Recycling of waste tyre rubber into oil absorbent
B. Wua, b,
M.H. Zhoua
a
,
,
College of Environmental Science and Engineering, Donghua University, Shanghai 201620, People’s
Republic of China
b
Department of Polymer Material and Engineering, Donghua University, State Key Laboratory for Modification
of Chemical Fiber and Polymer Materials, Shanghai 201620, People’s Republic of China
Abstract The abundant and indiscriminant disposal of waste tyres has caused both health and environmental problems. In this work, we provide a new way to dispose off waste tyres by reusing the waste tyre rubber (WTR) for oil absorptive material production. To investigate this feasibility, a series of absorbents were prepared by graft copolymerization-blending method, using waste tyre rubber and 4-tert-butylstyrene (tBS) as monomers. Divinylbenzene (DVB) and benzoyl peroxide (BPO) were employed as crosslinker and initiator, respectively. The existence of graft-blends (WTR-g-tBS) was determined by FTIR spectrometry and verified using thin-layer chromatography (TLC). In addition, the thermal properties of WTR-g-tBS were confirmed by a thermogravimetric analyzer (TGA). Oil absorbency of the grafted-blends increased with increases in either feed ratio of WTR to tBS or DVB concentration. This absorbency reached a maximum of 24.0 g g−1 as the feed ratio and DVB concentration were 60/40 and 1 wt%, respectively, after which it decreased. At other ratios and concentrations the absorbency decreased. The gel fraction of grafted-blends increased with increasing concentration of DVB. Oil-absorption processes in pure toluene and crude oil diluted with toluene were found to adhere to first-order absorption kinetics. Furthermore, the oil-absorption rate in diluted crude oil was observed to be lower than pure toluene.
1. Introduction Oil pollution of marine environments is becoming a more serious issue with the growth of the off-shore petroleum industry and the necessity of marine oil transportation. One of the methods to solve this problem is by using oil absorptive resins, which can collect and remove the oil spilled on water, and some have proved to be highly efficient (Atta and Arndt, 2005, Jang and Kim, 2000, Shimizu et al., 1997, Suni et al., 2004, Teas et al., 2001 and Zhou and Cho, 2001). However, the high cost of the absorptive materials limits their applications. Therefore, it is necessary to seek a cost-effective way to produce an absorptive material for oil cleanup.
As the biggest share amongst waste polymers in world, waste tyre rubber does not decompose easily owing to its crosslinked structure and presence of stabilizers and other additives. Waste tyres traditionally have been disposed by incineration or landfilling (Chen and Qian, 2003 and Warith and Rao, 2006). However, this poses two problems: wasting of valuable rubber and environmental pollution. Therefore, reuse or recycling of scrap tyres becomes an important social subject. Recycling of waste rubber by means of blending with polymeric materials has become an important topic in recent years (Luo and Isayev, 1998 and Mull et al., 2002). Recently, surface-modification techniques have been adopted for recycling of scrap rubber powder, particularly through various irradiation techniques (AbdelBary et al., 1997, Adam et al., 1990, Bagheri et al., 1997, Lee and Ryu, 1998 and Yu and Ryu, 1999). There have been limited studies on the potential use of waste tyres as oil absorbents. Recycled rubber from waste tyres has an intrinsic ability to absorb hydrocarbons. However, this absorption capacity of four to six times its weight is significantly lower than other absorbent materials currently in use (Aisien et al., 2003). Consequently, the first objective of this work is to synthesize an oil absorptive material by graft copolymerization-blending method with waste tyre rubber (WTR) and 4-tert-butylstyrene (tBS). This not only provides a way for waste disposal, and reduces the growing menace of ground tyre rubber to the environment, but also reuses the waste rubber and lowers the cost for oil absorbent production. A further goal is to investigate the quantitative influence of synthesis conditions of the oil absorptive resins on their oil absorptive properties. In both cases, the final objective is to prove the merit of reuse of waste, which would be considered as a reasonable starting point in the choice of the most appropriate sorbent for a specific absorption process.
