Chain Dynamics and Strain-Induced Crystallization of Pre- and Postvulcanized Natural Rubber Latex Using Proton Multiple Quantum NMR and Uniaxial Deformation by in Situ Synchrotron X-ray Diffraction

A Study on the Preparation of a Vulcanizing Mixture and Its Application in Natural Rubber Latex

The traditional preparation process of natural rubber latex requires tedious treatment of a variety of rubber additives. In this paper, a new process of wet mixed grinding was used to prepare a reinforced vulcanization mixture and a rapid vulcanization effect. The effect of different amounts of vulcanization mixtures on the mechanical properties of natural latex film was studied, and the pre-vulcanization process of latex and the vulcanization process of film were optimized. The results showed that with the increase in the amount of vulcanization mixture, the tensile strength increased from 5.96 MPa to 29.28 MPa, and the tear strength increased from 7.59 kN/m to 52.81 kN/m. When the vulcanization temperature of the latex film is heated from 80 °C to 100 °C, the vulcanization time is shortened by 5~6 times. The new vulcanization mixture prepared in this work has the characteristics of simple production and fast vulcanization speed, which provides a new solution for the development of the latex product industry.

Dynamic Permittivity Measurement of Ground-Tire Rubber (GTR) during Microwave-Assisted Devulcanization

Many efforts are being made to find innovative ways of recycling rubber from end-of-life tires (ELTs), also called ground tire Rubber (GTR). Recycling through devulcanization allows the reintroduction of rubber back into the manufacturing industry. Such a process requires providing enough energy to break the sulfur links, while preventing damage to the polymeric chain. Microwave heating is controllable, efficient, and it does not rely on conventional heating mechanisms (conduction, convection) which may involve high heating losses, but rather on direct dielectric heating. However, to adequately control the microwave-assisted devulcanization performance, a thorough knowledge of the GTR permittivity versus temperature is required. In this work, GTR permittivity was monitored during its devulcanization. A resonant technique based on a dual-mode cylindrical cavity was used to simultaneously heat rubber and measure its permittivity at around 2 GHz. The results show sharp changes in the GTR permittivity at 160 and 190 °C. After the GTR cooled down, a shifted permittivity evidences a change in the GTR structure caused by the devulcanization process. Microwave-assisted devulcanization effectiveness is proven through time-domain nuclear magnetic resonance (NMR) measurements, by verifying the decrease in the cross-link density of processed GTR samples compared to the original sample.

ADVANCED CHARACTERIZATION OF RECYCLED RUBBER FROM END-OF-LIFE TIRES

There are currently many well-established applications for recycled rubber from end-of-life tires (ELT), but it is essential to investigate and seek new approaches to enhance the value of these products. Recent developments in new technologies and innovative recycling and devulcanization processes have opened up new perspectives for ELT crumb rubber. To promote the use of these products in newly added value applications, it is essential to develop and optimize methods that allow the characterization of parameters related to the ultimate properties of potential final applications. In this respect, a novel characterization methodology based on advanced 1H double-quantum (DQ) nuclear magnetic resonance experiments has been applied for the first time to quantify the key parameters that characterize the structure of ELT crumb rubber after diverse recycling processes: from simple mechanical grinding to complex devulcanization methods. This experimental approach enables the quantification of parameters that define the network structure of rubber, such as the nonelastic network defects (sol fraction, dangling chain ends, loops), the cross-link density, and the heterogeneity of the network, directly from rubber granulate and powder (without any additional sample preparation steps), overcoming most of the drawbacks and uncertainties that limit the application of traditional rubber characterization methods (e.g., equilibrium swelling experiments). By applying this experimental approach, it is possible to identify and quantify the actual technical limits for a complete selective devulcanization process of ELT crumb rubber.

