A novel reprocessable chloroprene rubber based on dynamic disulfide metathesis

ABSTRACT

For sustainable application of chloroprene rubber (CR), a new technology is developed by the vulcanization of CR using 2,2’-dithiodipyridine (DPD) as a cross-linking agent with the reprocessing performance owing to disulfide metathesis. When DPD was incorporated into CR vulcanization, the Menschutkin reaction between allyl chloride group and pyridine group occurred with a maximum exothermic peak at 184°C. The number of effective cross-linking bond at 0.5 phr DPD vulcanizate was higher than that at high DPD content. This vulcanizate showed high tensile strength (11.12 MPa) and elongation at break (1253 ± 120%) owing to the exchangeable disulfide bond in the system. Under the catalysis of triphenylphosphine, the metathesis of disulfide compound was improved obviously, which endowed CR/DPD vulcanizates with good recyclability performance. Disulfide cross-linkage maintains its stability at low temperature, thus ensuring the mechanical stability of CR/DPD vulcanizate under the ambient conditions. Vulcanization and reprocessing of CR/DPD vulcanizate can be conducted with common industrial rubber processing equipment. Such reprocessable chloroprene rubber could have potential application in CR industry, also serve to significantly improve environmental sustainability.

GRAPHICAL ABSTRACT

KEYWORDS:

• Chloroprene rubber
• reprocessable
• Menschutkin reaction
• disulfide

1. Introduction

Due to their unique elasticity, good mechanical properties as well as solvent resistance, rubbers are widely used in tires [1], shock absorbers [2], belts [3], seals [4], electromagnetic shielding [5], flame retardant layer [6], actuators [7] and other functional materials [8–10]. For engineering application, rubbers need to be vulcanized or cross-linked with the formation of 3D network structure. However, the covalent network prevents the reshaping or recycling of rubbery objects. As estimated, approximate 1000 million tires reach the end of their useful lives every year, of which 80% are reclaimed by grinding or pyrolysis process [11], and the remainder is either burned or discarded in landfill or the ocean, causing a serious threat to ecological and environmental system [12,13].

With the deepening of the concept of sustainable development, rubber recycling has attracted widespread attention in the industry and has been continuously studied by researchers [13,14]. In this regard, it is of great significance to explore new strategies for manufacturing rubbers with high mechanical performance as well as the potential reprocess. Adaptable networks exhibiting reversible adaptive networks appear to be a possible way to achieve this goal. Vitrimers, recently introduced by Leibler and coworkers [15], are fully processable materials, because their network contains chemical bonds that can undergo exchangeable reactions such as transesterification [16,17], olefin metathesis [18,19], disulfide exchange [20–22], boronic ester exchange [23–25], imine metathesis [26,27], and transcarbamoylation [28]. For the industrialized rubbers, the dynamic chemical bond is convenient and economical to crosslink rubber to meet the requirements of recycling and reprocessing.

Chloroprene rubber (CR, Polychloroprene or Neoprene) is a traditional rubber, which was first successfully industrialized by DuPont in the 1920s [29]. In industry, 2-chloroprene is used as the monomer to synthesize CR by free radical polymerization [30]. C-Cl and C=C bonds in the polymer chain endow CR with some basic properties. For example, the strong polarity of C-Cl bond leads to high interchain force and excellent mechanical properties of CR. Meanwhile, chlorine atom contributes to the high limiting oxygen index (LOI) of CR due to the flame retardancy of halogens [31,32]. With its excellent mechanical properties and flame retardant properties, CR has been widely used in automobiles, aircrafts, protective layers of cables and wires, conveyor belts, V-belts, heat-resistant conveyor belts, chemical-resistant hoses, and other fields [33].

