Continuous recycling of vulcanisates

Transcription

1 Gummi Fasern Kunststoffe, No., 2, p. 48 Continuous recycling of vulcanisates K. Fukumori, M. Mouri, N. Sato, H. Okamoto and M. Matsushita * Translation submitted by C. Hinchliffe Selected from International Polymer Science and Technology, 28, No. 2, 2, reference GK //48; transl. serial no SUMMARY Vulcanised EPDM rubber waste produced during the manufacturing process is devulcanised using a screw granulator under controlled conditions of shear stress, temperature and internal stress. During this recycling process, carbon-sulphur bonds are broken selectively so that a devulcanised rubber consisting of a sol component and a gel component is obtained which can be processed like new rubber and revulcanised with an accelerated sulphur curing system. The revulcanisates exhibit nearly the same properties as cured new rubber. EPDM rubber which has been devulcanised using the new process is used for the production of rubber products for the automobile industry.. INTRODUCTION In recent times, increased importance has been attached to the recycling of waste in industries all over the world. As far as rubber products are concerned, the automobile industry and transport industry are the greatest consumers of new rubber. Rubber waste is produced during the manufacturing processes and by the consumers, in the form of worn out products, for example worn tyres. In Japan, about million tonnes of old tyres are produced each year. From the point of view of the energy balance, the material recycling of rubber waste should be given preference over all other recycling processes. However, in 997, for example, material recycling in the form of ground tyres and reclaim was only used for about 2% of old tyres (ref. ). Technological developments in the tyre sector, such as steel-belted tyres and radial tyres have resulted in limits being placed on the quantity of recycled rubber used in high-quality rubber compounds for tyres. One of the oldest and simplest devulcanisation processes in the rubber industry is known as the ladle process (ref. 2). In this process, fine ground rubber crumbs mixed with oil and devulcanisation agents are heated with steam in a pressurised vessel for h to a temperature of about 2 C. This process should usually be followed by other types of treatment (refining and straining) to obtain the regenerated rubber in its final state. The physical properties of the regenerated rubber produced using this procedure are greatly inferior to those of new rubber as the bonds are broken completely randomly in both the crosslink points and in the main molecule chain (C C bonds). For this reason, new technologies have been developed for material recycling, such as microwave devulcanisation (ref. ) and ultrasound devulcanisation (ref. 4) with the objective of short reaction times. However, the quality of the rubbers regenerated with these procedures is not good enough for them to be used on a large scale in practical products. In addition to tyres, numerous other rubber products are used in cars, for example door and window seals, hoses, vibration insulators and many small rubber parts whose weight accounts for only about % of the overall weight of the car. Here, the use of ethylene-propylene-diene rubber (EPDM) accounts for approximately half of the total amount of polymer used (ref. ). For this reason, in this study we developed a new continuous recycling process for vulcanised EPDM waste with the objective of obtaining high-quality, regenerated, i.e. devulcanised rubber. * K. Fukumori, M. Mouri, N. Sato, H. Okamoto, M. Matsushita: all from Toyota Central Research & Dev. Labs, Inc. Aichi, Japan H. Honda, K. Nakashima, Totoda Gosei Co. Ltd., Aichi Y. Suzuki, M. Owaki, Toyota Motor Corp. Aichi International Polymer Science and Technology, Vol. 28, No. 6, 2 T/

