This work deals, in the first part, with the preparation and characterization of a type of pyrolysis char (PC) from waste tires. Best conditions were chosen to obtain the optimal PC with the largest Brunauer-Emmett-Teller (BET) surface area. The structure of PC was characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The second part focuses on preparation of novel room-temperature vulcanizated silicone rubber (SR)/PC composites. Properties such as thermal stability, flame-retardance, tensile strength, and elongation at break were researched and compared. Flame-retardance test showed that the vertical burning time and limiting oxygen index (LOI) values of different SR composites were increased. The last part evaluates the reinforcing and flame-retardant mechanisms of PC in SR composites. Polar light microscopy (PLM) and SEM analysis demonstrated that the mixture of PC and ammonium polyphosphate (APP) had better compatibility in SR matrix.
As one of the most important high-temperature-resistant synthetic rubbers, silicone rubber (SR) possesses good thermal stability, electrical-insulating behavior, and low-temperature toughness. This polymer has been widely used in electronic and electric industries (1, 2). The flame-retardant materials used in these fields are highly required. However, SR can constantly burn after being ignited (3). Thus, flame-retardant SR needs to be developed.
One way to improve the flame-retardancy of SR is to introduce a flame-retardant additive. Both fire properties and mechanical behavior of the materials can be improved with the addition of intumescent additives (4, 5). A cellular charred layer on the surface of the material can be produced by intumescent flame-retardant (IFR) agent during heating process. This char layer can protect the underlying material from the action of the heat flux. Thus, the diffusion of combustible volatile products toward the flame and of oxygen toward the polymer is limited (6, 7). IFR system usually contains three main substances. The typical example is ammonium polyphosphate (APP) as an acid source, pentaerythritol (PER) as a carbon source, and melamine (ME) as a gas source (8).
To develop the applications, judicious routes compounding APP and other inorganic fillers were used and polymeric composites were being continuously manufactured. Wang et al. (9) synthesized a novel type of carbonific. The carbonization and flame-retardant properties of polyurethane (PU) varnish were increased with the addition of APP and the carbonific. Zheng and Wang (10) added organically-modified montmorillonite (OMMT) as an additional filler in the APP-triphenyl phosphate system. Due to the particular nano-layer structure of OMMT, the flame-retardancy of PU was improved. This composite possessed the longest combustion duration time, slowest heat release rate, and lowest total smoke production. However, till now, the research on application properties of pyrolysis char (PC) from waste tires as a type of carbonific has not been conducted.
Nowadays, waste is known as a big challenge in people’s lives. One well-known waste is scrap tires. About 3.4 and 4.6 million tons of discarded tires are produced every year in Europe and the USA, respectively (11–13). These tires are usually produced by non-biodegradable material, i.e. styrene-butadiene rubber (SBR). Thus, attention has been paid to the harmful effects of these tires on the environment in developed countries (14). In order to attain this aim, pyrolysis of the tires has recently been considered as an useful recycling process. The products of the lysates from waste tires are valuable oil, PC, and gaseous products. The PC mainly consists of carbon black, inorganic compounds initially present in the tire, and carbonaceous deposits formed during the pyrolysis. The production and utilization of PC has been investigated by many studies (15–18). Some suggested that PC from scrap tires could be upgraded to an activated char through an activation process. It can be used as a pollutant adsorbent in both the gas phase and liquid phase separation processes (19, 20).
In this paper, PC from waste tires was prepared and used as a carbonific to prepare flame-retardant SR composites. The structure and properties of SR composites were characterized by thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET), polar light microscopy (PLM), and scanning electron microscopy (SEM) together with the tensile properties, thermal stability, and flame-retardant analysis. Results showed that the combination of APP and PC from waste tires had obvious effects on the mechanical properties of SR, which resulted in the improvement of tensile strength, thermal stability, and flame-retardant behavior of SR composites.
SR, two components (A:B, 1:1, 30% aerosilica, component A: hydrogenous silicane, Si-(O-Si-(CH3)2-H)4; component B: ethylene-terminated polysiloxane, CH2=CH-Si(CH3)2 O(Si(CH3)2O)n-Si(CH3)2-CH=CH2, and the organic platinum catalyst), industrial grade, were supplied by Bluestar Silicones Shanghai Company (Shanghai, China). To prepare PC, waste tires were collected from the college market (rubber matrix, 63 wt.%, carbon black, 30 wt.%, and other fillers, 7 wt.%). APP, chemical pure, was supplied by Shanghai Guoyao Chemical Agent Group (Shanghai, China).