2. Experimental 2.1. Materials Waste tyre rubber (WTR) of particle size 100 mesh used in this investigation was devulcanized and obtained from Zhejiang Lvhuan Rubber Powder Engineering Co. Ltd. The element content of WTR is: C = 80.92%, H = 6.80%, O = 4.30%, N = 0.38%, S = 1.66%; the ash content is 5.82%. The 4-tert-butylstyrene (tBS; Aldrich Chem, USA) and divinylbenzene (DVB; Fluka) were each extracted with a 5% aqueous sodium hydroxide solution and water, dried over anhydrous sodium sulfate, and distilled under reduced pressure prior to use. Benzoyl peroxide (BPO; Aldrich Chem) was recrystallized from methanol. All other chemicals were used as received.
2.2. Sample preparation The monomers, with the feed ratios of WTR to tBS of 80/20, 60/40, 50/50, 40/60 and 20/80, were immersed initially in liquid paraffine used as dilution, to which the initiator (BPO) of
1 wt% in relation to the monomers and a determined amount of crosslinker (DVB) were added (Wang, 2007). To hinder coalescence and break-up of droplets in the course of polymerization, gelatine of 4 wt%, tricalcium phosphate of 0.5 wt% and sodium dodecylsulfate (SDS) of 0.5 wt% were used as suspending agents. The ratio of organic phase to water phase was 1:5. The copolymerization-blending process was then conducted at 80 °C for 8 h in a 250-ml fournecked reaction flask fitted with a variable mechanical stirrer, thermometer, nitrogen gas inlet and reflux condenser. After the reaction was completed, the resulting copolymer beads were filtered and washed with hydrochloric acid and then hot water, and then dried in a vacuum oven at 45 °C. The dried samples were extracted with tetrahydrofuran to remove the diluent, residual monomers and uncrosslinked copolymers (Zhou and Cho, 2000). Following extraction, the samples were dried to a constant weight. The final product consisted of spherical beads 0.5–1 mm in diameter.
2.3. Oil-absorption test Oil-absorptivities of oil-absorption resins were determined by ASTM (F726-81; ASTM F726, standard method of testing sorbent performance of adsorbents E1-1993): 0.1 g of product was put in a stainless steel mesh (4 cm×4 cm×2 cm), and immersed in pure or in crude oil solution (crude oil diluted with toluene, 10% oil). The swollen gels were taken out at regular time intervals, blotted with tissue paper gently, weighed on a balance and replaced into the same bath. This procedure of swelling and weighing was continued until the sample achieved a constant final weight. The equilibrium oil-absorbency of crosslinked composite particles Qeq can be calculated by equation(1)
Turn MathJaxon
where W0 is the initial weight of the sample, and We is the weight at equilibrium. Experiments were run in triplicate and averaged values were reported.
2.4. Measurements FTIR spectrometer (Nicolet NEXUS-670, US) was used to confirm the basic components of WTR and identify the existence of WTR-g-tBS, which was also verified by thin-layer chromatography (TLC). The thin-layer chromatographic method of samples was developed by use of an iodine–ethanol solution (Shanghai Chemical Reagents Company) as mobile phase, and silica gel plate (Shanghai Chemical Reagents Company) as solid phase. Experiments were run in triplicate and average values were reported. The thermal degradation behavior of the prepared samples was studied using a Schimadzu DT 30A thermogravimetric analyzer
(TGA) instrument. Experiments were conducted in the temperature range of 40–600 °C under a nitrogen gas purge at scanning rate of 10 °C min−1.
3. Results and discussion 3.1. Characterization Fig. 1 shows the FTIR spectra of WTR and WTR-g-tBS, and signals attribution are listed in Table 1. Fig. 1a shows the characteristic peak at 2915 cm−1 corresponds to the aliphatic C– H bond unsymmetrical stretching vibration of CH3– groups. The peak at 2856 cm−1 is due to aliphatic C – H symmetrical stretching vibration of – CH2 – groups (Zhang, 1994), which indicates the unmodified WTR may contain natural rubber (NR). The peak at 1448 cm − 1
corresponds to scissoring vibration of –CH2– affected by S atoms in –CH2–S–CH2– group in
NR ( Yi, 1998 and Zhang, 1990). Therefore, the presence of a peak at 1448 cm−1 confirms that unmodified WTR contains NR. The weak absorption peak at 1726 cm−1 corresponding to aldehyde group is markedly the result of oxidation in the course of NR blending (Zhang, 1990), and thereby further supports the previous explanation. Interestingly, it can be observed that the characteristic absorption bands at 500–800 cm−1 are very weak, which indicates the C–S–C and S–S bonds have partially been cleaved in the course of reclamation.
