so far.
In our studies, instead of using expensive Pd or Pt catalysts, we used much cheaper Fe, Ni, Mn, Cu, or Co acetylacetonates. Moreover, triethanolamine was used to ensure the alkaline environment of the reaction. The base in Heck-type reactions is responsible for the regeneration of the catalyst through the bonding of HCl, which is produced during the reaction. To ensure halogenation and unsaturation of the bonds, which are necessary in this reaction, CR was used. The predicted mechanism of the CR crosslinking reaction with the new curing system is presented in Scheme 2, based on the mechanism proposed by Yao et al.21 for the Heck reaction catalyzed by Pd complex.
The greatest advantage of using metal complexes as curing agents is the high activity with a very small amount of crosslinking agent. Both metal complexes and triethanolamine are relatively inexpensive; combined with the amount used in the system, this new curing system is cheaper than conventional metal oxides system used for CR. Another advantage is the simple recipe; the components are active enough, and there is no need to add toxic activators and accelerators to the proposed system. Moreover, preliminary studies confirmed the activity of the proposed curing system also for halogenated butyl rubbers. Because of the low level of unsaturation, butyl rubber requires a special composition of curing systems to provide the best possible rate and state of vulcanization.22 High activity of the proposed curing system can enable the elimination of harmful vulcanization activators and accelerators from the composition of rubber products based on halogenated butyl rubbers. Lastly, the versatility of the method is promising for co-curing between halogenated rubber and conventional diene rubber, which is especially important in the tire industry. Because of this, the proposed curing system can be a breakthrough in the rubber/tire industry.
CR modified with xantogen disulfide (XD grades, “Baypren”) was used as the polymer matrix. CR was provided by LANXESS (Cologne, Germany). Selected metal acetylacetonates, such as iron (II) acetylacetonate (Fe[acac]), manganese (II) acetylacetonate (Mn[acac]), nickel (II) acetylacetonate (Ni[acac]), and cobalt (II) acetylacetonate (Co[acac]), were used as crosslinking agents. All acetylacetonates were purchased from Sigma Aldrich (Poznań, Poland). Fumed silica (SiO2, Aerosil 380 from Degussa A.G, Essen, Germany) and carbon black (CB; N-550 supplied from Konimpex, Konin, Poland) were used as fillers. To ensure the alkaline environment of the crosslinking process, triethanolamine (TEOA) provided by Chempur (Piekary Śląskie, Poland) was used. All chemicals were used as received without further purification. The composition of a typical elastomer blend was as follows: CR 100 phr, Me(acac) 0.1 phr, TEOA 4 phr, SiO2 or CB 30 phr.
Preparation and characterization of rubber compounds
The mixing procedure was carried out using a two-roll mill (with roll dimensions of D = 200 mm and L = 450 mm) maintained at 30 °C with a friction of 1.15. Rubber compounding was performed with a two-stage mixing technique. In the first mixing stage, a part of the filler was incorporated into raw CR after 4 min of plasticizing. In the second stage, the rest of the filler was mixed with the rubber and curing systems for an additional 6 min. The total time of mixing was 10 min, and the actual mixing temperature varied in the range of 27–37 °C. A small amount of filler was required to incorporate liquid TEOA into the rubber matrix. Table I illustrates the formulations of CR compounds used in this work. All compositions are presented as parts per hundred of rubber (phr).
CR composites were vulcanized at the optimal vulcanization time measured during the rheometrical tests. Cure characteristics were measured at 160 °C using Mon-Tech D-RPA 3000 rheometer according to PN-ISO 3417:1994. The vulcanization (at 160 °C) was carried out using a hydraulic press with electrical heating. The plates obtained had a thickness of approximately 1 mm.
Degree of Crosslinking. —
The degree of crosslinking of the vulcanizates was calculated based on solvent-swelling measurements in toluene or of toluene in chloroform vapor, according to standard PN-ISO 817:2001/ap1:2002.
The degree of crosslinking (α) of the vulcanizates was calculated from Eq. 1:
where Qv is the volume swelling determined based on Eq. 2:
where Qw is the equilibrium swelling determined using Eq. 3, ρr is the rubber density (g/cm3), and ρs is the solvent density (g/cm3).
where msw is the weight of the swollen sample (mg) and md is the weight of the dried sample after swelling (mg).
Mechanical Properties. —
The tensile tests were carried out using a Zwick 1435 universal testing machine with a crosshead speed of 500 mm/min, according to standard PN-ISO 37:1998. The samples had a standard dumbbell shape.