2.2 Preparation of PC and SR composites
One hundred grams of waste tires was placed in the Muffle furnace (atmosphere: air) and treated at different temperatures and times. Then, they were grounded and dried in the oven at 120°C for 6 h, and thus PC was obtained.
Different amounts of APP and PC were mixed with hydrogenous silicane (hydrogen content: 2%; viscosity: 20 mm2/s, 25°C), one component of SR. This mixture was then vigorously stirred at room temperature for 3 h. It was blended with ethylene-terminated polysiloxane, the other component of SR, and stirred for 0.5 h. The percentage of APP and PC in SR was 20%. The ratios of APP and PC (APP/PC) were 20/0, 15/5, 10/10, 5/15. The mixture was molded in a Teflon mold. Last, elastic films were obtained after curing at room temperature 20°C for 24 h.
BET surface area of PC was measured with a nitrogen adsorption instrument (Micrometrics ASAP 2020). Prior to the measurements, all samples were degassed at 300°C for 1 h.
FTIR of PC was obtained using a Nicolet spectrometer (model Avatar 370). The resolution was 2 cm-1 and the scanning range was from 4000 to 700 cm-1. The spectra were obtained using 64 scans.
TGA and its differential (DTG) of SR composites were performed under air with a flow rate 5×10-7 m3/s at 10°C/min using a Linseis equipment (PT1000). The mass of the samples used were 10 mg. The precision of the temperature measurements was 1°C.
The vertical burning tests of SR composites were carried out on a horizontal and vertical burning tester (5400, Kunshan, China) according to the standards ASTM D635, respectively, on sheet (125±5 mm)*(13.0±0.5 mm)*3.0(0.0+0.2 mm). In the result of this test, tign means the ignition time of the first burning, tres means the after-flame time for the first burning, tsec ign means the ignition time of the second burning, tsec res means the after-flame time for the second burning, tres light means the after-glow time. Limiting oxygen index (LOI) values were obtained using a Stanton Redcroft instrument on sheets (100*10*3 mm3) according to ASTM 2863.
Tensile test of SR composites was conducted using a TCR-2000 instrument at room temperature with a crosshead speed of 500 mm/min. The samples were prepared in a standard dumbbell-shape. All measurements were repeated five times and a median value was obtained.
PLM of SR composites was observed using a XPF-300 polar light microscopy. SEM of PC and SR composites was obtained using a Hitachi S-2150 scanning electron microscope. The electron beam potential used in this test was 25 kV.
3 Results and discussion
3.1 Preparation of PC from waste tires
Table 1 presented the factors and calculated results of BET test results. It was clear that the most satisfactory conditions for preparation of PC with the largest BET surface area was 600°C and 6 h. Calcining temperature had no close relationship with the surface area of PC. At high temperature such as 800°C, more oxidation of the carbon black in PC appeared and large coacervated particles were formed. This can lead to macroscopic aggregations with low surface areas. In addition, we can see that the longer the calcining time, the larger the surface area was. The entire degradation solid residues, PC, had these characteristics (21). The preparation mechanism of PC was shown in Scheme 1. The calcining temperature was closely related to the pyrolysis degree. At 800°C, the weight loss was about 80%, and this was 25% higher than that of 400°C, about 55%. Also, the longer the calcining time, the more the weight loss was.
|Calcining temperature (°C)||Calcining time (h)||Weight loss (%)||BET surface area (m2/g)|
In order to see the properties of the best combination, PC prepared in 600°C and 6 h was characterized and applied into SR composites.
FTIR spectrum of PC is shown in Figure 1. It shows weak peaks at 2800~3000 cm-1 and a strong peak at 1469 cm-1. This was due to the C-H stretching and bending absorptions in the organic materials which still existed in PC coming from the waste tires. The absorptions at 800~1200 cm-1 were ascribed to stretching and bending behavior of other organic materials in the waste tires. The removal of most organic materials and the formation of mesoporous structure took place during the calcination process. However, some high-temperature-resistant materials still remained in PC.
Figure 2 shows the SEM images of the microscopic morphology of PC. Figure 2(A), PC was made up of large powders with irregularly shaped organic shells. The size of most powder was about 30~50 μm. Figure 2(B), PC was covered with irregular and flocculent organics. This illustrated that the components of PC had not only carbon black but also the remaining rubber residues.