Fig. 1. FTIR spectra of (a) WTR and (b) WTR-g-tBS. Figure options Table 1. Analysis of signals attribution Analysis of signals
WTR (cm−1)
WTR-g-tBS (cm−1)
Unsymmetrical stretching vibration of aliphatic C–H
2915
2923
Symmetrical stretching vibration of aliphatic C–H
2856
2854
Scissoring vibration
1448
1452
Characteristic absorption peak of aldehyde group
1726
1727
Bend vibration of aliphatic C–H
1371
Out-of-plane vibration of aromatic C–H
725
Table options
All peaks mentioned above are observed in spectrum b (Fig. 1), however, a prominent peak in spectrum b at 1371 cm−1, which is corresponding to aliphatic C–H bend vibration of CH3– groups present in 4-tert-butyl group on tBS, and the peak at 725 cm−1 is due to out-of-plane vibration of aromatic C–H of substituent on tBS. These peaks confirm the existence of the graft of tBS onto the surface of WTR. Furthermore, the disappearance of peak at 1630 cm−1, which corresponds to the vinyl stretching vibration of tBS, further supports the fact that tBS exists on the surface of WTR. Fig. 2 illustrates a schematic of the TLC of homopolymer of tBS (P(tBS)), WTR, P(tBS)/WTR, and WTR-g-tBS combinations. The position of extension in WTR-g-tBS was different from P(tBS), WTR, P(tBS)/WTR, and the average values of Rf for WTR-g-tBS and P(tBS) were 0.56 and 0.73, respectively. Therefore, the graft of tBS onto WTR was verified.
Fig. 2. Result of TLC. Figure options
3.2. Thermal property The weight loss of samples due to thermal degradation was obtained from the thermogravimetric analysis (TGA) results shown in Fig. 3. These results provide helpful information regarding the composition and phase structure of the samples. The TGA traces of samples in this figure show that two major weight loss events are observed in two distinct regions. The first weight loss behavior was due to degradation of P(tBS) that occurred in the temperature range of 200–300 °C, and the second weight loss was due to degradation of WTR (342–468 °C). Carbon black and some other additives remain in the form of residues at the temperatures above 500 °C. By comparing both TGA traces, it is evident that the amounts
of residues are different, which also supports the conclusion that the graft of tBS onto the WTR has occurred.
Fig. 3. Weight loss of WTR-g-tBS compared to WTR. Figure options
3.3. Oil absorptivity The effect of WTR concentration on oil absorbency is shown in Fig. 4. It is observed that the oil swelling equilibrium can be obtained within 1 h or so, and the maximum value of oil absorbency is 24.0 g g−1. This value is slightly lower compared to 30 times its weight for commercial oil sweep (http://www.dawginc.com). From Fig. 4, it can be seen that the oil absorbency of the composite particles increases with increasing feed ratio of WTR to tBS up to 60/40, after which it starts to decrease. This can be explained by the supposition that the carbon black and ash contained in WTR may act as filler, and play a role in physical crosslinks (Cai and Salovey, 1999 and Cai and Salovey, 2001). The physical crosslinking may weaken the chemical crosslinking strength by forming a relaxed network structure which has positive influence on the oil absorbency. On the other hand, the more the WTR, the more is the carbon black and ash, which may block the pores on the surface of the products, leading to a decrease of oil absorbency. Additionally, further increase of the feed ratio of WTR to tBS after 60/40 indicates the concentration of tBS is too small, and this will cause the structure of 4-tert-butyl group on the styrene to undergo reduced swelling (Zhou and Cho, 2001).
