Continuous ultrasonic devulcanization of unfilled NR vulcanizates

Sulfur-cured unfilled natural rubber (NR) is successfully devulcanized in a continuous extrusion process under the application of high-power ultrasonic energy. The die characteristics and ultrasonic power consumption are measured. A unique correlation is found between the crosslink density and gel fraction of the devulcanized NR. This correlation is independent of the processing parameters, such as barrel temperature, die gap, flow rate, and amplitude of ultrasound. However, these parameters do influence the degree of devulcanization. In most cases, the degree of devulcanization is found to pass through a maximum at an intermediate level of ultrasonic energy. It is hypothesized that simultaneous breakup and reformation of crosslinks occur during the devulcanization of NR, with the relative contribution of each being determined by the process parameters. The cure curves and mechanical properties of the revulcanized NR are studied. The mechanical properties are found to depend on the revulcanization recipe. On optimizing it, tensile strength as high as 14.2 MPa is achieved, which is about 70% of that of the virgin NR vulcanizate. Ultimate elongation as high as 670% is obtained, which is the same as that of the virgin NR vulcanizate. Such stress–strain behavior is an indication that the devulcanized NR maintains the strain-induced crystallization characteristics inherent to the virgin NR vulcanizates. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2007–2019, 1998     

Continuous ultrasonic devulcanization of unfilled butyl rubber

The devulcanization of resin‐cured unfilled butyl rubber with a grooved‐barrel ultrasonic reactor under various processing conditions was carried out. The experiments indicated that, because of the lower unsaturation and good thermal stability of butyl rubber, its devulcanization could be successfully accomplished only under severe ultrasonic‐treatment conditions. Gel permeation chromatography measurements were carried out for the virgin gum and sol part of devulcanized samples to study the changes in the rubber network during the devulcanization process. The obtained data showed a significant molecular weight reduction and a broadening of the molecular weight distribution upon devulcanization, which indicated that the devulcanization and degradation of butyl rubber occurred simultaneously. The rheological properties showed that devulcanized butyl rubber was more elastic than the virgin gum. The vulcanizates of the devulcanized butyl rubber showed mechanical properties comparable to those of the virgin vulcanizate. The thermal behaviors of the virgin and devulcanized butyl rubber were different and were correlated to the double‐bond content. The structural characteristics of the devulcanized butyl rubber were simulated with the Dobson–Gordon theory of rubber network statistics. A fairly good agreement between the experimental data and theoretical prediction was achieved. The simulation of devulcanized butyl rubber indicated that the rate of crosslink rupture was much higher than that of the main chain. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1316–1325, 2004     

Continuous ultrasonic devulcanization of unfilled butadiene rubber

The devulcanization of sulfur‐cured unfilled butadiene rubber (BR) with a grooved‐barrel ultrasonic reactor under various processing conditions was carried out. The experiments indicated that BR had a narrow devulcanization window. Outside this window, significant degradation or no devulcanization occurred. Gel permeation chromatography (GPC) measurements were carried out with the sol part of virgin and devulcanized samples to study the breakdown of the polymeric chains. The GPC data showed a significant molecular weight reduction and a broadening of the molecular weight distribution upon devulcanization, indicating that the devulcanization and degradation of BR occurred simultaneously. The rheological properties showed that devulcanized BR was more elastic than the virgin gum. The vulcanizates of the blends of virgin and devulcanized BR showed a considerable enhancement of the mechanical properties. The thermal behaviors of the virgin and devulcanized BR were found to be different. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1166–1174, 2004     

Evaluation of fatigue performance of filled and unfilled natural rubber/styrene-butadiene rubber composites

This paper deals with the fatigue life analysis of a blend of natural rubber (NR) and styrene-butadiene rubber (SBR) with and without nanoclay particles. Various damage parameters based on strain are investigated. A nonlinear finite element analysis is carried out by using ABAQUS. To formulate the life prediction models, the measured fatigue life is used together with various damage parameters. It is shown that all the damage parameters can estimate the fatigue lives effectively with correlation coefficients greater than 0.9. There is a good agreement between the obtained fatigue live predictions and the measured fatigue results. The effect of various parameters such as true strain and nanoparticles' loading is also investigated. The results of sensitivity analysis show that the strain has a greater effect on the variation of the rubber compounds' fatigue life. The test samples' fracture surface is assessed via scanning electron microscopy (SEM). SEM results show that as the strain increases, the test samples softly fail while the fracture surface of the nanocomposite is roughened by the addition of nanoclay.     