The cross-linkages formed by CR vulcanization methods are mainly covalent bonds, which make it difficult to be recycled. Presently, the most common vulcanization method for the industrial application of CR is to use metal oxides (zinc oxide, magnesium oxide) for vulcanization [34,35], due to the advantages of low energy consumption, light environmental pollution and little harm to operators. Moreover, lead oxide and antimony trisulfide were also reported for its vulcanization; however, due to the environmental pollution of lead and antimony elements, these vulcanization methods are rarely used in the industrial application of CR [33,36]. Imidazole compounds were also reported in patents, but their vulcanization mechanism is not very clear. When the chlorine atom is directly connected to the C=C bond (C=C-Cl), the reactivity of allyl hydrogen group of CR is significantly different with that of the traditional rubber, so CR cannot be vulcanized with traditional sulfur [37]. To enable CR with recyclability, Zhang [38] prepared CR crosslinking agents containing dynamic bonds, and utilized the disulfide metathesis between disulfide and polysulfide in polymer chain under the catalysis of copper(II) methacrylate to realize the recycling of covalently bond crosslinked CR. Due to few S-S bonds in CR chain, the reprocessing process of CR vulcanizate is very cumbersome.

The reaction between pyridine and alkyl or allyl chloride group occurs easily and smoothly [39–42]. Inspired by the reactions with these functional groups, it is envisioned that the reaction of allyl chloride group in CR molecular chain could also react with the pyridine group. In this study, 2,2’-dithiodipyridine (DPD) was first used as a vulcanizing agent for CR. The effects of DPD on the vulcanization reaction, mechanical properties, and recyclability of CR were studied in detail. The CR/DPD reaction is energy-saving and environmentally friendly. In line with the concept of green development of rubber industry, this vulcanization has potential application value.

2. Materials and methods

2.1 Materials

Chloroprene rubber CR3221 was supplied by Chongqing Changshou Chemical Co., Ltd, China. 2,2’-Dithiodipyridine (DPD) was supplied by Shanghai Yien Chemical Technology Co., Ltd, China. Zinc oxide and other chemical reagents were obtained from Aladdin.

2.2. Preparation of CRs

The X(S) K-160 double-roll mill was used for kneading at room temperature. Firstly, CR3221 rubber was put into the open mill and plasticized through the rolls for five times. Then, zinc oxide, DPD, triphenylphosphine and other additives were added and mixed evenly. Finally, the roll distance was adjusted to 2 mm to produce rubber sheets. The vulcanization and DSC analysis of rubber samples were performed after parking for 8 h.

2.3. Measurement and characterization

¹H and ¹³C NMR spectra were recorded at 27°C using a JEOL JNM-ECA600 spectrometer (600 MHz). Chemical shift was referenced to tetramethylsilane (δ = 0), and the deuterated benzene was used as the solvent.

The vulcanization characteristics were tested according to ASTM D2084–2001 using a moving Die Rhometer GT-M2000-A (Gaotie Test Instruments Factory, China) at different temperatures. If not stated, the vulcanization temperature was 160°C.

The tensile properties of the rubber samples were tested according to GB/T 528–2009 using a GT-AI-7000-S high temperature stretching machine with a speed of 500 mm/min at room temperature. The average value of three individual samples was recorded for each sample.

Stress-relaxation experiments were carried out using Anton Paar MCR301 in a parallel-plate mode. Circular specimens with a diameter of 25 mm were punched out of the sample sheets. When the temperature reached the set ones, samples were initially compressed by a strain of 5% and the strain was maintained during the stress-relaxation experiments. Meanwhile, the stress and modulus were recorded as a function of time.

To evaluate the self-healing ability of rubber, the samples were completely cut in half and then manually recombined. After rehabilitation for a preset time period at 40°C, 60°C, 80°C, 100°C, and 120°C, respectively. The healed samples were again subjected to tensile tests. Healing efficiency (η) was calculated from the ratio of the tensile strength of the healed specimens to that of virgin specimens.

