So-Hyeon Leea,
Jun-Hyun Kim*b and
Hyun-Ho Park*a
aDepartment of Chemistry, Keimyung University, Daegu 42601, South Korea. E-mail: rubchem@kmu.ac.kr
bDepartment of Chemistry, Illinois State University, Normal, Illinois 61790-4160, USA. E-mail: jkim5@ilstu.edu
First published on 26th October 2022
This study reports the effects of recovered carbon black (produced in a clean and sustainable way) as a reinforcing agent on the physicochemical properties of a styrene–butadiene rubber (SBR) matrix. SBR-based composite materials are prepared with recovered green carbon black (GCB), and these are thoroughly compared to the composite materials containing conventional virgin carbon black (VCB) (produced by the incomplete combustion of petroleum products). The GCB–SBR composite materials generally show detectably inferior properties compared to the VCB–SBR composite under the same preparation conditions due to the limited functionality of the GCB filler. However, the introduction of a small amount of crosslinker, acrylate-functionalized POSS (polyhedral oligomeric silsesquioxane), into the GCB–SBR composite materials effectively enhances the overall physical properties, including the tensile strength, fracture elongation, and thermal stability. The degree of the crosslinking efficiency, thermal stability, and mechanical properties of the composite materials are optimized and thoroughly examined to demonstrate the possibility of replacing typical VCB with GCB, which can allow for upcycling the inexpensive and ecofriendly carbon black materials as effective reinforcing fillers.
Styrene–butadiene rubber (SBR) is an important synthetic material due to its abrasion resistance and good aging stability.1,14,15 For diverse applications, SBR-based materials often require fillers to reinforce their original physicochemical properties. Among various reinforcing fillers, carbon black and silica-derived materials have shown positive effects on the overall properties of the resulting SBR-based composite materials.16–18 This is because simply blending two or more different organic and/or polymeric materials with SBR substances has shown a limited degree of improvement when considering the interactions between the inorganic filler and rubber and/or filler–filler across composite materials.15,19 The filler–rubber interactions are related to the occlusion degree of the rubber, which is physically combined at fine scales in the filler structure (e.g., carbon black). Such interactions may be observed in a bound rubber that includes aggregates regardless of the elastic part of the matrix.20,21 However, filler–filler interactions could primarily influence the rigidity of the polymer matrix, where the rigidity could systematically increase as a function of the filler amount. These overall interactions are determined by new chemical bonds between the filling particle surfaces (filler–filler or filler–rubber matrix), physical attractive forces (e.g., van der Waals forces, hydrogen bonding, etc.), the shape of the filler network, and the volume of the fillers.14,22–24 Even after the introduction of carbon black fillers into SBR materials, the utilization of additional crosslinking agents could further enhance the chemical and physical interactions across the composite materials. Polyhedral oligomeric silsesquioxanes (POSS) are an important class of nanostructure materials that have been successfully introduced in the design of polymeric rubber-based complex systems.25–29 POSS typically have a 3-D cubic cage structure composed of a Si–O backbone with surface functional groups that can be easily modified with organic moieties to become compatible with a polymer matrix.30 Upon the incorporation of POSS into polymeric networks, the physical and mechanical properties have shown great improvements in the resulting composite materials due to the reinforcement at the molecular level.31–33 In addition, surface-modified POSS with unsaturated double bonds (e.g., vinyl and acryl groups) could create new chemical bonds with the rubber matrix and fillers via free radical reactions. As such, the effective integration of POSS could potentially improve the performance of rubber-derived composite materials without sacrificing the mechanical properties. It was reported that the use of POSS molecules significantly improves various properties of polymeric materials, including the decomposition temperature, surface hardening, flammability, hydrophobicity, and viscosity reduction.26,27,34 As such, a fundamental understanding of the nature of interactions between POSS molecules and rubber-based polymer matrices, as well as their impact on thermal, mechanical, and morphological properties, is of great importance.
