Crosslink densities and phase morphologies in thermoplastic vulcanizates

Polyolefin Blends with Selectively Crosslinked Disperse Phase Based on Silane-Modified Polyethylene

Polypropylene-based multiphase compositions with a disperse elastomer phase provide superior impact strength. Making this property indifferent to processing steps requires stabilization of the morphology of these materials. Various approaches have been tested over time, each of which shows limitations in terms of performance or applicability. Using polyethylene (PE) homo- and copolymers capable of silane-based crosslinking as modifiers was explored in the present study, which allows decoupling of the mixing and crosslinking processes. Commercial silane-copolymerized low-density PE (LD-PEX) from a high-pressure process and silane-grafted high-density PE (HD-PEX) were studied as impact modifiers for different types of PP copolymers, including non-modified reference PE grades, LDPE and HDPE. Blends based on ethylene-propylene random copolymers (PPR) and based on impact- (PPI) and random-impact (PPRI) copolymers show improvements of the stiffness-impact balance; however, to different degrees. While the absolute softest and most ductile compositions are achieved with the already soft PPRI copolymer base, the strongest relative effects are found for the PPR based blends. Modifiers with lower density are clearly superior in the toughening effect, with the LD-PEX including acrylate as second comonomer sticking out due to its glass transition around -40 °C. The impact strength improvement found in most compositions (except at very high content) results, however, not from the expected phase stabilization. For comparable systems, particle sizes are normally higher with crosslinking, probably because the process already starts during mixing. Thermoplastic processability could be retained in all cases, but the drop in melt flow rate limits the practical applicability of such systems.

The Effect of Cross-Linking Type on EPDM Elastomer Dynamics and Mechanical Properties: A Molecular Dynamics Simulation Study

The cross-linking structure of the Ethylene-propylene-diene monomer (EPDM) is made up of a number of cross-linking types, including carbon atoms from the main chain or monomer and ether crosslinks formed during degradation. Through molecular dynamic simulations, the contribution of each type of cross-linked structure to the dynamics and mechanical properties of EPDM, the study’s focus, were investigated. Cross-linking between the tertiary carbons of two main chains, cross-linking at the monomer’s unsaturated position, ether cross-linking after oxidation, and other combinations of target cross-linked carbon atoms from different positions, totaling eight types of cross-linked types, were mixed with EPDM free chains in a 1:1 ratio to form eight types of cross-linked EPDMs. These varieties of cross-linked EPDMs were then compared to an uncross-linked EPDM in terms of density, radius of gyration, free volume, mean square displacement, and uniaxial tensile stress-strain curves. It was found that the cross-linking was always proven to have a favorable influence on mechanical characteristics; however, the relaxation inhibition effect varied. The cross-linking between the diene monomer at the C9 position resulted in a more flexible molecular shape and was more than double the free volume of the uncross-linked EPDM, resulting in an improved diffusion ability. The ether cross-linking produced by the oxidation of the side chain cross-linking improved the positive contribution to stiffness and enhanced the inhibitory impact on diffusion properties, whereas the main chain cross-linking had the opposite effect. The research presented in this study leads to a better knowledge of the microscopic aspects underlying EPDM performance.

Green Thermoplastic Vulcanizates Based on Silicone Rubber and Poly(butylene succinate) via In Situ Interfacial Compatibilization

Presenting a combination of sustainability and environmental friendliness, a new class of green and non-petroleum-based thermoplastic vulcanizates (TPVs) was successfully developed from silica-filled silicone rubber (FSR) and poly(butylene succinate) (PBS) via dynamic vulcanization. The phase morphology, interfacial compatibilization, and microstructural properties of FSR/PBS TPVs were investigated. Notably, a large number of FSR microparticles were observed and were dispersed in the continuous PBS phase, indicating complete phase inversion during the dynamic vulcanization. The fine phase morphology of FSR/PBS TPVs was achieved by a fine phase morphology of the SR/PBS premix, the good interfacial compatibility between the PBS phase and the cross-linked FSR phase, and complete phase inversion. The as-prepared TPVs possessed high tensile strength, good elastic behavior, easy processability, and reprocessability. These novel non-petroleum-based TPVs have potential applications in packagings, biomedical devices, and three-dimensional (3D) printing materials.

Morphology Evolution and Rheological Behaviors of PP/SR Thermoplastic Vulcanizate

The thermoplastic vulcanizates (TPVs) of polypropylene (PP)/silicone rubber (SR) were prepared by dynamic vulcanization (DV) technology. The mixing torque, morphology, viscoelasticity, and creep response of PP/SR TPVs were investigated by torque rheometer, scanning electron microscope (SEM), transmission electron microscope (TEM), rotational rheometer, and dynamic mechanical analysis (DMA). A mixing-torque study showed that torque change and dynamic-vulcanization time increased with SR content increasing in the DV process, but DV rate was independent of SR content. TEM images indicated that the phase inversion of PP/SR-60 TPV from bicontinuous to a sea⁻island structure took place in the DV process, and a hot press would break the rubber aggregates and shrink a large SR phase. Dynamic-strain measurement demonstrated that PP/SR TPVs exhibit a distinct "Payne effect", which can be attributed to the destruction and reconstruction of SR physical networks. Complex viscosity indicated that SR content did not affect the processability of PP/SR TPVs at high shear rates. Furthermore, the creep deformation and recovery of PP/SR TPVs at solid and melt states were studied, respectively.