(1) $\mathit{\eta }=\frac{{\mathit{T}}_{\mathit{he}}}{T_{\mathit{vir}}}$(1)

where $T_{\mathit{vir}}$ is the tensile strength of virgin specimens, $T_{\mathit{he}}$ is the tensile strength of the healed specimens.

For the recycling of CR-DPD, the specimens were cut into small particles (about 3 mm) by scissors. The particles were then pulverized at room temperature using a X(S)K-160 double-roll mill to produce sheets. Finally, the compression molding was carried out with a hydraulic press of 10 MPa at 170°C for 30 min. The mechanical properties of the recycled rubber were measured by tensile test to evaluate the recycling ability.

ATR-FTIR spectra were recorded on a TENSOR27 spectrometer (Bruker). Samples were characterized by signal averaging 32 scans at a resolution of 4 cm⁻¹ in the wavenumber range of 500–4000 cm⁻¹.

Differential scanning calorimetry (DSC) analysis was carried out to study the crosslinking behavior of samples using a TA equipment (SDTQ20). The samples were heated to 200°C at a rate of 10°C/min under N₂ flow.

The thermal degradation behavior was studied using a thermal gravimetric analyzer (DTG-60 H, Shimadzu) at a heating rate of 10°C/min (from room temperature to 900°C) under nitrogen atmosphere. Each pulverized sample was measured in an open alumina crucible and weighed approximately 10 mg.

3. Results and discussion

For the convenience of discussion, the blend designations and compositions are shown in Table 1.

Table 1. Blend designations and compositions.

Ingredient (phr)	Sample ID
	C-ZnO	C-D1	C-D2	C-D3	C-D4	C-D5	C-D6
CR	100	100	100	100	100	100	100
DPD	—	0.5	1	3	6	12	0.5
ZnO	3	0	0	0	0	0	0
Antioxidant 1010	1	1	1	1	1	1	1
Triphenylphosphine	0	0	0	0	0	0	1

3.1 The crosslinking reaction between DPD and CR

The Menschutkin reaction (quaternization reaction) between pyridine and allyl chloride active group of geranyl chloride can occur smoothly at room temperature in chloroform with high yield [42]. Therefore, it can be expected that the allyl chloride group of CR can also react with the pyridinyl group of DPD under mild conditions. To confirm the existence of allyl chloride group in CR molecular chain, NMR experiment was carried out. The ¹H NMR and ¹³C NMR spectra of CR are shown in Figure 1. The appearance of the signal at 5.75 ppm and 1.39 ppm in the ¹H NMR spectrum and the signal at 135.2 ppm and 38.8 ppm in the ¹³C NMR spectrum confirmed that the CR polymer chain contains a small amount of allyl chloride groups, which was consistent with the results reported in the literatures [43–45]. The primary goal is to investigate the Menschutkin reaction between the allyl chloride group of CR and the pyridine group of DPD. The CR/DPD vulcanizate cannot be completely dissolved in toluene at room temperature, indicating that the crosslinking reaction did occur during the vulcanization process. When the temperature of the mixture was raised to 90°C for 1 h, the CR/DPD gel gradually disappeared in the solution and it was difficult to observe the gel visually, which indicates that the disulfide bonds in solution occurred exchange reaction.

Figure 1. The NMR spectra of CR. (a) ¹H NMR, (b) ¹³C NMR.

To demonstrate the feasibility of using DPD as a crosslinker for CR, the reaction between CR and DPD was verified by DSC tests with CR/ZnO as a control sample. As shown in Figure 2, it can be found that the crosslinking reaction between CR and ZnO started at 125°C with an exothermic peak located at 137°C. The crosslinking reaction temperature is higher than the kneading temperature of CR compounds and much lower than the thermal decomposition temperature of the CR vulcanizate. Therefore, ZnO is widely used as the vulcanizing agent for CR in industrial application.

Figure 2. DSC curves of CR/DPD and CR/ZnO during heating.