亚临界流体挤出法对天然橡胶基轮胎胶粉脱硫化反应的影响

采用亚临界流体挤出法对天然橡胶基轮胎胶粉(GTR)与三元乙丙橡胶(EPDM)进行熔融共混脱硫,得到脱硫轮胎胶粉(DGTR)/EPDM共混物,然后与丁苯橡胶(SBR)共混制备了再硫化材料,考察了亚临界流体品种和双螺杆挤出机螺杆转速、挤出温度对GTR脱硫效果与再硫化材料物理机械性能的影响,并通过傅里叶变换红外光谱和扫描电子显微镜表征了脱硫共混物和再硫化材料。结果表明,在相同挤出温度和螺杆转速条件下,采用亚临界水挤出制得的DGTR/EPDM共混物的凝胶含量和门尼黏度较采用其他亚临界流体挤出共混物低;在3种亚临界醇状态下,采用亚临界甲醇挤出DGTR/EPDM共混物的凝胶含量较低,门尼黏度最高;采用亚临界醇所得DGTR/EPDM/SBR再硫化材料的拉伸性能均明显优于采用亚临界水所得的再硫化材料,其中,采用亚临界乙醇且在挤出温度为180℃、螺杆转速为500 r/min、反应压力为2.0 MPa的条件下,所制得DGTR/EPDM/SBR再硫化材料的拉伸强度和扯断伸长率分别达到19.4 MPa和456%;采用不同亚临界流体脱硫后制得再硫化材料中未熔融凝胶粒子尺寸大小不等,其中采用亚临界水时最小,小于1μm,分别采用亚临界甲醇、乙醇时较小,约为1μm,而采用亚临界丙醇时较大;随着反应温度的升高或螺杆转速的增加,再硫化材料中凝胶粒子的数目及尺寸均明显减小。     

Proton NMR studies of molecular-level mobility in vulcanised and epoxidised natural rubber

Proton spin-lattice relaxations in the laboratory and rotating frames, together with transverse relaxation (free induction decay), have been measured for natural and epoxidised rubber, plus a series of vulcanised samples. The data have been analysed by fitting procedures to give characteristic times which are discussed in terms of the mobility of the various parts of the samples, and of their compositions (including the effects of paraffin wax and oil additives). Interpretation has been aided by recording free induction decays following a period of spin-locking. Carbon-13 magic-angle spinning spectra have also been measured.     

Comparative analysis of ultrasonically devulcanized unfilled SBR,NR, and EPDM rubbers

The comparative study of the continuous ultrasonic devulcanization of various unfilled rubbers [natural rubber, styrene–butadiene rubber (SBR), ethylene–propylene–diene rubber (EPDM)] is carried out by means of a ultrasonic reactor. The power consumption, gel fraction, crosslink density, cure behavior, and physical properties of devulcanized rubbers were measured. The glass transition temperatures of virgin, vulcanized, and devulcanized rubbers were determined in order to characterize the difference in the mobility of rubber molecules for each rubber before and after devulcanization. Thermogravimetric analysis was also used to determine thermal stability of the various rubbers. A unique correlation between gel fraction and crosslink density indicated significant differences in the efficiency of devulcanization of various rubbers. Under certain devulcanization conditions, the mechanical properties of revulcanized SBR and EPDM rubbers were found to improve compared to those of the original rubbers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 434–441, 2003     

The structure change of dynamically fatigued unfilled natural rubber vulcanizates