This study initially involves the characterization of VCB, GCB, and POSS components, as well as their intrinsic roles as reinforcing agents, upon the preparation of SBR-based composite materials. Particularly, the GCB powder used in this experiment was obtained from waste passenger car radial tires that underwent anaerobic digestion and pyrolysis at relatively low temperature, followed by a controlled pulverization process. The proper utilization of recycled carbon black (i.e., GCB) could offer very attractive aspects. For example, GCB as a filler can be economical (e.g., cheaper than VCB), environmentally friendly (e.g., significant reduction of CO2 during the preparation process), and sustainable supply of carbon materials (e.g., cost is not impacted by the price of crude oil). We also established an effective method to incorporate the reinforcing agents into the SBR. Given the limited surface functionality around GCB, the mechanical properties of GCB-containing SBR were examined to be detectably inferior to VCB-based SBR. After understanding the limited control of physical properties by simply using GCB, properly introducing an additional crosslinker, methacrylate functionalized-POSS, enabled several properties (i.e., crosslinking efficiency, thermal stability, and mechanical properties) of the resulting GCB-containing SBR to be comparable to those of the VCB–SBR composite materials. As such, this study demonstrated the capability of upcycling eco-friendly GCB as an effective filler to reinforce SBR-based materials.
Material | T-1 (SBR–VCB) | T-2 (SBR–GCB) | T-3 (SBR–GCB–POSS1) | T-4 (SBR–GCB–POSS2) |
---|---|---|---|---|
a Unit: phr – part per hundred rubber (e.g., 100 g SBR, 40 g VCB, 4 g F40KEP, and 1 g POSS). | ||||
SBR raw material | 100 | 100 | 100 | 100 |
GCB | — | 40 | 40 | 40 |
VCB | 40 | — | — | — |
POSS | — | — | 1 | 2 |
Crosslinker (F40KEP) | 4 | 4 | 4 | 4 |
A Banbury mixer (Kobe, Japan) was used to thoroughly mix all components prior to the preparation of the composite materials (the detailed mixing process and parameters are shown in Table 2). The initial step involved the addition and mixing of SBR and reinforcing agents in the mixer, after which the resulting mixture was matured at room temperature for 24 h. Finally, the compounds were mixed with a peroxide-based crosslinker and/or POSS in the curing step.
Masterbatch preparation |
---|
Banbury operating conditions |
Mixing speed: 77 rpm |
Ram pressure: 3.0 kgf cm−2 |
Temperature: 60 °C |
Cooling water temperature: 18 °C |
Fill factor: 0.7 |
Mixing procedure |
1. Add SBR (0.5 min) |
2. Add VCB or GCB reinforcing agents (1.5 min) |
3. Add remaining additives (1.5 min) |
4. Discharge (3.0 min) |
5. Cool the samples overnight after removal from the mill |
Curing agent addition on a mill (at 60–70 °C) |
---|
1. Set the mill opening at 4 mm and add SBR masterbatches from step A (0.5 min) |
2. Add peroxide and POSS curing agents (1.5 min) |
3. Set the mill opening at 2 mm (1.5 min) and repeatedly cut the sample (∼4 times) on each side |
4. Set the mill opening at 5 mm (4.5 min) |
5. Cool the samples overnight after removal from the mill |
(Tmax − Tmin) × crosslinking rate (%) + Tmin = Tc90. |
The optimal crosslinking time was determined when the crosslinking rate reached 90%. The scorch time (ts2) was the moment when the minimal torque (Tmin) increased to 2 lb-in, indicating that the rubber began to undergo the crosslinking reaction.
To determine the presence of the POSS component in the SBR samples, infrared spectra were measured under attenuated total reflection (ATR) conditions. Additional characterizations of POSS were carried out by powder X-ray diffractometry (PXRD; Rigaku RINT 2000, Japan) and nuclear magnetic resonance spectrometry (29Si-NMR, AVANCE III 500; Bruker). A thermogravimetry analyzer (STA 409; Netzsch, Japan) was used to examine the thermal stability of composite materials as a function of time. The temperature-dependent weight loss patterns were monitored under a heating rate of 10 °C min−1 from 25 °C to 600 °C in air.