For the CR/DPD composites, the initial exothermic temperature of C-D1 reaction began at 173°C and the maximum exothermic peak located at 184°C (Figure 2). When the DPD content increased to 12 phr (C-D5), the exothermic onset temperature remained unchanged, but two exothermic peaks appeared in the curve, corresponding to 191.8°C and 201.6°C, respectively. Excitingly, the crosslinking reaction temperature of CR/DPD was located between kneading temperature and vulcanization temperature. Therefore, the general rubber processing equipment is suitable for kneading and vulcanization of CR/DPD.

Here, ATR-FTIR was used to characterize the reaction of CR with different DPD proportions (vulcanized at 160°C for 30 minutes). As shown in Figure 3, the characteristic absorption bands of DPD were located at 1570 and 1558 cm⁻¹, which is consistent with the literature reported data [46]. The characteristic absorption bands of CR (Figure 3a) mainly presented at 2916 cm⁻¹(stretching vibration of C-H), 1659 cm⁻¹ (the stretching vibration of C=C), and 667 cm⁻¹ (the stretching vibration of C-Cl) [47]. Pure CR had almost no absorption at 1570, 1558 cm⁻¹ in the FTIR spectra. Taking the characteristic absorption peaks of DPD at 1570, 1558 cm⁻¹ as the reference peak, the reaction degree can be evaluated by the shift of these characteristic absorption bands.

Figure 3. ATR-FTIR spectra of different compounds. (a) CR, (b) DPD, (c) C-D1, (d) C-D2, (e) C-D3, (f) C-D4, (g) C-D5.

As shown in Figure 3c&d, the intensity of characteristic absorption peaks at 1570 and 1558 cm⁻¹ of C-D1 and C-D2 samples became very weak, and shifted to a higher wavenumber at 1580 cm⁻¹. This is due to the increase in stiffness of the pyridine ring arising from the formation of the quaternary ammonium salt between pyridine group and C=C-C-Cl bond of CR [48–50], indicating the occurrence of Menschutkin reaction. When the DPD content was further increased in CR, the intensity of the characteristic peaks at 1570 and 1558 cm⁻¹ gradually increased due to the excess of pyridine groups. Simultaneously, new absorption bands of the obtained the quaternary ammonium salt further shifted to higher wavenumbers, at 1597 cm⁻¹ for C-D4 and 1615 cm⁻¹ for C-D5, respectively. With the increase of DPD content, the excess of pyridine groups promoted the formation of quaternary ammonium salts, so the intensity of new absorption bands also increased accordingly. These results further confirmed that the crosslinking reaction did occur between CR and DPD.

Due to the interaction between π and lone pair electron, the chlorine atom attached to the C=C bond inactivates the reactivity of the C=C bond. Correspondingly, the C=C bond also reduces the polarity of the C-Cl bond, resulting in the inability of CR to be vulcanized with sulfur. Benefiting from the structure of the presence of a small amount of allyl chloride in the CR polymer chain, C=C-C-Cl is more active than C=C-Cl due to the formation of a p-π conjugated system of allyl cation, which provides the active site for the CR vulcanization reaction. DSC and FTIR results confirmed the occurrence of vulcanization reaction between CR and DPD. As shown in Figure 4, two possible types of mechanisms were proposed involving crosslinking reaction between CR and DPD. The two types of Menschutkin reactions exhibit the difference of the nucleophilic attack of pyridine group toward different carbocations of allyl chloride [51]. The routes look reasonable, since it cannot be distinguished the detailed chemical structures of the cross-linkages of CR/DPD from FTIR spectra, we might expect faster attack at the terminated carbon of C=C bond. The $N^{2^{,}}$ reaction in the right-hand is preferred to the $N^2$ in the left-hand box due to steric hindrance effect.

Figure 4. Proposed possible mechanism for the crosslinking reaction between CR and DPD. (a) $N^2$ reaction; (b) $N^{2^{,}}$ reaction.