3.2 Properties analysis of SR/PC composites
PC was used as a carbon source and was combined with APP as an IFR agent. They were added with different ratios in SR composites.
3.2.1 Thermal stability
Figure 3 provides TGA and DTG curves of different SR composites. Three parameters were measured from the above curves. That is, the onset temperature of thermal degradation (Tonset, the temperature at which weight loss is 5 wt.%), the center temperature of thermal degradation (Tmax, the temperature at which weight loss is the fastest), and the yield of charred residue at 600°C (22). The results are given in Table 2.
|Tonset (°C)||Tmax (°C)||Residual mass (%)|
In a word, the thermal stability of most SR composites was not better than that of pure SR. At a loading of 20% APP, Tonset and Tmax of SR/APP-20 exhibited a decreased value, 150°C and 345°C, 250°C and 215°C lower than that of pure SR. Ai and Ma (23) investigated a type of flame-retardant room temperature vulcanizates (RTV) prepared with dihydroxy dimethylsiloxane, fumed silica, and inorganic flame-retardant APP. The effect of APP content on the flame-retardant and physical properties as well as thermal stability of RTV was studied. Results also showed that the initial degradation temperature of RTV was decreased. The reason for the decreased thermal stability of SR or RTV with the addition of APP was attributed to the early decomposition catalyzed by the degradation product from APP, such as phosphoric acid, metaphosphate, and polymetaphosphate, etc. In this experiment, when 5% PC was added with 15% APP into SR, Tonset and Tmax of SR/APP-15/PC-5 were increased to 335°C and 450°C, 185°C and 105°C higher than that of SR/APP-20 composite. Compared with that of pure SR, the intensity of degradation peaks of SR/APP-15/PC-5 was obviously decreased. This showed an improved thermal stability of these composites which was mainly attributed to the carbonaceous and synergetic effect of PC in SR. This also illustrated 3:1 of APP with PC was the most suitable combination ratio and had the best effect on inhibiting thermal degradation in the SR matrix. Moreover, SR/APP-5/PC-15 and SR/APP-20 showed almost the same degradation behavior especially befor 350°C at very different compositions. This phenomenon may be attributed to the following two reasons. First, these two composites possessed 80% of their composition with the same silicone matrix. Second, 5% or an even lower amount of APP in SR/APP-5/PC-15 may be enough to catalyze the early decomposition of SR, and 20% APP exhibited the same catalyzation effect in SR/APP-20 composite.
Due to the difference in the preparation method, the level of impurities, the residual catalyst, and the degradation of conditions, four mechanisms were put forward by Chen et al. (24). Molecular mechanism, the end-initiated unzipping mechanism, the random main chain scission mechanism, and the externally catalyzed mechanism were included in these mechanisms. It can be deduced that SR was mainly decomposed from chain ends with a contribution of random degradation. In addition, with the increase of temperatures, the extent of random degradation also increased.
3.2.2 Flame-retardant behavior
The vertical burning time and LOI values of different SR composites are summarized in Table 3. SR was completely burnt after the first burning time. By adding 20% APP into pure SR, the burning rate was decreased. tres was increased to 8.2 s after the first burning time, and tsec res was increased to 3.9 s after the second burning time. Moreover, when 5% PC and 15% APP were added into SR, this composite cannot be ignited for the first and the second burning time. However, when more PC were added with APP, the burning time was increased. The flame-retardant property was decreased. As shown in Table 3, the LOI value of pure SR was 22.0%. This illustrated that this polymer was flammable in an air environment. When 5% PC and 15% APP were incorporated into SR, the LOI value was increased to 28.0%. The improvement was about 27%. It showed the best fire protection. In this system, PC acted as a carbonific agent and showed a synergetic effect with the acid source, APP. This can improve the flame-retardant behavior of SR matrix. Du and Wang (25) researched the flame retardancy of SR/melamine polyphosphate (MP) composites. Results showed that the LOI value was increased from 24% to 29.5% or 31.5% after the addition of 30 or 40 phr of MP into SR matrix. The improvement was about 23% and 31%, respectively. Sheng et al. (26) added 15 phr of aluminum hydroxide in the SR matrix. The LOI value was increased from 24% to about 30%, and the increase was 25%.