The POSS crosslinker was also examined using PXRD and 29Si-NMR prior to preparing SBR–GCB composite materials (Fig. 3). The PXRD of POSS shows two broad 2θ peaks at ∼8° and ∼22°, which correspond to the cage-like structure and the amorphous siloxane backbone (porous structure), respectively (also explained by previous reports).29,44–46 Specifically, the calculated d-spacing of the first broad peak was around 1.1 nm, which possibly corresponds to the core diameter of POSS molecules. The d-spacing for the second broad peak was around 0.40 nm, which could be the distance of the Si–O–Si bond. In addition, the 29Si-NMR spectrum of POSS clearly showed a T2 peak (partial opening of silicone cage) near −67 ppm and a T3 peak (silicone cage) near −69 ppm. The presence of these two peaks (T2 and T3) from 29Si-NMR is often used to explain the cage-like structure of POSS derivatives.28,47,48
The characteristics of SBR composite materials upon the addition of the reinforcing agents (VCB and GCB) and crosslinking agent (POSS) as a function of content are shown in Table 3. The corresponding Mooney viscosity is also shown in Table 4. When the GCB-reinforcing agent was added to SBR, the crosslinking concentration and Mooney viscosity were examined to be higher than when the VCB reinforcing agent was added. With the increasing POSS content from the T-3 to T-4 sample, the crosslinking concentration further increased. When GCB was added, the optimal crosslinking time was extended compared to that of the VCB-reinforcing agent. The increase of the crosslinking degree and viscosity of the GCB-containing SBR composite could be due to a simple physical combination of the SBR matrix and the reinforcing agent caused by the irregular cross-section of GCB. The increase in the crosslinking density caused by adding slightly more POSS content improved the crosslinking efficiency across the raw material SBR matrix. In addition, the increase in the POSS content was expected to influence the viscosity (Mooney viscosity; ML1+4) and scorch time (T5: initial reaction time) of the composite materials. Compared to VCB, the optimal crosslinking time in the presence of GCB was slightly delayed due to the lack of surface functional groups. However, the crosslinking time could be shortened by increasing the amount of the POSS content. It is important to remember that the use of POSS greatly influenced the crosslinking degree and viscosity for GCB-containing composite materials. Further increasing the POSS content (e.g., ≥3 phr) readily resulted in poor miscibility across the SBR matrix when preparing homogeneous composite materials.
Mix no. | T-1 (SBR–VCB) | T-2 (SBR–GCB) | T-3 (SBR–GCB–POSS1) | T-4 (SBR–GCB–POSS2) | |
---|---|---|---|---|---|
MDR 160 °C for 50 min | Tmax (lb-in) | 71.0 | 81.4 | 79.1 | 80.0 |
Tmin (lb-in) | 17.6 | 21.4 | 15.6 | 13.7 | |
Tmax − Tmin | 53.4 | 60.0 | 63.5 | 66.3 | |
Tc90 (min) | 24.43 | 29.59 | 32.33 | 31.53 | |
ts2 (min) | 2.04 | 2.09 | 2.08 | 1.54 |
Mix no. | T-1 | T-2 | T-3 | T-4 | |
---|---|---|---|---|---|
a ML1+4: 1 min of preheating and 4 min of rotation, T5: initial reaction time. | |||||
Mooney viscosity | ML1+4 (MU) | 45.1 | 55.8 | 52.5 | 50.7 |
T5 (min) | 9.25 | 10.47 | 10.44 | 9.27 |
After the formation of carbon black containing the SBR composite materials (T-1: VCB–SBR, T-2: GCB–SBR, T-3: GCB–SBR–POSS1, and T-4: GCB–SBR–POSS2), FT-IR spectra were collected (Fig. 4). Although it was somewhat difficult to identify the presence of POSS, the T-3 and T-4 composite materials displayed a slightly stronger peak at 1722 cm−1 (CO stretching) and several peaks at 800–1030 cm−1 (Si–O associated stretching and bending). The small shift of the CO stretching peak could be due to the free-radical polymerization of the acrylate groups in POSS, which is also explained by other group.33 This observation clearly suggested the crosslinking of POSS across the GCB–SBR samples (i.e., T-3 and T-4).