3.2. The vulcanization characteristics of CR/DPD

Vulcanization of rubber affects its industrial application. The vulcanization characteristics of CR/DPD were investigated with CR/ZnO as a control sample. Pure CR is unstable at high temperature and self-crosslinking reaction between polymer chains may occur [52,53]. Therefore, 1 phr antioxidant 1010 was added to eliminate the indiscernibility effect of self-crosslinking on vulcanization properties of samples. Taking C-D3 as an example, the vulcanization data of the C-D3 and C-ZnO at different temperatures are shown in Table 1 and Figure 5. Using ZnO as a vulcanizing agent, the vulcanization speed of CR was relatively high. As expected, the vulcanization rate of CR obviously increased with the increasing temperature. The scorch time (T₁₀) at 160°C was 1.04 min, and it was shortened to 0.7 min at 180°C. Meanwhile, the vulcanization time (T₉₀) was 2.93 min at 160°C, shortened to 1.95 min at 180°C. When DPD was used as a crosslinker for CR, the vulcanization reaction rate was slow, the vulcanization time was 16.8 min at 160°C, decreased to 6.33 min at 180°C.

Figure 5. The vulcanization curves of CR/ZnO and C-D3 at different temperatures.

For CR/ZnO, the maximum torque (M_H) increased with increasing vulcanization temperature, because more chemical cross-linkages were formed at high temperature. For CR/DPD, the M_H also increased with increasing vulcanization temperature, but not significantly, mainly due to the combined effect of increased crosslinking density and enhanced the exchange reaction of disulfide bonds. As shown in Figure 5 and Table 2, the torque of C-ZnO was higher than that of C-D3. C-ZnO and C-D3 form covalent crosslinking network and dynamic cross-linking network, respectively. Due to the reversible metathesis of disulfide bonds at high temperature, the M_H of C-D3 samples was significantly lower than that of C-ZnO.

Table 2. Curing parameters and results of CR with DPD or ZnO.

Test items	Sample ID
	C-ZnO	C-ZnO	C-ZnO	C-D3	C-D3	C-D3
Temperature,°C	160	170	180	160	170	180
ML, dN.m	0.30	0.42	0.56	0.17	0.14	0.14
MH,dN.m	4.73	6.30	8.67	1.44	1.98	2.31
T10, min	1.04	0.70	0.61	12.51	7.32	4.26
T90, min	2.93	2.16	1.95	16.83	9.55	6.33

Furthermore, the effect of DPD content on the vulcanization properties of CR was investigated. As shown in Figure 6, unexpectedly, the CR vulcanization rate (derived from the slope of the vulcanization curve) decreased with the increasing the content of DPD. It can be also found that the lower of the DPD content, the higher of the M_H. The M_H of C-D1 achieved the maximum value of 2.9 dN·m, which was 1.5 times that of the C-D3. These unexpected results of the vulcanization can be explained by the schematic diagram in Figure 7. When the molar ratio of pyridine⁄(C=C-C-Cl) was higher than 1 (the number of pyridine group is greater than that of allyl chloride groups), only one pyridine group in most DPD molecules reacted with CR after vulcanization reaction, resulting in many pendant side chains (ineffective crosslinking bonds) located on the CR chain, therefore the number of effective crosslinking bonds becomes low. When the molar ratio of pyridine⁄(C=C-C-Cl) is less than 1, both pyridine groups of DPD could be involved in the Menschutkin reaction during vulcanization, thus forming more effective crosslinking bonds. Therefore, the vulcanization curves showed a higher crosslinking density and a faster vulcanization rate.

Figure 6. The vulcanization curves of CR with different DPD content at 170°C.

Figure 7. Schematic diagram of the crosslinking reaction between CR and DPD at different molar ratios of pyridine⁄(C=C-C-Cl).