Properties of EPDM/PP thermoplastic vulcanizates produced by an intermeshing-type internal mixer comparing with a co-rotating twin-screw extruder

A unique technology for producing thermoplastic vulcanizate (TPV) has been developed using an intermeshing-type internal mixer (ITM). TPV produced by the ITM was compared with that produced using a co-rotating twin screw extruder (Co-TSE) to assess the former’s commercial possibilities. TPV, originating from ethylene-propylene-diene monomer (EPDM) and polypropylene (PP), was produced by both machines with equal filled volumes, same shear rate, and same specific mechanical energy. Results indicate that ITMs can be used to produce TPV with mechanical properties comparable to those of TPV produced by Co-TSE. TPV can be produced with a lower shear rate with ITM compared to Co-TSE. A long residence time can be maintained in the mixing chamber of the ITM, allowing high conversion of the cross-linking reaction and resulting in better elastic recovery properties due to the higher cross-linking density. However, this resulted in higher viscosity of TPV produced by ITM.

Thermoplastic vulcanizates based on waste truck tire rubber and copolyester blends reinforced with carbon black

Devulcanized rubber (DR) was prepared from waste truck tire rubber via thermo-chemical devulcanization process. Thermoplastic vulcanizates (TPVs) based on blending of DR and copolyester (DR/COPE) were prepared. Effects of carbon black loading on microstructure, mechanical properties and elastomeric behaviors of dynamically cured DR/COPE blends were investigated. It was found that increasing of the carbon black loadings leads to transformation from co-continuous phase structure to dispersion of smaller vulcanized rubber domains in the COPE matrix. Furthermore, the carbon black was well dispersed in the DR/COPE matrix up to 10 wt% and then the aggregates slightly occurred with increasing of carbon black loadings. In addition, dynamic experiments proved that a progressive non-linear behavior was more pronounced with increasing of carbon black loadings. Also, tan δ_max of the DR phase decreases with increasing carbon black concentration indicating mainly localization of filler in rubber phase. Moreover, it was found that increases of CB loadings resulted in increase of tensile strength and hardness while the elongation at break was slightly decreased. Additionally, the rate of stress relaxations was found to be increased with increasing CB loadings.

FATIGUE CHARACTERIZATION OF A THERMOPLASTIC ELASTOMER

The capacity to resist crack development in an olefinic thermoplastic elastomer (TPE) has been measured via a set of experiments that quantify (1) the fracture mechanical strength of the material under quasi-static loads, (2) the rate of growth of a crack under dynamic solicitations as a function of the energy release rate, and (3) the size of crack precursors in new material. Because the subject TPE exhibited strong inelasticity in the stress–strain response, it also was necessary to characterize the development of an inelastic set under cyclic loading as a function of the applied strain. Combined with the multiplicative kinematic split, this additional measurement yields the elastic part of the strain. It also enables engineering calculations to be made of fatigue life.

HIGH-TEMPERATURE THERMOPLASTIC ELASTOMERS FROM RUBBER–PLASTIC BLENDS: A STATE-OF-THE-ART REVIEW

This article reviews different types of high-temperature thermoplastic elastomers and thermoplastic vulcanizates from rubber-plastic blends. Preparation, structure, and properties of these materials are discussed briefly. Strategies to further improve the high-temperature performance of these materials are presented herein. A synopsis of the applications of these high-performance materials in the automotive industry is reported, pointing out the gaps to motivate potential research in this field.

Unique microstructure of an oil resistant nitrile butadiene rubber/polypropylene dynamically vulcanized thermoplastic elastomer

This paper reports on the microstructure, morphological evolution and the properties of oil resistant nitrile butadiene rubber (NBR)/polypropylene (PP) thermoplastic vulcanizates (TPVs) prepared by dynamical vulcanization (DV).

REVIEW OF THE RECLAIMING OF RUBBER WASTE AND RECENT WORK ON THE RECYCLING OF ETHYLENE–PROPYLENE–DIENE RUBBER WASTE

Rubbers do not decompose easily, and therefore, disposal of rubber waste is a serious environmental concern. Raw material costs, diminishing natural resources, and the growing awareness of environmental issues and sustainability have made rubber recycling a major area of concern. Reclaiming and recycling rubber waste is a major scientific and technological challenge facing rubber scientists today. This article reviews a number of important areas related to the reclaiming, characterizing, testing, and recycling of rubber waste. These include chemical and microbial devulcanization with particular emphasis on main chain scission and kinetics of chemical devulcanization reactions; the cutting-edge techniques for reclaiming devulcanized rubber waste by the action of large shearing forces, heat, and chemical agents: and analytical techniques and methods for characterizing composition and testing of devulcanized rubber waste, respectively. In addition, some aspects of the recycling of devulcanized ethylene–propylene–diene rubber (EPDM) waste will be reported. EPDM is used extensively in automotive components worldwide, and recycling the rubber at the end of its useful service life is of major importance to manufacturers of automotive components.