Fig. 4. Oil absorbency for grafted-blends having different feed ratios of WTR to tBS as a function of immersion time using 1 wt% DVB crosslinker in 10% diluted crude oil in toluene at 298 K. Figure options
To investigate the effect of the concentration of chemical crosslinking agent on the oil absorbency and gel fraction, we prepared various products with DVB contents (from 0% to 6% of the total monomer weight), with all other variables constant. Fig. 5 illustrates the relationship between oil absorbency and the amount of DVB. As shown in Fig. 5, when the amount of DVB is zero, the oil absorbency of 21.8 g g−1 is the second largest value. This can be explained by the C–S–C and S–S bonds almost been cleaved in the course of production WTR, and as a result there were little restorable vinyl groups on NR. In other words, because of its main component (NR), WTR may not only be used as comonomer, but act as the chemical crosslinker, resulting in the formation of slightly crosslinked network (Wu et al., 2006). There is a maximum point of oil absorbency, as the amount of DVB reaches 1 wt%, after which the oil absorbency decreased with increasing DVB amount. It is a well known fact that an increase in the amount of a chemical crosslinking agent causes the formation of a denser network of the copolymer and reduces the chain length between crosslinks, and thereby diminishes oil absorbency. Meanwhile, Fig. 5 shows that the gel fraction increases with an increase in DVB amount. The reason is that the higher the DVB concentration, the higher is the crosslinking efficiency, and the smaller is the soluble polymer; consequently, the gel fraction increases.
Fig. 5. Effect of the concentration of DVB on oil absorbency (■) and gel fraction of the graft-blends (○ ). Figure options
3.4. Absorption kinetics The swelling of the oil-absorption resin beads is determined by the properties of the solvent used as diluent during polymerization and the degree of crosslinking. A good solvent and slightly crosslinked polymer will give a high degree of swelling; a poor solvent and highly crosslinked polymer will cause a small degree of swelling. The kinetics of oil-absorption was determined by a direct measurement of the weight of the polymer beads as a function of time. The absorption kinetics of the samples can be written with the first-order kinetics equation (Yao and Zhou, 1994): equation(2)
Turn MathJaxon
where Qmax is the maximum oil absorbency; Qt is the oil absorbency at time t; and K is the swelling kinetic constant. Integration from Eq. (2) gives equation(3)
-ln(Qmax-Qt)=Kt+C Turn MathJaxon
where t is the swelling time, and C is the integration constant. When t = 0, Qt = 0, and C = − ln Qmax, the equation simplifies to equation(4)
Turn MathJaxon
The swelling kinetic constant K was determined from the slope in ln {Qmax/(Qmax−Qt)} versus t plots that are shown in Fig. 6. It can be seen that the oil-absorption processes in pure toluene and crude oil diluted with toluene basically obey the first absorption kinetics. Interestingly, the value of K for the for pure toluene is larger than that for crude oil diluted with toluene, which means the oil-absorption rate of grafted-blends in toluene is larger than toluene diluted crude oil. This is believed to be due to the fact that the viscosity of pure toluene is smaller than that of crude oil, and toluene molecules can easily diffuse into the crosslinked network.
Fig. 6. Variation of ln {Qmax/(Qmax-Qt)} with absorption time in (a) crude oil diluted with toluene (b) pure toluene. Figure options
4. Conclusions From the above-mentioned results, it can be concluded that the grafting of tBS onto WTR was feasible. In addition, this work provides another way for disposal of waste rubber. The modified WTR can be reused as oil absorbent by means of graft copolymerization-blending
method. It was found that the optimized reaction condition for maximum oil absorbency of WTR-g-tBS was 60/40 for the feed ratio of WTR to tBS, and 1% for DVB concentration. The gel fraction of grafted-blends increased with increasing concentration of DVB. Additionally, the oil-absorption processes in pure toluene and crude oil diluted with toluene were found to obey first-order absorption kinetics. Furthermore, the oil-absorption rate in diluted crude oil was observed to be lower than pure toluene. Despite the fact that the oil absorbency is lower than commercial products, the potential use of waste tyres for an oil absorbent product has been demonstrated.
Acknowledgements Our work was financially supported by the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, China (No. 0601).
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[SD-008] Corresponding author. Address: Department of Polymer Material and Engineering, Donghua University, State Key Laboratory for Modification of Chemical Fiber and
Polymer Materials, Shanghai 201620, People’s Republic of China. Tel.: +86 21 67796158.