3.3. Tensile properties of CR/DPD

Before carrying out a comparative study on the static mechanical properties of DPD-crosslinked CR elastomers mentioned above, a covalently crosslinked CR sample was prepared as a benchmark by the crosslinking reaction between CR and ZnO (3phr). As shown in Figure 8, it can be found that the tensile strength of the covalent bond crosslinked vulcanizate (C-ZnO) was higher than that of the dynamic bond crosslinked vulcanizate (C-D3), while the elongation at break of C-ZnO vulcanizate was lower than that of C-D3 vulcanizate. These results were consistent with the expected results of rubber crosslinking theory.

Figure 8. The stress and strain curves of C-Ds with dynamic crosslinking bond and C-ZnO with covalent crosslinking bond.

The effect of DPD content on the tensile properties of CR vulcanizate was investigated in detail. As shown in Figure 8 and Table 3, the tensile strength of CR vulcanizate decreased with the increasing DPD content. Surprisingly, the elongation at break of samples also decreased with increasing DPD content. Compared with CR vulcanizate with high DPD content, the C-D1 vulcanizate obtained the excellent mechanical properties, and its tensile strength and elongation at break were simultaneously improved. On the one hand, the crosslinking density of C-D1 vulcanizate was higher (illustrated in Figure 7), and during the stretching process, the continuous breaking and regeneration of the disulfide bond in the cross-linkage was favorable for the gradual orientation of the polymer chains along the stretching direction, effectively avoiding the stress concentration. Therefore, C-D1 vulcanizate has the highest tensile strength (11.12 ± 1.83 MPa) and elongation at break (1253 ± 120%), which was consistent with the results of the vulcanization curves (Figure 6). Because most of the DPD participated in the formation of effective crosslinking bonds in the C-D1 vulcanizate. During the stretching process, the disulfide bond at the stress concentration position may preferentially break to generate sulfur radicals, which underwent a metathesis reaction with the other disulfide bond to form new disulfide bonds. In the system, the crosslinking network structure was constantly optimized while maintaining the crosslinking density of the network unchanged. When the DPD content was in excess, the sulfur radicals generated at the stress concentration position preferentially react with disulfide bond of ineffective cross-linkages. Thus, the density of the crosslinked network in the system cannot be maintained stable, and the structure of the crosslinked network cannot be well optimized. Therefore, the elongation at break of CR/DPD decreased with the increasing DPD content. The tensile modulus of covalently crosslinked CR-ZnO was basically consistent with that of dynamically crosslinked samples, and the tensile modulus of all samples at the initial stage was relatively low, indicating that the tensile modulus of low crosslinking degree CR samples is mainly related to the characteristics of CR itself.

Table 3. Mechanical properties of CR composites.

Test items	Sample ID
	C-ZnO	C-D1	C-D2	C-D3	C-D4
Tensile strength, MPa	15.51 ± 2.21	11.12 ± 1.83	9.63 ± 1.75	5.68 ± 0.73	4.81 ± 0.66
Elongation at Break, %	958 ± 43	1253 ± 120	1060 ± 114	970 ± 118	942 ± 115
Tensile strength at 25%, MPa	0.37 ± 0.01	0.36 ± 0.01	0.45 ± 0.01	0.26 ± 0.01	0.31 ± 0.01
Tensile strength at 100%, MPa	0.66 ± 0.02	0.58 ± 0.02	0.64 ± 0.02	0.47 ± 0.02	0.53 ± 0.02
Tensile strength at 300%, MPa	0.83 ± 0.02	0.65 ± 0.02	0.82 ± 0.02	0.65 ± 0.02	0.67 ± 0.02
Modulus, MPa	9.3 ± 1.3	8.9 ± 1.2	10.5 ± 1.3	6.5 ± 0.9	7.7 ± 1.1