|tign (s)||tres (s)||tsec ign (s)||tsec res (s)||tres light (s)||LOI (%)|
|SR||10 (completely burnt)||/||/||/||/||22.0|
3.2.3 Tensile properties
Figure 4 illustrates the tensile properties of different SR composites. The tensile strength was decreased from 3.4 to 3.0 MPa at a loading of 20% APP. The decrease was about 12%. With the addition of 15% APP and 5% PC, the composite possessed the highest tensile strength, 3.8 MPa. The increase was about 12%. The improvement of the tensile strength was attributed to two facts: (1) a high stress bearing capability and efficiency due to the uniform dispersion of PC at low addition ratios; (2) an effective constraint of the motion of rubber chains resulting from the strong interactions and large contact surface between PC, APP, and SR chains (27). However, the tensile strength was decreased when the amount of PC was increased to 10% and 15%. This was ascribed to the formation of aggregates and the reduced interface area between polymer and the fillers. In addition, no obvious changes in the elongation at break when APP and PC were added.
3.3 Mechanism analysis of SR/PC composites
3.3.1 Reinforcing mechanism
The PLM of rubber composites after the addition of APP and PC were shown in Figure 5. SR (Figure 5A) filled with 20% APP (Figure 5B), had obvious interfaces, and their compatibility was not satisfactory. However, the interface of SR filled with 15% APP and 5% PC (Figure 5C) was not obvious. This indicated that it had good compatibility and possessed greater tensile strength (28). Furthermore, the SR composites filled with 5% APP and 15% PC (Figure 5D) had obvious interfaces between fillers and rubbers, and their compatibility was poor. This composite showed decreased tensile properties.
SEM supplied further evidence of dispersion of APP and PC in SR composites. Compared with pure SR (Figure 6A), scattered fillers can be clearly seen in the fractured surfaces of SR/APP-15/PC-5 (Figure 6B). The good mechanical properties of this composite can be confirmed by the fractured surface with no voids and no deformed portions. However, more holes were shown in the section surface when more PC was added. This was due to the aggregation of these fillers in the silicone matrix (29).
3.3.2 Flame-retardant mechanism
Here, the present study described the application of PC together with APP in the SR matrix. PC was composed of residual rubber molecules and carbon blacks. It behaved like a carbonific for the IFR system. The flame-retardant effect of APP and PC can be used to fabricate a new type of enhanced flame-retardant composites.
The scheme of the flame-retardant mechanism in these SR composites is shown in Scheme 2. Regarding the increase of flame-retardance of SR composites, these SR composites may have following flame-retardant mechanism. First, the presence of the carbonific may increase the interactions and reactions between the polymer and additives, and this can improve the flame-retardance of SR composites. That is to say, PC may activate the intumescent and carbonization process (9). Second, PC may be assumed to remain at the surface when SR is burnt, and thus an excellent barrier effect is shown. During combustion, PC can slow down the degradation of polymer chains to flammable small molecules together with the obstruction of movement of the combustion interface. Third, the degradation products coming from phosphorus, nitrogen, and carbon elements in APP and PC may produce aromatic compounds. These compounds had high thermal stability, and thus can slow down combustion. Fourth, the degradation products of SR and methylene groups in the fillers may produce char layers. The outside oxygen penetrating into the internal composite may be slowed down by these char layers, and thus the combustion at interface may be inhibited (30).
PC was successfully prepared by calcining of waste tires. Results showed the BET surface area of PC was influenced by the calcining temperature and calcining time. Results showed that the most satisfactory conditions for preparation of PC with the largest BET surface area were 600°C and 6 h. The structure and morphology of PC were characterized by FTIR and SEM.
Thermal stability, flame-retardance, and tensile tests demonstrated the effect of PC in SR composites. When 5% PC was added with 15% APP into SR, Tonset and Tmax of SR/APP-15/PC-5 was increased to 335°C and 450°C, 185°C and 105°C higher than that of SR/APP-20 composite. By adding 5% PC and 15% APP into SR, the LOI value was increased to 28.0%. This composite also possessed the highest tensile strength. Its tensile strength was increased from 3.4 to 3.8 MPa, and the improvement was about 12%. The efficiently reinforcing and flame-retardant abilities of PC were confirmed by the improved mechanical and physical properties. PLM and SEM analysis revealed that PC had good compatibility with SR matrix.
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