The submicrostructure of dynamically fatigued unfilled natural rubber vulcanizates was investigated by using scanning electron microscope (SEM) and atomic force microscope (AFM). AFM photographs showed the sample surface roughness became worse after tensile fatigue and the largest surface undulation was as twice that of the unfatigued sample. SEM photographs showed that many micropores of 10¹–10² nm, a sort of defect, occurred on the cross section of samples after tensile fatigue. The surface roughness became weaker and the size of the micropore was reduced to a few to dozens of nanometers with the addition of antiaging agent N‐(1,3‐dimethyl butyl butyl)‐N′‐phenyl‐p‐phenylene diamine (4020); furthermore, the mechanical properties and dynamic viscoelastic properties in the later period of fatigue changed much. E′ decreased greatly and tan δ increased obviously with the extension of fatigue. It indicated that 4020 was only effective in the early period of tensile fatigue.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Transport of styrene monomer through natural rubber

The diffusion and transport of inhibitor-free styrene through crosslinked natural rubber (NR) have been studied at various temperatures. NR has been vulcanized by conventional, efficient, peroxide and mixed vulcanization techniques. The dependence of diffusion coefficient on the crosslinking system has been studied for all the systems. The influence of temperature on the sorption and the activation energies of sorption have been calculated. The interaction parameter, permeability, sorption coefficient and molecular weight between crosslinks have been evaluated using the diffusion data. The effect of degree of crosslinking on the sorption characteristics of styrene through NR has also been investigated for the different crosslinking systems. The peroxide system showed lowest uptake and the conventional system showed highest uptake.     

Influence of reversion in compounds containing recycled natural rubber: In search of sustainable processing

The influence of rubber devulcanization by microwaves in the reversion behavior is still modestly explored in the literature. The reversion occurs due to thermal degradation of unstable crosslinkings formed during the vulcanization process. This phenomenon results in poor final mechanical properties of the artifacts. In this work, some formulations based on natural rubber (NR) with the incorporation of NR devulcanized by microwaves at different exposure times to the microwaves were vulcanized by compression molding at the same temperature and time used for the study of their vulcanization characteristics, in order to correlate the properties obtained and understand the influence of the reversion on the mechanical properties. The results showed that levels of devulcanization/degradation of the recycled phase, as well as the additional heating time influenced on the behavior of reversion and, consequently, on the mechanical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45325.     

复合促进剂对双螺杆诱导废轮胎胶粉力化学脱硫反应的影响

利用自行组装的双螺杆挤出机对废轮胎胶粉进行力化学剪切脱硫,考察了过氧化物、烷基酚多硫化物及其复合促进剂对脱硫产物门尼黏度、凝胶含量以及共混天然橡胶硫化胶力学性能的影响。结果表明,过氧化物或烷基酚多硫化物均可促进力化学脱硫反应,当以过氧化物/烷基酚多硫化物为复合促进剂时可产生协同效应,起到共同促进力化学脱硫反应和保护产物双键的作用。在主机转速100 r/min、喂料与主机螺杆转速比1.4和180℃的脱硫条件下,以过氧化二异丙苯-烷基酚多硫化物480为复合促进剂时,二者的协同作用可使产物的门尼黏度和凝胶质量分数分别达到29.9和68.1%,脱硫产物共混天然橡胶硫化胶的拉伸强度和扯断伸长率分别达到19.3 MPa和588%。此外,根据红外光谱的表征结果推测了复合促进剂对废轮胎胶粉脱硫反应的可能机理。     

Properties of natural rubber vulcanizates containing mechanochemically devulcanized ground tire rubber

The devulcanization of ground tire rubber (GTR) was carried out with a self-designed pan-mill type mechanochemical reactor. Gel fraction and crosslink density measurements confirmed the occurrence of stress induced mechanochemical devulcanization of GTR. The partially devulcanized GTR (dGTR) was blended with virgin natural rubber (NR) at different ratios. The curing characteristics and mechanical properties of these composites were investigated and compared with those composites of raw ground tire rubber (rGTR) and NR. The results showed that the tensile properties of the dGTR/NR vulcanizates were much better than those of the rGTR/NR vulcanizates, which are comparable to or even better than the virgin vulcanizate, indicating the significant benefit of mechanochemical devulcanization. At the GTR content of 10%, the tensile strength of the dGTR/NR blends increased to 23.2 MPa from 13.7 MPa of the rGTR/NR blends, enhanced by 69% through partial devulcanization of GTR, and the elongation at break increased by 47%.     