Resistance to Thermo-oxidative Aging. —
The investigation of the resistance of the CR vulcanizates to thermo-oxidative aging was carried out in accordance with standard PN-88/C-04207. The vulcanizates were subjected to circulating air at a temperature of 70° C for 168 h. Tensile properties, such as tensile strength (TS) and elongation at break (EB), were measured before and after the aging procedure. The aging coefficient (Af) was calculated from the following relationship:
RESULTS AND DISCUSSION
Cure characteristic and the crosslinking density of vulcanizates
The effect of the metal acetylacetonates on the crosslinking process of the CR composites was investigated (Table II). It has been widely known that the minimum torque (ML) is directly related to the viscosity of rubber compounds at the test temperature. The maximum torque (MH) is closely related to the modulus of rubber vulcanizates. The difference between MH and ML (torque increment) can be used as an indirect indication of the crosslinking density of the vulcanizates.23
According to data presented in Table II, it can be concluded that the viscosity of the uncured prepared compounds depends significantly on the type of filler used. SiO2-filled compounds exhibited higher values of ML for both curing systems compared with compounds filled with CB. When comparing the influence of the curing system for the SiO2-filled compounds, it is evident that compounds crosslinked with the standard curing system exhibit higher viscosity at 160 °C than compounds cured with metal acetylacetonates. In the case of CB-filled compounds, the viscosity value was much lower, and an opposite trend was observed: compounds cured with metal acetylacetonates exhibited higher values of ML.
Comparing the maximum torque values, the influence of both the curing system and the filler is even more evident. For compounds either with or without CB as filler, new crosslinking agents give higher values of MH compared with compounds cured with metal oxides. For SiO2-filled compounds, higher MH can be observed for CR cured with metal oxides. This can result from the adsorption of the metal complexes on the polar surface of SiO2. Only CR crosslinked with Fe(acac) exhibited a similar value of MH as that of the standard MgO/ZnO system. It was confirmed that the Fe(acac) had the highest curing activity in comparison with other metal acetylacetonates.
Considering the torque increment during vulcanization of the CR filled with CB, the metal complexes were effective crosslinkers, which resulted in higher ΔM values than those of compounds containing the ZnO/MgO system, which is commercially used in the vulcanization of CR. The highest torque increment was determined for rubber compounds with Fe(acac), which confirmed its high activity in the vulcanization of CR. It should be noticed that in the case of SiO2-filled rubber compounds, only this acetylacetonate resulted in a higher torque increment compared with that of the reference compound containing metal oxides. Mn(acac) and Ni(acac) seemed to be less active metal complexes, which resulted in lower torque increment during CR vulcanization.
Taking into account the influence of the filler, a higher activity of the metal acetylacetonates was achieved for compounds filled with CB. All rubber compounds containing metal complexes exhibited a torque increment close to 20 dNm; for the reference sample, the torque increment was approximately 13 dNm. A lower activity was found when the metal complexes were used as curatives for the SiO2-filled compounds, which, as mentioned before, can result from the adsorption of the metal acetylacetonates on the surface of SiO2.
Applying new cross-linkers affected the scorch time of the CR compounds. The greatest difference in scorch time was seen for unfilled rubber compounds. CR cured with metal oxides exhibited a longer scorch time, which is preferred in terms of the safety of the processing. However, compounds filled with SiO2 and cured with metal acetylacetonates were characterized with a similar scorch time as that of the reference sample cured with metal oxides.
Comparing the activity of metal oxides and Fe(acac) for unfilled rubber compounds, a significant difference in optimal vulcanization time was observed. This confirms that Fe(acac) is a much more active crosslinking agent for CR than metal oxides. Fe(acac) gives an optimal vulcanization time close to 10 min; for metal oxides, it is more than 80 min.
Furthermore, the results presented in Table II indicate the different influences of the fillers on the optimal vulcanization time of rubber compounds containing the proposed curatives. According to those results, the optimal vulcanization time was considerably shorter for compounds filled with CB, which is very beneficial from an industrial point of view since a shorter vulcanization time (TC90) is strongly connected with lower costs of the vulcanization process.
Regarding the data presented in Table II, it is evident that the degree of crosslinking of the obtained vulcanizates depends on the type of acetylacetonate used. Furthermore, it is worth noting that the differences between the degree of crosslinking measured based on equilibrium swelling in toluene (α(T)) and toluene in chloroform vapor (α(T+CH3Cl)) indicates the presence of noncovalent network nodes in the structure of the vulcanizates, which may improve the mechanical strength of the CR composites.
Taking into consideration the degree of crosslinking of the vulcanizates crosslinked with acetylacetonates, Fe(acac) was the most effective crosslinker. This confirmed the results of the rheometrical measurements. In addition, CR vulcanizates crosslinked with metal acetylacetonates without filler or with CB were characterized by a higher degree of crosslinking (α(T)), contrary to the reference samples cured with metal oxides. This confirmed the high activity of the metal complexes in the process of CR crosslinking. Only vulcanizates filled with SiO2 and cured with metal acetylacetonates exhibited lower degrees of crosslinking than that of the reference sample. This probably resulted from the partial adsorption of the metal complexes on the surface of this filler. Considering the amount of metal complexes in the rubber compounds (0.1 phr), their partial adsorption could considerably affect the efficiency of crosslinking.