2 2. EXPERIMENTAL PART 2. Material Waste from vulcanised EPDM, such as that which typically occurs during the production of window and door seals and other articles was used in this study. The waste from vulcanised rubber was broken up roughly into pieces of approximately mm in size for the subsequent recycling process. 2.2 Equipment A newly developed recycling procedure for rubber is based on the controlled shear-flow reaction (shear-flow stage reaction control technology SRCT). With this technology, different types of chemical reactions may be effectively controlled to optimise shear stress, temperature and pressure (ref. ). In the experimental studies, continuous devulcanisation of EPDM waste was performed in a reactor fitted with a modular screw with a diameter of mm and a length of 26 mm, as shown in Fig.. The screw geometry and the configuration with modular screw elements (dextro-rotary screw, kneading discs, etc.) were arranged suitably for the continuous devulcanisation process. In the first zone of this reactor, coarsely shredded rubber material is crushed into fine particles by a strong shear stress and then heated to the temperature at which devulcanisation takes place. Care is taken to ensure that the dwell time in this zone is sufficient to conclude the devulcanisation reaction under the shear flow stress in the next zone. In this reaction zone, the particles are greatly elongated by compaction and the shear stress from the kneading discs and hence possibly plasticised. The devulcanisation temperature and the screw speed were varied with the aim obtaining a good surface quality of the extrudate from the devulcanised extrudate. The feeding rate was approximately kg/h. After the devulcanisation process, the devulcanised rubber is extruded from the head of the reactor, cooled in a water bath and continuously pelleted. The overall duration of the process is approximately minutes. In some cases, a process oil (PW-8, Idemitsu Petrochemicals Co.) and a devulcanisation agent (diphenyl disulphide) were added. Table gives an overview of the conditions during the devulcanisation of the EPDM waste. Table Devulcanisation conditions Test no Tests Devulcanisation agent, phc. Process oil, phc Temp., C Note: phc parts per parts of compound Screw speed, rpm Measurements of the Mooney viscosity and the molecular weight distribution of the (soluble in toluene) sol component and the crosslink density (i.e. the effective density of the network chains) of the (insoluble) gel component were performed on the devulcanised rubbers obtained under different conditions (Table ); here we used conventional methods of investigation. An accelerated sulphur vulcanisation system corresponding to the formulation shown in Table 2 for new rubber was added to the devulcanised rubbers. We analysed the vulcanising characteristics at 6 C and deformation properties, such as compression moulding, extrusion and extrusion-foam behaviour in the rubber compounds produced using standard machines. The tensile properties and rubber hardness were determined on the revulcanised rubber systems under normal ambient conditions. Shredded rubber waste Cooling section Pelleting Grinding process Devulcanisation Fig. Schematic diagram of the reactor for devulcanisation T/6 International Polymer Science and Technology, Vol. 28, No. 6, 2

3 Table 2 Mixing formulation for devulcanised rubber Table Characteristic values of the devulcanised rubber Devulcanised Zinc oxide Stearic acid Sulphur TMTD MBT mixture phr The time-dependent changes, which take place during the devulcanisation process under typical test conditions in the rubber matrix, were determined by quickly removing the screw from the reactor cylinder during operation and taking samples of material from along the screw length. The samples of material taken from the different points were analysed to determine viscosity (using capillary rheometer measurements), the proportions of the sol and gel components, the molecular mass distribution of the sol component and the effect network chain density of the gel component. In this study, we also analysed the changes to the network structure of these materials when using chemicals such as thiol/amine reagents, which are used to break up sulphur bonds. A mixture of propan-2-thiol and piperidine in heptane was used to break up polysulphide bonds. Similarly, hexan--thiol in piperidine may be used to break up polysulphide and disulphide bonds (ref. 7). These two chemicals and swelling measurements in the solvent may be used to obtain a complete characterisation of the de-crosslinked structure formed during the devulcanisation process. Test no A B Mooney viscosity, ML Gel content Effective network chain density of the gel component x mol/cm , Note: A new rubber used; B vulcanised new rubber (systems Nos. to No. and No. 8). It was found that the curves for the molecular weight distribution of the rubber devulcanised at temperatures of less than C are virtually identical to those of the curve obtained for new rubber.. RESULTS AND DISCUSSION. General properties of the devulcanised mixtures The Mooney viscosity, the gel content and its effect network chain density for the devulcanised mixtures are listed in Table. The Mooney viscosity increases as the temperature rises during the devulcanisation process. The Mooney values for the devulcanised rubbers treated at about C without the addition of oil and devulcanisation agent seem to be almost the same as those of a mixture with new rubber. Increasing the screw speed causes a slight drop in the viscosity. The addition of a devulcanisation agent also causes a drop in viscosity. The gel content and its effective network chain density fall as the temperature increases. At the same temperature, the addition of oil or devulcanisation agent causes a decrease in the effective network density, while the gel content remains almost constant. Figure 2 shows the change in the molecular weight distribution of the sol component as the temperature increases Intensity New rubber Sol components of the devulcanised rubber Molecular mass Test No Test No 2 Test No Test No 4 Fig. 2 Molecular mass distribution of the sol component International Polymer Science and Technology, Vol. 28, No. 6, 2 T/7