Continuous ultrasonic devulcanization of NR/SBR blends

The ultrasonic devulcanization of sulfur‐cured natural rubber (NR)/styrene–butadiene rubber (SBR) blends was studied with the goal of understanding the devulcanization of rubber vulcanizates in which two networks of different natures were present. Also, similarities and differences in the devulcanization behaviors of NR, SBR, and their blends were found. During the devulcanization of cured NR/SBR blends, we observed that, as for NR, the ultrasonic power consumption for 75/25 and 50/50 (w/w) NR/SBR blends passed through a maximum at 7.5 μm. For SBR and 25/75 (w/w) NR/SBR blends, the power consumption increased with increasing ultrasonic amplitude. The higher power consumption led to a higher degree of devulcanization. The crosslink densities of the devulcanized 25/75, 50/50, and 75/25 (w/w) NR/SBR blends were lower than those of the devulcanized NR and SBR, possibly because of the reduced degree of unsaturation. The tensile properties of the revulcanized blends were lower than those of the virgin vulcanized blends. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 160–168, 2002     

Study on thermal degradation of sol and gel of natural rubber

The thermal degradation of natural rubber (NR) in air at a constant heating rate was studied by using of the thermogravimetry (TG) and thermogravimetry and differential thermal analysis (TG–DTA) simultaneous techniques. It indicates that the temperature of thermal degradation of gel and sol of NR rises linearly along with the increment of the heating rate, whereas the heating rate has little effect on the degree of thermal degradation. Accompanying other side reactions, the thermal degradation of NR is not a simple random chain scission process, and it is an exothermic reaction. The dynamic variation of molecular structure of NR during the thermal degradation was studied using Fourier transform infrared spectroscopy (FTIR). It shows that the products of the thermal degradation of both sol and gel of NR are hydroperoxide, carbonyl, and hydroxyl compounds. The formation of gel makes the temperature of the thermal degradation of NR decrease and the rate of the thermal degradation increase; thus, the thermal stability of NR is reduced. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1339–1344, 2000     

影响废旧轮胎胶粉热-机械剪切脱硫再生效果的因素

利用自行研制的长径比为60/1的强剪切型双螺杆挤出机对废旧轮胎胶粉进行热-机械剪切脱硫,用傅里叶变换红外光谱对脱硫胶粉进行了表征,考察了胶粉粒径和喂料螺杆转速与主机转速之比对脱硫胶粉凝胶含量、交联密度及再生胶力学性能的影响,以及脱硫过程中胶粉相对分子质量及其分布的变化。结果表明,随着胶粉粒径的增大,胶粉所受到的剪切力增强,脱硫效果也越好。随着转速比的增加,胶粉的脱硫程度提高;脱硫过程的第1个阶段是交联网络的破坏,第2个阶段除交联网络的破坏外分子主链也发生降解,再生胶的力学性能先增大后减小;当转速比为1.4时再生胶的扯断伸长率和弹性最好,拉伸强度也相对较高。     

Ozonation of unstretched natural rubber film from Hevea brasiliensis studied by ozone consumption and 13C NMR

Unstretched films of natural rubber (NR) from Hevea brasiliensis were exposed to ozone flow of 15 ml min^?1 from 4 to 300 min. The efficiency of reaction was determined by ozone consumption of NR films. Plots of reacted ozone mass versus film thickness show that the ozone penetration and the ozone reaction progressed into deeper layers (170 µm) than described in the literature (～0.5 µm). The previous proposed model based on viscometry measurements was corroborated by ozone consumption results. The effect of thickness on the O₃/NR stoichiometric ratio indicated that the diffusion process that controls the ozonation in unstretched film does not consist of the boundary progression behind which all reactive sites have been saturated. Ozonation in unstretched rubber film, while being less efficient than ozonolysis in solution, does have a reaction efficiency of the same order of magnitude. NMR spectroscopy was used to characterise the products formed by ozonation. Copyright © 2004 Society of Chemical Industry     