Thermogravimetric analysis revealed the two degradation stages of the composites in the temperature range of 100°C to 900°C (Figure 9, Table 4). The first-stage degradation peak temperatures of C-D3 and C-ZnO were 364.5°C and 304°C, it may correspond to the dehydrochlorination reaction of CR [54,55]. The weight loss of C-D3 and C-ZnO-3 in the first stage were 50.6% and 46.6%, respectively. Table 4 shows that the peak temperature for the second stage degradation of C-D3 and C-ZnO occurred around 450°C, corresponding to the continuous degradation of CR via cracking and pyrolysis, resulting in the formation of low hydrocarbons with linear or cyclic structures. Over 30% weight loss of C-D3 and C-ZnO occurred at the second stage, indicating the completion of the degradation process. The remaining weight of C-D3 (16.9%) was lower than that of C-ZnO (19.4%). The possible reason is that DPD cross-linkages can be degraded and volatilized into the environment, while ZnO crosslinkages remain in the degraded product. Compared with the traditional ZnO crosslinked CR, the thermal stability of DPD crosslinked CR was not significantly reduced.

Figure 9. Thermogravimetric curves of different compounds (a) C-D3, (b) C-ZnO.

Table 4. Thermogravimetric analysis and char yield of unfilled and CR composites.

Blend	First stage				Second stage		T(50)(℃)
	Onset temperature, ℃ (To)^a,	Temperature range, ℃	Peak temperature, ℃ (Tmax)^b	Wt, loss (%)	Peak temperature, ℃	Wt, loss (%)
C-D3	284.3	284.3 – 422.5	364.5	50.6	453.2	32.49	451.4
C-ZnO	301.6	301.6 – 403.2	304.6	46.6	453.5	34.05	429,6

^aTemperature at 5% weight loss. ^b Temperature at maximum weight loss.

3.4. Self-healing and recyclability performances of CR/DPD vulcanizates

With a dynamic disulfide bond in each cross-linkage of CR system, the self-healing performance CR/DPD system was investigated. As shown in Figure 10, it can be found that increasing the temperature and prolonging the healing time can promote the self-healing ability of C-D3, which was consistent with the phenomenon reported in the literature [38]. However, the self-healing efficiency of C-D3 was poor, which may be due to the lack of reactive sites on the particle surface and the low density of effective cross-linkages in the C-D3 vulcanizate. The metathesis of disulfide bonds across the interface of C-D3 is difficult to occur even at high temperature (120°C), because the diffusion of the molecular segment movement at both sides of the interface is greatly restricted without external stimulation and the disulfide bond cannot exchange smoothly without catalyst at low temperature.

Figure 10. (a) effect of temperature on self-healing properties of C-D3 (healing time is 30 min). (b) effect of time on self-healing properties of C-D3 (healing temperature is 70°C).

To further conform the dynamic properties of the CR/DPD samples with the reversible disulfide bonds, stress relaxation experiments were carried on at different temperatures (namely, 150, 160, 170°C). The normalized relaxation modulus as a function of time is shown in Figure 11a-c. The normalized relaxation modulus deceased rapidly with increasing temperature, but the characteristic relaxation time was longer. For example, the characteristic relaxation time of C-D3 was 26.7 min at 170°C, in agreement with the poor self-healing efficiency at lower temperature. The disulfide metathesis was enhanced by triphenylphosphine, as shown in Figure 11d，the characteristic relaxation time of C-D6 was reduced to 5.1 min at 170°C.

Figure 11. Stress relaxation curves of CR/DPD samples. The dashed line indicates the characteristic relaxation time where the normalized modulus decreases to 1/e. (a) C-D3, 150°C, (b) C-D3, 160°C, (c) C-D3, 170°C, (d) C-D6, 170°C.