4 Linear polymer chain (sol component) Loosely crosslinked network chain (gel component Crosslink point New rubber Devulcanised rubber Fig. Model for the structure in the devulcanised rubber The curve obtained for system No. (devulcanised at C) has shifted to the area of lower molecular masses due to the occurrence of chain splitting. This indicates that chain splitting (the splitting of C C bonds) cannot occur during the devulcanisation processes of systems Nos., 2 and 8, but that in the main selected breaks occur at the crosslink points (C S or S S bonds). The devulcanised rubbers consist of the sol component, which is soluble in toluene, and the insoluble gel component. In the case of system No. 8, which was devulcanised without additives, the contents of the sol and gel components were determined as 44 % and 6 % respectively. It was also established that the effective network chain density of the gel component was a twentieth of the value of the original vulcanised rubber. Fig. is a schematic diagram of the spatial heterogeneity in the vulcanised rubber, which seems to consist of mixtures of uncrosslinked polymer chains, assigned to the sol component, and loosely crosslinked polymer chains, assigned to the gel component. Figure 4 shows typical vulcanisation curves for the mixtures of devulcanised rubber compared to a mixture containing new rubber. System No. 9 (devulcanised without additives) seems to have normal vulcanisation properties, which are very similar to those of the mixture containing new rubber. In the case of systems Nos. and (devulcanised using additives), on the other hand the increase in the torque during the vulcanisation process is less pronounced and the vulcanisation process takes place more slowly than it does in the mixture containing new rubber..2 Tensile properties The tensile strength and breaking strength of the re-vulcanised systems of devulcanised mixtures are shown in Table 4. Table 4 Mechanical properties of revulcanised rubbers Test no. Hardness, JIS A Tensile strength, MPa Elongation at break, % Stress, MPa Vulcanised new rubber Fig. 4 Rheometer torques Elongation, % Revulcanised rubber VNk * Note: Vnk* vulcanised new rubber T/8 International Polymer Science and Technology, Vol. 28, No. 6, 2

5 Mixture containing new rubber (a) Compression moulding Torque, N m (b) (c) Extrusion Extrusion foaming Fig. 6 External properties of rubber products Vulcanisation time, min Fig. Typical stress elongation curves for revulcanised rubbers Figure contains typical stress-strain curves for revulcanised systems Nos. 9, and. System No. 9 has excellent tensile properties, almost comparable with the system with new rubber. Systems Nos. and on the other hand have lower values for the modulus of elasticity tensile strength and high breaking strength values. The results shown in Fig. reveal that if the devulcanisation reactions are suitably controlled, it is possible to obtain revulcanised systems with a wide range of tensile properties.. Thermoformability To take into account engineering aspects, we investigated the moulding properties of the mixture with devulcanised rubber in system No 8 during the compression moulding, extrusion and extrusion-foam processes shown in Fig. 6. With the compression moulding procedure shown in Fig. 6a, the plasticity and other properties of these mixtures were found to be quite comparable with those of new rubber. The surface quality and the density of the products are also the same as those of new rubber. With the extrusion process shown in Figure 6b, the surface quality of the products is also the same as it is with new rubber, while the injection swelling is slightly too high. In addition, it was determined that the extrusionfoam process is also possible using a blowing agent (Fig. 6c) and that the density of the foam produced may be controlled in a suitable way. This confirms that devulcanised rubber produced with the procedure developed may be used in a similar way to new rubber in the different moulding processes..4 Time-dependent change to the network structure during the devulcanisation process The time-dependent changes which occur in the rubber matrix and network structure during the devulcanisation process were determined on the material samples taken at different places along the axial length of the screw (Fig. 7). In the early stage of the process, after the material has been fed into the machine (Fig. 7, position A), the coarsely shredded pieces of rubber are ground into fine particles and heated to devulcanisation temperature by the addition of heat and shear stress from the screw rotation. In the middle stage of the process, shown as position B in Fig. 7, the vulcanised rubber powder is plasticised by compaction and kneading stress and the dwell time is set so it is sufficient for the devulcanisation reaction. In the last stage of the process, shown as positions C and D, the devulcanised rubber containing both soluble and insoluble components is converted to a homogeneous mixture by the kneading stress. Material feed Grinding zone Devulcanisation zone Position: A B C D Position A Position B Position C Position D Fig. 7 Sampling points along the screw axis International Polymer Science and Technology, Vol. 28, No. 6, 2 T/9