Ultra-Tough, Strong, and Defect-Tolerant Elastomers with Self-Healing and Intelligent-Responsive Abilities

Mechanical strength, toughness, and defect tolerance are usually exclusive in most artificial materials. Herein, inspired by many biomaterials that overcome this tradeoff by integrating soft and hard ingredients through elaborate structural designs, we report a facile latex-assembly method to fabricate ultra-tough, strong, and defect-tolerant elastomers. The elastomers are featured by a microscopic inverse opal-mimetic rigid skeleton of dynamically cross-linked chitosan and a continuous soft matrix of vulcanized natural rubber. Such structural design enables the load-bearing capability, sacrificial property, and self-healing ability of the skeleton, the stress redistribution and extensibility of the matrix, and the stiffness variation between hard and soft ingredients, thereby imparting the elastomers with outstanding mechanical strength and defect tolerance, as well as extremely high toughness of 122 KJ m^-2, which is even higher than that of the current state-of-the-art titanium alloys. Moreover, the elastomers show prominent humidity sensitivity due to the hydrophilic nature of the chitosan skeleton. Harnessing these advantages, we fabricate a walking robot triggered by humidity variation and shoes that are able to regulate temperature and humidity. The concept of designing a rigid sacrificial skeleton within a soft continuous matrix on the microscale is quite general, enabling the development of high-performance and intelligent materials for emerging applications.

Time Domain NMR in Polymer Science: From the Laboratory to the Industry

Highly controlled polymers and nanostructures are increasingly translated from the lab to the industry. Together with the industrialization of complex systems from renewable sources, a paradigm change in the processing of plastics and rubbers is underway, requiring a new generation of analytical tools. Here, we present the recent developments in time domain NMR (TD-NMR), starting with an introduction of the methods. Several examples illustrate the new take on traditional issues like the measurement of crosslink density in vulcanized rubber or the monitoring of crystallization kinetics, as well as the unique information that can be extracted from multiphase, nanophase and composite materials. Generally, TD-NMR is capable of determining structural parameters that are in agreement with other techniques and with the final macroscopic properties of industrial interest, as well as reveal details on the local homogeneity that are difficult to obtain otherwise. Considering its moderate technical and space requirements of performing, TD-NMR is a good candidate for assisting product and process development in several applications throughout the rubber, plastics, composites and adhesives industry.

VULCANIZATION CHARACTERISTICS OF NATURAL RUBBER COAGULATED BY MICROORGANISMS

The network variations of natural rubber (NR) during the vulcanization process were investigated by 1H chemical shift by liquid-state 1H nuclear magnetic resonance (NMR) spectroscopy. NR latex coagulated by microorganisms (NR-m) was contrasted with NR latex coagulated by acid (NR-a). The influences of the coagulation process on the structures, vulcanization characteristics, and mechanical properties of NR were analyzed. The results show that the cross-link density (XLD) and mass percentage of cross-link network (A(Mc)) can be increased with the increment of the vulcanization time; while the mass percentage of dangling free ends of the hydrocarbon and small molecules (A(T2)), the longitudinal relaxation time (T1), transverse relaxation time (T2), and molecular mass of inter–cross-link chains (Mc) decreased with the prolonging of vulcanization time both NR-m and NR-a. NR-m exhibits shorter scorch times (ts1, ts2) and optimum cure time (t90) and shows higher maximum torque (MH) and minimum torque (ML) than that of NR-a. It is obvious that the higher XLD and A(Mc) and lower A(T2), T1, T2, and Mc values of NR-m result in higher stress, tensile strength, and tear strength of NR compounds.

INVESTIGATION OF THE VULCANIZATION CHARACTERISTICS OF NATURAL RUBBER COAGULATED BY MICROORGANISMS

The network variations of NR during the vulcanization process were investigated by 1H chemical shift by liquid-state 1H-NMR spectroscopy. NR latexes coagulated by microorganisms (NR-m) or acid (NR-a) were contrasted. The influences of coagulation on the structures, vulcanization characteristics, and mechanical properties of NR were analyzed. The results show that the cross-link density (XLD) and mass percentage of cross-link network [A(Mc)] increased with the increment of the vulcanization time, whereas the mass percentage of bangling free ends of the hydrocarbon and small molecules [A(T2)], the longitudinal relaxation time (T1), the transverse relaxation time (T2), and the molecular mass of inter–cross-link chains (Mc) decreased with the prolonging of vulcanization time for both NR-m and NR-a. Although NR-m exhibits shorter scorch times and optimum cure time, it shows higher maximum torque and minimum torque than that of NR-a. It is obvious that the higher XLD and A(Mc), the lower the A(T2), T1, T2, and Mc values of NR-m, resulting in higher stress, tensile strength, and tear strength of NR compounds.