Mechanical properties of vulcanizates
Regarding the results presented in Table III, CR composites cured with metal acetylacetonates exhibited good TS within the range of 11 to 20 MPa. The highest elongation at break was demonstrated by SiO2-filled CR crosslinked with Mn(acac), whereas the lowest value of this parameter was exhibited by vulcanizates containing Fe(acac), which confirmed, respectively, the lowest and the highest crosslinking degrees of these vulcanizates.
Comparing the mechanical properties of unfilled vulcanizates, significant differences in TS and elongation at break were observed. For the reference compound, both parameters were more than twice that of the vulcanizate cured with Fe(acac). For the filled compounds, the differences were smaller. For CB-filled CR cured with metal complexes, the TS was approximately 2–4 MPa lower than that of the standard curing system; for SiO2-containing vulcanizates, the difference in TS was less than 2 MPa.
The elongation at break of the unfilled vulcanizate or of those containing CB indicates that CR cured with the new crosslinking agents had a lower elasticity, which results from the significantly higher degree of crosslinking of the vulcanizates. For vulcanizates filled with SiO2, the elongation at break was higher for samples crosslinked with metal acetylacetonates. This was a result of curative adsorption on the SiO2 surface, leading to a lower degree of crosslinking of the CR in comparison with those of the reference vulcanizates cured with metal oxides.
Moreover, it is worth noting that the effect of the applied curatives on stress at 100% relative elongation (SE100) strongly depends on the kind of filler and relates to the degree of crosslinking of the vulcanizates. Therefore, CB-filled vulcanizates cured with metal complexes exhibited higher SE100 than the reference vulcanizate; for SiO2-containing vulcanizates, the opposite influence was achieved by the novel curatives on the SE100 module.
The last parameter presented in Table III is the aging coefficient (Af). Samples crosslinked with metal complexes are sensitive to thermo-oxidative aging. Nevertheless, all vulcanizates were characterized with quite good resistance to thermo-oxidative aging, with an aging coefficient in the range of 0.4–0.8. A value of aging coefficient close to 1 represents a sample with perfect aging resistance, indicating that the aging process had no effect on TS or elongation at break. The applied fillers seemed to have a significant effect on the aging resistance of CR in both curing systems. The vulcanizates filled with CB exhibited higher resistance to thermo-oxidative aging. The reference vulcanizate filled with SiO2 showed the lowest resistance to aging. Using metal complexes instead of metal oxides increased the aging coefficient of SiO2-containing vulcanizates.
The effect of thermo-oxidative aging on TS and elongation at break of the CR vulcanizates is presented in Table III. The lower TS of the CR composites after aging may indicate both degradation and crosslinking processes. The reduction in elongation at break after aging indicates an increase in the degree crosslinking, which confirms that crosslinking takes place during aging.
The selected metal acetylacetonates are effective CR crosslinking agents, which was confirmed by the high torque increment during vulcanization and the degree of crosslinking of the vulcanizates. CR vulcanizates crosslinked with metal coordination compounds are characterized by good mechanical properties and a higher degree of crosslinking than those of the reference compound crosslinked with metal oxides.
Vulcanizates filled with fumed SiO2 were characterized by a slightly longer scorch time than that of CR filled with CB. However, SiO2-filled vulcanizates exhibited much longer optimal vulcanization time. Lower activity of metal acetylacetonates in the crosslinking of SiO2-filled CR resulted from their partial adsorption on the surface of SiO2. Using metal complexes for CR filled with CB resulted in a shortened optimal vulcanization time, which is preferred in the rubber industry because of the lower costs of the crosslinking process. All vulcanizates containing metal acetylacetonates exhibit quite good resistance to aging, which was confirmed by the aging coefficient (Af).
The most active crosslinking agent was Fe(acac), which yielded the highest torque increment during vulcanization, the shortest optimal vulcanization time, and the highest degree of crosslinking of the vulcanizates.
According to the presented results, the application of Heck-type reactions in the rubber industry can be a considerable breakthrough. The greatest advantage of using the metal acetylacetonates is the possibility of reducing the content of curatives, which is necessary to obtain the final product with the required properties. The content of the metal acetylacetonate system in the CR compounds is more than two times smaller than the conventionally used ZnO/MgO system.
Young Scientists' Fund at the Faculty of Chemistry, Lodz University of Technology, Grant W-3D/FMN/34G/2016.