6 Viscosity, Pa s 6 Not measurable 2 C Deformation rate /s Gel content, % 4 6 Effective network chain density, log (v/mol cm ) 4 A B C D,E Position Vulcanised rubber A B C D,E Position Fig. Changes to the gel content and the network chain density of the gel along the screw axis 7 Fig. 8 Changes in viscosity value along the screw axis Mean molecular mass of the sol component x 4 Sol content ~ A B C D,E New rubber Position used Fig. 9 Changes in the mean molecular mass of the sol components along the screw axis Figures 8 and 9 show the changes to the viscosity values and the mean molecular weight of the sol component during the process. From position B, at which the devulcanisation starts, the viscosity starts to decrease, while from position B to position D the mean molecular weight of the sol component is virtually constant and comparable with that of new rubber. Fig. shows the changes in the gel component and in its network chain density during process. The proportion of the gel component falls as the devulcanisation reaction progresses, which is particularly clear from position C, and finally remains virtually constant. The network chain density of the gel component falls as the devulcanisation progresses and at position D is about a twentieth of the value of the vulcanised rubber. Figure a shows the changes in the contents of mono-, diand polysulphide bonds in the network structure during the process. With vulcanised rubber, originally the contents of mono-, di- and polysulphide bonds were determined as 2%, 2% and 6% respectively. In the early stage of the process, the number of disulphide and the number of polysulphide bonds fall, while the number of monosulphide bonds increases. In the last stage of the process, the number of monosulphide bonds falls and is finally negligibly small. If, on the other hand, the vulcanised rubber is only heated in a pressurised vessel under nitrogen, the number of polysulphide and disulphide bonds falls, while the number of monosulphide bonds rises. During the devulcanisation process, the disulphide and polysulphide bonds are first converted to monosulphide bonds, and then the monosulphide bonds are split under high shear stress and internal pressure, as shown in Fig. b. Effective network density, x - mol/cm Polysulphide bond Polysulphide bonds Disulphide bonds Original material Disulphide bond main chain Vulcanised bond Monosulphide bonds A B C D,E Monosulphide bond Heated only Fig. Changes to the network structure: (a) number of mono-, di- and polysulphide bonds; (b) devulcanisation mechanism a) b) Devulcanised rubber T/ International Polymer Science and Technology, Vol. 28, No. 6, 2

7 (a) (b) Bonding energy Elastic constant (relative value) k CC (~) k CS (k CC >k CS >k SS ) k SS (~) E CC :7 kj/mo E CS kj/mo E SS :27 kj/mo Shear flow stress Fig. 2 Breaks in the crosslink points with strong shear flow: (a) model of the network chain; (b) deformation of the network chain (particularly the S S bonds) due to shear stress Figure 2 is a basic representation of the splitting processes in the transverse bonds under strong shear stress. There is a slight difference in the bonding energy between C C bonds and C S or S S bonds. Therefore, in the case of simple heating in a pressurised vessel, the splitting of C C and C S or S S bonds may take randomly, i.e. unselectively. This results in a deterioration of the physical properties in the rubbers regenerated with the conventional procedure. If, one the other hand, we consider the spring constant k for these bonds (estimated approximately on the basis of the values for crystals), the k value for the S S bonds is determined as approximately one thirtieth of the value for the C C bonds, as shown in Fig. 2a. In general, the physical behaviour of the vulcanised rubbers may be viewed as being determined in the main by the entropy term in the deformation energy. Contrary to this entropy-determined deformation behaviour, in the case of extremely high shear stresses, caused by compaction and kneading stress in the reactor, it may happen that the majority of the rubber-type molecules are fully stressed to the limit of their extensibility. Under these conditions, the spring with the lower spring constant (the S S bonds) may be extended more than the spring with the higher spring constant (the C C bonds), as shown in Fig. 2b in an elastic model. This means that the elastic energy generated by the high shear stress may be particularly concentrated on the S S bonds, which results on the selective breaking of the (corresponding) crosslink points. 4. CONCLUSIONS This study developed a continuously recycling procedure for vulcanised rubber using a reactor with a modular screw. The devulcanised rubber obtained using this procedure has excellent physical properties, which are comparable with those of the corresponding natural rubber. In the reactor equipped with an optimised screw geometry and configuration, the action of heat, shear stresses and internal pressure may predominantly lead to the fracture of crosslink points (C S or S S bonds). This kind of reaction contributes to the improved properties of the regenerated rubber. In 977, this technology was put into practical use for the material recycling of waste from door and window seals. The devulcanised rubbers are currently being used to produce rubber parts for cars. It is also expected that this procedure may be used for recycling various other types of rubber, such as NR, SBR, IIR etc. REFERENCES. Tire Industry of Japan, Jatma, 'Recycling of Rubber', Rapra Report 99, 9, 997. Goodyear Tire & Rubber Co. Ltd., Plast. Rubb. News, Sept. 9, A.I. Isayev, J. Chen and A. Tukachinsky, Rubber Chem. Technol., 68, 99, p Y. Suzuki, M. Owaki, M. Mouri, N. Sato, H. Honda and K. Nakahima, Toyota Technical Review, 48, 998, p. 6. N. Sato, H. Takahashi and T. Kurauchi, SAE Paper No. 9629, Monsanto Report, July, 98 (No date given) International Polymer Science and Technology, Vol. 28, No. 6, 2 T/