Properties and unique morphological evolution of dynamically vulcanized bromo-isobutylene-isoprene rubber/polypropylene thermoplastic elastomer

We studied the microstructure, morphological evolution and the corresponding mechanism, and the properties of BIIR/PP TPV.

Novel heat and oil-resistant thermoplastic vulcanizates based on ethylene-vinyl acetate rubber/poly(vinylidene fluoride)

Thermoplastic vulcanizates (TPVs) combine the excellent elasticity of conventional vulcanized rubbers and the easy processability and recyclability of thermoplastics.

Effect of cross-linking degree of EPDM phase on the electrical properties and formation of dual networks of thermoplastic vulcanizate composites based on isotactic polypropylene (iPP)/ethylene–propylene–diene rubber (EPDM) blends

Thermoplastic vulcanizates (TPVs), as a special class of high-performance thermoplastic elastomers, have been widely used in the automotive industry, building, and electronics due to their good processability and recyclability.

Microstructure and properties of bromo-isobutylene–isoprene rubber/polyamide 12 thermoplastic vulcanizate toward recyclable inner liners for green tires

Bromo-isobutylene–isoprene rubber (BIIR)/polyamide 12 (PA 12) thermoplastic vulcanizate (TPV) were prepared by dynamic vulcanization (DV) and the microstructure and properties of BIIR/PA 12 TPV toward recyclable green inner liners were studied.

EPDM RECYCLED RUBBER POWDER CHARACTERIZATION: THERMAL AND THERMOGRAVIMETRIC ANALYSIS

Investigations on ethylene–propylene–diene rubber (EPDM) with varying cross-link densities and carbon black contents, as well as commercial EPDM waste ground rubber (WGR), have been performed to improve characterization. The aim is to provide additional quality-control methods in the field of rubber recycling, as WGR is transformed and reused in the form of rubber mats mainly for playground, agricultural, sport, and automobile applications. Inductively coupled plasma optical emission spectrometry, atomic absorption spectrometry, and elemental (CHNS) analyses were used to determine the content of various chemical elements. Thermogravimetric analysis (TGA) was used to determine the amount of carbon black and inorganic material in the sample. Mass loss at 400 °C was related to cross-link density. TGA coupled with mass spectrometry showed that this mass loss corresponds to the loss of SO2 as cross-links are destroyed during heating. Melt point and glass transition temperatures determined by differential scanning calorimetry are mainly proportional to the ethylene/propylene weight ratio.

New understanding of microstructure formation of the rubber phase in thermoplastic vulcanizates (TPV)

The breakup of the rubber phase in an ethylene-propylene-diene monomer (EPDM)/polypropylene (PP) blend at the early stage of dynamic vulcanization is similar to that in an unvulcanized EPDM/PP blend because of the low crosslink density of the EPDM phase. In this work, the minimum size of the rubber phase in the unvulcanized EPDM/PP blend was first calculated by using the critical breakup law of viscoelastic droplets in a matrix. The calculated results showed that the minimum size of the rubber phase in the unvulcanized blend was in the nanometer scale (25-46 nm), not the micrometer scale as reported in many works. Meanwhile, the actual size of the rubber phase in the thermoplastic vulcanizate (TPV) at both the early stage and the final stage of dynamic vulcanization was observed by using peak force tapping atomic force microscopy (PF-AFM). The results indicated that the EPDM phase indeed broke up into nanoparticles at the early stage of dynamic vulcanization, in good agreement with the calculated results. More interestingly, we first revealed that the micrometer-sized rubber particles commonly observed in TPV were actually the agglomerates of rubber nanoparticles with diameters between 40 and 60 nm. The mechanism for the formation of rubber nanoparticles and their agglomerates during dynamic vulcanization was then discussed. Our work provides guidance to control the microstructure of the rubber phase in TPV to prepare high performance TPV products for a wide range of applications in the automobile and electronic industries.

MORPHOLOGY EVOLUTION AND THERMOMECHANICAL CHARACTERISTICS OF TPV NANOCOMPOSITES BASED ON PP/EPDM PREPARED BY REACTIVE EXTRUSION

The rubberlike behavior of thermoplastic vulcanizates (TPVs), based on polypropylene/ethylene–propylene–diene M-class rubber (PP/EPDM), and their corresponding nanocomposites was investigated. The temperature Scanning Stress Relaxation (TSSR) method was used. Polypropylene nanocomposites, with different nanoclay dispersion levels, ranging from intercalated structures to a mixture of intercalated montmorillonite tactoids and exfoliated layers were used as the thermoplastic phase. A phenolic curing system was used to cross-link the rubber phase during the reactive extrusion process. The TSSR method provided useful information about the extent of the cross-linking reaction. Based on mechanical properties and the different criteria obtained by TSSR analysis, it was found that the presence of nanoclay would be more beneficial at high EPDM content in which the morphological studies suggested the existence of a network consisting of irregular-shaped rubber particles near a few rubber droplets.