Characterization of epoxidized natural rubber by 2D NMR spectroscopy

Assignment of signals in aliphatic region of ¹H NMR spectrum for epoxidized natural rubber was carried out through NMR spectroscopy. The epoxidized natural rubber was prepared by epoxidation of purified natural rubber with peracetic acid in latex stage followed by degradation with propanal and ammonium persulfate. The resulting liquid epoxidized natural rubber was characterized through 1D- and 2D-NMR spectroscopy. The unknown signals in the aliphatic region of the ¹H NMR spectrum were assigned through ¹³C NMR and two-dimensional heteronuclear shift correlation (HETCOR) measurement. The assignments were proved by two-dimensional inverse detected heteronuclear long-range shift correlation (HMBC) and two-dimensional homonuclear shift correlation (COSY) measurements, and they were supported with epoxidized squalene as a model compound through NMR spectroscopy.     

Effect of natural coagula maturation on the processability,cure, and mechanical properties of unfilled vulcanizates of Hevea natural rubber

Fresh latex from different Hevea brasiliensis clones was naturally coagulated, subjected to different durations of maturation, processed into solid rubber, and compounded into pure gum stocks and vulcanized. Coagula maturation had clone‐specific effects on the processability of the raw rubber: reduced for some clones, while others was less sensitive. The cure and mechanical behaviors of the compounded stocks and vulcanizates, respectively, were not sensitive to the clonal origin of coagula and their duration of maturation. Although coagula maturation could be associated with leaching, deactivation of inherent antioxidants in Hevea latex, as well as crosslinking and/or oxidation of polyisoprene chains, these results show that compounding with a standard pure gum recipe compensates for the Hevea latex constituents affected by maturation. Hence, extended maturation of Hevea coagula, for economic or other reasons, would influence much more the bulk behavior of raw rubber and have insignificant effects on the compounded stocks and vulcanizates. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2359–2363, 2007     

Complete devulcanization of sulfur‐cured butyl rubber by using supercritical carbon dioxide

Sulfur‐cured butyl rubber was devulcanized completely in supercritical CO₂ by using diphenyl disulfide (DD) as a devulcanizing reagent. The optimum devulcanizing conditions were studied and the sol fraction of the reclaimed rubber obtained was up to 98.5%. The possible devulcanizing mechanism was investigated. Then, the sol component of reclaimed rubber was characterized by gel permeation chromatography, ¹H‐NMR, and differential scanning calorimetry, and the reclaimed rubber was characterized by TGA. Because of the substitution of a large portion of allylic hydrogen by sulfurated functional groups during vulcanization, the signal of the olefinic proton shift. As a result of the numerous decreases in the active crosslinking sites and the remaining DD, reclaimed rubber could not be cured by sulfur. At last, the blends of virgin butyl rubber and different contents of reclaimed rubber were revulcanized and their mechanical properties investigated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Mechanochemical devulcanization of ground tire rubber and its application in acoustic absorbent polyurethane foamed composites

The use of recycled rubber in preparation of acoustic absorbent materials will help to combat the existing environmental problems of both waste disposal and noise pollution. The focus of this work is to investigate the influence of mechanochemical pretreatment of ground tire rubber (GTR) on the acoustic absorption properties of polyurethane (PU)/GTR foamed composites. GTR subjected to pan‐milling could be mechanochemically devulcanized by breaking up the crosslinked structures through inducing fairly strong shearing and compressing forces. The significant increase in sol fraction of GTR confirmed the partial devulcanization during pan‐milling. Moreover, thermal gravimetric analysis indicated that rubber content in the soluble part of GTR was also remarkably increased. The devulcanization increased flexible chains of the GTR particles, which could help to improve damping properties as well as acoustic absorption ability of the PU/GTR foamed composites. Dynamic mechanical analysis and acoustic absorption measurements well confirmed this hypothesis. The loss modulus and sound absorption coefficient of PU/GTR foamed composites were remarkably increased through the mechanochemical pretreatment of GTR. The mechanochemical pretreatment also enhanced foamability of the composites as revealed by cell morphology. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013