Table 5 shows the pyrolysis temperature of the composite materials as a function of weight loss (%), which was examined by a thermogravimetric analyzer. All composite materials exhibited somewhat similar weight loss patterns, but the GCB-containing composites generally slowed down their decomposition rates. Although the presence of POSS did not significantly change the decomposition process of the SBR matrix, increasing the POSS content slightly raised the initial decomposition temperature. This observation indicated that POSS crosslinking could enhance the thermal stability of the SBR matrix. As expected, the composite materials containing GCB had a large amount of residue (i.e., ash) because recovered GCB from waste tires often contains additional impurities (e.g., Al, Cl, Zn, and Si) from the manufacturing process.43,49–51 The composition of typical VCB is reported by the manufacturers to be more than 95% carbon, with minimal quantities of O, H, N, and S.
Composite sample | 5 wt% loss temp. (°C) | 10 wt% loss temp. (°C) | 50 wt% loss temp. (°C) | Residue (%) |
---|---|---|---|---|
T-1 | 432 | 439 | 468 | 4.5 |
T-2 | 439 | 444 | 473 | 5.7 |
T-3 | 439 | 446 | 476 | 6.5 |
T-4 | 441 | 445 | 474 | 5.9 |
The tensile strength and elongation at the break for the composite materials is summarized in Fig. 5. The addition of GCB caused the slight reduction of the tensile strength and elongation of the composite materials. However, these properties gradually increased upon the introduction of POSS, presumably due to the crosslinking effect.25,52–54 Unlike the use of VCB, the limited surface functionality of GCB could induce weaker attractive interactions across the SBR matrix to unfavourably influence the reinforcing effect. Upon adding POSS, the overall mechanical properties slowly recovered, possibly because the polymerization of POSS could increase the crosslinking density throughout the SBR matrix. The utilization of a small amount of acrylate-functionalized POSS greatly improved the physicochemical properties of rubber-based composite materials containing inexpensive and recovered GCB as a filler that can possibly replace conventional VCB.
The tensile strength of the aged SBR composite containing VCB, GCB, and POSS is summarized in Table 6. After aging at 100 °C for 70 h, the composite materials containing GCB exhibited detectably larger changes of tensile strength. However, the strength changes of these composite materials notably decreased as a function of the POSS content, implying a greatly improved heat resistance. As we mentioned above, the limited functionality around GCB resulted in the less efficient physical interactions with the SBR matrix, where the external heat treatment (i.e., aging test) could easily harden the SBR matrix itself to greatly decrease the tensile strength (i.e., larger changes of the tensile strength before and after aging). In contrast, the composite materials containing POSS showed a lower degree of hardness after aging, where the tensile strength fluctuation rate before and after aging was ∼30%. As the peroxide-based crosslinker (C–C bond energy between molecules is ∼345 kJ mol−1) could play an important role in the preparation of carbon black–SBR composite materials, the use of additional POSS crosslinker possessing Si–O bond (∼445 kJ mol−1) across the SBR matrix could result in a slightly higher heat resistance.55,56 However, the SBR matrix containing POSS still maintained its flexibility, which could be a unique feature for SBR–GCB composite materials (i.e., high flexibility and heat resistance). This observation clearly implied that the use of POSS minimizes the flexibility changes of composite materials, even after 70 h of aging at 100 °C.
Composite sample | T-1 | T-2 | T-3 | T-4 |
---|---|---|---|---|
Tensile strength change rate (%) | 52 ± 8 | 60 ± 9 | 35 ± 5 | 31 ± 5 |
This journal is © The Royal Society of Chemistry 2022 |