To obtain the excellent recyclability of CR-DPD through efficient exchange of disulfide cross-linkage, the high temperature and high pressure are essential factors. Triphenylphosphine (1 wt%) was also added to promote the metathesis of disulfide bonds. Therefore, the recyclability performance of C-D6 was investigated under a pressure of 10 MPa at 170°C. As shown in Figure 12 (route 1), the particles of C-D6 sample cannot be directed compress molded to produce a sheet at 170°C for 30 min, since the content of disulfide linkage in the system is low, the disulfide bond is stable below 170°C. As shown in Figure 12 (route 2), the particles of C-D6 sample were pulverized by double-roll mill at room temperature, and followed by a compress molding at 170°C for 30 min. C-D6 sample can be compressed and molded to produce a sheet via route 2. The smaller particles mean the larger specific surface area and the higher surface concentration of exposed disulfide linkages. Higher pressure also can reduce interstitial space of functional groups. The disulfide bonds of pulverized C-D6 sample could contact with triphenylphosphine to perform the reaction.

Figure 12. Pictures of reprocessing of C-D6 sample. (a) Particles of C-D6 sample. (b) Particles were compress molded under 10 MPa, at 170°C for 30 min. (c) Particles were pulverized to powders by a double-roll mill. (d) Powders were compress-molded under 10 MPa, at 170°C for 30 min.

As shown in Figure 13, the tensile strength (4.1 MPa) and elongation at break (830%) of the first-recycled C-D6 sample were reduced compared with those of the virgin sample presumably due to the formation of a less uniform network structure and the fracture of CR chain during hot-pressing. The second-recycled sample obtained the almost similar mechanical properties with the once-recycled sample, confirming the possibility for repeated recycling of CR. Clearly, the recycled C-D6 can regain its mechanical properties to a great extent due to disulfide metathesis enabling crossover bonding among the polymer chains.

Figure 13. Mechanical properties of the virgin and recycled C-D6 by route 2.

3.5. The mechanism for disulfide metathesis

The disulfide metathesis mechanism of CR/DPD is proposed, which is consistent with the mechanism reported in the literature [56–58]. As shown in Figure 14. the exchange reaction between disulfide compounds occurred via a radical-mediated mechanism without catalyst. In CR/DPD sample, the disulfide metathesis based on free radicals is insufficient due to low degree of crosslinking and low self-healing temperature, resulting in a poor self-healing performance. As shown in Figure 14, under the catalysis of triphenylphosphine, the exchange reaction between disulfide compounds occurs via an ionic mechanism, the reaction involves a nucleophilic attack of triphenylphosphine on disulfide compounds, forming thiolate and the Ph₃P⁺–S cation [58]. Subsequently, based on two possible mechanisms, the disulfide exchange reaction is prone to occur, endowing C-D6 with good reprocessing performance.

Figure 14. The metathesis of disulfides without catalyst (a), and (b) Triphenylphosphine-catalyzed metathesis of disulfides.

4. Conclusions

Taking inspiration from the reaction of small molecule model compounds, a new technology was developed for the crosslinking of CR using DPD (2,2’−dithiodipyridine) as a crosslinking agent. When DPD was incorporated into CR fabricated following a vulcanization method commonly used in the rubber industry, the Menschutkin reaction between allyl chloride (C=C-C-Cl) group of CR and pyridine group of DPD occurred with a maximum exothermic peak temperature at 184°C. The number of effective crosslinking bond of C-D1 vulcanizate is higher than that of CR/DPD vulcanizates with high DPD content. When DPD is in excess, only one pyridine group of some DPDs is involved in the reaction with CR, resulting in many pendant side chains (ineffective crosslinking bonds). C-D1 vulcanizate obtained an excellent mechanical property, and its tensile strength and elongation at break were simultaneously improved. Compared with the traditional CR/ZnO vulcanizate, the thermal stability of DPD crosslinked CR is not significantly reduced. Disulfide bond in the cross-linkages maintains its stability at low temperatures, ensuring the mechanical stability of CR/DPD vulcanizate under ambient conditions. The metathesis of disulfide bond is improved obviously under the catalysis of triphenylphosphine, which endows CR/DPD vulcanizates with a good recyclability performance. These unique properties lead to the development of energy saving and environmentally friendly CR vulcanizates, which is essential for the green development of CR.

Disclosure statement

No potential conflict of interest was reported by the author(s).